194 results on '"Timoshenko, Janis"'
Search Results
152. Tracking the Evolution of Single-Atom Catalysts for the CO2Electrocatalytic Reduction Using Operando X-ray Absorption Spectroscopy and Machine Learning
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Martini, Andrea, Hursán, Dorottya, Timoshenko, Janis, Rüscher, Martina, Haase, Felix, Rettenmaier, Clara, Ortega, Eduardo, Etxebarria, Ane, and Roldan Cuenya, Beatriz
- Abstract
Transition metal-nitrogen-doped carbons (TMNCs) are a promising class of catalysts for the CO2electrochemical reduction reaction. In particular, high CO2-to-CO conversion activities and selectivities were demonstrated for Ni-based TMNCs. Nonetheless, open questions remain about the nature, stability, and evolution of the Ni active sites during the reaction. In this work, we address this issue by combining operando X-ray absorption spectroscopy with advanced data analysis. In particular, we show that the combination of unsupervised and supervised machine learning approaches is able to decipher the X-ray absorption near edge structure (XANES) of the TMNCs, disentangling the contributions of different metal sites coexisting in the working TMNC catalyst. Moreover, quantitative structural information about the local environment of active species, including their interaction with adsorbates, has been obtained, shedding light on the complex dynamic mechanism of the CO2electroreduction.
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- 2023
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153. Shape-Dependent CO2Hydrogenation to Methanol over Cu2O Nanocubes Supported on ZnO
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Kordus, David, Jelic, Jelena, Lopez Luna, Mauricio, Divins, Núria J., Timoshenko, Janis, Chee, See Wee, Rettenmaier, Clara, Kröhnert, Jutta, Kühl, Stefanie, Trunschke, Annette, Schlögl, Robert, Studt, Felix, and Roldan Cuenya, Beatriz
- Abstract
The hydrogenation of CO2to methanol over Cu/ZnO-based catalysts is highly sensitive to the surface composition and catalyst structure. Thus, its optimization requires a deep understanding of the influence of the pre-catalyst structure on its evolution under realistic reaction conditions, including the formation and stabilization of the most active sites. Here, the role of the pre-catalyst shape (cubic vs spherical) in the activity and selectivity of ZnO-supported Cu nanoparticles was investigated during methanol synthesis. A combination of ex situ, in situ, and operandomicroscopy, spectroscopy, and diffraction methods revealed drastic changes in the morphology and composition of the shaped pre-catalysts under reaction conditions. In particular, the rounding of the cubes and partial loss of the (100) facets were observed, although such motifs remained in smaller domains. Nonetheless, the initial pre-catalyst structure was found to strongly affect its subsequent transformation in the course of the CO2hydrogenation reaction and activity/selectivity trends. In particular, the cubic Cu particles displayed an increased activity for methanol production, although at the cost of a slightly reduced selectivity when compared to similarly sized spherical particles. These findings were rationalized with the help of density functional theory calculations.
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- 2023
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154. A combined theoretical and experimental EXAFS study of the structure and dynamics of Au147nanoparticles
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DuanThese authors contributed equally to this work., Zhiyao, Li, Yuanyuan, Timoshenko, Janis, Chill, Samuel T., Anderson, Rachel M., Yancey, David F., Frenkel, Anatoly I., Crooks, Richard M., and Henkelman, Graeme
- Abstract
In this study, we present a framework for characterizing the structural and thermal properties of small nanoparticle catalysts by combining precise synthesis, extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) calculations. We demonstrate the capability of this approach by characterizing the atomic structure and vibrational dynamics of Au147. With the combination of EXAFS spectroscopy and DFT, the synthesized Au147nanoparticles are determined to have an icosahedral structure. A decrease in the Einstein temperature of the Au147particles compared to their bulk value was observed and interpreted in terms of softer vibration modes of surface bonds.
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- 2016
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155. Local structure and dynamics of wurtzite-type ZnO from simulation-based EXAFS analysis.
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Timoshenko, Janis, Anspoks, Andris, Kalinko, Aleksandr, and Kuzmin, Alexei
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MULTIPLE scattering (Physics) , *MOLECULAR dynamics , *EVOLUTIONARY algorithms , *WURTZITE , *ZINC oxide , *PIEZOELECTRIC materials research , *NANOSTRUCTURED materials - Abstract
Conventional methods of EXAFS data analysis are often limited to the nearest coordination shells of the absorbing atom due to the difficulties in accurate accounting for the so-called multiple-scattering effects. Besides, it is often difficult to resolve the non-equivalent groups of atoms in a single coordination shell due to strong correlation between structural parameters. In this study we overcome these problems by applying two different simulation-based methods, i.e., classical molecular dynamics (MD) and reverse Monte with evolutionary algorithm (EA), to the analysis of the Zn K-edge EXAFS data for wurtzite-type bulk ZnO. The RMC/EA-EXAFS method allowed us to separate the contributions of thermal disorder and the effect of noncentrosymmetric zinc oxide structure, being responsible for its piezoelectrical and pyroelectrical properties. The MD-EXAFS method allowed us to test the accuracy of several available force-field models, which are commonly used in the MD simulations of ZnO nanostructures. [ABSTRACT FROM AUTHOR]
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- 2014
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156. Molecular dynamics simulations of EXAFS in germanium.
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Timoshenko, Janis, Kuzmin, Alexei, and Purans, Juris
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Classical molecular dynamics simulations have been performed for crystalline germanium with the aim to estimate the thermal effects within the first three coordination shells and their influence on the single-scattering and multiple-scattering contributions to the Ge K-edge extended x-ray absorption fine structure (EXAFS). [ABSTRACT FROM AUTHOR]
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- 2011
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157. Role of Nanoscale Inhomogeneities in Co2FeO4Catalysts during the Oxygen Evolution Reaction
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Haase, Felix Thomas, Rabe, Anna, Schmidt, Franz-Philipp, Herzog, Antonia, Jeon, Hyo Sang, Frandsen, Wiebke, Narangoda, Praveen Vidusha, Spanos, Ioannis, Friedel Ortega, Klaus, Timoshenko, Janis, Lunkenbein, Thomas, Behrens, Malte, Bergmann, Arno, Schlögl, Robert, and Roldan Cuenya, Beatriz
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Spinel-type catalysts are promising anode materials for the alkaline oxygen evolution reaction (OER), exhibiting low overpotentials and providing long-term stability. In this study, we compared two structurally equal Co2FeO4spinels with nominally identical stoichiometry and substantially different OER activities. In particular, one of the samples, characterized by a metastable precatalyst state, was found to quickly achieve its steady-state optimum operation, while the other, which was initially closer to the ideal crystallographic spinel structure, never reached such a state and required 168 mV higher potential to achieve 1 mA/cm2. In addition, the enhanced OER activity was accompanied by a larger resistance to corrosion. More specifically, using various ex situ, quasi in situ, and operandomethods, we could identify a correlation between the catalytic activity and compositional inhomogeneities resulting in an X-ray amorphous Co2+-rich minority phase linking the crystalline spinel domains in the as-prepared state. OperandoX-ray absorption spectroscopy revealed that these Co2+-rich domains transform during OER to structurally different Co3+-rich domains. These domains appear to be crucial for enhancing OER kinetics while exhibiting distinctly different redox properties. Our work emphasizes the necessity of the operandomethodology to gain fundamental insight into the activity-determining properties of OER catalysts and presents a promising catalyst concept in which a stable, crystalline structure hosts the disordered and active catalyst phase.
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- 2022
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158. Origin of Pressure‐Induced Metallization in Cu3N: An X‐ray Absorption Spectroscopy Study (Phys. Status Solidi B 11/2018).
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Kuzmin, Alexei, Anspoks, Andris, Kalinko, Aleksandr, Timoshenko, Janis, Nataf, Lucie, Baudelet, François, and Irifune, Tetsuo
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PHASE transitions ,METALLIZING ,X-ray absorption - Abstract
X‐ray absorption spectroscopy is a powerful tool to probe in situ changes in the local structure of a material induced by pressure. In article no. 1800073 it was used by Kuzmin et al. to study the behavior of copper nitride (Cu3N) crystal lattice at high pressure (0–26.7 GPa). The analysis of the Cu K‐edge X‐ray absorption near‐edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS) suggests that at all pressures the local atomic environment around copper atoms remains close to that in cubic Pm‐3m phase. At the same time, a reduction of the lattice parameter of Cu3N by about 2% was found upon increasing pressure up to 26.7 GPa. These results indicate that the transition to the metal state in Cu3N above 5 GPa, observed previously using pressure‐dependent electrical resistance and optical absorption measurements, is due to the band gap collapse with a decrease of the unit cell volume. [ABSTRACT FROM AUTHOR]
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- 2018
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159. Selectivity Control of Cu Nanocrystals in a Gas-Fed Flow Cell through CO$_2$ Pulsed Electroreduction
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Jeon, Hyo Sang, Timoshenko, Janis, Rettenmaier, Clara, Herzog, Antonia, Yoon, Aram, Chee, See Wee, Oener, Sebastian, Hejral, Uta, Haase, Felix T., and Roldan Cuenya, Beatriz
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13. Climate action - Abstract
Journal of the American Chemical Society 143(19), 7578-7587 (2021). doi:10.1021/jacs.1c03443, In this study, we have taken advantage of a pulsed CO$_2$ electroreduction reaction (CO$_2$RR) approach to tune the product distribution at industrially relevant current densities in a gas-fed flow cell. We compared the CO$_2$RR selectivity of Cu catalysts subjected to either potentiostatic conditions (fixed applied potential of −0.7 V$_{RHE}$) or pulsed electrolysis conditions (1 s pulses at oxidative potentials ranging from $E_{an}$ = 0.6 to 1.5 V$_{RHE}$, followed by 1 s pulses at −0.7 VRHE) and identified the main parameters responsible for the enhanced product selectivity observed in the latter case. Herein, two distinct regimes were observed: (i) for $E_{an}$ = 0.9 V$_{RHE}$ we obtained 10% enhanced C$_2$ product selectivity (FE$_{C_2H_4}$ = 43.6% and FE$_{C_2H_5OH}$ = 19.8%) in comparison to the potentiostatic CO$_2$RR at −0.7 V$_{RHE}$ (FE$_{C_2H_4}$ = 40.9% and FE$_{C_2H_5OH}$ = 11%), (ii) while for $E_{an}$ = 1.2 V$_{RHE}$, high CH$_4$ selectivity (FE$_{CH_4}$ = 48.3% vs 0.1% at constant −0.7 V$_{RHE}$) was observed. Operando spectroscopy (XAS, SERS) and ex situ microscopy (SEM and TEM) measurements revealed that these differences in catalyst selectivity can be ascribed to structural modifications and local pH effects. The morphological reconstruction of the catalyst observed after pulsed electrolysis with $E_{an}$ = 0.9 V$_{RHE}$, including the presence of highly defective interfaces and grain boundaries, was found to play a key role in the enhancement of the C$_2$ product formation. In turn, pulsed electrolysis with $E_{an}$ = 1.2 V$_{RHE}$ caused the consumption of OH$^–$ species near the catalyst surface, leading to an OH-poor environment favorable for CH$_4$ production., Published by American Chemical Society, Washington, DC
160. Thermal disorder and correlation effects in anti-perovskite-type copper nitride
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Timoshenko, Janis, Anspoks, Andris, Kalinko, Alexandr, and Kuzmin, Alexei
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3. Good health - Abstract
Acta materialia 129, 61 - 71 (2017). doi:10.1016/j.actamat.2017.02.074, Reverse Monte Carlo simulations coupled with evolutionary algorithm were employed for the analysis of the temperature dependent (10–300 K) Cu K-edge extended X-ray absorption fine structure (EXAFS) spectra of polycrystalline copper nitride (Cu$_3$N) with the goal to extract information on the thermal disorder and interatomic correlations in anti-perovskite-type crystal lattice. The obtained results are discussed in comparison with metallic copper and perovskite-type rhenium trioxide. The analysis of EXAFS spectra suggests that the anisotropy of copper atom vibrations is significantly enhanced upon increasing temperature, leading to pronounced tilting motion of NCu$_6$ octahedra. Strong correlation in the motion of atoms was found along –N–Cu–N– atomic chains but it reduces rapidly with an increase of interatomic distance. Finally, anticorrelated motion of neighboring Cu atoms occurs along Cu–Cu bonds and is consistent with breathing-type motion of NCu$_6$ octahedra., Published by Elsevier Science, Amsterdam [u.a.]
161. Temperature-dependent EXAFS study of the local structure and lattice dynamics in cubic Y2O3.
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Jonane, Inga, Lazdins, Karlis, Timoshenko, Janis, Kuzmin, Alexei, Purans, Juris, Vladimirov, Pavel, Gräning, Tim, and Hoffmann, Jan
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LATTICE dynamics , *X-ray absorption , *TEMPERATURE - Abstract
The local structure and lattice dynamics in cubic Y2O3 were studied at the Y K-edge by X-ray absorption spectroscopy in the temperature range from 300 to 1273 K. The temperature dependence of the extended X-ray absorption fine structure was successfully interpreted using classical molecular dynamics and a novel reverse Monte Carlo method, coupled with the evolutionary algorithm. The obtained results allowed the temperature dependence of the yttria atomic structure to be followed up to ∼6 Å and to validate two force-field models. [ABSTRACT FROM AUTHOR]
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- 2016
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162. Local structure of A-atom in ABO3 perovskites studies by RMC-EXAFS.
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Anspoks, Andris, Marini, Carlo, Miyanaga, Takafumi, Joseph, Boby, Kuzmin, Alexei, Purans, Juris, Timoshenko, Janis, and Bussmann-Holder, Annette
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X-ray absorption spectra , *EXTENDED X-ray absorption fine structure , *X-ray absorption near edge structure , *RADIAL distribution function , *MONTE Carlo method , *X-ray absorption - Abstract
The ferroelectric distortions in perovskites were a subject of numerous investigations for a long time. However, some controversial results still exist, coming from the analysis of diffraction (X-ray, neutron or electron) data and X-ray absorption spectra. In this study, our goal was to revisit these classical materials using recently developed methods without imposing any predefined structural model. Local environment around A-type atom in ABO 3 perovskites (SrTiO 3 , BaTiO 3 , EuTiO 3) was studied by X-ray absorption spectroscopy (XAS) in a wide range of temperatures (20–400 K). Using reverse Monte Carlo method enhanced by evolutionary algorithm, the 3D structure was extracted from the extended X-ray absorption fine structure (EXAFS) and interpreted in terms of the radial distribution functions (RDFs). Our findings show that both diffraction and XAS data are consistent, but reflect the structure of the material from different points of view. In particular, when strong correlations in the motion of certain atoms are present, the information obtained by XAS might lead to a different from expected shape of the RDF. At the same time, the average positions of all atoms are in good agreement with those given by diffraction. This makes XAS an important technique for studying interatomic correlations and lattice dynamics. • X-ray absorption spectra for BaTiO 3 , SrTiO 3 , EuTiO 3 analysed by reverse Monte Carlo. • Strong interatomic correlations lead to a different from expected shape of the RDF. • The average positions of all atoms agree with data from diffraction. [ABSTRACT FROM AUTHOR]
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- 2020
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163. Reversible metal cluster formation on Nitrogen-doped carbon controlling electrocatalyst particle size with subnanometer accuracy.
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Timoshenko J, Rettenmaier C, Hursán D, Rüscher M, Ortega E, Herzog A, Wagner T, Bergmann A, Hejral U, Yoon A, Martini A, Liberra E, Monteiro MCO, and Cuenya BR
- Abstract
Copper and nitrogen co-doped carbon catalysts exhibit a remarkable behavior during the electrocatalytic CO
2 reduction (CO2 RR), namely, the formation of metal nanoparticles from Cu single atoms, and their subsequent reversible redispersion. Here we show that the switchable nature of these species holds the key for the on-demand control over the distribution of CO2 RR products, a lack of which has thus far hindered the wide-spread practical adoption of CO2 RR. By intermitting pulses of a working cathodic potential with pulses of anodic potential, we were able to achieve a controlled fragmentation of the Cu particles and partial regeneration of single atom sites. By tuning the pulse durations, and by tracking the catalyst's evolution using operando quick X-ray absorption spectroscopy, the speciation of the catalyst can be steered toward single atom sites, ultrasmall metal clusters or large metal nanoparticles, each exhibiting unique CO2 RR functionalities., (© 2024. The Author(s).)- Published
- 2024
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164. Revealing the structure of the active sites for the electrocatalytic CO 2 reduction to CO over Co single atom catalysts using operando XANES and machine learning.
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Martini A, Timoshenko J, Rüscher M, Hursán D, Monteiro MCO, Liberra E, and Roldan Cuenya B
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Transition-metal nitrogen-doped carbons (TM-N-C) are emerging as a highly promising catalyst class for several important electrocatalytic processes, including the electrocatalytic CO
2 reduction reaction (CO2 RR). The unique local environment around the singly dispersed metal site in TM-N-C catalysts is likely to be responsible for their catalytic properties, which differ significantly from those of bulk or nanostructured catalysts. However, the identification of the actual working structure of the main active units in TM-N-C remains a challenging task due to the fluctional, dynamic nature of these catalysts, and scarcity of experimental techniques that could probe the structure of these materials under realistic working conditions. This issue is addressed in this work and the local atomistic and electronic structure of the metal site in a Co-N-C catalyst for CO2 RR is investigated by employing time-resolved operando X-ray absorption spectroscopy (XAS) combined with advanced data analysis techniques. This multi-step approach, based on principal component analysis, spectral decomposition and supervised machine learning methods, allows the contributions of several co-existing species in the working Co-N-C catalysts to be decoupled, and their XAS spectra deciphered, paving the way for understanding the CO2 RR mechanisms in the Co-N-C catalysts, and further optimization of this class of electrocatalytic systems., (open access.)- Published
- 2024
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165. Synergizing Fe 2 O 3 Nanoparticles on Single Atom Fe-N-C for Nitrate Reduction to Ammonia at Industrial Current Densities.
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Murphy E, Sun B, Rüscher M, Liu Y, Zang W, Guo S, Chen YH, Hejral U, Huang Y, Ly A, Zenyuk IV, Pan X, Timoshenko J, Cuenya BR, Spoerke ED, and Atanassov P
- Abstract
The electrochemical reduction of nitrates (NO
3 - ) enables a pathway for the carbon neutral synthesis of ammonia (NH3 ), via the nitrate reduction reaction (NO3 RR), which has been demonstrated at high selectivity. However, to make NH3 synthesis cost-competitive with current technologies, high NH3 partial current densities (jNH3 ) must be achieved to reduce the levelized cost of NH3 . Here, the high NO3 RR activity of Fe-based materials is leveraged to synthesize a novel active particle-active support system with Fe2 O3 nanoparticles supported on atomically dispersed Fe-N-C. The optimized 3×Fe2 O3 /Fe-N-C catalyst demonstrates an ultrahigh NO3 RR activity, reaching a maximum jNH3 of 1.95 A cm-2 at a Faradaic efficiency (FE) for NH3 of 100% and an NH3 yield rate over 9 mmol hr-1 cm-2 . Operando XANES and post-mortem XPS reveal the importance of a pre-reduction activation step, reducing the surface Fe2 O3 (Fe3+ ) to highly active Fe0 sites, which are maintained during electrolysis. Durability studies demonstrate the robustness of both the Fe2 O3 particles and Fe-Nx sites at highly cathodic potentials, maintaining a current of -1.3 A cm-2 over 24 hours. This work exhibits an effective and durable active particle-active support system enhancing the performance of the NO3 RR, enabling industrially relevant current densities and near 100% selectivity., (© 2024 The Authors. Advanced Materials published by Wiley‐VCH GmbH.)- Published
- 2024
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166. Electrocatalytic Nitrate and Nitrite Reduction toward Ammonia Using Cu 2 O Nanocubes: Active Species and Reaction Mechanisms.
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Bai L, Franco F, Timoshenko J, Rettenmaier C, Scholten F, Jeon HS, Yoon A, Rüscher M, Herzog A, Haase FT, Kühl S, Chee SW, Bergmann A, and Beatriz RC
- Abstract
The electrochemical reduction of nitrate (NO
3 - ) and nitrite (NO2 - ) enables sustainable, carbon-neutral, and decentralized routes to produce ammonia (NH3 ). Copper-based materials are promising electrocatalysts for NOx - conversion to NH3 . However, the underlying reaction mechanisms and the role of different Cu species during the catalytic process are still poorly understood. Herein, by combining quasi in situ X-ray photoelectron spectroscopy (XPS) and operando X-ray absorption spectroscopy (XAS), we unveiled that Cu is mostly in metallic form during the highly selective reduction of NO3 - /NO2 - to NH3 . On the contrary, Cu(I) species are predominant in a potential region where the two-electron reduction of NO3 - to NO2 - is the major reaction. Electrokinetic analysis and in situ Raman spectroscopy was also used to propose possible steps and intermediates leading to NO2 - and NH3 , respectively. This work establishes a correlation between the catalytic performance and the dynamic changes of the chemical state of Cu, and provides crucial mechanistic insights into the pathways for NO3 - /NO2 - electrocatalytic reduction.- Published
- 2024
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167. Enhanced Methanol Synthesis from CO 2 Hydrogenation Achieved by Tuning the Cu-ZnO Interaction in ZnO/Cu 2 O Nanocube Catalysts Supported on ZrO 2 and SiO 2 .
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Kordus D, Widrinna S, Timoshenko J, Lopez Luna M, Rettenmaier C, Chee SW, Ortega E, Karslioglu O, Kühl S, and Roldan Cuenya B
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The nature of the Cu-Zn interaction and especially the role of Zn in Cu/ZnO catalysts used for methanol synthesis from CO
2 hydrogenation are still debated. Migration of Zn onto the Cu surface during reaction results in a Cu-ZnO interface, which is crucial for the catalytic activity. However, whether a Cu-Zn alloy or a Cu-ZnO structure is formed and the transformation of this interface under working conditions demand further investigation. Here, ZnO/Cu2 O core-shell cubic nanoparticles with various ZnO shell thicknesses, supported on SiO2 or ZrO2 were prepared to create an intimate contact between Cu and ZnO. The evolution of the catalyst's structure and composition during and after the CO2 hydrogenation reaction were investigated by means of operando spectroscopy, diffraction, and ex situ microscopy methods. The Zn loading has a direct effect on the oxidation state of Zn, which, in turn, affects the catalytic performance. High Zn loadings, resulting in a stable ZnO catalyst shell, lead to increased methanol production when compared to Zn-free particles. Low Zn loadings, in contrast, leading to the presence of metallic Zn species during reaction, showed no significant improvement over the bare Cu particles. Therefore, our work highlights that there is a minimum content of Zn (or optimum ZnO shell thickness) needed to activate the Cu catalyst. Furthermore, in order to minimize catalyst deactivation, the Zn species must be present as ZnOx and not metallic Zn or Cu-Zn alloy, which is undesirably formed during the reaction when the precatalyst ZnO overlayer is too thin.- Published
- 2024
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168. Role of Fe decoration on the oxygen evolving state of Co 3 O 4 nanocatalysts.
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Haase FT, Ortega E, Saddeler S, Schmidt FP, Cruz D, Scholten F, Rüscher M, Martini A, Jeon HS, Herzog A, Hejral U, Davis EM, Timoshenko J, Knop-Gericke A, Lunkenbein T, Schulz S, Bergmann A, and Roldan Cuenya B
- Abstract
The production of green hydrogen through alkaline water electrolysis is the key technology for the future carbon-neutral industry. Nanocrystalline Co
3 O4 catalysts are highly promising electrocatalysts for the oxygen evolution reaction and their activity strongly benefits from Fe surface decoration. However, limited knowledge of decisive catalyst motifs at the atomic level during oxygen evolution prevents their knowledge-driven optimization. Here, we employ a variety of operando spectroscopic methods to unveil how Fe decoration increases the catalytic activity of Co3 O4 nanocatalysts as well as steer the (near-surface) active state formation. Our study shows a link of the termination-dependent Fe decoration to the activity enhancement and a significantly stronger Co3 O4 near-surface (structural) adaptation under the reaction conditions. The near-surface Fe- and Co-O species accumulate an oxidative charge and undergo a reversible bond contraction during the catalytic process. Moreover, our work demonstrates the importance of low coordination surface sites on the Co3 O4 host to ensure an efficient Fe-induced activity enhancement, providing another puzzle piece to facilitate optimized catalyst design., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)- Published
- 2024
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169. Reversible Structural Evolution of Metal-Nitrogen-Doped Carbon Catalysts During CO 2 Electroreduction: An Operando X-ray Absorption Spectroscopy Study.
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Hursán D, Timoshenko J, Ortega E, Jeon HS, Rüscher M, Herzog A, Rettenmaier C, Chee SW, Martini A, Koshy D, and Roldán Cuenya B
- Abstract
Electrochemical CO
2 reduction (CO2 RR) is a rising technology, aiming to reduce the energy sector dependence on fossil fuels and to produce carbon-neutral raw materials. Metal-nitrogen-doped carbons (M-N-C) are emerging, cost-effective catalysts for this reaction; however, their long-term stability is a major issue. To overcome this, understanding their structural evolution is crucial, requiring systematic in-depth operando studies. Here a series of M-N-C catalysts (M = Fe, Sn, Cu, Co, Ni, Zn) is investigated using operando X-ray absorption spectroscopy. It is found that the Fe-N-C and Sn-N-C are prone to oxide clusters formation even before CO2 RR. In contrast, the respective metal cations are singly dispersed in the as-prepared Cu-N-C, Co-N-C, Ni-N-C, and (Zn)-N-C. During CO2 RR, metallic clusters/nanoparticles reversibly formed in all catalysts, except for the Ni-N-C. This phenomenon, previously observed only in Cu-N-C, thus is ubiquitous in M-N-C catalysts. The competition between M-O and M-N interactions is an important factor determining the mobility of metal species in M-N-C. Specifically, the strong interaction between the Ni centers and the N-functional groups of the carbon support results in higher stability of the Ni single-sites, leading to the excellent performance of Ni-N-C in the CO2 to CO conversion, in comparison to other transition metals., (© 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.)- Published
- 2024
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170. Dynamic behaviour of platinum and copper dopants in gold nanoclusters supported on ceria catalysts.
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Müller N, Banu R, Loxha A, Schrenk F, Lindenthal L, Rameshan C, Pittenauer E, Llorca J, Timoshenko J, Marini C, and Barrabés N
- Abstract
Understanding the behaviour of active catalyst sites at the atomic level is crucial for optimizing catalytic performance. Here, the evolution of Pt and Cu dopants in Au
25 clusters on CeO2 supports is investigated in the water-gas shift (WGS) reaction, using operando XAFS and DRIFTS. Different behaviour is observed for the Cu and Pt dopants during the pretreatment and reaction. The Cu migrates and builds clusters on the support, whereas the Pt creates single-atom active sites on the surface of the cluster, leading to better performance. Doping with both metals induces strong interactions and pretreatment and reaction conditions lead to the growth of the Au clusters, thereby affecting their catalytic behaviour. This highlights importance of understanding the behaviour of atoms at different stages of catalyst evolution. These insights into the atomic dynamics at the different stages are crucial for the precise optimisation of catalysts, which ultimately enables improved catalytic performance., (© 2023. The Author(s).)- Published
- 2023
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171. Operando insights into correlating CO coverage and Cu-Au alloying with the selectivity of Au NP-decorated Cu 2 O nanocubes during the electrocatalytic CO 2 reduction.
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Rettenmaier C, Herzog A, Casari D, Rüscher M, Jeon HS, Kordus D, Luna ML, Kühl S, Hejral U, Davis EM, Chee SW, Timoshenko J, Alexander DTL, Bergmann A, and Cuenya BR
- Abstract
Electrochemical reduction of CO
2 (CO2 RR) is an attractive technology to reintegrate the anthropogenic CO2 back into the carbon cycle driven by a suitable catalyst. This study employs highly efficient multi-carbon (C2+ ) producing Cu2 O nanocubes (NCs) decorated with CO-selective Au nanoparticles (NPs) to investigate the correlation between a high CO surface concentration microenvironment and the catalytic performance. Structure, morphology and near-surface composition are studied via operando X-ray absorption spectroscopy and surface-enhanced Raman spectroscopy, operando high-energy X-ray diffraction as well as quasi in situ X-ray photoelectron spectroscopy. These operando studies show the continuous evolution of the local structure and chemical environment of our catalysts during reaction conditions. Along with its alloy formation, a CO-rich microenvironment as well as weakened average CO binding on the catalyst surface during CO2 RR is detected. Linking these findings to the catalytic function, a complex compositional interplay between Au and Cu is revealed in which higher Au loadings primarily facilitate CO formation. Nonetheless, the strongest improvement in C2+ formation appears for the lowest Au loadings, suggesting a beneficial role of the Au-Cu atomic interaction for the catalytic function in CO2 RR. This study highlights the importance of site engineering and operando investigations to unveil the electrocatalyst's adaptations to the reaction conditions, which is a prerequisite to understand its catalytic behavior., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)- Published
- 2023
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172. Spatially and Chemically Resolved Visualization of Fe Incorporation into NiO Octahedra during the Oxygen Evolution Reaction.
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Yang F, Lopez Luna M, Haase FT, Escalera-López D, Yoon A, Rüscher M, Rettenmaier C, Jeon HS, Ortega E, Timoshenko J, Bergmann A, Chee SW, and Roldan Cuenya B
- Abstract
The activity of Ni (hydr)oxides for the electrochemical evolution of oxygen (OER), a key component of the overall water splitting reaction, is known to be greatly enhanced by the incorporation of Fe. However, a complete understanding of the role of cationic Fe species and the nature of the catalyst surface under reaction conditions remains unclear. Here, using a combination of electrochemical cell and conventional transmission electron microscopy, we show how the surface of NiO electrocatalysts, with initially well-defined surface facets, restructures under applied potential and forms an active NiFe layered double (oxy)hydroxide (NiFe-LDH) when Fe
3+ ions are present in the electrolyte. Continued OER under these conditions, however, leads to the creation of additional FeOx aggregates. Electrochemically, the NiFe-LDH formation correlates with a lower onset potential toward the OER, whereas the formation of the FeOx aggregates is accompanied by a gradual decrease in the OER activity. Complementary insight into the catalyst near-surface composition, structure, and chemical state is further extracted using X-ray photoelectron spectroscopy, operando Raman spectroscopy, and operando X-ray absorption spectroscopy together with measurements of Fe uptake by the electrocatalysts using time-resolved inductively coupled plasma mass spectrometry. Notably, we identified that the catalytic deactivation under stationary conditions is linked to the degradation of in situ-created NiFe-LDH. These insights exemplify the complexity of the active state formation and show how its structural and morphological evolution under different applied potentials can be directly linked to the catalyst activation and degradation.- Published
- 2023
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173. Tracking the Evolution of Single-Atom Catalysts for the CO 2 Electrocatalytic Reduction Using Operando X-ray Absorption Spectroscopy and Machine Learning.
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Martini A, Hursán D, Timoshenko J, Rüscher M, Haase F, Rettenmaier C, Ortega E, Etxebarria A, and Roldan Cuenya B
- Abstract
Transition metal-nitrogen-doped carbons (TMNCs) are a promising class of catalysts for the CO
2 electrochemical reduction reaction. In particular, high CO2 -to-CO conversion activities and selectivities were demonstrated for Ni-based TMNCs. Nonetheless, open questions remain about the nature, stability, and evolution of the Ni active sites during the reaction. In this work, we address this issue by combining operando X-ray absorption spectroscopy with advanced data analysis. In particular, we show that the combination of unsupervised and supervised machine learning approaches is able to decipher the X-ray absorption near edge structure (XANES) of the TMNCs, disentangling the contributions of different metal sites coexisting in the working TMNC catalyst. Moreover, quantitative structural information about the local environment of active species, including their interaction with adsorbates, has been obtained, shedding light on the complex dynamic mechanism of the CO2 electroreduction.- Published
- 2023
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174. Elucidating electrochemical nitrate and nitrite reduction over atomically-dispersed transition metal sites.
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Murphy E, Liu Y, Matanovic I, Rüscher M, Huang Y, Ly A, Guo S, Zang W, Yan X, Martini A, Timoshenko J, Cuenya BR, Zenyuk IV, Pan X, Spoerke ED, and Atanassov P
- Abstract
Electrocatalytic reduction of waste nitrates (NO
3 - ) enables the synthesis of ammonia (NH3 ) in a carbon neutral and decentralized manner. Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts demonstrate a high catalytic activity and uniquely favor mono-nitrogen products. However, the reaction fundamentals remain largely underexplored. Herein, we report a set of 14; 3d-, 4d-, 5d- and f-block M-N-C catalysts. The selectivity and activity of NO3 - reduction to NH3 in neutral media, with a specific focus on deciphering the role of the NO2 - intermediate in the reaction cascade, reveals strong correlations (R=0.9) between the NO2 - reduction activity and NO3 - reduction selectivity for NH3 . Moreover, theoretical computations reveal the associative/dissociative adsorption pathways for NO2 - evolution, over the normal M-N4 sites and their oxo-form (O-M-N4 ) for oxyphilic metals. This work provides a platform for designing multi-element NO3 RR cascades with single-atom sites or their hybridization with extended catalytic surfaces., (© 2023. The Author(s).)- Published
- 2023
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175. Deciphering the Structural and Chemical Transformations of Oxide Catalysts during Oxygen Evolution Reaction Using Quick X-ray Absorption Spectroscopy and Machine Learning.
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Timoshenko J, Haase FT, Saddeler S, Rüscher M, Jeon HS, Herzog A, Hejral U, Bergmann A, Schulz S, and Roldan Cuenya B
- Abstract
Bimetallic transition-metal oxides, such as spinel-like Co
x Fe3- x O4 materials, are known as attractive catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. Nonetheless, unveiling the real active species and active states in these catalysts remains a challenge. The coexistence of metal ions in different chemical states and in different chemical environments, including disordered X-ray amorphous phases that all evolve under reaction conditions, hinders the application of common operando techniques. Here, we address this issue by relying on operando quick X-ray absorption fine structure spectroscopy, coupled with unsupervised and supervised machine learning methods. We use principal component analysis to understand the subtle changes in the X-ray absorption near-edge structure spectra and develop an artificial neural network to decipher the extended X-ray absorption fine structure spectra. This allows us to separately track the evolution of tetrahedrally and octahedrally coordinated species and to disentangle the chemical changes and several phase transitions taking place in Cox Fe3- x O4 catalysts and on their active surface, related to the conversion of disordered oxides into spinel-like structures, transformation of spinels into active oxyhydroxides, and changes in the degree of spinel inversion in the course of the activation treatment and under OER conditions. By correlating the revealed structural changes with the distinct catalytic activity for a series of Cox Fe3- x O4 samples, we elucidate the active species and OER mechanism.- Published
- 2023
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176. Shape-Dependent CO 2 Hydrogenation to Methanol over Cu 2 O Nanocubes Supported on ZnO.
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Kordus D, Jelic J, Lopez Luna M, Divins NJ, Timoshenko J, Chee SW, Rettenmaier C, Kröhnert J, Kühl S, Trunschke A, Schlögl R, Studt F, and Roldan Cuenya B
- Abstract
The hydrogenation of CO
2 to methanol over Cu/ZnO-based catalysts is highly sensitive to the surface composition and catalyst structure. Thus, its optimization requires a deep understanding of the influence of the pre-catalyst structure on its evolution under realistic reaction conditions, including the formation and stabilization of the most active sites. Here, the role of the pre-catalyst shape (cubic vs spherical) in the activity and selectivity of ZnO-supported Cu nanoparticles was investigated during methanol synthesis. A combination of ex situ , in situ , and operando microscopy, spectroscopy, and diffraction methods revealed drastic changes in the morphology and composition of the shaped pre-catalysts under reaction conditions. In particular, the rounding of the cubes and partial loss of the (100) facets were observed, although such motifs remained in smaller domains. Nonetheless, the initial pre-catalyst structure was found to strongly affect its subsequent transformation in the course of the CO2 hydrogenation reaction and activity/selectivity trends. In particular, the cubic Cu particles displayed an increased activity for methanol production, although at the cost of a slightly reduced selectivity when compared to similarly sized spherical particles. These findings were rationalized with the help of density functional theory calculations.- Published
- 2023
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177. Role of Nanoscale Inhomogeneities in Co 2 FeO 4 Catalysts during the Oxygen Evolution Reaction.
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Haase FT, Rabe A, Schmidt FP, Herzog A, Jeon HS, Frandsen W, Narangoda PV, Spanos I, Friedel Ortega K, Timoshenko J, Lunkenbein T, Behrens M, Bergmann A, Schlögl R, and Roldan Cuenya B
- Abstract
Spinel-type catalysts are promising anode materials for the alkaline oxygen evolution reaction (OER), exhibiting low overpotentials and providing long-term stability. In this study, we compared two structurally equal Co
2 FeO4 spinels with nominally identical stoichiometry and substantially different OER activities. In particular, one of the samples, characterized by a metastable precatalyst state, was found to quickly achieve its steady-state optimum operation, while the other, which was initially closer to the ideal crystallographic spinel structure, never reached such a state and required 168 mV higher potential to achieve 1 mA/cm2 . In addition, the enhanced OER activity was accompanied by a larger resistance to corrosion. More specifically, using various ex situ , quasi in situ , and operando methods, we could identify a correlation between the catalytic activity and compositional inhomogeneities resulting in an X-ray amorphous Co2+ -rich minority phase linking the crystalline spinel domains in the as-prepared state. Operando X-ray absorption spectroscopy revealed that these Co2+ -rich domains transform during OER to structurally different Co3+ -rich domains. These domains appear to be crucial for enhancing OER kinetics while exhibiting distinctly different redox properties. Our work emphasizes the necessity of the operando methodology to gain fundamental insight into the activity-determining properties of OER catalysts and presents a promising catalyst concept in which a stable, crystalline structure hosts the disordered and active catalyst phase.- Published
- 2022
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178. Multi-scale microscopy study of 3D morphology and structure of MoNi 4 /MoO 2 @Ni electrocatalytic systems for fast water dissociation.
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Zschech E, Topal E, Kutukova K, Gluch J, Löffler M, Werner S, Guttmann P, Schneider G, Liao Z, and Timoshenko J
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The 3D morphology of hierarchically structured electrocatalytic systems is determined based on multi-scale X-ray computed tomography (XCT), and the crystalline structure of electrocatalyst nanoparticles is characterized using transmission electron microscopy (TEM), supported by X-ray diffraction (XRD) and spatially resolved near-edge X-ray absorption fine structure (NEXAFS) studies. The high electrocatalytic efficiency for hydrogen evolution reaction (HER) of a novel transition-metal-based material system - MoNi
4 electrocatalysts anchored on MoO2 cuboids aligned on Ni foam (MoNi4 /MoO2 @Ni) - is based on advantageous crystalline structures and chemical bonding. High-resolution TEM images and selected-area electron diffraction patterns are used to determine the crystalline structures of MoO2 and MoNi4 . Multi-scale XCT provides 3D information of the hierarchical morphology of the MoNi4 /MoO2 @Ni material system nondestructively: Micro-XCT images clearly resolve the Ni foam and the attached needle-like MoO2 micro cuboids. Laboratory nano-XCT shows that the MoO2 micro cuboids with a rectangular cross-section of 0.5 × 1 µm2 and a length of 10-20 µm are vertically arranged on the Ni foam. MoNi4 nanoparticles with a size of 20-100 nm, positioned on single MoO2 cuboids, were imaged using synchrotron radiation nano-XCT. The application of a deep convolutional neural network (CNN) significantly improves the reconstruction quality of the acquired data., (Copyright © 2022 Elsevier Ltd. All rights reserved.)- Published
- 2022
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179. Efficient Electrochemical Nitrate Reduction to Ammonia with Copper-Supported Rhodium Cluster and Single-Atom Catalysts.
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Liu H, Lang X, Zhu C, Timoshenko J, Rüscher M, Bai L, Guijarro N, Yin H, Peng Y, Li J, Liu Z, Wang W, Cuenya BR, and Luo J
- Abstract
The electrochemical nitrate reduction reaction (NITRR) provides a promising solution for restoring the imbalance in the global nitrogen cycle while enabling a sustainable and decentralized route to source ammonia. Here, we demonstrate a novel electrocatalyst for NITRR consisting of Rh clusters and single-atoms dispersed onto Cu nanowires (NWs), which delivers a partial current density of 162 mA cm
-2 for NH3 production and a Faradaic efficiency (FE) of 93 % at -0.2 V vs. RHE. The highest ammonia yield rate reached a record value of 1.27 mmol h-1 cm-2 . Detailed investigations by electron paramagnetic resonance, in situ infrared spectroscopy, differential electrochemical mass spectrometry and density functional theory modeling suggest that the high activity originates from the synergistic catalytic cooperation between Rh and Cu sites, whereby adsorbed hydrogen on Rh site transfers to vicinal *NO intermediate species adsorbed on Cu promoting the hydrogenation and ammonia formation., (© 2022 Wiley-VCH GmbH.)- Published
- 2022
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180. Covalent Organic Framework (COF) Derived Ni-N-C Catalysts for Electrochemical CO 2 Reduction: Unraveling Fundamental Kinetic and Structural Parameters of the Active Sites.
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Li C, Ju W, Vijay S, Timoshenko J, Mou K, Cullen DA, Yang J, Wang X, Pachfule P, Brückner S, Jeon HS, Haase FT, Tsang SC, Rettenmaier C, Chan K, Cuenya BR, Thomas A, and Strasser P
- Abstract
Electrochemical CO
2 reduction is a potential approach to convert CO2 into valuable chemicals using electricity as feedstock. Abundant and affordable catalyst materials are needed to upscale this process in a sustainable manner. Nickel-nitrogen-doped carbon (Ni-N-C) is an efficient catalyst for CO2 reduction to CO, and the single-site Ni-Nx motif is believed to be the active site. However, critical metrics for its catalytic activity, such as active site density and intrinsic turnover frequency, so far lack systematic discussion. In this work, we prepared a set of covalent organic framework (COF)-derived Ni-N-C catalysts, for which the Ni-Nx content could be adjusted by the pyrolysis temperature. The combination of high-angle annular dark-field scanning transmission electron microscopy and extended X-ray absorption fine structure evidenced the presence of Ni single-sites, and quantitative X-ray photoemission addressed the relation between active site density and turnover frequency., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)- Published
- 2022
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181. Tracking heterogeneous structural motifs and the redox behaviour of copper-zinc nanocatalysts for the electrocatalytic CO 2 reduction using operando time resolved spectroscopy and machine learning.
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Rüscher M, Herzog A, Timoshenko J, Jeon HS, Frandsen W, Kühl S, and Roldan Cuenya B
- Abstract
Copper-based catalysts are established catalytic systems for the electrocatalytic CO
2 reduction reaction (CO2 RR), where the greenhouse gas CO2 is converted into valuable industrial chemicals, such as energy-dense C2+ products, using energy from renewable sources. However, better control over the catalyst selectivity, especially at industrially relevant high current density conditions, is needed to expedite the economic viability of the CO2 RR. For this purpose, bimetallic materials, where copper is combined with a secondary metal, comprise a promising and a highly tunable catalyst for the CO2 RR. Nevertheless, the synergy between copper and the selected secondary metal species, the evolution of the bimetallic structural motifs under working conditions and the effect of the secondary metal on the kinetics of the Cu redox behavior require careful investigation. Here, we employ operando quick X-ray absorption fine structure (QXAFS) spectroscopy coupled with machine-learning based data analysis and surface-enhanced Raman spectroscopy (SERS) to investigate the time-dependent chemical and structural changes in catalysts derived from shape-selected ZnO/Cu2 O nanocubes under CO2 RR conditions at current densities up to -500 mA cm-2 . We furthermore relate the catalyst transformations observed under working conditions to the catalytic activity and selectivity and correlate potential-dependent surface and subsurface processes. We report that the addition of Zn to a Cu-based catalyst has a crucial impact on the kinetics of subsurface processes, while redox processes of the Cu surface layer remain largely unaffected. Interestingly, the presence of Zn was found to contribute to the stabilization of cationic Cu(i) species, which is of catalytic relevance since Cu(0)/Cu(i) interfaces have been reported to be beneficial for efficient electrocatalytic CO2 conversion to complex multicarbon products. At the same time, we attribute the increase of the C2+ product selectivity to the formation of Cu-rich CuZn alloys in samples with low Zn content, while Zn-rich alloy phases result in an increased formation of CO paralleled by an increase of the parasitic hydrogen evolution reaction., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)- Published
- 2022
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182. Creation of Exclusive Artificial Cluster Defects by Selective Metal Removal in the (Zn, Zr) Mixed-Metal UiO-66.
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Feng X, Jena HS, Krishnaraj C, Arenas-Esteban D, Leus K, Wang G, Sun J, Rüscher M, Timoshenko J, Roldan Cuenya B, Bals S, and Voort PV
- Abstract
The differentiation between missing linker defects and missing cluster defects in MOFs is difficult, thereby limiting the ability to correlate materials properties to a specific type of defects. Herein, we present a novel and easy synthesis strategy for the creation of solely "missing cluster defects" by preparing mixed-metal (Zn, Zr)-UiO-66 followed by a gentle acid wash to remove the Zn nodes. The resulting material has the reo UiO-66 structure, typical for well-defined missing cluster defects. The missing clusters are thoroughly characterized, including low-pressure Ar-sorption, iDPC-STEM at a low dose (1.5 pA), and XANES/EXAFS analysis. We show that the missing cluster UiO-66 has a negligible number of missing linkers. We show the performance of the missing cluster UiO-66 in CO
2 sorption and heterogeneous catalysis.- Published
- 2021
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183. Selectivity Control of Cu Nanocrystals in a Gas-Fed Flow Cell through CO 2 Pulsed Electroreduction.
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Jeon HS, Timoshenko J, Rettenmaier C, Herzog A, Yoon A, Chee SW, Oener S, Hejral U, Haase FT, and Roldan Cuenya B
- Abstract
In this study, we have taken advantage of a pulsed CO
2 electroreduction reaction (CO2 RR) approach to tune the product distribution at industrially relevant current densities in a gas-fed flow cell. We compared the CO2 RR selectivity of Cu catalysts subjected to either potentiostatic conditions (fixed applied potential of -0.7 VRHE ) or pulsed electrolysis conditions (1 s pulses at oxidative potentials ranging from Ean = 0.6 to 1.5 VRHE , followed by 1 s pulses at -0.7 VRHE ) and identified the main parameters responsible for the enhanced product selectivity observed in the latter case. Herein, two distinct regimes were observed: (i) for Ean = 0.9 VRHE we obtained 10% enhanced C2 product selectivity (FEC = 43.6% and FE2 H4 C = 19.8%) in comparison to the potentiostatic CO2 H5 OH2 RR at -0.7 VRHE (FEC = 40.9% and FE2 H4 C = 11%), (ii) while for E2 H5 OHan = 1.2 VRHE , high CH4 selectivity (FECH = 48.3% vs 0.1% at constant -0.7 V4 RHE ) was observed. Operando spectroscopy (XAS, SERS) and ex situ microscopy (SEM and TEM) measurements revealed that these differences in catalyst selectivity can be ascribed to structural modifications and local pH effects. The morphological reconstruction of the catalyst observed after pulsed electrolysis with Ean = 0.9 VRHE , including the presence of highly defective interfaces and grain boundaries, was found to play a key role in the enhancement of the C2 product formation. In turn, pulsed electrolysis with Ean = 1.2 VRHE caused the consumption of OH- species near the catalyst surface, leading to an OH-poor environment favorable for CH4 production.- Published
- 2021
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184. Operando Investigation of Ag-Decorated Cu 2 O Nanocube Catalysts with Enhanced CO 2 Electroreduction toward Liquid Products.
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Herzog A, Bergmann A, Jeon HS, Timoshenko J, Kühl S, Rettenmaier C, Lopez Luna M, Haase FT, and Roldan Cuenya B
- Abstract
Direct conversion of carbon dioxide into multicarbon liquid fuels by the CO
2 electrochemical reduction reaction (CO2 RR) can contribute to the decarbonization of the global economy. Here, well-defined Cu2 O nanocubes (NCs, 35 nm) uniformly covered with Ag nanoparticles (5 nm) were synthesized. When compared to bare Cu2 O NCs, the catalyst with 5 at % Ag on Cu2 O NCs displayed a two-fold increase in the Faradaic efficiency for C2+ liquid products (30 % at -1.0 VRHE ), including ethanol, 1-propanol, and acetaldehyde, while formate and hydrogen were suppressed. Operando X-ray absorption spectroscopy revealed the partial reduction of Cu2 O during CO2 RR, accompanied by a reaction-driven redispersion of Ag on the CuOx NCs. Data from operando surface-enhanced Raman spectroscopy further uncovered significant variations in the CO binding to Cu, which were assigned to Ag-Cu sites formed during CO2 RR that appear crucial for the C-C coupling and the enhanced yield of liquid products., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)- Published
- 2021
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185. Operando high-pressure investigation of size-controlled CuZn catalysts for the methanol synthesis reaction.
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Divins NJ, Kordus D, Timoshenko J, Sinev I, Zegkinoglou I, Bergmann A, Chee SW, Widrinna S, Karslıoğlu O, Mistry H, Lopez Luna M, Zhong JQ, Hoffman AS, Boubnov A, Boscoboinik JA, Heggen M, Dunin-Borkowski RE, Bare SR, and Cuenya BR
- Abstract
Although Cu/ZnO-based catalysts have been long used for the hydrogenation of CO
2 to methanol, open questions still remain regarding the role and the dynamic nature of the active sites formed at the metal-oxide interface. Here, we apply high-pressure operando spectroscopy methods to well-defined Cu and Cu0.7 Zn0.3 nanoparticles supported on ZnO/Al2 O3 , γ-Al2 O3 and SiO2 to correlate their structure, composition and catalytic performance. We obtain similar activity and methanol selectivity for Cu/ZnO/Al2 O3 and CuZn/SiO2 , but the methanol yield decreases with time on stream for the latter sample. Operando X-ray absorption spectroscopy data reveal the formation of reduced Zn species coexisting with ZnO on CuZn/SiO2 . Near-ambient pressure X-ray photoelectron spectroscopy shows Zn surface segregation and the formation of a ZnO-rich shell on CuZn/SiO2 . In this work we demonstrate the beneficial effect of Zn, even in diluted form, and highlight the influence of the oxide support and the Cu-Zn interface in the reactivity.- Published
- 2021
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186. Publisher Correction: Key role of chemistry versus bias in electrocatalytic oxygen evolution.
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Nong HN, Falling LJ, Bergmann A, Klingenhof M, Tran HP, Spöri C, Mom R, Timoshenko J, Zichittella G, Knop-Gericke A, Piccinin S, Pérez-Ramírez J, Cuenya BR, Schlögl R, Strasser P, Teschner D, and Jones TE
- Published
- 2021
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187. Enhanced Formic Acid Oxidation over SnO 2 -decorated Pd Nanocubes.
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Rettenmaier C, Arán-Ais RM, Timoshenko J, Rizo R, Jeon HS, Kühl S, Chee SW, Bergmann A, and Roldan Cuenya B
- Abstract
The formic acid oxidation reaction (FAOR) is one of the key reactions that can be used at the anode of low-temperature liquid fuel cells. To allow the knowledge-driven development of improved catalysts, it is necessary to deeply understand the fundamental aspects of the FAOR, which can be ideally achieved by investigating highly active model catalysts. Here, we studied SnO
2 -decorated Pd nanocubes (NCs) exhibiting excellent electrocatalytic performance for formic acid oxidation in acidic medium with a SnO2 promotion that boosts the catalytic activity by a factor of 5.8, compared to pure Pd NCs, exhibiting values of 2.46 A mg-1 Pd for SnO2 @Pd NCs versus 0.42 A mg-1 Pd for the Pd NCs and a 100 mV lower peak potential. By using ex situ, quasi in situ, and operando spectroscopic and microscopic methods (namely, transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption fine-structure spectroscopy), we identified that the initially well-defined SnO2 -decorated Pd nanocubes maintain their structure and composition throughout FAOR. In situ Fourier-transformed infrared spectroscopy revealed a weaker CO adsorption site in the case of the SnO2 -decorated Pd NCs, compared to the monometallic Pd NCs, enabling a bifunctional reaction mechanism. Therein, SnO2 provides oxygen species to the Pd surface at low overpotentials, promoting the oxidation of the poisoning CO intermediate and, thus, improving the catalytic performance of Pd. Our SnOx -decorated Pd nanocubes allowed deeper insight into the mechanism of FAOR and hold promise for possible applications in direct formic acid fuel cells., Competing Interests: The authors declare no competing financial interest., (© 2020 American Chemical Society.)- Published
- 2020
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188. Key role of chemistry versus bias in electrocatalytic oxygen evolution.
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Nong HN, Falling LJ, Bergmann A, Klingenhof M, Tran HP, Spöri C, Mom R, Timoshenko J, Zichittella G, Knop-Gericke A, Piccinin S, Pérez-Ramírez J, Cuenya BR, Schlögl R, Strasser P, Teschner D, and Jones TE
- Abstract
The oxygen evolution reaction has an important role in many alternative-energy schemes because it supplies the protons and electrons required for converting renewable electricity into chemical fuels
1-3 . Electrocatalysts accelerate the reaction by facilitating the required electron transfer4 , as well as the formation and rupture of chemical bonds5 . This involvement in fundamentally different processes results in complex electrochemical kinetics that can be challenging to understand and control, and that typically depends exponentially on overpotential1,2,6,7 . Such behaviour emerges when the applied bias drives the reaction in line with the phenomenological Butler-Volmer theory, which focuses on electron transfer8 , enabling the use of Tafel analysis to gain mechanistic insight under quasi-equilibrium9-11 or steady-state assumptions12 . However, the charging of catalyst surfaces under bias also affects bond formation and rupture13-15 , the effect of which on the electrocatalytic rate is not accounted for by the phenomenological Tafel analysis8 and is often unknown. Here we report pulse voltammetry and operando X-ray absorption spectroscopy measurements on iridium oxide to show that the applied bias does not act directly on the reaction coordinate, but affects the electrocatalytically generated current through charge accumulation in the catalyst. We find that the activation free energy decreases linearly with the amount of oxidative charge stored, and show that this relationship underlies electrocatalytic performance and can be evaluated using measurement and computation. We anticipate that these findings and our methodology will help to better understand other electrocatalytic materials and design systems with improved performance.- Published
- 2020
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189. Electrocatalytic CO 2 Reduction on CuO x Nanocubes: Tracking the Evolution of Chemical State, Geometric Structure, and Catalytic Selectivity using Operando Spectroscopy.
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Möller T, Scholten F, Thanh TN, Sinev I, Timoshenko J, Wang X, Jovanov Z, Gliech M, Roldan Cuenya B, Varela AS, and Strasser P
- Abstract
The direct electrochemical conversion of carbon dioxide (CO
2 ) into multi-carbon (C2+ ) products still faces fundamental and technological challenges. While facet-controlled and oxide-derived Cu materials have been touted as promising catalysts, their stability has remained problematic and poorly understood. Herein we uncover changes in the chemical and morphological state of supported and unsupported Cu2 O nanocubes during operation in low-current H-Cells and in high-current gas diffusion electrodes (GDEs) using neutral pH buffer conditions. While unsupported nanocubes achieved a sustained C2+ Faradaic efficiency of around 60 % for 40 h, the dispersion on a carbon support sharply shifted the selectivity pattern towards C1 products. Operando XAS and time-resolved electron microscopy revealed the degradation of the cubic shape and, in the presence of a carbon support, the formation of small Cu-seeds during the surprisingly slow reduction of bulk Cu2 O. The initially (100)-rich facet structure has presumably no controlling role on the catalytic selectivity, whereas the oxide-derived generation of under-coordinated lattice defects, can support the high C2+ product yields., (© 2020 The Authors. Published by Wiley-VCH GmbH.)- Published
- 2020
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190. Operando Insight into the Correlation between the Structure and Composition of CuZn Nanoparticles and Their Selectivity for the Electrochemical CO 2 Reduction.
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Jeon HS, Timoshenko J, Scholten F, Sinev I, Herzog A, Haase FT, and Roldan Cuenya B
- Abstract
Bimetallic CuZn catalysts have been recently proposed as alternatives in order to achieve selectivity control during the electrochemical reduction of CO
2 (CO2 RR). However, fundamental understanding of the underlying reaction mechanism and parameters determining the CO2 RR performance is still missing. In this study, we have employed size-controlled (∼5 nm) Cu100- x Znx nanoparticles (NPs) supported on carbon to investigate the correlation between their structure and composition and catalytic performance. By tuning the concentration of Zn, a drastic increase in CH4 selectivity [∼70% Faradaic efficiency (F.E.)] could be achieved for Zn contents from 10 to 50, which was accompanied by a suppression of the H2 production. Samples containing a higher Zn concentration displayed significantly lower CH4 production and an abrupt switch in the selectivity to CO. Lack of metal leaching was observed based on quasi in situ X-ray photoelectron spectroscopy (XPS). Operando X-ray absorption fine structure (XAFS) spectroscopy measurements revealed that the alloying of Cu atoms with Zn atoms takes place under reaction conditions and plays a determining role in the product selectivity. Time-dependent XAFS analysis showed that the local structure and chemical environment around the Cu atoms continuously evolve during CO2 RR for several hours. In particular, cationic Zn species initially present were found to get reduced as the reaction proceeded, leading to the formation of a CuZn alloy (brass). The evolution of the Cu-Zn interaction with time during CO2 RR was found to be responsible for the change in the selectivity from CH4 over Cu-ZnO NPs to CO over CuZn alloy NPs. This study highlights the importance of having access to in depth information on the interplay between the different atomic species in bimetallic NP electrocatalysts under operando reaction conditions in order to understand and ultimately tune their reactivity.- Published
- 2019
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191. Selective CO 2 Electroreduction to Ethylene and Multicarbon Alcohols via Electrolyte-Driven Nanostructuring.
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Gao D, Sinev I, Scholten F, Arán-Ais RM, Divins NJ, Kvashnina K, Timoshenko J, and Roldan Cuenya B
- Abstract
Production of multicarbon products (C
2+ ) from CO2 electroreduction reaction (CO2 RR) is highly desirable for storing renewable energy and reducing carbon emission. The electrochemical synthesis of CO2 RR catalysts that are highly selective for C2+ products via electrolyte-driven nanostructuring is presented. Nanostructured Cu catalysts synthesized in the presence of specific anions selectively convert CO2 into ethylene and multicarbon alcohols in aqueous 0.1 m KHCO3 solution, with the iodine-modified catalyst displaying the highest Faradaic efficiency of 80 % and a partial geometric current density of ca. 31.2 mA cm-2 for C2+ products at -0.9 V vs. RHE. Operando X-ray absorption spectroscopy and quasi in situ X-ray photoelectron spectroscopy measurements revealed that the high C2+ selectivity of these nanostructured Cu catalysts can be attributed to the highly roughened surface morphology induced by the synthesis, presence of subsurface oxygen and Cu+ species, and the adsorbed halides., (© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)- Published
- 2019
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192. Mapping XANES spectra on structural descriptors of copper oxide clusters using supervised machine learning.
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Liu Y, Marcella N, Timoshenko J, Halder A, Yang B, Kolipaka L, Pellin MJ, Seifert S, Vajda S, Liu P, and Frenkel AI
- Abstract
Understanding the origins of enhanced reactivity of supported, subnanometer in size, metal oxide clusters is challenging due to the scarcity of methods capable to extract atomic-level information from the experimental data. Due to both the sensitivity of X-ray absorption near edge structure (XANES) spectroscopy to the local geometry around metal ions and reliability of theoretical spectroscopy codes for modeling XANES spectra, supervised machine learning approach has become a powerful tool for extracting structural information from the experimental spectra. Here, we present the application of this method to grazing incidence XANES spectra of size-selective Cu oxide clusters on flat support, measured in operando conditions of the methanation reaction. We demonstrate that the convolution neural network can be trained on theoretical spectra and utilized to "invert" experimental XANES data to obtain structural descriptors-the Cu-Cu coordination numbers. As a result, we were able to distinguish between different structural motifs (Cu
2 O-like and CuO-like) of Cu oxide clusters, transforming in reaction conditions, and reliably evaluate average cluster sizes, with important implications for the understanding of structure, composition, and function relationships in catalysis.- Published
- 2019
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193. Nanoporous Copper-Silver Alloys by Additive-Controlled Electrodeposition for the Selective Electroreduction of CO 2 to Ethylene and Ethanol.
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Hoang TTH, Verma S, Ma S, Fister TT, Timoshenko J, Frenkel AI, Kenis PJA, and Gewirth AA
- Abstract
Electrodeposition of CuAg alloy films from plating baths containing 3,5-diamino-1,2,4-triazole (DAT) as an inhibitor yields high surface area catalysts for the active and selective electroreduction of CO
2 to multicarbon hydrocarbons and oxygenates. EXAFS shows the co-deposited alloy film to be homogeneously mixed. The alloy film containing 6% Ag exhibits the best CO2 electroreduction performance, with the Faradaic efficiency for C2 H4 and C2 H5 OH production reaching nearly 60 and 25%, respectively, at a cathode potential of just -0.7 V vs RHE and a total current density of ∼ - 300 mA/cm2 . Such high levels of selectivity at high activity and low applied potential are the highest reported to date. In situ Raman and electroanalysis studies suggest the origin of the high selectivity toward C2 products to be a combined effect of the enhanced stabilization of the Cu2 O overlayer and the optimal availability of the CO intermediate due to the Ag incorporated in the alloy.- Published
- 2018
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194. Size dependent behavior of Fe 3 O 4 crystals during electrochemical (de)lithiation: an in situ X-ray diffraction, ex situ X-ray absorption spectroscopy, transmission electron microscopy and theoretical investigation.
- Author
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Bock DC, Pelliccione CJ, Zhang W, Timoshenko J, Knehr KW, West AC, Wang F, Li Y, Frenkel AI, Takeuchi ES, Takeuchi KJ, and Marschilok AC
- Abstract
The iron oxide magnetite, Fe
3 O4 , is a promising conversion type lithium ion battery anode material due to its high natural abundance, low cost and high theoretical capacity. While the close packing of ions in the inverse spinel structure of Fe3 O4 enables high energy density, it also limits the kinetics of lithium ion diffusion in the material. Nanosizing of Fe3 O4 to reduce the diffusion path length is an effective strategy for overcoming this issue and results in improved rate capability. However, the impact of nanosizing on the multiple structural transformations that occur during the electrochemical (de)lithiation reaction in Fe3 O4 is poorly understood. In this study, the influence of crystallite size on the lithiation-conversion mechanisms in Fe3 O4 is investigated using complementary X-ray techniques along with transmission electron microscopy (TEM) and continuum level simulations on electrodes of two different Fe3 O4 crystallite sizes. In situ X-ray diffraction (XRD) measurements were utilized to track the changes to the crystalline phases during (de)lithiation. X-ray absorption spectroscopy (XAS) measurements at multiple points during the (de)lithiation processes provided local electronic and atomic structural information. Tracking the crystalline and nanocrystalline phases during the first (de)lithiation provides experimental evidence that (1) the lithiation mechanism is non-uniform and dependent on crystallite size, where increased Li+ diffusion length in larger crystals results in conversion to Fe0 metal while insertion of Li+ into spinel-Fe3 O4 is still occurring, and (2) the disorder and size of the Fe metal domains formed when either material is fully lithiated impacts the homogeneity of the FeO phase formed during the subsequent delithiation.- Published
- 2017
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