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Cobalt doping of Au25 nanoclusters led to an enhancement in the catalytic behaviour. The evolution of the doped Au clusters to nanoalloys with controlled surface sites was revealed by combined XAFS, DRIFTS and XPS studies.

A cross-disciplinary approach is employed to study working nanocomposite room temperature methanol sensors via a combination of operando and in situ spectroscopy techniques, paralleled by resistance measurements.

Spatio-temporal nonuniformities in H2 oxidation on individual Rh(h k l) domains of a polycrystalline Rh foil were studied in the 10–6 mbar pressure range by photoemission electron microscopy (PEEM), X-ray photoemission electron microscopy (XPEEM), and low-energy electron microscopy (LEEM). The latter two were used for in situ correlative microscopy to zoom in with significantly higher lateral resolution, allowing detection of an unusual island-mediated oxygen front propagation during kinetic transitions. The origin of the island-mediated front propagation was rationalized by model calculations based on a hybrid approach of microkinetic modeling and Monte Carlo simulations.

Syngas can be produced from biomethane via Partial Oxidation of Methane (POM), being an attractive route since it is ecofriendly and sustainable. In this work, catalysts of Ni supported on MgO–ZrO2 solid solutions, prepared by a one-step polymerization method, were characterized by HRTEM/EDX, XRD, XPS, H2-TPR, and in situ XRD. All catalysts, including Ni/ZrO2 and Ni/MgO as reference, were tested for POM (CH4:O2 molar ratio 2, 750 ºC, 1 atm). NiO/MgO/ZrO2 contained two solid-solutions, MgO–ZrO2 and NiO-MgO, as revealed by XRD and XPS. Ni (30 wt%) supported on MgO–ZrO2 solid solution exhibited high methane conversion and hydrogen selectivity. However, depending on the MgO amount (0, 4, 20, 40, 100 molar percent) major differences in NiO reducibility, growth of Ni0 crystallite size during H2 reduction and POM, and in carbon deposition rates were observed. Interestingly, catalysts with lower MgO content achieved the highest CH4 conversion (~ 95%), high selectivity to H2 (1.7) and CO (0.8), and low carbon deposition rates (0.024 g carbon.gcat−1 h−1) with Ni4MgZr (4 mol% MgO) turning out to be the best catalyst. In situ XRD during POM indicated metallic Ni nanoparticles (average crystallite size of 31 nm), supported by MgO–ZrO2 solid solution, with small amounts of NiO–MgO being present as well. The presence of MgO also influenced the morphology of the carbon deposits, leading to filaments instead of amorphous carbon. A combustion-reforming mechanism is suggested and using a MgO–ZrO2 solid solution support strongly improves catalytic performance, which is attributed to effective O2, CO2 and H2O activation at the Ni/MgO–ZrO2 interface.

Die Oberflächenstruktur von Metall-Nanopartikel auf Oxidträgern lässt sich über charakteristische Schwingungen von adsorbierten Sondenmolekülen wie CO bestimmen. Üblicherweise konzentrieren sich spektroskopische Untersuchungen auf die Peak-Position und -Intensität, die mit der Bindungsgeometrie bzw. der Anzahl der Adsorptionsplätze zusammenhängen. Anhand zweier unterschiedlich präparierter Modellkatalysatoren wird gezeigt, dass die polarisationsabhängige Summenfrequenzspektroskopie (SFG) die gemittelte Oberflächenstruktur und Form von Nanopartikel beleuchten kann. SFG-Ergebnisse für verschiedene Partikelgrößen und Morphologien werden mit direkter Realraum-Strukturanalyse mittels TEM und STM verglichen. Die beschriebene Anwendung von SFG kann zur in situ Detektion der Partikelstruktur verwendet werden und könnte ein wertvolles Werkzeug in der operando Katalyse werden.

The surface structure of oxide-supported metal nanoparticles can be determined via characteristic vibrations of adsorbed probe molecules such as CO. Usually, spectroscopic studies focus on peak position and intensity, which are related to binding geometries and number of adsorption sites, respectively. Employing two differently prepared model catalysts, it is demonstrated that polarization-dependent sum-frequency-generation (SFG) spectroscopy reveals the average surface structure and shape of the nanoparticles. SFG results for different particle sizes and morphologies are compared to direct real-space structure analysis by TEM and STM. The described feature of SFG could be used tomonitor particle restructuring in situ and may be a valuable tool for operando catalysis.

A surface science based approach was applied to model carbon supported Pd nanoparticle catalysts. Employing physical vapour deposition of Pd on sputtered surfaces of highly oriented pyrolytic graphite (HOPG), model catalysts were prepared that are well-suited for characterization by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Analysis of the HOPG substrate before and after ion-bombardment, and of Pd/HOPG before and after annealing, revealed the number of “nominal” HOPG defects (~ 1014 cm−2) as well as the nucleation density (~ 1012 cm−2) and structural characteristics of the Pd nanoparticles (mean size/height/distribution). Two model systems were stabilized by UHV annealing to 300 °C, with mean Pd particles sizes of 4.3 and 6.8 nm and size/height aspect ratio up to ~ 10. A UHV-compatible flow microreactor and gas chromatography were used to determine the catalytic performance of Pd/HOPG in ethylene (C2H4) hydrogenation up to 150 °C under atmospheric pressure, yielding temperature-dependent conversion values, turnover frequencies (TOFs) and activation energies. The performance of Pd nanocatalysts is compared to that of polycrystalline Pd foil and contrasted to Pt/HOPG and Pt foil, pointing to a beneficial effect of the metal/carbon phase boundary, reflected by up to 10 kJ mol−1 lower activation energies for supported nanoparticles. Graphical Abstract

A hybrid catalyst (Ni/Al2O3 and K–Co/Al2O3) is studied for CH4 conversion to C2+. The hybrid catalyst works effectively at a relatively low temperature (490 °C).

Driving metal-cluster-catalyzed high-temperature chemical reactions by sunlight holds promise for the development of negative-carbon-footprint industrial catalysis, which has yet often been hindered by the poor ability of metal clusters to harvest and utilize the full spectrum of solar energy. Here, we report the preparation of Mo

Cascade catalysis of reverse water gas shift (RWGS) and well-established CO hydrogenation holds promise for the conversion of greenhouse gas CO2 and renewable H2 into liquid hydrocarbons and methanol under mild conditions. However, it remains a big challenge to develop low-temperature RWGS catalysts with high activity, selectivity, and stability. Here, we report the design of an efficient RWGS catalyst by encapsulating ruthenium clusters with the size of 1 nm inside hollow silica shells. The spatially confined structure prevents the sintering of Ru clusters while the permeable silica layer allows the diffusion of gaseous reactants and products. This catalyst with reduced particle sizes not only inherits the excellent activity of Ru in CO2 hydrogenation reactions but also exhibits nearly 100% CO selectivity and superior stability at 200–500 °C. The ability to selectively produce CO from CO2 at relatively low temperatures paves the way for the production of value-added fuels from CO2 and renewable H2.

Co-doped Au

Self-sustained oscillations in H2 oxidation on a Rh nanotip mimicking a single catalytic nanoparticle were studied by in situ field emission microscopy (FEM). The observed spatio-temporal oscillations result from the coupling of subsurface oxide formation/depletion with reaction front propagation. An original sophisticated method for tracking kinetic transition points allowed the identification of local pacemakers, initiating kinetic transitions and the nucleation of reaction fronts, with much higher temporal resolution than conventional processing of FEM video files provides. The pacemakers turned out to be specific surface atomic configurations at the border between strongly corrugated Rh{973} regions and adjacent relatively flat terraces. These structural ensembles are crucial for reactivity: while the corrugated region allows sufficient oxygen incorporation under the Rh surface, the flat terrace provides sufficient hydrogen supply required for the kinetic transition, highlighting the importance of interfacet communication. The experimental observations are complemented by mean-field microkinetic modeling. The insights into the initiation and propagation of kinetic transitions on a single catalytic nanoparticle demonstrate how in situ monitoring of an ongoing reaction on individual nanofacets can single out active configurations, especially when combined with atomically resolving the nanoparticle surface by field ion microscopy (FIM).

Imaging reactions across facets Metal nanoparticles used in heterogeneous catalysis can bear different facets with different reaction kinetics. Suchorski et al. used field electron microscopy with high spatial (∼2 nanometers) and time (∼2 milliseconds) resolution to study hydrogen oxidation on a curved rhodium crystal that displayed individual nanofacets. They also performed field ion microscopy of the water products. Periodic formation and depletion of subsurface oxygen blocked or allowed hydrogen adsorption, respectively, and led to oscillatory kinetics that could frequency lock between facets but at different frequencies. Surface reconstructions could also induce collapse of spatial coupling of oscillations. Science , abf8107, this issue p. 1314

In an effort to combine the favorable catalytic properties of Co3 O4 and CeO2 , nanocomposites with different phase distribution and Co3 O4 loading were prepared and employed for CO oxidation. Synthesizing Co3 O4 -modified CeO2 via three different sol-gel based routes, each with 10.4 wt % Co3 O4 loading, yielded three different nanocomposite morphologies: CeO2 -supported Co3 O4 layers, intermixed oxides, and homogeneously dispersed Co. The reactivity of the resulting surface oxygen species towards CO were examined by temperature programmed reduction (CO-TPR) and flow reactor kinetic tests. The first morphology exhibited the best performance due to its active Co3 O4 surface layer, reducing the light-off temperature of CeO2 by about 200 °C. In contrast, intermixed oxides and Co-doped CeO2 suffered from lower dispersion and organic residues, respectively. The performance of Co3 O4 -CeO2 nanocomposites was optimized by varying the Co3 O4 loading, characterized by X-ray diffraction (XRD) and N2 sorption (BET). The 16-65 wt % Co3 O4 -CeO2 catalysts approached the conversion of 1 wt % Pt/CeO2 , rendering them interesting candidates for low-temperature CO oxidation.

Methane, the main component of natural gas and considered a greenhouse gas, was converted to value-added hydrocarbons (C2+) using synthesized Na2WO4 and MnTiO3 supported on SBA-15 through the oxidative coupling of methane. The MnTiO3 co-impregnated with Na2WO4 on SBA-15 (MnTiO3-NW/SBA-15) was more active than catalysts that were prepared using a simple one-pot impregnation of Mn2+, Ti4+, and Na2WO4 over fumed-SiO2, MCM-41, or SBA-15. Characterizations of the catalysts suggested that the presence of MnTiO3 was more important in the activation of methane relative to Mn2O3. The highest C2+ yield obtained from MnTiO3-NW/SBA-15 was 24.9 % with 63.0 % C2+ selectivity and 39.4 % CH4 conversion at a reaction temperature of 700 °C, a feed ratio of CH4:O2:N2 = 3:1:4, and a space velocity of 18,500 h−1. A time-on-stream test of MnTiO3-NW/SBA-15 over 25 h revealed that the C2+ yield slowly decreased from 24.9%–20.2% at the end of the 25 h run, because of a decrease in the catalyst’s surface area, a loss of active Na2WO4 from the catalyst, and the destruction of α-cristobalite. Experiments using in-situ diffuse reflectance infrared Fourier transform spectroscopy with mass spectrometry, operated at 475 °C, revealed that the MnTiO3-NW/SBA-15 catalyst enhanced the formation of CH3 and H radicals, while no products or intermediates were found when only SBA-15 was tested.

The kinetic behavior of individual Rh(

Scanning photoelectron microscopy (SPEM) and photoemission electron microscopy (PEEM) allow local surface analysis and visualising ongoing reactions on a µm-scale. These two spatio-temporal imaging methods are applied to polycrystalline Rh, representing a library of well-defined high-Miller-index surface structures. The combination of these techniques enables revealing the anisotropy of surface oxidation, as well as its effect on catalytic hydrogen oxidation. In the present work we observe, using locally-resolved SPEM, structure-sensitive surface oxide formation, which is summarised in an oxidation map and quantitatively explained by the novel step density (SDP) and step edge (SEP) parameters. In situ PEEM imaging of ongoing H2 oxidation allows a direct comparison of the local reactivity of metallic and oxidised Rh surfaces for the very same different stepped surface structures, demonstrating the effect of Rh surface oxides. Employing the velocity of propagating reaction fronts as indicator of surface reactivity, we observe a high transient activity of Rh surface oxide in H2 oxidation. The corresponding velocity map reveals the structure-dependence of such activity, representing a direct imaging of a structure-activity relation for plenty of well-defined surface structures within one sample., Surface oxide formation under reaction conditions may change the catalytic activity of a catalyst. Here, the authors explore the effect of atomic structure of Rh surfaces on the surface oxide formation and its influence on catalytic activity in hydrogen oxidation, revealing a high transient activity.

Sum frequency generation (SFG) vibrational spectroscopy is a unique surface/interface-sensitive method, enabling the identification of chemical species and molecular structures, densities and orientations. SFG has been proven to be a powerful probe to examine adsorbates and reactions at solid–gas interfaces related to heterogeneous catalysis, employing well-defined ultra-high vacuum (UHV) grown model catalysts and UHV-compatible high-pressure reaction cells, enabling bridging both the materials and pressure gaps. SFG was thus among the first methods for ambient pressure surface science, enabling the characterization of “high pressure adsorbates”. In this mini-review, we provide an overview of SFG studies of CO-related processes in heterogeneous model catalysis. This includes pressure- and/or temperature-dependent CO adsorption on single crystals (platinum, palladium, rhodium, iridium, copper, nickel) and oxide/graphene-supported (palladium, platinum) nanoparticles, as well as CO reactions (oxidation/hydrogenation) simultaneously monitored by SFG and mass spectrometry. The adsorption of isotopic CO mixtures on single crystals and nanoparticles provides information on the individual contributions of vibrational coupling and chemical interactions to the adsorbate–adsorbate interactions. Altogether, SFG helps to identify various adsorption sites, adsorbate structures, molecular orientations and CO reactions on prototypical catalyst surfaces of increasing complexity. Specifically, the analysis of molecular orientation (tilt angles) can be carried out by polarization-dependent SFG.

Converting CO2 directly from the air to fuel under ambient conditions is a huge challenge. Thus, there is an urgent need for CO2 conversion protocols working at room temperature and atmospheric pressure, preferentially without any external energy input. Herein, we employ magnesium (nanoparticles and bulk), an inexpensive and the eighth-most abundant element, to convert CO2 to methane, methanol and formic acid, using water as the sole hydrogen source. The conversion of CO2 (pure, as well as directly from the air) took place within a few minutes at 300 K and 1 bar, and no external (thermal, photo, or electric) energy was required. Hydrogen was, however, the predominant product as the reaction of water with magnesium was favored over the reaction of CO2 and water with magnesium. A unique cooperative action of Mg, basic magnesium carbonate, CO2, and water enabled this CO2 transformation. If any of the four components was missing, no CO2 conversion took place. The reaction intermediates and the reaction pathway were identified by 13CO2 isotopic labeling, powder X-ray diffraction (PXRD), nuclear magnetic resonance (NMR) and in situ attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), and rationalized by density-functional theory (DFT) calculations. During CO2 conversion, Mg was converted to magnesium hydroxide and carbonate, which may be regenerated. Our low-temperature experiments also indicate the future prospect of using this CO2-to-fuel conversion process on the surface of Mars, where CO2, water (ice), and magnesium are abundant. Thus, even though the overall process is non-catalytic, it could serve as a step towards a sustainable CO2 utilization strategy as well as potentially being a first step towards a magnesium-driven civilization on Mars.

Alloying has been studied actively for thiolate (SR)-protected metal nanoclusters (NCs) because heteroatom mixing enables the creation of novel electronic structures, physical/chemical properties, ...

Evaluation of the catalytic performance of CeO2 typically requires dedicated (GC or MS) analysis over extended times. Herein, we present a simpler and fast method of assessing the catalytic activity of CeO2 through 3,3′,5,5′-tetramethylbenzidine (TMB) color change. Five different morphologies (nanoflakes (NF), nanorods (NR), nanoparticles (NP), nanocubes (NC) and nano-octahedra (NO)) of CeO2 were used as test models, and their catalytic activity can be rapidly predicted within 10 min by the developed TMB method. Depending on the CeO2 catalytic activity, which is related to O2 dissociation under strongly acidic conditions (0.05 M H2SO4, pH = 1), more or less of highly reactive hydroxyl species are produced, which can subsequently oxidize TMB from colorless to yellow. This change can be observed even by the naked eye or, preferentially, by a UV–vis spectrometer. The feasibility of the TMB-H+ system to assess catalytic activity was verified by the imine synthesis, methylene blue degradation and CO oxidation. The TMB method can thus rapidly and accurately indicate the catalytic activity of CeO2 with different morphologies, and can be used for pre-screening of synthesized materials, avoiding time-consuming and/or more complex characterization of CeO2 samples with poor catalytic activity.

The shape of nanosized CeO2, obtained via polyvinylpyrrolidone (PVP) micelles, was controlled by microwave (MW)-aided synthesis combined with different combinations of energy input/transfer, including ultrasound (US), ultraviolet (UV) and pressure (P). Whereas ceria nanoflakes resulted from standard solvothermal synthesis, CeO2 nanoparticles were obtained from MW, MW + US and MW + UV. New CeO2 “nanospindles” (with aspect ratio of 2) resulted from MW + US + UV, and nanorods (with aspect ratio of 11) emerged from MW + P. All ceria morphologies, even nanospindles and nanorods, were mesoporous agglomerates of small CeO2 nanocrystals (6–8 nm size), but they still exhibited different specific surface area (SSA) and Ce3+/Ce4+ ratio. Underlying reasons of how the different synthesis routes affect the ceria morphology are discussed. Among the six types, CeO2 nanorods (MW + P) exhibited the highest SSA (196 m2 g−1) and the most surface defects (Ce3+: 26.4%), resulting in excellent catalytic performance in imine synthesis and CO oxidation.

Monolayer protected Au nanocluster catalysts are known to undergo structural changes during catalytic reactions, including dissociation and migration of ligands onto the support, which strongly affects their activity and stability. To better understand how the nature of ligands influences the catalytic activity of such catalysts, three types of ceria supported Au nanoclusters with different kinds of ligands (thiolates, phosphines and a mixture thereof) have been studied, employing CO oxidation as model reaction. The thiolate-protected Au

CO adsorption and dissociation on "perfect" and "defect-rich" Ir(111) surfaces were studied by a combination of surface-analytical techniques, including polarization-dependent (PPP and SSP) sum frequency generation (SFG) vibrational spectroscopy, low-energy electron diffraction (LEED), Auger electron spectroscopy, X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. CO was found to be ordered and tilted from the surface normal at high coverage on the "perfect" surface (e.g., θ = 30° at 0.70 ML), whereas it was less ordered and preferentially upright (θ = 4-10°) on the "defect-rich" surface for coverages of 0.55-0.70 ML. SFG, LEED, and XPS revealed that CO adsorption at low pressure/high temperature and high pressure/low temperature was reversible. In contrast, upon heating to ∼600 K in near mbar CO pressure, "perfect" and even more "defect-rich" Ir(111) surfaces were irreversibly modified by carbon deposits, which, according to DFT, result from CO disproportionation.

The bimetallic nanocluster catalyst structure can change during pretreatment and reaction, thus in situ characterization techniques are required for a proper analysis of the active sites. In situ XAFS and DRIFTS were used to study the dynamic evolution of the metal active sites in bimetallic AgAu nanoclusters supported on ITQ2 zeolite during CO catalytic oxidation. The activity of the bimetallic nanocluster catalyst in this reaction was significantly higher than those of supported monometallic Ag and Au nanoclusters. These results were explained by the formation of AgAu alloyed nanoparticles, which favoured reactant adsorption and reaction. Furthermore, the initial activity depended on the catalyst pretreatment, obtaining better conversion, at lower temperatures, with the catalyst pretreated with hydrogen than with the catalyst pretreated with oxygen. This was also associated with an easier formation of a AgAu alloy under hydrogen pretreatment at 150 °C. However, the alloying process seemed to be completed after reaction in both cases, i.e. for the catalyst pretreated with oxygen and with hydrogen, obtaining the same catalytic performance with both catalysts upon reuse. The activity is constant in successive reaction runs, indicating high stability of the active species formed under reaction conditions. The results have shown that the combination of catalytic studies with in situ characterization techniques provides insight into the structural dynamics of the catalysts during activation and reaction., The authors thank the Spanish Ministry of Economy and Competitiveness through RTI2018-101784-B-I00 (MINECO/FEDER) and SEV-2016-0683 projects for the financial support. I. López Hernández is grateful to Generalitat Valenciana and European Social Fund for the pre doctoral grant ACIF2017. C.W.L. thanks PRH 50.1 – ANP/FINEP Human Resources Program (Brazil) for the Visiting Researcher Fellowship. We gratefully acknowledge ALBA synchrotron for allocating beamtime (proposal 2015091414) and the CLÆSS beamline staff for their help and technical support during our experiment. We acknowledge support by the Austrian Science Fund (FWF) via grants DK+Solids4Fun (W1243), Single Atom Catalysis (I 4434-N) and Elise Richter (V831-N).