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The solid solution of a reducible oxide with a (non or) less reducible oxide may open the way to incorporate substantial amounts of hydrogen by the simple exposure to H2 at elevated temperatures, as exemplified by the mixture of RuO2 and TiO2.

The solid solution of a reducible oxide with a (non or) less reducible oxide may open the way to incorporate substantial amounts of hydrogen by the simple exposure to H2 at elevated temperatures, as exemplified by the mixture of RuO2 and TiO2. We are able to incorporate 17.6 mol% hydrogen into the mixed oxide Ru0.3Ti0.7O2 by H2 exposure at 250 °C, while this is not possible for pure RuO2 and rutile TiO2 that is either reduced to metallic Ru or does not allow for hydrogen absorption, respectively. Hydrogenated Ru0.3Ti0.7O2 may be utilized in hydrogenation catalysis. In this study, however, we demonstrate that hydrogen-incorporated Ru0.3Ti0.7O2 improves substantially the catalytic performance in oxidation reactions such as the propane combustion and HCl oxidation reaction. Hydrogen induced lattice strain in Ru0.3Ti0.7O2 accompanied with altered electronic properties is likely to be the reason for the observed enhanced catalytic activity. Hydrogen treatment can be performed in the reactor, thus providing an additional parameter to fine-tune in situ the catalytic performance of a mixed oxide catalyst.

The electrochemical growth of Au nanowires in a template of nano-porous anodic aluminum oxide was investigated in situ by means of grazing-incidence transmission small- and wide-angle x-ray scattering (GTSAXS and GTWAXS), x-ray fluorescence (XRF) and 2-dimensional surface optical reflectance (2D-SOR). The XRF and the overall intensity of the GTWAXS patterns as a function of time were used to monitor the progress of the electrodeposition. Furthermore, we extracted powder diffraction patterns in the direction of growth and in the direction of confinement to follow the evolution of the direction-dependent strain. Quite rapidly after the beginning of the electrodeposition, the strain became tensile in the vertical direction and compressive in the horizontal direction, which showed that the lattice deformation of the nanostructures can be artificially varied by an appropriate choice of the deposition time. By alternating sequences of electrodeposition to sequences of rest, we observed fluctuations of the lattice parameter in the direction of growth, attributed to stress caused by electromigration.. Furthermore, the porous domain size calculated from the GTSAXS patterns was used to monitor how homogeneously the pores were filled., Short communication manuscript. Four figures

The effect of the carrier material on the intrinsic activity of three catalytic total oxidation reactions, namely CO oxidation and methane and propane combustion, over highly dispersed RuO2 is studied. The carrier materials are systematically varied from ZrO2 to CeO2 using ultrathin CeO2 films on ZrO2 particles (CeO2@ZrO2) as the linking carrier materials that were subjected to different calcination and redox treatments. CeO2@ZrO2 is subjected to annealing to 900 °C, transforming the CeO2 layer into a mixed CexZr1−xO2 layer, and finally, after reductive treatment with H2 at 950 °C and mild re-oxidation at 500 °C a kappa phase layer is formed on the ZrO2 core. The catalytic CO oxidation and the propane combustion reveal pronounced dependencies on the carrier material. The variation in propane combustion activity with the carrier material is correlated with the varying surface concentration of vacant surface Ce sites, while the activity in CO oxidation is correlated with the oxygen storage capacity (OSC). Quite in contrast, the intrinsic activity and the apparent activation energy in methane combustion are less affected by the carrier material than in the other two reactions. For comparison we chose rutile TiO2 as the carrier that is known to disperse RuO2 quite efficiently, revealing, however, by far the lowest activity in all total oxidation reactions studied, due likely to strong metal–support interaction (SMSI).

Down to a cathodic potentials of -1.20 V versus the reversible hydrogen electrode, the structure of IrO2(110) electrodes supported by TiO2(110) is found to be stable by in situ synchrotron-based X-ray diffraction. Such high cathodic potentials should lead to reduction to metallic Ir (Pourbaix diagram). From the IrO2 lattice parameters, determined during cathodic polarization in a H2SO4 electrolyte solution (pH 0.4), it is estimated that the unit cell volume increases by 1% due likely to proton incorporation, which is supported by the lack of significant swelling of the IrO2(110) film derived from X-ray reflectivity experiments. Ex situ X-ray photoelectron spectroscopy suggests that protons are incorporated into the IrO2(110) lattice below -1.0 V, although Ir remains exclusively in the IV+ oxidation state down to -1.20 V. Obviously, further hydrogenation of the lattice oxygen of IrO2(110) toward water is suppressed for kinetic reasons and hints at a rate-determining chemical step that cannot be controlled by the electrode potential.

ChemCatChem (2020). doi:10.1002/cctc.202000907, Published by WILEY-VCH Verlag, Weinheim

The Pechini method allows for compositional and structural control of mixed ruthenium–iridium powder samples. Extensive characterization reveals that the calcination step leads to agglomerates with a metallic core that is encapsulated by the mixed RuxIr1–xO2 oxides. In the composition range of 21 up to 74 mol % ruthenium, the metal core reveals a miscibility gap with an Ir-rich fcc structure and a Ru- enriched hcp structure. Quite in contrast, the mixed oxide particles form a well-defined RuxIr1–xO2 solid solution throughout the entire composition range. Catalytic activity tests of the mixed RuxIr1–xO2 oxide catalysts are conducted with the prototypical CO oxidation reaction under oxidizing and under both stoichiometric reaction conditions ; note that the metallic core is buried and does therefore not contribute to the activity. The least active catalyst is pure IrO2. At moderate reaction temperatures above 100 °C, the most active oxidation catalyst is identified with the Ir0.125Ru0.875O2 mixed oxide. All mixed RuxIr1–xO2 oxide catalysts are bulk-stable under the reaction conditions considered. However, upon CO oxidation reaction, the Ir4+ concentration at the mixed oxide surface is significantly enhanced with respect to its bulk composition. This information may be important for other catalytic oxidation reactions as well, such as the anodic oxygen evolution reaction in the electrocatalytic water splitting.

CeO2 is a promising material in the catalytic recovery of Cl2 from HCl by oxidation with O2 due to its beneficial redox properties (Ce3+/Ce4+) and accompanied oxygen storage capacity (OSC). Two options to tailor the OSC are the usage of dopants and nanostructuring. In this work, we report the synthesis of mesoporous doped Ce0.9M0.1O2 (M = Zr, Gd, Pr, and Tb) powders with identical mesopore size, shape, and specific surface area (≈90 m2 g– 1) to investigate the impact of aliovalent (Gd), isovalent (Zr), and aliovalent/isovalent (Pr and Tb) dopants on the OSC as well as on the catalytic activity/stability. The cubic arrangement of spherical 12 nm mesopores surrounded by a crystalline framework is obtained by using a diblock copolymer as structure directing agent in the sol-gel-based preparation approach, indicated by small angle X-ray scattering (SAXS) and scanning electron microscopy (SEM). Pr-doped CeO2 revealed the highest oxygen storage capacity but the lowest catalytic activity/stability in the HCl oxidation together with the Tb- and Gd-doped samples. By contrast, the Zr-doped mesoporous CeO2 powders showed the highest catalytic performance and stability in the CO and HCl oxidation of all samples but a lower OSC. These findings suggest that the OSC should therefore be treated with caution when relating to the catalytic performance in oxidation reactions. The differences in the stability are in agreement with recent studies of the HCl oxidation on model cerium oxide films with defined stoichiometry: the low stability of the aliovalent doped CeO2 materials in the HCl oxidation reaction can be attributed to a large number of oxygen vacancies.

The interaction of ultrathin single-crystalline IrO2(110) films with the gas phase proceeds via the coordinatively unsaturated sites (cus), in particular Ircus, the undercoordinated oxygen species ...

Stoichiometric cerium oxide nanoparticles with different sizes, ranging between 2 and 12 nm, were prepared by a rational design of the synthetic concept, based on a special hydrothermal procedure. ...

Long-term stability of heterogeneous catalysts is an omnipresent and pressing concern in industrial processes. Catalysts with high activity and selectivity can be searched for by high-throughput screening methods based maybe on educated guesses provided by ab initio thermodynamics or scaling relations. However, high-throughput screening is not feasible and is hardly able to identify long-term stable catalyst so that a rational and knowledge-driven approach is called for to identify potentially stable and active catalysts. Unfortunately, our current microscopic understanding on stability issues is quite poor. We propose that this gap in knowledge can be at least partly closed by investigating dedicated model catalyst materials with well-defined morphology that allow for a tight link to theory and the application of standard characterization methods. This topic is highly interdisciplinary, combining sophisticated inorganic synthesis with catalysis research, surface chemistry, and powerful theoretical modeling. In this Account, we focus on the stability issues of Deacon catalysts (RuO2 and CeO2-based materials) for recovering Cl2 from HCl by aerobic oxidation and how to deepen our microscopic insight into the underlying processes. The main stability problems under harsh Deacon reaction conditions concomitant with a substantial loss in activity arise from deep chlorination of the catalyst, leaching of volatile chlorides and oxychlorides, and decrease in active surface area by particle sintering. In general, powder materials with undefined particle shape are not well suited for examining catalyst stability, because changes in the morphology are difficult to recognize, for instance, by electron microscopy. Rather, we focus here on model materials with well-defined starting morphologies, including electrospun nanofibers, shape-controlled nanoparticles, and well-defined ultrathin crystalline layers. CeO2 is able to stabilize shape-controlled particles, exposing a single facet orientation so that comparing activity and stability studies can reveal structure sensitive properties. We develop a quasi-steady-state kinetic approach that allows us to model the catalyst chlorination as a function of temperature and gas feed composition. For the case of pure CeO2 nanocubes, this simple approach predicts chlorination to be efficiently suppressed by addition of little amounts of water in the reaction feed or by keeping the catalyst at higher temperature. Both process parameters have great impact on the actual reactor design. Thermal stabilization of CeO2 by intermixing Zr has been known in automotive exhaust catalysis for decades, but this does not necessarily imply also chemical stabilization of CeO2 against bulk-chlorination since Zr can readily form volatile ZrCl4 and may quickly lose its stabilizing effect. Nevertheless, with model experiments the stabilizing effect of Zr in the Deacon process over mixed CexZr1-xO2 nanorods is clearly evidenced. Even higher stability can be accomplished with ultrathin CeO2 coatings on preformed ZrO2 particles, demonstrating the great promise of atomic layer deposition (ALD) in catalysis synthesis.

We investigate with in situ surface X-ray diffraction (SXRD) and X-ray reflectivity (XRR) experiments the cathodic stability of an ultrathin single-crystalline IrO2(110) film with a regular array of mesoscopic rooflike structures that is supported on a RuO2(110)/Ru(0001) template. It turns out that the planarity of the single-crystalline IrO2(110) film is lost in that IrO2(110) oxide domains delaminate at a cathodic potential of −0.18 V. Obviously, the electrolyte solution is able to reach the RuO2(110) layer presumably through the surface grain boundaries of the IrO2(110) layer. Subsequently, the single-crystalline RuO2(110) structure-directing template is reduced to amorphous hydrous RuO2, with the consequence that the IrO2(110) film loses partly its adhesion to the template. From in situ XRR experiments we find that the IrO2(110) film does not swell upon cathodic polarization down to −0.18 V, while from in situ SXRD experiments, the lattice constants of IrO2(110) are shown to be not affected. The rooflike mesostructure of the IrO2(110) flakes remains intact after cathodic polarization to −0.18 V, evidencing that the crystallinity of IrO2(110) is retained.

Review of scientific instruments 93(6), 065111 (2022). doi:10.1063/5.0087864, Characterizing electrode surface structures under operando conditions is essential for fully understanding structure-activity relationships in electrocatalysis. Here, we combine in a single experiment, high-energy surface x-ray diffraction as characterizing technique with a rotating disk electrode to provide steady state kinetics under electrocatalytic conditions. Using Pt(111) and Pt(100) model electrodes, we show that full crystal truncation measurements are readily possible up to rotation rates of 1200 RPM. Furthermore, we discuss possibilities for both potentiostatic as well as potentiodynamic measurements, demonstrating the versatility of this technique. These different modes of operation, combined with the relatively simple experimental setup, make the combined rotating disk electrode - surface x-ray diffraction experiment a powerful technique for studying surface structures under operando electrocatalytic conditions., Published by American Institute of Physics, [S.l.]

The journal of physical chemistry / C 126(48), 20243 - 20250 (2022). doi:10.1021/acs.jpcc.2c06429, In situ stability studies of an IrO$_2$(110)–TiO$_2$(110) model electrode are carried out under acidic water electrolysis conditions, employing synchrotron-based techniques including surface X-ray diffraction (SXRD) and X-ray reflectometry (XRR) with a photon energy of 21.5 keV. These experiments are complemented by ex situ scanning electron microscopy (SEM), scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS) experiments. Even at an anodic current density of 250 mA·cm$^{-2}$ during electrochemical water splitting, the IrO2(110)–TiO$_2$(110) model electrode turned out to be stable against Ir dissolution. However, radiation-induced damages of the IrO$_2$(110) film are observed: Part of the IrO2(110) film delaminates upon heavy exposure to the synchrotron beam, while subsequently the uncovered TiO$_2$(110) is subject to further (photon-induced) corrosion. We propose that the X-ray photons induce oxygen vacancy formation by displacing O$^{2–}$ ions of TiO$_2$ from regular to interstitial sites, while the potential drop across the TiO$_2$(110) substrate leads to a migration of interstitial O$^{2–}$ ions from interface toward bulk TiO$_2$. This reduction step at the interface between IrO$_2$(110) and TiO$_2$(110) weakens the adhesion of the epitaxially grown IrO$_2$(110) film to the TiO2(110) substrate so that the strained IrO$_2$(110) film is partially delaminated. Higher X-ray photon energies of 60–90 keV mitigate this degradation process., Published by Soc., Washington, DC

Electrochemical water splitting is a key technology for moving toward a promising energy scenario based on renewable (regenerative) energy resources in that wind and solar energy can be stored and ...

ACS catalysis 9(7), 6530 - 6539 (2019). doi:10.1021/acscatal.9b01402, Sophisticated $\mathrm{IrO_{2}(110)}$-based model electrodes are prepared by deposition of a 10 nm thick single-crystalline $\mathrm{IrO_{2}(110)}$ layer supported on a structure-directing $\mathrm{IrO_{2}(110)-RuO_{2}(110)/Ru(0001)}$ template, exposing a regular array of mesoscopic rooflike structures. With this model electrode together with the dedicated in situ synchrotron based techniques (SXRD, XRR) and ex situ characterization techniques (SEM, ToF-SIMS, XPS), the corrosion process of $\mathrm{IrO_{2}(110)}$ in an acidic environment (pH 0.4) is studied on different length scales. Potential-induced pitting corrosion starts at 1.48 V vs SHE and is initiated at so-called surface grain boundaries, where three rotational domains of $\mathrm{IrO_{2}(110)}$ meet. The most surprising result is, however, that even when the electrode potential is increased to 1.94 V vs SHE 60–70% of the $\mathrm{IrO_{2}}$ film still stays intact down to the mesoscale and atomic scale and no uniform thinning of the $\mathrm{IrO_{2}(110)}$ layer is encountered. Neither flat$\mathrm{IrO_{2}(110)}$ terraces nor single steps are attacked. Ultrathin single-crystalline $\mathrm{IrO_{2}(110)}$ layers seem to be much more stable to anodic corrosion than hitherto expected., Published by ACS, Washington, DC

The growth of a flat, covering, and single-crystalline IrO2(110) film with controlled film thickness on a single-crystalline TiO2(110) substrate is reported. The preparation starts with a deposition of metallic Ir at room temperature followed by a post-oxidation step performed in an oxygen atmosphere of 10-4 mbar at 700 K. On this surface, additional Ir can be deposited at 700 K in an oxygen atmosphere of 10-6 mbar to produce a IrO2(110) layer with variable thicknesses. To improve the crystallinity of the resulting IrO2(110) layer, the final film was post-oxidized in 10-4 mbar of O2 at 700 K for 5 min. The surface-sensitive techniques of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED) are employed to characterize the morphology, crystallinity, and electronic structure of the prepared ultrathin IrO2(110) films and how these films decompose upon annealing under ultrahigh vacuum (UHV) conditions. STM provides evidence that the IrO2(110) films start already to reduce at 465 K under UHV conditions. Upon annealing to 605 K under UHV the reduction of IrO2 intensifies (XPS), but the oxide film can readily be restored by re-oxidation in 10-4 mbar of O2 at 700 K. Thermal decomposition at 725 K leads, however, to severe reduction of the IrO2(110) layer (XPS, STM) that cannot be restored by a subsequent re-oxidation step. The utility of the IrO2(110)-TiO2(110) system as model electrodes is exemplified with the electrochemical oxygen evolution reaction in an acidic environment.

A strategy is presented to deposit defined layers of TiO2 onto the pore surface within ordered mesoporous, crystalline CeO2-ZrO2 thin films using atomic layer deposition (ALD). As structure-directing agent, a special diblock copolymer, poly(isobutylene)-block-poly(ethylene oxide), was used, resulting in a three-dimensional arrangement of spherical pores with diameter around 13 nm. High resolution transmission electron microscopy investigations evidence the presence of anatase TiO2 coatings within the mesopores, while ellipsometric porosimetry studies together with time-of-flight secondary-ion mass spectrometry depth profiles indicate the deposition inside the mesopores up to 50 ALD cycles. Afterward, the interconnecting channels between the mesopores are filled completely prohibiting further transport of the gaseous TiO2 precursor into the ordered structure of mesopores and hence, limit the layer growth on the surfaces of the pores. Therefore, the size of the mesopores and their connections are decisive w...

The journal of physical chemistry / C 123(7), 3979 - 3987 (2019). doi:10.1021/acs.jpcc.8b10741, With in situ surface X-ray diffraction (SXRD)and X-ray reflectivity (XRR) in combination with ex situcharacterization by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and cyclic voltamme-try, the electrochemical reduction of an ultrathin (1.66 nmthick) single-crystalline RuO$_2$(110) layer supported onRu (0001) is studied in an acidic environment, providingclear-cut evidence and mechanistic details for the trans-formation of RuO$_2$ to hydrous RuO$_2$ and metallic Ru. The reduction process proceeds via proton insertion into the RuO$_2$ (110)lattice. For electrode potentials (0 to−50 mV vs standard hydrogen electrode), the layer spacing of RuO2(110) increased,maintaining the octahedral coordination of Ru (SXRD). Continuous proton insertion at−100 to−150 mV leads to thetransformation of the lattice oxygen of RuO$_2$ to OH and water, which destroys the connectivity among the Ru-O6octahedronsand eventually leads to the loss of crystallinity (SXRD) in the RuO$_2$ (110)film at−200 mV accompanied by a swelling of thelayer with a well-defined thickness (XRR). During the protonation process, soluble Ru complexes may form. With XPS thetransformation of RuO$_2$ (110) to a hydrous RuO$_2$ layer is followed, a process that proceedsfirst homogeneously and at highercathodic potentials heterogeneously by re-deposition of previously electrochemically dissolved Ru complexes., Published by ACS Publ., Washington, DC

Down to a cathodic potentials of -1.20 V versus the reversible hydrogen electrode, the structure of IrO

RuO 2 is commercially employed as an anodic catalyst in the chlor-alkali process. It is also one of the most active electrocatalysts for the oxidation of water, relevant to electrochemical water splitting. However, the use of RuO 2 is limited by its low anodic stability under acidic conditions, especially at high overpotentials. In the present work, the electrochemical stability of model RuO 2(110)/Ru(0001) anodes was investigated in order to gain a deeper understanding of the relation between structure and performance in Cl 2 and O 2 evolution reactions (CER and OER, respectively). Online electrochemical mass spectrometry was used to determine the onset potential of CER and OER in HCl and H 2SO 4 electrolytes, respectively. The onset potential of OER was higher in HCl than in H 2SO 4 due to competition with the kinetically more favorable CER. A detailed stability evaluation revealed pitting corrosion of the electrode surface with exposure of Ru(0001) metal substrate concomitant with the formation of a hydrous RuO 2 in some areas regardless of the applied electrochemical treatment. However, despite local pitting, the RuO 2(110) layer preserves its thickness in most areas. Degradation of the electrode was found to be less severe in 0.5 M HCl due to a decrease in the faradaic efficiency of RuO 2 oxidation caused by competition with the kinetically more favorable CER.

Mixed Ce1-xZrxO2 nano-rod particles with varying Zr doping levels of up to 20% were exposed to Deacon reaction mixtures with high HCl concentration at a reaction temperature of 430 °C. The mixed Ce1-xZrxO2 nano-rod samples were characterized before and after Deacon reaction by x-ray diffraction (XRD), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS). Pure CeO2 nano-rods are shown to suffer from bulk-chlorination of the catalyst (CeCl3x6H2O) for reaction mixtures HCl:O2 > 2 that is accompanied by dramatic activity losses. For a reaction mixture of HCl:O2 = 2.5:1, already 5% Zr doping suffices to stabilize the structure Ce1-xZrxO2 nano-rods against bulk chlorination. Although ZrCl4 is volatile above 330 °C, we do not encounter discharge of Zr from the fixed bed flow reactor as long as severe bulk chlorination of the Ce1-xZrxO2 catalysts in the form of CeCl3 is not observed in XRD. With a simple quasi steady-state model we can corroborate the stabilizing effect of Zr assuming that ZrCl4 evaporation is limited by diffusion of Zr4+ in the ceria-zirconia subsurface. As qunatified with XPS the chlorine concentration of stable and active Ce1-xZrxO2 nano-rods with 5% Zr is too high to be solely ascribed to on-surface chlorine. The incorporation of Cl into the surface-near region is further supported by XPS experiments performed after oxygen plasma treatment. For even more harsh reaction conditions HCl:O2 = 3:1 none of the Ce1-xZrxO2 nano-rods was stable at 430 °C. However, an increase of the reaction temperature to 500 °C enables even pure CeO2 nano-rods to be stable under such harsh reaction conditions.

Ultrathin crystalline CeO2(111) films were grown on Ru(0001) in order to study the interaction of HCl with this surface as a first step of the Deacon reaction with experimental techniques, including low energy electron diffraction (LEED), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and thermal desorption spectroscopy (TDS) in combination with density functional theory calculations (DFT+U). HCl molecules adsorb dissociatively on CeO2(111), forming a well-ordered (3x3)R30° overlayer structure with one Cl and one H per surface unit cell. DFT calculations indicate that HCl adsorption is exothermic by 1.15 eV and proceeds via an acid–base reaction. The mixed overlayer structure is stabilized by Lewis acid–base pairing (∼0.4 eV). Stoichiometric CeO2(111) films are likely to be not very active in the Deacon process since at 800 K the recombination of adsorbed H* and Cl* to form HCl is far more preferred over Cl* + Cl* recombination to form the desired product Cl2.

A template-assisted growth of a flat, covering, and single-crystalline IrO2(110) film with controlled film thickness is reported that is suitable for use in model catalysis and as model electrodes in electrocatalysis. The template consists of a single-crystalline covering RuO2(110) layer grown on Ru(0001). In the first step, we formed IrO2 seeds on the RuO2(110) layer, which then continue to grow by deposition of Ir in an oxygen atmosphere of 3 × 10–7 mbar at a sample temperature of 700 K. The IrO2 seeds are prepared by depositing nanometer size metallic Ir particles on RuO2(110) (in total 0.3–0.5 monolayer (ML) of Ir) at room temperature. Subsequently, the Ir particles are oxidized in 10–5 mbar of O2 at a sample temperature of 700 K. The techniques of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, and low-energy electron diffraction (LEED) are employed to characterize the morphology, crystallinity, and electronic structure of the prepared ultrathin IrO2(110) films. Thermal desorpt...

Extended Tafel plots at various temperatures for an electrocatalyzed reaction and (possibly) its reversed reaction on single-crystalline model electrodes allow for constructing the (essential part of the) free energy surface, in particular the free energies of the transition states (TS). Free energies of the reaction intermediates (RIs) including the chemical nature of active surface sites (S) are hardly accessible to experiment and need therefore to be taken from constrained ab initio thermodynamics calculations. The compact compilation of experimental kinetic data in the form of a free energy diagram enables a critical assessment and validation of theoretical free energy landscapes based on first-principles kinetics. For three prototypical electrocatalyzed reactions, namely the chlorine evolution reaction (CER) and oxygen evolution reaction (OER) over RuO2(110) as well as hydrogen evolution reaction (HER) on Pt(111), we exemplify this universal approach and discuss potential benefits for theoretical mod...

CeO2 nanoparticles with diameters of ca. 6 nm were synthesized using a microwave-based synthetic route, enabling dispersions in various unipolar solvents. From these dispersions, CeO2 nanofibers were prepared by electrospinning followed by heat treatment at 550 °C in air, possessing mesoporosity with BET surface areas larger than 100 m2 g−1. This mesoporosity is due to a nanoscale separation between the nanoparticles and the spinning polymer. The well-defined CeO2 fibers were used as catalysts in the HCl oxidation reaction (Deacon process), and the catalytic parameters (space-time yield (STY), oxygen storage capacity (OSC), and the so-called complete oxygen storage capacity (OSCc)) were compared with recently reported nanoscopic CeO2 materials. It is found that the fibers and also the particles themselves show comparably high activity (STY), which correlates with a high OSCc value, in comparison with CeO2 materials possessing larger particle sizes. This correlation implies that the entire particles, not only the surface, are involved in oxidation reactions. In the HCl oxidation reaction, the fiber morphology is degraded and the surface area is substantially decreased, but the activity is still quite high after 60 h on stream, and no chlorination is detectable by X-ray Diffraction, in contrast to CeO2 materials with larger particle sizes. These findings demonstrate that the stability and activity of CeO2-based catalysts can only be scrutinized by a material comprising both a high surface area and well-defined morphology.

We studied the stability of CeO 2 nano-cubes with preferentially (1 0 0)-oriented facets in the HCl oxidation reaction (Deacon process) for various reaction temperatures and the addition of small concentrations of water in the gas feed. For a reaction mixture HCl:O 2 = 1:2 we find that CeO 2 is substantially chlorinated below 380 °C, revealing a low catalytic activity. At 390 °C both the activity and the chlorination degree change abruptly: activity becomes high and chlorination is not detectable by XRD. The experimental results are rationalized by a kinetic model which allows us to study catalyst chlorination as a function of temperature and gas feed composition. The model predicts that chlorination sets in at the inlet of the catalyst bed and propagates then slowly along the catalyst bed towards the reactor outlet to fully chlorinate the CeO 2 catalyst bed. This process has been confirmed by a dedicated experiment employing two separate catalyst layers, where only the first layer is shown to be chlorinated while exposed to the Deacon gas feed. Our model attributes the excessive chlorination at the reactor inlet to the absence of H 2 O in the gas feed, as the formation of H 2 O by the reaction of HCl with CeO 2 is the prevailing driving force for catalyst chlorination. When running the Deacon process at 375 °C, the chlorination of CeO 2 nano-cubes is efficiently suppressed by the addition of 1% water to the reaction mixture. This extrinsic stabilization of oxide catalysts towards chlorination by water is a general concept, which may enable the identification of new oxide materials that have previously been ruled out as Deacon catalysts due to their low stability under reaction conditions.

Transient spectroscopic surface-chemistry experiments in combination with spatially resolved kinetic Monte Carlo (KMC) simulations offer great potential to gain a wealth of molecular information on the kinetics of catalytic surface reactions as exemplified by the CO oxidation reaction over RuO2(110). This approach surpasses the common problem that in the steady-state reactions, the prevailing species detectable by in operando surface-sensitive spectroscopy are frequently spectator species, thereby obscuring the reactive surface species. Our experiment is sensitive to the relative activity of different oxygen species by saturating the surface with loosely bound oxygen, leaving only single vacancies where CO can adsorb and recombine with oxygen. With in situ reflection–absorption infrared spectroscopy (RAIRS) in combination with ab initio based KMC simulations, we follow the time evolution toward steady state (transient experiment). In this way, we are able to resolve a long-standing controversy about the a...

Ultrathin single-crystalline RuO 2(110) films supported on Ru(0001) are employed as model electrodes to extract kinetic information about the industrially important chlorine evolution reaction (CER) in a 5M concentrated NaCl solution under well-defined electrochemical conditions and variable temperatures. A combination of chronoamperometry (CA) and online electrochemical mass spectrometry (OLEMS) experiments provides insight into the selectivity issue: At pH = 0.9, the CER dominates over oxygen evolution, whereas at pH = 3.5, oxygen evolution and other parasitic side reactions contribute mostly to the total current density. From temperature-dependent CA data for pH = 0.9, we determine the apparent free activation energy of the CER over RuO 2(110) to be 0.91 eV, which compares reasonably well with the theoretical value of 0.79 eV derived from first-principles microkinetics. The experimentally determined apparent free activation energy of 0.91 eV is considered as a benchmark for assessing future improved theoretical modeling from first principles.

The growth of a flat, covering, and single-crystalline IrO

Oxygen storage in solid catalyst is very important for industrial oxidation reactions such as HCl oxidation reaction (Deacon process). Ceria nanoparticles act as promising catalysts for HCl oxidation reaction. Ceria possesses high OSC (oxygen storage capacity) which is a measure of the oxygen quantity that material can store and release. This makes ceria suitable for redox reactions which usually follow Mars-van Krevelen mechanism (surface oxygen atoms directly involved in reactions). CexZr1−xO2 solid solutions were prepared as a function of the composition x with constant specific surface area in order to explore the relationship between oxygen storage capacity (OSC) and activity of the oxidation reactions of CO and HCl. The as-prepared CexZr1−xO2 solid solutions were characterized by X-ray diffraction (XRD), Raman spectroscopy (Raman), and X-ray photoelectron spectroscopy (XPS). The complete (or total) oxygen storage capacity (“complete” OSC: OSCc) at 430 °C is shown to be linearly correlated to the CO oxidation activity at 430 °C as a function of the Ce concentration x, thus being compatible with the expected Mars–van-Krevelen mechanism. For the catalytic HCl oxidation reaction at 430 °C the activity is also shown to be linearly correlated to the OSCc with a maximum activity realized with Ce0.8Zr0.2O2. From the linear relationship of oxidation activity of HCl and OSCc (that in turn is linear to the activity of CO oxidation) we conclude that the HCl oxidation reaction over CexZr1−xO2 solid solutions proceeds via the Mars–Van-Krevelen mechanism with the reduction of CexZr1−xO2 being rate-determining.

Dry impregnation of CeO2 on ZrO2 particles (CeO2@ZrO2) and calcination at 600 °C for 5 h results in a highly dispersed layer of the catalytically active phase CeO2 with an enriched concentration of Ce3+. The distinct morphology of CeO2@ZrO2 is responsible for the high (specific) activity in the gas-phase oxidation of HCl (Deacon reaction). Surprisingly, the CeO2@ZrO2 catalyst calcined at 600 ºC is chemically even more stable towards chlorination under harsh Deacon conditions than that calcined at 900 °C, where a solid solution of ZrO2 and CeO2 is formed at the solid interface. The improved stability of the CeO2 layer calcined at 600 ºC is traced to its sharp interface formed with ZrO2 support. We expect that this kind of active and stable layer morphology of CeO2 on ZrO2 is beneficial for other catalytic oxidation reaction as well, either as active component or as supporting material for another active metal component.

Ab initio kinetic Monte Carlo (KMC) is successfully applied to simulate the experimentally observed promoting effect of O2 on the HCl oxidation reaction (Deacon process) catalyzed by RuO2(110). Density functional theory (DFT) calculations provide, in addition to the adsorption energies of reaction intermediates and activation energies, also interaction energies between the adsorbates within the cluster expansion approach. KMC simulations with this extended set of energy parameters were analyzed employing the concept of “degree of rate control”. In contrast to previous propositions, our simulations indicate that neither the dissociative O2 adsorption (the sterically hindered first reaction step) nor the associative desorption of chlorine (the step with the highest activation energy) are rate determining under typical Deacon conditions. Instead, the hydrogen transfer in the water formation determines the rate of the overall reaction. These hydrogen transfer processes are not highly activated but turn out to...

Illumination by ultraviolet light and subsequent annealing to 600–700 K lead to characteristic morphological changes of a fully covering four to five monolayer thick RuO2(110) film on TiO2(110) and even to a decomposition of RuO2(110) islands on TiO2(110). These photo-induced degradation processes in a water atmosphere of 10−6 mbar and without water were studied with scanning tunneling microscopy and supplemented by X-ray photoelectron spectroscopy. The photo-assisted process is considered to be mediated by the underlying semiconducting TiO2(110) substrate through photo-generated hot electron–hole pairs. Due to energy band bending formed at the RuO2(110)/TiO2(110) heterojunction (Schottky barrier) the holes are selectively drawn towards the interface, where these can conduct oxidation reactions leading to modifications of the RuO2(110) surface and to the observed morphological degradation in the subsequent annealing step to 600 or 700 K.

We examined the interaction of oxygen with ultrathin Ru layers deposited on a Au(111) substrate using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and low-energy electron diffraction. The deposition of pure Ru below one monolayer (ML) at room temperature leads to the formation of clusters on the Au(111) surface, preferentially located at the elbow sites of the herringbone reconstruction. Subsequent exposure of molecular oxygen to such a Ru-covered Au(111) surface at 680 K results in the growth of two-layer-thick Ru islands that are embedded in the top Au(111) layer. This structural reorganization of Ru is driven by the minimization of surface energy and mediated by a mobile RuOx species. Deposition of an increasing amount of Ru at 620 K (0.5-10 ML, ML = monolayer) leads to a rough Ru film on Au(111). Subsequent oxygen treatment (10(-5) mbar) at 680 K creates either a porous Ru film (4 ML) or a flat RuO2(110) film (6 ML) depending on the thickness of the Ru film.

In our paper(1) the N 1s core level energy as obtained by Al Kα excited X-ray photoelectron spectroscopy (XPS) of boron nitride on Ru(0001) is not correctly reproduced, it is 398.63 ± 0.15 eV and not 389.63 ± 0.15 eV. Table 1 reproduces the B 1s and N 1s binding energies and work functions for h-BN on Rh(111), Ru(0001), Ni(111), and Pd(111).(2)Figure 1 reproduces the corresponding core level data with the correct energy scale. We are grateful to W. Auwarter for spotting these inconsistencies.

We report on a scanning tunneling microscopy (STM) study of the nanostructuring of the Au/Ru(0001) thin film system for the cases of 5 monolayers (ML) and 9 ML of Au deposited at 300 K and subsequently annealed at 1050 K. A new laterally periodic superstructure is observed at the surface of the 9 ML film, which is essentially a rippling in height of the surface atomic layer with the magnitude up to 0.03 ± 0.01 nm and in-plane periodicity of 4.6 ± 0.4 nm, the long-range order being absent.

In this perspective we discuss how an intimate interaction of experiments with theory is able to deepen our insight into the catalytic reaction system on the molecular level. This strategy is illustrated by discussing various examples from our own research of surface chemistry and model catalysis. The particular examples were carefully chosen to balance the specific strength of both approaches – theory and experiment – and emphasize the benefit of this combined approach. We start with the determination of complex surface structures, where diffraction techniques in combination with theory are clear-cut. The promoter action of alkali metals in heterogeneous catalysis is rationalized with theory and experiment for the case of CO coadsorption. Predictive power of theory is limited as demonstrated with the apparent activity of chlorinated TiO2(110) in the oxidation of HCl: Even if we know all elementary reaction steps of a catalytic reaction mechanism, the overall kinetics may remain elusive and require the application kinetic Monte Carlo simulations. Catalysts are not always stable under reaction conditions and may chemically transform as discussed for the CO oxidation reaction over ruthenium. Under oxidizing reaction conditions ruthenium transforms into RuO2, a process which is hardly understood on the molecular level. Lastly we focus on electrochemical reactions. Here theory is clearly ahead since spectroscopic methods are not available to resolve the processes at the electrode surface.

Long-term stability is a major issue in the development of new catalysts and in the improvement of existing catalysts. In general, high-surface-area materials in catalysis are not well suited as model materials for testing catalyst stability, because changes in the morphology are difficult to recognize with electron microscopy. Therefore, we developed a new type of model catalyst in the form of nanofibers for assessing structural changes of CeO 2 -based catalysts under highly corrosive Deacon reaction conditions. In the Deacon process, chlorine is recycled by heterogeneously catalyzed gas-phase oxidation of HCl, using metal oxide catalysts. The Deacon process generally suffers from degradation of the catalyst due to in-depth chlorination. One-dimensional CeO 2 -based nanofibers with a well-defined morphology and surface enable direct visualization of structural changes before and after the Deacon reaction, simply employing scanning electron microscopy supported by X-ray diffraction and physisorption measurements. Preparing solid solutions of CeO 2 and ZrO 2 has shown to improve both the chemical stability and the activity of the catalyst.

The structural modification of the Ru(0001) surface is followed in real-time using low-energy electron microscopy at elevated temperatures during exposure to molecular oxygen. We observe the nucleation and growth of three different RuO2 facets, which are unambiguously identified by single-domain microspot low-energy electron diffraction (μLEED) analysis from regions of 250 nm in diameter. Structural identification is then pushed to the true nanoscale by employing very-low-energy electron reflectivity spectra R(E) from regions down to 10 nm for structural fingerprinting of complex reactions such as the oxidation of metal surfaces. Calculations of R(E) with an ab initio scattering theory confirm the growth of (110), (100), and (101) orientations of RuO2 and explain the shape of the R(E) spectra in terms of the conducting band structure. This methodology is ideally suited to identify the structure of supported ultrathin films and dynamic transformations at multicomponent interfaces down to few nanometer lateral resolution at elevated temperature and in reactive environments.

With scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) we studied the redox properties of Au islands supported on Ru(0001) as a function of the island thickness. Both t...