Recently, there has been increasing evidence that CO oxidation on TiO2 supported Au catalysts proceeds predominantly via a Au-assisted Mars-van Krevelen mechanism for reaction temperatures of 80 degrees C and above. We here present results of a combined experimental and theoretical study, aiming at the identification of activated steps in this reaction. O-2 multipulse experiments, performed in a temporal analysis of products (TAP) reactor at different temperatures between -80 and +240 degrees C, revealed that the replenishment of surface lattice oxygen vacancies at perimeter sites, at the perimeter of the interface between TiO2 support and Au nanoparticles, proceeds with essentially constant efficiency, independent of the reaction temperature. Hence, this reaction step is barrier-free. Previous studies (Widmann and Behm Angew. Chem. Int. Ed. 2011, 50, 10241) had shown that the preceding step, the formation of a surface lattice oxygen vacancy at these sites, is activated, requiring temperatures above room temperature. Density functional theory based calculations, performed on a Au nanorod supported on a TiO2 anatase (101) substrate confirmed that the presence of the Au nanorod leads to a significant reduction of the vacancy formation energy at these sites, resulting in a barrier of only similar to 0.9 eV for vacancy formation by reaction with adsorbed CO. The reverse process, replenishing the vacancies by reaction with O-2, was found to be activated in the case of individual vacancies but essentially barrier-free for the case of pairs of neighbored vacancies. Consequences of these findings for the mechanism of the CO oxidation reaction on these catalysts, which can be considered as a model system for Au catalysts supported on reducible oxides, are discussed.

AbstractNach neueren Berichten sind Au/Mg(OH)2‐Katalysatoren bei niedrigen Temperaturen, unterhalb Raumtemperatur, weitaus aktiver in der katalytischen CO‐Oxidation als die häufig untersuchten Au/TiO2‐Katalysatoren. Mithilfe kinetischer und in‐situ‐IR‐spektroskopischer (DRIFTS‐)Messungen wird gezeigt, dass die vergleichsweise schwache Wechselwirkung von Au/Mg(OH)2 mit in der Reaktion gebildetem CO2 der hauptsächliche Grund für die höhere Aktivität dieser Katalysatoren bei niedrigen Reaktionstemperaturen ist. Dies ermöglicht die schnelle Desorption des Reaktionsprodukts CO2 und damit eine effiziente kontinuierliche Oxidation von CO bei Temperaturen deutlich unterhalb 0 °C. Bei diesen Temperaturen katalysiert Au/TiO2 zwar auch die Bildung von CO2, hier ist aber kaum CO2‐Desorption möglich, und dies führt zu einer Selbstvergiftung des Katalysators. Bei höheren Temperaturen (oberhalb 0 °C) ist dagegen die Bildung von CO2 geschwindigkeitsbestimmend, was in einer deutlich höheren Aktivität von Au/TiO2‐Katalysatoren unter diesen Bedingungen resultiert.

Electronic metal-support interactions (EMSIs) are demonstrated to severely affect the CO oxidation activity and the CO adsorption properties of Au/TiO2 catalysts. Bulk oxygen vacancies, generated by a strongly reductive pretreatment of Au/TiO2 at elevated temperature in 10% CO/N-2, significantly lower the catalytic activity for CO oxidation at 80 degrees C. With time on stream, the activity slowly increases until reaching the same steady-state value as that obtained for a previously calcined and, hence, defect-poor Au/TiO2 catalyst (activation period), where the time required for the activation period decreases with reaction temperature but is independent of the oxygen partial pressure. Considering the similar Au particle sizes and Au loadings, we conclude that the different activities originate from the presence of bulk oxygen vacancies generated during pretreatment, which are slowly replenished during reaction. In situ IR spectroscopy measurements reveal that the lower activity in the presence of bulk defects is coupled with and likely results from a strong modification of the CO adsorption strength on the reduced Au/TiO2 catalysts due to EMS's. A possible mechanism explaining how these EMSIs may be induced by the presence of bulk defects is discussed.

Au/Mg(OH)2 catalysts have been reported to be far more active in the catalytic low-temperature CO oxidation (below 0 °C) than the thoroughly investigated Au/TiO2 catalysts. Based on kinetic and in situ infrared spectroscopy (DRIFTS) measurements, we demonstrate that the comparatively weak interaction of Au/Mg(OH)2 with CO2 formed during the low-temperature reaction is the main reason for the superior catalyst performance. This feature enables rapid product desorption and hence continuous CO oxidation at temperatures well below 0 °C. At these temperatures, Au/TiO2 also catalyzes CO2 formation, but does not allow for CO2 desorption, which results in self-poisoning. At higher temperatures (above 0 °C), however, CO2 formation is rate-limiting, which results in a much higher activity for Au/TiO2 under these reaction conditions.

Changes of the geometric and electronic structure of gold on Au/CeO2 catalysts induced by different pre-treatments (oxidative and reductive) and by the CO oxidation reaction at 80°C were followed by operando XANES / EXAFS measurements. The results showed that i) oxidative pre-treatment (O2) leads to larger Au nanoparticles than reductive pre-treatment (CO), that ii) Au is predominantly metallic during CO oxidation, irrespective of the preceding pre-treatment, and that iii) there is a reaction induced Au particle growth. Correlations with the activity of the respective catalysts and its temporal evolution give insights into the origin of deactivation of these catalysts under reaction conditions, in particular on reaction induced changes in the Au particle size.

Aiming at an improved mechanistic understanding of the preferential oxidation of CO on supported Au catalysts, we have investigated the competition between CO and H2 for stable, active oxygen (Oact) species on a Au/TiO2 catalyst during the simultaneous exposure to CO and H2 with various CO/H2 ratios at 80 °C and 400 °C by quantitative temporal analysis of products (TAP) reactor measurements. It is demonstrated that, at both higher and lower temperature, the maximum amount of active oxygen removal is (i) independent of the CO/H2 ratio and (ii) identical to the amount of active oxygen removal by CO or H2 alone. Hence, under preferential CO oxidation (PROX) reaction conditions, in the simultaneous presence of CO and H2, CO and H2 compete for the same active oxygen species. In addition, also the dependency of the selectivity towards CO oxidation on the CO/H2 ratio was evaluated from these measurements. Consequences of these findings on the mechanistic understanding of the PROX reaction on Au/TiO2 will be discussed.

As part of concepts for chemical energy storage of excess electrical energy produced from renewable sources, we have investigated the performance of a Au/ZnO catalyst for the formation of methanol from CO2 and H-2 at pressures between 5 and 50 bar and at 240 degrees C, focusing on the effect of total pressure and on the influence of CO, and compared it to the activity and selectivity of commercial CuanO/Al2O3 catalysts. Kinetic measurements reveal that Au/ZnO catalysts have similar methanol formation rates and superior selectivity toward methanol compared to Cu/ZnO/Al2O3 over the whole range of pressure. CO, which can be formed during CO2 hydrogenation via the reverse water-gas shift (RWGS) reaction, has little effect on the methanol formation for low CO concentrations up to 1%, while higher CO concentrations significantly diminish the methanol formation rate, at constant CO2 and H-2 partial pressures. From these kinetic data and from additional in-situ infrared (IR) spectroscopy measurements, performed during CO2 hydrogenation as well as during CO hydrogenation at 5 bar, we conclude that methanol formation from CO2 and CO proceeds via different, independent reaction pathways and therefore CO is not an intermediate in the hydrogenation of CO2. Consequences of these results on possible applications of Au/ZnO in renewable energy concepts will be discussed. (C) 2015 Elsevier Inc. All rights reserved.

Despite enormous breakthroughs in our understanding of the reaction mechanism of the low-temperature CO oxidation on gold catalysts, in particular on Au/TiO2 and down to temperatures as low as -150 degrees C, there are still many contradictory proposals about the dominant reaction pathway. In this work, we will demonstrate that these discrepancies often originate from the rather different reaction conditions applied in numerous studies, most notably from different reaction temperatures. By combining temporal analysis of products reactor measurements with electron paramagnetic resonance spectroscopy, we will show that removal of TiO2 surface lattice oxygen from a Au/TiO2 catalyst upon exposure to CO (i) readily takes place at 120 degrees C, where it represents the active oxygen species for CO oxidation, (ii) is still possible at -20 C, although much slower and to a much lower extent, and (iii) is completely inhibited at -90 degrees C. Consequences of these findings for our understanding of the dominant reaction pathway for the CO oxidation on Au/TiO2 catalysts, in particular its dependency on the reaction temperature, will be discussed.

Understanding the redox properties of metal oxide based catalysts is a major task in catalysis research. In situ electron paramagnetic resonance (EPR) spectroscopy is capable of monitoring the change of metal ion valences and formation of active sites during redox reactions, allowing for the identification of ongoing redox pathways. Here in situ EPR spectroscopy combined with online gas analysis, supported by ex situ X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), temporal analysis of product (TAP), and mass spectrometry (MS) studies, was utilized to study the redox behavior of CuO-CeO2 catalysts under PROX conditions (preferential oxidation of carbon monoxide in hydrogen). Two redox mechanisms are revealed: (i) a synergetic mechanism that involves the redox pair Ce4+/Ce3+ during oxidation of Cu0/Cu+ species to Cu2+ and (ii) a direct mechanism that bypasses the redox pair Ce4+/Ce3+. In addition, EPR experiments with isotopically enriched 17O2 established the synergetic mechanism as the major redox reaction pathway. The results emphasize the importance of the interactions between Cu and Ce atoms for catalyst performance. With the guidance of these results, an optimized CuO-CeO2 catalyst could be designed. A rather wide temperature operation window of 11 K (from 377 to 388 K), with 99% conversion efficiency and 99% selectivity, was achieved for the preferential oxidation of CO in a H2 feed.

Methanol synthesis for chemical energy storage, via hydrogenation of CO2 with H2 produced by renewable energies, is usually accompanied by the undesired formation of CO via the reverse water-gas shift reaction. Aiming at a better mechanistic understanding of methanol formation from CO2/H2 on highly selective supported Au/ZnO catalysts we have investigated the role of CO in the reaction process using isotope labelling experiments. Using (13)C-labelled CO2, we found for reaction at 5 bar and 240 °C that (i) the methanol formation rate is significantly higher in CO2-containing gas mixtures than in a CO2-free mixture and (ii) in mixtures containing both CO2 and CO methanol formation from CO increases with the CO content up to 1% CO, and then remains at 20% of the total methanol formation up to a CO2/CO ratio of 1/1, making CO2 the preferred carbon source in these mixtures. A shift in the preferred carbon source for MeOH from CO2 towards CO is observed with increasing reaction temperatures between 240 °C and 300 °C. At even higher temperatures CO is expected to become the dominant carbon source. The consequences of these findings for the application of Au/ZnO catalysts for chemical storage of renewable energies are discussed.

Applying quantitative temporal analysis of products reactor measurements, we studied the reactive removal of active oxygen present on Au/TiO2 catalysts after calcination at elevated temperatures (400 degrees C) by CO pulses and its replenishment by O-2 pulses at 80 degrees C, focusing on the nature of the active oxygen species. In contrast to previous studies, which mainly focused on and clarified the nature of the active oxygen species for the catalytic CO oxidation, which is reversibly formed and replenished under typical reaction conditions, this study demonstrates that directly after calcination an additional oxygen species is present. This species is also active for the CO oxidation, but it is not or only very little formed under typical reaction conditions. Implications of these results on the mechanistic understanding of the CO oxidation on Au/TiO2, in particular on the role of different active oxygen species, will be discussed. (C) 2016, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

To better understand the role of water in the selective methanation of CO in CO2-rich reformate gases on Ru/Al2O3 catalysts, the influence of exposing these catalysts to H2O-rich reformate gases on their reaction characteristics in transient experiments was investigated by employing kinetic and in situ spectroscopic measurements as well as ex situ catalyst characterization. Transient exposure of the ruthenium catalyst to wet reaction gas (5 or 15% H2O) results in significantly enhanced activity and selectivity for CO methanation in subsequent reactions in dry reformate compared with activation and reaction in dry reformate directly. Operando X-ray absorption spectroscopy results reveal that this is in accordance with a significant decrease in ruthenium particle size, which is stable during subsequent reaction in dry reformate. The implications of these data and additional results from in situ IR spectroscopy on the role and influence of H2O on the reaction, also in technical applications, are discussed.

The potential of metal oxide supported Au catalysts for the formation of methanol from CO2 and H2 under conditions favorable for decentralized and local conversion, which could be concepts for chemical energy storage, was investigated. Significant differences in the catalytic activity and selectivity of Au/Al2 O3 , Au/TiO2 , AuZnO, and Au/ZrO2 catalysts for methanol formation under moderate reaction conditions at a pressure of 5 bar and temperatures between 220 and 240 °C demonstrate pronounced support effects. A high selectivity (>50 %) for methanol formation was obtained only for Au/ZnO. Furthermore, measurements on Au/ZnO samples with different Au particle sizes reveal distinct Au particle size effects: although the activity increases strongly with the decreasing particle size, the selectivity decreases. The consequences of these findings for the reaction mechanism and for the potential of Au/ZnO catalysts for chemical energy storage and a "green" methanol technology are discussed.

The Inside Back Cover, which was hand-drawn by a theoretical chemist who is also an art enthusiast, illustrates the pathway of the synthesis of green methanol (MeOH) on Au nanoparticles supported on ZnO. The green MeOH is produced using H2 generated by the electrolysis of water using excess electrical energy from renewable sources and CO2 obtained by CO2 capture and sequestration. This concept enables the storage of electrical energy in the form of chemical energy as well as the utilization of CO2 leading to the reduction of overall CO2 emissions, both of which are major challenges today. More details are available in the Communication by Behm et al. on page 456 (DOI: 10.1002/cssc.201402645).

The catalytic properties of nanostructured Au and their physical origin were investigated by using the low-temperature CO oxidation as a test reaction. In order to distinguish between structural effects (structure-activity correlations) and bimetallic/bifunctional effects, unsupported nanoporous gold (NPG) samples prepared from different Au alloys (AuAg, AuCu) by selective leaching of a less noble metal (Ag, Cu) were employed, whose structure (surface area, ligament size) as well as their residual amount of the second metal were systematically varied by applying different potentials for dealloying. The structural and chemical properties before and after 1000 min reaction were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The catalytic behavior was evaluated by kinetic measurements in a conventional microreactor and by dynamic measurements in a temporal analysis of products (TAP) reactor. The data reveal a clear influence of the surface contents of residual Ag and Cu species on both O2 activation and catalytic activity, while correlations between activity and structural parameters such as surface area or ligament/crystallite size are less evident. Consequences for the mechanistic understanding and the role of the nanostructure in these NPG catalysts are discussed.

In bester Lage: Aktiver Sauerstoff für die CO-Oxidation über Au/TiO2-Katalysatoren ist eine sehr stabile Sauerstoffspezies, deren Bildung leicht und kaum aktiviert ist. Es handelt sich dabei um Oberflächengittersauerstoff am Rand der Au-TiO2-Grenzfläche, der durch die Au-Nanopartikel aktiviert ist. Bei höheren Temperaturen können wegen thermisch aktivierter Oberflächendiffusion von OGitter und Leerstellen auch weiter entfernte Spezies an der Reaktion teilnehmen.

Ideal location: The active oxygen for CO oxidation on Au/TiO2 catalysts is a highly stable oxygen species, whose formation is facile and hardly activated. This species is proposed to be surface lattice oxygen at the perimeter of the Au–TiO2 interface, activated by the presence of the Au nanoparticle. At higher temperatures, thermally activated Olatt migration also gives access to adjacent Olatt species. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.