Electrifying model catalysts for understanding electrocatalytic reactions in liquid electrolytes.

Electrocatalysis is at the heart of our future transition to a renewable energy system. Most energy storage and conversion technologies for renewables rely on electrocatalytic processes and, with increasing availability of cheap electrical energy from renewables, chemical production will witness electrification in the near future1-3. However, our fundamental understanding of electrocatalysis lags behind the field of classical heterogeneous catalysis that has been the dominating chemical technology for a long time. Here, we describe a new strategy to advance fundamental studies on electrocatalytic materials. We propose to 'electrify' complex oxide-based model catalysts made by surface science methods to explore electrocatalytic reactions in liquid electrolytes. We demonstrate the feasibility of this concept by transferring an atomically defined platinum/cobalt oxide model catalyst into the electrochemical environment while preserving its atomic surface structure. Using this approach, we explore particle size effects and identify hitherto unknown metal-support interactions that stabilize oxidized platinum at the nanoparticle interface. The metal-support interactions open a new synergistic reaction pathway that involves both metallic and oxidized platinum. Our results illustrate the potential of the concept, which makes available a systematic approach to build atomically defined model electrodes for fundamental electrocatalytic studies.

Charge transfer and spillover phenomena in ceria-supported iridium catalysts: A model study.

Iridium-based materials are among the most active bifunctional catalysts in heterogeneous catalysis and electrocatalysis. We have investigated the properties of atomically defined Ir/CeO2(111) model systems supported on Cu(111) and Ru(0001) by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, near ambient pressure X-ray photoelectron spectroscopy (NAP XPS), scanning tunneling microscopy, and temperature programmed desorption. Electronic metal-support interactions in the Ir/CeO2(111) system are accompanied by charge transfer and partial reduction of CeO2(111). The magnitude of the charge transfer depends strongly on the Ir coverage. The Ir/CeO2(111) system is stable against sintering upon annealing to 600 K in ultrahigh vacuum (UHV). Annealing of Ir/CeO2(111) in UHV triggers the reverse oxygen spillover above 450 K. The interaction of hydrogen with Ir/CeO2(111) involves hydrogen spillover and reversible spillover between 100 and 400 K accompanied by the formation of water above 190 K. Formation of water coupled with the strong reduction of CeO2(111) represents the dominant reaction channel upon annealing in H2 above 450 K. The interaction of Ir/CeO2(111) with oxygen has been investigated at moderate and NAP conditions. Additionally, the formation and stability of iridium oxide prepared by deposition of Ir in oxygen atmosphere was investigated upon annealing in UHV and under exposure to H2. The oxidation of Ir nanoparticles under NAP conditions yields stable IrOx nanoparticles. The stability of Ir and IrOx nanoparticles under oxidizing conditions is hampered, however, by encapsulation by cerium oxide above 450 K and additionally by copper and ruthenium oxides under NAP conditions.

Counting electrons on supported nanoparticles.

Electronic interactions between metal nanoparticles and oxide supports control the functionality of nanomaterials, for example, the stability, the activity and the selectivity of catalysts. Such interactions involve electron transfer across the metal/support interface. In this work we quantify this charge transfer on a well-defined platinum/ceria catalyst at particle sizes relevant for heterogeneous catalysis. Combining synchrotron-radiation photoelectron spectroscopy, scanning tunnelling microscopy and density functional calculations we show that the charge transfer per Pt atom is largest for Pt particles of around 50 atoms. Here, approximately one electron is transferred per ten Pt atoms from the nanoparticle to the support. For larger particles, the charge transfer reaches its intrinsic limit set by the support. For smaller particles, charge transfer is partially suppressed by nucleation at defects. These mechanistic and quantitative insights into charge transfer will help to make better use of particle size effects and electronic metal-support interactions in metal/oxide nanomaterials.

Reactivity of atomically dispersed Pt(2+) species towards H2: model Pt-CeO2 fuel cell catalyst.

The reactivity of atomically dispersed Pt(2+) species on the surface of nanostructured CeO2 films and the mechanism of H2 activation on these sites have been investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy in combination with density functional calculations. Isolated Pt(2+) sites are found to be inactive towards H2 dissociation due to high activation energy required for H-H bond scission. Trace amounts of metallic Pt are necessary to initiate H2 dissociation on Pt-CeO2 films. H2 dissociation triggers the reduction of Ce(4+) cations which, in turn, is coupled with the reduction of Pt(2+) species. The mechanism of Pt(2+) reduction involves reverse oxygen spillover and formation of oxygen vacancies on Pt-CeO2 films. Our calculations suggest the existence of a threshold concentration of oxygen vacancies associated with the onset of Pt(2+) reduction.

Surface sites on Pt-CeO2 mixed oxide catalysts probed by CO adsorption: a synchrotron radiation photoelectron spectroscopy study.

By means of synchrotron radiation photoemission spectroscopy, we have investigated Pt-CeO2 mixed oxide films prepared on CeO2(111)/Cu(111). Using CO molecules as a probe, we associate the corresponding surface species with specific surface sites. This allows us to identify the changes in the composition and morphology of Pt-CeO2 mixed oxide films caused by annealing in an ultrahigh vacuum. Specifically, two peaks in C 1s spectra at 289.4 and 291.2 eV, associated with tridentate and bidentate carbonate species, are formed on the nanostructured stoichiometric CeO2 film. The peak at 290.5-291.0 eV in the C 1s spectra indicates the onset of restructuring, i.e. coarsening, of the Pt-CeO2 film. This peak is associated with a carbonate species formed near an oxygen vacancy. The onset of cerium oxide reduction is indicated by the peak at 287.8-288.0 eV associated with carbonite species formed near Ce(3+) cations. The development of surface species on the Pt-CeO2 mixed oxides suggests that restructuring of the films occurs above 300 K irrespective of Pt loadings. We do not find any adsorbed CO species associated with Pt(4+) or Pt(2+). The onset of Pt(2+) reduction is indicated by the peak at 286.9 eV in the C 1s spectra due to CO adsorption on metallic Pt particles. The thermal stability of Pt(2+) in Pt-CeO2 mixed oxide depends on Pt loading. We find excellent stability of Pt(2+) for 12% Pt content in the CeO2 film, whereas at a Pt concentration of 25% in the CeO2 film, a large fraction of the Pt(2+) is converted into metallic Pt particles above 300 K.

Hydrogen activation on Pt-Sn nanoalloys supported on mixed Sn-Ce oxide films.

We have studied the interaction of H2 with Pt-Sn nanoalloys supported on Sn-Ce mixed oxide films of different composition by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy. The model catalysts are prepared in a three step procedure that involves (i) the preparation of well-ordered CeO2(111) films on Cu(111) followed by subsequent physical vapor deposition of (ii) metallic Sn and (iii) metallic Pt. The formation of mixed Sn-Ce oxide is accompanied by partial reduction of Ce(4+) cations to Ce(3+). Pt deposition leads to the formation of Pt-Sn nanoalloys accompanied by the partial re-oxidation of Ce(3+) to Ce(4+). Subsequent annealing promotes further Pt-Sn alloy formation at expense of the Sn content in the Sn-Ce mixed oxide. Adsorption of H2 on Pt-Sn/Sn-Ce-O at 150 K followed by stepwise annealing results in reversible reduction of Ce cations caused by spillover of dissociated hydrogen between 150 and 300 K. Above 500 K, annealing of Pt-Sn/Sn-Ce-O in a hydrogen atmosphere results in irreversible reduction of Ce cations. This reduction is caused by the reaction of hydrogen with oxygen provided by the mixed oxide substrate via the reverse spillover to Pt-Sn nanoalloy. The extent of the hydrogen and oxygen spillover strongly depends on the amount of Sn in the Sn-Ce mixed-oxide. We observe an enhancement of hydrogen spillover on Pt-Sn/Sn-Ce-O at low Sn concentration as compared to Sn-free Pt/CeO2. Although the extent of hydrogen spillover on Pt-Sn/Sn-Ce-O with high Sn concentration is comparable to Pt/CeO2, the reverse oxygen spillover is substantially suppressed on these samples.

Maximum noble-metal efficiency in catalytic materials: atomically dispersed surface platinum.

Platinum is the most versatile element in catalysis, but it is rare and its high price limits large-scale applications, for example in fuel-cell technology. Still, conventional catalysts use only a small fraction of the Pt content, that is, those atoms located at the catalyst's surface. To maximize the noble-metal efficiency, the precious metal should be atomically dispersed and exclusively located within the outermost surface layer of the material. Such atomically dispersed Pt surface species can indeed be prepared with exceptionally high stability. Using DFT calculations we identify a specific structural element, a ceria "nanopocket", which binds Pt(2+) so strongly that it withstands sintering and bulk diffusion. On model catalysts we experimentally confirm the theoretically predicted stability, and on real Pt-CeO2 nanocomposites showing high Pt efficiency in fuel-cell catalysis we also identify these anchoring sites.

Redox-mediated C-C bond scission in alcohols adsorbed on CeO2-xthin films.

The decomposition mechanisms of ethanol and ethylene glycol on well-ordered stoichiometric CeO2(111) and partially reduced CeO2-x(111) films were investigated by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, and temperature programmed desorption. Both alcohols partially deprotonate upon adsorption at 150 K and subsequent annealing yielding stable ethoxy and ethylenedioxy species. The C-C bond scission in both ethoxy and ethylenedioxy species on stoichiometric CeO2(111) involves formation of acetaldehyde-like intermediates and yields CO and CO2accompanied by desorption of acetaldehyde, H2O, and H2. This decomposition pathway leads to the formation of oxygen vacancies. In the presence of oxygen vacancies, C-O bond scission in ethoxy species yields C2H4. In contrast, C-C bond scission in ethylenedioxy species on the partially reduced CeO2-x(111) is favored with respect to C-O bond scission and yields methanol, formaldehyde, and CO accompanied by the desorption of H2O and H2. Still, scission of C-O bonds on both sides of the ethylenedioxy species yields minor amounts of accompanying C2H4and C2H2. C-O bond scission is coupled with a partial recovery of the lattice oxygen in competition with its removal in the form of water.

Quantitative Analysis of the Oxidation State of Cobalt Oxides by Resonant Photoemission Spectroscopy.

Quantitative assessment of the charge transfer phenomena in cobalt oxides and cobalt complexes is essential for the design of advanced catalytic materials. We propose a method for the evaluation of the oxidation state of cobalt oxides with mixed valence states using resonant photoemission spectroscopy. The method is based on the calculation of the resonant enhancement ratio (RER) from the heights of the resonant features associated with the Co3+ and Co2+ states. The nature of the corresponding states was corroborated by means of density functional calculations. We employed a well-ordered Co3O4(111) film to calibrate the RER with respect to the atomic Co3+/Co2+ ratio. The method was applied to monitor the reduction of a well-ordered Co3O4(111) film to CoO(111) upon annealing under exposure to isopropanol. We demonstrate that this method yields the stoichiometry of cobalt oxides at a level of accuracy that cannot be achieved when fitting the Co 2p core level spectra.

The mechanism of hydrocarbon oxygenate reforming: C-C bond scission, carbon formation, and noble-metal-free oxide catalysts.

Towards a molecular understanding of the mechanism behind catalytic reforming of bioderived hydrocarbon oxygenates, we explore the C-C bond scission of C2 model compounds (acetic acid, ethanol, ethylene glycol) on ceria model catalysts of different complexity, with and without platinum. Synchrotron photoelectron spectroscopy reveals that the reaction pathway depends very specifically on both the reactant molecule and the catalyst surface. Whereas C-C bond scission on Pt sites and on oxygen vacancies involves intermittent surface carbon species, the reaction occurs without any carbon formation and deposition for ethylene glycol on CeO2(111).