Origin of the surface-orientation dependence of the reduction kinetics of ultrathin ceria

Performance of catalytic redox reactions depends crucially on the oxygen storage and release capability of the catalyst and with that the catalyst’s defect chemistry. Here, we show that the surface defect chemistry of cerium oxide, a prototypical reducible oxide, differs markedly between two surface terminations. The results are in good agreement with density functional theory calculations and provide important guiding factors for rational design of industrially relevant catalysts. The study is conducted by preparing (100) and (111) terminated nanoislands of cerium oxide next to each other on Cu(111). Leveraging the benefits of full-field imaging capability of photoemission electron microscopy (PEEM), we follow the structural and chemical properties of the nanoislands under reducing hydrogen atmosphere simultaneously and in situ. The results, summarized in Figure 1, directly reveal different overall reducibility that can be traced to equilibrium oxygen vacancy concentrations via a kinetic model. The density functional theory calculations provide further details regarding the equilibrium co-ordination of oxygen vacancies for both surface planes. Conjoining the two, the unique simultaneous nature of the PEEM-facilitated structure–activity relationship study allows us to separate the thermodynamics of reduction from the kinetics of oxygen exchange, revealing the fact that the difference in reducibility of the two surfaces of ceria is not determined by the kinetic rate constants of the reduction reaction, but rather by the equilibrium concentration of oxygen vacancies, an information that has not been provided by the isolated model system approach to date. Surprisingly, the reason for the different reducibilities is a purely geometric one: the creation of nearest neighbor oxygen vacancies. Please click Additional Files below to see the full abstract

Atomic species identification at the (101) anatase surface by simultaneous scanning tunnelling and atomic force microscopy

Anatase is a pivotal material in devices for energy-harvesting applications and catalysis. Methods for the accurate characterization of this reducible oxide at the atomic scale are critical in the exploration of outstanding properties for technological developments. Here we combine atomic force microscopy (AFM) and scanning tunnelling microscopy (STM), supported by first-principles calculations, for the simultaneous imaging and unambiguous identification of atomic species at the (101) anatase surface. We demonstrate that dynamic AFM-STM operation allows atomic resolution imaging within the materiala € s band gap. Based on key distinguishing features extracted from calculations and experiments, we identify candidates for the most common surface defects. Our results pave the way for the understanding of surface processes, like adsorption of metal dopants and photoactive molecules, that are fundamental for the catalytic and photovoltaic applications of anatase, and demonstrate the potential of dynamic AFM-STM for the characterization of wide band gap materialsWork supported by the NIMS (AA002 and AF006 projects), by the MEXT KAKENHI Grant Number 26104540, by the Charles University (GAUK 339311) and by the Spanish MINECO (projects PLE2009-0061, MAT2011- 023627 and CSD2010-00024). Computer time was provided by the Spanish Supercomputing Network (RES, Spain) at the MareNostrum III Supercomputer (BCS, Barcelona), and by the PRACE initiative (project RA0986) at the Curie Supercomputer (CEA, France). O.S and V.M. thank the Charles University-NIMS International Cooperative Graduate School Program. J.W.R. thanks NIMS for funding through the NIMS Internship Program and ICIQ for his ICIQ Fellowshi

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 Ce3+ 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 Pt4+ or Pt2+. The onset of Pt2+ 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 Pt2+ in Pt–CeO2 mixed oxide depends on Pt loading. We find excellent stability of Pt2+ for 12% Pt content in the CeO2 film, whereas at a Pt concentration of 25% in the CeO2 film, a large fraction of the Pt2+ 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 Ce4+ cations to Ce3+. Pt deposition leads to the formation of Pt–Sn nanoalloys accompanied by the partial re-oxidation of Ce3+ to Ce4+. 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

Excitons at the B K edge of boron nitride nanotubes probed by x-ray absorption spectroscopy

We have performed a near-edge x-ray absorption fine-structure (NEXAFS) investigation of multi-walled boron nitride nanotubes (BNNTs). We show that the one-dimensionality of BNNTs is clearly evident in the B K edge spectrum, while the N K edge spectrum is similar to that of layered hexagonal BN (h-BN). We observe a sharp feature at the Ã* onset of the B K edge, which we ascribe to a core exciton state. We also report a comparison with spectra taken after an ammonia plasma treatment, showing that the B K edge becomes indistinguishable from that of h-BN, due to the breaking of the tubular order and the formation of small h-BN clusters

Reactivity of atomically dispersed Pt2+ species towards H2: model Pt–CeO2 fuel cell catalyst

The reactivity of atomically dispersed Pt2+ 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 Pt2+ 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 Ce4+ cations which, in turn, is coupled with the reduction of Pt2+ species. The mechanism of Pt2+ 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 Pt2+ reduction

Selective electrooxidation of 2-propanol on Pt nanoparticles supported on Co3O4: an in-situ study on atomically defined model systems

2-Propanol and its dehydrogenated counterpart acetone can be used as a rechargeable electrofuel. The concept involves selective oxidation of 2-propanol to acetone in a fuel cell coupled with reverse catalytic hydrogenation of acetone to 2-propanol in a closed cycle. We studied electrocatalytic oxidation of 2-propanol on complex model Pt/Co3O4(111) electrocatalysts prepared in ultra-high vacuum and characterized by scanning tunneling microscopy. The electrocatalytic behavior of the model electrocatalysts has been investigated in alkaline media (pH 10, phosphate buffer) by means of electrochemical infrared reflection absorption spectroscopy and ex-situ emersion synchrotron radiation photoelectron spectroscopy as a function of Pt particle size and compared with the electrocatalytic behavior of Pt(111) and pristine Co3O4(111) electrodes under similar conditions. We found that the Co3O4(111) film is inactive towards electrochemical oxidation of 2-propanol under the electrochemical conditions (0.3–1.1 VRHE). The electrochemical oxidation of 2-propanol readily occurs on Pt(111) yielding acetone at an onset potential of 0.4 VRHE. The reaction pathway does not involve CO but yields strongly adsorbed acetone species leading to a partial poisoning of the surface sites. On model Pt/Co3O4(111) electrocatalysts, we observed distinct metal support interactions and particle size effects associated with the charge transfer at the metal/oxide interface. We found that ultra-small Pt particles (around 1 nm and below) consist of partially oxidized Pt δ + species which show minor activity towards 2-propanol oxidation. In contrast, conventional Pt particles (particle size of a few nm) are mainly metallic and show high activity toward 2-propanol oxidation