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Perovskite-like oxides present huge chemical variability and a wide range of applications as catalysts for oxidation reactions. The interaction of several small gas molecules with the surface of LaCoO3 and LaMnO3 perovskite-like oxides was studied by Near Ambient Pressure X-ray photoelectron spectroscopy (NAP-XPS) and CO Temperature Programmed Desorption (CO-TPD). Surface chemical changes such as the Osurf/Olattice and cation B oxidation state ratios were analyzed as a function of temperature (400 K, 450 K, 500 K, 550 K, and 650 K) under different gas atmospheres like Ar, CO, H2, and O2. It was found that there was a partial surface reduction when H2 and CO were used in the reaction, and therefore, the cation B oxidation state (Mn4+/Mn3+ and Co3+/Co2+) ratio decreased. Under the CO stream, carbonate species were formed, presenting a C1s signal between 284.5 eV and 287 eV. The CO2 evolution during the reaction at temperatures greater than 500 K was associated with CO activation over or near to surface oxygen species. A Mars-van Krevelen mechanism was proposed for the process, finding significant differences between LaCoO3 and LaMnO3 perovskite-like solid catalysts behavior. [ABSTRACT FROM AUTHOR]

The adsorption of CO2 on the Fe3O4(001)-(√2 ×√2)R45° surface was studied experimentally using temperature programmed desorption (TPD), photoelectron spectroscopies (UPS and XPS), and scanning tunneling microscopy. CO2 binds most strongly at defects related to Fe2+, including antiphase domain boundaries in the surface reconstruction and above incorporated Fe interstitials. At higher coverages, CO2 adsorbs at fivefold-coordinated Fe3+ sites with a binding energy of 0.4 eV. Above a coverage of 4 molecules per (√2 ×√2)R45° unit cell, further adsorption results in a compression of the first monolayer up to a density approaching that of a CO2 ice layer. Surprisingly, desorption of the second monolayer occurs at a lower temperature (≈84 K) than CO2 multilayers (≈88 K), suggestive of a metastable phase or diffusion-limited island growth. The paper also discusses design considerations for a vacuum system optimized to study the surface chemistry of metal oxide single crystals, including the calibration and characterisation of a molecular beam source for quantitative TPD measurements. [ABSTRACT FROM AUTHOR]

Interactions between catalytically active metal particles and reactant gases depend strongly on the particle size, particularly in the subnanometer regime where the addition of just one atom can induce substantial changes in stability, morphology, and reactivity. Here, time-lapse scanning tunneling microscopy (STM) and density functional theory (DFT)-based calculations are used to study how CO exposure affects the stability of Pt adatoms and subnano clusters at the Fe3O4(001) surface, a model CO oxidation catalyst. The results reveal that CO plays a dual role: first, it induces mobility among otherwise stable Pt adatoms through the formation of Pt carbonyls (Pt1-CO), leading to agglomeration into subnano clusters. Second, the presence of the CO stabilizes the smallest clusters against decay at room temperature, significantly modifying the growth kinetics. At elevated temperatures, CO desorption results in a partial redispersion and recovery of the Pt adatom phase. [ABSTRACT FROM AUTHOR]

Metal-support interactions are frequently invoked to explain the enhanced catalytic activity of metal nanoparticles dispersed over reducible metal oxide supports, yet the atomicscale mechanisms are rarely known. In this report, scanning tunneling microscopy was used to study a Pt1-6/Fe3O4 model catalyst exposed to CO, H2, O2, and mixtures thereof at 550 K. CO extracts lattice oxygen atoms at the cluster perimeter to form CO2, creating large holes in the metal oxide surface. H2 and O2 dissociate on the metal clusters and spill over onto the support. The former creates surface hydroxy groups, which react with the support, ultimately leading to the desorption of water, while oxygen atoms react with Fe from the bulk to create new Fe3O4(001) islands. The presence of the Pt is crucial because it catalyzes reactions that already occur on the bare iron oxide surface, but only at higher temperatures. [ABSTRACT FROM AUTHOR]

The adsorption of Ni, Co, Mn, Ti, and Zr at the (√2 × √2)R45°-reconstructed Fe3O4(001) surface was studied by scanning tunneling microscopy, x-ray and ultraviolet photoelectron spectroscopy, low-energy electron diffraction (LEED), and density functional theory (DFT). Following deposition at room temperature, metals are either adsorbed as isolated adatoms or fill the subsurface cation vacancy sites responsible for the (√2 × √2)R45° reconstruction. Both configurations coexist, but the ratio of adatoms to incorporated atoms depends on the metal; Ni prefers the adatom configuration, Co and Mn form adatoms and incorporated atoms in similar numbers, and Ti and Zr are almost fully incorporated. With mild annealing, all adatoms transition to the incorporated cation configuration. At high coverage, the (√2 × √2)R45° reconstruction is lifted because all subsurface cation vacancies become occupied with metal atoms, and a (1 × 1) LEED pattern is observed. DFT+U calculations for the extreme cases, Ni and Ti, confirm the energetic preference for incorporation, with calculated oxidation states in good agreement with photoemission experiments. Because the site preference is analogous to bulk ferrite (XFe2O4) compounds, similar behavior is likely to be typical for elements forming a solid solution with Fe3O4. [ABSTRACT FROM AUTHOR]