More than skin deep: In spite of their identical 1:1 surface composition, the geometric and electronic structures of a multilayer and monolayer PdZn surface alloy are different, as are their catalytic selectivities. The CO2 selective multilayer alloy features surface ensembles of PdZn exhibiting a “Zn‐up/Pd‐down” corrugation (see picture). These act as “bifunctional” active sites both for water activation and for the conversion of methanol into CO2. On the monolayer alloy CO and not CO2 is produced.

Discussed are the recent experimental and theoretical results on palladium-based catalysts for selective hydrogenation of alkynes obtained by a number of collaborating groups in a joint multi-method and multi-material approach. The critical modification of catalytically active Pd surfaces by incorporation of foreign species X into the sub-surface of Pd metal was observed by in situ spectroscopy for X=H, C under hydrogenation conditions. Under certain conditions (low H2 partial pressure) alkyne fragmentation leads to formation of a PdC surface phase in the reactant gas feed. The insertion of C as a modifier species in the sub-surface increases considerably the selectivity of alkyne semi-hydrogenation over Pd-based catalysts through the decoupling of bulk hydrogen from the outmost active surface layer. DFT calculations confirm that PdC hinders the diffusion of hydridic hydrogen. Its formation is dependent on the chemical potential of carbon (reactant partial pressure) and is suppressed when the hydrogen/alkyne pressure ratio is high, which leads to rather unselective hydrogenation over in situ formed bulk PdH. The beneficial effect of the modifier species X on the selectivity, however, is also present in intermetallic compounds with X=Ga. As a great advantage, such PdxGay catalysts show extended stability under in situ conditions. Metallurgical, clean samples were used to determine the intrinsic catalytic properties of PdGa and Pd3Ga7. For high performance catalysts, supported nanostructured intermetallic compounds are more preferable and partial reduction of Ga2O3, upon heating of Pd/Ga2O3 in hydrogen, was shown to lead to formation of PdGa intermetallic compounds at moderate temperatures. In this way, Pd5Ga2 and Pd2Ga are accessible in the form of supported nanoparticles, in thin film models, and realistic powder samples, respectively

In situ FT-IR spectroscopy was exploited to study the adsorption of CO2 and CO on commercially available yttria-stabilized ZrO2 (8 mol % Y, YSZ-8), Y2O3, and ZrO2. All three oxides were pretreated at high temperatures (1173 K) in air, which leads to effective dehydroxylation of pure ZrO2. Both Y2O3 and YSZ-8 show a much higher reactivity toward CO and CO2 adsorption than ZrO2 because of more facile rehydroxylation of Y-containing phases. Several different carbonate species have been observed following CO2 adsorption on Y2O3 and YSZ-8, which are much more strongly bound on the former, due to formation of higher-coordinated polydentate carbonate species upon annealing. As the crucial factor governing the formation of carbonates, the presence of reactive (basic) surface hydroxyl groups on Y-centers was identified. Therefore, chemisorption of CO2 most likely includes insertion of the CO2 molecule into a reactive surface hydroxyl group and the subsequent formation of a bicarbonate species. Formate formation following CO adsorption has been observed on all three oxides but is less pronounced on ZrO2 due to effective dehydroxylation of the surface during high-temperature treatment. The latter generally causes suppression of the surface reactivity of ZrO2 samples regarding reactions involving CO or CO2 as reaction intermediates.

The adsorption, bonding, defect formation, and reactivity of hydrogen on different In 2 O 3 powder samples were studied by a combination of volumetric adsorption, thermal desorption, diffraction, and spectroscopic techniques. Surface reduction was observed in dry hydrogen up to 400 K, followed by reduction of surfacenear regions. Above 500 K bulk reduction, along with the formation of metallic In, sets in. Raman spectra indicate a considerable reordering of the In 2 O 3 structure in this temperature regime. Despite their TPD proven presence, the related adsorbed H-containing species were not detectable by Fourier transform infrared spectroscopy and/or Raman spectroscopy, in strong contrast to related experiments on -Ga 2 O 3 . Hydrogeninduced oxygen vacancies were found to be easily replenished by traces of water in the gas feed.

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Together we are strong: Immediately after high‐temperature reduction, ZnPd/ZnO catalysts are not notably CO2 selective in the methanol steam reforming process. Under methanol steam reforming conditions, high CO2 selectivity can only be achieved by the rather slow formation of ZnO patches on the surface of intermetallic ZnPd nanoparticles to create a highly synergistic interface.

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Quantitative in situ X-ray diffraction in combination with catalytic tests in dry reforming of methane (DRM) has been performed to unveil the strong structural dynamics of LaNiO 3 catalysts during the DRM reaction. Structure−activity correlations reveal polymorphic changes of the rhombohedral LaNiO 3 structure first into cubic LaNiO 3 and further into transient oxygen-deficient triclinic LaNiO 2.7 and monoclinic LaNiO 2.5 . These changes occur up to 620 °C and already cause considerable DRM activity. Another intermediate structure, the Ruddlesden−Popper phase La 2 NiO 4 with moderate DRM activity, is formed in parallel with the decomposition of monoclinic LaNiO 2.5 . The decay of La 2 NiO 4 directly goes along with the appearance of crystalline metallic Ni and monoclinic La 2 O 2 CO 3 and a drastic enhancement of DRM activity. The formation of monoclinic La 2 O 2 CO 3 and decomposition of La 2 NiO 4 proceed exactly alike up to 670 °C with the accumulation of metallic Ni. At 670 °C and up to 750 °C, monoclinic La 2 O 2 CO 3 is directly transformed into hexagonal La 2 O 2 CO 3 , and no further Ni exsolution is observed. Only above 750 °C, hexagonal La 2 O 3 is observed and apparently formed directly from a drastically accelerated decomposition of monoclinic La 2 O 2 CO 3 alongside another small increase in metallic Ni. Our direct structure−activity correlation unambiguously shows that the active phase in DRM is a mixture of metallic Ni in contact with monoclinic La 2 O 2 CO 3 . The roles of the latter phase are twofold: acting as the CO 2 -activated species and stabilizing the metallic Ni particles. Naturally, this implies a perfect carbon removal ability of the metallic Ni/La 2 O 2 CO 3 interface, which directly relates to an enhanced coking resistance and, most probably, long term stability. Heating LaNiO 3 in hydrogen yields a similar sequence of structural transformations with the striking difference of the missing transient La 2 NiO 4 structure, corroborating its crucial role in the formation of the DRM-active Ni/monoclinic La 2 O 2 CO 3 interface. The final structural fate is a metallic Ni/hexagonal La 2 O 3 phase mixture. Exemplified for the DRM reaction and the initial LaNiO 3 structure, only the knowledge about the sheer complexity of the structural dynamics allows the unequivocal assignment of participating structures and phases to their respective catalytic performance, and therefore, allows definite conclusions about the formation and the properties of the final active phase.