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

Pd/Ga2O3 methanol steam reforming (MSR) catalysts were characterized in detail by utilizing a range of in situ techniques of varying surface sensitivity. Correlating the nature of the intermetallic Pd-Ga compound (IMC; formed upon reduction) with the corresponding activity/selectivity revealed pronounced differences. Generally, a dynamic response of the catalyst surface to the surrounding gas environment was observed. Special attention was paid to the bulk and surface stability of the Pd-Ga IMCs. Whereas the bulk was stable in O2, decomposition of the surface occurred resulting in a partial coverage by GaxOy islands. In addition, we focused on the formation mechanism of undesired CO and were able to identify the reasons limiting the selectivity to MSR. We observed a detrimental effect of CO on the selective Pd-Ga intermetallic compound, causing partial decomposition of the IMC to metallic Pd at the surface. Consequently, patches of Pd metal are present under reaction conditions, catalyzing the unwanted parallel methanol decomposition reaction.

A systematic study of the nature, stability, and dynamics of surface species present under methanol steam reforming (MSR) conditions over Pd/Ga 2 O 3 and Pd 2 Ga/ Ga 2 O 3 was performed by combining steady state and concentration modulation FTIR spectroscopy. This powerful combination allowed us to obtain novel mechanistic insights into the selective pathway leading to the formation of H 2 and CO 2 and thus to contribute to the understanding of the remarkably different catalytic properties of Pd/Ga 2 O 3 and Pd 2 Ga/Ga 2 O 3 . Strongly enhanced formation of adsorbed formates at low temperatures was detected on Pd 2 Ga/ Ga 2 O 3 . We ascribe the facilitated formation of these species to the presence of reactive oxygen sites in the Ga 2 O 3 surface, which are formed during high-temperature reduction and formation of the intermetallic compound Pd 2 Ga. While the stability of involved formates is high under reaction conditions of methanol decomposition (i.e., in the absence of H 2 O), the entire adsorption system behaves more dynamically in the presence of water. We propose that the introduction of H 2 O into the system converts stable bridging-and bidentate formates into more reactive, monodentate species. These react either with adsorbed methoxy to methyl formate (MFO) in the absence of water or with OH groups supplied by H 2 O to CO 2 and H 2 . The reaction with OH is faster, leading to a smaller concentration of intermediate monodentate formate under MSR conditions. MFO is easily decomposed into CO and CH 3 OH and therefore, it is unlikely to be an intermediate in the selective MSR reaction to CO 2 and H 2 . While the formation of intermetallic particles by high-temperature reduction is a prerequisite to achieving high MSR selectivity, our results suggest that the reaction sequence predominantly proceeds on the Ga 2 O 3 surface, that is modified by the high temperature reduction and the formation of Pd 2 Ga, and is only promoted by the presence of the intermetallic particles.

In this work, the structure and stability of Pd–Ga intermetallic nanoparticles under various reactive conditions was investigated by combining in situ X-ray absorption spectroscopy (XAS), FTIR of CO adsorption, and XRD. By in situ XAS we followed in detail the formation of Pd–Ga intermetallic compounds (IMC) upon reduction of Pd/Ga2O3, which was observed to be a rather slow process that depends on the availability of reduced Ga formed by the atomic H provided by Pd. Using crystal structures of a variety of Pd–Ga IMCs, we have identified Pd2Ga as the compound that is formed during reduction at 623 K. In contrast to Pd/Ga2O3, β-hydride formation did not occur once Pd2Ga particles are formed, as evidenced by the absence of lattice expansion in hydrogen atmosphere. However, XAS revealed that Pd2Ga is not stable in oxygen already at room temperature. Although XRD showed no bulk structural modification, CO adsorption on an oxygen exposed catalyst detected a metallic Pd surface, partly decorated with oxidic Ga. Only in situ XAS provided clear indications on the structural modification occurring upon oxygen exposure, showing that the overall state of the sample is a mixture of Pd or a Ga-depleted IMC and Pd2Ga. Based on these observations, Ga segregation from the surface-near region to the surface, followed by oxidation, was concluded. The intermetallic surface is easily reformed by reduction, due to remaining Pd at the surface activating H2.

Ligand-capped Pt nanowires, prepared by colloidal synthesis and deposited on a high surface area γ-Al(2)O(3) support, were subjected to surface characterization by electron microscopy and FTIR spectroscopy using CO as a probe molecule. The structural, adsorption, and catalytic reaction properties of the colloidal Pt nanowires were compared to those of conventional, impregnated Pt nanoparticles on the same Al(2)O(3) support. In situ FTIR spectroscopy indicated ligand effects on the CO resonance frequency, irreversible CO-induced surface roughening upon CO adsorption, and a higher resistance of colloidal catalysts toward oxidation (both in oxygen and during CO oxidation), suggesting that the organic ligands might protect the Pt surface. Elevated temperature induced a transformation of Pt nanowires to faceted Pt nanoparticles. The colloidal catalyst was active for hydrodechlorination of trichloroethylene (TCE), but no ligand effect on selectivity was obtained.