Oxide-supported Pd and Pd alloys in methanos steam reforming

In addition to a range of well-studied copper catalysts, a group of highly CO 2 -selective Pd-based methanol steam reforming catalysts supported on ZnO, Ga2 O3 , and In 2 O3 has been identified recently. They all exhibit the common feature of formation of nanoparticulate Pd intermetallic phases, which are in contact with the respective reducible oxide. A combination of structural and electronic influences is likely to lead to high activity and selectivity towards additional hydrogen and CO 2 , due to the applied reductive activation procedure. The question is whether these "stereoelectronic" effects are limited to a certain bimetallic surface state for all these systems, or if they also affect the contact area between the bimetallic nanoparticles and the partially reduced oxide support, and the support itself. Even for the best-studied system, PdZn/ZnO x , the overall mechanistic role of the PdZn bimetallic surface in the reforming process toward CO 2 and the influence of metal-support interactions is unclear. The presence of a bimetallic PdZn surface appears to be crucial for the steering of dehydrogenation steps in between methanol and CO. The most important open questions regard the essential intermediates leading selectively toward CO 2 , the mechanism and site of water activation, the role of the reduced support and the metal-oxide phase boundary, the stability of the bimetallic (nano)particles under "real" reaction conditions, their surface state and composition depending on size, chemical environment and temperature, i.e. the topic of chemically and thermally induced segregation trends. All these open questions have only in part been addressed for PdZn/ZnOx but not at all for the related Pd-Ga and Pd-In systems. From an argument of chemical analogy we suggest that also Pd/SnOx and Pd/GeOx are promising candidates for enhanced reforming selectivity, and we propose to include the analogous investigation of both the Pd-Sn and Pd-Ge model catalysts. One of the central goals of this project is therefore to identify (or disprove) a potential "common reasoning" for the high CO 2 selectivity of these systems, using a directional model catalyst approach to distinguish the influences of the different involved phases and their boundaries. We will put particular effort on the characterization of the catalytically most relevant surface composition of the models (purely metallic, oxidic, and with defined phase boundary) before and after reaction using a combination of electron spectroscopies and ion scattering techniques, and during reaction under "real" conditions, using in-situ spectroscopy techniques such as high-pressure XPS with tunable photon energy. Synergistic structural and electronic effects are already expected for the unsupported bi- or multi-metallic catalyst surfaces, by alteration of the d-band density of states (e.g. the DOS of Cu can be simulated due to the Zn-Pd "ligand effect") and formation of bifunctional Pd-Zn surface ensembles acting as a structural promoter. A model catalyst approach is chosen which allows not only to distinguish the catalytic function of the bimetallic ligand- and ensemble-modified surfaces, but also that of the contact area of these with the respective oxide support in a defined state of surface and/or bulk reduction. The final goal is to identify the (combination of) micro- or rather "nanoscopic" catalytic units which enable highly selective low-temperature steam reforming and methanol synthesis in a variety of systems.

In addition to a range of well-studied copper catalysts, a group of highly CO 2 -selective Pd-based methanol steam reforming catalysts supported on ZnO, Ga2 O3 , and In 2 O3 has been identified recently. They all exhibit the common feature of formation of nanoparticulate Pd intermetallic phases, which are in contact with the respective reducible oxide. A combination of structural and electronic influences is likely to lead to high activity and selectivity towards additional hydrogen and CO 2 , due to the applied reductive activation procedure. The question is whether these "stereoelectronic" effects are limited to a certain bimetallic surface state for all these systems, or if they also affect the contact area between the bimetallic nanoparticles and the partially reduced oxide support, and the support itself. Even for the best-studied system, PdZn/ZnO x , the overall mechanistic role of the PdZn bimetallic surface in the reforming process toward CO 2 and the influence of metal-support interactions is unclear. The presence of a bimetallic PdZn surface appears to be crucial for the steering of dehydrogenation steps in between methanol and CO. The most important open questions regard the essential intermediates leading selectively toward CO 2 , the mechanism and site of water activation, the role of the reduced support and the metal-oxide phase boundary, the stability of the bimetallic (nano)particles under "real" reaction conditions, their surface state and composition depending on size, chemical environment and temperature, i.e. the topic of chemically and thermally induced segregation trends. All these open questions have only in part been addressed for PdZn/ZnOx but not at all for the related Pd-Ga and Pd-In systems. From an argument of chemical analogy we suggest that also Pd/SnOx and Pd/GeOx are promising candidates for enhanced reforming selectivity, and we propose to include the analogous investigation of both the Pd-Sn and Pd-Ge model catalysts. One of the central goals of this project is therefore to identify (or disprove) a potential "common reasoning" for the high CO 2 selectivity of these systems, using a directional model catalyst approach to distinguish the influences of the different involved phases and their boundaries. We will put particular effort on the characterization of the catalytically most relevant surface composition of the models (purely metallic, oxidic, and with defined phase boundary) before and after reaction using a combination of electron spectroscopies and ion scattering techniques, and during reaction under "real" conditions, using in-situ spectroscopy techniques such as high-pressure XPS with tunable photon energy. Synergistic structural and electronic effects are already expected for the unsupported bi- or multi-metallic catalyst surfaces, by alteration of the d-band density of states (e.g. the DOS of Cu can be simulated due to the Zn-Pd "ligand effect") and formation of bifunctional Pd-Zn surface ensembles acting as a structural promoter. A model catalyst approach is chosen which allows not only to distinguish the catalytic function of the bimetallic ligand- and ensemble-modified surfaces, but also that of the contact area of these with the respective oxide support in a defined state of surface and/or bulk reduction. The final goal is to identify the (combination of) micro- or rather "nanoscopic" catalytic units which enable highly selective low-temperature steam reforming and methanol synthesis in a variety of systems.