The water-gas shift (WGS) reaction is one of the most important processes in the context of industrial hydrogen production. Supported Ni catalysts would be attractive as the abundant Ni typically has high activity, especially with zirconia (ZrO2) as active and stable support. Carbon supports are interesting because of high specific surface area, but Ni/C exhibits only low WGS activity. However, this may be overcome by alloying Ni with a second transition metal such as Zr, i.e., by building the oxide functionality already into the nanoparticles, and then using, e.g., graphene (GR) as support.
The adsorption sites, molecular structures and orientations of CO and H2O, their intermolecular interactions, and the WGS reaction will be systematically studied at different pressures (UHV to 1 bar) and temperatures (100-600 K). Hollow, bridge and on-top CO, H-bonds and “free”-OH, and potential intermediates such as carboxyl or formate will be spectroscopically identified. The reactants (CO, H2O), products (CO2, H2), and byproducts (CH4, C) will be quantified to determine catalytic performance. Catalyst complexity will be increased stepwise, employing single crystals, thin films, and supported nanoparticles as model systems, with the goal to explain the activity and selectivity of NiZrOx/GR catalysts via molecular level structure-function relationships.
The approach is based on ambient pressure surface science, using an ultra-high vacuum surface preparation/analysis chamber, which is connected to an atmospheric pressure cell for surface specific sum frequency generation (SFG) spectroscopy and mass spectroscopy (MS) under reaction conditions. Additional UHV characterization will be carried out by low energy electron diffraction (LEED), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM), with surface X-ray diffraction (SXRD) and density functional theory (DFT) performed within collaborations.
The combination of UHV-grown bimetallic nanoparticle model catalyst and operando polarization-dependent-SFG surface spectroscopy at ambient pressure is unique. A thin film oxide support will be compared with a GR support, for which a promotional effect by interfacial hydrogen was observed for powder GR nanoplatelets. In the current model system, CO and/or H2 intercalation between the GR-layer and the Ir substrate is possible, depending on the GR morphology (i.e., grain boundaries, defects, etc.). The designed “hybrid catalyst” of NiZrOx/GR, a composition that has not yet been attempted, combines the benefits of activation at metal/oxide interfaces and of carbon supports.
- Primary researchers involved
Dr. Xia Li, a postdoctoral researcher at TU Wien, is mainly responsible (PD-SFG, LEED/AES, XPS). She will be supported by Prof. Günther Rupprechter (mentor) and the collaboration partners Prof. Andreas Stierle (SXRD) and Dr. Alexander Genest (DFT).
