Molecular Insights in Water Gas Shift on NiZrOx/GR Catalysts

Hydrogen production can be carried out via the so-called water gas shift (WGS) reaction, using CO and H2O to produce CO2 and H2. It is industrially performed in two steps: a high temperature shift at 350- 500 C and a low temperature shift at 180-250 C using Fe-Cr oxide and Cu-ZnO based catalysts, respectively. Generally, a CO molecule adsorbs on the metal nanoparticle surface (CO*), whereas H2O is activated/dissociated at the oxide support (OH*). Due to the carcinogenic nature and toxicity of Cr and the sensitivity to air and condensed water of Cu catalysts, scientists have put great efforts to find alternative transition metal nanoparticles as catalysts supported on the oxides. Traditionally, there are two mechanisms: associative and redox. In the former one the adsorbed CO* and OH* react to form carboxyl (COOH*), formate (HCOO*) and/or carbonate (CO3*) intermediates or spectators. Decomposition of carboxyl or formate then yields CO2 and H2 products. In the latter one, there is no reaction between the adsorbed CO* and OH*, and CO directly reacts with an oxygen atom of the oxide support to form CO2 and H2O dissociates on the oxide support to liberate H2. My project will prepare and characterize a novel model catalyst in ultra-high vacuum by physical vapor deposition: bimetallic NiZrOx nanoparticles supported by a graphene layer on Ir(111). This catalyst has the support oxide functionality already built into the nanoparticles. Several electron-based surface analysis techniques will be applied to characterize the surface structure and composition of catalysts, such as low energy electron diffraction (LEED), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), surface X-ray diffraction (SXRD), and scanning tunneling microscopy (STM). To test its catalytical activity, adsorption of CO, water, coadsorption of CO+H2O, as well as low temperature WGS reaction will be studied by sum frequency generation (SFG) laser spectroscopy and mass spectroscopy (MS). Utilizing the advantages of surface specificity and selectivity and the polarization dependence of SFG, the surface structure, adsorption sites (on-top, bridge, hollow) and orientation (up, down, or tilted) of adsorbed molecules can be determined. Most importantly, the reaction pathways (associative or redox mechanism) can be achieved according to the species measured by SFG (OH: 3100-3500 cm-1; CO: 1800-2150 cm-1; formate and carboxyl: 1300-1750 cm-1) combined with computational modeling of characteristic surface species.

This project aimed at the water-gas shift (WGS) reaction, in which CO and HO react to form CO and H, using bimetallic NiZrO nanoparticles supported on graphene/Ir(111). To elucidate the reaction mechanism, we first investigated CO and HO adsorption on several reference surfaces. For CO on Ir(111), the resonant sum frequency generation (SFG) signal of the C-O stretch on a rough Ir surface was approximately eight times stronger than on a smooth Ir surface. We systematically evaluated possible origins, including coverage, lateral interactions, molecular hyperpolarizability, adsorption geometry, Fermi resonances, hot vibrational bands, and surface plasmons. We concluded that sputter-induced surface roughness, evidenced by scanning tunneling microscopy (STM), promotes localized surface plasmon resonances (LSPRs), resulting in the observed SFG enhancement. CO adsorption on Gr/Ir(111) showed no evidence of intercalation, confirming the integrity of the graphene layer. For atomic layer deposition (ALD) ZrO, AlO, and TiO oxide support films, no OH groups were detected at room temperature, even after heating ZrO in 10 mbar HO at 520 K. At 93-95 K, residual HO produced a free-OH signal at 3720 cm. To avoid this interference, we used DO to form amorphous solid water (ASW). At low coverage, the SFG spectra showed free OD (~2730 cm) and a broad feature (~2650 cm) tentatively assigned to water hydrogen-bonded to the oxide surface. With increasing coverage, water-water hydrogen bonding became dominant. HO-ASW exhibited a stronger and narrower surface-hydrogen-bonded OH signal but a weaker free-OH signal, indicating fewer free OH groups. No significant differences in the DO hydrogen-bonding networks were observed among the ALD films. For Niclusters deposited on ZrO films, no CO SFG signal was detected so far at 100-200 K up to 26 mbar, suggesting either a low CO surface density or poor ordering on the Ni clusters. DO-ASW on Ni/ZrO also produced much weaker free-OD and hydrogen-bonded OD signals than on the ALD oxides, while the absence of the ~2650 cm feature could further support our assignment to water-oxide hydrogen bonding.