The conversion of carbon dioxide to liquid fuels, such as methanol, is an important reaction both to our present day industry and to a future renewable economy. Methanol is advantageous for its uses in transportation, fuel cells, olefin production, and other chemical synthesis. Presently, the methanol synthesis reaction is performed at high-pressures (50-100 bar) using copper-zinc oxide catalysts and a CO/CO2/H2 feed. It would be advantageous to perform this reaction at lower pressures such that lower cost or decentralized production becomes possible. However, for the currently used catalysts, lower pressures shift reaction selectivity towards carbon monoxide via the reverse water-gas shift (rWGS) reaction. Thus, new catalysts must be developed which can maintain high activity and selectivity at low pressure conditions. Prior work by F. Studt et al. (Nature Chem. 2014) showed that nickel-gallium based catalysts, particularly Ni5Ga3, had comparable activity to CuZnO catalysts at ambient pressure while significantly repressing the rWGS reaction. To accelerate development of this catalyst, a deeper understanding of the catalytic sites is desired. Operando spectroscopy holds the promise of delivering this understanding.
In collaboration with the Stanford Synchrotron Radiation Lightsource (SSRL), methods were developed for the characterization of catalysts operating under reaction conditions. Reactions were carried out in a quartz capillary with simultaneous characterization by transmission x-ray microscopy and x-ray absorption near edge spectroscopy (TXM-XANES). The structure and electronic state of the catalyst was monitored as the reaction progressed. The spectroscopy set-up is based on previous experiments by J. Andrews and B. Weckhuysen (ChemPhysChem2013) with some key performance modifications. The current system is able to operate up to 350°C and up to 10 bar over several hours of testing.
Using TXM-XANES and other tools, nickel-gallium catalysts were synthesized and evaluated for methanol synthesis. Via an incipient wetness impregnation technique, we produced roughly 10 nm Ni-Ga alloy nanoparticles supported on silica. Catalytic testing showed that the catalysts made in this study performed similarly to those previously reported. The only detected products were methanol and CO, with methanol activity of 0.31 mol MeOH/(mol metal*hr) and a CO/MeOH ratio of 1.3. Ex situ pre- and post-reaction characterization by transmission electron microscopy (TEM) and x-ray diffraction (XRD) did not show significant changes after reaction. However, the catalysts were also tested in the operando set-up at SSRL. This preliminary experiment investigated the nickel K-edge and demonstrated an increase in the nickel oxidation state during operation. The most active catalyst (Ni5Ga3) transitioned from a metallic spectra before reaction to a Ni1.15+ fit during reaction. This suggests that there are changes to the catalyst which have been previously unseen in ex situ measurements, potentially involving coordination changes or adsorbate interactions with the surface. Additional beamline studies are being planned to elucidate the root cause of the changes and to expand the work to include the extended x-ray absorption fine structure as well as the gallium spectra. With this additional data, the future work will round out our understanding of the active sites present in this catalyst. By knowing how these catalysts work, models can be improved, leading to more active targets for future catalyst research.