Converting CO2 to CO via the photocatalyzed reverse water gas shift (RWGS) reaction satisfies a critical step in the production of C1 solar fuels (e.g., CH4, CH3OH) and longer hydrocarbons via the Fischer-Tropsch process. Oxide-supported nickel phosphides (NixPy) are an emerging group of photocatalysts with a number of accessible phases. Additionally, the broad-spectrum absorption of NixPy materials affords photo-induced local heating to drive catalytic processes at mild operating temperatures. In the current project, NixPy with different P/Ni molar ratios are being investigated as RWGS catalysts on two types of support materials: 1) silica (SiO2), a relatively inert support, and 2) a reducible oxide support (e.g., In2O3, TiO2). NixPy/SiO2 catalysts (2.5 wt%) were synthesized via incipient wetness impregnation followed by temperature-programmed reduction (TPR); crystalline phases (Ni2P, Ni12P5, Ni3P) were identified using X-ray diffraction and compositions were determined by energy dispersive X-ray spectroscopy (EDS). Ongoing RWGS studies are being carried out in a batch photoreactor using a Xe lamp (300 W) with the gas composition monitored by gas chromatography. Initial studies were executed using a SiO2 support, which is inactive for the RWGS reaction under light exposure, enabling the photocatalytic properties of the different NixPy phases to be probed. Trends of increasing CO selectivity and decreasing CO2 conversion have emerged as the P/Ni molar ratio increases, while the photo enhancement (light/dark ratio) decreases. Studies are underway to investigate the same NixPy phases on reducible oxide supports for comparison with the silica-supported counterparts to gain insight on the roles of the support and NixPy-support interface in determining the RWGS photocatalytic properties.

Nickel phosphide photocatalyst development for CO2-derived solar fuels

ACS Spring 2022

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