Experimental Optimization of Ni/P Atomic Ratio for Nickel Phosphide Catalysts in Reverse Water-gas Shift

Abstract

Nickel phosphide catalysts show a high level of selectivity for the reverse water-gas shift (RWGS) reaction, inhibiting the competing methanation reaction. This work investigates the extent to which suppression of methanation can be controlled by phosphidation and tests the stability of phosphide phases over 24-hour time on stream. Herein the synthesis of different phosphide crystal structures by varying Ni/P atomic ratios (from 0.5 to 2.4) is shown to affect the selectivity to CO over CH4 in a significant way. We also show that the activity of these catalysts can be fine-tuned by the synthesis Ni/P ratio and identify suitable catalysts for low temperature RWGS process. Ni12P5-SiO2 showed 80–100% selectivity over the full temperature range (i.e., 300–800 °C) tested, reaching 73% CO2 conversion at 800 °C. Ni2P-SiO2 exhibited CO selectivity of 93–100% over a full temperature range, and 70% CO2 conversion at 800 °C. The highest CO2 conversions for Ni12P5-SiO2 at all temperatures among all catalysts showed its promising nature for CO2 capture and utilisation. The methanation reaction was suppressed in addition to RWGS activity improvement through the formation of nickel phosphide phases, and the crystal structure was found to determine CO selectivity, with the following order Ni12P5 >Ni2P > Ni3P. Based on the activity of the studied catalysts, the catalysts were ranked in order of suitability for the RWGS reaction as follows: Ni12P5-SiO2 (Ni/P = 2.4) > Ni2P-SiO2 (Ni/P = 2) > NiP-SiO2 (Ni/P = 1) > NiP2-SiO2 (Ni/P = 0.5). Two catalysts with Ni/P atomic ratios; 2.4 and 2, were selected for stability testing. The catalyst with Ni/P ratio = 2.4 (i.e., Ni12P5-SiO2) was found to be more stable in terms of CO2 conversion and CO yield over the 24-hour duration at 550 °C. Using the phosphidation strategy to tune both selectivity and activity of Ni catalysts for RWGS, methanation as a competing reaction is shown to be no longer a critical issue in the RWGS process for catalysts with high Ni/P atomic ratios (2.4 and 2) even at lower temperatures (300–500 °C). This opens up potential low temperature RWGS opportunities, especially coupled to downstream or tandem lower temperature processes to produce liquid fuels.