CO2 conversion through Reverse Water Gas Shift over Molybdenum and Tungsten Carbides: Catalytic Performance Evaluation

Abstract

Mitigating climate change requires reducing CO2 emissions and conserving energy, but current CO2 capture and storage methods are complex and costly. An alternative is the thermocatalytic hydrogenation of CO2 using the Reverse Water–Gas Shift (RWGS) reaction to produce value-added chemicals. Transition metal carbides (TMCs) offer significant potential in this area. This research aims to assess the catalytic performance of Molybdenum carbide (Mo2C) and Tungsten Carbide (WC) to enhance CO2 conversion and CO selectivity in the RWGS reaction. Using the Temperature Programmed Reduction (TPR) method (15% CH4/H2, 600°C), Mo2C, WC, and five mixed carbides (MoWxC with x=0.25, 0.5, 0.75, 1, and 1.5) were synthesized and compared. The catalyst with the highest selectivity and good conversion was chosen to optimize the RWGS reaction.

The performance of TMCs was assessed in terms of both conversion and selectivity by varying four parameters such as Temperature (500°C - 350°C), Pressure (1 to 9 bar), GHSV (20,000 to 400,000 mlg-1*h-1), H2:CO2 Feed Ratio (1:1 to 5:1). To evaluate the catalyst resilience, stability tests have been also performed. Moreover, the structure of pre and post reaction catalyst has been investigated. The resulting reaction products were monitored using an in-line Infrared Analyzer to identify the concentration of CO, CH4, and CO2.

The results indicated that at 500°C, CO2 conversions approaching equilibrium for most carbide samples, categorized into three main groups. Absence of W (Mo2C) resulted in higher conversion but lower selectivity (Group 1). Higher W concentrations in MoW1.5C and WC led to higher selectivity but lower CO2 conversion (Group 3). In Group 2, MoWxC (x=0.25, 0.5, 0.75, and 1) showed better conversion and selectivity, with MoW0.25C and MoW0.5C exhibiting higher CO2 conversion and CO selectivity than MoW0.75C and MoWC. WC was chosen for its high CO selectivity and good CO2 conversion for optimizing the RWGS reaction. Under optimized conditions (500°C, GHSV = 20,000 ml g⁻¹ h⁻¹, Feed Ratio= 5:1, atmospheric pressure), WC showed 100% selectivity and 33% conversion, maintaining stability for 100 hours with full CO selectivity and stable CO2 conversion. Increasing the feed ratio for WC increased conversion with full CO selectivity, while for Mo2C, more hydrogen led to more methane formation.

This study serves as a foundation for the optimization of Mo2C and WC, aiming to convert the global challenge of CO2 into an opportunity by producing renewable value-added chemicals and feedstock.