(509r) Tungsten Carbides As Active Catalysts for CO2 Hydrogenation

Transition metal carbides are an attractive substitute for noble metal catalysts for carbon dioxide hydrogenation due to their low cost and high selectivity to desired products (CO, CH_4, or hydrocarbons). Supported Mo₂C and WC exhibit high selectivity toward CO during reverse water-gas shift (RWGS),¹ however WC-based catalysts show relatively lower catalytic activity due to the high synthesis temperature resulting in nanoparticle agglomeration and low availability of active surface sites. Previous work suggests the W₂C phase is more active for RWGS,² although individual carbide phases have not yet been isolated and examined for CO₂ hydrogenation.

Our investigations aim to isolate the RWGS active phase of WC_x-based catalysts by adjusting synthetic and carburization parameters. Nano-WC_x is synthesized via an adapted micelle synthesis method³ and contrasted to an incipient wetness impregnation (IWI) control to investigate effects of particle size and carburization profiles on the tungsten phase, and in turn, RWGS performance. Based on our prior results over alkali-promoted WC_x, we hypothesize that maximizing the active surface area and W₂C phase will result in the highest performing catalyst. TEM images indicate a particle size of ~10 nm for the nano-WC_x, in contrast to ~ 30 nm over the IWI control as shown in Figure 1. The tungsten phase is confirmed as W₂C over the nano-WCx and as a mixed carbide (W₂C and WC) over the IWI control via X-ray characterization techniques (XRD and XPS). Pulse chemisorption and temperature programmed desorption experiments show increased CO uptake over nano-WC suggesting a greater availability of active sites with smaller W particle size. Packed bed reactor studies illustrate nano-WC_x is more active compared to the IWI control (CO space-time yields of 65.9 µmol CO/g_Tungsten/s and 11.8 µmol CO/g_Tungsten/s respectively). This increased activity observed over nano-WC_x could be attributed to a greater predominance of W₂C.