Water-gas shift (WGS) is one of the hydrogen generating reactions in aqueous phase reforming of biomass derived molecules. We have measured the WGS turnover rate (TOR) at liquid phase conditions and determined the kinetics. The reaction was carried out in a stirred tank reactor at about 40 bar in a semi-batch manner, with continuous flow of gases (Ar, H2, CO and CO2) and water pre-loaded along with the catalyst. The percentage Pt loading was measured by ICP-OES and the Pt dispersion was estimated by transmission electron microscopy (TEM). The minimum value of the agitation speed for which external mass transfer was not the controlling step was measured and all experiments were conducted above that speed. The Koros-Nowak criterion was applied for 1% and 5% Pt/C and the TOR* was found to be 5.8x10-3 s-1 and 4.7x10-3 s-1 respectively. This test suggests that the actual kinetics was being measured.
The liquid phase WGS reaction (LWGS) was carried out on two different catalysts – Pt/C and PtMo/C. The TOR* obtained for PtMo/C (~0.05s-1) was observed to be 10 times higher than that for Pt/C (~0.005s-1). A factor of about 10 was also observed for both gas-phase WGS and the glycerol reforming reaction. The gas-phase WGS TOR was extrapolated to LWGS conditions and vice-versa in order to compare the two. The TOR obtained in LWGS was found to be about 10 times higher than that for the gas-phase WGS. In LWGS on Pt/C, a higher CO order (~0.5) as compared to gas-phase WGS (~0.2) was observed, indicating lower coverage of CO. This can be explained by competitive adsorption between CO and H2O. Carbon dioxide was found to be significantly more inhibiting in LWGS (~-0.3) as compared to gas-phase WGS (~-0.1). We attribute this to a lower temperature of operation as well as the higher partial pressure range of CO2 over which the order was measured. The H2 orders in LWGS and gas-phase WGS are similar (~-0.33). The kinetic data for WGS at liquid phase reforming conditions are the first reported and the results are consistent with predictions made by extrapolation from gas-phase conditions.
*All the TOR values have been normalized to T=160°C, PCO=4bar, PH2=4bar, PCO2=0.2bar, PH2O=4.7bar