In situ ATR-IR spectroscopic and reaction kinetics studies of water-gas shift and methanol reforming on Pt/Al2O3 catalysts in vapor and liquid phases.
Reaction kinetics measurements of the water-gas shift reaction were carried out at 373 K on Pt/Al2O3 in vapor phase to investigate the effects of CO, H2, and H2O partial pressures. Results of in situ ATR-IR studies conducted in vapor phase under similar conditions suggest that the Pt surface coverage by adsorbed CO is high (approximately 90% of the saturation coverage), leading to a negligible effect of the CO pressures on the rate of reaction. The negative reaction order with respect to the H2 pressure is caused by the increased coverage of adsorbed H atoms, and the fractional positive order with respect to the water pressure is consistent with non-equilibrated H2O dissociation on Pt. Results of in situ ATR-IR studies carried out at 373 K show that the presence of liquid water leads to a slight decrease in the Pt surface coverage by adsorbed CO (approximately 80% of the saturation coverage) when the CO partial pressure is the same as in the vapor-phase studies. The rate of the WGS reaction in the presence of liquid water is comparable to the rate under complete vaporization conditions when other factors (such as CO partial pressure) are held constant. Reaction kinetics measurements of methanol reforming were carried out at 423 K over a total pressure range of 1.36-5.84 bar. In situ ATR-IR studies were conducted at 423 K to determine the Pt surface coverage by adsorbed CO in completely vaporized methanol feeds and in aqueous methanol solutions. The decomposition of methanol is found to be slower during the reforming of methanol in liquid phase than in vapor phase, which leads to a lower rate of hydrogen production in liquid phase (0.08 min(-1) at 4.88 bar) than in vapor phase (0.23 min(-1) at 4.46 bar). The lower reaction order with respect to methanol concentration observed for vapor-phase versus liquid-phase methanol reforming (0.2 versus 0.8, respectively) is due to the higher extent of CO poisoning on Pt for reforming in vapor phase than in liquid phase, based on the higher coverage by adsorbed CO observed in completely vaporized methanol feeds (55-60% of the saturation coverage) than in aqueous methanol feed solutions (29-40% of the saturation coverage).