Methanol Synthesis from H2/CO2 Over Ga2O3-Pd Silica-Supported Catalysts: Effect of CO Addition in the Feed and the RWGS Reaction

J. BARRANDEGUY, D.L. CHIAVASSA, S.E. COLLINS, A.L. BONIVARDI, M.A. BALTANÁS

Cancún, México

Congreso; 18th Noth American Catalysis Society Meeting; 2003

American Catalysis Society

The catalytic activity over a Ga-Pd(2%)/SiO2 catalyst (Ga to Pd atomic ratio equal to 3) was evaluated in a downflow, differential microreactor (523 K, 3 MPa), fed with different proportions of CO2 and CO, but keeping a constant H2/(CO2+CO) = 3 mol ratio. In other experiments, CO was replaced by He, still maintaining the H2/(CO2+He) = 3 mol ratio in the feeding mixture. A monotonic reduction of the initial reaction rate to methanol (RMeOH) upon decreasing the CO2 mole fraction, in either ternary mixture fed, was found. Vis-à-vis, for the same pCO2 in the system, this reduction became far-reaching when H2/(CO2+CO) was used instead of H2/(CO2+He). Previous FTIR results offer a direct proof that the decrease in RMeOH cannot be ascribed to competitive chemisorption of CO onto gallia. Conversely, CO is strongly chemisorbed on Pd at process conditions, which hampers the ability of the metal to dissociate H2 and, consequently, decreases the rate of hydrogenation of CO2 (via H-spillover) on gallia. Further evidence to sustain this hypothesis was produced by analizing the coverage of CO on Pd (qCO) as a function of pCO. A Temkin isotherm at reaction temperature was generated using the Qad of CO measured on Pd/SiO2 by FTIR. It is showed that there is a direct correspondence beween qCO increases and RMeOH diminution when the pCO in the reaction mixture is made higher.