Methanol synthesis from CO2/H2 using Ga2O3–Pd/silica catalysts: Kinetic modeling

The synthesis of methanol from CO2/H2 on a Ga2O3–Pd/silica catalyst, together with the reverse water gas shift reaction, was modeled for a wide range of temperatures (508–538 K), pressures (1–4 MPa), compositions (H2/CO2 = 1, 3 and 6) and space velocity conditions. The kinetic information was combined with relevant spectroscopic (FT-IR) data. The rate determining steps (rds) of the reactions were the hydrogenation of the formate intermediate, and its decomposition on the gallia surface, respectively.A competitive adsorption mechanism, where adsorbed atomic hydrogen occupies the same active sites as other oxygenated surface intermediates on the gallia, was found as the most satisfactory, in terms of physicochemical significance of the parameter estimates. Minimal residuals were found when considering as kinetically relevant the simultaneous surface occupancy by formate, methylenebisoxy, hydroxyl and atomic hydrogen intermediates.The deleterious impact of CO for certain process conditions, such as high conversion and/or ternary H2/CO2/CO mixture feeds, was also studied. In these cases, CO competes with H2 on the Pd crystallites, severely limiting the availability of atomic hydrogen to the gallia surface. Using the steady-state approximation, the supply and demand of atomic hydrogen were then balanced to find the best model interpretation of the observed reactivity.