Revealing Origins of Methanol Selectivity Loss in CO2 Hydrogenation over CuZn-Containing Catalysts

CO2 hydrogenation to methanol (CH3OH) is widely accepted to proceed through two parallel reactions: (i) CH3OH formation and (ii) the reverse water gas shift (RWGS) reaction to CO. The latter reaction causes the loss of CH3OH selectivity. Our spatially resolved analysis of rates of product formation over a classical CuZnAlOx catalyst in a broad range of CO2 conversion degrees (from 0 to 90% of equilibrium conversion) suggests revisiting this concept. In comparison with the RWGS reaction, CH3OH decomposition to CO mainly contributes to the loss of CH3OH selectivity with a rising degree of CO2 conversion. Separate CH3OH decomposition tests in a broad range of experimental conditions proved that this side reaction is accelerated by H2O but negatively affected by H2 and rising total pressure. Moreover, the decomposition should occur on other sites rather than those participating in CH3OH synthesis from CO2 and can be practically suppressed above certain partial pressures of CH3OH and H2O due to site saturation. The sites responsible for the hydrogenation of CO2 to CH3OH, however, are not saturated. Thus, this product can be further produced in downstream catalyst layers. As this concept is valid for several CuZn-based catalysts, we provide fundamentals for the design of selective CH3OH synthesis catalysts and for optimizing reaction conditions. Operating under conditions, where the undesired CH3OH decomposition reaction is hindered, enabled us to achieve 93% CH3OH selectivity at 19% CO2 conversion (55% equilibrium conversion) at 50 bar and 200 °C using a feed with the ratio of H2/CO2 of 3.