Enhanced Methanol Production over Non-promoted Cu-MgO-Al2O3Materials with Ex-solved 2 nm Cu Particles: Insights from an Operando Spectroscopic Study

Abstract

Enhanced methanol production is obtained over a non-promoted Cu-MgO-Al2O3 mixed oxide catalyst derived from a Cu-Mg-Al hydrotalcite precursor (HT) containing narrowly distributed small Cu NPs (2 nm). Conversions close to the equilibrium (∼20%) with a methanol selectivity of 67% are achieved at 230 °C, 20 bar, and a space velocity of 571 mL·gcat-1·h-1. Based on operando spectroscopic studies, the striking activity of this Cu-based catalyst is ascribed to the stabilization of Cu+ ions favored under reaction conditions due to lattice reorganization associated with the "HT-memory effect"promoted by water. Temperature-resolved infrared-mass spectrometry experiments have enabled the discernment of monodentate formate species, stabilized on Cu+ as the intermediate in methanol synthesis, in line with the results of density functional theory calculations. These monodentate formate species are much more reactive than bridge formate species, the latter ones behaving as intermediates in methane and CO formation. Moreover, poisoning of the Cu0 surface by strongly adsorbed species behaving as spectators is observed under reaction conditions. This work presents a detailed spectroscopic study highlighting the influence of the reaction pressure on the stabilization of active surface sites, and the possibility of enhancing methanol production on usually less active non-promoted nano-sized copper catalysts, provided that the proper support is selected, allowing the stabilization of doped Cu+. Thus, a methanol formation rate of 2.6 × 10-3 molMeOH·gcat-1·h-1 at 230 °C, 20 bar, and WHSV = 28 500 mL·gcat-1·h-1 is obtained on the Cu-MgO-Al2O3 HT-derived catalyst with 71% methanol selectivity, compared to 2.2 × 10-4 molMeOH·gcat-1·h-1 with 54% methanol selectivity obtained on a reference Cu/(Al2O3/MgO) catalyst not derived from a HT structure.