Quantification of Formate and Oxygen Coverages on Cu Under Industrial Methanol Synthesis Conditions

Abstract

A method for quantifying the surface coverage of adsorbate species for reactions at high pressure and high temperature is presented. Methanol was synthesized from CO2/CO/H2 (3/29/68) at 523 K and 50 bar on Raney Cu, which yielded a turnover frequency (TOF) per Cu surface atom of (6.22 ± 1.04) × 10–3 s–1. Rapid quenching by submerging the catalytic reactor in ice water during operation allowed the formate surface coverage on Cu (θCu-HCOO) at reaction conditions to be determined in a subsequent temperature programmed desorption experiment. This yielded θCu-HCOO = 0.071 ± 0.012 ML and a corresponding TOF per adsorbed HCOO of 0.088 ± 0.021 s–1. CO2 was essentially observed to be the exclusive source of methanol formation over Raney Cu, as the CO in the syngas could be replaced by N2 without impact on the methanol formation rate. Based on this observation and the considerable support in the literature for a CO2-pathway via surface formate on Cu the TOF per surface formate species should represent the true rate of the catalytic cycle in Cu-catalyzed methanol synthesis. Temperature programmed hydrogenation of Raney Cu and unsupported Cu after quenching during high-pressure operation did not indicate a presence of oxygen species (O or OH) on the working Cu surface.