In Situ Quantification of Reaction Adsorbates in Low-Temperature Methanol Synthesis on a High-Performance Cu/ZnO:Al Catalyst

The industrial low-temperature process has been applied for 50 years; however, in situ data under relevant conditions are rare. We report on the in situ quantification of the surface adsorbates present under industrially relevant conditions by high-pressure thermogravimetry. In addition, high-pressure IR spectroscopy is applied for the identification of carbon-based adsorbates. On a high-performance Cu/ZnO:Al catalyst it has been shown that during CO2 hydrogenation adsorbates of up to 1.9 wt % of the catalyst are reversibly accumulated, and 70% of the ad-layer consists of H2O-derived species. Under CO2 hydrogenation conditions, the weight accumulation on the surface is limited to an increase of 1.2 wt % mainly due to the absence of H2O. The stable adsorbate layers from different feeds are qualitatively assigned by surface titration experiments and spectroscopic insights. In accordance with the literature, it is clearly illustrated that, on the basis of the feed-dependent coverage of the surface, different reaction mechanisms for the methanol formation are involved. These investigations under realistic conditions finally show the importance of Zn-OH groups, likely located at the Cu/ZnO interface, as being crucial for activation and hydrogenation of CO2-derived intermediates to CH3OH. The accumulation of H2O- and carbon-derived species on the surface of the catalyst might explain the poor activity in CH3OH formation at low temperatures and consequently limits the application of Cu/ZnO-based catalysts under mild, thermodynamically preferred conditions.