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

Abstract

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 CO-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.

Methanol‐Assisted Autocatalysis in Catalytic Methanol Synthesis

“…The data in Figure illustrate that even minute amounts of water lower the activity substantially, which strongly indicates that water is not the source of the autocatalytic effect. Competitive adsorption of water or its dissociation products is a likely explanation for this kinetic inhibition, as water adsorption isotherm measurements on Cu/ZnO/Al 2 O 3 suggest a high coverage on the Cu surface at temperatures and H 2 O partial pressures similar to those where major water inhibition is observed in Figure . The increase in TOF with rising product concentrations in Figure is even more remarkable when considering the strong inhibition caused by the co‐produced water.…”

Cu−Zn Alloy Formation as Unfavored State for Efficient Methanol Catalysts

“…As a consequence, the loss in catalytic activity is explained by the temperature induced sintering and phase segregation tendencies decreasing the number of active sites. Our group recently published a study highlighting the strong impact of H 2 O under CH 3 OH synthesis conditions . Coverages of 1–2 monolayers of H 2 O were identified as being present under industrially relevant conditions.…”

“…The decrease of the reaction rate coupled to an increase of the CuZn‐alloy formation and E A might be explained by the multiple synergisms focused at the in Cu−ZnO interface (structurally, electronically and mechanistically). A CuZn‐alloy might be an active structure for CH 3 OH synthesis, but not all synergistic effects are observable (e. g. unable to activate H 2 , too stable oxygenates, no OH‐groups to activate CO 2 ‐derived intermediates). Besides, the formation of a small fraction of CuZn‐alloy (likely on the surface/interface of Cu−ZnO) is, under relevant conditions, structurally a reversible phenomenon (re‐oxidation to Cu−O−Zn).…”

Bifunctional Synergy in CO Hydrogenation to Methanol with Supported Cu

“…At lower temperatures (< 473 K) a few kPa of water partial pressure is sufficient to eliminate the majority of the activity for Cu-based methanol synthesis catalysts [6,7]. As H2O adsorption isotherms on Cu [10] or Cu/ZnO/Al2O3 [10][11][12] also suggest that a high coverage is reached at H2O pressures of a few kPa the inhibition can largely be attributed to competitive adsorption (illustrated in supplementary information, figs. S1 & S2).…”