High-Pressure Pulsing of Ammonia Results in Carbamate as Strongly Inhibiting Adsorbate of Methanol Synthesis over Cu/ZnO/Al2O3

The active site of the industrially used Cu/ZnO catalyst for the formation of methanol is controversially debated to date. Ammonia has been used as a probe molecule as it was found to inhibit methanol synthesis in CO2-containing synthesis gas. Herein, we investigate the poisoning effect using an industrial-type Cu/ZnO/Al2O3 catalyst synthesized by co-precipitation. During steady-state methanol synthesis in a CO2/CO/H2 synthesis gas, isobaric trimethylamine (TMA) and ammonia injections poisoned methanol formation reversibly, with the poisoning of ammonia being significantly stronger than that of TMA. Based on DFT calculations, a mechanism of ammonia poisoning was derived: ammonia activation takes place on adsorbed oxygen or hydroxyl groups, followed by the reaction with CO2 forming a stable carbamate on the active site. Further hydrogenation of the carbamate to methylamine was calculated to exhibit high barriers, thus being rather slow, explaining why ammonia poisoning has a long-term effect, as TMA cannot form a strongly bound carbamate species exclusively acting as weakly bound site-blocking species.