Surface-Bound Intermediates in Low-Temperature Methanol Synthesis on Copper: Participants and Spectators

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• 作者：Yong Yang ; Donghai Mei ; Charles H. F. Peden ; Charles T. Campbell ; Charles A. Mims
• 刊名：ACS Catalysis
• 出版年：2015
• 出版时间：December 4, 2015
• 年：2015
• 卷：5
• 期：12
• 页码：7328-7337
• 全文大小：501K
• ISSN：2155-5435

文摘

The reactivity of surface-adsorbed species present on copper catalysts during methanol synthesis at low temperatures was studied by simultaneous infrared spectroscopy (IR) and mass spectroscopy (MS) measurements during 鈥渢itration鈥?(transient surface reaction) experiments with isotopic tracing. The results show that adsorbed formate is a major bystander species present on the surface under steady-state methanol synthesis reaction conditions, but it cannot be converted to methanol by reaction with pure H₂ or with H₂ plus added water. Formate-containing surface adlayers for these experiments were produced during steady-state catalysis in (a) H₂:CO₂ (with substantial formate coverage) and (b) moist H₂:CO (with no IR visible formate species). These reaction conditions both produce methanol at steady state with relatively high rates. Adlayers containing formate were also produced by (c) formic acid adsorption. Various 鈥渢itration鈥?gases were used to probe these adlayers at modest temperatures (T = 410鈥?50 K) and 6 bar total pressure. Methanol gas (up to 鈭?% monolayer equivalent) was produced in 鈥渢itration鈥?from the H₂:CO₂ catalytic adlayers by H₂ plus water, but not by dry hydrogen. The decay in the formate IR features accelerated in the presence of added water vapor. The H₂:CO:H₂O catalytic adlayer produced similar methanol titration yields in H₂ plus water but showed no surface formate features in IR (less than 0.2% monolayer coverage). Finally, formate from formic acid chemisorption produced no methanol under any titration conditions. Even under H₂:CO₂ catalytic reaction conditions, isotope tracing showed that preadsorbed formate from formic acid did not contribute to the methanol produced. Although non-formate species exist during low-temperature methanol synthesis on copper which can be converted to methanol gas by titration with pure H₂ plus water in sufficient quantities for that intermediate to be observable by IR, formate itself is only a 鈥渟pectator鈥?in this reaction and gives no observable methanol upon any titration we performed with H₂ or H₂ plus water.