Simultaneous MS-IR Studies of Surface Formate Reactivity Under Methanol Synthesis Conditions on Cu/SiO2

• Published in: Topics in catalysis 2009-09, Vol.52 (10), p.1440-1447
• Main Authors: Yang, Y., Mims, C. A., Disselkamp, R. S., Peden, C. H. F., Campbell, C. T.
• Format: Article
• Language: English
• Subjects: Catalysis; Characterization and Evaluation of Materials; Chemistry; Chemistry and Materials Science; Industrial Chemistry/Chemical Engineering; Original Paper; Pharmacy; Physical Chemistry
• ISSN: 1022-5528; 1572-9028
• DOI: 10.1007/s11244-009-9320-3

The coverages and surface lifetimes of copper-bound formates on Cu/SiO 2 catalysts, and the steady-state rates of reverse water-gas shift and methanol synthesis have been measured simultaneously by mass (MS) and infrared (IR) spectroscopies under a variety of elevated pressure conditions at temperatures between 140 and 160 °C. DCOO lifetimes under steady state catalytic conditions in CO 2 :D 2 atmospheres were measured by 12 C– 13 C isotope transients (SSITKA). The values range from 220 s at 160 °C to 660 s at 140 °C. The catalytic rates of both reverse water gas shift (RWGS) and methanol synthesis are ~100-fold slower than this formate removal rate back to CO 2  + 1/2 H 2 , and thus they do not significantly influence the formate lifetime or coverage at steady state. The formate coverage is instead determined by formate’s rapid production/decomposition equilibrium with gas phase CO 2  + H 2 . The results are consistent with formate being an intermediate in methanol synthesis, but with the rate-controlling step being after formate production (for example, its further hydrogenation to methoxy). A 2–3 fold shorter life time (faster decomposition rate) was observed for formate under reactions conditions, with both D 2 and CO 2 present, than in pure Ar or D 2  + Ar alone. This effect, due in part to the effects of the coadsorbates produced under reaction conditions, illustrates the importance of using in situ techniques in the study of catalytic mechanisms. The carbon which appears in the methanol product spends a longer time on the surface than the formate species, 1.8 times as long at 140 °C. The additional delay on the surface is attributed in part to readsorption of methanol on the catalyst, thus obscuring the mechanistic link between formate and methanol.