Interaction of CO with OH on Au(111): HCOO, CO < inf> 3 , and HOCO as key intermediates in the water-gas shift reaction

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We have investigated the role of formate (HCOO), carbonate (CO 3), and carboxyl (HOCO) species as possible intermediates in the OHads + COgas → CO2,gas + 0.5H 2,gas reaction on Au(111) using synchrotron-based core level photoemission, near-edge X-ray absorption fine structure (NEXAFS), and infrared absorption spectroscopy (IR). Adsorbed HCOO, CO3, and OH species were prepared by adsorbing formic acid, carbon dioxide, and water on a Au(111) surface precovered with ̃0.2 ML of atomic oxygen, respectively. HCOOH interacts weakly with Au(111), but on O/Au(111) it dissociates its acidic H to yield adsorbed formate. The results of NEXAFS, IR, and density-functional calculations indicate that the formate adopts a bidentate configuration on Au(111). Since the HCOO groups are stable on Au(111) up to temperatures near 350 K, it is not likely that formate is a key intermediate for the OHads + COgas → CO2,gas + 0.5H2,gas reaction at low temperatures. In fact, the formation of this species could lead eventually to surface poisoning. When compared to a formate species, a carbonate species formed by the reaction of CO2 with O/Au(111) has low stability, decomposing at temperatures between 100 and 125 K, and should not poison the gold surface. Neither HCOO nor CO3 was detected during the reaction of CO with OH on Au(111) at 90-120 K. The results of photoemission and IR spectroscopy point to HO ↔ CO interactions, consistent with the formation of an unstable HOCO intermediate which has a very short lifetime on the gold surface. The possible mechanism for the low-temperature watergas shift on gold catalysts is discussed in light of these results. © 2009 American Chemical Society.

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Journal of Physical Chemistry C