Interaction of CO with OH on Au(111): HCOO, CO₃, and HOCO as Key Intermediates in the Water-Gas Shift Reaction

We have investigated the role of formate (HCOO), carbonate (CO 3 ), and carboxyl (HOCO) species as possible intermediates in the OH ads + CO gas → CO 2,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, CO 3, 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 OH ads + CO gas → CO 2,gas + 0.5H 2,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 CO 2 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 CO 3 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 water-gas shift on gold catalysts is discussed in light of these results.