The Double-Edged Sword Effect of Water in the Low-Temperature CO Oxidation on Pt(111) Surface

The positive and negative effects of water on low-temperature CO oxidation were comprehensively investigated on a Pt(111) surface by thermal desorption spectroscopy, X-ray photoelectron spectroscopy, and polarization-modulated infrared reflection absorption spectroscopy. At 110 K, the interaction between H2O and preadsorbed O2 forms the {O2(a)·(H2O)n} complex on the Pt(111) surface through hydrogen bonding. Upon heating the surface to 170 K, the O2 in the {O2(a)·(H2O)n} complex is subject to dissociation. On the O2-saturated Pt(111) surface (50 L O2/Pt(111)), part of the chemisorbed O2 on Pt(111) can be displaced by H2O and forms {O2(g)·(H2O)n} trapped by H2O molecules at high H2O coverage (>0.44 ML H2O). At this point, high H2O coverage also weakens the molecular oxygen dissociation. Regarding the CO oxidation, a new CO2 production channel by the interaction of CO with the {O2(a)·(H2O)n} complex at ∼153 K has been discovered by exposing H2O to the CO/O2/Pt(111) surface at 110 K, which dominates the low-temperature (<200 K) CO reaction processes. On the other hand, high H2O coverage can also weaken the CO2 production by replacing adsorbed O2 on Pt(111). These results provide deep insights into the fundamental understanding for the effect of water in low-temperature CO oxidation.