Copper catalysts are used in the water gas shift reaction to convert mixtures of carbon monoxide and water to hydrogen and carbon dioxide. Conversely, the reverse water gas shift reaction utilizes carbon dioxide as a resource. The mechanisms of these reactions involve several surface intermediates that are difficult to characterize with spectroscopy. We have used reflection absorption infrared spectroscopy (RAIRS) to study these reactions under ambient pressures in the range of 0.1 to 150 Torr. In the presence of 10 Torr of CO, the Cu(111) surface undergoes an irreversible reconstruction to yield a surface on which CO is stably adsorbed at room temperature. In contrast, under ultrahigh vacuum conditions, from temperature programmed desorption studies CO is known to desorb below 190 K. The results are in accord with a recent study with scanning tunneling microscopy that showed formation of Cu nanoclusters on a Cu(111) surface in response to ambient pressures of CO. A multitude of different C–O stretching frequencies are observed for CO adsorbed on the reconstructed surface, which is indicative of the diversity of sites available on the Cu clusters. The CO-induced reconstruction also leads to a higher stability for adsorbed water than is observed on a flat Cu(111) surface. In the presence of 150 Torr of carbon dioxide, a peak is observed at 1294 cm-1, which is assigned to the symmetric stretch of an adsorbed CO2 molecule that is bent through its interaction with the surface. Upon addition of 150 Torr of H2(g) at 300 K, the 1294 cm-1 peak of CO2 is greatly diminished, while new peaks appear due to gas phase water. By performing the experiment with polarized infrared radiation, the gas phase and surface species are easily distinguished. When the Cu(111) surface is annealed to higher temperatures, numerous changes occur in the spectra revealing the formation of a series of surface intermediates containing C–H bonds. The results demonstrate that CO2 hydrogenation is readily achieved over a Cu(111) model catalyst and that surface intermediates can be easily detected with RAIRS.

Infrared spectroscopy of the water gas shift reaction over the Cu(111) surface under ambient pressure conditions

255th National Meeting (2018)

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