Intermediates Arising from the Water–Gas Shift Reaction over Cu Surfaces: From UHV to Near Atmospheric Pressures

Abstract

The water–gas shift (WGS) reaction $({\text{CO}} + {\text{H}}_{ 2} {\text{O}} \to {\text{CO}}_{ 2} + {\text{H}}_{ 2} )$ is a key process to the production of high purity H₂ from gas streams rich in CO. The identification of the WGS reaction mechanism and the probable stable intermediates is critical to design the catalyst structure, optimize composition and tune reaction kinetics/thermodynamics to achieve the optimum selectivity and activity. In this study, first the WGS reaction steps on Cu(111) have been studied using X-ray photoelectron spectroscopy (XPS) and infrared reflection absorption spectroscopy under ultra-high vacuum (UHV) conditions. Then the interactions of H₂O with CO on Cu(111) have been studied under elevated pressures (90 mTorr CO + 30 mTorr H₂O) at 300–575 K with ambient pressure XPS. Under UHV conditions, non-dissociative adsorption of H₂O on Cu(111) and Cu₂O/Cu(111) was observed. Whereas H₂O readily dissociates, by breaking the O–H bond on a chemisorbed O layer on Cu(111) to form OH species. Even though this OH interacts with adsorbed CO, it does not react to form any associative intermediate and simply desorbs as H₂O at 275 K under UHV conditions. At ambient pressures, no associative intermediates species, only CO and OH, were observed in the reaction of CO with H₂O although the catalytic production of H₂ can be detected under these conditions. Since intermediate species other than CO and OH were not observed on Cu(111) under reaction conditions, we concluded that the redox mechanism is the dominant WGS pathway on Cu(111). The coupling of Cu to an oxide, Cu–CeO₂ catalyst, or a carbide, Cu–TiC catalyst, favors an associative mechanism and produces a very large increase in the rate for the production of H₂ through the WGS.