Kinetics and mechanism of the water-gas shift reaction catalysed by the clean and Cs-promoted Cu(110) surface: a comparison with Cu(111)

Abstract

The structural sensitivity of the water-gas shift (WGS) reaction (CO + H₂O → H₂+ CO₂) over metallic copper is addressed here by comparing its kinetics over the atomically clean Cu(110) surface with prior results for Cu(111). The surfaces were prepared and characterized with UHV surface analysis (AES, LEED, XPS), then transferred to an attached microreactor for medium-pressure [10–1000 Torr (1 Torr = 101 325/760 Pa)] kinetic measurements and finally returned to UHV for post-reaction surface analysis. For both surfaces, the rate is nearly first-order in H₂O pressure and zero-order in CO. Depending upon the temperature, Cu(110) is four- to ten-fold more active than the more densely packed Cu(111) surface. The apparent activation energy is also ca. 7 kcal mol^–1(1 cal = 4.184 J) lower on Cu(110). This is attributed to a lower barrier for O—H bond cleavage in the rate-determining step: i.e. the dissociative adsorption of water. Strong evidence for a ‘surface redox’ mechanism involving oxygen adatoms is provided by comparing the known kinetics of reverse WGS with the rate of dissociative CO₂ adsorption on Cu(110). A potential-energy diagram is presented which explains the known kinetics and energetics for the elementary steps as occurring in the forward or reverse direction, as well as the overall WGS reaction on clean Cu(110). The influence of adsorbed Cs on the reaction kinetics is also presented, and compared to earlier results on Cs/Cu(111). In both cases, Cs strongly accelerates the reaction. In the case of Cu(110), the rate-determining step is even changed. The reaction mechanism involves Cs · CO_{3, a} and Cs · O_a.