Self-sustained combustion of carbon monoxide over CuCe0.75Zr0.25Oδ catalyst: Stability operation and reaction mechanism

Abstract

The self-sustained combustion of carbon monoxide (CO) has been studied over the CuCe0.75Zr0.25Oδ catalyst by sol–gel method, and compared with the CuCe0.75Zr0.25Oδ(H) and Ce0.75Zr0.25Oδ to investigate sensitivity of different active sites, such as dispersed CuO, Cu–Ce and Ce–Zr solid solution. The CuCe0.75Zr0.25Oδ(H) is obtained via CuCe0.75Zr0.25Oδ sonicated in nitric acid to remove surficial CuO species, and the Ce0.75Zr0.25Oδ is prepared as the reference catalyst. Using the temperature programmed oxidation of CO together with an infrared camera (CO-TPO + FLIR), the catalytic activity for CO self-sustained combustion is determined not only by the ignition temperatures, following the decreasing order CuCe0.75Zr0.25Oδ (81 °C) > CuCe0.75Zr0.25Oδ(H) (131 °C) > Ce0.75Zr0.25Oδ (167 °C) at the flow rate of 200 mL/min, but also by corresponding limits of lean combustion (equivalence ratio Φ) of 0.06–0.09, 0.10–0.13, 0.24–0.37, respectively, under the flow rate of 100–1000 mL/min. The M-K mechanism, in which adsorbed CO reacts with lattice oxygen, is crucial for all the catalysts via temperature programmed surficial reaction and in situ infrared analysis (TPSR + DRIFT). The sensitivity of active sites as follows: well-dispersed CuO > Cu–Ce solid solution > Ce–Zr solid solution. As rate-determining step for CO self-sustained combustion, CO is preferentially adsorbed on surficial dispersed CuO to yield carbonyls, and then the carbonyls interact with lattice oxygen to form CO2 release. CO is secondly adsorbed on the surficial oxygen of copper and cerium sites in solid solution to yield carbonates. The carbonates formed are more stable and thus CO2 is produced at the lower rate than carbonyls, indicating that the solid solution is less active than dispersed CuO. Exposed Ce3+ favors to form vacancies on the Cu–Ce solid solution surface, which is beneficial to adsorbing both CO and O2, thus presenting the higher activity than Ce–Zr solid solution.