The Synergetic Effect of Cu ⁺ /Cu ²⁺ in Unsupported Cu _<i>x</i> O on the Activity and Mechanism of Low-Temperature CO Oxidation: A Transient Kinetic and Isotopic Study

High Resolution Image Download MS PowerPoint Slide This work has been motivated by the lack of knowledge concerning the influence of the surface Cu + /Cu 2+ molar ratio on the composition and reactivity of the adsorbed phase established on the unsupported Cu 2 O-CuO solids during CO oxidation in excess oxygen (1% CO/20% O 2 /He) and at low temperatures ( T < 200 °C). Variation of the Cu + /Cu 2+ ratio was achieved under controlled calcination conditions applied on an initially c-Cu 2 O synthesized by the colloidal method. The influence of the Cu + /Cu 2+ molar ratio on the mechanism of CO oxidation (Langmuir–Hinshelwood, Mars–van Krevelen, or Eley–Rideal), the surface concentration of active CO-s and COO-s reaction intermediates, and the thermal stability of carbonate-type adsorbed inactive species formed on the working catalytic surface were derived using the SSITKA technique combined with mass spectrometry and in situ DRIFTS. It was found that for high Cu + /Cu 2+ molar ratio values (ca. ∼4.9), CO oxidation follows an Eley–Rideal mechanism, where gaseous CO reacts mainly with labile lattice oxygen species. Reversibly chemisorbed CO (θ ∼ 0.15) was measured, but this was not in the CO oxidation path. On the other hand, for low Cu + /Cu 2+ molar ratio values (ca. 0.01–0.025), the mechanism shifts toward M-vK type, where the concentration of active COO-s was found to increase with increasing Cu 2+ /Cu + molar ratio. Two types of linear adsorbed CO-s were identified by SSITKA-DRIFTS under working reaction conditions, and these were associated with Cu 2+ and Cu + sites. The ratio of the two adsorbed CO species did not vary proportionally with the Cu + /Cu 2+ ratio. The specific CO oxidation rate (per gram or m 2 of Cu x O) was found to increase with increasing Cu + /Cu 2+ ratio, while the opposite is true for the apparent activation energy ( E app, kJ mol –1 ). An E app of 80 kJ mol –1 for Cu + /Cu 2+ ∼ 4.9 compared to 125 kJ mol –1 for Cu + /Cu 2+ ∼ 0.01 was measured. TOF CO (s –1 ) values were estimated based on the active sites associated with reaction intermediates truly participating in the CO oxidation reaction path. The TOF CO was also found to increase with increasing Cu + /Cu 2+ ratio, largely due to the increased concentration of Cu 2 O-CuO heterojunctions of the hollow structure formed (Cu + /Cu 2+ ∼ 4.9), and likely to their enhanced site activity. Dynamic rates of lattice oxygen reduced and its amount (He → 1%CO/He step-gas switch) and those for oxygen refilling (He → 2% O 2 /He step-gas switch) over the reduced Cu 2 O-CuO, the chemical structures of CO-s and CO 2 -s formed, and the thermal stability of CO 2 -s in the oxygen atmosphere were determined. HRTEM highlighted the morphology change of Cu 2 O particles with varying calcination temperature of c-Cu 2 O NPs. X-ray photoelectron spectroscopy allowed us to estimate the surface Cu + and Cu 2+ composition as a function of calcination temperature. In situ CO-DRIFTS enabled the examination of variations in the surface Cu + /Cu 2+ ratio under the CO oxidation reaction conditions.