Adsorption of CO on the copper-precovered ZnO(0001) surface: a molecular-beam scattering study.

Auger electron spectroscopy (AES), thermal-desorption spectroscopy (TDS), and, in particular, molecular-beam scattering techniques have been combined to address particle size effects in the adsorption of CO on Cu-on-ZnO(0001). AES and TDS lead to a Cu coverage, theta(Cu), calibration. The TDS curves, as a function of theta(Cu) and CO exposure, revealed three structures at 150, 220, and 260-280 K, in agreement with prior studies. However, a unique assignment of the TDS structures to a particular Cu face was not possible. An enhancement of the initial adsorption probability, S0, with respect to the support indicates the effect of the Cu nanoparticles. Despite that the shape of S0 versus impact energy curves was independent of theta(Cu) and agreed with Cu single-crystal reference systems, distinct particle size effects were present with regard to the adsorption mechanism. It was possible to observe a crossover from Langmuir-type adsorption dynamics to more precursor-assisted adsorption dynamics with increasing theta(Cu). Thus, a dynamic structure-activity relationship was evident, i.e., the energy-transfer mechanism depends on the Cu morphology.