Surface photochemistry of CO adsorbed on alumina supported nanoparticulate platinum

Laser induced desorption of CO adsorbed on platinum nanoparticles on an epitaxial alumina support grown on NiAl(110) is reported for nanosecond laser excitation at λ = 355 nm. The nominal amount of platinum deposited was 0.1 nm, resulting in platinum particles with an average diameter of a few nanometres. The laser fluence was systematically varied between 6.4 and 25.5 mJ cm-2 per pulse. Fourier transform infrared reflection absorption spectra have been recorded as a function of CO coverage, the laser fluence and the number of photons impinging on the surface. Laser desorption is observed, in contrast to the case for experiments on Pt(111) for the same laser wavelength. For laser fluences below 12.7 mJ cm-2 per pulse, a cross section of (1.1 ± 0.2) x 10-19 cm2 can be estimated from the measurements. At elevated fluences a second desorption channel occurs with a cross section more than an order of magnitude larger, scaling linearly with the laser fluence. In all cases desorption ends at a critical coverage beyond which no desorption occurs and which depends on the laser fluence. Laser induced particle morphology changes are observed for higher laser fluences which are not apparent for bare particles. A model implying energy pooling within adsorbates at hot spots and even spillover between the metal nanoparticles and the oxidic support is discussed. Implications for the design of photocatalysts with possible use in chemical solar energy conversion are pointed out.