The Characterization of thin Platinum Films on Alumina

Industrial metal catalysts are usually in the form of small metal particles supported on a porous oxide. The typical size of these metal particles ranges between 1.0 and 10.0 nm and it is well known that the particle size and the oxide substrate can affect the catalytic properties of the metal for some important reactions[l]. Previous work with adsorption on small particles has indicated that desorption temperatures[2–4] and the ability to dissociate CO[5,6] can also be affected by the particle size. To further investigate these size and substrate effects, we have examined the adsorption properties of several simple gases on small Pt particles supported on alumina using temperature programmed desorption (TPD). We will show that the desorption curves for CO, H2, and NO on these particles are very similar to curves measured on single crystals. The details of sample preparation and characterization have been described previously[7]. Samples were prepared by vapor depositing Pt onto an alumina substrate which was formed by exposing an Al film to O2. Auger electron spectroscopy was used to monitor surface cleanliness and to characterize the growth of the Pt on the substrate. As reported previously, the Pt grew in a layer-by-layer manner at both 90K and 300K. This film aggregated into clusters when the sample was heated above 650K and the average size of these particles could be determined from the saturation coverage of CO and from the quantity of metal deposited, as measured using a film thickness monitor. All adsorption measurements reported in this paper were made on samples heated above 650K. The TPD results for CO were described in detail elsewhere[7]. For particles with a diameter of 1.1 nm, a single desorption state was observed at 510K. For larger particles, a second desorption event at 400K was also observed and the relative amounts of CO in this lower desorption state increased with particle size. Peaks at 400 and 510K are also observed for CO desorption from single crystals of Pt. On Pt(111), the desorption peak temperature at saturation occurs at 400K[8]. On more open crystal planes like Pt(110), an additional desorption state is observed at 510K[9]. By comparing this work on single crystals to our results on small particles, we concluded that adsorption on small particles must be very similar to adsorption on bulk metals. This implies that adsorption must be very localized. The 1.1-nm particles we studied had approximately 90% of their atoms available for adsorption at the surface and did not contain enough atoms to give them a “metallic” band structure[10]. We hypothesized that the differences we observed in the desorption curves as the particle size was changed was due to the types of sites available for adsorption, with the large particles having a higher concentration of high coordination sites like those found on the (111) surface. TPD results for NO on the small particles[11] were also similar to those reported for Pt crystals. We observed complete dissociation of the NO on small particles, with only N2 being observed at 450K during desorption. For larger particles, a fraction of the NO desorbed undissociated at 420K. These desorption temperatures are in reasonable agreement with those reported for Pt crystals[12]. Also, it has been shown that the fraction of NO dissociating on single crystals during TPD is a strong function of crystal plane, with no dissociation occurring on the low-index Pt(111) surface[12] and complete dissociation occurring on the more open Pt(410) surface[13]. These results again imply that the differences we observed in TPD as the particle size was changed may be due to changes in the coordination of the available sites. The TPD results for H2 from the small particles displayed three desorption peaks at 200K, 280K, and 370K[11]. These peaks were present for all particle sizes studied but were clearly resolved only for the continuous film. The relative populations of these states did not change significantly with particle size. Again, desorption states at these three temperatures are also observed for Pt foils[14] and single crystals[15,16]. However, the relative populations of the desorption features on single crystals do not follow simple trends. The interpretation is made more difficult by the fact that the TPD curves for single crystals can change with the adsorption temperature[15] and with the perfection of the crystal planes[16]. Since the presence of different desorption peaks in TPD can be due to molecular interactions and compression of the adsorbate overlayer, as well as the coordination of the metal site, it is probable that the reason we do not observe changes in the TPD curves with particle size for H2 like we did for CO and NO may be simply due to differences in the effects leading to the various desorption states for each adsorbate. However, the similarity in the desorption curves for H2 from small particles and from bulk metals, both in the fact that several desorption peaks are observed and in the temperatures of those peaks, implies that adsorption must be similar and, therefore, the adsorption bonding must be localized.