Single crystal and supported nano-cluster copper-palladium/platinum model catalysts
Reilly, James P (2000) Single crystal and supported nano-cluster copper-palladium/platinum model catalysts. PhD thesis, Dublin City University.
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This work involved the experimental investigation of a variety of model copperpalladium/ platinum heterogeneous catalysts in the form of single-crystal surfaces (Cu(110)/Pd and Cu(100)/Pt) and highly orientated pyrolytic graphite (HOPG) supported nano-clusters. The surface structural arrangement was primarily examined using scanning tunnelling microscopy (STM) and low energy electron diffraction (LEED) while X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were used to determine surface composition. In addition, temperature-programmed desorption (TPD) has been employed to probe the reactivity of the surfaces via the decomposition of formic acid (HCOOH).
In the case of the Cu(l 10)/Pd system XPS measurements backed up by STM/LEED clearly demonstrate the formation of regions of a Cu(l 10)-p(2xl)-Pd surface alloy at low Pd coverages (0pd<l ML) with considerable disorder in the form of monolayer deep pits and islands. Higher Pd coverages led to the formation of a granular film of epitaxial densely packed flat topped Pd clusters with largely a rectangular shape of average size 75x150 A. The favoured growth mechanism is of multilayered Pd islands above a mixed (2x1) CuPd interface of two to three atomic layers thickness. Only after annealing to 600 K is the granular structure of the higher coverage Pd films disrupted due to inter-diffusion into the Cu substrate, leading to a surface with irregularly shaped flat domains separated by mono-atomic steps. Higher temperature (720 K) annealing led to further flattening and the appearance of regular parallel lines in STM images whose spacing varies with Pd loading, which is assigned to strain due to lattice mismatch between a “capping” outermost copper monolayer and the underlying mixed CuPd alloy. High Pd coverage samples annealed to 500 K displayed substantial destabilisation of the formate intermediate relative to clean Cu(l 10), attributed to formate adsorption on mixed CuPd sites.
The nucleation and growth of both monometallic and bimetallic Pd and Cu clusters on HOPG have been investigated. STM, and XPS measurements of monometallic films of Cu and Pd verified a Volmer-Weber growth mechanism with formation of hemispherical monometallic clusters of average diameter ~4 nm (Pd) and ~8 nm (Cu).
Bimetallic films of Cu and Pd formed by sequential deposition revealed properties dependent on the sequence of metal deposition. In the case of Cu deposition on predeposited Pd clusters, preferential coating of Pd by a Cu thin film (themodynamically favourable in surface energy terms) at low Cu coverages is observed, followed by the formation of phase separated Cu clusters as the Cu coverage is increased. XPS measurements rule out substantial alloy formation. STM indicates the majority of hemispherical bimetallic clusters are 5 to 7 nm in diameter, intermediate in size compared to that found for both monometallic Cu and Pd films. For Pd deposition on pre-deposited Cu, XPS data indicate a growth mechanism whereby the simultaneous growth of Pd clusters on the HOPG surface and the alloying/capping of areas already covered by Cu clusters occurs. Shifts in the Cu 2p core-level to lower binding energies for increased Pd loading on all levels of Cu pre-dosed samples illustrate a strong Pd-Cu interaction and fully supports this theory. Again an intermediate cluster-size distribution was observed by STM. In the course of this work a new simple and reliable method for the absolute calibration of surface coverage was developed, based on the ex-situ analysis of the absolute amount of the metal evaporated using graphite furnace atomic adsorption spectroscopy (GF-AAS). This method is shown to have an absolute sensitivity of better than 0.1 ML.
The Cu(100)/Pt bimetallic combination has been examined by LEED, AES and TPD. An underlayer c(2><2) CuPt alloy capped by a Cu monolayer may be formed by deposition of ~0.6 ML Pt followed by thermal activation. In contrast, higher coverages of Pt (1.0-2.5 ML) followed by annealing to 550 K led to a local order c(2x2) CuPt alloy with a mixed CuPt top layer. A top layer CuPt alloy led to a destabilisation of the formate intermediate due to adsorption on a mixed CuPt site although the surface chemistry is still similar to pure Cu(100). The underlayer CuPt c(2x2) also exhibits destabilisation of the formate intermediate but to a lesser extent than the c(2x2) mixed CuPt top layer.
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