In Situ Studies of Surface Mobility on Noble Metal Model Catalysts Using STM and XPS at Ambient Pressure
High Pressure Scanning Tunneling Microscopy (HP-STM) and Ambient Pressure X-ray Photoelectron Spectroscopy were used to study the structural properties and catalytic behavior of noble metal surfaces at high pressure. HP-STM was used to study the structural rearrangement of the top most atomic surface layer of the metal surfaces in response to changes in gas pressure and reactive conditions. AP-XPS was applied to single crystal and nanoparticle systems to monitor changes in the chemical composition of the surface layer in response to changing gas conditions.
STM studies on the Pt(100) crystal face showed the lifting of the Pt(100)-hex surface reconstruction in the presence of CO, H2, and Benzene. The gas adsorption and subsequent charge transfer relieves the surface strain caused by the low coordination number of the (100) surface atoms allowing the formation of a (1x1) surface structure commensurate with the bulk terminated crystal structure. The surface phase change causes a transformation of the surface layer from hexagonal packing geometry to a four-fold symmetric surface which is rich in atomic defects. Lifting the hex reconstruction at room temperature resulted in a surface structure decorated with 2-3 nm Pt adatom islands with a high density of step edge sites. Annealing the surface at a modest temperature (150 ˚C) in the presence of a high pressure of CO or H2 increased the surface diffusion of the Pt atoms causing the adatom islands to aggregate reducing the surface concentration of low coordination defect sites.
Ethylene hydrogenation was studied on the Pt(100) surface using HP-STM. At low pressure, the lifting of the hex reconstruction was observed in the STM images. Increasing the ethylene pressure to 1 Torr, was found to regenerate the hexagonally symmetric reconstructed phase. At room temperature ethylene undergoes a structural rearrangement to form ethylidyne. Ethylidyne preferentially binds at the three-fold hollow sites, which are present on the Pt(100) hex reconstructed phase, but not the (100)-(1x1) surface. The increase in ethylene pressure caused the adsorbate interactions to dominate the crystal morphology and imposed a surface layer structure that matched the ethylidyne binding geometry. The STM results also showed that the surface was reversibly deformed during imaging due to increases in Pt mobility at high pressure.
The size dependence on the activity and surface chemistry of Rh nanoparticles was studied using AP-XPS. The activity was found to increase with particle size. The XPS spectra show that in reaction conditions the particle surface has an oxide layer which is chemically distinct from the surface structure formed by heating in oxygen alone. This surface oxide which is stabilized in the catalytically active CO oxidation conditions was found to be more prevalent on the smaller nanoparticles.
The reaction-induced surface segregation behavior of bimetallic noble metal nanoparticles was observed with APXPS. Monodisperse 15 nm RhPd and PdPt nanoparticles were synthesized with well controlled Rh/Pd and Pd/Pt compositions. In-situ¬ XPS studies showed that at 300 ˚C in the presence of an oxidizing environment (100 mTorr NO or O2) the surface concentration of the more easily oxidized element (Rh in RhPd and Pd in PdPt) was increased. Switching the gas environment to more reducing conditions (100 mTorr NO and 100 mTorr CO) caused the surface enrichment of the element with the lowest surface energy in its metallic state. Using in-situ¬ characterization, the redox chemistry and the surface composition of bimetallic nanoparticle samples were monitored in reactive conditions. The particle surfaces were shown to reversibly restructure in response to the gas environment at high temperature.
The oxidation behavior of the Pt(110) surface was studied using surface sensitive in-situ characterization by APXPS and STM. In the presence of 500 mTorr O2 and temperatures between 25 and 200 ˚C, subsurface oxygen was detected in the surface layer. STM images show that these conditions were found to cause a roughened surface decorated with 1 nm islands. The formation of this surface oxide is a high pressure phenomenon and was not detected in 50 mTorr O2. After forming the surface oxide at high pressure, its chemical activity was measured through the reaction with CO at low pressure while continuously monitoring the oxygen species with XPS. The subsurface oxygen was removed by CO oxidation at a comparable rate to the chemisorbed oxygen at 2 ˚C. Repeating the experiment at -3 ˚C reduced the reaction rate, but not the relative activity of the two chemical species suggesting that neither species is significantly more active for the CO oxidation reaction.
These studies use molecular level surface characterization in the presence of gases to show the structural changes induced by gas adsorption at high pressure. The in-situ¬ results show that both the adsorbed gases and the metal surface are dynamic layers which change in structure in response to changing reaction environments.