In Situ Studies of Surface Mobility on Noble Metal Model Catalysts Using STM and XPS at Ambient Pressure

Abstract

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, H 2 , 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 H 2 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. 2Ethylidyne 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 con...

In Situ Scanning Tunneling Microscopy and X-ray Photoelectron Spectroscopy Studies of Ethylene-Induced Structural Changes on the Pt(100)-hex Surface

“…37 The CO peak assignment is rationalized by the presence of two O 1s peaks at 532.5 and 531.4 eV, which are assigned to CO on top and bridge sites, respectively. 39,40 The presence of chemisorbed CO, along with the fact that the islands in Figure 4a look similar to pure CO-induced islands on Pt(100) under STM, 14,17 strongly suggests that CO plays an important role in lifting the hexagonal reconstruction. The CO originates from background gases in the chamber, which increases due to displacement from the chamber walls and pump memory effects upon introduction of a new gas species.…”

“…The surface strain that originates from the lattice mismatch between the hexagonal overlayer and the square bulk termination is balanced by the energy gain through the close packing . When heating above 1100 K, the Pt(100)-hex rotates by 0.7° with respect to the second layer, forming a superstructure named as Pt(100)-hex-R0.7°. , Both Pt(100)-hex and Pt(100)-hex-R0.7° reconstructions can be lifted by exposure to CO, − O 2 , − NO, ,, and benzene, leading to the formation of islands by the excess Pt atoms from the initial hexagonally reconstructed Pt(100) surface.…”

“…10 When heating above 1100 K, the Pt(100)-hex rotates by 0.7°with respect to the second layer, forming a superstructure named as Pt(100)-hex-R0.7°. 6,7 Both Pt(100)-hex and Pt(100)-hex-R0.7°reconstructions can be lifted by exposure to CO, 11−18 O 2 , 18−20 NO, 15,21,22 and benzene, 17 leading to the formation of islands by the excess Pt atoms from the initial hexagonally reconstructed Pt(100) surface.…”