Funda Aksoy

Evolution of Structure and Chemistry of Bimetallic Nanoparticle Catalysts under Reaction Conditions

Three series of bimetallic nanoparticle catalysts (Rh(x)Pd(1-x), Rh(x)Pt(1-x), and Pd(x)Pt(1-x), x = 0.2, 0.5, 0.8) were synthesized using one-step colloidal chemistry. X-ray photoelectron spectroscopy (XPS) depth profiles using different X-ray energies and scanning transmission electron microscopy showed that the as-synthesized Rh(x)Pd(1-x) and Pd(x)Pt(1-x) nanoparticles have a core-shell structure whereas the Rh(x)Pt(1-x) alloys are more homogeneous in structure. The evolution of their structures and chemistry under oxidizing and reducing conditions was studied with ambient-pressure XPS (AP-XPS) in the Torr pressure range. The Rh(x)Pd(1-x) and Rh(x)Pt(1-x) nanoparticles undergo reversible changes of surface composition and chemical state when the reactant gases change from oxidizing (NO or O(2) at 300 degrees C) to reducing (H(2) or CO at 300 degrees C) or catalytic (mixture of NO and CO at 300 degrees C). In contrast, no significant change in the distribution of the Pd and Pt atoms in the Pd(x)Pt(1-x) nanoparticles was observed. The difference in restructuring behavior under these reaction conditions in the three series of bimetallic nanoparticle catalysts is correlated with the surface free energy of the metals and the heat of formation of the metallic oxides. The observation of structural evolution of bimetallic nanoparticles under different reaction conditions suggests the importance of in situ studies of surface structures of nanoparticle catalysts.

New ambient pressure photoemission endstation at Advanced Light Source beamline 9.3.2

During the past decade, the application of ambient pressure photoemission spectroscopy (APPES) has been recognized as an important in situ tool to study environmental and materials science, energy related science, and many other fields. Several APPES endstations are currently under planning or development at the USA and international light sources, which will lead to a rapid expansion of this technique. The present work describes the design and performance of a new APPES instrument at the Advanced Light Source beamline 9.3.2 at Lawrence Berkeley National Laboratory. This new instrument, Scienta R4000 HiPP, is a result of collaboration between Advanced Light Source and its industrial partner VG-Scienta. The R4000 HiPP provides superior electron transmission as well as spectromicroscopy modes with 16 microm spatial resolution in one dimension and angle-resolved modes with simulated 0.5 degrees angular resolution at 24 degrees acceptance. Under maximum transmission mode, the electron detection efficiency is more than an order of magnitude better than the previous endstation at beamline 9.3.2. Herein we describe the design and performance of the system, which has been utilized to record spectra above 2 mbar.

Intrinsic Relation between Catalytic Activity of CO Oxidation on Ru Nanoparticles and Ru Oxides Uncovered with Ambient Pressure XPS

Recent progress in colloidal synthesis of nanoparticles with well-controlled size, shape, and composition, together with development of in situ surface science characterization tools, such as ambient pressure X-ray photoelectron spectroscopy (APXPS), has generated new opportunities to unravel the surface structure of working catalysts. We report an APXPS study of Ru nanoparticles to investigate catalytically active species on Ru nanoparticles under oxidizing, reducing, and CO oxidation reaction conditions. The 2.8 and 6 nm Ru nanoparticle model catalysts were synthesized in the presence of poly(vinyl pyrrolidone) polymer capping agent and deposited onto a flat Si support as two-dimensional arrays using the Langmuir-Blodgett deposition technique. Mild oxidative and reductive characteristics indicate the formation of surface oxide on the Ru nanoparticles, the thickness of which is found to be dependent on nanoparticle size. The larger 6 nm Ru nanoparticles were oxidized to a smaller extent than the smaller Ru 2.8 nm nanoparticles within the temperature range of 50-200 °C under reaction conditions, which appears to be correlated with the higher catalytic activity of the bigger nanoparticles. We found that the smaller Ru nanoparticles form bulk RuO(2) on their surfaces, causing the lower catalytic activity. As the size of the nanoparticle increases, the core-shell type RuO(2) becomes stable. Such in situ observations of Ru nanoparticles are useful in identifying the active state of the catalysts during use and, hence, may allow for rational catalyst designs for practical applications.

In Situ Oxidation Study of Pt(110) and Its Interaction with CO

Many interesting structures have been observed for O(2)-exposed Pt(110). These structures, along with their stability and reactivity toward CO, provide insights into catalytic processes on open Pt surfaces, which have similarities to Pt nanoparticle catalysts. In this study, we present results from ambient-pressure X-ray photoelectron spectroscopy, high-pressure scanning tunneling microscopy, and density functional theory calculations. At low oxygen pressure, only chemisorbed oxygen is observed on the Pt(110) surface. At higher pressure (0.5 Torr of O(2)), nanometer-sized islands of multilayered α-PtO(2)-like surface oxide form along with chemisorbed oxygen. Both chemisorbed oxygen and the surface oxide are removed in the presence of CO, and the rate of disappearance of the surface oxide is close to that of the chemisorbed oxygen at 270 K. The spectroscopic features of the surface oxide are similar to the oxide observed on Pt nanoparticles of a similar size, which provides us an extra incentive to revisit some single-crystal model catalyst surfaces under elevated pressure using in situ tools.

In Situ Observation of Water Dissociation with Lattice Incorporation at FeO Particle Edges Using Scanning Tunneling Microscopy and X-ray Photoelectron Spectroscopy

The dissociation of H2O and formation of adsorbed hydroxyl groups on FeO particles grown on Au(111) were identified with in situ X-ray photoelectron spectroscopy (XPS) at water pressures ranging from 3 × 10(-8) to 0.1 Torr. The facile dissociation of H2O takes place at FeO particle edges, and it was successfully observed in situ with atomically resolved scanning tunneling microscopy (STM). The in situ STM studies show that adsorbed hydroxyl groups were formed exclusively along the edges of the FeO particles with the O atom becoming directly incorporated into the oxide crystalline lattice. The STM results are consistent with coordinatively unsaturated ferrous (CUF) sites along the FeO particle edge causing the observed reactivity with H2O. Our results also directly illustrate how structural defects and under-coordinated sites participate in chemical reactions.

Effect of O2, CO, and NO on Surface Segregation in a Rh0.5Pd0.5 Bulk Crystal and Comparison to Rh0.5Pd0.5 Nanoparticles†

We present an in situ study of the interaction of a bimetallic Rh(0.5)Pd(0.5) bulk crystal with O(2), CO, and NO using ambient pressure X-ray photoelectron spectroscopy (APXPS) and compare it to results for 15 nm nanoparticles with the same overall composition. The bulk crystal surface has less Rh present under both oxidizing and reducing conditions than the surface of nanoparticles under identical conditions. Segregation and oxidation/reduction proceeds faster and at lower temperature for nanoparticles than for the bulk crystal. The near surface of the Rh(0.5)Pd(0.5) bulk crystal after high temperature vacuum annealing is ca. 9% Rh measured by APXPS. Heating in 0.1 Torr O(2) to 350 °C increases the Rh surface composition to ca. 40%. The surface can then be reduced by heating in H(2) at 150 °C, leading to a chemically reduced surface with 30% Rh. Titration of CO by gas-phase O(2) from this Rh-rich surface proceeds at a much lower pressure than that on the Rh-deficient starting surface.

A Step toward the Wet Surface Chemistry of Glycine and Alanine on Cu{110}: Destabilization and Decomposition in the Presence of Near-Ambient Water Vapor

The coadsorption of water with organic molecules under near-ambient pressure and temperature conditions opens up new reaction pathways on model catalyst surfaces that are not accessible in conventional ultrahigh-vacuum surface-science experiments. The surface chemistry of glycine and alanine at the water-exposed Cu{110} interface was studied in situ using ambient-pressure photoemission and X-ray absorption spectroscopy techniques. At water pressures above 10(-5) Torr a significant pressure-dependent decrease in the temperature for dissociative desorption was observed for both amino acids, accompanied by the appearance of a new CN intermediate, which is not observed for lower pressures. The most likely reaction mechanisms involve dehydrogenation induced by O and/or OH surface species resulting from the dissociative adsorption of water. The linear relationship between the inverse decomposition temperature and the logarithm of water pressure enables determination of the activation energy for the surface reaction, between 213 and 232 kJ/mol, and a prediction of the decomposition temperature at the solid-liquid interface by extrapolating toward the equilibrium vapor pressure. Such experiments near the equilibrium vapor pressure provide important information about elementary surface processes at the solid-liquid interface, which can be retrieved neither under ultrahigh vacuum conditions nor from interfaces immersed in a solution.

Iron-resonant valence band photoemission and oxygen near edge x-ray absorption fine structure study on La1−xSrxFe0.75Ni0.25O3−δ

Iron resonant valence band photoemission spectra (VB PES) of Sr substituted LaFe0.75Ni0.25O3−δ have been recorded across the Fe 2p-3d absorption threshold to obtain Fe specific spectral information on the 3d projected partial density of states. Comparison with La1−xSrxFeO3 resonant VB PES literature data suggests that substitution of Fe by Ni forms electron holes which have mainly O 2p character. Substitution of La by Sr increases the hole concentration to an extent that the eg structure vanishes. The variation in the eg and t2g structures is paralleled by the changes in the electrical conductivity.