AVS2011 Session SS2-WeM: Chemisorption on Metal & Oxide Nanoparticles

Energy research has increased in importance in the past decade due to our growing understanding of climate science and rising oil prices. Many catalysts center around expensive and rare transition metals, such as Pt and Pd, supported on oxide substrates. However, copper, a relatively cheap and abundant metal, supported on metal oxides has been used as a heterogeneous catalyst in industrial settings for various chemical processes.^1,2 Recent works have shown that copper nanoparticles supported on metal oxides (ZnO, CeO₂, TiO₂) have higher activity for the water gas shift reaction (WGSR) as well as other important chemical reactions when compared to their individual components.^3,4 Understanding how these complex catalysts work on a fundamental level will allow for the design and implementation of more efficient and selective systems in the future. Here, using a homemade thermal evaporator, a model system of copper nanoparticles deposited on CeO₂ films (200 nm thick) grown on YSZ (111) single crystals is used. X-ray photoelectron spectroscopy (XPS) is used to characterize the oxidation state of supported copper nanoparticles and temperature programmed desorption (TPD) is used to probe their reactivity and thermal stability. Copper coverages ranging from 0.25ML to 1ML are investigated. Carbon monoxide and water are used as probe molecules since they are the reactants involved in the WGSR. We have found that copper’s stability is highly temperature dependent and have found evidence of its encapsulation by the support.

1. K. Klier, Adv. Catal., 1982, 31, 243.

2. J. C. Bart and R. P. A. Sneedon, Catal. Today, 1987, 2, 124.

3. J. A. Rodriguez, P. Liu, J. Hrbek, J. Evans, M. Pérez, Angew. Chem. Int. Ed.. 2007, 46, 1351.

4. X. Zhao, J. A. Rodriguez, J. Hrbek, M. Pérez, Surface Science, 2005 , 600, 229.

The atomic and electronic structure of MoS₂ nanoclusters is of considerable interest due to the catalytic application of MoS₂ in e.g. hydrotreating catalysis of crude oil and in photocatalysts and hydrogen evolution reactions. Previous atom-resolved STM results have shown in great detail that both the overall morphology and in particular the edge structure of MoS₂ nanoclusters, which are known to contain the most catalytically active sites for hydrotreating and H₂ dissociation, adopt a structure which is very dependent on the conditions under which the cluster are kept. Under sulfiding conditions, atom-resolved STM images show that the MoS₂ nanoclusters expose fully sulfide edges, whereas activation by H₂ or mixed H₂/H₂S exposures show that sulfur vacancies and S-H form on the cluster edges reflecting the MoS₂ catalyst in its active state. To dynamically follow such structural changes at the MoS₂ edges induced e.g. by hydrotreating reaction conditions we have here combined high-resolution x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) studies of single-layer MoS₂ nanoclusters with a well known structure. The XPS studies done on well-characterized samples reveal a set of edge specific core level shifts in the Mo3d photoemission peak that can be uniquely associated with the fully sulfide edges, edge with S vacancies or fully reduced edges. The XPS fingerprint thus allows us to dynamically follow changes between the catalytically active states of MoS₂ when exposed to sulfiding of sulforeductive conditions. Preliminary in-situ XPS results on the same MoS₂ samples obtained under a 10^-2 torr H₂ atmosphere on the Ambient Pressure X-ray Photoelectron Spectroscopy on beamline 11.0.2 of the Advanced Light Source Berkeley show a characteristic sequence of sulfur reduction steps on the catalytically interesting edges followed by decomposition of MoS₂ at higher temperatures. The present studies thus successfully shows that XPS in combination with STM can be successfully used as a tool to characterize the chemistry of highly dispersed active sites of well-defined nanoclusters, such as the active edges on MoS₂.

Single-layer graphene supported on transition metals provides a unique substrate for synthesizing metal nanostructures due to the high crystallographic quality, thermal stability, and chemical inertness of the graphene. Contrary to its formation of three-dimensional (3-D) nanoclusters on graphene supported on a SiO₂ substrate, Au forms two-dimensional (2-D) islands on graphene moiré/Ru(0001). These Au islands maintain their 2-D structures up to 1 monolayer (ML) equivalent of Au dosage and are stable at room temperature. Our scanning tunneling microscopic study further shows that the 2-D Au islands are most likely two layers high, and conform to the graphene moiré in the lateral direction. Spin- and angle-resolved photoemission studies indicate even though these Au islands are largely electronically isolated, a weak through-graphene coupling exists between the Au islands and the Ru(0001) substrate. The structure for these 2-D Au islands and the corresponding electronic band structures are proposed based on DFT calculations.

Reducibility of pure and doped CeO₂ is of interest in emission control catalysts because of the ability of the CeO₂ to store and supply oxygen during oxidation catalysis. But, it is not known how the structure or crystallographic termination of the CeO₂ affects the catalytic reaction rates and selectivity. Using CeO₂ nanoparticles with controlled shapes including cubes, octahedra and rods that are terminated on (100), (111) and (110) surfaces respectively, we have investigated this structure dependence. Temperature programmed desorption, temperature programmed reaction, flow reactor rates, and in situ DRIFTS were used to probe adsorption states, desorption, reaction, oxidation rates and product selectivity for CO and ethanol oxidation. Results show pronounced differences between the three different morphologies. All morphologies show evidence of surface ethoxide species at room T, but during subsequent TPD, the DRIFTS exhibits variation in surface species between the different surfaces with evidence for formation of adsorbed acetaldehyde and acetate. Temperature induced changes in the C-H stretching regions, different also for each polymorph, suggest competing dehydrogenation and dehydration of surface species. Desorption temperatures and product distributions also vary. The ratio of H₂/H₂O, and the H₂ peak desorption temperature is highest for the octahedra, consistent with its highest vacancy formation energy and therefore least available oxygen. This ratio is lowest for high surface area multi-faceted nanoparticles, and its variability has important implications for tailoring and understanding CeO₂ catalysts or supports for production of H₂ in ethanol fuel cells. Product profiles during TPR of ethanol in O₂ were also dependent upon the surface structure. Octahedra show the highest selectivity to acetaldehyde and an onset of H₂ evolution above 400 °C while the cubes and rods showed lower temperatures for the onset of H₂, indicating that the hydrogen is evolved by two different pathways on different shaped ceria. Similarly, in a steady state flow reactor, the ratio of selective oxidation product (acetaldehyde) to the total oxidation product (CO₂) followed the order (111) > (100) > (110). Such results provide a basis for fundamental understanding of how surface coordination, bonding, decomposition and reaction are affected by the atomic structure of an oxide surface, especially important for reducible oxides.

Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, US DOE.

The doping of metal oxides has been explored in several investigations with the aim to prepare better materials for catalysis, optics and electronic applications. Doping of wide-gap oxide materials can be realized by the controlled introduction of various types of lattice defects, including point defects such as oxygen vacancies, line defects (e.g. grain boundaries and dislocations) and impurity atoms. In this study, we have exploited photon-scanning tunneling microscopy used in imaging as well as in cathode-luminescence mode to investigate Au clusters supported on thin, single crystalline MgO(001) and Cr-doped MgO(001) films grown on Mo(001). First of all, we have prepared Cr-doped MgO films on a Mo(001) support as a model system for a transition-metal doped wide-gap insulator with interesting applications in catalysis. To elucidate the role of the Cr in the MgO matrix, the morphological and optical properties of the system were analyzed as a function of the Cr load, using the STM. The Cr was incorporated into the film either by Cr-Mg co-deposition in oxygen or post-evaporation followed by an annealing step. From the distinct light emission properties of the doped oxide, a detailed picture has been developed on the Cr⁺³ position inside the MgO lattice and the associated modifications in the electronic structure. The role of the Cr dopants on the adsorption behaviour of the oxide film was investigated by depositing small amounts of Au. While on pristine MgO films, Au nucleates into 3D particles, mainly 2D aggregates form on the doped oxide support. We assign this change in the Au growth mode to charge transfer processes from the Cr centres into the Au clusters and will discuss possible consequences on the chemical activity of the doped metal-oxide system.

We report a combined experimental and theoretical investigation of the reaction of molecular water with terminal hydroxyls (OH_t’s) on reduced TiO₂(110)-(1x1) surface at 300 K. We show that OH_t’s have a significant effect on the water reactivity and extract molecular-level details about the underlying reaction mechanisms. By tracking the same surface area with high-resolution scanning tunneling microscopy before and after water exposure, we demonstrate that there are two distinctive reaction pathways involving multiple proton transfers [1]. For water interaction with OH_t on an adjacent Ti row, the proton can be transferred through bridging oxygen to OH_t, which leads to the formation of a new water molecule and apparent across-row motion of OH_t due to O scrambling. This process further manifests the existence of the equilibrium between molecular and dissociated states of water on TiO₂(110) [2]. If H₂O interacts with OH_t along the same Ti row, fast multi-step OH_t motion along the Ti row is observed. Our density functional theory results show that this process is caused by the fast diffusion of (OH_t + H₂O) pairs, whereby the underlying mechanism involves proton transfer and H₂O hopping over OH_t.

[1] Y. Du, N. A. Deskins, Z. Zhang, Z. Dohnálek, M. Dupuis, and I. Lyubinetsky, Phys. Chem. Chem. Phys. 12 (2010) 6337.

[2] Y. Du, N. A. Deskins, Z. Zhang, Z. Dohnálek, M. Dupuis, and I. Lyubinetsky, Phys. Rev. Lett. 102 (2009) 096102.

This work was supported by the U.S. Department of Energy (DOE) Office of Basic Energy Sciences, Division of Chemical Sciences, and performed at EMSL, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at PNNL.