Donna A. Chen

FUNDAMENTAL STUDIES OF HYDRODESULFURIZATION BY METAL SURFACES

Abstract Fundamental studies of desulfurization chemistry induced by single-crystal transition-metal surfaces are reviewed with emphasis on recent work. Specifically, the chemistry associated with clean, modified and multimetallic systems is addressed. A brief of our experimental approach, including the use of systematic variation in CS bond enthalpies for mechanistic studies, is discussed. Studies of thiol reactions on Mo(110) have shown that desulfurization occurs via homolytic CS bond dissociation in the thiolate intermediate. Furthermore, the relative rates of desulfurization can be correlated with CS bond strengths. The product distributions for molecules that form radical or cation intermediates and undergo various rearrangement reactions are used to elucidate reaction mechanisms. Adsorbates like oxygen and sulfur passivate the surface toward CS and CH bond scission in most cases. However, oxygen substitution also occurs when thiols react on some oxygen-covered surfaces. No synergistic reactivity was observed for thiol chemistry on bimetallic CoMo surfaces.

Decomposition of Dimethyl Methylphosphonate on Pt, Au, and Au−Pt Clusters Supported on TiO2(110)

The decomposition of dimethyl methylphosphonate (DMMP) was studied by temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) on TiO(2)-supported Pt, Au, and Au-Pt clusters as well as on TiO(2)(110) itself. In agreement with previous work, TPD experiments for DMMP on TiO(2)(110) showed that methyl and methane were the main gaseous products. Multiple DMMP adsorption-reaction cycles on TiO(2)(110) demonstrated that active sites for DMMP decomposition were blocked after a single cycle, but some activity for methyl production was sustained even after five cycles. Furthermore, the activity of the TiO(2) surface could be regenerated by heating in O(2) at 800 K or heating in vacuum to 965 K to remove surface carbon and phosphorus, which are byproducts of DMMP decomposition. On 0.5 ML Pt clusters deposited on TiO(2)(110), TPD studies of DMMP reaction showed that CO and H(2) were the main gas products, with methyl and methane as minor products. The Pt clusters were more active than TiO(2) both in terms of the total amount of DMMP reaction and the ability to break C-H, P-O, and P-OCH(3) bonds in DMMP. However, the Pt clusters had no sustained activity for DMMP decomposition, since the product yields dropped to zero after a single adsorption-reaction cycle. This loss of activity is attributed to a combination of poisoning of active sites by surface phosphorus species and encapsulation of the Pt clusters by reduced titania after heating above 600 K due to strong metal support interactions (SMSI). On 0.5 ML Au clusters, CO and H(2) were also the main products detected in TPD experiments, in addition to methane and methyl produced from reaction on the support. The Au clusters were less active for DMMP decomposition to CO and H(2) as well as P-O bond scission, but surface phosphorus was removed from the Au clusters by desorption at approximately 900 K. Au-Pt bimetallic clusters on TiO(2)(110) were prepared by depositing 0.25 ML of Pt followed by 0.25 ML of Au, and the bimetallic surfaces exhibited activity intermediate between that of pure Pt and pure Au in terms of CO and H(2) desorption yields. However, there is evidence that the production of methane from DMMP decomposition occurs at Au-Pt sites.

Decomposition of dimethyl methylphosphonate on TiO2(110): principal component analysis applied to X-ray photoelectron spectroscopy

The surface chemistry of dimethyl methylphosphonate (DMMP) has been studied on a TiO2(1 1 0) surface using X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). The application of principal component analysis (PCA) to the C(1s) and P(2p) XPS data showed that at least two linearly independent carbon-containing species and two linearly independent phosphorous-containing species are present on the surface between room temperature and 700 K. At room temperature, the main surface species contains intact P–OCH3 bonds and is most likely molecularly adsorbed DMMP. Between room temperature and 700 K, the adsorbed DMMP decomposes to produce methane and H 2 as the primary gaseous products. Molecular DMMP also desorbs from the surface below 550 K. After heating to 800 K, all of the carbon is removed from the surface, whereas a single phosphorous-containing species is detected even after heating to 1000 K.

Structure and reactivity of thin-film oxides and metals

The structure and reactivity of thin-film oxides of molybdenum and Co metal supported on oxidized Mo(110) is discussed. Reactions of interest in heterogeneous oxidation catalysis, in particular hydrocarbon oxidation is the focus of the work. A combination of electron energy loss, infrared, and X-ray photoelectron is used to characterize the structures of the oxides and Co films. Oxidation conditions are used to control the nature of the oxygen coordination sites available as well as the thickness and morphology of the oxide. Accordingly, the reactivity of specific types of oxygen coordination sites was investigated. In the case of the Co overlayers, thermal treatment was used as a means of varying the structure and morphology of the metal supported on the oxidized Mo. Oxygen bound to Mo(110) in low-symmetry, high-coordination sites was found to play an important role in the hydrocarbon oxidation process. For example, gaseous methyl radicals selectively add to oxygen in these sites, but not to terminal oxygen. In the microscopic reverse of methyl radical oxidation, vacancies at high-coordination sites are necessary for methanol reaction to methoxy to occur. The site-specific oxidation chemistry is modeled in selected cases using first-principles electronic structure calculations. The reactions of alcohols on various Co thin films were also investigated. The selectivity for alcohol reaction is altered by electronic and structural modification of the film. The reactions of ethanol and methanol were used to illustrate these principles.

Adsorbate-induced structural changes of metal thin films : cobalt-oxygen and cobalt-sulfur overlayers on Mo(110)

Abstract The oxygen- and sulfur-induced aggregation of cobalt in mixed Co-oxygen and Co-sulfur overlayers on Mo(110) have been studied using X-ray photoelectron and temperature-programmed desorption spectroscopies and low-energy electron diffraction. X-ray photoelectron data indicate that deposition of oxygen on 1 monolayer of Co results in the formation of Co oxide, which begins to decompose above 400 K. Only minor amounts of Co oxide are formed when 1 ML of Co is deposited on an oxygen overlayer. After annealing to 965 K for 60 s, the attenuation of the Co X-ray photoelectron signal and the temperature-programmed desorption data provide evidence for migration of oxygen to Mo and formation of three-dimensional Co clusters. Sulfur in mixed CoS overlayers also induces Co island formation as sulfur bonds to Mo, although to a lesser extent than oxygen. The Co aggregation is explained in terms of the thermodynamic preference for oxygen and sulfur to bond to Mo over Co. These studies provide a means of constructing phases that can be used to investigate structural effects in surface reactions.

Design of a heating-cooling stage for scanning tunneling microscopy and temperature programmed desorption experiments

A heating-cooling stage with detachable thermocouple contacts and an electron beam heater has been designed and tested for use in temperature programmed desorption and scanning tunneling microscopy (STM) experiments. This sample stage is compatible with the Omicron variable temperature STM, and the thermocouple contacts can be used with both the standard Ta plates and the variable temperature sample holder. The stage can also be retrofitted to the standard Omicron manipulation stage without requiring any additional electrical connections.

Desulfurization of benzenethiol on CoMo(110) phases

Abstract The desulfurization of benzenethiol on Co-covered Mo(110) ( θ Co = 0.25–1.3 ML) produces benzene, H 2 and adsorbed carbon and sulfur. Benzene is formed via adsorbed phenyl thiolate, which is identified by X-ray photoelectron and electron energy loss spectroscopies. Benzene production is attributed to reaction on Co, since it is evolved at a temperature well below that required for sulfur-induced aggregation of Co. The reaction products and mechanisms are qualitatively similar on Co-covered Mo(110) and a range of other transition metal surfaces. The selectivity for benzene production on the 1.3 ML Co overlayer, ∼ 65%, is higher than on many surfaces. Furthermore, benzene evolution occurs at a very low temperature, 125 K, on the 1.3 ML Co film. Notably, we did not observe any new products or a dramatic change in reaction temperature or selectivity as a result of CoMo interactions or of structural changes in the Co layer. Comparison of the benzene evolution temperature for benzenethiol reaction with that of methane from methanethiol reaction on the close-packed Co overlays indicates that homolytic CS bond scissions do not control the hydrogenolysis rate, in contrast to reaction on Mo(110) but similar to Ni(111). This difference may be due to hydrogen-induced structural transformations of the Co layer or to hydrogen-assisted CS bond breaking.

Novel recirculating loop reactor for studies on model catalysts: CO oxidation on Pt/TiO2(110).

A novel recirculating loop microreactor coupled to an ultrahigh vacuum (UHV) chamber has been constructed for the kinetic evaluation of model catalysts, which can be fully characterized by UHV surface science techniques. The challenge for this reactor design is to attain sufficient sensitivity to detect reactions on model single-crystal surfaces, which have a low number of active sites compared to conventional catalysts of equivalent mass. To this end, the total dead volume of the reactor system is minimized (32 cm(3)), and the system is operated in recirculation mode so that product concentrations build up to detectable levels over time. The injection of gas samples into the gas chromatography column and the refilling of the recirculation loop with fresh feed gas are achieved with computer-controlled, automated switching valves. In this manner, product concentrations can be followed over short time intervals (15 min) for extended periods of time (24 h). A proof of principle study in this reactor for CO oxidation at 145-165 °C on Pt clusters supported on a rutile TiO2(110) single crystal yields kinetic parameters that are comparable to those reported in the literature for CO oxidation on Pt clusters on powdered oxide supports, as well as on Pt(100). The calculated activation energy is 16.4 ± 0.7 kcal/mol, the turnover frequency is 0.03-0.06 molecules/(site·s) over the entire temperature range, and the reaction orders in O2 and CO at 160 °C are 0.9 ± 0.2 and -0.82 ± 0.03, respectively.

Understanding Uptake of Pt Precursors During Strong Electrostatic Adsorption on Single-Crystal Carbon Surfaces

The formation of Pt clusters deposited by strong electrostatic adsorption (SEA) has been studied on highly oriented pyrolytic graphite (HOPG) as well as on higher surface area graphene nanoplatelets (GNPs), which have well-defined surface structures. Both surfaces can be hydroxylated by treatment with hydrochloric acid, and in the case of HOPG, the surface functional groups are exclusively hydroxyls. When SEA is carried out with the anionic precursor PtCl62− under acidic conditions, the dominant cationic sites for electrostatic adsorption are protonated aromatic rings rather than protonated hydroxyl groups, and therefore the unfunctionalized surfaces exhibit high uptake compared to the HCl-treated surfaces. In contrast, for SEA with the cationic precursor Pt(NH3)42+ under basic conditions, surface hydroxylation leads to higher Pt uptake since deprotonated hydroxyl groups act as negatively charged adsorption sites for the cations. After reduction of the adsorbed ionic precursors by heating in H2, the metallic Pt clusters on HOPG produced from PtCl62− SEA form dendritic aggregates associated with high Pt diffusion rates and irreversible Pt–Pt bonding. For Pt(NH3)42+ SEA on HOPG, subsequent reduction resulted in clusters preferentially located at step edges on the untreated surface, whereas smaller clusters more uniformly distributed across the terraces were observed on the hydroxylated surface. Similar trends in particle sizes and densities were observed on the GNP surfaces, demonstrating that understanding of nucleation and growth on single-crystal HOPG surfaces can be extended to the high surface area GNPs. Oxidative adsorption to Pt4+ was observed for SEA of Pt2+ precursors on both HOPG and GNP surfaces.

Copper-Coated Self-Assembled Monolayers: Alkanethiols and Prospective Molecular Wires

The study of the effects of metal overlayers on organic self-assembled monolayers (SAMs) is of interest in many scientific areas. The self-assembly process leads to a well-defined substrate whereas the numerous combinations of end-groups and metals allows for the fine-tuning of the chemistry at the monolayer/metal interface. These unique features make metal/SAM interfaces a valuable tool for probing fundamental processes. SAM/metal interfaces, have been proposed as templates to elucidate details on, for example, metal/“organic host” interactions which are known to occur in biologically active metallic reaction sites [1]; reaction mechanisms at organic/metal/gas interfaces, such as those involved in heterogeneous catalysis and environmental chemistry [2]; the solvation and electron-transfer reactions of metals in molecular solvents or within molecular clusters [3, 4]; and the insertion of zero-valent metals into chemical bonds frequently found in homogeneous catalysis reactions [3, 4, 5, 6].