D. Wayne Goodman

Synthesis of dimethyl ether (DME) from methanol over solid-acid catalysts

The catalytic conversion of methanol to dimethyl ether (DME) has been studied over a series of solid-acid catalysts, such as γ-Al2O3, H-ZSM-5, amorphous silica-alumina, as well as titania modified zirconia. All the catalysts are active and selective for DME formation. The apparent activation energy for DME formation over γ-Al2O3 is ca. 25 kcal/mol, a value that increases to ca. 37 kcal/mol upon the addition of 23 Torr of H2O to the reagent. The rate of methanol dehydration decreases with increasing acidity (silica content) over the amorphous silica-alumina catalysts. Although H-ZSM-5 with Si/Al = 25 is the most active among the catalysts tested, the DME selectivity is only 20% at 280°C, a typical temperature used in the syngas-to-methanol process. An amorphous silica-alumina catalyst with 20 wt.-% silica content (SIRAL20) exhibits the best catalytic performance of those tested at 280°C.

High pressure catalytic reactions over single-crystal metal surfaces

Abstract Studies dealing with high-pressure catalytic reactions over single-crystal surfaces are reviewed. The coupling of an apparatus for the measurement of reaction kinetics at elevated pressures with an ultrahigh vacuum system for surface analysis allows detailed study of structure sensitivity, the effects of promoters and inhibitors on catalytic activity, and, in certain cases, identification of reaction intermediates by post-reaction surface analysis. Examples are provided which demonstrate the relevance of single-crystal studies for modeling the behaviour of high-surface-area supported catalysts. Studies of CO methanation and CO oxidation over single-crystal surfaces provide convincing evidence that these reactions are structure insensitive. For structure-sensitive reactions (ammonia synthesis, alkane hydrogenolysis, alkane isomerization, water-gas shift reaction, etc.) model single-crystal studies allow correlations to be established between surface structure and catalytic activity. The effects of both electronegative (S and P) and electropositive (alkali metals) impurities upon the catalytic activity of metal single crystals for ammonia synthesis, CO methanation, alkane hydrogenolysis, ethylene epoxidation and water-gas shift are discussed. The roles of “ensemble” and “ligand” effects in bimetallic catalysts are examined in light of data obtained using surfaces prepared by vapor-depositing one metal onto a crystal face of a dissimilar metal.

Nonoxidative Activation of Methane

Effective utilization of methane remains one of the long-standing problems in catalysis. Over the past several years, various routes, both direct and indirect, have been considered for the conversion of methane to value-added products such as higher hydrocarbons and oxygenates. This review will focus on the range of issues dealing with thermal and catalytic decomposition of methane that have been addressed in the last few years. Surface science studies (molecular beam methods and elevated-pressure reaction studies) involving methane activation on model catalyst systems are extensively reviewed. These studies have contributed significantly to our understanding of the fundamental dynamics of methane decomposition. Various aspects of the nonoxidative methane to higher hydrocarbon conversion processes such as high-temperature coupling and two-step low-temperature methane homologation have been discussed. Decomposition of methane results in the production of COx-free hydrogen (which is of great interest to state-of-the-art low-temperature fuel cells) and various types of carbon (filamentous carbon, carbon black, diamond films, etc.) depending on the reaction conditions employed; these issues will be briefly addressed in this review.

Synthesis and characterization of ultra-thin MgO films on Mo(100)

Abstract Ultra-thin MgO films have been synthesized under UHV conditions by evaporating Mg onto Mo(100) in various background pressure of oxygen. Low-energy electron diffraction (LEED) studies show that MgO films grow epitaxially in the 200–600 K substrate temperature range with the (100) face of MgO oriented parallel to Mo(100). The one-to-one stoichiometry of the MgO films has been confirmed using Auger electron spectroscopy (AES) and temperature programmed desorption (TPD). A typical loss pattern, characteristic of single-crystal MgO, has been obtained by high-resolution electron energy-loss spectroscopy (HREELS). At low oxygen pressures, the MgO film grows via a mechanism of island nucleation with domains that coexist with metallic Mg particles. The heat of sublimation of three-dimensional MgO islands is dependent on the oxygen pressure during growth and relates to the coordination number of the Mg cations.

CO adsorption on Pd(111) and Pd(100): Low and high pressure correlations

The adsorption of CO on Pd(111) and Pd(100) have been studied using infrared reflection–absorption spectroscopy over a wide range of CO pressures and temperatures. A strong dependence of CO adsorption on the initial conditions was found for Pd(111) while CO adsorption on Pd(100) was essentially independent of the conditions of adsorption. Initial isosteric heats of adsorption of 30 and 38 kcal/mol were determined for Pd(111) and Pd(100), respectively. For Pd(111) and equilibrium phase diagram was constructed on the basis of the infrared (IR) data. The excellent correspondence among IR data for the single crystal Pd surfaces and a supported Pd catalyst suggests that the Pd particles in the supported catalyst consist primarily of low index [(111) and (100)] crystal faces.

The role of surface structure and dispersion in CO hydrogenation on cobalt

Abstract The effects of surface structure on the CO hydrogenation reaction have been investigated by comparing the steady-state activity and selectivity of submonolayer cobalt deposited on W(110) and W(100) with those of carbonyl-derived Co/alumina catalysts of varying dispersion and extent of reduction. The Co/W surfaces have highly strained and different geometries ( 1 ) but have similar activity. The activity matches that of the highly active, highly reduced Co/alumina catalysts, suggesting that the steady-state activity of cobalt surfaces is independent of surface structure. AES spectra show the after-reaction Co/W surfaces to have high coverages of both carbon and oxygen, with carbon lineshapes characteristic of carbidic carbon. Carbonyl-derived Co/dehydroxylated alumina catalysts have high extents of reduction, high dispersions, and good activity stability. Increasing the dehydroxylation temperature of the alumina support increases metal dispersion while decreasing CO 2 and olefin selectivities. Specific CO hydrogenation activity is constant over the range of dispersion of 5–37% for highly reduced 3 and 5% Co/alumina catalysts and over the entire range of dispersion (0–100%) if polycrystalline Co and Co/W surfaces are included. The specific activity of carbonyl-derived catalysts appears to be more closely related to the extent of reduction and the support dehydroxylation temperature than to the dispersion. Thus, the chemical nature of the support surface appears to be the controlling factor in determining the specific activity of supported cobalt catalysts.

XPS characterization of ultra-thin MgO films on a Mo( 100) surface

Abstract The oxidation of ultra-thin Mg films supported on a Mo(100) surface has been studied using X-ray photoelectron spectroscopy (XPS) in the 90–1300 K sample temperature range. Upon adsorption of oxygen onto Mg thin films or deposition of Mg in the presence of oxygen, a Mg(2p) XPS feature at ~ 50.5–50.8 eV is observed. The binding energy of this peak is higher than that of metallic Mg(2p) (at 49.6 eV) and is assigned to oxidized magnesium. The associated O(1s) XPS spectra exhibit two peaks which can be attributed to a dioxygen species concluded to be magnesium peroxide and the lattice oxygen in MgO. Upon annealing the peroxide containing film to ~ 700 K, the magnesium peroxide is reduced to MgO through the loss of oxygen and metallic magnesium existing within the film is subsequently oxidized to MgO. Mg deposition in an oxygen background (~ 10 −6 Torr) onto the Mo(100) surface at 300 K produces essentially stoichiometric MgO films.

Calcium Phosphate Phase Identification Using XPS and Time-of-Flight Cluster SIMS

Reproducible time-of-flight cluster static secondary ion mass spectra (ToF-SSIMS) were obtained for various standard calcium phosphate (CP) powders, which allowed for phase identification. X-ray diffraction was not able to detect signals from microscopic amounts of CP (∼15 mmol m(-)(2)). The phases studied were α-tricalcium phosphate [α-Ca(3)(PO(4))(2)], β-tricalcium phosphate [β-Ca(3)(PO(4))(2)], amorphous calcium phosphate [Ca(3)(PO(4))(2)·xH(2)O], octacalcium phosphate [Ca(8)H(2)(PO(4))(6)·H(2)O], brushite (CaHPO(4)·2H(2)O), and hydroxyapatite [Ca(10)(PO(4))(6)(OH)(2)]. The SIMS spectra were obtained via bombardment with (CsI)Cs(+) projectiles. X-ray photoelectron spectroscopy (XPS) core levels of the P 2p, Ca 2p, and O 1s orbitals and the relative O 1s loss intensity were examined. The PO(3)(-)/PO(2)(-) ratios from ToF-SSIMS spectra in conjunction with XPS of the CP powders showed much promise in differentiating between these phases at microscopic CP coverages on the metal oxide surface.

CO adsorption on Pd(111): the effects of temperature and pressure

Infrared reflection-absorption spectroscopy (IRAS) has been used to study the adsorption of carbon monoxide on a Pd(111) surface. IRAS spectra were collected at temperatures from 100 to 1000 K and at pressures from 1.0 × 10−7 to 10.0 Torr. The IRAS data at temperatures > 250 K showed the coverage versus temperature behavior anticipated from the data acquired under UHV conditions. However, at temperatures < 250 K, multiple CO adsorption structures were observed which were dependent upon the adsorption temperature and pressure. The low temperature/low pressure adsorption data extrapolate to the high temperature/high pressure regime only if appropriate adsorption conditions are employed.

CO Oxidation over AuPd(100) from Ultrahigh Vacuum to Near-Atmospheric Pressures : The Critical Role of Contiguous Pd Atoms

It is demonstrated that gas-phase CO pressure higher than approximately 0.1 Torr is required to segregate a sufficient amount of Pd to the surface of a well-annealed AuPd(100) sample to form contiguous Pd sites. These contiguous sites are critical in dissociating O(2) for low-temperature CO oxidation, where CO chemisorbed on Au sites clearly participates in the reaction at temperatures below approximately 400 K. Measured reaction kinetics demonstrates that the higher reaction rate is achieved on a surface with higher coverages of contiguous Pd sites.