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Active sites for hydrocarbon catalysis on metal surfaces

Description: There is a great deal of experimental evidence associating selective bond breaking ability with low coordination number surface sites on transition metal surfaces. Atomic steps break H-H and C-H bonds efficiently while kinks in the steps are required for additional C-C and C=O bond scissions. The blockage of some of the ''active sites'' yields the reaction selectivity commonly observed as a result of alloying or upon introduction of certain promoters. Another important parameter in controlling transition metal catalytic activity is the formal oxidation state of the surface metal atom. Oxidation of the surface atoms by oxygen or halogens or their reduction by electron donors (carbon, alkali metals, etc.) changes markedly the catalytic surface chemistry. H/sub 2/-D/sub 2/ exchange, hydrocarbon conversion reactions and the hydrogenation of CO are examples to demonstrate the importance of low coordination number sites and surface oxidation state in controlling catalytic activity and selectivity.
Date: October 18, 1977
Creator: Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

Molecular ingredients of heterogeneous catalysis

Description: The purpose of this paper is to present a review and status report to those in theoretical chemistry of the rapidly developing surface science of heterogeneous catalysis. The art of catalysis is developing into science. This profound change provides one with opportunities not only to understand the molecular ingredients of important catalytic systems but also to develop new and improved catalyst. The participation of theorists to find answers to important questions is sorely needed for the sound development of the field. It is the authors hope that some of the outstanding problems of heterogeneous catalysis that are identified in this paper will be investigated. For this purpose the paper is divided into several sections. The brief Introduction to the methodology and recent results of the surface science of heterogeneous catalysis is followed by a review of the concepts of heterogeneous catalysis. Then, the experimental results that identified the three molecular ingredients of catalysis, structure, carbonaceous deposit and the oxidation state of surface atoms are described. Each section is closed with a summary and a list of problems that require theoretical and experimental scrutiny. Finally attempts to build new catalyst systems and the theoretical and experimental problems that appeared in the course of this research are described.
Date: June 1, 1982
Creator: Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

Surface science and heterogeneous catalysis

Description: The catalytic reactions studied include hydrocarbon conversion over platinum, the transition metal-catalyzed hydrogenation of carbon monoxide, and the photocatalyzed dissociation of water over oxide surfaces. The method of combined surface science and catalytic studies is similar to those used in synthetic organic chemistry. The single-crystal models for the working catalyst are compared with real catalysts by comparing the rates of cyclopropane ring opening on platinum and the hydrogenation of carbon monoxide on rhodium single crystal surface with those on practical commercial catalyst systems. Excellent agreement was obtained for these reactions. This document reviews what was learned about heterogeneous catalysis from these surface science approaches over the past 15 years and present models of the active catalyst surface.
Date: May 1, 1980
Creator: Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

Surface science and catalysis

Description: Modern surface science studies have explored a large number of metal catalyst systems. Three classes of catalytic reactions can be identified: (1) those that occur over the metal surface; (2) reactions that take place on top of a strongly adsorbed overlayer and (3) reactions that occur on co-adsorbate modified surfaces. Case histories for each class are presented. 44 refs., 13 figs., 3 tabs.
Date: February 1, 1985
Creator: Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

Structure of solid surfaces and of adsorbates by low-energy electron diffraction

Description: LEED theory has developed to the point where the diffraction beam intensities can be computed using the locations of the surface atoms as the only adjustable parameters. The position of atoms in many clean monatomic solid surfaces and the surface structures of ordered monolayers of adsorbed atoms have been determined this way. Surface crystallography studies are now extended to small hydrocarbon molecules that are adsorbed on metal surfaces. These studies are reviewed. (GHT)
Date: October 18, 1977
Creator: Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

Adsorption of energy in photocatalytic reactors

Description: The dissociation of water to hydrogen and oxygen requires energy ..delta..G/sub 298/ = 228 kJ/mole. By irradiating a semiconductor with light of energy greater than this amount, one may produce electrons in the excited state and electron vacancies at the surface that can perform the photochemical reduction (2H/sup +/ + 2e/sup -/ ..-->.. 2H ..-->.. H/sub 2/) and oxidation (20H/sup -/ + 2/sup +/ ..-->.. H/sub 2/O/sub 2/ ..-->.. H/sub 2/O + (1/2)O/sub 2/). There are several semiconductors, SrTiO/sub 3/, TiO/sub 2/, CdS, and Fe/sub 2/O/sub 3/ among them, that can photodissociate water. Some possess sites for both reduction and oxidation, while others carry out the two processes at different surfaces. A reversible solid state reaction that involves changes in the transition metal and ion oxidation state must accompany the splitting of water. Platinum, rhodium, and ruthenium oxide, when deposited on the semiconductor, serve as catalysts that accelerate the water photodissociation. These additives accelerate the recombination of hydrogen and oxygen atoms, shift the semiconductor Fermi level to a more favorable position that improves the thermodynamic feasibility for the process, accelerate electron transport, and inhibit side reactions like the photoreduction of oxygen. Many of the elementary reaction steps leading to photoproduction of hydrogen and oxygen over SrTiO/sub 3/ and Fe/sub 2/O/sub 3/ have been identified and will be discussed.
Date: July 1, 1985
Creator: Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

Molecular surface science of heterogeneous catalysis. History and perspective

Description: A personal account is given of how the author became involved with modern surface science and how it was employed for studies of the chemistry of surfaces and heterogeneous catalysis. New techniques were developed for studying the properties of the surface monolayers: Auger electron spectroscopy, LEED, XPS, molecular beam surface scattering, etc. An apparatus was developed and used to study hydrocarbon conversion reactions on Pt, CO hydrogenation on Rh and Fe, and NH/sub 3/ synthesis on Fe. A model has been developed for the working Pt reforming catalyst. The three molecular ingredients that control catalytic properties are atomic surface structure, an active carbonaceous deposit, and the proper oxidation state of surface atoms. 40 references, 21 figures. (DLC)
Date: August 1, 1983
Creator: Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

Auger electron spectroscopy

Description: A review of Auger electron spectroscopy is presented. Methods, resolution, sensitivity, and uses are discussed. 30 references, 10 figures. (GHT)
Date: December 1, 1980
Creator: Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

Energy redistribution in diatomic molecules on surfaces

Description: Translational and internal degrees of freedom of a scattered beam of NO molecules from a Pt(111) single crystal surface were measured as a function of scattering angle and crystal temperature in the range 450 to 1250K. None of the three degrees of freedom were found to fully accommodate to the crystal temperature, the translational degree being the most accommodated and the rotational degree of freedom the least. A precursor state model is suggested to account for the incomplete accommodation of translational and vibrational degrees of freedom as a function of crystal temperature and incident beam energy. The vibrational accommodation is further discussed in terms of a competition between desorption and vibrational excitation processes, thus providing valuable information on the interaction between vibrationally excited molecules and surfaces. Energy transfer into rotational degrees of freedom is qualitatively discussed.
Date: April 1, 1984
Creator: Asscher, M. & Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

Molecular Studies of Surfaces under Reaction Conditions; Sum Frequency Generation Vibrational Spectroscopy, Scanning Tunneling Microscopy and Ambient Pressure X-Ray Photoelectron Spectroscopy

Description: Instruments developed in our laboratory permit the atomic and molecular level study of NPs under reaction conditions (SFG, ambient pressure XPS and high pressure STM). These studies indicate continuous restructuring of the metal substrate and the adsorbate molecules, changes of oxidation states with NP size and surface composition variations of bimetallic NPs with changes of reactant molecules.
Date: November 11, 2009
Creator: Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

Interaction of molecular beams with solid surfaces

Description: From proceedings of the International Summer Institute in surface science; Milwaukee, Wisconsin, USA (28 Aug 1973). Molecular beam - surface scattering experiments, the nature of elastic and inelastic scattering, and the information obtained from detection of the angular distribution and kinetic energy of the scattered particles are described. The types of energy exchange that take place between a gas atom or molecule and the surface atoms and the theories that have been developed to explain some of these energy transfer processes are discussed. Finally the results of some experiments are reviewed, and directions for future research are pointed out. (JFP)
Date: August 1, 1973
Creator: Somorjai, G.A. & Brumbach, S.B.
Partner: UNT Libraries Government Documents Department

Hydrogenation of CO and CO/sub 2/ on clean rhodium and iron foils. Correlations of reactivities and surface compositions

Description: An experimental arrangement consisting of an ultrahigh vacuum bell jar equipped with an internal sample isolation cell was used to investigate the hydrogenation of CO over Fe and Rh surfaces. This apparatus permitted both UHV surface characterization (Auger electron spectroscopy, low-energy electron diffraction) and high pressure (1-20 atm) catalytic reactions to be carried out. Small surface area (approximately 1 cm/sup 2/) metal samples, both single crystals and polycrystalline foils, were used to catalyze the H/sub 2//CO reaction at high pressures (1-6 atm). Reaction products were monitored with a gas chromatograph equipped with a flame ionization detector. The surface compositions of the metal samples were determined before and after the reaction and the results correlated with the observed product distributions and reaction rates. In addition, the influence of various surface additives (carbon, oxygen, potassium) was also investigated. Iron was the more reactive of the two metals studied and was found to produce C/sub 1/-C/sub 5/ straight chain hydrocarbons but it poisoned rapidly. The catalytically active surface of both metals was covered with a carbonaceous monolayer. The carbonaceous monolayer was stable on the rhodium surface and produced C/sub 1/-C/sub 4/ hydrocarbons at a steady rate even after several hours of reaction. The absolute rates on rhodium samples were, however, substantially lower than those observed for the catalytically active iron samples. Differences in the poisoning characteristics and product distributions of the initially clean metal surfaces and the promoted rhodium and iron catalysts indicate the importance of additives and the formation of surface compounds in controlling the activity and selectivity.
Date: December 16, 1977
Creator: Dwyer, D.; Yoshida, K. & Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

Structure of adsorbed monolayers. The surface chemical bond

Description: This paper attempts to provide a summary of what has been learned about the structure of adsorbed monolayers and about the surface chemical bond from molecular surface science. While the surface chemical bond is less well understood than bonding of molecules in the gas phase or in the solid state, our knowledge of its properties is rapidly accumulating. The information obtained also has great impact on many surface science based technologies, including heterogeneous catalysis and electronic devices. It is hoped that much of the information obtained from studies at solid-gas interfaces can be correlated with molecular behavior at solid-liquid interfaces. 31 references, 42 figures, 1 table.
Date: June 1, 1984
Creator: Somorjai, G.A. & Bent, B.E.
Partner: UNT Libraries Government Documents Department

Fundamental studies of catalytic gasification

Description: Previous work has shown that chars and coal can be gasified with steam in the presence of alkali-transition metal oxide catalysts or alkali-earth alkali catalysts at relatively low temperatures. These studies are to be extended to the investigation of the amounts of catalysts required and whether a throw away catalyst can be used. Fresh versus stored char will be gasified to determine the role of oxidation of the char on gasification rates. Less expensive catalyst materials such as sodium instead of potassium and iron instead of nickel will be explored. Reaction rates will be determined in the presence of nitrogen, Co and CO{sub 2}. Reactions of methane and carbon solids in the presence of an oxidizing agent such as water, oxygen, and/or carbon dioxide will be explored in the presence of similar catalyst. This quarter, additional experiments on catalytic gasification of coal were carried out. Major emphasis, however, was on the production of C{sub 2} and higher hydrocarbons from methane at very high selectivities. Catalysts studied include KCaNiO. 6 figs., 1 tab.
Date: June 1, 1990
Creator: Heinemann, H. & Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

The puzzles of surface science and recent attempts to explain them

Description: Surfaces have several unique properties that remain unexplained on the atomic level. These include (1) the unique chemical activity of rough surfaces, (2) the breaking of chemical bonds in narrow temperature ranges; and (3) the role of co-adsorption and strongly adsorbed monolayers during catalysis and tribological change (friction, lubrication). The dynamic surface restructuring model and surface structure induced variations in local density of states that have been suggested to rationalize the surface behavior await experimental confirmation. 24 refs., 10 figs., 2 tabs.
Date: July 1, 1990
Creator: Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

Modulated molecular beam scattering of CO and NO from Pt(111) and the stepped Pt(557) crystal surfaces

Description: The modulated molecular beam scattering of CO and NO from Pt(111) and Pt(557) have been studied in the temperature range of 350-1100 K. For CO scattered from Pt(111), an adsorption-desorption model with constant sticking coefficient fits the data well above 500°K, The best rate parameters are: {nu}=2.9x10{sup 13}, E=29.9 kcal/mole, and S=0.74. For CO scattered from Pt(557) the same model fits the data well above 550°K and the best rate parameters are {nu}=7.9x10{sup 13}, E=33.6 kcal/mole, and S=0.74. The higher activation energy for desorption from the stepped Pt(557), as compared to the flat Pt(111), suggests that while the incident molecules can be adsorbed at the step as well as at the terrace, their desorption energy is influenced by the presence of steps. An adsorption-desorption model with coverage-dependent sticking coefficient fits the data obtained at all temperatures well for both surfaces without change of the kinetic parameters. For NO scattered from Pt(111), the adsorption-desorption model with constant sticking coefficient fits the data well above 525°K using the rate parameters of {nu}=6.2x10{sup 13}, E=28.6 kcal/mole, and S=0.65. For NO scattered from Pt(557), the same model fits the data well above 525°K with {nu}=1.2x10{sup 14}, E=32.3 kcal/mole, and S=0.71. The higher activation energy suggests again that NO desorption is influenced by the presence of steps. Below 525°K, the NO scattering results are quite different from those of CO and can not be simulated with the simple models that were tried. The data can be used, however, to rule out several models of surface interactions. NO does not dissociate detectably between 400 K ~1200 K under our experimental conditions. The sticking coefficient decreases with increasing coverage.
Date: November 1980
Creator: Lin, T. H. & Somorjai, G. A.
Partner: UNT Libraries Government Documents Department

Hydrogenation of the alpha,beta-Unsaturated Aldehydes Acrolein, Crotonaldehyde, and Prenal over Pt Single Crystals: A Kinetic and Sum-Frequency Generation Vibrational Spectroscopy Study

Description: Sum-frequency generation vibrational spectroscopy (SFG-VS) and kinetic measurements using gas chromatography have been used to study the surface reaction intermediates during the hydrogenation of three {alpha},{beta}-unsaturated aldehydes, acrolein, crotonaldehyde, and prenal, over Pt(111) at Torr pressures (1 Torr aldehyde, 100 Torr hydrogen) in the temperature range of 295K to 415K. SFG-VS data showed that acrolein has mixed adsorption species of {eta}{sub 2}-di-{sigma}(CC)-trans, {eta}{sub 2}-di-{sigma}(CC)-cis as well as highly coordinated {eta}{sub 3} or {eta}{sub 4} species. Crotonaldehyde adsorbed to Pt(111) as {eta}{sub 2} surface intermediates. SFG-VS during prenal hydrogenation also suggested the presence of the {eta}{sub 2} adsorption species, and became more highly coordinated as the temperature was raised to 415K, in agreement with its enhanced C=O hydrogenation. The effect of catalyst surface structure was clarified by carrying out the hydrogenation of crotonaldehyde over both Pt(111) and Pt(100) single crystals while acquiring the SFG-VS spectra in situ. Both the kinetics and SFG-VS showed little structure sensitivity. Pt(100) generated more decarbonylation 'cracking' product while Pt(111) had a higher selectivity for the formation of the desired unsaturated alcohol, crotylalcohol.
Date: November 26, 2008
Creator: Kliewer, C.J. & Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

Monodisperse metal nanoparticle catalysts on silica mesoporous supports: synthesis, characterizations, and catalytic reactions

Description: The design of high performance catalyst achieving near 100% product selectivity at maximum activity is one of the most important goals in the modern catalytic science research. To this end, the preparation of model catalysts whose catalytic performances can be predicted in a systematic and rational manner is of significant importance, which thereby allows understanding of the molecular ingredients affecting the catalytic performances. We have designed novel 3-dimensional (3D) high surface area model catalysts by the integration of colloidal metal nanoparticles and mesoporous silica supports. Monodisperse colloidal metal NPs with controllable size and shape were synthesized using dendrimers, polymers, or surfactants as the surface stabilizers. The size of Pt, and Rh nanoparticles can be varied from sub 1 nm to 15 nm, while the shape of Pt can be controlled to cube, cuboctahedron, and octahedron. The 3D model catalysts were generated by the incorporation of metal nanoparticles into the pores of mesoporous silica supports via two methods: capillary inclusion (CI) and nanoparticle encapsulation (NE). The former method relies on the sonication-induced inclusion of metal nanoparticles into the pores of mesoporous silica, whereas the latter is performed by the encapsulation of metal nanoparticles during the hydrothermal synthesis of mesoporous silica. The 3D model catalysts were comprehensively characterized by a variety of physical and chemical methods. These catalysts were found to show structure sensitivity in hydrocarbon conversion reactions. The Pt NPs supported on mesoporous SBA-15 silica (Pt/SBA-15) displayed significant particle size sensitivity in ethane hydrogenolysis over the size range of 1-7 nm. The Pt/SBA-15 catalysts also exhibited particle size dependent product selectivity in cyclohexene hydrogenation, crotonaldehyde hydrogenation, and pyrrole hydrogenation. The Rh loaded SBA-15 silica catalyst showed structure sensitivity in CO oxidation reaction. In addition, Pt-mesoporous silica core-shell structured NPs (Pt{at}mSiO{sub 2}) were prepared, where the individual Pt NP is encapsulated by ...
Date: September 14, 2009
Creator: Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

Fundamental studies of catalytic gasification

Description: The major purpose of this project was to finding catalysts which will permit steam gasification of carbonaceous material at reasonable rates and at lower temperatures than currently practiced. This quarter, experiments with steam gasification of graphite were performed. Catalysts studied were potassium, calcium, and nickel oxides. Reaction kinetics are discussed. 28 refs., 34 figs., 8 tabs.
Date: June 1, 1991
Creator: Heinemann, H. & Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

Fundamental studies of catalytic gasification

Description: Our previous work has shown that chars and coal can be gasified with steam in the presence of alkali-transition metal oxide catalysts or alkali-earth catalysts at relatively low temperatures. These studies are to be extended to the investigation of the amounts of catalysts required and whether a throw away catalyst can be used. Fresh versus stored char will be gasified to determine the role of oxidation of the char on gasification rates. Less expensive catalyst materials such as sodium instead of potassium and iron instead of nickel will be explored. Reaction rates will be determined in the presence of nitrogen, CO and CO{sub 2}. Reactions of methane and carbon solids in the presence of an oxidizing agent such as water, oxygen, and/or carbon dioxide will be explored in the presence of similar catalysts. It is expected that hydrocarbon liquids and C{sub 2} plus gases will be produced along with hydrogen. These studies will be performed over a wide range of reaction pressures and reaction temperatures in a flow reactor using a GC-MS detector. Pure paraffinic and cyclic hydrocarbons of high carbon to hydrogen ratio will be designed to have an indication of the ease of gasification of different carbonaceous materials. Results on the performance of Na, Cs, and BaO{sub x} as catalysts are briefly discussed. 3 figs.
Date: September 1, 1990
Creator: Heinemann, H. & Somorjai, G.A.
Partner: UNT Libraries Government Documents Department

Report of the surface science workshop

Description: A three-day workshop was held to review the various areas of energy development and technology in which surface science plays major roles and makes major contributions, and to identify the major surface-science-related problem areas in the fields with ERDA's mission in the fossil, nuclear, fusion, geothermal, and solar energy technologies and in the field of environmental control. The workshop activities are summarized. (GHT)
Date: March 1, 1977
Creator: Somorjai, G.A.; Yates, J.T. Jr. & Clinton, W.
Partner: UNT Libraries Government Documents Department

Conversion of ethane and of propane to higher olefin hydrocarbons

Description: Purely thermal reactions for the conversion of ethane were carried out in an empty and in a quartz chip filled reactor over a temperature range of 300--800{degrees}C in the absence and presence of oxygen and oxygen plus water. Ethane alone shows no conversion below 600{degrees}C and some conversion to CH{sub 4} and very little C{sub 2}H{sub 4} at 700{degrees} and 800{degrees}C. Ethane and oxygen produce CO{sub 2} as the major product above 400{degrees}C. The additional presence of water does not appreciably change this picture. Converting ethane with oxygen and water over a Ca{sub 3}Ni{sub 1}K{sub 0.1} catalyst at very low space velocity gave increasing conversion with temperature, primarily CO{sub 2} production and a small amount of C{sub 3+} hydrocarbons. The CO{sub 2} production was decreased and slightly more C{sub 3} hydrocarbons were produced when the potassium concentration of the catalyst was increased. Activation energies have been calculated for the various ethane conversion reactions. It appears that the CaNiK oxide catalyst is not suited for oxidative ethane coupling at the conditions thus far investigated. The indications are that much shorter contact times are required to prevent oxidation of intermediates. Blank runs with propane and oxygen in the absence of a catalyst have shown significant reaction at temperatures as low as 400{degrees}C. 12 figs., 3 tabs.
Date: October 1, 1991
Creator: Heinemann, H. & Somorjai, G.A.
Partner: UNT Libraries Government Documents Department