The role of water in propylene partial oxidation: thermal desorption studies on Ag(110)

The desorption and reactions of propylene and propylene oxide adsorbed on atomic oxygen covered and hydroxyl covered Ag(110) were investigated to elucidate the effect of water on the oxidation of propylene over silver catalysts. Previous studies clearly indicate enhancement of propylene partial oxidation by the addition of water to reactor feed streams. Propylene combustion by oxygen adatoms on Ag(110) is completely passivated by water coadsorption on the oxygen atom covered surface (water adsorption on O-Ag(110) results in hydroxyl groups). The desorption activation energy of propylene and propylene oxide is increased by up to 30% by adsorbed oxygen atoms on Ag(110). The desorption activation energy for propylene and propylene oxide is reduced on the hydroxyl covered surface relative to desorption from atomic oxygen covered Ag(110). These results suggest that the inhibition of deep oxidation plays an important role in the previously observed water enhancement. In addition, the decreased desorption activation energies for both propylene, the reactant, and propylene oxide, the desired product, may influence the selectivity of this complex reaction system. Potential changes in catalytic reactivity and selectivity caused by water addition are discussed in terms of a general catalytic reaction rate law. This revised version was published online in July 2006 with corrections to the Cover Date.     

Oxygen States at Magnesium and Copper Surfaces Revealed by Scanning Tunneling Microscopy and Surface Reactivity

By a combination of STM and XPS a study of the dynamics of oxygen chemisorption at Mg(0001) at 295?K has revealed oxygen states involving nucleation sites and the development of hexagonal and square lattice structures; the hexagonal structures develop epitaxially with the Mg(0001) surface. There is extensive surface mobility with, at the early stage of chemisorption, oxygen states being observed at steps at the (1×1)-O adlayer and overlapping magnesium atoms. These are active in ammonia and hydrocarbon oxidation whereas at higher oxygen coverage the surface is inactive. The dissociative chemisorption of nitric oxide generates a surface characterized by the hexagonal oxide structure; nitrogen adatoms known to be present from the N(Is) spectra are disordered. The chemisorption of hydrogen chloride at a Cu(110) surface results in a c(2×2)–Cl structure with at high coverage domains 1.8 nm wide. A sub-surface oxygen state at Cu(110), although unreactive to ammonia, undergoes a chemisorption replacement (corrosive chemisorption) reaction with HCl at 295 K.     

The effect of particle size on nitric oxide decomposition and reaction with carbon monoxide on palladium catalysts

The effect of palladium particle size on its catalytic activity was investigated by the decomposition of chemisorbed nitric oxide and the reaction of nitric oxide with carbon monoxide in flow conditions. Palladium particles (30–500 Å) were prepared on silica thin films (100 Å) which were supported on a Mo(110) surface. The reactivity of the supported palladium varied with the metal particle size. On large palladium particles, nitric oxide (NO) reacts to form nitrous oxide (N₂O), dinitrogen (N₂) and atomic oxygen during temperature-programmed reaction, whereas on small particles (< 50 Å), nitrous oxide is not formed. Similarly, reactions of NO with CO on large particles, in flow conditions produce N₂O, N₂ and CO₂, whereas N₂O is not produced on small particles. In addition, more extensive NO decomposition is observed on the smaller particles.     

Insights from Theory on the Relationship Between Surface Reactivity and Gold Atom Release

Density functional theory, informed by experimental studies, is used to investigate the interplay of surface morphology, the adsorption site of reactants, the nature of the interaction between adsorbates and the surface, the potential energy landscape for adsorbates on the surface, adsorbate coverage, temperature, and the dynamic evolution of these factors during adsorption and reaction. We summarize our current understanding of Au atom release on the (111) surface and the corresponding effects on adsorption and reactivity. Gold was selected for these investigations because of the recent intense interest in the activity of gold nanoparticles for several important catalytic reactions. Fundamental experimental studies on Au single-crystal surfaces have established that atomic O is extremely active for oxidation of CO and olefins, that the local bonding of O is an important factor in determining the reactivity and selectivity for oxidation, and that Au atom release is induced by electronegative adsorbates, such as O, Cl, and S. These experimental results guided our theoretical studies. Density functional theory is an extremely useful tool since it evaluates the energetics associated with the incorporation of gold into the adsorbate layer, while providing fundamental physical insight into the underlying cause of gold incorporation. We use our results from static DFT calculations along with ab initio molecular dynamics simulations to understand the effect of surface morphology on the activity of gold for CO oxidation. Our investigation of Au atom release and incorporation induced by electronegative atoms clearly illustrates the importance of using experiments in combination with theory to establish the importance of and the underlying reasons for metal atom release and the affect on bonding and reactivity.     

Reaction of Au(111) with Sulfur and Oxygen: Scanning Tunneling Microscopic Study

The reaction of sulfur and oxygen with the gold surface is important in many technological applications, including heterogeneous catalysis, corrosion, and chemical sensors. We have studied reactions on Au(111) using scanning tunneling microscopy (STM) in order to better understand the surface structure and the origin of gold’s catalytic activity. We find that the Au(111) surface dynamically restructures during deposition of sulfur and oxygen and that these changes in structure promote the reactivity of Au with respect to SO₂ and O₂ dissociation. Specifically, the Au(111) herringbone reconstruction lifts when either S or O is deposited on the surface. We attribute this structural change to the reduction of tensile surface stress via charge redistribution by these electronegative adsorbates. This lifting of the reconstruction was accompanied by the release of gold atoms from the herringbone structure. At high coverage, clusters of gold sulfides or gold oxides form by abstraction of gold atoms from regular terrace sites of the surface. Concomitant with the restructuring is the release of gold atoms from the herringbone structure to produce a higher density of low-coordinated Au sites by forming serrated step edges or small gold islands. These undercoordinated Au atoms may play an essential role in the enhancement of catalytic activity of gold in reactions such as oxygen dissociation or SO₂ decomposition. Our results further elucidate the interaction between sulfur and oxygen and the Au(111) surface and indicate that the reactivity of Au nanoclusters on reducible metal oxides is probably related to the facile release of Au from the edges of these small islands. Our results provide insight into the sintering mechanism which leads to deactivation of Au nanoclusters and into the fundamental limitation in the edge definition in soft lithography using thiol-based self-assembled monolayers (SAMs) on Au. Furthermore, the enhanced reactivity of Au after release of undercoordinated atoms from the surface indicate a relatively insignificant role of an oxide support for high reactivity.     

Insights into surface reactivity: formic acid oxidation on Cu(110) studied using STM and a molecular beam reactor

Using a combination of STM and molecular beam reactor data we summarise some important features of a model reaction (formic acid oxidation on Cu(110)) which is of general significance to surface reactivity and to catalysis. Three such features are highlighted here. The first concerns the role of weakly held species (possibly physisorbed) in surface reactions. These species, although of very short lifetime on the surface, can, nevertheless, diffuse over long distances to “find” a sparse distribution of active sites. Thus a very low coverage of oxygen on the surface of Cu(110) increases the sticking probability of all the formic acid molecules which strike the surface to high value (0.82), even though the clean surface is relatively unreactive. The important concept here is the “diffusion circle” or “collection zone” which represents the area of surface visited by the molecule in its short sojourn in the weakly held state. The second theme concerns the concept of the “flexible surface”. We show that the involvement of surface atoms in reactions directs the structure and reactivity for a particular reaction. For formic acid oxidation the liberation of Cu atoms during the removal of oxygen as water leads to gross restructuring of the surface and can lead to “compression” of one reactant (the oxygen in this case) into a lower area, higher local coverage, unreactive state (the c(6×2) oxygen structure). Thirdly, and finally, it is proposed that, for many surface reactions, the surface acts in an analogous way to a solvent, supporting a “dissolved” (highly mobile and fluxional) phase of intermediates at low coverage, which crystallise out above a critical coverage (the 2D “solubility limit”). This revised version was published online in July 2006 with corrections to the Cover Date.     

Reactions on Single-Crystal Metal Surfaces Studied with Scanning Tunneling Microscopy

A review is given of scanning tunneling microscopy (STM) studies of reactions between different chemical species on metal surfaces. Results presented for a variety of systems show that STM can give unique insight at the atomic level into the mechanistic details of surface reactions and decomposition of molecules. This is illustated with a number of examples of reactions occuring on single-crystal metal surfaces, such as Ni(110), Cu(110), Ag (110), Rh (110), and Pt(111). The ability of the local STM probe to study the influence of defect sites on reactivity is highlighted, and similarities and differences in reactivity of different systems are discussed. Finally, the application of STM to enhance reactivity via tip interactions is addressed.     

Theoretical Insight into the Nature of Ammonia Adsorption on Vanadia-Based Catalysts for SCR Reaction

Density functional theory (DFT) calculations were carried out on monomeric and oligomeric vanadium oxide clusters to probe the factors leading to the formation of NH₄ species from the adsorption of ammonia. The interaction of ammonia with monomeric vanadium oxide clusters leads to the formation of hydrogen-bonded NH₃ species, with energy changes for ammonia adsorption near -50 kJ/mol. The interaction of ammonia with oligomeric vanadium oxide clusters leads to the formation of bidentate NH_4 species, where the ammonium cation is coordinated between two V=O groups on adjacent vanadium cations. The energy change for ammonia adsorption in this mode is near -100 kJ/mol. Adsorption of ammonia as NH₄ species was not observed when the oligomeric vanadium oxide clusters were reduced by addition of hydrogen atoms, i.e., in clusters where the formal oxidation state of the vanadium cations was 4+. Based on our findings, a model for the generation of Brönsted acidity through the interaction of vanadium oxide oligomers with the titanium oxide support is proposed.     

Investigation of Ethylene Oxide on Clean and Oxygen-Covered Ag(110) Surfaces

Temperature-programmed desorption (TPD) and Density Functional Theory (DFT) were used to investigate the reactions of oxametallacycles derived from ethylene oxide on clean and oxygen-covered Ag(110) surfaces. Ethylene oxide ring-opens following adsorption at 250 K on both clean and O-covered Ag(110) to form a stable oxametallacycle. On the clean Ag(110) surface, the oxametallacycle reacts to reform the parent epoxide at 280 K during TPD, while the aldehyde isomer, acetaldehyde, is observed at higher oxametallacycle coverages. In the presence of coadsorbed oxygen atoms, a portion of the oxametallacycles dissociate to release ethylene. However, of those that react to form oxygen-containing products, the fraction forming ethylene oxide is similar to that on the clean surface. The acetaldehyde product of oxametallacycle reactions combusts via formation of acetate species; the acetates react to form CO₂ at temperatures as low as 360 K on the O-covered surface. No evidence was observed for other combustion channels. This work provides experimental evidence for the connection of oxametallacycles to combustion via acetaldehyde formation as well as to ring-closure to form ethylene oxide.     

A comparative theoretical study of Au,Ag and Cu adsorption on TiO<Subscript>2</Subscript> (110) rutile surfaces

The adsorption properties of Au, Ag and Cu on TiO₂ (110) rutile surfaces are examined using density functional theory slab calculations within the generalized gradient approximation. We consider five and four different adsorption sites for the metal adsorption on the stoichiometric and reduced surfaces, respectively. The metal-oxide bonding mechanism and the reactivity of metal atoms are also discussed based on the analyses of local density of states and charge density differences. This study predicts that Au atoms prefer to adsorb at the fourfold hollow site over the fivefold-coordinated Ti(5c) and in-plane and bridging O(2c) atoms with the adsorption energy of ≈0.6 eV. At this site, it appears that the covalent and ionic interactions with the Ti(5c) and the O(2c), respectively, contribute synergistically to the Au adsorption. At a neutral F _s ⁰ center on the reduced surface, Au binds to the surface via a rather strong ionic interaction with surrounding sixfold-coordinated Ti(6c) atoms, and its binding energy is much larger than to the stoichiometric surface. On the other hand, Ag and Cu strongly interact with the surface bridging O(2c) atoms, and the site between two bridging O(2c) atoms is predicted to be energetically the most favorable adsorption site. The adsorption energies of Ag and Cu at the B site are estimated to be ≈1.2 eV and ≈1.8 eV, respectively. Unlike Au, the interaction of Ag and Cu with a vacancy defect is much weaker than with the stoichiometric surface. °This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Density functional theory studies of mechanistic aspects of the SCR reaction on vanadium oxide catalysts

Density functional theory (DFT) calculations were carried out on a vanadium oxide cluster containing four vanadium atoms to probe the mechanism of the selective catalytic reduction (SCR) of NO with ammonia. The interaction of ammonia with Brønsted acid sites on this V₄-cluster leads to the formation of NH₄ species bonded to two vanadyl (VO) groups, with a bonding energy of −110 kJ/mol. This adsorbed NH₄ species reacts with NO in a series of steps to form an adsorbed NH₂NO species, which subsequently undergoes decomposition to form N₂, H₂O, and a reduced vanadium oxide cluster (V₄H). The latter reaction occurs via a series of hydrogen-transfer steps by a “push–pull” mechanism with adjacent VO and VOH groups on the vanadium oxide cluster. The rate limiting process in this conversion of NO and NH₃ to give N₂, H₂O, and V₄H involves the reaction of an adsorbed NH₃NHO adduct to form NH₂NO species. The transition state of this step may be stabilized through hydrogen bonding with surrounding vanadia and/or titania moieties.     

New Perspectives on Direct Heterogeneous Olefin Epoxidation

Important commercial direct oxidation processes include the epoxidation of ethylene to ethylene oxide and the newer epoxidation of butadiene to epoxybutene (EpB), both carried out with silver catalysts. However, detailed reaction mechanisms for these processes are still matters of debate. The guiding hypothesis of our research is that surface oxametallacycles are key intermediates in selective olefin epoxidation. By a combination of surface science experiments and density functional theory (DFT) calculations, we have synthesized the first stable surface oxametallacycles and have verified their identities and structures. In the case of EpB chemistry, we have been able to demonstrate direct connections between surface oxametallacycles and epoxide products. The EpB ring opens with an activation energy of 8.4 kcal/mol on Ag(110) to form a stable surface oxametallacycle. Spectroscopic results for this species are in excellent agreement with DFT calculations for an oxametallacycle bound to three silver atoms of a seven-atom cluster. This oxametallacycle undergoes 1,2 and 1,4 ring-closure reactions during temperature programmed desorption to form EpB and 2,5-dihydrofuran, respectively. This reaction represents the first demonstration of surface oxametallacycle ring closure to form an epoxide, and we suggest that surface oxametallacycles are of general importance in silver-catalyzed olefin epoxidation.     

Variations in Reactivity on Different Crystallographic Orientations of Cerium Oxide

Cerium oxide is a principal component in many heterogeneous catalytic processes. One of its key characteristics is the ability to provide or remove oxygen in chemical reactions. The different crystallographic faces of ceria present significantly different surface structures and compositions that may alter the catalytic reactivity. The structure and composition determine the number of coordination vacancies surrounding surface atoms, the availability of adsorption sites, the spacing between adsorption sites and the ability to remove O from the surface. To investigate the role of surface orientation on reactivity, CeO₂ films were grown with two different orientations. CeO₂(100) films were grown ex situ by pulsed laser deposition on Nb-doped SrTiO₃(100). CeO₂(111) films were grown in situ by thermal deposition of Ce metal onto Ru(0001) in an oxygen atmosphere. The chemical reactivity was characterized by the adsorption and decomposition of various molecules such as alcohols, aldehydes and organic acids. In general the CeO₂(100) surface was found to be more active, i.e. molecules adsorbed more readily and reacted to form new products, especially on a fully oxidized substrate. However the CeO₂(100) surface was less selective with a greater propensity to produce CO, CO₂ and water as products. The differences in chemical reactivity are discussed in light of possible structural terminations of the two surfaces. Recently nanocubes and nano-octahedra have been synthesized that display CeO₂(100) and CeO₂(111) faces, respectively. These nanoparticles enable us to correlate reactions on high surface area model catalysts at atmospheric pressure with model single crystal films in a UHV environment.     

Structure Se nsitivity of Photochemical Oxidation and Reduction Reactions on SrTiO3 Surfaces

The photochemical reduction of Ag⁺ and oxidation of Pb²⁺ from aqueous solution by SrTiO₃ leave insoluble reaction products (silver and PbO₂, respectively) on the surface. Microscopic analysis has been used to relate the rates of these two reactions to the structure and orientation of SrTiO₃ surfaces. The nonpolar (100) surface is the most reactive for silver reduction and the composition of the termination layer does not influence this reaction. On the polar (111) surface, the reduction and oxidation reactions occur on terraces with different terminations and opposite charges; this leads to a nonuniform distribution of reaction products. The polar (110) surface is the least reactive, and the majority of the reaction products are observed at steps along <100> directions where the more reactive {100} surfaces are exposed. The distribution of oxidation products found on (110) terraces is also influenced by the composition and charge of the surface termination. The results show that the photochemical reactivity of SrTiO₃ is anisotropic and that, on polar surfaces, dipolar fields arising from charged surface domains influence the transport of photogenerated charge carriers and promote spatially selective oxidation and reduction reactions.     

Reactivity of surface alkoxy species on acidic zeolite catalysts

[Reaction: see text]. A solid understanding of the mechanisms involved in heterogeneously catalyzed reactions is of fundamental interest for modern chemistry. This information can help to refine modern theories of catalysis and, in a very practical way, can help researchers to optimize existing industrial processes and develop new ones. To understand the mechanisms of heterogeneous catalysis, we need to observe and identify reaction intermediates on a working catalyst. Motivated by this goal, we have monitored the catalytic events in heterogeneous systems using in situ magic-angle-spinning (MAS) NMR under flow conditions. In this Account, we describe the reactivity and possible intermediate role of surface alkoxy species in a variety of zeolite-catalyzed reactions. First, we isolate the surface alkoxy species on a working zeolite catalyst and then investigate the chemical reactivity with different probe molecules under reaction conditions. Finally, we investigate reaction mechanisms facilitated by these intermediate surface alkoxy species. We examined the reactivity of surface methoxy species (SMS) in terms of C-O bond and C-H bond activation. SMS on acidic zeolite catalysts act as an effective methylating agent when reacted with different probe molecules (including methanol, water, ammonia, alkyl halides, hydrochlorides, aromatics, carbon monoxide, and acetonitrile) through C-O bond activation. At higher reaction temperatures (ca. 523 K and above), the C-H bond activation of SMS may occur. Under these conditions, intermediates such as surface-stabilized carbenes or ylides are probably formed. This C-H bond activation is directly related to the initiation mechanism of the methanol-to-olefin (MTO) process and invites further investigation. Based on our experimental results, we also discuss the reactivity and the carbenium-ion-like nature of surface alkoxy species and recent theoretical investigations in this area.     

The interaction of rutile (TiO2) surface with some catalytically active transition metal oxides during heating,studied by electrochemical methods

The interaction of the surface of a rutile monocrystal (110 oriented) with the oxides of some transition metals (V, Mo, Cr, Mn and Nb) during heating of the crystal with these oxides was investigated by electrochemical methods. In all cases insertion of the metal atoms in the rutile surface was observed, the degree of insertion depending strongly on the metal in question and the conditions of experiment. The interaction of rutile surface with metal oxides changes dramatically the conditions of charge transfer at the rutile surface, which may influence the course of catalytic reactions occurring at the surface of rutile-supported transition metal oxide catalysts. This revised version was published online in June 2006 with corrections to the Cover Date.     

Reactivity of Hydroxyl Species from Coadsorption of Oxygen and Water on Ni(110) Single-Crystal Surfaces

The formation and reactivity of hydroxyl species originating from coadsorption of water and oxygen on Ni(110) single-crystal surfaces have been studied by using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). The resulting surface population of hydroxyl intermediates at a given water–oxygen coverage combination was found to be temperature-dependent. This was demonstrated by the differences in hydroxyl coverages determined by TPD and XPS: while the TPD data were determined to mostly reflect the maximum coverages that can be reached for a given set of gas exposures at low temperatures, the XPS results measure the OH coverages formed at the temperature of dosing. Our results indicate that, besides the stoichiometric and reversible H₂O(ads) + O(ads) = 2OH(ads) step, a second water-decomposition reaction on the oxygen-precovered surface deposits additional hydroxyl adsorbates. Depletion of surface oxygen can be induced by thermal reaction with coadsorbed ammonia as well, a result that provides direct evidence for the OH(ads) disproportionation reaction.     

Oxygen chemisorption at Cu(110) at 120 K: dimers, clusters and mono-atomic oxygen states

Three distinct states of oxygen have been observed at a Cu(110) surface at 120 K by scanning tunnelling microscopy (STM): isolated oxygen adatoms; pairs or dimers, separated by about 6 Å and clusters of five or six atoms arranged anisotropically. There is also evidence for oxygen atoms undergoing ballistic motion as might be expected from “hot” oxygen atoms. Such states of oxygen have been central to the mechanistic models proposed earlier, and based on surface spectroscopic studies, for the oxidation of ammonia at copper surfaces under ammonia-rich conditions.     

Self-organised synthesis of Rh nanostructures with tunable chemical reactivity

Nonequilibrium periodic nanostructures such as nanoscale ripples, mounds and rhomboidal pyramids formed on Rh(110) are particularly interesting as candidate model systems with enhanced catalytic reactivity, since they are endowed with steep facets running along nonequilibrium low-symmetry directions, exposing a high density of undercoordinated atoms. In this review we report on the formation of these novel nanostructured surfaces, a kinetic process which can be controlled by changing parameters such as temperature, sputtering ion flux and energy. The role of surface morphology with respect to chemical reactivity is investigated by analysing the carbon monoxide dissociation probability on the different nanostructured surfaces.     

Complementary structure sensitive and insensitive catalytic relationships

The burgeoning field of nanoscience has stimulated an intense interest in properties that depend on particle size. For transition metal particles, one important property that depends on size is catalytic reactivity, in which bonds are broken or formed on the surface of the particles. Decreased particle size may increase, decrease, or have no effect on the reaction rates of a given catalytic system. This Account formulates a molecular theory of the structure sensitivity of catalytic reactions based on the computed activation energies of corresponding elementary reaction steps on transition metal surfaces. Recent progress in computational catalysis, surface science, and nanochemistry has significantly improved our theoretical understanding of particle-dependent reactivity changes in heterogeneous catalytic systems. Reactions that involve the cleavage or formation of molecular pi-bonds, as in CO or N(2), must be distinguished from reactions that involve the activation of sigma-bonds, such as CH bonds in methane. The activation of molecular pi-bonds requires a reaction center with a unique configuration of several metal atoms and step-edge sites, which can physically not be present on transition metal particles less than 2 nm. This is called class I surface sensitivity, and the rate of reaction will sharply decrease when particle size decreases below a critical size. The activation of sigma chemical bonds, in which the activation proceeds at a single metal atom, displays a markedly different size relationship. In this case, the dependence of reaction rate on coordinative unsaturation of reactive surface atoms is large in the forward direction of the reaction, but the activation energy of the reverse recombination reaction will not change. Dissociative adsorption with cleavage of a CH bond is strongly affected by the presence of surface atoms at the particle edges. This is class II surface sensitivity, and the rate will increase with decreasing particle size. Reverse reactions such as hydrogenation typically show particle-size-independent behavior. The rate-limiting step for these class III reactions is the recombination of an adsorbed hydrogen atom with the surface alkyl intermediate and the formation of a sigma-type bond. Herein is our molecular theory explaining the three classes of structure sensitivity. We describe how reactions with rates that are independent of particle size and reactions with a positive correlation between size and rate are in fact complementary phenomena. The elucidation of a complete theory explaining the size dependence of transition metal catalysts will assist in the rational design of new catalytic systems and accelerate the evolution of the field of nanotechnology.