Active Sites In Heterogeneous Catalysis Research Articles

Revisiting active sites in heterogeneous catalysis: Their structure and their dynamic behaviour

This short review article aims at revisiting the description of active sites in heterogeneous catalysis on solid surfaces and their role in catalyst activity and selectivity. Special emphasis is brought to: (i) structure of the solid surface; (ii) importance of active site isolation on the surface; (iii) dynamic behaviour under reaction conditions and (iv) importance of active phase-support interaction. After a general view on active sites in heterogeneous catalyst, this article describes different case studies of metal oxide catalysts, namely MoO3 and VPO catalysts, to exemplify the concepts, including active site description, structure and/or electronic sensitivity, synergy effects and dynamic surface behaviour during catalytic reaction. Static and dynamic models are discussed considering the many and different views expressed during the years and all considering active sites as atoms or ensembles of atoms as suggested by Taylor in 1925 and instable catalytic reaction intermediates as proposed by Sabatier in 1913. However, with the years, complex structures and dynamic behaviour with interaction between the active sites and the adsorbates (reactants and products) during catalytic reactions have been identified and characterised. In the latter case surface metalloinorganic or metalloorganic entities, designated as chemadphase, constitute the basis of reaction intermediates and are submitted to continuous oscillations on the surface under steady state. Recent developments in physical techniques used to characterise catalysts, in particular during catalytic reaction, and theoretical approach and modelling are also presented and discussed in view of improved description of active sites.

Theoretical Study of the Role of a Metal–Cation Ensemble at the Oxide–Metal Boundary on CO Oxidation

Identification of the active sites in heterogeneous catalysis is important for a mechanistic understanding of the structure–reactivity relationship and rationale of the design of new catalysts, but remains challenging. Among others, the boundaries at metal nanoparticles and supported oxides were found to be important and attributed to the active sites in various catalytic reactions. To reveal the nature of the active sites at the boundaries, the catalytic role of the inverse 3d transition-metal oxide nanoislands on Pt(111) for low-temperature CO oxidation was studied by density functional theory calculations. A characteristic Pt–cation ensemble at the oxide/metal boundaries as the active sites is identified. In Pt–cation ensembles, coordinate-unsaturated (CUS) cations exposed at the edges of oxide nanoislands are highly active for O2 adsorption and dissociation, and less-reactive Pt binds modestly with dissociated O responsible for the facile CO oxidation. Inverse VIIIB-oxide/Pt boundaries exhibit high ac...

ChemInform Abstract: Insights into Reaction Mechanisms in Heterogeneous Catalysis Revealed by in situ NMR Spectroscopy

AbstractReview: 58 refs.

Insights into reaction mechanisms in heterogeneous catalysis revealed by in situ NMR spectroscopy

This tutorial review intends to show the possibilities of in situ solid state NMR spectroscopy in the elucidation of reaction mechanisms and the nature of the active sites in heterogeneous catalysis. After a brief overview of the more usual experimental devices used for in situ solid state NMR spectroscopy measurements, some examples of applications taken from the recent literature will be presented. It will be shown that in situ NMR spectroscopy allows: (i) the identification of stable intermediates and transient species using indirect methods, (ii) to prove shape selectivity in zeolites, (iii) the study of reaction kinetics, and (iv) the determination of the nature and the role played by the active sites in a catalytic reaction. The approaches and methodology used to get this information will be illustrated here summarizing the most relevant contributions on the investigation of the mechanisms of a series of reactions of industrial interest: aromatization of alkanes on bifunctional catalysts, carbonylation reaction of methanol with carbon monoxide, ethylbenzene disproportionation, and the Beckmann rearrangement reaction. Special attention is paid to the research carried out on the role played by carbenium ions and alkoxy as intermediate species in the transformation of hydrocarbon molecules on solid acid catalysts.

Adsorption of Chitosan into SiO2 Monoliths Materials: Physical and Chemical Properties

AbstractThe dispersion of biopolymer chitosan on silica polymer matrix, prepared by the sol‐gel method under conditions of nearly linear chain growth, has allowed us obtain the beneficial properties of the large surface area of the dispersed SiO2 porous substrates. This study primarily aims to explore the chitosan adsorption in different weight ratios on silica monoliths by means solvent‐aided dispersion. The chitosan on SiO2 system has been investigated to know certain polymer properties, such as morphology, structure and thermal stability. The surface of the SiO2 monolith disks calcined (500 °C) employed as matrixes for chitosan were fractures free. SBET surfaces areas of these monoliths range from 452.99 m2/g. According to the FTIR analysis, the hydrogen bond there is an attractive intermolecular force between chitosan and SiO2. In contrast, the amino groups of the biopolymer are still accessible as active sites for heterogeneous catalysis. In addition, the thermostability of chitosan deposited on the monoliths was better than chitosan isolate.

Graftable chiral ligands for surface organometallic materials: calixarenes bearing asymmetric centers directly attached to the lower rim

A family of chiral lower-rim substituted calixarene ligands 1–14 is synthesized and characterized as prospective ligands for synthesis of asymmetric active sites for heterogeneous catalysis and adsorption using a surface organometallic approach. The ligands possess asymmetric centers directly attached to lower-rim oxygens; the asymmetric centers are tailored with sterically bulky naphthyl and electron withdrawing CF3 substituents. 1H and 13C NMR spectroscopies demonstrate that mono- and 1,3-di-alkylated calixarenes synthesized using this procedure adopt the cone-shaped conformation, which is stabilized by multiple hydrogen bonds at the lower rim. Smaller pendant groups and polar solvents increase the dialkylation/monoalkylation ratio. The reaction proceeds with full inversion of configuration as established viasingle crystal X-ray diffraction of an N-Cbz-protected aminocyclopentoxy-modified calixarene. The calixarenes synthesized by this approach are characterized using circular dichroism (CD) spectroscopy and ab initio modeling, which is used to identify the minimal molecular fragment responsible for the observed lowest energy Cotton effect in the CD spectrum. Factors that influence the intramolecular induction of asymmetry throughout the calixarene scaffold are investigated by systematically varying substituent steric bulk, connectivity of the asymmetric center to the calixarene core, and the extent of hydrogen bonding at the calixarene lower rim. The asymmetry of the calixarene core is quantified using 1H NMR spectroscopic shifts of aromatic meta and methylene bridge hydrogens and supported using ab initio calculations. The results demonstrate that calixarene core asymmetry is most strongly induced for rigid calixarene cores—an attribute that is expected to be preserved upon metal complexation and anchoring of these prospective ligands.

Identification of Active Edge Sites for Electrochemical H 2 Evolution from MoS 2 Nanocatalysts

The identification of the active sites in heterogeneous catalysis requires a combination of surface sensitive methods and reactivity studies. We determined the active site for hydrogen evolution, a reaction catalyzed by precious metals, on nanoparticulate molybdenum disulfide (MoS2) by atomically resolving the surface of this catalyst before measuring electrochemical activity in solution. By preparing MoS2 nanoparticles of different sizes, we systematically varied the distribution of surface sites on MoS2 nanoparticles on Au(111), which we quantified with scanning tunneling microscopy. Electrocatalytic activity measurements for hydrogen evolution correlate linearly with the number of edge sites on the MoS2 catalyst.

On the Role of Oxygen Defects in the Catalytic Performance of Zinc Oxide

Metal oxides are highly important as components in heterogeneous catalysts. It is also well established that as the building blocks of nanoscaled materials get smaller, surfaces become increasingly important in determining their properties. Very early on, researchers found that catalytic activity is only indirectly related to the surface area; in fact it depends on the density of active sites. Although solid-state defects have been proposed to be the active sites in heterogeneous catalysis, active centers have rarely been conclusively identified and the rational design of catalysts is still out of reach. Processes in heterogeneous catalysis associated with the production of energy-storage molecules, for example, methanol as a storage molecule for hydrogen, is presently of great interest. Low-pressure production of methanol is conducted industrially with catalysts containing Cu and ZnO as the active phases and alumina as the support. Composites (Cu + ZnO), however, exhibit higher activities than that expected based on the performance of the single components. This was attributed to a strong metal–support interaction (SMSI) effect. Nevertheless, the active sites in Cu/ZnO and even in the pure ZnO have not yet been identified experimentally. Some important information concerning pure ZnO catalysts is already available: Generally, the catalytic activity of ZnO does not increase linearly with the increasing BET surface area. It was concluded that the catalytic formation of methanol over ZnO is a structure-sensitive reaction requiring the presence of polar ZnO facets. According to the most recent models of the methanol-synthesis reaction on the polar ZnO(0001) surface, oxygen vacancies formed on this surface of ZnO crystals serve as the active sites (Scheme 1). However, there has been no definite experimental proof for this assumption.

Unusual Coordination Behavior of Cr3+ in Microporous Aluminophosphates

A CrAPO-5 molecular sieve has been investigated with X-ray absorption spectroscopy (EXAFS-XANES) as dehydrated material and after loading with water and ammonia to unravel the coordination geometries of Cr3+ in the framework of a microporous crystalline aluminophosphate, more particularly of the AFI-type. A comparison of the XANES data, a preedge analysis with crystal field multiplet calculations and EXAFS data, pointed toward the presence of framework Cr3+ which, on dehydration, takes on a distorted tetrahedral coordination state. Due to the 3d3 configuration of Cr3+, this unusual tetrahedral coordination environment strongly tends to transform into the more stable 6-fold coordination geometry by binding two extraframework water molecules during hydration. In the presence of ammonia, tetrahedral Cr3+ readily transforms into a 5-fold coordination geometry by binding one ammonia molecule. Therefore, depending on the environmental conditions, the Cr3+ ions can occur in a 4-, 5-, or 6-fold coordination. This observation underlines the flexibility of transition metal ions, such as Cr3+, to cope with unusual coordination geometries in inorganic hosts, making them interesting as potential active sites in heterogeneous catalysis.

Chemisorbed Atoms and Molecules of Reactants As Active Sites in Heterogeneous Catalysis

In some heterogeneous chemical processes occurring on the solid/gas interface, reactant atoms and molecules chemisorbed during the reaction are not directly involved in chemical transformations but act as catalytic sites accelerating this reaction.

Active sites for heterogeneous catalysis by functionalisation of internal and external surfaces

Molecular modelling has the ability to provide the atomistic understanding of processes in catalysis. To be able to investigate the mechanisms of reactions we must first be certain of the active site of the catalyst. In this paper, we outline our recent work on methanol synthesis in which we used the polar surfaces of zincite as a model for the industrial catalyst, before detailing our investigation of Group 11 metals supported on metal oxides. Copper is known to promote industrial catalysis, while silver and gold have also been shown to be active at a much lower weight percent of metal (with gold being the most promising) for both methanol synthesis and the water gas shift reaction. We also present the first systematic study of the stability of Groups 4 and 14 metal impurities in the framework of silicalite. Throughout we assess the effect of the metal dopant on the atomic and electronic structure of the oxide support. All the work presented here has employed an embedded cluster approach suitable for studying impurities in solid lattices both at internal and external surfaces, which has been implemented in the software ChemShell.

Porous Chitosan-Silica Hybrid Microspheres as a Potential Catalyst

Chitosan/silica hybrids were synthesized and shaped as microspheres, which may or may not present a core/shell structure, depending on the experimental conditions. The core is constituted by a homogeneous hybrid, while the shell is pure silica. The amino groups of the biopolymer are still accessible as active sites for heterogeneous catalysis.

Active Sites in Heterogeneous Catalysis: Development of Molecular Concepts and Future Challenges

AbstractFor Abstract see ChemInform Abstract in Full Text.

Active Sites in Heterogeneous Catalysis: Development of Molecular Concepts and Future Challenges

The concept that catalytic turnover occurs at a small fraction of the surface sites dates back to the 1920s. The application of modern surface science techniques and model catalysts confirmed the presence of active sites and identified their structures in some cases. Low coordination defect sites on transition metals, steps and kinks, or open “rough” crystal faces that make high coordination metal sites available have been uniquely active for breaking H–H, C–H, C–C, C=O, O=O and N≡N bonds. Oxide–metal interfaces provide highly active sites for reactions of C–H and C=O bonds. Electron acceptor and proton donor sites are implicated in hydrocarbon conversion (acid–base catalysis), and sites where metal ion–carbon bonds can form are active for polymerization. The observations of dynamic restructuring of catalytic surfaces upon adsorption of reactants indicate that many catalytic sites are created during the chemical reaction. Similar restructuring is detected for enzyme catalysts. The high mobility of both surface metal atoms and adsorbed molecules during the catalytic process observed recently bring into focus the dynamic nature of active sites that may have a finite lifetime as they form and disassemble. The development of techniques that provide improved time resolution and spatial resolution, and can be employed under catalytic reaction conditions will provide information about the time dependent changes of active site structure and molecular intermediates at these active sites as the reaction products form.

Use of organic probes in detecting active sites in heterogeneous catalysis

Use of organic probes in detecting active sites in heterogeneous catalysis

A Computer Program Package for the Analysis, Creation, and Estimation of Generalized Reactions—GRACE. I. Generation of Elementary Reaction Network in Radical Reactions—GRACE(I)

Abstract The system GRACE generates elementary reaction networks for simple reactions including free radicals, ions and even active sites in heterogeneous catalysis, and also predicts the overall reaction rates and the product distributions in radical reactions in the gas phase where the Arrhenius parameters of elementary reactions are available. In this paper, A/GRACE (part A of GRACE) is explained, which prepares elementary reaction networks for radical reactions. The reactant and the product are represented by square symmetric matrices. The off-diagonal elements represent bond multiplicity, or the number of localized electrons, between corresponding atoms, whereas the diagonal elements imply the number of unshared electrons on radical atoms. A reacting system, an ensemble of the participating molecules, is composed of atom groups, each of which consists of a center atom (usually non-hydrogen) and attached hydrogen atom(s), if any. Firstly, all the possible changes in the atom groups are selected, and the permitted combinations are preapred bearing in mind the equivalent groups in the system and the restrictions optioned. The major restriction is the complexity, i.e., the number of ruptured or formed bonds. A direct sum of matrices are prepared from one of the combinations, and the elementary reaction is completed by setting the appropriate numerals, 1 or −1, to the outside elements. Secondly, the procedures are repeated until all the feasible networks are prepared. As an illustration the hydrogenation of ethylene over a heterogeneous catalyst has been cited.

Active Sites for Heterogeneous Catalysis

The catalytic decomposition of carbon monoxide on (100), (110), and (111) monocrystalline nickel thin films has been studied. Reacted nickel films and extraction replicas were examined by transmission electron microscopy to determine the influence of orientation, lattice defects, and topographical imperfections on the catalytic activity of these films. The order of decreasing catalytic activity, determined by the rate of graphite deposition, was (111) > (110) > (100). The results on (100) films indicated that a Ni–C phase was formed initially and subsequently decomposed to give polycrystalline graphite in nickel-depleted regions, generally associated with lattice defects or strain paths along 〈001〉 directions. On (110) and (111) films, oriented graphite deposits were found on the nickel surfaces, and the results indicated that nucleation occurred at active sites for the catalytic reaction with no intermediate Ni–C phase detectable. No correlation was found between lattice defects and graphite nucleation sites. These nucleation sites in general corresponded to specific steps and kink sites on the nickel surface. It was proposed that the important factors governing the activity of steps for a given catalytic reaction included the step height and the nature of the step base and step face.

Dislocations as ``Active Sites'' in Heterogeneous Catalysis

Experiments on the dependence of catalytic activity of nickel foils as a function of deformation and annealing are interpreted as evidence that dislocations are active sites in heterogeneous catalysis.