Transition Metal Surface Chemistry

Both the cluster, as well as periodic approaches, will likely play invaluable roles in the future toward the quantitative prediction of transition metal surface chemistry. Herein, we discuss some of the recent developments on the application of DFT-cluster calculations to chemisoiption and reactivity of adsorbates on metal surfaces. We demonstrate how these results can subsequently be used to begin to model overall catalytic cycles and interpret different selective oxidation chemistries. 

Vojvodic A, Nprskov JK, AbUd-Pedersen F. Electronic structure effects in transition metal surface chemistry. Top Catal 2013 57 25-32. 

Gaussian-type orbitals, the computational requirements grow, in the limit, with the fourth power in the number of basis functions on the SCF level and with even a higher power for methods including correlation. Both the conceptual and the computational aspects prevent the computational study of important problems such as the chemistry of transition metal surfaces, interfaces, bulk compounds, and large molecular systems. 

Gas phase transition metal cluster chemistry lies along critical connecting paths between different fields of chemistry and physics. For example, from the physicist s point of view, studies of clusters as they grow into metals will present new tests of the theory of metals. Questions like How itinerant are the bonding electrons in these systems and Is there a metal to non-metal phase transition as a function of size are frequently addressed. On the other hand from a chemist point of view very similar questions are asked but using different terminology How localized is the surface chemical bond and What is the difference between surface chemistry and small cluster chemistry Cluster science is filling the void between these different perspectives with a new set of materials and measurements of physical and chemical properties. 

The description of bonding at transition metal surfaces presented here has been based on a combination of detailed experiments and quantitative theoretical treatments. Adsorption of simple molecules on transition metal surfaces has been extremely well characterized experimentally both in terms of geometrical structure, vibrational properties, electronic structure, kinetics, and thermo-chemistry [1-3]. The wealth of high-quality experimental data forms a unique basis for the testing of theoretical methods, and it has become clear that density functional theory calculations, using a semi-local description of exchange and correlation effects, can provide a semi-quantitative description of surface adsorption phenomena [4-6]. Given that the DFT calculations describe reality semi-quantitatively, we can use them as a basis for the analysis of catalytic processes at surfaces. 

The political justification for transition metal cluster chemistry is the assumption that clusters are models in which metallic properties may be more easily studied than in the metals themselves. These properties include electronic phenomena such as color and conductivities as well as surface phenomena, such as atom arrangements and catalytic activities. Thus, there are two main lines of cluster research. The more academic line leads to the search for new types of clusters and their structure and bonding, whereas the more practical line leads to the investigation of reactivities with the hope that clusters may open catalytic pathways that neither plain metals nor mononuclear catalysts can provide. The interdependence of both lines is obvious. 

Considerable advances in the field of transition metal cluster chemistry have been made during the last five years. They have confirmed that in many respects a cluster complex is comparable to a metallic surface. They have also shown that clusters allow reactions which are not observed with simple metal complexes. And they have finally demonstrated that structural and bonding properties of clusters require new concepts for their description. 

Very recently, spectroscopic investigations have provided evidence that at least two types of molecular NO can exist simultaneously on transition metal surfaces (12,15.19,20, 21, 22), thus supporting the interesting possibility that the rich diversity of nitrosy1-metal chemistry extends from metal complexes to metal surfaces. Two states of molecular NO have been observed on Ru(001) by electron energy loss spectroscopy (EELS) (13), and also by X-ray photo-... 

Over the past several years, the area of gas-phase transition metal ion chemistry has been gaining increasing attention from the scientific community [1-16]. Its appeal is manifold first, it has broad implications to a spectrum of other areas such as atmospheric chemistry, corrosion chemistry, solution organometallic chemistry, and surface chemistry secondly, an arsenal of gas phase techniques are available to study the thermochemistry, kinetics, and mechanisms of these "unusual" species in the absence of such complications as solvent and ligand... 

Whereas the CO insertion reaction has been investigated extensively in organometallic chemistry and homogeneous catalysis, there are only few first-principles investigations available for this reaction on transition metal surfaces (32,33,60). 

A few examples may demonstrate this. The approximation has been used to treat the photodissociation processes of NOHCl [33] and NO2 [34] which include three internal modes. Five dimensional time dependent calculations were performed on the photodissociation of CH3I [35, 36]. The state-to-state chemistry has been investigated for the reactive scattering of H-1-H2 (v=0,l) —> H2 (v=0,l) -1- H collinear system [37]. The MCTDSCF approach has also been applied to surface chemistry, in particular H2 dissociation on a transition metal surface [38] the photodissociation of CH3I on MgO surface [39, 40] and to inelastic molecule-surface scattering [41, 42]. Recently, the MCTDSCF method has been used to investigate multimode effects in the absorption spectrum of pyrazine taking into account 14 vibrational modes [43]. 

Zaera F (1995) An organometalhc guide to the chemistry of hydrocarbon moieties on transition metal surfaces. Chem Rev 95 2651... 

Shustorovich, E., Sellers, H. The UBI-QEP method A practical theoretical approach to understanding chemistry on transition metal surfaces. Surf. Sci. Rep. 1998, 31, 5-119. 

The chemistry of acetate on transition metal surfaces is important for a variety of selective oxidation processes. Methanol and vinyl acetate syntheses are two such important oxidation chemistries where acetate intermediates have been postulated. In VAM synthesis, acetate is a critical intermediate in both VAM formation, as well as in its decomposition to CO2. The latter unselective decarboxylation path becomes important at higher operating temperatures. Understanding the mechanism for decarboxylation and VAM synthesis may ultimately aid in the design of new catalyst formulations on new operating conditions. 

The presence of oxygen can open up a number of additional reaction pathways that can control the actual surface chemistry. Madix has demonstrated that adsorbed atomic oxygen can behave as a nucleophillic center and attack surface bound hydrocarbon intermediates or as a Brqnsted base for hydrogen transfer reactions [63]. Chemisorbed atomic oxygen can also act as a poison on different transition metal surfaces. 

Perhaps the best way to illustrate the diverse surface-structure chemistry of organic monolayer adsorbates is to review the adsorption behavior of ethylene and benzene on various transition metal surfaces. These two molecules are described in detail below. 

Much of the justification for the extensive study of transition metal cluster chemistry is embedded in the assumption that reactions of metal clusters are realistic structural models for reactions at metal surfaces in such processes as heterogeneous catalysis (9,10,11). For example, the metal carbonyl clusters, Ir4(CO)i2 and Os3(CO)i2, were demonstrated to be effective homogeneous catalysts for methanation (12). Additionally, Demitras and Muetterties (13) have found Ir4(CO)i2 to be a homogeneous catalyst in the Fischer-Tropsch synthesis of aliphatic hydrocarbons. Homogeneous catalysis of the water gas shift reaction by metal carbonyl clusters (e.g., Ru3(CO)i2) in alkaline solution has been reported by Laine, Rinker, and Ford (14), and more recently by Pettit s group (15). Nevertheless, mononuclear metal carbonyls (e.g., Fe(CO)s and the group VIb metal hexacarbonyls) have been demonstrated to have considerable activity above 120°C as soluble catalysts for Reaction 2 (16),... 

Cluster compounds have been included iu previous sections of this chapter and earlier chapters. Transition-metal cluster chemistry has developed rapidly since the 1980s. Beginning with simple dimeric molecules, such as Co2(CO)g and Fe2(CO)9, chemists have developed syntheses of far more complex clusters, some with interesting and unusual structures and chemical properties. Large clusters have been studied with the objective of developing catalysts that may improve on the properties of heterogeneous catalysts the surface of a cluster may mimic the behavior of the surface of a solid catalyst. 

The Quantum Chemistry of Transition Metal Surface Bonding and Reactivity... 

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