Sg Sharankumar Shetty

Reactivity theory of transition-metal surfaces: a Brønsted-Evans-Polanyi linear activation energy - free-energy analysis

The exponential increase in computational processor speed, the development of novel computational architectures, together with the tremendous advances in ab initio theoretical methods that have emerged over the past two decades have led to unprecedented advances in our ability to probe the fundamental chemistry that occurs on complex catalytic surfaces. In particular, advances in density functional theory (DFT) have made it possible to elucidate the elementary steps and mechanisms in surface-catalyzed processes that would be difficult to explore experimentally. The advanced state of plane wave DFT has made it possible to rapidly examine systematic changes to the metal or the reactant in order to establish structure-property relationships. As a result, extensive data based on the energetics for various different surface-catalyzed reactions has been generated. This invites a detailed theoretical analysis of the factors that control reaction paths and corresponding potentialenergy surfaces of surface reactions. Such a theoretical analysis will not only provide interesting new insights into the intricate relationship between the chemical bonding features, structure, and energies of transition states but also serve as a basis for the development of analytical expressions that relate transitionstate properties to more easily accessible thermodynamic properties. The Brønsted-Evans-Polanyi (BEP) relationship is one such example which has been widely applied in the analysis of surface elementary reaction steps.1-8 δEact )RδEr (1)

Direct versus Hydrogen-Assisted CO Dissociation

The mechanism of CO dissociation is a fundamental issue in the technologically important Fischer-Tropsch (F-T) process that converts synthesis gas into liquid hydrocarbons. In the present study, we propose that on a corrugated Ru surface consisting of active sixfold (i.e., fourfold + twofold) sites, direct CO dissociation has a substantially lower barrier than the hydrogen-assisted paths (i.e., via HCO or COH intermediates). This proves that the F-T process on corrugated Ru surfaces and nanoparticles with active sixfold sites initiates through direct CO dissociation instead of hydrogenated intermediates.

Structure sensitivity of the Fischer-Tropsch reaction; molecular kinetics simulations

Strong interest in the Fischer–Tropsch reaction that converts synthesis gas into hydrocarbons is reappearing because it is basic to one of the major routes that convert natural gas into liquid energy carriers. For catalytic science it provides amongst others an opportunity to revisit still open mechanistic issues of the Fischer–Tropsch conversion reaction. New approaches as computational advances and development of model systems are tools that may provide new insights. In this paper we will review our current understanding of the kinetics and its relation to catalyst structural parameters that determine the selectivity of the reaction. In the introductory section we formulate the key questions that we will address. Especially we will discuss the reason for particle size dependence of metallic catalysts as Co and Ru when particles are in the nanosize range and also the apparent paradox that step–edge sites are necessary for the chain growth reaction, where the CO molecule has to dissociate but that such sites should not be poisoned by the presence of the growing hydrocarbon chains or deactivating carbonaceous residue. One of the main selectivity issues of this reaction is the desire to produce long chain hydrocarbon molecules, without co-production of light gas molecules as methane. We will begin the presentation with the elementary kinetic expressions that enable calculation of the selectivity from a microkinetics reaction scheme. This will highlight that the rate of chain growth termination has to be one of the slow reaction steps and also that CO dissociation has to be fast. Since the past decade has seen major advances in the understanding of the structure sensitivity of transition metal catalysed surface reactions, the kinetic analysis helps to understand how structure sensitivity affects Fischer–Tropsch selectivity. Quantum chemical computational studies now can be used to analyse reaction paths and estimate reaction intermediate adsorption energies. Also activation free energies can be deduced for elementary surface reactions. We will illustrate this by discussing a so-called dual site model of the reactive catalyst center. On this reaction center we will discuss in detail CO dissociation and initiation of the chain growth reaction. It appears that synchronized subsequent reaction events involving reaction intermediate diffusion to different positions at the reaction center leads to accommodation of hydrocarbon chain growth while CO dissociation is not suppressed. Ultimately the mechanistic model deduced from the quantum-chemical studies will have to be used in kinetic equations to predict overall catalytic conversion rates. We will demonstrate how this can be done in a Kinetic Monte Carlo scheme, in which no assumption on the rate limiting step has to be made. We will present the results of some initial simulations where we allow for growth of short hydrocarbon chains. These results can be used in an insightful way using the kinetic model equations presented earlier in the paper. The paper is concluded by discussing the implications of these model results for our general mechanistic understanding of the Fischer–Tropsch reaction.

Theoretical investigation of CO adsorption on Rhn (n = 3-13) clusters

The adsorption of CO onto Rhn (n = 3–13) clusters has been investigated using the density functional approach. Stable active sites for CO adsorption such as top, bridge, and hollow have been identified on these clusters. Our results show that CO mostly prefers the bridge or top site, except on the Rh4 and Rh11 clusters where it prefers hollow sites. Rh6 demonstrates two different active sites of almost equal energies for CO adsorption. Highly stable clusters show weak CO adsorption behavior. We also observe that the magnetic moment of the clusters is usually reduced after the CO adsorption. The preference of the active sites for CO adsorption has been analyzed using the charge density difference plots.