Catalysts, electronic structure calculations

Keith Gubbins I want to add a comment about ab initio calculations I m certainly not an expert in that, but there is a lot of industrial interest now in combining electronic structure calculations with large-scale simulations. There was a conference on this in Britain in January that I went to, called Industrial Applications of Computer Simulation, and I was surprised to see many more people from industry than academia. As a result of the industrial interest, their next conference in January will get together the electronic structure people with the simulators, but I wouldn t hazard a guess as to when this would be sufficiently practical, say, to design a new catalyst. Whether we re close to that in the next five or ten years, I m not really certain. 

The aim of this chapter is to provide the reader with an overview of the potential of modern computational chemistry in studying catalytic and electro-catalytic reactions. This will take us from state-of-the-art electronic structure calculations of metal-adsorbate interactions, through (ab initio) molecular dynamics simulations of solvent effects in electrode reactions, to lattice-gas-based Monte Carlo simulations of surface reactions taking place on catalyst surfaces. Rather than extensively discussing all the different types of studies that have been carried out, we focus on what we believe to be a few representative examples. We also point out the more general theory principles to be drawn from these studies, as well as refer to some of the relevant experimental literature that supports these conclusions. Examples are primarily taken from our own work other recent review papers, mainly focused on gas-phase catalysis, can be found in [1-3]. 

Since all the quantities in Eqs (2.86)-(2.88) can be obtained from electronic structure calculations and the exploration of the potential energy surface for a given transformation is also feasible, it is possible to obtain in silico kinetic data both in the gas phase and in solution. In addition, it is possible to test mechanistic hypotheses and compare the computational results with the experimental outcome in order to understand the observed behavior and, at least in favorable cases, to predict the behavior of untested reactants or catalysts. 

In previous years, platinum has been widely used as electrocatalyst for the ORR because it is the most efficient pure catalyst material. However, its scarcity and associated high cost motivated the development of alloy materials to reduce the load of Pt. Moreover, the ORR kinetics is slow on pure Pt catalysts, requiring a high overpotential due to the presence of oxygen and hydroxyl strongly bonded to the surface as it was determined by electronic structure calculations [18]. Therefore, there is a need for catalysts with low cost and enhanced ORR activity to replace pure Pt. 

Electronic structure calculations on the isomerization and epimerization of xylose to xylulose and lyxose by a zeolite Lewis acid catalyst suggest lyxose is formed from a stable intermediate and that xylulose is thermodynamically and kinetically favoured... 

Thompson, D. and Hodnett, B. (2008). Hydrocarbon Selective Oxidation on Vanadium Phosphorus Oxide Catalysts Insights from Electronic Structure Calculations, Top. Catal, 50, pp. 116-123. 

Thanks to a plethora of efforts in theory and experiment, the ORR has lost some of its enigmatic appearance. Especially, electronic structure calculations at the level of DFT have brought about tremendous advances in understanding of surface electrochemical processes at metallic catalysts. In this section, the following questions will be explored Does a cohesive picture of the ORR pathway and mechanisms exist Is the significance of electronic structure effects, formation of surface-adsorbed reaction intermediates, and kinetic limitations for the overall process understood ... 

Bis(imino)pyridine iron complex 5 as a highly efficient catalyst for a hydrogenation reaction was synthesized by Chirik and coworkers in 2004 [27]. Complex 5 looks like a Fe(0) complex, but detailed investigations into the electronic structure of 5 by metrical data, Mossbauer parameters, infrared and NMR spectroscopy, and DFT calculations established the Fe(ll) complex described as 5 in Fig. 2 to be the higher populated species [28]. 

Coloma F, Marquez F, Rochester CH, Anderson JA (2000) Determination of the nature and reactivity of copper sites in Cu-Ti02 catalysts. Phys Chem Chem Phys 2 5320-5327 Umebayashi T, Yamaki T, Itoh H, Asai K (2002) Analysis of electronic structures of 3d transition metal-doped Ti02 based on band calculations. J Phys Chem Solids 63 1909-1920 Yamashita H, Ichihashi Y, Takeuchi M, Kishiguchi S, Anpo M (1999) Characterization of metal ion-implanted titanium... 

Electronic structure theory has developed to a point where realistic bond energies and activation barriers can be calculated. Typically the model catalysts used in such calculations are even more idealized than in the surface science experiments (perfect surfaces, ordered overlayers etc.), but the insight into the details of the potential energy surface of the reaction is much greater. 

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