Suib New and Future Developments in Catalysis

Catalysis by Nanoparticles
1. Auflage 2013
ISBN: 978-0-444-53875-8
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 512 Seiten

New and Future Developments in Catalysis is a package of seven books that compile the latest ideas concerning alternate and renewable energy sources and the role that catalysis plays in converting new renewable feedstock into biofuels and biochemicals. Both homogeneous and heterogeneous catalysts and catalytic processes will be discussed in a unified and comprehensive approach. There will be extensive cross-referencing within all volumes.The use of catalysts in the nanoscale offers various advantages (increased efficiency and less byproducts), and these are discussed in this volume along with the various catalytic processes using nanoparticles. However, this is not without any risks and the safety aspects and effects on humans and the environment are still unknown. The present data as well as future needs are all part of this volume along with the economics involved. - Offers in-depth coverage of all catalytic topics of current interest and outlines future challenges and research areas - A clear and visual description of all parameters and conditions, enabling the reader to draw conclusions for a particular case - Outlines the catalytic processes applicable to energy generation and design of green processes

Autoren/Hrsg.

Suib, Steven L

Leseproben

Chapter 2 Structural and Electronic Properties of Group 6 Transition Metal Oxide Clusters
Shenggang Li and and David A. Dixon,     Chemistry Department, The University of Alabama, Shelby Hall, Box 870336, Tuscaloosa, AL 35487-0336, USA, [e-mail] Acknowledgments
This work was supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, US Department of Energy (DOE) under Grant No. DE-FG02-03ER15481 (catalysis center program) and was performed, in part, in the W.R. Wiley Environmental Molecular Sciences Laboratory including the Molecular Science Computing Facility, a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory, operated for DOE by Battelle. D.A.Dixon thanks the Robert Ramsay Chair Fund of the University of Alabama and Argonne National Laboratory for support. 2.1 Introduction
Early transition metal oxides (TMOs) have a broad range of technological applications [1]. They form a class of important catalyst materials due to their rich acid-base and redox chemistries [2–6]. Examples of the redox reactions catalyzed by TMOs are the selective oxidations of alkanes, alkenes, and alcohols for feedstocks for the chemical industry [7–11]. Many TMOs are semiconductors with sizable band gaps (MoO3: 3.2 eV; WO3: 2.6 eV) [12]. They have been extensively studied for their photocatalytic activities toward H2O splitting and CO2 conversion for the efficient utilization of solar energy [13], especially for TiO2 (band gap: 3.1 eV) and its mixed or doped oxides [14–23]. In addition, CrO2 is widely used in magnetic recording systems [24–26]. ZrO2 and especially HfO2 have recently been used to replace the SiO2 gate dielectric due to their high dielectric constant (high-?) in order to reduce current leakage as transistor size continues to shrink [27,28]. Metal oxides can also be used as solid state gas sensors [29]. Because of the interest in TMOs for heterogenous processes, the properties of gas phase TMO clusters, which have well-defined structures (atomic connectivity with controllable stoichiometries) are being studied to gain insight into the complex catalytic processes occurring on surfaces [30]. TMO clusters have been used as models of actual catalysts, and, in some cases such as the polyoxometallates [31–33], are the actual catalysts. The focus of the current chapter is the properties of small TMO clusters of the Group 6 transition metals. A key to understanding these clusters has been the interplay between theory and experiment. Most of the free clusters have been observed in matrices or as ions in the gas phase. They have been characterized by mass spectroscopy, photoelectron spectroscopy, and infra-red spectroscopy, all of which do not provide structural details. Electronic structure calculations often at the density functional theory (DFT) level [34] have been used to predict the molecular structures and energetics. The structures have been checked by comparing the predicted spectroscopic properties with experiment. Catalytic acid-base and redox chemistries account for over 90% of practical applications of metal oxides as catalysts. Thus, knowledge of acid-base properties is important in developing a fundamental understanding of TMO catalyzed reactions. A key reaction to generate acidic sites is the hydrolysis of TMO species. The gas phase molecular acidity, a well-studied property from both theory and experiment [35], is given by the free energy of Reaction (2.1). (2.1) The gas phase basicity is defined as the negative of the free energy of Reaction (2.2). (2.2) The temperature for these definitions is usually taken as 298 K. As an example, there is an excellent correlation between the calculated gas phase acidities at the DFT level and the solution phase acidity constants for the first row transition metal and inorganic oxyacids [36,37]. The very strong gas phase acidity scale has been corrected on the basis of high level ab initio calculations using correlated molecular orbital theory at the G3MP2 [38] and coupled cluster theory [39–42] CCSD(T)/CBS (complete basis set) levels [43]. Self consistent reaction field (SCRF) approaches [44], for example with the conductor-like screening model (COSMO) [45], can be used to predict the pKa values of acids in aqueous and other solutions to connect the gas phase values with those of the condensed phase. A quantitative Lewis acidity scale based on calculated fluoride affinities (FAs) where the FA is defined as the negative of the enthalpy of Reaction (2.3) (2.3) has been developed [46,47]. Due to its high basicity and small size, the fluoride anion Lewis base reacts with essentially all Lewis acids. This chapter describes the use of a variety of computational electronic structure methods to predict the properties and reactions of Group 6 TMOs. The first section deals with the accurate calculation of the thermodynamic properties of TMO compounds for the Group 4 and 6 metals. Subsequent sections deal with specific examples of the properties of the Group 6 TMO clusters. 2.2 Accurate Thermochemistry for Transition Metal Oxide Clusters
Extensive studies have shown that one must take care in the calculation of the geometries and vibrational frequencies of transition metal compounds with ab initio molecular orbital (MO) methods. In general, one needs to use some type of correlation treatment, often beyond the second-order Møller-Plesset perturbation theory (MP2) level. The DFT method, with a variety of exchange-correlation functionals from local to gradient-corrected to hybrid ones, yields good predictions of the structures and frequencies of transition metal complexes at a fraction of the computational cost of correlated MO methods [48]. The B3LYP [49,50] hybrid exchange-correlation functional provides reasonable structures and energies especially for M = Mo and W [51–57]. Care must be taken when using hybrid functionals which contain a certain amount of Hartree-Fock exchange in calculations of compounds containing first row transition metals as Hartree-Fock has significant difficulties with these metals. The high atomic numbers for the second and especially the third row transition metals imply that relativistic effects must be properly included to attain even semi-quantitative accuracy [58,59]. For most of the properties of interest, the core electrons are chemically inert so one can eliminate the core contributions from direct consideration by using pseudopotentials (PPs) or effective core potentials (ECPs) [58,60–62]. One can obtain excellent results for the structures and frequencies of a broad range of transition metal complexes using ECPs and DFT [63–65]. A challenge for DFT is the choice of a exchange-correlation functional suitable for the problem of interest [66], and a range of exchanged-correlation functionals for the DFT method have been benchmarked for various applications [67], which can help one to choose the appropriate functional for the problem of interest. The accurate prediction of cluster thermodynamic properties and reaction energies is critical to the successful modeling of the behavior of TMO catalysts. An important thermodynamic property needed for any reaction is the heat of formation. Although heats of formation for compounds of main group elements can now be somewhat routinely predicted with high accuracy [68–72], for example, with composite methods including the Gaussian-N (Gn) methods [73–76], the Weizmann-N (W-n) methods [77–79], the complete basis set (CBS) methods [80], the Feller-Peterson-Dixon (FPD) approach developed at Washington State University and The University of Alabama [68,81,82], and the HEAT method [83,84], such approaches have not been as widely used for transition metal compounds. One of the reasons is that these methods were developed for compounds of main group elements. This is evident from the test set for the Gaussian-N methods [85] and the latest G4 method [76], which are based on energetics of compounds involving the first three main row group elements. The exclusion of the compounds of the heavier main group elements and especially those of the transition elements is partially due to the lack of accurate experimental data. Very recently, the correlation consistent composite approach (ccCA) [86–89] has been tested for the calculation of heats of formation for a set of first row transition metal compounds [90–92]. The ccCA method is an approximation to the FPD approach. The FPD approach and the ccCA approach were found to have mean absolute deviations of 3.1 and 3.4 kcal/mol, respectively, which are close to the initial goal of “chemical accuracy” of ±3 kcal/mol for transition metal model chemistries defined by the ccCA group [90]. This differs from the definition of chemical accuracy of ±1 kcal/mol usually used for the predicted heats of formation of main group compounds. Coupled cluster methods, especially CCSD(T), have...