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CHAPTER 1
Introduction to the Application of Nitrides, Carbides, Phosphides and Amorphous Boron Alloys in Catalysis
KEVIN J. SMITH
Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, BC V6T 1Z3, Canada
Email:
[email protected]1.1 Introduction
Catalysts are used to enhance the rates of thermocatalytic, electrocatalytic and photocatalytic reactions.1 Catalysts function by interacting with reacting species to generate new reaction intermediates that only exist on the catalyst surface, thereby providing alternative, faster reaction pathways to the desired products. Key to the catalytic reaction is the breaking and forming of new bonds between the catalyst and the reacting molecules. Consequently, the efficacy of a particular catalyst is determined by its surface chemistry and new developments in catalysis science and technology are driven in part by the development of new materials with well controlled surface properties. Increasing attention has been given to the discovery of heterogeneous catalysts that have high thermal and chemical stability, using materials that are not strategically limited and are of low cost.2 Catalysts are used in a wide range of applications including motor vehicle emissions control, upgrading and refining of crude oils, and they are responsible for approximately 90% of the chemical processes currently operating world-wide in the chemical industry.1,3
Controlling the reactivity of a catalyst surface plays a key role in obtaining catalysts with high activity and selectivity. Sabatier's principle indicates that the interactions between reacting species and a catalyst surface must be of the appropriate strength—interactions that are too strong or too weak yield less active catalysts.1 Hence, in the classic paper of Levy and Boudart,4 C added to W to produce W2C, was shown to decrease the high reactivity of W toward O and thereby yield an effective catalyst for the H2+O2→H2O reaction at room temperature, a reaction typically catalysed by Pt. Similarly, the W2C was active for the isomerisation of 2,2-dimethylpropane to 2-methylbutane, a reaction also catalysed by Pt.4 This work spawned a large number of studies in which new materials with metallic character, formed by the incorporation of C, N, P and B into the lattices of early transition metals, have been investigated as potential catalysts for various reactions.5–7 These interstitial alloys adopt simple crystal structures with the carbides and nitrides forming face-centered cubic (fcc), hexagonal close packed (hcp) or simple hexagonal (hex) structures, as summarised in Table 1.1. Many metal phosphides are also known7 and their crystal structures as shown in Figure 1.1. The very rich chemistry of these interstitial alloys provides an opportunity for the development of new catalysts with a wide potential for application, especially with the synthesis of these materials as well dispersed nanoparticles.5
Table 1.1 Common Crystal Structures of Selected Carbides and Nitrides. Adapted from ref. 5 and 8.
| Crystal structure | Compound |
| Fcc | TiC, ZrC, HfC, VC, NbC, and TaC |
| TiN, VN, NbN |
| γ-Mo2N, β-W2N, Re2N |
| Hcp | β-Mo2N, W2C, Re2C |
| Hexagonal | WC, MoC, δ-WN |
Figure 1.1 Crystal structures of metal-rich phosphides.9
Reprinted from Catal. Today,143, S. T. Oyama, T. Gott, H. Zhao and Y. Lee, Transition metal phosphide hydroprocessing catalysts: A review, 94–107. Copyright (2009) with permission from Elsevier.
The unique catalytic behaviour of metals bound to C, N, P or S may be attributed to the changes in the electronic properties of the metal surface induced by the ligands and/or the geometry by which the metal and the ligands are arranged at the catalyst surface. Hence, the ligands passivate the metal surface reactivity and these effects have been quantified in some cases using molecular simulation.10 For example, Liu and Rodriguez11 have shown, by DFT calculation, that the CO adsorption and S adsorption energies on the (001) surfaces of MoN, MoC, MoP and Mo decrease with an increase in their d-band centre energy, as shown in Figure 1.2.
Figure 1.2 Calculated adsorption energy of CO (g) and S (l) as a function of the d-band centre for Mo in clean surfaces of Mo(001), γ-MoC(001), δ-MoN(001) and MoP(001). Here, the d-band centres are relative to the Fermi energy.11 P. Liu and J. A. Rodriguez, Catalysis Letters, 2003, 91, 247–252. With permission of Springer.
These observations suggest the possibility that metal surface reactivity can be controlled and tuned using interstitial atoms.12 Consequently, there exist several detailed reviews describing the incorporation of C,5,13 N,6,14 P7,9 and B15 into metals and the use of the resulting interstitial alloys for catalysis, with varying degrees of success. Since, in nearly all cases, the interstitial atoms are incorporate...
