Cell Systems Explained, 2e. Wiley.55 55 Hutchings, G.J. (1985). Vapor phase hydrodechlorination of acetylene: correlation of catalytic activity of supported metal chloride catalysts. J. Catal. 96: 292.
56 56 Haruta, M., Kobayashi, T., Sano, H., and Yamada, N. (1987). Novel gold catalysts for the oxidation of carbon monoxide at a temperature far below 0 °C. Chem. Lett. 16: 405.
57 57 Chen, M.S. and Goodman, D.W. (2004). The structure of catalytically active gold on titania. Science 306: 252.
58 58 Okazaki, K., Ichikawa, S., Maeda, Y. et al. (2005). Electronic structures of Au supported on TiO2. Appl. Catal., A 291: 45.
59 59 Bezemer, G.L., Bitter, J.H., Kuipers, H.P.C.E. et al. (2006). Cobalt particle size effects in the Fischer–Tropsch reaction studied with carbon nanofiber supported catalysts. J. Am. Chem. Soc. 128: 3956.
60 60 Li, Y., Boone, E., and El‐Sayed, M.A. (2002). Size effects of PVP−Pd nanoparticles on the catalytic Suzuki reactions in aqueous solution. Langmuir 18: 4921.
61 61 Zhu, J., Yang, M.‐L., Yu, Y. et al. (2015). Size‐dependent reaction mechanism and kinetics for propane dehydrogenation over Pt catalysts. ACS Catal. 5: 6310.
62 62 Murray, R.W. (2008). Nanoelectrochemistry: metal nanoparticles, nanoelectrodes, and nanopores. Chem. Rev. 108: 2688.
63 63 Xu, F., Chen, J., Kalytchuk, S. et al. (2017). Supported gold clusters as effective and reusable photocatalysts for the abatement of endocrine‐disrupting chemicals under visible light. J. Catal. 354: 1.
64 64 Weng, B., Lu, K.‐Q., Tang, Z. et al. (2018). Stabilizing ultrasmall Au clusters for enhanced photoredox catalysis. Nat. Commun. 9: 1543.
65 65 Maschmeyer, T., Rey, F., Sankar, G., and Thomas, J.M. (1995). Heterogeneous catalysts obtained by grafting metallocene complexes onto mesoporous silica. Nature 378: 159.
66 66 Samantaray, M.K., D'Elia, V., Pump, E. et al. (2019). The comparison between single atom catalysis and surface organometallic catalysis. Chem. Rev. https://doi.org/10.1021/acs.chemrev.9b00238.
67 67 Yang, X.‐F., Wang, A., Qiao, B. et al. (2013). Single‐atom catalysts: a new frontier in heterogeneous catalysis. Acc. Chem. Res. 46: 1740.
68 68 Liu, L. and Corma, A. (2018). Metal catalysts for heterogeneous catalysis: from single atoms to nanoclusters and nanoparticles. Chem. Rev. 118: 4981–5079.
69 69 Kim, W., Edri, E., and Frei, H. (2016). Hierarchical inorganic assemblies for artificial photosynthesis. Acc. Chem. Res. 49: 1634.
2 Facets Engineering on Catalysts
Jian (Jeffery) Pan
The University of New South Wales, School of Chemical Engineering, Department of Particles and Catalysis Research Group, Sydney, NSW, 2052, Australia
2.1 Introduction
Crystal facets engineering has become one of the most effective strategies to enhance performance of nanomaterials in many applications, such as heterogeneous catalysis, gas or liquid sensing, photocatalysis, electrocatalysis, fuel cell, solar cell, and lithium‐ion batteries [1–4]. The reactions occurred at the surface or interface of nanomaterials are extremely sensitive to the exposed surface atomic structures and their respective physical and chemical properties. The well‐defined crystal surface has unusual properties compared to the bulk, due to the termination of periodic crystal lattices. The various properties of different facets of a single crystal are attributed to crystal anisotropy. As a consequence, the whole behavior of a faceted nanomaterial would be dramatically affected by the surface, especially when the particle size shrinks to a nanoscale and the surface/bulk atomic ratio can no longer be negligible.
This chapter is mainly focused on faceted single crystals of metals and semiconductors in heterogeneous catalysis. Metal catalysts are mainly used in thermal catalysis and electrocatalysis, such as producing chemicals, petroleum refining, and in fuel cells. Studies of metal catalyst surfaces are comprehensive and began much earlier than that of semiconductors in photo‐related catalysis. In the past two decades, significant progress has been made to synthesize metal nanocrystals in a variety of shapes and enhanced performance [5]. Although the miniaturization of catalysts to single (metal) atom catalysts and metal cluster catalysts have become popular in recent years due to the strong desire to attain 100% atom utilization efficiency especially when using some of the least abundant elements [6, 7], the advantage of facets engineering of metal catalysts continues to be an essential and indispensable tool. For example, in photocatalysis where optical absorption is a bulk property that determines the rate of charge carriers generation, it is desirable to apply facets engineering to the bulk crystals to enhance surface charge transfer efficiencies.
Being the core fundamental of heterogeneous catalysis, the studies of surface reactivities have long been appreciated by the community. However, it is only in the last two decades that the tremendous development in the synthesis and characterization of crystal facets further paved the way for significant achievements of facets engineering in catalytic applications. As such, this chapter briefly introduces the mechanisms of facets engineering, the anisotropic properties of crystal facets, and the effects of facets engineering.
2.2 Mechanisms of Facets Engineering
The faceted nanocrystals, whether metals or semiconductors, can be achieved through many synthesis methods, including solution‐phase, vapor‐phase, and solid‐phase methods. The solid‐phase methods include gas oxidation route, topotactic transformation method, and crystallization transformation method [8, 9]; the vapor‐phase methods include thermal decomposition method, metal–organic chemical vapor deposition [10, 11]; and the solution‐phase methods are more powerful and versatile than others. It includes basic wet chemical route, sol–gel method, hydrothermal [12, 13] and solvothermal [14] methods, microwave treatments [15, 16], electrochemical [17] and photochemical [18, 19] methods. It should be noted that the synthesis of metal crystals is quite different from semiconductor crystals, although they sometimes share the same methods. Metal crystal catalysts are composed of single or binary metal atoms. Their synthesis process, including the steps from ions to nuclei or cluster, to seed, and then to nanocrystals, might be far different from the synthesis of semiconductors, which are composed of metal and nonmetal elements. However, no matter metals or semiconductors, and regardless of the synthesis methods, the one common feature is the spontaneous reaction that dictates the crystal formation is under thermodynamic control.
The excess energy at the surface of a material compared with the bulk is defined as the surface energy. Without facets engineering, the most stable free surface will occupy the greatest portion of the crystal surface as it attempts to minimize the total surface energy in a given growth condition. According to the Gibbs–Wulff theorem, the facets with higher surface energies always grow faster and end up with a tiny fraction or even vanish [20]. The Wulff construction method predicts the shape of a crystal with the lowest surface energy, and the most stable form is called the equilibrium shape of a crystal. Owing to the anisotropic surface energy of crystal facets, the final shape of a single crystal is usually enclosed with the facets of lowest surface energy and smallest surface area in a given volume. For example, for a metal crystal that contains NA atoms and each bulk (i.e., nonsurface) atom that is defined by a coordination number of CN, there will be (NA × CN)/2 bonds in the crystal. As such, the energy of each bond can be calculated as
