Heterogeneous Catalysts

Care should be taken not to confuse zeolites with well‐ordered mesoporous catalysts (e.g., MCM‐41, SBA‐15, KIT‐6), which belong to a different class of porous materials and, by definition, consist of pores in the range of 2–50 nm. The MCM‐41 (tunable pore size of 2–9 nm) and SBA‐15 (tunable pore size of 5–10 nm), discovered by Charles T. Kresge et al. at the Mobil Oil Corporation in 1992 [45] and Galen D. Stucky and coworkers at the University of Santa Barbara in 1998 [46], respectively, are arguably the gold standards for this class of catalytic materials. These mesoporous catalysts are templated through the addition of bulky micelles such as those formed by cetyltrimethylammonium bromide (CTAB) surfactant and Pluronic P123 triblock copolymer, and sol–gel silica particles will precipitate in between these self‐assembled soft templates. Because the micelles serve as long‐range structural templates (and none at short range like those used for the synthesis of zeolites), well‐ordered mesopores can be obtained, but the silica walls are basically amorphous. These glassy walls are catalytically inactive, in stark contrast with the crystalline walls of zeolites. Nevertheless, the mesoporous materials are attractive as high‐surface‐area supports with mesoporous channels large enough for the deposition of a wide range of active metals without pore blocking and at the same time accessible to bulky reactant molecules that otherwise could not penetrate the zeolite micropores. Because there is no requirement for short‐range ordering, these surfactant templates can be flexibly used to fabricate a plethora of other mesoporous metal oxides including TiO 2, WO 3, and Al 2O 3. Furthermore, the mesoporous silica can be used as hard templates for the synthesis of mesoporous carbon and metal oxide nanorods [47]. An area that is actively being pursued is the synthesis of hierarchical zeolites, where mesoporous channels are introduced in zeolites, in such a way that the wall of the mesoporous catalyst is no longer amorphous silica but that of catalytically active, microporous crystalline silicate. This allows the accessibility of acid sites by large reactant molecules while overcoming the mass diffusion limitation associated with the narrow micropores of zeolites during catalytic reactions. More details on the design and synthesis of such hybrid micro‐/mesoporous catalysts are presented in Chapter 7.

Metal–organic framework (MOF) is a term first coined by Omar Yaghi in 1995 to describe a class of crystalline porous solids formed by a continuous network of multivalent metal cations/clusters and organic linkers of at least two coordination positions [48]. It is analogous to the zeolites, except with different set of building blocks. The elegance of MOFs arises from the simplicity of the template‐free synthesis, and the micropore size can be easily tunable by adjusting the length of the organic linker. A classic example is the fabrication of UiO‐66 that involves the simple hydrothermal reaction between zirconyl chloride and 1,4‐benzenedicarboxylic acid (BDC) linker. By replacing the BDC with a longer 1,4‐biphenyldicarboxylic acid (BPDC), one can obtain UiO‐67 and an extension of the pore size from 7.5 and 12 Å to 12 and 16 Å, respectively. In fact, the design of MOFs is so flexible that it can be extended to fabricate mesoporous catalysts by manipulation of the linkers or using SDAs [49]. The catalytic active sites of MOFs may originate from the active metal atoms or compounds covalently functionalized on the linkers or the framework metal cation centers if made coordinatively unsaturated (without affecting the rigidity of the MOF structure). An elegant account on the different strategies in designing MOF catalysts can be found in Chapter 8. To date, MOFs find wide applications in organic synthesis, biomass conversion, photocatalysis, and electrocatalysis, among others. Because of their organic frameworks, MOFs are normally used in mid‐ to low‐temperature applications below 500 °C. A more recent sister class of compound is the covalent organic frameworks (COFs), first discovered by Yaghi in 2005, that are built entirely based on nonmetal centers [50]. In their pristine forms, some COFs are effective in catalyzing photocatalytic and electrocatalytic reactions, while their tunable porous structures can also be functionalized with the desired metal catalysts similar that of the mesoporous silica structure to catalyze a wider range of reactions, e.g., the Suzuki–Miyaura coupling reaction when deposited with the Pd 2+single‐atom catalyst.

The synthesis of porous anisotropic catalysts received significant interests since 2005 or so, especially for photocatalytic reactions such as solar water splitting, abatement of environmental pollutants, and CO 2reduction. Photocatalysts are composed of semiconductor materials, that is, they can photoexcited with photons equal to or larger than their bandgaps to produce usable charges for surface redox reactions. Photocatalytic reactions can be carried out in two ways: particulate photocatalysis where the redox reactions as mediated by the electron–hole pairs take place on the same photocatalyst particle/aggregate (see Chapter 11on the art of photocatalysts design) and photoelectrocatalysis where the photocatalyst is made into a photoelectrode and connected with a counter electrode in such a way that the electron–hole pairs are separated across the two electrodes (see Chapter 36on the basics of photoelectrocatalysis) [51]. One‐dimensional (1D) photocatalysts such as nanorod and nanotube arrays are particularly attractive to capitalize on the high surface‐to‐bulk ratio as well as the much sought‐after vectorial charge transport for efficient photocharge separation during photoelectrocatalytic reactions. A variety of synthesis techniques to obtain such structures have been developed, ranging from chemical vapor deposition, spray pyrolysis, and hydro/solvothermal synthesis to electrochemical anodization, producing efficient anisotropic photocatalysts of TiO 2nanotubes, WO 3nanosheets, Nb 2O 5nanorods, Ta 2O 5nanotubes, α ‐Fe 2O 3nanotubes, etc. The electrochemical synthesis of these fascinating array photocatalysts can be found in Chapter 3. In recent years, the interest has expanded to two‐dimensional (2D) photocatalysts such as the graphitic carbon nitride, molybdenum disulfide, tungsten disulfide, and MXenes. Besides maximizing the surface‐to‐bulk ratio, these materials exhibit unique quantum electronic properties seen only when made into atomic‐thin layers [52].

A core criterion in the design of catalysts is to maximize the active metal dispersion (the ratio of surface atom to bulk), such that the highest reactivity per amount of metal loading on the catalyst can be achieved. This is especially relevant when precious metals are used, which is indeed the case for a large number of catalytic reactions. In the abovementioned historical overview, the strategies for maximizing dispersions include making metal sheets into metal sponges and gauzes of fine wires, as well as depositing active metals onto high‐surface‐area supports to make very fine deposits or thin atomic layers.

The synthesis of supported catalysts capitalizes on the strong interfacial interactions between the active metal and the (usually oxide) support to allow the former to exist as stable and small size deposits. Without the strong interfacial interactions, the initially small deposits tend to diffuse on the support surface and coalesce with another deposit until its surface energy (i.e., a function of surface area) decreases to that of the interfacial energy. Incipient wetness impregnation is by far the most common procedure for the preparation of supported catalysts, where metal precursor solution is drawn into the pores of the support by means of capillary effect. To prevent overflowing of the solution to the external surface of the support, the solution volume introduced in each impregnation step should not exceed that of the pore volume (typically maintained at 80–90%). More liquid solution can be introduced repeatedly upon complete drying of the liquid solvent, leaving behind more metal salt within the pore during each repetition. The advantage of the incipient wetness impregnation is that it does not require very strong interactions between the oxide support surface and the coordinated metal cation from the precursor to reach the desired loading amount. On the contrary, wet impregnation is when the porous support is immersed in the metal precursor solution and the amount that penetrates the pores depends on the metal precursor–support interactions. If the interaction is strong, the impregnated concentration would be higher than that of the bulk, and vice versa. Further drying to remove the solvent from the pores and calcination yield the supported catalysts in both cases of impregnation. To minimize coalescence between the metal deposits during the calcination step, it is essential to remove moisture and oxygen by flowing inert gas and introducing a small amount of NO, respectively [53]. Other techniques such as deposition–precipitation, chemical vapor deposition, and the one‐step flame synthesis (see Chapter 10) have also become popular alternatives for producing supported metal catalysts. The ability to produce small Pt deposits on carbon support has been one of the major breakthroughs that led to popularity of low‐temperature H 2‐polymer electrolyte membrane (PEM) fuel cell. In fact, the amount of the ∼3.5 nm Pt used is so small (0.2 mg/cm 2of fuel cell, compared with 28 mg/cm 2in the early days) that it significantly reduced the device cost and thus popularising the H 2‐PEM fuel cell [54]. Chapter 32introduces the design of electrocatalysts for PEM fuel cell applications, while Chapter 4complements nicely the strategies of using carbon supports for such purpose.