Heterogeneous Catalysts

Besides maximizing the metal dispersions, further miniaturization of metal deposits to or approaching the quantum‐related level can result in altered electronic properties not otherwise seen in larger particles. Gold catalysis is an intriguing example of such a phenomenon, which was led notably by the independent efforts of Graham J. Hutchings and Masatake Haruta since the mid‐1980s. They showed that gold, which was classically believed to be almost inactive, can be made extremely active in the hydrochlorination of acetylene [55] and the oxidation of carbon monoxide (at −77 °C!) [56], respectively, when made less than 25 nm. The latter, which gold size was 4.5 ± 1.6 nm, was first prepared by the coprecipitation technique but was later superseded by the deposition–precipitation technique in which dissolved gold precursor was precipitated by raising the pH of the medium in the presence of suspended oxide support. Over time, the commercial flame‐synthesized P25 TiO 2became the preferred support. Many new reactions by gold catalysis followed in the next three decades, ranging from the oxidation of aqueous polyalcohols to carboxylic acids, selective oxidation of cyclohexane to cyclohexanol and cyclohexanone, epoxidation of propylene, water‐gas shift, to the selective hydrogenation of 3‐nitrobenzene and the hydrogenation of alkynes to alkenes. Size‐dependent turnover frequencies (i.e., conversion rate per active site) is typically observed due in part to the variation of electronic interactions, with the optimum gold deposit size for CO oxidation in the range of 2–4 nm [57, 58]. The size‐dependent activity is a general phenomenon as observed readily on different metal deposits including cobalt for FTS [59], palladium for Suzuki coupling [60], and platinum for propane dehydrogenation [61].

Synthesizing ultrasmall size deposits of less than 2 nm (<100 atoms), or so‐called metal clusters (or nanoclusters as a more appealing terminology), can be quite challenging because of their high surface energies. At such a size, the surface energy can become so overwhelming that even when deposited onto high‐surface‐area supports, the metal deposits prefer to exist as larger sizes so as to minimize the exposed surface area (and hence the total energy). In such cases, stabilizing ligands such as glutathione, cetyl trimethyl ammonium bromide (CTAB), and poly(vinylpyrrolidone) (PVP) that bind to the surface of the small deposits can be added during the synthesis procedure. With the ligands being exposed and having lower surface energy than the bare metals, they protect the metal clusters from coalescing or dissolving. Metal clusters are so called not just to distinguish them from the larger nanosized particles, but importantly they reach a state where they no longer behave like metals. As a result of the size quantization effect that gives rise to the discrete orbitals and formation of an energy gap, they essentially behave more as semiconductors. The effect is not unlike the size quantization phenomenon commonly observed for semiconductor photocatalysts with the diameter smaller than the Bohr excitonic radius. Although the term quantum dot (commonly abbreviated as Q‐dot) refers exclusively to such semiconductor particles, by the same definition, metal clusters should also be termed quantum metals! [62]. In that respect, Chapter 5readily lays out the physics as well as the design principles of different metal clusters catalysts. Metal clusters, with or without ligand bound, have been shown to exhibit catalytic properties different from that of larger nanoparticles, for example, the highly selective oxidation of cyclohexane to cyclohexanone over Ag 6/graphene oxide, 100% selectivity of 4‐nitrobenzaldehyde to 4‐nitrobenzyl alcohol over Au 99(SPh) 42/CeO 2, and electrocatalytic reduction of carbon dioxide to carboxylic acid Cu 32H 20L 12(L = dithiophosphate ligand). Because of the semiconductor nature of metal clusters, they can even function as photocatalysts, for example, in the photocatalytic degradation of aqueous organic micropollutants over glutathione‐protected gold clusters [63, 64].

Single‐atom catalysts (SACs) represent the ultimate extreme end of catalyst miniaturization. First demonstrated by John Meurig Thomas of the Davy‐Faraday Research Laboratory in 1988 by surface grafting Ti (from titanocene) onto the silanol sites of mesopores of MCM‐41, the SAC of Ti showed high catalytic oxidation activities, albeit, rather short‐lived [65]. Other forms of SACs include ion‐exchanged zeolites or mesoporous silica, unsaturated framework metal sites, coordinated metal ions around the pyridinic sites graphene or carbon nitride, unsaturated metal centers in MOFs, supported organometallics, and isolated surface‐exposed metal atoms dispersed in the form of alloy or supported on metal oxide. The physical criterion of SACs requires the catalytic site (usually referring to a metal atom) exists in full isolation from another metal atom of the same type. The ability of SAC to function in silo as a catalytic site reminisces that of freestanding homogeneous organometallic catalyst. In that sense, catalysis on SACs is often touted as a convergence of heterogeneous and homogenous catalyses [66]. Although the research on SACs has progressed reasonably since the report by Thomas, explicit interest picked up substantially since the mid‐2010s in conjunction with the advancement of aberration‐corrected high‐angle annular dark field scanning transmission electron microscopy (HAADF‐STEM) that enabled the direct visualization and interrogation of SACs. Producing SAC sites is particularly meaningful for precious but otherwise highly active metals (e.g. Pt, Ir, Rh, Ru, Os) since the unity dispersion means that every single atom is on the surface and thus can be utilized for reactions. Elegant account on the synthesis of SACs is well covered in Chapter 6, while the carbon‐supported SACs are partly covered in Chapter 4. Compared with the metal clusters and their nanoparticle counterparts, SACs tend to have lower coordination numbers (i.e., less neighboring atoms that are directly bonded with the SAC), although the extent of which depends greatly on their syntheses. At the same time, the effects of quantum confinement and metal–support interactions are amplified in SACs [67]. These effects contributed to the unique catalytic properties of SACs as demonstrated for CO oxidation, water‐gas shift reaction, photocatalytic hydrogen evolution, electrochemical oxygen reduction reaction, etc. [68]. Intriguingly, by using a similar surface grafting technique as that reported by Thomas but using paired heteroatoms, e.g. Co 2+/Zr 4+, Cu +/Zr 4+, Cu +/Ti 4+, Cr 3+/Ti 4+, and Co 2+/Ti 4+on MCM‐41, Heinz Frei created a class of new photocatalytic system based on metal‐to‐metal charge transfer (MMCT). In MMCT, each binuclear unit is composed of a photoexcitable donor cation and an acceptor cation connected by an oxo bridge, e.g., Co 2+–O–Zr 4++ hv (light) → Co 3+–O–Zr 3+. When loaded with cocatalysts such as Pt, Cu, and IrO 2clusters, these MMCT systems show extremely high activities in water splitting and carbon dioxide reduction under visible‐light activations [69].

In the course of 120 years since the discovery of modern heterogeneous catalysis, almost the entire physical length scale of catalyst design with compositions across the periodic table has been explored. From the use of bulk metals in the form of wires and metal strips during the Faraday era to metal nanoparticles and metal clusters and all the way to SACs, the primary objective has always been to identify the most active, selective, and durable catalysts and at the same time economically feasible (not necessarily low cost) and environmentally benign ones. While the classical techniques for catalyst preparation such as impregnation, precipitation, sol–gel, hydro/solvothermal syntheses, and solid‐state sintering continue to be relevant to this day, both industrially and fundamentally, many new syntheses and design strategies have since emerged. They include electrochemical anodization, supramolecular assembly, microwave synthesis, vapor deposition, spray pyrolysis, flame and plasma synthesis, etc. At the same time, design strategies including those with soft and hard templating to induce highly ordered pore structures, engineering of crystal facets and anisotropy (see Chapter 2on how they can be creatively used to manipulate the target catalytic reactions), ligand‐capping to obtain well‐defined metal clusters, and surface grafting of single‐atom sites were developed to tune the physicochemical characteristics and hence the reactivities of catalysts. It is such a process of continuously pushing the boundaries of catalyst design that led to many new catalytically usable properties, e.g., size and spatial selective pores, localized surface plasmon resonance, size quantization effects, non‐Newtonian metal–support interactions, and low coordination active sites. The ability to uncover and utilize these new catalytic properties for targeted reactions is what constitutes the frontier in the field.