1. G ENERAL I NTRODUCTION
1.2. A TOMICALLY D ISPERSED C ATALYSTS
1.2.4. Catalytic Properties of Atomically Dispersed Catalysts
Figure 1.10. (a) Trends in relative turnover frequency (TOF) upon reducing the atomic population of metal NPs (■) to SAC (△). (b) Schematic illustration of the coordination effect of Fe–NxCy catalytic sites for the benzene oxidation reaction. (c) The polarization curves for the ORR before and after stability tests and corresponding H2O2 selectivity for 0.2 wt % Pt1/TiC and 0.2 wt % Pt1/TiN. Reprinted with permission from refs. 5, 85, and 86, respectively. Copyright © 2018 WILEY-VCH Verlag GmbH
& Co. KGaA, Weinheim. Copyright © 2019 Springer Nature. Copyright © 2017 American Chemical Society.
Inheriting the advantages of both homo- and heterogeneous catalysts, atomically dispersed catalysts, in many instances, exhibit superior catalytic activity for various thermo-,28 photo-,49 and electrocatalytic77 reactions. Atomically dispersed catalysts have demonstrated great promise in numerous reactions, including CO oxidation, WGS,26,28 alkene hydrogenation,27 alcohol oxidation,78 methane conversion,79 hydrochlorination,80 hydrogen evolution,81 and oxygen reduction reactions.82 However, the catalytic activity of atomically dispersed catalysts is not always better than that of their metal NP counterparts.5 The superiority of catalytic activity can be altered between atomically dispersed catalysts and metal NPs depending on the type of catalyst and reaction required (Figure 1.10a). Different catalytic trends can evolve even for the same catalytic reaction using the same type of metal catalyst.
Rossel and co-workers showed that atomically dispersed Pd catalysts supported on magnetite do not (b)
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catalyze the hydrogenation reaction, but Pd clusters can participate in the reaction.83 In contrast, the Zheng group reported that a TiO2-supported single Pd catalyst exhibited superior conversion during the hydrogenation of styrene when compared to a commercial Pd/C catalyst.60 Hence, it is critical to investigate the optimal coordination environments of single metal sites and their electronic structure to obtain high-performance atomically dispersed catalysts.
In this regard, Li and co-workers reported the modulation of the coordination number of a series of atomically dispersed Co–N/C catalysts to form Co–N2, Co–N3, and Co–N4 by varying the pyrolysis temperature used in their synthesis. The Co–N/C catalyst with Co–N2 coordination showed the best CO2
electroreduction activity with a CO formation Faradaic efficiency of 94%.84 In a similar manner, the Chen group regulated the coordination structure of various atomically dispersed Fe–N/C catalysts to form Fe–N4, Fe–N3C1, and Fe–N2C2 by controlling the thermal activation temperature (Figure 1.10b).84 As a result, Fe single atoms ligated with four N atoms exhibited the highest benzene oxidation reaction performance with a phenol selectivity of 100%.85 Zhang and co-workers tuned the coordination number of Pt single atoms supported on Fe2O3 to unravel the structure–performance relationships observed for a series of atomically dispersed catalysts. They found that increasing the Pt–O coordination number reduced the oxidation state of Pt, and the catalytic activity observed in the hydrogenation reaction of 3- nitrostyrene was enhanced without any loss in the chemoselectivity toward 3-vinylaniline. The interface formed between single metal atoms and support surface sites can be tuned by controlling the type of support used. Lee and co-workers controlled the interface formed between the catalyst and support by preparing single Pt atoms on two different TiC and TiN supports.86 The ORR activity and H2O2
selectivity of the TiC-supported single Pt catalyst were higher than those observed for the TiN-supported catalyst, highlighting the important role of the support in establishing the catalytic properties of atomically dispersed catalysts. The Lu group also investigated the role of the support in atomically dispersed Pt catalysts.69 Among Co3O4, CeO2, ZrO2, and graphene, the Co3O4 support induced the largest depletion in the 5d states of Pt, promoting the highest activity observed in the hydrolytic dehydrogenation reaction.
The geometrically isolated nature of atomically dispersed catalysts is beneficial for their catalytic selectivity (Figure 1.11a).87 In the acetylene hydrogenation reaction, Li and co-workers demonstrated that Pd NP-based catalysts showed low ethylene selectivity due to the occurrence of a secondary hydrogenation reaction toward ethane, whereas the hydrogenation of ethylene to ethane was forbidden using atomically dispersed Pd catalysts, which exhibited an ethylene selectivity of 100% (Figure 1.11b).45 The Lee group successfully performed the methane activation reaction and prepared methanol and ethane using an atomically dispersed Rh catalyst (Figure 1.11c).88 On the surface of the metal NPs (5% Rh/SiO2), the C–H bonds are completely dissociated to form C + 4H due to their successive dissociation on the metal surface, thereby generating CO2 as the main product. In contrast, on the Rh single sites supported on ZrO2 (0.3% Rh/ZrO2), the CH3 intermediate can be stabilized and methanol
and ethane can be produced from methane in the presence of an appropriate oxidant. Lee and co-workers demonstrated the enhanced selectivity observed in various electrochemical reactions when using isolated metal sites.86,89 Atomically dispersed Pt sites can inhibit the side-on adsorption of O2 during the ORR, which leads to the four-electron (4e−) pathway generating H2O. Instead, they prefer the end-on adsorption of O2, prompting the two-electron (2e−) pathway ORR to produce H2O2.42,86,89 In the formic acid oxidation reaction, while Pt NP catalysts drive the indirect dehydration pathway generating H2O and CO, an atomically dispersed Pt catalyst induced the direct hydrogenation pathway and produced CO2 because of the absence of any adjacent metal atoms.
Figure 1.11. (a) Schematic illustration of improvement of butenes selectivity on single atom Pd1/graphene catalyst. (b) Catalytic reactivity for semi-hydrogenation of acetylene. Pd-SAs-900 and PdNPs/CN indicate atomically dispersed Pd and Pd NP-based catalysts, respectively. (c) Direct methane oxidation results on the 0.3 wt % Rh/ZrO2, 2 wt % Rh/ ZrO2, and 5 wt % Rh/SiO2 catalysts, and recyclability test results performed with the 0.3 wt % Rh/ZrO2 catalyst. Reprinted with permission from refs. 87, 45, and 88, respectively. Copyright © 2015 American Chemical Society. Copyright © 2018 Springer Nature. Copyright © 2017 American Chemical Society.
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The Achilles heel of atomically dispersed catalysts is their catalytic stability. Industrially important catalytic processes are often conducted at significantly high temperatures under oxidative or reductive reaction conditions. Under such harsh conditions, the agglomeration of metal particles or coking on the catalyst surface are unavoidable, which undermines the superb catalytic properties of atomically dispersed catalysts. In this context, it is of paramount importance to develop atomically dispersed catalysts that are stable under the harsh reaction conditions conventionally used in industrial processes. To obtain highly stable atomically dispersed catalysts, Yan and co-workers fabricated Pt SACs firmly anchored onto the internal surface of mesoporous Al2O3.90 The Pt/m-Al2O3 catalysts withstood 60 cycles of an CO oxidation stability test performed between 100 and 400 °C for over one month, suggesting a method to address the stability issues of atomically dispersed catalysts. The Hosono group stabilized single Pt atoms within the sub-nanometer cavities of 12CaO7·Al2O3 (C12A7).91 The interactions between the metal anions and positively charged surface cavities of C12A7 permit the stable operation of the selective hydrogenation of nitroarenes under reducing conditions up to 600 °C.
Zhang and co-workers reported the use of coke-resistant atomically dispersed Ni catalysts in the dry reforming reaction of methane (DRM).92 Due to selective activation of the first C–H bond in methane with negligible carbon deposition, the DRM could be operated for 100 h.