Abstract Much of industrial chemical processing (in the petrochemicals industry, for example), and a great deal of laboratory chemical synthesis, involves catalysts that both lower the energy barrier to reaction and may help steer a reaction along a particular path. Traditionally, catalysts have come in two classes: heterogeneous, typically meaning that the catalyst is an extended solid; and homogeneous, where the catalyst is a small molecule that shares a solvent with the reactants. In heterogeneous catalysis, the reaction generally takes place on a surface, involving molecules attached there by covalent bonds. Homogeneous catalysts are often organometallic compounds, in which a metal atom or small cluster of atoms supplies the active site for reaction. In recent years, these distinctions have become somewhat blurred thanks to the advent of single-atom catalysis, where the catalytic site consists of a single atom (as in many homogeneous catalysts) attached to or embedded in a surface. The emergence of this field might be regarded as the logical conclusion of the use of ‘supported metal clusters’—small metal particles of nanometer scale and below, containing perhaps hundreds, tens or just a few atoms. It has became clear that such clusters can sometimes provide greater product selectivity and activity than macro-sized particles or powders of the same metal, partly because the active sites might be atoms at particular locations (such as edges and corners) in the nanoscale particles. By reducing their scale down to the level of single atoms, one can optimize these properties. At the same time, the potential uniformity of the atoms’ environments makes such catalysts more amenable to rational design and modeling to understand mechanism. This field represents an appealing blend of fundamental chemistry and physics—from the quantum-mechanical level upwards—and applied research aimed at producing many of the products vital to society, such as fuels and materials. Researchers in China have been strongly active in this field in recent years (see, for example, refs [1–5]). Jean Marie Basset of the King Abdullah University of Science and Technology in Thuwal, Saudi Arabia, is one of the leading practitioners in the area, and National Science Review spoke to him about the development and prospects of the field. NSR: When was it first appreciated that catalysis by supported metals could involve single atoms? Basset: In work that my team and I published in 1998  we discovered that when we fully characterize a platinum particle modified with tin deposited by the surface organometallic chemistry (SOMC) technique, we found that each Pt atoms is surrounded by tin atoms, and this increased the selectivity of the surface for catalysing dehydrogenation of isobutane to almost 100%. At that time we said that this increase in selectivity could be explained by a ‘site isolation effect’—that the important factor was that the platinum atoms were individually isolated. At about the same time, we discovered that a single zirconium atom attached to a silica surface by a triple bond to a Si-O- group could achieve low-temperature hydrogenolysis (splitting of the carbon backbone using hydrogen) of alkanes. The method we used to prepare the lone Zr atoms started with an organometallic alkyl precursor, and created Zr atoms with a hydrogen attached . Hydrogenolysis of alkane was a known reaction in heterogeneous catalysis (for example, on nickel particles) but here the temperature was much lower (close to room temperature) than in heterogeneous catalysis (above 200°C). Besides that, the mechanism could be unambiguously determined, and shown to occur on a single Zr atom—it was no longer necessary to invoke any ‘ensemble effect’, a common notion in heterogeneous catalysis on small metal particles. View largeDownload slide Jean Marie Basset, distinguished professor at King Abdullah Universityof Science and Technology, Saudi Arabia. (Courtesy of Prof. Basset) View largeDownload slide Jean Marie Basset, distinguished professor at King Abdullah Universityof Science and Technology, Saudi Arabia. (Courtesy of Prof. Basset) NSR: I understand that one of the difficulties with supported metal nanoparticles is that they are often inhomogeneous. Is one of the attractions of single-atom catalysis that homogeneity becomes possible again? Basset: That's the right question, and the answer is yes. With single atoms, we can have a situation where all the active sites are almost identical. The discovery of low-temperature hydrogenolysis of alkanes was part of the origin of surface organometallic chemistry. Since then, a huge variety of metal atoms have been attached homogeneously on the surfaces of oxides, with the same structure for each atom. SOMC is not particularly familiar within the heterogeneous catalysis community, but has led to the discovery of new catalytic reactions such as Ziegler-Natta depolymerization , alkane metathesis , non-oxidative coupling of methane  and cyclo-alkane metathesis . Furthermore it has improved the activity, selectivity or lifetime of known reactions such as alkene metathesis and epoxidation, and imine metathesis. In these cases, the majority if not all of the active sites are identical. Because the structure of these grafted atoms are known at the atomic and molecular level, we can use the familiar concepts of molecular chemistry (organic, organometallic, coordination chemistry) to explain how bonds can be broken and reformed. The reactivity of surface organometallic fragments (SOMF) or surface coordination fragments (SCF) is pivotal to the outcomes. NSR: In your own work, you have promoted the idea of SOMC, which seems to aim at applying the concepts of homogeneous organometallic chemistry to surface-bound species. Can you explain more about what this entails? What advantages does a surface provide, relative to homogeneous catalysts? Basset: As I said earlier, SOMC is not really an extension of homogeneous catalysis. Rather, it is a new discipline of heterogeneous catalysis. It uses organometallic compounds to prepare well-defined heterogeneous catalysts in which a single atom is linked to a surface. It is quite distinct from homogeneous catalysis, in which typically the catalysts are metal atoms with ligands attached, because here the ‘ligand’ is a rigid surface, which creates completely different reactivity. The concept is actually much closer to heterogeneous catalysis, because it is like a supported metal on an oxide. Or one might better say, it is closest to the concept of surface-active catalysts (SAC). You can see the strong difference with classical homogeneous catalysis, and the consequent benefits, in the way that many new reactions which do not exist in homogeneous catalysis (or for that matter in heterogeneous catalysis) can be achieved in SOMC. Here's a simple way of picturing the comparison between a heterogeneous SOMF used in SOMC and a surface-active catalyst: View largeDownload slide The comparison between SOMF and SAC. View largeDownload slide The comparison between SOMF and SAC. The image on the left shows a SAC where a metal atom M’ is linked to an oxide. On the right is a SOMF. In SOMC we have a ‘preconceived’ mechanism, and the fragments are just possible intermediates in the catalytic cycle. In SAC it is intuitively assumed that the metal will, under the influence of the reagents A and B, adopt the right coordination sphere. But in SOMC the fragments A and B are components of the SOMF, already attached to the metal atom before grafting. Another difference between SOMC and SAC is the presence of a predetermined spectator ligand X in the former to tune the coordination number, the electron density and ultimately the steric control around the metal atom. The concepts of molecular chemistry are used to determine A, B and X, as in homogeneous catalysis—but the surface acts as a ligand that brings rigidity, pincer properties, acid-base and redox properties. NSR: How easy is it to prepare and characterize well-defined catalysts of this type? How much do we know, and not know, about the precise environment of the metal atoms? Basset: The preparation of SOMC is becoming ever easier. We have improved the methods. At the beginning it was necessary to use fragile organometallic components under a well controlled atmosphere, but now there are techniques to adsorb simple coordination complexes on oxides, as was done in many cases in classical heterogeneous catalysis (for example, WCl6 and TiCl4 on silica) and then to alkylate (say) in situ to achieve the right coordination sphere. The characterization is becoming easier, because the sites are mostly identical. The classical techniques can be applied, such as surface microanalysis, IR and UV spectroscopy, extended X-ray absorption fine structure spectroscopy (EXAFS), X-ray absorption near-edge structure (XANES) and density functional theory for calculations. The most useful method is solid-state NMR, which has played a decisive role in identifying the SOMFs with the accuracy of molecular chemistry. When we write a formula on a surface, it is no longer a cartoon but is very close to the real structure of most of the sites. NSR: That single metal atoms can be important and versatile catalytic centers is of course a well-established idea in bioinorganic chemistry too. Is there any overlap with this field in terms of an understanding of the mechanisms involved? Basset: SOMC can let us create well-defined SOMFs, thanks to the conceptual overlap with organometallic chemistry. But it can also lead to well-defined surface coordination compounds thanks to the overlap with bioinorganic chemistry, inorganic and coordination chemistry. This is the direction in which we are currently obtaining the most spectacular results—unpublished as yet! NSR: What kinds of reactions can be catalysed by these surface organometallic single-atom systems? What are some of the most useful and/or important? Basset: See the scheme below. Some recent advances include using CO2 to make cyclic carbonates, alkane metathesis, converting methane to ethane and hydrogen and to aromatics, oxidation chemistry of epoxides and aldehydes, and direct transformation of ethylene to propylene. The case of alkane metathesis is particularly important. When we discovered this reaction the turnover number [number of times the catalyst can repeat the reaction] was 60 using hydrogenated tantalum atoms. Now we can reach turnover numbers of 20 000 with bimetallic systems (tungsten/titanium) . Our target is 100 000, which is what we need for such a process to become commercial. View largeDownload slide Reactions catalysed by surface organometallic single-atom systems. View largeDownload slide Reactions catalysed by surface organometallic single-atom systems. NSR: How predictable and amenable to rational design are these systems? Do we have the computational methods that we need? Basset: We use the scheme below to do ‘catalysis by design’ and discover new reactions or to improve existing ones. The most important aspect is to transfer the elementary steps known in organic, organometallic and coordination chemistry to write a priori a catalytic cycle. Based on these elementary steps, we choose the metal, the support and the ligands X, A and B as mentioned earlier, and fully characterize the coordination sphere. View largeDownload slide Catalysis by design. View largeDownload slide Catalysis by design. NSR: How did your own interest in this field evolve? Basset: My first experiment in this area was to chemisorb an iron carbonyl complex Fe3(CO)12 on alumina in order to make iron nanoparticles. We were surprised to find formation of the species [HFe3(CO)11]− Al+ . This was a shock for us. It seemed that a new kind of chemistry was emerging from the overlap between organometallic and surface chemistry. We progressively developed this chemistry in many directions, with many metals and diverse supports: porous, non-porous, acidic, redox and so on. Then we discovered that this field could also apply to nanoparticles of zero-valent metals and we adapted the tools to characterize such materials. The first catalytic reaction was also a second shock: the low-temperature hydrogenolysis of alkanes with atomic Zr on hydrogenated silica, which I mentioned earlier. This opened the way to predict Ziegler-Natta depolymerization. Moving from Zr to Ta, we discovered how to conduct metathesis of alkanes  and plenty of new reactions too. Density functional theory was a crucial tool to understand what was going on, in particular using the tools developed by Luigi Cavallo here at KAUST. The strategy to develop new catalytic reactions slowly emerged, This area has a fantastic future because it allows us to get out of the ‘black box’. —Jean Marie Basset and the concepts are still evolving as we discover new catalytic reactions. NSR: There seems to be a strong interest in this topic in China. From where do you think the most important results are emerging in China? Do you feel they are building on a strong tradition of inorganic chemistry in China? Basset: Catalysis requires diversified approaches. I’m not sure that the SOMC concept has yet been so much explored in China. But Xuxu Wang from Fuzhou, one of my former students in Lyon, has made big impact in photocatalysis via SOMC. Nevertheless, there have been some great advances in catalysis in China in the last 20 years or so. Many impressive homogeneous and heterogeneous systems have been developed: heterogeneous by Tao Zhang, Can Li, Xinhe Bao, Yuhan Sun, Wei Wei and others; homogeneous by Xiaoming Feng, Kuiling Ding, Zhenfeng Xi, Qilin Zhou, Zhangjie Shi, Aiwen Lei, Guosheng Liu, Shuli You, Zhixiang Yu and many others. However, I’d like to see more emphasis given to fundamental understanding at the molecular level. For example, studies on the reactivities of organometallic species seem less popular, but they build the basis for catalytic applications. There is, however, very good work in this area from people like Zuowei Xie, Shaowu Wang, Yaofeng Chen, Ming-Hua Zeng and many others. NSR: Where do you feel the field is now heading? Are there potential types of analytical/characterization techniques that would make a big difference to our fundamental understanding of the processes involved? Basset: I feel that this area has a fantastic future because it allows us to get out of the so-called ‘black box’. This is due to the fact that we have the conceptual and experimental tools to predict any reaction, just by transferring concepts from molecular chemistry to surfaces. The science of molecular chemistry, whether it is inorganic, organometallic or organic, teaches us how to create or cleave bonds. Then the choice of metals, ligands, surfaces and so on is becoming more understood. We have some spectacular new results that will explain my optimism when we publish them in the near future. NSR: Do you think that this is one area of chemistry in which the links between fundamental research and industrial applications are particularly strong? Basset: I believe that CO2 chemistry, photocatalytic dissociation of water, CH4 and alkane chemistry, and oxidation are the areas where industry will benefit most from SOMC. NSR: Who were your own key influences in your early career, and why? Basset: When I was in France, I recruited Yves Chauvin when he retired from the French Petroleum Institute [where he worked from 1960 to 1995]. Nine years later he was awarded the Nobel Prize in chemistry. Not only was he a friend but I learned a lot from his broad knowledge of homogeneous catalysis and industrial processes. I want to mention his strong influence on me, and I will always be thankful to him: he was a modest, curious but fantastic scientist. Besides Yves Chauvin, I would like to mention Renato Ugo from Milan, who in the 1980s was developing analogies between homogeneous and heterogeneous catalysis; Paolo Chini, also from Milan, who made me dream about large clusters; Bob Grubbs from Caltech for his work on olefin metathesis; Wolfgang Herrmann for collaboration on SOMC; and all the community in homogeneous and heterogeneous catalysis, from whom I have learnt a lot in two disciplines that have tended to develop their own concepts separately. REFERENCES 1. 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National Science Review – Oxford University Press
Published: Sep 1, 2018
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