Special Topic : Catalysis — Facing the Future Catalysis : facing the future — an interview with Gerhard Ertl and Avelino Corma

Most of the chemical reactions used to produce the molecules and materials that our societies need—for example, in the petrochemical and pharmaceutical industries, the synthesis of plastics and other materials, and the production of foods and drinks—make use of catalysts. These speed up the rate at which atoms and molecules rearrange themselves into new forms, and provide a degree of control over the shape and form of those rearrangements. Catalysts let us drive a chemical reaction in a selected direction, in preference to others that could occur. In this way they turn chemistry from crude cookery into a rational and precise form of molecular engineering. And always we can draw inspiration, and sometimes borrow tricks, from the delicate and precise catalytic processes that occur in nature, where enzymes carry out processes in aqueous solution and at mild temperatures and pressures that often we struggle to achieve with far more extreme conditions—such as the fixation of atmospheric nitrogen into useful forms. It is often claimed that this particular catalytic process—the Haber–Bosch process for converting nitrogen into ammonia, discovered just over a century ago—has, by making possible the synthesis of artificial fertilizers, had a greater effect on humankind than any other single chemical innovation. It is what allows us to feed the world. Yet while nature performs this reaction using soluble molecules (enzymes) as catalysts, the Haber–Bosch process uses powdered iron (plus some additives). The reactions between nitrogen and hydrogen take place on the surface of iron particles: this is so-called heterogeneous catalysis, involving surface chemistry, rather than the homogeneous catalysis of enzyme reactions, in which the catalysts are soluble molecules. Both homogeneous and heterogeneous catalysis are essential to the chemical industries. National Science Review spoke with two of the foremost practitioners of the latter field—Nobel laureate Gerhard Ertl of the Fritz Haber Institute in Berlin, Germany, and Avelino Corma of the Institute of Chemical Technology (ITQ) at the Polytechnic University of Valencia, Spain—about the current status of research in catalysis and prospects for the future.


NSR:
To what extent has catalysis evolved from a trial-and-error form of engineering into a science based on rational design? And what has made this possible? Ertl: Catalysis on an industrial scale is still strongly reliant on experience and trial and error. However, thanks on the one hand to experimental progress in the characterization of the catalysts' surfaces and in the identification of the molecular processes involved, and on the other hand to better theoretical understanding on the basis of advanced quantum-chemical computation methods, we are coming closer to a rational design of the strategies for industrial research. Corma: Heterogeneous catalysis has always involved trial and error, assisted by the accumulated knowledge and basic principles of physical chemistry, solid-state chemistry and tools for characterizing what is going on in the reactions. We shouldn't overlook the important efforts made by people working on heterogeneous catalysis in the past to rationalize their field. Today we are still learning from those earlier studies, and even sometimes reinventing the concepts developed by them.
I believe that today we can rationalize the discipline still further: to predict and design solid catalysts ever more reliably due to the development of more powerful computational methods that allow better simulation of catalyst surfaces and their interaction with reactants and products. As computational methods are combined with spectroscopic and microscopic techniques, which allow us to 'see' atomic-scale interactions under (or closer to) real reaction conditions, we can better bridge the gap between the microscale and macroscale models that help to understand catalytic phenomena.

GETTING CLOSER TO REAL LIFE
NSR: In heterogeneous catalysis, until recently one often heard the concern that there was a big divide between the idealized systems used for fundamental experimental and computational studies, such as single crystals and ultrahigh vacuum environments, and the much messier systems typically employed in practice. Has this gap narrowed? Corma: Those concerns or limitations still exist in many cases. But researchers are getting better at identifying the specific active sites on the surface of a catalyst, so we can bridge the gap and transfer what has been learned from the 'ideal' cases to more realistic ones. What's more, with the knowledge gained through material science, we are finding ways of avoiding some of the more messy catalytic systems. Our ultimate objective is to design and synthesize solid catalysts with uniform, well-defined single or multiple sites to maximize their selectivity. Ertl: Despite the large difference between 'real' and idealized systems, there are some examples for which this gap could be closed. Perhaps the most spectacular case is the synthesis of ammonia over iron catalysts (the Haber-Bosch process). This was achieved by systematic studies, with ideal model systems, of the various factors governing the catalytic activity.

WHERE ARE WE NOW, AND WHERE ARE WE GOING?
NSR: Where do you feel some of the most important questions lie today in research on heterogeneous catalysis? What are some of the most exciting recent developments? Corma: We have urgent problems to solve in the fields of energy generation, especially sustainable energy. In the short term, we have to make the best use of fossil fuels, by developing more efficient processes that allow us to maximize the yield of products or energy and minimize fuel consumption and CO 2 I am a strong believer in unified principles in catalysis.
-Avelino Corma emissions. In using fossil fuels-as we have had to in order to get this far-we are buying time for developing sustainable energy and finding ways to store it. What we need most of all are efficient and clean ways of activating and forming carbon-carbon bonds. How do we activate alkanes in a selective way? Can we design catalysts to transform alkanes into more reactive forms under mild reaction temperatures and with environmentally friendly conditions? One particular outstanding problem is the efficient activation of methane. Sustainable sources of energy, such as wind and solar, will be increasingly used to produce electricity. But there is a problem in storing that energy so that it can be used on demand. One possibility is to use solar electricity to produce hydrogen from water. Then the question is how to store the hydrogen-to find efficient adsorbers of the gas. Or we might combine the hydrogen with carbon atoms from CO or CO 2 to synthesize high-energycontent molecules like methanol that can be stored and transported as liquids. Of course, batteries will also play a major role.
For sustainable production of chemical feedstocks, we need solid catalysts that can use oxygen for chemo-, region-and enantioselective oxidation. We need multifunctional solid catalysts that can integrate many steps of a multistep synthesis. This might entail a merging of enzymatic and synthetic catalysts. Ertl: Examples of recent progress include the increasing use of ambient-pressure techniques for characterizing catalysts under working conditions, and studies that look with atomic resolution at processes on nanoparticles.

UNIFYING THE FIELD
NSR: Do you see a narrowing of the traditional distinctions between homogenous and heterogenous catalytic systems, for example in terms of ways to achieve stereochemical control of reactions, or the use of nanoporous and nanoparticle systems? Corma: Absolutely. I am a strong believer in unified principles in catalysis. The chemistry is the same, we are just looking at the different sides of the same coin. My paper 'Attempts to Fill the Gap Between Enzymatic, Homogeneous, and Heterogeneous Catalysis' [1] tried to find points of connection between enzymatic, homogeneous and heterogeneous catalysis and catalysts. I have always been looking at this issue in my work on zeolites, in which well-defined single or multiple catalytically active sites can be introduced in a molecular-sieve system, with controlled adsorption properties. Zeolites lack the flexibility of the enzymes, but nevertheless we and others could show the benefits of zeolites for stabilizing reaction transition-state complexes by weak interactions with the walls of the nanopores, much as enzyme active sites do. In an analogous way, we are trying to work with MOFs (metal-organic frameworks) for catalysis. Maybe their 204 Natl Sci Rev, 2015, Vol. 2, No. 2

INTERVIEW
As a scientist you should never stop asking questions.
-Gerhard Ertl flexibility and diversity will help us go further in catalysis. I'm also enjoying very much working with metal nanoclusters of a few (3-15) atoms (e.g. [2]

CATALYSIS AND THE BIG ISSUES
NSR: Would it be fair to say that some of the most urgent scientific/technological questions we are facing-for example, in green technologies such as photocatalytic water splitting, battery technology, green methods of producing and converting new and renewable chemical feedstocks-are ones in which catalysis has a central role? Corma: I fully agree with that. I said earlier that energy and green chemistry were key issues. Finding efficient catalysts for water splitting and even photocatalysts for reacting CO 2 and H 2 O to produce alcohols and hydrocarbons, and developing better catalysts for electrochemically based process, are timely research subjects. We should also pay more attention to developing new processes for using the building blocks already present in biomass to generate new chemical compounds with improved performances. We can already convert biomass into liquid fuels, and there are efforts to design genetically modified living systems from which more and better liquid fuels can be obtained. Enzymatic and chemical catalytic processes are also being developed for improving the yields and quality of fuels from biomass. And it would make sense to use the elaborate molecules nature has synthesized as the building blocks for making new chemicals. Nevertheless-and this relates to your two previous questions-I believe that something still missing in heterogeneous catalysis by solids is to achieve 'intrinsic' enantioselectivity. It is true that some enantioselective solid catalysts have been observed by adsorbing or anchoring chiral molecules on solid surfaces, or by synthesizing structured hybrid organic-inorganic catalysts. However, the development of enantiomerically pure, intrinsically chiral solids that can perform reactions with high chiral selectivity would be a major breakthrough. Ertl: Some of the most urgent problems waiting for a solution from science are concerned with the conversion and storage of energy and with the protection of the environment. Here catalytic processes will surely play a decisive role.

CATALYSIS IN CHINA
NSR: Could you comment on the contributions to the field that are coming from China? Where are some of the most important research groups, in your view? Corma: The quality and originality of the work in heterogeneous catalysis coming from China has increased greatly in the last 15 years. We are now seeing work from China at the forefront of catalysis. We collaborate with groups in, for example, Dalian, Changchun, Shanghai, Nanjing, Beijing and Xiamen. In my opinion, it's urgent for China now to use its research capabilities to solve serious environmental problems and to develop new environmentally friendly chemical processes. Work on the development of sustainable energies could match very well the current needs of the country. This should be possible, given the high level of imagination and strength that many Chinese researchers have. Ertl: The growing activities of strong laboratories working on catalysis in China is reflected, for example, in the increasing number of publications in leading journals and in participation in international conferences. In my opinion, the Dalian Institute of Chemical Physics (Chinese Academy of Sciences) is at the forefront here.

ADVICE FOR YOUNG SCIENTISTS
NSR: What suggestions would you make to young researchers entering this field? Corma: This is a field full of opportunities, in which much fundamental knowledge still needs to be developed. Heterogeneous catalysis is a multidisciplinary field that involves (at least) physical chemistry, inorganic chemistry, organic chemistry, chemical engineering and materials science. To make important advances, one needs to acquire a good knowledge of those fields, and must build collaborative teams that can deal with all the aspects of catalytic phenomena.
I also would say that there is much intellectual enjoyment in this field, together with an important social reward: the knowledge developed helps to solve technical and environmental problems in our society. Ertl: Catalysis research is a multidisciplinary field, requiring experimental and theoretical contributions ranging from chemistry to physics to engineering. There are a never-ending number of open problems, and as a scientist you should never stop asking questions.
Philip Ball writes for NSR from London.