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Special Issue Preface Cite This: J. Phys. Chem. C 2019, 123, 7521−7526

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Autobiography of Joachim Sauer

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statement, but it pointed in the right direction. There were great opportunities ahead applying theory in chemistry. I started my chemistry studies at Humboldt University in September 1967, where I was again subject to an “experiment” connected to the higher education reform in East Germany and again to my advantage. The reform was targeted at completing the communist party control over universities and replacing “bourgeois professors” with “socialistic cadres”, but it also had “collateral” effects to my advantage. First, there was the option of a personalized curriculum including early participation in research and deeper studies on some subjects while skipping others. For example, in a small group with a professor as a tutor, I studied Cotton’s Chemical Applications of Group Theory, and I learned normal-mode analysis from Nakamoto’s book on infrared and Raman spectra of inorganic compounds. Second, instead of doing diploma studies (1 year) and subsequent doctoral studies, the latter with teaching assistant obligations, successful students could directly enter the doctoral studies and submit the thesis after 3 years. So, it happened that in 1974, at the age of 25, I had a doctorate in chemistry (Dr. rer. nat.) from Humboldt University. Third, a unique opportunity had been created in which the doctoral thesis could be coauthored by two students based on joint research work. The advantage was that there was always a colleague to discuss with who had the same interest and aim. This was crucial because together with my fellow student Christoph Jung, we were given a vaguely defined problem to work on, and we had to be autodidacts in quantum chemistry. The advantage was that we became very independent from the beginning, and the disadvantage was that our studies were not very targeted. But there was even something positive in the latterwe acquired a rather broad education in quantum chemistry. To get a firm basis, we enrolled in the quantum mechanics course for physics students and read original articles, for example, that of Fock that he had published in German in Zeitschrif t f ür Physik and that of Roothaan for which I prepared a handwritten translation. Together with other doctoral students and guided by Reinhart Stößer, a highly educated expert in EPR spectroscopy, we studied Methods of Quantum Mechanics by McWeeny and Sutcliffe, and we also actively participated in the weekly seminars of the theoretical chemistry group at the Academy Institute in Berlin-Adlershof headed by the Lutz Zülicke. Our supervisor, Karl-Heinz Heckner, was a physical chemist who was lecturing mathematics, and since both of us did well, he invited Christoph and me to do doctoral studies with him. His interest in electrochemoluminescence, specifically the recombination of radical cations with radical anions of polycyclic aromatic hydrocarbons, was fueled by the East German attempts to develop devices for optoelectronic

orn in 1949, the year when the East German State was replacing the Soviet Occupation Zone, I was growing up in a community in an area in Brandenburg that had rural roots and was shaped by open brown coal mines, briquettes plants, and lignite power plants. Not far from there, brown coal coke for industrial use was produced (“Großkokerei Lauchhammer”), and in a close neighborhood there was also a Fischer− Tropsch plant (“Synthesewerk Schwarzheide”) already erected by the Nazis to produce fuel for their war. One of the chemical plant workers, Herbert Schulz, decided to become a chemistry teacher, and I was lucky that he became my teacher. After school had finished in the early afternoon, we did not have much to do, but he offered to do additional experiments with us once a week. He also brought us to the Chemistry Olympiad competitions that were held at the laboratories of the “Synthesewerk”. I will never forget an evening lecture by a leading chemical engineer of the Fischer−Tropsch plant. He not only was telling interesting things but also was what today would be called “a cool guy”, whose career you would like to follow. I am always remembering this when I go and lecture to high school students, for example, as part of the outreach activities of our Berlin-Brandenburg Academy of Science and Humanities. After grade eight, at the age of 14, I changed to high school. Within 4 years, it would lead to a school leaving certificate qualifying me to enter university (“Abitur”). Not only was I again lucky with an inspiring chemistry teacher, Günter Holzhaus, but also my age group was chosen to participate in an educational experiment in East Germany, to my advantage. The dual vocational education and training system typically leading within 3 years to a skilled workers certificate, for example, as a lab assistant in chemistry, was always, and still is, a strength of the German educational system. Just in the years relevant to me (1963−1967), within the same period of 4 years, one would attend both high school (3 weeks a month) and get a vocational training (1 week a month). So it happened that at the age of 18, I had both the school leaving certificate (“Abitur”) and a skilled workers’ certificate as a lab assistant in chemistry from “Großkokerei Lauchhammer”. Like most young students, I had many interests. I also liked mathematics, and I was participating in the Olympiads in Mathematics. When, as a high school student, I read an article about the chemical bond in the popular East German science magazine Wissenschaf t und Fortschritt, I was fascinated. There was this Schrödinger equation, a mathematical equation that embraces all chemistry with all its sensual aspects: freezing water, beautiful crystals, changing colors, vigorous reactions all in one equation. At some point I had to decide about my university studies: Shall I go for mathematics or chemistry? The decisive advice came from a friend who was already studying physics and philosophy. He told me, “If you like mathematics, you should study chemistry. The chemists urgently need people who know more mathematics than the ‘rule of three’.” This may not have been a completely fair © 2019 American Chemical Society

Special Issue: Hans-Joachim Freund and Joachim Sauer Festschrift Published: April 4, 2019 7521

DOI: 10.1021/acs.jpcc.9b01380 J. Phys. Chem. C 2019, 123, 7521−7526

The Journal of Physical Chemistry C

Special Issue Preface

H3Si−O−SiH3, which I used as a model for Si−O−Si bonds in silica and zeolites. The first paper was published in 1979. Subsequently, I built analog models for Si−O−Al linkages in zeolites with and without charge-compensating Na+ ions, and I studied the interaction of water with all of these models. To make them more realistic and distinguish between binding sites (Na+ ions) in different crystallographic positions, I was surrounding the models with arrays of point charges. This research program “in statu nascendi” was presented by Wolfgang Schirmer, the director of the Central Institute, at the fifth International Zeolite Conference in Naples 1980 under the title “What Can Be Expected from Quantum Chemistry in the Investigation of Adsorption in Zeolites” and was also published in the proceedings. Although there was no doubt that the Brønsted acidity of protonated zeolites originates from “structural” hydroxyl groups, their nature and local structure were an open question in zeolite chemistry. The reason was that the concentration of these active sites is low, and they are not ordered into unit cells. I came up with H3Si−O(H)−AH3 and H3Si−OH as models for bridging and terminal hydroxyl groups, respectively. I predicted bond distances and angles and also vibrational wavenumbers. As an acidity measure, I calculated deprotonation energies, which allowed comparison with gas-phase acidity scales and indicated that zeolitic Brønsted sites are even more acidic than HJ. I had completed these calculations when Wilfried Mortier from Leuven visited the NMR group of Harry Pfeifer in Leipzig, and he also wanted to meet with me. Getting a Western foreigner in the Central Institute was a major effort, but I could easily go to LeipzigI was already cooperating with them. It turned out that my calculations were in agreement with the IR results of Johannes Lercher and Hans Noller in Vienna and reinforced the structural arguments Wilfried had developed together with the colleagues in Vienna. We decided to publish a joint paper in The Journal of Physical Chemistry (1984). This was the beginning of a scientific friendship not only with Wilfried Mortier but also with Johannes Lercher, whom I met in person a year later in Hungary. He became one of my most trustworthy experimental colleagues in catalysis, and I could not say no when he invited me to join the editors of Journal of Catalysis in 2015. I visited Pavel Hobza and Rudolf Zahradnı ́k in Prague, and this resulted in a common study of hydrogen bond formation with different sites at silica surfaces (The Journal of Physical Chemistry, 1981). The hydrophobicity of all-silica zeolites was shown to be an entropy effect. We used molecular statistics in addition to quantum chemical energy calculations to predict free energies. For a proper description of van der Waals interactions, we were augmenting Hartree−Fock results with semiempirical dispersion terms long before it became common practice with density functional theory (DFT) after the year 2000. To properly account for dispersion, I also applied wave function methods that included electron correlation, manybody perturbation theory (MP2, MP4) in collaboration with the colleagues in Prague (Peter Č ársky, Pavel Hobza), and multireference configuration interaction methods (MR-CI) in collaboration with Reinhart Ahlrichs (Karlsruhe) and Gianfranco Pacchioni (West Berlin at that time). We found that “a substantial share of these energies is due to intersystem correlation effects (dispersion)” (The Journal of Physical Chemistry, 1986) and concluded “that the binding of water

applications. He set the research goal for our thesis in an unusual way. In the library, he showed us two journal issues, both in ugly colors (but great science in them), very different from the covers of today’s journals: one pale blue, The Journal of Chemical Physics, and the other dark red, The Journal of Physical Chemistry. He pointed in the former to an article of Paldus and Č ı ́žek about coupled-cluster theory and in the latter to an article of Č ársky and Zahradnik about UV−vis spectra of polycyclic aromatic hydrocarbons, and he said, “Try to do something similar that is relevant to our experiments.” With respect to Paldus and Č ı ́žek, we did not succeed, but with respect to Č ársky and Zahradnik, we did. We used semiempirical methods, PPP, and CNDO and assigned electronic spectra of radical anions and cations. This resulted in several publications with experimentalists (in German). From the quantum chemical point of view, we paid attention to the proper application of Koopman’s theorem in restricted Hartree−Fock calculations for open-shell systems. To our pride, this manuscript (in German) with only us as authors was accepted in Theoretica Chimica Acta, published by Springer in West Germany, and to my amusement, it has been recently cited not only by the eminent quantum chemist E. R. Davidson but also by my surface science theory colleagues Paul Bagus and Francesc Illas. As a student who had just finished his fourth year of studies, it was a very stimulating experience to participate in the annual meeting of East German quantum chemists “Arbeitstagung Quantenchemie” in Kühlungsborn at the Baltic coast. This was the place to meet other active quantum chemists from abroad, not only from “socialist” neighbor countries but also from the West. There I was particularly excited to meet Rudolf Zahradnı ́k from the Heyrovský Institute in Prague, whose work we were supposed to follow. He invited Christoph and myself to Prague, and this was the beginning of a life-long mentorship and friendship. Another person who impressed me with the deep and broad scientific knowledge he showed in the discussions was Reiner Radeglia, an NMR specialist at the Academy of Sciences. I often understood the point of an oral contribution only after he had asked a question. Both Rudolf Zahradnı ́k and Reiner Radeglia have been very influential for my career in chemistry, not only because of their being great chemists and inspiring minds but also because of their personality and political independence. Their friendship provided encouragement in difficult times when nobody could think of the wall coming down. In 1977, I joined the Central Institute of Physical Chemistry of Academy of Sciences in Berlin-Adlershof. There was a major effort on zeolites, in particular, with respect to gas adsorption and separations. There were modeling activities on different scales, including atomistic simulations with simple force fields. They were looking for somebody who would be able to get deeper understanding of molecule−surface interactions using quantum chemistry and, on this basis, would develop improved force fields. At that time, I was fascinated by the upcoming ab initio calculations, and I managed to get a tape with Pople’s Gaussian70 code (via Toruń, Poland). My idea was to use small models for building blocks of zeolite structures. I started with silica, which−as zeolites−consists of corner-sharing SiO4 tetrahedra. Physicists had used the (linear) Si−O−Si molecule for electronic structure calculations, but it was clear to me that I needed to do something to enforce the tetrahedral sp3 hybridization on Si. So, I was saturating the dangling bonds with hydrogen atoms and obtained the disiloxane molecule, 7522




Planck Society started in November 1990 and that gave selected researchers 7 years of funding at an East German university. The host university had to agree to hire the researchers as regular professors based on a joint appointment committee and transfer all group members as regular staff. My host university became Humboldt University. Equally important as the generous financial support was the administrative support by the partner institute, the Max Planck Institute of Solid State Research in Stuttgart. The key person behind my appointment as a Max Planck Research group leader was Hans Georg von Schnering, not only an outstanding solid state chemist and crystallographer but also a great character, to whom I owe a lot for this wonderful start in the new time. My quantum chemistry group started in the beginning of 1992, and since 1993, I have been a full professor for physical and theoretical chemistry at Humboldt University. As computational quantum chemists, we heavily rely on computer resources. In the East, we were always lagging behind by about one computer generation. This changed quickly after the fall of the wall. I received one powerful RISC workstation from the Volkswagen Foundation based on a joint proposal with Reinhard Ahlrichs and another one (Silicon Graphics) from BIOSYM. The funny thing was that they could not arrive in East Berlin before the currency union on June 1, 1990, when the U.S. embargo on advanced technology was lifted. Further money for workstations came from the “Fonds der Chemischen Industrie”, and after the Max Planck investment, my group had a pool of workstations that was absolutely competitive with any other group in the world. The more so, as the Max Planck group came with a position for a system administrator dedicated to my group. I was lucky to hire Bernhard Küpper just after he had finished his diploma studies in computer science at the Technical University in Dresden, and he did a great job. All of this gave me the opportunity to expand my research program, and I did it in three directions: (i) More accurate calculations, and on larger and more realistic models. When I visited Karlsruhe in 1988/1989, the group had just completed the first version of a new ab initio code that exploited the most recent computer technology (workstations with RISC processors) and allowed Hartree− Fock calculations on much larger molecules than had been accessible before. The dependence of predictions on the model size could now be examined, and with an efficient MP2 module (MP2, second-order Møller−Plesset perturbation theory), electron correlation effects on energies could now also be obtained on a routine basis. After the invention of functionals that depend not only on the density but also on the density gradient, DFT became a useful tool in computational chemistry, and with the efficient implementation in TURBOMOLE, DFT calculations, in particular, with the B3LYP functional, became our workhorse for large zeolite models. (ii) Making the models more realistic by embedding in a periodic environment. With respect to not only understanding but also rational design, it is crucial to know how the location of a given active site (hydroxyl group or transition metal ion) in different zeolite frameworks or in different crystallographic positions within a given zeolite affects its local structure, its properties, and its catalytic activity. The cluster models used in those days were generic models for active sites in zeolites and did not capture the variation of the active sites with their surroundings. Simulations of the full periodic structure of zeolites, which includes several hundred atoms in the unit cell,

molecules on copper and nickel surfaces is dominated by vdW forces”. The research program that emerged from all of these studies resulted in a Chemical Reviews article with the title “Molecular Models in ab Initio Studies of Solids and Surfaces: From Ionic Crystals and Semiconductors to Catalysts” (1989). In November 1989, the East German wall came down, which changed not only my personal life but also the conditions for doing science and being successful. At the East German Academy Institute, I was scientifically independent and could follow my own ideas. In 1985, I received the Dr. sc. nat. degree, the East German equivalent to “Habilitation”. I enjoyed the interactions with knowledgeable colleagues at the institute, among them Günter Engelhardt (solid-state NMR), Klaus Fiedler (Monte Carlo simulations), Reiner Radeglia (NMR), and also Harry Pfeifer and his NMR group in Leipzig. As far as possible for someone who was not considered a socialist cadre, I also had the support of the directors, first, Wolfgang Schirmer and, later, Gerhard Ö hlmann. There were one or two staff members working with me, and I had some technical support, but I was denied any further promotion. I was also denied traveling to conferences in the West, but there were frequent Western visitors in the institute. I was presenting my results at conferences in the Eastern bloc, where I would also meet colleagues from the West. Together with my publication record, this made me a visible member of the international community. Among those who would always visit and were very helpful in the transition period were the zeolite researchers Jacek Klinowski and Lovat Rees as well as the quantum chemists, Werner Kutzelnigg and Reinhart Ahlrichs. One year before the fall of the wall, I was finally allowed to travel West, and two times I spent 3 months with Ahlrichs in Karlsruhe (fall 1988 and fall 1989). I was also allowed to attend the International Zeolite Conference in Amsterdam in summer 1989 and was financially able to do so thanks to Wilfried Mortier and Rutger van Santen. There I met Richard Catlow for the first time and also people from BIOSYM (later MSI and Accelrys) who considered launching a Catalysis and Sorption consortium to develop simulation software. I was visiting in San Diego in December 1989 and came back to East Berlin with a job offer. I thought that this would be a great opportunity to implement and expand the research program I had in mind, to build cooperation between academic groups, and to cast all of this in a uniform software with the financial support from industry. However, I did not want to leave Germany for good in such interesting times. What came out was a (part-time) employment until the end of 1991, and then I continued as an advisor and cooperation partner. I very much enjoyed the creative atmosphere in the Catalysis and Sorption team headed by John Newsam, the dialogue with other advisors (Alex Bell, Tony Cheetham), and the regular meetings with the consortium members from industry. A very fruitful cooperation developed with Richard Catlow, who was in a similar role as me in the consortium. Those who want to know more about my life as a scientist before and after the fall of the wall can have a look at an interview that appeared in Humboldt Kosmos no. 96/2010, the magazine of the Alexander von Humboldt Foundation (http://www.humboldt-foundation.de/web/kosmos-coverstory-96-3.html). What happened to me next was the best thing that could happen to an East German scientist after the unification. My group became part of the short-term program that the Max 7523




could be done only with force fields, which have their own limitations. They require experimental data for parametrization, which is a severe obstacle for the Brønsted sites in zeolites because they are not ordered into unit cells and their concentration is low. To overcome these problems, we parametrized two different types of force fields based on ab initio data for zeolite fragments. Jörg Hill parametrized BIOSYM’s “consistent force field” (cff), and Klaus-Peter Schröder parametrized the ion-pair shell-model potential that had long been successfully used by Richard Catlow and others in the U.K. and had the advantage of including polarization. Moreover, we developed a hybrid QM/MM method that combines quantum mechanics (QM) for the reaction site with molecular mechanics (MM) for the full periodic structure. In 1994, at the 10th International Zeolite Congress, I presented the first results obtained with a code that Christoph Kölmel had written as part of the BIOSYM software (SOLIDS_EMBED). Key persons in later implementations were Uwe Eichler and Marek Sierka (QMPOT). (iii) Quantum chemical calculations with periodic boundary conditions. In the early 1990s, Parrinello had shown that DFT calculations can be performed with periodic boundary conditions for realistic models of condensed-phase systems when GGA-type functionals are applied with plane waves as basis sets and pseudopotentials for the cores. We performed the first zeolite simulations with his CPMD code in 1996. The same year, two other groups published such calculations with their own codes, Karlheinz Schwarz in Vienna and Mike Payne in Cambridge. As far as the accuracy for adsorption energies and energy barriers was concerned, DFT with gradient-corrected functionals was inferior to MP2, and we used the QMPOT code to perform hybrid QM/QM calculations, using MP2 as the highlevel method for the reaction site and DFT as a low-level method for the full periodic structure (Christian Tuma). We use this hybrid MP2/DFT approach to optimize structures and then perform single-point coupled-cluster (CCSD(T)) calculations on small models of the reaction site. Together with the enormous increase in computing power over time, today this enables us to reach chemical accuracy (4 kJ/mol) for molecule surface interactions (binding energies and energy barriers) for unit cells with several hundred atoms. Based on the available methods and those we were developing, we could significantly contribute to the understanding of zeolite structures and their reactivity. Hybrid QM/ MM calculations for zeolites with different framework structures and compositions (SiO2 vs AlPO4) confirmed that the deprotonation energy is a suitable measure of acidity strength for Brønsted sites and hence of their catalytic activity. The acidity was found to decrease with increasing aluminum content, while the OH vibrational frequency is primarily determined by the framework structure. The prediction that silicon-substituted aluminum phosphate frameworks are less acidic than aluminum-substituted silica with the same framework was confirmed experimentally for industrially used catalysts. The deprotonation energy became a popular descriptor used in many subsequent studies, but early on we pointed out that different descriptors may predict different acidity sequences. We explained how small molecules like water, methanol, and ammonia interact with Brønsted sites. It was, and to some extent still is, controversial if, and under which conditions, a

particular molecule is protonated in a zeolite. For complexes of one molecule per Brønsted site, we were able to assign the observed IR spectra for water and the NMR signals for methanol to molecularly adsorbed species. When Tony Cheetham and John Thomas believed to have evidence of protonated water molecules from neutron diffraction, we showed that this was an effect of water loading. Three water molecules are needed to observe proton transfer, and two methanol molecules, whereas with ammonia, one molecule is sufficient. The crucial parameter is the proton affinity of the adsorbed species, as had been concluded before by Jim Haw from his NMR studies. We did not find evidence that protonated zeolites behave as superacids, as some colleagues were speculating. The crucial question was, “Do zeolites protonate isobutene, and is the tertbutyl cation a stable species?” Our hybrid MP2/DFT calculations showed that yes, the tert-butyl cation may exist as a (meta) stable species in zeolite cavities, but the isobutoxide is more stable, and the most stable species is adsorbed isobutene, with the proton staying at the zeolite. We also calculated the barrier for the conversion of the tert-butyl cation back into adsorbed isobutene and concluded that its lifetime (10−450 ms), should it be formed in a hydrocarbon reaction as an intermediate, would not be sufficient to be probed by NMR but perhaps by UV−vis. When the wall was still up, I had benefited tremendously from my contacts with the colleagues in Prague, in particular, those at the Heyrovský Institute. Therefore, when Blanka Wichterlová was asking me in the 1990s for help to understand their experimental results on Cu-zeolites, I was more than happy to cooperate on this project. The Volkswagen Foundation was funding joined German−Czech proposals, and the work with Dana Nachtigallová and Petr Nachtigall turned out to be pleasant and productivewe published six papers together. We applied the hybrid QM/MM approach and screened possible copper(I) sites in zeolites. This became a popular reference point for many subsequent studies. We managed to identify two different types of sites for which there was evidence from UV−vis spectroscopy, and we could explain why one of them is particularly active in NO decomposition. Starting from there, Petr Nachtigall, who today is a professor at Charles University, continued to be successful with his own quantum chemical studies on the interaction of small molecules with binding sites in zeolites. The use of molecules as models for fragments of solids or the use of clusters as models for bulk metals or metal oxides, as outlined in my Chemical Reviews article, has an important advantage: Gas-phase experiments can be performed, and the results can be directly compared with quantum chemical calculations. In this way, the accuracy and reliability of computational methods can be assessed. In addition, and even more importantly, these molecules and clusters represent “experimental” models of reduced complexity compared with “real” (powder) catalysts, which contain a distribution of active species of different sizes anchored differently on a supporting oxide. This we had in mind when Klaus Hermann and I organized a meeting on “Quantum Chemical Aspects of Heterogeneous Catalysis” in Berlin in 1994. Among the speakers were the cluster chemists A. W. Castleman, Manfred Irion, and Helmut Schwarz as well as Hans-Joachim (Hajo) Freund, who was talking about another type of model catalysts: thin oxide films studied with surface science techniques in ultrahigh vacuum. 7524




the reader) and tried to involve everybody who could contribute, even if not always to the same extent. This strategy proved to be highly motivating for solving problems together. A good example is our joint paper of three research groups (Baerns/Kondratenko, Freund/Bäumer, Sauer/Döbler) and one international cooperation partner (Peter Stair) in Journal of Catalysis, in which we showed that a “generally accepted” assignment of vibrational features to polymeric vanadia species on oxide supports has to be revised. Another success story is the thin silica films supported on metal substrates that are shown on the cover of this “Festschrift”. Only with DFT calculations could the observed IR spectra and STM images be assigned to hexagonal monolayers and bilayers of SiO4 tetrahedra. The first structural models were found by “intelligent design”; subsequently, “evolution” (genetic algorithms) and calculated phase diagrams predicted additional structures depending on the oxygen partial pressure. Our experimental colleagues trusted us, went back to the laboratory, and found evidence of the predicted structures. What was even more rewarding for me was that our FHI colleagues managed to introduce Al into the SiO2 film and created a Brønsted site in the film that could be imaged by STM. They also measured the OH vibrational frequency and its shift on CO adsorption. With the acidity measures I had introduced decades ago, we could show that Brønsted sites in these two-dimensional structures are similarly acidic as the strongest sites found in the three-dimensional zeolite frameworks (The Journal of Physical Chemistry C, 2013). In addition to Jens Döbler (31 publications), three senior members of my group (Habilitation) were involved in these collaborations: Veronica Ganduglia-Pirovano, Joachim Paier, and Marek Sierka. With Veronica Ganduglia-Pirovano, I coauthored 34 papers. At some point, I had to convince her to work on ceria as a support. This work was so important because our colleagues managed to prepare and to clearly identify monomeric and polymeric vanadia species on ceria. The calculations turned out to be successful not only scientifically but also citation-wise (because ceria is such a popular material). At the end, Veronica liked it so much that she kept working on ceria after she had left for a permanent position in Madrid. Her successor is Joachim Paier, with whom I have published 22 papers since 2013. He has a strong methodological background with respect to periodic plane wave calculations, in particular, with hybrid functionals, and he is the group expert on the topic. With Marek Sierka, I published 60 papers between 1997 and 2014, and over many years he was the key person for the success of my group. He did his doctorate with me after he had already done research for his master’s thesis in Berlin. After a postdoctoral stay with Reinhard Ahlrichs in Karlsruhe, he came back on a permanent position and did Habilitation in 2009. Since 2012, he has been a professor in Jena. Marek Sierka not only was involved in the joint work with experimentalists but also made important method contributions. He parametrized shell-model ion-pair potentials for zeolites and wrote the hybrid QM/MM code QMPOT, which included an efficient optimizer for minima and saddle points. He also designed, implemented, and programmed a genetic algorithm for structure optimizations of gas-phase clusters and later extended it to surface problems (doctoral thesis of Radoslaw Włodarczyk). The catalysis research in the Berlin area got another boost with the Excellence Initiative managed by the German

Not too long after this meeting, in 1996, Hajo Freund moved to Berlin as a scientific member and director at the Fritz Haber Institute (FHI) of the Max Planck Society (MPG), as a colleague of Robert Schlögl, an expert in heterogeneous catalysis. Helmut Schwarz was also in Berlin at the Technical University, at Free University was Ludger Wöste, doing spectroscopy on clusters, and my colleague at Humboldt University, Klaus Rademann, was also interested in clusters. With Manfred Baerns as director, there was additional catalysis expertise in the Institute of Applied Chemistry in BerlinAdlershof. After some discussion, we agreed to apply for a Center of Colloborative Research (CRC) with the German Research Foundation. We proposed a study on the support effect of metal-oxide catalysts relying on model catalysts that would include, in addition to gas-phase clusters, single-crystal surfaces and thin films and also inorganic model compounds. Theory and computation would play a major role in connecting the different experiments on different types of model systems. Our proposal got approved, and in the summer of 1999, 14 research groups started working jointly on “Structure, Dynamics, and Reactivity of Transition Metal Oxide Aggregates”. I was elected as chair (spokesman), and, as much work as it was, I enjoyed very much the scientific part and that I had to go into a deep scientific dialogue with all of the individual projects, not only those with whom I had an active collaboration. There was yet another reason why I liked it. As challenging and rewarding as it was solving zeolite problems with quantum chemical tools, there was a new dimension, a new challenge with transition metal oxides: their complex electronic structure and their redox activity as the origin of their catalytic activity in selective oxidations. We managed to write convincing renewal proposals and got funded until 2011, the maximum period of 12 years. Over the years, some colleagues left, and others joined. The needs of our CRC became very relevant when hiring new faculty. This was the case with Christian Limberg, who contributed substantially to the success with his inorganic model compounds. Later, he was taking the lead for another CRC on “Metal Oxide/Water Systems at the Molecular Scale” (2014−2018), in which we followed a similar model system approach. As seen from the number of joint publications, my collaboration with Helmut Schwarz and Detlef Schröder (9 publications) as well as with Knut Asmis (19 publications) has been particularly intense. For the gas-phase complexes, I performed TURBOMOLE calculations myself, and I wrote many of these papers from scratch, a joyful luxury that I had not experienced for some time. My collaboration with Hajo Freund and his group leaders Markus Heyde, Helmut Kuhlenbeck, Niklas Nilius, Shamil Shaikhutdinov, and Martin Sterrer was most productive. To date, we have published more than 50 papers together, and we solved a number of problems that neither theory nor experiment could have solved alone. People who have been engaged in such experiment−theory collaboration know that it is very rewarding but also very challenging. To be successful, both sides have to know about each other’s methods and be aware of possible gaps between what is measured and what is calculated. Often, it is not easy to synchronize experimental and computational work. Experimentalists would like to publish as quickly as possible when they succeed with an experiment, but calculations need time and sometimes can only start then. We always looked for opportunities to publish together (by the way, for the profit of 7525




Research Foundation. Our proposal “Unifying Concepts in Catalysis” got funded in 2007, and 24 principal investigators with experience in molecular (homogeneous), enzymatic, and heterogeneous catalysis were part of the team. Over 10 years, with Matthias Driess and Peter Hildebrandt from Technical University as coordinators, we worked on the conversion of small energy-relevant molecules. I was coordinating the research area “Bridging the materials gap in complex catalysis”, and we focused on methane activation. The chemical engineer in the team was Reinhard Schomäcker, and Raimund Horn and Robert Schlögl synthesized “real” powder catalysts and conducted catalytic experiments, whereas model systems were studied by Gerard Meijer, Helmut Schwarz (gas-phase clusters), and Hajo Freund (thin films). Theory was represented by Matthias Scheffler with Sergey Levchenko and myself. We focused on oxidative coupling of methane (OCM) and approached the problem from two different directions. The reactor and process engineers built a mini-plant (with the support of BASF) and used the most promising catalyst to start with. The others started with Li-doped MgO, the simplest material that is active for this reaction, and aimed at getting an atomistic understanding. According to the well-established Lunsford mechanism, Li-doping creates oxygen radical sites (Li+O•−) that abstract hydrogen from CH4 and release methyl radicals in the gas phase. For this elementary step, our calculations yielded barriers that were much lower than the range of experimental values. We concluded that the Li+O•− site may not be the active site. The crucial experiment (temperature-programmed reaction) was done by Raimund Horn, who found that pure MgO is also active in OCM and that the same sites are responsible for the activation of CH4 on both Li-doped MgO and pure MgO catalysts. This showed that the Lunsford mechanism needs to be revised, and it stimulated further calculations. It turned out that CH4 chemisorbs heterolytically on morphological defects. The release of methane into the gas phase happens only when O2 is present on the surface. O2 picks up an electron, forming a superoxo species. Hence, the quantum chemical calculations, in combination with experiments, suggested a new role of the oxide catalysts in the oxidative coupling reaction. They do not provide and receive back electrons as transition metal oxide catalysts do in selective oxidations, rather they stay inert with their own electronic system and just bring together the reactants, allowing them to exchange electrons (redox equivalents) directly between themselves. It was more than these joint endeavors that made me proud and happy when the Max Planck Society appointed me as an external scientific member of the Fritz Haber Institute (FHI) in 2005 and I could celebrate this with Hajo Freund, Gerald Meijer, Matthias Scheffler, and Robert Schlögl with a pleasant dinner. And no less honored did I feel when my FHI colleagues organized a symposium on the occasion of my 60th birthday in 2009. The Fritz Haber Institute as a lighthouse in catalysis research always maintained excellent relationships with all three universities in Berlin, Free University, Technical University, and Humboldt University in the former East. All of the department directors were involved in active collaborations and supported their colleagues at the universities. They were and are holding honorary professorships, some at all three universities. It was a great motivating moment for my students when the Nobel Prize winner Gerhard Ertl showed up and gave a lecture in one of my courses. When the Max Planck

Society introduced International Max Planck Research Schools to promote doctoral students, our colleagues at the Fritz Haber Institute were among the first to use this instrument and involve their university colleagues. In 2002, under the directorship of Hajo Freund, the IMPRS “Complex Surfaces in Materials Science” started and became a great success. Fruitful collaborations developed directly between students. The lectures we were giving in biannual bloc courses helped to establish uniform knowledge across the different backgrounds, for example, between physicists and chemists. My scientific career after the fall of the wall would not have been the same without the Max Planck Research group, and my work in catalysis would not have had the same impact without the fruitful cooperation and the support of my colleagues at the FHI. Therefore, I would never say no when the Max Planck Society asked for service for the community. It was my scientific pleasure to serve on the advisory board (Fachbeirat) of the Max Planck Institute for Coal Research for 12 years. Since 2012, I have been a member of the Minerva Center Committee (my second period; the first was 1998− 2003). I am glad that in this role I can help maintain the very good relationships between Israel and German scientists. I do this from conviction, and also from conviction was I more than ready to accept when the MPG president Martin Stratmann asked me to chair the scientific committee for the Dioscuri Centers of Excellence in Poland. I feel I can give something back to the scientific community there and the many friends who helped me to survive as a scientist and honest person in difficult times before the fall of the wall. In the fall of 2017, after I had reached the age limit of 68, I gave my farewell lecture as a professor at Humboldt University. Now I am happy to continue with research as an employee of Humboldt University as a “Senior Researcher”. A couple of years ago, I was successful with an application for a Reinhart Koselleck grant “Ab initio Free Energy Calculations with Chemical Accuracy for Molecule−Surface Interactions” (German Research Foundation), and I am grateful to the university leadership and the colleagues at the Institute of Chemistry for support. I feel privileged because I am doing what was my dream as a studentto use quantum mechanics and solve chemical problems. I am grateful to all of the great colleagues and friends I found, first, among quantum chemists and zeolite researchers and, later, in the catalysis, surface science, and gasphase cluster communities. I would like to thank all who contributed to this Special Issue and especially the Guest Editors, Markus Bäumer and Marek Sierka.

Joachim Sauer

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Autobiography of Joachim Sauer - ACS Publications - American

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