Special Issue Preface Cite This: J. Phys. Chem. C 2019, 123, 7495−7498
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Hans-Joachim Freund and Joachim Sauer Preface
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s there a magic substance that can turn metals into gold? Can it bring immortality? And how can one find it? Since more than 2500 years ago, this quest has fascinated alchemists. But the legendary “philosopher’s stone” has remained mysteriously elusive. A beautiful description was penned by the alchemist Zosimos, who lived in the early fourth century A.D.: “This stone which is not a stone, this precious thing which has no value, this polymorphous object which has no form, this unknown which is known to all, which has several names and which has no name.” Modern-day science has no place and no patience for such miraclesan achievement and a loss. But one field like no other has preserved the fascination for agents that can affect transformations of matter: heterogeneous catalysis. Commodity and fine chemicals, polymers, building materials, pharmaceuticals, and many more are mainly produced using heterogeneously catalyzed processes. The importance of heterogeneous catalysis for industry and, by extension, for society is undoubted. Similar to the quest for the philosopher’s stone, scientists are seeking catalysts that are highly active while producing the desired product with high selectivity. However, in contrast with the alchemists, the noble goal of today’s scientists is not only finding the catalyst itself; rather, it is the desire to understand the operating mechanisms and underlying principles, thus paving the way for a rational design of heterogeneous catalysts. The reality of today, however, tells a different story. Screening approaches still dominate the development in industry: good old alchemists’ methodsjust without the magic. Thankfully, there have been pioneers in the history of heterogeneous catalysis who would not resign to the dogma of “whatever works, works”. Ostwald, who received the Nobel Prize in 1909, partly for his work in heterogeneous catalysis, set the start point. While his work laid the phenomenological foundations, the atomistic understanding gradually gained importance until this development was crowned almost 100 years later by the Nobel Prize for Gerhard Ertl in 2007, who was able to explain the mechanism of the heterogeneously catalyzed synthesis of ammonia at the atomic level. This Special Issue pays tribute to two further eminent researchers who contributed tremendously to the achievements in this era: Hans-Joachim “Hajo” Freund, who has advanced and pushed the field experimentally with the unprecedented potential that experimental surface science offers. The other is Joachim Sauer, who ingeniously took advantage of modern quantum chemistry to drive the progress from a theoretical perspective. Both are prolific researchers with over 1000 scientific publications between them. Their importance for the systematic advancement of the field can be gleaned from the numerous prizes each of them has obtained. For Joachim Sauer, some of the most prestigious awards are the Chemistry Award of the Academy of Sciences of Göttingen, the Alexander von Humboldt-Award of
Photo Credit: Manuela Misch
Photo Credit: Peter Himsel
Special Issue: Hans-Joachim Freund and Joachim Sauer Festschrift Published: April 4, 2019
© 2019 American Chemical Society
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DOI: 10.1021/acs.jpcc.9b01395 J. Phys. Chem. C 2019, 123, 7495−7498
Special Issue Preface
The Journal of Physical Chemistry C
has pervaded all aspects of his professional life. When an idea captures his mind, he pursues it unrelentingly, whatever the obstacles. He would demand the highest standards of anyone who chose to work with him, and he had an intensity that could be daunting. But the harshest judgment would always be reserved for himself, which makes him a scientist of absolute integrity. He comes from a personal background that always inspired him to follow his own convictions, but he intuitively understood that life does not owe you anything: Heterogeneous catalysis does not yield its secrets easily. It is presumably this feature that drew him to this discipline to begin with. Because such experimental problems could not hold him back, before long he devised an ingenious solution. He surmised that the preparation of well-ordered thin oxide films on single-crystalline metal supports would overcome the analytical problems associated with oxide single-crystal surfaces. But it was not an art that could be done by just anybody. Atomically ordered films in epitaxy with a metal have to be prepared with great care, but finally the prize was worth the effort. With the successful connection to the conducting support, all of a sudden almost all surface analytical techniques could be used. With this breakthrough, many important results with respect to the adsorption of small molecules on such surfaces could be achieved. Eventually, this approach was the foundation for a new research field: “model catalysis”. Driven by the idea to extend the hitherto existing limits of surface science studies to unravel catalytic questions, he envisioned that the deposition of metal nanoparticles on top of such thin-film oxide surfaces would lead to more suitable models for heterogeneous catalysts because they mimic the higher structural complexity of real catalysts more adequately than well-ordered single-crystalline metal surfaces. This is commonly known as bridging the “materials gap”. The technique could be tuned so precisely that it was now possible to include typical defects and the abundance of lowcoordinated surface atoms in the model systems. This was extremely valuable because these features are characteristic of a type of heterogeneous catalysts that is very commonplace in industry: supported metal catalysts. Depositing the active catalyst in the form of nanoparticles on a cheap (porous) oxide support allows using a comparatively small amount of the usually expensive metal to still achieve high surface areas. Now it was possible to study the adsorption and reactions on such complex surfaces and thus to arrive at a profound atomistic understanding. Naturally, this extensive body of work required the collaboration of many coworkers; it has laid the foundation for many academic careers, including that of one of the two Guest Editors of this Special Issue (M.B.). In this context, another quality of Hajo Freund became important for his success and, even more importantly, for the success of his ideas. He has the personality of an inspiring leader. His is a leadership based on expertise and his extraordinarily profound knowledge. But he is not self-centered; his assertiveness and self-assurance allowed him to see the scientific potential and talents in others. He always highly appreciated the contributions of his coworkers and trained them to become critical and independent researchers on their own. It is perhaps apt to say that Hajo Freund has a demanding mentoring style, but he sees more in the person than just the scientist. He never forgets an act of loyalty, and loyalty is a most precious value for himself as well. He ensured that his mentees were welcomed in the scientific communities he was privy to, and he guided them, for instance, through the application process for positions, a rite of passage that was as grueling then as it is today. And, if possible, he facilitated permanent contracts for
the Belgian National Fonds for Scientific Research, the Kolos Medal and Lecture Award of the University of Warsaw and the Polish Chemical Society, and the Liebig Commemorative Medal of the German Chemical Society. Hajo Freund received the highly prestigious Leibniz-Award from the German Research Foundation, the Centenary Award of the Royal Society of Chemistry, the Gabor A. Somorjai Award of the American Chemical Society, the Karl-Ziegler-Award of the German Chemical Society, the Gaede-Langmuir Award of the American Vacuum Society, the Bunsen Memorial Medal of the German Bunsen Society for Physical Chemistry, and, as the latest honor, the ACS Award in Surface Chemistry this year. Both are fellows of the Berlin-Brandenburg Academy of Sciences and Humanities, the German National Academy of Sciences Leopoldina, and the European Academy of Sciences, to name but a few. Honorary doctorates and visiting professorships were bestowed on them, and the readers of this journal will be well acquainted with their published work and their numerous contributions to conferences. In this Preface, however, we would like to shed a very subjective light on the milestones of their scientific careers and share our personal appreciation for Hajo Freund and Joachim Sauer since we have had the privilege of working with them directly. The challenges in such a rich and diverse field as heterogeneous catalysis are manifold, for example, the issue of scale. In heterogeneous catalysis, processes spanning many orders of magnitude in length as well as pressure and temperature ranges need to be taken into account. Not only do the observed rates of reactions depend on the fundamental chemical transformation, but also transport phenomena (diffusion) and fluid dynamics must be considered for a “perfect” process. Therefore, the field is inherently interdisciplinary and requires the input of chemists, physicists, materials scientists, and engineers alike. Arguably, however, the most fundamental issue of heterogeneous catalysis revolves around the question of how a surface or a material brings about a desired reaction with a high turnover rate while suppressing undesired side reactions. The lifetime achievement of both Hajo Freund and Joachim Sauer was to bring light into the darkness of the complex network of elementary steps in catalysis. The work of Hajo Freund is characterized by a deep interest in a mechanistic and atomistic understanding of reactions at surfaces, andwhat makes him outstanding as a researcher in heterogeneous catalysishe had the unwavering conviction that every atom counts. This interest can perhaps be traced to his early research in theoretical chemistry in Cologne under the supervision of Georg Hohlneicher. The interest in surfaces was continued in his first post as an Associate Professor at Erlangen University, where he started his first detailed studies of adsorbates on metal single-crystal surfaces. Here he was also inspired by the late Gerd Wedler, who was one of the first in Germany who studied the adsorption and reaction of small molecules on metal surfaces, but still in the form of polycrystalline metal films. For Freund’s career, perhaps the most defining move was accepting an appointment of the Ruhr-University Bochum as a chair; it was here where he shifted his focus from metal surfaces to oxide surfaces. This was a step that carried a high risk: There are substantial experimental problems in preparing and studying such surfaces. After all, oxides are insulators and cannot be easily addressed with electron spectroscopy or STM. However, Hajo Freund had just the right personality to seek out such a challenge and to tackle it; he has an exceptionally strong work ethic that 7496
DOI: 10.1021/acs.jpcc.9b01395 J. Phys. Chem. C 2019, 123, 7495−7498
Special Issue Preface
The Journal of Physical Chemistry C
and is driven by the deep-rooted ambition to go to the true bottom of a scientific problem. Joachim Sauer’s research is in many ways avant-garde, and he has made impressive contributions in the field of heterogeneous catalysis. In particular, he developed a long-standing interest in zeolites, which are among the most important catalysts in the chemical industry. He belongs to the pioneers who established quantum-chemical methods to study the structure and properties of catalytically active centers in zeolites as well as their interaction with small molecules. He was also one of the first researchers who initiated the development and applications of hybrid quantum mechanics/ molecular mechanics methods. These methods allow for a significant reduction of the complexity of quantum-chemical calculations without a significant loss of accuracy while at the same time taking into account the complete (periodic) structure of a material. In 1999, Joachim Sauer and Hajo Freund initiated the Collaborative DFG Research Center 546 “Structure, Dynamics and Reactivity of Transition Metal Oxide Aggregates” and brought together other scientists from the research institutions and universities in Berlin. This long-term research project was planned with much vision and foresight. It is apt to say that this very successful research consortium can be considered a forerunner for the establishment of a “Cluster of Excellence” in 2007 in Berlin, which is a highly competitive and prestigious funding program by the German state. Joachim Sauer and Hajo Freund were founding members of this Cluster called “Unifying Concepts in Catalysis” (UniCat). Since 2007, UniCat has been supported with funds of up to 5.5 million euros annually from the German Excellence Initiative. Currently, UniCat has more than 50 chemistry, physics, biology, and engineering groups working on catalyst research and development. Perhaps the sheer size and breadth of these research consortia and the honorees’ driving role in their establishment can give an indication of the deep understanding and the appreciation they have for the importance of other disciplines beyond their own approaches. It is therefore no surprise that the research of both has always cherished the close mutual collaboration between theory and experiment. In this combination, it was possible for them to jointly elucidate the relationship between the structure, properties, and reactivity of many model systems with catalytic relevance in the past. A very recent collaborative and very impressive work, also highlighted by the cover art, concerns ultrathin, two-dimensional silicon dioxide layers that could be generated and characterized experimentally by Hajo Freund, but only by the calculations in the group around Joachim Sauer could a convincing structural model be established. This work eventually led to the discovery of a metal-supported, glassy ultrathin silica filmthe “world’s thinnest glass”and welldefined, two-dimensional aluminosilicate filmsthe “thinnest zeolite in the world”. The work of Hajo Freund and Joachim Sauer is not just remarkable for the systems they analyzed. Such a limited impact would not have sufficed for their probing, inquisitive minds they are men with a clear vision. Their lives’ ambition has been to uncover the deeply hidden, to transcend the simple, and to challenge the conventional: They have been seeking their personal philosopher’s stone. On this quest, they would not suffer fools gladly, and they could be formidable to work with. But their passion for their field and their crystal clear intellects were a special experience for anyone who had the fortune to
his coworkers in the precarious phase before they were qualified for a full academic position, not for any personal gain but as a proof of care for his coworkersan act of human decency which by far cannot be taken for granted. It is a tribute to him that ten of the Ph.D. students he mentored went on to pursue successful careers as professors in academia. It was not before long that his original approach to heterogeneous catalysis attracted related disciplines and he became the co-founder of a DFG research consortium (“Forschergruppe”) called “MODELLKAT” at the Ruhr-University Bochum. This was an era of intense collaboration between physical, theoretical, and technical chemistry as well as physics. Particular highlights of this period were very successful collaborations with theory that demonstrated the absolute necessity to combine experiment and theory. This would later also become important for his joint work with Joachim Sauer. After many fruitful years in Bochum and broad recognition of his work in the scientific community, he eventually became a Max Planck Society director at the Fritz Haber Institute in Berlin. Thanks to the unrivaled research conditions at this place, he was able to extend his visions and to push the limits of his approach even further. Most importantly, he started to develop strategies to study welldefined model systems not only under UHV conditions but also at ambient pressures to approach conditions under which catalysts usually operate in real-life catalytic processes. This discrepancy between a UHV environment and real catalytic working conditions is known as the “pressure gap”, and he made it his business to overcome this gap as well. His visionary approach to science that he could pursue at the FHI in an ideal manner and his desire to inspire the next generation of catalysis scientists has made him a true pleasure to work with. In such a complex field as heterogeneous catalysis, a sound theoretical basis is pivotal in order to truly appreciate these systems in their entirety. Only if the two disciplines of theory and experiment are intertwined can the meaning of new results and findings be interpreted beyond the mere example and instead become exemplary for general understanding. Therefore, the other scientist we wish to honor with this special issue is Joachim Sauer. Joachim Sauer was born in Eastern Germany in 1949. He always harbored critical views of the socialist regime in spite of the difficult political circumstances before the unification of Germany. Scientists in the West are so used to the privilege of airing their personal and political opinions in public that they easily forget that freedom of opinion is indeed a privilege. In Eastern Germany, regime-critical personal opinions could lead to the exclusion from higher education, repercussions in one’s professional career, and a creeping invasion into one’s personal life. Accordingly, the highest respect is due to Joachim Sauer, who started his successful career in such a difficult professional environment without losing sight of his moral compass. This inner strength was certainly a hallmark of his character then as it is today: His interest is not the limelight but the essence of science itself. His area of expertise is in the complex field of quantummechanical studies of catalytic processes. Perhaps because computer technology in Eastern Germany was behind the technology available in the West, he had to compensate for this drawback by becoming the visionary thinker that he is known as today. Driven by his persistence and purposefulness, which may be traced back to his personal background, Joachim Sauer’s research has two characteristics. It is always of the highest quality 7497
DOI: 10.1021/acs.jpcc.9b01395 J. Phys. Chem. C 2019, 123, 7495−7498
Special Issue Preface
The Journal of Physical Chemistry C work with or for them, and they have left a profound impact on many. Their legacy in heterogeneous catalysis is proof that it is possible to embark on a journey from model materials to realworld system. They are the joint architects of two bridges: the first across the materials gap and the second across the pressure gap. Many future generations will walk across. So, can the philosopher’s stone of Freund and Sauer turn metals into gold? Probably not. Has it brought them immortality? As much as a scientist can hope for.
Marcus Bäumer
University of Bremen
Marek Sierka
University of Jena
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DOI: 10.1021/acs.jpcc.9b01395 J. Phys. Chem. C 2019, 123, 7495−7498
