Joachim Sauer

2009 – Joachim Sauer


Mrs. President, Mr. President,
Mr. Dean, Dear Joachim,
Ladies and Gentlemen,

It is a delight to present this award to Professor Joachim Sauer. He has been a long time colleague and friend of many of us at the Warsaw theoretical chemistry laboratory and of the Polish theoretical community. Our relationship goes back to the early 1970s. Over the past almost 40 years he has become an outstanding scholar in the field of catalysis, respected, invited, and cited by many researchers all over the world. Today, by presenting him this medal we celebrate Joachim’s mastery of his field, his rigorous mind, his hard work and his achievements, and last but not least, his friendly links to Poland.

Joachim Sauer obtained his Diploma in Chemistry at age 23, and only two years later received, with honors, the Doctoral Degree – both from Humboldt University in Berlin. Humboldt University had been his employer at that time, until 1976, when he moved to the Academy of Sciences, Central Institute of Chemistry, also in Berlin. As a citizen of East Germany, DDR, he could not go for a post-doctoral work to the West. His opportunities to travel were restricted to Soviet-block countries until the end of the 1980s. He tried to make the best of it by visiting the gurus of Eastern Europe’s quantum chemistry: Rudolf Zahradnik in Praha and Włodzimierz Kołos in Warsaw. Fortunately, in his research he has always been very independent and needed peers rather than mentors. He often visited Poland and Czechoslovakia, where he could collaborate with Polish, Czech and Slovak researchers, and also attend numerous conferences and meet and talk to the best western scientists of the time. In 1985 he obtained Dr. Sc. nat. at the Academy of Sciences in Berlin, a title in DDR equivalent to the habilitation degree in Poland and Germany.

Once the ordeal of the communist epoch came to an end, he went for one year, in 1990/91 to the renowned center of computational technology, BIOSYM in San Diego, USA. In 1992 he became the head of a theory group affiliated with Max Planck Society, and a year later, in 1993, he was appointed C4 professor of physical and theoretical chemistry at his Alma Mater, Humboldt. His group began to rapidly grow, his scientific activity flourished, his reputation became firmly established. He has been awarded numerous awards and prizes, including Johann-Gottlieb-Fichte Preis, Friedrich-Woehler Prize and Alexander von Humboldt Award. Altogether he has published above 260 articles and given more than 280 invited lectures. Since 1995 he has been a member of the Brandenburg Academy of Sciences and since 2007 a member of the German Academy of Sciences Leopoldina.

The trademark of Sauer’s research has always been, as Richard Catlow, director of the Davy Faraday Research Lab, put it: „rigor and relevance” – two factors that are particularly hard to combine for such complex systems as are involved in catalysis. The realm of catalysis is not only the kingdom of rather large molecules, but also of surfaces and heterogeneous bulk, interconnected in a richness of ways – an area thus particularly hard for a fine theoretical quantum mechanical analysis. Not surprisingly, for a long time theoretical research in this field was plagued by both irrelevance and a lack of rigor: one either studied small clusters that could hardly represent the true chemical stage and its players, or ventured to bold and simplistic models that often might give a right answer for the wrong reason. The path between illegitimate bold claims and „overproduction of truth” (the term Joachim particularly liked to describe a research without novelty) is narrow and treacherous. Joachim has trod this path with great success.

The area of catalytic function and properties of zeolites has been central to Sauer’s research for a long time – one may call him a „Zealot of zeolites”. For a very good reason, since zeolites are both industrially used and interesting from a fundamental perspective. Sauer has pioneered the application of quantum methods to zeolites and explained how small molecules interact with the zeolite in terms of the active site concept. He pointed to the crucial role of the proton affinity while the nanoporous framework structure turned out to play only second fiddle.

The question of how various structures and positions of active sites within a given framework influence its spectroscopic properties and catalytic activity demanded the development of a variant of hybrid quantum mechanics/molecular mechanics method, QM/MM. Such methods are meant to cope with complex systems that are too large for exact quantum chemistry, yet only their fragments actually require a quantum description. Sauer is one of the pioneers of such an approach. His group developed a code that is at present the only one that applies the boundary conditions. Crucial for the success in zeolites were the shell-model pair potentials which have been derived from DFT.

This method gave Sauer’s group an edge over other researchers in heterogenous catalysis. Predictions of acidity were made for a number of different zeolites of different structures and with charming exotic names such as chabasite, faujasite, modernite and ferrite. Interestingly, different measures of acidity were shown to lead to somewhat different acidity sequences. Criticism of such parameters as NMR proton chemical shifts and OH vibrational frequency as reactivity descriptors was another result of this work. The acidity was shown to fall with the growing content of aluminum while the vibrations of OH turned out as primarily related to the structure of the framework. Proton mobility proved different for different zeolites, and the details of the water assisted proton transfer were elucidated.

Sauer‘s interest in developing methods has often been driven by the ultimate application in zeolites as model catalysts. DFT is a powerful tool for systems of that size, but currently used functionals poorly account for the Van der Waals – dispersion – interaction. Yet, Van der Waals interaction is hardly negligible and far from constant, not only at particular catalytic sites, but all along the reaction path. Sauer has proposed a combined approach, MP2:DFT, that treats finite size clusters at the MP2 level of theory while modeling the periodic catalyst structure with DFT. This approach enabled reliable calculations of catalytic hydrocabon conversion processes – the dispersion factor is crucial for hydrocarbons which do not possess low-rank electric moments. The specific questions posed pertained to the stablility of carbenium ions – purported intermediates – and the shape selectivity as a unique feature of zeolite catalysis.

Singularly challenging has been the project on transition metal oxides. The programme has been carried out in collaboration with experimentalists. It included the comparison of gas phase clusters with bulk oxides, and the comparison of surface science studies on experimental model catalysts with studies on powder catalysts under real conditions. The presence of weakly interacting d-symmetry electrons require more advanced methods: DFT with broken symmetry and ultimately verification by the most demanding multi-reference and coupled-cluster methods. A spectacular example is the prediction of the unusual structure of small aluminum oxide clusters that have no feature in common with the bulk alumina phase, in agreement with the IR photodissociation experiment.

Another significant prediction, subsequently verified spectroscopically, was that the most stable isomers of vanadium oxide in the gas phase have cage-like structures and in that dramatically differ from bulk solids. It is often claimed that gas phase experiments can tell us a lot about transition metal oxide clusters on supports and their reactivity. The problem with these experiments is that they use the charged species, cations and anions. Sauer has investigated both size and structure effects as well as charge effects on the reactivity of transition metal oxide species. As a result, substantial differences in reactivity were found due to the open-shell and charged character of the cluster cations.

To identify the cluster of transition metal oxides one needs an assignment of spectroscopic lines. Model calculations of vibrational spectra of transition metal oxideson deposited on silica and alumina supports revealed an important role of vibrations at the active metal oxide-supporting interface. This led to revision of Raman band to di- and polymeric vanadia species. Ultra-thin silica films on a metal substrate had been prepared and characterised by two experimental groups, but only after the computations of Sauer’s group was a convincing structure established.

So much for a brief presentation of some of the major accomplishments of Professor Sauer, higlights of his ample research record. These works earn him today the Kolos‘ Medal and Lecture. We may also be sure that a lot is to come in the near future. The issues that have been his favorite for many years have become hot now. His excellent expertise on catalysis and nanostructures can hardly have better opportunities.

This presentation would not be complete without recalling also a second, more personal story, that of a strong friendship and, yes, of solidarity. We went through the adventure of socialism together helping each other in many ways. Because in dire circumstances, as they were in the totalitarian world, fundamental human virtues are of singular value. Joachim visited Poland many times, and had colleagues and made friends in Warsaw, Torun, Wrocław. There was of course science. But also a free political debate along with a song and a joke, shared in the evening at the lake, at the camp fire, or in the mountains. Such little things were very important. They gave us strength of hope, and nourished our belief in honesty, beauty, justice and science. As aptly put into verse by Zbigniew Herbert:

„It didn’t require great character at all:
our refusal, disagreement, and resistance,
we had a shred of necessary courage,
but fundamentally it was a matter of taste,
Yes, taste
that orders us to stand out and make a wry face, draw out a sneer”…

… and Achim both has taste and is master of a sneer.

Ladies and gentlemen. Thanks you for your kind attention.

And Joachim, congratulations and many thanks again for your outstanding contribution to quantum chemistry and also for building the understanding and friendly ties between the scientific communities of our countries.

Grzegorz Chałasiński