Topics in Catalysis 1 (1994) 189-192 189

Personal perspectives from fifty years in catalysis research

Anders Nielsen

Haldor Topoe A / S, Nymollevej 5 5 , D K-2800 L yngby , Denmark

Keywortkq: catalysis research; ammonia synthesis; sulfuric acid; catalyst characterization

My first exposure to catalysis was during my studies of reactions in solution at the Institute of Physical Chemistry in Copenhagen with Professor J.N. Bronsted. My lifelong experience in catalysis has, of course, been with Haldor Topsoe.

Let me, as an illustration of the very limited equipment available around 1940, ment ion that, while I was at the Institute, the Institute received its first pH meter.

The first catalytic reaction I worked with in the Topsoe company was vanadium catalysts for sulfuric acid. I noticed in one of the recent numbers of Chemical Engi- neering News that it is listed as the number one priority in chemical catalysis today to develop a catalyst that will allow executing this reaction at a considerable lower temperature.

Our work was done essentially at atmospheric pressure and in glass equipment. I believe this was most fortunate since it allowed us to follow the different colors of the vanadium associated with the different valence states. This was during the sec- ond world war, and while vanadium catalysts for sulfuric acid were known the Swedish company which gave us the project was unable to secure vanadium cata- lysts from previous suppliers, so we had to develop the catalyst and produce it within two years based on their crude vanadium pentoxide and whatever support materials we could find in Denmark and Sweden.

We did develop the theory that the vanadium catalyst in its active state is a sup- ported liquid catalyst, and actually carried out experiments in which we circulated the active liquid in a tower and used this to convert SO2 to SO3.

Today, when we develop a new catalyst we have a complete mini manufacturing plant, which we call the "catalyst pilot", to test out all of the individual steps in the manufacture of the catalyst. We had nothing of the sort at that time. We had to rely on Dr. Haldor Topsoe's contacts to test individual steps and ended up buying the biggest restaurant model meat grinding machine available in town placing it at a vertical position with an electric motor on the shaft to carry out the catalyst extru- sion. But prior to that, we had fully designed a very ingenious machine that simu-

© J.C. Baltzer AG, Science Publishers

190 A. Nielsen / Personalperspectives

lated the way we made pellets in the laboratory and in which the catalyst paste would be put in a layer on a table and an arm equipped with 1000 small tubes at its end would come down to shape a 1000 small cylinders, which were subsequently ejected by individual pistons. I still believe this would be a most fantastic tool to manufacture good catalysts since it combines the best aspects of tabletting and extrusion, but I am afraid we would have had all sorts of problems if we had oper- ated that way. The only "physical" test we had was to write with a catalyst pellet on the blackboard. I f it could write, it was good.

Work on ammonia catalysts was started in 1944--45. We did get available shortly thereafter a high pressure test equipment but the only physical examination of catalyst preparations we could carry out was X-ray diffraction, which was car- ried out at the Royal Veterinarian and Agricultural College with Professor A. Tovborg Jensen.

Dr. Hans Bohlbro joined the company in 1945 and shortly thereafter he started to build up a BET unit that could also be used for chemisorption. This was a tremen- dous step forward in acquiring some knowledge on what we were doing. We could also carry out pore size distribution by a modified Pierce method, while mercury penetration for pore size distribution was added several years later when Aminco developed a commercial instrument.

In work on nitrogen adsorption phenomena in the range from liquid nitrogen to liquid oxygen temperature, Dr. Bohlbro found a first-order phase transition of adsorbed nitrogen at pressures in the range of (2-7)x 10 -4 mm Hg. We found that only about 5% of the total surface is involved in this transition and believe the tran- sition is limited to the iron surface. It may be mentioned that Professor J.H. de Boer on a theoretical basis expected a first-order phase transition at this pressure and attributed it to nitrogen molecules adsorbed with the axis perpendicu- lar to the catalyst surface since an adsorption with the axis parallel to the surface would call for a lower temperature to allow a first-order phase transition.

A few years later I was engaged with Professor C. Knakkergaard Mailer in try- ing to elucidate the electronic promotion of ammonia catalyst by the potassium promoter. We were studying the K edge by X-ray absorption studies. We worked with very well monochromatized X-rays and long exposure times but, unfortu- nately, the work was never completed and, of course, it may have been futile because of the equipment available at the time. However, with today's equipment it could possibly lead to some results.

I never met Henry Eyring but spent many thoughts on the work he had carried out in the mid and late 1930s on the development of the theory of absolute reaction rates. The assumption of intermediate molecules in chemical reactions really goes back to Arrhenius and description of the reaction complex in terms of a potential energy surface goes back to van Hoff and Marcelin. I was puzzled by the universal frequency kT/h and believe it had a fundamental background. It can derive directly from the Heisenberg Uncertainty Principle AEAt t> h/2.

If we assume that the energy difference that decides whether a molecule is in the

A. Nielsen / Personalperspectives 191

transition state or has reacted to product is indeed in the order of the energy k T or a few kT, then we may learn from the Heisenberg relation that At is in the order of h /kT or the frequency in the order ofkT/h.

In the 60s and well into the 70s something extremely important happened for us who manufacture catalysts and design reactors and that was the advent of compu- ter programs that allowed us to kind of determine the reaction rate on larger parti- cles from the rate on small particles. I believe in the past we often measured rates on large particles but what we were measuring was indeed a composite of reaction rates and mass and heat transfer.

For the design and performance evaluation of industrial units, where you can- not always work with small particles due to pressure drop limits, the use of high pressure mercury porosimetry, combined with detailed study of the pore structure by electron microscopy and by diffusion measurements to determine tortuosity, allow us by use of computers to separate the mass and heat transfer from the cataly- tic reaction itself and to calculate the concentrations of reactants through6ut the pore system. It was like separation of variables when you solve a partial differential equation. You can also by this approach determine the temperature gradients inside the particle and on its external surface where you can have significant tem- perature variations.

When we consider energy dissipation in catalytic reactions we assume that the energy is transferred to the surface of the catalyst and conducted back to the gas or liquid if we have a stationary situation. A very interesting question is whether the product molecules may carry excess vibrational energy with them when they leave the surface (phosphorescence, momen tum rule).

I have considered whether in fact the heat flux itself may contribute to re-crystal- lization, not only temperature. But then, on the other hand, we are not able to define the temperature of the site if we have a very exothermic reaction going on.

Today we use a large multitude of techniques to study catalysts and form an opinion of the active sites and details of the reaction. When you study just some 5-6 catalytic reactions you often use 15-20 different kinds of structure and surface analysis as well as computer calculations (in addition to measurements which are used to characterize the physical properties of catalyst particles, such as surface area measurements or pore structure determination).

One group of techniques are applied to the study of clean surfaces. However, for us the most promising techniques are those that can be applied to the working surface of the catalyst (under the conditions under which the catalysts will be used or under other conditions under which the surface is not too different from this). These are in situ techniques like: FTIR, EXAFS, M6ssbauer, X-ray diffraction and many others. A second group are those where a surface reaction is stopped and the surface swept with various gases analyzing what the surface gives off and of course isotope studies during reaction. If I were to point to one technique that for us has been most versatile, I would point to FTIR. However, to put the picture together you need to combine a number of techniques.

192 A. Nielsen / Personal perspectives

It has been a wonderful experience to follow catalysis research through a good fifty years but the most wonderful of all of it were the people whom I met on the way, and I should like to mention some very few who have meant something special to me: Professor J.N. Br~nsted, Dr. Haldor Topsoe, Professor J.H. de Boer, Professor Victor La Mer from Columbia University, Sir Hugh S. Taylor, Professor Paul Emmett , Professor Michel Boudart, and Professor Kenzi Tamaru. But these are all people in my age group, or older. Around the table I see many young and younger scientists who already contributed greatly to catalysis and ensure that the future will bring new great revelations.