The impact of Jens Nørskov’s scientific contributions on industrial catalysis

By Henrik Topsøe, former chairman of the board, Haldor Topsøe A/S & Poul Georg Moses, director of exploratory R&D, Haldor Topsoe A/S

Modern society depends on industrial processes for converting natural resources into useful products and intermediates such as fuels and chemicals. A majority of these industrial processes require heterogeneous catalysts to facilitate the chemical reactions involved in the conversions. One prime example is the production of ammonia for use in fertilizers. Jens Nørskov’s research is in very general terms concerned with understanding the underlying physics of heterogeneous catalysis and using this understanding to design and develop improved catalysts.

Jens Nørskov has made several contributions to the fundamental understanding of catalysis, among the most important are three theoretical concepts. The d-band model of chemical bonding at transition metal surfaces, a quantitative explanation of the qualitative Sabatier principle, and the identification of linear scaling relations in heterogeneous catalysis and their physical origin.

In addition to these concepts, Jens Nørskov’s research has demonstrated the strength of combining microkinetic and thermodynamic modelling with data from computational methods based on quantum mechanics. Computational methods to solve the quantum mechanical equation have progressed tremendously since the beginning of the early 1990s. This progress has been driven by the general increase in computational power, by software packages based on improved algorithms, and by more accurate approximations to the full quantum mechanical problem. Jens Nørskov’s research has over the years contributed to method development relevant to heterogeneous catalysis including the development of open-source software needed for large-scale computational studies.

The impact of the concepts and methodologies developed by Jens Nørskov cannot be overstated, and essentially Jens Nørskov’s work is the major reason why many of the leading catalyst companies today have established in-house capabilities to perform computational quantum chemistry calculations.

Jens Nørskov’s studies of NH3 synthesis catalysts serve as an excellent example of the impact of his research on industrial catalysis. Jens Nørskov’s early work on NH3 catalysts shed light on the nature of the active sites and identified the physical origin of the promotion effect of alkali promoters. Insights that provided valuable input on how to optimize NH3 catalysts. In addition to these insights, the NH3 synthesis also served as the model reaction used to develop the above mentioned quantitative explanation of the qualitative Sabatier principle. The Sabatier principle states that the interaction between the reactants and the catalysts should be just right, neither too weak nor too strong to provide the optimal catalysts. In itself a useful guideline, but Jens Nørskov’s work provides a model that explains why this guideline exists and shows that adsorption energies calculated based on computational quantum chemistry provide a quantitative measure of the optimal interaction.

In our view, one particular study marks a paradigm shift in computational quantum chemistry for heterogeneous catalysis. In 2005, Jens Nørskov and co-workers demonstrated that it was possible with remarkable accuracy to calculate reaction rates based on first-principle results from computational quantum chemistry with the only experimental input being the particle size distribution. The fact that computational tools based on quantum mechanics had reached a level of accuracy, where they had predictive power, changed the research field of computational chemistry for catalysis from a focus on describing and understanding catalysis to predicting and ultimately designing catalytic materials.

With the introduction of this new paradigm, Jens Nørskov developed the linear scaling relations identified for transition metal catalysts into a tool for catalyst design. Linear scaling relations refer to correlations between adsorption energies and between adsorption energies and activation energies. Scaling relationships ultimately provide the means to calculate an entire reaction network based on just a few descriptors. Typically, these descriptors would be the atomic adsorption energies of the main atoms involved in a reaction. The simplest example being a NH3 synthesis, where the rate is given by the adsorption energy of the nitrogen atom. This allows for fast screening of a large set of materials to identify possible new catalyst candidates to be synthesized and tested. Maybe even more importantly, once a database of adsorption energies has been established, it is possible to quickly investigate entirely new catalytic reactions.

The realization that linear scaling relations exist, also makes it possible to establish what the theoretical maximum performance of a catalyst would be and what underlying physics limit the performance. One example of this is the electrochemical water splitting reaction, which sets the limit for the efficiency of water electrolysis for H2 production. Water electrolysis for hydrogen production is a key technology for long-term energy storage and for production of hydrogen from renewable sources in general. This has spurred intense experimental efforts aimed at developing a catalyst that could split water without any losses. However, an analysis of the underlying reaction network reveals that the existence of linear scaling relations means that it is not possible to make a catalyst, which can split water at the thermodynamic limit. Excess energy has to be provided. At first this may appear as a depressing result; however, given that the physics behind the linear scaling relations are understood, routes can be devised to break the scaling relations. Therefore, the linear scaling relations provide a rational framework for the design of catalysts for the water splitting reaction.

Transition metal-sulfide based catalysts provide an example, where Jens Nørskov’s research has impacted the development of industrial catalysts. Transition metal sulfides are widely used for so-called hydrotreating of oil, which is reactions of oil with hydrogen to remove unwanted impurities such as sulfur. For this system of catalysts, Jens Nørskov has provided valuable atomic-scale insight, and based on this insight he has identified the activity descriptor of hydrotreating activity as the sulfur vacancy formation energy. In addition, as an example of the very broad impact of Jens Nørskov’s concepts, he identified transition metal sulfides as a very promising catalyst for hydrogen evolution in water electrolysis based on the almost optimal adsorption energy of hydrogen on the very same catalysts systems used for hydrotreating.

To conclude, the concepts developed by Jens Nørskov for heterogeneous catalysis have changed the way we understand heterogeneous catalysis, and his research has matured computational methods based on quantum mechanics to a stage, where these tools and methodologies are used in the industry for catalysts development.