(Nanowerk NewsBefore they consider a breakthrough in computational chemistry research, University of Wisconsin-Madison chemical engineers have developed a model of how catalytic reactions work on the atomic scale. This understanding allows engineers and chemists to develop more efficient catalysts and harmonize industrial processes — potentially with enormous energy savings, given that 90% of the products we encounter in our lives are produced, at least in part, through catalysis.
Catalysts speed up chemical reactions without changing themselves. They are essential for the refining of petroleum products and for the manufacture of pharmaceuticals, plastics, food additives, fertilizers, eco-friendly fuels, industrial chemicals, and more.
Scientists and engineers have spent decades perfecting catalytic reactions — but because it’s currently impossible to directly observe them at the extreme temperatures and pressures often involved in industrial-scale catalysis, they don’t yet know exactly what happens at the nano- and atomic scales. This new research helps unravel the mystery with the potential to have major ramifications for the industry.
In fact, just three catalytic reactions — steam-methane reforming to produce hydrogen, ammonia synthesis to produce fertilizer, and methanol synthesis — use nearly 10% of the world’s energy.
“If you lowered the temperature at which you have to run these reactions by just a few degrees, there would be a huge reduction in the energy demands we face today,” said Manos Mavrikakis, a professor of chemical and biological engineering at UW–Madison who led the research. “By reducing the energy requirement to carry out all of these processes, you also reduce their environmental footprint.”
Mavrikakis and postdoctoral researchers Lang Xu and Konstantinos G. Papanikolaou with graduate student Lisa Je publish news of their progress in the journal Science (“Formation of active sites in transition metals via reaction-driven migration of surface atoms”).
In their research, UW–Madison engineers developed and used powerful modeling techniques to simulate catalytic reactions at the atomic scale. For this study, they looked at reactions involving transition metal catalysts in the form of nanoparticles, which include elements such as platinum, palladium, rhodium, copper, nickel, and others that are important in industry and green energy.
According to the current rigid surface catalysis model, the dense transition metal catalyst atoms provide a 2D surface to which the chemical reactants attach and participate in the reaction. When enough pressure and heat or electricity are applied, the bonds between the atoms in the chemical reactants break, allowing the fragments to recombine into new chemical products.
“The prevailing assumption is that these metal atoms are tightly bound to each other and provide only ‘landing points’ for the reactants. What everyone assumed was that the metal-metal bond remained intact during the reaction it catalyzed,” said Mavrikakis. “So here, for the first time, we are asking the question, ‘Can the energy for breaking bonds in a reactant be equal to the energy required for breaking bonds in a catalyst?’”
According to the Mavrikakis model, the answer is yes. The energy provided for many catalytic processes to take place is sufficient to break bonds and allow single metal atoms (known as adatoms) to break off and begin walking across the surface of the catalyst. These adatoms join into groups, which serve as sites on the catalyst where chemical reactions can proceed much more easily than the rigid catalyst surface.
Using a custom set of calculations, the team looked at the important industrial interactions of eight transition metal catalysts and 18 reactants, identifying the energy levels and temperatures that tend to form these tiny metal clusters, as well as the number of atoms in each cluster, which can also greatly affect reaction rates.
Their experimental collaborators at the University of California, Berkeley, used an atomically resolved scanning tunneling microscope to look at the adsorption of carbon monoxide on nickel (111), a stable, crystalline form of nickel useful in catalysis. Their experiments confirmed models showing that various defects in the catalyst structure can also affect how single metal atoms escape, as well as how reaction sites form.
Mavrikakis says the new framework challenges the foundations of how researchers understand catalysis and how it occurs. This may apply to other non-metallic catalysts as well, which he will investigate in future work. It is also relevant for understanding other important phenomena, including corrosion and tribology, or the interaction of moving surfaces.
“We are revisiting some very well-established assumptions in understanding how catalysts work and, more generally, how molecules interact with solids,” said Mavrikakis.