In a significant breakthrough for the field of computational chemistry, chemical engineers at the University of Wisconsin-Madison have created a model that explains how catalytic reactions work at the atomic level. This new understanding could allow engineers and chemists to design improved catalysts and optimize industrial procedures, which could lead to huge energy savings, as catalysis is involved in the production of 90% of the products we use daily.
Catalytic substances accelerate chemical reactions without undergoing any changes themselves. They play a crucial role in the processing of petroleum products and the production of a wide range of items, including pharmaceuticals, plastics, food additives, fertilizers, environmentally friendly fuels, and various industrial chemicals.
Scientists and engineers have spent decades refining catalytic reactions – but since it is currently impossible to directly observe these reactions at the extreme temperatures and pressures often involved in industrial-scale catalysis, they don’t know exactly what happens on the nano and atomic scales. This new research helps unravel that mystery with potentially major ramifications for the industry.
In fact, just three catalytic reactions — steam methane reforming to produce hydrogen, synthesis of ammonia to produce fertilizers, and synthesis of methanol — use nearly 10% of the world’s energy.
“If you lower the temperatures at which you have to run these reactions by just a few degrees, there will be a huge decrease in the energy demands that we face as humanity today,” says Manos Mavrikakis, professor of chemical engineering. and biological at UW-Madison who led the research. “By decreasing the energy requirements to run all of these processes, you also decrease their environmental footprint.”
Mavrikakis and postdoctoral researchers Lang Xu and Konstantinos G. Papanikolaou along with graduate student Lisa Je published news of their progress in the April 7, 2023 issue of the journal Science.
In their research, UW-Madison engineers develop and use powerful modeling techniques to simulate atomic-scale catalytic reactions. For this study, they looked at reactions involving transition metal catalysts in the form of nanoparticles, which include elements like platinum, palladium, rhodium, copper, nickel and others important in industry and green energy.
According to the current model of rigid surface catalysis, the tightly packed atoms of transition metal catalysts provide a 2D surface to which chemical reactants adhere and participate in reactions. When enough pressure and heat or electricity is applied, the bonds between the atoms of the chemical reactants break, allowing the fragments to recombine into new chemicals.
“The prevailing assumption is that these metal atoms are tightly bound to each other and simply provide ‘landing points’ for the reactants. What everyone assumed was that metal-metal bonds remain intact during the reactions they catalyze,” says Mavrikakis. “So here, for the first time, we asked the question, ‘Could the energy needed to break the bonds in the reactants be similar to the energy needed to break the bonds in the catalyst?'”
According to Mavrikakis’ modeling, the answer is yes. The energy provided for many catalytic processes to take place is sufficient to break the bonds and allow single metal atoms (called adatoms) to detach and begin moving across the surface of the catalyst. These adatoms combine into clusters, which serve as sites on the catalyst where chemical reactions can take place much more easily than the original rigid surface of the catalyst.
Using a set of special calculations, the team examined the industrially important interactions of eight transition metal catalysts and 18 reactants, identifying the energy levels and temperatures likely to form such small metal clusters. , as well as the number of atoms in each cluster, which can also significantly affect reaction rates.
Their experimental collaborators at the University of California, Berkeley used atomic-resolution scanning tunneling microscopy to examine the adsorption of carbon monoxide on nickel (111), a stable crystalline form of nickel useful in catalysis. Their experiments confirmed models that showed that various defects in the catalyst structure can also influence how single metal atoms break apart, as well as how reaction sites form.
Mavrikakis says the new framework challenges how researchers understand catalysis and how it takes place. This may also apply to other non-metallic catalysts, which he will investigate in future work. It is also relevant to understanding other important phenomena, including corrosion and tribology, or the interaction of moving surfaces.
“We are re-examining some very well-established assumptions to understand how catalysts work and, more generally, how molecules interact with solids,” says Mavrikakis.
Reference: “Formation of active sites on transition metals through reaction-driven migration of surface atos” by Lang Xu, Konstantinos G. Papanikolaou, Barbara AJ Lechner, Lisa Je, Gabor A. Somorjai, Miquel Salmeron Manos Mavrikakis, April 6, 2023, Science.
The authors acknowledge support from the US Department of Energy, Basic Energy Sciences, Chemical Sciences Division, Catalysis Science Program, Grant DE-FG02-05ER15731; the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the US Department of Energy under contract no. DE-AC02-05CH11231, via the Structure and Dynamics of Materials Interfaces program (FWP KC31SM).
Mavrikakis acknowledges the financial support of the Miller Institute at UC Berkeley through a Miller Visiting Professorship in the Department of Chemistry.
The team also used the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the U.S. Department of Energy Office of Science under Contract No. DE-AC02-05CH11231 using award NERSC BES-ERCAP0022773.
Some of the computational work was performed using supercomputing resources at the Center for Nanoscale Materials, a DOE Office of Science user facility located at Argonne National Laboratory, supported by DOE contract DE-AC02-06CH11357 .
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