Even two articles by authors from the Czech Advanced Technology and Research Institute (CATRIN) were published in the latest issue of Advanced Materials Interfaces. In addition, an illustration by Martin Pykal from CATRIN adorns the cover of the special edition focused on single-atomic catalysis.
Graphene derivatives serve as a suitable platform for anchoring individual metal atoms, which can be used as stable and efficient so-called single-atom catalysts (SACs). A prerequisite for their stable function is the formation of a strong bond between the metal atoms and the carbon substrate, thus preventing the metal atoms from clustering into larger particles and the associated reduction in the material’s catalytic activity. Computational chemists, who can predict both the stability and reactivity of these catalysts, assisted in helping to design the SACs.
In an article entitled Anchoring of Transition Metals to Graphene Derivatives as an Efficient Approach for Designing Single‐Atom Catalysts computational chemists from CATRIN focused on the bond strength between the graphene derivatives and three groups of transition metals, namely the iron triad, light platinum metals and coin metals.
“We found that the bond strength between these metals and the graphene derivatives was closely related to the transfer of charge between them. Graphene acts as a kind of reservoir of electrons that can move into unoccupied, lower-energy-level orbitals of metal atoms or, on the other hand, move from those orbitals to graphene. The amount of charge transferred in this way depends on the specific metal and its oxidation state. Unoccupied orbitals of metals with a higher oxidation state are in lower energy levels than those of metals with a lower oxidation state; therefore, more charge can be transferred to their orbitals and their bond with the substrate is more solid,” explained the first author of the paper, Dagmar Zaoralová.
Simply, the process can be imagined as connected vessels: electrons travel from the π-conjugated bonds of graphene to the metal’s orbitals, or vice versa, until the level (energy of the highest occupied orbitals of substrates and metals) equals. In this way, for most metals, bonding to graphene derivatives changes the oxidation number to a particular value, regardless of the metal’s original oxidation number.
“This metal-substrate communication could be very interesting for application of these materials as catalysts, but also for other electrochemical and spintronic applications, for example,” added Zaoralova.
Scientists from CATRIN, along with colleagues from abroad, participated in the paper entitled The Hallmarks of Copper Single Atom Catalysts in Direct Alcohol Fuel Cells and Electrochemical CO2 Fixation, in which they designed a SAC using copper atoms.
A single-atom catalyst consisting of copper covalently bound to cyanographene offers an effective acceleration of, for example, electrochemical reduction of carbon dioxide. “This is a significant reaction allowing the conversion of a greenhouse gas by electricity into a useful chemical that can be stored for long periods and used later, for example, as fuel. Research conducted in this area fits into our long-term strategy of pursuing sustainable and carbon-neutral energy,” said Michal Otyepka.
The possibility of firmly anchoring individual metal atoms on a graphene skeleton and subsequent use in catalysis was, until recently, a sheer illusion for scientists. But scientists at CATRIN have found a universal way to anchor individual atoms onto graphene and use them for catalysis. They were able to combine the benefits of homogeneous and heterogeneous catalysis and, in turn, suppress some of their weaknesses. They first published this technology in 2019 in the journal Advanced Materials (Bakandritsos A. et al., Advanced Materials 2019, 31, 1900323).