Effective coordination number: A simple indicator of activation energies for NO dissociation on Rh(100) surfaces

Abstract

We have used density-functional theory to compute the activation energy for the dissociation of NO on two physical and two hypothetical systems: unstrained and strained Rh͑100͒ surfaces and monolayers of Rh atoms on strained and unstrained MgO͑100͒ surfaces. We find that the activation energy, relative to the gas phase, is reduced when a monolayer of Rh is placed on MgO, due both to the chemical nature of the substrate and the strain imposed by the substrate. The former effect is the dominant one, though both effects are of the same order of magnitude. We find that both effects are encapsulated in a simple quantity which we term as the "effective coordination number" ͑n e ͒; the activation energy is found to vary linearly with n e . We have compared the performance of n e as a predictor of activation energy of NO dissociation on the above-mentioned Rh surfaces with the two well-established indicators, namely, the position of the d-band center and the coadsorption energy of N and O. We find that for the present systems n e performs as well as the other two indicators.Catalysts are vital to the present-day chemical industry and are also important in the search for alternative and cleaner energy sources. However, most catalysts that are in use today have been developed by a process of trial and error, and replacing this by a program of rational design is one of the grand challenges in chemistry today. Theoretical calculations using density-functional theory ͑DFT͒ can help provide insight and guiding principles since they can supply detailed microscopic information about the various elementary steps in a reaction. 1 They also allow one to explore hypothetical systems that, even if impossible to create in the laboratory, can help one to discern patterns that can then provide guidelines for the design process.In this Brief Report, we focus on one particular reaction: the dissociation of NO, which is one of the steps in converting undesirable NO x gases to N 2 , for example, in catalytic converters in automobiles; Rh has been shown to be one of the best catalysts for this process. [2][3][4] In catalytic converters, small particles of the metal catalyst are placed on a ceramic substrate. This situation is quite different from the ideal and clean single-crystal metal surfaces that have traditionally been studied using the techniques of surface science. In recent years, however, there has been increasing attention paid to more realistic situations, such as the presence of asperities and defects, finite particle sizes, and the influence of the substrate. The substrate can affect the operation of a catalyst in various ways since it changes the environment of the metal catalyst atoms: they can have different neighbors, and may also be strained due to the presence of the substrate.Here we consider four model systems that enable us to explore the effects of changing the local environment of catalyst atoms: ͑a͒ a Rh͑100͒ surface, ͑b͒ a Rh͑100͒ surface where both the surface and substrate have been stretched ͑by 9.9%͒ to the larg...

Ab Initio Study of the Adsorption of NO on the Rh6+ Cluster

“…If the determination of both the geometry and electronic structure is being achieved at the same level, which is desirable due to the strong interdependence of both kinds of properties, DFT has been demonstrated to be a very efficient and reliable approach for a large variety of systems. Thus, Ghosh and co-workers 16 calculated recently, using DFT, the activation energies for NO dissociation on unstrained and strained Rh(100) surfaces and monolayers of Rh atoms on strained and unstrained MgO(100) surfaces. These authors were able to establish a new indicator of activation energies that collect both the chemical nature of the substrate and the strain imposed by the substrate.…”

Local structure and spin transition inFe2O3hematite at high pressure

“…4), while the structural transition (red squares) is only completed at ∼10 % and ∼40 %, respectively. This shows again that the electronic transition actually precedes the structural transition, and leads to the following description of the HP transition of Fe 2 O 3 : i) volume and bond distances decrease with pressure until a "volume threshold" value is reached at ∼50 GPa, in which the low-spin phase is more stable as predicted by some theoretical calculations [8,9,34]. The spin crossover transition from HS to LS states is thus activated ii) the full HS to LS transition occurs between ∼50 and 55 GPa.…”

“…Another important and controversial point refers to the nature of the phase-transition at ∼50 GPa: is it the structural transition that drives the electronic transition or viceversa? Some authors stated that the structural transition precedes the change in the electronic properties of Fe 2 O 3 [2,3], while other authors proposed the opposite scenario [5][6][7][8][9]. Different theoretical approaches lead to different and controversial results.…”

“…We now address the controversial issue of the nature of the phase-transition [2,3,[5][6][7][8][9], i.e., do the electronic properties of Fe 2 O 3 change only after the structural transition with a decrease in volume and a change in the lattice symmetry, or viceversa? In this regard, the evolution of the Fe 3d electronic structure vs pressure can be investigated from the pre-edge region of the Fe K-edge absorption spectrum (top panel of Fig.…”

“…At ambient conditions, hematite crystallizes in the rhombohedral corundum-type structure, space group R3c, and is a wide-band antiferromagnetic insulator. By increasing pressure at room temperature, the corundum structure of hematite is progressively distorted and, above ∼50 GPa, a series of physical changes occur [1][2][3][4][5][6][7][8][9][10][11]: the unit cell volume drops down by about 10 %, the crystal symmetry changes completely, the electrical resistivity decreases drastically due to the breakdown of the d-electron correlation (Mott insulator-metal transition), the magnetic moments collapse [transition of iron ions from high-spin (HS) to low-spin (LS) state] and the long-range magnetic order disappears. Besides being interesting from the viewpoint of solid-state physics, the phenomena are also important in geophysics for modeling materials behavior in deep Earth's mantle [12][13][14][15][16].…”