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A molecular view of heterogeneous catalysis
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Image of FIG. 1.
FIG. 1.

Heterogeneous catalysis at different length scales: from reactor design to molecular level insight.

Image of FIG. 2.
FIG. 2.

Left: in situ images of a supported Ru catalyst recorded at and in a gas composition of 3:1 (from Ref. 15). Right: STM image of Ru nanoparticles supported on graphite. The catalytically active steps can be seen directly (from Ref. 16).

Image of FIG. 3.
FIG. 3.

Left: Calculated energy diagram for synthesis over a Ru surface. Energetics for both the flat surface (dashed lines) and the stepped surface (solid lines) are shown. All energies are shown relative to and in the gas phase. Starting from the left, is first adsorbed on the active site , with a weak bond. Then, it dissociates with a sizable activation barrier which depends strongly on the surface structure. adsorbed dissociatively with no activation barrier. When atomic nitrogen and hydrogen atoms are present on the surface, they start combining to form adsorbed NH, , and which finally desorb from the surface. The geometry of the transition state structures for dissociation on the two surfaces are shown in the insets. Right: ammonia synthesis productivity under industrial conditions calculated directly from the potential energy diagram compared to measurements on a real supported Ru catalyst (from Ref. 18).

Image of FIG. 4.
FIG. 4.

(a) Illustration of the catalytically active adsorbed oxygen species and on a surface. (b) Rates of formation as a function of the CO pressure for a fixed pressure. Experimental values are shown in black (Ref. 26) and the calculated results in green (Ref. 23). (c) A snapshot of the surface during turnover showing adsorbed O (red) and CO (blue). Reaction takes place at the boundary where O and CO meet (from Ref. 23).

Image of FIG. 5.
FIG. 5.

Left: calculated activation energies for dissociation plotted as a function of the calculated dissociative chemisorption energy for a number of metals. Right: calculated ammonia synthesis rate per site per second (turn-over frequency, TOF). The values for the individual elemental metals are shown together with the value expected for an alloy consisting of Co and Mo (from Refs. 28 and 31).

Image of FIG. 6.
FIG. 6.

Left: Pareto plot of catalysts predicted to be good compromises with respect to cost and activity for methanation. The positions of the catalysts are determined by the cost of their constituent elements vs their distance from the optimal dissociative chemisorption energy for CO with respect to the experimentally observed optimum (see inset). Right: measured rate of methanation for different Ni–Fe alloy catalysts (adapted from Ref. 32).

Image of FIG. 7.
FIG. 7.

(a) Identification of the active site and transition state configurations for epoxidation (EO) and complete oxidation of ethylene over Ag(111). (b) Alloying affects the two transition states differently. (c) Calculated change in the difference in activation barriers for the two competing reactions by alloying Cu, Pd, and Au into Ag(111). Cu changes the selectivity toward epoxidation most (from Ref. 34).

Image of FIG. 8.
FIG. 8.

(a) Schematic illustration of the formation of a chemical bond between an adsorbate valence level and the and states of a transition metal surface. The coupling to the states merely leads to a broadening of the adsorbate state, while the coupling to the narrows metal states results in the formation of bonding and antibonding states. (b) Calculated dissociative dinitrogen, carbon monoxide, and dioxygen chemisorption energies over different transition metals plotted as a function of the center of the transition metal bands. (c) Top: comparison of the x-ray emission (XES) (occupied states) and x-ray absorption (XAS) (unoccupied states) spectra of atomic N adsorbed on Ni(100) and Cu(100) with separated components. Bottom: calculated density of states for the same systems. States below the Fermi level are shown in red, while empty states are shown in blue. In order to make the comparison to the experimental results clearer, the spectrum has been broadened by the experimental energy resolution of (from Ref. 42).


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: A molecular view of heterogeneous catalysis