Origin of Extraordinary Stability of Square‐Planar Carbon Atoms in Surface Carbides of Cobalt and Nickel

Abstract

Surface carbides of cobalt and nickel are exceptionally stable, having stabilities competitive with those of graphitic C on these surfaces. The unusual structure of these carbides has attracted much attention: C assumes a tetracoordinate square‐planar arrangement, in‐plane with the metal surface, and its binding favors a spontaneous p4g clock surface reconstruction. A chemical bonding model for these systems is presented and explains the unusual structure, special stability, and the reconstruction. C promotes local two‐dimensional aromaticity on the surface and the aromatic arrangement is so powerful that the required number of electrons is taken from the void M4 squares, thus leading to Peierls instability. Moreover, this model predicts a series of new transition‐metal and main‐group‐element surface alloys: carbides, borides, and nitrides, which feature high stability, square‐planar coordination, aromaticity, and a predictable degree of surface reconstruction.

Synergy Effects of the Mixture of Bismuth Molybdate Catalysts with SnO2/ZrO2/MgO in Selective Propene Oxidation and the Connection between Conductivity and Catalytic Activity

“…They have been the subject of many studies since the 1960s. − Recent publications show that ongoing research has resulted in the discovery of new properties and applications of these mixed oxides. − The synergy effect (the enhancement of the catalytic activity in the catalysts mixed from different components) in bismuth molybdate based catalysts has been observed and studied since the 1980s. − This synergy effect was interpreted differently by different research groups, such as the elimination of excess Bi at the surface of the gamma phase, the elimination of Mo at the surface of the alpha phase, or the spillover of oxygen. , However, no decisive conclusion on the origin of the synergy effect has been obtained. Actually, adding some additives to bismuth molybdate catalysts could result in many unprecedented performance enhancements in the activity and selectivity by modifying the electronic properties of active sites, generating the “remote control effect” to facilitate the redox Mars and van Krevelen mechanism, or could even lead to reconstruction of the surface structure during the reaction. − The discussion about the catalytic activities of bismuth molybdate based catalysts seemed to cease in the early 2000s. Since the synergy effects of different compositions of gamma bismuth molybdate (γ-Bi 2 MoO 6 ) in the mixture with beta bismuth molybdate (β-Bi 2 Mo 2 O 9 ) were reported by Vieira Soares et al and followed by our group, , the discussions on the synergy effect in bismuth molybdate based catalysts were continued with more distinct interpretations .…”

Divide-and-Conquer Chemical Bonding Models for Materials: A Tool for Materials Design at the Electronic Level

“…First of all, again, the X-binding states lie deep below E F (Figure 3). Our localized AdNDP states directly correspond to these delocalized states of the solid, as can be proven by finding them among the true plotted wave functions across the Brillouin zone, 26 or via DOS projections on AOs. Secondly, tetracoodinated C is usually tetrahedral, sp 3 -hydridized, and participates in 4 covalent bonds.…”

“…We will start from the surface alloys: borides, carbides, and nitrides, of Co and Ni. 26 These materials are important in catalysis. Carbon embedding in the surfaces of the Co and Ni metals initiates coke deposition and eventual blocking of all sites on the metal and poisoning of the catalysis, Co -in Fischer-Tropsch synthesis, and Ni -in steam reforming.…”

“…Therefore, the M 4 X unit, M=Co,Ni, X=C,B, in the surface alloy (Figure 3A) is a representative cluster that should contain the localized bonding elements responsible for strong binding of C/B, in the unusual square planar configuration. In our original publication 26 we simply analyzed the delocalized wave functions of the surface alloy to draw our conclusions. However, we subsequently further supported the conclusions through the AdNDP analysis.…”

“…Lastly, as a tribute to the stability of the aromatic arrangement, we showed that upon replacement of M and X (M = Co, Ni, Cu, Rh, Pd, Ag, and X = B, C, N), the states that bind the main group element remain exactly the same. 26 The alloys hold onto their triply-aromatic coordination of X, even though the charge distribution may vary drastically, e.g. B is nearly neutral, while C has the charge of -1.…”