Search

The possible reaction mechanisms for CO oxidation on the perfect Cu2O(111) surface have been investigated by performing periodic density functional theoretical calculations. We find that Cu2O(111) is able to facilitate the CO oxidation with different mechanisms. Four possible mechanisms are explored (denoted as MER1, MER2, MLH1, and MLH2, respectively): MER1 is CO(gas)+O2(ads)→CO2(gas); MER2 is CO(gas)+O2(ads)→CO3(ads)→O(ads)+CO2(gas); MLH1 refers to CO(ads)+O2(ads)→O(ads)+CO2(ads); and MLH2 refers to CO(ads)+O2(ads)→OOCO(ads)→O(ads)+CO2(ads). Our transition state calculations clearly reveal that MER1 and MLH2 are both viable; but MER1 mechanism preferentially operates, in which only a moderate energy barrier (60.22 kJ/mol) needs to be overcome. When CO oxidation takes place along MER2 path, it is facile for CO3 formation, but is difficult for its decomposition, thereby CO3 species can stably exist on Cu2O(111). Of course, the reaction of CO with lattice O of Cu2O(111) is also considered. However, the calculated barrier is 600.00 kJ/mol, which is too large to make the path feasible. So, we believe that on Cu2O(111), CO reacts with adsorbed O, rather than lattice O, to form CO2. This is different from the usual Mars-van Krevene mechanism. The present results enrich our understanding of the catalytic oxidation of CO by copper-based and metal-oxide catalysts. [ABSTRACT FROM AUTHOR]

Coadsorption of carbon monoxide (CO) and nitric oxide (NO) on the Cu2O(111) surface was studied using periodic density functional theory calculations. It is interesting to find that CO+NO on Cu2O(111) could react to form adsorbed NCO surface species. Coadsorption of CO and NO could give rise to the formation of a O–C...N–O complex well bound to the Cu2O(111) surface, in which both the C–O and N–O bonds are greatly activated and the C–N bond is formed. Consequently, the reaction of CO with NO to form adsorbed NCO and CNO species may occur, for which it is disclosed that NCO formation is more possible than CNO formation both thermodynamically and kinetically. In addition, our calculations of searching transition states reveal that it is facile for NCO formation both kinetically and thermodynamically when CO+NO reaction takes place at CuCUS site, and is impossible when this reaction takes places at Ovac site. Moreover, CO2 species cannot form when CO+NO reaction occurs at Ovac site. Therefore, oxygen vacancy on Cu2O(111) does not play a positive role on CO+NO reaction to forming NCO, CNO, or CO2 species. [ABSTRACT FROM AUTHOR]

Thermoelectrics interconvert heat to electricity and are of great interest in waste heat recovery, solid-state cooling and so on. Here we assessed the potential of SnS2 and SnSe2 as thermoelectric materials at the temperature gradient from 300 to 800 K. Reflecting the crystal structure, the transport coefficients are highly anisotropic between a and c directions, in particular for the electrical conductivity. The preferred direction for both materials is the a direction in TE application. Most strikingly, when 800 K is reached, SnS2 can show a peak power factor (PF) of 15.50 μW cm−1 K−2 along the a direction, while a relatively low value (11.72 μW cm−1 K−2) is obtained in the same direction of SnSe2. These values are comparable to those observed in thermoelectrics such as SnSe and SnS. At 300 K, the minimum lattice thermal conductivity (κmin) along the a direction is estimated to be about 0.67 and 0.55 W m−1 K−1 for SnS2 and SnSe2, respectively, even lower than the measured lattice thermal conductivity of Bi2Te3 (1.28 W m−1 K−1 at 300 K). The reasonable PF and κmin suggest that both SnS2 and SnSe2 are potential thermoelectric materials. Indeed, the estimated peak ZT can approach 0.88 for SnSe2 and a higher value of 0.96 for SnS2 along the a direction at a carrier concentration of 1.94 × 1019 (SnSe2) vs. 2.87 × 1019 cm−3 (SnS2). The best ZT values in SnX2 (X = S, Se) are comparable to that in Bi2Te3 (0.8), a typical thermoelectric material. We hope that this theoretical investigation will provide useful information for further experimental and theoretical studies on optimizing the thermoelectric properties of SnX2 materials. [ABSTRACT FROM AUTHOR]

Abstract: First-principles calculations based on density functional theory (DFT) and the generalized gradient approximation (GGA) have been used to study the adsorption of CO and NO molecules on the Cu2O(111) surface in the presence of oxygen vacancy. The calculations employ slab geometry and periodic boundary conditions with partial relaxation of atom positions. Two molecular orientations, X- and O-down (X=C, N), at two distinct sites, Cu1C and oxygen vacancy sites, have been considered. Total energy calculations indicate that the Cu1C position is relatively more favored than the oxygen vacancy site. The predicted binding energies are 144.5kJmol−1 (CO) and 124.1kJmol−1 (NO), respectively. The C–O and N–O stretching frequencies are unequally red-shifted upon adsorption. Upon adsorption at Cu1C site, CO molecule was found to bind to Cu1C atoms in vertical configuration whereas NO molecule adsorption in tilted mode. While upon adsorption at oxygen vacancy site, CO and NO molecules are both vertical to the Cu2O(111) surface. Interestingly, we found that their adsorption properties on oxygen vacancy site are dependent on the defect density. As the density of defective sites increased, the adsorption energies of the defect-XO configuration increase and the N–O bond is continuously weakened whereas the C–O bond remains constant. Therefore, such a process favors the dissociation of the NO molecule and has a small influence on the adsorbed CO molecule. [Copyright &y& Elsevier]

Abstract: Density functional theory has been employed to investigate the adsorption and the dissociation of an N2O at different sites on perfect and defective Cu2O(111) surfaces. The calculations are performed on periodic systems using slab model. The Lewis acid site, CuCUS, and Lewis base site, OSUF are considered for adsorption. Adsorption energies and the energies of the dissociation reaction N2O→N2 +O(s) at different sites are calculated. The calculations show that adsorption of N2O is more favorable on CuCUS adsorption site energetically. CuCUS site exhibits a very high activity. The CuCUS–N2O reaction is exothermic with a reaction energy of 77.45kJmol−1 and an activation energy of 88.82kJmol−1, whereas the OSUF–N2O reaction is endothermic with a reaction energy of 205.21kJmol−1 and an activation energy of 256.19kJmol−1. The calculations for defective surface indicate that O vacancy cannot obviously improve the catalytic activity of Cu2O. [Copyright &y& Elsevier]