1,721,056 research outputs found
On the effects of doping on the catalytic performance of (La,Sr)coo 3 . A DFT study of CO oxidation
Full text linkThe effects of modifying the composition of LaCoO3 on the catalytic activity are predicted by density functional calculations. Partially replacing La by Sr ions has benefical effects, causing a lowering of the formation energy of O vacancies. In contrast to that, doping at the Co site is less effective, as only 3d impurities heavier than Co are able to stabilize vacancies at high concentrations. The comparison of the energy profiles for CO oxidation of undoped and of Ni-, Cu-m and Zn-doped (La,Sr)CoO3(100) surface shows that Cu is most effective. However, the effects are less spectacular than in the SrTiO3 case, due to the different energetics for the formation of oxygen vacancies in the two hosts
Ab Initio Modeling of TiO2 Nanosheets
No full textWe present density functional calculations on 1–6 monolayer (ML) thick TiO2 films peeled off from the main low-index surfaces of anatase. The structure of the films is optimized both by constraining the lattice constants to those of bulk anatase, and by allowing them to relax. It is found that the stability order of the films does not follow that of the surfaces from which they are derived, and does not increase monotonously with film thickness. Furthermore, relaxing the lattice constants can induce large modifications in the film structure. In particular, two anomalously stable films are found. One derives from the 2 ML (001) film, and rearranges to a lepidocrocite-TiO2 nanosheet. The other one derives from a 4 ML (101) film, and gives rise to a novel phase, where all the Ti ions are fivefold coordinated
A Theoretical Study of the Chemisorption of H2O and H2S on the Ti2O3(10-12) Non-Polar Surface
No full textDensity functional molecular cluster calculations have been used to investigate the interaction of two Brønsted acids (H2X, X = O, S) with the Ti2O3(10-12) non-polar surface. Adsorbate geometries, vibrational parameters and chemisorption enthalpies are computed and discussed. According to experimental outcomes [R. L. Kurtz and V. E. Henrich, Phys. Rev. B, 1982, 26, 6682], H2O is molecularly adsorbed, even if one of the O–H bonds is significantly lengthened as a consequence of a short hydrogen bonding between the H atom and a surface Lewis base site (Lbs). This interaction determines the peculiar arrangement of the molecule on the surface. At variance with water, H2S is partially deprotonated upon chemisorption giving rise to Las-SH (Las = surface Lewis acid site) and Lbs-H surface species. Independently of the adsorption character, molecular or dissociative, the valence band maximum of Ti2O3(10-12) is negligibly perturbed upon chemisorption, while the conduction band minimum extensively participates to the H2X–Ti2O3(10-12) interaction. The H2O–substrate bonding is dominated by a donation from the adsorbate in-plane and out-of-plane lone pairs into Las empty levels, whereas all the valence orbitals of the HS fragment participate in the Las-S bond to a similar extent
DFT modelling of the NO reduction process at the Cu-doped SrTiO3(1 0 0) stepped surface
No full textThree-way catalytic converters are used to convert the toxic CO and NO automotive emissions into more environmentally sustainable products as CO2 and N2/N2O. In the recent years, the strict control of NOX emission has stimulated research on the catalytic NO reduction processes. Our aim is to use the DFT to investigate the capabilities of doped, rare earth element free perovskites (SrTiO3) step to catalyze the NO reduction process. Here we are focusing the attention on the role of the Cu doping and on the structural defects on the reactivity. In this paper, we investigate the ability of steps to catalyze the complete (to N2) and partial (to N2O) NO reduction considering three mechanisms of the Langmuir-Hinshelwood and of the Eley-Rideal type also in the presence of Cu dopants. Both the number of the oxygen vacancies and the dopant play a role on the process. Energy profiles show that the reduction of NO to N2 is possible at both pure and doped steps, even if with different mechanisms. The partial reduction to N2O is not favored. The Cu doping can modify the mechanism, but its ability to improve the catalytic properties of the step seems to be limited to the capability of stabilizing oxygen vacancies
First Principles Studies of Vanadia−Titania Monolayer Catalysts: Mechanisms of NO Selective Reduction
No full textThe selective reduction of NO with NH3 catalyzed by isolated VOx species grafted onto TiO2 (anatase) is studied by means of periodic density functional calculations. NH3 is adsorbed molecularly by the bare support both as a Lewis-bonded complex at (101) 5-fold coordinated Ti sites, and as a H-bonded complex at (001) Ti−OH sites. Analogous interactions are predicted for stable submonolayer VOx species, which provide V5+ Lewis acid sites and V−OH sites. Neither Ti−OH nor submonolayer V−OH groups act as Brønsted acids toward NH3. Reaction pathways where both Lewis-bonded and H-bonded NH3 complexes yield a NH2NO intermediate are found. In the former case, a (rate-determining) deprotonation step of NH3 is required, whereas, in the latter, NH2NO is formed directly through a concerted mechanism. This suggests that many channels may contribute to the NO reduction process
LCAO-LDA Study of the Chemisorption of Formate on Cu(110) and Ag(110) Surfaces
No full textThe coordination of formate on the Cu(110) and Ag(110) surfaces has been investigated by coupling density functional theory to the molecular-cluster approach. Two adsorption sites, the bidentate bridging (BB) and the bidentate chelating (BC), have been considered for both surfaces. In the BB arrangement, the HCOO oxygen atoms bridge two adjacent metal (M) atoms in the (1[1 with combining macron]0) direction, while in the BC form they chelate a single M atom. Adsorption energies, optimized geometries and vibrational frequencies of the surface HCOO at the BB and BC sites have been computed. Furthermore, the molecular orbitals involved in the adsorbate–substrate interaction have been identified. Independently of the chemisorption site geometry, the HCOO–Cu(110) bond is computed to be stronger and more covalent than that of HCOO–Ag(110). Total energy calculations indicate that the BB coordination site of the Cu(110) surface is favoured with respect to that of BC by ca. 1.0 eV. Despite the nearest neighbour Ag–Ag internuclear distance being longer than that of Cu–Cu, theoretical results pertaining to HCOO on Ag(110) again indicate that the BB site is more stable than that of BC by ca. 0.7 eV
A Comparative Study of the CO Chemisorption on Ti2O3(10-12) and V2O3(10-12) Non-Polar Surfaces
No full textDensity functional molecular cluster calculations have been used to investigate the coordination of CO to Lewis acid sites (Lsa) available on Ti2O3(10-12) and V2O3(10-12) non-polar surfaces. The electronic structure of the clean substrates, the adsorbate geometry and chemisorption enthalpies are computed and discussed. Properties of the clean surfaces are well described by the chosen cluster models. Moreover, the Lsa–CO bonding is found to be very similar to that holding for transition metal carbonyls: i.e., a two-way electron flow implying a σ donation from the CO 5σ HOMO into empty Lsa AOs, assisted by a π backdonation from Lsa occupied orbitals into the CO 2π LUMO. Both the electronic and molecular structure of the adsorbate are significantly perturbed upon chemisorption and, consistently with experimental data, the C–O bond results strongly weakened. The chemisorption enthalpy of CO on V2O3(10-12) (∼−30 kcal/mol) is about twice that computed for CO on Ti2O3(10-12) (∼−16 kcal/mol)
Interstitial O3 in Silica: a Molecular Cluster Density Functional Study
No full textThe electronic and molecular properties of interstitial O3 in SiO2 have been theoretically studied by coupling the molecular cluster model to the density functional theory. We find that, on passing from the free species to the interstitial one, electronic and molecular structures of O3 are only slightly perturbed. Moreover, in agreement with the experimental assignment of Skuja et al., it is confirmed that the ubiquitous absorption band at 4.8 eV characterizing 7.9 eV photon-irradiated SiO2 samples includes a contribution due to excitations between O3 based occupied and unoccupied MOs
Theoretical Investigation of the Chemisorption of H2 and CO on the ZnO(10-10) Surface
No full textDensity functional molecular cluster calculations have been used to study the adsorption of CO and H2 on the ZnO(10-10) surface. Substrate and adsorbate geometry modifications, adsorption energies and adsorbate vibrations are computed in good agreement with experiment. For CO, the influence of Cu surface impurities has been also considered. Despite the limited size of the adopted clusters, surface relaxations computed for the clean and undoped ZnO(10-10) agree well with experimental measurements. The chemisorption of CO on ZnO(10-10) relieves some of the relaxation of the Lewis acid site (Las ); nevertheless, the electronic structure is negligibly affected by the interaction with CO. At variance to that, the stronger interaction of CO with copper impurities significantly influences both the geometry and the electronic structure of , extending its effects to the adjacent Lewis base site (Lbs ). The dissociative adsorption of H2 is found to be exothermic by 23 kcal/mol, and it implies the Las−Lbs bond breaking
A Comparative Study of CO Chemisorption on Al2O3 and Ti2O3 Nonpolar Surfaces
No full textDensity functional molecular cluster calculations have been used to investigate the interaction of CO with the M2O3(10-12) (M = Al and Ti) nonpolar surface. The electronic structure of the clean surface, the adsorbate geometry, vibrational parameters, and chemisorption enthalpies are computed and discussed. Theoretical results pertaining to the clean surface agree quite well with experimental measurements and other theoretical investigations. As far as the adsorbate−substrate interaction is concerned, our data indicate that the CO−M2O3(10-12) bonding is charaterized, in both Al2O3 and Ti2O3, by a two-way electron flow involving both donation from CO based σ levels into virtual orbitals of the unsaturated surface Lewis acid site and back-donation from surface states into the CO π* virtual levels. However, the nature of surface orbitals involved in back-donation and the concomitant effects on the adsorbate structure are very different in the two cases. CO is only slightly affected upon chemisorption on Al2O3(10-12), while perturbations induced into the CO electronic and molecular structure by the interaction with the Ti2O3(10-12) surface are very intense and, consistently with experimental data, the C−O bond becomes strongly weakened
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