•Wavefunctions at the band edges of four prototypical perovskites are constructed.•Trends are analyzed on the relation between composition and band gap of perovskites.•Incorporation of BH4− or pseudohalides in MAPbX3 maintains its direct band gap.•Replacing Pb2+ in MAPbX3 with non-s 2 metals weakens its optoelectronic properties.Perovskites are diverse materials to explore for advanced functional applications, but experimental or computational characterization of individual perovskite compositions is inefficient considering nearly endless possible combinations of the constituent ions. We analyze the band edges of a semiconducting perovskite by constructing electronic wavefunctions based on orbital symmetries, and then extract such information as electron wavevectors, band-edge transition, and chemical bonding. Using MAPbX3 (MA = methylammonium; X = Br, I), CsCdBr3, CsCaBr3, and TMASn(N3)3 (TMA = tetramethylammonium) as prototypical perovskites, we propose a set of trends on whether ionic substitution changes MAPbX3 from a direct band gap to an indirect one. Compositions containing an s2 metal cation and a (pseudo)halide are found to exhibit a direct band gap as MAPbX3 does. The broad applicability of these trends, verified by an extensive range of perovskite compositions, indicates that pseudohalide perovskites should be explored for novel functional materials, and that substitution of Pb2+ in MAPbI3 by non-s2 metal cations will probably deteriorate the optoelectronic properties of MAPbI3.

Periodic density functional theory calculations were performed to study the surface structures and stabilities of the La2O3 catalyst in CO2 and O2 environments, relevant to the conditions of the oxidative coupling of methane (OCM) reaction. Thermodynamic stabilities of the clean surfaces were predicted to follow the order of (001) ≥ (011) ≫ (110) > (111) > (101) > (100), with their direct band gaps at the Γ point following the similar order of (001) > (011) > (110) > (111) > (100) > (101). Hubbard U corrections to the La 4f and 5d orbitals do not qualitatively change the predictions of surface energies and band gaps. For the most stable (001) surface, CO2 chemisorption to form carbonate species is exothermic by −0.60 eV with a negligible energy barrier of 0.07 eV, whereas O2 chemisorption to form peroxide species is endothermic by 0.64 eV with a considerable energy barrier of 1.29 eV. For the slightly less stable (011) surface, both CO2 and O2 chemisorption can occur at different surface sites, and the same applies to the other studied surfaces. Dissociation temperatures of surface carbonate species range from 300 to 1000 K at pCO2 of 1 bar, which follow the order of (101) ≈ (110) > (111) ≈ (100) ≈ (011) ≫ (001), showing their strong sensitivity to the surface structure. Dissociation temperatures of surface peroxide species are mostly lower than the room temperature except for those of the (011) and (111) surfaces, although the significant kinetic barriers predicted should prevent their facile dissociation. Insights into the temperature-programmed desorption experiments and the methane reactivity of La2O3 in the OCM reaction were also given based on the results of our calculations.

Co3O4 nanocatalysts have been experimentally shown to have excellent performance in catalyzing CH4 combustion. These nanocatalysts of different morphologies, such as nanoparticle/nanocube, nanorod/nanobelt, and nanoplate/nanosheet, were previously synthesized and characterized to mainly expose the (001), (011), and (112) surfaces, respectively, with distinct reactivities. In this study, rigorous first principles calculations were performed to investigate CH4 reactivities of the above Co3O4 surfaces of different terminations. CH4 dissociation was predicted to occur at the Co–O pair site on these surfaces. For each surface, the most reactive Co–O pair site was identified based on calculated energy barriers of the different active sites, which should contribute most significantly to the reactivity of that surface. The lowest energy barriers for the (001), (011), and (112) surfaces were predicted to be 0.96, 0.90, and 0.79 eV, respectively, suggesting CH4 reactivity to increase in that order for the different Co3O4 surfaces, consistent with the trend found experimentally for Co3O4 nanocatalysts of different morphologies. Direct comparison between the estimated and experimental CH4 reaction rates per gram of the nanocatalysts at 325 °C further indicate that their relative ratios were well reproduced by considering three main factors: the effective energy barrier for CH4 dissociation, the surface area of the nanocatalyst, and the number of independent active sites per unit surface area. The important influence of surface area on CH4 reactivity is also demonstrated by the significant difference in the reactivities of the nanocatalysts when exposing the same facet but with distinct surface areas.

First principles calculations using both molecular cluster and periodic slab models were performed to reveal the mechanism for the formation of dimethoxymethane (DMM) from methanol over V2O5/TiO2-based catalysts. Two different pathways were found, and the formation of DMM was predicted to be initiated by methanol chemisorption followed by a dehydration reaction with hemiacetal catalyzed by acidic sites. For unpromoted V2O5/TiO2 catalysts, we predicted the energy barrier for the rate determining step (RDS) to follow the order formaldehyde (FA) > methyl formate (MF) > DMM, consistent with the experimental observation for the preferential formation of DMM at a relatively low temperature and that of MF at a relatively high temperature. For sulfate-promoted catalysts, the energy barriers were calculated to follow the order FA > DMM > MF, so the sulfate promoter was predicted to mainly enhance the selectivity of MF, consistent with our previous experiment in which very high yield of MF was obtained with the sulfate-promoted catalyst. Calculated rate constants for the RDS were further used for semi-quantitative predictions of the product selectivities, which were found to be in quite good agreement with some of the recent experimental data in the literature, showing the validity of our approach. We also investigated the effects of the titania support and the polymerization of the vanadia species on the reactivity of the V2O5/TiO2 catalyst. Finally, we benchmarked several popular exchange–correlation functionals for calculating the reaction energies for the formation of FA, MF, and DMM from methanol oxidation, and the M06 hybrid functional was found to be superior to other semi-local and hybrid functionals studied.

Density functional theory and coupled cluster theory calculations were carried out to study the effects of the acid–base properties of the La2O3 catalyst on its catalytic activity in the oxidative coupling of methane (OCM) reaction. The La3+–O2− pair site for CH4 activation is considered as a Lewis acid–Brönsted base pair. Using the Lewis acidity and the Brönsted basicity in the fluoride affinity and proton affinity scales as quantitative measures of the acid–base properties, the energy barrier for CH4 activation at the pair site can be linearly correlated with these acid–base properties. The pair site consisting of a strong Lewis acid La3+ site and a strong Brönsted base O2− site is the most reactive for CH4 activation. In addition, the basicity of the La2O3 catalyst was traditionally measured by temperature-programmed desorption of CO2, but the CO2 chemisorption energy is better regarded as a combined measure of the acid–base properties of the pair site. A linear relationship of superior quality was found between the energy barrier for CH4 activation and the CO2 chemisorption energy, and the pair site favorable for CO2 chemisorption is also more reactive for CH4 activation, leading to the conflicting role of the “basicity” of the La2O3 catalyst in the OCM reaction. The necessity for very high reaction temperatures in the OCM reaction is rationalized by the requirement for the recovery of the most reactive acid–base pair site, which unfortunately also reacts most readily with the byproduct CO2 to form the very stable CO32− species.

Vanadia–titania–sulfate nanocatalysts for methanol oxidation to methyl formate (MF) were prepared by coprecipitation. When calcinated at 400 °C, both methanol conversion and MF selectivity reached ∼98.5% at reaction temperatures of 140–145 °C. Characterizations with several experimental techniques revealed the catalysts as highly dispersed vanadia supported by anatase titania with acidic sites of significant strength and density. The catalysts also showed very high stability with lifetime exceeding 4500 h. Extensive density functional theory calculations using both cluster and surface models revealed MF to form via the hemiacetal mechanism, involving the condensation of methanol and formaldehyde at acidic sites and methanol and hemiacetal oxidations at redox sites. The alternative formyl mechanism was predicted kinetically much less favorable, showing these catalysts to work in a distinct mechanism from the rutile titania photocatalyst. Interestingly, methanol chemisorption at redox sites leads to the formation of acidic sites capable of catalyzing the condensation reaction.

Density functional theory calculations were carried out using both cluster and slab models to investigate the catalytic reaction network and the effect of sulfate promoter on methanol selective oxidation using the V2O5/TiO2 catalyst. The hemiacetal mechanism was reaffirmed to be the most favorable reaction pathway for the formation of methyl formate (MF) by the prediction of another reaction pathway involving formic acid. Molecular oxygen was found to assist the desorption of the reaction products from the reduced catalyst active site, both formaldehyde and methyl formate (MF). The mechanism of catalyst regeneration was elucidated based solely on first-principles calculations, which involve the conversion of another methanol molecule to formaldehyde over a peroxo species. This leads to our formulation of a complete catalytic reaction network for methanol selective oxidation to formaldehyde and MF on the V2O5/TiO2 catalyst. In addition, the detailed mechanism for the formation of formaldehyde and MF was also predicted on the sulfate-promoted V2O5/TiO2 catalyst. Based on the calculated reaction networks, the preferred formation of MF on the V2O5/TiO2-based catalysts was attributed to the lower energy barrier of the oxidative dehydrogenation (ODH) of the CH3OCH2O* intermediate than that of CH3O*. Furthermore, our calculated energy barriers also suggest that the sulfate-promoted V2O5/TiO2 catalyst has not only higher catalytic activity for methanol conversion but also higher selectivity of MF over CH2O, consistent with previous experimental observations. The sulfate promoter was found to increase the positive charge at the V site, leading to a lower energy barrier for the ODH of CH3O* than the unpromoted catalyst.

Density functional theory and coupled cluster theory calculations were carried out to study the formation of the carbonate species on La2O3 catalyst using the cluster model and its effect on subsequent CH4 activation. Physisorption and chemisorption energies as well as energy barriers for the reaction of CO2 and La2O3 clusters, and the reaction of CH4 with the CO32– site on the resulting clusters, were predicted. Our calculations show that CO2 chemisorption at the La3+–O2– pair sites is thermodynamically and kinetically very favorable due to the strong basicity of the O2– site on La2O3, which leads to the formation of the La3+–CO32– pair sites. In addition, CH4 activation at the La3+–CO32– pair sites is similar to that at the La3+–O2– pair sites, which results in the formation of the bicarbonate species and the La–CH3 bond, although the La3+–CO32– pair sites are much less reactive with CH4 in terms of both thermodynamics and kinetics. Further thermodynamical calculations show that the CO32– species in these clusters dissociate between 500 to 1250 K, with half of them completely dissociated at 873 K, consistent with the experimental observation. Our studies suggest that the CO32– site is unlikely to be the active site in La2O3-catalyzed oxidative coupling of methane, and CO2 as a major byproduct is likely to act as a poison to the La2O3-based catalysts especially at modest reaction temperature.

Density functional theory and coupled cluster theory [CCSD(T)] calculations reveal an important pathway for the one-step CH3OH formation upon CH4 activation at the peroxide (O22–) site of La2O3-based catalysts for the oxidative coupling of methane (OCM) reaction. Using modest-sized La4O7 and La6O10 clusters as catalyst models, two types of structures for the O22– site were predicted, with the less stable structure (type II) more reactive with CH4 than the more stable structure (type I). CH4 activation at the O22– site can always occur via the above pathway, and for the smaller La2O4 cluster and the type I structure of La4O7, an alternative pathway leading to La–CH3 bond formation was also predicted, similar to that at the oxide (O2–) site from our previous study. For the type I structure of La4O7, the energy barrier for La–CH3 bond formation is lower than that for CH3OH formation, but both are higher than that for CH3OH formation for the type II structure of La4O7. The O22– site was predicted to be much less reactive with CH4 than the oxide (O2–) site, and can lead to CH3OH formation, which is considered as a side reaction. Thus, our calculations do not appear to support the central role previously proposed for the O22– site for La2O3-based catalysts for the OCM reaction. However, considering the catalytic and redox nature of this reaction, both the O2– and O22– sites may still play important roles in the whole catalytic cycle.

Density functional theory and coupled cluster theory were employed to study the activations of CH4 by neutral lanthanum oxide clusters (LaO(OH), La2O3, La3O4(OH), La4O6, La6O9) as models for the La2O3 catalysts for the oxidative coupling of methane (OCM) reaction. The physisorption energies (ΔH298 K) of CH4 on the lanthanum oxide clusters were predicted to be −4 to −3 kcal/mol at the CCSD(T) level. CH4 is activated by hydrogen transfer to one of the O sites on the lanthanum oxide clusters, and the energy barriers (ΔE0 K) from the physisorption structures were calculated to be modest at ∼20 kcal/mol for La2O3 and ∼25 kcal/mol for the other clusters. This is accompanied by the formation of a La–CH3 bond, whose bond dissociation energy (ΔE0 K) was calculated to be 53 to 60 kcal/mol. CH4 chemisorption is slightly exothermic on LaO(OH) and La2O3, whereas it becomes increasingly endothermic for the larger lanthanum oxide clusters. The formation of the CH3 radical was predicted to be substantially endothermic, by ∼50 kcal/mol for LaO(OH) and La2O3 and 64 to 76 kcal/mol for La3O4(OH) and La4O6 (ΔH298 K). Calculations on the activation of CH4 by La6O9 with a higher coordination number for both the La and O sites than La4O6 yield an energy barrier slightly higher by <1 kcal/mol, suggesting that the effects of the coordination numbers on the reaction energetics are rather small. The energy barrier for hydrogen abstraction does not correlate well with the negative charge on the O site, and a linear relation between the energy barrier and the chemisorption energy was not found for all the lanthanum oxide clusters, which is attributed to the strong dependency of their correlation on the specific chemical environment of the reactive site. Cluster reaction energies, physisorption and chemisorption energies, energy barriers, and La–CH3 bond energies calculated at the DFT level with the B3LYP and PBE functionals were compared with those calculated at the CCSD(T) level showing that the B3LYP functional yields better cluster reaction energies, chemisorption energies, and energy barriers. Although the PBE functional yields better physisorption energies, the DFT results can deviate substantially from the CCSD(T) values. Although the O2– sites in these cluster models were predicted to be less reactive toward CH4 than the O– sites modeled by the nonstoichiometric La2O3.33(001) surface (Palmer, M. S. et al. J. Am. Chem. Soc. 2002, 124, 8452), they are more reactive than the O22– site modeled on the stoichiometric La2O3(001) surface, which suggests the relevance of the lattice oxygen sites on the La2O3 catalyst surfaces in the OCM reaction.

Periodic density functional theory calculations were carried out to investigate CO dissociation pathways on the Fe(100) surfaces covered with up to one monolayer of Cu atoms, which serve as the simple models for the Cu/Fe catalysts for higher alcohol synthesis (HAS) from syngas. For all the model catalyst surfaces, H-assisted CO dissociation was predicted to have lower energy barriers than direct CO dissociation. The difference in the energy barriers between the two dissociation pathways increases as Cu surface coverage increases, suggesting reduced contribution of direct CO dissociation on Cu-rich surfaces. A further thermodynamic analysis also reaches the same conclusion. Several reaction properties for CO dissociation, including CO physisorption and chemisorption energies, and energy barriers for direct and H-assisted CO dissociations, were found to scale linearly with Cu surface coverage, and these reaction properties were predicted to depend largely on the structure of the surface layer, which can be expected to also apply to other metal alloy catalysts. Cu doping was found to reduce the activity of the Fe(100) surface in catalyzing direct and H-assisted CO dissociations, so CO dissociations should occur primarily on Fe-rich surfaces, leading to CHx formation, whereas Cu-rich surfaces are potential sources for physisorbed CO molecules. This is also expected to apply to other Cu/M catalysts and is consistent with the dual site mechanism previously proposed for these bimetallic catalysts. A synergy between these two types of active sites is beneficial for the formation of higher alcohols, which may be the reason for the superior performance of the Cu/Fe catalysts for the HAS reaction.

Density functional theory and coupled cluster theory calculations were carried out to study the effects of the acid–base properties of the La2O3 catalyst on its catalytic activity in the oxidative coupling of methane (OCM) reaction. The La3+–O2− pair site for CH4 activation is considered as a Lewis acid–Brönsted base pair. Using the Lewis acidity and the Brönsted basicity in the fluoride affinity and proton affinity scales as quantitative measures of the acid–base properties, the energy barrier for CH4 activation at the pair site can be linearly correlated with these acid–base properties. The pair site consisting of a strong Lewis acid La3+ site and a strong Brönsted base O2− site is the most reactive for CH4 activation. In addition, the basicity of the La2O3 catalyst was traditionally measured by temperature-programmed desorption of CO2, but the CO2 chemisorption energy is better regarded as a combined measure of the acid–base properties of the pair site. A linear relationship of superior quality was found between the energy barrier for CH4 activation and the CO2 chemisorption energy, and the pair site favorable for CO2 chemisorption is also more reactive for CH4 activation, leading to the conflicting role of the “basicity” of the La2O3 catalyst in the OCM reaction. The necessity for very high reaction temperatures in the OCM reaction is rationalized by the requirement for the recovery of the most reactive acid–base pair site, which unfortunately also reacts most readily with the byproduct CO2 to form the very stable CO32− species.

First principles calculations using both molecular cluster and periodic slab models were performed to reveal the mechanism for the formation of dimethoxymethane (DMM) from methanol over V2O5/TiO2-based catalysts. Two different pathways were found, and the formation of DMM was predicted to be initiated by methanol chemisorption followed by a dehydration reaction with hemiacetal catalyzed by acidic sites. For unpromoted V2O5/TiO2 catalysts, we predicted the energy barrier for the rate determining step (RDS) to follow the order formaldehyde (FA) > methyl formate (MF) > DMM, consistent with the experimental observation for the preferential formation of DMM at a relatively low temperature and that of MF at a relatively high temperature. For sulfate-promoted catalysts, the energy barriers were calculated to follow the order FA > DMM > MF, so the sulfate promoter was predicted to mainly enhance the selectivity of MF, consistent with our previous experiment in which very high yield of MF was obtained with the sulfate-promoted catalyst. Calculated rate constants for the RDS were further used for semi-quantitative predictions of the product selectivities, which were found to be in quite good agreement with some of the recent experimental data in the literature, showing the validity of our approach. We also investigated the effects of the titania support and the polymerization of the vanadia species on the reactivity of the V2O5/TiO2 catalyst. Finally, we benchmarked several popular exchange–correlation functionals for calculating the reaction energies for the formation of FA, MF, and DMM from methanol oxidation, and the M06 hybrid functional was found to be superior to other semi-local and hybrid functionals studied.