Density functional study of the C atom adsorption on the α-Fe 2O3 (001) surface

The adsorption of C atoms on the α-Fe₂O₃ (001) surface was studied based on density function theory (DFT), in which the exchange-correlation potential was chosen as the PBE (Perdew, Burke and Ernzerhof) generalized gradient approximation (GGA) with a plane wave basis set. Upon the optimization on different adsorption sites with coverage of 1/20 and 1/5 ML, it was found that the adsorption of C atoms on the α-Fe ₂O₃ (001) surface was chemical adsorption. The coverage can affect the adsorption behavior greatly. Under low coverage, the most stable adsorption geometry lied on the bridged site with the adsorption energy of about 3.22 eV; however, under high coverage, it located at the top site with the energy change of 8.79 eV. Strong chemical reaction has occurred between the C and O atoms at this site. The density of states and population analysis showed that the s, p orbitals of C and p orbital of O give the most contribution to the adsorption bonding. During the adsorption process, O atom shares the electrons with C, and C can only affect the outermost and subsurface layers of α-Fe₂O₃; the third layer can not be affected obviously. Copyright © 2008 Chinese Journal of Structural Chemistry.

Density Functional Study on Reaction Activity of Steam Decomposition Products Adsorbed on Carbonaceous Surface

A systematic theoretical study on the adsorption of steam and its thermal decomposition products on carbon both zigzag and armchair surface was performed to provide molecular-level understanding of the reaction activity of all these reactants in biomass steam gasification process. All the calculations were carried out using density functional theory (DFT) at the B3LYP/6-31+g(d,p) level. The structures of carbonaceous surfaces, all reactants and surface complexes were optimized and characterized. Based on the value of adsorption heat been obtained from the calculation, the activity of all reactants can be ordered as: O > O2 >H2 >H >OH >H2O for both zigzag and armchair surface, and the adsorption style is physisorption to water molecule and chemisorption to the other dissociated components.

Identifying the distinct features of geometric structures for hole trapping to generate radicals on rutile TiO2(110) in photooxidation using density functional theory calculations with hybrid functional

Using density functional theory calculations with HSE 06 functional, we obtained the structures of spin-polarized radicals on rutile TiO2(110), which is crucial to understand the photooxidation at the atomic level, and further calculate the thermodynamic stabilities of these radicals. By analyzing the results, we identify the structural features for hole trapping in the system, and reveal the mutual effects among the geometric structures, the energy levels of trapped hole states and their hole trapping capacities. Furthermore, the results from HSE 06 functional are compared to those from DFT + U and the stability trend of radicals against the number of slabs is tested. The effect of trapped holes on two important steps of the oxygen evolution reaction, i.e. water dissociation and the oxygen removal, is investigated and discussed.

Density functional theory study of mixed-phase TiO2: heterostructures and electronic properties

In this work, density functional theory calculations have been performed to study the geometric, electronic, and energetic properties of two-phase TiO2 composites built by joining two single-phase TiO2 slabs, aiming at verifying possible improvement of the photo-activities of the composites through phase separation of excitons. We find that such desired electronic properties can be determined by several factors. When both the HOMO and LUMO levels of one of the two single-phase TiO2 slabs are higher than the corresponding ones of the other, the composite may have native electronic structures with phase-separated HOMO-LUMO states, especially when the two slabs exhibit highly matched surface lattices. For those pairs of TiO2 slabs with the HOMO and LUMO levels of one phase being within the range of those of the other, though the energetically favored composite give HOMO-LUMO states within one phase, one may still be able to separate them and move the HOMO state to the interface region by destabilizing the interactions between the two slabs.

Activity and coke formation of nickel and nickel carbide in dry reforming: A deactivation scheme from density functional theory

Dry reforming is a promising reaction to utilise the greenhouse gases CO2 and CH4. Nickel-based catalysts are the most popular catalysts for the reaction, and the coke formation on the catalysts is the main obstacle to the commercialisation of dry reforming. In this study, the whole reaction network of dry reformation on both flat and stepped nickel catalysts (Ni(111) and Ni(211)) as well as nickel carbide (flat: Ni3C(001); stepped: Ni3C(111)) is investigated using density functional theory calculations. The overall reaction energy profiles in the free energy landscape are obtained, and kinetic analyses are utilised to evaluate the activity of the four surfaces. By careful examination of our results, we find the following regarding the activity: (i) flat surfaces are more active than stepped surfaces for the dry reforming and (ii) metallic nickel catalysts are more active than those of nickel carbide, and therefore, the phase transformation from nickel to nickel carbide will reduce the activity. With respect to the coke formation, the following is found: (i) the coke formation probability can be measured by the rate ratio of CH oxidation pathway to C oxidation pathway (r(CH)/r(C)) and the barrier of CO dissociation, (ii) on Ni(111), the coke is unlikely to form, and (iii) the coke formations on the stepped surfaces of both nickel and nickel carbide can readily occur. A deactivation scheme, using which experimental results can be rationalised, is proposed. 

Density functional theory study on the activation of molecular oxygen on a stepped gold surface in an aqueous environment: a new approach for simulating reactions in solution

The activation of oxygen molecules is an important issue in the gold-catalyzed partial oxidation of alcohols in aqueous solution. The complexity of the solution arising from a large number of solvent molecules makes it difficult to study the reaction in the system. In this work, O-2 activation on an Au catalyst is investigated using an effective approach to estimate the reaction barriers in the presence of solvent. Our calculations show that O-2 can be activated, undergoing OOH* in the presence of water molecules. The OOH* can readily be formed on Au(211) via four possible pathways with almost equivalent free energy barriers at the aqueous-solid interface: the direct or indirect activation of O-2 by surface hydrogen or the hydrolysis of O-2 following a Langmuir-Hinshelwood mechanism or an Eley-Rideal mechanism. Among them, the Eley-Rideal mechanism may be slightly more favorable due to the restriction of the low coverage of surface H on Au(211) in the other mechanisms. The results shed light on the importance of water molecules on the activation of oxygen in gold-catalyzed systems. Solvent is found to facilitate the oxygen activation process mainly by offering extra electrons and stabilizing the transition states. A correlation between the energy barrier and the negative charge of the reaction center is found. The activation barrier is substantially reduced by the aqueous environment, in which the first solvation shell plays the most important role in the barrier reduction. Our approach may be useful for estimating the reaction barriers in aqueous systems.

Mn ion dissolution from MnS:a density functional theory study

The dissolution of MnS inclusions could induce pitting corrosion in stainless steels, but its dissolution mechanism is poorly understood at the atomic scale. With the help of ab initio molecular dynamics calculations, one inevitable step in the dissolution of MnS is studied by simulating the process of one Mn ion leaving the surface. The reaction mechanism is determined to contain three steps with two large barriers and a small one, leading to two slow steps in the Mn ion dissolution. Comparing to the Na ion dissolution from NaCl, the barriers of the Mn ion dissolution are much larger, which is a reflection of their different electronic structures.

A density functional theory study of hydrogen dissociation and diffusion at the perimeter sites of Au/TiO2

Reactivity of supported gold catalysts is a hot topic in catalysis for many years. This communication reports an investigation on the dissociation of molecular hydrogen at the perimeter sites of Au/TiO2 and the spillover of hydrogen atoms from the gold to the support using density functional theory calculations. It is found that the heterolytic dissociation is favoured in comparison with homolytic dissociation of molecular hydrogen at the perimeter sites. However, the surface oxygen of the rutile TiO2(110) surface at these sites can be readily passivated by the formed OH, suggesting that further dissociation of molecular hydrogen may occur at pure gold sites.

Density Functional Theory Studies of Ethanol Decomposition on Rh(211)

Density functional theory calculations were carried out to examine the mechanism of ethanol decomposition on the Rh(211) surface. We found that there are two possible decomposition pathways: (1) CH(3)CH(2)OH -> CH(3)CHOH -> CH(3)COH -> CH(3)CO -> CH(3) + CO -> CH(2) + CO -> CH + CO -> C + CO and (2) CH(3)CH(2)OH -> CH(3)CHOH -> CH(3)COH -> CH(2)COH -> CHCOH -> CHCO -> CH + CO -> C + CO. Both pathways have a common intermediate of CH(3)COH, and the key step is the formation of CH(3)CHOH species. According to our calculations, the mechanism of ethanol decomposition on Rh(211) is totally different from that on Rh(111): the reaction proceeds via CH(3)COH rather than an oxametallacycle species (-CH(2)CH(2)O- for Rh( 111)), which implies that the decomposition process is structure sensitive. Further analyses on electronic structures revealed that the preference of the initial C(alpha)-H path is mainly due to the significant reduction of d-electron energy in the presence of the transition state (TS) complex, which may stabilize the TS-surface system. The present work first provides a clear picture for ethanol decomposition on stepped Rh(211), which is an important first step to completely understand the more complicated reactions, like ethanol steam reforming and electrooxidation.

Oxygen reduction reaction mechanism on nitrogen-doped graphene:A density functional theory study

Nitrogen-doped graphene (N-graphene) was reported to exhibit a good activity experimentally as an electrocatalyst of oxygen reduction reaction (ORR) on the cathode of fuel cells under the condition of electropotential of similar to 0.04 V (vs. NNE) and pH of 14. This material is promising to replace or partially replace the conventionally used Pt. In order to understand the experimental results. ORR catalyzed by N-graphene is studied using density functional theory (DFT) calculations under experimental conditions taking the solvent, surface adsorbates, and coverages into consideration. Two mechanisms, i.e., dissociative and associative mechanisms, over different N-doping configurations are investigated. The results show that N-graphene surface is covered by O with 1/6 monolayer, which is used for reactions in this work. The transition state of each elementary step was identified using four different approaches, which give rise to a similar chemistry. A full energy profile including all the reaction barriers shows that the associative mechanism is more energetically favored than the dissociative one and the removal of O species from the surface is the rate-determining step. (C) 2011 Elsevier Inc. All rights reserved.

Acrolein hydrogenation on Pt(211) and Au(211) surfaces:a density functional theory study

Partial hydrogenation of acrolein, the simplest alpha, beta-unsaturated aldehyde, is not only a model system to understand the selectivity in heterogeneous catalysis, but also technologically an important reaction. In this work, the reaction on Pt(211) and Au(211) surfaces is thoroughly investigated using density functional theory calculations. The formation routes of three partial hydrogenation products, namely propenol, propanal and enol, on both metals are studied. It is found that the pathway to produce enol is kinetically favoured on Pt while on Au the route of forming propenol is preferred. Our calculations also show that the propanal formation follows an indirect pathway on Pt(211). An energy decomposition method to analyze the barrier is utilized to understand the selectivities at Pt(211) and Au(211), which reveals that the interaction energies between the reactants involved in the transition states play a key role in determining the selectivity difference.

Some Understanding of Fischer-Tropsch Synthesis from Density Functional Theory Calculations

The combination of density functional theory (DFT) calculations and kinetic analyses is a very useful approach to study surface reactions in heterogeneous catalysis. The present paper reviews some recent work applying this approach to Fischer-Tropsch (FT) synthesis. Emphasis is placed on the following fundamental issues in FT synthesis: (i) reactive sites for both hydrogenation and C-C coupling reactions; (ii) reaction mechanisms including carbene mechanism, CO-insertion mechanism and hydroxyl-carbene mechanism; (iii) selectivity with a focus on CH(4) selectivity, alpha-olefin selectivity and chain growth probability; and (iv) activity.

Density Functional Theory Study of Iron and Cobalt Carbides for Fischer-Tropsch Synthesis

Carbides are important phases in heterogeneous catalysis. However, the understanding of carbide phases is inadequate: Fe and Co are the two commercial catalysts for Fischer-Tropsch (FT) synthesis, and experimental work showed that Fe carbide is the active phase in FT synthesis, whereas the appearance of Co carbide is considered as a possible deactivation cause, TO understand very different catalytic roles of carbides, all the key elementary steps in FT synthesis, that is, CO dissociation, C(1) hydrogenation, and C(1)+C(1) coupling, are extensively investigated on both carbide surfaces using first principles calculations. In particular, the most important issues in FT synthesis, the activity and methane selectivity, on the carbide surfaces are quantitatively determined and analyzed. They are also discussed together with metallic Fe and Co surfaces. It is found that (i) Fe carbide is more active than metallic Fe and has similar methane selectivity to Fe, being consistent with the experiments; and (ii) Co carbide is less active than Co and has higher methane selectivity, providing evidence on the molecular level to support the suggestion that the formation of Co carbide is a cause of relatively high methane selectivity and deactivation on Co catalysts.

An Energy Descriptor To Quantify Methane Selectivity in Fischer-Tropsch Synthesis:A Density Functional Theory Study

Selectivity is a fundamental issue in heterogeneous catalysis. In this study, the CH(4) selectivity in Fischer-Tropsch synthesis is chosen to be investigated: CH4 selectivity on Rh, Co, Ru, Fe, and Re surfaces is computed by first-principles methods. In conjunction with kinetic analyses, we are able to derive the effective barrier difference between methane formation and chain growth (Delta E(eff)) to quantify the CH(4) selectivity. By using this energy descriptor, the ranking of methane selectivity predicted from density functional theory (DFT) calculations is consistent with experimental work. Moreover, a linear correlation between Delta E(eff) and the chemisorption energy of C + 4H (Delta H) is found. This fundamental finding possesses the following significance: (i) it shows that the selectivity, which appears to have kinetic characteristics, is largely determined by thermodynamic properties; and (ii) it suggests that an increase of the binding strength of C + 4H will suppress methane selectivity.