Riguang Zhang

A density functional theory investigation on the mechanism and kinetics of dimethyl carbonate formation on Cu2O catalyst

A theoretical analysis about the mechanism and kinetics of dimethyl carbonate (DMC) formation via oxidative carbonylation of methanol on Cu2O catalyst is explored using periodic density functional calculations, both in gas phase and in solvent. The effect of solvent is taken into account using the conductor‐like screening model. The calculated results show that CO insertion to methoxide species to produce monomethyl carbonate species is the rate‐determining step, the corresponding activation barrier is 161.9 kJ mol−1. Then, monomethyl carbonate species reacts with additional methoxide to form DMC with an activation barrier of 98.8 kJ mol−1, above reaction pathway mainly contributes to the formation of DMC. CO insertion to dimethoxide species to form DMC is also considered and analyzed, the corresponding activation barrier is 308.5 kJ mol−1, suggesting that CO insertion to dimethoxide species is not competitive in dynamics in comparison with CO insertion to methoxide species. The solvent effects on CO insertion to methoxide species involving the activation barriers suggest that the rate‐determining step can be significantly affected by the solvent, 70.2 kJ mol−1 in methanol and 63.9 kJ mol−1 in water, which means that solvent effect can reduce the activation barrier of CO insertion to methoxide species and make the reaction of CO insertion to methoxide in solvents much easier than that in gas phase. Above calculated results can provide good theoretical guidance for the mechanism and kinetics of DMC formation and suggest that solvent effect can well improve the performance of DMC formation on Cu2O catalyst in a liquid‐phase slurry.

A DFT theoretical study of CH4 dissociation on gold-alloyed Ni(111) surface

Abstract A density-functional theory (DFT) method has been conducted to systematically investigate the adsorption of CH x (x = 0∼4) as well as the dissociation of CH x (x = 1∼4) on (111) facets of gold-alloyed Ni surface. The results have been compared with those obtained on pure Ni(111) surface. It shows that the adsorption energies of CH x (x = 1∼3) are lower, and the reaction barriers of CH 4 dissociation are higher in the first and the fourth steps on gold-alloyed Ni(111) compared with those on pure Ni(111). In particular, the rate-determining step for CH 4 dissociation is considered as the first step of dehydrogenation on gold-alloyed Ni(111), while it is the fourth step of dehydrogenation on pure Ni(111). Furthermore, the activation barrier in rate-determining step is higher by 0.41 eV on gold-alloyed Ni(111) than that on pure Ni(111). From above results, it can be concluded that carbon is not easy to form on gold-alloyed Ni(111) compared with that on pure Ni(111).

Insight into the mechanism of methane synthesis from syngas on a Ni(111) surface: a theoretical study

The mechanism of methane synthesis from syngas on a Ni(111) surface has been systematically investigated using the density functional theory method together with the periodic slab model, which covers the main existence form of CHx (x = 1–3) species and all possible formation pathways. Our results show that CO hydrogenation to a HCO intermediate is more favorable than CO desorption and CO direct dissociation on Ni(111); starting from HCO, six possible formation pathways of CHx (x = 1–3) species are considered: (i) HCO → CH, (ii) HCO → HCOH → CH, (iii) HCO → HCOH → CH2OH → CH2, (iv) HCO → CH2O → CH2, (v) HCO → CH2O → CH2OH → CH2, (vi) HCO → CH2O → CH3O → CH3, followed by CHx (x = 1–3) successive hydrogenation to CH4, suggesting that CH is the dominate existing form of CHx (x = 1–3) species involving in CH4 formation from syngas, which is dominantly formed by two parallel reaction pathways (i) and (ii), followed by the sequential hydrogenation of CH species to CH4. Meanwhile, surface C is mainly formed by the pathway of CO → HCO (HCOH) → CH → C. Furthermore, beginning with the key CH intermediate, CH preferred to be hydrogenated to CH2 rather than being dissociated into C.

The effect of γ-Al2O3 surface hydroxylation on the stability and nucleation of Ni in Ni/γ-Al2O3 catalyst: a theoretical study

Using recent well-defined models of γ-Al2O3 surfaces, the interactions of Nin(n = 1–7) clusters with different γ-Al2O3 surfaces have been investigated in order to illustrate, by density functional theory periodic calculations, the effect of γ-Al2O3 surface hydroxylation on the stability and nucleation of Ni in Ni/γ-Al2O3 catalyst. Three types of γ-Al2O3 surfaces, dehydrated γ-Al2O3(100), dehydrated γ-Al2O3(110) and hydrated γ-Al2O3(110) were considered. Our results show that for the adsorption of Nin(n = 3–7) clusters, the γ-Al2O3(110) surface is more favorable than the γ-Al2O3(100) surface, however, for single Ni atoms and Ni2 clusters, the reverse becomes true. Meanwhile, for the adsorption of Nin(n = 2–7) clusters, the hydrated (110) surface is not favorable compared to the dehydrated (110) surface, due to the presence of surface hydroxyls on the former. The reverse is true for single Ni atoms due to weaker surface deformation. Further, the support stabilizes Nin(n = 2–7) clusters well in the supported state, in which the presence of surface hydroxyls reduces the stability of the supported Nin clusters. On the other hand, the nucleation ability of Nin clusters on different γ-Al2O3 surfaces, is more favorable on the γ-Al2O3(110) surface than on the γ-Al2O3(100) surface, and the dehydrated (110) surface is more favorable than the hydrated (110) surface due to the presence of surface hydroxyls, namely, surface hydroxylation reduces the nucleation ability of Nin clusters on the γ-Al2O3 surface. More importantly, the exothermicity of supported Nin(n = 2–7) clusters on different γ-Al2O3 surfaces is lower than that of isolated Nin clusters, indicating that the support is not favorable for the nucleation of Nin(n = 2–7) clusters, as a result, the support can inhibit the aggregation of clusters, and favors the formation of small clusters.

The effect of Si/Al ratios on the catalytic activity of CuY zeolites for DMC synthesis by oxidative carbonylation of methanol: a theoretical study

Cu-exchanged Y zeolites with different Si/Al ratios have been investigated using a density functional theory method in order to determine the effect of Si/Al ratios on the catalytic activity for producing DMC by the oxidative carbonylation of methanol. The stable structures and catalytic activity of the CuY zeolites with different Si/Al ratios are identified. In addition, CO adsorption and the effect of CH3O on CO adsorption, including the changes in the adsorption energy and stretching vibrational frequency for the adsorbed CO, are obtained. Our results indicate that the CuY zeolite with a Si/Al ratio = 6.5 has the highest catalytic activity for producing DMC by oxidative carbonylation of methanol among all the Si/Al ratios considered in this work, which is supported by the experimental facts. Finally, our results suggest that theoretical calculations can be used as a useful tool and provide good theoretical guidance for the experimental design of CuY zeolites.

The adsorption and dissociation of methane on cobalt surfaces: thermochemistry and reaction barriers

The adsorption and dissociation of methane (CH4) on a variety of cobalt (Co) surfaces have been assessed using density functional theory. The adsorption of CHx (x = 0–4) and H species, as well as the sequential dissociation of CH4 on Co(111), (100) and (110) surfaces, have been studied. Using the calculated adsorption energies, the preferred adsorption sites for CHx (x = 0–4) and H species on different Co surfaces have first been located, and then the stable co-adsorption configurations of CHx and H (x = 0–3) on these surfaces obtained. In addition the mechanism for the sequential dissociation of CH4 on Co surfaces has been investigated and compared in terms of the thermodynamics and kinetics involved. Our results suggest that Co(100) is the preferred surface for CH4 dissociation, both thermodynamically and kinetically, rather than (111) and (110) surfaces.

A theoretical study on the hydrolysis mechanism of carbon disulfide.

AbstractThe hydrolysis mechanism of CS2 was studied using density functional theory. By analyzing the structures of the reactant, transition states, intermediates, and products, it can be concluded that the hydrolysis of CS2 occurs via two mechanisms, one of which is a two-step mechanism (CS2 first reacts with an H2O, leading to the formation of the intermediate COS, then COS reacts with another H2O, resulting in the formation of H2S and CO2). The other is a one-step mechanism, where CS2 reacts with two H2O molecules continuously, leading to the formation of H2S and CO2. By analyzing the thermodynamics and the change in the kinetic function, it can be concluded that the rate-determining step involves H and OH in H2O attacking S and C in CS2, respectively, causing the C=S double bond to change into a single bond. The two mechanisms are competitive. When performing the hydrolysis of CS2 with a catalyst, the optimal temperature is below 252°C. FigureThe hydrolysis mechanism of CS2

Insight into C + O(OH) reaction for carbon elimination on different types of CoNi(111) surfaces: a DFT study

A density-functional theory (DFT) method has been performed to investigate the reaction of C + O(OH) on three types of bimetallic alloy CoNi(111) surface, and the results obtained are compared with those on the pure Ni(111) surface. Our results show that the introduction of Co into the Ni catalyst is beneficial for the adsorption of C, O and OH species, while it weakens the adsorption of CO. Moreover, O(OH) absorbs preferentially on the CoNi(111) surfaces with the surface enrichment of Co compared with the homogeneous CoNi(111) surface; the increased degree of O adsorption energy outweighs the corresponding values of C on the pure Ni(111) and three types of bimetallic alloy CoNi(111) surfaces, indicating that Co has a stronger affinity for oxygen species than for carbon species. On the other hand, the mechanism of the C + O(OH) reaction and the corresponding rate constants at different temperatures show that OH species have a stronger ability to eliminate carbon than O species on Ni(111) and CoNi(111) surfaces; on the CoNi(111) surface, when the Co surface coverage is equal to 1 monolayer (ML), compared to Ni(111), the C + O reaction can be accelerated. When the Co surface coverage is equal to 3/4 ML, the C + OH reaction is the most favorable; further, the rate constant for the C + OH reaction on a CoNi(111) with Co surface coverage of 3/4 ML is much larger than that of the C + O reaction on a CoNi(111) with Co surface coverage of 1 ML. As a result, for carbon elimination on the CoNi alloy surface, OH species should serve as the key species for carbon elimination, and the Co surface coverage of CoNi(111) surface should be kept at 3/4 ML.

Density functional theory analysis of carbonyl sulfide hydrolysis: effect of solvation and nucleophile variation

AbstractThe detailed mechanisms of the hydrolysis of carbonyl sulfide (OCS) by nucleophilic water and hydroxide ion in both the gas phase and bulk water solvent have been investigated using density functional theory. Various reaction channels on the potential surface have been identified. The thermodynamic results demonstrate that the hydrolysis of OCS by nucleophilic water and hydroxide ion should proceed more favorably at low temperature. The hydrolysis of OCS by the hydroxide ion is the main reaction channel from thermodynamic and kinetic perspectives, and the bulk solvent can influence the rate-determining step in this channel. However, the solvent barely modifies the activation energy of the rate-determining step. For the hydrolysis of OCS by nucleophilic water, the solvent does not modify the rate-determining step, and the corresponding activation energy of the rate-determining step barely changes. This bulk solvent effect suggests that most of the contribution of the solvent is accounted for by considering one water molecule and a hydroxide ion. FigureReaction energy profiles for the H2O and OH- attack on OCS

Catalytic selectivity of Rh/TiO2 catalyst in syngas conversion to ethanol: probing into the mechanism and functions of TiO2 support and promoter

The catalytic selectivity, the functions of a TiO2 support and promoter, and the mechanism of ethanol synthesis from syngas on a Rh/TiO2 model catalyst have been fully identified. Our results show that all species preferentially interact with Rh7 clusters of a Rh/TiO2 catalyst, rather than the support and cluster–support interface. CO → CHO → CH2O → CH3O is an optimal pathway. CH3 formed via the CH3O → CH3 + O route is the most favored CHx (x = 1–3) monomer, and this route is more favorable than methanol formation by CH3O hydrogenation; CO insertion into CH3 can then form CH3CO, followed by successive hydrogenation to ethanol. Methane is formed by CH3 hydrogenation. The Rh/TiO2 catalyst exhibits better catalytic activity and selectivity toward CH3 than CH3OH formation. Starting from the CH3 species, CH4 formation is more favorable than CH3CO formation; thus, ethanol productivity and selectivity on a Rh/TiO2 catalyst with a support is determined only by CH4 formation, which is similar to that on a pure Rh catalyst without a support. Introducing an Fe promoter into the Rh/TiO2 catalyst effectively suppresses methane production, and promotes CH3CO formation. Therefore, compared to a pure Rh catalyst without a support, the TiO2 support serves only to promote the activity and selectivity of CH3 formation, and provide more CH3 species for ethanol formation; methane formation is independent of the Rh catalyst support, and depends only on the promoter. In order to achieve high ethanol productivity and selectivity, an effective Rh-based catalyst must contain a suitable combination of supports and promoters, in which the promoter, M, should have characteristics that draw the d-band center of the MRh/TiO2 catalyst closer to the Fermi level compared to the Rh7/TiO2 catalyst; as a result, the MRh/TiO2 catalyst can suppress CH4 production and facilitate C2 oxygenate formation.