[en] Highlights: • DRIFTS and DFT calculations were employed for CH₄ dry reforming. • Dual-path of CO₂ activation over the reduced NiO-MgO was proposed. • E-R type mechanism and surface hydrogenation and dissociation had been confirmed in CO₂ activation. - Abstract: The interaction mechanisms of dry reforming of methane, especially in the part of CO₂ activation on the reduced NiO-MgO catalyst, have been systematically investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations. Based on the results, it is indicated that, what more favorable for CO generation is gaseous CO₂ reaction with deposited C intermediates, namely E-R type mechanism, rather than CO₂ direct dissociation to form CO and atomic O. In addition, with the help of H species, monodentate carbonate, which derives from the adsorbed CO₂ on the MgO surface, can be also activated and hydrogenated dissociation to generate CO.

[en] The catalytic activity of Mo-based catalysts prepared from (NH_4)_6Mo_7O_2_4 and (NH_4)_2MoS_4 was compared in the sulfur resistant methanation process. The catalyst using oxide precursor had relatively higher activity than the catalyst using sulfide precursor, and the presulfidation procedure almost had no effect on the catalytic performance of the catalyst using oxide precursor. In view of the characterization results, it could be supposed that the amorphous MoS_2 was more active for sulfur-resistant methanation than the crystalline MoS_2. The molybdenum sulfides and oxides with lower valence states (Mo"4"+, Mo"5"+) could be responsible for the catalytic activity and make a possible contribution to the carbon monoxide methanation in the reaction condition

[en] In light of the good performance of Mo-based catalysts for sulfur-resistant CO methanation, we investigated the reaction mechanism over pure MoS₂ in a previous study, in which the Mo-edge from (0 1 0) surface of MoS₂ was found inactive due to the difficulty in S-vacancy creation. It was generally recognized that Co is a good additive on Mo-based catalyst for CO methanation. Thus, we focused on promoting the Mo-edge by presenting a group of cobalt substituted surface models via DFT simulation. These models were all reconstructed by performing a thermodynamic investigation on the numbers of the created S-vacancies according to the reaction condition of CO methanation from experimental data. Based on the discussion of reaction pathways over four determined surfaces, we found that a substitute ratio of 0.25 and 0.50 in ortho-position can exhibit the highest catalytic activities, but a substitute ratio of 0.50 in meta-position exhibits the optimum stability in the overall reaction.

[en] The basal plane of MoS₂ possessing the highest specific surface area was always neglected in catalysis due to its inert surface. While in a biaxial strained MoS₂, the surface can possibly be activated by creating stable S-vacancies. By analyzing the electronic properties of the S-vacancies, we found that the catalytic activity for CO methanation could be exploited. In view of this aspect, the mechanisms of S-vacancies formation, CO and H₂ adsorption and CO methanation have been investigated through DFT approach. The results indicated that CO methanation on the activated basal plane of MoS₂ can be proceeded more easily once it was being activated. The activated basal plane of MoS₂ is even more active than the active sites over some specific edge surfaces.

[en] Unsupported MoS₂ catalysts were synthesized by a one-step hydrothermal method using ammonium heptamolybdate and thiourea at different temperatures ranging from 150 °C to 180 °C. With the decrease of hydrothermal temperature, the obtained MoS₂ catalyst showed an increased sulfur-resistant methanation performance. The physical structure and chemical characteristics of the catalysts were analyzed by N₂-physisorption, XRD, SEM, HRTEM, Elemental Analyzer, H₂-TPR, XPS and Raman techniques. Combining the catalytic performance trend with the characterization analysis results, we found that the catalysts obtained at lower temperature contain more bridging ${\text{S}}_{\text{2}}^{\text{2-}}$ groups, that is beneficial for H₂ dissociation and methanation reaction. We deduced that over-stoichiometric sulfur in the catalyst existed in the form of bridging ${\text{S}}_{\text{2}}^{\text{2-}}$ groups, which increased with the decrease of hydrothermal temperature. The finding that the positive role of the bridging ${\text{S}}_{\text{2}}^{\text{2-}}$ groups rather than the S vacancies in MoS₂ catalyst is helpful for designing high efficient MoS₂ catalysts not only for methanation but also for other hydrogenation reactions.

[en] Highlights: • Ce_xZr_1-xO₂ solid solution carriers were prepared by one-step co-precipitation method. • Cubic Ce_xZr_1-xO₂ supported MoO₃ catalyst achieves higher methanation activity than tetragonal one. • The cubic MoO₃/Ce_0.8Zr_0.2O₂ possesses more reducible Ce³⁺ and higher reducibility than the tetragonal one. • The enhanced mobility of active oxygen and increased amounts of O_II vacancies on the cubic MoO₃/Ce_0.8Zr_0.2O₂. • The active oxygen and more Ce³⁺ species resulted in the higher methanation performance. In this paper, two kinds of Ce_xZr_1-xO₂ solid solution carriers with different Ce/Zr ratio were prepared by one-step co-precipitation method: the cubic Ce_0.8Zr_0.2O₂ and the tetragonal Ce_0.2Zr_0.8O₂ support. The MoO₃/Ce_0.8Z_r0.2O₂ and MoO₃/Ce_0.2Zr_0.8O₂ catalysts were prepared by incipient wetness impregnation method for comparative study on sulfur-resistant methanation reaction. The N₂ adsorption/desorption, X-ray diffraction (XRD), Raman spectroscopy (RS), X-ray photoelectron (XPS), transmission electron microscopy (TEM), temperature-programmed reduction by hydrogen (H₂-TPR) were undertaken to characterize the physico-chemical properties of the samples. The results indicated that the prepared MoO₃/Ce_xZr_1-xO₂ catalysts have a mesoporous structure with high surface area and uniform pore size distribution, achieving good MoO₃ dispersion on Ce_xZ_r1-xO₂ supports. As for the catalytic performance of sulfur-resistant methanation, the cubic MoO₃/Ce_0.8Zr_0.2O₂ exhibited better than the tetragonal MoO₃/Ce_0.2Zr_0.8O₂ catalyst at reaction temperature 400 °C and 450 °C. CO conversion on the cubic MoO₃/Ce_0.8Zr_0.2O₂ catalyst was 50.1% at 400 °C and 75.5% at 450 °C, which is respectively 7% and 20% higher than that on the tetragonal MoO₃/Ce_0.2Zr_0.8O₂ catalyst. These were mainly attributed to higher content of active MoS₂ on the surface of catalyst, the enhanced oxygen mobility, increased Mo-species dispersion as well as the excellent reducibility resulted from the increased amount of the reducible Ce³⁺ on the cubic MoO₃/Ce_0.8Zr_0.2O₂ catalyst.

[en] The CeO₂-Al₂O₃ supports prepared with impregnation (IM), deposition precipitation (DP), and solution combustion (SC) methods for MoO₃/CeO₂-Al₂O₃ catalyst were investigated in the sulfur-resistant methanation. The supports and catalysts were characterized by N₂-physisorption, transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy (RS), and temperature-programmed reduction (TPR). The N₂-physisorption results indicated that the DP method was favorable for obtaining better textural properties. The TEM and RS results suggested that there is a CeO₂ layer on the surface of the support prepared with DP method. This CeO₂ layer not only prevented the interaction between MoO₃ and γ-Al₂O₃ to form Al₂(MoO₄)₃ species, but also improved the dispersion of MoO₃ in the catalyst. Accordingly, the catalysts whose supports were prepared with DP method exhibited the best catalytic activity. The catalysts whose supports were prepared with SC method had the worst catalytic activity. This was caused by the formation of Al₂(MoO₄)₃ and crystalline MoO₃. Additionally, the CeO₂ layer resulted in the instability of catalysts in reaction process. The increasing of calcination temperature of supports reduced the catalytic activity of all catalysts. The decrease extent of the catalysts whose supports were prepared with DP method was the lowest as the CeO₂ layer prevented the interaction between MoO₃ and γ-Al₂O₃.

[en] Density functional theory (DFT) calculations are used to investigate the oxidative carbonylation of ethanol occurring on Cu(I)/β or Pd(II)/β. The thermochemistry and activation energy for all elementary steps involved in the formation of diethyl carbonate are presented. Upon calculation, we identify that the mechanisms for the formation of surface O atom are varying on different catalysts. Ethanol can react with the surface O atom to produce (C₂H₅O)(OH)-M/β (M = Cu⁺, Pd²⁺) species. And this intermediate can further react with carbon monoxide or ethanol to give monoethyl carbonate or di-ethoxied species ((C₂H₅O)₂-M/β). Diethyl carbonate can form via two distinct reaction pathways: (I) ethanol reacts with monoethyl carbonate or (II) carbon monoxide inserts into di-ethoxide species. Upon calculation, we confirmed that both reaction pathways for the formation of DEC are accessible on Cu(I)/β catalyst, whereas only Path II is achievable on Pd(II)/β catalyst.

[en] A highly efficient Ag catalyst supported on a novel Ti-decorated spherical fibrous silica (Ag/Ti-KCC-1) was synthesized via a facile approach. Characterization of catalysts, including FT-IR, N₂ physisorption, TEM, ICP-OES and DRUV–vis was carried out to investigate the physicochemical properties of the catalyst, XPS, H₂-TPR, H₂-TPD and DMO-TGA were conducted to further elucidate the effect of Ti dopant. It is shown that the Ti additive is able to, (1) inhibit the aggregation of Ag; (2) induce electron transfer from Ti to Ag, which effect the Ag dispersion and the adsorption ability of the reactants on the catalyst surface. Proper amount of Ti additive provided a balanced coverage of both hydrogen and dimethyl oxalate (DMO) on catalyst surface, which is essential in promoting the performance of the catalyst in the hydrogenation of DMO. By using the novel 10Ag/0.02Ti-KCC-1 catalyst, a high MG yield of 93.0% under extremely high WHSV of 1.75 h⁻¹ is achieved.

[en] In this paper, a facile and controllable method for in-situ synthesis of the magnetic carbon nanotubes nanocomposites (Fe/CNTs) in water-ethylene glycol (EG) mixed solvents is reported by the deposition–precipitation method following annealing. The effect of water/EG ratio on the physico-chemical properties of magnetic Fe/CNTs is investigated by X-ray diffraction, transmission electron microscope, scanning electron microscopy, thermogravimetric analysis and physical property measurement system. The results indicate that the iron particle size distribution and grain size can be well-tuned by adjusting the water/EG ratio. With the variation of EG fraction in the mixed solvent, the nucleation, growth and crystallization of magnetic iron oxides with a controllable morphologies and particle sizes attached on the exterior surface of CNTs can be achieved. The as-prepared Fe/CNTs nanocomposites display superparamagnetic property at room temperature and the water/EG ratio determines the magnetization of the sample. Possible formation mechanism for magnetic Fe/CNTs is proposed based on the characterization results. - Highlights: • In-situ synthesis of monodisperse magnetic Fe/CNTs nanocomposites was reported. • Water/EG ratio determined the dispersion and distribution of magnetic particles. • The as-prepared Fe/CNTs nanocomposites were superparamagnetic at room temperature. • Formation mechanism for magnetic Fe/CNTs was proposed.