Search

On the basis of spin-polarized periodic density functional theory including the latest dispersion correction (PBE-D3), the mechanisms of H 2 , O 2 , H 2 O, CO 2 , CO, CH 4 and C 2 H x dissociative adsorption on the hexagonal Mo 2 C surface have been computed. In our study we used the metallic Mo 2 C(001) surface as well as the Mo 2 C(101) surface with Mo/C = 1/1 ratio. It is found that the dissociative adsorptions of these small molecules are exothermic and have low barriers; and the Mo 2 C(001) surface has much stronger dissociative adsorption than the Mo 2 C(101) surface. In contrast to the Mo 2 C(001) surface, OH + H and O + 2H can form equilibrium on Mo 2 C(101) surface. For C 2 H x dissociative adsorption on the Mo 2 C(001) surface, C H bond dissociation is kinetically much more favorable than the C C bond dissociation, and the optimum C 2 H 6 dissociation route follows the order of C 2 H 6 → CH 3 CH 2 + H → C 2 H 4 + 2H → CH 2 CH + 3H → C 2 H 2 + 4H → C 2 H + 5H→ C 2 + 6H. Due to the very strong dissociative adsorption energies, both surfaces can be oxidized easily by using H 2 O; and high oxygen coverage can be expected. The Mo 2 C(001) surface can uptake more surface O atoms than the Mo 2 C(101) surface. These surfaces can also be carburized by using CH 4 , albeit in less extent. On the surface with co-adsorbed CO 2 + 4H; CO 2 dissociation (CO 2 → CO + O → C + 2O) is more favorable than the hydrogenation of CO 2 (CO 2 + H → HCOO or COOH) and CO (CO + H → HCO or COH). It is noted that CO 2 hydrogenation towards CH 4 formation is unlikely on the Mo 2 C(001) surface, while the effective barrier of surface C hydrogenation on the Mo 2 C(101) surface can be reduced by 2O and 2OH pre-covered surfaces. [ABSTRACT FROM AUTHOR]

Highlights: [•] Surface terminations of orthorhombic Mo2C catalysts have been computed. [•] (110)-Mo and (100)-Mo terminations are stable at low carbon chemical potential. [•] (111)-Mo/C and (110)-Mo/C terminations are stable at high carbon chemical potential. [•] CO activation on Mo terminations is dissociation controlled zero-order kinetic. [•] CO activation on Mo/C terminations is adsorption controlled first-order kinetic. [ABSTRACT FROM AUTHOR]

Novel Cu/ZnO-Al 2 O 3 composites had been constructed and applied in furfural hydrogenation to produce furfuryl alcohol in a fixed-bed reactor. As the result, conversion of FFA over Cu/ZnO catalyst (within 16–24 h TOS) could be improved from 84.9% to 98.0% at 120 °C and 51.4% to 72.1% at 100 °C with a high selectivity of FOL (≈98%) over the novelly constructed Cu/ZnO-Al 2 O 3 catalyst. The role of Al components derived from different precursors like Al(NO 3 ) 3 , as-prepared Al(OH) 3 and pseudo-boehmite had been systemically investigated by various techniques such as XRD, Raman spectroscopy, H 2 -TPR, N 2 O titration, XPS, AES experiments and confirmed to be both improving dispersion of copper/zinc species and hindering Cu-ZnO interaction of the aurichalcite derived catalysts. Preparation of the industrial Cu/ZnO-Al 2 O 3 catalysts had been revealed and explained more clearly. [ABSTRACT FROM AUTHOR]

Periodic density functional theory (revised PBE; RPBE) and ab initio atomistic thermodynamics were used to assess the dissociative adsorption of H 2 O and CO 2 on a (5 × 5) Ru(0001) surface model. For H 2 O dissociation on clean Ru(0001) [H 2 O → OH + H → O + 2H], the first step has lower barrier than the second step (0.75 vs. 0.80 eV). On O pre-covered Ru(0001), H 2 O dissociation [H 2 O + O → 2OH] has lower barrier (0.62 eV) and is more exothermic (−0.29 vs. −0.15 eV). CO 2 dissociation on clean Ru(0001) [CO 2 → CO + O] needs low barrier (0.23 eV) and is highly exothermic by 1.47 eV. In turn, CO 2 formation from surface O removal by CO has barrier of 1.70 eV, in agreement with the experimentally detected 1.80 ± 0.15 eV. That the computed CO desorption energy (1.55 eV) is smaller than CO oxidation barrier is in agreement with the experimental finding. The computed desorption temperatures of H 2 O, CO and CO 2 under ultrahigh vacuum conditions agree perfectly with the experiments. In addition, high oxygen and OH pre-coverage do not significantly affect the energetics of the dissociation of H 2 O and CO 2 . The excellent agreement between theory and experiment confirms the applied models and methods, and in turn, gives the reliability and confidence about the further predicted results. [ABSTRACT FROM AUTHOR]

The mechanisms of CO hydrogenation with the formation of C 1 surface species on the clean and K 2 O-doped χ-Fe 5 C 2 (111) surfaces with carbon vacancies have been comparatively elucidated. On both surfaces, CO direct dissociation (CO → C + O) is not favored. On the clean surface, the preferred H-assisted pathway is CO → HCO → CH 2 O → CH 3 O → CH 3 OH. On the K 2 O-doped surface, the preferred paths are HCO → CHOH → CH + OH and HCO → CH 2 O → CH 2 OH → CH 2 + OH due to the barrier decrease of the C O bond cleavage of COH, CHOH and CH 2 OH by K 2 O promotion. These results might explain the K 2 O promoted carburization process and the formation of long-chain hydrocarbons. [ABSTRACT FROM AUTHOR]

The effect of Cu promoter on CH 4 formation from CO hydrogenation on Fe(100) has been investigated using spin-polarized density functional theory computations on periodic slab models. Two pathways of CH 4 formation are considered: (a) direct dissociation of CO followed by successive C hydrogenation and (b) successive hydrogenation of CO followed by deoxygenation of CH x O (x = 1–3). On the clean and Cu doped surfaces, direct dissociation of CO followed by successive C hydrogenation is more favorable kinetically and the overall CH 4 formation is endothermic. Compared with the clean surface, the Cu doped surface can suppress CH 4 formation by raising the barrier from the co-adsorbed CO + 4H. Methanol formation is unfavorable on both surfaces kinetically and thermodynamically. Reversely, methanol decomposition is favorable. It is also noted that Cu doping lowers the adsorption ability of the Fe(100) surface and therefore the adsorption energies of all species on the potential energy surfaces. [ABSTRACT FROM AUTHOR]

Highlights: [•] Most, medium and least stable Fe5C2(111), (−411) and (001) surfaces are used. [•] Adsorption of H2O molecules and O atoms was calculated at high coverage. [•] Surface stability correlates with the adsorption energies of H2O and O. [•] High coverage H2O prefers hydrogen bonding instead of adsorption on iron. [•] The number of surface O atoms depends on temperature and H2O/H2 ratio. [ABSTRACT FROM AUTHOR]

Highlights: [•] H2O dissociative adsorption on the χ-Fe5C2(010) surface was investigated. [•] The χ-Fe5C2(010) surface has both iron and carbon regions. [•] Surface iron region is active for H2O dissociative adsorption and H2 formation. [•] Pre-adsorbed oxygen atoms play a significant role for further H2O adsorption. [•] Surface carbon region is inactive for H2O dissociative adsorption. [ABSTRACT FROM AUTHOR]

Abstract: A comprehensive Fischer–Tropsch synthesis (FTS) kinetics model including the hydrocarbon and oxygenate formation reactions as well as water gas shift (WGS) reaction over an Fe–Mn catalyst is developed in a continuous spinning basket reactor. On the basis of CH2 insertion alkyl mechanism, the kinetics expressions for paraffins, olefins, alcohols and acids are derived. Kinetics calculation results indicate that the formation of paraffins, olefins, alcohols and acids over the Fe–Mn catalyst are parallel competitive reactions. Reasonable and strict analysis in mathematics is given to interpret the results which the comprehensive kinetics model cannot predict the olefin/hydrocarbon ratio well though the olefin readsorption and secondary reactions are involved in the kinetics model. Oxygenates might readsorb over the catalyst surface and take part into the corresponding secondary reactions (chain growth and hydrogenation to the corresponding hydrocarbons), which leads to the exponentially decrease of the experimental alcohol/hydrocarbon and acid/hydrocarbon ratios with the increase of carbon number. [Copyright &y& Elsevier]

Abstract: A systematic study has been carried out to investigate the impacts of SiO2 content, incorporation manner of SiO2 and drying process on the physico-chemical and catalytic performances of a precipitated iron–manganese catalyst for Fischer–Tropsch synthesis (FTS) in a fixed bed reactor (H2/CO=2.0, T=265°C, P=2.5MPa and GHSV=1000h-1). Characterization technologies of N2 physisorption, X-ray diffraction (XRD), Mössbauer effect spectroscopy (MES), temperature-programmed reduction (TPR) and scanning electron microscopy (SEM) were used to study the textural properties, bulk phase composition, reduction behavior and morphologies of the catalysts, and the SiO2 framework in the catalysts. The results of characterization showed that the incorporation of SiO2 leads to the increase in surface area. The catalyst with incorporated with precipitated SiO2 has the highest surface area and exhibits a stronger interaction between iron and SiO2 matrix than the interaction with binder SiO2. The strong Fe–SiO2 interaction restrains the reduction in H2 and the carburization in syngas of the catalysts. The addition of both binder and precipitated SiO2 greatly influences the activity and selectivity of FTS, and obviously improves its stability. For the FTS reaction tests, it was found that the catalyst activity is decreased with the increase of the amount of SiO2 incorporated. For the catalysts incorporated with the same levels of SiO2, the spray-dried catalyst has lower activity than the normal dried one. In addition, the catalyst incorporated with precipitated SiO2 has higher activity than that with binder SiO2. The FTS stability of the catalysts is improved with the incorporation of SiO2. The selectivity to gaseous hydrocarbons (C1–C4) and olefins decreases with the increase of SiO2 content. The spray-dried catalysts incorporated with either precipitated SiO2 or binder SiO2 produce more gaseous hydrocarbons and fewer olefins, while the normal-dried process and the addition of binder SiO2 cause a great increase in selectivity to light hydrocarbons. The selectivity to oxygenates in product decreases with the increase of SiO2 content in the catalyst. For the catalysts incorporated with binder SiO2, the spray-dried catalyst (SPUW) produces more oxygenates than the normal dried catalyst (FMSC), whereas the spray-dried catalyst incorporated with precipitated SiO2 (FSCP) produces the least oxygenates among all catalysts. [Copyright &y& Elsevier]

A systematic study has been carried out to investigate the impact of potassium promoter on the performance of a precipitated iron-manganese catalyst for Fischer–Tropsch synthesis (FTS). Characterization technologies of N2 physisorption, X-ray diffraction (XRD), Mössbauer effect spectroscopy (MES) and H2 thermal gravimetric analysis (H2-TGA) were used to study the effect of potassium on the textural properties, bulk phase composition and reduction behavior. FTS reaction test was performed in a fixed bed reactor. The results of characterization showed that the addition of potassium leads to the relatively large crystallite size of α-Fe2O3 and inhibits the reduction of catalyst. The carbonization of the catalyst is enhanced with the increase in both the potassium content and the reaction temperature. A maximum in FTS and water-gas shift (WGS) activity is noted upon increasing K content (0.7 wt.% K), followed by a sharp decline in activity at the potassium level in excess of the maximum. It is found that potassium is an effective promoter to suppress the hydrogenation function of the catalyst. The selectivity to olefins is promoted and the formation of methane and light hydrocarbons is restrained with the increasing potassium level. The selectivity to oxygenates shows a rapid and monotonic decrease with the increase of potassium loading and passes through a minimum at potassium loading of 0.7 wt.%. After the point, it increases slowly with further increasing in potassium content. At the same time, increasing reaction temperature results in a monotonic decrease in the weight percent of oxygenates over the un-promoted and potassium-promoted catalysts. [Copyright &y& Elsevier]