A Review of Theoretical Studies on Carbon Monoxide Hydrogenation via Fischer-Tropsch Synthesis over Transition Metals.

The increasing demand for clean fuels and sustainable products has attracted much interest in the development of active and selective catalysts for CO conversion to desirable products. This review maps the theoretical progress of the different facets of most commercial catalysts, including Co, Fe, Ni, Rh, and Ru. All relevant elementary steps involving CO dissociation and hydrogenation and their dependence on surface structure, surface coverage, temperature, and pressure are considered. The dominant Fischer-Tropsch synthesis mechanism is also explored, including the sensitivity to the structure of H-assisted CO dissociation and direct CO dissociation. Low-coordinated step sites are shown to enhance catalytic activity and suppress methane formation. The hydrogen adsorption and CO dissociation mechanisms are highly dependent on the surface coverage, in which hydrogen adsorption increases, and the CO insertion mechanism becomes more favorable at high coverages. It is revealed that the chain-growth probability and product selectivity are affected by the type of catalyst and its structure as well as the applied temperature and pressure.

Carbon dioxide hydrogenation over the carbon-terminated niobium carbide (111) surface: a density functional theory study.

Carbon dioxide (CO2) hydrogenation is an energetic process which could be made more efficient through the use of effective catalysts, for example transition metal carbides. Here, we have employed calculations based on the density functional theory (DFT) to evaluate the reaction processes of CO2 hydrogenation to methane (CH4), carbon monoxide (CO), methanol (CH3OH), formaldehyde (CH2O), and formic acid (HCOOH) over the carbon-terminated niobium carbide (111) surface. First, we have studied the adsorption geometries and energies of 25 different surface-adsorbed species, followed by calculations of all of the elementary steps in the CO2 hydrogenation process. The theoretical findings indicate that the NbC (111) surface has higher catalytic activity towards CO2 methanation, releasing 4.902 eV in energy. CO represents the second-most preferred product, followed by CH3OH, CH2O, and HCOOH, all of which have exothermic reaction energies of 4.107, 2.435, 1.090, and 0.163 eV, respectively. Except for the mechanism that goes through HCOOH to produce CH2O, all favourable hydrogenation reactions lead to desired compounds through the creation of the dihydroxycarbene (HOCOH) intermediate. Along these routes, CH3* hydrogenation to CH4* has the highest endothermic reaction energy of 3.105 eV, while CO production from HCO dehydrogenation causes the highest exothermic reaction energy of -3.049 eV. The surface-adsorbed CO2 hydrogenation intermediates have minimal effect on the electronic structure and interact only weakly with the surface. Our results are consistent with experimental observations.

The effect of impregnated autogenous epidural adipose tissue with bupivacaine, methylprednisolone acetate or normal saline on postoperative radicular and low back pain in lumbar disc surgery under spinal anesthesia; A randomized clinical trial study.

BACKGROUND: Low Back Pain (LBP) and radicular leg pain (RLP) after lumbar disc surgery are great challenges that prevent patients and neurosurgeons in making a surgical decision. By spinal anesthesia, LBP and RLP diminish up to 2 to 3 hours postoperatively. The aim of this study was to determine the effect of impregnated epidural adipose tissue (EAT) with bupivacaine or methylprednisolone acetate on reduction of late postoperative pain after spinal anesthesia. METHODS: This study was performed on lumbar disc herniation surgery under spinal anesthesia. Sixty six patients entered our study who were divided into three groups, EAT impregnated with bupivacaine (group 1), methylprednisolone acetate (group2) and normal saline (control group). The LBP and RLP were evaluated during the first 24 hours postoperatively and 14 days later by visual analogue scale (VAS). RESULTS: Of 66 patients, 53% were female and 47% male. The average (SE) LBP in the first 6 hours after surgery based on VAS were 1.59 ± 0.90 in group one, 2.36 ± 2.38 in group 2 and 3.09 ± 1.41 in control group but the VAS for RLP in this period were 1.95 ± 1.13, 1.31 ± 1.39 and 2.40 ± 1.09, respectively. The average LBP and RLP did not show any differences after 14 days postoperatively. CONCLUSIONS: According to our data bupivacaine was effective on LBP relief and steroid was effective on RLP relief during the first 12 hours after surgery.