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Chemical looping reforming of methane to syngas (CO and H2) is one of the most promising routes for methane utilization, where the further reaction of CO on oxygen carrier surfaces is a primary determinant of CO selectivity. In this work, the effects of oxygen vacancy (Vo) on CO desorption, CO oxidation, and CO dissociation are systematically studied by using density functional theory calculattions. Our calculated results reveal that increasing Vo concentration can weaken CO desorption at Fe sites due to the enhanced localization of electrons in the Fe atoms. Also, the increase in Vo concentration from 1/12 ML to 1/6 ML leads to a dramatic increase of activation energy in the CO oxidation from 0.64 eV to 1.10 eV. Moreover, the increase in Vo concentration is conducive to CO dissociation, but the dissociation is still almost impossible due to the high reaction energies (large than 3.00 eV). Considering these three reaction paths, CO desorption can proceed spontaneously at reaction temperatures above 900 K. Increasing Vo concentration can improve the selectivity of syngas production due to the less favorable CO oxidation compared with CO desorption at high Vo concentrations (1/6 ML). This work reveals the microscopic mechanism that CO selectivity rises in the CLRM as the degree of Fe2O3 reduction increases. [ABSTRACT FROM AUTHOR]

The development of a single-atom iron catalyst (Fe©SiO2) for the direct conversion of methane to olefins, aromatics, and hydrogen is a breakthrough in the field of nonoxidative conversion of methane (NCM). However, the optimization of the catalyst remains desirable for industrial applications. Herein, 25 transition metals, including Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, Pt, and Au, are selected to replace the central Fe atom for screening out better single-atom catalysts for the NCM. Using the performance on the activation of methane, such as the adsorption energy of methane, the dissociation energy, and the barrier of methane as the screening descriptors, Mn©SiO2, Fe©SiO2, W©SiO2, and Re©SiO2 are first screened out. The remarkable performance of the four catalysts on methane activation is attributed to the unique geometric structure and the d z 2 orbitals of the central metal crossing over the Fermi level, which can benefit the interaction between methane and the catalysts. By considering the catalytic performance on the whole pathway of methane to ethylene, W©SiO2 is finally selected as the most active catalyst for the NCM, which has the lowest rate-determining barrier of 1.62 eV and the smallest free energy span (1.06 eV) of the overall catalytic cycle. [ABSTRACT FROM AUTHOR]

The one-step conversion of ethanol to 1,3-butadiene has achieved a breakthrough with the development of beta zeolite supported dual metal catalysts. However, the reaction mechanism from ethanol to butadiene is complex and has not yet been fully elucidated, and no catalyst screening effort has been done based on central metal atoms. In this work, density functional theory (DFT) calculations were employed to study the mechanism of one-step conversion of ethanol to butadiene over Zn-Y/BEA catalyst. The results show that ethanol dehydrogenation prefers to proceed on Zn site with a reaction energy of 0.77 eV in the rate-determining step, and the aldol condensation to produce butadiene prefers to proceed on Y site with a reaction energy of 0.69 eV in the rate-determining step. Based on the mechanism revealed, six elements were selected to replace Y for screening superior combination of Zn-M/BEA (M=Sn, Nb, Ta, Hf, Zr, Ti; BEA: beta polymorph A) for this reaction. As a result, Zn-Y/BEA (0.69 eV) is proven to be the most preferring catalyst compared with the other six ones, and Zn-Zr/BEA (0.85 eV), Zn-Ti/BEA (0.87 eV), and Zn-Sn/BEA (0.93 eV) can be potential candidates for the conversion of ethanol to butadiene. This work not only provides mechanistic insights into one-step catalytic conversion of ethanol to butadiene over Zn-Y/BEA catalyst but also offers more promising catalyst candidates for this reaction. [ABSTRACT FROM AUTHOR]