Effect of the synthesis conditions of Ni/Al2O3 catalysts on the biogas decomposition to produce H2-rich gas and carbon nanofibers
Abstract The utilization of biogas as carbon source for the co-production of filamentous-like materials and syngas is proposed. Catalysts with different Ni:Al molar ratios, Al 2 O 3 supports and preparation methods were synthesized. The suitability of the catalysts was analyzed considering activity, stability and carbon yield. Catalysts showed a good performance and similar results according to syngas compositions were obtained. However, some differences related to stability over time and carbon yields were detected. TPR analysis and TEM micrographs of the fresh catalysts revealed the presence of two different Ni particles: large Ni particles with low metal support interaction (MSI) that favoured the formation of encapsulating carbon and small Ni particles inserted in the Al 2 O 3 structure with a greater MSI that lead to nanofilamentous carbons formation. Generally both kinds of Ni particles were observed in all catalysts, however their relative abundance was dramatically affected by the Al 2 O 3 employed, the preparation method and the Ni:Al molar ratio.
Slurry phase methanation of carbon monoxide over nanosized Ni–Al2O3 catalysts prepared by microwave-assisted solution combustion
Abstract The clean transformation of coal into natural gas is an interesting topic in green chemistry, as well as a promising process for the production of natural gas. In the present work, the methanation of CO was performed in a slurry-bed reactor with a series of nanosized Ni–Al2O3 catalysts prepared by microwave-assisted solution combustion with different fuels (urea, glycine, ethylene glycol and citric acid). As compared with other fuels, a majority of urea and nitrates simultaneously decompose and occur in extensive overlapped temperature ranges during heating, which is conducive to the formation of a moderate and sustained combustion reaction. In addition, the thermodynamic calculation and thermogravimetric analysis indicate that the amount of heat generated during combustion is the minimum when urea was used as fuel. By the formation of a stable combustion and a relative low combustion temperature, the NiAl-U catalyst exhibits the largest BET area and metal surface area, the smallest Ni particle size and more dispersive Ni species, as evidenced by XRD, TEM, N2 adsorption–desorption, H2-TPR and H2 pulse chemisorption characterizations. In the slurry methanation of CO, the conversion of CO and selectivity for CH4 over NiAl-U catalyst reach up to 95.7% and 96.2% at 300 °C, 1.0 MPa and 3000 mL/gcat h, respectively, which are superior to those of other combustion-synthesized catalysts and commercial catalyst. In addition, The NiAl-U catalyst exhibits significant improvement in anti-sintering during 200 h lifetime test, mainly because of smaller Ni particle size, high dispersity of Ni active species on support and high stability of support. The findings presented here are expected to provide new approaches for rational design of nanosized Ni–Al2O3 catalysts for CO methanation reactions in slurry phase.
TEM evidence for factors affecting the genesis of carbon species on bare and K-promoted Ni/MgO catalysts during the dry reforming of methane
Abstract The structure and morphology of carbon species generated under dry reforming of methane (DRM) at 650 and 800°C on ‘bare’ and ‘K-doped’ Ni/MgO catalysts have been comparatively investigated by Transmission Electron Microscopy (TEM) analyses of ‘used’ samples. K-addition (Kat/Niat, 0.125) strongly improves the resistance of the Ni/MgO catalyst to coking and sintering phenomena at any temperature. At 650°C, an extensive formation of filamentous (whisker carbon) carbon species on bare Ni/MgO catalyst causes the detachment of a large number of Ni particles from the support with a consequent destruction of the structure and remarkable sintering phenomena of the active phase. Considerably lower amounts of carbon deposits with a shell-like (encapsulating carbon) morphology, forming at 800°C on both catalysts, point to the Bouduard reaction as the main route of carbon deposition on Ni-based catalysts during DRM. The electronic effect induced by potassium on the active phase of the Ni/MgO system, timely monitored by a rise in Eapp of DRM from 50 to 70 kJ/mol, markedly hinders the rate of coking also affecting the morphology of carbon whiskers, by inhibiting the processes of C diffusion and nucleation across Ni particles under steady-state conditions.