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Abstract
Steam reforming is the most economical and widely-used route for the conversion of light hydrocarbon (such as natural gas) to various valued-added products. This process is commonly carried out over a low-cost alumina-supported nickel catalyst, which often suffers from carbon deposition resulting in loss of active sites, flow and thermal maldistribution, as well as excessive pressure drop.
A bimetallic catalyst with improved anti-coking properties was formulated by incorporating the nickel-based system (15% loading) with cobalt metal (5% loading). Two-level factorial design was employed to investigate the effect of major preparation variables, namely impregnation pH value (2-8), calcination temperature (873-973 K), heating rate (5-20 K min-1) and time (1-5 h).
The catalysts prepared were subjected to various characterisation techniques to determine key physicochemical properties (i.e. BET area, H2-chemisorption and NH3- TPD acidity). X-ray diffraction revealed that NiO, Co3O4, NiCo2O4 and a proportion of Ni(Co)Al2O4 aluminates were transformed during H2-reduction to active Co and Ni crystallites. TEM images showed an egg yolk profile in the low-pH catalyst suggesting that main deposition site was located in the particle centre, while metal deposition occurred primarily around the particle exterior for the high-pH catalyst. Temperature programmed experiments were carried out to examine the extent of conversion, type of surface species and solid-state kinetics (using the Avrami-Erofeev model) involved during various stages in catalyst life-cycle (calcination, reduction, oxidation and regeneration). Steam reforming analysis suggested that enhanced catalyst activity may be due to synergism in the Co-Ni catalyst. Specifically, the low-pH catalyst exhibited better resistance towards carbon-induced deactivation than the high-pH formulation. The study also provided the first attempt to develop a quantitative relation between catalyst preparation conditions and its performance (activity, product selectivity and deactivation) for steam reforming reaction.
Deactivation and reforming kinetic coefficients were simultaneously evaluated from propane reforming conversion-time data under steam-to-carbon ratios of 0.8-1.6 and reaction temperatures between 773-873 K. The time-dependent optimum operational policy derived based on these rate parameters gave better conversion stability despite the heavy carbon deposit. Thermal runs further showed that the catalysts regenerated via two-stage reductive-oxidative coke burn-off exhibited superior surface properties compared to those rejuvenated by a single-step oxidation.