Hydrocarbon Steam Reforming Catalysts - Alkali Induced Resistance to Carbon Formation

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Summary

The effect of the addition of a potassium promoter to a nickel steam reforming catalyst has been probed in terms of the propensity of the catalyst to resist carbon formation. It has been found that potassium facilitates a reduced accumulation of carbon by decreasing the rate of hydrocarbon decomposition on the catalyst and by increasing the rate of steam gasification of filamentary carbon from the catalyst. The effect of the promoter on the carbon removal reaction is evident in an enhancement of the pre-exponential factor in the rate equation by promotion of water adsorption on the catalyst surface.

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Cited by (15)

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    Indeed, during catalytic tests, alkali oxides could migrate and cover the surface of the Ni(0) particles. This covering could have beneficial influences because it favors the adsorption-dissociation of H2O and CO2 molecules, and therefore increases the direct oxidation of the carbonaceous species cracked [24,59–61]. However, for the steam and dry reforming of CH4, the presence of alkali at the surface of Ni(0) could also block the access to some active sites and slightly decrease the catalytic activity of the materials.

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    According to the literatures [32–36], the presence of basic additives or promoters (e.g. Li, Na, K, Mg) on supported nickel catalysts can enhance the resistivity toward carbon deposition. Haddenet al. [37] proved that the K promoted the adsorption ability of water under steam reforming reaction. The main effect of alkali dopants is suppression of carbon deposition with improved stability favoring water adsorption or neutralizing acid sites of the catalyst.

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    Density functional theory (DFT) simulations have indicated that the barrier for CO formation is lower than that for CC formation on a Ni–Sn alloy compared to monometallic Ni [41,42]. It has been also found that the addition of alkaline oxides [40,43] or alkaline earth oxides [44] prevents the accumulation of carbon by decreasing the rate of hydrocarbon decomposition while increasing the rate of steam gasification of filamentous carbon on the catalyst [43–47]. However, in all cases, the incorporation of these alkaline additives decreases the overall reforming activity [48,49].

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    The mechanism of the potassium effect on the performance of nickel catalysts for hydrocarbons steam reforming has not been fully explained so far [11,48]. However, the most often the following points of the potassium influence have been postulated: removal of acidic centres on the alumina surface [14,22], increase of water vapour adsorption [49], “spillover” of dissociated water over the catalyst surface [11], constraint on hydrocarbon dissociation [50,51], increase in coke gasification [52–56], significant reduction of the sticking probability for methane chemisorption on the Ni(1 0 0) and Ni(1 1 1) planes [11,18]. In the case of the potassium-promoted nickel-rich catalysts, a decrease both of the amount of chemisorbed hydrogen and of the steam reforming activity was observed.

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