Abstract
<p>The role of MgO as a factor improving the resistance to coking of the alumina supported nickel catalysts in the steam reforming of hydrocarbons is discussed. A series of catalysts containing variable amounts of MgO, NiO and a constant amount of Al<sub>2</sub>O<sub>3</sub> was prepared by the co-precipitation method. It was found that the specific activity of the catalysts exhibits a broad but not deep minimum for the MgO contents from 8.5 to 27.3 wt.%. At the same time these catalysts reveal a high resistance to coking either in the reaction with methane or with <em>n</em>-butane. The most promising composition, in terms of the activity and simultaneous resistance to the coke formation, was found to be 27.3 wt.% of MgO and 39.0 wt.% of NiO. The analysis of various factors controlling the activity and resistance to coking leads to the conclusion that MgO reduces the catalysts acidity what, in consequence, reduces the rate of coke formation during the reforming reactions. Furthermore, The resistance to coking correlates well with the mean size of nickel crystallites, the same is observed for the specific catalyst activity.</p><p> </p>
Highlights
The problem of the carbon deposits formation on catalysts surfaces in processes involving the synthesis gas or hydrogen production has been attracting a considerable interest in the field of nickel catalysts with an improved resistance to coking.One of the methods of improving the catalyst quality is a modification of the carrier composition, leading to the systems exhibiting enhanced resistance either to coking or sintering [1,2,3]
The increase of the activity for mean sizes of nickel crystallites in the range 3.7–8 nm (Fig. 11) confirms the results found in literature [46]
It was found that all the catalysts containing MgO exhibited a higher reduction temperature than the standard alumina-supported nickel catalysts
Summary
The problem of the carbon deposits formation on catalysts surfaces in processes involving the synthesis gas or hydrogen production (steam reforming of higher hydrocarbons, steam reforming of natural gas at low steam to methane ratio, or carbon dioxide reforming of hydrocarbons) has been attracting a considerable interest in the field of nickel catalysts with an improved resistance to coking. One of the methods of improving the catalyst quality is a modification of the carrier composition (typically -Al2O3), leading to the systems exhibiting enhanced resistance either to coking or sintering [1,2,3]. The Ni/Al2O3 catalysts are often modified by the incorporation of MgO It may be introduced into a catalyst by utilizing magnesium aluminum spinel as a carrier or by doping the Al2O3 structure with MgO using various preparation techniques and MgO contents. A complex influence of MgO on the catalyst properties is a result of the formation of NiO–MgO solid solutions [8, 9], changes of the NiO reduction degree and nickel dispersion [7, 10], changes of the surface acidity [11] as well as an interaction of the catalyst with components of the reaction mixture [12]
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