Synthesis Of Mesoporous Alumina Supported Nickel Catalysts And Their Catalytic Properties For Pre-reforming Of Liquefied Petroleum Gas

Liquefied petroleum gas(LPG) is potential source for stationary and mobile hydrogen production applications because of their higher energy density than natural gas, easy storage and well-established distribution infrastructures. Nickel-based catalysts especially supported on alumina have been recognized as promising catalysts for the steam reforming of hydrocarbons because of their high catalytic activity and low cost. In this work, mesoporous γ-Al2O3 supported Ni–Mg and Ni–Ce oxides were first prepared through one-pot hydrolysis of inorganic salts without surfactants, and used for low temperature steam reforming(pre-reforming) of LPG. The influences of prepare methods, catalyst components, calcination temperatures on the catalyst structure and catalytic performance were investigated in detail as follows:(1) Mesoporous γ-alumina supported Ni–Mg oxides(Ni O–Mg O/γ-MA) with various Mg/Al molar ratios were prepared by one-pot hydrolysis method of inorganic metal nitrates.After calcined at 400 oC, the prepared materials possessed mesoporous structures with high surface areas, large pore sizes and narrow pore size distributions; Ni and Mg oxides species were homogeneously distributed in mesoporous γ-alumina framework mainly in the form of highly dispersed Ni(Mg)O, and produced uniformly dispersed Ni nanoparticles with relatively narrow particle size distributions upon H2 reduction. The comparative investigation for the pre-reforming of LPG over the Ni–Mg O/Al2O3 catalysts obtained by different routes revealed that the high stability and coke resistance ability of the Ni–Mg O/γ-MA catalysts were mainly due to the formation of smaller Ni nanoparticles and stable support structures.(2) Influences of Mg O addition on structure, surface properties, interaction between Ni species and support, reducibility of Ni2+ ions, and dispersion of Ni crystallites of the Ni O–Mg O/γ-MA materials and the catalyst performance for the LPG pre-forming were investigated in detail. The results showed that Mg O could improve surface basicity of the catalyst and lower Ni crystallite sizes, resulting in significant enhancements in the activity, stability and resistance to coke deposition for the pre-reforming of LPG.(3) Influences of calcination temperature on physicochemical properties of the Ni O–Mg O/γ-MA materials and the catalyst performance for the LPG pre-forming were systematically analyzed. The Ni crystallite sizes increased with calcination temperature due to the Ni crystallite growth by Ostwald ripening rather than by migration of Ni nanoparticles. The reaction results for the pre-reforming of LPG over the Ni O–Mg O/γ-MA catalysts showed that smaller Ni nanoparticles facilitated LPG reforming, methanation of carbon oxides and water gas shift, but lowered the rate of coke deposition; mesopores with uniform pore sizes were more favorable for diffusion of reactants and products to active sites, improving catalytic activity and stability.(4) Influences of nickel content on physicochemical properties of the Ni O–Mg O/γ-MA materials and the catalyst performance for the LPG pre-forming were discussed in detail. The dominant Ni O species were homogeneously dispersed with an attenuated interaction by the support with the increase in Ni content, producing uniform Ni nanoparticles throughout γ-alumina frameworks after H2 reduction. The Ni crystallite sizes decreased with increasing Ni content and showed a minimum at 18 wt%. The reaction results demonstrated that Ni surface areas were mainly responsible for catalytic activities; smaller Ni nanoparticles benefited steam reforming of hydrocarbons, methanation of carbon oxides, and water gas shift, but inhibited hydrocracking of hydrocarbons and lowered rate of coke deposition, improving catalytic activity and stability.(5) Cerium-doped mesoporous γ-alumina supported nickel oxides(Ni O/Ce–γ-MA) were prepared via one-pot hydrolysis of inorganic salts without surfactants and applied for the pre-reforming of LPG under a low steam to carbon molar ratio of 1.2. Influences of Ce contents on catalyst structure, interaction between Ni species and support, dispersion of nickel species and Ni particle sizes were investigated in detail. The addition of Ce O2 could significantly promote the dispersion of nickel species and lower Ni crystallite sizes, improving the catalytic properties of the reduced Ni/Ce–γ-MA catalyst. The Ni/Ce–γ-MA catalyst containing 7 wt% Ce produced the smallest sub-4-nm Ni crystallites, resulting in the most excellent catalytic activity and stability with less than 1 wt% carbon deposition during the 100-h reaction time for LPG pre-reforming.