|dc.description.abstract||Rice husk ash (RHA) was impregnated with aluminium nitrate and calcined to form silica alumina oxides (RHA-Al2O3) and used as catalyst support; it is further loaded with nickel by impregnation and deposition-precipitation. Characterization was made with nitrogen adsorption, ICP-AES, SEM, TEM, XRD, XPS, TGA, DSC, H2-TPD, ammonia adsorption, and TPR while the catalytic performance was determined by the hydrogenation of carbon dioxide under normal pressure. Based on the experiment results, model is established to describe the preparation of Ni/RHA-Al2O3 from RHA.
Silica content is different for RHA of different origin and ranges from 9 to 14wt.% in terms of SiO2. Highly porous RHA with SiO2 content greater than 99.98% can be obtained by hydrolysis following leaching with 3N HCl. Such obtained RHA is of amorphous and mesopore enriched and merits attention of being used as catalyst support.
For the composite support (RHA-Al2O3), it is found that impregnation of aluminum salt modified the pore size resulting in uniform pores, which is believed in favor of selectivity. In the mean time, the specific surface area of RHA-Al2O3 decreased with increasing Al2O3 content. The acid amount of RHA-Al2O3 is proportional to the surface area。
In the preparation of I-Ni/RHA-Al2O3 catalysts by incipient wetness impregnation method, three types of nickel oxide can exist, namely, bulk-NiO and two types of NiAl2O4-like, one with Ni-ion deposited on tetrahedral lattice of Al2O3 and the other on octahedral lattice. That on octahedral lattices was reduced easily, that on tetrahedral lattice was much difficult to be reduced. NiAl2O4-like of tetrahedral was observed in lower nickel loading below 10wt.%, when nickel loading went beyond 10wt.% bulk-NiO formed gradually and all three types of nickel oxide were observed. The crystal size of nickel oxide grew with increasing of nickel loading. The dried precursor, complex of nickel aluminum nitrate from incipient wetness impregnation of nickel nitrate, was of net work structure and decomposed at 400ºC and above. From TPR, the solid dissolution was observed; Ni-ions diffused from bulk-NiO to octahedral lattices of Al2O3 then to tetrahedral lattices and became more difficult to be reduced. It is also seen from XRD, the NiO converted to spinel along with the increasing of calcination temperature.
The specific surface area of I-Ni/RHA-Al2O3 increased with nickel loading during impregnation to the maximum of 200 m2/g at 15wt% nickel loading and then decreased. Though the specific surface area of I-Ni/RHA-Al2O3 is lower than that of commercial I-Ni/SiO2-Al2O3, its pores are larger than that of I-Ni/SiO2-Al2O3 but shallower giving less pore resistance. Results of methanation experiment confirm that I-Ni/RHA-Al2O3 exhibits better activity and selectivity as well than I-Ni/SiO2-Al2O3. The supported nickel catalyst possess good thermal stability, hence the activity of catalysts for the CO2 methanation was not affected by calcination and reduction temperature. However, the catalyst activity increases with reaction temperature until 500 ºC then decreases, therefore, the optimum temperature for CO2 methanation is 500 ºC.
For the P-Ni/RHA-Al2O3 catalysts prepared by deposition- precipitation, both increasing the Ni-ion concentration and deposition- precipitation time increased the nickel loading. Increasing the deposition- precipitation time also increased the amount of nickel-support interaction. With constant deposition-precipitation time of 24 hours, controlling the nickel loading by adjusting Ni-ion concentration, nickel dispersed very well when nickel loading below 26.4wt.% but gradually formed bulk-NiO when go beyond 26.4wt.%. When keeping the Ni-ion concentration at 0.14M and controlling the nickel loading by deposition-precipitation time, it gave good dispersion when nickel loading was below 12.0wt.%. Therefore, controlling the nickel loading by adjusting Ni-ion concentration gives better dispersion of nickel on catalyst.
The stability of precipitate from deposition-precipitation is better than Ni-ion, it react only with hydroxides on the support surface and will not diffuse into the support lattice. P-Ni/RHA-Al2O3 from deposition-precipitation method only presented two types of nickel oxides, bulk-NiO and nickel aluminate. With low nickel loading, only nickel aluminate was observed. Both types of nickel oxides were found at higher nickel loading. Similar to the incipient wetness impregnation method, crystal size of nickel oxide increased with nickel loading. But when the deposition-precipitation time extended over 48 hours, the crystal size tended to shrink slightly. The catalyst precursor after drying also presents net work structure and converts to nickel oxide at the calcination temperature of 500 ºC. Calcination of the catalyst precursor gave NiAl2O4 spinel which is very difficult to reduce. From XRD it is found that NiO transformed to spinel gradually along with increasing of calcination temperature but with smaller crystal size than that found in incipient wetness impregnation method. It shows that the X-ray diffraction spectrum of phase change is more obvious with incipient wetness impregnation method than that of deposition-precipitation method.
The BET specific surface area of P-Ni/RHA-Al2O3 from deposition-precipitation increased with precipitation time ranging from 151 to 176 m2/g, but decreased with increasing of Ni-ions concentration. The surface structure is of mesopore and the pore size can be controlled by Ni-ion concentration. Experimental result shows that the activity of catalyst increases with reaction temperature until 500 ºC and then decreases. Hence the optimum reaction temperature for CO2 methanation is also 500 ºC. Based on the reaction performance on the CO2 methanation, I-Ni/RHA-Al2O3 gives both better activity and selectivity than P-Ni/SiO2-Al2O3 does.
The RHA-alumina composite oxide prepared in this experiment is a catalyst support with highly promotive. Whether the supported nickel catalyst made from incipient wetness impregnation method or deposition-precipitation, all gives better than 90% yield in the CO2 methanation and is a superior catalyst.||en_US|