Preparation Of Pyrochlore Catalysts With High Performance For CO And CH₄ Oxidation

Rare earth stannates, Ln2Sn2O7, have a chemical formula of A2B2O7, which belong to pyrochlore compounds. They generally exhibit high thermal stability and excellent conductivity. Furthermore, the A and B sites in the structure can be replaced according to the requirements to prepared desired materials, thus attracting more and more attentions over recent years. However, their properties as environmental catalysts have been rarely investigated and still lack systematic study. Previous work demonstrated that with traditional methods, it generally required high temperature to form Ln2Sn2O7 compounds, thus generating phase impurity easily and limiting the surface areas of the prepared samples, which in turn influenced their properties as catalysts or as catalyst supports.This paper mainly aims to optimize the preparation method of Ln2Sn2O7 pyrochlores, with the expectation to prepare samples with low aggregations and high surface area at relatively low preparation temperatures. Moreover, the factors affecting the structure of Ln2Sn2O7 pyrochlores, and the relations between the structures and the CO oxidation performance of the Ln2Sn2O7 samples have been investigated. Finally, based on monolayer dispersion theory, the Y2Sn2O7 pyrochlore modified γ-Al2O3 supports with both high surface areas and pyrochlore properties have been prepared and used to support Pd for CO oxidation. The main results of the thesis are summarized here: 1 The mesoporous La2Sn2O7-HT prepared by hydrothermal method and Pd/La2Sn2O7-HT catalysts show evidently higher activity for CO oxidation in comparison with the other two non-mesoporous pyrochlores prepared by co-precipitation(La2Sn2O7-CP) and sol-gel(La2Sn2O7-SG) methods. N2-BET, H2-TPR and CO-TPD results indicate that La2Sn2O7-HT has higher surface area and contains more active surface oxygen species. As a support, it can improve Pd dispersion and strengthen the interaction between Pd and the support. Therefore, La2Sn2O7-HT is a superior support for Pd for CO oxidation. 2 Hydrothermal method was successfully used to prepare A2Sn2O7 pyrochlores with different A sites(A=Y、Pr、Sm), among which, Y2Sn2O7 pyrochlore shows the best CO oxidation activity. Compared with Y2Sn2O7-CP pyrochlore prepared by co-precipitation method, Y2Sn2O7-HT, the sample prepared by hydrothermal method and Y2Sn2O7-HT-800(calcined at 800 oC) show superior CO oxidation activity. TEM results testify that the high activity of Y2Sn2O7-HT is mainly attributed to the preferentially exposure(111) lattice planes. As a support, Y2Sn2O7-HT can improve the dispersion of Pd, thus achieving catalysts with enhanced activity. 3 Stable Y2(Sn0.7M0.3)2O7 pyrochlore compounds were successfully prepared by hydrothermal method, in which Sn in the B site can be partially replaced by Al or Ce cations, as confirmed by XRD, H2-TPR and Raman results. It is revealed that with partially B site replacement, the crystallization of the catalysts can be impeded, hence improving their surface areas evidently. In addition, the amount of the acidic sites of the catalysts was reduced. These are believed to be the reasons accounting for the improved CH4 oxidation activity of the catalysts. 4 With XRD extrapolation, XPS correlation and H2-TPR method, the monolayer dispersion capacity of Y2Sn2O7 pyrochlore on γ-Al2O3 support was evaluated, which is about 7% Y2Sn2O7(equals to 0.109 mmol Y2Sn2O7/100 m2 γ-Al2O3). The results indicate the CO oxidation activity of the catalysts improves with the increasing of the Y2Sn2O7 loading. Compared with the individual Y2Sn2O7 and γ-Al2O3 supports, Pd supported on Y2Sn2O7/γ-Al2O3 composite support showed much better reaction performance. 1%Pd/15%Y2Sn2O7/γ-Al2O3 sample displays the highest CO oxidation activity.