B-P-O系催化剂表面酸性的吸附量热研究

Microcalorimetric Study on Surface Acidity of B-P-O Catalysts

制备了不同P／B比的W-P-O系催化剂，XRD和Raman光谱表明各样品中P和B都处于四面体结构中，但Raman光谱显示P／B＜1的样品表面有B的三配位物相B2O3。存在．用吸附量热法考察了NH3、H2O及二甲醚（DME）等探针分子的吸附行为，研究了样品的表面酸性，NH3在P／B＞1的样品上发生解离吸附，而在P／B＜1的样品上，其起始吸附热与在γ-Al2O3上的相似．进一步的研究表明，二甲醚可以作为合适的探针分子表征B-P-O系催化剂的表面酸性．

The B-P-O catalysts with different P/B ratios from 0.8 to 1.3 were prapared and characterized. X-ray diffraction showed that B and P were of tetrahedral structure and Raman spectroscopy indicated that the catalysts with P/B>1 are abundant in P2O5 on the surface while the catalyst with P/B<1 is abundant in B2O3 on the surface. Microcalorimetric adsorption of ammonia, water and dimethyl ether(DME) was performed respectively to evaluate the surface acid properties of these catalysts. It was revealed that the catalyst with P/B<1 exhibited similar initial heat of adsorption of ammonia as γ-A12O3. However, the catalysts with P/B>1 displayed unusual low heat for the adsorption of ammonia. In addition, the curve of differential heat versus coverage of the ammonia passed through a maximum for the catalysts with P/B>1. According to the results of microcalorimetric adsorption of water, the above behavior can be explained by the dissociative adsorption of ammonia. This demonstrated that ammonia may not be a proper probe molecule for titration of acidity on B-P-O catalysts. Further study by using DME for the microcalorimetric adsorption showed that the surface arid strength measured in terms of initial adsorption heat increased with P content in the B-P-O catalysts.