Pd-B/SiO_2非晶态合金的晶化过程及其催化加氢活性的研究

Study on the Crystallization Process of Pd-B/SiO_2 Amorphous Alloy Catalyst and its Hydrogenation Activity

用Ｒａｍ ａｎ、ＸＲＤ、ＳＥＭ 等技术， 研究了Ｐｄ－Ｂ／ＳｉＯ２ 非晶态合金的晶化过程及其对催化活性的影响． 研究发现， Ｐｄ２＋ 能高度分散在ＳｉＯ２ 载体上， 并与载体发生作用． 但Ｐｄ－Ｂ合金在载体上形成颗粒微细的原子簇． 温度低于６７３ Ｋ时， 这些原子簇在ＳｉＯ２ 载体上呈非晶态结构， 并随着温度的增加而发生团聚． 温度高于６７３ Ｋ时， Ｐｄ－Ｂ合金发生晶化并分相， 活性比表面积和催化活性随温度增加而下降． 由于Ｐｄ 是催化活性物种， 因此Ｂ的促进作用主要归因于其与Ｐｄ 合金化作用形成对加氢有利的非晶态结构和表面电子态； 载体ＳｉＯ２ 对Ｐｄ－Ｂ非晶态合金的分散作用， 不仅有利于提高催化剂的比表面积， 而且可改善非晶态合金的热稳定性．

The crystallization process of Pd\|B/SiO 2 amorphous alloy and its effect on the hydrogenation activity were investigated based on the various characterizations, such as Raman, XRD, SEM, hydrogen chemisorption. It was found that Pd 2+ is highly dispersed on the silica support and the Pd\|B amorphous alloy is formed as atom clusters in ultrafine particles at temperature lower than 673 K. The alloy particles gradually gathered with the increase of temperature . The phase decomposition and crystallization of Pd\|B alloy occur at temperature around 673 K. Its active surface area decreased in crystallization process, which, in turn, resulted in the decrease in its nitrobenzene hydrogenation activity. As Pd has been proved to be the active centers for hydrogenation, the promoting effect of B on the hydrogenation activity can be mainly attributed to its alloying effect with Pd to form the favorite amorphous structure and surface electronic states. The promoting effect of silica support could be understood by considering the increase of the surface area of the catalyst and improvement of thermal stability of the Pd\|B amorphous structure owing to the high dispersion of the Pd\|B alloy particles on the support.