Reaction-relevant gold structures in the low temperature water-gas shift reaction on Au-CeO2
Reaction-relevant gold structures in the low temperature water-gas shift reaction on Au-CeO2
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DOI：
10.1021/jp800075y
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发表时间：
2008-08-21
影响因子：
3.7
通讯作者：
Flytzani-Stephanopoulos, Maria
中科院分区：
文献类型：
--
作者：
Deng, Weiling;Frenkel, Anatoly I.;Flytzani-Stephanopoulos, Maria
结合原位X射线吸收近边结构（XANES）和扩展X射线吸收精细结构（EXAFS）研究已经进行以跟踪低含量（<1% Au）金-氧化铈催化剂在水煤气变换（WGS）反应测试中在100和200 ℃下以及在氧气中在150 ℃下加热用过的催化剂之后的金结构变化。新鲜（400 ℃煅烧）材料中的金原子分散在氧化铈中。在WGS反应条件下，观察到氧化金物种的还原，伴随着逐渐的金聚集。新鲜材料的Au-Au配位数为零，但随着反应温度的增加，在氦气中含有5%CO-3%H2O的气体混合物中，Au-Au配位数增加到6.5 +/- 2.4（在100 ℃下使用后）和8.7 +/- 1.5（在200 ℃后）。第二个重要参数是反应气体组成，它决定了Au-O还原的程度。反应气体混合物的还原电位越低，所用催化剂中金的氧化程度越高，其活性越高。Au-CeO_2的最大活性是完全分散的Au-O-Ce新鲜材料的活性。用H-2-TPR法测定了反应过程中样品的表面氧损失，结果表明，表面氧损失与活性损失相当。通过在氧气中在150摄氏度下加热来再氧化和再分散金的尝试是无效的。然而，我们在此报道，在对100 ℃和200 ℃使用的催化剂样品进行400 ℃氧处理后，可以完全恢复表面氧量和金在二氧化铈中的再分散，同时恢复初始催化剂活性。这些测试通过在相同的微型反应器中连续的H-2-TPP/稳态催化剂活性测量进行。
Combined in situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) studies have been conducted to follow gold structural changes of low-content (< 1 % Au) gold-ceria catalysts in water-gas shift (WGS) reaction tests at 100 and 200 degrees C; and after heating the used catalysts in oxygen gas at 150 degrees C. Gold in the fresh (400 degrees C-calcined) material was atomically dispersed in cerium oxide. Under WGS reaction conditions, reduction of the oxidized gold species was observed, accompanied by gradual gold aggregation. The Au-Au coordination number is zero for the fresh material, but increases with the reaction temperature, to 6.5 +/- 2.4 (after use at 100 degrees C) and to 8.7 +/- 1.5 (after 200 degrees C) in a gas mixture containing 5% CO- 3% H2O in helium. The second important parameter is the reaction gas composition which determines the extent of Au-O reduction. The lower the reduction potential of the reaction gas mixture, the more oxidized the gold is in the used catalyst, and the higher its activity. The maximum activity of Au-CeO2 was that of the fully dispersed Au-O-Ce fresh material. Loss of surface oxygen took place during reaction, as measured by H-2-TPR of the used samples, and it was commensurate with the activity loss. Attempts to reoxidize and redisperse the gold by heating in oxygen gas at 150 degrees C were not effective. However, we report here that complete recovery of the surface oxygen amount and redispersion of gold in ceria was possible after a 400 degrees C- oxygen treatment of both the 100 degrees C- and 200 degrees C- used catalyst samples, with concomitant recovery of the initial catalyst activity. These tests were conducted by consecutive H-2-TPP/steady-state catalyst activity measurements in the same microreactor.