In situ UV-vis-NIR diffuse reflectance and Raman spectroscopy and catalytic activity studies of propane oxidative dehydrogenation over supported CrO3/ZrO2 catalysts.
In situ UV-vis-NIR diffuse reflectance and Raman spectroscopy and catalytic activity studies of propane oxidative dehydrogenation over supported CrO3/ZrO2 catalysts.
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DOI：
10.1021/la049590v
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发表时间：
2004-07
期刊：
影响因子：
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通讯作者：
T. V. Malleswara Rao;G. Deo;J. Jehng;I. Wachs
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文献类型：
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作者：
T. V. Malleswara Rao;G. Deo;J. Jehng;I. Wachs
在不同条件下考察了初湿浸渍制备的3%和6% CrO3/ZrO2 催化剂的分子结构、氧化态和反应活性。脱水条件下的原位拉曼光谱研究表明，3% 和 6% CrO3/ZrO2 催化剂具有等量的单色酸盐和多铬酸盐物质。因此，ZrO2 载体上的单层覆盖率约为 3% CrO3。由于 Cr2O3 晶体与剩余的 3% CrO3 相对应，因此 6% CrO3/ZrO2 具有额外的拉曼带。此外，在反应条件下，多铬酸盐物质优先被还原，单色酸盐物质受到轻微影响，而Cr2O3晶体不受影响。原位紫外-可见-近红外漫反射光谱结果表明，在稳态反应条件下，对于 3% 和 6% CrO3/ZrO2 样品，表面 Cr6+ 阳离子的还原程度和边缘能量位置随着还原环境的增加而增加。丙烷氧化脱氢 (ODH) 研究表明，两种催化剂的催化活性（以每秒每克催化剂转化的丙烷摩尔数表示）相似，这与存在等量的分子分散氧化铬一致。然而，6% CrO3/ZrO2 催化剂的周转频率小于 3% CrO3/ZrO2 样品的周转频率，因为 Cr2O3 晶体的存在对丙烷 ODH 相对不活跃。对于该催化系统和所使用的实验条件，丙烯、CO 和 CO2 是主要产物。此外，两种催化剂的 33-39% 丙烯选择性不受 C3H8/O2 比例的影响。结构反应性研究表明，分子分散的物质在 3% 和 6% CrO3/ZrO2 样品中以等量存在，因为 Cr6+ 单铬酸盐和多铬酸盐物质是参与丙烷 ODH 反应的最有效的催化活性位点。
The molecular structures, oxidation states, and reactivity of 3 and 6% CrO3/ZrO2 catalysts prepared by incipient wetness impregnation were examined under different conditions. The in situ Raman spectroscopic studies under dehydrated conditions reveal that the 3 and 6% CrO3/ZrO2 catalysts possess equal amounts of monochromate and polychromate species. Consequently, monolayer coverage on this ZrO2 support is about 3% CrO3. The 6% CrO3/ZrO2 possesses an additional Raman band due to Cr2O3 crystals corresponding to the remaining 3% CrO3. Furthermore, during reaction conditions the polychromate species is preferentially reduced, the monochromate species are slightly affected, and the Cr2O3 crystals are not affected. The in situ UV-vis-NIR diffuse reflectance spectroscopy results reveal that under steady-state reaction conditions the extent of reduction and edge energy position of surface Cr6+ cations increase with an increase in reduction environment for the 3 and 6% CrO3/ZrO2 samples. Propane oxidative dehydrogenation (ODH) studies reveal that the catalytic activity expressed in moles of propane converted per gram catalyst per second is similar for the two catalysts, which is consistent with equal amounts of molecularly dispersed chromia present. The turnover frequency for the 6% CrO3/ZrO2 catalyst is, however, smaller than that for the 3% CrO3/ZrO2 sample due to the presence of Cr2O3 crystals, which are relatively inactive for propane ODH. For this catalytic system and for the experimental conditions used, propene, CO, and CO2 are primary products. Furthermore, the 33-39% propene selectivity is not affected by the C3H8/O2 ratio for both catalysts. Structure-reactivity studies suggest that the molecularly dispersed species are present in equal amounts in the 3 and 6% CrO3/ZrO2 samples as Cr6+ monochromate and polychromate species are the most effective catalytic active sites taking part in the propane ODH reaction.