Subject Area物理化学
Comparative study of CO adsorption over the supported iridium catalysts by employing microcalorimetry and FT-IR methods
Wang XT(王兴棠); Wang XD(王晓东); Li L(李林); Lin J(林坚); Wang AQ(王爱琴); Zhang T(张涛); T.Zhang
Conference NameThe 5th International and 7th China-Japan Joint Symposium on Calorimetry and Thermal Analysis & Exhibition
Conference Date2008-5-18
Conference PlaceChina
Funding OrganizationDICP
AbstractThe supported iridium catalysts have widely been used for various chemical processes, including hydrazine decomposition, hydrogenation, hydrocarbon activation, and deNOx. Many research results show that the catalytic performance of iridium-based catalysts depends on the metal particle size (structure-sensitive reaction) and on the support employed [1-2]. Therefore, the surface chemistry study of supported iridium seems to be necessary so as to understand how the surface sites are modified, for instance, by the iridium loadings or pretreatment conditions. In the present work, microcalorimetry and FTIR were employed to study the effects of the support (Al2O3, SiO2), the iridium loadings (2%, 10% and 20%), as well as the reduction temperature (573K and 773K) on the surface chemistry of the iridium catalysts. It was found that the profiles of microcalorimetry curves are similar for all the investigated iridium catalysts with different support and iridium loadings. At coverage of =0-0.6, the profile of differential heat presents one plateau with almost constant value. When the coverage exceeds 0.6, the differential heat decreases quickly until the saturation coverage is reached. The initial heats of CO adsorption on iridium catalysts (120-160kJ/mol) are in agreement with the values reported in literature [3-4]. Interestingly, the iridium loadings and reduction temperature impose a different effect on the initial heat of CO on Al2O3 and SiO2 supported catalysts. For the Ir/Al2O3 catalysts reduced at 573K, the initial heat rose from 129 to 145 kJ/mol with an increase in the iridium content from 2% to 20 wt%. However, in the case of Ir/SiO2, increasing iridium loadings was found to have little effect on the initial heat of CO adsorption. Such a different behavior in adsorption heat could be accounted for in terms of chemical state of Ir on the two catalysts. As revealed by H2-TPR, the cationic Ir on the Ir/SiO2 was totally reduced to Ir0, whereas both cationic Ir and metallic Ir species existed on the reduced Ir/Al2O3, and the ratio of metallic to cationic Ir increased with the Ir loading. The increase in the reduction temperature from 573K to 773K resulted in the higher initial heat as well as the differential heat plateau on Al2O3 supported catalysts, while the corresponding values on the Ir/SiO2 catalysts are lowered slightly. The results of FTIR for CO adsorption on Ir/Al2O3 catalysts exhibited the simultaneous appearance of linear (Ir-CO) and gem-dicarbonyl (Ir+-(CO)2) species [5], suggesting that the differential heat plateau could be an average heat resulting from the two different modes. Since the bond strength of gem-dicarbonyl species is lower than that of linear species [6] and the transformation of Ir+ to metallic Ir is promoted at a higher reduction temperature, a higher differential heat was observed on the samples reduced at a higher temperature. In contrast, the differential heat plateau on the Ir/SiO2 catalysts could be attributed to the adsorption heat of linear CO species. The increase in the reduction temperature leads to the decrease in the low-coordinated Ir sites, which binds CO more strongly [7]. As a result, the initial heat as well as the differential heat plateau is lowered slightly
Document Type会议论文
Corresponding AuthorT.Zhang
Recommended Citation
GB/T 7714
Wang XT,Wang XD,Li L,et al. Comparative study of CO adsorption over the supported iridium catalysts by employing microcalorimetry and FT-IR methods[C],2008:364/2.
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