Evolution of intermetallic GaPd2/SiO2 catalyst and optimization for methanol synthesis at ambient pressure
Evolution of intermetallic GaPd2/SiO2 catalyst and optimization for methanol synthesis at ambient pressure
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DOI：
10.1080/14686996.2019.1603886
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发表时间：
2019-12-31
影响因子：
5.5
通讯作者：
Damsgaard, Christian D.
中科院分区：
文献类型：
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作者：
Fiordaliso, Elisabetta M.;Sharafutdinov, Irek;Damsgaard, Christian D.
采用初湿浸渍法制备纳米分散金属间化合物GaPd2/SiO2催化剂，在常压下高效催化CO2加氢制甲醇。在这里，我们优化催化剂的金属含量和还原温度的关系，其催化活性。我们发现，与金属负载为23 wt.%和7 wt.%的催化剂相比，金属负载为13 wt.%的GaPd2/SiO2催化剂的本构活性更高，这表明反应的最佳粒径约为8 nm。为了揭示活性相的形成，我们使用了x射线衍射（XRD）、x射线吸收近边缘精细结构（XANES）和扩展x射线吸收精细结构（EXAFS）等原位技术的组合，研究了23 wt.%和13 wt.%的GaPd2/SiO2催化剂的催化活性。我们发现，在室温下，金属含量较高的催化剂在H-2/Ar混合物中还原为金属Pd，而金属含量较低的催化剂在高达140摄氏度的温度下仍保持PdO和Pd的混合物。两种催化剂在300摄氏度以上形成GaPd2相，尽管金属含量较高的催化剂的晶体中间Pd纳米颗粒的比例在更高的温度下减少。在最终状态下，高金属负载的催化剂含有部分非合金金属Pd，而低金属负载的催化剂是相纯的。选择25℃、320℃和550℃三种温度，讨论了导致催化剂活性相形成的合金化机理。
The CO2 hydrogenation to methanol is efficiently catalyzed at ambient pressure by nanodispersed intermetallic GaPd2/SiO2 catalysts prepared by incipient wetness impregnation. Here we optimize the catalyst in terms of metal content and reduction temperature in relation to its catalytic activity. We find that the intrinsic activity is higher for the GaPd2/SiO2 catalyst with a metal loading of 13 wt.% compared to catalysts with 23 wt.% and 7 wt.%, indicating that there is an optimum particle size for the reaction of around 8 nm. The highest catalytic activity is measured on catalysts reduced at 550 degrees C. To unravel the formation of the active phase, we studied calcined GaPd2/SiO2 catalysts with 23 wt.% and 13 wt.% using a combination of in situ techniques: X-ray diffraction (XRD), X-ray absorption near edge fine structure (XANES) and extended X-ray absorption fine structure (EXAFS). We find that the catalyst with higher metal content reduces to metallic Pd in a mixture of H-2/Ar at room temperature, while the catalyst with lower metal content retains a mixture of PdO and Pd up to 140 degrees C. Both catalysts form the GaPd2 phase above 300 degrees C, albeit the fraction of crystalline intermediate Pd nanoparticles of the catalyst with higher metal loading reduces at higher temperature. In the final state, the catalyst with higher metal loading contains a fraction of unalloyed metallic Pd, while the catalyst with lower metal loading is phase pure. We discuss the alloying mechanism leading to the catalyst active phase formation selecting three temperatures: 25 degrees C, 320 degrees C and 550 degrees C.