Instituto de Tecnología Química (Universitat Politècnica de València - Consejo Superior de Investigaciones Científicas), Avda. de Los Naranjos s/n, Valencia 46022, Spain.
Phys Chem Chem Phys. 2018 May 9;20(18):12700-12709. doi: 10.1039/c8cp00517f.
The activation of precursor-dependent Pd nanoparticles was comprehensively followed by in situ X-ray absorption spectroscopy on two inorganic supports for rationalizing the final catalytic activity. Two series of Pd-based catalysts (7 wt% Pd) were prepared by impregnation of γ-Al2O3 and activated carbon supports varying the metal precursor (Pd(NO3)2, PdCl2 and Pd(OAc)2). The most relevant physicochemical properties of the studied catalysts were determined by several techniques including ICP-OES, XRD, N2 adsorption and XAS. The results indicate that the thermal stability of the metal precursor plays an important role in the size and speciation of the formed Pd nanoparticles after the activation process. The Cl-based precursor, which presents high thermal stability, passes through a PdOxCly mixed phase when submitted to calcination on Pd/Al2O3 and leaves Cl-species after metal reduction on Pd/C (which can be detrimental to catalytic reactions). Differently, Pd(OAc)2 and Pd(NO3)2 promote the formation of larger species due to different precursor decomposition pathways. Ordered PdO is observed even before calcination when Pd(NO3)2 was used as a metallic source, which translates into large nanoparticles after reduction in H2. By using the average coordination numbers of Pd species obtained from EXAFS data of the as-reduced catalysts, a correlation was observed comparing the three precursors: PdCl2 generates smaller nanoparticles than Pd(OAc)2, which in turn generates smaller nanoparticles than Pd(NO3)2, regardless of the support used for catalyst preparation.
通过在两种无机载体上进行原位 X 射线吸收光谱学研究，全面跟踪了前体依赖性 Pd 纳米颗粒的活化，以合理推断最终的催化活性。通过浸渍 γ-Al2O3 和活性炭载体，制备了两种系列的 Pd 基催化剂（7wt%Pd），所用金属前体分别为 Pd(NO3)2、PdCl2 和 Pd(OAc)2。采用 ICP-OES、XRD、N2 吸附和 XAS 等多种技术确定了所研究催化剂的最相关物理化学性质。结果表明，金属前体的热稳定性在活化过程后形成的 Pd 纳米颗粒的尺寸和形态中起着重要作用。Cl 基前体具有较高的热稳定性，在 Pd/Al2O3 上进行煅烧时会经历 PdOxCly 混合相，并在 Pd/C 上进行金属还原后留下 Cl 物种（这可能对催化反应有害）。相比之下，Pd(OAc)2 和 Pd(NO3)2 由于不同的前体分解途径，促进了较大物种的形成。当使用 Pd(NO3)2 作为金属源时，甚至在煅烧前就观察到有序的 PdO，这转化为 H2 还原后的大纳米颗粒。通过使用 EXAFS 数据还原后催化剂中 Pd 物种的平均配位数，可以观察到三种前体之间的相关性：PdCl2 生成的纳米颗粒比 Pd(OAc)2 小，而 Pd(OAc)2 生成的纳米颗粒又比 Pd(NO3)2 小，无论用于催化剂制备的载体如何。
