Anin situXAS study of the activation of precursor-dependent Pd nanoparticles

Abstract

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.

The Influence of the Support Nature and the Metal Precursor in the Activity of Pd‐based Catalysts for the Bromate Reduction Reaction

“…This is due to the formation of Pd x O y Cl z species during the synthesis of the catalyst prepared with PdCl 2. As it was discussed in previous papers, [35,44] these species were formed during catalyst calcination preventing palladium agglomeration during catalyst activation with hydrogen. After the reduction procedure, the Cl‐atoms disappeared and well dispersed Pd nanocrystals were formed.…”

“…This did not occur with the catalyst prepared from nitrate because Pd was completely reduced and all the non‐metallic atoms from the precursor were removed during the activation with hydrogen at 200 °C. These differences were not observed with the catalysts supported on alumina as they were calcined in air at 450 °C, previously to the hydrogen activation, removing in this step all the non‐metallic atoms from the different precursors [35] …”

“…As it can be seen all the materials had a Pd content close to the established target and a surface area around 75–95 m 2 /g. The surface areas of the Pd catalysts were quite similar to that of the support or slightly higher due to the partial modification of the support surface by the highly acidic media used during the metal impregnation [35] . The materials presented type II isotherms typical of mesoporous materials.…”

“…Other Pd reference compounds were also measured by XAS during the experiment. The samples were in situ analyzed with a cell built in our laboratory [35] . XAS measurements reproduced the conditions used for the catalyst preparation.…”

Characterization of metallic species on porous materials by in situ XAS

“…For example, nanoparticle growing can be avoided by restriction of support pore size, 33 distinct metal reduction pathways and blockage of determined surface atoms can be observed due to strong metal-support interactions, [20][21]36 including other effects. Moreover, using the same support but just changing the metal impregnation approach (by choosing different metal precursors or methods) [37][38] or activation parameters (calcination and/or reduction processes) 37,39 could result in a metal-support system completely distinct.…”