Combined macroscopic, nanoscopic and atomic‐scale characterization of highly dispersed bimetallic particles supported on ceria‐zirconia mixed oxide catalysts

Abstract

Highly dispersed gold nanoparticles appear as promising catalysts in the field of fine chemistry. However, the relatively low resistance to sintering of gold particles under reaction conditions often leads to a significant loss of catalytic activity. One strategy to overcome this limitation is combining gold with another metal to form a bimetallic system with enhanced stability, activity and/or selectivity. [1] The catalytic behavior of bimetallic particles is determined by their structure, size, morphology and chemical composition. Undoubtedly, to design efficient and high‐quality catalyst requires controlling all these features during the synthesis as well as a careful characterization of the synthesized catalysts at the finest scales. Such a characterization poses demanding challenges to STEM techniques if we want it to be representative of the actual macroscopic state of the surface of the catalyst. In this work, the capabilities and limitations of both the macroscopic (XPS, XRD, ICP‐AES, and chemisorption) and atomic scale (STEM‐XEDS) techniques in the characterization of highly dispersed bimetallic particles, AuRu and AuPd, supported on a ceria‐zirconia mixed oxide (CZ) have been evaluated. In particular, the effects of the catalyst activation pretreatment on nanostructure and catalytic performance for the selective oxidation of glycerol to glyceric acid of both bimetallic systems are investigated in comparative terms. Following the approach described in [2], the relationship between composition (Au at.%), as determined by quantitative STEM‐XEDS analysis, and size, as determined by HREM and HAADF, of a large ensemble of individual nanoparticles on the two types of catalysts was established as a function of calcination temperature, Figures 1 and 2. According to size‐composition maps shown there, Figures 1(a) and 2(a), the monometallic gold particles exhibit a wider larger of sizes and a larger average size, whereas monometallic Ru or Pd particles are smaller in average. Bimetallic entities are found in the intermediate size range. Raising the temperature up to 700ºC, Figures 1 and 2 (b), induces a compositional homogenization in the case the AuPd catalysts which is not observed in the case of the AuRu bimetallics. From this point of view both catalysts behave in a quite different manner, in spite of Ru and Pd being elements very close in the Periodic Table. Comparison of data obtained on the corresponding monometallic reference catalysts clearly indicates that the second metal, Ru or Pd, moderates the sintering behavior of Au, i.e. the stability of this type of catalysts. Moreover, a significant improvement in the catalytic activity takes place in the bimetallic catalysts, which is related to the presence of bimetallic entities in both catalysts. STEM data suggest that these bimetallic nanoparticles are formed by decoration of the surface of Au nanoparticles with 3D, nanosized, domains of the second metal, instead of forming actual Au‐Ru or Au‐Pd alloys. [3,4] The limitations of STEM studies to reach a precise representation of the composition on both bimetallic systems determined by macroscopic techniques (like XPS or ICP) will also be addressed, Figure 1and 2 (c). The likely origin of such discrepancies as well as strategies to overcome this limitation will also be discussed.