Evidence of Pd segregation and stabilization at edges of AuPd nano-clusters in the presence of CO: A combined DFT and DRIFTS study

Abstract

International audienceCombined Density Functional Theory (DFT) calculations and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) were performed to study the distribution of Pd atoms in bimetallic AuPd nanoparticles in the presence of adsorbed CO. Compared to vacuum condition, the results showed evidence of Pd surface enrichment where both Pd monomers and Pd dimers could exist. The energetic stability calculated for several alloy configurations evidenced the preference of Pd to occupy undercoordinated edge sites in the presence of CO gas. Moreover, the calculation of the vibrational frequencies of adsorbed CO for the first time allowed the fine assignment of the complex experimental DRIFTS bands of CO interacting with the bimetallic nanoparticles and their evolution with time exposure. Electronic structure analysis shows preponderant p-back-donation from under-coordinated Pd to CO inducing strong bonding on edge sites

Carbon Monoxide Poisoning Resistance and Structural Stability of Single Atom Alloys

“…This has previously been noted experimentally for dilute Pd/Cu SAA nanoparticles whereby exposure to CO pulls Pd to the surface and consequently enhances the activity of these nanoparticles towards acetylene hydrogenation [26]. Moreover, several theoretical studies have demonstrated the phenomenon of adsorbate induced segregation [27][28][29][30][31][32][33]. For example a study by Sansa et al on the CO induced segregation of single transition metal dopant atoms in Au reveals that the adsorption energy of CO is sufficient to promote dopant atom segregation to both the Au(111) and Au(100) surfaces from the bulk [32].…”

“…Additionally, we recognize that the presence of adsorbates may induce structural changes in binary alloy materials, such as segregation of atoms from the bulk into the surface layer, as well as promoting aggregation and island formation [26][27][28][29][30][31][32][33][34]. Such changes are caused by differences in adsorption behaviour between an adsorbate on each metallic component of the alloy; these differences can offset or increase the energy change upon restructuring of the material.…”

Application of a parallel genetic algorithm to the global optimization of medium-sized Au–Pd sub-nanometre clusters

“…The small difference in electronegativity values of Pd(2.2) and Au(2.4) can lead to weak (Pd to Au) s electron transfer [34,38] (favouring mixing in larger clusters), but favouring surface Au in smaller clusters, as the more electronegative Au atom can better stabilise the negative charge that tends to build up on the cluster surface [71]. However, it should be noted that for larger clusters reverse (Au to Pd) d -band electron donation can also occur [72,73].…”

Theoretical insights on the effect of reactive gas on the chemical ordering of gold-based alloys

“…In a recent work devoted to the calculation of CO-Pd frequencies and IR experimental assignments, we have demonstrated that in gold-palladium nanoalloys (with very low Pd loading), only isolated or dimer palladium could exist on the surface [17]. This combined approach allowed identification of different CO alloy-bonding and quantification of all Pdtype coordinations of the surface in the presence of gas (Fig.…”

“…Moreover, the adsorbate-induced segregation of metal alloys under the reaction conditions and thus the changes in local atomic composition and surface structure have been predicted and demonstrated to occur for a number of gold alloy systems [12,13]. In particular, for Au-Pd nanoalloys (that will be more detailed bellow) although the gold surface enrichment is predicted to be thermodynamically favourable under vacuum conditions [14], a reversed segregation of Pd as a more active component to the surface is reported to occur in the presence of adsorbates [15][16][17]. Concerning the adsorbateinduced surface reconstruction, Yoshida et al [18] have recently reported the results of visualizing gas molecules interacting with supported nanoparticle catalysts at reaction conditions.…”