Resonant-XRD Characterization of Nanoalloyed Au-Pd Catalysts for the Direct Synthesis of H2O2: Quantitative Analysis of Size Dependent Composition of the Nanoparticles †

Paolo Centomo,Patrizia Canton,Marco Zecca,Claudio Burato,Carlo Meneghini

doi:10.3390/app9152959

Abstract

The focus of this work is on the relationship between the quantitative structural characterization of bimetallic Au-Pd nanoparticles dispersed in an amorphous polymer matrix and their catalytic activity in the direct synthesis of hydrogen peroxide (DS reaction). Resonant X-ray powder diffraction with synchrotron radiation was employed to probe selectively and to reveal fine details of the structure of bimetallic nanoparticles embedded in the support. The semi-quantitative analysis of the resonant X-ray powdered diffraction data, made on a large number of metal nanoparticles, shows that in one of the polymer-supported Au-Pd catalyst for the DS reaction (P75) featured by an overall molar Pd/Au of about 5.54, the smallest metal nanoparticles (MNPs), which account for more than 99.9% of the total MNPs number and for more than 95% of the metal surface, are formed by practically pure palladium. The relative number of bimetallic alloyed nanoparticles is very small (less than 4 × 102 ppm) and they contribute to only about 2% of the total metal surface. In a second gold-enriched catalyst (P50) with an overall molar Pd/Au of 1.84, the proportion of the bimetallic alloyed nanoparticles increased to about 97% and they account for about 99% of the metal surface. As a result of the metal intermixing, the catalytic productivity for the DS reaction increased from 97 to 109 mmolH2O2/molH2, owing to the gold-promotion of palladium.

Highlights

Metal nanoparticles (MNPs) have received much attention for years due to their peculiar properties, which are appealing to basic sciences and innovative technologies
The above results show that a reliable estimate of the population of the phases and of the composition and size of their MNPs can be achieved with A-XRD analysis
Here, we reported the first investigation where the composition of different families of bimetallic MNPs was determined in DS catalysts as a function of their size distribution down to

Summary

Introduction

Metal nanoparticles (MNPs) have received much attention for years due to their peculiar properties, which are appealing to basic sciences and innovative technologies. This technique exploits the ample variation of the anomalous scattering factor (the atomic scattering factor for the atom, j, is given by fj = f0 + f’ + if”, where f0 corresponds to the atomic scattering factor for a spherically symmetric collection of free electrons in the atom, and f’ and f” are the dispersion correction or “anomalous contributions”) [35] of a resonant atomic species close to its atomic absorption edge as a contrast factor to selectively extract its contribution to the overall scattering intensity from the sample For this purpose, in the A-XRD experiment, the diffraction pattern is first measured with the X-ray energy tuned near (EN ) the absorption edge of one of the metals in the bimetallic sample

Discussion

Relative

Conclusions