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

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 × 10 2 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 mmol H 2 O 2 /mol H2 , owing to the gold-promotion of palladium.shown that gel-type cross-linked functional polymers (gel-type resins) are effective in the multi-gram scale preparation of MNPs [4]. In fact, on the basis of the nanometer size porosity of gel-type resins in the swollen state, we can control both the size and the size distribution of metal and metal oxide NPs through the template controlled synthesis (TCS) [5][6][7][8], thus showing that TCS is a reliable method for the preparation of very small metal particles (<10 nm) entangled within the polymer framework. Most importantly, this framework, apart from providing an efficient and reliable size and dispersion control, stabilizes the MNPs against coalescence, oxidation, and early ageing while preserving a remarkable chemical activity [9]. As a matter of fact, resins proved to be useful scaffolds for the preparation of nanostructured inorganic phases both in catalysis and material science (see, for example, [10][11][12][13][14]) and the knowledge of their morphology is important in this respect [9,15].The great challenge in the frame of MNPs-based materials is the ability to understand and employ bimetallic systems in which the specific properties and characteristics of two different kinds of metals can be used and modulated. In several cases, bimetallic systems outperform the monometallic ones; however, very often, the specific reasons for this better behavior are not yet unequivocally understood [16][17][18]. In this regard, Au-Pd MNPs are exploited as catalysts for many reactions, such as, for example, the photoinduced generation of hydrogen molecules from water [19], oxidation of CO to CO 2 [20...

A magnetically retrievable air and moisture stable gold and palladium nanocatalyst for efficient C−C coupling reactions

“…However, in the 2 θ = 37°–40° region a low intensity broad amorphous halo is detected. This type of amorphous halo is usually detected in the formation of irregular metal alloys [ 32 ]. We propose that this broad region is the result of the lowered 2 θ values of palladium (111) and the gold (111) reflections overlapping upon the formation of an amorphous alloy on the surface of the AMNPs through reduction of Au 3+ and Pd 2+ .…”

Comparing Catalysts of the Direct Synthesis of Hydrogen Peroxide in Organic Solvent: is the Measure of the Product an Issue?

“…Our research group has been active on DS for 14 years [71–78,87,88] and in this period we had occasion to test different methods for the analysis of H 2 O 2 in the reaction mixtures. After having routinely used the iodometric titration for long, recently we have opted for the colorimetric measure with TiOSO 4 and started to wonder whether this change could be a reason of concern in comparing our data.…”