Influence of Metal Precursors and Reduction Protocols on the Chloride-Free Preparation of Catalysts for the Direct Synthesis of Hydrogen Peroxide without Selectivity Enhancers

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

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.

Direct Synthesis of Hydrogen Peroxide under Semi-Batch Conditions over Un-Promoted Palladium Catalysts Supported by Ion-Exchange Sulfonated Resins: Effects of the Support Morphology

Palladium catalysts supported by a mesoporous form of sulfonated poly-divinylbenzene, Pd/µS-pDVB10 (1%, w/w) and Pd/µS-pDVB35 (3.6% w/w), were applied to the direct synthesis of hydrogen peroxide from dihydrogen and dioxygen. The reaction was carried for 4 h out in a semibatch reactor with continuous feed of the gas mixture (H2/O2 = 1/24, v/v; total flow rate 25 mL·min−1), at 25 °C and 101 kPa. The catalytic performances were compared with those of a commercial egg-shell Pd/C catalyst (1%, w/w) and of a palladium catalyst supported by a macroreticular sulfonated ion-exchange resin, Pd/mS-pSDVB10 (1%, w/w). Pd/µS-pDVB10 and Pd/C showed the highest specific activity (H2 consumption rate of about 75–80 h−1), but the resin supported catalyst was much more selective (ca 50% with no promoters). The nanoparticles (NP) size was somewhat larger in Pd/µS-pDVB10, showing that either the reaction was structure insensitive or diffusion limited to some extent over Pd/C, in which the support is microporous. The open pore structure of Pd/µS-pDVB10, possibly ensuring the fast removal of H2O2 from the catalyst, could also be the cause of the relatively high selectivity of this catalyst. In summary, Pd/µS-pDVB10 was the most productive catalyst, forming ca 375 molH2O2·kgPd−1·h−1, also because it retained a constant selectivity, while the other ones underwent a more or less pronounced loss of selectivity after 80–90 min. Ageing experiments showed that for a palladium catalyst supported on sulfonated mesoporous poly-divinylbenzene storage under oxidative conditions implied some deactivation, but a lower drop in the selectivity; regeneration upon a reductive treatment or storage under strictly anaerobic conditions (dry-box) lead to an increase of the activity but to both a lower initial selectivity and a higher drop of selectivity with time.