Pd-Au and Pd-Pt catalysts for the direct synthesis of hydrogen peroxide in absence of selectivity enhancers

Authors:  S. Sterchele, P. Biasi, P. Centomo, P. Canton, S. Campestrini, T. Salmi, M. Zecca

Abstract:
Organic polymers are suitable candidates as catalytic materials for the direct synthesis of hydrogen peroxide (DSR)from dihydrogen and dioxygen, in view ofthe low temperature applied. In particular, strongly acidic ion-exchange resins have been already proposed for this reaction, either as carriers of Pd^II ions or Pd⁰ nanoparticles, but not for bimetallic Pd/Pt and Pd/Au catalysts, which are also active in this reaction. The introduction of PdII, Pt^II and Au^III precursors into the commercial ion-exchangers Lewatit K2621 and their subsequent reduction with an aqueous formaldehyde solution produced a library of bimetallic catalysts with 1% Pd (w/w) and a variable content of either Pt (0.1–1%, w/w) or Au (0.25–1%, w/w). The catalysts were tested with no selectivity enhancer in the DSR under batchwise condition at 2 ^◦C in MeOH at 2 MPa, with a CO₂/O₂/H₂ (72/25.5/2.5) mixture. In the Pd-Pt catalysts the addition of the smallest amount of Pt (0.1%, w/w) gave the best results in terms of the initial selectivity (43%) and the top concentration of H₂O₂ achieved in solution increased to 14.3 mM from 10.8 mM with the monometallic catalyst. With higher platinum amount the initial selectivity was lower, but in all these catalysts the addition of platinum limited the decrease of selectivity with time in comparison with the monometallic one. The addition of platinum in increasing amount seems also to favour poisoning of the catalysts by the products (most likely by H₂O₂). The presence of small amount of gold is also effective in achieving a comparatively high initial selectivity. With the addition of only 0.25% Au (w/w), the initial selectivity towards H₂O₂ and the top concentration of the product were boosted up to 61% and 17 mM, respectively. Gold in relatively little amount is also useful to limit the decrease of selectivity with time. In particular, it seems to limit the combustion of hydrogen and to be much more effective than platinum in this respect. Preliminary TEM analysis of the size and size distribution of the metal nanoparticles in the Au-Pd bimetallic catalysts shows that relatively large nanoparticles are preferable to achieve higher and stable selectivity, but also that size is likely not the most important parameter in controlling the catalyst performance. 

Keywords:
H₂O₂ direct synthesis
Palladium
Pd-Pt
Pd-Au
Lewatit K2621
Absence of promoters

Published in: Applied catalysis A: General   (Volume 468, Pages 1-474, 5 November 2013)

Publisher: Elsevier  

ISSN Information: 0926-860X

Pd-Au and Pd-Pt catalysts for the direct synthesis of hydrogen peroxide in absence of selectivity enhancers

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