Gold-Palladium Catalysts: Towards H2O2 Production
Authors: Pizzutilo Enrico , Freakley Simon , Geiger Simon , Polymeros George , Baldizzone Claudio , Hutchings Graham J. , Mayrhofer Karl J.J. 
Published: February 2020

Hydrogen peroxide (H2O2) is an important chemical, commonly employed for disinfection, water treatment and bleaching. Whilst H2O2 itself is considered as an environmental friendly chemical oxidant, whose sole side product is water, its current synthesis method, the anthraquinone process, can be considered less so. Considering that most of end-use applications require concentrations between 2 to 5 wt%, a continuous on-site production of H2O2 would provide an attractive alternative and is therefore highly desirable.  Both the electrochemical reduction of oxygen to hydrogen peroxide [1-2] and the direct synthesis in an autoclave [3] have attracted considerable research interest into catalysts which could provide a more efficient alternative to the current industrial process.

Since the first patent claiming Pd catalysts active for the peroxide synthesis [4], several studies have been carried out to investigate the role of different acids and halides [5-6] as well of different supports [3] in the productivity [mol/kgcat/h] and hydrogen selectivity for Pd based catalysts.

Despite the state of the art characterization techniques, being applied to determine the surface structure of mono and bimetallic catalysts, there is still a lack of detailed understanding of the active sites of the metal nanoparticle.

The promotional effect of Au addition to Pd catalysts for the direct synthesis of H2O2in a reaction medium free of acid and halide promoters has been extensively studied by Hutchings and co-workers over the last decade [3]. In particular, one major breakthrough is related to the role of the acid pre-treated activated carbon [7].

Starting from these previous studies, in this work different molar ratio of Au/Pd catalysts were synthesized [8] with a final metal loading of 10 wt% on activated carbon and the influence of bimetallic nanoparticles composition for Oxygen Reduction Reaction (ORR) and Hydrogen Oxydation Reaction (HOR) was studied.

The change in activity, selectivity towards hydrogen peroxide as well as in H2O2 decomposition was characterized in both an electrochemical cell, using a Rotating Ring Disk Electrode (RRDE), and in an autoclave. Whilst the addition of Au to Pd increases the overall selectivity, the results for pure Au showed low peroxide productivity during direct synthesis.

Furthermore, the stability of these catalysts in acidic media and the catalyst degradation  consequences on the performances for H2O2 synthesis represents also a major interest of this work. The online dissolution was characterized with a Scanning Flow Cell and Inductively Coupled Plasma Mass Spectrometer (SFC-ICPMS), while the microscopic stability was studied with Identical Location TEM (IL-TEM).

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