The effect of heat treatment on the performance and structure of carbon-supported Au–Pd catalysts for the direct synthesis of hydrogen peroxide

Abstract

a b s t r a c tThe direct synthesis of hydrogen peroxide using supported gold palladium catalysts prepared by incipient wetness impregnation is described and discussed. The effect of an acid pre-treatment step on the activated carbon support prior to the deposition of the metals, together with the effect of the calcination temperature, has been investigated. The acid pre-treated samples all show superior activity to those materials prepared with the omission of this acid pre-treatment stage. The calcination temperature affects both the re-usability and hydrogenation activity of the catalysts. Detailed characterisation using X-ray photoelectron spectroscopy and aberration-corrected scanning transmission electron microscopy is described. The enhanced activity is associated with a higher surface concentration of palladium in the acid pre-treated samples which is principally present as Pd 2+ . Calcination of the catalysts at 400°C is required to achieve re-usable and stable catalysts, and this is associated with the morphology and dispersion of the metal nanoparticles. The surface ratio of Pd 0 /Pd 2+ is found to be an important factor controlling the hydrogenation of hydrogen peroxide, and a series of controlled reduction and re-oxidation of a sample show how the Pd 0 /Pd 2+ surface ratio can influence the relative rates of hydrogen peroxide synthesis and hydrogenation.

Engineering in direct synthesis of hydrogen peroxide: targets, reactors and guidelines for operational conditions

“…It was reported by Landon et al 47,130 that the addition of small amounts of Au or Pt to Pd leads to a significant enhancement, especially in terms of selectivity in the hydrogen peroxide direct synthesis; however, sometimes the same trend was also observed for the hydrogenation of the hydrogen peroxide that was formed. This effect has been subsequently confirmed in many studies by Edwards et al 50,93,120,124,126 , Bernardotto et al, 92 Menegazzo et al 44,113 and Hutchings et al 125 Notwithstanding, the combination of bimetallic catalysts with a proper acidified support leads to a decrease of the hydrogenation reaction. It is interesting to see that the addition of Au or Pt improves the selectivity while at the same time reducing the rate of H 2 conversion.…”

“…131 The synergetic behaviour of the two metals is usually considered to originate from an electronic effect rather than from a morphological effect, but the precise origin is yet to be determined. 44,92,113,124 Since there are no significant differences in the particle size distribution and particle morphology for these Au-Pd nanoalloy catalysts prepared using different Pd/Au ratios, it is believed, based on the results published in recent years, that the positive effects observed when alloying Pd and Au are caused by an electronic effect where the addition of a small amount of Au affects the electronic nature of Pd. 125,126 However, other authors found that the effect can be ascribed to both the effects: morphological and electronic.…”

“…92 It seems clear that the different preparation methods studied over the years gave different supported nanostructures with different properties for the direct synthesis reaction. 44,64,120,[124][125][126]131 Core shell or alloys of Pd-Au metals were investigated by many authors. The electronic effect between the two metals and between the metals and the support can enhance the catalytic properties making the catalyst more selective towards H 2 O 2 , but the real mechanisms of the modifications and of the direct synthesis reaction are still unclear.…”

“…The electronic effect between the two metals and between the metals and the support can enhance the catalytic properties making the catalyst more selective towards H 2 O 2 , but the real mechanisms of the modifications and of the direct synthesis reaction are still unclear. 44,50,64,92,93,120,[123][124][125][126] The addition of halides/acids enhances selectivity but it is not yet clear how, and if, their effects are similar to the addition of Au or Pt.…”

Continuous H2O2 direct synthesis process: an analysis of the process conditions that make the difference

“…The main issue is still the design of appropriated catalysts [5] (active metal or support [6][7][8][9][10][11][12][13][14][15][16][17]), followed by the focus on the addition of promoters (acids and halides) [6,11,16,17], selection of liquid solvent (water, methanol, ethanol or a mixture of them) and optimization of reaction conditions since appropriate selection of operational parameters are crucial in aiming at high selectivities in the H 2 O 2 direct synthesis [18][19][20][21][22][23][24][25][26][27][28][29]. Finally, the gas phase concentration is limited by flammability limits of hydrogen-oxygen mixtures.…”