The Effect of Bromide Pretreatment on the Performance of Supported Au–Pd Catalysts for the Direct Synthesis of Hydrogen Peroxide

Abstract

The effect of pretreating Au–Pd catalysts on MgO and C supports with aqueous bromide solution, prior to using them for the direct synthesis of hydrogen peroxide, has been investigated. These two supports were selected since the parent materials exhibit contrasting microstructures and activities. The carbon‐supported catalysts comprise homogeneous Au–Pd alloy nanoparticles, which give high activity, whereas the MgO‐supported catalyst has Au–Pd alloys with a Pd‐rich surface and a Au‐rich core, which result in lower activity. Pretreatment of these catalysts with bromide was found to enhance H2O2 productivity and the degree of enhancement was largely dependent on the nature of the Au–Pd nanoparticles. Whereas bromide pretreatment significantly enhanced H2O2 productivity over the MgO‐based catalysts, the carbon‐based catalyst only showed a subtle promotional effect. Very low loadings of bromide (0.00034–0.044 wt %) were required to yield a significant positive effect. Higher bromide loadings (0.5–8.3 wt %) proved deleterious. The promotional effect has been correlated to selective poisoning of sites responsible for H2O2 hydrogenation and decomposition. In view of the limited effect of bromide pretreatment on the yield of H2O2 coupled with the effective performance of the carbon supported Au‐Pd catalysts in the absence of halides, for practical processes the addition of halides is not considered advisable with this catalyst system.

Revealing the role of bromide in the H2O2 direct synthesis with the catalyst wet pretreatment method (CWPM)

“…Typically, 0. . Previously, Ntainjua et al 28,29 reported that a Br:Pd mean molar ratio of 0.2 largely improved the performance of the pristine catalyst. In light of this, we chose to treat the pristine catalyst with a methanolic solution of NaBr with a Br:Pd molar ratio equal to 0.2.…”

“…In particular, the influence of the addition of the halides and acids was studied on a macroscopic level as recently reported 27,28 . For instance, Choudhary et al 27 studied the impregnation of bromide on a 5% Pd/Al 2 O 3 catalyst and found an optimum, in terms of the catalyst activity/selectivity.…”

“…In the first report, Ntainjua et al 28 reported the results of bromide impregnation of the 2.5 wt.% Au- Until now, only one publication mention a procedure to wet pre-treat the catalyst with halides 30 .…”

In Situ X‐ray Absorption Fine Structure Spectroscopy of a Palladium Catalyst for the Direct Synthesis of Hydrogen Peroxide: Leaching and Reduction of the Metal Phase in the Presence of Bromide Ions

“…The addition of suitable promoters, such as mineral acids and halide ions (Cl − , Br − ), to the reaction mixture is known to enhance the selectivity . Halide ions, which generally decrease the reaction rate and enhance the selectivity, allegedly prevent the formation of surface oxide layers or avoid the dissociative chemisorption of O 2 upon blocking the sites responsible for it. Although the hypothesis of site blocking received some support from theoretical studies based on DFT calculations, the mechanism is still unclear and sound direct evidence on the nature of their action has not been achieved yet.…”

“…In spite of this, the problem of leaching has been paid relatively little attention, in particular in academic investigation, in which experiments are not designed specifically to detect it. Although the issue of the leaching of the active metal phase has seldom been addressed, it was shown to be remarkable in carbon‐supported catalysts and in the presence of bromide ions as the selectivity enhancers …”