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

Abstract

For the first time a Pd catalyst for the direct synthesis (DS) of H2O2 from H2 and O2 was investigated using in situ Pd K‐edge X‐ray absorption fine structure spectroscopy. The catalyst was loaded into a continuous reaction cell and fed at room temperature and pressure with a mixture of CO2, O2, H2 (86:10:4, v/v), and methanol. Pd and PdO phases are observed in similar and steady proportions (62 and 28 %, respectively). The same is found if the catalyst is either dry or treated with pure methanol only. With 10 ppm of NaBr in the reaction mixture both the Pd edge jump and the fraction of PdO decrease steadily during the DS. This shows that Br− ions promote both the leaching and the reduction of Pd in a monometallic DS catalyst. It also suggests that they are connected to each other and to the ability of Br− ions to promote Pd in the DS. An unprecedented mechanism of protection and remediation supported by Br− ions against the overoxidation of Pd is proposed and discussed in this context.

Dynamic structural changes of supported Pd, PdSn, and PdIn nanoparticles during continuous flow high pressure direct H2O2synthesis

“…Transformation and leaching of Pd species in methanol solution (excess O 2 ) in the presence of halide ions at ambient pressure was also observed by Centomo et al 61 using in situ XAS. Furthermore, Pd complexes leached in the presence of halogens are generally accepted as active species of C-C coupling reactions in the basic media.…”

Advanced Development Strategy of Nano Catalyst and DFT Calculations for Direct Synthesis of Hydrogen Peroxide

“…In Figure 8i, the X-ray absorption near edge structure (XANES) spectra of the bimetallic NCs showed similar features assigned to metallic Pd but with distinguishable shifts, implying small changes in electronic/geometric structure. [74,75] In Figure 8j, the dominant backscattering of Pd-Pd neighbors observed at the extended X-ray absorption fine structure (EXAFS) provided a clear evidence for the majority of metallic Pd 0 species, which is in good agreement with the XRD data. [74,75] For interplanar spacings of the NCs, Pd-Ni, and Pd-Zn were found to have significantly smaller Pd-Pd distances compared to pure Pd, a result of doping with the second metal.…”

Comparing Catalysts of the Direct Synthesis of Hydrogen Peroxide in Organic Solvent: is the Measure of the Product an Issue?

“…For this paper we have reviewed 79 papers dealing with the DS and published since 2000 to date [8–80] (Figure 1). Out of them three regrettably did not report how the amount of H 2 O 2 was measured [81–83] .…”

“…Our research group has been active on DS for 14 years [71–78,87,88] and in this period we had occasion to test different methods for the analysis of H 2 O 2 in the reaction mixtures. After having routinely used the iodometric titration for long, recently we have opted for the colorimetric measure with TiOSO 4 and started to wonder whether this change could be a reason of concern in comparing our data.…”