Effects of bromide adsorption on the direct synthesis of H2O2 on Pd nanoparticles: Formation rates, selectivities, and apparent barriers at steady-state

Enhancing Pd Catalytic Activity by Amine Group Modification for Efficient Direct Synthesis of H2O2

A great deal of research has been carried out on the design of Pd-based catalysts in the direct synthesis of H2O2, mainly for the purpose of improving the H2O2 selectivity by weakening the activation energy on the Pd active site and thus inhibiting the dissociation of the O-O bonds in O2*, OOH*, and HOOH*. However, this often results in insufficient activation energy for the reaction between H2 and O2 on Pd, leading to difficulties in improving both the selectivity and productivity of H2O2 simultaneously. Based on this, this study reports an efficient catalyst composed of amine-functionalized SBA-15-supported Pd. The strong metal-support interaction not only makes the PdNPs highly dispersed with more Pd active sites but also improves the stability of the catalyst. The amine group modification increases the proportion of Pd0, further enhancing Pd activity and promoting the adsorption and conversion of H2 and O2 on Pd, thereby significantly increasing H2O2 productivity. Additionally, the density-functional theory simulation results showed that due to the hydrogen-bonding force between the amine group and H2O2, this particular anchoring effect would make the hydrogenation and decomposition of H2O2 effectively suppressed. Ultimately, both the selectivity and productivity of H2O2 are improved simultaneously.

Iron oxide-promoted photochemical oxygen reduction to hydrogen peroxide (H2O2)

Hydrogen peroxide (H2O2) is a valuable green oxidant with a wide range of applications. Furthermore, it is recognized as a possible future energy carrier achieving safe operation, storage and transportation. The photochemical production of H2O2 serves as a promising alternative to the waste- and energy-intensive anthraquinone process. Following the 12 principles of Green Chemistry, we demonstrate a facile and general approach to sustainable catalyst development utilizing earth-abundant iron and biobased sources only. We developed several iron oxide (FeOx) nanoparticles (NPs) for successful photochemical oxygen reduction to H2O2 under visible light illumination (445 nm). Achieving a selectivity for H2O2 of >99%, the catalyst material could be recycled for up to four consecutive rounds. An apparent quantum yield (AQY) of 0.11% was achieved for the photochemical oxygen reduction to H2O2 with visible light (445 nm) at ambient temperatures and pressures (9.4-14.8 mmol g-1 L-1). Reaching productivities of H2O2 of at least 1.7 ± 0.3 mmol g-1 L-1 h-1, production of H2O2 was further possible via sunlight irradiation and in seawater. Finally, a detailed mechanism has been proposed on the basis of experimental investigation of the catalyst's properties and computational results.

Selective Oxidation Using In Situ-Generated Hydrogen Peroxide

ConspectusHydrogen peroxide (H2O2) for industrial applications is manufactured through an indirect process that relies on the sequential reduction and reoxidation of quinone carriers. While highly effective, production is typically centralized and entails numerous energy-intensive concentration steps. Furthermore, the overhydrogenation of the quinone necessitates periodic replacement, leading to incomplete atom efficiency. These factors, in addition to the presence of propriety stabilizing agents and concerns associated with their separation from product streams, have driven interest in alternative technologies for chemical upgrading. The decoupling of oxidative transformations from commercially synthesized H2O2 may offer significant economic savings and a reduction in greenhouse gas emissions for several industrially relevant processes. Indeed, the production and utilization of the oxidant in situ, from the elements, would represent a positive step toward a more sustainable chemical synthesis sector, offering the potential for total atom efficiency, while avoiding the drawbacks associated with current industrial routes, which are inherently linked to commercial H2O2 production. Such interest is perhaps now more pertinent than ever given the rapidly improving viability of green hydrogen production.The application of in situ-generated H2O2 has been a long-standing goal in feedstock valorization, with perhaps the most significant interest placed on propylene epoxidation. Until very recently a viable in situ alternative to current industrial oxidative processes has been lacking, with prior approaches typically hindered by low rates of conversion or poor selectivity toward desired products, often resulting from competitive hydrogenation reactions. Based on over 20 years of research, which has led to the development of catalysts for the direct synthesis of H2O2 that offer high synthesis rates and >99% H2 utilization, we have recently turned our attention to a range of oxidative transformations where H2O2 is generated and utilized in situ. Indeed, we have recently demonstrated that it is possible to rival state-of-the-art industrial processes through in situ H2O2 synthesis, establishing the potential for significant process intensification and considerable decarbonization of the chemical synthesis sector.We have further established the potential of an in situ route to both bulk and fine chemical synthesis through a chemo-catalytic/enzymatic one-pot approach, where H2O2 is synthesized over heterogeneous surfaces and subsequently utilized by a class of unspecific peroxygenase enzymes for C-H bond functionalization. Strikingly, through careful control of the chemo-catalyst, it is possible to ensure that competitive, nonenzymatic pathways are inhibited while also avoiding the regiospecific and selectivity concerns associated with current energy-intensive industrial processes, with further cost savings associated with the operation of the chemo-enzymatic approach at near-ambient temperatures and pressures. Beyond traditional applications of chemo-catalysis, the efficacy of in situ-generated H2O2 (and associated oxygen-based radical species) for the remediation of environmental pollutants has also been a major interest of our laboratory, with such technology offering considerable improvements over conventional disinfection processes.We hope that this Account, which highlights the key contributions of our laboratory to the field over recent years, demonstrates the chemistries that may be unlocked and improved upon via in situ H2O2 synthesis and it inspires broader interest from the scientific community.

Improving Catalytic Activity towards the Direct Synthesis of H2O2 through Cu Incorporation into AuPd Catalysts

With a focus on catalysts prepared by an excess-chloride wet impregnation procedure and supported on the zeolite ZSM-5(30), the introduction of low concentrations of tertiary base metals, in particular Cu, into supported AuPd nanoparticles can be observed to enhance catalytic activity towards the direct synthesis of H2O2. Indeed the optimal catalyst formulation (1%AuPd(0.975)Cu(0.025)/ZSM-5) is able to achieve rates of H2O2 synthesis (115 molH2O2kgcat−1h−1) approximately 1.7 times that of the bi-metallic analogue (69 molH2O2kgcat−1h−1) and rival that previously reported over comparable materials which use Pt as a dopant. Notably, the introduction of Cu at higher loadings results in an inhibition of performance. Detailed analysis by CO-DRFITS and XPS reveals that the improved performance observed over the optimal catalyst can be attributed to the electronic modification of the Pd species and the formation of domains of a mixed Pd2+/Pd0 oxidation state as well as structural changed within the nanoalloy.

N-Heterocyclic Carbene Modified Palladium Catalysts for the Direct Synthesis of Hydrogen Peroxide

Heterogeneous palladium catalysts modified by N-heterocyclic carbenes (NHCs) are shown to be highly effective toward the direct synthesis of hydrogen peroxide (H₂O₂), in the absence of the promoters which are typically required to enhance both activity and selectivity. Catalytic evaluation in a batch regime demonstrated that through careful selection of the N-substituent of the NHC it is possible to greatly enhance catalytic performance when compared to the unmodified analogue and reach concentrations of H₂O₂ rivaling that obtained by state-of-the-art catalysts. The enhanced performance of the modified catalyst, which is retained upon reuse, is attributed to the ability of the NHC to electronically modify Pd speciation.

Silica Bifunctional Supports for the Direct Synthesis of H2O2: Optimization of Br/Acid Sites and Pd/Br Ratio

We have studied the direct synthesis of hydrogen peroxide using a catalytic system consisting of palladium supported on silica bifunctionalized with sulfonic acid groups and bromide in the absence of acid and halide promoters in solution. Catalysts with several bromide substituents were employed in the catalyst synthesis. The prepared samples were characterized by TXRF, XPS, and hydrogen peroxide decomposition. Catalysts characterization indicated the presence of only palladium (II) species in all of the samples, with similar values for surface and bulk of Pd/Br atomic ratio. The catalysts were tested via direct synthesis, and all samples were able to produce hydrogen peroxide at 313 K and 5.0 MPa. The hydrogen peroxide yield and selectivity changed with the Pd/Br ratio. A decrease in the Pd/Br ratio increases the final hydrogen peroxide concentration, and the selectivity for H2O2 reaches a maximum at a Pd/Br ratio around 0.16 and then decreases. However, the maximum hydrogen peroxide concentration and selectivity occur at slightly different Pd/Br ratios, i.e., resp. 0.4 vs. 0.16.

Effect of Pd Coordination and Isolation on the Catalytic Reduction of O2 to H2O2 over PdAu Bimetallic Nanoparticles

The direct synthesis of hydrogen peroxide (H₂ + O₂ → H₂O₂) may enable low-cost H₂O₂ production and reduce environmental impacts of chemical oxidations. Here, we synthesize a series of Pd₁Au_x nanoparticles (where 0 ≤ x ≤ 220, ∼10 nm) and show that, in pure water solvent, H₂O₂ selectivity increases with the Au to Pd ratio and approaches 100% for Pd₁Au₂₂₀. Analysis of in situ XAS and ex situ FTIR of adsorbed ¹²CO and ¹³CO show that materials with Au to Pd ratios of ∼40 and greater expose only monomeric Pd species during catalysis and that the average distance between Pd monomers increases with further dilution. Ab initio quantum chemical simulations and experimental rate measurements indicate that both H₂O₂ and H₂O form by reduction of a common OOH* intermediate by proton-electron transfer steps mediated by water molecules over Pd and Pd₁Au_x nanoparticles. Measured apparent activation enthalpies and calculated activation barriers for H₂O₂ and H₂O formation both increase as Pd is diluted by Au, even beyond the complete loss of Pd-Pd coordination. These effects impact H₂O formation more significantly, indicating preferential destabilization of transition states that cleave O-O bonds reflected by increasing H₂O₂ selectivities (19% on Pd; 95% on PdAu₂₂₀) but with only a 3-fold reduction in H₂O₂ formation rates. The data imply that the transition states for H₂O₂ and H₂O formation pathways differ in their coordination to the metal surface, and such differences in site requirements require that we consider second coordination shells during the design of bimetallic catalysts.