Carbon-supported palladium catalysts for the direct synthesis of hydrogen peroxide from hydrogen and oxygen

Metal-support spin orders: Crucial effect on electrocatalytic oxygen reduction

Magnetic property (e.g. spin order) of support is of great importance in the rational design of heterogeneous catalysts. Herein, we have taken the Ni-supported ferromagnetic (FM) CrBr3 support (Nix/CrBr3) to thoroughly investigate the effect of spin-order on electrocatalytic oxygen reduction reaction (ORR) via spin-polarized density functional theory calculations. Specifically, Ni loading induces anti-FM coupling in Ni-Cr, leading to a transition from FM-to-ferrimagnetic (FIM) properties, while Ni-Ni metallic bonds create a robust FM direct exchange, benefiting the improvement of the phase transition temperature. Interestingly, with the increase in Ni loading, the easy magnetic axis changes from out-of-plane (2D-Heisenberg) to in-plane (2D-XY). The adsorption properties of Nix/CrBr3, involving O2 adsorption energy and configuration, are not governed by the d-band center but strongly correlate with magnetic anisotropy. It is noteworthy that the applied potential and electrolyte acidity triggers spin-order transition phenomena during the ORR and induces the catalytic pathway change from 4e- ORR to 2e- ORR with the excellent onset potential of 0.93 V/reversible hydrogen electrode, comparable to the existing most excellent noble-metal catalysts. Generally, these findings offer new avenues to understand and design heterogeneous catalysts with magnetic support.

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.

Tailored Exfoliation of Polymeric Carbon Nitride for Photocatalytic H2O2 Production and CH4 Valorization Mediated by O2 Activation

The exfoliation of bulk C3N4 (BCN) into ultrathin layered structure is an effective strategy to boost photocatalytic efficiency by exposing interior active sites and accelerating charge separation and transportation. Herein, we report a novel nitrate anion intercalation-decomposition (NID) strategy that is effective in peeling off BCN into few-layer C3N4 (fl-CN) with tailored thickness down to bi-layer. This strategy only involves hydrothermal treatment of BCN in diluted HNO3 aqueous solution, followed by pyrolysis at various temperatures. The decomposition of the nitrate anions not only exfoliates BCN and changes the band structure, but also incorporates oxygen species onto fl-CN, which is conducive to O2 adsorption and hence relevant chemical processes. In photocatalytic O2 reduction under visible light irradiation, the H2O2 production rate over the optimal fl-CN-530 catalyst is 952 μmol g-1 h-1, which is 8.8 times that over BCN. More importantly, under full arc irradiation and in the absence of hole scavenger, CH4 can be photocatalytically oxidized by on-site formed H2O2 and active oxygen species to generate value-added C1 oxygenates with high selectivity of 99.2 % and record-high production rate of 1893 μmol g-1 h-1 among the metal-free C3N4-based photocatalysts.

Construction of a Novel Cascade Electrolysis-Heterocatalysis System by Using Zeolite-Encaged Ultrasmall Palladium Catalysts for H2 O2 Generation

In situ generation of hydrogen peroxide (H₂ O₂ ) has attracted extensive attention, especially in water treatment. However, traditional anthraquinones can only produce high-concentration H₂ O₂ and its transportation and storage are not convenient and dangerous. Herein, an in situ and on-demand strategy to produce H₂ O₂ by using a cascade water electrolysis together with a heterocatalysis system is provided. Beginning with water, H_2, and O₂ can be generated via electrolysis and then react with each other to produce H₂ O₂ immediately on efficient zeolite-encaged ultrasmall Pd catalysts. Significantly, the H₂ O₂ generation rate in the optimized cascade system reaches up to 0.85 mol L^-1 h^-1 g_Pd ^-1 , overcoming most of the state-of-the-art catalysts in previous literature. The confinement effect of zeolites is not only beneficial to the formation of highly dispersed metal species, promoting the H₂ O₂ generation, but also inhibits the H₂ O₂ decomposition, enhancing the production yield of H₂ O₂ . In addition, the effect of electrolytes, sizes of Pd species, as well as zeolite acidity are also systematically studied. This work provides a new avenue for H₂ O₂ generation via a highly efficient cascade electrolysis-heterocatalysis system by using zeolite-supported metal catalysts. The high catalytic efficiency and green process for H₂ O₂ generation make it very promising for further practical applications.

Synthesis and performance of PdMulti@HCS catalysts with Pd nanoparticles partially embedded in the inner wall of hollow carbon spheres for the direct synthesis of hydrogen peroxide from hydrogen and oxygen

PdMulti@HCS catalysts ensure the maximum exposure of Pd active sites and optimal transfer and diffusion ability for H2O2 synthesis.

Pd/CNT with controllable Pd particle size and hydrophilicity for improved direct synthesis efficiency of H2O2

Comparison of Pd catalyst performance before and after N/O doping. Schematic diagram of anchoring Pd nanoparticles on the surface of N and O doped CNT.

The synergistic effect of the carbon shell pore volume and core Pd size of Pd@hollow@C-X for the synthesis of H2O2

Yolk–shell Pd@hollow@C-X (X = 1.5, 3.2, 4.5 and 6) catalysts with Pd as the core and porous carbon as the shell were prepared via the inverse microemulsion method.

Hydrogenation on Palladium Nanoparticles Supported by Graphene Nanoplatelets

Pd nanoparticles (1 wt %; mean size ∼4 nm) were supported on ∼2 μm sized, but few nanometers thick, graphene nanoplatelets (GNPs) and compared to 1 wt % Pd on activated carbon or γ-alumina. Catalyst morphology, specific surface area, and Pd particle size were characterized by SEM, BET, and TEM, respectively. H2-TPD indicated that GNPs intercalated hydrogen, which may provide additional H2 supply to the Pd nanoparticles during C2H4 hydrogenation. Whereas the two types of Pd/GNPs (NaOH vs calcinated) catalysts were less active than Pd/C and Pd/Al2O3 below 40 °C, at 55 °C they were about 3-4 times more active. As for example Pd/GNPs (NaOH) and Pd/Al2O3 exhibited not too different mean Pd particle size (3.7 vs 2.5 nm, respectively), the higher activity is attributed to the additional hydrogen supply likely by the metal/support interface, as suggested by the varying C2H4 and H2 orders on the different supports. Operando XANES measurements during C2H4 hydrogenation revealed the presence of Pd hydride. The Pd hydride was more stable for Pd/GNPs (NaOH) than for Pd/C, once more pointing to a better hydrogen supply by graphene nanoplatelets.

Enhancing the selectivity of Pd/C catalysts for the direct synthesis of H2O2 by HNO3 pretreatment

The acid functional groups of activated carbon improve the H₂O₂ selectivity.

Looking for the “Dream Catalyst” for Hydrogen Peroxide Production from Hydrogen and Oxygen

The reaction between hydrogen and oxygen is in principle the simplest method to form hydrogen peroxide, but it is still a “dream process”, thus needing a “dream catalyst”. The aim of this review is to analyze critically the different heterogeneous catalysts used for the direct synthesis of H2O2 trying to determine the features that the ideal or “dream catalyst” should possess. This analysis will refer specifically to the following points: (i) the choice of the metal; (ii) the metal promoters used to improve the activity and/or the selectivity; (iii) the role of different supports and their acidic properties; (iv) the addition of halide promoters to inhibit undesired side reactions; (v) the addition of other promoters; (vi) the effects of particle morphology; and (vii) the effects of different synthetic methods on catalyst morphology and performance.

Boosting the Characterization of Heterogeneous Catalysts for H2O2 Direct Synthesis by Infrared Spectroscopy

Infrared (IR) spectroscopy is among the most powerful spectroscopic techniques available for the morphological and physico-chemical characterization of catalytic systems, since it provides information on (i) the surface sites at an atomic level, (ii) the nature and structure of the surface or adsorbed species, as well as (iii) the strength of the chemical bonds and (iv) the reaction mechanism. In this review, an overview of the main contributions that have been determined, starting from IR absorption spectroscopy studies of catalytic systems for H2O2 direct synthesis, is given. Which kind of information can be extracted from IR data? IR spectroscopy detects the vibrational transitions induced in a material by interaction with an electromagnetic field in the IR range. To be IR active, a change in the dipole moment of the species must occur, according to well-defined selection rules. The discussion will be focused on the advancing research in the use of probe molecules to identify (and possibly, quantify) specific catalytic sites. The experiments that will be presented and discussed have been carried out mainly in the mid-IR frequency range, between approximately 700 and 4000 cm−1, in which most of the molecular vibrations absorb light. Some challenging possibilities of utilizing IR spectroscopy for future characterization have also been envisaged.

Gold-Palladium Nanoalloys Supported by Graphene Oxide and Lamellar TiO2 for Direct Synthesis of Hydrogen Peroxide

Hybrid catalysts composed of gold-palladium nanoalloys that are sandwiched between layers of graphene oxide (GO) and lamellar TiO₂ are synthesized via the deposition-reduction method. The resulting AuPd catalysts with different compositions of metal and support are fully characterized by a series of techniques, including X-ray diffraction, scanning transmission electron microscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma mass spectrometry. The catalysts are also optimized against Au, Pd, GO, and TiO₂ contents and employed in the direct synthesis of hydrogen peroxide (DSHP) from H₂ and O₂. The sandwich-like AuPd nanoalloy comprising 1 wt % nanoparticle of an equimolar mixture of Au and Pd with 6 wt % GO and 93 wt % TiO₂ supports shows a promising catalytic performance toward the DSHP reaction with H₂O₂ productivity and selectivity of 5.50 mol H₂O₂ g_metal^-1 h^-1 and 64%, respectively. The catalyst is found to be considerably more active than those reported in the literature. Furthermore, the H₂O₂ selectivity of the catalyst is found to improve considerably to 88% when the TiO₂ support is pretreated by HNO₃. It is found that the perimeter sites of the interface of AuPd alloy and TiO₂ are deemed as catalytically active sites for the DSHP reactions and the acidic property of TiO₂ can retard the other overreactions and the decomposition of yielded H₂O₂. Results of the present study may provide a design strategy for partially covered catalysts that are confined by 2D materials for selective reactions.

Direct and Selective Synthesis of Hydrogen Peroxide over Palladium-Tellurium Catalysts at Ambient Pressure

Highly selective hydrogen peroxide (H₂ O₂ ) synthesis directly from H₂ and O₂ is a strongly desired reaction for green processes. Herein a highly efficient palladium-tellurium (Pd-Te/TiO₂ ) catalyst with a selectivity of nearly 100 % toward H₂ O₂ under mild conditions (283 K, 0.1 MPa, and a semi-batch continuous flow reactor) is reported. The size of Pd particles was remarkably reduced from 2.1 nm to 1.4 nm with the addition of Te. The Te-modified Pd surface could significantly weaken the dissociative activation of O₂ , leading to the non-dissociative hydrogenation of O₂ . Density functional theory calculations illuminated the critical role of Te in the selective hydrogenation of O₂ , in that the active sites composed of Pd and Te could significantly restrain side reactions. This work has made significant progress on the development of high-selectivity catalysts for the direct synthesis of H₂ O₂ at ambient pressure.

Dispersing Pd nanoparticles on N-doped TiO2: a highly selective catalyst for H2O2 synthesis

The selectivity of H₂O₂ synthesis directly from H₂ and O₂ over Pd/TiO₂ catalysts was found to be improved greatly if TiO₂ was doped by nitrogen before dispersing Pd nanoparticles.

A review on research progress in the direct synthesis of hydrogen peroxide from hydrogen and oxygen: noble-metal catalytic method, fuel-cell method and plasma method

The direct synthesis of H₂O₂ from H₂ and O₂ using Pd catalyst, fuel cell and plasma methods have been reviewed systematically.

Direct synthesis of hydrogen peroxide from hydrogen and oxygen over activated-carbon-supported Pd–Ag alloy catalysts

A series of bimetallic PdAg catalysts on an activated carbon support were prepared for the direct synthesis of hydrogen peroxide from hydrogen and oxygen. The addition of Ag to Pd caused an increase in selectivity due to ensemble and electronic effects.

Carbon Nanotubes as Support in the Platinum-Catalyzed Hydrolytic Dehydrogenation of Ammonia Borane

We report remarkable support effects for carbon nanotubes (CNTs) in the Pt/CNT-catalyzed hydrolytic dehydrogenation of ammonia borane. The origin of the support-dependent activity and durability is elucidated by combining the catalytic and durability testing with characterization by a range of spectroscopy and high-angle annular dark-field scanning transmission electron microscopy techniques and ICP analysis. The effects mainly arise from different electronic properties and different abilities for the adsorption of boron-containing species on platinum surfaces and changes in size and shape of the platinum particles during the reaction. Defect-rich CNTs in particular are a promising support material, as it not only enhances the platinum binding energy, leading to the highest hydrogen generation rate, but also inhibits the adsorption of boron-containing species and stabilizes the platinum nanoparticles to resist the agglomeration during the reaction, leading to the highest durability. The insights revealed herein may pave the way for the rational design of highly active and durable metal/carbon catalysts for the hydrolytic dehydrogenation of ammonia borane.