(340bc) Design of Selective Palladium-Based Catalysts for Direct Synthesis of Hydrogen Peroxide

Direct synthesis of hydrogen peroxide (DSHP) (H₂+O₂=H₂O₂/solvent+catalyst) is a promising green alternative to the current industrial method for H₂O₂ production (Riedl-Pfleiderer process). The process only requires a reactor unit without any separation and purification step and operates at room temperature. The product can be directly obtained after the separation of H₂O₂ solution from the solid catalyst. Coupling of the DSHP process with the downstream industrial processes in situ (Fenton process, selective oxidation of hydrocarbons (methane, propylene, cyclohexane)) is an attractive process modification. Two critical issues impede the industrial application of DSHP, including the significant secondary reactions (H₂O₂ decomposition and hydrogenation) and the low primary selectivity (H₂O₂ synthesis and H₂ combustion).

Mechanism study of Pd-catalyzed H₂O₂ decomposition

Skills: batch reactor setup; kinetic measurement; XRD; TEM/STEM; Chemisorption; ICP; UV-VIS; HPLC; XPS; catalyst preparation (impregnation; strong electrostatic adsorption)

Pd is the most selective among all the monometallic catalyst for DSHP but the selectivity still fails to satisfy industrial requirements. The mechanism study of H₂O₂ decomposition over Pd catalyst gives insight into the rational catalyst design by demonstrating the catalyst properties that can suppress secondary reactions during DSHP. We propose a solvent-assisted mechanism involving kinetically relevant proton electron transfer (PET) reactions. The mechanism work predicts higher oxidation state of Pd or alloying Pd with high electronegativity element suppresses the secondary H₂O₂ decomposition reaction due to the increase of Pd work function.

Highly selective Pd carbide catalyst for direct synthesis of H₂O₂

Skills: three-phases reaction (reactant gas, liquid solvent, solid catalyst); reaction engineering; GC; TEM/STEM; LabVIEW for reactor system control; Phidget box and software; semi-batch reactor; plug flow reactor setup (applies to all kinds of kinetic reactions involving gas-liquid-solid phases reaction and gas-solid phases reaction; operates at high pressure (30 bar) or atmospheric pressure and at either higher or lower than room temperature.)

We carbonized the Pd catalyst through heat treatment in C₂H₂ to synthesize the interstitial Pd-C catalyst. We found the selectivity of the carbide catalyst can achieve up to 50% as compared to the metallic Pd catalyst (14%) at the expanse of activity. The proposed carbonization treatment is much simpler and cheaper than preparing the state-of-the-art bimetallic catalyst for increasing H₂O₂ selectivity. For industrial application, Pd carbide catalyst shows better potential as compared to bimetallic catalyst since the carbide catalyst is recyclable. The conversion between metallic Pd and Pd carbide is reversible as reducing Pd carbide with H₂ forms metallic Pd catalyst.

Future work:

Since Pd carbide shows high selectivity but low activity, we plan to optimize the catalyst through pre-treatment (identity and concentration of gas, temperature, time, etc.) and post-treatment of catalyst (heat treatment with H₂ to clean surface carbon species).

We plan to study the multiple roles of carbonization treatment in increasing selectivity (inhibition of Pd hydride formation, decreased adsorption energy of H₂ and O₂, and surface modification effect).

We plan to incorporate other non-metallic elements (B, S, etc.) into Pd lattice and try to unlock another series of candidate catalysts for DSHP.

CO₂ Hydrogenation to value-added chemicals and fuels

Skills: TPD/TPR; physisorption; high pressure gas-solid flow reactor

In my undergraduate thesis research, I studied the effect of support pore size on CO₂ hydrogenation to hydrocarbons over Fe/Al₂O₃ catalyst. I found support pore size controls the size of particle size and optimum Fe₂O₃ particle size will lead to the highest selectivity to C₅⁺ hydrocarbons from C-C chain growth. I studied the effect of precursor, ligand on cobalt-based catalyst preparation for CO₂ methanation.

Abstract:

Direct synthesis of hydrogen peroxide (DSHP) from H₂ and O₂ is a promising alternative to the current industrial method for the manufacture of H₂O₂, the Riedl-Pfleiderer process. Pd-based catalysts are widely acknowledged as the most selective for DSHP.

H₂O₂ does not accumulate to the required industrial-grade concentration with current catalytic technology because of significant secondary reactions (H₂O₂ decomposition and hydrogenation). The mechanism of Pd-catalyzed H₂O₂ decomposition is still debated. We propose a solvent-assisted mechanism involving kinetically relevant proton electron transfer (PET) reactions rather than the typical Langmuir-Hinshelwood mechanism. The proposed mechanism is consistent with the observed saturation kinetics, primary kinetic isotope effect, influence of Pd particle size, and influence of proton concentration. Kinetic coupling of Pd-catalyzed H₂O₂ decomposition with the Fenton reaction demonstrates the reactive surface species. The study will support the rational design of selective catalyst for DSHP by demonstrating catalyst properties that can suppress secondary degradation reactions to produce targeted industrial grade H₂O₂.

We developed a cheaper and simpler approach for increasing H₂O₂ selectivity for DSHP as compared to the state-of-the-art bimetallic catalysts. We carbonized Pd catalyst through heat treatment in C₂H₂ to form an interstitial Pd-C phase. We found the carbonization treatment increases H₂O₂ selectivity by four-fold for DSHP at the expense of activity. With increasing time on stream, Pd carbide catalysts show stable selectivity using the water as solvent while the selectivity decreases using methanol as solvent. Our work demonstrates the possibility and unlocks the potential of incorporating inexpensive non-metallic elements into Pd lattice to achieve high selectivity for DSHP.