Decomposition Of Methanol Into Synthesis Gas Over Supported Palladium Catalysts

Author : Tianze Xie
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (134 download)

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Download or read book Design of Selective Palladium-based Catalyst for Direct Synthesis of Hydrogen Peroxide written by Tianze Xie and published by . This book was released on 2022 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Direct synthesis of hydrogen peroxide (DSHP) is a promising alternative to the current industrial method for H2O2 production (Riedl-Pfleiderer process). Pd-based catalyst is known to be highly active for DSHP while the selectivity fails to satisfy industrial requirement. Pd-metal alloys receive most attention to achieve high H2O2 selectivity, whereas to best of our knowledge, no effort has been devoted to the incorporation of light element (e.g., B, C, P, and S) into Pd for DSHP. The achievable concentration of H2O2 from DSHP process is far below the industrial requirement, primarily due to the significant extent of degradation reactions. Decomposition of H2O2 to water and oxygen is a dominant secondary reaction in DSHP and critical in other oxidation chemistries. The mechanism of Pd-catalyzed H2O2 decomposition is still debated. This dissertation mainly covers two topics: (i) development and optimization of selective Pd carbide catalyst for DSHP and (ii) mechanism study of Pd-catalyzed H2O2 decomposition. We proposed a carbonization method to prepare interstitial Pd carbide catalyst as an alternative to the use of bimetallic catalyst for H2O2 synthesis. We carbonized silica-supported palladium catalyst through heat treatment in acetylene flow and the synthesized Pd-C material shows a 4-fold increase in H2O2 selectivity but only with a half reduction in H2O2 formation rate compared with the parent metallic Pd catalyst. Both bulk carbon and surface carbon species contribute to the increased selectivity towards H2O2. The fragments of hydrocarbon species adsorb on catalyst surface after C2H2 treatment and break up contiguous Pd ensembles. The isolated Pd active site is more selective for H2O2 formation. DFT calculations demonstrate the subsurface carbon has an electronic effect on Pd and increases the activation barrier for the O-O bond breaking. Consistently, the measured apparent activation energy increases for both H2O2 formation and H2O formation over Pd carbide catalyst, while the impact of carbonization is more significant on H2O formation. The Pd carbide catalyst is simple and cheap to prepare, facile to regenerate by O2 and H2 and also exhibits a comparable selectivity increase as the state-of-the-art bimetallic catalyst. We optimized the kinetic performance of Pd carbide catalyst by pre-treatment (carbonization time, temperature, and gas identity) and post-treatment (H2 and O2 treatment). With increased carbonization time or temperature, the selectivity towards H2O2 increases while the yield of H2O2 initially increases and then decreases. Pd carbide catalyst shows higher H2O2 selectivity at the same bulk carbon content after C2H2 treatment than using CO and CH3OH vapor as carbonization gas. We attribute this observation to the different identity of the carbon species adsorbed on catalyst surface. Reduction of Pd carbide catalyst by H2 enhances the activity for H2O2 formation whereas decreases the H2O2 selectivity. We demonstrated the regenerability of Pd and Pd carbide catalyst through oxygen and hydrogen post-treatment. Addition of methanol into solvent (water) increases the H2O2 selectivity on both Pd and Pd carbide, while the impact is more significant on Pd than Pd carbide. We propose the role of organic alcohol as solvent resembles the carbonization treatment to the Pd catalyst that surface adsorbed carbon species isolate the Pd active site and exert an ensemble effect on inhibiting O-O bond breaking. We summarize the activity and selectivity of Pd carbide catalysts optimized in this work follow a volcano relationship. We proposed a water-assisted mechanism involving kinetically relevant proton electron transfer (PET) reactions for Pd-catalyzed H2O2 decomposition. The proposed mechanism is consistent with the observed saturation kinetics, primary kinetic isotope effect, particle size effect, and influence of proton concentration. Larger particles have increased reactivity towards H2O2 decomposition due to increased electron transfer to H2O2 relative to smaller Pd particles. Insignificant influence of proton concentration on decomposition rate indicates water molecule rather than H+ serves as proton donor and participates the kinetically relevant PET step. Kinetic coupling of Pd-catalyzed H2O2 decomposition with the Fenton reaction (reactive oxygen species oxidation) demonstrates surface adsorbed *H2O2 and *HOO species are the most abundant reactive intermediate. These results 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.