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Title:Understanding the effects of promoters on the direct synthesis of hydrogen peroxide over supported palladium catalysts
Author(s):Priyadarshini, Pranjali
Director of Research:Flaherty, David W
Doctoral Committee Chair(s):Flaherty, David W
Doctoral Committee Member(s):Kenis, Paul; Seebauer, Edmund; Rodríguez-López, Joaquín
Department / Program:Chemical and Biomolecular Engineering
Discipline:Chemical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
hydrogen peroxide
Abstract:H2O2 is an active and selective oxidant that can be an environment-friendly replacement for toxic industrial oxidants such as Cl2 and other chlorine-based oxidants. The current industrial H2O2 production process, the anthraquinone autoxidation (AO), requires significant energy and capital expenditure due to the extensive purification and concentration steps. This makes H2O2 cost-prohibitive for many oxidation processes as compared to Cl2 which is inexpensively obtained from the chlor-alkali process. The direct synthesis of H2O2 (H2 + O2 → H2O2) is a potentially less expensive and energy-consumptive alternative to the incumbent anthraquinone autoxidation process. However, the formation of H2O by the reaction of H2 and O2 is thermodynamically favored over the formation of H2O2 on most transition metal catalysts which lowers the H2O2 selectivity of direct synthesis. Hence, significant research has been directed towards improving H2O2 selectivity through various methods (addition of promoters, alloying Pd with other transition metals). The addition of promoters (e.g., inorganic acids, halide salts) is a common method to improve H2O2 selectivity and several studies have investigated the effect of the addition of a cocktail of promoters on H2O2 rates and selectivities. However, very few studies have illustrated the fundamental way in which these promoters affect catalysis on the surface. The aim of this work is to provide a fundamental understanding of how certain promoters affect H2O2 rates and selectivities through rigorous experimental procedures and analysis. The addition of inorganic acids (e.g., HCl, H2SO4, H3PO4) improve H2O2 selectivities, yet they also lead to the dissolution of Pd from the support. Under acidic conditions, Pd can exist in heterogeneous (Pd0) as well as homogeneous (Pd2+) forms and both these forms of Pd can potentially contribute to catalysis. In chapter 2, we demonstrate, by a combination of kinetic measurement and in situ UV-Vis spectroscopy, that the heterogeneous Pd0 nanoparticles are responsible for the H2O2 formation and H2O2 hydrogenation reactions under semi-batch conditions in presence of HCl. Introducing HCl into the reaction solution decrease the rates of H2O2 hydrogenation. Chloride adsorbs on the surface of the dispersed Pd nanoparticles, modifying the surface properties of the catalytically active Pd0 species, which are responsible for the reduction in H2O2 hydrogenation and hence increase the selectivity of H2O2. The homogeneous complexes (e.g., [PdCl4]2- and [PdCl3(H2O)]-) do not play a significant role in improving the selectivity of the reaction in both water and methanol solvent. These findings will help in understanding the role played by different promoters such as acids and halides in improving the H2O2 selectivity for direct synthesis. In chapter 3, we examine another ubiquitous promoter for direct synthesis reaction, NaBr. Many studies exist that demonstrate that H2O2 selectivities are greater in presence of NaBr and an inorganic acid. Chapter 3 shows that variation of NaBr concentrations (in water solvent in the absence of a second promoter) increases H2O2 selectivities. Contact with NaBr solutions irreversibly modifies Pd nanoparticles, yielding higher H2O2 selectivities, due to the presence of strongly bound Br*-atoms. Bromide adsorption isotherms show that reduced Pd nanoparticles adsorb well over a monolayer of Br*-atoms, which suggests Br saturates surface Pd and subsequently intercalates within the near-surface region of Pd nanoparticles. Infrared spectra of adsorbed CO imply that Br atoms bind preferentially to under coordinated sites while ex situ X-ray photoelectron spectroscopy (XPS) indicates that these Br*-atoms withdraw charge from Pd atoms and yield larger fractions of Pd2+. H2O2 and H2O, both in the presence and absence of Br*-atoms, form via elementary steps that involve H2O-mediated proton-electron transfer (PET). Consequently, increased selectivities on Br*-modified surfaces reflect differences in apparent activation enthalpies for H2O2 (Δ〖H^‡〗_(H_2 O_2 )) and H2O (Δ〖H^‡〗_(H_2 O)) formation. Δ〖H^‡〗_(H_2 O_2 ) and Δ〖H^‡〗_(H_2 O) increase systematically with [NaBr], although with different sensitivities on [NaBr] indicating that Br*-atoms alter the electronic structure of Pd. The irreversible adsorption of Br atoms on Pd and their effects on H2O2 and H2O rates and H2O2 selectivities provide valuable insights into the role of Br in affecting direct synthesis. We continue the exploration of different strategies to improve H2O2 selectivities and investigate the effect of incorporating methylphosphonic acid (MPA) on the catalyst and in the solvent in chapter 4. SiO2 supported Pd nanoparticles modified with methylphosphonic acid gave higher H2O2 selectivities as compared to unmodified Pd while H2O2 turnover rates remained almost constant. The H2O2 selectivity was further enhanced when MPA was introduced into the water solvent indicating that MPA in the solvent can act as a promoter and improve H2O2 selectivity. MPA binds irreversibly to the catalyst over the timescales of the experiment (~60 h) since the selectivity remained constant after flushing with water. MPA-functionalized SiO2 and MPA functionalized Pd-SiO2 both show increase in the density of acidic functions, however, there is a greater distribution of acidic functions on SiO2 as compared to Pd-SiO2 indicating that majority of the ligand was bound to the support and not adsorbed on Pd. The activation enthalpies for H2O2 and H2O formation do not change with the introduction of ligands either on the support or in the solvent (or both) indicating that the electronic structure of Pd is not perturbed due to the presence of the MPA ligand. Instead, a decrease in the local pH of the catalyst is likely responsible for the increase in H2O2 selectivities. The rigorous kinetic measurements complemented with the thorough characterizations of catalyst provide insight into how different promoters alter the local environment and coordination of catalytically active Pd and influence the direct synthesis of H2O2. The combination of these techniques shows how surface properties of catalysts control the rates and selectivities, thus guiding the rational design of active, selective, and stable catalysts for the direct synthesis of hydrogen peroxide.
Issue Date:2020-10-30
Rights Information:Copyright 2020 Pranjali Priyadarshini
Date Available in IDEALS:2021-03-05
Date Deposited:2020-12

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