Organic species, alloys, and support identity control H2 and O2 activation to H2O2 over metal nanoparticles

Adams, Jason Shelby

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https://hdl.handle.net/2142/115408 Copy

Description

Title

Organic species, alloys, and support identity control H2 and O2 activation to H2O2 over metal nanoparticles

Author(s)

Adams, Jason Shelby

Issue Date

2022-04-21

Director of Research (if dissertation) or Advisor (if thesis)

Flaherty, David W

Doctoral Committee Chair(s)

Flaherty, David W

Committee Member(s)

• Seebauer, Edmund G
• Guironnet, Damien
• Rodríguez-López, Joaquin

Department of Study

Chemical & Biomolecular Engr

Discipline

Chemical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Catalysis
• Sustainable Chemistry

Abstract

Herein, we examine the role of solvent molecules, alloys, metal-support interfaces, and organic adsorbates during the reactions of H2 and O2 over noble metal (Pd, Pt, Au) surfaces, which form either H2O2 or H2O. Moreover, we investigate these effects over a broad range of reactant activities, temperatures, and electrochemical potentials to understand reaction mechanisms and the connections of these systems to related electrochemical driving forces. Overall, this understanding guides the design of materials and conditions that specifically drive rates and selectivities of H2O2 formation. Solvent molecules influence reactions of H2 and O2 on Pd nanoparticles. Organic solvents activate to form reactive surface intermediates that mediate oxygen reduction through pathways distinct from reactions in pure water. Kinetic measurements and ab initio quantum chemical calculations indicate that methanol and water co-catalyze oxygen reduction by facilitating proton-electron transfer reactions. Methanol generates hydroxymethyl intermediates on Pd surfaces that efficiently transfer protons and electrons to oxygen to form hydrogen peroxide and formaldehyde. Formaldehyde subsequently oxidizes hydrogen to regenerate hydroxymethyl. Water, however, heterolytically oxidizes hydrogen to produce hydronium ions and electrons that reduce oxygen. These findings suggest that reactions of solvent molecules at solid-liquid interfaces can generate redox mediators in situ and provide new opportunities to increase rates and selectivities for catalytic reactions substantially. Moreover, H2O2 and H2O form over other metal nanoparticles through the electrochemical oxygen reduction reaction, analogously seen during the thermochemical direct synthesis of H2O2 from H2 and O2. The similar mechanisms for these reactions suggest catalysts should exhibit similar reaction rates and selectivities at equivalent electrochemical potentials, determined by reactant activities, electrode potential, and temperature. We quantitatively compare kinetic parameters for twelve nanoparticle catalysts obtained in a thermocatalytic fixed-bed reactor and a ring-disk electrode cell. Koutecky-Levich and Butler-Volmer analyses yield electrochemical rate constants and transfer coefficients, which informed mixed potential models that treat each nanoparticle as a short-circuited electrochemical cell. These models require that the hydrogen oxidation and oxygen reduction reactions occur at equal rates to conserve charge on nanoparticles. These kinetic relationships predict that nanoparticle catalysts operate at potentials that depend on reactant activities (H2, O2), H2O2 selectivity, and rate constants for hydrogen oxidation and oxygen reduction, confirmed by measurements of the operating potential during the direct synthesis of H2O2. The selectivities for H2O2 formation during thermocatalysis and electrocatalysis correlate across all catalysts when operating at equivalent electrochemical potentials. This analysis provides quantitative relationships that guide the optimization of H2O2 formation rates and selectivities. Catalysts achieve the greatest H2O2 selectivities when operating at high H-atom coverages, low temperatures, and potentials that maximize electron transfer towards stable species of OOH* and H2O2* while preventing excessive occupation of O-O antibonding states that lead to H2O formation. These findings guide the design and operation of catalysts that maximize H2O2 formation, and these concepts may inform other liquid-phase chemistries. Of these materials, Au-based catalysts typically show the greatest selectivities toward H2O2 formation. Indeed, supported Au nanoparticles effectively oxidize and reduce organic substrates by activating O2 and H2 through water-assisted heterolytic pathways. Au nanoparticles size and chemical functions of supports influence barriers of these pathway, implying that sites located at the Au-support interface catalyze these steps. Here, we examine steady-state rates of H2O2 and H2O formation by coupled oxygen reduction and hydrogen oxidation reactions as a function of the mean diameter of Au nanoparticles (2-25 nm) upon several supports, informing how chemical functions at the interface influence reactions of H2 and O2. Rates and barriers depend strongly upon support identity for Au nanoparticles, suggesting that nucleophilic functions at the Au-support interface reduces barriers for kinetically relevant activation and oxidation of H2. Au nanoparticles on supports that strongly bind O2 (e.g., La2O3) show lower barriers to dissociate O-O bonds (16-22 kJ mol-1) compared to weakly binding catalysts like Au-SiO2 (72-85 kJ mol-1). Moreover, the resulting proton-electron pairs combine with O2-derived species coordinated to Au atoms. Infrared spectra of adsorbed CO estimate the relative fractions of Au atoms exposed at interfacial sites and at metallic regions of Au nanoparticles. Deconvolutions of rates on each site indicate that metallic Au atoms are highly selective to H2O2 (~95%) compared to interfacial sites (~10-20%), consistent with increasing H2O2 selectivities with increasing Au nanoparticle diameters. These findings also explain differences in reactivity between PdAu nanoparticles on different supports. Alloying Pd significantly lowers barriers of hydrogen oxidation and increases rates of oxygen reduction; however, unselective Au substrates lower barriers of H2O formation. Therefore, this understanding guides the design of selective and reactive Au-based catalysts for H2O2 formation, applicable to other redox chemistries. Finally, aromatic and carbonylic ligands adsorb to Pd nanoparticles and influence the catalysis of molecular hydrogen and oxygen at solid-liquid interfaces by forming new reactive intermediates that mediate oxygen reduction. Specifically, quinones and related species inhibit H2O formation paths, increasing selectivities of H2O2 formation (~65-85%) relative to unmodified palladium (~45%). Fourier transform infrared spectroscopy and temperature-programmed oxidation measurements show that these species persist over extended periods of catalysis, consistent with irreversible adsorption of the ligand. Moreover, Ab initio calculations combined with kinetic and isotopic measurements suggest the carbonyl groups of quinones react heterolytically with hydrogen and generate O-H functions. Then, the resulting hydroquinone transfers protons and electrons to O2-derived species adsorbed to the surface of Pd. Theoretical calculations and temperature-dependence measurements suggest these species present favorable barriers to form H2O2 while obstructing O-O dissociation paths, increasing H2O2 selectivities. Notably, the use of hexaketocyclohexane leads to the greatest selectivity of H2O2 formation (85 ± 8%) with high stability over 130 h on stream using DI H2O as the solvent in the absence of other promoter molecules. Overall, these paths mimic the anthraquinone auto-oxidation used to produce most of the global supply of H2O2. Quinone ligands, however, facilitate similarly high rates and selectivities of H2O2 formation as auto-oxidation but on a single catalytic surface without using organic solvents. Thus, these species effectively mediate hydrogen transfer and oxygen reduction catalysis. Furthermore, this understanding provides strategies to avoid the high deactivation rates and environmental costs of using organic solvents while maintaining high rates and selctivities within aqueous solutions. Overall, each of these approaches present strategies of tailoring catalysis at solid-liquid interface, enabling a broad range of rates and selectivities during reactions of H2 and O2, which may inform the understanding of other chemistries involving H2 and O2 activation.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Jason Adams

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