Active site requirements for selective catalytic O2 reduction with H2

Ricciardulli, Tomas

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https://hdl.handle.net/2142/117627 Copy

Description

Title

Active site requirements for selective catalytic O2 reduction with H2

Author(s)

Ricciardulli, Tomas

Issue Date

2022-09-12

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

Flaherty, David W

Doctoral Committee Chair(s)

Flaherty, David W

Committee Member(s)

• Peters, Baron G
• Rodríguez-López, Joaquín
• Yang, Hong

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
• spectroscopy
• peroxide
• h2o2
• nanoparticles

Abstract

The catalytic direct synthesis of hydrogen peroxide (H¬2 + O2 → H2O2) may enable low-cost H2O2 production and reduce environmental impacts of chemical oxidations. However, no catalysts with sufficiently high activity and selectivity towards the desired H2O2 product have been reported. Moreover, the site requirements to selectively reduce O2 to H2O2 remain elusive. Developing strategies for rational catalyst design will require greater understanding of the roles played by the H2O2 and H2O formation mechanisms, the coverages of reactive intermediates on the catalyst surface, pH as well as the electronic and geometric structures of active sites. Here, we synthesize and characterize different types of active sites and examine their reactivity in aqueous O2 reduction reactions to study the site requirements for O2 reduction with H2 to form H2O2. We synthesize a series of Pd1Aux nanoparticles (where 0 ≤ x ≤ 220, ~9 nm) and show that, in pure water solvent, H2O2 selectivity increases with the Au to Pd ratio and approach 100% for Pd1Au220. Analysis of in situ X-ray Absorption Spectroscopy (XAS) and ex situ infrared spectroscopy of adsorbed 12CO and 13CO show that materials with Au to Pd ratios of ~40 and greater expose only monomeric Pd species during catalysis and that the average distance between Pd monomers increases with further dilution. Ab initio quantum chemical simulations and experimental rate measurements indicate that both H2O2 and H2O form by reduction of a common OOH*-intermediate by proton-electron transfer (PET) steps mediated by water molecules over Pd and Pd1Aux nanoparticles. Measured apparent activation enthalpies and calculated activation barriers for H2O2 and H2O formation both increase as Pd is diluted by Au, even beyond the complete loss of Pd-Pd coordination. These effects impact H2O formation more significantly, indicating preferential destabilization of transition states that cleave O-O bonds reflected by increasing H2O2 selectivities (19 % on Pd; 95 % on PdAu220) but with only a three-fold reduction in H2O2 formation rates. The data imply that the transition states for H2O2 and H2O formation pathways differ in their coordination to the metal surface, and such differences in site requirements require that we consider second coordination shells during design of bimetallic catalysts. We examine the direct synthesis of H2O2 on bimetallic Pt1Aux (0 ≤ x ≤ 230) and Pt catalysts at steady-state in pure water and relate kinetic parameters for H2O2 and H2O formation to possible active site structures informed by complementary characterization methods. X-ray photoelectron spectra show significant Pt surface enrichment compared to the bulk composition. Analysis of infrared spectra of mixed monolayers of 12CO* and 13CO* indicate that Pt and Au form substitutional surface alloys. The Pt1Aux nanoparticles with the greatest mole fractions of Au predominantly expose Pt monomers (i.e., isolated Pt atoms), yet Pt atoms exposed upon all these nanoparticles possess electronic structures distinct from bulk Pt. Despite these differences, rate measurements are consistent with product formation through proton-electron transfer pathways for all Pt1Aux catalysts. In situ XAS indicate that Pt remains metallic during H2O2 synthesis. Under the most oxidizing conditions, selectivities toward H2O2 increase strongly with the Au to Pt ratio from 2% for monometallic Pt to 85% for Pt1Au170. However, selectivities are similar among all catalysts within reducing conditions. Comparisons of apparent activation enthalpies for the formation of H2O2 and H2O across these catalysts and the range of conditions suggest that Pt monomers within Au provide the greatest selectivities for H2O2 formation, because these active sites present high barriers for O-O bond rupture. Selectivities decrease with increasing ratios of H2 to O2 pressures, because Pt atoms aggregate and form oligomers that readily dissociate dioxygen intermediates. Dilute alloys may exhibit emergent properties in comparison to their monometallic counterparts, but many bulk metals are considered immiscible. We hypothesize that small nanoparticles may better stabilize alloy dispersions and present scalable synthesis techniques to form clean small (< 2 nm) Au clusters on SiO2 and deposit dilute (Au/M=12-20, M=Pd, Pt, Rh, Ir, Ni) quantities of a second metal. Infrared spectra of CO adsorbed to the catalysts after oxidative or reductive pretreatments reveal distinct spectroscopic features in the bimetallic materials and provide evidence for the formation of alloyed catalyst structures. These materials are applied to O2 reduction with H2, which occurs slowly on Au but with high selectivity towards the desired H2O2 product. Under excess H2 pressure, the PdAu, PtAu and RhAu catalysts show comparably high H2O2 formation rates (>10x higher than Au) and PdAu is more selective (~80%) towards H2O2 than unmodified Au (~75%). However, compared to Au, IrAu is only slightly more reactive and NiAu is less reactive. The small PdAu catalyst shows ~75% H2O2 selectivity, a substantial improvement over ~40% on a 9 nm material of similar composition. Activation barrier measurements reveal that the addition of Rh, Pd or Pt to Au creates new active sites involved in both H2 and O2 activation and reaction and facilitate the formation of both H2O2 and H2O. The differences in the catalytic rates and selectivities of both mono- and bimetallic catalysts are rationalized in terms of the oxygen and hydrogen binding energies. Current Pd-based direct H2O2 synthesis catalysts are insufficiently active and selective towards H2O2 to enable low-cost H2O2 production. Reaction mixtures with lower pH are widely reported to promote H2O2 synthesis, but this effect is not well-understood. Here, we synthesize Pd-loaded acidic zeolites to analyze the impact of acidity and confinement on H2O2 synthesis. Infrared spectra of adsorbed CO reveal features associated with both Pd single-atoms and nanoparticles, but comparisons of turnover rates and apparent activation enthalpies for H2O2 and H2O formation indicate that Pd nanoparticles catalyze these pathways and Pd single atoms exchanged at Brønsted acid sites do not contribute measurably to rates. Support material screening experiments show that Pd supported on medium and large-pore zeolites (FAU, BEA) is ~10x more reactive for H2O2 synthesis than small-pore zeolites (MFI, CHA) but similarly selective towards H2O2 (60-70%). Zeolite catalyst supports show comparable selectivities to other Brønsted acid materials (sulfonic acid resin, Al-MCM-41) which are significantly higher than on conventional catalyst supports like SiO2 and Al2O3 (40%). Physical mixture experiments and point of zero charge measurements reveal that a significant fraction of the increase in H2O2 selectivity when Pd is supported on Brønsted acid materials can be attributed to decreases in fluid-phase pH. Acidic environments destabilize H2O2 formation transition states by 5-10 kJ mol-1 and H2O formation transition states by 25-50 kJ mol-1 on Pd. Transition states which form H2O are more sensitive to pH because they are larger and exchange more charge with the metal surface. Overall, decreasing solvent pH decreases the rate of hydrogen consumption but increases the fraction of oxygen reduced to H2O2 rather than H2O.

Graduation Semester

2022-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Tomas Ricciardulli

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