Phenol is an important model compound to understand the thermocatalytic (TCH) and electrocatalytic hydrogenation (ECH) of biomass to biofuels. Although Pt and Rh are among the most studied catalysts for aqueous-phase phenol hydrogenation, the reason why certain facets are active for ECH and TCH is not fully understood. Herein, we identify the active facet of Pt and Rh catalysts for aqueous-phase hydrogenation of phenol and explain the origin of the size-dependent activity trends of Pt and Rh nanoparticles. Phenol adsorption energies extracted on the active sites of Pt and Rh nanoparticles on carbon by fitting kinetic data show that the active sites adsorb phenol weakly. We predict that the turnover frequencies (TOFs) for the hydrogenation of phenol to cyclohexanone on Pt(111) and Rh(111) terraces are higher than those on (221) stepped facets based on density functional theory modeling and mean-field microkinetic simulations. The higher activities of the (111) terraces are due to lower activation energies and weaker phenol adsorption, preventing high coverages of phenol from inhibiting hydrogen adsorption. We measure that the TOF for ECH of phenol increases as the Rh nanoparticle diameter increases from 2 to 10 nm at 298 K and −0.1 V vs the reversible hydrogen electrode, qualitatively matching prior reports for Pt nanoparticles. The increase in experimental TOFs as Pt and Rh nanoparticle diameters increase is due to a larger fraction of terraces on larger particles. These findings clarify the structure sensitivity and active site of Pt and Rh for the hydrogenation of phenol and will inform the catalyst design for the hydrogenation of bio-oils.
Chiral modifiers of heterogeneous catalysts can function as activity promotors to minimize the influence of unmodified sites on the enantiomeric excess to obtain highly enantioselective catalysts. However, the origin on this effect is not well understood. It is investigated using a model catalyst of R‐(+)‐1‐(1‐naphthyl)‐ethylamine (R‐1‐NEA)/Pd(111) for the hydrogenation of methyl pyruvate (MP) to methyl lactate (ML). The activity of the model catalyst remains constant for multiple turnovers. No rate enhancement is found for R‐1‐NEA coverages below ∼0.5 monolayer (ML), but a significant increase is found at R‐1‐NEA coverages of ∼0.75 ML, with a rate approximately twice that of the unmodified catalyst. This is investigated using infrared spectroscopy to distinguish between MP monomers and dimers. MP titration experiments with hydrogen show a half‐order hydrogen pressure dependence, with the monomer reacting at twice the rate as the dimer. It is found that the dimer is the most abundant species on clean Pd(111), but the ratio of monomers to dimers increases as the R‐1‐NEA coverage increases due to surface crowding. The monomeric species is also found to be more stable on the crowded surface than on clean Pd(111); the chiral modifier also serves to stabilize the reactant. Finally, this model nicely explains the unusual 1‐NEA‐covergae dependence of the reactivity.
more » « less- Award ID(s):
- 1855199
- NSF-PAR ID:
- 10390441
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- ChemCatChem
- Volume:
- 15
- Issue:
- 1
- ISSN:
- 1867-3880
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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