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H2 activation is fundamental in catalysis. Single-atom catalysts (SACs) can be highly selective hydrogenation catalysts due to their tunable geometric and electronic properties. In this work, H2 activation (adsorption, splitting, and diffusion) on the anatase TiO2-supported SAC has been modeled in detail. The stable configurations of 14 transition metals from 3d to 5d (Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Cd, Os, Ir, Pt, and Au) and Sn have been screened. We compared H and H2 adsorption and H2 heterolytic and homolytic splitting on SA/TiO2. H on the SAC in neutral, hydridic, and proton forms and the preferred H2 dissociation paths are revealed. We found that the metal adatoms strengthen the Brønsted acids via forming the SA-O bonds and promote the H adsorption on Ti sites via forming the Ti3+ sites. The electronic descriptor using the energy level of the frontier d orbital, referenced to vacuum, can predict the single H and H2 dissociative adsorption energies on the metal site. As the SA-Hδ- interaction is stronger than Ti-Hδ-, the activation barriers for heterolytic paths over SA-O sites are lower than over Ti-O sites. H2 adsorption is activated on Au, Ru, Rh, Pd, and Ir in a dihydrogen complex structure with an elongated H-H bond. Homolytic splitting over SA sites is favored thermodynamically and kinetically on Rh, Pd, Os, Ir, and Pt. In contrast, for the remaining SA/TiO2, H-H splitting at the SA-O is kinetically favored compared to the Ti-O sites, but the products are less thermodynamically favored.
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Explaining the structure sensitivity of Pt and Rh for aqueous-phase hydrogenation of phenol
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.
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- Award ID(s):
- 1919444
- PAR ID:
- 10363705
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 156
- Issue:
- 10
- ISSN:
- 0021-9606
- Page Range / eLocation ID:
- Article No. 104703
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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