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An empirical approach based on density functional theory (DFT)+U is proposed to provide adsorption energies for atoms of transition metals adsorbed on ceria that are more accurate than those found in the existing DFT literature. It involves applying an intermediate Hubbard U value to the Ce 4f states and another U value to the d states of the metal adatom that is optimized for each metal to match its calculated bulk cohesive energy with the experimental value. The results compare favorably with Campbell’s calorimetric measurements of metal adsorption heats on CeO2(111). Thus, this approach produces metal adsorption energies on ceria that are more accurate in both the single-atom and the bulk limits. Furthermore, it preserves a common reference for Ce so that calculations involving different metals adsorbed on ceria, e.g., bimetallic and multimetallic clusters, can be analyzed on the same footing.
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Growth of Uniquely Small Tin Clusters on Highly Oriented Pyrolytic Graphite
Sn clusters have been grown on highly oriented pyrolytic graphite (HOPG) surfaces and investigated by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. At low Sn coverages ranging from 0.02-0.25 ML, Sn grows as small clusters that nucleate uniformly on the terraces. This behavior is in contrast with the growth of transition metals such as Pd, Pt, and Re on HOPG, given that these metals form large clusters with preferential nucleation for Pd and Pt at the favored low-coordination step edges. XPS experiments show no evidence of Sn-HOPG interactions, and the activation energy barrier for diffusion calculated for Sn on HOPG (0.06 eV) is lower or comparable to those of Pd, Pt and Re (0.04, 0.22, and 0.61 eV, respectively), indicating that the growth of the Sn clusters is not kinetically limited by diffusion on the surface. DFT calculations of the binding energy/atom as a function of cluster size demonstrate that the energies of the Sn clusters on HOPG are similar to that of Sn atoms in the bulk for Sn clusters larger than 10 atoms, whereas the Pt, Pd, and Re clusters on HOPG have energies that are 1-2 eV higher than in the bulk. Thus, there is no thermodynamic driving force for Sn atoms to form clusters larger than 10 atoms on HOPG, unlike for Pd, Pt, and Re atoms, which minimize their energy by aggregating into larger, more bulk-like clusters. In addition, annealing the Sn/HOPG clusters to 800 K and 950 K does not increase the cluster size but instead removes the larger clusters, while Sn deposition at 810 K induces the appearance of protrusions that are believed to be from subsurface Sn. DFT studies indicate that it is energetically favorable for a Sn atom to exist in the subsurface layer only when the Sn atom is located at a subsurface vacancy.
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- Award ID(s):
- 1764164
- PAR ID:
- 10547425
- Publisher / Repository:
- Journal of Physical Chemistry C
- Date Published:
- Journal Name:
- The Journal of Physical Chemistry C
- Volume:
- 128
- Issue:
- 8
- ISSN:
- 1932-7447
- Page Range / eLocation ID:
- 3567 to 3577
- Subject(s) / Keyword(s):
- Platinum tin scanning tunneling microscopy X-ray photoelectron spectroscopy highly oriented pyrolytic graphite titanium dioxide clusters
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1021/acs.jpcc.3c08215
