Surface (a) and subsurface (b) structures of idealized (cartoon) bimetallic catalysts. (c) and (d) Actual structures of Ni/Pt nanoparticles reconstructed with reverse Monte Carlo analysis of in situ EXAFS data during aqueous phase reforming of ethylene glycol. Structures consist of Pt core with Ni enriched surface and subsurface. Redrawn from Ref. 34 .
(a)–(f) Side view of N adsorption on various microstructures: Ni (yellow balls), Pt (blue balls), and N atoms (red balls). (a) Ni and Pt atoms are intermixed in the 1st layer. (b) Ni and Pt atoms are intermixed in the 1st layer and Ni atoms exist also in the 2nd layer. (c) N adsorption on Pt with Ni atoms in the 2nd layer. For (a)–(c), N atoms are on fcc hollow sites of Ni, Pt, or both. (d) N adsorption on Ni adatoms. (e) N adsorption on Ni adatoms with Ni atoms in the first layer of Pt. (f) N atoms on Pt hollow sites with nearby Ni adatoms.
Pairwise plot of the N* binding energies calculated using Eq. (1) vs. DFT data. The dashed (parity) line indicates the exact representation of DFT binding energies. The black circles correspond to structures shown in Figs. 2(a)–2(c) , the red squares for structures in Figs. 2(d) and 2(e) , and green triangles for structures in Fig. 2(f) .
Binding energies of N on Pt, Ni, Ni-Pt-Pt, Pt-Ni-Pt, and other high symmetry configurations in Fig. 2 . The nitrogen coverage is 1/16. The solid black line is the linear regression of the black circles (Pt/Ni/Pt is excluded).
Surfaces used for coverage-dependent N binding energy calculations. Panels (a)–(g) stand for pure Pt, pure Ni, 6NiPt-Pt-Pt, 6adNi-Pt-Pt, 1NiPt-Pt-Pt, Pt-Ni-Pt, and Ni-Pt-Pt, respectively. For 6NiPt-Pt-Pt (c) and 1NiPt-Pt-Pt (e) structures, the Ni atoms are in the same (first) layer with Pt. For the 6adNi-Pt-Pt surface (d), the six Ni adatoms form an island on top of Pt(111). Other surfaces are (111) terrace structures. N atoms (not shown) reside at the most stable fcc hollow sites.
Average N binding energies as function of N coverage. Curves (a)–(g) correspond to surfaces indicated in Figs. 5(a)–5(g) .
Average N binding energy on Pt(111) from DFT and N-N pair interaction model as function of coverage. 1st, 2nd, and 3rd nearest N-N pairs are considered.
(a) 1st, 2nd, and 3rd nearest N-N pair interaction energies. (b) 1st nearest N-N pair interaction energies as function of the N binding energy at low coverage (θ = 1/16). There are 2 configurations for 6adNi-Pt-Pt surface, one with 3 fcc and the other with 3 hcp hollow sites. The 6adNi-Pt-Pt(3hcp) structure was also considered, in which the 3 hcp-hollow sites are the most favorable for N adsorption. The insets illustrate the configurations of the 1st nearest N-N pairs across the step of the 6NiPt-Pt-Pt cluster and the step edges of the 6adNi-Pt-Pt structures. The red dots represent N atoms. The solid line indicates the fitted equation of the black circles. “ad” stands for adatoms of Ni on Pt and cross for N atoms being across a step, as shown in the insets.
The 4 × 3 matrix element of (eV/atom) calculated from the least-squares fit of the DFT data.
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