Elucidation of the active surface and origin of the weak metal-hydrogen bond on Ni/Pt(111) bimetallic surfaces: a surface science and density functional theory study
John R. Kitchin, Neetha A. Khan, Mark A. Barteau, Jingguang G. Chen, Boris Yakshinskiy and Theodore E. Madey, “Elucidation of the active surface and origin of the weak metal-hydrogen bond on Ni/Pt(111) bimetallic surfaces: a surface science and density functional theory study”, Surface Science 544(2-3), 295-308 (2003). https://doi.org/10.1016/j.susc.2003.09.007
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Question
Ni/Pt alloys are known for better selectivity in partial hydrogenation than either pure metal, and Pt-rich Pt-Ni(111) single crystal alloys release hydrogen at lower temperature than Pt(111) or Ni(111). Earlier work from the Chen group had found that annealed Ni films on Pt(111) bond cyclohexene differently and are more selective in thiophene hydrodesulfurization. This paper asks what the active Ni/Pt(111) surface actually looks like after the thermal treatments used in those studies, and why it binds hydrogen more weakly than either parent metal (Kitchin et al. 2003).
Approach
Ni was evaporated onto Pt(111) in ultrahigh vacuum either at 300 K, at 300 K followed by annealing near 600 K, or directly at 600 K (640 K in the STM chamber). The “1 ML” label refers to a Ni(LMM)/Pt(237 eV) Auger ratio near unity, not a verified uniform monolayer. Hydrogen temperature programmed desorption (TPD) after saturation exposure probed chemistry, and scanning tunneling microscopy (STM), low energy He+ ion scattering (LEIS), Auger electron spectroscopy and normal versus grazing emission XPS probed structure and composition. Heats of dissociative H2 adsorption were estimated from peak temperatures with a Redhead-type second-order analysis. Density functional theory (Dacapo, PW91, three-layer 2x2 slabs, 350 eV cutoff) gave hydrogen binding energies (HBE) at 0.25 ML in the fcc site on Pt, Ni, surface alloys (Ni1Pt3, Ni3Pt1), a Ni monolayer on Pt, and a Pt-Ni-Pt sandwich with Ni in the second layer, along with the surface d-band center from projected densities of states (Kitchin et al. 2003).
Key findings
Hydrogen desorbed from the 600 K and 300 K/600 K surfaces at about 234 to 241 K, below the 285 K peak for clean Pt(111) and the 346 K peak for a thick (>5 ML) Ni film, while the unannealed 300 K surface gave a peak near 353 K. The estimated heats of adsorption were 14.6 kcal/mol for the annealed surfaces, 17.6 for Pt(111), 21.9 for the 300 K surface and 21.5 for thick Ni, in line with literature values for Pt-Ni alloy crystals (Kitchin et al. 2003).
STM showed that Ni grown near room temperature nucleates as islands at Pt step edges with sharp island-step boundaries, starts new layers before lower ones are complete, and displays dislocation networks attributed to the roughly 11% Ni-Pt bond length mismatch. Annealing to 640 K erased these features, lowered the Ni/Pt Auger ratio (for example from 1.35 to 0.82), and produced surfaces resembling those grown directly at 640 K, including a long-range line network with threefold symmetry. LEIS and Auger showed Ni signals falling from about 700 K and becoming nearly undetectable by 900 K, and a 650 K anneal lowered the Auger ratio from 0.9 to 0.2 over 20 h. Grazing to normal XPS intensity ratios of about 1.35 at 300 K and 0.74 at 600 K indicated Ni concentrated at the surface in the first case and distributed below it in the second. The authors conclude Ni diffuses into the subsurface and bulk on annealing, leaving a Pt-enriched top layer (Kitchin et al. 2003).
In DFT the HBE tracked the surface d-band center roughly linearly. Ni in the surface layer raised the d-band center and strengthened H binding, and the Ni monolayer bound H slightly more strongly than pure Ni, matching the 300 K surface (which LEIS put above 90% Ni). Subsurface Ni instead shifted surface Pt d states down and weakened binding below both Pt and Ni, matching the annealed surfaces. The Pt-Ni-Pt sandwich was 0.65 eV per Ni atom more stable than the Ni monolayer. With spin polarization and top-layer relaxation the ordering Ni monolayer > Ni > Pt > Pt-Ni-Pt persisted (for example -2.99, -2.84, -2.76 and -2.56 eV), and a 2% smaller lattice constant shifted HBEs without changing the trend. Computed heats of adsorption overestimated experiment but reproduced the trend, giving 13.4 kcal/mol for the subsurface model (Kitchin et al. 2003).
Significance
The paper identifies the catalytically interesting Ni/Pt(111) surface as a Pt-terminated structure with Ni underneath rather than a Ni overlayer, and explains its weak metal-hydrogen bond as a ligand effect of subsurface Ni on surface Pt d states. This experiment-theory link between subsurface 3d metals, d-band downshifts and weakened adsorption was the starting point for the group’s later systematic DFT studies of Pt-skin and sandwich structures (Kitchin et al. 2003).