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Mechanical work makes important contributions to surface chemistry at steps.

Francis MF, Curtin WA - Nat Commun (2015)

Bottom Line: The mechanism driving the trend is mechanical, arising from the relaxation of stored mechanical energy.The mechanical energy change can be larger than, and of opposite sign than, the energy changes due to electronic effects and leads to a violation of trends predicted by the widely accepted electronic 'd-band' model.This trend has a direct impact on catalytic activity, which is demonstrated here for methanation, where biaxial tension is predicted to shift the activity of nickel significantly, reaching the peak of the volcano plot and comparable to cobalt and ruthenium.

View Article: PubMed Central - PubMed

Affiliation: 1] École Polytechnique Fédérale de Lausanne, EPFL STI IGM LAMMM, ME C1 399 (Bâtiment ME), Station 9, Lausanne CH-1015, Switzerland [2] Brown School of Engineering, 182 Hope Street, Providence, Rhode Island 02912, USA.

ABSTRACT
The effect of mechanical strain on the binding energy of adsorbates to late transition metals is believed to be entirely controlled by electronic factors, with tensile stress inducing stronger binding. Here we show, via computation, that mechanical strain of late transition metals can modify binding at stepped surfaces opposite to well-established trends on flat surfaces. The mechanism driving the trend is mechanical, arising from the relaxation of stored mechanical energy. The mechanical energy change can be larger than, and of opposite sign than, the energy changes due to electronic effects and leads to a violation of trends predicted by the widely accepted electronic 'd-band' model. This trend has a direct impact on catalytic activity, which is demonstrated here for methanation, where biaxial tension is predicted to shift the activity of nickel significantly, reaching the peak of the volcano plot and comparable to cobalt and ruthenium.

No MeSH data available.


Related in: MedlinePlus

Binding energy versus strain for CO*/Cu(211) under σ[−111] and σ[01–1].(a), Change in binding energy ΔEBE versus strain, showing that compressive stress normal to the step increases the binding energy. (b), Change in electronic contribution to binding, ΔEelec (red), which are small but show the expected increase in binding energy under tensile strain for both (111) and (211), and correlate with the negative of the shift of the d-band centre, −ΔEd (blue), under both σ[−111] and σ[01–1] loading states. (c), Change in mechanical contribution to binding ΔEmech, which controls the overall energy change (part(a)) for both σ[−111] and σ[01–1] loading states, with σ[−111] loading causing increased binding under compression. Vertical axes are in units eV.
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f3: Binding energy versus strain for CO*/Cu(211) under σ[−111] and σ[01–1].(a), Change in binding energy ΔEBE versus strain, showing that compressive stress normal to the step increases the binding energy. (b), Change in electronic contribution to binding, ΔEelec (red), which are small but show the expected increase in binding energy under tensile strain for both (111) and (211), and correlate with the negative of the shift of the d-band centre, −ΔEd (blue), under both σ[−111] and σ[01–1] loading states. (c), Change in mechanical contribution to binding ΔEmech, which controls the overall energy change (part(a)) for both σ[−111] and σ[01–1] loading states, with σ[−111] loading causing increased binding under compression. Vertical axes are in units eV.

Mentions: We first use the cases of CO* on Ni and Cu to illustrate the scope of new phenomena and new understanding emerging from this work. Figure 2a shows the computed binding energy change due to biaxial stress on CO*/Ni(211) and CO*/Ni(111). The response for Ni(111) shows the expected trend: tensile loading causes stronger binding. In contrast, the response for CO*/Ni(211) is the opposite: compressive loading causes stronger binding, and the magnitude of the effect is comparatively large. Therefore, the chemical response to stress/strain of an adsorbate at a step site can vary from that at a terrace. Figure 3a shows the computed binding energy changes for CO*/Cu(211) for various stress states. For the same step site, the binding energy change has two different trends for the two different loading directions. Figures 2a and 3a show clear departures from the expectation that ‘tensile strain causes stronger binding’. This new trend at steps is found for various adsorbates and LTMs (Supplementary Note 1; Supplementary Figs 1–3). Even when the net effect of strain on binding at the step is small, the difference in response between step and terrace can be important in many reactions (Supplementary Note 2; Supplementary Figs 3 and 4). Generalizing to other adsorbates, Fig. 4 shows the binding energy changes of CO, SH, OH, and NH to Ni(211) and Cu(211) under σ[−111] loading. The trend of stronger adsorbate-binding under compression is found for all these species. The nonvalence binding of a species AX to a surface through A is controlled by A regardless of the X31. Thus, the results in Fig. 4 suggest that the new trend will exist for the huge number of catalytic reactions involving binding through C, S, O and N. While we explain the origin of these effects below, the observation that changes in binding due to applied stress are different between terraces and steps and depend on the direction of the applied stress are the first two main results of this paper.


Mechanical work makes important contributions to surface chemistry at steps.

Francis MF, Curtin WA - Nat Commun (2015)

Binding energy versus strain for CO*/Cu(211) under σ[−111] and σ[01–1].(a), Change in binding energy ΔEBE versus strain, showing that compressive stress normal to the step increases the binding energy. (b), Change in electronic contribution to binding, ΔEelec (red), which are small but show the expected increase in binding energy under tensile strain for both (111) and (211), and correlate with the negative of the shift of the d-band centre, −ΔEd (blue), under both σ[−111] and σ[01–1] loading states. (c), Change in mechanical contribution to binding ΔEmech, which controls the overall energy change (part(a)) for both σ[−111] and σ[01–1] loading states, with σ[−111] loading causing increased binding under compression. Vertical axes are in units eV.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4339914&req=5

f3: Binding energy versus strain for CO*/Cu(211) under σ[−111] and σ[01–1].(a), Change in binding energy ΔEBE versus strain, showing that compressive stress normal to the step increases the binding energy. (b), Change in electronic contribution to binding, ΔEelec (red), which are small but show the expected increase in binding energy under tensile strain for both (111) and (211), and correlate with the negative of the shift of the d-band centre, −ΔEd (blue), under both σ[−111] and σ[01–1] loading states. (c), Change in mechanical contribution to binding ΔEmech, which controls the overall energy change (part(a)) for both σ[−111] and σ[01–1] loading states, with σ[−111] loading causing increased binding under compression. Vertical axes are in units eV.
Mentions: We first use the cases of CO* on Ni and Cu to illustrate the scope of new phenomena and new understanding emerging from this work. Figure 2a shows the computed binding energy change due to biaxial stress on CO*/Ni(211) and CO*/Ni(111). The response for Ni(111) shows the expected trend: tensile loading causes stronger binding. In contrast, the response for CO*/Ni(211) is the opposite: compressive loading causes stronger binding, and the magnitude of the effect is comparatively large. Therefore, the chemical response to stress/strain of an adsorbate at a step site can vary from that at a terrace. Figure 3a shows the computed binding energy changes for CO*/Cu(211) for various stress states. For the same step site, the binding energy change has two different trends for the two different loading directions. Figures 2a and 3a show clear departures from the expectation that ‘tensile strain causes stronger binding’. This new trend at steps is found for various adsorbates and LTMs (Supplementary Note 1; Supplementary Figs 1–3). Even when the net effect of strain on binding at the step is small, the difference in response between step and terrace can be important in many reactions (Supplementary Note 2; Supplementary Figs 3 and 4). Generalizing to other adsorbates, Fig. 4 shows the binding energy changes of CO, SH, OH, and NH to Ni(211) and Cu(211) under σ[−111] loading. The trend of stronger adsorbate-binding under compression is found for all these species. The nonvalence binding of a species AX to a surface through A is controlled by A regardless of the X31. Thus, the results in Fig. 4 suggest that the new trend will exist for the huge number of catalytic reactions involving binding through C, S, O and N. While we explain the origin of these effects below, the observation that changes in binding due to applied stress are different between terraces and steps and depend on the direction of the applied stress are the first two main results of this paper.

Bottom Line: The mechanism driving the trend is mechanical, arising from the relaxation of stored mechanical energy.The mechanical energy change can be larger than, and of opposite sign than, the energy changes due to electronic effects and leads to a violation of trends predicted by the widely accepted electronic 'd-band' model.This trend has a direct impact on catalytic activity, which is demonstrated here for methanation, where biaxial tension is predicted to shift the activity of nickel significantly, reaching the peak of the volcano plot and comparable to cobalt and ruthenium.

View Article: PubMed Central - PubMed

Affiliation: 1] École Polytechnique Fédérale de Lausanne, EPFL STI IGM LAMMM, ME C1 399 (Bâtiment ME), Station 9, Lausanne CH-1015, Switzerland [2] Brown School of Engineering, 182 Hope Street, Providence, Rhode Island 02912, USA.

ABSTRACT
The effect of mechanical strain on the binding energy of adsorbates to late transition metals is believed to be entirely controlled by electronic factors, with tensile stress inducing stronger binding. Here we show, via computation, that mechanical strain of late transition metals can modify binding at stepped surfaces opposite to well-established trends on flat surfaces. The mechanism driving the trend is mechanical, arising from the relaxation of stored mechanical energy. The mechanical energy change can be larger than, and of opposite sign than, the energy changes due to electronic effects and leads to a violation of trends predicted by the widely accepted electronic 'd-band' model. This trend has a direct impact on catalytic activity, which is demonstrated here for methanation, where biaxial tension is predicted to shift the activity of nickel significantly, reaching the peak of the volcano plot and comparable to cobalt and ruthenium.

No MeSH data available.


Related in: MedlinePlus