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Activity descriptor identification for oxygen reduction on platinum-based bimetallic nanoparticles: in situ observation of the linear composition-strain-activity relationship.

Jia Q, Liang W, Bates MK, Mani P, Lee W, Mukerjee S - ACS Nano (2015)

Bottom Line: Despite recent progress in developing active and durable oxygen reduction catalysts with reduced Pt content, lack of elegant bottom-up synthesis procedures with knowledge over the control of atomic arrangement and morphology of the Pt-alloy catalysts still hinders fuel cell commercialization.Despite their different atomic structure, the oxygen reduction reaction (ORR) activity of PtxCo/C and Pt/C NPs is linearly related to the bulk average Pt-Pt bond length (RPt-Pt).These linear correlations together demonstrate that (i) the improved ORR activity of PtxCo/C NPs over pure Pt NPs originates predominantly from the compressive strain and (ii) the RPt-Pt is a valid strain descriptor that bridges the activity and atomic composition of Pt-based bimetallic NPs.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and Chemical Biology and ‡Department of Biology, Northeastern University , Boston, Massachusetts 02115, United States.

ABSTRACT
Despite recent progress in developing active and durable oxygen reduction catalysts with reduced Pt content, lack of elegant bottom-up synthesis procedures with knowledge over the control of atomic arrangement and morphology of the Pt-alloy catalysts still hinders fuel cell commercialization. To follow a less empirical synthesis path for improved Pt-based catalysts, it is essential to correlate catalytic performance to properties that can be easily controlled and measured experimentally. Herein, using Pt-Co alloy nanoparticles (NPs) with varying atomic composition as an example, we show that the atomic distribution of Pt-based bimetallic NPs under operating conditions is strongly dependent on the initial atomic ratio by employing microscopic and in situ spectroscopic techniques. The PtxCo/C NPs with high Co content possess a Co concentration gradient such that Co is concentrated in the core and gradually depletes in the near-surface region, whereas the PtxCo/C NPs with low Co content possess a relatively uniform distribution of Co with low Co population in the near-surface region. Despite their different atomic structure, the oxygen reduction reaction (ORR) activity of PtxCo/C and Pt/C NPs is linearly related to the bulk average Pt-Pt bond length (RPt-Pt). The RPt-Pt is further shown to contract linearly with the increase in Co/Pt composition. These linear correlations together demonstrate that (i) the improved ORR activity of PtxCo/C NPs over pure Pt NPs originates predominantly from the compressive strain and (ii) the RPt-Pt is a valid strain descriptor that bridges the activity and atomic composition of Pt-based bimetallic NPs.

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Pt L3 edge (left) and Co K edge (right) EXAFS spectra collected at 0.54 V in N2-saturated 0.1 M HClO4 electrolyte and the corresponding least-squares fits for BOL and 5k-cycled (Figure S6) PtxCo/C and Pt/C NP catalysts. The fit of the Co reference foil data is also included for comparison. The vertical black lines are drawn as guides to the eye. As the Co/Pt atomic ratio decreases, the Pt–Co scattering peak intensity decreases and the Co–Pt scattering peak intensity increases, accompanied by a right shift of the main FT peaks.
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fig4: Pt L3 edge (left) and Co K edge (right) EXAFS spectra collected at 0.54 V in N2-saturated 0.1 M HClO4 electrolyte and the corresponding least-squares fits for BOL and 5k-cycled (Figure S6) PtxCo/C and Pt/C NP catalysts. The fit of the Co reference foil data is also included for comparison. The vertical black lines are drawn as guides to the eye. As the Co/Pt atomic ratio decreases, the Pt–Co scattering peak intensity decreases and the Co–Pt scattering peak intensity increases, accompanied by a right shift of the main FT peaks.

Mentions: Comprehensive Fourier transform (FT) EXAFS analysis, which involves analyzing the Pt L3 edge, involving Pt–Pt and Pt–Co scattering, and the Co K edge, involving Co–Co and Co–Pt scattering, was conducted on beginning-of-life (BOL) and 5k-cycled PtxCo/C and Pt/C catalysts to explore their bulk structures at the atomic scale. For PtxCo/C catalysts, first-shell coordination number fitting is applied to both the Pt and Co edges concurrently. EXAFS data collected at 0.54 V (double-layer region where no evidence of electrochemical adsorbates such as Hupd, Oads, and OHads was found on these Pt-based catalysts) and the first-shell fits for the Pt L3 edge and Co K edge of the samples are shown in Figure 4 (the corresponding data in k space are provided in Figures S4 and S5). The fitting results are listed in Table 3. Basic constraints as follows are introduced for the fitting:12Here RPt–Co = RCo–Pt is the bond distance for Pt–Co (or Co–Pt) scattering; σ2Pt–Co and σ2Co–Pt are uncertainties in the bond lengths as suggested by the fitting.


Activity descriptor identification for oxygen reduction on platinum-based bimetallic nanoparticles: in situ observation of the linear composition-strain-activity relationship.

Jia Q, Liang W, Bates MK, Mani P, Lee W, Mukerjee S - ACS Nano (2015)

Pt L3 edge (left) and Co K edge (right) EXAFS spectra collected at 0.54 V in N2-saturated 0.1 M HClO4 electrolyte and the corresponding least-squares fits for BOL and 5k-cycled (Figure S6) PtxCo/C and Pt/C NP catalysts. The fit of the Co reference foil data is also included for comparison. The vertical black lines are drawn as guides to the eye. As the Co/Pt atomic ratio decreases, the Pt–Co scattering peak intensity decreases and the Co–Pt scattering peak intensity increases, accompanied by a right shift of the main FT peaks.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4492796&req=5

fig4: Pt L3 edge (left) and Co K edge (right) EXAFS spectra collected at 0.54 V in N2-saturated 0.1 M HClO4 electrolyte and the corresponding least-squares fits for BOL and 5k-cycled (Figure S6) PtxCo/C and Pt/C NP catalysts. The fit of the Co reference foil data is also included for comparison. The vertical black lines are drawn as guides to the eye. As the Co/Pt atomic ratio decreases, the Pt–Co scattering peak intensity decreases and the Co–Pt scattering peak intensity increases, accompanied by a right shift of the main FT peaks.
Mentions: Comprehensive Fourier transform (FT) EXAFS analysis, which involves analyzing the Pt L3 edge, involving Pt–Pt and Pt–Co scattering, and the Co K edge, involving Co–Co and Co–Pt scattering, was conducted on beginning-of-life (BOL) and 5k-cycled PtxCo/C and Pt/C catalysts to explore their bulk structures at the atomic scale. For PtxCo/C catalysts, first-shell coordination number fitting is applied to both the Pt and Co edges concurrently. EXAFS data collected at 0.54 V (double-layer region where no evidence of electrochemical adsorbates such as Hupd, Oads, and OHads was found on these Pt-based catalysts) and the first-shell fits for the Pt L3 edge and Co K edge of the samples are shown in Figure 4 (the corresponding data in k space are provided in Figures S4 and S5). The fitting results are listed in Table 3. Basic constraints as follows are introduced for the fitting:12Here RPt–Co = RCo–Pt is the bond distance for Pt–Co (or Co–Pt) scattering; σ2Pt–Co and σ2Co–Pt are uncertainties in the bond lengths as suggested by the fitting.

Bottom Line: Despite recent progress in developing active and durable oxygen reduction catalysts with reduced Pt content, lack of elegant bottom-up synthesis procedures with knowledge over the control of atomic arrangement and morphology of the Pt-alloy catalysts still hinders fuel cell commercialization.Despite their different atomic structure, the oxygen reduction reaction (ORR) activity of PtxCo/C and Pt/C NPs is linearly related to the bulk average Pt-Pt bond length (RPt-Pt).These linear correlations together demonstrate that (i) the improved ORR activity of PtxCo/C NPs over pure Pt NPs originates predominantly from the compressive strain and (ii) the RPt-Pt is a valid strain descriptor that bridges the activity and atomic composition of Pt-based bimetallic NPs.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and Chemical Biology and ‡Department of Biology, Northeastern University , Boston, Massachusetts 02115, United States.

ABSTRACT
Despite recent progress in developing active and durable oxygen reduction catalysts with reduced Pt content, lack of elegant bottom-up synthesis procedures with knowledge over the control of atomic arrangement and morphology of the Pt-alloy catalysts still hinders fuel cell commercialization. To follow a less empirical synthesis path for improved Pt-based catalysts, it is essential to correlate catalytic performance to properties that can be easily controlled and measured experimentally. Herein, using Pt-Co alloy nanoparticles (NPs) with varying atomic composition as an example, we show that the atomic distribution of Pt-based bimetallic NPs under operating conditions is strongly dependent on the initial atomic ratio by employing microscopic and in situ spectroscopic techniques. The PtxCo/C NPs with high Co content possess a Co concentration gradient such that Co is concentrated in the core and gradually depletes in the near-surface region, whereas the PtxCo/C NPs with low Co content possess a relatively uniform distribution of Co with low Co population in the near-surface region. Despite their different atomic structure, the oxygen reduction reaction (ORR) activity of PtxCo/C and Pt/C NPs is linearly related to the bulk average Pt-Pt bond length (RPt-Pt). The RPt-Pt is further shown to contract linearly with the increase in Co/Pt composition. These linear correlations together demonstrate that (i) the improved ORR activity of PtxCo/C NPs over pure Pt NPs originates predominantly from the compressive strain and (ii) the RPt-Pt is a valid strain descriptor that bridges the activity and atomic composition of Pt-based bimetallic NPs.

Show MeSH
Related in: MedlinePlus