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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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Δμ spectra of PtxCo/C and Pt/C catalysts obtained by subtracting the XANES at the Pt L3 edge collected at 0.54 V from various potentials in N2-pruged 0.1 M HClO4 (Δμ = μ(V) – μ(0.54 V)). The Δμ spectra of Pt2Co/C and Pt7Co/C are derived from the Pt L3 edge spectra in Figure 2.
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fig3: Δμ spectra of PtxCo/C and Pt/C catalysts obtained by subtracting the XANES at the Pt L3 edge collected at 0.54 V from various potentials in N2-pruged 0.1 M HClO4 (Δμ = μ(V) – μ(0.54 V)). The Δμ spectra of Pt2Co/C and Pt7Co/C are derived from the Pt L3 edge spectra in Figure 2.

Mentions: Δμ analysis is conducted only on the Pt L3 edge XANES, as the Co K edge XANES does not change significantly with applied potentials. The Δμ, defined as Δμ = μ(A/Pt) – μ(Pt), is surface sensitive because it involves the difference between the XAS absorption when adsorbates are present, μ(A/Pt), and the signal from a nearly adsorbate-free Pt surface, μ(Pt), thus highlighting the small changes occurring on the surface caused by the adsorption. It provides information about the nature of the adsorbate, the adsorbate coverage, and specific adsorption sites on the Pt.46−49 As shown in Figure 3, all the PtxCo/C and Pt/C catalysts exhibit a Δμ peak with characteristic features of a Pt surface, which can be well mimicked by the Δμ signal given by the theoretical multiple scattering FEFF850 calculations using a Pt6 or Pt25 cluster with or without adsorbates, as reported many times previously in the literature.46−49,51,52 This further confirms the Pt-dominant surface of all the PtxCo/C catalysts. The monotonic increase in Δμ magnitude with increasing potential reflects the increase in adsorbate coverage with applied potentials. This adsorbate coverage measured in O2-free solutions has been widely related to the ORR activities of Pt–alloy catalysts, as the adsorbates are believed to act as “poisons” to the active sites.20,51,53


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)

Δμ spectra of PtxCo/C and Pt/C catalysts obtained by subtracting the XANES at the Pt L3 edge collected at 0.54 V from various potentials in N2-pruged 0.1 M HClO4 (Δμ = μ(V) – μ(0.54 V)). The Δμ spectra of Pt2Co/C and Pt7Co/C are derived from the Pt L3 edge spectra in Figure 2.
© Copyright Policy
Related In: Results  -  Collection

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

fig3: Δμ spectra of PtxCo/C and Pt/C catalysts obtained by subtracting the XANES at the Pt L3 edge collected at 0.54 V from various potentials in N2-pruged 0.1 M HClO4 (Δμ = μ(V) – μ(0.54 V)). The Δμ spectra of Pt2Co/C and Pt7Co/C are derived from the Pt L3 edge spectra in Figure 2.
Mentions: Δμ analysis is conducted only on the Pt L3 edge XANES, as the Co K edge XANES does not change significantly with applied potentials. The Δμ, defined as Δμ = μ(A/Pt) – μ(Pt), is surface sensitive because it involves the difference between the XAS absorption when adsorbates are present, μ(A/Pt), and the signal from a nearly adsorbate-free Pt surface, μ(Pt), thus highlighting the small changes occurring on the surface caused by the adsorption. It provides information about the nature of the adsorbate, the adsorbate coverage, and specific adsorption sites on the Pt.46−49 As shown in Figure 3, all the PtxCo/C and Pt/C catalysts exhibit a Δμ peak with characteristic features of a Pt surface, which can be well mimicked by the Δμ signal given by the theoretical multiple scattering FEFF850 calculations using a Pt6 or Pt25 cluster with or without adsorbates, as reported many times previously in the literature.46−49,51,52 This further confirms the Pt-dominant surface of all the PtxCo/C catalysts. The monotonic increase in Δμ magnitude with increasing potential reflects the increase in adsorbate coverage with applied potentials. This adsorbate coverage measured in O2-free solutions has been widely related to the ORR activities of Pt–alloy catalysts, as the adsorbates are believed to act as “poisons” to the active sites.20,51,53

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