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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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Coordination numbers and J values of the Co (A) and Pt (C) absorbers, and the Co–Co (B) and Pt–Pt (D) bond distances in the fresh (solid line) and 5k-cycled (dashed line) PtxCo/C and/or Pt/C catalysts as a function of the nominal Co/Pt atomic ratio.
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fig5: Coordination numbers and J values of the Co (A) and Pt (C) absorbers, and the Co–Co (B) and Pt–Pt (D) bond distances in the fresh (solid line) and 5k-cycled (dashed line) PtxCo/C and/or Pt/C catalysts as a function of the nominal Co/Pt atomic ratio.

Mentions: The main output of the EXAFS analysis is summarized in Figure 5, where the coordination numbers, the J values, and the bond distances of the BOL (solid lines) and 5k-cycled (dashed) PtxCo/C and Pt/C catalysts are plotted as a function of the initial Co/Pt ratio. The PtCo core morphology is probed by analyzing the EXAFS data at the Co K edge (Figure 5A and B). When nCo/Pt is ∼0.15 (Pt6Co/C and Pt7Co/C), the Co atoms are surrounded by mostly Pt atoms (NCo–Co ≈ 1 and NCo–Pt ≈ 11) and statistically distributed in the cores without Co segregation (JCo ≈ 1). When nCo/Pt is increased to ∼0.25, NCo–Co sharply increases to 3.6 while NCo–Pt drastically decreases to 8.5, with consequent drop of JCo. This indicates the formation of concentrated Co in the cores with increasing Co content. Interestingly, further increase of nCo/Pt does not change the values of NCo–Co, NCo–Pt, and JCo significantly, and these values are close to those of a perfectly disordered Pt3Co model (NCo–Co = 3, NCo–Pt = 9, JCo = 1). This appears to suggest the formation of a disordered Pt3Co core for these catalysts. However, the JCo and NCo–Pt of the Pt2Co/C and Pt3Co/C catalysts drop significantly upon 5k cycles, which indicates that the Co atoms in the inner cores (survived the cycling) are surrounded by less Pt neighbors than the Co atoms in the near-surface region (removed by cycling). This is further confirmed by the drastic reduction in RCo–Co upon cycling for these two catalysts (Figure 5B), as the RCo–Co in the Co-rich cores is expected to be shorter than that in the Pt-rich near-surface region according to Vegard’s law. Also we noticed the JCo, NCo–Pt, and RCo–Co of the cycled samples generally decrease with increasing nCo/Pt, which clearly shows that the Co concentration in the inner cores generally increases with increasing nCo/Pt. Thus, we conclude that the Pt2Co/C and Pt3Co/C catalysts possess a Co concentration gradient such that some Co is concentrated in the inner cores and gradually depletes within the Pt-rich near-surface region. On the contrary, the JCo, NCo–Pt, and RCo–Co of the PtxCo/C catalysts with low Co content (x ≥ 4) do not change significantly with voltage cycling, as these catalysts do not possess Co-populated near-surface regions and Co-concentrated inner cores, and thus exhibit a relatively uniform distribution of Co rather than a Co concentration gradient.


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)

Coordination numbers and J values of the Co (A) and Pt (C) absorbers, and the Co–Co (B) and Pt–Pt (D) bond distances in the fresh (solid line) and 5k-cycled (dashed line) PtxCo/C and/or Pt/C catalysts as a function of the nominal Co/Pt atomic ratio.
© Copyright Policy
Related In: Results  -  Collection

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

fig5: Coordination numbers and J values of the Co (A) and Pt (C) absorbers, and the Co–Co (B) and Pt–Pt (D) bond distances in the fresh (solid line) and 5k-cycled (dashed line) PtxCo/C and/or Pt/C catalysts as a function of the nominal Co/Pt atomic ratio.
Mentions: The main output of the EXAFS analysis is summarized in Figure 5, where the coordination numbers, the J values, and the bond distances of the BOL (solid lines) and 5k-cycled (dashed) PtxCo/C and Pt/C catalysts are plotted as a function of the initial Co/Pt ratio. The PtCo core morphology is probed by analyzing the EXAFS data at the Co K edge (Figure 5A and B). When nCo/Pt is ∼0.15 (Pt6Co/C and Pt7Co/C), the Co atoms are surrounded by mostly Pt atoms (NCo–Co ≈ 1 and NCo–Pt ≈ 11) and statistically distributed in the cores without Co segregation (JCo ≈ 1). When nCo/Pt is increased to ∼0.25, NCo–Co sharply increases to 3.6 while NCo–Pt drastically decreases to 8.5, with consequent drop of JCo. This indicates the formation of concentrated Co in the cores with increasing Co content. Interestingly, further increase of nCo/Pt does not change the values of NCo–Co, NCo–Pt, and JCo significantly, and these values are close to those of a perfectly disordered Pt3Co model (NCo–Co = 3, NCo–Pt = 9, JCo = 1). This appears to suggest the formation of a disordered Pt3Co core for these catalysts. However, the JCo and NCo–Pt of the Pt2Co/C and Pt3Co/C catalysts drop significantly upon 5k cycles, which indicates that the Co atoms in the inner cores (survived the cycling) are surrounded by less Pt neighbors than the Co atoms in the near-surface region (removed by cycling). This is further confirmed by the drastic reduction in RCo–Co upon cycling for these two catalysts (Figure 5B), as the RCo–Co in the Co-rich cores is expected to be shorter than that in the Pt-rich near-surface region according to Vegard’s law. Also we noticed the JCo, NCo–Pt, and RCo–Co of the cycled samples generally decrease with increasing nCo/Pt, which clearly shows that the Co concentration in the inner cores generally increases with increasing nCo/Pt. Thus, we conclude that the Pt2Co/C and Pt3Co/C catalysts possess a Co concentration gradient such that some Co is concentrated in the inner cores and gradually depletes within the Pt-rich near-surface region. On the contrary, the JCo, NCo–Pt, and RCo–Co of the PtxCo/C catalysts with low Co content (x ≥ 4) do not change significantly with voltage cycling, as these catalysts do not possess Co-populated near-surface regions and Co-concentrated inner cores, and thus exhibit a relatively uniform distribution of Co rather than a Co concentration gradient.

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