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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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Specific activity (red triangle) and RPt–Pt (blue dots) of PtxCo/C and Pt/C NPs are displayed as a function of the Co content determined by EXAFS. The specific activity (green triangle) and RPt–Pt (green dots) of dealloyed PtCo/C and PtCo3/C catalysts reported in refs (33) and (38) are also included. The RPt–Pt calculated using the in situ Co/Pt atomic ratio based on Vegard’s law is shown (blue dashed line) for comparison. The slight deviations observed for Pt/C are caused by the shorter RPt–Pt of small Pt NPs (∼2.75 Å) compared to that of bulk Pt (2.77 Å)59 used for Vegard’s law calculations. The red dashed line is a guide to the eye.
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fig8: Specific activity (red triangle) and RPt–Pt (blue dots) of PtxCo/C and Pt/C NPs are displayed as a function of the Co content determined by EXAFS. The specific activity (green triangle) and RPt–Pt (green dots) of dealloyed PtCo/C and PtCo3/C catalysts reported in refs (33) and (38) are also included. The RPt–Pt calculated using the in situ Co/Pt atomic ratio based on Vegard’s law is shown (blue dashed line) for comparison. The slight deviations observed for Pt/C are caused by the shorter RPt–Pt of small Pt NPs (∼2.75 Å) compared to that of bulk Pt (2.77 Å)59 used for Vegard’s law calculations. The red dashed line is a guide to the eye.

Mentions: Figure 7 clearly shows that the PtxCo/C NPs with initially higher Co content are subjected to bigger Co loss during the acid pretreatment. This is because (i) the PtxCo/C NPs with initially high Co content contain some Co in the near-surface region that is prone to acid dissolution, as evidenced by Δμ-XANES and EXAFS analysis, and (ii) the Co dissolution can retreat from the surface region toward the inner core of the PtxCo/C NPs with high enough Co content that the leaching rate of Co exceeds the surface diffusion of Pt.29,30,57 As a result, the residual Co contents in dealloyed PtCo/C and PtCo3 are comparable to those of Pt3Co/C and Pt2Co/C despite their higher initial Co content. Correspondingly, the RPt–Pt(s) in these NPs are very close (Figure 8). This unambiguously confirms that the RPt–Pt of PtxCo/C NPs is related to the residual rather than initial Co content. Indeed, Figure 8 shows that for all the Pt–Co/C NPs the experimental RPt–Pt values match well with those calculated using the residual Co content based on Vegard’s law, despite their dissimilar atomic structures. This also suggests that the bulk RPt–Pt of PtM NPs, or the compressive strain, is predominantly determined by the atomic composition and insensitive to the atomic distribution. The particle size effects on RPt–Pt wear out for Pt/C NPs bigger than 3 nm58,59 and are thus negligible here given that the majority of the PtxCo/C NPs are greater than 3 nm (Table 1 and Figure S2).


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)

Specific activity (red triangle) and RPt–Pt (blue dots) of PtxCo/C and Pt/C NPs are displayed as a function of the Co content determined by EXAFS. The specific activity (green triangle) and RPt–Pt (green dots) of dealloyed PtCo/C and PtCo3/C catalysts reported in refs (33) and (38) are also included. The RPt–Pt calculated using the in situ Co/Pt atomic ratio based on Vegard’s law is shown (blue dashed line) for comparison. The slight deviations observed for Pt/C are caused by the shorter RPt–Pt of small Pt NPs (∼2.75 Å) compared to that of bulk Pt (2.77 Å)59 used for Vegard’s law calculations. The red dashed line is a guide to the eye.
© Copyright Policy
Related In: Results  -  Collection

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

fig8: Specific activity (red triangle) and RPt–Pt (blue dots) of PtxCo/C and Pt/C NPs are displayed as a function of the Co content determined by EXAFS. The specific activity (green triangle) and RPt–Pt (green dots) of dealloyed PtCo/C and PtCo3/C catalysts reported in refs (33) and (38) are also included. The RPt–Pt calculated using the in situ Co/Pt atomic ratio based on Vegard’s law is shown (blue dashed line) for comparison. The slight deviations observed for Pt/C are caused by the shorter RPt–Pt of small Pt NPs (∼2.75 Å) compared to that of bulk Pt (2.77 Å)59 used for Vegard’s law calculations. The red dashed line is a guide to the eye.
Mentions: Figure 7 clearly shows that the PtxCo/C NPs with initially higher Co content are subjected to bigger Co loss during the acid pretreatment. This is because (i) the PtxCo/C NPs with initially high Co content contain some Co in the near-surface region that is prone to acid dissolution, as evidenced by Δμ-XANES and EXAFS analysis, and (ii) the Co dissolution can retreat from the surface region toward the inner core of the PtxCo/C NPs with high enough Co content that the leaching rate of Co exceeds the surface diffusion of Pt.29,30,57 As a result, the residual Co contents in dealloyed PtCo/C and PtCo3 are comparable to those of Pt3Co/C and Pt2Co/C despite their higher initial Co content. Correspondingly, the RPt–Pt(s) in these NPs are very close (Figure 8). This unambiguously confirms that the RPt–Pt of PtxCo/C NPs is related to the residual rather than initial Co content. Indeed, Figure 8 shows that for all the Pt–Co/C NPs the experimental RPt–Pt values match well with those calculated using the residual Co content based on Vegard’s law, despite their dissimilar atomic structures. This also suggests that the bulk RPt–Pt of PtM NPs, or the compressive strain, is predominantly determined by the atomic composition and insensitive to the atomic distribution. The particle size effects on RPt–Pt wear out for Pt/C NPs bigger than 3 nm58,59 and are thus negligible here given that the majority of the PtxCo/C NPs are greater than 3 nm (Table 1 and Figure S2).

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