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Detecting and utilizing minority phases in heterogeneous catalysis

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ABSTRACT

Highly active phases in carbon monoxide oxidation are known, however they are transient in nature. Here, we determined for the first time the structure of such a highly active phase on platinum nanoparticles in an actual reactor. Unlike generally assumed, the surface of this phase is virtually free of adsorbates and co-exists with carbon-monoxide covered and surface oxidized platinum. Understanding the relation between gas composition and catalyst structure at all times and locations within a reactor enabled the rational design of a reactor concept, which maximizes the amount of the highly active phase and minimizes the amount of platinum needed.

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There are structural changes in a 2 wt% platinum on Al2O3 catalyst during the switch from carbon monoxide to the catalytic mixture of carbon monoxide and oxygen as observed by transient quick XANES and EXAFS at the Pt L3 edge. (A) Whiteline intensity of normalized Pt L3 XANES spectra versus time during the switch indicates that right after the switch, starting from a carbon monoxide-poisoned surface, a short-lived phase forms, as indicated by a temporary decrease in whiteline intensity followed by a rapid increase. This rapid increase is indicative of the formation of a surface oxide36. (B) Corresponding XANES spectra at 0 s (green), 31 s (blue), and 43 s (red). (C) Spectra of the carbon monoxide-covered platinum (green), surface platinum oxide (red) and the intermediate (blue) determined by the genetic algorithm procedure. (D) Fractions of the surface occupied by carbon monoxide (green), the intermediate species (blue) and a surface oxide (red) as determined by linear combination fitting of the spectra in (C) (crosses) as a function of time. The estimated error was 5% based on a systematic analysis of the sensitivity of the fit towards variation in relative amounts of each component.
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f1: There are structural changes in a 2 wt% platinum on Al2O3 catalyst during the switch from carbon monoxide to the catalytic mixture of carbon monoxide and oxygen as observed by transient quick XANES and EXAFS at the Pt L3 edge. (A) Whiteline intensity of normalized Pt L3 XANES spectra versus time during the switch indicates that right after the switch, starting from a carbon monoxide-poisoned surface, a short-lived phase forms, as indicated by a temporary decrease in whiteline intensity followed by a rapid increase. This rapid increase is indicative of the formation of a surface oxide36. (B) Corresponding XANES spectra at 0 s (green), 31 s (blue), and 43 s (red). (C) Spectra of the carbon monoxide-covered platinum (green), surface platinum oxide (red) and the intermediate (blue) determined by the genetic algorithm procedure. (D) Fractions of the surface occupied by carbon monoxide (green), the intermediate species (blue) and a surface oxide (red) as determined by linear combination fitting of the spectra in (C) (crosses) as a function of time. The estimated error was 5% based on a systematic analysis of the sensitivity of the fit towards variation in relative amounts of each component.

Mentions: Figure 1A illustrates the change in maximum whiteline intensity of the individual XAS spectra versus time, which is indicative of platinum nano-particles changing their structure upon rapid switching of the gas phase from carbon monoxide to a carbon monoxide/oxygen mix (1:1), which induces the catalytic reaction. The whiteline intensity increased as expected for the transition from a carbon monoxide covered, reduced platinum surface to a surface oxide as determined by EXAFS analysis26. However, before the increase, a small decrease in the whiteline intensity occurred. This identifies the existence of a third phase during the transition, which is confirmed by principal component analysis of the full XANES dataset (Figure S2, details in the SI). Principle component analysis is a well-established tool to quantify all existing components in a XAS spectrum2728. It enables a quantitative view of the entire data set not provided by whiteline intensities or areas. The maximum whiteline intensity was used here for illustrative purposes. In addition to the increased whiteline intensity in the spectra recorded beyond 30 s, there is a small shift towards lower edge energies. This shift is the result of the removal of carbon monoxide and the associated back bonding interaction2429.


Detecting and utilizing minority phases in heterogeneous catalysis
There are structural changes in a 2 wt% platinum on Al2O3 catalyst during the switch from carbon monoxide to the catalytic mixture of carbon monoxide and oxygen as observed by transient quick XANES and EXAFS at the Pt L3 edge. (A) Whiteline intensity of normalized Pt L3 XANES spectra versus time during the switch indicates that right after the switch, starting from a carbon monoxide-poisoned surface, a short-lived phase forms, as indicated by a temporary decrease in whiteline intensity followed by a rapid increase. This rapid increase is indicative of the formation of a surface oxide36. (B) Corresponding XANES spectra at 0 s (green), 31 s (blue), and 43 s (red). (C) Spectra of the carbon monoxide-covered platinum (green), surface platinum oxide (red) and the intermediate (blue) determined by the genetic algorithm procedure. (D) Fractions of the surface occupied by carbon monoxide (green), the intermediate species (blue) and a surface oxide (red) as determined by linear combination fitting of the spectra in (C) (crosses) as a function of time. The estimated error was 5% based on a systematic analysis of the sensitivity of the fit towards variation in relative amounts of each component.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5121607&req=5

f1: There are structural changes in a 2 wt% platinum on Al2O3 catalyst during the switch from carbon monoxide to the catalytic mixture of carbon monoxide and oxygen as observed by transient quick XANES and EXAFS at the Pt L3 edge. (A) Whiteline intensity of normalized Pt L3 XANES spectra versus time during the switch indicates that right after the switch, starting from a carbon monoxide-poisoned surface, a short-lived phase forms, as indicated by a temporary decrease in whiteline intensity followed by a rapid increase. This rapid increase is indicative of the formation of a surface oxide36. (B) Corresponding XANES spectra at 0 s (green), 31 s (blue), and 43 s (red). (C) Spectra of the carbon monoxide-covered platinum (green), surface platinum oxide (red) and the intermediate (blue) determined by the genetic algorithm procedure. (D) Fractions of the surface occupied by carbon monoxide (green), the intermediate species (blue) and a surface oxide (red) as determined by linear combination fitting of the spectra in (C) (crosses) as a function of time. The estimated error was 5% based on a systematic analysis of the sensitivity of the fit towards variation in relative amounts of each component.
Mentions: Figure 1A illustrates the change in maximum whiteline intensity of the individual XAS spectra versus time, which is indicative of platinum nano-particles changing their structure upon rapid switching of the gas phase from carbon monoxide to a carbon monoxide/oxygen mix (1:1), which induces the catalytic reaction. The whiteline intensity increased as expected for the transition from a carbon monoxide covered, reduced platinum surface to a surface oxide as determined by EXAFS analysis26. However, before the increase, a small decrease in the whiteline intensity occurred. This identifies the existence of a third phase during the transition, which is confirmed by principal component analysis of the full XANES dataset (Figure S2, details in the SI). Principle component analysis is a well-established tool to quantify all existing components in a XAS spectrum2728. It enables a quantitative view of the entire data set not provided by whiteline intensities or areas. The maximum whiteline intensity was used here for illustrative purposes. In addition to the increased whiteline intensity in the spectra recorded beyond 30 s, there is a small shift towards lower edge energies. This shift is the result of the removal of carbon monoxide and the associated back bonding interaction2429.

View Article: PubMed Central - PubMed

ABSTRACT

Highly active phases in carbon monoxide oxidation are known, however they are transient in nature. Here, we determined for the first time the structure of such a highly active phase on platinum nanoparticles in an actual reactor. Unlike generally assumed, the surface of this phase is virtually free of adsorbates and co-exists with carbon-monoxide covered and surface oxidized platinum. Understanding the relation between gas composition and catalyst structure at all times and locations within a reactor enabled the rational design of a reactor concept, which maximizes the amount of the highly active phase and minimizes the amount of platinum needed.

No MeSH data available.


Related in: MedlinePlus