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Investigation of the Kinetics of a Surface Photocatalytic Reaction in Two Dimensions with Surface-enhanced Raman Scattering.

van Schrojenstein Lantman EM, Gijzeman OL, Mank AJ, Weckhuysen BM - ChemCatChem (2014)

Bottom Line: Catalytic reactions within a self-assembled monolayer are confined within two dimensions, as the molecules involved do not leave the surface.As a proof of principle, we study the photocatalytic reduction of p-nitrothiophenol.A study of the reaction rate and dilution effects leads to the conclusion that a dimerization must take place as one of the reaction steps.

View Article: PubMed Central - PubMed

Affiliation: Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht (The Netherlands).

ABSTRACT

Heterogeneous catalysis is a surface phenomenon. Yet, though the catalysis itself takes place on surfaces, the reactants and products rapidly take the form of another physical state, as either a liquid or a gas. Catalytic reactions within a self-assembled monolayer are confined within two dimensions, as the molecules involved do not leave the surface. Surface-enhanced Raman spectroscopy is an ideal technique to probe these self-assembled monolayers as it gives molecular information in a measured volume limited to the surface. We show how surface-enhanced Raman spectroscopy can be used to determine the reaction kinetics of a two-dimensional reaction. As a proof of principle, we study the photocatalytic reduction of p-nitrothiophenol. A study of the reaction rate and dilution effects leads to the conclusion that a dimerization must take place as one of the reaction steps.

No MeSH data available.


a) Time‐dependent SERS measurements at λ=532 nm from a pNTP‐coated silver island film; each horizontal line represents a spectrum (blue–red: low–high signal intensity). b) The first (top) and last (bottom) spectrum of part a. The colored bands depict peak integration area for quantification of pNTP (=1293–1373 cm−1 and product (=1412–1473 cm−1) over time. c) Time traces for pNTP (—) and product (—) taken from the peak areas marked in part b. d) Relative peak area as function of time. Spectra were taken with 5 s integration time at 6×103 W cm−2.
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fig2: a) Time‐dependent SERS measurements at λ=532 nm from a pNTP‐coated silver island film; each horizontal line represents a spectrum (blue–red: low–high signal intensity). b) The first (top) and last (bottom) spectrum of part a. The colored bands depict peak integration area for quantification of pNTP (=1293–1373 cm−1 and product (=1412–1473 cm−1) over time. c) Time traces for pNTP (—) and product (—) taken from the peak areas marked in part b. d) Relative peak area as function of time. Spectra were taken with 5 s integration time at 6×103 W cm−2.

Mentions: With green laser excitation, pNTP will be reduced if it is adsorbed onto a SERS‐active substrate,21e as illustrated in Figure 2 a. All experiments were performed by using a low NA objective, to limit the effect of any local heterogeneities in the catalytic activity. The two characteristic Raman peaks for tracking the reaction are 1335 cm−1 (reactant pNTP, asymmetric NO2 stretch)22 and 1440 cm−1 (product, assigned to pATP as a b2 mode23 or to DMAB as a N=N stretch).13a Tracking both peak areas as a function of time gives an idea of the surface concentration of both species (Figure 2 b). The concentration of pNTP decays over time, while the product is formed synchronously. The postulated multistep reaction mechanism24 is expected to have a single rate‐determining step. Therefore, we approximate the reaction to have one step and identify the order of reaction of this rate‐limiting step.


Investigation of the Kinetics of a Surface Photocatalytic Reaction in Two Dimensions with Surface-enhanced Raman Scattering.

van Schrojenstein Lantman EM, Gijzeman OL, Mank AJ, Weckhuysen BM - ChemCatChem (2014)

a) Time‐dependent SERS measurements at λ=532 nm from a pNTP‐coated silver island film; each horizontal line represents a spectrum (blue–red: low–high signal intensity). b) The first (top) and last (bottom) spectrum of part a. The colored bands depict peak integration area for quantification of pNTP (=1293–1373 cm−1 and product (=1412–1473 cm−1) over time. c) Time traces for pNTP (—) and product (—) taken from the peak areas marked in part b. d) Relative peak area as function of time. Spectra were taken with 5 s integration time at 6×103 W cm−2.
© Copyright Policy - open-access
Related In: Results  -  Collection

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fig2: a) Time‐dependent SERS measurements at λ=532 nm from a pNTP‐coated silver island film; each horizontal line represents a spectrum (blue–red: low–high signal intensity). b) The first (top) and last (bottom) spectrum of part a. The colored bands depict peak integration area for quantification of pNTP (=1293–1373 cm−1 and product (=1412–1473 cm−1) over time. c) Time traces for pNTP (—) and product (—) taken from the peak areas marked in part b. d) Relative peak area as function of time. Spectra were taken with 5 s integration time at 6×103 W cm−2.
Mentions: With green laser excitation, pNTP will be reduced if it is adsorbed onto a SERS‐active substrate,21e as illustrated in Figure 2 a. All experiments were performed by using a low NA objective, to limit the effect of any local heterogeneities in the catalytic activity. The two characteristic Raman peaks for tracking the reaction are 1335 cm−1 (reactant pNTP, asymmetric NO2 stretch)22 and 1440 cm−1 (product, assigned to pATP as a b2 mode23 or to DMAB as a N=N stretch).13a Tracking both peak areas as a function of time gives an idea of the surface concentration of both species (Figure 2 b). The concentration of pNTP decays over time, while the product is formed synchronously. The postulated multistep reaction mechanism24 is expected to have a single rate‐determining step. Therefore, we approximate the reaction to have one step and identify the order of reaction of this rate‐limiting step.

Bottom Line: Catalytic reactions within a self-assembled monolayer are confined within two dimensions, as the molecules involved do not leave the surface.As a proof of principle, we study the photocatalytic reduction of p-nitrothiophenol.A study of the reaction rate and dilution effects leads to the conclusion that a dimerization must take place as one of the reaction steps.

View Article: PubMed Central - PubMed

Affiliation: Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht (The Netherlands).

ABSTRACT

Heterogeneous catalysis is a surface phenomenon. Yet, though the catalysis itself takes place on surfaces, the reactants and products rapidly take the form of another physical state, as either a liquid or a gas. Catalytic reactions within a self-assembled monolayer are confined within two dimensions, as the molecules involved do not leave the surface. Surface-enhanced Raman spectroscopy is an ideal technique to probe these self-assembled monolayers as it gives molecular information in a measured volume limited to the surface. We show how surface-enhanced Raman spectroscopy can be used to determine the reaction kinetics of a two-dimensional reaction. As a proof of principle, we study the photocatalytic reduction of p-nitrothiophenol. A study of the reaction rate and dilution effects leads to the conclusion that a dimerization must take place as one of the reaction steps.

No MeSH data available.