Reactions of Methyl Radicals with Pd° Nanoparticles in Aqueous Solutions

Ronen Bar-Ziv, Chemistry, Ben-Gurion Univ. and Nuclear Research Centre Negev (N.R.C.N), Beer-Sheva, Israel
Israel Zilbermann, Chemistry, Ben-Gurion Univ. and Nuclear Research Centre Negev (N.R.C.N), Beer-Sheva, Israel
Guy Yardeni, Chemistry, Ben-Gurion Univ. and Nuclear Research Centre Negev (N.R.C.N), Beer-Sheva, Israel
Tomer Zidki, Biological Chemistry , Ariel University Center of Samaria, Ariel, Israel
Dan Meyerstein, Biological Chemistry , Ariel University Center of Samaria and Ben-Gurion Univ., Jerusalem, Israel

Alkyl radicals are important intermediates in electro-catalytic and heterogeneous catalytic processes. Nevertheless only few studies concerning the kinetics and mechanisms of reaction of alkyl radicals with M°-NPs suspended in solvents are available. All the reactions of M°-NPs and semi-conductor NPs suspended in aqueous solutions are very fast ^{[1, 2]}:

NPs + nR∙ → (NPs)-R_n    k > 1x10⁸ M^-1s^-1

The mechanism of decomposition of the (NPs)-R_n transients thus formed depends on the nature of ·R and the NP. For R = ∙CH₃ and NP = Ag°; Au°; TiO₂; CuO the transients decompose via:

(NPs)-(CH₃)_n → (NPs)-(CH₃)_n-2 + C₂H₆

For Cu°-NPs the transients decompose via:

(Cu°-NPs)-(CH₃)_n + H⁺ → (Cu°-NPs)⁺-(CH₃)_n-1 + CH₄

On the other hand (Pt°-NPs)-(CH₃)_n is stable in aqueous solution though traces of C₂H₆; C₂H₄ and CH₄  are formed when methyl radicals react with Pt°-NPs.

As a consequence of the results obtained in the presence of Pt°-NPs, it was decided to investigate the reactions of methyl radicals with Pd°-NPs a metal with similar chemical properties. Indeed, the results obtained show that there are similarities between the two; however the yield of methane and ethane is considerably larger in the presence of Pd°-NPs and the number of methyls remaining bound to the Pd°-NPs is considerably smaller than for the Pt°-NPs.

In addition, the surface coverage causes a change in the electron density on the NPs causing a gradual change in the UV-Vis spectrum.

References

1.  R. Bar-Ziv, I. Zilbermann, O. Oster, G. Yardeni, H. Cohen, D. Meyerstein, Chemistry-a European Journal,  2012, 18, 4699-4805.

2.  R. Bar-Ziv, I. Zilbermann, T. Zidki, G. Yardeni , V. Shevchenko, D. Meyerstein,  Chemistry-a European Journal,  2012, 18, 6733-6736.