Reduction Kinetics of the Three Nitrothiophenol Isomers on Pt-Coated Gold Nanorods

4-Nitrothiophenol (4-NTP) has been widely used in surface-enhanced Raman scattering (SERS)-based reduction studies for testing the catalytic performance of a variety of noble-metal nanoparticles using hydride or hydrogen as reducing agent. The catalytic conversion of 4-NTP to 4-aminothiophenol (4-ATP) strongly depends on the catalyst’s morphology, size, and material composition. This reduction reaction proceeds via a Langmuir–Hinshelwood mechanism, involving the formation of active Pt–H species. Two alternative reaction pathways with respect to the transferred hydrogen species are in principle possible. The first possible pathway describes the direct transfer of a hydrogen radical or hydride to the molecule (hypothesis 1), while the second pathway describes an electron transfer from the platinum to the 4-NTP followed by a proton transfer from the active species (hypothesis 2). To falsify one of these two contradicting hypotheses, we varied the position of the nitro group relative to the thiol groups and thereby the distance of the nitro group to the metal surface. The reduction kinetics of the three isomers of NTP (2-NTP, 3-NTP, and 4-NTP) on platinum-coated gold nanorods was monitored by SERS with molecular hydrogen as the reducing agent. On the basis of the experimental SERS kinetic data giving first-order rate constants in the order k2-NTP > k3-NTP > k4-NTP, we conclude that the reduction does not proceed via an electron transfer mechanism, but via hydrogen/hydride transfer.