| AVS 57th International Symposium & Exhibition | |
| Surface Science | Monday Sessions |
| Session SS1-MoA |
| Session: | Nanocluster Reactivity |
| Presenter: | I.L. Zorić, Chalmers University of Technology, Sweden |
| Authors: | C. Langhammer, Chalmers University of Technology, Sweden M.K. Larsson, Chalmers University of Technology, Sweden B.H. Kasemo, Chalmers University of Technology, Sweden I.L. Zorić, Chalmers University of Technology, Sweden |
| Correspondent: | Click to Email |
Nanocalorimetric studies of reaction rates for the H2 +O2 -> H2O reaction on supported Pd nanoparticles were performed, under realistic (T, p) conditions, by measuring local temperature of catalytic nanoparticles using the indirect nanoplasmonic sensing method1, 2. This method utilizes a sensing platform consisting of a 2D array of nanoplasmonic Au sensor nanodisks, coated with a few nm thick SiO2 spacer layer onto which Pd nanoparticles, with an average size of 2.2, 3.9, 6.5 and 18 nm, were prepared. The chemical power generated by the above exothermic reaction causes a temperature rise of the nanocatalyst and the sensing Au nanodisks, which in turns leads to a spectral shift of the localized surface plasmon resonance (LSPR) of the latter. During the experiments the surrounding temperature was scanned from RT to 3000C for each reactant concentration. By subtracting the calibrated temperature dependence of the Au LSPR from the observed spectral shifts during the reaction we obtain local temperature changes on the Pd nanocatalysts, which is directly proportional to the reaction rate, as a function of surrounding temperature for different reactant concentration a=H2/[H2+O2] in the feed gas and for Pd nanocatalysts of different sizes. A clear transition in the measured reaction rate, from the kinetically limited, low temperature regime, via a light-off, into the mass transport limited regime was observed. In the low temperature kinetically limited regime an Arrhenius analysis yields apparent activation energies, for a given catalyst size, that vary as a function of reactant concentrations in agreement with the data in the literature. A shift of the light-off temperature towards higher values, observed at higher a values, is accounted for by an increased catalyst self poisoning by hydrogen. By following a nonmonotonic change of the light-off temperature, at a given a value, for four different sizes of the Pd nanocatalyst, we were able to extract information about the size dependence of the catalytic activity for our systems. This was achieved by substracting away the size dependence of the surface area as obtained from the TEM pictures of the nanocatalysts. The catalytic activity of the smallest nanoparticles (2.2 nm) shows an almost three fold increase compared to the activity of 18nm nanoparticles.
1. Larsson, E.M., Langhammer, C., Zorić, I. & Kasemo, B. Nanoplasmonic Probes of Catalytic Reactions. Science 326, 1091-1094 (2009).
2. Langhammer, C., Zhdanov, V.P., Zorić, I. & Kasemo, B. Size-Dependent Kinetics of Hydriding and Dehydriding of Pd Nanoparticles. Physical Review Letters 104, 135502 (2010).