The Curious Case of Au Nanoparticles

Transcription

1 The Curious Case of Au Nanoparticles Industrial reactions performed by metals 1

2 Low Au reactivity Predictions are typically based on d-band model Hold well for polycrystalline materials Coinage metals have fully occupied d-bands, but Ag and Cu have relatively low ionization potentials J. Phys. Chem. B 108, (2004) Volcano plot of the measured catalytic activity of the oxygen reduction reaction for various transition metal electrodes as a function of the calculated binding energy of oxygen atoms from DFT calculations. Gold Nanocatalysts Catalytic properties discovered in the 1970s, but not cloesly investigated until the work of Masatake Haruta in the 1980s Size effect: activity for nanoparticles (NPs) larger than 5 nm is low quantum size effect Morphology effect: low-coordination sites Support effect: interfacial atoms tend to have electronic properties modified by the substrate Currently pursued for low-t CO oxidation and selective hydrocarbon oxidation Tends to be poison resistant Melting temp is around 600 K 2

3 Quantum effect Dependence of catalytic activity of Au/Al 2 O 3 catalyst for (a) CO oxidation and (b) acetylene hydrogenation on the diameter of Au particles Catalyst support affects reaction T 3

4 Co-precipitation Metal salt solutions and support precursors are combined together, then calcined Typically used for Au/iron oxide systems, where the iron oxide is easily precipitated out of aqueous solution Calcination = thermal treatment process Deposition precipitation: deposition NaOH Urea J. Phys. Chem. B, 2002, 106 (31), pp

5 Deposition precipitation: calcination Journal of Catalysis, Volume 222, Issue 2, 10 March 2004, Pages Impregnation Nanoparticles are preformed, and then loaded onto the support of choice 5

6 Impregnation: metal reduction in soln. Reaction determined by electrochemical potential Handbook of Chemistry & Physics ev Au NPs HAuCl 4 + NaBH 4 Au + B(OH) 3 + NaCl + H + Fast reduction with sodium borohydride Anal. Chem., 1965, 37 (9), pp J. Phys. Chem. B, 2001, 105, Ag + + C 6 H 5 O 7 3- Ag + C 5 H 4 O 5 + CO 2 + H + Turkevich Method: Slow reduction using sodium citrate Discuss. Faraday Soc., 1951, 11, Chem. Rev., 2004, 104 (1), pp Impregnation: loading = C 16 H 33 SH J. Am. Chem. Soc., 2006, 128 (44), pp TEM images of 6.3 nm (A G) and 3.5 nm (H) gold nanoparticles (5% in weight) supported on different oxides: (A) zeolite (CBV600); (B) α-fe 2 O 3 ; (C) TiO 2 (P25); (D) hydroxyapatite; (E) Al 2 O 3 ; (F) ZnO; (G) fumed SiO 2 ; (H) SiO 2. The images were taken from the samples after thermal treatment at 300 C in air for 1 h. All scale bars are 20 nm. 6

7 Mesoporous supports SBA-15 Monodispersed platinum nanoparticles are encapsulated in mesoporous silica with a channel structure (SBA-15) to form a 3D model catalyst system. Somorjai G A et al. PNAS 2006;103: by National Academy of Sciences 7

8 Au NPs for CO oxidation Pollution Remediation Reactions at Different Fuel Cell Electrodes Reaction products are formed at the anode Solid oxide Molten carbonate Alkaline the cathode Phosphoric acid Proton exchange membrane 8

9 Mechanism for CO oxidation Independent of [CO] down to 0.1%vol and slightly higher dependency on [O 2 ] Cluster Model Ignores effect of support Single atomic site can activate reactants 9

10 Titania support Volume 6, Number 3, , DOI: /A: Possible reaction mechanism 10

11 Interfacial sites? Proposed reaction mechanism 11

12 Reactivity of Au: active site? Au on TiO 2, ML model: Au is bonded to Ti Au (1x3) structure is more active thatn the (1x1) monolayer Access to the Au/Ti interface is hindered M S Chen, D W Goodman Science 2004;306: Conclusions Extraordinarily high catalytic activity of supported Au catalysts for CO oxidation at room temperature arises from the reaction of CO adsorbed on the step, edge, and corner sites of metallic Au particles with oxygen molecules adsorbed at the perimeter sites on the support surfaces. But maybe not, according to newer studies. It is unlikely that oxidic Au species and non-metallic small Au clusters are responsible for the extraordinarily high catalytic activity, although they are active to a certain extent. 12

13 Au/Co 3 O 4 13

14 High-throughput catalysis testing 14