Gold nanoparticles show an excellent catalytic performance in energy production related processes such as low temperature CO oxidation, PROX and WGS reactions [1-2]. The effect of the support, like titania or ceria, and/or the presence of different kinds of promoters on the catalytic performance of gold have also been investigated, and a general agreement about the advantages of using reducible oxides is deduced from a revision of the most recent literature. Gold catalysts have also proven to be active in environmental reactions as those occurring in the catalytic converters (TWC) for depuration of exhaust gases in vehicles. The use of gold as a component of the current TWC formulations, substituting the more critical Pt and Pd, would only be feasible if the excellent catalytic properties of this metal could be maintained after exposure at high temperature operating conditions. However due to its relative low melting point gold suffers a severe sintering process starting at temperatures of around 473 K [3].

In order to investigate the stabilization of gold nanoparticles a reference system, 1.5 wt% Au/TiO2World Gold Council catalyst, was modified by depositing on its surface a monolayer of CeO2by incipient wetness impregnation to a final ceria molar loading of 5.4%.Scanning Transmission Electron Microscopy (STEM) studies show that the dispersion of the gold nanoparticles remained unchanged after the impregnation in a value about 36%. Afterwards, both catalysts were tested in consecutive CO oxidation reaction loops at increasing final temperatures. In these cycles, the ceria-modified catalyst showed not only a higher activity but, more importantly, a largely enhanced stability against deactivation.

Scanning Transmission Electron Microscopy has provided key information to rationalize the origin of the stabilization effect provided by ceria. In particular, STEM-HAADF images haveclearly revealed the presence of nanometer-sized ceria rafts, less than 1 nm thick, on the surface of the fresh CeO2(5.4%)/Au(1.5%)/TiO2catalysts. After the CO oxidation test at the highest temperature, 1223 K, the conventional WGC catalyst suffered from a very severe Au nanoparticle sintering, Figure 1(a),whereas Au nanoparticle growth was very much limited in the ceria-modified catalyst after the same aging test, Fire 1(b).

Cs-corrected STEM results reveal that a major fraction of the Au nanoparticles (75%), comprising all the smaller ones (< 5 nm), was contacting the ceria nanolayers. This evidences an important stabilizing effect of the proposed surface modification. Moreover, these results open up possibilities for gold catalysts in applications where high temperatures are reached under working conditions.

(1) G.J. Hutchings, Catalysis by gold, Catal. Today. 100 (2005) 55–61. doi:10.1016/j.cattod.2004.12.016.

(2) M. López-Haro, J.J. Delgado, J.M. Cies, E. Del Rio, S. Bernal, R. Burch, et al., Bridging the gap between CO adsorption studies on gold model surfaces and supported nanoparticles, Angew. Chemie – Int. Ed. 49 (2010) 1981–1985. doi:10.1002/anie.200903403

(3) H. Zhu, Z. Ma, S.H. Overbury, S. Dai, Rational design of gold catalysts with enhanced thermal stability: post modification of Au/TiO2 by amorphous SiO2 decoration, Catal. Letters. 116 (2007) 128–135. doi:10.1007/s10562-007-9144-3.

Figures:

HAADF-STEM image representative of the structure of Au(1.5%)/TiO2 catalyst after exiting the last CO oxidation cycle performed at 1223 K.

HAADF-STEM image representative of the structure of CeO2(5.4%)/Au(1.5%)/TiO2 catalyst after exiting the last CO oxidation cycle performed at 1223 K.

« Back to The 16th European Microscopy Congress 2016

EMC Abstracts - https://emc-proceedings.com/abstract/stem-investigation-of-titania-supported-gold-nanoparticles-stabilized-by-ceria/