Encapsulation Of Gold Nanoparticles With Metallic Oxides For Ehanced Catalytic Activity And Increased Thermal Stability

Gold is a key catalyst that is invaluable in many important industrial processes such as CO oxidation in catalytic converters, selective reduction of NOx and the reactions in fuel cells. The catalytic performance of the nano Au catalysts is closely related to the Au particles size. Generally, the smaller particles size results in an improved catalytic activity. Unfortunately, the aggregation and sintering of Au nanoparticles with small particle size seems to be easy, which is a dominant cause for the deactivation of Au catalysts and their loss of selectivity. Accordingly, preparation of excellent stability and highly efficient Au nanocatalyst is still highly desirable and technologically important. Herein, we focused on the improvement of thermal stability and catalytic activity of Au nanocatalyst, and fabricated a series of novel Au catalysts that encapsulated by metallic oxides. It was shown that the unique mesoporous structure could be formed in the high crystallized metallic oxides shells by accurately controlling the crystallization process of metallic oxides through a "protection-calcination" strategy. Additionally, we combined the advantages of the encapsulation by metallic oxides and the formation of alloy nanostructure for improving the thermally stability of noble metal nanoparticles, and synthesized the supported Au-Pt alloy catalyst coated by Tin oxides, and mainly investigated the influence of the co-incorporation of alloy nanoparticles and metallic oxides to the stability of Au nanoparticles. Finally, we also made a systematic comparison of the improvement of various metallic oxides support on Au-Pt bimetallic alloy catalysts. The photocatalytic degradation of organic pollutants or selective reduction of p-nitrophenol were chosen as the probe reaction for investigating of the beneficial effects of these distinct constructions on the catalytic activity. Moreover, the intrinsic relations between their structural characteristic, thermal stability and catalytic performance were also studied.1). The mechanism for the controlling of the crystallinity and pore properties in the "protection-calcination" process was investigated first. This method involved preparation of SiO2@TiO2 core-shell colloidal templates, sequential deposition of carbon and then silica layers through solvothermal and sol-gel processes, crystallization of TiO2 by calcination and finally removal of the inner and outer silica to produce hollow anatase TiO2 shells. The prepared samples were characterized by TEM, XRD, N2 adsorption-desorption isotherms and UV-vis absorption spectroscopy. The results showed that the combustion of carbon offered the space for the TiO2 to grow into large crystal grains, and the outer silica layer served as a barrier against the excessive growth of anatase TiO2 nanocrystals. When used as photocatalysts for the oxidation decomposition of Rhodamine B and phenol in aqueous solution under UV irradiation, the hollow TiO2 shells that obtained by roasting under 650℃ for 16 h showed an even higher performance than commercial P25 TiO2.2). A novel "core-satellites" structure Au/TiO2 hollow catalyst, in which sub-10 run Au nanoparticles were coated with a mesoporous anatase TiO2 shell, was then prepared based on the above "protection-calcination" by carbon layer and SiO2 shell. We emphasized the study on the influence of crystallizing temperature on the Au particles size. When used as the catalyst for the reduction of 4-nitrophenol, the synthesized Au/TiO2 catalysts exhibited significantly enhanced catalytic performance. Moreover, the obtained Au/TiO2 hollow spheres showed a superior thermal stability, as it resisting sintering during the additional calcination at 500℃, whereas the sample prepared by deposited Au nanoparticles on commercial P25(Au/P25) was found to sinter severely.3). Eccentric Au@(SiO2,TiO2) core-shell nanostructure that consisted of a 20 nm Au core coated with SiO2-TiO2 composites oxides shell was fabricated by a facile sol-gel method. The results showed that incorporation of SiO2-TiO2 composites shells led to an improvement in the thermal stability of Au catalyst comparing with the pure TiO2 coated one, which was attributed to the "SiO2-protection calcination". The composites oxides coated Au catalysts exhibited significantly enhanced catalytic performance compared with the Au@SiO2 catalyst. In particular, the calcined Au@(SiO2,TiO2) particles showed the highest catalytic activity due to the easy mass transfer, improved thermal stability and increased synergy effect of Au with TiO2. Finally, a possible reaction mechanism for the reduction reaction on Au@(SiO2,TiO2) was also proposed.4). A facile method was developed for the synthesis of highly active Au-Pt nanoalloys supported on the surface of ellipsoidal Fe@SiO2 nanoparticles and covered with Tin oxides. This method involved the loading of Pt NPs on the nanocapsules via Sn2+linkage and reduction, then in-situ fabricating of Au nanoparticles by the galvanic replacement reaction between Au and Pt, and finally calcination and reduction to convert the nonmagnetic Fe2O3 to Fe core with high saturation magnetization. XRD and XPS analysis demonstrated the alloy structure of Au-Pt nanoparticles in the final samples. TEM images suggested that the supported Au-Pt nanocatalyst possessed the smallest Au particle size after roasting under air and hydrogen. The obtained Fe@SiO2/Au-Pt samples exhibited a remarkably higher catalytic activity in comparison with the supported monometallic counterparts toward reduction of 4-nitrophenol to 4-aminophenol by NaBH4. The catalyst could be reused for several cycles with convenient magnetic separation.5). Finally, we demonstrated the convenience of the in-situ depositing of Au-Pt alloy nanoparticles on various oxides surface (mSiO2、TiO2、CeO2 and ZrO2). The structure of the supported Au-Pt nanoalloys were characterized in detail by TEM, XRD, and N2 physical absorption, and their catalytic activity in 4-NP reduction was evaluated. The supported Au-Pt alloy catalysts exhibited an improvement thermal stability comparing with respect to that supported on SiO2, which was attributed to the mesopore in the oxides and the strong interaction between noble metal and metal oxides that prevented the growth of Au-Pt NPs. The reduction reaction indicated that incorporation of TiO2 or CeO2 in the bimetal nanocatalyst could significantly improve the catalytic activity as compared with other inorganic oxides(ZrO2 and m-SiO2). Possible mechanisms were proposed to explained the synergistic effects in the supported bimetallic AuPt nanocatalyst.