Heterostructured catalysts prepared by dispersing Au@Fe2O3 core–shell structures on supports and their performance in CO oxidation

doi:10.1016/j.cattod.2010.05.013

Catalysis Today

Volume 160, Issue 1, 2 February 2011, Pages 87-95

https://doi.org/10.1016/j.cattod.2010.05.013 Get rights and content

Abstract

Herein, we report novel gold catalysts made by dispersing Au@Fe₂O₃ core–shell structures on solid supports. In the synthesis of Au@Fe₂O₃ core–shell structures, dodecanethiol-capped gold nanoparticles were used as the seed and Fe(CO)₅ was used as the precursor to Fe₂O₃ shell. The Au@Fe₂O₃ core–shell particles were deposited onto SiO₂ support to obtain Au@Fe₂O₃/SiO₂ catalysts that were highly active for low-temperature CO oxidation. The catalytic activity was even higher than that of Au/SiO₂ or Au/Fe₂O₃ prepared by colloidal deposition with comparable gold loadings. The influences of thermal pretreatment, shell thickness, and different supports (e.g., SiO₂, TiO₂, C, Fe₂O₃) were investigated, and relevant characterization using TG/DTG, XRD, TEM, HAADF, and EDX was conducted.

Introduction

Gold nanoparticles dispersed on solid supports have many potential applications in environmental catalysis, chemical synthesis, and clean energy processing [1], [2], [3], [4], [5], [6], [7], [8]. These catalysts are usually prepared by deposition–precipitation and colloidal-deposition [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19] methods and the supports often used are TiO₂, CeO₂, Fe₂O₃, Al₂O₃, ZrO₂, SiO₂, and C. More complicated gold catalysts may be desirable for achieving better performance and stability in catalysis [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. Along this line, our group has previously explored the use of Au–Fe₃O₄ dumbbell heterostructures on SiO₂, TiO₂, and C supports to stabilize gold nanocatalysts and demonstrated the high activities and thermal stability of these catalysts in CO oxidation [31]. We have also dispersed colloidal NiAu particles onto SiO₂ supports and demonstrated that upon oxidative pretreatment at elevated temperatures, NiAu nanoparticles could transform into Au nanoparticles whose surfaces were decorated by amorphous NiO [32], [33]. This Au–NiO/SiO₂ catalyst was active for low-temperature CO oxidation. In the current contribution, we report the application of Au@Fe₂O₃ core–shell structures [34] as the precursor for the preparation of supported catalysts highly active for low-temperature CO oxidation (Scheme 1).

Great effort has been made toward the designing of metal@oxide core–shell structures [34], [35], [36], [37], [38], [39]. However, only limited attention has been paid to the use of core–shell structures [40], [41], [42], [43], [44], [45], [46], [47], [48], [49] or their supported versions [49], [50], [51], [52] in making new catalysts [53], [54]. Notably, Tsang and co-workers prepared a CeO₂-encapsulated-Pt catalyst (denoted as Pt@CeO₂) via a microemulsion method [43], [44]. The catalyst showed activity in water–gas shift reaction comparable to Cu/ZnO and Pt/CeO₂ catalysts, and there was no formation of undesirable methane. In another work, Tsang and co-workers prepared a Pt@SiO₂/Al₂O₃ catalyst for the hydrogenation of toluene [52]. The resulting catalyst showed much higher thermal stability than Pt/SiO₂–Al₂O₃. More recently, Somorjai and co-workers designed Pt@SiO₂ core–shell structures for CO oxidation [47]. This catalyst had not only mesopores caused by the removal of residual organic species, but also good thermal stability due to the presence of SiO₂ shell.

Little effort has been paid to the use of Au@oxide core–shell structures in catalysis. Others have prepared new catalysts featured by the strategic location of small gold nanoparticles in much bigger hollow ZrO₂ or SiO₂ shells [55], [56], [57], [58] or by the entrapment of a number of gold nanoparticles in amorphous SiO₂ matrix [59]. These catalysts showed enhanced thermal stability due to the encapsulation of gold nanoparticles in inorganic matrixes. Recently, Xie and co-workers synthesized Au@SnO₂ core–shell structures by an intermetallics-based dry-oxidation approach [60]. There were several steps in their approach: (1) Gold nanoparticles were prepared by reducing HAuCl₄ by a sodium citrate solution; (2) AuSn nanoparticles were prepared by reducing SnCl₂ by NaBH₄ in the presence of the pre-synthesized gold nanoparticles; (3) The Au@SnO₂ core–shell structure was formed via a three-step oxidation procedure. As a result, gold nanoparticles with a mean size of 15 nm were encapsulated by the SnO₂ shell with a thickness of 6–7 nm. The core–shell structured catalyst still showed 50% CO conversion when the reaction temperature was 230 °C, even though the catalyst was pretreated at 850 °C prior to reaction testing. The authors proposed that the relatively high activity of Au@SnO₂ was due to a synergetic confinement effect, the electron transfer from the oxide support to gold nanoparticles, and larger interaction areas [60]. Nevertheless, the size of gold nanoparticles was still too big for catalysis, and the preparation method was relatively complicated. In another work, Xu and co-workers synthesized bimetallic Au–Ni nanoparticles embedded in SiO₂ spheres, and demonstrated that Au–Ni@SiO₂ showed higher catalytic activity and better durability than monometallic Au@SiO₂ and Ni@SiO₂ in the hydrolysis of ammonia borane [48].

In this contribution, we designed supported Au@Fe₂O₃ catalysts and tested their activity in CO oxidation. We have chosen Fe₂O₃ as the shell based on below considerations. First, the synthesis of Au@Fe₂O₃ has been established in the materials community [34]. Second, Au/Fe₂O₃ with Au–Fe₂O₃ interface is known to be active in CO oxidation [61], [62], [63], [64], [65], [66], [67]. Therefore, we expect that supported Au@Fe₂O₃ catalysts should also be active in CO oxidation. We are interested in investigating the following questions. First, is there any requirement for catalyst pretreatment in order to obtain high catalytic activity? Second, what is the effect of shell thickness on catalytic activity? Third, what effect can be induced if the underlying support is different? Finally, how do supported Au@Fe₂O₃ catalysts compare to Au/SiO₂, Au/TiO₂, Au/C, Au/Fe₂O₃, and Au@Fe₂O₃? The results from our work provide insights for the preparation and application of supported gold core-oxide shell catalysts.

Section snippets

Experimental

Gold nanoparticles were synthesized according to a classic method [68], [69], [70]. HAuCl₄·3H₂O (0.63 g) was added into 50 mL deionized water, and the aqueous solution was added, with vigorous stirring, into a solution containing 3.0 g tetraoctylammonium bromide and 160 mL toluene. The yellow HAuCl₄ solution became clear quickly and the toluene phase became orange-brown as the gold precursor was transferred into it. The organic phase was isolated, 80 mg dodecanethiol was added, and the resulting

Case study of Au@Fe₂O₃/SiO₂: effect of pretreatment temperature

Fig. 1 shows the light-off curves of Au@Fe₂O₃/SiO₂ catalysts under different pretreatment conditions. The thickness of the Fe₂O₃ shell is estimated as 2 nm, as described in Supporting Information. The as-synthesized Au@Fe₂O₃/SiO₂ catalyst is not active for CO oxidation when the reaction temperature is below 200 °C. The conversion increases sharply in between 230 and 250 °C, and the T₅₀ (temperature required for 50% CO conversion) value is 238 °C. After the catalytic test was finished, we repeated

Conclusions

Au@Fe₂O₃ core–shell heterostructures were deposited on SiO₂, TiO₂, C, and Fe₂O₃ supports by colloidal deposition, and the performance in CO oxidation was studied. It is necessary to pretreat the catalysts at elevated temperatures such as 300 °C, in order to remove residual organic species. The burning of organic species could result in the creation of pores for the entrance of CO and O₂ while allowing for the exit of CO₂. However, the catalytic activity decreases further when the pretreatment

Acknowledgements

Research sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. This research was also supported by the appointment for H.F. Yin to the ORNL Research Associates Program, administered by Oak Ridge Associated Universities.

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Heterostructured catalysts prepared by dispersing Au@Fe2O3 core–shell structures on supports and their performance in CO oxidation

Abstract

Introduction

Section snippets

Experimental

Case study of Au@Fe2O3/SiO2: effect of pretreatment temperature

Conclusions

Acknowledgements

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Heterostructured catalysts prepared by dispersing Au@Fe₂O₃ core–shell structures on supports and their performance in CO oxidation

Case study of Au@Fe₂O₃/SiO₂: effect of pretreatment temperature