Letter pubs.acs.org/acscatalysis
Toward the Design of a Hierarchical Perovskite Support: UltraSintering-Resistant Gold Nanocatalysts for CO Oxidation Chengcheng Tian,† Xiang Zhu,*,† Carter W. Abney,‡ Xiaofei Liu,† Guo Shiou Foo,‡ Zili Wu,‡ Meijun Li,† Harry M. Meyer, III,§ Suree Brown,† Shannon M. Mahurin,‡ Sujuan Wu,⊥ Shi-Ze Yang,§ Jingyue Liu,# and Sheng Dai*,†,‡ †
Department of Chemistry, University of Tennessee-Knoxville, Tennessee 37996-1600, United States Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States # Department of Physics, Arizona State University, Tempe, Arizona 85287, United States ⊥ Electron Microscopy Center of Chongqing University, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China ‡
S Supporting Information *
ABSTRACT: An ultrastable Au nanocatalyst based on a heterostructured perovskite support with high surface area and uniform LaFeO3 nanocoatings was successfully synthesized and tested for CO oxidation. Strikingly, small Au nanoparticles (4−6 nm) are obtained after calcination in air at 700 °C and under reaction conditions. The designed Au catalyst not only possessed extreme sintering resistance but also showed high catalytic activity and stability because of the strong interfacial interaction between Au and the heterostructured perovskite support.
KEYWORDS: gold nanopaticle, perovskite, sintering-resistance, high temperature, heterogeneous catalysis
I
Perovskite-supported precious metal catalysts have recently been successfully developed and used industrially as a three-way catalyst for gasoline automotive emission control.8 It is worth noting that these three-way catalysts have to withstand temperatures in excess of 800 °C.9 A wide body of literature has demonstrated the ideal ABO3-type crystal structure of perovskite oxides has a profound effect on the stability of precious metals, and even the activity of the catalytic system.10 Therefore, perovskite-type compounds, especially those which contain rare earth metals at A-site, are interesting materials for catalytic applications and for fundamental studies as well.11 Although Pd, Rh, and Pt have been integrated within perovskite supports and display excellent sintering-resistant capabilities,12 thus far there have been no reports regarding similar developments for Au catalysts, with the limited studies on gold-perovskites only reporting the essential activity data for catalytic oxidation.13 In the present work, we present a new strategy for the preparation of perovskite-confined Au nanoparticle catalysts
t is well-known that at the nanoscale, materials exhibit unprecedented physicochemical and catalytic properties that differ drastically from their bulk counterparts. This is the case for gold, which is inert and inactive in catalysis in the bulk form. In contrast, gold nanoparticles supported on oxide supports exhibit an excellent catalytic activity for CO oxidation as first demonstrated in the research of Haruta1 and Hutchings.2 Supported Au catalysts have subsequently drawn extensive and ever increasing attention due to their unique catalytic performance for numerous types of important chemical reactions.1−3 As is well-known, the catalytic performance of supported Au catalysts is highly dependent on the Au particles’ size. Unfortunately, due to the low Tammann temperature (395 °C),4 Au nanoparticles sinter easily under reaction conditions and modest temperatures (>400 °C), thereby losing their catalytic activity.5 Although some supported Au catalysts have shown resistance to calcination at temperatures of 500−600 °C,6 Au nanocatalysts appear to be incapable of withstanding calcination temperatures above 600 °C.7 Clearly it remains a formidable challenge to design hierarchical catalyst supports with controlled morphologies and nanoarchitectures, which can stabilize gold nanoparticles against sintering. © XXXX American Chemical Society
Received: February 13, 2017 Revised: March 23, 2017 Published: April 10, 2017 3388
DOI: 10.1021/acscatal.7b00483 ACS Catal. 2017, 7, 3388−3393
Letter
ACS Catalysis
X-ray diffraction (XRD) patterns of LaFeO3-MCF (Figure 1a) revealed broad peaks corresponding to the crystalline LaFeO3 phase, as displayed in Figure 1a. The size of LaFeO3 nanocrystals was calculated from X-ray line broadening of the peaks using the Scherrer equation and reveals crystal sizes of 10−12 nm. The mesoporosity of LaFeO3-MCF was evaluated by the nitrogen adsorption−desorption isotherms, which exhibited Type IV isotherms (Figure S1), with a BET surface area and total pore volume of 465 m2 g−1 and 1.71 cm3 g−1, respectively (Table 1). Importantly, a reduction in overall surface area, pore volume, and hysteresis loop following LaFeO3 inclusion on the MCF suggests a uniform coating of the LaFeO3 onto the inner walls of the MCF, without significant distortion of pore shape or blockage. As presented in the TEM images, the dark contrast indicated by the black arrows in Figure 1b and Figure S2 is attributed to LaFeO3 in LaFeO3-MCF. On the basis of the above characterization, it appears that the LaFeO3 in LaFeO3-MCF coats on the pore surface of MCF in the form of thin layered nanostructures. As measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES), the La and Fe loadings in LaFeO3MCF are 9.5 and 3.7 wt %, respectively, affording an atomic ratio of La to Fe of almost precisely 1:1. After introduction of Au, a very broad Au XRD (111) peak at about 2θ = 38.3° is visible for Au-LaFeO3-MCF-0.6 and AuLaFeO3-MCF-1.0 (Figure 1a). The size of Au species estimated from X-ray line broadening increases from 4 to 6 nm with an increase in Au loading. We further examined the samples by transmission electron microscopy (TEM). Figure 1 shows two typical Z-contrast scanning transmission electron microscopy (STEM) images of Au-LaFeO3-MCF samples and their size distributions. The small and uniform Au NPs (about 4−5 nm) are clearly observed in both of the Au-LaFeO3-MCF-0.6 (Figure 1c) and Au-LaFeO3-MCF-1.0 samples (Figure 1d), although some relatively larger Au NPs (about 6−7 nm) are also observed in Au-LaFeO3-MCF-1.0. These STEM results are consistent with the XRD results, confirming good dispersion of nanoscale Au species on LaFeO3-MCF. It is worth noting that all the catalysts were calcined at 700 °C, compared to a typical Au-TiO2 catalyst in previous report,6 suggesting that these Au catalysts exhibited remarkable resistance to sintering. Even though the calcination time was increased to 6 h at 700 °C, we still cannot observe the distinct change for the size of Au based on the XRD (Figure S3) and TEM results (Figure S4). The remarkable sintering resistance of the Au-LaFeO3-MCF catalysts suggests that these perovskite-supported gold catalysts can have great utility for gas-phase catalytic reactions. CO oxidation was employed as a probe reaction for these supported Au catalysts due to its great importance in both fundamental study and practical applications.18 Figure 2a presents a comparison of the light-off curves for CO oxidation by AuLaFeO3-MCF catalysts after calcination at 700 °C in air for 2 h. Before the introduction of Au, the nascent LaFeO3-MCF material showed no activity for CO oxidation below 200 °C. In contrast, all the Au-LaFeO3-MCF catalysts showed enhanced activity. For the Au-LaFeO3-MCF-0.6 and Au-LaFeO3-MCF1.0 samples, almost complete conversion of CO was observed even at room temperature. Because of the smaller nanoparticles (4−5 nm) on the Au-LaFeO3-MCF-0.6, it showed greater activity for low-temperature CO oxidation than Au-LaFeO3MCF-1.0. Upon calcination at 800 °C for 2 h, as displayed in Figure 2b, the Au-LaFeO3-MCF catalysts still presented very high activity, although a certain loss of activity is clearly
with high thermal stability. The hierarchical perovskite support with a unique nanoarchitecture is composed of uniform LaFeO3 nanocoatings (10−12 nm) on 3D mesoporous cellular foam (MCF, silica), which serves as an efficient support to disperse and stabilize the small Au nanoparticles. These Au−LaFeO3− MCF composite systems exhibit exceptionally high activity for low-temperature CO oxidation. More importantly, the Au nanoparticles maintain their size and exhibit high sinteringresistant capabilities, even after calcination at 700 °C. Surprisingly, catalysts annealed at 800 °C also retained highly dispersed small gold nanoparticles. Although a few larger Au nanoparticles can be also observed, the catalyst still displays complete CO conversion at 50 °C. The activity in terms of specific rate is even higher than that of typical Au/TiO2 catalysts (World Gold Council). This work significantly lowers the barrier to practical applications of supported Au nanocatalysts, especially for high-temperature catalytic reactions. The diagram shown in Scheme 1 depicts the facile synthesis of the mesoporous perovskite support and the ensuing Scheme 1. Schematic Illustration of the Au-LaFeO3-MCF Preparation Process
preparation of the Au catalyst. A modified Pechini-method was used to prepare heterostructured LaFeO3−MCF hybrid materials with uniform LaFeO3 nanocrystals coated on the MCF surface, similar to the nanoscopic barium sulfate14 and vanadium phosphate15 coatings on MCF in our previous report. Although there are many well-established synthetic protocols for the preparation of perovskite oxides, such as coprecipitation16 and citrate sol−gel,17 samples derived from such synthetic methods are bulk materials with relatively low surface areas (
