Chinese Journal of Chemical Physics  2016, Vol. 29 Issue (5): 571-577

The article information

Wang Chunlei, Lu Junling
王春雷, 路军岭
Sub-nanometer-thick Al2O3 Overcoat Remarkably Enhancing Thermal Stability of Supported Gold Catalysts
亚纳米厚的氧化铝包裹层可以显著提高负载型金纳米颗粒催化剂的热稳定性
Chinese Journal of Chemical Physics, 2016, 29(5): 571-577
化学物理学报, 2016, 29(5): 571-577
http://dx.doi.org/10.1063/1674-0068/29/cjcp1604065

Article history

Received on: April 2, 2016
Accepted on: May 18, 2016
Sub-nanometer-thick Al2O3 Overcoat Remarkably Enhancing Thermal Stability of Supported Gold Catalysts
Wang Chunlei, Lu Junling     
Dated: Received on April 2, 2016; Accepted on May 18, 2016
Department of Chemical Physics, Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), CAS Key Laboratory of Materials for Energy Conversion, University of Science and Technology of China, Hefei 230026, China
Author: E-mail:junling@ustc.edu.cn
Abstract: Supported gold nanoparticle catalysts show extraordinarily high activity in many reactions. While the relative poor thermal stability of Au nanoparticles against sintering at elevated temperatures severely limits their practical applications. Here atomic layer deposition (ALD) of TiO2 and Al2O3 was performed to deposit an Au/TiO2 catalyst with precise thickness con-trol, and the thermal stability was investigated. We surprisingly found that sub-nanometer-thick Al2O3 overcoat can su ciently inhibit the aggregation of Au particles up to 600 C in oxygen. On the other hand, the enhancement of Au nanoparticle stability by TiO2 overcoat is very limited. Di use reffectance infrared Fourier transform spectroscopy (DRIFTS) of CO chemisorption and X-ray photoelectron spectroscopy measurements both con rmed the ALD overcoat on Au particles surface and suggested that the presence of TiO2 and Al2O3 ALD overcoat on Au nanoparticles does not considerably change the electronic properties of Au nanoparticles. The catalytic activities of the Al2O3 overcoated Au/TiO2 catalysts in CO oxidation increased as increasing calcination temperature, which suggests that the embed-ded Au nanoparticles become more accessible for catalytic function after high temperature treatment, consistent with our DRIFTS CO chemisorption results.
Key words: Au/TiO2     Sintering     Atomic layer deposition     Al2O3 overcoat     Nanoparticle stability    
I. INTRODUCTION

Since the first discovery that oxide supported gold nanoparticles could be catalytically active in CO oxidation, gold catalysts have drawn extensive attention in many reactions, including CO oxidation [1, 2], hydrocarbons oxidations [3], water-gas-shift (WGS) reaction [4], H$_2$O$_2$ synthesis [5], preferential CO oxidation in rich hydrogen [6] and selective epoxidation [7]. In these reactions, supported Au nanoparticle catalysts show remarkably high activity and selectivity [8, 9].

Nonetheless, the applications of Au catalysts in industrial processes are still very limited due to several issues [10, 11]. One of the most important limitations is sintering of gold nanoparticles under reaction conditions, which causes severe catalyst deactivation due to the decreased number of exposed Au atoms [12] and the much lower activity of Au nanoparticles larger than $\sim$5 nm due to the well-known strong size dependence of Au catalysts [3, 13]. For instance, Valden et al. observed that sintering of the Au nanoparticles occurs rapidly, once the Au/TiO$_2$ (110) catalyst was exposed to a mixture of CO and O$_2$ at room temperature [14]. Lu et al. reported that Ostwald riping of the Au nanoparticles on CeO$_2$(111) mainly proceeds along the step edges of the CeO$_2$(111) support in a mixture of CO and O$_2$ at much lower pressure ($\sim$1 ubar) at room temperature according to the scanning tunneling microscopy (STM) [15]. Luengnaruemitchai et al. observed a dramatic decrease of CO conversion from 60% to about 10% within 48 h over CeO$_2$ supported Au catalyst in the WGS reaction. In the spent catalyst, an increase of average size of Au nanoparticles from 4 nm to 5. 5 nm observed by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) was suggested to be the main reason for the deactivation [16].

In order to improve the thermal stability of Au/TiO$_2$ catalyst, two different strategies have been reported. One is to modify the support to strengthen the metal-support interaction between the Au particles and the support [17-19]. For instance, Yan and his co-workers achieved an ultra-stable Au catalyst up to 500 ℃ by modifying the TiO$_2$ surface with aluminum oxide via a surface sol-gel process [13]. Zhang et al. modified a FeO$_x$ support with hydroxyapatite (HAP, Ca$_{10}$(PO$_4$)$_6$(OH)$_2$), wherein Au nanoparticles were able to be stabilized by the OH$^-$ and PO$_4$$^{3-}$ group of HAP \newpage \hspace{-0. 3cm}for calcination at as high as 600 ℃ [20]. Another common method is to deposit an oxide protective layer onto Au nanoparticles. Encapsulation of metal nanoparticles with an oxide protecting layer using techniques such as chemical vapor deposition and sol-gel chemistry provides an effective way to improve the stability of metal nanoparticles [21-25]. However, the thick protective layer (usually tens of nanometers thickness) often results in a large decrease in catalytic activity due to mass transfer resistance. A method of atomically precise control over the thickness and composition of protective layers is highly desirable to preserve catalytic activity to the greatest extent while improving the stability. For instance, Zhu et al. reported an Au/TiO$_2$ based catalyst with enhanced thermal stability through post modification of Au by amorphous SiO$_2$ layer using sol-gel chemistry [26]. In this case, the thickness of oxide films is often lack of precise control which might cause a large decrease in catalytic activity due to mass transfer resistance. As a consequence, it could be very critical to achieve precise control over the oxide overcoat thickness to preserve catalytic activity to the greatest extent while improving the stability.

Atomic layer deposition (ALD), a thin film growth technique through self-limiting binary reactions between gaseous precursors and the substrate [27, 28], has shown great advantages in applying ultrathin and conformal coatings on supported metal catalysts to improve the thermal stability of metal catalysts [29-31]. Nonetheless, applying ALD oxide overcoat onto supported Au catalysts has been very limited. Ma et al. first explored to apply SiO$_2$ overcoat on Au/TiO$_2$ using ALD, therein, the stability of Au nanoparticles was remarkably improved by the thin SiO$_2$ overcoat, however, sintering of Au nanoparticles still happened once the sample was calcined at above 500 ℃ [32]. Therefore, SiO$_2$ overcoat is not a proper material to improve the thermal stability of supported Au particles. Biener et al. demonstrated that an Al$_2$O$_3$ film grown by ALD with even 1 nm thickness can stabilize the nanoporous bulk gold up to 1000 ℃, much more sufficient than an 2 nm thick TiO$_2$ overcoat grown by ALD which can stabilize the nanoporous bulk gold up to 600 ℃ [33]. We recently demonstrated that TiO$_2$ ALD overcoat on Au/Al$_2$O$_3$ catalyst significantly improved the CO oxidation activity, where we found that TiO$_2$ overcoat was preferentially grown at the low-coordination sites of Au nanoparticles [34]. However, applying TiO$_2$ and Al$_2$O$_3$ ALD overcoat onto supported Au catalysts for improving thermal stability of Au nanoparticles has not been reported.

In the present work, we applied different cycles of TiO$_2$ and Al$_2$O$_3$ overcoat on an Au/TiO$_2$ catalyst using ALD and compared the stability of the resulting TiO$_2$ and Al$_2$O$_3$ overcoated Au/TiO$_2$ catalysts after calcination at 500 and 600 ℃, respectively. We surprisingly found that sub-nanometer-thick Al$_2$O$_3$ overcoat can sufficiently inhibit the aggregation of Au particles during calcination at as high as 600 ℃. On the other hand, the enhancement of Au nanoparticle stability by TiO$_2$ overcoat is not obvious. Al$_2$O$_3$ overcoated Au/TiO$_2$ catalysts in CO oxidation reaction showed that the catalytic activity was almost completely lost, which suggests that the ultrathin Al$_2$O$_3$ ALD overcoat on Au nanoparticles is very dense, making the embedded Au nanoparticles almost inaccessible for catalytic function. Nonetheless, high temperature calcination could make the Al$_2$O$_3$ ALD overcoat become porous and activate the catalyst to a large extent.

II. Experiments A. Au/TiO$_2$ catalyst synthesis

An Au/TiO$_2$ catalyst was prepared using the deposition-precipitation (DP) method with urea [35, 36], Titania Degussa P25 (BET surface area $S_{\textrm{BET}}$=45 m$^2$/g, nonporous, 70% anatase and 30% rutile, purity>99. 5%) were used as the support. HAuCl$_4$$\cdot$4H$_2$O (Sinopharm Chemical Reagent Co, Ltd. , Au content of 47. 8%) was used as the Au precursor. Typically, 3 g of TiO$_2$ was added to 100 mL aqueous solution, which contained HAuCl$_4$ (4. 2$\times$10$^{-3}$ mol/L) and urea (0. 42 mol/L) at an initial pH of 2. The mixture was vigorously stirred at 75 ℃ for 8 h in the absence of light to avoid the gold precursor decomposition. Next, the suspension was centrifuged and thoroughly washed with deionized water for at least six times to remove chlorine residual. Finally, the obtained material was dried at 80 ℃ in air overnight and further calcined at 250 ℃ in 10% O$_2$ in He to obtain the catalyst. The gold contents in Au/TiO$_2$ catalyst were 2. 5% determined by an inductively coupled plasma atomic emission spectrometer (ICP-AES).

B. TiO$_2$ and Al$_2$O$_3$ ALD overcoating

TiO$_2$ ALD overcoating on Au/TiO$_2$ catalysts was performed in a viscous flow reactor (GEMSTAR-6TM Benchtop ALD, Arradiance) at 150 ℃ by alternatively exposing titanium isopropoxide (TTIP, 99. 7%, Sigma-Aldrich) and Millipore water for different ALD cycles. The TTIP precursor was heated to 80℃ to get a reasonable vapor pressure, while the water source was kept at room temperature [34]. The timing sequence of TiO$_2$ ALD was 10, 200, 8, and 250 s for TTIP exposure, N$_2$ purge, water exposure, and N$_2$ purge, respectively. The resulting catalysts are denoted as $x$cTi-Au/TiO$_2$ (the number of ALD cycles, $x$=20 and 50, and Ti was used to represent the TiO$_2$ overcoat). Al$_2$O$_3$ ALD coating on the Au/TiO$_2$ catalyst was performed by alternative exposures to trimethylaluminum (TMA, Sigma Aldrich, 99%) and Millipore water at 150 ℃ for different cycles [29, 30, 37], the TMA precursor was kept at room temperature. The timing sequence of Al$_2$O$_3$ ALD was 8, 200, 6, and 250 s for TMA exposure, N$_2$ purge, water exposure, and N$_2$ purge, respectively. The resulting catalysts are denoted as $x$cAl-Au/TiO$_2$ (the number of ALD cycles, $x$=2 and 4, and Al was used to represent Al$_2$O$_3$ overcoat). All the catalysts with and without overcoats were calcined at 500 and 600 ℃ in 10% O$_2$ in Ar for 1 h to examine the thermal stability of Au catalysts.

C. Characterization

TEM measurements were performed on a JEOL-2010 instrument operated at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were taken on a Thermo-VG Scientific Escalab 250 spectrometer equipped with an aluminum anode (Al K$\alpha$=1486. 6 eV). The binding energies in XP spectra were referenced to the C 1s binding energy at 284. 8 eV. These three facilities, as well as ICP-AES, belong to the Instruments' Center for Physical Science, University of Science & Technology of China.

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements of CO chemisorption were carried out on a Nicolet iS10 spectrometer, equipped with an MCT detector and a low temperature reaction chamber (Praying Mantis Harrick). After loading a sample to the reaction chamber, the sample was first calcined at a specified temperature in 10% O$_2$ in He for 30 min and then reduced in 10% H$_2$ in He for another 30 min. A background spectrum was collected after cooling the sample to room temperature in He. Subsequently, the sample was exposed to 10% CO in He at a flow rate of 20 mL/min for about 15 min until saturation. In order to avoid any variations caused by the decline of CO chemisorption peak during He purge [34], here we collected the DRIFT spectra of all samples during the flow of 10% CO in He. The CO chemisorption spectra were obtained by subtracting the CO chemisorption DRIFT spectrum of TiO$_2$ to remove the gas phase CO. All DRIFT spectra were collected with 256 scans at a resolution of 4 cm$^{-1}$.

D. Catalytic activity

The activities of Au/TiO$_2$ and $x$cAl-Au/TiO$_2$ catalysts in CO oxidation reaction were conducted using a fixed-bed tubular quartz reactor at atmospheric pressure. The inner diameter of the reactor was about 0. 8 cm. For the reaction test, 50 mg of uncoated Au/TiO$_2$ catalyst was diluted by 1 g of fine quartz chips (60/80 mesh) to avoid any hot spots. A thermal couple protected by a one-end closed quartz-tube was directly inserted into the catalyst bed to measure the sample temperature. For other $x$cAl-Au/TiO$_2$ catalysts, the amount of catalyst was adjusted to keep the same Au content with the uncoated Au/TiO$_2$ catalyst. After loading a catalyst to the quartz reactor, the catalyst was first calcined in 10% O$_2$ in He at a flow rate of 40 mL/min at a specified temperature for 1 h. Next, the reaction gas consisting of 2% CO and 8% O$_2$ balanced in He was fed to the reactor at a total flow rate of 50 mL/min. The reaction products were analyzed by an on-line gas chromatograph (Fuli, GC-9790II) with a thermal conductivity detector. The CO conversion was calculated based on the ratio of the consumed CO to the fed CO.

III. RESULTS AND DISCUSSION A. Thermal stability of $x$cTi-Au/TiO$_2$ and $x$cAl-Au/TiO$_2$ catalysts

An Au/TiO$_2$ catalyst with a particle size of 3±0. 4 nm was synthesized using the DP method [38], as shown in Fig. 1(a). The Au loading of the catalyst was determined to be 2. 5% using ICP-AES. Remarkable sintering by increasing the Au particle size to 6. 8±0. 4 and 10. 3±2. 1 nm was observed after calcining the Au/TiO$_2$ catalyst in 10% O$_2$ in Ar at 500 and 600 ℃ for 1 h, respectively (Fig. 1 (b) and (c)), consistent with previous observation [13]. Applying TiO$_2$ overcoating onto the Au/TiO$_2$ catalyst by ALD at 150 ℃ would not change the Au particle size, indicated by Fig. 1(d) [34]. The insert of Fig. 1(d) shows that the thickness of TiO$_2$ overcoat on 50cTi-Au/TiO$_2$ was about 1. 5 nm, which indicates the growth rate of TiO$_2$ ALD was about 0. 3 Å, consistent with reports in Ref. [34]. After calcining 10cTi-Au/TiO$_2$ (not shown here) and 50cTi-Au/TiO$_2$ in 10% O$_2$ in Ar at 500 and 600 ℃ for 1 h, significant sintering occurred, showing nearly identical Au particle sizes with the uncoated Au/TiO$_2$ catalyst after the same treatment (Fig. 1 (e) and (f)). Obviously, the TiO$_2$ overcoat did not considerably improve the stability of Au nanoparticles, which is different from the observation by Biener et al. , where TiO$_2$ ALD overcoat can stabilize nanoporous bulk Au up to 600 ℃. This might not be surprising, since Au nanoparticle is more unstable than bulk Au.

FIG. 1 TEM images of (a) uncoated Au/TiO$_2$ catalyst as-prepared, calcined at (b) 500 ℃ and (c) 600 ℃, respectively. TEM images of (d) 50cTi-Au/TiO$_2$ catalyst as-prepared, calcined at (e) 500 ℃ and (f) 600 ℃, respectively. The TiO$_2$ overcoat is shown in the inset of (d).

On the contrary, the stability of Au nanoparticles was largely improved by even two ALD cycles of Al$_2$O$_3$ overcoat (2cAl-Au/TiO$_2$), where calcination at 500 ℃ only slightly increased the Au particle size from 3±0. 4 nm to 4±0. 74 nm (Fig. 2 (a) and (b)). And very surprisingly, we found that four ALD cycles of Al$_2$O$_3$ overcoat completely inhibited the sintering of Au particles up to 600 ℃, where no changes in Au particle size were observed (Fig. 2 (d-f)). According to the growth rate reported in Refs. [27, 39], two and four cycles of Al$_2$O$_3$ ALD should produce approximately 0. 3 and 0. 5 nm thickness of amorphous alumina films respectively, even though we did not observe the Al$_2$O$_3$ overcoating layer on our samples under current resolutions.

FIG. 2 TEM images of (a) 2cAl-Au/TiO$_2$ catalyst as-prepared, calcined at (b) 500 ℃ and (c) 600 ℃, respectively. TEM images of (d) 4cAl-Au/TiO$_2$ catalyst as-prepared, calcined at (e) 500 ℃ and (f) 600 ℃, respectively.
B. DRIFTS of CO chemisorption

DRIFTS of CO chemisorption measurements were further carried out to investigate the accessibility of embedded Au nanoparticles under the TiO$_2$ and Al$_2$O$_3$ overcoat. As shown in Fig. 3, the uncoated Au/TiO$_2$ sample showed a strong CO peak at 2103 cm$^{-1}$, which is assigned to linear CO on low-coordination neutral Au sites [40-43]. On both 10cTi-Au/TiO$_2$ and 2cAl-Au/TiO$_2$ catalysts after calcination at 250 ℃, the intensity of CO peak both decreased dramatically, directly implying the presence of oxide overcoats on Au nanoparticles (Fig. 3). In both cases, the shifts of CO peak by overcoat were minor, thus the electronic properties of Au nanoparticles are not considerably altered by these overcoats. Meanwhile, we also noticed that increasing calcination temperatures could increase the intensity of CO chemisorption peaks, which suggests that high temperature treatment caused the change in the structures of TiO$_2$ and Al$_2$O$_3$ overcoats and allow the embedded Au nanoparticles more accessible. Such observation is consistent with the results of alumina overcoated Pd and Cu catalysts [29, 44].

FIG. 3 DRIFT spectra of CO chemisorption on (a) 10cTi-Au/TiO$_2$ and (b) 2cAl-Au/Al$_2$O$_3$ catalysts after calcination at different temperatures of 250, 350, and 450 ℃.
C. XPS studies

XPS was also carried out to study the electronic properties of Au nanoparticles after applying different cycles of ALD overcoat. As shown in Fig. 4, the Au 4f binding energy remained nearly constant at 83. 4 eV after either TiO$_2$ or Al$_2$O$_3$ ALD overcoat, implying that the ALD overcoat does not considerably change the electronic properties of Au nanoparticles, similar to the case of TiO$_2$ overcoated Au/Al$_2$O$_3$ catalysts [34] and others reported in literature [45, 46]. This results also explain the minor shifts of CO chemisorption peak on both 10cTi-Au/TiO$_2$ and 2cAl-Au/TiO$_2$ in Fig. 3. Moreover, the gradual decrease of the intensity of Au 4f binding energy with the increasing number of ALD cycles again confirms the successful deposition of TiO$_2$ and Al$_2$O$_3$ overcoat on Au nanoparticles.

FIG. 4 XPS spectra of (a) as-prepared $x$cTi-Au/TiO$_2$ and (b) $x$cAl-Au/TiO$_2$ in the Au 4f region.
D. Catalytic activity

Our results have clearly demonstrated that sub-nanometer Al$_2$O$_3$ ALD overcoat can remarkably increase the thermal stability of Au/TiO$_2$ catalyst up to 600 ℃. Here, the catalytic activity of 2cAl-Au/TiO$_2$ and 4cAl-Au/TiO$_2$ catalyst was further examined using CO oxidation as a probe reaction as shown in Fig. 5. Compared with the uncoated Au/TiO$_2$ catalyst, the activity largely decreased on 2cAl-Au/TiO$_2$ and became almost inactive on 4cAl-Au/TiO$_2$, which is likely due to the blockage of perimeter sites at the Au-TiO$_2$ interface, since the perimeter sites are suggested to be the active sites [47, 48]. Nearly no activity of 4cAl-Au/TiO$_2$ in CO oxidation also implies that $\sim$0. 5 nm thickness of Al$_2$O$_3$ ALD overcoat on Au nanoparticles is very dense, making the embedded Au nanoparticles almost inaccessible for catalytic function. This result is consistent with the observation of 1 nm thick Al$_2$O$_3$ overcoat on nanoporous gold by Biener et al. [33]. Nonetheless, calcining the catalysts at higher temperatures of 350, 500, and 600 ℃, respectively could make the Al$_2$O$_3$ ALD overcoat become porous and activate the catalyst to a large extent. It is worthy to note that the 4cAl-Au/TiO$_2$ catalyst calcined at 600 ℃ showed a very similar catalytic activity with an Au/Al$_2$O$_3$ catalyst (Au particle size: 2. 9±0. 45 nm) in our previous work [34].

FIG. 5 Catalytic activities of (a) 2cAl-Au/TiO$_2$ and (b) 4cAl-Au/TiO$_2$ catalysts in CO oxidation reaction after calcination at different temperatures of 250, 350, 500 and 600 ℃. The catalytic activity of uncoated Au/TiO$_2$ was shown in both (a) and (b) for comparison.
IV. CONCLUSION

In order to improve the stability of Au nanoparticles, TiO$_2$ and Al$_2$O$_3$ ALD were performed in this work to precisely deposit an oxide overcoating layer to an Au/TiO$_2$ catalyst. We found that the enhancement of Au nanoparticle stability by TiO$_2$ ALD overcoat (<1.5 nm thickness) is very limited below 600 ℃. On the contrary, sub-nanometer-thick Al$_2$O$_3$ overcoat can sufficiently inhibit the aggregation of Au particles up to 600 ℃ during calcination in oxygen. DRIFT of CO chemisorption and XPS measurements both confirmed the ALD overcoat on Au particles surface according to the attenuated intensity of CO peak and Au 4f peak, respectively, and suggested that the presence of TiO$_2$ and Al$_2$O$_3$ ALD overcoat on Au nanoparticles does not considerably change the electronic properties of Au nanoparticles. In CO oxidation, the nearly complete loss of catalytic activity of 4cAl-Au/TiO$_2$ implies that the $\sim$0. 5 nm thickness of Al$_2$O$_3$ ALD overcoat on Au nanoparticles is very dense, making the embedded Au nanoparticles almost inaccessible for catalytic function. Nonetheless, high temperature calcination could make the Al$_2$O$_3$ ALD overcoat become porous and activate the catalyst to a large extent. In summary, our work has demonstrated an effective method to stabilize supported Au catalyst using Al$_2$O$_3$ ALD overcoat, which might open new opportunities to apply the stabilized Au catalysts in other reactions under severe conditions.

V. ACKNOWLEDGMENTS

This work was supported by the One Thousand Young Talents Program under the Recruitment Program of Global Experts, the National Natural Science Foundation of China (No. 51402283 and No. 21473169), the Fundamental Research Funds for the Central Universities (No. WK2060030014, No. WK2060190026 and No. WK2060030017), and the Startup Funds from University of Science and Technology of China.

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亚纳米厚的氧化铝包裹层可以显著提高负载型金纳米颗粒催化剂的热稳定性
王春雷, 路军岭     
中国科学技术大学化学物理系, 合肥微尺度国家科学实验室, 能源材料化学协同创新中心, 中科院能量转换材料重点实验室, 合肥 230026
摘要: 负载型金纳米颗粒催化剂在许多催化反应中展现出非常好的催化活性,但是金纳米颗粒在高温等反应条件下容易烧结团聚,极大地限制了金催化剂的应用。利用原子层沉积技术在Au/TiO2催化剂表面分别精确沉积了一层超薄的二氧化钛和氧化铝包裹层,并对比研究了包裹层对金纳米颗粒的热稳定性影响。原位红外漫反射CO吸附和x-射线光电子能谱数据证实了氧化物包裹层的存在。发现亚纳米厚的氧化铝包裹层能够在600 C完全避免金纳米颗粒的团聚;相反,二氧化钛包裹层对金纳米颗粒稳定性的提高没有明显效果。通过CO氧化探针反应的活性测试,发现随着煅烧温度的升高氧化铝包裹的Au/TiO2 催化剂的活性逐渐提高,表明高温处理可以促进被包裹金原子的暴露并表现出催化活性。提供了提高金纳米颗粒稳定性的有效方法,为拓展金催化剂在条件苛刻的反应中的应用奠定了技术基础.
关键词: Au/TiO2     烧结     原子层沉积     氧化铝包裹     颗粒稳定性