Î±-Fe2O3-Hollow Catalysts: General Synthesis

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

CO Oxidation on Au/#-Fe2O3-hollow Catalysts: General Synthesis and Structural Dependence Liangpeng Zeng, Kongzhai Li, Hua Wang, He Yu, Xing Zhu, Yonggang Wei, Peihong Ning, Congzhi Shi, and Yongming Luo J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 23, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

CO Oxidation on Au/α-Fe2O3-hollow Catalysts: General Synthesis and Structural Dependence Liangpeng Zeng,a Kongzhai Li,a,b,* Hua Wang,a,b He Yu,a Xing Zhu,a,b Yonggang Wei,a,b Peihong Ning,a Congzhi Shi,b and Yongming Luoc a

State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, Yunnan, China

b

Faculty of Metallurgy and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, Yunnan, China

c

Faculty of environmental engineering, Kunming University of Science and Technology, Kunming 650093, Yunnan, China

*Corresponding author. E-mail: [email protected] or [email protected] (Kongzhai Li)

Abstract: Gold nanoparticles supported on iron oxides are highly active catalysts for the CO oxidation even at low temperatures, and their activity strongly depends on the nature of the Fe2O3 support. In the present work, we found a facile route to synthesize α-Fe2O3 nanoparticles with hollow architecture via a hydrothermal-thermal decomposition process without utilization of templates and surfactants. The key of this synthetic strategy is to induce the formation of hole-like surface defects on the β-FeOOH nanorod precursors with relatively small diameters (20–30 nm) and give a suitable temperature gradient to enhance the volume diffusion of such defects during heating. Finally, the hollow structure can be evolved from the growth of the surface defect clusters on the nanorods. For comparison, α-Fe2O3 nanoparticles with other typical morphologies (i.e., spindle, rod,

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and rod-hollow) were also prepared. Au nanoparticles (2–5 nm) were well dispersed on the different α-Fe2O3 supports via a colloid-deposition method. The prepared Au/α-Fe2O3-hollow catalyst exhibits superior catalytic performance for CO oxidation than others. This can be attributed to the relatively high exposure degree of surface defects in the unique embedded hollow structure, which could strongly improve the gold-support interaction. The investigations with the aid of XPS and in situ DRIFT suggested that the intimate gold-support interaction brings about more active surface species (i.e., –OH group and adsorbed oxygen) for CO oxidation.

1. Introduction The emissions of carbon monoxide (CO) from industrial activities and automobile exhaust are emerging as an important issue due to its harmful effects on the environment and human health. For the abatement of CO emissions, oxidizing CO to carbon dioxide (CO2) efficiently by capable catalyst is regarded as one of the most viable option.1 Searching for a highly active catalyst in catalytic oxidation of CO is the crucial issue. Among different types of catalysts, the supported gold (Au) nanoparticle (NP) catalysts have attracted the most attention due to the exceptional catalytic activity at low-temperature.2–4 The nature of supports is one of the key factors in determining the catalytic activity. It is generally accepted that reducible metal oxides (e.g., Fe2O3, CeO2, MnOx, and Co3O4) supported Au NPs can offer the additional active sites for CO oxidation in comparison to the irreducible oxide supports (e.g., SiO2, Al2O3, and MgO), which results in enhanced catalytic performance.5−8 As a result, the exploration focusing on various reducible oxides-supported gold catalysts has attracted increasing interests. Wang et al.9 compared the Au/MnO2, Au/Mn2O3 and Au/Mn3O4 catalysts for CO oxidation and found that the catalytic activity strongly depends on the

2


Page 2 of 57

Page 3 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nature of the manganese oxide support. Venezia et al.10 investigated the catalytic activity of Au/CeO2 catalysts prepared by three different methods (i.e., solvated metal atom dispersion, co-precipitation and deposition-precipitation). They proposed that the small size of gold particles is not the main requisite for a surprisingly high activity towards CO oxidation, but the surface oxygen reducibility of the ceria support should be the determining factor. Not coincidentally, similar conclusion was also reported by Arena et al.11 in the CO oxidation over Au/CeO2 catalysts, and they suggested that the catalytic activity is related to various parameters included enhanced reducibility of the active CeO2 phase. Liotta et al.12 conducted a comparative study on the activity for CO oxidation over Au/CeO2 and Au/Co3O4 catalysts. They observed an obvious support effect over different catalysts and suggested that the peculiar oxygen mobility of ceria resulted in higher activity over Au/CeO2 than over Au/Co3O4. Recently, Xie et al.13 reported a highly active nanocatalyst (xAu/3DOM Co3O4) for CO oxidation. In their study, it was concluded that the excellent catalytic activity of Au/3DOM Co3O4 was closely related to oxygen adspecies concentration, low-temperature reducibility, and the interaction between metal and support oxide. Hematite (α-Fe2O3) is one of the most important and widely available transition metal oxides in many catalytic systems due to its excellent stability, non-toxic, low cost and outstanding redox properties.14−16 α-Fe2O3 is also widely used as an active support to fabricated oxide-supported gold catalysts. Of particular interest is that various novel α-Fe2O3 nanostructures at the nanosize level can be fabricated by a wide variety of methods, which give rise to intriguing shape- and size-dependent properties in many application areas.17−24 On account of the different crystal structures, the Fe2O3 with tunable crystal-phase, size, and shape could be used to promote the performance of iron oxide-supported gold catalysts. Naturally, investigations on the Fe2O3 supported Au catalyst with

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

tunable morphology, size and composition are of great interest in this area. Park et al.25 investigated the activity for CO oxidation with different pretreatment conditions over Au/Fe2O3 catalyst. They pointed out that the particles size of gold and the fraction of oxidic trivalent gold (Au2O3) is critical for determining the catalytic activity. Almost at the same time, Tripathi et al.26 found that the enhanced catalytic activity of CO depends on the promotional effect of the defective structural sites at the interface of nanosized Au crystallites and Fe2O3 support. Guczi et al.27 showed that a well-developed interface of iron oxide and gold formed in Au/Fe2O3 catalyst can result in obvious changes in morphology and electron structure. It suggested that the enhanced activity of CO oxidation is attributed to the acceleration of CO adsorption caused by the electron charge flow from the O2− to the gold particles. Numbers of studies also suggest that the α-Fe2O3 with a novel structure or morphology as the support of Au catalysts plays an essential role in determining the catalytic activity. 28–30 The α-Fe2O3 with porous nanostructures supported Au catalyst was prepared and investigated by Liu et al..31 The results showed that a great number of pores distributed on unique α-Fe2O3 accommodated numerous Au NPs, providing augmented active sites to improve the activity of Au/α-Fe2O3 catalyst. Yin et al.28 fabricated Au@Fe2O3 core-shell structure catalysts with controlled core size (2.1 nm) and shell thicknesses for CO oxidation. For this type of catalyst, more active sites are formed on the interfacial structure between Au NPs and Fe2O3 support, resulting in much higher catalytic activity compared with the conventional colloidal-deposition-derived Au/α-Fe2O3 catalyst. An enhanced catalytic activity towards CO oxidation was also observed on the porous α-Fe2O3 nanorods supported Au catalysts by comparing with the commercial Au/α-Fe2O3 catalyst.32 More interestingly, a likeness of embedded structure was constructed via the insert of small Au particles

4


Page 4 of 57

Page 5 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


into the pores with a size range of 1–5 nm, which strongly enhanced the interaction between the Au particles and the porous α-Fe2O3 support. Larsen et al.16 synthesized active hybrid Au/Fe2O3 catalysts with various morphologis (e.g., spheres, rings and tubes), and they suggested that the performance of these catalysts was determined by the novel morphology and multifunctional nanostructures of Fe2O3 support. Furthermore, Zhang et al.33 obtained Au-functionalized α-Fe2O3 hybrid nanospindles. The Au NPs with an average size of ca. 3 nm were well anchored on the support, and the strong-metal support interaction was deemed to the main contribution for the high activity. Based on above-mentioned works, there is considerable scientific and technological value for optimizing the fine structure of α-Fe2O3 supports to improve the catalytic activity of Au/α-Fe2O3 catalysts and for elucidating the correlation between the support structure and the catalytic performance. In this work, a special nanohollow α-Fe2O3 was prepared, which, as a support of Au catalyst, was compared with some conversional α-Fe2O3 supports (i.e., nanorod, nanospindle and hollow-rod). To explore the structural susceptibility of hematite support in the catalytic CO oxidation, the physicochemical properties of different Au/α-Fe2O3 catalysts were investigated by diverse means. The catalytic mechanism was also discussed on base of the in situ tool. We find that the Au@α-Fe2O3-hollow catalyst showed superior catalytic activity for CO oxidation due to its unique structure.

2. Experimental 2.1. Preparation of the α-Fe2O3 crystals All of the chemical reagents used in the experiment were analytical grade without further purification, and the deionized water was used throughout the experiments. The iron oxides (α-Fe2O3) support with various shapes (i.e., nanorod, nanorod-hollow and hollow) were synthesized by a

5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

hydrothermal-thermal decomposition method. Typically, 4.33 g aqueous iron (III) chloride (FeCl3·6H2O) and 1.44 g urea (NH2CONH2) were dissolved in 80 mL deionized water, respectively, and the mixed solution stirred vigorously for at least 30 min under different water bath temperatures. After that, the resulting solution was sealed in a Teflon-lined autoclave and kept at 100 °C for 24 h in a temperature controlled oven. After cooling, the resulting precipitate was filtered and washed with deionized water and absolute ethanol several times, respectively, and then dried at 60 °C for 12 h. Finally, the α-Fe2O3 supports were obtained by calcining the dry precursor at 500 °C with varying heating rates for 2 h in air. In order to modify the shapes of the α-Fe2O3 support, the preparation parameters (i.e., water bath temperature, hydrothermal reaction temperature, and molar ratio of urea and FeCl3) were varied in a systematic fashion, as shown in Table 1. The α-Fe2O3 nanospindles were prepared with a similar procedure. Under the water bath temperature of 20 °C, an 80-mL portion of aqueous solution containing 5 mM NH4H2PO4 and 0.3 M NH2CONH2 was added into the 0.2 M FeCl3 at a constant rate. After further stirring for at least 30 min, the mixed solution was heated at 100 °C for 24 h. The resulting precipitate was filtered and washed several times with deionized water and absolute ethanol, and then dried at 60 °C for 12 h. Finally, the product was calcinated at 500 °C for 2 h. 2.2. Preparation of Au/α α-Fe2O3 catalysts The Au/α-Fe2O3 catalysts were prepared using a colloidal deposition method.33,34 A 0.2-g portion of already preformed α-Fe2O3 crystals with different shapes were suspended in 10 mL deionized water under stirring. Subsequently, 3 mL of 1 mM lysine aqueous solution and an appropriate volume of 1 mM HAuCl4·3H2O water solution were added to the suspension, followed by adjusting to pH at 5–6 with 1 mM sodium hydroxide (NaOH) solution. After further stirring for 30 min, 0.1 M fresh

6


Page 6 of 57

Page 7 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NaBH4 solution was quickly added to the resulting mixture with the retention of pH at 8–10 under ultrasonic processing. After further ultrasound treatment for about 10 min, the precipitates were recovered by centrifugation followed by washing with deionized water and absolute ethanol until free of Cl–, detected by the silver nitrate titration, and then dried at 60 °C overnight. Finally, the hematite-supported Au catalysts were obtained by calcination in air at 300 °C for 2 h. Table 1 Summary of synthetic parameters for the four kinds of α-Fe2O3 nanocrystals with various shapes.

Water bath

Hydrothermal

Hydrothermal

Calcination conditions

Molar ratio of

NH4H2PO4

temperature [°C]

temperature [°C]

time [h]

[°C/min, °C, h]

urea and FeCl3

concentration [mM]

Spindle

20

120

24

5 °C/min, 500 °C for 2 h

1.5

5

Hollow

20

100

24

1~2 °C/min, 400/500 °C for 2 h;

1.5

/

Rod

20

100

24

5 °C/min, 500 °C for 2 h

1.0

/

0

80

24

5 °C/min, 500 °C for 2 h

1.5

/

Sample

Hollow-rod

2.3. Characterizations The powder X-ray diffraction (XRD) data of the prepared samples were recorded on a Rigaku D/max-ШB X-ray diffractometer at a scanning rate of 2 °/min with 2θ ranging from 10 to 90° (Cu Kα radiation λ = 0.15406 nm). The microstructures and morphologies of the prepared samples were investigated by a JEOL JEM-2100 (UHR) transmission electron microscope (TEM) instruments using a LaB6 filament operating at 200 kV and a Tecnai G2 TF30 S-Twin field emission transmission electron microscopy (FETEM, produced by FEI Co., Netherlands) instruments operating at 300 kV. The actual gold loading of various α-Fe2O3-supported Au catalysts was determined by the

7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

inductively coupled plasma atomic emission spectrometer (ICP-AES) on an IRIS Intrepid ER/S instrument (Thermo Electron Corporation). The nitrogen adsorption-desorption isotherms was performed at −196 °C using a Quantachrome Autosorb-iQ physisorption instrument. Specific surface area of as-synthesized α-Fe2O3 support and Au/α-Fe2O3 was calculated according to the Brunauer-Emmett-Teller (BET) method by the N2 adsorption isotherm. The pore size distributions and average pore diameter were calculated based on the Barrett-Joyner-Halenda (BJH) method by using the desorption branch. X-ray photoelectron spectroscopy (XPS) studies were carried out on a PHI–5500 system equipped with a monochromatic Mg Kα (1253.6 eV) X-rays source. Spectra were registered at ambient temperature in vacuum (residual pressure < 10–7 Pa). Sample charging during the measurement was compensated by an electron flood gun. The standard deviation in the binding energies values of the XPS line is 0.10 eV. Binding energies (BE) were determined by computer fitting of the measured spectra and calibrated using the C 1s peak (284.8 eV) as a reference. The surface composition and chemical state were evaluated based on the areas and binding energies of the Fe 2p, O 1s and Au 4f photoelectron peaks. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis was taken on a FTIR spectrometer (vertex 70, Bruker, Germany) to evaluate the adsorbed species on the catalyst under reaction conditions. An FTIR absorbance spectrum at 4 cm−1 resolution in the range of 4000 to 1000 cm−1 was recorded by the FTIR spectrometer equipped with a liquid N2 cooled Mercury-Cadmium-Telluride (MCT) detector. Catalysts were placed in a Pike Technologies HC-900 DRIFTS cell (nominal cell volume of 6 cm3) equipped with SeZn windows. Prior to the in situ DRIFTS experiments, the sample was pretreated in the DRIFTS cell in flowing 10%O2/Ar (25

8


Page 8 of 57

Page 9 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mL/min) at 300 °C for 1 h and then cooled to RT before switching to Ar. 2.4. CO oxidation test The catalytic activity of CO oxidation over different Au/α-Fe2O3 catalysts was performed in a fixed-bed quartz tubular reactor at atmospheric pressure under steady-state conditions. Before the testing, the samples (100 mg, 40–60 mesh) were pretreated in 10%O2/Ar flow (50 mL/min) at 300 °C for 30 min. After being cooled to 20 °C in O2/Ar atmosphere, the argon (99.999% purity) was switched to purge for 30 min. Then, the reactant gases (1% CO and 10% O2 balanced with argon) were passed continuously through the catalyst bed at a flow rate of 50 mL/min (giving a space velocity (SV) of 30000 mL/(gcat.·h)). The reactor was heated through a temperatureprogrammed route with a heating rate of 5 °C/min. The amounts of CO, CO2 and O2 in the outlet streams were measured by a gas chromatograph (Agilent 7890A GC System, produced by Agilent Co.) equipped with HP-Plot 5A and HP-Plot-Q column. The kinetic experiment was conducted to investigate the intrinsic activity of the catalysts, which was defined as a reasonable correlation with the active site density of catalysts. The reaction rate of CO oxidation was determined by an isothermal reaction at 60 °C in the kinetic regime, which the influences of internal and external mass and heat transfer were all excluded. The reaction gas composition and total flow rate were the same as the aforesaid CO oxidation reaction. The CO conversion (x) was calculated from the changes in CO concentration between the inlet gases and outlet gases (Eq.(1)), where [CO]in and [CO]out were the CO concentration of inlet and outlet streams, respectively. For better comparison on the catalytic activity, the parameters of reaction rate (r, mol/(g·s)) and turnover frequency (TOF, s−1) were defined and calculated at a specified temperature of 40 °C according to the equations (2) and (3).35,36 Where C0 (mol/s) is the

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

initial CO concentration per second, xCO is the CO conversion at 40 °C, FCO is the flow rate of CO in mol·s−1, mcat. is the amount of Au/α-Fe2O3 catalyst, XAu is the actual Au loading in the catalyst, DAu is the dispersion of Au estimated by chemical adsorption method, and MAu is the molar weight of Au (196.967 g·mol−1).

x=

[CO]in − [CO]out ×100% [CO]in

(1)

r=

xCO × c0 mcat.

(2)

TOF =

xౙ౥ Fౙ౥ MAu

(3)

mcat. XAu DAu

3. Results 3.1. Synthesis of α-Fe2O3 supports TEM images provide initially insight into the microstructure and morphology of the products obtained under specified conditions, and XRD is used to verify their crystal phases. Figure1 shows the TEM images and XRD patterns of the hydrothermal synthesized precursor and further calcinated material. The precursor, which is synthesized at the water bath temperature of 20 °C and the hydrothermal temperature of 100 °C exhibits a rod-like morphology with a width of 80–200 nm and a length of 0.9–1.05 µm (aspect ratio ≈ 11, see Figure 1a and b). The XRD pattern (see Figure 1f red line) can unambiguously be assigned to pure tetragonal β-FeOOH with good crystallization (JCPDS No. 34-1266). The TEM images in Figure 1c–e show the nanoparticles after calcinations at 500 °C in air with a heating rate of 5 °C/min, which maintains the rod shape of β-FeOOH precursor. But numbers of defects (resembled holes) are clearly observed on the surface of nanorods. Such hole-like defects

10


Page 10 of 57

Page 11 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


formed in the structure of α-Fe2O3 nanorods after calcination treatment on FeOOH nanorods were also reported in the literatures.37,38 The XRD pattern in Figure 1f (blue line) demonstrates that the nanoparticles after calcinations agrees well with the hematite phase (JCPDS No. 33-0644). The sharp and strong peaks indicate that the α-Fe2O3 particles are highly crystalline. It is clear that the calcination treatment results in a complete transformation of β-FeOOH to α-Fe2O3 with the formation of defective structure.

Figure 1. TEM images and XRD patterns of the hydrothermal synthesized β-FeOOH precursor and the thermal heated α-Fe2O3 rods: TEM images of (a–b) β-FeOOH precursor, (c–e) α-Fe2O3 product after calcination at 500 °C with 5 °C/min, and (f) XRD patterns of the precursor (red line) and the heated product (blue line). The rod-like β-FeOOH precursor with smaller diameters (20–25 nm) was obtained under the nearly same synthetic procedure with a slightly lower hydrothermal temperature (80 °C) and higher molar ratio of urea and FeCl3 (1.5). As observed in Figure 2a and b, the as-prepared precursor mainly composed of uniform sized nanorods with average diameter of ~25 nm and length of ~350 nm (aspect ratio ≈ 14) with serious agglomeration. After the thermolysis treatment of 500 °C in air with a 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

heating rate 5 °C/min, the size of the sample remained primitively, but its surface morphology and crystal phase changed greatly. As can be seen in Figure 2d and e, the rods grew into nanohollow-rods structure. Apparently, the XRD pattern shown in Figure 2f (blue line) reveal that α-Fe2O3 as a single phase with very fine crystallite is detected after calcination. These results suggest the formation of α-Fe2O3 with a hollow-rod structure.

Figure 2. TEM images and XRD patterns of the β-FeOOH precursor and the prepared α-Fe2O3 hollow-rods: TEM images of (a–b) β-FeOOH precursor, (c–e) α-Fe2O3 hollow-rods after calcinations at 500 °C with 5 °C/min, and (f) XRD patterns of the precursor (red line) and the as-prepared product (blue line). Figure 3 shows the third type of β-FeOOH precursor. As observed in Figure3 a and b, the β-FeOOH also shows a nanorod structure with a

width of approximate 20–30 nm and a length

100–500 nm, which is similar with the as-obtained β-FeOOH nanorods displayed in Fig 2a and b. The main difference between the two precursors is that the agglomeration of the present nanorods is reduced. After calcination treatment, a completely different morphology with interconnected hollows are formed, as shown in Figure 3c–g. When the heating rate of calcination treatments is as high as

12


Page 12 of 57

Page 13 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5 °C/min, the α-Fe2O3 product exhibits irregular-nanostructure with some immature hollows and partial hollow-like rods (see Figure 3c). Because of the rapid temperature rising, the α-Fe2O3 crystals remain the same defects on the surface of β-FeOOH nanorods precursor to form the hollow-like rods, and other defects further grow to immature hollows. Presumably, these hollow-like rods can be considered as the forerunner of nanohollows. As the heating rate decreases to 2 °C/min (see Figure 3d), the hollow-like structure is widely formed in the α-Fe2O3 crystals, indicating the growth and evolution of particles under thermal treatment. Driven by the growth tendency of the hollows structure, the heating rate of calcination treatments is further reduced. With a decrease of the heating rate to 1 °C/min, more regular and mature hollow structure with clear inside volume forms and cross-links to each other. A highly uniform hollow structure with enlarged sizes (20–30 nm) can be clearly observed from the images of the product prepared with a low heating rate (1 °C/min for 500 °C, see Figure 3f and g). Interestingly, Figure 3e shows the TEM images of a typical nanohollows α-Fe2O3 crystals with the inner diameters of 15–20 nm, which was calcined at 400 °C with heating rate of 1 °C/min. The above observations indicate that the calcination conditions, especially the heating rate, play a very critical role in the fabrication of nanohollow structure. The representative N2 adsorption-desorption isotherms and pore-size distributions of the hollow-rods and hollows samples obtained after calcination treatments are provided in Figure S1, and their textural properties are shown in Table S1 (see the Supplementary Information). Pore size distributions and specific surface area were calculated using the Barrett-Joyner-Halenda (BJH) formula and BET equation, respectively. The specific surface area for the both samples are very low (10.9 and 9.30 m2/g for the hollows and hollow-rods samples, respectively). This suggests that there is no mesoporous structure in the hollows-structured samples. It seems that the hollows sample is

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

formed via the interlacement of Fe2O3 filaments, without formation of long holes.

Figure 3. TEM images and XRD patterns of the β-FeOOH precursor and the prepared α-Fe2O3 hollows: TEM images of (a–b) β-FeOOH precursor, α-Fe2O3 hollows after calcinations (c) at 500 °C with 5 °C/min, (d) at 500 °C with 2 °C/min, (e) at 400 °C with 1 °C/min, (f and g) at 500 °C with 1 °C/min, and (f) XRD patterns of the precursor (red line) and the as-prepared product (blue line). Based on the above interesting findings, the synthetic strategy deserves some attentions. In the present work, urea is used as the precipitating agent in the hydrothermal synthesis reaction. In a typical synthesis process of rod-like precursor, iron chloride salt (FeCl3) initially dissociates to yield Fe3+, and then the urea plays a pivotal role in precipitation of Fe3+ to the iron oxyhydroxide (FeOOH) via a dissolution/recrystallization mechanism. The hydrolysis rate of urea strongly affects the nucleation and growth of FeOOH particles. An appropriate high water bath temperature is beneficial to the breakage of various reactants and the coordination-assisted of ion in the solution. The crystal growth and nucleation-assembly recrystallization is occurred in the hydrothermal process. With the constant collision and combination of protons, the crystals are obtained by the original nucleus and extend further outward. Therefore, a mechanism for the formation of β-FeOOH nanorods was

14


Page 14 of 57

Page 15 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


proposed as the following. In the beginning, the hydrolysis of a transparent FeCl3 aqueous solution and urea yields Fe3+ and OH−, which are followed by the formation of primary β-FeOOH crystal nucleus. Subsequently, driven by the rise of temperature of the reaction solution system, the β-FeOOH crystal particles constantly aggregate along the major axis direction and grew up to rod-shaped nanoparticles. However, different particle matching rates also result in slightly difference of assembly. Because the hydrolysis is an endothermic reaction, the amount of β-FeOOH crystal nucleus formed instantaneously increases with the temperature rising, leading to the smaller crystal particles. Comparing with these TEM images of the nanorods prepared under various reaction conditions, there are some significant difference on the diameter, length and dispersibility of the as-prepared nanorods. Wei et al.39 also reported that the growth rate of β-FeOOH nanorods is strongly related to the concentration of urea and the reaction temperature.

Figure 4. TEM images and XRD patterns of the hydrothermal α-Fe2O3 precursor and the calcinated α-Fe2O3 spindles: TEM images of precursor (a–b) and α-Fe2O3 spindles after calcinations (c–e), and XRD patterns of α-Fe2O3 precursor (red line) and calcinated α-Fe2O3 spindles (blue line). The further thermal decomposition of rod-shaped β-FeOOH particles results in various 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

morphological α-Fe2O3 products. In the case of heating in air atmosphere, with the decomposition transformation of β-FeOOH to α-Fe2O3 phase, the water molecules and specific adsorption of carbonate groups remove from β-FeOOH nanorods. As a consequence, the mechanism for the formation of hole-like defects on the surface may be attributed to the generation of a gas (vapour) cavity in the calcination process. Thereafter, it becomes the nucleation center of the single crystal growth or polycrystalline gather heterogeneous and promotes the formation of final α-Fe2O3 products. The heating rate of thermal treatments plays a vital role in changes of microstructure and morphology, because it strongly affects the diffusion of the defects. The relatively lower heating rate would give enough time for the diffusion of defects, which is beneficial for the formation of hollow-rod or hollow morphology. α-Fe2O3 nanospindles could be synthesized at 120 °C for 24 h with the presence of additional NH4H2PO4 reactant. Figure 4 shows TEM images of the as-synthesized spindles-like α-Fe2O3. NH4H2PO4 is added as a surfactant to mediate the nucleation of original crystal and anisotropic growth, owing to its polybasic feature. In general, needle-like β-FeOOH primary nanoparticles are initially nucleated by the forced hydrolysis of iron (III) chloride (FeCl3) and urea solutions. With increasing the temperature of solution system, the dissolution of the β-FeOOH phase occurred according to Ostwald’s rule of stages, which leads to the release of Fe3+ anions back into solution to supply the growth of α-Fe2O3 nanoparticles.40 A nucleation-aggregation-dissolution mechanism is demonstrated in the case of the monodisperse hematite nanocrystals formed by iron (III) chloride in the phosphate ions reaction system.21 Because of the specific adsorption of phosphate anions on α-Fe2O3, the aggregation and growth of the crystal seeds along the longitudinal axis direction is further developed, but the growth of horizontal axis direction is inhibited. In addition, the crystal

16


Page 16 of 57

Page 17 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


faces of the α-Fe2O3 crystals grew at different rates arises from the extent of phosphate adsorption on hematite due to its intrinsic crystal structure, which results in the produce of final α-Fe2O3 spindles. As seen in Figure 4 (TEM and XRD observations), the spindles-like α-Fe2O3 nanoparticles firsthand with very uniform size were directly obtained after the hydrothermal process, without the formation of FeOOH as precursor. Furthermore, no obvious changes on the morphology was observed over the sample after calcination treatment at 500 °C for 2 h. Specifically, the XRD pattern of the calcinated oxide is featured with stronger and sharper diffraction peaks, which indicates a more excellent crystallinity than the hydrothermal precursor oxide. 3.2. Characterizations of Au/α α-Fe2O3 catalysts After the deposition of gold, the Au-containing samples with α-Fe2O3 support morphologies of spindles, rods, hollow-rods and hollows could be clearly identified by the TEM technology (see Figure 5). The Au particle size distributions established from the measurement of 200 Au particles for each sample based on the TEM images are also presented in Figure 5. For all of the four catalysts, the α-Fe2O3 supports still perfectly retained their original morphologies after the Au NPs deposition. In the high-magnification TEM images (Figures. 5-a3, b3, c3, and d3), highly dispersed gold NPs can be obviously observed on the surface of support oxides. In particular, it should be noted that some of the gold particles have embedded into the α-Fe2O3-hollows nanostructure (Figure 5c), and a structure of nanorods containing hole-like defects is still observed in the Au/α-Fe2O3-hollow-rods sample.

Indeed,

it

is

a

distinctive

structural

feature

of

Au/α-Fe2O3-hollows

and

Au/α-Fe2O3-hollow-rods, which is different from Au/α-Fe2O3-spindles and Au/α-Fe2O3-rods. From the size distributions of Au NPs shown in the insets of Figure 5, one can see that the average Au particle size towards Au/α-Fe2O3-spindles was 4.1 nm with a size range of 1–6 nm (see inset in

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 57

Figure 5-a3) and the average size of Au NPs on the Au/α-Fe2O3-rods, Au/α-Fe2O3-hollow-rods and Au/α-Fe2O3-hollows is 3.7, 4.0, and 3.4 nm, respectively. The average Au particle size of each catalyst is well within the classical size range of catalytically active gold in the oxide supported gold system, which is in the optimum size range of 3–5 nm to obtain high catalytic activity of CO oxidation.2,3,41 The lattice fringes in a typical HRTEM image of α-Fe2O3-hollow support (Figure 4-c1) are separated by 0.25 nm, which agrees well with the (110) crystal plane of hematite.

Figure 5. TEM, STEM, and HRTEM images and Au particle size distributions of the as-prepared Au/α-Fe2O3

catalysts:

(a1–4)

Au/α-Fe2O3-spindles,

(b1–4)

Au/α-Fe2O3-rods,

(c1–4)

Au/α-Fe2O3-hollow-rods, (d1–4) Au/α-Fe2O3-hollows. The real gold loadings of the four different Au/α-Fe2O3 catalysts as determined by ICP-AES 18


Page 19 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


technique are at ca. 2.0%, as shown in Table 2. Figure S1 shows the XRD patterns of the as-obtained α-Fe2O3-supported Au catalysts. Compared with the corresponding XRD pattern of the parent α-Fe2O3 support, the deposition of Au did not give rise to much change in the crystal structure of α-Fe2O3 supports. The crystal structure of the four α-Fe2O3-supported Au catalysts could be perfectly indexed to α-Fe2O3 phase, while no significant diffraction peaks of Au or Au oxides were observed. The absence of diffraction peaks of Au or Au oxides in the XRD measurement should be attributed to the low content (only ca. 2 wt% determined by ICP-AES) and/or small particle sizes (1–6 nm) of the highly dispersed gold nanoparticles. In addition, the crystallite size of α-Fe2O3 in each sample was calculated according to the Scherrer’s formula using the FWHM of the {104} and {110} lines of α-Fe2O3, as shown in Table 2. All the samples show similar particle size of α-Fe2O3 support. The specific surface area and pore size distribution of the Au-containing samples were determined by nitrogen adsorption-desorption isotherms, which was summarized and shown in Table S2 and Figure S3 (Supplementary Information), respectively. The isotherms for the Au/α-Fe2O3-spindles sample can be classified as type-II of BET's classification with a type H3 hysteresis loop in the relative pressure (p/p0) range of 0.8–1.0, characteristic of particulate materials. For the other three samples, a small hysteresis loop in the p/p0 range of 0.4–1.0 was observed, which is an indication of the presence of mesopores with limited content. The specific surface areas are 41.0 m2/g for Au/α-Fe2O3-hollows, 36.0 m2/g for Au/α-Fe2O3-hollow-rods, 29.8 m2/g for Au/α-Fe2O3-rods, and 23.0 m2/g for Au/α-Fe2O3-spindles, showing a close connection to the intrinsic structure characteristics of materials. It is interesting that the specific surface area of the catalysts with hollow-rod or hollow structure is much higher than the as-synthesized α-Fe2O3 support (which is only ca. 10.0 m2/g, as shown in Table S1).

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

For the nitrogen adsorption-desorption isotherms shown in Figures S2 and S3, a hysteresis loop in the p/p0 range of 0.4–1.0 was observed over the Au/α-Fe2O3-hollows catalyst, but it is absence for the bare α-Fe2O3-hollows support. This indicates the presence of mesoporous structure after Au loading. In the deposition-themolysis process, the nucleation and further growth of the starting α-Fe2O3 precursor may take place with colloid gold deposition under the aqueous environment. It is also possible that there are some blind holes in the supports, which may be opened during the colloid-deposition process. These phenomena could result in an increase in the specific surface area. On the other hand, the contribution of gold localization on the prerogative α-Fe2O3 is also suggested as the reason for the increased specific surface area. According to the data from both the TEM and specific surface area measurements, it is found that the changes of specific surface area over the four Au/α-Fe2O3 catalysts is in accordance with the order of the average size of Au particles. In spite of the Au content is only ca. 2 wt %, the average particle size of gold on the α-Fe2O3-hollows is as small as 3.4 nm. This would result in abundant interface between the Au particles and Fe2O3 support. The gold is not always deposited in assumptive inerratic spherical forms, which would lead to the formation of slit pores. In summary, the increased specific surface area after loading of Au can be attributed to the possible structure evolution of the Fe2O3 support and the formation of attached structure owning from the dispersion of Au particles on the Fe2O3 support. The surface element compositions, metal oxidation states, and oxygen species of various Au/α-Fe2O3 catalysts were investigated by XPS technique. As shown in Figure 6A, two distinct peaks at binding energies of 710.8 eV for Fe 2p3/2 and 724.8 eV for Fe 2p1/2 with a shake-up satellite at 717.5 eV are observed in the Fe 2p XPS spectrum of each Au/α-Fe2O3 sample, suggesting a contribution of Fe species with varying electronic states.24,33,42 After the Fe 2p curve fitting, the

20


Page 20 of 57

Page 21 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


spectrum of Fe 2p3/2 consists of a pair of distinct peaks at ca. 710.4 (labelled as U′) and 712.9 eV (labelled as U''), which are assignable to the surface Fe2+ and Fe3+ species, respectively.43,35 The weak shake-up satellite peak detected at BE = 717.5 eV (labelled as satellite) is also characteristic of Fe2+ in Fe2O3.44 The Fe 2p1/2 XPS peak is perfectly fitted to the high-BE component at the same position (BE = 724.7 eV, labelled as V''), which could be attributed to Fe3+.33 A quantitative analysis of the fitted peak shows that the Fe3+/Fe2+ molar ratio ((V''+U'')/U′) of each sample follows the order of Au/α-Fe2O3-hollows < Au/α-Fe2O3-hollow-rods < Au/α-Fe2O3-spindles < Au/α-Fe2O3-rods (see Table 2). Table 2 Average crystallite sizes (Dα-Fe2O3), average Au particle sizes, real Au contents, and surface element compositions of the Au NPs samples with different supports. Dα-Fe2O3a

Au particle size b

Au content c

Dispersion

OII/OI

Fe3+/Fe2+

Auδ+/Au0

(nm)

(nm)

(wt%)

of gold (%)

molar ratio

molar ratio

molar ratio

Au/α-Fe2O3-spindles

68

4.10

2.04

28.60

0.23

1.41

0.16

Au/α-Fe2O3-rods

66

3.70

2.01

31.67

0.20

1.44

0.17

Au/α-Fe2O3-hollows

62

3.40

2.05

34.47

0.50

1.21

0.23

Au/α-Fe2O3-hollow-rods

67

4.00

2.02

29.30

0.39

1.25

0.20

Sample

a

The values determined according to the Scherrer equation using the FWHM of the {104}and {110}line of

α-Fe2O3. b Data estimated according to the TEM images, and the standard deviation in particle size measurement is ±0.5 nm. c Data determined by the ICP−AES technique.

For all kinds of samples, the O 1s XPS spectrum (see Figure 6B) could be decomposed into three fitted components centered at binding energies of ca. 529.5 eV (denoted as OI), 531.5 eV (denoted as OII), and 533.4 eV (denoted as OIII), respectively. As generally suggested in the literatures,14,42,35,45

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the OI component at the low binding energy is characteristic of the lattice oxygen (O2-) in iron(III) oxide, and the OII component is assigned to the oxygen species adsorbed in the regions of oxygen vacancies. The OIII component can be attributed to –OH group stretching of the adsorbed water species. It is well known that the oxygen species (e.g., adsorbed oxygen) on the surface of catalysts play an important role in the catalytic oxidation reactions. The molar ratio of surface oxygen and lattice oxygen (OII/OI) is evaluated based on the intensity of corresponding components in the O 1s XPS peak. The Au/α-Fe2O3-hollows sample emerges the highest OII/OI molar ratio (0.50, see Table 2) among the four samples, suggesting abundant active oxygen on this catalyst. 5,18,32

Figure 6. Fe 2p (A), O 1s (B), and Au 4f (C) XPS spectra of (a) Au/α-Fe2O3-spindles, (b) Au/α-Fe2O3-hollows, (c) Au/α-Fe2O3-hollow-rods, (d) Au/α-Fe2O3-rods. Figure 6C displayed the fitted spectra of Au 4f core-level. For the Au 4f7/2 spectrum, the first peak at bind energy of 83.7 is typical of surface metallic Au0 species, and the second peak with a binding energy shift of ca. 0.7 eV is associated with the existence of oxidized surface Au species such as Au+ or Au3+(represented by Auδ+ species).32,46–48 The same components are also analyzed in the Au 4f5/2 contributions, which the peak at low-BE position is assigned to metallic Au0 species and the high-BE

22


Page 22 of 57

Page 23 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


peak is attributed to Auδ+ species.48 As the results reported in previous studies,49 the relatively higher Auδ+/Au0 molar ratio towards the Au/α-Fe2O3-hollows catalyst (shown in Table 2) indicates a stronger interaction between Au NPs and α-Fe2O3 support. This deduction can be confirmed by the above analysis on the oxygen species, because the generation of abundent surface oxygen species may be owing to the changes of metal valence resulting from the metal-support interaction. It is worth mentioning that a strong metal-support interaction is general considered to be an important cause resulting in the highly active toward CO oxidation.33,45,50 3.3. Catalytic activity for CO oxidation The catalytic performances of the Au/α-Fe2O3 catalysts for CO oxidation at a space velocity (SV) of 30000 mL/(gcat.·h) are shown in Figure 7A, where the CO conversion is presented as a function of temperature (light-off test). Further details on the evaluation of the catalytic activities were also performed, as shown in Table 3. From the CO conversion curves, one can see that the activity of Au/α-Fe2O3-hollows is markedly higher than other samples under the tested conditions. Specifically, among the four samples, the Au/α-Fe2O3-hollow sample was first to reach the 10% conversion at around 36 °C. The CO conversion of 50% (T50) for the Au/α-Fe2O3-hollow sample is 62 °C, which is 36 °C lower than that for the Au/α-Fe2O3-rods sample. Furthermore, the complete oxidation of CO over Au/α-Fe2O3-hollows can be achieved at 80 °C, while it delays to 140 °C for the Au/α-Fe2O3-spindles sample. Among the four samples, the catalytic activity follows the sequence of Au/α-Fe2O3-hollows > Au/α-Fe2O3-hollow-rods > Au/α-Fe2O3- spindles > Au/α-Fe2O3-rods. The normalized reaction rate (r) and turnover frequency (TOF) for CO oxidation presented in Table 3 were obtained with the CO conversion lower than 15%. To minimize the effect of diffusion and/or mass-transfer limitation, the reaction temperature at 40 °C was identified as the optimized

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

conditions. In the Au/α-Fe2O3 catalyst system, Au nanoparticles are well accepted to be most important active sites to participate in the CO oxidation. Therefore, the calculation of TOF was performed based on the dispersion of Au NPs in each catalyst for a better comparison of catalytic activity. The CO reaction rate over Au/α-Fe2O3-hollows is 5.21 ×10-7 mol g–1 s–1, which is 2.4 times higher that for the Au/α-Fe2O3-spindles (2.23×10-7 mol g-1 s-1). The reaction rate is also much higher than that towards the Au/α-Fe2O3-rods (3.72×10-7 mol g-1 s-1) and Au/α-Fe2O3-hollow-rods (3.34× 10-7 mol g-1 s-1) samples. For the TOF, the values among different samples decrease in the order of Au/α-Fe2O3-hollows (1.45 s–1) > Au/α-Fe2O3-rods (1.15 s–1) > Au/α-Fe2O3-hollow-rods (1.11 s–1) > Au/α-Fe2O3-spindles (0.76 s–1). Based on the above results, it is reasonable to propose that the hollow structure of α-Fe2O3 is beneficial for enhancing the activity of Au/α-Fe2O3 catalyst.

Figure 7. (A) CO conversion as a function of reaction temperature and (B) Arrhenius plots for CO oxidation over various Au/α-Fe2O3 catalysts. Reaction condition: CO concentration = 1 vol%, CO/O2 molar ratio = 1/10, SV = 30000 mL/(gcat.·h). It is generally acknowledged that the CO oxidation in the presence of excessive oxygen (CO/O2 volume ratio = 1/10) over the Au/α-Fe2O3 catalysts could be reasonably considered to obey a first-order reaction mechanism in regard to CO concentration.35 Hence, the apparent activation 24


Page 24 of 57

Page 25 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


energy can be calculated according to relational expression of r = –kc = [–A exp(–Ea/RT)]c, where r, k, A, c, and Ea are the reaction rate (mol/(g⋅s)), rate constant (s–1), pre-exponential factor (s–1), CO concentration, and apparent activation energy (kJ/mol), respectively. The Arrhenius plots of the reaction rate against the reciprocal of the temperature for CO oxidation over each catalyst are shown in Figure 7B. Based on the data, the corresponding apparent activation energy (Ea) is obtained and summarized in Table 3. Table 3 Catalytic activities, TOF, reaction rate (r) activation energies (Ea), and the amount of reactive oxygen of different samples. T10

T50

T90

TOFAu a

rb

Ea

Amount of reactive

(°C)

(°C)

(°C )

(s–1)

(×10–7 mol g–1 s–1)

(kJ/mol)

oxygen c (µmol)


41

77

120

0.76

2.23

77.57

0.76

Au/α-Fe2O3-rods

46

98

136

1.15

3.72

65.86

0.90

Au/α-Fe2O3-hollows

36

62

75

1.45

5.21

51.65

1.19


48

82

95

1.11

3.34

53.26

1.03

Samples

a

The TOF (converted CO per molar amount of Au/α-Fe2O3 catalysts per second) values was evaluated based on the

dispersion of Au particles at low conversions under a kinetically controlled regime for CO oxidation. b The reaction rate values was estimated at 40 °C for CO oxidation. c The amount of reactive oxygen was calculated according to the sequential pulses of CO and O2 test.

One can see that the apparent activation energy increases in the sequence of Au/α-Fe2O3-hollows (51.65 kJ/mol) < Au/α-Fe2O3-hollow-rods (53.26 kJ/mol) < Au/α-Fe2O3-rods (65.86 kJ/mol) < Au/α-Fe2O3-spindles (77.57 kJ/mol). The results suggest that CO oxidation proceeds more readily over Au/α-Fe2O3-hollows sample than other counterparts. Also, an on-stream CO catalytic durability

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

test (see Figure S5 in Supplementary Information) indicats that the Au/α-Fe2O3-hollows catalyst is capable of maintaining ca. 90% CO conversion over more than 50 h at 80 °C. All the above results highlight the novelty of hollow structure in presented Au/α-Fe2O3 catalysts, achieving extremely high CO conversion, reaction rate and TOF value. 3.4. In situ DRIFTs analysis In order to gain further insight into the CO oxidation behavior of highly active Au/α-Fe2O3-hollows, in situ DRIFTs were performed under simulated reaction conditions of CO oxidation at various temperatures. Au/α-Fe2O3-rods sample was used as a counterpart.

Figure 8. In situ DRIFT spectra of (A) Au/α-Fe2O3-rods and (B) Au@α-Fe2O3-hollows in CO oxidation at different temperatures. As shown in Figure 8A, the carbonate-type region with a wavenumber of 1800~1200 cm−1, a band of –OH groups at 3662 cm−1, and a doublet characteristic for adsorbed CO with two bands at 2171 and 2112 cm−1for Au/α-Fe2O3-rods is visible throughout CO oxidation process.8,51–56 These observations indicate that there are significant quantities of carbonate-like species and hydroxyls groups on the surface of this sample. The band at 2112 cm−1 is assigned to the CO chemisorption on metallic Au0 particles, and the band at 2171 cm−1 can be attributed to CO adsorbed on the support

26


Page 26 of 57

Page 27 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


or/and gold cations.8,56 The band at approximately 1448 cm−1 was observed in the initial reaction stage (the reaction temperature as low as 30 °C), which is reported to be typical for the formation of bicarbonates.51,52 Rise of the reaction temperature results in the increase of the band at 1448 cm−1 and the appearance of a new band at 1246 cm−1, indicating the growing of bicarbonates species. Smit et al.52 deemed that the initial formed bicarbonates are attributed to the adsorption of CO2 at –OH groups from air. For the Au catalysts, the reactive Au-hydroxycarbonyl as intermediates can be formed through the direct reaction between CO and –OH groups, which may produces bicarbonates via its further oxidation by the adsorbed oxygen from oxide support.51,58 Consequently, the possible contribution of bicarbonates can be ascribed to the overlap of the adsorption of CO2 at –OH groups and the transformation of hydroxycarbonyl into bicarbonates by assistance of adsorbed oxygen. It is known that the bicarbonate is stable and difficult to completely remove even at temperature over 475 K.24 Considering the more intensive and broad intensity of the peaks at 1448 and 1246 cm−1 at slightly higher temperature (80 and 100 °C), it can be concluded that the stable bicarbonate contained on the surface of catalyst go on accumulating with increasing the temperature of reaction system. Nonetheless, further rising of reaction temperature also leads to the appearance and moderate increase of the doublet for gaseous CO2 (2362 and 2316 cm−1), with an accompanied increase in –OH groups region. The possible contribution of the two bands should be ascribed to the oxidation of CO by activated oxygen adsorbed on gold particles and/or from the support,29,52,53,57 which can be confirmed by the corresponding decrease of adsorbed CO (2171 and 2112 cm−1, especially the first one). The incremental quantities of –OH groups may be explained by a reason of H2O production or less pronounced absorptions of –OH during CO oxidation at higher temperature. As explained by earlier studies,59,60 the deprotonation of bicarbonate to a carbonate result in the increase in the

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

concentration of surface H2O. Indeed, a moderate increase in the band at approximately 1358 cm−1, typical for carbonate species, is observed for this Au/α-Fe2O3 sample with increasing temperature. Subsequently, rapid dissociation of water as proposed by Busca et al.61 is occurred on the surface of iron oxide, which could keep continuous supply of –OH groups with increasing system temperature, giving evidence of a more intense peak of –OH groups and the little distinguished characteristic band of H2O at 3466 cm−1 all the time. Therefore, we can infer that the increased absorbance at wavenumbers at 1448 and 1246 cm−1 are associated with the replacement or overlapping of bicarbonates by some addtional carbonyl species. Similarly, the species observed in the Au/α-Fe2O3-rods sample are also detected over the Au@α-Fe2O3-hollows catalyst (see Figure 8B). However, the Au/α-Fe2O3-hollows shows a more intensive and distinguished bands for the characteristic of carbonate-type species at low temperature. This indicates more considerable formation of carbonate-type species produced by the adsorption or interaction between CO/CO2 and –OH groups, proving increased CO conversion of this catalyst in this temperature range. Moreover, a new band at 1612 cm−1, indicative of bicarbonates, is recorded on this sample, but the peak is not so intense and strengthened with temperature increasing. In addition, it should be highlighted that there are two new bands at 2933 and 2866 cm−1, as an evidence of citrates production,52 in the CO oxidation over Au/α-Fe2O3-hollows when temperature increased to 80 °C. Upon this reaction condition, it can be found that the sample shows almost 100% of CO conversion, as shown in Figure 7A. With respect to the unique observation on the Au/α-Fe2O3-hollows, the contribution for the presence of this new doublet bands can be connected with a strong pronounced of an oxidation process between carboxylate group and superoxide in the case of oxygen-enriched atmosphere.62 Boccuzzi et al.63 suggested that the presence of gas-phase

28


Page 28 of 57

Page 29 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


oxygen is conducive to the formation of carbonate-like species. Nonetheless, the distinct difference between the Au/α-Fe2O3-hollows and Au/α-Fe2O3-rods catalysts should be related to the concentration of surface oxygen species. Combined with the XPS results, more abundant oxygen vacancies are formed on the surface of Au/α-Fe2O3-hollows due to the lower Fe3+/Fe2+ molar ratio. Namely, it can be concluded that the existence of oxygen vacancy may facilitate the distinguished conversion of CO.

4. Discussion 4.1. Formation of hollow-like structure As shown in Figures1–3, rod-like β-FeOOH particles with different lengths and diameters are obtained via a hydrothermal method by changing the experimental conditions. After thermal treatments, the rod-like β-FeOOH precursors transform to α-Fe2O3 nanoparticles, which is confirmed by the XRD patterns. Meanwhile, along with the formation of hematite phase, some hole-like structural defects appear and develop gradually on the surface of rod with calcination proceeding. For some rod precursors, the evolution of hole-like defects to hollow structure can be achieved by controlling the heating rate in the thermal treatment, which result in the formation of hollow structural α-Fe2O3 (see Figure 3). It seems that the formation of hollow-like structure is proposed as a result of gradual growth and transformation of the hole-like surface defect under calcination. Coincidentally, a similar pothole-shape structural defect is found in the surface of α-Fe2O3 nanorods reported by Chaudhari et al.,38 which is obtained by heating β-FeOOH precursor at 250 °C. According to their finding, the tunnel sites is more easily occupied by chloride ions than other ions during the formation of β-FeOOH, and the further growth of the resulting channels upon heating

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

treatments leads to the porous structure of α-Fe2O3 nanorods. A similarly situation could also be observed in other literatures, 14,37 whereas the well-preserved α-Fe2O3 with pothole-like or porous surface structural defects is obtained by calcining the β-FeOOH precursor between 300 and 600 °C. It is known that the thermal treatment plays a crucial role in the evolution of surface texture and phase structure by a complex diversification process of phase and internal structure during heat absorption. In the present work, no template agent or surfactant in addition to ferric chloride and urea were used to obtain uniform rod-shaped β-FeOOH precursor via a hydrothermal method, and no defects or channels can be observed in surface of as-synthesized precursor. But the subsequent thermal treatment with various heating conditions led to the evolution of surface texture even final morphology of the products. From TEM images shown in Figure1, we can see that a high proportion of pothole-like structural defects being produced in the α-Fe2O3 nanorods after a rapid heating process (5 °C/min). However, mesoporous structure is not formed in the α-Fe2O3 nanocrystals, which is confirmed by N2-adsorptiondesorption isotherms and specific surface areas of this sample (Figure S2 and Table S1). As mentioned in the Section 3.1, the formation of defects may attribute to the removing of impurities and moisture through the intrinsic structure gap of material. Interestingly, two kinds of different-shaped hematite nanostructures (i.e., hollow and hollow-rod, see Figures 2 and 3) are defined after heating the β-FeOOH nanorods with relatively samll diameter (20–30 nm). As a comparison, the hollow-rods could be considered as an intermedium of defect-contained rods to perfect hollow-structured α-Fe2O3 hollows. Li et al.64 proposed that the hollow structural hematite spindle was evolved from the holes defects on its tip due to the corresponding high surface energy and low coverage of ethylene glycol. Here the heating rate and rod diameter are the main influence factors in the formation of hematite with hollow structural

30


Page 30 of 57

Page 31 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


feature. For the β-FeOOH nanorods with relatively large diameters (e.g., the rods with 80–200 nm diameters shown in Figure1), the growth of defects cannot destroy the mature bulk structure. In the case of the nanorods with relatively small diameters (20–30 nm), the diffusion of such defects would strongly affect the final structure of α-Fe2O3. The heating rate becomes the key factor to determine the final morphology and structure of calcinated α-Fe2O3 via influencing the diffusion rate of defects. To regulate and control the heating rate as low as 1 °C/min, the complete and internal hollows can be achieved. Presumably, the slow rise in temperature is beneficial to provide a relatively stable thermal environment for the development of nanocrystals, which facilitated the well-organized squeezing of particles from the hole-defects center to outer surface and further to form the hollow structure. In other word, for α-Fe2O3 hollow-rods, duration time of heating might not be enough to grow to the final hollow morphology. By prolonging the temperature-rise period, the hollow-like structure in bulk of the rods will split along crosswise to form intertwining hollow, as shown in Figure 3e–g. 4.2. Structure dependence of Au/α α-Fe2O3 catalysts On account of iron oxide supported gold systems in CO oxidation, the effect of structure or morphology of oxide support on catalytic performance have been investigated in the previous literatures.7,8,65,66 It is found that the crystallite size of Au NPs and the nature of the supports are the key factor to determine the catalytic activity. Generally, smaller particle size of gold would enhance the CO oxidation activity. In the present work, it is a little surprising that the notable advantages of CO catalytic activities over different samples are not completely associated with the corresponding size order of Au particles deposited on α-Fe2O3 supports (see Table 2, Figure 5 and Figure 7). As pointed out by Haruta et al.,1,41 a high CO oxidation activity can be readily achieved over gold catalysts with the gold particles size less than approximately 6 nm. Recently, numerous

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

investigations suggested the tiny promoting effect of gold particles size on the catalytic activity of reducible oxides supported Au catalysts when the diameter of gold particles was in the range of 3–5 nm.2,3,41 Based on the above facts, the size effect on the prepared Au catalysts is generally believed to be minor for a high activity in CO oxidation, because the distribution of gold particles on each support is quite homogeneous and the main Au particles are all in the optimize size (2–6 nm, see Figure 5). The results described above demonstrate that the specific morphology of α-Fe2O3 support and the interaction between Au particles and support play a significant role in determining the catalytic activity. Among the four samples in the present work, Au catalyst supported on α-Fe2O3-hollows shows the most excellent catalytic activity for CO oxidation, and that on α-Fe2O3-hollow-rods with abundant hollow-like defect follows (Figure 7A and Table 3). Clearly, the novel α-Fe2O3 with well-defined hollow structure is highly advantageous. Based on previous discussion, the nanohollow structure is obtained by controlling the growth of hole-like surface defects under calcinations treatment. The irregular surface-deficient structure possesses more edges and corners, which is regarded as active sites for the adsorption of reactants. In the deposition process of gold particles, the hollow-like structural defects may provide a residence to sustain and adhere gold particles. Moreover, it is observed that a mass of small-sized Au NPs are embedded in the corners and the gaps of mutual cross-linked hollows for Au/α-Fe2O3-hollows catalyst (see Figure 5). Sun et al.29 recently illustrated that the unique walls and cavities benefited from the hollow structure is responsible for uniform dispersion of Au NPs and also suppresses the aggregate of Au NPs. The migration and growth of Au NPs on the surface of support was also effectively suppressed under calcination treatments, and meanwhile give an intensive gold-support interaction. This also demonstrates that, as the support of

32


Page 32 of 57

Page 33 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Au NPs, the surface-deficient structure is beneficial for improving catalytic activity due to the special surface property. The XPS data has confirmed that more abundant Fe2+, adsorbed oxygen as well as active Auδ+ species were detected on the α-Fe2O3 hollows surface. The enhanced formation of such preponderant active species could be ascribed to the intimate gold-support interaction, which results in the reduction of Fe3+ to Fe2+ and oxidation of metallic gold to cationic gold (Au+ and Au3+). The charge transfer may also result in the formation of oxygen vacancies. As revealed in XPS results, a comparatively abundant adsorbed oxygen species was included in the hollow-structured Au/α-Fe2O3 samples (i.e., Au/α-Fe2O3-hollow-rods and Au/α-Fe2O3-hollows). It is widely accepted that the active oxygen species are of vital importance in the CO oxidation reaction because the oxygen is suggested to react with CO adsorbed on gold particles nearby the boundary of gold contacted with support. To make a quantitative analysis on the content of adsorbed oxygen, the sequential pulses of CO and O2 over each catalyst were performed. Figure S6 illustrates the consumption of CO and O2 as well as the production of CO2 during sequential pulses of CO and O2. The content of reactive oxygen species was calculated accordingly, as shown in Table 3. It can be seen that, because of the adsorption of CO2 in the form of carbonate species, the amount of CO2 production over each sample is lower than the CO consumption. By this test, there is 1.19 µmol available oxygen assessed in the Au/α-Fe2O3-hollows sample, 1.03 µmol for Au/α-Fe2O3-hollow-rods, 0.90 µmol for Au/α-Fe2O3 –rods, and a lowest 0.76 µmol for the Au/α-Fe2O3-spindles, respectively. It can be found that much more oxygen species could participate in the CO oxidation for the Au/α-Fe2O3-hollows, suggesting the momentous contributions to CO catalytic oxidation. Combined with in situ DRIFT analysis, the distinct density of bicarbonate species was recorded on the Au/α-Fe2O3-hollows in the overall CO

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

oxidation period, which can be corresponding to a more outstanding oxidation ability of carboxylate groups. The finding could support the indication of the difference between the CO consumption and CO2 production obtained by sequential pulses study. Clearly, it is also coincident with the inference that more intimate gold-iron oxide interaction occurred in hollow-structured Au catalyst. On the other hand, the unique structure sensitivity associated with exposed surface planes of support is also observed. The formation of oxygen vacancies closely related to the exposed surface planes because of the different formation energy of vacancies for various planes. Huang et al.67 disclosed that the priority exposed planes {110} on the surface of ceria nanorods is beneficial for the creation of oxygen vacancies, which could anchor and disperse very fine gold NPs, resulting in the strong morphology effect of support for catalytic CO oxidation. A recent report by Liu et al.68 has suggested that the dominant exposed plane {110} of α-Fe2O3 provided more oxygen vacancies and active sites for catalytic CO oxidation. In the present case, as shown in Figure 5, the hollow structured α-Fe2O3 support is exposed huge quantity of planes {110}. The percentage of {110} facet exposed on different Fe2O3 support (S110/S, where S and S110 are the total surface area and that contributed by the {110} facet, respectively) was therefore estimated based on the statistical analysis of TEM images (see Figure S4 and Table S2, Supporting Information). As can be seen, the estimated average percentage of {110} facets of Au/α-Fe2O3-hollows sample is ca. 32%, which is much higher than other samples. This indicates that the preferentially exposed {110} facet may also contribute to the high activity of Au/α-Fe2O3-hollows sample. In addition, hydroxyl groups on the support surface may also play a significant role in enhanced catalytic activity.29,51 Applying the comparative analysis of in situ DRIFTs between Au@α-Fe2O3-hollows and Au/α-Fe2O3-rods (Figure 8), the conspicuous acceleration of hydroxyl

34


Page 34 of 57

Page 35 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


groups is straightforward authenticated. As discussed in Section 3.3, the handsome hydroxyl groups on the surface of Au/α-Fe2O3-hollows would favor the adsorption of CO at –OH groups, and further generate carbonate and additional –OH groups via the deprotonation of bicarbonate and H2O dissocation procedure. By comparison of Figure 7, a higher catalytic activity may be associated with richer hydroxyl groups contained on the surface of Au/α-Fe2O3-hollows sample. According to the literatures,7,69–71 the hydroxyl groups play a pronounced role in the activation and stabilization of gold nanoparticles in CO oxidation reaction, assisting CO oxidation to form carbon-containing species (such as carboxylate) via a new reaction channel.

Figure 9. Schematic diagram of the reaction mechanism of CO oxidation over the Au/α-Fe2O3-hollows catalyst. Based on the XPS and in situ DRIFT results, together with the previous discussions, we propose a possible reaction mechanism for CO oxidation over the Au/α-Fe2O3-hollows catalyst. The schematic is shown in Figure 9. In this process, the O2 is firstly adsorbed on the interface between NPs and α-Fe2O3 hollow support and creates active oxygen species, which could directly react with the adsorbed CO to generate gas-phase CO2. In addition, the oxygen species adsorbed on the oxygen vacancies are active for the further oxidation of hydrocarbon species formed by the reaction of adsorbed CO with hydroxyl groups. The oxygen vacancies could be easily be occupied by the oxygen from gas-phase oxygen, and then the oxygen activation and migration will occur on the Au 35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

NPs to vacate the oxygen vacancies. Such a cycle was completed to catalytic CO oxidation. Clearly, the creation of the reactive oxygen species as well as their diffusion and replenishment play a key role in the enhanced CO oxidation activity. It is certain that the contribution of the strong gold-support interaction and dominant exposed plane {110} to create more oxygen vacancies is significant for Au/α-Fe2O3-hollows to enhance CO catalytic activity.

5. Conclusions This study indicates that the surface structure and morphology of α-Fe2O3 support markedly affect the physicochemical properties of α-Fe2O3-suppotred Au catalysts. By controllable synthesis of typical β-FeOOH-rods precursors and an elaborate calcination step (especially controlling the heating rate of calcination), the α-Fe2O3 with a hollow structure is obtained. The α-Fe2O3 oxides with other well-preserved morphologies (i.e., spindles, rods and hollow-rods) are also synthesized for comparison. It is found that the hollow structure is evolved from the hole-like surface structural defect on the nanorods. By the colloid-deposition method, the Au NPs (2–6 nm) are highly dispersed on the α-Fe2O3 supports. Appling to the CO oxidation, the Au/α-Fe2O3 catalyst with hollow structural feature exhibits a much higher catalytic activity than other counterparts. Fortunately, the hollow structured α-Fe2O3 support provides a very fine substrate to restrain the migration and further growth of Au NPs, giving a strong interaction between Au NPs and α-Fe2O3 support. The characterizations by XPS and In situ DRIFT suggested that the intimate gold-support interaction brings about more active surface species (i.e., –OH group and adsorbed oxygen) for CO oxidation. The creation of the reactive oxygen species as well as their diffusion and replenishment play a key role in the catalytic oxidation process. The gas-phase CO could be efficiently converted on the catalyst surface via the adsorption-oxidation process involving the formation of hydrocarbon species 36


Page 36 of 57

Page 37 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


as an intermediate.

Supplementary Information Complementary characterization of the α-Fe2O3 support and Au/α-Fe2O3 samples (XRD, N2 adsorption-desorption, BET surface area, average pore size, durability test of CO catalytic oxidation, and CO/O2 pulse test). The Supplementary Information is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Project Nos. 51374004 and 51604137), the candidate talents training fund of Yunnan Province (2012HB009, 2014HB006), and the Applied Basic Research Program of Yunnan Province (No. 2014FB123). Also, the authors would like to acknowledge the Scientific and Technological Leading Talent Projects in Yunnan Province (No. 2015HA019).

References 1. Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Novel gold catalysis for the oxidation of carbon-monoxide at a temperature for below 0 °C. Chem. Lett. 1987, 16, 405–408. 2. Herzing, A. A.; Kiely, C. J.; Carley, A. F.; Landon, P.; Hutchings, G. J. Identification of active gold nanoclusters on iron oxide supports for CO oxidation. Science 2008, 321, 1331–1335. 3. Green, I. X.; Tang, W.; Neurock, M.; Yates, J. T. Spectroscopic observation of dual catalytic sites during oxidation of CO on a Au/TiO2 catalyst. Science 2011, 333, 736–739. 4. Okumura, M.; Fujitani, T.; Huang, J. H.; Ishida, T. A career in catalysis: Masatake Haruta. ACS Catal. 2015, 5, 4699–4707. 5. Schubert, M. M.; Hackenberg, S.; Veen, A. C.; Muhler, M.; Plzak, V.; Behm, R. J. CO oxidation

37



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

over supported gold catalysts—“inert” and “active” support materials and their role for the oxygen supply during reaction. J. Catal. 2001, 197, 113–122. 6. Klyushin, A. Y.; Greiner, M. T.; Huang, X.; Lunkenbein, T.; Li, X.; Timpe, O.; Friedrich, M.; Hävecker, M.; Knop-Gericke, A.; Schlögl, R. Is nanostructuring sufficient to get catalytically active Au? ACS Catal. 2016, 6, 3372−3380. 7. Widmann, D.; Liu, Y.; Schüth, F.; Behm, R. J. Support effects in the Au-catalyzed CO oxidation – correlation between activity, oxygen storage capacity, and support reducibility. J. Catal. 2010, 276, 292–305. 8. Li, L.; Wang, A.; Qiao, B.; Lin, J.; Huang, Y.; Wang, X.; Zhang, T. Origin of the high activity of Au/FeOx for low-temperature CO oxidation: Direct evidence for a redox mechanism. J. Catal. 2013, 299, 90–100. 9. Wang, L. C.; Liu, Q.; Huang, X. S.; Liu, Y. M.; Cao, Y.; Fan, K. N. Gold nanoparticles supported on manganese oxides for low-temperature CO oxidation. Appl. Catal. B 2009, 88, 204–212. 10. Venezia, A. M.; Pantaleo, G.; Longo, A.; Carlo, G. D.; Casaletto, M. P.; Liotta, F. L.; Deganello, G. Relationship between structure and CO oxidation activity of ceria-supported gold catalysts. J. Phys. Chem. B 2005, 109, 2821–2827. 11. Arena, F.; Famulari, P.; Trunfio, G.; Bonura, G.; Frusteri, F.; Spadaro, L. Probing the factors affecting structure and activity of the Au/CeO2 system in total and preferential oxidation of CO. Appl. Catal. B 2006, 66, 81–91. 12. Liotta, L. F.; Carlo, G. D.; Longoa, A.; Pantaleo, G.; Venezia, A. M. Support effect on the catalytic performance of Au/Co3O4-CeO2 catalysts for CO and CH4 oxidation. Catal. Today 2008, 139, 174–179.

38


Page 38 of 57

Page 39 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


13. Xie, S.; Dai, H.; Deng, J.; Liu, Y.; Yang, H.; Jiang, Y.; Tan, W.; Ao, A.; Guo, G. Au/3DOM Co3O4: highly active nanocatalysts for the oxidation of carbon monoxide and toluene. Nanoscale 2013, 5, 11207–11219. 14. Lian, J. B.; Duan, X. C.; Ma, J.; Peng, P.; Kim, T.; Zheng, W. J. Hematite (α-Fe2O3) with various morphologies: Ionic liquid-assisted synthesis, formation mechanism, and properties. ACS Nano. 2009, 3, 3749–3761. 15. Sun, M.; Dong, Y.; Zhang, G.; Qu, J.; Li, J. α-Fe2O3 spherical nanocrystals supported on CNTs as efficient non-noble electrocatalysts for the oxygen reduction reaction. J. Mater. Chem. A 2014, 2, 13635–13640. 16. Larsen, G. K.; Farr, W.; Murph, S. E. H. Multifunctional Fe2O3–Au nanoparticles with different shapes: Enhanced catalysis, photothermal effects, and magnetic recyclability. J. Phys. Chem. C 2016, DOI: 10.1021/acs.jpcc.6b03733. 17. Mou, X.; Wei, X.; Li, Y.; Shen, W. Tuning crystal-phase and shape of Fe2O3 nanoparticles for catalytic applications, Crystengcomm 2012, 14, 5107−5120. 18. Shodiya, T.; Schmidt, O.; Peng, W.; Hotz, N. Novel nano-scale Au/α-Fe2O3 catalyst for the preferential oxidation of CO in biofuel reformate gas. J. Catal. 2013, 300, 63–69. 19. Gao, Q. X.; Wang, X. F.; Di, J. L.; Wu, X. C.; Tao, Y. R. Enhanced catalytic activity of α-Fe2O3 nanorods enclosed with {110} and {001} planes for methane combustion and CO oxidation. Catal. Sci. Technol. 2011, 1, 574–577. 20. Ma, J.; Lian, J. B.; Duan, X. C.; Liu, X. D.; Zheng, W. J. α-Fe2O3: Hydrothermal synthesis, magnetic and electrochemical properties. J. Phys. Chem. C 2010, 114, 10671–10676. 21. Hu, X.; Yu, J. C.; Gong, J.; Li, Q.; Li, G. α-Fe2O3 nanorings prepared by a microwave-assisted

39



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

hydrothermal process and their sensing properties. Adv. Mater. 2007, 19, 2324–2329. 22. Setiawan, A.; Kennedy, E. M.; Dlugogorski, B. Z.; Adesina, A. A.; Stockenhuber, M. The stability of Co3O4, Fe2O3, Au/Co3O4 and Au/Fe2O3 catalysts in the catalytic combustion of lean methane mixtures in the presence of water. Catal. Today 2015, 258, 276–283. 23. Xu, Y.; Yang, S.; Zhang, G.; Sun, Y.; Gao, D.; Sun, Y. Uniform hematite α-Fe2O3 nanoparticles: Morphology, size-controlled hydrothermal synthesis and formation mechanism. Mater. Lett. 2011, 65, 1911–1914. 24. Yang, Y.; Ma, H.; Zhuang, J.; Wang, X. Morphology-controlled synthesis of hematite nanocrystals and their facet effects on gas-sensing properties. Inorg. Chem. 2011, 50, 10143–10151. 25. Park, E. D.; Lee, J. S. Effects of pretreatment conditions on CO oxidation over supported Au catalysts. J. Catal. 1999, 186, 1–11. 26. Tripathi, A. K.; Kamble, V. S.; Gupta, N. M. Microcalorimetry, adsorption, and reaction studies of CO, O2, and CO + O2 over Au/Fe2O3, Fe2O3, and polycrystalline gold catalysts. J. Catal. 1999, 187, 332–342. 27. Guczi, L.; Horváth, D.; Pászti, Z.; Petõ, G. Effect of treatments on gold nanoparticles: Relation between morphology, electron structure and catalytic activity in CO oxidation. Catal.Today 2002, 72, 101–105. 28. Yin, H.; Ma, Z.; Chi, M.; Dai, S. Heterostructured catalysts prepared by dispersing Au@Fe2O3 core–shell structures on supports and their performance in CO oxidation. Catal. Today 2011, 160, 87–95. 29. Sun, B.; Feng, X.; Yao, Y.; Su, Q.; Ji, W.; Au, C. T. Substantial pretreatment effect on CO

40


Page 40 of 57

Page 41 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


oxidation

over

controllably

synthesized

Au/FeOx

hollow

nanostructures

via

hybrid

Au/β-FeOOH@SiO2. ACS Catal. 2013, 3, 3099−3105. 30. Wang, G. H.; Li, W. C.; Jia, K. M.; Spliethoff, B.; Schüth, F.; Lu, A. H. Shape and size controlled α-Fe2O3 nanoparticles as supports for gold-catalysts: Synthesis and influence of support shape and size on catalytic performance. Appl. Catal. A 2009, 364, 42−47. 31. Liu, X.; Zhang, J.; Guo, X.; Wu, S.; Wang, S. Porous α-Fe2O3 decorated by Au nanoparticles and their enhanced sensor performance. Nanotechnology 2010, 21, 095501–7. 32. Zhong, Z.; Ho, J.; Teo, J.; Shen, S.; Gedanken, A. Synthesis of porous α-Fe2O3 nanorods and deposition of very small gold particles in the pores for catalytic oxidation of CO. Chem. Mater. 2007, 19, 4776–4782. 33. Zhang, J.; Liu, X.; Wang, L.; Yang, T.; Guo, X.; Wu, S.; Wang, S.; Zhang, S. Au-functionalized hematite hybrid nanospindles: General synthesis, gas sensing and catalytic properties. J. Phys. Chem. C 2011, 115, 5352–5357. 34. Li, S. Y.; Liu, G.; Lian, H. L.; Zhao, G. M.; Jiang, D. Z.; Zhang, W. X. Low-temperature CO oxidation over supported Pt catalysts prepared by colloid-deposition method. Catal. Commun. 2008, 9, 1045−1049. 35. Han, W.; Deng, J.; Xie, S.; Yang, H.; Dai, H.; Au, C. T. Gold supported on iron oxide nanodisk as efficient catalyst for the removal of toluene. Ind. Eng. Chem. Res. 2014, 53, 3486−3494. 36. Jia, A. P.; Jiang, S. Y.; Lu, J. Q.; Luo, M. F. Study of catalytic activity at the CuO-CeO2 interface for CO Oxidation. J. Phys. Chem. C 2010, 114, 21605–21610. 37. Pradhan, G. K.; Parida, K. M. Fabrication, growth mechanism, and characterization of α-Fe2O3 nanorods. ACS Appl. Mater. Interfaces. 2011, 3, 317–323.

41



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

38. Chaudhari, N. K.; Yu, J. S. Size control synthesis of uniform β-FeOOH to high coercive field porous magnetic α-Fe2O3 nanorods. J. Phys. Chem. C 2008, 112, 19957–19962. 39. Wei, C.; Nan, Z. Effects of experimental conditions on one-dimensional single-crystal nanostructure of β-FeOOH. Mater. Chem. Phys. 2011, 127, 220–226. 40. Almeida, T. P.; Fay, M. W.; Zhu, Y.; Brown, P. D. Hydrothermal growth mechanism of α-Fe2O3 nanorods derived by near in situ analysis. Nanoscale, 2010, 2, 2390–2399. 41. Haruta, M. When gold is not noble: Catalysis by nanoparticles. Chem. Rec. 2003, 3, 75−87. 42. Ouyang, J.; Pei, J.; Kuang, Q.; Xie, Z.; Zheng, L. Supersaturation-controlled shape evolution of α-Fe2O3 nanocrystals and their facet-dependent catalytic and sensing properties. ACS Appl. Mater. Interfaces 2014, 6, 12505−12514. 43. Ismail, H. M.; Cadenhead, D. A.; Zaki, M. I. Surface reactivity of iron oxide pigmentary powders toward atmospheric components: XPS, FESEM, and gravimetry of CO and CO2 adsorption. J. Colloid Interf. Sci. 1997, 194, 482−488. 44. Wagner, C. D.; Moulder, J. F.; Davis, L. E.; Riggs, W. M. Handbook of X-ray photoelectron spectroscopy. Perkin-Elmer: Waltham, MA, 2008. 45. Dupin, J. C.; Gonbeau, D.; Vinatier, P.; Levasseur, A. Systematic XPS studies of metal oxides, hydroxides, and peroxides. Phys. Chem. Chem. Phys. 2000, 2, 1319−1324. 46. Liu, X.; Liu, M. H.; Luo, Y. C.; Mou, C. Y.; Lin, S. D.; Cheng, H.; Chen, J. M.; Lee, J. F.; Lin, T. S. Strong metal−support interactions between gold nanoparticles and ZnO nanorods in CO oxidation. J. Am. Chem. Soc. 2012, 134, 10251−10258. 47. Landon, P.; Ferguson, J.; Solsona, B. E.; Garcia, T.; Al-Sayari, S.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Makkee, M.; Moulijn, J. A. et al. Selective oxidation of CO in the presence of H2,

42


Page 42 of 57

Page 43 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H2O and CO2 utilising Au/α-Fe2O3 catalysts for use in fuel cells. J. Mater. Chem. 2006, 16, 199–208. 48. Huang, X. S.; Sun, H.; Wang, L. C.; Liu, Y. M.; Fan, K. N.; Cao, Y. Morphology effects of nanoscale ceria on the activity of Au/CeO2 catalysts for low-temperature CO oxidation. Appl. Catal. B 2009, 90, 224–232. 49. Zhang, X.; Corma, A. Supported gold (III) catalysts for highly efficient three-component coupling reactions. Angew. Chem. Int. Ed. 2008, 47, 4358–61. 50. Wu, B. H.; Zhang, H.; Chen, C.; Lin, S. C.; Zheng, N. F. Interfacial activation of catalytically inert Au (6.7 nm)–Fe3O4 dumbbell nanoparticles for CO oxidation, Nano. Res. 2009, 2, 975–983. 51. Daniells, S. T.; Overweg, A. R.; Makkee, M.; Moulijn, J. A. The mechanism of low-temperature CO oxidation with Au/Fe2O3 catalysts: a combined Mössbauer, FT-IR, and TAP reactor study. J. Catal. 2005, 230, 52–65. 52. Šmit, G.; Strukan, N.; Crajé, M. W. J.; Lázár, K. A comparative study of CO adsorption and oxidation on Au/Fe2O3 catalysts by FT-IR and in situ DRIFTS spectroscopies. J. Mol. Catal. A 2006, 252, 163–170. 53. Martı́nez-Arias, A.; Fernández-Garcı́a, M.; Iglesias-Juez, A.; Hungrı́a, A. B.; Anderson, J. A.; Conesa, J. C.; Soria, J. Influence of thermal sintering on the activity for CO–O2 and CO–O2–NO stoichiometric reactions over Pd/(Ce, Zr)Ox/Al2O3 catalysts. Appl. Catal. B 2002, 38, 151–158. 54. Silberova, B. A. A.; Mul, G.; Makkee, M.; Moulijn, J. A. DRIFTS study of the water–gas shift reaction over Au/Fe2O3. J. Catal. 2006, 243, 171–182. 55. Green, I. X.; Tang, W.; McEntee, M.; Neurock, M.; Yates, J. T. Inhibition at perimeter sites of Au/TiO2 oxidation catalyst by reactant oxygen. J. Am. Chem. Soc. 2012, 134, 12717–12723.

43



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

56. Mihaylov, M.; Knozinger, H.; Hadjiivanov, K.; Gates, B. C. Characterization of the oxidation states of supported gold species by IR spectroscopy of adsorbed CO. Chem. Ing. Tech. 2007, 79, 795–806. 57. Guo, L. W.; Du, P. P.; Fu, X. P.; Ma, C.; Zeng, J.; Si, R.; Huang, Y. Y.; Jia, C. J.; Zhang, Y. W.; Yan, C. H. Contributions of distinct gold species to catalytic reactivity for carbon monoxide oxidation. Nat. Commun. 2016, DOI: 10.1038/ncomms13481. 58. Bond, G.; Thompson, D. Formulation of mechanisms for gold-catalysed reactions. Gold Bull. 2009, 42, 337–342. 59. Costello, C. K.; Yang, J. H.; Law, H. Y.; Wang, Y.; Lin, J. N.; Marks, L. D.; Kung, M. C.; Kung, H. H. On the potential role of hydroxyl groups in CO oxidation over Au/Al2O3. Appl. Catal. A 2003, 243, 15–24. 60. Kung, H. H.; Kung, M. C.; Costello, C. K. Supported Au catalysts for low temperature CO oxidation. J. Catal. 2003, 216, 425–432. 61. Busca, G. Spectroscopic characterization of the acid properties of metal oxide catalysts. Catal. Today 1998, 41, 191–206. 62. Bond, G. C.; Thompson, D. T. Gold-catalysed oxidation of carbon monoxide. Gold Bull. 2000, 33, 41–50. 63. Boccuzzi, F.; Chiorino, A.; Tsubota, S.; Haruta, M. FTIR study of carbon monoxide oxidation and scrambling at room temperature over gold supported on ZnO and TiO2. J. Phys. Chem. 1996, 100, 3617–3624. 64. Li, X.; Yu, X.; He, J.; Xu, Z. Controllable fabrication, growth mechanisms, and photocatalytic properties of hematite hollow spindles. J. Phys. Chem. C 2009, 113, 2837–2845.

44


Page 44 of 57

Page 45 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


65. Guo, Y.; Gu, D.; Jin, Z.; Du, P. P.; Si, R.; Tao, J.; Xu, W. Q.; Huang, Y. Y.; Senanayake, S.; Song, Q. S. et al. Uniform 2 nm gold nanoparticles supported on iron oxides as active catalysts for CO oxidation reaction: structure–activity relationship. Nanoscale 2015, 7, 4920–4928. 66. Wang, G. H.; Li, W. C.; Jia, K. M.; Spliethoff, B.; Schüth, F.; Lu, A. H. Shape and size controlled α-Fe2O3 nanoparticles as supports for gold-catalysts: Synthesis and influence of support shape and size on catalytic performance. Appl. Catal. A 2009, 364, 42–47. 67. Huang, X. S.; Sun, H.; Wang, L. C.; Liu, Y. M.; Fan, K. N.; Cao, Y. Morphology effects of nanoscale ceria on the activity of Au/CeO2 catalysts for low-temperature CO oxidation. Appl. Catal. B 2009, 90, 224–232. 68. Liu, X.; Liu, J.; Chang, Z.; Sun, X.; Li, Y. Crystal plane effect of Fe2O3 with various morphologies on CO catalytic oxidation. Catal. Commun. 2011, 12, 530–534. 69. Qian, K.; Zhang, W.; Sun, H.; Fang, J.; He, B.; Ma, Y.; Jiang, Z.; Wei, S.; Yang, J.; Huang, W. Hydroxyls-induced oxygen activation on ‘‘inert’’ Au nanoparticles for low-temperature CO oxidation. J. Catal. 2011, 277, 95–103. 70. Ganesh, P.; Kent, P. R. C.; Veith, G. M. Role of hydroxyl groups on the stability and catalytic activity of Au clusters on a rutile surface. J. Phys. Chem. Lett. 2011, 2, 2918–2924. 71. Singh, J. A.; Overbury, S. H.; Dudney, N. J.; Li, M.; Veith, G. M. Gold nanoparticles supported on carbon nitride: Influence of surface hydroxyls on low temperature carbon monoxide oxidation. ACS Catal. 2012, 2, 1138−1146.

FIGURES:

45



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. TEM images and XRD patterns of the hydrothermal synthesized β-FeOOH precursor and the thermal heated α-Fe2O3 rods: TEM images of (a–b) β-FeOOH precursor, (c–e) α-Fe2O3 product after calcination at 500 °C with 5 °C/min, and (f) XRD patterns of the precursor (red line) and the heated product (blue line).

Figure 2. TEM images and XRD patterns of the β-FeOOH precursor and the prepared α-Fe2O3 hollow-rods: TEM images of (a–b) β-FeOOH precursor, (c–e) α-Fe2O3 hollow-rods after calcinations at 500 °C with 5 °C/min, and (f) XRD patterns of the precursor (red line) and the as-prepared product (blue line).

46


Page 46 of 57

Page 47 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. TEM images and XRD patterns of the β-FeOOH precursor and the prepared α-Fe2O3 hollows: TEM images of (a–b) β-FeOOH precursor, α-Fe2O3 hollows after calcinations (c) at 500 °C with 5 °C/min, (d) at 500 °C with 2 °C/min, (e) at 400 °C with 1 °C/min, (f and g) at 500 °C with 1 °C/min, and (f) XRD patterns of the precursor (red line) and the as-prepared product (blue line).

Figure 4. TEM images and XRD patterns of the hydrothermal α-Fe2O3 precursor and the calcinated α-Fe2O3 spindles: TEM images of precursor (a–b) and α-Fe2O3 spindles after calcinations (c–e), and XRD patterns of α-Fe2O3 precursor (red line) and calcinated α-Fe2O3 spindles (blue line).

47



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 57

Figure 5. TEM, STEM, and HRTEM images and Au particle size distributions of the as-prepared Au/α-Fe2O3

catalysts:

(a1–4)

Au/α-Fe2O3-spindles,

(b1–4)

Au/α-Fe2O3-rods,

(c1–4)

Au/α-Fe2O3-hollow-rods, (d1–4) Au/α-Fe2O3-hollows.

Figure 6. Fe 2p (A), O 1s (B), and Au 4f (C) XPS spectra of (a) Au/α-Fe2O3-spindles, (b) Au/α-Fe2O3-hollows, (c) Au/α-Fe2O3-hollow-rods, (d) Au/α-Fe2O3-rods.

48


Page 49 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. (A) CO conversion as a function of reaction temperature and (B) Arrhenius plots for CO oxidation over various Au/α-Fe2O3 catalysts. Reaction condition: CO concentration = 1 vol%, CO/O2 molar ratio = 1/10, SV = 30000 mL/(gcat.·h).

Figure 8. In situ DRIFT spectra of (A) Au/α-Fe2O3-rods and (B) Au@α-Fe2O3-hollows in CO oxidation at different temperatures.

Figure 9. Schematic diagram of the reaction mechanism of CO oxidation over the Au/α-Fe2O3-hollows catalyst. 49



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 50 of 57

TABLES: Table 1 α-Fe2O3 supports with various shapes obtained by varying experimental conditions.

Sample

Water bath

Hydrothermal

Hydrothermal

Calcination conditions

Molar ratio of

NH4H2PO4

temperature [°C]

temperature [°C]

time [h]

[°C/min, °C, h]

urea and FeCl3

concentration [mM]

Spindle

20

120

24

5 °C/min, 500 °C for 2 h

1.5

5

Hollow

20

100

24

1~2 °C/min, 400/500 °C for 2 h;

1.5

/

Rod

20

100

24

5 °C/min, 500 °C for 2 h

1.0

/

0

80

24

5 °C/min, 500 °C for 2 h

1.5

/

Hollow-rod

Table 2 Average crystallite sizes (Dα-Fe2O3), average Au particle sizes, real Au contents, and surface element compositions of the Au NPs samples with different supports. Dα-Fe2O3a

Au particle size b

Au content c

Dispersion

OII/OI

Fe3+/Fe2+

Auδ+/Au0

(nm)

(nm)

(wt%)

of gold (%)

molar ratio

molar ratio

molar ratio


68

4.10

2.04

28.60

0.23

1.41

0.16

Au/α-Fe2O3-rods

66

3.70

2.01

31.67

0.20

1.44

0.17

Au/α-Fe2O3-hollows

62

3.40

2.05

34.47

0.50

1.21

0.23


67

4.00

2.02

29.30

0.39

1.25

0.20

Sample

a

The values determined according to the Scherrer equation using the FWHM of the {104}and {110}line of

α-Fe2O3. b Data estimated according to the TEM images, and the standard deviation in particle size measurement is ±0.5 nm. c Data determined by the ICP−AES technique.

Table 3 Catalytic activities, TOF, reaction rate (r) activation energies (Ea), and the amount of reactive oxygen of

50


Page 51 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


different samples. T10

T50

T90

TOFAu a

rb

Ea

Amount of reactive

(°C)

(°C)

(°C )

(s–1)

(×10–7 mol g–1 s–1)

(kJ/mol)

oxygen c (µmol)


41

77

120

0.76

2.23

77.57

0.76

Au/α-Fe2O3-rods

46

98

136

1.15

3.72

65.86

0.90

Au/α-Fe2O3-hollows

36

62

75

1.45

5.21

51.65

1.19


48

82

95

1.11

3.34

53.26

1.03

Samples

a

The TOF (converted CO per molar amount of Au/α-Fe2O3 catalysts per second) values was evaluated based on the

dispersion of Au particles at low conversions under a kinetically controlled regime a for CO oxidation.

b

The

reaction rate values was estimated at 40 °C for CO oxidation. c The amount of reactive oxygen was calculated according to the sequential pulses of CO and O2 test.

51



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TOC Graphic:

52


Page 52 of 57

Page 53 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphic for manuscript

In the present work, we prepared a novel -Fe2O3 with nanohollow structure via a hydrothermal-thermal decomposition process without utilization of templates and surfactants. As a support of Au catalyst, the -Fe2 O3 -hollow shows much higher catalytic activity for CO oxidation than other supports (e.g., spindle, rod, and rod-hollow). The superior catalytic performance can be attributed to the unique embedded hollow structure, which strongly improves the gold-support interaction, resulting in more abundant active species (i.e., –OH group and adsorbed oxygen) on the catalyst.

Figure 1. TEM images and XRD patterns of the hydrothermal synthesized -FeOOH precursor and the thermal heated -Fe2O3 rods: TEM images of (a–b) -FeOOH precursor, (c–e) -Fe2O3 product

1



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 54 of 57

after calcination at 500 °C with 5 °C/min, and (f) XRD patterns of the precursor (red line) and the heated product (blue line).

Figure 2. TEM images and XRD patterns of the -FeOOH precursor and the prepared -Fe2O 3 hollow-rods: TEM images of (a–b) -FeOOH precursor, (c–e) -Fe2O3 hollow-rods after calcinations at 500 °C with 5 °C/min, and (f) XRD patterns of the precursor (red line) and the as-prepared product (blue line).

Figure 3. TEM images and XRD patterns of the -FeOOH precursor and the prepared -Fe2O 3 hollows: TEM images of (a–b) -FeOOH precursor, -Fe2O3 hollows after calcinations (c) at 500 °C with 5 °C/min, (d) at 500 °C with 2 °C/min, (e) at 400 °C with 1 °C/min, (f and g) at 500 °C with

2


Page 55 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 °C/min, and (f) XRD patterns of the precursor (red line) and the as-prepared product (blue line).

Figure 4. TEM images and XRD patterns of the hydrothermal -Fe2O3 precursor and the calcinated -Fe2O3 spindles: TEM images of precursor (a–b) and -Fe2O3 spindles after calcinations (c–e), and XRD patterns of -Fe2O3 precursor (red line) and calcinated -Fe2O3 spindles (blue line).

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 56 of 57

Figure 5. TEM, STEM, and HRTEM images and Au particle size distributions of the as-prepared Au/-Fe2O3

catalysts:

(a1–4)

Au/-Fe2O3 -spindles,

(b1–4)

Au/-Fe2O3 -rods,

(c1–4)

Au/-Fe2O3 -hollow-rods, (d1–4) Au/-Fe2 O3 -hollows.

Figure 6. Fe 2p (A), O 1s (B), and Au 4f (C) XPS spectra of (a) Au/-Fe2O3 -spindles, (b) Au/-Fe2O3 -hollows, (c) Au/-Fe2O3 -hollow-rods, (d) Au/-Fe2O3 -rods.

4


Page 57 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. (A) CO conversion as a function of reaction temperature and (B) Arrhenius plots for CO oxidation over various Au/α-Fe2O3 catalysts. Reaction condition: CO concentration = 1 vol%, CO/O 2 molar ratio = 1/10, SV = 30000 mL/(gcat.·h).

Figure 8. In situ DRIFT spectra of (A) Au/-Fe2O3 -rods and (B) Au@-Fe2O3 -hollows in CO oxidation at different temperatures.

Figure 9. Schematic diagram of the reaction mechanism of CO oxidation over the Au/-Fe2O3 -hollows catalyst. 5


Î±-Fe2O3-Hollow Catalysts: General Synthesis

Recommend Documents