A Rational Design of the Sintering-Resistant Au-CeO₂ Nanoparticles Catalysts for CO Oxidation: The Influence of H₂ Pretreatments

Abstract

The redox pretreatment of samples is one of the crucial ways of altering the catalytic properties of the supported noble metal materials in many heterogeneous reactions. Here, H₂-reducing pretreatment is reported to enhance the thermal stability of Au-CeO₂ catalysts prepared by the deposition–precipitation method and calcination at 600 °C for CO oxidation. In order to understand the improved activity and thermal stability, a series of techniques were used to characterize the physico-chemical changes of the catalyst samples. H₂ pretreatment may lead to: (i) a strong metal–support interaction (SMSI) between Au nanoparticles (NPs) and CeO₂, evidenced by the particular coverage of Au NPs by CeO₂, electronic interactions and CO adsorption changes. (ii) the production of surface bicarbonates which can accelerate CO oxidation. As a result, the H₂ pretreatment makes the Au NPs more resistant to sintering at high temperature and enhances the CO oxidation activity. Furthermore, this reduction pretreatment strategy may provide a potential approach to enhance the thermal-stability of other supported noble metal catalysts.

1. Introduction

In heterogeneous catalytic reactions, the Au nanoparticles (NPs), as the active species with superior catalytic activities, have been attracted considerable interests in recent decades. They are active in a great deal of oxidation and reduction reactions, especially in the oxidation of CO [1]. Most intensely studied are not only Au/CeO₂ catalysts, which have emerged as one of the best candidates for the low temperature CO oxidation [2,3], but also other catalysts such as Au/HAP [4], Au/Fe₂O₃ [5], Au/TiO₂ [6] and Au/ZrO₂ [7] have been investigated. However, the major problem of supported Au catalysts is the quick deactivation due to the low thermal stability during the catalytic reactions, thus, hindering their practical application. Au NPs are thermodynamically unstable and tend to be easily sintered when calcined at elevated temperatures or a long period of time. Since then, the active supported Au catalysts with the sintering-resistant property have been covered widely by numerous studies. As a result, it has been confirmed that the catalytic activity strongly depends on the change of Au particle size [8], the appropriate choice of support materials (oxides/non-oxides) [9], the formation of carbonates adsorbed on the active sites [10] and the metal-support interactions [11].

Several strategies have been developed to stabilize Au NPs for CO oxidation processes. Chen’s group reported that the enhancing of catalytic activity and the various carbonate species are contributed by the different small Au particle sizes [8]. Putla et al. introduced various dopants to Au/CeO₂, and found that the appropriate support materials is able to improve the catalytic properties of Au based catalysts [12]. Qiao et al. demonstrated a classical strong metal–support interaction (SMSI) for Au/TiO₂, which markedly depended on the reversible encapsulation of Au NPs by TiO₂ support following high-temperature redox pretreatments [13]. Park’s group performed an oxidation atmosphere pretreatment on Au/TiO₂ catalysts, and found that the oxidation pretreatment enhanced SMSI with the improved activity of CO oxidation [14]. However, the structural effects associated with the pretreatment process are not well understood. Because the Au NPs calcined at high temperatures are easily agglomerated after pretreatments [15], there are few efficient methods to improve the thermal stability of Au NPs with reduction conditions.

Here, inspired by previous research results, H₂-reducing pretreatment method was used to fabricate SMSI in the Au-CeO₂ catalysts further to trace whether the Au NPs can be stabilized while holding their high activity. So, in our study, the Au-CeO₂ spheres calcined at 600 °C for various times, followed by pretreated upon H₂ atmosphere at 200 °C. These products were characterized by X-ray diffraction (XRD), N₂ adsorption–desorption, high-resolution transmission electron microscope (HRTEM), in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) and X-ray photoelectron spectra (XPS) to elucidate the effect of H₂ pretreatments on the catalytic performance of CO oxidation. After H₂ pretreatment, the introduction of SMSI in the Au-CeO₂ catalysts could provide the enhanced activity and thermal stability until the calcination times increased to 12 h. Thus, the H₂ pretreatment makes the Au NPs more resistant to sintering at high temperatures, which may be extended to other supported noble metal catalysts.

2. Experimental

2.1. Materials

Cerium nitrate (Ce (NO₃)₃·6H₂O, 99.5%) was purchased from Shanghai Macklin Biochemical Reagent Factory, Shanghai, China. Polyvinyl pyrrolidone (PVP, K30), ethylene glycol and chloroauric acid (HAuCl₄·4H₂O, 99%) were obtained from Sino pharm Chemical Reagent Factory, Shanghai, China. All reagents were used without further purification. Deionized water and absolute alcohol were used throughout.

2.2. The Synthesis of Au-CeO₂ Samples

The CeO₂ nanospheres were synthesized with minor alterations according to our previous reports [16]. In a representative experiment, Ce (NO₃)₃·6H₂O (1.0 g) and PVP (0.4 g) were dissolved in ethylene glycol (30 mL) and distilled water (2 mL). The mixture was kept for another 20 min at atmospheric temperature. The resulting clear liquor was transferred to a 100 mL Teflon-lined autoclave and heated at 160 °C for 8 h. When the autoclave was cooled at atmospheric temperature, the mauve products were collected and washed separately with deionized water and ethanol in rinse-centrifuge cycles. The CeO₂ products were dried at 60 °C in an oven overnight.

Au NPs were placed onto CeO₂ by a deposition–precipitation method. At atmospheric temperature, 10 mL CeO₂ precursor solution (0.145 mol/L) was added to 10 mL HAuCl₄·4H₂O (0.024 mol/L) whose pH was adjusted to 9 by NaOH (0.1 mol/L) in advance. The pH of the mixture solution was kept at ~9 for 1 h by NaOH (0.1 mol/L) at atmospheric temperature. Then the mixture was heated to 60 °C and stirred for 1 h. The precipitates were collected and washed with deionized water and dried at 60 °C for 12 h. The sample was calcined in a muffle furnace at 600 °C for 3 h, 6 h, 9 h, 12 h and 24 h. After that, the catalysts were pretreated at 200 °C for 0.5 h in H₂ (5% H₂ in N₂) atmosphere, and denoted as AC-3-H, AC-6-H, AC-9-H, AC-12-H and AC-24-H, respectively. Apart from, the samples without pretreatment were denoted as AC-3, AC-6, AC-9, AC-12 and AC-24.

2.3. Characterization

Powder X-ray diffraction patterns (XRD) were carried on a BRUKER-AXS D8 Advance X-ray Diffractometer (Berlin, Germany) with Cu-Kα radiation (λ = 0.15418 nm) in the 2θ range from 10° to 90°. BET surface areas and size distribution of catalysts were determined by N₂ adsorption-desorption isotherms using a Micromeritics ASAP 2020 surface area analyzer (Norcross, GA, USA). JEM-2100F microscope (JEOL Ltd., Tokyo, Japan) operating at 300 kV was employed to acquire high-resolution transmission electron microscope (HRTEM). The existence of surface elements and their valence states were confirmed by X-ray photoelectron spectra (XPS, Thermo Scientific Escalab 250Xi, Hongkong, China).

The in-situ diffuse reflectance infrared Fourier-transform spectra of CO adsorbed on solid samples were performed on a Nicolet IS50 (Thermo Fisher Scientific, Hongkong, China). Before the tests, the samples were pretreated at 200 °C for 0.5 h in H₂ (5% H₂ in N₂) atmosphere. After cooling to atmospheric temperature, the background spectrum was recorded. Afterwards, a feed gas (0.2% CO/N₂) was brought into the sample cell at the flow rate of 50 mL/min for 20 min and then the spectra were recorded until there was no variation inside. The spectra are displayed in Kubelka–Munk units, with the vertical axis implicating absorbance.

2.4. Catalytic Tests

The activities of catalysts in CO oxidation were carried out in a fixed-bed quartz reactor with a length of 240 mm and a diameter of 8 mm. The catalyst weight was 50 mg, sieved into 40–80 mesh and the catalyst was not diluted in any diluent. A feed gas (0.2% CO in N₂ + 5% O₂ in He) was introduced into the catalyst equipment at the total flow rate of 100 mL/min, resulting in a space velocity (SV) of 120,000 mL·g_cat⁻¹·h⁻¹. Then, the samples were heated from 30 °C at the rate of 4°/min. The products were detected with an online gas chromatograph (GC-2080, Ruipeng Chemical Instrument Corporation, Tengzhou, China) equipped with a thermal conductivity detector (TCD). The stability tests were measured with the same reactor and the same feed gas at 100 °C.

3. Results and Discussion

Catalytic CO oxidation is used as a probe reaction to investigate the relations between the pretreatment conditions and the catalytic properties of the Au-CeO₂ catalysts. Figure 1 describes the CO conversion profiles of the various Au-CeO₂ samples calcined at 600 °C for 3–24 h. The T₅₀ and T₉₀ (temperatures for 50% and 90% CO conversion) of different samples were summarized in (Table S1) to compare the activity of the catalysts. For the Au-CeO₂ samples calcined at different prolonged times without H₂ pretreatments, the CO oxidation activities decrease in the order: AC-6 > AC-3 > AC-9 > AC-12 > AC-24 (Figure 1a). Obviously, the deactivation could occur in the Au-CeO₂ samples without H₂ pretreatment as the prolonged calcination time. Before the catalytic process, introducing the H₂ pretreatments to the samples causes a significant enhancement of the catalytic activity (Figure 1b). The catalytic activity of AC-3 with uncovered Au NPs is significantly lower than that of AC-3-H with partially covered Au NPs. However, the AC-6-H sample (T₅₀ = 60 °C, T₉₀ = 106 °C) has a similar catalytic activity on AC-6 sample (T₅₀ = 70 °C, T₉₀ = 101 °C), indicating the active site of the AC-6 sample after H₂ pretreatment is blocked. When the calcination time arises to 12 h, the samples with H₂ pretreatment also provides stable catalytic performance (T₅₀ = 46 °C, T₉₀ = 111 °C). Especially, the AC-9-H sample has better catalytic activities, which gives the T₅₀ of 45 °C and T₉₀ of 77 °C.

To better evaluate the thermal sintering performances, the temporal evolution profiles of the CO oxidation over the Au-CeO₂ samples with and without H₂ pretreatments (reaction at 100 °C for 48 h) are illustrated in Figure 2. There was no obvious deactivation in CO conversion, demonstrating its sintering-resistant catalytic performance. Overall, it can be found that H₂ pretreatment could obtain thermally stable Au-CeO₂ samples with a higher activity in comparison to samples without H₂ pretreatments.

The XRD profiles of different Au-CeO₂ samples are shown in Figure 3 (with H₂ pretreatment) and Figure S1 (without H₂ pretreatment). The reference pure CeO₂ sample in Figure S1 showed weak diffraction peaks with a lower average crystallite size of 5.0 nm (summarized in Table S2). All Au-CeO₂ samples with or without H₂ pretreatment clearly show obvious diffraction peaks at 2θ value of 28.5° (111), 33.1° (200), 47.4° (220), 56.3° (311), 59.0° (222), 69.4° (400), 79.1° (331) and 88.4° (420), which are the fluorite-type cubic structure of CeO₂ [16]. Besides, the diffuse diffraction peaks of Au (111) located at 38.2°, suggesting the characteristic of Au NPs [17].

In order to know the influence of the calcination time on the textural properties of CeO₂, average crystallite sizes were calculated from the Debye–Scherrer equation and specific surface areas were determined from N₂ adsorption-desorption test (

Table 1. Structural information of the different samples.

Sample	CeO2 Average Crystallite Size (nm) a	SBET (m2/g) b	Vp (cm3/g) c
AC-3-H	10.2	55.1	0.20
AC-6-H	10.9	39.8	0.18
AC-9-H	10.0	45.0	0.14
AC-12-H	12.8	36.1	0.14
AC-24-H	13.3	33.2	0.18

^a Estimated from XRD studies; ^b Brunauer–Emmett–Teller (BET) specific surface area; ^c Total pore volume.

). It clearly shows that the specific surface area of Au-CeO₂ samples is about 30–60 m²/g with pore volume of 0.14–0.20 cm³/g (Table S1). For the unpretreated Au-CeO₂ samples, the prolonged calcination time is accompanied by the increase of CeO₂ average crystallite size (Table S2). After H₂ pretreatment, the CeO₂ average crystallite size for all Au-CeO₂ samples is decreased, implying that H₂ pretreatment may restrain the size growth of CeO₂. There is a decrease trend in surface area with the prolonged calcination time which correlates well with the increase of average crystallite size except the AC-9-H sample. The meticulous correlation of crystallite size and surface area of Au-CeO₂ samples reveals the unusual crystallite size and surface area of AC-9-H sample. H₂ pretreatment causes an obvious variation of average crystallite size for Au-CeO₂ samples, which may lead to the unusual surface area for AC-9-H sample.

Transmission electron microscope (TEM) is further used to identify microscopic structures of Au-CeO₂ with or without H₂ pretreatment. As shown in Figure 4, highly dispersed Au NPs are successfully attached on the CeO₂ hollow nanospheres. For all Au-CeO₂ samples, the CeO₂ nanospheres have a particle size of ~120 nm (Table S3). With the increase of calcination time at 600 °C, there is no obvious agglomeration for CeO₂ nanospheres. Significantly, it is clearly found that the sizes of Au NPs increase with the prolonged calcination time following in the order: AC-3-H (9.8 nm) < AC-6-H (10.1 nm) < AC-9-H (10.2 nm) < AC-12-H (12.0 nm) < AC-24-H (13.0 nm) (Table S3).

As can be seen in Figure 5, the typical HRTEM micrographs of the single Au-CeO₂ nanosphere reveal the presence of Au particles, in contact with the CeO₂ surfaces. The fringe period of ~0.312 nm and ~0.271 nm is respectively in agreement with the (111) and (200) lattice spacing of CeO₂, the lattice fringes of ~0.236 nm and ~0.204 nm are consistent with the (111) and (200) crystal plane of metallic Au. As shown in Figure 5a, Au NPs on the AC-3 sample without H₂ pretreatment were naked. After H₂ pretreatment, several important characters can be distinguished for the different samples in Figure 5b–f: (i) the naked and covered Au NPs coexist for AC-3-H, AC-9-H, AC-12-H and AC-24-H samples; (ii) whereas for AC-6-H sample, the coverage increased rapidly and the Au NPs are almost completely covered. Interestingly, AC-6-H is the only sample with Au NPs completely covered, which may be due to the increase of Au NPs and CeO₂ crystallite size with the prolonged calcination time and/or the reduction of H₂ pretreatment. The coverage of oxides support on metal NPs is a typical feature of SMSI phenomenon [13]. As a result, the coverage of CeO₂ support on the Au NPs indicates the formation of SMSI in our Au-CeO₂ catalysts with H₂ pretreatments.

The SMSI generally results in fewer CO adsorption sites on Au NPs, because the CO adsorption sites are covered by the support layer [17]. The in-situ DRIFTS measurements of CO adsorption with a suitable probe molecule were used to explore the CO adsorption change and/or electron transfers of the Au-CeO₂ catalysts. As shown in Figure 6a, two bands are detected on the 2168–2177 cm⁻¹ and 2104–2112 cm⁻¹ for all the samples. The former is assigned to gaseous CO [18], and the later can vest in CO at the metallic Au (CO-Au⁰) [19]. Obviously, the unpretreated AC-3 sample exhibits the highest peak intensities for the two bands with CO-Au⁰ centered at 2112 cm⁻¹. Additionally, for the other unpretreated Au-CeO₂ samples, the CO-Au⁰ band is also detected at around 2112 cm⁻¹ (shown in Figure S2a). After H₂ pretreatment, the CO-Au⁰ band for AC-3-H sample is blue shifted to 2104 cm⁻¹ with the peak intensity largely decreased. As the calcination time increased, there is a blue shift for CO-Au⁰ absorbed species on Au-CeO₂ samples, giving 2108 cm⁻¹ over AC-9-H, and 2112 cm⁻¹ over AC-6-H, AC-12-H and AC-24-H. After H₂ pretreatment, the CO-Au⁰ band on Au-CeO₂ samples is blue shifted, indicating more electropositive Au species can be formed on Au-CeO₂ with H₂ pretreatment. This blue-shift of the CO adsorption suggests the electron transfer from the CeO₂ support to Au NPs [20]. Besides, the AC-6-H sample gives a lower intensity at the CO-Au⁰ band. According to the previous reports [17], the decreased CO adsorption is related to the loss in CO adsorption sites mainly deriving from the partial cover of Au NPs by CeO₂ supports. In our work, the H₂ pretreatment could be a powerful approach to alter the electron interactions between Au NPs and CeO₂.

The in-situ DRIFT spectra of CO adsorption in the region of 2000–800 cm⁻¹ (Figure 6b and Figure S2) shows bands corresponding to various carbonate surface substances, which are basically chemisorbed on CeO₂. The presence of various carbonate species is summarized in Table S4. Further, Several important carbonate species can be distinguished for the Au-CeO₂ catalysts: tridentate carbonates (bands at 1048–1073 cm⁻¹ and 1266–1276 cm⁻¹ and 1460–1550 cm⁻¹), bidentate carbonates (bands at 1014–1028 cm⁻¹ and ~1319 cm⁻¹) and bicarbonate species (bands at 1600–1616 cm⁻¹ and 1618–1638 cm⁻¹) [21]. The negative bands at 1600–1700 cm⁻¹ in Figure 6b and Figure S2b may be ascribed to the adsorption of water vapor on the sample before IR tests. However, only traces amount of tridentate carbonates and bidentate carbonates are formed on AC-3 sample. The carbonate surface species are more easily formed after H₂ pretreatment. It can be noted from HRTEM results that the H₂ pretreatment leads to a closer connection between Au NPs and CeO₂ which facilitates CO oxidation at low temperatures [9]. Thus, the produced CO₂ may be adsorbed on the ceria surfaces forming the large amounts of carbonates. When the Au-CeO₂ samples are exposed to CO, the CO molecule can be adsorbed on the active Au NPs, forming CO-Au⁰ species. The lattice oxygen atoms on the surface of CeO₂ are more reactive and can react with the adsorbed CO molecule, producing carbonates, biocarbonates species and oxygen vacancy. Finally, the surface carbonates species decomposed to CO₂, with the remaining oxygen atom filling the vacancy. In fact, the proposed reaction mechanisms have already been widely accepted by other researches [22].

For the Au-CeO₂ samples with H₂ pretreatments, the band intensity of these carbonate species decreases in the order: AC-9-H > AC-3-H > AC-6-H > AC-12-H > AC-24-H. Among all samples with H₂ pretreatments, AC-9-H and AC-3-H samples exhibit relatively larger band intensity of carbonate species, which may be ascribed to their higher surface areas compared with other samples. Significantly, bicarbonate species have been detected for all samples and account for the vast majority of these carbonates species. Moreover, when the calcination time increased to 24 h, the peak intensity of the bicarbonates species decreased sharply. It is well known [18,23] that the surface bicarbonates are less thermally stable than carbonates species, which facilitates faster desorption of CO₂, allowing the surface effective for further chemisorption. Therefore, the presence of bicarbonates on the Au-CeO₂ under H₂ pretreatments can accelerate CO oxidation.

X-ray photoelectron spectroscopy (XPS) experiments were conducted in order to confirm whether there is a change in the valence states of the elements in the Au-CeO₂ samples with or without H₂ pretreatment. Figure 7 shows the XPS spectra of Au 4f and Ce 3d for different catalysts with H₂ pretreatment, while Figure S3 shows the XPS spectra of AC-3 without H₂ pretreatment. In the Ce 3d region, the peaks marked as u’ (916.0 eV), v’ (900.3 eV), u’’ (898.0 eV), v’’’ (882.0 eV), u’’’ (907.0 eV) and v’’’ (888.5 eV) correspond to the Ce⁴⁺ state, whereas that denoted as u (902.1 eV) and v (884.5 eV) are assigned to Ce³⁺ [24]. In Table S5, the results of the primary binding energies Ce 3d achieved by the XPS quantitative analysis, are reported for the different samples. There is no difference in the Ce 3d spectra of these samples except AC-6-H, indicating their similar electronic properties. Clearly, the over encapsulation of Au NPs for AC-6-H may result in a lower content of Ce³⁺.

In the Au 4f region, for quantitative evaluation, the Au 4f peaks with binding energies (BEs) were determined to be two different states. After curve fitting, the Au 4f peaks with binding energies of about 84.3 and 87.8 eV in AC-3 sample are attributed to the presence of Au⁰, while two weak BE peaks at 88.4 and 84.6 eV indicate the existence of the Au^δ+ species [21]. After H₂ pretreatment, a visible shift of BE to 83.5 eV–83.8 eV was observed on Ae-CeO₂ samples, implying the Au NPs supported on CeO₂ become electron-rich [11]. As a consequence of this electron-rich effect, the atomic ratio of the Au^δ+ species for Au-CeO₂ after H₂ pretreatment decreases rapidly (Table S6). Overall, the electron transfers between Au NPs and CeO₂ are created to make their interface more closely connected, leading to electron-rich Au after H₂ pretreatment [4]. According to the in-situ DRIFTS, XPS and HRTEM results, the particular coverage of Au nanoparticles by CeO₂, the electron transfers and the CO adsorption changes are identical to those characteristics in SMSI.

There are many efficient strategies for stabilizing Au NPs on various supports, such as the synthetic of yolk–shell nanoparticles [25], the coverage of Au NPs substrate coatings and the surface-modified of supports before Au NPs loading [26]. After H₂ pretreatment, our CO oxidation results imply that the Au-CeO₂ catalysts provide the enhanced activity and thermal stability until the elevated calcination times increased to 12 h. This is due to the occurrence of SMSI and the presence of bicarbonates. After H₂ pretreatment, the construction of SMSI can be proposed for Au-CeO₂ catalysts where the particular coverage of Au nanoparticles by CeO₂, the electron transfers and the CO adsorption changes are in good agreement with those in classic SMSI. As for the non-linearity in the catalytic performance, the possible reason can be ascribed to the different coverage degree of Au NPs on the CeO₂ support and the size variation of Au NPs. The partially exposed Au NPs for AC-3-H and AC-9-H, which are in direct contact with CeO₂, the show the better catalytic activities for CO oxidation. On the contrary, the over-encapsulation of Au NPs on AC-6-H can suppress the active sites, resulting in relatively low activities. Moreover, the AC-24-H sample has poorer catalytic activities than others due to the relatively larger Au NPs. Besides, the surface bicarbonates would favor a faster desorption of CO₂, through which the CO oxidation is accelerated. Moreover, the AC-24-H sample has poorer catalytic activities than others due to the relatively larger Au NPs and less bicarbonates surface species, which are still an important factor to limit the catalytic activity.

4. Conclusions

In summary, we have unambiguously demonstrated that the resulting Au-CeO₂ samples calcined at 600 °C are highly stable under a H₂ environment at elevated calcination times. Owing to the construction of SMSI and the presence of bicarbonates, it results in a remarkable high catalytic activity, which makes it possible to obtain the sintering-resistant Au catalysts. Furthermore, it is expected that these discoveries may provide a novel way for supported Au catalysts with thermal stability and may be expanded to other supported noble metal catalysts.