Catalytic oxidation of CO is a classical and crucial reaction, concerning the practical applications such as the gas sensors of trace amount of CO, automobile exhaust purification, and safety masks, which attracted enormous research interests. The supported Au nanoparticles (NPs), which play an important role in many catalytic reactions for the unusual catalytic performances [1], show unexpected performances in CO oxidation [2, 3]. Currently, the supported Au nanoparticles have been regarded to be promising for potential wide applications, but these catalysts still suffer from the challenges of stability and deactivation, where the sintering of NPs at high reaction temperature usually occurs [4].

To overcome the aforementioned issues, various strategies have been developed, including isolating the Au NPs on the surfaces [5, 6] or in the mesopores of supports [7], covering the Au NPs with metal oxide/silica/carbon overlayers [8, 9], encapsulating them in the rigid solids [10-14]. Particularly, construction of the strong metal-support interaction (SMSI) [15] to cover or encapsulate metal NPs by metal oxides, has been shown as an efficient route to stabilize the NPs. Several reducible oxides have been chosen to prepare these supported Au NP catalysts, including TiO₂ [16-18], Nb₂O₅ [19], ZnO [20], and CeO₂ [21, 22]. These strategies have effectively improved the stability of supported Au catalysts, but relatively complicated procedures for these strategies has a challenge for the industrial applications.

Herein, we developed a simple and efficient route to improve the catalytic activity and durability of Au NPs on an inert support of SiO₂ modified with CeO_x. Key to this success is to anchore a slight amount of CeO_x NPs onto the surface of Au NPs in the presence of ethylenediaminetetraacetic acid (EDTA), which was denoted as CeO_x@Au/SiO₂. Owing to the modification by CeO_x, abundant Au-CeO_x interfaces were reasonably achieved, leading to increase of positively-charged Au (Au^δ+) concentration. Catalytic tests in CO oxidation show that the CeO_x@Au/SiO₂ exhibits much better activity and stability than conventional Au nanoparticles supported on silica support (Au/SiO₂).

Synthesis of Au NPs. As a typical run, 1.0 g of polyvinyl pyrrolidone (PVP, K-30) was added to 100 mL of HAuCl₄ aqueous solution (0.05 mmol of HAuCl₄). After stirring the mixture in an ice bath for 0.5 h, 10 mL of NaBH₄ aqueous (0.01 mmol of NaBH₄) was added quickly with vigorous stirring. After stirring for another 2 h, the Au NPs colloid were finally obtained.

Synthesis of CeO_x@Au/SiO₂. In a typical run, 0.4 mmol of Ce(NO₃)₃ was added to the 100 mL of Au NPs colloid with stirring, followed by addition of 0.8 mL of NH₃·H₂O and 5 mL of EDTA aqueous solution (0.4 mmol of EDTA). After stirring for 0.5 h, 1.0 g of amorphous SiO₂ was impregnated with the colloid. The liquid mixture was continuously stirred for another 3 h at room temperature. After distilling under vacuum condition to remove the water, drying at 105 ℃ overnight, and calcining at 400 ℃ for 4 h, the CeO_x@Au/SiO₂ was finally obtained.

Synthesis of CeO_x-Au/SiO₂. 0.04 mmol CeO₂ was added to the 100 mL of Au NPs colloid directly. After stirring for 0.5 h, 1.0 g of amorphous SiO₂ was impregnated with the colloid. The liquid mixture was continuously stirred for another 3 h at room temperature. After distilling under vacuum condition to remove the water, drying at 105 ℃ overnight, and calcining at 400 ℃ for 4 h, the CeO_x-Au/SiO₂ was obtained.

Synthesis of Au/SiO₂. The Au/SiO₂ was prepared via the similar impregnation without the addition of Ce species.

Synthesis of CeO₂/SiO₂. The CeO₂/SiO₂ was synthesized by physically mixing 0.08 mmol of CeO₂ and 1.0 g of amorphous SiO₂.

The CO oxidation was carried out in a continuous fixed-bed glass vertical reactor (length at 450 mm and inner diameter at 6 mm). As a typical run, quartz sands were placed into both ends of the catalyst to maintain the bed height and reduce the dead volume. 0.15 g of catalyst (40-60 mesh) was diluted with 0.3 g of quartz sands (40-60 mesh) in the catalyst bed. Before the reaction, the catalyst was pretreated by O₂ (20% in He) at 300 ℃ for 1 h. CO/O₂/He (2%/16%/82%) was introduced to the upper inlet of the reactor with a rate of 40 mL/min. The reactor temperature was programmed by a temperature-controlled instrument. The composition of effluent gas was analyzed with a Fu Li-9790 gas phase chromatography (GC) equipped with a thermal conductivity detector (TCD).

X-ray diffraction (XRD) patterns were collected on a Rigaku D/MAX 2550 diffract meter with Cu K_α radiation (λ = 1.5406 Å). The composition of catalysts was measured with an inductively coupled plasma (ICP) analysis (Perkin-Elmer 3300DV). Transmission electron microscopy (TEM) images were obtained on a JEM-2100F electron microscopy with an acceleration voltage of 200 kV and a FEI Tecnai G² F20 S-TWIN electron microscopy with an acceleration voltage of 200 kV. X-ray photoelectron spectra (XPS) of the samples were recorded using a Kratos AXIS SUPRA with Al K_α X-ray radiation as the X-ray source. The binding energies were calibrated on the basis of the C 1s (284.8 eV) peak. H₂-temperature programmed reduction (H₂-TPR) was performed on a Finesorb-3010. Infrared (IR) spectra were recorded using a Bruker Vestor 22 FT-IR spectrometer equipped with a MCT/A detector and ZeSe windows and a high temperature reaction chamber. As a typical run, 50 mg of solid sample was localized in the chamber and pretreated at 200 ℃ for 30 min in flowing pure Ar (20 mL/min). Then, the chamber was adjusted to desired temperature (200 ℃), and CO (10% CO in Ar) was flowed to the sample for 30 min. After removing the physically adsorbed CO by pure Ar gas, the FTIR spectra of CO adsorbed on the samples were recorded. Then O₂ (10% O₂ in Ar) was introduced, the spectra were collected when gas was flowed through the chamber.

By ICP-AES analysis, the Au and Ce loading on the CeO_x@Au/SiO₂ were measured to be 0.9 and 5.0 wt%, respectively. For comparison, the CeO_x-Au/SiO₂ was synthesized with similar method in the absence of EDTA, and the CeO₂/SiO₂ was prepared from a physical mixture of CeO₂ and SiO₂.

Fig. 1 shows XRD patterns of Au/SiO₂, CeO_x@Au/SiO₂ and CeO_x-Au/SiO₂, giving peaks at 38.2°, 44.3°, 64.6° and 77.5°, which are characteristics of (111), (200), (220) and (311) planes of metallic Au, respectively. In addition, CeO_x@Au/SiO₂ and CeO_x-Au/SiO₂ samples also show diffraction peaks at 28.5°, 33.0°, 47.5° and 56.3° associated with of CeO₂, indicating the successful loading of CeO_x species on the Au/SiO₂ catalyst. The CeO₂ signals of CeO_x-Au/SiO₂ are stronger than those of CeO_x@Au/SiO₂, suggesting the relatively larger CeO_x particles on CeO_x-Au/SiO₂ (Fig. S1).

TEM images give direct observation of the Au and CeO_x distribution on silica support. As shown in Fig. 2(a), the Au NPs are uniformly distributed on the SiO₂ support, displaying narrow diameter distribution at 6.0-15.0 nm and mean size at 10.6 nm. Very interestingly, small CeO_x crystals (lattice distance at 0.31 nm, corresponding to the (111) plane) with diameters at ~2 nm were observed on the surface of Au nanoparticle (Fig. 2(b)).

Furthermore, XPS characterization was employed to analyze the surface composition of the catalysts. As shown in Fig. 3, Au 4f_7/2 XPS spectra give binding energy signals at 83.6, 84.5 and 85.7 eV [23]. These signals can be assigned to the metallic Au NPs and surface positive Au species of Au⁺ and Au³⁺, respectively. Notably, the positively charged Au concentration of CeO_x@Au/SiO₂ catalyst is much higher than those of Au/SiO₂ and CeO_x-Au/SiO₂ catalysts, which could reasonably be attributed to the modification of Au/SiO₂ by CeO_x species, where the electronic interaction occurred between Au species and CeO_x nanocrystals (Fig. S2). This interaction should be favorable for the catalytic oxidations since the positively charged Au species have been considered as an active site for the activation of oxygen [24, 25].

Fig. 4 shows CO oxidation light-off curves over CeO_x@Au/SiO₂, CeO_x-Au/SiO₂, CeO₂/SiO₂, and Au/SiO₂ catalysts, exhibiting quite different catalytic performances. The Au/SiO₂ catalyst shows low CO conversion (4.0%) at 160 ℃ and complete conversion of CO at 340 ℃. In contrast, the CeO_x@Au/SiO₂ catalyst exhibits CO conversion of 98.8% at 160 ℃ and the complete conversion is achieved at 180 ℃. When the CeO_x-Au/SiO₂ is employed, the complete CO conversion is at 300 ℃, which is slightly better than the Au/SiO₂ catalyst. In addition, the CeO₂/SiO₂ catalyst without Au is also active for CO oxidation, but the CO conversion is much lower (half CO conversion at 460 ℃). Considering the CeO_x@Au/SiO₂, CeO_x-Au/SiO₂, and Au/SiO₂ have similar Au nanoparticles and loading as well as the same silica support, a significant enhancement of catalytic activity should be directly assigned to the CeO_x modification rather than other factors. In addition, the CeO_x-Au/SiO₂ is much less active than the CeO_x@Au/SiO₂, therefore it is suggested that the EDTA might be also important. One possibility is that the EDTA serves as a ligand to maximize the interaction between CeO_x nanocrystals and Au nanoparticles.

In attempt to understand the promotion of CeO_x, we have performed in situ DRIFT experiments over the CeO_x@Au/SiO₂ and Au/SiO₂ catalysts. For CO adsorption (Fig. 5(a)), the Au/SiO₂ catalyst gives IR bands at 2156 and 2117 cm^-1 [26, 27] associated with the CO adsorbed on positive and metallic Au species, but the CeO_x@Au/SiO₂ catalyst shows additional IR bands at 2230, 1599, 1493 and 1449 cm^-1. The band at 2230 cm^-1 is correspondent to [Au(CO)₂]^δ+ species [28], which formed by two CO species adsorbed on the same positive Au nanoparticle. The bands at 1599, 1493 and 1449 cm^-1 are assigned to the bidentate, polydentate, and monodentate carbonate (CO₃^2-) species. These species might mean that the CO indeed interacts with the active oxygen species on the catalyst surface. The invisibility of these bands over the Au/SiO₂ suggests the lack of active oxygen species. In addition, it is also observed a strong band at 2173 cm^-1 on the CeO_x@Au/SiO₂, which is attributed to the adsorption of CO on Ce⁴⁺ species. In a word, the existence of Au-CeO_x interface strengthens the CO adsorption on the CeO_x@Au/SiO₂, which benefits for the CO conversion.

When oxygen was introduced to the CO-adsorbed catalysts, as shown in Fig. 5(b), new bands at 2232, 1492, and 1447 cm^-1 are observed on the Au/SiO₂, which are correspondent to [Au(CO)₂]^δ+ species, polydentate, and monodentate CO₃^2- species, respectively. These signals indicate the generation of CO₂. Besides, bands at 2156 and 2117 cm^-1 on the CO-adsorbed Au/SiO₂ shift to 2162 and 2120 cm^-1, meaning that the CO adsorption become weak. For the CeO_x@Au/SiO₂ catalyst, bands at 2361 and 2334 cm^-1 assigned to gas phase CO₂ are observed, demonstrating higher CO oxidation activity of CeO_x@Au/SiO₂ than that of Au/SiO₂. Part of [Au(CO)₂]^δ+ species (2241 and 2231 cm^-1) and CO species adsorbed on the Au NPs (2175, 2137 and 2120 cm^-1) still exist, but their intensities are significantly reduced. In contrast, the bands at 1602, 1492 and 1447 cm^-1 are obviously strengthened, suggesting that the CO₃^2- species are mainly derived from CO adsorbed on the CeO_x@Au/SiO₂. Moreover, it is observed a weak band at 2343 cm^-1 assigned to CO₂ adsorbed on Ce⁴⁺, confirming strong oxidation activity from CO to CO₂ over the CeO_x@Au/SiO₂ catalyst, in good agreement with those shown in the catalytic results.

Fig. 6 shows the durability of the CeO_x@Au/SiO₂ and Au/SiO₂ catalysts. In the beginning of reaction at 180 ℃ over the CeO_x@Au/SiO₂, the CO is completely converted. Changing the temperature to 140 ℃, it is given the CO conversion at 78%. To evaluate the stability of the Au NPs, we employed a high temperature of 260 ℃. After reaction at 260 ℃ for 18 h, we cannot observe any decrease for the CO conversion. After cooling to 180 ℃ for another 12 h, full CO conversion is also achieved, confirming very high durability of the CeO_x@Au/SiO₂ catalyst. In contrast, the CO conversion of the Au/SiO₂ is remarkably reduced after reaction at 260 ℃ for 10 h, which is reasonably attributed to the aggregation of Au NPs [4]. In addition, we also performed the calcination treatment of the catalysts at 600 ℃ for 3 h (Fig. 7), which is harsh for the survival of normal Au NPs. Interestingly, it is almost unchangeable for the Au NPs of CeO_x@Au/SiO₂ (10.8 nm) before and after the calcination, but it is obviously observed the aggregation of the Au NPs on the calcined Au/SiO₂.

In summary, we report a significant enhancement of the catalytic activity and durability over silica supported Au NP catalyst by CeO_x modification in the presence of EDTA. The existence of CeO_x nanocrystals on the Au NPs surface increases positively charged Au^δ+ concentration, improving the CO oxidation activity. More importantly, the CeO_x@Au/SiO₂ catalyst shows excellent durability. In situ IR spectra reveal that CeO_x modification greatly promotes the CO adsorption on the catalyst, leading to the efficient conversion of CO to CO₂ in the oxidative atmosphere. The strategy in this work might open an alternative way to develop more active and stable catalysts for the catalytic oxidations in the future.

Acknowledgments

Technical assistance, material support, and other help or advice may be acknowledged briefly in this section (excluding financial support, which should appear in the footnote on the title page).