Carbon-protected Au nanoparticles supported on mesoporous TiO₂ for catalytic reduction of p-nitrophenol

Abstract

With the introduction of carbon on Au/TiO₂, the reaction rate of C/Au/TiO₂ increased by 29% and the stability was enhanced by about 3 times more than Au/TiO₂ in the p-nitrophenol reduction reaction. Carbon species enhanced the stability of Au nanoparticles and also increase the organic reactants adsorptive ability.

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Supported gold catalysts have drawn tremendous attention due to their high catalytic activity and efficiency for a broad range of chemical reactions including CO oxidation,^1,2 selective hydrogenation,^3,4 methanol synthesis^5,6 and water–gas shift.^7–9 Numerous studies have found that the activity of gold catalysts was largely dependent on their particle size and dispersion.¹⁰ However, with the decrease of the size of Au NPs, the nanosized Au NPs easily move and grow on most of the metal oxide surfaces. Once NPs aggregation occurs and the particle size increases to micro-metric scale, the activity of Au NPs could disappear completely. Thus, Au NPs aggregation is still an obstacle to the industrial application particularly in reaction systems.¹¹ In order to ameliorate the stability of Au NPs on the support surface, researchers focus on incorporating additional components (SiO₂, TiO₂, FeO_X et al.) which attempt to anchor Au NPs and prevent aggregation.^12–15 Compared with these metal oxides, carbon is affordable, easily prepared, and chemical inertness, which would not bring unexpected side reactions in industrial reaction processes. Another important role of carbon is the adsorption ability of organics in the solution which could promote mass transfer in catalyst-related process.¹⁶ Here, we report an in situ carbonization method to improve the p-nitrophenol reduction rate and stability of Au/TiO₂ catalyst.

In our previous work, mesoporous TiO₂ was prepared from potassium titanate via hydration and ion-exchange.¹⁷ The corresponding gold catalyst exhibited excellent catalytic performance as well as good stability due to mesoporous structure.^18,19 In order to improve catalyst performance and stability, we introduced a layer of carbon on the surface of gold catalyst. As Fig. 1(a) shows, the Au/TiO₂ was coated with a layer of glucose, followed by in situ carbonization of the glucose at 500 °C to leave a base of carbon at the catalyst surface. Compare with the Au/TiO₂ calcinated at the same temperature, the aggregation of Au NPs was limited and the size of Au NPs in C/Au/TiO₂ smaller than that in Au/TiO₂. The detailed carbon modification processes are described in ESI.†

Fig. 1 (a) Schematic illustration of the C/Au/TiO₂ and Au/TiO₂ preparation. (b) Raman spectrum and (c) TG curve of C/Au/TiO₂.

Raman analysis of C/Au/TiO₂ was used to identify the carbon species on TiO₂ (Fig. 1(b)). It shows a characteristic G-band at around 1600 cm⁻¹ and a weak D-band at around 1340 cm⁻¹, indicating that the carbon species is graphitized carbon.^20,21 Carbon content in the C/Au/TiO₂ catalysts was measured by TG. As illustrated in Fig. 1(c), the sample weight loss before 300 °C was ascribed to the removal of physically adsorbed water and impurity. The weight loss from 300 °C to 500 °C is mainly due to the burn out of carbon species.²² The carbon content could be less than 1.1 wt%.

XRD analyses show the crystalline structures of TiO₂, Au/TiO₂, and C/Au/TiO₂ (Fig. S1 in ESI†). Seven distinct diffraction peaks of three samples are indexed to anatase (JCPDF 21-1272) with high crystallinity, while weak peaks corresponding to the TiO₂ (B) phase (JCPDF 35-0088) can be observed. These results are consistent with previously report.¹⁸ The average TiO₂ crystal size was about 11 nm, as reckoned from the peak width of the anatase (101) peak by using the Scherrer equation. Such common characteristics of these three samples show that they have the same crystal structure. It reveals that the carbon species may not influence the catalysts structure and dispersion of active species. However, the presence of diffraction peaks of Au was dim probably owing to its low content and small particle size. The Barrett–Joyner–Halenda (BJH) pore-size analyses from the N₂ desorption curve reveals that three samples exhibit almost identical pore-size distributions ranging from 6 to 12 nm with a mean pore size around 9 nm (Fig. S2 in ESI†). Table 1 summarized the specific surface area of each sample, 112.5 m² g⁻¹ for TiO₂, 112.1 m² g⁻¹ for Au/TiO₂, and 108.5 m² g⁻¹ for C/Au/TiO₂, which was calculated by the multi-point Brunauer–Emmett–Teller (BET) method. These observations demonstrate that catalyst structure was maintained even after introducing carbon materials.

Table 1 Physical structure properties and catalytic performance of samples

Sample	Crystal size/nm	SBET/m^2 g^−1	VP/cm^3 g^−1	Au loading^a/wt%	dAu^b/nm	k/min^−1
a The content of Au was analyzed by XRF.b The size of Au NPs was evaluated by FESEM statistics.
TiO2	11.2	112.5	0.19	—	—	—
Au/TiO2	11.2	112.1	0.19	0.871	5.5 ± 1.5	0.44
C/Au/TiO2	11.3	108.5	0.16	0.861	4.1 ± 1.0	0.57

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The morphology of Au NPs was further studied by TEM. As shown in the Fig. 2(a) and (b), Au NPs dispersed on mesoporous TiO₂ were observed. The lattice fringe of 0.35 nm corresponds to the (101) plane of anatase.²³ A representative high-resolution TEM (HRTEM) image (Fig. 2(c)) shows that Au NPs are highly crystalline as evidenced from the well resolved Au lattice fringes of 0.24 nm ((111) plane of Au).²³ With the incorporation of carbon, Au NPs size in C/Au/TiO₂ became smaller than that of Au/TiO₂ catalyst, and an uniform particle size distribution at 4 nm is observed in the inset of Fig. 2(a) and (b).

Fig. 2 (a) TEM images of Au/TiO₂. Inset: size distribution of Au NPs; (b) TEM images of C/Au/TiO₂. Inset: size distribution of Au NPs; (c) HRTEM images of C/Au/TiO₂; (d) UV-vis spectra of C/Au/TiO₂ and Au/TiO₂.

The Au NPs dispersion was also performed by UV-vis absorption spectroscopy. Fig. 2(d) shows that C/Au/TiO₂ and Au/TiO₂ both have an absorption peak in the visible region (from 550 to 580 nm). This peak can be attributed to the surface plasma absorption (SPA) of the Au NPs.²⁴ For C/Au/TiO₂, the absorption peak is at around 550 nm, which is lower than that of Au/TiO₂ (around 579 nm). The entire absorption peak in the visible region of Au/TiO₂ displays a larger red shift compared with that of C/Au/TiO₂. This red-shift was attributed to the gather of Au NPs as reported in the literature.²³ Au NPs on the surface of TiO₂ agglomerated after 500 °C, which cause the decreased distance among them. Apparently, this red shift demonstrates that Au NPs of C/Au/TiO₂ have a better dispersions and smaller sizes, which were also previously confirmed by TEM statistics analysis.

Reduction of p-nitrophenol by an excess amount of NaBH₄ is chosen as a model reaction to evaluate the catalytic performances of the Au catalysts. It is well-known that the reaction is simple and steady in the presence of metallic surfaces. The Au NPs played a key role as an electron transfer mediator between BH₄⁻ and PNP molecules. The maximum UV-vis absorption peak of the aqueous mixture of PNP and NaBH₄ stayed unaltered at 400 nm with time. Reaction mixture causes fading as the catalyst added. The reaction kinetics could be analyzed and confirmed from the time-dependent absorption spectra, which showed the gradually decrease of p-nitrophenolate ions.^25,26 Fig. 3(a) shows linear relation of ln(C_t/C₀) versus reaction time by using C/Au/TiO₂, Au/TiO₂ and Au/C. The ratio of C_t and C₀, where C_t and C₀ represent PNP concentrations at time t and 0, respectively, was measured from the relative intensity of the respective absorbencies. The linear relations of ln(C_t/C₀) versus time were observed for all the tested catalysts, indicating that the PNP reduction reaction followed the pseudo-first-order kinetics. In this study, C/Au/TiO₂ exhibited a better PNP reduction performance than that of Au/TiO₂. The reaction rate constant is 0.44 min⁻¹ calculated from the slope of Au/TiO₂, which is comparable to other gold catalysts reported recently.^27–30 However, with the introduction of carbon, the reaction rate constant of the C/Au/TiO₂ increased to 0.57 min⁻¹, which is 29% higher than Au/TiO₂. Surface carbon species on gold catalyst is beneficial to the improvement in the PNP reduction by NaBH₄, which is partly related to the smaller size and better dispersion of Au NPs.

Fig. 3 (a) Plot of ln(C_t/C₀) versus reaction time of aqueous phase reduction of PNP by NaBH₄ over different gold catalysts. (b) Reuse cycles of newly catalysts for PNP reduction.

Besides, Au/C shows relatively low reaction rate. The reaction rate constant is only 0.06 min⁻¹ and almost reaches to zero after 6 min. It suggests that interaction between C and Au is not the important reason of the performance enhancement. But carbon on the surface of the catalyst might accelerate efficiency of mass transfer (Fig. S3 in ESI†). Obviously, the PNP absorption on C/Au/TiO₂ is about 37% after 1 min, while it is only 11% on Au/TiO₂. Although both of absorption amount are about the same when reach absorptive equilibrium, PNP adsorptive velocity of catalysts is increased by carbon. The carbon might change the hydrophilic performance of catalyst surface, which make nitrophenol easily transfer from aqueous phase to catalyst surface. Hence, carbon species on the surface of gold catalysts enhanced the activity, which could be attributed to two factors: (1) the Au NPs of C/Au/TiO₂ have the smaller size and the better dispersion and (2) the increase of p-nitrophenol adsorptive ability by carbon.

For industrial applications of view, reusability and recovery performance is the main concern of a heterogeneous catalyst. Fig. 3(b) shows that reuse cycles of novel catalysts were evaluated for PNP reduction. The catalyst particles were immediately separated from the reaction mixture by simple centrifugation at 6000 rpm. Then experiments were performed by keeping all other factors constant. The results revealed that C/Au/TiO₂ shows good activity after 100 reaction cycles. The reaction rate constant of the C/Au/TiO₂ is 0.52 min⁻¹, which only dropped by 4%. However, the reaction rate constant of Au/TiO₂ dropped 12.5% from 0.48 min⁻¹ to 0.42 min⁻¹. It can be seen that the stability of C/Au/TiO₂ catalysts enhanced about 3 times than Au/TiO₂ catalysts. Therefore, carbon species further enhance the stability of catalyst and prevent the deactivation of the catalyst.

Fig. 4 shows the change in sizes of Au NPs for the catalysts before and after 100 cycles PNP reduction reaction. The size distributions of Au particles are summarized from six images of each sample, shown in the insets of Fig. 2. After reaction, Au NPs on Au/TiO₂ agglomerated and become larger as demonstrated by a size distribution from 5.5 ± 1.5 nm to 7.05 ± 1.0 nm, while the size distribution of Au NPs on C/Au/TiO₂ showed good stability with a minor change from 4.1 ± 1.0 nm to 4.5 ± 2.0 nm. Therefore, carbon species help to enhance the stability of Au NPs on the TiO₂ surface.

Fig. 4 TEM images of Au/TiO₂ before (a) and after 100 cycles reaction (b), C/Au/TiO₂ before (c) and after 100 cycles reaction (d), the insets are the size distribution of Au NPs corresponding to each change of catalysts.

In summary, we prepared a carbon surface modification of Au nanoparticles supported on TiO₂ catalysts. Carbon species has no influence on the structure of catalyst but can enhance stability and activity of Au catalysts in the hydrogenation of p-nitrophenol reaction. With less than 1.1% carbon protected, the activity of C/Au/TiO₂ was enhanced 29% by carbon introduction. In addition, we also found the presence of carbon promotes the adsorption of reactant during reaction. After 100-cycle PNP reduction reaction, C/Au/TiO₂ presented more stable activity than Au/TiO₂. The carbon modification on nanocrystal supports can be a viable route to tailor the stability and activity of supported catalysis system. The detailed structural mechanism for the enhanced stability is currently under investigation.

This work was financially supported by the Major Program of National Natural Science Foundation of China (91334202), Chinese National Key Technology Research and Development Program (Grant no. 2006AA03Z455) and the National Natural Science Foundation of China (Grant no. 21136001, 21136004, 20976080, 21206070).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and additional characterization data. See DOI: 10.1039/c4ra04412f

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