Indium oxide nanocluster doped TiO₂ catalyst for activation of molecular O₂

Abstract

The In₂O₃ nanocluster doped faceted nanosize anatase TiO₂ can activate molecular O₂ for styrene epoxidation reaction. The {001} planes of anatase TiO₂ are exposed for 550 °C calcined samples whereas {101} planes are predominantly observed for 450 °C calcined samples. The computational studies highlight that In₂O₃ is better stabilized on {001} planes of TiO₂ resulting in efficient activation of molecular oxygen on In₂O₃ nanocluster doped {001} faceted TiO₂ nanostructures. From the kinetic measurements, it is found that styrene epoxidation reaction is of pseudo-zero order and the corresponding rate constant (k) for the reaction calcined at 450 °C is 0.188 h⁻¹ and at 550 °C it is 0.366 h⁻¹. The activation energy for the reaction is found to be 28.44 kcal mol⁻¹.

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Introduction

The oxidation reaction of organic substrates using molecular oxygen has always remained a challenging task because of its high atom efficiency. Epoxidation of olefins is a difficult job as epoxides are highly unstable in nature.¹ After the commercial production of ethylene epoxide using ethylene and molecular O₂ over a silver catalyst, the epoxidation reactions of several higher homologues e.g. propylene,² 1,3-butadiene,³ cyclohexene,⁴ etc. are being investigated worldwide. The styrene epoxide, an important monomer, is synthesized by using several catalyst systems e.g. TS-1,⁵ VS-1, mixed metal oxides,⁶ cobalt doped mesoporous silica⁷ etc. as reported in the literature. It is found that a cobalt catalyst can activate molecular O₂ effectively for the styrene epoxidation reaction; however the use of several additives have complicated the fundamental understanding of the reaction mechanism to develop better catalysts.⁸

The surface engineering of exposed facets of nanoparticles is a highlighted research area in this decade. Reduction of surface energy by surface restructuring and surface relaxation hinder the growth of nanoparticles to the bigger particles. Recently In₂O₃/TiO₂ composites have drawn attention for photo catalytic degradation of organic molecules.⁹ Though there are several studies reported for micro sized anatase titania,¹⁰ the study of the nanosized titania has not been done extensively. The efficient charge separation can be possible through reactive selectivity photo generated charge carriers on {101} and {001} crystallographic facets in anatase TiO₂, which is highly desirable for solar energy driven phenomena.¹¹

The development of semiconductor/TiO₂ hetero-structures nanocomposites with {101} and {001} exposed facets can be a key to construct new catalysts. Research on indium oxide (In₂O₃) nanoparticles has attracted much attention in the recent years^12,13 because of its applications in solar cell, field emission display, lithium ion battery, nanoscale bio-sensor, gas sensor, optoelectronics and photocatalysis.^14–17 In this paper, for the first time, we report that indium oxide nanoclusters supported on nanosized anatase titania can activate molecular oxygen, demonstrating epoxidation of styrene at mild condition. The thorough characterization of the catalyst by different techniques e.g. N₂ physisorption, XRD, HRTEM, XPS along with computational and kinetic studies demonstrate interesting correlation with catalyst activity results of the In₂O₃/TiO₂ catalyst.

The XRD analysis confirmed the presence of anatase TiO₂ phase of the synthesized materials. No peaks due to indium is observed in XRD study (Fig. S1†) and it indicates that indium oxide nanoclusters are highly dispersed over TiO₂ surface. The particle size of the In₂O₃/TiO₂ catalyst is increased for 550 °C calcined sample in comparison with 450 °C calcined sample.

The surface area is decreased by half (60 m² g⁻¹) for the sample calcined at higher temperature of 550 °C, than that for the sample calcined at lower temperature of 450 °C (123 m² g⁻¹). The decrease of surface area and increase of mean pore diameter (Fig. S2† & Table 1) of 1% In/TiO₂ catalyst at higher calcination temperature are mainly due to the coarsening of nanoparticles as well as neck formation among the particles. The change in morphology is occurred from spherical to tetragonal bipyramidal geometry which is consisted with the minimization of surface energy during the thermal treatment.

Table 1 Nitrogen physisorption analysis and morphological study of 1 wt% In/TiO₂ (calcined at two different temperatures) and 2 wt% In/TiO₂

Catalyst	BET S. A. (m^2 g^−1)	Mean pore diameter (nm)	Pore volume (cm^3 g^−1)	Type of isotherm
1 wt% In/TiO2 (calcined at 450 °C)	123	8.0	0.25	IV
1 wt% In/TiO2 (calcined at 550 °C)	60	30	0.45	IV
2 wt% In/TiO2 (calcined at 550 °C)	57.3	16.5	0.24	IV

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The sample calcined at 450 °C temperature had predominantly {101} exposed planes (5–15 nm size nanoparticles, from HRTEM), while another sample calcined at 550 °C had some exposed {001} planes in addition to the {101} planes (10–15 nm size nanoparticles, from HRTEM). The highly crystalline, well faceted, rice-grain shaped (truncated tetragonal by-pyramidal) nanoparticles of TiO₂ are observed from HRTEM studies of In₂O₃/TiO₂ (550 °C) catalyst (Fig. 1a). The shape of anatase titania calcined at 550 °C temperature is slightly truncated tetragonal bi-pyramidal having majority of {101} and minority of {001} as revealed by Wulff construction from surface energy considerations.¹⁸ The insets of the Fig. 1 showed the SAED pattern with two distinct lattice fringes which correspond to {101} (0.35 nm spacing) and {001} (0.48 nm) for 550 °C calcined sample (Fig. 1a, inset), while single lattice fringe corresponding to {101} plane is observed for 450 °C calcined sample (Fig. 1b, inset). The percentage of {001} planes are 5.6% as reported earlier.¹⁷

Fig. 1 HRTEM of 1% In₂O₃/TiO₂ catalyst calcined (a) at 550 °C temperature and (b) at 450 °C temperature. The inset image shows SAED pattern of the materials.

It is observed from XPS results that the indium is present in its +3 oxidation state in the In₂O₃/TiO₂ catalyst.¹⁷ After indium doping there is a little shift in the Ti2p and O1s binding energies (Table 2) which is probably due to the formation of Ti–O–In species on the TiO₂ surface.¹⁹

Table 2 Quantitative XPS analysis

Species	Catalyst	TiO2		1 wt% In/TiO2 (calcined at 550 °C)
		Binding energy (eV)	Remarks	Binding energy (eV)	Remarks
Ti	2p3/2	458.2	Ti^4+	458.4	Little shift of binding energy implies interaction with indium
	2p1/2	463.9		464.2
O	1s	529.5	Ti–O–Ti	529.6	Absence of surface –OH indicates formation of Ti–O–In
		531.4	Surface –OH	—
In	3d5/2	—	—	451.1	In2O3 species
	3d3/2	—	—	458.1

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The catalytic activity of 1 wt% In₂O₃/TiO₂ (calcined at 450 °C), 1 wt% In₂O₃/TiO₂ (calcined at 550 °C) and 2 wt% In₂O₃/TiO₂ (calcined at 550 °C) towards styrene epoxidation reaction are presented in Table 3. From the results it is found that styrene conversion is more for 1 wt% In₂O₃/TiO₂ (calcined at 550 °C). The styrene epoxide selectivity is comparable with that for 1 wt% In₂O₃/TiO₂ (calcined at 450 °C). For the sample 2 wt% In₂O₃/TiO₂ (calcined at 550 °C) styrene conversion is less compared to other two catalysts. This may be due to the fact that lower loading of indium the clusters are highly dispersed compare to higher loading of indium. Recently Nijhuis et al. observed similar phenomenon in propylene epoxidation reaction over supported gold nanocluster catalyst.²⁰ It is interesting to be noted that the products are obtained only DMF was used as a solvent. The epoxidation reaction of unsaturated C=C occurs probably by following peroxo radical mechanism as reported in the literature.²¹ The formation of metal solvent complex in presence of molecular oxygen and subsequent transformation to a metal superoxo complex was reported in the case of styrene epoxidation reaction over cobalt doped mesoporous silica catalyst.²² As it is observed that indium is present in its stable +3 oxidation state so the mechanism established for cobalt catalyst are unlikely to be followed in this case. The 1% In₂O₃/TiO₂ catalyst calcined at 550 °C has nearly half the surface area than that for the 450 °C calcined sample however the activity of former is much higher. This may be attributed to the fact that the sample calcined at 550 °C has some {001} planes of anatase titania exposed on the surface (as evident from HRTEM study), which can better stabilize the indium oxide nanoclusters substantially (as discussed later based on computational calculations).

Table 3 Result of catalytic activity of In/TiO₂ catalyst towards styrene oxidation reaction

Catalyst	Conversion^a (%)	Selectivity (%)		TOF^b (h^−1)
		Styrene oxide	Benzaldehyde
a Reaction conditions: DMF (10 mL), styrene (6.5 mmol), dodecane (0.1 mL), catalyst (0.1 g), O2 (10 mL min^−1), 150 °C, 8 h.b Moles of styrene converted per mol of indium per hour (Fig. S9).
1 wt% In/TiO2 (calcined at 450 °C)	27.8	74.2	25.8	25.9
1 wt% In/TiO2 (calcined at 550 °C)	52	82	18	48.4
2 wt% In/TiO2 (calcined at 550 °C)	10.6	100	—	4.6
In2O3 (nanopowder)	18	79.3	20.7	0.2

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From theoretical calculation it is clear that, {101} crystal facet of TiO₂ is more stabilized (lower energy) compared to TiO₂ {001} facet (Fig. S3; Table S1†). After geometry optimization, final energy of TiO₂ {101} surface has been calculated as 12 130 kcal mol⁻¹, compared to 13 677 kcal mol⁻¹ for {001} surface (Table S2†). Before calculating binding energy of the feed (styrene and oxygen), the feed is stabilized on In₂O₃/TiO_2{101} (Fig. S4†) and In₂O₃/TiO_2{001} (Fig. S5†). To simulate real experimental condition, the ratio of oxygen to styrene is kept as 20 : 1 in adsorption module. Calculations for binding energy confirm that the feed (styrene and oxygen) has much higher affinity (higher binding energy) to In₂O₃/TiO_2{001} system compared to In₂O₃/TiO_2{101} system (Fig. 2; Table 4).

Fig. 2 Geometry optimized structure showing adsorption of styrene and oxygen (1 molecule styrene and 20 molecules O₂, the ratio selected for computational studies is in accordance with experimental conditions used in our work) on (a) In₂O₃/TiO₂ {101} surface, and (b) In₂O₃/TiO₂ {001} surface.

Table 4 Binding energy calculation on different TiO₂ surfaces {001} and {101}

System	Binding energy (kcal mol^−1) calculated upon insertion of molecule on In2O3–TiO2{101} system	Binding energy (kcal mol^−1) calculated upon insertion of molecule on In2O3–TiO2{001} system
In2O3	−12 169.8	−13 717.3
Styrene : oxygen (1 : 20)	−5119.04	−5126.98
Benzaldehyde and styrene epoxide	−5117.16	−5120.5

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The higher affinity of feed towards In₂O₃/TiO_2{001} system suggests higher conversion for the system (Table 3) which is in accordance to the experimental results. It is found that the sample calcined at 550 °C (though has lower surface area) has better catalytic activity. This clearly indicates that better stabilization of indium oxide on {001} planes of titania is responsible for higher activity.

To scrutinize the affinity of the product, similar to the reactant, product molecules benzaldehyde and styrene-epoxide are stabilized on In₂O₃/TiO_2{001} (Fig. S6†) and In₂O₃/TiO_2{101} (Fig. S7†). It is found from Table S3† that isosteric heat of adsorption both for feed styrene and oxygen and products (benzaldehyde and styrene-epoxide) are higher in case of In₂O₃/TiO_2{001} nanostructures compared to In₂O₃/TiO_2{101} nanostructures.

So it can be concluded that the feed has more affinity to the surface where isosteric heats are higher. The difference in binding energy for products is lower compared to feed (3 kcal mol⁻¹ compared to 7 kcal mol⁻¹), which is quite necessary condition for product diffusion.

For the kinetic studies styrene is taken in a batch reactor at temperature of 150 °C and O₂ is passed continuously at flow rate of 10 mL min⁻¹. Linear regression method is used for the kinetics evaluation of styrene epoxidation reaction. Reaction is carried out over both the catalysts calcined at 450 °C and 550 °C. For different styrene reaction orders, the rate constant is calculated and it is found that for order n = 0 fits the data obtained experimentally (Fig. 3) and corresponding rate constant (k) for the reaction over catalyst calcined at 450 °C is 0.188 h⁻¹ (ESI Table S4 and Fig. S8†) and calcined at 550 °C is 0.366 h⁻¹ (ESI Table S5 and Fig. S9†), corresponding R² values are 0.908 and 0.920 respectively. Conversion profile for pseudo-zero order reaction is also predicted in this case which is verified theoretically and simulated results (Fig. 3). As reported in the literature²³ reaction order for styrene epoxidation varies within 0–1 which is similar to our work. Another interesting observation is that no deactivation is occurred of the catalyst which was found in other system reported earlier. The deactivation occurs mostly due to ring opening of the catalyst forming styrene glycol which covers the catalyst surface.²⁴ The activation energy obtained for the reaction is 28.44 kcal mol⁻¹.

Fig. 3 Plot of simulated and experimental conversion (%) vs. time (h) for styrene oxidation over 1 wt% In₂O₃/TiO₂ catalyst, (calcined at 450 °C temperature) in a batch reactor at 150 °C reaction temperature.

Conclusions

The results demonstrate that the unique features of the material such as synergism between TiO₂ surface and In₂O₃ nanoclusters, high crystallinity and presence of high energy {001} facets make it an excellent catalyst for activation of molecular oxygen, which is resulted in good activity towards catalytic epoxidation of styrene. Our results open a new avenue to develop high-performance nanocatalysts.

Acknowledgements

BC would like to acknowledge the DST Govt. of India for research grant (scheme SB/S1/PC-10/2012). CS and SR would like to acknowledge UGC for his fellowship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Materials and reagents, synthesis of In₂O₃/TiO₂, theoretical calculation method, characterization techniques and catalytic activity study. See DOI: 10.1039/c5ra13104a

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