Promoting effects of lanthanum on the catalytic activity of Au/TiO₂ nanotubes for CO oxidation

Abstract

Lanthanum-modified TiO₂ nanotube (NT) supported gold catalysts were prepared. The linear relationship between actual and nominal lanthanum concentrations was found. The possible formation mechanism of La-modified TiO₂ NTs was suggested. The effects of calcination temperature, La concentration and gold loading on the catalytic activity for CO oxidation were investigated. Au/La₂O₃–TiO₂-NTs (Au: 4.18%, La: 1.62%) calcined at 300 °C could completely convert CO at 30 °C. T_100% of La-free catalyst was 60 °C. The catalytic activity of Au/TiO₂-NTs could be improved by modification with lanthanum.

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Gold has long been disregarded for catalytic purposes because of its inert nature. However, when gold is supported on certain metal oxides in a dispersed state, it shows a surprisingly high activity for several important reactions, including CO oxidation, the water-gas shift, and water pollutant removal.^1–3 Extensive literature describes the ability of nanosized gold supported on titania (Au/TiO₂) to catalyse the low temperature oxidation of carbon monoxide.^4–6 It has been well established the catalytic properties of gold for CO oxidation are not only determined by the particle size but also largely dependent on the nature of the support.

The influence of metal oxides on the catalytic activity has obtained great scientific interest. The surface modification of nanocrystal supports can be a viable route to tailor the activity and stability of the supported catalysis systems. It's believed that different metal oxide modifier may have different redox properties, and may even change the oxidation states of gold and create new active sites.⁵ The modification strategy has been used to prepare TiO₂-based gold catalysts.^7–9 Sheng Dai' group loaded various metal oxide additives onto the TiO₂ support via excess-solution impregnation of soluble precursors followed by calcination, and established that Au/M_xO_y/TiO₂ all retained significant activity in CO oxidation at ambient temperature even after pretreatment at 500 °C.^10,11 Nruparaj Sahu et al. reported that the doping of TiO₂ with rare earth could provide additional adsorption/reaction sites for oxygen adsorption and interaction with adsorbed CO, and significantly improved the catalytic activity of Au/TiO₂ under ambient condition.¹² Peter D. Clark et al. studied the performance of Au/TiO₂ modified with La₂O₃, and found that the resulting materials showed very high CO conversion above 323 K and remained active over prolonged periods in the presence of SO₂ and other sulfur compounds.¹³ Jun Yu et al. prepared gold catalyst supported on La or Ce-modified TiO₂, and found that modification with ceria or lanthanum oxide improved the synergistic interaction between the support and gold particles, and enhanced the reactivity of the surface oxygen species of the catalyst, resulting in good activity for CO oxidation.¹⁴ There have been several reports on the metal-modified to enhance catalytic performance. The majority of reports related to CO oxidation focused on the thermal stability. The effect of the metal ion modification, especially metal ion concentration, on catalytic activity have not been studied in detail.

The shape or morphology of the support has been found to influence the catalytic activity of the resulting gold catalysts. Some studies have been conducted to use TiO₂ NTs as support material because of NTs' novel properties, such as unique shape, size confinement in radial-direction and large specific surface area.^15,16 Despite several studies on Au/TiO₂ NTs for CO oxidation, there are no examples of CO oxidation catalysis on metal oxide-modified Au/TiO₂ NTs.

The metal ion modified TiO₂ was often used as photocatalyst for degradation of organic compounds, and was emphasized in recent years.^17–20 In our previous work, we have prepared La-modified TiO₂ nanotubes, and found that the photocatalytic activity of TiO₂ NTs could be improved by modification with lanthanum ions.²¹ Hydrothermal treatment was generally applied to prepare TiO₂ NTs, However, few reports referred to the study on the controlled synthesis of catalyst with predetermined metal ion concentration.

The aim of the present report was to investigate the situation of La in Au/TiO₂ NTs and the effects of lanthanum oxide on the catalytic activity of CO oxidation. La-modified TiO₂-NTs supported gold catalysts (denoted as Au/La₂O₃–TiO₂-NTs) were prepared by hydrothermal treatment and deposition–precipitation method. The catalysts were characterised by ICP, XRD, TEM, XPS and BET. Their catalytic activities for CO oxidation were evaluated.

Au/La₂O₃–TiO₂-NTs were prepared via the combination of sol–gel process with hydrothermal treatment. La₂O₃–TiO₂ powder was prepared by the sol–gel route using tetrabutyl titanate and lanthanum nitrate as the precursors. Under constant stirring, ethanol solution of tetrabutyl titanate was added drop-wise to the mixture of ethanol and lanthanum nitrate solution. The transparent sol was obtained when the mixture was hydrolyzed at room temperature for 6 h under vigorous stirring. The resulting gel was dried at 80 °C for 12 h. After the dried material was calcined in air at 500 °C for 3 h, La₂O₃–TiO₂ powder was obtained. A series of La-modified TiO₂ catalysts with different concentrations of La were prepared by changing the amount of lanthanum nitrate added in the ethanol solution. La₂O₃–TiO₂-NTs were synthesized via a simple hydrothermal chemical process. 1.6 g La₂O₃–TiO₂ powder was mixed with 70 mL 10 mol L⁻¹ NaOH aqueous solution in a Teflon vessel and maintained at 150 °C for 12 h. Obtained materials were washed with 0.1 mol L⁻¹ HNO₃ solution and distilled water, respectively, and then dried at 80 °C overnight. After the prepared material was calcined at 300 °C for 2 h, La₂O₃–TiO₂-NTs were obtained. La₂O₃/TiO₂ NTs were dispersed in HAuCl₄ solution with different concentration. The suspension was adjusted to various pH = 4 with ammonia solution then agitated for 12 h, and refluxed for 6 h. Then the suspension was centrifuged and washed with water to remove Cl⁻ ions. After dried at 80 °C overnight, and calcined for 2 h, Au/La₂O₃–TiO₂-NTs were obtained. Au/TiO₂-NTs without modification was synthesized with the same process except that distilled water instead of lanthanum nitrate solution was used in sol–gel process.

Elemental analysis was performed on an Optima 2000 DV model inductively coupled plasma optical emission spectrometer (ICP-OES). The concentrations of La and Au were expressed as wt%. The powder X-ray diffraction (XRD) experiments were carried out at room temperature using a Rigaku D/Max-2500 X-ray diffractometer (CuKα λ = 0.154 nm) to identify the crystal phase of the samples. Transmission electron microscopy (TEM) images of were obtained using a JEM-2100 transmission electron microscopy working at 200 kV. The chemical composition and oxidation state on the surface of sample were investigated by X-ray photoelectron spectroscopy (XPS) using an Al X-ray source (Al Kα-150 W, Kratos Axis Ultra DLD). The Brunauer–Emmett–Teller (BET) surface areas were recorded by JW-K surface area analyzer.

Catalytic tests of Au/La₂O₃–TiO₂-NTs for CO oxidation were carried out in a fixed-bed flow reactor under atmospheric pressure using 200 mg Au/La₂O₃–TiO₂-NTs. A stainless steel tube with an inner diameter of 8 mm was chosen as the reactor tube. The samples were diluted with chemically inert quartz sand. Reaction gas mixture consisting of 10% CO balanced with air was passed through the catalyst bed at a total flow rate of 36.3 mL min⁻¹. The temperature dependence of the sample catalytic activity was recorded in the range of 30–80 °C with a ramping rate of 5 °C min⁻¹. After holding at the reaction temperature for 30 min, effluent gases were analyzed on-line by GC-508A gas chromatography. The activity was expressed by the degree of conversion of CO.

The ICP-OES results shown in Fig. 1a reveal the actual and nominal amount of lanthanum in Au/La₂O₃–TiO₂-NTs (Au: 4.18%). The actual lanthanum concentration was lower than the nominal one for all catalysts. According to the preparation procedure, all hydrolysates of tetrabutyl titanate and lanthanum nitrate were completely retained in sol–gel process. This indicated that lanthanum should be lost during hydrothermal treatment. The concentration of lanthanum in TiO₂ was found to increase linearly with increasing concentration of La(NO₃)₃ solutions used during sol–gel process. The average utilization efficiency of lanthanum was 60.25%.

Fig. 1 Relationships between actual and nominal content of lanthanum (a) and gold (b) in Au/La₂O₃–TiO₂-NTs.

ICP was also used to determine the amount of gold present in each of Au/La₂O₃–TiO₂-NTs (La: 1.62%). The relationship between actual and nominal gold content is depicted in Fig. 1b. The actual loading of gold in the catalysts was not a linear function of the nominal value. The samples formed from 2.5, 5.0, 7.5, 10.0 and 12.5% gold precursors contained 2.27, 3.7, 4.18, 4.66, and 5.26% gold respectively. This showed that gold loading efficiency decreased (from 91% to 42%) with increasing amount of gold precursors.

Fig. 2 depicts the XRD patterns of Au/La₂O₃–TiO₂-NTs (Au: 4.18%, La: 1.62%) without calcination and calcined at different temperatures. Diffractions that were attributed to anatase TiO₂ were detectable in all samples (JCPDS 21-1272). For the sample without calcination, the peak at 2θ = 25.3° was asymmetric, one shoulder peak at lower 2θ could be identified, which implied that there was a small amount of titanate in the sample. titanate was also confirmed by the very weak peak at 2θ = 9.2° in the sample.²³ However, titanate did not present in calcined Au/La₂O₃–TiO₂-NTs which indicated that titanate phase could be transformed into anatase phase by calcination treatment. The sharpness of anatase reflection peaks increased with increasing calcination temperature without a change in the overall patterns, which was attributed to improvement of crystallinity and increase of particle size (collapse of TiO₂ nanotube) with increasing calcination temperature. For all samples, peaks at 2θ = 44°, 64° and 77° were designated to the 200, 220 and 311 plane of gold (JCPDS 4-0784), respectively, which proved that TiO₂ nanotubes were modified by gold particles. The 111 diffraction peak of gold was at 2θ = 38°, which might be overlaid by the 103 diffraction peak of anatase TiO₂. No independent phase originating from La₂O₃ or La₂Ti₂O₇ was detected. It indicated that the lanthanum ions were highly dispersed on TiO₂, and XRD was not sensitive enough to detect such minor changes to TiO₂. The average gold nanoparticle sizes were estimated from X-ray line broadening analysis applying the Debye–Scherrer equation on the 111 diffraction of gold (2θ = 44°). The sizes of gold of Au/La₂O₃–TiO₂-NTs without calcination, calcined at 200, 300, 400, and 500 °C were 2.1, 3.1, 3.4, 3.6, and 6.9 nm, respectively. Obviously, the gold nanoparticle sizes increased with increasing calcination temperatures. The gold size increased only slightly with increasing calcination temperature from 200 to 400 °C, which indicated that lanthanum oxide could depress the movement and growth of gold nanoparticles.^11,14 The stabilizing effect appears also to be dependent on the nature support. When the sample was calcined at 500 °C, accompanying with the break of nanotubes, gold nanoparticles dramatically grew.

Fig. 2 XRD patterns of Au/La₂O₃–TiO₂-NTs (Au: 4.18%, La: 1.62%) without calcination and calcined at different temperatures for 2 h.

The possible formation mechanism of La-modified TiO₂ NTs could be described as follows: in sol–gel process, La³⁺ were homogeneously dispersed in TiO₂ sol and gel. After heating treatment, TiO₂ amorphous phase was transformed into anatase. The ionic radius of La³⁺ (0.115 nm) is bigger than that of Ti⁴⁺ (0.068 nm). Therefore, it is difficult for La³⁺ to really enter the lattice of TiO₂. La³⁺ ions were most likely to be found as dispersed metal oxides within the crystal matrix or on the surface of TiO₂.^13,19,22 The amount of La₂O₃ within TiO₂ matrix should be proportional to total amount of lanthanum. The structure of anatase TiO₂ can be described in terms of chains of distorted TiO₆ octahedra. Each Ti⁴⁺ is surrounded by an octahedron of six O²⁻ ions. Two Ti–O bonds are longer, while the other four are shorter. In hydrothermal treatment, reacting with concentrated NaOH, the longer Ti–O bonds will be attacked by OH⁻ ions and break, but the shorter ones will not, thus to form larger planar nanosheets with (Ti–O)_n netlike structure.^24,25 La₂O₃ within TiO₂ matrix was embedded in nanosheets, whereas La₂O₃ on the surface of TiO₂ was not. When the fragments grew up into an adequate size, tubulating process took place. With the exfoliating, curling and scrolling of nanosheets, lanthanum modified TiO₂ NTs were formed.

The TEM image of Au/La₂O₃–TiO₂-NTs is shown in Fig. 3. For samples without calcination (Fig. 3a) and calcined at 300 °C (Fig. 3b), nanotubular structures were clearly observed. The nanotubes were open-ended, and their length was more than hundreds of nanometers. The nanotube had inner and outer diameters of about 8 nm and 10 nm, respectively. For the sample without calcination, gold nanoparticles were distributed homogeneously on the TiO₂ NTs, showing no preference of anchoring sites. The size of monodisperse spherical gold nanoparticle was about 2 nm, which was in agreement with XRD data. At higher magnification (Fig. 3c), a detail examination of the nanotubes revealed that gold nanoparticles were both encapsulated in and deposited on the nanotubes. The La₂O₃–TiO₂ nanotubes were dispersed in HAuCl₄ solution during deposition–precipitation process. The nanotubes were open-ended and their inner diameter was about 8 nm, enabling some gold ion species to enter their tube cavities. Further heating treatment induced growth of gold particles within the tubes, which was responsible for the result of Fig. 3c. Fig.3d shows the TEM image of Au/La₂O₃–TiO₂-NTs calcined at 500 °C for 2 h. It could be observed the nanotubes broke. It could be also seen that accompanying with the break of nanotubes, gold particles obviously grew. It was also confirmed by XRD data. The gold nanoparticle size given by XRD had a dramatic increase when the sample was calcined at 500 °C.

Fig. 3 TEM images of Au/La₂O₃–TiO₂-NTs (Au: 4.18%, La: 1.62%) without calcination (a) calcined at 300 °C (b and c) and 500 °C (d).

Fig. 4 shows the XPS spectra of Au/La₂O₃–TiO₂-NTs (Au: 4.18%, La: 1.62%) calcined at 300 °C for 2 h. There were O, Ti, Au, La and C on the surface. The residual carbon resulted from the organic precursors used in the sol–gel method. The adventitious hydrocarbon from XPS itself might also cause the presence of C. The concentration of lanthanum analyzed by XPS was 1.81 wt%. The surface lanthanum concentration was slightly higher than actual concentration (1.62 wt%) determined by ICP, which could be attributed to the surface enrichment of La₂O₃ in Au/La₂O₃–TiO₂-NTs due to calcination. The atomic ratio of O to Ti analyzed by XPS was ca. 2.76. Li et al. reported 2.17 and 2.70 of the atomic ratio of O to Ti for 0.7 and 1.2 wt% La³⁺–TiO₂ powders.¹⁹ X. J Quan et al. also found that the atomic ratio of O to Ti was ca. 2.49 for the 1.0 wt% La³⁺–TiO₂.²⁰ In our previous work, O/Ti surface ratios was measured to be ca. 2.31 for the 0.75 wt% La/TiO₂ NTs.²¹ The O/Ti surface ratio obtained in this work agreed well with that reported previously because it was found that the ratio of [O]/[Ti] on the surface of TiO₂ increased with the increase of lanthanum concentration. The concentration of gold analyzed by XPS was 2.82 wt%. The surface gold content was much lower than actual content of gold (4.18 wt%) determined by ICP, which implied that some gold nanoparticles encapsulated in the tube cavities. It was also confirmed by TEM observations.

Fig. 4 XPS spectra of Au/La₂O₃–TiO₂-NTs (Au: 4.18%, La: 1.62%) calcined at 300 °C for 2 h: (a) O 1s peaks; (b) Ti 2p peaks; (c) Au 4f peaks; (d) La 3d peaks.

Fig. 4a shows the O 1s XPS spectra of Au/La₂O₃–TiO₂-NTs. The O 1s peak was asymmetric, besides the main peak of O1s located at about 529.3 eV corresponding to lattice oxygen of TiO₂, two shoulder peaks at higher and lower binding energy could be identified. The peak at 530.8 eV should be attributed to the surface species such as Ti–OH and Ti–O–O–. Generally, narrow scan O1s XPS spectrum requires only two peaks to fit the curve.²⁶ In this, the small shoulder peak at 527.6 eV was attributed to La–O.^13,27 Fig. 4b shows the high resolution XPS spectra of Ti 2p. The Ti 2p_1/2 and Ti 2p_3/2 spin–orbital splitting photoelectrons were located at binding energies of 464.9 and 459.3 eV respectively, which was in accord with the reported literature values, showing the presence of Ti⁴⁺.²³ Fig. 4d shows the high solution XPS spectra of the La 3d region of Au/La₂O₃–TiO₂-NTs. The signals of La were very weak, due to the low lanthanum concentration. It could be observed that both the spin–orbit split 3d_5/2 and 3d_3/2 levels showed double-peak structures. The spin–orbit splitting between the 3d_3/2 and 3d_5/2 levels was 17.1 eV, and the separation between the satellite and main peak was 4.5 eV, which agreed with reported values for La³⁺ compounds.^28,29 The presence of La³⁺ in the catalysts was also revealed by the small peak at 527.6 eV (O linked to La).^13,27 Fig. 4c shows the Au 4f XPS spectra of Au/La₂O₃–TiO₂-NTs. The Au 4f_7/2 peak was located at binding energy of 84.2 eV. The binding energy was characteristic for zero valent gold and closed to the ones observed for bulk metallic gold (84.0 eV).³⁰ No peaks corresponding to oxidized gold species, which would be located around 85.5 and 86.3 eV, have been detected.

The BET surface areas of La-free Au/TiO₂-NTs and 1.62% La-modified Au/La₂O₃–TiO₂-NTs (calcined at 300 °C for 2 h, Au: 4.18%) were 166.3 and 192.7 m² g⁻¹, respectively, which indicated that the modification with lanthanum resulted in an enhanced surface area surface area of the TiO₂ support. This is consistent with the results of previous studies.^12,14,31 Compared with La-free TiO₂-NTs, Au/La₂O₃–TiO₂-NTs had larger adsorption capacity for oxygen because of their larger surface area. The spillover process of adsorbed oxygen could took place easily and efficiently. The higher surface area of Au/La₂O₃–TiO₂-NTs might play a role for their higher catalytic activity than La-free TiO₂-NTs.

Calcination temperature significantly affects the activity of catalyst. The higher the calcination temperature, the stronger interaction exists between gold particles and support. Catalytic property of titania-supported gold catalyst is associated with the strong metal–support interaction. CO oxidation activities of Au/La₂O₃–TiO₂-NTs (Au: 4.18%, La: 1.62%) calcined at different temperatures were shown in Fig. 5a. The results showed that CO conversion increased with increasing calcinations temperature; however, when calcination temperature was increased from 300 °C to 400 °C, the CO conversion slightly decreased. The sample calcined at 300 °C possessed the best catalytic activity, and it could convert CO to CO₂ completely at 30 °C. It is worth to point out that some gold catalysts described in literature achieved complete CO conversions at or below room temperature, but it has to be taken into account that most studies in literature used 1% CO or less (while we used 10% of this gas). The activity of the catalyst calcined had significantly lessened with complete conversion of CO at the temperature of 70 °C, and only 52% of CO was converted at 30 °C. The main reason might be that TiO₂ nanotubes collapsed during the high-temperature treatment, resulting in increased mobility and growth of gold nanoparticles, as seen in the TEM images, which correspondingly led to the loss of catalytic activity. It could be concluded that the appropriate calcination temperature was 300 °C.

Fig. 5 Catalytic activities of Au/La₂O₃–TiO₂-NTs calcined at different temperatures (a); with different Au loadings (b) and La concentration (c).

Au/La₂O₃–TiO₂-NTs were prepared with different gold loadings of 1.86%∼5.26% (La: 1.62%). All catalysts were calcined at 300 °C for 2 hours. Fig. 5b shows the catalytic activities of samples as a function of gold loading. The CO conversion increased with increasing gold content from 1.86% to 4.18%, and while gold loading was increased from 4.18% to 5.26% the CO conversion decreased. The 4.18% gold-loading catalyst exhibited the highest activity among the catalysts studied. In order to investigate the influence of lanthanum on the catalytic activity of Au/TiO₂ NTs, the catalytic activities of samples with different La contents (Au: 4.18%) were evaluated for CO oxidation. As seen in Fig. 5c, the sample with 1.62% lanthanum had the best catalytic activity. At 30 °C, CO could be completely converted to CO₂ for 1.62% La-modified catalyst, while only 9.1% of CO was converted for La-free sample. T_100% of catalyst without La-modification was 60 °C.

The results revealed that the catalytic activity of Au/TiO₂-NTs could be improved by modification with lanthanum ions. According to the in situ FTIR study reported previously, the presence of La³⁺ could promote the reactivity of CO adsorbed on the Ti⁴⁺ site, and gave rise to certain new O₂ adsorption sites at Au/TiO₂ interfaces for the adsorption and activation of O₂ molecules, which was attributed to the increase in the content of surface oxygen vacancies and defects after lanthanum modification.^12,13 Thus, lanthanum could improve the catalytic activity of Au/TiO₂-NTs for CO oxidation.

In summary, La-modified TiO₂-NTs supported gold catalysts were prepared by hydrothermal treatment and deposition–precipitation method. The actual concentration of lanthanum was found to increase linearly with increasing nominal concentration, while the actual gold loadings were not a linear function of the nominal loading. The gold loading efficiency decreased with increasing amount of gold precursor. The possible formation mechanism of La-modified TiO₂ NTs was proposed. The nanotubes were open-ended, and their length was more than hundreds of nanometers. The size of monodisperse spherical gold particle was about 2 nm. Gold nanoparticles were both encapsulated in and deposited on the TiO₂ nanotubes. Au/La₂O₃–TiO₂-NTs (Au: 4.18%, La: 1.62%) calcined at 300 °C possessed the best catalytic activity, and it could convert CO to CO₂ completely at 30 °C, while only 9.1% of CO was converted for La-free sample at 30 °C. T_100% of catalyst without lanthanum modification was 60 °C. The catalytic activity of Au/TiO₂-NTs could be improved by modification of lanthanum ions.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (21271110, 21373120 and 21301098), the Applied Basic Research Programs of Science and Technology Commission Foundation of Tianjin (12JCYBJC13100 and 13JCQNJC02000) and MOE Innovation Team (IRT13022) of China.

Notes and references

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