Monolith-Like TiO2 Nanotube Array Supported Pt Catalyst for HCHO

Apr 7, 2014 - The superior performance is related to the specific monolith-like structure, confinement effect and metal–support interaction in Pt/Ti...
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Monolith-Like TiO2 Nanotube Array Supported Pt Catalyst for HCHO Removal under Mild Conditions Huayao Chen, Zebao Rui, and Hongbing Ji* Department of Chemical Engineering, School of Chemistry & Chemical Engineering, and The Key Lab of Low-Carbon Chemistry & Energy Conservation of Guangdong Province, Sun Yat-Sen University, Guangzhou 510275, People’s Republic of China ABSTRACT: Highly ordered pore-through TiO2 nanotube arrays (TiNT) were prepared by an electrochemical anodization method and used as the support for a Pt/TiNT catalyst. The Pt/TiNT was then applied to the trace HCHO oxidation. The effect of Pt/TiO2 structural properties on its performance was studied with respect to the behavior of adsorbed species on the catalyst surface using in situ DRIFTS. In comparison with the commercial TiO2 powders (P25 and anatase) supported Pt catalysts, Pt/ TiNT shows higher activity under parallel preparation and test conditions. A HCHO conversion of 95% with a more than 100 h stable performance is achieved over Pt/TiNT at 30 °C with a low 0.4 wt % Pt loading amount. The superior performance is related to the specific monolith-like structure, confinement effect and metal−support interaction in Pt/TiNT. Finally, the reaction mechanism is presented based on the in situ DRIFTS study and XPS characterization to explain the performance difference among the Pt/TiO2 samples with different morphologies.

1. INTRODUCTION Formaldehyde is one of the most common toxic volatile organic compounds (VOCs) classified as carcinogenic, causing leukemia.1 Many methods have been proposed to remove trace HCHO in air, such as adsorption, photocatalysis, and catalytic oxidation. Among them, catalytic oxidation is one of the most effective and economically feasible technologies because HCHO can be oxidized into CO2 over catalysts at much lower temperature than that of thermal oxidation.2−4 Many groups have been working on this subject and significant progress has been achieved, especially over the TiO2 supported catalysts.2−4 Zhang and He3 reported that HCHO could be completely oxidized into CO2 and H2O at room temperature over the Pt/TiO2 with a Pt loading amount of 1 wt %. Huang et al.4 found that reduction treatment had great influence on the structural properties of the Pt/TiO2 catalysts and the resulting catalytic activity for HCHO oxidation. They proposed that well-dispersed and negatively charged metallic Pt nanoparticles, and rich chemisorbed oxygen were probably responsible for their high catalytic activities. A nearly 100% HCHO conversion was achieved over sodium borohydride reduced Pt/TiO2 catalysts even with a 0.1 wt % Pt loading amount, however, only the stability of 1 wt % Pt/TiO2 was presented. The reactor design for the catalytic combustion process of VOCs is also very important, especially when the catalyst with high activity is applied. The reactors with packed beds are easy to construct, but they cause a high-pressure drop and do not allow reduction of the particle size to obtain high specific surface area.5 Hence, a structured catalyst with high surface area and low pressure-drop is favorable. Recently, anodic oxidation technology has frequently been applied by many groups to produce structured catalysts.6−12 Wang and Kameyama6 loaded Pt/TiO2 catalyst on structured anodic alumite supports for the catalytic decomposition of formaldehyde at room temperature. Such a design showed many advantages over the powder catalysts, however, the poor interaction between the support and the active layer may lead a stability problem. © 2014 American Chemical Society

Highly ordered TiO2 nanotube array (TiNT) fabricated by electrochemical anodization method have attracted widely and continuously increasing interest, owing to its precisely controllable nanoscale features, unique optical and electronic characteristics as well as a superior physical topology.13,14 A wide range of functional applications of the TiNT have been explored.15−21 It has been commonly recognized that an enhanced catalytic activity can be obtained from nanostructured TiO2 supported catalyst, which exhibits a high surface area and short solid state diffusion paths.15 In our previous work,10−12 monolith-like structured Ru/TiNT catalyst was successfully applied to the combination reaction of carbon dioxide reforming and partial oxidation of methane for syngas production. It was found that Ru/TiNT exhibited high activity and good stability during the harsh reaction conditions. Considering the superior catalytic properties of Pt/TiO2 for HCHO oxidation and the monolith-like structure of TiNT together, Pt/TiNT holds the possibility to meet the requirement of the catalyst for HCHO oxidation. This work reports the preparation and application of highly efficient monolith-like structured Pt/TiNT catalyst for trace HCHO oxidation in detail. In comparison with the commercial TiO2 powders (P25 and anatase) supported Pt catalysts, Pt/ TiNT shows higher activity under parallel preparation and test conditions. A HCHO conversion of 95% with a more than 100 h stable performance is achieved over Pt/TiNT at 30 °C with a low 0.4 wt % Pt loading amount. The mechanism leading to its high catalytic activity and stability is studied by various characterizations, especially in situ DRIFTS study, which is a powerful technology for the investigation of surface species under realistic conditions.3,22 Received: Revised: Accepted: Published: 7629

February 24, 2014 April 4, 2014 April 7, 2014 April 7, 2014 dx.doi.org/10.1021/ie5004009 | Ind. Eng. Chem. Res. 2014, 53, 7629−7636

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Figure 1. SEM images of the top and bottom view of TiNT: (a) top view before calcination, (b) bottom view before calcination, (c) top view after calcination, (d) bottom view after calcination, (e) cross section view before calcination, and (f) cross section view after calcination.

Then the as-synthesized TiNT was dried at 120 °C, and subsequently, calcined at 500 °C for 5 h with a heating rate of 10 °C/min in air. Pt/TiO2 catalyst was prepared by an impregnation method. The support TiNT, Degauss P25 (51.1 m2/g), or anatase TiO2 (Alfa Aesar, 97.0 m2/g), was uniformly dispersed into the H2PtCl6 solution (5.4 mg/mL, Alfa Aesar) with an appropriate volume to obtain a Pt loading amount of 0.12, 0.4, or 0.8 wt %, which was confirmed by ICP. The samples were then dried at 120 °C overnight to evaporate the solvent and finally calcined at 400 °C for 4 h with a heating rate of 10 °C/min in air. The catalysts were reduced in the H2 stream at 300 °C for 3 h before reaction and characterization. The as-prepared catalysts were respectively denoted as Pt/TiNT, Pt/P25 and Pt/ATiO2 (for Pt/anatase TiO2) with a Pt loading amount number in front for simplicity. 2.2. Characterization. In situ Diffuse Reflectance Infrared Fourier Transformed Spectroscopy (DRIFTS) was carried out on EQVINOX-55 FFT spectroscope apparatus (Bruker), equipped with a diffuse reflectance accessory and a MCT detector. Finely ground sample (ca. 10 mg) was placed in a ceramic crucible in the in situ chamber. The total gas flow rate

2. EXPERIMENTAL PROCEDURE 2.1. Catalysts Preparation. The procedure for the TiNT preparation by an electrochemical anodization method follows the well-established procedures in our previous paper with some modifications.10 In a typical preparation experiment, titanium foil (99.7%) with thickness of 0.3 mm and a size of 200 × 50 mm2 was used as the anode and a Pb plate with a size of 200 × 50 mm2 was served as the cathode. The distance between the two electrodes was 10 mm. The entire course of anodization was conducted at 20 °C. In order to improve the orderliness and alignment of the TiO2 nanotubes, the Ti foil was prepatterned by anodization at 20 V for 20 min in 1 M H2SO4 aqueous solution.23 The sample was then annealed at 700 °C in air for 1 h. Subsequently, the second anodization was conducted at 60 V for 42 h. The electrolyte was composed of 0.3 wt % NH4F (>96.0%), 2.0 wt % distilled water, and 97.7 wt % ethylene glycol (EG, 99.7%). Finally, the sample was anodically oxidized at 130 V for 10 min to obtain a throughpore membrane.24 The as-anodized samples were cleaned in distilled water to remove surface debris, and the nanotube arrays could be isolated from the titanium foil at the same time. 7630

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confirmed by XRD and shown in Figure 2. As presented, the as-anodized TiNT is amorphous, while the sample calcined at

was 100 mL/min. HCHO was bubbled into the chamber with N2. The spectra under reaction conditions were recorded after 64 scans with a resolution of 4 cm−1. SEM morphologies were observed on FEI Quanta 400 FEG at a 20 kV accelerating voltage. Metal nanoparticle size distribution was observed by a JEOL 2100F transmission electron microscopy (TEM). The phase purity and crystal structure of the samples were examined by XRD using a D-MAX diffractometer with Cu Kα radiation at a scanning rate of 10°/min and a step size of 0.02°. BET surface area of catalysts was determined by N2 adsorption isotherms at 77 K, operated on ASAP 2020 adsorption equipment. The samples were degassed at 300 °C for 2 h in vacuum before N2 adsorption experiment. The loading amount of Pt was confirmed by Inductively Coupled Plasma-atomic Emission Spectrometry (TJ IRIS). X-ray photoelectron spectra (XPS) were recorded on a ESCALAB 250 spectrometer (Thermo Fisher Scientific, Al Kα, hν = 1486.6 eV) under a vacuum of ∼2 × 10−7 Pa. Charging effects were corrected by adjusting the main C 1s peak to a position of 284.8 eV. 2.3. Catalytic Experiment. The catalytic oxidation of HCHO was performed in a quartz tubular (i.d.= 6 mm) fixedbed reactor under atmospheric pressure with a homemade setup. Approximately 0.2 g of the catalyst was packed in the reactor. A simulated air stream (N2/O2 = 4, 100 mL/min) containing ∼50 ppm of HCHO and water vapor (∼35 % relative humidity) was introduced as the reactants. Gaseous HCHO was generated by passing a stream of simulated air through a bubbler containing an HCHO solution (35 wt % HCHO). The gas hourly space velocity, defined as the total flow rate of gas at STP per unit weight of catalyst, is 30 000 mL· h−1·g−1. HCHO concentration in the reactant or product gas stream was analyzed by phenol spectrophotometric method. The gas stream containing trace HCHO was bubbled through 5 mL phenol reagent (C6H4SN(CH3)C/NNH2·HCl, Alfa Aesar) solution (1 × 10−4 wt %) for 30 s to collect HCHO by absorption. Then, 0.4 mL (1 wt %) ammonium ferric sulfate (NH4Fe(SO4)2·12H2O, Tianjin Fuchen Chemical Reagent Company) solution was added as the coloring reagent. After being shaken for 5 s and staying for 15 min in the dark, HCHO concentration in the gas stream was then determined by measuring light absorbance at 630 nm with a spectrophotometer (UV-240, Shimadzu Co. Ltd., Japan). The conversion of HCHO was calculated based on its concentration change. Each datum was measured in triplicates, and the standard error was evaluated and presented with error bars in the figures.

Figure 2. XRD patterns of TiNT and Pt/TiNT.

500 °C is of anatase phase (JCPDS file no. 21-1272). The phase evolution for the anodized TiO2 during calcinaion has also been reported in the literature.11,25 XRD patterns of Pt/ TiNT, Pt/ATiO2, and Pt/P25 are also shown in Figure 2. The characteristic peaks of PtO2 or Pt are too weak to be detected during the XRD characterizations for all the samples. Pt/ATiO2 consists of pure anatase, and Pt/P25 consists of both anatase and rutile TiO2 (JCPDS file no. 65-0190). Since the Pt particles are too small to be detected by TEM employed when the Pt loading amount is 0.4 wt %, TEM images and Pt particle size distribution of Pt/TiNT, Pt/ATiO2, and Pt/P25 with a Pt loading amount of 0.8 wt % are compared in Figure 3. It can be found that small and homogeneous Pt nanoparticles uniformly present on all the reduced catalysts. Approximately 500 Pt nanoparticles were calculated to obtain the histogram of metal particle size distribution for each sample. As shown in Figure 3b, a Pt nanoparticle size range of 3−8 nm are observed from Pt/P25, with an average value of 7.0 nm. While a Pt nanoparticle size range of 1−7 nm with an average value of 3.2 nm are calculated from Pt/TiNT, as listed in Figure 3d. Figure 3f shows that Pt/ATiO2 has an wide Pt nanoparticle size range of 1−9 nm with an average value of 3.6 nm. Thus, although the TiNT holds the smallest BET-area among the three supports, Pt particles can be well dispersed in the ordered tubes, which results in a good dispersion and a small average Pt particle size over Pt/TiNT. 3.2. XPS characterization. The XPS analysis was carried out to identify the surface elements chemical states, as shown in Figure 4 and listed in Table 1. Here, the Pt loading amount is 0.4 wt %. Figure 4a shows that binding energy (BE) negative shift of Pt 4f was observed over Pt/TiNT, Pt/P25, and Pt/ ATiO2, i.e., 70.2, 70.3, and 70.5 eV for Pt/TiNT, Pt/P25, and Pt/ATiO2, respectively, in comparison with the BE of 71.2 eV for bulk metallic Pt.29 The electron transfer from TiO2 to Pt was proposed to be responsible for this negative shift.30,31 The main peaks of O 1s (OI) and Ti 2p3/2 on the reduced catalysts are located at 530.0−530.3 and 458.7−459.0 eV, respectively. While the main peaks of O 1s (OI) and Ti 2p3/2 on the PtO/ TiO 2 catalysts were located at 530.3 and 459.0 eV, respectively.4 A BE negative shift for O 1s and Ti 2p3/2 on the reduced Pt/TiO2 catalysts (except for Pt/ATiO2) occurs. In addition, a significant shoulder peak of O 1s (OII) appears at 532.3, 531.8, and 532.0 eV for Pt/TiNT, Pt/P25, and Pt/ ATiO2, respectively. The chemisorbed oxygen OII can be activated on the metal−support interface, forming highly active

3. RESULTS AND DISCUSSION 3.1. Structural Properties. Figure 1 depicts the top, bottom, and cross-sectional morphologies of the as-anodized and annealed TiNT samples. As shown, the pore-through highly ordered and vertically oriented tube arrays have been successfully synthesized. The as-anodized TiNT is of structure with an average inner diameter, wall thickness, and tube length of about 110 nm, 22 nm, and 160 μm, respectively. The top and bottom morphologies of the calcined TiNT with a BET surface area of 31.6 m2/g are shown in Figure 1c,d, which indicates that the TiNT structure keeps well after calcination at 500 °C. Obviously, the surface roughness of the as-anodized and annealed TiNT samples are completely different, with more surface defects for the latter. The appearance of surface defects and their increased density at a high annealing temperature may be ascribed to the large stress occurring during crystallization, heating and cooling.12,25−28 The phase transformation is 7631

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Figure 3. TEM images and Pt nanoparticle size distribution of 0.8% Pt/P25 (a,b), 0.8% Pt/TiNT(c,d), and 0.8% Pt/ATiO2(e,f).

oxygen species that are involved in the oxidation reaction.4 The results also show that the structural properties of the TiO2 support have a significant influence on the surface Pt and active oxygen (OII) concentrations. For example, the Pt/Ti surface atom ratio of Pt/TiNT, Pt/P25, and Pt/ATiO2 are 0.0016, 0.0010, and 0.0011, respectively. 3.3. Catalysts Activity Test. The catalytic activities of TiNT, Pt/TiNT, Pt/P25, and Pt/ATiO2 are compared in Figure 5. The results show that the activity has a sequence of Pt/TiNT > Pt/ATiO2 > Pt/P25 when the Pt loading amount is 0.4 wt %. For example, the initial HCHO conversions at 30 °C are 95.9%, 16.3%, and 15.9%, respectively, over Pt/TiNT, Pt/ P25, and Pt/ATiO2. Even with a very low Pt loading amount, 0.12% Pt/TiNT has a comparable activity with 0.4% Pt/P25. Figure 5b shows that 0.4% Pt/TiNT holds a good stability over the 100 h test at 30 °C, with a HCHO conversion around 95%. These results are convincing regarding the good performance

and great application potential of monolith-like Pt/TiNT catalysts for HCHO oxidation, and that the structural properties of the TiO2 support significantly affect the performance of the catalysts. 3.4. In Situ DRIFT Study. In situ DRIFTS study was carried out to compare the different performance of Pt/TiNT, Pt/P25, and Pt/ATiO2 for HCHO oxidation with respect to the behavior of adsorbed species on the catalyst surface. Figure 6 shows the dynamic changes in the DRIFTS spectra of the different catalysts (0.4 wt % Pt) as a function of time in a flow of O2 + HCHO + He at 30 °C. After exposing the catalyst to O2 + HCHO + He mixture gas, four bands appear at 2062, 1657, 1570, and 1359 cm−1. According to previous studies, two strong bands at 1570 and 1359 cm−1 are ascribed to υas(COO) and υs(COO) on the active sites,32,33 which indicates that the adsorbed HCHO is converted into formate species. The weak band appearing at 2062 cm−1 is assignable to linear CO 7632

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Figure 5. (a) Dependence of HCHO conversion on reaction temperature for TiNT, 0.12% Pt/TiNT, 0.4% Pt/ATiO2, and 0.4% Pt/P25, and (b) Long-term test for trace HCHO oxidation over 0.4% Pt/TiNT at 30 °C.

support TiNT is small and negligible during the exposure process. The amount of formate species over the catalyst surface is determined by two factors, i.e., the formation rate from adsorbed HCHO and its decomposing rate. The overall effect of the two factors can increase or decrease the formate species amount, depending on their relative dominance. In combination with the activity test in Figure 5, we can propose that the low formate species amount over TiNT is due to the slow formate species formation rate. Both the formate species formation rate and decomposing rate over Pt/TiNT are the fastest among these catalysts, resulting in a smaller formate species amount but higher activity in comparison with those over Pt/P25 and Pt/ATiO2. The amount of accumulated formate species over these catalysts has a sequence of Pt/P25 > Pt/ATiO2 > Pt/TiNT, which is inverse to the activity sequence of the catalysts. This phenomenon indicates that the controlling step of the HCHO oxidation over these catalysts should be the formate species decomposing step, which is consistent with the viewpoints in the literature.3,7 3.5. Discussion. Zhang and He3 proposed a reaction scheme for HCHO oxidation over powder TiO2 supported noble metal catalysts involving the behavior of surface adsorbed species obtained from in situ DRIFTS. In this mechanism, HCHO was first oxidized into formate surface species. The formate surface species then directly decomposed into adsorbed CO species and H2O, and the CO species finally reacts with O2 to produce gas phase CO2. The in situ DRIFTS study in this work indicates that HCHO oxidation over the monolith-like Pt/TiNT generally follows this mechanism, e.g., adsorbed formate speicies and adsorbed CO are the main intermediate species. In combination with the O2 activation

Figure 4. XPS of the reduced 0.4% Pt/TiO2 catalysts: Pt 4f (a), Ti 2p (b), and O 1s (c).

Table 1. XPS Data for the Reduced 0.4% Pt/TiO2 Catalysts BE (eV)

surface atom ratio

catalysts

Pt 4f7/2

OII (OI)

Ti 2p

OII/OIa

Pt/Ti

Pt/TiNT Pt/P25 Pt/ATiO2

70.2 70.3 70.5

532.3(530.1) 531.8 (530.0) 532.0 (530.3)

458.8 458.7 459.0

0.10 0.14 0.14

0.0016 0.0010 0.0011

Calculated from the corresponding areas of fitted peaks done by XPSPEAK 4.1 with Shirley background.

a

adsorbed on Pt, and the band at 1657 cm−1 is due to water adsorbed on the catalyst,3,34 which originates from the water in reactant gases or the products of the HCHO oxidation. The intensities of these bands increase with increasing exposure time and reach a steady level after 60 min of exposure. A semiquantitative analysis by calculating the integrated area of the formate species band at 1570 cm−1 is given to illustrate the activity difference among these samples. As shown in Figure 6e, both the band intensities and the speed of intensity change with exposure time over Pt/P25 are the largest, while both are smallest over Pt/TiNT among the three catalysts during the whole 60 min on stream time. The band intensity over the 7633

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Figure 6. In situ DRIFTS spectra of O2 + HCHO + He gas mixture adsorption over (a) 0.4% Pt/TiNT, (b) 0.4% Pt/P25, (c) 0.4% Pt/ATiO2, (d) TiNT at 30 °C, and (e) intensity of the corresponding bands at 1570 cm−1.

process, or the involvement of the chemisorbed oxygen in the process, the complete reaction process is illustrated in Figure 7. This process can be generally separated into two parts, i.e., storage step and oxidation step. In the storage step, HCHO is first chemisorbed and react with the chemisorbed oxygen to form formate surface species and then decomposes into adsorbed CO species and H2O. In the oxidation step, the adsorbed CO species reacts with O2 or chemisorbed oxygen to produce gas phase CO2, and the gas O2 is reactivated by the active sites to form chemisorbed oxygen. In situ DRIFTS study indicates that the controlling step of the HCHO oxidation over the Pt/TiNT, Pt/P25, and Pt/ATiO2 studied is the formate species decomposing in the storage step. Thus, although the density of the chemisorbed oxygen over Pt/TiNT is smaller than those over Pt/ATiO2 and Pt/P25, and the chemisorbed oxygen has been reported to be very important during the oxidation process,4,35,36 the activity of Pt/TiNT is higher. It should be noted that the detailed reaction mechanism still needs to be clarified, e.g., how the decomposing of formate species happens, whether it is the gas O2 or chemisorbed oxygen that reacts with adsorbed CO, and so forth.

Figure 7. Reaction scheme for the catalytic oxidation of HCHO over the TiO2 supported Pt catalyst.

The high activity of Pt/TiNT can be attributed to its special structural properties. First, well-dispersed and the surface 7634

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enrichment of Pt (or high surface Pt/Ti atom ratio) due to the confinement effect of the regular nanotube structure is beneficial for the storage of HCHO, i.e., chemisorbed of HCHO and the decomposing of the formate species. In addition, such a confinement effect of the Pt particles in the nanotube and the specific monolith-like structure provides the resistance ability against catalyst sintering and deactivation.10−12 Second, negative BE shift of Pt 4f was observed over all the reduced Pt/TiO2 catalysts.29 The electron transfer from TiO2 support to Pt particles is the reason for the negatively charged Pt particles.4,37 This phenomenon may become more intense for smaller Pt particles due to the decreased coordination number of surface Pt atoms.37 Thus, stronger metal−support interactions (or larger negative shift of Pt 4f) are observed over Pt/TiNT in comparison with Pt/P25 and Pt/ATiO2. The negatively charged Pt (electron donor) can enhance the decomposition of the chemisorbed formate species. In all, small and well-dispersed Pt, surface enrichment of Pt and the strong-metal−support-interaction, which originate from the ordered monolith-like structure of TiNT, together lead to the good performance of the Pt/TiNT catalyst.

4. CONCLUSIONS Highly ordered pore-through TiO2 nanotube arrays (TiNT) prepared by an electrochemical anodization method were used as the support for a Pt/TiNT catalyst. The Pt/TiNT was then applied to the trace formaldehyde oxidation. The regular and monolith-like structure led to well-dispersed metallic Pt nanoparticles, surface enrichment of Pt, and the strongmetal−support-interaction over Pt/TiNT. Hence, in comparison with the commercial TiO2 powders (P25 and anatase TiO2) supported Pt catalysts, Pt/TiNT showed higher activity under parallel preparation and test conditions. A HCHO conversion of 95% with a more than 100 h stable performance was achieved over Pt/TiNT at 30 °C with a low 0.4 wt % Pt loading amount. In situ DRIFTS study illustrated that HCHO was first chemisorbed and reacted with the chemisorbed oxygen to form formate surface species and then decomposed into adsorbed CO species and H2O. The fast formate surface species forming and decomposing rate over Pt/TiNT led to a higher HCHO oxidation activity in comparison with the Pt/P25 and Pt/anatase catalysts. Shortly, monolith-like Pt/TiNT was demonstrated to be an effective catalyst for the removal of HCHO under mild conditions.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 20 84113658. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work has been supported by the Natural Science Foundation of China (21106189 and 21036009), the Natural Science Foundation of Guangdong Province, China (S2011040001767), and the Fundamental Research Funds for the Central Universities(12lgpy11).

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REFERENCES

(1) Bianchi, F.; Careri, M.; Musci, M.; Mangia, A. Fish and food safety: Determination of formaldehyde in 12 fish species by SPME extraction and GC−MS analysis. Food Chem. 2007, 100, 1049−1053. 7635

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dx.doi.org/10.1021/ie5004009 | Ind. Eng. Chem. Res. 2014, 53, 7629−7636