Removal of Volatile Organic Compounds Driven by Platinum Supported on Amorphous Phosphated Titanium Oxide

HUANG Xieyi, WANG Peng, YIN Guoheng, ZHANG Shaoning, ZHAO Wei,WANG Dong, BI Qingyuan, HUANG Fuqiang,3,4

Removal of Volatile Organic Compounds Driven by Platinum Supported on Amorphous Phosphated Titanium Oxide

HUANG Xieyi1,2, WANG Peng2,3, YIN Guoheng1, ZHANG Shaoning1, ZHAO Wei1,WANG Dong1, BI Qingyuan1, HUANG Fuqiang1,3,4

(1. State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China; 2. University of Chinese Academy of Sciences, Beijing 100049, China; 3. School of Physical Science and Technology, ShanghaiTech University, Shanghai 200031, China; 4. State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China)

Development of high efficiency catalyst is the key factor to catalytic combustion of volatile organic com-pounds (VOCs). Herein, amorphous mesoporous phosphated TiO2(ATO-P) with high specific surface area supported platinum catalyst was successfully fabricated. P-dopant can increase the surface area (up to 278.9 m2?g?1) of ATO-P, which is 21 times higher than that of pristine TiO2, and make the amorphous titanium oxide structure. The supported Pt catalyst with amorphous mesoporous feature shows impressive performance and excellent thermostability for VOCs oxidation. The Pt/ATO-P catalyst exhibits outstanding catalytic efficiency, the50and90(temperatures required for achieving conversions of 50% and 90%) are respectively 130 ℃and 140 ℃, for toluene oxidation under high gas hourly space velocity (GHSV) of 36000 mL·h?1·g?1and toluene concentration of 10000 mL·m?3. The performance is superior to the reference Pt/TiO2and comparable with the state-of-the-art catalysts. These findings can make a significant contribution on the new applications of amorphous mesoporous phosphated materials in VOCs removal.

amorphous mesoporous structure; phosphated TiO2; Pt nanoparticle; toluene oxidation; VOCs removal

Volatile organic compounds (VOCs), like toluene, benzene, esters and hydrocarbons, are emitted from vari-ous industrial sources which can cause serious envi-ronmental pollution and health problems[1?2]. Toluene, one kind of toxic and strong carcinogenic chemical, is frequently used in making paints, adhesives, rubbers, and leather tanning processes because of its excellent ability to dissolve organic substances[3-4]. However, toluene is difficult to degrade due to its stable structure[5]. Several techniques, such as physical and chemical adsorption, photocatalytic and catalytic oxidation methods, are widely used for the combustion of VOCs[6-7]. Among them, catalytic oxidation is regarded as a promising approach owing to its high efficiency and convenient operating conditions[8].

Researches on catalysts for toluene oxidation have been conducted, including noble metal and metal oxides catalysts[9-10]. Due to the significant reduction on acti-vation energy during the catalytic oxidation process, noble metal based catalysts, such as Pt, Pd, Au, Rh, and Ir have shown impressive performance in toluene remo-val[11-13]. It was found that supported Pt catalysts showed the best catalytic performance compared with other noble metals[14-15]. It should be pointed out that the supports play an important role in the catalytic reaction pro-cesses[16-18]. Many works have focused on the metal- support interac-tion by studying the catalytic properties of TiO2, Al2O3, ZrO2, and ZnO supported Au nanopar-ticles[19], and the shape effect of Pt/CeO2catalysts[10]. Nevertheless, most supports suffer from low specific surface area and few active sites, which are crucial for the overall catalytic activity.

Due to high specific surface area and variable valence, amorphous materials have attracted increasing interests in VOCs oxidation. And the numerous defects in amo-rphous structures can offer large quantities of oxygen vacancies, which are beneficial for the adsorption of oxygen and organic molecules. Lee,[20]reported that carbon black supported amorphous MnOis highly efficient for oxygen involved reaction. Wang,[21]found that amorphous MnOmodified Co3O4can en-hance the catalytic activity for the VOCs oxidation. It was demonstrated that the amorphous structure of bimetallic Pd-Pt/CeO2-Al2O3-TiO2could provide more vacancies and active sites for catalytic combustion[22]. Therefore, the amorphous catalysts show a tremendous potential in practical catalytic reactions. However, it is still a challenge to develop highly active and robust catalysts based on the amorphous materials for the oxidation of VOCs.

Herein, we demonstrate an efficient Pt/ATO-P catalyst for the catalytic removal of VOCs under high gas hourly space velocity (GHSV)and high substrate concentration. It should be pointed out that incorporating phosphorus into the framework of TiO2is a widely applied strategy for obtaining amorphous mesoporous feature[23-24]. And the P element can stabilize the TiO2framework and significantly increase the specific surface area[24].

1 Experimental

1.1 Preparation of sample

1.1.1 Preparation of support

All reagents were of analytical grade and were used without any purification. 3 mL of tetrabutyl titanate was dissolved in 30 mL of ethanol at room temperature, which was marked as solution A. Then 0.125 mL of phosphoric acid (H3PO4) was subsequently dropwisely added into solution A with stirring to form a homogenous mixture, and kept stirring for 24 h. The obtained white solid products were separated by centrifuge, and washed by deionized water and ethanol several times, followed by freeze drying overnight. The as-prepared products were calcined at 400 ℃in air for 4 h at a heating rate of 5 ℃?min?1.

1.1.2 Preparation of catalyst

The ATO-P supported platinum (Pt/ATO-P) sample was preparedimpregnation method. A desired amount of ATO-P was transferred into aqueous solution containing appropriate amount of chloroplatinic acid (H2PtCl4). Subsequently, the samples were impregnated at room temperature for 12 h. After drying out the H2O at 80 ℃, the samples were treated at 350 ℃ for 2 h with a H2/Ar mixture (5/95,/).

1.2 Characterization

XRD characterization of the samples was carried out on a German Bruker D8 Advance X-ray diffractometer (XRD) using the Ni-filtered Cu Kα radiation at 40 kV and 40 mA. Nitrogen adsorption-desorption isotherms were measured at –196 ℃ on a Micromeritics ASAP 2460 analyzer. Samples were degassed at 120 ℃ for 24 h prior to the measurement. The specific surface area of the samples was calculated using the Brunauer–Emmett– Teller (BET) method with the adsorption data at the relative pressure (/0) range of 0.05–0.2. The total pore volumes were estimated at/0=0.99. The pore size distribution (PSD) curves were calculated from the adsorption branch using Barrett-Joyner-Halenda (BJH) model. The prepared materials were pressed into tablets with KBr powder and then detected by FT-IR (Perkin Elmer, USA) in the scanning range from 400 to 4000 cm–1. SEM images were obtained by Hitachi-S4800. A JEOL 2011 microscope operating at 200 kV equipped with an EDX unit (Si(Li) detector) was used for the transmission electron microscope (TEM) and high resolution trans-mission electron microscope (HRTEM) investigations. The samples for TEM testing were prepared by dis-persing the powder in ethanol and applying a drop of highly dilute suspension on carbon-coated grids. XPS data were recorded with a Perkin Elmer PHI 5000 C system equipped with a hemispherical electron energy analyzer. The spectrometer was operated at 15 kV and 20 mA, and a magnesium anode (Mg Kα,=1253.6 eV) was used. The C1s line (284.6 eV) was used as the reference to calibrate the binding energies (BE). TG measurements were conducted on a Netzsch STA 449C TG-DSC thermoanalyzer. The flow rate of the carrier gas (air) was 30 mL?min–1. The temperature was raised from room temperature to 800 ℃ at a ramp rate of 10 ℃?min–1. Prior to H2-TPR test, the sample (100 mg) was pretreated at 200 ℃ for 2 h and cooled to 50 ℃ in the flowing He. TPR experiment was carried out in 5vol% H2/He flowing at 30 mL?min–1, with a ramping rate of 5 ℃?min–1to a final temperature of800 ℃. The signal was monitored using a TCD detector.

1.3 Catalytic activity test

The catalytic activity of samples was evaluated in a continued-flow fixed-bed quartz reactor with 50 mg catalyst. Toluene was introduced into the reactor with bubbling toluene solution in ice bath with pure air. The concentration of toluene was about 104mL?m?3, and the flow rate was kept at 30 mL?min–1by a mass controller, equivalent to a gas hour space velocity (GHSV) of 36000 mL?h–1?g–1. After steady operation for 100 min, the activity of the catalyst was tested. Toluene con-cen-tration was detected by a gas chromatograph equi-pped with a flame ionization detector. The toluene conversion (toluene) was calculated according to the equation:

toluene(inout)/in·100% (1)

whereinandoutare the inlet and outlet toluene concentrations, respectively.

2 Results and discussion

2.1 Physicochemical properties of ATO-P support

Fig. 1 displays the schematic diagram of amorphous ATO-P preparedfacile co-precipitation. XRD patterns of ATO-P and TiO2are shown in Fig. 2. All diffraction peaks of basic TiO2sample are indexed to anatase phase (JCPDS 21-1276). Interestingly, there is no TiO2crystal phase observed for ATO-P sample (Fig. 2), suggesting that ATO-P sample is typically amorphous and phosphorus dopant can markedly restrain the crystallization of anatase[25?26].

According to the TGA-DSC thermograms (Fig. 3), a thermal decomposition of ATO-P took place in the temperature range of 20?900 ℃. The first DSC peak at 30?80 ℃ is due to the release of physical adsorbed water. When all the water is released, Ti?OH and HPO42?groups start to condense[27]. These processes occur simultaneously in the temperature range of 100?220 ℃ (1.927% of weight loss) and 220?516 ℃ (0.7% of weight loss), resulting in an overlap of the TG data. There is no further weight loss up to 516 ℃. The DSC curve shows two exothermic peaks at 704and 781 ℃, corresponding to a two-step exothermic transformation of ATO-P into a crystalline phase.

Fig. 1 Structure of amorphous ATO-P prepared via facile co-precipitation

Fig. 2 XRD patterns of TiO2 and ATO-P samples

Fig. 3 TG (solid line) and DSC (dashed line) curves for ATO-P

Fig. 4(a,b) show the SEM images of ATO-P. The ATO-P nanoparticles are homogeneously dispersed with the particle size of ~20 nm, and the sizes are similar to that of TiO2(Fig. S1(a)). HRTEM was employed to characterize the nanostructure of samples. No porous structure is observed in the HRTEM image of TiO2(Fig. S1(b)), while various porous structure is shown in ATO-P (Fig. 4(c)). Moreover, the pores of ATO-P are uniform, and the average diameter is around 10 nm. EDS elemental mappings indicate that the P element homo-geneously distributes in ATO-P (Fig. 4(d)). It is found that H3PO4owns unique effects for synthesizing amorphous mesoporous phosphated TiO2[28-29].

Fig. 4 SEM (a, b) and HRTEM (c) images, and EDS elemental mapping (d) of ATO-P

As shown in Fig. 5, the obtained ATO-P sample shows a characteristic type-IV isotherm with clear hysteresis loop locates at the/0range of 0.45?1.0, showing the existence of a large amount of mesopore. Notably, the specific surface area of 278.9 m2·g?1for ATO-P is 21 times higher than that of pristine TiO2. The pore diameters of ATO-P center around 10 nm (Fig. 5 and Table 1), which is consistent with HRTEM result (Fig. 4(c)).

The results of EDX are listed in Table 1. The actual P concentration is much less than the initial addition amount of H3PO4, suggesting that partial H3PO4is leached during the preparation process.

FT-IR spectra of TiO2and ATO-P samples are depicted in Fig. 6. The wide absorption bands around 3440 and 1620 cm?1are attributed to the surface adsorbed water and/or hydroxyl groups[30-31]. The bands at 1100 cm?1are ascribed to the stretching vibration of Ti?O?P species, which are absent in TiO2. The weak bands at 610 cm?1are due to the vibration of Ti?O?Ti bond[22]. Compared with TiO2, a weak peak appears in series ATO-P, which may result from the incorporating effect of phosphorus dopant. There is no distinct peak over the range of 700?800 cm?1(Fig. 6), indicating the absence of P?O?P groups in the amorphous mesoporous phosphated TiO2. Therefore, the P element is incorporated into the frameworks of ATO-P by forming Ti?O?P bonds[24].

Fig. 5 N2 adsorption-desorption isotherms (a) and pore size distributions (b) of ATO-P and TiO2

Table 1 Textural properties and elemental compositions ofTiO2 and ATO-P samples

[a] Weight fraction (wt%) are determined by EDX analysis

Fig. 6 FT-IR spectra of TiO2 and ATO-P

As shown in Fig. 7(a), the full XPS spectra indicate the existence of P in ATO-P. High-resolution XPS spectra of P 2p, Ti 2p and O 1s are depicted in Fig. 7(b?d). The peak of P 2p of ATO-P is at 134.0 eV, suggesting that phosphorus in ATO-P gives a pentavalent oxidation state of P5+. No peak observed at 128.6 eV, which is the characteristic binding energy of P2p in TiP, indicating the absence of Ti?P bonds in ATO-P samples. As depicted in Fig. 7(c), the peaks of Ti2p3/2and Ti2p1/2in ATO-P show remarkable blue-shift owing to the incorporation effect of phosphorus element. Fig. 7(d) shows the XPS spectra of O1s signals of TiO2and ATO-P. The single peak at 529.5 eV is corresponded to the oxygen in Ti?O bond of TiO2. However, the O1s spectrum of ATO-P contains two peaks at 531.4 and 532.9 eV, which are contributed to Ti?O?P and O?H bond, respectively[32-33].

2.2 Physicochemical properties of Pt/ATO-P catalysts

Fig. 8(a) shows that the Pt nanoparticles are well dis-persed over the ATO-P support, and the size is relatively uniform with the average parameter of (1.8±0.3) nm (insert in Fig. 8(a)). Fig. 8(b) and S2 demonstrate a-spacing of 0.23 nm, attributed to the (111) plane of the highly crystalline Pt nanostructure. Furthermore, the actual Pt content was also confirmed by inductively coulped plasma atomic emission spectra (ICP-AES). The mass loadings of Pt in Pt/TiO2and Pt/ATO-P catalysts are 0.90 and 0.92, respectively, which are close to the nominal composition of 1wt%.

Fig. 8(c) shows the XRD patterns of Pt/ATO-P and Pt/TiO2catalysts. The amorphous structure is still remained for Pt/ATO-P sample. However, no diffraction pattern of Pt nanoparticles is observed, indicating that the Pt nanoparticles are quite small and/or the Pt species are highly dispersed on the ATO-P surface. These results are well consistent with the HRTEM data above mentioned in Fig. 8(a, b).

Fig. 7 Full XPS spectra (a) of TiO2 and ATO-P; High-resolution XPS P2p (b), Ti2p (c), and O1s (d) of TiO2 and ATO-P

Fig. 8 TEM (a) and HRTEM (b) images of Pt/ATO-P with insert in (a) indicating the particle size distribution of Pt nanoparticles, XRD patterns (c) and XPS Pt4f (d) of Pt/ATO-P

The results of XPS analysis of Pt/ATO-P and Pt/TiO2samples are depicted in Fig. 8(d). It is known that the positions of Pt4f7/2binding energy at 71.1, 72.4, and 74.2 eV are attributedto Pt0, Pt2+, and Pt4+species, respec-tively[34]. Similiar XPS profiles arerendered as the indication of a mixture of various valence states for Pt species overthe small Pt nanoparticles. The exisence of Pt+species reflects the strong metal-support interaction (Pt?ATO-P), especially the prominent electronic intera-ction between active Pt and underlying phosphated TiO2support[35]. This is probably due to the changes of the metal- support interaction by doping phosphorus atoms which can make an obvious effect onTi?O?P frameworks.

The H2-TPR profiles depicted in Fig. S3 show that there are two H2-consumption peaks at low and high temperature attributed to weak and strong interaction of Pt and supports, respectively[36]. Notably, two reduction peaks of Pt/ATO-P catalyst at 78 and 601 ℃ show stro-nger intensity than that of Pt/TiO2at 72 and 433 ℃, indicating strongPt-support interaction for Pt/ATO-P. These results are consistent with the XPS data.

2.3 Removal of VOCs by Pt/ATO-P catalysts

The catalytic efficiencies are depicted in Fig. 9. It is clearly observed that reaction temperature can enhance the performance of Pt/ATO-P catalyst. The50and90are widely used to evaluate the catalytic performance[37]. As shown in Fig. 9(a), Pt/ATO-P shows the excellent catalytic activity.50and90values for toluene com-bustion are 130 and 140 ℃, which are much lower than those of Pt/TiO2with50and90of 160 and 190 ℃, res-pectively. Combined with the above XPS data (Fig. 8(d)), it can be concluded that the existance of phosphorus component plays an important role in electronic structure of the active Pt species underlying amorphous meso-porous ATO-P support and thus the catalytic oxidation removal of toluene over Pt/ATO-P catalyst.

Fig. 9 Toluene conversion (a) of 1wt% Pt/ATO-P with respect to reaction temperature, and thermal stability (b) of Pt/ATO-P at 180 ℃

It is well known that noble metal loading significantly affects the catalytic behavior for many reactions. Pt/ATO-P catalysts with different Pt loadings were examined, and the results are depicted in Fig. 10. Compared with 0.5wt% and 2wt%, the Pt loading of 1wt% shows better performance (lower50and90) for toluene oxidation. The low catalytic activity of 0.5 wt% Pt/ATO-P results from low density of active platinum nanoparticles anchoring on the surface of ATO-P support. For the Pt/ATO-P catalyst with Pt loading up to 2wt%, larger size of Pt nanopartices (~5 nm) can be obtained (Fig. S4). Larger Pt particles can not only decrease the dispersion of Pt species[38], but also lead to a weaker metal-support (Pt/ATO-P) interactions, thus resulting in the poor activity.

Stability is critical for the catalysts on the practical application. 1wt% Pt/ATO-P exhibits excellent thermal stability for toluene oxidation over a 50-h period on stream at 180 ℃ without visible loss of activity, as shown in Fig. 9(b). The toluene conversion remains a high level of 95.4% at the end of reaction process and maintains near full selectivity to final products of CO2and H2O. The excellent stability of Pt/ATO-P catalyst is attributed to the unique geometric structure of crystalline Pt nanoparticles and amorphous mesoporous phosphated TiO2with prominent electronic interaction. For the used 1wt% Pt/ATO-P, TEM measurement and XPS analysis (Fig. S5 and Fig. S6) demonstrate no significant change on the morphology, average size of Pt nanoparticles, and the chemical oxidation state of active Pt species. These results suggest the robustness of Pt/ATO-P catalyst for toluene oxidation removal under a relatively mild the-rmal process.

Given the superb thermocatalytic performance for 1wt% Pt/ATO-P catalyst toward toluene oxidation, we were curious to examine whether the engineered material would also catalyze the removal of a class of VOCs, especially the complete oxidation of benzene,-hexane, ethyl acetate, and mesitylene. As depicted in Fig. 11, the90values for the catalytic oxidation of benzene, ethyl acetate,-hexane, and mesitylene are 216, 331, 271, and 200 ℃, respectively. Notably, high tem-perature is requ-ired for ethyl acetate conversion at 90% due to its strong structural stability[39-40]. These results show a broad scope toward catalytic combustion invo-lving trouble-some organic compounds over Pt/ATO-P and indicate that the Pt/ATO-P catalysts can provide a new insight for the oxidation of VOCs.

Fig. 10 Toluene conversion over Pt/ATO-P catalysts with different Pt loadings

Fig. 11 Catalytic activity of Pt/ATO-P for the conversion of benzene (a), ethyl acetate (b), n-hexane (c), and mesitylene (d) with respect to reaction temperature

3 Conclusions

In summary, we successfully fabricated the amorphous mesoporous phosphated TiO2supported platinum catalysts for efficient removal of volatile organic compounds. The electronic modifications of supported Pt nanoparticles for the underlying amorphous ATO-P material and Pt loading for the whole catalyst were systematically investigated. The phosphorus dopant played an important role for stabilizing the inflated Ti?O?P frameworks as well as the electronic structure of Pt species. Compared with pristine TiO2, ATO-P with high specific surface area showed signi-ficant enhancement for Pt/ATO-P samples for catalytic overall oxidation of toluene under practical conditions. The performance of the engineered Pt/ATO-P for toluene combustion was superior to the reference Pt/TiO2and comparable with the state-of-the-art catalysts. Additionally, Pt/ATO-P catalyst exhibited excellent stability for toluene oxidation removal under a relatively mild thermal process and could be potentially applied in a broad scope of VOCs. The present work is expected to make a significant contribution on the new application of amorphous mesoporous phosphated material in VOCs removal.

Supporting Materials

Supporting Materials related to this article can be found at https://doi.org/10.15541/jim20190154.

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摻磷非晶氧化鈦負(fù)載鉑用于高效催化氧化揮發(fā)性有機(jī)化合物

黃謝意1,2, 王鵬2,3, 尹國恒1, 張紹寧1, 趙偉1, 王東1, 畢慶員1, 黃富強(qiáng)1,3,4

(1. 中國科學(xué)院 上海硅酸鹽研究所, 高性能陶瓷和超微結(jié)構(gòu)國家重點(diǎn)實(shí)驗(yàn)室, 上海 200050; 2. 中國科學(xué)院大學(xué), 北京 100049; 3. 上?？萍即髮W(xué) 物理科學(xué)與技術(shù)學(xué)院, 上海 200050; 4. 北京大學(xué) 化學(xué)與分子工程學(xué)院, 稀土材料化學(xué)及應(yīng)用國家重點(diǎn)實(shí)驗(yàn)室, 北京 100871)

高活性催化劑是揮發(fā)性有機(jī)化合物(VOCs)催化氧化消除的關(guān)鍵因素。本研究通過簡單的共沉淀法成功制備了具有高比表面積的非晶介孔磷摻雜氧化鈦負(fù)載鉑催化劑(Pt/ATO-P)。通過P摻雜, 既可獲得非晶介孔結(jié)構(gòu), 又可獲得高ATO-P比表面積(可達(dá)278.9 m2?g?1)。非晶介孔Pt/ATO-P催化劑顯示出優(yōu)異的VOCs催化氧化性能和良好的熱穩(wěn)定性。Pt/ATO-P樣品在空速為36000 mL?h?1?g?1、甲苯濃度為10000 mL?m?3的反應(yīng)條件下, 對甲苯催化氧化的50和90(實(shí)現(xiàn)50%和90%轉(zhuǎn)化率所需的溫度)分別為130 ℃和140 ℃, 明顯優(yōu)于無磷催化劑Pt/TiO2。這些發(fā)現(xiàn)可以為拓展非晶介孔磷化材料在環(huán)境凈化和能源轉(zhuǎn)化等領(lǐng)域的應(yīng)用提供重要參考。

非晶介孔材料; 磷摻雜非晶氧化鈦; 鉑納米顆粒; 甲苯催化氧化; VOCs消除

O782

A

2019-04-12;

2019-05-24

National Key Research and Development Program of China (2016YFB0901600); National Natural Science Foundation of China (21872166); Science & Technology Commission of Shanghai (16ZR1440400, 16JC1401700); The Key Research Program of Chinese Academy of Sciences (QYZDJ-SSW-JSC013 and KGZD-EW-T06)

Huang Xieyi (1994–), male, Master candidate. E-mail: huangxieyi@student.sic.ac.cn

黃謝意(1994–), 男, 碩士研究生. E-mail: huangxieyi@student.sic.ac.cn

BI Qingyuan, associate professor. E-mail: biqingyuan@mail.sic.ac.cn;

HUANG Fuqiang, professor. E-mail: huangfq@mail.sic.ac.cn

畢慶元, 副研究員. E-mail: huangfq@mail.sic.ac.cn; 黃富強(qiáng), 研究員. E-mail: huangfq@mail.sic.ac.cn

1000-324X(2020)04-0482-09

10.15541/jim20190154