Ziqing Yuan, Ziyu Chen, Jianxin Mao, Renxian Zhou

Institute of Catalysis, Zhejiang University, Hangzhou 310028, China

Keywords:Environment Particle size distribution Catalysis Thermal stability Size effect of Pt

A B S T R A C T Pt/Al2O3 catalysts with smaller size of Pt nanoparticles were prepared by ethylene glycol reduction method in two different way and their oxidation activities for three typical VOCs (volatile organic compounds) were evaluated. The catalyst prepared by first adsorption and then reduction procedure is denoted as L-Pt/Al2O3 while the catalyst prepared by first reduction and then loading procedure is defined as R-Pt/Al2O3.The results show that L-Pt/Al2O3 with the stronger interaction between Pt species and Al2O3 exhibit smaller size of Pt nanoparticles and favorable thermal stability compared with R-Pt/Al2O3.L-Pt/Al2O3 is favor of the formation of more adsorbed oxygen species and more Pt2+species,resulting in high catalytic activity for benzene and ethyl acetate oxidation.However,R-Pt/Al2O3 catalysts with higher proportion of Pt0/Pt2+and bigger size of Pt particles exhibits higher catalytic activity for n-hexane oxidation. Pt particles in R-Pt/Al2O3 were aggregated much more serious than that in L-Pt/Al2O3 at the same calcination temperature. The Pt particles supported on Al2O3 with ～10 nm show the best catalytic activity for n-hexane oxidation.

1. Introduction

Volatile organic compounds (VOCs) are considered as a major class of pollutants emitted from chemical industries because they have harmful effects on human health and induce many environment problems [1]. Among the various techniques for eliminating VOCs,catalytic combustion is identical as one of the most promising ways because of its high efficiency, low energy consumption and low production of secondary pollutants [2-6]. Catalysts like noble metals especially platinum supported Al2O3are widely used for VOCs combustion [7-11]. Although Pt/Al2O3catalysts are highly active for the oxidation of VOCs, those catalysts are very expensive in the industrial field [12]. So the development of catalysts presenting high catalytic activity with a small amount of noble metals has been paid great attention and the nano-sized Pt supported Al2O3catalysts are strongly desired. Nanosized Pt supported catalysts own high dispersion of Pt and abundant unsaturated coordination surface sites, which dramatically improves the surface-to-volume ratios of Pt subsequently increasing surfaceactive atoms available for VOCs to adsorb[7-9].For example,Yanget al.[13]confirmed that the 0.3%Pt/10%Ce-10%V/γ-Al2O3catalyst represents good catalytic activity for benzene combustion mainly because of the high dispersion of Pt species to be sufficiently exposed to the reactant molecule.Moreover,solution-based reduction method is also widely used to prepare supported nanosized metallic catalysts by heating corresponding metal precursor in ethylene glycol,followed by the loading of the metal nanoparticles onto a support [14-17].

However, the nano-sized Pt particles are easier to aggregate under higher reaction temperature which may severely affects catalytic activity. So strategies to stabilize Pt nanoparticles are reported by many academic groups.One is to form barriers to inhibit the migration of noble metal particles. The other is to use mutual interaction between noble metal nanoparticles and the support to avoid them sintering.Zhanet al.[18]demonstrated that surface-bound surfactants could be used to stabilize Au nanoparticlesviaan efficient thermal annealing way. The Au/TiO2is annealed at 500°C in a N2flow to carbonize the surfactants to form architectural conformal coatings on Au and then remove the coatings by post-thermal treatment in air, thus effectively stabilizing the Au particles.Liet al.[19]reported a well-defined cuboctahedral MgAl2O4spinel support that was capable of stabilizing platinum particles in the range of 1-3 nm on its abundant 111 facets during aging at 800°C in air.Hatanakaet al.[20]synthesized Pt/CeO2with various Pt loading and found that the formation of Pt-O-Ce bond on the surface of a catalyst support inhibited Pt particles sintering and promoted the redispersion of agglomerated Pt particles. On the whole, it is of great practically significant to develop a feasible method to synthesize nano-sized Pt supported catalysts with high thermal stability. In addition, the particle size effects of noble metal supported on various supports were reported in more literatures, and both positive and negative size effects were revealed.Kamiuchiet al.[21] reported that the activity for ethyl acetate of Pt/SnO2catalysts was deteriorated due to the particle growth.Padilla-Serranoet al.[22] revealed a negative particle size effect for the combustion ofo-xylene andm-xylene over carbon aerogel supported Pt (1%, by mass) catalyst. At present, the influence of metal particle size on the catalytic activity towards different reactants is still under discussion.

In this paper, the effects of two different loading approaches of nano-sized Pt onto Al2O3by ethylene glycol reduction method were introduced. The catalysts were calcined at various temperatures to investigate particle size effects of noble metal and their thermal stability. The catalytic performance of the catalysts towards the oxidation of benzene,ethyl acetate andn-hexane were evaluated. A range of characterizations, including HRTEM, CO chemisorption,in situDRIFTS, XPS and H2-TPR were adopted to acquire more information about the relationship between their physical-chemical properties and the catalytic performance.

2. Experimental

2.1. Chemicals

Al2O3(4.1%La2O3, BET surface area: 160 m2·g-1) was supplied by SOLVAY USA INC. It was calcined in air at 500 °C for 2 h before use. H2PtCl6·6H2O, ethylene glycol, hydrochloric acid, NaOH, benzene,ethyl acetate andn-hexane were purchased from Sinopharm Chemical Reagent Limited Company and used without further treatment.

2.2. Catalyst preparation

Two different methods were used to synthesize Pt/Al2O3catalyst. The catalyst was prepared with EG (ethylene glycol) method[11](abbreviated as L-method),which was similar to the preparation of Pt/MWNT(A)[16]. In this way,the support was suspended in ethylene glycol solution and mixed with hexachloroplatinic acid EG solution to make H2PtCl6adsorbed in the support, the solution was then heated until H2PtCl6was fully reduced to Pt and loaded on to the support. The detail process is as follows: 2 g Al2O3was added into a solution containing 50 ml ethylene glycol and H2-PtCl6·6H2O(corresponding to 0.01 g Pt),which was vigorously stirred for 3 h at room temperature to ensure PtCl62-deposited onto the surface of Al2O3. NaOH solution was added to adjust the pH value to 11.0. The solution was heated under reflux at 160 °C for 6 h in flowing N2,subsequently cooled down to room temperature,centrifuged and washed with deionized water and ethanol, finally dried at 100°C in air for 12 h.The catalyst is labeled as L-Pt/Al2O3-100. The corresponding calcined catalysts are labeled as L-Pt/Al2O3-T, T is the calcination temperature.

Another comparison preparation method(named as R-method)was designed to check the interaction between Pt and the support.In this way,nanosized Pt particles were firstly produced in EG solution, then deposited on to a support to get a comparison catalyst[14,15]. The typical preparation procedure is as follows: a glycol solution of H2PtCl6·6H2O (0.01 g Pt in 50 ml) was adjusted to 11.0 by NaOH solution, which was then heated at 160 °C for 6 h with nitrogen flow passing through the reaction system.After cooling in air, the pH of the mixture was adjusted to lower than 4 by HCl solution, at the same time, the black platinum nanoclusters in glycol were precipitated [17]. Then 2 g of Al2O3was added into the solution and heated to 100°C for 3 h to make Pt nanoparticles loaded on the substrates completely. The subsequent treatment procedure was the same as that stated above. The as-synthesized and calcined catalysts are denoted as R-Pt/Al2O3-100 and R-Pt/Al2O3-T respectively.

The Inductively Coupled Plasma(ICP) analysis results show the actual Pt loading is 0.48%and 0.49%for L-Pt/L-Pt/Al2O3-100 and RPt/L-Pt/Al2O3-100 respectively.

2.3. Catalytic performance evaluation

The oxidation performance of the catalysts was evaluated in a fixed-bed flow quartz reactor (i.d. = 8 mm) at atmospheric pressure. 140 mg catalysts (40-60 meshes) were fixed by upper and lower layers of quartz wool in the reactor. In the case of benzene,ethyl acetate andn-hexane oxidation, continuous feed composed of ～0.28% (volume) benzene, 0.65% (volume) ethyl acetate and 0.35% (volume)n-hexane with dry air as the balance gas,corresponding to a GHSV (Gas Hourly Space Velocity) of 32,000 ml·gc-at1·h-1.The inlet and outlet gases were analyzed on line by a chromatograph (GC-5190) equipped with a flame ionization detector (FID) and a packed column of 15% SE-54/Chromosorb-101.The conversions of VOCs oxidation were calculated according to the ratios of the concentration of reactants in inlet and outlet flow.

2.4. Catalyst characterization

High resolution transmission electron microscopic (HRTEM),High angle annular dark field (HAADF) and Energy Dispersive Xray (EDX)-mapping images of the catalysts were obtained on a JEM-2100F (JEOL Ltd.,Japan)apparatus with the acceleration voltage of 200 kV.

X-ray photoelectron spectra (XPS) were obtained using a Thermo spectrometer equipped with AlKα radiation source(1486.6 eV) operating at 150 W. The charge compensation of the samples was corrected by referring to the C 1s line at 284.6 eV.

In situdiffuse reflectance infrared Fourier transform spectroscopy(DRIFTS)data were collected on a Nicolet 6700 apparatus equipped with an MCT detector and the resolution of 32 scans and 4 cm-1.Prior to experiment,the sample was pretreated with H2at 350°C for 0.5 h and then cooled to 30°C in order to reduce Ptx+to Pt0.The reduced samples were contacted with CO for 1 h,followed by collecting IR spectra.

The platinum dispersion of the catalysts was assessed using a CHEMBET-3000 (Quantachrome Co.) instrument. 0.10 g catalyst was pretreated by H2heated at 400°C for 1 h,then treated by pure He(30 ml·min-1)at 400°C for 0.5 h,and at last cooled down to 30°C. CO pulse chemisorption measurements were performed at this temperature.The Pt dispersion was calculated from the amount of CO chemisorption by assuming a stoichiometric ratio of CO/Pt=1/1.

Redox property of the catalysts was measured by H2-Tempera ture-Programmed-Reduction(H2-TPR).100 mg catalyst was placed in a U-shaped quartz reactor,pretreated with 30 ml·min-1of N2at 200°C for 30 min,and then cooled to below 0°C by the mixture of liquid nitrogen and ethanol.The reduction experiment was carried out in a gas mixture of 5%(volume)H2/Ar(40 ml·min-1)along with raising temperature from 0 °C to 550 °C at a rate of 10 °C·min-1.Hydrogen consumption was followed by gas chromatograph equipped with thermal conductivity detector (TCD).

3. Results and Discussion

3.1. HRTEM characterization

Fig.1 shows TEM images of R,L-Pt/Al2O3catalysts calcinated at various temperatures.As the background of Al2O3makes it difficult to count the size of Pt particles, typical HRTEM-HAADF images of the Pt/Al2O3catalysts are also presented in Fig. 1, where the platinum particles can be distinguished as bright white objects due to the higher Z number of Pt in comparison with Al. From Fig. 1(A) and (D), it is obvious that L-Pt/Al2O3-100 catalyst prepared by L-method exhibits smaller size of Pt particles finely distributed on the Al2O3supports and the average diameter of Pt particles only is about 1.3 nm. While the size of Pt particles in R-Pt/Al2O3-100 catalyst possesses a wider size distribution ranging from 0.5 nm to 3.0 nm,and the average diameter is 1.8 nm.It indicates that supported nano-sized Pt catalysts could be synthesized by ethylene glycol reduction method, due to the glycolic acid resulting from ethylene glycol oxidation being thought as a good stabilizer for the Pt colloids by forming chelate-type complex in alkaline solutions [11,23].

Fig.1. HRTEM images of L,R-Pt/Al2O3 catalysts calcined at various temperatures and the corresponding particle size histograms of Pt.(A)and(a),L-Pt/Al2O3-100;(B)and(b),L-Pt/Al2O3-350;(C)and(c),L-Pt/Al2O3-550;(D)and(d),R-Pt/Al2O3-100;(E)and(e),R-Pt/Al2O3-350;(F)and(f),R-Pt/Al2O3-550.Inserts in A-F are annular dark field images.

EDX-mapping images (Fig. 2) of the overlapped elements (Al(green), O (blue) and Pt (red)) show us the unique distribution of Pt on Al2O3for L-Pt/Al2O3-100, while some single Pt particles(marked with white rectangle) existed in R-Pt/Al2O3-100, which may be related to the loading manner of Pt nanoparticles. H2PtCl6species are reduced after adsorbed onto the surface of Al2O3support for L-Pt/Al2O3-100, thus the stronger adsorption between PtCl62-species and Al2O3would inhibit the increase of Pt particle sizes in the process of reduction and dry. As for R-method, Pt nanoparticles could also be produced in the ethylene glycol solution, they were deposited on the carrier by adding high acidic HCl solution. Some particles might be precipitated directly and mixed in the sample; therefore, single Pt particles were detected by EDX-mapping.

Fig. 1 (continued)

L-Pt/Al2O3-100 and R-Pt/Al2O3-100 were calcined at higher temperature to check the temperature effect. It can be seen that from Fig. 1(B), (C), (E) and (F), the average diameter of Pt particles in the L-Pt/Al2O3-100 catalyst increases from 1.3 to 1.8 nm, as the calcination temperature was raised to 550°C.It increases from 1.8 to 2.0 nm for R-Pt/Al2O3-100 catalyst.No serious agglomeration of Pt particles is observed and it still has a narrower size distribution ranging from 1.0 nm to 3.0 nm.While some Pt particles with larger particle sizes(＞4.0 nm)appear in R-Pt/Al2O3-550 catalysts.It indicates that the thermal stability of noble metal catalysts greatly depends on the interaction between the metal Pt and the supports,which is affected by the order of the adsorption and reduction process during the preparation of catalysts.

3.2. In situ CO-DRIFTS and Pt dispersion characterizations

Thein-situCO adsorption DRIFTS measurement was used to determine the types of CO adsorption on Pt sites and the results are shown in Fig. 3, and the corresponding information about the sites and the intensity of peaks are summarized in the Table 1.Four kinds of vibration absorption bands of CO species adsorbed on Pt0are observed, which are marked asW1-W4. Two bands at about 2073-2057 cm-1(W1) and 1999-1990 cm-1(W3) can be ascribed to symmetry and asymmetry stretching vibration absorption of twin-adsorbed CO species. While bands at about 2039-2019 cm-1(W2) and 1824-1820 cm-1(W4) can be ascribed to linear-adsorbed (CO)n-Pt0species and bridge-adsorbed (CO)n-Pt0species [24,25], respectively. It is noteworthy that the area of W1band over L-Pt/Al2O3-100 is obviously larger than that over R-Pt/Al2O3-100, and shift to lower wavenumber. Generally, it is regarded as the index of the particle size of Pt because corner,steps and edges of the Pt particles with smaller particle size tend to be adsorbed by more than one CO molecules [26], indicating that LPt/Al2O3-100 has smaller size of Pt particles, in good agreement of the result got from HRTEM.After calcined at higher temperature,the peak (W1) of twin-adsorbed CO species becomes obviously weaker and all the peaks shift to higher wavenumber with the increase of calcination temperature, which is attributed to the weakened bond force on account of the increase of electron cloud density in anti-bonding orbital from Pt particles to CO molecule due to aggregation of Pt particles [24]. Moreover, from Table 1, it can be seen that the proportion of twin adsorbed CO (W1) with regard to the linear and bridge-adsorbed CO(W2andW4)decreases with the increase of calcination temperature, also indicating that the size of Pt particles increases. The sums of the area of all the CO adsorption bands declines in the following order: L-Pt/Al2O3-100 ＞L-Pt/Al2O3-350 ＞R-Pt/Al2O3-100 ＞L-Pt/Al2O3-550 ＞R-Pt/Al2O3-350 ＞R-Pt/Al2O3-550, demonstrating that more CO can be adsorbed on the active sites of the L-catalysts than R-catalysts,which means Pt dispersion of L-Pt/Al2O3catalysts is much higher.The results are in agreement with the dispersion estimated from CO pulse chemisorption listed below.

Fig. 2. EDX-mapping images of the overlapped elements (Al (green), O (blue) and Pt (red)) for L, R-Pt/Al2O3-100 sample respectively.

Fig. 3. In situ CO adsorption DRIFTS spectra of L, R-Pt/Al2O3 catalysts calcined at different temperatures.

Fig. 4. XPS spectra of L, R-Pt/Al2O3 catalysts calcined at different temperatures: Pt 4f and (b) O 1s.

Table 1 The position and area of CO adsorption peaks obtained by in situ CO adsorption DRIFTS

The CO uptakes estimated by pulse chemisorption method and the calibrated results using data from CO-in situDRIFTS are displayed in Table 2.A1represents the CO adsorption value according to the assumption of CO/Pt=1/1,the corresponding Pt dispersion is listed in the column ofD1. It is noticed thatD1for L-Pt/Al2O3-100 catalyst is more than 100%. It might be due to the twin-adsorbed CO and bridge-adsorbed species as stated above. Therefore, CO/Pt = 2/1 and CO/Pt = 1/2 are also considered to recalculate Pt dispersion (D2) based on the distribution percent of three kinds of adsorption species in Table 1. The corresponding calibrated data are listed in columnA2,D2andd2.From Table 2,it can be seen that L-Pt/Al2O3catalysts calcined at various temperatures all exhibit larger Pt dispersion and CO chemisorptions measured by the two kinds of methods aforementioned than L-Pt/Al2O3catalysts, and the size of Pt particles in the L-Pt/Al2O3catalysts is obviously smaller than the latter. Moreover, the changes of amount of CO chemisorption, Pt dispersion and average size of Pt particles are the same trends with increasing calcination temperature. The results of Pt particle size and Pt dispersion are consistent with that derived from TEM observation (d3) andin situDRIFTS (d2).

Table 2 The dispersion and averaged size of Pt particles in the L, R-Pt/Al2O3 catalysts.

3.3. XPS characterization

The Pt 4f and O 1s spectra of R, L-Pt/Al2O3catalysts calcined at different temperatures were measured by XPS and shown in Fig.4(a) and (b), respectively. Pt 4f band could be further divided into two valance states with four binding energies at 71.2-71.6 eV,74.5-74.8 eV, 73.2-73.6 eV and 76.4-76.8 eV. The former two bands could be assigned to metallic Pt (Pt0) and the latter two is typical for Pt2+species [10,27]. It is obvious that L-Pt/Al2O3-100 catalyst owns more Pt2+species than R-Pt/Al2O3-100 and the ratio of Pt0/Pt2+is 0.48 and 0.7,respectively.It may be related to smaller Pt particles for L-Pt/Al2O3-100 catalyst, which is more easily oxidized in the air. Compared with L-Pt/Al2O3-100 catalysts, the content of Pt0species in the catalysts calcined at 350°C(0.45 for L-Pt/Al2O3-350 and 0.65 for R-Pt/Al2O3-350) decrease slightly, possibly due to the oxidation of some Pt0species in the heating treatment procedure. But the content of Pt0species in the catalysts calcined at 550 °C dramatically increases and, the ratio of Pt0/Pt2+is 0.63 in L-Pt/Al2O3-550 and 0.71 in R-Pt/Al2O3-550, respectively, which could be attributed to the decomposition of PtO at relatively high calcination temperature. The values of Pt0/Pt2+follow the order:R-Pt/Al2O3-550 (0.71) ＞ R-Pt/Al2O3-100 (0.7) ＞ R-Pt/Al2O3-350(0.65) ＞ L-Pt/Al2O3-550 (0.63) ＞ L-Pt/Al2O3-100 (0.48) ＞ L-Pt/Al2O3-350 (0.45). Moreover, the binding energy for Pt7/2of catalysts with smaller Pt particle size is higher than that of catalysts with bigger Pt size, 71.5-71.6 eV for L-Pt/Al2O3catalysts and 71.2-71.4 eV for R-Pt/Al2O3catalysts, respectively. This indicates that the existence of strengthened interaction between noble metal and supports is in favor of electronic transportation from Pt to supports, leading to the formation of more Pt2+species. As for O 1s spectra (Fig. 4(b)), the broad peak could be deconvoluted to two peaks at 531.10-531.34 eV and 532.6-533.6 eV, which could be assigned to lattice oxygen species (Olatt) and absorbed oxygen species (Oads, O2-, O-and O22-) respectively. The Oads/Olattratios decrease as the sequence of R-Pt/Al2O3-550 (0.12) ＜R-Pt/Al2O3-350 (0.16) ＜ L-Pt/Al2O3-550 (0.18) ＜ R-Pt/Al2O3-100(0.20) ＜L-Pt/Al2O3-350 (0.23) ＜L-Pt/Al2O3-100 (0.29), implying that Pt/Al2O3catalysts with smaller Pt particle size have more absorbed oxygen species. Moreover, the binding energy of Oadsfor L-Pt/Al2O3catalysts (532.6-532.9 eV) is obviously lower than that for R-Pt/Al2O3catalysts (532.7-533.6 eV), indicating that the stronger interaction between Pt species and Al2O3support also facilitates the electron to transfer from Pt to Oads,making Oadsspecies more active.For VOCs oxidation,it is generally considered that adsorbed oxygen plays an important role and the formation of more adsorbed oxygen species on the surface of catalysts is favorable for accelerating the catalytic activity.

3.4. H2-TPR characterization

Redox properties of the catalysts were measured by H2-TPR technique and showed in Fig. 5. As shown in Fig. 5, two reduction peaks are observed over the Pt/Al2O3catalysts at the temperature range of 0-550 °C. The weak peak at ～45 °C is assigned to the reduction of PtOxspecies highly dispersed on the surface of Al2O3, making it more easily reduced at lower temperature, and the stronger peak at 425-465 °C can be ascribed to the reduction of oxygen species (O2-, O-and O22-) adsorbed on the nano-sized Pt particles [8,10]. The reduction peaks of the absorbed oxygen species over L-Pt/Al2O3catalysts in intensity are stronger than those of R-Pt/Al2O3catalysts, indicating that the former forms more absorbed oxygen species due to smaller Pt particle size,which is in accordance with the result of XPS above. Moreover,the absorbed oxygen species obviously decreases with increasing calcination temperature, however, it is also worth noting that the reduction peak is shifted to a higher temperature.The temperature of reduction peak decreases as follows: L-Pt/Al2O3-100 (425°C) ＜L-Pt/Al2O3-350 (430 °C) ＜R-Pt/Al2O3-100 (444 °C) ＜L-Pt/Al2O3-550 (447 °C) ＜R-Pt/Al2O3-350 (456 °C) ＜R-Pt/Al2O3-550(465 °C). The results suggest that the oxygen adsorbed species on the surface of Pt/Al2O3catalysts with smaller Pt particle size are more easily reduced, in other words, it is more reactive, which has been confirmed by XPS characterization above.

Fig.5. H2-TPR profiles of L,R-Pt/Al2O3 catalysts calcined at different temperatures.

3.5. Catalytic activity of catalysts

Fig. 6 depicts the catalytic activities of benzene, ethyl acetate andn-hexane oxidation over R or L-Pt/Al2O3catalysts calcined at different temperatures, and no signals of any byproducts are detected over all the samples.

It can be seen from Fig.6(A)and(B)that L-Pt/Al2O3-100 catalyst exhibits higher catalytic activities for benzene and ethyl acetate oxidation and the complete conversion temperatures (T99.9%) are only 165 °C for benzene and 255 °C for ethyl acetate, respectively,which is lower than that of R-Pt/Al2O3-100 catalyst (180 °C and 275 °C). But forn-hexane oxidation, the catalytic activity of L-Pt/Al2O3-100 (T99%= 252 °C, Fig. 6(C)) is obviously lower than R-Pt/Al2O3-100 (T99%= 220 °C, Fig. 6(D)).

Fig. 6. Catalytic activities of benzene (A), ethyl acetate (B) and n-hexane (C, D) oxidation over L, R-Pt/Al2O3 catalysts calcined at different temperatures.

Moreover, with the increase of the calcination temperature,T99.9%of benzene over L-Pt/Al2O3and R-Pt/Al2O3catalysts obviously increases and follow the sequence of R-Pt/Al2O3-550 (200°C) ＞R-Pt/Al2O3-350 (195 °C) ＞L-Pt/Al2O3-550 (185 °C) ＞R-Pt/Al2O3-100 (180 °C) ＞L-Pt/Al2O3-350 (175 °C) ＞L-Pt/Al2O3-100(165 °C). And for ethyl acetate oxidation, the activities (T99.9%)order for L, R-Pt/Al2O3catalysts is as follows: L-Pt/Al2O3-100 (255°C) ≈L-Pt/Al2O3-350 (259 °C) ＞L-Pt/Al2O3-550 (268 °C) ＞R-Pt/Al2O3-100 (275 °C) ≈R-Pt/Al2O3-350 (274 °C) ＞R-Pt/Al2O3-550(282°C).Thus based on the catalyst characterization results above,it indicates that L-Pt/Al2O3catalysts with smaller Pt particle size exhibits improved activity for benzene and ethyl acetate oxidation compared to R-Pt/Al2O3catalysts with higher proportion of Pt0/Pt2+,declaring that Pt particle size is the decisive factor of benzene and ethyl acetate oxidation over Pt/Al2O3catalyst with highly dispersed Pt particles. Pt/Al2O3catalysts with smaller Pt particle size own more PtOxspecies with high oxidation state and more active oxygen species, which is in favor of the improved activity for benzene and ethyl acetate oxidation.Generally,the oxidized Pt species are regarded as active sites for the oxidation of ethyl acetate, and thus the content of Pt2+species may have more influence than the size of Pt particles on the catalytic activity of ethyl acetate oxidation [21].

However, it is interesting that a different phenomenon with benzene and ethyl acetate oxidation can be observed for the oxidation ofn-hexane.From Fig.6(C)and(D),it can be seen that the catalytic activities of the Pt/Al2O3catalysts for the oxidation ofnhexane, except for R-Pt/Al2O3-350 and R-Pt/Al2O3-750 catalyst,are obviously promoted at low-temperature. In comparison withT50%values of the catalysts (the reaction temperature at which then-hexane conversion is 50%), the catalytic activity follows the sequence of L-Pt/Al2O3-750 (165 °C) ≈ R-Pt/Al2O3-550 (164°C) ＞R-Pt/Al2O3-100 (170 °C) ＞R-Pt/Al2O3-350 (174 °C) ≈R-Pt/Al2O3-750 (174 °C) ≈L-Pt/Al2O3-550 (176 °C) ≈L-Pt/Al2O3-350(178°C)＞L-Pt/Al2O3-100(184°C),which is on the whole in accordance with the orders of their Pt particle size except R-Pt/Al2O3-750 and the ratio values of Pt0/Pt2+in the catalysts.HAADF images of L, R-Pt/Al2O3-750 samples (Fig. 7) show that the Pt particle size of R-Pt/Al2O3-750(～20 nm)is much larger than that of L-Pt/Al2O3-750 (～10 nm). This indicates that Pt particles prepared by Rmethod would be aggregated much more serious than that by Lmethod at the same calcination temperature.The Pt particles supported on Al2O3with ～10 nm might be the best size to get the best catalytic activity forn-hexane oxidation.

Fig. 7. HAADF images of L, R-Pt/Al2O3-750 samples.

The different Pt size effects for different reactants on the same catalyst may be clarifiedviathe comparison in the different adsorption fashion and reactivities of reactants on Pt particles.Benzene molecular is easily adsorbed on Pt in a structure parallel to the surface,namely that associated with the σ donation of electron from the carbon sp2orbitals into anti-bonding Pt orbitals(with some π-backing bonding as well). Ethyl acetate with double bond(C＝O)can also form the similar bond between the molecular and Pt as that of benzene.Therefore,there is no significant selectivity for benzene and ethyl acetate whether adsorbing on lager Pt particles or on smaller Pt particles,and the catalytic performances of Pt/Al2O3catalysts for oxidation of benzene and ethyl acetate are prominently determined by the surface catalytic active sites on the catalyst surface and adsorbed oxygen species.However,long-chain hydrocarbon tends to develop three Pt-C σ bonds through a sp3-hybridized carbon [28], therefore, it is much more difficult fornhexane adsorbing on the Pt compared with benzene and ethyl acetate. While lager Pt particles with suitable size own more contact area with reactants, which is beneficial to the adsorption ofnhexane,and thus promoting the catalytic activity ofn-hexane combustion, but the catalytic activity would decrease as the serious aggregation of Pt makes the number of activity centers decreased.

4. Conclusions

A series of L, R-Pt/Al2O3catalysts were prepared by ethylene glycol reduction method and calcined at different temperatures to assess particle size effects of noble metal and their thermal stability.The catalytic activities of these catalysts for the oxidation of benzene, ethyl acetate andn-hexane were evaluated. The results show that the chemical state and size of Pt nanoparticles as well as the amount and redox of adsorbed oxygen species in Pt/Al2O3catalyst play different important role in various VOCs. L-Pt/Al2O3catalysts with the stronger interaction between Pt species and Al2O3exhibit a narrower size distribution, smaller size of Pt nanoparticles and favorable thermal stability compared with RPt/Al2O3catalysts prepared by first reduction and then loading procedure.L-Pt/Al2O3is favor of the formation of more adsorbed oxygen species and more PtOxspecies with high oxidation state,resulting in high catalytic activity for benzene and ethyl acetate oxidation. Moreover, Pt2+species play more important role than the size of Pt particles on the catalytic activity of ethyl acetate oxidation.Pt/Al2O3catalysts possess higher proportion of Pt0/Pt2+and bigger size of Pt particles due to that Pt0species are main active site ofn-hexane oxidation, resulting in promoting the catalytic activity forn-hexane oxidation. The optimal Pt particle size supported on Al2O3forn-hexane oxidation is about 10 nm.

Acknowledgements

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The financial support from the National Key Research and Development Program of China (2016YFC0204300) is gratefully acknowledged.