ARTICLE pubs.acs.org/IECR
Low-Temperature Performance of Pt/TiO2 for Selective Catalytic Reduction of Low Concentration NO by C3H6 Xinyong Liu, Zhi Jiang, Minxia Chen, Jianwei Shi, Zhixiang Zhang, and Wenfeng Shangguan* Research Center for Combustion and Environmental Technology, Shanghai Jiao Tong University, Shanghai, 200240, People's Republic of China ABSTRACT: Pt/TiO2 catalysts prepared by solgel and wetness impregnation methods were investigated for the selective catalytic reduction (SCR) of NO by C3H6 in the presence of excess oxygen. The characteristics of the prepared catalysts were analyzed through XRD, BET surface area, NO temperature programmed desorption (TPD), and NO/C3H6 temperature programmed oxidation (TPO). The effects of the crystalline phase of TiO2, O2/C3H6 concentration, and Pt loading amount have been studied. It was found that when bare support was impregnated with the noble metal Pt, the NO removal efficiency increased significantly. In particular, under the condition of 150 ppm NO, 150 ppm C3H6, and 18 vol % O2, balanced with Ar, 0.5 wt % Pt loaded TiO2 calcined at 500 °C could achieve a complete C3H6 conversion and 47.03% NOx reduction simultaneously at 180 °C. This enhanced activity may be associated with its outstanding activities in the TPO processes of NO to NO2 and C3H6 to CO2. The pure anatase crystal form together with its relatively high specific surface area for TiO2 sample calcined at 500 °C was suggested to be responsible for such an enhancement. The research results also suggested that higher concentration of O2 and higher concentration of C3H6 favored NO removal, and Pt loading played an important role in the SCR process.
1. INTRODUCTION NOx (NO þ NO2) exhausted from vehicles and stationary combustion engines is one of the important causes of photochemical smog, acid rain, and ozone depletion, which possess serious challenges to human health and environmental protection. Selective catalytic reduction (SCR), which is recognized as a promising method of deNOx, has received much attention due to its simplicity and high efficiency.16 Usually, SCR technology needs high temperature (more than 300 °C), but in places such as road tunnels and underground parking lots, under cold start conditions, and so on, the temperature is quite low (always below 200 °C) and NOx concentration is in a low range as well. Therefore, it is necessary to develop novel catalysts which can work effectively for low concentration pollutants at low temperature. It is well-known that NH3 is a quite effective reducing agent for SCR of NOx (NH3-SCR),48 but problems such as storage, transportation/leakage, and corrosion of NH3 are inevitable. Also, hydrocarbons (HCs) are a natural component in combustion exhaust, so it is quite convenient and competitive to use HCs as reducing agent (HC-SCR). Previous studies showed that C3H6 is a well-used reducing agent for SCR of NO,914 but the operation temperature is always in the high range. Rodríguez and co-workers9 used Pd-integrated perovskite as a C3H6-SCR catalyst and obtained a maximum 35% NO removal efficiency at 250 °C. Recently, Nguyen et al.15 developed an Au/TiO2 catalyst which could exhibit less than 40% NOx reduction to N2 at 325 °C. Moreover, TiO2 is a well-known photocatalyst and is also a good catalyst support for SCR reaction.1518 Our previous investigation19 revealed that commercial available Degussa P25 (TiO2) supported Pt was a quite effective deNOx catalyst compared with other noble metals (Pd, Rh, and Ru). In the r 2011 American Chemical Society
present work, we prepared and compared a series of Pt/TiO2 samples as C3H6-SCR catalysts to remove low concentration NO in a low temperature range between 100 and 340 °C. Special emphasis is given to the possible correlations among the properties of TiO2 support, O2 concentration, C3H6 concentration, Pt loading amount, and NO conversion efficiency of the catalysts.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The TiO2 samples were synthesized by the solgel method. Butyl titanate (10 mL), anhydrous ethanol (60 mL), and 3638% chlorhydric acid (1 mL) were mixed under vigorous stirring at room temperature to yield a yellowish transparent sol. The sol was dried at 80 °C for 24 h to form xerogel, followed by calcination in air at different temperatures for 2 h. All prepared TiO2 samples were denoted as Tx, where x denoted the calcination temperature (°C). Thermal analysis of the TiO2 gel powder was performed in a TGA 2050 thermogravimetric analyzer (TA Instruments, USA) in an air atmosphere by a temperature rise of 20 °C/min. All Pt/TiO2 catalysts were prepared by impregnating weighted TiO2 supports with calculated amounts of Pt(NH3)2(NO2)2 aqueous solution, followed by 30 min of ultrasonic dispersion. Then all catalysts were dried at 110 °C for 12 h followed by calcination in air at 450 °C for 4 h. All the above catalysts were ground and sieved to 4060 mesh for evaluation. The 0.5 wt % Pt loaded catalysts were denoted as PTx, where x represented the calcination temperature (°C). Received: November 26, 2010 Accepted: May 6, 2011 Revised: April 5, 2011 Published: May 06, 2011 7866
dx.doi.org/10.1021/ie1023854 | Ind. Eng. Chem. Res. 2011, 50, 7866–7873
Industrial & Engineering Chemistry Research
ARTICLE
Figure 1. Schematic diagram of experimental apparatus.
2.2. Catalyst Characterization. The powder X-ray diffraction (XRD) measurements were carried out with a Rigaku D/max2200/PC X-ray diffractometer with Cu KR radiation. Measurements of the BET surface area were performed by nitrogen adsorption data from Quantachrome NOVA1000a Sorptomatic apparatus. Pt dispersion in the catalysts was determined by CO chemisorption (Micromeritics Instrument Corp., AutoChem II 2920). Transmission electron microscopic (TEM) images were obtained by employing a JEM-2010 (JEOL) device with a 200 kV accelerating voltage. 2.3. Catalytic Activity Test. Catalytic activity of the catalysts for NO reduction was determined in a continuous fixed bed U-type quartz reactor (4 mm i.d.) using 0.2 g of catalyst of 4060 mesh (Figure 1). The standard reactant composition was as follows: 150 ppm C3H6, 150 ppm NO, 18 vol % O2, and Ar balance at a total flow rate of 120 cm3/min (gas hourly space velocity (GHSV) = 72 000 h1). The concentrations of NO, NO2, and NOx were monitored by a chemiluminescent NO/ NO2 analyzer (Thermo Environmental Instruments Inc., 42i LS). C3H6 and CO2 were determined by a gas chromatograph (GC) equipped with a flame ionization detector (FID), and a quadrupole mass spectrometer (Ametek Process Instruments, Dycor DM 100M) was also used combined with a GC. The data were collected when the reactions reached the steady state around 40 min at each temperature point. A bypass line was installed to measure the inlet NO concentration. Unless otherwise specified, the gas flow rates in this paper were all fixed at 120 cm3/min. The NOx removal efficiency was calculated as follows:
NOx removal efficiency ¼
½NOx in ½NOx out 100% ½NOx in
where [NOx]in is the inlet concentration of NOx and [NOx]out is the outlet concentration of NOx. 2.4. NO Temperature Programmed Desorption (NO-TPD). Before the TPD experiment, the samples were pretreated in pure Ar flow at 350 °C for 30 min and then cooled to 50 °C. Adsorption of NO was carried out over 0.2 g of catalyst by passing a flow of 150 ppm NO and 18 vol % O2 balanced with Ar through the sample bed at 50 °C for 1 h. After the sample was purged with Ar for 20 min at 50 °C, the TPD experiment was performed with a heating rate of 4 °C/min. 2.5. NO þ O2 and C3H6 þ O2 Temperature Programmed Oxidation (NO/C3H6-TPO). NO/C3H6-TPO was also carried out with a heating rate of 4 °C/min. For NO/C3H6-TPO, the feed gas mixture was 150 ppm NO/150 ppm C3H6 and 18 vol % O2 balanced with Ar; 0.2 g of catalyst was used as well.
Figure 2. Thermogravimetric (TG) curves of dried TiO2 gel in air atmosphere.
3. RESULTS AND DISCUSSION 3.1. Thermal Analysis, BET, and XRD Results. The results of thermal analysis on TiO2 gel powder are shown in Figure 2. The weight loss process which includes dehydration, decomposition of organics, and the transition of amorphous TiO2 to anatase TiO2 ends before heating in 500 °C; with further increase in temperature the weight remained constant. The BET surface areas and platinum dispersions of the prepared catalysts are shown in Table 1. Noble metal Pt impregnation brought a slight decrease of BET surface area compared to bare support, which is a normal phenomenon after the impregnation process due to the coverage of the surface by low surface area clusters of active component. The results in Table 1 also show that, with the increase of calcination temperature, the catalyst BET surface areas decreased considerably. It should be noted that, corresponding to the decrease of BET surface areas, the dispersion of Pt decreased as well, indicating that higher surface area of TiO2 support was favorable for the dispersion of Pt. The XRD patterns of all the catalysts are presented in Figure 3. No diffraction peak corresponding to the noble metal Pt or the oxide was observed, indicating that Pt was highly dispersed over the support or that the formed particles were too small to be detected by XRD. In addition, it can be clearly seen from Figure 3 that the crystalline phase of TiO2 has a close relationship with the calcination temperature. Increasing the calcination temperature caused the transformation of the crystalline phase from anatase to rutile, which also corresponded to the decrease of BET surface areas. PT450, PT500, and PT550 were 100% anatase phase, while PT800 with single rutile phase could be obtained. For PT650, the anatase phase was the main phase, with only a little rutile phase; by contrast, rutile was the main phase of PT750, with only a little anatase phase. 3.2. Catalytic Activity Tests. The NOx conversion curves as a function of reaction temperature using as-made catalysts in the C3H6-SCR process are presented in Figure 4. T500 showed the lowest activity, giving only 16.54% NOx reduction at 240 °C (Figure 4a), whereas the activity increased markedly after the impregnation of the noble metal Pt. It is clear that PT500 performed the most efficiently among all PT samples and could achieve 47.03% NOx reduction (Figure 4a) and 100% C3H6 7867
dx.doi.org/10.1021/ie1023854 |Ind. Eng. Chem. Res. 2011, 50, 7866–7873
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Physicochemical Properties of Prepared Catalysts BET surface
amt of NO desorbed
amt of NO2 desorbed
amt of NOx desorbed
during NO-TPD (μmol/g)
during NO-TPD (μmol/g)
during NO-TPD (μmol/g)
9.43
3.59
13.02
32.52
34.91
67.43
33.75
49.58
83.33
71.5
20.83
32.04
52.87
31.7
6.15
1.58
7.73
5.32
29.5
1.95
0.29
2.24
3.99
15.5
1.81
0.15
1.96
sample
area (m2/g)
Pt dispersion (%)
T500
67.31
PT450
71.02
PT500
62.54
79.8
PT550
40.77
PT650
11.21
PT750 PT800
Figure 3. XRD patterns of synthesized catalysts.
conversion (Figure 4c) simultaneously at 180 °C. Moreover, it also gave the widest temperature range for the NOx removal. By contrast, the maximum NOx conversion efficiency of PT450, PT550, PT650, PT750, and PT800 were lower than that of PT500. PT550 and PT650 reached maximum NOx conversion efficiencies of 41.7 and 34.2% at 180 °C, whereas PT450, PT750, and PT800 showed 40.33, 19.7, and 18.6% maximum NOx conversion at 200 °C. It is of interest to note that the decreasing order of the SCR activity for all these samples was in accordance with the order of the surface area except for PT450. TG analysis and XRD results showed that the main crystalline phase of PT450 was anatase; however, it still contained amorphous intermediate and undecomposed organics, which may in part explain its lower catalytic activity compared with PT500. The results also show that maximum NOx conversion efficiency of these samples increased with the increase of Pt dispersion. The high dispersion degree of PT500 can also be confirmed by TEM images (Figure 5), on which Pt particles are uniformly distributed with a size smaller than 2 nm. Moreover, with the increase of reaction temperature from 120 to 340 °C, all samples exhibited “volcano-shape” behavior in NOx removal as shown in Figure 4a, namely, the reduction efficiency increased until the maximum was reached and then decreased. The temperature dependence of NO conversion to NO2 in C3H6-SCR is shown in Figure 4b.The conversion of NO to NO2 in SCR of PT500 was much higher than that of the others especially in the higher temperature range. All catalysts except T500 oxidized C3H6 completely before 220 °C (Figure 4c).
3.3. NO Temperature-Programmed Desorption (NO-TPD). The amounts of NOx desorbed in the NO-TPD experiments are summarized in Table 1.The total amounts of NOx desorbed from the PT500 and PT550 catalysts (83.33 and 52.87 μmol/g, respectively) are far larger than that from T500 (13.02 μmol/g). This suggests that Pt loading can enhance the adsorption of NOx over catalysts although the Pt loading brought a little decrease of the BET surface area by impregnation. Owing to the small BET surface area, PT800 has limited capacity of NOx adsorption as presented in Table 1. Figure 6 illustrates the TPD profiles of NO and NO2 over T500, PT500, and PT550. Obviously, T500 shows three main desorption peaks (100, 140, 260 °C) on the NO curve (Figure 6a), while PT500 exhibits four desorption peaks (94, 126, 178, 322 °C) (Figure 6b). Additionally, PT550 displays similar similar to those of PT500 (Figure 6c). However, the amount of NO2 desorbed from PT500 and PT550 apparently exceeds that of NO (Figure 6 b,c, Table 1). NOx desorbed from the catalysts in the low temperature range can be attributed to the physical adsorption on the surface of catalysts,while it could be assigned to the decomposition of nitrate species formed on the catalyst surface at higher temperature.2022 3.4. NO þ O2 and C3H6 þ O2 Temperature Programmed Oxidation. NO-TPO activities of all prepared catalysts were investigated and compared. As shown in Figure 7, the oxidative activities of NO to NO2 on catalysts increased with the temperature increasing approximately. T500 had very poor oxidizing ability in the whole temperature range, while PT650 showed the best oxidation activity in the low temperature range (
