- Email: [email protected]
TiO2 catalysts for NO reduction by CO in an oxygen-rich condition
Applied Surface Science 230 (2004) 94–105
Studies of sol–gel TiO2 and Pt/TiO2 catalysts for NO reduction by CO in an oxygen-rich condition J.A. Wanga, A. Cuanb, J. Salmonesc,*, N. Navab, S. Castillob, M. Mora´n-Pinedab, F. Rojasd a
Laboratorio de Cata´lisis y Materiales, ESIQIE, Instituto Polite´cnico Nacional, Col. Zacatenco, C.P. 07738, Me´xico, D.F., Mexico Programa de Ingenierı´a Molecular, Instituto Mexicano del Petro´leo, Eje La´zaro Ca´rdenas 152, C.P. 07730, Me´xico, D.F., Mexico c Programa de Tratamiento de Crudo Maya, Instituto Mexicano del Petro´leo, Eje La´zaro Ca´rdenas 152, C.P. 07730, Me´xico, D.F., Mexico d Departamento de Quı´mica, Universidad Auto´noma Metropolitana-Iztapalapa, P.O. Box 55-534, C.P. 09340, Me´xico, D.F., Mexico b
Received 6 November 2003; received in revised form 30 January 2004; accepted 3 February 2004 Available online 27 April 2004
Abstract The textural properties, morphological features, surface basicity and oxygen reduction behaviours of titania and Pt supported titania catalysts synthesized via a sol–gel method were studied by means of N2 physisorption, SEM, TEM, CO2-TPD and H2TPR techniques. Mesostructured TiO2 shows a very narrow pore size distribution that uniformly centred at about 4 nm. High resolution TEM images confirmed that most of Pt particles on Pt/TiO2-SG had a size smaller than 2 nm. Both the titania support and Pt loaded catalysts chiefly contained weak basic sites with small amount of strong basic sites. Loading Pt did not significantly alter the surface reduction characters of titania, indicating a weak interaction between Pt metals and titania support. Catalytic evaluation revealed that the selectivity of NO reduction over titania was insensitive to variation of textural property. On the bare titania, low NO conversion but high selectivity to N2O was obtained. However, the Pt/TiO2-SG catalysts exhibited high NO conversion and high selectivity to N2, which is assumed to relate to NO dissociation catalysed by the metallic Pt clusters. In addition, when the reaction temperature was above 200 8C, 3–11% NO2 was yielded over the Pt/TiO2-SG catalysts, which was discussed on a basis of reaction competition, metal-support interaction and NO dissociation. # 2004 Elsevier B.V. All rights reserved. Keywords: Titania; NO reduction; Sol–gel synthesis; Pt/TiO2; Catalysts
1. Introduction The emissions of NOx (N2O, NO and NO2) from both mobile and fixed stationary sources remain a problem considerable interest because of the contribution of these pollutants to rain acidification, global warming and depletion of the stratospheric ozone * Corresponding author. Tel.: þ52-5-9175-8444. E-mail address: [email protected] (J. Salmones).
layer [1,2]. In most cases, NOx emissions together with other contaminates and oxygen are discharged to the air, for example, in the automobile exhausts, NOx, CO, hydrocarbons and O2 coexist [3–5]. Particularly, nowadays, more and more internal combustion engines are designed to run in a fuel-lean conditions that lead to the activity of current three-way catalysts (TWCs) decreasing due to remarkable deterioration of NOx reduction efficient in the presence of rich oxygen. In the other practical applications like selective catalytic
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.02.057
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
reduction (SCR), ammonia is usually used as reducing agent for NOx reduction, however, ammonia usually causes the secondary problems such as corrosion of equipment. Therefore, NOx reduction by other reducing agent such as CO or hydrocarbons in the presence of excess oxygen has become an increasingly interesting subject attracting a lot of attention of researchers [4–9]. Platinum, palladium and rhodium are the key components for the current TWCs catalysts since these catalysts always show high catalytic activity and selectivity. Catalyst support, of course, is also very important in the catalyst design due to several wellknown functions such as providing high thermal stability and high active metal dispersion. Alumina, ZSM-5 and ceria-doped alumina are the common catalyst supports in the environmental catalysis [10–12]. Titanium oxide (TiO2) has also been widely employed for NO emission reduction since the first TiO2-based catalyst was applied commercially in air pollution control in 1970s [13–18]. The various forms of titania obtained from different preparation methods usually exhibit different surface structure arrangements thus different surface reactivity [19]. By using a sol–gel method with various hydrolysis catalysts, Bokhimi et al. prepared nanosized titania in those cationic defects are formed in the crystalline structures [20]. When Au, Pt or Rh metals were introduced on the sol–gel titania or second metal oxide doped titania, it exhibits high activity for NO reduction or CO oxidation [21–24]. Usually, a weak or strong interaction between the supported noble metals and titania may occur [25,26], depending on reduction temperature and preparation methods, that strongly affects not only the surface reduction properties and acid-basic character, but the catalytic activity also. Although titania was intensively studied including its synthesis and various applications in the past years, however, effects of surface structures and textural properties as well as metal–titania interaction on catalytic activity and selectivity are still interesting research topics. In this work, by mains of a sol–gel method, titanium oxide (TiO2-SG) showing very narrow and uniformed pore size distribution was prepared. The Pt/TiO2-SG catalysts with different Pt concentrations used for NO reduction in an
95
oxygen-rich condition were obtained by impregnation Pt precursor on TiO2. The textural properties and surface basicity as well as surface oxygen reduction behaviour of TiO2 and Pt/TiO2-SG catalysts were characterised by N2-physisoption, CO2-TPD and H2-TPR techniques. Furthermore, activity and selectivity of NO reduction by CO over the TiO2 and Pt/TiO2 catalysts were measured in oxygen-rich condition and the possible reaction mechanism was discussed.
2. Experimental 2.1. Preparation of TiO2-SG TiO2-SG was prepared by sol–gel method: 7.1 g of Ti(O-Pri)4 was added into a 30 g solvent of isopropanol at 28 8C with a continuous stirring for 20 min. Afterwards, 36.1 ml water and 0.75 ml concentric nitric acid were slowly dropped into the above mixture to remain the pH value at 5. A white gel was formed after 24 min of gelation. The extra solvent was evaporated and then the precipitated solid was dried at 90 8C for 24 h. Finally, it was first calcined at 200 8C for 2 h and then at 500 8C for 2 h. 2.2. Preparation of Pt/TiO2-SG catalysts For obtaining the 1 wt.% Pt/TiO2-SG catalyst, dinitro tetra-amino platinum(II) was used as platinum precursor and the above calcined titania powder used as support. The support was impregnated with a solution of dinitro tetra-amino platinum(II) to obtain 1 wt.% Pt on the support, then it was calcined at 500 8C in air. In the case of 3 wt.% Pt/TiO2-SG, a method of consecutive impregnation Pt was used: the reduced 1 wt.% Pt/TiO2-SG sample was impregnated again with a solution of dinitro tetra-amino platinum(II) to obtain a new sample with 2 wt.% Pt. This new sample was calcined at 500 8C and then reduced at 300 8C by H2 for 30 min. The impregnation procedure was repeated, finally the 3 wt.% Pt/TiO2-SG catalyst was obtained. The platinum on both Pt/TiO2 catalysts are in oxidized states. Part of them were reduced by hydrogen at 300 8C for 1 h in order to compare variation of the metal particle size before and after reduction.
96
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
2.3. Textural analysis The surface area and pore size distribution of the samples were measured in a Digisorb 2600 equipment by analysing N2 physisorption isotherms. 2.4. Temperature programmed desorption of CO2 (TPD-CO2) To measure basicity of the TiO2 and Pt containing catalysts, TPD-CO2 experiments were carried out in a Zeton Altamire Model AMI-3 equipment. The catalysts were thermal treated in a flow of He (30 ml/min) at 500 8C for 1 h, and then the temperature cooled down to room temperature in a He atmosphere. The CO2 was allowed to the sample cell for 30 min of adsorption. Afterwards, the temperature rose to 700 8C in a rate of 5 8C/min in a flow of He. The desorption signal was recorded by a thermal conductivity detector (TCD). 2.5. Temperature programmed reduction of hydrogen (H2-TPR) The titania support and PtO/TiO2-SG samples (without reduction) were first pertreated in a flow of He (30 ml/min) at 550 8C for 1 h in order to remove the adsorbed water and other species, and then they were cooled down to room temperature in He atmosphere. After the thermal treatment, the catalysts were reduced in a mixture of 10% H2 in He at a rate of 10 8C increasing temperature programmed up to 750 8C. Catalysts weight used for TPR measurements was about 0.1 g. The TPR experiments were carried out on a Zeton Altamire Model AMI-3 equipment. 2.6. Surface feature images Transmission and scanning electron microscopy (TEM and SEM) were used to image the structure and growth characteristics of platinum on TiO2 supports. The TEM and SEM observations were carried out on a Jeol Model 100 CX-11 and JEOL 35CF, respectively.
and 3 ml volume). In all the cases, 50 mg of catalysts (80–120 mesh) was used. Before the reaction, the Pt/TiO2 catalysts were activated by treatment with 99.9% H2. The temperature of was raised to 300 8C with a heating rate 2 8C/min and the samples were reduced for 1 h at 300 8C. The reaction mixture containing 1.5 vol.% CO, 0.5 vol.% NO and 1.5 vol.% O2. Helium was used as carrier gas and the inlet flow rate was 30 ml/min. The products were analysed by FTIR (Nicolet-8220 Gas Analyser) in coupled with gas chromatography (HP-5890 Chemical Station).
3. Results and discussion 3.1. Textural properties of TiO2 and Pt/TiO2-SG Fig. 1a–c shows the N2 adsorption–desorption isotherms of the sol–gel TiO2-SG support and 1% Pt/TiO2SG and 3% Pt/TiO2-SG catalysts. For comparison, the nitrogen isotherm of a reference sample TiO2-D (Degussa) is also shown in Fig. 1d. The hysteresis loops of the sol–gel samples are characteristics of the typical mesopore structures in type IV, which are very different from the one of the reference. The reference sample TiO2-D exhibits a hysteresis loop of type II with almost vertical and nearly parallel adsorption and desorption branches. The limiting adsorption at p=p0 ¼ 1 in Fig. 1d attains an extremely large value of 140 cc/g. The pore size distributions of the above samples are shown in Fig. 2a–d and the pore volumes and average pore diameters are reported in Table 1. It is clearly observed that the titania sample prepared with sol–gel method has a uniform pore size distribution in a very narrow range with a maximum of about 4.2 nm, which significantly differs from the commercial TiO2-D (Fig. 2d), which shows a very wide distribution ranging from 1 to 100 nm. Besides, the platinum loaded samples exhibit a similar pore size distribution as that of the bare TiO2-SG support, however, Pt metals loading leads to a slight decrease of both surface area and pore diameter (Table 1).
2.7. Catalytic test 3.2. Morphology of TiO2 and Pt/TiO2-SG Catalytic activity in the NO reduction by CO was tested from 100 to 500 8C under atmospheric pressure in a quartz reactor of fixed bed (9 mm diameter
It is observed by SEM that either TiO2-SG or TiO2-D sample contains both the primary and secondary
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
97
Fig. 1. Profiles of N2 isotherms adsorption–desorption: (a) TiO2-SG; (b) 1% Pt/TiO2-SG; (c) 3% Pt/TiO2-SG and (d) TiO2-D.
particles. The elementary particles are roughly spherical in shape with a size in 0.1–0.3 mm. These fine grains are coalesced to form secondary particles with a size of about 2 mm (Fig. 3a–c). To observe the Pt particles located on the surface of the titania support, high resolution TEM was applied (Fig. 4a and b). The very fine particles that are absent Table 1 Textural properties of TiO2-SG, TiO2-D and Pt/TiO2-SG catalysts Samples
Specific area (m2/g)
Pore volume (cc3/g)
Mean pore ˚) diameter (A
TiO2-D TiO2-SG 1% Pt/TiO2-SG 3% Pt/TiO2-SG
53.8 90.0 67.0 46.4
0.1537 0.1435 0.0783 0.0476
500.3 42.1 37.1 36.7
in the electron micrographs of the bare support taken in the same magnification and similar focus conditions, are observed on the Pt/TiO2-SG, which are attributed to Pt or PtOx particles. After reduction by Table 2 Platinum particle distribution on the oxidised and reduced 1% Pt/ TiO2-SG determined by HRTEM ˚) Diameter (A
Oxidised 1% Pt/TiO2-SG
Reduced 1% Pt/TiO2-SG
20 40 60 80 100 120 140
64.9 26.5 5.4 1.6 0.9 0.0 0.7
74.9 18.2 4.0 1.7 0.9 0.0 0.0
98
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
Fig. 2. Pore diameter distribution of the titania and Pt/TiO2-SG catalysts: (a) TiO2-SG; (b) 1% Pt/TiO2-SG; (c) 3% Pt/TiO2-SG and (d) TiO2-D.
hydrogen, the crystallite size of the metallic Pt on the Pt/titania was obviously reduced (Tables 2 and 3). For example, on the oxidised 1% Pt/TiO2-SG, a statistical analysis on the dimension of the Pt particles confirms that about 64.9% of the Pt crystals show a size smaller than 2 nm and 26.5% in between 2 and 4 nm; while, Table 3 Platinum particle distribution on the oxidised and reduced 3% Pt/ TiO2-SG catalysts determined by HRTEM ˚) Diameter (A
Oxidised 3% Pt/TiO2-SG
Reduced 3% Pt/TiO2-SG
20 40 60
86.1 13.7 0.2
99.7 0.3 0
after reduction by hydrogen, the percentage of Pt crystals with a size of 2 nm increased to 74.9%. On the oxidised 3% Pt/TiO2-SG, 13.7% Pt clusters show a size of 4 nm. However, on the reduced 3% Pt/TiO2SG, only 0.3% Pt particles are around 4 nm. It is interesting to observe that the crystallite size of the metallic Pt on the 3% Pt/TiO2-SG is smaller than the one on the 1% Pt/TiO2-SG, showing that the consecutive impregnation is a useful method to obtain small metal crystals on the support. These observations are in good agreement with experimental results of Pt average size determined by CO chemisorption, for example, on the 1% Pt/TiO2-SG, the average crystallite size of Pt is 2.6 nm which is very close to the mean crystallite size of 2.5 nm determined by TEM observation. Formation of smaller Pt clusters
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
99
Fig. 4. High resolution TEM images: (a) oxidised 1% Pt/TiO2-SG and (b) reduced 1% Pt/TiO2-SG. Fig. 3. SEM images of titania and Pt/TiO2-SG catalysts: (a) TiO2SG; (b) 1% Pt/TiO2 and (c) 3% Pt/TiO2.
obtained from the consecutive impregnation method may associate to consecutive reduction steps. The reduction treatment after the first impregnation step increases the metal dispersion that may inhibit metal crystals growth in the following impregnation. Since most of Pt crystals are smaller than the pore diameter of TiO2 support, making them possible to not only locate outside of the pore opening, but also on inner surfaces of channels and particle interfaces, thus highly increasing the metal dispersion.
3.3. Basicity of TiO2 and Pt/TiO2 Fig. 5 shows the CO2-TPD profiles of TiO2-SG, 1% Pt/TiO2-SG and 3% Pt/TiO2-SG samples. The basicity increases in a sequence as follows: TiO2 -SGð273:95 mmol=gÞ < 1% Pt=TiO2 -SGð282:09 mmol=gÞ < 3% Pt=TiO2 -SGð297:69 mmol=gÞ From this result, it is concluded that loading metal on the titania support develops more basic sites.
100
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
Fig. 5. TPD-CO2 profiles of TiO2-SG, 1% Pt/TiO2-SG and 3% Pt/TiO2 catalysts: (a) TiO2-SG; (b) 1% Pt/TiO2-SG and (c) 3% Pt/TiO2-SG.
However, the peak shape of the TPD-CO2 profiles is very similar in the three samples: a big desorption peak located between 30 and 200 8C and a small one in the range between 200 and 430 8C. These results indicate that the samples have weak basic sites together with some strong basic sites. Obviously, the former is dominant. Compared to bare titania, when Pt metals were supported TiO2-SG, the strength of both strong and weak basicity increased as evidenced by the temperature corresponding to the peak maximum shifting 15 8C to higher range. However, the strength of basicity on the Pt/TiO2-SG catalysts is independent of metal concentration. 3.4. Reduction behaviours of TiO2 and PtO/TiO2 Fig. 6 shows TPR profiles of different samples. On the TiO2-SG, a main TPR peak centred at 618 8C was observed (Fig. 6a). This indicates that in a reducing atmosphere, some Ti4þ ions were reduced to Ti3þ, leading to TiO2 changing to TiOx (x < 2); the reduction of Ti4þ to Ti3þ starts from about 400 8C and its
Fig. 6. TPR spectra of the TiO2-SG, 1% Pt/TiO2 and 3% Pt/TiO2-SG catalysts: (a) TiO2-SG; (b) 1% Pt/TiO2-SG and (c) 3% Pt/TiO2-SG.
maximum rate of reduction is achieved at about 618 8C. The TPR profile of 1% Pt/TiO2-SG catalyst shows a big peak at the temperature between 450 and 620 8C with a shoulder at 448 8C. Based on the TPR character of titania, the prominent peak with a maximum at 614 8C in TPR profile is assigned to the reduction of TiO2 support, while, the shoulder one should correspond to the reduction of the platinum oxide to metallic Pt. Similar to 1% Pt/TiO2-SG, the catalyst 3%/Pt/TiO2-SG had a dominant peak at about 616 8C with a shoulder appearing at 467 8C, which are, respectively, attributed to the reduction of the titania support and platinum oxide. Fig. 6 clearly shows that below 400 8C, no any reduction reaction takes place on both the bare TiO2 and Pt-supported catalysts during the TPR procedure. This result demonstrates that though our TiO2 sample is reducible, however, its lattice oxygen reduction is not so easy. Some reports show that on the Pt/TiO2 sample, the supported metallic Pt can catalyse
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
101
Table 4 NO conversion over the different catalysts at various reaction temperatures Catalysts
TiO2-D TiO2-SG 1% Pt/TiO2-SG 3% Pt/TiO2-SG
NO conversion (%) 100 8C
200 8C
250 8C
300 8C
350 8C
400 8C
0.0 0.0 2.7 2.5
2.6 1.3 15 19
5.1 2.0 42 44
5.3 2.1 69 73
9.7 4.4 70 75
15 12 100 100
molecular H2 dissociation, and might induce atom hydrogen spillover from metal to titania [27], this may lower the reduction temperature of support. However, in our case, such a hydrogen spillover behaviour was not observed. It is noted that before and after metal loading, the reduction character of titania support remained almost the same. This result strongly indicates that the reduction of oxygen species on titania does not blocked by loading metal and the interaction between platinum oxide and support is very weak. The latter differs from other claims of strong interaction occurring between metal and titania in the M/TiO2 (M: Pt, Rh) [25,28,29]. Although we could not find a satisfactory explanation for the weak interaction between Pt and TiO2 support in our Pt/TiO2 catalyst, however, we may suggest that the different interaction occurring between Pt and titania is probably related to the variation of the surface structures resulting from different preparation methods and treatment conditions such as reduction/ calcination temperature. 3.5. Catalytic activity Table 4 shows that bare titania-SG support is inactive for NO reduction at 100 8C, and even at high reaction temperature, i.e., 400 8C, it still shows very low activity with a NO conversion less than 15%. The conversion of NO over the commercial TiO2-D is almost half of that of the sol–gel one. As reported in Table 1, the surface area of the TiO2-SG is about two times larger than the one of the commercial TiO2D. More active sites due to bigger surface area of the TiO2-SG sample may be responsible for the higher activity of NO reduction. However, when compared the selectivity of NO reduction over these two different titania, one may find that they have very similar selectivity to N2O and to N2 although their pore size
distribution and surface area are significantly different (Tables 1 and 5). This result shows that the selectivity of NO reduction over TiO2 is insensitive to the textural property. This might be interpreted by the diffusion of NO on the titania: NO is very small molecule, its diffusion from the surface to the pore network of the TiO2 is rapid, it is not the limiting elementary step. Therefore, the selectivity of NO reduction is independent of the textural property of the TiO2. Compared to TiO2, the Pt/TiO2-SG catalysts behave very differently in NO reduction, high activity is shown (Table 4). For example, at 250 8C, NO conversion is higher than 40%, and at 400 8C, it reaches 100%. The variation of the product distribution provides clues to the surface reaction processes. Our results orient toward to an indication that the exposed metallic Pt and weak metal-support interaction play essential roles in NO conversion. When the Pt/TiO2 catalyst was reduced at a temperature above 500 8C, migration of the titanium oxide over the surface of the metal particles would become possible, resulting in a thin covering TiO2 layer on the metal particles, thus blocking the active centres at the metal surface [28–30]. Thereby the interaction between the metals and the TiO2 layer and the support may become strong, this finally reducing the chemisorption capacity and catalytic activity/selectivity. However, the Table 5 Selectivity (%) of NO reduction via CO on the TiO2-SG and TiO2D samples Reaction temperature (8C) 100 200 250 350
TiO2-SG
TiO2-D
N2
N2O
N2
N2O
– 36 44 46
– 64 56 54
– 39 40 41
– 61 60 59
102
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
Pt/TiO2-SG catalysts reported herein were reduced at 300 8C before the catalytic reaction evaluation, in such case, the decoration effect does not occur. Therefore, the catalytic active centres on the Pt/TiO2-SG catalysts are not blocked after reduction treatment and the metal-support interaction in our Pt supported TiO2 catalysts is relatively weak. The catalytic activity of our Pt/TiO2 catalyst is hence high. This results demonstrates that a proper reduction temperature is in favour of the NO adsorption and further dissociation, thus enhancing the catalytic activity. From the acid-base reaction point of view, as a strong acidic gas, NO may preferentially adsorb on basic centres on the solid. Although both the basicity and NO conversion on the TiO2-SG and Pt/TiO2-SG show a similar increasing sequence, however, the catalytic behaviour of Pt/TiO2 seems to indicate there is no salient correlation between basicity of the catalysts and catalytic activity because the variations of the basicity between the Pt/TiO2 catalysts and the TiO2 support are quilt small, which disagrees with the significant difference of reaction activity between them. On the bare titania, the basic sites possibly serve as active centres for NO adsorption, while, in the platinum loaded catalysts, besides the active sites on the support, the Pt metal catalytic action is mainly
responsible for the significant enhancement of reactivity. When the support TiO2-SG was used for NO reduction, below 400 8C of reaction, NO2 was not formed, for example, at 250 8C, only N2O (56%) and N2 (44%) were yielded. However, on the Pt/TiO2-SG catalysts, no matter the Pt content is 1% or 3%, NO2 always was formed with an increasing concentration at 3–11% in the reaction temperature between 150 and 400 8C. In the outlet stream, N2, NO2, NO and N2O were detected as products (Figs. 7–9). In comparison with TiO2-SG, lower selectivity to N2O over Pt/TiO2-SG catalysts was obtained (Figs. 7 and 8). For example, at 250 8C, the N2O selectivity is 34% on the 1% Pt/TiO2-SG catalyst, which is 37% lower than the one achieved on TiO2-SG support at the same reaction temperature. On the other hand, the selectivity to N2 on Pt/TiO2-SG is enhanced relative to that on titania (Fig. 9). The increment of selectivity to N2 is likely a result of reduction of N2O selectivity. It is possible that N2O is an important intermediate leading to N2. These results seem to indicate that metallic Pt promote N2 formation by catalysing the reaction N2 O þ CO ¼ N2 þ CO2 , which leads to N2O concentration decreasing and N2 formation. Based on above results and discussion, NO reduction via CO on
Fig. 7. Selectivity of NO reduction via CO over 1% Pt/TiO2-SG catalyst as a function of reaction temperatures.
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
103
Fig. 8. Selectivity of NO reduction via CO over 3% Pt/TiO2-SG catalyst as a function of reaction temperatures.
the Pt/TiO2-SG catalysts in the presence of excess oxygen probably performs through a mechanism as follows: NO þ 12 O2 ¼ NO2
(1)
2NO þ CO ¼ N2 O þ CO2
(2)
N2 O þ CO ¼ N2 þ CO2
(3)
Based on the above assumption, NO2 is formed through reaction (1) catalysed by metallic Pt. The
Fig. 9. Comparison of the selectivity of NO reduction at 250 8C over different catalysts.
104
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
elementary steps of NO by CO have been suggested to include molecular adsorption, dissociation of NO, reassembling of the surface species and finally desorption of the production [31,32]. On the Pt/TiO2-SG catalysts, it is possible that NO first adsorb on the metallic Pt clusters, and then the adsorbed NO species dissociatively decomposed, yielding adsorbed N and O intermediates [33]. In the case of oxygen presence, on the one hand, oxygen may attack N to form NO2; likewise, NO may react with adsorbed O species to produce NO2 or/and react with N to N2O which further reacts with CO to produce N2 and CO2 via reaction (3). In the dissociation step, metallic Pt takes a key role. The ability of Pt to dissociate the adsorbed NO, from an electronic point of view, might be resulted from the d-electrons from 5d atomic orbital in Pt atoms back-donation to the 2p-antibonding orbital of NO molecule, that activates the NO–Pd complex and weakens the bond of N–O and finally leads to N–O bond destibalisation and breaking. This Pt association ability is expected to be promoted by increasing temperature. At low reaction temperature, i.e., 100 8C, due to low association ability of Pt to NO, no NO2 was formed on the Pt/TiO2 catalysts. Without the metallic Pt, dissociative decomposition of NO on TiO2-SG is quilt difficult; therefore, it is impossible for NO2 formation on blank TiO2-SG below 400 8C of reaction. It is noteworthy that Pt particles are an active oxidation catalyst, it is usually used as a catalytic promoter for CO combustion. It is not surprising that on the Pt/TiO2-SG catalysts, when oxygen is present, two parallel reactions, CO oxidation by oxygen to CO2 and NO reduction by CO, simultaneously take place. There may exist a competition between the two reactions. It has been found that the chemisorption of NO on the catalysts at temperature 300 8C is remarkable that strongly inhibits the CO þ O2 reaction under oxidising condition [34,35]. However, at a reaction temperature higher than 300 8C, the activity of CO oxidising to CO2 on Pt/TiO2-SG is greatly improved, closing to a conversion of 100%, the CO oxidation to CO2 strongly consumes CO and oxygen, this, in turn, leads to inhibition of NO þ CO reaction due to diminishing of CO concentration as shown in the suggested reactions pathway, as a result, NO2 formation is relatively enhanced. Therefore, NO2 formation is
likely also a result of above competition of reactions.
4. Conclusions The following conclusions have been drawn from this work: (1) Titania with uniformed pore size distribution concentrating at about 4 nm could be obtained by using sol–gel method, which consists of many primary spherical particles with a size ranging 0.1–0.3 mm and secondary spherical particles with a size of about 2 mm. (2) The consecutive impregnation method with a reduction treatment between the impregnation steps as reported herein is a useful method to obtain small metal Pt crystals dispersed on the support. On the reduced 3% Pt/TiO2 catalyst prepared by the consecutive impregnation method, 99.7% of metallic Pt crystals with a size less than 2 nm are located on both the surface and inner pores of the support. (3) Interaction between metallic Pt and titania support in the Pt/TiO2 catalysts is very week before and after metal leading, which is probably due to the relatively low reduction temperature. (4) One of the new contributions is the finding of the selectivity of NO reduction over TiO2 in the oxygen-rich condition being insensitive to the variation of textural properties. (5) Although both the titania support and Pt/TiO2-SG catalysts present similar basicity, however, they show very different reaction activity and selectivity for NO reduction. A great improvement of catalytic activity of NO reduction together with high selectivity to N2 on the Pt loaded titania is obtained in comparison to bare titania support, which is explained by assumption different reaction mechanisms.
Acknowledgements Dr. J. Navarrete Bolan˜ os is gratefully acknowledged for fruitful discussion. The authors also thank the Mexican Institute of Petroleum and National Poly-
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
technic Institute for financial support (Grant No. FIES-98-29-III and CGPI-IPN-20021187). References [1] J. Pasel, V. Speer, C. Albrecht, F. Richter, H. Papp, Appl. Catal. B: Enveron. 25 (2000) 105. [2] A. Dandclear, M.A. Vannice, Appl. Catal. B: Environ. 22 (1999) 179. [3] A. Crucq (Ed.), Catalysis and automotive pollution control II, Study of Surface Science and Catalysis, vol. 71, Elseiver, Amsterdam, 1991. [4] A. Martı´nez, M. Ferna´ ndez-Garcı´a, A. Iglesias-Juez, A.B. Hungrı´a, J. A: Anderson, J.C. Conesa, J. Soria, Appl. Catal. B: Environ. 31 (2001) 51. [5] M. Ferna´ ndez-Garcı´a, A. Marrı´nez, A. Iglesias-Juez, A.B. Hungrı´a, J. A: Anderson, J.C. Conesa, J. Soria, Appl. Catal. B: Environ. 31 (2001) 39. [6] D. Ciuparu, A. Bensalem, L. Pfefferle, Appl. Catal. B: Environ. 26 (2000) 241. [7] S.E. Golunski, H.A. Hatcher, R.R. Rajaram, K. Jansson, L. Truex, SAE Technical Paper Series, No. 950408, 1995. [8] U.S. Ozkan, M.W. Kumthekar, G. Karakas, Catal. Today 40 (1998) 3. [9] A. Ali, W. Alvarez, C.J. Loughran, Appl. Catal. B: Environ. 14 (1997) 13. [10] M. Thammachat, V. Meeyoo, T. Risksomboon, S. Osuwan, Catal. Today 68 (2000) 53. [11] G.W. Graham, H.W. Jen, R.W. McCabe, A.M. Straccia, L.P. Hack, Catal. Lett. 67 (2000) 99. [12] H. Muraki, G. Zhang, Catal. Today 63 (2000) 337. [13] S. Matsuda, A. Kato, Appl. Catal. 8 (1983) 149. [14] H. Bosch, F. Jansen, Catal. Today 2 (1988) 369. [15] M.A. Vanince, J. Catal. 74 (1982) 199. [16] M.W. Kumthekar, U.S. Ozkan, J. Catal. 171 (1997) 45.
105
[17] M.W. Kumthekar, U.S. Ozkan, J. Catal. 171 (1997) 54. [18] U.S. Ozkan, M.W. Kumthekar, G. Karkas, J. Catal. 171 (1997) 67. [19] G. Martra, Appl. Catal. 200 (2000) 275–285. [20] X. Bokhimi, A. Morales, O. Novaro, T. Lopez, E. Sa´ nchez, R. Gomez, J. Mater. Res. 10 (1995) 2788. [21] M. Mora´ n-Pineda, S. Castillo, R. Go´ mez, React. Kinet. Catal. Lett. 76 (2002) 375. [22] S. Castillo, R. Go´ mez, M. Mora´ n-Pineda, React. Kinet. Catal. Lett. 79 (2003) 271. [23] S. Castillo, M. Mora´ n-Pineda, R. Go´ mez, T. Lo´ pez, J. Catal. 172 (1997) 263. [24] S. Castillo, M. Mora´ n-Pineda, V. Molina, R. Go´ mez, T. Lo´ pez, Appl. Catal. B: Environ. 15 (1998) 203. [25] J.A. Horsley, J. Am. Chem. Soc. 101 (1979) 2870. [26] S.J. Tauster, S.C. Fung, R.L. Garten, J. Am. Chem. Soc. 100 (1978) 70. [27] J.C. Conesa, J. Sorla, J. Phys. Chem. 86 (1982) 1392. [28] S. Bernal, J.J. Calvino, M.A. Cauqui, J.M. Gatica, C. Lo´ pez Cartes, J.A. Pe´ rez Omil, J.M. Pintado, Catal. Today 77 (2003) 385. [29] S. Bernal, F.J. Botana, J.J. Calvino, C. Lo´ pez, J.A. Pe´ rez Omil, J.M. Rodrı´guez-Izquierdo, J. Chem. Soc., Faraday Trans. 92 (1996) 2799. [30] N. Nava Entzana, Doctor Thesis, Unversidad Autonoma Metropolitana-Iztapalapa, 2002, p. 96. [31] S.B. Schwartz, G.B. Fisher, L.D. Schmidt, J. Phys. Chem. 92 (1988) 389. [32] R.L. Klein, S. Schwartz, L.D. Schmidt, J. Phys. Chem. 89 (1985) 4908. [33] M. Shelef, G.W. Graham, Catal. Rev.-Sci. Eng. 36 (1994) 433. [34] S.H. Oh, C.C. Eickel, J. Catal. 128 (1991) 526. [35] G.B. Fisher, S.H. Oh, C.L. DiMaggio, S.J. Schmieg, D.W. Goodman, C.H.F. Peden, in: M.J. Phillips, M. Ternan (Eds.), Proceedings of the 9th International Congress on Catalysis, vol. 3, Chemical Institute of Canada, Ottawa, 1988, p. 1355.
Studies of sol–gel TiO2 and Pt/TiO2 catalysts for NO reduction by CO in an oxygen-rich condition J.A. Wanga, A. Cuanb, J. Salmonesc,*, N. Navab, S. Castillob, M. Mora´n-Pinedab, F. Rojasd a
Laboratorio de Cata´lisis y Materiales, ESIQIE, Instituto Polite´cnico Nacional, Col. Zacatenco, C.P. 07738, Me´xico, D.F., Mexico Programa de Ingenierı´a Molecular, Instituto Mexicano del Petro´leo, Eje La´zaro Ca´rdenas 152, C.P. 07730, Me´xico, D.F., Mexico c Programa de Tratamiento de Crudo Maya, Instituto Mexicano del Petro´leo, Eje La´zaro Ca´rdenas 152, C.P. 07730, Me´xico, D.F., Mexico d Departamento de Quı´mica, Universidad Auto´noma Metropolitana-Iztapalapa, P.O. Box 55-534, C.P. 09340, Me´xico, D.F., Mexico b
Received 6 November 2003; received in revised form 30 January 2004; accepted 3 February 2004 Available online 27 April 2004
Abstract The textural properties, morphological features, surface basicity and oxygen reduction behaviours of titania and Pt supported titania catalysts synthesized via a sol–gel method were studied by means of N2 physisorption, SEM, TEM, CO2-TPD and H2TPR techniques. Mesostructured TiO2 shows a very narrow pore size distribution that uniformly centred at about 4 nm. High resolution TEM images confirmed that most of Pt particles on Pt/TiO2-SG had a size smaller than 2 nm. Both the titania support and Pt loaded catalysts chiefly contained weak basic sites with small amount of strong basic sites. Loading Pt did not significantly alter the surface reduction characters of titania, indicating a weak interaction between Pt metals and titania support. Catalytic evaluation revealed that the selectivity of NO reduction over titania was insensitive to variation of textural property. On the bare titania, low NO conversion but high selectivity to N2O was obtained. However, the Pt/TiO2-SG catalysts exhibited high NO conversion and high selectivity to N2, which is assumed to relate to NO dissociation catalysed by the metallic Pt clusters. In addition, when the reaction temperature was above 200 8C, 3–11% NO2 was yielded over the Pt/TiO2-SG catalysts, which was discussed on a basis of reaction competition, metal-support interaction and NO dissociation. # 2004 Elsevier B.V. All rights reserved. Keywords: Titania; NO reduction; Sol–gel synthesis; Pt/TiO2; Catalysts
1. Introduction The emissions of NOx (N2O, NO and NO2) from both mobile and fixed stationary sources remain a problem considerable interest because of the contribution of these pollutants to rain acidification, global warming and depletion of the stratospheric ozone * Corresponding author. Tel.: þ52-5-9175-8444. E-mail address: [email protected] (J. Salmones).
layer [1,2]. In most cases, NOx emissions together with other contaminates and oxygen are discharged to the air, for example, in the automobile exhausts, NOx, CO, hydrocarbons and O2 coexist [3–5]. Particularly, nowadays, more and more internal combustion engines are designed to run in a fuel-lean conditions that lead to the activity of current three-way catalysts (TWCs) decreasing due to remarkable deterioration of NOx reduction efficient in the presence of rich oxygen. In the other practical applications like selective catalytic
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.02.057
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
reduction (SCR), ammonia is usually used as reducing agent for NOx reduction, however, ammonia usually causes the secondary problems such as corrosion of equipment. Therefore, NOx reduction by other reducing agent such as CO or hydrocarbons in the presence of excess oxygen has become an increasingly interesting subject attracting a lot of attention of researchers [4–9]. Platinum, palladium and rhodium are the key components for the current TWCs catalysts since these catalysts always show high catalytic activity and selectivity. Catalyst support, of course, is also very important in the catalyst design due to several wellknown functions such as providing high thermal stability and high active metal dispersion. Alumina, ZSM-5 and ceria-doped alumina are the common catalyst supports in the environmental catalysis [10–12]. Titanium oxide (TiO2) has also been widely employed for NO emission reduction since the first TiO2-based catalyst was applied commercially in air pollution control in 1970s [13–18]. The various forms of titania obtained from different preparation methods usually exhibit different surface structure arrangements thus different surface reactivity [19]. By using a sol–gel method with various hydrolysis catalysts, Bokhimi et al. prepared nanosized titania in those cationic defects are formed in the crystalline structures [20]. When Au, Pt or Rh metals were introduced on the sol–gel titania or second metal oxide doped titania, it exhibits high activity for NO reduction or CO oxidation [21–24]. Usually, a weak or strong interaction between the supported noble metals and titania may occur [25,26], depending on reduction temperature and preparation methods, that strongly affects not only the surface reduction properties and acid-basic character, but the catalytic activity also. Although titania was intensively studied including its synthesis and various applications in the past years, however, effects of surface structures and textural properties as well as metal–titania interaction on catalytic activity and selectivity are still interesting research topics. In this work, by mains of a sol–gel method, titanium oxide (TiO2-SG) showing very narrow and uniformed pore size distribution was prepared. The Pt/TiO2-SG catalysts with different Pt concentrations used for NO reduction in an
95
oxygen-rich condition were obtained by impregnation Pt precursor on TiO2. The textural properties and surface basicity as well as surface oxygen reduction behaviour of TiO2 and Pt/TiO2-SG catalysts were characterised by N2-physisoption, CO2-TPD and H2-TPR techniques. Furthermore, activity and selectivity of NO reduction by CO over the TiO2 and Pt/TiO2 catalysts were measured in oxygen-rich condition and the possible reaction mechanism was discussed.
2. Experimental 2.1. Preparation of TiO2-SG TiO2-SG was prepared by sol–gel method: 7.1 g of Ti(O-Pri)4 was added into a 30 g solvent of isopropanol at 28 8C with a continuous stirring for 20 min. Afterwards, 36.1 ml water and 0.75 ml concentric nitric acid were slowly dropped into the above mixture to remain the pH value at 5. A white gel was formed after 24 min of gelation. The extra solvent was evaporated and then the precipitated solid was dried at 90 8C for 24 h. Finally, it was first calcined at 200 8C for 2 h and then at 500 8C for 2 h. 2.2. Preparation of Pt/TiO2-SG catalysts For obtaining the 1 wt.% Pt/TiO2-SG catalyst, dinitro tetra-amino platinum(II) was used as platinum precursor and the above calcined titania powder used as support. The support was impregnated with a solution of dinitro tetra-amino platinum(II) to obtain 1 wt.% Pt on the support, then it was calcined at 500 8C in air. In the case of 3 wt.% Pt/TiO2-SG, a method of consecutive impregnation Pt was used: the reduced 1 wt.% Pt/TiO2-SG sample was impregnated again with a solution of dinitro tetra-amino platinum(II) to obtain a new sample with 2 wt.% Pt. This new sample was calcined at 500 8C and then reduced at 300 8C by H2 for 30 min. The impregnation procedure was repeated, finally the 3 wt.% Pt/TiO2-SG catalyst was obtained. The platinum on both Pt/TiO2 catalysts are in oxidized states. Part of them were reduced by hydrogen at 300 8C for 1 h in order to compare variation of the metal particle size before and after reduction.
96
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
2.3. Textural analysis The surface area and pore size distribution of the samples were measured in a Digisorb 2600 equipment by analysing N2 physisorption isotherms. 2.4. Temperature programmed desorption of CO2 (TPD-CO2) To measure basicity of the TiO2 and Pt containing catalysts, TPD-CO2 experiments were carried out in a Zeton Altamire Model AMI-3 equipment. The catalysts were thermal treated in a flow of He (30 ml/min) at 500 8C for 1 h, and then the temperature cooled down to room temperature in a He atmosphere. The CO2 was allowed to the sample cell for 30 min of adsorption. Afterwards, the temperature rose to 700 8C in a rate of 5 8C/min in a flow of He. The desorption signal was recorded by a thermal conductivity detector (TCD). 2.5. Temperature programmed reduction of hydrogen (H2-TPR) The titania support and PtO/TiO2-SG samples (without reduction) were first pertreated in a flow of He (30 ml/min) at 550 8C for 1 h in order to remove the adsorbed water and other species, and then they were cooled down to room temperature in He atmosphere. After the thermal treatment, the catalysts were reduced in a mixture of 10% H2 in He at a rate of 10 8C increasing temperature programmed up to 750 8C. Catalysts weight used for TPR measurements was about 0.1 g. The TPR experiments were carried out on a Zeton Altamire Model AMI-3 equipment. 2.6. Surface feature images Transmission and scanning electron microscopy (TEM and SEM) were used to image the structure and growth characteristics of platinum on TiO2 supports. The TEM and SEM observations were carried out on a Jeol Model 100 CX-11 and JEOL 35CF, respectively.
and 3 ml volume). In all the cases, 50 mg of catalysts (80–120 mesh) was used. Before the reaction, the Pt/TiO2 catalysts were activated by treatment with 99.9% H2. The temperature of was raised to 300 8C with a heating rate 2 8C/min and the samples were reduced for 1 h at 300 8C. The reaction mixture containing 1.5 vol.% CO, 0.5 vol.% NO and 1.5 vol.% O2. Helium was used as carrier gas and the inlet flow rate was 30 ml/min. The products were analysed by FTIR (Nicolet-8220 Gas Analyser) in coupled with gas chromatography (HP-5890 Chemical Station).
3. Results and discussion 3.1. Textural properties of TiO2 and Pt/TiO2-SG Fig. 1a–c shows the N2 adsorption–desorption isotherms of the sol–gel TiO2-SG support and 1% Pt/TiO2SG and 3% Pt/TiO2-SG catalysts. For comparison, the nitrogen isotherm of a reference sample TiO2-D (Degussa) is also shown in Fig. 1d. The hysteresis loops of the sol–gel samples are characteristics of the typical mesopore structures in type IV, which are very different from the one of the reference. The reference sample TiO2-D exhibits a hysteresis loop of type II with almost vertical and nearly parallel adsorption and desorption branches. The limiting adsorption at p=p0 ¼ 1 in Fig. 1d attains an extremely large value of 140 cc/g. The pore size distributions of the above samples are shown in Fig. 2a–d and the pore volumes and average pore diameters are reported in Table 1. It is clearly observed that the titania sample prepared with sol–gel method has a uniform pore size distribution in a very narrow range with a maximum of about 4.2 nm, which significantly differs from the commercial TiO2-D (Fig. 2d), which shows a very wide distribution ranging from 1 to 100 nm. Besides, the platinum loaded samples exhibit a similar pore size distribution as that of the bare TiO2-SG support, however, Pt metals loading leads to a slight decrease of both surface area and pore diameter (Table 1).
2.7. Catalytic test 3.2. Morphology of TiO2 and Pt/TiO2-SG Catalytic activity in the NO reduction by CO was tested from 100 to 500 8C under atmospheric pressure in a quartz reactor of fixed bed (9 mm diameter
It is observed by SEM that either TiO2-SG or TiO2-D sample contains both the primary and secondary
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
97
Fig. 1. Profiles of N2 isotherms adsorption–desorption: (a) TiO2-SG; (b) 1% Pt/TiO2-SG; (c) 3% Pt/TiO2-SG and (d) TiO2-D.
particles. The elementary particles are roughly spherical in shape with a size in 0.1–0.3 mm. These fine grains are coalesced to form secondary particles with a size of about 2 mm (Fig. 3a–c). To observe the Pt particles located on the surface of the titania support, high resolution TEM was applied (Fig. 4a and b). The very fine particles that are absent Table 1 Textural properties of TiO2-SG, TiO2-D and Pt/TiO2-SG catalysts Samples
Specific area (m2/g)
Pore volume (cc3/g)
Mean pore ˚) diameter (A
TiO2-D TiO2-SG 1% Pt/TiO2-SG 3% Pt/TiO2-SG
53.8 90.0 67.0 46.4
0.1537 0.1435 0.0783 0.0476
500.3 42.1 37.1 36.7
in the electron micrographs of the bare support taken in the same magnification and similar focus conditions, are observed on the Pt/TiO2-SG, which are attributed to Pt or PtOx particles. After reduction by Table 2 Platinum particle distribution on the oxidised and reduced 1% Pt/ TiO2-SG determined by HRTEM ˚) Diameter (A
Oxidised 1% Pt/TiO2-SG
Reduced 1% Pt/TiO2-SG
20 40 60 80 100 120 140
64.9 26.5 5.4 1.6 0.9 0.0 0.7
74.9 18.2 4.0 1.7 0.9 0.0 0.0
98
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
Fig. 2. Pore diameter distribution of the titania and Pt/TiO2-SG catalysts: (a) TiO2-SG; (b) 1% Pt/TiO2-SG; (c) 3% Pt/TiO2-SG and (d) TiO2-D.
hydrogen, the crystallite size of the metallic Pt on the Pt/titania was obviously reduced (Tables 2 and 3). For example, on the oxidised 1% Pt/TiO2-SG, a statistical analysis on the dimension of the Pt particles confirms that about 64.9% of the Pt crystals show a size smaller than 2 nm and 26.5% in between 2 and 4 nm; while, Table 3 Platinum particle distribution on the oxidised and reduced 3% Pt/ TiO2-SG catalysts determined by HRTEM ˚) Diameter (A
Oxidised 3% Pt/TiO2-SG
Reduced 3% Pt/TiO2-SG
20 40 60
86.1 13.7 0.2
99.7 0.3 0
after reduction by hydrogen, the percentage of Pt crystals with a size of 2 nm increased to 74.9%. On the oxidised 3% Pt/TiO2-SG, 13.7% Pt clusters show a size of 4 nm. However, on the reduced 3% Pt/TiO2SG, only 0.3% Pt particles are around 4 nm. It is interesting to observe that the crystallite size of the metallic Pt on the 3% Pt/TiO2-SG is smaller than the one on the 1% Pt/TiO2-SG, showing that the consecutive impregnation is a useful method to obtain small metal crystals on the support. These observations are in good agreement with experimental results of Pt average size determined by CO chemisorption, for example, on the 1% Pt/TiO2-SG, the average crystallite size of Pt is 2.6 nm which is very close to the mean crystallite size of 2.5 nm determined by TEM observation. Formation of smaller Pt clusters
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
99
Fig. 4. High resolution TEM images: (a) oxidised 1% Pt/TiO2-SG and (b) reduced 1% Pt/TiO2-SG. Fig. 3. SEM images of titania and Pt/TiO2-SG catalysts: (a) TiO2SG; (b) 1% Pt/TiO2 and (c) 3% Pt/TiO2.
obtained from the consecutive impregnation method may associate to consecutive reduction steps. The reduction treatment after the first impregnation step increases the metal dispersion that may inhibit metal crystals growth in the following impregnation. Since most of Pt crystals are smaller than the pore diameter of TiO2 support, making them possible to not only locate outside of the pore opening, but also on inner surfaces of channels and particle interfaces, thus highly increasing the metal dispersion.
3.3. Basicity of TiO2 and Pt/TiO2 Fig. 5 shows the CO2-TPD profiles of TiO2-SG, 1% Pt/TiO2-SG and 3% Pt/TiO2-SG samples. The basicity increases in a sequence as follows: TiO2 -SGð273:95 mmol=gÞ < 1% Pt=TiO2 -SGð282:09 mmol=gÞ < 3% Pt=TiO2 -SGð297:69 mmol=gÞ From this result, it is concluded that loading metal on the titania support develops more basic sites.
100
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
Fig. 5. TPD-CO2 profiles of TiO2-SG, 1% Pt/TiO2-SG and 3% Pt/TiO2 catalysts: (a) TiO2-SG; (b) 1% Pt/TiO2-SG and (c) 3% Pt/TiO2-SG.
However, the peak shape of the TPD-CO2 profiles is very similar in the three samples: a big desorption peak located between 30 and 200 8C and a small one in the range between 200 and 430 8C. These results indicate that the samples have weak basic sites together with some strong basic sites. Obviously, the former is dominant. Compared to bare titania, when Pt metals were supported TiO2-SG, the strength of both strong and weak basicity increased as evidenced by the temperature corresponding to the peak maximum shifting 15 8C to higher range. However, the strength of basicity on the Pt/TiO2-SG catalysts is independent of metal concentration. 3.4. Reduction behaviours of TiO2 and PtO/TiO2 Fig. 6 shows TPR profiles of different samples. On the TiO2-SG, a main TPR peak centred at 618 8C was observed (Fig. 6a). This indicates that in a reducing atmosphere, some Ti4þ ions were reduced to Ti3þ, leading to TiO2 changing to TiOx (x < 2); the reduction of Ti4þ to Ti3þ starts from about 400 8C and its
Fig. 6. TPR spectra of the TiO2-SG, 1% Pt/TiO2 and 3% Pt/TiO2-SG catalysts: (a) TiO2-SG; (b) 1% Pt/TiO2-SG and (c) 3% Pt/TiO2-SG.
maximum rate of reduction is achieved at about 618 8C. The TPR profile of 1% Pt/TiO2-SG catalyst shows a big peak at the temperature between 450 and 620 8C with a shoulder at 448 8C. Based on the TPR character of titania, the prominent peak with a maximum at 614 8C in TPR profile is assigned to the reduction of TiO2 support, while, the shoulder one should correspond to the reduction of the platinum oxide to metallic Pt. Similar to 1% Pt/TiO2-SG, the catalyst 3%/Pt/TiO2-SG had a dominant peak at about 616 8C with a shoulder appearing at 467 8C, which are, respectively, attributed to the reduction of the titania support and platinum oxide. Fig. 6 clearly shows that below 400 8C, no any reduction reaction takes place on both the bare TiO2 and Pt-supported catalysts during the TPR procedure. This result demonstrates that though our TiO2 sample is reducible, however, its lattice oxygen reduction is not so easy. Some reports show that on the Pt/TiO2 sample, the supported metallic Pt can catalyse
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
101
Table 4 NO conversion over the different catalysts at various reaction temperatures Catalysts
TiO2-D TiO2-SG 1% Pt/TiO2-SG 3% Pt/TiO2-SG
NO conversion (%) 100 8C
200 8C
250 8C
300 8C
350 8C
400 8C
0.0 0.0 2.7 2.5
2.6 1.3 15 19
5.1 2.0 42 44
5.3 2.1 69 73
9.7 4.4 70 75
15 12 100 100
molecular H2 dissociation, and might induce atom hydrogen spillover from metal to titania [27], this may lower the reduction temperature of support. However, in our case, such a hydrogen spillover behaviour was not observed. It is noted that before and after metal loading, the reduction character of titania support remained almost the same. This result strongly indicates that the reduction of oxygen species on titania does not blocked by loading metal and the interaction between platinum oxide and support is very weak. The latter differs from other claims of strong interaction occurring between metal and titania in the M/TiO2 (M: Pt, Rh) [25,28,29]. Although we could not find a satisfactory explanation for the weak interaction between Pt and TiO2 support in our Pt/TiO2 catalyst, however, we may suggest that the different interaction occurring between Pt and titania is probably related to the variation of the surface structures resulting from different preparation methods and treatment conditions such as reduction/ calcination temperature. 3.5. Catalytic activity Table 4 shows that bare titania-SG support is inactive for NO reduction at 100 8C, and even at high reaction temperature, i.e., 400 8C, it still shows very low activity with a NO conversion less than 15%. The conversion of NO over the commercial TiO2-D is almost half of that of the sol–gel one. As reported in Table 1, the surface area of the TiO2-SG is about two times larger than the one of the commercial TiO2D. More active sites due to bigger surface area of the TiO2-SG sample may be responsible for the higher activity of NO reduction. However, when compared the selectivity of NO reduction over these two different titania, one may find that they have very similar selectivity to N2O and to N2 although their pore size
distribution and surface area are significantly different (Tables 1 and 5). This result shows that the selectivity of NO reduction over TiO2 is insensitive to the textural property. This might be interpreted by the diffusion of NO on the titania: NO is very small molecule, its diffusion from the surface to the pore network of the TiO2 is rapid, it is not the limiting elementary step. Therefore, the selectivity of NO reduction is independent of the textural property of the TiO2. Compared to TiO2, the Pt/TiO2-SG catalysts behave very differently in NO reduction, high activity is shown (Table 4). For example, at 250 8C, NO conversion is higher than 40%, and at 400 8C, it reaches 100%. The variation of the product distribution provides clues to the surface reaction processes. Our results orient toward to an indication that the exposed metallic Pt and weak metal-support interaction play essential roles in NO conversion. When the Pt/TiO2 catalyst was reduced at a temperature above 500 8C, migration of the titanium oxide over the surface of the metal particles would become possible, resulting in a thin covering TiO2 layer on the metal particles, thus blocking the active centres at the metal surface [28–30]. Thereby the interaction between the metals and the TiO2 layer and the support may become strong, this finally reducing the chemisorption capacity and catalytic activity/selectivity. However, the Table 5 Selectivity (%) of NO reduction via CO on the TiO2-SG and TiO2D samples Reaction temperature (8C) 100 200 250 350
TiO2-SG
TiO2-D
N2
N2O
N2
N2O
– 36 44 46
– 64 56 54
– 39 40 41
– 61 60 59
102
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
Pt/TiO2-SG catalysts reported herein were reduced at 300 8C before the catalytic reaction evaluation, in such case, the decoration effect does not occur. Therefore, the catalytic active centres on the Pt/TiO2-SG catalysts are not blocked after reduction treatment and the metal-support interaction in our Pt supported TiO2 catalysts is relatively weak. The catalytic activity of our Pt/TiO2 catalyst is hence high. This results demonstrates that a proper reduction temperature is in favour of the NO adsorption and further dissociation, thus enhancing the catalytic activity. From the acid-base reaction point of view, as a strong acidic gas, NO may preferentially adsorb on basic centres on the solid. Although both the basicity and NO conversion on the TiO2-SG and Pt/TiO2-SG show a similar increasing sequence, however, the catalytic behaviour of Pt/TiO2 seems to indicate there is no salient correlation between basicity of the catalysts and catalytic activity because the variations of the basicity between the Pt/TiO2 catalysts and the TiO2 support are quilt small, which disagrees with the significant difference of reaction activity between them. On the bare titania, the basic sites possibly serve as active centres for NO adsorption, while, in the platinum loaded catalysts, besides the active sites on the support, the Pt metal catalytic action is mainly
responsible for the significant enhancement of reactivity. When the support TiO2-SG was used for NO reduction, below 400 8C of reaction, NO2 was not formed, for example, at 250 8C, only N2O (56%) and N2 (44%) were yielded. However, on the Pt/TiO2-SG catalysts, no matter the Pt content is 1% or 3%, NO2 always was formed with an increasing concentration at 3–11% in the reaction temperature between 150 and 400 8C. In the outlet stream, N2, NO2, NO and N2O were detected as products (Figs. 7–9). In comparison with TiO2-SG, lower selectivity to N2O over Pt/TiO2-SG catalysts was obtained (Figs. 7 and 8). For example, at 250 8C, the N2O selectivity is 34% on the 1% Pt/TiO2-SG catalyst, which is 37% lower than the one achieved on TiO2-SG support at the same reaction temperature. On the other hand, the selectivity to N2 on Pt/TiO2-SG is enhanced relative to that on titania (Fig. 9). The increment of selectivity to N2 is likely a result of reduction of N2O selectivity. It is possible that N2O is an important intermediate leading to N2. These results seem to indicate that metallic Pt promote N2 formation by catalysing the reaction N2 O þ CO ¼ N2 þ CO2 , which leads to N2O concentration decreasing and N2 formation. Based on above results and discussion, NO reduction via CO on
Fig. 7. Selectivity of NO reduction via CO over 1% Pt/TiO2-SG catalyst as a function of reaction temperatures.
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
103
Fig. 8. Selectivity of NO reduction via CO over 3% Pt/TiO2-SG catalyst as a function of reaction temperatures.
the Pt/TiO2-SG catalysts in the presence of excess oxygen probably performs through a mechanism as follows: NO þ 12 O2 ¼ NO2
(1)
2NO þ CO ¼ N2 O þ CO2
(2)
N2 O þ CO ¼ N2 þ CO2
(3)
Based on the above assumption, NO2 is formed through reaction (1) catalysed by metallic Pt. The
Fig. 9. Comparison of the selectivity of NO reduction at 250 8C over different catalysts.
104
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
elementary steps of NO by CO have been suggested to include molecular adsorption, dissociation of NO, reassembling of the surface species and finally desorption of the production [31,32]. On the Pt/TiO2-SG catalysts, it is possible that NO first adsorb on the metallic Pt clusters, and then the adsorbed NO species dissociatively decomposed, yielding adsorbed N and O intermediates [33]. In the case of oxygen presence, on the one hand, oxygen may attack N to form NO2; likewise, NO may react with adsorbed O species to produce NO2 or/and react with N to N2O which further reacts with CO to produce N2 and CO2 via reaction (3). In the dissociation step, metallic Pt takes a key role. The ability of Pt to dissociate the adsorbed NO, from an electronic point of view, might be resulted from the d-electrons from 5d atomic orbital in Pt atoms back-donation to the 2p-antibonding orbital of NO molecule, that activates the NO–Pd complex and weakens the bond of N–O and finally leads to N–O bond destibalisation and breaking. This Pt association ability is expected to be promoted by increasing temperature. At low reaction temperature, i.e., 100 8C, due to low association ability of Pt to NO, no NO2 was formed on the Pt/TiO2 catalysts. Without the metallic Pt, dissociative decomposition of NO on TiO2-SG is quilt difficult; therefore, it is impossible for NO2 formation on blank TiO2-SG below 400 8C of reaction. It is noteworthy that Pt particles are an active oxidation catalyst, it is usually used as a catalytic promoter for CO combustion. It is not surprising that on the Pt/TiO2-SG catalysts, when oxygen is present, two parallel reactions, CO oxidation by oxygen to CO2 and NO reduction by CO, simultaneously take place. There may exist a competition between the two reactions. It has been found that the chemisorption of NO on the catalysts at temperature 300 8C is remarkable that strongly inhibits the CO þ O2 reaction under oxidising condition [34,35]. However, at a reaction temperature higher than 300 8C, the activity of CO oxidising to CO2 on Pt/TiO2-SG is greatly improved, closing to a conversion of 100%, the CO oxidation to CO2 strongly consumes CO and oxygen, this, in turn, leads to inhibition of NO þ CO reaction due to diminishing of CO concentration as shown in the suggested reactions pathway, as a result, NO2 formation is relatively enhanced. Therefore, NO2 formation is
likely also a result of above competition of reactions.
4. Conclusions The following conclusions have been drawn from this work: (1) Titania with uniformed pore size distribution concentrating at about 4 nm could be obtained by using sol–gel method, which consists of many primary spherical particles with a size ranging 0.1–0.3 mm and secondary spherical particles with a size of about 2 mm. (2) The consecutive impregnation method with a reduction treatment between the impregnation steps as reported herein is a useful method to obtain small metal Pt crystals dispersed on the support. On the reduced 3% Pt/TiO2 catalyst prepared by the consecutive impregnation method, 99.7% of metallic Pt crystals with a size less than 2 nm are located on both the surface and inner pores of the support. (3) Interaction between metallic Pt and titania support in the Pt/TiO2 catalysts is very week before and after metal leading, which is probably due to the relatively low reduction temperature. (4) One of the new contributions is the finding of the selectivity of NO reduction over TiO2 in the oxygen-rich condition being insensitive to the variation of textural properties. (5) Although both the titania support and Pt/TiO2-SG catalysts present similar basicity, however, they show very different reaction activity and selectivity for NO reduction. A great improvement of catalytic activity of NO reduction together with high selectivity to N2 on the Pt loaded titania is obtained in comparison to bare titania support, which is explained by assumption different reaction mechanisms.
Acknowledgements Dr. J. Navarrete Bolan˜ os is gratefully acknowledged for fruitful discussion. The authors also thank the Mexican Institute of Petroleum and National Poly-
J.A. Wang et al. / Applied Surface Science 230 (2004) 94–105
technic Institute for financial support (Grant No. FIES-98-29-III and CGPI-IPN-20021187). References [1] J. Pasel, V. Speer, C. Albrecht, F. Richter, H. Papp, Appl. Catal. B: Enveron. 25 (2000) 105. [2] A. Dandclear, M.A. Vannice, Appl. Catal. B: Environ. 22 (1999) 179. [3] A. Crucq (Ed.), Catalysis and automotive pollution control II, Study of Surface Science and Catalysis, vol. 71, Elseiver, Amsterdam, 1991. [4] A. Martı´nez, M. Ferna´ ndez-Garcı´a, A. Iglesias-Juez, A.B. Hungrı´a, J. A: Anderson, J.C. Conesa, J. Soria, Appl. Catal. B: Environ. 31 (2001) 51. [5] M. Ferna´ ndez-Garcı´a, A. Marrı´nez, A. Iglesias-Juez, A.B. Hungrı´a, J. A: Anderson, J.C. Conesa, J. Soria, Appl. Catal. B: Environ. 31 (2001) 39. [6] D. Ciuparu, A. Bensalem, L. Pfefferle, Appl. Catal. B: Environ. 26 (2000) 241. [7] S.E. Golunski, H.A. Hatcher, R.R. Rajaram, K. Jansson, L. Truex, SAE Technical Paper Series, No. 950408, 1995. [8] U.S. Ozkan, M.W. Kumthekar, G. Karakas, Catal. Today 40 (1998) 3. [9] A. Ali, W. Alvarez, C.J. Loughran, Appl. Catal. B: Environ. 14 (1997) 13. [10] M. Thammachat, V. Meeyoo, T. Risksomboon, S. Osuwan, Catal. Today 68 (2000) 53. [11] G.W. Graham, H.W. Jen, R.W. McCabe, A.M. Straccia, L.P. Hack, Catal. Lett. 67 (2000) 99. [12] H. Muraki, G. Zhang, Catal. Today 63 (2000) 337. [13] S. Matsuda, A. Kato, Appl. Catal. 8 (1983) 149. [14] H. Bosch, F. Jansen, Catal. Today 2 (1988) 369. [15] M.A. Vanince, J. Catal. 74 (1982) 199. [16] M.W. Kumthekar, U.S. Ozkan, J. Catal. 171 (1997) 45.
105
[17] M.W. Kumthekar, U.S. Ozkan, J. Catal. 171 (1997) 54. [18] U.S. Ozkan, M.W. Kumthekar, G. Karkas, J. Catal. 171 (1997) 67. [19] G. Martra, Appl. Catal. 200 (2000) 275–285. [20] X. Bokhimi, A. Morales, O. Novaro, T. Lopez, E. Sa´ nchez, R. Gomez, J. Mater. Res. 10 (1995) 2788. [21] M. Mora´ n-Pineda, S. Castillo, R. Go´ mez, React. Kinet. Catal. Lett. 76 (2002) 375. [22] S. Castillo, R. Go´ mez, M. Mora´ n-Pineda, React. Kinet. Catal. Lett. 79 (2003) 271. [23] S. Castillo, M. Mora´ n-Pineda, R. Go´ mez, T. Lo´ pez, J. Catal. 172 (1997) 263. [24] S. Castillo, M. Mora´ n-Pineda, V. Molina, R. Go´ mez, T. Lo´ pez, Appl. Catal. B: Environ. 15 (1998) 203. [25] J.A. Horsley, J. Am. Chem. Soc. 101 (1979) 2870. [26] S.J. Tauster, S.C. Fung, R.L. Garten, J. Am. Chem. Soc. 100 (1978) 70. [27] J.C. Conesa, J. Sorla, J. Phys. Chem. 86 (1982) 1392. [28] S. Bernal, J.J. Calvino, M.A. Cauqui, J.M. Gatica, C. Lo´ pez Cartes, J.A. Pe´ rez Omil, J.M. Pintado, Catal. Today 77 (2003) 385. [29] S. Bernal, F.J. Botana, J.J. Calvino, C. Lo´ pez, J.A. Pe´ rez Omil, J.M. Rodrı´guez-Izquierdo, J. Chem. Soc., Faraday Trans. 92 (1996) 2799. [30] N. Nava Entzana, Doctor Thesis, Unversidad Autonoma Metropolitana-Iztapalapa, 2002, p. 96. [31] S.B. Schwartz, G.B. Fisher, L.D. Schmidt, J. Phys. Chem. 92 (1988) 389. [32] R.L. Klein, S. Schwartz, L.D. Schmidt, J. Phys. Chem. 89 (1985) 4908. [33] M. Shelef, G.W. Graham, Catal. Rev.-Sci. Eng. 36 (1994) 433. [34] S.H. Oh, C.C. Eickel, J. Catal. 128 (1991) 526. [35] G.B. Fisher, S.H. Oh, C.L. DiMaggio, S.J. Schmieg, D.W. Goodman, C.H.F. Peden, in: M.J. Phillips, M. Ternan (Eds.), Proceedings of the 9th International Congress on Catalysis, vol. 3, Chemical Institute of Canada, Ottawa, 1988, p. 1355.