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TiO2 for NO reduction with CH4
Catalysis Communications 3 (2002) 199–206 www.elsevier.com/locate/catcom
In situ DRIFTS characterization of wet-impregnated and sol–gel Pd=TiO2 for NO reduction with CH4 Gurkan Karakas 1, Junko Mitome-Watson, Umit S. Ozkan
*
Department of Chemical Engineering, The Ohio State University, 140 W. 19th Ave., Columbus, OH 43210, USA Received 30 January 2002; accepted 25 March 2002
Abstract The adsorption/desorption behavior of 2%Pd/TiO2 catalysts synthesized by wet-impregnation and modified sol–gel techniques were examined in NO–CH4 –O2 reaction using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The catalyst prepared by the modified sol–gel method showed significantly higher resistance toward oxygen while maintaining a 100% NO conversion. Under NO þ CH4 þ O2 flow, the main adsorbed NO species was identified as the linearly adsorbed NO on metallic palladium (Pd0 –NO) at high temperatures. On the oxidized sample, the major NO species was found to be a bridged nitrate species. Pd0 –NO species was suggested to react with CHx to form surface NHx species. Over the sol–gel catalyst, the peak intensity for Pd0 –NO species was much higher than that over the impregnated catalyst. The DRIFTS data seem to suggest that this species could play a role in reduction of NO with CH4 over Pd=TiO2 catalysts. Ó 2002 Published by Elsevier Science B.V.
1. Introduction NO reduction with methane in the presence of oxygen has been widely investigated following the pioneering work by Li and Armor, who used metal exchanged zeolitic catalysts [1–3]. Numerous studies using different zeolitic systems have been investigated including Pd–H–ZSM-5, Pd–Ce-H–ZSM-5 and H-zeolite [4–6]. NO decomposition and reduction with methane on non-zeolitic catalysts (La2 O3 , * Corresponding author. Tel.: +1-614-292-6623; fax: +1-614292-3769. E-mail address: [email protected] (U.S. Ozkan). 1 Present address: Department of Chemical Engineering, Middle East Technical University, Inonu Bulvari, Ankara 06531, Turkey.
CeO2 , Nd2 O3 , Sm2 O3 , Tm2 O3 , Y2 O3 , Sc2 O3 and Lu2 O3 ) [7–9] and precious metals supported on silica and alumina [10,11] have also been investigated. We have reported use of Pd-based catalysts supported on titania for the NO reduction with methane in the presence of oxygen [12–18]. The extent of the reaction was found to be highly dependent on the oxidation state of palladium and the metallic phase of palladium was necessary for the reduction of NO to N2 [12–16]. We have also shown that the Pd=TiO2 catalyst prepared by the modified sol–gel method was more tolerant to oxygen deactivation than the catalyst prepared by wet-impregnation technique [18]. Using in situ FTIR spectroscopy several reaction mechanism have been proposed in the literature for the selective catalytic reduction of NOx
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with lower alkanes. These mechanisms include: (i) adsorption of NO on the surface as ðNO2 Þads , and the subsequent reaction of ðNO2 Þads with lower alkanes [19–21]; (ii) the lower alkane forming an intermediate with ðNO2 Þads (e.g. isocyanites, oximes, etc.) followed by the oxidative conversion of the intermediate into N2 , O2 , CO, CO2 and H2 O [22–26]. The first mechanism is typical for the selective catalytic reduction of NO with methane while the latter is more common in the reduction with C2þ hydrocarbons. However, the nature of the reaction intermediate that is formed with methane and ðNOx Þads species is still not clear. In this paper, we present the characterization of 2%Pd/TiO2 catalysts that were prepared by two different methods, namely, wet-impregnation and modified sol–gel methods. The surface adspecies under reaction conditions were studied using DRIFTS to obtain clues that could help to explain the different catalytic performances observed for NO–CH4 –O2 reaction in the two synthesis methods.
generated using a copper anode, thus producing ). The samples were Ka radiation (k ¼ 1:5432 A scanned from a 2h value of 20° to 60° at a rate of 1° per minute. DRIFTS experiments were performed using a Bruker IFS66 equipped with a DTGS detector and a KBr beamsplitter. For adsorption spectra obtained at several different temperature levels, background spectra were taken at each temperature level under helium prior to the introduction of adsorption gas. Each spectrum was averaged over 1000 scans in the mid-IR range ð400–4000 cm1 Þ to a nominal 2 cm1 resolution.
3. Results 3.1. X-ray diffraction Fig. 1(a) shows the XRD pattern on oxidized 2%Pd/TiO2 prepared by the wet-impregnation technique. The major features exhibited on this
2. Methods 2.1. Catalyst preparation Catalysts were synthesized using wet-impregnation and modified sol–gel method as described elsewhere [18]. The metal loading of the catalysts were 2%Pd/TiO2 by weight. The BET surface area measurement by N2 adsorption resulted in the specific surface areas of 19 and 80 m2 /g for wetimpregnated and modified sol–gel catalysts, respectively. All catalysts were calcined at 500 °C under oxygen following the synthesis procedure. Some catalysts were reduced in situ prior to a reaction or characterization experiment. The term ‘‘reduced catalyst’’ refers to samples which have gone through this pre-reduction step, ‘‘oxidized catalyst’’ refers to samples which have not undergone any pre-reduction. 2.2. Catalyst characterization The diffractometer used in this study was a Scintag PAD-V diffractometer. The X-rays were
Fig. 1. X-ray diffraction patters of oxidized (a) 2%Pd/TiO2 prepared by wet-impregnation technique, (b) TiO2 prepared by sol–gel, (c) 2%Pd/TiO2 prepared by sol–gel method.
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XRD pattern corresponded to the anatase form of TiO2 . There was also a weak signal from (1 0 1) reflection of PdO observed at a d spacing of 2.644 . Fig. 1(b) shows the XRD pattern obtained over A a bare TiO2 support synthesized by the modified sol–gel method without the impregnation of Pd. The XRD pattern of the sol–gel TiO2 indicates that its crystallinity is very similar to that for wetimpregnated Pd/TiO2 except for negligible broadening of the peaks. The XRD pattern on oxidized 2%Pd/TiO2 synthesized by the modified sol–gel method is shown in Fig. 1(c). We observed a much more pronounced broadening of the XRD features for the anatase on the sol–gel Pd/TiO2 suggesting the bulk TiO2 to consist of smaller particles and possibly to contain poorly crystallized or amorphous titania domains. The PdO peak intensity
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was much smaller compared to that over wet-impregnated Pd/TiO2 , indicating that palladium is more dispersed on the sol–gel sample. 3.2. In situ DRIFTS under NO þ CH4 þ O2 flow 3.2.1. Wet-impregnated Pd=TiO2 Surface adspecies formed during NO þ CH4 þ O2 flow were investigated over the in situ reduced catalyst samples. The background spectra were obtained at every temperature level under He flow. Under flow of reaction mixture NO (1780 ppm), CH4 (2.13%), and O2 (1000 ppm) in balance He, DRIFT spectra were taken at different temperatures as shown in Fig. 2. The catalyst was first reduced in 33% H2 in He at 200 °C for 30 min and flushed in helium before exposing it to the reaction
Fig. 2. DRIFT spectra taken under NO þ CH4 þ O2 (1780 ppm NO, 2.13% CH4 , 1000 ppm O2 ) on reduced 2%Pd/TiO2 (wet-impregnation) at: (a) 27 °C, (b) 67 °C, (c) 112 °C, (d) 154 °C, (e) 197 °C, (f) 230 °C, (g) 270 °C.
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mixture. The gas phase CH4 has characteristic IR bands at 3010 and 1301 cm1 . There are two possible assignments for the peak formed at 1337 cm1 . The first possibility is that this band is due to the C–H bending vibrations. The fact that this is IR-inactive in the gas phase would suggest that this band arises from methane dissociatively adsorbed on the catalysts surface [27]. Although they were not observed under flow conditions, DRIFTS experiments performed under vacuum following NO þ CH4 adsorption at room temperature showed two bands at 2868 and 2960 cm1 , which could be assigned to surface CH and CH2 stretching vibrations [28]. This observation confirms the presence of methyl-type species, and supports the assignment of the 1337 cm1 band to C–H bending vibrations on the surface. One cannot, however, rule out other possibilities such as the assignment of this band to the nitrate species which show characteristic bands at 1438 and 1337 cm1 on TiO2 [29,30]. The IR band observed at 1775 cm1 was attributed to linearly adsorbed NO on Pd [11,31]. As the surface temperature was increased, the peak intensities of linearly adsorbed NO on Pd were decreased where there was a development of an IR band at 1666 cm1 that might be attributed to the bended form of adsorbed NO on Pd. The bended NO species on Pd0 are more stable adspecies than the linearly adsorbed NO. Moriki et al. [31] suggested that there are three possible types of adspecies of NO on Pd in the form of linear bended and bridged structure. According to their study, bridged form of NO adspecies are converted into bent adspecies and linearly adsorbed adspecies progressively with increasing surface coverage or the concentration of NO in the gas phase. The fact that 1775 cm1 band decreases with temperature while the 1666 cm1 increases could be explained by Moriki’s model since higher temperatures would lead to NO desorption and hence decrease the surface coverage. This, in turn, would allow more of the bent NO species to form. The broad band observed in 1370–1554 cm1 is a characteristic feature of free nitrates [29]. Due to numerous peaks overlapping with each other, it is difficult to identify specific types of nitrate species.
Also, there were two peaks at 2362 and 2335 cm1 corresponding to gas phase CO2 above 197 °C. The negative CO2 peaks are a result of residual CO2 in the background spectrum being subtracted from the sample spectrum under reaction flow below 154 °C, at which product CO2 concentration was lower than the background CO2 concentration. Fig. 3 represent DRIFT spectra taken under flow conditions similar to those of Fig. 2. The only difference was that the oxygen concentration used was 10 times higher. At lower temperatures, the relative band intensity of the nitrate species to linearly adsorbed NO was higher compared to that in Fig. 2. This seems to indicate that the presence of higher O2 concentration enhances NO adsorption as nitrate species rather than linear NO species on Pd. However, the spectra taken at higher temperatures were very similar to those taken under lower O2 concentration (1000 ppm). 3.2.2. Sol–gel Pd/TiO2 DRIFT spectra similar to Fig. 2 were taken for reduced sol–gel catalysts under a mixture, NO (1780 ppm), CH4 (2.13%), and O2 (1000 ppm) in balance He (Fig. 4(a)). When Figs. 2 and 4(a) are compared, a significant difference in types of surface species is noticed. There were well-resolved bands in the nitrate region on the sol–gel Pd/TiO2 . With increasing temperature, the peaks at 1513 and 1154 cm1 that were assigned to bridged NO on Pd decreased in intensity [29]. As they disappeared, peaks at 1600, 1561 and 1538 cm1 grew larger. The band at 1600 cm1 was assigned to bridged bidentate with associated peak at 1240 cm1 [29,30]. The band at 1561 cm1 was assigned to chelated bidentate [30]. Both bridged and chelated bidentate nitrate species were seen to desorb at lower temperatures than the monodentate nitrate species, which are represented by the band at 1537 cm1 with associated peak at 1232 cm1 [32,33]. The IR band for linearly adsorbed NO on Pd grew with temperature also and it was the most stable species at higher temperatures. The behavior of surface species on the sol–gel catalyst was quite different from the wet-impregnated catalyst under the reaction conditions. It is possible that although the composition of catalyst is the same, the surface
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Fig. 3. DRIFT spectra taken under NO þ CH4 þ O2 (1780 ppm NO, 2.13% CH4 , 10,000 ppm O2 ) on reduced 2%Pd/TiO2 (wet-impregnation) at (a) 27 °C, (b) 67 °C, (c) 112 °C, (d) 154 °C, (e) 197 °C, (f) 230 °C, (g) 270 °C.
reaction is quite different under the same reaction conditions. Similar DRIFT spectra were taken on the oxidized sol–gel sample as shown in Fig. 4(b). In this case the bridged nitrate species (1600 and 1243 cm1 ) are already formed even at room temperature and stable up to 230 °C. The band for bidentate nitrate increased in intensity significantly with increasing temperature. When Figs. 4(a) and (b) are compared, it is clear that the formation of monodentate nitrate species is preferred over the reduced sample and bridged nitrate on the oxidized sample because there is more surface oxygen available for bridge formation. Also, there was a significantly less linearly adsorbed NO present on the oxidized surface compared to the reduced
surface. This indicates that the linear NO is preferentially adsorbed on Pd in zero oxidation state or partially reduced state. Even on the catalyst which was initially oxidized, under the reaction conditions at higher temperatures, the surface becomes partially reduced; therefore, NO can adsorb on Pd linearly. 3.3. In situ DRIFTS in NO ! NO þ CH4 ! NO þ CH4 þ O2 sequential adsorption experiments DRIFT spectra were taken over reduced and oxidized sol–gel Pd/TiO2 catalysts following exposure to NO, NO þ CH4 and NO þ CH4 þ O2 (low O2 concentration) NO þ CH4 þ O2 (high O2 concentration) flow in sequence. The concentra-
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Fig. 4. DRIFT spectra taken under NO þ CH4 þ O2 (1780 ppm NO, 2.13% CH4 , 1000 ppm O2 ) on 2%Pd/TiO2 prepared by modified sol–gel: (a) reduced, (b) oxidized.
tions were NO ¼ 1780 ppm, CH4 ¼ 2:13%, O2 ¼ 1000 and 50,000 ppm, respectively. Spectra were taken after the catalysts were exposed to each gas mixture for 30 min at 300 °C surface temperature. The spectrum taken under NO flow only on reduced sol–gel Pd/TiO2 (Fig. 5(a)) shows the dominant surface species to be the linearly adsorbed NO on Pd (1794 cm1 ). Other species observed include bidentate and monodentate nitrate on the support (1578 and 1537 cm1 ). When CH4 was introduced to the NO flow, the linearly adsorbed NO species disappeared rapidly and the formation of a multiplet in the N–H stretching region ð3400–3200 cm1 Þ was observed. Also, a characteristic band for the symmetric deformation of NH3 coordinated on Lewis acid sites was
Fig. 5. DRIFT spectra taken on 2%Pd=TiO2 prepared by sol– gel method: (a) reduced, (b) oxidized.
observed at 1184 cm1 [34]. Disappearance of the linear NO on Pd coincides with the introduction of the CH4 to the gas phase and the appearance of the NHx species. While this may suggest that a reaction of linear NO and CH4 could be contributing to the formation of NH3 , it is also conceivable that NH3 could be reacting with linearly adsorbed NO. When 1000 ppm O2 was introduced to the NO þ CH4 flow, there were no significant changes observed on the surface (Fig. 5(a)). The formation of CO2 was enhanced due to the presence of O2 , as expected. In the presence of 5% O2 , the bands for NHx species disappeared and the linearly adsorbed NO on Pd returned. It should be noted that even in the presence of 5% O2 , the gas phase was never completely depleted of methane.
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Fig. 5(b) shows the spectra taken under the same flow over freshly calcined Pd/TiO2 . Over this catalyst, we did not observe the formation of NHx species as we did over the reduced catalyst. Also, the band that corresponded to the linearly adsorbed NO on Pd (1794 cm1 ) was much lower in intensity; however, it did not disappear when CH4 was introduced.
4. Discussion Previously, we have shown that Pd/TiO2 catalyst prepared by wet-impregnation technique showed a lower resistance to deactivation through oxidation during NO þ CH4 þ O2 reaction while the catalyst with the same composition prepared by the modified sol–gel method showed significant improvement in the resistance to oxidation while maintaining a 100% NO conversion to N2 with high selectivity (>96%) [17,18]. The modified sol–gel method offers better control of catalyst properties such as particle size, surface area and pore size distribution. The surface area of the sol–gel Pd/ TiO2 was improved by 4-folds. The XRD patterns indicated that palladium was more dispersed in the sol–gel sample compared to the wet-impregnated sample. It is also conceivable that the ‘‘anchoring’’ of the Pd species achieved through the sol–gel technique helps to keep them in the zero oxidation state under more oxidizing environments. In order to gain insight for different activities observed over the two catalysts, in situ DRIFT spectra were taken under reaction conditions. Over both catalysts, there were several NO adspecies such as linearly adsorbed NO, bidentate nitrate, monodentate nitrate and bridged nitrate species. At higher temperatures, the dominant species under the flow of NO–CH4 –O2 on the better performing catalyst (sol–gel) was the linearly adsorbed NO on Pd. This leads us to suggest that this species plays an important role during NO–CH4 – O2 reaction. Lobree et al. [5] investigated NO reduction by methane on Pd–H–ZSM-5 catalyst using in situ IR spectroscopy. They observed a band at 1881 cm1 after NO adsorption on the catalyst and assigned this band to NO adsorbed on Pd cations adjacent to two framework Al atoms.
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Kikuchi and co-workers [35] observed a peak at 1910 cm1 under the reaction conditions of 1000 ppm NO, 1000 ppm CH4 and 10% O2 . They assigned this band to be NO adsorbed on Pd2þ . In our study, the difference in adsorption behavior between the reduced and oxidized catalysts in Figs. 4(a) and (b) clearly shows the linear NO species is formed more preferentially on metallic or partially reduced Pd. Hoost et al. [11] observed similar results during NO adsorption on reduced and oxidized 2%Pd/Al2 O3 samples. They suggested that oxidation of Pd to PdO might result in steric hindrances of NO adsorption sites on Pd. The XRD results and the NO adsorption in DRIFTS suggest that there is more Pd in zero oxidation state available for NO adsorption on the sol–gel Pd/ TiO2 compared to the wet-impregnated catalyst under reaction conditions. Another difference between the two catalysts was that there were more of the nitrate species, such as bidentate and monodentate nitrate, observed over the sol–gel Pd/TiO2 compared to wetimpregnated Pd/TiO2 . As mentioned earlier, the dominant species at higher temperatures was the linearly adsorbed NO on Pd and nitrate species seemed to diminish with temperature. Ramis et al. studied the nitrate formation on TiO2 . They reported that the formation of nitrates takes place through the oxidation of NO over Ti4þ centers to NO2 followed by the oxidation of NO2 to NO 3 over Ti4þ sites [34]. The fact that nitrate species can be formed easily at room temperature and decompose at higher temperatures could suggest that they act as temporary NO storage sites that could release NO and allow it to adsorb on Pd sites as they get reduced by CH4 . In the sequential adsorption studies over the reduced and oxidized sol–gel catalysts, we saw a very interesting phenomenon regarding the formation of NHx species. Over the pre-reduced catalyst, linearly adsorbed NO species completely disappeared upon CH4 introduction and formation of adsorbed NH3 on Lewis acid sites took place. However, when the catalyst was fully oxidized, the linearly adsorbed NO remained on the surface and NH3 could not be formed. This seems to show that methane first gets activated on Pd0 sites and subsequently react with linear NO to
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form NH3 . But this is not possible when the catalyst is oxidized. Previously, using controlled-atmosphere XPS technique, we have reported that a Pd 3d X-ray photoelectron spectrum of Pd/TiO2 (sol–gel), over which our standard reduction step was applied, showed palladium to be mostly in zero oxidation state [18]. Moreover, the post-reaction characterization of the catalysts that remained active throughout the reaction showed Pd to be mostly in metallic state, while the catalysts that were deactivated during the course of the reaction showed Pd to be in +2 oxidation state. In investigating the oscillatory behavior observed in Pd phase by in situ XRD and in reaction products by mass spectrometry during NO þ CH4 þ O2 reaction over Pd/TiO2 , it was shown that NO reduction was favored on metallic Pd whereas CH4 combustion leading to CO2 and CO formation was favored on Pd oxide [16]. It was proposed that the activation of methane through dissociation takes place on Pd in zero oxidation state. Once CHx species are formed, it is possible that linear NO and CHx species react together to form NH3 . In a subsequent paper, we will present DRIFTS results combined with temperature-programmed desorption to provide further clues to the adsorption behavior and surface intermediates during reaction [28].
Acknowledgements The financial contributions from the National Science Foundation and the Ohio Coal Development Office are gratefully acknowledged.
References [1] [2] [3] [4] [5]
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In situ DRIFTS characterization of wet-impregnated and sol–gel Pd=TiO2 for NO reduction with CH4 Gurkan Karakas 1, Junko Mitome-Watson, Umit S. Ozkan
*
Department of Chemical Engineering, The Ohio State University, 140 W. 19th Ave., Columbus, OH 43210, USA Received 30 January 2002; accepted 25 March 2002
Abstract The adsorption/desorption behavior of 2%Pd/TiO2 catalysts synthesized by wet-impregnation and modified sol–gel techniques were examined in NO–CH4 –O2 reaction using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The catalyst prepared by the modified sol–gel method showed significantly higher resistance toward oxygen while maintaining a 100% NO conversion. Under NO þ CH4 þ O2 flow, the main adsorbed NO species was identified as the linearly adsorbed NO on metallic palladium (Pd0 –NO) at high temperatures. On the oxidized sample, the major NO species was found to be a bridged nitrate species. Pd0 –NO species was suggested to react with CHx to form surface NHx species. Over the sol–gel catalyst, the peak intensity for Pd0 –NO species was much higher than that over the impregnated catalyst. The DRIFTS data seem to suggest that this species could play a role in reduction of NO with CH4 over Pd=TiO2 catalysts. Ó 2002 Published by Elsevier Science B.V.
1. Introduction NO reduction with methane in the presence of oxygen has been widely investigated following the pioneering work by Li and Armor, who used metal exchanged zeolitic catalysts [1–3]. Numerous studies using different zeolitic systems have been investigated including Pd–H–ZSM-5, Pd–Ce-H–ZSM-5 and H-zeolite [4–6]. NO decomposition and reduction with methane on non-zeolitic catalysts (La2 O3 , * Corresponding author. Tel.: +1-614-292-6623; fax: +1-614292-3769. E-mail address: [email protected] (U.S. Ozkan). 1 Present address: Department of Chemical Engineering, Middle East Technical University, Inonu Bulvari, Ankara 06531, Turkey.
CeO2 , Nd2 O3 , Sm2 O3 , Tm2 O3 , Y2 O3 , Sc2 O3 and Lu2 O3 ) [7–9] and precious metals supported on silica and alumina [10,11] have also been investigated. We have reported use of Pd-based catalysts supported on titania for the NO reduction with methane in the presence of oxygen [12–18]. The extent of the reaction was found to be highly dependent on the oxidation state of palladium and the metallic phase of palladium was necessary for the reduction of NO to N2 [12–16]. We have also shown that the Pd=TiO2 catalyst prepared by the modified sol–gel method was more tolerant to oxygen deactivation than the catalyst prepared by wet-impregnation technique [18]. Using in situ FTIR spectroscopy several reaction mechanism have been proposed in the literature for the selective catalytic reduction of NOx
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with lower alkanes. These mechanisms include: (i) adsorption of NO on the surface as ðNO2 Þads , and the subsequent reaction of ðNO2 Þads with lower alkanes [19–21]; (ii) the lower alkane forming an intermediate with ðNO2 Þads (e.g. isocyanites, oximes, etc.) followed by the oxidative conversion of the intermediate into N2 , O2 , CO, CO2 and H2 O [22–26]. The first mechanism is typical for the selective catalytic reduction of NO with methane while the latter is more common in the reduction with C2þ hydrocarbons. However, the nature of the reaction intermediate that is formed with methane and ðNOx Þads species is still not clear. In this paper, we present the characterization of 2%Pd/TiO2 catalysts that were prepared by two different methods, namely, wet-impregnation and modified sol–gel methods. The surface adspecies under reaction conditions were studied using DRIFTS to obtain clues that could help to explain the different catalytic performances observed for NO–CH4 –O2 reaction in the two synthesis methods.
generated using a copper anode, thus producing ). The samples were Ka radiation (k ¼ 1:5432 A scanned from a 2h value of 20° to 60° at a rate of 1° per minute. DRIFTS experiments were performed using a Bruker IFS66 equipped with a DTGS detector and a KBr beamsplitter. For adsorption spectra obtained at several different temperature levels, background spectra were taken at each temperature level under helium prior to the introduction of adsorption gas. Each spectrum was averaged over 1000 scans in the mid-IR range ð400–4000 cm1 Þ to a nominal 2 cm1 resolution.
3. Results 3.1. X-ray diffraction Fig. 1(a) shows the XRD pattern on oxidized 2%Pd/TiO2 prepared by the wet-impregnation technique. The major features exhibited on this
2. Methods 2.1. Catalyst preparation Catalysts were synthesized using wet-impregnation and modified sol–gel method as described elsewhere [18]. The metal loading of the catalysts were 2%Pd/TiO2 by weight. The BET surface area measurement by N2 adsorption resulted in the specific surface areas of 19 and 80 m2 /g for wetimpregnated and modified sol–gel catalysts, respectively. All catalysts were calcined at 500 °C under oxygen following the synthesis procedure. Some catalysts were reduced in situ prior to a reaction or characterization experiment. The term ‘‘reduced catalyst’’ refers to samples which have gone through this pre-reduction step, ‘‘oxidized catalyst’’ refers to samples which have not undergone any pre-reduction. 2.2. Catalyst characterization The diffractometer used in this study was a Scintag PAD-V diffractometer. The X-rays were
Fig. 1. X-ray diffraction patters of oxidized (a) 2%Pd/TiO2 prepared by wet-impregnation technique, (b) TiO2 prepared by sol–gel, (c) 2%Pd/TiO2 prepared by sol–gel method.
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XRD pattern corresponded to the anatase form of TiO2 . There was also a weak signal from (1 0 1) reflection of PdO observed at a d spacing of 2.644 . Fig. 1(b) shows the XRD pattern obtained over A a bare TiO2 support synthesized by the modified sol–gel method without the impregnation of Pd. The XRD pattern of the sol–gel TiO2 indicates that its crystallinity is very similar to that for wetimpregnated Pd/TiO2 except for negligible broadening of the peaks. The XRD pattern on oxidized 2%Pd/TiO2 synthesized by the modified sol–gel method is shown in Fig. 1(c). We observed a much more pronounced broadening of the XRD features for the anatase on the sol–gel Pd/TiO2 suggesting the bulk TiO2 to consist of smaller particles and possibly to contain poorly crystallized or amorphous titania domains. The PdO peak intensity
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was much smaller compared to that over wet-impregnated Pd/TiO2 , indicating that palladium is more dispersed on the sol–gel sample. 3.2. In situ DRIFTS under NO þ CH4 þ O2 flow 3.2.1. Wet-impregnated Pd=TiO2 Surface adspecies formed during NO þ CH4 þ O2 flow were investigated over the in situ reduced catalyst samples. The background spectra were obtained at every temperature level under He flow. Under flow of reaction mixture NO (1780 ppm), CH4 (2.13%), and O2 (1000 ppm) in balance He, DRIFT spectra were taken at different temperatures as shown in Fig. 2. The catalyst was first reduced in 33% H2 in He at 200 °C for 30 min and flushed in helium before exposing it to the reaction
Fig. 2. DRIFT spectra taken under NO þ CH4 þ O2 (1780 ppm NO, 2.13% CH4 , 1000 ppm O2 ) on reduced 2%Pd/TiO2 (wet-impregnation) at: (a) 27 °C, (b) 67 °C, (c) 112 °C, (d) 154 °C, (e) 197 °C, (f) 230 °C, (g) 270 °C.
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mixture. The gas phase CH4 has characteristic IR bands at 3010 and 1301 cm1 . There are two possible assignments for the peak formed at 1337 cm1 . The first possibility is that this band is due to the C–H bending vibrations. The fact that this is IR-inactive in the gas phase would suggest that this band arises from methane dissociatively adsorbed on the catalysts surface [27]. Although they were not observed under flow conditions, DRIFTS experiments performed under vacuum following NO þ CH4 adsorption at room temperature showed two bands at 2868 and 2960 cm1 , which could be assigned to surface CH and CH2 stretching vibrations [28]. This observation confirms the presence of methyl-type species, and supports the assignment of the 1337 cm1 band to C–H bending vibrations on the surface. One cannot, however, rule out other possibilities such as the assignment of this band to the nitrate species which show characteristic bands at 1438 and 1337 cm1 on TiO2 [29,30]. The IR band observed at 1775 cm1 was attributed to linearly adsorbed NO on Pd [11,31]. As the surface temperature was increased, the peak intensities of linearly adsorbed NO on Pd were decreased where there was a development of an IR band at 1666 cm1 that might be attributed to the bended form of adsorbed NO on Pd. The bended NO species on Pd0 are more stable adspecies than the linearly adsorbed NO. Moriki et al. [31] suggested that there are three possible types of adspecies of NO on Pd in the form of linear bended and bridged structure. According to their study, bridged form of NO adspecies are converted into bent adspecies and linearly adsorbed adspecies progressively with increasing surface coverage or the concentration of NO in the gas phase. The fact that 1775 cm1 band decreases with temperature while the 1666 cm1 increases could be explained by Moriki’s model since higher temperatures would lead to NO desorption and hence decrease the surface coverage. This, in turn, would allow more of the bent NO species to form. The broad band observed in 1370–1554 cm1 is a characteristic feature of free nitrates [29]. Due to numerous peaks overlapping with each other, it is difficult to identify specific types of nitrate species.
Also, there were two peaks at 2362 and 2335 cm1 corresponding to gas phase CO2 above 197 °C. The negative CO2 peaks are a result of residual CO2 in the background spectrum being subtracted from the sample spectrum under reaction flow below 154 °C, at which product CO2 concentration was lower than the background CO2 concentration. Fig. 3 represent DRIFT spectra taken under flow conditions similar to those of Fig. 2. The only difference was that the oxygen concentration used was 10 times higher. At lower temperatures, the relative band intensity of the nitrate species to linearly adsorbed NO was higher compared to that in Fig. 2. This seems to indicate that the presence of higher O2 concentration enhances NO adsorption as nitrate species rather than linear NO species on Pd. However, the spectra taken at higher temperatures were very similar to those taken under lower O2 concentration (1000 ppm). 3.2.2. Sol–gel Pd/TiO2 DRIFT spectra similar to Fig. 2 were taken for reduced sol–gel catalysts under a mixture, NO (1780 ppm), CH4 (2.13%), and O2 (1000 ppm) in balance He (Fig. 4(a)). When Figs. 2 and 4(a) are compared, a significant difference in types of surface species is noticed. There were well-resolved bands in the nitrate region on the sol–gel Pd/TiO2 . With increasing temperature, the peaks at 1513 and 1154 cm1 that were assigned to bridged NO on Pd decreased in intensity [29]. As they disappeared, peaks at 1600, 1561 and 1538 cm1 grew larger. The band at 1600 cm1 was assigned to bridged bidentate with associated peak at 1240 cm1 [29,30]. The band at 1561 cm1 was assigned to chelated bidentate [30]. Both bridged and chelated bidentate nitrate species were seen to desorb at lower temperatures than the monodentate nitrate species, which are represented by the band at 1537 cm1 with associated peak at 1232 cm1 [32,33]. The IR band for linearly adsorbed NO on Pd grew with temperature also and it was the most stable species at higher temperatures. The behavior of surface species on the sol–gel catalyst was quite different from the wet-impregnated catalyst under the reaction conditions. It is possible that although the composition of catalyst is the same, the surface
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Fig. 3. DRIFT spectra taken under NO þ CH4 þ O2 (1780 ppm NO, 2.13% CH4 , 10,000 ppm O2 ) on reduced 2%Pd/TiO2 (wet-impregnation) at (a) 27 °C, (b) 67 °C, (c) 112 °C, (d) 154 °C, (e) 197 °C, (f) 230 °C, (g) 270 °C.
reaction is quite different under the same reaction conditions. Similar DRIFT spectra were taken on the oxidized sol–gel sample as shown in Fig. 4(b). In this case the bridged nitrate species (1600 and 1243 cm1 ) are already formed even at room temperature and stable up to 230 °C. The band for bidentate nitrate increased in intensity significantly with increasing temperature. When Figs. 4(a) and (b) are compared, it is clear that the formation of monodentate nitrate species is preferred over the reduced sample and bridged nitrate on the oxidized sample because there is more surface oxygen available for bridge formation. Also, there was a significantly less linearly adsorbed NO present on the oxidized surface compared to the reduced
surface. This indicates that the linear NO is preferentially adsorbed on Pd in zero oxidation state or partially reduced state. Even on the catalyst which was initially oxidized, under the reaction conditions at higher temperatures, the surface becomes partially reduced; therefore, NO can adsorb on Pd linearly. 3.3. In situ DRIFTS in NO ! NO þ CH4 ! NO þ CH4 þ O2 sequential adsorption experiments DRIFT spectra were taken over reduced and oxidized sol–gel Pd/TiO2 catalysts following exposure to NO, NO þ CH4 and NO þ CH4 þ O2 (low O2 concentration) NO þ CH4 þ O2 (high O2 concentration) flow in sequence. The concentra-
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Fig. 4. DRIFT spectra taken under NO þ CH4 þ O2 (1780 ppm NO, 2.13% CH4 , 1000 ppm O2 ) on 2%Pd/TiO2 prepared by modified sol–gel: (a) reduced, (b) oxidized.
tions were NO ¼ 1780 ppm, CH4 ¼ 2:13%, O2 ¼ 1000 and 50,000 ppm, respectively. Spectra were taken after the catalysts were exposed to each gas mixture for 30 min at 300 °C surface temperature. The spectrum taken under NO flow only on reduced sol–gel Pd/TiO2 (Fig. 5(a)) shows the dominant surface species to be the linearly adsorbed NO on Pd (1794 cm1 ). Other species observed include bidentate and monodentate nitrate on the support (1578 and 1537 cm1 ). When CH4 was introduced to the NO flow, the linearly adsorbed NO species disappeared rapidly and the formation of a multiplet in the N–H stretching region ð3400–3200 cm1 Þ was observed. Also, a characteristic band for the symmetric deformation of NH3 coordinated on Lewis acid sites was
Fig. 5. DRIFT spectra taken on 2%Pd=TiO2 prepared by sol– gel method: (a) reduced, (b) oxidized.
observed at 1184 cm1 [34]. Disappearance of the linear NO on Pd coincides with the introduction of the CH4 to the gas phase and the appearance of the NHx species. While this may suggest that a reaction of linear NO and CH4 could be contributing to the formation of NH3 , it is also conceivable that NH3 could be reacting with linearly adsorbed NO. When 1000 ppm O2 was introduced to the NO þ CH4 flow, there were no significant changes observed on the surface (Fig. 5(a)). The formation of CO2 was enhanced due to the presence of O2 , as expected. In the presence of 5% O2 , the bands for NHx species disappeared and the linearly adsorbed NO on Pd returned. It should be noted that even in the presence of 5% O2 , the gas phase was never completely depleted of methane.
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Fig. 5(b) shows the spectra taken under the same flow over freshly calcined Pd/TiO2 . Over this catalyst, we did not observe the formation of NHx species as we did over the reduced catalyst. Also, the band that corresponded to the linearly adsorbed NO on Pd (1794 cm1 ) was much lower in intensity; however, it did not disappear when CH4 was introduced.
4. Discussion Previously, we have shown that Pd/TiO2 catalyst prepared by wet-impregnation technique showed a lower resistance to deactivation through oxidation during NO þ CH4 þ O2 reaction while the catalyst with the same composition prepared by the modified sol–gel method showed significant improvement in the resistance to oxidation while maintaining a 100% NO conversion to N2 with high selectivity (>96%) [17,18]. The modified sol–gel method offers better control of catalyst properties such as particle size, surface area and pore size distribution. The surface area of the sol–gel Pd/ TiO2 was improved by 4-folds. The XRD patterns indicated that palladium was more dispersed in the sol–gel sample compared to the wet-impregnated sample. It is also conceivable that the ‘‘anchoring’’ of the Pd species achieved through the sol–gel technique helps to keep them in the zero oxidation state under more oxidizing environments. In order to gain insight for different activities observed over the two catalysts, in situ DRIFT spectra were taken under reaction conditions. Over both catalysts, there were several NO adspecies such as linearly adsorbed NO, bidentate nitrate, monodentate nitrate and bridged nitrate species. At higher temperatures, the dominant species under the flow of NO–CH4 –O2 on the better performing catalyst (sol–gel) was the linearly adsorbed NO on Pd. This leads us to suggest that this species plays an important role during NO–CH4 – O2 reaction. Lobree et al. [5] investigated NO reduction by methane on Pd–H–ZSM-5 catalyst using in situ IR spectroscopy. They observed a band at 1881 cm1 after NO adsorption on the catalyst and assigned this band to NO adsorbed on Pd cations adjacent to two framework Al atoms.
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Kikuchi and co-workers [35] observed a peak at 1910 cm1 under the reaction conditions of 1000 ppm NO, 1000 ppm CH4 and 10% O2 . They assigned this band to be NO adsorbed on Pd2þ . In our study, the difference in adsorption behavior between the reduced and oxidized catalysts in Figs. 4(a) and (b) clearly shows the linear NO species is formed more preferentially on metallic or partially reduced Pd. Hoost et al. [11] observed similar results during NO adsorption on reduced and oxidized 2%Pd/Al2 O3 samples. They suggested that oxidation of Pd to PdO might result in steric hindrances of NO adsorption sites on Pd. The XRD results and the NO adsorption in DRIFTS suggest that there is more Pd in zero oxidation state available for NO adsorption on the sol–gel Pd/ TiO2 compared to the wet-impregnated catalyst under reaction conditions. Another difference between the two catalysts was that there were more of the nitrate species, such as bidentate and monodentate nitrate, observed over the sol–gel Pd/TiO2 compared to wetimpregnated Pd/TiO2 . As mentioned earlier, the dominant species at higher temperatures was the linearly adsorbed NO on Pd and nitrate species seemed to diminish with temperature. Ramis et al. studied the nitrate formation on TiO2 . They reported that the formation of nitrates takes place through the oxidation of NO over Ti4þ centers to NO2 followed by the oxidation of NO2 to NO 3 over Ti4þ sites [34]. The fact that nitrate species can be formed easily at room temperature and decompose at higher temperatures could suggest that they act as temporary NO storage sites that could release NO and allow it to adsorb on Pd sites as they get reduced by CH4 . In the sequential adsorption studies over the reduced and oxidized sol–gel catalysts, we saw a very interesting phenomenon regarding the formation of NHx species. Over the pre-reduced catalyst, linearly adsorbed NO species completely disappeared upon CH4 introduction and formation of adsorbed NH3 on Lewis acid sites took place. However, when the catalyst was fully oxidized, the linearly adsorbed NO remained on the surface and NH3 could not be formed. This seems to show that methane first gets activated on Pd0 sites and subsequently react with linear NO to
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form NH3 . But this is not possible when the catalyst is oxidized. Previously, using controlled-atmosphere XPS technique, we have reported that a Pd 3d X-ray photoelectron spectrum of Pd/TiO2 (sol–gel), over which our standard reduction step was applied, showed palladium to be mostly in zero oxidation state [18]. Moreover, the post-reaction characterization of the catalysts that remained active throughout the reaction showed Pd to be mostly in metallic state, while the catalysts that were deactivated during the course of the reaction showed Pd to be in +2 oxidation state. In investigating the oscillatory behavior observed in Pd phase by in situ XRD and in reaction products by mass spectrometry during NO þ CH4 þ O2 reaction over Pd/TiO2 , it was shown that NO reduction was favored on metallic Pd whereas CH4 combustion leading to CO2 and CO formation was favored on Pd oxide [16]. It was proposed that the activation of methane through dissociation takes place on Pd in zero oxidation state. Once CHx species are formed, it is possible that linear NO and CHx species react together to form NH3 . In a subsequent paper, we will present DRIFTS results combined with temperature-programmed desorption to provide further clues to the adsorption behavior and surface intermediates during reaction [28].
Acknowledgements The financial contributions from the National Science Foundation and the Ohio Coal Development Office are gratefully acknowledged.
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