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Possible role of spillover processes in the operation of NOx storage and reduction catalysts.
Studies in Surface Science and Catalysis 138 A. Guerrero-Ruiz and I. Rodriguez-Ramos (Editors) 9 2001 Elsevier Science B.V. All rights reserved.
429
Possible role of spillover processes in the operation of NOx storage and reduction catalysts. Allan J. Paterson, Daniel J. Rosenberg and James A. Anderson Department of Chemistry, The University, Dundee, DD 1 4HN, Scotland, UK
A series of catalysts containing Pt-BaO-A1203, have been prepared and characterised. Baria content was varied between 1 and 10 wt%. The processes which might involve spillover and thus contribute to the storage of NOx were studied using in-situ DRIFT and simultaneous temperature programmed reaction of the stored NOx. Experiments performed with NO2 in place of NO and comparison of Pt containing with Pt-free samples were designed to show the role which is played by Pt in forming barium nitrate as the initial storage step. Even after NO2 is formed, and is adsorbed as a nitro species at exposed baria surfaces, the subsequent transformation to nitrate species does not proceed readily in the absence of Pt suggesting that spillover of activated oxygen and/or an activated form of NO2 may be required for the storage step and for the samples to behave as effective storage materials. Other differences between Pt and Pt-free sample observed during in-situ reaction, are the consequence of the Pt catalysed enhanced decomposition of barium carbonate, which otherwise acts as a poison for NO2 adsorption and subsequent storage. 1. INTRODUCTION Storage and reduction catalysts offer one possibility of controlling NOx emissions from automobile sources while allowing operation under predominantly lean-burn conditions [1]. Alkaline earth oxide components may be used to store NOx under lean conditions which is then released during intermittent rich/stoichiometric periods. The NOx released is then reduced by CO or HC over the noble metal component. Although certain mechanistic details of the processes involved are now emerging [2-4], much detail is still required. For example, it is uncertain to what extent the stored NOx is stable under warm-up conditions and to what extent spillover processes are involved in transferring NOx from the noble metal to the storage compound and vice versa. To address these aspects, combined FTIR-TPD studies were performed over Pt and Pt-free BaO/A1203 catalyst under different conditions using samples that have been previously exposed to NO2. 2. EXPERIMENTAL BaO/A1203 samples containing between 1 to 10 wt% BaO were prepared by precipitating the hydroxide from a barium nitrate solution using an ammonia solution onto 7-alumina (Degussa Aluminoxid C). The precipitate was filtered, washed and dried at 363 K for 16 h before being calcined in air (100 cm 3 rain -1) at 773 K (2 h). A fraction of each sample was
430 retained whilst the remainder was wet impregnated with 1 wt% Pt (H2PtC16). Excess solvent was removed by heating under continuous stirring and the resulting powder dried overnight at 363 K prior to calcination in air (100 cm 3 min -1) at 773 K (2 h). BET surface areas measurements were performed using Ar as adsorbate on samples outgassed at 573 K. Pt dispersion was measured using CO pulsed chemisorption on samples reduced in H2 at 573 K. Barium dispersion was determined from CO2 adsorption isotherms at 298 K on samples heated to 1013 K under N2. DRIFT spectra were recorded at 4 cm -~ resolution (100 scans) using an MCT detector and Harrick environmental cell. A computer-controlled gas blender fed the desired composition of reactant gases to the IR cell, with the exit gases passed to a chemiluminescence detector for NO/NOz/total NOx analysis. A quadropole mass spectrometer and 2.5 m path length gas-phase IR cell were also available for on-line product analysis. 80 mg of powdered sample was calcined in situ at 673 K (1 h) in the DRIFT cell in a flow of dry air (50 cm 3 min-l), and either exposed to NO2 at 673 K, or cooled to 298 K and exposed to NO2. Previous in-situ XRD studies [5] indicate that at 673 K in an air/NO2 flow, barium carbonate species undergo decomposition. The IR cell was then flushed in air before a TPD of NOx from the samples was performed under air or air/propene/N2 flows (50 cm 3 mini) between 298 to 873 K at 3 K min l. Spectra recorded in transmittance mode were obtained by heating samples in a quartz cell at the desired temperature followed by lowering the sample to the optical compartment, exposing the sample to the required adsorbate and measuring the spectra at beam temperature. NO2 (99.5 %) was used as supplied. 3. RESULTS AND DISCUSSION 3.1 Sample characterisation Table 1 Sample characteristics Sample A1203 1 % BaO/A1203 2.5 % BaO/A1203 5 % BaO/A1203 7.5 % BaO/A1203 10% BaO/AI203 Pt, 1 % BaO/A1203 Pt, 5 % BaO/AI203 Pt, 10 % BaO/AI203
BET (mZgl )
Pt Dispersion (%)
CO2:BaO Ratio
93.5 88.3 91.1 87.1 92.7 101.7 93.8 92.6 90.9
35.2 46.3 51.8
0.45 0.36 0.27 0.16 0.09 0.22 0.23 0.21
XRD patterns of as prepared samples, indicated (24 and 42 ~ 20) that crystalline Ba(CO3) was present at all baria loadings, with the l wt% sample showing the weakest peaks. BET surface areas (Table 1) were reasonably constant with little change following the addition of baria, or subsequent addition of Pt and further calcination. BaO dispersion showed an almost linear decrease as a function of loading. Platinum dispersion (assuming Pt:CO =1) unexpectedly increased as the BaO loading was increased, even though the fraction of surface covered by baria was greater at higher loadings leaving less of the alumina surface free for interaction with the Pt salt. The subsequent addition of Pt appeared to influence the dispersion
431 of baria with a homogenisation of the baria apparent. Previous studies [6] suggest that this results from the use of the acidic H2PtC16 solution which effectively redisperses the baria. To eliminate any possible differences in behaviour between Pt and Pt-free samples based on baria dispersion, results presented in this study were largely performed using 5%Ba/A1203 and Pt5%Ba/A1203 which present very similar Ba dispersions. 3.2 Are spillover processes involved in the formation of stored NOx?
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Fig. 1. TPD o f N O 2 desorbed in air from (a) A1203, (b) 5% B a O / A I 2 0 3 and (c) Pt-5% BaO/A1203 and (d) NO desorbed from Pt-5% BaO/A1203 after samples were previously exposed to NO2 at 298 K Fig.1 shows profiles of NOx desorbed from samples, which, according to XRD patterns may contain barium carbonates. FTIR spectra of the Pt-free samples show a band at c a . 1450 cm -1 prior to exposure to NO2 that may be attributed to such a species (Fig. 2a). However, exposure to NO2 at 298 K led to the appearance of a triplet of bands around 1600 c m -1 and another around 1300 c m l which compare with those observed by Parkyns following exposure of alumina alone to NO2 [7]. These bands are similarly attributed in the main to nitrate species on the alumina surface. The fact that the main contribution in IR spectra of NO2 on Ba/A1203 is due to species on the alumina support explains why TPD profiles are similar for A1203, Ba/A1203 and Pt/Ba/A1203 (Fig. 1) with maxima around 350 and 650 K. The only desorption peak unique to the baria containing samples appeared at 500 K, suggesting that exposed baria sites, even though in the main covered with carbonate species, were available for adsorption of NO2 after a 673 K calcination treatment.
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Fig. 2. FTIR of 5%BaO/A1203 (a) calcined in air at 673 K then (b) exposed to NO2 at 298 K followed by heating in air (50 cm 3 min l ) at (c) 345, (d) 440, (e) 530, (f) 605 and (g) 673 K.
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Fig. 3. FTIR spectra of Pt5%Ba/A1203 after (a) calcination in air at 673 K then (b) exposure to NO2 at 298 K and then heating in air at (c) 363, (d) 400, (e) 523, (f) 623 and (g) 673 K.
433 Consistent with spectra of samples in the absence of Pt, a sample of Pt5%Ba/A12Os after calcination at 673 K showed bands in the 1600-1200 cm -1 region (1558, 1469, 1351 and 1242 cm ~ indicating that this treatment was not effective in completely removing all traces of carbonates from the sample. This is consistent with the work of others concerning the formation of stable carbonates on NOx storage catalysts [9]. Exposure to NO2 generated similar species to those obtained in the absence of Pt with bands at ca. 1600 and 1300 cm -1 (Fig. 3b) mainly due to adsorption on the alumina support. However an additional feature, at 1447 cm l was present in the temperature range 298-523 K (Fig. 3 b-e) which was not detected for the 673 K calcined Ba/AI203 sample (Fig. 2) in the absence of Pt. To determine whether the presence of Pt was essential for the generation of this species, for example, by producing an activated form of NO2 which is spiltover to exposed baria sites, samples of 5%Ba/A1203 were calcined over a range of temperatures and then exposed to NO2. Fig 5 shows spectra of 5%Ba/A1203 outgassed at 1023 K and then exposed to NO2 at 298 K.
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1800
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Fig. 4. FTIR spectra of 5%Ba/A1203 outgassed at 1023 K then exposed to NO2 at 298 K at pressures up to 1.0 Torr. Spectra clearly show the growth as a function of NO2 pressure of a feature at 1445 cml (Fig. 4). It would appear that the ability to observe the form of adsorbed NO2 responsible for the band at ca 1447 cm -1 is a function of the extent to which carbonate and other species are removed from the baria surface. The presence of platinum (compare Figs 2 and 3) apparently plays a role in assisting removal of these surface contaminants. The above experiment was conducted in vacuum thus indicating that the presence of oxygen is not required for formation of such an adsorbed nitro species. Fridell et al. [2] and Mestl et al [10-12] have proposed that oxygen is essential, however, for the nitrate formation process and that the presence of oxygen was required to convert ligated nitro species to nitrate. Fridell et al. [2] suggest that Pt plays a role in this process by supplying dissociated oxygen atoms. Initial evidence from Fig 3 might suggest that this baria adsorbed NO2 species (1447 cm -1 band) may have been a requirement for the changes observed at higher temperatures (Fig. 3 f,g) when bands at 1413 and 1362 cm -1 became apparent. It is unclear as to whether the adsorbed NO2 species (1447 cm -1 band) was a precursor for this high temperature species.
434 However, a distinct shoulder (ca. 1450 cm -~) of considerable intensity was still present in spectra recorded at 673 K (Fig. 3g), showing that such species were not significantly depleted. A similar feature (shoulder at 1460 cm -~) was observed for our Ba/SiO2 catalysts [3] and this was still present when intense bands at 1418 and 1360 cm -~ were detected. The latter pair were assigned to the formation of barium nitrate [3] and so maxima for Pt/BaO/AI203 at 1413/1362 cm -1 are similarly assigned. The maxima at 1413 and 1362 cm 1 were depleted between 730 and 771 K [13], consistent with in-situ Raman studies [9] which indicate that crystalline Ba(NO3)2 was stable up to 770 K before being transformed into other species. Initial analysis of results would tend to suggest that the adsorbed nitro species giving the 1447 (1460 sh) cm -1 band is not a precursor for barium nitrate formation and that the absence of such nitrate species for Pt-free samples (Fig. 1) results from other factors such as carbonate/other surface contamination whose removal is Pt assisted. In-situ XRD studies of these catalysts [5] indicate that under a NOz/air flow, 673 K is the minimum temperature required to decompose bulk barium carbonate in the presence of Pt. FTIR spectra (Fig. 3f, g) indicate that under similar conditions, onset of nitrate formation occurs around 600 K, which might indicate bulk carbonate begins to decompose in the presence of NO2 before reaching 673 K.
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Fig.5 Transmittance IR spectra of 5%Ba/A1203 sample (a) outgassed at 1023 K and then (b) exposed to 1.0 Torr NO2 at 298 K. Sample then heated to (c) 673 K in NO2 before (d) addition of air (10 Torr) at 673 K and (e) evacuation at 673 K.
435 An alternative postulation, which would account for the co-existence of both the adsorbed nitro species and the nitrate species on the baria surface, involves a possible role of platinum in converting adsorbed nitro to nitrate. Only nitro species adsorbed on baria in intimate contact with Pt might be transformed to nitrate while nitro species adsorbed on isolated patches of baria, free from platinum are not transformed to nitrate. Note that Fridell et al. [2] report that no effective NOx storage occurs over pure BaO or BaO/AI203, even if NO2 is used. To check this proposal, an experiment was conducted for a Pt-free sample of 5%Ba/AlzO3,which following high 1023 K outgassing (Fig. 5a), exhibited the 1445 cm -1 band in the presence of NO2 (Fig.5b) confirming the presence of adsorbed nitro species on the cleaned baria surface. The sample was then heated in the presence of NO2 to determine whether conversion to nitrate in the absence of oxygen and Pt could proceed. Spectra recorded after heating at 673 K (Fig. 5c) show two maxima at 1417 and 1329 cm ~ which could be candidates for barium nitrate. However, in addition to differences in band positions, these band were much less dominant than those observed in spectra of Pt5%Ba/A1203 recorded at 673 K (Fig. 3g) which had shown such a clear, well resolved doublet at 1413/1362 cm -1 due to barium nitrate formation. One must conclude that spectra in figure 5 do not represent conversion of nitro to significant quantities of nitrate. Heating to 723 K did not lead to further significant changes in the spectrum. In an attempt to induce nitrate formation, dry air was introduced to the sample. Heating at 673 K in air/NO2 (Fig. 5d) or by evacuation at 673 K (to eliminate re-adsorption of NO2 on the alumina)(Fig. 5e) failed to increase the yield of these species or to shift the band positions towards those observed for the Pt5%Ba/A1203 sample (Fig. 3g). One plausible explanation for the above results is that bands at 1417/1329 cm -~ for Ba/AI203 were due to a surface type nitrate and that only in the presence of platinum is bulk barium nitrate formation (1413/1362 cm -1) possible. Note that Fridell et aL [2] suggest that Pt plays a role in nitrate formation by supplying dissociated oxygen atoms and that no effective NOx storage can occur over pure BaO or BaO/AI203, even if NO2 is used. Our spectral interpretation would be consistent with these findings. As the dispersion of baria was similar for both Pt containing and Pt-free samples (Table 1), an interpretation of the different band frequencies in terms of different baria structures is dismissed. If effective nitrate formation does require the presence of platinum, how is the surface nitrate species generated in Fig 5? It is worth considering other mechanisms of nitrate formation. Transmittance spectra in Fig. 4 clearly show an additional feature not readily observed in spectra obtained in the diffuse reflectance mode. The band at 1945 cm 1 has been observed in previous studies of NO2 adsorbed on alumina and alumina supported catalysts [7,8] and attributed to the nitrosonium ion (NO+). The dimmer of NO2 is known to undergo self-ionisation to produce nitrosonium and nitrate ions: 2NO2 + .~ N204 -'~ "~ NO + + NO3-. The adsorption of both types of ions would clearly shift the above equilibria towards the formation of such species. The direct formation of nitrate from N204 was used to explain barium nitrate formation over Pt/BaC12/SiO2 catalysts [3]. While such a reaction would readily justify the formation of surface type nitrates formed on alumina, where the nitrate ion would coordinate to exposed aluminium cations at the surface [7], the above reaction is unlikely to account for the formation of barium nitrate species of longer range 3-D order, as this would leave the structure with a distinct oxygen ion excess. It is quite plausible that such a process is responsible for the formation of the surface nitrate species on baria (Fig. 5) formed by heating a cleaned baria surface in the presence of NO2 but in the absence of
436 platinum. In the presence of platinum, a supply of, most likely some form of activated oxygen [2], allows the process to continue one step further, leading to the formation of bulk barium nitrate. Only on completion of this step does effective storage take place. Note that the same catalysts when examined by in-situ XRD [5] did not reveal the presence of 3-D nitrate species although similar treatments lead to the detection of IR bands at 1413 and 1362 cm -1. This was interpreted in terms of NO2 diffusion being limited to the first few surface layers. 4. CONCLUSIONS
NO2 formed by NO oxidation over platinum is adsorbed by clean baria surfaces in the form of an adsorbed nitro species. The presence of carbonate blocks nitro formation on low temperature treated baria, although the presence of Pt in the process assists in the decomposition process. In the absence of Pt, conversion of nitro species to adsorbed nitrate is possible, however Pt is required for bulk nitrate formation, probably by providing some form of activated oxygen via spillover to the baria sites. REFERENCES
1. N. Takahashi, H.Shinjoh, T. Iijima, T. Suzuki, K. Yamazaki, K. Yokota, H. Suzuki, S. Matsumoto, T. Tanizawa, T. Tanaka, S. Tateishi and K. Kashara, Catal. Today, 27 (1996) 63. 2. E. Fridell, M. Skogludh, B. Westerberg, S. Johansson and G. Smedler, J. Catal., 183 (1999) 196. 3. J.M. Coronado and J.A. Anderson, J. Molec. Catal., 138 (1999) 83. 4. J.A. Anderson, A.J. Paterson and M. Femfindez-Garcia, Stud. Surf. Sci. and Catal., 130 (2000) 1331. 5. J.A. Anderson and M. Fernfindez-Garcia, Trans IChemE., 78 (2000) 935 6. A. Sepulveda-Escribano, M. Primet and H. Praliaud, App. Catal. A., 108 (1994) 221. 7. N.D. Parkyns, Proc. 5th Inter Cong. Catal., p-255, J.W. Hightower (ed.) North Holland, Amsterdam, 1973. 8. J.A. Anderson, G.J. Millar and C.H. Rochester, J.C.S. Faraday Trans, 86 (1990) 86. 9. S. Hodjati, P. Bernhardt, C. Petit, V. Pichon and A. Kiennemann, Appl. Catal., 19 (1998) 209 and 221. 10. H.Kn6zinger and G.Mestl, Topics in Catal., 8 (1999) 45. 11. G.Mestl, M.P.Rosynek and J.H.Lunsford, J. Phys. Chem. B, 101 (1997) 9321 12. G.Mestl, M.P.Rosynek and J.H.Lunsford, J. Phys. Chem. B, 101 (1997) 9329. 13. A.J. Patterson, M. Fernfindez-Garcia and J.A. Anderson, manuscript in preparation.
429
Possible role of spillover processes in the operation of NOx storage and reduction catalysts. Allan J. Paterson, Daniel J. Rosenberg and James A. Anderson Department of Chemistry, The University, Dundee, DD 1 4HN, Scotland, UK
A series of catalysts containing Pt-BaO-A1203, have been prepared and characterised. Baria content was varied between 1 and 10 wt%. The processes which might involve spillover and thus contribute to the storage of NOx were studied using in-situ DRIFT and simultaneous temperature programmed reaction of the stored NOx. Experiments performed with NO2 in place of NO and comparison of Pt containing with Pt-free samples were designed to show the role which is played by Pt in forming barium nitrate as the initial storage step. Even after NO2 is formed, and is adsorbed as a nitro species at exposed baria surfaces, the subsequent transformation to nitrate species does not proceed readily in the absence of Pt suggesting that spillover of activated oxygen and/or an activated form of NO2 may be required for the storage step and for the samples to behave as effective storage materials. Other differences between Pt and Pt-free sample observed during in-situ reaction, are the consequence of the Pt catalysed enhanced decomposition of barium carbonate, which otherwise acts as a poison for NO2 adsorption and subsequent storage. 1. INTRODUCTION Storage and reduction catalysts offer one possibility of controlling NOx emissions from automobile sources while allowing operation under predominantly lean-burn conditions [1]. Alkaline earth oxide components may be used to store NOx under lean conditions which is then released during intermittent rich/stoichiometric periods. The NOx released is then reduced by CO or HC over the noble metal component. Although certain mechanistic details of the processes involved are now emerging [2-4], much detail is still required. For example, it is uncertain to what extent the stored NOx is stable under warm-up conditions and to what extent spillover processes are involved in transferring NOx from the noble metal to the storage compound and vice versa. To address these aspects, combined FTIR-TPD studies were performed over Pt and Pt-free BaO/A1203 catalyst under different conditions using samples that have been previously exposed to NO2. 2. EXPERIMENTAL BaO/A1203 samples containing between 1 to 10 wt% BaO were prepared by precipitating the hydroxide from a barium nitrate solution using an ammonia solution onto 7-alumina (Degussa Aluminoxid C). The precipitate was filtered, washed and dried at 363 K for 16 h before being calcined in air (100 cm 3 rain -1) at 773 K (2 h). A fraction of each sample was
430 retained whilst the remainder was wet impregnated with 1 wt% Pt (H2PtC16). Excess solvent was removed by heating under continuous stirring and the resulting powder dried overnight at 363 K prior to calcination in air (100 cm 3 min -1) at 773 K (2 h). BET surface areas measurements were performed using Ar as adsorbate on samples outgassed at 573 K. Pt dispersion was measured using CO pulsed chemisorption on samples reduced in H2 at 573 K. Barium dispersion was determined from CO2 adsorption isotherms at 298 K on samples heated to 1013 K under N2. DRIFT spectra were recorded at 4 cm -~ resolution (100 scans) using an MCT detector and Harrick environmental cell. A computer-controlled gas blender fed the desired composition of reactant gases to the IR cell, with the exit gases passed to a chemiluminescence detector for NO/NOz/total NOx analysis. A quadropole mass spectrometer and 2.5 m path length gas-phase IR cell were also available for on-line product analysis. 80 mg of powdered sample was calcined in situ at 673 K (1 h) in the DRIFT cell in a flow of dry air (50 cm 3 min-l), and either exposed to NO2 at 673 K, or cooled to 298 K and exposed to NO2. Previous in-situ XRD studies [5] indicate that at 673 K in an air/NO2 flow, barium carbonate species undergo decomposition. The IR cell was then flushed in air before a TPD of NOx from the samples was performed under air or air/propene/N2 flows (50 cm 3 mini) between 298 to 873 K at 3 K min l. Spectra recorded in transmittance mode were obtained by heating samples in a quartz cell at the desired temperature followed by lowering the sample to the optical compartment, exposing the sample to the required adsorbate and measuring the spectra at beam temperature. NO2 (99.5 %) was used as supplied. 3. RESULTS AND DISCUSSION 3.1 Sample characterisation Table 1 Sample characteristics Sample A1203 1 % BaO/A1203 2.5 % BaO/A1203 5 % BaO/A1203 7.5 % BaO/A1203 10% BaO/AI203 Pt, 1 % BaO/A1203 Pt, 5 % BaO/AI203 Pt, 10 % BaO/AI203
BET (mZgl )
Pt Dispersion (%)
CO2:BaO Ratio
93.5 88.3 91.1 87.1 92.7 101.7 93.8 92.6 90.9
35.2 46.3 51.8
0.45 0.36 0.27 0.16 0.09 0.22 0.23 0.21
XRD patterns of as prepared samples, indicated (24 and 42 ~ 20) that crystalline Ba(CO3) was present at all baria loadings, with the l wt% sample showing the weakest peaks. BET surface areas (Table 1) were reasonably constant with little change following the addition of baria, or subsequent addition of Pt and further calcination. BaO dispersion showed an almost linear decrease as a function of loading. Platinum dispersion (assuming Pt:CO =1) unexpectedly increased as the BaO loading was increased, even though the fraction of surface covered by baria was greater at higher loadings leaving less of the alumina surface free for interaction with the Pt salt. The subsequent addition of Pt appeared to influence the dispersion
431 of baria with a homogenisation of the baria apparent. Previous studies [6] suggest that this results from the use of the acidic H2PtC16 solution which effectively redisperses the baria. To eliminate any possible differences in behaviour between Pt and Pt-free samples based on baria dispersion, results presented in this study were largely performed using 5%Ba/A1203 and Pt5%Ba/A1203 which present very similar Ba dispersions. 3.2 Are spillover processes involved in the formation of stored NOx?
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Fig. 1. TPD o f N O 2 desorbed in air from (a) A1203, (b) 5% B a O / A I 2 0 3 and (c) Pt-5% BaO/A1203 and (d) NO desorbed from Pt-5% BaO/A1203 after samples were previously exposed to NO2 at 298 K Fig.1 shows profiles of NOx desorbed from samples, which, according to XRD patterns may contain barium carbonates. FTIR spectra of the Pt-free samples show a band at c a . 1450 cm -1 prior to exposure to NO2 that may be attributed to such a species (Fig. 2a). However, exposure to NO2 at 298 K led to the appearance of a triplet of bands around 1600 c m -1 and another around 1300 c m l which compare with those observed by Parkyns following exposure of alumina alone to NO2 [7]. These bands are similarly attributed in the main to nitrate species on the alumina surface. The fact that the main contribution in IR spectra of NO2 on Ba/A1203 is due to species on the alumina support explains why TPD profiles are similar for A1203, Ba/A1203 and Pt/Ba/A1203 (Fig. 1) with maxima around 350 and 650 K. The only desorption peak unique to the baria containing samples appeared at 500 K, suggesting that exposed baria sites, even though in the main covered with carbonate species, were available for adsorption of NO2 after a 673 K calcination treatment.
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1400
1300
1200
W a v e n u m b e r / c m -~
Fig. 3. FTIR spectra of Pt5%Ba/A1203 after (a) calcination in air at 673 K then (b) exposure to NO2 at 298 K and then heating in air at (c) 363, (d) 400, (e) 523, (f) 623 and (g) 673 K.
433 Consistent with spectra of samples in the absence of Pt, a sample of Pt5%Ba/A12Os after calcination at 673 K showed bands in the 1600-1200 cm -1 region (1558, 1469, 1351 and 1242 cm ~ indicating that this treatment was not effective in completely removing all traces of carbonates from the sample. This is consistent with the work of others concerning the formation of stable carbonates on NOx storage catalysts [9]. Exposure to NO2 generated similar species to those obtained in the absence of Pt with bands at ca. 1600 and 1300 cm -1 (Fig. 3b) mainly due to adsorption on the alumina support. However an additional feature, at 1447 cm l was present in the temperature range 298-523 K (Fig. 3 b-e) which was not detected for the 673 K calcined Ba/AI203 sample (Fig. 2) in the absence of Pt. To determine whether the presence of Pt was essential for the generation of this species, for example, by producing an activated form of NO2 which is spiltover to exposed baria sites, samples of 5%Ba/A1203 were calcined over a range of temperatures and then exposed to NO2. Fig 5 shows spectra of 5%Ba/A1203 outgassed at 1023 K and then exposed to NO2 at 298 K.
'• 1945
2000
1800
1600 Wavenum
ber/cm
1400
1200
-~
Fig. 4. FTIR spectra of 5%Ba/A1203 outgassed at 1023 K then exposed to NO2 at 298 K at pressures up to 1.0 Torr. Spectra clearly show the growth as a function of NO2 pressure of a feature at 1445 cml (Fig. 4). It would appear that the ability to observe the form of adsorbed NO2 responsible for the band at ca 1447 cm -1 is a function of the extent to which carbonate and other species are removed from the baria surface. The presence of platinum (compare Figs 2 and 3) apparently plays a role in assisting removal of these surface contaminants. The above experiment was conducted in vacuum thus indicating that the presence of oxygen is not required for formation of such an adsorbed nitro species. Fridell et al. [2] and Mestl et al [10-12] have proposed that oxygen is essential, however, for the nitrate formation process and that the presence of oxygen was required to convert ligated nitro species to nitrate. Fridell et al. [2] suggest that Pt plays a role in this process by supplying dissociated oxygen atoms. Initial evidence from Fig 3 might suggest that this baria adsorbed NO2 species (1447 cm -1 band) may have been a requirement for the changes observed at higher temperatures (Fig. 3 f,g) when bands at 1413 and 1362 cm -1 became apparent. It is unclear as to whether the adsorbed NO2 species (1447 cm -1 band) was a precursor for this high temperature species.
434 However, a distinct shoulder (ca. 1450 cm -~) of considerable intensity was still present in spectra recorded at 673 K (Fig. 3g), showing that such species were not significantly depleted. A similar feature (shoulder at 1460 cm -~) was observed for our Ba/SiO2 catalysts [3] and this was still present when intense bands at 1418 and 1360 cm -~ were detected. The latter pair were assigned to the formation of barium nitrate [3] and so maxima for Pt/BaO/AI203 at 1413/1362 cm -1 are similarly assigned. The maxima at 1413 and 1362 cm 1 were depleted between 730 and 771 K [13], consistent with in-situ Raman studies [9] which indicate that crystalline Ba(NO3)2 was stable up to 770 K before being transformed into other species. Initial analysis of results would tend to suggest that the adsorbed nitro species giving the 1447 (1460 sh) cm -1 band is not a precursor for barium nitrate formation and that the absence of such nitrate species for Pt-free samples (Fig. 1) results from other factors such as carbonate/other surface contamination whose removal is Pt assisted. In-situ XRD studies of these catalysts [5] indicate that under a NOz/air flow, 673 K is the minimum temperature required to decompose bulk barium carbonate in the presence of Pt. FTIR spectra (Fig. 3f, g) indicate that under similar conditions, onset of nitrate formation occurs around 600 K, which might indicate bulk carbonate begins to decompose in the presence of NO2 before reaching 673 K.
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1800
I
I
1700
1600
I
1500
1400 -1
I
I
1300
1200
Wavenumber/cm
Fig.5 Transmittance IR spectra of 5%Ba/A1203 sample (a) outgassed at 1023 K and then (b) exposed to 1.0 Torr NO2 at 298 K. Sample then heated to (c) 673 K in NO2 before (d) addition of air (10 Torr) at 673 K and (e) evacuation at 673 K.
435 An alternative postulation, which would account for the co-existence of both the adsorbed nitro species and the nitrate species on the baria surface, involves a possible role of platinum in converting adsorbed nitro to nitrate. Only nitro species adsorbed on baria in intimate contact with Pt might be transformed to nitrate while nitro species adsorbed on isolated patches of baria, free from platinum are not transformed to nitrate. Note that Fridell et al. [2] report that no effective NOx storage occurs over pure BaO or BaO/AI203, even if NO2 is used. To check this proposal, an experiment was conducted for a Pt-free sample of 5%Ba/AlzO3,which following high 1023 K outgassing (Fig. 5a), exhibited the 1445 cm -1 band in the presence of NO2 (Fig.5b) confirming the presence of adsorbed nitro species on the cleaned baria surface. The sample was then heated in the presence of NO2 to determine whether conversion to nitrate in the absence of oxygen and Pt could proceed. Spectra recorded after heating at 673 K (Fig. 5c) show two maxima at 1417 and 1329 cm ~ which could be candidates for barium nitrate. However, in addition to differences in band positions, these band were much less dominant than those observed in spectra of Pt5%Ba/A1203 recorded at 673 K (Fig. 3g) which had shown such a clear, well resolved doublet at 1413/1362 cm -1 due to barium nitrate formation. One must conclude that spectra in figure 5 do not represent conversion of nitro to significant quantities of nitrate. Heating to 723 K did not lead to further significant changes in the spectrum. In an attempt to induce nitrate formation, dry air was introduced to the sample. Heating at 673 K in air/NO2 (Fig. 5d) or by evacuation at 673 K (to eliminate re-adsorption of NO2 on the alumina)(Fig. 5e) failed to increase the yield of these species or to shift the band positions towards those observed for the Pt5%Ba/A1203 sample (Fig. 3g). One plausible explanation for the above results is that bands at 1417/1329 cm -~ for Ba/AI203 were due to a surface type nitrate and that only in the presence of platinum is bulk barium nitrate formation (1413/1362 cm -1) possible. Note that Fridell et aL [2] suggest that Pt plays a role in nitrate formation by supplying dissociated oxygen atoms and that no effective NOx storage can occur over pure BaO or BaO/AI203, even if NO2 is used. Our spectral interpretation would be consistent with these findings. As the dispersion of baria was similar for both Pt containing and Pt-free samples (Table 1), an interpretation of the different band frequencies in terms of different baria structures is dismissed. If effective nitrate formation does require the presence of platinum, how is the surface nitrate species generated in Fig 5? It is worth considering other mechanisms of nitrate formation. Transmittance spectra in Fig. 4 clearly show an additional feature not readily observed in spectra obtained in the diffuse reflectance mode. The band at 1945 cm 1 has been observed in previous studies of NO2 adsorbed on alumina and alumina supported catalysts [7,8] and attributed to the nitrosonium ion (NO+). The dimmer of NO2 is known to undergo self-ionisation to produce nitrosonium and nitrate ions: 2NO2 + .~ N204 -'~ "~ NO + + NO3-. The adsorption of both types of ions would clearly shift the above equilibria towards the formation of such species. The direct formation of nitrate from N204 was used to explain barium nitrate formation over Pt/BaC12/SiO2 catalysts [3]. While such a reaction would readily justify the formation of surface type nitrates formed on alumina, where the nitrate ion would coordinate to exposed aluminium cations at the surface [7], the above reaction is unlikely to account for the formation of barium nitrate species of longer range 3-D order, as this would leave the structure with a distinct oxygen ion excess. It is quite plausible that such a process is responsible for the formation of the surface nitrate species on baria (Fig. 5) formed by heating a cleaned baria surface in the presence of NO2 but in the absence of
436 platinum. In the presence of platinum, a supply of, most likely some form of activated oxygen [2], allows the process to continue one step further, leading to the formation of bulk barium nitrate. Only on completion of this step does effective storage take place. Note that the same catalysts when examined by in-situ XRD [5] did not reveal the presence of 3-D nitrate species although similar treatments lead to the detection of IR bands at 1413 and 1362 cm -1. This was interpreted in terms of NO2 diffusion being limited to the first few surface layers. 4. CONCLUSIONS
NO2 formed by NO oxidation over platinum is adsorbed by clean baria surfaces in the form of an adsorbed nitro species. The presence of carbonate blocks nitro formation on low temperature treated baria, although the presence of Pt in the process assists in the decomposition process. In the absence of Pt, conversion of nitro species to adsorbed nitrate is possible, however Pt is required for bulk nitrate formation, probably by providing some form of activated oxygen via spillover to the baria sites. REFERENCES
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