J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2815-2820 FTIR Study of the Influence of Sulfate Species on the Adsorption of NO, CO and NH, on CuO/AI,O, Catalysts Mohamed Waqif and Mahjoub Lakhdar Universite Hassan 11, Faculte des Sciences I, Casablanca, 02 Maroc Odette Saur and Jean-Claude Lavalley* Laboratoire Catalyse et Spectrochimie, URA CNRS 4 14-ISMRA-Universite, 6,Boulevard du Marechal Juin-14050 Caen-Cedex , France CuO/AI,O, catalysts have been sulfated by oxidation of various amounts of SO,. The surface properties of the samples were studied using probe molecules. The IR spectra of CO species adsorbed on the non-sulfated sample show a strong band at 2125 cm-' with a shoulder at 2160 cm-', which shift to 2165 and 2180 cm-', respectively, with a concomitant decrease in intensity as the amount of sulfate increases.Adsorbed NO species on the non-sulfated catalyst give rise to two weak bands at 1754 and 1873 cm-'. On sulfated samples, the first band disappears whereas that at 1873 cm-' shifts towards higher wavenumber and reaches 1917 cm-' for large amounts of sulfate. These results show that the copper electronic state is affected by sulfation. The positive charge on the copper sites increases with the amount of sulfate. NH, adsorption is also influenced by sulfation. Sulfate species are adsorbed on basic sites and high sulfation prevents NH, dissociative adsorption while it favours coordinated and protonated species on acid-base pair sites. Elimination of NO, from flue gas is an important subject because the emission of these oxides is one of the causes of acid rain and air pollution.The main methods for removal of NO, have been reviewed recently.' Selective catalytic reduction by NH, seems to be the most efficient process. Many catalysts based on noble metals (Pd, Ru, Pt) or transition-metal oxides have been studied. ' Amongst the latter, vanadia on titania has received much attention.'-6 The presence of sulfur oxides in an oxidizing atmosphere is inevi- table and their emission also contributes to acid rain and air pollution. Moreover, the SO, chemisorbed species are oxi- dized and stable sulfate species may remain on catalysts and influence the reduction of nitric oxides.'** Methods for simul- taneous removal of NO, and SO, have been developed and for about 30 years copper-based catalysts were tested for such a reaction.'.' Supported or unsupported copper oxide is used in a wide variety of other processes in the chemical industry.For instance, it has been shown that copper is an active com- ponent in the oxidation of hydrocarbons" or alcohols.' '-13 Various physical methods (XPS, EPR, TPR, IR) have been used to characterize copper and its interaction with the support in these copper-based materials, especially on A1,0, .l4-I7 IR spectroscopic studies using probe molecules such as CO and NO have been used in particular to deter- mine the oxidation state of copper.' '-" When copper/alumina was used as a sorbent catalyst for SO, removal, sulfate species linked to A1-0 sites and CuO were observed by IR and XPS.30*31 It has been shown that the presence of anions, such as sulfate species, modifies the acid-base character of the o~ides~'-~~ and we have recently reported that the enhancement of acidity depends on the amount of sulfate.36 The aim of this work was to study the effect of sulfate species on the electronic properties of copper.Apart from CO, which is generally used as a probe for such IR spectros-copy characterization, we have also studied the adsorption of the reactants, NO and NH, . Experimental The CuO/Al,O, sample (112 m2 g-', 4.88% w/w CuO on AI,O,) was prepared by wet adsorption of copper acetate on industrial y-Al,03, followed by a thermal decomposition in air at 450°C.Sulfate ions were introduced on the sample evacuated at 450°C by heating known amounts of SO, with a large excess of 0, at 450°C for about 14 h. Then the sample was again evacuated at 450°C for 2 h. The sulfate content was determined by an elementary microanalysis using coulometry. The notation used, the amounts of SO, added and the result of S analysis are reported in Table 1. For IR studies, self-supported discs of about 15 mg cm-, were used. They were evacuated at 450°C for 2 h. The gases were introduced at room temperature. The time required for equilibrium was very short and the spectra were scanned after ca. 10 min. All the spectra were recorded at room tem- perature using a Nicolet MX-1 FTIR spectrometer. Spectra of adsorbed CO, NO and NH, species were obtained by sub- tracting the absorbance of the activated catalyst and that of the gas phase when applicable.Results Spectra of Evacuated Sample The spectra of the evacuated samples show broad bands in the 3900-3500 cm-' and 1500-1000 cm-' ranges due to OH and sulfate groups, respectively (Fig. 1 and 2). Analysis of the broad band in the 3900-3500 cm-' range by a curve-fitting program leads to the appearance of four bands at 3780, 3725, 3650 and 3580 cm-' for the S-0 sample. The intensity of the highest-wavenumber bands decreases while the amount of sulfate increases. Only a very broad band is observed on the spectra of the S-3 sample. In the 1500-1000 cm-' range, Table 1 Notation of the samples, amounts of SO, used for sulfation and S content notation amount of SO,/pmol g- s (wt.%) s-0 - - s-0.2 50 0.15 S-0.3 100 0.3 S-0.6 200 0.6 S-1.3 400 1.3 s-3 lo00 3.2 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I 3800 3700 3600 3500 wavenumber/crn-l Fig. 1 IR spectra in the 3850-3500 cm-' range of: (a) S-0, (b)S-0.3, (c)S-0.6, (6)S-3 spectra show the v(S=O) band near 1375 cm-' and v(S-0) ones near 1070-1030 cm-', as previously observed on alumina.35 Increasing the amount of sulfate leads to the width of the first band owing to the superposition of another one since different sulfate species were formed on alumina when the amount of sulfate is higher than 2 pmol m-*. Moreover, for the S-3 sample other bands appear near 1230 and 1170 cm-' which had been assigned to sulfate species on copper ions.30 NO Adsorption After introduction of successive doses of ca.15 pmol g-' of NO onto the unsulfated sample, two weak bands were observed. In the presence of gas (P, = 1.3 x lo3 Pa) their maxima are at 1873 and 1754 cm-'. Their intensity increases in unison (Fig. 3), but the amount of chemisorbed NO is too low to determine the integrated molar absorption coefficient of these bands by adding successive portions of NO. The adsorbed species are easily removed by evacuation (2 min at room temperature sufficed) showing that they are very loosely held on surface. The presence of only 50 pmol g-' of sulfate leads to the disappearance of the 1754 cm-' band, whereas the band at 1873 shifts to 1882 cm-'. Fig.4 shows that the shift of this band increases with the amount of sulfate; the band wave- number reaches 1917 cm-' for the S-3 sample. The intensity I , I I I 1900 1700 wavenurn ber/crn -Fig. 3 IR spectra of species adsorbed on S-0 after addition of (a)17 pmol g-* NO, (b)150 pmol g-' NO (c) 850 pmol g-' NO, and (d)in the presence of NO gas (P, = 1.3 x lo3Pa) of the band is enhanced in the presence of sulfate species (Fig. 4). In Fig. 5, we report the variation of the absorbance between 1950 and 1825 cm-' as a function of the amount of added NO on the various samples; a linear increase is h I I 1950 1850 1750 wavenurnber/cm-' Fig. 4 IR spectra of adsorbed NO in presence of gaseous NO (P, = 1.3 x lo3Pa) on (a)S-0, (b)S-0.2, (c)S-0.3, (6)S-1.3, (e) S-3 L i 1400 1100 0 100 200 300 wavenurnber/cm-' amount of added NO/pmol g-l Fig.2 IR spectra of the sulfate species obtained by oxidation of Fig. 5 v(N0) band area us. added amounts of NO on (a) S-0, (b) various amounts of SO, :(a) S-0.2,(b)S-0.3, (c)S-0.6, (6)S-1.3, (e)S-3 S-0.2, (c) S-0.3, (d)S-1.3, (e)S-3 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 evident followed by saturation. The linear portions of the curves exhibit nearly the same slope for the four samples, and could allow evaluation of the integrated molar absorption coeficient (E = 14 f2 cm pmol-'). It is almost constant showing that it does not vary with v(N0). From E and the area of the v(N0) band at saturation, we can estimate the number of NO adsorption sites as 48 pmol g-' on S-0.3, 58 pmol g-' on S-0.6, 66 pmol g-' on S-1.3 and 95 pmol g-' on S-3 (Fig.5). These numbers can be compared with the amounts of sulfate and copper. The adsorbed NO species formed on sulfated samples are more stable than those on pure CuO/Al,O, and, for example, on S-3 they resist evac- uation at room temperature for more than 1 h. CO Adsorption Fig. 6 shows the spectra of adsorbed species on the different samples in the presence of CO gas (P, = 1.3 x lo3 Pa). A strong band is observed at 2125 cm-' accompanied by a shoulder at 2160 cm-', in the spectrum of the non-sulfated sample [Fig. 6(a)]. The spectrum of CO adsorbed on the sul- fated samples shows a band in this spectral region.Its wave- number has been progressively shifted from 2125 to 2165 cm-' as the amount of sulfate increases. The shoulder also shifts from 2160 to 2180 cm-'. Conversely, the intensity decreases when the amount of sulfate increases (Fig. 6). v(C0) is correlated to v(S-0) (Fig. 7) which increases with the amount of sulfate.30 The CO adsorption is partly reversible at room temperature (Fig. 8). The overall amount of CO adsorbed and the irreversible part decrease with increasing amount of sulfate. However, the fraction of CO remaining adsorbed after evacuation increases with sulfate content. In order to characterize the origin of the band at 2125 cm-' with a shoulder at 2160 cm-' in the spectrum of the unsulfated sample, treatment with H, has been carried out at various temperatures before introducing CO.v(C0) does not change even after H, treatment at 400°C. However, its inten- sity decreases from 45 to 10 in arbitrary units and reoxida- tion under 0, at 400°C does not fully restore it (20 arb. units). The integrated molar absorption coefficient of the 2 125 cm -'band can be evaluated as 23 cm pmol -'. CO and NO Coadsorption On the S-0.6 sulfated sample, 1.3 x lo3 Pa of NO were intro- duced. As previously observed, a strong band appeared at m cu F cu 1400 /1392 r I EY 1384 II s. 1376 1368 2 120 2130 2140 2150 2160 2170 v ( CO)/cm -' Fig. 7 v(C0) us. v(S-0) 1892 cm-'. Then 1.3 x lo3 Pa of CO were added and gave rise to one v(C0) band at 2135 cm-'.The absorption bands due to CO or NO adsorbed alone and those due to NO and CO coadsorption are compared in Fig. 9. Note that the shoulder due to v(C0) near 2180 cm-' is absent and that the intensity of v(C0) at 2135 cm-' is slightly decreased (35%) relative to that observed with CO alone. As for NO, the intensity of the 1892 cm-band is almost unaffected (< 10%) when NO is coadsorbed with CO. ujn--. ?? -0 10 n h 0 0, I I 0 25 50 75 100 evacuation ti me/m in Fig. 8 Variation of the v(C0) band area during evacuation at room temperature: (0)S-0, (+) S-0.2, (0)S-0.3, (V)S-0.6, (A)S-1.3, (+)s-3 I 2200 1900 2200 2050 wavenum ber/cm -' 'waven um ber/cm -Fig.9 IR spectra of adsorbed species on the S-0.6 sample: (a)after Fig. 6 IR spectra of adsorbed CO (P, % 1.3 x lo3 Pa) on (a)S-O,(b) CO addition in the cell; (b) after NO addition; (c) after NO then CO S-0.2, (c)S-0.3, (d) S-0.6, (e)S-1.3,O S-3 addition I* B 3700 3100 wavenumber/cm-' '' 3700 3100 1550 1250 wavenumber/cm-' wavenumber/cm-' Fig. 10 IR spectra of adsorbed NH,: A, B, on the S-0 sample after NH, addition (pol g-'): (a)55, (b) 133, (c) 1.3 x lo3 Pa NH,, fol- lowed by evacuation at (d)room temperature (e) 100 "C,(f) 200 "C. C, D, on the S-3 sample after NH, addition (pmol g-'): (a) 133, (b) 425. (c),(d),cf)as for parts A and B. NH, Adsorption The first doses adsorbed on S-0 gave rise to the spectra shown in Fig.10A and B with bands at 3370,3260cm-'and 3150 cm-', and 1620 and 1240 cm-'. Such bands were assigned to coordinated NH, species on alumina.37 Other bands appear at 1450, 1490 and 3530 cm-' (very broad) with increasing amount of adsorption. The NH,-treated sample was then heated to 100, 200 or 400°C under vacuum or in the presence of NH, gas phase. The intensity of the bands at 1620 and 1240 cm-' decreases, whereas the temperature increases. Conversely, the bands at 1450 and 1490 cm-' remained even after evacuation at 400°C. Moreover, new bands appeared at 2280, 2250 and 2060 cm-' when the sample had been evacuated at 200 or 400"C (Fig. 11). The adsorption of successive doses of NH, on the S-3 sample has been studied. The spectra of the adsorbed NH, species on that sample are reported in Fig.1OC and D. Bands at 1620 cm-', 3360, 3260 and 3150 cm-' are observed as in the case of the unsulfated sample, but their intensity is higher. The 1240 cm-' band is masked by absorption bands due to the sulfate species. The v(S=O) sulfate band normally near 1400 cm-' is highly perturbed and wide bands appear near 0a3I_ M 2250 2050 wavenumber/cm-' Fig. It IR spectra of adsorbed species after NH, addition on the S-0 sample (P, x 5 x lo3 Pa), followed by evacuation at (a)200, (b) 400 "C J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1250 cm-'. No bands appear at 1450, 1490 and near 3530 cm-' in contrast with the spectra of NH, adsorbed on the non-sulfated sample. The intensity of the various bands decreases concomitantly when the sample was heated under vacuum at high temperature, but we did not observe new bands in the 2300-2000 cm-' range.The sulfate band at 1400 cm-' was progressively restored. Heating the sample under NH, pressure decreases the intensity of the sulfate band while a band develops at 1450 cm-'. This can be explained by the formation of ammonium sulfate. Discussion IR Study of OH Groups and Sulfate Species The spectra of the samples show bands due to OH groups in the 3800-3600 cm-range and bands in the 1400-1000cm-region assigned to sulfate species. The bands due to the type I, I1 and I11 OH groups3* of pure alumina could be observed on the S-0 sample spectrum although they are not well resolved.This poor resolution could be explained by a lack of crystallinity of the alumina.39 The interaction of Cu2+ ions with the alumina OH groups was studied recently and it was shown that it is not specific to one type.40 These authors concluded that for samples with <6-7 wt.% CuO, copper ions are well dispersed on the alumina surface. AI-0-Cu-0-A1 links have been formed by exchange of the hydrogen of the hydroxy groups and Cu atoms. Sulfation leads first to a decrease of the v(0H) absorbance between 3780 and 3730 cm-' when the amount of sulfate increases. A single broad band remains for the highly sulfated sample from 3750 to 3600 cm-' which could be assigned to internal OH groups4' or hydrogen-bonded OH groups linked to sulfate species.The same broad band was observed on the spectrum of highly sulfated alumina without copper. The sulfate species spectra observed in the 1400-1000 cm-' range (Fig. 2)on pure al~mina~~and on CuO/Al,O, 30 have previously been discussed. In the pure alumina spectra, a band at 1380 cm-' assigned to v(S=O) was observed for low sulfate coverage,35 while for higher amounts, another species was characterized by a band near 1405 cm-'. The latter was assigned to S20,2-42 or to SO, species on Al-0.30 On CuO/Al,O,, other species on Cu-0-A1 or CuO sites had been characterized by IR bands in the 1220-1080cm-' region.,' The spectra of the various samples (Fig. 2) show that the relative amount of these different sulfate species depends on the amount of sulfate.NO Adsorption NO has widely been used as a probe molecule to study properties of various catalysts, for instance V205/Ti02 and Fe/Si02,43 and especially the oxidation state of Cu on pure Cu02' or supported on silica or al~mina.'~*'~,~~ The energy diagram of the NO molecule shows that the 7c* orbital is higher in energy than that of the 0 and N atoms and this orbital is therefore unstable. During the interaction between NO and surface ions, electron transfer preferentially occurs from NO towards the metallic ions. As Cu2+ is an electron acceptor while Cu+ a donor (the d level is full), bonds are expected between NO and Cu2+ ions. Such an interaction had been demonstrated by Gandhi and Shelef'* on copper oxide by thermogravimetric methods.Recently, Lin and co- worker~~~used low-temperature IR spectroscopic methods to study adsorption of NO on CuO/y-Al,O,. They observed three bands at 1888, 1862 and 1800 cm-'.They attributed the first two bands to the vibration of NO molecules J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 adsorbed on two Cu2+ sites in different surroundings (CuO/Al,O, or CuAl,O,). The third band was attributed to coordination of NO on Cu' sites, although no such Cu+ sites were reported in their XPS study. In Fig. 3, we observe at room temperature only two bands at 1873 and 1754 cm-' on the non-sulfated sample which could be due to NO adsorbed in a dimeric form as on reduced oxide or zeolites.22 However, two bands were also observed on Fe/SiO, at 1805-1825 and 1755 cm-' and were, respectively, attributed to NO monomer on Fe2 + and FeO.,, On sulfated samples, the intensity of the band at 1873 cm-' increases with the amount of sulfate and shifts towards higher wavenumbers while that at 1754 cm- ' disappears. As the SO,' -group attracts electrons, its presence must increase the positive charge on copper ions.We consider that the 1880 cm-' band is due to NO adsorbed on Cu2+ sites and that at 1754 cm- 'to NO on Cu+ sites. On an oxidized A120, sample containing 10% w/w CuO, Knozinger and co-workers26 observed only one band at 1865 cm-', also assigned to NO-Cu2+. These authors also observed bands in the range 1500-1600 cm-' upon heating, which indicates the formation of nitrito and nitrato surface complexes.We did not observe bands in this range at room temperature, without heating the sample. CO Adsorption On pure CuO, Busca" observed a rather strong band near 2115 cm-' due to linearly coordinated CO on Cu+ ions exposed on the CuO surface. Bands due to carbonates in the 1700-1200 cm-l range were also noted. Lin and co-worker~~~observed three bands at 2170,2148 and 2120 cm-' by IR at low temperature. They assigned the first band to CO adsorbed on Cu'+ and the last to CO on a Cu+ site. Since this last band is less sensitive to the increase in temperature, they concluded that the Cu+ adsorption sites of CO were stronger than the CU" sites. Primet and co-~orkers~~ also observed a v(C0) band near 2120 cm-' when CO was adsorbed on CuO/Al,O, , but they assigned it to CO coordi- nated to a Cu2' site.The band assignments for CO interacting with copper oxides vary widely in the literature, but it is generally report- ed that CO adsorption leads to bands appearing in the 2105- 2130 cm-' range for CO on metallic copper, or in the 2115-2130 and 2120-2140 cm-' ranges for CO on Cu+ and Cu2+,respectively. On CuO/Al,O, without sulfate, we have noted a band at 2125 cm- ',which we assign to CO adsorption on Cu+ sites in agreement with ref. 20 and 29. This band shifts towards higher wavenumber and its intensity decreases with sulfate loading. Fig. 7 shows a correlation between v(C0) and v(S=O), showing the positive charge of the Cu sites increas- ing with sulfate loading.Our assignment of the 2125 cm-' band to CO adsorption on Cu+ sites accounts for the energy levels of CO. The CT orbital of CO is lower in energy than that of the 0 and C atoms and this orbital is therefore relatively stable. Conse- quently, it is difficult to transfer the CT electron of CO. Cu+ is a better electron donor than Cu2+ (3d"). Therefore, the transfer of a 3d electron to the n* orbital of CO is easier for Cu+ than Cu2+. The decrease in the intensity is due to the attractive effect of SO,,-groups which favours an increase of the positive charge on the Cu atoms. It could also be partly explained by the poisoning of the Cu2+ ions by sulfate groups, since the IR spectra of these SO,2- species show bands near 1150 cm-' which were assigned to in interaction with Cu or CU-A~.~' The experiment with H2-reduced CuO/A1,0, did not show any new band due to metallic copper but a decrease of the intensity of the v(C0) band assigned to the Cu' site.This may be due to a decrease in the dispersion of copper with H, reduction. We did not observe a band of significant intensity resulting from the adsorption of CO on Al,O, since the bands expected in the 2200-2240 cm-' range for CO coordination on activated alumina were not detected. Note that the sulfate species poison the strongest Lewis-acid sites, A13+, and emphasize the acidity of the least acidic sites.36 The spectrum obtained by CO adsorption after intro- duction of NO (Fig.9) shows simultaneously the band char- acteristic of NO coordinated to Cu2+ ions (band at 1892 cm-') and that due to CO coordinated to Cu+ ions (band at 2135 cm-'). The presence of the two different sites (well dis- tinguished by the use of the two probes) is confirmed. The shoulder observed at 2160 cm- 'upon CO adsorption (Fig. 6) and assigned to some Cu2+ ions does not appear when CO and NO are coadsorbed, which confirms that the adsorption of NO on Cu2+ ions is stronger than that of CO. NH, Adsorption IR studies of NH, adsorbed on activated A1,0, have been the subject of many Apart from species coordi- nated to Lewis-acid sites, characterized by IR absorption bands at 1620 and 1240 cm-' and hydrogen-bonded species giving rise to broad bands near 3300 cm-', some authors observed other bands in the 1550-1450 cm-' range whose assignment was not easy.Hall and co-~orkers~~ thought that these bands could result from dismutation of NH, on acid- base pair sites leading to NH,+and NH,-. On sulfated alumina, activated up to 400 "C, a strong band at 1620 cm-due to coordinated species was observed, but there were no bands in the 1550-1450 cm-I range. This is in agreement with Hall's interpretation, since sulfate species poison the basic sites and the dismutation could not occur. Moreover, NH, coordination shifts the v(S=O) band towards lower wavenumber as observed for pyridine ad~orption.~~ On CuO/Al,O,, we observe the same bands as on pure alumina [Fig. 10A, B]. They are also assigned to the forma- tion of coordinated species (bands at 1620 and 1240 cm-') and to NH,' and NH,- ions (bands at 1450 and 1490 cm-').The NH,-treated S-0sample has been heated under vacuum and this treatment leads to an increase in the inten- sity of the NH4+ and NH2- absorption bands (1450, 1490, ca. 3500 cm-') and to the appearance of new bands in the 2200-2050 cm-' range (Fig. 11). The last bands could be assigned to species resulting from NH, dissociation and espe- cially to Cu-NSN species, in agreement with Rochester and co-~orkers~~ or to surface-bound a~ide.~' On spectra of sulfated CuO/Al,O, samples (Fig. lOC, D) only one band (1620 cm-') due to NH, coordinated species is observed, since the bands due to sulfate species overlap the 1240 cm-' band.NH, adsorption sites are close to these species since the sulfate absorption bands are highly per- turbed. Heating in the presence of NH, gas leads to the for- mation of NH,' species (band at 1450 cm-') and ammonium sulfate, as previously observed.46 Some hydrogen-bonded species are also formed (negative absorb- ance near 3700 cm- ') but dissociation of NH, does not occur. This could be explained by the poisoning of basic sites and copper sites by sulfate. Conclusion This IR study of copper on alumina catalysts containing various amounts of sulfate ions shows that the presence of 2820 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 23 A. A. Davydov, in Infrared Spectroscopy of Adsorbed Species on such anions increases the positive charge of the copper sites. the Surface of Transition Metal Oxides, Wiley, Chichester, 1990.Moreover, owing to sulfate adsorption on basic sites, high 24 M.C. Marion, E. Garbowski and M. Primet, J. Chem. SOC.,sulfation of the catalysts prevents NH, dissociative adsorp- Faraday Trans., 1990,86,3027.tion. Only coordinated and protonated species are then 25 R. Hierl, H. 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