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ELSEVIER Applied Catalysis B: Environmental 13 (1997) 45-58

An FT-IR study of ammonia adsorption and oxidation over anatase-supported metal oxides

Jo& Manuel Gallardo Amoresa, Vicente Sanchez Escribanoa, Gianguido Ramisb, Guido Buscab’*

a Departamento de Quimica Inorganica, Faculdad de Quimica, Universidad, Plap de la Mewed, E-37008 Salamanca, Spain

b Istituto di Chimica, FacoltA di Ingegneria, Universitir, Pie Kennedy, I-16129 Geneva, Italy

Received 15 May 1996; received in revised form 19 August 1996; accepted 7 September 1996

Abstract

The adsorption and the oxidation of ammonia over sub-monolayer Ti02-anatase supported chromium, manganese, iron, cobalt, nickel and copper oxides, has been investigated using FT-IR spectroscopy. These materials are models of catalysts active in the Selective Catalytic Reduction of NO, by ammonia (SCR process) and in the Selective Catalytic Oxidation of ammonia to dinitrogen (SC0 process). For comparison, the adsorption of ammonia and hydrazine over the TiOz-anatase support has also been studied. CrO-Ti02 adsorbs ammonia both in a co-ordinated form over Lewis acid sites and in a protonated form over Bronsted acid sites, involving high-valence chromium (chromyl species). However, simple outgassing at r.t. causes the desorption of ammonia from Bronsted acid sites showing that they are very weak. All other catalysts do not present any Bronsted acidity. Co-ordinated ammonia gives rise to several oxidation products over Fe#-TiOz, CrO,-Ti02, Coo,-Ti02 and CuO-Ti02, among which hydrazine is likely present. Other species have been tentatively identified as imido species, NH, nitroxyl species, HNO, and nitrogen anions, NT. NiO,-Ti02 and MnO,-Ti02 appear to be even more active in ammonia oxidation, because the adsorbed species disappeared completely at lower temperature (473 K) than in the other cases. However, possibly just due to their excessive activity, no adsorbed species different from co-ordinated ammonia can be found in significant amounts over these surfaces. Based on these data, the mechanism of the SCR and SC0 processes over these catalytic materials is discussed. In particular, it is concluded that Bronsted acidity is not a requirement for SCR and SC0 activity.

1. Introduction by ammonia, through the main reaction:

The so-called SCR process is now well established 4N0 + 4NH3 + O2 = 4N2 + 6H20.

and is widely used for the abatement of nitrogen oxides from waste gases of stationary sources [ 1,2]. It is based on the selective catalytic reduction of NO,

*Corresponding author.

The industrial catalysts are based on V205-Ti02- anatase with addition of either W03 or Moo3 [ 1,2]. These formulations are optimal in order to have sufficient catalyst stability and catalytic activity in the SCR reaction, but with limited activity in some

0926-860X/97/$37.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO926-3373(96)00092-6

46 J.M.G. Amores et al./Applied Catalysis B: Environmental 13 (1997) 45-58

concurrent reactions, i.e. the over-oxidation of NH3 to N20 and NO and the oxidation of SO2 (present in waste gases from power stations) to SO3 [3]. The use of Ti02-anatase as the support is related to its ability to give rise to vanadium oxide “monolayers” [4] with enhanced catalytic activity in oxidation reactions and to its behaviour towards SO,, that causes its reversible sulphation which does not result in catalyst deactivation [S]. Other catalyst formulations give rise to high or very high rates of the SCR reaction, but with lower selectivity to N2 or with higher conversions of SOz to SO3 [l] or, finally, with fast catalyst deactivation upon reaction. In particular, other transition metal oxides supported on TiOz-anatase, such as CuO-TiOz [6-81, Fe203-Ti02 [9] and Cr20s--TiOz [lo] have been investigated and appear to give good to very good performances. Also supported manganese, cobalt and nickel oxides are active SCR catalysts [l,l I].

Several Ti02-supported metal oxides active in SCR, like V205-Ti02 [12], WOs-Ti02, Moos-TiOz [13], CuO-Ti02 and MnO,-TiOz [ 141 are also active in the Selective Catalytic Oxidation of NH3 to N2 (SC0 process), following the reaction:

2NHs + 3/202 = N2 + 3H20 (2)

where N20 can also be a by-product [12-141. This process has been proposed recently as an industrial process for the abatement of slipped ammonia after the SCR reactors.

Several investigations on the mechanism of the SCR process have been published recently. It certainly occurs by reaction of strongly adsorbed ammonia with gas phase or weakly adsorbed NO [l]. However, authors disagree on the activation mode of ammonia over the catalysts. Takagi et al. [15] and Miyamoto et al. [16] suggested that the active ammonia species over vanadia-based catalysts are ammonium ions, so activation of ammonia would occur on the catalyst surface Bronsted acid sites. Several other authors supported a key role of Bronsted acid sites in produ- cing ammonium ions as active ammonia species, although with different reaction schemes [ 17-201. Other authors suggested that ammonia activation should alternatively occur at vanadyl sites. Janssen et al., on the basis of isotopic labelling experiments proposed ammonia activation over V=O groups giving rise to V-O-NH2 species [21], i.e. formally an adsorbed form of hydroxylamine. This hypothesis has

been recently considered also by Ozkan et al. [22]. Based on IR experiments over bulk V205 [23], V205- Ti02 [24] and V20s--WOs-Ti02 catalysts [25], a mechanism involving ammonia activation over vanadyl centres giving rise to V-NH2 amide species has been suggested by some of us. Other authors used this mechanism for kinetic modelling [26].

The adsorption and oxidation of ammonia on CuO- Ti02 and FezOs-based bulk catalysts, that are quite active in both SCR and SC0 [8,27], were investigated previously by some of us showing that they do not present significant Bronsted acidity. It has been concluded that Bronsted acidity is not a necessary requirement for SCR and SC0 catalytic activity.

IR spectroscopic methods are used since decades to characterise the surface acidity of catalytic materials [28-301. Among other probe molecules, ammonia can be successfully used to reveal the existence of both Lewis and Bronsted acid sites and to evaluate their strength [28-301.

In the present paper we summarise our results concerning the study of the interaction of ammonia with different Ti02-anatase supported transition metal oxides active in SCR and SC0 catalysis. The aim of this work is to obtain a broader picture on the interaction of ammonia over SCR and SC0 catalysts, to determine the type and the strength of the surface acidity of the catalysts that perform such reactions and to obtain further data on the activation of ammonia over such catalysts.

2. Experimental

Some data on the catalysts are summarised in Table 1. The M,O,-Ti02 catalysts have been pre- pared by impregnation of Degussa P25 Ti02 pigment (anatase with nearly 20% of rutile) followed by calcination at 673 K. The BET surface areas of the resulting catalysts are, within experimental error, the same of the support, 48 m2 g-l. The metal oxide loaded amount was calculated to have a lower coverage with respect to that of the full monolayer. The area per metal atom is of the order of 10 A2 in all cases.

FT-IR spectra were recorded with a Nicolet SZDX Fourier Transform spectrometer (4 cm-’ resolution) using self-supporting pressed disks of the pure catalyst

J.M.G. Amores et al./Applied Catalysis B: Environmental 13 (1997) 45-58

Table 1 3. Results Composition of catalysts investigated here

Catalyst

TiOz Fe2O3-Ti02 Coo,-Ti02 NiO,-Ti02 MnO,-Ti02 CrO,-TiOz CuO-TiO?

M,O, loading % wt/wt

- 5.9 as FezOx 5.9 as Co0 5.9 as NiO 6.0 as MnO 5.9 as Crz03 5.9 as CuO

Impregnating salt

- Fe(III) nitrate Co(I1) nitrate Ni(I1) nitrate Mn(I1) acetate Cr(II1) nitrate Co(I1) nitrate

powders, previously pre-treated by calcination in the IR cell at 673 K for 2 h and outgassing at 673 K for 30 min. Ammonia was taken from commercial cylin- ders from SIAD (Milano, Italy). Hydrazine water solution was purchased from Carlo Erba (Milano, Italy).

3.1. Ammonia and hydra&e adsorption on TiOz (anatase)

The spectrum of ammonia adsorbed on TiOz-anatase after outgassing at r.t. is shown in Fig. l(a). As already reported [28,31], ammonia is adsorbed on pure titania anatase at r.t. in co-ordinated form over two surface Lewis acid sites of different strength, according to the splitting of the symmetric deformation mode 6,,,NH3 at 1225 cm-‘, shoulder, and 1190 cm-‘, very strong. The position of these two components, definitely shifted upwards with respect to the value of liquid ammonia, 1054 cm-‘, indicates that Ti02 is a medium-strong Lewis acid solid [28- 30]. Also the bands in the NH stretching region (v,,NHs, vsymNH3 and the overtone of the &,,NHs

1

Fig, 1. FT-IR spectra of adsorbed species on TiOZ (anatase) after contact with (a) ammonia (20 Torr) and outgassing at room temperature 3 min; (b) after contact with hydrazine-water vapours (10 Torr) and following outgassing at (c) room temperature, (d) 373 K, (e) 473 K.

48 J.M.G. Amores et al./Applied Catalysis B: Environmental I3 (1997) 45-58

asymmetric deformation mode, interacting with vSymNH3 via a Fermi resonance) split in part, giving rise to bands at 3393, 3348, 3255, 3195 and 3 154 cm-‘. The 6,,NHs at 1603 cm-’ is not sensitive enough to split.

Traces of bands of ammonium cation can be observed, on our sample, according to shoulders at 1455 cm-’ and near 1650 cm-’ (Fig. la), due to the presence of silica impurities that provide weak Bronsted acid sites [31]. However, these shoulders desappear quickly by outgassing even at room temperature, showing that these sites and that TiO;?-anatase (even impure by silica) lacks of significant Bronsted acidity.

Additionally, a very weak band at 2197 cm-‘, quite resistant to outgassing even at rather high temperature, can also be found. According to Martra et al. [32] this band can be due to N; ions (that can be taken as an adsorbed form of molecular nitrogen [8,27]), while Matyshak and Krylov assigned a similar band found at 2200 cm-’ upon ammonia oxidation on ZnO to adsorbed N20 [33]. However, we previously investigated the interaction of N20 with Ti02-anatase and we found bands in the region 2235-2300 cm-‘, due to the asymmetric N-N-O stretching modes of molecularly adsorbed N20, readily desorbed upon outgassing at room temperature [34]. Thus, we are inclined to accept the assignment given by Martra et al. [32] to dinitrogen anions. By evacuation at increasing temperature the absorptions of co-ordinated ammonia disappear slowly. No other bands form. These data nicely agree with TPD data that showed that most of ammonia desorbs intact from Ti02-anatase, only small amounts of nitrogen being also produced [35,36], revealed by the detection of N; anions in our spectra.

The possible intermediacy of hydrazine in ammonia oxidation was previously proposed [8]. The spectra recorded after adsorption of a vapour arising from a hydrazine water solution and after following evacuation at increasing temperature on TiO*-anatase are reported in Fig. l(b)-(e). In the presence of vapours (Fig. lb), two strong absorptions at 1624 cm-’ (with shoulder at lower frequency) and 1564 cm-’ and weaker bands at 1416 cm-’ (weak, broad and with shoulder at higher frequency), 1355 cm-’ (very weak) and 1177 cm-’ (sharp) are detected. Evacuation at r.t. (Fig. lc) and at 373 K (Fig. Id) cause relevant changes in the spectrum: (i) the intensities of the

couple of bands at 1624 and 1564 cm-’ strongly decrease down to disappear, leaving a sharp absorption at 1610cm-‘, probably masked before outgassing; (ii) the band at 1416 cm-’ completely desappers, while a new band is detected at 1444 cm-’ (sharp and strong); and (iii) a new weak band appears at 1225 cm-’ and the band at 1177 cm-’ strongly increases in intensity and shifts downward first, and upwards later to 1186 cm-‘. After further evacuation at increasing temperature (Fig. le) weak sharp bands are detected at 1610 (apparently split), 1444, 1225 and 1180 cm-‘, with a further weak additional component near 1380 cm-‘. In the NH stretching region, before outgassing the spectral feature is dominated by a strong and broad absorption centred near 3300 cm-‘. After evacuation at room temperature the intensity of the broad band partially decreases and several bands can be detected at 3415,3387,3356, 3265, 3246, 3143, 3071, 2968 and 2928cm-‘. By outgassing at increasing temperature, all these bands decrease in intensity and totally desapper after evacuation at 473 K. No bands are observed in the region 2400-1800 cm-’ except again a weak band near 2200 cm-‘.

The positions of the observed IR bands in liquid and co-ordinated hydrazine are summarised in Table 2. According to the literature [37-391, the spectrum of co-ordinated hydrazine is characterised by bands in the region 1610-1560 cm-’ (NH2 scissorings), 135s 1300 cm-’ (NH2 waggings) and 122CL1150 cm-’ (N-N stretching and NH2 rockings). From the comparison of these data with Fig. 1, it seems evident that hydrazine is essentially adsorbed on titania in a co- ordinated form after outgassing at 373 K. However, the sharp band at 1444 cm-’ is hardly related to molecular hydrazine, and should be due to a transformation product. A band just in this position is observed frequently after both ammonia and hydrazine adsorption (see below and ref. [S]). A tentative assignment to the NH deformation of an imido species was given previously [8].

Instead, after contact with hydrazinelwater vapours the spectrum observed is dominated by different features, that can be assigned to hydrazinium ions. The absorptions at 1624 and 1564 cm-’ are due S,,NHa and 6,,,NHa of the -NH,f group, in agreement with the spectra of NH,-NH: Cl- [40] and of Cll +NHs-+NHa Cl- [41]. Also the band at

Table 2

J.M.G. Amores et al. /Applied Catalysis B: Environmental I3 (1997) 45-58 49

Wavenumbers (cm-‘) of the IR absorption bands spectra of hydrazine species

CrO-TiO2

NH3 ads

1613

1350

1221 1160

t.w.

Coo,-TiOZ

NH3 ads

1611

1341

t.w.

Fe203-Ti02

NH, ads

1612 1580 1350

1225 1160

t.w.

CuO-Ti02

NH3 ads

1611 1560 1350 1280 1220 1180

8

TiOz

N2& ads

1610

1380

1225 1180

t.w.

ZnClzx2 Nz& NZ&

Complex Liquid

1610 1608 1570 1608 1345 1324 1310 1283 1170 1098 1150 1042 975 871

39 37-39

Assignment

NH2 scissoring NH2 scissoring NH? wagging NH2 wagging N-N stretching NH?; rocking NH* rocking references

1416 cm-’ is reported for hydrazinium cations [40], likely due to another deformation mode. We can mention that hydrazine is slightly less basic than ammonia (pK, NH,f = 9.24; pK, NH*-N+Hs = 7.49 [42]). Previous experiments showed that the co-adsorption of water and pyridine (pK, CsHsNH+ = 5.25 [42]) on titania caused the appearance of pyridinium ions [43]. This is due to the Bronsted acidity of water, than can be even increased by molecular adsorption. The same occurs with hydrazine, which is more basic than pyridine, with the formation of hydrazinium ions in the presence of water.

Thus, our data suggests that after contact with hydrazine/water vapours the titania surface is necessarily hydrated and hydrazine is mainly adsorbed in a protonated form. Outgassing at room and higher temperatures causes water to desorb and the equilibrium:

NH2 -NH; OH- -+ NH2 - NH2 + HZ0 J’ (3)

to be shifted towards co-ordinated hydrazine. The interaction of hydrazine on the dry surface is, instead, molecular, in a co-ordinated form. Additionally, part of hydrazine likely undergoes a transformation giving rise to a new species characterised by a single band at 1444 cm-’ , possibly associated to NH species. It is not excluded that hydrazine can in part decompose according to the following reaction, proposed on tentative bases:

NH2 - NH2 -+ NH + NH3 (4)

The spectra shown in Fig. 1 show that adsorbed hydrazine and adsorbed ammonia on Ti02-anatase cannot be easily distinguished. In fact, most bands

are nearly superimposed. However, the band due to the &NH3 of co-ordinated ammonia is found quite broad invariably in the region 16051595 cm-’ [28], while the most intense 6NH2 mode of adsorbed hydrazine is located at or above 1610 cm-‘, is definitely sharper and pointed, and frequently is coupled to a weak sharp peaknear 1580 cm-’ . Also the lower frequency strong band due to adsorbed hydrazine is sharper than the corresponding one due to co-ordinated ammonia. Finally, weak bands could be observed in between these modes only in the case of adsorbed hydrazine. The spectrum of adsorbed hydrazine in the NH stretching region is certainly more complex than that of adsorbed ammonia. However, the bands are weaker and the radiation in this region can be severely scat- tered, so that the analysis of these modes is quite difficult. In any case, our study of ammonia adsorption on TiOz-anatase do not provide evidence of the formation of detectable amounts of hydrazine from ammonia on this surface even at higher temperatures.

3.2. Adsorption and oxidation of ammonia on Fe2O3-Ti02

The spectrum recorded after adsorption of ammonia on Fe@-TiOZ followed by evacuation at r.t. (Fig. 2a) is characterised by a band with a maximum at 1600 cm-’ and an evident sharp component near 1610 cm-‘, and also a tail at its lower frequency side. Additionally, a strong band centred at 1152 cm-’ with a weaker component at 1223 cm-’ are observed. Finally, two weak peaks are found at 1490 and 1443 cm-‘. Evacuation at increasing temperatures (Fig. 2b) causes a relevant sharpening of the band

50 J.M.G. Anores et al. /Applied Catalysis B: Environmental 13 (1997) 45-58

r

Fig. 2. FI-IR spectra of adsorbed species arising from contact of ammonia vapor (20 Torr) with Fe203-Ti02 (anatase) at room temperature and following outgassing at 300 K (a), 373 K (b) and 473 K (c).

around 1600 cm- ‘, with an inversion of the relative intensities of the maxima at 1610 cm-‘, now more intense, and at 1600 cm-‘, now weaker. A sharp peak can be envisaged also at 1580 cm-‘. In the lower frequency band the main maximum is now detected at 1172 cm-‘, while that at 1152 cm-’ is now evident as a shoulder. After outgassing at even higher temperature (473 K, Fig. 2c) bands are detected at 2200 and 1800 cm-‘, both weak, 1612cm-‘, sharp, 1477 and 1445 cm-‘, strong and sharp doublet, 1350 cm-‘, weak, 1225 and 1160 cm-‘.

The spectra described above show that Fe20s-TiO2 is a medium strength Lewis acid solid, as indeed expected on the basis of previous experiments on ammonia adsorption and oxidation on Fez03 [8,33,44] and on MgFezOd [8]. In fact, ammonia adsorbs co-ordinatively on Lewis sites giving rise to species characterised by the bands at 1600 and 1150 cm-‘, &NH3 and &,NHa, respectively. No

Bronsted acidity is found, like on pure TiOZ and on Fe203 WI.

On the other hand, the data also show that ammonia deeply transforms over this solid. The reactions observed here necessarily mainly involve the supported Fe203 species, because they are not found at all on the bare Ti02 surface. The results are consistent with the idea that co-ordinated ammonia species react at room and higher temperatures giving rise to hydrazine (bands at 1612,1580,1350,1225 and 1160 cm-‘, see table). Other products of ammonia transformation are those characterised by the bands at 2200, 1800, 1477 and 1445 cm-‘. These bands behave independently from each other, so that they should be due to four different species. Reasonable although tentative assignments [8,28,45] are to N; nitrogen anions (2200 cm-‘, NN stretching); adsorbed nitric oxide NO (1800 cm-‘, NO stretching), adsorbed nitroxyl HNO (band at 1477 cm-‘, NO stretching) and NH

J.M.G. Amores et al/Applied Catalysis B: Environmental 13 (1997) 45-58 51

imido species (1443 cm-‘, NH deformation). At even higher temperatures, all species disappear.

All the above species are possible products of ammonia oxidation at the surface, and some of them can be intermediates in the catalytic oxidation of NH3 to N2 or to NO,. On the other hand, ferric oxides are known to act as catalysts for ammonia oxidation either to nitrogen (SC0 reaction), at lower temperature, or to NO at higher temperatures [46,47]. The present data can be taken and an evidence of the high oxidising ability of the oxidised Fe203-Ti02 surface towards ammonia.

3.3. Ammonia adsorption and oxidation on Coo,--Ti02

Contact of ammonia with Coo,-Ti02 at room temperature (Fig. 3a) causes the formation of bands

at 1602 cm-’ (sharp and strong), 1585 cm-’ (weak), 1444 cm-’ (medium, sharp) and 1150 cm-’ (with shoulders at higher frequency). Evacuation at 373 K (Fig. 3b) causes, as the main result, the band near 1600 cm-’ to shift up to 1610 cm-‘, and sharpen, and the overall transmittance of the disk to decrease so that the region below 1150 cm-’ becomes obscured. Out- gassing at higher temperature (573 K, Fig. 3c) results in the growth of sharp bands at 1611, 1476, 1443 and 1341 cm-‘. These new absorption disappear by a further heating (Fig. 3d), while in the region 2400- 1700 cm-’ only a weak band near 2030 cm-’ can be detected. A similar band was previously observed on CuO-Ti02 and assigned to N; ions [8].

These data show that also Coo,-TiOz is a medium- strength Lewis acid solid (bands at 1600 and 1150 cm-‘, F,,NH3 and 6,,,NH3, respectively, of co-ordinated ammonia), with no detectable Brgnsted

Fig. 3. FT-IR spectra of adsorbed species arising from contact of ammonia vapor (20 Torr) with Coo,-Ti02 (anatase) at room temperature and following outgassing at 300 K (a), 373 K (b), 573 K (c) and 773 K (d).

52 J.M.G. Amores et al. /Applied Catalysis B: Envimnmental 13 (1997) 45-58

acidity. On the other hand cobalt oxides were characterised previously as quite basic solid materials [48]. Furthermore, also on Coo,-Ti02 ammonia transformation to oxidised products is found. It seems reasonable to propose that co-ordinated ammonia oxidises to hydrazine (bands at 1611 and 1341 cm-‘, with additional components at lower frequencies, not detectable). Additionally, the typical band doublet at 1476 and 1443 cm-’ is also found. However, the lower component of this doublet (assigned to imido- species) is already detectable after adsorption at room temperature, while the higher frequency component (assigned to nitroxyl) only appears after heating.

The chemistry of ammonia adsorption and oxidation observed on Coo,-TiOZ is similar to that observed on CuO-Ti02 [8] and Fe2O3-Ti02 and also

agrees with the high activity of Cos04 in ammonia oxidation to N2 and to NO [46,47]. It was previously shown by some of us that ammonia is deeply oxidised at the Cos04 surface giving rise to nitrogen oxides already at room temperature, while it is simply molecularly adsorbed on Co0 [48]. So, we can reasonably assign the activity in ammonia oxidation to Co3+ sites at the surface of the Coo,-TiO2 catalyst.

3.4. Ammonia adsorption on NiO,-Ti02 and MnO,-TiOz

The spectra of ammonia adsorbed on NiO,-Ti02 and MnO,-TiOz are similar (Fig. 4). In both cases the features of co-ordinatively adsorbed species (1602 and 1160 cm-‘, 6,,NHs and 6,,,NHs, respectively) are observed and r.t. and decrease down to disappear after

1

Fig. 4. Fl-IR spectra of adsorbed species arising from contact of ammonia vapor (20 Torr) with MnO,-TiOZ (anatase) (a and b) and NiO,- Ti02 (anatase) (c and d) at room temperature and following outgassing at 300 K (a and c), and at 473 K (b and d).

J.M.G. Amores et al./Applied Catalysis B: Environmental 13 (1997) 45-58 53

outgassing at 473 K. In both cases, this treatment leads to the appearance of a weak band at 2190 cm-‘. In the case of MnO,-Ti02 a very weak band at 1445 cm-’ is detected at room temperature up to 473 K.

These data show that also NiO,-Ti02 and MnO,- TiOz are medium-strength Lewis acids, with no detectable Bronsted acidity. We do not observe intense bands due to partial oxidation products of ammonia here, but adsorbed ammonia species are completely disappeared already after heating at so low a temperature as 473 K, so at a definitely lower temperature with respect to those observed on the above catalysts. This should indicate that these oxides are even more active than the others investigated here in ammonia oxidation to nitrogen, that is per se not detectable but is revealed by the weak band at 2190cm-‘, assigned to N; anions. This can be related to the highest activity found, among transition metal oxides, by Mn02 in ammonia oxidation [46,47].

3.5. Ammonia adsorption and oxidation on CrO,-Ti02

The spectrum of ammonia adsorbed on CrO,-Ti02 recorded in presence of gas (Fig. 5a) is significantly different with respect to all those recorded in contact with the previous catalysts. In fact in addition to the bands due to co-ordinated ammonia near 1601 and 1160 cm-’ (Z&NH3 and 6,,,NH3, respectively) a broad absorption at 1460 cm-’ due to ammonium ion (6,,NH4) and a shoulder near 1650 cm-’ @,,,NHJ are also clearly detected. Evacuation at r.t. (Fig. 5b) causes the disappearance of these absorptions, while the symmetric deformation of co-ordinated ammonia contains three components at 1223, 1172 and 1149 cm-‘. A weak sharp band at 1446 cm-’ is also detected.

The spectrum obtained after evacuation at 373 K is similar to the previous one, although the maximum of

Fig. 5. FLIR spectra of adsorbed species arising from contact of ammonia vapor (20 Torr) with CrO,-TiO* (anatase) at room temperature in contact with the gas (a) and following outgassing at 300 K (b) 373 K (c) 473 K (d) and 673 K (e).

54 J.M.G. Amores et al./Applied Catalysis B: Environmental I3 (1997) 45-58

JJ,oBo 0.40 I

2sJo 2200 2100 2ow loo0 1040 1020 loa, SW OBO w*-tarn(cm-1) W*wmmlbm(c4n-1)

Fig. 6. Absorption bands in the FT-IR spectrum of GO,-Ti02 (anatase) after activation, assigned to Cl=0 stretching modes.

the strong low frequency peak is shifted to 1172 cm-’ and, at the higher frequency side of the i&NH3 mode, a sharp component at 1610 cm-’ becomes evident. After outgassing at 473 K (Fig. 5d) the spectrum definitely transforms. Bands at 1613 (sharp), at 1479,1443, 1350, 1221 and 1160 cm-’ are observed. They can be assigned as above to hydrazine, nitroxyl and imido species. These bands persist, although weaker, after outgassing at 673 K.

Difference spectra show that ammonia adsorption cause the perturbation of sharp peaks observed weak in the spectrum of the activated catalyst at 2019,2000, 1012, and 1000 cm-’ (Fig. 6). These bands are due to the Cr=O stretchings (two overtones and two funda- mental modes) that have been previously fully characterised using Raman spectrometry by Wachs [49] for similar CrO,-TiOz catalysts. According to this author, they are due to two different species, both having a single chromyl Cr=O bond. These species are perturbed by ammonia adsorption possibly because the co-ordination sphere at chromium can expand or

because they are sensitive to the interaction with nearer Ti4+ or Cr3+ cations.

These data show that, among the TiOz-supported transition metal oxides investigated here, CrO,-TiO2 is the only one showing, besides medium-strength Lewis acidity, also detectable Bransted-type acidity, although very weak. On the other hand, this catalyst is also the only one showing the features of covalently- bonded M=O bonds, associated to high valence, probably hexavalent, chromium species. As discussed else- where [50], these two features are related, Bronsted- type acidity being associated to covalence of the metal-oxygen bond to the availability of double-bonds able to delocalize the anionic charge after dissociative adsorption. Surface hexavalent chromium species in the form of chromyl species are also found on oxidised bulk chromia and are also associated to weak Bronsted-type acidity [5 11.

On the other hand, and probably independently from this, the chemistry related to ammonia oxidation found on the CrO,-TiOz surface seems to be closely


related to those found on CuO-Ti02 [8] and on FezOs- Ti02 and Coo,-TiO2.

4. Discussion

The results of the present study can be discussed in relation to the catalytic activity, described in the literature for some of the materials investigated here in the SCR and in the SC0 reaction. These data can also be related to previous studies concerning the interaction of ammonia with materials like VzOs- Ti02 [23,24,52], Moos-Ti02 [53], WOs-Ti02 [54] and CuO-TiOz [8], which are all active catalysts in both SCR and SCO.

V205-Ti02, Moos-Ti02 and WOs-Ti02 are all both quite strong Lewis acids and quite strong Bronsted acids, as demonstrated by the adsorption of ammonia occurring both coordinatively and in a protonated form. This leads several authors to propose that the previous protonation of ammonia to ammonium ions provides the activation towards SCR [15- 201 and possibly also towards SCO. We show here that Fe203-Ti02, which is certainly an active catalyst for SCR [9], is, like Cu&Ti02 [8] not Bronsted acidic. Also CrO,-Ti02, which is reported to be highly active in both SCR and ammonia oxidation [lo], is only very weakly Bransted acidic. No Bronsted acidity is found also on MnO,-Ti02, like previously on MnO,-A120s active SCR catalysts [55].

Water is certainly present in the reaction conditions of both SCR and SCO, being a reaction product. The reaction of water with ammonia can give rise (at room temperature) to ammonium ions because of the Bronsted acidity of water, not because of the Bransted acidity of the catalyst. As shown here in the case of hydrazine adsorption on Ti02 in the presence of water, the base + water equilibrium is displaced towards the reactants by mild heating, so that the protonated base disappears just above room temperature. The ammonia adsorption experiments discussed here have been performed with hydroxylated catalyst surfaces free of adsorbed molecular water. Thus, we give the precise information on whether the catalyst Bronsted acidity is sufficiently strong to protonate ammonia or not. Even in the presence of water, ammonium species cannot be observed just above room temperature. On the other hand, we previously showed [24]

that even on quite strong Bronsted acidic surfaces, in conditions approaching those of the reaction, ammonium species are largely disappeared. These data further support our conclusion that Bronsted acidity and ammonium ions are directly involved neither in SCR nor in SCO.

The data reported here show that catalysts like Fe2O3-Ti02, CrO,-Ti02 and Coo,--TiOZ have a high activity in the surface oxidation of ammonia, as demonstrated by the complex spectra arising from ammonia adsorption, certainly due to the formation of several different oxidation products. This activity is not observed on the bare TiOa support, and this definitely demonstrates that it is due to the supported transition metal oxide species. On the other hand, it has been shown that iron, chromium and cobalt oxides are active catalysts for ammonia oxidation giving rise either to dinitrogen or to nitrogen oxides at different reaction temperatures [46,47] while Ti02 is far less active [46].

More than one species are formed from ammonia over these catalyst surfaces and it seems strongly supported the idea that hydrazine is one of them, like previously proposed for ammonia oxidation over CuO-Ti02 [8]. Looking at the possible mechanisms for the ammonia oxidation to dinitrogen (SC0 process), the intermediate formation of hydrazine as the first N-N bond containing species seems indeed quite reasonable. On the other hand, it also seems quite supported in the literature that over oxidizing surfaces, ammonia can easily pose one hydrogen giving rise to amide species NHZ, which have been actually observed on SCR catalysts [23,24]. We previously speculated that, to be an oxidation product of ammonia, amide species must have a radical-like character [23,24]. It seems consequently very reasonable to suppose that such species tend to undergo a radical-like coupling, giving easily rise to hydrazine. On the other hand, hydrazine is produced industrially through oxidation of ammonia via amide species: the Rasching process employs sodium hypochlorite NaClO and involves chloramine NH2Cl as an intermediate [56]. Moreover, inorganic chemistry says that hydrazine is easily oxidised to nitrogen by high-valence cations like Cr(VI), Co(III), Mn(III), Cu(I1) and Mo(V1) [57]. This supports our proposal that, over the surfaces of Fe2O3-Ti02, CrO,-TiOZ, Coo,-Ti02 and CuO-TiOZ the following

56 J.M.G. Amores et al. /Applied Catalysis B: Environmental 13 (1997) 45-58

mechanism is active:

NH3 + M”+ + NH2 + MC”-‘I+ + H+ (5)

2NHz 4 N&L, (6)

N2H4 + 4M “+ + N2 + 4M(“-‘)+ + 4H+ (7)

Under SC0 catalytic reaction conditions the following reoxidation reaction closes a Mars-Van Krevelen-type redox mechanism:

2M(“-‘I+ + 2H+ +1/202 + 2M”+ + H20 (8)

In reactions (5) to (8) only one electron exchanges at cations are considered, but other possibilities are not excluded.

As already proposed, it seems also likely that amide species are common intermediates in SC0 and in SCR, which are certainly very closely related reactions. In fact, amide species can react, via a radical- like coupling, with NO [8,23,24] with the intermediacy of a nitrosamide-like species NH2-NO, to finally give again dinitrogen and water, as shown below:

NH2 + NO + NH2-NO + N2 + H20 (9)

Other species besides hydrazine are also found to be produced from ammonia in our experiments. In particular we observe upon different experiments the formation of a sharp band near 1445 cm-’ and of another sharp band near 1480 cm-‘. The band near 1445 cm-’ is observed to be formed from hydrazine on Ti02, but it is not observed from ammonia on the same surface. On the contrary, this band is observed to be formed also from ammonia on all other surfaces, which have an oxidising character. We previously assigned, tentatively, this band to the NH deformation mode of an adsorbed imido species. We proposed above that this species can form from hydrazine on bare titania through a non-oxidative way, reaction (4). On the oxidation catalysts, instead, imido species can form from deeper oxidation of ammonia, through amide:

NH2 + M”+ -+ NH + MC”-‘)+ + H+ (10)

This can represent the first step of the way giving rise finally to NO,, which is competitive with respect to the main ways for both SC0 (steps (5), (6) and (7)) and SCR (steps (5) and (9)).

On all surfaces that produce hydrazine from ammonia (i.e. on Fe2O3-Ti02, Coo,-Ti02, CrO,-Ti02 and CuO-Ti02) we detect, at relatively higher tempera-

ture, a new strong band near 1480 cm-‘. In effect, when this band is formed, also the band near 1445 cm-’ is enhanced in intensity. This can indicate that either two species are actually existing at relatively high temperature, that responsible for the band at 1480 cm-’ and the imido species responsible for the band at 1445 cm-‘, or, alternatively, one only species responsible for both bands (one of which is superimposed to the band of imido species) is existing at high temperature. Catalytic ammonia oxidation studies showed that N2 is generally the predominant product of ammonia oxidation at low temperatures while NO is produced at higher temperatures. As for example, over bulk Mn02, Cos04, CuO and Fe20s, N2 is the main product in the temperature range 373- 573 K, while NO predominates above near 673 K [47]. Also on catalysts similar to those have been investigated here, like e.g. V205-Ti02 [ 121 and CuO- Ti02 [8,58], ammonia oxidation gives rise mainly to N2 at low temperature and to NO at higher temperature. For this reasons, it seems quite reasonable to suppose that the species responsible for the strong band at 1480 cm-’ is associated to NO formation. We previously assigned, tentatively, this band to the NO stretching of an adsorbed nitroxyl species HNO. HNO has been isolated in matrices and its IR spectrum recorded; in these conditions this species has been found to be responsible for three bands near 3400 cm-’ (asymmetric stretching, mainly NH stretching), 1565 cm-‘, symmetric stretching, mainly NO stretching) and near 1100 cm-’ (deformation) [59]. However, when adsorbed this species can be responsible for a stretching band similar to that of other N=O containing species (typically in the 1400- 1550 cm-’ range) and for a NH bending like for secondary amines, in the 1450-1400 cm-’ region. So, a species similar to nitroxyl can be responsible for both bands we observe to grow upon heating at 1480 and 1445 cm-‘. This species contains nitrogen in a more oxidised state than hydrazine, so that in can be formed either by ammonia and by hydrazine oxidation, as it has been found indeed.

The data reported here show that on catalytic systems like Fe203-TiOz, Coo,-Ti02 and CrO,- TiOa ammonia oxidation experiments give rise to a picture which is definitely similar to that we previously described for CuO-TiOZ [8]. It seems likely that the situation is also similar MnO,-TiO2 and


NiO,-TiOz although, possibly due to the even faster oxidation of ammonia, intermediates were not found there. The data presented here allowed to precise some details of the mechansim proposed previously for the SCR and SC0 reactions, as well as for the oxidation of ammonia to NO,.

We propose a Mars-van Krevelen-type redox mechanism for both SCR and SC0 and also for the oxidation of ammonia to NO. Amide species, NHz, and hydrazine, N&, would be intermediate in SCO, while NH and NH0 species would be intermediates in NO, production. Amide species, NH2, would be common intermediates for SCR and SCO. This picture at least partially agrees with previous proposals from the literature. Among others, Williamson et al. [60] suggested that amide species NH*, formed by ammonia oxidation on Cu2+ sites, are key intermediates in NH3 oxidation to N2 on CuNaY zeolites. More recently, Biermann and Janssen [61] reported that common intermediates exist for SCR and SC0 over molybdena-silica. They identified such intermediates as NH20 species (i.e. hydroxylamine-type com- plexes). They proposed a Mars-van Krevelen-type redox mechanism for SCO, suggesting NH20 species to “dimerize” give rise to a O-NH-NH-O intermediate. A key role of NH and NH0 species in ammonia oxidation over transition metal oxides was proposed many years ago by Zawadski [62], and supported more recently by Germain and Perez [46]. These authors suggested that these species can react each other giving rise to nitrogen, while NH0 would be intermediate in NzO formation and NH species would react with molecular oxygen to give NO. To a partial support to a possible role of molecular oxygen in ammonia oxidation to NO,, we can cite a paper from Escalona-Plater0 et al. [63] who observed spectro- scopically the oxidation of ammonia to nitrates by surface superoxide species on Coo-MgO catalysts, and a recent observation that virtually irreducible non- transition metal oxides can have remarkable activity in ammonia oxidation [64].

The data presented here and in refs. [8], [23] and [24] are, to our knowledge, the first giving “direct” evidence for some or all surface intermediates in ammonia oxidation catalysed by transition metal oxides, although, at the moment, the identification of most of these species must be taken as largely tentative. In any case, the overall picture concerning

the data available in the literature for the reaction concerned here seems to converge into what we can call a “ generalised” mechanism active over metal oxide catalysts, whose different behaviour should mainly be related to the relative rates of the different steps.

5. Conclusions

The conclusions from the present study are the following: 1. Fe203-Ti02 catalysts do not present Bronsted

acidity while CrO,-Ti02 only present a very weak Bronsted acidity involving high valence chromium species (chromyl species). According to the catalytic activity of these systems in SCR and SCO, this confirms the previous conclusion that Bronsted acidity is not directly involved in the SCR and SC0 activities.

2. Over Fe203-Ti02, Coo,-Ti02 and CrO,-TiOz catalysts ammonia is oxidised in the temperature range 300-673 K giving rise to hydrazine and to other species identified as imido NH and nitroxyl HNO species. Hydrazine is thought to be intermediate in ammonia oxidation to dinitrogen (SC0 process) while imido and nitroxyl species are thought to be intermediates in ammonia oxidation to NO.

3. The reactivity of FezOX-Ti02, Coo,-Ti02 and CrO,-Ti02 catalysts towards ammonia is closely similar to that observed previously for CuO-TiOz catalysts.

4. NiO,-TiOz and MnO,-Ti02 appear to be even more active in ammonia oxidation than other anatase-based metal oxides, and, just for this reason, no intermediate adsorbed species are found over their surfaces.

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