J O U R N A L O F C H E M I S T R Y Materials A study of Mn–Ti oxide powders and their behaviour in propane oxidation catalysis Fabio Milella,*a Jose� Manuel Gallardo-Amores,a,b Marco Baldic and Guido Buscaa* aIstituto di Chimica, Facolta` di Ingegneria, Universita`, P.le J.F. Kennedy, I-16129, Genova, Italy bDepartamento de Quý�mica Inorga�nica, Universidad Complutense, Ciudad Universitaria, E-28040-Madrid, Spain cDipartimento di Ingegneria Idraulica e Ambientale, Universita`, via Ferrata 1, I-27100 Pavia, Italy Received 28th May 1998, Accepted 23rd July 1998 Mn–Ti mixed oxides with composition Ti1-xMnxOy (x=0, 0.1, 0.2, 0.5, 0.8, 0.9 and 1) have been prepared.A Mn–TiO2 monolayer type sample has also been prepared by impregnation, for comparison. Manganese is found to speed up the anatase-to-rutile phase transition, more clearly in the impregnated sample, while titanium tends to slightly hinder the thermodynamically reversible hausmannite-to-bixbyite phase transition upon cooling.The catalytic activity of all samples in propene oxidation decreases by increasing the Ti content. Conversely, the catalytic activity in propane oxidation shows a maximum at intermediate composition.tion with the required amount of manganese acetate to an 1 Introduction aqueous suspension of TiO2 (Degussa). Then, it was stirred Manganese based mixed oxides, such as perovskite-type at 373 K until total removal of water, and treated thermally manganites,1 Mn-containing b-aluminas,2 Mn-containing spi- as above. nels,3 Mn oxides supported on alumina as powders4 and on Nitrates and residual organic compounds were decomposed, cordierite monoliths5 and the pure oxides MnO2,6–8 Mn2O39,10 in air, in an electronically controlled furnace at 723 K for 4 h.and Mn3O411 have been proposed as cheap, environmentally The heating and cooling rate before and upon calcination was friendly and active catalysts for volatile organic compound 40 K min-1.(VOC) and methane total oxidation processes. They are, XRD spectra were recorded on a Philips PW 1710 however, less active than the more expensive and environmen- diVractometer (Cu-Ka radiation, Ni filter; 45 kV, 35 mA) and tally demanding catalysts based on noble metals.5 To increase cell parameters calculated using dedicated least square the activity of oxidation catalysts, supporting the active phase software.FTIR spectra were recorded using a Nicolet Magna on oxide carriers or mixing it with other oxides is sometimes 750 Fourier Transform instrument. For the region useful. In particular, TiO2-anatase is reported to activate V2O5 4000–350 cm-1 a KBr beam splitter has been used with a catalysts for several oxidation reactions12,13 and vanadia– DTGS detector.For the FIR region (600–50 cm-1) a ‘solid titania based catalysts are used industrially for alkyl aromatic substrate’ beam splitter and a DTGS polyethylene detector oxidations14,15 as well as for reducing NOX with ammonia in were used. KBr pressed disks (IR region) or polyethylene the selective catalytic reduction (SCR) process.16,17 Titania pressed disks and samples deposed on Si disks (FIR region) was also reported to activate molybdena and tungsta based were used.catalysts for oxidation18 and MoS2 and WS2 sulfide catalysts FT-Raman spectra were recorded using a Bruker RFS100 for hydro-treating.19 The nature of such activating eVects is Instrument, with an Nd-YAG laser (1064 nm), using 30 mW still under debate, being either due to the ability of anatase to laser power, 2000 scans and 4 cm-1 resolution.DiVuse reflecdisperse appropriately the active phases or to optimise their tance spectra in the range 2500–200 nm were obtained with a acid–base properties,20 or due to electronic eVects.21,22 Jasco V-570 spectrophotometer at room temperature using a Supporting or mixing Mn oxides with titania seemed to be polymer as reference.BET surface areas were measured with a reasonable option to try to improve Mn oxides for oxidation a conventional volumetric instrument by nitrogen adsorption catalysis and to further study the eVect of oxide supports with respect to supported oxide catalysis. Mn–Al oxides have been deeply investigated as SCR catalysts.23,24 Mn–Ti oxides are industrially produced as inorganic pigments25 and are reported to behave as fairly active and selective catalysts in the oxidation of NH3 to N2.26 In the present paper we will summarise our results on the preparation, characterisation and testing of Mn–Ti complex oxides. 2 Experimental The preparation of the mixed oxide samples, TixMn1-xOy (x=0, 0.1, 0.2, 0.5, 0.8, 0.9, 1), was carried out by mixing carefully Mn(CH3COO)3·2H2O (Acros, 98%) and Ti[OCH(CH3)2]4 (Aldrich, 97%) hydrolysing with water, then drying the gel at 393 K for several hours.The subscript x in the notation corresponds to the atomic fraction of Ti in the whole metal content. Another sample was synthesised by a Fig. 1 XRD patterns of the powders after calcination at 773 K. conventional impregnation method, adding an aqueous solu- J.Mater. Chem., 1998, 8(11), 2525–2531 2525at liquid nitrogen temperature. DTA–TG experiments were performed in air, with a Setaram TGA 92-12 apparatus, from room temperature to 1273 K, with heating and cooling rates of 10 K min-1. Catalytic tests were carried out at atmospheric pressure in a continuous flow tubular glass reactor. Variable amounts of catalysts calculated to have the same exposed total surface area (1.83 m2) were loaded in the form of fine powder (60–70 mesh) mechanically mixed with a predetermined amount of inert, low surface area, material (quartz) to avoid preferential gas flow paths and hot spots in the catalytic bed.The total gas flow was 330 ml min-1 and the feed composition was ca. 1.5% of hydrocarbon in oxygen-containing helium.The hydrocarbon/oxygen molar ratio in the feed was 156. The reactants and the reaction products were analysed using two on-line gas chromatographs (HP 5890), working in diVerent analysis conditions in order to give a better resolution of inorganic and organic species. 3 Catalyst characterisation 3.1 Structural characterisation of the mixed oxide catalysts after calcination at 773 K Fig. 1 shows the XRD powder patterns of the mixed oxide samples after calcination at 773 K. The observed crystal phases with the measured unit cell parameters are summarised in Table 1. The Mn oxide sample is constituted by the random tetragonal spinel phase a-Mn3O4 (hausmannite). This phase alone is also found in the sample Ti0.1Mn0.9 and is present in the patterns of all samples up to Ti0.8Mn0.2 where it is still Fig. 2 FTIR/FTFIR skeletal spectra of the powders after calcination detected in traces. From Ti0.2Mn0.8 the TiO2-anatase phase is at 773 K. (a) TiO2, (b) Ti0.9Mn0.1, (c) Ti0.8Mn0.2, (d) Ti0.5Mn0.5, also observed, and is the only phase detectable in the sample (e) Ti0.2Mn0.8, (f ) Ti0.1Mn0.9, (g)Mn3O4, (h) 6%Mn-TiO2 (Degussa), Ti0.9Mn0.1.Pure titania is constituted by anatase with small (i) TiO2 (Degussa). amounts of brookite. Traces of rutile are detected for Ti0.5Mn0.5 only. The cell volume of the anatase phase appears to increase with dissolution of Mn in the samples Ti0.9Mn0.1 typical of spinels20,29 and reported explicitly for hausmannand Ti0.8Mn0.2. Conversely, the volume of the hausmannite ite,20,30 for pure Mn oxide [Fig. 3(a)] and for samples up to phase appears to decrease in the samples Ti0.2Mn0.8 and Ti0.8Mn0.2 [Fig. 3(b)–(e)]. In the Raman spectrum of Ti0.5Mn0.5 suggesting dissolution of Ti in Mn3O4. Ti0.2Mn0.8 the main peak of anatase21 at 142 cm-1 starts to The skeletal IR and FIR spectra clearly show the typical be present. For the sample Ti0.9Mn0.1 only the peaks of anatase absorptions of the hausmannite spinel phase for the pure Mn are found at 638, 512, 395, 322, 236 and 142 cm-1, while for oxide [Fig. 2(g)] with main bands at 611, 524, 421, 245, 165 pure TiO2 traces of brookite (450 and 359 cm-1) are detectable and 124 cm-1, in good agreement with the results reported by within the pattern of anatase.21 Thus, both vibrational spectro- Lutz et al.27 Theabsorptions are also observed in the scopic techniques fully confirm the data arising from XRD spectra of the Ti-containing samples up to Ti0.5Mn0.5 summarised in Table 1.[Fig. 2(d)–(f )], although traces of these absorptions can also The specific surface areas, reported in Table 1, show an be found for Ti0.8Mn0.2. In parallel, the broad absorptions of almost monotonic decrease upon increasing Mn content. TiO2 (anatase)28 are clearly found already for Ti0.2Mn0.8 [Fig. 2(a)–(c)]. In the case of Ti the complexity in the range 3.2 Study of the thermal stability of mixed oxide catalysts 600–400 cm-1 is attributed to the presence of some traces of brookite.21 DTA and XRD studies. The DTA curves for mixed oxide catalysts are shown in Fig. 4. The observed crystal phases with Similarly, the Raman spectra show a peak at 655 cm-1, Table 1 XRD data for TixMn1-xOy samples at 773 K Cell parameters/A° Sample Tcalc./K XRD phase(s) a c Volume/A° 3 SBET/m2 g-1 Ti 773 Anatase 3.777(2) 9.460(6) 135.0 94 (Brookite) Ti0.9Mn0.1 773 Anatase 3.790(1) 9.446(5) 135.6 83 Ti0.8Mn0.2 773 Anatase 3.790(1) 9.493(4) 136.4 53 (Hausmannite) Ti0.5Mn0.5 773 Anatase 3.809(6) 9.434(27) 136.9 42 Hausmannite 5.763(1) 9.429(5) 313.0 (Rutile) Ti0.2Mn0.8 773 Hausmannite 5.761(0) 9.427(3) 313.0 25 (Anatase) Ti0.1Mn0.9 773 Hausmannite 5.768(0) 9.446(1) 314.3 18 Mn 773 Hausmannite 5.759(1) 9.443(5) 314.3 11 2526 J.Mater. Chem., 1998, 8(11), 2525–2531Table 2 XRD data for TixMn1-xOy samples at 1273 K. Cell parameters/A° Tcalc./ XRD Volume/ Sample K phase(s) a c A° 3 Ti 1273 Rutile 4.592(0) 2.959(0) 62.4 Ti0.9Mn0.1 1273 Rutile 4.589(0) 2.957(0) 62.3 Pyrophanite 5.139(0) 14.277(5) 326.5 Ti0.8Mn0.2 1273 Rutile 4.585(0) 2.956(0) 62.1 Pyrophanite 5.125(1) 14.270(5) 324.6 (Bixbyite) Ti0.5Mn0.5 1273 Pyrophanite 5.136(1) 14.279(3) 326.2 Rutile 4.596(5) 2.943(9) 62.1 (Bixbyite) Ti0.2Mn0.8 1273 Bixbyite 9.425(5) 9.425(5) 837.2 Pyrophanite 5.118(2) 14.229(24) 322.8 (Rutile) Ti0.1Mn0.9 1273 Bixbyite 9.425(1) 9.425(1) 837.2 (Rutile) Mn3O4 1273 Hausmannite 5.761(0) 9.450(2) 313.6 Mn2O3 a Bixbyite 9.430(2) 9.430(2) 838.6 aReference sample.the measured unit cell parameters after DTA are summarised in Table 2. At the end of the DTA cycle, the Mn oxide is still in the form of Mn3O4 (hausmannite), although with a large decrease in the crystal size.By contrast the sample Ti0.1Mn0.9 only shows features that must be attributed to a-Mn2O3 (bixbyite) with small traces of TiO2-rutile. A further phase, the mixed oxide MnTiO3 (pyrophanite) present at a trace level in Ti0.2Mn0.8, is the predominant phase in Ti0.5Mn0.5 and is a minor phase for the sample Ti0.8Mn0.2. In the latter sample rutile is the major phase.Unit cell parameters of rutile slightly decrease by increasing Mn content in the mixed phase powder, in agreement with the behaviour for MnO2–TiO2 solid solu- Fig. 3 FT-Raman spectra of the powders after calcination at 773 K. (a) Mn3O4, (b) Mn0.9Ti0.1, (c) Mn0.8Ti0.2, (d) Mn0.5Ti0.5, tions.31 By contrast, bixbyite parameters decrease with respect (e) Mn0.2Ti0.8, (f )Mn0.1Ti0.9, (g) TiO2 (its intensity value is 10 times to a pure reference sample, possibly indicating that some greater than for the others).titanium is dissolved in it. For TiO2 [Fig. 4(a)] the DTA curve shows a sharp exothermic peak at 1013 K, due to the anatase-to-rutile transition, 32,33 preceded by a broad exothermic feature in the range 873–973 K due to anatase sintering.34 The anatase-to-rutile phase transition is shifted to ca. 963 K in catalysts Ti0.9Mn0.1 and Ti0.8Mn0.2 [Fig. 4(b) and (c)], and is associated with a very small weight loss detectable in the TG curve (Table 3). A pronounced endothermic peak associated to a weight loss is observed additionally at 1203 K in the DTA curve (Table 3) of catalysts Ti0.9Mn0.1 and Ti0.8Mn0.2. According to XRD analysis of the samples after the DTA runs up to 1073 and Table 3 TG data on TixMn1-xOy samples Temperature/ Weight Weight Sample K loss (%) gain (%) TiO2 873–973 — — 1013 — — Ti0.9Mn0.1Oy 963 — — 1203 0.65 — Ti0.8Mn0.2Oy 963 — — 1203 0.92 — Ti0.5Mn0.5Oy 873–963 0.80 — 963 — 0.45 1203 2.61 — Ti0.2Mn0.8Oy 863–1023 — 0.90 1203 0.45 — Ti0.1Mn0.9Oy 863–1057 — 1.70 1203 0.30 — Mn3O4 863 — 2.05 1057 1.40 — TiO2 (Degussa) 1083 — — Fig. 4 DTA curves of the powders after calcination at 773 K. (a) TiO2, 6%Mn-TiO2 (Degussa) 973 — 0.15 (b) Ti0.9Mn0.1, (c) Ti0.8Mn0.2, (d) Ti0.5Mn0.5, (e) Ti0.2Mn0.8, (f) 1213 0.35 — Ti0.1Mn0.9, (g) Mn3O4. J. Mater. Chem., 1998, 8(11), 2525–2531 25271273 K, these features are due to the reactions first giving rise to Mn2O3 from Mn3O4 (in fact Mn3O4 is metastable with respect to Mn2O3 below ca. 1253 K)35 and later producing MnTiO3 (pyrophanite), according to the following stoichiometry: TiO2+1/2 Mn2O3�MnTiO3+1/4 O2 In the case of Ti0.5Mn0.5 [Fig. 4(d)] we again find the features due to the anatase-to-rutile phase transition (963 K) and pyrophanite formation (1203 K). However, we also find a complex situation with weight loss and an exothermic phenomenon, in the range 873–973 K followed by a weight gain during the anatase-to-rutile transition phase.In agreement with the above experiments, during the exothermic anatase sintering, Mn3+ oxide species are segregated as Mn3O4, with oxygen loss. Later Mn3O4 is oxidised to Mn2O3 in the range 973–1013 K before reacting with TiO2 to give pyrophanite. In fact, the only way to explain a weight gain is that part of Mn is oxided.For the samples with a large content of Mn, no phase transition of anatase is observable while the endothermic peak associated with pyrophanite formation is still observed. Another endothermic peak due to the Mn2O3-to-Mn3O4 phase transition is found at 1057 K mainly for pure Mn oxide. In fact, according to the thermodynamic phase diagram,33 Mn3O4 is thermodynamically stable above 1253 K.In agreement with this the TG–DTA (Table 3) runs for Mn3O4 show first a weight gain at 863 K (metastable-to-stable phase transition) and later a weight loss due to the thermodynamically driven Fig. 5 UV–VIS spectra of the powders after calcination at 773 K. inverse reaction [Fig. 4(g)]. (a) TiO2 (Degussa), (b) 6%Mn-TiO2 (Degussa), (c) TiO2, (d) These data show that Mn favours the anatase-to-rutile phase Ti0.9Mn0.1, (e) Ti0.8Mn0.2, (f ) Ti0.5Mn0.5.transition. Previous studies showed that other cations such as Cu2+36 and V5+32 also favour anatase sintering and the anatase-to-rutile phase transition. Conversely, we reported that Mo6+, Con +,33 W6+,37 and Si4+38 tend to hinder both these phenomena.We observed that this behavior is predominantly found when the cations are impregnated at the anatase surface. We interpreted these data by a sintering-induced phase transition mechanism.33 The data presented here fully agree with previous observations, indicated that Mnn+ behaves similarly to V5+ and Cu2+. On the other hand, Ti seems to hinder slightly the thermodynamically reversible hausmanniteto- bixbyite phase transition upon cooling. 3.3 Electronic characterisation of mixed oxide catalysts UV–VIS diVuse reflectance spectra of TiO2, Ti0.9Mn0.1, Ti0.8Mn0.2 and Ti0.5Mn0.5 are shown in Fig. 5. The electronic spectra of the TiO2 samples [Fig. 5(a) and (c)] correspond to those reported previously for similar samples39,40 and other oxide binary systems such as Ti–Sr41 and Ti–Al.42 This spectrum is characterized by a strong absorption edge in the range 200–400 nm with two main absorptions at ca. 220 and 305 nm which are attributed an O 2p�Ti 3d charge-transfer transition.43 In the samples containing Mn, additional absorption appears at higher wavelengths, i.e. in the visible region. As a result, the absorption at 305 nm increases in intensity progressively and its position shifts towards higher wavelengths, up to 345 nm for the sample Ti0.8Mn0.2 [Fig. 5(e)] before falling to ca. 320 nm for Ti0.5Mn0.5 [Fig. 5(f )]. Meanwhile, the absorption at 220 nm is shifted to 255 nm for same samples. Above 400 nm a broad tail appears which becomes more predominant upon increasing the Mn content, with the appearance of higher wavelength components as shoulders at 570 and 760 nm, which is more evident for the sample Ti0.8Mn0.2.These two components become indistinguishable when the Mn content is 0.5. Fig. 6 UV–VIS spectra of the powders after calcination at 773 K. UV–VIS spectra of samples with greater Mn content are (a) Ti0.5Mn0.5, (b) Ti0.2Mn0.8, (c) Ti0.1Mn0.9, (d) Mn3O4, (e) Mn2O3 (reference sample). shown in Fig. 6, together those of the pure manganese oxides, 2528 J. Mater. Chem., 1998, 8(11), 2525–2531hausmannite and bixbyite (the latter calcined at 1073 K). In these spectra, the absorption at 255 nm decreases in intensity without shifting till it almost disappears, while the other at 320 nm becomes the predominant component in the range 320–305 nm in the spectrum of Mn3O4 [Fig. 6(d)]. As for the components in the region above 400 nm, at least three clear shoulders can be observed at 460, 565 and 740 nm for the Mn–Ti mixed oxides, the last decreasing in intensity with the Mn content. This component disappears in the Mn3O4 electronic spectrum, which however also shows a very broad absorption in the NIR region (see insert in Fig. 6). The a- Mn2O3 spectrum is formed by three bands at ca. 370, 485 and 755 nm. In the a-Mn2O3 structure, Mn3+ ions (d4) occupy octahedral Fig. 7 DTA curves of impregnated samples after calcination at 773 K. sites and, if highly symmetric, a single spin-allowed absorption (a) TiO2 (Degussa), (b) 6%Mn-TiO2 (Degussa). band in the d–d transition region is expected similarly to [Mn(H2O)6]3+ at 500 nm.44 However, a distortion of the octahedral coordination sphere can give rise to a diVerent and TiO2 (Degussa) samples.The TiO2 (Degussa) run is characterised by an exothermic peak near 1083 K without splitting of d levels and so other d–d transitions can occur.45,46 Thus, the absorptions in the bixbyite spectrum can be assigned, appreciable weight loss, as reported in Table 3, due to the anatase-to-rutile phase transition.The addition of Mn pro- in order of increasing wavelength, to a O2-AMn3+ chargetransfer transition, to superimposed 5B1gA5B2g and 5B1gA5Eg duces a notable shift of this peak up to 973 K, indicating that the presence of Mn favours the anatase-to-rutile phase trans- crystal-field d–d transitions, and to a 5B1gA5A1g crystal-field d–d transition, respectively.45,46 ition, not only in the mixed oxide samples but also if impregnated on the surface.Also, a new endothermic peak appears The interpretation of the hausmannite spectrum is complex because it shows random cation distribution in the spinel at 1203 K, which can be associated with pyrophanite formation. structure. However, some considerations should be taken into account: for instance, Mn2+ (d5) d–d transitions are expected The FTIR spectra (Fig. 2) of the Degussa support and of the Mn containing sample prepared from it appear to be very to be weak in both octahedral and tetrahedral sites, since they are, in principle, both spin and orbitally forbidden.47 similar and both show the strong absorptions of TiO2.The spectrum in the FIR region does not reveal appreciable traces According to previous studies48–51 the absorption band at 255 nm in the spectrum of Mn3O4 is associated with a of Mn oxide phases.The electronic spectra of the TiO2 (Degussa) support and O2-�Mn2+ charge-transfer transition and that at 320 nm with O2-�Mn3+ charge-transfer. In the near-IR region, a of the impregnated catalyst are compared in Fig. 5(a) and (b). When Mn is added, a significant absorption above 400 nm broad band with a maximum at ca. 1750 nm is only found in the hausmannite spectrum. Previous studies45,46,52 have into the visible region is observed in addition to that of the TiO2 edge. Components can be found near 440 nm, just at the reported that this band in the spinel structure can be assigned to a 5E�5T2 d–d transition of octahedral Mn3+ and its major lower energy side of the TiO2 gap transition, and in the 750 nm region.As discussed above, by comparison with the Mn2O3 energy with respect to that of the crystal field Do is due to a distortion from octahedral coordination. spectrum, these new features are likely to be due to octahedral Mn3+ species. In this case, however, the TiO2 band gap does In the UV–VIS spectra of Mn–Ti mixed oxide samples, the variation of Mn content explains clearly the decrease of not seem to be substantially modified, in contrast with what occurs for mixed oxide samples.This can be associated to the intensity and the shifting of the bands, since these features relate to TiO2 charge-transfer transitions. The component near existence, in the case of mixed oxides only, of Mn species dissolved in the TiO2 bulk. 460 nm and the broad absorption centred around 750–760 nm can be related to the absorptions present in the spectrum of a-Mn2O3 at ca. 485 and 755 nm, suggesting that the majority 4 Catalytic tests of manganese is Mn3+ in octahedral coordination. However, according to previous studies the apparent shift of the absorp- All Mn-containing samples are found to be active in the catalytic oxidation of both propane and propene.Propene tion edge in the range near 400 nm in the sample Ti0.9Mn0.1 (which only shows the anatase phase with, possibly, dissolved oxidation is total in all cases giving rise almost exclusively to CO2 with very small amounts of CO (selectivities always below Mn ions, according to XRD) could also be associated to transitions of Mn4+ in the TiO2 phase.49 The other weak and 5%) and negligible traces of ethylene (<0.1% selectivity).However, the sample activity measured by using the same sharp component near 565 nm can be tentatively assigned to the 6A1�4T1 forbidden d–d transition of Mn2+, either in an catalyst surface areas and flow rates, is very sensitive to composition (Fig. 8). Conversion above 97% is achieved on octahedral or tetrahedral site.47 bixbyite at 540 K while on hausmannite such levels of conversion are attained above 600 K. On mixed and supported 3.4 Characterisation of the impregnated catalyst after Mn–Ti oxides 6% or lower conversions are obtained at 700 K. calcination at 773 K However, comparison of the activities at lower temperatures and conversions (Fig. 8), where the kinetic regime is chemical Similar experiments have been performed on a sample prepared by impregnation of a commercial TiO2 support, from Degussa (as shown by the calculated activation energies of 19 kcal mol-1), shows similar behaviour for the two Mn (anatase+30% rutile mixture), for comparison. The loaded Mn amount (6% wt./wt.) was calculated to be approximately oxides.This apparently contradicts a previous study from our group, where Mn3O4 was reported to be significantly more that needed to cover the overall support surface with a complete ‘monolayer’. In the XRD powder pattern of a sample active than a-Mn2O3.10 However, in that case we measured the activity of samples diVerent than those described here, and after calcination at 773 K, anatase and rutile phases are present with relative ratios very similar to those of pure support TiO2, characterised by very diVerent surface area and pre-treatment temperatures. and traces of the bixbyite phase.53 Fig. 7 compares DTA curves of the 6%Mn–TiO2 (Degussa) The catalytic activities of both Mn–Ti mixed oxides and of J. Mater. Chem., 1998, 8(11), 2525–2531 2529Fig. 10 Product reaction upon propane oxidation as a function of the Fig. 8 Propene conversion over manganese and titanium oxides as a reaction temperature over 6%Mn-TiO2 (Degussa). (2) O2 (conv.), function of the reaction temperature. (2) Mn3 O4, (&) Mn2 O3, (1) CO2 (sel.), (&) C3H8 (conv.), (%) C3H6 (sel.), (#) CO (sel.), (+) Ti0.1Mn0.9, (1) Ti0.2Mn0.8, (%) Ti0.5Mn0.5, (#) 6%Mn-TiO2 (6) C2H4 (sel.).(Degussa). sample Ti0.5Mn0.5 and a maximum in selectivity to propane for Ti0.2Mn0.8. Conclusions Conclusions from the above data are as follows. 1 The addition of Ti to Mn oxide the formation of the thermodynamically stable phase a-Mn2O3 (bixbyite) with respect to that of the metastable phase (at room temp.) Mn3O4 (hausmannite). 2 Ti apparently enters the bixbyite phase in small amounts. 3 Mn addition, both in the samples of mixed oxides and the supported catalyst, favours the anatase-to-rutile phase transitions.This eVect is particularly evident for impregnation, in agreement with the sintering-induced phase-transition eVect, previously proposed by some of us.33 4 Mn enters the TiO2 anatase phase in small amounts, in Fig. 9 Propane conversion over manganese and titanium oxides as the samples of mixed oxides. a function of the reaction temperature. (2) Mn3 O4, (&) Mn2 O3, 5 The pyrophanite phase MnTiO3 is produced by heating (+) Ti0.1Mn0.9, (1) Ti0.2Mn0.8, (%) Ti0.5Mn0.5, ($) Ti0.8Mn0.2, (6) Ti0.9Mn0.1, (#) 6%Mn-TiO2 (Degussa). Mn-mixed oxides above 1200 K. 6 The surface areas of mixed Mn–Ti oxides tend to decrease upon increasing the Mn content. the supported catalysts are by far lower than those of the pure 7 Characterisation of a supported Mn-TiO2 catalyst shows Mn oxides.Temperatures of 580 K are needed to obtain 10% that Mn oxide species are well dispersed, with a-Mn2O3 propene conversion for the samples Ti0.1Mn0.9 and Ti0.2Mn0.8, (bixbyite) particles only detectable in traces.while the same conversion is attained only above 620 K for the 8 UV–VIS spectra suggest that surface Mn oxide species on sample Ti0.5Mn0.5 and by the supported sample. TiO2 are mainly constituted of octahedrally coordinated The conversion of propane is, as expected, lower than that of Mn3+ species. propene in all cases, under the same conditions (Fig. 9). In this 9Mixed and supported Mn–Ti oxides are active and selective case, however, oxydehydrogenation to propene is predominant catalysts for the total oxidation of propene to CO2.By at very low conversion and still remains significant up to total contrast, they give rise to substantial partial oxidation products propane conversion. CO2 is the predominant product when like propene and CO upon oxidation of propane after partial propane conversion becomes significant while CO is also formed conversion.later. Traces of ethylene are also observed. The behaviour of the 10 Mixing Mn oxides with, or supporting on, TiO2 strongly supported catalyst 6% Mn-TiO2 (Degussa) is shown in Fig. 10, deactivates them for propene oxidation. As for propane and can be regarded as representative of all Mn-containing oxidation, mixed Mn–Ti oxides are by far less active than samples.A similar behaviour has been described previously for a-Mn2O3 (bixbyite) but can be more active than Mn3O4 bulk Mn oxides.10,11 By comparing again the activities at lower (hausmannite). conversions (Fig. 9), we find that the catalytic activity of a- 11 The activation eVect of TiO2 on vanadia, molybdena and Mn2O3 (bixbyite) is in this case far higher than that of Mn3O4 tungsta oxidation catalysts does not occur on Mn oxides.This (hausmannite).Mn3O4 is, however, more selective towards pro- can be interpreted assuming that this eVect is not due to the pene at similar conversion, giving yields of propene of the order ability of TiO2 anatase to disperse the active phase (occurring of 5% under these conditions.Again CO is produced in very in both cases), but rather to diVerent electronic eVects that small amounts (selectivity <5%), CO2 being predominant. distinguish oxides of d0 cations (e.g. vanadia, molybdena and The mixed oxide samples show catalytic activity higher than tungsta) from the oxides of d-electron containing cations. Mn3O4 although lower than for Mn2O3. The catalytic activity of the supported catalyst does not diVer significantly from those Acknowledgements of mixed oxide catalysts, and also the sample Ti0.9Mn0.1 shows significant oxidation activity.The trends for propane conversion Part of this work has been supported by NATO activity and propene selectivities are quite diYcult to rationalise (CRG-960316).J. M. G. A. acknowledges MEC for a FPI grant. for Mn–Ti mixed oxides, with a maximum in activity for the 2530 J. Mater. Chem., 1998, 8(11), 2525–2531Escribano and P. Piaggio, J. Chem. Soc., Faraday Trans., 1994, References 90, 3181. 29 M. I. Baraton, G. Busca, V. Lorenzelli and R. J. Willey, J. Mater. 1 P. E. Marti, M. Maciejewski and A. Baiker, Appl. Catal., B: Sci. Lett., 1994, 13, 275.Environ., 1994, 4, 225. 30 M. C. Bernard, A. Hugot-Le GoV, B. Vu Thi and S. Cordoba de 2 G. Groppi, M. Bellotto, C. Cristiani and P. Forzatti, Appl. Catal., Torresi, J. Electrochem. Soc., 1993, 140, 3065. 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Sanchez Paper 8/03994A J. Mater. Chem., 1998, 8(11), 2525–2531 2531