Development of strategies in order to limit theemission of toxic gases such as volatile organic compounds (VOC) fromindustrial processes is one of the major challenges of the present time. It isdesirable to develop environmentally friendly technologies to eliminatepollutants without resulting in further toxic by-products. Thermal combustionis effective; however, using a catalyst can result in much better conversionrate at a comparatively lower temperature by altering the kinetics. For thisreason, various scientific efforts were made to develop novel catalytic materialsfor that purpose ^{[1, 2]}. Even though noble metals were found to be effective inmost of the reactions concerned, it is not at all a cost effective solutionfrom an industrial perspective. Interestingly, perovskite structured mixedmetal oxides (ABO₃) were found to be as effective as noble metalsfor various catalytic partial or total oxidation reactions especially those ofhydrocarbons and volatile organic compounds ^{[3, 4]}. However, the applicabilityof these materials is not yet fully exploited since the high temperature (> 700 °C) conditionsused in the synthesis of these materials result in very low specific surfacearea (< 30 m²/g) ^{[4, 5, 6, 7, 8]}. Hence a major milestone that needs to beachieved for the effective utilization of these materials on an industrialscale is the development of a synthesis strategy that helps to achieve higher surfaceareas. On this regard, Kaliaguine et al. ^[9] successfully synthesizedperovskite oxides with higher surface areas (100 m²/g) wherecalcination is performed at temperatures around 200 °C. However, in this casealso, the specific surface area was found to decrease at higher calcinationtemperatures.

The discovery of ordered mesoporous silica and thedevelopments that followed on the research focusing on various mesoporousmaterials in the past two decades have made it possible to also synthesize variousnon-siliceous composition materials (carbon, metal oxides, carbides, etc) withextremely high values of specific surface areas ^{[10, 11]}. Out of the methodsavailable for the synthesis of mesoporous materials, nanocasting enjoysa unique position. This method is found to be efficient for developingmono-metallic and/or mixed oxides with high specific surface areas ^{[12, 13, 14, 15]},which cannot be obtained using other methods. Various studies were performedutilizing the nanocasting approach for the synthesis of a variety ofcompositions which were successfully utilized for a wide range of applications[16-20, see also literature cited in 15]. Even though these nanocast oxideswere examined for a large number of catalytic reactions, most of such studiesfocused on the measurement of temperature dependent conversions as a functionof catalyst composition or surface area ^{[14, 21, 22, 23, 24, 25]}. For the successfulemployment of these materials in the industry advanced knowledge on the surfacereactions as well as the reaction kinetics is required.

We have recently reported the synthesis andcatalytic studies of mesoporous perovskite oxides with high specific surfacearea synthesized using the method of nanocasting ^[15]. Our studies clearlydemonstrated the higher catalytic efficiency of the nanocast perovskitescompared to their bulk counterparts synthesized using reactive grinding methodand conventional citrate method for various gas phase reactions. To supplementour previous study ^[15], we report here on the synthesis of high surface areaLaMnO₃ materials using SBA-15 silica aged at different temperatures(35, 100 and 140 °C) as thehard templates, and discuss the influence of the porosity parameters of thethus-obtained perovskites on their catalytic activity and kinetics. Thecatalytic properties of these high surface area materials were studied for thetotal oxidation of methanol. The surface and redox properties of the materialswere analyzed using temperature-programmed characterization methods. Detailedkinetic data processing was performed for these materials to achieve a betterunderstanding of the high catalytic efficiencies observed for these newmaterials.

Ordered mesoporous silica SBA-15 hard templateswere synthesized according to the previously reported procedure, using PluronicP123 as the structure-directing agent and tetraethylorthosilicate (TEOS) as thesilicon source ^[26]. In a typical synthesis, 4.0 g of P123 was dissolved in 76g of deionized water and 2.3 g of hydrochloric acid (37%) at 35 °C under magneticstirring. To the obtained homogeneous solution, 8.6 g of TEOS was rapidly addedwith continued stirring for 24 h at 35 °C and subsequently subjected tohydrothermal treatment at a desired temperature (35, 100 and 140 °C) for an additional 24 h to ensure furtherframework condensation. After cooling, the resulting solution was filtered andthe solid products were dried at 100 °C for 24 h. Finally, the powders werecalcined at 550 °C in order to remove the organic copolymer template.

Nanocasting of the mesoporous perovskites wasperformed by the previously reported procedure using a citrate complex of metalcations as the perovskite precursor and ordered mesoporous silica SBA-15 as thehard template ^[15]. The precursor was impregnated into the template by usingthe wet impregnation method. In a typical synthesis, La(NO₃)₃·6H₂Oand Mn(NO₃)₂·xH₂O (3 mmol each) weredissolved in an ethanolic solution of citric acid (10 mL) to obtain anequimolar solution, which was added slowly to SBA-15 (1 g) dispersed in water(10 mL). The molar ratio of total metal ions and citric acid was kept at 2:1.The mixture was stirred for a few hours at room temperature, and then thesolvent was evaporated under vacuum with a rotary evaporator. The powder thusobtained was further dried at 80 °C for 24 h, ground well in a mortar, and calcined at 500 °C for 4 h to remove the organic part. Impregnationwas repeated twice, using for the second time one half of the amount of theprecursor, to achieve higher loadings. The final powder was calcined at 700 °C for 6 h, and the silica template was thenremoved by treating the composite 3 times with NaOH (2 mol/L) at roomtemperature. The obtained product was washed with water and ethanol and driedovernight at 80 °C. Threesyntheses were performed using template SBA-15 aged at 35, 100 and 140 °C and here after, these samples will be denotedas LaMnO₃-35, LaMnO₃-100 and LaMnO₃-140, wherethe numbers indicate the aging temperature of the SBA-15 template used.

Wide-angle powder XRD was performed with a SiemensModel D5000 diffractometer using Cu K_α radiation (λ = 0.15496nm). N₂ physisorption analyses were performed at -196 °C with an ASAP 2010 sorption analyzer. Prior to analysis, the sampleswere degassed overnight at 150 °C. Specific surface areas of nanocast perovskites were calculatedusing the BET method on the lower relative pressure region of the isotherm(0.05-0.2). Pore size distributions were obtained by using the NLDFT methodassuming cylindrical pore geometry (applying the kernel of metastable NLDFTadsorption isotherm, i.e., adsorption branch) supplied by the Autosorb-1 1.55software from Quantachrome Instruments ^{[27, 28, 29]}. The total pore volume was calculated from the N₂sorption capacity at p/p₀ = 0.95. Elemental analysiswas performed using an M1100 B Perkin-Elmer atomic absorption spectrophotometer.For TEM images, the samples were first dispersed in ethanol, deposited oncarbon grids and analyzed on a JEOL JEM 1230 microscope.

A RXM-100 multicatalyst testing and characterizationsystem (Advanced Scientific Design Inc.) was used to perform O₂-TPDand H₂-TPR. For TPR experiments, the catalyst (50 mg) was placed ina quartz reactor and pretreated under a flow of 20 mL/min (20% O₂ inHe) at 500 °C for 1 h. The TPRwas performed under a flow of 10 mL/min (5% H₂ in Ar) with a temperatureramp of 5 °C/min from 25 to 900°C. The consumption of H₂was monitored and quantified with a thermal conductivity detector (TCD). For O₂-TPD,the same pre-treatment was performed as for the TPR experiments and the sameamount of the catalyst was used under a flow of 10 mL/min of He. A TCD was usedfor the quantification of the O₂ desorbed.

The catalytic bed was set up with the catalyst(200 mg) inserted between two quartz wool plugs in a U-shaped quartz reactor(internal diameter = 5 mm). The temperature was controlled using a K-typethermocouple placed in the reactor. To purge the catalytic system, thecatalysts were first flushed with He for 1 h at room temperature and thenpretreated for 1 h at 200 °Cbefore performing the catalytic tests. The feed, composed of 0.5% CH₃OHand 5% O₂ in He, was passed through the reactor and the temperaturewas increased. Gas samples were collected in the steady-state regime after aninterval of 2 h at constant conversion, and the products were analyzed using agas chromatograph (HP 6890 series) equipped with a TCD. Reactants and productswere separated using a Haye-Sep T column (internal diameter = 1 mm, L = 2-5 m).

To obtain kinetic data, methanol steady-stateconversions were monitored for the nanocast mesoporous LaMnO₃catalyst at the reactor outlet at different flow rates (10-40 mL/mincorresponding to a gas hourly space velocity of 19500-78200 h^-1).The obtained experimental conversions were cross-plotted as a function ofpseudo-contact time over selected reaction temperatures. The values of reactionrates obtained by the analytical derivatization of the equation correspondingto the fitted curves were evaluated using a simplified equation proposedoriginally by Arai et al. ^[4] for the total oxidation of methane.

The ordered mesoporous SBA-15 silicas synthesizedat different aging temperatures used as the hard templates were characterizedusing N₂ physisorption performed at -196 °C (see SupportingInformation Fig. S.1). Clear indication of variations in pore size, pore volumeand surface area was observed in close agreement with the literature ^{[26, 29, 30, 31]}. Using these silica materials as hardtemplates, a series of LaMnO₃ mesoporous perovskites weresynthesized. After the removal of the silica template, the presence of theperovskite structure was confirmed in all these materials from the wide angleXRD patterns shown in Fig. 1. No peaks corresponding to crystalline impuritiessuch as mono-metallic oxides of lanthanum or manganese were observed in thewide angle region. Intensity of reflections was higher for LaMnO₃-35,most probably because this sample was dominated by non porous bulk particles. Alow intensity broad peak around 2θ = 28° was observed for all the samples indicating the presence of anX-ray amorphous species; most probably silicates resulting from the incompleteremoval of the SBA-15 template. Atomic absorption analysis confirmed thepresence of approximately 10% Si. This might have occurred due to enhancedinteractions between the template silica and the rare earth metal in theperovskite structure. The presence of such residual silicate species innanocast perovskites was documented previously ^{[14, 15]}. Information regardingmesostructural order was obtained from the TEM images, and representativeimages for all samples are shown in Fig. 2. Well-ordered mesostructural domainswere clearly observed for LaMnO₃-100 and LaMnO₃-140 alongwith some less defined and disordered nanoporous regions. When SBA-15 aged at35 °C was used as the hardtemplate, the presence of ordered regions was comparatively lesser than theother nanocast perovskites.

Fig. S1. N2 physisorption isotherms (-196 °C) and the corresponding pore size distributions (bottom) of ordered mesoporous SBA-15 silica hard templates. Pore size distributions were calculated from the adsorption branch of the isotherm using the NLDFT method.

Fig. 1. Wide angle powder XRD patterns of mesoporous LaMnO3 perovskite oxides synthesized using ordered mesoporous SBA-15 aged at 35, 100 and 140 °C as the hard templates.

Further information regarding the porosity andtextural parameters was obtained by performing N₂ physisorption analysisat -196 °C on mesoporous LaMnO₃perovskites synthesized using SBA-15 hard templates aged at different temperatures.The adsorption-desorption isotherms and the corresponding pore sizedistributions for all these materials are shown in Fig. 3. A type IV behaviorwas observed for the isotherms. Hysteresis loops appear in the relativepressure range from 0.5 to 1.0 for LaMnO₃-100 and LaMnO₃-140,which is typical of such mesoporous metal oxides obtained from nanocasting ^{[12, 32]}.For LaMnO₃-35 capillary condensation occurs at comparatively higherrelative pressure (0.6). Brunauer-Emmett-Teller (BET) specific surface areasobtained for nanocast perovskites are exceptionally high when compared withthose of materials obtained by using conventional methods, especially consideringthe high calcination temperature of 700 °C (Table 1). Variations in specific surface area were observed in thecase of nanocast LaMnO₃ with respect to the aging temperature of thehard template. A similar trend was observed in the case of NLDFT average poresizes and pore volumes calculated from the volume adsorbed at the relativepressure of 0.95. An excellent agreement was observed between the theoreticallyderived NLDFT isotherms and the experimental isotherms for all the nanocastperovskites in the present study, which validates the use of this method forthe pore size determination of the nanocast perovskites (Supporting InformationFig. S.2).

Fig. S2. N2 physisorption isotherms (-196 °C) and the corresponding NLDFT theoretical isotherms (colour) of nanocast LaMnO3 perovskite oxides synthesized by use of ordered mesoporous SBA-15 silica hard templates.

Fig. 3. N2 physisorption isotherms (-196 °C) (a) and the corresponding pore size distributions (b) of nanocast LaMnO3 perovskite oxides synthesized by use of ordered mesoporous SBA-15 silica hard templates. Pore size distributions were calculated from the adsorption branch of the isotherm using the NLDFT method.

Table 1 Structural parameters of the SBA-15 templates and nanocast perovskites obtained by performing N2 physisorption analysis at -196 °C. Sample SBET a (m2/g) DP b (nm) VP c (cm3/g) SBA-15-35 738 5.8 0.66 SBA-15-100 960 7.3 1.1 SBA-15-140 852 9.0 1.7 LaMnO3-35 80 4.8 0.15 LaMnO3-100 140 5.1 0.21 LaMnO3-140 190 6.0 0.29 a Calculated by using the BET method on the relatively low-pressure region (0.05-0.2); b NLDFT pore size; c Pore volume.

Table 1 Structural parameters of the SBA-15 templates and nanocast perovskites obtained by performing N2 physisorption analysis at -196 °C.

Studies examining the effect of template porestructure on the final nanocast replica were performed by various authors.Rumplecker et al. ^[32] found that the structure of Co₃O₄replica can be tuned from randomly arranged rods to highly ordered mesoporousnetwork structures depending on interconnectivity of the SBA-15 template,loading of the precursor and impregnation procedure. These authors alsoconfirmed that better replicas were observed by using microwave digested silicatemplate owing to the presence of higher fraction of interconnecting mesoporesin the same. Further, Jiao et al. ^[33] synthesized ordered mesoporousNiO with a bimodal pore size distribution consisting of a series of small (3.3nm) and large pores (11 nm). In this case, the bimodal porosity in nanocast NiOreplica was achieved by varying the degree of microporous bridging between thetwo sets of mesopores in the ordered mesoporous KIT-6 hard template. Tuysuz etal. ^[34] performed studies on nanocasting of Co₃O₄using KIT-6 as the hard template. These authors have shown that the texturalparameters of the final replica strongly depend on the structure parameters ofthe parent template used. In short, an inverse correlation between the templateaging temperatures with the BET surface area, pore volume and pore size wasobserved. More recently, similar results were obtained by Yen et al.^[35] for Cu-CeO₂ mixed oxides. In all these studies the authors usedmetal nitrates as the precursors for respective oxides. We believe that theopposite trends obtained for perovskites in the present study are probably dueto the variation in pore size, pore volume and enhanced interconnectivity ofthe SBA-15 template, resulting from the increase in aging temperature. Largerpore volume and interconnectivity seems to be needed to facilitate a moreadequate loading of the complexed precursor which consisting of a bulky largeorganic molecule (e.g., metal cations consist with citric acid). This resultsin a comparatively better structural order after the removal of the templateand hence the highest value of surface area when SBA-15 aged at 140 °C was used as the hard template. Note thatthe isotherm of LaMnO₃-140 shows a particularly well-developed mesoporosity,expressed by a sharp capillary condensation step and hysteresis loop,characteristic of large ordered cylindrical-like mesopores.

The reduction behavior of metal cations innanocast mesoporous perovskites was examined using temperature-programmed reductionby hydrogen (H₂-TPR). Because the A-site metal is non reducibleunder the present conditions of H₂-TPR, the observed H₂consumption peaks correspond to the reduction behaviour of Mn ions in theperovskite structure and the observed profiles are shown in Fig. 4. Differentfrom other perovskite compositions, complete reduction to Mn⁰ doesnot occur for LaMnO₃, under the present analysis conditions ^{[36, 37]}.For all the three samples, a broad main peak with a high temperature and a lowtemperature shoulders were observed. The main peak can be assigned to thereduction of Mn³⁺ to Mn²⁺ (0.5 mol H₂ per atomof Mn). The low temperature shoulder clearly indicates the presence of Mn⁴⁺resulting from the presence of over-stoichiometric oxygen as observed previously^[38], whereas the high temperature shoulder suggests the presence of Mn ionsthat are not easily accessible. Interestingly, it was also observed that the Mn⁴⁺content was higher for LaMnO₃-140. Absence of other noticeable peaksin the TPR pattern, especially at higher temperatures indicates that a verynegligible interaction, if any, exists between the reducible metal (Mn in thiscase) and the residual silicon species. Also significant changes in thereduction behaviour of Mn were hardly observed for any of the samples in thepresent study. The amounts of hydrogen consumed during the reduction of nanocastperovskites are given in Table 2. The values of Mn⁴⁺ show a clearvariation from 3% to 15%, which reflects the variations in the amount of excessoxygen in the structure.

Fig. 4. H2-TPR (a) and O2-TPD (b) profiles of nanocast mesoporous LaMnO3 perovskite oxides synthesized by use of ordered mesoporous SBA-15 silica hard templates aged at different temperatures.

Table 2 Amount of H2 consumed during H2-TPR. Perovskite H2 consumed (molH2/Mn) Mn4+ (%) LaMnO3-35 0.516 3 LaMnO3-100 0.536 7 LaMnO3-140 0.576 15

Table 2 Amount of H2 consumed during H2-TPR.

Information regarding the nature of oxygen atomson nanocast LaMnO₃ perovskites was obtained by recording the temperature-programmeddesorption of oxygen (O₂-TPD) profiles. Two types of oxygen speciesare generally found to be desorbed from perovskite oxides. Desorption of oxygenbound to the surface takes place below 700 °C (designated as α-O₂), and desorption of lattice oxygentakes place at a higher temperature (designated as β-O₂). In thepresent work two peaks were observed for LaMnO₃-100 and LaMnO₃-35,whereas in the case of LaMnO₃-140, the high temperature peakcorresponding to the β-O₂ was negligible. Very clear increase in theintensity of the low temperature peaks (α-O₂) was observed for thenanocast LaMnO₃ with respect to the increase in surface area of thematerial (see Fig. 4). The amounts of desorbed oxygen were calculated and arelisted in Table 3. Assuming 4 µmol/m² of oxygen amounts to onemonolayer, the number of desorbed monolayers were also calculated.

Table 3 Amount of O2 desorbed during O2-TPD. Perovskite α-O2 (µmol/g) β-O2 (µmol/g) α-O2 monolayer β-O2 monolayer LaMnO3-35 264 372 0.82 1.16 LaMnO3-100 506 220 0.82 0.38 LaMnO3-140 638 — 0.83 —

Table 3 Amount of O2 desorbed during O2-TPD.

In our previous work, we clearly showed thatnanocast perovskite materials were better oxidation catalysts than materials ofsimilar compositions synthesized by other methods ^[15]. The catalytic oxidationactivity of the mesoporous perovskites was examined using methanol as the modelcompound, under the flow conditions corresponding to a gas hourly space velocityof 39100 h^-1. The comparison of mesoporous LaMnO₃ catalystssynthesized by varying the aging temperature of the hard template, and thusvarying porosity and surface area parameters, was performed and the obtainedtemperature dependent conversion curves are given in Fig. 5. Clear variationsin the conversion efficiencies with the values of the catalyst specific surfacearea were observed. For the catalyst with largest specific surface area fullconversion was observed at 145 °C. As expected, an increase in full conversion temperature wasobserved for lower value of specific surface area. Even though some conversionwas already observed near room temperature for nanocast LaMnO₃-100and LaMnO₃-140, the values of conversion were slightly higher forthe latter under the same conditions of temperature. Also, CO₂ wasthe only product detected for all the catalysts, without any presence of formaldehydeor CO, which indicates that all of these nanocast materials are highlyefficient methanol oxidation catalysts.

Fig. 5. Methanol conversion profiles as a function of temperature over nanocast LaMnO3 perovskites synthesized using SBA-15 silica hard template aged at different temperatures (GHSV = 39100 h-1).

Kinetic data processing was performed for thetotal oxidation of methanol over all three nanocast LaMnO₃catalysts. For this purpose, the temperature dependent conversions were monitoredfor these catalyst materials at different flow rates which resulted in gashourly space velocities ranging from 19500 to 78200 h^-1.The corresponding conversion curves obtained in the case of LaMnO₃-35,LaMnO₃-100 and LaMnO₃-140 are shown in SupportingInformation Fig. S.3. The conversion values at several selected temperatureswere cross-plotted against the pseudo-contact time W/F (in whichW is the mass of the catalyst and F is the molar flow rate of methanol). Thisyielded a series of isothermal curves as shown in Fig. 6 (and SupportingInformation Figs. S.4 and S.5). The resultant data were then fitted using thesigmoidal Eq.(1), the analytical derivatization of which gave the values ofreaction rates:

Fig. s3. Temperature dependent conversion profiles for the total oxidation of methanol over nanocast LaMnO3-35 (top) LaMnO3-100 (middle) and LaMnO3-140 (bottom) at different space velocities.

Fig. s4. Cross-plotting the values of experimental conversions at selected temperatures as a function of pseudo-contact time obtained for nanocast LaMnO3-35. Points represent experimental data, and lines are calculated by using Eq. (1). The numerical values of rates obtained in each case are represented as a function of the partial pressure of methanol (bottom).

Fig. s5. Cross-plotting the values of experimental conversions at selected temperatures as a function of pseudo-contact time obtained for nanocast LaMnO3-140. Points represent experimental data, and lines are calculated by using Eq. (1). The numerical values of rates obtained in each case are represented as a function of the partial pressure of methanol (bottom).

Fig. 6. Cross-plotting the values of experimental conversions at selected temperatures as a function of pseudo-contact time obtained for nanocast LaMnO3-100 (a). Points represent experimental data, and lines are calculated by using Eq (1). The numerical values of rates obtained in each case are represented as a function of the partial pressure of methanol (b).

in which X is theconversion of methanol and a, b, and c are nonlinearregression parameters.

The numerical values of the reaction rates (r)thus obtained were further fitted using Eq.(2) and the values of rate constants(k) were obtained. This equation which was previously proposed by Arai etal. ^[4] assumes the participation of only lattice oxygen in the catalyticprocess and the corresponding fit is shown in Fig. 6.

The values of pre-exponential factor (A) andactivation energy (E_a) for the nanocast catalysts weredetermined from the Arrhenius plot by performing linear regression analysis asshown in Fig. 7. These values are reported in Table 4. The values of E_awere found to remain essentially constant irrespective of the surface area ofthe catalyst used. Interestingly, a linear correlation was found to existbetween the pre-exponential factors and the values of specific surface area ofthe catalysts as shown in Fig. 7. This indicates that the specific activity perunit surface area remains the same for all the catalysts used in the presentstudy.

Fig. 7. Arrhenius plots for the rate constant k obtained for nanocast LaMnO3-35, LaMnO3-100 and LaMnO3-140 (a); The linear correlation between the pre-exponential factor and the specific surface area (b).

Table 4 Kinetic parameters obtained for total oxidation of methanol over nanocast perovskites. Perovskite a Ea /(kcal/mol) b A /(mol·g·0.1 MPa)-1 LaMnO3-35 13.49 9.7e4 LaMnO3-100 13.52 12.3e4 LaMnO3-140 13.51 24.2e4 aActivation energy; b Pre-exponential factor.

Table 4 Kinetic parameters obtained for total oxidation of methanol over nanocast perovskites.

The residual Si species remaining in thesenanocast perovskites, that cannot be completely removed after the multipletemplate removal steps (3 times using NaOH (2 mol/L)), can cause severaleffects towards the efficiency of these materials. Previous studies on suchresidual species indicated a negative contribution on the value of specificsurface area ^[39]. On the other hand, these residual Si species could as wellinduce a positive contribution by providing improved stability to thesenanoporous framework structures. Also, it can interact with the active phaseand thereby affect the efficiency of these materials. However, to confirm theexistence of either of these aforementioned effects on the catalytic activityof the nanocast perovskites, more information regarding the exact nature of theresidual Si species is necessary. Studies are being conducted in thisdirection.

As mentioned above, Arai et al. ^[4] Eq. (2)assumes that lattice oxygens, usually designated as b-oxygens, are utilized inthe catalytic oxidation process. However as shown in Table 3, the b-oxygen contentof the nanocast LaMnO₃ catalysts decreases with the ageingtemperature of the template, even reaching the zero value for LaMnO₃-140which is the most active catalyst. This suggests that this high temperaturedesorbing oxygen species is not the active oxygen. On the other hand, the otheroxygen species designated here as a-oxygen desorbs over a temperature rangecomprised between 580 and 720 °C (Fig. 4). This is quite high for an adsorbedoxygen species, which suggests that here the so-called a-oxygen is likely to be amere surface lattice oxygen. This is corroborated by the number of monolayersof this species remaining essentially constant and close to one in the threesolids (Table 3). These oxygens being the active ones in the oxidation processwould thus also be coherent with the observed proportionality of reaction rateconstants being linearly correlated to specific surface area (Fig. 7).

As indicated in Table 4, the observed activationenergy is not affected by the template ageing temperature and therefore notaffected by specific surface area. The value of 13.5 kcal/mol seems a littlelow for a process having surface reaction as the rate limiting step. Thisraises the possibility of internal diffusion being controlling the overallreaction rate. Careful examination of our kinetic results allows howeverrejecting this hypothesis. The occurrence of a limitation by internal diffusionis represented using an effectiveness factor h in the rate equation:

In absence of a significant methanol concentrationgradient, the effectiveness factor is equal to one and Eq. (3) is reduced toEq. (2). The value of h is determined by the Thiele moduluswhich for a first order reaction is expressed as

where R is theparticle diameter, k₀ is the rate constant expressed per unit volume(mol/(s·m³), S₀the specific surface area (m²/m³) and D_eff(m/s) is the effective diffusion coefficient of methanol in the solid catalyst.

The effectiveness factor h is close toone for j sufficiently small (typically <1). An estimate of j using the highest rate value given in Fig. 6 yields of value of 0.3.It is therefore concluded that internal diffusion is not affecting the ratevalues reported.

This conclusion is coherent with the observationof a constant reaction rate per unit surface area in the three catalysts. Thelarge differences in pore volume (Table 1 and Fig. 3) which would affect theeffective diffusion coefficient would change the Thiele modulus in thedifferent catalysts. Thus if j was higher than one its different valueswould affect internal diffusion and the effectiveness factor.

In conclusion, we have synthesized a series ofmesoporous perovskite oxides with lanthanum in the A site and manganese in theB site by using the nanocasting method and SBA-15 aged at three differenttemperatures as hard templates. These materials were found to display extremelyhigh specific surface area. Also a correlation between the aging temperature ofthe template and the specific surface area was observed. The observed values ofactivation energy for the catalysts were low and remained constant for all thecatalysts under the present conditions of study. Furthermore, apart from thevariations in the textural parameters of these materials, the specific activityfor methanol oxidation per unit surface area remains the same for all thesenanocast perovskites. Further work is in progress to determine the nature ofresidual Si species on these nanocast perovskites that cannot be removed duringthe template removal step. We believe that this is necessary to determine therole of these residual species on the efficiency of such nanocast materials.

N₂ physisorption isotherms andcorresponding pore size distributions determined by the NLDFT method forordered mesoporous SBA-15 templates, Comparison between the experimental N₂physisorption isotherms with the theoretical ones calculated using the NLDFTmethod for nanocast LaMnO₃, Temperature dependent methanol conversionprofiles at various space velocities, cross plotting of experimentalconversions as a function of pseudo contact time for LaMnO₃-35 andLaMnO₃-140, representation of reaction rates as a function ofmethanol partial pressure for LaMnO₃-35 and LaMnO₃-140.