Mechanism of CO+ N2O Reaction via Transient CO32â Species over

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Mechanism of CO + N2O Reaction via Transient CO32- Species over Crystalline Fe-Substituted Lanthanum Titanates Mrinal R. Pai,† Atindra M. Banerjee,† Krishnan Kartha,‡,§ Rajesh V. Pai,⊥ Valmik S. Kamble,† and Shyamala R. Bharadwaj*,† Chemistry DiVision, Fuel Chemistry DiVision, Bhabha Atomic Research Centre, Mumbai 400085, India, and Department of Applied Chemistry, Cochin UniVersity of Science and Technology, Cochin 682022, India ReceiVed: NoVember 13, 2009

Some newer mechanistic aspects investigated by in situ Fourier transform infrared (FTIR) in conjunction with catalytic activity under similar conditions over crystalline lanthanum titanates as a function of Fe substitution at the B-site for the CO + N2O reaction are reported for the first time in the present communication. La2Ti2(1-x)Fe2xO7-δ (0.0 e x e 1.0) was synthesized by gel combustion where Fe3+ substitution effectively enhanced the conversion rates for N2O reduction as compared to the pristine La2Ti2O7 (LTOGC). Among all samples, maximum conversion over La2Ti0.8Fe1.2O7-δ [LF(0.6)GC] catalyst was observed. In situ FTIR results reveal that substitution-induced anionic vacancies/defects provide additional sites on the surface of LF(0.6)GC for CO chemisorptions, whereas a perfect stoichiometric lattice like LTOGC is devoid of such sites. Surfaceadsorbed CO reacts with surface lattice oxygen in the case of nonstoichiometric LF(0.6)GC to produce carbonates (M-CO32-) at a much lower temperature. The reaction proceeds via carbonate formation, leaving the catalytic surface oxygen deficient in LF(0.6)GC, and therefore facilitates the reduction of preadsorbed, N2O [N2O(g) + * f N2 + *-O) by easily adsorbing the oxygen species (*-O) generated in N2O reduction, which is subsequently driven away by adsorbed/gas phase CO, whereas in the case of LTOGC, progress of the reaction was sluggish in the absence of transient carbonate species. Dissociative chemisorptions of N2O are not facilitated on stoichiometric oxygen excess titanate, as there is no vacancy in the surface to accommodate another oxygen atom. The redox mechanism via CO32- species is proposed for CO + N2O reaction over La2Ti2(1-x)Fe2xO7-δ, as against the associative mechanism observed in the unsubstituted sample, La2Ti2O7, as suggested by in situ FTIR in conjunction with catalytic activity results. 1. Introduction Dinitrogen oxide, more accurately nitrous oxide, N2O, popularly known as laughing gas, poses a threat to our environment, as it is a potent greenhouse gas with global warming potential (GWP) 310 and 21 times of CO2 and CH4, respectively, and is also involved in stratospheric ozone destruction.1-3 The reduction of N2O by CO is a much preferred and required reaction for lowering of N2O levels emitted in the environment, especially in processes where CO is already present in the waste gas, particularly by automobile exhaust and chemical processes like the production of oxalic acid (end of pipe solutions).1-7 N2O is also produced as an intermediate in the NO/CO reaction1-3 involved in vehicle emission control.4,5 Heterogeneous catalytic decomposition/reduction of N2O provides an energy-efficient and suitable solution in many of the anthropogenic emission sources. Various catalytic formulations have been proposed thus far, including supported8-10 and unsupported metals,3,5,11 pure and mixed oxides,8,12-17 and zeolitic systems.2,3,18 Kapteijn et al.3 have extensively reviewed a variety of catalysts, such as metal oxides, mixed metal oxides for N2O decomposition. * To whom correspondence should be addressed. Tel: 91 22 25595100. Fax: 9122 25505151. E-mail: [email protected]. † Chemistry Division, Bhabha Atomic Research Centre. ‡ Cochin University of Science and Technology. § Present address: Regional Research Laboratory (RRL), Industrial Estate P.O. Pappanamcode, Trivandrum - 695 019. Kerala, India. ⊥ Fuel Chemistry Division, Bhabha Atomic Research Centre.

Concerning charge transfer involved in the processes of nitrogen oxide decompositions, multiple oxidation states of catalytic metal cations have been found to be instrumental in many oxide systems,19-22 such as perovskites where nonstoichiometry and the valency of the metal ions can be controlled in a regular way. Wang et al.14 reported the catalytic decomposition of dinitrogen oxide (N2O) over perovskites, La2-xSrxCuO4 (x ) 0-1), LaMO3 (M ) Cr, Mn, Fe, Co, and Ni), and La2MO4 (M ) Co, Ni, and Cu) mixed oxides and correlated the catalytic activity and the average oxidation number of copper (AON) as observed for La2-xSrxCuO4 and explained it by a Cu2+/Cu3+ redox mechanism. LaMnO3, a perovskite, better known as nonstoichiometric metal ion-deficient LaMnO3+δ, was found to be less active than oxygen-deficient substituted mixed metal ion oxides for the N2O decomposition reaction.23 LaCoO3 is stoichiometric with a tendency to be oxygen-deficient and shows a high catalytic activity for the N2O and NO decomposition reaction as reported previously.24 The catalytic decomposition of N2O on the oxygen excess, spinels, MxCo1-xCo2O4 (M ) Ni2+ and Mg2+, x ) 0.0-0.99) was also reported by Yan et al.16 in the temperature range of 200-300 °C. The partial replacement of Co2+ by Ni2+ or Mg2+ in Co3O4 spinel oxide led to a significant rise in the catalytic activity as compared to unsubstituted samples, which was correlated with ease in the removal of adsorbed surface oxygen from partially substituted samples, Ni0.74Co0.26Co2O4 and Mg0.54Co0.46Co2O4. Belapurkar et al.25 have also reported the role of oxygen vacancies in the decomposition of N2O over YBa2Cu3O7-δ and Gd2CuO4 oxide

10.1021/jp102306u  2010 American Chemical Society Published on Web 05/05/2010

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TABLE 1: Identification of Phase in La2Ti2(1-x)Fe2xO7-δ Samples Synthesized by the Gel Combustion (GC) Route no.

nominal composition La2Ti2O7 La2Ti1.8Fe0.2O7-δ La2Ti1.6Fe0.4O7-δ La2Ti1.2Fe0.8O7-δ La2Ti0.8Fe1.2O7-δ La2Ti0.4Fe1.6O7-δ LaFeO3

1 2 3 4 5 6 7

Fe content (x)

abbreviation LM(x)R (M ) Ti/Fe, x ) 0.1-1.0)

synthesis route

0.0 0.1 0.2 0.4 0.6 0.8 1.0

LTOGC LF(0.1)GC LF(0.2)GC LF(0.4)GC LF(0.6)GC LF(0.8)GC LFOGC

GC GC GC GC GC GC GC

phase identification by XRD single phase, La2Ti2O7 La2Ti2O7 and LaFeO3 La2Ti2O7 and LaFeO3 single phase LaFeO3 single phase LaFeO3 single phase LaFeO3 single phase LaFeO3

TABLE 2: Crystallite Size and N2-BET Surface Area of La2Ti2(1-x)Fe2xO7-δ Samples as Calculated from XRD Line Width Using the Scherrer Equation no.

value of x

sample

particle size (nm)

surface area (m2g-1)

pore size (Å)

1. 3 4 5. 6. 7.

0.0 0.2 0.4 0.6 0.8 1.0

LTOGC LF(0.2)GC LF(0.4)GC LF(0.6)GC LF(0.8)GC LFOGC

60-62 14-16 15-17 40-42 44-46 77-80

12 14 68 47 9 7

∼400 ∼300 ∼20 ∼20

systems, where stoichiometric compounds have shown much poorer activity as compared to oxygen-deficient compounds. To understand the fundamental processes occurring at the molecular level, the adsorption studies of N2O and CO are extremely important. Several reports have been published on the adsorption studies by in situ IR of N2O and CO on various oxides, metals, and supported metals, namely, MgO,26 Ir,27 Cu/ ZnO,28 SnO2,29 Cu/Al2O3,30 Pt/Al2O3,31-33 Rh(111), Rh/Al2O3,34 and ZnO.35 The CO + N2O reaction reportedly proceeds either by associative or by redox mechanism or by both, which are discussed in detail in section 3.3.4 of this report. However, in multimetal mixed oxide systems, in situ Fourier transform infrared (FTIR) results are scantily reported, particularly as a function of substitution for the CO + N2O reaction. Recently, Pacultova et al.36 have reported the carbonate formation in the CO + N2O reaction and mentioned that its role was unclear with an uncertain mechanism over mixed oxides, calcined Co-Mn-Al hydrotalcite. It is obvious that the N2O + CO reaction over a mixed metal oxide catalyst is very complex, and exact elucidation of the reaction mechanism is very difficult. More insights into this mechanism could be acquired by experiments with isotopically labeled molecules or in situ FTIR measurements or by providing transient pulse experiments. Here, therefore, we have attempted to delineate the role of carbonate formation in the mechanism of the CO + N2O reaction over Fe-substituted/unsubstituted lanthanum titanates by in situ FTIR measurements. Earlier, we have reported structure-activity correlations and thermophysical properties of Th(VO3)4, LaMnO3, In2Ti1-xFexO5δ, In2Ti1-xCrxO5-δ (0.0 e x e 0.02), etc., mixed metal oxides catalysts.37-39 We have dealt in detail elsewhere40 with the preparation and characterization by X-ray diffraction (XRD), N2-BET, FTIR, scanning electron microscopy (SEM), Mo¨ssbauer spectroscopy, and temperature-programmed reduction (TPR) profiles of Fe-substituted oxygen excess layered perovskites, lanthanum titanates, La2Ti2(1-x)Fe2xO7-δ. The consequences of iron substitution in the crystal lattice of lanthanum titanate, rather than being dispersed for the N2O reduction by CO and also knowing the intermediates produced in the course of the reaction, have yet to be verified. The present work thus deals with in situ IR experiments carried out under CO alone, N2O alone, and a CO + N2O gas mixture to delineate the role of Fe substitution at the B-site in enhancement of the catalytic activity of substituted lanthanum titanate, La2Ti2O7, for the catalytic CO + N2O reaction.

2. Experimental Section 2.1. Sample Preparation and Characterization. La2Ti2(1-x) Fe2xO7-δ samples with nominal compositions of x ) 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 were synthesized by gel combustion using citric acid and calcined at 800 °C; this has been discussed in detail in our earlier work.40 Samples were characterized by XRD, N2-BET, IR, SEM, Mo¨ssbaeur spectroscopy, and TPR. The results of these characterization techniques have been explained in detail in our previous paper.40 Table 1 lists the abbreviations and phases identified in all substituted and unsubstituted combustion-synthesized samples calcined at 800 °C. Henceforth, in the present study, all of the samples will be referred to by the abbreviation used in Table 1. The surface area, pore size by N2-BET, and crystallite size estimated by the Scherrer equation using the XRD line width of La2Ti2(1-x)Fe2xO7-δ samples are mentioned in Table 2. 2.2. Catalytic Activity. Temperature-dependent catalytic activity of all La2Ti2(1-x)Fe2xO7-δ samples was evaluated for the CO + N2O reaction in a flow through reactor under continuous flow of reactant mixtures (CO:N2O:He ) 2:2:16) at a feed rate of 18 L h-1 g-1 using 100 mg of sample in the range of 50-600 °C, by holding for 1 h at each temperature. The effluent gas was analyzed by a gas chromatograph equipped with a Poropaq Q column and thermal conductivity detector (TCD). The catalytic conversions for the CO + N2O reaction were calculated on the basis of product yield CO2. To ascertain the stability and reproducibility of every composition, the activity evaluation over spent samples was repeated, and average conversion rates were plotted. Prior to activity measurements, each sample was initially pretreated at 350 °C for 2 h under N2 flow. 2.3. In Situ FTIR. In situ infrared spectroscopy experiments were performed using an indigenously developed high-temperature, high-pressure, stainless steel cell equipped with two watercooled CaF2 windows as explained previously.41 A schematic of this cell is presented in Figure 1. A self-supporting sample pellet (80 mg, 25 mm diameter, and 0.8 mm thickness) was mounted in the IR cell, and the spectra were recorded in transmission mode using a JASCO 610 FTIR spectrophotometer in tandem with a DTGS detector. To achieve a good signal-tonoise ratio, 300 scans were collected for recording each spectrum at a resolution of 4 cm-1. Prior to its exposure to reactant gases, the sample pellets were heated in a vacuum of about 10-4 Torr for 4 h at a temperature of 300 °C. For IR experiments as a function of temperature studies, the sample was exposed to a

Mechanism of CO + N2O Reaction

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Figure 1. Front and side view of a stainless steel cell for the in situ FTIR study of a catalyst surface during catalytic reactions. (A) Top flange with sample heater block and (B) cell body: 1, rectangular stainless steel body; 2, cartridge heater block; 3 and 4, flanges; 5, sample wafer; 6, sample holder block; 7, CaF2 window; 8 and 9, cooling water jackets; 10, manifold with gas inlet/outlet and evacuation facility; and 11, thermocouple to measure sample temperature.

reactant gas mixture to maintain a final pressure of 100 mbar. Unless mentioned otherwise, the final spectra were recorded with an unexposed sample wafer as a reference, and the IR bands due to unadsorbed gaseous spectra were subtracted wherever applicable. The values given in the parentheses in some figures represent the absorbance values of individual IR bands so as to give an estimate of the relative intensity. The samples used in the present study are La2Ti2(1-x)Fe2xO7-δ samples having x ) 0 and 0.6 and have been prepared by the gel combustion method.40 Henceforth, in this paper, the above samples will be referred to as LTOGC and LF(0.6)GC, respectively. The samples chosen as LF(0.6)GC exhibited the highest catalytic activity for the N2O + CO reaction in the present study.40 So, the adsorption behavior of N2O + CO on the surface of the host Fe-substituted and unsubstituted lanthanum titanate samples was studied for delineating the role of Fe doping in enhancement of activity. The gases used for adsorption study are CO alone, N2O alone, and a mixture of N2O:CO:He ) 1:1:17. 3. Results and Discussion 3.1. Characterization. We have dealt in detail40 elsewhere with the preparation and characterization by XRD, N2-BET, FTIR, SEM, Mo¨ssbauer spectroscopy, and TPR profiles of Fesubstituted oxygen excess lanthanum titanates, La2Ti2(1-x)Fe2xO7δ. Here, we are restating some of the characterization results for better understanding and establishing correlations in structure-activity-in situ FTIR studies. Phases identified in all substituted and unsubstituted combustion-synthesized samples calcined at 800 °C are listed in Table 1. The XRD patterns of samples (0.0 e x e 1.0) synthesized by gel combustion reaction calcined at 800 °C are shown in Figure 2. The XRD pattern of sample with x ) 0.0 (Figure 2a) matches with monoclinic La2Ti2O7 (JC-PDS card no. 28-0517). As expected, the substitution of lower-valent Fe3+ in place of Ti4+ decreased the extra oxygen in the La2Ti2O7 phase. This is manifested in the change of layered perovskite to perovskite for the samples having a Fe

Figure 2. XRD patterns of La2Ti2(1-x)Fe2xO7-δ samples synthesized by citrate combustion calcined at 800 °C for different values of x, LTOGC (a), LF(0.1)GC (b), LF(0.2)GC (c), LF(0.4)GC (d), LF(0.6)GC (e), LF(0.8)GC (f), and LFOGC (g). *XRD peak at 2θ ) 32.17° is attributed to perovskite LaFeO3, and #XRD peak at 2θ ) 29.8° corresponds to La2Ti2O7 phase.

content more than 40%. The samples having x g 0.4 crystallizes in a single rhombohedral LaFeO3 phase (Figure 2d-g) having unit cell parameters a ) 5.566 Å, b ) 7.854 Å, and c ) 5.56 Å, vol ) 242.81 Å3 (JC-PDS card no. 37-1493). The unsubstituted samples LTOGC and LFOGC are single phasic La2Ti2O7 and LaFeO3, respectively, without any impurity phase. Samples with a Fe content g40% crystallized in single phase LaFeO3. The in between (from 10-40%) substituted samples are mixtures of two extreme phases. The room temperature Mo¨ss-

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Figure 3. Temperature-dependent activity of La2Ti2(1-x)Fe2xO7-δ samples for CO + N2O reaction.

bauer spectra of La2Ti2(1-x)Fe2xO7-δ samples40 reveal distortion in symmetry in the LF(0.4)GC sample, which may arise due to anisotropic deformation of the environment around the Fe3+ ions due to oxygen vacancies generated by substitution at Ti4+ sites.40 3.2. Catalytic Activity. 3.2.1. CO + N2O Reaction. The temperature-dependent catalytic activity of La2Ti2(1-x)Fe2xO3+δ samples synthesized by a gel combustion route calcined at 800 °C for the CO + N2O reaction is exhibited in Figure 3. The reactant mixture of CO:N2O:He ) 2:2:16 was fed to the catalyst at a rate of 18 L h-1 g-1. Partially Fe-substituted samples have shown better activity as compared unsubstituted samples. Unsubstituted LTOGC and LFOGC with single phase La2Ti2O7 and LaFeO3, respectively, have shown poor activity with a mere ∼20 and ∼55% conversion at 400 °C, respectively, in curves a and c of Figure 3. LF(0.2)GC and LF(0.8)GC (curves b and d) samples have shown 100% conversion at ∼475 and ∼450 °C, respectively, higher than unsubstituted LTOSS sample with 100% conversion at above 550 °C. The LF(0.4)GC sample resulted in a considerable rise in activity as shown in curve e with 100% conversion at 425 °C in comparison to other samples. Among all samples, LF(0.6)GC has shown a maximum activity of 100% conversion at ∼395 °C with an onset at ∼275 °C for the CO + N2O f CO2 + N2 reaction (Figure 3f). The light off temperature of substituted LF(0.6)GC sample was at a much lower temperature of 275 °C as compared to 400 °C in the case of unsubstituted LTOGC. LFOGC and LF(0.8)GC samples have shown similar activities. T50 and T100 suggest the temperatures at which 50 and 100% conversion for the CO + N2O reaction achieved oversubstituted and unsubstituted samples. Both T100 and T50 values are lowest for LF(0.4)GC and LF(0.6)GC, suggesting that 40-60% Fe substitution leads to catalytically active compositions for the CO + N2O reaction. The difference in T50 and T100 values of LF(0.6)GC sample is ∼28 °C (395 minus 365 °C from curve f), much lower as compared to unsubstituted pristine LTOGC (91 °C from curve a) in Figure 3. Thus, once reaction onsets, the rate increases steeply with a rise in temperature over LF(0.6)GC, as evident from Figure 3. 3.3. In Situ FTIR. Linear molecules, CO and N2O (N-N-O), conforming to 3N-5 (where N is the number of atoms in the molecule), show a singlet and three doublets with P and R branches as fundamental vibration modes, respectively, in their gaseous FTIR spectra. The N2O spectrum is also marked with

Figure 4. In situ FTIR spectra of CO gas in the presence and absence of La2Ti2O7 (LTOGC) and LF(0.6)GC samples recorded under ambient and evacuated conditions. The subtraction spectrum: spectrum minus the gaseous CO spectrum. A distinct extra band at 2056 and 2124 cm-1 is attributed to the presence of carbonyl species of the type M-CO and M-(CO)n type, respectively.

TABLE 3: Fundamental IR Bands of Gaseous CO and N2O as Recorded in in Situ IR Cells cm-1 state

CO

gaseous

2170/2118

chemisorbed

2056 (M-CO) 2124 (M-COn) 1700-1000 (CO32- like species) 1230 (M-CO32- in LTOGC sample) 1323 [M-CO32- in LF(0.6)GC sample]

N 2O 3498/3464 2583/2547 2238/2208 2197 2204 2217 2235

a number of overtones and combination bands in addition to fundamental modes. Table 3 lists vibrational bands of gaseous and chemisorbed CO and N2O, observed in in situ FTIR spectral measurements. It is pertinent to mention here that catalytic activity evaluation and in situ FTIR were recorded for the CO + N2O reaction under similar conditions in the absence of any gaseous oxygen in feed gas (CO + N2O + He). 3.3.1. CO Adsorption. Transmission mode spectra of the species resulting from adsorption of CO (100 Torr) over a disk of LTOGC and LF(0.6)GC at ambient temperature are shown in Figure 4. Prominent IR signals due to gaseous CO and various overlapping signals in the 2000-2300 cm-1 region due to chemisorbed CO along with a shoulder at ∼2056 cm-1 attributed to stretching of CO in the M-CO bond over La2Ti2O7 (LTOGC), where M ) Ti4+, were observed in Figure 4. Unlike the parent sample, in the case of Fe-substituted La2Ti0.8Fe1.2O7δ, (LF(0.6)GC) sample, there is a shift in the IR band at ∼2124 cm-1 (from 2118 cm-1, Figure 4), which is attributed to the formation of multicarbonyl species of the type M-(CO)n, where n g 2 and M can be Fe3+, Ti4+, anionic vacancy, or pores present in the sample. In general, it is anticipated that the molecule



Figure 5. In situ IR spectra of adsorbed CO on LTOGC at different temperatures (A) and time (B).

will be bound more strongly to defect sites (which are coordinative unsaturated) than to normal, terrace ones,42,43 resulting in a shift to higher vibration wavenumbers. The spectra obtained after subtracting the gas phase CO are also shown, where it is observed that there are residual bands at 2122 cm-1 due to multicarbonyl species in LF(0.6)GC and a sharp peak at 2056 cm-1 in LTOGC. Evacuation of the in situ cell for 5 min at room temperature, after the exposure of CO over LTOGC, resulted in complete removal of gaseous CO, adsorbed CO, and an extra band at 2056 cm-1 (Figure 4). This confirms the weak adsorption of CO on LTOGC. On the contrary, strong chemisorptions on LF(0.6)GC are confirmed by the presence of residual bands on its surface by the presence of 2124 cm-1 peak even after evacuation (Figure 4). When the LTO sample disk is exposed to 100 mbar of CO gas, then the peaks that are obtained as a function of temperature are shown in Figure 5A. It is evident from Figure 5A that there is a slight decrease in the intensity of the CO peak as the temperature increases from room temperature to 200 °C with the appearance of a broad and strong band at 1230 cm-1 at 200 °C. This band can be attributed to the formation of surface carbonates, that is, M-CO32- or bicarbonates type species, which remain present even after evacuation, as shown in Figure 6. The bands observed in 1700-1000 cm-1 can be assigned to carbonate-like species, M-CO32- adsorbed as reported on Au-supported ZnO and TiO2 catalysts.44 At 350 °C, the broad band vanishes completely, and the CO2 peak appears; that is, the decomposition of carbonates evolves CO2 with an increase in temperature. The CO2 peak at 350 °C is observed as a function of time in Figure 5B, where it is observed that the CO2 peak increases in intensity with time along with the decrease in intensity of the CO peak, and no carbonate peaks are observed. Thus, CO adsorbs at a surface adsorption site, which at higher temperatures (200 °C) form surface carbonates or bicarbonates, and then, at 350 °C, these carbonates decompose, producing CO245,46 as per the following mechanism.

Figure 6. Presence of the CO32- band even after evacuation of LTOGC after exposure to 100 mbar CO at 200 °C.

CO(g) + * f *-CO

(a)

*-CO + 2*-O2- f *-CO32- + 2* + 2e-

(b)

*-CO32- f CO2(g) + *-O2-

(c)

Under similar conditions when the LF(0.6)GC sample disk is exposed to 100 mbars of CO, as the temperature is increased to 100 °C, the carbonate peak appears at 1323 cm-1 with a decrease in intensity of adsorbed CO, but no CO2 is formed, as shown in Figure 7A. This again indicates the formation of

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Figure 7. In situ FTIR spectra of 100 mbar of CO adsorbed on LF(0.6)GC as a function of temperature (A) and time (B).

surface carbonates or M-CO32- type species. In the case of LF(0.6)GC, the carbonate formation is observed at a much lower temperature (100 °C) than LTO (200 °C), and also, the nature of carbonate is also different as the peak position (1230 cm-1) and broadness also vary. Carbonate formation at lower temperature in LF(0.6)GC is probably due to greater participation of the lattice oxygen atoms on the surface in the substituted samples. The substitution-induced nonstoichiometry generated in the lattice makes the lattice oxygen more mobile, and thus, a surface-adsorbed CO can react with surface lattice oxygen to produce carbonates at a much lower temperature. In other words, the lattice oxygen atoms on the surface must be reactive enough to allow the adsorption and subsequent formation of carbonates at these sites. Such sites are lacking in a perfect stoichiometric lattice like LTOGC but are present in a defect structure43 like LF(0.6)GC. The CO binds to two surface oxygen-forming carbonates at the same time, reducing the cation, and this process is also facilitated in the iron-substituted sample due to the presence of more reducible cation Fe3+. It is also seen that the carbonate peak appears at 1323 cm-1 at a much higher wavenumber, which can be at an iron-substituted site. Thus, in addition to increasing the oxygen mobility, Fe substitution has resulted in the formation of additional surface sites for the adsorption of CO. Subsequently, with a further increase in temperature to 300 °C, the CO2 peak appears, and the intensity of carbonates increases, and after that, the intensity of carbonate remains constant, but that of CO2 increases drastically at 350 °C (Figure 7A). Another interesting feature is that the absorbance value of CO also decreases at this point. The effect of time at 350 °C has also been observed and is shown in Figure 7B, where it is seen that the carbonate peak intensity remains constant throughout 60 min, while that of CO decreases and CO2 increases. Although the decrease of carbonate intensity was less even with an increase in time, the decomposition of surface carbonates does appear to have occurred.47 The amount of CO2 formed is also much higher as noticed from the absorbance values for LF(0.6)GC than LTOGC. This result is in agreement with the catalytic activity evaluated on LTOGC and LF(0.6)GC samples for the CO + O2 reaction.

3.3.2. N2O Adsorption. Let us now consider the effect of exposure of 100 mbar of N2O alone on a pellet of LTOGC and LF(0.6)GC. The transmission mode spectra of the species resulting from adsorption of N2O (100 Torr) as a function of temperature over a disk of LTOGC and LF(0.6)GC are shown in Figure 8A,B, respectively. The bands at 2112 and 2235 due to P and R branches centered at 2223 cm-1 correspond to the antisymmetric NN stretching (ν3 mode) and 1274 and 1301 (P and R) centered at 1285 cm-1 correspond to symmetric NO stretching vibrations (ν1 mode) of the N2O molecule.48-50 In LTOGC, it is seen that with temperature there is a decrease in the peak intensity of N2O. Because LTOGC is an oxygen excess titanate, it is not expected that any dissociative chemisorptions of N2O can occur on its surface as any further oxidation of the surface is impossible. Thus, the decrease in intensity of N2O absorbance with an increase in temperature can be attributed to desorption of the physisorbed N2O with a rise in temperature. On the contrary, on LF(0.6)GC, the N2O absorbance values remain mostly constant. Upon evacuation of the sample cell after exposure to 100 mbar of N2O, we observe from Figure 9B that there are residual bands at 2197 and 2204 cm-1 on LF(0.6)GC in addition to 2217 cm-1, whereas a broad band centered at 2235 cm-1 of negligible intensity is observed in the spectrum on LTOGC (Figure 9A). Thus, on the Fe-doped sample, LF(0.6)GC, the adsorption of N2O is more strengthened as compared to LTOGC when their spectra were compared after evacuation of the IR cell. Thus, in LTOGC, N2O is primarily weakly adsorbed on the surface, which undergoes desorption at higher temperatures, but Fe substitution in LF(0.6)GC generates surface sites for chemisorptions of N2O. 3.3.3. CO + N2O Adsorption. The transmission mode spectra of the species resulting from adsorption of N2O + CO (100 Torr) as a function of temperature over a disk of LTOGC are shown in Figure 10A. Both N2O and CO get adsorbed on the surface of LTOGC, as is evident from Figure 10A, where we see a strong peak due to NN vibration of N2O in comparison to a weak band resulting from CO stretching, which is reflected in the gas phase spectra as well. This difference in absorption intensity of the two gases can be attributed to the higher



Figure 8. Temperature-dependent in situ FTIR spectra of LTOGC (A) and LF(0.6)GC (B) upon exposure to 100 mbar of N2O.

Figure 9. In situ FTIR spectra of 100 mbar of N2O adsorbed on LTOGC (A) and LF(0.6)GC (B) at room temperature after evacuation in the 216-2260 cm-1 region.

extinction coefficient for the NN stretching vibration of N2O. The differences in chemisorptive behavior between the two species have been discussed by de Paola et al.51,52 However, the extent of N2O adsorption/desorption and the rate of reaction of CO + N2O are definitely influenced by Fe doping. When the LTOGC disk is exposed to 100 mbar of CO + N2O as a function of temperature, no change in peak position and intensity is observed until 200 °C, and no carbonate or nitrate peaks are observed. At 350 °C, a slight decrease in N2O peak intensity along with meager formation of CO2 is observed. The effect of exposure of the gas mixture on LTOGC at 350 °C as a function of time was also observed and is plotted in Figure 10B. It is seen from Figure 10B that there is no change in peak intensity

of the adsorbed CO and N2O with time, nor there is any evidence for the increase in CO2 concentration even at so high temperatures. Thus, the CO + N2O reaction proceeds very slowly on the LTO surface. In contrast, in the case of LF(0.6)GC, the N2O peak intensity decreases drastically with temperature along with the formation of carbonate at 200 °C at 1337 cm-1, as shown in Figure 11A. The decrease in peak intensity of N2O can be attributed to dissociative chemisorptions of N2O producing gas phase N2 and adsorbed oxygen. Because the N2 molecule is IR inactive, it is not seen in the spectrum. Considerable gas phase CO2 formation is also observed from 300 °C (Figure 11A) over the LF(0.6) sample. Thus, in the presence of N2O, CO oxidation proceeds with simultaneous N2O reduction in both of the

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Figure 10. In situ FTIR spectra of 100 mbar of CO + N2O adsorbed on LTOGC as a function of temperature (A) and time (B).

Figure 11. In situ FTIR spectra of 100 mbar of CO + N2O adsorbed on LF(0.6)GC as a function of temperature (A) and time (B).

samples. In LF(0.6)GC, the carbonate formation assists the reduction of preadsorbed N2O, while in the case of LTO, the absence of carbonate makes the reaction progress sluggishly as inferred by the following results. In either of the samples, nitrate formation is not observed, but the decrease in N2O peak intensity with temperature definitely suggests the dissosciative adsorption of N2O on the LF(0.6)GC surface. When the temperature is increased from 200 to 300 °C in LF(0.6)GC (Figure 11A), then the CO32-, N2O, and CO2 peak absorbance values increase, decrease, and increase, respectively. At an even higher temperature at 350 °C, the carbonate peak disappears, and moreover, the CO2 and the N2O peak intensities gradually increase and decrease, respectively, with time at 350 °C, as is observed from the time-dependent curve (Figure 11B), indicating that the reaction proceeds with time at 350 °C. On the basis of these

results, a mechanism is proposed for reduction of N2O by CO via transient CO32- species, which is explained in detail in the next section. 3.3.4. Mechanism. The mechanism of the N2O decomposition reaction (Scheme 1, eqs 1.1-1.4) is seen from published kinetic studies,53,54,12 where it is interpreted that desorption of adsorbed oxygen (eqs 1.3 and 1.4) is the rate-limiting step in the decomposition. The surface-adsorbed oxygen (eq 1.3) could be removed more easily in the presence of a reducing agent, for example, light hydrocarbons,55 NH3,56 CO,6,7 and H2.7 Several different mechanisms of N2O reduction with CO were proposed.57,58 The most often published mechanism is “redox” (Scheme 2), when the surface is oxidized by N2O and consequently reduced by CO with simultaneous formation of CO2 (eq 2.1-2.4). The “redox”

Mechanism of CO + N2O Reaction SCHEME 1: Mechanism of N2O Decomposition

J. Phys. Chem. B, Vol. 114, No. 20, 2010 6951 SCHEME 4: Mechanism Proposed for CO + N2O Reaction Proceeding via CO32- Formation over Fe-Substituted Lanthanum Titanate, LF(0.6)GC, Sample

SCHEME 2: Redox Mechanism

SCHEME 3: Associative Mechanism

reaction could proceed by two ways: Adsorbed oxygen can react with adsorbed CO (eq 2.3)27 or with CO from the gas phase according to the Eley-Rideal mechanism (eq 2.4).28,29 The redox mechanism was published over several oxides and supported metal oxides.29-36 The other mechanism is called “associative” (Scheme 3). The simultaneous oxidation and reduction are considered, and the reaction proceeds via species adsorbed on the catalyst surface (eqs 3.1-3.4). The reaction can proceed between both reactants in adsorbed form (eq 3.3) or between adsorbed CO and N2O from the gas phase (eq 3.4). The associative mechanism was described over Ag/Al2O3,28 MgO,59,60 NiO,61 and Fe2O3/SiO2.62 In some cases, both mechanisms take part in the reaction, and contribution to the overall reaction rate depends on the experimental conditions such as temperature and the extent of catalyst reduction, or each mechanism takes place at different sites. The combination of redox and associative mechanisms was considered over calcined hydrotalcites and Fesilicalite. Apart from a simple reduction, the simultaneous reduction and direct N2O decomposition can also proceed.

The replacement of Ti4+ by Fe3+ has considerably modified the surface properties of the oxides. On the basis of activity data and the in situ FTIR spectra recorded for the LTOGC and LF(0.6)GC samples, it is suggested that Fe substitution provides a different pathway for the CO + N2O reaction as compared to unsubstituted lanthanum titanate. We have already discussed that when the samples LTOGC and LF(0.6)GC are exposed to CO alone, then the lattice oxygen atoms on the surface of LF(0.6)GC are more reactive and react with an adsorbed CO molecule, producing carbonates. The carbonate formation is accompanied by the reduction of the cation, which is again facilitated in the substituted sample by the presence of transition metal cation Fe3+, which is more reducible than any of the other cation-Ti4+ or La3+.40 Furthermore, iron substitution provides additional surface sites for the adsorption of CO. On the substituted sample, CO is chemisorbed as compared to weakly adsorbed CO in LTOGC. The catalytic activity results support the in situ FTIR findings, where we find that the activity for the CO + N2O reaction is much higher in the substituted samples, LF(0.6)GC, as compared to unsubstituted LTOGC. From the in situ FTIR studies for N2O alone, we have seen that N2O is primarily weakly adsorbed on the LTOGC surface, while Fe substitution provides anionic vacant sites for the stronger adsorption of N2O but in neither case is any dissociative chemisorptions of N2O observed. On exposure to both of the gases N2O and CO simultaneously, we observe carbonate formation in LF(0.6)GC but not in the stoichiometric LTOGC, as a perfect stoichiometric lattice lacks reactive surface lattice oxygen atoms on the surface. Aliovalent Fe3+ substitution in place of Ti4+ generates anionic vacancies, which results in increased lattice oxygen mobility and thus enhanced availability of lattice oxygen O2- on the surface. Thus, in LF(0.6)GC, a chemisorbed CO can react with surface lattice oxygen, producing surface carbonates (eq 4.2), which decompose, producing CO2, and this process creates surface oxygen vacant sites for dissociative chemisorptions of N2O producing molecular N2 and adsorbed oxygen (O*) (eq 4.3). The anionic vacancies present in the lattice are responsible for the production of vacant surface active sites for further oxygen adsorption (eq 4.3). Thus, in the presence of CO, N2O reduction on LF(0.6)GC proceeds via surface activation for dissociative chemisorptions of N2O by CO2 formation from carbonates. Then, either adsorbed CO or gas phase CO forms CO2 using up the adsorbed oxygen (O*, eq 4.4). So, a mechanism is proposed as shown in Scheme 4, where the rate-determining step is CO2 formation via carbonates (K2′, eq 4.2). As soon as a CO molecule is removed, producing CO2 via carbonate, the catalytic surface becomes oxygendeficient, and dissociative chemisorptions of N2O (eq 4.3)

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proceed spontaneously and the adsorbed oxygen generated in the process is driven away by a gas phase CO or adsorbed CO. Thus, in LF(0.6)GC, the carbonate formation assists the reduction of preadsorbed N2O, while in the case of LTO, the absence of carbonate makes the reaction progress sluggishly. The proposed mechanism is essentially a redox phenomenon, where the surface is initially reduced by CO and then the reduced surface is oxidized by dissociative N2O chemisorption. The catalytic activity data exactly corroborate the in situ FTIR results where we find that for the CO + N2O reaction, the LF(0.6)GC sample shows the highest activity. However, on LTOGC, CO cannot activate the surface for any dissociative chemisorptions of N2O. This incapability is probably due to a stoichiometric surface resulting in weak physisorption of CO, and poor lattice oxygen diffusion does not lead to any carbonate formation. Furthermore, a stoichiometric oxygen excess titanate will not in any way facilitate dissociative chemisorptions of N2O, as there is no vacancy in the surface to accommodate another oxygen atom. Thus, the reduction of N2O with CO over LTOGC proceeds via an associative mechanism (Scheme 3). 4. Conclusion To the best of our knowledge, this is the first work of its kind on the crystalline iron-substituted lanthanum titanates, La2Ti2(1-x)Fe2xO7-δ, using in situ FTIR spectroscopy to investigate the surface species and to predict the most probable mechanism involved in the vapor phase catalytic reduction of N2O by CO in conjunction with catalytic activity under similar conditions. The present study concludes that the catalytic properties of La2Ti2O7, unsubstituted phase, were considerably enhanced by partially substituting Fe3+ at the B-site in place of Ti4+. Among all, samples with the composition La2Ti0.8Fe1.2O7-δ [LF(0.6)GC], prepared by a gel combustion route, have shown maximum activity for the CO + N2O reaction. The catalytic activity data exactly corroborate the in situ FTIR results recorded under similar conditions. The anionic vacancies/defects or pores generated in Fe-substituted samples provide additional surface active sites for chemisorption of CO and N2O adsorption and also are active sites for dissociative chemisorptions of N2O. The CO binds to two surface oxygen-forming carbonates at the same time, reducing the cation, and this process is also facilitated in the iron-substituted sample due to the presence of more reducible cation Fe3+. Pristine lanthanum titanate, La2Ti2O7, lacks such sites on the stoichiometric surface, leading to physisorption of reactants; therefore, no carbonate formation was observed. Investigations by in situ FTIR reveal that both the position and the intensity of product CO2 band correlate exceptionally well with the enhanced intensities of the carbonate species and also the reduced intensity of the adsorbed N2O on the LF(0.6)GC sample. The proposed mechanism is essentially a redox phenomenon, where the surface is initially reduced by CO, and then, the reduced surface is oxidized by dissociative N2O chemisorption as against an associative mechanism observed in unsubstituted sample, La2Ti2O7. Acknowledgment. We thank Prof. Dr. T. Mukherjee, Director, Chemistry Group, and Dr. D. Das, Head, Chemistry Division, Bhabha Atomic Research Centre, Mumbai, India, for their kind support and encouragement. References and Notes (1) Theimens, M. H.; Trogler, W. C. Science 1991, 251, 932–934. (2) Roy, P. K.; Pirngruber, G. D. J. Catal. 2004, 227, 164–174. (3) Kapteijn, F.; Mirasol, J. R; Moulijn, J. A. Appl. Catal., B 1996, 9, 25–64.

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