Support-Induced Effects of LaFeO3 Perovskite on the Catalytic

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Support-Induced Effects of LaFeO3 Perovskite on the Catalytic Performances of Supported Pt Catalysts in DeNOx Applications J.P. Dacquin, C. Lancelot, C. Dujardin, C. Cordier-Robert,‡ and P. Granger* Universite de Lille Nord de France, Unite de Catalyse et de Chimie du Solide, UMR CNRS 8181, Bâtiment C3, 59650 - Villeneuve d’Ascq Cedex, France ‡ Universite de Lille Nord de France, Unite Materiaux et Transformations UMR CNRS 8217 - groupe metallurgie Bâtiment C6, 59650 - Villeneuve d’Ascq Cedex, France

bS Supporting Information ABSTRACT: A comparative investigation of the catalytic performance in the simultaneous conversion of NOx and N2O has been achieved on supported nanosized Pt particles interacting with conventional alumina and perovskite based materials. Particular attention has been paid to successive thermal treatments under reductive and oxidative atmospheres which induce bulk and surface reconstructions. Those modifications considerably alter the catalytic behavior of Pt in interaction with LaFeO3 or γ-Al2O3 in terms of activity and selectivity toward the selective transformation of NOx to nitrogen at low temperature. Changes in physicochemical properties have been examined using appropriate techniques, such as H2-temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and Fourier transform infrared (FTIR) of CO adsorption. It has been found that oxidic Pt4þ species initially stabilized on LaFeO3 lead after subsequent H2 reduction to the formation of metallic nano-Pt particles in stronger interaction than on γ-Al2O3 support and then become more resistant to sintering during thermal aging in 1000 ppm NO, 1000 ppm N2O, 3 vol % O2, 0.5 vol % H2O, and 0.5 vol % H2 at 500 °C. Correlatively, significant improvements have been observed in the selective reduction of NOx to nitrogen. This study opens new prospects in the development of supported catalysts containing low Pt loadings because of the existence of stronger interactions with perovskite supports.

’ INTRODUCTION There is currently a growing interest in developing supported catalysts containing lower amounts of noble metals and maintaining high performances in terms of stability and selectivity particularly for environmental applications where the use of noble metals is widespread because of their much higher intrinsic activity. In this scientific context, it has been reported that perovskite materials can be adequate as support material for stabilizing the dispersion of noble metals1-5 for high-temperature catalytic applications. By way of illustration, three-way catalysts usually deactivate irreversibly because of the continuous particle growth under cycling conditions. Such a process usually speeds up when noble metals weakly interact with conventional alumina supports. On the other hand, reducible materials such as ceria can be profitably used to strengthen the metal/support interface and then to attenuate thermal sintering phenomena. In a recent investigation, Hatanaka et al.6 showed that the formation of Pt-O-Ce bonds on Pt/CeO2 stabilizes Pt against sintering r 2011 American Chemical Society

and favors the redispersion of agglomerated Pt particles. Recent investigations also reported significant rate enhancement in the water-gas shift and DeNOx reactions on ceria-zirconia and reducible mixed oxides with noble metals substituted in ionic form.7,8 Now, returning to the peculiar properties of perovskites, previous investigations pointed out their catalytic activity in DeNOx reactions under lean conditions ascribed to the mobility of surface lattice oxygen, the concentration of anionic vacancies, and the mixed valence of metal.9-11 Surface reconstructions which currently occur under typical three-way conditions might also induce redispersion processes and then stabilize noble metals inside the perovskite structure as isolated oxidic species in unusual oxidation state.3,4 Such a feature leads to considerable Received: July 26, 2010 Revised: November 12, 2010 Published: January 6, 2011 1911

dx.doi.org/10.1021/jp1069779 | J. Phys. Chem. C 2011, 115, 1911–1921

The Journal of Physical Chemistry C enhancement in the catalytic conversion of NOx, CO, and unburned hydrocarbons in the automotive exhaust gas which represents a significant progress from a practical point of view.12,13 In fact, reversible dynamic changes take place at the surface under successive reductive/oxidative atmosphere14 particularly when noble metals are incorporated into the network of the perovskite structure. In that case, it is suggested that oxidic noble metal species go out the structure to segregate into nanosized noble metal particles under reductive exposure.12,13 On the other hand, the reverse trend takes place under oxidative conditions. Hence, those reversible processes would protect noble metals from thermal sintering and enhance the lifetime of the catalyst. In fact, the versatility of perovskite structure is well-known and has been put forward in various catalytic applications in mostly oxidation reactions.15 It is obvious that the extent of those surface processes that might occur under successive reductive/oxidative atmosphere strongly depends on the reducibility of the perovskite support but also on the sensitivity of noble metal particles to reoxidation. Probably, the conjunction of both chemical processes determines the extent of noble metal redispersion.4,5,16 Recently, we developed an alternative strategy4,5 from that earlier implemented1-3,6,12 for the introduction of noble metals. It consists in impregnating the surface of the perovskite and redispersing the agglomerated oxidic noble metal clusters by successive reductive/oxidative thermal treatments rather than incorporating noble metals inside the structure during the sol-gel preparation. This latter method obtains solids with noble metal homogeneously distributed in the bulk but with correlatively significant lower accessible metal concentrations than those obtained by using a classical wet impregnation. By applying this latter methodology, we recently obtained significant rate enhancement in the reduction of NO to nitrogen by hydrogen at low temperature on 4 wt % Pt on LaFeO3 which has been ascribed to the formation of epitaxially orientated Pt particles on characteristic LaFeO3 crystallite planes stable at high temperature under rich and lean conditions.5 Therefore, such a finding opens new practical interest in minimizing the noble metal content as earlier reported for the NO/H2/O2 reactions on Pt/La0.5Ce0.5MnO317 and Pt/La 0.7 Sr 0.2 Ce 0.1 FeO3 . 18 Herein, it will be demonstrated that optimal preactivation thermal treatment on supported Pt/LaFeO 3 containing a significant lower amount (1 wt % Pt) could lead to significant improvement in terms of stability and selectivity toward the transformation of NO to nitrogen.

’ EXPERIMENTAL SECTION Catalyst Preparation and Characterization. LaFeO3 was synthesized via an experimental protocol described elsewhere19,20 involving a sol-gel method using citric acid as a complexing agent. Typically, iron and lanthanum nitrate salts were dissolved in the presence of citric acid (CA) according to the molar CA/(Fe þ La) ratio equal to 1. Then, solvent evaporation was processed at 60 °C under vacuum to obtain a precursor gel further dried at 80 °C. Prior to calcination in air at 600 °C for 8 h leading to the formation of LaFeO3, nitrates were decomposed at 200 °C in a muffle furnace. LaFeO3 (25 m2 g-1) and γ-Al2O3 (100 m2 g-1) were impregnated using hexachloroplatinic acid solutions with adjusted concentrations in order to obtain 1 wt % Pt. The impregnated precursors were successively calcined in air at 400 °C for 8 h and were reduced at 300 or 500 °C in pure H2

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overnight. The samples were labeled, respectively, Pt/LaFe(C400) and Pt/LaFe(RX) with X = 300 or 500 according to the prereductive thermal treatment temperature. The same nomenclature was adopted in the case of Pt/γ-Al2O3. Hydrogen temperature-programmed reduction (H2-TPR) was performed in a Micromeritics Autochem II 2920 apparatus (5 vol % H2/Ar). X-ray photoelectron spectroscopy (XPS) experiments were performed using a Vacuum Generator Escalab 220XL spectrometer equipped with a monochromatized aluminum source for excitation in the analysis chamber under ultra high vacuum (∼10-10 Torr). Binding energy (BE) values were referenced to the binding energy of the C 1s core level (285.1 eV). A mixed Gaussian/Lorentzian peak fit procedure was used to simulate the experimental photopeaks according to the software supplied by VG Scientific. Semiquantitative analysis accounted for a nonlinear Shirley background subtraction.21,22 Transmission electron microscopy (TEM) studies were performed on a Tecnai 20 microscope operating at an accelerating voltage of 200 kV. Prior to TEM observations, samples were deposited from ethanolic solution onto holey-carbon copper grids. CO chemisorption followed by IR spectroscopy was performed at room temperature (RT), and the catalyst was placed in a diffuse reflectance Fourier transform (DRIFT) cell (Harrick). A Nicolet 460 Fourier transfrom infrared (FTIR) spectrometer fitted with a mercury cadmium telluride (MCT) detector in DRIFT mode (64 scans) was used to analyze the nature of adsorbed CO species at room temperature under 1 vol % CO diluted in He. The M€ossbauer spectra were recorded at room temperature using a standard constant acceleration M€ossbauer spectrometer. Gamma resonance absorption measurements were performed with a 57Co(Rh) source. Velocity scale was calibrated with an iron-metal absorber. Catalytic Measurements. Temperature-programmed reaction experiments were carried out in a fixed-bed flow reactor using 0.7 g of prereduced catalysts in powder form. Gas hourly space velocity of approximately 10 000 h-1 was calculated on the basis of the volume of the catalytic bed and corresponded to a total flow rate of 15 L h-1. The reactant mixture was typically composed of 1000 ppm N2O, 1000 ppm NO, 3 vol % O2, 0.5 vol % H2O, and 0.5 vol % H2 as reducing agent. Inlet and outlet gas mixtures were analyzed with a μGC Varian CP-4900 chromatograph fitted with two thermal conductivity detectors. NO2 was detected by using a Balzer mass spectrometer. Prior to quantification, reactants and products were separated on two 5 Å molecular sieve and Poraplot Q columns. The procedure implemented for the characterization of the catalytic performances included a preliminary temperature-programmed reaction experiment from room temperature to 500 °C (TPR-1) on prereduced Pt/LaFe(RX) and Pt/Al(RX) catalysts in H2 at 300 or 500 °C. The temperature was gradually increased with a constant heating rate of 3 °C min-1. At the final temperature of 500 °C, the catalyst was maintained under steady-state reactive conditions overnight. After cooling down at RT, a second temperature-programmed experiment (TPR-2) was achieved to evaluate the catalytic performance of aged samples. Aged catalysts were systematically prereduced in H2 at 300 or 500 °C before temperature-programmed reaction experiments (TPR-2). According to this experimental protocol, aged catalysts were labeled Pt/LaFe(RXA500RX) and Pt/Al(RXA500RX), with X = 300 or 500. TPR experiments were also repeated on aged samples in the absence of postreductive thermal treatment (TPR-3). 1912

dx.doi.org/10.1021/jp1069779 |J. Phys. Chem. C 2011, 115, 1911–1921

The Journal of Physical Chemistry C

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Figure 1. H2-temperature-programmed reduction on LaFeO3 and 1 wt % Pt/LaFeO3: (a) LaFeO3; (b) Pt/LaFe(C400); (c) Pt/LaFe(R300A500); (d) Pt/LaFe(R500A500); (e) sol-gel LaFe0.95Pt0.5O3(C600).

’ RESULTS Reducibility of Oxidic Pt Species on Calcined and Aged Catalysts. Earlier investigations on highly loaded supported

Pt catalysts5 containing 4 wt % Pt have shown that H2-TPR experiments might provide interesting information on the nature of oxidic species and on the extent of interaction with the support. Figure 1 reports similar experiments on calcined and aged Pt/LaFeO3 samples containing 1 wt % Pt. As shown, a weak and broad signal for LaFeO3 is discernible at ∼350 °C which emphasizes the poor reducibility of the perovskite structure up to 700 °C. Further incorporation of Pt results in the appearance of a low-temperature H2 consumption range on calcined samples with a maximum at 197 °C ascribed to the reduction of oxidic Pt species predominantly stabilized as PtO2 according to atomic H/ Pt ratio ∼ 4.0. Previous H2-TPR experiments on highly loaded samples containing 4 wt % Pt revealed that the reduction process operated at lower temperature with a maximum at 128 °C.5 Nevertheless, in all cases, the results seem to be in agreement with a preferential formation of PtO2 species interacting with LaFeO3 after calcination. Such experiments were repeated on aged Pt/LaFe(R300A500) and Pt/LaFe(R500A500). Interestingly, no significant hydrogen uptake is detectable which suggests that metallic Pt particles once formed during the reductive pretreatment in H2 at 300 or 500 °C preserve their bulk metallic character during thermal aging under reactive conditions (in the presence of oxygen excess). The outlet gas was analyzed by mass spectrometry in the course of H2-TPR experiments. As exemplified in Figure 2, a prominent deviation of the signal m/z = 44 appears during H2-TPR experiments on aged catalysts probably because of N2O in the outlet gas mixture coming from the reduction of accumulated ad-NOx. One cannot completely rule out the presence of gaseous CO2 from stable carbonates species which would be incompletely removed during the perovskite synthesis. Changes on the relative mass at m/z = 28 can be assigned to cracking of N2O but also to N2 production. It is noticeable that different profiles characterize Pt/LaFe(R300A500) and Pt/LaFe(R500A500) as shown in Figure 2A which might reflect a different extent of surface changes during thermal aging depending on the temperature of the prereductive thermal

Figure 2. Mass spectroscopy analysis of the outgas during H2-temperature-programmed reduction experiments on (a) aged Pt/LaFe(R300A500) and (b) Pt/LaFe(R500A500). (A) m/z = 44 (gaseous CO2 or N2O); (B) m/z = 28 (CO or N2).

Figure 3. TEM micrographs on (a) freshly prepared Pt/LaFe(R300); (b) after aging Pt/LaFe(R300A500); (c) Pt/LaFe(R300A500R300); (d) particle size distribution plot of Pt/LaFe(R300) (9), Pt/LaFe(R300A500) (0), and Pt/LaFe(R300A500R300) (shaded square); number of particles: 200.

treatment (300 or 500 °C). Similar experiments were repeated on Pt/Al(C400) and on Pt/Al(R300A500). The estimates of the atomic H/Pt ratio of approximately 3.1 on Pt/Al(C400) are 1913



Figure 4. TEM micrographs on (a) freshly prepared Pt/LaFe(R500); (b) after aging Pt/LaFe(R500A500); (c) Pt/LaFe(R500A500R500); (d) particle size distribution plot of Pt/LaFe(R500) (9), Pt/LaFe(R500A500) (0), and Pt/LaFe(R500A500R500) (shaded square); number of particles: 200.

concomitant with previous results obtained on highly loaded samples demonstrating the coexistence of Pt2þ and Pt4þ species in interaction with alumina on calcined samples. All those observations seem to be in relative good agreement with previous findings highlighting that more extensive interactions with the support is accompanied with a preferential formation of Pt4þ on calcined samples.5 Transmission Electronic Microscopy Observations (TEM). TEM observations have been performed on Pt/LaFeO3 catalysts after the previously described thermal treatments. Representative micrographs of the catalysts with reducing steps carried out at 300 °C are shown in Figure 3 together with the corresponding Pt particle size distribution. On Pt/LaFe(R300), small Pt particles are present with a mean diameter of 1.0 nm, which is smaller than that previously observed on the catalyst containing 4 wt % Pt and that were reduced at 300 °C (1.5 nm).5 On this latter sample, a few larger particles with diameters around 10-20 nm were also present which is not the case on its equivalent with 1 wt % Pt in relation with the lower amount of platinum that was dispersed on the support. Subsequent thermal aging treatment led to an enlargement of the particle diameter with a mean value of 1.8 nm. As previously observed on 4 wt % Pt samples, a redispersion process occurs after the postreductive thermal treatment in H2 highlighted by a decrease of the mean particle size from 1.8 to 1.2 nm. As exemplified in Figure 4, a similar behavior is observed on the samples treated in pure H2 at 500 °C during the reducing step with an enlargement of the particle size during aging (1.3 nm for Pt/LaFe(R500) and 1.5 nm for Pt/LaFe(R500A500)) followed by a redispersion after subsequent reduction with a mean particle size of 1.1 nm on Pt/ LaFe(R500A500R500). However, the size variation due to aging is much smaller in this case than on the samples reduced at 300 °C. Overall, TEM observations confirm the absence of sintering phenomenon on platinum when supported on LaFeO3 through-

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Figure 5. XPS spectra of Pt 4d on 1 wt % Pt/Al2O3: (a) Pt/LaFe(R300), (b) Pt/LaFe(R300A500), (c) Pt/Al(R300A500R300).

out the thermal treatment process whatever the temperature of the reducing step. Surface Properties: X-ray Photoelectron Spectroscopy versus Fourier Transformed Infrared Spectroscopy (FTIR) of CO Adsorption. Surface analysis by XPS was performed on calcined Pt/Al2O3 and Pt/LaFeO3 without prior in situ thermal treatment. In the former case, the Pt 4d photopeak was examined in Figure 5. Subsequent quantifications are summarized in Table 1. As indicated, no significant change in the BE values is noticeable irrespective of the reductive and oxidative atmosphere during thermal aging. The slightly higher BE value in comparison with the current ones (314.4 eV23) is characteristic of those recorded on bulk metallic Pt surfaces. In fact, those slight shifts in BE values can be explained by the presence of chemisorbed O atoms that are on reduced catalysts because of storage in ambient atmosphere or that are accumulated during thermal aging treatment. Subsequent perturbations of the electronic properties of Pt are characteristic of the usual electron-withdrawing effect from chemisorbed hydroxyl groups or from O atoms or from partial stabilization of PtO film at the topmost surface of metallic Pt particles. A significant decrease of the surface atomic Pt/Al ratio from 2.3 10-3 to 1.4 10 -3 on Pt/Al(R300) and Pt/Al(R300A500R300), respectively, underlines the detrimental effect of the thermal aging on the surface Pt concentration which could be associated with the occurrence of thermal sintering processes as previously evidenced on highly loaded Pt samples.5 The Pt 4f core level has been examined on Pt/LaFeO3. XPS spectra are reported in Figure 6A and B. The BE value recorded on Pt/LaFe(C400) at 75.0 eV characterizes the predominant formation of Pt4þ in PtO223 in accordance with H2-TPR observations. A shift to lower BE values is discernible after reduction in H2 at 300 °C, but the values remain higher than the characteristic one for metallic Pt as previously explained on Pt/Al2O3 because of storage in ambient atmosphere (72.7 vs 71.0 eV for Pt0).23 Subsequent exposure under lean conditions leads inevitably to a partial oxidation but probably is limited to the topmost surface as indicated by H2-TPR experiments which did not reveal significant H2 uptake because of bulk oxidation. 1914



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Table 1. XPS Analysis of 1 wt % Pt/Al2O3 and 1 wt % Pt/LaCoO3 after Exposure under Oxidative and Reductive Conditions catalysts

surface compositionb

BE (eV) Pt 4d5/2

Pt 4f7/2

Pt/Al

Pt/La

Fe/La

bulk composition

H2 uptake a

Pt/M (M = Al or La)

(μmol g-1 cata)

2.6 10-3

80

3.1

102

4.0

Pt/Al(C400)

a

H/Pt a

-3

Pt/Al(R300)

315.5

2.3 10

Pt/Al(R300A500)

315.5

1.4 10-3

Pt/Al(R300A500R300)

315.4

1.4 10-3

Pt/LaFe(C400)

75.1

Pt/LaFe(R300)

72.7

0.049

0.66

0.013

Pt/LaFe(R300A500) Pt/LaFe(R300A500R300)

74.7 72.1

0.060 0.046

0.69 0.64

Pt/LaFe(R500)

71.2

0.033

0.49

Pt/LaFe(R500A500)

71.9

0.030

0.48

Pt/LaFe(R500A500R500)

71.4

0.030

0.49

Calculated from H2-TPR experiments. b Relative accuracy (20%.

Table 2. Estimate of the Relative Surface Composition of Oxidic Pt Species Stabilized in Different Oxidation States on 1 wt % Pt/LaFeO3 catalysts

relative surface composition 0

Pt2þ

Pt/LaFe(R300)

4.6

87.2

8.2

Pt/LaFe(R300A500)

5.6

38.6

55.8

Pt


Figure 6. XPS spectra of Pt 4f on 1 wt % Pt/LaFeO3: (A) (a) Pt/ LaFe(C400), (b) Pt/LaFe(R300), (c) Pt/LaFe(R300A500), (d) Pt/ LaFe(R300A500R300). (B) (a) Pt/LaFe(C400), (b) Pt/LaFe(R500), (c) Pt/LaFe(R500A500), (d) Pt/LaFe(R500A500R500).

Successive H2 exposure restores the metallic character and highlights the existence of reversible changes with no significant changes on the relative surface Pt concentration according to the margin of error. The decomposition of XPS Pt 4f7/2 photopeaks was performed on 1 wt % Pt/LaFeO3 according to the same procedure reported elsewhere.5 Three different contributions have been identified with maxima at 71.3, 72.2, and 74.0 eV assigned to surface Pt0, Pt2þ, and Pt4þ species, respectively. As previously discussed, the BE values lower than those currently reported for

Pt4þ

9.8

89.1

1.1

Pt/LaFe(R500)

63.8

36.2

0

Pt/LaFe(R500A500) Pt/LaFe(R500A500R500)

37.3 47.5

60.8 52.5

1.9 0

Pt2þ as PtO (73.6-74 eV)23 and for Pt4þ as PtO2 (74.8 eV) may reflect the stabilization of a PtOx monolayer on Pt0 particles, the presence of subsurface oxygen species, and a stronger metal/ support interface inducing disturbance on the electronic properties of Pt. An estimate of the relative amount of each Pt surface species according to the nature of thermal treatment under reductive or oxidative atmosphere can be achieved on the basis of this decomposition. Results in Table 2 show the partial restoration of Pt4þ species during thermal aging while they completely disappear when the catalyst is exposed to hydrogen at 300 °C. A second series including prereduction at 500 °C prior to thermal aging has been assessed. Spectra collected in Figure 6B underline a different behavior during thermal aging because of the absence of a significant contribution at higher BE values assigned to oxidic Pt species on Pt/LaFe(R500A500). Thus, Pt would completely preserve its metallic character. Also, lower BE values are observed after reduction at higher temperature (500 °C vs 300 °C). Hence, such a comparison would highlight that a possible accumulation of oxygen at the surface during storage is not responsible for the BE shift observed but would possibly reflect the different extent of Pt/support interaction and subsequent surface reduction of LaFeO3 generating intermetallic microstructures at the interface. Fe 2p spectra remained unchanged irrespective of the nature of the thermal treatment under reductive or oxidative atmosphere with BE value, varying within the margin of error, and the typical shake-up structure of Fe3þ has been preserved in all cases (results not shown). However, it is 1915



Figure 7. DRIFT spectra of CO adsorbed on 1 wt % Pt/Al2O3. Prior to CO adsorption, all catalyst samples were heated under He at 250 °C: (a) Pt/Al(R300), (b) Pt/Al(R300A500), and (c) Pt/Al(R300A500A300).

obvious that XPS is not sensitive enough to detect accurately local surface reduction particularly at the metal/support interface accounting for a very low Pt content. In fact, more accurate information was obtained from Mossbauer spectroscopic measurements performed on calcined LaFeO3 and Pt/LaFe(R500). Spectra reported in Figure S1 of the Supporting Information show essentially a sextet pattern on LaFeO3. Subsequent analysis by means of computer-assisted-least-squares fitting procedures, assuming Lorentzian line shapes, leads to the calculation of the hyperfine parameters, such as the isomer shift IS = 0.35 mm s-1, the quadrupole splitting QS = -0.028 mm s-1, and the hyperfine field H = 51.8 T which essentially characterize Fe3þ in the perovskite structure.24 Relative abundance of 3 ( 1% Fe2þ iron sites can also be deduced. Interestingly, an increase of the relative Fe2þ concentration from 3 to 14 ( 1 is clearly distinguishable on Pt/LaFe(R500) which suggests that Pt interacting with LaFeO3 could promote partial reduction of Fe3þ to Fe2þ. Returning to XPS analysis, subsequent examination of the atomic Pt/La ratio emphasizes the fact that a prereductive thermal treatment at higher temperature slightly affects the surface Pt concentration (see Table 1). Nevertheless, Pt concentration remains remarkably stable irrespective of the thermal treatment process correlatively to the preservation of the metallic character of Pt. Infrared Spectroscopy of CO Adsorption. Additional infrared spectroscopic measurements were performed to visualize different absorption modes for chemisorbed CO molecules on Pt species in connection with the oxidation state and the structural properties of Pt species. In fact, XPS analyses on a depth of approximately 5-10 nm cannot reflect the real surface properties that are directly involved in catalysis typically for well-dispersed supported catalysts. In this sense, CO adsorption measurements might bring complementary information. CO adsorption on supported polycrystalline Pt catalysts and Pt(111) surface has been widely investigated.25-29 Generally, carbonyls stabilized on cationic Pt sites are observed above 2100 cm-1.25,29 Linear Pt3þ-CO species on TiO2 stable up to 100 °C was earlier characterized by an IR band at 2183 cm-1.25 More stable Pt2þ-CO appears up to 450 °C on the same supported catalyst

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Figure 8. DRIFT spectra of CO adsorbed on 1 wt % Pt/LaFeO3. Prior to CO adsorption, all catalyst samples were heated under He at 250 °C: (a) Pt/LaFe(R300), (b) Pt/LaFe(R300A500), and (c) Pt/LaFe(R300A500A300).

on titania.25 Similar spectral features characterize Pt/Al2O3 with usual observations of linearly coordinated CO species on Pt2þ above 2100 cm-1 while CO adsorption on partially oxidized platinum Ptδþ yields a 2082 cm-1 IR band. Generally speaking, CO linearly coordinated to metallic Pt sites leads to the observation of IR bands in the range 2060-2075 cm-1 whereas bridged CO species are currently detected near 1845 cm-1.23 Figure 8 shows infrared spectra recorded on Pt/Al(R300), Pt/ Al(R300A500), and Pt/Al(R300A500R300). Prior to adsorption, catalyst samples were systematically exposed under flowing He at 250 °C. CO adsorption was achieved at room temperature until saturation. As observed in Figure 7a, three distinct contributions arise with apparent maximum at 2074, 2090, and 2125 cm-1. The 2074 cm-1 IR band can be ascribed to CO linearly coordinated to Pt0. The 2090 cm-1 IR band corresponds to CO coordinated to Pt sites preadsorbed by chemisorbed oxygen whereas CO adsorption on Pt2þ occurs at higher wavenumber according to previous assignments. A weak CO adsorption is also discernible at low wavenumber 1844 cm-1 ascribed to bridged CO species on Pt0. Interestingly, the high frequency IR band related to the presence of Pt2þ species disappears after aging. Subsequent reductive thermal treatment of Pt/Al(R300A500R300) induces a significant attenuation of the 2098 cm-1 IR band because of the consumption of strongly chemisorbed O atoms on Pt accumulated during thermal aging. No alteration in intensity of the corresponding bridged CO species IR band is discernible. Different spectral features characterize Pt/LaFeO3. As seen in Figure 8a, weak and broad IR bands appear on Pt/ LaFe(R300) with apparent maximum at 2130 and 2099 cm-1 assigned to CO on oxidic and metallic Pt species. The contribution at 2130 cm-1 attenuates after aging. Finally, subsequent, reductive thermal treatment leads to a prominent IR band at 2077 cm-1 which essentially characterizes the presence of linear CO species bound to Pt0 on Pt/LaFe(R300A500R300). Those observations cannot be completely correlated to XPS 1916



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Table 3. Catalytic Performances of 1 wt % Pt/Al2O3 and 1 wt % Pt/LaFeO3 at Low Temperature for the NO Reduction by H2 to N2 and N2O T (°C)

XNO (%)

SN2 (%)

TMa (°C)

Pt/Al(R300)

75

12.4

45.8

105

100

32.6

Pt/Al(R300A500) Pt/Al(R300A500R300)

40

16.3

0

65

100

5.3

Pt/LaFe(R300)

95

33.7

47.0

122

Pt/LaFe(R300A500)

87

37.7

50.8

132

100

43.3


82

28.0

34.1

113

100

31.1

Pt/LaFe(R500)

132

29.8

19.3

162

Pt/LaFe(R500A500)

118

23.1

44.0

162


127

28.7

21.0

141

catalyst

a

XNOb (%)

75.2

60.7 100 42.8

SN2c (%)

47.8

29.2 44.9 23.7

Temperature at the maximum NO conversion. b Maximum NO conversion. c Selectivity at the maximum NO conversion.

observations because He pretreatment before adsorption might induce partial desorption of surface adsorbates. In addition, the reductive effect of CO has to be taken into account even if adsorption measurements were performed at room temperature. However, it is obvious that two different trends arise from the comparison of IR with XPS data related to an increase of the reducibility of oxidic Pt species at the surface and the development of the relative intensity of IR bands on Pt/LaFe(R300A500R300) contrarily to Pt/Al(R300A500R300). This is well in line with the preservation of the surface Pt concentration as reported in Table 1 emphasizing a greater resistance of Pt to thermal sintering process once deposited on perovskites as demonstrated on the basis of TEM observations. Catalytic Performances in the Simultaneous Reduction of NO and N2O by H2. Most of the results obtained within this study for the catalytic NO/H2 reaction can be explained from the following set of overall reactions. Equation 1 represents the target reaction associated with the formation of nitrogen. Side products can be obtained related to the incomplete reduction of NO to nitrous oxide and to the competitive NO oxidation to NO2 in the presence of 3 vol % O2. 2NO þ 2H2 ¼ N2 þ 2H2 O

ð1Þ

N2 O þ H2 ¼ N2 þ H2 O

ð2Þ

2NO þ H2 ¼ N2 O þ H2 O

ð3Þ

2NO þ 5H2 ¼ 2NH3 þ 2H2 O

ð4Þ

2NO þ O2 ¼ 2NO2

ð5Þ

2H2 þ O2 ¼ 2H2 O

ð6Þ

Generally speaking, gaseous oxygen usually has a detrimental impact on the reduction of NO over noble metals inducing strong inhibiting effect on the competitive adsorption of NO and a poor selectivity with eq 6 occurring more readily than for NO/ H2 reaction. On the other hand, different behavior might characterize noble metals that are supported on reducible supports or that exhibit oxygen storage properties. Previous investigations reported the contribution of gaseous oxygen in the overall reduction of NO to nitrogen on Pt/MgO-CeO230,31 and Pt/LaCeMnO3.32 It was found from Steady-State-IsotopicTransient Kinetic Analysis (SSITKA-MS) experiments that oxygen does not participate in the formation of N2O on Pt/MgOCeO231 according to eq 8 but likely participates only in the

formation of N2 via eq 7. However, the participation of oxygen in the production of N2O was evidenced on Pt/LaCeMnO3.32 2NO þ 3H2 þ O2 ¼ N2 O þ 3H2 O

ð7Þ

2NO þ 4H2 þ O2 ¼ N2 þ 4H2 O

ð8Þ

This presumably involves the participation of ad-NOx species stabilized on the support as possible intermediates in the reduction of NO to nitrogen. Catalytic measurements have been assessed by temperatureprogrammed reaction experiments carried out on as-prepared supported Pt on γ-Al2O3 and LaFeO3 (TPR-1) and on aged catalysts (TPR-2) after exposure to the reactive conditions overnight at 500 °C. As earlier mentioned, prior to TPR experiments, the catalyst samples were systematically reduced in pure H2 at 300 or 500 °C. Additional TPR experiments were repeated on aged catalysts without any postreductive thermal treatment in H2 (TPR-3). Catalytic measurements are summarized in Table 3. Figure 9 illustrates TPR experiments performed on Pt/γ-Al2O3. As exemplified in Figure 9A, two NO conversion ranges are distinguishable. At low temperature, a narrow range essentially leads to the production of N2O on Pt/Al(R300). Subsequent increase in temperature leads to a strong enhancement of the competitive H2/O2 reaction. In parallel, NO2 formation intensifies above 300 °C. Thermal aging under reactive conditions overnight at 500 °C induces significant changes in the kinetic behavior of Pt/Al(R300A500). As observed in Figure 10B, the low temperature NO conversion range broadens and shifts to a lower temperature. However, parallel to that observation, the production of N2O is favored at the expense of N2 with a selectivity shifting from 32.6 to 5.3% at complete NO conversion. Pt/LaFe(R300) exhibits the same kinetic behavior with the appearance of two NO conversion ranges, but the simultaneous N2O conversion is completely inhibited at high temperature (see Figure 10A). In those conditions, the production of NO2 is preferentially activated between 250 and 350 °C exhibiting the predominant oxidative properties of LaFeO3 in this temperature range. Thermal aging processes induce an increase of the low temperature NO conversion on Pt/LaFe(R300A500) and a relatively stable N2 selectivity contrarily to previous observations on Pt/Al2O3. The examination of Figure 10C also reveals a preservation of the catalytic activity above 200 °C but an accentuation of the NO2 formation. Contrary to previous observations on Pt/Al(R300A500), the selectivity to N2 formation is not significantly altered at the maximum NO conversion on Pt/LaFe(R300A500) showing a better thermal stability. 1917



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Figure 9. Temperature-programmed reaction experiments TPR-1 on Pt/Al(R300) (A); TPR-3 on Pt/Al(R300A500) (B) in the presence of 1000 ppm NO, 1000 ppm N2O, 3% O2, 0.5 vol.% H2 and 0.5% H2O diluted in helium: (a) NO conversion (O) and N2O concentration ((), (b) N2 concentration (2) and NO2 MS intensity signal (b), (c) O2 concentration (b) and H2 conversion (0).

Postreductive thermal treatment in H2 has no significant effect except at low temperature with an enlargement of the NO conversion window on Pt/LaFe(R300A500R300). The most significant results are obtained on the catalysts prereduced at 500 °C associated to the suppression of the high temperature NO2 formation. The temperature-programmed conversion profile reported in Figure 11A on Pt/LaFe(R500) reveals a strong attenuation of the low temperature conversion range, but it considerably broadens at high temperature associated with a promotion of the simultaneous N2O conversion. It is noticeable that the absence of NO2 production is still observable on aged Pt/LaFe(R500A500) which seems to be in relative agreement with XPS analysis showing that Pt retains its metallic character. However, subsequent postreductive thermal treatment leads to a significant loss of activity.

’ DISCUSSION A recent investigation that focused on the existence of peculiar interactions between Pt and LaFeO3 has been highlighted in our laboratory by HR-TEM observations showing the formation of epitaxially orientated Pt particles on the characteristic planes of LaFeO3.5 Such developments differ from those earlier published by Uenishi and co-workers2,3 who prepared solids with noble metals stabilized as cationic species homogeneously distributed in the perovskite lattice corresponding to stronger metal/support interactions. In this latter way, the use of significant amount of noble metals is required to get sufficient surface concentration even if subsequent chemical environment changes of isolated oxidic noble metal species drastically enhance their intrinsic activity. In our case, microstructures earlier found on Pt/LaFeO3 considerably improve the catalytic performance particularly the 1918



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Figure 10. Temperature-programmed reaction experiments: (A) TPR-1 on Pt/LaFe(R300); (B) TPR-2 on Pt/Al(R300A500R300); (C) TPR-3 on Pt/Al(R300A500) in H2 at 300 °C in the presence of 1000 ppm NO, 1000 ppm N2O, 3% O2, 0.5 vol % H2, and 0.5% H2O diluted in helium: (a) NO conversion (O) and N2O concentration ((), (b) N2 concentration (2) and NO2 MS intensity signal (b), (c) O2 concentration (b) and H2 conversion (0).

resistance to thermal sintering opening new prospects to lower the noble metal content while preserving the noble metal dispersion in severe reactive conditions. Hence, we developed in the present work an alternative strategy to that previously implemented by Uenishi and co-workers2,3 which consisted in redispersing Pt clusters previously deposited by classical impregnation method by means of successive reductive/oxidative thermal treatments. In this sense, the previous investigation proposed by Hatanaka et al.,6 demonstrating that the formation of Pt-O-Ce bond on Pt/CeO2 stabilizes Pt against sintering and favors the redispersion of agglomerated Pt particles, can be considered as a precursor study and might provide some guidelines to explain our results. The methodology developed in this study consisted in successive reductive/oxidative preactivation thermal treatment to optimize the metal/support interface chemical structure. Among the different parameters which might influence the growth of the Pt/LaFeO3 interface, the temperature of the reductive thermal treatment was found to be a key parameter. Our results also suggest that the operating window to optimize the surface properties gaining in activity and selectivity is probably very narrow. In fact, the stability of surface properties and the catalytic behavior after aging remain stable after a postreductive thermal treatment in H2 at moderate temperature

(T = 300 °C) whereas a postreductive thermal treatment at 500 °C induces a detrimental effect both on the NO conversion and on the selectivity to N2 production. However, the absence of NO2 after reduction at 500 °C is noticeable and of practical interest emphasizing the fact that surface Pt2þ/Pt4þ species probably catalyze the preferential oxidation of NO to NO2. These catalytic observations can be tentatively related to the physicochemical characterization. Interestingly, CO infrared spectroscopic measurements show on Pt/Al2O3 and Pt/LaFeO3 a 2125 cm-1 IR band assigned to cationic Pt species which progressively disappears on aged samples and particularly after a postreductive thermal treatment in H2. In parallel, TEM observations showed that a different degree of dispersion characterized both catalysts with a dramatic particle growth observed on Pt/Al(R300A500R300) whereas no sintering effect is discernible on Pt/LaFe(R300A500R300). XPS data corroborate TEM observations with a decrease of the surface Pt/Al ratio which seems to be consistent with a Pt particle sintering. On the other hand, the Pt/La ratio remains unchanged according to the margin of error on Pt/LaFe(R300) and Pt/LaFe(R300A500R300) and slightly increases on Pt/LaFe(R300A500) which underlines the occurrence of a redispersion process and a greater stabilization of metallic Pt species on the reduced samples. Such 1919



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Figure 11. Temperature-programmed reaction experiments: (A) TPR-1 on Pt/LaFe(R500); (B) TPR-2 on Pt/Al(R500A500R500); (C) TPR-3 on Pt/Al(R500A500) in the presence of 1000 ppm NO, 1000 ppm N2O, 3% O2, 0.5 vol % H2, and 0.5% H2O diluted in helium: (a) NO conversion (O) and N2O concentration ((), (b) N2 concentration (2) and NO2 MS intensity signal (b), (c) O2 concentration (b) and H2 conversion (0).

tendencies seem to be in relative good agreement with previous observations reported by Hatanaka et al. on Pt/CeO2.6 In fact, thermodynamic calculations agree with the fact that bulk reoxidation of metallic Pt particles to PtO and PtO2 should be avoided at high temperature under oxidative conditions in the absence of interaction between Pt and the support. According to this, activity loss induced by the postreductive thermal treatment on Pt/LaFe(R500A500R500) correlated to the absence of a significant amount of Pt4þ is consistent with the suppression of the Pt/support interaction. In fact, Hatanaka et al.6 explain the discrepancy between predictions and experiment by the formation a Pt-O-Ce bond which would decrease the total energy of Pt/CeO2 further stabilizing dispersed Pt species on the support. Such an explanation corroborates previous observations on highly loaded supported Pt catalysts on LaFeO35 and seems to be still consistent on 1 wt % Pt/LaFeO3 thus opening the discussion on possible surface nanocomposite oxide that involves the formation of Pt-O-(Fe-O-La)/Pt-O-(La-O-Fe) bond. Hence, the greater stabilization of Pt4þ species on Pt/ LaFe(C400) than on Pt/Al(C400) could also be related to stronger metal/support interactions. Regarding the existence of two NO conversion ranges, both might reflect the existence of different kinetic behaviors involving different intermediates and active sites.30-33 As a matter of fact,

both aspects are the subject of strong debates earlier opened by Ueda et al.33 who suggested a direct NO/H2 reaction over metallic noble metal sites whereas extensive oxygen accumulation at higher temperature would inhibit the dissociation of NO and promote the production of NO2 acting as key intermediate. However, such an explanation partly disagrees with more recent investigations which claimed that the formation of NO2 at low temperature might originate a strong activity enhancement which has been explained by the involvement of surface reaction between ad-NOx species and atomic hydrogen at the metal/ support interface of Pt/LaCeMnO332 and Pt/MgO-CeO2.30,31 In fact, Costa and Efsthatiou30-32 suggested that the rate of the H-spillover process from the Pt surface to the vicinity of the support would depend on the Pt particle size and the morphology. Hence, subsequent reductive/oxidative thermal treatment might act differently according to the H2 reduction temperature. Clearly, the metal/support interface would be partly altered on Pt/LaFe(R500A500R500). The loss of interactions probably modifies the electronic properties of Pt underlined by lower BE values for the Pt 4f core level. As shown, the preservation of its metallic character at high temperature correctly explains the absence of NO2 formation but also a subsequent loss of activity in the NO/H2 reaction at low temperature. Hence, optimal surface properties of Pt/LaFeO3 probably require the preservation of the 1920


The Journal of Physical Chemistry C metallic character of Pt to suppress the production of NO2 at high temperature and a high dispersion of Pt to enhance the metal/support interface.

’ CONCLUSION This investigation dealt with the influence of successive oxidative/reductive thermal treatments on the extent of redispersion/agglomeration processes of Pt species deposited by classical wet impregnation and related impact on the catalytic performances in the NO/H2/O2 reactions. It was found that the support composition and the temperature of pre- and postreductive treatment in H2 after aging under lean conditions determine the extent of those surface reconstructions. As exemplified, metallic Pt particles in weak interaction with inert alumina support exhibit a poor thermal resistance to particle sintering. Consequently, irreversible deactivation takes place at high temperature with the disappearance of the simultaneous conversion of NO and N2O and the promotion of NO2 formation. Conversely, NO conversion is improved at low temperature but essentially leads to the formation of N2O at the expense of N2. Regarding Pt/LaFeO3, different catalytic behaviors have been observed. It seems obvious that successive oxidative and reductive thermal treatment at moderate temperature strengthens the metal/support interaction with the stabilization of Pt4þ species on calcined samples. Subsequent reduction of those oxidic Pt species to metallic particles promotes the selectivity to nitrogen production at low temperature and preserves the Pt dispersion. On the other hand, H2 reduction at 500 °C suppresses this beneficial interaction particularly after aging. ’ ASSOCIATED CONTENT

bS

Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ33 320 434 938. Fax: þ33 320 436 561. E-mail: pascal. [email protected].

’ ACKNOWLEDGMENT We gratefully acknowledge the Region Nord-Pas-de-Calais through the Institut de Recherche en Environnement Industriel and the Ademe for a PhD fellowship (J.P. Dacquin). We thank Mrs. M. Trentesaux and Mr. O. Gardoll who conducted XPS and thermal analysis experiments, respectively. The TEM facility in Lille (France) is supported by the Conseil Regional du Nord-Pas de Calais and by the European Regional Development Fund (ERDF).

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Support-Induced Effects of LaFeO3 Perovskite on the Catalytic

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