Improvement of Catalytic Activity of LaFe0.95Pd0.05O3 for Methane

Dec 16, 2010 - ... industry in the use of perovskite-type oxides in catalytic converters has been promoted by the research at Daihatsu(18) and General...
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J. Phys. Chem. C 2011, 115, 1231–1239

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Improvement of Catalytic Activity of LaFe0.95Pd0.05O3 for Methane Oxidation under Transient Conditions† Arnim Eyssler,‡ Evgueny Kleymenov,§ Andre´ Kupferschmid,| Maarten Nachtegaal,§ M. Santhosh Kumar,‡ Paul Hug,‡ Anke Weidenkaff,‡ and Davide Ferri*,‡ Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Solid State Chemistry and Catalysis, Ueberlandstrasse 129, CH-8600 Du¨bendorf, Switzerland, Paul Scherrer Institute, CH-5232 Villigen, Switzerland, and Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Electronics and Metrology, Ueberlandstrasse 129, CH-8600 Du¨bendorf, Switzerland ReceiVed: July 14, 2010; ReVised Manuscript ReceiVed: NoVember 8, 2010

The catalytic performance of methane oxidation catalysts, i.e., LaFe0.95Pd0.05O3, 2 wt % Pd/LaFeO3, and 2 wt % Pd/Al2O3, has been compared at 500 °C under periodic red-ox conditions. The state of palladium has been followed under operando conditions using XANES and QEXAFS at the Pd K-edge combined with mass spectrometry (MS) for product detection. Online MS data reveal that in correspondence to every change of feed composition (O2 pulses) CO2 production is enhanced over LaFe0.95Pd0.05O3 that is associated with a drop in methane concentration. This is not the case for the samples where PdO nanoparticles are deposited on the support material (Pd/LaFeO3 and Pd/Al2O3). The time-resolved QEXAFS spectra have been treated with a modulation excitation spectroscopy approach. Phase sensitive detection (PSD) enabled to highlight the subtle changes in the whiteline region. Continuous reduction-oxidation of Pd occurs in all samples at every change of feed composition. However, by this process Pd in LaFe0.95Pd0.05O3 reversibly emerges on the LaFeO3 surface under reducing conditions and enters the LaFeO3 structure under oxidizing conditions. On the contrary, Pd oscillates between the reduced and partially oxidized state in Pd/Al2O3 and Pd/LaFeO3 in which welldefined Pd nanoparticles are already available. This structural difference is responsible for the activity enhancement in correspondence of each switch and is attributed to the self-regenerative property of perovskitetype oxides. 1. Introduction There has been an increase in popularity of natural gas for application in stoichiometric and lean combustion engines as a reasonable alternative to gasoline. Natural gas (>90% CH4) offers some advantages over both gasoline and diesel, e.g., lower CO2 and NOx emissions and the practical absence of particulate matter.1,2 Moreover, it can be easily mixed with biogas from biomass valorization. However, unburned methane needs to be abated from the exhaust gas. This is not an easy task as methane is stable compared to other typical hydrocarbon pollutants.3,4 Though conventional three-way catalysts (TWCs) comprised of Pd-Rh and/or Pt-Pd-Rh precious metal components are typically applied, palladium is the most promising Pt-group metal to control the emissions of exhaust gases from engines working under stoichiometric conditions. Pd-only catalysts are also economically advantageous.5 Besides, they are ideal materials for the deep oxidation of methane.6,7 Although the chemistry of this reaction has been largely studied, it has been comparably less understood in the stoichiometric operation of TWCs for the control of exhaust gases from natural gas fuelled vehicles. The chemistry of methane under rich or lean conditions can be different from that found in stoichiometric engines with gasoline †

Part of the “Alfons Baiker Festschrift”. * Corresponding author. E-mail: [email protected]. Phone: +41 44 823 4609. Fax: +41 44 823 4019. ‡ Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Solid State Chemistry and Catalysis. § Paul Scherrer Institute. | Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Electronics and Metrology.

fuel.8 Methane reforming was suggested to be the dominant process under rich conditions, and the reduction of NO by CO, i.e, the major reaction for NOx abatement in gasoline engines, was not observed. TWCs work under continuously changing conditions of gas composition and temperature that are associated with the driving conditions. Continuous fluctuations in the concentration of pollutants and oxygen near the optimal stoichiometric air-tofuel ratio ensure the pollutants CO, NOx, and hydrocarbons to be efficiently abated. Such conditions also avoid deactivation of the TWC under steady state operation of either continuous reducing or oxidizing conditions. Reproducing these fluctuations at a lab scale in combination with spectroscopic methods enables the simulation of the operating conditions experienced by the catalyst and the simultaneous detection of the state of the catalyst at every instant of the reaction. Besides possible improvements in product yields and selectivity generated by periodic changes of feed composition,9 perturbations of steady-state conditions are of particular interest from an analytical point of view for the determination of reaction mechanisms.10-12 In such an approach, modulation excitation spectroscopy (MES)13 makes use of phase sensitive detection (PSD) to enhance the subtle changes stimulated by the continuous perturbation of, for example, feed composition.14-16 Thus, additional insight can be gained on the involvement of specific components to a reaction mechanism. Under periodic operating conditions, perovskite-type mixed oxides17 enable the noble metal (Pd, Pt, and Rh) to reversibly exit and re-enter into their structure.18 A solid solution is formed under oxidative conditions, whereas well dispersed nanoparticles

10.1021/jp106537v  2011 American Chemical Society Published on Web 12/16/2010

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of reduced metal can emerge on the surface of the perovskite oxide under reductive conditions. This “self-regenerative property” can lead to retarding sintering effects typically recognized on a TWC. Therefore, this class of materials is a potential alternative to conventional TWCs19,20 and also for methane emission abatement.21-23 The interest of the automotive industry in the use of perovskite-type oxides in catalytic converters has been promoted by the research at Daihatsu18 and General Motors.24 Pd/LaFeO3 is more active than LaFe0.95Pd0.05O3 for the deep oxidation of methane.25 The presence of PdO particles appears a necessary condition to impart activity to LaFeO3. Formation of a solid solution between Pd and LaFeO3 has no substantial influence on the catalytic activity. Besides, LaFeO3-based materials are in general the least active for methane oxidation among the perovskite-type catalysts.26 Improved activity has been observed in Pd-LaMnO327 and LaFe0.95Pd0.05O325 when the catalysts were reacted at temperatures above the threshold temperature of the PdO T Pd equilibrium.28 X-ray absorption spectroscopy (XAS) has been largely used to demonstrate the reversible migration of Pd in and out of the perovskite lattice. However, most studies have been performed ex situ with samples reduced and oxidized at the same temperature.29 Energy dispersive (ED) EXAFS with 10 ms time resolution was used to show the kinetics of the segregationincorporation mechanism under H2 and O2 flows compared to Pd/Al2O3.30 Recently the dynamic structural changes of Pd particles in Pd/LaFeO3 and Pd/Al2O3 were followed also under periodic redox CO-NO fluctuations.31 A stable oxidic layer was found to form on Pd/LaFeO3 which is most likely responsible for the low NO reduction activity. Otherwise the influence of the occurrence of the self-regenerative property on the catalytic performance of perovskite-type oxides using spectroscopic methods has been addressed only on a very limited basis.31,32 Our aim is to provide first knowledge on the structure of the active component under the simulated reaction conditions of a TWC for natural gas applications. We use the methane oxidation reaction under oscillating conditions as a model reaction to describe that operation. In this work we show that a practical activation protocol of LaFe0.95Pd0.05O3 could be that of running the Pd-based perovskite-type oxide under periodic red-ox conditions. This does not simply allow mimicking the conditions under which the catalyst works but is also of relevance to gain crucial structural information of the active phases during the reaction. Preliminary results are shown that are based on modulation excitation QEXAFS measurements during continuous fluctuations in the oxygen concentration of the CH4 + O2 feed.

Eyssler et al. The detailed characterization of the catalysts has been reported in ref 25. The 2 wt % Pd/Al2O3 was kindly provided by Umicore (average Pd particle size, 3.5 nm; Pd dispersion, 20%). In situ XAS spectra were recorded in transmission mode at the Pd K-edge (E0 ) 24.35 keV) at the SuperXAS beamline at the Swiss Light Source (SLS, Paul Scherrer Institute, Villigen, Switzerland). Spectra were energy calibrated, background corrected and normalized using the WinXAS 3.1 software package.35 A capillary reactor cell36 was used for the experiments in which the feed to the catalyst was switched between reducing and oxidizing conditions. The reactor was heated using a gas blower and was connected to a gas manifold. A thermocouple was installed between the reactor and the blower away from the X-ray beam. A mass spectrometer (Omnistar, Pfeiffer) was installed at the exit of the capillary reactor to monitor the exhaust gas online. The following m/z signals were monitored: m/z ) 2 (H2), 4 (He), 15 (CH4), 18 (water), 28 (CO), 32 (O2), 44 (CO2) and 40 (Ar). Prior to the experiments, the sample was activated in flowing 5 vol% O2/He (50 mL/min) at 500 °C for 30 min. In the case of CH4 + O2 f H2 f CH4 + O2 f CH4 f CH4 + O2 f CH4 sequences each pulse was ca. 20 min long and the feed flow was changed between 1 vol % CH4/4 vol % O2/He and 1 vol % CH4/1 vol % Ar/He or 5 vol % H2/He. In situ XANES data were collected in this experiment. The catalyst (ca. 20 mg, 50-100 µm) was firmly fixed in the capillary reactor (quartz, di ) 1.5 mm, Hilgenberg) between two quartz wool plugs. In the case of CH4 + O2 f CH4 sequences, each pulse was 60 s long and the sequences were repeated 10 times. Two electrically activated valves positioned in front of the reactor allowed to repeatedly and rapidly switch the gas atmosphere provided to the catalyst. A capillary reactor with di ) 3 mm (70 mg sample, 50 mL/min, GHSV ) ca. 70 000 h-1) was used to ensure that no pressure drop was built during the measurement and to avoid consequent artifacts as a result of valve switching. Time-resolved QEXAFS spectra were collected at 1 Hz rate. The sequence of CH4 + O2 T CH4 pulses is named modulation experiment, because a parameter, i.e. in this case the oxygen concentration, is periodically varied. The timeresolved spectra measured over the modulation periods (defined as one full CH4 + O2 f CH4 sequence) were processed using the phase sensitive detection (PSD) method.13,16 Briefly, they were first averaged over the modulation periods, thus intrinsically providing already improved signal-to-noise levels. Then, the averaged spectra were transformed into a set of phaseresolved spectra according to eq 113 PSD

Aφk k (e) )

2 T

∫0T A(e, t) sin(kωt + φPSD k ) dt

(1)

2. Experimental Section All perovskite-type oxides were prepared according to the amorphous citrate method.33 Aqueous solutions of La(NO3)3 · 6H2O (puriss p.a.> 99%, Fluka), Fe(NO3)3 · xH2O (x ) 7.5, estimated by TGA; 98+% A.C.S., Sigma-Aldrich) and citric acid (assay 99.5-100.5%, Riedel-de Hae¨n) with the total amount of metal ions and citric acid at 1:1 (slight excess of citric acid) were dried first in a rotary evaporator at 60 °C and then in a vacuum oven overnight at 80 °C to obtain a dry foam. After calcination at 700 °C, LaFeO3 was impregnated with Pd(NO3)2 · 2H2O to obtain 2 wt % Pd/LaFeO3 and was calcined at 500 °C for 2 h. This material exhibited 26% Pd dispersion.34 LaFe0.95Pd0.05O3 was prepared by adding Pd(NO3)2 · 2H2O to the precursors solution to satisfy the final stoichiometry and was calcined at 700 °C. The Pd content was confirmed by ICP-OES.

where A(e,t) is the set of time-resolved spectra over the energy scale e, k determines the demodulation frequency, k ) 1 being the fundamental frequency, ω is the stimulation frequency, φPSD is the demodulation phase angle, and T is the modulation period. PSD enables extracting information on those components of the system that are reversibly responding to the external stimulus (fluctuation in the oxygen concentration). The phase-resolved spectra resemble conventional difference spectra and spectra obtained by the ∆µ XANES method because the “chemically unreactive bulk signal”37 is completely eliminated and additionally display considerably enhanced signal-to-noise ratio. In fact, the noise has a different frequency compared to the stimulation frequency and is canceled out. Phase-resolved spectra better reflect the subtle changes occurring on the catalyst surface when

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TABLE 1: Textural and Catalytic Properties of Perovskite-Type Samples and 2 wt % Pd/Al2O3 LaFe0.95Pd0.05O3 Pd/LaFeO3 LaFeO3 Pd/Al2O3

Tcalc (°C)

SSA (m2/g)

XRDa

state of Pdb

T50% (°C)c

700 700 + 500d 700 500d

14 13 14 135

P P P PdO + Al2O3 phases

s.s. s.s. + n.p.

565-495 485-466 585-680 375-354

n.p.

a P ) orthorhombic perovskite. b From XAS data (ref 25): s.s. ) solid solution, n.p. ) PdO nanoparticles. c Temperature of 50% CH4 conversion during heating (1st value) and cooling (2nd value). d The second temperature value refers to the calcination temperature subsequent to wet impregnation with Pd(NO3)2 · H2O.

stimulating the catalyst with an external fluctuation of for example gas composition.16 Catalytic activity was determined under steady state conditions in a lab scale quartz reactor (di ) 6 mm) installed in an electrically heated furnace and connected to a gas manifold.25 The catalyst (100 mg, 150-200 µm) was mixed with quartz glass in the ratio 1:4 and firmly fixed between two quartz wool plugs. Prior to reaction, the catalyst was activated at 500 °C for 2 h in flowing 20 vol % O2/He (50 mL/min). Then, the feed was changed to 1 vol % CH4/4 vol % O2/He (130 mL/min, GHSV ) 18 400 h-1) and the reaction was monitored online using a gas chromatograph (3000A micro GC, Agilent Technologies) while heating (or cooling) at a rate of 5 °C/min. The overall catalytic activity was evaluated for each sample during a full heating-cooling run (200 f 900 f 200 °C) under identical reaction conditions. Activity is defined in terms of the temperature at which 50% conversion was achieved (T50%) and conversion was determined as X(CH4) ) 100(CH4in - CH4out)/ CH4in, where CH4in is the initial concentration of methane at 200 °C. 3. Results and Discussion 3.1. Catalytic Activity. The textural and catalytic properties of the catalysts are reported in Table 1. Details of the characterization of the perovskite-based materials has been extensively provided before.25 Evidence has been given that Pd exists either in solid solution with LaFeO3 (LaFe0.95Pd0.05O3) or both in solid solution and deposited in the form of finely dispersed PdO nanoparticles (Pd/LaFeO3). The catalytic activity data for the oxidation of methane on Pd/LaFeO3, LaFe0.95Pd0.05O3, Pd/Al2O3, and LaFeO3 are reported in Table 1 and are expressed in terms of temperature at which methane conversion attains 50% (T50%). Two values are provided for each sample, corresponding to T50% obtained while heating under the 1 vol % CH4-4 vol % O2-He gas mixture followed by cooling in the same atmosphere. It is evident that Pd/LaFeO3 is the most active catalyst among the perovskite-type materials and displays T50% always below 500 °C. The activity of LaFe0.95Pd0.05O3 is approximately equal to that of LaFeO3 during heating indicating that Pd in octahedral coordination within the perovskite structure is not active. An important difference emerges during cooling in the presence of the reactants: LaFe0.95Pd0.05O3 gains activity by 70 °C in the T50% and becomes as active as Pd/LaFeO3. Heating up to 900 °C in the reactant mixtures causes Pd to exit the perovskite structure of LaFe0.95Pd0.05O3 as the PdO T Pd equilibrium intervenes at temperatures higher than 700 °C.25 We have shown that at the conclusion of the activity run the fraction of Pd still occupying the Fe-sites of the perovskite-type oxide is diminished suggesting that at this point PdO-like species are found at the surface of the catalyst and might be considered the active species for methane oxidation.38,39 Therefore, heating LaFe0.95Pd0.05O3 in the reactant mixture to higher temperatures than 700 °C allows the PdO T Pd equilibrium to take place. A catalyst is thus

produced with improved catalytic activity, because the local structure of Pd changes as the result of the emergence of Pd from the solid solution with LaFeO3. Measuring the catalytic performance under steady state conditions reveals that LaFe0.95Pd0.05O3 is not a good catalyst among the series, but its performance can be improved by increasing the reaction temperature over the limit reported for the PdOTPd equilibrium in order to promote the dispersion of Pd on LaFeO3. Since the high temperature also induces structural changes in the perovskite-type oxide, like for example particle growth (the samples have been all calcined at 700 °C), a better strategy is needed to maintain the perovskite structure intact but with an improvement of the activity of LaFe0.95Pd0.05O3 towards methane oxidation. This strategy is based on the exploitation of the “self-regenerative property” of perovskitetype oxides that has been invoked for the same catalyst formulation in the case of TWCs for gasoline fuelled vehicles.18 This process is reversible for LaFe0.95Pd0.05O3. Provided the possible application of these materials for the treatment of exhaust gases of natural gas vehicles as three-way catalysts, experiments were performed in which the samples were subjected to continuous reduction-oxidation cycles. 3.2. Long Red-Ox Sequences. Figure 1a shows the result of an experiment where LaFe0.95Pd0.05O3 was subjected to the CH4 + O2 f H2 f CH4 + O2 f CH4 f CH4 + O2 f CH4 sequence of switches in a capillary reactor at 500 °C while recording in situ X-ray absorption near edge structure (XANES) spectra at the Pd K-edge and monitoring the exhaust of the reactor using the mass spectrometer (MS). The temperature was chosen based on the T50% values for the heating segment. The XANES spectra have been measured before and after each switch and therefore represent the state of Pd before and after each switch. Under CH4+O2 (Figure 1a) the local structure of Pd in LaFe0.95Pd0.05O3 is identical to that found in the fresh catalyst. The whiteline is broader than in the case of PdO-like structures and possesses a doublet (∆E ∼ 11.5 eV) associated to Pd occupying Fe sites and exhibiting distorted octahedral coordination (Figure S1).25,40,41 The doublet is not particularly well-resolved in Figure 1a. This feature is very different from that of Pd in Pd/LaFeO3 (Figure 1b), which in turn resembles the Pd/Al2O3 catalyst (Figure S1). The difference in the whiteline is provided by the different local structure of Pd in the two catalysts, in the solid solution with LaFeO3 in the case of LaFe0.95Pd0.05O3 and deposited as PdO nanoparticles in Pd/ LaFeO3 (see Table 1).25 The difference between the XANES spectra of LaFe0.95Pd0.05O3 and Pd/LaFeO3 is shown in Figure 1b. The spectrum of LaFe0.95Pd0.05O3 displays lower amplitude at the edge energy but an additional feature ca. 30 eV higher than in the spectrum of Pd/LaFeO3 that is evidence for the doublet. Though these spectra measured in a capillary reactor are less well resolved than previously reported XANES spectra,25,40 the features best expressed by the difference spectrum in Figure 1b are retained (see Figure S1).

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Figure 1. (a) Normalized in situ XANES spectra of LaFe0.95Pd0.05O3 recorded during CH4 + O2 f H2 f CH4 + O2 f CH4 f CH4 + O2 f CH4 sequences (from bottom to top) in a capillary reactor. Spectra were measured before switching to the next atmosphere. The dotted spectrum corresponds to Pd/LaFeO3 reduced at 500 °C in an equivalent capillary reactor. (b) Normalized in situ XANES spectra of fresh LaFe0.95Pd0.05O3 and Pd/LaFeO3 in a capillary reactor and difference spectrum I(LaFe0.95Pd0.05O3) - I(Pd/LaFeO3). The arrow indicates the signal ca. 30 eV higher than the edge energy. All spectra are offset for the sake of clarity.

After changing to diluted H2, the characteristic whiteline of octahedrally coordinated Pd vanishes completely suggesting the reduction to metallic Pd (Figure 1a). This is also evident from the shift of the edge position to ca. 24.350 keV. The disappearance of the whiteline under reducing conditions (H2 or CH4) during the sequence of Figure 1a demonstrates that Pd has emerged from the perovskite-type structure and deposited on LaFeO3. The phenomenon, described as the self-regenerative property of perovskite-type oxides allows Pd to reversibly enter and exit the perovskite structure under oxidizing and reducing conditions, respectively.18 However, the spectrum of the reduced state is rather different from that of well-defined Pd nanoparticles. Such a spectrum (dotted profile) is shown for comparison in Figure 1a as well and corresponds to the spectrum of Pd/ LaFeO3 reduced at 500 °C in an identical capillary reactor. The striking difference and the particular shape of the spectrum of reduced LaFe0.95Pd0.05O3 suggest that after reduction Pd is found in the form of finely dispersed entities which provide damped EXAFS oscillations.42 Further, all CH4 f CH4 + O2 switches (Figure 1a) restored the whiteline of octahedral Pd and therefore the LaFe0.95Pd0.05O3 structure. Therefore, the spectra of Figure 1a clearly show that Pd reversibly exits and enters the perovskite structure under these conditions in agreement with the expected self-regenerative property of perovskites.18 It should be mentioned that bulk LaFeO3 is stable at 500 °C against reducing conditions25 and only the process forcing Pd to exit and enter the perovskitetype structure is considered to be responsible for the observed changes. The online MS signals recorded during the same experiment witness the reaction occurring at each switch and will be described in detail in the next section for shorter CH4 + O2 f CH4 sequences. 3.3. Short Red-Ox Sequences. LaFe0.95Pd0.05O3 was subjected to repeated and shorter CH4 + O2 f CH4 switches of equal length (60 s). Quick-EXAFS (QEXAFS) spectra were collected simultaneously and continuously at the Pd K-edge at 1 Hz acquisition rate. The transient response of the catalyst in the MS (Figure 2) was compared to that of Pd/Al2O3 (Figure

Figure 2. Online MS data recorded during CH4 + O2 f CH4 sequences on LaFe0.95Pd0.05O3 at 500 °C. The effective pulse time is provided in the top panel.

3) as the benchmark catalyst and of Pd/LaFeO3 (Figure 4). Ar (m/z ) 40) was used as internal standard; this allowed us to determine the beginning and conclusion of the oxygen pulse.

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Figure 3. Online MS data recorded during CH4 + O2 f CH4 sequences on Pd/Al2O3 at 500 °C. The effective pulse time is provided in the top panel.

Figure 4. Online MS data recorded during CH4 + O2 f CH4 sequences on Pd/LaFeO3 at 500 °C. The effective pulse time is provided in the top panel.

A close inspection of the Ar signal in correspondence to the CH4 f CH4 + O2 switch reveals that Ar was removed from the reactor within 4 s. The close resemblance of the profiles of the Ar signal in Figures 3-5 confirms that changes occurring in the other signal profiles are not artifacts. Under methane oxidation conditions (1 vol % CH4 + 4 vol % O2), LaFe0.95Pd0.05O3 displays activity as demonstrated by the CO2 signal (m/z ) 44). When the atmosphere was continuously changed between CH4+O2 and CH4 the production of CO2 was boosted in correspondence to all switches. The CH4 f CH4 + O2 switch produced more CO2. In correspondence to every CO2 maximum, the signal m/z ) 15 representing the CH4 pollutant dropped demonstrating that the CO2 spikes at each switch correspond to rapid activity variations. After increase of CO2 production, (CH4+O2, >20 s after the switch) the activity level recovered to a steady state value under oxidizing conditions. Turning off oxygen in the feed caused again an increase in activity. After ca. 20 s, H2 and CO breakthrough in the gas phase was observed indicating that the catalyst was experiencing reforming conditions. The MS data collected for LaFe0.95Pd0.05O3 reveal that a virtual control of methane emissions could be achieved by running shorter pulses than 60 s.43 Pd/Al2O3 displayed 100% conversion at 500 °C under steady state conditions and is therefore far more active than the two perovskite-type catalysts for methane oxidation. Its high surface area certainly contributes to the overall activity compared to the perovskite-type samples (Table 1). Figure 3 shows that under periodic operation the activity of Pd/Al2O3 intensified under oxidizing conditions (signal of CO2) and attained a stable level within 10 s. A few differences can be found compared to the

experiment with LaFe0.95Pd0.05O3. The transient CO2 maximum was observed only at the CH4 f CH4 + O2 switch, which is however difficult to associate with an activity enhancement because the signal corresponding to CH4 quickly vanishes within 5 s. Moreover, contrarily to LaFe0.95Pd0.05O3, the level of H2 produced by Pd/Al2O3 increased rapidly when changing to CH4 and dropped continuously during the pulse. For Pd/LaFeO3 (Figure 4), the behavior of the CO2 and CH4 signals resembled that observed for Pd/Al2O3. This observation suggests that the short activity increase at each switch in the case of LaFe0.95Pd0.05O3 needs to be attributed to the evolution of a short-living state of the catalyst under these conditions which is not observed in the case of Pd/Al2O3 and Pd/LaFeO3. Although the atmosphere has been varied between two extremes by turning on and off the oxygen flow, the continuous change between reducing and oxidizing conditions is a simulation of the lambda oscillations experienced by the catalyst in natural gas fuelled vehicles adopting methane oxidation as the test reaction. Figure 2 demonstrates that the activity of LaFe0.95Pd0.05O3 can be enhanced by repeatedly cycling between reducing and oxidizing conditions. Moreover, comparison of Figure 2 with Figure 4 indicates that the consumption of methane at every switch (within 20 s) for LaFe0.95Pd0.05O3 approached that of Pd/LaFeO3, which is otherwise more active under steady state conditions (Table 1). As the state of Pd is a major difference between LaFe0.95Pd0.05O3 and Pd/LaFeO3 or Pd/Al2O3, the different catalytic behavior of LaFe0.95Pd0.05O3 is attributed to the different states of Pd exhibited during reaction. This state can be followed by XAS. Selected time-resolved QEXAFS spectra of the experi-

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Figure 5. (a) Normalized time-resolved QEXAFS spectra of LaFe0.95Pd0.05O3 under CH4+O2 (red, average of 20 spectra) and CH4 (blue, average of 10 spectra) atmosphere, their difference spectrum I(CH4) - I(CH4 + O2) and the corresponding phase-resolved spectra obtained by PSD (bottom). For the sake of clarity, the phase-resolved spectra are offset. (b) Detail of the phase-resolved spectra. The dashed spectrum is the difference spectrum between the reference samples, Pd foil and bulk PdO.

ments conducted with LaFe0.95Pd0.05O3 are shown in Figure 5a. Additionally, Figure 5 shows the phase-resolved spectra obtained from these time-resolved spectra after processing the data using the PSD method.13 The time-resolved transmission spectra recorded for LaFe0.95Pd0.05O3 (and Pd/LaFeO3) are of poor quality, one reason being the compromise to be met between time resolution and strong X-ray absorption of the perovskite LaFeO3. Averaging as in the case of the spectra of Figure 5a partially improves the data quality. It is apparent that significant changes occur at the whiteline when changing from CH4 + O2 to CH4 and back. Subtraction of the time-resolved spectrum recorded under CH4+O2 from that recorded under CH4 (I(CH4) - I(CH4 + O2)) eliminates the nonreactive bulk signal (for example the absorption jump). The resulting spectrum exhibits a profile associated with the relative changes of oxidation state. Figure 5a shows that the signal-to-noise ratio can be sensibly improved by the PSD method compared to this difference spectrum. The set of phase-resolved spectra provides information on the relative periodic changes occurring during the CH4 + O2 vs CH4 experiment described above. Similarly to the difference spectrum, the absorption jump is filtered out so as most of the noise, as these do not respond to the external stimulation of O2-concentration variation. The phase-resolved spectra of Figure 5 (and the difference spectrum) exhibit three distinct features, at the edge energy threshold (24.350 keV) and above (ca. +24 and +40 eV). The shape and the evolution of the signals in the phase resolved spectra indicate that the first and third absorption signals belong to the same Pd species. Because the intensity of the first signal (24.350 keV; corresponding to the edge energy) increases under reducing conditions, they are indicative of the reduced state of Pd in LaFe0.95Pd0.05O3, and intensify during reduction by CH4.44 The middle signal is anticorrelated to the other two signals, i.e. it is negative when the other two are positive, and is representative of Pd in the oxidized state as this signal corresponds approximately to the whiteline. Hence, a reversible oxidationreduction process is occurring when the oxygen is turned on and off, respectively.

The time-resolved spectra recorded for the same experiment on 2 wt % Pd/Al2O3 shown in Figure 6a reveal that Pd was mostly in a reduced state during the experiment and partially reoxidized during the CH4 + O2 pulses, though no reduction pretreatment was applied. This may mean that Pd/Al2O3 was not able to completely recover its oxidic state after the reducing CH4 pulse. Spectra are in this case of better quality but the gain in signal-to-noise ratio from the difference spectrum of Figure 6a to the phase-resolved spectra is still evident. Moreover, it is clear from the comparison of the time-resolved spectra of Figures 5a and 6a that the amplitude changes in the whiteline intensity are larger for LaFe0.95Pd0.05O3 than for Pd/Al2O3. Pd in LaFe0.95Pd0.05O3 periodically changes from a fully oxidized state to a reduced state, whereas Pd in Pd/Al2O3 varies from reduced to partly reoxidized. This striking difference under reducing conditions relates to the presence of well-defined Pd particles on Al2O3, whereas finely dispersed entities form in the case of LaFe0.95Pd0.05O3 as it was shown in Figure 1b. The phase-resolved spectra of Pd/Al2O3 of Figure 6b appear better resolved than those of LaFe0.95Pd0.05O3 (Figure 5b) and allow observing further features above 24.4 keV. This is not only attributable to the better quality of the time-resolved spectra but also to the fact that well-defined Pd(O) particles are present in Pd/Al2O3. The phase-resolved spectra of Pd/Al2O3 also bear three major signals in the same energy region of Figure 5b. Additionally, a fourth signal appears ca. 6 eV above the edge threshold. Two weak features ca. 60 and 70 eV higher than the edge threshold are likely a remainder of the formation of PdCx species under the reducing CH4 pulse. In fact, it is evident that the two features do not belong to a reduction-oxidation process. The latter signal is positive in the phase-resolved spectra when the difference spectrum obtained from Pd foil and PdO, taken as the reference spectrum for the simple red-ox process, is negative. On the contrary, below 24.40 keV, the phaseresolved spectra exhibit similar profiles to the reference difference spectrum. PdCx has been seen to form on Pd/Al2O3 at 300 °C under CO-NO fluctuations by XRD45 and PSD applied to EXAFS.16 It was also inferred to form in self-sustaining

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Figure 6. (a) Normalized time-resolved QEXAFS spectra of 2 wt % Pd/Al2O3 under CH4 + O2 (red, average of 10 spectra) and CH4 (blue, average of 10 spectra) atmosphere, their difference spectrum I(CH4) - I(CH4 + O2) and the corresponding phase-resolved spectra obtained by PSD (bottom). For the sake of clarity, the phase-resolved spectra are offset. (b) Detail of the phase-resolved spectra. The dashed spectrum is the difference spectrum between the reference samples, Pd foil and bulk PdO (scaled by factor 2).

oscillations during methane oxidation under methane-rich conditions.46 The more elevated temperature used for this study most likely hinders formation of carbide-like species.47 If formation of PdCx occurs during the CH4 pulse, its subsequent oxidation to CO2 at the beginning of the CH4 + O2 pulse could contribute to the transient maximum in CO2 production observed for Pd/ Al2O3 in Figure 3. The activity maximum cannot be directly attributed to improved methane oxidation, in contrast to LaFe0.95Pd0.05O3, because the transient CO2 production is not associated with the drop in the methane concentration. Figure 6b suggests that also in the case of Pd/Al2O3 a reversible oxidation-reduction process is taking place during the CH4 + O2 T CH4 switches, which is partly accompanied by formation of PdCx species. Pd/LaFeO3 (not shown) displays similar behavior to that of Pd/Al2O3 with respect to the red-ox properties. 3.4. Structural Interpretation under Periodic Conditions. The QEXAFS data together with the corresponding phaseresolved spectra are able to capture the changes of oxidation state of Pd under the periodic pulses. However, provided the quality of the spectra presented in this work, the association of the activity enhancement observed for LaFe0.95Pd0.05O3 when turning on and off oxygen in the feed with the self-regenerative property of the perovskite oxide is not unambiguous. Clearly, a reversible oxidation-reduction process occurs for all samples. In the case of Pd/Al2O3 (and Pd/LaFeO3) this appears simply a reduction of Pd particles followed by their partial reoxidation. To show that Pd exits LaFeO3 under CH4 reducing conditions and enters it under CH4 + O2 oxidizing conditions, i.e. not a simple reduction-oxidation but a structural change imposed by the perovskite structure, we need to compare the features of the phase-resolved spectra with the difference spectra obtained from reference samples. A closer look at the phase-resolved spectra of LaFe0.95Pd0.05O3 and Pd/Al2O3 reveals some subtle differences which could be related to the different state of Pd in the two materials under cyclic operation. The phase-resolved spectrum of LaFe0.95Pd0.05O3 (Figure 7a) exhibits a larger ratio between the signal

Figure 7. (a) Phase-resolved spectrum obtained for the CH4 + O2 T CH4 pulses on LaFe0.95Pd0.05O3 at 500 °C; (b) phase-resolved spectrum obtained for the CH4 + O2 T CH4 pulses on Pd/Al2O3 at 500 °C; (c) difference spectrum between the reference samples, Pd foil and bulk PdO; (d) difference spectrum obtained from a H2 temperature programmed experiment on LaFe0.95Pd0.05O3 and (e) difference spectrum obtained from the same experiment on Pd/LaFeO3 (both experiments from ref 25). Spectra are offset for the sake of clarity.

corresponding to the edge threshold and the one at ca. +40 eV than that of Pd/Al2O3, which is displayed in Figure 7b. A difference spectrum obtained by subtracting the transmission

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spectrum of bulk PdO from the Pd foil shows that the two signals have comparable intensity (Figure 7c). We take the ratio observed in this spectrum as the signature of the reductionoxidation process that should occur if reduced Pd oxidizes to PdO-like species. Furthermore, Figure 7, panels d and e, displays the difference spectra obtained from transmission H2-TPR experiments of LaFe0.95Pd0.05O3 and Pd/LaFeO3, respectively, followed by in situ XANES by subtracting the spectrum of the fresh catalyst from that of the catalyst after reduction in a capillary reactor.25 Also in this case, the ratio between the two signals is larger for LaFe0.95Pd0.05O3 than for Pd/Al2O3. These observations suggest that the features observed in the phaseresolved spectra of LaFe0.95Pd0.05O3 must be characteristic of Pd in solid solution with LaFeO3. Therefore, we hypothesize that, besides indicating reversible oxidation-reduction of Pd, these spectra are also reflecting the continuous migration of Pd in and out of the LaFeO3 structure under these reactive conditions. The exact nature of the signals requires further analysis and will be subject of a future study. The combination of MS data (Figures 2-4) and XAS spectra (Figure 5) suggests that when the migration of Pd in and out of LaFeO3 takes place enhanced activity is observed. This is not the case for the reduction-oxidation of Pd in Pd/Al2O3 and Pd/ LaFeO3. Because the experiments presented in Figures 2-4 are ideal pulses of oxygen in a methane flow, the enhanced activity at all CH4 + O2 f CH4 and CH4 f CH4 + O2 switches could be related to an optimal oxygen surface coverage.48,49 However, activity enhancements were not observed for Pd/Al2O3 and Pd/ LaFeO3 in which the state of Pd is similar and Pd nanoparticles are expected to be present on the surface.25 Steady state lean conditions seem to be beneficial for Pd/Al2O3,49 and probably also Pd/LaFeO3, as this allows creating the right reaction conditions involving PdO-species. On the contrary, well-defined Pd nanoparticles are missing in oxidized LaFe0.95Pd0.05O3 as Pd occupies the coordination positions of iron. Pd in this state is not active for methane oxidation. Only under reducing conditions metallic Pd entities can appear at the LaFeO3 surface. In this state, the catalyst cannot express activity for methane oxidation because of the absence of oxygen in the feed. On the basis of these preliminary spectroscopic data, we tentatively interpret the activity enhancements observed for LaFe0.95Pd0.05O3 as the result of the change of the coordination of Pd with the possible formation of an intermediate transient Pd species between fully oxidized Pd within LaFeO3 and metallic Pd. This might be a different oxidation state or a different local structural environment than the initial and final ones. The formation of this transient species is not favored on Pd/Al2O3 and Pd/LaFeO3 as activity is likely determined by the surface PdO species. 4. Conclusions LaFe0.95Pd0.05O3 and 2 wt % Pd/LaFeO3 have been prepared by the amorphous citrate method and tested for methane deep oxidation. The state of Pd in the as-synthesized materials is significantly different. In LaFe0.95Pd0.05O3 Pd occupied Fe position in the perovskite framework, whereas in Pd/LaFeO3 it is largely present in the form of PdO nanoparticles. Under steady state lean conditions, 2 wt % Pd/LaFeO3 is more active than LaFe0.95Pd0.05O3 toward methane oxidation. Under non steadystate conditions of reducing-oxidizing pulses (CH4 vs CH4 + O2) at 500 °C the activity of LaFe0.95Pd0.05O3 is shortly enhanced at every feed composition change. This behavior is not observed for Pd/LaFeO3 and Pd/Al2O3, as the reference catalyst. Therefore, LaFe0.95Pd0.05O3 can be activated by the periodic change of feed composition between reducing and oxidizing conditions.

Eyssler et al. Operando QEXAFS combined with online mass spectrometry (MS) demonstrated that the coordination and oxidation state of Pd change under cyclic reduction-oxidation conditions (CH4 vs CH4 + O2) at 500 °C. LaFe0.95Pd0.05O3 has been compared to 2 wt % Pd/Al2O3 that is significantly more active than the LaFeO3-based perovskite-type oxides for methane oxidation. Under reducing conditions Pd is clearly in the reduced state in the samples where well-defined Pd nanoparticles are exposed at the surface of the catalyst (Pd/LaFeO3 and Pd/Al2O3). Under the same conditions, the state of Pd in LaFe0.95Pd0.05O3 rather reflects its fine dispersion as the result of migration from the LaFeO3 crystal structure to the surface. Under oxidation conditions (CH4 + O2), Pd reoxidizes in all samples. However, Pd nanoparticles partly reoxidize. Phase sensitive detection (PSD) applied to the operando QEXAFS data indicates that Pd reverts to the distorted octahedral coordination within the perovskite framework in LaFe0.95Pd0.05O3, which is a reversible process. Therefore, the activity of Pd incorporated within the perovskite framework can be increased by forcing it to periodically exit the perovskite structure. This is the evidence that the reversible migration of the precious metal effectively influences catalytic activity. The simultaneous and short activity enhancement is attributed to the formation of a transient Pd species that cannot form when well-defined Pd nanoparticles are already present. Acknowledgment. The authors are grateful to Empa and the Swiss National Science Foundation (SNF, National research Programme NRP 62 Smart Materials, project nr. 406240126127) for financial support. The Swiss Light Source (SLS, Villigen, Switzerland) is kindly acknowledged for providing beam time and Dr. M. Harfouche for valuable support during beam time. Supporting Information Available: Normalized XANES spectra of LaFe0.95Pd0.05O3, 2 wt % Pd/LaFeO3, and 2 wt % Pd/Al2O3. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) http://www.erdgasfahren.ch/30.html. (2) Bach, C.; Schreiber, D.; Alvarez, R.; Lienin, S. “Characterization of exhaust gas and particle emissions of modern gasoline, diesel and natural gas vehicles”; EET-2008 European Ele-Drive Conference, International Advanced Mobility Forum, 2008, Geneva. (3) Gonzalez-Velasco, J. R.; Botas, J. A.; Gonzalez-arcos, J. A.; Gutierrez-Ortiz, M. A. Appl. Catal. B: EnViron. 1997, 12, 61. (4) Lampert, J. K.; Kazi, M. S.; Farrauto, R. J. Appl. Catal. B: EnViron. 1997, 14, 211. (5) http://www.platinum.matthey.com/index.html/. Platinum Today, Web-based Realtime PGM Prices, (Johnson Matthey Precious Metal marketing). (6) Ciuparu, D.; Lyubovsky, M. R.; Altman, E.; Pfefferle, L. D.; Datye, A. Catal. ReV.-Sci. Eng. 2002, 44, 593. (7) Ge´lin, P.; Primet, M. Appl. Catal. B: EnViron. 2002, 39, 1. (8) Salau¨n, M.; Kouakou, A.; Da Costa, S.; Da Costa, P. Appl. Catal. B: EnViron. 2009, 88, 386. (9) Matros, Y. S. Can. J. Chem. Eng. 1996, 74, 566. (10) Marwood, M.; Doepper, R.; Prairie, M.; Renken, A. Chem. Eng. Sci. 1994, 49, 4801. (11) Ortelli, E. E.; Wokaun, A. Vibr. Spectrosc. 1999, 19, 451. (12) Thullie, J.; Renken, A. Chem. Eng. Sci. 1993, 48, 3921. (13) Baurecht, D.; Fringeli, U. P. ReV. Sci. Instrum. 2001, 72, 3782. (14) Bu¨rgi, T.; Baiker, A. J. Phys. Chem. B 2002, 106, 10649. (15) Urakawa, A.; Baiker, T. B. A. Chem. Eng. Sci. 2008, 63, 4902. (16) Ferri, D.; Santosh Kumar, M.; Wirz, R.; Eyssler, A.; Korsak, O.; Hug, P.; Weidenkaff, A.; Newton, M. A. Phys. Chem. Chem. Phys. 2010, 12, 5634. (17) Pena, M. A.; Fierro, J. L. G. Chem. ReV. 2001, 101, 1981.

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