Active Oxygen Species in Lan+1NinO3n+1 Layered Perovskites for

Article pubs.acs.org/JPCC

Active Oxygen Species in Lan+1NinO3n+1 Layered Perovskites for Catalytic Oxidation of Toluene and Methane Qingjie Meng,† Wanglong Wang,† Xiaole Weng,*,† Yue Liu,† Haiqiang Wang,† and Zhongbiao Wu†,‡ †

Key Laboratory of Environment Remediation and Ecological Health, Ministry of Education, College of Natural Resources and Environmental Science, Zhejiang University, 310029 Hangzhou, China ‡ Zhejiang Provincial Engineering Research Centre of Industrial Boiler & Furnace Flue Gas Pollution Control, 388 Yuhangtang Road, 310058 Hangzhou, P. R. China S Supporting Information *

ABSTRACT: Layered perovskites possess frameworks that can sustain a larger extent of oxygen nonstoichiometry than perovskites, which makes them possibly more suitable for catalytic applications. However, there is little work in literature concerning the catalytic behaviors of them. In this paper, Lan+1NinO3n+1 layered perovskites were selected as potential catalysts for catalytic oxidation of toluene and methane. The activation and transmission of active oxygen species over the layered structure were explored by using a range of analytical techniques, e.g., O2-TPD, XPS, and H2-TPR. It was noted that the LaNiO3 (n = ∞) that possessed the most chemisorbed oxygen species had the lowest activation energy (Ea, ca. 70.1 kJ mol−1) in toluene oxidation and the La4Ni3O10 (n = 3) that owned a superior lattice oxygen mobility showed the lowest Ea (ca. 111.0 kJ mol−1) in methane oxidation. The La2NiO4 (n = 1) that lacked chemisorbed and superficial lattice oxygen species yielded the highest Ea in either toluene or methane oxidation. It is expected that the work conduct herein could generate more awareness onto the potential catalyst application for layered perovskites and would also give promising insights into the correlation between structural framework and catalytic behaviors for extraordinary materials.

1. INTRODUCTION Developments on advanced catalysts toward superior activity, high mechanical and thermal stability, and acceptable cost are still in great demand for the current catalyst market. Among the developed catalysts, perovskite oxides have recently attracted tremendous interest for applications in diesel exhaust treatment,1 methane oxidation,2 volatile organic compounds (VOCs) oxidation,3 etc. The low cost, high thermal stability, and mechanical strength, particularly the ability to sustain a large amount of structural distortion (generally via A or B site substitutions) make the perovskites a very promising alternative for catalytic applications.4,5 Layered perovskites also have high mechanical and thermal stabilities. Their unique layered framework could induce extraordinary electronic and ionic transmission behaviors,6 which have been utilized as cathode materials for solid oxide fuel cells,7 ferroelectric materials, superconductors, photocatalysts,8,9 etc. Taking Ruddlesden−Popper (RP) layered perovskites for example, the materials with the formula of An+1MnO3n+1 possess the framework that shares a wellorganized n-dimensional intercalated structure with an ordered rock salt layer (AO) along the c axis.10 This framework can sustain an even larger oxygen nonstoichiometry than perovskite, which makes the layered perovskites possibly more suitable for catalytic application. However, in the literature, © 2016 American Chemical Society

studies on catalytic behaviors of layered perovskites are still rare, which have been severely hindered by their inherent difficulty in syntheses. Generally, the syntheses of layered perovskites, e.g. La3Ni2O7 and La4Ni3O10, require a prolonged homogenization and reheating process, the length of which could be up to 14 days.10 Such a long period and large effort in obtaining pure phase have blocked up the discovery of their potential catalyst applications. By using a supercritical water (referred as sc-H2O) reaction environment, the authors have developed a facile route to synthesize layered perovskites in a relatively short heattreatment period (within several hours).11,12 Thereafter, Du et al.13 had reported a modified citric acid route that had further shortened the calcination period. Both of the efforts have made the study of the catalytic behaviors of layered perovskites become feasible. In this paper, Lan+1NinO3n+1 layered perovskites (n = 1, 3, and ∞) were selected as potential catalysts for catalytic oxidation of toluene and methane. Their catalytic performances were evaluated via both apparent toluene (or methane) conversion and kinetic measurements. The transmission and activation of oxygen species (e.g., chemisorbed Received: September 6, 2015 Revised: January 27, 2016 Published: January 27, 2016 3259

DOI: 10.1021/acs.jpcc.5b08703 J. Phys. Chem. C 2016, 120, 3259−3266

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The Journal of Physical Chemistry C

of temperature were evaluated by using a gas chromatograph (GC, Agilent Technologies 7890A) equipped with a flame ionization detector (FID). For kinetic measurements, reactions were conducted in kinetic region as proposed in the literature.15,16 Reaction rate constant (k) was calculated by using conversion rate (less than 10%) and GHSV values. Apparent activation energy (Ea) was evaluated by applying Arrhenius plots.

oxygen, superficial lattice oxygen, bulk lattice oxygen, etc.) as generated from the layered framework were extensively explored.

2. EXPERIMENTAL SECTION 2.1. sc-H2O Syntheses. Lan+1NinO3n+1 layered perovskites were synthesized by using a sc-H2O flow reactor, followed by a series of heat-treatments at the desired temperature and time.11 In brief, a solution that contained La(NO3)3 and Ni(NO3)2 (with corresponded La/Ni molar ratio) was pumped to meet a flow of NaOH, which then reacted with sc-H2O (at T = 387 °C and P = 23.0 MPa) to induce a rapid coprecipitation of La3+ and Ni2+ ions, leading to a formation of La(OH)3 and NiO mixture. The mixture was then calcinated at 750, 1000, and 1075 °C respectively for 6 h in static air to yield phase pure LaNiO3, La2NiO4, and La4Ni3O10. 2.2. Characterizations. X-ray powder diffraction was conducted by using a Rigaku D/Max RA diffractometer with a Cu Kα radiation (λ = 0.15418 nm) at 2θ = 10−80°. Kr physisoprtion at the liquid nitrogen temperature was conducted via a static volumetric adsorption analyzer JW-BK132F. The data was analyzed based on Brunauer−Emmett−Teller (BET) model. Prior to measurement, all samples were degassed at 300 °C for 12 h in vacuum.14 Surface elemental analysis was carried out by using a Thermo ESCALAB 250 X-ray photoelectron spectroscopy (XPS) instrument with Al Kα radiation (photon energy 1486.6 eV) at 150 W. The signal of adventitious carbon (binding energy at 284.6 eV) was used to calibrate the binding energy scale for the measurement. Temperature-programmed desorption (TPD) and temperature-programmed reduction (TPR) were conducted by using an automatic multipurpose adsorption instrument TP-5079 (Tianjin, Xianquan) equipped with a thermal conductivity detector (TCD) and a HIDEN QGA portable mass spectrometry (MS). For O2-TPD, accurately measured 0.10 g sample was pretreated at 500 °C for 2 h with a pure flow of O2, which was then cooled to room temperature at a ramp of 2 °C min−1. Thereafter, He purge was switched in with a flow rate of 50 mL min−1. Samples were then heated to 100 °C for 40 min to remove the residue O2 and to stabilize the apparatus and further heat-treated to 1000 °C at a ramp of 10 °C min−1. The oxygen desorption was recorded by using MS and the desorption amounts were calibrated by using a pulse of pure O2 with 86.5 μL volume. For H2-TPR, accurately measured 0.05 g samples were heated to 400 °C under a purge of 5 vol. % O2/He (flow rate at 50 mL min−1) with a dwelling time of 1 h and then naturally cooled to room temperature. Thereafter, samples were heated to 100 °C for 40 min and further heated to 900 °C at a rate of 10 °C min−1 under a purge of 6 vol. % H2/N2 (flow rate at 35 mL min−1). The H2 consumption amounts were recorded by using TCD and calibrated by a standard reagent of CuO. 2.3. Activity Measurements. Catalytic activities in toluene and methane oxidation were evaluated by using a fixed-bed reactor (i.d. = 8 mm). Samples (0.50 g, particle size at 0.25− 0.42 mm) were mixed with quartz sands (particle size at 0.25− 0.42 mm) to reach a height of 2.3 cm. The corresponded mass ratios between samples and sands were at ca. 1.5, 1.7, and 1.9 for LaNiO3, La2NiO4, and La4Ni3O10, respectively. The feed gas (total flow rate at 160 mL min−1) consisted of a nitrogen carrier gas, a reactant gas (1 vol. % methane or 500 ppm toluene), and a 10 vol. % oxygen with a gas hourly space velocity (GHSV) at 19200 mL gcat−1 h−1. The conversions of reactants as a function

3. RESULTS AND DISCUSSION Phase identity and purity of Lan+1NinO3n+1 were evaluated by using X-ray powder diffraction (XRD), which was directly referenced to JCPDS files. As shown in the Supporting Information, Figure S1, the reflections of Lan+1NinO3n+1 were all indexed to LaNiO3 (JCPDS 34-1028), La4Ni3O10 (JCPDS 50-0243), and La2NiO4 (JCPDS 34-0314), respectively. No reflections corresponding to the NiO or La2O3 phases were identified, which was in accordance with our previous reports.11,12 With the increase in calcination temperature, the Lan+1NinO3n+1 experienced a sharp drop in specific surface area where the LaNiO3, La2NiO4, and La4Ni3O10 had revealed the surface areas at ca. 13.5, 3.1, and 1.3 m2 g−1, respectively. The low surface area would profoundly inhibit adequate exposure to catalytic reaction, making the comparison barely based on apparent activities become inappropriate. As such, kinetic measurements had been carried out throughout the study. It is well-known that La2NiO4 is apt to possess hyperstoichiometric structure on account of its interstitial oxygen sites in LaO salt-rock layer17 while LaNiO3 was preferable with an oxygen deficient structure.11 The La4Ni3O10 is however able to sustain both oxygen hyper-stoichiometric and deficient structures.18,19 Since the lack of oxygen in structure (i.e., oxygen deficiency) would provide abundant oxygen vacancies at the surface, it is expected that the LaNiO3 (or La4Ni3O10) might have higher oxidizing abilities due to the enhanced active chemisorbed oxygen adsorption. Besides, the presence of a layered framework in La2NiO4 and La4Ni3O10 would induce sufficient interstitial oxygen species in interlayers, the transmission and activation of which might also affect the redox potential for catalysts. As such, to get a better understanding on the catalytic function of active oxygen species over Lan+1NinO3n+1, their desorption and transmission behaviors were first evaluated by using O2-TPD, XPS, and H2-TPR techniques. 3.1. O2-TPD Analyses. O2-TPD was conducted within the temperature range of 100−1000 °C (see Figure 1). The corresponding O2 desorption amounts were illustrated in Table 1. In general, oxygen desorption peaks in the O2-TPD profile can be classified into three regions, denoted as α-O (T < 300 °C), α′-O (300 °C < T < 600 °C), and β-O (T > 600 °C). The α-O belongs to chemisorbed oxygen species that are weakly bonded onto the vacancies of perovskites at surface,20 which could be O2− or O− as evidenced by Electron Spin Resonance (ESR) and thermal desorption measurement.21 The α′-O can be ascribed to superficial lattice oxygen species (e.g., O2− and O−) that is generated from lattice defects (e.g., dislocations and grain boundaries).3,22 In Lan+1NinO3n+1, the α′-O might also involve interstitial oxygen species (hereafter referred as α″-O) that are present as point defects in rock-salt layers. The β-O is liberated mostly from the bulk structure, which is accompanied by a valence reduction of B site metal ion in perovskites.23,24 As shown in Figure 1, the La2NiO4 was shown with an absence of α-O desorption peak in the temperature range of 3260


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Figure 1. O2-TPD profiles of (a) La2NiO4, (b) La4Ni3O10, and (c) LaNiO3. Condition: 50 mL min−1 He, 0.10 g catalyst, ramping rate at 10 °C min−1.

Figure 2. O 1s XPS spectra of (a) La2NiO4, (b) La4Ni3O10, and (c) LaNiO3. The shift of Olat peak was highlighted.

100−300 °C while the LaNiO3 and La4Ni3O10 revealed distinct peaks centered at ca. 175 °C and ca. 284 °C, respectively. Krylova et al. had reported that the O2− radical anion was stable at T ≤ 175 °C while the O− ion was at T = 225−300 °C.21,25 Accordingly, it was proposed that the α-O in LaNiO3 would be O2− radical anion and that in La4Ni3O10 would be O− ion. There was a slight fluctuation at ca. 290 °C in LaNiO3, which might imply a coexistence of O− ion in the catalyst. In the temperature range of 300−600 °C, the La2NiO4 had yielded a very intense peak centered at ca. 336 °C, which could be assigned to interstitial oxygen ions (i.e. α″-O) due to its presence of hyper-stoichiometric structure (see Table 1). As the order n increased, this peak was shifted toward higher temperature range for La4Ni3O10 at ca. 386 °C and for LaNiO3 at ca. 453 °C. Since the O2− ion was more stable than the O− ion,26 the α′-O in the two catalysts were proposed to be O2− ions. However, it can not be excluded that the α′-O in La4Ni3O10 would possibly involve α″-O ion as the catalyst also possessed layered framework as to La2NiO4 (this will be further verified in the following H2-TPR analyses). Furthermore, as compared with LaNiO3, the La4Ni3O10 had revealed a lower desorption temperature for α′-O, which implied that the transmission of superficial lattice oxygen in La4Ni3O10 was much easier than that in LaNiO3. This was consistent with previous report,27 which indicated that the La4Ni3O10 had a superior oxygen ionic conductivity and could be utilized as cathode materials in solid oxide fuel cells (SOFCs). O 1s XPS measurements were also conducted to correlate with the O2-TPD analyses, the results of which were illustrated in Figure 2 and Table 2. In the literature, the O 1s spectra of La2NiO4 was evaluated with three XPS binding energy (BE) peaks as ascribed in sequence to lattice oxygen O2−, OH− or CO32− and chemisorbed H2O.28 This was consistent with our result (see Figure 2). However, for LaNiO3 and La4Ni3O10, an additional shoulder (centered at ca. 529 eV) was appeared,

Table 2. O 1s XPS Data of Lan+1NinO3n+1 catalyst

OH2O BE/ eV

OOH− BE/ eV

La2NiO4 La4Ni3O10 LaNiO3

532.0 532.2 532.3

530.4 530.3 530.4

Oad BE/ eV

Olat BE/ eV

Oad/Olat

529.0 528.8

528.1 528.1 527.8

0.28 0.29

which should be ascribed to the chemisorbed oxygen species (i.e., O− as reported previously29,30). This observation was in agreement with O2-TPD analyses (see Figure 1), which revealed that the La2NiO4 lacked chemisorbed oxygen species while the LaNiO3 and La4Ni3O10 didn’t. A slight higher BE shift for the Olat peak (at ca. 528.1 eV) was also observed in La2NiO4 and La4Ni3O10, which could be due to their interstitial oxygen species that had a higher BE than lattice oxygen species.31 In general, the vacancy formation over Lan+1NinO3n+1 layered perovskites followed a Kroger−Vink notation as follows32 1 × 2Ni ×Ni + OO → 2Ni′Ni + V •• O2 O + (1) 2 In a low temperature range, the weakly chemisorbed oxygen species are formed via a rapid adsorption of molecular oxygen upon anion vacancies (see eq 2) and/or via a comparatively slow dissociation of the adsorbed oxygen species as shown in eq 3.23,33 × •• Ni′Ni + V •• O + O2 → Ni Ni + O′2ad [V O ] · •• Ni ×Ni + V •• O + O2 → NiNi + O′2ad [V O ]

Ni′Ni + V •• O +

(2)

1 O2 → Ni ×Ni + O′O 2

•• × • Ni′Ni + V •• O + O′2ad [V O ] → Ni Ni + 2h + 2O′O

(3)

Table 1. Desorption Amounts of Various Oxygen Species in Lan+1NinO3n+1 α-O (600 °C)

catalyst

nonstoichiometry δ

peak (°C)

O2 (mmol mol−1)

peak (°C)

O2 (mmol mol−1)

peak (°C)

O2 (mmol mol−1)


+0.18 −0.46 −0.09

290 182(263)

1.7 2.3

350 389 445

45.7 1.7 1.8

>1000 957

20.3 >203.7 128.2

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Figure 3. Structural configurations of (a) La2NiO4, (b) La4Ni3O10, and (c) LaNiO3. The illustrative box shows the representative atoms and the isolated octahedral structure.

According to either eq 2 or eq 3, the desorption amounts of α-O in Lan+1NinO3n+1 were all linearly dependent on the oxygen vacancy concentration at surface. Since the oxygen vacancies of Lan+1NinO3n+1 perovskites were preferable to form at the vertex site that was shared by two MO6 octahedra34,35 and in view of the structural configurations of Lan+1NinO3n+1 (see Figure 3), the LaNiO3 was with the most shared vertex sites, it was unsurprising to note that this catalyst had revealed the highest amounts of oxygen vacancies at surface (hence yielded the most α-O in O2-TPD, see Table 1). In contrast, the La2NiO4 was absence of shared vertex sites, which was hence a lack of α-O, consistent with the O2-TPD result. For α′-O, the La2NiO4 had revealed a distinct desorption peak, which could mainly involve interstitial oxygen ions in interlayer36 (i.e., α″-O, this had been further confirmed by H2TPR analyses, see latter section). For La4Ni3O10 and LaNiO3, their α′-O were proposed to be originated from the diffusion of surface oxygen vacancies into the bulk structure.37,38 However, it can not be excluded that the α′-O in La4Ni3O10 might also involve interstitial oxygen ions (i.e., α″-O) as a shift in Olat XPS peak (due to the presence of α″-O) was also observed in the catalyst (see Figure 2). Furthermore, the onset desorption temperature for α″-O was observed much lower in La2NiO4 than that in LaNiO3. This implied that the transmission of interstitial oxygen ion in layered structure was faster than that of the surface vacancy.39,40 For β-O, only LaNiO3 showed a distinct desorption peak centered at ca. 958 °C. This was unsurprising as the vast majority of Ni ions in LaNiO3 were Ni(III), the reduction of which was generally accompanied by the formation of β-O.41 3.2. H2-TPR Study. H2-TPR was employed to evaluate the redox potential of Lan+1NinO3n+1. The corresponding H2 consumption amounts were illustrated in Table 3. As shown in Figure 4, the LaNiO3 had revealed a relatively weak peak centered at ca. 334 °C (with two shoulders at ca. 235 °C and ca. 297 °C, respectively) and an intense peak at ca. 567 °C. The former one was proposed to be originated from the reduction of Ni(III) to Ni(II)42 (where the shoulders should be ascribed to the H2 reaction with chemisorbed oxygen species43). The latter one could be assigned to a further reduction of Ni(II) to elemental nickel.44 For La4Ni3O10, the H2-TPR profile had shown the peaks centered at ca. 390 °C (with a shoulder at ca. 293 °C), 430 °C and ca. 604 °C. As compared with LaNiO3, a

Table 3. H2 Consumption Amounts of Lan+1NinO3n+1 peak I

peak II

catalysts

e− mol−1

H2 consumptiona mmol mol−1

e− mol−1

H2 consumptiona mmol mol−1


0.3 1.7 0.7

153.9 874.7 365.5

2.0 5.5 1.5

979.2 2745.6 757.3

a

The quantitative calculation was calibrated based on the amount of H2 consumption by a standard reagent CuO.

Figure 4. H2-TPR profiles of (a) La2NiO4, (b) La4Ni3O10, and (c) LaNiO3 Condition: 6 vol. % H2 in N2 with a flow rate at 50 mL min−1, 0.05 g catalyst, ramping rate at 10 °C min−1.

higher reduction temperature shift was observed in La4Ni3O10, which implied a lower redox potential for the catalyst. In La2NiO4, a weak peak at 390 °C and an intense peak at ca. 757 °C were observed. The presence of the 390 °C peak implied that there was Ni(III) involved in La2NiO4, the presence of which was to compensate the interstitial oxygen ions as existed in LaO rock-salt layer.31,45 In particular, the La2NiO4 did not yield low temperature shoulders as LaNiO3 and La4Ni3O10 did, which implied that this catalyst did indeed lack α-O, consistent with the O2-TPD result (see Figure 1). 3262


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The Journal of Physical Chemistry C In view of H2 reaction with active oxygen species, it was proposed that the shoulders at ca. 235 °C and ca. 297 °C in LaNiO3 should be originated from the H2 reaction with α-O (i.e., O2− and O−, respectively) and the peak at ca. 332 °C was from the reaction with α′-O.46 The peak at ca. 567 °C was attributed to the reaction with β-O. For La4Ni3O10, the shoulder at ca. 293 °C (similar to that at ca. 297 °C in LaNiO3) corresponded to the reaction with α-O (i.e., O−) and the one at ca. 390 °C should be ascribed to the reaction with interstitial oxygen ions (i.e., α″-O)31 as a similar reduction peak was also observed in La2NiO4 (mainly involved α″-O). This result further confirmed the existence of interstitial oxygen ions in the La4Ni3O10. The peak at ca. 430 °C was attributed to the reaction with superficial lattice oxygen species (i.e., α′-O). For β-O, the reactivity47 followed the sequence of LaNiO3 > La4Ni3O10 > La2NiO4, which was in agreement with O2-TPD result (see Figure 1). Based on O2-TPD and H2-TPR analyses, it can be concluded that with the progressive increase in perovskite layers, the amounts of oxygen vacancies were increased owing to the increase in shared octahedral vertex sites (see Figure 3). The LaNiO3 with n = ∞ possessed the most α-O while the La2NiO4 with n = 1 lacked them. The La4Ni3O10 that had a superior oxygen ionic conductivity (i.e., high lattice oxygen mobility) showed a faster α′-O transmission rate than the LaNiO3 although it was with a lower redox potential. The La2NiO4 had the vast majority of α″-O in its desorbed α′-O while the La4Ni3O10 was partially involved. Since the role of α-O in oxidation process has been extensively studied25,48,49 and those of α′-O, α″-O, and β-O are still lack, in this paper, toluene and methane oxidation were selected as probe reactions to evaluate the catalytic functions of α-O, α′-O, α″-O, and β-O. 3.3. Catalytic Performances. The catalytic performances of Lan+1NinO3n+1 were evaluated via both reactant conversion rate and kinetic measurement. The kinetic calculation was based on the assumption that both toluene and methane oxidation obeyed a first-order reaction mechanism as follows50,51 ⎛ E ⎞ r = −kc = −A exp⎜ − a ⎟c ⎝ RT ⎠

Figure 5. (a) Conversion rates and (b) Arrhenius plots of La2NiO4 (□), La4Ni3O10 (○), and LaNiO3 (△) in toluene oxidation. Condition: toluene (500 ppm), O2 (10 vol. %), 0.5 g of catalyst, GHSV = 19200 mL gcat−1 h−1.

that were Ni(III) and α-O, it was proposed that in toluene oxidation, the Ni(III) would act as an active site for toluene adsorption, where the absorbed delocalized π bond was then dissociated and oxidized by α-O (e.g., the electrophile O−), leading to the formation of CO2 (or CO) and H2O. One might argue that the Ni(II) might also act as an active site for toluene oxidation. This seems to not happen as Pecchi et al.52 had reported that the Ni(II) was not active for catalytic oxidation of toluene. In kinetic measurements, the Arrhenius plots for Lan+1NinO3n+1 all had linear dependence within the investigated temperature range (see Figure 5b). The calculated kinetic parameters are summarized in Table 4. It was noted that the Table 4. Kinetic Parameters of La2NiO4, La4Ni3O10, and LaNiO3 in Toluene Oxidation toluene k ( × 10−2 s−1) catalyst

(4)


where r is the reaction rate (mol mL−1 s−1), k is the reaction rate constant (s−1), A is the pre-exponential factor (s−1), and Ea is the apparent activation energy (kJ mol−1). 3.3.1. Toluene Oxidation. In toluene oxidation measurements, the Lan+1NinO3n+1 were subjected to a feed stream containing a nitrogen carrier gas, 500 ppm toluene and 10 vol. % oxygen at a GHSV = 19 200 mL gcat−1 h−1. As shown in Figure 5a, the LaNiO3 had shown the highest activity with the T90 (i.e., 90% toluene conversion) at ca. 250 °C, which was much lower than that of La4Ni3O10 (at ca. 310 °C). However, if taking the surface area into consideration, the areal conversion rates for LaNiO3 and La4Ni3O10 were relatively similar, both revealing a conversion of ca. 0.003 μmol s−1 m−2 toluene at the temperature of ca. 250 °C (see the Supporting Information, Figure S2a). The La2NiO4 revealed the lowest activity with the T90 at ca. 350 °C. This is unsurprising as the catalyst was proven lack of α-O (see O2-TPD in Figure 1), which played significant role in toluene oxidation.48,49 In toluene, the π bond had a high density of electron cloud, which was inclined to be attacked (or adsorbed) by electrophile. Since the Lan+1NinO3n+1 had two electrophiles

180 °C 200 °C 220 °C 240 °C 1.0 5.4 7.5

2.7 12.6 16.5

6.6 27.3 34.0

15.5 55.8 66.3

Ea (kJ mol−1)

ln A (s−1)

89.0 75.0 70.1

19.1 16.7 16.3

LaNiO3 had revealed the lowest Ea in toluene oxidation, which was similar to those of LaMnO3 and mesoporous Cr2O3 as reported in literature.3,53 The La2NiO4 had shown the highest Ea, which is unsurprisingly given its presence of majority Ni(II) and lack of α-O. 3.3.2. Methane Oxidation. In methane oxidation measurements, the Lan+1NinO3n+1 were subjected to a feed stream containing a nitrogen carrier gas, 1 vol. % methane and 10 vol. % oxygen at a GHSV = 19 200 mL gcat−1 h−1. As shown in Figure 6a, the LaNiO3 still showed the highest activity in methane oxidation with the T90 at ca. 530 °C. However, if the surface area is taken into consideration, the La4Ni3O10 was surprisingly given a higher activity than the LaNiO3 (see the Supporting Information, Figure S2b). The La2NiO4 again showed the worst activity with the T90 at ca. 680 °C, implying that the α″-O seemed not active in methane oxidation.54 In kinetic measurements, the Arrhenius plots for Lan+1NinO3n+1 were all linear dependence within the inves3263


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the La4Ni3O10 with a much higher α′-O desorption rate than the LaNiO3, it explained why this catalyst had yielded the lowest Ea in methane oxidation although its redox potential was relatively lower than that of LaNiO3 (see Figure 4). In addition, a TPR experiment with only CH4 involved (hereafter referred as CH4-TPR) was also conducted to further evaluate the functions of α′-O, α″-O, and β-O in methane oxidation. The resulting products, CO (rather than CO2 due to the lack of gaseous O2) and H2O, were in situ monitored by using the MS equipment. As shown in Figure 7, the desorption

Figure 6. (a) Conversion rates and (b) Arrhenius plots of La2NiO4 (□), La4Ni3O10 (○), and LaNiO3 (△) in methane oxidation. Condition: methane (1 vol. %), O2 (10 vol. %), 0.5 g catalyst, GHSV = 19 200 mL gcat−1 h−1.

tigated reaction region (see Figure 6b). The calculated kinetic parameters were summarized in Table 5. Unlike toluene Table 5. Kinetic Parameters of La2NiO4, La4Ni3O10, and LaNiO3 in Methane Oxidation Figure 7. CH4-TPR plots for La2NiO4 (■), La4Ni3O10 (○), and LaNiO3 (▲). Condition: methane flow rate at 25 mL min−1, 0.05 g catalyst, ramping rate at 10 °C min−1.

methane k (×10−2 s−1) catalyst La2NiO4 La4Ni3O10 LaNiO3

370 °C 390 °C 410 °C 430 °C 6.3 17.3

0.2 11.8 35.4

0.4 21.3 69.5

1.0 37.1 131

Ea (kJ mol−1)

ln A (s−1)

173.3 111.0 126.8

25.5 17.6 22.3

of CO and H2O were both detected at the temperature higher than 500 °C for Lan+1NinO3n+1. The LaNiO3 had revealed the lowest CO desorption peak temperature at ca. 550 °C while the La2NiO4 and La4Ni3O10 both showed that at ca. 650 °C. Clearly, the α′-O (desorbed at 300 °C < T < 600 °C, see Figure 1) in LaNiO3 was involved into methane oxidation where only β-O (desorbed at T > 600 °C) was involved for La2NiO4 and La4Ni3O10. It was noted that the CH4-TPR result was inconsistent with methane oxidation measurements (see Figure 6a), which revealed that the La4Ni3O10 was able to induce distinct methane conversion in the temperature range of 400− 600 °C. Since the methane oxidation test was involved gaseous oxygen, it is proposed that the La4Ni3O10 might be able to store gaseous oxygen as active α′-O in structure (as evidenced by O2TPD, see Figure 1), which was then released to oxidize methane at elevated temperatures. This storage capacity was proposed to be originated from its ability in sustaining both oxygen hyper-stoichiometric and deficient structures. As such, in CH4-TPR, the lack of gaseous oxygen had blocked up the storage process, leading to the inactivation of La4Ni3O10 at the temperature range of 400−600 °C.

oxidation, the La4Ni3O10 had yielded the lowest Ea in methane oxidation, which was consistent with areal conversion measurement (see the Supprting Information, Figure S2b). This Ea value was even lower than that of CuO(or MnOx)/Al2O3 (Ea = 144 kJ mol−1) and La0.6Sr0.4Co0.2Fe0.8O3‑δ (Ea = 128 kJ mol−1),55,56 revealing that the La4Ni3O10 could be a promising alternative for catalytic oxidation of methane. Accoriding to Alifanti et al.,57 the oxidation of methane generally followed that the methane was first attacked by nucleophilic superficial lattice oxygen O2− (i.e., α′-O), leading to the cleavage of C−H bond. The resulting methyl radical (CH3·) was then oxidized by reactive superficial lattice oxygen that was coordinated with the electron acceptor of Ni(III), leading to the formation of CO2 (or CO) and H2O and the reduction of Ni(III) to Ni(II). The left superficial vacancies were then filled by labile lattice oxygen (could be α′-O or β-O), which reoxidized the Ni(II) to Ni(III) to complete the redox cycle.

4. CONCLUSIONS In this paper, the active oxygen species in Lan+1NinO3n+1 (n = 1, 3, and ∞) for catalytic oxidation of toluene and methane were studied. The main results were summarized as follows, (1) The LaNiO3 that possessed the most chemisorbed oxygen species had yielded the highest activity and lowest Ea (ca. 70.1 kJ mol−1) in toluene oxidation.

As such, it is known that the transmission of labile lattice oxygen plays an important role in affecting the performance of catalysts. Since the O2-TPD analyses (see Figure 1) had proven 3264


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(2) The La4Ni3O10 that had superior lattice oxygen mobility showed the lowest Ea (ca. 111.0 kJ mol−1) in methane oxidation. (3) The La2NiO4 that lacked chemisorbed and superficial lattice oxygen species had shown the worst performance either in toluene or methane oxidation. (4) The interstitial oxygen species as existed in the interlayer of Lan+1NinO3n+1 seems not active in methane oxidation. (5) The La4Ni3O10 might be able to store gaseous oxygen as active α′-O in structure due to its ability in sustaining both oxygen hyper-stoichiometric and deficient structures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b08703. X-ray powder diffraction and areal conversion rates for catalytic oxidation of toluene and methane (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 571 88982034. Fax: +86 571 88982034. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 51208458), the Program for Zhejiang Leading Team of S&T Innovation (Grant No. 2013TD07), and the Major Science and Technology Project of Zhejiang Province (2012C03003-3).

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NOTE ADDED AFTER ASAP PUBLICATION This article was published ASAP on February 4, 2016, without all of the corrections. The corrected article was published ASAP on February 9, 2016.

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Active Oxygen Species in Lan+1NinO3n+1 Layered Perovskites for

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