Catalytic Activity and Stability over Nanorod-Like Ordered Mesoporous

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Catalytic activity and stability over nanorod-like ordered mesoporous phosphorus-doped alumina supported palladium catalysts for methane combustion Xiaohua Chen, Yong Zheng, Fei Huang, Yihong Xiao, Guohui Cai, Yongchun Zhang, Ying Zheng, and Lilong Jiang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02420 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 17, 2018

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ACS Catalysis

Catalytic activity and stability over nanorod-like ordered mesoporous phosphorus-doped alumina supported palladium catalysts for methane combustion ‡

‡

‡

Xiaohua Chen †, Yong Zheng , Fei Huang †, Yihong Xiao , Guohui Cai , Yongchun Zhang †, ,

,

Ying Zheng * †, and Lilong Jiang * ‡ †

College of Chemistry and Materials Science, Fujian Provincial Key Laboratory of Advanced

Materials Oriented Chemical Engineering, Fujian Normal University, Fuzhou, Fujian 350007, P. R. China ‡

National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University,

Fuzhou, Fujian 350002, P. R. China *Corresponding authors: Prof. Ying Zheng Tel: +86 591 83464353, Fax: +86 591 83464353, E-mail: [email protected] Prof. Lilong Jiang Tel: +86 591 83731234-8201, Fax: +86 591 83709796, E-mail: [email protected]

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ABSTRACT

Nanorod-like phosphorus-doped ordered mesoporous γ-alumina (OMA), which had abundant ordered pore channels in the nanorods, was rapidly synthesized through a modified solgel strategy without using any mineral acids. Highly dispersed Pd-based catalysts were synthesized taking as-obtained phosphorus-doped OMA materials as carriers for methane combustion. The crystallization temperature of γ-Al2O3 was increased by phosphorus-doping. The surface acidity properties of γ-Al2O3 were modified upon phosphorus incorporation which had a significant effect on catalyst activities, and this influence was much more conspicuous for the supports calcined at high temperature. The incorporation of phosphorus adjusted the distribution of palladium active species and the reducibility of catalysts, synergistically affecting the low-temperature catalytic performance. Pd/6P-OMA catalyst demonstrated enhanced lowtemperature catalytic property and stability in the 13 cycling stability and long-term stability tests. During the reaction cycles, the total CH4 conversion temperature for Pd/6P-OMA catalyst was as low as 345 °C, which could be reduced down to 321 °C via hydrogen reduction treatment. Compared with the catalyst without dopant, Pd/6P-OMA catalyst also exhibited higher hydrothermal stability in the presence of excess water vapor in the feed.

Keywords: Nanorod-like; ordered mesostructure; methane combustion; palladium-based catalyst; phosphorus-doped.


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1. INTRODUCTION Natural gas has been viewed as a promising candidate for future energy systems, especially its application for automotive fuels.1,2 Methane is the chief constituent in natural gas, however, the direct emission of the unburned methane from natural-gas-fueled vehicles will cause environmental issues, since that methane is also a greenhouse gas with an effect which is about 20 times greater than CO2.3 To date, the catalytic combustion of methane using heterogeneous catalysts is regarded as an effective route to lower releases of methane.3,4 However, to meet the more strict guidelines of energy conservation and emission reduction, it is urgently needed to design and develop a catalyst for methane combustion with higher reactivity and stability. Various catalysts have been utilized for catalytic combustion of methane, mainly including perovskite oxides,5–6 hexaaluminates,7–8 and noble metal-based catalysts.9–14 Although perovskite oxides and hexaaluminates show lower price and good thermal stability, their catalytic activity in low temperature range needs to be further improved. Considering their low ignition temperature and high activity at low temperatures, noble metal-based catalysts, especially supported palladium catalysts are more promising for this reaction compared to perovskite oxides and hexaaluminates.11–13 As for supported palladium catalysts, the nature of support has a significant influence on the microstructure and properties of the catalysts used for methane oxidation.11–15 Among the numerous supports used, γ-Al2O3 is viewed as the most suitable support in practice.12,13 However, some defect of γ-Al2O3 supported palladium catalysts should not be ignored. The wide pore-size distribution of γ-Al2O3 was not beneficial for efficient catalytic reactions,16,17 for instance, not salutary for confining the palladium nanoparticles, hence leading to their migration and aggregation into inactive large particles under the real working condition. In the previous


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studies, various strategies have been exploited to stabilize the noble metal nanoparticles of supported noble metal nanocatalysts in order to preserve their catalytic performances at hightemperature reaction processes. Recently, Zhan et al. demonstrated several approaches, such as the enhancement of metal-support interaction through a sacrificial coating strategy, fine-tuning of the NPs’ crystal structure, as well as a surfactant-assisted synthetic strategy aiming at the stabilization of noble metals, which have achieved good results and provided novel methods to stabilize precious metal particles.18–20 However, as for Pd/Al2O3 catalysts, the sintering of palladium nanoparticles still remains a problem to be solved. In term of alumina support, exploiting an alumina material with appropriate pore size to anchor palladium nanoparticles in the pores by a more suitable synthetic method is a promising strategy to reduce the sintering problem. Ordered mesoporous alumina (OMA) materials are prominent candidates for catalyst supports, due to their desirable properties such as controlled morphologies, well-defined and interconnected mesopores, which are favorable for the mass and heat transport, hence promoting catalytic activities of supported-catalysts.21–23 In particular, taking advantage of tunable mesoporosity of OMA to adjust the pore dimensions into an optimal size range will be expected to disperse the metal nanoparticles and prevent metal nanoparticles from migrating and aggregating.22 At present, OMA was prepared mainly through nanocasting and modified sol-gel self-assembly methods.24,25 In comparison with the first strategy, the latter was confirmed to be a more practicable means to obtain OMA, due to their controllable synthesis conditions and simple synthesis process. Actually, OMA materials as catalyst carriers have been employed to construct novel catalysts, which demonstrated high catalytic properties in different reactions.21–23 Nonetheless, in the reported synthesis system of sol-gel process, hydrochloric acid or nitric acid


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was normally used to act as pH adjustor. While, the residual Cl– in the body of OMA after calcination may bring about catalyst poisoning, and the formation of NOx due to decomposition of nitric acid during calcination process may cause pollution to the environment.26 Another issue is related to their relatively longer synthesis period. On the other hand, the inhibitory effect of water from the reaction feed over Pd/Al2O3 catalysts needs to be taken into account for methane oxidation especially at temperatures below 450 ºC, which can be explained by the competition adsorption between water and methane over the active sites.12,27 Recently, a doping strategy of alumina by metal oxides has been developed in methane combustion to improve the hydrothermal stability of the catalyst.12,27 Non-metal phosphorus element has been proven to serve as a structure stabilizer of alumina.28–30 Meanwhile, the incorporation of phosphorus into alumina brings about the decoration of hydroxyl groups on the surface of alumina accompanied by the formation of P-OH groups. The dehydroxylation between P-OH groups results in the formation of P=O groups, which will rehydrate into P-OH groups in the presence of water and consequently reduce the adsorption of water on the adsorption sites of the catalysts, hence enabling the improvement of the hydrothermal stability for the Pd/Al2O3 catalysts. In this work, through adjusting the proportion of solvents to control the hydrolytic rate of alumina salt and the formation rate of gel, the sol-gel self-assembly approach was further modified for synthesizing phosphorus-doped ordered mesoporous γ-alumina (OMA). And not any mineral acid was added to the synthesis system for the purpose of eliminating the adverse effect of HCl and HNO3. Phosphorus element was incorporated into the framework of γ-alumina to modify the thermal and hydrothermal stability, as well as surface acidity properties of γalumina. The influence of phosphorus incorporation on the surface acidity properties of OMA


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carriers, the distribution of palladium active species as well as the reducibility of the catalysts taking the as-obtained OMA as supports were investigated, all of which were correlated with the catalytic activities for methane combustion. Encouragingly, the catalyst with highly dispersed nanoparticles showed good low-temperature catalytic activity and stability, along with hydrothermal stability for CH4 combustion reaction. This study enabled the design of an ordered mesoporous catalyst with highly catalytic activities and stability for methane oxidation under the severe conditions.

2. EXPERIMENTAL 2.1. Synthesis of catalyst carriers All chemicals used were of analytical grade. P-doped alumina was prepared via a modified sol-gel method, in which not any mineral acid was used. 1.50 g P123 (EO20PO70EO20, Mav = 5800) was dissolved in the mixed solution of 30 mL absolute ethanol, 10 mL isopropanol and 0.18 mL acetic acid at ambient temperature. Then, a required amount of (NH4)H2PO4 and 0.015 mol of Al(OiPr)3 were introduced into the above solution in sequence under vigorous stirring for 4 h. The resulting milky solution was transferred into a dish to age for 6 h in air and then 60 °C in an oven for 8 h. The obtained white xerogels were calcined in air at 500 °C for 4 h (2 °C min–1 heating rate), and then treated at a desired temperature of 800 and 1000 °C for 1 h (10 °C min–1 heating rate). The final sample was marked as xP-OMA(T), where x represents the P content (wt%) (x = 0, 3, 6, 8, 11 and 13), T is the final pyrolysis temperature of carriers.

2.2. Synthesis of catalysts


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Taking the as-obtained P-doped OMA samples as carriers, supported palladium catalysts with a constant Pd loading amount of 0.5 wt% (theoretic value) were synthesized by the incipient wetness impregnation method with Pd(NO3)2 aqueous solution (0.028 g/mL) as metal precursors. After dried in a 100 °C drying oven for 12 h, the catalyst precursor was calcined at 500 °C for 1 h (10 °C min–1 heating rate). The as-synthesized catalyst was herein denoted as Pd/xP-OMA(T), where x represents the P content (wt%) (x = 0, 3, 6, 8, 11 and 13), T is the final pyrolysis temperature of carriers.

2.3. Characterization The powder X-ray diffraction patterns were obtained on a Philips X’pert Pro MPD diffractometer with Cu Kα radiation (λ = 0.15406 nm). The nitrogen adsorption-desorption isotherms at 77 K were measured with a Quanta chrome Nova 4200 analyzers, and the BET surface area was estimated by the Brunauer-Emmett-Teller (BET) theory. The pore size distributions were determined by using the adsorption branch of the isotherm with the BarrettJoyner-Halenda (BJH) method. Transmission electron microcopy (TEM) images were taken over a FEI Tecnai G2 F20 S-TWIN transmission electron microscope operated at 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Thermo Scientific ESCALAB 250 spectrometer with Al Kα X-ray radiation (hν = 1486.6 eV). For correcting charging effects, all of the binding energies were referenced to C 1s peak at 284.8 eV. The XPS data were analyzed and fitted with “Thermo Avantage” software. Gaussian(70%)/Lorentzian(30%) curvefitting method after subtracting a Shirley background was performed to deconvolute the spectra for the sake of determining the peaks areas and positions. Diffuse reflectance UV-vis spectra


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(DRS) of the supports were performed on a Perkin Elmer Lambda 950 in the 200–800 nm range, with BaSO4 as the reference substance. Raman spectra of the catalysts were performed with a Laser Micro-Raman spectrometer. An Ar-laser excitation source at λ = 532 nm with a power of 20 mW and spectral resolution of 1 cm–1 was used. Temperature-programmed reduction of CH4 (CH4-TPR-MS), temperature-programmed reduction of H2 (H2-TPR) and temperature programmed desorption of NH3 (NH3-TPD) were implemented on a Micromeritics AutoChem II 2910 equipment (for more experimental details see Page S2, Supporting Information).

2.4. Catalytic activity test The catalytic activity of the catalyst (100 mg) was measured in a quartz U-shaped (Φ 8 mm) fixed-bed reactor system under atmospheric pressure. The test gas consisted of 1 vol% CH4, 5 vol% O2 and balancing N2, and the gas hourly space velocity (GHSV) was 50 000 mL g–1 h–1. The concentrations of the inlet and outlet gases were monitored by online gas chromatography (Agilent 7820A GC). For investigating the effect of GHSV, gas hourly space velocity was controlled at 20 000, 30 000, 40 000, 50 000 and 60 000 mL g–1 h–1. Comparative tests with water vapor (3%, 5%, 8% and 10%) atmosphere were performed to investigate the effect of water vapor on the catalytic activities at a gas hourly space velocity of 50 000 mL g–1 h–1. The apparent activation energy (Ea) was calculated when the methane conversion was lower than 20% (GHSV = 80 000 mL g–1 h–1) with the Arrhenius empirical equation: lnr = –(Ea/RT) + lnA, where r (mol s–1 g–1 ) is the reaction rate, R is the universal gas constant, T is the Kelvin temperature and A is the pre-exponential factor.


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3. RESULTS AND DISCUSSION 3.1. Characterization of the carriers

Figure 1. Small- and Wide-angle XRD patterns for the xP-OMA carriers calcined at 800 °C (A, C) and 1000 °C (B, D).

The formation of ordered mesoporous structure was confirmed by the small-angle X-ray diffraction (SXRD). As shown in Figure 1 (A), all the samples calcined at 800 ºC displayed strong (100) diffraction peaks at around 0.82°, indicating that hexagonally ordered mesoporous


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structures with p6mm hexagonal symmetry were inherited. The values were in good agreement with the literature.31 The peak positions shifted to relatively higher 2θ degree with the increasing P amount, related to a contraction in lattice cell parameter (Table S1, Supporting Information) due to the shorter bond length of P-O than that of Al-O, which manifested that phosphorus was incorporated into the alumina lattice. This similar phenomenon was also found in the Co-doped OMA.32 It was noted that the ordered mesoporous structures were still maintained even after pyrolysis at 1000 °C (Figure 1(B)), while, the (100) diffraction exhibited a slight shift to a higher angle sides with increasing calcination temperature, indicating a further small shrinkage of the lattice cell parameter.33 The wide-angle X-ray diffraction (WXRD) patterns of xP-OMA calcined at 800 °C were displayed in Figure 1 (C). For the samples with P content of 0–6 wt%, the peaks at 2θ of 45.9° and 67.0° which were ascribed to (400) and (440) reflections of the γ-Al2O3 were detected, indicating the amorphous framework was partly converted to γ-Al2O3.34 However, the samples with P content of 8–13 wt% performed no apparent diffraction peaks indexed to γ-Al2O3, suggesting amorphous frameworks were maintained. As presented in Figure 1(D), all samples after 1000 ºC calcination exhibited the typical diffraction peaks attributable to γ-Al2O3 phase, implying their higher degree of crystallinity than those treated at 800 ºC. In general, the intensity of diffraction peaks ascribed to γ-Al2O3 gradually declined with increasing phosphorus content, demonstrating that phosphorus doping increased the crystallization temperature of γ-Al2O3. Moreover, regardless of the calcination temperature, all the samples exhibited no characteristic diffraction peaks associated with α-Al2O3 and other phases, proving the high thermal stability of the obtained samples.


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Table 1. Textural parameters of the samples calcined at different temperatures. 800 ºC

1000 ºC

sample SBET/m2 g–1

Vp/cm3 g–1

Dp/nm

SBET/m2 g–1

Vp/cm3 g–1

Dp/nm

0P-OMA

255.1

0.78

10.3

149.7

0.49

11.9

3P-OMA

300.7

0.73

8.9

167.2

0.47

11.3

6P-OMA

332.7

0.83

9.0

177.6

0.54

11.4

8P-OMA

279.2

0.77

9.2

161.3

0.53

11.6

11-POMA

256.0

0.61

8.8

159.6

0.41

9.9

13P-OMA

258.1

0.71

10.0

156.2

0.41

10.2

The textural properties of xP-OMA calcined at 800 and 1000 °C were given in Table 1. The samples with P content of 3–6 wt% possessed larger specific surface area, especially for the sample 6P-OMA, which displayed high surface area up to 332.7 and 177.6 m2 g–1 at 800 and 1000 °C, respectively. However, no significant increase in the BET surface area was detected when the P content exceeded 11wt% compared with that of un-doped OMA sample. The results demonstrated that the incorporation of adequate content of phosphorus could improve the textural parameters, while the excessive addition of phosphorus was likely to block the pore of the support leading to a drop of specific surface area. It was obvious that the increase in the treatment temperature resulted in the dip of BET surface area, due to the growth of alumina particles under higher pyrolysis temperature. This observation agreed well the previous reports.35


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Figure 2(A) and (B) displayed the nitrogen adsorption-desorption isotherms of the carriers treated at different temperatures, which could be classified as a typical type IV isotherm, consistent with the reported mesoporous materials.33 As for the carriers calcined at 1000 ºC, twowell defined peaks on the pore size distribution curve could be observed (Figure 2(D)), which located in the range of 2.1–5.5 nm and 6.2–27 nm, respectively, indicating their bimodal mesomesoporous structures. The behavior may be attributed to a small section of partial filling effect in some fields.36 However, the bimodal structure was less obvious for the samples calcined at 800 ºC (Figure 2(C)). Regardless of the calcination temperatures, the peaks on the pore size distribution curve shifted to lower values as the phosphorus percentage increased, indicating the increase of pores with small dimension.


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Figure 2. N2 sorption isotherms and the pore size distribution curves for the xP-OMA carriers calcined at 800 °C (A, C) and 1000 °C (B, D).

Figure 3. TEM and HRTEM images of 6P-OMA carrier calcined at 800 °C. (a, b) nanorod-like structure; (c, d, f) Ordered mesoporous pore channels on the various nanorods; (e) Lattice fringes of γ-Al2O3 obtained from the further amplification of (c).

To further confirm the well-organized mesoporous structures of xP-OMA, the TEM images taking 6P-OMA(800) for example was performed and provided in Figure 3. It presented a group


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of well-distributed nanorod-like structure (Figure 3(a, b)), the rod diameter of which was homogeneous and in the range of 20–40 nm. As shown in Figure 3(c, d, f), the highly ordered cylindrical mesopores viewed along [110] orientation for different parts in the nanorods were observed. The average diameter of the pores was about 3.4 nm, which was in line with the pore size distribution based on the nitrogen adsorption-desorption result. When it went to higher resolution, the sample exposed the lattice fringes of γ-Al2O3, indicating the crystallization of the framework (Figure 3(e)), which was in good line with that of WXRD measurement. The XPS analyses were performed to investigate the surface valence state of the assynthesized carriers calcined at 800 °C. The characteristic peaks for Al, P, O and C elements could be clearly detected in the full spectra of XPS (Figure S1, Supporting Information), suggesting that P element was successfully incorporated into the lattice matrix of OMA. The result could be further confirmed by the UV-Vis DRS spectra (Figure S2, Supporting Information). Un-doped alumina sample essentially did not absorb in the range of 200–800 nm.37 Conversely, P-doped alumina sample appeared a strong signal located at ca. 250 nm, ascribed to P5+ oxo-species in tetrahedral coordination, representing that the presence of phosphorus enhanced the absorption in near-ultraviolet region. As displayed in Figure 4 (A), for the undoped carrier, the binding energy of Al 2p was ca. 74.1 eV, which was consistent with the reported value of pure Al2O3.38 It could be found that the Al 2p BE (74.2–74.4 eV) of P-doped carrier was upshifted compared with that in the un-doped carrier, which could be derived from the different electronegativity between P and Al element existing within the mesoporous skeleton, implying that the incorporation of phosphorus led to the change of coordination state of Al and the formation of Al-O-P bond. Since P element possesses more electronegativity than that in Al element, the addition of P into the framework induces the decline in the outer electron


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density of the nucleus for Al atom, leading to the increase of shielding effectiveness. The P 2p BE was ca. 134.2 eV (Figure 4 (B)), which matched well with the value reported for P5+ previously, indicating that P element existed mainly in P5+ oxide state.39 The smaller ionic radius of P5+ than that of tetra-coordinated Al3+ ionic makes it possible for P5+ to enter the matrix of alumina to partly take place of tetra-coordinated Al3+ ions, hence stabilizing the structure of OMA.28–30

Figure 4. (A) Al 2p and (B) P 2p XPS spectra of the xP-OMA carriers calcined at 800 °C.

The acidity properties of the carriers calcined at different temperatures were evaluated by the NH3-TPD measurements and shown in Figure 5. It was noticed that the TPD profiles for xP-OMA(800) carriers were similar in shape (Figure 5(A)), which exhibited a broad NH3desorption peak with a maximum temperature around 278 °C attributed to the medium-strength acidic sites.40,41 It was observed that the temperatures of desorption peaks were close in spite of addition of P, indicating that P doping had little influence on the acidic site strength, which may


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be that the alumina species were mostly amorphous leading to this delicate difference. According to the desorbed NH3 amounts (Table S2, Supporting Information), P doping affected the amount of surface acidity, which were gradually increased with increasing the P content. This increasing effect of acidity after P addition has also been observed in the other reported P-modified alumina materials, which may be ascribed to the formation of P-OH groups on the surface.42 The effect of calcining temperature on acidity of alumina was evident. The total acid amount of alumina was reduced with raising calcination temperature (Table S2). Furthermore, together with the retained moderate acid sites around 276 °C, all supports treated at 1000 °C demonstrated an increase of weak acid sites (100–200 °C).41 And the support without dopant displayed weaker acid strength belonging to weak acid sites (Figure 5(B)). The different acidity properties of supports had an obvious influence on catalyst performance.

Figure 5. NH3-TPD profiles of xP-OMA carriers calcined at 800 °C (A) and 1000 °C (B).


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3.2. Catalyst characterization The XRD patterns of Pd/xP-OMA catalysts were displayed in Figure S3 (Supporting Information). All samples still showed characteristic peak indexed to (100) reflection (2θ = 0.86°), representing the ordered mesoporous structure, which could provide more accessible active centers for the gaseous reactants, accounting for better catalytic activities.33 In comparison with their precursors, the results of the cell parameter (a0) and d-spacing (d100) values (Table S3, Supporting Information) for the catalysts demonstrated a further small shrinkage of the unit cell parameter. Besides, the catalysts exhibited more distinct diffraction peaks ascribed to γ-Al2O3 phase, indicating their higher degree of crystallinity. The diffraction peaks corresponding to the reflections for α-Al2O3 were undetectable. Moreover, no diffraction peaks representing palladium oxide and palladium phases were detected, indicating that palladium species were finely dispersed with small particles beyond the limit of detection. The result was in contrast with the reported ones demonstrating palladium species in the WXRD profiles, which may be attributed to the different dispersity and particle sizes of palladium species in the supports.1


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Figure 6. TEM image (A) and Pd particle size distributions (B) of Pd/6P-OMA(1000) catalyst. The textural parameters of the catalysts were listed in Table S4 (Supporting Information). In general, compared with their corresponding supports xP-OMA, Pd/xP-OMA(800) catalysts experienced a distinct decline in BET surface area, while, no obvious dip in the surface area for Pd/xP-OMA(1000) catalysts was observed combining with their excellent thermal stability. As presented in Figure S4 (Supporting Information), the isotherms for all the catalysts remained IV type isotherms, ascribed to mesoporous materials. Besides, the pore size distribution characterization of the as-prepared catalysts exhibited no obvious changes as compared with their corresponding carriers. The TEM image of the representative catalyst Pd/6P-OMA(1000) was displayed in Figure 6(A). The lattice spacing of 0.23 nm was attributed to the corresponding Pd (111) crystal plane.43 The as-synthesized nanoparticles presented uniform size distribution with average size of 3.3 nm (Figure 6(B)), suggesting that the nanoparticles were highly dispersed on the support structure, consistent with the result of WXRD. Furthermore, the size of metallic nanoparticles was similar with the smaller pore size (2.1–6.2 nm) of the catalysts shown above.

Figure 7. Raman spectra (A), H2-TPR profiles (B) and CH4-TPR profiles (C) of Pd/xP-OMA(1000) catalysts.


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Figure 7(A) showed the Raman spectrum of Pd/xP-OMA(1000) catalysts. All samples exhibited a characteristic band around 633 cm–1 assigned to the B1g vibration mode of tetragonal PdO, which was in line with those reported in the literature.44 The band at 633 cm–1 comes from the first-order scattering processes and exhibits the motion of oxygen atoms along the c-axis.44 The characteristic band upshifted slightly to higher values as the P doping amount increased from 3 to 6 wt%, and then shifted to lower wave number positions with further increasing P content. That is, Pd/xP-OMA (x = 3–6) exhibited higher wavenumbers, corresponding to the stronger interaction between PdO and support, which played an important role on the reduction behavior of PdO species. To investigate the reduction characteristics of the Pd/xP-OMA(1000) catalysts, H2-TPR analysis was performed. As shown in Figure 7(B), one low-temperature reduction peak ɑ in the region of 90–105 ºC was detected, which could be ascribed to the reduction of PdO species highly dispersed on Al2O3.1 In addition, two broad peaks (designated as β and γ) ranging from 250 to 800 ºC were attributed to the reduction of surface oxygen and subsurface oxygen.45,46 Based on the reduction temperatures of peak ɑ, the low-temperature reducibility decreased in the sequence Pd/6P-OMA(1000) > Pd/3P-OMA(1000) > Pd/8P-OMA(1000) > Pd/0P-OMA(1000) > Pd/11P-OMA(1000) > Pd/13P-OMA(1000). This behavior verified that adding an appropriate amount of phosphorus into alumina could facilitate the reduction of PdO species, owing to their different metal-support interactions as discussed in Raman results. As for the Pd/xP-OMA(800) catalysts (Figure S5(A) and Table S5, Supporting Information), the low-temperature reduction peak ɑ located in the range of 140–200 ºC, which


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upshifted to larger value with the increasing of phosphorus content, indicating the decline of reducibility. The result may be ascribed to the lower crystallization degree of γ-Al2O3 for Pdoped supports especially for Pd/xP-OMA(800) (x = 8–13), leading to the weaker metal-support interaction. On the other hand, it was noted that with the increase of calcination temperature of supports from 800 to 1000 ºC, the reduction temperature of PdO species (peak ɑ) was obviously decreased, suggesting their better reduction behavior.45,47 And this was beneficial for the catalytic properties over Pd/xP-OMA(1000) catalysts. The reduction characteristics of the catalysts were further investigated by CH4-TPR. The results (Figure 7(C), Figure S5(B) and Figure S6–S7) demonstrated that the reduction properties of all catalysts obeyed the similar orders as those obtained in the H2-TPR profiles, which proved equally that Pd/6P-OMA(1000) exhibited the optimal reduction behavior among the catalysts involved, contributing to its higher initial catalytic activity. It was noted that some clear differences were detected in the profiles among the samples. In term of Pd/xP-OMA(1000) (x = 6–13) catalysts, the CH4 conversion dominated by two different steps (Figure S6, Supporting Information). The start-up reduction temperature of CH4 in the first stage occurred at around 300 ºC accompanied by the production of CO2 and H2O, which could be attributed to the reaction between CH4 and PdO species (CH4 + PdO → Pd + CO2 + H2O).12 Afterwards, CH4 was continuously consumed in the 330–500 ºC temperature region, accompanied by the release of CO2, CO, H2 and H2O. While the signals of H2 and CO delayed behind those of CO2. An explanation for this was that, CH4 was first completely oxidized and then partially oxidized by surface/subsurface oxygen species (CH4 + 3Osurface/subsurface → CO2 + 2H2O, CH4 + 2Osurface/subsurface → CO + 2H2O, CH4 + Osurface/subsurface → CO + 2H2).5,48 However, in Pd/xPOMA(1000) (x = 0–3) catalysts, the situation was distinctly different. In addition to the reduction


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of PdO species and surface/subsurface oxygen species, massive conversion of CH4 was detected in the third stage, with simultaneous H2 production. This phenomenon could be derived from the CH4 cracking reaction over the catalysts (CH4 → 2 C + 2H2).5,48 The curve features in Pd/xPOMA(800) (x=0–6) catalysts (Figure S7, Supporting Information) was similar with those in Pd/xP-OMA(1000) (x = 6–13) catalysts. While, for temperatures higher than 550 ºC, small amounts of methane over Pd/xP-OMA(800) (x = 8–13) was consumed in the third process, only accompanied by the formation of CO and H2, which might be attributed the continued partial oxidation of CH4 with oxygen species. The chemical states of Pd and its concentration on the surface of the catalysts were explored by X-ray photoelectron spectroscopy (XPS). As displayed in Figure 8 (A), the asymmetric Pd 3d peaks for both Pd 3d5/2 and Pd 3d3/2 components of the as-prepared catalysts were deconvoluted into two types of palladium species by Gaussian/Lorentzian functions, of which the highintensity Pd 3d5/2 peaks were selected to analyze. The peaks at BE = 335.32–335.72 eV are ascribed to metallic Pd, and those at BE = 336.89–337.39 eV to PdO species (Table 2), indicating the coexistence of Pd and Pd2+ states on all the catalysts.49,50 The Pd2+/Pd0 ratios were calculated from the Pd 3d5/2 peak areas (Table 2). It could be seen that the higher incorporation amount of phosphorus, the higher proportion of Pd2+, suggesting the P-dopant can stabilize Pd2+. The result indicated that the electronic state of palladium species changed due to the different interaction between palladium and supports by P-doping. For Pd-based catalysts, the oxidation state of Pd particles influences the catalytic activity for methane combustion reaction. Pd2+ particles are normally regarded as the catalytic active sites, however, metallic Pd is also reported to catalyze CH4 oxidation.12,51 As shown in Figure 8(B), both Pd0 and Pd2+ components could still be observed for all the catalysts after the first run of catalytic activity testing. However,


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comparing with the corresponding as-prepared catalysts, the Pd2+/Pd0 ratios in the reacted catalysts increased obviously (Table 2). It was not unexpected that the Pd0 was oxidized to PdO phase during the methane oxidation process, resulting in the increase of Pd2+ concentration.52 The behavior provided a strong evidence that active sites experimented redispersion and reconstruction during the catalytic tests, resulting in the regulation of catalytic activities. As shown in Figure S8, the P 2p BE of the catalysts Pd/xP-OMA(1000) before and after reaction still located at around 134.1 eV, coinciding with those for the carriers calcined at 800 ºC, suggesting that P-doped Al2O3 materials were stable at high temperature and reaction conditions.

Figure 8. Pd 3d XPS spectra of the Pd/xP-OMA(1000) catalysts (A) before reaction and (B) after the first run of catalytic activity testing.


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Table 2. XPS spectra analysis of Pd 3d5/2 and calculated activation energies (Ea) for the catalysts Pd/xP-OMA(1000). Pd0 BE (ev) sample

a

Pd2+ BE (ev)

b

a

Pd2+ / Pd0

b

a b

Ea (kJ mol–1)

Pd/0P-OMA(1000)

335.45 335.93

336.89 337.03

0.89 1.72

93

Pd/3P-OMA(1000)

335.72 336.02

337.04 337.06

1.03 2.05

86

Pd/6P-OMA(1000)

335.35 335.97

337.39 337.42

1.21 2.40

84

Pd/8P-OMA(1000)

335.41 335.51

337.12 337.48

2.65 3.51

90

Pd/11P-OMA(1000)

335.34 335.77

337.08 337.41

2.94 3.81

96

Pd/13P-OMA(1000)

335.32 335.58

337.16 337.43

3.32 4.52

99

(a) for the catalysts before reaction; (b) for the catalysts after the first run catalytic activity testing.

3.3. Catalytic performance In order to investigate the influence of phosphorus loading on the catalytic activity, the methane oxidation experiments over Pd/xP-OMA(1000) catalysts were performed and displayed


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in Figure 9 (A), showing that low-temperature catalytic activity varied with the phosphorus amount. Among the catalysts, Pd/xP-OMA(1000) (x = 3–8) exhibited better catalytic performance, with lower CH4 conversion temperature (T99) than that of unmodified catalyst. While, excessive phosphorus (x = 11–13) content may decrease the catalytic activity. This result demonstrated that appropriate amount of phosphorus could enhance the catalytic activity of the Pd/xP-OMA(1000) catalysts. In combination with the results of the above analysis, one of the influence factors of the enhanced activities could be attributed to the improved reducibility of the catalysts confirmed by H2-TPR and CH4-TPR. According to the results of XPS, with the P content increasing, the Pd2+/Pd ratio increased steadily, however, the catalytic activity of the catalysts increased at first and then turned to decrease. The consequence confirmed that both PdO and metallic Pd species synergistically contributed to the methane combustion, and P doping with proper content could induce the appropriate distribution of PdO and Pd active site pairs, indicating that mixed valent states of Pd with a suitable partition ratio gave rise to higher catalytic performance. The results provided evidence that the nature of the active sites was crucial for the catalytic properties of Pd catalysts during methane reactions, which was in line with the report by Weaver et al..53 On the other hand, if the catalytic performance only depended on the acidity of the catalyst supports, the Pd/0P-OMA(1000) catalyst might the most active catalyst due to the weaker acid strength belonging to weak acid sites and less total acid sites of its corresponding support, while it was not the case. So it could be concluded that the catalytic performances for methane oxidation appeared to be ascribed to the synergetic effect of the distribution of PdO and Pd active site pairs of the catalysts, the reducibility, as well as the acidity properties of the catalyst supports.54 The apparent activation energies (Ea) over Pd/xPOMA(1000) catalysts were calculated at CH4 conversions lower than 20% to avoid the mass


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diffusion limitations from the Arrhenius plots (Figure 9 (B)). As shown in Table 2, Pd/xPOMA(1000) (x = 3, 6) catalysts displayed lower Ea values of 86 and 84 kJ mol-1, respectively, indicating that Ea values could be reduced via the incorporation of appropriate amount of phosphorus, confirming their higher catalytic performances. It was noted that the results of Ea was in good line with the results of catalytic activity.

Figure 9. Initial activity at GHSV = 50 000 mL h–1 g–1 (A) and Arrhenius plots at GHSV = 80 000 mL h–1 g–1 (B) of Pd/xP-OMA(1000) catalysts. Reaction condition: 1 vol% CH4, 5 vol% O2 in N2 as balance gas.

As displayed in Figure S9 (Supporting Information), the variation of CH4 conversion for Pd/xP-OMA(800) catalysts showed a different trend from Pd/xP-OMA(1000) catalysts. It could be observed that the catalytic activity of Pd/xP-OMA(800) catalysts decreased with the increase of phosphorus content, then with the decline of reducibility of the catalysts due to the different metal-support interactions resulting from different crystallization degree of γ-Al2O3. Additionally, it was obvious that the increase of the treatment temperature of support from 800 to 1000 ºC resulted in the enhancement of catalytic properties (Figure S10, Supporting


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Information). Similar change was consistent with that reported in the literature concerning the Pd/γ-Al2O3 system.55 This consequence could be explained by that the crystal structure of Al2O3 treated at high temperature was more stable, which resulted in a less change during the calcination and reaction process of catalysts.55 This behavior was also resulted from the lower acidity properties of catalyst carriers xP-OMA(1000) and higher reducibility of Pd/xPOMA(1000) catalysts. When correlating the catalytic activities with the different pore size distribution of the catalysts, it was reasonable to infer that more obviously bimodal mesostructured Pd/xPOMA(1000) catalysts were more favorable for methane combustion than Pd/xP-OMA(800) catalysts. It has been previously reported the presence of secondary mesoporous pores was usually desirable in catalysis science.56 The smaller pores were helpful to anchor and stabilize metallic nanoparticles with proper sizes. And the larger ones was favorable for molecular diffusion by providing the large pathway for the reactants to contact with the active sites in the small pores, hence contributing to higher reaction activity.56 On the other hand, the Pd/xPOMA(1000) (x = 11–13) catalysts possessing smaller average pore dimensions exhibited lower catalytic activity among the Pd/xP-OMA(1000) catalysts, indicating that the catalytic performance could be enhanced only when the ratio of smaller mesopores to larger mesopores achieved appropriate region. The similar behavior was also found in the Pd/xP-OMA(800) catalysts (x = 11–13). Therefore, it could be concluded that the pore structure properties of the catalysts could be optimized by regulating the synthesis conditions, thereby promoting high catalytic performance. Based on the analysis stated above, the Pd/6P-OMA(1000) catalyst exhibited better lowtemperature activity towards CH4 oxidation, so was selected as a representative in order to


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investigate the stability of this sort of materials. As shown in Figure 10 and Figure S11 (Supporting Information), during the reaction cycles, the full-conversion temperature (T99) of CH4 was decreased to 395 ºC in the second activity testing, exhibiting better catalytic performance than its initial activity in the first run (T99 = 430 ºC). When the second activity testing finished, the temperature of the reactor was fixed at 390 ºC for 20 h to accomplish a longterm stability test, and the CH4 conversion of the catalyst was retained 99% for the entire period, demonstrating its favorable durability. Next, the used catalyst was inspected again in six successive recycling experiments. It could be seen that the catalytic activity was steadily improved, confirmed by that the temperature required for 50% conversion and 99% conversion of CH4 was gradually decreased to 305 and 345 ºC in the 8th cycling test. The enhancement in low-temperature combustion activity might be resulted from the restructuring and coalesce of active nanoparticles during the reaction cycles, as well as the strong metal-support interactions which inhibited the sintering of active sites.57,58 Then the CH4 conversion at 345 ºC was monitored over the catalyst for another 20 h to look for the long-term effect. The result showed that no appreciable decline in the catalytic conversion of CH4 was detected. After the long-term stability test, the catalyst was pre-treated over the flow of 10 vol% H2/Ar at 400 ºC for 30 min, and then the cyclic stability tests continued to be performed. It was worth noting that the catalytic performance of the catalyst was improved obviously (T99 = 321 ºC and 332 ºC, respectively) in the 9th and 10th run. The reason might be that the partial reduction of the PdO phase by H2 re-adjusted the active sites for methane combustion. As evidenced by Figure S12 (Supporting Information), the Pd 3d binding energies and the Pd2+/Pd0 ratio (1.38) over the catalyst after reduction treatment downshifted in comparison with those after the first run of catalytic activity testing. The consequence further indicated that adequate proportion of Pd2+/Pd0


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could contribute to the improvement of methane conversion,51 which matched well with the result mentioned above. Finally, the 11th–13th activity tests were conducted, showing that the catalyst still maintained high catalytic activity and stability. Generally speaking, the Pd/6POMA(1000) catalyst performed well during the long-term and cyclic reaction, which demonstrated its competitive activity for the methane oxidation with comparison to the reported Pd-based catalysts (Table S6, Supporting Information).

Figure 10. Cyclic stability test and Long term stability test over Pd/6P-OMA(1000) catalyst. Reaction condition: 1 vol% CH4, 5 vol% O2 in N2 as balance gas; GHSV of 50 000 mL h–1 g–1.

The influence of CH4/O2 molar ratio on the catalytic oxidation of CH4 (GHSV = 50 000 mL h–1 g–1) was investigated over the used-catalyst Pd/6P-OMA(1000) under 350 ºC (Figure 11(A)). It could be observed that the sample displayed an 88% conversion of CH4 when the CH4/O2 molar ratio is 1:2.5. With the CH4/O2 ratio increased to 1:5, the CH4 conversion reached up to 99%. However, further increase in the O2 concentration led to a slight dip of CH4 conversion (about 93% conversion). The consequence demonstrated that the O2 proportion in the


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mixture affected the CH4 conversion, which might be related to a change of the oxidation state of Pd, resulting in different active phase.1 The result also proved that the active sites could be adjusted under different conditions as those aforementioned.

Figure 11. Effect of the CH4/O2 ratio (A) and gas hourly space velocity (GHSV) (B) on catalytic activity for Pd/6P-OMA(1000) catalyst.

Gas hourly space velocity (GHSV) was also an influence factor in determining the catalytic performance.22 Therefore, the CH4 conversion under different space velocities was investigated over the used-catalyst Pd/6P-OMA(1000) under the condition of CH4/O2 = 1:5. Figure 11(B) exhibited that the full-conversion temperature (T99 = 340, 345, 360, 365 and 368 ºC, respectively) rose gradually with the increase of the feed flow from 20 000 to 60 000 mL g–1 h–1 and the CH4 conversion declined at the same reaction temperature of 340 ºC with increasing GHSV (Figure S13). To evaluate the stability of the catalyst to water, the influence of different amounts of H2O on the CH4 conversion of the Pd/xP-OMA(1000) (x = 0, 6) catalysts was also investigated and


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the results were displayed in Figure 12(A) and (B). The light-off and full-conversion temperatures were not significantly increased in the presence of 3–8 vol% water vapor, indicating that the inhibitory effect of small amount of water vapor could be negligible for the two catalysts. When a high H2O content of 10 vol% was added to the feed stream, the catalytic activities of the catalysts were decreased obviously. The negative influence was also reported over NiO@PdO/Al2O3 catalyst.50 The reason could be attributed to that water adsorbed on the active sites limited both oxygen and methane adsorption, hence inhibiting both oxygen and methane activation, which was previously reported by Weaver et al..59 However, after the introduction of 10 vol% H2O, the increased value of full conversion temperature (T99) for Pd/6POMA(1000) was lesser than that for Pd/0P-OMA(1000). The result indicated that the addition of phosphorus into Al2O3 promoted the catalyst more resistant to deactivation under water vapor atmosphere. The reason might be that P=O groups formed by the dehydroxylation between P-OH groups could rehydrate into P-OH in the presence of water, hence reducing the adsorption of water on the active sites over the catalysts, and then improving the hydrothermal stability of the catalyst. After removal of water, the catalyst Pd/6-POMA(1000) achieved 50% and 99% conversion at 328 °C and 383 °C, respectively (Figure 12C), indicating that the catalytic activity could be recovered. Afterwards, 50 h stability tests were performed at T50 (328 °C) and T95 (376 °C), respectively. The result demonstrated that T50 was quite stable and the methane conversion at T95 fluctuated between 91% and 95% during 50 h on stream reaction (Figure 12 (C)), once again, confirming its high activity stability.


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Figure 12. The curves of CH4 conversion of Pd/0P-OMA(1000) (A) and Pd/6P-OMA(1000) (B) under different amounts of water vapor, the CH4 conversion curve and long term stability test over Pd/6P-OMA(1000) catalyst at T50 and T95 after removal of water (C). Reaction condition: 1 vol% CH4, 5 vol% O2 in N2 as balance gas; GHSV of 50 000 mL h–1 g–1.

4. CONCLUSIONS In conclusion, nano-sized clubbed P-doped ordered mesoporous alumina (OMA) was rapidly synthesized through a modified sol-gel self-assembly approach by adjusting the proportion of solvents to control the hydrolytic rate of alumina salt and the formation rate of gel. The as-obtained OMA materials represented a promising support for preparing highly-dispersed supported palladium catalysts with highly low-temperature catalytic activities for methane combustion. Doping of phosphorus and different thermal treatments enabled to regulate the surface acidity properties of supports, which affected the catalytic performance for CH4 combustion obviously. The distribution of PdO/Pd active species and redox properties were modified own to the incorporation of phosphorus, resulting in the enhancement of low-


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temperature catalytic activities. This kind of catalysts demonstrated an excellent inhibition effect against sintering of palladium nanoparticles, showing good thermal stability. By P addition, the inhibition effect of excess water vapour can be reduced significantly compared to the catalyst without doping. Owning to these favorable properties, the catalysts can be considered as promising candidates for methane catalytic combustion. Overall, this work should stimulate further studies of more active supported Pd/Al2O3 catalysts for methane completed combustion reaction.

ASSOCIATED CONTENT Supporting Information Detailed experimental procedures of CH4-TPR-MS, H2-TPR and NH3-TPD; Lattice cell parameter, XPS full spectra, UV-vis DRS spectra and additional NH3-TPD data of supports; XRD patterns, textural properties, N2 sorption isotherms and pore size distribution curves of catalysts; CH4-TPR-Ms profiles, H2-TPR analysis and catalytic activity of catalysts; Cycling stability test; Performance comparison of some alumina supported palladium catalysts; Effect of gas hourly space velocity (GHSV); XPS spectra of P2p and Pd 3d.

AUTHOR INFORMATION Corresponding author *E-mail: [email protected] (Y. Zheng) *E-mail: [email protected] (L.L. Jiang)


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful to financial support from the National Natural Science Foundation of China (No. 21872027), the Natural Science Foundation of Fujian Province (No. 2018J01669 and 2017J01414), Innovation Foundation of Fujian Province Development and Reform Commission (No. 2016-482).

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Table of Contents Graphic and Synopsis


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Catalytic Activity and Stability over Nanorod-Like Ordered Mesoporous

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