Improved methane oxidation activity of P-doped γ-Al2O3 supported palladium catalysts by tailoring the oxygen mobility and electronic properties

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Improved methane oxidation activity of P-doped g-Al2O3 supported palladium catalysts by tailoring the oxygen mobility and electronic properties Xiaohua Chen a, Yong Zheng b, Yelin Chen a, Yalan Xu a, Fulan Zhong b, Wen Zhang a, Yihong Xiao b,*, Ying Zheng a,** a

College of Chemistry and Materials Science, Fujian Provincial Key Laboratory of Advanced Materials Oriented Chemical Engineering, Fujian Normal University, Fuzhou 350007, Fujian, People's Republic of China b National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, 350002, Fujian, People's Republic of China

highlights

graphical abstract


Cluster nanostructured P-doped ordered

micro-mesoporous

g-

Al2O3 was obtained.
The ultrasonic cavitation effect helped in reducing the reaction time.
P-doping promoted the mobility and reducibility of oxygen species.
The status and electronic structure of palladium species was tuned by P-doping.
The methane oxidation activity of P-containing

catalysts

was

enhanced.

article info

abstract

Article history:

Micro-mesoporous P-doped g-Al2O3 with cluster morphology was obtained via an efficient

Received 25 July 2019

ultrasound-assisted sol-gel process and taken as carrier to construct palladium catalysts

Received in revised form

for methane oxidation. It was revealed that the structure and properties of catalysts were

19 August 2019

significantly influenced by the phosphorus precursors with diverse valence and acidity.

Accepted 30 August 2019

Dissimilar reducibility of surface hydroxyl and oxygen species is observed in the catalysts

Available online 23 September 2019

derived from different phosphorus sources, indicating the difference in the oxygen mobility and the capacity of the catalysts to convert intermediate CO. The behavior of

Keywords:

charge-transfer transition and d-d transition, the transfer ability of electrons from palla-

Electronic structure

dium particles into the antibonding 2p* orbitals of CO, together with the surface acidity and

Methane oxidation

electronic density of palladium species was likewise tailored, which demonstrated the

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Xiao), [email protected] (Y. Zheng). https://doi.org/10.1016/j.ijhydene.2019.08.237 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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Oxygen mobility

metal-support interaction could be tuned, making palladium species behave with diverse

Phosphorus-doped

status and electronic structures. The optimized properties cooperatively provided an

Pd/Al2O3

enhancement in catalytic performance of P-containing catalysts. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Natural gas as a clean fuel has been extensively used in automobiles, power plants and other fields, but the direct emission of the unburned methane in the exhaust gas will contribute to serious greenhouse effect. The complete catalytic oxidation of CH4 is considered as an eco-friendly technology to abate the emissions of the unburned CH4. Among the catalysts designed for CH4 combustion, noble metal-based catalysts, especially palladium based, demonstrate highly catalytic performance and have therefore attracted intensive attention in recent decades [1e6]. The palladium precursor can be supported over various supports, such as Al2O3, SiO2, MgO, ZrO2 and zeolites. However, due to the high surface free energy, the active metal nanoparticles are prone to agglomerating and sintering to form larger particles under high temperatures, resulting in the decline of metal surface area and then the deactivation of metal nanoparticles for supported catalysts [7]. Hence, an increment in catalytic activity and stability by tuning the structures and properties of supported Pd-based catalysts is of great significance. To this aim, various strategies have been widely explored, of which alloying the primary active component with a second metal species is a commonly used approach. The reported systems contain Pd-Pt, Pd-Co, and Pd-Au and so forth [8e10]. Besides in thermos-catalytic, the bimetallic systems have been manifested to contribute to the improved activity and stability in electro-catalysis as well due to the strong synergistic effect [11]. In addition to the development of active component itself, the support nature is also vital, which significantly affects the status and electronic structure of supported palladium particles through the interaction between palladium species and the support; and thus, influences the adsorption and activation of reactants over the catalysts. The optimization strategies of support properties mainly include the hydrophobic modification [12], the incorporation of a foreign element into the support framework [13], the modulation of surface acid-base properties [6,8], and exposure of special crystal planes [14]. Peng et al. demonstrated that the doping of Mg2þ into ZnO support increased the metal-support interaction induced by interfacial electron transfer from ZnO to palladium particles, maintaining the stability of the active components, and hence improved the catalytic activity and stability of the catalyst [13]. He et al. pointed out that the electron density of palladium particles could be increased on the surface basic sites, while, the acidic sites generated electron-deficient surface metallic Pd [15]. Osman and co-workers have observed the addition of TiO2 to Al2O3 and zeolite (ZSM-5) can balance the electrophilicity of Pd0 species and the supply of oxygen, and thus favors the

methane oxidation process [8]. Lou et al. have proposed that the electronic properties of supported Pd can be efficiently modified by the surface acidic sites of H-ZSM-5, which can catalyze the C-H bond activation and correspondingly benefit the activity for CH4 oxidation [6]. Thereby, the nature of the support plays an important part in determining the catalytic performance of the catalysts. Amongst, the doping strategy through which dopant adjusts the properties of support materials and then modifies effectively the activity and stability of host catalysts has been gradually exploited for catalysis [16,17]. And the support properties varied with the difference of the chosen doping elements. The doping of metal atoms has been employed to tailor the performance of porous materials including surface acid-base properties and electronic structure in many researches [8,13]. Non-metals such as nitrogen and phosphorus have also been adopted as a dopant to facilitate the catalytic activity [18,19]. The phosphorus element has five electrons in its outer electronic orbit, which may change the outer electron distribution of the active metal species through electronic interaction. Li et al. reported that the addition of phosphorus into carbon can generate electronic palladium-support interaction by facilitating the electronic transfer from supported palladium to phosphorus; as a result, the activity of Pd/P-C-X catalysts for direct oxidation of COOH was enhanced [19]. The transfer of electron density from supported metal particles to phosphorus has also been observed for nickel supported Pdoped alumina catalysts [20]. In addition, as compared with metal elements, rich valence electrons and high electronegativity of phosphorus element could affect the surface properties of nickel-based catalysts via the formation of a defective and asymmetrical electronic structure [20]. On the other hand, as a structural stabilizer, the incorporation of phosphorus is known to resist crystal phase transformation from g-Al2O3 to a-Al2O3 by hindering the atomic diffusion [21]. Chen et al. supported that the modification of phosphorus was contributed to the stabilization of palladium species through the electronic interaction of phosphorus and palladium [22]. The modification of phosphorus can also effectively modulate the acid-base properties of support materials [23]. Our recent work confirmed that Pd/P-Al2O3 catalysts exhibited enhanced catalytic activity towards conversion of hydrocarbon (HC), CO and NOx due to the improvement of adsorption properties and reducibility through modifying ordered mesoporous alumina (OMA) with phosphorus [24]. Very recently, our group exploited a modified sol-gel method to rapidly synthesize nanorodlike phosphorus-doped OMA, and observed that the surface density of the acidic sites over g-Al2O3 seemed to be one of the important factors in determining the catalytic activity of Pd/ xP-OMA for methane combustion [25]. Hence, phosphorus element is a promising candidate for the dopants of g-Al2O3.

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On the basis of our previous research, a more important progress was attained in this work. The intervention of ultrasonic cavitation effect in the sol-gel system of fabricating phosphorus-doped g-Al2O3 materials not only controlled the hydrolytic equilibrium and the reaction time, but also modulated the morphology. More importantly, over the palladiumbased catalysts constructed with the as-obtained phosphorusdoped g-Al2O3, the reducibility and mobility of hydroxyl groups and active oxygen species could be manipulated by using different precursors of phosphorus modifiers with diverse valences and acidity, and thus altered the methane oxidation activity. What's more, the status and electronic properties of palladium species were revealed in depth by the combined investigation of the ability of charge-transfer transition and dd transition, the strength of Pd-CO 2p* bonding, the redox properties of PdO, the electronic-deficiency of palladium species induced by the surface acidic properties. Results supported the features of palladium species could be tuned through the modification of metal-support interaction by using different phosphorus sources, which synergistically affected the catalytic performance. This work can guide the design of advanced catalysts for methane catalytic combustion.

Materials and methods Samples preparation An ultrasound-assisted sol-gel approach was applied for the preparation of supports. 1.5 g of triblock copolymer P123 (Mav ¼ 5800, EO20P70EO20) was dissolved in 15 mL anhydrous ethanol containing 0.12 mL glacial acetic acid, and then phosphorus precursor with the theoretical content of phosphorus on P-doped alumina being 6 wt% was added. 3.06 g of aluminum isopropoxide was dispersed in 15 mL of isopropanol by sonicating for 30 min, which was slowly added into the above solution. After that, the resultant mixture was sonicated for another 1.5 h, and then transferred to a dish and underwent solvent evaporation at room temperature for 6 h, thermal condensation treatment in a drying oven at 80  C for 8 h. The solid was successively calcined at 500  C for 4 h with a heating rate of 2  C min1 and then 900  C (10  C min1) for 1 h. Three different phosphorus sources (NH4)H2PO4, H3PO3 and H3PO4 have been used as molecular precursors, respectively, and the obtained samples were correspondingly denoted as ADP-OMA, OP-OMA, and P-OMA. The un-doped ordered mesoporous alumina was named as OMA. Taking the as-obtained alumina as support, a palladiumbased catalyst was synthesized by incipient wetness impregnation method with 0.5 wt% of Pd as loading (theoretic value) from Pd(NO3)2 aqueous solution. After impregnation, the catalyst was dried at 60 and 110  C for 5 h, respectively, and then was treated at 500  C (10  C min-1) for 1 h. The assynthesized catalysts were labeled as Pd/ADP-OMA, Pd/OPOMA, Pd/P-OMA and Pd/OMA, respectively.

Catalysts characterization X-ray diffraction patterns (XRD) of the samples were recorded on a Philips X'pert Pro MPD diffractometer using Cu Ka

radiation (1.5406
A, 45 kV and 40 mA). The 2q was scanned in the range of 10e80 for wide-angle XRD (WXRD) and 0.5e5 for low-angle XRD (SXRD), respectively. N2 adsorption-desorption isotherms were measured by using a Micromeritics Tristars 3000 analyzer at liquid nitrogen temperature. Prior to the measurement, the catalysts were vacuumed at 300  C for 3 h. The specific surface was calculated by the Brunauer-Emmett-Teller method; pore volume was determined by the adsorbed amount of N2 at a relative pressure (P/P0) of 0.995; pore size distribution curve was obtained from isotherm desorption branch and Barrett-Joyner-Halenda (BJH) theory; and the average pore radius was estimated by using BJH theory. UV-visible diffuse spectra (UV-DRS) were recorded in the range of 200e800 nm (scanning rate ¼ 200 nm min1) with a PerkinElmer Lambda 950 spectrophotometer, using BaSO4 as the reference substance. Transmission electron microcopy (TEM) examinations were carried out on a FEI Tecnai G2 F20 S-TWIN transmission electron microscope operating at 200 kV. X-ray photoelectron spectroscopy (XPS) measurement was performed using a Thermo ESCALAB 250 spectrometer with monochromatic Al Ka X-ray source (hn ¼ 1486.6 eV). The C 1s peak with the binding energy of 284.8 eV was used as a reference for correcting charging effects. Hydrogen temperature programmed Reduction (H2-TPR) experiments were performed on a Micromeritics Autochem II 2910 instrument equipped with a thermal conductivity detector (TCD) to monitor the consumed H2. A 100 mg sample was pretreated in N2 (30 mL min1) for 60 min at 300  C. After cooling down to room temperature, the sample was exposed to a flow of 10 vol% H2/Ar (30 mL min1) and then heated to 900  C with a heating rate of 10  C min1. CO temperature programmed Reduction (CO-TPR) experiments were carried out on a Micromeritics Autochem II 2910 instrument equipped with a mass spectrometer (Hiden QIC20). Typically, the sample (150 mg) was pretreated with He (30 mL min1) for 60 min at 300  C and then was cooled down to room temperature. Subsequently, a gas flow containing 1 vol% CO/He (50 mL min1) was switched into the system, and the sample was heated to 900  C at 10  C min1. The consumption of CO, and the formation of CO2, H2 and H2O were identified by the signals of m/z ¼ 28, m/z ¼ 44, m/z ¼ 2 and m/ z ¼ 18, respectively. The In situ diffuse reflectance FTIR spectra (DRIFT) were collected from 800 to 4000 cm1 using a Nicolet IS50 FT-IR spectrometer with a high-sensitive MCT detector. All spectra were obtained at a spectral resolution of 4 cm1 (number of scans: 32). The DRIFT spectrum of KBr was used as the background spectrum. Before each DRIFT experiment, the sample was pretreated in an Ar stream at 300  C for 30 min, and then was reduced in a 10 vol% H2/Ar stream at 300  C for 60 min. Subsequently, the system was cooled to 30  C in Ar, and the CO adsorption DRIFTS experiments were carried out at 30  C in a 1 vol% CO/Ar stream for 30 min, and finally the spectra were monitored again after the sample was purged by Ar for 30 min. CO pulse chemisorption and temperature-programmeddesorption (CO-TPD) measurements were performed on a Micromeritics Autochem II 2910 instrument with a TCD detector. Before testing, 100 mg of the sample was purged with

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He (flow rate: 30 mL min1) at 300  C for 30 min, and then prereduced at 300  C for 60 min under 10 vol% H2/Ar (flow rate: 30 mL min1), then the sample was cooled down to room temperature and kept for 60 min to adsorb CO under 5 vol% CO/He (flow rate: 30 mL min1). Finally, the sample was purged by He (30 mL min1) for another 30 min, then the COTPD tests were carried out by heating the catalyst to 900  C at a rate of 10  C min1. The adsorption saturation capacity of CO was recorded by a dynamic pulse method. The Pd dispersion and average particle size of Pd were calculated by using a CO/ Pd average stoichiometry of 1. The procedure of ammonia temperature-programmeddesorption (NH3-TPD) experiment applied to the catalysts is alike CO-TPD analysis. A 50 mg sample was pretreated in He (30 mL min1) for 60 min at 300  C, and then 8.3 vol% NH3/He gas flow (30 mL min1) was fed to the samples for 1 h after cooling to room temperature. Then the sample was flushed by He gas flow (30 mL min1) for another 1 h. Finally, TPD spectra were obtained via linearly raising the temperature from room temperature to 900  C under He gas flow (30 mL min1) at 10  C min1.

Catalytic evaluation The catalysts (30e80 mesh) were tested in a fixed-bed quartz reactor with an internal diameter of 8 mm. A K-type thermocouple which located at the center of the catalyst bed was used to monitor the catalyst temperature. The activity measurement for methane combustion was performed under atmospheric pressure where the reaction mixture was composed of 1 vol% CH4, 5 vol% O2 and N2 (balance) and a total gas hourly space velocity (GHSV) was controlled in 50 000 mL h1 g1 (100 mg of catalyst was used for each experiment). The inlet and outlet gas concentrations were then analyzed online by a gas chromatograph (Agilent 7820A GC) equipped with a thermal conductivity detector. The conversion of methane (c) was calculated by the following equation: c¼

½CH4 in  ½CH4 out  100% ½CH4 out

where [CH4]in and [CH4]out represented the inlet concentration and outlet concentration of CH4, respectively.

Results and discussion Textural and structural properties Powder X-ray diffraction (XRD) analysis of the catalysts is displayed in Fig. S1. The SXRD patterns (Fig. S1A) of all catalysts present a well-resolved (100) peak at around 0.83 indexed to P6mm hexagonal symmetry [25]. The absence of other scattering peaks can be ascribed to the formation of non-uniform micelles caused by the fast formation rate of mesoporous structure. The evidence indicates that all samples possess short-rang ordered pore structure. In comparison with un-modified Pd/OMA, the (100) diffraction peak for Pcontaining catalysts shifts to a higher degree, which suggests that phosphorus entered into the matrix of alumina and

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caused the lattice shrinkage. The WXRD patterns (Fig. S1B) of Pd/OMA show well-defined peaks ascribed to the (111), (311), (222), (400), (511) and (411) reflections of g-Al2O3 [16]. Relative to Pd/OMA, P-containing catalysts presented weaker and fewer diffraction peaks, indicating the decrease of crystallinity of g-Al2O3, which is possibly caused by the structure disorder of alumina due to the incorporation of phosphorus. Especially for H3PO4 derived Pd/P-OMA, the degree of crystallinity is much lower, which may have a negative effect on the construction of metal-support interaction. No diffraction peaks indexed to palladium species are detected possibly due to small-sized Pd-containing phases or low loading amount. The nitrogen adsorption isotherms and the pore size distribution (PSD) are shown in Fig. S2. From Fig. S2A, all catalysts show type-IV isotherms with apparent hysteresis loops, supporting the existence of mesoporous structure [26]. The PSD curves of all samples (Fig. S2B) demonstrate a bimodal property, which is favorable for catalytic reaction for that the internal void space can provide an efficient transport routes for reaction gases [27]. While, the bimodal property is different from the single pore distribution found in P-modified Pd-Ce catalyst [22], indicating that the pore structure is affected by the synthesis method. Comparatively, the average pore diameter of Pd/P-OMA is smaller among the samples (Table S1). On the whole, in comparison with the un-modified Pd/ OMA, the addition of phosphorus when using (NH4)H2PO4 and H3PO3 as precursors results in the increase of surface area, which is significantly dropped for H3PO4 derived catalyst Pd/POMA. The substantial decline of textural properties for Pd/POMA may be caused by the formation of phosphate species blocking the pore channels, although which is not detected in the WXRD patterns likely due to its high dispersion and amorphous form. The large surface area can provide better dispersion of metal nanoparticles and strong metal-support interface interaction, which is beneficial to preventing nanoparticles from sintering [28]. As confirmed from CO pulse chemisorption in Table S1, the palladium nanoparticles on Pd/ADP-OMA and Pd/OP-OMA can be more homogeneously dispersed than those on Pd/OMA; while, Pd/P-OMA displayed the poorest dispersity of palladium particles. Moreover, it can be seen that P-addition from (NH4)H2PO4 and H3PO3 precursors contributes to maintain higher exposed metal surface area, which may help to reduce the energy needed for the C-H bond activation elementary step to form related transition states [29]. In general, higher palladium dispersion and larger exposed metal surface area of the catalysts are favorable for their catalytic activity [30]. It can be reasonably judged that Pd/ADP-OMA and Pd/OP-OMA will demonstrate higher catalytic activity. Among the catalysts, Pd/OP-OMA presents better textural properties, so OP-OMA is selected as a representative carrier to investigate the pore structure by using TEM (Fig. 1A). OPOMA prepared through an ultrasonic-assistant sol-gel approach exhibits a cluster-like structure (Fig. 1A(a)), which is different from the nanorod-like structure of 6P-OMA obtained with a conventional sol-gel approach using magnetic stirring in our previous work [25]. The different morphologies detected in the supports of Pd/OP-OMA and Pd/6P-OMA probably have an effect on the performance of mass and heat transfer, and then may be one of the factors for their different catalytic

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Fig. 1 e Representative TEM and HRTEM images of (A) OP-OMA prepared with H3PO3 as a phosphorus precursor: (a) cluster structure, (bed) ordered pore structure, (e) lattice fringes of g-Al2O3; (B) Pd/P-OMA catalyst. activity. The parallel ordered channels are clearly visible (Fig. 1A(b)e(d)), associated with the SXRD observation. The average diameter of pores exposed is slightly smaller than 2.0 nm (Fig. 1A(c),(e)), indicating that a certain amount of micropores exist in the sample. The lattice fringes of g-Al2O3 can be detected in Fig. 1 A(e), demonstrating the crystallization of the framework. Fig. 1B presents TEM and HRTEM images of the Pd/P-OMA catalyst. It can be seen that palladium nanoparticles with dimensions of around 7.8 nm exist in the catalyst, which is almost in consistent with the result of CO chemisorption. The lattice fringe spacing of 0.26 nm is attributable to the (101) crystal plane of PdO [31].

Catalytic activity for CH4 oxidation Fig. 2 and Table S2 present the conversion of CH4 over all catalysts, indicating the catalytic activity is dependent on the type of phosphorus precursors. From Fig. 2A, as compared with Pd/OMA (T90 ¼ 465  C), the higher initial catalytic activities are obtained with Pd/ADP-OMA and Pd/OP-OMA, which demonstrate a lower T90 of 430 and 440  C, respectively. While,

Pd/P-OMA presents the poorest activity with its T90 being 500  C among the four catalysts. The change trend in catalytic activity is consistent with that in palladium dispersity and exposed metal surface area, confirming these factors are vital in influencing the catalytic performance for CH4 oxidation as inferred from the above. In order to understand the relationship between the catalytic properties and the palladium active sites on the as-synthesized catalysts for CH4 oxidation, turnover frequency (TOF, defined as the number of converted CH4 molecules per exposed active palladium site per second) of the catalysts is measured at a CH4 conversion lower than 15% under 330  C. From Table S2, TOF values (13e31  103 S) increase monotonically with the increase of dispersion. Higher TOF values are obtained on Pd/ADP-OMA and Pd/OP-OMA, confirming the increased accessibility of the active sites by reactant molecules over these catalysts, which contributes to the increase in catalytic activity [32]. The cycle of CH4 oxidation reaction is then iterated under a parallel reaction condition (Fig. 2B, Table S2). It results that all catalysts demonstrate a significantly enhanced activity with the T90 being reduced by 20e40  C in the second run, which

Fig. 2 e Light-off curves of CH4 oxidation over different P-doped alumina supported palladium catalysts in the first run test (A), in the second run test (B), and at 370  C with time on stream (C). (1 vol % CH4 þ 5 vol % O2 balanced with N2, GHSV ¼ 50 000 mL h¡1 g¡1).

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can be ascribed to the electronic rearrangement of active components over the catalysts. The conclusion will be validated through UV-Vis DRS, H2-TPR and XPS experiments next. And it therefore suggests that activation takes place over all catalysts under cyclic process examined herein. The similar phenomenon has also been elucidated by Demoulin et al. [33] and Hong et al. [2] in CH4 oxidation reaction over Pd/Al2O3 and Pd/ZrO2 catalysts. To further investigate the activation behavior, the isothermal reaction over all catalysts after the second run is conducted for 30 h at 370  C. From Fig. 2C, the CH4 conversion on Pd/OMA gradually increases with time; while, for the Pcontaining catalysts, the CH4 conversion tends to increase at the preceding stage and then almost keep constant over time. The result demonstrates that different activation phenomenon occurs between the unmodified Pd/OMA and the P-containing catalysts in the later stage. For all this, the reactivity order of the catalysts is not changed at 370  C.

Analysis of UVeVis DRS spectra The chemical structure of the catalysts is investigated by UVevisible spectra. As shown in Fig. 3A, the absorption band at about 209 nm is assigned to the charge-transfer transition of O2 to Pd2þ belonging to finely dispersed PdO particles [34], the broadband in the region of 320e540 nm is related to the dd transitions [35]. For Pd/ADP-OMA and Pd/OP-OMA, the adsorption bands migrate slightly toward lower wavelengths and the intensities decrease with respect to Pd/OMA, indicates that the ability of charge-transfer transition is weakened and the band gap energy is enhanced due to the enhancement of metal-support interaction [36,37]. However, the peak position of Pd/P-OMA exhibits an obvious red shift and the adsorption band intensity grows compared with that of Pd/OMA, corresponding to its lower gap energy. On the basic of quantummechanical theory, the band gap energy is inversely proportional to the nanoparticle sizes [37]. Namely, the particle size of palladium species is decreased in (NH4)H2PO4 derived catalyst Pd/ADP-OMA and H3PO3 derived catalyst Pd/OP-OMA, while the addition of phosphorus from H3PO4 cause the increase of nanoparticle size. The finding is in accord with that of CO chemisorption. The evidence indicates the electronic metal-support interaction can be mediated by using different

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phosphorus precursors, and thus affects the electronic structure of palladium species. As shown in Fig. 3B, all the catalysts after the first catalytic run present a new adsorption band at about 280 nm associated with PdO charge transfer. And the broadband attributed to d-d transition of PdO has been widened in comparison with that before reaction. The different results of UVeVis DRS spectra before and after reaction support the electronic rearrangement of palladium species during the reaction process, hence leading to the improvement of catalytic activity in the second run.

Redox properties H2-TPR characterizations are further performed to demonstrate the metal-support interaction and redox properties of the impregnated PdO species. From Fig. 4A and Table S3, the TPR profiles of all catalysts before reaction show a broadened H2 consumption peak at 110e180  C, representing the reduction of stable PdO species [38]. In comparison with that on Pd/ OMA, the reduction peak of PdO on Pd/ADP-OMA and Pd/OPOMA shifts toward lower temperature region, indicating higher reducibility of PdO species which derives from the higher palladium dispersion due to the stronger metalsupport interaction, and thus leads to superior initial catalytic activity. While, the reduction of PdO by H2 for Pd/P-OMA occurs at a higher temperature, corresponding to the poorer redox property and initial activity. Since the electronic structure of metal particles is related with the dissociation H2 adsorption capability of the corresponding metal catalyst [30]. The result reveals that P-doping from different phosphorus precursors modifies the electronic structure of palladium, which adjusts the catalytic performance. In addition, the higher-temperature reduction peaks at 300e450  C and 700e900  C may be assigned to the reduction of surface oxygen and subsurface oxygen [39]. It can be found the reducibility of oxygen species and PdO reducibility appear the similar variation tendency. As indicated in Fig. 4B, each catalyst after the first cycle shows one desorption peak of H2 at ~80  C assigned to the dissociation of PdHx [11,30]. The evidence indicates that PdO species have been reduced below ambient temperature, which is associated to their improved reducibility after the first catalytic run. It confirms that the electronic structure of palladium species undergoes

Fig. 3 e UV-Vis DRS spectra of palladium supported catalysts (A) before reaction and (B) after the first catalytic run.

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Fig. 4 e H2-TPR profiles of palladium supported catalysts (A) before reaction and (B) after the first catalytic run. readjustment during the reaction process, therefore enabling the improvement of the catalytic performance for all catalysts in the second reaction run. The reducibility of the catalysts is further evaluated by COTPR technique (Fig. 5), which is considered as one of the best probes for the characterization of surface oxygen/hydroxyl species [40]. CO-TPR experiment is also performed on pure alumina support for comparison (Fig. S3). In the case of Pd/ OMA (Fig. 5A), a shoulder CO consumption peak centered at around 452  C is detected, accompanied by the simultaneous production of CO2 and H2. The result involves in the reaction between the adsorbed CO and oxygen/hydroxyl species of the support (COads þ 2OH(support) / CO2 þ H2 þ OL, COads þ 2OL / CO2) [40]. In addition, the appearance of small amount of water can be attributed to hydrogen carbonate decomposition [41]. It can be detected that bare Al2O3 support exhibits higher temperature reduction behavior (>600  C), accompanied by a smaller amount of CO consumption and CO2 formed. The result confirms that the reduction of oxygen/hydroxyl species must have been enhanced by the addition of palladium species over Pd/OMA catalyst in comparison with bare Al2O3, which coincides with that reported in the literature [42]. As displayed in Fig. 5B and Fig. S3, with respect to Pd/OMA, the reduction peaks of Pd/ADP-OMA and Pd/OP-OMA shift to a lower temperature, while positively shifts for Pd/P-OMA, which is in coincided with the result of H2-TPR (Table S3). The result reveals that the reducibility of oxygen/hydroxyl

species is improved on Pd/ADP-OMA and Pd/OP-OMA, indicating the increased oxygen mobility caused by the strengthened metal-support interaction [43]. The consequence is just opposite in the case of Pd/P-OMA. It is acknowledged that CH4 combustion follows the Mars-van Krevelen (M-vK) redox-type mechanism, involving that PdO is first reduced by CH4 to produce oxygen vacancies at the PdO surface and then metallic Pd was re-oxidized by oxygen species from the support or from the gas phase [2,44]. Hence, the activity of oxygen species from the support, and the oxygen exchange rate between support and PdO particles play a vital role in catalytic activity for CH4 oxidation. It therefore signifies that the higher oxygen mobility of Pd/ADP-OMA and Pd/OPOMA than the other two catalysts can be considered as one of the factors in accelerating their catalytic activity. On the other hand, CO can be formed as a reaction intermediate during the catalytic combustion of CH4, hence more active oxygen/hydroxyl species which are available in Pd/ADP-OMA and Pd/OP-OMA are conducive to the rapid conversion of CO to CO2 from surface/active sites, which is also likely responsible for their better catalytic activity.

In situ infrared spectra of chemisorbed CO and CO-TPD Fig. 6A and Table 1 display the CO-DRIFT spectra of the catalysts at ambient temperature. All catalysts show a band at about 2167 cm1, which is assigned to the weak adsorption of

Fig. 5 e CO-TPR-MS profiles for (A) Pd/OMA and (B) all palladium supported catalysts.

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gaseous CO [22,45]. The band at 2085 cm1 and 1913 cm1 can be attributed to linearly adsorbed and threefold hollowbonded CO on metallic Pd0 sites, respectively [46]. The migration in the position of CO adsorption band is usually considered as evidence of the alteration in the electronic structure of supported metals. It clearly comes out that the frequencies of both chemisorbed CO on P-containing catalysts are slightly shifted to a higher wavenumber with respect to Pd/OMA. The phenomenon is related to a weaker electron back-donation from atomic palladium particles into the antibonding 2p* orbitals of the C-O bond and a weaker CO adsorption energy, thanks to the withdrawing of the Pd 3d electron density induced by phosphorus doping [19]. Contrastively, a shift of the CO adsorption bands to higher wavenumbers is much more evident on Pd/P-OMA, which may be caused by the aggregation of Pd particles with the poorer dispersity [47]. From Fig. S4, after removing CO from the feed gas and purging by argon for 30 min, the adsorption peak at 2170 cm1 faded away, which may be due to the relatively low enthalpy adsorption, resulting in easier desorption of adsorbate. However, the bands for linearbonded and threefold hollow-bonded CO are still retained, owing to high enthalpy adsorption on Pd0 active sites. The observation above is further confirmed by CO-TPD analysis. As displayed in Fig. 6B, two well-defined peaks centered around 150 and 410  C for all tested catalysts can be ascribed to the desorption of CO molecules in Pd-linear sites and Pd 3-fold sites [48]. In the case of P-containing samples, the desorption of CO species arises at a lower temperature when compared with Pd/OMA, demonstrating the weakening of Pd-CO 2p* bonding. The desorption temperature decreases in the sequence of Pd/OMA, Pd/ADP-OMA, Pd/OP-OMA and Pd/ P-OMA, which is in line with the change order of the adsorption peak migration shown in Fig. 6A. From Table 1, the amount of adsorbed CO on Pd/ADP-OMA and Pd/OP-OMA catalysts is relatively higher than that on Pd/P-OMA and Pd/ OMA, indicating more accessible active sites on Pd/ADP-OMA and Pd/OP-OMA. From Fig. 6A, the bands detected at ca. 1452 cm1 for Pd/ OMA and Pd/OP-OMA, and 1638 cm1 for all catalysts may be attributable to surface bidentate carbonate-type species, which are created upon the interaction of oxygen species with chemisorbed CO [31,49]. A broad absorption band at 1068 cm1

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for Pd/OMA originates from the stretching vibration of Al-O-H bending modes [31], which is shifted to lower wavenumber on P-containing catalysts. Additionally, P-modified samples show a new band centered in the region of 1170e1250 cm1, whose position and intensity varies somewhat with the phosphorus precursor. In the case of Pd/ADP-OMA and Pd/OPOMA, the absorption at 1165 and 1190 cm1 can be assigned to the vibration of P-O single bond; whereas the band at 1250 cm1 with strong adsorption intensity for Pd/P-OMA is the characteristic vibration of double-bonded P¼O groups [50]. The finding confirms the formation of phosphate species in Pd/P-OMA. Phosphate is not detected by XRD analysis, but its presence is verified by In situ-DRIFT measurement. And the coverage of phosphate on the alumina surface together with the low crystallinity of g-Al2O3 interferes with the metalsupport interaction, and thus lowers the catalytic activity of Pd/OMA catalyst.

XPS analysis The surface chemical states of the four catalysts are investigated by XPS. Fig. S5 depicts the Al 2p and P 2p XPS spectra of the samples. The symmetric peak of Al 2p at about 74.0 eV is attributed to Al3þ species in Al2O3 [25]. The peak of P 2p located at a binding energy of around 133.7 eV can be assigned to P5þ species [25]. For Pd/OP-OMA catalyst, it is noteworthy that although phosphorus exists in the form of P3þ in the raw material of the synthesized P-doped alumina, it has been transformed into P5þ ion during the process of drying and calcination. The similar phenomenon that low-valence phosphorus is oxidized into high-valence phosphorus during calcination has also been reported in the literature [22]. As displayed in Fig. S5(A), the Al 2p signals of P-containing catalysts are positively shifted relative to the unmodified catalyst Pd/OMA, which can be explained by that the more electronegativity of phosphorus than that of aluminum induces the decrease of the outer electron density in the nucleus of aluminum atom, and then the increasing of shielding effectiveness [25]. From Fig S5(B), it can be detected that the binding energies of P 2p vary with phosphorus precursors, indicating that the type of phosphorus sources affects the bonding state of phosphorus. Amongst, Pd/ADP-OMA and Pd/ OP-OMA present higher BE of P 2p (133.7 and 133.8 ev), which

Fig. 6 e (A) In situ-DRIFT spectra of CO adsorbed over palladium supported catalysts for 30 min and (B) CO-TPD profiles of palladium supported catalysts.

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Table 1 e Adsorption properties of the synthesized catalysts. Sample

Pd/OMA Pd/ADPOMA Pd/OPOMA Pd/P-OMA a b

Adsorption of gaseous COa (cm1)

Pd0eCOa (cm1)

Pd03eCOa (cm1)

Bidentate carbonate-type speciesa (cm1)

AleOa (cm1)

PeOa (cm1)

Quantity of adsorbed COb (umol g1)

2167 2170

2085 2092

1913 1918

1641、1459 1639

1068 1010

e 1165

15.2 18.9

2172

2095

1920

1634、1459

983

1190

17.4

2175

2112

1925

1634

963

1250

7.6

Obtained from CO-DRIFT spectra. Obtained from CO pulse chemisorption (CO/Pd ¼ 1).

can be assigned to stronger P-O bonding strength deriving from the stronger interaction between P, Al, O and Pd. This strong interaction is conductive to the enhancement in catalytic activity. Nevertheless, in comparison with Pd/ADP-OMA and Pd/OP-OMA, a shift of P 2p towards lower BE is observed on Pd/P-OMA (133.5 ev). This shift is attributed to the weakening of interaction between palladium and P-O-Al units, which is caused by the inhibiting effect of the formation of surface phosphate species. XPS spectra of O 1s and Pd 3d, which are de-convoluted into different oxygen species and palladium oxidation states for the catalysts before reaction are displayed in Fig. 7. The peaks of O 1s spectra at around 530, 531 and 532 eV are ascribed to the lattice oxygen (OI), surface-adsorbed oxygen (OII), and hydroxyl groups or adsorbed water (OIII) [11]. The results show that the concentration of oxygen species varies with precursors (Table S4). Among the oxygen species, the OII species has been reported to play an important part in the combustion reaction of CH4, for it exhibits higher oxygen mobility and can take part in the activation of CH4 [44,51]. In comparison with that for un-modified Pd/OMA, the content of OII species for Pd/ ADP-OMA and Pd/OP-OMA is significantly increased; while the corresponding value for Pd/P-OMA is decreased arising from the reducing specific surface area. As for the Pd 3d spectra of Pd/OMA, the two peaks centered at ca. 335.8 and 341.0 eV, are corresponding to Pd0 3d5/2 and 3d3/2; and the other two ones

appearing at ca. 337.5 and 342.8 eV can be ascribed to Pd2þ 3d5/ 2 and 3d3/2 [36]. These values match well with those previously reported [52]. All the binding energies of Pd 3d5/2 species in the P-containing catalysts exhibit a positive shift with respect to that of Pd/OMA (Table S5), further indicating the electron transfer from the Pd nanoparticles to the support. Chen et al. also demonstrated that the shift of binding energies is correlated with the alteration of atomic structures [52]. In addition, it is known that the binding energies of supported palladium nanoparticles migrate to lower values upon increasing size [19]. Hence, in comparison with Pd/ADP-OMA and Pd/OPOMA, the lower binging energy shift detected in Pd/P-OMA can be explained by its larger size of palladium particles. The results demonstrate the different interaction strength between the support and the Pd NPs which agrees well with that obtained from UV-Vis DRS, H2-TPR and CO-DRIFTS analyses, complementarily proving that phosphorus affects the electronic structure of palladium species. In comparison with that of Pd/OMA, the Pd2þ/Pd0 ratios of P-containing catalysts are increased. Correlation with the activity results, it is clear seen that Pd/ADP-OMA has an optimal atom ratio of Pd2þ/Pd0 and higher concentration of OII species, which cooperatively contributes to its higher activity for CH4 combustion. After the first run testing, the valance states of palladium species in all catalysts remain unchanged (Fig. S6A). It means that PdO and Pd0 phases are considered as the catalytically active sites

Fig. 7 e XPS spectra of O1s (A) and Pd 3d (B) for all catalysts before reaction.

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well as available surface hydroxyl groups arising from the coverage of phosphate species.

The discussion of catalytic mechanism

Fig. 8 e NH3-TPD profiles of palladium supported catalysts.

throughout the reaction in CH4 combustion. Usually, Pd0 sites are needed to adsorb CH4, while PdO sites are required to oxidize methane in the mechanism of methane oxidation reaction [8]. Additionally, the Pd2þ/Pd0 ratios of all samples are changed after the first catalytic run as compared with those before reaction (Fig. S6B), leading to their higher catalytic activity. This once again confirms the redistribution of palladium nanoparticles and alteration in electronic structure of palladium species during the reaction process.

Surface acidity properties For gaining more insights into the effect of phosphorus doping from different phosphorus sources, the surface acidity of the catalysts is characterized by NH3-TPD (Fig. 8). All TPD curves present two desorption peaks centered at around 120  C and 425  C, which can be corresponding to the desorption of NH3 adsorbed on weak and strong acidic sites, respectively [27,53]. From Table S6, it can be seen that the total acidity of Pd/ADPOMA and Pd/OP-OMA is increased in comparison with that of Pd/OMA, which indicates that more acidic sites are formed due to the addition of phosphorus from (NH4)H2PO4 and H3PO3 into g-Al2O3. However, the acidity on Pd/P-OMA is the lowest among the four catalysts. And it is known that palladium is positively charged by the acidic sites, namely, acidic sites tend to generate an electron-deficient surface [15]. Therefore, the higher positive shift of binding energy in Pd/ADP-OMA and Pd/ OP-OMA confirmed by XPS analysis is also probably associated with these stronger acidic sites. When the phosphorus precursors were added before the formation of alumina gel framework, phosphorus element may be selectively located in the lattice or on the surface of alumina, hence tuning the coordination structure of Al3þ and properties of surface OH groups. Since, the acidic sites on the Al2O3 support are related to surface defects with accessible Al3þ cations and surface hydroxyl groups [54]. In the case of Pd/ADP-OMA and Pd/OP-OMA catalysts, the generation of POH groups corresponding to medium-strong acid sites may contribute to the increase in the number and strength of acid sites. However, the decline of surface acidity on Pd/P-OMA can be attributed to the decrease of exposed alumina atoms as

The P-containing catalysts basically follow the M-vK redoxtype pathways, furthermore, the multifunctional properties of the catalysts especially in terms of the oxygen mobility and functional P-OH groups play an important role in catalytic process. The CH4 molecules are first adsorbed and dissociated on a site pair of Pd*/Pd-O (Pd* stands for an oxygen vacancy) to yield Pd-OH and Pd-CH3 [55,56]. Subsequently, the condensation of OH groups generates surface vacancy and H2O, which is followed by the step involving in the regeneration of PdO [57]. The adsorbed methane (PdCH3) reacts with the oxygen species to produce CO2 and H2O. It is essential that the incorporation of phosphorus promotes the oxygen change between Al2O3 and O-vacancy by accelerating the oxygen mobility, facilitating the desorption of OH groups and re-oxidation rate of the vacancy, and reducing the re-adsorption of H2O on the vacancy. In addition, the addition of phosphorus into alumina gives rise to the generation of P-OH groups. The recombination of P-OH yields P¼O groups during the CH4 oxidation reaction, which can be rehydrated into P-OH groups by H2O, and hence decreases the adsorption of H2O on the adsorption sites as well [25]. The improved electronic metal-support interaction invoked by the modification of phosphorus endows the Pcontaining catalyst with larger exposed metal surface area and more accessible active centers for CH4 adsorption. All these boost the catalytic activity of Pd/g-Al2O3 for CH4 oxidation.

Conclusions Cluster-like P-doped ordered micro-mesoporous g-Al2O3 was prepared using sol-gel process based on an ultrasonic-assisted approach. The ultrasonic cavitation effect was confirmed to provide advantage in promoting the hydrolytic equilibrium of alumina salts and reducing the reaction time significantly. The modification of phosphorus from (NH4)H2PO4 and H3PO3 made active hydroxyl groups and oxygen species behave with higher reducibility and mobility, which contributed to the oxygen exchange between PdO and support, and the removal of intermediate CO, thus caused higher activity in CH4 oxidation. As evidenced by the blue shift of CO adsorption frequency, the decreased ability of charge-transfer transition and d-d transition, and more electronic-deficiency of palladium species induced by stronger surface acidic property, the incorporation of phosphorus from (NH4)H2PO4 and H3PO3 simultaneously favored the establishment of stronger metalsupport interaction. This not only gave rise to higher palladium dispersion, larger exposed metal surface area and superior reducibility of PdO, but also to more suitable distribution of palladium species over the P-modified catalysts, in line with the optimized status and electronic properties of palladium species, which cooperatively amplified the catalytic performance.

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Acknowledgements 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 2017J01408).

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.237.

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