H-ZSM-5: Active Pd

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Low-temperature methane combustion over Pd/H-ZSM-5: active Pd sites with specific electronic properties modulated by acidic sites of H-ZSM-5 Yang Lou, Jian Ma, Wende Hu, Qiguang Dai, Li Wang, Wangcheng Zhan, Yanglong Guo, Xiao-Ming Cao, Yun Guo, Peijun Hu, and Guanzhong Lu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01801 • Publication Date (Web): 19 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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Low-temperature methane combustion over Pd/HZSM-5: active Pd sites with specific electronic properties modulated by acidic sites of H-ZSM-5 Yang Lou‡, Jian Ma‡, Wende Hu, Qiguang Dai, Li Wang, Wangcheng Zhan, Yanglong Guo, Xiao-Ming Cao*, Yun Guo*, P. Hu and Guanzhong Lu Key Laboratory for Advanced Materials and Research Institute of Industrial Catalysis, School of Chemistry & Molecular Engineering, East China University of Science and Technology, Shanghai, 200237, P. R. China

ABSTRACT: Pd/H-ZSM-5 catalysts could completely catalyze CH4 to CO2 at as low as 320 oC while there is no detectable catalytic activity for pure H-ZSM-5 at 320 oC and only the conversion of 40% could be obtained at 500 oC over pure H-ZSM-5. Both of the theoretical and experimental results prove that surface acidic sites could facilitate the formation of active metal species as the anchoring sites, which could further modify the electronic and coordination structure of metal species. PdOx interacting with the surface Brönsted acid sites of H-ZSM-5 could exhibit Lewis acidity and lower oxidation states as proved by the XPS, XPS valence band, CO-DRIFTS, Pyridine FT-IR and NH3-TPD. The density functional theory calculations suggest PdOx as the active sites for methane combustion is in the form of [AlO2]Pd(OH)-ZSM-5. The stronger Lewis acidity of coordinatively unsaturated Pd and the stronger basicity of oxygen from

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anchored PdOx species are two key characteristics of the active sites ([AlO2]Pd(OH)-ZSM-5) for methane combustion. As a result, the PdOx species anchored by Brönsted acid sites of H-ZSM-5 exhibit high performance for catalytic combustion of CH4 over Pd/H-ZSM-5 catalysts.

KEYWORDS: Methane combustion, Pd, H-ZSM-5, Surface acidity, Lewis acidity

Introduction The natural gas has been widely used in power generation and other heating applications due to its large quantities throughout the world and high calorific value. Methane (CH4) as the main component of natural gas has higher than 20 times greenhouse effect of CO2[1]. Hence, the incomplete combustion of natural gas not only releases of methane and lead to the waste of energy, but also produces atmosphere pollution. Compared with conventional flame combustion, the catalytic combustion of CH4 could increase combustion efficiency and provide ultra low emissions of air pollutants, such as CO, NOx, and unburned hydrocarbon[2, 3]. Because CH4 has very stable and highly symmetrical structure, how to decrease ignition temperature and promote the activation of CH4 at low temperature is a big challenge. There is the general consensus that Pd-based catalysts are considered as the most active catalysts for CH4 combustion and activation[2-6]. For CH4 combustion over Pd-based catalysts, the state of Pd species including dispersion, coordination and electronic structure[7, 8], properties of support[3] and the interaction between the Pd and support determine the catalytic activity. Such as, Pd species with +2[9,

10]

and +4

valence[11] are considered as the active species and metallic Pd is inactive. The electronic metal support interaction (EMSI) between Pd and supports like metal oxides (MOs) could not only


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stabilize and downsize the Pd particles but also modulate the properties of Pd particles[1]. In general, the large surface area, fine particle size and/or exposing specific crystal plane of metal oxides are considered to play the crucial roles in modifying the chemical and/or electronic structure of supported noble metal[12-16], which could affect catalytic activity of Pd/metal oxides for CH4 combustion significantly. Generally, considering the poor redox ability of zeolite, the catalytic performance of zeolite catalysts for methane combustion is attributed to the surface acidity[17-20]. Sommer et al.[18] and Truitt et al.[21] propose strong solid acids/Brönsted acidic sites could directly protolyze the C-H bond and therefore activate short-chain alkanes at low temperature. Lu and his co-workers propose that the [AlO]Pd2+ sites could catalyze the heterolytic dissociation of methane[20]. Okumura et al.[22] reports that the surface acid sites could benefit the anchoring of PdO species, which reveals that the acid property of zeolite plays an important role in determining the properties of supported Pd, such as dispersion and oxidation state. However, whether the surface acid sites of zeolites could directly involve the methane combustion or not is still unclear from the molecular level. Meanwhile, the comprehensive study on the acidic and electronic properties of supported Pd for methane combustion is still seldom from the aspects of density functional theory (DFT) calculation or experiment. In order to identify the roles of electronic and coordination structure of supported Pd in the activation of methane, a series of Pd/H-ZSM-5 catalysts are synthesized under different conditions. Our comprehensive analysis of DFT and experimental results confirms that surface acid sites of H-ZSM-5 are not involved in the catalytic reaction but could efficiently modulate the electronic and coordination structure of Pd species, which enables Pd/H-ZSM-5 catalysts to behave fabulous catalytic activity and long-term stability for methane combustion at low


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temperature. 2. Experimental Section 2. 1 Catalyst preparation The different Pd/H-ZSM-5 samples were synthesized by deposition methods[23]. Typically, 1.0 g H-ZSM-5 powder (Zeolyst®, SiO2/Al2O3 = 80:1, activated at 600 oC in air for 6 h) was dispersed in 200 mL deionized water in a flask under vigorous stirring. 0.0167g PdCl2 (SigmaAldrich, > 99.9%) was dissolved in 2 mL 0.1 M HCl solution and then diluted to 20 mL with deionized water. 0.2g NaOH (Alfa Aesar, 98%) was dissolved in 50 mL deionized water, respectively. The separated PdCl2 and NaOH solution were simultaneously dropped into the HZSM-5 aqueous solution. After the deposition of PdCl2 on the H-ZSM-5 powder, the mixture was further aged for 2 h. During the whole process of preparation, the pH of the solution was kept 6.0, 7.0, 8.0, and 9.0 (by controlling the amount of NaOH) respectively, to adjust the existing forms of PdClx(OH)y[24, 25]. After filtering, the samples were dried at 80 °C for 24 h and then calcined at 400 °C in air for 2 h. The prepared Pd/H-ZSM-5 catalysts were denoted by Pd/H-ZSM-5 (x), where x represented the deposition pH value. In addition, a series of H-ZSM-5 samples were synthesized following the same method as preparing the above catalysts of Pd/H-ZSM-5 under different four pH values (pH = 6.0, 7.0, 8.0, and 9.0) except adding PdCl2, which were correspondingly named as H-ZSM-5 (x), where x represented the deposition pH value. In order to distinguish the H-ZSM-5 (x) catalysts, the HZSM-5 sample without any treatment was named as pure H-ZSM-5. 2. 2 Evaluation of the catalytic performance The catalytic activity of Pd/H-ZSM-5 and H-ZSM-5 samples for CH4 combustion was tested in a quartz fixed-bed reactor (Ф 3.5 mm) at atmospheric pressure and 200 mg of catalyst (40 - 60


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mesh) was used. The feed gas with 1 vol.% CH4 and 20 vol.% O2 balanced with N2 passed through the catalyst bed at a flow rate of 50 ml/min. The temperature of catalyst bed was programmed from 150 oC to 800 oC at the rate of 4 oC/min and the conversion of CH4 was measured at a temperature interval of 20 oC by an on-line gas chromatograph (Aglient GC 7890A). The stability of Pd/H-ZSM-5 (8.0) for methane combustion at 290 oC was evaluated for 100 h under the same conditions. After the stability test of 100 h, the activity of used catalyst was evaluated again. The high temperature stability test of Pd/H-ZSM-5 (8.0) for methane combustion was evaluated under the same condition. The conversion of CH4 was measured from 150 oC to 850 oC with the heating rate of 4 oC/min, and then cooling to 150 oC at the same speed. After that, another 2 cycles were repeated, and the conversion of CH4 was detected. 2. 3 Catalytic Characterization 2. 3. 1 Structural Characterization (XRD) The step-scanning X-ray diffraction patterns were recorded on a Bruke D8 focus diffraction spectrometer using a Cu Kα radiation (1.54056 Å, 40 kV and 40 mA), scanning from 5 to 80o with a speed of 10 s/step, and the step size was equal to 0.02o. The average crystalline sizes of catalysts were calculated by the Scherrer equation. 2. 3. 2 Temperature-Programmed Desorption of NH3 (NH3-TPD) NH3-TPD experiments were carried out using a quartz tube, manufactured by our laboratory, equipped with a thermal conductivity detector (TCD). Firstly, 100 mg of the catalyst was pretreated under Ar (30 ml/min) at 400 oC for 1 h. After cooling to 100 oC, the catalysts were saturated with pure NH3 flow for 15 min, and then the catalyst was purged with Ar for 30 min. Afterwards, the catalyst was heated to 600 oC at the rate of 10 oC/min in Ar (30 ml/min).


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2. 3. 3 Pyridine-FT-IR Pyridine-adsorbed FT-IR spectra was carried out on a Nicolet Model 710. Firstly, 50 mg of the catalyst was grounded and pressed into a thin wafer with a diameter of 16 mm and then placed in a quartz IR cell (manufactured by our laboratory) equipped with a CaF2 window in a vacuum system. The catalysts were in-situ treated at 350 oC for 1 h under vacuum, then cooled to room temperature. After that, the pyridine vapor was introduced into the cell for 1 h, And the physically adsorbed pyridine was pumped out by evacuating for another 1 h. Finally the sample was heated to 50, 150, 250 and 350 oC in vacuum for 1 h, and a spectrum was recorded at each set temperature. 2. 3. 4 XPS Valence Band Spectra The valence band photoemission spectra were conducted on a Thermo ESCALAB 250 using a mono-chromated Al Kα X-ray source (hν = 1486.6 eV). The pass energy was 30 eV. The Tougaard background was subtracted from the measured spectrum and the peaks were fitted before calculating the d-band center according to previous references[26, 27]. The p center of the dband calculation was defined as

µp =

∫

E EF

N(ε ) ε p dε

, where N(ε) was the density of states (DOS),

EF was the Fermi level and p was the order of moment. 2. 3. 5 Temperature-programmed Reduction of CH4 (CH4-TPR) CH4-TPR experiment was carried out in a quartz flow microreactor on a chemisorption analyzer (Micromeritics Autochem Ⅱ 2920) equipped with a HPR-20 QIC mass spectrometer (MS). Firstly, 50 mg of the catalyst was treated at 400 oC for 40 min in 3 vol.% O2/He with a flow of 50 min/ml, and then the catalyst was cooled to room temperature. The TPR tests were conducted by heating the samples to 850 oC with a heating speed of 10 oC/min with a 50 mL/min mixture of 1 vol.% CH4/He. The signal of CH4 was identified by the fragment at m/e = 15 to


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avoid the interference of the oxygen fragment (m/e = 16) from CO2 or H2O, and the products (m/e = 2, 28, 44) were detected as well. 2. 3. 6 Temperature-Programmed Desorption of O2 (O2-TPD) O2-TPD experiment was performed using the same reaction apparatus with CH4-TPR. Firstly, 100 mg of the catalyst was pretreated at 400 oC for 60 min under 3 vol.% O2/He with a flow of 50 ml/min, then the catalyst was cooled to room temperature and kept for 1 h to adsorb O2. After purging the sample with He for another 1 h, the sample was heated to 900 oC at a rate of 10 o

C/min. The O2 signals (m/e = 32) were detected by a mass spectrometer (MS) at the outlets.

2. 3. 7 Nuclear Magnetic Resonance of 27Al (27Al-NMR) 27

Al MAS NMR experiments were conducted on Bruker AVANCE III 600 spectrometer at a

resonance frequency of 156.4 MHz. 27Al MAS NMR spectra were recorded on a 4 mm probe by small-flip angle technique with a pulse length of 0.5 µs (

H-ZSM-5: Active Pd

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