Investigation of supported palladium catalysts for combustion of methane: The activation effect caused by SO2

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Investigation of supported palladium catalysts for combustion of methane: The activation effect caused by SO2 ⁎

Ya Dinga,b, Sheng Wanga, , Lei Zhanga,b, Lirong Lva,b, Dekang Xua,b, Wei Liua, Shudong Wanga a b

Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

catalytic oxidation performance • The of Pd/CZ was promoted after SO 2

• • •

treatment. The high activity can be sustained on a 50 h time-on-stream test. Appropriate acidity helped to prevent the formation of deactivated bulk sulfates. SO2 was selectively combined with cerium to form sulfates preferentially.

A R T I C LE I N FO

A B S T R A C T

Keywords: Ceria-zirconia solid solution Pd Catalytic combustion SO2 Activation Acidity

Catalytic combustion of methane was studied over Ce0.5Zr0.5O2 and CeO2 supported Pd (Pd/CZ and Pd/Ce) catalysts in the presence of SO2. The effect of sulfation (air including 100 ppm SO2 for 6 h at 500 °C) on the sulfur-tolerant performance of the catalysts was also investigated. Results indicated that there is a considerable promotion effect for CH4 complete oxidation on Pd/CZ catalyst, while the activity of Pd/Ce catalyst was inevitably inhibited. Further, the superior sulfur-tolerant behavior of Pd/CZ was verified through a 50 h time-onstream stability test. Mechanistic insights into the synergistic effect between the support and Pd were unveiled by Raman, H2-TPR, XPS, NH3-TPD, DRIFTS, HAADF-STEM, etc. It was deduced that the formation of sulfates at the Pd-support interface could promote the dissociative adsorption of methane on Pd/CZ catalyst. Meanwhile, the higher relative acid strength and the acid amount were also decisive factors for the high sulfur-resistant performance. The enhanced acid strength and acid amount prevented the further formation of bulk-like sulfate species for the SO2 treated Pd/CZ catalyst, thus enhancing the sulfur-tolerant ability of the catalyst.

1. Introduction Catalytic combustion was considered as one of the most efficient ways for end-of-pipe diluted volatile organic compounds (VOCs) emission control [1]. And the study of the catalytic combustion of alkanes is of fundamental importance since they are emitted from a

number of different industries and automotive vehicles [2]. Although CH4 does not belong to VOCs species according to some definitions, it is the most stable alkane. Catalytic combustion of CH4 was usually used as a probe reaction to screen the combustion catalyst. On the other hand, the catalytic combustion of CH4 is extremely important because of its higher greenhouse contribution compared to CO2 [3–5]. Supported

⁎ Corresponding author at: Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China. E-mail address: [email protected] (S. Wang).

https://doi.org/10.1016/j.cej.2019.122969 Received 15 June 2019; Received in revised form 20 September 2019; Accepted 25 September 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Ya Ding, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.122969

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SO2 during combustion. In this regard, SO2 was used as the probe molecule to investigate the sulfur poisoning effect of the catalysts. In order to reveal the promoting effect by the formed surface sulfates, the Pd/CZ-fresh and Pd/Ce-fresh samples were pretreated by SO2. First, 150 mg of the fresh samples were loaded in a U-shaped tube, which was then put into an electronic furnace. The furnace was ramped to 500 °C with an air flow of 100 mL·min−1. Then the gas involving 100 ppm SO2, 4 vol% of H2O and balanced air was supplied to pretreat the fresh catalyst at 500 °C for 6 h. Afterward, the SO2-containing gas was stopped and the sample was cooled down to room temperature. The obtained products were denoted as Pd/CZ-100SW-6h and Pd/Ce100SW-6h, respectively.

palladium catalysts are widely applied in the catalytic removal of methane under lean-fuel conditions [6–10]. However, it is generally recognized that palladium-based catalysts are sensitive to sulfur. Generally sulfur element in the sulfur-containing VOCs would be converted to SO2 during catalytic oxidation. Thus SO2 was used as the model compound to investigate the sulfur-tolerant characteristics of catalysts. SO2 could inevitably deactivate oxidation catalysts [11–19]. And the formation of stable palladium sulfates is the primary reason for the deactivation [20]. However, the sulfates can spill over onto the sulfated support. So the sulfur-tolerant behavior of palladium catalysts is strongly dependent on the supports [21]. The resistance to SO2 poisoning over sulfated support outperformed analogous non-sulfating support catalysts [22]. Considerable studies have been carried on the deactivation mechanism owing to sulfur poisoning [23,24]. The promotion of SO2 to catalytic combustion was also previously proposed [25–28]. The surface sulfates formed on γ-Al2O3 can enhance the catalytic oxidation of propane on Pt/Al2O3 by increasing its dissociative chemical adsorption [25]. Ceria-zirconia supported catalysts have been previously studied in the system of catalytic combustion of CH4 [23,29–31]. The formed sulfate over Pt/CeO2 enriched the oxygen vacancies in the ceria that governed the oxygen spillover. And the dissociative adsorption of methane was facilitated. However, the inhibiting effect by SO2 exposure would emerge when the ceria was gradually saturated with sulfates [32]. For Pt/Ce-Zr catalysts, sulfate (SO42−) improved significantly their catalytic activities of propene and CO oxidation [33]. To the best of our knowledge, most researches on the promotion effects by sulfur were related to the supported Pt catalysts. While for Pd catalysts, hardly any related studies were proposed because Pd is easier to be deactivated owing to sulfur poisoning than Pt [34]. Furthermore, most promotion effects referred above merely exhibited to be a transient process [32], i.e. introducing SO2 for a duration of several minutes to the reaction mixture. In this study, we investigated the performance of Pd supported on ceria-zirconia solid solution during CH4 catalytic combustion under sulfur poisoning. And non-transient promotional effect by sulfur dioxide was also discussed in detail. The mechanistic insights into the relationships between the catalytic performance and physicochemical properties of the fresh and used catalysts are provided. Expectedly, this kind of in-depth knowledge can provide the guidelines for the design and fabrication of the catalyst with superior sulfur-tolerant behaviors.

2.3. Characterization X-ray diffraction (XRD) patterns of the samples were recorded on a PANalytical Empyrean diffractometer with Cu Kα radiation (λ = 1.5418 Å) from 10 to 90° with a step size of 0.03° and the scan speed was 11.6°·min−1, operated at 40 kV and 40 mA. And the crystalline size was calculated using the Scherrer formula. Raman spectra were recorded using a Renishaw inVia Raman microscope with 532 nm incident radiation. The specific surface areas (SBET) of the catalysts were calculated by the BET (Brunauer-Emmett-Teller) method from N2 adsorption isotherms obtained at −196 °C using a Quantachrome NOVA 2200e instrument. The samples were degassed at 250 °C under vacuum for 2 h prior to the measurement. And the pore size distributions were measured by the Barrett-Joyner-Halenda (BJH) method from N2 desorption isotherms. The reducibility of the catalysts was analyzed by the hydrogen temperature programmed reduction (H2-TPR) experiments with a Quantachrome CHEMBET pulsar adsorption instrument equipped with a TCD detector. The samples with a mass of 50 mg were pretreated at 300 °C under Ar flow at 40 mL·min−1 for 30 min. Then the samples were cooled by N2 prechilled by liquid N2. A mixture of 10% H2/Ar flowed at 40 mL·min−1 through the sample until stabilization of the baseline. Then the furnace was heated to 900 °C at a ramp of 10 °C·min−1. The surface Pd atoms were calculated through CO chemical adsorption (CO chemisorption) through the same instrument with H2TPR. CO can be oxidized by CeO2 to form carbonates and adsorbed on the surface. Excessive CO consumption will overestimate the Pd dispersion. Therefore, CO2 was pre-adsorbed on the CeO2 surface before CO adsorption to eliminate the deviation [36]. The samples were first reduced in a 10% H2/Ar mixture at 300 °C for 1 h. Then it was purged with He at the same temperature for 0.5 h to remove the H2 adsorbed on the surface of the sample, and cooled to room temperature under He atmosphere. Subsequently, it was purged for 5 min under 3% O2/He atmosphere, purged for 5 min under CO2 atmosphere, purged for 5 min under He atmosphere, and purged with 10% H2/Ar for 5 min. Finally, the carrier gas was switched to He for 30 min, and after the TCD baseline was stabilized, the CO gas was pulsed into the system until the adsorption was saturated. When calculating the dispersion, it is assumed that one CO molecule is adsorbed on one Pd atom. The acidity of the catalysts was analyzed by the ammonia temperature programmed desorption (NH3-TPD) experiments through the same instrument with H2-TPR. The samples with a mass of 150 mg were pretreated at 500 °C under He flow at 40 mL·min−1 for 30 min. Then the sample was cooled down to 100 °C. Afterward, the sample was kept in a 10% NH3/He flowing of 40 mL·min−1 for 30 min. After that, He was fed to purge the spare NH3. And the samples were heated from ambient temperature to 700 °C at a ramp of 5 °C·min−1. X-ray photoelectron spectra (XPS) were measured using an ESCALAB 250Xi spectrometer with an aluminum anode for Kα (hν = 1484.6 eV) radiation. Powder samples were pressed into tablets, and vacuum treatment was performed before testing. Detailed spectra

2. Experimental 2.1. Catalyst preparation The Ce0.5Zr0.5O2 (CZ) and CeO2 (Ce) supports were prepared by homogeneous precipitation method [35]. In a typical procedure, Zr (NO3)4·5H2O, Ce(NO3)3·6H2O and urea were dissolved in deionized water. The mixture was then heated to 100 °C and kept boiling for 2 h. Afterward, the mixture was filtered and washed with deionized water and isopropanol several times, followed by dried in a vacuum oven at 60 °C overnight. Finally, the sample was calcined at 550 °C for 2 h. In the impregnation process, the supports were pressed to a wafer before crushed into 40–60 mesh particles. Catalysts were prepared by incipient wetness impregnation from an aqueous solution of Pd(NO3)2 with Pd loading of 0.8 wt%, Pd(NO3)2 solution was quantified by ICP in advance. The solid was then dried at 120 °C for 10 h and calcined at 550 °C for 2 h with a heating rate of 2 °C·min−1. The catalysts of Pd supported on Ce0.5Zr0.5O2 and CeO2 were marked as Pd/ Pd/CZ-fresh and Pd/Ce-fresh, respectively. 2.2. Pretreatment of the catalyst Combustible compounds involving sulfur would be converted into 2

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diffusion limitations. And heat transfer limitations have also been excluded [37]. Turnover frequency (TOF) of CH4 was calculated by the following equation:

were recorded for the region of C1s, Ce3d, S2p and O1s photoelectrons with a 0.05 eV step. The analysis was performed by the Avantage software, and all binding energies were referenced to C 1s at 284.8 eV. Thermogravimetric analysis (TGA) was carried out under air flow (20 mL·min−1) at a heating rate of 10 °C·min−1 by using a Netzsch STA 449 F3 analyzer, and the temperature range was 30–1200 °C. Inductively coupled plasma optical emission spectrometry (ICPOES) analysis was carried out on a PerkinElmer Optima 8000 to measure the contents of Pd, S, Ce and Zr elements in the soluble components of all the samples. The catalysts were washed with deionized water and filtrated for five times, and the filtrate was used to take the ICP-OES analysis. Pyridine-adsorbed FT-IR spectra were obtained on a Nicolet 6700 spectrometer with an MCT detector. First, 25 mg of the catalyst was ground and pressed into a thin wafer with a diameter of 13 mm and placed in a quartz cell (homemade) equipped with a CaF2 window in a vacuum system. The catalysts were treated in situ at 400 °C for 2 h under vacuum, and the pressure was below 10−2 Pa. The background spectra were collected after the samples were cooled to room temperature. After that, the pyridine vapor was introduced into the cell for 5 min, then the physically adsorbed pyridine was pumped out by evacuating for another 1 h. Finally, the sample was heated to 150, 250, and 350 °C under vacuum for 1 h, respectively. The spectrum was recorded after vacuum treatment at each temperature. In situ Diffuse Reflectance Infrared Fourier Transform (DRIFT) of CO adsorption was performed on the same spectrometer as Pyridine-adsorbed FT-IR, which was equipped with a PIKE DiffusIR in situ sample cell with ZnSe windows. The DRIFT spectra were collected with a resolution of 4 cm−1 and 32 scans in Kubelka-Munk units. Typically, the samples were pretreated at 300 °C for 120 min by Ar, the background spectra were collected after the samples were cooled to 30 °C in the same atmosphere. And then 5 vol% CO/Ar (30 mL·min−1) was introduced for 30 min, followed by Ar flow for another 30 min to purge gas phase CO, after that in situ DRIFT spectra were obtained. The high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) analysis, as well as energy-dispersive X-ray spectroscopy (EDS) characterization, were performed on a JEM ARM200F microscope with a probe Cs-corrector working at 200 kV. The samples were prepared by ultrasonically dispersing the sample into ethanol, and droplets of the resulting suspensions were dropped onto a carbon-coated copper grid and then dried in air.

TOF =

where the Vm (L·mol−1) is the molar volume of gas, F (L·s−1) is the flow rate of CH4, and α is the conversion of CH4. The number of active sites N (mol·g−1) equals to the surface Pd atoms, which was calculated through CO chemisorption. In order to eliminate the mass and heat transfer limitations, normally, the temperature was chosen when the CH4 conversion was below 10% [38]. Thus, in this study, CH4 conversion at 400 °C and 450 °C during the absence of water in the feed gas were chosen for Pd/CZ and Pd/Ce, respectively. To study the poisoning effect of SO2 on the catalysts, a time-onstream test was carried out in the reaction gases involving 10 ppm SO2 at 550 °C. 3. Results and discussion 3.1. Catalytic performance The effects involving SO2 were verified during a time-on-stream test. And the synergetic effects of water were also investigated on the stabitility of catalyst. Generally, water would accelerate the deactivation of catalysts during sulfur poisoning [39]. And water always participates in the combustion reaction in the feed gas. In view of practical application, 5 vol% water was added to the feed gas. As can be seen in Fig. 1a. There was a rapid decline on CH4 conversion over both Pd/Cefresh and Pd/CZ-fresh as H2O was introduced. It is widely recognized that the competitive adsorption of H2O on supported Pd catalysts would block the access of CH4 to the PdO sites and thus cause deactivation [9]. However, the two catalysts demonstrated entirely different trends when 10 ppm SO2 was introduced in the feed gas. As can be seen from Fig. 1b, there was a continuous decline on CH4 conversion for Pd/Ce-fresh during a 50 h time-on-stream test. On the contrary, higher activity was sustained and there was a significant boost in the activity for the Pd/CZfresh sample. To clarify the effect of the Ce0.5Zr0.5O2 carrier, its stability under SO2 was also measured in a time-on-stream test (Fig. S1). By comparison with Pd/CZ-fresh, it can be deduced that the sulfur-induced activation performance might be attributed to a synergistic effect between the support and Pd. Water inhibition and SO2-induced activation were further confirmed from Fig. S2. As can be seen from Fig. S2, SO2-induced activation was independent on whether or not water occurred. So the SO2-induced catalysts were collected in the absence of water and characterized to unveil the intrinsic mechanism (Fig. 1c). In line with Fig. 1b, the CH4 conversion almost approached to zero for Pd/Ce-fresh as the reaction proceeded. Correspondingly there was a significant increase in CH4 conversion for the Pd/CZ-fresh counterpart. Moreover, at all specified temperatures, i.e., 450, 500, and 550 °C in Fig. 2, the same trends were exhibited. There was almost no deactivation over Pd/CZ-fresh in the presence of SO2. In the beginning, the activity was gradually enhanced in the presence of SO2. Although this promotion effect would get gradually less pronounced as the reaction proceeded, as can be seen in Fig. 2. By comparison with the maximum induced activity, there was a slight decay on the activity after a long time-on-stream test in Fig. 1 and Fig. 2. It was possibly attributed to the formation of the deactivated bulk-like sulfate species and the sintering of Pd species under the reaction conditions, which was also evidenced in the following section. To further understand the intrinsic mechanism of the promoting effect, the reaction was terminated when CH4 conversion achieved a relatively higher value and kept increasing. Then the catalyst was cooled and collected under air atmosphere. From Fig. 1c, CH4

2.4. Catalytic evaluation The catalytic activities for CH4 combustion were measured in a continuous flow fixed-bed quartz tubular reactor (i.d. 6 mm) mounted in a tube furnace. 0.15 g sample was packed into the reactor. The feed gas was composed of 1% CH4, 4% O2 and N2 (balance gas). To examine the influence of water, 5 vol% water vapor was added to the reactor by bubbling through the water tank at 33 °C. The total flow rate was set to 100 mL·min−1, thus the GHSV (gas hourly space velocity) sustained at 40,000 mL·g−1·h−1. The feed gas and products were analyzed by a gas chromatograph (Agilent 7890A) equipped with a Flame Ionization Detector (FID) and Thermal Conductivity Detector (TCD). The CH4 conversion (X) was calculated as follows:

X=

Product molecules F×α = Active sites × Time Vm·N

CH4 in − CH4 out × 100% CH4 in

where CH4 in and CH4 out are concentrations of CH4 at the inlet and outlet, respectively. All the evaluation tests have been repeated three times to confirm the repeatability of our experimental results. The experimental conditions were obtained, in which internal and external diffusions were eliminated (see the details in Supporting Information). In this study, CH4 conversion was collected in case of no internal and external 3

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Fig. 3. Catalytic performances of Pd/Ce-fresh, Pd/Ce-100SW-6h, Pd/CZ-fresh, and Pd/CZ-100SW-6h for CH4 oxidation.

As the reaction proceeded, it rose gradually. To determine the relationship between the amount of formed sulfate over the Pd/CZ-fresh and SO2 aging time, the aged catalysts were analyzed by TGA. As can be seen from Fig. S3b, the formation rate of sulfate over the Pd/CZ-fresh slowed down as the time prolonged, which is in line with the result of SO2 adsorption. It is possible owing to the increase of acidity, which will be discussed in section 3.4. And the laboratory-scale accelerated treatments using higher concentrations of H2O and SO2 was done to further illustrate the SO2 induced activation. To validate that the in-situ state can be sustained for Pd/CZ-100SW-6h during cooling, two samples were collected after cooling down to 250 °C and room temperature. And there was little difference in activity for the sample obtained by two cooling methods. The catalytic activities of Pd/Ce-fresh, Pd/Ce-100SW-6h, Pd/CZ-fresh, and Pd/CZ-100SW-6h for CH4 combustion in the absence of water are shown in Fig. 3. To quantitatively assess the induced activation, the TOF values were calculated for the fresh and aged samples in Table.1. The TOF for Pd/CZ-100SW-6h sample was almost 11 times higher than the Pd/CZ-fresh catalyst. Meanwhile, the TOF of Pd/Ce-100SW-6h catalyst was also higher than the fresh catalyst, even though its activity was lower than the Pd/CZ-fresh catalyst. It is recognized that the introduction of Zr into the ceria lattice could enhance the oxygen storage capacity of Ce0.5Zr0.5O2. On the other hand, the thermal stability of ceria will be improved in the presence of doping Zr. To further validate the catalytic stability of Pd/CZ, Pd/ZrO2 with the same Pd loading were also evaluated and compared. And there was a negative effect for Pd/ZrO2 catalyst when SO2 was involved in the reaction (Fig. S16). For all the reactions mentioned above, only CO2 and unconverted CH4 were detected in the flue gases. And there were no other by-products such as CO and H2. The selectivity to CO2 and H2O was both 100%. This kind of catalyst we reported has good potential in the application of sulfur-containing hydrocarbons pollution abatement.

Fig. 1. CH4 conversion as a function of time on stream over Pd/CZ-fresh and Pd/Ce-fresh catalysts at 550 °C in the presence (a, b) and absence (c) of 5 vol% water vapor and 10 ppm SO2 exposure (when used).

Fig. 2. CH4 conversion as a function of time on stream with 10 ppm SO2 exposure with Pd/CZ-fresh carried out at different temperatures.

3.2. Structure and composition analysis The structures of fresh and pre-sulfated samples were studied by XRD and Raman. Pure tetragonal Ce0.5Zr0.5O2 phase (JCPDS No. 381436) and cubic CeO2 phase (JCPDS No. 43-1002) were presented from the XRD patterns in Fig. 4a. It is worth mentioning that in the XRD profiles of the pre-sulfated samples, no evidence of structural changes and S-containing phases were observed, suggesting that the sulfated species might exist as either surface species or amorphous bulk species in the samples [33,40]. The absence of significant diffraction peak of Pd species could be due to the low Pd loading for all the catalysts.

conversion reached a maximum value when the reaction proceeded at 550 °C for 20 h under the reactants involving SO2, then the sample was collected and denoted as Pd/CZ-R-20 h. In order to get insights into the adsorption of SO2 onto Pd/CZ-fresh catalyst, the outlet SO2 concentration was detected when the catalyst was kept at 550 °C in the mixture of air and 50 ppm SO2 (see the details in the Supporting Information). It can be seen from Fig. S3a that the initial outlet SO2 concentration was lower than the inlet concentration. 4

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Table 1 BET surface area, average pore sizes, total pore volumes, ICP-OES, Pd dispersion, and TOF results of the catalysts. Sample

Pd/CZ-fresh Pd/CZ-100SW-6h Pd/CZ-R-20 h Pd/Ce-fresh Pd/Ce-100SW-6h

SBET

Pore size

Pore volume

ICP (wt%)

(m2·g−1)

(nm)

(cm3·g−1)

Ce

S

Zr

113 87 107 100 56

9.7 7.9 9.7 3.8 7.7

0.34 0.28 0.33 0.13 0.12

ND 5.22 1.47 ND 9.25

ND 2.33 0.54 ND 3.65

ND ND ND ND ND

Weight lossa

Pd dispersionb

Reaction ratec

TOFc

Pd

(%)

(%)

(*10−7 mol/g-cat . s−1)

(*10−2 s−1)

ND ND ND ND ND

– 6.96 2.16 – 7.36

44.2 13.1 16.9 23.9 14.2

1.22 4.42 – 2.10 1.68

0.4 4.5 – 1.2 1.6

ND = Not Detectable. a Determined by TGA at the range of 700–1100 °C. b Determined by CO pulse absorption. c Reaction rate and TOF based on CH4 conversion at 400 °C and 450 °C for Pd/CZ and Pd/Ce, respectively.

presence of the band was due to the perturbation of M−O bond symmetry generated by oxygen vacancy [41], which is beneficial for CH4 combustion. As can be seen in Fig. 4b, the Pd/CZ-fresh possessed more oxygen vacancies than Pd/Ce-fresh. And it was possibly associated with the higher activity over Pd/CZ-fresh. For CH4 oxidation reaction over the Pd/CZ-fresh, oxygen was adsorbed and activated on the oxygen vacancy. While CH4 could be adsorbed on the PdOx sites. What is more, the oxygen vacancy increased after SO2 treatment for Pd/CZ-fresh sample. The other bands at ~1020 and ~1125 cm−1 could be assigned to the vibration of sulfate, the former could be the symmetric S-O stretch, and the latter belonged to the asymmetric S-O stretch [40]. The Raman results fairly suggested that there were sulfur-containing species in some of the samples. Textural properties and the element abundance from ICP-OES are summarized in Table 1. BET analysis indicated that the surface area of all the sulfated samples decreased because of the formed sulfates covering on the surface. There was a more remarkable decrease (about 44%) on its surface area for Pd/Ce-fresh, which meant that the catalyst suffered from a more devastating pore blocking. The influence of SO2 poisoning for Pd/CZ-R-20h sample was negligible which could be in line with the best catalytic performance. The sulfate deposited on the catalysts might be soluble cerium sulfate, zirconium sulfate or palladium sulfate. So the catalysts after the reaction were washed with deionized water to determine the concentration of Ce, S, Zr, and Pd elements (Table 1). It can be seen that cerium was the only metal element detectable in all these filtrates. This illustrated that SO2 was preferred to adsorb on cerium and formed Ce2(SO4)3 during the reaction. It was associated with the capabilities of ceria to oxidize SO2 and adsorb the product as sulfate [43]. Whereas, in the Ce0.5Zr0.5O2 support, cerium and zirconium share the same crystal lattice in the form of solid solution. But the SO2 was selectively combined with cerium during the reaction. Thus, we propose that sulfates formed preferentially over the ceria and accumulated onto the surface. Ultimately, the interaction between SO2 and Pd/CZfresh catalyst diminish because of the increased acidity, which will be discussed in the next section. To further confirm that cerium sulfate was preferentially formed. TGA was used to detect the thermal decomposition property of the sulfated catalysts. And Ce2(SO4)3·5H2O was used as reference material to identify the decomposition behaviors of the sulfate radicals chemically bonded to the catalysts. And the results are shown in Fig. S4. The weight losses at 100 and 300 °C were derived from the dehydration of Ce2(SO4)3·5H2O. The decomposition of the sulfate radical in Ce2(SO4)3 occurred above 700 °C, which is in accordance with the decomposition of the sulfated catalysts in this work. From Table 1, the relative amount of sulfate species formed on the Pd/Ce-100SW-6h samples should be a little more than that on Pd/CZ-100SW-6h.

Fig. 4. XRD patterns for Pd/CZ-fresh, Pd/CZ-100SW-6h, Pd/Ce-fresh, and Pd/ Ce-100SW-6h samples (a), Raman spectra of Pd/CZ-fresh, Pd/CZ-100SW-6h, Pd/CZ-R-20 h, Pd/Ce-fresh and Pd/Ce-100SW-6h samples (b).

As an effective tool to determine the crystalline phase of ceria and zirconia polymorphs, Raman spectra in the range of 1700–150 cm−1 were recorded for all the investigated samples (Fig. 4b). The weak band at 305 cm−1 of Pd/CZ-fresh samples could be attributed to the displacement of oxygen atoms from their ideal fluorite type lattice of cerium oxide [40]. And the dominant feature near 470 cm−1 could be assigned to the F2g mode Raman shifts of CeO2 that originates from a symmetrical stretching of the CeO8 cubes [40–42]. The concentration of oxygen vacancy could be effectively determined through Raman spectroscopy [41]. For Pd/CZ-fresh samples, additional band occurred at 550–650 cm−1 was frequently confirmed to belong to the non-degenerate Raman inactive longitudinal optical (LO) mode of ceria. The 5

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for Pd/CZ-100SW-6h sample. This was probably due to the formation of Ce2(SO4)3 on the surface after SO2 treatment. From the XPS spectrum of O 1s (Fig. 5b), qualitative and quantitative information of the oxygen species could be gained [47]. The O1s spectrum could be divided into lattice oxygen (Olatt), surface adsorbed oxygen (Osurf) and chemisorbed water (Oα), respectively [46,48]. Among them, surface adsorbed oxygen such as defect-oxide or hydroxyl-like group is often thought to be more efficient in oxidation reactions because of its higher mobility [47]. The concentration of surface adsorbed oxygen calculated by Osurf/(Olatt + Osurf + Oα) can be seen from Table 2. In our opinion, the surface sulfate contributes to the Osurf to some extent. Compared with Pd/CZ-fresh counterpart, the relatively higher concentration of Osurf of Pd/CZ-100SW-6h sample, which demonstrated better activity, indicated the existence of surface sulfate. The phenomenon coincided well with the reported conclusion that the presence of S-O components at the interface between the active component and the support may promote the catalytic combustion of alkanes [27]. Meanwhile, the Pd/Ce-fresh sample underwent a decrease in the concentration of Osurf after SO2 treatment and demonstrated a deactivation for CH4 combustion. It is possibly attributed to the formation of a strongly adsorbed sulfate. Finally, the surface sulfate will spillover and form bulk sulfate in the Pd/Ce-fresh sample. According to the XPS spectrum of S 2p (Fig. S6), the binding energy of the S 2p1/2 was 170 eV and S 2p3/2 was 169 eV, which indicated that sulfur is present in the form of +6 valence state only [49]. Based on the XPS results, it was deduced that the formed sulfur-containing species should be SO42−. And under the high temperature and rich oxygen conditions, the reaction mechanism of SO2 to SO42− might be 2CeO2 + 3SO2 + O2 → Ce2(SO4)3, i.e. SO2 would react with CeO2 to form sulfates [50]. Furthermore, the relative intensity of Pd/Ce-100SW6h is much weaker than the Pd/CZ-100SW-6h, but there was more SO2 adsorb on Pd/Ce-100SW-6h, which proved the accumulation of sulfate species on the surface of Pd/CZ-100SW-6h. The Pd loading on these samples were relatively low for XPS detection. Besides, there existed signals overlap between Pd 3d (3d3/2: 339 eV, 3d5/2: 333.6 eV) and Zr 3p (3p1/2: 343 eV, 3p3/2: 330 eV) [51]. The spectra at the range of 347–329 eV were provided in Fig. S6 for reference, and there were no discernable Pd 3d peak. Thus, the XPS spectrum of Pd 3d was not resolved here. The redox property of supported Pd catalysts is an important factor and could influence the catalytic combustion performance [52]. In order to study the redox properties of the related catalysts, H2-TPR was carried out in this study (Fig. 6). Pd/CZ-fresh demonstrated a major reduction peak at 82 °C with a shoulder at 121 °C, which was attributed to the reduction of PdOx to metallic Pd and the reduction of the surface Ce4+ interacting with Pd species to Ce3+ [53]. In this work, there was no individual reduction peak of PdOx species, which indicated the PdO species were mainly dispersed and stabilized on the Ce sites [54]. The peak at 527 °C could assign to the reduction of lattice oxygen in bulk CeO2 [55]. H2-TPR profiles were quantified by fitting the peak areas from 50 to 300 °C and 300–850 °C respectively for all the samples (Table 2). After pre-sulfation, the first peak in the Pd/CZ-100SW-6h sample is much smaller than the fresh one. This is related to the decrease of surface

Fig. 5. XPS profiles for Ce 3d(a) and O 1 s(b) of Pd/CZ-fresh, Pd/CZ-100SW-6h, Pd/Ce-fresh, and Pd/Ce-100SW-6h samples.

3.3. XPS analysis and H2-TPR measurement XPS is an effective technique to investigate ceria-containing systems [44], for it provides immediate information about the redox properties of the Ce4+/Ce3+ couple [45]. According to the literature [46], the complex spectrum of Ce 3d can be resolved into ten peaks with the assignment defined in Fig. 5(a). These peaks are corresponding to five pairs of spin–orbit doublets, and could be donated as V0, V, V’, V”, V”’, U0, U, U’, U”, and U”’. The peaks labeled by V0, V’, U0, and U’ are generally identified as Ce3+ species, while those represented as V, V”, V”’, U, U”, and U”’ are assigned to Ce4+ species. The relative percentage of related species was calculated from the area ratio of Ce3+ to (Ce4+ + Ce3+) from the XPS spectrum of Ce 3d, and the results were listed in Table 2. For Pd/CZ-fresh sample, the Ce3+/(Ce4+ + Ce3+) ratio is 0.38, but the value increased to 0.42 Table 2 Grain sizes, XPS, and H2-TPR data analysis of related samples. Sample

Pd/CZ-fresh Pd/CZ-100SW-6h Pd/Ce-fresh Pd/Ce-100SW-6h a

Grain size

Binding energy (eV)

Osurf/(Olatt + Osurf + Oα)

(nm)

Olatt

Osurf

Oα

8.1 7.8 5.1 5.3

529.7 530.2 529.4 529.8

531.7 532.1 531.2 531.9

533.5 533.7 533.8 533.4

0.26 0.42 0.39 0.34

Ce3+/(Ce4+ + Ce3+)

0.38 0.42 0.27 0.28

Peak area

(a.u.)

Low

High

86.0 68.7 56.6 41.7

10.1 318.6 44.4 354.0

“Low” and “High” represent the low temperature (50–300 °C) and high temperature (300–850 °C) region in H2-TPR profiles, respectively. 6

a

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Fig. 6. TPR profiles of Pd/CZ-fresh, Pd/CZ-100SW-6h, Pd/Ce-fresh, and Pd/Ce100SW-6h samples.

Fig. 7. NH3-TPD profiles for Pd/CZ-fresh, Pd/CZ-100SW-6h, Pd/CZ-R-20 h, Pd/ Ce-fresh, and Pd/Ce-100SW-6h samples.

Ce4+ during sulfation, which is in accordance with the XPS results. It is worth noting that the shift to relatively higher reduction temperature indicates stronger metal-support interactions [56]. The large peak at around 514 °C could be the reduction of sulfate radical and traces of lattice oxygen species in bulk CeO2 [33]. The reduction of Pd/Ce-fresh indicates three reduction peaks at 114, 245, and 768 °C, respectively. The first peak is assigned to the reduction of the species related to the PdO-CeO2 interaction. And the peak at 245 °C is assigned to traces of surface-capping oxygen of CeO2 that is not in contact with Pd. Herein the reduction of lattice oxygen species in bulk CeO2 occurs at 768 °C. While in the TPR curve of Pd/Ce-100SW6h, the first peak weakens a lot and shift to a higher temperature due to the decrease of surface Ce4+, the peak at 220 °C gets more distinct. In our opinion, the increase in the peak at 220 °C proved that there is more surface-capping oxygen of CeO2 which is not in contact with Pd. In other words, pre-sulfation weakened the metal-support interactions in the Pd/Ce-100SW-6h catalyst. Similarly, the large peak at 474 and 550 °C could be the reduction of sulfate radical and traces of lattice oxygen species in bulk CeO2.

Table 3 Quantification of the acidic sites according to the NH3-TPD profiles.a Sample

Pd/CZ-fresh Pd/CZ100SW6h Pd/CZ-R20 h Pd/Ce-fresh Pd/Ce100SW6h a

Weak acidity

Moderate acidity

Strong acidity

Amount

T(°C)

Amount

T(°C)

Amount

T(°C)

Total acidity Amount

0.40 0.49

216 215

0.35 0.60

283 283

0.25 0.62

350 370

1.00 1.70

0.37

215

0.44

283

0.63

350

1.44

0.14 0.21

215 216

0.25 0.26

283 284

0.16 0.16

372 361

0.55 0.63

The total acidity amount of Pd/CZ-fresh sample is assumed to be 1.00.

by the adsorption of pyridine followed by FTIR spectroscopy. In general, the band at around 1560 cm−1 is assigned to Brønsted acid sites. And that at around 1450 cm−1 can be assigned to Lewis acid sites [58]. It is clear from Fig. 8 that Lewis acid sites were present on the surface, corresponding to the band at around 1444 cm−1. After the reaction occurred at 550 °C involving 10 ppm SO2 for 20 h, the peak related to Lewis acid sites become much stronger. However, the bands at 1575 cm−1 nearly remain the same. Further explanation was given in the next section.

3.4. Acidity of the catalysts To shed lights on the role of acidic sites involved in sulfur resistance experiment, also determining the abundance of the surface adsorption sites on the various investigated catalysts, NH3-TPD study was carried out. The desorption profiles are shown in Fig. 7. Typically, the NH3-TPD profiles could be deconvoluted into three peaks, which were assigned to weak, moderate and strong acidic site in view of desorption temperatures at the range of 150–250, 250–350 and 350–450 °C, respectively [33,40,57]. The total area of all the peaks is proportional to NH3 uptake. So the total acidity amounts of other samples were listed relative to the reference Pd/CZ-fresh in Table 3. The total acidity amounts of both Pd/Ce-fresh and Pd/CZ-fresh samples increased correspondingly after SO2 treatment. Especially there was a significant increase on moderate and strong acidity for Pd/CZ-100SW-6h sample compared with the fresh one. Meanwhile, the Pd/Ce-fresh counterpart exhibited much fewer acidic sites, and there was no apparent strong acidity even after the treatment of SO2. In our opinion, the stronger acidity of Pd/ CZ-fresh and its increased acidity after interacting with SO2 could decrease the adsorption and deposition of SO2 on the catalyst and could slow down the further accumulation of sulfate, which might be the reason for the invariant CH4 conversion in the end. Likewise, the relatively weaker acidity and fewer acidic sites of Pd/Ce-fresh might contribute to the poisoning effect of SO2. To discern further the contribution of different acid sites to CH4 catalytic combustion, the fresh and spent catalysts were characterized

3.5. In situ DRIFT spectroscopy measurements 3.5.1. Surface hydroxyl group The DRIFT spectrum of Pd/Ce-fresh (Fig. 9) displays three main bands, ca 3495 cm−1, 3633 cm−1 and 3704 cm−1. The first band was assigned to cerium oxyhydroxide impurities within the ceria pores, and the peak centered at 3633 cm−1 was assigned to Ce2(OH) doubly bridging groups on different facets of ceria crystallites and/or in different local environments[59]. The last band centered at 3704 cm−1 was assigned to Ce-OH terminal hydroxyls [59]. As can be seen from the spectrum, there was barely any bridge or terminal OH group signals for Pd/Ce-100SW-6h. It reveals that OH species on the surface of ceria was consumed during SO2 pretreatment. Different from the Pd/Ce-fresh samples, there are two bands between 3700 and 3400 cm−1 for Pd/CZ-fresh catalysts, i.e bands centered at 3660 and 3735 cm−1. Similarly, they are attributed to Ce2(OH) or Zr2(OH) [59]. However, these species are not easily distinguished from bridged OH existing on ceria or zirconia. By contrast, for the Pd/ 7

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Fig. 8. FTIR spectra of adsorbed pyridine on Pd/CZ-fresh sample (a), and Pd/CZ-R-20 h sample (b).

O

O H

H

O

O M

S SO 2 M

O +

M

O M+

Scheme 1. Possible representations of different kinds of Brønsted acid sites on Pd/CZ-fresh and their transformation into Lewis acid sites upon SO2 involved in the catalytic reaction. M = Ce [59].

consumed by SO2. Therefore, the decreased Lewis acidity and increased Brønsted acidity might contribute to the improvement of the activity of the Pd/CZ-fresh to some extent. As reported, carbonate and carboxylate species are preferentially formed on the surface of ceria containing samples [63]. In the 1700–1100 cm−1 region (Fig. S7), the bands centered at 1460, 1390–1365, 1067 and 1033 cm−1 are assigned to carbonate species (Table 4). The bands in the range of 1400–1340 cm−1 could be attributed to surface sulfate species, and the broadband near 1200 cm−1 could be assigned to bulk-like sulfate species [64]. As can be seen from Fig. S7, bulk-like sulfate species are dominant over Pd/Ce100SW-6h sample, while both two kinds of sulfate species coexist on Pd/CZ-100SW-6h displayed (Fig. S8). This provided the evidence for our assumption that sulfates formed preferentially over the ceria and accumulation onto the surface.

Fig. 9. DRIFT spectra for Pd/CZ-fresh, Pd/CZ-100SW-6h, Pd/Ce-fresh, and Pd/ Ce-100SW-6h samples.

CZ-100SW-6h sample, the main band at 3640 cm−1, which has shifted to lower wavenumbers by about 20 cm−1 compared with the band at 3660 cm−1 from the fresh sample. The red shift of υ(OH) bands might be caused by the sulfates on the surface [60]. Yet, there is still no consensus on the accurate assignment of these bands. Binet et al. [61] attribute the different υ(OH) bands to different number of Ce4+ ions coordinated with hydroxyl species. From the differential spectra in Fig. S8, there were obvious negative peaks attributed to the υ(OH) bands, which proved that the overall amount of OH groups has decreased after SO2 treatment. This is inconsistent with the reported ceria-zirconia samples, which were sulfated by H2SO4 solution [59]. Combined with the results from pyridine adsorbed FTIR, an assumption can be proposed. As is shown in Scheme 1, surface hydroxyl groups could be different forms of Brønsted acid sites, in our opinion, sulfates would firstly form on the surface of the catalyst when SO2 was involved in the reaction. And the electron withdrawing effect of sulfate groups on neighboring metal coordination sites generate Lewis acidity on these sites. Furthermore, the coordinatively unsaturated cations on side-terminations could also contribute to the strong Lewis acidic sites [59]. The surface hydroxide was assumed to be inactive for breaking C–H bonds [62], In situ DRIFT spectroscopy and the pyridine-adsorbed FT-IR proved that surface hydroxyl groups on the surface could be

3.5.2. In situ CO adsorption Extensive information about CO chemisorption on Pd could be measured using DRIFTS [65]. After CO adsorption, as shown in Fig. 10a, the fresh sample exhibited two strong peaks at 1970 and Table 4 Attribution of bands of carbonaceous and sulfate species.

8

Species

Frequency (cm−1)

Ref.

Bidentate carbonates Bridged carbonates Monodentate carbonates Hydrogen carbonates Formates Bulklike sulfate Surface sulfate

1581, 1301 1700–1750, 1180 1514, 1333, 1472, 1392 1608, 1411, 1220 1329, 1369, 1558, 1587 1400–1340 1200–1060

[53] [54] [53] [53] [55] [52] [52]

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shown in Fig. 10b were assigned to carbonate species. Carbonate species were still clearly formed on the Pd/Ce-100SW-6h sample, but on Pd/CZ- −100SW-6h sample, carbonate species were barely detectable. The adsorption sites for CO would diminish when the surface is occupied by sulfates, hence in our opinion, the sulfates were accumulated on the surface of Pd/CZ-l00SW-6h sample. This is in accordance with the stronger symmetric S-O stretch and the asymmetric S-O stretch in Raman spectrum of Pd/CZ-l00SW-6h sample compared with Pd/Cel00SW-6h sample. And the negative bands at about 1381 cm−1 was probably due to the replacement of sulfate during CO adsorption. 3.6. Morphology of the catalysts The micro-structural of the Pd/CZ-fresh and Pd/CZ-100SW-6h catalysts were studied using electron microscopy. HAADF-TEM images indicated that the particles sizes of the catalysts are in the range of 5–10 nm in diameter (Fig. S11,). HAADF-STEM images can still not provide enough contrast to distinguish dispersed noble metals on ceria, and STEM-EDS mapping could be a useful tool to help to determine the distribution of noble metals and the support [68]. Herein, the EDS spectrum and EDS elemental mapping were used to distinguish the distribution of different elements on the catalysts in this work. From the EDS analysis of Pd/CZ-fresh samples in Fig. S12, in the larger region randomly selected, Pd content is 0.93 wt%, which is very close to the theoretical value of 0.8 wt%. The atomic ratio of Ce to Zr (16.58:13.06) approached the nominal ratio. In the meantime, the Pd La and Pd Lb peak (enlarged in Fig. S12b) is obviously stronger than the noise interference, which indicated the information on Pd might be convincing. From the element mapping diagram of Fig. 11, it illustrated that not only Ce and Zr but also the active component Pd uniformly dispersed. There is no obvious aggregation of Pd signals, which could prove that Pd should be dispersed on the support with minimal size. However, according to CO DRIFTS results, there existed both linearly and bridge adsorption of CO on Pd. It can be deduced that Pd sites existed in multiple patterns including single atoms and nanoparticles. It is more likely to occur as sub-nano particles on the support. From Fig. S11b, no apparent changes in morphology were observed on the Pd/CZ-100SW-6h sample. EDS results in Fig. 12 demonstrated that in the boxed micro-region, O, Ce, and Zr elements demonstrated strong stereoscopic image. The even distribution of S was independent on the size of particles selected. It was possible that S element was uniformly distributed on the surface of Pd/CZ-100 SW-6h sample, which further proved the formation of surface sulfate on the Pd/CZ-100 SW-6h sample. The high-resolution atom image, shown in the left of Fig. 12, indicated clearly the representative crystal lattice of Ce0.5Zr0.5O2. The Ce substrate is heavier and far more than Pd and S, the relatively light elements like Pd and S are almost indistinguishable. Furthermore, the relative atomic concentrations of Pd and S in the mapping area are 0.23% and 0.36%, respectively. Atoms with such low concentrations tended to be invisible. As can be seen in the element mapping, Pd and S element still existed in this area, which fully proved that Pd was uniformly dispersed on the surface of Pd/CZ-100SW-6h sample. Furthermore, Fig. S13 demonstrated the line scan result on a small domain of Pd/CZ-100SW-6h sample, it intuitively proved the enrichment of Ce on the surface and the uniform distribution of S and Pd.

Fig. 10. DRIFT spectra of CO chemisorption over the Pd/CZ-fresh, Pd/CZ100SW-6h, Pd/Ce-fresh, and Pd/Ce-100SW-6h samples.

2082 cm−1. These peaks can be assigned to bridge-bonded CO and linear CO on the corner atoms or edges of Pd, respectively. After SO2 treatment, there was no obvious difference for the Pd/Ce-100SW-6h sample, while the two peaks weaken for the Pd/CZ-100SW-6h sample. And the band at ca. 2082 cm−1 almost vanished. There could be two possible reasons for this disappearance, i) the Pd particles got larger [66] after SO2 pretreatment, ii) sulfates close to palladium covered its corner atoms and/or edges [65,67]. To further determine the mechanism, Pd/CZ-100SW-6h sample was washed several times with deionized water to remove the sulfates, the sample was denoted as Pd/CZ-100SW-6h-W, and the spectra were collected after CO adsorption. From Fig. S9, the peak at 2090 cm−1 reappeared after washing, hence we can deduce the latter assumption, i.e. sulfates formation caused the covering of palladium. Furthermore, the relatively higher wave number of adsorbed CO could be related to the sulfates close to palladium. As can be seen in Fig. S10, there is still about 2% weight loss which is related to the residual sulfates. The sulfates close to palladium would withdraw electron density from the palladium particles and hence decrease back donation into the adsorbed CO [65]. Thus we could speculate, On the Pd/CZ-100SW-6h, because of the synergistic effect between PdOx and the surface surfates at the interface, sulfates close to palladium would withdraw electron density from the metal during the reaction, cooperative pairs of Pdδ+ and (SO4)δ− species can activate CH4 for dissociation more efficiently through polarization of the C–H bond [32], which could promote the dissociative adsorption of CH4. And the abstraction of the first hydrogen in the dissociative adsorption of methane is generally considered to be the rate-limiting step for methane oxidation [62]. In the 1700–1100 cm−1 region, the bands with moderate intensity

4. Conclusions The activation effect caused by SO2 were both confirmed over the pretreated and fresh Pd/CZ catalysts for the catalytic combustion of CH4. Meanwhile, the high activity can be sustained during a 50 h timeon-stream. It was possibly attributed to a synergistic effect between the Ce0.5Zr0.5O2 support and Pd during SO2 exposure. Combined with characterization and experiments, it is deduced that SO2 preferentially 9

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Fig. 11. Representative HAADF TEM mages (a), EDS spectrum(b) and EDS mapping of Pd /CZ-fresh. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

further formation of the deactivated bulk-like sulfate species. This kind of in-depth knowledge is vital in the development of sulfur-tolerant catalysts.

interacted with Ce in Pd/CZ-fresh sample during the reaction, meanwhile, oxygen vacancy was increased for Pd/CZ-fresh catalyst after SO2 treatment. Moreover, a certain amount of sulfate was deposited on the surface of Pd/CZ-fresh catalyst. Thus the new active sites were formed at the Pd-support interface, which could promote the dissociative adsorption of methane on Pd/CZ-fresh. The activation of the first C–H bond of CH4 occurred possibly at the perimeter of Pd-sulfate interface. There was an accumulation of surface sulfate, which was stable under reaction conditions on Pd/CZ-fresh sample. Furthermore, appropriate acidity amount and acidity strength were crucial for SO2 to achieve a dynamic equilibrium between adsorption and desorption on the catalyst. Hence it can avoid the further deposition of SO2 and prevent the

Acknowledgments This work was funded by the National Key Scientific Research Project (2016YFC0204302), the National Natural Science Foundation of China (21676267), and Dalian Institute of Chemical Physics (DICP I201937).

Fig. 12. Representative HAADF TEM mages and EDS mapping of Pd /CZ-100SW-6h, the left is the enlarged image of the region indicated with a box in panel (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 10

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Appendix A. Supplementary data

oxidation over Pt/Al2O3, Catal. Commun. 3 (2002) 533–539. [29] C. Bozo, N. Guilhaume, J.-M. Herrmann, Role of the Ceria-Zirconia Support in the Reactivity of Platinum and Palladium Catalysts for Methane Total Oxidation under Lean Conditions, J. Catal. 203 (2001) 393–406. [30] S. Specchia, E. Finocchio, G. Busca, P. Palmisano, V. Specchia, Surface chemistry and reactivity of ceria-zirconia-supported palladium oxide catalysts for natural gas combustion, J. Catal. 263 (2009) 134–145. [31] A. Toso, S. Colussi, S. Padigapaty, C. de Leitenburg, A. Trovarelli, High stability and activity of solution combustion synthesized Pd-based catalysts for methane combustion in presence of water, Appl. Catal. B 230 (2018) 237–245. [32] L. Kylhammar, P.-A. Carlsson, M. Skoglundh, Sulfur promoted low-temperature oxidation of methane over ceria supported platinum catalysts, J. Catal. 284 (2011) 50–59. [33] L. Gu, X. Chen, Y. Zhou, Q. Zhu, H. Huang, H. Lu, Propene and CO oxidation on Pt/ Ce-Zr-SO42-diesel oxidation catalysts: Effect of sulfate on activity and stability, Chin. J. Catal. 38 (2017) 607–615. [34] H.N. Sharma, V. Sharma, A.B. Mhadeshwar, R. Ramprasad, Why Pt Survives but Pd Suffers From SOx Poisoning? J. Phys. Chem. Lett. 6 (7) (2015) 1140–1148. [35] X. Du, D. Gao, Z. Yuan, N. Liu, C. Zhang, S. Wang, Monolithic Pt/Ce0.8Zr0.2O2/ cordierite catalysts for low temperature water gas shift reaction in the real reformate, Int. J. Hydrogen Energy 33 (2008) 3710–3718. [36] T. Takeguchi, S. Manabe, R. Kikuchi, K. Eguchi, T. Kanazawa, S. Matsumoto, W. Ueda, Determination of dispersion of precious metals on CeO2-containing supports, Appl. Catal. A 293 (2005) 91–96. [37] S. Oyama, X. Zhang, J. Lu, Y. Gu, T. Fujitani, Epoxidation of propylene with H2 and O2 in the explosive regime in a packed-bed catalytic membrane reactor, J. Catal. 257 (2008) 1–4. [38] Q. Zhang, Y. Li, R. Chai, G. Zhao, Y. Liu, Y. Lu, Low-temperature active, oscillationfree PdNi(alloy)/Ni-foam catalyst with enhanced heat transfer for coalbed methane deoxygenation via catalytic combustion, Appl. Catal. B 187 (2016) 238–248. [39] D.L. Mowery, R.L. McCormick, Deactivation of alumina supported and unsupported PdO methane oxidation catalyst: the effect of water on sulfate poisoning, Appl. Catal. B 34 (2001) 287–297. [40] B. De Rivas, C. Sampedro, M. García-Real, R. López-Fonseca, J. Gutiérrez-Ortiz, Promoted activity of sulphated Ce/Zr mixed oxides for chlorinated VOC oxidative abatement, Appl. Catal. B 129 (2013) 225–235. [41] M. Kurnatowska, L. Kepinski, W. Mista, Structure evolution of nanocrystalline Ce1xPdxO2-y mixed oxide in oxidizing and reducing atmosphere: Reduction-induced activity in low-temperature CO oxidation, Appl. Catal. B 117–118 (2012) 135–147. [42] Y. Lee, G. He, A.J. Akey, R. Si, M. Flytzani-Stephanopoulos, I.P. Herman, Raman Analysis of Mode Softening in Nanoparticle CeO2-δ and Au-CeO2-δ during CO Oxidation, J. Am. Chem. Soc. 133 (33) (2011) 12952–12955. [43] A. Trovarelli, Catalytic Properties of Ceria and CeO2-Containing Materials, Catalysis Rev. 38 (1996) 439–520. [44] M. Romeo, K. Bak, J. El Fallah, F. Le Normand, L. Hilaire, XPS study of the reduction of cerium dioxide, Surf. Interface Anal. 20 (1993) 508–512. [45] E. Paparazzo, Some notes on XPS Ce3d spectra of cerium-bearing catalysts, Chem. Eng. J. 170 (2011) 342–343. [46] S. Ali, L. Chen, F. Yuan, R. Li, T. Zhang, S.u.H. Bakhtiar, X. Leng, X. Niu, Y. Zhu, Synergistic effect between copper and cerium on the performance of Cux-Ce0.5-xZr0.5 (x = 0.1-0.5) oxides catalysts for selective catalytic reduction of NO with ammonia, Appl. Catalysis B: Environ. 210 (2017) 223-234. [47] Y. Ding, S. Wang, L. Zhang, Z. Chen, M. Wang, S. Wang, A facile method to promote LaMnO3 perovskite catalyst for combustion of methane, Catal. Commun. 97 (2017) 88–92. [48] W. Shan, F. Liu, H. He, X. Shi, C. Zhang, A superior Ce-W-Ti mixed oxide catalyst for the selective catalytic reduction of NOx with NH3, Appl. Catal. B 115 (2012) 100–106. [49] B. Darif, S. Ojala, M. Kärkkäinen, S. Pronier, T. Maunula, R. Brahmi, R.L. Keiski, Study on sulfur deactivation of catalysts for DMDS oxidation, Appl. Catal. B 206 (2017) 653–665. [50] Z. Ma, L. Sheng, X. Wang, W. Yuan, S. Chen, W. Xue, G. Han, Z. Zhang, H. Yang, Y. Lu, Y. Wang, Oxide Catalysts with Ultrastrong Resistance to SO2 Deactivation for Removing Nitric Oxide at Low Temperature, Adv. Mater. (2019), https://doi.org/ 10.1002/adma.201903719. [51] M. Monai, T. Montini, M. Melchionna, T. Duchoň, P. Kúš, C. Chen, N. Tsud, L. Nasi, K.C. Prince, K. Veltruská, V. Matolín, M.M. Khader, R.J. Gorte, P. Fornasiero, The effect of sulfur dioxide on the activity of hierarchical Pd-based catalysts in methane combustion, Appl. Catal. B 202 (2017) 72–83. [52] L. Zhang, W. Zou, K. Ma, Y. Cao, Y. Xiong, S. Wu, C. Tang, F. Gao, L. Dong, Sulfated Temperature Effects on the Catalytic Activity of CeO2 in NH3-Selective Catalytic Reduction Conditions, J. Phys. Chem. C 119 (2015) 1155–1163. [53] H. Peng, C. Rao, N. Zhang, X. Wang, W. Liu, W. Mao, L. Han, P. Zhang, S. Dai, Confined Ultrathin Pd-Ce Nanowires with Outstanding Moisture and SO2 Tolerance in Methane Combustion, Angew. Chem. 130 (29) (2018) 9091–9095. [54] M. Sun, W. Hu, T. Cheng, Y. Chen, P. Yao, M. Zhao, S. Yuan, Y. Chen, A novel insight into the preparation method of Pd/Ce0.75Zr0.25O2-Al2O3 over high-stability close coupled catalysts, Appl. Surf. Sci. 467 (2019) 723–739. [55] H. Zhu, Z. Qin, W. Shan, W. Shen, J. Wang, Pd/CeO2-TiO2 catalyst for CO oxidation at low temperature: a TPR study with H2 and CO as reducing agents, J. Catal. 225 (2004) 267–277. [56] S. Hinokuma, H. Fujii, M. Okamoto, K. Ikeue, M. Machida, Metallic Pd nanoparticles formed by Pd-O-Ce interaction: a reason for sintering-induced activation for CO oxidation, Chem. Mater. 22 (2010) 6183–6190. [57] L. Li, L. Zhang, K. Ma, W. Zou, Y. Cao, Y. Xiong, C. Tang, L. Dong, Ultra-low loading of copper modified TiO2/CeO2 catalysts for low-temperature selective catalytic

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122969. References [1] R. Prasad, L.A. Kennedy, E. Ruckenstein, Catalytic combustion, Catalysis Rev.-Sci. Eng. 26 (1984) 1–58. [2] T. Garcia, B. Solsona, S.H. Taylor, The catalytic oxidation of hydrocarbon volatile organic compounds, Handbook Of Advanced Methods And Processes In Oxidation Catalysis: From Laboratory To Industry, World Scientific, 2014, pp. 51–90. [3] M. Cargnello, J.D. Jaén, J.H. Garrido, K. Bakhmutsky, T. Montini, J.C. Gámez, R. Gorte, P. Fornasiero, Exceptional activity for methane combustion over modular Pd@CeO2 subunits on functionalized Al2O3, Science 337 (2012) 713–717. [4] G. Landi, A. Di Benedetto, P.S. Barbato, G. Russo, V. Di Sarli, Transient behavior of structured LaMnO3 catalyst during methane combustion at high pressure, Chem. Eng. Sci. 116 (2014) 350–358. [5] V. Di Sarli, P.S. Barbato, A. Di Benedetto, G. Landi, Start-up behavior of a LaMnO3 partially coated monolithic combustor at high pressure, Catal. Today 242 (2015) 200–210. [6] D. Ciuparu, E. Perkins, L. Pfefferle, In situ DR-FTIR investigation of surface hydroxyls on γ-Al2O3 supported PdO catalysts during methane combustion, Appl. Catal. A 263 (2004) 145–153. [7] J.B. Miller, M. Malatpure, Pd catalysts for total oxidation of methane: Support effects, Appl. Catal. A 495 (2015) 54–62. [8] K. Persson, A. Ersson, S. Colussi, A. Trovarelli, S.G. Järås, Catalytic combustion of methane over bimetallic Pd-Pt catalysts: The influence of support materials, Appl. Catal. B 66 (2006) 175–185. [9] K. Persson, L.D. Pfefferle, W. Schwartz, A. Ersson, S.G. Järås, Stability of palladiumbased catalysts during catalytic combustion of methane: the influence of water, Appl. Catal. B 74 (2007) 242–250. [10] H. Yoshida, T. Nakajima, Y. Yazawa, T. Hattori, Support effect on methane combustion over palladium catalysts, Appl. Catal. B 71 (2007) 70–79. [11] S. Ordóñez, Methane catalytic combustion over Pd/Al2O3 in presence of sulphur dioxide: development of a deactivation model, Appl. Catal. A 259 (2004) 41–48. [12] S. Ordóñez, J.R. Paredes, F.V. Díez, Sulphur poisoning of transition metal oxides used as catalysts for methane combustion, Appl. Catal. A 341 (2008) 174–180. [13] F. Arosio, S. Colussi, G. Groppi, A. Trovarelli, Regeneration of S-poisoned Pd/Al2O3 catalysts for the combustion of methane, Catal. Today 117 (2006) 569–576. [14] S. Colussi, F. Arosio, T. Montanari, G. Busca, G. Groppi, A. Trovarelli, Study of sulfur poisoning on Pd/Al2O3 and Pd/CeO2/Al2O3 methane combustion catalysts, Catal. Today 155 (2010) 59–65. [15] P. Gélin, L. Urfels, M. Primet, E. Tena, Complete oxidation of methane at low temperature over Pt and Pd catalysts for the abatement of lean-burn natural gas fuelled vehicles emissions: influence of water and sulphur containing compounds, Catal. Today 83 (2003) 45–57. [16] X. Zi, L. Liu, B. Xue, H. Dai, H. He, The durability of alumina supported Pd catalysts for the combustion of methane in the presence of SO2, Catal. Today 175 (2011) 223–230. [17] G. Di Carlo, G. Melaet, N. Kruse, L.F. Liotta, G. Pantaleo, A.M. Venezia, Combined sulfating and non-sulfating support to prevent water and sulfur poisoning of Pd catalysts for methane combustion, Chem. Commun. 46 (34) (2010) 6317–6319. [18] L.S. Escandon, S. Ordonez, A. Vega, F.V. Diez, Sulphur poisoning of palladium catalysts used for methane combustion: effect of the support, J. Hazard. Mater. 153 (1–2) (2008) 742–750. [19] Z. Yang, J. Liu, L. Zhang, S. Zheng, M. Guo, Y. Yan, Catalytic combustion of lowconcentration coal bed methane over CuO/γ-Al2O3 catalyst: effect of SO2, RSC Adv. 4 (2014) 39394. [20] L.S. Escandón, S. Ordonez, A. Vega, F.V. Díez, Sulphur poisoning of palladium catalysts used for methane combustion: effect of the support, J. Hazard. Mater. 153 (2008) 742–750. [21] D. Bounechada, S. Fouladvand, L. Kylhammar, T. Pingel, E. Olsson, M. Skoglundh, J. Gustafson, M. Di Michiel, M.A. Newton, P.A. Carlsson, Mechanisms behind sulfur promoted oxidation of methane, PCCP 15 (22) (2013) 8648–8661. [22] J.K. Lampert, M.S. Kazi, R.J. Farrauto, Palladium catalyst performance for methane emissions abatement from lean burn natural gas vehicles, Appl. Catal. B 14 (1997) 211–223. [23] B. Azambre, L. Zenboury, P. Da Costa, S. Capela, S. Carpentier, A. Westermann, Palladium catalysts supported on sulfated ceria-zirconia for the selective catalytic reduction of NOx by methane: Catalytic performances and nature of active Pd species, Catal. Today 176 (2011) 242–249. [24] Y. Ryou, J. Lee, H. Lee, J.W. Choung, S. Yoo, D.H. Kim, Roles of ZrO2 in SO2poisoned Pd/(Ce-Zr)O2 catalysts for CO oxidation, Catal. Today 258 (2015) 518–524. [25] H. Yao, H. Stepien, H. Gandhi, The effects of SO2 on the oxidation of hydrocarbons and carbon monoxide over Ptγ-Al2O3 catalysts, J. Catal. 67 (1981) 231–236. [26] A. Hinz, M. Skoglundh, E. Fridell, A. Andersson, An Investigation of the Reaction Mechanism for the Promotion of Propane Oxidation over Pt/Al2O3 by SO2, J. Catal. 201 (2001) 247–257. [27] A.F. Lee, K. Wilson, R.M. Lambert, C.P. Hubbard, R.G. Hurley, R.W. McCabe, H.S. Gandhi, The origin of SO2 promotion of propane oxidation over Pt/Al2O3 catalysts, J. Catal. 184 (1999) 491–498. [28] G. Corro, R. Montiel, C. Vázquez, Promoting and inhibiting effect of SO2 on propane

11

Chemical Engineering Journal xxx (xxxx) xxxx

Y. Ding, et al.

sulfation, Appl. Catal. B 11 (1997) 193–205. [65] T. Lear, R. Marshall, J.A. Lopez-Sanchez, S.D. Jackson, T.M. Klapotke, M. Baumer, G. Rupprechter, H.J. Freund, D. Lennon, The application of infrared spectroscopy to probe the surface morphology of alumina-supported palladium catalysts, J. Chem. Phys. 123 (17) (2005) 174706. [66] M.S. Wilburn, W.S. Epling, SO2 adsorption and desorption characteristics of Pd and Pt catalysts: precious metal crystallite size dependence, Appl. Catal. A 534 (2017) 85–93. [67] J. Lu, B. Fu, M.C. Kung, G. Xiao, J.W. Elam, H.H. Kung, P.C. Stair, Coking-and sintering-resistant palladium catalysts achieved through atomic layer deposition, Science 335 (2012) 1205–1208. [68] J. An, Y. Wang, J. Lu, J. Zhang, Z. Zhang, S. Xu, X. Liu, T. Zhang, M. Gocyla, M. Heggen, R.E. Dunin-Borkowski, P. Fornasiero, F. Wang, Acid-Promoter-Free Ethylene Methoxycarbonylation over Ru-Clusters/Ceria: The Catalysis of Interfacial Lewis Acid-Base Pair, J. Am. Chem. Soc. 140 (11) (2018) 4172–4181.

reduction of NO by NH3, Appl. Catal. B 207 (2017) 366–375. [58] T. Barzetti, E. Selli, D. Moscotti, L. Forni, Pyridine and ammonia as probes for FTIR analysis of solid acid catalysts, J. Chem. Soc., Faraday Trans. 92 (1996) 1401–1407. [59] B. Azambre, L. Zenboury, J.V. Weber, P. Burg, Surface characterization of acidic ceria-zirconia prepared by direct sulfation, Appl. Surf. Sci. 256 (2010) 4570–4581. [60] C. Morterra, G. Cerrato, F. Pinna, M. Signoretto, Crystal phase, spectral features, and catalytic activity of sulfate-doped zirconia systems, J. Catal. 157 (1995) 109–123. [61] C. Binet, M. Daturi, J.-C. Lavalley, IR study of polycrystalline ceria properties in oxidised and reduced states, Catal. Today 50 (1999) 207–225. [62] R. Burch, M. Hayes, C-H bond activation in hydrocarbon oxidation on solid catalysts, J. Mol. Catal. A: Chem. 100 (1995) 13–33. [63] A. Bensalem, F. Bozon-Verduraz, IR studies of cerium dioxide: influence of impurities and defects, J. Chem. Soc., Faraday Trans. 90 (1994) 653–657. [64] M. Waqif, P. Bazin, O. Saur, J. Lavalley, G. Blanchard, O. Touret, Study of ceria

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