Superior catalytic activity of a Pd catalyst in methane combustion by fine-tuning the phase of ceria-zirconia support

Journal Pre-proof Superior catalytic activity of a Pd catalyst in methane combustion by fine-tuning the phase of ceria-zirconia support Yuanqing Ding (Conceptualization) (Methodology) (Data curation) (Investigation) (Writing - original draft), Qingqing Wu (Methodology), Bo Lin (Validation) (Visualization), Yanglong Guo (Supervision), Yun Guo (Writing - review and editing), Yunsong Wang (Resources), Li Wang (Resources), Wangcheng Zhan (Supervision) (Writing review and editing)

PII:

S0926-3373(20)30046-1

DOI:

https://doi.org/10.1016/j.apcatb.2020.118631

Reference:

APCATB 118631

To appear in:

Applied Catalysis B: Environmental

Received Date:

5 October 2019

Revised Date:

5 January 2020

Accepted Date:

11 January 2020

Please cite this article as: Ding Y, Wu Q, Lin B, Guo Y, Guo Y, Wang Y, Wang L, Zhan W, Superior catalytic activity of a Pd catalyst in methane combustion by fine-tuning the phase of ceria-zirconia support, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118631

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier.

Superior catalytic activity of a Pd catalyst in methane combustion by fine-tuning the phase of ceria-zirconia support Yuanqing Ding, Qingqing Wu, Bo Lin, Yanglong Guo, Yun Guo, Yunsong Wang, Li Wang, Wangcheng Zhan* [email protected]

Key Laboratory for Advanced Materials and Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science

ro of

and Technology, Shanghai 200237, P.R. China. *

Corresponding Author: Wangcheng Zhan, Fax: +86-21-64252923, (W.C. Zhan)

Jo

ur

na

lP

re

-p

Graphical abstract

Highlights: 

CeZrO4 support with κappa phase and large surface area was prepared successfully.



The Pd/κ-CZ catalyst exhibited an excellent activity for CH4 combustion.



Smaller particle size and higher amount of active Pd sites are present on κ-CZ. 1



κ-CZ can prompt the redox cycle of active Pd sites during combustion reaction.

Abstract: Methane, one of the greenhouse gases, is controlled by the catalytic combustion that is the most efficient method. Herein, ceria-zirconia solid solutions (CeZrO4) with high specific surface areas and different phases (κappa and tetragonal) were synthesized and then used to construct the supported Pd catalyst for methane combustion. Compared to the tetragonal CeZrO4 support (t-CZ), the smaller particle

ro of

sizes of PdOx and the higher amount of active PdO species are present on the κappa CeZrO4 support (κ-CZ), leading to a higher ability of the Pd/κ-CZ catalyst for

methane activation. More critically, the oxygen mobility of the Pd/κ-CZ catalyst is

much higher than that of the Pd/t-CZ catalyst. Consequently, an excellent and stable

-p

redox cycle of PdO species is achieved on the Pd/κ-CZ catalyst compared to the Pd/tCZ catalyst, prompting the oxidation of hydrocarbon fragments. These two factors

re

result into a superior catalytic activity of the Pd/κ-CZ catalyst for methane combustion.

lP

Keywords: Ceria-zirconia solid solution; Kappa-phase; Methane combustion; Chemical state of Pd; Oxygen mobility

na

1. Introduction

Methane is the largest constituent of natural gas, which is widely used in power generation and heating applications [1-5]. However, the release of unburned CH4

ur

during flame combustion can pose serious environmental problems due to the strong greenhouse effect of CH4 [6,7]. To date, the catalytic combustion of CH4 has become

Jo

a leading technology to reduce the emissions of CH4 in industrial processes owing to the high combustion efficiency and few emissions of other toxic products, such as NOx [8-12]. How to decrease the ignition temperature and promote the purification efficiency of CH4 at low temperature remains a large challenge due to the stable and highly symmetrical structure of CH4 molecules. Pd-based catalysts are universally acknowledged to present highest catalytic activity for CH4 combustion [13-15]. their catalytic activities are dependent on the dispersion and electronic structure of Pd [16, 17], the properties of the support [18-20] 2

and the interaction between Pd and the support [21-23]. In the reaction process, PdO is the active site to dissociate CH4 molecules and the support can affect the state of Pd active species. A variety of supports are utilized to load Pd, such as transition metal oxides (NiO [24], CoOx [25] and ZrO2 [26]), zeolites (ZSM-5) [27] and CeO2-based oxides [14, 28-32]. On the supported Pd catalysts, the reaction mechanism of CH4 combustion usually follows the M-vK mechanism [5, 31]. Therefore, the extracting efficiency of active oxygen species and the replenishment rate of oxygen vacancies could influence the catalytic activity for CH4 combustion. As a result, Pd/CeO2 has received particular attention due to the excellent oxygen storage/release capacities

ro of

(OSC) of CeO2. However, CeO2 is easily sintered at high temperature, resulting in its OSC decrease. Fortunately, it is well known that the addition of ZrO2 to CeO2 to form ceria-zirconia solid solution (CexZr1-xO4) can significantly improve both the high

temperature stability and OSC of CeO2 [33-37]. Thus, CexZr1-xO4 instead of CeO2 has been widely used as the support for the catalysts in industrial applications.

-p

The OSC of CexZr1-xO4 is dependent on its composition and phase. The phases of

CexZr1-xO4 can be divided into tetragonal (t, t’ and t’’), monoclinic (m) and cubic (c)

re

[38]. Based on the results of density functional theory (DFT) [39, 40] and experimental researches [41-44], the kappa-phase CexZr1-xO4 with x=0.5 (cubic

lP

system, generally assigned as κ-Ce2Zr2O8) possesses the highest OSC. The cerium efficiency (the ratio of Ce3+/Ce after typical reduction processes) of κ-Ce2Zr2O8 approaches 89%, but that of t-CeZrO4 only reaches 52% with the same chemical

na

stoichiometry [45]. However, the high-temperature reduction treatment during the preparation of κ-Ce2Zr2O8, which is essential to the formation of κ-Ce2Zr2O8, generally causes serious sintering and extremely low surface area (< 1 m2/g) of the

ur

product [46-48]. These drawbacks obstruct the extensive applications of the activephase κ-Ce2Zr2O8 in catalytic combustion reactions, since the low surface area of the

Jo

support is not conducive to the dispersion of active metal. Hence, it is urgent to develop an efficient method for synthesizing κ-Ce2Zr2O8 with high specific surface area.

Herein, κ-Ce2Zr2O8 was synthesized by a solvent-thermal method with CMK-3 as a

hard template to enlarge the surface area, and then Pd was loaded by a reductiondeposition method. The effects of CexZr1-xO4 supports with different phases on the catalytic activity of Pd/CexZr1-xO4 for CH4 combustion were evaluated. Meanwhile, based on the various characterizations, how the phase of CexZr1-xO4 affects the 3

activities of catalysts was discussed in details. Through the researches above, we aim to provide a new strategy to synthesize a Pd-based catalyst with excellent catalytic performance for CH4 combustion.

2. Experimental section 2.1 Catalyst preparation SBA-15 was synthesized by the procedures described in the literature [49]: 2.00 g of EO20PO70EO20 (Pluronic P123) was dissolved into 60 mL of HCl solution (2 mol/L) at 40 ℃. Then, 4.00 g of tetraethoxysilane (TEOS) as the silica source was added into the solution. After stirring for 24 h, the solution was transferred to a

ro of

Teflon-lined autoclave and incubated at 100 ℃ for 24 h. The product was obtained by filtration and washed with deionized water. Finally, the white powder was dried at 100 ℃ overnight and calcined at 550 ℃ for 4 h.

CMK-3 was prepared by the procedures described in the literature [50]: 5 mL of

-p

H2O was mixed with 0.08 g of concentrated H2SO4, and then, 0.8 g of sucrose as the carbon source was added into the acid solution. A total of 1.00 g of SBA-15 was

re

added into the solution, and the mixture was incubated at 100 ℃ for 6 h and subsequently at 160 ℃ for another 6 h. After heating at 850 ℃ for 3 h under N2

lP

atmosphere, the obtained black powder was treated with 1 M NaOH solution to remove the silica. Finally, CMK-3 was obtained by filtration and dried at 100℃ overnight.

na

The support of κ-Ce2Zr2O8 was prepared according to the steps below: 2.00 g of P123, 1.00 g of Ce(NO3)3·6H2O and 0.54 g of ZrO(NO3)2·xH2O were dissolved into 35 mL of ethyl alcohol. A total of 0.30 g of CMK-3 was dispersed in the solution in a

ur

three-necked flask with constant stirring for 3 days. After adding 3.00 g of PEG-600, the mixture was heated at 100 ℃ for 12 h with vigorous stirring. The mixture was then

Jo

transferred into a Teflon-lined autoclave and incubated at 100 ℃ for 24 h. The precursor was collected by centrifugation and dried at 100 ℃ for 12 h. The black solid was reduced at 1400 ℃ for 4 h under 10% H2/Ar, followed by calcination at 600 ℃ in air for 4 h to simultaneously oxidize the pyrochlore phase into the kappa phase and to remove CMK-3. The yellow powder obtained is κ-Ce2Zr2O8 (abbreviated as κ-CZ). The support of t-Ce2Zr2O8 (abbreviated t-CZ) was prepared by a coprecipitation method. A total of 2.00 g of Ce(NO3)3·6H2O and 1.06 g of ZrO(NO3)2·xH2O were dissolved in 30 mL of deionized water and treated ultrasonically for 30 min. The 4

equivalent amount of CMK-3 was added into the solution and stirred for 24 h. After 10 mL of ammonium hydroxide and 30 mL of deionized water were added dropwise, the mixture was stirred for 5 h. The sample was obtained by filtration and washing. Finally, the obtained sample was dried at 110 °C and then calcined at 600 °C for 4 h, respectively. The supported Pd catalysts were prepared by a reduction-deposition method using CZ as support. A total of 0.35 g of an aqueous solution of Pd(NO3)2 (0.273 mol/L) was dispersed into 20 mL of distilled water containing 1.00 g of CZ. After stirring for 30 min, the reductant of N2H4 (16 μL) was added into the mixture at 100 ℃ with

ro of

vigorous stirring. The samples were obtained by filtration and washing with distilled water. Finally, the sample was dried at 60 °C for 12 h, and then calcined at 500 °C for 3 h. The obtained catalysts are labelled as Pd/κ-CZ and Pd/t-CZ. In addition, the

catalysts with low Pd content (0.5 wt.%) were also synthesized by the same method,

-p

and they are designated as 0.5Pd/κ-CZ and 0.5Pd/t-CZ, respectively.

2.2 Catalytic performance

re

The catalytic activity of the supported Pd catalysts for CH4 combustion was tested in a fixed bed quartz reactor. A total of 100 mg of catalyst (40–60 mesh) was used. The

lP

feed gas consisted of 1 vol.% CH4, 20 vol.% O2 and N2 balance with a flow rate of 50 mL/min. The gas hourly space velocity (GHSV) was 30,000 mL/(g•h). The temperature of the catalyst bed was programmed from 200 to 600 °C at a rate of 4

na

°C/min. The CH4 concentrations in reaction gas were measured by an online gas chromatograph (Agilent GC 7890A) equipped with TCD detectors. The conversion of

ur

CH4 (XCH4) was calculated using the following equation: 𝑋𝐶𝐻4 =

[𝐶𝐶𝐻4 ]𝑖𝑛 − [C𝐶𝐻4 ]𝑜𝑢𝑡 × 100% [𝐶𝐶𝐻4 ]𝑖𝑛

Jo

where [C𝐶𝐻4 ]𝑖𝑛 and [𝐶CH4 ]𝑜𝑢𝑡 are the CH4 concentrations in the inlet and outlet gas, respectively.

The reaction rate was tested using 5 mg of catalyst in the feed gas of 1 vol.% CH4 +

20 vol.% O2 + 79 vol.% N2 with GHSV of 600000 mL/ (g•h). The conversion of CH4 was kept within 5% to 15%. The reaction rate of CH4 combustion (rCH4) was calculated by the following equation: 𝑟𝐶𝐻4 =

[𝐶𝐶𝐻4 ][𝑋𝐶𝐻4 ]𝑉[𝑃𝑎𝑡𝑚 ] [𝑚𝑐𝑎𝑡. ][𝜔𝑃𝑑 ]𝑅𝑇

(mol/g•s)

5

where [CCH4] is the concentration of CH4 in the feed gas; [XCH4] is the conversion of CH4; V is the total gas rate; [mcat.] is the mass of catalyst used; [ωPd] is the Pd content on catalyst; [Patm] is the atmosphere pressure, equaled to 101.3 kPa; R is molar gas constant, equaled to 8.314 Pa•m3/mol•K; T is the room temperature, equaled to 298 K. The turnover frequency (TOF) was calculated by the following equation: TOF =

[r𝐶𝐻4 ]•[M𝑃𝑑 ] [D𝑃𝑑 ]

(s-1)

where [MPd] is the atomic weight of Pd, equaled to 106.4 g/mol; [DPd] is the Pd

ro of

dispersion, which is measured by CO-Pulse.

2.3 Catalyst characterization

Powder X-ray diffraction (XRD) patterns were performed on a Bruker D8 Focus diffractometer with Cu Kα radiation (λ=1.54056 Å, operated at 40 kV and 40 mA)

-p

and the scanning speed was 6 o/min. Nitrogen adsorption-desorption isotherms were obtained on Quantachrome Nova Touch LX3 instrument at -196 ℃ and the surface

re

areas of samples were calculated by the Brunauer-Emmett-Teller (BET) method. Scanning Electron microscopy (SEM) images were taken on a JSM-6360LV scanning electron microscope. Transmission electron microscopy (TEM) images were recorded

lP

on a JY/T 011-1996 electron microscope operated at 200 kV. The sample was firstly dispersed in ethanol and then collected on copper grids covered with carbon film. The

na

content of Pd on the catalysts was detected by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) on a Perkin-Elmer Optima 2100 DV spectrometer. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a PHI-Quantera

ur

SXM spectrometer with Al Kα (1486.6 eV) radiation as the excitation source at ultrahigh vacuum (6.7×10-8 Pa). All binding energies (BE) were determined with

Jo

respect to the C1s line (284.4 eV) originating from adventitious carbon. The powder samples were pressed into self-supporting disks loaded in the sub-chamber and evacuate for 4 h. The XPS spectra were deconvoluted and fitted by a Gaussian function with the XPSPEAK 4.1 software. H2 temperature-programmed reduction (H2-TPR) was carried out on a TPDRO 1100 (CE Instruments). After 100 mg of catalyst was swept in a flow of 5 vol.% H2/N2 (40 mL/min) at room temperature for 30 min, the samples were heated from 30 to 800 ℃ at a rate of 10 ℃/min in a flow of 5 vol.% H2/N2 (40 mL/min). The hydrogen 6

consumption was quantitatively evaluated by the TCD signal. The Pd dispersion was tested by CO pulse on a Micromecitics AutoChem Ⅱ2920 chemisorption analyzer. The catalyst (30 mg) were placed in a U-shaped quartz reactor and pretreated with 5 vol.% H2/Ar at 400 ℃ for 60 min. After cooling to 25 ℃, 1 vol.% CO/He (50 mL/min) was injected into the sample cell every two minutes until CO signal was kept constant. Temperature programmed reactions were measured by chemisorption analyzer (Micromecitics AutoChem Ⅱ2920) with mass spectrometer (HPR-20 QIC). For temperature programmed desorption of O2 (O2-TPD), the catalysts (50 mg) were

ro of

pretreated in 3 vol.% O2/He (50 mL/min) at 400 ℃ for 45 min, and then cooled to room temperature and kept in the gas flow for 45 min. After purging in He (50

mL/min) for 30 min, the catalysts were heated to 900 ℃ at a rate of 10 ℃/min. For

temperature programmed reduction of CH4 (CH4-TPSR), the catalysts (50 mg) were pretreated in 3 vol.% O2/He (50 mL/min) at 400 ℃ for 45 min, and then cooled to

-p

room temperature and kept in the gas flow for 45 min. The samples were heated from 30 to 900 ℃ at a rate of 10 ℃/min in a flow of 1 vol.% CH4/He (50 mL/min). The

re

effluent gas was monitored by a mass spectrometer (HPR-20 QIC) and MS signal of CH4 (m/z=15), CO2 (m/z=44) and H2O (m/z=16).

lP

CH4/O2 pulse experiments were carried out on a Micromeritics Model AutoChem Ⅱ 2920 instrument. The sample (50 mg) was placed in a U-shaped quartz reactor and a He gas flow of 40 ml·min−1 was used as the carrier gas. Prior to the experiments, the

na

catalysts were pretreated in a 3 vol% O2/He with a flow rate of 40 mL·min-1 at 500 oC for 30 min. After cooling down to 320 oC, the catalyst was purged with a 40 mL·min-1 He carrier gas flow for 30 min at 320 oC. Then a stream of 1 vol% CH4/He was

ur

injected into the catalyst for ten pulses with a precise analytical syringe. The total amount of CH4 consumption for ten pulses was calculated and represented by a

Jo

column. Then a stream of 3 vol% O2/He was injected into the catalyst instead of 1 vol% CH4/He. For the pulse experiments, CH4/O2 pulse measurements were conducted by repeatedly dosing 1 vol% CH4/He or 3 vol% O2/He (loop volume 0.5173 mL) into the catalyst bed at 320 oC.

3. Results 3.1 Catalytic performance for CH4 combustion 7

The catalytic performances of the Pd/CZ catalysts for CH4 combustion are shown in Fig. 1. Both κ-CZ and t-CZ supports exhibit poor catalytic activity for CH4 combustion, on which T90 (reaction temperature for 90% conversion of CH4) is nearly 530 °C. After Pd is loaded on the CZ supports, the catalytic performances of the Pd/CZ catalysts for CH4 combustion are significantly improved. Furthermore, the Pd/κ-CZ catalyst exhibits a much higher catalytic activity than the Pd/t-CZ catalyst. T90 on the Pd/κ-CZ catalyst is approximately 345 ℃, which is 80 ℃ lower than that of the Pd/t-CZ catalyst. As listed in Table 1, the reaction rate of the Pd/κ-CZ catalyst at 320 ℃ is 2.3 times higher than that of the Pd/t-CZ catalyst, and the TOF for the

ro of

former is almost as twice as that of the latter. The Pd/κ-CZ catalyst also maintains the superior catalytic activity at a low Pd content (0.5 wt.%) and even with 5% water

vapor in the reactant gas. Meanwhile, the catalytic activities of the Pd/κ-CZ and Pd/tCZ catalysts were compared to those of catalysts reported in references, and the

details are presented in Table S1. Interestingly, both the reaction rate and TOF of the

-p

Pd/κ-CZ catalyst are much higher than those of previously reported Pd-based catalysts (Pd/CeO2, Pd/Co3O4, Pd/ZrO2 and Pd/Al2O3) [21-28], highlighting the superior

re

catalytic performance of the Pd/κ-CZ catalyst for CH4 combustion. The stability of the Pd/CZ catalysts for CH4 combustion was also evaluated. As

lP

shown in Fig. 2A, both the Pd/CZ catalysts exhibit similar catalytic activity within five reaction cycles, indicating the high stability of the Pd/CZ catalysts for CH4 combustion. This property can also be confirmed by long-term reaction experiments.

na

As shown in Fig. 2B, the conversions of the Pd/κ-CZ and Pd/t-CZ catalysts for CH4 combustion are maintained at 85% and 28%, respectively, for 24 h at 330 ℃. However, the water-resistance stabilities of both Pd/CZ catalysts are low. After

ur

adding 5 vol.% water into the reactant gas, the conversion of CH4 on the Pd/κ-CZ catalyst declines to approximately 60%, while that on the Pd/t-CZ catalyst decreases

Jo

to 8%. As water was removed from the reactant gas, the conversion of CH4 can recover completely on the Pd/κ-CZ catalyst, even a slight increase in CH4 conversion on the Pd/t-CZ catalyst. To evaluate the potential applications, the catalytic activities of the Pd/κ-CZ and

Pd/t-CZ catalysts were further investigated under different GHSV conditions. As shown in Fig. 3A, the catalytic activities of both catalysts slightly decrease when GHSV increases from 30,000 to 120,000 mL/g·h, while the catalytic activity of the Pd/κ-CZ catalyst is still much better than that of the Pd/t-CZ catalyst. Meanwhile, Pd 8

utilization efficiencies of the two catalysts were calculated by the ratio of the converted CH4 amount to total Pd amount, as shown in Fig. 3B. It can be observed that the Pd utilization efficiency of the Pd/κ-CZ catalyst is much higher than that of the Pd/t-CZ catalyst regardless of GHSVs.

3.2 Structural characterization of the catalyst 3.2.1 XRD The XRD patterns of the CZ supports and Pd/CZ catalysts are shown in Fig. 4. The diffraction peaks of the t-CZ support are attributed to the tetragonal phase (t-phase) with a space group of P42/nmc (137) (JCPDS File Card No. 38-1436), while the

ro of

diffraction peaks of the κ-CZ support are assigned to the cubic fluorite-type structure with a space group of P213 (JCPDS File Card No. 90-465). Compared to the t-CZ support, a weak diffraction peak appears at 14° (Fig. 4B) for the κ-CZ support,

indicating the formation of a typical κ phase and the superlattice structure [46].

-p

Furthermore, based on the Scherrer equation, the corresponding crystallite sizes of κCZ and t-CZ supports are 13.4 and 5.2 nm, respectively.

re

Additionally, the Pd/κ-CZ and Pd/t-CZ catalysts exhibit the same patterns as the corresponding CZ supports and the diffraction peaks of Pd or PdO are absent due to

lP

the high Pd dispersion on the surface [51-53]. However, compared to CZ supports, the crystallite sizes of the Pd/κ-CZ and Pd/t-CZ catalysts slightly increase to 15.5 nm and

na

6.3 nm, respectively. 3.2.2 SEM and TEM

SEM images of the Pd/κ-CZ and Pd/t-CZ catalysts are shown in Fig. 5. There is a

ur

marked difference in the morphology of the two catalysts. The Pd/κ-CZ catalyst is stacked with spherical particles and possesses numerous pore structure. The

Jo

high porosity ratio is caused by the removal of CMK-3 during the preparation process, and leads to a larger specific surface area of the Pd/κ-CZ catalyst (22 m2/g) than that reported in references (< 2 m2/g) [46-48]. Different from the Pd/κ-CZ catalyst, the Pd/t-CZ catalyst exhibits a rod-like morphology, which is similar to the morphology of CMK-3. Furthermore, the specific surface area of the Pd/t-CZ catalyst is as 2.3 times as that of the Pd/κ-CZ catalyst, and a similar relationship between t-CZ and κCZ supports is observed (Table 1). The differences in the structure between the Pd/κ-CZ and Pd/t-CZ catalysts were 9

also detected by TEM. As shown in Fig. 6, interplanar spacing of ~0.304 nm for the Pd/κ-CZ is assigned to the (222) plane of Ce2Zr2O8 and interplanar spacing of ~0.305 nm for Pd/t-CZ is assigned to the (101) plane of CeZrO4. The interplanar spacing of 0.265 nm is assigned to the (101) plane of PdO. The average particle sizes of PdO on the Pd/κ-CZ and Pd/t-CZ catalysts are 5.5 and 7.4 nm, respectively, confirming the higher dispersion of Pd particles on κ-CZ support than that on t-CZ support, as listed on Table 1. The relatively higher Pd dispersion on κ-CZ support might be attributed to the stronger metal-support interaction between PdO and κ-CZ compared to that between PdO and t-CZ [53]. It is well known that Ce exhibits the properties of 4f

ro of

orbit and structural relaxation. The electron charge transfer between loading metal atoms and Ce atoms on the surface can strengthen the interaction between metal and Ce-based supports [54, 55]. Since the localized relaxation in κ-CZ decreases the

relaxation energy due to its special cation arrangement, the interaction between Pd

-p

and κ-CZ is enhanced for the Pd/κ-CZ catalyst compared to the Pd/t-CZ catalyst [40].

3.3 XPS

re

The XPS spectra of CZ supports and Pd/CZ catalysts are shown in Fig. 7. As shown in Fig. 7A, the peaks labelled as v and u are attributed to the Ce 3d5/2 and Ce

lP

3d3/2, respectively. The Ce 3d XPS spectra can be fitted with five pairs of peaks, and the spin-orbit splitting is 18.4 eV [56-58]. The doublets of v0 (881.5-881.7 eV)/u0 (899.9-900.1 eV) and v2 (885.3-885.5 eV)/u2 (903.7-903.9 eV) can be assigned to

na

Ce3+, while the peaks of v1 (882.6-883.0 eV)/u1 (901-901.4 eV), v3 (889.2-889.5 eV)/u3 (907.6-907.9 eV) and v4 (898.4-898.7 eV)/u4 (916.8-917.1 eV) are assigned to Ce4+ [59-61]. The ratio of Ce3+/(Ce3+ + Ce4+) on the surface of the Pd/CZ catalysts

ur

could be determined from the corresponding peak areas of v and u. As listed in Table 2, the ratio of Ce3+/(Ce3+ + Ce4+) on the surface of κ-CZ support is 0.16, which is as

Jo

same as that on t-CZ support. After Pd loading, the ratios of Ce3+/(Ce3+ + Ce4+) on the surface of the Pd/κ-CZ and Pd/t-CZ catalysts increase slightly to 0.19 and 0.22, respectively. It can be found that the ratio of Ce3+/(Ce3+ + Ce4+) for the two catalysts is not significantly different. Fig. 7B shows O 1s XPS spectra of the CZ support and Pd/CZ catalysts. The peak (Oα) at approximately 529.6 eV is assigned to lattice oxygen. The peak (Oβ) at approximately 531.5 eV represents surface oxygen, including surface-adsorbed oxygen belonging to defect oxide (such as O22- or O-) and hydroxyl-like groups (such 10

as OH) [62-64]. The proportion of surface oxygen species (Oβ/Oα+Oβ) is calculated by the peak area ratio of Oβ to the total of Oβ and Oα. The ratios of (Oβ/Oα+Oβ) are 0.28 and 0.29 for κ-CZ and t-CZ supports, respectively. After Pd loading, the ratio of (Oβ/Oα+Oβ) for the Pd/κ-CZ catalyst significantly increases to 0.46, while the ratio for the Pd/t-CZ catalyst slightly increases to 0.31. The increase in the amount of surface oxygen species can be ascribed to the increase in the number of oxygen-defect sites on the Pd/CZ catalysts. It is proposed that the length of the interface between Pd and CZ supports is crucial to the quantities of oxygen defects [65, 66]. With respect to the results of TEM and Pd dispersion, the Pd/κ-CZ catalyst exhibits smaller size of Pd particles due to the stronger interaction between Pd and κ-CZ supports than the Pd/t-

ro of

CZ catalyst. As a result, the total perimeter of the Pd−CZ interface for the Pd/κ-CZ

catalyst is higher than that of the Pd/t-CZ catalyst, leading to the significant increase in the ratio of (Oβ/Oα+Oβ) for the Pd/κ-CZ catalyst.

Fig. 7C shows Pd 3d XPS spectra of the Pd/CZ catalysts. The peak at 337.0 eV is

-p

attributed to Pd2+ in PdO particles [67, 68], and the peak at 338.0 eV is attributed to

Pdδ+ species with high oxidation valence in PdxCe1-xO2 [69, 70]. The Pd2+/Pd ratio is

re

calculated by the corresponding peak area ratio. The ratio for the Pd/κ-CZ catalyst is 0.57, and that of the Pd/t-CZ catalyst is 0.48. As PdO is the active site for methane

lP

combustion [16, 17], the large amount of PdO species for the Pd/κ-CZ catalyst can contribute much to the improvement of its catalytic activity for methane combustion.

na

3.4 Redox properties of catalysts

Fig. 8 shows the O2-TPD profiles of the Pd/CZ catalysts. For the Pd/κ-CZ catalyst, there are two desorption peaks with strong intensity at 550 and 640 ℃, accompanying

ur

a peak with low intensity at 860 ℃. The first two peaks are attributed to surface oxygen of the catalyst and the oxygen species at the interface between PdO and κ-CZ

Jo

support, respectively, while the last one is assigned to lattice oxygen in the CZ bulk [20, 71]. Three broad peaks are also present at 505, 600 and 799 °C for the Pd/t-CZ catalyst. Compared to the Pd/κ-CZ catalyst, the intensity of the first two peaks for the Pd/t-CZ catalyst is much lower, while that of the last peak is higher. Therefore, the Pd/κ-CZ catalyst can release a much larger amount of oxygen species at a relatively low temperature and participate in CH4 oxidation during the reaction process [72]. H2-TPR is utilized to detect the reducibility of CZ supports and the Pd/CZ catalysts. As shown in Fig. 9A, a broad reduction peak of Ce4+ to Ce3+ in CZ solid solution can 11

be observed for both κ-CZ and t-CZ supports. The complete oxygen storage capacity (OSCC) based on H2-TPR profiles is listed in Table 3. The OSCC values are 1.58 and 0.97 mmol/g for κ-CZ and t-CZ supports, respectively. According to the reaction equilibrium of Ce2Zr2O8 + H2 → Ce2Zr2O7 + H2O, the corresponding ratio of generating Ce3+ from Ce4+, assigned as the cerium efficiency [45], reaches 93% for κCZ and 57% for t-CZ supports. These results indicate that κ-CZ support has the higher OSCC and better reducibility due to the lower formation energy of oxygen vacancies in κ-CZ support compared to t-CZ support [40]. After loading Pd, the reducibility of the Pd/CZ catalyst is significantly improved [72]. For the Pd/κ-CZ catalyst (in Fig. 9B), the peak assigned as α1 is attributed to the

ro of

reduction of highly dispersed PdO that interacts strongly with κ-CZ support to

produce Pd0 [73]. The amount of H2 consumption for this peak is 160 μmol·g-1, which is almost one time larger than the theoretical amount for reducing PdO to Pd0 (89

μmol·g-1). The extra H2 consumption is caused by the oxygen back-spillover from CZ

-p

support to Pd [74]. The α2 peak at 150 ℃ for the Pd/κ-CZ catalyst can be attributed to the reduction of the surface oxygen of CZ around PdO particles due to the hydrogen

re

spillover from Pd to the support, and the γ peak is ascribed to the reduction of bulk CZ [20]. Different from the Pd/κ-CZ catalyst, there are two reduction peaks of α2 and

lP

γ for the Pd/t-CZ catalyst. The absence of α1 peak for the Pd/t-CZ catalyst is attributed to the case that the highly dispersed PdO interacting weakly with t-CZ support can be reduced at room temperature [75, 76]. Furthermore, compared to the Pd/κ-CZ

na

catalyst, the α2 reduction peak for the Pd/t-CZ catalyst shifts to lower temperature. In contrast, the γ reduction peak for the Pd/t-CZ catalyst shifts to slightly higher temperature than that of the Pd/κ-CZ catalyst. As shown in Table 3, the total amount

ur

of H2 consumption for the α1 and α2 reduction peaks on the Pd/κ-CZ catalyst is much higher than that on the Pd/t-CZ catalyst. During the reduction process, the lattice

Jo

oxygen in the CZ support can diffuse to the catalyst surface and is consumed via the channel Pd reduction. Therefore, the co-reduction process can deliver the oxygen mobility of the catalyst. In other words, compared to the Pd/t-CZ catalyst, the high amount of H2 consumption on the Pd/κ-CZ catalyst at low temperature demonstrates its excellent oxygen mobility. 3.5 Temperature programmed reduction of CH4 (CH4-TPSR) To further investigate the reactivity of oxygen species for methane oxidation on the 12

surface of the Pd/CZ catalysts, CH4-TPSR measurement was conducted and shown in Fig. 10. One peak in the temperature range of 180 - 280 ℃ appears for the Pd/κ-CZ catalyst, accompanying the simultaneous generation of CO2 and H2O. For the Pd/t-CZ catalyst, the peak of methane consumption starts at approximately 200 ℃ and centres at 225 ℃. Furthermore, the peak area for the Pd/t-CZ catalyst is approximately onehalf that of the Pd/κ-CZ catalyst. This result reveals that the Pd/κ-CZ catalyst possesses a higher amount of active oxygen species on the surface than the Pd/t-CZ catalyst, which is consistent with O2-TPD and H2-TPR results. As the temperature exceeds 400 ℃, a broad peak for methane consumption centred at 630 ℃ is present for

ro of

two Pd/CZ catalysts, and the production of CO is detected instead of CO2, indicating partial oxidation of CH4. However, the intensity of the consumption peak for the

Pd/t-CZ catalyst is significantly lower than that for the Pd/κ-CZ catalyst. In summary, the Pd/κ-CZ catalyst can provide more active oxygen species to oxidize CH4 than the Pd/t-CZ catalyst, promoting the activity of the Pd/κ-CZ catalyst for methane

-p

combustion.

re

4. Discussion

Pd-based catalyst generally possesses an excellent catalytic activity for the complete

lP

oxidation of methane, especially at low temperature [20, 21]. Herein, the remarkable enhancement of catalytic performance for methane combustion is achieved by finetuning the phase of CZ support, i.e., the tetragonal (t-CZ) and kappa phases (κ-CZ).

na

Compared to the Pd/t-CZ catalyst, the Pd/κ-CZ catalyst exhibits a much higher activity for methane combustion. T90 of the Pd/κ-CZ catalyst is 345 oC, while that of the Pd/t-CZ catalyst increases to 425 ℃, and the corresponding TOF value of the

ur

Pd/κ-CZ catalyst at 320 ℃ is approximately twice that of the Pd/t-CZ catalyst. Meanwhile, the Pd/κ-CZ catalyst exhibits a high activity for methane combustion

Jo

under the reagent gas containing water and high GHSV. Furthermore, the reaction rate and TOF of the Pd/κ-CZ catalyst are higher than those of the supported Pd catalysts in the references (Table S1), demonstrating the superior activity of the Pd/κ-CZ catalyst for methane combustion. Generally, methane combustion follows the Mars-van Krevelen (M-vK) mechanism [77-80]. CH4 molecules are first adsorbed and dissociated on the active sites of PdO [31]. Subsequently, CH3- is gradually oxidized by oxygen from PdO to form CO2 and H2O, and PdO is reduced to Pdδ+ (0<δ<2). Finally, Pdδ+ species is re-oxidized back to 13

PdO by both active oxygen on the surface of the CZ support, accompanying the formation of the oxygen vacancy on the CZ support, and the gas phase O2 directly [81]. The oxygen vacancy is later replenished by gas phase oxygen. Therefore, both the activation of methane and the redox cycle of PdO species are crucial steps in the process of methane combustion. The activation of methane is mainly dependent on the particle size and chemical valence of Pd. Murata et al. reported that the relationship between the sizes of Pd particles on Al2O3 supports and their catalytic activities for methane combustion displays a volcano shaped curve, and a high activity is obtained on Pd/Al2O3 with Pd

ro of

particles of 5-10 nm [23]. However, Stakheev [82] and Fujimoto et al. [77] reported that the TOF values of the Pd/Al2O3 and Pd/ZrO2 catalysts for methane combustion increase with the increase in the particle sizes of Pd within the range of 1-20 nm.

Muller’s work also revealed that the smaller particle size of PdO corresponds to the

lower catalytic activity of Pd/ZrO2 for methane combustion when the particle sizes of

-p

Pd are in the range of 5-15 nm [83]. This observation can be ascribed to the stronger

interaction between smaller PdO particles and support, which limits the reducibility of

re

PdO by methane. Though no unanimous conclusion can be drawn on the optimized size of PdO particle for supported Pd catalysts in methane combustion, the higher

lP

activity of the Pd/κ-CZ catalyst should be not mainly dependent on the smaller particle size of PdO compared to the Pd/t-CZ catalyst. There are two reasons for supporting this speculation. Firstly, the particle size of PdO on the Pd/κ-CZ and Pd/t-

na

CZ catalysts is relatively close (5.6 nm vs 7.2 nm). Secondly, the particle size of PdO on both catalysts is in the size range that affords the high catalytic activity of supported Pd catalyst for CH4 combustion.

ur

Meanwhile, it is widely accepted that PdO generally plays a decisive role in methane activation and oxidation [16, 17]. The XPS spectra indicates that both Pd2+ in

Jo

PdO and Pdδ+ in PdxCe1-xO2 (2<δ<4) exist on the Pd/κ-CZ and Pd/t-CZ catalysts. Nevertheless, the Pd2+/Pd ratio of the Pd/κ-CZ catalyst is higher than that of the Pd/tCZ catalyst, which is beneficial to improving the catalytic activity of the former for methane combustion. Based on the reaction mechanism of methane combustion over the supported Pd catalysts, another crucial step of methane combustion, in addition to the activation of methane, is the redox cycle of PdO species. As a material with a superior oxygen mobility, CZ support attracts more attention in methane combustion. Compared to t14

CZ support, the special crystalline structure of κ-CZ support, with an ordered array of cations, makes it easier to release the lattice oxygen, leading to an extremely high oxygen OSC and oxygen mobility for κ-CZ support [47]. In our work, the high oxygen mobility of κ-CZ support can be confirmed by the H2-TPR results of CZ supports. As shown in Fig. 7, the amount of H2 consumption for the κ-CZ support is almost as 1.6 times to that of the t-CZ support. Furthermore, the inherent factor of the superior oxygen mobility for the κ-CZ support is proposed by DFT calculation results. It has been concluded that the average formation energy of oxygen vacancies increases to 2.09 eV for the t-CZ support, which is almost three times as much as that

ro of

for the κ-CZ support (0.72 eV) [40]. The advantage of oxygen mobility for κ-CZ support is reserved after Pd loading, originating from both the nature of CZ support and the interaction between Pd and CZ support.

The oxygen mobility of the Pd/CZ catalyst is systematically evaluated by H2-TPR, O2-TPD and CH4-TPSR. The XPS results show that the ratio of surface oxygen

-p

species (Oβ/Oα+Oβ in XPS results) for the Pd/κ-CZ catalyst is higher than that for the

Pd/t-CZ catalyst, which can be ascribed to the strong interaction between Pd and κ-CZ

re

support [50]. The H2-TPR results indicate that the total amount of H2 consumption for the reduction peaks (α1 and α2) at the temperature lower than 250 oC on the Pd/κ-CZ

lP

catalyst is much higher than that on the Pd/t-CZ catalyst. Furthermore, the oxygen back-spillover from CZ support to Pd species appears during the reduction process. In other words, the lattice oxygen in the CZ support can diffuse to the catalyst surface

na

and is consumed via the channel Pd reduction. Therefore, the high amount of H2 consumption on the Pd/κ-CZ catalyst at low temperature proves its excellent oxygen mobility compared to the Pd/t-CZ catalyst. O2-TPD and CH4-TPSR experiments

ur

demonstrate that the Pd/κ-CZ catalyst can release more active oxygen species during only the heating process or methane injection, originating from the high amount of

Jo

surface oxygen species and the diffusion of oxygen from the bulk to the catalyst surface. These results indicate that the Pd/κ-CZ catalyst possesses a more outstanding oxygen mobility compared to the Pd/t-CZ catalyst, which can prompt the redox cycle of PdO species and result in the high catalytic activity of the Pd/κ-CZ catalyst for methane combustion. To confirm this conclusion, the CH4/O2 pulse experiments have been carried out and the results is presented in Fig. 11. Based on CH4 pulses in the first cycle, the total amount of CH4 consumption on the Pd/κ-CZ catalyst (27 μmol/g) for ten pulses is significantly higher than that on the Pd/t-CZ catalyst (0.6 μmol/g), 15

confirming the higher oxygen mobility of the Pd/κ-CZ catalyst compared to the Pd/tCZ catalyst. More importantly, the oxygen species that consumed in CH4 pulses can be recovered during O2 pulses on the Pd/κ-CZ catalyst, i.e the re-oxidation of Pd species and even CZ support, leading to the reservation of a high amount of CH4 consumption for CH4 pulses in the second cycle. In contrast, within four cycles of the CH4/O2 pulse experiments, the total amounts of both CH4 and O2 consumption decrease gradually on the Pd/t-CZ catalyst. Therefore, it can be concluded that the Pd/κ-CZ catalyst possesses an excellent and stable redox cycle of PdO species compared to the Pd/t-CZ catalyst.

ro of

On the other hand, the XPS spectra of the Pd/CZ catalysts after stability testing for 8 h in methane combustion are detected. The results (Fig. S1 and Table S2) reveal

that the ratios of Pd2+/Pd and Oβ/O for the Pd/κ-CZ catalyst change slightly, but these ratios drop significantly for the Pd/t-CZ catalyst. In addition, the catalytic activity of

the Pd/CZ catalysts is evaluated in the feed gas with stoichiometry ratio of CH4:O2 =

-p

1:2. As shown in Fig. S2, under this condition, the catalytic activity of the Pd/κ-CZ

catalyst decreases slightly as T90 increases from 345 to 367 ℃, but T90 of the Pd/t-CZ

re

catalyst increases from 425 to 470 ℃. These results indicate that the Pd/κ-CZ catalyst possesses not only a high amount of oxygen species on the surface but also an

lP

excellent oxygen mobility, thus leading to a higher catalytic activity of the Pd/κ-CZ

5. Conclusion

na

catalyst for CH4 combustion.

The CeZrO4 supports with a high specific surface area and different phases were synthesized by a solvent-thermal method with CMK-3 as hard template, and then the

ur

Pd/CeZrO4 catalysts were prepared. The Pd/κ-CZ catalyst exhibits a much higher catalytic activity for methane combustion than the Pd/t-CZ catalyst, whether the

Jo

reaction atmosphere contains water or not. Because methane combustion on noble metal catalysts generally follows the Mars-van Krevelen (M-vK) mechanism, the effects of the phase (κappa and tetragonal phases) of the CeZrO4 support on the state of PdOx active sites and oxygen mobility of Pd/CZ catalyst were investigated in details. Compared to the t-CZ support, the relatively smaller particle size of PdOx and higher ratio of Pd2+/Pd on the surface are present on the κ-CZ support due to the strong interaction between Pd and κ-CZ support. Hence, there are more PdO active sites for methane combustion on the κ-CZ support compared to those on the t-CZ 16

support. In addition, the Pd/κ-CZ catalyst has more surface oxygen species, higher reducibility, and greater ability to adsorb oxygen than the Pd/t-CZ catalyst, indicating the excellent oxygen mobility of the Pd/κ-CZ catalyst. These properties can prompt the redox cycle of Pd species, and then the catalytic activity of the Pd/κ-CZ catalyst for methane combustion. This path based on constructing a κ-CZ solid solution will provide an opportunity for us to prepare the supported noble metal catalysts with excellent performance for the catalytic combustion of hydrocarbons.

Credit author statement

ro of

Yuan Qing Ding: Conceptualization, Methodology, Data curation, Investigation, Writing-original draft. Qing Qing Wu: Methodology. Bo Lin: Validation,

Visualization. Yang Long Guo: Supervision. Yun Guo: Writing-review & editing. Yun Song Wang: Resources. Wang Li: Resources. Wang Cheng Zhan:

re

-p

Supervision, Writing-review & editing.

Declaration of interests

ur

na

lP

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Jo

This work was financially supported by the National Key Research and Development Program of China (2016YFC0204300), the National Natural Science Foundation of China (21922602, 21577034), and Fundamental Research Funds for the Central Universities (222201717003).

17

References 1. X. Wang, Y.B. Zhang, S.Y. Song, X.G. Yang, Z. Wang, R.C. Jin, H.J. Zhang, LArginine-Triggered self-assembly of CeO2 nanosheaths on palladium nanoparticles in water, Angew. Chem. Int. Ed. 55 (2016) 4542-4546. 2. Z. Ahmad, O. Kaario, C. Qiang, V. Vuorinen, M. Larmi, A parametric investigation of diesel/methane dual-fuel combustion progression/stages in a heavy-duty optical engine, Appl. Energ. 251 (2019) 113191. 3. A.H. Sohi, A. Eslami, A. Sheikhi, R. Sotudeh-Gharebagh, Sequential-based

ro of

process modeling of natural gas combustion in a fluidized bed reactor, Energ. Fuel 26 (2012) 2058-2067.

4. S. Specchia, M.A.A. Irribarra, P. Palmisano, G. Saracco, V. Specchia, Aging of premixed metal fiber burners for natural gas combustion catalyzed with

-p

Pd/LaMnO3·2ZrO2, Ind. Eng. Chem. Res. 46 (2007) 6666-6673.

5. Z.Y. Zhao, B.W. Wang, J. Ma, W.C. Zhan, L. Wang, Y.L. Guo, Y. Guo, G.Z. Lu,

re

Catalytic combustion of methane over Pd/SnO2 catalysts, Chinese J. Catal. 38 (2017) 1322-1329.

6. E.S. Rubin, R.N. Cooper, R.A. Frosh, T.H. Lee, G. Marland, A.H. Rosenfeld,

lP

D.D. Stine, Realistic mitigation options for global warming, Science 257 (1992) 148-149.

na

7. G.C. Rhoderick, W.D. Dorko, Standards development of global warming gas species: methane, nitrous oxide, trichlorofluoromethane, and dichlorodifluoromethane, Environ. Sci. Technol. 38 (2004) 2685-2692.

ur

8. X. Zhu, K.Z. Li, L. Neal, F.X. Li, Perovskites as geo-inspired oxygen storage materials for chemical looping and three-way catalysis: a perspective, ACS Catal.

Jo

8 (2018) 8213-8236.

9. C. He, J. Cheng, X. Zhang, M. Douthwaite, S. Pattisson, Z.P. Hao, Recent advances in the catalytic oxidation of volatile organic compounds: A review based on pollutant sorts and sources, Chem. Rev. 119 (2019) 4471-4568.

10. D. Ferri, M. Elsener, O. Krocher, Methane oxidation over a honeycomb Pd-only three-way catalyst under static and periodic operation, Appl. Catal., B 220 (2018) 67-77. 11. X. Wang, B.F. Jin, R.X. Feng, W. Liu, D. Weng, X.D. Wu, S. Liu, A robust core18

shell silver soot oxidation catalyst driven by Co3O4: Effect of tandem oxygen delivery and Co3O4-CeO2 synergy, Appl. Catal., B 250 (2019) 132-142. 12. L.Y. Yang, X. Yang, S.Y. Lin, R.X. Zhou, Insight into the role of a structural promoter (Ba) in three-way catalyst Pd/CeO2-ZrO2 using in situ DRIFT, Catal. Sci. Technol. 5 (2015) 2688-2695. 13. E.D. Goodman, S. Dai, A.C. Yang, C.J. Wrasman, A. Gallo, S.R. Bare, A.S. Hoffman, T.F. Jaramillo, G.W. Graham, X.Q. Pan, M. Cargnello, Uniform Pt/Pd bimetallic nanocrystals demonstrate platinum effect on palladium methane combustion activity and stability, ACS Catal. 7 (2017) 4372-4380.

ro of

14. M. Cargello, J.J. Delgado Jaén, J.C. Hernández Garrido, K. Bakhmutsky, T. Montini, J.J. Calvino Gámez, R.J. Gorte, P. Fornasiero, Exceptional activity for

methane combustion over modular Pd@CeO2 subunits on functionalized Al2O3, Science 337 (2012) 713-717.

15. X.W. Yang, Q. Gao, Z.Y. Zhao, Y.L. Guo, Y. Guo, L.Wang, Y.S. Wang, W.C.

-p

Zhan, Surface tuning of noble metal doped perovskite oxide by synergistic effect of thermal treatment and acid etching: A new path to high-performance catalysts

re

for methane combustion, Appl. Catal., B 239 (2018) 373-382.

16. J. Ma, Y. Lou, Y.F. Cai, Z.Y. Zhao, L. Wang, W.C. Zhan, Y.L. Guo, Y. Guo, The

lP

relationship between the chemical state of Pd species and the catalytic activity for methane combustion on Pd/CeO2, Catal. Sci. Technol. 8 (2018) 2567-2577. 17. F. Huang, J.J. Chen, W. Hu, G.X. Li, Y. Wu, S.D. Yuan, L. Zhong, Y.Q. Chen, Pd

na

or PdO: catalytic active site of methane oxidation operated close to stoichiometric air to fuel for natural gas vehicles, Appl. Catal., B 219 (2017) 73-81. 18. H. Yoshida, T. Nakajima, Y. Yazawa, T. Hattori, Support effect on methane

ur

combustion over palladium catalysts, Appl. Catal., B 71 (2007) 70-79. 19. E. Hong, C.S. Kim, D.H. Lim, H.J. Cho, C.H. Shin, Catalytic methane

Jo

combustion over Pd/ZrO2 catalysts: Effect of crystalline structure and textural properties, Appl. Catal., B 232 (2018) 544-552.

20. 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. 21. X.W. Yang, Q. Li, E.J. Lu, Z.Q. Wang, X.Q. Gong, Z.Y. Yu, Y. Guo, L. Wang, Y.L. Guo, W.C. Zhan, J.S. Zhang, S. Dai, Taming the stability of Pd active phases through a compartmentalizing strategy toward nanostructured catalyst supports, 19

Nat. Commun. 10 (2019) 1611-1619. 22. Y.D. Cao, R. Ran, X.D. Wu, B.H. Zhao, J. Wan, D. Weng, Comparative study of ageing condition effects on Pd/Ce0.5Zr0.5O2 and Pd/Al2O3 catalysts: Catalytic activity, palladium nanoparticle structure and Pd-support interaction, Appl. Catal., A 457 (2013) 52-61. 23. K. Murata, Y. Mahara, J. Ohyama, Y. Yamamoto, S. Arai, A. Satsuma, The metal-support interaction concerning the particle size effect of Pd/Al2O3 on methane combustion, Angew. Chem. Int. Ed. 56 (2017) 15993-15997. 24. X.L. Zou, Z.B. Rui, H.B. Ji, Core-shell NiO@PdO nanoparticles supported on

1615-1625.

ro of

alumina as an advanced catalyst for methane oxidation, ACS Catal. 7 (2017)

25. Z.P. Chen, S. Wang, Y. Ding, L. Zhang, L.R. Lv, M.Z. Wang, S.D. Wang, Pd

catalysts supported on Co3O4 with the specified morphologies in CO and CH4 oxidation, Appl. Catal., A 532 (2017) 95-104.

-p

26. C. Chen, Y.H. Yeh, M. Cargnello, C.B. Murray, P. Fornasiero, R.J. Gorte,

Methane oxidation on Pd@ZrO2/Si-Al2O3 is enhanced by surface reduction of

re

ZrO2, ACS Catal. 4 (2014) 3902-3909.

27. Y. Lou, J. Ma, W.D. Hu, Q.G. Dai, L. Wang, W.C. Zhan, Y.L. Guo, X.M. Cao, Y.

lP

Guo, P. Hu, G.Z. Lu, Low-temperature methane combustion over Pd/H-ZSM-5: Active Pd sites with specific electronic properties modulated by acidic sites of HZSM-5, ACS Catal. 6 (2016) 8127-8139.

na

28. M. Jeong, N. Nunotani, N. Moriyama, N. Imanaka, Effect of introducing Fe2O3 into CeO2-ZrO2 on oxygen release properties and catalytic methane combustion over PdO/CeO2-ZrO2-Fe2O3/γ-Al2O3 catalysts, Catal. Sci. Technol. 7 (2017) 1986-

ur

1990.

29. A. Gomathi, S.M. Vickers, R. Gholami, M. Alyani, R.W.Y. Man, M.J.

Jo

MacLachlan, K.J. Smith, M.O. Wolf, Nanostructured materials prepared by surface-assisted reduction: New catalysts for methane oxidation, ACS Appl. Mater. Interfaces 7 (2015) 19268-19273.

30. H. Peng, C. Rao, N. Zhang, X. Wang, W.M. Liu, W.T. Mao, L. Han, P. Zhang, S. Dai, Confined ultrathin Pd-Ce nanowires with outstanding moisture and SO2 tolerance in methane combustion, Angew. Chem. Int. Ed. 57 (2018) 8953-8957. 31. H. Stotz, L. Maier, A. Boubnov, A.T. Gremminger, J.D. Grunwaldt, O. Deutschmann, Surface reaction kinetics of methane oxidation over PdO, J. Catal. 20

370 (2019) 152-175. 32. P. Yang, S.S. Yang, Z.N. Shi, Z.H. Meng, R.X. Zhou, Deep oxidation of chlorinated VOCs over CeO2-based transition metal mixed oxide catalysts, Appl. Catal., B 162 (2015) 227-235. 33. P. Fornasiero, E. Fonda, R. Di Monte, G. Vlaic, J. Kašpar, M. Graziani, Relationship between structural/textural properties and redox behavior in Ce0.6Zr0.4O2 mixed oxides, J. Catal. 187 (1999) 177-185. 34. A. Trovarelli, F. Zamar, J. Llorca, C. de Leitenburg, G. Dolcetti, J.T. Kiss, Nanophase fluorite-structured CeO2-ZrO2 catalysts prepared by high-energy

ro of

mechanical milling, J. Catal. 169 (1997) 490-502. 35. J. Fan, X.D. Wu, X.D. Wu, Q. Liang, R. Ran, D. Weng, Thermal ageing of Pt on low-surface-area CeO2-ZrO2-La2O3 mixed oxides: Effect on the OSC performance, Appl. Catal., B 81 (2008) 38-48.

36. P. Yang, S.S. Yang, Z.N. Shi, F. Tao, X.L. Guo, R.X. Zhou, Accelerating effect of

-p

ZrO2 doping on catalytic performance and thermal stability of CeO2-CrOx mixed oxide for 1,2-dichloroethane elimination, Chem. Eng. J. 285 (2016) 544-553.

re

37. C.W. Li, Y. Sun, F. Hess, I. Djerdj, J. Sann, P. Voepel, P. Cop, Y.L. Guo, B.M. Smarsly, H. Over, Catalytic HCl oxidation reaction: Stabilizing effect of Zr-

lP

doping on CeO2 nano-rods, Appl. Catal., B 239 (2018) 628-635. 38. M. Yashima, T. Hirose, S. Katano, Y. Suzuki, M. Kakihana, M. Yoshimura, Structural changes of ZrO2-CeO2 solid solutions around the monoclinic-tetragonal

na

phase boundary, Phys. Rev. B 51 (1995) 8018-8026. 39. M. Yashima, Crystal structures of the tetragonal ceria-zirconia solid solutions CexZr1-xO2 through first principles calculations (0≤x≤1), J. Phys. Chem. C 113

ur

(2009) 12658-12662.

40. H.F. Wang, X.Q. Gong, Y.L. Guo, G.Z. Lu, P. Hu, A model to understand the

Jo

oxygen vacancy formation in Zr-doped CeO2: electrostatic interaction and structural relaxation, J. Phys. Chem. C 113 (2009) 10229-10232.

41. J. Kašpar, P. Fornasiero, M. Graziani, Use of CeO2-based oxides in the three-way catalysis, Catal. Today 50 (1999) 285-298. 42. M. Boaro, A. Trovarelli, J. Hwang, T.O. Mason, Electrical and oxygen storage/release properties of nanocrystalline ceria-zirconia solid solutions, Solid State Ionics 147 (2002) 85-95. 43. Y. Nagai, T. Yamamoto, T. Tanaka, S. Yoshida, T. Nonaka, T. Okamoto, A. 21

Suda, M. Sugiura, X-ray absorption fine structure analysis of local structure of CeO2-ZrO2 mixed oxides with the same composition ratio (Ce/Zr=1), Catal. Today 74 (2002) 225-234. 44. J. Li, X.F. Liu, W.C. Zhan, Y. Guo, Y.L. Guo, G.Z. Lu, Preparation of high oxygen storage capacity and thermally stable ceria-zirconia solid solution, Catal. Sci. Technol. 6 (2016) 897-907. 45. S. Lemaux, A. Bensaddik, A.M.J. van der Eerden, J.H. Bitter, D.C. Koningsberger, Understanding of enhanced oxygen storage capacity in Ce0.5Zr0.5O2: The presence of an Anharmonic pair distribution function in Zr-O2

ro of

subshell as analyzed by XAFS spectroscopy, J. Phys. Chem. B 105 (2001) 48104815.

46. S. Urban, P. Dolcet, M. Moller, L. Chen, P. Klar, I. Djerdj, S. Gross, B.M.

Smarsly, H. Over, Synthesis and full characterization of the phase-pure pyrochlore Ce2Zr2O7 and the κ-Ce2Zr2O8 phases, Appl. Catal., B 197 (2016) 23-34.

-p

47. Y.Q. Ding, Z. Wang, Y.L. Guo, Y. Guo, L. Wang, W.C. Zhan, A novel method

for the synthesis of CexZr1-xO2 solid solution with high purity of kappa phase and

re

excellent reactive activity, Catal. Today 327 (2019) 262-270.

48. M. Tada, S.H. Zhang, S. Malwadkar, N. Ishiguro, J. Soga, Y. Nagai, K. Tezuka,

lP

H. Imoto, S. Otsuka-Yao-Matsuo, S. Ohkoshi, Y. Iwasawa, The active phase of nickel/ordered Ce2Zr2Ox catalysts with a discontinuity (x=7-8) in methane steam reforming, Angew. Chem. Int. Ed. 124 (2012) 9495-9499.

na

49. D.Y. Zhao, J.L. Feng, Q.S. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores, Science 279 (1998) 548-552.

ur

50. J. Lee, S.H. Joo, R. Ryoo, Synthesis of mesoporous silicas of controlled pore wall thickness and their replication to ordered nanoporous carbons with various pore

Jo

diameters, J. Am. Chem. Soc. 124 (2002) 1156-1157.

51. P. Yang, S.F. Zou, Z.N. Shi, F. Tao, R.X. Zhou, Elimination of 1,2-dichloroethane over (Ce,Cr)xO2/Moy catalysts (M=Ti, V, Nb, Mo, W and La), Appl. Catal., B 191 (2016) 53-61. 52. P. Bera, K.R. Priolkar, A. Gayen, P.R. Sarode, M.S. Hegde, S. Emura, R. Kumashiro, V. Jayaram, G.N. Subbanna, Ionic dispersion of Pt over CeO2 by the combustion method: structural investigation by XRD, TEM, XPS and EXAFS, Chem. Mater. 15 (2003) 2049-2060. 22

53. Z. Hu, Z. Wang, Y. Guo, L. Wang, Y.L. Guo, J.S. Zhang, W.C. Zhan, Total oxidation of propane over a Ru/CeO2 catalyst at low temperature, Environ. Sci. Technol. 52 (2018) 9531-9541. 54. P. Tereshchuk, R.L.H. Freire, C.G. Ungureanu, Y. Seminovski, A. Kiejna, J.L.F. da Silva, The role of charge transfer in the oxidation state change of Ce atoms in the TM3-CeO2(111) systems (TM=Pd, Ag, Pt, Au): a DFT+U investigation, Phys. Chem. Chem. Phys. 17 (2015) 13520-13530. 55. S. Tosoni, G. Pacchioni, Trends in adhesion energies of gold on MgO(100), Rutile TiO2(110), and CeO2(111) surfaces: A comparative DFT study, J. Phys. Chem. C

ro of

121 (2017) 28328-28338. 56. C. Li, Y. Sun, I. Djerdj, P. Veopel, C. Sack, T. Weller, R. Ellinghaus, J. Sann, Y. Guo, B. Smarsly, H. Over, Shape-controlled CeO2 nanoparticles: stability and

activity in the catalyzed HCl oxidation reaction, ACS Catal. 7 (2017) 6453-6463. 57. A. Nelson, K. Schulz, Surface chemistry and microstructural analysis of CexZr1model catalyst surfaces, Appl. Surf. Sci. 210 (2003) 206-221.

-p

xO2-y

58. Z. Hu, S. Qiu, Y. You, Y. Guo, Y.L. Guo, L. Wang, W.C. Zhan, G.Z. Lu,

re

Hydrothermal synthesis of NiCeOx nanosheets and its application to the total oxidation of propane, Appl. Catal., B 225 (2019) 110-120.

lP

59. Q.G. Dai, Z.Y. Zhang, J.R. Yan, J.Y. Wu, G. Johnson, W. Sun, X.Y. Wang, S. Zhang, W.C. Zhan, Phosphate-functionalized CeO2 nanosheets for efficient

13437.

na

catalytic oxidation of dichloromethane, Environ. Sci. Technol. 52 (2018) 13430-

60. Q.G. Dai, J.Y. Wu, W. Deng, J.S. Hu, Q.Q. Wu, L.M. Guo, W. Sun, W.C. Zhan, X.Y. Wang, Comparative studies of P/CeO2 and Ru/CeO2 catalysts for catalytic

ur

combustion of dichloromethane: From effects of H2O to distribution of chlorinated by products, Appl. Catal., B 249 (2019) 9-18.

Jo

61. Z. Wang, Z.P. Huang, J.T. Brosnahan, S. Zhang, Y.L. Guo, Y. Guo, L. Wang, Y.S. Wang, W.C. Zhan, Ru/CeO2 catalyst with optimized CeO2 support morphology and surface facets for propane combustion, Environ. Sci. Technol. 53 (2019) 5349-5358. 62. X.F. Tang, J.H. Ji, L. Sun, H.M. Hao, Origination of N2O from NO reduction by NH3 over β-MnO2 and α-Mn2O3, Appl. Catal., B 99 (2010) 156-162. 63. D.M. Meng, W.C. Zhan, Y. Guo, Y.L. Guo, L. Wang, G.Z. Lu, A highly effective catalyst of Sm-MnOx for the NH3-SCR of NOx at low temperature: Promotional 23

role of Sm and its catlaytic performance, ACS Catal. 5 (2015) 5973-5983. 64. H. Arandiyan, H.X. Dai, K. Ji, H. Sun, J.H. Li, Pt nanoparticles embedded in colloidal crystal template derived 3D ordered macroporous Ce0.6Zr0.3Y0.1O2: highly efficient catalysts for methane combustion, ACS Catal. 5 (2015) 17811793. 65. B. Wang, D. Weng, X.D. Wu, R. Ran, Modification of Pd-CeO2 catalyst by different treatments: Effect on the structure and CO oxidation activity, Appl. Surf. Sci. 257 (2011) 3878-3883. 66. H.L. Wang, S. Liu, Z. Zhao, X. Zou, M.H. Liu, W. Liu, X.D. Wu, D. Weng,

ro of

Activation and deactivation of Ag/CeO2 during soot oxidation: Influences of interfacial ceria reduction, Catal. Sci. Technol. 7 (2017) 2129-2139.

67. K.S. Kim, A.F. Gossmann, N. Winograd, X-ray photoelectron spectroscopic

studies of palladium oxides and the palladium-oxygen electrode, Anal. Chem. 197 (1974) 46.

-p

68. T. Pillo, R. Zimmermann, P. Steiner, S. Hüfner, The electronic structure of PdO found by photoemission (UPS and XPS) and inverse photoemission (BIS), J.

re

Phys. Condens. Matter 9 (1997) 3987-3999.

69. O. Pozdnyakova, D. Teschner, A. Wootsch, J. Kröhnert, B. Steinhauer, H. Sauer,

lP

L. Toth, F.C. Jentoft, A. Knop-Gericke, Z. Paál and R. Schlögl, Preferential CO oxidation in hydrogen (PROX) on ceria-supported catalysts, part Ⅱ: Oxidation states and surface species on Pd/CeO2 under reaction conditions, suggested

na

reaction mechanism, J. Catal. 237 (2006) 17-28. 70. Z. J. Zuo, W. Huang, P.D. Han, Z.H. Li, Theoretical and experimental investigation of the influence of Co and Pd on the titanium dioxide phase

ur

transition by different calcined temperature, J. Mol. Struct. 936 (2009) 118-124. 71. L. Lan, S. Chen, Y. Cao, M. Zhao, M.C. Gong, Y.Q. Chen, Preparation of ceria-

Jo

zirconia by modified coprecipitation method and its supported Pd-only three-way catalyst, J. Colloid Interf. Sci. 450 (2015) 404-416.

72. H.L. Wang, B.F. Jin, H.B. Wang, N.M. Ma, W. Liu, D. Weng, X.D. Wu, S. Liu, Study of Ag promoted Fe2O3@CeO2 as superior soot oxidation catalysts: The role of Fe2O3 crystal plane and tandem oxygen delivery, Appl. Catal., B 237 (2018) 251-262. 73. S.Y. Lin, L.Y. Yang, X. Yang, R.X. Zhou, Redox properties and metal-support interaction of Pd/Ce0.6Zr0.33O2-Al2O3 catalyst for CO, HC and NOx elimination, 24

Appl. Surf. Sci. 305 (2014) 642–649. 74. H. Zhu, Z. Qin, W. Shan, 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. 75. M.Q. Shen, M. Yang, J. Wang, J. Wen, M.W. Zhao, W.L. Wang, Pd/support interface-promoted Pd/Ce0.7Zr0.3O2-Al2O3 automobile three-way catalysts: Studying the dynamic oxygen storage capacity and CO, C3H8 and NO conversion, J. Phys. Chem. C 113 (2009) 3212-3221. 76. A. Iglesias-Juez, A. Martínez-Arias, M. Fernández-García, Metal-promoter

ro of

interface in Pd/(Ce,Zr)Ox/Al2O3 catalysts: effect of thermal aging, J. Catal. 221 (2004) 148-161.

77. K. Fujimoto, F.H. Ribeiro, M. Avalos-Borja, E. Iglesia, Structure and reactivity of PdOx/ZrO2 catalysts for methane oxidation at low temperatures, J. Catal. 179 (1998) 431-442.

-p

78. B.V. Devener, S.L. Anderson, T. Shimizu, H. Wang, J. Nabity, J. Engel, J. Yu, D. Wickham, S. Williams, In situ generation of Pd/PdO nanoparticle methane

re

combustion catalyst: Correlation of particle surface chemistry with ignition, J. Phys. Chem. C 113 (2009) 20632-20639.

lP

79. W.R. Schwartz, L.D. Pfefferle, Combustion of methane over palladium-based catalysts: Support interactions, J. Phys. Chem. C 116 (2012) 8571-8578. 80. H. Stotz, L. Maier, A. Boubnov, A.T. Gremminger, J.D. Grunwaldt and O.

na

Deutschmann, Surface reaction kinetics of methane oxidation over PdO, J. Catal. 370 (2019) 152-175.

81. J.J. Willis, A. Gallo, D. Sokaras, H. Aljama, S.H. Nowak, E.D. Goodman, L. Wu,

ur

C.J. Tassone, T.F. Jaramilo, F. Abild-Pedersen, M. Cargnello, Systematic structure-property relationship studies in palladium-catalyzed methane complete

Jo

combustion, ACS Catal. 7 (2017) 7810-7821.

82. A.Y. Stakheev, A.M. Batkin, N.S. Teleguina, G.O. Bragina, V.I. Zaikovsky, I.P. Prosvirin, A.K. Khudorozhkov, V.I. Buykhtiyarov, Particle size effect on CH4 oxidation over noble metals: Combustion of Pt and Pd catalysts, Top. Catal. 56 (2013) 306-310. 83. C.A. Muller, M. Maciejewski, R.A. Koeppel, A. Baiker, Combustion of methane over palladium/zirconia: effect of Pd-particle size and role of lattice oxygen, Catal. Today 47 (1999) 245-252. 25

26

ro of

-p

re

lP

na

ur

Jo

27

ro of

-p

re

lP

na

ur

Jo

ro of -p re lP

Fig. 1 Catalytic activity of the Pd/CZ catalysts for CH4 combustion: (A) CH4 conversion as a function of reaction temperature; (B) Reaction rate of the Pd/CZ

Jo

ur

na

catalysts at different reaction temperatures.

28

ro of -p re lP

Fig. 2 The stability of the Pd/CZ catalysts for CH4 combustion: (A) Recycling experiments of the Pd/CZ catalysts; (B) Reaction stability of the Pd/CZ catalysts at

Jo

ur

na

330 ℃ with and without water.

29

ro of

Fig. 3 Effect of the space velocity on the catalytic activity of the Pd/CZ catalysts and

Jo

ur

na

lP

re

-p

Pd utilization efficiency.

30

ro of

Fig. 4 XRD patterns (A) and an enlarge view of the corresponding pattern (B) of CZ

Jo

ur

na

lP

re

-p

support and Pd/CZ catalysts.

31

A

B

Jo

ur

na

lP

re

-p

ro of

Fig. 5 SEM images of the Pd/κ-CZ (A) and Pd/t-CZ (B) catalysts.

32

ro of -p re lP na ur

Jo

Fig. 6 TEM images of the Pd/κ-CZ (A, B) and Pd/t-CZ (C, D) catalysts, and the corresponding particle size distributions of the Pd/κ-CZ (E) and Pd/t-CZ (F) catalysts.

33

ro of -p re lP na ur Jo

Fig. 7 Ce 3d (A), O 1s (B) and Pd 3d (C) XPS spectra of CZ support and Pd/CZ catalysts.

34

O2 Uptake/release Intensity (a.u.)

Pd/-CZ Pd/t-CZ o

640 C o

550 C

o

855 C

100

200

300

400

o

o

600 C

500

600

502 C

o

799 C

700

800

900

Temperature (oC)

Jo

ur

na

lP

re

-p

ro of

Fig. 8 O2-TPD profiles of the Pd/κ-CZ and Pd/t-CZ catalysts.

35

H2 Uptake/release Intensity (a.u.)

A 560 oC

-CZ

545 oC t-CZ

200

300

400

500

600

Temperature (oC)

700

800

ro of

H2 Uptake/release Intensity (a.u.)

100

2

B

1 

Pd/-CZ



100

200

300

400

Pd/t-CZ

500

600

700

800

lP

0

re

-p

2

Temperature (oC)

Jo

ur

na

Fig. 9 H2-TPR profiles of CZ supports (A) and Pd/CZ catalysts (B).

36

Jo

ur

na

lP

re

-p

ro of

Fig. 10 CH4-TPSR profiles of the Pd/κ-CZ and Pd/t-CZ catalysts.

37

CH4 pulse

Pd/-CZ

30

O2 pulse

25

25

20

20 Pd/t-CZ

15

15

10

10

5

5

0

1

2

3

1

4

3

2

4

O2 Consumption (mol/g)

CH4 Consumption (mol/g)

30

0

Cycles

ro of

Fig. 11 CH4 and O2 consumption during the CH4/O2 pulse experiments within four cycles. The specific column is the total amount of CH4 or O2 consumption for ten

Jo

ur

na

lP

re

-p

pulses during each cycle.

38

Table 1. The structural parameters and catalytic performances of CZ supports and Pd/CZ catalysts Pd

Surface

Crystallite

Lattice

T90

Pd

rCH4×105 e

TOF×

content a

area

size b

parameters c

(℃)

dispersion d

(mol·g-1·s-

103 e (s-

(wt%)

(m2/g)

(nm)

(nm)

(%)

1

1

κ-CZ

/

37

13.4

1.054

/

/

/

/

Pd/κ-CZ

0.95

22

15.5

1.052

345

36.2

32.2

94.9

t-CZ

/

85

5.2

0.376/0.534

/

/

/

/

0.93

50

6.3

0.376/0.542

430

21.3

9.8

48.9

Sample

Pd/t-CZ

)

a

measured by ICP; calculated by Scherrer formula; c calculated by Bragg equation; d measured by CO-pulse; e measured at 320 ℃.

Jo

ur

na

lP

re

-p

ro of

b

39

)

Table 2. Surface elemental compositions of CZ supports and Pd/CZ catalysts Sample

Surface composition

Ce/Zr

Ce3+/Ce

Oβ/(Oα+Oβ)

Pd2+/Pd

(atom/%) Pd

O

κ-CZ

15.2

-

74.4

1.5

0.16

0.28

-

Pd/κ-CZ

11.2

2.3

76.4

1.1

0.19

0.46

0.57

t-CZ

17.4

-

74.7

2.2

0.16

0.29

-

Pd/t-CZ

15.1

1.4

72.7

1.4

0.22

0.31

0.48

Jo

ur

na

lP

re

-p

ro of

Ce

40

Table 3. The amounts of H2 consumption for CZ supports and Pd/CZ catalysts Sample

The total amount -1

(mmol·g )

The α1

The α2

The γ amount

amount

amount

(μmol·g-1)

(μmol·g-1)

(μmol·g-1)

1.58

--

--

--

Pd/κ-CZ

0.99

160

807

21

t-CZ

0.97

--

--

--

Pd/t-CZ

0.59

--

470

120

Jo

ur

na

lP

re

-p

ro of

κ-CZ

41