Facile Design of Highly Effective CuCexCo1âxOy Catalysts with

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Materials and Interfaces

Facile Design of Highly Effective CuCexCo1-xOy Catalysts with Diverse Surface/Interface Structures towards NO Reduction by CO at Low Temperatures Xinyang Wang, Xinyong Li, Jincheng Mu, Shiying Fan, Liang Wang, Guoqiang Gan, Meichun Qin, Ji Li, Zeyu Li, and Dongke Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01636 • Publication Date (Web): 30 Jul 2019 Downloaded from pubs.acs.org on August 10, 2019

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Industrial & Engineering Chemistry Research

Facile Design of Highly Effective CuCexCo1-xOy Catalysts with Diverse Surface/Interface Structures towards NO Reduction by CO at Low Temperatures Xinyang Wang†, Xinyong Li*†, Jincheng Mu†, Shiying Fan†, Liang Wang†, Guoqiang Gan†, Meichun Qin†, Ji Li†, Zeyu Li†, Dongke Zhang*‡ †State Key Laboratory of Fine Chemicals, Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China ‡ Centre for Energy (M473), The University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia

* Corresponding authors: Xinyong Li, Email: [email protected] Dongke Zhang, Email: [email protected]

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: A series of highly effective CuCexCo1-xOy catalysts with diverse surface/interface structures were synthesised and applied to NO reduction by CO from100 to 400 ˚C. The CuCe0.2Co0.8Oy catalyst exhibited superior catalytic performance, achieving 100% NO reduction at 175 oC. The structural, physicochemical properties, and the intermediate species were systematically characterized. The results indicated that the surface dispersity of CuO species, the generation of low-oxidation-state metal species (Cu+, Co2+) and surface oxygen vacancies (SOVs) were greatly enhanced due to stronger interaction between highly dispersed CuO species and supports, which favours for the enhancement of consequent redox properties and the regeneration of SOVs and low-oxide-state metal ions during NO reduction by CO. The in situ FT-IR results illustrated that the adsorption bands of CO2 and N2O on CuCe0.2Co0.8Oy surface could emerge at lower temperatures, revealing that the CO adsorption/conversion and NO dissociation were significantly promoted by Ce modification.

KEYWORDS: CuCexCo1-xOy; NO reduction by CO; oxygen vacancy; synergetic effect


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1. INTRODUCTION Nitrogen oxide (NOx) is one of poisonous and high reactive gas pollutants, which leads serious environmental problems such as acid rain, photochemical smog, ozone depletion.1,

2

Carbon

monoxide (CO) is always discharged with NOx from transportation and industrial combustion devices.3 In a previous study, CO as a high ecological toxic pollutant was always eliminated by catalytic oxidation.4,

5

Currently, NO reduction with CO has been considered as an efficient

process that was developed based on the three-way catalyst (TWC) technology, and could remove two pollutants simultaneously without carbon deposits.6 Platinum group metals (PGMs) have been widely identified as one of the efficient catalysts for this reaction.7-12 Unfortunately, the inevitable disadvantages restricted their widespread applications, such as high cost, limited resources and low thermal stability.10 Therefore, it is highly desired to develop PGM-free catalysts, especially the transition metal oxides with robust performance and environmental friendliness in the NO reduction.13-15 Enhancing the activation/generation of surface oxygen vacancies (SOVs) has proven to be one of the most advantageous methods to synthesize an effective metal oxide catalyst, since NO species dissociation on the SOVs is considered as the key step in NO reduction by CO.16,

17

Currently, cobalt-based catalysts have been widely applied in NO reduction with CO and NO/N2O decomposition owing to the outstanding oxygen mobility, which has proven to be conducive to the activation of SOVs by CO.18 Co3O4 nanorods were applied in NO reduction by CO and exhibited superior NO conversion, it was found that the activated SOVs that produced through the restructuring into nonstoichiometric CoO1-x played significant roles.19 Moreover, Co2+ and SOVs have been proposed to be significant active sites participated in the activation of



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the N-O bond to promote the dissociation of NOx species dissociation on cobalt based catalysts,20 which is the rate-determination step in NO reduction with CO.21 In addition, highly dispersed copper oxides also acted as an efficient active component in NO reduction by CO, since the strong interaction between highly dispersed CuO and supports favours for the adsorption of CO and the activation of SOVs.22-25 The catalytic domains were considered to consist of low-oxide-state metal ions and SOVs for NO reduction with CO.21, 26 Moreover, the synergetic effect of the exposed catalytic domains in CuO-CoOx catalysts is crucial for the performance enhancement through promoting the dissociation of NO.18,

27

Although much progress has been achieved in the development of CuO-CoOx over NO reduction with CO, there were still some limitations, for instance low NO conversion and N2 selectivity, especially at low temperatures (< 200 oC).28 To improve the catalytic performance of CuO-CoOx catalysts, a suitable promoter could be added to boost NO conversion and N2 selectivity.29, 30 Commonly, it is accepted that ceria was used as a catalyst promoter or carrier in NO reduction by CO and CO oxidation due to outstanding redox property and high oxygen storage/release capacity.31-33 Additionally, it has been proposed that the introduction of Ce could greatly enhance the interaction between highly dispersed CuO species and supports, which is beneficial for the enhancement of redox properties and regeneration of surface oxygen vacancies and low-oxide-state metal ions during NO reduction by CO.34-36 Therefore, it is definite that the construction of CuCexCo1-xOy catalysts with stronger interaction between CuO and supports, abundant low-oxide-state metal and SOVs via Ce doping is a reliable way to enhance low-temperature NO conversion and N2 selectivity in NO reduction by CO. In the present work, a series of CuCexCo1-xOy catalysts were prepared for NO reduction by CO. The physicochemical properties of the obtained samples were


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systematically investigated through various systematic characterizations, such as SEM, XRD, TPR/TPD and XPS, EPR. This study is mainly focused on the interactions between the surface/interface structure and catalytic performance for NO reduction by CO. More importantly, in situ FT-IR was adopted to extensively explore the adsorption-desorption behaviors as well as the detailed mechanism of NO reduction by CO. Combining with the integrated analysis of in situ FT-IR spectra and various characterization results, the structure-activity relationship and reaction mechanism about CuCe0.2Co0.8Oy towards NO reduction by CO were deeply investigated. 2. EXPERIMENTAL SECTION 2.1. Catalysts Preparation The CexCo1-xOy supports (x is the molar ratio of Ce/(Ce+Co), x = 0.1, 0.2, 0.3, 0.4) were prepared from metal nitrates in aqueous solutions by co-precipitation method. In this synthesis process, the desired molar ratio of cerium (Ⅲ) nitrate and cobalt (Ⅱ) nitrate were dissolved in 100 mL distilled water and stirred vigorously, then oxalic acid solution was slowly added into with the molar ratio of oxalic acid and metal ions to 3:1. The resulting suspension was continuous stirred for 3 h and aged for 6 h at room temperature. Finally, the precipitates were collected by centrifugation after washed several times with deionized water and ethanol until pH = 7. The obtained precursors were dried at 80 ℃ in vacuum and calcined at 500 ℃ for 3 h in air atmosphere to obtain CexCo1-xOy supports. In addition, Co3O4 and CeO2 were also prepared by a similar method for comparison. The CuCexCo1-xOy catalysts were obtained by wet-impregnation method. The CexCo1-xOy supports were impregnated with solution of copper (Ⅱ) nitrate which concentration is equivalent to 5% CuO loaded, the mixtures were treated under ultrasonic condition for 5 min and placed



still for 12 h at room temperature. After that, mixed materials were vaporized water and dried at 80 oC for 10 h, and finally calcined at 450 oC for 3 h in air to obtain the CuCexCo1-xOy catalysts. Then the samples CuO loaded on Co3O4 and CeO2 were labeled as CuCoOy and CuCeOy, respectively. In order to investigate the physicochemical properties of the catalyst after reaction, the CuCe0.2Co0.8Oy catalyst was used in NO reduction by CO below 350 oC was named as used CuCe0.2Co0.8Oy. 2.2. Catalytic Performance Test The catalytic performance of CuCexCo1-xOy catalysts for NO reduction by CO was evaluated in a fixed-bed reactor under simulated reaction condition that involving a mixed composition of 1000 ppm NO, 2000 ppm CO, and He balance at a Gas Hour Space Velocity (GHSV) of 50000 h-1. 200 mg of the samples were sieved with 20-40 mesh, and then loaded in the quartz tube reactor. The catalysts were pretreated at 100 oC for 1 h under high purity He stream, and the mixed gases were switched on after the quartz tube reactor cooled to room temperature. Subsequently, the reaction was carried out at different desired temperatures between 100 and 400 ˚C. The concentration of NO and CO in the inlet and outlet gases was measured by flue gas analyzer (Testo 350) after stabilizing for an hour and recorded corresponding concentration when it was unchanged for 15 min. N2 concentration in products was measured by a gas chromatograph (TECHCOMP 7890 II) equipped with thermal conductivity detectors (TCD, T = 80 °C) and two chromatographic columns (length = 1.75 m, diameter = 3 mm). N2 concentration in outlet gas was measured for at least five times at each target temperature to reach a within 5% errors, and the average data was adopted to calculate the N2 selectivity. NO conversion, N2 yield, N2 selectivity were calculated from concentration of the inlet and outlet gases as eq (1) - (3), respectively.


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NO Conversion 

N 2 Yield =

N 2 Selectivity 

 NOin -  NOout  NOin 2   N 2 out

 NOin

 100%

 

2   N 2 out

 NOin -  NOout

 100%

(1)

(2)

(3)

[NO]in was the inlet NO concentration, [NO]out was the outlet NO concentration, [N2]out was the outlet N2 concentration. 2.3. Catalyst Characterization X-ray diffraction (XRD) patterns of CuCexCo1-xOy catalysts were performed on the Rigaku D/Max-2400 X-ray diffractometer (12 KW, Cu Kα, λ = 0.1541 nm) from 10° to 80°. FT-IR spectra were obtained on a Bruker VERTEX 70 FT-IR spectrometer. All samples were pressed into self-supporting disks after mixed with KBr. Field-emission scanning electron microscopy (FE-SEM) images were carried out on a Hitachi SU-8020 instrument, to obtain the surface morphologies of the CuCoOy and CuCe0.2Co0.8Oy. Textural properties of all catalysts were measured by N2 adsorption-desorption at 77 K using a Quantachrome NOVA 4200e instrument. The samples were degassed under vacuum at 300 oC for 4 h before experiments. The specific surface area was calculated using the Brunauer-Emmet-Teller (BET) method while the pore distribution and pore volumes were calculated using the Barrett-Joyner-Halenda (BJH) method. Temperature-programmed reduction (TPR) and Temperature-programmed desorption (TPD) were carried out on the Quantachrome ChemBET TPR/TPD chemisorption analyzer instrument equipped with a thermal conductivity detector (TCD). Before TPR/TPD analysis, the samples were pretreated at 300 oC with heating rate of 10 oC/min with 30 mL/min high purity He stream 7 Environment ACS Paragon Plus


for 1 h. The samples (50 mg) were reduced under 10% H2/Ar with a flow of 30 mL/min after cooling to room temperature, and H2-TPR were analyzed with heating from 50 to 600 oC at a rate of 10 oC/min. For Temperature-programmed desorption with CO (CO-TPD), the samples (100 mg) were exposed in 5000 ppm CO/He stream for 100 min at room temperature after pretreatment. After that, the samples were purged in high purity He stream for 1 h. And then COTPD analysis was carried out in 30 mL/min high purity He stream when the samples were heated from 50 to 600 oC at a rate of 10 oC/min. X-ray photoelectron spectroscopy (XPS) analysis was recorded with a Thermo Fisher Scientific ESCALAB 250Xi spectrometer using Al Kα X-rays radiation with an energy step size of 0.05 eV. In addition, the binding energy was calibrated for surface charging by taking the adventitious C 1s peak at 284.6 eV as a reference. The Electron Paramagnetic Resonance (EPR) signals of oxygen vacancies were recorded with a Bruker EC106 X-band spectrometer at the condition of 100K. In situ FT-IR spectra were recorded from 4000 to 400 cm-1 at a resolution of 4 cm-1 (number of scans = 16) on a Bruker VERTEX 70 FT-IR spectrometer. The samples (40 mg) were pressed into self-supporting disks and pretreated in high purity He stream at 350 oC for 1 h to take off the adsorbed gas and moisture. Then the background spectra were collected as baselines at each desired temperature in the process of cooling to room temperature. After that, the catalysts were exposed to a controlled stream of 1000 ppm NO/He or/and 2000 ppm CO/He with a total flow rate of 100 mL/min until saturated. In situ NO or/and CO adsorption spectra were obtained at different target temperature at a heating rate of 10 oC/min from 25 to 350 oC by subtraction of the corresponding baselines. 3. RESULTS and DISCUSSION 3.1. Catalytic Performance for NO reduction by CO


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The CuCexCo1-xOy catalysts were applied in NO reduction with CO, and the corresponding catalytic performances are exhibited in Figure 1. As shown in Figure 1a, the pristine CuCoOy shows inferior NO conversion, merely reaching 50% and 100% at 125 oC, 250 oC, respectively. Satisfactorily, the catalytic performance over CuCexCo1-xOy was significantly improved in the whole temperature range through CeO2 modification with the rank of CuCe0.2Co0.8Oy > CuCe0.1Co0.9Oy > CuCe0.3Co0.7Oy > CuCe0.4Co0.6Oy > CuCoOy. Typically, CuCe0.2Co0.8Oy exhibits the best performance in all catalysts, achieving more than 50% NO conversion at 100 oC while CuCoOy just reached about 23%, and the 100% NO conversion temperature of CuCe0.2Co0.8Oy was obviously lower than CuCoOy. Additional, the catalytic performance of the CuCe0.2Co0.8Oy over NO reduction by CO was compared with catalysts reported in previous literatures (Table S1). The catalytic activity of CuCe0.2Co0.8Oy catalysts constructed in this work is superior to other catalysts in the previous reports. To investigate the stability of CuCe0.2Co0.8Oy catalyst, the stability test was performed at 175 oC and 300 oC (Figure S1). NO conversion of CuCe0.2Co0.8Oy kept above 95% in the stability test lasting for 24 h, indicating that CuCe0.2Co0.8Oy catalyst possesses outstanding stability for NO reduction by CO reaction. As shown in Figure 1b, c, CuCoOy catalysts exhibits poor N2 yield and selectivity at low temperatures (< 200 oC), demonstrating that NO is mainly reduced to N2O at low temperatures. In contrast, the N2 yield and selectivity of CuCexCo1-xOy catalysts were significantly enhanced at low temperatures with Ce addition, further increased with temperature and reached nearly 100% at 300 oC. As shown in Figure 1d, CO conversion significantly increased with NO conversion owing to the rapid reaction between two reactants, achieving approximate 50% at 250 oC.37 With the reaction temperature further rise to 300 oC, the CO conversion further grows to nearly 100% owing to the reduction of Co3O4 by CO above 250 oC.19 More importantly, it was found that CO



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conversion at 300 oC (Figure S1) in stability test decreases to approximate 50% to reach a dynamic equilibrium on the CuCe0.2Co0.8Oy catalyst between NO and CO.37 To investigate the effect of different pretreatment conditions for catalytic behaviors, the CuCe0.2Co0.8Oy catalyst was pretreated in 40 mL/min 5000 ppm CO/He (rich-burn condition), 5000 ppm NO/He (lean-burn condition) and 10% H2O stream at 200 oC for 3 h. The NO conversion was displayed in Figure S2. The lower NO conversion of CO-pretreated CuCe0.2Co0.8Oy catalyst owing to the surface carbonate species accumulation and coverage of active sites during CO pretreatment, which inhibits NO adsorption on catalyst surface.38,

39

Moreover, the catalytic performance of catalyst pretreated under NO atmosphere was slightly declined, it should be ascribed to preferential adsorption of the NO on catalyst surface during NO treatment, and the adsorbed NOx species could react with CO as rising temperature. The catalytic behavior of catalyst pretreated under 10% H2O steam was restrained at low temperatures, as a result of the active sites coverage by H2O during pretreatment.40 3.2. XRD XRD patterns of all catalysts are shown in Figure 2. It could be seen that the main diffraction peaks at 2θ values of 31.27°, 36.85° and 59.36° are attributed to (220), (311) and (511) planes, respectively, which can be ascribed to the cubic phase Co3O4 (JCPDS #42-1467).41 Additionally, the diffraction peaks at 2θ values of 28.55°, 47.48° and 56.33° should be attributed to the (111), (220) and (311) planes in cubic fluorite phase CeO2 (JCPDS #34-0394).42 The results indicate that the CexCo1-xOy nanocomposites were successfully prepared. More importantly, no obvious diffraction peaks of CuO could be detected in all CuCexCo1-xOy catalysts, indicating that the supported CuO species should be highly dispersed on the surface of supports. The XRD pattern of used CuCe0.2Co0.8Oy is exhibited in Figure S3, the diffraction peaks of CeO2 are unchanged


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significantly compared with fresh CuCe0.2Co0.8Oy, which is favourable for the thermal stability. In addition, most of Co3O4 phase was reduced to CoO (JCPDS #65-2902), indicating that CuCe0.2Co0.8Oy sample lost part of lattice oxygen in NO reduction by CO.19 FT-IR spectra of all samples are shown in Figure S4, and all spectra displayed two obvious bands at (a) 663 cm-1 and (b) 568 cm-1 except for CuCeOy, which could be related to the tetrahedral and octahedral domains of the Co3O4 spinel lattice.41 Moreover, the bands for CuO could not be detected in all spectra, meaning that the CuO species are highly dispersed on supports, as proved by XRD results.43, 44 3.3. Morphologies The surface morphologies of the prepared catalysts are shown in Figure 3. As displayed in Figure 3a, b, CuCoOy consists of irregular particles to be a block-like structure and exhibited obviously aggregation on the surface. CuCe0.2Co0.8Oy catalyst exhibits rod-like structure with orderly lines assembled by some nanoparticles with a diameter of 40nm, and the surface dispersity of CuCe0.2Co0.8Oy sample has been obviously improved after Ce doping (Figure 3c, d). As revealed in previous literature, CeO2 modification is beneficial for the enhancement of dispersity, thermal stability, smaller crystallites size and larger surface areas of catalysts, which could promote the adsorption and conversion of the reactants.45 3.4. N2 Adsorption-Desorption The N2 adsorption-desorption isotherms and BJH pore size distribution curves of obtained catalysts are shown in Figure 4. According to the IUPAC classification, the catalysts exhibit Ⅲtype isotherms with obvious H3-type hysteresis loops in Figure 4a, corresponding to the capillary condensation of N2 gas, which is ascribed to the formation of mesoporous structure. Meantime, the BJH pore size distribution curves in Figure 4b illustrate that the samples possess mesoporous



structure and the pore size mainly distributed in the range of 2~5 nm. Pore volume and average pore size of CuCexCo1-xOy catalysts are shown in Table 1. Moreover, the BET surface area of catalysts increased with higher Ce content, which is favourable for the adsorption of the reactant molecules. 3.5. XPS and EPR To study the surface nature of the active sites on the fresh CuCexCo1-xOy and used CuCe0.2Co0.8Oy catalyst, XPS spectra were recorded and the results of Co 2p, Ce 3d, Cu 2p and O 1s are displayed in Figure 5. As shown in Figure 5a, Co 2p3/2 and Co 2p1/2 for two samples appears at about 775-792, 792-812 eV. The main peak located at about 779.5 and 794.6 eV are labeled as Co3+ species and the split peak located at 781.5 and 796.5 eV are noted as Co2+ species.3 Two enhanced shake-up satellite peaks are exhibited in CuCe0.2Co0.8Oy with the increase of Co2+ ratio owing to the charge transfer from Ce to Co.46 Furthermore, two groups of spin-orbital multiplets in the binding energy range of 874-921 eV in Ce 3d spectra, corresponding to 3d3/2 is labeled as u and 3d5/2 is labeled as v (Figure 5b). The sub-bands labeled u′ and v′ are attributed to the 3d104f1 initial electronic state corresponding to Ce3+, and those labeled u, u″, u‴, v, v″ and v‴ are ascribed to Ce4+.47 The relative proportion of Ce3+ of CuCeOy and CuCe0.2Co0.8Oy catalysts are 22% and 18%, respectively.34, 47 The corresponding lower and higher shift of binding energy for Co 2p and Ce 3d in CuCe0.2Co0.8Oy could be ascribed to the electron transfer from Ce to the far more electrophilic Co due to strong interaction between Co3O4 and CeO2.34, 48 In addition, used CuCe0.2Co0.8Oy (Figure S5a) exhibits higher Co2+ content due to the reduction of Co3O4 phase to CoO phase in sample, which is consistent with the XRD results.


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The spectra of Cu 2p exhibited Cu 2p3/2 peak at approximately 932.5 eV with a shake-up satellite peaks located at the binding energy range of 937.5~947 eV, and the peak at 932.3 eV could be ascribed to Cu+ or Cu0 species while the peak of Cu2+ species located at 933.9 eV (Figure 5c). As exhibited in Table 2, the Cu+/(Cu+ + Cu2+) for CuCe0.2Co0.8Oy (48%) is higher than CuCoOy (42%), indicating that the addition of Ce benefits to the formation of Cu+, which is available to promote the CO adsorption and the activation of SOVs on catalysts surface. The increase of Cu+ content mainly owing to the strong synergistic effect between CuO and supports and the subsequent redox equilibrium (Cu2+ + Ce3+ ↔ Ce4+ + Cu+).49 Furthermore, the used CuCe0.2Co0.8Oy catalyst possesses more Cu+ due to the reduction of Cu2+ at higher temperature (Figure S5b, c). To study the various oxygen species on the surface of catalysts, XPS results of O 1s are shown in Figure 5d. All curves were fitted with two deconvoluted peaks from 526 eV to 536 eV, the split peak at higher binding energy of 531.2 eV is ascribed to surface adsorbed oxygen (Oads), and the dominant peak at 529.3 eV is considered as lattice oxygen (Olat).50 As shown in Table 2, the higher Oads content of CuCe0.2Co0.8Oy indicates that the catalysts possess more surface adsorbed oxygen, which could be attributed to the generation of more oxygen vacancies due to Ceria modification. Moreover, the content of oxygen vacancies increased with the rising of Co2+ content because of the electroneutrality principle and oxygen vacancies are closer to the surface of catalyst.51, 52 Low temperature EPR spectra of CuCoOy and CuCe0.2Co0.8Oy were recorded at the condition of 100 K to further confirm the creation of more oxygen vacancies (Figure 6),53 and the signal at g = 2.003 could be ascribed to the oxygen vacancies.54 The increased signal intensity indicates that the higher oxygen vacancies content on the surface of CuCe0.2Co0.8Oy catalyst compared to CuCoOy, which is consistent with the XPS results. More oxygen vacancies



on the catalysts surface could be beneficial for the enhancement of catalytic performance through promoting the dissociation of NO/N2O and the proceeding of redox reaction in NO reduction by CO.16 In addition, the used CuCe0.2Co0.8Oy (Figure S5d) possesses more surface adsorbed oxygen species could be ascribed to the reduction of catalyst.26 3.6. H2-TPR H2-TPR experiments were performed to investigate the reduction behavior about CexCo1-xOy supports and CuCexCo1-xOy catalysts, and the profiles are displayed in Figure 7. As displayed in Figure 7a, Co3O4 exhibits two reduction peaks at 396, 450 oC corresponding to the reduction of Co3+ to Co2+ and Co2+ to Co0, respectively.45 Additionally, it is remarkable that the reduction peaks of CeO2 appears at 526 oC, supporting that the reduction behavior of CeO2 have little direct influence on NO reduction with CO occurred at the temperature range of 100 to 400 oC in this work.55 The reduction peaks of Ce0.2Co0.8Oy significantly shifted to lower temperature when compared with Co3O4 and mixed CeO2-Co3O4 (mechanism mixed with a Co3O4/CeO2 ratio 4), demonstrating that the redox property was enhanced by the strong interaction between CeO2 and Co3O4 interfaces.35 TPR profiles of CuCexCo1-xOy catalysts are shown in Figure 7b, four reduction peaks (noted as α1, α2, β, γ) were recorded for samples. Peak α1 and α2 could be ascribed to the reduction of highly dispersed CuO and clustered stated CuO species on CuCexCo1-xOy surface, respectively.56, 57

Consequently, the reduction peak of Co3+ to Co2+ is labeled as peak β, and this reduction

process is a crucial step for the formation of more oxygen vacancies and active sites for NO/N2O decomposition in NO reduction by CO.20 The last peak that named as γ appears for the reduction of Co2+ to Co0 at elevated temperature.35 Peak α1 appears on the CuCexCo1-xOy surface and exhibits the relatively higher peak areas compared to CuCoOy, indicating that CeO2 doping


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enhanced dispersity of CuO species.35 The relatively lower temperature shift of reduction peaks β, γ compared to CuCoOy demonstrates that the redox properties of catalysts were enhanced due to the stronger interaction between highly dispersed copper oxides and supports after Ce addition, which is beneficial for the regeneration of surface oxygen vacancies with surface reduction and conversion of NO during NO reduction by CO at low temperatures.57 It is worth noting that the CuCe0.2Co0.8Oy displayed the best catalytic performance when the reduction curve exhibits the lowest reduction temperature for peak α and β, demonstrating that the catalytic activity closely related to the redox properties of catalysts. Combining XPS results, the higher content of Cu+ species and SOVs are beneficial for the promotion of performance through enhancing redox properties of catalysts.

22, 35

TPR profile of used CuCe0.2Co0.8Oy sample is

shown in Figure S6, the peaks of α1 disappeared and the intensities of peaks α2, β, γ reduced obviously, meaning that highly dispersed CuO and the most of Co3+ were corresponding reduced to metallic Cu+/Cu0 species and Co2+ species in NO reduction by CO, respectively. 3.7. CO-TPD To investigate the CO chemisorption properties of catalysts, CO-TPD experiments were employed and the profiles of CuCexCo1-xOy samples are exhibited in Figure 8. No obvious desorption peak was detected for CuCoOy sample, meaning that CO adsorption on CuCoOy is weak. In contrast, apparent desorption peaks from 50 to 300 oC were recorded in the curves of other CuCexCo1-xOy catalysts. The intensities of TCD signal and CO desorption peak areas are increased with higher Ce content, pointing out that the CO chemisorption ability of CuCexCo1xOy

samples is significantly enhanced due to higher Cu+ contents after Ce doping.44 Although the

stronger CO chemisorption is favourable for the activation of SOVs and promotion of CO reaction with NO, the excessive CO adsorption could lead to severe accumulation of surface



carbonate species to restrain the reaction of adsorbed CO with activated oxygen species, the desorption of CO2 and the further regeneration of SOVs, which inhibit the proceeding of NO reduction with CO.38 CO-TPD result of used CuCe0.2Co0.8Oy catalyst is shown in Figure S7. The CO-TPD curve of used CuCe0.2Co0.8Oy exhibits relatively larger desorption area from 50-350 oC, and the extraordinary CO desorption peak emerged at 350~500 oC could be ascribed to decomposition of surface carbonate species.35 The results demonstrate that stronger CO adsorption occurred on the used CuCe0.2Co0.8Oy catalyst owing to the higher Cu+ proportion on the used CuCe0.2Co0.8Oy catalyst. 3.8. In situ FT-IR Results To further investigate the surface-adsorbed species and probe the proposed mechanism for NO reduction with CO. In situ FT-IR spectra of CO and/or NO adsorption on the surface of CuCexCo1-xOy catalysts were recorded at different temperature. 3.8.1. In situ FT-IR Spectra of CO Adsorption In situ FT-IR spectra of CO adsorption on the surface of CuCoOy and CuCe0.2Co0.8Oy are exhibited in Figure 9. For CuCoOy catalyst, the band at 1040 cm-1 is assigned to vas(C-O) vibration mode of monodentate coordination disappeared at 200 oC. The bands at about 1303, 1489 and 1547 cm-1 are assigned to vs(CO32-), v(C-O) in monodentate coordination and v(C=O) in bidentate coordination.26, 30 For CuCe0.2Co0.8Oy sample, the bands at 1058 cm-1 and 1360 cm-1 is assigned to vas(C-O) vibration mode of monodentate coordination and the vs(COO-) in bidentate coordination, respectively. The band at 1475 cm-1 is assigned to vas(CO32-) in monodentate coordination, and it became weaken and even disappeared with the increasing temperature.58 The bands at about 1548 cm-1 can be ascribed to v(C=O) in bidentate coordination, and the band of vas(COO-) appears at 1580 cm-1 at 150 oC.30


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For both of samples, the adsorption bands at 2180 cm-1 and 2110 cm-1 were observed in the temperature range of 25~350 oC, which can be attributed to the P branch and R branch of gaseous CO.16,

59

The bands at 2300~2400 cm-1 that appeared at 200 oC are assigned to the

formation of physically adsorbed or gaseous CO2.26 The bands of physically adsorbed or gaseous CO2 for CuCe0.2Co0.8Oy appeared at lower temperature even 25 oC, and the intensities of gaseous CO2 and surface carbonate species of CuCe0.2Co0.8Oy are stronger than CuCoOy. It should be attributed to stronger CO adsorption and better oxidation performance due to more Cu+ and surface active oxygen on the surface of CuCe0.2Co0.8Oy, which are favourable for the activation of SOVs at low temperatures.60 3.8.2. In situ FT-IR Spectra of NO Adsorption The in situ FT-IR spectra of NO adsorption on the surface of CuCoOy and CuCe0.2Co0.8Oy catalysts are exhibited in Figure 10. The bands at about 1906 cm-1 and 1845 cm-1 could be ascribed to the vibration of the adsorbed NO species in bent and linear coordination modes for both of catalysts, respectively, and the intensities decreased with elevated temperature.16 For CuCoOy catalyst (Figure 10a), the bands at about 995 cm-1 and 1033 cm-1 can be assigned to bridged bidentate nitrate; the two bands of symmetric and asymmetric vibration at 1547 cm-1 and 1210 cm-1 are assigned to chelated bidentate nitrate, and the latter shift to 1171 cm-1 at 100 oC.58, 61

The band at 1274 cm-1 is ascribed to the linear nitrites (M-O-N=O);58 the band at 1479 cm-1

should be ascribed to monodentate nitrate, and the shoulder band at 1436 cm-1 should be ascribed to bridging monodentate nitrates below 200 oC.21 In addition, the intensity of bridged monodentate nitrates decreased with the increase of temperature. For CuCe0.2Co0.8Oy (Figure 10b), the bands at about 1005 cm-1, 1033 cm-1 are attributed to bridged bidentate nitrate; the bands at 1560 cm-1 and 1210 cm-1 can be assigned to chelated bidentate nitrate. The linear



nitrites at 1276 cm-1 eliminated below 200 oC. Furthermore, the bridged monodentate nitrates at about 1445 cm-1 decreased with the increasing temperature, and even disappeared at 300 oC,18 the bridged bidentate nitrates and chelated bidentate nitrates get weaken but do not eliminate because of strong stability of adsorbed species on the catalysts surface. All of the results demonstrate that the adsorbed NO species are easier to desorb, convert or dissociate on CuCe0.2Co0.8Oy.47, 50, 58 3.8.3. In situ FT-IR Spectra of NO + CO co-adsorption In situ FT-IR spectra of NO + CO co-adsorption on the surface of CuCoOy and CuCe0.2Co0.8Oy catalysts are exhibited in Figure 11. For CuCoOy catalyst, the bands at about 985 cm-1, 1031 cm-1 can be assigned to bridged bidentate nitrate; the bands at 1134 cm-1 and 1163 cm-1 are ascribed to nitrite;62 the band at 1213 cm-1 is assigned to chelated bidentate nitrate; the band at 1276 cm-1 is ascribed to the linear nitrites (M-O-N=O); the band at 1303 cm-1 is ascribed to vs(CO32-); and the band at 1487 cm-1 should be ascribed to monodentate nitrates. Moreover, the band at 1604 cm-1 can be assigned to bridged bidentate nitrate. Additionally, the bands for surface carbonate and carboxylate species adsorbed on catalysts are not found at temperature below 200 oC, indicating that NO species preferentially adsorbed on the surface of CuCoOy and the adsorption of CO is inhibited, which is not conducive to the activation of surface oxygen vacancies.63 The bands corresponding to CO2 and N2O at 2300~2400 cm-1 and 2239 cm-1 are only detected above 200 oC,

meaning that the reaction between CO and NO is not obvious below 200 oC, which is

consistent with the catalytic activity and N2 yield. For CuCe0.2Co0.8Oy catalyst, the bands at 1196 cm-1 could be ascribed to chelated bidentate nitrate was detected in spectra and disappeared at higher temperature. And the bands at about 995 cm-1, 1037 cm-1 can be assigned to bridged bidentate nitrate were detected at 150 oC; the


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bands at 1134 cm-1 and 1163 cm-1 are ascribed to nitrite, and the relatively intensities are weaker than CuCoOy at the corresponding temperature. Most of adsorbed NO species disappear or show low intensity at equivalent temperature on CuCe0.2Co0.8Oy, demonstrating that the adsorbed NO species are easier to desorb, convert or dissociate compared with CuCoOy due to more lowoxidation-state metal species and SOVs.21, 36, 58 In addition, the bands at about 2230 and 2239 cm-1 can be attributed to the formation of gaseous N2O as intermediate product in NO dissociation process. It could be explained that NO molecules are dissociated to be radical [N] and [O] under the catalytic effect of SOVs, and then the [N] species recombine with NO molecules to form N2O.26 The bands of gaseous N2O are observed at 150 oC for CuCe0.2Co0.8Oy and the intensities are stronger than CuCoOy, demonstrating that the reaction between NO and CO takes place evidently at lower temperature due to the synergetic effect between lowoxidation-state species and SOVs in CuCe0.2Co0.8Oy catalyst, which is supported by catalytic performance.58 With the increase of temperature, more N2 would be generated due to the decomposition of NO/N2O. Combining the in situ NO and/or CO adsorption FT-IR spectra, the performance enhancement could mainly be attributed to the less NO predominant adsorption and more CO adsorption on catalysts surface at low temperatures due to the generation of more Cu+, which are conducive to the activation of more SOVs and the proceeding of surface reaction. 3.9 Reaction Mechanism Based on the above characterization and in situ FT-IR spectra results, the reaction mechanism of NO reduction by CO occurred on the surface of CuCe0.2Co0.8Oy was deeply investigated under different temperature ranges. When CuCe0.2Co0.8Oy is exposed in reaction atmosphere at room temperature, NO molecules are adsorbed on the surface of the CuCe0.2Co0.8Oy catalyst to generate several kinds of nitrate and nitrite species, and the adsorption of CO molecules were



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inhibited.21 With the increasing of temperature, CO molecules are preferentially adsorbed on the CuCe0.2Co0.8Oy catalyst surface with more Cu+ generation to form surface carbonate species, and more SOVs are activated due to the reaction of adsorbed CO with the surface active oxygen species. It is well known that NO molecules could be dissociated to be radical [N] and [O] under the promotion of surface OVs, which could weaken the N-O bond.19, 21, 63 Furthermore, the NOx species reacted with CO to form N2O, CO2 and N2 as detected by in situ FT-IR spectra at 150 oC. With the further increase of temperature, high-oxidation-state species (Cu2+ and Co3+) could be reduced to low-oxidation-state species (Cu+ and Co2+) by CO molecules with the generation of more SOVs. The synergetic effect between low-oxidation-state species and SOVs is conducive to the performance enhancement for CuCexCo1-xOy catalysts over NO reduction with CO via promoting the activation and conversion of the reactant gas.21,

26

The regeneration of low-

oxidation-state species and SOVs would be further promoted at elevated temperatures, resulting in remarkable enhancement of the catalytic performance. And CO molecules are converted to CO2; NO molecules and intermediate product N2O are converted to nontoxic and pollution free product N2. 4. CONCLUSIONS In this study, a series of CuCexCo1-xOy (x = 0.1, 0.2, 0.3, 0.4) catalysts were prepared for NO reduction with CO. The CuCexCo1-xOy catalyst in particular exhibited outstanding catalytic properties for NO reduction at low temperatures, achieving 100% NO reduction at 175 oC. The characterization of the CuCexCo1-xOy catalysts confirmed the presence of intermediate active species and products in NO reduction with CO over the catalysts. The following conclusions have been drawn:


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(1) According to the N2 adsorption-desorption, SEM and H2-TPR results, surface area and particle size of CuCexCo1-xOy catalysts were evidently improved, and Ce addition promoted the dispersity of CuO species on the surface of CuCexCo1-xOy catalysts, which is conducive to the enhancement of the interaction between the highly dispersed Cu species and the support. The enhanced redox properties of CuCexCo1-xOy catalysts contributes to the regeneration of SOVs on supports during NO reduction by CO at low temperatures. (2) Supported by the H2-TPR, XPS and EPR results, the Ce introduction significantly promoted the redox properties of CuCexCo1-xOy catalysts with more low-oxidation-state (Cu+, Co2+) and SOVs generated due to the redox equilibrium and electronic transfer between metal ions, which played a crucial role in conversion of CO and NO at low temperatures and the enhancement of low-temperature catalytic performance over CuCexCo1-xOy catalyst. (3) The addition of Ce enhanced the adsorption/conversion of CO, the activation of SOVs and the dissociation of NO on the catalysts surface. The adsorption bands of CO2 and N2O emerged at 150 oC in the in situ FT-IR spectra of CuCe0.2Co0.8Oy, revealing that the reaction between NO and CO takes place evidently at the low temperatures. ASSOCIATED CONTENT Supporting Information Catalytic stability test of CuCe0.2Co0.8Oy catalyst at 300 oC for 24 h. FT-IR spectra of CuCexCo1-xOy catalysts. Comparison of XRD pattern, H2-TPR, CO-TPD between fresh and used CuCe0.2Co0.8Oy. XPS spectra of used CuCe0.2Co0.8Oy. AUTHOR INFORMATION Corresponding Author



*E-mail: [email protected] *E-mail: [email protected]. ORCID Xinyong Li: 0000-0002-3182-9626 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported financially by the Key Project of the National Ministry of Science and Technology (No. 2016YFC0204204), the Major Program of the National Natural Science Foundation of China (No. 21590813), the National Natural Science Foundation of China (Nos. 21377015 and 21577012), the Program of Introducing Talents of Discipline to Universities (B13012), the Fundamental Research Funds for the Central Universities(DUT19LAB10) and the Key Laboratory of Industrial Ecology and Environmental Engineering, China Ministry of Education. RERERENCES (1) Wu, Y.; Zhang, S. J.; Li, M. L.; Ge, Y. S.; Shu, J. W.; Zhou, Y.; Xu, Y. Y.; Hu, J. N.; Liu, H.; Fu, L. X.; He, K. B.; Hao, J. M., The challenge to NOx emission control for heavy-duty diesel vehicles in China. Atmos. Chem. Phys. 2012, 12, 9365. (2) Skalska, K.; Miller, J. S.; Ledakowicz, S., Trends in NOx abatement: a review. Sci. Total Environ. 2010, 408, 3976.


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(58) Yao, X. J.; Gao, F.; Yu, Q.; Qi, L.; Tang, C. J.; Dong, L.; Chen, Y., NO reduction by CO over CuO-CeO2 catalysts: effect of preparation methods. Catal. Sci. Technol. 2013, 3, 1355. (59) Morales, F.; Desmit, E.; Degroot, F.; Visser, T.; Weckhuysen, B., Effects of manganese oxide promoter on the CO and H2 adsorption properties of titania-supported cobalt FischerTropsch catalysts. J. Catal. 2007, 246, 91. (60) Gholami, Z.; Luo, G., Low-Temperature Selective Catalytic Reduction of NO by CO in the Presence of O2 over Cu:Ce Catalysts Supported by Multiwalled Carbon Nanotubes. Ind. Eng. Chem. Res. 2018, 57, 8871. (61) Stakheev, A. Y.; Mashkovsky, I. S.; Bragina, G. O.; Baeva, G. N.; Telegina, N. S.; Malmstrøm Larsen, K.; Kustov, A. L.; Thøgersen, J. R., Mechanism of Low-Temperature NOx Storage for Reducing NOx Cold Start Emission. Top. Catal. 2016, 59, 931. (62) Symalla, M. O.; Drochner, A.; Vogel, H.; Philipp, S.; Göbel, U.; Müller, W., IR-study of formation of nitrite and nitrate during NOx-adsorption on NSR-catalysts-compounds CeO2 and BaO/CeO2. Top. Catal. 2007, 42-43, 199. (63) Zou, W.; Liu, L.; Zhang, L.; Li, L.; Cao, Y.; Wang, X.; Tang, C.; Gao, F.; Dong, L., Crystal-plane effects on surface and catalytic properties of Cu2O nanocrystals for NO reduction by CO. Appl. Catal., A 2015, 505, 334.



List of Figures: Figure 1. (a) NO conversion, (b) N2 yield, (c) N2 selectivity and (d) CO conversion of CuCexCo1-xOy catalysts for NO reduction by CO. Reaction condition: 1000 ppm NO, 2000 ppm CO, He balance, GHSV = 50,000 h-1. Figure 2. XRD patterns of CuCexCo1-xOy catalysts. Figure 3. SEM images of (a, b) CuCoOy, (c, d) CuCe0.2Co0.8Oy samples. Figure 4. (a) N2 adsorption/desorption isotherms and (b) BJH pore size distribution curves of CuCexCo1-xOy catalysts. Figure 5. XPS profiles of (a) Co 2p, (b) Ce 3d, (c) Cu 2p and (d) O 1s of CuCexCo1-xOy catalysts. Figure 6. Low temperature EPR spectra of CuCe0.2Co0.8Oy and CuCoOy at the condition of 100K. Figure 7. H2-TPR profiles of (a) CexCo1-xOy supports and (b) CuCexCo1-xOy catalysts. Figure 8. CO-TPD profiles of CuCexCo1-xOy catalysts. Figure 9. In situ FT-IR spectra of CO adsorption for (a) CuCoOy and (b) CuCe0.2Co0.8Oy catalysts. Figure 10. In situ FT-IR spectra of NO adsorption for (a) CuCoOy and (b) CuCe0.2Co0.8Oy catalysts. Figure 11. In situ FT-IR spectra of NO + CO co-adsorption for (a) CuCoOy and (b) CuCe0.2Co0.8Oy catalysts.


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Figure 1. (a) NO conversion, (b) N2 yield, (c) N2 selectivity and (d) CO conversion of CuCexCo1-xOy catalysts for NO reduction by CO. Reaction condition: 1000 ppm NO, 2000 ppm CO, He balance, GHSV = 50,000 h-1.



Figure 2. XRD patterns of CuCexCo1-xOy catalysts.


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Figure 3. SEM images of (a, b) CuCoOy, (c, d) CuCe0.2Co0.8Oy samples.



Figure 4. (a) N2 adsorption/desorption isotherms and (b) BJH pore size distribution curves of CuCexCo1-xOy catalysts.


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Figure 5. XPS profiles of (a) Co 2p, (b) Ce 3d, (c) Cu 2p and (d) O 1s of CuCexCo1-xOy catalysts.



Figure 6. Low temperature EPR spectra of CuCe0.2Co0.8Oy and CuCoOy at the condition of 100K.


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Figure 7. H2-TPR profiles of (a) CexCo1-xOy supports and (b) CuCexCo1-xOy catalysts.



Figure 8. CO-TPD profiles of CuCexCo1-xOy catalysts.


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Figure 9. In situ FT-IR spectra of CO adsorption for (a) CuCoOy and (b) CuCe0.2Co0.8Oy catalysts.



Figure 10. In situ FT-IR spectra of NO adsorption for (a) CuCoOy and (b) CuCe0.2Co0.8Oy catalysts.


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Figure 11. In situ FT-IR spectra of NO + CO co-adsorption for (a) CuCoOy and (b) CuCe0.2Co0.8Oy catalysts.



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List of Tables: Table 1. Textural Data of CuCexCo1-xOy Catalysts. Table 2. Surface Composition of fresh CuCexCo1-xOy and used CuCe0.2Co0.8Oy Catalysts.

Table 1. Textural Data of CuCexCo1-xOy Catalysts. Sample

SBET (m2/g)

Pore volume (cm3/g)

Average pore size (nm)

CuCoOy

10.7

0.043

11.33

CuCe0.1Co0.9Oy

16.9

0.091

22.97

CuCe0.2Co0.8Oy

18.7

0.082

18.55

CuCe0.3Co0.7Oy

22.9

0.069

11.97

CuCe0.4Co0.6Oy

29.5

0.082

11.33

CuCeOy

36.9

0.095

13.48

Table 2. Surface Composition of fresh CuCexCo1-xOy and used CuCe0.2Co0.8Oy Catalysts. Sample

Cu+/(Cu+ + Cu2+)

Co2+/(Co2+ + Co3+)

Ce3+/(Ce4+ + Ce3+)

Oads/(Oads + Olat)

used CuCo0.2Ce0.8Oy

71%

64%

22%

48%

CuCo0.2Ce0.8Oy

48%

54%

18%

44%

CuCoOy

42%

47%

24%

33%


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Graphical Abstract:



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