Balance between Reducibility and N2O Adsorption Capacity for the

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Balance between reducibility and N2O adsorption capacity for the N2O decomposition: CuxCoy catalysts as an example Shangchao Xiong, Jianjun Chen, Nan Huang, Shijian Yang, Yue Peng, and Junhua Li Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b02892 • Publication Date (Web): 05 Aug 2019 Downloaded from pubs.acs.org on August 5, 2019

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Environmental Science & Technology

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Balance between reducibility and N2O adsorption capacity for

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the N2O decomposition: CuxCoy catalysts as an example

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Shangchao Xiong1, 2, Jianjun Chen*1, 2, Nan Huang1, 3, Shijian Yang4, Yue Peng1, 2, Junhua Li1, 2

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1State

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Environment, Tsinghua University, Beijing 100084, PR China

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2National

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Equipment, Beijing, 100084, China

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3School

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Technology, Nanjing, 210094 PR China

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Key Joint Laboratory of Environment Simulation and Pollution Control, School of

4School

Engineering Laboratory for Multi Flue Gas Pollution Control Technology and

of Environmental and Biological Engineering, Nanjing University of Science and

of Environment and Civil Engineering, Jiangnan University, Wuxi 214122, PR China

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*Corresponding author.

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Phone: +86 010 62771093

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Email address: [email protected] (Jianjun Chen)

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ACS Paragon Plus Environment


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Abstract:

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CuxCoy (CuO−Co3O4 mixed oxides) catalysts were prepared via co−precipitation for the N2O

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decomposition reaction. They exhibited a higher N2O decomposition activity than that of pure

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CuO and Co3O4 due to the balance of redox property and N2O adsorption capacity. Co3O4

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presented a large number of surface oxygen vacancies, increasing the N2O chemical adsorption as

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“□−Co−ON2” on the catalyst surface, whereas CuO was dispersed around Co3O4 and presented

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high reducibility on the interface of Co3O4−CuOx for the N−O break of N2O, healing oxygen

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vacancies by leaving one oxygen atom in the vacancy. Based on kinetic studies, the rate constant

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of N2O decomposition was related to the number of surface vacancy sites ([Mn+]) and the rate of

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N−O break (k3), while the rate determining step is the N−O break. Therefore, the N2O

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decomposition rate is first order to the N2O concentration. Overall, both the DFT calculations and

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kinetic results indicate that the quantities of adsorption and activation sites derived from the

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interaction between Co and Cu (dual−function mechanism) were accounted for the excellent N2O

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decomposition performance of CuxCoy catalysts.

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2


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Table of Contents

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1.

Introduction

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Nitrous oxide (N2O) emitted from the production of adipic acid and nitric acid, as well as the

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processes using nitric acid as an oxidant, contributes to the ozone hole and greenhouse effect.1, 2

37

Its global warming potential (GWP) is ~310 times and ~31 times higher than that of CO2 and CH4,

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respectively, and the lifetime of N2O is ~114 years.3 Moreover, N2O can deplete the ozone layer

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by a reaction pathway similar to that of chlorofluorocarbons (CFCs). Previous studies reported

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that N2O would be the dominant ozone−depleting substance in the 21st century.4 Thus, the

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reduction in anthropogenic N2O emissions is urgently required. Several techniques were proposed

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to control anthropogenic N2O emissions, whereas the direct catalytic decomposition of N2O is

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regarded as the most promising alternative technique.5,

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treatment process to incorporate this technique is relatively convenient and can minimize the

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economic demands.

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Retrofitting the existing flue gas

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A series of noble metals and nonnoble metals were used to catalyze the decomposition of N2O.6

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Noble metals (e.g., Rh and Ru) show a satisfactory N2O decomposition performance at low

48

temperature, but their high cost and poor tolerance to various influential factors (e.g., oxygen and

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water vapor) extremely restrict their widespread applications.7-9 Iron−based zeolites (especially

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Fe−ZSM−5) are another type of N2O decomposition catalyst, which attracted great interest

51

because of their tolerance to O2 and H2O.10, 11 The N2O decomposition activity of Fe−ZSM−5 is

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even promoted by the presence of NO in flue gas.12 However, the reaction temperature of

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iron−based zeolites is quite high, and it is difficult to meet the actual flue gas conditions.

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Metal oxides, especially transition metal oxides, are widely researched and employed in the

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N2O decomposition reaction, which are consequences of their low price, excellent reducibility and

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adequate catalytic characteristics.13, 14 Particularly, metal oxides exhibiting the spinel structure are

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efficient catalysts to decompose N2O.15 The metal cations in the spinel structure are in the mixed

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valence state, which frequently consists of divalent and trivalent states. The divalent and trivalent

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cations in the spinel structure are located in tetrahedral and octahedral coordination centers and are

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represented as AIIBIII2O4. Since the key step in the N2O decomposition reaction is generally

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regarded as the charge transfer from the active sites to the antibonding orbital of N2O, spinels can

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decompose N2O at a relatively low temperature due to their excellent redox property attributed to 4


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the divalent and trivalent cations in the spinel structure.16

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with the spinel structure were systematically investigated in the decomposition of N2O.6 Russo et

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al. investigated several spinel−type catalysts and found that Co−based spinels can provide the

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most efficient N2O decomposition performance.18 However, the redox properties of Co−based

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spinels are not the best among those of spinel catalysts. Consequently, there must exist another

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crucial property that significantly affects the catalytic performance of N2O decomposition. Many

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researchers used DFT methods to calculate the reaction pathway of N2O decomposition and

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proposed that N2O adsorption is the first step in N2O decomposition.15, 19 The chemical adsorption

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of N2O generally follows “N−N−O−□”.20,

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vacancies can contribute to the chemical adsorption of N2O, and this is probably the main reason

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for the superior N2O decomposition performance of the Co3O4 spinel. Given this perspective,

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improving the reducibility without blocking oxygen vacancies is the most efficient way to

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improve the N2O decomposition performance of the Co3O4 spinel. Cu−based catalysts are another

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type of N2O decomposition catalysts that possess a superior redox property22. Combining the

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advantages of both the Cu−based catalysts and Co3O4 spinel can certainly improve the N2O

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decomposition performance. Therefore, in this work, a series of CuxCoy (CuO−Co3O4 spinel

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mixed oxides) catalysts were synthesized to decompose N2O.

21

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Given this perspective, metal oxides

This result suggests that abundant surface oxygen

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Kinetic study is an important approach to investigate the key factors of the N2O decomposition

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reaction. Freek Kapteijn et al. accomplished a comparative kinetic analysis over Co−, Fe−, and

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Cu−ZSM−5 and found that the effects of O2, NO and CO were influenced by their partial

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pressure.23 L. Obalova and V. Fıla established a novel kinetic model over hydrotalcites, which

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proposes that N2O chemisorption determines the rate of N2O decomposition at low O2 partial

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pressures, whereas the reaction between active O atoms and N2O is the rate−determine step at high

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O2 partial pressures.24 However, kinetic studies of N2O decomposition are rarely reported among

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recent studies and are even absent from recent reviews.6,

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between the kinetic model and physicochemical properties exists and greatly limits the design of

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efficient catalysts for N2O decomposition.

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Furthermore, a lack of connection

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Herein, the N2O decomposition mechanism and the key roles of CuO and Co3O4 spinel in

91

CuxCoy mixed oxides were systematically investigated by a kinetic study combined with DFT, in 5



92

situ DRIFTs, N2O−TPD, H2−TPR and XPS studies. The crucial properties of CuxCoy catalysts and

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the connection between the physicochemical properties and the kinetic study were proposed.

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2.

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2.1 Catalyst preparation

Experimental

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Co3O4, CuxCoy and CuO catalysts were prepared via the coprecipitation method. CuxCoy

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represents Cu1Co2, Cu1.5Co1.5 and Cu2Co1 catalysts, corresponding to molar ratios of Cu to Co of

98

1:2, 1:1 and 2:1, respectively. Suitable amounts of cupric nitrate and cobaltous sulfate were added

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to a solution with an excess of sodium hydroxide followed by continuous stirring for 3 h. The

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suspension was separated by centrifugation and washed with deionized water. The process of

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centrifugation and washing was repeated 5 times to remove any residual substances. The obtained

102

particles were dried at 105 °C for 12 h and then calcinated at 500 °C for 3 h in air.

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2.2 Characterization

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The BET surface area and X−ray diffraction (XRD) data were determined on a physisorption

105

analyzer (Blesorp max II) and an X−ray diffractometer (Rigaku D/max−2500). The surface

106

analyses (XPS and AES) were carried out on an X−ray photoelectron spectroscopy microprobe

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(EscaLab 250 Xi). Temperature program desorption (TPD) of O2 and N2O were both conducted

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on a chemisorption apparatus (Autochem II 2920), and N2O−TPD were further analyzed by a

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mass spectrum (MS, HPR−20 R&D). H2−temperature program reduction (H2−TPR) was also

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performed on the chemisorption apparatus. After H2−TPR studies, the coefficient between H2

111

consumption rate and peak intensity was gotten by the test results of standard sample (standard

112

CuO). Therefore, the dependences of H2 consumption rates versus 1/T (T=130~165 oC) were

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obtained as the initial H2 consumption rates.

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2.3 DFT calculation details

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Previous experimental data indicated that the Co3O4 spinel mainly exposes the (100) and (111)

116

planes, with only a minor exposure of the (110) plane.25 Additionally, the (100) plane is more

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stable than the (110) and (111) planes in a wide range of temperatures.26 Thus, the (100) plane of

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the Co3O4 spinel was reconstructed by a [2×2] supercell to generate the slab model. For CuO, the

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(111) plane is regarded as the most stable plane.27 Therefore, a slab model of the CuO (111) plane 6


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was also reconstructed by a [2×2] supercell.

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All calculations were conducted by the Vienna ab initio simulation package (VASP 5.4.4). The

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Perdew, Burke, and Ernzerhof (PBE) functional within the generalized gradient approximation

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plus Hubbard model (GGA+U) was used to calculate the electronic exchange and correlation. The

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Ueff of Cu and Co in this study were 7.0 eV and 3.5 eV, respectively.28, 29 The cutoff energy was

125

500 eV, and a Monkhorst−Pack grid of 2×2×1 k−points were employed due to the large size of the

126

slab (~12 Å×12 Å). The thickness of the slab was ~8 Å, with a 15 Å vacuum gap. Moreover, all

127

the slabs were relaxed until the atomic forces were reduced below 0.05 eV/Å.

128

The adsorption energies of N2O (Ead) were estimated by the following equation:

129

Ead= Esurf+N2O−Esurf−EN2O

130

where Esurf represents the energy of the clean surface, EN2O denotes the energy of a free N2O

131

molecule in the vacuum, and Esurf+N2O is the energy of N2O adsorbed on the surface. It is

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noteworthy that a negative value for Ead indicates a stable adsorption.

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2.4 Activity test

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The catalytic decomposition of N2O was performed in a fixed−bed reactor with 100 ml min−1 of

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flue gas containing 1000 ppm N2O, 2% O2 (when used), 200 ppm NO (when used), 0.5% H2O

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(when used) and the balance as N2. The catalyst mass was 100 mg, and the corresponding GHSV

137

was 60,000 cm3 g−1 h−1. The N2O concentration at the outlet was monitored online by a

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MultiGas™ 2030 FTIR continuous gas analyzer.

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The steady-state kinetic study of N2O decomposition was also performed in the fixed−bed

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reactor. The flue gas contained 500−1500 ppm N2O with the balance as N2. An extremely high

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GHSV of 60,000−6,000,000 cm3 g−1 h−1 was used to ensure that the N2O decomposition was less

142

than 20%, thus, overcoming the diffusion limitation.30-32

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3.

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3.1 Performance of N2O decomposition

Results

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The N2O decomposition performance of Co3O4, CuxCoy and CuO catalysts is shown in Figure

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1a. The N2O decomposition activity of Co3O4 was superior to that of CuO. Furthermore, all the

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CuxCoy catalysts showed superior activity to that of Co3O4 and CuO. Interestingly, although CuO

148

presented the lowest N2O decomposition activity, high amounts of Cu promoted the performance 7



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more significantly. For Cu2Co1, the reaction temperature for full N2O decomposition was ~375 °C

150

under idealized reaction conditions, which was significantly lower than the ~450 °C for Co3O4 and

151

~500 °C for CuO.

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Generally, O2, NO and water vapor exist in flue gas and often interfere with N2O

153

decomposition.33 The effects of O2, NO and water vapor on N2O decomposition over Cu2Co1 were

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investigated (Figure 1b). O2, NO and water vapor all interfered with the N2O decomposition

155

performance of Cu2Co1 at low temperatures, and the influencing degree increased according to the

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following sequence: NO < O2 < H2O. These results were also observed in Co3O4, Cu1Co2 and

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CuO catalysts (shown in Figure S1). Previous studies have shown that NO and H2O have

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completely different inhibiting mechanisms in the N2O decomposition. H2O prefers to bind with

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oxygen vacancy sites and then blocks the oxygen transfer, whereas NO shows a competitive

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oxidation effect, which consumes labile oxygen and decelerates the regeneration of active sites.34

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Further, O2 possibly can inhibit the recombination of residual O during N2O decomposition

162

reaction, leading to the inhibition of the regeneration of active sites. On the whole, N2O

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decomposition dramatically decreased from ~90% to ~6% at 350 °C when O2, NO and water

164

vapor coexisted; however, their influencing degrees decreased with increasing temperature. The

165

N2O decomposition of Cu2Co1 in the presence of O2, NO and H2O increased form ~6% to ~95%

166

when the temperature increased from 350 °C to 450 °C. This result suggests that Cu2Co1 still

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showed a superior N2O decomposition performance in the simulated flue gas.

168

3.2 Characterization

169

3.2.1 XRD and BET surface area

170

As shown in Figure S2, the XRD pattern of Co3O4 corresponded well to that of the cubic spinel

171

(JCPDS: #43−1003), and the XRD pattern of CuO was assigned to tenorite (JCPDS: #48−1548).

172

The XRD patterns of the CuxCoy catalysts showed characteristic peaks corresponding to both the

173

Co3O4 and CuO, and the peak positions were nearly unchanged. The crystal sizes and crystal

174

parameters of CuO and Co3O4 clusters in the Co3O4, CuxCoy and CuO catalysts were calculated on

175

the basis of the XRD patterns, and the results are shown in Table 1. The crystal parameters of

176

Co3O4 clusters of CuxCoy catalysts (a=b=c=~0.8097) were all slightly higher than those of pure

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Co3O4 (a=b=c=0.8090), meanwhile the crystal parameters of CuO clusters of CuxCoy catalysts 8


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were slightly different with those of pure CuO. These results indicate that a small amount of

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Cu−Co solid solution was generated, whereas most of CuO and Co3O4 existed in crystal form. The

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Co3O4 and CuO catalysts exhibited the maximum crystal sizes, whereas the crystal size of Cu2Co1

181

was the smallest. These results are in accordance with the results of BET surface area. The BET

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surface area of Cu2Co1 was higher than those of Co3O4, CuO and other CuxCoy catalysts.

183

Moreover, The BET surface area of CuO was only 8.1 m2 g−1, which was significantly lower than

184

those of the Co3O4 and CuxCoy catalysts. This difference might have been one of the reasons

185

responsible for the poor N2O decomposition performance of CuO.

186

3.2.2 Redox properties

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The reducibility of active sites (e.g., Cu,21 Ni,35 Co,17 and Fe36) would strongly affect the N2O

188

decomposition performance. H2−TPR studies were performed to investigate the enhancement in

189

reducibility of CuxCoy catalysts (Figure 2a). The H2 reduction peaks of the CuxCoy catalysts were

190

similar to those of CuO, which are situated at lower temperatures than those of Co3O4. That result

191

means the reductions of CuO and Co3O4 in CuxCoy catalysts occurred at approximately the same

192

time. Consequently, the data indicate the Cu species in CuxCoy catalysts played a dominant role in

193

the redox reaction, which could promote the reduction of Co species by a charge interaction in the

194

CuxCoy catalysts.37 Moreover, the H2 reduction peaks and the initial H2 consumption temperature

195

of all the Cu-containing catalysts seem identical (~158 °C). To further identify the reducibility of

196

Cu-containing catalysts, the initial H2 consumption rates were determined and are shown in Figure

197

2b. The initial H2 consumption rate of Cu2Co1 was clearly faster than those of the other

198

Cu-containing materials. This result indicates that Cu2Co1 represented the optimal reducibility,

199

which could facilitate the N2O decomposition of the Cu2Co1 mixed oxide.

200

3.2.3 Surface analysis

201

Generally, the redox cycles between the Cu2+/Cu+ and Co3+/Co2+ play important roles in N2O

202

decomposition.38 Thus, the ratios of Cu+/(Cu++Cu2+) and Co2+/(Co2++Co3+) are crucial to N2O

203

decomposition.15 The AES and XPS studies were used to determine the surface components of the

204

Co3O4, CuxCoy, and CuO catalysts, and the surface chemical compositions are listed in Table 2. In

205

Figure 3a, the AES spectra of Cu-containing catalysts over the spectral region of the Cu LMM

206

contained features mainly centered at ~916.0 eV and ~918.0 eV, which were assigned to Cu+ and 9



207

Cu2+, respectively.39, 40 Among the Cu-containing samples, the ratios of Cu+/(Cu++Cu2+) for all the

208

CuxCoy catalysts were higher than those for CuO. This result was further confirmed by the XPS

209

spectra for the Cu 2p3/2 spectral region (shown in Figure S3). Additionally, the Cu 2p3/2 spectral

210

region of pure CuO was situated at higher binding energies, whereas the peaks of Cu 2p3/2 for the

211

CuxCoy catalysts were shifted to lower binding energies. These results imply the electron cloud of

212

Cu species in the CuxCoy catalysts were altered due to the charge interaction between Cu and Co.

213

Corresponding with the shift in Cu 2p3/2, Figure 3b shows that the Co 2p3/2 spectral region of

214

pristine Co3O4 was also located at higher binding energies, and the peaks moved to lower binding

215

energies as Cu was added, further confirming the existence of the charge interaction between Cu

216

and Co. These results are in accordance with those of the H2−TPR study. The XPS spectra of

217

Co-containing samples for the spectral region of the Co 2p3/2 contained peaks mainly centered at

218

780.5 eV and 779.3 eV, which were attributed to Co2+ and Co3+, respectively. The ratios of

219

Co2+/(Co2++Co3+) for the CuxCoy catalysts were also higher than that for Co3O4. Overall, part of

220

the metal elements on the surface of the CuxCoy catalysts transformed from high valence states to

221

relatively low valence states, which was mainly due to the charge interaction between Co and

222

Cu.41 The facilitation of redox cycles between Cu2+/Cu+ and Co3+/Co2+ could play an important

223

role in the N2O decomposition performance of the CuxCoy catalysts.

224

3.2.4 N2O adsorption capacities

225

DFT calculations were employed to identify the characteristics of N2O adsorption on the

226

CuxCoy catalysts. Considering that the CuxCoy catalysts were mixed oxides of CuO and Co3O4, the

227

N2O adsorption configurations on their slab models were calculated. A negative value for Ead

228

indicates a stable adsorption in this study. As shown in Figure 4a, N2O could be weakly adsorbed

229

on Cu2+ to form a Cu−ON2 species. The bond length of Cu−O in the Cu−ON2 species was 2.87 Å,

230

and the corresponding N2O adsorption energy (Ead) was only −0.1 eV. N2O could also be weakly

231

adsorbed on a CuO surface with an oxygen vacancy to form a □−Cu−ON2 species (Figure 4b).

232

The bond length of Cu−O in the □−Cu−ON2 species was quite short (2.14 Å), whereas the

233

corresponding Ead was slightly lower than that of the Cu−ON2 species. These results suggest that

234

oxygen vacancies slightly promoted N2O adsorption on CuO, but these adsorption configurations

235

remained very unstable. For Co3O4, N2O could hardly be adsorbed on the complete surface 10


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structure (Figure 4c). The Ead of Co−ON2 was even higher than 0. However, the Ead of N2O

237

adsorbed on the Co3O4 surface with an oxygen vacancy (Figure 4d−f) was lower than those of

238

N2O adsorbed on Co3O4 and CuO, which suggests that N2O could be strongly adsorbed on the

239

Co3O4 surface with oxygen vacancies to form the □−Co−ON2 species. Consequently, N2O is more

240

likely to be adsorbed on a Co3O4 surface with oxygen vacancies than on a CuO or CuO−□ surface

241

in the CuxCoy catalysts.

242

N2O−TPD was performed to determine the capacity of N2O adsorption at 50 oC. In the

243

N2O−TPD study, all the samples were first treated under He atmosphere at 400 °C for 1 h and

244

then cooled to 50 °C to adsorb 2% N2O/He for 30 min. Finally, the original N2O−TPD profiles

245

(Figure S4a) were recorded by a TCD detector at a heating rate of 10 °C/min, and the detailed

246

desorption species were analyzed by MS (Figures S4b−4f). Therefore, the desorption amounts of

247

N2O (Figure 4g), N2 (Figure S4g) and NO (Figure S4h, a by-product of N2O decomposition) were

248

obtained by the integration of MS spectra. It is worth mentioning that the desorption amounts of

249

O2 generated by N2O decomposition during N2O−TPD could not be obtained, due to the

250

influences of adsorbed oxygen and/or the crystal oxygen on/in the catalysts.

251

Interestingly, although the N2O decomposition performance of Co3O4 was weaker than those of

252

the CuxCoy catalysts, the catalyst showed a relatively high capacity for N2O adsorption. This

253

property is mainly originated from the abundant surface oxygen vacancies of Co3O4 (shown in

254

Figure S5), which could promote the N2O adsorption through a □−Co−ON2 style (shown in Figure

255

4d−f). The high N2O adsorption capacity was probably responsible for the excellent N2O

256

decomposition performance of Co3O4 and the other Co−based spinels. In contrast, CuO exhibited

257

the worst N2O adsorption capacity, which was mainly due to the unstable Cu−ON2 and

258

□−Cu−ON2 species (shown in Figure 4a−b) and the lowest BET surface area, and therefore, this

259

catalyst showed the poorest N2O decomposition performance. The N2O adsorption capacities of

260

the CuxCoy catalysts increased with the Cu doping amount, which is in excellent accordance with

261

their N2O decomposition performance and O2−TPD profiles (Figure S5). Consequently, the

262

capacity of N2O adsorption played an important role in the N2O decomposition reaction.

263

4. Discussion 11



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4.1 Reaction mechanism and kinetic study Many researchers proposed that the N2O decomposition process can be generally described as:17, 19, 21, 42, 43

267

N 2 O(g) +M n+  M n+  ONN

(1)

268

M n+  ONN  M (n+1)+  O  +N 2(g)

(2)

269

2M (n+1)+  O   2M n+  O 2(g)

(3)

270

First, N2O is adsorbed at an active site on the surface (Reaction 1). Then, the adsorbed N2O can

271

decompose to N2 and a residual O atom (Reaction 2). Finally, two residual O atoms on the surface

272

can combine to generate O2, and the active sites are regenerated (Reaction 3).

273

Thus far, the rate limiting step of N2O decomposition was comprehensively discussed using the

274

DFT calculations and experimental study. However, the conclusions were completely

275

inconsistent.15 Some researchers found that the splitting of N2O (Reaction 2) is the rate

276

determining step, but others considered that the recombination of O2 (Reaction 3) determines the

277

N2O decomposition rate.44-47

278

If the rate-determining step is the splitting of N2O (Reaction 2), the rate of N2O decomposition

279

can be described as:

280

N O  

281 282

2

d[M n+  ONN] =k2 [M n+  ONN] dt

(4)

where vN2O, k2 and [Mnn+−ONN] represent the N2O decomposition rate, the kinetic constant of Reaction 2 and the concentration of adsorbed N2O, respectively.

283

While the GHSV is extremely high and the gaseous N2O concentration is relatively low,

284

Reaction 1 can be simply considered as a reversible reaction. Therefore, the concentration of the

285

adsorbed N2O can be approximately described as:

286

[M n+  ONN]=K1[M n+ ][N 2 O(g) ]

287 288 289

(5)

where K1, [Mn+] and [N2O(g)] represent the equilibrium constant of Reaction 1, the quantity of active sites and the concentration of gaseous N2O, respectively. Combined with Equations 4 and 5, the N2O decomposition rate can de depicted as:

12


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290


 N O  K1k2 [M n+ ][N 2 O(g) ]

(6)

2

291

In Equation 6, K1 and k2 are only related to the reaction temperature over the same catalyst.

292

Therefore, K1 and k2 can be regarded as constants if the reaction reaches steady state. Meanwhile,

293

the quantity of active sites (Mn+) can be rapidly regenerated through Reaction 3. This action

294

suggests that the quantity of active sites (Mn+) can also be regarded as a constant. Therefore,

295

Equation 6 can be simplified to:

296

 N O  k N O [N 2 O(g) ]

(7)

297

k N2O  K1k2 [M n+ ]

(8)

2

2

298

where kN2O denotes the reaction rate constant of the N2O decomposition.

299

Overall, the N2O decomposition is a first-order reaction when the rate-determining step is the

300 301

splitting of N2O. If the recombination of O2 (Reaction 3) determines the N2O decomposition rate, then the rate of

302

N2O decomposition can be described as:

303

N O  

304 305

2

d[M (n+1)+  O  ] =2k3 [M (n+1)+  O  ]2 dt

(9)

where k3 and [M(n+1)+−O−] denote the reaction rate constant of Reaction 3 and the concentration of M(n+1)+−O−, respectively.

306

Reactions 1 and 2 can be regarded as opposing reactions while the GHSV is extremely high;

307

consequently, the gaseous N2O concentration is relatively low, and the recombination of O2 is the

308

rate-determining step. Therefore, Equation 5 remains workable in this case, and the concentration

309

of M(n+1)+−O− can be described as:

310

[M (n+1)+  O  ]  K 2 [M n+  ONN]

(10)

311

where K2 represents the equilibrium constant of Reaction 2 in this case.

312

Combining Equations 5, 9 and 10, the rate of N2O decomposition can be formulated as:

d[M (n+1)+  O  ] =2k3 [M (n+1)+  O  ]2 2 dt  2k3 K 2 2 [M n+  ONN]2  2k3 K 2 2 K12 [M n+ ]2 [N 2 O(g) ]2

N O   313

 k N2O [N 2 O(g) ]2 13


(10)


314 315

k N2O  2k3 K 2 2 K12 [M n+ ]2

Page 14 of 32

(11)

Overall, the N2O decomposition is a second-order reaction when the rate-determining step is the

316

recombination of O2.

317

4.2 Model verification

318

A steady-state kinetic study was performed to judge the rate-limiting step of N2O

319

decomposition. An extremely high GHSV and a relatively low gaseous N2O concentration were

320

employed to satisfy the assumptions of the kinetic equations and to overcome the diffusion

321

limitations. The results of the steady-state kinetic study over the Co3O4, CuxCoy and CuO catalysts

322

are shown in Figure S6. All the materials showed significant linear relationships between the

323

gaseous N2O concentration and the N2O decomposition rate from 350−500 °C, with all lines going

324

through the origin of the coordinates. This result indicates that the N2O decomposition reaction

325

was a first-order reaction in this case, which is in good accordance with Equation 7. Therefore, the

326

splitting of N2O (Reaction 2), rather than the recombination of O2 (Reaction 3), was the

327

rate-determining step of N2O decomposition over the Co3O4, CuxCoy and CuO catalysts. Because

328

if the recombination of O2 was the rate-determining step, then the reaction order of N2O

329

decomposition would be 2. Thus, the linear regression presented in Figure S6 was performed to

330

obtain the reaction rate constant of N2O decomposition, and the results are shown in Figure 5.

331

Hinted by Equations 7 and 8, the reaction rate constant of N2O decomposition (kN2O) positively

332

correlated with the equilibrium constant of N2O adsorption (K1), the kinetic constant of N2O

333

splitting (k2) and the quantity of active sites (Mn+). Generally, the kinetic constant of N2O splitting

334

(k2) increases with the temperature. Therefore, the N2O decomposition rate of all the materials

335

increased with the temperature (shown in Figure 5). The kinetic constant of N2O splitting (k2)

336

relates to the redox ability,21, 38 whereas the quantity of active sites (Mn+) mainly relates to the

337

metal cations in a low valence state (e.g., Cu+ and Co2+) on the surface. The results of the H2−TPR

338

study (Figure 2) suggest that the sequence of redox ability followed Cu2Co1>Cu1.5Co1.5≈Cu1Co2≈

339

CuO≫Co3O4. The AES and XPS results (Figure 3) demonstrate that the percentages of metal

340

cations in a low valence state on the surface of the CuxCoy catalysts were almost the same, but

341

these percentages were significantly higher than those for CuO and Co3O4. The equilibrium

342

constant of N2O adsorption (K1) is mainly related to the ability to adsorb N2O. The adsorptivity of 14


Page 15 of 32


343

N2O was determined by N2O−TPD (Figure 4g), and the results show that the adsorptivity of N2O

344

decreased in the following sequence: Cu2Co1 > Co3O4 > Cu1.5Co1.5 > Cu1Co2 > CuO. CuO

345

presented the lowest N2O adsorptivity and redox cyclability, as well as a medium quantity of

346

active sites and redox property. Therefore, the lowest N2O decomposition performance of CuO

347

(shown in Figure 5) mainly resulted from its poor N2O adsorptivity and redox cycling between the

348

Cu2+/Cu+. Co3O4 presented a relatively high N2O adsorptivity as well as a medium quantity of

349

active sites. However, its redox ability was the weakest, which was mainly responsible for the

350

inferior N2O decomposition performance of Co3O4 (shown in Figure 5). For the CuxCoy catalysts,

351

the redox ability derived from CuO, the N2O adsorptivity derived from Co3O4, and the higher

352

quantities of active sites derived from the charge interaction between Co and Cu were all

353

responsible for the excellent N2O decomposition performance. Meanwhile, these properties in

354

Cu1.5Co1.5 and Cu1Co2 were almost the same, except the N2O adsorption capacity of Cu1.5Co1.5 was

355

slightly higher than that of Cu1Co2. This result proves that the N2O decomposition performance of

356

Cu1.5Co1.5 was slightly higher than that of Cu1Co2 (shown in Figure 5). Cu2Co1 sustained the

357

highest N2O adsorptivity, redox ability and quantity of active sites. Consequently, Cu2Co1 showed

358

the optimum performance for N2O decomposition (shown in Figure 5).

359

Based on the above results and conclusions, the N2O decomposition mechanism over CuxCoy

360

catalysts, and the key roles of CuO and Co3O4 in CuxCoy catalysts for N2O decomposition were

361

proposed (Scheme 1). The DFT calculation results suggest that Co3O4 provided abundant surface

362

oxygen vacancies, and thus, served as the major adsorption site of N2O. CuO was dispersed

363

around Co3O4 and provided high reducibility on the interface of Co3O4−CuOx, which promoted

364

the rate-determining step (N−O break) of N2O decomposition and left O in the defect sites.

365

Meanwhile, the charge interaction became stronger with increasing Cu content and promoted the

366

formation of Cu+ and Co2+, which performed as the active sites and adsorption sites, respectively.

367

Finally, the residual O in the defect sites recombined to release O2.

368

15



369

Acknowledgements:

370

This work was financially supported by the National Key Research and Development Program

371

(2017YFC0210700 and 2017YFC0212804), and the National Natural Science Foundation of

372

China (21876093 and 21777081).

373

Supporting Information Available

374

This information is available free of charge via the Internet at http://pubs.acs.org/.

375

N2O decomposition performances of Co3O4, Cu1Co2, and CuO under different conditions. XRD

376

patterns of the Co3O4, CuxCoy, and CuO catalysts. XPS spectra of the CuxCoy and CuO catalysts

377

for the spectral region of Cu 2p3/2. TCD signals of N2O−TPD profiles over Co3O4, CuxCoy, and

378

CuO catalysts. MS spectra of N2O, O2, NO, and N2 over Co3O4, CuxCoy, and CuO during

379

N2O−TPD. N2 and NO desorption amounts during N2O−TPD over Co3O4, CuxCoy, and CuO

380

catalysts. Dependence of the N2O decomposition rate on the N2O concentration over the Co3O4,

381

CuxCoy, and CuO catalysts at 350−500 °C.

382

16


Page 16 of 32

Page 17 of 32


383

References:

384

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385

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Decomposition of Nitrous Oxide over ZSM-5 Catalysts. J. Catal. 1997, 167, 256-265.

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Koscheev, S. V.; Chupakhin, A. P.; Boronin, A. I. In situ XRD, XPS, TEM, and TPR study of

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highly active in CO oxidation CuO nanopowders. J. Phys. Chem. C 2013, 117, 14588-14599.

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Rico-Pérez, V.; Bueno-López, A.; Kotarba, A. Strong dispersion effect of cobalt spinel active

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Computational and experimental investigations into N2O decomposition over MgO nanocrystals

498

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500

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501

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502

(45) Yamashita, T.; Vannice, A. N2O decomposition over manganese oxides. J. Catal. 1996, 161,

503

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504

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505

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506

H2O on recombination of oxygen. J. Catal. 2008, 256, 183-191.

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(47) Wu, L. N.; Qin, W.; Hu, X. Y.; Ju, S. D.; Dong, C. Q.; Yang, Y. P. Decomposition and

508

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509

22


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Page 23 of 32


510

Table Captions

511

Table 1. Crystal sizes, crystal parameters and BET surface areas of the Co3O4, CuxCoy, and CuO

512

catalysts.

513

Table 2. Surface chemical compositions of the Co3O4, CuxCoy, and CuO catalysts.

514

23



Page 24 of 32

515

Table 1. Crystal sizes, crystal parameters and BET surface areas of the Co3O4, CuxCoy, and CuO

516

catalysts. crystal size a /nm

Co3O4

517

BET surface area /m2 g−1

CuO

Co3O4

CuO

Co3O4

−

21

−

a=b=c=0.8090, α=β=γ=90o

27

a=b=c=0.8098, α=β=γ=90o

30

a=b=c=0.8097, α=β=γ=90o

28

a=b=c=0.8095, α=β=γ=90o

40

−

8.1

Cu1Co2

22

18

Cu1.5Co1.5

24

17

Cu2Co1

14

13

CuO

27

−

a Calculated

crystal parameter a /nm

a=0.4689, b=0.3433, c=0.5137, α=γ=90o, β= 99.44o a=0.4678, b=0.3446, c=0.5127, α=γ=90o, β= 99.47o a=0.4687, b=0.3399, c=0.5104, α=γ=90o, β= 99.49o a=0.4687, b=0.3420, c=0.5135, α=γ=90o, β= 99.37o

from the XRD patterns.

518

24


Page 25 of 32

519


Table 2. Surface chemical compositions of the Co3O4, CuxCoy, and CuO catalysts /%. Cua

520 521 522

Cob Cu+

Co3+

Co2+

Co3O4

−

−

13.2

7.7

79.1

Cu1Co2

9.8

3.8

11.6

9.5

65.3

Cu1.5Co1.5

13.4

4.7

10.3

8.5

63.1

Cu2Co1

17.1

6.9

8.5

6.6

60.9

CuO

36.5

4.8

−

−

58.7

a Calculated b Obtained

Ob

Cu2+

from the AES spectra of Cu LMM. from the XPS spectra of Co 2p3/2.

25



523

Figure Captions

524

Figure 1. (a) N2O decomposition performance of the Co3O4, CuxCoy, and CuO catalysts. (b) N2O

525

decomposition performance of Cu2Co1 under different conditions. Reaction conditions: [N2O] =

526

1000 ppm, [O2] = 2% (when used), [NO] = 200 ppm (when used), [H2O] = 0.5% (when used),

527

catalyst mass = 100 mg, flow rate = 100 ml min−1, and GHSV=60,000 cm3 g−1 h−1.

528

Figure 2. (a) H2−TPR profiles of the Co3O4, CuxCoy, and CuO catalysts. (b) The initial H2

529

consumption rates of the CuxCoy and CuO catalysts in the H2−TPR study.

530

Figure 3. (a) AES spectra of the CuxCoy and CuO catalysts for the spectral region of the Cu

531

LMM. (b) XPS spectra of the Co3O4 and CuxCoy catalysts for the spectral region of the Co 2p3/2.

532

Figure 4. Model structures of N2O adsorbed on CuO: (a) CuO with an oxygen vacancy (b), Co3O4

533

(c), and Co3O4 with an oxygen vacancy (d−f). The white balls represent N, red balls represent O,

534

the blue balls represent Cu, and the navy-blue balls represent Co. (g) N2O desorption amounts

535

during N2O−TPD over Co3O4, CuxCoy, and CuO catalysts.

536

Figure 5. N2O decomposition rate constants of the Co3O4, CuxCoy, and CuO catalysts.

537

Scheme 1. The key roles of CuO and Co3O4 in the CuxCoy catalysts for N2O decomposition.

538 539 540 541

26


Page 26 of 32

Page 27 of 32


N2O decomposition/%

100 80 60

Co3O4 Cu1Co2 Cu1.5Co1.5 Cu2Co1 CuO

40 20 0

N2O decomposition/%

300

100

350

400

450

Temperature/oC (a)

no addition with H2O

500

with O2 with NO with O2+NO+H2O

80 60 40 20 0

350

400

Temperature/oC

450

542

(b) Figure 1. (a) N2O decomposition performance of the Co3O4, CuxCoy, and CuO catalysts. (b) N2O

543

decomposition performance of Cu2Co1 under different conditions. Reaction conditions: [N2O] =

544

1000 ppm, [O2] = 2% (when used), [NO] = 200 ppm (when used), [H2O] = 0.5% (when used),

545

catalyst mass = 100 mg, flow rate = 100 ml min−1, and GHSV=60,000 cm3 g−1 h−1.

546

27



Page 28 of 32

215 158

Intensity/a.u.

Cu2Co1

213 184

Cu1.5Co1.5 Cu1Co2

367 320

100

Initial H2 consumption rate/mol g-1 s-1

CuO

202

200

300

Co3O4 400

Temperature/oC (a)

4.0

500

600

Cu1Co2 Cu1.5Co1.5 Cu2Co1 CuO

3.2 2.4 1.6 0.8 0.0

2.28

2.32

2.36

2.40

2.44

2.48

547

1000 T-1/K-1 (b) Figure 2. (a) H2−TPR profiles of the Co3O4, CuxCoy, and CuO catalysts. (b) The initial H2

548

consumption rates of the CuxCoy and CuO catalysts in the H2−TPR study.

549

28


Page 29 of 32


550 Co 2p3/2

Cu LMM +

Cu 

Satellites

+

Cu1.5Co1.5Cu Cu1Co2

Cu2Co1

Intensity/a.u.

Intensity/a.u.

Cu2Co1



Cu+ 

Co2+ 

Cu1.5Co1.5

Satellites

Cu1Co2

Cu 

Co3O4

CuO

Co2+ 

Satellites

+

908

Co2+  779.2 eV

Co2+ 

779.7 eV

Satellites

912

916

792

920

787

782

777

Binding Energy/eV

Kinetic Energy/eV

551

(a) (b) Figure 3. (a) AES spectra of the CuxCoy and CuO catalysts for the spectral region of the Cu

552

LMM. (b) XPS spectra of the Co3O4 and CuxCoy catalysts for the spectral region of the Co 2p3/2.

553

29



554

555 556

Figure 4. Model structures of N2O adsorbed on CuO: (a) CuO with an oxygen vacancy (b), Co3O4

557

(c), and Co3O4 with an oxygen vacancy (d−f). The white balls represent N, red balls represent O,

558

the blue balls represent Cu, and the navy-blue balls represent Co. (g) N2O desorption amounts

559

during N2O−TPD over Co3O4, CuxCoy, and CuO catalysts.

30


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561 562

Figure 5. N2O decomposition rate constants of the Co3O4, CuxCoy, and CuO catalysts.

563

31



564 565

Scheme 1. The key roles of CuO and Co3O4 in the CuxCoy catalysts for N2O decomposition.

566

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Balance between Reducibility and N2O Adsorption Capacity for the

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