Tuning the K+ - ACS Publications - American Chemical Society

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Tuning the K concentration in the tunnels of #-MnO to increase the content of oxygen vacancy for ozone elimination Guoxiang Zhu, Jinguo Zhu, Wenlu Li, Wenqing Yao, Ruilong Zong, Yongfa Zhu, and Qianfan Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01594 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 3, 2018

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Environmental Science & Technology

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Tuning the K+ concentration in the tunnels of α-MnO2 to increase the content of oxygen

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vacancy for ozone elimination

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Guoxiang Zhua, Jinguo Zhub, Wenlu Lia, Wenqing Yaoa, Ruilong Zonga, Yongfa Zhua* and Qianfan Zhangb*

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a Department of Chemistry, Tsinghua University, Beijing 100084, China

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b School of Materials Science and Engineering, Beihang University, Beijing 100191, China

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ACS Paragon Plus Environment


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ABSTRACT

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α-MnO2 is a promising material for ozone catalytic decomposition and the oxygen vacancy is

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often regarded as the active site for ozone adsorption and decomposition. Here, α-MnO2 nanowire

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with tunable K+ concentration was prepared through a hydrothermal process in KOH solution. High

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concentration K+ in the tunnel can expand crystal cell and break the charge balance, leading to a

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lower average oxidation state (AOS) of Mn, which means abundant oxygen vacancy. DFT

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calculation has also proven that the samples with higher K+ concentration exhibit lower formation

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energy for oxygen vacancy. Due to the enormous active oxygen vacancies existing in the α-MnO2

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nanowire, the lifetime of the catalyst (corresponding to 100 % ozone removal rate, 25 ºC) is

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increased from 3 h to 15 h. The FT-IR results confirmed that the accumulation of intermediate

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oxygen species on the catalyst surface is the main reason why it is deactivated after long time

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reaction. In this work, the performance of the catalyst has been improved because the abundant

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active oxygen vacancies are fabricated by the electrostatic interaction between oxygen atoms inside

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the tunnels and the introduced K+, which offers us a new perspective to design a high efficiency

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catalyst and may promote manganese oxide for practical ozone elimination.

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TOC Art

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1. INTRODUCTION

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In recent years, ozone is given increased attention as a common air pollutant due to the potential

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impact on public health and our living environment. A long-term exposure to even low level of ozone

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would cause serious health problems, including cardiopulmonary disease,1 respiratory disease2 and

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cardiovascular disease.3, 4 Generally, ozone is formed near the ground in the photochemical reaction

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referring to volatile organic compounds (VOCs), NOx and carbon monoxide.5 However, household

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electrostatic equipments6 and ultraviolet disinfection equipments7 also cannot be ignored for indoor

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ozone production. Besides, ozonation and plasma process also cause serious ozone pollution in

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factory. Apart from the hazard of ozone, the indoor residual ozone also leads to the formation of

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VOCs by the ozone-surface reaction with building materials and indoor furnishing,8-10 and the

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produced compounds may form the secondary organic aerosol (SOA).11, 12 Thus, the Occupational

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Safety and Health Administration (OSHA, US) stipulates the maximum allowable exposure to ozone

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in the terms of eight-hour is 0.10 ppm.13 The regulations of Chinese indoor air quality standard’

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(GB/T 18883 - 2002) also require that the indoor ozone concentration should not exceed 0.08 ppm.

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Therefore, the research on the ozone elimination is significant for environmental protection and

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public health.

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Various methods have been applied for ozone removal, such as plants purification,14

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adsorption,15 thermal decomposition and catalytic decomposition.16-19 Compared with other methods,

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the catalytic decomposition has been recognized as a promising approach, due to its higher efficiency,

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safety and lower cost. The catalyst for ozone elimination can be divided into noble metal,20, 21

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transitional metal oxide16-19,

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large-scale application, the research about ozone catalytic decomposition mainly focus on

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and noble-transitional compounds. Considering the cost and

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transitional metal oxide, especially manganese oxide. Jia et al.24 compared the activity of

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α, β and γ−MnO2 for ozone removal and observed the highest ozone removal rate with α−MnO2.

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They found that the catalytic performance of manganese oxide strongly depends on the density of

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oxygen vacancies. In our previous study, surface oxygen vacancy was proved to be the active site for

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ozone adsorption and further decomposition.19 The formation of oxygen vacancy would change the

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original charge distribution, resulting in a lower ozone adsorption energy and better catalytic

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activity.25 For example, Wang et al.26 reported that an cryptomelane-type manganese oxide (OMS-2)

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catalyst synthesized with MnAc2 as Mn2+ precursor could form abundant Mn3+, leading to a higher

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catalytic performance. Ma et al.18 also reported that a higher catalytic performance could be obtained

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in transition mental doped OMS-2, resulting in the abundant oxygen vacancies. Therefore, the

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enhancement of oxygen vacancy is regarded as an effective strategy to improve the catalytic

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performance.

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α−MnO2 is consisted of 2 × 2 edge-share MnO6 octahedral chains, which corner-connected to

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form 0.46 × 0.46 nm tunnels. Abundant MnO6 edges exposes in the structure, leading to easy release

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of lattice oxygen.27 Manganese is located in the framework and the guest cation, such as Li+, Na+, K+,

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NH4+, is filled in the tunnels to stabilized the structure. Because of the unique tunnel structure,

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various cations, especially potassium ion, can be introduced into the tunnels to tailor its chemical and

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physical properties. For example, Yuan et al.28 reported that the presence of potassium ions inside the

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tunnels of α−MnO2 would increase its electronic conductivity. Tseng et al.29 pointed out that

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magnetic properties of α−MnO2 nanotube is closely connected with the K+ doping concentration.

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Hou et al.30 also demonstrated that K+-doped OMS-2 nanowire has better catalytic activity for

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benzene oxidation. Generally, these cations are located in the center of tunnels and the charge on the 5



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cations would be balanced by substitution of Mn3+ for some of the Mn4+.31 So, oxygen vacancies are

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easy to form at the MnO6 edges and a higher ozone removal rate is expected when the K+ entered

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into the tunnels. Herein, we developed a simply post-processing method to prepare K-rich α-MnO2

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nanowire for ozone catalytic decomposition. The effect of K+ concentration in the tunnels of α-MnO2

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on the performance for ozone removal has been studied theoretically and experimentally.

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2. EXPERIMENTAL SECTION

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Preparation of K-rich MnO2

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α-MnO2 nanowires were synthesized by a hydrothermal process using KMnO4 and MnSO4.

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Specifically, 3.042 g MnSO4·H2O and 1.896 g KMnO4 (Mn7+ : Mn2+ = 2 : 3) were added into 80 mL

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deionized (DI) water with constant stir for 20 mins to form a suspension. Subsequently, the

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suspension was transformed into 100 mL Teflon-lined stainless steel autoclave and heated at 150 °C

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for 12 h. The products were collected by centrifugation, then washed with DI water and dried at

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80 °C for 10 h. The obtained nanowire was identified as MnO2. The K+ concentration was tuned by a

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hydrothermal process in KOH solution as illustrated in Figure 1. 1.0 g synthesized nanowire was

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added in 80 mL KOH solution accompanied with ultrasonic dispersion for 20 mins, and then

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transformed into 100 mL Teflon-lined stainless steel autoclave and heated at 150 °C for several hours.

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Then the collected products were washed with DI water until the filtrate was neutral (PH ≈ 7).

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Finally, the K-rich MnO2 nanowire was obtained after drying at 80 °C for 10 h.

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Figure 1. Schematic illustration of the preparation of K-rich α-MnO2.

Catalyst Characterization.

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X-ray diffraction (XRD) patterns of the samples were obtained by a Rigaku D/max-2400 X-ray

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diffractometer with Cu Kα (λ = 1. 5406 Å) radiation at 40 kV and 150 mA. Morphologies of the

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samples were captured by a Field Emission Gun Scanning Electron Microscopy (FESEM, Hitachi

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SU-8010) and a transmission electron microscopy (TEM, Hitachi 7700) with an accelerating voltage

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of 100 kV. High resolution transmission electron microscopy (HRTEM) images were obtained by a

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JEM 2100F field emission transmission electron microscope at an accelerating voltage of 200 kV.

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The element composition and distribution were recorded by ICP-MS (iCAPQ, Thermo Fisher

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SCientific) and an energy dispersive (EDS) detector in Hitachi SU-8010 and JEM 2100F. X-ray

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photoelectron spectroscopy (XPS) was conducted in a PHI Quantera SXMTM system and the binding

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energy was calibrated with the signal for adventitious carbon at 284.8 eV. Fourier transform infrared

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(FT-IR) spectra and in situ diffused Fourier transform infrared spectroscopy (DFTIR) was recorded

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by Bruker V70 spectrometer. Thermogravimetric analysis (TGA) was carried out in a TGA/DSC1

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STARe system (METTLER TOLEDO). Temperature programmed desorption (TPD) was carried out

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on a Cat-Lab (BEL Japen, Inc.) eqiupped with an online QIC-200 quadrupole mass (Inprocess

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Instruments, GAM 200) as a detector.

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Catalytic Activity. 7



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The performance of catalytic ozone decomposition was evaluated in a continuous fixed bed

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reactor (11 mm inner diameter) at room temperature (25 ºC). 0.1 g catalyst was used for each test

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with the space velocity of 5.4 × 105 mL·g-1·h-1. The total gas flow through the reactor was kept at

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900 mL/min and the inlet ozone concentration maintained at 50 ± 1 ppm. The relative humidity was

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measured by a hygronom (605-H1 Testo) and controlled by tuning the gas flow through a humidifier

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without changing the total gas flowrate. An ozonator (model 1000BT-12, Shanghai Enaly

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Mechanical and Electrical Technology Company) was applied to generate ozone by arc discharge in

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O2 stream, where the O2 flow and the discharge voltage can be tuned. The generated ozone mixed

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adequately with air in a mixing drum and then transported to the reactor. The inlet and outlet ozone

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concentration were measured by an ozone detector (model 202, 2B Technologies). The ozone

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removal rate can be calculated from the following equation:

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Ozone removal rate = 100 % × (Cin - Cout)/Cin.

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where Cin and Cout are inlet and outlet ozone concentration respectively.

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3. RESULTS AND DISCUSSION

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3.1 The Characterization of Structure and Morphology.

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Figure 2. XRD patterns of α-MnO2 before and after treated in 1M KOH solution (a); the enlarged

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view of the α-MnO2 (200) peak (b).

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The structure of α-MnO2 nanowire before and after the treatment in KOH solution were

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analyzed by XRD as shown in Figure 2a. All the obtained samples have the same peaks,

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corresponding to a pure tetragonal cryptomelane type MnO2 (JCPDS No. 29-1020), which means the

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crystalline phase is well maintained during the treatment.32 In order to confirm the influence of the

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post-processing on the composition of the samples, the bulk contents of K and Mn were detected by

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ICP-MS (Table 1). After hydrothermal treatment in 1 M KOH solution for 4 h, the K/Mn molar ratio

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is increased from 13.44 % to 20.75 %. The K/Mn molar ratio on the catalyst surface has also been

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measured by XPS and the treatment can increase the ratio from 13.46 % to 22.89 % compared with

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pure MnO2, as shown in Figure S3. The same results can also be obtained by the EDS equipped in

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FESEM and HRTEM, indicating that the K+ content has been indeed enhanced.

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In the α-MnO2 crystal structure, the crystal radii of K+ (1.65 Å) is significantly larger than that

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of Mn3+ (high spin, 0.785 Å; low spin, 0.72 Å) and Mn4+ (0.67 Å), suggesting that K+ entered into

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the 2 × 2 tunnels of α-MnO2 rather than the octahedral framework.18 Further observation of XRD

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spectra shows that all the peaks gradually shift to lower angles with the extension of hydrothermal

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time and the trend is more clearly for (200) peak (Figure 2b), which means the tunnel is expanded.28,

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29, 33

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maintained a pure cryptomelane structure and possess of high crystallinity. However, the lattice

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spacing of (200) plane has increased from 0.476 nm to 0.489 nm after treated, corresponding to the

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expansion of the tunnel structure. In addition, the mapping images (Figure S4) suggest that the K and

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Mn is well distributed in the nanowire before and after hydrothermal treatment. Therefore, it can be

High-resolution TEM (HRTEM) images (Figure 3c, f) indicate that MnO2 and KOH-4h

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concluded that abundant K+ were entered into the tunnel structure and well dispersed in the

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hydrothermal process.

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Table 1. Atomic ratio of K/Mn, BET surface areas, and AOS of Mn of the manganese oxide samples. Atomic ratio of K/Mn Sample

BET surface areas AOS of Mn

ICP-MS

FESEM-EDS

[m2/g]

MnO2

13.44

11.45

61.00

3.76

KOH-2h

19.24

16.19

52.50

3.49

KOH-4h

20.75

16.93

52.43

3.43

KOH-6h

23.18

17.16

51.94

3.37

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The morphologies of as-prepared samples were investigated by FESEM and TEM. As shown in

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Figure 3a, b, the as-synthesized MnO2 presents a typical nanowire morphology with a diameter in the

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range of 20-40 nm and a length for about 2-5 µm. However, after hydrothermal treatment in 1 M

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KOH solution for 4 h, the original isolated nanowire arranges along the diameter direction and forms

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a bunchy structure. TEM images (Figure 3d, e) also confirmed that the length of KOH-4h bundle has

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no obvious change but the width is beyond 60 nm. As shown in FT-IR spectra (Figure 4a), the peaks

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value at 3430 cm-1 and 1630 cm-1, attributed to the surface absorbed water and hydroxyl groups

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respectively,34 become larger with longer treatment time, suggesting abundant surface hydroxyl

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groups is formed during the post-processing. Hu et al.35 pointed out that abundant surface hydroxyl

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groups are produced from the hydroxylation of the surface oxides to balance the overall charge after

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K+ entered into the tunnels of α-MnO2 nanowire. Therefore, the existing of abundant surface

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hydroxyl groups means a higher K+ content in the tunnels. From Figure S5, it can also be found that

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the bundled trend become serious with increased hydrothermal time. Table 1 also has shown that the

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treatment in KOH solution leads to the decrease of surface area from 61 m2/g to 51.91 m2/g, 10


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corresponding to the change in morphology. Besides, the bundled extent of treated samples reduces

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partly after calcination at 300 ºC for 2 h, while the crystal structure has no obvious difference (Figure

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S7, 8). Hence, it can be concluded that the bunchy structure is formed with the help of the abundant

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surface hydroxyl groups, which induce hydrogen bond interaction among isolated nanowires. The

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bunchy structure also reconfirms the enhancement of K+ concentration in the treated samples.

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Figure 3. FESEM images of α-MnO2 (a) and KOH-4h (d); TEM images of α-MnO2 (b) and

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KOH-4h (e); HRTEM images of α-MnO2 (c) and KOH-4h (f).

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The evolution of oxygen from the samples was investigated by temperature programmed

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desorption (TPD-MS) as shown in Figure 4b. The desorption process can be divided into four parts.

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The peaks lower than 200 ºC corresponds to the physical absorbed water molecule.36 The peak

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between 200 ºC and 400 ºC is assigned as the desorption of surface hydroxyl radical. For KOH-4h,

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the peak at 233 ºC shift to 278 ºC, suggesting the existence of a stronger bond between surface 11



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hydroxyl radical and KOH-4h. The peaks between 400 ºC to 650 ºC is assigned as the release of the

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lattice oxygen close to the surface19 and the crystalline phase will completely transform to Mn2O3

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while the temperature rises to 800 ºC. Compared with MnO2, the peaks of KOH-4h at 475 ºC and

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586 ºC shift to 531 ºC and 617 ºC respectively and the peak area has an obvious decrease, indicating

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that a large amount of lattice oxygen is more stable after treated in KOH solution.33 As shown in

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Figure 4c, the TGA curve under N2 atmosphere has been recorded to characterize the thermostability.

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After the treatment in KOH solution, the weight loss from 45 ºC to 800 ºC is reduced from 11.65 %

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to 9.97 %, as a result of the higher K+ content in KOH-4h. Interestingly, the weight loss between 400

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ºC to 650 ºC decreases form 5.72 % to 3.26 % while the ratio between 650 ºC to 800 ºC increases

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from 3.44 % to 4.53 %, corresponding to the TPD-MS results, suggesting a more stable structure is

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indeed induced by increasing K+ concentration in tunnels. Based on the elaborate analysis of the

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crystal structure of cryptomelane type MnO2 (Figure S9), it can be found that the increase of K+

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concentration in the tunnels can induce the electrostatic interaction between K+ and the lattice

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oxygen with a sp3 hybridization [O (sp3)] to form a balanced force distribution, which leads to a

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higher temperature for the evolution of oxygen.

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Considering the vital impact of Mn-O bond on the catalytic activity, the average oxidation state

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(AOS) of surface Mn atom was analyzed via XPS. As shown in Figure S11, the signals of Mn, O and

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K can be found in all the samples. After treated in KOH solution, the charge on the tunnel cation (K+)

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is balanced by substitution of Mn3+ for some of the Mn4+ in the framework.31 Further observation

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(Figure 4d) has shown that the binding energy difference (∆‫ܧ‬௦ ) between the two peaks of Mn 3s

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varies with the increase of K+ content and the AOS of surface Mn has been estimated via the

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following formula37,

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: AOS=8.956-1.126∆‫ܧ‬௦ . After treated in KOH solution, the AOS of Mn 12


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gradually decreases from 3.76 to 3.37 with the extension of treatment time, meaning a lower average

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coordination number and higher oxygen vacancy content. In addition, the decrease of AOS means

200

that Jahn-Teller (JT) distortion can occur to lower the system’s ground state37, which has the same

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conclusion with XRD results.

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Figure 4. The FTIR spectra (a), the TPD-MS profiles (b), TGA curve (c) and Mn 3s XPS spectra (d)

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of the samples.

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3.2 Performance of Ozone Removal

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Figure 5. The ozone removal rate of the samples treated with different concentration KOH solution

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(a) and the samples treated for different time in 1M KOH solution (b); The ozone removal rate of the

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samples treated with different alkali solution for 4h (c); The ozone removal rate of the samples

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treated with different potassium salt solution for 4h (d); The ozone removal rate on KOH-4h at

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different RH (e); The ozone removal rate on KOH-4h at alternate humidity condition (f); All the

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reaction was carried out at 25 ºC.

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Based on the characterized analysis, K+ entered into the tunnels of α-MnO2 with the treatment,

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which can generate abundant oxygen vacancy for ozone decomposition. Therefore, a higher ozone

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removal rate is expected for α-MnO2 with higher K+ content. As shown in Figure 5a, the ozone

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removal rate of the samples treated in different concentration KOH solution have been evaluated. For

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α-MnO2 without treatment, the ozone removal rate begins to decrease while the reaction runs for 3 h

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in the gas flow with 50 ppm ozone and it decreases to 25 % when the reaction maintained for 12 h.

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The bunchy structure in the treated samples, with a smaller surface area, provides a poor activity if 14


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the density of active site has no obvious increase. However, when the as-synthesized nanowire was

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treated in 0.25 M KOH solution for 4 h, the time possessed of 100 % ozone removal rate increased

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from 3 h to 10 h and it could extend to 15 h if the concentration of KOH solution keeps at 1.00 M. It

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indicates that the treatment in KOH solution has indeed improved the catalytic performance for

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ozone removal. The atomic K/Mn ratio detected by EDS detector equipped in FESEM also shows

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there is a positive correlation between the catalytic performance and the K+ content, suggesting that

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the increase of K+ content in the tunnels is beneficial for ozone decomposition. The performance of

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the catalyst is also affected by hydrothermal time. The optimal catalytic activity can be obtained

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when α-MnO2 nanowire is treated in 1.00 M KOH solution for 4 h (Figure 5b). However, if the

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hydrothermal time or the concentration of KOH solution continuously increased, the increase of K+

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content will be slow (Figure S12a, b) and the effect of bunchy structure on the surface area will be

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obvious, leading to an almost unchanged catalytic activity.

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In order to further confirm the reason why the ozone removal rate has been improved, the

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α-MnO2 nanowire was treated in different alkali and kali salt solution respectively. As can be seen in

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Figure 5c, the samples treated in LiOH, NaOH or NH3·H2O solution are all possessed of a lower

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performance compared with untreated MnO2, indicating that it is K+ content in the tunnels rather

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than surface hydroxyl group that improves the ozone removal rate. When the α-MnO2 nanowire was

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treated in KNO3, K2SO4 and CH3COOK solution respectively (the K+ concentration keeps at 1M),

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the activity for ozone removal has different levels of increase except for the sample treated in KNO3

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solution (seeing Figure 5d), suggesting ozone decomposition is also effected by the anion absorbed

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on the surface. High-resolution XPS spectra of KNO3-4h, K2SO4-4h, CH3COOK-4h also has

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confirmed the existence of absorbed anions on the surface of the samples (Figure S14). Besides, the 15



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atomic K/Mn ratio has been further analyzed by the EDS detector, as shown in Figure S12c. The

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samples treated in different kali salt solution are possessed of various K+ content (Figure S12d),

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resulting in different catalytic performance for ozone removal. It can be concluded that the anion in

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the solution will affect the ability of K+ entering the α-MnO2 tunnel structure, leading to various

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content of oxygen vacancy. In general, the ozone catalytic decomposition can be facilitated by

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increasing the K+ content in the tunnels and it is also influenced by the corresponding anion in the

249

solution.

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The catalyst is easy to be deactivated in humid environment.17, 39 As can be seen in Figure 5e,

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when the relative humidity (RH, 25 °C) increases from 22 % to 30 %, the ozone removal rate at 15 h

252

decreases from 99 % to 78 % and the removal rate continues to decreasing with further increase of

253

RH, suggesting that the performance of the catalysts is also sensitive to the humidity of the gas flow.

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In the high humidity gas flow, a large amount of surface hydroxyl group would form under the

255

combined action of water vapor and ozone molecule. The hydrogen bond interaction between surface

256

hydroxyl group and water molecule would adverse to the ozone uptake on the catalyst.40 For

257

KOH-4h, due to the abundant hydroxyl group on the surface, a strong hydrogen bond interaction has

258

already exist in the initial stage of the reaction, resulting in a dramatically decrease of ozone removal

259

rate at the beginning (Figure S16). For untreated MnO2, the inherent surface hydroxyl group is finite,

260

so the ozone removal rate decreased gradually. However, KOH-4h is possessed of higher activity at

261

12 h than MnO2, which suggests more active sites existing in KOH-4h. As can be seen in Figure 5f,

262

the ozone removal rate has been evaluated at alternate humidity conditions. When the catalyst was

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placed into the wet gas flow (50 % RH), the ozone removal rate has a sharp decrease due to the

264

competitive adsorption of water molecule. However, once the relative humidity was decreased, the 16


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ozone removal rate recover to 100 % quickly, indicating that the deactivation resulting from water

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molecule is temporary and the catalytic activity in the dry gas flow has not been affected by surface

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hydroxyl group. When the relative humidity of the gas flow was increased to 50 % again, the ozone

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removal rate decreases to the level of the latest cycle quickly and then continues at a slower rate,

269

suggesting the increase of the surface hydroxyl group can exacerbate the competitive adsorption

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which leads to a lower performance for ozone removal.

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3.3 The Formation Energy of Oxygen Vacancy

272 273

Figure 6. The calculation results of oxygen vacancy formation energies at specific sites with varied

274

K content.

275

The process of oxygen vacancy formation often plays a key role in catalytic activity.41, 42 In

276

order to deeply understand the effect of K+ in the tunnels on the formation of oxygen vacancy, the

277

first-principles calculations are performed based on density functional theory as implemented in the

278

VASP code. The elaborated calculation method can be found in the Supporting Information. As

279

shown in Figure S9, in the framework of α-MnO2, each oxygen atom is coordinate with three 17



280

manganese atoms.

281

three nearby Mn atoms to form the sp2 hybridization [O (sp2)], while other half locate out of plane to

282

form the sp3 hybridization [O (sp3)] with nearest manganese atoms. The oxygen vacancy formation

283

energy (Eform) at two specified sites (inset of Figure 6), with sp2 and sp3 hybridization respectively,

284

has been simulated in the absence or presence of different K+ concentration in the tunnels to present

285

us a tendency of oxygen vacancy formation energies. As shown in Figure 6, the oxygen vacancy

286

formation energy at the sp2 site dramatically decreases from 3.001 eV to 2.018 eV after 1 K+ is

287

constructed in the tunnel structure, and the formation energy keeps decreasing with the increase of

288

K+ concentration, corresponding to Hou’s results.30 The results indicate that the filled K+ can lead to

289

the electrostatic interaction between K+ and the lattice oxygen which is beneficial to the formation of

290

active oxygen vacancy. Besides, the effect of K+ is closely related to the distance between K+ and the

291

oxygen site. However, different results were obtained at the specific sp3 oxygen site. Without K+ in

292

the tunnels, the sp3 oxygen atom is faced with an asymmetrical force distribution which exhibits

293

lower Eform. However, when two K+ is constructed in the neighborhood of the specified oxygen atom,

294

a symmetrical force distribution forms, leading to a significant increase of oxygen vacancy formation

295

energies at sp3 oxygen atoms. So, the Eform first decreases and then increases with the growth of K+

296

content. Overall, the formation energy of oxygen vacancy is smaller at sp3 oxygen site, indicating

297

that the oxygen vacancy preferentially forms at sp3 oxygen site, corresponding to our former works.19

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Therefore, tuning the K+ concentration in the tunnels appropriately will be an effective method to

299

enhance the content of oxygen vacancy.

300

3.4 The Schematic of Ozone Decomposition and Catalyst Regeneration.

Among them, half of the oxygen atom locate roughly in the same plane with the

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Figure 7. The schematic of ozone catalytic decomposition on the K-rich MnO2 (a); FT-IR spectra of

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KOH-4h after reaction for different time (b); FT-IR spectra of MnO2 after reaction in different gas

304

flow (c); The TPD-MS profiles of KOH-4h before and after reaction (d); In situ DFTIR spectra of the

305

sample after reaction (e); High-resolution XPS spectra of KOH-4h before and after reaction (f).

306

The oxygen vacancy mechanism24, 43, 44 is generally accepted and our previous works also have

307

elaborated the effect of oxygen vacancy on the ozone decomposition.19 Firstly, one of the oxygen

308

atoms in the ozone molecule inserts into the surface oxygen vacancy and electrons transform from

309

Mn atom to ozone molecule, leading to the decomposition of ozone molecule into oxygen molecule

310

and oxygen species (O2-). Then, another ozone molecule reacts with the oxygen species and forms an

311

oxygen molecule and peroxide species (O22-). Finally, the peroxide species transforms into oxygen

312

molecule and the oxygen vacancy recovers. As shown in Figure 7a, the hydrothermal treatment in

313

KOH solution will induce the formation of oxygen vacancy. The more oxygen vacancy form, the

314

more active sites for ozone decomposition exist, resulting in a higher performance for ozone 19



315

removal.

316 317

Figure 8. The FT-IR spectra of KOH-4h before reaction, KOH-4h after reaction and regenerated

318

KOH-4h (a); The ozone removal rate of the KOH-4h and regenerated KOH-4h (b).

319

However, the catalytic performance decreases with the reaction on, so the reason why the

320

catalyst deactivates is important for designing more effective catalyst. As can be seen in Figure 7b,

321

only the peaks around 3430 cm-1 and 1630 cm-1 can be found in the FT-IR spectra of fresh KOH-4h.

322

However, a new peak appears at 1380 cm-1 after the catalytic reaction, suggesting the surface

323

composition has changed in the process.45, 46 And the intensity of that peak increases with the extend

324

of the reaction time, indicating the peak at 1380 cm-1 results from the accumulation of intermediate

325

species. To further confirm the intermediate species, pure O2 was taken as the source gas to evaluate

326

the activity of KOH-4h and the FT-IR spectra of treated KOH-4h was shown in Figure 7c. The peak

327

appears whether the source gas is O2 or air, indicating that the intermediate species belongs to

328

surface oxygen species. The results of TPD-MS (Figure 7d) show that an oxygen desorption peaks

329

appear at 364 ºC, suggesting the intermediate species indeed belongs to oxygen species. To further

330

understand the thermostability of the intermediate oxygen species, in situ DFTIR has been carried

331

out. As shown in Figure 7e, the peak at 1380 cm-1 always exist even the temperature increased to 200 20


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332

ºC, suggesting that the intermediate oxygen species is relatively stable. However, Jia et al.24 reported

333

that the peroxide significantly decomposes at temperature less than 100 ºC from Raman spectrum.24

334

So, we speculate that the peroxide species (O22-)43, 47, 48 would evolve into a relatively stable oxygen

335

species, corresponding to the peak at 1380 cm-1, leading to the deactivation of the catalyst. The same

336

results can be obtained through the increase of the content of surface adsorbed oxygen (Figure S18)

337

and the AOS of Mn atom (Figure 7f). Therefore, the higher performance of KOH-4h results from the

338

abundant oxygen vacancy induced by K+, but the accumulation of intermediate oxygen species

339

causes the inactivation of the catalyst.

340

To reuse the catalyst, the deactivated KOH-4h has been calcinated in Ar atmosphere at 400 ºC

341

for 2 h to remove the accumulation oxygen species. As shown in figure 8a, the peak at 1380 cm-1

342

disappear after the deactivated KOH-4h has been calcinated. At the same times, the performance also

343

recovers partly (figure 8b), indicating that calcination in Ar atmosphere is an effective method to

344

regenerate the catalyst.

345 346

3.5 Implications and Future Direction of the Research.

347

In this work, the K+ content in the tunnel structure of α-MnO2 nanowire is increased dramatically

348

via a simply post-processing with KOH solution, generating abundant oxygen vacancy which leads

349

to a higher performance for ozone removal. Theoretical results show that the increase of K+ content

350

in the tunnels can decrease the formation energy of oxygen vacancy, which is beneficial for the

351

formation of oxygen vacancy in the crystal lattice. Experimental data also shows the AOS of Mn

352

decreases as the K+ content in the tunnels increases, meaning the formation of abundant oxygen

353

vacancy. The existence of active oxygen vacancy accelerates the decomposition of ozone. The K+ 21



354

content can be controlled by adjusting KOH concentration and hydrothermal conditions. When the

355

as-synthesized α-MnO2 nanowire is treated in 1 M KOH solution for 4 h, the time possessed of 100 %

356

ozone removal rate at high space velocity (5.4 × 105 mL·g-1·h-1) can be increased from 3 h to 15 h.

357

In addition, the deactivation in high humidity is temporary and the activity would recover once the

358

humidity decrease. So, a high performance for ozone decomposition can be obtained by adding a

359

dehumidifier before the reactor. More importantly, the deactivated catalyst can be regenerated by

360

calcination in Ar atmosphere, which is significant for practical application.

361

Here, abundant active surface oxygen vacancy was fabricated by the electrostatic interaction

362

between oxygen atoms inside of the tunnels and introduced K+, which offer us a new perspective to

363

improve the catalytic performance for ozone removal. However, the results of FT-IR spectra show

364

peroxide species (O22-) can accumulate on the surface of the catalyst and evolve into a relatively

365

stable oxygen species, causing a decrease of the ozone removal rate. Therefore, the accumulation of

366

intermediate oxygen species is a key factor limiting ozone decomposition efficiently and sustainably

367

and need to be further researched.

22


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368

4. ASSOCIATED CONTENT

369

Supporting Information Available. Materials characterization methods, DFT calculation method

370

and other supplementary date are provided in the Supporting Information. The detail information

371

about the calculation in this study can be provided by contacting the corresponding author Qianfan

372

Zhang ([email protected]). These materials are available free of charge via the Internet at

373

http://pubs.acs.org.

374 375

AUTHOR INFORMATION

376

Corresponding Author

377

a* Email: [email protected]

378

b* Email: [email protected]

379

Note

380

The authors declare no competing financial interest.

381 382

ACKNOWLEDGEMENTS

383

This work was partly supported by Chinese National Science Foundation (21437003, 21673126,

384

21621003, 21761142017) and Collaborative Innovation Center for Regional Environmental Quality.

385

23



386

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Tuning the K+ - ACS Publications - American Chemical Society

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