Nonthermal Plasma-Assisted Catalytic Oxidation of Carbon Monoxide

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Cite This: J. Phys. Chem. C 2019, 123, 16721−16730

Nonthermal Plasma-Assisted Catalytic Oxidation of Carbon Monoxide over CuOx@γ-Al2O3: Understanding Plasma Modification of Catalysts and Plasma−Catalyst Synergy Jian Zhang, Xiaohong Yao, Qi Shao, Bowen Xu, Xiaoyang Liang, and Chao Long* State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, 163 Xianlin Avenue, Nanjing 210023, China Downloaded via BUFFALO STATE on July 28, 2019 at 22:24:21 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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ABSTRACT: Nonthermal plasma (NTP)-enhanced CO catalytic oxidation over CuOx@γ-Al2O3 was investigated in a coaxial dielectric barrier discharge reactor at ambient temperature under dry and humid conditions. The CO conversion of 100% was obtained at about 600 J/L under dry conditions. CuOx@γ-Al2O3 had a stronger resistance to water in the NTP-catalytic system in comparison to the thermal-catalytic system. The CO conversion kept stable under humid conditions (RH = 80%) within 10 h in plasma mode, whereas the CO conversion decreased by 12% in thermal mode. It is found that the catalyst’s bed temperature inside the NTP reactor at 600 J/L was not high enough to completely oxidize CO into CO2, suggesting that CO plasma-catalytic oxidation was not dependent only on the thermal activation caused by the plasma-induced heating. In addition, the CuOx@ γ-Al2O3 catalysts after plasma exposure for different times were characterized by N2 adsorption−desorption, X-ray diffraction, scanning electron microscopy− energy-dispersive X-ray, H2-TPR, O2-TPD, and X-ray photoelectron spectroscopy. The results show that plasma treatment decreased the ratio of Cu+/Cu2+ and the surface-adsorbed oxygen species of the CuOx@γ-Al2O3 catalyst. On the basis of the characterization of virgin and plasma-treated CuOx@γ-Al2O3 catalysts as well as the catalytic performance tests, a plausible mechanism of plasma-assisted catalysis of CO is discussed. The Eley−Rideal and Mars− van Krevelen mechanism besides the Langmuir−Hinshelwood mechanism may be responsible for enhancing carbon monoxide oxidation in the plasma-catalytic system. This result brings a new insight into utilizing plasma activation to effectively enhance the catalytic performance.

1. INTRODUCTION Considered harmful to public health and the environment, the oxidation of carbon monoxide (CO) has become important in the field of air purification and automotive exhaust cleaning. However, it is almost impossible to achieve CO elimination through homogeneous combustion of CO because of the lower concentration (usually within the range of 50−50 000 ppm).1 Alternatively, catalytic oxidation is more effective in removing carbon monoxide. Copper-based catalysts have attracted much attention because of the high catalytic activity of CuOx and low cost compared with precious metals. For example, Cu-based catalysts are applied widely in preferential oxidation2,3 and water gas shift reactions,4 and show a greater CO selectivity than noble-metal catalysts.5,6 However, a notable disadvantage of Cu-based catalysts is lack of activity below 200 °C.7 Moreover, the lesser resistance to deactivation by water is a key restriction for practical use of Cu-based catalysts. In recent years, alternative methods to the thermal activation process have been explored to improve the operation efficiency at low temperatures. Among these methods, plasma catalysis has proven to be highly effective in promoting the activity in VOC oxidation,8−11 automotive catalysis,12,13 and reforming/ hydrogenation reactions.14,15 Compared with traditional © 2019 American Chemical Society

thermal catalytic reactions, nonthermal plasma (NTP) activation could provide high conversion.16−18 A few works have been conducted for the catalytic oxidation of CO in an NTP reactor. Chang and Lin.19 studied CO oxidation in a dielectric barrier discharge (DBD) reactor combined with an Mn catalyst in a single-stage process and two-stage process. Kim et al.20 found that NTP greatly enhanced the CO oxidation over Au/TiO2 catalysts. Kirkpatrick et al.21 reported a significant augmentation of the oxidation rate of CO when plasma was placed upstream of a diesel oxidation catalyst. However, although water vapor is ubiquitous in the real exhaust gases, the effect of water vapor on CO oxidation in a plasma catalytic reactor was not investigated in these studies. Moreover, it is suggested that the mechanism of CO oxidation in plasma catalytic systems was attributed mainly to the reaction between carbon monoxide and the active species because of ozone decomposition on the surface of catalysts. However, it is found that NTP could lead to an increase in the reactor temperature,22 and may modify the catalyst by Received: April 1, 2019 Revised: June 16, 2019 Published: June 16, 2019 16721

DOI: 10.1021/acs.jpcc.9b03010 J. Phys. Chem. C 2019, 123, 16721−16730

Article

The Journal of Physical Chemistry C

Figure 1. Schematic of a plasma-catalytic and a thermal-catalytic system.

catalyst bed temperature measured by the thermocouple placed in the catalyst bed without plasma, shown in Figure S1a. The discharge power of the NTP reactor was measured using the charge−voltage (Q−U) Lissajous method and calculated as reported in ref 26. The thermal-activation experiment was carried out in the same cylindrical reactor by replacing the ground electrode with a K-type thermocouple (diameter of 3 mm) placed in the center of the catalyst bed to measure the reaction temperature. The temperature of the reactor was controlled using an electric furnace. The concentration of carbon monoxide was controlled at about 1500 ppm. The total gas flow rate was set at 1 L/min, corresponding to the gas hourly space velocity of 130 000 h−1 in the dielectric barrier discharge region. The inlet and outlet concentrations of CO were detected using a gas chromatograph (GC-9890A, China) equipped with a flame ionization detector and a methane converter. The stability of the catalyst under dry and wet (RH 80%) conditions was tested over 600 min at a constant temperature or NTP conditions, respectively. The conversion of CO can be calculated as the following equation

changing the oxidation state or metal surface area of the components,23−25 which might have an effect on CO oxidation. To the best of our knowledge, few studies have been conducted systematically on the mechanism of CO oxidation in plasma catalytic systems, especially the contributions of modification and heating of the catalyst to the catalytic reaction because of the plasma. In this study, the CuOx@γ-Al2O3 catalyst was first prepared via the incipient-wetness impregnation method. The catalytic oxidation of CO over CuOx@γ-Al2O3 with plasma and conventional heating under dry and humid conditions was studied. In addition, the thermal oxidation performance of CO over CuOx@γ-Al2O3 catalysts exposed to plasma for different times was evaluated. The effect of plasma on structural changes and surface properties of the catalyst were also investigated. On the basis of the characterization of virgin and plasmatreated CuOx@γ-Al2O3 as well as the catalytic performance tests, the mechanism of significantly promoted activity of CO oxidation over the CuOx@γ-Al2O3 catalysts in the plasmacatalytic system would be discussed.

2. EXPERIMENTAL SECTION 2.1. Plasma−Catalysis Hybrid System. As shown in Figure 1, the experimental setup included four parts: (i) gas distributing system, (ii) two power supplies (one for conventional heating, another for producing plasma), (iii) fixed bed flow reactor using conventional heating or NTP, and (iv) gas analysis instruments. All gas streams (N2, O2, CO) were regulated by mass flow controllers and premixed before entering the reactor. The relative humidity (RH) was regulated by bubbling the water and detected using a hygrometer (Testo 605-H1). The plasma was ignited by AC high voltage power (Suman CTP2000-K, China), and its electrical characterization was measured by a digital oscilloscope (Tektronix TBS1020). In the plasma-activation experiment, a cylindrical DBD reactor was adopted to produce NTP. The DBD reactor consists of a quartz tube (inner diameter of 10 mm, outer diameter of 12 mm), a stainless-steel rod (diameter of 3 mm, ground electrode) placed along the axis of the quartz tube, and a stainless-steel mesh (length of 30 mm, discharging electrode) on the outer wall of the reactor connected with the highvoltage power. The catalyst was filled in the discharge zone to ensure a direct contact with the plasma-generated species and to make use of any possible local heating effects. The outside wall temperature of the reactor was measured by a noncontact infrared thermometer (Smart sensor, AR862A). The catalyst bed temperature was obtained using the calibration curve between the outside wall temperature of the reactor and the

η =

Cin − Cout × 100% Cin

2.2. Catalyst Preparation. CuOx/γ-Al2O3 catalysts were prepared by an impregnation method with a copper nitrate aqueous solution as the metal precursor. The γ-Al2O3 with 20− 40 meshes was used as support. In order to investigate the effect of NTP on the structure of catalysts, the catalysts were placed in the DBD reactor and modified by NTP at 526.3 J/L for 5, 30, and 60 min. The gas condition of plasma modification simulates the reaction condition, that is, 1 L/ min, 20% O2 + 80% N2. The plasma-treated catalysts were denoted as CuOx/γ-Al2O3-a, where a is the modification time. 2.3. Catalyst Characterization. The nitrogen adsorption−desorption isotherms on a Micromeritics ASAP 2020 at −196 °C was used to measure the specific surface area and pore volume of the catalysts. Prior to the measurements, the catalysts were degassed at 200 °C for 8 h under vacuum to remove moisture and other adsorbed gases. The actual Cu loading amount of the catalysts was measured by an inductively coupled plasma optical emission spectrometer (PerkinElmer USA) and found to be 9.3 wt %. The crystalline structure of the catalysts was studied by Xray diffraction (XRD, X′TRA, ARL, Switzerland) on a diffractometer with Cu Kα radiation at 40 kV and 40 mA. The surface morphology and elemental distribution of the 16722

DOI: 10.1021/acs.jpcc.9b03010 J. Phys. Chem. C 2019, 123, 16721−16730

Article

The Journal of Physical Chemistry C

Figure 2. CO conversion over γ-Al2O3 support, CuOx@γ-Al2O3 catalyst, and blank sample under plasma conditions (a), and over the CuOx@γAl2O3 catalysts with different plasma-modification times (0, 5, 30, 60 min) under thermal conditions (b).

Figure 3. Effect of RH on CO conversion over the virgin CuOx@γ-Al2O3 catalysts under thermal (a) and plasma conditions (b); stability test of thermal and plasma catalytic CO conversion over 10 h under dry and wet conditions (c).

catalysts were investigated by scanning electron microscopy (SEM, S-4700, Hitachi). X-ray photoelectron spectroscopy (XPS) measurement was performed using an Al Kα radiation (1486.6 eV) on a PHI5000 VersaProbe system. The binding energies were calibrated using the C 1s peak at 284.8 eV as a reference. The H2-temperature programmed reduction (TPR) experiments of the catalysts were conducted in an AutoChem II 2920 apparatus. At first, 100 mg of the sample was pretreated at 120 °C and purged with helium gas for 1 h. Subsequently, the catalyst was reduced with 10% H2/Ar, while the temperature was increased from 25 to 500 °C (10 °C/min). The O2temperature programmed desorption (TPD) of the catalysts

was performed using the same apparatus. The catalyst (100 mg) was first preheated to 120 °C and purged with He for 1.5 h, and then cooled down to 25 °C. After that, the catalyst was purged with 5% O2/He at 25 °C for 2 h. Finally, the catalyst was purged with He and the desorption profile of O2 was recorded online at a heating rate of 10 °C/min to 900 °C.

3. RESULTS AND DISCUSSION 3.1. Catalytic Performance with Plasma and Conventional Heating. 3.1.1. Catalytic Activity. Figure 2a shows the CO conversions in a plasma reactor under different conditions (plasma alone, packed with γ-Al2O3 or CuOx@γ-Al2O3 catalyst). The plasma alone gave a very low CO conversion 16723

DOI: 10.1021/acs.jpcc.9b03010 J. Phys. Chem. C 2019, 123, 16721−16730

Article

The Journal of Physical Chemistry C Table 1. Textural Properties and Redox Characteristics of the Various CuOx@γ-Al2O3 Catalysts H2 consumption (mmol H2 g−1)

peak temperature (°C)

sample

SBET (m2 g−1)a

dp (nm)b

Vp (cm3 g−1)c

Cu crystallite size (nm)d

peak α

peak β

peak (α + β)

peak α

peak β

CuOx@γ-Al2O3 CuOx@γ-Al2O3-5 CuOx@γ-Al2O3-30 CuOx@γ-Al2O3-60

147.2 153.4 153.8 155.7

7.72 7.74 7.72 7.76

0.42 0.41 0.42 0.42

27.6 27.2 28.4 28.8

0.70 0.60 0.52 0.53

0.38 0.58 0.71 0.79

1.08 1.18 1.23 1.32

150.2 152.4 153.2 159.5

189.4 192.6 202.7 213.1

a

BET specific surface area. bAverage pore diameter by applying the Barret−Joyner−Halenda method to the desorption branch of the isotherm. Average pore volume. dCalculated from the Scherrer equation according to the [111] diffraction peaks of CuO.

c

to the catalyst surface than oxygen or carbon monoxide, thus poisoning its catalytic performance.28,29 Also, in the plasmacatalytic system, the increase of water limits the electron density and quenches the active species.30 In addition, the stability of the CuOx@γ-Al2O3 catalyst under dry and wet (RH = 80%) conditions was tested over 10 h under thermal and plasma conditions, respectively. For comparison, similar starting CO conversions of approximately 90% were obtained under thermal (180 and 250 °C) and plasma (526.3 and 677 J/L) conditions under dry and wet conditions, respectively. As shown in Figure 3c, the CO conversion under plasma condition kept stable at 90% no matter for dry or wet gas; however, the CO conversion with thermal activation decreased gradually under wet condition. The catalysts used in the thermal and plasma activation under wet condition were characterized using XRD, XPS, Fourier transform infrared (FTIR), and thermogravimetry (TG)/derivative TG (DTG) (Figures S2−S5), respectively. After 10 h of reaction under wet condition in the NTP and thermal systems, the XRD results show no meaningful changes for the catalysts (Figure S2), but the XPS results (Figure S3) show that the Cu2+/(Cu+ + Cu2+) surface ratio (from 0.41 to 0.55) in the NTP test was higher than that (from 0.41 to 0.48) in the thermal test. The surface oxidation of the catalyst is not conducive to the thermal catalytic activity, but has little effect on the plasma catalytic activity. The detailed discussion about surface property changes has been shown in Section 3.2.4. The FTIR results show the presence of surface carbonates in the region 1532 cm−1.31,32 The peak intensity for catalysts used in the thermal catalytic reaction was stronger than that in the plasma catalytic reaction. Also, an obvious weight loss occurred at nearly 249 °C over the CuOx@γ-Al2O3 catalyst used in the plasma catalytic reaction. The results indicate that the deactivation of the CuOx@γ-Al2O3 catalyst in thermal catalytic oxidation under wet conditions was due to the formation of carbonate, which blocked the redox sites of the catalyst.33,34 Stronger CO2 adsorption easily gives rise to the formation of surface carbonate species.35 As plasma can promote the desorption process through ion and electron bombardment,36,37 the CO2 was adsorbed less in the NTP system than in the thermal system. Namely, rapid CO2 desorption avoids the accumulation of surface carbonate species in the plasma catalytic system. Therefore, the formation of carbonate in the catalyst is less in the plasma catalytic reaction. Overall, the above results suggest that plasma could enhance CuOx@γ-Al2O3 catalytic oxidation of CO, and CuOx@γ-Al2O3 had a stable catalytic activity for CO conversion under wet conditions in the plasma-activated system. In comparison to the traditional thermal activation, plasma activation is a promising method for CO conversion.

of about 8% over the tested specific input energy (SIE) range. The result is in accord with the previous study.20 Generally, it is hard to oxidize carbon monoxide to carbon dioxide using NTP alone by reason of the limited rate coefficients of gasphase CO-radical reactions. However, in a packed-bed discharge plasma reactor with γ-Al2O3, the CO conversion increased to a maximum of 30% first, and then declined with the increase of SIE. This is because γ-Al2O3 possesses the capability for decomposing the plasma-induced ozone into reactive atomic oxygen species, which can directly oxidize CO into CO2.27 However, because of the self-decomposition of O3 at a high temperature caused by high SIE (the reactor temperature at different SIE, Figure S1b), the CO conversion did not increase and instead decreased with the further increase of SIE. In addition, a significant improvement of CO conversion was observed when the CuOx@γ-Al2O3 catalyst was packed in the plasma reactor, reaching 100% at 600 J/L. On the basis of these results, it is strongly suggested that the CuOx@γ-Al2O3 catalysts play a predominant role in the oxidation of carbon monoxide. It is well-known that NTP could result in the increase of reactor temperature.22 In order to investigate the effect of plasma-induced heating on plasma activation, the temperature of the catalyst bed was measured during NTP reactions (Figure S1b), and then thermally activated light-off experiments of the unmodified and plasma-treated CuOx@γ-Al2O3 catalyst were carried out (Figure 2b). The reactor temperature measured during the NTP was 133 °C at an SIE of 526.3 J/L. Under NTP activation, the CO conversion was close to 90% at a SIE of 526.3 J/L; however, there was only 20% CO conversion at 133 °C under thermal activation. A similar thermal property requires temperatures above 180 °C (Figure 2b). Under a lower SIE ( CuOx@γAl2O3-5 > CuOx@γ-Al2O3. It is because the partial Cu+ was oxidized to Cu2+ in the plasma-treating process (Figure 6c). In summary, in combination with the results of H2-TPR, O2-TPD, and XPS, it can be concluded that plasma-treating decreases the ratio of Cu+/Cu2+ and the amount of surface-adsorbed oxygen species of the CuOx@γ-Al2O3 catalyst. Thus, compared to the virgin CuOx@γ-Al2O3 catalyst, the plasma-modified CuOx@γ-Al2O3 exhibited lower thermal catalytic performance for CO oxidation. 3.3. Plasma−Catalyst Synergy. Plasma−catalyst interactions in plasma catalysis can be described as resulting either from the catalyst changing the discharge characteristics or the plasma affecting the performance of the catalyst. Figure 7a shows the voltage waveforms measured at the capacitor and the NTP reactor with and without packing materials (γ-Al2O3 or CuOx@γ-Al2O3). The spikes on the capacitor voltage are characteristic for the formation of the micro-discharges in the NTP reactor.48 It is found that micro-discharges were less and weaker in the NTP-CuOx@γ-Al2O3 reactor compared to those in the NTP alone and the NTP-γ-Al2O3 reactor, indicating that CuOx@γ-Al2O3 catalysts suppressed the formation of microdischarges and may result in the formation of less amount of high-energy species to involve in the reaction. In addition, it is

may lower the thermal catalytic activity of CuOx@γ-Al2O3 for CO oxidation. The relationship between the thermal catalytic activity and the integrated peak area of peroxy species and the total of peroxy species O2−(ad) and monatomic species O−(ad), which may be involved in CO catalytic oxidation because of low desorption temperature, is displayed in Figure 5c. It could be observed that the amount of the O2− is an approximately linear function of the CO conversion of thermal oxidation, whereas a less straightforward correlation is recognized between the activity and monatomic species O−(ad), indicating that the chemisorbed peroxy species O2−(ad) is the determining factor. The participation of the adsorbed O2− species in copper-based catalysts for CO oxidation at low temperatures has been studied.42 More surface-adsorbed oxygen produces more reactive oxygen species, which could lead to a high catalytic activity via surface reactions.44 From these results, we learn that plasma modification decreased the formation of oxygen vacancies and the mobility of surface oxygen, resulting in the lower thermal catalytic activity of plasma-treated CuOx@γAl2O3 for CO conversion compared to the virgin CuOx@γAl2O3 catalysts (Figure 2b), which is in compliance with the H2-TPR results. 3.2.4. Surface Analysis of the Catalysts. XPS was performed to further investigate the chemical compositions in the surface region of all the catalysts. The XPS spectra of Cu 2p3/2 for the virgin and plasma-modified CuOx@γ-Al2O3 catalysts are shown in Figure 6a. There are obvious satellite peaks positioned at about 940.5 and 943.4 eV, which indicates the presence of Cu(II) species.43 Moreover, two characteristic peaks of the main Cu 2p3/2 signal are observed at 934.7 and 932.1 eV (Figure 6a), which can be ascribed to Cu(II) and Cu(I) species, respectively. In order to clarify the change of Cu(II) and Cu(I) species in the plasma process, the Cu 2p3/2 peaks of all the CuOx@γ-Al2O3 catalysts are fitted by Gaussian curves and the area ratio between Cu+ and Cu2+ is compared. It is worth mentioning that the relative ratio of Cu+/Cu2+ decreased with the increase of plasma modification time (Figure 6c), indicating that the oxidation reaction (Cu+ → 16727

DOI: 10.1021/acs.jpcc.9b03010 J. Phys. Chem. C 2019, 123, 16721−16730

Article

The Journal of Physical Chemistry C

activation of bulk lattice oxygen could effectively improve lattice oxygen mobility and ensure the fast regeneration of available sites, thus facilitating CO oxidation and enhancing the catalytic activity.

noted that three CuOx@γ-Al2O3 catalysts with different plasma exposure times had a negligible effect on the intensity of spikes (Figure 7b). The results suggest that the improvement of CO conversion in the NTP reactor packed with CuOx@γ-Al2O3 compared with that in the NTP alone reactor is not due to the enhancement of micro-discharges, but depends on the catalytic ability of CuOx@γ-Al2O3. On the other hand, plasma discharge may influence catalyst properties. It is accepted generally that three pathways could account for the assistant effects of plasma on catalytic reactions: (1) the temperature rise because of plasma, (2) the plasma-modification of catalysts, and (3) the new reaction pathways because of plasma-generated species.22 In the present study, plasma did result in an increase in temperature of the catalyst bed, but the temperature is not high enough to achieve a complete conversion, especially the temperatures at low SIE, as seen from the light-off curve of the thermal reaction for CuOx@γ-Al2O3 (Figure 2b). That is to say, the NTP can heat the CuOx@γ-Al2O3 catalysts but the temperature of the catalysts is too low to obtain high thermal CO conversion. In addition, as discussed before, plasma-treating decreased the formation of oxygen vacancies and the mobility of surface oxygen in the surface of CuOx@γ-Al2O3. As reported in the literature, carbon monoxide oxidation over the copper-based catalyst at the low- and medium-temperature (