In Situ DRIFTs Investigation of the Low-Temperature Reaction

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In Situ DRIFTs Investigation of the Low-Temperature Reaction Mechanism Over Mn-Doped CoO for the Selective Catalytic Reduction of NO With NH 3

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Hang Hu, Sixiang Cai, Hongrui Li, Lei Huang, Liyi Shi, and Dengsong Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06057 • Publication Date (Web): 17 Sep 2015 Downloaded from http://pubs.acs.org on September 20, 2015

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The Journal of Physical Chemistry

In Situ DRIFTs Investigation of the Low-Temperature Reaction Mechanism over Mn-Doped Co3O4 for the Selective Catalytic Reduction of NOx with NH3 Hang Hu,† Sixiang Cai,† Hongrui Li,† Lei Huang,† Liyi Shi, †,‡ and Dengsong Zhang*,† †

Research Center of Nano Science and Technology, ‡Department of Chemistry, Shanghai University, Shanghai 200444, China.

ABSTRACT:

The Co3O4 and Mn-doped Co3O4 nanoparticle were synthesized by a

co-precipitation method and used as selective catalytic reduction of NO with NH3 (NH3-SCR) catalysts. After the doping of manganese oxides, the NH3-SCR activity of Mn0.05Co0.95Ox catalyst is greatly enhanced. The NO oxidation ability of two catalysts is compared and the X-ray diffraction results demonstrate that Mn has been successfully doped into the lattice of Co3O4. The X-ray photoelectron spectroscopy, temperature-programmed reduction with H2 results confirmed that there is a strong interaction between Mn and Co in the Mn0.05Co0.95Ox catalyst. Their adsorption and desorption properties were characterized by temperature-programmed desorption with NH3 or NO+O2 and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTs). These results indicated that the doping of manganese could provide more acid sites on the catalysts, and bidentate nitrates species originated from NOx adsorption are obviously activated on the Mn0.05Co0.95Ox catalyst surface. Moreover, the transient reaction studied by in situ DRIFTs found that the “fast SCR” reaction participated by gaseous NO2 and the standard SCR reaction participated

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by bidentate nitrates contribute to the low-temperature SCR activity.

1. INTRODUCTION With the development of industry and the growth in the number of motor vehicles, nitrogen oxides (NOx) emissions have been brought under public’s spotlight.1, 2 NOx is one of the serious pollutants that cause environmental problems such as acid rain, photochemical smog, ozone depletion and greenhouse effect which are harmful to all living species. To control NOx emission, lots of techniques were developed among which the selective catalytic reduction (SCR) of NO with NH3 or hydrocarbon is the most effective way.3-5 The NH3-SCR is the most promising method to reduce the harmful impacts of the NOx containing exhaust gas from stationery resource. Cobalt oxide catalysts with a spinel structure have been intensively studied due to their low price, thermodynamic stability, and oxygen evolution/reduction ability.6, 7 The cation distribution of the stoichiometric spinel Co3O4 is demonstrated to be Co2+[Co23+]O42-, whereas the Co3+ cations are octahedrally while Co2+ cations are tetrahedrally coordinated with oxygen ions. In some certain catalytic reactions, like CO oxidation, hydrocracking process of crude fuels, and photocatalytic reactions,8-10 Co3O4 shows good performance as well as in the SCR of NO.11-13 Meng et al. compared the Co3O4 nanorods with nanoparticles in NH3-SCR reaction, pointing out the morphology of catalyst may affect the adsorption behavior of the reactants in gas phase, and further leading to the difference in activity.12 Recently, many researchers redirected their study focused on catalyst with single component to bimetallic catalyst.14-18 It has been reported that the doping of the Co3O4 spinel structure produces substoichiometric oxides with excess oxygen which are balanced 2


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by the increase of Co3+ ions.19 Higher content of Co3+ involved in the whole redox circle can enhance the catalytic ability.20, 21 Among these dopants, manganese oxide (MnOx) had been proven to be an active component for low temperature SCR.22, 23 Many kinds of Mn-based catalysts including MnOx-CeO2, Mn-Fe spinel, and MnOx-NiO/TiO2 were developed to improve their SCR activity.14, 24, 25 The excellent redox properties of the mixed metal oxides play a positive role in completing a catalytic cycle. Thirupathi et al.25 prepared a series of Mn-M’/TiO2 (M’ = Cr, Fe, Co, Ni, Cu, Zn, Ce, Zr) catalysts and found that the activity of Ni doped Mn/TiO2 (Ni/Mn = 0.4) catalyst was greatly improved when compared with Mn/TiO2. Chang et al.26 found the addition of Sn into MnOx-CeO2 could lead to the improvement of activity and SO2 tolerance. Despite the extensive study of catalyst preparation and characterization, the reaction mechanism of NH3-SCR is also one important aspect.27-30 However, there are several different hypotheses and reaction pathways. By differentiating the NOx species adsorbed on the catalyst, we could find out possible reactive sites and deduce the reaction process and pathway. Zhang et al.31 observed the adsorption and reaction process in DRIFT spectra and concluded that the cis-N2O22- formed on CeO2 reacted more favorably with NH3 than other nitrate species. Shi et al.32 used the same methods finding out that NH3 could react with bridging nitrate species adsorbed on Mn/TiO2. In our previous work,17 MnxCo3-xO4 nanocages catalyst with good deNOx performance was synthesized. To investigate the synergetic effect between Mn and Co and hopefully to develop a supported catalyst which is fit for practical use, Mn0.05Co0.95Ox as a target catalyst was compared with Co3O4 using various characterization methods. Besides, in situ diffuse reflectance infrared Fourier 3



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transform spectroscopy (in situ DRIFTS) was conducted to evaluate the active sites of NH3-SCR and illuminate the mechanism of Mn-Co interaction.

2. EXPERIMENTAL SECTION Catalysts Preparation. The Mn-doped Co3O4 and Co3O4 nanoparticles were prepared by a co-precipitation method. Oxalic acid and precursor salts Mn(CH3COO)2·4H2O and Co(CH3COO)2·4H2O (AR grade) were purchased from Sinopharm Chemical Reagent Company and used without further purification. For Mn0.05Co0.95Ox nanoparticles which represents the molar ratio of Mn:Co is 1:19, 0.2451 g of Mn(CH3COO)2·4H2O and 4.7325 g of Co(CH3COO)2·4H2O were dissolved in 100 mL deionized water, and then a solution of oxalic acid (5.0428 g dissolved in 100 mL deionized water) was added dropwise as precipitant. The mixture was filtered after stirring for 1 h. The product was washed with deionized water, dried at 60 oC for 12 h and then calcined at 500 oC for 4 h in air. The Co3O4 nanoparticles were synthesized without adding manganese salt. Catalytic Activity Evaluation. NH3-SCR activity measurements were carried out in a fixed-bed quartz reactor (internal diameter 8 mm) using 0.3 g sample (40-60 mesh). The following reaction conditions were used: 500 ppm of NO, 500 ppm of NH3, 5 vol% O2, and N2 balance. The total flow rate was 210 mL/min, corresponding to a gas hourly space velocity (GHSV) of 60 000 h-1. The concentration of the feed gases and the effluent streams were analyzed continuously by a KM9106 flue gas analyzer. The NO conversion percentage was calculated using the following equation:

NO conversion % =

NOin -NOout -NO2 out ×100% NOin 4


(1)

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Where the subscripts in and out denote the inlet and outlet gas concentration of reactant, respectively. NO oxidation tests were performed in the same fixed-bed quartz reactor using 0.3 g sample under the following reaction conditions: 500 ppm of NO, 5 vol% O2, and N2 balance. The total flow rate was 210 mL/min. The concentration of the feed gases and the effluent streams were analyzed continuously by a KM9106 flue gas analyzer. The NO to NO2 conversion percentage was calculated using the following equation:

NO to NO2 conversion % =

NO2 out ×100% 2 NOin

Catalyst Characterization. The specific surface areas and pore volume measurements were carried out at 77 K on a Quantachrome instrument by nitrogen adsorption/desorption. The X-ray diffraction (XRD) measurements were carried out on a computerized Rigaku D/MAS-RB X-ray diffactometer employing Cu Kα radiation operated at 40 kV and 40 mA. XRD patterns were recorded in the 2θ range of 10 to 90o at a scan rate of 8o/min. The X-ray photoelectron spectroscopy (XPS) data were obtained on a RBD upgraded PHI-5000C ESCA system with Mg Kα radiation. The binding energy of Co and O were referenced to the C 1s line at 284.6 eV from contaminant carbon. The high-resolution transmission electron microscopy (HRTEM) images were taken by a JEOL JEM-2100F operating at 200 kV. The temperature-programmed reduction with hydrogen experiment (H2-TPR) was performed on a Tianjin XQ TP5080 autoadsortion apparatus. Prior to the reduction, the samples (80 mg) were first heated to 300 oC for 0.5h at a ramping rate of 10

o

C/min under constant N2 flow

rate of 30 mL/min, then the samples were cooled down to room temperature. During the reduction process, the samples were heated to 800 oC at a ramping rate of 10 oC/min under a 5



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flow of 5 vol% H2/N2 (30 ml·min-1). The consumption of hydrogen was monitored by a thermal conductivity detector (TCD). The temperature-programmed desorption with NH3 or NOx (NH3-TPD or NOx-TPD) experiments were performed on a same Tianjin XQ TP5080 autoadsorption apparatus. Prior to each experiment, the samples (150 mg) were first heated to 300 oC for 0.5 h at a ramping rate of 10 oC/min under constant He flow rate of 30 mL/min. For NH3-TPD, the adsorption process was carried out at 100 oC, the samples were exposed to 500 ppm of NH3 for 1 h, followed by He purging for 0.5 h to remove physisorbed NH3. For NOx-TPD, the adsorption process was carried out at 25 oC, the samples were exposed to 500 ppm of NO + 5% O2 for 1 h, followed by He purging for 0.5 h to remove physisorbed NOx. During the desorption process, the samples were heated to 800 oC at a ramping rate of 10 oC/min under a flow of He (30 ml·min-1). In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) experiments were carried out on a Nicolet 6700 spectrometer

equipped

with

a

Harrick

Scientific

DRIFT

cell

and

a

mercury-cadmium-telluride (MCT) detector cooled by liquid N2. The DRIFT spectra were collected in the range of 2000 to 800 cm-1, accumulating 64 scans at 4 cm-1 resolution in Kubelka-Munk format. Prior to each test, all samples were held at 300 oC under N2 flow (50 mL/h) for 0.5 h and cooled to desired temperature to get a background spectrum, and this spectrum was then subtracted from the sample spectra for each measurement.

3. RESULTS AND DISCUSSION Catalytic Performance. The catalytic performances over Co3O4 and Mn0.05Co0.95Ox are shown in Figure 1. For the NH3-SCR reaction, an important factor for evaluating catalyst 6


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performance is the light-off temperature (T50), whereas the T50 of Mn0.05Co0.95Ox is about 45 o

C lower than that of Co3O4. Meanwhile, the NO conversion of Mn0.05Co0.95Ox is much

higher than that of Co3O4 in the whole temperature range (90-210 oC) under the same operating conditions. Co3O4 shows its maximum conversion with 68% at 180 oC, while Mn0.05Co0.95Ox exhibits more than 80% conversion in the temperature range of 150-195 oC. It can be seen that only a small proportion of manganese oxide doped in cobalt oxide could greatly promote the NH3-SCR activity. The deactivation of both catalysts above 180 oC is generally believed to result from the unselective NH3 oxidation.33, 34 The properties of NO oxidation to NO2 are shown in Figure 2. During the reaction process, a larger amount of NO2 formed on Mn0.05Co0.95Ox than Co3O4, indicating that the addition of manganese oxide enhanced the NO oxidation ability. Therefore, it is reasonable to deduce that the Mn doping can facilitate the “fast SCR” (NO + NO2 + 2NH3→2N2 + 3H2O) reaction, and resulting in excellent low temperature activity.35, 36 Characteristics of the Catalysts. The XRD patterns of Co3O4 and Mn0.05Co0.95Ox catalysts are shown in Figure 3. The reflections of both samples provide typical diffraction patterns for the cobalt oxide (JCPDS NO. 42-1467) with (111), (220), (311), (400), (511), and (440) crystal faces,12 indicating that manganese oxide was well dispersed in cobalt oxide and did not change the crystalline phase of cobalt oxide. It is worth noting that the characteristic peaks of Co3O4 become weaker and broader after the doping of manganese oxide. On the other hand, elemental mapping results (see Supporting Information) confirmed the high dispersion of manganese oxides in the catalyst sample. As shown in Table 1, the substitution of Mn in the Co lattice decreases the lattice constant α, making the crystal unit 7



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more closely aligned. The considerable shrinkage of the unit cell volume is beneficial to smaller grain size and higher dispersion degree. The average crystallite sizes of the catalysts calculated by Scherrer equation are also listed in Table 1. These results conclude that the Mn dopant plays a role in suppressing the cobalt oxide grain growth and refining the grain. Table 1 also summarizes the catalyst structural parameters including specific surface area, BJH desorption pore volume, and pore diameter. The addition of manganese oxide to the Co3O4 apparently increased the surface area from 16.9 to 31.9 m2/g. The smaller particle of Mn0.05Co0.95Ox achieved higher specific surface area than that of Co3O4, which is agreed well with XRD analysis. The larger surface area can provide more adsorptive sites for gas reactants and benefit the mass transfer process,37 thus becoming a significant aspect for achieving high catalytic efficiency. The XPS technique was used to illuminate the chemical composition of Co and O elements on the catalyst surface and determine the surface element concentrations. The XPS spectra of Co 2p and O 1s are presented in Figure 4, and the corresponding surface atomic concentrations and relative concentration ratios are listed in Table 2. As shown in Figure 4a, two distinct peaks centered at 780.6 and 795.5 eV can be ascribed to Co 2p1/2 and Co 2p3/2 spin-orbit peaks, respectively.17, 20, 38 The spin orbit splitting is ∆E = 15.1 eV.20 The Co 2p3/2 spectra are deconvoluted into two peaks at 779.9 eV and 781.5 eV which ascribed to Co3+ in octahedral coordination and Co2+ in tetrahedral coordination, respectively. The relative atomic concentration of Co3+/Co on Co3O4 is 44.80%, lower than that on Mn0.05Co0.95Ox (47.02 %) suggesting that the addition of Mn changes the oxidation state of Co and made more Co3+ concentrated on the catalyst surface. Since Co3+ has a stronger oxidation ability 8


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and be considered as a promoter of the catalyst redox properties,7, 12, 17 higher content of Co3+ is one important requisite for achieving excellent catalytic activity. The O 1s XPS spectra are presented in Figure 4b. The O 1s peaks can be fitted into two peaks referred to surface chemisorbed oxygen O- or O2- (denoted as Oα) and lattice oxygen O2- (denoted as Oβ) at binding energies of 531.6-532.0 eV and 529.6-530.2 eV, respectively.39,

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As listed in Table 2, the relative atomic concentration of Oα/O on

Mn0.05Co0.95Ox is 70.30%, higher than that on Co3O4 (66.21%). It is generally accepted that Oα is more active than Oβ because of its high mobility and low energy bonding to the surface, so that could accelerate the oxidation process during the whole redox reaction.41-44 Moreover, due to the strong electron affinity of O-, α-oxygen can be considered as a convenient model of electrophilic oxygen.45 The abundant Oα could facilitate the oxidation of NO to NO2. This is the intrinsic reason why Mn0.05Co0.95Ox shows a better NO oxidation ability than Co3O4. The NO2 molecular could take part in the “fast SCR” reaction and proved to be a very important intermediate specie for low-temperature SCR reaction.46, 47 H2-TPR measurements were used to study the influence of manganese oxide doped in cobalt oxide on reduction properties. As shown in Figure 5, Co3O4 exhibits two reduction peaks at 358 and 458 oC attributed to Co3+→Co2+ and Co2+→Co0, respectively.7, 12 For Mn0.05Co0.95Ox, two reduction peaks were also observed. The peak at 338 oC could be Co3+→Co2+, and the peak at 505 oC is due to Co2+→Co0. These results show that the reduction of Co3+ to Co2+ for Mn0.05Co0.95Ox shifted to lower temperature while the reduction of Co2+ to Co0 shifted to higher temperature. The position of the reduction peaks directly reflects the reduction properties of the catalyst. It suggests that when doped into 9



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Co3O4, Mn not only improved the reducibility of Co3+, making it easier to reduce to Co2+, but also hindered the formation of metallic cobalt. Therefore, a complete redox circle is enhanced. Besides, the total amount of H2 consumption of Mn0.05Co0.95Ox (0.121 mmol) is higher than that of Co3O4 (0.112 mmol), indicating there are more surface oxygen species on the Mn0.05Co0.95Ox catalyst.48 Adsorption and Desorption Properties. The adsorption and desorption of reactant gas on the catalyst is an important aspect of the whole catalysis process. The NH3-TPD patterns of two catalysts are presented in Figure 6a. Co3O4 and Mn0.05Co0.95Ox both displayed two broad NH3 desorption peaks at 110-250 oC and 260-460 oC. The peak observed at low temperature is ascribed to weak adsorption of ammonia on the catalyst. The peak at high temperature is caused by NH3 strongly adsorbed on the catalyst.33, 49 The amount of NH3 adsorbed on Mn0.05Co0.95Ox obviously increased after the incorporation of Mn into the Co3O4 spinel structure. These results indicate that the addition of manganese oxide to cobalt oxide can increase the adsorptive capacity of NH3 molecular on the catalyst. The promoting effect of Mn on Mn0.05Co0.95Ox is due to the fact that the Mn additive could provide more active acid sites. The in situ DRIFT spectra of NH3 desorption on Co3O4 and Mn0.05Co0.95Ox catalysts at different temperature are shown in Figure 7. The bands at 1418, 1649 cm-1 in Figure 7a and those at 1418, 1649 cm-1 in Figure 7b were assigned to asymmetric and symmetric bending vibrations of NH4+ species on Brønsted acid sites. The bands at 1206, 1595 cm-1 in Figure 7a and those at 1206, 1595 cm-1 in Figure 7b were assigned to asymmetric and symmetric bending vibrations of NH3 coordinated on Lewis acid sites. The bands at 1542, 1558 cm-1 in 10


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Figure 7a and those at 1542, 1558 cm-1 in Figure 7b might be assigned to –NH2 species.50-52 When the temperature rises from 25 to 210 oC, NH4+ species on Brønsted acid sites diminished before 150 oC while NH3 coordinated on Lewis acid sites still exist on catalyst at 210 oC. These results indicate that NH3 coordinated on Lewis acid sites is more stable. Meanwhile, the incorporation of manganese oxide to cobalt oxide did not result in the considerable changes in the desorption strength of ammonia which is accord fully with the NH3-TPD results. The NO+O2-TPD evaluation was also carried out to illustrate the desorption status of adsorbed NOx species (Figure 6b). Several peaks were observed by performing peak-fitting deconvolutions. The peaks around 120, 210, 300, and 380 oC are ascribed to physisorbed NOx, monodentate nitrites, bridged nitrates, and bidentate nitrates, respectively.53, 54 And this is in increasing order of thermodynamic stability.50 The peak area of low temperature (

In Situ DRIFTs Investigation of the Low-Temperature Reaction

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