Effect of Potassium-Precursor Promoters on Catalytic Oxidation Activity

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Effect of potassium-precursor promoters on catalytic oxidation activity of Mn-CoOx catalysts for NO removal Xiaolong Tang, Fengyu Gao, Ying Xiang, Honghong Yi, Shunzheng Zhao, Xiao Liu, and Yuening Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02062 • Publication Date (Web): 31 Aug 2015 Downloaded from http://pubs.acs.org on August 31, 2015

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Industrial & Engineering Chemistry Research

Effect of potassium-precursor promoters on catalytic oxidation activity of Mn-CoOx catalysts for NO removal Xiaolong Tang, Fengyu Gao, Ying Xiang, Honghong Yi*, Shunzheng Zhao, Xiao Liu, Yuening Li

Department of Environmental Engineering, Civil and Environmental Engineering School, University of Science and Technology Beijing, Beijing, 100083, PR China. ∗

Corresponding author: HongHong Yi

Tel./fax:+86 010 62232747; E-mail address: [email protected]

ACS Paragon Plus Environment


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Abstract: Manganese cobalt oxide catalysts prepared by the low-temperature solid phase reaction were modified by potassium-precursor promoters, and used for the oxidation of nitric oxide at low temperature. The activity of modified catalysts had a significant improvement in the following: Mn-Co-KOH >Mn-Co-K2CO3 >Mn-Co-KNO3, which was in good agreement with the order of alkalinity. NH3/CO2-TPD results presented that potassium-precursor promoters could regulate the acid-basic and adsorption property of catalyst through the neutralization and intervention of K+/basic groups. The XRD, XPS and NO-TPD results showed that the potassium-precursor enhanced the dispersion, the ratio of Mn4+/Mn3+ and Co2+/Co3+ to some extent, especially for the chemisorbed oxygen and adsorption performance of NO. The main reasons of improvement may be attributed to the possessed enhancement of NO adsorption performance and the synergetic effect between Co and Mn ions. Finally, the possible reaction mechanisms are proposed.

Keywords: Mn-Co catalysts; potassium-precursor promoters; catalytic oxidation; nitric oxide; stimulative mechanisms

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1 Introduction Nitrogen oxides (NOx: NO and NO2) exhausted mainly from the stationary and mobile sources of coal and fossil fuel have been resulting in many environmental problems, such as the acid rain, the photochemical smog, the ozone depletion, and the particulate matter, etc, and are also harmful to human health.1-3 More than 90% of emitted NOx from stationary sources is in the form of nitric oxide (NO).4 Various post-combustion techniques and efforts have been developed for eliminating NO, such as selective catalytic reduction (SCR),5 selective non-catalytic reduction (SNCR),6 direct decomposition,7 and non-thermal plasma (NTP).8,9 As an ideal, potential and alternative approach, selective catalytic oxidation (SCO) of NO to NO2 is a key step for many De-NOx techniques, and many efforts are focused on developing catalysts for the oxidation of NO.10-12 With the appropriate ratio of NO/NOx = 50%-60% by efficient SCO-catalysts, the great NOx removal efficiency could be achieved with further absorbed by the adsorbent or the absorption liquid.13 Till now, many catalysts have been investigated to oxidize NO to NO2,14-18 among them, transition metals such as manganese and cobalt based catalysts showed their potential for NO catalytic oxidation.19-22 However, the catalytic oxidation activity of those catalysts at low temperature needs to be further improved activity. Alkaline metals have been investigated as promoters to modified and improve the catalytic activity of SCO-catalysts. Studies of Chen et al.23 on coal catalytic gasification with CO2 and H2O showed that the catalyst activity of alkaline metals follow the order K > Na > Li. Pecchi et al.24 studied alkaline niobates as catalysts for soot oxidation, and found the highest catalytic activity was obtained by KNbO3, which suggested that K+ ions are the active sites for soot combustion. Alkaline metals, especially potassium metals have been widely studied for catalysts modification due to the better activity of alkali-promoted catalysts.25-27 But it is seldom used for the catalytic oxidation of NO, let alone the potassium-precursor promoters. In this work, the lack of studies in the current literature about the effect of the potassium precursor as catalytic additives for catalysts modification of NO oxidation was investigated. The effect of different potassium precursors (potassium nitrate, 3



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potassium hydroxide, and potassium carbonate) on the catalytic activity of Mn-Co catalysts for NO catalytic oxidation are studied and discussed, taking into account the characterization results by X-ray diffraction (XRD), X-ray photoelectron spectroscope (XPS), the temperature programmed desorption of NH3 (NH3-TPD), CO2 (CO2-TPD) and NO + O2 (NOx -TPD). 2 Experimental 2.1 Catalyst preparation As a fundamental research, one-component metal oxide such as MnOx, CrOx, CoOx, NiOx, CuOx and FeOx was prepared by a precipitation method and studied for NO oxidation, respectively. A certain amount of manganese acetate, nitrate salts of Cr, Co, Ni, Cu and Fe was dissolved in water, then ammonia solution was added to each above solution with vigorous stirring until pH is ca. 8, respectively. The mixtures were filtered and washed 3~4 times with deionized water. The powder was dried in the air at 120 oC for 12 h and then calcined in air at 400oC for 2 h. Finally, the samples were crushed and sieved to 40-60 mesh for testing. Mn-Co catalysts were prepared by the low temperature solid phase reaction method (SP). Firstly, the manganese acetate, potassium permanganate and cobalt nitrate were mixed and ground fully for 30 minutes. The manganese acetate/ potassium permanganate mole ratio was 2:3, and Mn/Co ratio was 9:1. The above ratio was based on the best results of the earlier experiment selection. The mixture was placed in the baking oven at 70 oC for 48 h. The product was washed 3~4 times with deionized water and 2~3 times with absolute ethanol, then filtered and dried at 100 oC. Finally, the catalyst sample was crushed and sieved to a 40~60 mesh for testing. Mn-Co-X (X=KOH, K2CO3 or KNO3) catalyst was prepared by impregnation method. Mn-Co catalyst particles and X (X = KOH, K2CO3 or KNO3) were ultrasonic impregnated for 30min, and then dried at 100 oC. The mass fraction of KOH, K2CO3 and KNO3 were 5%. 2.2 Catalytic activity measurements A steady-state reaction experiments were performed in a Pyrex tube fixed-bed 4


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flow reactor of 10 mm internal diameter, and the temperature was controlled by a K-type thermocouple in direct contact with the reactor. Sample (0.18 g) was loaded into the reactor. The feed gas compositions were as follows: 500ppm NO, 5% O2 and the balance N2. The total flow rate was fixed at 200 ml/min, and corresponded to a typical space velocity of 30000 h–1. On-line analysis of the feed and effluent gas was performed using Flue Gas Analyzer (KM9106, Kane International LTD). Since a balance of total NOx ( the sum of NO and NO2) between the inlet and the outlet gases, NO conversion was obtained as follows: NO conversion (%)= ([NO]in-[NO]out)/ [NO]in×100% where the [NO]in and [NO]out indicate the inlet and outlet concentration at steady state, respectively. 2.3 Catalyst characterization X-ray diffraction (D/MAX-2200) patterns were recorded with a Rigaku diffractometer operated at 36 kV and 30 mA by using Ni-filtered Cu Kα radiation (λ=0.15406 nm) at a rate of 5º/min from 2θ=5º~90º. X-ray photoelectron spectroscope (XPS) (PHI 5500) analysis used Al Kα radiation with energy of Al target and power 200W. The photoelectron spectra were calibrated using the C 1s signal detected at a binding energy of 284.8 eV from adventitious carbon. Atomic compositions were calculated with the corrected Scofield coefficients of the transmission function of the analyzer and/or with experimental coefficients determined for a reference compound. The continuum spectrum was fitted according to the Gaussian–Lorentzian files. The acid/base properties of catalysts was obtained by the temperature programmed desorption of NH3 and CO2, respectively. Typical sample of 0.1 g and gas flow rate of 100 mL/min were used for the experiments, and the response signal was recorded by the MAX300 mass spectrometer (Extrel, USA). The experiment consists of four stages: (1) Pretreatment: degasification of the sample in N2 at 250 oC for 1 h; (2) Adsorption: adsorption of 1%NH3 ( or pure CO2) at room temperature for 1 h ( or 0.5h); (3) Purge: desorption in N2 with 100 mL/min at room temperature until the signal was steady; (4) TPD stage: temperature-programmed desorption in N2 With a linear heating rate of 10 oC/min from 30 oC to 500 oC ( or 600 oC). 5



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Temperature-programmed desorption of NO + O2 (NOx -TPD) was carried out in a fixed-bed quartz reactor, and the response signal (the NO concentration) was obtained by KM9106. 0.18 g typical catalyst and gas flow rate of 200 mL/min (500ppm NO, 5% O2 and the balance N2) were used for the experiments, respectively. Firstly, the sample was blowed in N2 at 250oC for 1 h; Then, adsorbed NO (500 ppm) at 50oC for 1 h; Third, isothermal desorption in N2 at 50oC till there was no signal of NO concentration; Finally, temperature-programmed desorption in N2 (TPD step) at 10 oC/min up to 500oC was carried out. 3. Results and discussion 3.1 Catalytic activity of one- and two-component metal oxide catalysts Fig. 1 shows the MnOx catalyst has the highest catalytic oxidation activity of NO than other one-component metal oxides, which presented better NO conversion at any temperature point and reached 50.7% at 250oC. The catalytic activities of various samples are ordered as follows: MnOx > CrOx > CoOx > CuOx > FeOx > NiOx. By contrast, the catalytic activity of manganese-cobalt oxide and manganese-chromium oxide prepared by a co-precipitation method were studied and the results presented manganese-cobalt oxide obtained higher NO conversion than manganese-chromium oxide in Fig. 2(a). In addition, as shown Fig. 2(b), manganese-cobalt oxide synthesized by the low temperature solid phase reaction method (SP) presented better catalytic activity for NO oxidation than which prepared by the co-precipitation method (CP). 3.2 Catalytic performances of potassium-modified catalysts Based on the results above, the manganese-cobalt oxide catalyst synthesized by the low temperature solid phase reaction method (SP) was used in the following studies. The tests of catalytic activity for NO oxidation were performed over Mn-Co-KOH, Mn-Co-K2CO3 and Mn-Co-KNO3 catalysts individually, and the results are shown in Fig.3. In comparison with the un-modified Mn-Co catalyst, the catalytic oxidation activities could be improved slightly by loading K2CO3 and KNO3. Obviously, loading KOH could significantly improve the NO catalytic oxidation 6


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activity of Mn-Co catalyst with about 63.8%, 40.3%, 31.2%, 28.6% and 24.7% increase at each temperature point, and the highest NO conversion was obtained nearly 80% at 250 °C. The activity of modified catalysts have a significant improvement in the following: Mn-Co-KOH > Mn-Co-K2CO3 > Mn-Co-KNO3 > Mn-Co catalyst, which was in good agreement with the order of alkalinity. Fig. 4 presents the catalytic activities of 5%, 10%, 20% KOH-modified Mn-Co catalysts for NO oxidation. The 10%KOH-modified manganese-cobalt oxide catalyst showed the best NO catalytic oxidation activity, especially from 100 to 200 °C, which was about 82.9%, 88.8% and 89.1%, respectively. It clearly demonstrates that potassium precursor could significantly promote the NO catalytic activity over Mn-Co catalysts, and KOH-promoted catalyst obtained the best NO conversion. With the appropriate load of KOH over manganese-cobalt oxide catalyst, the great NOx removal efficiency could be achieved. 3.3. Physio-chemical Characterization of potassium-modified catalysts 3.3.1 XRD analysis The XRD patterns of samples are shown in Fig. 5. All of the catalysts have extremely poor crystallinity. In addition, most of the diffraction lines do not fully comply with the PDF standard cards, and therefore the constitution of compounds could not be determined accurately, which was similar to our previous studies.28-29 According to the literature,30-32 the peaks at 2θ=28.7°, 37.5°, 28.7°, 42.4° and 66.8° are matched to the values of MnO2, and the peak at 2θ=23.7°, 26.0° and 38.5° is attributed to Co2O3, Mn2O3 and Co3O4, respectively. As shown in Fig. 5, the diffraction peaks of the K-modified catalyst are weaker than which of normal catalysts. Therefore, adding K could decrease the crystallinity of catalysts and make the particles more dispersed. Moreover, it can be seen that Mn-Co-KOH catalyst has broader and weaker diffraction peaks than those of other catalysts. It is inferred that the metal oxides particles are higher dispersed in KOH modified Mn-Co catalyst. 3.3.2 XPS analysis In order to investigate the surface chemical states and the atomic compositions of catalysts, the XPS spectra of O1s, Mn2p, and Co2p in all samples were obtained, as 7



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shown in Fig. 6. The elemental compositions (at.%) and the relative percent of differential valence state (%) of Mn and Co ions were listed in Table 1 and 2, respectively. Table 1 shows that the surface concentration of K and Co element were increased, while Mn concentration was decreased, which was mainly due to the impregnation and load of potassium precursors. Besides, it was found that the atom concentration of oxygen element showed a significant increase after modification, which was contributed to the form of nitrate, carbonate and hydroxyl, respectively. From Fig. 6 (A), the O1s peaks could be split to two peaks: the chemisorbed oxygen at 531.3-532.2eV (denoted as Oα, such as O22-,O- ,OH- ,CO32-, etc) and the lattice oxygen at 529.2-530.0eV (denoted as Oβ, such as O2-).33 The content order of Oα/(Oα+Oβ) ratio was shown as following: Mn-Co-KNO3 (51.5%) > Mn-Co-K2CO3 (44.0%) > Mn-Co-KOH (41.9%) > Mn-Co (34.1%), and the trend was in good agreement with the oxygen (mole) content of the loaded potassium precursors (KNO3 > K2CO3 > KOH), while was not very consistent to the catalytic activity for modified catalysts (shown in Fig. 3). However, the atom concentration of oxygen element showed a significant increase after modification (see Table 1). This observation clearly implies that high concentration of surface chemisorbed oxygen (as the most active oxygen) is previously found to be preferable for catalysis, and play an important role in increasing the activity of catalysts in oxidation reactions.3,34 As shown in Fig. 6 (B), two main peaks due to Mn 2p3/2 (peak at 641.7 eV) and Mn 2p1/2 (peak at 653.5 eV) were observed. By performing a peak-fitting de-convolution, the Mn 2p3/2 spectra can be divided into three peaks: the peak position at 640.8-640.9 eV was attributed to Mn3+, at 642.1-642.8eV was assigned to Mn4+,1,35-37 and at 646 eV was accepted as satellite to obtain the best fit. As presented in Fig. 6 (C), the prominent peak of Co 2p3/2 level was de-convoluted into two peaks centered at 780.0-780.6 eV and 783.5 eV attributed to Co3+ 2p3/2 and Co2+ 2p3/2 configuration,35,38,39 and the other spin-orbit component, the Co 2p1/2, appeared at 795.2-795.6 eV and 797.5-797.8 eV corresponding to Co3+ 2p1/2 and Co2+ 2p1/2 configuration,38 respectively. The small peak at 788.5 eV was subjected to Co2+ shake-up satellite peak of Co3O4.38 8


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After modification, the ratio of Mn4+/Mn3+ showed a slight increase with the increase of both Mn4+ and Mn3+ concentration (listed in Table 2), and was calculated as following: Mn-Co-KOH (0.581) > Mn-Co-K2CO3 (0.576) > Mn-Co-KNO3 (0.570) > Mn-Co (0.562). Besides, the ratio of Co2+/Co3+ showed a slight increase with the increase of Co2+ at the expense of Co3+, and was shown as following: Mn-Co-KNO3 (0.555) > Mn-Co-K2CO3 (0.522) = Mn-Co-KOH (0.522) > Mn-Co (0.497). It has been reported that Mn4+ plays a key role in the catalytic redox process and Co2+ is the main active bit in Co3O4.1,39 Although it can be proposed that both high Mn4+/Mn3+ and Co2+/Co3+ are preferable for the oxidation reaction, the slight increases are not enough in the noticeable improvement of catalytic oxidation activity. 3.3.3 NH3-TPD and CO2-TPD analysis As we known that the potassium nitrate, potassium carbonate, and potassium hydroxide is the neutral, slightly alkaline and strong alkaline salt, respectively. To better understand the acid-basic properties of modified catalysts, the temperature programmed desorption of NH3 and CO2 experiments was performed, respectively. It is clear that the adsorbate adsorbed on weaker sites is desorbed at lower temperature and that adsorbed on stronger sites is desorbed at higher temperature. Therefore, the strengths of the acid or basic sites can be expressed in the temperature range wherein the NH3 or CO2 chemisorbed on the acid or basic sites is desorbed.40 The NH3-TPD was carried out to evaluate the acid site distributed of the catalysts, presented in Fig. 7, which was obtained in the temperature range of 30–500 °C. It has been reported that the peaks below 100 °C is caused by the desorption of physisorbed NH3,41 peaks at 100–200 °C and 200–350 °C were attributable to the desorption of NH3 by the weak Bronsted-acidic sites and chemisorbed to the strong Lewis-acidic sites.42 In the results of the NH3-TPD analysis, the intensity of desorption peak was gradually weaken and the peak position was shifted to the lower temperatures with the load of KNO3, K2CO3 and KOH, respectively. The decrease of acid amounts may be attributed to the neutralization of alkaline salts, which was in good agreement with the alkalinity order of potassium nitrate, potassium carbonate, and potassium hydroxide. Fig. 8 presented the CO2-TPD profiles of the fresh and modified catalysts in the 9



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temperature range of 30–600 oC. Peaks below 100 oC is caused by the desorption of physisorbed CO2, and the numbers of weak, medium, strong and very strong catalyst basic sites can be estimated from the area under their CO2-TPD curves for the temperature range of 100–250, 250–400, 400–650, and >650 oC.43 It can be seen that the total basic sites were increased in different degree with the load of potassium carbonate, and potassium hydroxide, while changed weakly with loading potassium nitrate. The results are understandable and prospective, taking into account alkalinity of promoters. For Mn-Co-KNO3 sample, the decrease of weak basic sites and the increase of medium basic sites was due to the surface covering and the adjustment of electronic promoter (K+) into the lattice of manganese-cobalt oxide. While for Mn-Co-K2CO3 and Mn-Co-KNO3 catalysts, the main reason for the significant increase of basic sites is not only due to the adjustment of electronic promoter (K+), but also ascribed to the introduction of basic groups, such as HCO3- and OH-, etc. 3.3.4 NOx-TPD analysis Fig. 9 presents the NOx-TPD curves of the samples given a temperature range of 30 to 500 °C. As observed from the figure, the amount of desorbed NOx increased at different degrees in the modified catalysts. There was a primary peak at 270 °C for Mn-Co catalyst, while moved to the left at 235 °C with a visible peak at 300 °C for Mn-Co-KNO3. For Mn-Co-K2CO3 sample, a main peak at 235 °C accompanied two peaks at 145 °C and 440 °C was obtained, which can be belonged to the physical, weak and strong chemical adsorption of nitric oxide, respectively. Besides, the NOx-TPD curve of Mn-Co-KOH sample showed two dominating peaks at 170-200 °C and 350 °C, respectively. By the integral operation, the amounts of NO desorbed by the samples are listed in the following sequence: Mn-Co-KOH(0.876 mmol/g) > Mn-Co-K2CO3 (0.555 mmol/g) > Mn-Co-KNO3 (0.531 mmol/g) > Mn-Co (0.382 mmol/g). This sequence was in good agreement with the trend of the catalytic activity shown in Fig. 3, which can be considered as one of the main reasons of the significant improvement of catalytic activity of potassium-precursor modified Mn-Co catalysts. 10


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3.4 Possible reaction mechanism It is well known that two reaction mechanisms had become consensus for the catalytic oxidation of NO.44,45 The first mechanism: (1) NO + O2 → NO3, (2) NO3 + NO → 2NO2; The second mechanism: (3) 2NO → (NO)2; (4) (NO)2 + O2 → 2NO2. The two mechanism maintain a competitive relationship for NO oxidation. At low reaction temperature, the reaction equilibrium constant of equation (3) decreases with the reaction temperature increasing due to the exothermic reaction, which leads to the decrease

of

total

equilibrium constant

of

equation

(3)

and

(4),

and

reaches the minimum at 327oC (600K). The last one has a obvious advantage at low temperature (below 327oC), while the first one will win with the temperature increasing.44,45 As P-type semiconductors, the outstanding catalytic oxidation activity MnO2 and Co3O4 catalysts is due to the strong adsorption capacity for O2, and the main form of the active oxygen is O－, which comes from the dissociation of adsorbed oxygen.45 Ruggeri et al.46 reported a mechanistic DRIFTS in-situ study of NO2, NO + O2 and NO adsorption on a commercial Cu-CHA catalyst, and found that nitrosonium cations (NO+) were identified as key surface intermediates in the process of NO oxidation to NO2. Hadjiivanov et al.47 and Szanyi et al.48 also demonstrated the NO+ (or related nitrites) as intermediates in NO oxidation. Chen et al.49 reported that the adsorbed NO can become NO+ads by capturing an electron from the cycle electronic transfer system and further react with O－ to NO2, this is very consistent with the second mechanism for NO oxidation with the differentiated adsorption states of NO (2NO → (NO)2; (NO)2 + O2 → 2NO2) at low temperature. From XPS results, the modification by potassium precursors resulted in the exposure of more chemisorbed oxygen, and slight increase of Mn4+/Mn3+ and Co2+/Co3+ on the catalyst surface, which are correlated with the catalytic oxidation performance of catalysts. It can be concluded that both Mn4+ and Co2+ must play an important role in the oxidation process. Thus, a possible redox reaction over Mn-Co catalyst and the synergetic effect between Mn and Co cations are proposed which has been presented in our previous study.50 A dynamic equilibrium is maintained through the electron transfer between Mn and Co ions in the catalytic oxidation process. Co3+ 11



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captures an electron from Mn3+ and they become Co2+ and Mn4+, which are responsible for the high catalytic oxidation activity. Such a process can be expressed as Co3+ + Mn3+ ↔ Co2+ + Mn4+, which is similar as Cr5+ + 2Mn3+ ↔ Cr3+ + 2Mn4+,49 Cu2+ + Mn3+ ↔ Cu+ + Mn4+.51,52 The NH3-TPD and CO2-TPD results present that potassium-precursor promoters could regulate both the acid-basic property and adsorption sites of catalyst through the neutralization and intervention of K+ and basic groups. It has reported that, as the electronic promoter, small amounts of K+ could invoke the changes of geometry and electronic structure, leading in the variations of adsorption capacity and activation energy for the target gas.24,53 As the NOx-TPD results shown, the adsorption performance of catalysts for NO was greatly improved by the effect of potassium-precursor promoters (KNO3, K2CO3 and KOH). Taking into account the proposed redox reaction and the synergetic effect between Mn and Co cations above, possible reaction mechanisms of the catalytic oxidation are proposed for the removal of nitric oxide over potassium-precursor modified Mn-Co catalysts at low temperature, shown in Fig. 10 ,which are summarized as follows: (1) The primary reaction: Co3+ captures an electron from Mn3+ and they become Co2+ and Mn4+, expressed as Co3+ + Mn3+ ↔ Co2+ + Mn4+. Mn4+ and Oads captures an electron from the adsorbed NO and Co2+, then translates to absorbed NO+ads and O-ads, respectively. Thus, NO2 is generated from the reaction of NO+ads and O-ads. (2) The secondary reaction: the adsorbed NO by Mn-CoOx and potassium -precursor promoters react with active oxygen ([O], generated from chemisorbed oxygen activation on the oxygen vacancies) to NO2 gas. (3) The stimulative reaction: the plentiful NO adsorbed by potassium-precursor promoters are translate to the Mn-CoOx system, as a result, the synergetic effect between Mn and Co cations for NO oxidation are stimulated significantly, as the primary reaction (1) shown above. Besides, the secondary reaction (2) of NOads and [O] is also promoted due to the increase of surface chemisorbed oxygen.

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4. Conclusions In this study, manganese cobalt oxide catalysts prepared by the low-temperature solid phase reaction were modified by potassium-precursor promoters (KNO3, K2CO3 and KOH) to improve the catalytic oxidation activity of nitric oxide at low temperature. The following conclusions were obtained: (1) The catalytic oxidation activity are improved significantly in the following: Mn-Co-KOH >Mn-Co-K2CO3 >Mn-Co-KNO3, which was in good agreement with the order of alkalinity. (2) Potassium-precursor promoters could regulate the acid-basic and adsorption property (adsorption sites and amounts) of catalyst through the neutralization and intervention of K+ and basic groups, which are benefit for the adsorption of NO. (3) The Mn-Co catalysts modified by potassium-precursors enhanced the dispersion, the ratio of Mn4+/Mn3+ and Co2+/Co3+ to some extent, especially for the chemisorbed oxygen and the adsorption performance of NO. (4) The possible reaction mechanisms are proposed for the low temperature oxidation of NO over the potassium-precursor modified Mn-Co catalysts, which are mainly attributed to the possessed enhancement of NO adsorption performance and the efficient synergetic catalytic effect between Co and Mn ions

Acknowledgments This work was supported by National Natural Science Foundation of China (20907018, 21177051), the Doctoral and New Teachers Foundation of Education Department (NECT-13-0667), the Special Project on Air Pollution Control of Beijing Municipal Science & Technology Commission (Z141100001014006), and the Fundamental Research Funds for the Central Universities (FRF-TP-14-007C1).

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Li(Ni1/3Co1/3Mn1/3)O2 as cathode for Li-ion batteries. Electrochim. Acta 2002, 48, 145. (53) Jiménez, R.; García, X.; López, T.; Gordon, A. L. Catalytic combustion of soot. Effects of added alkali metals on CaO–MgO physical mixtures. Fuel Process. Technol. 2008, 89, 1160.

19



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Figure captions Fig. 1 Catalytic activity of various one-component metal oxide catalysts Fig. 2 Catalytic activity of different two-component metal oxide Fig. 3 Influence of potassium precursor on the Mn-Co catalyst for NO oxidation Fig. 4 Influence of the KOH content on the Mn-Co catalyst for NO oxidation Fig. 5 XRD patterns of catalysts Fig.6 XPS spectra for O1s (A), Mn2p (B) and Co2p (C) of the catalysts Fig.7 NH3-TPD profiles of the catalysts Fig.8 CO2-TPD profiles of the catalysts Fig.9 NOx-TPD profiles of the catalysts Fig.10 The proposed mechanism of the catalytic oxidation reaction over the potassium-precursor modified Mn-Co catalysts

20


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60

NO Conversion (%)

Page 21 of 34

MnOx CoOx CrOx NiOx CuOx FeOx

40

20

0 50

100

150

200

250

Temperature (℃)

Figure 1 Catalytic activity of various one-component metal oxide catalysts. Reaction conditions: [NO]=500ppm, [O2]=5 vol%, N2 balance with the total flow rate of .2L/min, and corresponded to a typical space velocity of 30000 h–1.

21



80

80 (a)

Mn-Co-Ox (CP) Mn-Cr-Ox (CP)

(b)

Mn-Co-Ox (CP) Mn-Co-Ox (SP)

60

NO Conversion (%)

60

NO Conversion (%)

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Page 22 of 34

40

20

40

20

0

0 50

100

150

200

50

250

100

150

200

250

Temperature (℃)

Temperature (℃)

Figure 2 Catalytic activity of different two-component metal oxide Reaction conditions: [NO]=500ppm, [O2]=5 vol%, N2 balance with the total flow rate of .2L/min, and corresponded to a typical space velocity of 30000 h–1.

22


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100 Mn-Co Mn-Co-5% K2CO3

80

NO Conversion (%)

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Mn-Co-5% KNO3 Mn-Co-5% KOH

60 40

20

0 50

100

150

200

250

Temperature (℃)

Figure 3 Influence of potassium precursor on the Mn-Co catalyst for NO oxidation Reaction conditions: [NO]=500ppm, [O2]=5 vol%, N2 balance with the total flow rate of .2L/min, and corresponded to a typical space velocity of 30000 h–1.

23



100

80

NO Conversion (%)

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

60 40 Mn-Co Mn-Co-5% KOH Mn-Co-10% KOH Mn-Co-20% KOH

20

0 50

100

150

200

250

Temperature (℃)

Figure 4 Influence of the KOH content on the Mn-Co catalyst for NO oxidation Reaction conditions: [NO]=500ppm, [O2]=5 vol%, N2 balance with the total flow rate of .2L/min, and corresponded to a typical space velocity of 30000 h–1.

24


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Figure 5 XRD patterns of catalysts

25



A

O 1s

Oβ

Oα/(Oα+Oβ)

Oα Mn-Co

Intensity (a.u.)

34.1% Mn-Co-KNO3 51.5% Mn-Co-K2CO3 44.0% Mn-Co-KOH

536

41.9%

534

532

530

528

526

Binding Energy (eV)

B

4+

Mn 2p

3+

Mn /Mn

4+

3+

Mn 0.562

Mn-Co-KNO3

0.570

Mn-Co-K2CO3

0.576

Mn-Co-KOH

0.581

660

656

Mn

Sat.

Mn-Co

Intensity (a.u.)

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

Page 26 of 34

652

648

644

640

Binding Energy (eV)

26


636

Page 27 of 34

C

Co 2p

2+

2+

Co

Intensity (a.u.)

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


3+

3+

Co /Co

Co

0.497

Mn-Co-KNO3

0.522

Mn-Co-K2CO3

0.522

Mn-Co-KOH

0.556

800

796

Co

Co

Mn-Co

804

3+

2+

792

788

784

780

776

Binding Energy (eV)

Figure 6 XPS spectra for O1s (A), Mn2p (B) and Co2p (C) of the catalysts

27



Mn-Co

Intensity (a.u.)

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

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Mn-Co-5% KNO3

Mn-Co-5% K2CO3 Mn-Co-5% KOH

0

100

200

300

400

o

Temperature ( C)

Figure 7 NH3-TPD profiles of the catalysts

28


500

Page 29 of 34

Mn-Co

Intensity (a.u.)

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


Mn-Co-5% KNO3

Mn-Co-5% K2CO3 Mn-Co-5% KOH

0

100

200

300

400

500

o

Temperature ( C)

Figure 8 CO2-TPD profiles of the catalysts

29


600


Mn-Co Mn-Co-5% KNO3 170 ℃

Intensity (a.u.)

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

Page 30 of 34

Mn-Co-5% K2CO3

200 ℃

Mn-Co-5% KOH 235 ℃ 350 ℃

145 ℃

300 ℃ 440 ℃ 235 ℃ 270 ℃

0

100

200

300

400

Temperature (℃)

Figure 9 NOx-TPD profiles of the catalysts

30


500

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Figure 10 The proposed mechanism of the SCO reaction over the potassium-precursor modified Mn-Co catalysts

31



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Table captions Table 1 Atom concentrations on the surface of catalysts determined by XPS Table 2 Binding energies the percent of differential valence state determined by XPS

32


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Table 1 Atom concentrations on the surface of catalysts determined by XPS Catalyst

Metal content (at.%) C

K

O

Mn

Co

Mn-Co

29.3

3.9

36.2

29.0

1.59

Mn-Co-KNO3

26.6

3.6

46.6

20.0

1.62

Mn-Co-K2CO3

30.6

4.2

44.3

19.3

1.69

Mn-Co-KOH

27.2

6.1

45.9

19.1

1.71

33



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Page 34 of 34

Table 2. Binding energies the percent of differential valence state determined by XPS Element valence XPS spectra of

Mn 2p2/3

Co 2p

catalysts

Mn3+

Mn4+

Mn4+/Mn3+

Co2+

Co3+

Co2+/Co3+

Mn-Co

640.9eV

642.8eV

0.562

783.5eV, 797.5eV

780.6eV, 795.6eV

0.497

(51.4%)

(28.9%)

( 33.2%)

(66.8%)

641.0eV

642.8eV

783.5eV, 797.5eV

780.6eV, 795.6eV

(53.8%)

(30.7%)

( 34.3%)

(65.7%)

641.0eV

642.7eV

783.6eV, 797.6eV

780.5eV, 795.5eV

(56.1%)

(32.3%)

( 34.3%)

(65.7%)

641.1eV

642.7eV

783.5eV, 797.5eV

780.6eV, 795.5eV

(54.7%)

(31.7%)

( 35.7%)

(64.3%)

Mn-Co-KNO3

Mn-Co-K2CO3

Mn-Co-KOH

0.570

0.576

0.581

34


0.522

0.522

0.555

Effect of Potassium-Precursor Promoters on Catalytic Oxidation Activity

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