Direct Decomposition of NOx over TiO2 Supported Transition Metal

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Kinetics, Catalysis, and Reaction Engineering

Direct Decomposition of NOx over TiO2 Supported Transition Metal Oxides at Low Temperatures Devaiah Damma, Thirupathi Boningari, Padmanabha R. Ettireddy, Benjaram M. Reddy, and Panagiotis G. Smirniotis Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03532 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 18, 2018

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Industrial & Engineering Chemistry Research

Direct Decomposition of NOx over TiO2 Supported Transition Metal Oxides at Low Temperatures Devaiah Damma†, Thirupathi Boningari†, Padmanabha R. Ettireddy†,§, Benjaram M. Reddy‡, and Panagiotis G. Smirniotis† †

Chemical Engineering, College of Engineering and Applied Science, University of Cincinnati,

Cincinnati, OH 45221-0012, USA §

Cummins Inc, Columbus, IN 47201, USA

‡

Inorganic and Physical Chemistry Department, CSIR-Indian Institute of Chemical Technology

(IICT), Uppal Road, Hyderabad  500007, Telangana, India

Submitted to Industrial Engineering Chemistry Research

--------------------------------------------------* Corresponding author Tel.: (513) 556-1474. Fax: (513) 556-3473 E-mail: [email protected] (Panagiotis G. Smirniotis)

1 ACS Paragon Plus Environment

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ABSTRACT

TiO2 supported transition metal oxides were investigated for the direct decomposition of nitrogen oxides (NOx) at lower temperatures. The results showed that the catalytic performance strongly depends on the kind of transition metal oxide deposited on the TiO2. Among the various catalysts examined, Mn/TiO2 and Co/TiO2 exhibited relatively high NOx conversion at lower temperatures in the presence of 3 vol.% O2. The oxygen in the reaction stream had a positive impact on the NOx decomposition over the Mn/TiO2 catalyst. We have not observed any TiO2 phase conversion from anatase to rutile in the Mn/TiO2 during the NOx decomposition reaction at different temperatures (100‒350 oC). NOx decomposition activity was shown to be governed by the surface labile oxygen rather than the gas phase oxygen. The Mn/TiO2 catalyst exhibited a good resistance to 10 vol.% H2O and 100 ppm SO2.


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1.

INTRODUCTION

The abatement of nitrogen oxides (NOx) from the exhaust streams remains a significantly challenging task. Several methods for NOx removal using reducing agents have been investigated extensively and some of them have already been industrially used at medium and high temperatures.1,2 Particularly, low-temperature selective catalytic reduction of NOx with ammonia (NH3-SCR) is widely recognized as an effective method for controlling NOx emission from stationary and mobile sources.3‒5 However, the usage of ammonia in the SCR increases the process cost, since NH3 is an expensive reductant.6 The other major disadvantages associated with NH3-SCR technology is the catalysts deactivation by the formation of ammonium sulfates with the reaction of SOx and NH3.7 Excessive ammonia slip is also one of the major problems with the NH3-SCR technology used in power plants. The direct decomposition of NOx eliminates the retrofit (purchase, transportation, and storage) costs and operating problems associated with the use of an external reducing agent. Hence, there is a great interest in developing materials for the direct removal of NOx without using any reducing agent at low-temperatures. The decomposition of nitric oxide into nitrogen (2NO  N2 + O2) has been known and studied for years by employing noble metal and metal oxide catalysts at high temperatures.8‒13 However, this decomposition also leads to the NO2 formation via the rapid reaction NO + ½ O2  NO2 in the presence of oxygen. The direct decomposition of nitrogen oxides to form nitrogen and oxygen would be the most attractive solution to control the NOx emissions. Hypothetically, simple direct decomposition of NOx is thermodynamically favored at lower temperatures.14 Nevertheless, the NOx decomposition reaction is very slow owing to the high dissociation energy of NO (153.3 kcal/mol).



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The active research aims at developing a catalyst formulation that will avoid the need for using a reductant and simultaneously achieve the required NOx reduction. Although numerous catalysts were investigated to date, no material was found to be effective for the direct decomposition of NOx, which is able to overcome the kinetic barriers at lower temperatures. The discovery of Cu-exchanged ZSM-5 catalyst for the direct decomposition of NO at high temperatures by Iwamoto and co-workers was a considerable step forward in this research field.15,16 Iwamoto et al. observed that the NO directly decomposes to N2 and O2 over Cuexchanged zeolites at 750‒850 K. Afterwards, various other catalysts have been examined for this reaction such as, La2O3,17 Ba/MgO,18 Ba/Ba–Y–O,19 metal doped Co3O4,20 BaO/Y2O3, and BaO/Sc2O3.21 Doi et al. investigated the Ba-doped rare earth oxide (La2O3, CeO2, Pr6O11, Nd2O3, Sm2O3, Gd2O3, Tb4O7, Dy2O3, and Y2O3) catalysts for the direct decomposition of NO in the temperature range of 600‒900 oC.22 Recently, Chen et al. examined the La1.6Ba0.4NiO4-x%BaO (x = 5–20) materials for NO direct decomposition at higher temperature region (600‒850 oC).23 However, the major disadvantage associated with these catalysts is that they are inactive at lower temperatures (100250°C). Regardless of the setbacks, the search for an appropriate material is still active because finding a catalyst for the NOx decomposition without using a reducing agent would be a breakthrough with great environmental and economic benefits. In the present work, we report the various TiO2 supported transition metal oxide catalysts for the direct decomposition of NOx in the temperature range of 100‒350 oC. The decomposition activity was significantly varied with the different transition metals over the TiO2 support. Our isotopic labeling and in-situ FT-IR spectra studies over the best Mn/TiO2 catalyst revealed that the catalyst surface labile oxygen plays a decisive role during the direct decomposition of NOx. The oxygen in the reaction stream showed a positive impact in enhancing the NOx conversion


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over the Mn/TiO2 catalyst. The best catalyst (Mn/TiO2) also showed a very good tolerance to 10 vol.% H2O vapors and 100 ppm SO2.

2.

EXPERIMENTAL SECTION

2.1.

Preparation of catalysts

Various titania supported transition metal oxides were synthesized by a solution-impregnation method using metal nitrate precursors. The titania support used in our synthesis was a high BET surface area material (Hombikat UV 100 from Sachtleben Chemie). In a typical preparation of catalysts, 100 mL distilled water was added to a 500 mL beaker containing the support (2.0 g). The loading of transition metals (Cr, Mn, Fe, Co, Ni, Cu, Ni) was selected as 20 wt.%. The required quantity of metal nitrate precursors was dissolved in deionized water and added to the above-dispersed support under continuous stirring at 70 oC. The resultant mixture was evaporated to dryness. The weight amount of each metal is indicated with a number in front of each metal loaded on the support. Some amount of TiO2 anatase (Hombikat) support alone was also dispersed in deionized water for comparison purpose. Afterward, water was evaporated with continuous heating and stirring. Finally, the obtained materials were dried in an electric oven at 120 °C for 12 h, and were ground and sieved (100 mesh) to obtain the homogeneous powder. All the synthesized catalysts were calcined at 400 °C for 2 h under continuous air flow of 150 mL min–1.

2.2.

Characterizations

The X-ray diffraction (XRD) measurements were performed using a Phillips Xpert diffractometer equipped with a Cu-Kα (0.154056 nm) radiation source. The diffraction patterns



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were recorded over a 2θ range of 20–80° using a counting time of one second per point and a step size of 0.02°. Fourier transform infrared (FT-IR) spectra were executed on a BioRad (FTS40) instrument. Each normalized spectrum was obtained from the average of sixteen scans and the scans were recorded at a resolution of 2.0, an aperture opening of 2.0 cm1, and a scan speed of 5 kHz. Circular self-supporting thin wafers (8 mm diameter) with 12 mg of catalyst was placed in a hightemperature in-situ IR cell with CaF2 windows and pretreated at 200 oC in a flow of ultrahigh purity grade helium (30 mL min1, Wright Brothers) for 2 h followed by cooling to 50 oC. Subsequently, the sample exposed to NO (3.9 vol.% in He, ) with a flow rate of 30 mL min1 at 50 oC for 1 h. Prior to the measurement, the sample wafer was flushed with helium for 3 h at 50 o

C to eliminate the physisorbed gases. Finally, the in-situ FT-IR spectra were collected at 175

and 300 oC. In a distinct experiment, a similar procedure was used for NO+O2 co-adsorption. The isotopic labeling studies were performed using premixed gases such as oxygen (4% 16

O2 in He, Wright Brothers), isotopic oxygen (4%

18

O2 (99 atom%) in He, Isotech), and nitric

oxide (2.0% N16O in He, Matheson). The amount of catalyst material was 150 mg in these experiments. The reactions were carried out at 175 oC with a total flow rate of 50 mL/min. First, the catalyst was pretreated in a flow of O2 at reaction temperature for 2 h. Afterward, the reactions were performed with unlabeled components for 2 h under steady state condition before switched to the labeled gas. An online MKS PPT quadrupole mass spectrometer was used to analyze the effluent gas stream from the reactor at their respective atomic mass units (amu). Prior to performing the isotopic labeling studies, fragmentation patterns and their relative intensities were recorded for all labeled and unlabeled reaction species. Then, these data were used when correcting for interference from different fragments. For example, the transient for

14

N18O was


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obtained by monitoring the signal at m/e = 32. The interference to the m/e = 32 peak from oxygen was corrected using the relative intensities of m/e = 16 and m/e = 32 peaks for O2 that were obtained independently under similar conditions. After the switch of 16O2 to 18O2, the mass numbers for

16

O2 (m/e = 32),

16

O18O (m/e = 34), and

18

O2 (m/e = 36) were continuously

monitored. To detect the catalyst oxygen exchange with nitric oxide, the mass numbers for N16O (m/e = 30) and N18O (m/e = 32) were recorded.

2.3.

Catalytic experiments

The NOx direct decomposition activity of the catalysts was investigated in a quartz fixed-bed continuous flow reactor at atmospheric pressure. The reaction temperature was monitored using a type K thermocouple inserted directly into the catalyst bed. The reaction feed normally contained 1800 ppm NO, 200 ppm NO2, 0-3 vol.% oxygen, 10 vol.% H2O (when used), 100 ppm SO2 (when used) and He as balance. The premixed gases nitrogen oxides (2% in He, Matheson) and sulfur dioxide (1% in He, Wright Brothers) and the inert gas ultra-high purified (UHP) helium (Wright Brothers) were employed as received. Water vapor was fed using the ISCO series D pump controller. A Quadrupole mass spectrometer (MKS PPT-RGA) and a chemiluminescence detector (Eco Physics CLD 70S) were employed to analyze the reactants and products from the reactor effluent. The readings were taken when the reaction reached a steady state at each temperature step.

3.

RESULTS AND DISCUSSION

Figure 1 presented the XRD patterns of the M/TiO2 (M = Cr, Mn, Fe, Ni, Cu, and Zn) catalysts. For comparison purpose, we have also included the XRD of pure TiO2 (Hombikat), Mn-Co/TiO2



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and Mn-Co-Cu/TiO2 samples in Figure 1. The bare TiO2 support showed the reflections at 2θ = 25.3, 37.4, 47.9, 54.8, and 62.5 characteristics of the (101), (004), (200), (211), and (204) planes, respectively, of anatase phase (JCPDS #71-1169).24 The diffraction patterns of all TiO2supported transition metal oxides are very similar to that of the pure titania sample. Moreover, no peaks related to the rutile phase of titania were detected in all the supported samples. This indicates that the deposition of transition metals over the TiO2 does not favor the anatase to rutile phase transformation. Therefore, it can be concluded that the deposition of transition metals does not lead to significant changes in the crystal structure of TiO2. Furthermore, no additional peaks corresponding to the chromium oxide, iron oxide, and nickel oxide were appeared for the Cr/TiO2, Fe/TiO2, and Ni/TiO2 catalysts, respectively. This could be owing to the fine dispersion and/or poor crystalline state of the active transition metal oxides on the TiO2 support. However, low-intensity reflections related to the manganese oxide,25 cobalt oxide,26 copper oxide,27 and zinc oxides28 were observed in the diffraction patterns of Mn/TiO2, Co/TiO2, Cu/TiO2, and Zn/TiO2, respectively. This suggests that the presence of crystalline active metal oxides in the samples. Interestingly, no crystalline phases associated with the MnOx and other active metal oxides were observed in the Mn-Co/TiO2 and Mn-Co-Cu/TiO2 samples. This is because of the low concentration and/or well dispersion of the active transition metal oxides on the surface of the titania. In the first instance, we have screened the different TiO2 supported transition metal oxides for the direct decomposition of NOx at 200 °C in a continuous stream. The extent of NOx decomposition into N2 and O2 were analyzed when the reaction reached steady state after about 24 h. No decline of the efficiency of the catalyst was detected at a temperature range of 100350 °C even after 24 h of a continuous stream. The NOx decomposition activity of various catalysts,


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namely, Cr/TiO2, Mn/TiO2, Fe/TiO2, Co/TiO2, Ni/TiO2, Cu/TiO2, and Zn/TiO2 are shown in Figure 2. It can be seen that Mn/TiO2 and the Co/TiO2 catalysts demonstrated substantially high NOx conversions among all the investigated catalysts. The NOx decomposition activity increased in the sequence of Zn/TiO2 < Ni/TiO2 < Cu/TiO2 < Cr/TiO2 < Fe/TiO2

Direct Decomposition of NOx over TiO2 Supported Transition Metal

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