Yujie Fu1, Qi Xin1, Shuo Zhang1,2, Yang Yang This email address is being protected from spambots. You need JavaScript enabled to view it.1

1 Institute for Thermal Power Engineering, Zhejiang University Hangzhou, Hangzhou, Zhejiang, China
2 MCC Capital Engineering & Research Incorporation Ltd., Beijing, China


Received: August 23, 2021
Revised: October 18, 2021
Accepted: October 20, 2021

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.


Download Citation: ||https://doi.org/10.4209/aaqr.210214  


Cite this article:

Fu, Y., Xin, Q., Zhang, S., Yang, Y. (2021). Simultaneous Catalytic Removal of VOCs and NOx with the Dual-layered Catalyst of CoCeOx and V2O5/TiO2. Aerosol Air Qual. Res. 21, 210214. https://doi.org/10.4209/aaqr.210214


HIGHLIGHTS

  • Dual-layered catalysts were constructed for simultaneous removal of NO and propane.
  • Simultaneous removal mechanism of NO and propane was revealed by In-situ DRIFT.
  • The functions of the V2O5/TiO2 and CoCeOx in the catalytic reaction were revealed.
 

ABSTRACT


NO and volatile organic compounds (VOCs) are ones of exhaust with high irritation and toxicity, thus it is very necessary to develop a catalyst that can simultaneously remove them. In this study, a type of dual-layered catalyst has been prepared by combining CoCeOx and V2O5/TiO2 as different functional layers. The dual-layered catalyst was tested under NO, NH3, propane and the mixing atmosphere, achieving great NOx removal efficiency and good propane degradation under wide range temperature window. It is presented that NO conversion could reach 100% at 225°C and kept more than 80% at 175–300°C, while VOCs degradation reached 80% above 260°C. Moreover, the dual-layered catalyst with different construction is surveyed to reveal the mechanism included in this special construction. It is supposed that V2O5/TiO2 layer possesses rich acid sites which adsorbs NH3 as an ammonia buffer part and CoCeOx layer provides good oxidizing ability to oxidize NO and propane. The combination moderates the competitive adsorption between NH3 and propane which is the initial obstacle for multiple pollutants removal.


Keywords: Simultaneous removal of propane and NO, Dual-layered catalyst, Reaction mechanism, Cobalt cerium catalyst


1 INTRODUCTION


The exhaust gas emitted from coal-fired power plants and vehicle is very complex, among which nitrogen oxides (NOx) and volatile organic compounds (VOCs) are ones of the main air pollutants with the most severe irritation and toxicity (Zhao et al., 2013; Kamal et al., 2016). The emission of NOx is always mixed with VOCs because of the incomplete combustion of the fossil fuels, which is assumed as a contribution to the photochemical smog and atmospheric oxidative pollution (Zhang et al., 2018; Bilgen, 2014; Cheng et al., 2016), dealing great harm to human health and ecological environment (Boningari and Smirniotis, 2016; Gholami et al., 2020). Fortunately, selective catalytic reduction (SCR) is an effective method (Busca et al., 1998) for the reduction of NO, with several metal oxides like TiO2 (Liu et al., 2018), CeO2 (Zhang et al., 2015; Tang et al., 2016), Fe2O3 (Yu et al., 2021) researched to obtain good reactive activity under working condition. For VOCs in traditional combustion process, several techniques like chemical adsorption (Yang et al., 2019; Li et al., 2020), photochemical degradation (Tobaldi et al., 2021) are adopted effectively to remove organic air pollutants from flue gas, among which catalytic combustion is considered to be the most efficient way for VOCs abatement (Li et al., 2019; Wang et al., 2019). However, as widely accepted for most power plant only soluble VOCs is removed in wet technology, most of hydrophobic VOCs still remain to be released into atmosphere without any reduction (Li et al., 2011). Therefore, it is very promising to explore a kind of catalyst that can simultaneously remove NO and VOCs in the SCR reaction zone.

In the past two decades, due to the complex process involved in the multiple pollutant degradation with various reactive mechanisms, multiple pollutants removal in flue gas is a still common problem to deal with although a large number of catalysts removing NOx and VOCs separately have been developed very fast (Chen et al., 2020; Sun et al., 2020). In recent years, synergistic control of NOx and VOCs in flue gas could now be achieved by catalyst combined with chemical trapping. Lin et al. (2009) found that BaAlO4 catalyst could attain the reduction of NOx and oxidation of VOCs at the same time. The results showed that NOx adsorbed on the surface forms nitrate species, while VOCs and active oxygen formed C(O). Then the nitrate species oxidized VOCs to CO2, and reduced to N2 and N2O itself. Mori et al. (2014) reported that Cu/BaO/La2O3 could reduce NOx and VOCs simultaneously, in which soot could act as a solid-phase reducing agent to promote the reduction of NO. Some researchers have surveyed on the HC-SCR of NO with hydrocarbons as reducing agents in order to further investigate the simultaneous removal of VOCs and NOx. Habib et al. (2014) surveyed Cu/Y catalyst using propylene as the reducing agent, and the propylene oxidation activity at 290°C was close to 100% with 73% for NOx conversion, 98% for N2 selectivity. Colpini et al. (2013) tested the SCR denitration performance of propane on vanadium-based catalysts with different supports. It was found that 10 wt% V/TiO2 catalyst showed the best catalytic performance, but only 50.8% NOx could be reduced with very low (10%) conversion of propane as well at 400°C. Chansai et al. (2014) used octane and methanol to trigger the SCR process on Ag/Al2O3 catalysts. The results showed that the NO conversion rate at 250°C could reach to 53%, with 83% for methanol degradation. In summary, multi-pollutants removal can be achieved through the combination of active sites with different functions, but the efficiency of which still needs to be promoted with unclear reaction mechanism.

In our previous work, we found that improvement of SCR reactivity could be realized by utilizing the oxidizing properties of cobalt-cerium oxide (denoted as CoCeOx) catalyst and the acidity of the V2O5/TiO2. Moreover, we also revealed that oxidation ability was the dominant property to enhance the degradation performance for VOCs (Hu et al., 2021). Therefore, we propose a dual-layered structure to achieve an effective collocation with different functional areas, enhancing its total activity for NOx and VOCs removal.

In this work, we developed a new dual-layered structural catalyst to realize simultaneous removal of VOCs and NOx. We selected CoCeOx catalyst and V2O5/TiO2 catalyst for layered construction, investigated the synergistic effect to each other between two layers, and consequentially put forward an interaction mechanism between propane oxidation reaction and SCR. As a result, this kind of dual-layered structure made it better to remove both NO and propane compared with single catalyst, which achieved the severe demand for efficient and simultaneous removal of propane and NOx.

 
2 METHODS


 
2.1 Catalyst Preparation

The CoCeOx catalysts in this work were prepared by citric acid complexation method. Appropriate amounts of Co(NO3)3·6H2O and Ce(NO3)3·6H2O were dissolved in deionized water with stirring. Added citric acid in a molar ratio of 1.5:1 to the solution, then reacting at 80°C till the solution turned into a dark red gel. Then the mixture was dried at 110°C for 12 h, calcined at 450°C for 4 h.

The V2O5/TiO2 catalysts in this work were prepared by incipient wetness impregnation, with NH4VO3 (Sinopharm Chemical Reagent Co., Ltd.) being precursors. TiO2 (Degussa P25) was used as supports. Details of the impregnation procedure was presented as below. NH4VO3 and oxalic acid were dissolved in ionized water at a molar ratio of 1:2, aging overnight. Then, 5g P25 was added into NH4VO3 solution with vigorous stirring, then under ultrasonic treatment for 20 minutes. The mixture was aged overnight, centrifuged, dried at 110°C for 12 hours and calcinated at 500°C for 5 hours sequentially.

 
2.2 Dual-layered Catalyst Preparation

Dual-layered catalyst was built up in the Quartz tube with 6mm inner diameter with particle size controlled in 40-60 mesh. The samples were introduced with different ratio and structure. Different modes were presented in Fig. 1. The dual-layered set was detailed as below: 30 mg oxidation catalyst and 30 mg SCR catalyst were filled into two layers, separated by quartz wool.


Fig. 1. Different samples for dual-layered catalyst.
Fig. 1. Different samples for dual-layered catalyst.

Mode 1 referred to SCR catalyst as former part along the direction of gas flow, and oxidation catalyst in the latter layer. Mode 2 had the layered arrangement opposite to Mode 1. Since the two catalysts did not interact with each other, they would not affect the surface structure and chemical properties to one another directly.

 
2.3 Catalytic Characterization

X-ray photoelectron spectroscopy (XPS) spectra was tested using a Thermo ESCALAB 250 instrument (Al Kα X-ray radiation, hν = 1486.6 eV). C1s binding energy with (284.8 eV) was set as reference binding energy. H2 temperature-programmed reduction (H2-TPR) and O2 temperature-programmed desorption (O2-TPD) experiments were performed on a Micromeritics Autochem II 2920 analyzer. For H2-TPR, the prepared catalysts (45 mg) were pretreated in N2 at 450°C for 30 min. After cooling to 50°C, the temperature was increased linearly to 800°C at a heating rate of 10°C min1, where the atmosphere was 10% H2/Ar flow (30 mL min1). For O2-TPD, 50 mg catalyst were pretreated in 20% O2/He (30 mL min1) at 400°C for 30 min, then cooled down to room temperature. After continuously purging in a He flow for 30 min, the catalysts were heated up to 900°C in He flow at a rate of 10°C min1.

In-situ DRIFT was tested by Nicolet NEXUS 6700 infrared spectrometer to survey the information of the surface functional groups as well as the reactive species adsorbed on surface during the catalytic reaction. The spectrometer is equipped with an MCT/A detector with a resolution of 4 cm1 and scanning range of 4000–400 cm1. The spectral signal was averaged after 16–32 scans. The acquisition time of a single spectrum was about 1 min.

 
2.4 Catalytic Activity Test

The catalytic activity was tested by a fixed-bed reactor as mentioned in Section 2 that 150 mg catalyst was placed in a quartz tube reactor for every sample (i.d. = 6 mm). The reaction condition for SCR was consistent of 1000 ppm NH3, 1000 ppm NO and 10% O2 with GHSV = 90,000 mL g−1 h−1 in He atmosphere. Propane was used as the target gas for VOCs testing. The reaction condition for propane oxidation was consistent of 800 ppm propane and 10% O2 with GHSV = 90,000 mL g1 h1 under He atmosphere. The concentration of reactants and products were analyzed online by a Fourier transform infrared spectroscopy (FTIR) Gas Analyzer (Gasmet Dx4000, Finland).

In SCR reaction, the NO conversion rates, XNO, and selectivity of N2, SN2, were calculated according to the following formula:

 

where, [NO]in and [NO]out referred to NO concentration at inlet and outlet respectively, [NH3]in represented the NH3 concentration at inlet; [N2O]out and [NO2]out referred to concentration of N2O and NO2 at outlet.

In propane oxidation, conversion of propane, XC3H8 and selectivity of CO2, SCO2 were calculated as below:

  

where [C3H8]in and [C3H8]out refer to C3H8 concentration at influx and outflux, [CO2]out stands for CO2 concentration at outflux.

The reaction rate of propane oxidation, r, could be calculated by the conversion rate, XC3H8 according to the following formula (Ren et al., 2016):

  

where, FC3H8 stands for the velocity of (L s1), Wcat represented the mass of catalyst (g).

 
3 RESULTS AND DISCUSSION


 
3.1 Performance of Dual-layered Catalyst

Co-based catalyst had good redox performance under low-temperature range but with low selectivity in high-temperature, while vanadium-based catalyst had good acidity with low activity in low-temperature condition. By optimizing the superb activity of single layer detailed in Fig. S1, we chose the CoCeOx (with the molar ratio of Co and Ce equals to 9:1) as the oxidation catalyst in the upper layer and the V2O5/TiO2 (with 1%wt V2O5 loading amount) as the SCR catalyst in the lower layer with volume ratio at 1:1 referred to Mode 1 in Fig. 1.

The SCR performance for dual-layered catalyst was surveyed, where Mode 1 was adopted as ‘CoCeOx + V2O5/TiO2’ for this whole article below. Fig. 2 showed the activity results and product distribution of the dual-layered catalyst. At temperature of 150°C, NO conversion rate of CoCeOx + V2O5/TiO2 reached 65%. As temperature increased from 150°C to 225°C, the NO conversion gradually increases from 65%, reaching 100% at 225°C. For temperature between 175–300°C, CoCeOx + V2O5/TiO2 maintained good performance where NO conversion was kept higher than 80%. Compared with V2O5/TiO2, the active window for SCR moved nearly 150°C to the left. Furthermore, CoCeOx + V2O5/TiO2 attained far better activity than that of CoCeOx at 200–300°C. This might be due to the acidity of V2O5/TiO2 catalyst, which trapped a large amount of NH3 on the up-stream position. The inhibiting of NH3 oxidation gave sufficient reducing agent in reactive atmosphere above 250°C, helping dual-layered catalyst obtain a wide range of active temperature. The product distribution for CoCeOx + V2O5/TiO2 was shown in Fig. 2(b). It was obvious that all by-products were below 100 ppm between 175°C and 300°C, indicating that NO was mainly converted to N2.

Fig. 2. SCR performance of CoCeOx + V2O5/TiO2 on (a) NO conversion, (b) product distribution, NH3 concentration: 1000 ppm, NO concentration: 1000 ppm; GHSV: 90,000 mL g−1 h−1; catalyst dosage: 150 mg.Fig. 2. SCR performance of CoCeOx + V2O5/TiO2 on (a) NO conversion, (b) product distribution, NH3 concentration: 1000 ppm, NO concentration: 1000 ppm; GHSV: 90,000 mL g−1 h−1; catalyst dosage: 150 mg.

In addition, as oxidation ability was one of promotions for VOCs degradation under NH3 atmosphere, we surveyed the NH3 oxidation ability for every sample. The result was shown in Fig. 3(a). CoCeOx + V2O5/TiO2 performed a strong catalytic oxidation of NH3 ability above 200°C, extremely better than the V2O5/TiO2 catalyst but similar to CoCeOx. The NH3 oxidation product distribution for dual-layered was presented in Fig. 3(b). It was obvious that below 200°C, NH3 was gradually oxidized to N2, NO and N2O with increasing temperature; When temperature rose to 300°C, the remaining NO and N2O could be further oxidized to NO2. Compared with pure CoCeOx catalyst, the proportion of NO and N2O was much less with more NH3 oxidized to N2

Fig. 3. (a) NH3 oxidation performance of CoCeOx + V2O5/TiO2, (b) product distribution of NH3 oxidation for CoCeOx + V2O5/TiO2. NH3 concentration: 1000 ppm; GHSV: 90,000 mL g−1 h−1; catalyst dosage: 150 mg.  Fig. 3. (a) NH3 oxidation performance of CoCeOx + V2O5/TiO2, (b) product distribution of NH3 oxidation for CoCeOx + V2O5/TiO2. NH3 concentration: 1000 ppm; GHSV: 90,000 mL g−1 h−1; catalyst dosage: 150 mg.
 

Secondly, we tested another type pf dual-layered catalyst (the V2O5/TiO2 catalyst in the upper layer, the CoCeOx catalyst in the lower layer, with volume ratio at 1:2, denoted as Mode 4 for its propane oxidation reactivity under different atmosphere in order to investigate the performance of simultaneous removal and synergistic mechanism of dual-layered catalyst.

As shown in Fig. 4, with C3H8 fed only, T50 (the temperature that 50% reactant is reacted) and T90 (the temperature that 50% reactant is reacted) for propane oxidation were 232°C and 263°C, respectively. If we introduced NH3 simultaneously, the activity for propane oxidation would be greatly reduced and the temperature of T50, T90 rose 53°C and 47°C higher respectively. It was proposed that the activation of C-H bond in adsorbed propane was the rate-determining step in propane oxidation, while the activation of N-H bond breaking took the initial step in NH3 oxidation. The C-H bond energy in propane reached 401.3 kJ mol–1 higher than that of N-H bond in NH3 (391 kJ mol–1), which meant that propane might be more difficult to be activated than NH3. As a result, propane and NH3 were in competitive adsorption for the active sites when two reactants were added to the reactor at the same time. Thus, the conversion rates of propane and NH3 were both lower than single layer reacting separately.

 Fig. 4. The influence of gas components on propane oxidation performance of CoCeOx. NH3 concentration: 1000 ppm, NO concentration: 1000 ppm, propane concentration 800ppm; GHSV: 90,000 mL g−1 h−1; catalyst dosage: 150 mg.Fig. 4. The influence of gas components on propane oxidation performance of CoCeOx. NH3 concentration: 1000 ppm, NO concentration: 1000 ppm, propane concentration 800ppm; GHSV: 90,000 mL g−1 h−1; catalyst dosage: 150 mg.

In another perspective for NO influence, we introduced C3H8 with NO into the reactive atmosphere. As shown in Fig. 4, the T50 and T90 for propane oxidation were at 248°C and 276°C, performing slightly lower activity than C3H8 alone. The NO-deduced inhibition on propane oxidation was significantly weaker than that caused by NH3, because propane could be adsorbed and activated more easily than NO. Moreover, NO + O2 was a higher oxidizing atmosphere but the curve was shifted to higher temperature, indicating that the lattice oxygen was the relevant species in propane oxidation. If C3H8, NH3 and NO were all introduced in the reactive atmosphere, activity of dual-layered catalyst went back to a high level especially at 275–350°C. It might be attributed to that SCR react in different sites to propane oxidation, helped reducing the competitive adsorption of NH3 and NO with propane.

It was widely accepted that C3H8 could be used as a reducing agent to react with NO in HC-SCR (Moreno-González et al., 2017). Therefore, we proposed that C3H8 acted as reducing agent, beneficial for NO reduction at high temperature. It was shown in Fig. 5 that the dual-layered catalyst had excellent SCR activity no matter whether C3H8 existed. We supposed that NH3 was mainly adsorbed on acid sites while C3H8 was mainly adsorbed and activated on redox sites. In summary, the simultaneous existence of NH3 and NO could reduce the activity inhibition, caused by competitive adsorption, of propane oxidation, promoting the synergistic removal of multiple pollutants.

Fig. 5. HC-SCR activity (a) under propane and NO atmosphere, (b) without propane. NH3 concentration: 1000 ppm, NO concentration: 1000 ppm, propane concentration: 800 ppm; GHSV: 90,000 mL g−1 h−1; catalyst dosage: 150 mg.Fig. 5. HC-SCR activity (a) under propane and NO atmosphere, (b) without propane. NH3 concentration: 1000 ppm, NO concentration: 1000 ppm, propane concentration: 800 ppm; GHSV: 90,000 mL g−1 h−1; catalyst dosage: 150 mg.

 
3.2 Mechanism for NO Oxidation and Propane Degradation

From the results of propane oxidation and our previous work (Zhang et al., 2019), it could be seen that the CoCeOx layer in the dual-layered catalyst performed outstanding redox performance for propane oxidation and NH3 oxidation. Otherwise as widely accepted that acidity also played a vital role in the performance of the SCR reaction, NH3 adsorption and desorption experiment was adopted to detect the acidity of the catalyst surface shown in Fig. 6.

Fig. 6. The NH3-TPD experiment of the CoCeOx catalyst. (a) Detailed processes of the NH3 adsorption and desorption, (b) the result of NH3-TPD. NH3 concentration: 900 ppm, the heating rate of desorption is 10°C min−1.Fig. 6. The NH3-TPD experiment of the CoCeOx catalyst. (a) Detailed processes of the NH3 adsorption and desorption, (b) the result of NH3-TPD. NH3 concentration: 900 ppm, the heating rate of desorption is 10°C min−1.

At the beginning, we introduced 900 ppm NH3 at 50°C to be adsorbed till saturated, then purged N2 in to remove the weakly adsorbed NH3, subsequently increasing the temperature to 500°C at 10°C min–1. A strong NH3 desorption peak was observed near 175°C with concentration of 65 ppm. The peak I at 150–200°C represented the desorption of physically adsorbed NH3, while the peak II at 250°C referred to weakly chemically adsorbed NH, with the peak III at 350°C the strong chemisorption peak of NH3 (Gao et al., 2011; Hu et al., 2017). Peak I and peak II reached 60% and 35% respectively, which were much higher than 5% of peak III. Therefore, CoCeOx catalyst was mainly affected by physical adsorption and weak chemical adsorption with much fewer strong acid sites.

In-situ DRIFTS spectroscopy was adopted to perform NH3 adsorption and desorption experiments to survey the surface acidity on CoCeOx catalyst. The results were shown in Fig. 7(a). The peak at 1230 cm–1 and 3200 cm–1 was related to NH3 adsorbed on Lewis acid site, and that of 1480 cm–1 referred to Brønsted acid sites (Hadjiivanov, 2000; Chen et al., 2010). It was depicted that Lewis and Brønsted acid sites on the surface of CoCeOx catalyst had basically the same adsorption strength for NH3. In addition, the vibration peak of hydroxyl group was observed at 3600–3750 cm–1, which indicated that NH3 was firstly adsorbed on hydroxyl group forming NH4+.

Fig. 7. In-situ DRIFTS spectrum results of CoCeOx under (a) temperature programmed NH3 desorption, (b) pre-adsorbed NH3 introduced with NO + O2, (c) pre-adsorbed NO + O2 introduced with NH3. NH3 concentration: 1000 ppm, NO concentration: 1000 ppm; O2 concentration: 10%, GHSV: 90,000 mL g−1 h−1.Fig. 7. In-situ DRIFTS spectrum results of CoCeOx under (a) temperature programmed NH3 desorption, (b) pre-adsorbed NH3 introduced with NO + O2, (c) pre-adsorbed NO + O2 introduced with NH3. NH3 concentration: 1000 ppm, NO concentration: 1000 ppm; O2 concentration: 10%, GHSV: 90,000 mL g−1 h−1.

In order to study the entire SCR reaction mechanism, more experiments of in-situ DRIFTS were used to analyze the intermediate reaction species. Fig. 7(b) showed the in-situ infrared spectrum where NO and O2 were introduced with pre-adsorbed NH3 at 200°C. It was revealed that after purging with pure N2 for 30 minutes, a significant NH3 adsorption peak signal was still observed with both Brønsted acid site and Lewis acid sites. As 1000 ppm NO and 10% O2 fluxed in, NH3 adsorption peak signal decayed rapidly, where adsorbed NH3 species reacted with the NO. After the influx of NO + O2 for 5 minutes, new vibration peak signals began to appear at 1260 cm–1 and 1600 cm–1 increasingly, which indicated NH3 was totally consumed. Furthermore, the vibration peak at 1260 cm–1 could be regarded as nitrate species, while the peak at 1600 cm–1 could be attributed to nitrite and adsorbed NO2 (Ruggeri et al., 2014, 2015). It was obvious that at the moment adsorbed NH3 species was consumed completely, NO would be rapidly oxidized on the surface of the CoCeOx catalyst. Fig. 7(c) showed the in-situ infrared spectrum where NH3 were introduced with pre-adsorbed NO and O2 at 200°C. After NH3 introduction, the nitrate and nitrite peak showed negligible change. There was no reaction between nitrate and NH3 as the adsorption and activation of NH3 played the vital role in low temperature SCR.

In-situ infrared spectroscopy was adopted to for C3H8 temperature-programmed reaction experiment on the CoCeOx to identify the reactive species on the catalyst surface. The results were shown in Fig. 8(a). Introducing propane at 50°C for 30 minutes, it could be found that the peak corresponding to hydroxyl group at 3373 cm–1 was rising, as well as the peak between 1450–1550 cm–1 representing the COO group asymmetry vibration. In addition, the peak at 1175 cm–1 represented carbonate species with bidentate coordination, and the peak at 1259 cm–1 was related to the swing vibration of the C-H bond (Vayssilov et al., 2011). As the temperature rose, the C-H bond vibration gradually decreases, while the COO vibration peak firstly went up and then began to be vanished above 150°C. It was revealed that the activated C-H bond would be converted into COO and finally decomposed into CO2. In absence of gas-phase O2, the oxidative decomposition process of propane could still be observed. It was clear that the oxygen required for propane oxidation mainly came from the oxygen species adsorbed on the catalyst surface, following the MvK reaction mechanism.

Fig. 8. In-situ DRIFTS spectrum results of CoCeOx under (a) C3H8, (b) C3H8 + O2. Propane concentration: 800 ppm, NO concentration: 1000 ppm, O2 concentration: 10%; GHSV: 90,000 mL g−1 h−1.Fig. 8. In-situ DRIFTS spectrum results of CoCeOx under (a) C3H8, (b) C3H8 + O2. Propane concentration: 800 ppm, NO concentration: 1000 ppm, O2 concentration: 10%; GHSV: 90,000 mL g−1 h−1.


3.3 Analysis on Function of CoCeOx Layer in Dual-layered Catalyst

In order to reveal the influence of catalyst contact on the total activity, we added a quartz wool layer as the transforming barrier in the middle of the two catalysts. The layout structure is shown in Fig. 9(a). The corresponding SCR reaction performance was shown in Fig. 9(b). The results showed that the SCR activity of dual-layered catalyst with quartz wool (Mode 2) was significantly lower than that without quartz wool (Mode 1) below 300°C. It was worth noting that the result of Mode 2 was extremely similar to that of pure V2O5/TiO2, indicating that the synergistic effect between the CoCeOx and the V2O5/TiO2 catalysts was dominant for dual-layered SCR activity.

Fig. 9. (a) Layered structure before and after adding quartz wool. Mode 1: The V2O5/TiO2 in the upper layer and the CoCeOx in the lower layer. Mode 2: the same dual-layered structure with silica wool in the middle of two layers. (b) NO conversion with addition of Quartz wool. NH3 concentration: 1000 ppm, NO concentration: 1000 ppm; O2 concentration: 10%, GHSV: 90,000 mL g−1 h−1.Fig. 9. (a) Layered structure before and after adding quartz wool. Mode 1: The V2O5/TiO2 in the upper layer and the CoCeOx in the lower layer. Mode 2: the same dual-layered structure with silica wool in the middle of two layers. (b) NO conversion with addition of Quartz wool. NH3 concentration: 1000 ppm, NO concentration: 1000 ppm; O2 concentration: 10%, GHSV: 90,000 mL g−1 h−1.

To clarify the synergistic effect in this dual-layered structure, we surveyed the effects on the removal of propane and NOx by tuning filling construction of dual-layered catalyst shown in Fig. 10. Then we introduced C3H8 and NO mixed gas flow to test the comprehensive performance for multiple pollutant removal.

Fig. 10. Schematic diagram for dual-layered catalyst with different filling order. Mode 1: V2O5/TiO2 in the upper layer and CoCeOx in the lower layer. Mode 3: CoCeOx in the upper layer and V2O5/TiO2 in the lower layer.Fig. 10. Schematic diagram for dual-layered catalyst with different filling order. Mode 1: V2O5/TiO2 in the upper layer and CoCeOx in the lower layer. Mode 3: CoCeOx in the upper layer and V2O5/TiO2 in the lower layer.

Fig. 11(a) showed the propane oxidation activity of different modes. It could be found that both modes showed great and similar propane oxidation performance, with slight superior of Mode 1 than Mode 2. At 275°C, propane reduction of Mode 1 reached ghsv% which was much higher than 71% of Mode 2. We have known that during oxidation process, metal sites firstly gained electrons from the adsorbed gas molecular, then lattice oxygen provided O atoms to help break the chemical bond to complete the reactive cycle. So the good acidity of vanadium helped V2O5/TiO2 layer in Mode1 trap amounts of NH3 to prevent the competitive adsorption in CoCeOlayer between NO and propane. Fig. 11(b) depicted the SCR activity for two modes. The results showed that the impact of layer order on SCR was far stronger than on propane oxidation. At 175–325°C, Mode 1 had shown consistent superior SCR activity with NO conversion rate above 85%. But for Mode 2, it did not show any difference with Mode 1 Below 175°C, both reaching about 80%; For temperature at 225–275°C, NO conversion rate declined rapidly to 0% with temperature increasing.

Fig. 11. The influence of filling order to (a) propane oxidation, (b) SCR reaction; Byproduct generation of (c) N2O, (d) NO2. Propane concentration: 800ppm, NH3 concentration: 1000 ppm, NO concentration: 1000 ppm; O2 concentration: 10%, GHSV: 90,000 mL g−1 h−1.Fig. 11. The influence of filling order to (a) propane oxidation, (b) SCR reaction; Byproduct generation of (c) N2O, (d) NO2. Propane concentration: 800ppm, NH3 concentration: 1000 ppm, NO concentration: 1000 ppm; O2 concentration: 10%, GHSV: 90,000 mL g−1 h−1.

In order to verify why there was a significant SCR reactivity difference between two sequences, product distribution of the reaction process was analyzed shown in Fig. 11(c). It could be seen that the N2O and NO2 generated in Mode 1 kept in low concentration in the entire temperature range, and NO was mostly reduced to N2. On the contrary, Mode 2 produced a large amount of N2O at 225–275°C, which means a severe side reaction had been triggered and that NH3 was directly oxidized resulting in low SCR activity. It was worth noticing that although the conversion rate of NO returned up slightly above 300°C, the activity was still not satisfied because this increase was mainly due to the oxidation from NO to NO2. Obviously, the V2O5/TiO2 should be set before the CoCeOx for multiple pollutant removal.

As mentioned above, the V2O5/TiO2 layer inhibited the competitive adsorption on the CoCeOx to promote entire catalytic activity. Thus, we consequently adjusted the bed thickness of V2O5/TiO2 layer to further investigated the mechanism. Under the consistent mass of total catalyst, the ratio of the V2O5/TiO2 to CoCeOx was changed to 1:2 denoted Mode 4.

The catalytic activity with different bed thicknesses on propane is shown in Fig. 12. The results showed that T50 and T90 were at 240°C and 268°C, respectively, which was slightly better than Mode 1. The larger proportion of CoCeOx provided more active sites for propane oxidation, enhancing the oxidation activity. Fig. 13(b) showed the SCR result with different bed thicknesses. It was presented that both Mode 1 and Mode 2 had good SCR reactivity with NO conversion rate above 80% at 175–325°C. The superior activity was attributed to that the V2O5/TiO2 catalyst participated as a temporary NH3 buffer area, where adsorbed NH3 species was firstly trapped to avoid over flow of NH3. When these adsorbed NH3 species diffused downwards and reached the interface between layers, NH3 was quickly reacted on CoCeOx with abundant NO, rather than further deep oxidation. Therefore, dual-layered catalyst could attain superior low-temperature activity and N2 selectivity.

Fig. 12. Dual-Layered catalysts with different bed thicknesses.Fig. 12. Dual-Layered catalysts with different bed thicknesses.

Fig. 13. The influence of different bed thicknesses on (a) propane oxidation, (b) SCR reaction. Propane concentration: 800 ppm, NH3 concentration: 1000 ppm, NO concentration: 1000 ppm; O2 concentration: 10%, GHSV: 90,000 mL g−1 h−1.Fig. 13. The influence of different bed thicknesses on (a) propane oxidation, (b) SCR reaction. Propane concentration: 800 ppm, NH3 concentration: 1000 ppm, NO concentration: 1000 ppm; O2 concentration: 10%, GHSV: 90,000 mL g−1 h−1.

 
4 CONCLUSIONS


In this paper, we developed a new dual-layered CoCeOx + V2O5/TiO2 with high VOCs removal efficiency and good low-temperature denitration performance, of which the V2O5/TiO2 was set on the upper layer, and the CoCeOx on downside. NO conversion of dual-layered catalysts could reach 100% at 225°C and kept more than 80% below 300°C, while VOCs degradation reached 80% above 260°C.

In order to investigate the interaction mechanism for dual-layered catalyst, we examined both layers for their contribution to simultaneous removal under multiple pollutant atmosphere and various mode with different structure is surveyed such as catalyst packing form, bed thickness and particle size were adopted. It was revealed that V2O5/TiO2 possesses rich acid sites which could tunable attract NH4+ and CoCeOx provides good oxidizing ability to oxidize NO and propane. The combination of the two moderate the competitive adsorption between NH3 and propane which is the initial obstacle for CoCeOx to quickly remove multiple pollutants simultaneously. Moreover, compared with the lack of strong acid sites in the CoCeOx catalyst, the V2O5/TiO2 catalyst adsorbed more NH3 as a transparent buffer area to adjust the ammonia concentration in down-streamed CoCeOx oxidation layer, avoiding prior oxidation of NH3 on CoCeOx surface.

 
ACKNOWLEDGMENTS


This work is supported by the National Natural Science Foundation of China (No. 51836006 and No. 52006192). The authors wish to thank Shuyang Jiang for valuable discussion and accompany.

 
DISCLAIMER


The authors declare no conflict of interest.

 
SUPPLEMENTARY MATERIAL


Supplementary material for this article can be found in the online version at https://doi.org/10.4209/aaqr.210214


REFERENCES


  1. Bilgen, S. (2014). Structure and environmental impact of global energy consumption. Renewable Sustainable Energy Rev. 38, 890–902. https://doi.org/10.1016/j.rser.2014.07.004

  2. Boningari, T., Smirniotis, P.G. (2016). Impact of nitrogen oxides on the environment and human health: Mn-based materials for the NOx abatement. Curr. Opin. Chem. Eng. 13, 133–141. https://doi.org/10.1016/j.coche.2016.09.004

  3. Busca, G., Lietti, L., Ramis, G., Berti, F. (1998). Chemical and mechanistic aspects of the selective catalytic reduction of NOx by ammonia over oxide catalysts: A review. Appl. Catal., B 18, 1–36. https://doi.org/10.1016/s0926-3373(98)00040-x

  4. Chansaia, S., Burch, R., Hardacre, C., Norton, D., Bao, X., Lewis, L. (2014). Investigating the promotional effect of methanol on the low temperature SCR reaction on Ag/Al2O3. Appl. Catal., B 160, 356–364. https://doi.org/10.1016/j.apcatb.2014.05.040

  5. Chen, L., Li, J., Ge, M. (2010). DRIFT study on cerium-tungsten/titiania catalyst for selective catalytic reduction of NOx with NH3. Environ. Sci. Technol. 44, 9590–9596. https://doi.org/10.1021/es102692b

  6. Chen, L., Liao, Y., Chen, Y., Wu, J., Ma, X. (2020). Performance of Ce-modified V-W-Ti type catalyst on simultaneous control of NO and typical VOCS. Fuel Process. Technol. 207, 106483. https://doi.org/10.1016/j.fuproc.2020.106483

  7. Cheng, Y., Zheng, G., Wei, C., Mu, Q., Zheng, B., Wang, Z., Gao, M., Zhang, Q., He, K., Carmichael, G., Poschl, U., Su, H. (2016). Reactive nitrogen chemistry in aerosol water as a source of sulfate during haze events in China. Sci. Adv. 2, e1601530. https://doi.org/10.1126/sciadv.1601530

  8. Colpini, L.M.S., Lenzi, G.G., Martins, L., Urquieta Gonzalez, E.A., Andreo dos Santos, O.A., Macedo Costa, C.M. (2013). Selective catalytic reduction of NO with propane on V2O5/SiO2, V2O5/TiO2, and V2O5/Al2O3 catalysts obtained through the sol-gel method. Acta Sci.-Technol. 35, 139–145. https://doi.org/10.4025/actascitechnol.v35i1.11888

  9. Gao, X., Du, X., Fu, Y., Mao, J., Luo, Z., Ni, M., Cen, K. (2011). Theoretical and experimental study on the deactivation of V2O5 based catalyst by lead for selective catalytic reduction of nitric oxides. Catalysis Today 175, 625–630. https://doi.org/10.1016/j.cattod.2011.05.025

  10. Gholami, F., Tomas, M., Gholami, Z., Vakili, M. (2020). Technologies for the nitrogen oxides reduction from flue gas: A review. Sci. Total Environ. 714, 136712. https://doi.org/10.1016/j.scitotenv.2020.136712

  11. Habib, H.A., Basner, R., Brandenburg, R., Armbruster, U., Martin, A. (2014). Selective catalytic reduction of NOx of ship diesel engine exhaust gas with C3H6 over Cu/Y zeolite. ACS Catal. 4, 2479–2491. https://doi.org/10.1021/cs500348b

  12. Hadjiivanov, K.I. (2000). Identification of neutral and charged NxOy surface species by IR spectroscopy. Catal. Rev. Sci. Eng., 42, 71–144. https://doi.org/10.1081/cr-100100260

  13. Hu, W., Zhang, Y., Liu, S., Zheng, C., Gao, X., Nova, I., Tronconi, E. (2017). Improvement in activity and alkali resistance of a novel V-Ce(SO4)2/Ti catalyst for selective catalytic reduction of NO with NH3. Appl. Catal., B 206, 449–460. https://doi.org/10.1016/j.apcatb.2017.01.036

  14. Hu, W., Zou, R., Dong, Y., Zhang, Y., Ran, M., Xin, Q., Yang, Y., Song, H., Wu, W., Liu, S., Zheng, C., Gao, X. (2021). Mechanism and enhancement of the low-temperature selective catalytic reduction of NOx with NH3 by bifunctional catalytic mixtures. Ind. Eng. Chem. Res. 60, 6446–6454. https://doi.org/10.1021/acs.iecr.0c05214

  15. Kamal, M.S., Razzak, S.A., Hossain, M.M. (2016). Catalytic oxidation of volatile organic compounds (VOCs) - A review. Atmos. Environ. 140, 117–134. https://doi.org/10.1016/j.atmosenv.2016.05.031

  16. Li, J.J., Yu, E.Q., Cai, S.C., Chen, X., Chen, J., Jia, H.P., Xu, Y.J. (2019). Noble metal free, CeO2/LaMnO3 hybrid achieving efficient photo-thermal catalytic decomposition of volatile organic compounds under IR light. Appl. Catal., B 240, 141–152. https://doi.org/10.1016/j.apcatb.2018.08.069

  17. Li, L., Liu, S., Liu, J. (2011). Surface modification of coconut shell based activated carbon for the improvement of hydrophobic VOC removal. J. Hazard. Mater. 192, 683–690. https://doi.org/10.1016/j.jhazmat.2011.05.069

  18. Li, X., Zhang, L., Yang, Z., Wang, P., Yan, Y., Ran, J. (2020). Adsorption materials for volatile organic compounds (VOCs) and the key factors for VOCs adsorption process: A review. Sep. Purif. Technol. 235, 116213. https://doi.org/10.1016/j.seppur.2019.116213

  19. Lin, H., Li, Y., Shangguan, W., Huang, Z. (2009). Soot oxidation and NOx reduction over BaAl2O4 catalyst. Combust. Flame, 56, 2063–2070. https://doi.org/10.1016/j.combustflame.2009.08.006

  20. Liu, J., Guo, R., Li, M., Sun, P., Liu, S.M., Pan, W., Liu, S.W., Sun, X. (2018). Enhancement of the SO2 resistance of Mn/TiO2 SCR catalyst by Eu modification: A mechanism study. Fuel 223, 385–393. https://doi.org/10.1016/j.fuel.2018.03.062

  21. Moreno-González, M., Palomares, A.E., Chiesa, M., Boronat, M., Giamello, E., Blasco, T. (2017). Evidence of a Cu2+-Alkane Interaction in Cu-Zeolite Catalysts Crucial for the Selective Catalytic Reduction of NOx with Hydrocarbons. ACS Catal. 7, 3501–3509. https://doi.org/10.1021/acscatal.6b03473

  22. Mori, K., Iwata, Y., Yamamoto, M., Kimura, N., Miyauchi, A., Okamoto, G., Toyoshima, T., Yamashita, H. (2014). An efficient Cu/BaO/La2O3 catalyst for the simultaneous removal of carbon soot and nitrogen oxides from simulated diesel exhaust. J. Phys. Chem. C 118, 9078–9085. https://doi.org/10.1021/jp501940f

  23. Ren, Z., Wu, Z., Song, W., Xiao, W., Guo, Y., Ding, J., Suib, S.L., Gao, P.X. (2016). Low temperature propane oxidation over Co3O4 based nano-array catalysts: Ni dopant effect, reaction mechanism and structural stability. Appl. Catal., B 180, 150–160. https://doi.org/10.1016/j.apcatb.2015.04.021

  24. Ruggeri, M.P., Selleri, T., Colombo, M., Nova, I., Tronconi, E. (2014). Identification of nitrites/HONO as primary products of NO oxidation over Fe-ZSM-5 and their role in the Standard SCR mechanism: A chemical trapping study. J. Catal. 311, 266–270. https://doi.org/10.1016/j.jcat.2013.11.028

  25. Ruggeri, M.P., Selleri, T., Colombo, M., Nova, I., Tronconi, E. (2015). Investigation of NO2 and NO interaction with an Fe-ZSM-5 catalyst by transient response methods and chemical trapping techniques. J. Catal. 328, 258–269. https://doi.org/10.1016/j.jcat.2015.02.003

  26. Sun, X., Zhao, Y., Li, Z., Hong, P., Zhao, W. (2020). Advances on simultaneous removal of multiple pollutants from flue gas by oxidation method. Mod. Chem. Ind. 40, 23–27.

  27. Tang, C., Zhang, H., Dong, L. (2016). Ceria-based catalysts for low-temperature selective catalytic reduction of NO with NH3. Catal. Sci. Technol. 6, 1248–1264. https://doi.org/10.1039/c5cy01487e

  28. Tobaldi, D.M., Dvoranova, D., Lajaunie, L., Rozman, N., Figueiredo, B., Seabra, M.P., Skapin, A.S., Calvino, J.J., Brezova, V., Labrincha, J.A. (2021). Graphene-TiO2 hybrids for photocatalytic aided removal of VOCs and nitrogen oxides from outdoor environment. Chem. Eng. J. 405, 126651. https://doi.org/10.1016/j.cej.2020.126651

  29. Vayssilov, G.N., Mihaylov, M., St Petkov, P., Hadjiivanov, K.I., Neyman, K.M. (2011). Reassignment of the vibrational spectra of carbonates, formates, and related surface species on ceria: A combined density functional and infrared spectroscopy investigation. J. Phys. Chem. C, 115, 23435–23454. https://doi.org/10.1021/jp208050a

  30. Wang, Y., Yang, D., Li, S., Zhang, L., Zheng, G., Guo, L. (2019). Layered copper manganese oxide for the efficient catalytic CO and VOCs oxidation. Chem. Eng. J. 357, 258–268. https://doi.org/10.1016/j.cej.2018.09.156

  31. Yang, C., Miao, G., Pi, Y., Xia, Q., Wu, J., Li, Z., Xiao, J. (2019). Abatement of various types of VOCs by adsorption/catalytic oxidation: A review. Chem. Eng. J. 370, 1128–1153. https://doi.org/10.1016/j.cej.2019.03.232

  32. Yu, Y., Tan, W., An, D., Wang, X., Liu, A., Zou, W., Tang, C., Ge, C., Tong, Q., Sun, J., Dong, L. (2021). Insight into the SO2 resistance mechanism on gamma-Fe2O3 catalyst in NH3-SCR reaction: A collaborated experimental and DFT study. Appl. Catal., B 281, 119544. https://doi.org/10.1016/j.apcatb.2020.119544

  33. Zhang, L., Li, L., Cao, Y., Yao, X., Ge, C., Gao, F., Deng, Y., Tang, C., Dong, L. (2015). Getting insight into the influence of SO2 on TiO2/CeO2 for the selective catalytic reduction of NO by NH3. Appl. Catal., B 165, 589–598. https://doi.org/10.1016/j.apcatb.2014.10.029

  34. Zhang, S., You, J., Kennes, C., Cheng, Z., Ye, J., Chen, D., Chen, J., Wang, L. (2018). Current advances of VOCs degradation by bioelectrochemical systems: A review. Chem. Eng. J. 334, 2625–2637. https://doi.org/10.1016/j.cej.2017.11.014

  35. Zhang, S., Liu, S., Zhu, X., Yang, Y., Hu, W., Zhao, H., Qu, R., Zheng, C., Gao, X. (2019). Low temperature catalytic oxidation of propane over cobalt-cerium spinel oxides catalysts. Appl. Surf. Sci. 479, 1132–1140. https://doi.org/10.1016/j.apsusc.2019.02.118

  36. Zhao, B., Wang, S.X., Liu, H., Xu, J.Y., Fu, K., Klimont, Z., Hao, J.M., He, K.B., Cofala, J., Amann, M. (2013). NOx emissions in China: historical trends and future perspectives. Atmos. Chem. Phys. 13, 9869–9897. https://doi.org/10.5194/acp-13-9869-2013

Share this article with your colleagues 

 

Subscribe to our Newsletter 

Aerosol and Air Quality Research has published over 2,000 peer-reviewed articles. Enter your email address to receive latest updates and research articles to your inbox every second week.