Thermal oxidation of carbonmonoxide in air using various self-prepared catalysts

Use your smartphone to scan this QR code and download this article ABSTRACT Carbonmonoxide (CO) is a very toxic pollutant emitted fromwood fired boiler, which is widely used in small and medium enterprises in Vietnam. The treatment of CO containing flue gas faces many difficulties due to the inert property of CO that cannot be removed by traditional adsorption and absorption methods and one of the effective CO treatments is catalytic oxidation. Therefore, we aimed to prepare various catalysts on different carriers for treatment of CO in flue gas, including γAl2O3-based metal oxides (Co3O4/Al2O3 , Cr2O3/Al2O3 , and CuO/Al2O3), CuO–MnOx/OMS-2, and CuO-MnOx/zeolite. The CO removal tests were conducted in a continuous fixed bed reactor in laboratory scale with temperature range of 50 – 550 ◦C. The characteristics of catalytic materials were then determined by various methods such as Brunauer-Emmett-Teller measurement, X-ray diffraction, energy-dispersiveX-ray spectroscopy, Fourier transform infrared spectroscopy, scanning electronmicroscopy, and thermogravimetric analysis. Results showed that CuO-MnOx/OMS-2 was the best catalyst with high removal efficiency of 98.41% at reactor temperature of 250 ◦C while gas outlet temperature of < 50 ◦C, proving the suitability of this material for practical treatment of CO in flue gas. The reaction followsMars-Van-Krevelenmechanismwith the presence of Cu2+-O2−-Mn4+ ↔ Cu+-□-Mn3+ + O2 redox in the structure of the material. Moreover, the effect of environmental factors such as flow rate, inlet CO concentration, and catalysts amount on the CO removal efficiency were investigated and noted for designing and operation purposes. Concentration of outlet CO metwell QCVN 19: 2009/BTNMT National technical regulation on industrial emissions for dust and inorganic substances. Therefore, CuO-MnOx/OMS-2 catalyst material could be a potential catalyst for treatment of CO in flue gas of boiler.


INTRODUCTION
Every year, millions of tons of carbon monoxide (CO) emitted into the environment cause serious consequences for human health 1 . In addition to natural emission sources, carbon monoxide (CO) is also generated by incomplete combustion of carboncontaining substances in wood-fired boilers, waste incinerators and other processes. In Vietnam, many small and medium enterprises are using wood boilers as main heat energy source for their production process. Due to the limited budget and low technology, most of the wood boilers are cheap and inefficient and the incomplete combustion results in high concentration of CO in the flue gas. In order to meet the more and more strictly environmental regulations for emissions, several methods have been studied and some have been commercialized 2 . One of the most effective ways to treat CO is oxidation in the presence of a catalyst 3 . Noble metal containing catalysts such as Au/TiO 2 , Au/ZrO 2 and Pt/SnO 2 have been used for low temperature CO oxidation. However, due to the high cost and limited availability of noble and precious metals, the research group paid more attention to the goal of preparation and testing using popular metal catalysts for this application. In particular, Cu is widely mentioned because of its high activity for CO conversion at low temperatures 4 . Studies show that the catalytic activity of CuO depends on the oxidation state of Cu and especially on the nature of the carrier. A number of common carriers have been studied including CeO 2 , Al 2 O 3 , zeolite, and OMS-2. The increase in catalytic activity of Cu when carried on reducing oxides like CeO 2 is explained by the synergistic effect due to the good dispersion of CuO on CeO 2 lead to the possibility of reduction at lower temperatures than CuO 5 . Among new catalysts, octahedral molecular sieves (OMS) on the basis of manganese oxide which has a wide range of reactions, especially for oxidation reactions. Currently, there have not been many studies focusing on the treatment of CO in the waste biomass boiler (which has a temperature of about 250 • C) using catalysts that do not contain nobles metals. Therefore, in this study, the research group focused on the preparation of different catalytic materials and investigated the activity of these catalysts to apply for oxidation of CO in air at different temperatures. The effect of environmental factor such as inlet CO concentration, gas flowrate, and catalyst amount were also investigated.

Material synthesis and characterization
All chemical used are analytical-grade from China and Vietnam. Figures of all materials are displayed in Figure 1. Manganese oxide octahedral molecular sieves (OMS-2) were synthesized by a hydrothermal method 6 as follows: 11.33 g MnSO 4 .H 2 O dissolved in 120 mL of deionized water was added to a solution of 7.57 g KMnO 4 in 38 mL of deionized water and 4 mL of concentrated HNO 3 . The obtained mixture was transferred into a 165 mL Teflon-lined stainless steel autoclave and heated at 160 • C for 24 h. The product was then filtered, washed with deionized water and dried at 105 • C for 12 h. The material was obtained after calcination at 400 • C for 4 h and denoted as OMS-2. Catalyst of CuO-MnO x /OMS-2 with 15 wt.% of CuO-MnOx (Cu:Mn molar ratio of 6:4) was prepared by impregnation method. Calculated amounts of Cu(NO 3 ) 2 .3H 2 O and MnSO 4 .H 2 O were dissolved in deionized water and impregnated with OMS-2. The mixture was mixing and sonicated for 30 min, followed by stirring and heated at 80 • C for 15 min. After that, the material was dried at 105 • C for 12 h and finally ground and calcined at 400 • C for 4 h. CuO/Al 2 O 3 catalysts were prepared by impregnation method with copper nitrate aqueous solution of the desired concentrations 7 . At first, 2.82 g of Zn(NO 3 ) 2 .6H 2 O and 11.29 g of Al(OH) 3 were dissolved in distilled water. The mixture was then dried at 105 • C and calcination in air at 500 • C for 6 h. After that, CuO was impregnated by adding of 5.584 g Cu(NO 3 ) 2 into the mixture, followed by drying at 105 • C and calcination in air at 600 • C for 6 h.
were prepared by mixing a known mass of finely powdered Al(OH) 3 with a calculated amount of CrO 3 solid, followed by drying at 120 • C and calcination in air at 800 • C for 6 h 8 . The materials were characterized by Brunauer-Emmett-Teller (BET, Porous Materials, BET-202A), scanning electron microscopy (SEM) with energydispersive X-ray spectroscopy (EDS) (JEOL JSM-IT200), X-ray diffraction (XRD, D2 Phaser, Bruker),

Catalytic activity test
The micro-flow reaction system (Figure 2) consists of 3 parts: gas supply system, reaction system, and analysis system. Gas flow is supplied from clean gas pump and gas cylinder containing 5% CO balanced in N 2 . The CO gas is then diluted with clean air for reaching desire concentration of around 2000 ppm before feeding into the reaction. All gas flowrates were controlled by mass flow controller (MFC) with typical total flowrate was kept stably at around 1 L/min. Catalysts with amount of 0.4 or 1 g and particle size through a 20 -40 mesh sieve was placed inside the reactor to form a packed column inside the reaction tube. The catalyst was pre-activated in air stream at 250 • C for 30 min every first use of a new catalyst. All the experiments were conducted three times and the average values are presented in the manuscript. The concentration of CO in the inlet and outlet were continuously monitored by using a portable emissions analyzer (Testo 350 XL, Germany). The efficiency of CO treatment was then calculated as Equa-tion (1) and used as a criteria for evaluation the ability of the catalysts at different reaction temperatures.
Where H is the removal efficiency (%). C in and C out are the inlet and outlet CO concentration (ppm), respectively.

Material characterization
The BET surface area of catalysts are summarized in Table 1. It can be seen that surface area of OMS-2 support is 62.5 0 m 2 /g and that of CuO-MnOx/OMS-2 is 52.76 m 2 /g, which was slightly lower than that of OMS-2 support. The decrease in surface area of OMS-2 material after doping with copper oxidemanganese oxide mixture somehow demonstrates the dispersion of metal oxides on the surface of the support. SEM images of OMS-2 materials are presented in Figure 3. Both pure and doped OMS-2 materials exhibits nanowires morphology with diameters of about 13 nm and lengths of several µm. Figure 4 shows the XRD patterns of OMS-2 catalysts. The diffraction peaks at 2θ of 12.6 • , 17.9 • , 28.7 • , 37.5 • , 41.9 • , 49.9 • , and 60.1 • are attributed to the crystalline phase of cryptomelane (KMn 8 O 16 ), indicating that the nanowires OMS-2 materials has a cryptomelanetype structure 9 .   Thermogravimetric analysis of OMS-2 materials were also done, and the results of CuO CuO-MnOx/OMS-2 is presented in Figure 7. The weight loss of around 2% with temperature < 400 • C could be due to the water release while the loss at temperature range of 400 -700 • C could be the structural decomposition of the materials at high temperature.   % for CuO/Al 2 O 3 . In t he temperature range of 50 -250 • C, CO gas is inert and not converted to CO 2 . CO conversion started from 300 • C and increased at higher temperatures. For Cr 2 O 3 /Al 2 O 3 material as catalyst, CO conversion did not occur or was very limited even when increasing the reaction temperature. The highest performance at 400 • C temperature was only 11.62%. This proves that the catalytic activity of Cr 2 O 3 /Al 2 O 3 is very low and this material is not suitable for CO conversion in the exhaust gas.

Activity of catalysts based on OMS-2 and zeolite carrier
The ability of CuO/OMS-2, CuO-MnOx/OMS-2, and CuO-MnOx/zeolite catalysts for CO conversion are shown in Figure 9. Similar to Al 2 O 3 -based catalysts, CO conversion efficiencies by these catalysts was ineffective at 50 • C and increased with the reaction temperature. For CuO/OMS-2, the highest removal efficiency at 500 • C and reached 73.15%. At the temperature range of 50 -250 • C, low CO conversion of 6.44% was observed. And the conversion starts to be effective when the temperature was higher than 250 • C.
For CuO-MnOx/OMS-2 material, CO conversion increases with both the reaction temperature and the amount of catalyst. When more catalyst are used, CO conversion efficiency will be higher and more stable over time. With 0.4 g of material, the highest efficiency was 98.66% (at 600 • C). With 1 g of catalyst, the highest conversion efficiency was 99.96% only at 500 • C. Moreover, it is also observed from Figure 9 that, the conversion efficiency increases very fast at low temperatures from 50 -250 • C but the efficiency starts to slow down when temperature over 250 • C and become more stable. The results also proved that CuO-MnOx/OMS-2 materials was effective for CO removal and stable over time. On the octahedral molecular catalytic surface (OMS-2), the Mn-O bonds are relative weak, so that flexible surface oxygen atoms are able to participate in the reaction and restore to its original state when oxygen supply from gas flow is available. Compared to some other catalysts (e.g., Pt-catalytic conversion system reaches 50% at 290 • C 16 ), CuO-MnOx/OMS-2 has higher activity due to the combination of Mn and Cu can produce Hopcalite CuMn 2 O 4 with high oxidizing activity for CO removal. For CuO-MnOx/zeolite catalyst, the highest efficiency was at a temperature of 500 • C and reached 96.92%. In the temperature range of 50 -350 • C, CO conversion did not occurred. The conversion of CO starts to be effective at 400 • C but from this temperature or above, the ability of CO conversion increases slowly and stably over time. In the contrary, the treatment efficiency increased very rapidly from 0 to 94.63% in temperature range of 350 -450 • C. It is obvious that CuO-MnOx/OMS-2 had the highest removal efficiency of 98.41% at low temperature of 250 • C. Therefore, it was chosen as catalyst for further experiments.
In order to clarify the effect of CuO and MnOx on the OMS-2 support, the comparison experiments were conducted using OMS-2, CuO/OMS-2, and CuO-MnO x /OMS-2 materials. As seen in Figure 10, CO removal increases with the increase of temperature. For OMS-2, CO gas was relative inert at low temperature of 50 -100 • C while it started to be oxidized to CO 2 at temperature of 150 • C. This can be explained by the low activity of OMS-2 support without the presence of copper, which only reached the removal efficiency of 31.89% at 200 • C. Compared to OMS-2, CuO/OMS-2 and CuO-MnOx/OMS-2 had higher catalytic activity due to the doping of copper with Cu 2+ -O 2− -Mn 4+ bonds on the support surface. At catalyst surface, CO combines with oxygen in Cu 2+ -O 2− -Mn 4+ to form CO 2 and leaves an oxygen vacancy (Cu + -o -Mn 3+ ). These oxygen vacancies were then instantly occupied by free oxygen in the air thus act as active sites for reaction of CO 17 . Accordingly, the reaction of CO is continuously occurred on the surface of the catalyst due to the pres- The oxidation of CO to form CO 2 follows Mars-Van-Krevelen mechanism 6 . Figure 11 demonstrates the effect of calcination temperature on the activity of CuO-MnO x /OMS-2 material for CO oxidation. One can see that the catalytic activity gradually decreases with the increase of calcination temperature from 400 to 600 • C but significantly decreases with further increase of temperature to 700 and 800 • C. Thus, the activity of CuO-MnO x /OMS-2 depends on calcination temperature   Figures 12, 13, 14 and 15 illustrates the effect of operational condition on the CO removal using CuO-MnOx/OMS-2 material. In Figure 12, the CO removal efficiency increases with the increase of CO concentration from 526 to 2300 ppm, where it reaches maximum efficiency of 98.41%. This could be explained by the gas bulk mass transfer of CO increase in this low inlet concentration range. However, further increase of CO concentration slightly decreases its removal efficiency, possibly due to the limitation of catalyst surface active sites for CO adsorption and reaction. This should be noted when designing a catalytic system for practical application where CO concentration in flue gas fluctuates from 200 to 5000 ppm. The lower inlet concentration of CO could result in lower removal efficiency while the outlet concentration is required to meet emission standard (QCVN 19: 2009/BTNMT). In actual small wood boilers, the flowrate of flue gas usually fluctuates due to the variation in steam amount need of the production process. Figure 13 presents the effect of gas flowrate on the removal efficiency of CO. It is obvious that the removal efficiency continuously decreases with the increase of flowrate.

Effect of other environmental factors
The reason is mostly based on the gas retention time, where higher flowrate means shorter retention time for CO reaction on the catalyst surface. This result also indicates that if a designed catalyst system works well with the highest flue gas flowrate, it could even work better under lower flowrate condition. In order to clarify the effect of both flowrate and concentration on the removal ability of CO, the constant amount of CO was maintained while both flowrate and concentration was changed. The result is displayed in Figure 14 for flowrate range of 0.5 to 1.0 L/min and CO range of 4132 to 2045 ppm. It can be observed that the removal efficiency is more dependent on the flowrate than on the concentration in the investigated range. This implies that the gas retention time should be a more important designing parameter than the operational concentration of CO concentration in the practical application. The amount of catalyst used in a practical system is important design criteria. Too much catalyst would take more cost and high pressure drop while too less catalyst result in low efficiency. The effect of catalyst amount on CO removal is demonstrated in Figure 15.
It is obvious that the efficiency increases from 64.6 to 99.7% with the catalyst amount from 0.25 to 1.25 g due to the more active surface available for CO adsorption and reaction. However, the linear increase of efficiency was not observed for whole investigated range. The amount of 1 g could be the most suitable with the efficiency of 98.4% while 1.25 g is an excess amount with inefficient use by CO concentration of 2300 ppm and gas flowrate of 1 L/min.  Among them, CuO-MnOx/OMS-2 catalyst material had the highest efficiency at 250 • C and stable activity during reaction time. The most suitable operational condition was found to be inlet CO concentration of 2300 ppm CO, gas flowrate of 1 L/min, and catalyst amount of 1 g. These result shows a potential use of CuO-MnOx/OMS-2 material as an effective catalyst for treatment of CO-containing flue gas in practical application.