Catalytic Performance of NO Reduction by CO over Activated

Article pubs.acs.org/IECR

Catalytic Performance of NO Reduction by CO over Activated Semicoke Supported Fe/Co Catalysts Xingxing Cheng,† Luyuan Wang,† Zhiqiang Wang,*,† Mengze Zhang,‡ and Chunyuan Ma† †

National Engineering Lab of Coal-Fired Pollution Emission Reduction, School of Energy and Power Engineering, Shandong University, Jinan 250061, China ‡ Shandong Shenhua Shanda Energy & Environment Co., Ltd., Jinan 250014, China ABSTRACT: Performance of NO reduction by CO is studied over activated semicoke supported Fe/Co catalysts (FeCo/ ASC). The influence of oxygen and CO concentration is investigated systematically. It is found that excess CO, with a CO:NO ratio greater than 2:1, could significantly enhance NO reduction, but oxygen, even with a concentration less than 1%, could strongly inhibit NO conversion efficiency due to the oxidation of active surface metal sites. To explore the role of CO in NO reduction, reactions without CO addition are also conducted. By comparing NO reduction behavior in the presence and absence of CO, it is found that carbon support is the main reducing agent if oxygen is in great excess, even when CO is provided. At the same time, NO reduction efficiency increases at higher oxygen concentration due to the increase of surface oxygen complex. However, excess oxygen is strongly undesired because serious gasification of carbon support is inevitable. But if oxygen concentration is not very high, CO could be fed in excess to consume oxygen and excellent NO reduction could be achieved. Possible reactions over the FeCo/ASC catalyst are summarized and the onset temperatures of each reaction are also listed to figure out possible reaction condition for NO reduction. It is observed that a temperature around 200 to 250 °C is favorable for NO reduction by CO in the presence of oxygen because reactions relevant to CO are already active but reactions for carbon support gasification are not activated yet. A reaction mechanism is also proposed, providing some insights on the complex catalytic reactions among NO, O2, CO, and carbon.

1. INTRODUCTION Growing environmental awareness in recent years has resulted in the introduction of more rigorous and stringent environmental laws and regulations. NOx is considered as one of the primary pollutants of the atmosphere because it is responsible for the formation of photochemical smog, acid rain, ground level ozone,1 and the destruction of the stratosphere ozone layer. Carbon materials, either in the presence or absence of metal addition, are now widely used for the NOx treatment process, either in laboratories2 or in real applications.3 There are four categories of NOx treatment technologies over carbon materials. (1) Carbon materials, especially activated carbon, could be used as a sorbent for NO adsorption from the flue gas.4 (2) Direct NO decomposition could also be achieved over carbon catalysts.5 (3) Carbon support itself could react with NO to produce N2, CO, and CO2.6−11 (4) The most extensively studied and used process is NO reduction by a reducing agent over carbon supported catalysts, where NH3 is the most often used reducing agent.12−17 These carbonaceous catalysts were reported to be active at low temperatures (usually 150−300 °C)18 and have excellent resistance to SO2 poisoning.19 However, it would be of significant interest for the © XXXX American Chemical Society

industry if CO, as an alternative of NH3, could serve as a reducing agent for the catalytic reduction of NO over carbon catalyst because CO is much cheaper and is often a product or byproduct of the carbon related processes. There are some researchers investigating the reaction of NO and CO over carbon supported catalysts. Mehandjiev20 investigated the interaction of NO with CO over cobalt oxide supported on activated carbon (Co/AC) in the absence of oxygen at 100−550 °C. It was established that the prepared catalyst is active for NO reduction. At lower temperatures, N2O was the main product of CO−NO reaction, whereas the selectivity of N2 increases at higher temperatures. In later research by Mehandjiev,21 NO reaction over the Co/AC catalyst was tested without a gaseous reducing agent. NO decomposition into N2 was found to be active below 200 °C. At above 200 °C, NO reacts directly with the carbon support, catalyzed by cobalt oxide. In Stegenga’s research,22 a carbon supported copper−chromium catalyst was used as the catalyst Received: Revised: Accepted: Published: A

February 29, 2016 October 31, 2016 November 28, 2016 November 28, 2016 DOI: 10.1021/acs.iecr.6b00804 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Catalytic performance at different oxygen concentrations. (a) NO conversion; (b) outlet N2O concentration; (c) CO conversion; (d) carbon consumption. (Reaction conditions: initial NO = 1000 ppm, initial CO = 1000 ppm, GHSV = 20 000 h−1; stated error bars are exemplary.)

reactions include NO reduction by CO, NO reduction by carbon support, CO oxidation by oxygen and carbon support oxidation by oxygen. The reaction pathway and mechanisms will be proposed based on the systematic data. The onset temperatures of different reactions will also be compared, seeking a feasible reaction scheme for industrial applications.

and it was illustrated that the prepared catalyst was active for both NO reduction with CO and CO oxidation with O2. But O2 completely inhibited the NO reduction. A similar conclusion was also made for carbon supported copper catalysts.23 Recently, Rosas24,25 investigated NO reduction over activated carbon catalyst with and without chromium impregnated. The presence of chromium produced a considerable improvement of NO reduction compared to the performance over the carbon support. It was also found that addition of CO could considerably increase NO reduction over Cr/AC. But the main focus is the reduction of NO by activated carbon. CO was only considered as a promoting gas and is not systematically tested as a reducing agent. Other metals, such as potassium in Lopez and Calo’s work,26 also possesses an enhancing effect for the reaction of NO with carbon. Lopez and Calo explained this effect by an increasing number of reaction sites via the catalyst dispersion. CO was also added to the flue gas and it was found that the additional CO “catalyzes” NO reduction by the carbon support via the creation of labile surface complexes and facilitation of desorption of other oxygen complexes. The enhancement of CO on NO−carbon reaction was also observed by Aarna.9 In this paper, performance of CO as a reducing agent for NO reduction over carbonaceous catalysts will be systematically examined. Activated semicoke made from coal will be used as the carbon support. It has a higher mechanical strength than activated carbon, and is much cheaper and more easily available in industry, especially in steel factories or coal-fired power plants. Transition metals, ion and cobalt, are impregnated on the activated semicoke, expecting to promote the NO reduction activity. Reactions among different oxidants and reductants will be investigated in the presence and absence of oxygen. These

2. EXPERIMENTAL METHODOLOGY Commercial semicoke, with a density of 640 kg/m3, was used as the catalyst support. The semicoke was crushed into particles with an average diameter of 1.2−1.6 mm and then activated in an acid solution. After being immersed in 50 mL HNO3 (40 wt %) solution at 80 °C for 2 h, the particles (10 g) were then washed with deionized water, dried at 110 °C for 24 h, and calcinated in Ar at 700 °C for 4 h. Transitions metals, Fe and Co, were then loaded onto the activated semicoke (ASC) by a hydrothermal method. Fraction of metal loading was chosen after a preliminary selection. 3 g of ASC was immersed into 30 mL solution containing 12.51 g of Fe(NO3)3·9H2O and 9.02 g of Co(NO3)3·6H2O. The mixture was then enclosed in an autoclave and heated at 160 °C for 24 h. After that, the activated coke particles were washed by deionized water, and then dried at 120 °C for 5 h, followed by calcination in Ar at 500 °C for 4 h. The obtained catalyst has a density of 810 kg/m3, a metal content of 2.13% Fe and 1.69% Co acquired by ICP, and a surface atomic ratio of 0.89% Fe and 0.78% Co acquired by X-ray photoelectron spectroscopy. The morphologies of catalysts were observed by scanning electron microscopy (SEM) operated at 10 kV (Hitachi S4800). The specific surface area was investigated by nitrogen adsorption at 77 K (Quantachrome Autosorb 1C) using the BET method. The pore size distribution was derived from the B

DOI: 10.1021/acs.iecr.6b00804 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Table 1. Summary of Apparent Reactions over FeCo/ASC Number

Reaction

Desired (√) or undesired (×)

Activity

Reaction description

Onset temperature (°C)

R1

2NO → N2 + O2

√√

R2

2NO + CO → N2O + CO2

√√√

***

NO reduction by CO to produce N2O

150−200

R3

NO + CO → 1/2N2 + CO2

√√√√

***

NO reduction by CO to produce N2

∼200

R4

N2O + CO → N2 + CO2

√√√

***

N2O reduction by CO

∼200

R5

CO + 1/2O2 → CO2

××

*****

CO oxidation

∼150

R6

C + 1/2O2 → CO

××××

****

Carbon support oxidation to produce CO

∼250

R7

C + O2 → CO2

×××

****

Carbon support oxidation to produce CO2

∼250

×

**

NO reduction by carbon to produce CO

250−300

R9

1 NO + C → N2 + CO 2 2NO + C → N2 + CO2

√

**

NO reduction by carbon to produce CO2

250−300

R10

2NO + C → N2O + CO

√

*

NO reduction by carbon to produce N2O

250−300

R8

Direct NO decomposition

the reaction of NO and CO, carbon support gasification is also very important especially in the presence of oxygen. The intensity of carbon support gasification is evaluated by carbon consumption and plotted in Figure 1d. The carbon consumption is represented by the effluent concentrations of carbon-containing gases, CO and CO2, which are specifically released from carbon support gasification. The calculation of carbon consumption is shown as eq 3.

BJH method. The crystal structures were analyzed by using a powder X-ray diffractometer (XRD, Rigaku Dmax/2400) with Cu Kα radiation at a scanning rate of 8°/min over the 2θ range of 10−80°. The surface atomic states of the catalysts were studied by using X-ray photoelectron spectroscopy (XPS, AXIS ULTRADLD) with Al Kα radiation (hv = 1486.6 eV) at 150 W. The TGA experiments were performed using a Mettler Toledo TGA/DSC1 thermo analyzer under pure nitrogen or oxidative atmosphere. The catalyst samples were mortared and sieved into particles with diameters of less than 200 μm. The samples (10 mg) were heated from 25 to 900 °C at a heating rate of 20 °C/min. Catalytic activity of the prepared FeCo/ASC was investigated in a fixed bed reactor system, which consists of a stainless steel tubular reactor (12.7 mm I.D.), a gas supply and flow rate control unit, a gas heating unit (furnace), a gas analysis unit (FTIR flue gas analyzer, GASMET DX4000), and a data acquisition system. The model gas used in the experiment was a mixture prepared from several gas cylinders: 21% O2 balanced with N2, 0.6% NO balanced with N2, 1% CO balanced with N2, and pure N2 gas cylinders from Deyang Gas Inc. Total flow rate of gas mixture entering the reactor was kept at 500 mL/min. The bulk volume of the catalysts loaded was 1.5 mL to maintain a GHSV of 20 000 h−1. A blank reactor test was run before the experiments, and it was confirmed that the steel reactor did not contribute to the investigated reactions. During the activity test, each data point was taken 30 min after the outlet concentrations become stable. 3−5 runs were performed for each reaction condition with error bars provided for some data to show exemplary error ranges.

NO conversion = 100% −

Outlet NO + Outlet NO2 Initial NO

(1)

CO conversion = 100% −

Outlet CO Initial CO

(2)

Carbon consumption = Outlet CO + Outlet CO2 − Initial CO

(3)

The performance of NO reduction over FeCo/ASC is first discussed for the cases in the absence of oxygen. It could be seen from Figure 1a that NO conversion increases with temperature. At 150 °C, NO could be barely reduced. But when the temperature is increased to 200 °C and above, NO conversion increases significantly. Conversion of about 78.5% is achieved at 300 °C. Possible reactions over the prepared catalyst are summarized in Table 1. In the absence of oxygen, only NO, CO, and carbon support are involved in the NO related reaction. Possible reactions include NO direct decomposition,21 NO reduction by CO,24 and NO reduction by carbon support.5 It is commonly considered that NO reduction by carbon is far less active than CO−NO reaction.23 From Figure 1d, it could be concluded that the same amount of CO2 is produced when CO is consumed because carbon consumption is zero for all the tested oxygen-free conditions, implying that carbon support only played a role of catalyst and does not serve as a reducing agent. Therefore, possible reactions for the present cases are only NO direct decomposition to produce N2, represented by reaction R1, as shown in Table 1, and NO reduction by CO to produce N2O or N2, following reactions R2 and R3 (in Table 1). To evaluate the performance of NO direct decomposition over the prepared catalyst, NO reduction is also tested in the absence of CO and oxygen. In this additional test (results not shown here), little NO conversion is detected below 250 °C. At higher temperatures, it is found that the converted NO could well balance produced CO2 and CO following a NO−carbon reaction, implying that the converted NO is reduced by carbon rather than catalytically decomposed. Therefore, direct NO

3. RESULTS AND DISCUSSION 3.1. Influence of Oxygen Concentration. The NO reduction performance over FeCo/ASC catalyst is investigated in a temperature range of 150−350 °C. The influence of oxygen concentration in the flue gas is first studied. CO is fed in stoichiometrics with NO, e.g., CO:NO = 1:1. To evaluate the NO reduction and CO oxidation behavior, NO conversions and CO conversions are calculated following eqs 1 and 2 and then plotted in Figure 1. Very little NO2 was detected in the effluent gas stream. For NO reduction, it is observed in the experiments that NO could be reduced to either N2O or N2. Thus, N2O concentration in the outlet is also measured and presented in Figure 1b to illustrate the selectivity of NO reduction. Besides C


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Figure 2. Catalytic performance at different CO concentrations in the absence of oxygen. (a) NO conversion; (b) outlet N2O concentration; (c) CO conversion. (Reaction conditions: Initial NO = 1000 ppm, initial O2 = 0, GHSV = 20 000 h−1.)

to CO oxidation by reaction R5. At 150 °C, CO conversions are 23.6−48.3% at the oxygen concentration of 0.1−1%. Most of the converted CO is oxidized by oxygen instead of NO whose conversion remains almost zero as indicated in Figure 1a. Thus, the prepared FeCo/ASC catalyst could effectively catalyze CO−O2 reaction at a temperature as low as 150 °C. Similar performance is also found for carbon supported copper−chromium catalyst.22 Above 200 °C, CO could be totally oxidized to CO2. As shown in Figure 1a, no NO conversion is observed at 200 °C when oxygen is presented. It could be expected that the selectivity of CO oxidation by oxygen, R5, is much bigger than the selectivity of CO oxidation by NO, R2 or R3. At all the investigated initial oxygen concentration levels, the NO conversions are much smaller in the presence of oxygen than the conversions in the absence of oxygen. At above 300 °C, only about 40% NO conversion could be reached in the presence of oxygen, comparing to about 80% to 100% NO conversion in the absence of oxygen. The oxygen inhibition effect, caused by reducing agent consumption, is a well-known problem for technologies of catalytic NO reduction by CO27 or hydrocarbon28 over various catalysts. NO conversion always decreases significantly and continuously when oxygen concentration is increased.28 But in Figure 1a, NO conversions at different oxygen concentrations are similar, which is quite different with the trends obtained over other catalysts. It is interpreted that carbon support begins to serve as a reducing agent, represented by reactions R8, R9,9 or R10 (Table 1), after CO is totally consumed by oxygen. But the conversions in the presence of oxygen are much lower because NO−carbon reaction is much less active than NO−CO reaction. At 200 °C, NO conversion is 47.68% in the absence of oxygen and zero in

decomposition seems unlikely for the present experiments and reaction R1 could be neglected over the investigated catalyst. Either N2 or N2O could be produced from NO reduction, as illustrated by R2 and R3. There is also some debate over the role of N2O in the NO−CO reaction.20,22 One suggestion is that reaction R2 is a parallel reaction of R3. Another one is that N2O, followed by a further reduction step (R4) (Table 1), is an intermediate in the overall reduction of NO to N2. In the present study, only apparent reactions are discussed. N2O is considered as a parallel product along with N2 and reaction R4 will not be accounted. N2O concentrations in the experiments are measured and plotted in Figure 1b. It could be seen that significant N2O is produced at 200 and 250 °C. At 150 °C, NO conversion is only 1.16% and the production of either N2 or N2O is very low. At above 300 °C, higher conversion and reduction efficiency is expected and NO tends to be further converted into N2. Similar trend was also observed by Mehandjiev.20 Considering the amount of converted NO and produced N2O, selectivity of N2O is calculated according to eq 4 to be 89.3% and 35.4% at 200 and 250 °C, respectively. Above the temperature range of 200−250 °C, selectivity of N2 could achieve about 100%. The selectivity could also be confirmed by the mass balance of converted CO shown in Figure 1c. N2O selectivity = 2 × Outlet N2O/Reduced NO

(4)

In the presence of oxygen in the flue gas, there are more possible reactions over the catalyst, such as CO oxidation as reaction R5 (Table 1), and oxidation of the carbon supported to produce CO or CO2, represented by reactions R6 or R7 (Table 1). From Figure 1c, CO conversion in the presence of oxygen is much higher than that in the absence of oxygen, due D


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Figure 3. Catalytic performance at different CO concentrations and at O2 = 0.1%. (a) NO conversion; (b) CO conversion. (Reaction conditions: initial NO = 1000 ppm, initial O2 = 0.1%, GHSV = 20 000 h−1.)

reduction performance in excess CO. Figure 2 shows the experimental results of NO conversion, N2O production and CO conversion at different CO concentrations and in the absence of oxygen. In Figure 2a, it could be seen that the NO reduction performance over FeCo/ASC catalyst is enhanced when CO is fed in excess. When initial CO:NO is 2:1, NO conversion of 78.5% could be achieve at 250 °C. Further increasing the initial CO concentration could further increase the NO reduction performance, though the improvement is not very significant, possibly due to the saturation of active sites. In Figure 2b, the profiles of N2O at different inlet CO concentrations show similar trends. At 150 °C, N2O produced is less than 100 ppm at different CO concentrations due to small NO conversion. Maximum N2O is produced at 200 °C. Then, the outlet N2O concentrations decrease at higher temperatures because NO is further reduced to N2. The selectivity of NO reduction into N2O is further calculated according to eq 4 and plotted in the inset of Figure 2b. It could be seen that the selectivity into N2O decreases dramatically with temperature, from about 100% at 150 °C to around zero at 300 °C. The profiles of selectivity into N2O at different CO concentrations are similar, implying that the selectivity is not affected significantly by CO concentration. But higher CO concentration could somehow enhance the NO reduction into N2 because more reducing agent is available for the further reduction. Similar with NO conversion, CO oxidation in Figure 2c increases with temperature in the absence of oxygen due to NO−CO reaction. Oxidized CO could well balance reduced NO at different CO concentrations, implying no carbon deposition on the catalyst surface even when CO is fed in excess. Figure 3a illustrates the influence of CO concentration on NO reduction for oxygen-containing flue gas. The initial oxygen concentration is set as 0.1%. At initial CO = 1000 ppm, little NO conversion could be observed below 250 °C. A maximum of 37.6% conversion could be arrived at 350 °C. The flue gas is oxidizing because the molar fraction of oxidants is bigger than the fraction of reducing component (2 × O2 + NO > CO). At initial CO = 2000 ppm, NO conversion increases significantly because the flue gas is less oxidizing. The best NO reduction performance is observed for the cases at initial CO = 5000 ppm. In these cases the flue gas is reducing (2 × O2 + NO < CO and CO2 × O2 = 3000 ppm). But compared with NO conversions at CO = 2000 ppm and O2 = 0 in Figure 2a, NO conversions in the presence of oxygen are still lower though

the presence of oxygen. It is suggested that the onset temperature of NO−carbon reaction is higher than NO−CO and CO−O2 reactions. In Figure 1b, the N2O production in the presence of oxygen is much smaller than its production in the absence of oxygen because NO conversion is much lower. The profile of N2O concentration at O2 = 0.1% is similar to the profile for oxygenfree conditions, but with much smaller values. Peak of the curve shows up around 200−250 °C, where reaction R2 is considered to be predominant. Then at higher temperatures, little N2O could be detected although NO conversion increases significantly. Because the N2O production behavior at O2 = 0.1% is very similar to the behavior at oxygen-free conditions, it is assumed that NO−CO reaction is predominant. When initial oxygen concentration is further increased, behavior of N2O production changes a lot. N2O is mainly produced at higher temperatures in the presence of higher oxygen concentration. The maximum production of N2O shows up at around 300 °C. At 200−250 °C, when a large amount of N2O is produced at the oxygen-free conditions, little N2O is produced for the oxygen-containing conditions because NO conversion is very low. Then the outlet N2O concentration increases with temperature due to more NO reduced. But at 350 °C, N2O concentration decreases because more NO tends to be further reduced into N2. The peak of N2O production implies a higher onset temperature of NO reduction, in accordance with the higher onset temperature of NO−carbon reaction. The outlet N2O concentration increases with increased oxygen concentration. The trend is similar to NO conversions, both due to more abundant surface oxygen complex. Another important aspect of reactions over FeCo/ASC catalyst is the carbon support oxidation when oxygen is presented in the flue gas, represented by reactions R6 and R7. Because outlet CO concentration is always lower than inlet concentration, it could be assumed that CO produced from reaction R6 is instantly oxidized into CO2 or reaction R7 is predominant for the carbon support oxidation. From the carbon consumption data shown in Figure 1d, no extra carboncontaining gas, CO or CO2, is produced below 250 °C when oxygen is present in the flue gas. Thus, reactions involving carbon support are not active below 250 °C. But above 300 °C, significant amount of CO2 is produced and carbon support consumption becomes a serious problem for oxygen-rich conditions. 3.2. Influence of CO Concentration. Initial CO concentration is further changed to investigate the NO E


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anticipated that the onset temperature of NO−carbon reaction is around 250−300 °C and is higher than NO−CO reaction. N2O concentrations are observed to be very small as shown in Figure 5b, with values of less than 20 ppm for most investigated conditions. In the absence of oxygen, only 17 ppm of N2O could be detected at 250 °C. Considering the small NO conversion values for these conditions, selectivity toward N2O is expected to be higher than the selectivity toward N2. But at higher temperatures, detected N2O concentrations decrease sharply, presenting a higher selectivity toward N2, which is similar to NO−CO reaction. In the presence of oxygen, most of the detected N2O values are below 20 ppm, which is negligible considering the error range (±10 ppm). It is implied that oxygen could not only promote NO−carbon reaction but also enhance the selectivity toward N2. The outlet concentrations of produced CO2 and CO are presented in Figure 5c,d, respectively, to evaluate carbon support gasification. The trends of CO2 production profiles are similar to the NO conversion profiles. Very little CO2 is produced below 250 °C at different initial oxygen concentrations, indicating an onset temperature of about 250 °C for the carbon−oxygen reaction, which is similar to the NO− carbon reaction. Therefore, in the presence of oxygen, carbon support combusted by oxygen is inevitable when NO reduction by carbon is active. Moreover, significant carbon support could be consumed by combustion if oxygen is in excess. As illustrated in Figure 5c, the produced CO2 could almost balance inlet oxygen above 300 °C. Therefore, the prepared carbon material could be effectively used for NO reduction in the absence of CO only when oxygen concentration in the flue gas is relatively low. Otherwise, if oxygen is absent NO reduction efficiency will be very low, and if oxygen concentration is high carbon support consumption will be significant. In Figure 4d, it could be seen that there is no CO detected in the effluent gas until initial oxygen concentration is increased to be above 5%. For most cases, CO could be rapidly oxidized into CO2 by catalytic reaction,22 showing no CO in the outlet gas stream. But when initial oxygen concentration is higher, more CO will be produced, saturating the active metal site on the catalyst. Thus, the intermediate CO could not be fully oxidized and is detected in the outlet. 3.4. Catalyst Deactivation. The deactivation behavior is also invesigated. Sulfur and water effect are not considered in this paper. There are two possible sources of catalyst deactivation for the CO−NO reaction over ASC catalysts, carbon deposition from excess reducing agent and carbon loss due to carbon support gasification.5 To test the carbon deposition effect, NO reduction is performed for 300 min at an excess CO feeding, CO = 5000 ppm. The obtained conversions and N2 selectivity, as plotted in Figure 6a, are constant after 300 min run, implying no carbon deposition. Excess CO feeding into the reactor will not contribute to the catalyst deactivation. The other deactivation source, carbon support loss, is more important for the oxygen-containing conditions. The carbon loss is first evaluated by thermogravimetric analysis (TGA), and the results are shown in Figure 6b. It could be seen that strong carbon loss takes place at temperatures above 300 °C (at O2 = 10%). The onset tempeperature is higher when oxygen concentration is lower (420 °C at O2 = 5%). The weight loss is minor if oxygen is absent. In order to examine the effect of support loss, catalyst sample is gasified in oxygen-rich flue gas and the NO reduction performance is tested before and after

CO is in excess than oxidants. Thus, oxygen still inhibits NO reduction even though excess CO could fully consume the presented oxygen. This implies that the inhibition of oxygen is due the oxidation of metal sites instead of CO consumption because only reduced metal sites are considered to be effective for NO reduction.29,30 For CO conversions in Figure 3b, it could be seen that CO could be rapidly consumed if oxygen is above stoichiometrics. At above 200 °C, 100% conversion could be achieve for CO = 1000 ppm and CO = 2000 ppm. Even at 150 °C, significant amount of CO could be oxidized. For the cases of CO = 5000 ppm, the converted CO could well balance converted NO and initial oxygen, indicating that effective NO−CO reaction could only be achieved after oxygen is totally consumed. The initial oxygen concentration is further increased to 1% to investigate of influence of CO concentration when oxygen is in great excess, with the obtained NO conversions plotted in Figure 4. It could be seen that the profiles of NO conversion

Figure 4. NO conversion at different CO concentrations and at O2 = 1%. (Reaction conditions: initial NO = 1000 ppm, initial O2 = 1%, GHSV = 20 000 h−1.)

are similar at different CO concentrations. It is expected that CO is rapidly consumed by oxygen. Thus, the role of CO is not important for the NO reduction and NO−carbon reaction would be much more important than NO−CO reaction for these cases. 3.3. NO Reduction without CO Addition. NO reduction without the addition of reduction agent CO is also investigated in order to evaluate the performance and role of NO−carbon reaction among the reactions listed as R1−R10. The results are presented in Figure 5. In Figure 5a, it could be seen that NO conversion in the absence of oxygen is very low, implying an essential role of oxygen in the NO−carbon reaction. Little NO conversion could be observed below 300 °C. Only 10.81% NO conversion could be obtained at 300 °C. NO conversion increases significantly with initial oxygen concentration. The presence of oxygen could significantly improve NO reduction by carbon due to the enhancement effect of surface oxygen complex. At initial O2 = 1%, the obtained profile and values of NO conversion in Figure 5a are similar to the NO conversions in Figure 4 in the presence of 1% oxygen while CO concentration is varied. It could be concluded that the contribution of CO for the NO reduction is minor when oxygen is in excess. Thus, reaction R9 is predominant. At O2 = 8%, more than 95% NO conversion could be achieved at above 300 °C. However, less than 20% NO conversion could be obtained below 250 °C at different oxygen concentrations, indicating that NO−carbon reaction, although promoted by oxygen, is not active at temperatures below 250 °C. Thus, it is F


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Figure 5. Reaction performance without CO addition at different oxygen concentrations. (a) NO conversion; (b) outlet N2O concentration; (c) outlet CO2 concentration; (d) outlet CO concentration. (Reaction conditions: initial NO = 1000 ppm, GHSV = 20 000 h−1; stated error bars are exemplary.)

Figure 6. (a) Deactivation of catalytic performance, reaction conditions and catalysts: (A), CO = 5000 ppm, O2 = 0, T = 200 °C, fresh catalyst; (B) CO = 5000 ppm, O2 = 0, T = 200 °C, catalyst after 300 min run; (C) CO = 2000 ppm, O2 = 0, T = 250 °C, fresh catalyst; (D) CO = 2000 ppm, O2 = 0, T = 250 °C, catalyst after gasification for 30 min at O = 8% and T = 300 °C. (b) TG curve of FeCo/ASC at different oxygen concentrations.

gasification and pore volumes increase, confirming the observations of SEM images. Therefore, the NO reduction efficiency loss after gasification is not due to the surface area or pore volume change. XPS and XRD tests are then investigated for the information on surface chemicals and crystal phase change. The XPS diagrams of the samples, both as-synthesized and after gasification, are presented in Figure 9a. It could be seen that the peak of carbon (C 1s) decreased after gasification, due to carbon loss. But surface oxygen (O 1s) increased because oxygen compounds could be produced during the surface oxidation. At the same time, both Fe and Co peaks (Co 2p and Fe 2p) increased significantly. On the catalyst, fraction of metals increases whereas carbon fraction decreases, indicating

the gasification. The results are also shown in Figure 6a. It could be seen that after carbon support gasification the NO and CO conversion decreases by 27.92% and 15.75%, respectively. Moreover, N2 selectivity decreases by 29.77%, indicating that carbon support gasification strongly affect the further reduction of N2O into N2. SEM images, as shown in Figure 7, are taken for both the catalysts freshly synthesized and after gasification. Surface of carbon suppport is eroded after gasification. It is expected that big pores are opened due the carbon loss. Further nitrogen adsorption test shows that the BET suface area of the catalyst increased from 211.3 m2/g (as-synthesized) to 300.4 m2/g (after gasification). The BJH pore size distributions are shown in Figure 8. It is quite clear that more pores are opened after G


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Figure 7. SEM micrograph of the catalysts, (a) as synthesized, (b) after gasification for 30 min at O = 8% and T = 300 °C.

that more metal particles show up when carbon support is consumed. Although the number of active metal sites seems to increase, it is expected that the connection between metals and carbon supported is weakened. Because the NO reduction efficiency decreases a lot after gasification, it could be concluded that the catalytic activity of NO reduction not only depends on the number of active metal sites but also the interaction between metals and carbon support. The detail XPS spectra of Fe 2p, Co 2p, and O 1s are shown in Figure 9b−d. It could be seen from Fe 2p3/2 spectra that the peak shifted to higher binding energy after gasification (from 711.3 to 712 eV), indicating that more Fe is oxidized and shifted to a higher valence state. However, more Co is observed to process a lower valence state after gasification as the Co 2p3/2 peaks shifted from 781.5 to 780 eV. The O 1s peaks are deconvoluted into three peaks and the fractions of different oxygen species are

Figure 8. BJH pore size distribution of the catalysts, (a) as synthesized, (b) after gasification for 30 min at O = 8% and T = 300 °C.

Figure 9. XPS spectra of the catalysts as synthesized and after gasification for 30 min at O = 8% and T = 300 °C. (a) Survey scan XPS spectra; (b) detail XPS spectra of Fe 2p; (c) detail XPS spectra of Co 2p; (d) detail XPS spectra of O 1s and fitted spectra. H


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peaks of metal oxides are very small, implying a good distribution of small metal particles on the catalyst surface. After gasification, more metal oxide peaks appear and the signal intensity is much stronger. Thus, it could be expected that carbon loss helps to aggregate loaded metals and form bigger metal oxide clusters, leading to worse metal distribution over the catalyst surface. Similar with the discussion of XPS results, connection between active metal sites and carbon support could be weakened due to the aggregation of metal oxide clusters, which finally results in worse NO reduction performance. 3.5. Summary of NO Reduction Behavior over FeCo/ ASC Catalysts. Reactions over the prepared FeCo/ASC catalyst are complex even though detailed reaction pathways are not considered. Not only CO but also carbon support could serve as reductant, whereas oxidants are NO and oxygen. Possible apparent reactions over the catalyst are summarized in Table 1, including NO decomposition (R1), NO reduction by CO (R2, R3, and R4), NO reduction by carbon support (R8 and R9), CO oxidation (R5), and carbon support combustion (R6 and R7). Among these reactions, NO removal by either reduction or decomposition is the purpose of the NOx treatment process. But NO reduction by carbon support is not desired due to catalyst support consumption. Further NO reduction will be inhibited by both carbon loss and worse metal−carbon interaction. NO decomposition, although desired and having no catalyst consumption, is observed to

calculated based on the peak area. O′ at around 533.3 eV and O″ at around 531.3 eV are considered as the adsorbed oxygen and oxygen in carbonates, hydroxyl groups.31−33 O‴ at around 530.2 eV was assigned to the lattice oxygen bound to metal cations.31,34 After gasification, the fraction of surface adsorbed oxygen (O‴) decreased from 84.33% to 64.82%. The decrease of surface oxygen groups could result in the poorer performance of NO reduction. In the XRD diagram shown in Figure 10, change of signal peaks after gasification is very clear. For the fresh catalysts,

Figure 10. XRD diagram of the catalysts, (a) as synthesized, (b) after gasification for 30 min at O = 8% and T = 300 °C.

Table 2. Summary of Reaction Mechanism over FeCo/ASC Number

a

Reaction

Reaction site

Applicabilitya

b

√

Activation of metal site by CO

30

Description

Reference

R11

CO + *O ↔ CO2 + *

M

R12

* + NO ↔ *NO

M

√

NO adsorption

30

R13

*NO + CO ↔ *N + CO2

M

√

NO reduction

30

R14

*N + NO ↔ *N2O

M

√

N2O production

30

R15

*N2O ↔ N2O + *

M

√

N2O releasing

30

R16

*N2O ↔ N2 + *O

M

√

N2O reduction

30

R17

NO + 2* ↔ *O + *N

M

×

Direct NO decomposition

21, 22

R18

2NO + * ↔ *(NO)2

M

×

NO adsorption

31

R19

*(NO)2 ↔ N2O + *O

M

×

N2O production

31

R20

2C(NO) ↔ N2O + C(O) + C

Cc

×

N2O production

31

R21

C(O) ↔ CO

C

×

CO production

31

R22

*O + CO ↔ *O(CO)

M

√

CO adsorption

25

R23

*O(CO) + C(O) ↔ *O + C(CO2 )

M+C

√

CO oxidation

25

R24

C(CO2 ) ↔ C + CO2

C

√

CO2 releasing

25

R25

*O + C ↔ * + C(O)

M+C

√

Metal site activation

25

R26

* + NO ↔ *(NO)

M

×

NO adsorption

25

R27

*(NO) + C ↔ *O + C(N)

M+C

×

NO dissociation

25

R28

C(N) + NO ↔ N2 + C(O)

C

×

N2 production

25

R29

2C + O2 ↔ 2C(O)

C

√

Carbon−oxygen complex generation

25

R30

C(O) + *O ↔ CO2 + *

M+C

√

CO2 production

This work

R31

C + NO ↔ C(NO)

C

√

NO adsorption

This work

R32

* + NO ↔ *NO

M

√

NO adsorption

This work

R33

C(NO) + *NO ↔ CO + *N2O

M+C

√

NO reduction into N2O

This work

Applicability for the reactions over FeCo/ASC catalysts. bMetal site. cCarbon site. I


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Figure 11. NO conversion over ASC support. (a) CO = 1000 ppm; (b) CO = 2000 ppm; (c) CO = 5000 ppm.

for the NO reduction because CO could be rapidly consumed by oxygen. Then carbon support will play the role of reductant. The NO conversion will be much lower than the oxygen-free conditions because the activity of NO−carbon reaction is much lower. A quite different performance of NO−carbon reaction is that increased oxygen concentration could enhance NO reduction. However, higher oxygen is not desired because carbon−oxygen reactions, R8 and R9, are much more active and carbon combustion will become inevitably serious. By examining the summarized onset temperature of different reactions, temperatures between 200 and 250 °C are anticipated be favorable for NO−CO reaction. Because in this temperature range, NO reduction by CO is already active and selectivity toward N2 is becoming acceptable. At the same time, carbon support oxidation is not active, avoiding undesired carbon gasification. 3.6. Reaction Mechanism. Based on the performance of NO reduction over the FeCo/ASC catalyst, the reaction mechanism is further explored and summarized in Table 2. Some additional experiments are conducted for catalysts without metal loading and the results are shown in Figure 11. It should be noted that the GHSV is set as 5000 h−1, much lower than the experimental settings over FeCo/ASC. It is found that NO conversion is much lower comparing to the conversion over supported metal catalysts. Therefore, it could be concluded that metal sites are essential for the catalytic NO and CO reactions. A NO−CO reaction mechanism over metal oxides is proposed by Reddy and Khanna30 (for FexOy catalysts) based on molecular simulation study, represented by reaction R11− R16 (in Table 2). In these equations, *O is oxidized metal site, * is reduced metal site, or named as “activated” metal site, which could be effective for NO reduction. CO is first adsorbed

be inactive over the prepared catalyst. Thus, the effective and desired reaction is NO reduction by CO. On the other hand, the most undesired reaction is carbon support combustion, with either CO or CO2 produced. Another undesired reaction is CO oxidation, which consumes the reducing agent for NO reduction. But from another aspect, CO oxidation could consume oxygen that could otherwise oxidize the active metal sites and significantly inhibit catalytic NO reduction. Also, effective and fast oxidation of unreacted CO into CO2 could avoid the emission of toxic CO. For NO reduction, NO−CO reaction is very active. In the absence of oxygen, excellent NO reduction performance could be expected. Selectivity toward N2 increases with temperature. On another point of view, it could be considered that N2O is the intermediate for NO reduction into N2. Or reaction R3 is the overall reaction of sequence reactions R2 and R4. The onset temperature of reaction R4 is lower than that of R2, resulting in more N2O production at low temperatures. For the oxygen-free conditions, NO−carbon reactions, R8 or R9, are not active if CO is fed above stoichiometrics. In the presence of oxygen, NO reduction efficiency will be dramatically inhibited. CO tends to be oxidized by oxygen. And the activity of reaction R5 is observed to be much bigger than reaction R3. But a more important aspect is that active metal site tends to be oxidized, deactivating the CO−NO reaction. There are two types of apparently different performance of NO reduction for the oxygen-containing flue gas. If oxygen concentration is below the stoichiometrics of CO oxidation, the residue CO could be effectively used for NO reduction after oxygen is totally consumed. But the NO conversions will be slightly lower than conversions of the oxygen-free condition because some metal sites would be deactivated by oxygen. On the other hand, if excess oxygen is present in the flue gas, little CO will be used J


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Industrial & Engineering Chemistry Research onto an oxidized metal site, *O, producing CO2 and activating a reduced metal site (*). NO is then adsorbed onto the activated metal site and reduced by free CO, forming a nitrogen radical (*N). *N further reacts with NO in the gas phase to form *N2O, which finally be released from the catalyst surface as N2O or breaks down into nitrogen and regenerate oxidized metal sites (*O). In this mechanism, CO is required to initially activate the metal site * in R11. Then from R12 to R15, N2O is produced following an overall reaction 2NO + CO → N2O + CO2, which agrees well with reaction R4. In reaction R15, activated metal site * is regenerated for further NO adsorption (R12). At higher temperature, the endothermic decomposition of N2O will be easier, resulting in a higher N2 selectivity. This could also be confirmed by the experimental observation of higher selectivity into N2 at higher temperatures as presented in Figures 1b and 2b. Illustrated by this mechanism, CO and NO potentially occupy the same metal site in series. Thus, the site occupied by CO would not provide any form of competition toward site occupation of NO, confirmed by the enhancement of NO reduction by increased CO concentration shown in Figures 2a and 3a. A similar theory was also evaluated by Patel29 (for metal oxides supported on MCM-41), who suggested that CO interaction with the metal oxide only weakens the metal oxygen bond thereby causing only partial reduction of the catalyst surface. But based on the present experimental data, it is hard to tell whether the metal oxide sites are totally or partially reduced. There are some other theories proposed for NO reduction and N2O production. Some researchers21,22 (for Co/AC and CoCr/AC catalysts) suggested that NO directly decomposes into *N and *O when adsorbed onto the activated metal sites as R17. *N then reacts with gaseous NO to produce N2O, same as R14. But for the present experiment, this mechanism seems unlikely because direct NO decomposition is not observed. Alternatively, the production of N2O from NO was described by Busch35 (for Fe/AC catalysts) as R18 and R19 (in Table 2). In the both mechanisms of R17 and R18 to R19, each N2O produced requires two activated metal sites *. This implies that CO needed to activate the metal sites is twice of the amount of NO. It is not in accordance with the experiments, in which good stoichoimetrics balance is found for NO−CO reaction toward N2O as reaction R4. In all the above mechanisms, only metal sites are considered to be active for NO reaction. However, carbon support also plays an important role in NO reduction especially when oxygen is present. For the carbon supported catalysts, reactions R20 and R21 were proposed by Busch35 to describe the production of N2O over carbon sites. NO is supposed to be adsorbed onto carbon site to become carbon−nitrogen complex C(NO), which is then decomposed to produce N2O and CO. This mechanism is possible for NO−carbon reaction when oxygen is absent. Because only NO and carbon sites are involved, role of oxygen is not presented in this mechanism. However, in our experimental data, e.g., Figures 1a and 5a, oxygen is essential for NO−carbon reaction. This mechanism seems unlikely for the present experiments especially when oxygen is present. When both metal and carbon sites are considered, it is possible that CO and NO are adsorbed onto different sites of the catalyst, following a mechanism proposed by Rosas25 (for Cr/AC catalysts) for carbon supported catalysts as reactions

R22−R29. In these reactions, (C) is unstable carbon site. In reaction R22, CO is adsorbed onto a metal site. Then the metal site adsorbed with CO could further react with surface carbon− oxygen complex C(O) to oxidize CO into C(CO2) and regenerate the metal site (*O). The decomposition of C(CO2) could then release CO2 and produce an unstable carbon site (C). In reaction R25, the unstable carbon site could further activate the metal site (*). Reaction R26 represents the chemisorptions of NO onto the activated metal site. The dissociation of *(NO) takes place on a neighboring unstable carbon site (C) to produce a carbon−nitrogen complex C(N), which further react with NO to produce N2 and complete the CO−NO reaction. The purpose of reaction R29 is to generate unstable carbon site (C), whereas reaction R30 is to regenerate new carbon−oxygen complex and to close the carbon loop. In this proposed reaction pathway, reactions R22−R25 represent interaction of CO with the catalyst and generation of activated metal sites, following an overall reaction of CO + *O ↔ CO2 + *, which is the same as the reaction R11 proposed by Reddy and Khanna30 but better illustrates the interaction of carbon support and metal sites. Reactions R26−R28 illustrate NO reduction over metal and carbon sites. But production of N2O is not considered. An overall reaction of 2NO + CO + C ↔ N2 + CO2 + C(O) is given based on reactions R22−R28. In this mechanism, both CO and carbon support are reducing agents in the NO reduction. And the roles of CO and carbon are similarly important. However, from the present experimental observation, only one reducing agent is predominant for various conditions. CO is the main reductant when oxygen is absent or CO is in great excess. There is no carbon release observed when only NO−CO reaction takes place. But carbon support becomes more important for NO reduction when oxygen is in excess and metal sites tend to be oxidized. In these cases, CO is more likely to be oxidized by oxygen, instead of being involved in the NO reduction. Therefore, this mechanism does not fit well with the present experiments. But some considerations about the metal and carbon sites are inspiring. In addition to the mechanisms discussed, reactions R30−R33 are proposed here to illustrate the role of carbon sites, oxygen and metal sites in the NO reduction. In this additional mechanism, oxygen is first adsorbed into carbon site to form carbon−oxygen complex C(O) as R29. C(O) then get in touch with a neighboring oxidized metal site to produce CO2 and to activate the metal site * as R30. The activated metal site could later be used for NO reduction into either N2O or N2 following reactions R12−R16. In this reaction pathway, the metal site is essential for both carbon gasification and NO−carbon reaction. Also, oxygen in the flue gas could promote the production of a surface carbon−oxygen complex C(O) and then enhance the activation of metal site * and NO reduction. Because carbon is directly oxidized by both oxygen and oxidized metal sites, this mechanism could well explain why little CO was detected in the effluent gas. It is also clear about the role of supported metal on the elimination of possible emitted CO. Moreover, it could be expected that the frequency of site−site reaction as R30 is much lower than site-gas reaction as R11. Thus, this mechanism could well explain why activity of NO−CO reaction is significantly bigger than NO−carbon reaction. Reactions R31−R33 illustrate the pathway of NO−carbon reaction in the absence of CO addition. They are similar to the mechanism of R20 and R21, but carbon and metal sites interaction are considered. Summarizing the discussion of reaction mechaK


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based on literature review and experimental data, providing some insights on the complex reactions among NO, O2, CO, and carbon over the carbon supported catalyst. In the presently summarized mechanism, only one kind of metal site is considered. However, at least two kind of metal sites, ion site and cobalt site, exist on the FeCo/ASC catalyst. A more detailed mechanism should be proposed in the future to illustrate fully the different roles of Fe and Co in NO reduction. Moreover, detailed kinetic model needs to be developed in the future to illustrate quantitatively the role of each reaction.

nisms, reaction scheme over FeCo/ASC is proposed and illustrated in Figure 12.

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AUTHOR INFORMATION

Corresponding Author

*Z. Wang. Tel: (86) 531-88399372(605). Fax: (86) 53188385877. Email: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (No. 51406104), the National Key Technology R&D Program (No. 2014BAA02B03), the Project of Shandong Province Science and Technology Development Program (No. 2014GSF117034), Shandong Province Natural Science Foundation (No. ZR2016EEM17), and Independent Innovation Funds of Shandong Province (No. 2014ZZCX05201) for the financial support.

Figure 12. Reaction scheme of NO reduction over FeCo/ASC catalysts.

4. CONCLUSION Performance of NO reduction by CO is studied over the prepared FeCo/ASC catalyst. Influence of oxygen and CO concentrations is systematically investigated. It is found that excellent efficiency NO reduction by CO could be achieved at oxygen-free conditions. Excess CO could enhance NO reduction, implying no competition for the active sites. However, NO−CO reaction is strongly inhibited by the presence of oxygen, due the oxidation of active metal sites. If oxygen is in excess, CO will be rapidly consumed, and then NO−carbon reaction will be predominant. Oxygen could promote the NO−carbon reaction due to the formation of surface oxygen compounds. But these cases are strongly undesired because there will be serious carbon support consumption. Therefore, NO reduction by CO is only effective when there is no or little oxygen presented in the flue gas. Oxygen preferentially oxidizes CO than carbon support. If minor oxygen exists in the flue gas, carbon consumption could be avoided by feeding excess CO. But the consumption and cost of the excess CO should be evaluated first. Considering the catalytic behavior of FeCo/ASC, it is very difficult to achieve satisfactory performance of NO abatement in conventional fixed bed reactors in real applications in industry because there is oxygen concentrations are always as high as more than 5%. However, reactor design is a possible way to overcome the oxygen inhibition effect. For example, in the internal circulating fluidized bed (i-CFB) proposed by Yang and Bi,36−38 NOx adsorption and reduction were set into two different zones by separating the gas streams of flue gas and reducing gas. Thus, reducing gas will not get in touch with oxygen in the flue gas and the negative effect of oxygen could be avoided. The deNOx performance of FeCo/ASC catalysts will be tested in similar novel reactors in the future. But it is very important to maintain the temperature to be below 300 °C to avoid the gasification of carbon support. Possible reactions over FeCo/ASC are summarized with activity of each reaction qualitatively listed based on the experimental observation. Reaction mechanism is also proposed

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REFERENCES

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Catalytic Performance of NO Reduction by CO over Activated

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