Experimental and Kinetic Study of Low Temperature Selective

ARTICLE pubs.acs.org/IECR

Experimental and Kinetic Study of Low Temperature Selective Catalytic Reduction of NO with NH3 over the V2O5/AC Catalyst Zhigang Lei,* Aibin Long, Cuiping Wen, Jie Zhang, and Biaohua Chen State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Box 266, Beijing 100029, China

bS Supporting Information ABSTRACT: Selective catalytic reduction (SCR) of NO (nitric oxide) by NH3 (ammonia) at temperatures of 393523 K over AC (activated carbon) supported V2O5 (vanadium pentoxide) was investigated in this work. The V2O5/AC catalyst was prepared by the pore volume impregnation method. The intrinsic kinetics for this catalyst was first measured in the absence of internal and external diffusions in a fixed-bed microreactor. Then, five intrinsic kinetic models, i.e. the EleyRideal model, the LangmuirHinshelwood model, the Mars-van Krevelen model, the first-order model, and the power-rate law model, were applied to correlate the experimental data. Among them, the EleyRideal model is the most accurate, and the LangmuirHinshelwood model is unfeasible for describing SCR of NO by NH3 over the V2O5/AC catalyst. Therefore, the SCR of NO with NH3 over V2O5/AC is likely to follow the EleyRideal mechanism in light of intrinsic kinetic measurements. Finally, the kinetic equations were incorporated into a 3D mathematical model for monolithic honeycomb reactor, and it was found that the EleyRideal model is more suitable and can provide useful information for future application in the actual industrial plants.

1. INTRODUCTION The combustion of fossil fuels in massive amounts produces noxious substances such as SO2, NOX (mostly NO), volatile organic compounds (VOC), carbon particulates, and ashes, which contribute to serious world ecological problems including acidification of rain, formation of photochemical smog, green-house effect, and ozone depletion (Fortunately, it was reported on Sep., 2010 that ozone layer depletion has been halted; see http://www.cosmosmagazine.com/ news/3742/ ozone-layer-depletion-has-stopped-say-scientists.). As the growing concern about environmental pollution problems related to NOX and the introduction of stricter emission legislations, many catalytic technologies have been deeply investigated and tested in the recent years for their low cost and high efficiency.1,2 One of the most effective technologies is the SCR of nitrogen oxides with ammonia. The generally accepted (Bosch and Janssen, 1988) stoichiometry of this reaction on metal oxide catalysts appears to be 4NO þ 4NH3 þ O2 f 4N2 þ 6H2 O

ðR1Þ

Currently, SCR has been widely used for control of NOX emission in flue gases from stationary polluting sources such as thermal power plants, industrial boilers, furnaces, gas turbines, biomass incinerators, and gasifiers. Catalysts made of metal oxides, such as CuOAl2O3 TiO2, CuOSiO2TiO2, Cr2O3TiO2, and V2O5MoO3TiO2, are widely employed in the SCR process. However, the optimum temperature is within the range of 573673 K for the reaction, while the flue gas temperature for many burners is in the range of 393523 K, which results in a great energy consumption by the adding units for reheating stack gases.3 Carbon-supported catalysts have exhibited very high activity at low temperatures of flue gas emission and resistance to the remaining SO2 that can form ammonium bisulfate (NH4HSO4) or ammonium sulfate ((NH4)2SO4) deposits on the catalyst surface causing a significant decrease of NOX conversion and corrosion problems in the cold reactor equipment.46 More interestingly, SO2 in the flue gas does not deactivate the activated carbonr 2011 American Chemical Society

supported (AC) vanadium oxide catalyst but improves the activity.7,8 Furthermore, activated carbon has superior properties for removal of many flue gas pollutants, individually or collectively, though it is an uncomplicated and inexpensive material. Consequently, these advantages make carbon-supported catalysts an excellent option for SCR for NO removal. In the SCR process, monolithic structure is preferred over the conventional fixed-bed reactor packed with pellet catalyst particles due to its low pressure drop in the exhaust system, good thermal shock, and resistance to deposition of carbon and dust. However, the rigorous mathematical model for monolith honeycomb SCR reactor has to be established to understand the influence of operating and design parameters on the SCR performance. For this purpose, the intrinsic kinetic model is needed to incorporate into the mathematical model. On the other hand, we can deduce the most likely microscopic reaction mechanism from the macroscopic kinetic measurements by comparison of experimental data with the correlated values from the possible intrinsic kinetic models. Therefore, the objective of this work is to establish and identify the possible intrinsic kinetic models of SCR of NO by NH3 over V2O5/AC from the viewpoint of chemical reaction engineering. On this basis, a 3D mathematical model for monolith honeycomb SCR reactor, in combination with the intrinsic kinetic models obtained in this work, will be used to confirm the reliability of prediction of reaction performance.

2. EXPERIMENTAL SECTION 2.1. Catalysts Preparation. The desired particle sizes were triturated and sieved from a commercial honeycomb monolith AC which was used as the carbon supports. The proximate Received: October 18, 2010 Accepted: March 16, 2011 Revised: March 14, 2011 Published: March 25, 2011 5360

dx.doi.org/10.1021/ie102110r | Ind. Eng. Chem. Res. 2011, 50, 5360–5368

Industrial & Engineering Chemistry Research analysis of AC precursors is as follows: M (moisture content) 4.32 wt %, A (ash content) 39.97 wt %, and V (volatile content) 10.27 wt %, and the ultimate analysis gives the following element contents: C 49.68 wt %, H 0.29 wt %, O 5.32 wt %, and N 0.18 wt %. The V2O5/AC catalyst was prepared by the pore volume impregnation with an aqueous solution of ammonium metavanadate in oxalic acid. When the volume of the solution used in the impregnation is equal to the pore volume of porous carbon, a homogeneous distribution of the solute on the supporting surface will be achieved. Besides, the amount of V2O5 loaded into the carbon support can also be controlled by adjusting the concentrations of aqueous ammonium metavanadate.9 2.2. Catalyst Characterization. Specific surface area, maximum pore volume, micropore volume, and average pore size of AC and AC catalyst samples were measured by nitrogen adsorption at 77 K using an automated adsorption apparatus (Micromeritics ASAP 2020 micropore size analyzer). Prior to tests, the samples were degassed at 473 K up to a steady vacuum of 1 106 Torr. The specific surface area was calculated by BrunauerEmmettTeller (BET) equation. Maximum pore volume was derived by HorvathKawazoe method. The empirical t-plot method was applied to obtain the micropore volume. X-ray photoelectron spectroscopy (XPS) was used to determine the nature of the reactant species adsorbed on the catalyst surface. XPS spectra were measured with an ESCALAB 250 (Thermo Fisher Scientific) X-ray photoelectron spectrometer, using MgKR as the radiation source. The 1 wt % V2O5/AC catalysts were sulfated at 523 K in 1000 ppm SO2 þ 5% O2 in N2 for 3 h or subjected to a SCR reaction at 523 K in the absence of SO2 after previous operation. After that, the catalyst sample, including presulfide or reacted catalysts, was cooled in N2 to ambient temperature and then kept in a sealed vessel before taking the XPS detection.10 2.3. Experimental Apparatus. The steady state SCR reaction was performed in a fixed-bed microreactor constituted by a quartz tube, with 10 mm in internal diameter and 12 mm in external diameter. The catalyst was placed in the middle of the reactor, supported by quartz cotton. The temperature difference of the reaction tube between inside and outside the tube wall is less than 1.0 K measured by a thermocouple, so the reactor can be assumed to be an isothermal integral. The schematic diagram of experimental apparatus for activity and kinetic tests is shown in Figure 1, which consists of three parts: simulated fuel gas, fixed-bed reactor, and gas analyzer. Reaction gases (NH3, NO, and SO2) were controlled by mass flow meters and should be filtered before entering the reactor to avoid reactants corroding and blocking the pipeline. N2 and air were controlled by rotor-meter. The concentrations of simulated flue gas and reactants composition in the steady state were simultaneously determined by flue gas analyzer (KM9106 Quintox, Kane International Limited, England), which was equipped with NO, NO2, SO2, and O2 electrochemical sensors. To avoid any interference for NO determination, unreacted NH3 from reactor was scrubbed with a 5 wt % boric acid solution before flowing into the analyzer. Each experimental data point was measured at least three times to ensure the repeatability. 2.4. Activity Tests. NO conversion xNO is defined as follows ! C0NO CNO xNO ð%Þ ¼ 100 3 ð1Þ C0NO where C0NO and CNO are the measured initial NO concentration at the inlet and the NO concentration at the outlet, respectively, once the steady state is reached.

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Figure 1. Schematic diagram of experimental apparatus for kinetic experiment. 1- gas cylinder, 2- steam generator, 3- quartz cotton, 4catalyst particles, 5- quartz glass reaction tube, 6- boric acid solution, 7flue gas analyzer, 8- heat insulation furnace, 9- rotameter, 10- mass flow meter.

In the beginning, a diagnostic blank experiment was done in order to confirm that the contribution of the apparatus to the NO conversion could be safely neglected. The operating conditions were given as follows: the inlet gas volumetric flow rate 400 mL 3 min1, NO 500 ppm, NH3 500 ppm, and O2 5%. The blank AC particle was packed into the quartz tube reactor. It was observed that in this case NO conversion is about 6% at 373 K and decreases as the temperature increases. That is to say, the blank AC possesses a little catalytic activity for NO conversion. In previous works, V2O5/AC exhibits promotion of NO conversion and selectivity to N2 after a presulfide step.5,6,11 Before the activity measurement, the catalysts were sulfated at 523 K by 1000 ppm SO2 þ 5% O2 in N2 at a total flow rate of 400 mL 3 min1 for 3 h, followed by a purge of N2 to remove the physically adsorbed SO2 at the same temperature. Then the catalysts were assumed to be saturated by SO2 adsorption. The influence of the amount of V2O5 loading in the catalysts should be checked before the elimination of external and internal diffusions, since the ammonia chemisorption and promotion of SO2 are greatly related with the loading amount of vanadium compounds. The process of ammonium ions reacting continuously with NO effectively ensures the catalyst being promoted but not poisoned by SO2 at low V2O5 loading.1214 The V2O5 content in the catalysts can be controlled from 0.5 to 5.0 wt % by the pore volume impregnation method.

3. RESULTS AND DISCUSSION 3.1. BET and XPS Results for V2O5/AC. The BET surface area of V2O5/AC increases after loading 1.0 wt % V2O5 (see Table S1). It may be caused by charcoal loss leading to producing new pores, because V2O5 can react with the carbon, forming inactivity reduced vanadium oxide (i.e., the so-called preoxidization effect). Both the O (1s) XPS spectra are almost the same with only 1 eV difference (see Figure S1). We can speculate that it is carbonoxygen functional group for their binding energy exceeding 533 eV. For S (2p) XPS spectra, only the reacted sample exhibits a single peak with a binding energy of 172.91 eV, which may be attributed to S6þ species such as sulfate in Na2SO4, FeSO4, and Fe2(SO4)3.10 Therefore, it indicates that the element S exists on the V2O5/AC catalysts surface by the formation of SO42. 3.2. Influence of the Amount of V2O5 Loading. The influence of the amount of V2O5 loading on NO conversion is shown in Figure 2. It can be seen that when the amount of V2O5 loading reaches 1.0 wt %, the catalytic activity is the highest in the 5361

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Figure 2. Influence of the amount of V2O5 loading on NO conversion. The reacting stream compositions are as follows: NO 500 ppm, NH3 500 ppm, O2 5% and N2 as the balance gas; and the total volumetric flow rate 400 mL 3 min1.

Figure 3. Influence of different catalyst particle sizes on NO conversion. The reacting stream compositions are as follows: NO 500 ppm, NH3 500 ppm, O2 5% and N2 as the balance gas; and the total volumetric flow rate 400 mL 3 min1.

temperature range of 423523 K. This indicates that V2O5 at 1.0 wt % disperse the best on the AC surface. The lower activity at low amount of V2O5 loading, i.e. 0.5 and 0.8 wt %, may be attributed to the lower coverage of vanadium. But the high amount of V2O5 loading demonstrates a negative effect due to the overlapping of V2O5 on the AC surface. Therefore, the best amount of V2O5 loading is 1.0 wt %, which will be selected in subsequent kinetic experiments. Moreover, the 1.0 wt % V2O5/AC catalyst exhibits a high and stable activity during a 220 h operation at 523 K (see Figure S2), and it seems that 39.97 wt % ash in the catalyst does not influence the catalytic activity and stability. 3.3. Elimination of the Internal and External Diffusions. Figure 3 shows the influence of different catalyst particle sizes on NO conversion. Four kinds of particle size were tested, and there is almost no difference of NO conversion while the particle diameters are reduced to 0.20.3 mm. As the catalyst particle size is smaller to a certain extent, the resistance of the internal diffusion can be ignored. So the influence of internal diffusion is thought to be eliminated at the particle diameter of below 0.20.3 mm. Figure 4 shows the influence of weight hourly space velocity (WHSV) on NO conversion. Two catalyst weights, i.e. W1 = 0.3 g and W2 = 0.5 g, were respectively filled into the fixed-bed microreactor, and the gas volumetric flow rates ranged from

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Figure 4. Influence of external diffusion on NO conversion. The reacting stream compositions are as follows: NO 500 ppm, NH3 500 ppm, O2 5% and N2 as the balance gas; and the total volumetric flow rate 400 mL 3 min1.

400 to 700 mL 3 min1. It can be seen that two straight lines almost overlap each other at different WHSVs. Therefore, the influence of external diffusion is thought to be eliminated under the experimental conditions. The WHSV selected in subsequent kinetic experiments was 0.077 m3 3 h1 3 g1. 3.4. Kinetic Modeling. 3.4.1. The EleyRideal Model. There are two main microscopic mechanisms that have been recognized for SCR over metal oxide catalysts, providing some theoretical insight into the surface phenomena of the reaction compared to the empirical power-rate law model. An EleyRideal mechanism1519 is usually assumed that NH3 is strongly adsorbed on the catalyst and NO reacts from the gas phase. Another LangmuirHinshelwood mechanism15,16,20 involves adsorbed NO, offering a clue to the key mechanistic steps in the heterogeneous NO reduction with NH3. Ma et al.21 discovered that adsorption and oxidation of NH3 is the key in the SCR reaction of NO by temperature-programmed desorption (TPD) with online mass spectroscopy analysis. This work tried to identify which mechanism is more suitable from the macroscopic intrinsic kinetics. In terms of the EleyRideal mechanism, the reaction rate is proportion to the gas-phase concentration of NO and the surface fraction covered by adsorbed NH3 abiding by Langmuir isotherm equation. Thus, the rate expression for SCR reaction becomes rNO ¼

kKCNO CNH3 1 þ KCNH3

ð2Þ

where CNO and CNH3 are the concentrations of NO and NH3 in the units of mol 3 cm3, respectively, K is the adsorption equilibrium constant, and k is reaction rate constant related with Arrhenius law.22 The integrated expression corresponding to eq 2 is ln

1 W 1 M 1 MxNO ¼k ln ; ðM 6¼ 1, MxNO < 1Þ 1 xNO v0 K ð1 MÞC0NO 1 xNO

W M xNO ¼k 0 ; ðM ¼ 1Þ v0 KCNO 1 xNO

ð3Þ

where M = CNO0/CNH30 is the molar concentration ratio of NO to NH3 in the gas phase entering the fixed-bed microreactor, v0 is the total volumetric flow rate of the gas phase at the inlet, W is catalyst weight, and CNO0 and CNH30 are the molar concentrations of feeding NO and NH3, respectively. 5362



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for ln k versus 1/T and ln K versus 1/T are shown in Figure 5. It can be seen that adsorption equilibrium constant K is significantly larger than reaction rate constant k, indicating that surface reaction on this catalyst is the controlling step. The kinetic parameters of the EleyRideal model for SCR of NO with NH3 over sulfated V2O5/AC are listed in Table 1, which were determined by multiple linear regression (MLR). 3.4.2. The LangmuirHinshelwood Model. In the Langmuir Hinshelwood model, it is assumed that adsorption equilibrium of NO and NH3 is established at all times and the NO reduction occurs between the adsorbed species on the catalyst.23 It can be represented as Figure 5. Arrhenius plot for K and k in the EleyRideal model.

Table 1. Kinetic Parameters in Five Kinetic Models for SCR of NO with NH3 over the Sulfated V2O5/AC Catalyst parameters

units

estimated values

EleyRideal model A Ea Aads ΔHads

cm3 3 g1 3 s1 1

kJ 3 mol

cm3 3 mol1 kJ 3 mol1

1.131 108 57.38

Aads in KNO Aads in KNH3 ΔHads in KNO ΔHads in KNH3

cm3 3 g1 3 s1 cm3 3 g1 3 s1 cm3 3 g1 3 s1 kJ 3 mol1 kJ 3 mol1

kJ 3 mol1 Ea Mars-van Krevelen model Ao Ar Eo

mol1-m 3 cm3 m 3 g1 3 s1 mol1-n 3 cm3n 3 g1 3 s1 kJ 3 mol1

where >C() represents the empty sites, >C(NO) and >C(NH3) are the adsorbed species in which the active adsorbed species >C(NH3) exists at either Br€onsted acid sites as NH4þ or Lewis acid sites as NH3.24,25 Thus, the LangmuirHinshelwood rate expression becomes

where k is the rate constant of reaction R4, KNO and KNH3 are the equilibrium constants of reactions R2 and R3, respectively. They are defined in the similar forms as eqs 4 and 5. For the convenience of kinetic parameter regression, eq 6 is rearranged as follows ! rffiffiffiffiffiffiffiffiffi CNO 1 þ KNH3 CNH3 KNO ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi þ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi CNO rNO kKNO KNH3 CNH3 kKNO KNH3 CNH3

7.62 4107.21 61.05 38.88 49.17 1020.44

and ! rffiffiffiffiffiffiffiffiffiffi CNH3 1 þ KNO CNO KNH3 ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi þ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi CNH3 rNO kKNO KNH3 CNH3 kKNO KNH3 CNH3

30.04

dimensionless

0.80

N

dimensionless

0.60

first-order model A

cm3 3 g1 3 s1

72357.18

ð8Þ

32.68

power-rate law model 72357.18

Ea

kJ 3 mol1

32.68

x

dimensionless

0.8822

y

dimensionless

0.1023

According to the Arrhenius’ law, k and K are described by Ea k ¼ Aexp ð4Þ RT ΔHads K ¼ Aads exp RT

ð6Þ

ð7Þ

0.59

M

mol1-x-y 3 cm3(xþy) 3 g1 3 s1

ðR4Þ

0.81

10.38

A

ðR3Þ

KNO CNO KNH3 CNH3 rNO ¼ k ð1 þ KNO CNO þ KNH3 CNH3 Þ2

kJ 3 mol1

kJ 3 mol1

NH3 þ > CðÞ T > CðNH3 Þ

2028.98 44.13

Er

Ea

ðR2Þ

> CðNOÞ þ > CðNH3 Þ f 2 > CðÞ þ products

LangmuirHinshelwood model A

NO þ > CðÞ T > CðNOÞ

ð5Þ

where A and Aads are pre-exponential factors, Ea is activation energy, and ΔHads is adsorption heat. The experimental results

Therefore, linear plots of (CNO/rNO)1/2 versus CNO at constant CNH3 and (CNH3/rNO)1/2 versus CNH3 at constant CNO can be used to determine k, KNO, and KNH3 from the slopes and intercepts of these plots. The kinetic parameters of the Langmuir Hinshelwood model for SCR of NO with NH3 over sulfated V2O5/AC are listed in Table 1. The kinetic parameter dependence on the temperatures is shown in Figure 6. The adsorption equilibrium constants KNO and KNH3 exhibit the same trend, and KNH3 is larger than KNO, which reflects that NH3 is easier to be adsorbed into the empty active sites than NO. 3.4.3. The Mars-van Krevelen Model. Most catalytic processes can be represented by an oxidationreduction mechanism, e.g. Mars-van Krevelen model, and the reducing and oxidizing reagents are considered to be NH3 and NO in reaction R1, respectively. Consequently, the Mars-van Krevelen model may be chosen to interpret the reaction mechanism and correlate the kinetic data. 5363



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Figure 6. Kinetic parameter dependence on temperature in the LangmuirHinshelwood model.

Figure 8. Arrhenius plot for rate constant k0 in the first-order model.

it was also found that koCNOm > krCNH3n in the reaction concentration range. This indicates that there is a higher adsorption capacity for NH3 on the catalyst surface than for NO, and the reaction rate would have a much stronger dependence on CNO than on CNH3. 3.4.4. The First-Order Model. It is noted that eq 2 can reduce to a first-order model in the case of NH3 excess rNO ¼ k0 CNO

ð10Þ

v0 1 Ea ln k ¼ ¼ A exp W 1 xNO RT

ð11Þ

0

Figure 7. Kinetic parameter dependence on temperature in the Marsvan Krevelen model.

There are two consecutive steps involved in this mechanism: oxidized catalyst sites react with a reducing reagent to become reduced catalyst sites, followed by oxidation of the sites with an oxidizing reagent to restore its origin state. In the SCR reaction, the role of catalyst is that V5þ is reduced to V4þ, and then V4þ is oxidized by O2, or transition metals are oxidized by NO and then reduced by carbon.26,27 The SCR reaction can be expressed as kr

NH3 þ oxidized site sf reduced site þ other products ðR5Þ ko

NH3 þ reduced site sf oxidized site þ other products ðR6Þ According to reactions R5 and R6, the reaction rate can be derived to be rNO

ko CmNO kr CnNH3 ¼ ko CmNO þ kr CnNH3

ð9Þ

where m is the reaction order with respect to NO in reaction R6, n is reaction order with respect to NH3 in reaction R5, and kr and ko the rate constants of reactions R5 and R6, respectively. The kinetic parameters in this model are correlated by multiple nonlinear regression (MNR) and are listed in Table 1. It was found that the values of 0.8 and 0.6 for m and n, respectively, would give the best fitness to the experimental data. Figure 7 shows the kinetic parameter dependence on temperature. It can be seen that ko > kr in the reaction temperature range. Moreover,

The first-order model strongly depends on NO concentration and can only approximately describe the kinetic behavior of SCR catalysts. If full coverage of the surface with NH3 can be guaranteed under all conditions, the reaction order of NH3 is zero. Otherwise, taking the adsorption of both NH3 and NO into account would be a much better description.28 Figure 8 provides an Arrhenius-type plot of ln k versus 1/T. The kinetic parameters of the first-order model for SCR of NO with NH3 over sulfated V2O5/AC are listed in Table 1. 3.4.5. The Power-Rate Law Model. Although the power-rate law model is empirical and represents a possible approximation for kinetic data, it is always important since its form is simple and the model parameters can be directly input to the modern chemical process simulation software such as ASPEN Plus, PROII, CFX, and Fluent, etc. The temperature and reactant concentration dependence of intrinsic rate of NO reduction can be correlated by a power law expression Ea ð12Þ ðCNO Þx ðCNH3 Þy rNO ¼ A exp RT where rNO is the intrinsic rate of NO reduction in the unit of mol 3 g1 3 s1, A is the pre-exponential factor, Ea is the activation energy, and x and y are the reaction orders with respect to NO and NH3, respectively. The reaction order with respect to O2 can be considered to be zero because the amount of O2 is excessive. The fitting procedure was done using the kinetic parameters obtained from the first-order model as initial values since the orders of magnitude of A, Ea, x, and y are significantly different. The Marquardt method as in Press et al.29 was used for data correlation. Meanwhile, it was found in the calculation that A and Ea have almost no change and thus equal to those in the firstorder model. The kinetic parameters of the power-rate law model for SCR of NO with NH3 over sulfated V2O5/AC are listed in 5364



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3.4.6. Comparison among the Five Kinetic Models. The comparison of NO conversion between the experimental data and correlated results was done for the five kinetic models. The Supporting Information provides the experimental data, correlated results, and their comparison in detail. The average relative deviation is defined as N

ARDð%Þ ¼

Figure 9. Parity plots of NO conversion for the experimental data and correlated values: (a) the EleyRideal model; (b) the Mars-van Krevelen model; (c) the first-order model; and (d) the power-rate law.

Table 1. As expected, x is larger than y due to the stronger NH3 adsorption on the surface sites. The reaction order x is less than unity, indicating that NO does not react completely from the gas phase but a small part from an adsorbed state.

exp cor ∑ ðjxexp NO xNO j=xNO Þ i¼1

N

100

ð13Þ

The average relative deviations of NO conversion for the EleyRideal model, the Mars-van Krevelen model, the first-order model, and the power-rate law model are 11.06%, 18.37%, 20.24%, and 13.43%, respectively. However, it was found that almost all of the correlated NO conversion from the LangmuirHinshelwood model are overflow (i.e., above 100%), and thus this model is not feasible for describing SCR of NO by NH3 over the V2O5/AC catalyst. The parity plots for these kinetic models excluding the LangmuirHinshelwood model are shown in Figure 9. The fitness to the experimental data is in the order of the EleyRideal model > the power-rate law model > the Mars-van Krevelen model > the first-order model > the LangmuirHinshelwood model. Therefore, the EleyRideal model is the most accurate and could be a better choice to clarify the kinetic mechanism of SCR of NO with NH3 over the V2O5/AC catalyst compared to other models. That is to say, the strongly adsorbed NH3 on the catalyst is a premise for the SCR reaction for the V2O5/AC catalyst. 3.5. Activated Carbon Honeycomb Monolithic Catalyst for SCR Reaction. The commercial activated carbon honeycomb (ACH) monolith (made by Beijing Pacific Activated Carbon Products Co., LTD) with a wall thickness of 1.0 mm and a cell density of 50 cells per square inch (cpsi) was used to support V2O5 by the pore volume impregnation method, as proposed by Liu et al.30 With respect to the mathematical model of honeycomb monolithic catalyst, Tronconi31 have successfully made a one- and two-dimensional steady-state isotherm description of chemical kinetics and transport processes in catalytic monolithic SCR reactors, in which the Rideal-type rate equation was selected. Khodayari and Odenbrand32 used a two-dimensional model to describe poison accumulation and activity of a SCR catalyst and obtained some useful information about the poison resistance of SCR monolithic catalyst by reconfiguration of their pore structure and channel diameter. However, the one- and two-dimensional mathematical models cannot distinguish the concrete channel geometry and the hydraulic diameter Dh is taken on as the characteristics (size and shape) of the monolith channel. Therefore, we would like to establish a 3D mathematical model of ACH monolithic SCR reactor. The governing equations, boundary conditions, and algorithm selection for gas and solid phases are similar to those that have been described in our previous publication,31 except that the porous medium model was selected for solid phases because the active species V2O5 was immerged into activated carbon by pore volume impregnation. The ACH monolith was cut into small pieces, with L = 60 mm long and five channels distributed symmetrically (see Figure S3), and then placed in the quartz tube reactor. Before experiments, the monolith catalysts were saturated by 3%5% NH3. The experimental data were obtained by fixing CNO0 at 500 ppm but varying CNH30. The total volumetric flow rate was 450 mL 3 min1, with 5% O2 and N2 as the balance gas. The kinetic parameters of the EleyRideal model, the power-rate law, and the first-order model were respectively input into the 3D mathematical model of ACH 5365



Figure 10. Comparison between experimental data and the predicted values obtained from the 3D mathematical model of ACH monolith catalysts where the EleyRideal model is input. The operating conditions were as follows: CNO0 500 ppm but varying CNH30, O2 5% and N2 as the balance gas; and the total volumetric flow rate is 450 mL 3 min1. Experimental NO conversion: 2, M = CNO0/CNH30 (molar feed ratio of NO to NH3) = 1.6; b, M = 1; 9, M = 0.75. Predicted NO conversion by the EleyRideal model: Δ, M = 1.6; O, M = 1; 0, M = 0.75.

monolith. Comparison between experimental data and the predicted values obtained from the 3D mathematical model was made. It can be seen from Figure S4 that the predicted values by using the first-order model cannot distinguish the influence of different molar feed ratio of NO to NH3 on NO conversion. Therefore, the firstorder model is unfeasible for describing the reaction performance of ACH monolith. The predicted values by using the EleyRideal model and the power-rate law model both exhibit the similar trend as the experimental data (see Figures 10 and S4). The average relative deviations of NO conversion for the EleyRideal model and the power-rate law model are 17.5% and 24.2%, respectively. So the EleyRideal model is more suitable for describing the ACH monolith catalyst for SCR reaction than the power-rate law model. However, the difference between experimental data and the predicted values seems a little large at high temperatures. The reason may be attributed to two aspects: one is that the heat balance is difficult to be completely reached inside the ACH monolith reactor especially at high temperatures, which leads to the actual reaction temperature being overestimated; the other is that the product H2O would compete with NH3 to adsorb on the catalyst or react with NH3, both of which may decrease NO conversion. As a result, the predicted values overestimate NO conversion at high temperatures, but they are qualitatively consistent with the experimental data. In addition, the kinetic parameters for other AC catalysts, which come from the literature23 and are given in Table S3, were also input into the mathematical model of monolith catalysts in order to compare the reaction performance with V2O5/AC monolith catalyst prepared in this work (see Figure S5). It can be seen that the V2O5/AC monolith catalyst with smaller activation energy exhibits higher NO conversion than others.

4. CONCLUSIONS The V2O5/AC catalyst was prepared by the pore volume impregnation with an aqueous solution of ammonium metavanadate in oxalic acid and was characterized by nitrogen adsorption isotherm and XPS. The intrinsic kinetics was measured in the absence of internal and external diffusions. The optimal operation conditions are as follows: a presulfide step that will enhance the catalytic activity considerably; the best V2O5 loading

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1.0 wt %; the particle diameter chosen to eliminate the internal diffusion 0.20.3 mm; and the WHSV selected to eliminate the external diffusion 0.077 m3 3 h1 3 g1. The V2O5/AC catalyst prepared in this work exhibits a long life of high activity and stability. Five intrinsic kinetic models, i.e. the EleyRideal model, LangmuirHinshelwood model, the Mars-van Krevelen model, the first-order model, and the power-rate law model were applied to correlate the experimental data. Among them, the EleyRideal model is the most accurate. Therefore, it can be deduced from the macroscopic kinetic measurements that the SCR of NO with NH3 over V2O5/AC is likely to follow the EleyRideal mechanism. The LangmuirHinshelwood model is unfeasible for describing SCR of NO by NH3 over the V2O5/AC catalyst. On this basis, the kinetic equations were incorporated into a 3D mathematical model for ACH monolith. It was found that the predicted values by using the EleyRideal model are in good agreement with the experimental data except at high temperatures.

’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental date and correlated results from, the EleyRideal model, the Mars-van Krevelen model, the first-order model, and the power-rate law model as well as catalyst characterization and evaluation. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ86 10 64433695. E-mail: [email protected].

’ ACKNOWLEDGMENT This work is financially supported by the National Nature Science Foundation of China under Grant (Nos. 20736001, 20821004, and 21076008) and the Research Fund for the Doctoral Program of Higher Education of China (No. 20090010110002). ’ NOMENCLATURE A = pre-exponential factor in Arrhenius’ law in the power-rate law, the EleyRideal model, the first-order model, and the LangmuirHinshelwood model, same units as the specific reaction rate constant Aads = pre-exponential factor in Arrhenius’ law in the Eley Rideal model and the LangmuirHinshelwood model, cm3 3 g1 3 s1 Ao = pre-exponential factor in Arrhenius’ law in the Mars-van Krevelen model, mol0.2 3 cm2.4 3 g1 3 s1 Ar = pre-exponential factor in Arrhenius’ law in the Mars-van Krevelen model, mol0.4 3 cm1.8 3 g1 3 s1 C = molar concentration, kmol 3 m3 CNO0 = molar concentration of NO at the inlet of SCR reactor, kmol 3 m3 0 CNH3 = molar concentration of NH3 at the inlet of SCR reactor, kmol 3 m3 Ea = activation energy of NO in other models, kJ 3 mol1 Eo = activation energy of NO in the Mars-van Krevelen model, kJ 3 mol1 Er = activation energy of NH3 in the Mars-van Krevelen model, kJ 3 mol1 5366


Industrial & Engineering Chemistry Research F = total molar flow rate of the gas phase at the inlet of SCR reactor, mol 3 min1 ΔHads = adsorption heat, kJ 3 mol1 k = reaction rate constants in the EleyRideal model and LangmuirHinshelwood model, same units as the specific reaction rate constant k0 = reaction rate constants in the first-order model, s1 ko = reaction rate constants in the Mars-van Krevelen model, mol0.2 3 cm2.4 3 g1 3 s1 kr = reaction rate constants in the Mars-van Krevelen model, mol0.4 3 cm1.8 3 g1 3 s1 K = adsorption equilibrium constant in the EleyRideal model, cm3 3 mol1 KNO = adsorption equilibrium constant in the Langmuir Hinshelwood model, cm3 3 g1 3 s1 KNH3 = adsorption equilibrium constant in the Langmuir Hinshelwood model, cm3 3 g1 3 s1 0 M = CNO /CNH30 (molar feed ratio of NO to NH3), dimensionless m = reaction orders with respects to NO in the power-rate law model, dimensionless n = reaction orders with respects to NH3 in the power-rate law model, dimensionless N = number of experimental data, dimensionless rNO = intrinsic reaction rate of NO per unit catalyst weight, mol 3 g1 3 s1 T = reaction temperature, K v0 = total volumetric flow rate of the gas phase at the inlet of SCR reactor, mL 3 min1 W = catalyst weight, g WHSV = space velocity per unit time per unit catalyst weight, m3 3 h1 3 g1 x = reaction orders with respects to NO and in the Mars-van Krevelen model, dimensionless xNO = NO conversion, dimensionless y = reaction orders with respects to NH3 in the Mars-van Krevelen model, dimensionless Superscripts

cor = correlated NO conversion, dimensionless exp = experimental NO conversion, dimensionless

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