INTERFACIAL APPLICATIONS IN ENVIRONMENTAL ENGINEERING - CHAPTER 2 potx

Catalytic ap-proaches for conversion of NO to N2include 1 NO decomposition [25–28], 2 the NO–CO reaction [29–31], 3 the selective reduction of NO with hydrocar-bons, and 4 the selective

Transient In Situ IR Study of Selective Catalytic Reduction of NO on

Cu-ZSM-5

Akron, Ohio, U.S.A

NOx is a precursor to the formation of ground-level ozone and acid rain Concerns over the negative impact of NOx on the environment and human health led to regulation of NOx emission under the provisions of the 1990 Clean Air Act Amendments (CAAA) The CAAA require a significant decrease in the emission

of NOx, hydrocarbons, and CO over the next few years The key challenge for the automobile/truck industries and coal-fired power plants is to develop a cost-effective catalytic approach for control of NO emission [1–24] Catalytic ap-proaches for conversion of NO to N2include (1) NO decomposition [25–28], (2) the NO–CO reaction [29–31], (3) the selective reduction of NO with hydrocar-bons, and (4) the selective reduction of NO with ammonia [17,32] The direct decomposition of NO to N2and O2is the most attractive approach for NO emis-sion control However, no catalysts have been found to exhibit sufficient activities

in the oxidizing environment where a high concentration of O2is present in the exhaust stream O2poisons not only the NO decomposition catalysts but also the NO–CO reaction catalysts [26–28] In contrast, the presence of O2results in an increase in NO conversion and N2yields in the selective catalytic reduction (SCR)

of NO with hydrocarbons and NH3 The interesting role of O2in promotion NO conversion and N2formation has led to a large number of postulations [1–24]: (1) the reaction of O2with NO to form highly reactive NO2, (2) controlling the redox cycle of active sites and limiting coke formation, (3) the reaction of O2with hydrocarbons to form oxygenates that further reduce NO, and (4) enhancement of the rates of both formation and destruction nitrates—the SCR reaction intermedi-ates

To determine the role of O2 in the SCR and its raction pathway, we have employed in situ IR spectroscopy and mass spectrometry to study the dynamic behaviors of adsorbed species during the step-and-pulse switch of the NO, O2,

C3H6reactant flows during the SCR reaction over Cu-ZSM-5

The overexchanged Cu-ZSM-5-523 was prepared by repeated ion exchange of Cu-ZSM-5-83 with a 0.004 M copper acetate solution of pH⫽ 7

Cu-ZSM-5-83 was produced by Johnson Matthey and provided through the catalyst bank of Sandia National Laboratories The percentage copper exchange is defined as the molar ratio of the amount of Cu to that of Al multiplied by 2 [% Cu exchange

⫽ 2 (moles of Cu/moles of Al)%] [25] The ion-exchange Cu-ZSM-5 sample was filtered, washed by distilled water, dried overnight at 373 K, and then calcined at

733 K for two hours Inductive coupled plasma emission spectroscopy (Galbraith Laboratories, Knoxville, TN) determined Si/Al of 24.6 and Cu/Al of 2.6 in Cu-ZSM-5-523

cell [33], a Nicolet Magna 550 Fourier transform infrared (FTIR) spectrometer, and a Pfeiffer PRISMA mass spectrometer (MS) A gas distribution system delivered the reactants and inert gases to the IR reactor cell containing the cata-lyst To avoid the formation of NO2 from the gaseous reaction of NO with O2, the NO and O2 flows were mixed in the vicinity of the catalyst sample in the infrared reactor cell, as shown in the inset of Figure 1 One-hundred-mg catalysts were pressed into three to four thin disks, each weighing 25 mg, by a hydraulic press at 4000–5000 psi One of the disks was placed in the IR beam path inside the IR reactor cell, and the others were broken down into flakes and placed in the close vicinity of the IR beam path The MS determined the changes in the concentration of the reactants and products, while the FTIR monitored the changes in the concentration of the absorbates during the transient IR experiment

Dynamics of the formation of adsorbed species and products was studied by step and pulse experiments The step experiment involves switching the inlet flow from He to NO/C3H6/He and from NO/C3H6/He to NO/C3H6/O2/He He was used an an inert gas to dilute the reactant stream The switch of the flows creates

a step change in the reactant concentration while maintaining the total flow rate

FIG 1 Reaction system for HC-SCR.

constant at 50 cm3/min The pulse experiment uses a six-port gas chromatograph (GC) injection valve to introduce a C3H6pulse into a steady state of NO/O2/He flow over Cu-ZSM-5 at 373 and 623 K

Products

The mass-to-charge ratios (i.e., m/e or amu) for MS monitoring were m/e⫽ 30

for NO, m/e⫽ 32 for O2, m/e⫽ 46 for NO2, m/e⫽ 41 for C3H6, m/e⫽ 28 for

N2and CO, m/e ⫽ 12 for CO, m/e ⫽ 44 for N2O and CO2, and m/e ⫽ 22 for

CO2 The responding factor of each species was determined by injecting a known

amount of each species into the MS The relative responding ratio of m/e⫽ 28

to m/e ⫽ 12 for CO and that of m/e ⫽ 44 to m/e ⫽ 22 for CO2were further

determined to separate the contribution of CO to m/e⫽ 28 and that of CO2to

m/e⫽ 44 The responding factor obtained for each species allows for conversion

of the MS profile to the corresponding molar flow rate

Figure 2 shows MS profiles of reactants and products during the step experiments

at 623 K, providing an overall picture of the effect of O2on the reactant

conver-FIG 2 MS profiles during step switch from He to 800 ppm NO/2% C3H6/He and from

800 ppm NO/2% C3H6/He to 800 ppm NO/2% C3H6/2% O2/He (total flow rate 50 cm3/ min) at 623 K over Cu-ZSM-5

sion and product formation during the SCR reaction The MS intensity of each species is proportional to its concentration in the reactor effluent The most obvi-ous effect of adding 2% O2into the 800 ppm NO/2% C3H6/He stream at 623 K

is a significant decrease in the concentration of NO and C3H6as well as an in-crease in N2, CO2, and H2O concentrations The former reflects an increase in the conversion of NO and C3H6reactants; the latter indicates an increase in product formation

To gain an insight into the SCR reaction, the IR spectra taken during each step switch of the inlet flow were plotted along with the variation of molar flow rate of the reactant and products Figure 3 shows that exposure of Cu-ZSM-5 to

800 ppm NO/2% C3H6/He produced CH3COO⫺at 1452 cm⫺1, C3H7–NO2at 1547 and 1596 cm⫺1, Cu⫹–CO at 2155 cm⫺1, Cu0–CN at 2198 cm⫺1, and Cu⫹–NCO

at 2241 cm⫺1 These species have also been observed during the SCR over CuO/

Al2O3and Pt/Al2O3 [34,35] The variation of normalized infrared intensities of these IR-observable species with respect to time was plotted along with the changes in molar flow rate of a number of key species, such as NO, O2, CO2, and N2, in Figure 4 to illustrate the lead/lag relationships between adsorbates and gaseous products

FIG 3 IR spectra collected during step switch from He to 800 ppm NO/2% C3H6/He (total flow rate 50 cm3/min) at 623 K over Cu-ZSM-5

FIG 5 IR spectra collected during step switch from 800 ppm NO/2% C3H6/He to 800 ppm NO/2% C3H6/2% O2/He (total flow rate 50 cm3/min) at 623 K over Cu-ZSM-5

These lead/lag relationships may allow elucidation of the sequence of their formation Variation in the CO2molar flow rate followed very closely changes

in the Cu⫹–CO intensity, suggesting that CO2could be formed via Cu⫹–CO The formation of Cu⫹–CO prior to that of CH3COO⫺, C3H7– NO2, Cu0–CN, and

Cu⫹–NCO [shown inFigs 3and 4(a)] indicates that the pathway for Cu⫹–CO and CO2 formation is independent of that for the adsorbates (i.e., CH3COO⫺,

C3H7–NO2, Cu0–CN, and Cu⫹–NCO), since these species, lagging behind CO and containing N and/or H, are not originated from CO The same argument can also be used to conclude that the pathway for the formation of CH3COO⫺ is independent of C3H7–NO2

Figure 5 shows that the addition of O2 to the NO/C3H6/He flow resulted in

FIG 4 (a) Normalized IR intensity versus time and (b) formation rate of reactants and products during step switch from He to 800 ppm NO/2% C3H6/He (total flow rate 50

cm3/min) at 623 K over Cu-ZSM-5 (normalized IR intensity⫽ (I(t)–I0)/(I∞–I0), where

I0⫽ IR intensity at t ⫽ 0, I(t) ⫽ IR intensity at t, ⫽ I∞⫽ IR intensity at t ⫽ ∞, i.e., final

steady state)

an increase in IR intensity of C3H7–NO2, Cu⫹–NCO, Cu0–CN, and Cu⫹–CO The variation of IR intensity of these species with time, plotted in Figure 6(a), shows that the formation of Cu⫹–CO led that of Cu0–CN, which further led that

of C3H7–NO2, and Cu⫹–NCO The O2addition also resulted in enhancement of

CO2and N2formation as well as NO conversion, as shown in Figure 6(b) The absence of variation of CH3COO⫺ suggests that its formation and destruction rates were not affected by O2 The species may adsorb on the surface of

ZSM-5 and serve as a spectator The increase in the intensity of Cu⫹–CO and Cu0–

CN revealed that O2did not enhance oxidation of Cu0/Cu⫹to Cu2 ⫹site The key role of O2is to accelerate the rate of formation of C3H7–NO2, Cu⫹–NCO, Cu0–

CN, and Cu–CO adsorbates as well as gaseous N2and CO2products The forma-tion profile of CO2led that of N2, further supporting that CO2and N2formation

do not share the same reaction pathway

C3H6pulse into the NO/O2flow at 373 K Pulsing C3H6caused increases in IR intensities of CH3COO⫺at 1370 cm⫺1, Cu2 ⫹(NO3 ⫺) at 1575 and 1643 cm⫺1, and

C3H7–NO2at 1444 cm⫺1as well as increases in MS intensities of N2, CO2, H2O, and N2O, indicating reduction of NO/O2by C3H6 Reduction of NO by C3H6also caused the increase in the O2MS profile, suggesting that NO may have decom-posed to N2and O2in the presence of C3H6

cm3C3H6pulse into the NO/O2flow at 623 K Flowing NO/O2/He over

Cu-ZSM-5 produced bridged Cu2⫹(NO3 ⫺) This species has also been observed during the

NO decomposition reaction over Cu-ZSM-5 [26–28] Pulsing C3H6into the NO/

O2/He flow resulted in (1) the depletion of Cu2⫹(NO3 ⫺), (2) the emergence of

Cu⫹–CO, and (3) the formation of gaseous N2and CO2products The C3H6pulse also caused an initial increase in NO concentration, reflecting desorption of NO from the catalyst surface The differences in IR-observable species during the reaction at 373 and 623 K suggest the reaction pathway for N2 formation is strongly dependent on temperature

FIG 6 (a) Normalized IR intensity versus time, (b) formation rate of reactants and products, and (c) normalized formation rate during step switch from 800 ppm NO/2%

C3H6/He to 800 ppm NO/2% C3H6/2% O2/He (total flow rate 50 cm3/min) at 623 K over Cu-ZSM-5 (normalized formation rate⫽ (R(t)–R0) /(R∞–R0), where R0⫽ formation rate

at t ⫽ 0, R(t) ⫽ formation rate at t, and R∞⫽ formation rate at t ⫽ ∞, i.e., final steady

state)

FIG 7 (a) IR spectra collected and (b) MS profiles during 1 cm3C3H6pulse into steady flow of 800 ppm NO/2% O2/He (total flow rate 50 cm3/min) at 373 K over Cu-ZSM-5

reac-tion CO2can be formed by two pathways: (1) partial oxidation of CxHyto Cu⫹–

CO followed by oxidation and (2) oxidation of C3H7–NO2, Cu⫹NCO, and Cu0–

CN The former is much more rapid than the latter, as evidenced by the formation profiles of Cu⫹–CO/CO2leading to that of C3H7–NO2 C3H7–NO2may serve as

a precursor to the formation of both CO2and N2 In addition to C3H7–NO2, the intermediates for N2formation may include Cu0–CN and Cu⫹–NCO, of which the IR intensity profiles parallel the N2molar flow rate profiles during the step switch from He to NO/C3H6/He The nature of C3H7–NO2, Cu0

–CN, and Cu⫹– NCO intermediates can be further distinguished from the results of O2addition The difference in their IR profiles during the switch from NO/C3H6/He to NO/

O2/C3H6/He reflects their differences in reactivity toward O2 The variation in the sequence of C3H7–NO2 and Cu⫹–NCO formation during step switch from

FIG 8 (a) IR spectra collected and (b) MS profiles during 1 cm3C3H6pulse into steady flow of 800 ppm NO/2% O2/He (total flow rate 50 cm3/min) at 623 K over Cu-ZSM-5

FIG 9 Proposed pathways for the steady-state C3H6SCR reaction

FIG 10 Proposed pathways for the C3/H6pulse SCR reaction.

He to NO/C3H6and from NO/C3H6to NO/C3H6/O2/He indicates that Cu⫹–NCO does not have be formed via C3H7–NO2 The evolution of these species is in line with that of N2, suggesting that these species may serve as precursor for N2 forma-tion In fact, it has been shown that these species react with gaseous NO to pro-duce N2[36,37]

Figure 10 illustrates the postulated pathways for the C3H6-pulse SCR reaction where C3H6is pulsed into the NO/O2/He flow The reaction of NO with O2on Cu-ZSM-5 produced chelating nitrate at 373 K and bridged nitrate at 623K Puls-ing C3H6led to the formation of O2and N2and CO2at 373 K Formation of O2

indicates that NO can be decomposed to N2 and O2 through reduction of the catalyst surface C3H6 at 373 K At 623 K, O2 is highly reactive toward C3H6, producing CO2and H2O No direct evidence can be observed to support the NO decomposition pathway at 623 K The key reaction pathway for N2formation at this temperature is the direct reaction of nitrate with propylene, as shown in Fig

10 The nitrate–proplyene reaction led to the formation of not only CO2/N2but also CU⫹–CO The latter suggests that one of the key roles of C3H6is to keep

Cu in the Cu⫹state

An effective use of hydrocarbon in HC (hydrocarbon)-SCR is to develop a catalyst that accelerates the rate of the reaction between NO species and hydrocar-bon while inhibiting the direct reaction of hydrocarhydrocar-bons with oxygen The differ-ent pathways for CO2 and N2 formation suggest that it should be possible to devise a selective poisoning approach to inhibit CO2formation without affecting

N2formation Selective inhibition of CO2formation should limit the direct oxida-tion of hydrocarbons, resulting in a significant enhancement of the selectivity toward N2for the HC-SCR reaction

Infrared spectroscopy coupled with mass spectroscopy allows determination of the dynamic behavior of adsorbates, the reaction pathways for NO reduction and

C3H6oxidation during SCR reaction Transient IR studies of the NO/C3H6/O2

reaction at 623 K showed that CO2and N2were formed from different reaction pathways O2 enhanced the formation of Cu⫹–CO, C3H7–NO2, and Cu⫹–NCO

species and increased the overall rate of No conversion The different N2 and

CO2formation pathways suggest the SCR reaction process may be further im-proved by a selective poisoning approach that inhibits CO2 formation without interfering with N2formation

ACKNOWLEDGMENT

This work was supported by the National Science Foundation under Grant CTS-942111996

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