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Estimation of Effective Diffusivity of Stored NOx in the Barium Phase of Pt/BaO/ Al2O3 Catalysts using TAP Ashok Kumar, Michael P. Harold,* and Vemuri Balakotaiah* Department of Chemical and Biomolecular Engineering, UniVersity of Houston, Texas 77204-4404
A systematic study over Pt/BaO/Al2O3 powder catalyst is carried out using Temporal Analysis of Products (TAP) to estimate the effective diffusivity of stored NOx in the barium phase. The prenitration of Pt/BaO/ Al2O3 using sequential pulses of NO, followed by reduction with H2 results in the evolution of N2 and NH3. The reduction is carried out in the NOx transport limited regime in which diffusion of the stored NOx from BaO storage phase to Pt/BaO interface was determined to be the rate controlling process. The effluent profiles of N containing species (2N2 + NH3) were used to estimate stored NOx diffusivity in barium phase and the apparent activation energy. The activation energy (74-81 kJ/mol) is in good agreement with the estimated value of 75 kJ/mol from a recent NOx trap modeling study.1 1. Introduction NOx storage and reduction (NSR) is an emerging technology for NOx emission abatement in lean burn gasoline and diesel engines. It consists of two stages of operations; storage and reduction. The first stage involves storage of NOx (NO and NO2) on an alkaline earth oxide (BaO, CaO), mediated by precious metals (Pt, Rh), in the form of nitrate and/or nitrite. This is followed by injection of a rich pulse of a shorter duration to reduce the stored NOx to a mixture of N2, N2O, and NH3. This cycle is continuously repeated to achieve more than 90% conversion of the exhaust NOx. The reduction of stored NOx involves transport of stored NOx from the storage phase (BaO) to dispersed metal phase (Pt), followed by catalytic reaction of N containing species with adsorbed H and CO to form N2, NH3, N2O, CO2, and H2O. The transport of NOx in the storage phase occurs because of concentration gradients of NOx and can be the limiting process during the reduction process for lower Pt dispersion catalysts.1,2 The NOx species present in the barium phase near the Pt/BaO interface transfer to the Pt sites through a reverse spillover mechanism.3 The spillover transport is likely a result of a chemical potential gradient across the Pt/BaO interface. Forzatti et al.4 and Cant et al.5 showed the importance of close proximity of Pt and BaO on a support during NOx reduction. They showed that reduction rates were faster when Pt and BaO were dispersed on the same support than just a physical mixture of powders. In several studies,2,6,7 the reverse spillover of NOx from BaO to Pt was speculated to occur during the regeneration. James et al.6 utilized the reverse spillover mechanism of NOx to explain the decomposition of nitrates stored far from Pt. Zhou et al.7 suggested that the nitrate ions are mobile on the barium phase and the NOx reduction occurs at the Pt/Ba interface. Clayton et al.2 studied NOx reduction over Pt/BaO/Al2O3 catalysts having different Pt dispersion to quantify the effect of Pt surface area, Pt/BaO interfacial perimeter and distance of stored NOx from interface. A parallel route for NOx reduction may involve transfer of NOx from BaO to Pt by the decomposition of the stored nitrates/ nitrites, especially at higher temperatures, to NO and NO2 and their transport to Pt sites via the gas phase. The gaseous NOx transport is then followed by reaction on the Pt sites to form * To whom correspondence should be addressed. E-mail:
[email protected] (M.P.H);
[email protected] (V.B.).
N2, NH3, and N2O. In addition to these reduction mechanisms, the spillover of reductant from Pt to barium phase has been proposed.8,9 Such a mechanism involves activation of reductant on Pt with subsequent forward spillover to the storage component, where the reductant reacts directly with nitrates forming nitrites. This results in the release of the stored NOx as NO. However, debate persists over the direction of spillover and the identity of the spillover species during the regeneration of stored NOx. Few of the studies address the details and the effect of solidstate transport processes of NOx in the barium phase.10 The surface diffusion of NOx adspecies toward Pt sites has been proposed during reduction.11 Clayton et al.2 suggested that transport of NOx from the storage phase to the Pt/BaO interface is the limiting step during reduction, especially for Pt/BaO/Al2O3 catalyst with low Pt dispersion. Recently, Bhatia et al.1 developed a crystallite scale model for NOx reduction on Pt/ BaO/Al2O3 catalysts having different Pt dispersion. The model calculations help to explain H2 feed limitation for highdispersion catalyst (50%) and NOx transport limitation in barium phase for low dispersion catalyst (3.2%). They used diffusivity values of NOx in the barium phase for different temperatures based on characteristic times like reaction time, convection time, etc. An apparent activation energy was estimated to be 75 kJ/ mol with diffusivities in the range of 5 × 10-19 to 5 × 10-16 m2/s for temperatures 160-370 °C. Temporal Analysis of Products (TAP), developed by Gleaves and co-workers,12 has proven its utility in understanding a host of transient heterogeneous catalytic reaction systems (refs 13 and 14 and references within). The NSR is inherently a transient process that makes TAP a suitable tool. In our group, we have employed TAP to study NSR chemistry on Pt/Al2O3 and Pt/ BaO/Al2O3 catalysts using NO pulse and NO-H2 pump-probe experiments.15-17 Recently, TAP was coupled with isotopic labeling to gain understanding of spillover mechanisms and distribution of stored NOx in the barium phase.18 While spillover and reverse spillover processes have been proposed by the aforementioned studies to explain the NOx storage and reduction data, there have been no attempts to measure the diffusivity of NOx in the storage phase. In this study, we employ Temporal Analysis of Products (TAP) to estimate the effective diffusivity of stored NOx in barium phase. The diffusivity data at various temperatures is used to estimate stored
10.1021/ie100504q 2010 American Chemical Society Published on Web 07/22/2010
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Table 1. Properties of Catalyst Sample Used in TAP Rector parameter
notation
numerical value
Pt weight fraction BaO weight fraction Pt dispersion Pt crystallite radius mass of catalyst
wPt wBaO d ri mCat
0.0248 0.13 0.032 15.0 nm 41.4 mg
NOx diffusion activation energy in the barium phase of the catalyst. TAP experiments are carried out isothermally over the powder catalyst, thereby avoiding thermal and mass transport complications typical of atmospheric pressure reactors. 2. Experimental Section A generation-1 TAP reactor system was used following the methodologies described in more detail elsewhere.15 The Pt/ BaO/Al2O3 catalyst powder (Table 1) used in this study was provided by BASF Catalyst LLC (Iselin, NJ). It has a Pt dispersion of 3.2%, and contains 2.48 wt % Pt and 13.0 wt % BaO on the washcoat basis. About 41 mg of catalyst was sandwiched between two inert glass bead zones. The estimated number of exposed Pt sites in the catalyst sample was 1.0 × 1017. In a typical experiment, catalyst was first reduced by flowing H2 for 2 h at 400 °C, and was then bought down to reaction temperature (250-400 °C). The feed component (NO or H2) was injected into the reactor using fast pulsing valves. The spacing between two successive pulses was varied between 40 ms and 1 s. Effluent species including N2 (m/e ) 28), NO (m/e ) 30), N2O (m/e ) 44), and NH3 (m/e ) 16) were monitored with a calibrated UTI 100C quadrupole mass spectrometer. The NH3 signal was measured at m/e ) 16 to avoid the overlap with H2O at m/e ) 17. The gaseous species signals were calibrated with several hundred pulses on the inert bed reactor. The inlet pulse sizes were calculated by measuring the pressure drop in an isochoric bulb for each gaseous species. The pulse size was correlated with the mass spectrometer signal using a calibration number. Typically, the mass balances with these calibration numbers were accurate to within 10%. 3. Results and Discussion The process of reducing NOx stored on the Pt/BaO/Al2O3 catalyst with H2 involved three sequential processes: (i) supply of feed H2 from gas phase to Pt or Pt/BaO interface; (ii) transport of stored NOx from barium sites to Pt/BaO interface; (iii) reaction of NOx with H2 at Pt or Pt/BaO interface to produce gaseous N2 and NH3. To estimate an effective diffusivity of stored NOx in barium phase, the process of stored NOx reduction should operate under transport-limited conditions, that is, any limitation posed by other two processes needs to be eliminated. Nova et al.9 showed that surface nitrates react catalytically (in the presence of Pt) with H2 at low temperatures (∼150 °C). Moreover, the nitrate reactivity was reported very high, so that at 170 °C, H2 was completely consumed and the formation of the reaction products was limited by the H2 concentration. We show below that in order to avoid a H2 feed limiting condition, the spacing between consecutive H2 pulses should be shortened so as to sustain excess H2 in the vicinity of the Pt crystallites. The amount of H2 fed per pulse should be sufficient to ensure an excess supply. The exact amount depends on the amount of stored NOx, mass of the catalyst, and temperature for a catalyst with fixed Pt loading and dispersion. Experimental studies by Pihl et al.,19 Cumaranatunge et al.,20 Mulla et al.,21 Clayton et al.,22 and Lietti et al.23 showed that
Figure 1. Effluent H2 profiles during H2 pulsing on catalyst at 400 °C with pulse spacing of 1 s, 0.1 s, and 40 ms.
NH3 is an intermediate, as well as a hydrogen carrier during stored NOx reduction. The reduction of NOx was found to be limited by supply of reductant, which led to the conclusion that the identity of the reductant or the reaction kinetics was unimportant.21 Clayton et al.22 showed that kinetic limitations were important at lower temperatures because of reactivity differences between H2 and NH3. Bhatia et al.1 compared the characteristic reaction and diffusion times and concluded that the diffusion times of stored NOx in the barium phase are orders of magnitudes higher than reaction times for 3.2% Pt dispersed Pt/BaO/Al2O3 monolith catalysts and temperatures exceeding 160 °C. These results suggest that the reduction process would not be kinetically limited for reactor temperatures above 250 °C. The typical exit flux response of gaseous pulses of H2 from the reactor is shown in Figure 1. The spacing times between two consecutive pulses were fixed at 1 s, 0.1 s, or 40 ms. The longer spacing time of 1 s resulted in pulses that are well separated and have no overlap. The H2 exit flux response with 1 s spacing decreased to the baseline level in only 0.3 s. Under these conditions, the separated pulses may lead to a gaseous feed limited state because of the absence of H2 for some duration (i.e., from time 0.3 to 1.0 s). A shorter spacing time of 0.1 s leads to an overlap of two consecutive pulses in the reactor, such that H2 never decreases to the baseline level. A further shortening of the spacing time (40 ms) between two consecutive pulses results in significant overlap between consecutive exit fluxes of H2, whose minimum value is much higher than the baseline level. This high frequency pulsed feed mimics a continuous feed of H2. Moreover, it ensures that H2 is present in the reactor during the course of experiment and avoids feed limiting conditions. Therefore, all subsequent experiments had 40 ms spacing between consecutive reductant pulses. In a typical TAP experiment, Knudsen diffusion is the main transport process. In contrast, for the current experiments the assumption of Knudsen diffusion is not valid. Rather, the dominant transport process is convection, which helps to eliminate transport limitations. In order to estimate the effective diffusivity, the prereduced catalyst was first nitrated with sequential pulses of NO at temperatures between 250 and 400 °C. The spacing between consecutive NO pulses was kept at 40 ms. The NO was stored for a fixed time of 110 s (2750 pulses of NO) at each temperature. The amount of NOx stored at these temperatures is listed in Table 2. It is evident that the NOx storage increases with increase in temperature. A higher temperature facilitates N-O bond scission that leads to
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Table 2. Dependence of NOx Stored and NOx Reduced with Temperature temperature NOx stored reduction T (°C) ×1018 molecules pulses 400 375 350 325 300 275 250
3.02 3.04 2.98 3.03 1.53 1.31 1.03
3300 3300 3300 5500 5500 7700 7700
reduction NOx reduced time NNox tR (s) ×1018 molecules 132 132 132 220 220 307 307
2.99 2.77 2.39 2.38 1.21 0.96 0.72
formation of gaseous N2 and the adsorbed oxygen on the catalyst. The enrichment of surface oxygen results in a higher ratio of fractional coverages of surface O to N (or NO) that is essential for NOx storage in the forms of barium nitrate (O/N ) 2.5) and/or barium nitrite (O/N ) 1.5). In addition, the NOx storage is enhanced with an increase in temperature because this leads to an increase in the diffusivity of NOx in the barium phase at higher temperatures. The storage was followed by reduction in the presence of sequential pulses of H2 separated by aforementioned 40 ms. The reduction resulted in the production of N2 and NH3 along with the unreacted H2 in the effluent. The typical effluent profiles of N2, NH3, and 2N2 + NH3 are depicted in Figure 2 for two different H2 feed rates at 400 °C. A comparison of effluent N2 and NH3 for two H2 flow rates shows that the higher H2 flow rate (2.5 × 1018 molecules/s) favors NH3 production, while the smaller H2 flow rate (1.8 × 1018 molecules/s) favors N2 production. However, for both the cases, the response of total N-containing products (2N2 + NH3) remains the same. This result suggests that the production of N containing species is not limited by supply of H2 at these flow rates of H2. The variation of the effluent flux of N containing products with inlet H2 flow rate is shown in Figure 3. The exit N containing species flux profiles for H2 flow rates between 1.5 × 1018 and 2.5 × 1018 molecules/s are identical. However, the H2 flow rate of 1.0 × 1018 molecules/s resulted in a notable decrease in the product yield. This suggests that N-containing effluent response is limited by H2 supply for H2 flow rates lower than 1.0 × 1018 molecules/s, while H2 flow rate greater than 1.5 × 1018 molecules/s does not incur any H2 feed limitations. In order to understand the H2 feed limitation at lower temperatures, the reduction experiments (not shown here) were repeated
Figure 3. Exit flux of N-containing products () 2N2 + NH3) during different H2 flow rates at 400 °C.
Figure 4. Flux of effluent N containing products (2N2 + NH3) during reduction of stored NOx by H2 at (a) 400-350 and (b) 325-250 °C.
Figure 2. Effluent profiles of N2, NH3, and 2N2+NH3 during H2 flow of 1.8 × 1018 molecules/s and 2.5 × 1018 molecules/s at 400 °C.
at 350 °C. The feed limitations were found to be absent at H2 flow rates exceeding 8.7 × 1017 molecules/s. The corresponding H2 flow rate at 400 °C was 1.5 × 1018 molecules/s, which is a significantly higher value. Thus, a decrease in temperature resulted in the elimination of the feed limitations for a smaller H2 feed rate. This further implies that inlet H2 flow exceeding 1.5 × 1018 molecules/s would ensure the absence of feed limitations during NOx reduction for temperatures below 400 °C. The temporal variation of the total N containing species flux over temperatures between 250 and 400 °C is shown in Figure 4. The range of inlet H2 flux rate for these experiments was maintained
Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010
exposed Pt atoms in the catalyst in the sample, by
Table 3. List of Parameters Used for Crystallite Scale Pt parameter
notation
numerical value
total Pt Sites exposed Pt sites Avogadro number molecular weight of Pt number of Pt atoms in one crystallite number of crystallites in catalyst atomic packing factor for Pt radius of Pt atom surface density of BaO
Cat N Pt Exp N Pt NAv MPt Cryst N Pt
3.16 × 1018 1.01 × 1017 6.022 × 1023 195.08 g/mol 4.65 × 105
Cat N Cryst
6.81 × 1012
APF rPt SBaO
0.74 0.139 nm 1.89 × 1018 BaO molecules/ m2 exposed BaO
Exp Cat NPt ) dNPt
mCatwPtNAv MPt
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is estimated
(2)
where d is the fractional Pt dispersion. With an assumed hemispherically shaped Pt particle, the total number of Pt atoms in one Pt particle is obtained as
at 1.7-2.2 × 1018 molecules/s. This H2 flux rate was chosen such that reduction process was not limited by the supply of H2. The total N containing species profiles during reduction in the temperature range 400 °C -350 °C show an initial increase in N containing flux, followed by an intermediate maximum between 2 and 4 s, and then followed by a monotonic decrease (Figure 4a). A decrease in temperature resulted in lower peak flux and higher peak time of N containing species. A further decrease to 325 °C and lower resulted in a monotonic decrease in N species flux (Figure 4b). The flux of N containing species decreased with temperature, so the reduction time or corresponding number of H2 pulses was increased (Table 2). The effective diffusivity of stored NOx can be estimated from these data. Consider the crystalline scale schematic of Pt/BaO/Al2O3 in Figure 5, following the conceptual model used by Bhatia et al.1 Our approach is to estimate the effective distance of stored NOx from the Pt/BaO interface and an average reduction time. The schematic shows top view of a single crystallite with an assumed hemispherically shaped Pt particle. Here, ri is the radius of Pt particle, and ro is the radius over which stored NOx is reduced in reduction time, tR. A listing of parameters is provided in Table 3. Cat ) is The total number of Pt atoms present in the catalyst (N Pt estimated using the mass of catalyst (mCat) and Pt mass fraction (wPt) and is expressed as Cat NPt )
N Exp Pt
(1)
where, MPt is the molecular weight of Pt and NAv is Avogadro’s number. Using the above expression, the total number of
Cryst NPt
2 3 πr A 3 i PF ) 4 πr 3 3 Pt
(3)
Here, rPt is the radius of a single Pt atom, and APF is the atomic packing factor. A value of 0.74 is been used for APF, in line with the face centered cubic (FCC) structure of Pt lattice. The total number of Pt particles present in the sample is Cat NCryst )
Cat NPt
(4)
Cryst NPt
We assume that the stored NOx is stored in the proximity of Pt in the form of Ba(NO3)2 or Ba(NO2)2. In our recent NOx storage study over Pt/BaO/Al2O3, barium nitrate was found to be the primary stored species.18 The number of stored NOx molecules reduced during reduction time tR (i.e., NNOx) is equated to twice the number of exposed BaO sites, that is, Cat NNOx ) 2NCryst SBaO
∫
ro
ri
2πrσ(r)dr
(5)
Here, SBaO is the surface density of BaO (BaO molecules/m2 of exposed BaO), ro is the radius over which stored NOx is reduced during time tR, and σ(r) is the surface concentration of stored NOx in the barium phase. The value of SBaO is obtained from Bowker et al.,24 who reported that BaO (111) surface, prepared on thin film form on Pt (111), has a surface with twice the lattice parameter expected for that of the bulk termination, that is, a (2 × 2) reconstruction. Even though the actual BaO surface on the catalyst may not be an ideal (111) surface, this value of SBaO used as best possible value available in the literature.1 Sakamoto et al.25 imaged Pt/Ba thin film on a Si substrate using electron probe microanalysis (EPMA). They found that NOx strongly adsorbs around the edge of Pt crystallite and reduction by H2 preferentially occurs in the proximal region of Pt. Recently, Kumar et al.18 employed TAP coupled with isotopic labeling to experimentally verify the gradients in the vicinity of the Pt/BaO interface during NOx storage and reduction. In the absence of any prior stored NOx profiles in the barium phase, we assumed different forms for the function σ(r) as σ(r) ) 1.0
(6)
σ(r) ) 0.5
(7)
r
σ(r) ) e-0.05( ri -1) σ(r) ) e-0.001
Figure 5. Schematic diagram of Pt/BaO/Al2O3 catalyst showing ri, ro, and re.
(
r - 1 ri
(8)
)
2
(9)
Equations 6 and 7 represent uniform stored NOx distribution in the barium phase. Bhatia et al.1 assumed a uniform distribution (eq 6) in barium phase. Equations 8 and 9 represent exponential and normal distribution of NOx in the
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Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010 Table 4. Variation of Average Time, Effective Radius, and Effective Diffusivity with Temperature for the Special Case, σ(r) ) 1
Figure 6. Radial dependence of σ(r) for barium particle. The value of ro and re depend on both temperature and the functional form of σ(r).
barium phase. The maximum value of stored NOx is at Pt/ BaO interface (r ) ri), and it decreases monotonically with the distance in the barium phase (Figure 6). The coefficients of (r/ri) - 1) and (r/ri - 1)2 were chosen such that ro estimated using σ(r) and NNOx (eq 5) lies below the estimated barium particle radius (700 nm). Higher values of these coefficients would result into stored NOx radius greater than the radius of barium or the impossibility of NOx storage situation with defined σ(r) and NNOx. Using the stored NOx concentration function σ(r), the radius of stored NOx over which stored NOx is reduced during time tR, that is, ro can be calculated from eq 5. For a uniform NOx concentration for stored NOx (eq 6), ro can be simplified as ro )
NNOx
ri2 +
Cat 2NCryst πSBaO
(10)
Using the second moment of stored NOx in barium phase wrt Pt/BaO interface, the effective radius of stored NOx, re, is estimated as
re ) ri +
∫
ro
(r - ri)22πrσ(r)dr
ri
∫
ro
ri
(11) 2πrσ(r)dr
The above expression can be simplified for the special case, σ(r) ) 1, as re ) ri +
4ri(ro2 + ri2 + rori) ro2 + 3ri2 2 3(ro + ri)
temperature T (°C)
average time te (s)
reduced NOx radius ro (nm)
effective radius re (nm)
effective diffusivity De (m2/s)
400 375 350 325 300 275 250
20.2 21.8 26.0 72.8 97.4 121.2 116.3
193 186 173 172 123 110 96
108 103 93 93 59 49 39
3.7 × 10-16 3.2 × 10-16 2.3 × 10-16 8.0 × 10-17 2.8 × 10-17 1.7 × 10-17 1.3 × 10-17
R is the constant and its value is 2 for 1D diffusion, 4 for 2D diffusion, and 6 for 3D diffusion. Here, we consider radial diffusion of NOx in the barium phase during reduction, so a value of 2 is used for R. A sample calculation of the average time, effective radius and effective diffusivity of stored NOx are listed in Table 4 for a special case of σ(r) ) 1. Finally, the effective diffusivity is correlated to the exponential dependence of temperature as De ) De(To) exp
{ (
To ED 1RTo T
)}
(15)
This is Arrhenius type diffusivity expression, where De(To) is the effective diffusivity of NOx at temperature To, ED is diffusional activation energy, R is the universal gas constant, and T is temperature. The effective diffusivities are plotted against (1/RTo)(1 - (To/T)) using different stored NOx reduction models to estimate activation energy of NOx diffusion (ED) (Figure 7). The values of apparent diffusional activation energies and effective diffusivity at To ) 250 °C are tabulated in Table 5. This value is in good agreement with the reported value of 75 kJ/mol found in Bhatia et al.1 Figure 7 shows that effective diffusivity of stored NOx increases with increase in catalyst temperature. For a fixed profile of σ(r), the experimental data is scattered, but the order of data points is preserved when the σ(r) profile is changed using eqs 6-9. The average time of reduction, te, is independently estimated using exit N-containing flux profile and its value is fixed at particular NNOx (eq 13). On the other side, the value of re (or ro) depends on the assumed σ(r) profile (eq 5). A sample calculation of ro at 250 °C is shown in Table 5. For example, re (or ro) is minimum for σ(r) ) 1.0 for a fixed value of NNOx, that results in the smallest value of diffusivity for σ(r) ) 1.0 (eq 14). As the surface
(12)
An analogous derivation can be carried out to estimate the average time (te) over which the diffusion occurs. This is estimated from the first moment of exit flux of N-containing species as
∫ ∫
tR
te )
0
tR
0
tJ(t)dt
(13) J(t)dt
where, J(t) is the flux of effluent N-containing species (N2 and NH3) during reduction; J(t) is shown in Figure 4a and 4b. On the basis of the effective radius and average time, the effective diffusivity of stored NOx is estimated as De )
(re - ri)2 Rte
(14)
Figure 7. Effective diffusivity of stored NOx in barium phase estimated using eqs 6-9 with To ) 573.15 K.
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Table 5. Diffusional Activation Energy Estimation from Different Stored NOx Reduction Models equation
σ(r)
ro at T ) 573.15 K (nm)
De (To ) 573.15 K) (m2/s)
ED (kJ/mol)
coefficient of determination (R2)
eq 6 eq 7 eq 8 eq 9 Bhatia et al.1
1.0 0.5 e-0.05(r/ri - 1) e-0.001(r/ri - 1)2 1.0
96 134 104 96
9.34 × 10-18 2.10 × 10-17 1.10 × 10-17 9.45 × 10-18 1.41 × 10-17
75.8 74.5 80.7 76.5 75.4
0.96 0.96 0.96 0.96 0.99
concentration, σ(r), decreases (eq 8 and eq 9), the ro required to stored same amount of NNOx increases and hence the estimated value of diffusivity increases. This means that eq 6 gives the smallest values of effective diffusivity while eq 7 gives the largest values. But, the different initial surface concentration profiles of stored NOx in the barium phase, σ(r), can cause a maximum error by a factor of 2 for the diffusivity estimation. It should be pointed out that the low Pt dispersion (3.2%) catalyst was selected for this study because of larger average distance between stored NOx and Pt/BaO interface. The larger distance amplifies the extent of the limitations of stored NOx transported from the storage phase to Pt/BaO interface. In contrast, had a higher dispersion catalyst been used for this study, it would have been more difficult to achieve NOx transport limited conditions. The smaller distance between stored NOx and interface would enable the reduction process in a feed limited regime. In real practical automobile application, the Pt dispersion should be higher than used in this study. The diffusional transport limited reduction of stored NOx is more realistic during low temperature NOx reduction e.g. during conditions of cold start of the engine. Moreover, the prolonged operation and severe temperature conditions of the catalytic monolith can lead to sintering of the Pt and hence the dispersion of the Pt decreases. The transport limited conditions are more realistic at lower Pt dispersion case with a shift to feed limited conditions at high dispersion. The above finding suggests that for the case of transport limited conditions, it is beneficial to reduce NOx in frequent interval (smaller cycle time) with smaller reduction times than longer cycle time and larger reduction times. 4. Conclusions We have carried out stored NOx reduction study over Pt/BaO/ Al2O3 catalyst using TAP to estimate effective diffusivity of NOx in the barium phase. The spacing time between reductant pulses and reductant flow rates were manipulated such that the reduction was not feed limited and the temperature range for reduction was chosen to eliminate any kinetic limitation. The absence of feed and kinetic limitations ensured that reduction was carried out in NOx transport regime in the barium phase. The effluent profiles of N containing species (2N2 + NH3) were used to estimate NOx diffusivity and apparent activation energy in the barium phase. Disclaimer. This report was prepared as an account of work sponsored by an agency of the United State Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. References herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or favoring by the
United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Acknowledgment This study was jointly funded by BASF Catalysts LLC (formerly Engelhard Inc.) and the U.S. DOE National Energy Technology Laboratory (DE-FC26-05NT42630) with partial support from the National Science Foundation (CT50730824). We would like to thank Stan Roth and CZ Wan from BASF Catalysts LLC for engaging technical interactions and for the catalytic materials used in this study. Literature Cited (1) Bhatia, D.; Harold, M. P.; Balakotaiah, V. Modeling the effect of Pt dispesion and temperature during anaerobic regeneration of lean NOx trap catalyst. Catal. Today 2010, 151 (3-4), 314–329. (2) Clayton, R. D.; Harold, M. P.; Balakotaiah, V.; Wan, C. Z. Pt dispersion effects during NOx storage and reduction on Pt/BaO/Al2O3 catalysts. Appl. Catal. B: EnViron. 2009, 90 (3-4), 662. (3) Olsson, L.; Persson, H.; Fridell, E.; Skoglundh, M.; Andersson, B. A kinetic study of NO oxidation and NOx storage on Pt/Al2O3 and Pt/BaO/ Al2O3. J. Phys. Chem. B 2001, 105, 6895–6906. (4) Forzatti, P.; Castoldi, L.; Nova, I.; Lietti, L.; Tronconi, E. NOx removal catalysis under lean conditions. Catal. Today 2006, 117 (1-3), 316. (5) Cant, N. W.; Liu, I. O. Y.; Patterson, M. J. The effect of proximity between Pt and BaO on uptake, release, and reduction of NOx on storage catalysts. J. Catal. 2006, 243 (2), 309–317. (6) James, D.; Fourre´, E.; Ishii, M.; Bowker, M. Catalytic decomposition/ regeneration of Pt/Ba(NO3)2 catalysts: NOx storage and reduction. Appl. Catal. B: EnViron. 2003, 45 (2), 147. (7) Zhou, G.; Luo, T.; Gorte, R. J. An investigation of NOx storage on Pt-BaO-Al2O3. Appl. Catal. B: EnViron. 2006, 64 (1-2), 88–95. (8) Liu, Z.; Anderson, J. A. Influence of reductant on the thermal stability of stored NOx in Pt/Ba/Al2O3 NOx storage and reduction traps. J. Catal. 2004, 224 (1), 18–27. (9) Nova, I.; Lietti, L.; Castoldi, L.; Tronconi, E.; Forzatti, P. New insights in the NOx reduction mechanism with H2 over Pt-Ba/γ-Al2O3 lean NOX trap catalysts under near-isothermal conditions. J. Catal. 2006, 239 (1), 244–254. (10) Tuttlies, U.; Schmeisser, V.; Eigenberger, G. A mechanistic simulation model for NOx storage catalyst dynamics. Chem. Eng. Sci. 2004, 59 (22-23), 4731–4738. (11) Epling, W. C. L.; Yezerets, A.; Currier, N.; Parks, J. ,Overview of the Fundamental Reactions and Degradation Mechanisms of NOx Storage/ Reduction Catalysts. Catal. ReV. 2004, 46 (2), 163–245. (12) Gleaves, J. T.; Ebner, J. R.; Kuechler, T. C. Temporal Analysis of Products (TAP)- a unique catalyst evaluation system with submillisecond time resolution. Catal. ReV.sSci. Eng. 1988, 30 (1), 49–116. (13) Gleaves, J. T.; Yablonsky, G.; Zheng, X.; Fushimi, R.; Mills, P. L. Temporal analysis of products (TAP)--Recent advances in technology for kinetic analysis of multi-component catalysts. J. Mol. Catal A: Chem. 2010, 315 (2), 108. (14) Pe´rez-Ramı´rez, J.; Kondratenko, E. V. Evolution, achievements, and perspectives of the TAP technique. Catal. Today 2007, 121 (3-4), 160–169. (15) Kabin, K. S.; Khanna, P.; Muncrief, R. L.; Medhekar, V.; Harold, M. P. Monolith and TAP reactor studies of NOx storage on Pt/BaO/Al2O3: Elucidating the mechanistic pathways and roles of Pt. Catal. Today 2006, 114 (1), 72–85.
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ReceiVed for reView March 5, 2010 ReVised manuscript receiVed June 21, 2010 Accepted June 24, 2010 IE100504Q
