Microkinetic Modeling of the Effects of Oxygen on the Catalytic

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Microkinetic modelling of oxygen effects on the catalytic reduction of NO on Pt and Rh in automotive aftertreatment Vishnu S Prasad, and Preeti Aghalayam Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01717 • Publication Date (Web): 11 Aug 2016 Downloaded from http://pubs.acs.org on August 23, 2016

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Industrial & Engineering Chemistry Research

Microkinetic modelling of oxygen effects on the catalytic reduction of NO on Pt and Rh in automotive aftertreatment Vishnu S Prasad & Preeti Aghalayam* Dept. of Chemical Engineering, IIT Madras, Chennai, India 600 036 * Corresponding author. Tel: +91 44 2257 4153 E-mail: [email protected]

Abstract Nitric oxides and unburned hydrocarbons from automotive engines are major atmospheric pollutants. A strategy based on simultaneous reduction of NO and oxidation of hydrocarbon can be very effective in aftertreatment. In this study the various regimes of operation for this selective catalytic reduction process, choosing propene as a representative hydrocarbon, with and without the presence of oxygen, are delineated. Detailed kinetic modelling using quantitative microkinetics for Pt and Rh catalysts is performed. Interesting catalytic features including coking and oxygen poisoning, are clearly identified. An optimal operating regime where complete conversion of NO and C3H6 occurs in the presence of small amounts of oxygen, is highlighted.

1. Introduction Automobile exhausts contain pollutants including oxides of nitrogen and unburnt hydrocarbons. The presence of these harmful gases makes aftertreatment a necessity. NOx is a major pollutant in this category since it is linked with health and environmental issues including acid rain and photochemical smog. While the catalytic converter has been used extensively in modern automobiles, it is severely constrained in terms of the engine out compositions it can handle. Hydrocarbon -Selective Catalytic Reduction (HC-SCR), where NOx is catalytically reduced in the presence of hydrocarbons and oxygen, is one of the promising methods to reduce NOx and other pollutants at a wide range of engine out conditions. A significant amount of research is evident on HC-SCR, but achieving high conversions of NOx in the presence of high amounts of oxygen in the exhausts still remains a challenging problem. A variety of catalysts and reductants have been proposed for HC-SCR. Several platinum group metals (PGM) have been used as catalysts in the literature, in addition to non-noble metals. Comparison of the performance of different catalysts (Pt, Rh, Ir, Pd, etc.) for the reduction of NO by propene, at various operating conditions was performed in literature1, 2. Ag catalysts show a high amount of NOx reduction3,4, the influences of Ag loading and catalyst preparation methods were examined. Zeolite catalysts are also gaining popularity for HC-SCR of automobile exhaust gases and Cu, Co-exchanged zeolites were used with propane and methane as reductants5. Influence of several hydrocarbons for NO reduction using Pt catalysts was also studied - for a specific carbon number, the order of efficiency of NO reduction by HC was determined to be iso paraffins < aromatics < n paraffins < alcohols1. Furthermore, it has been established that compared to propane, propene works as a significantly better reductant for NO6. The influence of

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different supports of the catalyst (alumina and silica) for NO reduction in HC SCR was highlighted in literature7. Detailed catalytic surface reaction mechanisms are used in the literature to analyse the various NO reduction phenomena of relevance to automotive aftertreatment. For a three way catalytic converter, a large, detailed surface reaction mechanism was proposed in literature8, in this work, the effect of NO reduction for different exhaust conditions with lean, rich and stoichiometric mixtures was studied. Microkinetic modelling for different hydrocarbons on Ag catalysts is performed9, 10 and the influence of NO/HC concentrations on NO conversion, and also the effect of various hydrocarbons as reductants (including oxygenated species) were discussed. In this work, we closely examine the influence of inlet O2 concentration on the conversions of NO and propene at different temperatures, on Pt and Rh catalysts. A laboratory-scale packed bed reactor with small L/D ratio is simulated here, using an ideal reactor approximation. A microkinetic model based on literature on automotive catalytic converters is used in this simulations study. The overall aim is to identify various regimes for HC-SCR and explain the observed phenomena in each of these regimes via analysis of surface coverages. Ideal operating conditions are highlighted, and the drawbacks associated with some of the commonly encountered operating conditions are analysed.

2. Methodology The modelling and simulation is performed using the CHEMKIN software package (CHEMKIN-PRO Release 15131). The detailed surface reaction mechanism (provided in supporting information) is incorporated and the perfectly stirred reactor configuration in CHEMKIN is used. An isothermal perfectly stirred reactor of volume 2*10-3 m3, inlet gases flow at a rate of 5*10-2 m3/s, are considered. The pressure is 1 bar and the reactor temperature is varied from (200-650) 0C .The residence time11 in the reactor is 0.04 s. The catalysts studied here are Platinum and Rhodium, the internal surface area of both the catalysts is taken to be 180 m2/g, based on our earlier experimental work11. The surface site density of the Pt catalyst8 is assumed to be 2.04*10-5 mole/m2 while that of rhodium12 is 2.72x10-5 mole/m2. The input data used in our simulations is shown in Table 1. Table 1. Reactor and Catalyst input parameters

Volume of the reactor (V)

2*10-3 m3

Inlet volumetric flow rate (Q)

5*10-2 m3/s

Temperature of reactor (T)

(200-650) 0C

Pressure of the reactor (P)

1 bar

Surface area of catalysts (Pt/Rh)

70 m2

surface site density of Pt catalyst ( Γ / N A )

2.04*10-5mole/m2

surface site density of Rh catalyst ( Γ / N A )

2.72x10-5 mole/m2

In this work the inlet concentrations of HC (propene) and NO are constant while the O2 concentration is varied in order to study the behaviour of the system. The inlet feed compositions are HC 2000 ppm, NO 1000 ppm, with O2 varying from 0% to 3% and the remaining gas is N2.


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2.1. Detailed reaction mechanism The detailed surface reaction mechanism used here has 52 elementary reactions with 8 gaseous species and 18 surface species, as shown in (Table S1 in the supporting information). The surface reaction mechanism for the Pt catalyst is taken from literature8. All the reaction steps, pre-exponential factor/sticking coefficient and activation energy values for Pt catalysts shown in (Table S1 in the supporting information) are from literature8. However, in their work on three-way catalytic converters, they have included additional reactions to account for the presence of Rh catalyst alongside Pt catalyst, while in our work we examine Rh catalysts separately. The kinetic parameters for Rh catalysts are also taken from literature12, 13. The activation energy for 52nd reaction (which is not available in the literature sources) is calculated based on thermodynamic consistency. The kinetic parameters for reactions 10, 11, 18 - 22 and 31 - 34 are not available in the literature, and for this analysis are assumed to be same as Pt in the absence of other information. The detailed set of elementary reaction steps on Pt and Rh for the reduction of NO are expected to be the same, albeit with different kinetic parameters associated with each catalyst14, 15. The 8 gaseous species considered are O2, C3H6, H2, H2O, CO2, CO, NO and N2. The 18 surface species considered are Pt(S), O(S), C3H6(S), C3H5(S), OH(S), H(S), H2O(S), CO2(S), CO(S), C(S), CC2H5(S), C2H3(S), CH2(S), CH3(S), CH(S), CH3CO(S), NO(S) and N(S).There are 8 adsorption reactions, 9 desorption reactions and 35 surface reactions in the surface mechanism. The 18th reaction is a global reaction, it is considered as first order in O(S) and 0th order in vacant catalyst sites. In this work, we have neglected a couple of reaction mechanism effects that may prove to be important under certain conditions. In the literature8, some of the activation energies in the reaction mechanism are proposed to be coverage-dependent. We have neglected this aspect in order to be able to make more general conclusions. Furthermore, in automotive NO reduction, the undesirable formation of N2O has been discussed in our earlier work on NO-CO reactions16. This is an important effect that should be included in future work, and may be important at intermediate temperature conditions.

2.2. Reactor-Scale Model In this work, an isothermal, perfectly stirred reactor is simulated. Equations 1-3 represent the balance equations. The reactor-scale simulation is performed using the commercial software CHEMKIN, to obtain steady-state solutions of gas-phase species concentrations and surface coverages, at a range of reactor temperatures. Surface intermediates mass balance

Γ dθk nrxns = ∑ ν kj R j N A dt j =1

(k=1, 2,..nsurface)

(1)

(i=1, 2,...ngas)

(2)

Mass balance for gas-phase species nrxns dCi (Ci 0 − Ci ) = + av *( ∑ ν ij R j ) dt τ j =1

Site conservation nsurface

θ* = 1 −

∑

(3)

θk

k =1



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There are 17 surface intermediate mass balance equations, 8 PSR model gas-phase species equations and 1 site conservation equation. Here Γ is the site density of the catalyst, N A is the Avogadro number, θ k is the surface coverage of kth species, Ci0 is the inlet concentration of ith species, Ci is the outlet concentration of ith species, av is surface area/volume of the catalyst and τ is the space time. The rate constant for adsorption is calculated from kinetic theory of gases: 0.5

" RT % k j = si $ ' # 2ΠM i &

(4)

Here si is the sticking coefficient for the ith species, M i is the molecular weight of the ith species, T is the temperature in K, R is the universal gas constant in (g.m2)/ (s2.mol.k). The rate constants for desorption and surface reactions are calculated as

k j = k0 *

Γ ⎡ − Ea ⎤ *exp ⎢ NA ⎣ RT ⎥⎦

(5)

Here k0 and Ea are the pre-exponential factor and activation energy, respectively.

3. Results and Discussions The conversions of the reactants at the reactor outlet are of important consideration here. A systematic study of the reactant conversions at various operating conditions is undertaken. A detailed analysis of the surface coverages is also undertaken in order to understand the behaviour. The inlet concentrations of NO and C3H6 are maintained at 1000 and 2000ppm respectively, the O2 is varied systematically from (0-3) % with the balance being N2. Considerable differences in NO and HC conversions as the O2 is varied are observed. As far as automotive exhaust is concerned, both NO and C3H6 (which represents the unburned hydrocarbons from engines) are pollutants, ideal conditions in aftertreatment are when both pollutants demonstrate high conversions. Several interesting trends with respect to the conversions of NO and C3H6 when the inlet O2 % is varied are observed and discussed below.

3.1 NO and C3H6 Conversions It has been widely reported that the conversion of NO increases as the reactor temperature is increased16 with reductants like CO, C3H6 etc. However, unlike in the case of the NO-CO reaction, it has been reported that the conversion of NO for NO-C3H6 systems can decline at higher temperatures. (Figure 1a) shows the variation of NO conversion with reactor temperature obtained in our simulations, as the inlet O2 is varied from 0-3%. The corresponding conversion of C3H6 is shown in (Figure 1b). 0

The NO conversion for 0% O2 in the feed (Figure 1a) begins at 220 C and increases rapidly. The NO conversion reaches 98% at 420 0C and this conversion is maintained up to 480 0C, after which the NO reduction drops drastically. There is no NO reduction at temperatures beyond 600 0C. The HC oxidation shows a similar trend, though with a maximum of ~18% at 420 0C.


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NO Conversion (%)

100

a 0% O₂ .1% O₂ .5% O₂ .82% O₂ .9% O₂ 1 .9% O₂ 2 1% O₂ 1 1% O₂ 2 3% O₂

80 60 40 20 0 200

300

400 500 Temperature (0C)

600

700

100 HC Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b 0% O₂ 0.5% O₂ 0.9% O₂ 1 1% O₂ 1 3% O₂

80 60

0.1% O₂ 0.82% O₂ 0.9% O₂ 2 1% O₂ 2

40 20 0 200

300


600

700

Figure 1. a) NO conversion versus reactor temperature b) HC conversion versus reactor temperature

As the inlet O2 increases to 0.1% in the feed (see Figure 1a), the range of temperatures over which high NO conversion is observed expands, with a maximum of 100% conversion at 460 0C. The HC conversion for 0.1% O2 gradually increases from 220 0C and reaches a maximum of 49 % at 460 0C. There is a slight decrease in HC conversion after 640 0C. Further increase in the feed O2, from 0.5 to 0.82%, results in different behaviour. The NO conversion reaches a maximum of 100% in these cases, with the temperature of maximum being lower (~400 0C). The decrease in the NO conversion at higher temperatures is not observed in these cases. On the other hand, with the higher amount of O2 present in the system, the oxidation of the hydrocarbon is facilitated and 100% HC conversion is achieved, at these conditions, at higher temperatures.



If the feed O2 is increased beyond this, however, the system demonstrates multiple steady states - this is shown for the 0.9% and 1% O2 cases in Figure 1. In our study, two branches are traced (only the stable ones), without using any special bifurcation software. For 0.9% O2 in the feed, the 1st steady state branch is shown in (Figure 1a,b) using closed square symbols in the temperature range (200-360) 0 C. The 2nd steady state branch is obtained in the temperature range of (360-650) 0C (Figure 1a,b) and shown as open square symbols, with two clear and distinct steady states at 360 0C. Further delineation of turning points and oscillatory regimes is not undertaken here. The MSS feature is observable in both NO and HC conversion graphs, in (Figure 1a,b). Similar features are also observed at 1% inlet O2, with the MSS regime identified here to be in the temperature range of ~320-360 0C. As the O2% is increased further the multiple steady states vanish and for 3% O2 in the feed the NO conversion, although small with a maximum of 17%, occurs at low temperatures, with no associated multiple solutions. Because of the excess O2 present, the HC conversion reaches 100% in these cases, and remains high throughout the range of temperatures studied. These results point to several interesting features – (a) In the absence of oxygen, only moderate temperatures can yield high NO conversions (b) When O2 amounts are small, excellent conversions of both NO and C3H6 can be observed at high temperatures (c) When a large excess of O2 is present, a severe compromise occurs as far as the NO conversion is concerned, though the C3H6 conversion is high. Our results indicate a complex trade-off between reactant conversions and inlet O2 limits, which is analysed further in the subsequent sections.

3.2 Conditions for Maximum Conversion

100 Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Maximum HC conversion Maximum NO conversion

80 60 40 20 0 0.0

0.5

1.0

1.5 2.0 O2 inlet (%)

2.5

3.0

3.5

Figure 2. Maximum NO & HC conversions with respect to inlet O2 %

In this section, the maximum NO and hydrocarbon conversions are collated, in Figure 2. It is seen that, as the %O2 in the feed increases, the NO conversion decreases, from 100% conversion at 0.2% inlet O2 to 17% at 3% inlet O2. On the other hand, the maximum hydrocarbon conversion increases, from 50% at 0.1% inlet O2 to 100% at 3% inlet O2. In Figure 2, the maximum conversions predicted in steady state branch 1 alone are shown, for 0.9 and 1% inlet O2 – it is seen that these conversions are low – however these branches may not have been fully traced in our simulations. Figure 2 demonstrates that, at very low and very high inlet O2 conditions, there is a strong trade-off between NO and HC conversions with conditions that are appropriate for one being sub-optimal for the other.


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However, a practical window of opportunity at ~0.5-0.8% inlet O2 and ~500 0C is observed, where both conversions are high (~100%), and the catalyst is in an ideal state for HC-SCR.

3.3 Product Mole Fractions and Surface Coverages The surface coverage and mole fraction are analysed for five different feed compositions to know more about the behaviour as we increase the oxygen content. The five feed compositions chosen are 0% O2, 0.1 % O2, 0.6% O2, 1% O2 and 3% O2 in the feed, as each of them yields a specific distinguishable feature in terms of NO/HC conversions.

Molefraction

1.E-03

a

H₂O

8.E-04

CO₂

6.E-04

CO

4.E-04

H₂

2.E-04 0.E+00 200

300


600

1.E+00

700

b

1.E-03

Surface Coverage

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.E-06 Pt(S) H(S) NO(S) C(S)

1.E-09 1.E-12

O(S) CO(S) OH(S)

1.E-15 1.E-18 200

300


600

700

Figure 3. a) Exit mole fraction versus reactor temperature for 0% O2 b) Surface coverage versus reactor temperature for 0% O2

For 0% O2, the NO reduction starts at 220 0C while the HC oxidation starts at 280 0C (as seen in Figure 1a), initially the catalyst favours the reduction because the catalyst surface is mainly covered by CO(S) species and the surface coverage of O(S) and C(S) species are very low (Figure 3b). Beyond 440 0C when the temperature increases the surface coverage of CO(S) decreases and C(S) increases slightly till 580 0C. NO reduction and HC oxidation decreases after 580 0C (Figure 1a,b)



because the C(S) species cover the catalyst completely and all other surface species surface coverage drop rapidly, beyond 5800C (Figure 3b).The poisoning of the catalyst by carbon species takes place at temperatures higher than 580 0C, with all other surface species coverages reducing to low values - this demonstrates catalyst coking at higher temperatures. The main gaseous products formed are CO, H2, and H2O (Figure 3a). The formation of CO and H2O starts at 220 0C while the formation of H2 starts at 350 0C. After 350 0C the H2O formation decreases and H2 formation increases this is because the surface coverage of OH(S) drops while the surface coverage of H(S) increases. HC is getting converted mainly to CO and not CO2 in this case, because of the absence of O2 in the feed.

Molefraction

0.003

a

0.002

H₂

H₂O

CO₂

CO

0.001 O₂

0.000 200

300


600

1.E+00

700

b

1.E-02 Surface coverage

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.E-04 1.E-06 1.E-08

Pt(S) H(S) NO(S) C(S)

1.E-10 1.E-12

O(S) CO(S) OH(S)

1.E-14 1.E-16 200

300


600

700

Figure 4. a) Exit mole fraction versus reactor temperature for 0.1% O2 b) Surface coverage versus reactor temperature for 0.1% O2

For 0.1% O2 the NO reduction starts at 220 0C and the HC oxidation starts at 260 0C (Figure 1a,b), here catalyst is in the reductive state i.e. surface is mainly covered by the CO(S) species. At high temperatures the surface coverage of C(S) species increases and CO(S) decreases (Figure 4b) as a result the NO and HC conversions decrease after 600 0C, as seen earlier the presence of small amount of O(S) species delays the coking action of the catalyst by C(S) species to temperatures > 650 0C. The


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main gaseous products formed are CO, H2 and H2O (Figure 4a). The presence of small amount of O2 increases the CO, H2 and H2O production considerably. The H2O formation drops and H2 formation increases after 390 0C because OH(S) surface coverage drops considerably and H(S) species increases after 390 0C.

Molefraction

0.006

a

0.005

H₂

0.004

H₂O

0.003

CO₂

0.002

CO

0.001

O₂

0.000 200

300


600

1.E+00

700

b

1.E-02

Surface coverage

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.E-04 1.E-06 1.E-08 1.E-10 1.E-12 1.E-14 200

300

Pt(S)

O(S)

H(S)

NO(S)

OH(S)

C(S)


CO(S)

600

700

Figure 5. a) Exit mole fraction versus reactor temperature for 0.6% O2 b) Surface coverage versus reactor temperature for 0.6% O2

For 0.6% O2 (Figure 1a,b) the NO reduction and HC oxidation start at 260 0C, and reach 100% at ~460 0C. The high NO and HC conversions are maintained even at high temperatures till 650 0C as seen earlier. The catalyst surface is mainly covered by CO(S) species up to 600 0C (Figure 5b). At high temperatures above 600 0C the catalyst surface is mainly vacant (Figure 5b). The C(S) species surface coverage is low so poisoning by carbon species is not of concern here. The CO formation is high in the low and medium temperature ranges till 580 0C (Figure 5a) compared to CO2 formation but at high temperatures CO2 formation is higher. H2O formation is high at low and medium temperatures. H2 formation starts at 520 0C. At this operating condition, the catalyst is in an ideal



state, with high conversions of both the pollutants, particularly at higher temperatures where the HC is effectively oxidised to CO2 by a combination of NO and O2.

1.E+00 1.E-02 1.E-04 Surface coverage

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.E-06 1.E-08 1.E-10

b

1.E-12 1.E-14 1.E-16

Pt(S) 1 H(S) 2 OH(S) 1

1.E-18 1.E-20 200

Pt(S) 2 CO(S) 1 OH(S) 2

300

O(S) 1 CO(S) 2 C(S) 1


O(S) 2 NO(S) 1 C(S) 2

600

H(S) 1 NO(S) 2

700

Figure 6. a) Exit mole fraction versus reactor temperature for 1% O2 b) Surface coverage versus reactor temperature for 1% O2 [The inset in (a) shows a magnified view of mole fraction vs temperature in the MSS region]

When the inlet oxygen is increased to ~1%, the simulations indicate the presence of multiple steady state solutions for certain reactor temperature ranges, as shown in Figure 6a,b. A magnified view of the multiple steady state regime (320-350)0 C is shown in Figure 6a. In general, single steady state


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solutions of low NO and HC conversion are observed at low temperatures, as expected – up to ~320 0 C or so, similar to the other cases seen here. At temperatures >400 0C or so, the surface, in this case, the catalyst is covered predominantly by adsorbed oxygen, resulting in high HC conversions, and low NO conversions. This is similar to the behaviour at high temperatures described in the next regime, later. The most interesting features in this case occur at intermediate temperatures. Here, between ~ (320350) 0C, it is clearly observed that two steady state branches can be traced in the CHEMKIN reactor simulations. Continuation or other algorithms are not used in our study to identify turning points or bifurcation points. A “phase transition” on the catalyst surface is however observed, with the CO(S) covered surface switching to a O(S) covered one, over a very narrow temperature window in this range (Figure 6b). In earlier work, a fairly detailed study on the bifurcations, oscillations, and other related features observed in the reduction of NO by CO at automotive exhaust conditions was done17. Here, we do not delve into these details due to some of the uncertainties in the surface reaction mechanism used here, though we identify this operating regime – where NO conversions are small, and HC conversions may be high (with CO2 and H2O being the main products at high temperatures), as a practically viable, though undesirable, one.

0.03 a

0.03 Mole fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.02 0.02

CO

CO₂

H₂O

O₂

0.01 0.01 0.00 200

300



600

700


1.E+00

b

1.E-03

Surface coverage

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.E-06

Pt(S) H(S) NO(S) C(S)

1.E-09 1.E-12

O(S) CO(S) OH(S)

1.E-15 1.E-18 200

300


600

700

Figure 7. a) Exit mole fraction versus reactor temperature for 3% O2 b) Surface coverage versus reactor temperature for 3% O2

For 3% O2 the NO reduction is very low and the HC oxidation is very high, the NO conversion starts at 200 0C and reaches maximum conversion at 220 0C while the HC conversion starts at 200 0C and reaches maximum conversion at 380 0C. In this stage the catalyst is poisoned by O(S) species (Figure 7b) so the catalyst is oxidative in nature due to the high amount of O2, and as a consequence demonstrates poor performance towards NO reduction. The selective catalytic reduction of NO using HC is not possible under these conditions since there is no reductive environment to reduce NO. We label this as the oxygen poisoned case. The main products formed are H2O and CO2 (Figure 7a), a very small amount of CO is also obtained. Excess of O2 is obtained in the outlet unreacted.

3.4 Regimes of operation for Pt While the results presented here are for specific inlet feed O2 concentrations, the analysis in the previous section enables us to classify the conditions into four distinct regimes as far as the reduction of NO by HC, for automotive exhaust conditions is concerned. These can be labelled as – coking 1 (as seen here for 0% inlet O2), coking 2 (seen here at 0.1% O2), optimum operating (seen here at 0.6 – 0.8% O2) and oxygen poisoned (at 1% O2 multiple steady states and at 3 % O2) regimes. In the literature, various studies demonstrate one or other of these operating regimes1, 18, 19. The coking, optimal operating, and oxygen poisoned regimes are described in this section. The NO and HC conversions for Pt catalysts in various cases are presented in Figure 8. 3.4.1 Coking Regime 1 Coking is the phenomena where carbon species cover the catalyst surface completely, this occurs particularly when there is no O2 in the feed (Figure 8a). The coke formation is attributed by 9 reactions (18,23,24,29,30,39,40,49 and 50) in Table S1 in the supporting information, As the temperature increases at 0% O2 rate of production of coke (C(S) species) increases (Figure 3b).The NO conversion is high at low and medium temperatures but HC conversions are low at all temperatures for 0% O2. At high temperatures the carbon species [C(s)] occupies the catalyst surface completely (Figure 3b) and strongly, and thus no further reaction can take place. There is no conversion for all the species at high temperatures due to coking (Figure 3a). In literature, such coking phenomena is mentioned for a Ni exchanged mordenite catalyst13, where 1000 ppm NO and 1000 ppm propane are used with 0% O2. In Mosqueda’s experimental work NO conversion reaches 100% at 5000C, and maintains the same till 6000C. After 600 0C, it is suggested that carbon particles are


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deposited on the catalyst surface and coke formation occurs, and as a consequence, the NO and propane conversions are low. We label this as the Coking 1 regime, to indicate that this is in the absence of feed oxygen. 3.4.2 Coking Regime 2 Coking is observed even in situations when there is low O2 % in the feed, though it is seen that as the O2 % increases the temperature at which coking occurs, increases. For 0.1% O2 (Figure 8b) the coking process starts at 560 0C, as seen earlier in (Figure 4b). In this case, the small amount of feed O2 permits the catalyst to avoid coking up to a higher temperature than in Regime 1 discussed above, and provides a very interesting insight into the severe competition for vacant catalyst sites in the HC-SCR reaction.

Figure 8. a) NO and HC conversion with respect to temperature for 0% O2 b) NO and HC conversion with respect to temperature for .1% O2 c) NO and HC conversion with respect to temperature for .6% O2 d) NO and HC conversion with respect to temperature for 1% O2 e) NO and HC conversions with respect to temperature for 3% O2. (All the graph for Pt catalysts)



In earlier Pt foil experiments on methane oxidation, visual deposit of carbon on the Pt, leading to a drop in foil temperature, due to abstraction of hydrogens from CH4, is reported, in the presence of small amounts of oxygen20, 21. This is line with the conclusions of this work regarding coking in the presence of O2 in the feed. We label this regime as coking 2 regime, to indicate that this occurs even in the presence of O2. 3.4.3 Optimal Operating Regime With further increase in the O2 %, 100 % conversion for both NO and HC (Figure 8c) is observed, at higher temperatures - this is the most preferred condition for HC-SCR. The surface coverage of the species in this regime vary smoothly with temperature, with no abrupt changes (Figure 5b). The O2 in the feed is entirely consumed in the reactor, and CO2 and H2O are the main products at high temperatures (Figure 5a). In this study, such an ideal condition occurs for all inlet O2 concentrations in the range (0.3-0.82) %. In the literature, such a situation of high NO and C3H6 conversions, in the presence of small amounts of O2 in the feed, were observed experimentally19. The catalyst used in their case is Rh supported on ZrO2. The inlet feed consists of 500 ppm NO, 1167 ppm C3H6, 0.5% O2 and 10% H2O. It is observed that the conversions of NO and HC reach 100% at ~450 0C, with no decrease up to 600 0C. This is consistent with our findings using an inlet feed with similar composition, (with the same ratio of NO/HC, 0.5% O2, with the presence and absence of 10% H2O) as shown in Figure 9. The conversions of both NO and HC increase from 260 0C and reach 100% conversion at ~ 480 0C and remains high up to 600 0C. The presence of 10% H2O does not significantly affect the HC and NO conversions, in our simulations at these conditions. It must be remarked here that the literature experiments were performed with Rh/CeO2-ZrO2 catalysts, and the evidence of strong metal support interactions (SMSI) was found in their work, particularly when the molar ratio of Ce/Zr was 50/50. The results presented here do not capture the effect of the support, and can thus only qualitatively demonstrate the features seen in the experiments, under conditions of low SMSI (e.g. when the composition of the support is 0% CeO2).

100 Conversion (%)

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80 60 HC

NO

HC

NO

40 20 0 200

250

300

350 400 450 Temperature (0C)

500

550

600

Figure 9. NO and HC conversions versus reactor temperature for an inlet condition of HC 2000 ppm, NO 1000 ppm, O2 0.5% in the presence and absence of 10% H2O (closed symbols represents the presence of 10% H2O, open symbols represents the absence of H2O)


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Our analysis indicates that in such a situation, the catalyst surface conditions are ideal for the simultaneous reduction of NO and oxidation of propene, with a CO(S) covered surface leading to a surface that is predominantly vacant, as the temperature is increased (see Figure 5a). 3.4.4 Oxygen Poisoned Regime

When the inlet O2 % is increased to 0.9 % multiple steady states are observed in our simulations, by tracing steady state solutions with increasing and decreasing temperatures, separately. There are 2 branches in the MSS, one reductive steady state branch and the other an oxygen poisoned steady state branch (Figure 8d). The NO reduction is mainly taking place in the reductive steady state where CO(S) is the main surface coverage species and in the oxidative steady state O(S) is the main surface coverage species, HC oxidation is high in this oxidative steady state. In literature the experimental results allude to multiple steady states in a lean NOx trap operation22 and for the NO-CO reaction as well17. When the inlet O2 % is high (3%) the MSS vanishes and the catalyst is in fact completely “poisoned” by oxygen (Figure 8e). The catalyst is covered completely by O(S) species (Figure 7b) and NO conversion is very low in this region, for 3% O2 the maximum NO conversion is 17%. HC SCR cannot be used effectively if there is high oxygen content in the feed. In literature Burch and Millington1 have performed similar experiments with an inlet feed consisting of 500 ppm NO, 1000 ppm C3H6 and 5% O2, on Pt catalysts, and found results that are very similar to ours.

3.5 Regimes of Operation for Rh In this work, we have performed the above analysis (described in detail above for Pt catalysts), for Rh catalysts as well. The surface reaction mechanism in Table S1 in supporting information, for Rh, was incorporated in CHEMKIN PSR simulations, with identical reactor variables as Pt, and simulation results obtained at various conditions tabulated. The distinct operating regimes identified above were observed for Rh catalysts as well, as the inlet O2 was increased from 0 to 3%. The reaction mechanism parameters for Rh have a few uncertainties, and in some cases have been assumed to have the same value as Pt, as mentioned in section 2.1 earlier. The coking, optimal, and oxygen poisoned regimes are clearly described for Rh catalysts in this section, with the NO and HC conversions for Rh catalysts in various cases presented in Figure 10.



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Figure 10. a) NO and HC conversion with respect to temperature for 0% O2 b) NO and HC conversion with respect to temperature for 0.1% O2 c) NO and HC conversion with respect to temperature for0.8% O2 d) NO and HC conversion with respect to temperature for 1% O2 e) NO and HC conversion with respect to temperature for 3% O2. (All the graphs for Rh catalysts)

From Figures 8 and 10, it is observed that as the inlet oxygen concentration changes, various regions with different behaviours are seen. In particular • • • • •

Low XNO and Low XHC High XNO and Low XHC High XNO and High XHC Multiple steady states Low XNO and High XHC


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are observed. Figure 11 presents a pictorial summary of the various behaviours observed in these systems. The low XNO and Low XHC is obtained at high temperatures and low inlet oxygen conditions due to coking of the catalyst. High XNO and Low XHC are obtained at moderate temperatures and low oxygen conditions, whereas High XNO and High XHC are obtained in medium and high temperatures for optimum oxygen concentrations. Multiple steady states and oscillations may be expected at certain higher oxygen concentrations at medium temperatures due to phase transitions on the catalyst surface. Low XNO and High XHC are seen at medium and high temperatures for high oxygen concentrations, due to poisoning of the catalyst by oxygen. We believe that the exact temperature ranges and inlet oxygen concentrations at which these behaviours and regimes are observed will depend on various factors – including the nature of the catalyst, reactor operating conditions, inlet concentrations of NO and hydrocarbons, and so on. Furthermore, as discussed earlier in 2.1, a few other effects such as coverage dependent activation energies and N2O formation should be incorporated into the reaction mechanism, and may lead to some variations in the results. Nevertheless, these regimes are very likely to exist in practical HCSCR operation, particularly when Pt or Rh are used. The similarities in the results obtained with Pt and Rh also indicate that in TWC-like catalysts (which have both Pt and Rh present simultaneously), a similar set of operating regimes are likely to exist, though the exact details of temperature and inlet O2 % ranges may differ slightly.

Coking region

Oxygen poisoning

Low XNO, Low XHC High XNO, Low XHC

Ideal condition High XNO, High XHC

Low XNO, High XHC MSS / Oxygen poisoning

Low XNO, High XHC

Low XNO, Low XHC

Figure 11. Pictorial representation of operating regimes for NO reduction by HC-SCR

4. Conclusions The influence of O2 in NO reduction and HC oxidation is studied using isothermal reactor simulations incorporating detailed surface reaction mechanisms. Specific results on conversions and surface coverages is presented for Pt catalysts, and the information used to propose various practical operating regimes in HC-SCR. Results from reactor-scale simulations of Rh, and specific evidence from literature are used to highlight the generalisability of the result. The catalyst presents evidence of coking in the absence of oxygen, however, high inlet oxygen conditions are not ideal either due to the proclivity of the catalysts for oxygen poisoning. This



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indicates the challenges in aftertreatment for fuel-lean engine operation. The optimal conditions wherein NO is reduced to N2 and the unburned hydrocarbon is oxidised fully to CO2 and H2O, are identified at moderate levels of oxygen, in the temperature range of ~500 – 600 0C, for Pt catalysts.

Nomenclature av

surface area/volume of catalyst (m2/m3)

Ci

outlet concentration of gas species i (mol/m3)

Ci 0 Ea

inlet concentration of gas species i (mol/m3)

k0

pre-exponential factor (s-1)

kj

rate constant for reaction j

Mi NA

molecular weight of species i (g/mol)

P

pressure inside the reactor (bar)

Q

volumetric flowrate (m3/s)

R

gas constant (m2.g/ (s2.mol.K))

Rj

rate of surface reaction j ( mol/m2/s)

si

sticking coefficient for species i (dimensionless)

t

time (s)

T

Temperature (K)

V

volume of the reactor (m3)

activation energy (kJ /mol)

Avogadro number (6.022 x 1023 molecules/mol)

Greek letters

θk

fractional coverage of surface species k

θ* ν ij

fractional coverage of vacant site

ν kj

τ

stoichiometric coefficient of species k in surface reaction j residence time (s)

Γ

molar site density (molecules/m2)

stoichiometric coefficient of species i in surface reaction j

Supporting Information: Table comprising of reaction mechanism of NO reduction using Propene (HC-SCR) with detailed kinetic data (Activation energy and Pre-exponential factor) on Pt and Rh catalysts are provided.


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5 References (1) Burch, R.; Millington, P.J. Selective reduction of nitrogen oxides by hydrocarbons under leanburn conditions using supported platinum group metal catalysts. Catal. Today 1995, 26,185–206. (2) Nikolopoulos, A.A.; Stergioula, E.S.; Efthimiadis, E.A.; Vasalos, I.A. Selective catalytic reduction of NO by propene in excess oxygen on Pt- and Rh-supported alumina catalysts. Catal. Today 1999, 54,439–450. (3) Meunier, F.C.; Breen, J.P.; Zuzaniuk, V; Olsson, M; Ross, J.R.H. Mechanistic Aspects of the Selective Reduction of NO by Propene over Alumina and Silver – Alumina Catalysts. J. Catal. 1999, 187, 493–505. (4) Luo, Y; Hao, J; Hou, Z; Fu, L; Li, R; Ning, P; Zeng, X. Influence of preparation methods on selective catalytic reduction of nitric oxides by propene over silver-alumina catalyst. Catal. Today. 2004, 93-95,797–803. (5) Martins, L; Peguin, R.P.S; Urquieta-González E.A. Cu and Co exchanged ZSM-5 zeolites – activity towards NO reduction and hydrocarbon oxidation. Quim. Nov. 2006, 29,223–229. (6) Joubert, E; Courtois, X; Marecot, P; Duprez, D. NO reduction by hydrocarbons and oxygenated compounds in O2 excess over a Pt/Al2O3 catalyst: A comparative study of the efficiency of different reducers (hydrocarbons and oxygenated compounds). Appl. Catal. B Environ. 2006, 64,103–110. (7) Burch, R; Millington, P.J. Selective Reduction of NOₓ by Hydrocarbons in Excess Oxygen by Alumina- and Silica-Supported Catalysts. Catal. Today. 1996, 29, 37–42. (8) Chatterjee, D; Deutschmann, O; Warnatz, J. Detailed surface reaction mechanism in a three-way catalyst. R. Soc. Chem. 2001, 119,371–384. (9) Mhadeshwar, A.B; Winkler, B.H; Eiteneer, B; Hancu, D. Microkinetic modeling for hydrocarbon (HC)-based selective catalytic reduction (SCR) of NOx on a silver-based catalyst. Appl. Catal. B Environ. 2009, 89,229–238. (10) Sawatmongkhon, B; Tsolakis, A; Millington, P.J. Microkinetic modelling for selective catalytic reduction (SCR) of NOx by propane in a silver-based automotive catalytic converter. Appl. Catal. B Environ. 2012,111-112,165–177. (11) Prasad, V. S; Snigdha, R; Aghalayam, P. NO reduction using Pt and Ag catalysts. Chemcon, IIT Guwahati, Dec 27-30, 2015. (12) Deutschmann, O; Schwiedernoch, R; Maier, L.I; Chatterjee, D. Natural Gas Conversion VI. Stud. Surf. Sci. Catal. 2001, 136, 215-258. (13) Thormann, J; Maier, L; Pfeifer, P; Kunz, U; Schubert, K; Deutschmann, O. Int. J. Hydrogen Energy 2009, 34,5108-5120. (14) Ravikeerthi, T; Thyagarajan, R; Kaisare, N. S; Aghalayam, P. Microkinetic model for NO–CO reaction: Model reduction. Int. J. Chem. Kinet. 2012, 44,577–585. (15) Shustorovich, E; Bell, A.T. An analysis of fischer- tropsch synthesis by the bond-orderconservation-morse-potential approach. Surf. Sci. 1991, 248, 359-368 (16) Mantri, D; Aghalayam, P. Detailed surface reaction mechanism for reduction of NO by CO. Catal. Today. 2007, 119, 88–93. (17) Mantri, D; Mehta, V; Aghalayam, P. Bifurcation Analysis on Pt and Ir for the Reduction of NO by CO. Can. J. Chem. Eng. 2007, 85, 333–340.



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(18) Mosqueda-Jiménez, B.I; Jentys, A; Seshan, K; Lercher, J.A. On the surface reactions during NO reduction with propene and propane on Ni-exchanged mordenite. Appl. Catal. B Environ. 2003, 46,189–202. (19) Haneda, M; Shinoda, K; Nagane, A; Houshito, O; Takagi, H; Nakahara, Y; Hiroe, K; Fujitani, T; Hamada, H. Catalytic performance of rhodium supported on ceria-zirconia mixed oxides for reduction of NO by propene. J. Catal. 2008, 259,223–231. (20) Park, Y. K. Homogenous and catalytic oxidation of hydrogen and methane. Ph.D. Thesis, University of Massachusetts Amherst, 2000. (21) Aghalayam, P. Interactions of premixed flames with surfaces: Flame stability and pollutant abatement. Ph.D. Thesis, University of Massachusetts Amherst, 2000. (22) Sharma, M; Clayton, R; Harold, M.P; Balakotaiah, V. Multiplicity in lean NOX traps. Chem. Eng. Sci. 2007, 62, 5176–5181.


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291x271mm (96 x 96 DPI)


Microkinetic Modeling of the Effects of Oxygen on the Catalytic

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