Heterogeneous catalytic decomposition of nitrous oxide

i B EWlRONMENlAL

:

ELSWIER

Applied Catalysis B: Environmental

9 (1996) 25-64

Review

Heterogeneous

catalytic decomposition of nitrous oxide

Freek Kapteijn *, Jo& Rodriguez-Mirasol

‘, Jacob A. Moulijn

Industrial Catalysis, Department of Chemical Engineering, Delft University of Technology, Julian&an 2628 BL Del& Netherlands Received 30 October

1995; accepted 9 February

136,

1996

Abstract An overview

is given

on the ongoing

activities

in the area

of the decomposition

of nitrous

oxide, N,O, over solid catalysts. These catalysts include metals, pure and mixed oxides, supported as well as unsupported, and zeolitic systems. The review covers aspects of the reaction mechanism and kinetics, focusing on the role of surface oxygen, the inhibition by molecular oxygen, water and other species, poisoning phenomena and practical developments. Keywords; Nitrous Processes

oxide;

Decomposition;

Mechanism:

Oxides;

Zeolites;

Kinetics;

Inhibition;

Poisoning:

1. Introduction Nitrous oxide, N,O, has been long considered as a relatively harmless species and has suffered from a lack of interest from scientists, engineers and politicians, due to the underestimation and unawareness of the potential contribution of this species to environmental problems. During the last decade a growing concern can be noticed since nitrous oxide has been identified as a contributor to the destruction of ozone in the stratosphere and recognised as a relatively strong The estimated human contribution to the nitrous oxide greenhouse gas [l-3]. emission to the atmosphere amounts to 4.7-7 million ton per year [3,4], about 30-40% of the total emission including natural sources. The identified anthropogenic sources are given in Table 1 [3-71. They include adipic acid production, * Corresponding author. Fax: (+ 31-15) 2784384; e-mail: [email protected]. ’Present address: Department of Chemical Engineering, University of Malaga, Spain 0926-3373/96/$15.00 Copyright PII SO926-3373(96)00016-l

0 1996 Elsevier Science B.V. All rights reserved

26

F. Kapteijn et al./Applied

Catalysis B: Environmental 9 (1996) 25-64

nitric acid manufacture, fossil fuels and biomass combustion and land cultivation. Not all sources are identified yet at present, and the given values may change in the future when more accurate values become available. For example the regeneration of coked fluid cracking catalysts occurs at conditions at which in sewage sludge [8] and coal combustion [9] considerable amounts of N,O are being formed and may constitute a still unknown source. A sour message is that nitrous oxide is not only formed as a by-product but also as a consequence of measures to control the emission of other environmentally harmful species, like in the nonselective catalytic reduction of NO, (NSCR) with cyanuric acid or urea [6] and in three-way catalysis (TWC) to remove NO,, CO and hydrocarbons [4,6,10]. N,O emission has been identified only recently for aged TWCcatalysts [lo]. The human contribution has led to an imbalance between the total global sources and sinks, and a 70-80% reduction in the human emissions is necessary to stabilise the atmospheric N,O concentration at the present level of about 310 ppb [31. Emission reduction can be achieved principally in two ways, either by lowering the formation of N,O or by after-treatment (‘end-of-pipe’ solution). The choice of approach will strongly be influenced by economic considerations projected against legislation. Although at present no legislation exists with regard to N,O emissions, growing governmental awareness of the environmental impact of N,O can be noticed [7,11,12]. Hence, emission levels are expected to become regulated within a couple of years [4,7,11]. Control techniques will depend on the conditions under which N,O must be abated and the local (industrial) infrastructure. Table 2 gives typical concentration values of various sources [3-7,10,13,14]. In adipic acid industry for each molecule adipic acid one molecule of N,O is produced, e.g. Eq. (I), OH +2HN03

-

Ho&OH 0

+

N20 +2H20

yielding high concentration levels of N,O. In this respect all processes where nitric acid is used as oxidising agent should be evaluated on their N,O emissions. Nitric acid itself is produced by high temperature oxidation of ammonia over Pt-Rh wire gauzes. The non-selectively formed traces of N,O travel through the process without noticeable absorption and end up in the off-gas. The nonselective catalytic reduction techniques (NCSR), sometimes applied to reduce the NO, emissions of these plants may reduce the N,O levels [4], but, on the other hand, can yield N,O due to oxidation of the reducing agent [6,7]. In fluidized bed combustion the nitrogen of the coal or biomass ends up

F. Kapteijn et al./Applied Table I Estimated

Catalysis B: Erwironmental9 (1996) 25-64

27

amounts of N,O emitted by various human activities

Source

kton/year

Adipic acid production Nitric acid production Land cultivation, fertilizers Fossil fuels (stationary) Fossil fuels (mobile)

371 C.545) h 280-370 1000-2200 190-520 200,400-850

Biomass burning FCC regeneration Waste incineration Other chemicals

Point sources 23 255 > 1000 > 2.10s

500-1000

?‘c Man made ’

Refs.

5-8 4-8 14-45 4-10 4-15

151 [4,6] [3,41 L3.71 [3.4,71 [3,71

10-20 ? ? ?

i Total global man made emission taken 4.7-7 Total industrial production [5].

10’ kg N20 per year [4]

partially as N,O and NO, [3,6,&l 1,15,16]. The lower the temperature the more N,O is formed, while that of NO, decreases. The total conversion of coal nitrogen to NO, and N,O remains constant by a kind of trade-off mechanism [3,11]. CaO added to the fluidized bed to capture SO,, is active for the N,O decomposition [6,17], but the formed CaSO, results in increased NO, formation [3]. Selective catalytic reduction (SCR) catalysts, to reduce NO, with NH, in the presence of oxygen, also form N,O [6,18-201. At the optimal SCR conditions (temperature window) N,O is formed as a nonselective product of the reaction between NO and NH,, while at high temperatures this is due to NH, oxidation [ 191. The presence of water vapour suppresses the former production [ 181, but once formed the N,O is not reduced in the process [19]. Noble and base metal catalysts that are active for automotive emission control to reduce the emitted NO,, CO and hydrocarbons, exhibit a temperature window where the NO is converted (under reducing conditions) mainly to N,O instead of N,. At higher temperatures N, is the major product [6,21-261. Fresh commercial three-way catalysts (TWC) have the maximum production at low temperatures, but upon

Table 2 Typical min-max

concentrations

Source Adipic acid Nitric acid Three-way catalyst h Fluid-bed combustion Waste incineration NSCR FCC regeneration Ammonia combustion

of components

Temp. (“C) N20 200-300 180-200 25-800 700-900

(ppm) in N,O containing NO,

30-508 0.7% 300-3000 300-3000” o- 1000 O-2000 50-500 50-500 O-600 30-150 ‘J 200-500

i NO, /NO ratio around 1 [13]. Aged catalyst, values fluctuate due to air/fuel

off-gases

0,

H,O

4% 2-4% O-1000 2-10s

2-310 2-3s ca. 10% ca. 10%

CO

Refs.

so> 300 _

O-4000 10-1000

20-100 < 2000

151 ]4,131 [6,10] [3,6,14] [71

[61 171

control.

28

F. Kapteijn et al. /Applied Catalysis B: Enoironmental9 (1996) 25-64

aging (deactivation) this shifts to higher temperatures into the window where the catalysts frequently operate, inducing a continuous production of N,O. The NO conversion can even become nearly quantitative [lo], especially for Pd containing catalysts [27]. In addition, it is mentioned that new catalysts, which are being developed for the direct decomposition of NO or selective reduction with hydrocarbons under lean conditions, also might give rise to N,O formation [28-301, an aspect that should be considered in selection of the proper catalyst. Besides the known sources there are still unidentified ones. Potential sources may be FCC catalyst regeneration, oxidative destruction of volatile organic compounds (VOCs), ammonia oxidation, other processes that use nitric acid for oxidation, ore processing or metal surfaces etching, and even catalyst preparation processes whereby nitric acid (peptization for extrusion of supports) or nitrates (active phase precursors) are being used. Concentration data, however, are still unavailable for these cases. Since in many situations N,O formation is inevitable to the applied processes, catalysis offers a route for NiO abatement, either by improving selectivities of applied catalysts or by the direct decomposition of N,O into nitrogen and oxygen. The major sources known at present are suitable candidates for catalytic after-treatment, viz. adipic acid production, nitric acid plants, vehicles equipped with three-way catalysts and fluid bed combustion. From Tables 1 and 2 it is clear that N,O abatement can be most efficiently tackled in the adipic acid production due to the limited point sources and high concentrations. Most challenging, however, is the reduction of N,O emission for vehicles. Apart from abatement of N,O emissions on a global level, the catalytic decomposition of N,O is also an important local appliance, like from its use as anaesthetic in surgery rooms [31-331 and from the reprocessing of fuel rods using nitric acid [34]. The production of breathable air for astronauts is another application [35]. Furthermore, the N,O oxidation of catalysts has been applied extensively for characterization purposes, like in transient kinetics [36,37] and the determination of the specific copper surface area [38-411. The latter is based on the oxidation of only the copper surface by N,O at mild conditions, Eq. (2). Cu, + N,O + CuO, + N,

(2)

Some reviews exist on the N,O decomposition, which are either concerned with the early mechanistic studies or are part of another topic, limiting the extent devoted to N,O [42-461. Recently, a considerable number of papers has appeared in the open literature on new, highly active catalytic systems, which are interesting for practical application. Since most catalytic abatement techniques are still in a developmental stage this paper reviews the catalysts and catalysis involved in the destruction of N,O; included are mechanistic aspects and kinetics and challenges for further developments of catalysts and processes are indicated.

F. Kapteijn et al. /Applied Catalysis B: Environmental 9 (19961 25-64

29

2. Reactions Although thermodynamically unstable, the N,O molecule is quite stable at room temperature and has an estimated lifetime in the atmosphere of about 150 years. This implies that direct emission reduction measures will have noticeable effect only on the long term. In the asymmetric N-N-O molecule the N-N bond order is about 2.7 and that of N-O about 1.6, so the latter is most probable to be broken first. The activation energy for thermal fission of the N-O bond is ca. 250-270 kJ/mol [47-511 and temperatures above 900 K are required to achieve measurable conversions according to (3) 2N,O + 2N, + 0, (A,H”(298)

= - 163 kJ/mol)

(3)

Since in the N,O molecule all bonding orbitals are occupied, it is isoelectronic with CO,. The principal catalytic action can originate from charge donation into the antibonding orbitals, weakening the N-O bond and lowering the activation energy and reaction temperature. Reducing agents, like H,, CO, hydrocarbons and carbon, can be added to destroy N,O, e.g. (4) and (5) [49]. This parallels the role of N,O as intermediate in the selective catalytic reduction of NO by hydrocarbons in the presence of oxygen [28,30,52]. N,O + CO + N, + CO, (A, H”(298) = - 365 kJ/mol)

(4)

2N,O + C + 2N, + CO, (A,H”(298)

= - 557 kJ/mol)

(5)

N,O + NO + N, + NO, (A,H”(298)

= - 139kJ/mol)

(6)

N,O + SO, -+ N, + SO, (A,H”(298)

= - 181 kJ/mol)

(7)

2N0 + 0,

++ 2N0,

(~~~(298)

= - 114kJ/mol)

(8)

Carbon itself reacts above 600 K with N,O, but the addition of alkali or earth-alkaline elements enhances the rate considerably [53]. Unfortunately, this also holds for oxygen, so this is only useful at low or zero oxygen concentrations. In view of the various gas compositions (Table 2) other reactions might occur too, Eqs. (6)-(8), depending on the properties of the catalysts. Reactions (3)-(7) are all irreversible and all are exothermal. In case of high N,O concentrations large temperature rises may be expected, imposing demands on the thermal stability of the catalyst.

3. Catalysts In the early thirties already a lot of catalytic decomposition studies had been performed [49], where both gaseous and solid catalysts had been identified.

F. Kapteijn et al. /Applied

30 Table 3 Experimental

conditions

of various catalysts

System

Metals Pt Pt Pt Pt AU

Catalysis B: Enoironmental9

(1996) 25-64

for N,O decomposition

T-range

PN,O

PO,

Reactor system

(RI

&Pa)

&Pa)

Other gases

Refs.

760-840 870-1470 800-1500 1270-1370 720-880

2.6-66 6.7-53 0.01-0.5

< 66 < 15 _

5-22 lot-7000

30 _

500-700

6.7-35

13.26

batch (external recirculation)

[62,631

3, 6.7-35

13,26

batch

[61,631

batch flow batch flow

[1621 [661 [1631

batch batch flow batch flow

N,

[561 [551

[601 [571 I501

Pure oxides CaO, Fe,O,,

CuO, Rh,O,,

IrO,

MgO, NiO, SrO, Cr,O,, CeO,, ThO,, SnO,, MnO, BeO, ZnO, AlaOs, Ga,O,, HfO,, TiO, La,O,, Nd,O,, Sm,O,, Eu,O,, Gd,O,, Ho,O,, Tm,O,, Yb,O,,

700-800 800-1000 600-800

Lu*O3

800-950

Sa,O,, Y203, In,O,, Tb,O,, Dy,O,, Erz03 Nd,O,, Dy,O,, Er,O, MnO, Mn,O,, Mn,O,,

MgO

MnO,

-”

~

-

Co,O,, NiO cue Tho,, AlaO,, CdO, GO,, GO,, ZrO,, Fe,Os, ZnO, Ndz03, CraO, MgO, CaO, Sb,O,, WO,, BeO. U,O,, SiO,, GeO, COO, Cr,03, NiO, CuO CaO

700-900 570-670 600-850 550-650 650-750 700-900 900-l

0.03-12 5-20 25-30 100

0.02-2 0.5-8

_

[781

100

500-900 1120

0.001-o. 1 0.025-0.05

100 4

[@31

flOW

flow

co,,

[I71

H,O, so2

MnO,, V,O,, CuO, CeO, coo

CuO

Fe203 CaO SrO La203

Mixed oxides Solid solutions Co0 in MgO NiO in MgO Cr,O, in AlaO,

550-900 800-l 100 500-770 500-800 500-800 600-750 600-970

500-700 600-750 650-900

0.0078

_

6,12 0.013-0.052 15 0.3,6,7-35 0,12

_ _ _

8 8 2,40

_ _

flow

batch batch flow batch, flow

batch batch batch

El [I611 [lo61 [127,128]

[1641 [651

171,751 [69,741 [72,731

F. Kapteijn et al./Applied

31

Catalysis B: Environmental 9 (1996) 25-64

Table 3 (continued) System

Spinels MAl,O, (M = Co, Cu, Ni, Mg, Zn) MCr,O, (M = Co, Ca, Ni, Mg, Zn) MCo?O, (M = Co, Cu, Ni. Zn, Ni + Cu) cu,co,_,o,(x=o-1) Co,Mg,_,Al,O,(x=O-1)

T-range

PN,O

PO2

(K)

&Pa)

(!@a)

Reactor system

Other gases

Refs.

flOW

[781

&I,26

batch

[1211

700-800 600-750

6.7, 26 8

batch batch

[1651

700-900 600-800 600-800

0.1 6.7. 26 6.7.26

flow batch batch

[821 [1221 [431

650-750 670-750 700-900 610-750 610-750 700-900 650-900 690-750

6.7, 26 6.7. 26 0.1 6.7, 26

batch batch

[1661 [81,167]

750- 100 750-1000 400-520

100

[1201

Perouskite rypes AW LaMO, (M = Co, Ni, Cr, Mn, Fe) La,_,Sr,MnO, (x= O-1) MTiO, (M = Ca, Sr, Ba, Mg, Mn) MMnO, (M = La, Nd, Sm, Gd) LaMO, (M = Cr, Fe, Mn, Co, Ni) A,BQ M 1NiO, (M = La, Pr, Nd) M,CuO, (M = La, Pr, Nd, Sm. Gd) La,MO, (M = Co, Ni, Cu) La,CuO, La, 8&r &u%, La,- ,Sr,CuO, (x = O-1) La,_,Sr,CuO, (x = O-1) MSrFeO, (M = La, Pr, Nd, Sm, Gd) Ex-hydrotalcites M-Al-CO, -HT (M = Co, Ni) (M = Cu) M-Al-CO,-HT (M = Co, Ni, Cu, Co-La, Co-Pd, Co-Rh, Co-Mg, Co-Ru)

0.5- 1.o 3 6.7.26

0.25510

_ _

6.1

400-500 500-600 500-700

0.1

Fe&% Cub

800-950 600-900

33 varies

MM, RW,

550-650 470-720

0.08 0.02

Rh Ru Pd, CuO. Co0 Pt-Rh

550-650 600-700 650-800 500-700

0.1

0.0078

570, 820 500-800

100 0.013-0.052

Supported Alumina

flOW

2.5

batch

[821 [841

flow flow batch

[821 1831 [I681

batch

[851

flow

H,O

k361

systems

[89911

flOW

pulse reactor batch flow

1701

NO, coz,

[921 [141

so2

Silica Cr, Co, Ni, Fe Fez%

0.013

flOW

1881

flow

[61

flow

[261 [lo61

batch

32

F. Kapteijn et al./Applied

Catalysis B: Environmental 9 (1996) 25-64

Table 3 (continued) System

T-range

P&O

PO,

(K)

&Pa)

&Pa)

Reactor system

Zirconia Co, Cu, Ni, Fe, Ru Co/Ni

570-820 670-800

l-20 29

+?

Zeolites Fe-Mor Fe-Y Fe-ZSM-5 [Fe]-ZSM-5

600-900 600-900 570-850 500-700

I-10 l-10 0.013-0.052 0.1

l-10

Fe, Ru in ZSM-5 Co, Cu in: ZSM-5,ZSM-11, Y, L, Ferrierite, Beta, Mor, Erionite M-ZSM-5 (M = Co, Cu) (M = Ni, Mn, Fe) (M = Rh, Ru, Pd, Pt) Ru in ZSM-5, USY Cu, Co, Ni, Mn in Na-A

0.013-0.052 o-5

Other gases

Refs.

flow flow

k’fO91

flow flow batch flow

[lOOI [loll [94,106]

1951

ill01

450-800 600-800

l-3.5 0.1

O-10 _

(TPR) flow flow

600-700

0.02-O. 12

2.5

flow

NO, H,O

600-800 500-800

0.1

flow

-

400-600 700-900

11041 [881

[88,142, 1601 [881

_ 0.05-l 0.013-0.026

o-5

flow batch

11031 ilO21

Among these pioneers were famous names as M. Volmer, M. Bogdan, C.N. Hinshelwood, C.R. Prichard, G.M. Schwab, B. Eberle and E.W.R. Steacie [49]. Although not interesting for practical purposes, it is worth mentioning the gaseous catalysts for comparison with the solid systems. They include the halogens, Cl, Br, and I, in their atomic form [49,54] and mercury [49]. The decomposition activation energies, corrected for the heat of dissociation, are in the range 135-155 k.I/mol for the halogens. For bromine the three-step mechanism (9a-c) has been proposed, whereby the second step (9b) is the slowest step. Br, t) 2Br

(9a>

N,O + Br + N, + BrO

(9b)

2BrO + Br, + 0,

(9c> Many solid catalysts have been reported, and include supported and unsupported metals, pure and mixed oxides, and zeolitic systems. A collection of systems is presented in Table 3, together with specification of some experimental conditions. A lot of the listed studies have been performed in batch recirculation systems and/or under very low partial N,O pressures. The observed N,O disappearance rate may not necessarily reflect the stable steady state activity under prolonged operation, unlike the results of flow experiments. Therefore, the relevance of those catalysts in table 3 for practical application can vary considerably.

F. Kapteijn

et al. /Applied

Catalysis

/

I

12

1.4

B: Erwironmental9

1 16

33

(1996) 25-64

/ 18

:

1000 T-'I K-' Fig. 1. Decomposition rates (pmol/s m’) of selected pure oxides, illustrated IO kPa N,O and 0.1 kPa Oz pressure. calculated from [61,62.66.161].

as Arrhenius

plots calculated

for

The metal catalysts include Pt, Pd, Ag, Au and Ge where decomposition generally occurs above 650 K [49,50]. Especially the early studies focussed on Pt [55-591 that has been studied most amongst the metals. The reaction rate is proportional to pNzO and oxygen has an inhibiting effect [55-571 up to a certain partial pressure, above which the rate becomes independent of po, [57]. Also N, inhibits the reaction, although much less than 0, [55]. The apparent activation energy is around 135 kJ/mol [49]. Takoudis and Schmidt [60] studied the reaction at low N,O pressures (l-65 Pa) and arrived at an activation energy for the N,O dissociation at the surface of 146 kJ/mol, with a heat of adsorption of 89 kJ/mol. The activity of Au has been studied up to 70 bar, yielding a first order pNzO dependency and an activation energy around 142 kJ/mol. Pure oxides have been collected and reviewed in [42,61-631. The highest activities are exhibited by the oxides of the transition metals of group VIII (Rh, Ir, Co, Fe, Ni), by CuO and by some rare earth oxides (La) [42,62,64,65]. High activities per unit surface area are also claimed for CaO, SrO, V,O, and HfO, [6,61,62]. Moderate activities are found for elements of group III-VII (Mn, Ce, Th, Sn, Cr) and of group II (Mg, Zn, Cd). Fig. 1 gives a comparison of specific activities (per unit surface area) of various oxides. Other elements are less active. The valency of an element is also important. For manganese which can have various oxidation states the activity order (per unit surface area) was MnO < MnO, < Mn,O, < Mn,O, [66], so 3 + seems the optimal oxidation state. For vanadium V,O, is much more active than the nearly inactive V,O, [6]. It should be mentioned that, depending on the experimental conditions, some oxides are not stable and are partially converted, like MnO,, MnO [66], Cu,O [67] and Co0 [68]. The apparent activation energies range between 80 and 170 kJ/mol. The rate is usually proportional to pN,o or has a slightly lower order due to the inhibition of produced oxygen. The order in po, for strong inhibition amounts to -0.5. A

34

F. Kapteijn et al. /Applied Catalysis B: Enuironmental9 (19961 25-64 0.4 i

648

K

--_=

o.??----

I- Ol-

0

2.0

4.0

6.0

6.0

10.0

po2 / kPa

Fig. 2. Oxygen partial pressure dependency various temperatures, from [66].

for the N,O decomposition

rate over Mn,O,

at 10 kPa N,O and

typical moderate effect of molecular oxygen on the decomposition rate is given in Fig. 2 for Mn,O,. An important distinction is that some oxides seem not to be affected by the presence of oxygen, this holds for Ca, Sr, La, Ce, Zn, Hf [49,63]. For applications in oxygen containing environments this is an important issue. Much work has been done on mixed oxidic systems, like doped oxides or solid solutions, spinels and perovskites, not only for the N,O decomposition reaction as such, but also predominantly for a better mechanistic understanding of catalytic phenomena over oxidic transition metal (TM) systems in general. Nowadays the studies are focussed more on the development of more active and stable systems. Cimino, Stone and others have systematically studied the effect of various transition metal ion concentrations in relatively inert oxide matrices like MgO, Al,O,, MgAl,O, [45,69-751 on N,O decomposition. The catalytic activity

-7.0

I 1.3

I,

1.4

I 1.5

/

I

1.6

,

I,

1.7

,

1.8

,

1.9

2.0

1O3 T-’ / K-’ Fig. 3. Relative activities of 3d TM ion solutions in MgO (1 at.-%), illustrated as Arrhenius plots, from [45].

F. Kapteijn et al./Applied

Catalysis B: En~~ironmental9 (1996) 25-64

35

develops strongly already in very dilute solutions ( < 1 TM ion per 100 cations), where the activity per TM ion is the highest at the lowest dilution. Fig. 3 compares the activity of several transition metals at 1 atomic % concentration in MgO. The TM ions act specifically, i.e. the activity of different oxidation states varies widely. An example of the former is the study of Cimino and Indovina [76] who demonstrated that Mn3+ ions dispersed in a MgO matrix had the most active oxidation state compared to Mn2+ and Mn4+, in agreement with the results for the pure oxides [66]. The activation energies of the higher concentrations TM resemble those of the pure oxides of the same oxidation state. For Ni, Cr and Co ions in MgO the activation energy decreases with increasing dilution, ascribed to a weaker bonding of the adsorbed oxygen [45,69,71,74,77]. Although molecular oxygen can adsorb on these systems and exhibits an inhibiting effect, this adsorption is much less for the diluted system. Cr has been studied further in magnesium aluminate and cz-alumina, with similar results as for magnesia. Co-aluminate has been reported as the most active system of the pure TM aluminates [78]. Perovskites and other mixed oxides (e.g. spinels, pyrochlores) are of a similar interest as the solid solution systems. Perovskites can be represented by the general formula ABO, [43,79,80]. Also double (and multiple) structures exist, indicated by AA’BB’O,. The A ion is generally the larger one ( > 90 pm), often a rare earth element, mostly La, and catalytically relatively inactive, while B (> 51 pm) is mostly a 3d-transition metal element like Cu, Cr, Fe, Co, Ni, Mn, Ti, mostly responsible for the catalytic activity, although the A ion influences this. Partial substitution of the A and/or B ion with an ion of another valency gives rise to abnormal valencies of the B-site cation (Cu3+, Ni”+) and/or to oxygen vacancies. These structures can be represented by A, A,_ .Bl,B, _ ?O, _ x, where A is the oxygen vacancy to compensate for the cation charges. 8r and other alkaline-earth elements and Ce are often applied for A-site substitution, while B-site substitution is studied to a lesser extent. Thus, a wide variety of structures can be formed, explaining the enormous amount of studies on these materials. Perovskites are known for their structural and thermal stability, due to their high temperature preparation, resulting in low specific surface areas ( < 10 m’/g>. The wide compositional variety control over the oxygen defects and valency of metal ions make them suitable model compounds to study the relation between solid state chemistry and catalytic activity, including the N,O decomposition reaction [43,79-821. In spite of their low specific surface area the reactivities of these catalysts are relatively high. Upon variation of the B-site in LaMO, (M = Co, Ni, Mn, Fe, Cr) [43,82] good correlations were found between the activation energy for the decomposition reaction (35-130 kJ/mol), and the oxygen binding energy and the isotopic exchange reaction of oxygen. Clearly, the oxygen bond strength plays an important role. The activity order is Co > Ni, Cu > Fe > Mn > Cr (Fig.

36

F. Kaptegn et al./Applied

Cr

Mn

Catalysis B: Encironmental9

Fe

co

Ni

(1996) 25-64

cu

3d elements of B-site Fig. 4. Comparison of activities for N,O decomposition (% conversion) of mixed oxides of the type LaMO, and La,MO,. Conditions 723 K, 0.1 kF’a N,O and W/FNso = 7200 g s/mmol, from [82].

4). The oxide matrix has a considerable effect on the activity of the TM ions as is apparent by comparing Fig. 1, Figs. 3 and 4. The effect of oxygen varies considerably. The reaction at 6.6 kPa N,O over Fe is not, over Ni weakly and over Co-systems strongly inhibited by oxygen [43]. The reaction is first order in pN o, and -0.5 in po, for strong inhibition. It is noted that La,O, is active, too, in the decomposition [61,65], so the La ion may participate in the process. Variation of the A-site ion in MMnO, (M = La, Nd, Sm, Gd) results in a decreasing activation energy from 105 to 30 kJ/mol, which is also explained by an increased electron density on the Mn site, resulting in a facilitated desorption of oxygen [43]. For the related structure M,CuO, (M = La, Pr, Nd, Sm and Gd) the activation energies ranged non-systematically between 70 and 145 kJ/mol at 6.6 kPa, while at 26 kPa the values dropped to between 45 and 85 kJ/mol. Oxygen inhibited strongly. Partial substitution of the three-valent La by the two-valent Sr in La,CuO, induces an average Cu valency up to +2.3 and with increasing Sr substitution oxygen defects [82-841. The highest activity is obtained at an optimal fraction of Sr of 0.5, where the copper has its highest average oxidation state (Figs. 5 and 6). No correlation with the oxygen vacancy concentration is found, which is apparently not the dominating factor that determines the activity. The oxygen inhibition is the strongest at low pressures, but at higher po, (> 1 kPa) the rate does not decrease further, like that observed for Pt catalysts. Recently, very active mixed oxide catalysts were reported, prepared from thermal decomposition (ca. 700 K) of transition metal (Co, Cu, Ni, Rh, Ru, Pd, La) containing hydrotalcites, belonging to the class of clay minerals. Some of them are even more active than the zeolitic catalysts [SS-871. A typical reaction order found is Co-Rh > Co-La > Co-Mg > Co-ZSM-5. Conversion of

F. Kapteijn et al. /Applied Catalysis B: Erwironmental9 (1996) 25-64

%!c

0:2

014

016

31

Oi6

x in La,~xSrxCuO, Fig. 5. N,O conversion, normalised per unit surface area, over La,CuO, doped with Sr as a function of the fraction Sr for reaction at 723 K, 0.1 kPa N,O and W/F,?, = 7200 g s/mmol. Included is the average oxidation number (AON) of copper, from [82].

N,O already occurs below 500 K. The apparent activation energies amount to 45-55 kJ/mol and the reaction is first order in pNzO. The oxygen inhibition is not high, water inhibits strongly. The Co-containing calcined samples exhibit a sustained life at temperatures above 900 K in a wet and oxygen containing atmosphere with 10% N,O [86], keeping promises for practical application. Supported oxides are not as frequently studied as the pure and mixed oxides, but for practical applications they might be better suited due to the higher dispersion by combination with the larger specific surface area of the support. Often their behaviour is compatible with that of the pure oxides. On the other hand, the loading, the way of preparation and the temperature history determine the final catalyst performance and the distinction between supported oxide and solid solution may vanish. Most reports deal with alumina as carrier for Pd and

r

/I

:::I;:;i: . 723 K

0’ 1.8

I

1.9

I

2.0

/ 2.1

/ 2.2

’

2.3

AON of Cu Fig. 6. N,O conversion, normalised per unit surface area, as a function of the average oxidation number (AON) of copper in La,_ XSr,CuO, at 723 and 773 K, 0.1 kPa N,O and W/F,>, = 7200 g s/mmol, from [82].

38

F. Kapteijn et al. /Applied

g 00

Catalysis B: Environmental

9 (1996) 25-64

800

700

600

T/K Fig. 7. Comparison of N,O conversion over several ZSM-5 catalysts. time W/FNzo = 1455 g s/mmol, from [88].

Conditions

0.099 kPa N,O and space

oxides of Cu, Co, Mn, Rh, Ru, Fe, Cr [6,14,70,88-921, some with silica-supported oxides of Ni, Fe, Cr, Cu and Co [26,37,93,94], but an observed trend is that zirconia is applied more and more in combination with, e.g. Rh, Co/Ni, Cu [95-991, with as important property the hydrophobic character. Although the decomposition of N,O over zeolite catalysts was already known for some time for Fe-systems [93,100-1021, in recent years numerous catalysts have been identified with high activities for the reaction. They are mostly based on a transition metal ion (Fe, Co, Ni, Cu, Mn, Ce, Ru, Rh, Pd) exchange procedure with a suited zeolite (ZSM-5, ZSM-11, Beta, Mordenite, USY, Ferrierite, A, X) [88,90,94,100-1071, and some already exhibit activities below 600 K [88,103]. The combination of metal ion and zeolite type determines the activity for N,O decomposition. The activity order for the different elements can deviate considerably from that of the pure oxides. Pt itself has a good activity as a metal, but in a zeolite it is hardly active. Co is very active in ZSM-5, Beta, ZSM-11, Ferrierite and Mordenite (MOR), moderately in L and Erionite, but hardly active in Y. Fe in ZSM-5 is much more active than in MOR and Y [94,108]. For ZSM-5, the most studied zeolite, the activity order is Rh, Ru > Pd > Cu > Co > Fe > Pt > Ni > Mn [88,90,103], see Fig. 7. Data on variation of the metal content are scarce, but for Co-ZSM-5, Co-Mor and Fe-ZSM-5 the activity seems to be proportional to the TM content [88,106,107]. This is quite different from that for NO decomposition over Cu-ZSM-5 where a much stronger increase with Cu loading exists [109]. The reaction rate is mostly first order in pNzO, with apparent activation energies ranging between 75 and 170 kJ/mol, although for Ru values ranging between 46 (ZSM-51 and 220 kJ/mol (USY) were reported [103]. The oxygen inhibition varies from catalyst to catalyst. In ZSM-5, Pd, Fe and Co show hardly any, Rh a moderate, and Ru and Cu a strong inhibition, although a high

F. Kapteijn er al. /Applied Catalysis B: Enrironmental9

(19961 25-64

3’)

concentration of oxygen does not seem to lower the rate any further [90], a result also observed for Pt [57] and for Co-perovskite [82]. On the other hand, for Fe-ZSM-5 and Fe-MOR the complete absence of oxygen inhibition [94,100] and for [Fe]-ZSM-5 even a positive effect is reported [ 1 lo]. For Ru zeolite systems apparent orders in oxygen of -0.5 for Ru-USY and -0.2 for Ru-ZSM-5 are reported [ 1031. Recently, a Cu-ZSM-5 catalyst has been reported to be active in the photocatalytic decomposition of N,O. UV irradiation (A < 300 nm) of the catalyst stimulates the reaction already at ambient conditions [79,111- 1131, whereby oxygen is evolved. In earlier studies on this type of reaction over ZnO, TiO,, and Pt/TiO, oxygen was not always observed [114-1171. It is noted that N,O itself can decompose under UV radiation (X < 200 nm [49]). It is proposed that in the Cu-ZSM-5 catalyst the charge donation to the N,O is triggered by excited Cu+-Cu+ dimers, resulting in a fast decomposition [ 1111. The reaction turned out to be proportional to the Cu + concentration [ 1131, independent of the N,O pressure, but is inhibited by oxygen to a certain addition level. While evaluating the activity data on the reported N,O decomposition catalysts, it is noticed that of the first row of transition metals Co and Cu generally exhibit a very high activity, while Rh and Ru are the most active of the second row. Very active catalysts are based on calcined hydrotalcites, zeolites and alumina supported noble metals. Activity is, however, not the only factor that determines the choice for a specific catalyst, this is discussed further in Section 6.

4. Mechanistic aspects Several mechanisms have been proposed for the catalytic decomposition of N,O, and in view of the various observations that have been made with respect to partial pressures and temperature dependencies over the plethora of catalytic systems mentioned above, a generalization is of limited value. Nevertheless, some unification can be made. In its simplest form the reaction can be described as an adsorption of N,O at the active center, usually a coordinatively unsaturated surface (cus) TM ion, followed by a decomposition giving formation of N, and a surface oxygen. This surface oxygen can desorb by combination with another oxygen atom or by direct reaction with another N,O. These four steps are indicated by Eqs. (lo)-(13). Of course the surface oxygen can also be removed by a reducing agent. k, N,O+ * f, N,O* (101 km, N,O * 2

N,+O*

(11)

F. Kapteijn et al. /Applied Catalysis B: Environmental 9 (1996) 25-64

40

time / min Fig. 8. Concentration 711 K, from [36].

20*

response curves upon a step change from 13.5% N,O in He to pure He over MnO,

at

k,

++ 0,+2* k-3 k, + N,+O,+

N,O+O*

(12)

*

(13)

Steps (10) and (12) may be reversible, while (11) and (13) are irreversible. Many studies report the uptake of N,O by the catalyst in the initial stages of the experiment, which can be easily observed in batch systems, but the amounts involved are often small to negligible [42]. Moreover, a transient kinetic study over MnO, by step-response experiments [36], Fig. 8, demonstrated that this adsorbed N,O did not partake in the reaction, so not all the adsorption sites are active for the decomposition. To avoid this problem steps (10) and (11) are often combined to Eq. (14). N,O+

*

2 N,+O*

(14)

The reaction of N,O with the catalysts active centers is generally envisaged as a charge donation from the catalyst into the antibonding orbitals of N,O, destabilising the N-O bond and leading to scission. Metal surfaces, oxides with some local charge donation properties, and isolated transition metal ions with more than one valency can act as such centers, but also F-centers (vacancies in an oxide surface with a trapped electron) have been proposed for the interpretation of the activities of oxide surfaces. ESR studies have clearly demonstrated that O- species are primarily formed by reaction with N,O [45,75]. Most intriguing is the interaction of molecular oxygen with the surface. Many catalysts suffer from inhibition by oxygen to a certain extent, but some do not. Moreover, in some cases the inhibition by oxygen occurs to a certain concentration level above which no further rate decrease is noticed.

F. Kupteijn et al. /Applied Catalysis B: Enaironmental9 (1996) 25-64

41

Inhibition by oxygen can be simply accounted for by the reversible dissociative adsorption of oxygen, either directly, Eq. (12) backward, or via molecular adsorption of oxygen, Eq. (151, as has been proposed to occur over metal surfaces [57,118]. o*+*

ks

-+o;

:20*

fast

(15)

On oxides this molecular adsorption may result in 0; formation [ 1191 which can react to 20- by a second electron donation. With increasing temperatures this reactive O- species can be further transformed into the more strongly bonded 02- [ 118,119]. The extent of these transformations will strongly depend on temperature, the transition metal ion and its concentration in the surface, and the mobility of the oxygen. It is clear that if this dissociative inhibition occurs, close relations should exist with oxygen isotope exchange reactions and oxygen-surface binding energies. Indeed such relations have been reported. Winter [61-631 found for a series of oxides a good correlation between the activation energy for isotopic oxygen exchange and that for the N,O decomposition, Golodets observed a relation between the surface oxygen bond energy and the catalytic activity in N,O decomposition [42]. For perovskites these relations have been found, too [43]. Also, a correlation with the lattice parameter of the oxide type can be found [43,61,62], suggesting that a critical parameter in this 0, desorption must be the O-O distance in the surface. These relations should only hold when the desorption of oxygen is a difficult step in the process, otherwise other steps determine the overall rate and other activation energies can be observed. In this respect Winter [62] compared the activation energy for N,O decomposition with that for 0, desorption (deduced from isotopic oxygen exchange, where desorption is the difficult process) and found striking correspondence, mostly they were equal within less than 20 k.I/mol. Exceptions are SrO, TiO,, RhZ03, MnO, and h-0, where the desorption values were much lower, probably due to another exchange mechanism (molecular) for these oxides [62,118]. The studies of solid solutions of TM ions in oxide matrices [45,46,69,7 l74,76,120] has provided a more detailed picture of the active center in oxides related to Cu, Fe, Co, Cr, Mn and Ni. Some trends found are summarized as follows. At very low concentrations where the TM ions can be considered to be isolated, the activity per TM ion is very high and decreases with increasing concentration. With increasing concentration the TM ions form pairs or clusters with a lower activity and ultimately reflect the behaviour of the pure oxide. The activation energy has been found to increase with Co or Cr concentration (Figs. 9 and 10). Assuming that the oxygen desorption is rate determining this activation energy reflects the oxygen binding strength with the surface. So, isolation of the TM ion decreases the bond strength of the deposited

42

F. Kapteijn et al./Applied

Catalysis B: Enuironmental9

(1996) 25-64

-310’‘015’‘110’‘115’ ’’2.8’ x in Cr,AI,,O, Fig. 9. Specific activity k (cm Hg min-’ [Cr]- ’) and apparent activation energy I?, for the N,O decomposition over Cr, Al,_,O, as a function of the atomic fraction x of Cr, adapted from [73].

oxygen, lowers the activation energy and enhances the reaction rate. For Ni [69,74] a decreased oxygen inhibition was observed, in agreement with the results for dilute Cr and Co samples [71,72] that the oxygen uptake from 0, is much less than for the more concentrated samples or the pure oxides. On the other hand, exposure to N,O led to a much higher oxygen uptake per TM ion for dilute systems, i.e., for Cr even more than one oxygen atom per Cr ion. In the case of isolated TM ions it has been suggested that the oxygen can migrate via the matrix surface (via peroxide ion formation, 022-j until another adsorbed oxygen atom is found or a matrix oxygen favourable for the creation of an anion vacancy, i.e. desorption of 0, is encountered [71,73]. Alternatively, the oxygen can remain accommodated in the peroxide form at the oxygen anion bridging between the TM and the matrix cation until a second oxygen is deposited on the TM [73,76], as envisaged in Fig. 1la. These proposals have been confirmed later

I-

150-

,

1

E

,i

coo

IOO-

I!

NiO

50 -

O”“,‘l

10-3

I

“““I

10 -2

/“““I

IO -’

“‘I”’

100

molar fraction MO in MgO Fig. 10. Apparent activation energy for the N,O decomposition function of the atomic fraction Co, adapted from [69,71,74].

over solid solution

of Co0

in MgO as a

F. Kapteijn et al. /Applied Catalysis B: Emironmental9

v -

1

O-

_11I / CF”

43

(19961 25-64

N* 0,2-

j

crL

0,2,

/I

0,2-

T

0, N*

N,O_ o,z-_ CL

02.

/I -

“JY!z

04

bulk

Fig. 11. Reaction

schemes for N,O decomposition

over isolated Cr ions (a) and for bulk oxide (b), according

to [731.

by Indovina et al. through ESR measurements on highly diluted Co2+ ions in MgO [75,119]. N,O deposits an O- species on the Co ion leading to Co3+. This is a labile complex and the deposited oxygen subsequently moves to an adjacent matrix oxygen 02- under formation of Oi- and restoring Co2+ [75]. Molecular oxygen forms at similar conditions only the somewhat more stable Co3+-0, complexes that only dissociate into atomic species at higher temperatures [119]. The oxidation state of the TM ion determines the electron donation properties and the activity in the dissociation of N,O. For Co3+ in ZnCo,O, this is the rate limiting step, while for other spinels containing increasing amounts of Co2+ this shifts to the desorption of oxygen [121]. In these pictures the capacity of the matrix to accommodate oxygen as peroxide species Oz- is important. This has been used to explain the larger activity of dilute Cr ions in a-Al,O, and MgAl,O,. The latter spine1 structure is more reluctant to release oxygen than the corundum structure [73], accounting for the 40 kJ/mol lower activation energy for the N,O decomposition. This explanation has also been given for the difference in activation energies for diluted Co ions in MgO (71 kJ/mol) versus those for Co in ZnO and MgAl,O, (120 kJ/mol) [75,120].

44

F. Kapteijn et al/Applied

Catalysis B: Environmental 9 (1996) 25-64

O,8!0012 014 016

0:s’

x in La,_xSrxMnO, Fig. 12. Variation of the apparent activation La,_xSr,MnO,, from [122].

energy and atomic % Mn4+ as a function of the fraction Mn in

The coordination of the TM ions contributes to the performance. Octahedrally coordinated Co*+ in MgAl,O, is more active than the tetrahedrally coordinated ion, due to the distorsion of the structure which results in two longer Co-O bonds and, hence, a weaker bond strength [120]. The same is observed for Ni [46]. Apparently, the isolated sites cannot chemisorb 0, dissociatively, so the inhibition effect is lower for these systems, especially observed for the strongly inhibited NiO [69]. With increasing concentrations the TM ions pair-up and the O- is now held by such a pair (Fig. 1 lb). This reflects the O/Cr ratio of ca. 0.5 found [73]. These TM ions have d-electron density available, resulting in a stronger binding than in the case of a TM ion-matrix ion (A13+, Mg*+) pair, and reflect the increasing 0, desorption energy with TM ion concentration (Figs. 9 and 10). Reversely, 0, can now adsorb and dissociate at higher concentrations and, hence, inhibits more strongly. This pairing has been described for manganese in La, _XSr,Mn03 perovskites [122] and for cobalt in spinels like NiCo,O, and Co,O, [121]. In both cases the optimal activity was found where the Mn3+/Mn4+ or Co2+/Co3+ ratio was 1: 1, and the activation energy had dropped to 32 and 23 kJ/mol, respectively (Fig. 12). This emphasizes the participation of the neighbouring ion in the reaction, and a M-O-M system was assumed to constitute an active site for the reaction. Also in this case, the optimal charge transfers, from the TM to the 0 upon adsorption and back from O- to the TM upon 0, desorption, are used as interpretation for this phenomenon. It can be rationalized by the so-called Zener double exchange [80] where two ions, differing by one oxidation state and bonded via an oxygen ion, can exchange an electron through the 0 2p orbital. This leads to an average and easily adaptable oxidation state that facilitates the adsorption and desorption processes [ 123-1251.

F. Kapteijn et al/Applied

Catalysis B: Enuironmental9 (1996125-64

45

The doping of the perovskites with ions of another valency induces oxidation states which are usually not encountered in the pure oxides. Sr doping of La,CuO, [82,83] induces the CU*+/C.U~~ oxidation reduction cycle for copper during N,O decomposition. The absence of oxygen inhibition can be explained by two ways. Either reaction (12) is irreversible or reaction (13) takes place as a major route to destroy N,O, whereby 0, provides the oxidation of the sites, Eq. (12). For MnO, the reaction (13) could be clearly rejected on the basis of transient kinetic experiments [36], Fig. 8. In these flow reactor experiments the N,O-containing gas flow was nearly instantaneously switched to an inert gas flow and the response of the catalyst was followed by mass spectrometry. The production of N, ceased instantaneously, while the N,O was still observed for minutes. The oxygen desorption continued for a long period, so oxidized sites are available during this period. If reaction (13) would have taken place over this catalyst, the N, evolution should have followed the N,O evolution, which is clearly not the case. On the other hand (13) has been forwarded to explain the kinetics over zeolites [90,100,101]. More transient kinetics work is needed to shed more light on this aspect. The irreversibility of (12) can be justified for the dilute TM ion solution systems where the isolated TM ions are incapable of chemisorbing 0,. This is evidenced by their low 0, uptake capacity, only a few % of a monolayer, compared to the pure oxides, which generally can exchange half to a complete monolayer [65,71,72,126,127]. For pure oxides that exhibit no oxygen inhibition Winter [63] also rejected Eq. (13) as an interpretation and reasoned that two adjacent O- species that desorb as 0, results in the formation of an R, center (two adjacent F-centers). Subsequently, this center is rapidly destroyed by migration of a neighbouring O*- anion to one of these F-centers, yielding two non-neighbouring F-centers. Winter assumed that these three sites involved in this scheme are always the same isolated ‘triads’, confined to locations where the conditions are optimal to promote the rapid switching between these three sites. In fact, these types of sites are compatible with the isolated TM ion sites in dilute solid solutions which are also not accessible to dissociative chemisorption of 0,. By using r802 during the N,O decomposition experiments over these oxides (CaO, TiO,, A1203, Gd,O,) it was confirmed that the normal isotopic exchange reaction between 0, molecules was not affected by the reaction of N,O, indicating that these reactions occur over different types of sites [63]. The desorption of 0, from CaO and Li/MgO was studied in more detail [127-1291. After treatment in N,O considerable amounts of oxygen were present at the surface (O/Ca,,, = 0.3-0.5) and were identified as peroxide species. The deposition of this oxygen was the easier process, while the desorption of 0, required higher temperatures. From temperature programmed desorption (TPD) experiments, the process turned out to be second order in the

46

F. Kapteijn et al/Applied

Catalysis B: Environmental

9 (1996) 25-64

oxygen surface concentration with an activation energy of 130 kJ/mol for CaO and 141 kJ/mol for Li/MgO. This compares well with the apparent activation energy for N,O decomposition over CaO of 140 kJ/mol and 0, oxygen exchange of 130 kJ/mol [62], supporting the idea that in these cases the desorption of oxygen is the difficult step. The second order dependency indicates the mobility of the oxygen species over the surface. Oxygen itself did not adsorb on CaO, which agrees with the fact that N,O decomposition is not inhibited by 0,. 4.1. Zeolites With TM-ion exchanged zeolites a situation can also be created of a highly dispersed, isolated TM ion, like in dilute solid solution systems. So, similar results can be expected and are indeed found. Most studied with respect to N,O decomposition are the Fe- and Cu-ion exchanged ZSM-5, Mordenite, and Y [94,100,101,105,106,108,130,131]. The perfect isolation of these TM ions can be questioned, however, since copper is often used in an highly overexchanged form and also some migration abilities at the reaction temperatures are ascribed to copper and iron [loo]. The Cu and Fe ions may form pairs or clusters [ 108,109,130- 1321 and their performance may resemble more closely the less dilute solid solution systems. The most interesting studies related to N,O decomposition mechanism besides the reaction itself, are isotope step switching (e.g. from N,O to Ni*O), 0, isotope exchange reactions, and 0, desorption experiments. The reaction of N,O with the catalyst proceeds easily. The activation energy of Fe-ZSM-5 oxidation amounts to 40-45 kJ/mol [94] and the deposited amount of oxygen varies between about 0.2 to 0.5 (O/Fe or O/Cu ratio) [94,104,105,108,13 11. This oxygen is usually indicated as extra-lattice oxygen 100

2'

20-

80

Fe-ZSM-5 (773 K) I 1.0

I

2.0

3.0

pO,/kPa Fig. 13. N,O conversion and space time W/FNzo

as a function of oxygen partial pressure ZSM-5 catalysts. = 150 g s/mmol, from [90,135].

Conditions

0.1 kPa N,O

F. Kapteijn

Table 4 Observed activation energies ion-exchanged ZSM-5 [90]

et al. /Applied

in the N20

Apparent activation

co CU Fe

Catalysis

B: Encironmental9

decomposition

(1000

f 1996)

ppm) in various

25-64

gas mixtures

41

over TM

energies (kT/mol)

only N,O

CO/N,0

110 138 165

115 187 78

= 2

3% o2 118 170 187

(ELO). The latter value led, together with IR, NMR and Miissbauer results, to the conclusion that the EL0 was held between pairs of Fe3+ or Cu*+ ions [105,131,132]. For Fe and Cu the reported apparent activation energy for decomposition varies between 125 and 160 kJ/mol [90,94,100,106]. Some studies report no inhibition by 0, for Fe [94,100], but we observed some [90], depending on the temperature, and this is stronger for Cu [88,90], see Fig. 13. In the latter cases this is ascribed to the presence of small oxide clusters in addition to isolated ions [94,111,130] (some 0, adsorption is observed for the Cu systems [ 13 1,133], while corresponding Mn, Ni and Co systems do not). This also accounts for the leveling-off of this inhibition at higher partial pressures of 0,, when the contribution of the inhibited sites has vanished. The apparent activation energy is also pushed to higher values (see Table 4) and will be explained later (see Eq. (311). Intriguing results were obtained by step switching experiments from N,O to Ni’O over Fe-MOR [105]. Directly after the step change N,O evolved for an appreciable time, while the oxygen produced contained mainly unlabelled 0,. Only gradually increasing amounts of Ni*O and 1800 appeared in the product gas. The total amounts of unlabelled oxygen that evolved after the step was about 8 times the EL0 deposited on the catalyst, equal to about 6% of the total lattice oxygen of the zeolite. These results indicate that N,O intensely exchanges oxygen with the catalyst [loll, that the oxygen deposited by N,O can exchange with the lattice oxygen of the zeolite, and that the oxygen must have a certain mobility to achieve this. Similar results were observed for NO and ‘5N’80 step changes over Cu-ZSM-5 and Cu-Y [ 105,13 11. No 0, adsorption was observed for Fe-ZSM-5 [94,106], neither did it exchange with oxygen of the catalyst (‘heteromolecular exchange’ [ 1181) nor with ‘*02 (‘homomolecular exchange’ [I 181). After treatment of the catalyst at low temperatures in N,O, where the deposited oxygen remains on the catalyst, both types of exchange reactions occurred even at room temperature. About 60-80% of this oxygen was involved in the exchange processes. Apparently, the deposited oxygen plays a catalytic role. Molecular 0, was unable to produce these species. In analogy to the results for pure oxides, it was attempted to prove

48

F. Kapteijn et al. /Applied Catalysis B: Enoironmental9 (1996) 25-64

that this could be O- species by ESR but without success [94,106]. Nevertheless, it is assumed that by the N,O oxidation the state of the oxidized TM ions is changed from Fe*+ to Fe3+ and Cu+ to Cu*+ [130,131]. In the absence of N,O, the isotopic oxygen exchange in ZSM-5 zeolites generally requires temperatures between 700-800 K, much higher than the corresponding pure oxides [ 1341. Only Cu catalysed the exchange already around 500 K. In this study it turned out that a few % of the lattice oxygen was involved in the exchange: Cu (8%) > Nb (5%) > Co (1%). Especially Cu had a good activity for the ‘single step double exchange’ reaction (0, exchanges for 1802), a property also exhibited by the pure oxide [62,118]. Comparison of N,O decomposition results over Co-ZSM-5 with those over Cu- or Fe-ZSM-5 shows that the former operates with a much lower concentration of oxidized sites than the other two. The steady-state fractional oxygen coverage for Co is estimated to be 0.5, while those of Cu and Fe are between 0.9 and 1.0 [90,135], which indicates that the removal of oxygen is the difficult process for the latter. This is reflected by their higher activation energies (Table 4). Co-ZSM-5 takes an intermediate position, meaning that the catalyst oxidation step and the oxygen removal step are of equal importance in the overall process. If the oxygen removal is enhanced by e.g. CO, Eq. (16), the activation energy can drop considerably, as found for Fe-ZSM-5 (Table 4), pushing the slowest step towards the oxidation of the catalyst by N,O. In the case of copper the activation energy increases due to the strongly adsorbed CO at the Cu’+ sites. From in-situ infrared studies a value of ca. 60 kJ/mol has been found [135], considerably higher than on pure Cu,O (ca. 20-25 kJ/mol). Not only CO can remove the deposited oxygen, also NO and SO, are able to do so, Eqs. (17,18), and become oxidized [90]. co+o*

k, +co*+*

(16)

NO+O*

k, +N02+*

(17)

ka

so,+o* + so,+ *

(18)

The absence of oxygen inhibition has been explained by assuming that the steps (14) and (13) constitute the catalytic cycle [101,131]. Here, N,O acts as oxidising and reducing agent. This idea is supported by the fast exchange of oxygen between the N,O and the catalyst, which may be considered as a non-productive step (13). This reopens the description of the reaction for the isolated TM ions in oxide matrices, where a similar model could be applicable, but for which this kind of data is not available. The oxygen inhibition for Cu zeolites must be ascribed to the presence of pairs or clusters of oxidic Cu ions which can also accommodate molecular oxygen via the reverse of reaction (12).

F. Kapteijn et al. /Applied Catalysis B: Enoironmental9

f 1996125-64

49

This property of the Cu-zeolites is also responsible for its activity in the direct decomposition of NO [130]. Other zeolitic systems with high decomposition activities like Pd-, Rh- and Ru-ZSM-5 exhibit no, moderate and strong oxygen inhibition, respectively [88,103,104]. This is probably also due to the fact that the Rh and Ru catalysts merely contain oxide clusters and not isolated TM ions, as they are prepared by impregnation methods. This is corroborated by the fact that alumina-supported Rh and Ru exhibit similar behaviour towards oxygen [88]. Nevertheless, the dispersion should be high since the oxygen uptake, O/Ru, amounts to 0.13. The inhibition effect was strongest for Ru-USY [103], the most active zeolitic catalyst, where the rate is proportional to p&‘.‘, indicative of a dissociative adsorption model. From these results with zeolites, parallels can be drawn with those for diluted solid solutions. The isolated TM ions in zeolites can accommodate the oxygen from the N,O, whereby intensive exchange with lattice oxygen can take place. Tentatively it is suggested that, like in solid solutions, this can occur via accommodation of the oxygen on a bridging oxygen between the TM and the matrix, whereby the original identity of the oxygen is lost and exchange takes place. This confines the exchange to the immediate vicinity of the TM ion. The involvement of the lattice has become clear by the presented experimental evidence, and this might also point to the direction of an explanation why some zeolites result in more active catalysts than other, as has been given for the oxide matrices in solid solutions and perovskites. The bond strength of the lattice oxygen, as can be determined from the exchange rate in step-response experiments with labelled molecules, an ideal tool, could provide additional insight. The dispersed nature of the TM-ions explains why oxygen cannot adsorb dissociatively to inhibit the reaction, unless clusters of a TM oxide are present.

5. Kinetics Many studies, especially the early studies, were carried out in static reactor systems. This makes interpretation of the data in terms of kinetics difficult since the system does not work under steady-state conditions. Often it has been observed that in the initial stages of the experiment the catalyst is loaded with oxygen, apparent from a N,/O, product ratio much larger than 2. The low conversion data then merely represent the catalyst oxidation step rather than the overall reaction. For kinetic studies the use of flow reactors is preferred since the evolution of a catalyst towards the real steady state is automatically verified. Depending on the catalyst pretreatment this may take a few minutes to days, and an oxidative pretreatment is preferred [82]. Also the temperature programmed

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experiments with a relatively high heating rate [ 1 lo] suffer from transient phenomena. Most studies have been concerned with N,O partial pressures in the range between 0.1 and 10 kPa, and the effect of oxygen has been derived usually from the amount produced by the reaction. This limits the partial oxygen pressure range to that used for the N,O times its conversion level. Only few studies really added oxygen to the reaction mixture, thus covering a much wider partial pressure range. Various rate expressions have been proposed and the most important are summarised below. Based on reaction sequence (lo)--(121, with (10) and (12) at quasi-equilibrium the following expression is easily derived based on the steady state assumption and a constant number of active sites, represented by the active site concentration NT [ 1361. r=

kdwf 1 + K, 'PN,O

PN,O

(19)

The denominator represents the distribution between the catalyst sites that are empty and those that are occupied with N,O and 0, respectively. This expression has originally been proposed for oxides [61], but is applied in general. Adsorption of N,O and of 0, results in reaction inhibition. It could describe excellently the rate behaviour over Mn,O, [66]. For low nitrous oxide coverages the amount N,O adsorbed becomes negligible and reactions (10) and (11) can be represented by (141, resulting in: k; NT. I-=

PN,O

1+4po,//%

The remaining oxygen pressure term in the denominator is generally not negligible in view of the experimental results. For high oxygen coverages (high oxygen pressures) the number of vacant sites are negligible, yielding: r= k;&N,.

PN

0

--L PO, T--

(21)

Based on these expressions various pressure dependencies are identified and can be compared with the empirical power rate modelling. For N,O an order between 0 and 1 and for oxygen an order between 0 and -0.5 is easily recognised. Generally for N,O an apparent order of 1 or slightly lower is found. The latter is due to the inhibiting effect of produced oxygen which is not accounted for, so does not necessarily indicate an appreciable N,O adsorption. The oxygen inhibition varies from catalyst to catalyst and is often described to be zero, moderate or strong [43,62,77,81]. The phenomenon that some catalysts do not exhibit any dependency on the

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0, pressure does not imply that there are no sites occupied by oxygen, as would follow from expression (19). There is clear evidence from transient kinetic [36,37] and temperature programmed desorption experiments [127-1291 that considerable amounts of oxygen are present, indicating that oxygen desorption also determines the overall rate. Globally two alternatives have been proposed. The first is an irreversible reaction step (12), so dissociative 0, adsorption is forbidden. This yields a less simple expression, but has as limits that for low N,O pressures the reaction is first order in pNzO, while at high pressures the order becomes two, reflecting the fact that two oxygen atoms have to react at the surface to yield molecular oxygen. Such a second-order behaviour has indeed been found for the temperature programmed desorption of 0, from the surface of CaO [127,128], a catalyst showing no oxygen inhibition [63], and for Li/MgO [129]. This does not prove for the correctness of this model since under these desorption conditions no N,O was present, which plays a crucial role in the second explanation below. In this alternative, proposed for Fe-ZSM-5 [lOO,lOl], the removal of surface oxygen is effectuated by the reaction with N,O from the gas phase, according to (13). So, together with (14) the first order relation (22) for N,O decomposition is obtained. r=

2k;k,N, k; + k,

‘PN,O

(22)

Depending on the values of the rate constants k; and k, the following simplifications can be made. For k; < k, the slowest step is (14) and the catalyst will hardly contain surface oxygen: r= 2W,.p,zo For k, < k’, step (13) is the slowest and accumulation takes place:

(23) of oxygen at the surface

This is the case for Fe-ion exchanged zeolites [90,100,101] and to a smaller extent Cu-zeolites [90]. Co-ZSM-5 takes an intermediate position, with surface coverages between 40 and 50% [90,135]. This kinetic model is supported by diffuse reflectance FTIR with Co-ZSM-5. Here the surface oxygen removal proceeds faster than in the case of Fe-ZSM-5, and after removal of the N,O gas phase and purging with inert gas at 570 K the oxygen is expected to have disappeared if Eq. (12) holds. Subsequent introduction of CO, however, led to the formation of appreciable amounts of CO,, indicating that also for Co-ZSM-5 a reducing agent like CO or N,O is needed to remove the oxygen, according to Eq. (13) [90,135]. For metal catalysts the oxygen inhibition has been found to exhibit a pressure

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dependency order of - 1 and not - 0.5 [57]. Therefore, a molecular adsorption step, followed by a fast dissociation has been proposed, reaction (15). Now two reaction sequences give a closed catalytic cycle, namely steps (13) and (14) and steps (14) and (15). In the first cycle the surface oxygen is provided by N,O and in the second by 0,. In both cases the oxygen is removed by reaction with N,O [42,57]. The individual and combined rate expressions are given by (25-27).

w4w

Pi,0

‘(l)= +

(25)

k4) .pNzO + 2k, .po,

(k;

k&&

’PN,O . PO,

r(2) = + (k;

k4) -pNzO + 2k, .po, k; * pN20

r=k4NTePN20-

+

k5 . PO,

(k;+k4)~pN20+2k,~p02

(27)

This result implies that at low and high oxygen pressures the reaction is first order in N,O, and independent of O,, expressions (28) and (29), respectively, and in the intermediate range 0, inhibits the reaction. This accounts for the observation that with increasing oxygen pressure activity indeed seems to become constant [57]. r =

i-(l) =

%k4NT k’

.

2 r =

rQ)

=

k4NT 2

pNzO

4

. pN,O

From these equations it follows that k4 F-

w4

k; + k,

so: k;>k,

Similar oxygen inhibition phenomena have been found for perovskites and for zeolites [82,90]. In the latter case, due to the fact that in some cases no oxygen dependency is found, this is ascribed to a contribution of isolated sites, which oxygen cannot inhibit, and small oxidic clusters [130,137], where oxygen inhibition can occur. Molecular oxygen adsorption has been observed for Cu-Y up to a ratio O,/Cu = 0.05 [133]. Even two types have been identified by TPD with desorption activation energies of 135 and 200 kJ/mol. Ni-, Co- and Mn-Y hardly adsorbed oxygen. This clearly can account for the results of Cu-ZSM-5 in Table 4. For perovskites no clear preference for one of these models can be given on the basis of available data. For the models presented above, in which oxygen inhibition plays a role, it is assumed that step (12) is in quasi-equilibrium. From transient kinetic experiments it appeared, however, that this is not the case for NiO and MnO, [36,37].

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Here the observed amount of surface oxygen was clearly above the equilibrium value, based upon the obtained values of the rate constants k, and k_,. This illustrates the power of transient kinetics, it yields independently values for surface coverages and for rate constants and may assist in discrimination between rival models [36]. Moreover, steady-state rate expressions based on ( 13) not to be in quasi-equilibrium are rather complex and accurate parameter values are hard to obtain or are not obtainable at all from steady-state kinetic experiments. Lack of transient kinetic data for other catalysts hampers the evaluation of steady-state kinetic modelling for these systems. This is an opportunity for further research. The rate expressions clearly show that all reaction steps contribute to the overall rate and also determine the temperature dependency. Only in limiting cases can simplifications be made which give some insight in the meaning of the rate parameters. Two extremes of Eq. (19) are (23) and (21) with apparent rate constants k\A$ and kkyKA$. On the basis of these two extremes one can state for the limits of observed activation energies for this model [ 1361:

The temperature dependency of the second rate constant is higher since it contains a desorption enthalpy of oxygen, which is generally positive for thermodynamic reasons. Only a few values of activation energies for the reaction of N,O with the catalyst surface, Ea2, have been reported: 42 kJ/mol for Fe-ZSM-5 [94], lo-33 kJ/mol for solid solutions of Cr in alumina [72], 8 kJ/mol for Cr,O, [72], 84 kJ/mol for Cu,O [67] and 35 kJ/mol for NiO [37]. Winter assumed that for rare earth oxides the low N,O pressure data were representative for Ea2, [61] and reports values between 40 and 110 kJ/mol. These correspond to the 95 kJ/mol found for Mn,O, in a thorough kinetic modelling study [66]. So values up to 100 kJ/mol can be expected for this process. The other extreme includes the desorption enthalpy of oxygen, which can be identified with the activation energy for isotopic oxygen exchange (40-190 kJ/mol [62,118]) where the desorption is the difficult process. The low values of Ea2c explain the good correlation that Winter found between the observed activation energies for N,O decomposition and that for isotopic oxygen exchange, especially for systems that showed a strong oxygen inhibition [62]. The endothermal nature of the desorption explains why the oxygen inhibition decreases with increasing temperature. In the case of catalysts with no oxygen inhibition, where Eq. (28) applies, the temperature dependency is determined by Ea2, and Ea4. Whatever activation energy is observed depends on the absolute value of k,, and k,. That of the smallest rate constant over the temperature range studied will be found. A change from a higher to a lower value may be observed when the Arrhenius plots of the rate constants cross each other [136].

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By varying the experimental conditions the kinetics and, hence, the observed activation energy can change. This was demonstrated in Table 4 for zeolites and by others for low and high pressure data [60,61,81]. Therefore, comparison of catalysts on the basis of apparent activation energies is of limited value. Especially the attempt to relate differences in reported activation energies to vibrational energy levels of surface species is questionable [44]. Recently, isothermal oscillations have been observed for the N,O decomposition (1000 ppm) over Cu-ZSM-5 [138], indicating that sometimes the kinetic modelling given above is a limited simplification. An explanation for these oscillations could not yet be given, but it was demonstrated that addition of several ppm NO enhanced considerably the frequency and magnitude of the oscillations in the conversion. Above 20 ppm NO the oscillations were quenched, but the system stayed in a state of high conversion. It has been suggested that the formation of traces of NO during the decomposition triggers this effect [138]. 5.1. Other gases Other gases present in the mixtures that contain N,O can affect its destruction rate by reversible inhibition due to competitive adsorption, by deactivation due to irreversible blocking (poisoning), or by alternative reactions, like Eqs. (4-S). Important in this respect are H,O, NO, SO,, CO, CO, and halogens. Their effects are primarily important for practical applications, but far less studied than the ‘pure‘ reaction. In the rate expressions competitive adsorption can be accounted for by of expressions like including adsorption terms, K,pi, in the denominator (19)-(22). Deactivation or poisoning effects should be included through an expression for the active site density A$ that accounts for its rate of decline. For alternative reactions new expressions are needed. Water has different effects. It ranges from inhibiting the reaction [17,86,96,99,139] to hardly affecting the catalyst performance [97,98,108,140]. For perovskites and Cu/ZrO, also the hydroxylation of the surface was observed, through which the active site density decreased [80,96]. In most reported cases the effect of water turned out to be reversible, unless it caused a destruction of the catalyst, like for Cu-ZSM-5 [ 1391 and mordenites [ 1411 through dealumination. Since adsorption is exothermal, its inhibiting effect will be most pronounced at low temperatures, whereas it will cease at higher temperatures. On the other hand high temperatures favour hydrothermal deactivation. Co-ZSM-5 prepared from H-ZSM-5 is reported to have a better hydrothermal stability than those prepared from Na-ZSM-5 due to the presence of unexchanged Na ions [ 1421. Carbon dioxide generally hardly affects the decomposition rate, unless it

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leads to stable carbonate formation under the relevant conditions [80]. In a fluid-bed combustion atmosphere CaCO, is more stable than CaO, but is far less active [9,17,143]. Nitric oxide might exhibit some competitive adsorption and deactivation through nitrite and nitrate formation. For Cu/ZrO, the inhibition is pronounced, especially below 770 K [99], due to surface nitrates formation. On the other hand NO can react to NO, via (171, as demonstrated for Co- and Fe-ZSM-5 [90]. In the latter case it accelerated the N,O removal rate since for Fe-ZSM-5 the oxygen removal is a difficult process. For Co-ZSM-5 only a slight inhibition was reported [88], while for Cu-ZSM-5 there is no effect on the decomposition rate [88,90]. Sulphur dioxide is a well-known poison for oxidic catalysts through the formation of surface sulphates [80,144]. With respect to N,O decomposition, however, not much work has been done, and that which has is mainly in relation to fluid-bed coal combustion. Here CaO, a good N,O decomposition catalyst, is often added as limestone to capture SO,, yielding CaSO,, which is nearly inactive [17]. So, on the one hand the SO, emission is lowered, but the N,O emission increases. This ‘trade-off’ [9] is more often encountered in emission control as mentioned in the introduction. Of course some relevant data can be extracted from research in the area of oxidation catalysis, selective catalytic reduction, and three-way catalysis, where traces of sulphur are generally present [28,29,80,141,1441461. With zeolites a widely differing behaviour can be observed. Co-ZSM-5 exhibits mainly an inhibition by SO,, Cu-ZSM-5 is deactivated, and Fe-ZSM-5 shows an increased N,O removal rate, probably through reaction (18) [90]. A similar effect was found for the selective reduction of NO over Co-ZSM-5 [145] and Pt [28]. So, with respect to kinetics each catalyst has to be studied individually. An important target for catalyst development is the SO, tolerance. This has been investigated for perovskites for catalytic oxidation reactions [80]. The addition of dopants of elements with a low tendency for sulphate formation (Ti, Zr) increased the SO, poisoning resistance [80,147,148]. Only a few catalysts are reported with a good SO, tolerance, like Co-ZSM-5 [28,90,145] and oxidic systems mainly based on Rh,O, as the active phase [ 14,971. No data on halogen poisoning were found in the open literature. Patents of Mitsui Mining claim that catalysts based on Rh,O, and Co,O, are resistant against H,O, SO, and halogens [97] Cm-bon monoxide is a reducing agent and reacts as such with N,O, Eq. (4). It can easily remove the oxygen from the catalysts surface, Eq. (161, and enhances N,O destruction for catalysts that have a high steady state concentration of oxidized sites. This is exemplified by the results with ZSM-5 catalysts at which the reaction was enhanced by a factor of 2-3 for Co, and much more for Cu and

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Fe, where it took place at temperatures where the decomposition itself was hardly measurable [90]. The presence of this reaction requires additional kinetic studies, which are generally presented as CO oxidation reactions by N,O and related to three-way catalysts [14,26,58,108,149-1511. Apart from enhancing the N,O removal CO also inhibits over Cu-ZSM-5 when present in excess [90]. The strong adsorption of CO, even at temperatures of 600-700 K leads to a decreased activity and an increased activation energy (Table 4). Also for the N,O-CO reaction, steady-state multiplicity has been found over supported Pt such as that known for the reaction with oxygen [149].

6. Process

options

Not all human N,O emission sources can be tackled by end-of-pipe solutions. The most obvious that can be treated are adipic acid plants, nitric acid plants, fluid bed combustors and vehicles equipped with three-way catalysts. The first will yield relatively quick results due to the high concentrations and small amount of point sources, the last is most challenging. All catalysts will have to satisfy the requirements of high activity and stability at the conditions of application (see e.g. Table 2), which translates into a low sensitivity to 0, and H,O inhibition, tolerance to poisons like SO,, and (hydro)thermally stable. In dust-laden situations of FBC the catalyst must be attrition resistant. Additionally, for all targets catalyst application requires a low pressure drop. The high flow rates involved will generally require a solution like the use of monolithic structures, either as a washcoated ceramic monolith or one of the pure catalyst [ 1521, although also low pressure-drop packed-bed structures are being developed [153]. Washcoating monoliths is already standard technology, but coating with pure or mixed oxides, like perovskite materials, requires more development. A high activity per unit reactor volume is required, so a high activity per unit surface area is worth nothing without realization of an appreciable specific surface area, which is a typical topic in perovskite catalysts. Zeolites are usually applied in a binder matrix, which lowers the catalyst density in a reactor. Direct coating of the ceramic monolith walls or wire mesh gauze coating [ 153,154] is a promising development, reducing at the same time the diffusional length of the reactants in the catalyst structure. A typical flow rate in a nitric acid plant of 100000 N m3/h will require, for the most active zeolite catalyst [88] coated on a monolithic structure, volumes of ca. 30 m3 and temperatures between 600 and 700 K, without taking into account inhibition by water. This may double or triple the volume required, indicating that there is still room for improving catalyst activities to lower reactor volumes and operational temperatures.

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The reactions involved in N,O destruction, Eqs. (3)-(8), are all exothermic, so the thermostability of the catalyst should be taken into account, also in relation to the presence of H,O in most cases. This aspect seems mainly important for the high concentration levels in adipic acid plants. Here, levels of 30-50% (Table 2) lead to adiabatic temperature rises of 500-700 K. For reactions with reducing agents this will only increase more. Control of this exotherm is a notable design factor. The much lower concentration levels in the other sources limit the adiabatic temperature rise due to N,O decomposition to less than 10 K. 6.1. Adipic acid plants The AA producers (DuPont, ASAHI, BASF, Bayer and Rh8ne-Poulenc) have set up a kind of cooperation to develop processes to abate their N,O emissions, aiming at realization in 1998 [155]. DuPont and UOP (ElimiNox) have both developed a catalytic process to decompose N,O in adipic acid flue gases [156]. The UOP proprietary catalyst is based on a binary oxide system, claimed to be not too sensitive to the presence of 0,. Some precious metal is added to lower the light-off temperature. Due to the heat production at higher concentrations the reaction has an autocatalytic behaviour. In a packed bed N,O concentrations of > 5% could be treated, while in a monolithic form concentrations above 15% were needed. The latter is clearly due to the lower catalyst density in combination with better heat transfer [152]. The catalyst developed by DuPont is CoO/NiO on ZrO, [5] which also needs temperatures above 280°C to yield sufficient light-off. Air Products proposes their Co-ZSM-5 and ex-hydrotalcite catalysts as good candidates in view of their good thermostability [4,86,142]. Typical goals set out by DuPont for the catalyst development include > 98% N,O conversions at volume hourly space velocities of 20000 hh’, inlet concentration 10% (3 times diluted) and a life time better than 1 year [155]. Also AA producers BASF and ASAHI (Cu/Al,O,) aim at catalytic decomposition technologies. A typical process flowsheet does not differ much from that in Fig. 14. In the process part of the treated and cooled gas is recycled back to dilute the 30-50%

Adipic acid plant

---+

de-NO, (SCR)

-----+

Heat exchanger

de-N,0 reactor

+

Stack

Fig. 14. Flow scheme of adipic acid flue gas treatment, according

to [5,1.56]

58

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N,O in the plant off-gas flow (14 000-28 000 N m3/h> fed to the reactor thus avoiding high local temperature rises, and smearing out the heat production over the whole bed length. This cooling is performed with the feed in order to bring the feed at the desired level. Typical reactor in- and outlet temperatures are about 450 and 700°C and N,O concentrations below 12% IS]. The heat produced can be applied for steam generation. An optional deN0, unit lowers the NO, levels which gives better catalyst performance. Apparently, NO, inhibits the reaction over their catalyst. Other activities at DuPont, in cooperation with Rhone-Poulenc and EER, include the development of a process to reoxidise N,O to NO [5,155,157], which is recyclable to produce nitric acid which can be reused in the plant. About 0.15 mol NO is produced per mol N,O. A third alternative is the use as selective oxidising agent. Examples found in literature are the oxidation of benzene in phenol, methane to methanol, methanal or synthesis gas, ethane to ethene, ethanol or ethanal, propene to propanal, and the oxidative coupling of methane. Especially the oxidation of aromatics to phenols over zeolites is attractive [158,159]. 6.2. Nitric acid plants Not much data are available on efforts in this area. Air product developed an N,O decomposition application based on their own zeolite catalyst spectrum [4] and new catalysts from hydrotalcite structures [86,87]. Also the simultaneous selective reduction of NO by means of CH, and decomposition of N,O over Co-ZSM-5 has been shown to be feasible [160] and may form an alternative approach. Apart from decomposition a second alternative could be the thermodynamically favourable oxidation of NO by N,O, Eq. (81, and recycling the formed NO,. Fe- and Co-ZSM-5 catalyse this reaction [90]. 6.3. Fluid-bed combustion In fluid-bed combustion several locations can be identified where a catalyst can be applied [9]: (i) in the bed, (ii) above the freeboard, (iii> after the hot cyclone and (iv) after the air preheater (Fig. 15). These locations dictate the type of catalyst mainly by their temperature level. In (i) and (ii), 1000-1200 K, attrition resistant mixed oxide systems (perovskites) are preferred with a good thermostability and SO, tolerance, in (iii>, 800-l 100 K, therrnostability is required, and in (iv>, 500-600 K, a high low-temperature activity like those of zeolitic systems is needed. 6.4. Three-way catalysts In these catalysts many materials are already present that decompose N,O, viz. La,O,, CeO,, Pt and Rh. Also reducing agents are present that might help

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(iii)

(io

4----

Stack

Fig. 15. Options for N20 decomposition

in fluid bed combustion,

from [9].

destroy the N,O. N,O, however, is formed over these catalysts in the temperature range of 500-700 K. Above these temperatures it is either not formed or directly converted over the TWC-catalyst. The best option is to use a catalyst section after the TWC that converts the N,O at the required temperature range and which does not suffer from inhibition effects of the other gases. The major challenge is to combine a high activity at low N,O concentrations with SO, resistance, thermostability, and a low cost.

7. Conclusions The human contribution to N,O emissions has long been underestimated as a serious problem. Catalysis offers opportunities to reduce the emission of various sources. Although much research has been performed in the past for mechanistic studies, activities are still going on, focussing on the development of new catalysts for N,O decomposition. Like NO decomposition, the reaction is thermodynamically feasible, but also, contrary to NO decomposition, it has clearly been demonstrated to be practically feasible. Research is now targeting on catalysts active at low temperatures, as required by several applications. Each application imposes specific requirements for the catalyst and it is felt that the suitable catalyst for each application is either not completely optimized or still not identified. So, further developments are needed, especially for nitric acid plants, FBC units, and three-way catalysts. Important issues are 0, and H,O inhibition, thermostability and SO, poisoning.

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Especially work with zeolites and mixed oxide systems is promising, but a detailed understanding of their action has not yet been achieved. A striking parallel for oxidic systems is the clear involvement of the matrix in which the transition metal ion is embedded. It serves as oxygen storage, as a path for oxygen recombination and affects the sensitivity for oxygen inhibition. A promising tool for further elucidation is transient kinetic research, employing isotopically labelled molecules. It yields insight in the most probable and relevant reaction steps, their rate constants, and numbers of active surface species.

Acknowledgements The authors acknowledge support from the European Union within the framework of the JOULE II Extension Programme, under contract no. JOU2CT92-0229.

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