EXHIBITS TO DECLARATION OF GARY L. HALLER, PH.D. UNDER 37 C.F.R. § 1.132
`
`ELSEVIER
`
`Applied Catalysis A: General 132 ( 1995) 179-259
`
`APPLIED
`CATALYSIS
`
`A: GENERAL
`
`Review
`Nature of active species in copper-based catalysts
`and their chemistry of transformation of nitrogen
`oxides
`
`Gabriele Centi *, Siglinda Perathoner
`Department of Industrial Chemistry and Materials, Viale Risorgimento 4, 40I 36 Bologna, Italy
`
`Received 6 February 1995; revised 19 June 1995; accepted 23 June 1995
`
`Abstract
`
`Copper-based catalysts are active in a wide range reactions of transformation of nitrogen oxides
`and represent an useful model system to better understand the fundamental aspects of the chemistry
`and mechanism of reaction of catalytic transformation of these pollutants. After an introduction on
`the reactivity of copper-based catalysts (supported and unsupported copper oxide, Cu-zeolites, cupra(cid:173)
`tes and other copper compounds) in various reactions of conversion of nitrogen oxides, four main
`sub-topics are discussed in detail: (i) nature of copper species, (ii) chemisorption and surface
`transformations of NO, (iii) relationship between copper species and activity in the conversion of
`nitrogen oxides and (iv) mechanism of reduction of nitrogen oxides to N2• Five reactions of trans(cid:173)
`formation of nitrogen oxides are discussed in detail: ( i) decomposition of NO, (ii) reduction of NO
`with ammonia in the presence or not of oxygen, (iii) reduction of NO with hydrocarbons in the
`presence of oxygen, (iv) reduction of NO with CO and ( v) decomposition of N20. The mechanism
`of reduction of nitrite and N20 by copper enzymes is also discussed, with a view to provide some
`useful insights on the chemistry of transformation. In this review particular attention is directed
`towards controversial points in the literature, underestimated questions, and hypothesis and theories
`which do not allow interpretation of all sets of experimental data. Discussion is also focused on the
`presence of multiple and competitive pathways of transformation, the relative roles of which depend
`on reaction conditions.
`
`Keywords: Nitrogen oxides; NOx; N20; NO reduction; Copper-based catalysts; Cu-zeolites; Cu/ZSM-5; Reaction
`mechanism: Chemisorption
`
`Contents
`
`I. Introduction
`
`* Corresponding author. E-mail cicatal@boifcc.cineca.it, fax. ( + 39-51) 6443679.
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`2. Overview of the reactions of transformation of nitrogen oxides in which copper-based catalysts are
`················································· l~
`~~..........................................
`2.1. Reduction of NO with or without reducing agents: background and overview . . . . . . . . . . . . . . . . . . . . . . . . 182
`2.2. Other applications of copper-based catalysts for conversion of nitrogen oxides . . . . . . . . . . . . . . . . . . . . . . . 184
`3. Nature of copper species in copper-based catalysts
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`187
`3.1. Supported copper oxides
`. . . . . . . . . . . . . . .
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
`3.1.1. Copper-on-alumina catalysts . . . .
`. . . . . . . . . . . . . . . . . . . . . . .
`187
`3.1.2. Copper-on-silica catalysts . . . . . . . . . . . . . . . . . . . . . . . . . .
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
`3.1.3. Copper on titania and zirconia
`3.1.4. Open problems on the nature of copper species in supported catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
`197
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`3.2. Unsupported copper samples
`3.3. Zeolite-based copper catalysts . . . . . . . . . . . . . . . . . . .
`. . . . . . . . . . . . . . . . . . . . . . . . . . . .
`198
`198
`3.3.1. Different copper species present in copper-containing zeolites . . . . . . . . . . . . . . . . . . . .
`3.3.2. Localization and stability of copper ions which interact with the zeolite framework
`200
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`3.3.3. Valence state of copper during the catalytic reaction
`202
`3.3.4 Open problems in the characterization of the properties of Cu-zeolite samples . . . . . . . . . . . .
`206
`3.4. Cu prates and other copper compounds active in conversion of nitrogen oxide . . . . . . . . . . . . . . . . . . . . . . . . 207
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
`4. Chemisorption and surface transformations of NO
`4.1. Nature and role of nitrogen oxide adspecies in the decomposition of NO
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
`4.2. Effect of 0 2 on nitrogen oxide adspecies
`4.3. Influence of co-adsorbents on the nature and reactivity of surface copper complexes with nitrogen
`oxides ................................................................................................... 216
`5. Transformations of nitrogen oxides with enzymes
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`218
`6. Relationship between copper species and activity in conversion of nitrogen oxides . . . . . . . . . . . . . . . . . . . . . . 222
`. . . . . . . . . . . . . . . . . . . . . . 222
`6.1. Synergetic cooperation between active sites in the direct decomposition of NO
`6.2. Active sites in the reaction of NO with hydrocarbons and 0 2
`. • • • . • . • . . • . • . . . . • • • • • • • • • • • • • • • • • • • • • • . . • 226
`6.3. Influence of copper species on the competitive reactions during NO reduction with NH 3 /02
`•••••••• 228
`7. Mechanism of reduction of NO to N2 •.............................•..•.........•.••..•.•.••.•.••..•....... 232
`7.1. NO decomposition .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
`7.2. Influence of hydrocarbons and 0 2 on the pathways of NO reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233
`7.2.1. Role of oxygen and hydrocarbon
`. . . . . . . . . . . . .
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233
`7.2.2. Competitive surface phenomena and reaction pathways .......................................... 236
`7.2.3. Model of the reaction mechanism ................................................................ 242
`7.3. Surface transformations in the presence of NH, or CO ............................................... .
`243
`. ........... .
`7.4. Mechanism of N20 decomposition . .
`248
`8. Concluding remarks
`249
`.................... .
`References
`250
`
`1. Introduction
`
`The general term nitrogen oxides indicates the class of compounds of nitrogen
`and oxygen which includes N 20, NO/N20 2, N02 /N20 4 , N 20 3, N 20 5 and N03
`(unstable), where the/ symbol shows compounds in reversible equilibrium. NO
`emissions in the lower atmosphere are caused principally by combustion processes
`in stationary or mobile sources. Especially in industrialized countries, the latter is
`responsible for up to 60% of global atmospheric NO emissions. In contact with air
`and light, NO readily transforms to N02 • Both nitrogen oxides then give rise to a
`series of complex chemical/ photochemical reactions in the upper atmosphere
`which result in the formation on the one hand of nitric/nitrous acid, which signif(cid:173)
`icantly contributes to acid rain, and on the other hand in the formation of photo-
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`chemical smog. NO itself is not an irritant, but can react with haemoglobin to form
`methaemoglobin [ 1]. In respect to its toxicity, the TL V-TW A value for NO is 25
`ppm. N02 , on the contrary, is an irritant gas and causes pulmonary edema and
`exudative inflammation [ 1]. Chronic exposure to low doses results in coughing,
`headache and gastrointestinal disorder. The TL V-TW A value for N02 is 3 ppm.
`N20 forms mainly via microbial action in soil, but significantly high N20 emis(cid:173)
`sions occur in several chemical processes (for example, up to 30-50% in adipic
`acid production) [2]. N20 does not play a significant role in the troposphere, but
`contributes substantially to ozone depletion in the stratosphere as well as to the
`greenhouse effect [2-4]. N20 does not irritate the mucous membrane and has a
`powerful analgesic action; however chronic exposure may cause polyneuropathy
`and myelopathy [ 1]. The TL V-TW A value for N20 is 50 ppm.
`Several approaches are possible to reduce nitrogen oxides emissions into the
`atmosphere from stationary or mobile sources, but the catalytic approach is the
`most effective to meet current and future requirements. An early patent on this topic
`goes back to 1924 by Fauser, but it is from the beginning of the 1960' s that a great
`deal of research interest has been centred on the problem of the catalytic removal
`of nitrogen oxides. Current commercial catalytic systems are principally noble(cid:173)
`metal based three-way catalysts for the purification of car emissions (gasoline
`engines) and vanadium-on-titania based catalysts for the control of stationary(cid:173)
`source NO emissions by selective catalytic reduction ( SCR) in the presence of
`ammonia [ 5-7] .
`In recent years, a great deal of research has also been centred on the study of
`copper-based catalysts for the conversion of nitrogen oxides, principally for the
`possibility of developing new technologies of direct decomposition of NO to
`N2 + 0 2 or of selective NO reduction with hydrocarbons in an oxygen-rich atmos(cid:173)
`phere [ 8-12]. Copper-containing catalysts ( zeolite- and oxide-based samples) are
`active in a wider range of reactions of transformation of nitrogen oxides with respect
`to other catalytic systems. Copper is also the key component in the enzymes
`involved in the nitrogen cycle. Copper-based catalysts are thus an ideal model to
`understand the mechanism of transformation of nitrogen oxides because they give
`the opportunity to approach the problem from a multiplicity of points of view and
`to verify the validity of hypotheses and theories on analogous reactions and/ or
`catalysts.
`The scope of this review is to discuss and analyze critically the literature data on
`( i) the nature of copper species in supported copper oxides and copper ion(cid:173)
`exchanged zeolites and (ii) the mechanistic aspects of the chemistry of interaction
`and transformation of nitrogen oxides over these catalysts. A comparison with the
`mechanism of action of copper-based enzymes in the transformations of nitrogen
`oxides is also given to evidence the several analogies between these enzymes and
`solid catalysts. Scope of the review is not to compare the reactivity of the various
`catalysts to indicate which samples show superior performances, but instead to
`discuss the properties of all copper-based catalysts in the transformation of nitrogen
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`oxides. In fact, a significati ve limit of several of the hypotheses present in literature
`is that they cannot be generalized to explain the behaviour of other samples. The
`possibility offered by copper-based catalysts to analyze the chemistry of transfor(cid:173)
`mation of nitrogen oxides from various perspectives (different, but homogeneous
`series of samples; reactions involving different nitrogen oxides or reducing agents;
`reactivity in the presence or not oxygen; comparison with the behaviour of analo(cid:173)
`gous enzymes) is thus unique and may be very fruitful for the fundamental aim of
`a better understanding of the relationship between surface properties, reactivity and
`reaction mechanism.
`
`2. Overview of the reactions of transformation of nitrogen oxides in which
`copper-based catalysts are active
`
`2.1. Reduction of NO with or without reducing agents: background and overview
`
`Interest in the activity of copper-based catalysts for the conversion of NO began
`around the end of the sixties l 13-17] . At that time attention was focused on
`investigating possible alternative catalytic systems to those based on the use of
`noble metals for the purification of exhaust gas from gasoline engines. Supported
`copper oxides were found to have the highest activity among the tested transition
`metal oxides for the reduction of NO in the presence of CO [ 14]. Later, copper(cid:173)
`exchanged zeolites ( Y and X types) were also shown to have high activity in this
`reaction [ 18, 19] . Several studies have been reported on the characterization of
`these copper-exchanged zeolite catalysts [ 18-31], but these studies were focused
`especially on the investigation of the redox changes in the reaction with CO,
`hydrocarbons, ammonia and H 2 and not on the activity in the conversion of NO.
`The high activity of copper oxide in the reduction of NO with NH3 in the presence
`of 0 2 was also recognized early [ 32]. Later, copper-zeolites were also found to be
`highly active in this reaction. Partially Cull-exchanged Y-type zeolites [Cu11NaY],
`in particular, were shown to exhibit excellent as well as unique catalytic activities
`[ 33-39]. The key feature of these catalysts is the presence of a reversible maximum
`in the activity at very low temperature (about l 10°C) due to a reversible change
`in oxidation state of the copper. The low-temperature activity of this catalyst in the
`reduction of NO with NH3 in the presence of 0 2 is comparable to that of Pt-based
`catalysts [ 40), but for practical applications V 20 5 /TiOrbased catalysts are pref(cid:173)
`erable for the treatment of emissions from stationary sources due to their reduced
`sensitivity to poisoning and higher stability [ 7], albeit they are active at higher
`temperatures (usually in the 300-400°C range). It should be noted, however, that
`zeolite catalysts are commercialized for the reduction of NO with NH3 /02 [ 40,41]
`for particular high-temperature applications (above 400-450°C). Centi et al. [ 42]
`showed that Cu/ZSM-5 has a distinct advantage over V 20 5 /Ti02 catalysts in terms
`of a reduced rate of the side reaction of ammonia oxidation.
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`
`The interest in copper-zeolites and especially Cu/ZSM-5 increased considerably
`as a result of the finding of Iwamoto et al. [ 43,44] of the superior activity of Cul
`ZSM-5 in the direct decomposition of NO to N2 + 0 2 [ 43-48] in comparison with
`other copper ion-exchanged zeolites [ 49,50] or catalysts. Cu/ZSM-5 was found
`to be sensitive to poisoning by S02 , H20 and oxygen, decreasing the prospects for
`possible application. However, soon was discovered that the addition of hydrocar(cid:173)
`bons to the oxygen-rich feed leads to a drastic increase in the rate of the selective
`reduction of NO to N2 [ 48,51,52]. This discovery opened the field of applications
`for these catalysts to the treatment of oxygen-rich exhaust gas from mobile sources
`such as those deriving from two-stroke or lean-bum gasoline engines or diesel
`engines. The presence of excess oxygen in these emissions limits the efficiency of
`current three-way noble-metal catalysts for the reduction of NO to N2 • Alternative
`catalytic systems thus appear to be attractive [8,9]. Noble metal-based, in fact, are
`active in the selective reduction of NO in the presence of excess oxygen [ 53-55],
`but show a very sharp maximum in the conversion of NO increasing the reaction
`temperature. Indeed, several unresolved problems limit the outlook for successful
`use of zeolites in automotive converters: (i) hydrothermal stability, (ii) sensitivity
`to poisoning, (iii) possibility of manufacturing suitable shapes with sufficient
`mechanical resistance to thermal stress and vibrations, (iv) high light-off temper(cid:173)
`ature and limited temperature window, (v) possible formation of harmful byprod(cid:173)
`ucts, and (vi) necessity of post-engine hydrocarbon additions to reach the optimum
`hydrocarbon/NO ratio required to meet current and future legislative regulations
`on NO emissions. A low hydrothermal stability, in particular, is the more critical
`weakness of copper-containing zeolites.
`Interesting possibilities are also offered by a combination of noble metals and
`copper. Pd-Cu have been shown to be able to combine both advantages [ 56],
`eliminating the necessity for the use of Rh in automotive converters. Copper-based
`catalysts were proposed for automotive exhaust purification [57], when a low level
`of sulphur in gasoline is present. Recent developments from the Toyota research
`group [58,59] have also indicated the fruitful combination of a nitrogen oxide
`storage component to a noble metal component ( 'NOx storage-reduction cata(cid:173)
`lysts'). These catalysts store NOx under oxidized conditions and then reduce the
`stored NOx to nitrogen under stoichiometric and reduced conditions. The new
`catalyst is claimed to have higher NOx reduction activity in a wide temperature
`range. The NOx storage capacity of alumina may be promoted by a component such
`as copper which forms stable nitrates and enhances their rate of formation. Pt/Cu
`on alumina catalysts thus are potential catalysts for this application. A Pt/Cu
`catalyst also was found to have superior activity at room temperature in the reduc(cid:173)
`tion of NO with CO in an aqueous acid solution [60], even though the practical
`relevance is questionable.
`Various other metal-exchanged zeolites have been found to be active in the NO
`selective reduction by hydrocarbons/02 such as Ga/ZSM-5 [61,62], Co/ZSM-5
`[ 63], Ce/ZSM-5 [ 64], Ga/ferrierite [ 65], Co/ferrierite [ 66] as well as the zeolite
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`itself in the acid form [ 67], albeit at higher temperatures. Their activity, however,
`depends considerably on the nature of the hydrocarbon used and on the experimental
`conditions [ 9-12], differently from copper-zeolites which show good perform(cid:173)
`ances in a wider range of reaction conditions and using different kinds of hydro(cid:173)
`carbons. Metal oxides such as alumina have also been found to be active at high
`temperature in the selective reduction of NO with hydrocarbons/02 [68-70], but
`in general the addition of copper promotes the catalytic behaviour. The results,
`however, depend considerably on the oxygen concentration [ 70-72]. The presence
`of other co-cations such as Cs [73] or Ga [70] may further promote the activity.
`Copper-based catalysts are thus a reference catalyst family for the investigation of
`the mechanism of selective reduction of NO, albeit for practical applications their
`low hydrothermal stability may prevent a commercial use.
`The selective reduction of NO with hydrocarbons/02 (HC-SCR) over metal(cid:173)
`exchanged zeolites is interesting not only for the purification of car emissions, but
`also for the possibility of substituting ammonia as the selective reducing agent for
`the elimination of nitrogen oxides in emissions from stationary sources (flue gas,
`nitric acid plants, etc.). This would reduce risks and safety problems connected to
`the transport and handling of large amounts of ammonia, the secondary pollution
`derived from ammonia slip and equipment corrosion. Li and Armor [ 63, 7 4] showed
`that using methane as the reducing agent under net oxidizing conditions, Cu/ZSM-
`5 is ineffective, whereas other catalysts such as Co/ZSM-5 show a good activity.
`Ga/ZSM-5 is also active with methane [ 62]. Recent results indicate a dissociative
`chemisorption of methane without 0 2 chemisorption over Ga/ZSM-5 and the
`opposite behaviour over Cu/ZSM-5 [75]. This difference is probably responsible
`for the inactivity of Cu/ZSM-5 with methane, whereas using higher alkanes (pro(cid:173)
`pane) the hydrocarbon activation is easier and Cu/ZSM-5 shows superior catalytic
`performances with respect to Co/ZSM-5. It should be noted that significantly lower
`performances are obtained in the CH4-SCR reaction over Ga/ZSM-5 (maximum
`NO conversion about 40% [62]; similar results were observed using Co/ZSM-5
`[74]) as compared to those observed in C3H8-SCR reaction over Cu/ZSM-5 (NO
`conversion higher than 95%) [76]. The worse performances using methane in
`comparison to those using higher alkanes is a significative drawback for CH4-SCR
`technology for stationary sources.
`Summarized in Table 1 is an overview of the different reactions of nitrogen
`oxides conversions in which copper-based catalysts are active.
`
`2.2. Other applications of copper-based catalysts for conversion of nitrogen
`oxides
`
`Supported copper oxides, and copper oxide on alumina [77-80] or silica [81]
`in particular, are used for the combined removal of S02 and NOx from flue gas.
`S02 reacts with supported copper oxide to form sulphate species easily reduced in
`a separate stage by treatment with H2 or C~ [82-86], and at the same time
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`Table 1
`Examples of uses and applications of copper-based catalysts in transformation of nitrogen oxides
`
`Reaction or application
`
`2N0+2CO--> N2 +2C02
`
`2NO + 2NH3 + 0.502 --> 2N2 + 3 H20
`
`2NO --> N2 + 0 2
`
`NO+ hydrocarbons --> N2 , CO,, H20
`
`NO+ 0.502 --> N02
`Combined S02 and NOx removal
`
`NOx reduction in tail gas of nitric acid plants
`NOx and CO elimination in off gas of nuclear waste processing plants
`Photocatalytic NO decomposition
`
`Photocatalytic N20 decomposition
`N20 decomposition
`
`NO removal by selective adsorption
`Catalytic sensors for N02
`Enzymatic denitrification
`
`Catalyst
`
`Reference
`
`CuO
`CuOon Ti02
`CuO on Al 20 3
`supported Cu/Cr oxides
`Cu/Y
`Cu/ZSM-5
`cu prates
`CuO
`CuO on various oxides
`Cu/Y
`Cu/ZSM-5
`Cu-mordenite
`Cu/Y
`Cu/ZSM-5
`cuprates
`Cu/ZSM-5
`CuO on alumina
`Cu/Y
`CuO on alumina
`CuO on silica
`CuO on active carbon
`CuO-NiO on alumina
`Cu/ZSM-5
`CuO on silica
`Cu/ZSM-5
`Cu/ZSM-5
`Cu/ZSM-5
`cu prates
`BaO-CuO
`Cu0-Ni0-Sc20 3
`N20 reductase
`nitrite reductase
`
`[ 13,16]
`[113,114]
`[115]
`[57]
`[ 18]
`[116]
`[ 117]
`[32]
`[118]
`[34]
`[42]
`[ 119]
`[50]
`[43,48]
`[ 120]
`[9,47]
`[70,71]
`[ 105]
`[78-80]
`[81]
`[87]
`[90,91]
`[96]
`[97]
`[98]
`[99]
`[103]
`[ 121]
`[107]
`[ 108]
`[Ill]
`[ 112]
`
`catalyzes the reduction of NO with NH3/02 . The activity in the reduction of NO is
`influenced little by the progressive sulphation of supported copper species [ 84,85]
`differently from the V 20 5 /Ti02 catalysts [7]. The technology was tested in pilot
`plant facilities and technico-economical estimations indicate the promising outlook
`for the process.
`Similar technology for the combined removal of S02 and NOx is based on the
`use of copper oxide supported on carbon, but operates at lower temperatures (about
`150°C) as compared to about 350°C for the systems based on copper oxide sup(cid:173)
`ported on oxides. S02 is oxidized and adsorbed on carbon in the form of sulphuric
`acid/ ammonium bisulphate or sulphate instead of copper sulphate, whereas NO is
`converted to N2 in the presence of ammonia/ oxygen [ 87]. The addition of copper
`considerably enhances the activity of carbon for the latter reaction [ 88]. Demon(cid:173)
`stration plants of this technology exist [ 87] , but the critical aspects for wider
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`application are the cost of active carbon, rapid degradation of carbon by gasification,
`sample regeneration and by-product formation during regeneration. Carbon-sup(cid:173)
`ported copper-based catalysts also have been shown to have high activity in the
`reduction of N 0 with CO and simultaneous oxidation of CO and have been proposed
`as alternative catalysts for the removal of NO and CO from exhaust gas [ 89].
`A copper/nickel oxide on alumina catalyst is used for the abatement of NOx
`emissions in several nitric acid plants [90,91]. These emissions are characterized
`by a N02/NO ratio nearly unitary due to the low temperature of the gas and by the
`absence of S02 • In the Cu/Ni oxide on alumina catalysts, the promotion effect of
`nickel is mainly that of increasing the number of Cu2 + ions in tetrahedral positions
`owing to the displacement in the defective spinel-type surface of y-Al20 3 of Cu2 +
`from the octahedral positions by Ni2 + [92,93]. Blanco et al. [94] have also shown
`that the N02 /NO ratio in the feed has a considerable influence on the rate of reaction.
`Optimal performances were found for a 1: 1 ratio due to the higher rate of reoxidation
`of reduced Cu+ ions to Cu2 + shown by N02 as compared to 0 2 .
`Cu/ZSM-5 has been successfully applied for the control of both NOx and CO
`emissions from nuclear waste processing plants [95,96] the off-gas of which
`contains concentrations of about 1-3% of these pollutants. The original process
`uses two fixed-bed reactors for the selective catalytic reduction of NOk with ammo(cid:173)
`nia and oxidation of CO over a H-mordenite catalyst and a third reactor containing
`Pt and/ or Cu-based catalysts for clean-up of the ammonia slip and unconverted
`CO, but the use of Cu/ZSM-5 catalysts allows an increase in the overall perform(cid:173)
`ance of the technology.
`The photocatalytic decomposition of NO over copper oxide supported on Si02
`[97] and Cu/ZSM-5 [98] has also been demonstrated, but present results do not
`allow predictions regarding possible practical applications. The essence of the
`reaction is the reduction of isolated Cu2 + ions to Cu+. The formation of the latter
`is a maximum for evacuation temperatures of about 800-900°C. The Cu+ species
`react at room temperature with NO forming nitrosyl adducts which under UV
`irradiation give rise to an electron transfer reaction from an excited Cu+ ion to the
`7T-antibonding orbital of NO with back-donation to the vacant orbital of the Cu+
`ion. The local charge separation and weakening of the N-0 bond is the driving
`force for the decomposition of NO. The photocatalytic decomposition of N20 into
`N 2 and 0 2 over Cu I ZSM-5 has also recently been reported [ 99, 100] , butthe authors
`suggest that the reaction was related to the quenching of the excited state of the
`dimer of a monovalent copper ion (Cu+ -Cu+) by N 20 molecules.
`The outlook for practical applications is much better for the catalytic decompo(cid:173)
`sition of N20. Due to the greenhouse effect of N20 (over two orders of magnitude
`greater than C02 ) and its role in stratospheric ozone depletion, it was estimated
`that N20 emissions should be considerably reduced [2-4]. In E.U. countries, for
`example, anthropogenic N20 emissions should be reduced from about 1200 kton
`N20-N/year to about 200 kton N20-N/year in order to arrive to a climate goal of
`limiting future global warming to O.l°C/decade [2]. Two are the main sources
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`which can be reasonably controlled by catalytic treatments: (i) emissions from
`fluidized bed combustion (especially from municipal waste and sewage sludge
`incineration) and (ii) emissions from chemical processes using or producing nitric
`acid [ 4]. Various papers have been published on the catalytic decomposition of
`N20 [IOI], but few studies have addressed the question of the analysis of the
`behaviour of the catalyst under real conditions for the possible applications [ 102].
`Most of the catalysts reported in the literature either have reaction rates which are
`too low or deactivate very quickly. Copper-exchanged zeolites have been suggested
`to be promising catalysts [ 103, l 04], even though sensitive to poisoning by water
`and other components.
`Copper ion-exchanged zeolites have been reported to be the most active for the
`oxidation of NO to N02 [ l 05], a necessary step for the development of a process
`for removal of nitrogen oxide by adsorption on a solid adsorbent, since N02 is
`much more reactive and less difficult to remove. Indeed, the homogeneous gas
`phase oxidation of NO usually occurs with a too low rate under the temperature
`and NO concentration conditions necessary for practical applications. More
`recently, Arai and coworkers [ l 06, 107] have also proposed a process of nitrogen
`oxides removal using BaO-CuO binary oxides. Adsorbed NO is present in the final
`form as Ba nitrate, but probably the mechanism involves the NO oxidation over
`copper ions to form a copper nitrate species. The nitrate ion then shifts to Ba since
`the Ba compound is more stable.
`Copper oxide is also a key component for the preparation of catalytic sensors for
`nitrogen oxides. Imanaka et al. [ 108] have recently reported the good sensitivity
`properties of copper oxide and scandium oxide mixed with p-type semiconducting
`NiO for N02 detection especially in low concentrations. The role of copper is to
`enhance the N02 sensing characteristics.
`Finally, copper is the active element in the enzymatic processes of interconver(cid:173)
`sion of nitrogen compounds [ 109-112]. NO is produced and consumed via proc(cid:173)
`esses mediated by metalloproteins that contain iron or copper and it is a key
`intermediate in the global biological nitrogen cycle. Copper-containing enzymes
`play a central role in denitrification, whereby bacteria (nitrite and nitrous reduc(cid:173)
`tases) use nitrate and nitrite ions as terminal electron acceptors ultimately to pro(cid:173)
`duce gaseous nitrogenous products (NO, N20, and/or N2 ). Other useful Refs.
`[ 113-121] of reactions of nitrogen oxides over copper-based catalysts are listed
`in Table 1.
`
`3. Nature of copper species in copper-based catalysts
`
`3.1. Supported copper oxides
`
`3.1.1. Copper-on-alumina catalysts
`Alumina supported copper catalysts have been extensively characterized in the
`past for their wide range of applications in oxidation and hydrogenation reactions.
`Recently, renewed interest in these catalyst has derived from their activity in the
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`catalytic combustion of hydrocarbons [ 122, 123] and alcohols [ 124] and their
`performance in the combined removal of NOx and S02
`[ 78-80]. The alumina
`supported copper samples have been studied by a variety of techniques for the
`characterization of their bulk and surface properties [ 92,93, 122, 125-138]. The
`papers of Friedman et al. [ 126], Knozinger and co-workers [ 92,93], Strohmeier
`et al. [ 133] and Lo Jacono and co-workers [ 128,134] are those in which the
`problem of the identification of the surface copper species has been studied in more
`detail. The nature of the copper species depends clearly on the specific kind of
`alumina used, but almost all studies dealt with y-Al20 3 supported copper. y-Al20 3
`has a spinel-type structure in which the oxygens are cubic close-packed similar to
`the packing in MgA120 4 • The unit cell consists of 32 oxygens, 21 1I3 aluminiums
`and 2 2/3 cation vacancies distributed between the tetrahedral and octahedral sites.
`y-Al20 3 has a fairly well ordered oxygen lattice with considerable disorder in the
`tetrahedral Al lattice. Further details can be found in the classical review of Kno(cid:173)
`zinger and Ratnasamy [ 139] and in