In situ analytical investigation of redox behavior of Cu-ZSM-5catalysts

Carlo Dossi,*a Sandro Recchia,a Andrea Pozzi,a Achille Fusi,b Vladimiro Dalsantob andGiuliano Morettic

a Dipartimento di Scienze Chimiche, Fisiche e Matematiche, dellÏInsubria,UniversitaV ia L ucini, 3, 22100 Como, Italy. E-mail : dossic=csmtbo.mi.cnr.it

b Centro CNR ““CSSCMT BSOÏÏ and Dipartimento di Chimica Inorganica Metallorganica, eAnalitica, di Milano, V ia V enezian, 21 20133 Milano, ItalyUniversita

c Centro CNR ““SACSOÏÏ, Dipartimento di Chimica, UniversitaÏ ““L a SapienzaÏÏ,P. le A. Moro 5, 00185Roma, Italy

Received 10th May 1999, Accepted 2nd August 1999

The utilization of in situ DRIFT spectroscopy combined with advanced thermoanalytical techniques(temperature programmed decomposition, TPDE; temperature programmed oxidative decomposition, TPOD)allows the characterization of redox behavior of copper ions inside ZSM-5 zeolites. The oxidation ofcarbonaceous impurities inside the solid matrix to CO and is demonstrated to be responsible forCO2Cu(II)] Cu(I) reduction, since the formation of other oxidation products, namely or ions, wasO2 , H2O2 O2~not detected by mass spectrometry and DRIFT. The unusual kinetics of CO evolution in thermoanalyticalexperiments was attributed to the formation of a relatively stable Cu`É É ÉCO adduct characterized by a typicalIR band at 2157 cm~1, which acts as a speciÐc active site of a chromatographic adsorbent material.

Introduction

Since the discovery of the remarkable NO decompositionactivity of Cu-ZSM-5 catalysts by Iwamoto et al.,1 muche†ort has been devoted to characterize the behavior of suchmaterials in comparison to other copper-based systems ortransition metal ions exchanged in ZSM-5.

Following the vast body of literature that has appeared inthe last ten years,1h5 it is now clear that the most activesystems are those with the lowest Si/Al ratio and with Cuexchange levels above approximately 80È90%. On such activematerials, oligomeric copper polyoxocations, of simplestformula [CuÈOÈCu]2`, are believed to be the active sites, oractive site precursors.

The involvement of Cu(I) ions is now widely accepted, evenfrom theoretical studies.6 Formation of such entrapped ionsdoes not simply occur under the NO decomposition reaction,because the need for an activation time was not observed.Thus, the autoreduction of ion-exchanged Cu(II) ions has tooccur upon thermal preactivation of the freshly exchangedCu-ZSM-5 material, as was reported by several groups forCu-Y and Cu-mordenite materials.5,7h11

Di†erent processes have been proposed to explain thereduction of Cu(II) to Cu(I) upon heating under inert orvacuum conditions, either involving oxygen desorption,1,7 orformation of superoxide ions through reaction with adsorbedwater.12 Alternatively, removal of bridging oxygen within thebinuclear [CuÈOÈCu]2` sites, or involvement of O2~ frame-work ions of the ZSM-5 cage are the most likely processestaking place under typical reaction conditions to generateoxygen.

However, the real nature of the autoreduction process isstill not well established. For instance, the generalized use ofcopper acetate as a precursor of overexchanged catalysts5might suggest a possible role of acetate ions in the high-

temperature copper reduction. Equally, it is unlikely that highsurface area materials, such as ZSM-5 zeolites, might be con-sidered to be totally free from organic contamination, exceptfor model materials. Consequently, copper ions could act asoxidation catalysts for carbon impurities in the presence ofevolving oxygen or superoxide ions ; carbon monoxide orcarbon dioxide would thus be the end product in the redoxchemistry of Cu(II) reduction to Cu(I), although to date noclear evidence has been reported in the literature.

An in situ analytical investigation of the processes involvedin this redox chemistry is presented in this paper, by combin-ing advanced thermoanalytical techniques with di†use reÑec-tance infrared (DRIFT) spectroscopy. Such an in situanalytical approach appears to be of primary importance toeliminate any artifacts in the analytical characterization ofnarrow-pore structures such as those of the zeolite materialsstudied in the present work.

Experimental

Sample preparation

The zeolite H-ZSM-5 (Si/Al\ 25) used in this work wasobtained from by thermal treatment at 500 ¡C inNH4-ZSM-5air for 5 h. This zeolite has a surface area of 396 m2 g~1 and atotal pore volume of ca. 0.18 cm3 g~1.13 The amorphousmesoporous silicaÈalumina (MSA) with Si/Al\ 90 was pre-pared according to Bellussi et al.14 It has a surface area of 684m2 g~1 and a total pore volume of 0.53 cm3 g~1. MSA showsa narrow distribution of pores centered in the mesoporeregion, though a small amount of micropores (r \ 10 alsoÓ)contributed to the total pore volume.

The copper containing catalysts were prepared by ionexchange using dilute aqueous solutions of copper acetate(denoted A), and tetraammine copper(II) sulfate (TS). In

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general, a 2 g sample of the support (H-ZSM-5, MSA) wastreated with 250 ml of a copper solution at a given concentra-tion under stirring. The temperature in these preparations wasvaried in the range between room temperature and 80 ¡C. Thetime of the ion exchange was also modulated from minutes to24 h. Details about the preparation of the catalytic materialsand the relative Cu loadings are reported elsewhere.15 Thesamples after Ðltration were washed several times with dis-tilled water, dried at 110 ¡C for 2 h, and stored over a saturat-ed solution of at room temperature (relativeNH4Clhumidity \ 79%). Chemical analyses of the Cu content of allmaterials were performed by atomic absorption (VarianSpectrAA-30) upon chemical dissolution of the copper phasein concentrated Full details of the analytical method-HNO3 .ology for metal analysis in supported catalysts without com-plete dissolution of the siliceous matrix are reported in aprevious paper.16

Thermoanalytical techniques

Temperature programmed decomposition (TPDE17) studieswere done on fresh Cu-ZSM-5 and Cu-MSA materials byraising the temperature in Ñowing He from 25 to 500 ¡C at3 ¡C min~1 using a computer-controlled temperature pro-grammer (Ascon YM). Temperature programmed oxidativedecomposition (TPOD18) analyses were performed accord-ingly, but using a mixture as reaction carrier gas.O2(2%)ÈHeSpecially designed, Ñow-through glass reactors have been usedin order to minimize di†usional limitations and readsorptionphenomena during gas evolution. Sample weights between 40and 200 mg were used in order to have a constant weight ofcopper in all samples. For the investigation of the

complex as pure reference material,Cu2(CH3COO)3OH ÆH2Oa small amount of the pure microcrystalline powder, around5È10 mg, was dispersed in powdered quartz, in order to havethe desired height of sample bed in the TPDE sample holder.

Evolution products were monitored by an on-line quadru-pole mass spectrometer (VG Masstorr, 0È130 u) interfaced tothe Ñow system by a di†erentially pumped capillary inlet.Data acquisition was done in selected ion monitoring mode,and data will be presented by plotting the partial pressure (inmbar) of each m/z channel as a function of increasing tem-perature. A detailed description of the instrumental set-up wasreported elsewhere.19

DRIFT characterization

Di†use reÑectance infrared (DRIFT) spectra were recorded at4 cm~1 on a Digilab FTS-40 FTIR spectrophotometer. Theinstrument is equipped with a Harrick ScientiÐc DRA-3COdi†use reÑectance accessory, and a high pressure, high tem-perature chamber (Harrick ScientiÐc mod. HVC-DR3). Thecell is connected to a vacuum-gas line for catalyst pretreat-ment, and to a programmable temperature controller(Eurotherm 808) for programmed heating from room tem-perature to about 430 ¡C. The catalyst powder is loaded intothe DRIFT chamber and heated with the same heatingprogram used for TPDE studies, in order to compare thekinetics of gas evolution between the two techniques.

DRIFT spectra are periodically taken during the tem-perature ramp, and plotted as a function of increasing tem-perature. Evolution products are monitored by an on-line gaschromatograph (Intersmat IGC-12) equipped with a meth-anation converter and Ñame ionization detector. The instru-mental set-up and all experimental details are reported in aprevious paper.20

Chemisorption of CO on a Cu-ZSM-5 sample was doneprior the TPDE experiment by Ñowing pure CO (5 mLmin~1) at 25 ¡C in the DRIFT chamber until a constant inten-

sity DRIFT spectrum in the carbonyl stretching region wasobtained.

Results and discussionThe TPDE proÐle of pure H-ZSM-5 is shown in Fig. 1, wherethe evolution of CO at m/z\ 28, of at m/z\ 44 and ofCO2at m/z\ 18 are monitored as a function of the increasingH2Otemperature. The signal at m/z\ 28 slowly decreases withincreasing temperature. This instability could not be elimi-nated in a water-cooled, di†usion pumped quadrupole massspectrometer, since CO is generated by the hot Ðlament.21,22Fortunately, CO evolution peaks in Cu-ZSM-5 catalysts caneasily be distinguished on the slowly downsloping baseline(see for instance Fig. 2), although no quantitative data couldbe reliably obtained. In our instrumental set-up, where adi†entially-pumped jet separator inlet is used to sample aÑowing gas stream, semiquantitative data only can be reliablyobtained.

Apart from the large evolution of water occurring between30 and 250 ¡C, no other evolution peaks are observed in theTPDE proÐle of H-ZSM-5 ; the small deviation in the thermalproÐle of is in fact to be related to baseline instabilityCO2

Fig. 1 TPDE proÐles in Ñowing helium of a blank H-ZSM-5 zeolite.

Fig. 2 TPDE proÐles at selected m/z values of a Cu-ZSM-5 catalystat a copper exchange level of 81%.

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due to the large pressure rise upon desorption of physisorbedwater.

The proÐle of water evolution at m/z\ 18 remainsunchanged when Cu(II) ions are ion-exchanged within zeolitecages from copper acetate solutions up to 81% exchange level,and it is not reported in Fig. 2. Instead, a large peak of CO2evolution can be observed at 290 ¡C in the proÐle of m/z\ 44,with an evident shoulder around 250 ¡C (Fig. 2). The single,symmetric peak at 450 ¡C in the m/z\ 28 proÐle is to beascribed to the evolution of carbon monoxide. No signiÐcantevolution of or could be observed under our experi-O2 H2O2mental TPDE conditions ; the m/z traces at 32 and 34 showed,in fact, only instrumental noise around 10~11 mbar, and arenot reported in Fig. 2. However, oxygen evolution was report-ed to occur under thermal treatment,1,7 whereas the forma-tion of some could be related to the intermediateH2O2formation of as reported by Sarkany.12O2~,

Conversely, it is signiÐcant that hydrogen is evolved as abroad peak between 350 and 500 ¡C (Fig. 2). Any hydrogenformation between 100 and 250 ¡C is instead to be rejected,although the analytical signal at m/z\ 2 is well above thebaseline. Any peak in this temperature range is to be attrib-uted to instrumental artifacts linked to the large desorption ofphysisorbed water, as was observed with the H-ZSM-5 sample(compare Fig. 1 and 2).

There are actually several explanations for such evolutionof CO, and upon thermal activation of Cu-ZSM-5,CO2 H2ranging from the decomposition of surface acetate species23 toa copper-catalyzed oxidation of carbonaceous deposits.15

Therefore, any possible carry-over of organic contaminationof blank H-ZSM-5 upon ion-exchange with copper acetatewas investigated. A reference material was prepared byexchanging copper acetate on a mesoporous silicaÈaluminasupport (Cu-MSA), which was prepared analogously to aZSM-5 zeolite, but the synthesis was stopped prior to crys-tallization of the solÈgel phase.14,24 Surprisingly, no CO or

evolution was observed in the TPDE analysis up toCO2500 ¡C. Consequently, any carry-over of carbon contami-nation during the preparation of copper-containing materialfrom aqueous solutions of copper acetate can be safelyexcluded.

Intra-zeolite precipitation of some insoluble copper hydro-xoacetate species was then taken into consideration. However,this should be a rare occurrence in a sample with an exchangelevel of 81%, since precipitation of insoluble copper specieswas never observed at pH values below 6 and exchange tem-peratures lower than 50 ¡C.24 Nevertheless, we checked thishypothesis by preparing and investigating the thermal behav-ior of the pure model compound of formula

taken as reference material (Fig.Cu2(CH3COO)3OH ÆH2O 253). In this sample, CO and are evolved simultaneously inCO2the same temperature range between 180 and 270 ¡C, showingtwo unresolved peaks. Formation of CO was not observedabove 300 ¡C. The parallel formation of CO and isCO2attributed to the decomposition process of acetate ligands.Obviously, this is not the process which occurs in theCu-ZSM-5 materials. The carbon source for CO and CO2evolution during the thermal treatment of such materials isprobably located inside the zeolite structure. This hypothesisappears to be quite unlikely at a Ðrst glance, since blankH-ZSM-5 is prepared by air calcination of atNH4`-ZSM-5500 ¡C for 5 h. We thus studied the thermal behavior inÑowing oxygen (TPOD) of a Cu-ZSM-5 catalyst preparedfrom ion-exchange of In this way, any possibleCu(NH3)4SO4 .carry-over of acetates or carbonates in the coordinationsphere of copper ions deposited inside the zeolite structure iscompletely eliminated, as well as any formation of acetic acid,which would eventually lead to zeolite dealumination.

The TPOD experiment is shown in Fig. 4. A large negativepeak at 370 ¡C, with a small shoulder around 330 ¡C, is clearly

Fig. 3 TPDE proÐles in Ñowing helium for the puremodel complex.Cu2(CH3COO)3OH ÆH2O

evident in the mass trace of oxygen at m/z\ 32, which isindicative of a net oxygen consumption in this temperaturerange. Consequently, this oxygen consumption must beascribed to the oxidation of ammonia ligands to dinitrogencatalyzed by copper sites, according to the following reaction :

2NH3 ] 3/2O2 ] N2] 3H2O

Since no carbon monoxide or carbon dioxide would beexpected to evolve on a precalcined H-ZSM-5 zeolite, the sig-niÐcant evolution at m/z\ 28 centered around 370 ¡C isstraighforwardly attributed to formation. However, it isN2surprising to Ðnd the presence of a non-negligible evolution atm/z\ 44 as a large and asymmetric peak around 320 ¡C. Suchevolution of can be attributed to the oxygen combustionCO2of carbonaceous residues left within the zeolite cage upon aircalcination of fresh zeolite. Such a combustion process will,however, suggest that some carbon monoxide could beformed, as well ; therefore, the evolution peak at m/z\ 28could not directly be attributable to dinitrogen only, since

and CO` are isobaric fragments at m/z\ 28.N2`

Fig. 4 TPOD proÐles, in Ñowing mixture, of aO2(2%)ÈHeCu-ZSM-5 sample prepared from (Cu exchange[Cu(NH3)4]SO4level\ 90%).

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We tried to distinguish the two potential gas evolution pro-cesses inside the TPOD trace at m/z\ 28 by comparing itwith the trace at m/z\ 14, which is related to the N` frag-ment only (Fig. 4). The two evolutions at 14 and 28 look quiteidentical, making us conÐdent that the signal at mass 28 ispredominantly due to evolution of dinitrogen. All the ther-moanalytical evidence is in strong support of the role of thecarbonaceous residues as sources of CO and formationCO2with the parallel reduction of Cu(II) ions to Cu(I). These car-bonaceous residues must be present within the zeolite frame-work prior the introduction of copper. Although in ahigh-surface area material such as ZSM-5 zeolite, someincorporation of impurities from the outside atmosphere isnot unlikely, we think that the incomplete removal of organicmaterial upon calcination pretreatments is the most likelysource of carbonaceous deposits. Probably, the presencewithin the zeolitic framework of protonic sites of high aciditycould cause polymerization and eventually cokization ofalkanes and alkenes formed upon decomposition of alkyl-ammonium templating agents. The inÑuence of templatingagents in determining the physico-chemical and reactivitybehaviors of ZSM-5 zeolites was recently demonstrated byHall and coworkers.26

It is not evident why CO formation in TPDE conditionsunder an inert atmosphere is signiÐcantly delayed at muchhigher temperatures than In fact, the oxidation kineticsCO2 .of carbonaceous residues usually produce CO and simul-CO2taneously. Sometimes, CO formation is even observed atlower temperatures than carbon dioxide under TPDE condi-tions,17 where water gas shift reactions are necessarilyinvolved in formation from adsorbed CO onto metalCO2surfaces or metal clusters.

The utilization of the in situ DRIFT-GC technique20 wasthen tried in order to shed more light on this unusual andunexpected behavior. By this in situ technique, the thermaldecomposition of the catalytic material is directly investigatedinside the reaction chamber of the infrared spectrophotometer(Fig. 5). A sample at 147% exchange ratio of copper was usedfor this study, in order to compensate for the inherent lowersensitivity of the DRIFT technique. No substantial di†erenceswith TPDE studies are observed in the evolution proÐles ofCO and (compare with Fig. 2), apart for the lowerCO2resolution of the gas-chromatographic detection.21 In particu-lar, the evolution rate of carbon monoxide is conÐrmed toreach its maximum around 420 ¡C. It is noteworthy that theformation of hydrogen, which cannot be detected by a Ñameionization detector, was indirectly studied by monitoringmethane evolution. Methane is formed via copper-catalyzed,hydrogen reduction of carbon residues, as it is often observedfor supported catalysts under TPDE conditions.17

Fig. 5 TPDE proÐles of gas evolution inside the DRIFT cell asmonitored by on-line GC. Sample : Cu-ZSM-5 at 147% exchangelevel.

Fig. 6 Temperature dependence of DRIFT spectra in the carbonylstretching region. A : Cu-ZSM-5 sample of Fig. 5. B : A fresh Cu-ZSM-5 catalyst (Cu exchange level\ 280%) upon chemisorption of CO.

The DRIFT spectra have been recorded at increasing tem-peratures during the heating ramp and are reported in Fig.6A. The DRIFT spectrum of the fresh material at 25 ¡C indi-cates that no carbonyl bands are initially present in thesample. CO and formation has therefore to be ascribedCO2to combustion processes taking place during the thermaltreatment. By increasing the temperature above 300 ¡C, asingle absorption band at 2157 cm~1, which is attributed tolinear CO bonded on Cu(I) intrazeolitic ions,27 starts to formin the DRIFT spectrum. This band reaches its maximumintensity around 400 ¡C; then it decreases, and completely dis-appears upon heating for 1 h at 425 ¡C (Fig. 6A). This IRbehavior thus exactly parallels the proÐle of CO evolution inTPDE, suggesting the intermediate formation of a relativelystable adduct between Cu(I) sites and CO.

Such Cu(I)É É ÉCO adducts are formed already at room tem-perature if CO is Ñowed over Cu-ZSM-5 catalysts in theDRIFT chamber (Fig. 6B). Under these conditions, carbonmonoxide reduces Cu(II) to Cu(I) in the presence of wateraccording to the consecutive reactions :

2Cu2`] CO] H2O ] 2Cu`] CO2 ] 2H` (1)

Cu`] CO] Cu`É É ÉCO (2)

Two di†erent types of intrazeolite Cu(I) ions are observed tointeract with CO at room temperature, giving an intense bandat 2157 cm~1 and a shoulder at 2139 cm~1. This latter bandshows a much lower thermal stability ; it is totally depleted bysimple heating in argon Ñow around 100 ¡C. Consequently,this kind of Cu(I) ion cannot be involved in CO adsorptionduring the thermal pretreatment of fresh Cu-ZSM-5 materialssince there is no CO evolution in TPDE below 200 ¡C.Instead, the 2157 cm~1 band has a much higher thermal sta-bility ; complete decarbonylation is only observed by heatingabove 390 ¡C in DRIFT studies.

The delayed evolution of CO, with respect to duringCO2 ,the thermal heating in Ñowing argon of fresh samples, is due

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to the high stability of Cu(I)É É ÉCO adducts. In other words,the solid zeolitic material is acting much like an adsorptionmaterial for gasÈsolid gas chromatography. CO and areCO2formed simultaneously in the water-assisted oxidation of car-bonaceous residues, but is not retained at temperaturesCO2around 200 ¡C in the medium-pore structure of ZSM-5 zeolite,and is evolved in a single step. Instead, carbon monoxide istrapped onto forming Cu(I) ions ; consequently, the tem-perature of CO evolution becomes dependent on the strengthof COÉ É ÉCu(I) interactions. Such interactions are so strong asto increase to more than 200 ¡C the release temperature of COin the gas phase, and at the same time, to allow its parallelreduction to methane by hydrogen formed via water gas shiftreactions. This side reaction might be, however, of someimportance from an analytical point of view, since it allowsthe indirect detection of hydrogen in thermal programmedstudies when a FID-equipped gas chromatograph is used.

ConclusionThe combined DRIFT and thermoanalytical approach hasallowed us to shed more light on the redox chemistry of intra-zeolite copper species in Cu-ZSM-5 catalysts for NO decom-position.

The evolution of CO and during the thermal pretreat-CO2ment in an inert atmosphere to generate active Cu(I) ions hasbeen attributed to a copper-catalyzed oxidation of carbon-aceous deposits. The presence of traces of water is prominenthere, since they act as oxygen supply for CO and forma-CO2tion, with the parallel reduction of Cu(II) to Cu(I) ions and thecreation of protons for charge balance. The formation ofoxygen or superoxide ions is instead feasible only undermodel conditions, when all carbon contamination is elimi-nated either in the zeolite material and in the gas supply. Noevolution of or was in fact observed in our analyti-O2 H2O2cal investigation of a real Cu-ZSM-5 material.

The presence of carbonaceous material in Cu-ZSM-5 maybe due to residues left by incomplete decomposition of theorganic templating agent used in the ZSM-5 synthesis or, lesslikely, by hydrocarbon contamination, which is a processalways occurring when high surface area samples are exposedto the atmosphere.

By this in situ DRIFT investigation we have clearly shownthat the delayed evolution of CO, with respect to duringCO2 ,the thermal heating in Ñowing inert gas (TPDE experiment) isdue to a strong interaction of CO with speciÐc Cu(I) ions ofthe material, which acts as a chromatographic adsorbent.

Acknowledgementssupport from the Ministry of University and Scien-Financial

tiÐc Technological Research (MURSTÈ40% Projects 1998) is

gratefully acknowledged. We are deeply indebted to DrRinaldo Psaro and to Prof. Renato Ugo for useful discussionsand their interest in this research.

References1 M. Iwamoto, H. Furukawa, Y. Mine, F. Uemura, S. Mikuriya

and S. Kagawa, J. Chem. Soc., Chem. Commun., 1986, 1272 ; M.Iwamoto, H. Yahiro, N. Mizuno, W.-X. Zhang, Y. Mine, H. Fur-ukawa and S. Kagawa, J. Phys. Chem., 1992, 96, 9360.

2 W. Held and A. Koenig, Ger. O†en, DE 3642018 (1987) to Volks-wagen A. G; W. Held, A. Koenig, T. Richter and L. Puppe, SAEPaper 900496, 1990.

3 G. Centi and S. Perathoner, Appl. Catal. A, 1995, 132, 179.4 M. Shelef, Chem. Rev., 1995, 95, 209.5 G. Moretti, Catal. L ett., 1994, 28, 143.6 D. C. Sayle, C. R. A. Catlow, J. D. Gale, M. A. Perrin and P.

Nortier, J. Phys. Chem. A, 1997, 101, 3331.7 T. Beutel, J. Sarkany, G. D. Lei, J. Y. Yan and W. M. H. Sachtler,

J. Phys. Chem., 1996, 100, 845.8 R. Burch and P. J. Millington, Appl. Catal. B, 1993, 2, 101.9 S. C. Larsen, A. Ajol, A. T. Bell and J. A. Reimer, J. Phys. Chem.,

1994, 98, 11533.10 B. L. Trout, A. K. Chakraborty and A. T. Bell, J. Phys. Chem.

1996, 100, 4173.11 S. Hu, J. A. Reiner and A. T. Bell, J. Phys. Chem. B, 1997, 101,

1869.12 J. Sarkany, J. Mol. Struct., 1997, 410–411, 137 ; ibid., 1997, 410–

411, 95.13 M. J. Remy and G. Poncelet, J. Phys. Chem., 1995, 99, 773.14 G. Bellussi, C. Perego, A. Carati, S. Peratello, E. Previde Massara

and G. Perego, Stud. Surf. Sci. Catal., 1994, 84, 85.15 C. Dossi, A. Fusi, G. Moretti, S. Recchia and R. Psaro, Appl.

Catal. A, 1999, 4754, 1.16 C. Dossi, P. Losi and S. Calmotti, Ann. Chim. (Rome), 1993, 83,

233.17 C. Dossi, A. Fusi, R. Psaro and G. M. Zanderighi, Appl. Catal.,

1989, 46, 145.18 C. Dossi, A. Fusi, R. Psaro and D. Roberto, T hermochim. Acta,

1991, 182, 273.19 C. Dossi, A. Fusi and R. Psaro, T hermochim. Acta, 1994, 236,

165.20 C. Dossi, A. Fusi, G. Molteni, S. Recchia and R. Psaro, Analyst,

1997, 122, 279.21 C. Dossi and A. Fusi, Anal. Chim. Acta, 1989, 217, 197.22 N. S. Harris, Modern V acuum Practice, McGraw-Hill, London,

1989, p. 67.23 J. A. R. Van Veen, P. C. Jong-Versloot, G. M. M. Van Kessel and

F. J. Feels, T hermochim. Acta, 1989, 152, 359.24 G. Moretti, C. Dossi, A. Fusi, S. Recchia and R. Psaro, Appl.

Catal. B, 1999, 20, 67.25 N. Masciocchi, E. Corradi, A. Sironi, G. Moretti, G. Minelli and

P. Porta, J. Solid State Chem., 1997, 131, 252.26 W. K. Hall, X. Feng, J. Dumesic and R. Watwe, Catal. L ett.,

1998, 52, 13.27 G. Spoto, A. Zecchina, S. Bordiga, G. Ricchiardi, G. Martra, G.

Leofanti and G. Petrini, Appl. Catal. B, 1994, 3, 151.

Paper 9/05222D

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