The active sites of samarium orthovanadate (SmVO4) were studied by means of ESR, NO-TPD and temperature programmed O-18(2)-isotope exchange (TPIE) methods. The results of ESR and NO-TPD confirm the presence of V4+ in the catalyst. The TPIE revealed that the O-18(2)-isotope exchange was carried out through a single exchange procedure. The V4+ species associated with oxygen vacancies are the sites for O-2 activation

Au, CT

Li, JT

Wan, HL

Zhang, WD

万惠霖

Chemical Research in Chinese Universities

Xiamen University Institutional Repository

The Active Sites of the Reference Phase of SmVO4 asCatalyst for Propane Oxidative Dehydrogenation*ZHANG Wei-de* * , AU Chak-tong0 , L I Ji-tao and WAN Hui-lin(Depar tment of Chemistry and State K ey L aboratory f or Phy sical Chemistry of Solid Surf ace, X iamenUniver sity , X iamen, 361005; Department of Chemistry 0 , H ong K ong Bap tist Univ er sity , H ong K ong )( Received May 27, 1996)* Suppor ted by the National Natur e Sciences Foundation o f China .* * To w hom cor respondence should be addressed.　　T he activ e sites o f samar ium or thovanadat e( SmVO4 ) w er e studied by means of ESR, NO-TPD and t emperatur e pro g rammed 18O 2-isot ope exchange ( TPIE) methods. T he r esults of ESRand NO-T PD confirm t he presence o f V 4+ in the cata ly st . The TPIE revealed that the 18O 2-iso topeexchange w as car ried out t hrough a single exchange procedure. The V 4+ species a sso ciated w ithoxygen vacancies ar e the sites for O2 act iva tion.Keywords　Samar ium o rthovanadate, Activ e site, V 4+ species, Ox idative dehydrogenationIntroductionThe ut ilizat ion of alkane to produce intermediate chemicals is alw ay s att ractive. T hefunct ionalization of light paraf fin by catalyt ic ox idat ion to produce alkene and oxygenates is apr eferable method[ 1] . The oxidat ion o f n-butane to produce maleic anhydride catalyzed byVPO cataly st is the unique example which has been commer cialized in this field[ 2]. VM gOcatalysts have been claimed to be effect ive in the oxidat ive dehydrogenat ion of pr opane[ 3],butane[ 4]and cyclohexane[ 5]to produce the corresponding alkene. Kung and his colleaguessuggested M g 3V 2O 8 as the act ive phase in the ox idat iv e dehydrogenat ion react ion, while Vol-ta and co-w o rkers pointed out that A-Mg 2V 2O7 was the act ive phase w hereas the M g3V 2O 8was responsible for the to tal ox idat ion[ 6] . W e have prepared the pur e phase of rare earth or-thovanadates and studied their catalyt ic action on propane oxidat ive dehydrogenat ion[ 7]. T hepr esent paper covers the studies on the act ive sites of samarium o rthovanadate cataly st bymeans o f ESR, NO-T PD and18O 2-iso tope exchange methods.ExperimentalThe preparat ion of samarium orthovanadate( SmVO 4 ) has been described elsew here[ 7] .The pr epared sample has been confirmed by IR, Raman and XRD to be a pure phase of tet ra-hedral samarium orthovanadate. T he temperature prog rammed18O 2-iso tope exchange( T PIE)exper iment w as conducted in a f low sy stem connected to a Hew let t Packar d G1800A GCDmass quadrupole spectr ometer . T he sample( 200 mg ) was pretr eated in a flow of pure He at500 ℃ for 2 h and cooled down to 30 ℃. T he sample w as plugged w ith a mix ture gas o f1. 0% 18O 2 and 99. 0% He( in volume) for 30 min and then w as heated to 800 ℃ at a rate o f10 ℃/ min. T he 16O 2 and 18O16O wer e measured to detect the exchange ability of lat tice oxy-gen in the catalyst w ith the gaseous molecular ox ygen. The NO-T PD was performed in theVo l. 14 No. 3　1 9 9 8 　　　　　　　　　Chem ical Resear ch in Chinese Univer sities　　　　　　　　 294～296　sy stem as oxygen iso tope exchange and the sample w as pret reated in the same w ay. T he ESRwas conducted on a BRUKER 200D-SRC meter.Results and DiscussionFig . 1a shows the ESR spectum of fr esh SmVO 4 cataly st . It show s an intensive singal atg= 1. 98 which has been assigned to V4+io ns[ 8]. The hyperf ine st ructure due to V4+( S =1/ 2; I= 7/ 2) also appear s in a distorted tet rahedron st ructure, but it does no t show a verygood resonance. T he V4+ions in the cataly st can be easily o xidized by O2 . T he ESR spec-tr um o f SmVO 4 t reated by O 2 at 550 ℃ for 20 min and then sealed in a glass tube ( Fig . 1b)show s that the signal at g= 1. 98 is great ly reduced. The signal of V4+in the ESR spect ra al-so appear s after the react ion ( F ig. 1c) . The result indicates that V4+species existed in thecatalyst in the react ing pr ocedure.F ig . 1　ESR spectr a of SmVO 4.a. fr esh catalyst ; b. the catalys t w as pret reatedwith O 2 at 550℃ f or 20 min; c. af t er react ion.Fig. 2　NO-TPD pro files o f SmVO4 cat aly st.a. pretr eated w ith He at 550 ℃ f or 2 h; b. pre-t reated w ith O 2 at 550 ℃ f or 1 h .T he NO-TPD method can also be used to confirm the pr esence of V 4+ io ns [ 6] . T he cata-lyst w as first t reated w ith pure He for 2 h at 600 ℃ in order to eliminate the interference o fCO 2 which has the same m/ e( 44) as N 2O. T he NO-T PD prof ile of SmVO 4 pret reated by Heat 550℃ for 2 h show s tw o peaks at 205 and 450 ℃( F ig. 2a) . The peak at 205 ℃ is due tothe r eaction o f NO w ith the oxygen defect in the catalyst . The peak at ca. 440 ℃ corr e-sponds to the release of N 2O produced by the react ion o f NO w ith V 4+ ions. On the otherhand, the NO-T PD prof ile of SmVO 4 pret reated by O 2 at 550 ℃ for 1 h has no peak at therange, indicat ing that the oxygen vacancies in the cataly st and the V4+ions have been oxi-dized by O 2 in the pretr eatment pro cess. T his result is in good agreement w ith that o f ESR.Temperature prog rammed18O 2-isotope exchange( T PIE) w as employed to determ ine thedegr ee of isotope exchange betw een gas phase oxygen and the lat t ice oxygen o f the solid cata-lyst . T he type of isotope species formed( i . e. ,18O16O and16O2 ) in the pro cedure w as moni-to red. Our results of ox ygen isotope exchange on SmVO4 are show n in Fig . 3. The resultsshow that the18O16O pr ofile starts at 570 ℃ and increases w ith temperatur e increase. Also,a small peak appears at 738 ℃. Because the blank experiment ( w ithout cataly st ) show s nooccurrence of isotope exchange in the gaseous phase,18O16O must be produced by gaseous18O 2 exchanged w ith catalyst latt ice oxygen through a single exchange procedure, i. e . , thereis only one atom of the gaseous oxygen mo lecule exchanged fo r one lat t ice oxygen atom o fthe catalyst in a single step,295　No . 3 ZHANG Wei-de, AU Chak-t ong , L I Ji-t ao et al. 　　û 16Oû+ 18O 2( g ) û18Oû+ 18O 16O( g ) ( 1)Fig. 3　TP IE pro files o f SmVO4 .where û16Oû and û18Oû stand for lat t ice oxygenin the catalyst . The small peak at 738 ℃ cor -responds to an active site of the cataly st w hichcan exchange w ith gaseous oxygen at this tem-peratur e.The onset temperature for 16O 2 pro file isabout 610 ℃. The content of16O 2 is smal lerthan that o f18O16O below 657 ℃, but it in-creases ext remely above 650 ℃ and reachesthe max imum at 747 ℃, i . e. , at a high tem-peratur e, the content of16O 2 is much biggerthan that of18O1 6O. T he content o f16O 2 didnot increase in step w ith that of 18O 16O and theshape of 16O 2 pr ofile is no t sim ilar to that of 18O 16O. T he results indicate that 16O 2 was pro-duced due to the lat t ice oxygen desor pt ion. It did no t conceive with the isotope exchange pro-cedure. Because of the low concentrat ion o f 18O 16O in the gaseous phase, the 16O 2 producedby the exchange o f18O16O w ith lat tice o xygen(16O) must be ex t remely low , thus the cont inu-ous exchange of the produced18O16O in the f irst step w ith the lat t ice oxygen can be omit ted.The ESR and NO-T PD r esults clear ly show that there ex ist V4+species on the SmVO 4catalyst . In the V -based cataly sts, ther e are usually oxygen vacancies associated with V4+ .The r eact ion o f NO w ith the cataly st is as follow s:—O—V4+—[ e]— + 2NO —O—V5+—O— + N 2O ( 2)The18O-2 isotope exchange is car ried out as the fo llow ing process:— 16O—V 4+ —[ e]— + 18O 2 16O V4+O18 18O[ e]16O V5+O-18O18 —[ e]—V 4+ — 18O— + 18O 16O ( 3)w here [ e] stands for ox ygen vacancies in the cataly st . Based on the kinet ic scheme, one canreach a conclusion that the act iv ation of O 2 is on the site of V4+asso ciated w ith oxygen va-cancies. The adso rbed molecular ox ygen and O-species might be the act ive oxygen species inthe oxidative dehydro genat ion of propane.References[ 1]　Bell, A . T . , Boudard, M . and Ensley B. D. et al. , Catalysis Looks to t he Futur e, Nat ional Aca-demic P ress, Washingt on D. C. , 1992: 21[ 2]　Hodnet t, B. K . , Catal. Rev . Sci. Eng. , 27, 373( 1985)[ 3]　Nguyen, K. T . , Kung , H. H. , J. Cat al. , 122, 415( 1990)[ 4]　Patel, D. , Andersen, P . J. and Kung , H. H. , J. Catal. , 125, 132( 1990)[ 5]　Kung , M . C. and Kung, H. H. , J. Catal. , 128, 287( 1991)[ 6]　Guer rer o-Ruiz, A . , Rodr iguez-Ramos, I. and F ier ro , J. L . G . et al . , Studies in Sur face Science andCatalysis, Ed. by Ruiz, P . and Delm on, B. , Elsevier , Vol. 72, 1992: 203[ 7]　Au C. T . , Zhang W . D . , Wan H. L . , Catal. L ett . , 37, 241( 1996)[ 8]　Mar tini, G. , T rifiro , F . and Vaccar l, A . , J. Phy s. Chem. , 86, 1 573( 1982)296 Chemica l Resear ch in Chinese Univ ersities　　　　　　　　　　　　Vol. 14　

The active sites of the reference phase of SmVO4 as catalyst for propane oxidative dehydrogenation

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The active sites of the reference phase of SmVO4 as catalyst for propane oxidative dehydrogenation

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