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An ESR study of electron transfer and oxygen adsorption on 95Mo supported catalysts
CHEkfICAL
Volume 31, number 1
AN ESR STUDY OF ELECTRON ON %o
PHYSICS
TRANSFER
IS I~cbruxy
LEl’TL:.RS
J975
AND OXYGEN ADSORPTION :
SUPPORTED CATALYSTS
M. CHE =, J.C. McATEER** Imriru; de Reclrerches
scar la Catalyse; C.XR.S.,
69626
Villeurbanne,
France
and
A.J. TBNCH Chemistry Received
Division. /1. E. R. E., Hanvell, UK 13 November
1974
ESR studies using molybdenum enriched in g5hIo have confirmed that tOas is stabilised at hio6* sites at 77 K on silica and alumina supported molybdenum. Warming the MolAlzO3 to 300 ); leads to an incre-ase in the number of (O&ions observed due to electron transfer from seduced molybdenum sites to give additional (O&adsorbed at A13’ sites on the support_ Molybdenum is not involved at the adsorption site of (02, on the magnesia supported catalyst.
Samples of 2% mo!ybdenum supported on silica, r-alumina and magnesia were prepared by impregnation from ammonium paramolybdate solution. The g5Mo isotope was obtained as 95hfo03i and cowerted to ammoniumparamolybdate [lo] before impregnation. The materials were activated by heating under oxygen (500 torr) to 773 K, outgassed, and then re-
Chemisorbed oxygen molecules are reduced to (OF)s radical ions on molybdenum supported catalysts [l-3]. The charge of the cation present at the adsorption site can be determined from the gzz value of the adsorbed (Oy)s species [4,5]. By this proce‘dure Krylov et al. [2] and,Howe and Leith [G] found that the (Oy)s ion is adsorbed at Mo6+ sites at. 77 K on alumina suggested
supported th’at electron
molybdenum.
At 300 K they
duced
trznsfeeroccurs from (O,),
adsorbed at MO@ sites to give (OF)s species siabilised at A13+sites. However, it has been shown that, for adsorbed (OF& ions, a range ofg,,-values is observed with cations of the same charge 171, and this parameter does net give an unambiguous identification of the site involved. Studies using l?O, have confirmed the identity of the (O&species and indicate that some variations.exist in the adsorjtion sites on differ- ,, ent supports that are not.reflected in the gzz values [8,9]. In this work, we have studied the (Oz)s species adsorbed on supported samples enriched in g5Mo (I = S/2), in order to confirm the nature of, the adsorption site,.and to : study the electron traGfer pro-, cess.
.’
* To whom correspondence should be addressed.‘. *+ C.N.R.S. Postdoctoral Fellow 1973-1975. ,.
by hydrogen
(100
torr)
at 773 K. Oxygen
wac.
adsorbed at 77 K and the excess removed by evacuation at this temperature. Oxygen
adsorbs 6n the treated
95Mo/Si0,
sample
to give a signal which has been identified as (Oz), [S]: A second derivative analysis of the signal shows each principal value of the rtensor to be split into six component lines. The signal is characterised by the following magnetic parameters,gz, = 2.0176 g = 2.0105, Q.rxr = I gU = 2.0050 and a,, = 2, ayv = 1.9 (d< gauss. The same parameters were observed at 300 K. Oxygen is adsorbed on the %Io/Al~O~ material at 77 K to produce (Ol)s species, I; withg,a = 2.017, gyj = 2.010 and/jr= = 2.004:The second derivative :,spectrum revealed a sext¢red around the gYY value with aYY= 3.3 ? 0.2 gauss, but.no lines corret g%fo.in 97.43% abundance; supplied by the Electromagnetid Separator Unit, A.E.R.E., ~anvel!, UK.
.. .
8.
145
,Volkc
31. nthbcr~l
‘..
..
CIIEMICAL L’I~YSICS LEnERS
‘_ . . : sponding’tb the a,,,and QXXcompoq~nts were resolved.
At.300 K‘a new (C)T), speG$ II, appeared which was characterised by the pa&netdrsgzZ = 2.039, g_VY =, ” :.2.009, +&= 2.004 and tiiz T 5.4 (+0.2), aeV_,, = 3.6 (‘-0.2LQ_y, ,= 5.2 (kO.2) gauss. The same species was ‘observed on Cooling again to 77 K, and ;he magnetic l&meters femained unchanged. Samples containing molybdenum in natural abundance ‘gave no hyperfine ,. .interacti& at 77 K; while at 300 K the&R signal .. was identical with that of the isotopicallv enriched sample. A similar signal was obtained after y-irradia-. ._ t:on of -r-AI203 in the presence of oxygen. The corn centration
of (0~)~
radical
ion’s adsorbed
on tile
Mo/AlJ03 sample was 4etermined qt 77 K on,jniti$ adsorption and after wmning’to 300 K for 15 mln.’ utes. The integrated inthnsity of the radical ion signal increased,by appr,oxiiima\ely a factor of three on heating and signals from both types of (O?), species (I 31-&l II) &uId be seen. There was no decrease in the concentratiori of (Oy)s species I on warming. When the sam& was evacuated at 300 K for one hour, only the signal from (OT)s species II remained and the. .concentration of this species, determined at -77 K, was hzlf the maxjlnum rag&al ion concentration. The (OF); species with g,, = 2.070, g,,, = 2.004 and gxx = .2.001 formed on the treated MO/M,@ samples, but no hyperfine interaction was detected when enriched molybdenum samples.were used. A gzz value of 2.0176 on the silica suppbrted catalyst indicates integration of the oxygen with a highly ..charged ion [7]. The splitting of each principal value of the z tensor into six component lines when g5Mo. is present establishes that the (Oz)s is stabilised at an Mo6+io?, The observatidn of a splitting about giY on the’alumina supportad sample at 77 K show,s .the (Oz)s species I is adsorbed at an Mo6+site. The similarity of the inagnetic parameters for (Oz)s specks II adsorbed on g>Mo/A1203, MoAljOi and irradiated yA1203 confirms that this radical ion is stabilised at.27A13* sites
nt
:he surface.
It is important
td consider
the on‘&
-.
I5 fTcbru;uy 1975 .’
_
of the unpaired e&on which fois.the (O&stabilised at ,27A1?+sites. Previous workers [i] have suggested that &-Ielectron transfer occurs from the.(O,), adsorbed. at Mo6+ sites, but our results show that the total concentration of (Oy)s ions increases after warming to ;700 K and that the electron transfer must therefore im,olve some other species. The continued presence of (O,), adsorbed at Mo6+ sites after heating confin-m that th’ese ions are not involved in the electron transfer process and we attribute the formation of the (0~)~ species at Al 3+ sites to qn irreversible electron t,ransfer from reduced molybdenum sites that are not available at 77 K. The gzt value of 2.070 for (OS), adsorbed on Mo/MgO samples and the absence of -tisible hyperfijle interaction when g5Mo is present indicates that thi3 (Os)s species is adsorbed at Mg2+sites.
This work &as sponsored by the.C.N.R.S. (France) under A.T.P. no. 9107. J.C. McA. acknowledges the provision OFsupport and facilities from A.E.R.E., Hatieil.
References [l] X. i)ufau>r, M. Che and C. Naccache, Compt. Rend. Acsd.
Sci. (Paris)
268C
(1969)
2255.
[ 2] O.V. Krylov, G.B. Pariiski and K.N. Spiridonov, J. Catalysis 23 (1971) 301. [3J VA. Shvets and V.B. Knzanskii, J. Catalysis 25 (1972) 123: [4] W. ICanzig and hl.H. Cohen, Phys. Rev. Letters 3 (1959) 509. [Sl P.H. Kasai, J. Chem. Phys. 43 (1965) 3322. [6] R.F. Howe and I.R. Leith, J. Chcm. Sot. Faraday I69 ., U9:‘3) 1967. [7] J.H. Lunsford, Cztal. Rev. 8 (1973) 135. [8] hi. Che and A.J. Tenth, Chem. Phys LetIers.18 (i973) 199.
[ 91 M.‘Che,-A. J..Tench and C. Naccache, J. Chem. Sot Fnralny I70 (1974) 263. [ 101 P. Pcscal,
(himon,
ed.,,Nquvenu
Paris)$680,
trait& de chimie min&Iq
Vol.
14.
Volume 31, number 1
AN ESR STUDY OF ELECTRON ON %o
PHYSICS
TRANSFER
IS I~cbruxy
LEl’TL:.RS
J975
AND OXYGEN ADSORPTION :
SUPPORTED CATALYSTS
M. CHE =, J.C. McATEER** Imriru; de Reclrerches
scar la Catalyse; C.XR.S.,
69626
Villeurbanne,
France
and
A.J. TBNCH Chemistry Received
Division. /1. E. R. E., Hanvell, UK 13 November
1974
ESR studies using molybdenum enriched in g5hIo have confirmed that tOas is stabilised at hio6* sites at 77 K on silica and alumina supported molybdenum. Warming the MolAlzO3 to 300 ); leads to an incre-ase in the number of (O&ions observed due to electron transfer from seduced molybdenum sites to give additional (O&adsorbed at A13’ sites on the support_ Molybdenum is not involved at the adsorption site of (02, on the magnesia supported catalyst.
Samples of 2% mo!ybdenum supported on silica, r-alumina and magnesia were prepared by impregnation from ammonium paramolybdate solution. The g5Mo isotope was obtained as 95hfo03i and cowerted to ammoniumparamolybdate [lo] before impregnation. The materials were activated by heating under oxygen (500 torr) to 773 K, outgassed, and then re-
Chemisorbed oxygen molecules are reduced to (OF)s radical ions on molybdenum supported catalysts [l-3]. The charge of the cation present at the adsorption site can be determined from the gzz value of the adsorbed (Oy)s species [4,5]. By this proce‘dure Krylov et al. [2] and,Howe and Leith [G] found that the (Oy)s ion is adsorbed at Mo6+ sites at. 77 K on alumina suggested
supported th’at electron
molybdenum.
At 300 K they
duced
trznsfeeroccurs from (O,),
adsorbed at MO@ sites to give (OF)s species siabilised at A13+sites. However, it has been shown that, for adsorbed (OF& ions, a range ofg,,-values is observed with cations of the same charge 171, and this parameter does net give an unambiguous identification of the site involved. Studies using l?O, have confirmed the identity of the (O&species and indicate that some variations.exist in the adsorjtion sites on differ- ,, ent supports that are not.reflected in the gzz values [8,9]. In this work, we have studied the (Oz)s species adsorbed on supported samples enriched in g5Mo (I = S/2), in order to confirm the nature of, the adsorption site,.and to : study the electron traGfer pro-, cess.
.’
* To whom correspondence should be addressed.‘. *+ C.N.R.S. Postdoctoral Fellow 1973-1975. ,.
by hydrogen
(100
torr)
at 773 K. Oxygen
wac.
adsorbed at 77 K and the excess removed by evacuation at this temperature. Oxygen
adsorbs 6n the treated
95Mo/Si0,
sample
to give a signal which has been identified as (Oz), [S]: A second derivative analysis of the signal shows each principal value of the rtensor to be split into six component lines. The signal is characterised by the following magnetic parameters,gz, = 2.0176 g = 2.0105, Q.rxr = I gU = 2.0050 and a,, = 2, ayv = 1.9 (d< gauss. The same parameters were observed at 300 K. Oxygen is adsorbed on the %Io/Al~O~ material at 77 K to produce (Ol)s species, I; withg,a = 2.017, gyj = 2.010 and/jr= = 2.004:The second derivative :,spectrum revealed a sext¢red around the gYY value with aYY= 3.3 ? 0.2 gauss, but.no lines corret g%fo.in 97.43% abundance; supplied by the Electromagnetid Separator Unit, A.E.R.E., ~anvel!, UK.
.. .
8.
145
,Volkc
31. nthbcr~l
‘..
..
CIIEMICAL L’I~YSICS LEnERS
‘_ . . : sponding’tb the a,,,and QXXcompoq~nts were resolved.
At.300 K‘a new (C)T), speG$ II, appeared which was characterised by the pa&netdrsgzZ = 2.039, g_VY =, ” :.2.009, +&= 2.004 and tiiz T 5.4 (+0.2), aeV_,, = 3.6 (‘-0.2LQ_y, ,= 5.2 (kO.2) gauss. The same species was ‘observed on Cooling again to 77 K, and ;he magnetic l&meters femained unchanged. Samples containing molybdenum in natural abundance ‘gave no hyperfine ,. .interacti& at 77 K; while at 300 K the&R signal .. was identical with that of the isotopicallv enriched sample. A similar signal was obtained after y-irradia-. ._ t:on of -r-AI203 in the presence of oxygen. The corn centration
of (0~)~
radical
ion’s adsorbed
on tile
Mo/AlJ03 sample was 4etermined qt 77 K on,jniti$ adsorption and after wmning’to 300 K for 15 mln.’ utes. The integrated inthnsity of the radical ion signal increased,by appr,oxiiima\ely a factor of three on heating and signals from both types of (O?), species (I 31-&l II) &uId be seen. There was no decrease in the concentratiori of (Oy)s species I on warming. When the sam& was evacuated at 300 K for one hour, only the signal from (OT)s species II remained and the. .concentration of this species, determined at -77 K, was hzlf the maxjlnum rag&al ion concentration. The (OF); species with g,, = 2.070, g,,, = 2.004 and gxx = .2.001 formed on the treated MO/M,@ samples, but no hyperfine interaction was detected when enriched molybdenum samples.were used. A gzz value of 2.0176 on the silica suppbrted catalyst indicates integration of the oxygen with a highly ..charged ion [7]. The splitting of each principal value of the z tensor into six component lines when g5Mo. is present establishes that the (Oz)s is stabilised at an Mo6+io?, The observatidn of a splitting about giY on the’alumina supportad sample at 77 K show,s .the (Oz)s species I is adsorbed at an Mo6+site. The similarity of the inagnetic parameters for (Oz)s specks II adsorbed on g>Mo/A1203, MoAljOi and irradiated yA1203 confirms that this radical ion is stabilised at.27A13* sites
nt
:he surface.
It is important
td consider
the on‘&
-.
I5 fTcbru;uy 1975 .’
_
of the unpaired e&on which fois.the (O&stabilised at ,27A1?+sites. Previous workers [i] have suggested that &-Ielectron transfer occurs from the.(O,), adsorbed. at Mo6+ sites, but our results show that the total concentration of (Oy)s ions increases after warming to ;700 K and that the electron transfer must therefore im,olve some other species. The continued presence of (O,), adsorbed at Mo6+ sites after heating confin-m that th’ese ions are not involved in the electron transfer process and we attribute the formation of the (0~)~ species at Al 3+ sites to qn irreversible electron t,ransfer from reduced molybdenum sites that are not available at 77 K. The gzt value of 2.070 for (OS), adsorbed on Mo/MgO samples and the absence of -tisible hyperfijle interaction when g5Mo is present indicates that thi3 (Os)s species is adsorbed at Mg2+sites.
This work &as sponsored by the.C.N.R.S. (France) under A.T.P. no. 9107. J.C. McA. acknowledges the provision OFsupport and facilities from A.E.R.E., Hatieil.
References [l] X. i)ufau>r, M. Che and C. Naccache, Compt. Rend. Acsd.
Sci. (Paris)
268C
(1969)
2255.
[ 2] O.V. Krylov, G.B. Pariiski and K.N. Spiridonov, J. Catalysis 23 (1971) 301. [3J VA. Shvets and V.B. Knzanskii, J. Catalysis 25 (1972) 123: [4] W. ICanzig and hl.H. Cohen, Phys. Rev. Letters 3 (1959) 509. [Sl P.H. Kasai, J. Chem. Phys. 43 (1965) 3322. [6] R.F. Howe and I.R. Leith, J. Chcm. Sot. Faraday I69 ., U9:‘3) 1967. [7] J.H. Lunsford, Cztal. Rev. 8 (1973) 135. [8] hi. Che and A.J. Tenth, Chem. Phys LetIers.18 (i973) 199.
[ 91 M.‘Che,-A. J..Tench and C. Naccache, J. Chem. Sot Fnralny I70 (1974) 263. [ 101 P. Pcscal,
(himon,
ed.,,Nquvenu
Paris)$680,
trait& de chimie min&Iq
Vol.
14.