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Storage of reactive hydrogen species in CeMxOy (M = Cu, Ni; 0≤x≤1) mixed oxides
\ PERGAMON
International Journal of Hydrogen Energy 13 "0888# 0972Ð0981
Storage of reactive hydrogen species in CeMxOy "M Cu\ Ni^ 9 ¾ x ¾ 0# mixed oxides L[ Jalowiecki!Duhamel \ A[ Ponchel\ C[ Lamonier Laboratoire de Catalyse Heterogene et Homogene\ U[R[A[ C[N[R[S[ D93919\ Ba¼t[ C2\ Universite des Sciences et Technolo`ies de Lille\ 48544 Villeneuve d|Ascq Cedex\ France
Abstract Interaction of hydrogen with a series of mixed oxides CeMxOy "M Cu\ Ni^ 9 ¾ x ¾ 0# has been studied in the 299Ð 0962 K temperature range[ A dynamic method of titration has been used to provide evidence for reactive hydrogen H present in the solids[ This method of titration involves hydrogenation at 312 K of isoprene under helium ~ow in the absence of gaseous hydrogen[ The hydrogen reservoir capacity depends on the pretreatment temperature under H1 and is related to the creation of anionic vacancies in the solid[ All the CeMxOy mixed oxides are large catalytic hydrogen reservoirs with marked di}usion properties of hydrogen species "H \ OH−#[ In the series studied\ the highest amount of hydrogen H "18[4 09−2 mol g−0# is stored by CeNi9[6Oy pretreated under H1 at 412 K "H5[7CeNi9[6Oy#[ In the conditions in which the incorporation of hydrogen in the solid occurs\ X!ray di}raction analysis under H1 shows shifts depending on the treatment temperature and corresponding to non!negligeable expansion of the ceria!based lattice attributed to the insertion of hydrogen species of hydridic nature in the anionic vacancies[ Þ 0888 International Association for Hydrogen Energy[ Published by Elsevier Science Ltd[ All rights reserved[
0[ Introduction Intermetallic compounds as catalyst precursors have been widely explored[ They are essentially based on rare earth elements "RE# and a transition metal\ and are known to be able to absorb large quantities of hydrogen and form hydrides[ Recently by carrying out di}erent preparation procedures\ in our laboratory it has been shown that a CeNi4 oxide\ prepared by coprecipitation and reduced under H1 could store as much hydrogen as the CeNi4 intermetallic compound\ and was e.cient in hydrogenation of polyunsaturated hydrocarbons "iso! prene\ benzene and toluene# ð0Ł[ This last twenty years\ the key role of hydrogen has become widely evident for energy storage\ in metallurgy as well as in heterogeneous catalysis ð1Ł[ The nature of hydrogen is under controversy[ It is usually admitted that the hydrogen adsorption on metal or other surfaces able to adsorb hydrogen is dissociative[ The problem is if
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the rupture is heterolytic or homolytic[ Therefore\ four di}erent species can be created and transported] the rad! ical H=\ the bonded species H!\ or the charged species H¦ or H−[ This work concerns the interaction of hydrogen with a series of CeMxOy mixed oxides\ precursors of catalysts for hydrogenation reactions ð0Ł and oxydehydrogenation reactions ð2Ł\ using in!situ techniques[ The ability of the reduced solids to store hydrogen and the nature of the occluded hydrogen species will be discussed and a reduction mechanism will be proposed[
1[ Experimental The mixed oxides\ denoted CeMxOy\ in which M rep! resents Cu or Ni\ and x the M ] Ce ratio\ were prepared by coprecipitation of hydroxides from mixtures of cerium and copper or nickel nitrates using triethylamine "TEA# as precipitating agent\ drying at 252 K and calcination in air at 662 K "562 K for CeCu0Oy#[ The loading has been measured by microanalysis[ Pure cerium and nickel
9259!2088:88:,19[99 Þ 0888 International Association for Hydrogen Energy[ Published by Elsevier Science Ltd[ All rights reserved PII] S 9 2 5 9 ! 2 0 8 8 " 8 7 # 9 9 0 3 2 ! 7
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oxides were prepared according to the same method and used as reference compounds[ Thermogravimetric measurements were performed using a Sartorius 3091 microbalance equipped with a ~ow!gas system[ The oxides were treated under a hydro! gen ~ow "1[6 l h−0# and the temperature was increased at a rate of 0[6 K min−0 from room temperature to the _nal temperature of about 862 K[ Measurement of the amount of hydrogen stored in the solid is based on the ability of this class of compounds to hydrogenate 1!methylbut!0\2!diene "isoprene# in the absence of gaseous hydrogen\ according to a dynamic method described previously ð3Ł and applied in our lab! oratory to a large variety of compounds ð4\ 5Ł[ The pre! treatment and catalytic experiments were carried out in! situ at atmospheric pressure in an all!glass grease free ~ow apparatus[ The solid was treated _rst under a pur! i_ed hydrogen ~ow at various temperatures TT "09 h#\ then after elimination of molecular hydrogen at 312 K\ hydrogenation reaction was performed under iso! prene¦helium mixture "9[4) vol[#[ The hydrogenation reaction involves the participation of reactive hydrogen of the solid\ which can di}use from the bulk to hydro! genate isoprene at the surface[ The structure of the oxides during the treatment under H1 were analysed in a Siemens D4999 di}ractometer equipped with an Anton Paar HTK 09 chamber\ and connected to a gas introduction and puri_cation line[ A position sensitive detector was used^ the ~ow conditions and the rate of temperature increase were similar to those adopted in thermogravimetry[ Apart from Ka1 con! tribution eliminated by computer post!processing\ the patterns obtained were not subjected to further treat! ments[ Owing to the use of a platinum holder\ they show platinum di}raction peaks\ the evolution of which with TT was taken into account in determining the exact pos! ition of the peaks of the solids analysed[
2[ Results Interaction of hydrogen with some CeMxOy "M Cu\ Ni^ 9 ¾ x ¾ 4# has already been studied in the 299Ð0962 K temperature range[ XPS\ EPR\ in!situ XRD and ther! mogravimetric techniques have been used to characterize the processes occuring during the hydrogen treatment and the amount of hydrogen occluded when treating the solid at two di}erent temperatures under H1 ð6Ł[ The oxides of CeCuxOy and CeNixOy\ can be described as a mixture of copper oxide or nickel oxide and of ceria modi_ed by the insertion of a part of copper or nickel in its lattice[ The size of the copper oxide or nickel oxide varies considerably from clusters to a crystallized material\ depending on the x!value and on the exper! imental conditions[
2[0[ Anionic vacancies In a previous study\ we reported that after treatment under H1 at 362 or 462 K\ CeMxOy mixed oxides contain anionic vacancies produced by the elimination of H1O "OH groups#[ The thermograms under H1 have been per! formed\ and the results obtained are presented in Fig[ 0[ In agreement with the literature ð6Ł\ up to 262 K\ the weight loss observed corresponds to the elimination of physisorbed water\ while for higher TT anionic vacancies are created[ Moreover\ for temperatures higher than 312 K the observed phenomena are not the exact super! position of those obtained for the pure reference cerium "CeO1#\ and nickel "NiO# oxides and those reported for copper oxide "CuO#\ which has been previously studied ð6Ł[ The thermograms of all the samples show two domains of relatively signi_cant loss of weight\ marked out by the temperatures T0 and T1^ a _rst\ more distinct domain at about the temperature of reduction of NiO or CuO\ and the other one in the 712Ð762 K temperature range "Fig[0\ Table 0#[ The di}erence observed between MO reduction temperature and T0 probably results from the presence in the oxide state of clusters or small MO agregates which are more easily reduced[ Moreover\ in Table 0 has also been reported the reduction rate and it appears that the M1¦ reduction is modi_ed\ depending on the content of M1¦ in the ceria lattice ] redox processes between Ce3¦\ Ce2¦\ M and M¦ or M1¦ have already been demonstrated ð6Ł[ 2[1[ Hydro`en H After treatment under H1 at 362 or 462 K\ CeMxOy mixed oxides contain anionic vacancies and also some reactive hydrogen species able to hydrogenate alkadienes in absence of gaseous hydrogen ð0\ 6Ł[ These species are denoted H as we are not considering their exact charge[ At 312 K under a helium¦isoprene ~ow\ alkadiene hydrogenation occurs on H1 treated CeMxOy mixed oxides as presented in Fig[ 1 for H1 treated at 462 K CeNi9[1Oy[ It is important to note that the hydrogenation activity obtained on the untreated CeMxOy solid is null\ therefore\ these H moities are probably di}erent from OH groups which are known to be present on such cata! lysts[ As a function of time on stream\ the isoprene con! version or hydrogenation activity "HYD# is measured[ The ratio HYDrel AHt:AH9 "where AH9 and AHt are the initial hydrogenation and the hydrogenation activity at time t# can be plotted versus time^ this relative hydro! genation activity at 312 K under helium¦isoprene feed decreases with time[ For each solid a similar curve is obtained and by integrating this curve the extractable reactive hydrogen content of a solid can be determined if the product distribution is taken into account "1 H for monohydrogenation and 3 H for dihydrogenation#[ Moreover the extractable hydrogen content of the solid
L[ Jalowiecki!Duhamel et al[ : International Journal of Hydrogen Energy 13 "0888# 0972Ð0981
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Fig[ 0[ Thermogravimetric pro_les of the CeMxOy compounds treated under pure hydrogen[ NiO "#\ CuO "Ž#\ CeO1 "x#\ CeNi9[4Oy "Ž#\ CeCu9[4Oy "ž#[
Table 0 Temperatures relative to the di}erent weight losses in the mixed oxides and reference compounds Catalyst
T0 "K#
Reduction rate0 ) K−0
T1 "K#
CeO1 NiO CuO CeNi9[1Oy CeNi9[4Oy CeNi9[6Oy CeCu9[4Oy CeCu0Oy
Ð 372 322 337 362 343 282 262
Ð
768 Ð Ð 743 704 706 714 726
9[22 ¼9[33 9[91 9[97 9[02 ¼0 ¼9[7
0 Reduction rate related to weight loss starting at T0 and deduced from the slope of the curve[
is found to be dependent on the treatment temperature TT[ Figs 2 and 3 show the variation in the hydrogen H species concentration as a function of the treatment temperature TT for di}erent CeNixOy and CeCuxOy mixed oxides\ respectively[ In the series studied\ an opti! mum value H 18[4 09−2 mol g−0 is obtained on CeNi9[6Oy for TT 412 K corresponding to the H5[7CeNi9[6Oy compound as presented in Table 1[ The values obtained are very high\ as a matter of fact these mixed oxides are able to absorb higher quantities of hydrogen than the density of liquid hydrogen ð7Ł[ More! over\ for comparison\ one can recall that with this
Fig[ 1[ Relative hydrogenation activity at 312 K under helium! ¦isoprene ~ow versus time on CeNi9[1Oy treated in H1 at 462 K[
dynamic method\ the H content found with CeO1\ and CuO previously treated under H1 at 462 and 362 K\ respectively\ is nil while NiO treated under H1 at 462 K is able to store 12[8×09−2 mol g−0 of H species[ For the CeNixOy catalysts "with x 9[1\ 9[4 and 9[6# some reactive hydrogen species "less than 2×09−2 mol g−0# are inserted into the solid at 312 K "Fig[ 2#\ and at
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L[ Jalowiecki!Duhamel et al[ : International Journal of Hydrogen Energy 13 "0888# 0972Ð0981 Table 1 H content measured on the di}erent catalysts Catalyst CeMxOy
TT "K#
ðH Ł 092 mol g−0
H ] Ce0
CeNi9[1Oy CeNi9[4Oy CeNi9[6Oy CeCu9[4Oy CeCu0Oy
412 412 412 362 262
08[66 08[34 18[42 00[68 06[19
2[7 3[0 5[7 1[4 3[3
0 The atomic weight of the catalysts has been estimated assuming that y 1¦x ] M MCe¦xMM¦"1¦x#MO[
Fig[ 2[ Variation in the hydrogen H species concentration as a function of the treatment temperature TT of CeNixOy with x 9[1 "R#\ 9[4 "ž# and 9[6 "Ž#[
already very high\ maximum for the CeCu0Oy solid "06[1×09−2 mol g−0# and almost maximum for the CeCu9[4Oy compound "00[7×09−2 mol g−0#[ In addition\ between t and tf "time at which the hydro! genation activity is found to be zero# the percentage of reactive hydrogen still present in the solid can be esti! mated[ A plot of HYDrel versus H concentration can be obtained for each TT[ As an example\ in Fig[ 4 the results are reported for the CeNi9[1Oy solid treated at 312 K under H1 and the CeCu9[4Oy solid treated at 562 K under H1[ The curves obtained show clearly that there is no proportionality between the relative hydrogenation rate consuming H with the hydrogen content of the solid\ and therefore the kinetics of H consumption by the alkadiene is a complex phenomenon^ in particular a fast di}usional process of the H species within the solid must
Fig[ 3[ Variation in the hydrogen H species concentration as a function of the treatment temperature TT of CeCuxOy with x 9[4 "R#\ and 0 "Ž#[
this temperature the ability of the solid to store hydrogen decreases when the value of x increases[ For higher TT the hydrogen H storage ability increases when x increases[ Moreover\ the ability of the solids to store hydrogen species is maximum for a treatment temperature of about 412 K and then it decreases for temperatures higher than 512 K[ In the CeCuxOy compounds "Fig[ 3#\ for a treatment temperature TT 262 K the hydrogen H content is
Fig[ 4[ Relative hydrogenation activity at 312 K under helium! ¦isoprene ~ow versus the hydrogen H species concentration of CeNi9[1Oy treated in H1 at 312 K and of CeCu9[4Oy treated in H1 at 562 K[
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be considered ð3\ 8Ł[ A clear analogy exists between these results and those obtained on reduced copper based mixed oxides ð5\ 8Ł which were found to be hydrogen reservoirs[ The variation of isoprene hydrogenation activity as a function of time under helium "Fig[ 1# corresponds to a speci_c hydrogenated products distribution presented in Fig[ 5 for CeNi9[4Oy and in Fig[ 6 for CeCu9[4Oy[ Isoprene hydrogenation activity takes into account all the hydro! genated products\ which are isopentane "dihy! drogenated#\ 1!methylbut!0!ene "1MB0#\ 2!methylbut!0! ene "2MB0#\ and 1!methylbut!1!ene "1MB1# "monohy! drogenated#[ The direct analysis of the curves obtained in Figs 5 and 6 reveals di}erent areas\ as has been already observed and de_ned previously on di}erent copper! based oxides ð5Ł[ When molecular hydrogen disappears completely from the gas phase a sharp decrease of activity is observed[ At the same time the production of iso! pentane also decreases drastically\ whereas the pro! duction of monoenes formed changes[ Maxima are reached successively by 1!methylbut!1!ene and the two methylbut!0!enes[ No simple correlation is found between the time of isopentane disappearance or the appearance of maxima of formation of the various prod! ucts and either the H content[ It can be recalled that similar behaviour is also observed on sul_des "MoS1:Al1O2# for which a good correlation has been obtained between the selectivity and the site structure ð09Ł[ Similar variations of isoprene prod! ucts distribution under helium¦isoprene ~ow as a func!
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Fig[ 6[ Isoprene hydrogenated products distribution at 312 K as a function of time under helium on CeCu9[4Oy treated in H1 at 362 K[
tion of time have also been obtained with various H species concentrations[ It has been shown that a small number of active sites are su.cient to produce a large quantity of isopentane when the hydrogen reservoir is large enough[ However\ even though the amount of H hydrogen species in the solid is high\ the formation of isopentane is null when the corresponding active sites do not exist[ 2[2[ In!situ XRD
Fig[ 5[ Isoprene hydrogenated products distribution at 312 K as a function of time under helium on CeNi9[4Oy treated in H1 at 362 K[
The in!situ XRD has been performed on the CeMxOy mixed oxides which crystallize in the ~uorite type phase[ The evolution of the XRD spectra when treated the solid under H1 as a function of the temperature is presented\ as an example\ in Fig[ 7 for the CeNi9[4Oy mixed oxide[ No drastic crystallographic modi_cation is apparent\ indicating that neither the hexagonal Ce1O2 nor the rhombohedral CeO0[71 phase reported by Bevan is formed even at 0962 K ð00Ł[ A careful examination shows that each di}raction line is shifted towards lower angles for treatment tem! peratures higher than 312 K for the CeNixOy compounds and for treatment temperatures higher than 262 K for CeCuxOy compounds[ In order to specify the conditions in which the expanded phase has been formed during the reduction treatment\ the peak position has been plotted as a function of TT for the most intense lines[ Reported in Figs 8Ð01\ is the di}erence D"1u# between the initial peak position "299 K# and its position at TT\ corrected
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Fig[ 7[ XRD patterns during in situ reduction under pure hydrogen of CeNi9[4Oy] "P# CeO1\ "# NiO\ "R# platinum and "ž# nickel[
Fig[ 8[ Shift of the CeO1 di}raction peaks in CeNi9[1Oy during in!situ reduction treatment under hydrogen] "# 000\ "r# 199 and "# 200[
from the thermal expansion factor "the D"1u# value is given with an accuracy of 29[91>#[ Each curve has the same pro_le and three zones can be distinguished[ The cell parameter is rather unmodi_ed in zone I "299Ð262 or 362 K#\ while a lattice expansion occurs in zone II "262 or 362Ð732 K# and III "732Ð0962 K#[ Indeed\ the D"1u# values can be considered to be within the experimental error for zone I\ while the angular deviations are
su.ciently large in zones II and III to be linked to a bulk phenomenon[ To compare the thermal behaviour to the structural evolution of the solid\ the temperatures T0 and T1 presented in Table 0 have been shown again in Figs 8Ð01[ The temperature TM\ deduced from XRD\ at which the metallic phase appears has also been precised on the same _gures[ It is worth noting that for CeO1\ these temperatures
L[ Jalowiecki!Duhamel et al[ : International Journal of Hydrogen Energy 13 "0888# 0972Ð0981
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Fig[ 09[ Shift of the CeO1 di}raction peaks in CeNi9[4Oy during in!situ reduction treatment under hydrogen] "# 000\ "r# 199 and "# 200[
Fig[ 00[ Shift of the CeO1 di}raction peaks in CeCu9[4Oy during in!situ reduction treatment under hydrogen] "# 000\ "r# 199 and "# 200[
correspond to a more drastic change of the ceria cell parameter "462 and 752 K# or to the maximum of the lattice expansion in zone II "625 K#[ For a comparison of the di}erent D"1u# curves\ it appears that zone I shown for CeO1 has disappeared\ while the expansion of the ceria lattice at about 769 K reaches and remains at a similar level[ The main change occurs in the _rst domain of the expansion of the ceria lattice which is maintained "zone II#[ Considering in more detail zone II\ it is clear that several parameters in~uence the phenomena which occur in the bulk of the ceria!like phase] "i# the nature of the transition metal M[ While the NiÐCe association leads to only one maximum of D"1u#\ zone II shows two maxima for the CeCuxOy compounds[ This factor also a}ects the position of this zone in the temperature scale[ "ii# The
M ] Ce atomic ratio^ the x!value also a}ects the tem! perature corresponding to the maximum of D"1u#[ Besides\ the relative position of TM and T0 on the di}erent curves leads to the following observations] "i# Whatever the M ] Ce ratio\ T0 coincides with the beginning of the dilatation of ceria cell[ "ii# When the solid solution is the only phase detected by XRD in the oxide precursors\ the appearance of the metallic phase is associated with the end of zone II and sometimes to a decrease of the crys! tallographic parameter of ceria ð6Ł[ "iii# When MO is also present\ TM is shifted to T0\ and in addition to the reduction of the solid solution\ free MO transforms into metal[ The di}erence observed between T0 and TM implies that the metallic cation is preserved in the modi_ed ceria phase at least for temperatures lower than TM[ At higher temperatures\ a segregation of the solid solution occurs
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Fig[ 01[ Shift of the CeO1 di}raction peaks in CeCu0Oy during in!situ reduction treatment under hydrogen] "# 000\ "r# 199 and "# 200[
which explains the somewhat similar behaviour of CeMxOy and CeO1 above 762 K[
3[ Discussion The ceria!based catalysts are interesting for water gas shift reactions ð01Ł\ CO ð02Ł and hydrocarbon oxidation ð03\ 04Ł[ It appears that the common parameter for these applications is the participation of surface oxygen spe! cies:anionic vacancies ð05Ł[ As a matter of fact\ on ceria based oxides\ it has already been shown that the elim! ination of lattice oxygen under H1 leads to the creation of anionic vacancies ð06\ 07Ł[ And\ it is well known that CeO1 is able to store oxygen and to release it under reducing atmosphere leading to a CeO1!X compound ð08Ł[ Moreover\ the ceria reduction and reoxidation rate is increased on metals of group VIII supported on ceria catalysts ð19Ł[ Besides\ the interaction of hydrogen with cerium based oxides has also been largely studied[ It has been shown that ceria is able to store hydrogen and that the hydrogen storage depends on the reduction temperature under H1 ð0\ 6\ 8\ 03 Ð16Ł[ Moreover\ Otsuka et al[ have analysed the reduction and reoxidation rates on cerium oxide and a proportionality has been obtained between the quantity of H1 produced during the oxidation step and the con! centration of anionic vacancies created during the reduction step ð19Ł[ Fierro et al[ estimated that during the reduction process under H1\ the incorporation of hydrogen in ceria and the elimination of lattice oxygen can a}ect in an opposite way the mass of ceria ð06Ł[ On the CeMxOy mixed oxides\ the hydrogen treatment leads to the creation of anionic vacancies and to the insertion into the solid of hydrogen species[ Evidence is provided for the existence of particular reactive hydrogen
species of the solid[ Even\ if the hydrogen H uptake by the CeMxOy mixed oxides has already been reported in the literature ð0\ 6Ł\ only two treatment temperatures TT have been studied[ In the present study the evolution of the hydrogen H content has been analysed as a function of the treatment temperature TT and for slightly di}erent solids "coprecipitation in TEA and calcination at 662 K#[ The H species are able to hydrogenate alkadienes at 312 K without the presence of H1 when the solid has been previously treated under H1^ otherwise no H species is found[ Hence\ the hydroxyl groups "OH# always present on the solid cannot justify the results obtained[ Moreover the H species concentration depends on the treatment temperature under H1\ almost no H is found for TT lower than 242 K and the H content increases with the treatment temperature up to about 262Ð412 K depending on the solids[ For the CeMxOy mixed oxides\ the main experimental results observed in the temperature range corresponding to the _rst expansion of the ceria!based lattice "zone II] 262 or 362Ð732 K#\ can be summarized as follows] "i# the insertion of Cu1¦ or Ni1¦ in ceria favours the expansion of the ~uorite lattice under hydrogen\ by shifting the corresponding zone towards lower temperatures and by increasing the expansion degree[ The phenomenon is more marked for higher insertion of M1¦ in ceria[ "ii# The reduction degree of Ce3¦ is high when the solid solution is formed ð6Ł\ whereas the reduction of inserted Cu1¦ or Ni1¦ decreases[ "iii# For CeCuxOy compounds\ the reduction occurs in two steps and better interaction between Cu1¦ and Ce2¦ occurs at 362 K ð6Ł[ "iv# The amount of hydrogen extracted from the solids reduced at di}erent TT depends on the content of Ni1¦ or Cu1¦ in ceria in the oxide precursors[ Finally\ several hypothesis can be evoked to account for the variation of the cell parameter] the reduction of
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some metallic cations and:or the incorporation of hydro! gen[ Taking into account previous studies which have shown that the hydrogenation mechanism can be described by a nucleophilic attack of the diene which leads to the formation of anionic intermediates ð17Ł\ all the results are consistent with a heterolytic dissociation of H1[ One half of the hydrogen reservoir "H # consists of hydride ions H− and the other half is protons H¦ provided by the hydroxyl groups "OH−#[ Considering \ and 0[21 A \ the relative size of H−\ and O1− "0[43 A respectively#\ the lattice expansion observed in the reduction step under H1 corresponds to the substitution of an O1− species by an H− species even if the con! tribution of some reduced cations has also to be considered[ Therefore\ the insertion in the solid of particular hydrogen species created by heterolytic splitting of H1 "H− located in the anionic vacancy and H¦ forming with an O1− species an OH− group# can be summarized as follows] H1¦O1−¦ : H−¦OH−
"0#
Applied to pure ceria\ the following mechanism for the reduction of CeO1 under H1 has already been proposed ð6Ł] 1Ce3¦¦1O1−¦H1 : 1Ce2¦¦1OH−
"1#
Apart from the reduction of the free MO oxides\ when present\ into the corresponding metal\ the modi_cations registered in the reduction process when nickel or copper is added to the CeÐO system\ are related to the formation of anionic!defected solid solutions\ which can be written as Ce3¦0−XM1¦XO1−1−XX[ In addition to reaction "1# a series of reactions is proposed\ which we suggest accounts for the reduction mechanism that takes place in the solid solution[ For the cerium!nickel system] Ni1¦¦O1−¦H1 : Ni9¦H1O¦
"2#
1Ce3¦¦Ni9 t 1Ce2¦¦Ni1¦
"3#
and for the ceriumÐcopper system] 1Cu1¦¦O1−¦H1 : 1Cu¦¦H1O¦
"4#
Ce3¦¦Cu¦ t Ce2¦¦Cu1¦
"5#
1Cu¦¦O1−¦H1 : 1Cu9¦H1O¦
"6#
Ce3¦¦Cu9 t Ce2¦¦Cu¦
"7#
The reduction of Ni1¦\ Cu1¦ or Cu¦ in the solid solution is much easier than the Ce3¦ reduction\ which contributes to the creation of more anionic vacancies at lower tem! peratures than pure ceria and the hydrogen insertion is increased in the ceria matrix[
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It has been already reported that metallic nickel is able to adsorb hydrogen ð18Ł\ therefore\ for the cerium!nickel system\ in addition to the route described above\ the homolytic dissociation of H1 on Ni9 is also considered] H1 : 1H
"8#
which favours the Ce3¦ reduction according to] 1Ce3¦¦O1−¦1H : 1Ce2¦¦H1O¦
"09#
As the temperature increases the limit of stability of the complex ceria!based phase is reached\ and segregation of the solid solution occurs\ evidenced by the appearance of metallic nickel or copper and by a decrease in the cell parameter\ particularly in the CeCuxOy system[ For CeNixOy compounds\ the transition from zone II to zone III is less evident\ which is explained by the further reduction of Ce3¦ being made easier by the homolytic dissociation of H1[ Above 762 K\ the behaviour of the solids is controlled by the reduction of ceria\ the ~uorite lattice of which is maintained and submitted to a novel variation of the cell parameter while the reactive and extractable hydrogen content decreases[ Therefore\ for treatment temperatures higher than 712 K\ the increase of the lattice expansion is principally due to the reduction of the cations[ Moreover\ the hydrogen reservoir properties\ as pre! sented previously\ largely involve di}usion processes of reactive hydrogen species H [ The change of gas phase to a helium¦isoprene mixture involves a decrease of the activity\ which reaches zero in a time dependent on the reservoir capacity[ This variation of activity is accompanied by a particular variation of the products distribution "Figs 5 and 6#[ The presence of anionic vacancies has been seen necess! ary to obtain catalytic activity[ Evidence has been pro! vided of their role in lattice di}usion phenomena ð02Ł[ By analogy with homogeneous catalysis Siegel has de_ned three di}erent coordinatively unsaturated sites "CUS# with 0\ 1\ or 2 unsaturations on the cation "0M\ 1M\ or 2 M# in chromium and cobalt oxides ð29Ł[ The 2M and 1M sites have been respectively associated with alkadiene hydrogenation and isomerization reactions ð29Ł[ The con! cept has been validated and widely used by Tanaka and co!workers ð20Ł[ Thus if hydrogenation reaction is associ! ated with 2M!sites\ it appears reasonable to correlate isopentane coming from a double hydrogenation of iso! prene with a 2MÐ2M| ensemble\ where each cation is 2 CUS ð5Ł[ In fact\ the XMÐYM| ensemble composed of two cations "with x and y the unsaturation degrees of each cation# is correlated with the combination of the cor! responding reactions "hydrogenation\ isomerisation# occuring on each cation[ In the present study\ for the CeMxOy mixed oxides\ the 2 MÐ2M| ensembles exist because isopentane is obtained among the products[ The 2MÐ2M| ensembles are stable
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under a ~ow of H1¦isoprene at 312 K\ because the reac! tive hydrogen species H consumed by the hydrogenation reaction can be regenerated by heterolytic rupture of molecular hydrogen ð5Ł[ However\ it is highly probable that these elementary ensemble sites with some high degree of unsaturation are unstable in the absence of gaseous hydrogen^ therefore the substitution of H1 by He leads to a sharp decrease of isopentane which reaches zero[ The result obtained is a blocking of the sites associated with the production of isopentane "2MÐ2M|# sim! ultaneously with the creation of 2MÐ1M| sites correlated with 1!methylbut!1!ene formation resulting from a hydrogenation¦isomerisation reaction[ The con! centration of 1!methylbut!1!ene attains a maximum cor! responding to a maxima of 2MÐ1M| sites\ which become 2 MÐ0M| sites before a _nal transformation to 2MÐ9M| ensembles[ When the 2M!site disappears\ the hydro! genation activity cannot be anymore observed[ This phenomenon\ also observed on intermetallic systems ð21Ł\ has been detailed in copper based mixed oxides studies ð5Ł and related to OH group migration[ During the con! sumption of the H species the unsaturation degree of the site varies and decreases[ As a matter of fact\ if the hydrogen H species are for one half H− species\ they are also for the other half H¦ species "coming from OH groups#[ The H species are {pumped| to the surface by the hydrocarbon and consumed by hydrogenation reac! tion\ and this supposes the migration of H− and H¦ species[ It appears that it is the hydroxyl group which di}uses and once the H¦ species consumed\ the O1! spec! ies remains in the anionic vacancy and decreases the unsaturation degree of the site ð5Ł[
4[ Conclusions CeMxOy mixed oxides are shown to be large catalytic hydrogen reservoirs with marked di}usion properties for the hydrogen species[ The hydrogen storage depends on the treatment temperature under H1\ i[e[ to the creation of anionic vacancies as the structure is maintained in the range of the temperatures studied[ The insertion of Ni1¦ or Cu1¦ in ceria leads to a decrease of the reduction temperature of the host oxide and to the expansion of its lattice to a greater extent than the pure CeO1[ There is a correlation between the hydrogen content\ the amount of M1¦ inserted in the ceria matrix and the degree of reduction of the cations[ A mechanism has been proposed based on the formation of anionic vacancies in ceria\ facilitated by the incorporation of transition metal cations\ the heterolytic dissociation of H1 and redox reac! tions between Ce3¦ and the transition element[
References ð0Ł Sohier MP\ Wrobel G\ Bonnelle JP\ Marcq JP[ Appl[ Catal[ 0881^73]058[ ð1Ł Moyes RB[ Hydrogen e}ects in Catalysis\ Ed[ Z[ Paal and PG Menon\ M Dekker\ 0877\ p[0[ ð2Ł Jalowiecki!Duhamel L\ Ponchel A\ Barbaux Y[ Stud[ Surf[ Sci[ and Catal[ 0886^09]272[ ð3Ł Jalowiecki L\ Daage M\ Bonnelle JP\ Tchen A[ Appl[ Catal[ 0874^05]0[ ð4Ł Jalowiecki L\ Grimblot J\ Bonnelle JP[ J[ Catal[ 0889^015]090[ ð5Ł Sene A\ Jalowiecki!Duhamel L\ Wrobel G\ Bonnelle JP[ J[ Catal[ 0882^033]433[ ð6Ł Wrobel G\ Lamonier C\ Bennani A\ D|Huysser A\ Abou! ka(s A[ J[ Chem[ Soc[ Farad[ Trans[ 0885^81"00#]1990[ ð7Ł Diaz H\ Marcq JP\ Pinabiau M\ Barbaux Y[ European Patent no[ 7 497 109\ to BP France\ 0875[ ð8Ł Jalowiecki L\ Wrobel G\ Daage M\ Bonnelle JP[ J[ Catal[ 0876^096]264[ ð09Ł Duhamel!Jalowiecki L\ Sene A\ Wrobel G\ Grimblot J[ Int[ J[ Hydrogen Energy 0874^07]814[ ð00Ł Bevan DJM[ J[ Inorg[ Nucl[ Chem[ 0884^0]38[ ð01Ł Padeste C\ Cant NW\ Trimm DL[ Catal[ Letters 0882^07]294[ ð02Ł Sanchez MG\ Gazquez JL[ J[ Catal[ 0876^093]019[ ð03Ł Herrmann JM\ Hoang!Van C\ Dibansa L\ Harivololona R[ J[ Catal[ 0885^048]250[ ð04Ł de Leitenburg C\ Trovarelli A\ Liorca J\ Cavani F\ Bini[ Appl[ Catal[ 0885^028]050[ ð05Ł Trovarelli A\ de Leitenburg C\ Llorca J\ Dolcetti G[ J[ Catal[ 0884^040]000[ ð06Ł Fierro JLG\ Soria J\ Sanz J\ Rojo JM[ J[ Solid State Chem[ 0876^55]043[ ð07Ł Binet C\ Jadi A\ Lavalley JC[ J[ Chim[ Phys[ 0881^78]20[ ð08Ł Bavan D\ Kordis J[ J[ Inorg[ Nucl[ Chem[ 0853^15]0498[ ð19Ł Otsuka K\ Hatano M\ Morikawa A[ J[ Catal[ 0872^68]386[ ð10Ł Barrault J\ Alouche A\ Paul!Boncour V\ Hilaire L\ Per! cheron!Guegan A[ Appl[ Catal[ 0878^35]158[ ð11Ł Cunningham J\ O|Brien S\ Sanz J\ Rojo JM\ Soria JA\ Fierro JLG[ J[ Molec[ Catal[ 0889^46]268[ ð12Ł Laachir A\ Perrichon V\ Badri A\ Lamotte J\ Catherine E\ Lavalley JC\ Fallah J El\ Hilaire L\ Le Normand F\ Que! mere E\ Sauvion GN\ Touret O[ J[ Chem[ Soc[ Farad[ Trans[ 0880^76]0590[ ð13Ł Perrichon V\ Laachir A\ Bergeret G\ Frety R\ Tournayan L\ Touret O[ J[ Chem[ Soc[ Farad[ Trans[ 0883^89]662[ ð14Ł Ranga!Rao G\ Kaspar J\ Meriani S\ di Monte R[\ Graziani M[ Catal[ Letters 0883^13]096[ ð15Ł Bensalem A\ Bozon!Verduraz F\ Perrichon V[ J[ Chem[ Soc[ Faraday Trans[ 0884^80]1074[ ð16Ł Trovarelli A\ Zamar F\ Llorca J\ de Leitenburg C\ Dolcetti G\ Kiss JT[ J[ Catal[ 0886^058]389[ ð17Ł Daage M\ Bonnelle JP[ Appl[ Catal[ 0874^05]244[ ð18Ł Kacimi S\ Duprez D\ Dalmon JA[ J[ Chim[ Phys[ 0886^83]424[ ð29Ł Siegel S[ J[ Catal[ 0862^29]028[ ð20Ł Tanaka KI[ Adv[ Catal[ 0874^22]88[ ð21Ł Pinabiau M\ Ph[D[ thesis\ Lille\ France\ 0876[
International Journal of Hydrogen Energy 13 "0888# 0972Ð0981
Storage of reactive hydrogen species in CeMxOy "M Cu\ Ni^ 9 ¾ x ¾ 0# mixed oxides L[ Jalowiecki!Duhamel \ A[ Ponchel\ C[ Lamonier Laboratoire de Catalyse Heterogene et Homogene\ U[R[A[ C[N[R[S[ D93919\ Ba¼t[ C2\ Universite des Sciences et Technolo`ies de Lille\ 48544 Villeneuve d|Ascq Cedex\ France
Abstract Interaction of hydrogen with a series of mixed oxides CeMxOy "M Cu\ Ni^ 9 ¾ x ¾ 0# has been studied in the 299Ð 0962 K temperature range[ A dynamic method of titration has been used to provide evidence for reactive hydrogen H present in the solids[ This method of titration involves hydrogenation at 312 K of isoprene under helium ~ow in the absence of gaseous hydrogen[ The hydrogen reservoir capacity depends on the pretreatment temperature under H1 and is related to the creation of anionic vacancies in the solid[ All the CeMxOy mixed oxides are large catalytic hydrogen reservoirs with marked di}usion properties of hydrogen species "H \ OH−#[ In the series studied\ the highest amount of hydrogen H "18[4 09−2 mol g−0# is stored by CeNi9[6Oy pretreated under H1 at 412 K "H5[7CeNi9[6Oy#[ In the conditions in which the incorporation of hydrogen in the solid occurs\ X!ray di}raction analysis under H1 shows shifts depending on the treatment temperature and corresponding to non!negligeable expansion of the ceria!based lattice attributed to the insertion of hydrogen species of hydridic nature in the anionic vacancies[ Þ 0888 International Association for Hydrogen Energy[ Published by Elsevier Science Ltd[ All rights reserved[
0[ Introduction Intermetallic compounds as catalyst precursors have been widely explored[ They are essentially based on rare earth elements "RE# and a transition metal\ and are known to be able to absorb large quantities of hydrogen and form hydrides[ Recently by carrying out di}erent preparation procedures\ in our laboratory it has been shown that a CeNi4 oxide\ prepared by coprecipitation and reduced under H1 could store as much hydrogen as the CeNi4 intermetallic compound\ and was e.cient in hydrogenation of polyunsaturated hydrocarbons "iso! prene\ benzene and toluene# ð0Ł[ This last twenty years\ the key role of hydrogen has become widely evident for energy storage\ in metallurgy as well as in heterogeneous catalysis ð1Ł[ The nature of hydrogen is under controversy[ It is usually admitted that the hydrogen adsorption on metal or other surfaces able to adsorb hydrogen is dissociative[ The problem is if
Corresponding author
the rupture is heterolytic or homolytic[ Therefore\ four di}erent species can be created and transported] the rad! ical H=\ the bonded species H!\ or the charged species H¦ or H−[ This work concerns the interaction of hydrogen with a series of CeMxOy mixed oxides\ precursors of catalysts for hydrogenation reactions ð0Ł and oxydehydrogenation reactions ð2Ł\ using in!situ techniques[ The ability of the reduced solids to store hydrogen and the nature of the occluded hydrogen species will be discussed and a reduction mechanism will be proposed[
1[ Experimental The mixed oxides\ denoted CeMxOy\ in which M rep! resents Cu or Ni\ and x the M ] Ce ratio\ were prepared by coprecipitation of hydroxides from mixtures of cerium and copper or nickel nitrates using triethylamine "TEA# as precipitating agent\ drying at 252 K and calcination in air at 662 K "562 K for CeCu0Oy#[ The loading has been measured by microanalysis[ Pure cerium and nickel
9259!2088:88:,19[99 Þ 0888 International Association for Hydrogen Energy[ Published by Elsevier Science Ltd[ All rights reserved PII] S 9 2 5 9 ! 2 0 8 8 " 8 7 # 9 9 0 3 2 ! 7
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L[ Jalowiecki!Duhamel et al[ : International Journal of Hydrogen Energy 13 "0888# 0972Ð0981
oxides were prepared according to the same method and used as reference compounds[ Thermogravimetric measurements were performed using a Sartorius 3091 microbalance equipped with a ~ow!gas system[ The oxides were treated under a hydro! gen ~ow "1[6 l h−0# and the temperature was increased at a rate of 0[6 K min−0 from room temperature to the _nal temperature of about 862 K[ Measurement of the amount of hydrogen stored in the solid is based on the ability of this class of compounds to hydrogenate 1!methylbut!0\2!diene "isoprene# in the absence of gaseous hydrogen\ according to a dynamic method described previously ð3Ł and applied in our lab! oratory to a large variety of compounds ð4\ 5Ł[ The pre! treatment and catalytic experiments were carried out in! situ at atmospheric pressure in an all!glass grease free ~ow apparatus[ The solid was treated _rst under a pur! i_ed hydrogen ~ow at various temperatures TT "09 h#\ then after elimination of molecular hydrogen at 312 K\ hydrogenation reaction was performed under iso! prene¦helium mixture "9[4) vol[#[ The hydrogenation reaction involves the participation of reactive hydrogen of the solid\ which can di}use from the bulk to hydro! genate isoprene at the surface[ The structure of the oxides during the treatment under H1 were analysed in a Siemens D4999 di}ractometer equipped with an Anton Paar HTK 09 chamber\ and connected to a gas introduction and puri_cation line[ A position sensitive detector was used^ the ~ow conditions and the rate of temperature increase were similar to those adopted in thermogravimetry[ Apart from Ka1 con! tribution eliminated by computer post!processing\ the patterns obtained were not subjected to further treat! ments[ Owing to the use of a platinum holder\ they show platinum di}raction peaks\ the evolution of which with TT was taken into account in determining the exact pos! ition of the peaks of the solids analysed[
2[ Results Interaction of hydrogen with some CeMxOy "M Cu\ Ni^ 9 ¾ x ¾ 4# has already been studied in the 299Ð0962 K temperature range[ XPS\ EPR\ in!situ XRD and ther! mogravimetric techniques have been used to characterize the processes occuring during the hydrogen treatment and the amount of hydrogen occluded when treating the solid at two di}erent temperatures under H1 ð6Ł[ The oxides of CeCuxOy and CeNixOy\ can be described as a mixture of copper oxide or nickel oxide and of ceria modi_ed by the insertion of a part of copper or nickel in its lattice[ The size of the copper oxide or nickel oxide varies considerably from clusters to a crystallized material\ depending on the x!value and on the exper! imental conditions[
2[0[ Anionic vacancies In a previous study\ we reported that after treatment under H1 at 362 or 462 K\ CeMxOy mixed oxides contain anionic vacancies produced by the elimination of H1O "OH groups#[ The thermograms under H1 have been per! formed\ and the results obtained are presented in Fig[ 0[ In agreement with the literature ð6Ł\ up to 262 K\ the weight loss observed corresponds to the elimination of physisorbed water\ while for higher TT anionic vacancies are created[ Moreover\ for temperatures higher than 312 K the observed phenomena are not the exact super! position of those obtained for the pure reference cerium "CeO1#\ and nickel "NiO# oxides and those reported for copper oxide "CuO#\ which has been previously studied ð6Ł[ The thermograms of all the samples show two domains of relatively signi_cant loss of weight\ marked out by the temperatures T0 and T1^ a _rst\ more distinct domain at about the temperature of reduction of NiO or CuO\ and the other one in the 712Ð762 K temperature range "Fig[0\ Table 0#[ The di}erence observed between MO reduction temperature and T0 probably results from the presence in the oxide state of clusters or small MO agregates which are more easily reduced[ Moreover\ in Table 0 has also been reported the reduction rate and it appears that the M1¦ reduction is modi_ed\ depending on the content of M1¦ in the ceria lattice ] redox processes between Ce3¦\ Ce2¦\ M and M¦ or M1¦ have already been demonstrated ð6Ł[ 2[1[ Hydro`en H After treatment under H1 at 362 or 462 K\ CeMxOy mixed oxides contain anionic vacancies and also some reactive hydrogen species able to hydrogenate alkadienes in absence of gaseous hydrogen ð0\ 6Ł[ These species are denoted H as we are not considering their exact charge[ At 312 K under a helium¦isoprene ~ow\ alkadiene hydrogenation occurs on H1 treated CeMxOy mixed oxides as presented in Fig[ 1 for H1 treated at 462 K CeNi9[1Oy[ It is important to note that the hydrogenation activity obtained on the untreated CeMxOy solid is null\ therefore\ these H moities are probably di}erent from OH groups which are known to be present on such cata! lysts[ As a function of time on stream\ the isoprene con! version or hydrogenation activity "HYD# is measured[ The ratio HYDrel AHt:AH9 "where AH9 and AHt are the initial hydrogenation and the hydrogenation activity at time t# can be plotted versus time^ this relative hydro! genation activity at 312 K under helium¦isoprene feed decreases with time[ For each solid a similar curve is obtained and by integrating this curve the extractable reactive hydrogen content of a solid can be determined if the product distribution is taken into account "1 H for monohydrogenation and 3 H for dihydrogenation#[ Moreover the extractable hydrogen content of the solid
L[ Jalowiecki!Duhamel et al[ : International Journal of Hydrogen Energy 13 "0888# 0972Ð0981
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Fig[ 0[ Thermogravimetric pro_les of the CeMxOy compounds treated under pure hydrogen[ NiO "#\ CuO "Ž#\ CeO1 "x#\ CeNi9[4Oy "Ž#\ CeCu9[4Oy "ž#[
Table 0 Temperatures relative to the di}erent weight losses in the mixed oxides and reference compounds Catalyst
T0 "K#
Reduction rate0 ) K−0
T1 "K#
CeO1 NiO CuO CeNi9[1Oy CeNi9[4Oy CeNi9[6Oy CeCu9[4Oy CeCu0Oy
Ð 372 322 337 362 343 282 262
Ð
768 Ð Ð 743 704 706 714 726
9[22 ¼9[33 9[91 9[97 9[02 ¼0 ¼9[7
0 Reduction rate related to weight loss starting at T0 and deduced from the slope of the curve[
is found to be dependent on the treatment temperature TT[ Figs 2 and 3 show the variation in the hydrogen H species concentration as a function of the treatment temperature TT for di}erent CeNixOy and CeCuxOy mixed oxides\ respectively[ In the series studied\ an opti! mum value H 18[4 09−2 mol g−0 is obtained on CeNi9[6Oy for TT 412 K corresponding to the H5[7CeNi9[6Oy compound as presented in Table 1[ The values obtained are very high\ as a matter of fact these mixed oxides are able to absorb higher quantities of hydrogen than the density of liquid hydrogen ð7Ł[ More! over\ for comparison\ one can recall that with this
Fig[ 1[ Relative hydrogenation activity at 312 K under helium! ¦isoprene ~ow versus time on CeNi9[1Oy treated in H1 at 462 K[
dynamic method\ the H content found with CeO1\ and CuO previously treated under H1 at 462 and 362 K\ respectively\ is nil while NiO treated under H1 at 462 K is able to store 12[8×09−2 mol g−0 of H species[ For the CeNixOy catalysts "with x 9[1\ 9[4 and 9[6# some reactive hydrogen species "less than 2×09−2 mol g−0# are inserted into the solid at 312 K "Fig[ 2#\ and at
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L[ Jalowiecki!Duhamel et al[ : International Journal of Hydrogen Energy 13 "0888# 0972Ð0981 Table 1 H content measured on the di}erent catalysts Catalyst CeMxOy
TT "K#
ðH Ł 092 mol g−0
H ] Ce0
CeNi9[1Oy CeNi9[4Oy CeNi9[6Oy CeCu9[4Oy CeCu0Oy
412 412 412 362 262
08[66 08[34 18[42 00[68 06[19
2[7 3[0 5[7 1[4 3[3
0 The atomic weight of the catalysts has been estimated assuming that y 1¦x ] M MCe¦xMM¦"1¦x#MO[
Fig[ 2[ Variation in the hydrogen H species concentration as a function of the treatment temperature TT of CeNixOy with x 9[1 "R#\ 9[4 "ž# and 9[6 "Ž#[
already very high\ maximum for the CeCu0Oy solid "06[1×09−2 mol g−0# and almost maximum for the CeCu9[4Oy compound "00[7×09−2 mol g−0#[ In addition\ between t and tf "time at which the hydro! genation activity is found to be zero# the percentage of reactive hydrogen still present in the solid can be esti! mated[ A plot of HYDrel versus H concentration can be obtained for each TT[ As an example\ in Fig[ 4 the results are reported for the CeNi9[1Oy solid treated at 312 K under H1 and the CeCu9[4Oy solid treated at 562 K under H1[ The curves obtained show clearly that there is no proportionality between the relative hydrogenation rate consuming H with the hydrogen content of the solid\ and therefore the kinetics of H consumption by the alkadiene is a complex phenomenon^ in particular a fast di}usional process of the H species within the solid must
Fig[ 3[ Variation in the hydrogen H species concentration as a function of the treatment temperature TT of CeCuxOy with x 9[4 "R#\ and 0 "Ž#[
this temperature the ability of the solid to store hydrogen decreases when the value of x increases[ For higher TT the hydrogen H storage ability increases when x increases[ Moreover\ the ability of the solids to store hydrogen species is maximum for a treatment temperature of about 412 K and then it decreases for temperatures higher than 512 K[ In the CeCuxOy compounds "Fig[ 3#\ for a treatment temperature TT 262 K the hydrogen H content is
Fig[ 4[ Relative hydrogenation activity at 312 K under helium! ¦isoprene ~ow versus the hydrogen H species concentration of CeNi9[1Oy treated in H1 at 312 K and of CeCu9[4Oy treated in H1 at 562 K[
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be considered ð3\ 8Ł[ A clear analogy exists between these results and those obtained on reduced copper based mixed oxides ð5\ 8Ł which were found to be hydrogen reservoirs[ The variation of isoprene hydrogenation activity as a function of time under helium "Fig[ 1# corresponds to a speci_c hydrogenated products distribution presented in Fig[ 5 for CeNi9[4Oy and in Fig[ 6 for CeCu9[4Oy[ Isoprene hydrogenation activity takes into account all the hydro! genated products\ which are isopentane "dihy! drogenated#\ 1!methylbut!0!ene "1MB0#\ 2!methylbut!0! ene "2MB0#\ and 1!methylbut!1!ene "1MB1# "monohy! drogenated#[ The direct analysis of the curves obtained in Figs 5 and 6 reveals di}erent areas\ as has been already observed and de_ned previously on di}erent copper! based oxides ð5Ł[ When molecular hydrogen disappears completely from the gas phase a sharp decrease of activity is observed[ At the same time the production of iso! pentane also decreases drastically\ whereas the pro! duction of monoenes formed changes[ Maxima are reached successively by 1!methylbut!1!ene and the two methylbut!0!enes[ No simple correlation is found between the time of isopentane disappearance or the appearance of maxima of formation of the various prod! ucts and either the H content[ It can be recalled that similar behaviour is also observed on sul_des "MoS1:Al1O2# for which a good correlation has been obtained between the selectivity and the site structure ð09Ł[ Similar variations of isoprene prod! ucts distribution under helium¦isoprene ~ow as a func!
0976
Fig[ 6[ Isoprene hydrogenated products distribution at 312 K as a function of time under helium on CeCu9[4Oy treated in H1 at 362 K[
tion of time have also been obtained with various H species concentrations[ It has been shown that a small number of active sites are su.cient to produce a large quantity of isopentane when the hydrogen reservoir is large enough[ However\ even though the amount of H hydrogen species in the solid is high\ the formation of isopentane is null when the corresponding active sites do not exist[ 2[2[ In!situ XRD
Fig[ 5[ Isoprene hydrogenated products distribution at 312 K as a function of time under helium on CeNi9[4Oy treated in H1 at 362 K[
The in!situ XRD has been performed on the CeMxOy mixed oxides which crystallize in the ~uorite type phase[ The evolution of the XRD spectra when treated the solid under H1 as a function of the temperature is presented\ as an example\ in Fig[ 7 for the CeNi9[4Oy mixed oxide[ No drastic crystallographic modi_cation is apparent\ indicating that neither the hexagonal Ce1O2 nor the rhombohedral CeO0[71 phase reported by Bevan is formed even at 0962 K ð00Ł[ A careful examination shows that each di}raction line is shifted towards lower angles for treatment tem! peratures higher than 312 K for the CeNixOy compounds and for treatment temperatures higher than 262 K for CeCuxOy compounds[ In order to specify the conditions in which the expanded phase has been formed during the reduction treatment\ the peak position has been plotted as a function of TT for the most intense lines[ Reported in Figs 8Ð01\ is the di}erence D"1u# between the initial peak position "299 K# and its position at TT\ corrected
0977
L[ Jalowiecki!Duhamel et al[ : International Journal of Hydrogen Energy 13 "0888# 0972Ð0981
Fig[ 7[ XRD patterns during in situ reduction under pure hydrogen of CeNi9[4Oy] "P# CeO1\ "# NiO\ "R# platinum and "ž# nickel[
Fig[ 8[ Shift of the CeO1 di}raction peaks in CeNi9[1Oy during in!situ reduction treatment under hydrogen] "# 000\ "r# 199 and "# 200[
from the thermal expansion factor "the D"1u# value is given with an accuracy of 29[91>#[ Each curve has the same pro_le and three zones can be distinguished[ The cell parameter is rather unmodi_ed in zone I "299Ð262 or 362 K#\ while a lattice expansion occurs in zone II "262 or 362Ð732 K# and III "732Ð0962 K#[ Indeed\ the D"1u# values can be considered to be within the experimental error for zone I\ while the angular deviations are
su.ciently large in zones II and III to be linked to a bulk phenomenon[ To compare the thermal behaviour to the structural evolution of the solid\ the temperatures T0 and T1 presented in Table 0 have been shown again in Figs 8Ð01[ The temperature TM\ deduced from XRD\ at which the metallic phase appears has also been precised on the same _gures[ It is worth noting that for CeO1\ these temperatures
L[ Jalowiecki!Duhamel et al[ : International Journal of Hydrogen Energy 13 "0888# 0972Ð0981
0978
Fig[ 09[ Shift of the CeO1 di}raction peaks in CeNi9[4Oy during in!situ reduction treatment under hydrogen] "# 000\ "r# 199 and "# 200[
Fig[ 00[ Shift of the CeO1 di}raction peaks in CeCu9[4Oy during in!situ reduction treatment under hydrogen] "# 000\ "r# 199 and "# 200[
correspond to a more drastic change of the ceria cell parameter "462 and 752 K# or to the maximum of the lattice expansion in zone II "625 K#[ For a comparison of the di}erent D"1u# curves\ it appears that zone I shown for CeO1 has disappeared\ while the expansion of the ceria lattice at about 769 K reaches and remains at a similar level[ The main change occurs in the _rst domain of the expansion of the ceria lattice which is maintained "zone II#[ Considering in more detail zone II\ it is clear that several parameters in~uence the phenomena which occur in the bulk of the ceria!like phase] "i# the nature of the transition metal M[ While the NiÐCe association leads to only one maximum of D"1u#\ zone II shows two maxima for the CeCuxOy compounds[ This factor also a}ects the position of this zone in the temperature scale[ "ii# The
M ] Ce atomic ratio^ the x!value also a}ects the tem! perature corresponding to the maximum of D"1u#[ Besides\ the relative position of TM and T0 on the di}erent curves leads to the following observations] "i# Whatever the M ] Ce ratio\ T0 coincides with the beginning of the dilatation of ceria cell[ "ii# When the solid solution is the only phase detected by XRD in the oxide precursors\ the appearance of the metallic phase is associated with the end of zone II and sometimes to a decrease of the crys! tallographic parameter of ceria ð6Ł[ "iii# When MO is also present\ TM is shifted to T0\ and in addition to the reduction of the solid solution\ free MO transforms into metal[ The di}erence observed between T0 and TM implies that the metallic cation is preserved in the modi_ed ceria phase at least for temperatures lower than TM[ At higher temperatures\ a segregation of the solid solution occurs
0989
L[ Jalowiecki!Duhamel et al[ : International Journal of Hydrogen Energy 13 "0888# 0972Ð0981
Fig[ 01[ Shift of the CeO1 di}raction peaks in CeCu0Oy during in!situ reduction treatment under hydrogen] "# 000\ "r# 199 and "# 200[
which explains the somewhat similar behaviour of CeMxOy and CeO1 above 762 K[
3[ Discussion The ceria!based catalysts are interesting for water gas shift reactions ð01Ł\ CO ð02Ł and hydrocarbon oxidation ð03\ 04Ł[ It appears that the common parameter for these applications is the participation of surface oxygen spe! cies:anionic vacancies ð05Ł[ As a matter of fact\ on ceria based oxides\ it has already been shown that the elim! ination of lattice oxygen under H1 leads to the creation of anionic vacancies ð06\ 07Ł[ And\ it is well known that CeO1 is able to store oxygen and to release it under reducing atmosphere leading to a CeO1!X compound ð08Ł[ Moreover\ the ceria reduction and reoxidation rate is increased on metals of group VIII supported on ceria catalysts ð19Ł[ Besides\ the interaction of hydrogen with cerium based oxides has also been largely studied[ It has been shown that ceria is able to store hydrogen and that the hydrogen storage depends on the reduction temperature under H1 ð0\ 6\ 8\ 03 Ð16Ł[ Moreover\ Otsuka et al[ have analysed the reduction and reoxidation rates on cerium oxide and a proportionality has been obtained between the quantity of H1 produced during the oxidation step and the con! centration of anionic vacancies created during the reduction step ð19Ł[ Fierro et al[ estimated that during the reduction process under H1\ the incorporation of hydrogen in ceria and the elimination of lattice oxygen can a}ect in an opposite way the mass of ceria ð06Ł[ On the CeMxOy mixed oxides\ the hydrogen treatment leads to the creation of anionic vacancies and to the insertion into the solid of hydrogen species[ Evidence is provided for the existence of particular reactive hydrogen
species of the solid[ Even\ if the hydrogen H uptake by the CeMxOy mixed oxides has already been reported in the literature ð0\ 6Ł\ only two treatment temperatures TT have been studied[ In the present study the evolution of the hydrogen H content has been analysed as a function of the treatment temperature TT and for slightly di}erent solids "coprecipitation in TEA and calcination at 662 K#[ The H species are able to hydrogenate alkadienes at 312 K without the presence of H1 when the solid has been previously treated under H1^ otherwise no H species is found[ Hence\ the hydroxyl groups "OH# always present on the solid cannot justify the results obtained[ Moreover the H species concentration depends on the treatment temperature under H1\ almost no H is found for TT lower than 242 K and the H content increases with the treatment temperature up to about 262Ð412 K depending on the solids[ For the CeMxOy mixed oxides\ the main experimental results observed in the temperature range corresponding to the _rst expansion of the ceria!based lattice "zone II] 262 or 362Ð732 K#\ can be summarized as follows] "i# the insertion of Cu1¦ or Ni1¦ in ceria favours the expansion of the ~uorite lattice under hydrogen\ by shifting the corresponding zone towards lower temperatures and by increasing the expansion degree[ The phenomenon is more marked for higher insertion of M1¦ in ceria[ "ii# The reduction degree of Ce3¦ is high when the solid solution is formed ð6Ł\ whereas the reduction of inserted Cu1¦ or Ni1¦ decreases[ "iii# For CeCuxOy compounds\ the reduction occurs in two steps and better interaction between Cu1¦ and Ce2¦ occurs at 362 K ð6Ł[ "iv# The amount of hydrogen extracted from the solids reduced at di}erent TT depends on the content of Ni1¦ or Cu1¦ in ceria in the oxide precursors[ Finally\ several hypothesis can be evoked to account for the variation of the cell parameter] the reduction of
L[ Jalowiecki!Duhamel et al[ : International Journal of Hydrogen Energy 13 "0888# 0972Ð0981
some metallic cations and:or the incorporation of hydro! gen[ Taking into account previous studies which have shown that the hydrogenation mechanism can be described by a nucleophilic attack of the diene which leads to the formation of anionic intermediates ð17Ł\ all the results are consistent with a heterolytic dissociation of H1[ One half of the hydrogen reservoir "H # consists of hydride ions H− and the other half is protons H¦ provided by the hydroxyl groups "OH−#[ Considering \ and 0[21 A \ the relative size of H−\ and O1− "0[43 A respectively#\ the lattice expansion observed in the reduction step under H1 corresponds to the substitution of an O1− species by an H− species even if the con! tribution of some reduced cations has also to be considered[ Therefore\ the insertion in the solid of particular hydrogen species created by heterolytic splitting of H1 "H− located in the anionic vacancy and H¦ forming with an O1− species an OH− group# can be summarized as follows] H1¦O1−¦ : H−¦OH−
"0#
Applied to pure ceria\ the following mechanism for the reduction of CeO1 under H1 has already been proposed ð6Ł] 1Ce3¦¦1O1−¦H1 : 1Ce2¦¦1OH−
"1#
Apart from the reduction of the free MO oxides\ when present\ into the corresponding metal\ the modi_cations registered in the reduction process when nickel or copper is added to the CeÐO system\ are related to the formation of anionic!defected solid solutions\ which can be written as Ce3¦0−XM1¦XO1−1−XX[ In addition to reaction "1# a series of reactions is proposed\ which we suggest accounts for the reduction mechanism that takes place in the solid solution[ For the cerium!nickel system] Ni1¦¦O1−¦H1 : Ni9¦H1O¦
"2#
1Ce3¦¦Ni9 t 1Ce2¦¦Ni1¦
"3#
and for the ceriumÐcopper system] 1Cu1¦¦O1−¦H1 : 1Cu¦¦H1O¦
"4#
Ce3¦¦Cu¦ t Ce2¦¦Cu1¦
"5#
1Cu¦¦O1−¦H1 : 1Cu9¦H1O¦
"6#
Ce3¦¦Cu9 t Ce2¦¦Cu¦
"7#
The reduction of Ni1¦\ Cu1¦ or Cu¦ in the solid solution is much easier than the Ce3¦ reduction\ which contributes to the creation of more anionic vacancies at lower tem! peratures than pure ceria and the hydrogen insertion is increased in the ceria matrix[
0980
It has been already reported that metallic nickel is able to adsorb hydrogen ð18Ł\ therefore\ for the cerium!nickel system\ in addition to the route described above\ the homolytic dissociation of H1 on Ni9 is also considered] H1 : 1H
"8#
which favours the Ce3¦ reduction according to] 1Ce3¦¦O1−¦1H : 1Ce2¦¦H1O¦
"09#
As the temperature increases the limit of stability of the complex ceria!based phase is reached\ and segregation of the solid solution occurs\ evidenced by the appearance of metallic nickel or copper and by a decrease in the cell parameter\ particularly in the CeCuxOy system[ For CeNixOy compounds\ the transition from zone II to zone III is less evident\ which is explained by the further reduction of Ce3¦ being made easier by the homolytic dissociation of H1[ Above 762 K\ the behaviour of the solids is controlled by the reduction of ceria\ the ~uorite lattice of which is maintained and submitted to a novel variation of the cell parameter while the reactive and extractable hydrogen content decreases[ Therefore\ for treatment temperatures higher than 712 K\ the increase of the lattice expansion is principally due to the reduction of the cations[ Moreover\ the hydrogen reservoir properties\ as pre! sented previously\ largely involve di}usion processes of reactive hydrogen species H [ The change of gas phase to a helium¦isoprene mixture involves a decrease of the activity\ which reaches zero in a time dependent on the reservoir capacity[ This variation of activity is accompanied by a particular variation of the products distribution "Figs 5 and 6#[ The presence of anionic vacancies has been seen necess! ary to obtain catalytic activity[ Evidence has been pro! vided of their role in lattice di}usion phenomena ð02Ł[ By analogy with homogeneous catalysis Siegel has de_ned three di}erent coordinatively unsaturated sites "CUS# with 0\ 1\ or 2 unsaturations on the cation "0M\ 1M\ or 2 M# in chromium and cobalt oxides ð29Ł[ The 2M and 1M sites have been respectively associated with alkadiene hydrogenation and isomerization reactions ð29Ł[ The con! cept has been validated and widely used by Tanaka and co!workers ð20Ł[ Thus if hydrogenation reaction is associ! ated with 2M!sites\ it appears reasonable to correlate isopentane coming from a double hydrogenation of iso! prene with a 2MÐ2M| ensemble\ where each cation is 2 CUS ð5Ł[ In fact\ the XMÐYM| ensemble composed of two cations "with x and y the unsaturation degrees of each cation# is correlated with the combination of the cor! responding reactions "hydrogenation\ isomerisation# occuring on each cation[ In the present study\ for the CeMxOy mixed oxides\ the 2 MÐ2M| ensembles exist because isopentane is obtained among the products[ The 2MÐ2M| ensembles are stable
0981
L[ Jalowiecki!Duhamel et al[ : International Journal of Hydrogen Energy 13 "0888# 0972Ð0981
under a ~ow of H1¦isoprene at 312 K\ because the reac! tive hydrogen species H consumed by the hydrogenation reaction can be regenerated by heterolytic rupture of molecular hydrogen ð5Ł[ However\ it is highly probable that these elementary ensemble sites with some high degree of unsaturation are unstable in the absence of gaseous hydrogen^ therefore the substitution of H1 by He leads to a sharp decrease of isopentane which reaches zero[ The result obtained is a blocking of the sites associated with the production of isopentane "2MÐ2M|# sim! ultaneously with the creation of 2MÐ1M| sites correlated with 1!methylbut!1!ene formation resulting from a hydrogenation¦isomerisation reaction[ The con! centration of 1!methylbut!1!ene attains a maximum cor! responding to a maxima of 2MÐ1M| sites\ which become 2 MÐ0M| sites before a _nal transformation to 2MÐ9M| ensembles[ When the 2M!site disappears\ the hydro! genation activity cannot be anymore observed[ This phenomenon\ also observed on intermetallic systems ð21Ł\ has been detailed in copper based mixed oxides studies ð5Ł and related to OH group migration[ During the con! sumption of the H species the unsaturation degree of the site varies and decreases[ As a matter of fact\ if the hydrogen H species are for one half H− species\ they are also for the other half H¦ species "coming from OH groups#[ The H species are {pumped| to the surface by the hydrocarbon and consumed by hydrogenation reac! tion\ and this supposes the migration of H− and H¦ species[ It appears that it is the hydroxyl group which di}uses and once the H¦ species consumed\ the O1! spec! ies remains in the anionic vacancy and decreases the unsaturation degree of the site ð5Ł[
4[ Conclusions CeMxOy mixed oxides are shown to be large catalytic hydrogen reservoirs with marked di}usion properties for the hydrogen species[ The hydrogen storage depends on the treatment temperature under H1\ i[e[ to the creation of anionic vacancies as the structure is maintained in the range of the temperatures studied[ The insertion of Ni1¦ or Cu1¦ in ceria leads to a decrease of the reduction temperature of the host oxide and to the expansion of its lattice to a greater extent than the pure CeO1[ There is a correlation between the hydrogen content\ the amount of M1¦ inserted in the ceria matrix and the degree of reduction of the cations[ A mechanism has been proposed based on the formation of anionic vacancies in ceria\ facilitated by the incorporation of transition metal cations\ the heterolytic dissociation of H1 and redox reac! tions between Ce3¦ and the transition element[
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