rogen Adsorption on the Electron Paramagnetic Resonance
ysts Containing Chromium Oxidel Lurance M. Wehber 2% Chamialry Department, Universitu of California, Santa Barbara, California
(Received January 12, 1978)
A comparison of the epr spectra of reduced chromia gel, a-chromia, and 9.5 mol % chromia/alumiiia impregnated catalysts, measured in a vacuum and in a hydrogen atmosphere, showed that hydrogen adsorption had n strong influence on the Cr(I1I) ions believed to be responsible for the /3-phase signal. Adsorption of hydrogen on chromialalumina caused an increase in the epr line width, relative intensity, and Weiss constant, thus indicating that the number of Cr(I1I) ions had increased. Hydrogen adsorption on samples of chromia gel and a-chromia caused a decrease in the epr line width, relative intensity, and Weiss constant, indicating that the number of Cr(II1) ions had decreased. Therefore, it may be suggested that hydrogen adsorption on previousiy reduced and evacuated chromia/alumina causes surface oxidation of Cr(I1) to Cr(III), and that hydrogen adsorption on reduced and evacuated chromia gel and a-chromia causes the oxidation of Cr(II1) possibly to Cr(1V-j. The presence of an 27Alsuperhyperfinestructure on the 0-phase Cr(I1I) epr signal indicated that the lower oxidation state shown by chromia/alumina is related to direct interaction between chromium and adjacer t aluminum ions.
Introduction The epr spectra of catalysts containing chromium oxide have received attent)ion as shown by a review by Poole and ;\iIacIver.' RIost papers on this subject have dealt with chromium oxide supported on aluminum oxide. Most -c7-orkersagree that there are three major electronic configurations, or phases, that give rise to the epr speccrum of chromia/alumina catalysts prepared by impregnation. These are the 6 phase, which is thought to arise from Cr(l1I) ions that are isolated from other chromium ions; the y phase, attributed to Cr(V) ions; and the @ phase, attributed Cr(II1) ions which show a degree of antiferromagnetic coupling with adjacent Cr(ll1) ions. The p-phase signal dominates the spectrum of typical reduced chromia/alumina eatalysts. The only dgnal reported that can be attributed to chromium ions on unsupported chromia catalysts is thc ,E phase. This paper reports the results of expcriments undertaken to determine the influence of hydrogen on the @-phase resonance signal for some reduced chromium oxide catalyses.
~
x ~ ~ r i ~ ~ n ~ ~ ~ Pwparlztzon oj Samples. Chromia gcl was prepared ~~~~~~~
as described by Ciapetta and P a r k 3 Aqueous ammonia was added to a Cr(N03)Z solution. The resultiiig gelatinous precipitate was washed, dried, and heated in vacuo at ,180". The product had a specific surface (RET, N,) of about 200 m2, and it gave only a diffuse X-ray cliff raction pattern. The a-ohromia as prepared by heating the gel, prepared as described above, in vucuo a t 550" for 10 hr. The specific ~urfaccwas about 30 m2, and the diffraction pattern vvasthat of pure a-chromia. The Journal of Fh&cai
Chemistry, Vol. 76, N o . 19, 197f
Supported chromia was prepared by the impregnation of y-alumina with a Cr(XOs), solution. The alumina was obtained by the hydrolysis of triple-distilled aluminum ieopropoxide, followed by drying a t 625". The concentration of the Cr(N:O& solution was chosen to yield a 9.5 mol yochrornia content in the finished catalyst. The resulting emulsion was placed in a rotary evacuator and the moisture was drawn off by an aspirator. The specific surfacc was about 1208 m2, and the X-ray diffraction pattern that of y-alumina. Pretreatment. Furthcr pretreatment of all samples was done in situ. The chromia gel was heated in flowing hydrogen for 12 hr at 4QO", fallom-ed by evacuation for 12 hr a t 400". The a-chrornia was treated in exactly the same way. The chromia-alumina sample was evacuated for 2 hr a t 40Q0,treated in flowing hydrogen for 2 hr at 400", and finally evacuated again for 2 hr at 400". More severe pretreatment of one sample is described later. Chromia gei was pretreated for a, longer time because it was considerably more difficult to remove watcr and other adsorbed gases from this preparation. Increasing the time for chromia--alumina resulted in darkening the sample. Epr Measurements. Epr spectra were obtained on evacuated aamples after they had been cooled to the desired temperature. For measurements in a hydrogen atmosphere, the samples were exposed to hydrogen a t 1 atm, then heated to 300", and blowly cooled to (1) Extracted from the dissertation of the author, submitted in partial fulfillment of the requirements for the degree of Doct,or of Philosophy i n Chemistry at the University of California, Santa Barbara, 1970. (2) C. P. Poole and D. S. iMacIver, Advan.. CaEaZ., 17, 223 (1967). (3) E. C. Ciapetta and C. J. Park, "Catalysis," Vol. 1, P. W.Emmett, Ed., Reinhold, Baltimore, Md., 1954, p 342.
room t e m p e r a t ~ r ~fore the measurements were made. followed to ensure complete sureen the sample and hydrogen.4 ere inade on a Varian V-4502 milh ~ 1 ,Varian Vo ~ ~ and~ contrrol ~ tu ~ o ~ ~ ~ r ~ were t ~ obtai ~ e s ~ ~ i ~ e r a t ~cod ~ I rrol e unit. fled to permit evacu,.ttion Thn? epr signal of 4 in01
of ~
~~~~~~~~
~ spins c ~o n t r ~ b ~ t ~ton gthe~ by comparison ~ with a~ sample c~ilsojnria/alumina,which was treated by or 2 hr, followed by hydrosample was chosen as a percentage of chromium readily forxnetlj5and exhis is shown by the line which was 3100 G at 40". khat the line width of the pjpole-di pole intersetion
vlc :\J ~
v&
~
he epr spectra of samples of chromia gel, a-chromia, ere measured a t various The /?-reso~ ~ e ~ ~ ~in ~OUFUO ~ ~and a tin~hydrogen. r e s ~~a~~~ signal. present) in each case was symmetrical and of Imenteian line shape. The value of the g factor was nlcm 2 . Tlne1.e wa,s no variation of the g factor that (mdd be a t t r ~ ~to~ either t e ~ the variation of temper&turf?or to change of atmosphere. T h e spectrum of the chromia/alumina sample &ov ed weak 6 - a t i d v-phase resonance signals, and the ~ p of chromia ~ ~ and~of a-chromia r ~ contained weak gel migra;als which w m sttributed to color centerg. A of atmos~herehad no detectable effect on these in liize tempera lure ranges reported here. 'he ~ a r ~ a t ofi othe ~ epr line width with temperature -phase re:m;r$aiicesignals is shown by Figure 1. ataorn wm m w u r e d with the sample in vacuo drogeu aitorrusphere. The Pine width for the a l slairple ~ was~broadened ~ by~ the ad~ sorplioa of The line width was narrowed by h-cid~ogenfor chi*omiagel and for a-chromia. The inarrowing for a-d i i r m i i i f i was quite small owing, probably, Lo the ~rnailspaific surface of this sample. Figure 2 shows the garintion with temperature of the (3 iiitmnii,i~:s For all samples, the relative intensity, N was ~ ~ lrom the~ equation ~ c ~~~~~~~~~~~~
%
* I
K~AR2,,
t ~
~ ~
~
n
1
~ 1
~ ~
~
8
-200
0
2at
Ternparotwe P G )
Figure 1. Temperature dependence of t h e peak t o peak (derivative) line width of chromia gel (6 vac, 0 Hz), and 9.5 mol % chromi
sity for the chromia/alumina sample. relative intensity due to the adsorption of hydrogen on the chromia/alumina sample is S ~ Q W X I more readily by the reciprocal of the relative intensity shown in Figure 3. The epr relative intensity is a meamre of the total number of electronic spins in the system and is, therefore, a mcasure of the magnetic s u s c e p t i ~ ~ ~ ~Thus, ty. a plot of the reciprocal of the relative intensity should result in a straight line for a substance which follows the Curie-Weiss law. The reciprocal intensities of the /?-resonance signals plotted In Figure 3 were reasonably satisfactory straight lines, and thus, followed the Curie-Weiss law. a-Chrornia showed a deviation from linearity as the temperature of the NBeE point (34") was approached owing to the pesoisieence of antiferromagnetic properties.* The ~ ~ ~ ~ ~sam-~ n ~ a ple in hydrogen deviated from linearity below 40". ~When the Curie-Weiss law is followed, the slope and intercept) of the lines resulting from reciprocal plots are important constants which can aid in. the ~ ~ t ~ ~ ~ r e t ~ t i o n of magnetic data. The slope is khe Cur the intercept is the Weiss constant. intercepts of the linear portion of the r.eelproca1 relative intensity curves are shown in Table 1, The Curie constants were essentially unchanged by the adsorption of ~ ~ ~ ~ e ~ hydrogen. However, the adsorption of ~ ~ ~ 7 ~ r o g e ~
(1)
whew d is the signal amplitude, AH,,, is the peak to peak line width, and K 18 a Constant, 1.8, for a curve of Lorentzian ~ ~ s ~ r ~ Adsorption ~ ~ ~ i oof ~ hydrogen . ? decreased the ~ ~ ~ ofe then &resonance ~ ~ ~ signals y of chromia, gel and of n-dxwmia, and increased the inten-
~
(4) P. W. Selwood, 3. Amer. Chem. Sac,, QZ, 39 (1970). ( 5 ) R. P. Eischens and P. W. Selwood, ibid.,69,2698 (1947). (6) C. P. Poole and J. F. Itzel, Jr., J . Ch,em. Phus., 41, 287 (1964). (7) Varian Associates Technical Bulletin for tho V-4302 epr spec-
trometer. (8) Y . Y. Li, Phys. Rev., 84, 7 2 (1951). The Journal of Physical Chemistrg, Val. 76, No. 19, 1978
---Table I : Slopes and Intercepts of the Recipsorak oE the Relative Intensity Curves
Substance
Chromia gel a-Chromia Chromia/alumina
-200
200 Temperature PC)
Figure 2. Temperature dependence of the relative intensity of chromia gel (a vac, 0 Hz), a-chromia ( 0 vac, 0 €It), and 9.5 mol r0 chrornia/ali.imina (A vac, A HZ).
Slopc (Curie constant) -----.X 10 Vacuum Hydrogen
2.8 5.1 0.38
2.8 4.9 0.35
hteroept 4 W e i s s eonstant)--. Vacuum Hydrogen
-273 -300 -105
-310 -369 -38
hydrogen w a s probably not adsorbed. A slight flattening of the left lobe of the @-phaseresonance signal of this chromialalumina sample was observed as is shown by the spectrum of Figure 4. Examination of this spectrum under high amplification revealed a new signal which is shown by Figure 5. It was observed to be split into six lines representing the hyperfine splitting by a nuclear spin of 5 / 2 . It i R believed that this signal represents the superhyperfine couptirig of the Cr(lI1) unpaired electron spin with thc ‘E7AInucleus. Interactions between chromium electron spins and aluminum nuclei have been ~ ~ ~ ~ by e cnmr t ~techd niques, ¶
Discussion A comparison of the epr spectra of the chromium
l
e -
L--L.---L..-
-200
200
Temperature P C )
Figure 3. Temperature dependence of the reciprocal of the relative intensity of chromia gel ( a vac, n I&), a-chromia ( 0 vac, 0 Hz), and 9.5 mol % chromia/alumina (A vac, A 13%).
caused the WTeiss constant to become more negative for chromia gel and or-chromia and more positive for chromialalumina. The epr spectrum for the chromia/alumina sample was also measured after the following special pretreatment. The sample was heated in flowing hydrogen a t 500”, followed b,y evacuation a t 550” for 24 hr. The relative intensity of 1 he $-phase resonance signal now showed only a slight increase after hydrogen was readmitted. There was no change in the line width and no change in the Curie ox Weias constants. Thus, hydrogen had little influence on the epr spectrum of this sample and T A e Journal of Physical Chemistry, V o l . 76,No. 19, 1972
oxide catalysts measured in vacuo and i n a hydrogen atnzosphere shows that hydrogen bas an influence on the Cr(I1I) ions that are responsible for the p-phase resonance signals. However, this inAuevlce is a1tered by the presence of an alumina support. For the chromia/alumina sample the relative intensity, line width, and Weiss constant all irxcreased with the adsorption of hydrogen. The relative intensity is a measure of the number of spins of Cr(P11) ions that contribute to the signal. Thus, its increase indicates that the number of Cr(lI1) ions was increased. The line width of the p-phase ~ ‘ e s o ~ ~ ~signal z n c e is strongly influenced by the interaction between paramagnetic ions. The widths are broadened by dipoledipole interaction and narrowed by exchange interaction. The latter process accourits for the differences between chromia/alumina, chrornia gel, and or-chromia. Adsorption of hydrogen on the surface should have ljttle direct influencc on this process, but, if the sdsorption of hydrogen increases the number oi Cr(Id1) ions contributing to the signal, the signal could be broadened by the increase in the number of ion9 pa,rticipa,ting in the dipole-dipole broadening procem. The TVeiss constant calculated from the epr relative intensity for the chrornia/aluwaina sample became more positive on hydrogen adsorption. The Weiss constant, like the line width, is ~tronglydependent on exchange interaction, but the adsorption of hydrogen (9) D. E. O’Reilly and C. P. Poole, 6.Phg/s. Chem,, 67, 1762 (1963).
HYDIN~GEN ADSORPTION ON CATALYSTS CONTAINING CHROMIUM OXIIIE
2697
a-chromia influenced their epr spcctra in a rcvcrse manner from that on chromia/aluinirra. The liiir width, relative intensity, and Wrisr constant were all decreased by hydrogen adsorption. If tlir same arguments as applicd for chrornia-alumina arc urcd hcrc, then these decreases resultd iruin a d r a c m e in the number of Cr(II1) ions. Table II h m s a eoniparisoii of thc numFm of surface Cr(I1I) ions per gram ns tlderniirml from nims and vpr. The per cent surfact. From ~ m s s and surface was deterrriincd trom t>stnnatcs ol' thc number of nurfacc ions per gram a:\ hugg wood lo Thc total Cr(JI1) ion.i from ~p u 26 00 moo m o niincd from thc lotul calibrated rch,tivc, intensit y of H (gauss1 thr epr signals mcasurcd at SO', and ihc htirfacr wtiFigiue 4. 1Spr spectrum of 9.5 mol % chromia/alumina matt>': from cpr wen' bawd on thc ci-i;l.ngc~ili rclat ivc 4ww1ng the flattening of the left lobe of the p phase 5ignal intmnity rcwiting froni hydrog(m acf~orpticmat hO" mused b y the "A1 superflyperfine structure. 111 all case8 thc number of Cr(Ti1) ions from cur compares favorab!y ~ i t the h numbrr ~f ( ir(II!) i u w from inasr and surfacc.. Thv gren ditfcwncr i n surface perccritugi: i':that ~ h r i oby ~ thc u-clir~wia Thiq diffcrance is prubably dues t o ii; m a i l C,iirfaw arra and small difference in cipr intenhrtv requltang from )tion. The r.iomiw3 C J ~ thc3 p t CCLIT, ~ minctl by thr tn o mt.1 hod^, sliovvs that it ia reamiablr lo coiicludi~that thc etritnqc- in tIhc t.pr intensity rcwlring from hydrogtm ,zti*orption IS C ~ U P to a change in the numbtlr uf Cr(II1) ;oxis couiributing to the epr signal.
Table 11: Coinpttrisoii of the Number of Cr(I1I) 10119 per Grnni a? llctcrmined from Mas\ with Those I>eterniined f r o m the Epr Intensity Total
Cr(11I)
x
From mass and surface Chrornia gel
X 500
a-Chromia I_
I
1
I
2700 H
I
'
21300
(gaurr)
Figure 5. An amplification of the left lobe of the p phase epr signal of 9.5 mol chromia/rtlumii~~~ showing the Z'Al
wperhyperfine strrirture.
should not directly influence the interaction between chromium ions. Howver, since the Weiss constant is thc intercept of the reciprocal intensity, and an increase in the rc!ativv intcnsity results in a decrcasc in itr reciprocal, the intercept must become more positive if t4hc ~lopcsor Curie constant does riot change. Thus, the jncrraw i n the Wriss constant can also bc attributcd to art increase in the number of Cr(II1) ions which contributc to the signal. The adsorption of hydrogen on chromia gel and
Chromia/alumina From epr" intensity (80") Chromia gel
a-Chromia Chroinia/aluminu
lo-"
7.9 7.!+ 0.37
Surface Cr(III), rr,
8.2 1.2 :i
4.0
8.1
5.1 0.22
13.4 11.2
a Surface estimates from epr me based on a decrease of intensity on hydrogen adsorption for chrornia gel and U-chromia, an increase for supported.
Van Iiciien, et nl.," using magnrlic susceptibility and cpr methods, Fhowd that Cr(I1) ions wcrc present on thr surface of reduced chromia,lalurnina catalysts. The reduction of the catalyst produced Ck(I1) ions after rvacuation. Cr(1I) ITith its cvcm numbrr of (IO).'1 W. Selwood, J . Amer. Chrm. Soc., 88, 2676 (196W (11) L. I,. Van lieijen, \ti. M .l f . S:iclitler, 1;. Cossee, and I). M . Brower, I'roc. Int. Con/. Catal., I I , Yrd, 1963 (1965).
LTJRANCE M. WEBBER
2698 unpaired electrons cannot be detected by epr methods, but, if the evacuated surface chromium ions are in the 2-t- oxidation d a t e , then a change in oxidation stale would lead to an odd number of electrons which can be detected bv epr. The increase in the number of Gr(Pl1) ions indicates that this did happen. Thus, a rriechanism l o a the adsorption on previously reduced arid evacuated chromia- alumina surfaces can be proposed. It is suggested that the adsorption of hydrogen leads to the oxidation of Cr(1I) to Cr(II1) accompanied bs the production of a surhce hydride ion. Eii-
The increase in the number of Cr(lli1) ions due to this oxidative mechanism wouid result in the increase of the epr relative intensity, line width, and Weiss constant as describeo above. Further reduction of the CriII) to C'T(T) is not considered since Cr(I), which has an odd number of unpaired electrons, would be detected as a new signal by epr. This would not result in an increase in the p-phase signal The adsorption of hydrogen on the reduced and evacuated chromia gel and a-chromia surfaces caused a decrease ~rtthe number of CriITI) ions instead of an increase as was observed with chromia/alumina. The same oxid:itive mechanism can account for this decrease, that is, the oxidation of Cr(lI1) to Cr(1V) if the chromjum ions on the evacuated surface are in the 3$ oxidation state instead of 2 + . H(3)
The oxidation of Cr(lI1) to Cr(1V) would result in a decrease in the number of Cr(1II) ions and a decrease in the epr intensity, line width, and Weiss constant. The changes in the epr spectra due to hydrogen adsorption described in this paper suggest a gain or loss in the concentration of surface Cr(II1) ions. As suggested by Kazanskii and S h v ~ t s , ' ~these ~ ' ~ changes can also be broriglit about by changes in symmetry or coordination which would change the crystal field experienced by the Cr(lI1) ions. The relaxation time would also be changed and the epr spectrum of Cr(II1) may not be observable under these circumstances. They believe, through their interpretation of epr and optical spectra, that the reduced and evacuated chrornia/alumina surface ions are Cr(II1) in a square pyramid$ coordination rather than Gr(I1) as suggested by Van Keijen, et al., but, as pointed out by Burwell, et ~ 2 1 the . ~ square ~ ~ pyramidal Cr(lJ.1) should be easier to reduce to Cr(1I) which is more stable In a square planar coordination. Furthermore, the theory of Kazanskii does not explain the different effects on the The Journal of Physical Chemistry, Val. 76, NO.19, 1.972
epr spectra caused by the adsorption of hydrogen on chromia and chromia/alumina 8,s reportcd in 1his paper. Chromia gel and a-chromia interact with hydrogen in a different way than chromia/alunha. A clue to the reason for this difference C ~ be I found by the examination of the epr spectrum of the sample which gave rise to Figure 4. This sample was pretrcated at temperatures above 500" compared wiLh 400" for the samples described above. The action of' hydrogen on the epr spectrum of this sample, after evacuation, had little or no effect on the Rim width, relathe intensity, or FVVeiss constant showing that hydrogen ,~~ was probably not adsorbed. Voltz and W e l I e ~using volumetric methods, found li ttXe hydrogen adsorption on a chrornia/alumina sample wii h srmilar pretreatment. The hydrogen atmosplzc~lic*had no effect on the epr spectrum of this sample; however, there was an 27A1 hyperfine structure present on the 8-phase signal as shown by Figure 5 . 'The prescnee of a hyperfine structure shows that there i s a strong interaction betwecbn the chromium and aluminum ions. This hyperfine, though not present on the samples where the epr spectra were influenced by hydrogen, suggests that the alumina support may be respoahible for the lower oxidalion state experienced by c ~ ~ ~ i ~ ~ i ~ ~ ~ l u That I S , the interaction between the aiumaraum ions and the chromium ions may bring about conditions \\.hi& favor the formation of Cr(l1) ions. The mechanism of the process which brought about the interaction between chromium and the alumina support and the resulting hyperfine structure is not lmowxi, however, it is suggested lhat free electrons, which should have been present on hhe oxygen-deficient evacuated chromia/alumina rurface Esrming color centers, may have been transferred to the Cr(III1) ions forming Cr(1I) ions. This was indicated by the lack of an epr signal which could be aktributed to free electrons O K ~the chromia/alumina surface, unden conditions where it was found on pure y-aluminn and pure chromia. However, more research ir necessary to further characterize the hyperfine signal and deter/ a ~ ~ ~ n ~ mine i Ls influence on the c ~ r o ~ ~ ~ surface reactions.
Conclusion The results reported in this paper show that the @-phaseresonance signals of the epr spectra of chromia/ alumina and pure chromia are affected by the adsorp(12) J. A . Shvets and V. B. Kasanskii, Kinet. Catal. ( U S S R ) , 7, 627 (1966). (13) V". B. Kazanskii, ibid., 8, 960 (1967). (14) R. L. Burwell, Jr., G. L. Haller, K. C. Taylor, and J. 5.Read, Advan. Catal., 20, 1 (1969). (15) S. E. Voltz and $3. W. Weller, .I. Amer. Chem. Sac., 76, 4701 (1954).
Infrared Spectra of Hydrocarbons Adsorbed on Silica-Supported Met
svi and N. Sheppard" S ~ h of d Chemical Xciences, University of East Anglia, University Plain, Norwich, NOR 88C, England (Received A p r i l 4, 1872)
Infrared spectra have been obtained from the surface species resulting from the chemisorption of butene-1 , The first three hydrocarbons give very similar spectra from the adsorbed species. These are interpreted in terms of a, misture of associatively and dissociatively adsorbed species such as CH3CEE2CHMCH2M a4nd CB&H&ITM2 where M denotes a surface metal atom. The alternative, but less probable, species GH&H and CH&XMCHZCHMZwould be consistent with the observed spectra; the spectra are not, consistent with the presence of mainly associatively adsorbed species CHaCHMCHMCHI derived from butene-2's. The spectmm produced by the chemisorption of isobutene has less strong methyl absorption than would be expected from an associatively adsorbed species; the moderate absorption in the 2920-cm-"Iregion is such as would be expected from the presence of a greater proportion of CHz groups as in species such as CH~MGH(CHI)CBZM. 2-Methylbutene-2 gave less well-defined spectra from chemisorbed species, but again methyl groups are probably not more numerous than CH2 groups. Hydrogenation of the surface species from the linear butenes and from isobutene gave spectra expected from CI12CH2CH2CHzM and (CI?s~)zCHC€12M groups, respectively, whcn allowance had been made from contributions from physically adsorbed n-butane and isohutanc, partially released from the surface by the hydrogenation reaction. Deh.ydrogenation led to reductions in intensity of the speotra of the surface species which were partially restored by rehydrogenation. I n several cases heating of the hydrogenated surface species to 150' led to cracking of the hydrowrbon skeleton to give methane. Exchange reactions were observed with deuterium a t room temperature leslding to weak vCDand vOD bends. cis- and truns-butene-2, isobutene, and 2-methylbutene-2 on hydrogen-covered iridium
