Langmuir 1995,11, 2065-2070
2065
Microcalorimetric and Infrared Spectroscopic Studies of CO, C2H4, N20, and 0 2 Adsorption on Cu-Y Zeolite G. D. Borgard,? S. Molvik,* P. Balaraman, T. W. Root, and J. A. Dumesic" Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706 Received December 16, 1994. I n Final Form: March 20, 1995@ Microcalorimetric and infrared spectroscopic techniques were used to probe the interactions of carbon monoxide, ethylene, oxygen, and nitrous oxide with Cu-Y zeolite and CdSiOZ. Treatment of Cu-Y zeolite in hydrogen produces Cul+ cations that adsorb carbon monoxide and ethylene at 313 K with initial heats of 80 and 90 kJ/mol, respectively. Carbon monoxide is linearly adsorbed on Cul+ cations, and ethylene is n-adsorbed on these cations. The corresponding heats of adsorption of these molecules at 313 K on silica-supported metallic copper are lower than on Cul+ cations, equal t o 64 and 60 kJ/mol, respectively. Cuprous cations in Y zeolite do not decompose N2O and adsorb oxygen more weakly than metallic copper at temperatures up to 473 K. The heat of 0 2 adsorption on Cul+ cations is equal to 95-170 kJ/mol, and this value is considerably lower than the value of 400 kJ/mol for metallic copper estimated from results of N2O decomposition.
Introduction ployed to determine the heats of adsorption or decomposition of these molecules versus adsorbate coverage, and Copper-exchanged zeolites are promising materials for Fourier transform infrared spectroscopy was used to the direct decomposition and selective catalytic reduction monitor the nature of the various adsorbed species. of nitric oxide1-6 and the decomposition of nitrous oxidea7 Finally, the adsorptive properties of Cu2+and Cul+cations It was shown in these studies that the zeolite structure in Y zeolite were compared t o t h e behavior of metallic Cu plays an important role in determining catalytic activity. in a reduced CdSi02 sample. In addition, the adsorptive and oxidation-reduction properties of these catalysts have been suggested by various Experimental Section investigators to be important for these nitric oxide conversion reactions. For example, Hall and c o - w ~ r k e r s ~ ~ ~ Sample Preparation. The starting zeolite material was a a n d Liu and RobotalO have suggested that Cul+ cations Linde SK-40 Y zeolite in the Na form with a SUAl ratio of 2.4. are active centers, while Shelefll has proposed that Cu2+ This sample will be denoted as Na-Y. Approximately 10 g of cations may be involved. Accordingly, the present study Na-Y was first treated in 3.5 L of dilute N a 0 3 for 6 h, filtered, and subsequently ion-exchanged in 1.3 L of 0.048 M Cu(N03)~ was undertaken to examine the adsorptive properties of solution for 27 h at a pH of 4.4. The filtered sample was washed Cu2+ and CUI+ cations. with deionized water and dried overnight at 393 K. This sample Copper-exchanged Y zeolite was chosen for this study, will be denoted as Cu-Y. A third sample, designated H-Y, is the despite its lower activity compared to other Cu-exchanged protonated form of the Na-Y used in this study, and its zeolites, because the Y zeolite system is well-characterized preparation and acidic properties have been reported elseand has a high ion-exchange capacity. In addition, Cuwhere.16 Finally, a CdSiOZ sample was prepared following the exchanged Y zeolite catalysts are highly active for procedure of Sandoval and Be11.I6 The results of elemental hydrocarbon partial oxidation reactions,12 and are of analyses (Galbraith Laboratories) for these samples are shown commercial importance in the cyclodimerization of butain Table 1. diene to 4-~inylcyclohexene.~~J~ Ethylene, CO, NzO, and Catalyst Treatment. All samples were dehydrated at 573 0 2 were the molecules used to probe the adsorptive K under dynamic vacuum prior to oxidation or reduction treatments. Oxidized Cu-Ywas prepared by treating the sample properties of these cations. Microcalorimetry was emat 573 Kin 350-500 Torr of oxygen for 1-2 h. The sample was then cooled to 313 Kin oxygen and then held at this temperature * Author to whom correspondence should be addressed. for 1h. This sample will be denoted as Cu2+-Y. Evacuation at +Presentaddress: MonsantoAgriculturalCo.,800 N. Lindbergh higher temperatures could cause partial reduction ofthe ~ a t a l y s t . ~ Blvd, Mail Zone U21,St. Louis, MO 63167. Reduced Cu-Y was prepared by first oxidizingthe sample at *Present address: University of Trondheim, Department of 573 K for 1-2 h, followed by evacuations for 1 h at 573 and 673 Industrial Chemistry, N-7034 Trondheim, Norway. K. The sample was then cooled to 473 K in vacuo, treated in Abstract published in Advance ACS Abstracts, June 1, 1995. (1)Iwamot6 M.; Yokoo, S.; Sakai, K.; Kagawa, S: J. Chem. SOC., 400-500 Torr of hydrogen for 1 h, and evacuated at 473 K for Farad. Trans. 1981,77,1629. 1-2 h. This sample will be denoted as Cul+-Y. The H-Y and (2) Iwamoto, M; Hamada, H. Catal. Today 1991,10,57. Na-Y samples were treated in the same manner as Cul+-Y. (3) Sato, S.; Yu-u, Y.; Yahiro, H.; Mizuno, N.; Iwamoto, M. Appl. In a separate experiment, the degree of reduction of Cu-Y at Catal. 1991,70, L1. 473 K was estimated. The Cu-Y sample was initially oxidized (4) Iwamoto, M.: Yahiro, H.: Shundo. S.: Yu-u. Y.: Mizuno. N. ADDL .. Catal. 1991,69,L15. in flowing oxygen at 573 K, cooled to 313 K in flowing oxygen, ( 5 ) Hamada, H.; Kintaichi, Y.; Sasake, M.; Ito, T.; Tabata, M. Appl. and then purged with flowing helium. A trap downstream of the Catal. 1990,64,L1. sample was immersed in liquid nitrogen, and hydrogen flow was (6) Held, W.; Konig, A.;Richter, T.; Ruppe, L. SAE paperNo. 900496, initiated. The sample temperature was raised to 473 Kin flowing 1990. hydrogen and held at this temperature for 1 h. The water (7)Li, Y.; Armor, J. N. Appl. Catal B: Env. 1992,1, L21. produced during reduction was measured by subsequent expan(8) Hall, W. K.; Valyon, J. Catal. Lett. 1992,15,311. (9) Li, Y.; Hall, W. K. J. Catal. 1991,129,205. sion into a volumetric apparatus. (10) Liu, D.; Rohota, H. J. Catal. Lett. 1993,21,291. (11) Shelef, M. Catal. Lett. 1992,15,305. (12)Yu, J-s.;Kevan, L. J. Phys. Chem. 1991,95,6648. (15) Chen, D. T.; Sharma, S. B.; Filimonov,I.;Dumesic, J.A. Catal. (13) US Patent 3,444,253, 1969. Lett. 1992,12,201. (16) Sandoval, M. J.;Bell, A. T. J. Catal. 1993,144,227. (14) US Patent 3,497,462, 1970. @
0743-7463/95/2411-2065$09.00/0
0 1995 American Chemical Society
Borgard et al.
2066 Langmuir, Vol. 11, No. 6, 1995 Table 1. Elemental Composition of Catalysts sample Na bmol/g) Cu bmol/g) 1270 cu-Y 885 Na-Y 3770 H-Y 1330 A reduced CdSiOZ was prepared by treatment in 400-500 Torr of hydrogen at 623 K for 12-15 h, followed by evacuation at this temperature for 1h. Microcalorimetry. Differential enthalpy changes of adsorption, m a d s , versus adsorbate coverage were measured on a Setaram C-80 heat flux microcalorimeter. These values of m a d 8 are negative, and it is convenient to define the heat of adsorption as being equal to - m a d s . A detailed description of the microcalorimetric system is given e1~ewhere.l~ The apparatus contains a vacuum gas handling system attached to a sample cell and an empty reference cell. Catalyst samples were typically 100-700 mgin size. Adsorption isotherms are generated along with heats of adsorption as a functionof surface coverage through consecutive dosing of adsorbing gases. Adsorption was performed at 313 K unless otherwise mentioned. Ethylene, ammonia, and nitrous oxide were purified by several freeze-pump-thaw cycles prior to use. Oxygen was dried by passage at room temperature over activated molecular sieves (13x1. Hydrogen was passed through a heated palladium thimble. Carbon monoxide was passed over a trap at 573 K filled with quartz wool, followed by passage through activated molecular sieves at room temperature. Infrared Spectroscopy. Infrared spectra were collected at a resolution of 2 cm-l using a Mattson Galaxy 5020 FT-IR spectrometer with a DTGS detector and a KBr beamsplitter. Experiments for the adsorption of CO, ethylene, and NH3 were performed in a transmission geometry on thin, self-supporting wafers of Cul+-Y. The IR cell was attached to a vacuum system with calibrated dosing volumes and precision pressure gauges for the sequential addition of adsorbate to the catalyst at room temperature. Following exposure of the catalyst to a dose of the adsorbing gas, the cell was evacuated to remove weakly held and gaseous species. Equilibrium pressures ranged from 0.007 to 100 Torr for CO and 0.01 to 5 Torr for ethylene. Gases were purified in the same manner as for microcalorimetric studies.
90 I
I
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90
60 50 40
30 20
100 200 300 400 500 600 700
-
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4 6 8 1 0 1 2 1 4 CO adsorbed (pmol/g)
Figure 1. Differential heat of CO adsorption at 313 K on (a) Cul+-Y and (b) CdSiOZ.
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8
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Figure 2. Infrared spectra of CO adsorbed on Cul+-Yzeolite at room temperature. Spectra a-f were collected following Results evacuation for 10 min at room temperature. Spectrum g was Adsorption of CO. Figure 1 shows the differential recorded with 100 Torr of gas phase CO present. Surface coverage (after evacuation) of CO equals (a) 25, (b) 80, (c) 220, heat of CO adsorption versus coverage for the copper(d) 450, (e) 620, and (0 685 pmol/g. containing samples. The H-Y and Na-Y samples did not adsorb any CO under the conditions tested. The reduced Cul+-Ysample has approximately 625 pmollg of sites with broader a t lower wavenumbers, with new contributions heats of adsorption between 80 and 65 kJ/mol. The heats at 2138 and 2128 cm-l. The spectra recorded with CO near the lower end of this range are in agreement with pressures of 10,20, and 100 Torr in the IR cell show a pair the value of 62 kJ/mol estimated by HuanglBfrom CO ofpeaks at 2177 and 2117 cm-l due to weakly held species adsorption isotherms over a reduced Cu-Y sample. In that disappear upon evacuation a t room temperature. The contrast, the Cu2+-Ysample has very few sites for CO IR band a t 2160 cm-l has been observed previously for adsorption, and no sites exist with heats of adsorption CO adsorbed on reduced Cu-Y and has been assigned to above 45 kJ/mol. Thus the adsorptive properties of Cu-Y CO linearly adsorbed on Cul+cations.18 For comparison, are strongly influenced by sample pretreatment. The the stretching frequency of gas phase CO is 2143 cm-l, differential heat of CO adsorption on reduced CdSiOZ and CO on metallic copper gives IR bands a t 2077,2088, decreases with coverage from a value of 64 to nearly 46 and 2089 cm-l for C u ( l l l ) , Cu(llO), and Cu(lOO), rekJ/mol. These values correspond well with CO adsorption spe~tively.'~,~~ studies on copper crystal faces reported e l s e ~ h e r e . l ~ - ~ ~ Figure 3 shows the relationship between the IR peak FTIR experiments were conducted to probe the nature area and the amount of CO dosed onto the sample. A of CO adsorbed on Cul+-Y. The recorded spectra are shown nearly linear relationship is evident a t low coverages. This in Figure 2, where the contribution of the zeolite blank linear relationship does not hold a t higher coverages has been subtracted from each of these spectra. The first because evacuation of the gas phase between subsequent dose of CO on Cul+-Y gives IR bands a t 2160 and 2145 doses of CO onto the catalyst removes weakly adsorbed cm-l. As the CO coverage increases, the spectra become species. Extrapolating the linear portion of the curve a t low coverage indicates that 550-680pmolJg of strong sites (17) Cardona-Martinez, N.; Dumesic, J. A. Adu. Cutul. 1992, 38, are available for CO adsorption, in agreement with the 14918) Huang, Y . J. Cutul. 1973, 30, 187. microcalorimetric results. Nearly one-half of the ex(19)0vesen, C. V.; Stoltze, P.; N ~ r s k o vJ. , K.; Campbell, C. T. J. Cutul. 1992, 134, 445. changed Cu does not adsorb CO, presumablybecause these (20) Harendt, C.; Goschnick, J.; Hirschwald, W. Surf: Sci. 1985, sites are inaccessible to CO. 1521153, 453. Adsorption of Ethylene. Figure 4 shows the dif(21)Hollins, P.; Pritchard, J. Surf: Sci. 1979, 89, 486. (22) Kessler, J.; Thieme, F. Surf: Sci. 1977, 67, 405. ferential heat of ethylene adsorption versus coverage for
Studies of Adsorption on Cu-YZeolite
Langmuir, Vol. 11, No. 6, 1995 2067 0.60
0
1000
2000
3000
4000
Cumulative CO Dosed (pmol/g)
1
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Figure 5. Infrared spectrum of ethylene adsorbed on Cul+-Y zeolite at room temperature. The spectrum was collected following evacuation for 10 min at room temperature.
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Wavenumber (cm")
Figure 3. Integrated IR peak area versus cumulative amount of CO dosed on Cul+-Yzeolite prior to evacuation. 100,
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Ethylene adsorbed (pmol/g)
Figure 4. Differential heat of ethylene adsorption at 313 K on (a)Cul+-Y( 0 )and Na-Y (A)and (b) Cu2+-Y(W), CdSiOZ (VI, and H-Y(0).
various samples. In contrast to CO adsorption, significant extents of ethylene adsorption were observed on samples that did not contain copper. The Cul+-Y catalyst adsorbs about 625 pmollg of ethylene with a heat between 90 and 65 kJ/mol. Adsorption on these strong sites is followed by adsorption on nearly 68Opmollg of sites with heats from 55 to 35 kJ/mol. The Cu2+-Ysample, however, only showed 205 pmollg of sites, all of which had heats of adsorption below 55 kJ/ mol. The heat of ethylene adsorption on reduced Cu/SiOz decreases with coverage from 60 to 27 kJ/mol. These values agree well with the heats of adsorption between 47 and 35 kJ/mol determined in previous studies of ethylene adsorption on metallic Cu surfaces.23 Heats of ethylene adsorption were also measured on the Na-Y and H-Y samples. The majority of adsorption sites on both samples gave heats below 40 kJ/mol. These heats agree well with literature values for ethylene adsorbed on Na+ ions.24,25 Figure 5 shows IR spectra of ethylene adsorbed at room temperature on Cul+-Y. The contribution of the zeolite blank has been subtracted from each of these spectra. The major features of this spectrum are two bands a t 1546 and 1536 cm-l, a large peak at 1427 cm-', and a small peak a t 1923 cm-l. Similar features in the IR spectra of ethylene adsorbed on reduced Cu-Y have been observed (23) Jenks, C. J.; Bent, B. E.; Bernstein, N.;Zaera, F. Surf Sci. Lett. 1992,277, L89. (24) Huang, Y. J. Cutal. 1980, 61, 461. (25) Carter, J. L.; Yates, D. J. C.; Lucchesi, P. J.; Elliot, J. J.; Kevorkian, V. J.Phys. Chem. 1966, 70,1126.
Figure 6. Integrated IR peak areas versus the cumulative amount of ethylene dosed on Cul+-Yzeolite prior to evacuation for band at (a) 1427 cm-l (01,(b) 1540 cm-l ( x ) and (c) 1923 cm-l (a).
by H ~ a n g .Accordingly, ~~ the first two bands can be assigned to C-C stretching of n-adsorbed ethylene. As with CO, the appearance of two bands indicates that more than one type of adsorption site is present. The peak a t 1427 cm-l is assigned to the CH2 scissors vibration of adsorbed ethylene. This vibration is affected less strongly than the C=C stretch by the slight differences in the nature ofthe x-bond, so only one peak is detected. A combination of CH2 vibrations causes the band a t 1923 cm-l. Integrated areas of the IR bands are plotted in Figure 6 versus the cumulative amount of ethylene dosed onto the catalyst. A nearly linear relationship can be seen at low coverage. Extrapolation of the linear portion of the curve indicates the presence of 685-820 pmollg of strong adsorption sites. This result agrees with the number of strong sites observed by microcalorimetry. As with the adsorption of CO, nearly one-half of the exchanged Cu does not adsorb ethylene. Interactions of N20 and 0 2 with Copper. Microcalorimetric experiments employing 0 2 and NzO were conducted to probe the bonding of oxygen to copper. N20 decomposes over metallic copper according to reaction 1 below:
where represents a surface site. The reaction stops a t a surface oxygen coverage of approximately 30%.26The extent ofthis reaction can be monitored volumetrically by freezing the gaseous N2O and monitoring the amount of Nz remaining in the gas phase. (26) Giamello, E.; Fubini, B.; Lauro, P.; Bossi, A. J. Catul. 1984,87, 443.
Borgard et al.
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Figure 8. Infrared spectra of NH3 adsorbed on Cul+-Y and Cu2+-Yzeolite at room temperature. The spectrum was collected following evacuation at room temperature.
perature for 15 min to remove weakly held species. Ammonia adsorbed on Lewis acid sites gives characteristic IR bands near 1640 and 1290 cm-l, whereas ammonium ions associated with Bransted acid sites produce an IR band near 1450 cm-1.29,30 Figure 8 compares IR spectra of ammonia adsorbed on Cu2+-Yand Cul+-Ysamples. The Cu2+-Ysample contains a large number of Lewis acid sites and few Bransted acid sites, while the Cul+-Ysample has a substantial number of B r ~ n s t e dacid sites. Discussion Adsorption of CO. Na-Y, H-Y, and Cu2+-Yzeolites did not adsorb CO under the conditions of the present study. Therefore, Bransted acid sites, Na+, and Cu2+ cations are not responsible for the strong adsorption of CO on Cul+-Y. The large heat associated with NzO decomposition over metallic CdSiOZ was not observed for the Cul+-Y sample, indicating the absence of metallic copper on this sample. Thus, the high heat of adsorption seen for the Cul+-Y sample can be attributed to CO adsorption on Cul+ centers. The bonding of CO to transition metal centers involves formation of u and n bonds.31 The donation of electrons from the slightly anti-bonding 5u-orbital of CO into unfilled metal orbitals strengthens the C - 0 bond and shifts the stretchingvibration of adsorbed CO to higher frequencies. Donation of electrons from the filled metal d-orbitals into the vacant n*-anti-bonding orbitals of CO weakens the C-0 bond and shifts the stretching vibration of CO to lower frequencies. The stretching vibration frequency of the adsorbed CO thus depends on the relative contributions of the u- and n-bonding. For highly charged metal ions, the contribution of n-bonding is relatively small, and the metal-CO bond is mainly u in character. In contrast, n-bonding plays a significant role in the adsorption of CO to zero-valent metals. The present study shows that metallic Cu adsorbs CO less strongly than Cul+. Metallic Cu also adsorbs CO less strongly than metallic Ni (heat of adsorption of about 125 k J l m 0 1 ~), ~which has one electron fewer in the 3d shell. The work function of Cu metal decreases upon CO a d s o r p t i ~ nindicating , ~ ~ ~ ~ ~that electrons are donated from CO to Cu, in contrast to CO adsorption on reduced Ni.33 This behavior suggests that the bond between CO and Cu metal has more u character than CO bonding to other (29) Ward, J. J. Catal. 1968, 11, 251. (30) Ghosh, A.; Carthoys, G. J.Chem. SOC.Farad. Trans. 1 1984,80, 99
(27) Sbrkany, J.; d'Itri, J. L.; Sachtler, W. M. H. Catal. Lett. 1992, 16, 241. (28) Jacobs, P. A.; Tielen, M.; Linart, J.;Uytterhoeven, J. B. J. Chem. SOC.Farad. Trans. 1 1976, 72, 2793.
(31)Davydov, A. A. Infrared Spectroscopy of Adsorbed Species on the Surface ofTransition Metal Oxides; John Wiley & Sons: New York, 1990. (32) Doyen, G. and Ertl, G. Surf Sci. 1974,43, 197. (33)Tracy, J. C. J. Chem. Phys. 1972,56, 2736.
Studies of Adsorption on Cu-Y Zeolite
Langmuir, Vol. 11, No. 6, 1995 2069
Table 2. Heats of 0 2 Adsorption or Catalyst Oxidation transition metal catalysts like Ni, where the work function (kJ/mol of 0 2 ) on Copper Catalysts increases upon CO adsorption. NMR studies show that back-bonding is also minimal for Cu(1) carbonyl com4cu2cu- 2cuzopound~.~ Our ~ , NMR ~ ~ investigations of CO adsorption Cu/Zn026 CdSiOZ Cul+-Y ~ C U Z 2Cu040 O ~ ~ 4Cu040 on the Cul+-Y catalyst used in this study confirm that 02 360 95-170 339 315 291 back-bonding is relatively ~ n i m p o r t a n t .The ~ ~ bonding NzO 470 400 inactive of CO to both Cu metal and Cul+ thus appears to be predominantly a i n character, with electron donation from C-C bond, resulting in a shift of the C=C stretching the 50 orbital of CO to the 4s (or hybridized 4sp) orbital vibration to lower frequencies. A decrease in the work of Cu. However, the 4s (or 4sp) orbital is unfilled for Cul+ function is observed when ethylene adsorbs onto metallic and partially filled for Cu metal. Thus, Cul+ has a higher indicating the importance of a-donation from electron-withdrawing capacity, and consequently, a higher ethylene to copper. The formation of a n-adsorbed complex heat of CO adsorption is observed for Cul+ compared to as opposed to a di-a adsorbed complex on metallic copper Cu metal. also attests to the low back-bonding capacity of copper The blue shift in the IR band to 2160 cm-l from the gas since the formation of di-a complexes is thought to be phase value of 2143 cm-l for the adsorption of CO on accompanied by substantial b a ~ k - b o n d i n g .Thus, ~ ~ the Cul+-Y is consistent with the interpretation t h a t the bonding of ethylene on both Cul+ and metallic copper can Cul+-CO bond is predominantly a in character. Dobe interpreted as being predominantly a in character, as nation of electrons from the slightly anti-bonding 50 orconcluded above with respect to CO adsorption. The pair bital of CO to Cu results in stabilization of the C - 0 of IR bands observed for the adsorption of ethylene on bond and a shift to wavenumbers higher than t h a t for gas Cul+-Ycan be assigned to adsorption on Cul+cations with phase CO. Ab initio calculations of Bates and D ~ y e r ~ ~ different adsorption properties, e.g., a t different sites in show that a-bonding of CO with A13+ Lewis acid sites in the zeolite. zeolites results in a blue shift of the CO stretching Adsorption of Oxygen. The low temperature defrequency. sorption of oxygen from copper-exchanged zeolites has The IR spectra of CO adsorbed on Cul+-Y show a pair been suggested to have important implications for the of bands, even a t the lowest coverages studied (less than decomposition and reduction by hydrocarbons of NO. For 25 pmollg). Coupling between CO adsorbed on adjacent example, oxygen desorbs from Cu-Y or Cu-ZSM-5catalysts sites cannot explain the two bands observed at these low a t temperatures as low as 573 K.9 Table 2 presents the coverages. Therefore, these two bands are assigned to heat of 0 2 adsorption (per mole of 0 2 ) measured directly CO adsorbed on Cul+ centers at different sites in the zeolite and computed from the heat of N2O decomposition on with different adsorption properties. different catalysts. For comparison, heats of oxidation of The minimal back-bonding observed for both Cu metal Cu and Cu20 are also given per mole of 0 2 . It can be seen and Cul+ is possibly due to the extra stabilization that that the heat of oxygen adsorption is significantly lower accompanies a completely filled d shell (since both Cu on Cul+-Yzeolite than on metallic or bulk copper species. metal and Cul+ have a dl0configuration). In addition, Cu Specifically, the heat of 0 2 adsorption on Cul+ cations is has a smaller ionic radius and correspondingly lower 95-170 kJ/mol, considerably lower than the value of 400 electron-donating capacity than metals a t the beginning kJ/mol for metallic copper estimated from results of N2O of the transition metal block. decomposition. CO does not adsorb on Cu2+,despite the fact that Cu2+ The slow adsorption ofoxygen at 473 Kand the increased has a higher electron-withdrawing capacity than Cul+ oxygen uptake at 473 K compared to 313 K indicate that and Cu metal and should hence form a strong a-bond with oxygen adsorption on Cul+-Y is kinetically limited. The CO. This observation is consistent with the fact that no increase in the heat of adsorption with coverage can be stable carbonyl compounds of Cu2+exist at room temexplained by the presence of two different types of sites perature. The lack of d-electron density for back-bonding on the catalyst. While one site may adsorb oxygen by a is cited as a reason31 for the weak interaction of CO with nonactivated process, the other site may require activated Cu2+cations. In addition, we suggest t h a t coordinative adsorption. Benn and c o - w ~ r k e r sfound ~~ a similar saturation or inaccessibility of the Cu2+species in Y zeolite increase in oxygen deposition a t higher temperatures on may also be responsible for the inactivity of Cu2+cations reduced Cu-X zeolites and attributed this behavior to toward CO adsorption. kinetic limitations. Their studies also showed that while there was no appreciable NzO decomposition on these Adsorption of Ethylene. The present study shows catalysts a t 473 K, the amount of NzO decomposed that the bonding of ethylene to Cu centers is analogous increased with temperature and was the same as the to the adsorption of CO. Ethylene adsorbs more strongly amount of oxygen deposited a t 673 K and above. The on Cul+-Y than on metallic copper. Both Na-Y and H-Y observation made in this study that there is no appreciable zeolites adsorb ethylene very weakly. The weak adsorpamount of NzO decomposition a t 473 K on Cul+-Y is in tion observed on Cu2+-Y is probably due to ethylene agreement with the above study. adsorption on Na+ cations. As with the adsorption of CO, If the adsorption of oxygen is activated, the activation the strong adsorption of ethylene on Cul+-Y can thus be energy for desorption will be greater than the heat of assigned to adsorption on Cul+ cations. adsorption, resulting in smaller desorption rates than if Adsorption of ethylene on transition metals involves the adsorption was nonactivated. However, the smaller electron donation from the ethylene 2x orbital to empty heat of adsorption indicates that adsorbed oxygen species metal orbitals (a-bonding) and back-donation of electrons are still more weakly held than oxygen species on other from the metal d-orbitals to the ethylene 2n* orbital (ncatalysts, and possibly have increased reactivity with bonding). Both a- and n-bonding weaken the ethylene adsorbed hydrocarbons. In addition, the adsorption of (34) Geerts, R. L.; HuBnan, J. C.;Folting, IC; Lemmen, T. H.;Caulton, K. G. J. Am. Chem. SOC.1983,105, 3503. (35) Souma, Y . ;Iyoda, J.; Sano, H. Inorg. Chem. 1976,15 (4), 969. (36) Balaraman, P.; Dumesic, J. A.; Root, T. W. Manuscript in preparation. (37) Bates, S.; Dwyer, J. J. Chem. Phys. 1993,97,5897.
(38) Sheppard, N. Ann. Rev. Phys. Chem. 1988,39,589. (39) Benn, F. R.; Dwyer, J.;Esfahani, A,;Evmerides,N.P.;Szczepura, A. K.J . Catal. 1977,48,60. (40) CRC Handbook of Chemistry and Physics; Weast, R. C . , Ed.; CRC Press, Inc: Boca Raton, FL, 1987-88.
Borgard et al.
2070 Langmuir, Vol. 11, No. 6, 1995
NO on these catalysts is not activated. Therefore, the unique catalytic properties of Cu-zeolite materials may be at least partially due to increase in the adsorption rate of NO relative to 02. The heats of oxygen adsorption measured in the present study are in general agreement with the gravimetric results of Li and Hall,9 as shown below. The amount of oxygen that desorbs from Cu-ZSM-5 at 773 K was measured gravimetrically in flowing helium containing 0%,4%,and 21%oxygen. The weight losses corresponded to the desorption of 0.11,0.046,and 0.035 oxygen atoms, respectively, per Cu cation. Oxygen adsorbs dissociatively on Cul+-Yzeolite, as indicated by the -l/2 order influence of the oxygen pressure on the NO decomposition r e a ~ t i o n . ~ Assuming standard pre-exponentialfactors for adsorption and desorption of lo5Torr-l s-l and 1013s-l, respectively, the equilibrium constant for 0 2 adsorption, Ko2,can be estimated to be equal to 10-8exp(-AHad$RT). The value of KO2is estimated to be 0.026 Torr-l a t 773 K, for a heat of adsorption of 95 kJ/mol, as determined from microcalorimetry. If we assume that the loss of 0.11 O/Cu measured by Hall et al. for Cu-ZSM-5in the absence of gaseous oxygen corresponds to the maximum amount of labile oxygen at 773 K, then the fractional extents of oxygen removal in 4 and 21% 02 are equal to 0.046 and 0.035. Using the equilibrium constant for dissociative 02 adsorption estimated above, the predicted extents of oxygen removal in 4 and 21% 02 are equal to 0.058 and 0.036. The agreement between the results from these two different studies is good, considering that different catalysts were used under different reaction conditions. In short, the relatively low heat of oxygen adsorption on Cul+,directly measured for the first time, is an important property of copper ion-exchanged zeolites.
Summary Microcalorimetric and infrared spectroscopic measurements were made of the interactions of carbon monoxide, ethylene, oxygen, and nitrous oxide with Cu-Y and C d Si02 catalysts. Treatment of Cu-Y zeolite in hydrogen produces Cul+ ions that strongly adsorb carbon monoxide and ethylene. Carbon monoxide is linearly adsorbed on Cul+ cations, and ethylene is n-adsorbed to these cations. In both cases, the bond to the copper site is predominantly 0 in character, with electron donation from the adsorbate to the copper center. Cuprous cations have a higher electron-withdrawing capacity than metallic copper and, consequently, the adsorption of CO and ethylene on the former is accompanied by higher heats of adsorption. Nitrous oxide and oxygen interact with metallic copper to produce strongly held oxygen species on the surface. In contrast, Cul+-Y zeolite does not decompose NzO and adsorbs oxygen more weakly than metallic copper at temperatures up to 473 K. Oxygen uptake is higher at 473 K than at 313 K, indicating activated adsorption. Measured heats of oxygen adsorption on Cul+-Y are in agreement with the desorption characteristics reported in the literature for Cu-ZSM-5. The quantitative combination of adsorption microcalorimetry and infrared spectroscopy can be effective for the characterization of copper ion-exchanged zeolites. In particular, these techniques provide information about the concentration, location, bonding energetics, and reactivity of various copper species in the catalyst.
Acknowledgment. We wish to acknowledge the financial support of the National Science Foundation. LA941012Z
