13C High resolution NMR studies of adsorption and reaction of ethylene on silica-supported Ru and RuCu catalysts

Colloids and Surfaces, 45 (1990) 39-56 Elsevier Science Publishers B.V., Amsterdam -

39 Printed in The Netherlands

13CHigh Resolution NMR Studies of Adsorption and Reaction of Ethylene on Silica-Supported Ru arid Ru-Cu Catalysts M. PRUSKI’, J.C. KELZENBERG’,

M. SPROCK2, B.C. GERSTEIN3 and T.S. KING’

‘Ames Laboratory, 230 Spedding, Iowa State University, Ames, IA 50011 (U.S.A.) 2Department of Chemical Engineering and Ames Laboratory, 231 Sweeney Hall, Iowa State University, Ames, IA 50011 (U.S.A.) 3Department of Chemistry and Ames Laboratory, 229 Spedding, Iowa State University, Ames, IA 50011 (U.S.A.) (Received 7 August 1989; accepted 21 August 1989)

ABSTRACT The adsorption and subsequent reaction of ethylene on silica-supported Ru and Ru-Cu bimetallic catalysts have been studied via nuclear magnetic resonance (NMR ) employing a variety of experimental nuclear spin dynamics. NMR of 13Cusing cross polarization (CP) and magic angle spinning (MAS) allowed us to simultaneously observe transformations of chemisorbed species and weakly adsorbed molecules. Direct 13Cexcitation allowed quantitative measurements of various species on the surface. At room temperature the ethylene adsorbed on the ruthenium catalysts rapidly and converted to strongly adsorbed acetylide and alkyl species. Weakly adsorbed ethane, butane and cis- and trans-2-butene were observed. The butene slowly hydrogenated to form additional butane. When the temperature was raised to 140 ’ C propane and methane were observed suggesting hydrogenolysis of the product molecules. The introduction of copper into the catalysts reduced the hydrogenation capability while still maintaining the ability to form dimeric products. In addition to butene and butane, butadiene was observed on the bimetallic catalysts.

1. INTRODUCTION

The knowledge of the molecular structure, orientation and subsequent reaction of chemisorbed hydrocarbon species is considered to be important in the understanding of a variety of technologies including adhesion, lubrication and heterogeneous catalysis. Substantial progress has been made in fundamental investigations of chemisorption on metals by the use of vibrational spectroscopies such as transmission infrared (TIR) studies of supported metal systems and electron energy loss (EEL) investigations of species on singlecrystal metal surfaces [l-30]. These studies have given insight into the cata0166-6622/90/$03.50

0 1990 Elsevier Science Publishers B.V.

lytic phenomena occurring at a molecular level; consequently, they are proving to be useful in the tailoring of new industrial catalysts. EEL and TIR spectroscopies are both valuable techniques that, nonetheless, suffer some constraints. For example, EEL spectroscopies are limited to ultrahigh vacuum experiments using massive metal (mostly single-crystal) samples. TIR spectroscopy can investigate supported metal catalysts at higher pressures if gas phase interference is not too strong, however quantitative results are difficult to obtain with this technique. Nuclear magnetic resonance (NMR) is emerging as a powerful technique for the study of real catalyst systems. Duncan and Dybowski [ 311, Slichter [32] and Wang et al. [ 331 have reviewed much of the previous work and advances that have made NMR a valuable method for studying species adsorbed on metal surfaces. For example, Wang and Slichter [34] reported results describing the adsorbed structures and subsequent reactions of ethylene on Pt/ A1203 catalysts where spin-echo methods enabled observation of C-C and CH dipolar couplings in the double-labeled 13Cethylene. At room temperature all of the C-C bonds were intact with an internuclear distance of 1.49 A. Half of the carbon was found to have directly bonded hydrogen and C-H dipolar coupling consistent with a rotating -CH3 group. On the basis of this information, they proposed that ethylidyne is the sole species present at room temperature. On the other hand, in a recent study of ethylene on alumina- and silicasupported platinum catalysts, Gay [ 351 employed cross polarization (CP), magic angle spinning (MAS ) and proton decoupling to probe the chemical and structural environment of 13Cnuclei on the surface. In this work no signal from unprotonated surface carbons, such as ethylidyne, was detected. A species identified as a a-bonded olefin was observed on all samples. In our laboratory we have begun to use a variety of high resolution, solid state NMR experiments to investigate supported and unsupported catalytic systems. Of these, 13CNMR has proved particularly promising [ 361. It is the goal of this work to demonstrate the ability of solid state 13C NMR to probe the chemical and structural properties of adsorbed species as well as chemical decomposition products on supported metal catalysts at pressures higher than those possible in ultrahigh vacuum experiments and, consequently, closer to industrial conditions. We shall present a limited discussion of the details of the surface chemistry. The focus of this paper is to set forth examples of the kind of information which those techniques offer. To this end, some of our 13C NMR results obtained using CP, MAS at room and elevated temperatures with and without proton decoupling and single pulse experiments (Bloch decay), will be reported.

41 EXPERIMENTAL

Materials and preparation procedures Ruthenium catalyst samples used in this study were prepared by using an ion exchange technique described by Gay [ 371 with two modifications. First, Ru(N0) (NO,), (AESAR), not RuC13.nH,0, was used as the metal salt; second, instead of simply washing the catalyst with ammonia, ammonia was added to the solution to bring the pH to approximately 9-10. The catalyst was allowed to soak in that solution. The above technique allowed us to attain high metal dispersions (0.37) with a high ruthenium loading (12 wt%). The Ru-Cu/SiOz catalysts were prepared by co-impregnation of a ruthenium-copper impregnating solution in the same manner. The impregnating solutions were prepared by dissolving Ru(N0) (NO,), salt and CU(NO,)~*~H~O (AESAR), in distilled water. About 2.2 ml of impregnating solution per gram of SiOZ was sufficient to bring about incipient wetness. All catalysts were supported on Cab-0-Sil HS-5 amorphous, fumed silica (with a BET surface area of 300 m2 g-l). Additional samples containing only amorphous, fumed silica were also prepared by adding water to the silica (2 ml per gram Si02). The slurries obtained after impregnation and mixing were dried for 24 h at room temperature and 4 h in air at 383 K. The ruthenium loading for all ruthenium bimetallic catalysts was kept at 4% by total weight of the support and metals. The copper loading for a Cu/Si02 catalyst was 5 wt%. The amount of copper in each RuCu/Si02 and Cu/Si02 catalyst was determined by atomic adsorption spectroscopy. Reduction was performed either during dispersion measurements or when the NMR sample was prepared. In either case the sample (So-100 mg) was reduced for two hours in 15 seem hydrogen at 450” C. Ruthenium dispersions were measured using strong hydrogen chemisorption [ 381. Reduced and evacuated samples were exposed to hydrogen (O-60 torr) and allowed to equilibrate in order to generate a total hydrogen adsorption isotherm. The reversible hydrogen adsorption isotherm was collected under the same conditions after a 10 min evacuation period (to 10m6torr ) following the first adsorption step. The amount of strong hydrogen adsorption was found by taking the difference between the extrapolated values of the total and reversible isotherms at zero pressure. NMR sample preparation was done on a home-built adsorption apparatus consisting of a multiport glass manifold connected to a high vacuum system [39]. It was designed to have flow-through capability for catalyst reduction and to be capable of handling up to four samples simultaneously. Samples were prepared by loading 80-100 mg of catalyst into a 5.0 mm OD Norell XR-55 NMR sample tubes. The samples were reduced and then evacuated to a pressure of less than 10m5torr and cooled to room temperature. Samples were then dosed with ethylene (single 13C labeled, double 13C labeled, or natural abun-

42

dance) at either liquid nitrogen or room temperature, allowed to equilibrate, and then sealed. Prior to sealing, some samples were evacuated or given additional treatments.

Equipment and experimental techniques The CP/MAS experiments were carried out on a home-built spectrometer operating at 100.06 and 25.16 MHz for ‘H and 13C, respectively. A doubletuned, single-coil probe allowing for MAS of sealed samples at rates exceeding 5 kHz (&th an air drive) and at temperatures between 300 and 460 K was employed. The spinning assembly is a modified version of the ShoemakerApple design [40] with macor and torlon used as stator and rotor materials, respectively. For several samples a prolonged accumulation of free-induction decays (up to 150,000 scans) was required to achieve a satisfactory 13C signal-to-noise ratio. To attenuate baseline distortion associated with pulse breakthrough and receiver recovery, we used the proton spin temperature inversion with an addsubtract of 13C free-induction decays [41]. The ‘H rf power was adjusted to give a 90” pulse, 5.0 ps long, corresponding to a proton B1 field of 50 kHz. This field was kept at the same level during the cross-polarization and proton decoupling periods. The NMR signals were recorded with a dwell time of 10 p, resulting in a spectral range of 50 kHz. Throughout the work, all resonance line positions were determined in ppm with respect to tetramethylsilane (TMS) using the S scale with more positive numbers being downfield. The CP technique yields remarkable improvements of sensitivity in systems in which there exist polarization transfer mechanisms from higher-gyromagnetic-ratio, abundant spins (‘H), to the lower-gyromagnetic-ratio, diluted spins ( 13C). In solids the static dipolar interaction between nuclear spins is used for this polarization transfer. In liquids and liquid-like samples where the static dipolar coupling is averaged to zero through isotropic motion, 13C-lH cross polarization can still be observed [ 42,431. In this case the scalar coupling resulting from the hyperfine interactions of electrons and nuclei provides the means of polarization transfer (J cross-polarization). Despite the increased sensitivity, the cross polarization techniques may cause intensity distortions in the observed spectra, particularly when the J cross-polarization method is utilized. Thus, direct pulsed FT-NMR detection (Bloch decay) is advised for quantitative studies, provided that; (1) the 13Crelaxation times in the laboratory frame (spin lattice relaxation times) are known and (2) sufficient sensitivity can be achieved. The spectra of 13Cusing direct detection were taken with the delays of 5T,,, where TIL is the longest carbon spin lattice time constant in the sample. Also, 13Cspin counting was performed in order to determine the amounts of different types of carbon present on the catalyst surface

43

by comparing the integrated intensity of the 13Cabsorption peaks with that of a standard. For highly mobile molecules, the line narrowing techniques of strong proton decoupling and magic angle spinning did not lead to markedly increased resolution. For those species, the application of methods used in NMR of liquids proved helpful in identifying different types of carbon deposited on the surface of ruthenium via the observation of J-splittings. These measurements were carried out on the Bruker 300 WM spectrometer, operating at the 13C frequency of 75.41 MHz. RESULTS AND DISCUSSION

A series of i3C NMR experiments was performed on singly labeled (99% 13C) ethylene adsorbed at 80 K on a catalyst sample. The amount of ethylene present in the sample at room temperature was calculated to be greater than 400 torr. In Fig. 1, the CP/MAS proton decoupled spectra taken under the same experimental conditions for 1 day, 7 days and 30 days after preparation are presented. Each spectrum is the result of 150,000 cross polarizations taken with a contact time of 2 ms and a recycle time of 0.5 s between the scans (proton Tl relaxation time was determined to be less than 100 ms). Two types of resonance lines can be clearly distinguished: a group of narrow, Lorentzian peaks and a pair of broad features located between 200 and - 50 ppm. The narrow peaks are located at: (a) 126 2 1 ppm; (b) 26 + 1 ppm; (c) 18 & 1 ppm; (d) 14 2 1 ppm; and (e) 6 ? 1 ppm. It immediately became clear that these resonances represented species of high mobility that are not rigidly attached to the surface of ruthenium. This inference was confirmed by two experiments performed without line-narrowing techniques. These experiments were performed 8 and 9 days after preparation of the sample, and their results, presented in Figs 2a and 2b, are to be compared with the spectrum of Fig. lb. In the static experiment (Fig. 2a), where only strong proton decoupling was applied (no MAS), the linewidths of the sharp resonances remained almost unaffected. Only the width of peak (a) increased slightly to N 150 Hz. In Fig. 2b, a spectrum taken without either magic angle spinning or proton decoupling is shown. Although the broad features underlying high-resolution spectra were broadened almost beyond observation, the narrow peaks could still be seen. The width of peak (a) increased to about 500 Hz, and overlapping multiplets due to J-splitting were found for lines (b ) - (e ) . By performing a 13Cexperiment on the same sample at a higher magnetic field, further information on the nature of the J multiplets was achieved. In this experiment, a direct carbon excitation at a frequency of 75.4 MHz was applied. The upfield portions of high-resolution 13C spectra of the catalyst, taken with and without scalar decoupling, respectively, are shown in Figs 3a

t~““““1-“““‘.I-.“““‘I”“-“‘.I”“~~~’~I”~~.~~~

400

300

200

0

100 PPM

FROM

-100

-200

-31

TMS

Fig. 1. 13CCP/MAS spectra of species adsorbed on ruthenium surface as a result of decomposition of ethylene at room temperature: (a) 1 day; (b) 7 days; and (c) 30 days after sample preparation, respectively.

45

1.,,..,...1......,..1...,,.,.~1,.,,.,.,,1.,......,~ 400

300

200

0

100

PPU

FROM

-100

-200

-300

TUS

Fig. 2. 13C CP spectra of species adsorbed on ruthenium surface as a result of decomposition of ethylene at room temperature: (a) static (no MAS); (b) static without proton decoupling.

and 3b. The results of these experiments indicate that the line at 26 ppm represents CH, carbons, while the resonances at 14 and 6 ppm correspond to carbons in methyl groups. Note that line (c) is missing from the high-field spectra. This peak represents an intermediate of the catalytic reaction. The highfield measurements presented in Fig. 3 were performed more than a month after sample preparation when the intermediate had already been transformed to a product. Subsequent high-field measurements with a fresh sample revealed peak (c) to be due to the presence of a methyl group.

46

(a)

ethane

I

I

30

26

I 22

I

16

I 14

I

10

I 6

I 2

I -2

I -6

PPM FROM TMS

Fig. 3. High-resolution 13C spectra of weakly adsorbed species 30 days after sample preparation (75.4 MHz): (a) without proton decoupling; (b) with proton decoupling.

At this point, we may conclude that the narrow features in Fig. 1 represent weakly adsorbed hydrocarbon molecules that rotate fast enough at room temperature to reduce line-broadening interactions. For lines (b )-(e) the ‘H-13C dipolar interactions were averaged to nearly zero, which implies a frequency of rotation of at least lo5 Hz. This conclusion is supported by the result of a CP/ MAS experiment with a sample that was evacuated for 10 min after it had been adsorbed with ethylene for 20 min at room temperature. The narrow peaks were eliminated after the evacuation, indicating they were due to weakly adsorbed species. The assignment for the various species has been made in part by noting the chemical shift. For the weakly adsorbed species the chemical shifts observed can be directly related to the values reported in the literature for liquid samples. Small deviations from the literature values may be due to van der Waals interactions and bulk susceptibility of the metal [ 44,451. Strongly adsorbed species may also be affected by a Knight shift interaction; however, for adsorbed hydrocarbon species this appears to be a minor effect. On the basis of the information from the 13C NMR experiments, we made the following assignments for the narrow resonances [36]: peak (a) is a superposition of CH carbons of trans- and cis-2-butene; peak (b) represents a

41 TABLE 1 13Cchemical shifts (ppm) from liquid state NMR [46] C”

Cb

Ethane

C’H3 -C”H,

Butane

C’H3-CbHz-CbHz-CaH3

13.4

25.2

trans-Butene

H CBH3-Cb=Ca-CH3 H

17.6

126.0

cis-Butene

CeH3-Cb=Cb-CaH3 HH

12.1

124.6

Butadiene

bba tH2=C-~=~~Z HH

116.3

136.9

16.4

16.8

Propane

Methane

b :H,-CH,-:H,

5.7

-2.3

CH, carbon of butane; peak (c) corresponds to a CH, group of trans-2-butene; peak (d) represents methyl groups in butane and cis-2-butene; and peak (e) corresponds to CH3 groups of ethane. The identification of peaks was based on observed chemical shifts as compared to the data from liquid-state 13CNMR (see Table 1 [46 J) and from multiplicities of J-splittings and corresponding intensities (as obtained via direct 13CNMR measurements). The line narrowing due to molecular motion has no effect on the observed linewidth of those species that are rigid (strongly chemisorbed) on the surface of ruthenium. The chemisorbed species, under conditions of high resolution, solid-state NMR, are represented by the broad features of the 13CNMR spectra that do not disappear upon evacuation. Although MAS and strong proton decoupling lead to a considerable reduction of the observed linewidths (compare Figs la and 2b), a severe broadening ( N 3 kHz) remains even under highresolution conditions. The broadening observed is inhomogeneous in nature and is associated with the distribution of the local environments on the metal particles. In spite of the broadening, two different features can be distinguished: a downfield peak centered at N 85 ppm and a narrower peak located at N 15 ppm. The broad resonance at -85 ppm could represent acetylide. The assignment is suggested by chemical shift data of organometallic species [ 47,481. Acetylide formation was documented on both Ru(OO1) [l-3 ] and Ru(l,l,lO) [4]. On the basis of the shifts given by Carty et al. [ 481, a A-$ acetylide species best

48

tits our data. In this orientation, a a-bond is formed by the a-carbon with ruthenium, while a x-bond is formed with a second ruthenium atom. Whether acetylide is the species solely responsible for the intensity observed at -85 ppm still remains to be seen, however, the presence of some species may be discounted by examining available chemical shift data. These include carbidic species, carbynes (e.g. ethylidyne ) and carbenes, all of which exhibit resonances in the range 200 to 400 ppm. Further information on the nature of the rigid species may be obtained using resolution enhancement methods based on differences in relaxation processes in the sample. Two examples of those techniques are a variable contact time CP experiments and dipolar dephasing experiments. In Fig. 4 a plot of the intensity of the broad peak versus cross polarization contact time is shown. The experiment suggested that about 30% of the strongly adsorbed carbon contributing to this peak had a long cross polarization time indicating no directly bonded hydrogen. It must be noted that this type of experiment is time consuming. Also several effects, including intermolecular polarization transfer between different species on the surface, limit its accuracy. The broad resonances at O-40 ppm in Fig. 1 are typical of Sp3 hybridized carbon and are assigned to various, indistinguishable, surface-attached alkyl groups. This is indicated not only by the chemical shift but also by the fact that the distribution of resonances is narrower, probably because of their weaker, single-bond attachment to the metal. Ethyl group formation is possible either through the direct hydrogenation of an ethylene molecule or through the hydrogenation of a surface acetylide or ethylidyne. Another possible alkyl 150

100

Protonated

x L

E

al

--E 50 Non-protonated

0

__--

__-- ___--

____--

______________

I

I

0.5

Contact

1

time

1.

(ms)

Fig. 4. Intensity of the broad portion of the 13C spectrum versus cross polarization contact time. The solid line represents the fit to a bi-exponential function and the dashed lines are the components of that fit.

49

species, one that may account for the observed products butene and butane, is a metallocycle species. Metallocycle pentane can be formed from ethylene via an organometallic synthesis [49-511. In fact, it has been shown that the bis (ethylene)-to-transition-metal-metallocycle-pentane conversion is a reversible process [ 501. The presence of a metallocycle surface species has been postulated to be an intermediate in butane hydrogenolysis or iridium single crystals [52]. In addition, Basu and Yates [53] observed a C, metallocycle species on alumina-supported rhodium by using IR spectroscopy. One of the reasons NMR has been traditionally such a powerful tool is that the signals observed can be used for quantitative analysis. In our studies, direct excitation of 13C nuclei was used in spin-counting experiments to quantify the amount of carbon in the weakly and strongly adsorbed species. For the sample after 30 days at room temperature (so that all the butene had converted to butane), we found about one carbon atom in the strongly adsorbed layer for each surface ruthenium atom. In addition, there was about the same amount of weakly adsorbed carbon with an ethane-to-butane molar ratio of 2 : 1 [ 361. The 2 : 1 ratio is not seen directly in Fig. 1 due to the intensity distortions associated with the CP experiment (as discussed in the experimental section). It should be emphasized that no new resonances were observed using the Bloch decay experiment. In another example of the application of 13C NMR to the study of surfaces we used sequential dosing of labeled and unlabeled ethylene in order to probe the relation and possible interaction of the weakly adsorbed and chemisorbed species. One sample was exposed to labeled ethylene for a short time and then evacuated. Then the sample was exposed to unlabeled ethylene followed by evacuation. The exposure to unlabeled ethylene and evacuation was repeated several times. In all cases the structure of the strongly adsorbed layer formed initially by the labeled ethylene on the clean surface was unaltered by repeated “rinsing” with unlabeled (not observable) ethylene. The reverse of the above experiment, where unlabeled ethylene was adsorbed first followed by exposure to labeled ethylene, revealed that the labeled ethylene did not incorporate into the strongly adsorbed layer to an appreciable extent, but weakly adsorbed products were still formed. These results indicated that once the strongly adsorbed species were formed they were relatively inert. These results also raise the interesting possibility that the catalytic site may be associated with the chemisorbed layer itself and may not exist totally on the metal surface. Another set of experiments was performed to ascertain the location of product molecules ethane and butane on the catalyst. As was mentioned above, on the basis of results of spin counting, equal amounts of carbon in the narrow peaks (weakly adsorbed) and the broad base (strongly chemisorbed layer) were found. This result seems to indicate that the weakly adsorbed product molecules are in a liquid-like monolayer over the strongly chemisorbed species. Or, the weakly adsorbing species may be associated with the high surface-area

,50

(a)

400

300

200

100

0

-100

-200

(b)

400

300

200

iO0

0

- iO 0

- 200

(c)

• 00

300

200

IO 0 PPM

0 FROM

- IO 0

- 200

TMS

Fig. 5. 13C spectra of ethane adsorbed on: (a) pure silica; (b) freshly reduced 12% Ru/SiO~; (c) 12% Ru/giO2 with a chemi~rbed layer of ethylene.

51

support material. To test these ideas, we performed experiments in which N 50 torr of singly labeled (90% ) ethane was introduced to three samples at 295 K. The samples consisted of the following materials: (1) pure SiO,, (‘2) 12% Ru/ SiOz, and (3) 12% Ru/SiOz that had been exposed to unlabeled ethylene and evacuated. All samples were then exposed to ethane. The results of the CP/ MAS experiment presented in Figs 5a-c indicate the presence of only weakly adsorbed molecules. A resonance for ethane (5 t 1 ppm) was seen in all cases. This resonance corresponds to what was observed for the formation of ethane resulting from the adsorption of ethylene. In addition, a resonance assigned to methane, at - 3 ppm, is seen in Fig. 5b. This new feature in Fig. 5b shows that ethane experiences self-hydrogenolysis on a clean ruthenium surface at room temperature. The additional hydrogen that must be supplied to form the methane probably comes from dehydrogenated, strongly adsorbed species that are present in quantities too small to be adequately observed. It is also interesting to note that the ethane peak is about twice as broad on the clean Ru/SiOz sample (Fig. 5b) as it is on either the pure silica sample (Fig. 5a) or the Ru/SiO, sample that has first been exposed to unlabeled ethylene (Fig. 5~). The increased width is most likely due to interactions with the ruthenium metal. The peak width observed for the ethane resulting from ethylene adsorption (Fig. 1) is comparable to the ethane peak in Figs 5a and 5c. (Observed peak width for Fig. 1 refers to the width before a broadening function was applied to the free induction decay.) This result suggests that the ethane product formed upon ethylene adsorption is weakly adsorbed either on the silica support or on the chemisorbed layer rather than directly interacting with the metal. The effect of temperature on the nature of adsorption and reaction of ethylene on silica-supported ruthenium is shown in Fig. 6. For this experiment a 30 day old sample in which all butene had hydrogenated to butane was analyzed under CP/MAS at 140°C. After -8 h two distinct changes were noted. First, the amount of butane noticeably decreased at the same time that new peaks assigned to propane and methane were observed. Secondly, the nature of the strongly adsorbed species appears to be changing as evidenced by the change in the broad features in the spectrum. The broad peak at -85 ppm is more clearly distinguished or separated from the alkyl portion of the line. This change may be due to a loss in the amount of strongly adsorbed alkyl species from the surface or to their conversion to acetylide. The hydrogen produced from such a conversion could facilitate the hydrogenolysis reaction noted above. We have also studied ethylene adsorption of a series of Ru-Cu bimetallic catalysts because, due to the nature of the metal-metal mixing properties and pure component cohesive energies, we can selectively populate edge, corner and other defect-like sites with copper. Since copper is orders of magnitude

-100

400

PPM

FROM

00

TMS

Fig. 6. 13C CP/MAS spectra of species adsorbed on silica-supported ruthenium as a result of decomposition of ethylene at (a) room temperature for 30 days and (b) room temperature for 30 days plus 140°C for 8 h.

less reactive than ruthenium, we essentially eliminate specific catalytic sites. For example, the spectrum for ethylene adsorbed on a catalyst with copper comprising about 15% of the metal is shown in Fig. 7. Also shown is a schematic diagram of the way a bimetallic metal particle may look [ 54,551. The spectrum in Fig. 7 shows evidence for butadiene in both the cis- and trans-configurations. Two sets of double peaks (114 t- 1 and 139 + 1 ppm) straddle the peak associated with the butenes. Also, no ethane was observed. The effect of eliminating ruthenium from the edge and corner sites appears to be a great reduction in the hydrogenation capability of the catalyst. The eth-

53

I~~‘~~~~~‘I”“““~I”~‘~~~“I’~“~~~‘~I’~~~~’~~~I~’~~~~~~~I~~~”’~~r 0 200 100 300 400

PPM

FROM

-100

-260

-:

IO

TMS

Fig. 7. 13C CP/MAS spectrum of species adsorbed on silica-supported ruthenium-copper bimetallic catalysts as a result of ethylene adsorption and reaction. The catalyst is 4% by weight Ru and copper comprises 15% (atomic) of the metal. In the schematic representation of a bimetallic particle the dark atoms are ruthenium.

ylene is converting to molecules containing four carbon atoms and it would appear that butadiene is the primary product. Most likely some of the butadiene is converted to 2-butene and butane to account for those products. At higher copper loadings a similar experiment yielded another peak assigned to unreacted ethylene (Fig. 8). Some butadiene and butane were observed while the majority of the product consisted of 2-butene. Note that a similar experiment with a pure copper catalyst gave a spectrum with a very low intensity peak at a position corresponding to gaseous ethylene. The overall picture of the adsorption, decomposition and reaction of ethylene on silica-supported ruthenium inferred from the results of the present work is summarized below. At room temperature the ethylene adsorbs and rapidly forms strongly adsorbed species including acetylide and alkyl groups. In addition, ethane and a mixture of 2-butenes are rapidly formed and are weakly adsorbed (most likely on the support) or are experiencing restricted motion in the pores of the support. The butenes are then slowly hydrogenated to form butane. The sequential dosing experiments indicated that once formed, the rigid species identified as acetylide is no longer an active participant in any of the catalytic conversions.

54

400

300

200

100

PPM

0

FROM

-100

-200

-300

TMS

Fig. 8. 13C CP/MAS spectrum of species adsorbed on silica-supported ruthenium-copper bimetallic catalysts as a result of ethylene adsorption and reaction. The catalyst is 4% by weight Ru and copper comprises 30% (atomic) of the metal.

The hydrogenation capability of the catalyst appears to be associated with the edge and corner sites. When copper selectively poisons the defect-like sites no ethane is produced and a mixture of butane, 2-butene and butadiene are observed.

CONCLUSIONS

The application of 13Chigh-resolution NMR techniques allows the formation of a coherent picture of the chemistry of a relatively complex system by determining the structure and abundance of molecular species adsorbed on the catalyst surface under pressures close to industrial conditions. The 13C CP/ MAS technique allows simultaneous observation of the transformations of chemisorbed and weakly adsorbed molecules. Direct 13C excitation allows quantitative measurements of the various species present. From these experiments we observed the decomposition of ethylene at room temperature to form strongly adsorbed acetylide and alkyl species. Recombination of adsorbed species and hydrogenation of ethylene occurred rapidly at

55

room temperature and formed weakly adsorbed ethane and cis- and trans-2butene that subsequently hydrogenated to butane. The formation of C-C bonds is postulated to take place through an adsorbed metallocycle alkyl species. At 140’ C propane and methane are also formed by way of butane hydrogenolysis. Strongly adsorbed acetylide was not appreciably consumed in the formation of products, although it may have served as a host for other reactions. Spin counting revealed that there was one carbon in the strongly adsorbed layer for each surface ruthenium atom. The use of Ru-Cu bimetallic catalysts demonstrated that the hydrogenation capability of the catalyst is associated with the presence of defect-like ruthenium sites. Butadiene observed on the Ru-Cu bimetallic catalysts is most likely the primary C, product molecule. ACKNOWLEDGMENTS

This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Contract No W-7405ENG-82. One of the authors (JCK) wishes to acknowledge the financial support of the Amoco Foundation. Additional support was obtained from the Iowa State University Engineering Research Institute.

REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

M.M. Hills, J.E. Parmeter, C.B. Mullins and W.H. Weinberg, J. Am. Chem. Sot., 108 (1986) 3554. C.M. Greenlief, P.L. RadIoff, X.L. Zhouand J.M. White, Surf. Sci., 191 (1987) 93. M.A. Barteau, J.W. Broughton and D. Menzel, Appl. Surf. Sci., 19 (1984) 92. C. Egawa, S. Naito and K. Tamura, J. Chem. Sot. Faraday Trans. 1,82 (1986) 3197. G.H. Hatzikos and R.I. Masel, Surf. Sci., 185 (1987) 479. D. Godbey, F. Zaera, R. Yeates and G.A. Somoqjai, Surf. Sci., 167 (1986) 150. H. Steininger, H. Ibach and S. LehwaId, Surf. Sci., 117 (1982) 685. L.L. Kesmodel, L.H. Dubois and G.A. Somorjai, J. Chem. Phys., 70 (1979) 2180. J.E. Demuth, Surf. Sci., 84 (1979) 315. H. Ibach and S. Lehwald, J. Vat. Sci. Technol., 15 (1978) 407. E.M. Stuve and R.J. Madix, J. Phys. Chem., 89 (1985) 105. J.A. Gates and L.L. Kesmodel, Surf. Sci., 124 (1983) 68. L.L. Kesmodel, G.D. Waddill and J.A. Gates, Surf. Sci., 138 (1984) 464. W.T. Tysoe, G.L. Nyberg and R.M. Lambert, J. Phys. Chem., 88 (1984) 1960. E.M. Stuve, R.J. Madix and C.R. BrundIe, Surf. Sci., 152/153 (1985) 532. L.H. Dubois, D.G. Castner and G.A. Somorjai, J. Chem. Phys., 72 (1980) 5234. J.E. Demuth and H. Ibach, Surf. Sci., 78 (1978) L238. S. Lehwald and H. Ibach, Surf. Sci., 89 (1979) 425. J.C. Bertolini and J. Rousseau, Surf. Sci., 83 (1979) 425. W. Erly, A.M. Baro, P. McBreen and H. Ibach, Surf. Sci., 120 (1982) 273. M.R. Albert, L.G. Snedden and E.W. Plummer, Surf. Sci., 147 (1984) 127. M.M. Hills, J.E. Parmeter and W.H. Weinberg, J. Am. Chem. Sot., 108 (1986) 7215.

56 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

P.M. George, N.R. Avery, W.H. Weinberg and F.N. Tebbe, J. Am. Chem. Sot., 105 (1983) 1393. N. Sheppard, J. Electron Spectrosc. Relat. Phenom., 38 (1986) 175. R.J. Koestner, M.A. Van Hove and G.A. Somorjai, J. Phys. Chem., 87 (1983) 203. A.M. Baro and H. Ibach, J. Chem. Phys., 74 (1981) 4194. R.G. Windham, B.E. Koel and M.T. Paffett, Langmuir, 4 (1988) 1113. T.P. Beebe, M.R. Albert and J.T. Yates, J. Catal., 96 (1985) 1. T.P. Beebe and J.T. Yates, J. Phys. Chem., 91 (1987) 254. C. De la Cruz and N. Sheppard, J. Chem. Sot. Chem. Commun., (1987) 1854. T.M. Duncan and C. Dybowski, Surf. Sci. Rep., 1 (1981) 157. C.P. Slichter, Ann. Rev. Phys. Chem., 37 (1986) 25. P.K. Wang, J.P. Ansermet, S.L. Rudaz, Z. Wang, S. Shore, C.P. Slichter and J.H. Sinfelt, Science, 234 (1986) 35. P.K. Wang and C.P. Slichter, J. Phys. Chem., 89 (1985) 3606. I.D. Gay, J. Catal., 108 (1987) 15. M. Pruski, J.C. Kelzenberg, B.C. Gerstein and T.S. King, J. Am. Chem. Sot., submitted. I.D. Gay, J. Catal., 80 (1983) 231. X. Wu, B.C. Gerstein and T.S. King, J. Catal., 118 (1989) 238. X. Wu, B.C. Gerstein and T.S. King, J. Catal., in press. R.K. Shoemaker and T.M. Apple, J. Magn. Reson., 67 (1986) 367. E.G. Stejskal and J. Schaefer, J. Magn. Reson., 18 (1975) 560. S.R. Hartmann and E.L. Hahn, Phys. Rev., 128 (1962) 2042. R.D. Bertrand, W.B. Moniz, A.N. Garroway and G.C. Chingas, J. Am. Chem. Sot., 100 (1978) 5227. G. Engelhardt and D. Michel, High-Resolution Solid-State NMR of Silicates and Zeolites, Wiley, New York, 1987, pp. 368-478. T.M. Duncan, P. Winslow and A.T. Bell, J. Catal., 95 (1985) 305. W. Bremser, B. Franke and H. Wagner, Chemical Shift Ranges in Carbon-13 NMR Spectroscopy, VerIag Chemie, Weinheim, F.R.G., 1982. J. Evans and G.S. McNulty, J. Chem. Sot. Dalton Trans., (1984) 79. A.J. Carty, A.A. Cherkas and L.H. Randall, Polyhedron, 7 (1988) 1045. R.H. Grubbs, A. Miyashita, M. Liu and P. Burk, J. Am. Chem. Sot., 100 (1978) 2418. R.H. Grubbs and A. Miyashita, J. Am. Chem. Sot., 100 (1978) 1300. S.J. McLain, C.D. Wood and R.R. Schrock, J. Am. Chem. Sot., 99 (1977 ) 3519. J.R. Engstrom, D.W. Goodman and W.H. Weinberg, J. Am. Chem. Sot., 110 (1988) 8305. P. Basu and J.T. Yates, J. Phys. Chem., 93 (1989) 2028. M.W. Smale and T.S. King, J. Catal., 119 (1989) 441. J.K. Strohl and T.S. King, J. Catal., 116 (1989) 540.