Fresenius Z Anal Chem (1983) 314:315-318

Decomposition of Ethanol on Cu--ZnO and Pd--ZnO

Fresenius Zeitschrift ~r , ee

chemic ~" Springer-Verlag ]983

W. Mokwa, D. Kohl, and G. Heiland

2. Physikalisches Institut der Rheinisch-Westf/ilischen Technischen Hochschule Aachen, Templergraben 55, D-5100 Aachen, Federal Republic of Germany

Zerfall von Ethanol an Cu--ZnO und Pd--ZnO

Zusammenfassung. Der Zerfall von Ethanol auf kupfer- und palladiumbedeckten (000]-) ZnO-Oberflfichen wurde im UHV mit thermischer Desorptionsspektroskopie (TDS), Au- ger (AES) und durch Messung der Oberfliichenleitfiihigkeit untersucht.

Kupferbedeckte (0001) ZnO-Oberflfiche: Eine Monolage Kupfer unterdrtickt den Zerfall zu Ethylen und Wasser. Der Zerfall zu Acetaldehyd und Wasserstoff findet auf der kupferbedeckten Oberfl~iche bei niedrigeren Temperaturen statt als auf der reinen Fl~iche. Nach Angebot von Ethanol bei 130 K erreicht die Oberfliichenleitfiihigkeit nach Anlassen auf 300 K ein Maximum.

Palladiumbedeckte (0001) ZnO-Oberfl/iche: Nach dem ersten Anlassen auf 870 K verringert sich das Pd/Zn-Auger- Signalverhfiltnis auf die H/ilfte (Pd-Bedeckungen < 30 Mo- nolagen). Eine Desorption von Palladium wurde nicht gefun- den. Dies lfigt auf Clusterbildung wfihrend des Anlagprozes- ses schliel3en.

Auf Flgchen mit einer Palladiumbedeckung bis zu 6 Mo- nolagen werden nach Adsorption von Ethanol zwei verschie- dene Reaktionen gefunden. Reaktionsprodukte sind einmal Ethylen und Wasser, zum anderen Acetaldehyd und Wasser- stoff. Die palladiumbedeckten Fliichen zeigten ein zweites Desorptionsmaximum ftir Ethylen bei 400 K und ffir Wasser bei 650 K.

Bei gr6f3eren Pd-Bedeckungen wird eine zusfitzliche De- sorption yon Kohlenmonoxid gefunden. Dies weist auf einen vollsdindigen Zerfall des Ethanols an den Pd-Clustern hin.

Summary. The decomposition of ethanol on copper and palladium covered (0001) ZnO faces was investigated in UHV by thermal desorption spectroscopy (TDS), Auger (AES) and by measurement of surface conductivity.

Copper covered (0001) ZnO faces: One monolayer of copper strongly suppresses the decomposition to ethylene and water. With copper present on the surface ethanol decom- poses to acetaldehyde and hydrogen at lower temperatures as on the clean face. After exposure to ethanol at 130K the surface conductivity reaches a maximum upon annealing to 300 K.

Palladium covered (0001) Z n O faces: The first annealing to 870 K reduces the Pd/Zn-Auger signal ratio by a factor of two (for Pd coverages below 30monolayers). No Pd was found desorbing. This indicates the formation of clusters upon annealing.

After adsorption of ethanol two different reactions are observed on faces covered with up to 6 monolayers Pd: one

Offprint requests to : W. Mokwa

pair of reaction products consists of ethylene and water, the other pair consists of acetaldehyde and hydrogen. The Pd covered surfaces show a second peak for ethylene desorption at 400 K and a second peak for water at 650 K.

At higher Pd coverages an additional desorption of carbon monoxide is found indicating a complete decom- position at Pd clusters.

Introduction

ZnO is a well known catalyst, e.g. for methanol synthesis and for the water gas reaction [2].

On clean (000]-) and (1010) faces of ZnO the decom- position of ethanol and acetaldehyde has already been studied [7, 10]. Adsorption and decomposition of methanol, form- aldehyde, dimethyl ether and acetone on (1010) faces of ZnO have been investigated by UPS [7]. TDS, LEED and UPS studies have been done for benzene adsorbed on (10]-0) faces [13]. Catalytic reactions are strongly influenced by the presence of various metals on the surface [2].

ZnO faces covered by Pd and Ni have been characterized by LEED, AES, and UPS [1, 14].

The influence of catalytically active metals on reactions has not yet been investigated for ZnO single crystal faces. AES as well as observation of desorbing species by a mass spectrometer provide useful information about such an oxide supported catalyst. The surface conductivity monitors changes in the stoichiometry of the oxide surface [6, 9].

Experimental

The ZnO crystals were grown from the vapour phase. An addition of copper lowered the bulk conductivity to about 10 -3 (ohm cm) -1. Besides large as grown O faces with a diameter of 6mm also O faces cleaved in air were used. Pressure bonded gold leads served for resistive heating as well as for supporting the crystals. An A u - N i thermocouple monitored the crystal temperature. In UHV the crystals were annealed in oxygen (930 K, 5 × 10- 3 Torr, 30 rain) and within 5 rain cooled down to room temperature. Then the crystals were heated in vacuum to 800 K for one minute to remove adsorbed oxygen. The cleanliness of the surface was checked by AES.

The vacuum system was equipped with a cryogenic pump of 6,0001/s and a turbomolecular pump of 3501/s (Fig. 1). During gas exposure only a capacitance pressure meter was in use. In this way cracking of the organic molecules was avoided. For adsorption the gases were guided by a stainless

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The surface conductivity was measured by a four probe technique [4, 17] at 130 K after annealing for 3 min at various temperatures. Palladium was evaporated by electron impact [16] at pressures below 10 -9 Tort. Copper was deposited on a tungsten wire in a separate vacuum chamber. This copper source allowed a working pressure below 10-~0 Torr. For both metals evaporation rates between 0+1 to 1 monolayers/ min were used (1 monolayer -~ 2.2 x 1015 cm-2) . They were monitored by a quartz crystal (resolution 0.1 ML).

AES was performed with a cylindrical mirror analyzer (Varian). The copper coverage was determined by monitoring the Cu (920 eV)/Zn (994 eV) signal ratio. The detection limit is reached at a ratio of 0.15 because of the zinc Auger transitions at 906, 916, and 939 eV. The Pd 330eV Auger transition was used for detecting palladium.

Determination of surface contamination by carbon is limited to about 0.2 monolayers by the overlapping of the carbon Auger transition at 270 eV with a Pd Auger transition.

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Results

Deposition of Copper on (0007) ZnO

A pronounced increase of surface conductivity appears after deposition of Cu at 200K (Fig. 2e). Between 600K and 900 K the surface conductivity decreases to nearly the initial value. Also the Cu/Zn Auger ratio is lowered by annealing at 350 K to half of the former value (Fig. 2b). The ratio remains constant up to 700 K and decreases again at higher annealing temperatures. The Auger measurement did not change the surface conductivity. The desorption of CuO molecules is shown in Fig. la. Desorption starts at 500 K and shows a maximum at 750 K. Besides CuO there are also Cu and Zn found desorbing with an intensity smaller by a factor of 5, During further deposition/annealing cycles the number of des orbing CuO molecules remains below 2 x 1013 cm-2.

Deposition of Palladium on (0007) ZnO

After deposition of Pd at 300 K the Pd/Zn ratio decreases gradually during annealing (Fig. 3). At 850 K one half of the starting value is reached. TDS measurements show that after deposition of Pd there is no desorption of Pd.

Pd deposition up to 10 ML at 130 K does not change the surface conductivity. This holds still for subsequent anneal- ings up to 850 K (Detection limit of surface conductivity changes: 10-9 A/V).

steel nozzle onto the front face of the crystal. Ethanol was purified by degassing in vacuum [18]. The ethanol exposures were chosen between 500 and 10,000 Langmuirs. The pressure recovered to below 10-9 Torr within 7 rain from an ethanol exposure of 600 s at 10 -6 Torr. TDS was performed in UHV with a constant heating rate ft. The desorption products were monitored with a quadrupole mass spectrometer (Extra Nuclear Lab.). Six selected masses could be recorded simul- taneously. The observed products were identified by compar- ing their cracking patterns to those determined in the present

Ethanol on C u - ZnO

Before exposure to ethanol the crystals were annealed to 700 K for 3 rain. The desorption spectrum subsequent to the adsorption of ethanol on an O face without copper is given in the upper part of Fig. 4 [10]. Besides ethanol (maximum at 440 K), also acetaldehyde and molecular hydrogen (maxima at 520 K and 530 K) are found.

Desorption of water at 400 K and ethylene at 560 K is observed, too. The additional structure in the ascending part of the acetaldehyde signal reflects a contribution from the cracking of ethanol in the mass spectrometer. The lower part

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of Fig. 4 shows a desorpt ion spectrum after exposure to ethanol of the same O face now covered with one monolayer of copper. The desorption of ethanol shows two maxima at 410 K and 450 K, respectively. Molecular hydrogen is desorb- ing already at 390 K and has two further maxima at 520 K and 590 K. Acetaldehyde desorbs at 440 K. Water and ethylene are desorbing with an intensity smaller by a factor o f ten (not shown in Fig. 4). Adsorp t ion of ethanol at 130 K does not change the surface conductivity (Fig. 5). A value of 10- 7 A/V was reached after 3 Cu deposit ion/annealing cycles. Upon annealing to 300 K the surface conductivity increases by a factor of 10. The value remains constant up to 550K and

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Fig. 6a and b. TDS spectra after ethanol adsorption at 300K (5 x 10 -6 Torr, 200 s). a O face covered with 4 monolayers Pd. The first peak in the curve of the acetaldehyde reflects the desorption of ethanol. b Desorption of CO and ethylene after different Pd coverages. No sen- sitivity corrections

decreases to the former value at 850 K. Fo r comparison the results from the clean surface are also given [10].

Ethanol on P d - ZnO

The desorption products following an exposure to ethanol of an O face covered with 4 monolayers of Pd are given in part A of Fig. 6. Besides ethanol (not shown in the picture) also acetaldehyde (maximum at 550 K), molecular hydrogen (max- imum at 550 K), ethylene (maxima at 400 and 650 K) and water (maxima at 400 and 650 K) are found desorbing. The first maximum in the acetaldehyde signal is a contr ibution of ethanol cracked in the mass spectrometer. At a Pd coverage of 6 monolayers an addit ional peak at 420 K can be seen after exposure to ethanol in the desorption curve of ethylene (curve b in part B of Fig. 6). Because in the desorption of mass 26 (ethylene) this peak is missing, it has to be at tr ibuted to a

318

desorption of carbon monoxide. After increasing the Pd coverage to 8 monolayers the ttesorption of carbon monoxide dominates after exposure to ethanol. No change in surface conductivity was found after exposure to ethanol and sub- sequent annealing on O faces covered with different amounts of Pd.

Discussion

Deposition of Copper on (0001) ZnO

Deposition of copper increases the surface conductivity (Fig. 2c). This corresponds to an increase in surface electron density of 4 x 1 0 1 2 c m -2 (kt= 100cm//(Vs) [17]). The de- crease of surface conductivity above 700 K is connected to a decrease of the copper concentration in the surface (Fig. 2b). This is partly due to a reaction to cupric oxide which is found desorbing above 500K (Fig. 2a). Oxygen vacancies are produced which have been discussed as surface donors [3]. The number of desorbing molecules is too small (< 0.1 ML) to explain the decrease of the Cu/Zn Auger signal ratio com- pletely. Copper atoms possibly diffuse into the bulk. This implies that for copper atoms the diffusion near the surface is enhanced [11]. A migration of copper atoms on the surface with formation of clusters seems to be more probable.

Deposition of Palladium on (0007) ZnO

After deposition of palladium annealing steps at tempera- tures between 400 K and 850 K result in a steady decrease of the Pd/Zn Auger ratio (Fig. 3). Because no palladium is found desorbing the decrease of the Pd/Zn ratio points to a formation of clusters. After several deposition/annealing cycles the LEED pattern of a Pd (111) surface appears besides the ZnO pattern [5]. Layer growth is found on O faces not annealed after Pd deposition [5]. Gaebler et al. [1] studied Pd deposition on polished and sputtered polar ZnO faces with AES and UPS. They also found layer growth on surfaces which were not annealed, On annealed thick Pd layers (23 A) they could establish cluster formation with scanning electron microscopy.

Ethanol on C u - ZnO

Ethanol decomposes on the O face to acetaldehyde and hydrogen as well as to ethylene and water (Fig. 4a) [10]:

(a) C 2 H s O H ~ H(CH3)CO + H2,

(b) CzHsOH ~ CzH 4 + H 2 0 .

Reaction (a) has been found to be activated [10]. The increase in surface conductivity upon annealing subsequent to ethanol adsorption (Fig. 5) has been explained by a reaction of acetaldehyde with lattice oxygen at higher temperatures [10], An oxygen vacancy is formed acting as a surface donor [3]. Copper on the O face shifts the reaction to lower temperatures (Fig. 4b, Fig. 5). Acetaldehyde is desorbing at nearly the same temperature than subsequent to adsorption of acetaldehyde [10]. The second maximum in the desorption curve of ethanol as well as of hydrogen appears at the same temperature as on the clean O face. These desorption states are typical for the clean O face. The additional maxima in the desorption of ethanol and molecular hydrogen could arise from desorption states characterizing the copper covered surface. Studies of the ethanol decomposition on oxygen covered (110) and (100)

Cu faces show a desorption of the same decomposition products already below 420 K [15, 20].

The small intensities of the water and ethylene signals indicate that dehydrogenation is the main process on the Cu covered O face. Parrot found a dehydrogenation of ethanol on polycrystalline Cu20 [12]. He discussed a reduction of the oxide by the acetaldehyde. In Fig. 5 the increase of surface conductivity below 300 K points to the same reaction on the Cu covered O face, too. But it cannot be excluded that the increase in surface conductivity arises from hydrogen.

Ethanol on P d - Z n O

Both decomposition reactions (a) and (b) are found on the O face covered with 4monolayers of palladium (Fig. 6A). Acetaldehyde and molecular hydrogen are desorbing at nearly the same temperatures as from the surface without metal. The two additional maxima in the desorption curves of water and ethylene indicate that mainly reaction (b) is in- fluenced by the palladium coverage. No increase in surface conductivity is found upon annealing subsequent to ethanol adsorption. A reaction with lattice oxygen seems to be inhibited by the presence of palladium. However, after adsorption of atomic hydrogen on the palladium covered surface an increase in surface conductivity shows that the surface is still sensitive [5].

At higher coverages (more than 6 ML Pd) a desorption of carbon monoxide is found indicating an additional decom- position reaction (Fig. 6B). Ltith et al. [8] studied the decomposition of acetaldehyde on evaporated polycrystalline Pd films with UPS. They observed a decomposition down to carbon monoxide:

(c) H(CH3)CO(ads )--* CO(ads) + CH4(gas).

The desorption of CO from the O face with Pd clusters may arise from the decomposition reaction (a).

Acknowledgement. This work was supported by the Deutsche For- schungsgemeinschaft.

References

1. Gaebler W, Jacobi K, Ranke W (1978) Surf Sci 75 : 355 2. Gmelins Handbuch (1972) Kohlenstoff TeilC2, Chapter 1.6.1,

Verlag Chemic, Weinheim 3. G6pel W (1978) J Vac Sci Technol 15(4):1298 4. Heiland G, Kunstmann P (1969) Surf Sci 13:72 5. Jacobs H, Mokwa W, Kohl D, Heiland G (in press) 6. Kohl D, Heiland G (1977) Surf Sci 63:96 7. Lfith H, Rubloff GW, Grobman WD (1976) Solid State Commun

18:1427 8. Liith H, Rubloff GW, Grobman WD (1977) Surf Sci 63 : 325 9. Mokwa W, Kohl D, Heiland G (1980) Surf Sci 99:202

10. Mokwa W, Kohl D, Heiland G (1982) Surf Sci 117:659 11. Miiller G, Helbig R (1971) J Phys Chem Solids 32:1971 12. Parrot L, Rogers JW, White JM (1978) Appl Surf Sei 1:443 13. P6ss D, Ranke W, Jacobi K (1981) Surf Sci 105:77 14. Ranke W, Jacobi K (1979) Surf Sci 88:138 15. Sexton BA (1979) Surf Sci 88:299 16. Tesche B (1975) Vakuum-Technik 24:104 17. Veuhoff E, Kohl D (1981) J Phys C, Solid State Phys 14:2395 18. Wachs IE, Madix RJ (1978) J Catalysis 53:208 19. Wachs IE, Madix RJ (1978) Appl Surf Sci 1 : 303 20. Wachs IE, Madix ILl (1978) Appl Surf Sci 84:303