Amorphous alloy catalysis

Jmmn.al

of Molecular

Catalysis,

64 (1991)

41

41-51

Amorphous alloy catalysis Part 1. Dehydrogenation of 2-propanol on amorphous Cu61ZrSg alloy precursors &p&d

Molnti,

Department

Tam&s Katona,

of Organic

Chemistry,

Mih&ly Bart6k*

J&sef A. University,

Szeged

(Hungary)

and Khroly Varga Department

of Applied

Chemistry,

J&sef

A. University,

Szeged

(Hunga&

(Received October 27, 1989; revised July 16, 1990)

Abstract The dehydrogenation of 2-propanol to acetone on ChrZree alloy has been studied for the 6rst time. The dehydrogenation reaction (flow reactor, 573 K) on as-received or preactivated (hydrogen, 573 K, 3 h or 473 K, 16 h) alloy brings about a continuous activation of the catalyst. Other types of pretreatment (heat treatment at 800 K for 0.5 h, or treatment with water vapour at 500 K for 24 h) induce an initially high, but unstable, activity. The activity changes parallel changes in the copper surface area (determined via NaO decomposition), except for samples prepared by HF dissolution, which results in a highly active and stable catalyst with a low surface area. XRD and DSC indicate crystallisation of the amorphous alloy samples under the given reaction conditions and as a result of pretreatment.

Introduction

The recent high interest in the use of amorphous metal alloys in catalytic studies [ 1, 21 can be attributed to their exceptionally attractive physical, chemical and surface properties. Rapid quenching methods [3], which have made amorphous alloys readily available in large quantity, allow the preparation of alloys in wide composition ranges with the controlled and reproducible properties essential for catalytic studies and applications. Most effort has so far been devoted to studies of the hydrogenation of carbon monoxide on metal-metalloid alloys. These thorough and wide-ranging studies have already led to a surface model of the amorphous Fe-B catalyst [4]. Less attention has been paid to other reactions and to the use of metal-metal alloys, although initial observations immediately indicated their interesting behaviour. It was found that amorphous metal-zirconium alloys transform to highly active and selective catalysts during the hydrogenation of carbon monoxide [5-91 or carbon dioxide [lo], and under the reaction conditions in ammonia synthesis [ 111. Recent studies have concentrated on *Author to whom correspondence should be addressed.

0304-5102/91/$3.50

Q Elsevier Sequoia/Printed in The Netherlands

42

the use of glassy copper alloys, mainly with zirconium as the second metal, different pretreatments being applied to generate active catalysts via appropriate structural modifications. The catalysts were studied in the hydrogenation of carbon monoxide [ 12, 131, carbon dioxide [ 141, alkenes [ 15-171, dienes [ 17-19) and acetylenes [ 141, and in cycloamination [ 191. These results and our continuing interest in the properties of copper in catalytic transformations stimulated the present study. Since the high activity of copper in the dehydrogenation of alcohols is well known [ 201, the dehydrogenation of 2-propanol was chosen as a model to study amorphous Cu-Zr specimens. Different catalyst pretreatments, some used in previous studies of Cu-Zr alloys, were tested in this reaction. In parallel, the bulk and surface changes taking place during the pretreatment and the reaction were monitored by XRD and via the decomposition of NaO. Certain attempts have previously been made to correlate the changes in activity of amorphous alloy catalysts during reaction with the changes in their surface areas. However, the BET surfaces used to calculate turnover numbers in some studies of Cu - Zr alloys [ 12, 16-l 91 are not relevant to the catalytically active surface. The decomposition of NaO was used in the present study, as this provides a more accurate measure of the active surface, which might allow a comparison of the activating effects of pretreatment and reaction.

Experimental

Materials Amorphous ribbons with a nominal composition of CuBIZrsowere prepared in air by the melt-quenching (melt-spinning) method, using the rotating wheel technique. Two specimens were made at the same quenching rate (27 m s-’ disk velocity) but with different thicknesses and widths (sample A: 30-50 pm and 5 mm, sample B: 100 pm and 10 mm, respectively). Copper powder (99.99%) and copper foil (25 pm thick, 99.999%) were products of Alfa Co. A 1.5% CuLZrO, catalyst was prepared by the precipitation method described in [21]; a ceramic grade ZrOa (Aldrich) was used. 2-Propanol (Reanal, Hungary, 100% pure by GC) was used without further purification. Oxygen-free hydrogen was prepared with a Matheson 8326 generator, operating with a palladium membrane. Helium (99.99%) was purified by passage through an Oxy-Trap and Indicating Oxy-Trap (Alltech). The nitrogen used in the catalytic studies was purified with a Matheson 6046 gas purifier. Methods Catalytic studies The gas phase dehydrogenation of 2-propanol was performed in a flow microreactor, consisting of an 8 mm i.d. Pyrex tube with a fritted glass insert serving as catalyst holder. A saturator composed of a sinter-glass evaporator and a condenser provided with good precision 1.71 X lo- 7 mol

43

2-propanol vapour ml-‘, corresponding to the condenser temperature (273 K). A total flow of 30 ml min-’ was applied with hydrogen or nitrogen as carrier gas. 60 mg of alloy, in the form of 1 cmX l-2 mm strips, was used in the catalytic studies. The other copper catalysts studied for comparison were copper foil (250 mg), copper powder (20 mg) and Cu/ZrOz (10 mg). Catalyst charactwisation Surface measurements were carried out with NzO titration at 363 K by the pulse method [ 221, with the modifications suggested in [23]. To ensure high reliability and high accuracy, the purified helium carrier gas was further purified by passage through a 20 cm guard column of MnO operating at 423 K, activated with hydrogen at 723 K for 12 h before use. It was connected directly to the inlet of the sample holder and was switched off immediately before the NzO pulses were introduced. Since the catalyst samples were removed from the reactor after the catalytic studies, a 0.5 h activation at 573 K in flowing hydrogen (25 ml min-‘) was applied before the determination of surface areas. X-ray diffraction analyses with a DRON-3 powder diffractometer (USSR), using Cu-K, radiation, were carried out to detect changes in the bulk structure of the specimens. A Perkin Elmer DSC-2 instrument was used for differential scanning calorimetric (DSC) measurements. Pretreatment Heat treatment of the amorphous alloy to achieve crystallisation was performed by keeping the samples in oxygen-free helium at 800 K for 0.5 h. Partial dissolution of zirconium was carried out with a 1 mol dmm3 HF solution [ 171. Since our samples were fabricated in air no special care was taken to exclude oxygen during this treatment. For treatment with water vapour, a nitrogen stream (50 ml min-‘) saturated with water at room temperature was passed through the amorphous or crystallised alloy samples kept at 500 K [12].

Results and discussion The dehydrogenation of 2-propanol led selectively to acetone. However, on certain catalysts in the highly active state, a small amount of 4-methyl2-pentanone (up to 5%) was formed. This by-product is formed in a multistep process. Condensation of 2 molecules acetone yields 4-methyl-4-hydroxy-2pentanone. The ready elimination of water from this intermediate leads to the formation of 4-methyl-3-penten-2-one, which is transformed to 4-methyl2-pentanone by hydrogenation of the carbon-carbon double bond. In some cases, traces of the intermediates were also detected. XRD analysis of the as-quenched amorphous alloy samples indicates substantial differences between the two specimens (Fig. 1). Both sides of sample A exhibited the broad band centred at N 40” which is characteristic of amorphous materials. The shiny side, which was not in contact with the

I

-b

$0

40

20

20 (deg)

1. XRD diffractograms of as-received ribbons. (a) sample A, shiny side; (b) sample A, dull side; (c) sample B, shiny side; (d) sample B, dull side. Fig.

10

50

100

150 time on stream ( h I

F+ig.2. Changes in catalytic activity of amorphous and heat-treated (tryst.) samples.

rotating wheel (also called the outer face or free-surface side) showed traces of crystallinity, indicating the presence of crystalline ZrOz. In contrast, sample B exhibited a higher degree of crystallinity, pointing to a lower quenching rate. In addition to ZrCz crystals, the dull side (inner face or rapidly quenched side) also exhibited copper reflections. The two specimens gave similar DSC patterns, with a single exothermic peak at 765 K (heating rate: 10 deg min-1). As regards catalytic activities, a common feature of both samples is continuous activation during the dehydrogenation of Z-propanol (Fig. 2). A marked increase in activity was observed in the first 10 h, but further changes were still taking place even after 150 h on stream. However, the increase

45

in activity of the totally amorphous sample A was smaller. This observation parallels the results of earlier studies with different amorphous alloys, which also indicated higher catalytic activities of metastable samples with a certain low, controlled degree of crystallinity [IS, 18, 24, 251. After these initial observations, only the more promising sample B was used in further studies. When the reaction was carried out on a sample crystallised by heat treatment, the opposite trend was observed. After a small increase, the high initial activity slowly decreased, reaching values similar to those of the amorphous specimens (Fig. 2). The activation pattern of the amorphous Cu-Zr sample pretreated in hydrogen (573 K, 3 h) was very similar to those of the non-treated samples, except that it displayed higher initial and higher final activities (Fig. 3). Hydrogen pretreatment of the crystallised specimen caused a decreased activity. The polycrystalline copper catalysts (Cu foil, Cu powder and Cu/ ZrO,) studied for comparison exhibited decreasing activities after activation in hydrogen, behaviour characteristic of the crystallised alloy (Fig. 3). Similar tendencies, i.e. either activation or deactivation, were observed for samples pretreated in other ways. Water vapour is known to have a detrimental effect on the structure of zirconium-containing amorphous alloys [ 12, 261. Pretreatment of our Cu - Zr specimen with water vapour at elevated temperature according to [ 121 resulted in the formation of catalysts with high initial and slowly decreasing activities (Fig. 4). Another method of activation of metal-zirconium (and metal-titanium, -tantalum, -niobium) amorphous alloys is immersion of the sample in dilute HF solution. This treatment was first used to activate electrode materials for water electrolysis [ 27-301, methanol [ 31, 32) or formaldehyde electrooxidation [33-371 and chlorine evolution [38, 391. It has also been applied to prepare a copper catalyst highly active in carbon-carbon double bond hydrogenation [ 171.

time cn stream (h) Fig. 3. Changes in catalytic activity of amorphous and heat-treated samples and polycrystalline copper catalysts after pretreatment in hydrogen at 573 K for 3 h (reactions in hydrogen).

.I

20

ttme on stream

24

(h)

Fig. 4. Changes in Catalytic activity of amorphous and heat-fated samples after treatment with water vapour at 500 K for 24 h. (H): reaction in hydrogen, (N): reaction in nitrogen.

as-receive (HI

1

as-received IN)

1

20

timeon stream

24

J

( h)

Fig. 5. Changes in catalytic activity of amorphous and heat-treated samples after treatment with hydrogen fluoride (see also Experimental). (- . . . e): l-min treatment, (- - -, -): 5-min treatment.

The thermodynamically unstable nature of amorphous alloys is known to provide high chemical reactivity unless a passive fiIm is formed 138, 401. HF treatment results in vigorous hydrogen evolution and selective dissolution of the second metal. Stable crystalline alloys composed of multiple phases are not suitable for such preferential dissolution [38, 401. Scanning electron microscopy revealed surface roughening, while XPS indicated that a significant copper enrichment takes place during the brief HF treatment, which leaches out Zr and its oxide [ 17, 301. As a result, a Raney-type porous structure is formed on the surface of the amo~hous alloy. Our samples prepared in this way exhibited different behaviour, depending on the duration of HF treatment and the reaction conditions (Fig. 5). The rather varied activities observed

47

apparently identical treatment (dissolution for 5 min) demonstrates the difhculties involved in controlling the degree of dissolution. The X-ray diffractograms of the different samples reveal that the reaction itself and all forms of pretreatment result in drastic changes in the amorphous ribbon (Fig. 6). These include the partial or total crystallisation of the samples, the formation of metallic copper particles and the oxidation of zirconium to zirconium oxide. DSC measurements also pointed to crystallisation. After the reaction or pretreatment, the exothermic peak of crystallisation at 765 K was not detected. The results of surface measurements with N20 also indicate that significant changes in the copper surface area accompany the pretreatment and the reaction. Either a large increase or a decrease in surface area can be observed after

v

f qeaction

LA---

ezzYd”

&

d

L/-+---C

40

b

30

HF + reachon

HF

u

50

in N

Hz0

treatment

M

20

(degl

Fig. 6. XRD diiractograms of samples after different types of pretreatment and reaction. (a) heat treatment, (b) water vapour, (c) HF treatment, (d) HF treatment, after 24-h reaction, (e) as-received, after reaction in hydrogen, (f) as-received, after reaction in nitrogen.

48

(Table 1). For example, reactions in nitrogen always result in a smaller increase in area (for a non-treated sample) or a larger decrease in area (for pretreated catalysts) as compared to reactions in hydrogen. Surface measurements carried out at regular short time intervals during either water or HF treatment revealed a continuous increase in the surface area of the as-received sample (Figs. 7 and 8A). In contrast, but in harmony with the corrosion behaviour of crystallised alloys [38, 401, much smaller changes were exhibited by the heat-treated ahoy (Figs. 7 and 8A). Similar me~~ernen~ during reaction on the as-received alloy activated by water revealed a continuous decrease in surface area (Fig. 8s). A comparison of the activity data after reaction for 24 h with the corresponding surface areas does not allow clear-cut conclusions, although the as-received samples without pretreatment or after activation in hydrogen give very similar activities calculated on the basis of the number of active TABLE 1

Surface areas of catalysts before and after reaction (573 K, 24 h) determined by titration with N20 Catalyst

Wdk39

Pretreatment

Surface area (m” g-‘)

after treatment

after reaction

none

0.01&b

573 K/H/3 h

0.065

473 K/H/16 h water vapour’

0.077 2.22

HF/5 min’

1.33

0.19 (H) 0.09 (N)d 0.20’ (H) 0.20 (H) 0.17 (N) 0.18 (H) 0.91 (H) 0.19 (N) 0.10 (H) O.Olb (N)

none

o.o1*”

573 K/H/3 h water vapour’

0.12 0.27

HF/5 minf

0.06

0.45 0.02 0.35 0.38 0.04 0.28

573 K/H/3 h 573 K/H,‘3 h 573 K/H/3 h

0.19 O.Olb 0.40

-g -g 3

as-received

%2%39

heat-treated’

Cu powder cu foil CufirOs

“Measured after a 0.5 g sample was kept at 363 K in helium for 0.5 h. bEstimatedvalue, because of very low quantity of nitrogen evolved. ‘Reaction in hydrogen. dReaction in nitrogen. ‘Sample A; ali other experiments were carried out with sample B. rSee Experimental. Wnmeasurably low because of small sample size.

(H) (N) (H) (H) (N) (H)

49

SCU (rn2$)

+ 0.1 1.0 L!I

0

+ 1

2 3 4 5 time of treatment (min

1

Fig. 7. Changes in copper surface areas of amorphous (+) a function of the duration of HF treatment.

and heat-treated (0)

ribbons as

B

0

3

6

9 12 hme of

18

treatment (h)

24

0

3

6

12

tune :: stream21t(hl

Fig. 8. Changes in copper surface areas of amorphous (triangles) and heat-treatecl (hexagons) ribbons. (A): as a function of duration of water treatment. (B): as a function of the duration of the reaction after 24-h water treatment.

sites. Similarly, almost identical but lower activities are exhibited by the heat-treated samples. The most surprising observation is that the HF-treated sample exhibits a stable activity, despite an order of magnitude decrease in the copper surface area during reaction for 24 h (Table 1). A possible explanation for the exceptionally high and stable activity of HF-treated samples may be the high concentration of the ionic species formed during HF dissolution [ 171. These ions, together with metallic copper, have been proved to play an important role in alcohol dehydrogenation [ 4 l-431. Because of the complicated structure of the active sites, however, NaO decomposition alone is not an adequate means of characterising the working catalyst in this case.

50

Conclusions

Amorphous Cu61Zr39alloy was found to be active and selective in the dehydrogenation of 2-propanol to acetone. The as-received amorphous alloy exhibits a continuous activation during the reaction, which is a general characteristic of zirconium-containing alloys. Different types of pretreatment convert the amorphous alloy into a crystalline catalyst with a high initial, but rapidly decreasing activity. In contrast, the specimen prepared by HF treatment and the catalysts formed from the as-received amorphous alloys under the reaction conditions have the advantage of exhibiting stable or increasing activity. All these observations also direct attention to the extreme importance of surface characterization of the working catalysts in order to draw reliable conclusions on catalysts prepared from amorphous precursors. Further studies (DSC, SEM, EDAX, XPS, AES) are under way to acquire more detailed information about the structural changes and to clarify the nature of the active sites of the working catalysts.

Acknowledgements

The amorphous alloy samples used in the present study were supplied by Zolt6.n Hegediis (AGMI Institute for Material Testing and Quality Control, Budapest) and Csaba Kopasz (Csepel Metal Works, Budapest).

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