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Preparation of Amorphous Cu-Ti and Cu-Zr Alloys of High Surface area by Chemical Modification
Guczi, L et d.(Editors), New Frontiers in Catalysk Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
PREPARATION OF AMORPHOUS Cu-Ti AND Cu-Zr ALLOYS OF HIGH SURFACE AREA BY CHEMICAL MODIFICATION
S.Yoshidu,T.Kakehi, S.Matsumoto, T. Tanaka, H. Kanai and T. Funabiki Department of Hydrocarbon Chemistry and Division of Molecular Engineering, Faculty of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan
Abstract
Amorphous Cu-Ti and Cu-Zr alloys of high surface area (3-12 m 2 g-') have been prepared by chemical modification; doping of Zn atoms into the surface layers and leaching the Zn atoms with a diluted NaOH solution. A treatment of the alloys with 0, and H, under mild conditions were effective to obtain alloys of higher surface area and catalytic activity for methanol dehydrogenation 1. INTRODUCTION In the last decade, amorphous alloys (metallic glasses) have attracted much attention in many fields as newly appeared industrial materials. Although the studies on catalysis by metallic glasses have been, so far, not abundant, it is revealed that they are potential materials for catalysis attaining high activity and selectivity to desired chemicals, if we can utilize the unique structure and electronic state effectively [ll. In the series of study on catalysis by metallic glasses, we found that amorphous Cu-Ti alloys revealed high turnover frequencies in dehydrogenation of methanol to methyl formate 121. However, the surface area of the alloy ribbons produced by rapid quenching is small (ca. 0.1 m2 g-') and the activity normalized to a unit mass is substantially lower than that of conventional catalysts. Several methods have been proposed to enlarge the surface area. The simplest one is mechanical grinding of ribbons [3]. By this method, we can obtain alloys of 2-3 m 2 g-'. A more complicated physical procedure in a vacuum chamber produce alloys with much higher area [41, but these are not suitable for catalysts preparation, because the amount of obtained alloys is much limited. An alloy of huge surface area was reported by Shimogaki et al. [51. They found that the area was enlarged to 70 m2 g-' after a CO-H, reaction. However the alloy was crystallized. In the present study, the enlargement of surface area of alloys in an amorphous state has been attempted by a technique; doping and leaching of zinc atoms and the catalytic activity for methanol dehydrogenation was investigated.
* present address: Department
of Living Science, Kyoto Prefectural University, Kyoto 606
982
2. EXPERIMENTAL
Ribbon forms of amorphous Cu-Ti (Cu 67,Ti 33 in atom %) and Cu-Zr (Cu 62, Zr 38 in atom %) alloys were prepared by the rapid quenching method using a single steel roll, pulverized by a vibratory mill and sieved by 200 and 400 mesh screens (amorCuTi and amorCuZr). Doping of zinc atoms to amorCuTi or amorCuZr was conducted with a following manner; impregnation of alloy powder with a zinc formate solution, drying and heating the sample in uucuo at 200 "c, and then under a hydrogen atmosphere at 235 "C The zinc atoms doped in the powder were leached by a 1.3 M NaOH solution at 55 "c for a given time and washed by distilled water. These powders will be referred to as porCuTi and porCuZr. For comparison, the crystallized powder alloys (crysCuTi and crysCuZr) were prepared by heating amorCuTi (crystallization temperature, Tc= 415 "C 1 and amorCuZr (Tc= 490 "c) at 500 "C for 100 min in uucuo. The BET surface area of samples was estimated using krypton physisorption at 77 K The saturated amount of adsorbed CO was also measured a t room temperature as described elsewhere [61. Structural analyses were carried out by means of X-ray diffraction (XRD) with a Rigaku Geigerflex 2013, differential scanning calorimetry (DSC) with a Rigaku TG-DSC 808931. X-ray photoelectron spectroscopy (XPS) with a Phi Model 5500MT was employed for characterization of the surface state. Dehydrogenation of methanol was carried out with a conventional flow type reactor at 200 "C under atmospheric pressure using He as a carrier gas. The products were analyzed on line by two gaschromatographs. The one was attached with a FID detector and a Porapak T 2 m column for analysis of organic compounds and the other with a TCD detector and a n activated carbon 2 m column for analysis of CO a n d CO,.
3.RESULTS AND DISCUSSION 3.1. Structural change by doping and leaching of Zn At first, the effect of adsorbed amount of Zn(HCOO), and the condition of leaching Zn with a NaOH solution were investigated in detail for amorCuTi. As for the amount of Zn(HCOO),, the best result was obtained for a mol ratio; CuTi:Zn(HC00)~25:1.The effect of NaOH concentration (0.7 - 2.0 M) on the surface area was not significant, if the leaching time was the same, e.g. 100 min. Prolonged leaching (e.g. 300 min) with a dilute NaOH solution (e.g. 0.7 M) was effective to obtain a catalyst with higher surface area, but caused dissolution of main components of alloys. Thus, we settled the standard preparation condition as the initial ratio of CuTi(CuZr):Zn(HCOO)2 in the doping process = 25:l and the treatment of the doped samples with a 1.3 M NaOH solution at 55 "C for 100 min. Figure 1shows XRD patterns of amorCuTi , porCuTi and crysCuTi and those of CuZr analogs. As shown in the Figure, the amorphous state was kept after the doping and leaching process with the standard condition. In the Figure, besides the pattern of porCuZrl which was prepared with the standard
983 condition, a pattern of porCuZr4 prepared with the leaching time of 400 min is also included. The pattern indicates a slight crystallization.
I
30
I
40
I
I
60
diffraction angle12 8
1
L
30
I
I
40
I
I
60
I
diffraction angle12 0
Figure 1. XRD patterns of CuTi and CuZr alloy powder. (a) amorCuTi, (b) porCuTi, (c) crysCuTi, (d) amorCuZr, (e) porCuZrl, (0 porCuZr4, (g) crysCuZr. See text for sample labels. Thermal analysis by DSC revealed that there were two exothermic phase transitions at ca. 270 and 420°C for amorCuTi. The latter peak was a superposition of two peaks; a minor peak appeared as a shoulder in a lower temperature on the main peak. The first transition around 270°C was not observed for porCuTi and also a sample after doping but before leaching of Zn. Since the XRD patterns of amorCuTi heated at 300 "C and porCuTi did not show any peaks due to crystalline phases, the first transition could be ascribed to a structural relaxation in amorphous alloys [71. The second transition is crystallization as revealed by XRD. It is interesting that the DSC data shows a two-stepped crystallization. Leaching of Zn with a NaOH solution brought about a decrease in the heat of crystallization, i.e. for CuTi; before leaching and after leaching with a 1.3 M NaOH solution for 100 min, 17.0 and 16.2 calg-', respectively. This suggests that leaching of Zn causes some ordering in the texture of amorphous alloys. The same phenomena were also observed for CuZr alloys with 274 "C for the relaxation and 490 C for crystallization.
3 8 Effects of oxygen pretreatments on the surface properties 32.1. Surfacearea In previous studies on catalysis by amorphous alloys, we found that the surface properties is significantly affected by 0, pretreatments under a mild condition [S]. Thus, the comparison was made for catalysts pretreated with 0,
984
(P= 50 torr) at a given temperature for 1h, followed by H, treatment (P= 100 torr) at 200 "c for 1h. (referred to as the 0,-H, treatment hereafter). Table 1 shows the effects of the O,-H, treatment on the surface area and the saturated amounts of adsorbed CO for CuTi series. Obviously, the Zn doping-leaching increased the surface area and CO uptake significantly, indicating that the surface of porCuTi is much rough. The 02-H2 treatment is effective for further increase in the surface area except the crystallized crysCuTi. Although the effect was not appreciable to porCuTi, significant increase in CO uptake was observed, suggesting a change in the electronic state of the surface atoms. In the case of CuZr series, the surface area of amorCuZr alloy powder is already large, indicating that powder with significantly rough surface was prepared by the pulverization with a mill. It was noted that the amorphous CuZr ribbons as quenched were pulverized more easily than the CuTi ribbons. Leaching of Zn for 100 min (porCuZr1) was not enough for appreciable enlargement of the surface area as shown in Table 2. By elongation of the leaching time to 400 min, alloys of much high surface area (porCuZr4) were obtained, although some partial crystallization was accompanied a s mentioned above. Another remarkable characteristics of CuZr alloys was that the surface area increased with the temperature of oxygen treatment in the 0,-H, treatment,
Table 1 The effects of 0, treatment on surface area and CO adsorption of CuTi alloys Catalyst"' amorCuTi
crysCuTi
temperature of surface area (m2/g)
0,treatment("C1 noneb'
0.2
0.36
250 -300 noneb' 200
1.8 0.2 0.2
2.54 0.45
200
250
porCuTi
saturated amount of adsorbed CO (mmol g-cat.')
300 noneb'
200 250 300
0.8 1.1
0.2 0.2 3.5 3.7 3.7 3.1
All samples were treated with H, at 200 C for 1 h. a) See text. b)Without oxygen treatment.
1.52 1.74
0.59 0.65
0.76 0.92 3.3 3.92 3.27
985
Table 2 The effects of 0, treatment on surface area and CO adsorption of CuZr alloys Catalyst"' amorCuZr
temperature of surface area O? treatment('C1 (m2/g)
crysCuZr
noneb' 200 250 300 noneb'
porCuZrl
300 noneb'
porCuZr4
200 250
200 250
300 noneb' 200
250
300
2.9 4.5 7.1 10 1.0 1.1 1.7 3.3 2.4 3.7 6.1 10 4.9 6.4 9.5 12
saturated amount of adsorbed CO (mmol g-cat.') 0.75 2.55 3.22 7.91
0.50
0.87 1.04 1.46 1.20 1.26 3.44 4.09 2.41 3.17 2.94 3.84
See captions to Table 1for a ) and b). while the oxygen treatment at 300 "C brought about a decrease in the surface area in the case of CuTi alloys. This will relate with the difference in the stability of amorphous state as indicated in the crystallization temperature (415 "C for amorCuTi and 490 "Cfor amorCuZr).
3.2.2. Surface composition Surface composition and valence state of metallic species of the powder alloys were estimated by analysis of the XPS spectra. As the samples were contacted with air after the 02-H2 treatments, the surface was probably oxidized. Thus, the XPS measurements were carried out for samples sputtered by A r ions (3keV) for 12 sec. No sodium atoms were detected for all samples. The binding energies of T i ( 2 ~and ~ ~Zr(3d,,,) ~ ) were 485 and 183eV in all samples, indicating that these atoms in +4 state. The binding energy of Zn(2p3/,) was 1022 eV. Because of the small difference in binding energy of Zn(0)(1021.5eV) and Zn(II)(1021.7 eV) and the weak intensity of the peak, the valence state can not determined definitely by XPS,but the kinetic energy of Zn LMM Auger electrons (988 eV) indicates that Zn is in +2 state. The peak profile of Cu(2pPsla)was asymmetric, indicating a mixture of Cu(I1) and Cu(I) or Cu(0). As the binding energies of Cu(1) and
986
Cu(0) are very close, it is impossible to distinguish these species. Auger electron spectra, which afford the information about low valence states of Cu, showed the coexistence of Cu(I) and Cu(O), however the quantitative analysis was not possible. Thus, the quantitative analysis for Cu atoms was done by deconvolution of XPS spectra without distinction between Cu(0) and Cu(1). The results are shown in Table 3,and 4.
Table 3 Surface composition of CuTi powder alloys Composition (atom %) Cu (Cu(0)o r Cu(1) Cu(1I)) Ti(IV) Zn(I1)
Sample"' amorCuTi porCuTi noneb'
0,m
OCC'
0,250"c"
0,300 "C"
80
(16
64)
20
43
(37 (34 (49
6)
56 57 41 40
42 58
60
(50
8)
9) 10
1 1 1 ~
All samples were treated with H, at 200 "c for lh.
a) See text for the sample. b) Without oxygen treatment. c) Treated with 0, at the designated temperature for 1 h before the hydrogen treatment.
Table 4 Surface composition of CuZr powder alloys Composition (atom %) Cu (Cu(0)or Cu(I) Cu(II)) Zr(IV) Zn(II) Sample"' 52 (47 5) 48 amorCuZr porCuZrl 6) 69 4 noneb' 27 (21 9) 58 4 0,200 "C" 37 (28 15) 54 3 0,250 "C" 43 (28 15) 35 3 0,300 C"' 62 (47 porCuZr4 0,250 "C" 52 (37 15) 46 2 See captions to Table 3 for a), b) and c).
987
Generally, the doping and leaching of Zn process brought about a decrease in the concentration of surface Cu, suggesting that some Cu atoms dissolved into NaOH solutions in the leaching process. In fact, too much long time leaching was accompanied by dissolution of Cu ions. As seen in the Tables, doped Zn atoms were not completely leached under the present condition. Especially, the amount is large for CuZr alloys. Elongation of leaching time to 400 min reduced the amount, but still higher amount of Zn existed in the CuZr than that in CuTi. This would relate to the high surface area of amorCuZr, which is effective to dope Zn atoms in the surface layers. It is interesting that the amount of Zn decreased by the O,-H, treatment, and simultaneously, the amount of Cu increased, indicating that Zn atoms diffused into the deep layers, while C u atoms moved to the surface layers. A remarkable difference between CuTi and CuZr alloys is observed for the change in the concentration of Cu(I1) by the O,-H, treatment. In CuTi series, the change is small with the change in the temperature of oxygen treatment, while a significant increase was observed for the porCuZr treated by oxygen at above 250 "C. It should be noted that all samples were treated by H, at 200 "C after the first treatment by oxygen. The first oxygen treatment oxidizes Cu atoms in the surface layers being accompanied by enrichment of Cu ions and the subsequent hydrogen treatment reduces some part of Cu ions to CuU) or Cu(0). The results mentioned above indicate that Cu(1I) ions in porCuZr are more stable to the hydrogen treatment.
35. Methanol dehydrogenation
Previously, we reported the catalysis by amorCuTi for methanol dehydrogenation using a closed gas circulation reactor a t low pressure. A significant enhancement in the activity was observed by the O,-H, treatment [2]. In a preliminary experiment using the above reactor, the present porCuTi was more effective than amorCuTi, however, in the reaction by a flow type reactor, it was found that the activity decreased with time on stream gradually and a stationary activity was not obtained even after 2 h. As mentioned in the previous section, the surface states of porCuTi are not stable under a reducing atmosphere and this would the reason for the unstable catalytic
Table 5 Dehydrogenation of methanol by CuTi catalysts catalyst conversion (%) amorCuTi crysCuTi porCuTi
4.0 1.9 6.2
selectivity (%) HCOOCH, CO 74 20 --a6 53 6
CO, 6 14 41
reaction condition; reaction temperature: 200 "C, catalyst: 400 mg, feed rate: MeOH (74 mmol min-') + He (20 cm3min-')+ air (0.2 cm3rnin-')
988
activity; the surface will be reduced gradually by methanol and the products at
200 “c. In fact, a stationary activity after about 1h was ‘obtained by addition of
small amount of air to the feed stream (air:MeOH = 1:lO). A comparison among amor-, crys- and por-CuTi is shown in Table 5 for samples after the 0,-H, treatment with 0, treatment at 250 “C. The activity is the highest for porCuTi, but the selectivity to methyl formate is low and considerable amount of CO, was formed presumably by combustion by air. As the results are disappointed one, CuTi alloys are not considered a s promising catalysts for the reaction. Contrary to CuTi alloys, CuZr alloys exhibited stationary activities within 1h on stream without addition of air. The 02-H2treatment was very effective to improve the activity with high selectivity to methyl formate. The effect is shown in Figure 2 for amorCuZr, crysCuZr, porCuZrl and porCuZr4. Note that the amount of catalysts was smaller and the feed rate was higher than those for CuTi catalysts in these experiments. Evidently, the activity increases with the temperature of 0, treatment in the O,-H, treatment. Especially, the 0, treatment a t above 250 ‘C is remarkable for porCuZrl. The selectivity to methyl formate decreased gradually with the temperature. Generally, the order of activity is crysCuZr c porCuZrl c amorCuZr < porCuZr4 for catalysts after the 02-H2treatment under the same condition. The lower activity of porCuZrl than that of amorCuZr would result from the smaller surface area and residual Zn atoms (See Table 2 and 4). As expected, the activity is roughly related to the surface area, the total concentration of Cu and the saturated amount of adsorbed CO as shown in Table 2. The valence state of Cu should be essentially important to the reaction. Previously, we postulated Cu(1) ions as the active species 121. However, in the present work, no close relation between the valence state of Cu and the activity or the selectivity to methyl formate. Takagi et al. concluded that Cu(I1) ions are the active species for the selective formation of methyl formate in the dehydrogenation of methanol over Cu ion-exchagned fluorotetrasilicic mica [81. The present result, the higher activity of the CuZr catalysts treated with 0, at higher temperatures and the instability of CuTi catalysts, seems to support their conclusion. However, the decrease in the selectivity with the temperature suggests that the Cu(I1) ions alone are not responsible to the formation of methyl formate. It is more probably that a combination of Cu(I1) and Cu(1) or Cu(0) in a suitable ratio is essential, judging from W S data of the present work.
989
8 \
30n 50
A
8 \
30
40t
;20
20
10
0
(u) 200 250 300
(u) 200 250 300 temperature 1°C crysCuZr
temperature / "c amorCuZr
I¶ 10
0
(u) 200 250 300 350 temperature / "c porCuZrl
8 \
40
-
100
60
,$ 3
40 2
8
20 0
A
30 20 -
1om 0
(u) 200 250 300 temperature /"c amorCuZr4
0
Figure 2. Effects of the temperature of 0, treatment in the O,-H, treatment on the activity and selectivity of CuZr catalysts reaction condition; reaction temperature: 200 "C, catalyst: 300 mg, feed rate: MeOH(106 pmolmin-'1 + He(25.5 cm3min"). See text for the catalyst labels. conversion, A selectivity to HCOOCH,, selectivity to CO, selectivity to CO,. (u)without 0, treatment
990 4.
REFERENCES
1 A. Molnar, G.V. Smith an d M. Bartok, Adv. Catalysis, 36 (1989)329. 2 H. Yamashita, T. Kaminade, M. Yoshikawa, T. Funabiki and S. Yoshida, C1 Mol. Chem., 1 (1986)491. 3 H.Yamashita, T. Kaminade, T. Funabiki and S. Yoshida, J. Mater. Sci. Lett.,4 (198511241. 4 S. Ohnuma, Y.Nakanouchi and T. Masumoto, Rapidly Quenched Metals, Proceedings 5th. Int. Conference Vol 11, Elsevier Sci. (Netherland) (1985) 1117. 5 Y. Shimogaki, H. Komiyama, H. Inoue, T. Masumoto and H. Kimura, Chem. Lett.,(1985)661. 6 H.Yamashita, M. Yoshikawa, T. Funabiki and S. Yoshida, J. Chem. SOC., Faraday Trans. 1, 81 (1985)2485. 7 T.Masumoto and Maddin, Mater Sci. Eng.,19 (1975) 1. 8 K.Takagi, Y. Morikawa and T. Ikawa, Chem. Lett., (1985)527. DISCUSSION
Q: G. V. Smith (USA) I would like to suggest that these materials may not be entirely amorphous at their surface and especially after oxidation-reduction cycles. Although XRD shows no crystalline peaks, this method may not be sensitive enough to detect surface order. Indeed, your DSC data suggest some ordering after leaching Zn from CuTi, but you do not report seeing ordering from XRD of these samples. In part I base my suggestion on some of our recent results from decomposing trimethyl silane and silane over highly dispersed Pd/Si02 [l]. Although Si from the decomposition inhibits hydrogenation activity of Pd, such activity can be restored by oxidation-reduction cycles similar to those reported by you. We believe both components are oxidized but only the metal is reduced. (You show similar results). As a result, the reduced surface is composed of islands of Si02 and Pd. The fact that more than the initial hydrogenation activity can be restored for the smaller Pd particles reater than 50% exposed) suggests that Pd surface reconstruction may be occurring. f course, merely exposing these amorphous alloys to air results in surface oxidation, so it is reasonable to suggest surface reconstruction resulting in surface ordering under reducing conditions for all "amor hous" alloys which have been exposed to oxygen. . Tsandra, G. V. Smith, T. Wiltowski, Stoch, F. Notheisz, M. Bartbk, D. [l] Ostgard, in "Catalysisof Organic Reaction", (Ed. W . Pascoe) Marcel Dekker, Inc. (1992).
8
s
A: S. Yoshida I agree with your suggestion that our catalysts in use are not entirely amorphous (the state as quenched). We have already reported that conversion electron Mossbauer spectroscopy which is sensitive to surface states revealed some ordering of Fe in a Fe-Zr amorphous alloy even after pulverization of ribbons as quenched, although the conventional Mossbauer spectroscopy did not show no ordering (J. Mater. Sci. Letters, 6 (1986) 1163). However, we do not believe that the aggregate has the same geometrical structure as that of bulk crystals. The structure would be highly disordered compared with that of the bulk crystals. In the present case, the catalysts were treated by 0,and H, and it is possible that more ordering in the surface layers occurred. But I can not conclude whether the crystallites of complete order are generated or those are still in a disordered state.
991
Q: T. Katona (Hungary)
Amorphous Cu-Zr alloys are very well known to undergo substantial structural change upon H2-treatment even under relatively mild conditions. Thus, the copper surface area is increased considerably. Did you try to determine the Cu(0) surface area by N 0 titration ’? are working on the same Cu-Zr alloy system in the dehydrogenation of CH30H to methyl formate with the exception that we did not use Zn. We also obtained selectivity above 80 % with fairly good conversion. From this point of view the Zn, indeed, does not play role in the reaction, rather a delicate balance of ionic and metallic Cu is needed.
he
A. S. Yoshida We did not measure the N 2 0 uptake. However, as shown in the text, w e measured CO uptake as a measure of the surface area of copper, as Ti or Zr is in an oxidized stale. The results indicate that for original alloys, the increase in the surface area by the O2-Ht treatment is remarkable but not so significant in the alloys treated by Zn. Q: J. W. Geus (The Netherlands) My question is concerned with the state of your catalyst after the leaching of the zinc and the oxidation treatment. In our own research on alloy single crystals, we generally observe segregation of the less noble metal as an oxide on exposure to oxygen. Since the mobility oxygen in metallic copper is high, the uptake of oxygen will not be confined to a thin surface layer. Rather internal oxidation of the amorphous alloy will rapidly proceed and will lead to preferential oxidation of the titanium and zirconium within the alloy. The internal oxidation of titanium or zirconium and subsequent oxidation of copper will lead to porous material essentially consisting of copper oxide supported on titania and zirconia. Small oxidized copper particles d o not exhibit an ordered structure and d o not show X-ray or electron diffraction or diffraction contrast in transmission electron microscopy. My question therefore is d o your thermally treated catalysts show broadened X-ray diffraction pattern of titania or zirconia and is at least the surface layer of the alloy (highly) porous over a significant depth.
A: S . Yoshida The XRD patterns of the catalysts after the 0,-H2 treatment under the adopted condition are not so different from those before the treatment, although some indication of peak broadening was observed. I agree to your opinion that the oxidation is not confined to a thin surface layer, but I can not tell the depth of the oxidized layer. For other comment, see the answer to Professor Smith. Q: P. B. Wells (United Kingdom) In your reaction H2 is released in the dehydrogenation and 0, is present as oxidant to generate formic acid and methyl formate. Do you have any evidence for the occurrence of the H2/02 reaction, and whether this reaction corrodes the surface ? (H2/02 is known to corrode, say, Pt surface severely)
A S. Yoshida At first, w e did not feed oxygen in the reaction over Cu-Zr catalysts. In the reaction over Cu-Ti catalysts, w e fed a small amount of oxygen to obtain a stationary state in the activity as stated in the text. In this case, the H2/02 reaction may occur, however, the fact that the stationary state is achieved suggest the corrosion suggested by you is insignificant, if any.
Q: K. Klier (USA)
Efficient formation of HCOOCH, from methanol usually requires a dehydrogenation and a basic component in the catalyst. Do you have any evidence that the residual zinc, or
992 perhaps the undetected sodium could assume such a basic function '? In particular, is zinc with the counter ion essential for your activity and selectivity I?
A: S.Yoshida I think that the effect of zinc or undetected sodium as a basic component is not essential, As shown in Table 4, the contents of Zn in the catalysts are almost the same in spite of a remarkable difference in the activity among the catalysts. Q: J. L. G. Fierro (Spain) In order to elucidate what Cu-species are responsible for methanol dehydrogenation, there is a very simple procedure which consists in measuring the X-ray induced Auger parameters, aA, from the photoelectron spectrum of the spent catalysts, according to the equation
-
UA = hv + (KELMMOJ)
C~2~3/2)
in which hv is the energy of the excitation source, KEimM(Cu) is the kinetic energy of the X-ray induced Auger line (most intense peak), and KEi Cu 2p is the kinetic energy of the principal KE Cu 2p3n core level. Did you perform this type% measurements ? A: S.Yoshida We did the analysis by using the Auger parameters and concluded the coexistence of Cu(0) and Cu(l). However, the spectrum did not allow us a quantitative analysis by an exact deconvolution.
Q: R. Kieffer (France)
k
Promoting effect of Zn formate: have you com ared the promoted catalyst with a catalyst prepared from a ternary alloy containing Cu- r-Zn or Cu-Ti-Zn ?
A: S.Yoshida We prepared amorphous alloys from a Cu-Ti-Zn mother alloy in which 2 atom % of Zn was added. The catalysts were found to be crystallized more easily than those presented in the text. Thus, we did not investigate the catalysis in detail.
PREPARATION OF AMORPHOUS Cu-Ti AND Cu-Zr ALLOYS OF HIGH SURFACE AREA BY CHEMICAL MODIFICATION
S.Yoshidu,T.Kakehi, S.Matsumoto, T. Tanaka, H. Kanai and T. Funabiki Department of Hydrocarbon Chemistry and Division of Molecular Engineering, Faculty of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan
Abstract
Amorphous Cu-Ti and Cu-Zr alloys of high surface area (3-12 m 2 g-') have been prepared by chemical modification; doping of Zn atoms into the surface layers and leaching the Zn atoms with a diluted NaOH solution. A treatment of the alloys with 0, and H, under mild conditions were effective to obtain alloys of higher surface area and catalytic activity for methanol dehydrogenation 1. INTRODUCTION In the last decade, amorphous alloys (metallic glasses) have attracted much attention in many fields as newly appeared industrial materials. Although the studies on catalysis by metallic glasses have been, so far, not abundant, it is revealed that they are potential materials for catalysis attaining high activity and selectivity to desired chemicals, if we can utilize the unique structure and electronic state effectively [ll. In the series of study on catalysis by metallic glasses, we found that amorphous Cu-Ti alloys revealed high turnover frequencies in dehydrogenation of methanol to methyl formate 121. However, the surface area of the alloy ribbons produced by rapid quenching is small (ca. 0.1 m2 g-') and the activity normalized to a unit mass is substantially lower than that of conventional catalysts. Several methods have been proposed to enlarge the surface area. The simplest one is mechanical grinding of ribbons [3]. By this method, we can obtain alloys of 2-3 m 2 g-'. A more complicated physical procedure in a vacuum chamber produce alloys with much higher area [41, but these are not suitable for catalysts preparation, because the amount of obtained alloys is much limited. An alloy of huge surface area was reported by Shimogaki et al. [51. They found that the area was enlarged to 70 m2 g-' after a CO-H, reaction. However the alloy was crystallized. In the present study, the enlargement of surface area of alloys in an amorphous state has been attempted by a technique; doping and leaching of zinc atoms and the catalytic activity for methanol dehydrogenation was investigated.
* present address: Department
of Living Science, Kyoto Prefectural University, Kyoto 606
982
2. EXPERIMENTAL
Ribbon forms of amorphous Cu-Ti (Cu 67,Ti 33 in atom %) and Cu-Zr (Cu 62, Zr 38 in atom %) alloys were prepared by the rapid quenching method using a single steel roll, pulverized by a vibratory mill and sieved by 200 and 400 mesh screens (amorCuTi and amorCuZr). Doping of zinc atoms to amorCuTi or amorCuZr was conducted with a following manner; impregnation of alloy powder with a zinc formate solution, drying and heating the sample in uucuo at 200 "c, and then under a hydrogen atmosphere at 235 "C The zinc atoms doped in the powder were leached by a 1.3 M NaOH solution at 55 "c for a given time and washed by distilled water. These powders will be referred to as porCuTi and porCuZr. For comparison, the crystallized powder alloys (crysCuTi and crysCuZr) were prepared by heating amorCuTi (crystallization temperature, Tc= 415 "C 1 and amorCuZr (Tc= 490 "c) at 500 "C for 100 min in uucuo. The BET surface area of samples was estimated using krypton physisorption at 77 K The saturated amount of adsorbed CO was also measured a t room temperature as described elsewhere [61. Structural analyses were carried out by means of X-ray diffraction (XRD) with a Rigaku Geigerflex 2013, differential scanning calorimetry (DSC) with a Rigaku TG-DSC 808931. X-ray photoelectron spectroscopy (XPS) with a Phi Model 5500MT was employed for characterization of the surface state. Dehydrogenation of methanol was carried out with a conventional flow type reactor at 200 "C under atmospheric pressure using He as a carrier gas. The products were analyzed on line by two gaschromatographs. The one was attached with a FID detector and a Porapak T 2 m column for analysis of organic compounds and the other with a TCD detector and a n activated carbon 2 m column for analysis of CO a n d CO,.
3.RESULTS AND DISCUSSION 3.1. Structural change by doping and leaching of Zn At first, the effect of adsorbed amount of Zn(HCOO), and the condition of leaching Zn with a NaOH solution were investigated in detail for amorCuTi. As for the amount of Zn(HCOO),, the best result was obtained for a mol ratio; CuTi:Zn(HC00)~25:1.The effect of NaOH concentration (0.7 - 2.0 M) on the surface area was not significant, if the leaching time was the same, e.g. 100 min. Prolonged leaching (e.g. 300 min) with a dilute NaOH solution (e.g. 0.7 M) was effective to obtain a catalyst with higher surface area, but caused dissolution of main components of alloys. Thus, we settled the standard preparation condition as the initial ratio of CuTi(CuZr):Zn(HCOO)2 in the doping process = 25:l and the treatment of the doped samples with a 1.3 M NaOH solution at 55 "C for 100 min. Figure 1shows XRD patterns of amorCuTi , porCuTi and crysCuTi and those of CuZr analogs. As shown in the Figure, the amorphous state was kept after the doping and leaching process with the standard condition. In the Figure, besides the pattern of porCuZrl which was prepared with the standard
983 condition, a pattern of porCuZr4 prepared with the leaching time of 400 min is also included. The pattern indicates a slight crystallization.
I
30
I
40
I
I
60
diffraction angle12 8
1
L
30
I
I
40
I
I
60
I
diffraction angle12 0
Figure 1. XRD patterns of CuTi and CuZr alloy powder. (a) amorCuTi, (b) porCuTi, (c) crysCuTi, (d) amorCuZr, (e) porCuZrl, (0 porCuZr4, (g) crysCuZr. See text for sample labels. Thermal analysis by DSC revealed that there were two exothermic phase transitions at ca. 270 and 420°C for amorCuTi. The latter peak was a superposition of two peaks; a minor peak appeared as a shoulder in a lower temperature on the main peak. The first transition around 270°C was not observed for porCuTi and also a sample after doping but before leaching of Zn. Since the XRD patterns of amorCuTi heated at 300 "C and porCuTi did not show any peaks due to crystalline phases, the first transition could be ascribed to a structural relaxation in amorphous alloys [71. The second transition is crystallization as revealed by XRD. It is interesting that the DSC data shows a two-stepped crystallization. Leaching of Zn with a NaOH solution brought about a decrease in the heat of crystallization, i.e. for CuTi; before leaching and after leaching with a 1.3 M NaOH solution for 100 min, 17.0 and 16.2 calg-', respectively. This suggests that leaching of Zn causes some ordering in the texture of amorphous alloys. The same phenomena were also observed for CuZr alloys with 274 "C for the relaxation and 490 C for crystallization.
3 8 Effects of oxygen pretreatments on the surface properties 32.1. Surfacearea In previous studies on catalysis by amorphous alloys, we found that the surface properties is significantly affected by 0, pretreatments under a mild condition [S]. Thus, the comparison was made for catalysts pretreated with 0,
984
(P= 50 torr) at a given temperature for 1h, followed by H, treatment (P= 100 torr) at 200 "c for 1h. (referred to as the 0,-H, treatment hereafter). Table 1 shows the effects of the O,-H, treatment on the surface area and the saturated amounts of adsorbed CO for CuTi series. Obviously, the Zn doping-leaching increased the surface area and CO uptake significantly, indicating that the surface of porCuTi is much rough. The 02-H2 treatment is effective for further increase in the surface area except the crystallized crysCuTi. Although the effect was not appreciable to porCuTi, significant increase in CO uptake was observed, suggesting a change in the electronic state of the surface atoms. In the case of CuZr series, the surface area of amorCuZr alloy powder is already large, indicating that powder with significantly rough surface was prepared by the pulverization with a mill. It was noted that the amorphous CuZr ribbons as quenched were pulverized more easily than the CuTi ribbons. Leaching of Zn for 100 min (porCuZr1) was not enough for appreciable enlargement of the surface area as shown in Table 2. By elongation of the leaching time to 400 min, alloys of much high surface area (porCuZr4) were obtained, although some partial crystallization was accompanied a s mentioned above. Another remarkable characteristics of CuZr alloys was that the surface area increased with the temperature of oxygen treatment in the 0,-H, treatment,
Table 1 The effects of 0, treatment on surface area and CO adsorption of CuTi alloys Catalyst"' amorCuTi
crysCuTi
temperature of surface area (m2/g)
0,treatment("C1 noneb'
0.2
0.36
250 -300 noneb' 200
1.8 0.2 0.2
2.54 0.45
200
250
porCuTi
saturated amount of adsorbed CO (mmol g-cat.')
300 noneb'
200 250 300
0.8 1.1
0.2 0.2 3.5 3.7 3.7 3.1
All samples were treated with H, at 200 C for 1 h. a) See text. b)Without oxygen treatment.
1.52 1.74
0.59 0.65
0.76 0.92 3.3 3.92 3.27
985
Table 2 The effects of 0, treatment on surface area and CO adsorption of CuZr alloys Catalyst"' amorCuZr
temperature of surface area O? treatment('C1 (m2/g)
crysCuZr
noneb' 200 250 300 noneb'
porCuZrl
300 noneb'
porCuZr4
200 250
200 250
300 noneb' 200
250
300
2.9 4.5 7.1 10 1.0 1.1 1.7 3.3 2.4 3.7 6.1 10 4.9 6.4 9.5 12
saturated amount of adsorbed CO (mmol g-cat.') 0.75 2.55 3.22 7.91
0.50
0.87 1.04 1.46 1.20 1.26 3.44 4.09 2.41 3.17 2.94 3.84
See captions to Table 1for a ) and b). while the oxygen treatment at 300 "C brought about a decrease in the surface area in the case of CuTi alloys. This will relate with the difference in the stability of amorphous state as indicated in the crystallization temperature (415 "C for amorCuTi and 490 "Cfor amorCuZr).
3.2.2. Surface composition Surface composition and valence state of metallic species of the powder alloys were estimated by analysis of the XPS spectra. As the samples were contacted with air after the 02-H2 treatments, the surface was probably oxidized. Thus, the XPS measurements were carried out for samples sputtered by A r ions (3keV) for 12 sec. No sodium atoms were detected for all samples. The binding energies of T i ( 2 ~and ~ ~Zr(3d,,,) ~ ) were 485 and 183eV in all samples, indicating that these atoms in +4 state. The binding energy of Zn(2p3/,) was 1022 eV. Because of the small difference in binding energy of Zn(0)(1021.5eV) and Zn(II)(1021.7 eV) and the weak intensity of the peak, the valence state can not determined definitely by XPS,but the kinetic energy of Zn LMM Auger electrons (988 eV) indicates that Zn is in +2 state. The peak profile of Cu(2pPsla)was asymmetric, indicating a mixture of Cu(I1) and Cu(I) or Cu(0). As the binding energies of Cu(1) and
986
Cu(0) are very close, it is impossible to distinguish these species. Auger electron spectra, which afford the information about low valence states of Cu, showed the coexistence of Cu(I) and Cu(O), however the quantitative analysis was not possible. Thus, the quantitative analysis for Cu atoms was done by deconvolution of XPS spectra without distinction between Cu(0) and Cu(1). The results are shown in Table 3,and 4.
Table 3 Surface composition of CuTi powder alloys Composition (atom %) Cu (Cu(0)o r Cu(1) Cu(1I)) Ti(IV) Zn(I1)
Sample"' amorCuTi porCuTi noneb'
0,m
OCC'
0,250"c"
0,300 "C"
80
(16
64)
20
43
(37 (34 (49
6)
56 57 41 40
42 58
60
(50
8)
9) 10
1 1 1 ~
All samples were treated with H, at 200 "c for lh.
a) See text for the sample. b) Without oxygen treatment. c) Treated with 0, at the designated temperature for 1 h before the hydrogen treatment.
Table 4 Surface composition of CuZr powder alloys Composition (atom %) Cu (Cu(0)or Cu(I) Cu(II)) Zr(IV) Zn(II) Sample"' 52 (47 5) 48 amorCuZr porCuZrl 6) 69 4 noneb' 27 (21 9) 58 4 0,200 "C" 37 (28 15) 54 3 0,250 "C" 43 (28 15) 35 3 0,300 C"' 62 (47 porCuZr4 0,250 "C" 52 (37 15) 46 2 See captions to Table 3 for a), b) and c).
987
Generally, the doping and leaching of Zn process brought about a decrease in the concentration of surface Cu, suggesting that some Cu atoms dissolved into NaOH solutions in the leaching process. In fact, too much long time leaching was accompanied by dissolution of Cu ions. As seen in the Tables, doped Zn atoms were not completely leached under the present condition. Especially, the amount is large for CuZr alloys. Elongation of leaching time to 400 min reduced the amount, but still higher amount of Zn existed in the CuZr than that in CuTi. This would relate to the high surface area of amorCuZr, which is effective to dope Zn atoms in the surface layers. It is interesting that the amount of Zn decreased by the O,-H, treatment, and simultaneously, the amount of Cu increased, indicating that Zn atoms diffused into the deep layers, while C u atoms moved to the surface layers. A remarkable difference between CuTi and CuZr alloys is observed for the change in the concentration of Cu(I1) by the O,-H, treatment. In CuTi series, the change is small with the change in the temperature of oxygen treatment, while a significant increase was observed for the porCuZr treated by oxygen at above 250 "C. It should be noted that all samples were treated by H, at 200 "C after the first treatment by oxygen. The first oxygen treatment oxidizes Cu atoms in the surface layers being accompanied by enrichment of Cu ions and the subsequent hydrogen treatment reduces some part of Cu ions to CuU) or Cu(0). The results mentioned above indicate that Cu(1I) ions in porCuZr are more stable to the hydrogen treatment.
35. Methanol dehydrogenation
Previously, we reported the catalysis by amorCuTi for methanol dehydrogenation using a closed gas circulation reactor a t low pressure. A significant enhancement in the activity was observed by the O,-H, treatment [2]. In a preliminary experiment using the above reactor, the present porCuTi was more effective than amorCuTi, however, in the reaction by a flow type reactor, it was found that the activity decreased with time on stream gradually and a stationary activity was not obtained even after 2 h. As mentioned in the previous section, the surface states of porCuTi are not stable under a reducing atmosphere and this would the reason for the unstable catalytic
Table 5 Dehydrogenation of methanol by CuTi catalysts catalyst conversion (%) amorCuTi crysCuTi porCuTi
4.0 1.9 6.2
selectivity (%) HCOOCH, CO 74 20 --a6 53 6
CO, 6 14 41
reaction condition; reaction temperature: 200 "C, catalyst: 400 mg, feed rate: MeOH (74 mmol min-') + He (20 cm3min-')+ air (0.2 cm3rnin-')
988
activity; the surface will be reduced gradually by methanol and the products at
200 “c. In fact, a stationary activity after about 1h was ‘obtained by addition of
small amount of air to the feed stream (air:MeOH = 1:lO). A comparison among amor-, crys- and por-CuTi is shown in Table 5 for samples after the 0,-H, treatment with 0, treatment at 250 “C. The activity is the highest for porCuTi, but the selectivity to methyl formate is low and considerable amount of CO, was formed presumably by combustion by air. As the results are disappointed one, CuTi alloys are not considered a s promising catalysts for the reaction. Contrary to CuTi alloys, CuZr alloys exhibited stationary activities within 1h on stream without addition of air. The 02-H2treatment was very effective to improve the activity with high selectivity to methyl formate. The effect is shown in Figure 2 for amorCuZr, crysCuZr, porCuZrl and porCuZr4. Note that the amount of catalysts was smaller and the feed rate was higher than those for CuTi catalysts in these experiments. Evidently, the activity increases with the temperature of 0, treatment in the O,-H, treatment. Especially, the 0, treatment a t above 250 ‘C is remarkable for porCuZrl. The selectivity to methyl formate decreased gradually with the temperature. Generally, the order of activity is crysCuZr c porCuZrl c amorCuZr < porCuZr4 for catalysts after the 02-H2treatment under the same condition. The lower activity of porCuZrl than that of amorCuZr would result from the smaller surface area and residual Zn atoms (See Table 2 and 4). As expected, the activity is roughly related to the surface area, the total concentration of Cu and the saturated amount of adsorbed CO as shown in Table 2. The valence state of Cu should be essentially important to the reaction. Previously, we postulated Cu(1) ions as the active species 121. However, in the present work, no close relation between the valence state of Cu and the activity or the selectivity to methyl formate. Takagi et al. concluded that Cu(I1) ions are the active species for the selective formation of methyl formate in the dehydrogenation of methanol over Cu ion-exchagned fluorotetrasilicic mica [81. The present result, the higher activity of the CuZr catalysts treated with 0, at higher temperatures and the instability of CuTi catalysts, seems to support their conclusion. However, the decrease in the selectivity with the temperature suggests that the Cu(I1) ions alone are not responsible to the formation of methyl formate. It is more probably that a combination of Cu(I1) and Cu(1) or Cu(0) in a suitable ratio is essential, judging from W S data of the present work.
989
8 \
30n 50
A
8 \
30
40t
;20
20
10
0
(u) 200 250 300
(u) 200 250 300 temperature 1°C crysCuZr
temperature / "c amorCuZr
I¶ 10
0
(u) 200 250 300 350 temperature / "c porCuZrl
8 \
40
-
100
60
,$ 3
40 2
8
20 0
A
30 20 -
1om 0
(u) 200 250 300 temperature /"c amorCuZr4
0
Figure 2. Effects of the temperature of 0, treatment in the O,-H, treatment on the activity and selectivity of CuZr catalysts reaction condition; reaction temperature: 200 "C, catalyst: 300 mg, feed rate: MeOH(106 pmolmin-'1 + He(25.5 cm3min"). See text for the catalyst labels. conversion, A selectivity to HCOOCH,, selectivity to CO, selectivity to CO,. (u)without 0, treatment
990 4.
REFERENCES
1 A. Molnar, G.V. Smith an d M. Bartok, Adv. Catalysis, 36 (1989)329. 2 H. Yamashita, T. Kaminade, M. Yoshikawa, T. Funabiki and S. Yoshida, C1 Mol. Chem., 1 (1986)491. 3 H.Yamashita, T. Kaminade, T. Funabiki and S. Yoshida, J. Mater. Sci. Lett.,4 (198511241. 4 S. Ohnuma, Y.Nakanouchi and T. Masumoto, Rapidly Quenched Metals, Proceedings 5th. Int. Conference Vol 11, Elsevier Sci. (Netherland) (1985) 1117. 5 Y. Shimogaki, H. Komiyama, H. Inoue, T. Masumoto and H. Kimura, Chem. Lett.,(1985)661. 6 H.Yamashita, M. Yoshikawa, T. Funabiki and S. Yoshida, J. Chem. SOC., Faraday Trans. 1, 81 (1985)2485. 7 T.Masumoto and Maddin, Mater Sci. Eng.,19 (1975) 1. 8 K.Takagi, Y. Morikawa and T. Ikawa, Chem. Lett., (1985)527. DISCUSSION
Q: G. V. Smith (USA) I would like to suggest that these materials may not be entirely amorphous at their surface and especially after oxidation-reduction cycles. Although XRD shows no crystalline peaks, this method may not be sensitive enough to detect surface order. Indeed, your DSC data suggest some ordering after leaching Zn from CuTi, but you do not report seeing ordering from XRD of these samples. In part I base my suggestion on some of our recent results from decomposing trimethyl silane and silane over highly dispersed Pd/Si02 [l]. Although Si from the decomposition inhibits hydrogenation activity of Pd, such activity can be restored by oxidation-reduction cycles similar to those reported by you. We believe both components are oxidized but only the metal is reduced. (You show similar results). As a result, the reduced surface is composed of islands of Si02 and Pd. The fact that more than the initial hydrogenation activity can be restored for the smaller Pd particles reater than 50% exposed) suggests that Pd surface reconstruction may be occurring. f course, merely exposing these amorphous alloys to air results in surface oxidation, so it is reasonable to suggest surface reconstruction resulting in surface ordering under reducing conditions for all "amor hous" alloys which have been exposed to oxygen. . Tsandra, G. V. Smith, T. Wiltowski, Stoch, F. Notheisz, M. Bartbk, D. [l] Ostgard, in "Catalysisof Organic Reaction", (Ed. W . Pascoe) Marcel Dekker, Inc. (1992).
8
s
A: S. Yoshida I agree with your suggestion that our catalysts in use are not entirely amorphous (the state as quenched). We have already reported that conversion electron Mossbauer spectroscopy which is sensitive to surface states revealed some ordering of Fe in a Fe-Zr amorphous alloy even after pulverization of ribbons as quenched, although the conventional Mossbauer spectroscopy did not show no ordering (J. Mater. Sci. Letters, 6 (1986) 1163). However, we do not believe that the aggregate has the same geometrical structure as that of bulk crystals. The structure would be highly disordered compared with that of the bulk crystals. In the present case, the catalysts were treated by 0,and H, and it is possible that more ordering in the surface layers occurred. But I can not conclude whether the crystallites of complete order are generated or those are still in a disordered state.
991
Q: T. Katona (Hungary)
Amorphous Cu-Zr alloys are very well known to undergo substantial structural change upon H2-treatment even under relatively mild conditions. Thus, the copper surface area is increased considerably. Did you try to determine the Cu(0) surface area by N 0 titration ’? are working on the same Cu-Zr alloy system in the dehydrogenation of CH30H to methyl formate with the exception that we did not use Zn. We also obtained selectivity above 80 % with fairly good conversion. From this point of view the Zn, indeed, does not play role in the reaction, rather a delicate balance of ionic and metallic Cu is needed.
he
A. S. Yoshida We did not measure the N 2 0 uptake. However, as shown in the text, w e measured CO uptake as a measure of the surface area of copper, as Ti or Zr is in an oxidized stale. The results indicate that for original alloys, the increase in the surface area by the O2-Ht treatment is remarkable but not so significant in the alloys treated by Zn. Q: J. W. Geus (The Netherlands) My question is concerned with the state of your catalyst after the leaching of the zinc and the oxidation treatment. In our own research on alloy single crystals, we generally observe segregation of the less noble metal as an oxide on exposure to oxygen. Since the mobility oxygen in metallic copper is high, the uptake of oxygen will not be confined to a thin surface layer. Rather internal oxidation of the amorphous alloy will rapidly proceed and will lead to preferential oxidation of the titanium and zirconium within the alloy. The internal oxidation of titanium or zirconium and subsequent oxidation of copper will lead to porous material essentially consisting of copper oxide supported on titania and zirconia. Small oxidized copper particles d o not exhibit an ordered structure and d o not show X-ray or electron diffraction or diffraction contrast in transmission electron microscopy. My question therefore is d o your thermally treated catalysts show broadened X-ray diffraction pattern of titania or zirconia and is at least the surface layer of the alloy (highly) porous over a significant depth.
A: S . Yoshida The XRD patterns of the catalysts after the 0,-H2 treatment under the adopted condition are not so different from those before the treatment, although some indication of peak broadening was observed. I agree to your opinion that the oxidation is not confined to a thin surface layer, but I can not tell the depth of the oxidized layer. For other comment, see the answer to Professor Smith. Q: P. B. Wells (United Kingdom) In your reaction H2 is released in the dehydrogenation and 0, is present as oxidant to generate formic acid and methyl formate. Do you have any evidence for the occurrence of the H2/02 reaction, and whether this reaction corrodes the surface ? (H2/02 is known to corrode, say, Pt surface severely)
A S. Yoshida At first, w e did not feed oxygen in the reaction over Cu-Zr catalysts. In the reaction over Cu-Ti catalysts, w e fed a small amount of oxygen to obtain a stationary state in the activity as stated in the text. In this case, the H2/02 reaction may occur, however, the fact that the stationary state is achieved suggest the corrosion suggested by you is insignificant, if any.
Q: K. Klier (USA)
Efficient formation of HCOOCH, from methanol usually requires a dehydrogenation and a basic component in the catalyst. Do you have any evidence that the residual zinc, or
992 perhaps the undetected sodium could assume such a basic function '? In particular, is zinc with the counter ion essential for your activity and selectivity I?
A: S.Yoshida I think that the effect of zinc or undetected sodium as a basic component is not essential, As shown in Table 4, the contents of Zn in the catalysts are almost the same in spite of a remarkable difference in the activity among the catalysts. Q: J. L. G. Fierro (Spain) In order to elucidate what Cu-species are responsible for methanol dehydrogenation, there is a very simple procedure which consists in measuring the X-ray induced Auger parameters, aA, from the photoelectron spectrum of the spent catalysts, according to the equation
-
UA = hv + (KELMMOJ)
C~2~3/2)
in which hv is the energy of the excitation source, KEimM(Cu) is the kinetic energy of the X-ray induced Auger line (most intense peak), and KEi Cu 2p is the kinetic energy of the principal KE Cu 2p3n core level. Did you perform this type% measurements ? A: S.Yoshida We did the analysis by using the Auger parameters and concluded the coexistence of Cu(0) and Cu(l). However, the spectrum did not allow us a quantitative analysis by an exact deconvolution.
Q: R. Kieffer (France)
k
Promoting effect of Zn formate: have you com ared the promoted catalyst with a catalyst prepared from a ternary alloy containing Cu- r-Zn or Cu-Ti-Zn ?
A: S.Yoshida We prepared amorphous alloys from a Cu-Ti-Zn mother alloy in which 2 atom % of Zn was added. The catalysts were found to be crystallized more easily than those presented in the text. Thus, we did not investigate the catalysis in detail.