Recent advances in the catalytic properties of metastable materials

MATERIALS SCIENCE & EWGIWEERWG A

Materials Scienceand Engineering A226228 (1997) 891-899

Recent advances in the catalytic properties of metastable materials Koji Hashimoto Institute

fog Materials

Research,

Tohoku

University,

Sendai

980-77,

Japan

Abstract New catalysts and electrocatalysts which are amorphous alloys themselves or prepared from amorphous and other metastable alloys are reviewed in this paper. The most important characteristic of metastable materials including amorphous and nanocrystalline alloys from the chemical point of view is the homogeneous single phase nature consisting of a variety of elements, whose concentrations sometimes exceed the solubility limits at equilibrium. For the enhancement of the activity various treatments are carried out before catalytic reaction, such as oxidation-reduction and selective dissolution of alloy constituents. For example, after immersion in HF, amorphous Cu-Zr alloys show higher activity than amorphous Cu-Ti alloys for dehydrogenation of 2-propanol and hydration of acrylonitrile. This is due to dissolution of smaller amounts of titanium from Cu-Ti alloys than dissolution of zirconium from Cu-Zr alloys in HF, since the corrosion resistance of titanium in HF is higher than zirconium. The catalysts prepared from some amorphous nickel alloys are most effective for methanation of CO, at atmospheric pressure. Amorphous nickel-refractory metal alloys are the best cathode materials for electrolytic hydrogen evolution. The catalyst for CO2 methanation and the electrode for electrolytic hydrogen evolution are usedfor building a CO2 recycling plant to avoid global warming and to supply abundant energy. 0 1997Elsevier ScienceS.A. Keywords: Catalyst; Electrocatalyst; Hydrogen evolution

Amorphous

alloy; Nanocrystalline

1. Introduction The chemical application of materials generally requires the synergistic effect of various elements. Catalytic activity is often enhanced synergistically by coexisting elements. An electrocatalyst should possess high activity and durability. In order to provide such multiple functions it is necessary to form a single solid solution phase even if prescribed amounts of a variety of effective elements are contained within. Consequently, a chemically homogeneous single phase nature exceeding the solubility limits of alloying elements at equilibrium is the most important characteristic of amorphous alloys from the chemical point of view. The discovery of various extremely corrosion-resistant amorphous alloys was followed by investigations of electrode materials for the electrolysis of hot concentrated NaCl solutions in the late 197Os, since the electrode requires high corrosion resistance as well as high electrocatalytic activity. The anode performance of the amorphous alloys for the electrolysis of hot concentrated NaCl solutions was f&t reported in 1980 [l] and excellent anodes having high electrocatalytic activity 0921-5093/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved. DTTC -cl?, cnn3,nL\lno1” c

substance; Mechanical

milling;

CO, methanation;

CO, recycling;

and high corrosion resistance were tailored from amorphous precious metal-metalloid alloys containing palladium [2]. These investigations have been superseded by investigations of anode materials for use in the electrolysis of seawater; amorphous nickel-valve metal (Nb, Ta) alloys containing very small amounts of platinum group elements have been tailored for this purpose [3]. This investigation has progressed to the preparation of electrodes by laser and electron beam processing [4]. In the early 1980s basic studies of the hydrogen electrode reaction [5,6] and a study to use amorphous alloys as precursors for fuel cell electrodes [7,8] were also initiated. The activity of amorphous alloys for heterogeneous catalysis was at first examined in the hydrogenation of carbon monoxide [9]. A comparison of 15 different amorphous Ni-Fe-metalloid alloys and their crystalline counterparts reveals that amorphous alloys have higher activity, except for one particular alloy, Ni40Fe-20P. Because there is no difference in the reaction mechanism between amorphous and crystalline alloys, amorphous alloys seem to possess a higher density of active sites [9- 111.This interesting finding of

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/katerink

Science and Engineering

the high activity of amorphous alloys has stimulated a variety of studies all over the world into the catalytic activity of amorphous alloys. There have been several reviews of the catalytic properties of amorphous alloys [12-161. In this article recent results of the, performance of new catalysts and electrocatalysts which are amorphous alloys themselves or prepared from amorphous and other metastable alloys are presented. A brief description will be given of catalysis during mechanical milling.

2. Catalysis Amorphous alloys have been regarded as interesting from the catalytic point of view as follows: because of a lack of long range ordering, the surface of amorphous alloys is rich in low-coordination sites and defects which play an important role in catalysis [17]. The formation of a chemically homogeneous single phase solid solution exceeding the solubility limit at equilibrium enables us to modify readily the electronic properties and to tailor new homogeneous alloys in which alloying elements are supersaturated. Because of their metastable nature, amorphous alloys should be more reactive than their crystalline counterparts. It is, however, difficult to study the effect of the true amorphous alloy surface on catalytic reactions. The surface of amorphous alloys prepared by any method is generally covered by an air-formed oxyhydroxide film. Even if the surface oxyhydroxide can be reduced by conventional reduction methods without heating the specimen to higher temperatures than the crystallization temperature, the reduction treatment gives rise to a surface covered by the reduced metal or alloy which is not the same as the underlying bulk amorphous alloy. Accordingly, the characteristic of amorphous alloys affecting heterogeneous catalysis is mostly the topography of amorphous alloys, such as the density of terraces, kinks and ledges, the sizes of which are larger than the atomistic disorder in amorphous alloys. For instance, amorphous Pd-Si [IS] and Pd-Ge [19] are active for the deuteriumation of cis-cyclododecene producing more monodeutero trans-cyclododecene and dideuterocyclododecene at room temperature and 1 atm in comparison with palladium powder. The different selectivities were interpreted in terms of different surface topographies; the amorphous alloys are rougher than crystalline palladium and possess many sites of high coordinative unsaturation (20,211. This is not essential for all amorphous alloys but the surfaces of melt-spun amorphous alloys are rather inhomogeneous and sometimes contain crystalline-like regions. It may, therefore, be difficult to examine the effect of atomistitally uniform disordered structure on heterogeneous catalysis.

A226-228

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On the other hand, amorphous alloys consist of a highly reactive, homogeneous, single solid solution in a wide composition range exceeding the solubility limit at equilibrium and are used as new precursors of catalysts for specific reactions. In general, the direct use of amorphous alloys is not carried out but some surface activation treatments are applied for the enhancement of the catalytic activity, including reduction treatment in reducing atmospheres, such as PI2 [22], Hz-CO [23] and Hz-CO2 [24], and selective dissolution of alloy component(s) from the surface of the alloy specimen for surface roughening in addition to surface accumulation of active elements [25]. After selective dissolution in acids oxidation-reduction treatment is often carried out to further enhance catalytic activity. Even if activation treatment was applied previously, the surface of the amorphous alloy catalyst is often converted to the effective catalyst during the catalytic reaction. For instance, in early work using amorphous alloys as catalysts for the methanation of CO, the catalytic activity of amorphous Pd-Zr [26] and Ni-Zr [27,28] was shown to be greater than that of aluminasupported ruthenium and nickel catalysts. Their activity increases at a steady state due to the increase in the active sites during the catalytic reaction. This was interpreted differently for each alloy family as follows: the activity increase of the amorphous Ni-Zr alloys seems to be due to the significant increase in the effective surface area as a result of surface cracking, based on the formation of ZrO,, and due to the formation of the highly active NiZr compound. The catalytic reaction on the amorphous Pd-Zr catalysts does not lead to any increase in the effective surface area, but rather to the formation of a catalytically active new crystalline phase consisting of palladium, zirconium and oxygen. Accordingly, if the amorphous alloys are used as precursors, the catalytic properties are different depending upon alloy composition, catalytic reaction and surface activation method, although all catalysts are characterized by the fact that they have been prepared from a highly reactive, homogeneous, single solid solution In this paper two recent examples of catalytic reactions will be briefly reviewed. Dehydrogenation of 2-propanol on catalysts formed from amorphous copper alloys was studied by Katona and Molnar [29-331. Amorphous Cu-Zr alloys are active in the dehydrogenation of 2-propanol. Examples of the catalytic reaction are shown in Fig. 1. The dehydrogenation of 2-propanol is as follows: (CH,),CHOH

--+“’ (CH,),CO

(1) The activity of the Cu-Zr alloys in the dehydrogenation of 2-propanol is enhanced by the reaction itself [32], by oxidation-reduction treatment [32] and by treatment with H, [30], II,0 [30] and HF 1331. The reaction and these treatments result in the segregation

K. Hashinzoto /Materials

Science and Engineering A226-228

(1997) 891-899

893

duction treatment is applied to the Cu-Ti alloy, instead of dehydrogenation dehydration occurs as follows: (CH3)&HOH

Am

Cu-Ti

Am Cu-Ti

HF-tre

HF-treated

+

CH,CH=CH,

(2)

Only HF treatment is effective in enhancing the catalytic activity of Cu-Ti alloy in the dehydrogenation of 2-propanol, segregating copper on the surface beneath which TiO, is formed. However, the weight loss of Cu-Ti during immersion in 1 M HF is about one-third that of Cu-Zr and hence the surface segregation of copper on Cu-Ti is lower in comparison with Cu-Zr alloy. The difference between titanium and zirconium in the enhancement of catlytic activity by immersion in HF solutions is generally found not only in copper alloys but also other alloys such as nickel due to the higher corrosion resistance of titanium than zirconium in HF solutions [34]. The activation of crystallized Cu-Ti and Cu-Zr by immersion in HF is difficult because of selective dissolution of a phase from the crystalline two phase mixture instead of uniform selective dissolution of titanium and zirconium from homogeneous alloys. Similar results for amorphous copper alloys are observed for the hydration of acrylonitrile to acrylamide c351.

80 8 .

- Hz0

for 1 min

Fig. 1. Conversion of 2-propanol over amorphous and crystalline HF-treated Cu-Ti and Cu-Zr alloys as a function of reaction time [33] (from Tam&s Katona and Arp&d Molnar in Journal of Catalysis, Vol. 153 (1995) 334. Reprinted by permission of the publisher, Academic Press, Inc.)

CH,=CHCN

+z”

CH2=CHCONH,

(3)

Melt-spun amorphous Cu-33Ti and Cu-38Zr alloys were first pulverized by a vibratory mill, and then titanium and zirconium were leached out from the alloys by immersion in HF solutions. Table 1 shows the catalytic activities of these catalysts. The higher activity

of copper on the surface in addition to the formation of ZrOz in the interior. By contrast, Cu-Ti alloys are not active in the dehydrogenation of 2-propanol even after treatment with Hz and Hz0 [30]. When oxidation-re-

Table 1 Surface state and catalytic activity of catalysts prepared from amorphous and crystalline Cu-Ti and Cu-Zr alloys [35j (From T. Funabiki, H. Yamashita, M. Yase, T. Omatsu and S. Yoshida in Bull. Chem. Sot. Jpn., 66 (1993) 2134. Reprinted by permission of the publisher, the Chemical Society of Japan) Catalyst

HF cont.” (mol dmT3)

AN COIW.~(%)

AA sel.’ (%)

Surface aread m2 g-’

Amor. Cu-Ti

Nonef 0.2 1

Trace 1 11

-

0.2 2.3 4.2

43 74 86

Nonef 0.2 1 2

Trace 26 35 22

99 100 97

2.9 6.0 8.2 7.1

27 63 97 98

Amor. Cu-Zr

100 98

Cryst. Cu-Ti

1

3

93

6.8

80

Cryst. Cu-Zr

1 2

14 6

93 98

1.9 7.0

82 89

Powder Cu metal

Nonef

2

91

0.1

100

a Concentration of HF solution for pretreatment. b Acrylonitrile (AN) conversion after reaction for 4 h. c Conversion selectivity to acrylamide (AA). d Kr physisorption at 77 K. e Surface atomic ratio (%) calculated from XPS peak intensities of Cu 2p,,,, Ti 2p,,, and 2p,,,, and Zr 3d,,, and 3d3,2. ‘Samples were treated with H, at 473 K for 1 h.

894

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100

A

/Materials

, \

w

Science

and Etlgineet?ng

A226228

(1997)

100)

0

CH4

CH4

ao-

80-

8

891-899

Ni-I OZr

z; 60-

-

‘G > i! rz 40-

-

Ni-40Zr

8 ;60C> G ti z40v)

20-

20-

TH B 9 o----c----L -? 300 100 200 Temperature / “C

0

TH13 1 1 1 1 oo-200 300 Temperature / “C

100

100

I

I

I

Fe-70Zr

ao-

80

8

s

;SO .t: > ‘G

;60*Z ‘G > -

‘j8 40 co

z: 40(I)

0

0 Temperature

/ “C

co

I 100 200 Teniperature

300 / “C

Fig. 2. Selectivity of the reaction of 20% CO, and 80% H, on catalysts prepared from amorphous alloys [37].

of amorphous

Cu-Zr alloy in comparison with the Cu-Ti alloy can clearly be seen. In order to avoid global warming, the conversion of

the valve metals are all converted to oxides while the iron group elements are in the metallic state. Accord-

carbon dioxide has recently been seriously considered.

carried out at 1 atm of 0, at 500°C for titanium- and zirconium-containing alloys and at 600°C for niobiumand tantalum-containing alloys. Subsequent reduction of all specimens was performed in Hz at 1 atm and 300°C. The catalysts thus prepared consist of nanocrys-

If the large amount of carbon dioxide emitted globally is taken into account, the first requisite for its conversion is a very fast exothermic reaction occurring at atmospheric pressure using an inexpensive catalyst.

Methanation of carbon dioxide is the most promising reaction. CO, + 4H, -+ CH, + 2H,O

(4)

ingly, oxidation of these amorphous alloys was first

talline valve metal oxides on which iron group elements are finely dispersed. The catalysts have been used for methanation

of CO1 passing the reaction gas mixture of

20 mol.% CO1 and 80 mol.% H2 at atmospheric pres-

Amorphous iron group metal-valve metal (Ti, Zr, Nb or Ta) alloys have been used as catalyst precursors

sure. The conversion rate of CO2 on nickel alloy catalysts is generally more than two orders of magnitude

[36,37]. Under the condition for methanation of CO,

higher than that on iron alloy catalysts.The conversion

K. Hashbnoto /MateyiaB

Science and Engineering A226-228

rate of CO2 on cobalt alloy catalysts is located between those on iron and nickel alloys. An interesting fact is the reaction selectivity. As shown in Fig. 2 when the reaction is very fast on Ni-Zr catalysts CH, is formed with almost 100% selectivity but if the reaction rate is more than two orders of magnitude slower on Fe-Zr catalysts, the major product is carbon monoxide [37]. Among amorphous nickel-valve metal alloys Ni-Zr alloys show the highest activity for methanation of CO,. Fig. 3 shows the rate of formation of CH, on the catalyst prepared from amorphous Ni-40Zr alloy [38]. In this experiment a series of two reactors were used and before the reaction in the second reactor water formed by the reaction Eq. (4) in the first reactor was removed. It has also been reported [39] that the CH4 selectivity on amorphous Rh-80Zr alloy is almost 100% while no such high selectivity is found on crystalline rhodium catalyst supported on ZrO,. When pressurized conditions are used the selectivity is changed. Fig. 4 shows an example of the data obtained on Pd-Zr alloy at 15 atm [40]. Although the conversion is not high methanol is also formed. Similarly, when amorphous and crystalline powders prepared by the spark erosion method are used as the catalyst precursor at a pressurized condition such as 10 atm at 225”C, the formation rate of methanol in the gas form is 10.5 ml g-l h-’ for amorphous Cu-30Zr and 12 ml g-’ h-’ for crystalline Cu-30Zn [41]. The formation of CH, by the reaction of CO, and H2 on the catalyst prepared from amorphous Ni-Zr alloy is so fast that a COz recycling plant for the substantiation of our idea has been built on the roof of the Institute for Materials Research (IMR), Tohoku University using the catalyst and the amorphous alloy electrode mentioned later. It has been proved that CH4 formation by the reaction of CO, and H, on the catalyst prepared from the amorphous alloys is the

1st Reactor

160

11997) 891-899

180

EhO

Ni-4OZrAlloy Catalyst 300°C, 1 atm i

2000 qo -1 FAV I ml got h

6000

Fig. 3. Conversion to CH, at atmospheric pressure and 300°C as a function of the flow rate of the reactant gas mixture on 1 g of catalyst prepared from amorphous Ni-40Zr alloy [38].

260

Fig. 4. Selectivity and conversion of CO, at 15 atm on a Pd-67Zr alloy catalyst in-situ activated at 280°C [40]. (From Alfons Baiker and Danier Gasser in J. Chem. Sot. Faraday Trans, 1, Vol. 85 (1989) 1003. Reprinted by permission of the publisher, the Royal Society of Chemistry.)

most promising method to rapidly convert a large amount of CO, to a conventional fuel. Another interesting aspect in catalytic reactions is mechanical milling. Instead of thermal activation, mechanical activation during milling of amorphous alloys under a reactant gas atmosphere such as CO + H, results in a catalytic reaction in addition to conversion of the amorphous alloys [42-441. When mechanical milling of amorphous Ni-Zr alloys is conducted under the gas mixture of 1 mol CO and 3.3 mol H2 at a total pressure of 6 atm, CO dissociation leads to oxidation of zirconium and to separation of nickel which assists dissociative absorption of hydrogen. Accordingly the alloy was converted to nanocrystalline ZrO,, ZrO, Ni, and various Ni-Zr intermetallics and, at the same time, hydrocarbons are formed as shown in Fig. 5 [42]. If metal hydrides such as zirconium hydride are used for mechanical milling under a CO-H, atmosphere hydrocarbons are formed immediately without an induction period. Mechanical milling under a reactant gas atmosphere may be regarded as a new method for catalytic reaction.

3. Electrocatalysis 60

200 220 240 Temperature / “C

for hydrogen evolution

The hydrogen evolution reaction on metals has been extensively studied. It is known [45] that a periodic relationship between overvoltage for hydrogen evolution and atomic number exists. The overvoltage of each long period decreases with an increase in the number of d-electrons, then sharply increases when the d-orbitals are @led. Therefore, the second and third minima of the overvoltage for hydrogen evolution correspond to

896

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/Materials

Science

and Engineering

precious metals and the first one is nickel, although the overvoltage of nickel is higher than those of precious metals. It has been thought that alloying of nickel with a less electronegative element may result in charge transfer from the alloying element to nickel with a consequent decrease in the overvoltage for hydrogen evolution. The overvoltage of metallic elements for hydrogen evolution also changes with their boiling points showing a valley [46]; precious metals are located at the bottom of the valley. The boiling point of nickel is slightly lower than that of precious metals. Hence, alloying of nickel with a refractory element may decrease the overvoltage for hydrogen evolution. From these considerations the alloying of nickel with less electronegative refractory metals is carried out. Sputter-deposited Ni-Mo alloys [47] show an extremely high activity for hydrogen evolution in aqueous solutions and an interesting composition dependence, different from gas diffusion and mechanically milled electrodes. A Ni-Mo gas diffusion electrode was prepared as follows [48]. A suspension of PTFE and an ammoniacal solution containing nickel and molybdenum were sprayed on a nickel mesh and heated at 200-250°C. This procedure was repeated several times until all the perforations on the nickel mesh were fully coated with a metal oxide-PTFE mixture. The coated mesh was heated in air at 330°C and finally reduced under a hydrogen atmosphere at 330-350°C for 3 h. By this procedure the nickel mesh was covered by a metallic fee Ni-Mo solid solution. The overvoltage of the Ni-Mo gas diffusion electrodes for hydrogen evolution in a hot 100

I(

12

,“‘I1

1

1 mol CO : 3.3 mol H, at 6 atm

g ‘0

1 Id

11

l l

0

0

0

Ni-37Zr Ni-6OZr

0 0

2

4 Milling

Milling

6 Time / h

Time / h

8

IO

Fi& 5. Formation of hydrocarbons on Ni-Zr alloys in 1 mol CO and 3.3 mol H, at the total pressure of 6 atm as a function of milling time. The inset shows that CO decreases during the mechanical milling of Ni-60Zr powders [42]. (From G. Mulas, L, Conti, G. Scano, L. Schifini and G. Cocco in Materials Science and Engineering, Vol.A181/A182 (1994) 1088. Reprinted by permission of the publisher, Elsevier Science Ltd.)

5 E a

A226-228

(1997)

891-899

J

4003OwlO

KOH at 70°C

g300:

5 6 200: 5 g 100: G

0

* 5

10

15

20

MO Content

25

30

35

40

/ at%

Fig. 6. Hydrogen overvoltage of Ni-Mo gas diffusion electrode measured at 1 A cm-’ in 30 wt% KOH solution at 70°C [48]. (From D.E. Brown, M.N. Mahmood, M.C.M. Man and A.K. Turner in Electrochiiica Acta, Vol. 29 (1984) 1553. Reprinted by permission of the publisher, Elsevier Science Ltd.)

alkaline solution at 70°C is shown in Fig. 6 [48]. Brown et al. [49] attributed the high activity of the Ni-Mo coating for hydrogen evolution to the presence of the fee solid solution phase near the solubility limit of molybdenum in fee Ni. Schulz et al. [SO] examined the electrocatalytic activity of Ni-Mo alloys prepared by mechanical alloying for hydrogen evolution. They could not get a single amorphous phase alloy and have reported that the overvoltage for hydrogen evolution decreases linearly with a decrease in the size of fee crystal as shown in Fig. 7. They also have found that the milling in air is effective in decreasing the overvoltage for hydrogen evolution while the milling in argon is not effective, as shown in Fig. 8. Accordingly, they have concluded that the presence of oxygen in the alloy is essential for achieving a high activity for hydrogen evolution and that the active phase would be Ni-MO-0 rather than the Ni-Mo-fee solid solution. When Ni-Mo alloys were prepared by a sputter deposition technique on a nickel substrate they were converted to a single amorphous solid solution in the wide composition range from 21 to 70 at.% of molybdenum [47]. Ni-15 at.% MO alloy is composed of a single fee phase with very &ne grains. Since the electrode was tailored for the electrolysis of seawater the electrocatalytic properties were examined in 1 M NaOH at 30°C. As shown in Fig. 9 [47], the activities of sputter-deposited Ni-Mo alloys on nickel for hydrogen evolution are significantly higher than nickel metal. In particular decreasing molybdenum content increases the activity for hydrogen evolution in spite of the fact that molybdenum addition is necessary to decrease the hydrogen overvoltage. Because the adhesiveness of the sputter-deposit to the substrate nickel becomes poor with decreas-

K. Hashimoto /Materials

Science and Engineering A226228

ing molybdenum content the hydrogen evolution behavior of the alloys with lower than 15 at.% molybdenum content could not be measured. Instead of the preparation of low molybdenum alloys the molybdenum content of the surface of sputter-deposited alloys was lowered by leaching of molybdenum from the alloy surface by immersion in 10 M NaOH solution at 80°C. After leaching of molybdenum all alloys showed further higher activity for hydrogen evolution. For instance, the overvoltage for hydrogen evolution at lo3 A mm2 in 1 M NaOH at 30°C is only 80 mV. It can, therefore, be said that the formation of a single Ni-Mo solid solution containing a small amount of molybdenum is effective in enhancing hydrogen evolution regardless of whether they consist of amorphous or fee phase. Since the alloys containing 21 to 40 at.% molybdenum are not composed of the fee phase but of a single amorphous phase, the high electrocatalytic activity cannot be attributed to the formation of the fee Ni-Mo solid solution. In the present alloy preparation, after evacuation of the vacuum chamber to about 5 x 10v7 Torr sputter deposition of alloys was carried out at about 9 x 10 -’ Torr of high purity argon gas which was prepared by removal of oxygen, water and dust from argon gas of 99.9995% purity, and hence the

ot,,,,,,,““‘,,,,~,,‘, sss33q*1 10 20’ 30 0

‘5. 30%KOH .

Crystal

Size i nm

1

“‘-g. 0

30%KOH I 50 0 IO

at 70°C at 250 mA/cm’ I I I 20 30 40 Milling Time / h

i 50

Fig. 8. Hydrogen overpotential of mechanically alloyed Ni-Mo at 250 mA cm-* in 30 wt% KOH solution at 70°C as a function of milling time for a 75at.%Ni-25at.%Mo powder mixture milled under argon and air [SO]. (From R. Schulz, J.Y. Huot, M.L. Trudeau, L. Dignard-Bailey, Z.H. Yan, S. Jin, A. Lammarre, E. Ghali and A. Van Neste in J. Mater. Res., Vol. 9 (1994) p. 3006. Reprinted by permission of the publisher, Materials Research Society.)

oxygen content of the sputter deposits was lower than the detectable level. On the other hand, XPS analysis reveals that the formation of amorphous single Ni-Mo solid solution by sputter deposition results in charge transfer from Ni to MO. This indicates that the density of d-electrons of nickel tends to decrease by alloying with molybdenum. If the ovevoltage for hydrogen evolution decreases with the number of d-electrons, then the activity increases with decreasing molybdenum content. Although all sputter-deposited alloys are composed of single amorphous or fee phase, it is not known

at 70°C at 250 mA/cm 105

v

t

(1997) 891499

. . . ... . .. .. q

Ni-15Mo

i

q

Ni-25Mo 0 ‘i”““““““““‘t~t 0 10 20 Milling

._.I.

30 Time

40

1 50

/ h

Fig. 7. Hydrogen overvoltage of mechanically alloyed Ni-Mo at 250 mA crnm2 in 30 wt.% KOH solution at 70°C as a function of milling time. The inset shows the hydrogen overpotential change with the size of fee crystallites which changes with time of milling [50]. (From R. Schulz, J.Y. Huot, M.L. Trudeau, L. Dignard-Bailey, Z.H. Yan, S. Jin, A. Lammarre, E. Ghali and A. Van Neste in J. Mater. Res., Vol. 9 (1994) 3003. Reprinted by permission of the publisher, Materials Research Society.)

i 0.50

0.40

0.30

Hydrogen Overpotential

0.20

0.10

0.0

iV

Fig. 9. Polarization curves of sputter-deposited Ni-MO sured in 1 M NaOH at 30°C [47].

alloys mea-

898

K. Hashimoto /Materials

Science and Engineering A226-228

why the composition dependence of the sputter-deposited alloys for hydrogen evolution is different from that for heterogeneous alloys prepared by mechanical milling and chemical reduction of mixed oxide. Amorphous Ni-Mo alloys sputter-deposited on an expanded nickel metal have been used in a CO;, recycling plant for the substantiation of our idea built on the roof of IMR. CO, recycling has been illustrated at a previous conference, RQ 8 [51]: the necessary electricity is generated by solar energy in deserts and transmitted to the nearest coasts where the electricity is used for hydrogen (HZ) production by electrolysis of seawater. At the coasts, further methane (CH,) production is carried out by the reaction of H2 and C02, and CH4 is transported by tankers to energy consumers. The energy consumers recover CO2 after usage of CH, as the fuel and the recovered CO, is sent back to the coasts close to the deserts for production of CH,. It has been demonstrated using the CO, recycling plant that solar energy in deserts can be used by energy consumers in the form of CH, without emitting any CO, into the atmosphere. Catalysts prepared from amorphous alloys and the amorphous alloy electrodes have now been used for the CO2 recycling plant for the prevention of global warming and for abundant energy supply.

4. Conclusions The direct use of the homogeneous disordered structure of amorphous alloys is not easy because of a covering of air-formed oxyhydroxide films. It is, therefore, difficult to expect to utilize the high density of the surface defects characteristic of amorphous alloys. Nevertheless, metastable alloys including amorphous and nanocrystalline alloys are still attractive as catalyst precursors and electrocatalysts. This is mostly due to the formation of homogeneous single solid solutions containing prescribed amounts of a variety of necessary elements for specific chemical reactions. Tailoring of catalysts and electrocatalysts is often based on designing new combination of elements. The best catalysts for methanation of carbon dioxide have been tailored using amorphous alloys as precursors. Amorphous nickel base alloys are the best cathode materials for electrolytic hydrogen evolution. Consequently, metastable alloys are still potential candidates for new catalysts and electrocatalysts.

References [l] M. Hara, 25 (1980) [2] M. Hara, Solid, 54

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