Controlled formation of ultrafine nickel particles on well-defined hematite particles

Colloids and Surfaces A : Physicochemical and Engineering Aspects, 82 (1994) 29-35 0927-7757/94/507 .00 © 1994-Elsevier Science B .V. All rights reserved .

29

Controlled formation of ultrafine nickel particles on well-defined hematite particles" Atsushi Muramatsu*, Shin-ichiro Ichikawa, Tadao Sugimoto Institute for Advanced Materials Processing, Tohoku University, Katahira, Sendai 980, Japan

(Received 20 December 1992 ; accepted 1 July 1993) Abstract Ultrafine metallic Ni particles were precipitated on well-defined hematite particles of different shapes (spindles, pseudocubes, spheres, and platelets) by reducing nickel acetylacetonate with NaBH, in the presence of these carriers (molar ratio of Ni/hematite, 1/20) in 2-propanol at 355 K for 10 min . The deposited amorphous Ni particles on spindletype, pseudocubic and spherical hematites were about 4 nm in mean diameter and of narrow size distribution . They were free from nickel boride, as determined by X-ray diffractometry and electron microscopy in combination with heat treatment of the powder at 673 K for 2 h in N e atmosphere . On hexagonal hematite platelets, the deposited Ni particles were larger (i.e . about 20 nm) and of a much broader size distribution than those on the other types of hematite . It appears that the rough surfaces of the spindle-type, pseudocubic, and spherical carriers favor the stabilization of Ni particles in contrast to the smooth surface of the platelet-type hematite . The formation mechanism of the ultrafine Ni on colloidal hematite is discussed . Key words: Hematite particles; NaBH, ; Nickel particles ; Surface roughness

Introduction Ultrafine particles of the order of a nanometer have attracted special attention because of their specific functions as catalysts [1] . However, the catalytic activity of such particles in dispersions is normally limited because of their high activity in surface reactions which cause coagulation [2] . Furthermore, a simple addition of metal oxide particles as carriers to stabilize ultrafine metallic particles was mostly unsuccessful due either to the lack of affinity between particles of different kinds or inadequate control of the surface structure of the carriers . Some experiments involving impregnation of metallic particles into oxides have also been tried [2,3] ; however, the products synthesized in *Corresponding author. °Presented in part at the 63rd Spring Meeting of the Chemical Society of Japan, Osaka, March 28-31, 1992 . SSDI0927-7757(93)02592-3

the pores were often changed by succeeding reactions [4], which reduced the selectivity of the original compound . Thus it seems that the ultrafine catalysts should be dispersed on rather fine carrier particles, especially with a rough surface, which would provide a larger contact area and promote stabilization of the adhered metals . The objectives of the present study are to form stable ultrafine nickel on well-defined hematite particles using a reducing agent in a non-aqueous medium and to control their mean size and size distribution, with special emphasis of the effect of surface roughness of the substrates on the size of the deposited nickel . Finally, the formation mechanism of the ultrafine Ni particles on hematite will be discussed .

A . Muramarsu et al./Colloids Surfaces A : Physicochem . Eng. Aspects 82 (1994) 29-35

30

Experimental Preparation Preparation

and (E), obtained by transmission electron microscopy (TEM), X-ray diffraction (XRD), and N 2 adsorption measurements at 78 K . Except for particles (E), the particles used were monodisperse in size and shape . The spindle-type particles (A) and the platelet-type ones (E) were single crystals, while the pseudocubic ones (B, C) were polycrystals [9] . The spherical particles (D) were confirmed to be single crystal from our XRD analysis; this is consistent with Matijevie and Schemer's report [8] . Only the tabular particles (E) had very smooth surfaces while the other particles all had rough surfaces [9] . (D),

of hematite

particles

Hematite particles of different shapes were prepared as follows . Spindle-type hematite (A) was prepared by aging 2 .0 x 10-2 mol dm -3 FeCI, -4 solution in the presence of 3 .0 x 10 mol dm -3 NaH 2PO, at 373 K for 2 days as described earlier [5] . Pseudocubic hematite particles (B) were obtained by gradually adding 5 .4 mol dm -3 NaOH under magnetic agitation to the same volume of 2 .0 moI dm -3 FeC13 in a Pyrex bottle and then the resulting ferric hydroxide gel was aged in a laboratory oven at 373 K for 8 days in the same tightly closed container [6] . The solids were washed with doubly distilled water through seven cycles of centrifugal separation and ultrasonic redispersion . Different pseudocubic hematite particles (C) were prepared by aging an ethanol-water (1 :1) solution of 1 .9 x 10 - ' mol dm - ' FeCl 3 with 1 .2 x 10 - ' mol dm -3 HCI at 373 K for 2 days [7] . Spherical hematite particles (D) were precipitated by aging a solution 1 .8 x 10 z mol dm -3 in FeC13 and 1 .0 x 10 -t mol dm - ' in HCI [8] . Hexagonal platelet hematite particles (E) were synthesized by adding 8 .0 mol dm -3 NaOH to an equal volume of 2 .0 mol dm -3 FeCl 3 , followed by aging in a laboratory oven at 453 K for 2 h [9] . Table 1 lists shapes, sizes, specific surface areas, and crystallinity of hematite particles (A), (B), (C),

Synthesis of ultrafine Ni particles on hematite To form ultrafine Ni particles on the hematite surface, the powdered hematite was dispersed in 60 cm' of 2-propanol (which is chemically inert for NaBH 4 [10]) in a four-necked flask, fitted with a reflux condenser, under the influence of ultrasonic radiation . After adding Ni(AA)z (where AA is acetylacetonate CH,COCHCOCH 3 ) the oxygen dissolved in the solution was purged with N z for 1 h at 355 K, which is the boiling temperature of 2-propanol . A solution of NaBH 4 in 20 cm 3 of 2-propanol was then added under magnetic agitation, which caused a rapid change (in about 2 min) in the color of the solution from green to black, suggesting the formation of Ni metal . Ten minutes after the introduction of NaBH 4, a part of the suspension was withdrawn for TEM, powder XRD, and inductively coupled plasma (ICP) analysis .

Table I Properties of the hematite particles used in the present study Sample

Shape

Size (P in)

Specific surface area' (m2 g ,)

Structure

A B C D E

Spindle Pseudocubc Pseudocube Sphere Platelet

0.13 x 0.43 0.69 1 .22 0.11 1 .90b

19 3 .1 3 .3 18 4.5

Single crystal Polycrystal Polycrystal Single crystal Single crystal

' Nleasured by N, adsorption at 78 K using the BET equation . "Mean diameter .

A . Muramatsu et at . /Colloids Surfaces A : Physicochem. Eng. Aspects 82 (1994) 29-35

Under standard conditions, the quantities of reactants were as follows : hematite 1 .0 x 10 - ' mol, Ni(AA)2 5 .0 x 10 -5 mol, and NaBH, 2 .0 x 10 --4 mol in 80 cm' of 2-propanol. Characterization

The dried samples, separated from 2-propanol by filtration, were observed in a transmission electron microscope, and their composition and structure were determined by XRD using the Cu Ka line . The BET specific surface areas of the powders were measured by N 2 adsorption at 78 K . A Pyrex cell containing about 0 .4 g of hematite powder was evacuated to 1 x 10 -5 Torr at room temperature for 2 h and cooled to 78 K with liquid nitrogen, followed by introduction of nitrogen into the cell . Adsorption isotherms were obtained at 78 K at N 2 pressure ranging between 50 and 200 Torr . The contents of Fe, Ni and B were determined by ICP analysis . The samples, separated from the 2-propanol, were dried in a desiccator with silica gel, and then dissolved in hot I mol dm - ' HC1 . Wave numbers used for the elemental analysis were 259 .940 A, 231 .604 A, and 249 .678 A for Fe, Ni, and B, respectively. Results and discussion Formation of ultrafine Ni particles on spindle-type hematite particles

A sample of 1 .0 x 10 - ' mol monodisperse spindle-type hematite particles (0 .43 am in length and 0 .13 pm in width) with a specific surface area of 19 m 2 g -1 , was dispersed in 80 cm' 2-propanol containing 5 .0 x 10 -4 mol Ni(AA)2 and 1 .0 x 10 - ' mol NaBH 4 ([Ni ]/[a-Fe2O3]= 1/2) . Transmission electron micrographs, Figs. 1(a) and 1(b), show the original hematite and the Ni-carrying hematite particles, respectively . In the latter case most of the Ni particles were found deposited on hematite particles, but their size distribution was rather broad with a mean diameter

31

of 11 nm (Fig. 2(a)) . The total surface area of the added hematite was 0 .57 m2 and the total projected area of the Ni particles was calculated to be 0 .46 m 2, assuming 100% conversion of nickel ions into spherical metallic Ni . When the conditions were changed to 1 .0 x 10' mol hematite, 5 .0 x 10 -5 mol Ni(AA) 2 , and 2 .O x 10 -4 mot NaBH, ([Ni]/[a-Fe203]=1/20), the resulting Ni particles were only formed on the spindle-type hematite (Fig . 1(c)) and their mean size was considerably smaller (i.e . about 4 nm), while the size distribution was narrower (Fig . 2(b)) . The dried powder, prepared under the condition [Ni]/[a-Fe2 0 3 ]-1/2, was placed into a glass tube and heated at 673 K for 2 h under flowing N 2 . The TEM of the original and heated solids showed no significant change in the size and shape of Ni particles due to the heat treatment . The XRD patterns of the original and heated powders (Fig. 3) show only peaks corresponding to a-Fe 2O3 for the original sample and those of Ni metal and Fe 3 O, for the heated one . Obviously, the heat treatment caused the crystallization of the amorphous Ni particles without appreciable aggregation . The conversion of x-Fe 2 O 3 into Fe 3O, may be due to the reduction by residual NaBH, adsorbed onto the a-Fe 2 O3 particles . The molar ratios of Fe : Ni :B in the system (solid+liquid), and in the solid phase alone, before and after the reducing reaction and after the heat treatment at 673 K for 2 h in N 2 are listed in Table 2 . For the analysis of the compositions in the system and in the solid phase, solvent was removed by evaporation under reduced pressure at room temperature and by filtration through a membrane filter of pore size 0 .1 am, respectively . The remaining wet powders were dried in aa desiccator with silica gel. The final composition of the system shows that a considerable amount of boron escaped during the reducing and/or evaporation processes, which is probably due to the formation of gaseous boranes, such as B 2I-I6, since boron can react with H 2 occluded in Ni or Ni,B to form such a compound [11] . On the other hand, a considerable amount of B was left in the solid phase to



A . Muramatsu et aL/Colloids Surfaces A: Phvsicoehem. Eng. Aspects 82 (1994) 29-35

32

(a)

(c)

(b)

25nm Fig . 1 . Transmission electron micrographs of: (a) spindle-type hematite particles; (b) such particles with Ni particles on their surfaces prepared at [Ni]/[a-Fe 2O 3 ] ratio 1,2 (c) as (b) but at ratio 1/20 . I

I

(a) Before heat treatment

I (a)

60

-

O 1:a-Fe,O O

O

e d

40

N (h) After heat treatment at 673 K for 2b .

20 V Y e Y 7

a

Y C N

:Ni

O

a :Fe'O 4

0 60

A

(b) -

w 45

40

Fig.3 . X-ray diffraction patterns of Ni-carrying spindle-type hematite particles, prepared at [Ni]/[a-Fe 20 3 ] ratios, 1/2: (a) before; (b) after the heat treatment at 673 K for 2 h in N r,

20

0

5

20(deg . CuKa)

0

I 20 Diameter(nm) I

3t1

40

Fig. 2 . Histograms of the size distributions of Ni particles on the spindle-type hematite particles prepared at [Ni]/a-Fe 2 O3 ] ratios : (a) 1/2 ; (b) 1/20.

yield Ni 2 B [ 12] . However, no XRD peaks of Ni 2 B could be identified in Fig- 3(b), although amorphous nickel boride is known to be converted into the crystalline phase without changing the com-



A . Muramatsu et a[

,!Colloids

Surfaces A : Physicochem. Eng. Aspects 82

Table 2 The molar ratios of Fe : Ni : B in the system (solid + liquid) and in the solid phase, at the start and end of the reducing process and after the heat treatment at 673 K in N 2 Phases

Initial

Final'

After heat treatment`

Solid+liquid Solid

100 :25 :50 100 :25 :50

100 :25 :9.5 100 :25 :6.7

100 :25 :9.5 100 :25 :6.7

The initial quantities of the components were 1 .0 x l0 - ' mot spindle-type hematite, 5 .0 x 10 -0 mot Ni(AA)2, and 1 .0 x 10 -3 mot NaBH, . 'From ICP measurement .

position by heat treatment at 673K [13] . Therefore, the remaining boron in the solid phase was presumably adsorbed as unreacted NaBH 4 to Ni/a-Fe2 O3 particles .

(1994)

29-35

33

NaBH, ([Ni]/[x-Fe,,O3]=1/20) . Figure 4 shows that Ni particles on hematite samples (B), (C), and (D) had a diameter of about 4 nm, which is quite similar to the results shown in Fig . 1(c) . The mean diameter of the Ni particles on the basal planes of hematite (sample E) was much larger (i .e. about 20 nm) and of broad size distribution, in contrast to that on sample A (about 4 nm) with fairly narrow size distribution (Fig . 5) . These results indicate that a rough surface inhibits the coagulation of Ni particles by blocking the two-dimensional diffusion . It will be reported elsewhere that the Ni particles on the platelet-type hematite were much less active as a catalyst for hydrogenation of alkenes than those on the spindle-type, spherical or pseudocubic hematites .

The effect of surface roughness of hematite particles

The role of hematite as a carrier and the formation

of different shapes

mechanism of Ni

By a procedure similar to that described above, Ni particles were deposited on hematite particles (B)-(E) under conditions of 1 .0 x 10 -3 mol hematite, 5 .0 x 10 -5 mol Ni(AA) 2, and 2 .0 x 10 ' mol

We tried to synthesize Ni particles in the absence of hematite carriers . The addition of 20 cm' of -2 1 .0 x 10 mot dm -3 NaBH 4 solution to 60 cm' of 8 .3 x 10 -4 mol dm -3 Ni(AA) 2 solution with

(1)

(2)

(3)

100nm Fig. 4 . Transmission electron micrographs of hematite particles (B), (C), and (D) supporting Ni particles, prepared at [Ni],/[a-Fe 30 3 ] ratio 1/20 : (1) pseudocubic particles (B) : (2) pseudocuhic particles (C) ; (3) spherical particles (D) .

34

A . Muramatsu et al./Colloids Surfaces A : Phvsicochem . Eng. Aspects 82 ( 1994) 29-35

(a)

(b)

30nm Fig. 5 . Close-up TEM photographs of Ni particles supported on : (a) the spindle-type hematite; (b) the platelet-type hematite, prepared at [Ni]/[a-Fe2O,] ratio 1/20 .

agitation caused a change in color from green to black in about 5 s . The resulting particles coagulated in a few minutes into larger aggregates of subunits 5 nm in size, as shown in Fig . 6 . Since the average size of the ultrafine Ni particles on the

I- I

100nm Fig . 6 . Transmission electron micrograph of unsupported Ni particles .

spindle-type hematite particles was 4 .4 nm, the aggregation of the latter was avoided by their deposition on the carriers- Since the color change in the presence of hematite particles took a much longer time (see Experimental), it is suggested that either the reducing agent, NaBH 4 , or Ni(AA)2 was preferentially adsorbed on the surfaces of the carriers where the reduction of Ni(AA) 2 took place . In other words, it would seem that the deposition of Ni particles does not take place by adhesion of preformed particles in the liquid phase . Indeed, when hematite particles (1 .0 x 10' mol) were added 30 s after the introduction of NaBH 4 , corresponding to the time of complete reduction of Ni ions but before extensive aggregation of the Ni particles takes place, insignificant adhesion of the resulting Ni particles could be noted . In order to elucidate the mechanism further, the adsorption of Ni(AA)2 and NaBH4 , individually, on pseudocubic hematite (C) was measured using 1 .0 x 10 -3 mol of a-Fe2 O,t with 5 .0 x 10 -5 met of Ni(AA) 2 or with 2.0 x 10_ 4 mol of NaBH 4 in 80 cm' of 2-propanol at 355 K, respectively . In both cases, the resulting suspension was filtered through a membrane filter of 0 .5 .tm pore size



A. hluramatsu et al./Colloids Surfaces A : Physicochem_ Eng. Aspects 82 (1994) 29-35

3 min after the addition of the reactants and the content of B or Ni in the filtrate was determined by ICP. It was found that 8 .3% of the Ni(AA) 2 and 69% of NaBH, were found to be adsorbed to the hematite . Hence, it is likely that the reducing agent, NaBH4, is rapidly adsorbed onto the surface of hematite, followed by the reduction of Ni(AA)2.

4 5 6 7 8 9

References

10 11

1 2 3

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C.N . Satterfield, Heterogeneous Catalysis in Practice, McGraw-Hill, New York, 1980, p . 235 . M . Ozaki, S . Kratohvil and E . Matijevic, J . Colloid Interface Sci ., 102 (1984) 146 . T . Sugimoto and K . Sakata . J . Colloid Interface Set ., 152 (1992) 587 . S. Hamada and E . Matijevic. 1 . Colloid Interface Sci ., 84 (1981) 274. E . Matijevic and P. Schemer, J . Colloid Interface Sci ., 63 (1978) 509. T . Sugimoto, A . Muramatsu, K . Sakata and D . Shindo, l. Colloid Interface Sci ., 158 (1993) 420. H.O . House, Modern Synthetic Reactions, W .A . Benjamin, Menlo Park . CA, 1972 . p . 47 . J . Flechon and F.-A. Kuhnast, C .R . Acad. Set . Ser . 3, 274 (1972) 707 . H.C . Brown and C.A . Brown, J. Am . Chem . Sec ., 85 (1963) 1003 . H . Yamashita, M . Yoshikawa, T. Funabashi and S. Yoshida, J . Chem. Soc., Faraday Trans . 1, 81 (1985) 2485 .