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The effects of the environment on the growth of nickel metal particles supported on a range of Y zeolites prepared by ion exchange and impregnation
The effects of the environment on the growth of nickel metal particles supported on a range of Y zeolites prepared by ion exchange and impregnation Brendan Coughlan and Mark A. Keane Physical Chemist9 Laboratories, University College, Galway, Ireland The nature of the nickel metal phase supported on a range of alkali metal cation-exchanged Y zeolite catalysts was characterized by X-ray diffraction line broadening. The sintering process is shown to occur via a crystallite migration mechanism. Nickel crystallite size was measured and correlated with such diverse factors as reduction time, reduction temperature, heating rate, sample precalcination, and ammonia adsorption. The nature of the alkali metal co-cation (LP, Na+, K+, Rb+, or Cs’) is shown to strongly influence the dimensions of the supported metal, yielding the following order of decreasing crystallite size: NiLiY > NiNaY > NiKY > NiRbNaY > NiCsNaY. In the case of the CeNiNaY and CeNiKY samples, the introduction of an additional level of acidity, due to the presence of the highly polarizing Ce3+ ions, during thermal activation, serves to suppress crystallite growth. For comparative purposes, data on the sizes of nickel metal crystallites formed on the reduction of nickel-impregnated Y zeolites and silica and alumina supports are also presented; the nature of the alkali metal co-cation has a much greater influence on the dispersion of nickel supported on the impregnated zeolites. Under identical reduction conditions, the amorphous alumina and silica carriers exhibit the smallest supported crystallite sizes. Keywords:
Characterization;
NiY;
nickel
metal
crystallitea;
XRD line broadening;
INTRODUCTION Metal catalysts are commonly employed in the form of metal dispersed as small crystallites on high surface area supports. In catalyst preparation, maximum effort is expended on generating a well-dispersed metal with optimum crystallite size that is stable during catalysis. The unique structural characteristics exhibited by zeolites can be applied to the preparation of metal catalysts. Although the relationship between the degree of dispersion of the metal and the zeolite crystal chemistry is still not fully understood,’ the difficulty in obtaining a homogeneous, finely dispersed metal phase is well recognized.‘” Metal particle growth can occur during thermal treatment by two distinct mechanisms. The model developed by Pulvermacher and Ruckenstein,7 the Crystallite Migration Model, describes the sintering process as a migration of metal over the surface followed by coalescence with other metal crystallites upon collision. Mass transfer is proposed to proceed by a momentary accumulation of metal atoms on one side of a particle, the net effect being a migration of the Address reprint requests to Dr. Keane at (present address) Chemistry Department, The University, Glasgow, G12 900, Received 12 March 1990; accepted 31 July 1990 0 1991 Butterworth-Heinemann 2
ZEOLITES,
1991, Vol 11, January
the
UK.
Ce3+ exchange;
impregnation
particle as a whole in that direction.8 The second model, the Atomic Migration Model, proposed by Flynn and Wanke,g considers sintering to occur by the loss of single atoms from one particle that are then deposited on a growing crystallite. Mobility and agglomeration to form large crystallites during heat treatment has been one of the major problems associated with the reduction of Ni*+ cations supported on zeolites.‘0-‘8 Reduction of NiNaY has led to the formation of large ferromagnetic crystals on the external zeolite surface.“,’ On obtaining complete reduction of Ni*+ ions in NiNaY, Elliot and Lunsford*’ observed a broad particle distribution. Even when reduction is incomplete, part of the metal diffuses out of the zeolite pores to form large agglomerates.*t-** From temperatureprogrammed oxidation and reduction experiments on NiNaX and NiNaY systems, Jacobs et ~1.~~calculated the amount of nickel present inside and outside the zeolite lattice. They found that the size of the internal nickel was restricted to the size of the supercage and the Nt” formed on the external surface exhibited shape anisotropy. Penchev et al.*‘j obtained an optimum supported nickel metal surface area on reduction in the temperature range 723-773 K. Although the best distribution of metal has been
Effects
of environment
obtained when water is corn letely removed from the zeolite prior to reduction,’ P**’ contrary reports suggest that the addition of water vapor to the hydrogen stream promotes the acidic function and leads to the formation of more highly dispersed metal particles.28V2” Regardless, metal particles exceeding supercage dimensions have repeatedly been found embedded in the faujasite matrix; nickel aggregates of up to 15 nm have been reported in the internal structure. 5.15.22.25.30 The dispersion of Ni” particles depends strongly on the presence of a second cation or metal in the immediate environment. l4 Bager et ~1.~’ reported that although NiCeNaY and NiNH4Y exhibited similar degrees of Ni*+ cation reduction, the specific surface area of Ni” supported on NiNH4Y was three times greater than that supported on NiCeNaY. In complete contrast, Ce3+ addition to NiNaX samples has been reported to result in the formation of much smaller metal crystallites. 32-35 The trivalent cerium ions have been proposed to stabilize the nickel atom species (a) in limiting nickel oxidation by the electron acceptor sites of the support32.34.35 and (b) by generating sites for the preferential growth of metal particles,34 thereby inhibiting the migration of atomic species by the interaction of Ce3+ with Ni’. According to Klyueva et al., 36 the reduction of nickel zeolites prepared by impregnation results in the formation of metal particles that locate mainly on the outer surface of the zeolite. Chien et a1.,37 in contrast, reported that the average metal particle size for the ion-exchanged samples was larger than that for the impregnated samples. Nickel sintering also occurs on silica and alumina supports. 38 Using the same reduction conditions, Richardson l3 found that the nickel crystallite size increased in the order: NiA1203 < NiSi02/A1203 < Nisi02 < NiNaY. The size of the metal particles supported on the amorphous carriers also increased with increasing reduction temperature, hydrogen flow rate, and nickel loading and decreased on the introduction of a precalcination step.38S3g As part of a detailed catalytic and characterization study of Y zeolites, the present paper attempts to define the nature and extent of agglomeration of nickel metal supported on a range of Y zeolites under various conditions of pretreatment and reduction and to establish procedures for minimizing the extent of nickel agglomeration. Data for Si02 and Al203 supports are also included. This paper is the first to systematically probe the role of the alkali metal cocation in influencing the size of nickel metal particles supported on Y zeolites prepared by ion exchange and by impregnation.
EXPERIMENTAL The starting or Sieve LZY-52 (H20)260]. The (30-120 mesh) Ltd. and BDH, exchange) and
parent zeolite was Linde Molecular [formula: Nass(Al02)ss(Si02)~34 alumina (70-230 mesh) and silica supports were supplied by Labkem respectively. KY and LiY (100% RbNaY and CsNaY (ca. 68% ex-
on growth
of Ni particles:
B. Cough/an
and
M.A.
Keane
change) were prepared by exchanging out the parent Naf ions. A known weight of Nay, usually 250 g, was refluxed with 1 M solutions of KNOs, LiN03, C&l, and RbCl (ca. 400 cm3) for 24 h, after which the zeolite was filtered and thoroughly washed with hot deionized water to remove the occluded salt. The zeolite was then air dried at 383 K for 20 h. The partially exchanged samples, i.e., K, Li, Rb, WNaY, were exchanged a further nine times as described above. A fully exchanged NH4Y zeolite was prepared by refluxing 250 g of NaY with ca. 400 cm3 of a 0.5 M solution of NH4NOs over a period of 3 d, filtering and drying as before; after 10 such treatments, complete exchange was achieved. Nickel-exchanged samples were prepared by taking 100 g of Nay, KY, LiY, RbNaY, CsNaY, or NH4Y and refluxing with a 400 cm3 Ni(NO& solution for 24 h. In this study, dilute Ni(NOs) solutions (< 0.1 M) were employed in which the pH of the nitrate/zeolite suspension was in the range 6-7.5. Under these conditions, a single exchan e cycle resulted in a maximum exchange of ca. 7 Nl5 +/u.c., i.e., seven nickel cations per unit cell. Inzreparing samples with loadings greater than ca. 7 Ni +/u.c., repeated exchange was necessary. Quantitative impregnation of NaY, KY, LiY, RbNaY, CsNaY alumina, and silica was achieved with constant volumes of Ni(N03)* solutions by vacuum rotary evaporation to incipient wetness. The concentration of the Ni(NO& solution was chosen to achieve the desired metal content. All the samples were air dried at 373 K and stored over saturated NH4Cl prior to analysis. Atomic absorption (Ni*+ concentration) and flame emission (Na+, K+, Li+, Rb+, and Cs+ concentrations) techniques, using a Perkin-Elmer 360 atomic absorption spectrophotometer, were employed to determine the cation contents to within + 2%. Thermal analyses were also conducted on all the prepared samples using a Perkin-Elmer thermobalance operating in the t.g. mode to measure water loss as a function of temperature. The hydrated nickel samples (in pellet form, 1.181.70 mm diameter) were normally reduced in a fixed-bed catalytic reactor40’4’ in a 120 cm3 min-’ stream of hydrogen at 723 K for 18 h. However, these conditions were varied, the details of which will be described later. The heating system40.4’ normally employed only allowed for a variable rate of heating of the samples. In the experiments where a fixed rate of heating of the samples was desired, a temperature programmer/controller (Cambridge Process Controls Model 702) was used. The model 702 automatic controller is a combined sequence/ramp programmer and temperature control system. The microprocessor system in the unit allows fully automatic operation of the furnace through the complete operating cycle to a time and temperature sequence program stored in the unit’s memory. The reduced zeolites were also treated in a 120 cm3 min-’ stream of ammonia at 423 K for 15 min: hydrogen was then reintroduced, and the samples were flushed for 6 h at 473 K and treated for a further 24 h in hydrogen (120 cm3 min-‘) at 723 K.
ZEOLITES,
1991, Vol II, January
3
Effects
of environment
Table 1 Chemical ion exchange Zeolite sample NaY NiNaY-3.6 NiNaY-6.8 NiNaY-12.5 NiNaY-15.8 NiNaY-17.3 NiNaY-19.3 NiNaY-22.8 NiNaY-26.4 NiNaY-27.8 NiNaY-29.9 NiNaY-32.6 NiNaY-35.7 NiNaY-48.8 NiNaY-63.1 NiNaY-78.6 NiNaY-90.1 NiY
on growth composition
of Ni particles: of NiNaY
samples
B. Cough/an prepared
Water Na+/u.c. 58.0 55.7 53.7 50.7 48.8 47.5 46.1 44.0 41.7 40.9 41.0 38.3 36.0 30.0 22.3 14.6 6.9
Ni’+/u
.c .
1.0 f :: 4.6 5.0 5.6 6.6 7.7 8.1 8.7 9.5 10.4 14.1 18.3 22.8 26.1 29.0
H+/u.c.
and by
content (wt%) 25.1 25.5 25.3 26.3 26.5 26.4 26.5 26.6 26.8 26.8 26.8 27.2 27.6 28.6 29.1 29.5 30.0 30.4
0.3 0.3 0.1 0.5 0.7 0.8 0.9 0.9 0.6 0.7 1.2 -
Examination of the reduced samples for the presence of nickel crystallites was performed by studying the X-ray line broadening (Jeol JDX-85 diffractometer) around the nickel line at a 28 angle of 52.2” under the following conditions: time constant = 4; volta e = 30 kV; current = 20 mA; scan time = 0.5 min- 5 ; counts/s = 8 X 10’; radiation = CoK,. A correction for instrument broadening under identical conditions was made using Ni powder in either a NaY or KY matrix. Diffraction patterns of all the samples, before and after reduction, were obtained to ensure maintenance of sample crystallinity. Infrared spectroscopy in the range 1200-350 cm-’ was also used as a check on sample crystalhnity. The band at ca. 395 cm -’ has been assi ned to a breathing of the pore opening in zeolites 48 and is thus the most sensitive to changes in crystallinity. Table 2 Chemical exchange Zeolite sample
composition
of NiKY
samples
Water K+/u.c.
Ni’+/u.c.
H+/u.c.
KY NiKY-5.2 NiKY-8.0 NiKY-10.7 NiKY-17.9 NiKY-23.5 NiKY-26.9 NiKY-30.5 NiKY-35.6 NiKY-44.7 NiKY-47.0 NiKY-49.1 NiKY-54.5 NiKY-57.5 NiKY-62.5 NiKY-73.8 NiKY-82.0 NiKY-86.6 NiY
58.0 54.7 53.3 51.5 47.5 44.2 42.4 39.7 36.9 31.9 30.8 29.8 26.2 24.8 22.3 16.3 13.1 8.4 -
1.5 2.3 3.1 5.2 6.8 7.8 8.9 10.3 13.0 13.6 14.2 15.8 16.7 18.1 21.4 23.8 25.1 29.0
0.3 0.1 0.3 0.1 0.2 0.5 0.5 0.1 0.2 -
4
1991, Vol 11, January
ZEOLITES,
prepared
by ion
content (wt%) 22.4 22.6 22.3 22.7 23.6 23.5 24.2 24.4 24.8 25.5 26.0 26.4 26.9 27.3 27.6 28.0 28.4 29.3 30.2
M.A.
Keane
Table 3 Chemical NiNH4Y samples Zeolite sample
of NiLiY, NiRbNaY, by ion exchange
NiCsNaY,
Water AM+
LiY NiLiY-8.8 NiLiY-21.2 NiLiY-43.1 NiLiY-63.7 NiLiY-80.6 RbNaY NiRbNaY-7.3 NiRbNaY-18.6 NiRbNaY-27.3 NiRbNaY-35.1 NiRbNaY-47.4 NiRbNaY-59.1 CsNaY NiCsNaY-3.6 NiCsNaY-16.4 NiCsNaY-22.0 NiCsNaY-31.1 NiCsNaY-44.3 NiCsNaY-54.8 NH,,Y NiNH,Y-8.5 NiNH4Y-20.7 NiNH,Y-52.2 NiNH,Y-69.1 aAM’
composition prepared
7u.c. 58.0 52.3 44.6 32.0 20.7 14.2 40.2 35.8 29.4 24.3 19.8 15.2 10.3 39.8 36.6 29.2 26.0 20.7 13.6 11.2
Ni*+/u.c. 2.6 6.2 12.4 18.5 23.4 2.1 5.4 7.9 10.2 13.7 17.1 1.0 4.8 6.4 9.0 12.8 15.9 2.5 6.0 15.1 20.0
H+lu.c.
0.5 1.1 1.2 0.3 0.1 0.2 0.2 0.1 0.1 58.0 53.0 46.0 27.7 17.9
and
content (lNt%) 27.8 27.9 28.2 28.8 29.3 29.9 20.4 20.8 21.9 23.0 24.2 25.9 27.4 7.8 10.1 12.3 13.1 14.9 16.6 19.4 24.3 24.3 25.5 27.6 28.1
= Li+ , Rb+ # or Cs’
RESULTS AND DISCUSSION The chemical compositions of the ion-exchanged samples are given in Tables l-3. By and large, the exchange process was stoichiometric. The extent of hydrolysis, as evidenced by the number of protons present, was slight and occurred only for the lower exchanged samples. At higher nickel loadings (> 14 Ni*+/u.c.), the total number of exchanged ions exceeded the original 58 ions per unit cell. At these exchange levels, an excess amount of nickel has been incorporated into the zeolite framework in accordance with the findings of Schoonheydt et a1.43 Sample crystallinity, monitored by X-ray diffraction and i.r. spectroscopy, was maintained for each sample. The metal phase generated on reduction of the nickelloaded samples was characterized by X-ray line broadening using the Scherrer formula44: d=KAlBCos0 which relates the mean particle diameter (d) to the X-ray broadening (8) of the diffraction lines and K is a constant; in accordance with the work of Jenkins and deVries,44 a value of 0.9 was chosen for K. Metal dispersion can be related to crystallite size according tog: D = 0.505/d
(2)
where D is the dispersion or fraction of metal atoms exposed to the surface. The lower limit of detection
Effects
of environment
. E c 50
1,
5
A
Q E 40-
.-
0
. l
6
al 30. ?d = 3 220 t A .
.
.
.
l
on growth
of Ni particles:
B. Cough/an
and
M.A.
Keane
Elucidation of the sintering mechanism Two mechanisms have been proposed for metal crystallite growth, i.e., crystallite migration’ and atomic migration.g The former involves the migration of entire crystallites over the support surface followed by collision and coalescence, whereas the atomic migration mechanism involves the actual detachment of metal atoms from the crystallites that are ultimately captured by the larger crystallites. The power law rate equation for the crystallite migration model’,‘:
.
-dDldt 610
640
670
700
T/K Figure 1 Variation of the size of nickel crystallites supported on NiNaY-63.1 (A) and NiKY-62.5 (0) with increasing reduction temperature after 18 h on stream
for this method is ca. 5 nm, with the result that it gives no information on the state of the metal in the internal pore structure. X-ray line broadening is therefore diagnostic of the presence of nickel metal located on the external surface. Effect of reduction time and temperature The primary purpose of the present work was to elucidate the factors responsible for changes in supported nickel metal particle size. The main factors affecting crystallite size are the reduction temperature and time. The results obtained when varying these parameters are shown in Figures 1 and 2. The variation in crystallite diameter at various reduction temperatures for a NiNaY and NiKY sample of similar metal loading are depicted in Figure 1. Crystallite sizes proved to be larger at elevated temperatures; no metal particles were discernible at reduction temperatures lower than 573 K. The crystallite size vs. temperature plots closely mirror the reduction curves reported elsewhere,40 suggesting a direct correlation between the degree of Ni2+ reduction and the size of the Ni” metal particles that are formed. This is to be expected in that the higher the level of Ni2+ reduction, the greater the concentration of nickel metal particles with an associated higher probability of particle agglomeration resulting in the generation of larger particles. The effect of reduction time on particle size at constant reduction temperature is illustrated in Figure 2. The time-dependence relation is identical to that observed for the degree of Ni*+ reduction4’ in that both parameters increase with time and attain a fixed equilibrium value. Nickel metal crystallite size therefore increases with time because of the increased volume of metal formed during reduction. There is no evidence to suggest that crystallite sintering is dependent on the reduction time once the equilibrium reduction level has been attained.
= kD”
(3)
differs fundamentally migration modelg: -dDldt
from
that
for
the atomic
= kD5 exp (nDID0)
(4)
where D is nickel metal dispersion and n is the order of sintering. The identification of the prevailing mechanism is experimentally very difficult. Bartholomew and Sorensen45 adopted the following criteria to infer the predominant mechanism: crystallite migration results particle-size distribution;
(i)
8
in a log-normal
11 14 t i me I hrs.
17
. .
1,
.
.
b
.
s 36. 5 5 24. 3.=
A l . 0
39l 12. 5
b
I
8
14 11 time/hrs.
17
Figure 2 Variation of the size of nickel crystallites supported on NiNaY-63.1 (A) and NiKY-62.5 (0) with increasing reduction time at (a) 623 K and (b) 723 K
ZEOLITES,
1991, Vol 11, January
5
Effects
of environment
on growth
of Ni particles:
B. Cough/an
and
M.A.
Keane
observed activation energies are clearly lower than is the value of 431 kJ mol-’ quoted for nickel atom detachment,47 and, accordingly, the data presented in this study are consistent with a crystallite migration sintering process.
Ln(D1 /D2) Figure 3 Plot of In(t) vs. ln(D,/&) (A) and NiKY-62.5 (0)
for the reduction
of NiNaY-63.1
(ii) the sintering order (n) should lie between 3 and 5 for atomic migration and 2 and 13 for crystallite migration; (iii) the form of the rate equation for crystallite migration [Equation (3)] and atomic migration [Equation (4)] differ. Preliminary electron microscopic studies4’ on NiKY62.5 (18.1 Ni*+/u.c.) reduced at 723 K for 18 h in a 120 cm3 min- ’ hydrogen stream revealed a lognormal crystallite-size distribution (in the range 2565 nm) as well as a marked particle-shape anistropy. According to Wynblatt and Gjostein,46 the power law kinetics for sintering via crystallite migration can be expressed as: n ln(D1/D2) = C + In (t)
(5)
where D, and D2 represent the nickel dispersion at 623 K and 723 K, respectively, C is a constant, and t is the sintering time. If the sintering process follows the crystallite migration model, plots of ln(Di/Dz) vs. In(t) should result in straight lines from which the order of sintering (n) can be computed. The data presented in Figure 2 were interpreted according to the above rate equation; the straight-line plots of ln(D1/D2) as a function of In t for a NiNaY and NiKY sample of similar nickel loading (ca. 18 Ni*+/u.c.) are shown in Figure 3. Values for 72of 2.5 for NiNaY-63.1 and 3.1 for NiKY-62.5, however, fall within the range that corresponds to both sintering models.45 The apparent activation energy for sintering can be calculated fromg: E, = R(T,T2/T2
- T1) ln(AtilAt,)
ZEOLITES,
7997, Vol 11, January
. l
00
. A. c
:
. .
l
A
. . . . 0. A.0 . .
(6)
where At, and At, correspond to the time intervals necessary to obtain the same values of D1 and 02 at different temperatures T1 and T2. The experimental data yielded values of 20.2 kJ mol-’ for NiNaY-63.1 and 21.3 kJ mol-’ for NiKY-62.5 in the temperature range 623-723 K. This suggests a higher barrier for sintering in the case of the NiKY-62.5 sample. The
6
Effect of nickel loading Figure 4 illustrates the variation of crystallite size with nickel loading for a range of NiNaY and NiKY samples. It is immediately evident that the crystallite size increases for both systems with an increasing level of nickel exchange that can be explained by the increased probability of metal atoms being in close proximity to facilitate agglomeration, or, in terms of the model proposed by Law and Kenney,23 the greater number of protons generated on reduction of the nickel-rich samples48 promotes the counterdiffusion of nickel cations and protons, supplying more nickel ions close to the outer surface of the zeolite framework that are subsequently reduced to zerovalent nickel supported on the outer surface. From Equation (2), it can be readily observed that the higher loaded samples exhibited a lower dispersion of metal particles. As the lower limit of detection for the line-broadening technique is ca. 5 nm and the diameter of the large supercages is 1.2 nm, the date presented in Figure 4 represent the average crystallite size of the nickel metal supported on the external zeolite surface; the external nickel particles are therefore smaller when supported on the KY carrier. The generally observed bidispersities,10-25 i.e., large fractions of reduced metal at the outer surface with a smaller fraction of smaller clusters inside the zeolite, may result from the relatively low ion density in the migration path of Ni’. The larger K+ ions are therefore more effective than are the Na+ ions in blocking the path of the migrating nickel species and retard the aggregation process of metal particles on the external surface. Presumably, the larger K+ ions will also act to retard the growth of intracrystalline metal clusters. The observed trend of the larger alkali metal co-cation-exchanged Y zeolite generating smaller
l e * d
6 Figure NiNaY
4 Variation of crystallite size with (A) and NiKY (0) samples reduced
24 nickel loading at 723 K for
for the 18 h
Effects
of environment
on growth
of Ni particles:
B. Cough/an
Table 4 Variation of the size and dispersion supported on a range of alkali metal zeolites, with ammonia treatment
Zeolite
I
I
18 Ni2+/ UC
6
Figure 5 Variation NiLiY (Xl, NiRbNaY 723 K for 18 h
24
12
of crystallite (B), and
size with nickel loading for NiCsNaY (+) samples reduced
the at
nickel metal particles is also followed by the NiLiY, NiRbNaY, and NiCsNaY samples (Figure 5). Indeed, the order of decreasing nickel crystallite size is NiLiY > NiNaY > NiKY > NiRbNaY > NiCsNaY. As discussed elsewhere,4s treatment of the reduced samples with NHs poisons the surface Bronsted acid sites and results in the generation of a greater mass of nickel metal. From the present study, the ammonia treatment produces a larger average nickel particle size (Fipre 6 and Table 4). In agreement with the degree of reduction studies,4g the effect on crystallite size is not as marked for the NiRbNaY and NiCsNaY samples. The correlation of the mass of supported nickel metal generated during reduction4’ with the size of the crystallites formed during sintering is depicted in Figures 7-9. It can be seen that the particle diameter increases with the amount of available metal. Indeed, as is most clearly illustrated in Figure 8, there appears to exist a critical value above which the generation of further metal does not affect the crystallite size. Nevertheless, under ammonia pretreatment conditions, at a particular mass of supported
sample
NiLiY-8.8 NiNaY-6.8 NiKY-8.0 NiRbNaY-7.3 NiCsNaY-3.6 NiLiY-21.2 NiNaY-22.8 NiKY-23.5 NiRbNaY-18.6 NiCsNaY-22.0 NiLiY-43.1 NiNaY-48.8 NiKY-49.1 NiRbNaY-47.4 NiCsNaY-44.3 NiLiY-63.7 NiNaY-63.1 NiKY-62.5 NiRbNaY-59.1 NiCsNaY-54.8
Treated dlnm
with NH3 D . 1O-3
30.1 24.2 24.3 17.2 11.0 40.6 39.0 36.1 28.3 20.0 62.1 63.4 54.2 43.4 32.1 79.4 69.1 60.2 55.9 38.0
16.8 20.9 20.8 29.3 45.9 12.4 12.9 14.0 17.9 25.3 8.0 7.9 9.3 10.2 15.7 6.4 7.3 8.4 9.0 13.3
and
M.A.
Keane
of nickel crystallites, cation-exchanged
Y
Untreated dlnm
D . 1O-3
28.4 24.5 24.7 17.0 11.1 34.9 38.7 34.8 27.1 20.9 44.6 55.1 50.1 42.4 31.1 52.1 59.0 55.6 46.7 35.4
17.8 20.6 20.4 29.7 45.3 14.5 13.1 14.5 14.8 24.2 11.4 9.2 10.1 11.9 16.2 9.7 8.6 9.1 10.8 14.3
metal the nickel particle size increases in the order NiCsNaY < NiRbNaY < NiKY < NiNaY < NiLiY, which is consistent with the order of decreased sintering. The role of precalcination in the formation of nickel metal particles was also investigated. The X-ray diffraction line-broadening data are presented in Table 5. Heating the samples in nitrogen prior to reduction has been shown to lower the degree of Ni*+ reduction4’ with a resultant smaller volume of supported metal, which, as expected, is in the form of smaller crystallites. In the same way, heating the samples in hydrogen at higher rates generates smaller metal particles (Table 6).
Effect
of Ce’+ exchange
Diluting the NiNaY and NiKY samples with Ce3+ ions has resulted in the formation of much smaller
x’ x
*
. A
.
.
. n
+
I 6
12 Ni2’/ Ud8
I
24
Figure 6 Variation of crystallite size with nickel loading for the ammonia treated NiNaY (A), NiKY (0). NiLiY (Xl, NiCsNaY (+I, and NiRbNaY (m) samples reduced at 723 K for 18 h
Figure 7 Correlation gram of zeolite for 723 K for 18 h
I 44 36 28 Crystallite Diameter I nm of the mass the reduction
ZEOLITES,
of nickel of NiNaY
52
metal generated (A) and NiKY
1991, Vol II, January
(0)
per at
7
Effects
of environment
on growth
of Ni particles:
8. Cough/an
1
and
n x
4
To 28’ 7 -i3 ” 21 8
n
-
Keane
Table 5 The effect of precalcination stream) on the size of nickel crystallites ion-exchanged or impregnated samples at 723 K for 18 h.
x
Sample +
5 140.-
m +
Z 0 ’ 1
I
M.A.
n
+
+
Reduced
x
x
+
+ 46 / nm
Figure 8 Correlation of the mass of nickel metal generated per gram of zeolite for the reduction of NiLiY (X), NiRbNaY (m), and NiCsNaY (+I at 723 K for 18 h
nickel particles (Table 7) Jeanjean et a1.33 have explained the higher dispersion of nickel metal on the CeNaX support in terms of the strong electrostatic field associated with the Ce3+ cations that is capable of modifying the strength of interaction between metal and support and inhibits sintering. In the case of the Y zeolite support, the smaller nickel crystallites formed on the CeNaY and CeKY supports can be explained on the basis of a decrease in the mass of nickel metal generated per gram of the zeolite due to the introduction of additional acidity that increases with decreasing Ni2+/Ce3+ ratio. Treating the samples with NHs effectively neutralizes this acidity41s4 with a resultant increase in the metal particle size. A similar rationale can be applied to the NiNH4Y samples where the low levels of Ni*+ reduction4”4g generate such a small mass of nickel metal that nickel crystallites detectable by X-ray diffraction (> 5 nm) are apparent only at higher nickel loading (Table 8).
diameter/rim Precalcined
at 723 K
NiNaY-22.8 NiNaY-63.1 NiKY-23.5 NiKY-62.5 NiLiY-63.7 NiRbNaY-59.1 NiCsNaY-54.8
38.7 59.0 34.8 50.0 53.0 46.7 35.4
29.9 54.2 25.1 42.3 49.7 31.1 24.3
NiO-NaY/3.gs NiO-NaY/6.8 NiO-KY/3.6 NiO-KY/6.5 NiO-LiY/3.7 ND-RbNaY/4.4 NiO-CsNaY/4.0
43.2 50.1 39.1 45.3 36.7 38.4 26.9
30.1 36.2 28.4 33.6 24.2 28.3 19.4
NiAl,03/2.2 NiAl&/4.3 NiAl,03/9.2
13.9 30.7 46.3
10.2 24.5 37.0
‘Wt%
Ni
Table 6 Variation of the crystallite size of nickel supported on the ion-exchanged NiKY-23.5 and the impregnated NiO-KY/3.6 and NiAl,03/4.3 samples with the rate of heating in hydrogen up to a final temperature of 723 K that was maintained for 18 h Nickel Heating
rate/K
hr-’
25 50 100 300 500 600 800 1000
76 -
NiKY-23.5
exchange at 723
x Zeolite
sample
NiNaY-21.8 CeNiNaY-21.6 CeNiNaY-21.1 CeNiNaY-20.8 NiNaY-58.0 CeNiNaY-54.4 CeNiNaY-34.9 CeNiNaY-24.0
38-
26 42 58 Crystallite Diameter/
74 nm
Figure 9 Correlation of the mass of nickel metal generated per gram of zeolite for the reduction of the ammonia treated NiNaY (A), NiKY (01, NiLiY (XI, NiRbNaY (WI, and NiCsNaY (+) at 723 K for 18 h
1991, Vol 11, January
crystallite NiO-KYI3.6
38.1 36.3 35.4 35.0 32.2 30.2 27.0 25.3
Table 7 Effect of Ce3’ generated on reduction
ZEOLITES,
Nickel crystallite at 723 K
cm3 min-’ Nz on a range of on reduction
n
19 28 37 Crystallite Diameter
8
(in a 120 supported generated
NiKY-25.0 CeNiKY-24.8 CeNiKY-24.3 CeNiKY-23.9 NiKY-56.9 CeNiKY-55.2 CeNiKY-48.8 CeNiKY-35.7
NiZ+/Ce3+
2.8 0.8 0.6 6.7 1.4 0.6 4.2 0.9 0.6 7.3 2.1 1.2
sizelnm NiAl,03/4.3
41.3 40.5 39.6 38.4 37.1 37.0 36.3 36.0
on the
30.8 31.3 31.6 33.7 36.0 36.9 39.4 42.8
size of nickel
ctystallites
Nickel crystallite NH3 treated
size/rim Untreated
K
39.4 37.2 33.7 28.6 66.6 64.2 52.8 40.8
37.1 29.3 17.1 13.3 57.3 51.1 31.6 19.7
36.4 34.0 30.5 24.1 58.1 57.3 43.6 38.1
35.0 31.1 23.1 18.3 55.9 48.3 38.8 30.1
Effects Table 8 Variation of nickel nickel loading for a range NiNH,Y samples
Zeolite
crystallite size and of ammonia-treated
Treated dlnm
sample
NiNH,,Y-6.5 NiNH,Y-20.7 NiNH4Y-52.2 NiNH4Y-69.1
with NH3 D . 1O-3
14.2 16.4 26.1 33.1
’ Undetectable
by X-ray
line
dispersion with and untreated
Untreated dlnm
36.6 30.8 19.4 15.2 diffraction
of environment
D . 1O-3
a a
a *
14.0 20.4
36.1 24.7
on growth
of Ni particles:
9 Chemical composition by impregnation with
Zeolite sample NiGNaY NiO-NaY NiO-NaY NiO-NaY Ni(FKY ND-KY NiO-KY ND-KY NiGLiY NiO-LiY NiO-LiY NiO-LiY NiO-RbNaY NGRbNaY NiO-RbNaY NGRbNaY NiO-CsNaY NiO-CsNaY NiO-CsNaY NiO-CsNaY *Based
Ni”
of nickel-loaded nickel nitrate
Ni’+/u.c.
0.7 2.2 3.9 6.8 0.7 1.9 3.6 6.5 0.8 2.2 3.7 6.8 0.6 1.7 2.8 4.2 0.7 1.5 2.4 3.7 on hydrated
unit
2.0 6.5 11.7 20.3 2.3 6.0 11.4 20.3 2.2 6.2 10.4 19.3 2.3 6.1 10.2 15.1 2.7 6.2 9.8 15.2 cell
M.A.
Keane
broadening
Y zeolites
N i*+/ U.C
I
pre-
b 5
Water Wt%
and
the pore structure. Heating the nickel impregnated NH4Y support in hydrogen resulted in the formation of measurable metal crystallites (> 5 nm) at nickel loadings greater than ca. 15 Ni*+/u.c.. The reduction of the Ni/Si02 samples generated much smaller crystallites with only the higher loaded (> 9.7 wt% Ni) samples exhibiting X-ray detectable nickel particles.
Effect of preparation by impregnation The chemical compositions of the samples prepared by impregnation are given in Table 9. Nickel crystallite size as a function of nickel loading for representative NiLiY, NiKY, and NiCsNaY samples prepared by ion exchange and impregnation is plotted in Figure 10. It can be clearly observed that the impregnated samples exhibit a smaller average nickel crystallite size. The strong influence of the larger alkali metal co-cations in supressing the process of metal agglomeration observed for the ion exchanged is also manifest for the impregnated samples. Indeed, as the alkali metal ions are not exchanged out during the impregnation procedure (resulting in a net higher alkali metal ion concentration), this inhibitory effect is more pronounced in the case of the impregnated samples. Heating the supported nickel nitrate species in hydrogen results in the formation of water and possibly NO*, NO, or NHs as byproducts that are carried away in the hydrogen stream without the generation of any surface Bronsted acidity.41V4s In terms of the Law and Kenney mode1,23 the lower level of acidity associated with the impregnated samples may serve to stabilize smaller metal particles within Table pared
B. Cough/an
20
N i*+/ UC.
content (w-t%) 26.1 26.3 27.1 28.4 23.6 24.4 24.5 25.0 27.6 28.3 28.5 28.6 21.2 22.0 22.4 22.9 10.3 11.0 13.1 13.4
15
10
v
1
E 34 >
1
I
1 28-
. v
m 24 22.% $ 16?+I
v
.
. v
. 3.2
t 6.4
9.6
12.8
Ni*‘1 U.C. Figure 10 Variation of crystallite size (generated on reduction at 723 K for 18 h) with nickel loading for a range of (a) NiLiY samples [(O) ion exchanged; (M) impregnation)], (b) NiKY samples [(O) ion exchange; (0) impregnation], and (c) NiCsNaY samples [(V) ion exchange; (v) impregnation].
ZEOLITES, 1991, Vol 11, January
9
Effects
of environment
on growth
of Ni particles:
8. Cough/an
and
The metal particles supported on Al203 are of comparable size to those observed for the zeolite support and increase in magnitude with increasing nickel content, i.e., 13.9-46.3 nm in the loading range 2.2-9.2 wt% Ni. The effect of precalcination of the impregnated support was also considered; the relevant data are presented in Table 5. In common with the ion-exchanged samples, heating the impregnated samples in nitrogen prior to hydrogen reduction has been shown to lower the degree of Nizf cation reduction with the resultant generation of a smaller volume of nickel metal”‘.“” that is in the form of smaller crystallites. In the same manner, hydrogen treatinent of the impregnated zeolite samples at higher heating rates generates smaller nickel particles (Table 6). This is in complete contrast with the Ni/ A1203 system, where an increased heating rate results in the formation of larger metal particles (Table 6). The latter phenomenon must be due to the observed exothermic heat release during the heat treatment of Ni(N0&6 Hz0 in hydrogen”‘: A slower heating rate during the reduction of alumina-supported nickel nitrate prevents such exothermic temperature excursions, thereby suppressing the sintering process. This effect was not observed in the case of the zeolite support. The aluminosilicate framework must absorb the heat release during the high-temperature reduction of Ni(N0&.6 Hz0 without transmitting the effect to the supported metal phase.
M.A.
ACKNOWLEDGEMENTS We are indebted to Mr. Dermot McGrath, senior technician in these laboratories, for his help with the X-ray diffraction measurements.
REFERENCES
6 7 8 9 10 11 12 I3 14 15 I6
CONCLUSIONS Reduction of the nickel-exchanged zeolites results in the generation of a nickel metal phase exhibiting a wide particle-size distribution. As the reduction temperature is increased, crystallite growth occurs via crystallite migration, resulting in the formatiori of metal on the external surface. Crystallite size also increases with increasing nickel exchange. The larger alkali metal co-cations are the most effective in blocking the migration path of the nickel species and inhibit the aggregation of metal particles on the external surface yielding the following order of increasing crystallite size: NiCsNaY < NiRbNaY < NiKY < NiNaY < NiLiY. Nickel crystallite growth is also inhibited by precalcination in flowing nitrogen and higher heating rates in flowing hydrogen. The high polarizing power associated with Ce3+ ions generates considerable Brijnsted acidity that inhibits Ni’+ reduction and ultimately generates smaller supported nickel particles. Under identical reduction conditions, the nickel-impregnated zeolites exhibited smaller supported nickel crystallites when compared with their ion-exchange counterparts. Nevertheless, the zeolite lattice promotes the agglomeration of nickel metal particles to a greater extent than do the amorphous planar supports; the A1203 and, more particularly, the SiOp carriers exhibited the finest dispersions of nickel metal. The considerations reported here have a profound effect on the interpretation of catalysis over these samples and will be reported in due course.
10
ZEOLITES,
1991, Vol 11, January
Keane
I7 18 I9 20 21 22
23 24 25 26 27
28
29 30
31 32
33 34 35
Jacobs, P.A. Carboniooenic Activity Zeolites, Elsevier, Amsterdam, 1977 Jaeger, N., Plath, P. and Schulz-Ekloff, G. Acta Phys. Chem. Szeaed (Enalish Translation) 1985, 31, 198 Delifosse, I%, in Catalysis byzeolites, Stud. Surf. Sci. Catal. 1980,5, 237 Delafosse. D.. J. Chem. Phys. 1986. 83, 791 Jaeger, NJ., Ryder, P. and Schulz-Ekloff, G., in Strucrure and Reactivity of Modified Zeolites. Stud. Surf. Sci. Catal. 1984, 18,299 Narayanan, S. J. Sci. Ind. Res. 1985, 44, 314 Pulvermacher, B. and Ruckenstein, E. J. Caral. 1974,35, 115 Anderson, J.R. Structure of Metallic Catalysts, Academic Press, London, 1975 Flynn, P.C. and Wanke, S.E. Catal. Rev.-Sci. 1975, 12, 93 Narayanan, S., in Metal Microstructures in Zeolites, Stud. Surf. Sci. Catal. 1982, 12, 245 Brennan, J. Ph.D. Thesis, National University of Ireland, 1983 Riekert, L. Ber. Bunsenges Phys. Chem. 1969, 73, 331 Richardson, J.T. J. Catal. 1971, 21, 122 Lawson, J.D. and Rase, H.J. Ind, Eng. Prod. Res. Dev. 1970, 9,317 Exner, D., Jaeger, N., Nowak, R., Schrubbers, H. and SchulzEkloff, G. J. Caral. 1982, 74, 188 Davidova, N.P. Valcheva, M.L. and Shopov, D.M., in Caralysis by Zeolites, Stud. Surf. Sci. Caral. 1980, 5, 285 Schulz-Ekloff, G. and Blum, J.K. Surf. Sci. 1987, 183, 216 Reman, W.G., Ali, A.H. and Schuit, G.C.A. J. Catal. 1971,20, 374 Maskos, Z. and Vanhoof, J.H.C. J. Catal. 1980, 60, 73 Elliot, D.J. and Lunsford, J.H. J. Caral. 1979, 57, 11 Davidova, N., Peshev, N. and Shopov, D. J. Catal. 1979,58, 198 Minchev, Ch., Kanazirev, V., Kosova, L., Penchev, V., Gunsser, W. and Schmidt, F., in Proceedings of 5th international Conference on Zeolites, (Ed. L.V.C. Rees) Hevden, London, 1980, p. 355 Law, P.L. and Kenney, C.N. J. Catal. 1980, 64, 241 1984, 4, 231 Naravanan, S. Zeolites Jacobs, P.A., Nijs, H., Verdonck, J., Derouane, E.G., Gilson, J.P. and Simoens, A.J. Trans. Faraday Sot. I 1979, 5, 1196 Penchev, V., Davidova, N., Kanazirev, V., Minchev, Ch. and Neinska, Y Adv. Chem. Ser. 1973, 121, 461 Dalla-Betta, R.A. and Boudart, M. in Proceedings of 5th International Congress on Catalysis, Miami Beach, 1973, Vol 2, p. 1329 Penchev, V., Peshev, N.V. and Davidova, N.P., in Proceedings of 3rd International Symposium on Heterogeneous Catalysis, Varna, 7975 (Iza BAN, Sofia, 1978) p. 490 Penchev, V., Peshev, N.V. and Davidova, N.P. Do/d. Bolg. Akad. Nauk. (English Translation) 1975, 28, 347 Exner, D., Jaeger, N.i., Moller, K., Nowak, R., Schrubbers, H., Schulz-Ekloff, G. and Ryder, P., in Metal Microstructures in Zeolites, Stud. Surf. Sci. Catal. 1982, 12, 205 Bager, K.H., Vogt, F. and Bremer, H. ACS Symp. Sr. 40, Am. Chem. Sot.. Washington, DC, 1977, p. 528 Sauvion, G.N., Tempere, J.F., Guillieux, M.F., DjegaMariadassou, G. and Delafosse, D. J. Chem. Sot., faraday Trans. I 1985, 81, 1357 Jeanjean, J., Djemel, S., Guillieux, M.F. and Delafosse, D. J. Phys. Chem. 1981, 85,4145 Djemel, S., Guillieux, M.F., Jeanjean, J., Tempere F. and Delafosse, D. J. Chem. Sot., Faraday Trans. I 1982, 78, 835 Akalay, I., Guilleux, M.F., Tempere F. and Delafosse, D. J.
Effects
36 37 38 39 40 41 42
of environment
Chem. Sot., Faraday Trans. I 1987,83, 1137 Klyueva, N.V., Valcheva, M.L., Davidova, N.V., lone, K.G. and Shopov, D.M. React. Kinet. Katal. Lett 1981, 17, 315 Chien, S.H., Lu, K.L., Huang, H.W. and Hwang, J.M. Bull. Inst. Chem. Acad. Sinica 1986, 33, 81 Bartholomew, C.H. and Sorensen, W.L. J. Catal. 1983, 81, 131 Ueno, A., Suzuki, H. and Kotera, Y., J. Chem. Sot., Faraday Trans. I 1983, 79, 1137 Coughlan B. and Keane, M.A. J. Catal. 1990, 123, 364 Keane, M.A. Ph.D. Thesis (Vol. I and II), National Universin/ of Ireland, 1988 Flanigen, E.M., Khatami, H. and Szymauski, H.A. Adv. Chem. Ser. 101, Am. Chem. Sot., Washington, DC. 1971, p. 201
43 44
45 46 47 48 49 50
on growth
of Ni particles:
B. Cough/an
and
M.A.
Keane
Schoonheydt, R.A., Vandamme, L.J., Jacobs, P.A. and Uytterhoeven, J.B. J. Catal. 1976, 43, 292 Jenkins, R. and devries, J.L. Worked Examples of X-ray Analysis 2nd Ed. Phillips Technical Library, Macmillan, New York, 1978 Bartholomew, C.H. and Sorensen, W.L. J. Catal. 1983, 81, 131 Wynblatt, P. and Gjostein, N.A. Prog. So/id State Chem. 1975,9,21 Richardson, J.T. and Crump, J.G. J. Catal. 1979,57,417 Coughlan, B. and Keane, M.A. J. Colloid Interface Sci., 1990, 137,483 Coughlan, B. and Keane, M.A. J. Catal., in press Bartholomew, C.H. and Farrauto, R.J. J. Catal. 1976,45, 41
ZEOLITES,
1991, Vol 11, January
11
Characterization;
NiY;
nickel
metal
crystallitea;
XRD line broadening;
INTRODUCTION Metal catalysts are commonly employed in the form of metal dispersed as small crystallites on high surface area supports. In catalyst preparation, maximum effort is expended on generating a well-dispersed metal with optimum crystallite size that is stable during catalysis. The unique structural characteristics exhibited by zeolites can be applied to the preparation of metal catalysts. Although the relationship between the degree of dispersion of the metal and the zeolite crystal chemistry is still not fully understood,’ the difficulty in obtaining a homogeneous, finely dispersed metal phase is well recognized.‘” Metal particle growth can occur during thermal treatment by two distinct mechanisms. The model developed by Pulvermacher and Ruckenstein,7 the Crystallite Migration Model, describes the sintering process as a migration of metal over the surface followed by coalescence with other metal crystallites upon collision. Mass transfer is proposed to proceed by a momentary accumulation of metal atoms on one side of a particle, the net effect being a migration of the Address reprint requests to Dr. Keane at (present address) Chemistry Department, The University, Glasgow, G12 900, Received 12 March 1990; accepted 31 July 1990 0 1991 Butterworth-Heinemann 2
ZEOLITES,
1991, Vol 11, January
the
UK.
Ce3+ exchange;
impregnation
particle as a whole in that direction.8 The second model, the Atomic Migration Model, proposed by Flynn and Wanke,g considers sintering to occur by the loss of single atoms from one particle that are then deposited on a growing crystallite. Mobility and agglomeration to form large crystallites during heat treatment has been one of the major problems associated with the reduction of Ni*+ cations supported on zeolites.‘0-‘8 Reduction of NiNaY has led to the formation of large ferromagnetic crystals on the external zeolite surface.“,’ On obtaining complete reduction of Ni*+ ions in NiNaY, Elliot and Lunsford*’ observed a broad particle distribution. Even when reduction is incomplete, part of the metal diffuses out of the zeolite pores to form large agglomerates.*t-** From temperatureprogrammed oxidation and reduction experiments on NiNaX and NiNaY systems, Jacobs et ~1.~~calculated the amount of nickel present inside and outside the zeolite lattice. They found that the size of the internal nickel was restricted to the size of the supercage and the Nt” formed on the external surface exhibited shape anisotropy. Penchev et al.*‘j obtained an optimum supported nickel metal surface area on reduction in the temperature range 723-773 K. Although the best distribution of metal has been
Effects
of environment
obtained when water is corn letely removed from the zeolite prior to reduction,’ P**’ contrary reports suggest that the addition of water vapor to the hydrogen stream promotes the acidic function and leads to the formation of more highly dispersed metal particles.28V2” Regardless, metal particles exceeding supercage dimensions have repeatedly been found embedded in the faujasite matrix; nickel aggregates of up to 15 nm have been reported in the internal structure. 5.15.22.25.30 The dispersion of Ni” particles depends strongly on the presence of a second cation or metal in the immediate environment. l4 Bager et ~1.~’ reported that although NiCeNaY and NiNH4Y exhibited similar degrees of Ni*+ cation reduction, the specific surface area of Ni” supported on NiNH4Y was three times greater than that supported on NiCeNaY. In complete contrast, Ce3+ addition to NiNaX samples has been reported to result in the formation of much smaller metal crystallites. 32-35 The trivalent cerium ions have been proposed to stabilize the nickel atom species (a) in limiting nickel oxidation by the electron acceptor sites of the support32.34.35 and (b) by generating sites for the preferential growth of metal particles,34 thereby inhibiting the migration of atomic species by the interaction of Ce3+ with Ni’. According to Klyueva et al., 36 the reduction of nickel zeolites prepared by impregnation results in the formation of metal particles that locate mainly on the outer surface of the zeolite. Chien et a1.,37 in contrast, reported that the average metal particle size for the ion-exchanged samples was larger than that for the impregnated samples. Nickel sintering also occurs on silica and alumina supports. 38 Using the same reduction conditions, Richardson l3 found that the nickel crystallite size increased in the order: NiA1203 < NiSi02/A1203 < Nisi02 < NiNaY. The size of the metal particles supported on the amorphous carriers also increased with increasing reduction temperature, hydrogen flow rate, and nickel loading and decreased on the introduction of a precalcination step.38S3g As part of a detailed catalytic and characterization study of Y zeolites, the present paper attempts to define the nature and extent of agglomeration of nickel metal supported on a range of Y zeolites under various conditions of pretreatment and reduction and to establish procedures for minimizing the extent of nickel agglomeration. Data for Si02 and Al203 supports are also included. This paper is the first to systematically probe the role of the alkali metal cocation in influencing the size of nickel metal particles supported on Y zeolites prepared by ion exchange and by impregnation.
EXPERIMENTAL The starting or Sieve LZY-52 (H20)260]. The (30-120 mesh) Ltd. and BDH, exchange) and
parent zeolite was Linde Molecular [formula: Nass(Al02)ss(Si02)~34 alumina (70-230 mesh) and silica supports were supplied by Labkem respectively. KY and LiY (100% RbNaY and CsNaY (ca. 68% ex-
on growth
of Ni particles:
B. Cough/an
and
M.A.
Keane
change) were prepared by exchanging out the parent Naf ions. A known weight of Nay, usually 250 g, was refluxed with 1 M solutions of KNOs, LiN03, C&l, and RbCl (ca. 400 cm3) for 24 h, after which the zeolite was filtered and thoroughly washed with hot deionized water to remove the occluded salt. The zeolite was then air dried at 383 K for 20 h. The partially exchanged samples, i.e., K, Li, Rb, WNaY, were exchanged a further nine times as described above. A fully exchanged NH4Y zeolite was prepared by refluxing 250 g of NaY with ca. 400 cm3 of a 0.5 M solution of NH4NOs over a period of 3 d, filtering and drying as before; after 10 such treatments, complete exchange was achieved. Nickel-exchanged samples were prepared by taking 100 g of Nay, KY, LiY, RbNaY, CsNaY, or NH4Y and refluxing with a 400 cm3 Ni(NO& solution for 24 h. In this study, dilute Ni(NOs) solutions (< 0.1 M) were employed in which the pH of the nitrate/zeolite suspension was in the range 6-7.5. Under these conditions, a single exchan e cycle resulted in a maximum exchange of ca. 7 Nl5 +/u.c., i.e., seven nickel cations per unit cell. Inzreparing samples with loadings greater than ca. 7 Ni +/u.c., repeated exchange was necessary. Quantitative impregnation of NaY, KY, LiY, RbNaY, CsNaY alumina, and silica was achieved with constant volumes of Ni(N03)* solutions by vacuum rotary evaporation to incipient wetness. The concentration of the Ni(NO& solution was chosen to achieve the desired metal content. All the samples were air dried at 373 K and stored over saturated NH4Cl prior to analysis. Atomic absorption (Ni*+ concentration) and flame emission (Na+, K+, Li+, Rb+, and Cs+ concentrations) techniques, using a Perkin-Elmer 360 atomic absorption spectrophotometer, were employed to determine the cation contents to within + 2%. Thermal analyses were also conducted on all the prepared samples using a Perkin-Elmer thermobalance operating in the t.g. mode to measure water loss as a function of temperature. The hydrated nickel samples (in pellet form, 1.181.70 mm diameter) were normally reduced in a fixed-bed catalytic reactor40’4’ in a 120 cm3 min-’ stream of hydrogen at 723 K for 18 h. However, these conditions were varied, the details of which will be described later. The heating system40.4’ normally employed only allowed for a variable rate of heating of the samples. In the experiments where a fixed rate of heating of the samples was desired, a temperature programmer/controller (Cambridge Process Controls Model 702) was used. The model 702 automatic controller is a combined sequence/ramp programmer and temperature control system. The microprocessor system in the unit allows fully automatic operation of the furnace through the complete operating cycle to a time and temperature sequence program stored in the unit’s memory. The reduced zeolites were also treated in a 120 cm3 min-’ stream of ammonia at 423 K for 15 min: hydrogen was then reintroduced, and the samples were flushed for 6 h at 473 K and treated for a further 24 h in hydrogen (120 cm3 min-‘) at 723 K.
ZEOLITES,
1991, Vol II, January
3
Effects
of environment
Table 1 Chemical ion exchange Zeolite sample NaY NiNaY-3.6 NiNaY-6.8 NiNaY-12.5 NiNaY-15.8 NiNaY-17.3 NiNaY-19.3 NiNaY-22.8 NiNaY-26.4 NiNaY-27.8 NiNaY-29.9 NiNaY-32.6 NiNaY-35.7 NiNaY-48.8 NiNaY-63.1 NiNaY-78.6 NiNaY-90.1 NiY
on growth composition
of Ni particles: of NiNaY
samples
B. Cough/an prepared
Water Na+/u.c. 58.0 55.7 53.7 50.7 48.8 47.5 46.1 44.0 41.7 40.9 41.0 38.3 36.0 30.0 22.3 14.6 6.9
Ni’+/u
.c .
1.0 f :: 4.6 5.0 5.6 6.6 7.7 8.1 8.7 9.5 10.4 14.1 18.3 22.8 26.1 29.0
H+/u.c.
and by
content (wt%) 25.1 25.5 25.3 26.3 26.5 26.4 26.5 26.6 26.8 26.8 26.8 27.2 27.6 28.6 29.1 29.5 30.0 30.4
0.3 0.3 0.1 0.5 0.7 0.8 0.9 0.9 0.6 0.7 1.2 -
Examination of the reduced samples for the presence of nickel crystallites was performed by studying the X-ray line broadening (Jeol JDX-85 diffractometer) around the nickel line at a 28 angle of 52.2” under the following conditions: time constant = 4; volta e = 30 kV; current = 20 mA; scan time = 0.5 min- 5 ; counts/s = 8 X 10’; radiation = CoK,. A correction for instrument broadening under identical conditions was made using Ni powder in either a NaY or KY matrix. Diffraction patterns of all the samples, before and after reduction, were obtained to ensure maintenance of sample crystallinity. Infrared spectroscopy in the range 1200-350 cm-’ was also used as a check on sample crystalhnity. The band at ca. 395 cm -’ has been assi ned to a breathing of the pore opening in zeolites 48 and is thus the most sensitive to changes in crystallinity. Table 2 Chemical exchange Zeolite sample
composition
of NiKY
samples
Water K+/u.c.
Ni’+/u.c.
H+/u.c.
KY NiKY-5.2 NiKY-8.0 NiKY-10.7 NiKY-17.9 NiKY-23.5 NiKY-26.9 NiKY-30.5 NiKY-35.6 NiKY-44.7 NiKY-47.0 NiKY-49.1 NiKY-54.5 NiKY-57.5 NiKY-62.5 NiKY-73.8 NiKY-82.0 NiKY-86.6 NiY
58.0 54.7 53.3 51.5 47.5 44.2 42.4 39.7 36.9 31.9 30.8 29.8 26.2 24.8 22.3 16.3 13.1 8.4 -
1.5 2.3 3.1 5.2 6.8 7.8 8.9 10.3 13.0 13.6 14.2 15.8 16.7 18.1 21.4 23.8 25.1 29.0
0.3 0.1 0.3 0.1 0.2 0.5 0.5 0.1 0.2 -
4
1991, Vol 11, January
ZEOLITES,
prepared
by ion
content (wt%) 22.4 22.6 22.3 22.7 23.6 23.5 24.2 24.4 24.8 25.5 26.0 26.4 26.9 27.3 27.6 28.0 28.4 29.3 30.2
M.A.
Keane
Table 3 Chemical NiNH4Y samples Zeolite sample
of NiLiY, NiRbNaY, by ion exchange
NiCsNaY,
Water AM+
LiY NiLiY-8.8 NiLiY-21.2 NiLiY-43.1 NiLiY-63.7 NiLiY-80.6 RbNaY NiRbNaY-7.3 NiRbNaY-18.6 NiRbNaY-27.3 NiRbNaY-35.1 NiRbNaY-47.4 NiRbNaY-59.1 CsNaY NiCsNaY-3.6 NiCsNaY-16.4 NiCsNaY-22.0 NiCsNaY-31.1 NiCsNaY-44.3 NiCsNaY-54.8 NH,,Y NiNH,Y-8.5 NiNH4Y-20.7 NiNH,Y-52.2 NiNH,Y-69.1 aAM’
composition prepared
7u.c. 58.0 52.3 44.6 32.0 20.7 14.2 40.2 35.8 29.4 24.3 19.8 15.2 10.3 39.8 36.6 29.2 26.0 20.7 13.6 11.2
Ni*+/u.c. 2.6 6.2 12.4 18.5 23.4 2.1 5.4 7.9 10.2 13.7 17.1 1.0 4.8 6.4 9.0 12.8 15.9 2.5 6.0 15.1 20.0
H+lu.c.
0.5 1.1 1.2 0.3 0.1 0.2 0.2 0.1 0.1 58.0 53.0 46.0 27.7 17.9
and
content (lNt%) 27.8 27.9 28.2 28.8 29.3 29.9 20.4 20.8 21.9 23.0 24.2 25.9 27.4 7.8 10.1 12.3 13.1 14.9 16.6 19.4 24.3 24.3 25.5 27.6 28.1
= Li+ , Rb+ # or Cs’
RESULTS AND DISCUSSION The chemical compositions of the ion-exchanged samples are given in Tables l-3. By and large, the exchange process was stoichiometric. The extent of hydrolysis, as evidenced by the number of protons present, was slight and occurred only for the lower exchanged samples. At higher nickel loadings (> 14 Ni*+/u.c.), the total number of exchanged ions exceeded the original 58 ions per unit cell. At these exchange levels, an excess amount of nickel has been incorporated into the zeolite framework in accordance with the findings of Schoonheydt et a1.43 Sample crystallinity, monitored by X-ray diffraction and i.r. spectroscopy, was maintained for each sample. The metal phase generated on reduction of the nickelloaded samples was characterized by X-ray line broadening using the Scherrer formula44: d=KAlBCos0 which relates the mean particle diameter (d) to the X-ray broadening (8) of the diffraction lines and K is a constant; in accordance with the work of Jenkins and deVries,44 a value of 0.9 was chosen for K. Metal dispersion can be related to crystallite size according tog: D = 0.505/d
(2)
where D is the dispersion or fraction of metal atoms exposed to the surface. The lower limit of detection
Effects
of environment
. E c 50
1,
5
A
Q E 40-
.-
0
. l
6
al 30. ?d = 3 220 t A .
.
.
.
l
on growth
of Ni particles:
B. Cough/an
and
M.A.
Keane
Elucidation of the sintering mechanism Two mechanisms have been proposed for metal crystallite growth, i.e., crystallite migration’ and atomic migration.g The former involves the migration of entire crystallites over the support surface followed by collision and coalescence, whereas the atomic migration mechanism involves the actual detachment of metal atoms from the crystallites that are ultimately captured by the larger crystallites. The power law rate equation for the crystallite migration model’,‘:
.
-dDldt 610
640
670
700
T/K Figure 1 Variation of the size of nickel crystallites supported on NiNaY-63.1 (A) and NiKY-62.5 (0) with increasing reduction temperature after 18 h on stream
for this method is ca. 5 nm, with the result that it gives no information on the state of the metal in the internal pore structure. X-ray line broadening is therefore diagnostic of the presence of nickel metal located on the external surface. Effect of reduction time and temperature The primary purpose of the present work was to elucidate the factors responsible for changes in supported nickel metal particle size. The main factors affecting crystallite size are the reduction temperature and time. The results obtained when varying these parameters are shown in Figures 1 and 2. The variation in crystallite diameter at various reduction temperatures for a NiNaY and NiKY sample of similar metal loading are depicted in Figure 1. Crystallite sizes proved to be larger at elevated temperatures; no metal particles were discernible at reduction temperatures lower than 573 K. The crystallite size vs. temperature plots closely mirror the reduction curves reported elsewhere,40 suggesting a direct correlation between the degree of Ni2+ reduction and the size of the Ni” metal particles that are formed. This is to be expected in that the higher the level of Ni2+ reduction, the greater the concentration of nickel metal particles with an associated higher probability of particle agglomeration resulting in the generation of larger particles. The effect of reduction time on particle size at constant reduction temperature is illustrated in Figure 2. The time-dependence relation is identical to that observed for the degree of Ni*+ reduction4’ in that both parameters increase with time and attain a fixed equilibrium value. Nickel metal crystallite size therefore increases with time because of the increased volume of metal formed during reduction. There is no evidence to suggest that crystallite sintering is dependent on the reduction time once the equilibrium reduction level has been attained.
= kD”
(3)
differs fundamentally migration modelg: -dDldt
from
that
for
the atomic
= kD5 exp (nDID0)
(4)
where D is nickel metal dispersion and n is the order of sintering. The identification of the prevailing mechanism is experimentally very difficult. Bartholomew and Sorensen45 adopted the following criteria to infer the predominant mechanism: crystallite migration results particle-size distribution;
(i)
8
in a log-normal
11 14 t i me I hrs.
17
. .
1,
.
.
b
.
s 36. 5 5 24. 3.=
A l . 0
39l 12. 5
b
I
8
14 11 time/hrs.
17
Figure 2 Variation of the size of nickel crystallites supported on NiNaY-63.1 (A) and NiKY-62.5 (0) with increasing reduction time at (a) 623 K and (b) 723 K
ZEOLITES,
1991, Vol 11, January
5
Effects
of environment
on growth
of Ni particles:
B. Cough/an
and
M.A.
Keane
observed activation energies are clearly lower than is the value of 431 kJ mol-’ quoted for nickel atom detachment,47 and, accordingly, the data presented in this study are consistent with a crystallite migration sintering process.
Ln(D1 /D2) Figure 3 Plot of In(t) vs. ln(D,/&) (A) and NiKY-62.5 (0)
for the reduction
of NiNaY-63.1
(ii) the sintering order (n) should lie between 3 and 5 for atomic migration and 2 and 13 for crystallite migration; (iii) the form of the rate equation for crystallite migration [Equation (3)] and atomic migration [Equation (4)] differ. Preliminary electron microscopic studies4’ on NiKY62.5 (18.1 Ni*+/u.c.) reduced at 723 K for 18 h in a 120 cm3 min- ’ hydrogen stream revealed a lognormal crystallite-size distribution (in the range 2565 nm) as well as a marked particle-shape anistropy. According to Wynblatt and Gjostein,46 the power law kinetics for sintering via crystallite migration can be expressed as: n ln(D1/D2) = C + In (t)
(5)
where D, and D2 represent the nickel dispersion at 623 K and 723 K, respectively, C is a constant, and t is the sintering time. If the sintering process follows the crystallite migration model, plots of ln(Di/Dz) vs. In(t) should result in straight lines from which the order of sintering (n) can be computed. The data presented in Figure 2 were interpreted according to the above rate equation; the straight-line plots of ln(D1/D2) as a function of In t for a NiNaY and NiKY sample of similar nickel loading (ca. 18 Ni*+/u.c.) are shown in Figure 3. Values for 72of 2.5 for NiNaY-63.1 and 3.1 for NiKY-62.5, however, fall within the range that corresponds to both sintering models.45 The apparent activation energy for sintering can be calculated fromg: E, = R(T,T2/T2
- T1) ln(AtilAt,)
ZEOLITES,
7997, Vol 11, January
. l
00
. A. c
:
. .
l
A
. . . . 0. A.0 . .
(6)
where At, and At, correspond to the time intervals necessary to obtain the same values of D1 and 02 at different temperatures T1 and T2. The experimental data yielded values of 20.2 kJ mol-’ for NiNaY-63.1 and 21.3 kJ mol-’ for NiKY-62.5 in the temperature range 623-723 K. This suggests a higher barrier for sintering in the case of the NiKY-62.5 sample. The
6
Effect of nickel loading Figure 4 illustrates the variation of crystallite size with nickel loading for a range of NiNaY and NiKY samples. It is immediately evident that the crystallite size increases for both systems with an increasing level of nickel exchange that can be explained by the increased probability of metal atoms being in close proximity to facilitate agglomeration, or, in terms of the model proposed by Law and Kenney,23 the greater number of protons generated on reduction of the nickel-rich samples48 promotes the counterdiffusion of nickel cations and protons, supplying more nickel ions close to the outer surface of the zeolite framework that are subsequently reduced to zerovalent nickel supported on the outer surface. From Equation (2), it can be readily observed that the higher loaded samples exhibited a lower dispersion of metal particles. As the lower limit of detection for the line-broadening technique is ca. 5 nm and the diameter of the large supercages is 1.2 nm, the date presented in Figure 4 represent the average crystallite size of the nickel metal supported on the external zeolite surface; the external nickel particles are therefore smaller when supported on the KY carrier. The generally observed bidispersities,10-25 i.e., large fractions of reduced metal at the outer surface with a smaller fraction of smaller clusters inside the zeolite, may result from the relatively low ion density in the migration path of Ni’. The larger K+ ions are therefore more effective than are the Na+ ions in blocking the path of the migrating nickel species and retard the aggregation process of metal particles on the external surface. Presumably, the larger K+ ions will also act to retard the growth of intracrystalline metal clusters. The observed trend of the larger alkali metal co-cation-exchanged Y zeolite generating smaller
l e * d
6 Figure NiNaY
4 Variation of crystallite size with (A) and NiKY (0) samples reduced
24 nickel loading at 723 K for
for the 18 h
Effects
of environment
on growth
of Ni particles:
B. Cough/an
Table 4 Variation of the size and dispersion supported on a range of alkali metal zeolites, with ammonia treatment
Zeolite
I
I
18 Ni2+/ UC
6
Figure 5 Variation NiLiY (Xl, NiRbNaY 723 K for 18 h
24
12
of crystallite (B), and
size with nickel loading for NiCsNaY (+) samples reduced
the at
nickel metal particles is also followed by the NiLiY, NiRbNaY, and NiCsNaY samples (Figure 5). Indeed, the order of decreasing nickel crystallite size is NiLiY > NiNaY > NiKY > NiRbNaY > NiCsNaY. As discussed elsewhere,4s treatment of the reduced samples with NHs poisons the surface Bronsted acid sites and results in the generation of a greater mass of nickel metal. From the present study, the ammonia treatment produces a larger average nickel particle size (Fipre 6 and Table 4). In agreement with the degree of reduction studies,4g the effect on crystallite size is not as marked for the NiRbNaY and NiCsNaY samples. The correlation of the mass of supported nickel metal generated during reduction4’ with the size of the crystallites formed during sintering is depicted in Figures 7-9. It can be seen that the particle diameter increases with the amount of available metal. Indeed, as is most clearly illustrated in Figure 8, there appears to exist a critical value above which the generation of further metal does not affect the crystallite size. Nevertheless, under ammonia pretreatment conditions, at a particular mass of supported
sample
NiLiY-8.8 NiNaY-6.8 NiKY-8.0 NiRbNaY-7.3 NiCsNaY-3.6 NiLiY-21.2 NiNaY-22.8 NiKY-23.5 NiRbNaY-18.6 NiCsNaY-22.0 NiLiY-43.1 NiNaY-48.8 NiKY-49.1 NiRbNaY-47.4 NiCsNaY-44.3 NiLiY-63.7 NiNaY-63.1 NiKY-62.5 NiRbNaY-59.1 NiCsNaY-54.8
Treated dlnm
with NH3 D . 1O-3
30.1 24.2 24.3 17.2 11.0 40.6 39.0 36.1 28.3 20.0 62.1 63.4 54.2 43.4 32.1 79.4 69.1 60.2 55.9 38.0
16.8 20.9 20.8 29.3 45.9 12.4 12.9 14.0 17.9 25.3 8.0 7.9 9.3 10.2 15.7 6.4 7.3 8.4 9.0 13.3
and
M.A.
Keane
of nickel crystallites, cation-exchanged
Y
Untreated dlnm
D . 1O-3
28.4 24.5 24.7 17.0 11.1 34.9 38.7 34.8 27.1 20.9 44.6 55.1 50.1 42.4 31.1 52.1 59.0 55.6 46.7 35.4
17.8 20.6 20.4 29.7 45.3 14.5 13.1 14.5 14.8 24.2 11.4 9.2 10.1 11.9 16.2 9.7 8.6 9.1 10.8 14.3
metal the nickel particle size increases in the order NiCsNaY < NiRbNaY < NiKY < NiNaY < NiLiY, which is consistent with the order of decreased sintering. The role of precalcination in the formation of nickel metal particles was also investigated. The X-ray diffraction line-broadening data are presented in Table 5. Heating the samples in nitrogen prior to reduction has been shown to lower the degree of Ni*+ reduction4’ with a resultant smaller volume of supported metal, which, as expected, is in the form of smaller crystallites. In the same way, heating the samples in hydrogen at higher rates generates smaller metal particles (Table 6).
Effect
of Ce’+ exchange
Diluting the NiNaY and NiKY samples with Ce3+ ions has resulted in the formation of much smaller
x’ x
*
. A
.
.
. n
+
I 6
12 Ni2’/ Ud8
I
24
Figure 6 Variation of crystallite size with nickel loading for the ammonia treated NiNaY (A), NiKY (0). NiLiY (Xl, NiCsNaY (+I, and NiRbNaY (m) samples reduced at 723 K for 18 h
Figure 7 Correlation gram of zeolite for 723 K for 18 h
I 44 36 28 Crystallite Diameter I nm of the mass the reduction
ZEOLITES,
of nickel of NiNaY
52
metal generated (A) and NiKY
1991, Vol II, January
(0)
per at
7
Effects
of environment
on growth
of Ni particles:
8. Cough/an
1
and
n x
4
To 28’ 7 -i3 ” 21 8
n
-
Keane
Table 5 The effect of precalcination stream) on the size of nickel crystallites ion-exchanged or impregnated samples at 723 K for 18 h.
x
Sample +
5 140.-
m +
Z 0 ’ 1
I
M.A.
n
+
+
Reduced
x
x
+
+ 46 / nm
Figure 8 Correlation of the mass of nickel metal generated per gram of zeolite for the reduction of NiLiY (X), NiRbNaY (m), and NiCsNaY (+I at 723 K for 18 h
nickel particles (Table 7) Jeanjean et a1.33 have explained the higher dispersion of nickel metal on the CeNaX support in terms of the strong electrostatic field associated with the Ce3+ cations that is capable of modifying the strength of interaction between metal and support and inhibits sintering. In the case of the Y zeolite support, the smaller nickel crystallites formed on the CeNaY and CeKY supports can be explained on the basis of a decrease in the mass of nickel metal generated per gram of the zeolite due to the introduction of additional acidity that increases with decreasing Ni2+/Ce3+ ratio. Treating the samples with NHs effectively neutralizes this acidity41s4 with a resultant increase in the metal particle size. A similar rationale can be applied to the NiNH4Y samples where the low levels of Ni*+ reduction4”4g generate such a small mass of nickel metal that nickel crystallites detectable by X-ray diffraction (> 5 nm) are apparent only at higher nickel loading (Table 8).
diameter/rim Precalcined
at 723 K
NiNaY-22.8 NiNaY-63.1 NiKY-23.5 NiKY-62.5 NiLiY-63.7 NiRbNaY-59.1 NiCsNaY-54.8
38.7 59.0 34.8 50.0 53.0 46.7 35.4
29.9 54.2 25.1 42.3 49.7 31.1 24.3
NiO-NaY/3.gs NiO-NaY/6.8 NiO-KY/3.6 NiO-KY/6.5 NiO-LiY/3.7 ND-RbNaY/4.4 NiO-CsNaY/4.0
43.2 50.1 39.1 45.3 36.7 38.4 26.9
30.1 36.2 28.4 33.6 24.2 28.3 19.4
NiAl,03/2.2 NiAl&/4.3 NiAl,03/9.2
13.9 30.7 46.3
10.2 24.5 37.0
‘Wt%
Ni
Table 6 Variation of the crystallite size of nickel supported on the ion-exchanged NiKY-23.5 and the impregnated NiO-KY/3.6 and NiAl,03/4.3 samples with the rate of heating in hydrogen up to a final temperature of 723 K that was maintained for 18 h Nickel Heating
rate/K
hr-’
25 50 100 300 500 600 800 1000
76 -
NiKY-23.5
exchange at 723
x Zeolite
sample
NiNaY-21.8 CeNiNaY-21.6 CeNiNaY-21.1 CeNiNaY-20.8 NiNaY-58.0 CeNiNaY-54.4 CeNiNaY-34.9 CeNiNaY-24.0
38-
26 42 58 Crystallite Diameter/
74 nm
Figure 9 Correlation of the mass of nickel metal generated per gram of zeolite for the reduction of the ammonia treated NiNaY (A), NiKY (01, NiLiY (XI, NiRbNaY (WI, and NiCsNaY (+) at 723 K for 18 h
1991, Vol 11, January
crystallite NiO-KYI3.6
38.1 36.3 35.4 35.0 32.2 30.2 27.0 25.3
Table 7 Effect of Ce3’ generated on reduction
ZEOLITES,
Nickel crystallite at 723 K
cm3 min-’ Nz on a range of on reduction
n
19 28 37 Crystallite Diameter
8
(in a 120 supported generated
NiKY-25.0 CeNiKY-24.8 CeNiKY-24.3 CeNiKY-23.9 NiKY-56.9 CeNiKY-55.2 CeNiKY-48.8 CeNiKY-35.7
NiZ+/Ce3+
2.8 0.8 0.6 6.7 1.4 0.6 4.2 0.9 0.6 7.3 2.1 1.2
sizelnm NiAl,03/4.3
41.3 40.5 39.6 38.4 37.1 37.0 36.3 36.0
on the
30.8 31.3 31.6 33.7 36.0 36.9 39.4 42.8
size of nickel
ctystallites
Nickel crystallite NH3 treated
size/rim Untreated
K
39.4 37.2 33.7 28.6 66.6 64.2 52.8 40.8
37.1 29.3 17.1 13.3 57.3 51.1 31.6 19.7
36.4 34.0 30.5 24.1 58.1 57.3 43.6 38.1
35.0 31.1 23.1 18.3 55.9 48.3 38.8 30.1
Effects Table 8 Variation of nickel nickel loading for a range NiNH,Y samples
Zeolite
crystallite size and of ammonia-treated
Treated dlnm
sample
NiNH,,Y-6.5 NiNH,Y-20.7 NiNH4Y-52.2 NiNH4Y-69.1
with NH3 D . 1O-3
14.2 16.4 26.1 33.1
’ Undetectable
by X-ray
line
dispersion with and untreated
Untreated dlnm
36.6 30.8 19.4 15.2 diffraction
of environment
D . 1O-3
a a
a *
14.0 20.4
36.1 24.7
on growth
of Ni particles:
9 Chemical composition by impregnation with
Zeolite sample NiGNaY NiO-NaY NiO-NaY NiO-NaY Ni(FKY ND-KY NiO-KY ND-KY NiGLiY NiO-LiY NiO-LiY NiO-LiY NiO-RbNaY NGRbNaY NiO-RbNaY NGRbNaY NiO-CsNaY NiO-CsNaY NiO-CsNaY NiO-CsNaY *Based
Ni”
of nickel-loaded nickel nitrate
Ni’+/u.c.
0.7 2.2 3.9 6.8 0.7 1.9 3.6 6.5 0.8 2.2 3.7 6.8 0.6 1.7 2.8 4.2 0.7 1.5 2.4 3.7 on hydrated
unit
2.0 6.5 11.7 20.3 2.3 6.0 11.4 20.3 2.2 6.2 10.4 19.3 2.3 6.1 10.2 15.1 2.7 6.2 9.8 15.2 cell
M.A.
Keane
broadening
Y zeolites
N i*+/ U.C
I
pre-
b 5
Water Wt%
and
the pore structure. Heating the nickel impregnated NH4Y support in hydrogen resulted in the formation of measurable metal crystallites (> 5 nm) at nickel loadings greater than ca. 15 Ni*+/u.c.. The reduction of the Ni/Si02 samples generated much smaller crystallites with only the higher loaded (> 9.7 wt% Ni) samples exhibiting X-ray detectable nickel particles.
Effect of preparation by impregnation The chemical compositions of the samples prepared by impregnation are given in Table 9. Nickel crystallite size as a function of nickel loading for representative NiLiY, NiKY, and NiCsNaY samples prepared by ion exchange and impregnation is plotted in Figure 10. It can be clearly observed that the impregnated samples exhibit a smaller average nickel crystallite size. The strong influence of the larger alkali metal co-cations in supressing the process of metal agglomeration observed for the ion exchanged is also manifest for the impregnated samples. Indeed, as the alkali metal ions are not exchanged out during the impregnation procedure (resulting in a net higher alkali metal ion concentration), this inhibitory effect is more pronounced in the case of the impregnated samples. Heating the supported nickel nitrate species in hydrogen results in the formation of water and possibly NO*, NO, or NHs as byproducts that are carried away in the hydrogen stream without the generation of any surface Bronsted acidity.41V4s In terms of the Law and Kenney mode1,23 the lower level of acidity associated with the impregnated samples may serve to stabilize smaller metal particles within Table pared
B. Cough/an
20
N i*+/ UC.
content (w-t%) 26.1 26.3 27.1 28.4 23.6 24.4 24.5 25.0 27.6 28.3 28.5 28.6 21.2 22.0 22.4 22.9 10.3 11.0 13.1 13.4
15
10
v
1
E 34 >
1
I
1 28-
. v
m 24 22.% $ 16?+I
v
.
. v
. 3.2
t 6.4
9.6
12.8
Ni*‘1 U.C. Figure 10 Variation of crystallite size (generated on reduction at 723 K for 18 h) with nickel loading for a range of (a) NiLiY samples [(O) ion exchanged; (M) impregnation)], (b) NiKY samples [(O) ion exchange; (0) impregnation], and (c) NiCsNaY samples [(V) ion exchange; (v) impregnation].
ZEOLITES, 1991, Vol 11, January
9
Effects
of environment
on growth
of Ni particles:
8. Cough/an
and
The metal particles supported on Al203 are of comparable size to those observed for the zeolite support and increase in magnitude with increasing nickel content, i.e., 13.9-46.3 nm in the loading range 2.2-9.2 wt% Ni. The effect of precalcination of the impregnated support was also considered; the relevant data are presented in Table 5. In common with the ion-exchanged samples, heating the impregnated samples in nitrogen prior to hydrogen reduction has been shown to lower the degree of Nizf cation reduction with the resultant generation of a smaller volume of nickel metal”‘.“” that is in the form of smaller crystallites. In the same manner, hydrogen treatinent of the impregnated zeolite samples at higher heating rates generates smaller nickel particles (Table 6). This is in complete contrast with the Ni/ A1203 system, where an increased heating rate results in the formation of larger metal particles (Table 6). The latter phenomenon must be due to the observed exothermic heat release during the heat treatment of Ni(N0&6 Hz0 in hydrogen”‘: A slower heating rate during the reduction of alumina-supported nickel nitrate prevents such exothermic temperature excursions, thereby suppressing the sintering process. This effect was not observed in the case of the zeolite support. The aluminosilicate framework must absorb the heat release during the high-temperature reduction of Ni(N0&.6 Hz0 without transmitting the effect to the supported metal phase.
M.A.
ACKNOWLEDGEMENTS We are indebted to Mr. Dermot McGrath, senior technician in these laboratories, for his help with the X-ray diffraction measurements.
REFERENCES
6 7 8 9 10 11 12 I3 14 15 I6
CONCLUSIONS Reduction of the nickel-exchanged zeolites results in the generation of a nickel metal phase exhibiting a wide particle-size distribution. As the reduction temperature is increased, crystallite growth occurs via crystallite migration, resulting in the formatiori of metal on the external surface. Crystallite size also increases with increasing nickel exchange. The larger alkali metal co-cations are the most effective in blocking the migration path of the nickel species and inhibit the aggregation of metal particles on the external surface yielding the following order of increasing crystallite size: NiCsNaY < NiRbNaY < NiKY < NiNaY < NiLiY. Nickel crystallite growth is also inhibited by precalcination in flowing nitrogen and higher heating rates in flowing hydrogen. The high polarizing power associated with Ce3+ ions generates considerable Brijnsted acidity that inhibits Ni’+ reduction and ultimately generates smaller supported nickel particles. Under identical reduction conditions, the nickel-impregnated zeolites exhibited smaller supported nickel crystallites when compared with their ion-exchange counterparts. Nevertheless, the zeolite lattice promotes the agglomeration of nickel metal particles to a greater extent than do the amorphous planar supports; the A1203 and, more particularly, the SiOp carriers exhibited the finest dispersions of nickel metal. The considerations reported here have a profound effect on the interpretation of catalysis over these samples and will be reported in due course.
10
ZEOLITES,
1991, Vol 11, January
Keane
I7 18 I9 20 21 22
23 24 25 26 27
28
29 30
31 32
33 34 35
Jacobs, P.A. Carboniooenic Activity Zeolites, Elsevier, Amsterdam, 1977 Jaeger, N., Plath, P. and Schulz-Ekloff, G. Acta Phys. Chem. Szeaed (Enalish Translation) 1985, 31, 198 Delifosse, I%, in Catalysis byzeolites, Stud. Surf. Sci. Catal. 1980,5, 237 Delafosse. D.. J. Chem. Phys. 1986. 83, 791 Jaeger, NJ., Ryder, P. and Schulz-Ekloff, G., in Strucrure and Reactivity of Modified Zeolites. Stud. Surf. Sci. Catal. 1984, 18,299 Narayanan, S. J. Sci. Ind. Res. 1985, 44, 314 Pulvermacher, B. and Ruckenstein, E. J. Caral. 1974,35, 115 Anderson, J.R. Structure of Metallic Catalysts, Academic Press, London, 1975 Flynn, P.C. and Wanke, S.E. Catal. Rev.-Sci. 1975, 12, 93 Narayanan, S., in Metal Microstructures in Zeolites, Stud. Surf. Sci. Catal. 1982, 12, 245 Brennan, J. Ph.D. Thesis, National University of Ireland, 1983 Riekert, L. Ber. Bunsenges Phys. Chem. 1969, 73, 331 Richardson, J.T. J. Catal. 1971, 21, 122 Lawson, J.D. and Rase, H.J. Ind, Eng. Prod. Res. Dev. 1970, 9,317 Exner, D., Jaeger, N., Nowak, R., Schrubbers, H. and SchulzEkloff, G. J. Caral. 1982, 74, 188 Davidova, N.P. Valcheva, M.L. and Shopov, D.M., in Caralysis by Zeolites, Stud. Surf. Sci. Caral. 1980, 5, 285 Schulz-Ekloff, G. and Blum, J.K. Surf. Sci. 1987, 183, 216 Reman, W.G., Ali, A.H. and Schuit, G.C.A. J. Catal. 1971,20, 374 Maskos, Z. and Vanhoof, J.H.C. J. Catal. 1980, 60, 73 Elliot, D.J. and Lunsford, J.H. J. Caral. 1979, 57, 11 Davidova, N., Peshev, N. and Shopov, D. J. Catal. 1979,58, 198 Minchev, Ch., Kanazirev, V., Kosova, L., Penchev, V., Gunsser, W. and Schmidt, F., in Proceedings of 5th international Conference on Zeolites, (Ed. L.V.C. Rees) Hevden, London, 1980, p. 355 Law, P.L. and Kenney, C.N. J. Catal. 1980, 64, 241 1984, 4, 231 Naravanan, S. Zeolites Jacobs, P.A., Nijs, H., Verdonck, J., Derouane, E.G., Gilson, J.P. and Simoens, A.J. Trans. Faraday Sot. I 1979, 5, 1196 Penchev, V., Davidova, N., Kanazirev, V., Minchev, Ch. and Neinska, Y Adv. Chem. Ser. 1973, 121, 461 Dalla-Betta, R.A. and Boudart, M. in Proceedings of 5th International Congress on Catalysis, Miami Beach, 1973, Vol 2, p. 1329 Penchev, V., Peshev, N.V. and Davidova, N.P., in Proceedings of 3rd International Symposium on Heterogeneous Catalysis, Varna, 7975 (Iza BAN, Sofia, 1978) p. 490 Penchev, V., Peshev, N.V. and Davidova, N.P. Do/d. Bolg. Akad. Nauk. (English Translation) 1975, 28, 347 Exner, D., Jaeger, N.i., Moller, K., Nowak, R., Schrubbers, H., Schulz-Ekloff, G. and Ryder, P., in Metal Microstructures in Zeolites, Stud. Surf. Sci. Catal. 1982, 12, 205 Bager, K.H., Vogt, F. and Bremer, H. ACS Symp. Sr. 40, Am. Chem. Sot.. Washington, DC, 1977, p. 528 Sauvion, G.N., Tempere, J.F., Guillieux, M.F., DjegaMariadassou, G. and Delafosse, D. J. Chem. Sot., faraday Trans. I 1985, 81, 1357 Jeanjean, J., Djemel, S., Guillieux, M.F. and Delafosse, D. J. Phys. Chem. 1981, 85,4145 Djemel, S., Guillieux, M.F., Jeanjean, J., Tempere F. and Delafosse, D. J. Chem. Sot., Faraday Trans. I 1982, 78, 835 Akalay, I., Guilleux, M.F., Tempere F. and Delafosse, D. J.
Effects
36 37 38 39 40 41 42
of environment
Chem. Sot., Faraday Trans. I 1987,83, 1137 Klyueva, N.V., Valcheva, M.L., Davidova, N.V., lone, K.G. and Shopov, D.M. React. Kinet. Katal. Lett 1981, 17, 315 Chien, S.H., Lu, K.L., Huang, H.W. and Hwang, J.M. Bull. Inst. Chem. Acad. Sinica 1986, 33, 81 Bartholomew, C.H. and Sorensen, W.L. J. Catal. 1983, 81, 131 Ueno, A., Suzuki, H. and Kotera, Y., J. Chem. Sot., Faraday Trans. I 1983, 79, 1137 Coughlan B. and Keane, M.A. J. Catal. 1990, 123, 364 Keane, M.A. Ph.D. Thesis (Vol. I and II), National Universin/ of Ireland, 1988 Flanigen, E.M., Khatami, H. and Szymauski, H.A. Adv. Chem. Ser. 101, Am. Chem. Sot., Washington, DC. 1971, p. 201
43 44
45 46 47 48 49 50
on growth
of Ni particles:
B. Cough/an
and
M.A.
Keane
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