A Study of Some Parameters in Catalyst Preparation and their Influence on Catalyst Performance

B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Editors), Preparation of Catalysts IV

© 1987 ElsevierScience Publishers B.V., Amsterdam - Printed inThe Netherlands

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A STUDY OF SOME PARAMETERS IN CATALYST PREPARATION AND THEIR INFLUENCE ON CATALYST PERFORMANCE l• 2 A.W. NIENOW . 2 AND J.M. WINTERBOTTOM 3 P.T. CARDEW 1 • R.J. DAVEy P. ELLIOTT. 1I CI• Process Technology Group. New Science Group, PO Box 11. The Heath, Runcorn. Cheshire. WA7 4QE. U.K. ZICI pIc, Petrochemicals and Plastics Division. PO Box 8, The Heath, Runcorn, Cheshire. WAl 4QD. U.K. 3Department of Chemical Engineering, The University of Birmingham, PO Box 363, Edgbaston, Birmingham. B15 2TT, U.K. ABSTRACT The results are reported of an investigation into the influence of preparative conditions on catalyst performance, using silica-supported nickel as a model system. Nickel lII) dimethylglyoximate was precipitated in the presence of silica in a stirred mixing vessel; reduction with hydrogen then yielded metallic nickel supported on the silica. Metal surface area and specific activities of the catalysts were measured. Variation of the precipitation conditions enabled the relationship between preparative parameters and catalytic performance to be examined. 2 -1 Very high metal surface areas (ZOO-230cm g Ni) were obtained by this preparative route, but these were found to be almost independent of preparative conditions. The reduction of Nickel (II) dimethylglyoximate is accompanied by sublimation; this is believed to be responsible for both the high surface areas and the lack of correlation with the preparative conditions. INTRODUCTION Industrial catalysts are manufactured traditionally by established routes based upon empirical knowledge rather than upon a scientific understanding of how the preparation conditions affect catalyst performance.

Much is known about

the behaviour of catalysts once prepared. but little has been published which attempts a systematic analysis of catalyst preparation, or an identification of how preparative conditions might influence catalyst performance.

This paper

reports the results of a study of the way in which preparative conditions may affect the physical properties and catalytic performance of a model catalyst. The catalyst studied was metallic nickel on a silica support, prepared by a recently-developed route (ref.l) in which Nickel (II) dimethylglyoximate is precipitated in the presence of silica.

Following suitable filtration, washing

and drying. the organic complex is reduced in hydrogen to metallic nickel supported on the silica. This route gave highly dispersed nickel catalysts with metal surface areas as high as 250 m2 (gNi)-l; atomically dispersed nickel would have a surface area of 670 m2(gNirl(awrox). Of importance is the manner in which the components are brought together and a range of parameters was varied identifying those

16 of greatest importance in determining the properties & quality of the resulting catalyst. Measurements of catalytic activity, metal surface area and crystallite size, and metal-support interaction were used to characterise the catalysts thus prepared. Reported are the methods used in the preparation and characterisation of the catalysts, together with the results of efforts to correlate changes in preparative conditions with variations in catalyst properties and performance. Preliminary findings in this field have previously been reported (refs.2,3,4), while a more detailed study is currently in preparation (ref.S). CATALYST PREPARATION Supported nickel catalysts were prepared according to the following scheme: 1. Precipitation of Nickel (II) dimethylglyoximate in the presence of suspended silica. Z. Filtration, washing and drying of the precipitate and silica mixture. 3. Reduction of the nickel complex to metallic nickel by heating in hydrogen. Nickel was precipitated from an aqueous solution of Nickel (II) nitrate by the addition of an ethanolic solution of dimethylglyoxime (C4C8NZOZ) according to the following equation: Ni Z+ (aq) + ZHZDMG (EtOH) ~ Ni (HDMG)Z (s) + ZH+ (aq) dimethylglyoxime

Nickel (II)

in ethanol

dimethylglyoximate

The solubility product of Ni(HDMG)Z in water is reported to be 4.4 x 10- 18 (ref.6), so the precipitation is essentially complete under normal conditions. Indeed, dimethylglyoxime has long been the standard gravimetric reagent for the determination of nickel (ref.7).

However, the liberation of ~

ions

during precipitation necessitates the use of a pH buffer, since solubility increases significantly with decreasing pH.

Precipitations were performed in a stirred, baffled mixing vessel of 9dm 3

capacity, in which the dimethylglyoxime solution was added, by means of a metering pump, to the Ni Z+ solution containing the silica in suspension.

The

rate of addition of dimethylglyoxime was such that precipitation was complete Agitation was provided by a 7cm, 4-blade, 45 0 impeller,

after 20 minutes. pumping downards.

The following parameters were varied systematically in order to investigate the influence of each in turn on catalyst performance: 1. Ni Z+ concentration, varied from 5 x 10- 5 to 5 x 10- 3 mol dm-3; Z. pH, varied at unit pH intervals from 3.0 to 10.0 by means of buffers; 3. Temperature, varied in 100e steps from ooe to 400C; 4. Impeller speed, varied in 5 steps from 60 rpm to 900 rpm. After precipiation was complete,themixtureofNilHDMG)Z precipitate and

17

suspended silica was collected by filtration, washed with water, and dried. Two drying temepratures were used, 200 e and 80°C.

The effect of washing

additionally with ethanol prior to drying was also investigated. In all cases, Davison 95Z grade silica was used. range was

5-Z30~

The measured particle size

(by optical microscopy) and the nominal surface area of this

grade is 300-350m Z/g.

The ratio of nickel:silica was maintained such that

complete reduction of the Ni(HDMG)Z would yield a catalyst loading of 5% w/w on the silica support.

The HZDMG concentration in ethanolwasnormallyO.lmoldm 3•

For all precipitations, an induction time was measured, this being the delay between the beginning of HZDMG addition and the first appearance of the Ni(HDMG)Z precipitate, immediately recognisable from its distinctive pink coloration. Once dry, the catalyst precursor was heated at ZOoC/min to 500 0C in a stream of hydrogen at 40 cc/min to reduce the Ni(HDMG)Z complex to metallic nickel crystallites on the silica surface.

In order to isolate the effect of changes

only at the precipitation stage, the reduction step was generally identical for all catalysts.

However, asa result of developments in the work the effect of

reducing in hydrogen diluted to only 5% v/v by an inert gas (nitrogen), and of calcination prior to reduction, was additionally investigated in some instances. CATALYST PRECURSOR CHARACTERISATION When Ni(HDMG)Z is precipitated from aqueous solution, it forms well-documented, elongated needles approximately

10~

x

O.I~

in size (refs.8,9) (fig.l).

The

effect of some precipitation parameters on the size of these crystals has previously been studied by electron microscopy (refs.IO,II).

Since the

Ni(HDMG)Z crystal size could determine the metal crystallite size (and hence the metal surface area) of the final catalyst, it is desirable to know how the Ni(HDMG)Z

crystal size varies with precipitation conditions.

Beforereducing each precipitated catalyst precursor to nickel metal, the precipitates were themselves examined for any influence of all the parameters listed under Catalyst Preparation.

Ni(HDMG)Z crystal sizes were measured by

optical microscopy, scanning electron microscopy (SEM), and by means of a Coulter Counter.

Each technique of experimental characterisation gave similar

results relating precipitation conditions to crystal size and morphology. A complex set of interactions between all the variables were found. Therefore, it is essential to quote results for the variation of only one parameter at a time. One example for varations of Ni Z+ concentration with all other parameters held constant is given in fig.Z. The effect of Ni Z+ concentration was in accordance with expectations.nam~ly crystal size increases with decreasing Ni Z+ concentration, and hence with lower

18

Fig. 1 Electron micrograph of needle-like Ni(HDMG)Z crystals. (Scale; Smm = 1~; pH = 7; 250 rpm)

..c

"2'20

.!

~OIl 16

........

~12

!i! o %:

:i

8

g .. 4 x 5.10-2 5J.10·3 5.10"'4 5.10-5 Initial concentration of Ni2'mol dm-3)

Fig. 2 Variation of mean Ni(HDMG)2 crystal length with initial Ni Z+ concentration (Z80 rpm; temp = ZOoC, pH = 7).

19

levels of supersaturation (fig.2).

This is a familiar effect in precipitation

processes. Fig. 3 shows how mean Ni(HDMG)Z needle length varied with pH.

The pH-

dependence arises from the increasing solubility of Ni(HDMG)Z in water in acidic conditions.

Indeed, below pH 3 this nickel complex is appreciably water-soluble

and is not precipitated quantitatively.

At low pH values, therefore, the lower

supersaturation results in larger Ni(HDMG)Z crystals.

e

.:,.2 • :s....

.... c:

~14

o

III

>~10

..!:"

'" 58 =. %6 c: .,.

..

::1:

•

•

•

4 3

4

5

6

7

8

9

10

pH during preparation

Fig. 3 Variation of mean Ni(HDMG)2 crystal length with pH during precipitation. (Z80 rpm; temp ZooC; Ni Z+ concentration = 5 x 10- 3 M) The effect of temperature on crystal size was far less pronounced: the difference in mean cystal size between samples precipitated at ZOOC and SOOC was barely measurable. Crystals precipitated at extremes of pH and temperature were examined by X-ray diffraction (XRD) to detect any difference in the phase of the precipitated materials.

In fact, all samples gave identical XRD profiles.

Optical microscopy before and after drying the precipitate provided information on the degree of physical association between the Ni(HDMG)Z and the silica which had been in the tank during precipitation.

Inspection of the Ni(HDMG)2 - silica

mixture before drying revealed that there was no apparent association whatsoever between the two components.

This is consistent with the observation that the

induction times for precipitations with and without silica present were identical, showing that the silica plays no part in the precipitation and fails to act as a nucleation site for the precipitation of this nickel complex.

After drying,

however, a significant proportion of the Ni(HDMG)Z was seen to be physically

20

associated with the silica particles.

No difference in association was observed

between samples dried at ZOOC and those dried at SOOC, nor between those washed with ethanol and those washed only with water.

Very rapid drying, however

(e.g. by heating a thin film on a glass slide) precluded any accompanying association process.

The drying step therefore seems to be more important

than the precipitation process in determining the association between the catalyst precursor and its silica support. CATALYST CHARACTERISATION - PHYSICAL PROPERTIES Reduced catalysts were characterised according to metal surface area, metalsupport interaction, metal crystallite size and metal loading. Surface areas were measured by hydrogen chemisorption, using apparatus and methods described by Benesi et al. (ref.13) and Falconer and Schwarz (ref.14), except that hydrogen uptake during cooling was measured, rather than hydrogen desorption during heating.

About 100mg of each catalyst was weighed accurately

into a U-shaped tube, which was then placed in a programmable oven and purged with hydrogen. With a HZ flow rate over the sample of 40 cc/min, the oven was heated to 500 oC, reducing the nickel complex to metallic nickel. The gas flow was then changed to 5% HZ in Argon and the system allowed to equilibrate. A thermal conductivity detector (TCD) measured the composition of the gas at the exit from the tube, which was then cooled rapidly.

Hydrogen chemisorption onto

the metal surface depleted the HZ content of the exist gas passing through the TeD.

Integration of the TCD signal output determined the total HZ uptake by

the catalyst, from which the metal surface area was calculated using the BET equation, assuming monolayer coverage.

Calibration was achieved by injecting

pulses of the accurately known volume of fl2into the gas flow upstream of the TeD. Specific metal surface areas can be calculated if the exact loading of the catalyst is known.

Loadings were measured by dissolving away the Ni(HDMG)Z

from an accurately weighed sample of catalyst precursor using nitric acid, and measuring the Ni Z+ concentration of the resulting solution by atomic absorption spectroscopy.

Owing to the quantitative precipitation of Ni(HDMG)Z, measured

loadings were generally in good agreement with theoretical values. Metal-support interactions were investigated by temperature-programmed reduction (TPR) (refs.15,16). measurements.

The apparatus was the same as for HZ chemisorption

50mg of catalyst precursor was weighed into the U-tube, through

which 5% HZ in Argon was passed at 30 cc/min.

As the temperature was raised

at ZOoC/min, the progressive reduction of the catalyst removed HZ from the gas flow.

This change in composition was measured as before by the TCD.

Recording

the TeD output signal as a chart record resulted in a profile exhibiting two peaks, corresponding to a two-step reduction mechanism.

The temperature at

which these peaks occur reflects the ease of reducibility of the catalyst; any

21

shift in peak temperatures relative to unsupported Ni(HDMG)2 suggestive of metal-support interaction, rendering the reduction of the catalyst precursor more, or less, facile than for the unsupported material. Nickel crystallite sizes in the reduced catalysts were measured by transmission electron micrscopy (TEM), using magnification of 150 000 times. CATALYST CHARACTERISATION - CATALYTIC ACTIVITY Specific activities of catalysts were measured using a standard toluene hydrogenation reaction in a tubular furnace reactor operated in differential mode.

100mg of catalyst precursor was weighed accurately into a 1cm diameter

pyrex tube, forming a bed 4-5mm deep between two glass wool plugs. was flushed with HZ and heated to 5000C to reduce the catalyst.

The tube

The reactor

was then allowed to cool before switching the gas flow to 5% HZ in NZ previously saturated with toluene vapour.

The gas leaving the reactor was sampled and

analysed by gas chromatography. The fraction of toluene hydrogenated to methylcyclohexane (MCH) is depended on furnace temperature, and the relationship between catalyst temperature and % conversion (expressed as a reaction rate) gives a measure of the activation energy of the catalysed reaction, and hence of the catalyst activity. Conversions were maintained below 5% in order to employ the relatively simple kinetic analysis of the differential mode reactor. RESULTS Despite the known wide variations in Ni(HDMG)Z crystal size distribution (CSD) under different precipitation conditions, no significant differences were found between the properties and characteristics of the catalysts after reduction in HZ'

Metal surface areas were found to be ZOO-ZZO HZ (gNi)-l, but these were

similar regardless of precipitation conditions (table 1). Table 1 - HZ Chemisorption and TRP results for catalysts prepared at different impeller speeds Impeller Speed rpm

60 120 250 500 900 Unsupported Ni(HDMG)2

Measured

loading %w/w Ni

Lower TPR Peak Temp °C

Upper TPR Area Under Peak Temp TPR Profile Arbitrary °c Units

Metal Surface Area by H2 Chemisorption (g-1 Ni)

4.92 4.94 4.84 4.96 4.81

334 335 333 334 337

451 460 457 465 460

3.18 3.05 3.12 3.24 3.17

215 214 207 214 224

N/A

300

350

2.15

16.2

Temperature programmed reduction revealed a shift in the position of the two reduction peaks for supported relative to unsupported samples (fig.4), but the TPR profiles were similar for all catalysts tested (table 1). Fig. 5 shows nickel crystallites on the silica support, imaged by rEM, with

22

Ill-I o

~

." s-,

:t:

. .s

0'

c

"iii d

u

In)

40

40Q 350 Temperature • C

' 300

Fig. 4 Temperature programmed reduction profiles for Ni(HDMG)Z. (a) with no silica present. (b) with added silica to give a loading of 5% w/w nickel on silica catalyst.

Fig. 5 TEM photograph of a reduced catalyst. Nickel crystallites are clearly seen against the silica support. This sample was reduced firstly in 5%' HZ diluted with NZ before further reduction in pure HZ'

23

sizes ranging from lOA and 1501.

Again, no difference in size distributions

was apparent between the differently prepared samples. Likewise, activity data from the catalytic hydrogenation experiments showed no clear correlation with preparation conditions, and no pattern of activation energies emerged from the samples examined. Despite this lack of correlation, two potentially valuable relationships did emerge from this study.

Firstly, catalysts reduced in pure HZ exhibited

consistently lower surface areas than catalysts previously heated in 5% HZ in NZ' Prolonged calcination at 400°C in NZ prior to HZ-reduction, however, left the surface area unchanged. Ni(HDMG)Z

Secondly, the metal surface areas and TPR profile of

physically mixed with silica were the same as for samples where the

silica had been present during precipitation.

Hence no advantage is gained by

adding the silica at the precipitation stage.

Of particular significance,

mixtures of Ni(HDMG)2 and silica exhibited TPR profiles identical to those of supported Ni(HDMG)2, and quite different from the characteristic profile of unsupported Ni)HDMG)Z (fig.4), which might have been expected. DISCUSSION The lack of correlation between precipitation conditions and catalyst properties can be explained by the observation that, whereas the reduction of Ni(HDMG)Z in hydrogen commences at 3000C, Ni(HDMG)2 sublimes above 230 0C. Therefore, any variations in Ni(HDMG)2 CSD arising from different precipitation conditions will be eradicated during the reduction stage by the vapour-phase redistribution of the Ni(HDMG)2 at temperature just below the reduction temperature.

This also explains why physical mixtures of Ni(HDMG)2 and silica

produce similar TPR profiles, surface areas and catalytic activities to samples in which the silica was present during precipitation. The sublimation of Ni(HDMG)2 was examined by hot-stage microscopy: when rough mixtures of Ni(HDMG)2 + silica were heated on the microscope stage, the Ni(HDMG)2 was seen clearly to sublime above 230 0C, and the silica particles become well coated with Ni(HDMG)2 at 275°C.

The effect was observed again when

portions of this mixture, heated separately to 175, 2Z5, 250 and 275OC, were examined by SEM.

The progressive sublimation of the unassociated Ni(HDMG)2

crystals onto the surface of the silica support was clearly visible on comparing these samples. l~is

also seems to explain why physical mixtures of dry Ni(HDMG)2 crystals

+ silica powder exhibit TPR profiles identical to those of supported samples. of Ni(HDMG)2 precipitated in the presence of silica, and quite unlike the characteristic profile of unsupported Ni(HDMG)2: the Ni(HDMG)Z sublimes onto the silica surface and thereby becomes supported before reduction occurs. The measurement of lower surface areas for catalyst reduced in pure hydrogen,

24 rather than in 5% HZ in NZ' seems to be due to the sintering of small nickel crystallites when heated to 500 0C in pure hydrogen.

This is supported by TEM

results: the sample in fig. 5 was "pre-reduced" in 5% HZ -NZ before switching briefly to pure HZ' (below 100

R)

In samples reduced only in pure HZ, the small crystallites

are no longer seen by TEM, presumably having sintered into the

larger aggregates which are then clearly visible (fig.6).

The sintering and

mobility of Ni crystallites in HZ has been reported (refs.18,19,ZO), but it is not known whether in this case the process is one of genuine nickel crystallite migration, or a redistribution of the material during reduction by sublimation owing to the evolution of local heat, due to the exothermic reaction, while reducing in pure hydrogen.

Fig. 6 TEM photograh of a catalyst reduced in pure HZ at 500 oC. Crystallites of less than 100 A diameter are entirely absent, apparently due to sintering. CONCLUSIONS Supported nickel catalysts are difficult to prepare in a form which is both highly dispersed and highly reduced, on account of (a) sintering and (b) metalsupport interaction.

Nickel dimethylglyoximate as a catalyst precursor has been

reported to yield catalyst with a high degree of both reduction and dispersion (ref.Zl).

It has been found, however, that during reduction to metallic nickel

a vapour-phase redistribution of the catalyst precursor takes place.

Whereas

this precludes the modification of the catalyst by varying the precipitation

25

conditions, it also appears that this sublimation of the catalyst precursor prior to reduction to supported nickel is a vital factor in achieving the desired degree of dispersion in the final catalyst. Further work in this area might investigate the performance of catalysts prepared from different oxime complexes which might be more volatile than Ni(HDMG)2, or else examine alternative reduction conditions which specifically suppress, or enhance, the sublimation of the nickel complex.

A possible method

will be via sodium tetrahydroborate reduction in the liquid phase or via hydrazine vapour at temperatures less than 2000C i.e. below the sublimation temperature. REFERENCES 1 A.I. Thompson and J.M. Winterbottom, unpublished work. 2 A.J.S. Anderson and I.K. Minto, 3rd year research project, University of Birmingham. 3 G. Cambanis and M. Polack, 3rd year research project, University of Birmingham, 1982. 4 A.J.S. Anderson, G.C. Cambanis, P. Elliott, I.K. Minto, A.W. Nienow, M. Polack, A.I. Thompson and J.M. Winterbottom, Actas Simp. Iberoam. Catal., 9th, 1984, 2, 1609-10. 5 P. Eliott, Ph.D. Thesis, University of Birmingham (in preparation). 6 H. Christopherson and E.B. Sandell, Anal. Chim. Acta 10 (1954), 1-9. 7 A.F. Vogel, A Textbook of Quantiative Inorganic Analysis, 3rd ed. (1964),125. 8 K. Takiyoma and L. Gordon, Talanta 10 (1963), 1165-1167. 9 J.L. Jones and L.C. Hawick, Talanta 11 (1964), 757-60. 10 R.B. Fischer and S.H. Simonsen, Anal. Chem. 20 (1948), 1107-1109. 11 M. Ishibashi, E. Suito , K. Takayima and E. Sekido, Bull. Inst. Chem. Res. Kyoto Univ. 31 (1953), 365-367. 12 P.P. von Weirmarn, Chem. Rev. 2 (1926), 217-242. 13 H.A. Benesi, L.T. Atkins and R.B. Mosely, J.Catal. 23 (1971), 211-213. 14 J.L. Falconer and J.A. Schwarz, Catal. Rev. - Sci. Eng. 25(2), (1983), 141-227. 15 J.W. Jenkins, B.D. McNicol and S.D. Robertson, Chemtech., May 1977, 316-320. 16 S.D. Robertson, B.D. McNicol, J.H. de Baas and S.C. Kloet, J.Catal. 37 (1975), 424-431. 17 R.B. Anderson, "Kinetics of Catalytic Reactions" in B.R. Anderson and P.T. Davison, Experimental Methods in Catalytic Research (1976), 1-43. 18 t. Nakayama, M. Arai and Y. Nishiyama, J.Catal. 79 (1983), 497-500. 19 C.H. Bartholomew and W.L. Sorensen, J.Catal. 81 (1983), 131-141. 20 T. Nakayama, M. Arai and Y. Nishiyama, J.Catal. 87 (1984), 108-115. 21. E. Zogli and J.L. Falconer, Applied Catalysis 4 (1982), 135-143.

26

DISCUSSION L. GUCZI : Have you tried to calcine your catalyst precursor prior to hydrogenation? It may affect the sublimation of Ni(HDMG)2 simply accelerating the decomposition into Ni oxide. P. ELLIOTT: Brief calcination of samples at 300°C in nitrogen reduced the surface area measured by hydrogen chemisorption by 40%. The TPR profile exhibited two peaks in the same area ratio as uncalcined samples, but the total area under the TPR profile was also reduced by 40%. Calcination at 400°C gave only one TPR peak, and a surface area 30% lower than uncalcined samples. Prolonged calcination at these temperatures failed to produce a catalyst superior in surface area to material that has not been calcined. V. PERRI CHON : The 20°C min- l heating rate used for the reduction of the sample seems to me rather high. Don't you think that water vapour produced during the reduction can favour the sintering of the nickel particles? Did you try a lower heating rate to limit this phenomenon? P. ELLIOTT: The exact nature of the reaction between hydrogen and Ni(HDMG)2 is not yet known; the presence of water as a reaction product, although likely, has not been confirmed. Heating rates from SOC/min to 40°C/min were tried, and the heating rate was found not to have an effect upon catalyst surface area within this range. Very low heating rates would be expected to promote extensive sublimation of NlTHDMG)2 prior to reduction and, in a gas flow, result in removal of this volatile material altogether from the system. G.M. PAJONK : The fact that your Ni dimethyl glyoximate sublimates when on your silica support reminds me of course of the CVD (chemical vapor deposition) method. Do you expect to observe identical results if your silica was directly treated by your Ni l I complex in the vapour phase? P. ELLIOTT: Ni(HOMG)2-silica systems mixed mechanically by hand showed TPR profiles and metal surface areas (after reduction) which were identical to samples in which the nickel had been precipitated in the presence of the silica. Since we were concerned with the influence of mixing and precipitation conditions upon catalyst performance, no attempt was made to sublime bulk Ni(HDMG)2 onto the silica support in the conventional CVD manner. Nevertheless, this remains an option for future investigation, provided that dispersion and loading can be adequately controlled. J. B.NAGY : Did you use surfactant molecules to stabilize the Ni(II) ions or Ni particles? How the presence of surfactants influenced the size of the particles? Were the particles monodisperse in size? P. ELLIOTT: The presence of a surfactant at the precipitation stage resulted in Ni(HDMG)2 crystals 3-4 times smaller than in the absence of a surfactant. However, differences in Ni(HDMG)2 CSD at this stage are eradicated by the sublimation step which precedes reduction. The Ni(HDMG)2 size distribution was monodisperse with a clearly defined mean size. The nickel crystallites following reduction covered a broad size range, and the exact nature of the size distribution has not yet been elucidated. P. LANCASTER: How do nickel crystal sizes vary with time (activity variation with time), after reduction? Do different preparation nethods Wiich give specific crystal sizes show similar catalytic activity after reduction and usage for a short period of time?

27

P. ELLIOTT: The variation of nickel crystallite size or catalytic activity with time was not specifically investigated. Data reported above were measured immediately after catalyst preparation. However, when metal surface areas, determined by hydrogen chemisorption, were measured repeatedly on the same samples, the values initially decayed by about 10% before reaching stable values after five or six determinations. This behaviour was independent of preparation conditions.

J. KIWI: You buffer your solutions for Ni precipitation on 5i0 2 using dimethylglyoxime? Did you assess the competition of ions of the buffer (-ions) with the precursor of the catalyst at different pH ? If so, did you use straight acid and base to adjust the pH values during Ni-precipitation and compare the final catalyst produced? P. ELLIOTT: The use of cation buffer solutions is precluded by adverse chemical reactions; for example, phosphate-containing buffers precipitate nickel phosphate, whereas some organic molecules are said to complex with H2DMG. Beyond this, specific ionic contributions and interactions were not investigated, although the characteristics of catalysts prepared from unbuffered systems (adding base to maintain neutral pH) did not differ from those prepared in buffered systems.