THE TEXTURE OF
NICKEL-SILICA CATALYSTS
PROEFSCHRIFT
TER VERKRUGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAP-PEN AAN DE TECHNISCHE HOGESCHOOL TE DELFT, OP GEZAG VAN DE RECTOR MAG-NIFICUS IR. H.
J
.
DE WUS, HOOGLERAAR IN DE AFDELING DER MUNBOUWKUNDE, VOOR EEN COMMISSIE UIT DE SENAAT TE VERDEDIGEN OP WOENSDAG 9 DECEM-BER 1964 DES NAMIDDAGS TE 2 UURDOOR
BASTlAAN GER
ARDUS LINSEN
scheikundig ingenieur geboren te 's-Gravenhage
DIT PROEFSCHRIFT IS GOEDGEKEURD DOOR DE PROMOTOR PROF. DR. J. H. DE BOER
Aan mijn Ouders
DANKBETUIGING
Dit proefschrift is tot stand gekomen door samenwerking tussen het Unilever Research Laboratorium te Vlaardingen en de Re-searchgroep Katalyse te Delft.
Aan de Directie van het Unilever Research Laboratorium ben ik zeer veel dank ver-schuldigd voor de wijze waarop zij mij in de gelegenheid heeft gesteld, het materiaal voor dit proefschrift te bewerken en in deze vorm te publiceren.
CHAPTER I I.l 1.2 1.3 1.4 1.5 1.6 1.7 CHAPTER Il II.l 1I.2 11.3 11.4 1I.5 II.6 CHAPTER III lIl. 1 IIl.2 III.3 III.4 IIl.5 IIl.6 lIl. 7 lIl. 8 CHAPTER IV IV.1 IV.2 IV.3 IV.4 IV.5 CONTENTS page Introduction Preparation of Catalysts Introduction . . . . Preparation of NijSi02 catalysts by coprecipitation Preparation of NijSi02 catalysts by impregnation Reduction of the catalysts prepared.
Analysis of catalysts in reduced state Summary .
Literature. . . .
Adsorption of hydrogen on nickel at room temperature
Introduction .
Apparatus and experimental procedure
Comparison of X-ray crystallite size with that calculated from the hydrogen adsorption
Results and discussion . Summary .
Literature .
Texture of the catalysts investigated
Introduction .
Apparatus and experimental procedure
Determination of the specific surface area by means of the method according BRUNAUER, EMMETT and TELLER
Calcuhition of the pore volume and the average pore radius.
Shape and width of the pores calculated from nitrogen sorption iso-therms
Results and discussion . Summary.
Literature .
Determination of the texture of the catalysts by means of t-curve
Introduction .
Determination of t-curve
Determination of the texture of the catalysts by means of t-curve
Summary . Literature . 9 11 11 13 14 18 19 19 20 21 23 24 28 29 30 30 33 35 36 39 56 57 58 58 64 71 72
CHAPTER
V
V.I V.2 V.3 V.4 V.5 V.6 CHAPTER VI VI.! VI.2 VI.3 VI.4 VI.5External surface area of the catalysts
Introduction. . . . Experimental procedure. . . . Calculation of external surface and the average particle size from the particIe size distribution curve
Results and discussion . Summary .
Literature . . . .
Catalytic activity of the catalysts
Introduction. . . . Determination of the activity of the catalysts on sesame oil Results and discussion .
Summary.
Literature . . . .
CHAPTER VII Selectivity of catalysts VII. I Introduction. . . . VIl.2 Experimental procedure. VII.3 Results and discussion . VIlA Summary . VIl.5 Literature . . . . 73 73 76 77 80 81 82 82 84 90 90 91 92 92 103 103 Summary . . . lOS Samenvatting . . . 107 Appendix . . . . . . . . . . . . . . 109
INTRODUCTION
In 1897 SABATIER et al. discovered that hydrogen can be bound to ethylene
if these substances are passed over a platinum or nickel catalyst. SABATIER was of opinion that hydrogenation of double bonds could only take place in the gas phase, but in 1901 NORMANN succeeded in hydrogenating unsaturated liquid oils catalytically, using finely distributed nickel suspended in the oil as
catalyst.
Since that time, there is a tendency - as is the case with any other production process - to increase the quality of the products formed according to this pro-cess. In view of this, the need was feIt to carry out an extensive investigation into the catalytic reaction and the catalyst used.
As a rule, the technical catalyst used for the hydrogenation of liquid oils is prepared by mixing solutions of nickel sulfate and sodium carbonate in the presence of suspended kieselguhr.
In 1958 COENEN made a valuable contribution to the knowledge of the structure of this technical catalyst. X-ray investigations carried out by this au thor showed that the catalyst particle consists of an irregular, porous con
-glomerate of flat silica plates on which epitaxically grown and normally cubical nickel crystallites are attached. All the nickel- present in the catalyst -was in the crystalline state.
In heterogeneous catalysis, catalysts of ten owe their activity to the presence
of an extended "catalytically active" surface. In order to achieve the best
effect, efforts will therefore be made to obtain a large surface area per unit of volume or of weight of the catalyst.
Large specific surface areas may be obtained by using aporous system con -sisting of asolid substance having a network of pores. Such networks have sometimes been described in the literature as consisting of uniform cylindrically
shaped small tubes, all having the same diameter and being randomly distrib-uted through the matrix. Some networks may be regarded as being formed by interconnected open spaces between agglomerations of globules or by plane-parallel plates with open spaces in between, present throughout the crystallites.
In all such cases we speak of the texture of a catalyst being by definition the individual structure and arrangement of the coherent particles with open spaces in between. The catalytic behaviour of a catalyst is determined by this texture.
Since the texture of the NijSi02-catalysts used for the hydrogenation of liquid oils may have an important influence on the course ofthe hydrogenation process, an investigation into these catalysts will be described in this thesis.
In Chapter I a survey will be given of the preparation of the catalysts by coprecipitation as weU as by impregnation of a porous carrier material. In Chapters II-Y attention will be paid to the investigation and the analysis of the texture of the catalysts investigated, while in Chapters VI and VII the attention will be focussed on the influence of the texture on the catalytic be-haviour of the catalyst during the hydrogenation of a liquid oil, sesame oil.
CHAPTER I
PREPARATION OF CATALYSTS
1.1 Introduction
For the preparation of NijSi02 catalysts various methods are described in the literature, most of which are based on coprecipitation by mixing solutions of nickel salt and waterglass forming a mixture of nickel hydroxide, silica and nickel silicate. Af ter mixing, the coprecipitate can be filtered off rapidly or be boiled in the mother liquor for a certain time. According to the normal method of preparation of technical NijSi02 catalysts, suspended kieselguhr is used as
starting material as a rule, to which solutions of nickel sulfate and sodium carbonate are added at boiling temperature [1
J.
The fact that, according to this technical method, the structure of the guhr is strongly attacked and that in the preparation of coprecipitates basic nickel silicate is mostly formed, was shown by DE LANGE and VISSER [2] in 1946 and
confirmed later on by VAN EIJK VAN VOORTHUYSEN and FRANZEN [3], TRAM-BOUZE [4] and TEICHNER [5J. SCHUIT and VAN REIJEN [6] have published a detailed survey on this question. The foregoing should be reckoned with in the case of the reduction of the nickel compounds.
With catalysts prepared via coprecipitation, no correlation is generally found - aft er reduction - between the hydrogenation activity in sesame oil and the total nickel surface area. This is eXplained by COENEN [1] who assumes that the transport of molecules through the liquid to and from the surface of the catalyst is rate-determining or that the nickel surface is not quite accessible.
In order to render the nickel surface area completely accessible during hydrogenation, catalysts with extremely wide pores should be used. According to the literature, macroporous carrier material - covered with nickel via im-pregnation with nickel nitrate [7, 8] - is mostly used to this end.
The catalysts described in this thesis have been prepared via coprecipitation and by means of impregnation. Since the investigation described is not directly connected with catalyst preparation, this aspect will not be described in detail.
1.2 Preparation of NijSi02 catalysts by coprecipitation
1.2.1 "WANiG", "ALNiG", "ENiG!" and "ENiG2 "
acid we re added dropwise to distilled water with pH = 2 with vigorous stirring in such a way that this value remained constant. This was done because at this pH, the rate of condensation of Si(OH)4 is minimal, so that no gelation can occur during the preparation of the sol [9, 10, 11]. The final concentration of Si02 in the sol obtained was 5%. Subsequently, the pH was rapidly increased to about 7.5 by an extra addition of waterglass, af ter which the gel, which had formed, was aged at 80°C for 24 h. A solution of NiCb.6 H20 in distilled water was added to this aged gel with stirring, af ter which the pH was ad-justed to 8.5 by means of a concentrated NaOH-solution. The final concen-tration of Ni was about 6% and ofSi02 about 3%. The cake which formed was
washed chloride-free by means of distilled water and subsequently separated by means of a centrifuge.
One third part of this cake was dried at 120°C (WANiG -" water nick el gel).
The remaining 2/3 was washed 3 times with 96% ethyl alcohol. Half of this
latter portion was first dried at 50°C and then at 120°C (ALNiG - alcohol nickel gel). Since alcohol has a lower surface tension than water, wider pores may be expected in the latter samples [9].
In his thesis, VLEESKENS [12] described a preparation obtained by extracting
wet silicagel first with alcohol and then with ether. The ether wet gel was sub-sequently treated in an autoclave at 220°C. At this temperature, the ether vapour was blown off through a valve. The silica gel (aerogel) obtained in this way had an ave rage po re radius of 100
A.
With the aim to obtain wide pores in a similar way, the other half of the above-mentioned alcohol cake was treated likewise. To this end, the cake was extracted - in two batches because of the dimensions of the autoclave - with ether in a Soxhlet apparatus and subsequently heated in an autoclaveat 220°C and 40 atm., af ter which the ether was blown off (ENiG1, ENiG2 - ether nickel gel). The time of heating was in both cases about 30 min.
Af ter preparation, all the cakes were ground in an agate ball-mill.
1.2.2 "NiWAG", "NiALG"
Solutions of waterglass (26-27% Si02 , 8% Na20) and diluted hydrochloric acid were added dropwise to distilled water with pH = 2 with vigorous stirring
in such a way that this value remained constant. The final concentration of Si02 in the Si02 sol was 6%. To this sol, a 4 molar solution of NiCb.6 H20 with pH = 2 was added with vigorous stirring in a such amount that the fin al
ratio Ni/Si02 was 2. No gelation took place and the sol remained completely clear.
Subsequently, the pH was increased by means of a concentrated NaOH solution. At pH
=
6 gelation occurred while no formation of Ni(OH)z could be observed. Ni(OH)z was formed at a pH = 8.5. At this pH-value the additionof NaOH-solution was stopped. The substance thus obtained was aged at 80 oe for 24 h. The aged cake was centrifuged and washed chloride-free, af ter which half of it was dried at 120 oe (NiWAG - nickel water gel).
The other half was washed 3 times with 96% ethyl alcohol, first dried at 50 oe and th en at 120 oe (NiALG - nickel alcohol gel).
Af ter drying, the cakes we re ground in an agate ball-mill for 30 min.
1.2.3 Pr 21
For comparative purposes, a technical catalyst - indicated as Pr 21 - was also included in the series of samples to be investigated. As stated in the introduction of this chapter, this technical catalyst is prepared by adding solutions of nickel sulfate and sodium carbonate to a suspension of kieselguhr at boiling tem-perature.
1.3 Preparation of NijSi02 catalysts by iDlpregnation 1.3.1 Carrier material
Solutions of waterglass (26-27% Si02, 8% Na20 ) and diluted hydrochloric acid were added dropwise to distilled water with pH = 2 with vigorous stirring in such a way that this value remained constant. The final concentration of Si02 in the sol obtained was 5%. Subsequently, the pH was rapidly increased to a valuc of 7.5 by means of NH40H (1: 1), af ter which the gel formed was aged at 80 oe for 24 h. Af ter filtration and washing with distilled water, the gel was dried at 120 oe (Wipo - 3 NHT -wide pores - not hydrothermally treated).
Following a method also described by VLEESKENS [12], the dried silica gel was hydrothermally treated in an auto clave at 230 oe and a pressure of 30 atm. for 5 h, af ter which the gel was dried again at 120 oe (Wipo 1 HT, Wipo -2 HT).
~l was also used as a carrier material. This silica is a commercial product of Degussa (Germany) and is prepared by combustion of Sie14• A detailed description of this preparation has been given by WAGNER and BRÜNNER [13].
1.3.2 lmpregnation of the carrier material
Each type of carrier material- except the aerosil- was first ground in an agate ball-mill for 1 h. The ground product was then mixed with dry Ni(N03)z.
6 H20 in a mortar and ground again in ball-mill for 15 min. The mixing ratio carrierjnickel nitrate (weight to weight) varied from 0.4-4.0 inclusive. Af ter this treatment, the samples were heated in closed stoppered bottles, placed in a drying oven (75 Oe), for 4 days. (The melting point of nickel nitrate in its
crystal water is 56°C). The change in colour indicated th at a fairly homo-geneous covering of the carrier with nickel nitrate took place. Mter this melting
of the nickel nitrate in its crystal water, the samples were ground again in the ball-mill for 15 min.
The preparations cog 1-4 incl. (coated gel) have been prepared from Wipo 1 RT, the preparations DeOl-De06* incl. from Wipo 2 RT, the preparations Dell-De16* in cl. from Wipo 3 NRT and the preparations De22-De26* incl. from aerosil.
Another preparation was made by grinding lOg Wipo 1 RT with 38 g Ni(ORh/R20 in a ball-mill for 1 h (MIG - mixed gel).
1.4 Reduction of the catalysts prepared
1.4.1 Reduction of coprecipitates
As said in the introduction of this chapter, the cakes prepared by coprecipita-tion contain basic nicke1 silicate. FRANZEN and VAN EIJK VAN VOORTHUYSEN [3, 14, 15] and DE LANGE and VISSER [2] showed that this basic nickel silicate
is very difficult to reduce. Accordingly, the coprecipitates described in this thesis were reduced for 1 h in a tube furnace at 500
o
e
and in a hydrogen flowat a rate of 60 l/h.
When the sample - af ter reduction - was to be used for texture investigations,
a passivation was carried out, for if a freshly reduced catalyst is immediately
exposed to air, a considerable heat generation will occur as a result of which
practically all the metallic nickel is oxidized. In the literature [16, 17] various passivation methods for catalysts are described, which are based on a mild oxidation aft er which the catalyst remains reasonably sta bIe to air.
According to our method, the catalyst is cooled to room temperature in hydrogen af ter reduction. Subsequently, nitrogen is passed over the pyrophoric cata1yst. In the course of 11/2 h, air is slowly added to the nitrogen so that finally norm al undiluted air is passed over.
1.4.2 Reduction of impregnates
Various experiments were carried out in order to find a suitable reduction procedure.
The preparations cog 1-4 incl. and MIG were reduced in a tube furnace at 500°C. In the case of the preparations cog 1-4 incl., the nickel nitrate was
previously decomposed to NiO, N02 and O2 at the same temperature in a
nitrogen flow (rate 60 l/h).
* The
first figure of the indices of the preparations De (Delft) denotes the type of carrier material used (0 = Wipo HT; I = Wipo NHT; 2 = aerosil) and the second the prepara-tion number.As will be shown in Chapter 11, the nickel surface areas of these preparations are very small, which is probably caused by too strong a sintering of the nickel under these reduction conditions.
In order to obtain larger surface areas, better reduction conditions should be chosen, namely alowest possible reduction temperature and a rapid removal of the water formed during the reduction.
In order to determine the minimal reductiontemperature for the impregna-tion catalyst, whilst the degree of reduction is still practically optimal, the degree of reduction of one of the impregnation catalysts was determined as a function of the reduction temperature. The degree of reduction can be ca l-culated from the elementary nickel present in the reduced catalyst.
If the freshly reduced catalyst, cooled in hydrogen, is treated with acid, the amount of hydrogen developed during this treatment will on the one hand originate from the hydrogen adsorbed on the catalyst and on the other hand be the result of the reaction Ni
+
H2S04 -+ NiS04+
H2t
.
The latter amount of hydrogen must now be determined. To th is end, the
nitrogen hydrogen
air
D
Fig. 1.1 Schematic representation of the apparatus for the deterrnination of the degree of reduction.
amount of hydrogen adsorbed must be removed, in other words the catalyst must be degassed before treating it with acid.
A special apparatus has been designed for these experiments (Fig. 1.1).
The apparatus consists of:
(A) gasburette filled with a saturated KOH-solution; (B) burette filled with 4 N H2S04 ;
(C) hard glass reaction vessel placed in an oven (D).
Via g the reaction vessel (C) is fiUed with a certain amount of catalyst, af ter which g is sealed. Two sieve plates prevent th at catalyst is entrained by the gas flow during decomposition or reduction. Af ter the reaction vessel has been fiUed, the oven (D) - previously adjusted at the temperature desired - is placed around (C). The temperature of the catalyst bed is measured by means of a thermocouple, placed in a pocket in the reaction vessel.
By opening cocks d, c and b, nitrogen is passed over the catalyst for 1 h. The decomposition products are removed via cock a into a fume cup board. The total amount of nitrogen passed over is 60 l.
Af ter decomposition of the nickel nitrate, cock dis closed and subsequently hydrogen is passed over the catalyst - by opening cock e - at the same tem-perature for 1 h. The flow-rate of the hydrogen is 60 l/h.
Af ter the reduction, the catalyst can be cooled to room temperature in hydrogen, whereupon cocks e, c and a are closed.
Af ter the reaction vessel (C) has been connected to the gasburette (A) by means of cock a and atmospheric pressure has been adjusted, acid is introduced into the reaction vessel via cock c.
The amount of hydrogen formed is coUected in the gasburette (A). In this determination the degassing described above has, therefore, not been carried out. When all the nickel is dissolved, the reaction vessel (C) is fiUed with acid up to cock a, af ter which it is closed.
Mter equilibrium in the gasburette (A) has been reached, the volume (mI STP) ofthe hydrogen formed per g nickel can be calculated in the foUowing way:
100 273 B
V = - . .- [(g V2 -g VI) -(z V2 - z VI)] . . . 1.1 G·a (273+tk) 760
In this formula is
V = the volume of hydrogen in mI STP, formed by 1 g nickel of the catalyst investigated when dissolved in acid as weU as that of the hydrogen adsorbed;
a = percentage of nickel in the unreduced catalyst;
tk = room temperature in
o
e;
B = barometer in mm Hg;g VI = volume (mI) of gasburette at the beginning of experiment;
g V2
=
volume (mI) of gasburette at the end of experiment;Z VI = volume (mI) of sulfuric acid burette at the beginning of experiment;
z V2
=
volume (mI) of sulfuric acid burette at the end of experiment.The amount ofhydrogen in mI STP-corresponding with 1 g metallic nickel- is 381.7 mI. The degree of reduction is given by V/381. 7. (The degree of reduction defined in this way may be
>
1 because of hydrogen adsorbed on the nickel) .In order to remove the amount of hydrogen adsorbed, the catalyst is not cooled in hydrogen aft er reduction but, instead, nitrogen is passed over it at
the same high temperature. Subsequently, the catalyst is cooled in nitrogen
and dissolved in acid.
The gases used for the experiments were purified by passing over reduced BTS-catalyst ex B.A.S.F. (Badische Anilin- und Soda-Fabrik). Drying was
carried out by passing over KOH and by means of a cooling trap cooled with
liquid air.
In order to determine the time during which nitrogen must be passed over
before all the hydrogen adsorbed is removed, the degree of reduction as a function of the degassing time was determined for one of the catalysts (Deos) . The temperature at which the decomposition, reduction and degassing was carried out was 300
o
e.
A degree of reduction of 1.025 was found without degassing. Af ter degassing for 2 hours, a constant degree of reduction of 0.985 was calculated. From these
1.00 0,95 0.90 R 0.85 0.80 100 200 300 400 500 - - -___ temp·C
values it can be concluded that it is sufficient to pass over nitrogen for 2 h to remove the hydrogen adsorbed.
For the same catalyst (De06), the degree of reduction was determined as a function of the temperature af ter a reduction for 1 hour with always the same amount (100 mg) Ni in the vessel. After reduction, the catalyst was always degassed with nitrogen for 2 hours. The results obtained are shown in Fig. l.2. This figure shows that a practically optimal reduction is obtained at a
reduction temperature of 300°C or higher. UMEMURA [8J has drawn the same
conclusion from magnetic studies with impregnation catalysts.
On the basis of the experiments described above, the impregnation catalysts DeQl-De06 incl., Den-De16 incl. and De22-De26 in cl. were decomposed in a
nitrogen flow (60 l/h) at 300°C for 1 hand afterwards a reduction was carried
out in a hydrogen flow (60 l/h) at the same temperature for 1 h. If the sample had to be used for texture analyses, a passivation as described under I.4.1 was carried out.
The reduction of the preparations was always carried out in areaction
vessel as drawn in Fig. I.l.
1.5 Analysis of catalysts in reduced state
The reduced catalyst to be investigated was taken up in 4 N HCl. Mter all the nickel - being practically in metallic form (> 98%) as stated above - was
dissolved, the substance thus obtained was filtered over a paper filter.
The residue on the filter was washed with hot water and ignited at 1200 °C.
In the Si02 thus obtained, no impurities were determined.
Table I.l Analytical results of the catalysts in reduced state
Catalyst % Ni % SiO. NijSiO. Catalyst % Ni % SiO. NijSiO.
WANiG 64.0 32.0 2.00 Deo3 16.1 79.6 0.203 ALNiG 61.8 35.2 1.76 Deo. 21.7 73.9 0.294 ENiGl 61.8 34.0 1.82 Deo. 26.7 68.6 0.388 ENiG. 59.8 33.5 1.79 Deo. 31.7 64.4 0.492 NiWAG 60.7 30.7 1.98 NiALG 60.2 34.1 1.77 Del l 5.4 90.0 0.060 Pr 21 55.7 20.6 2.70 De12 10.0 88.9 0.113 De13 16.7 78.9 0.212 eog 1 9.2 89.3 0.103 Del( 22.5 74.0 0.304 eog 2 16.5 80.3 0.205 Del. 27.1 67.9 0.399 eog 3 22.9 74.6 0.307 DelG 33.1 64.0 0.517 eog4 27.5 69.6 0.395 9.0 85.0 MIG 58.0 37.4 1.55 De •• 0.105 De2• 23.2 72.2 0.321 Deol 4.9 91.8 0.053 De •• 28.5 68.6 0.415 Deo. 9.4 87.2 0.108 De2 • 33.4 65.4 0.515
In the filtrate collected, nickel was determined as Ni-dimethylglyoxime. The analytical results are given in Tabel 1.l.
1.6 SUDunary
In this chapter the preparation and reduction of the catalysts mentioned in this thesis are described.
Two groups of catalysts can be distinguished: one group prepared by
coprecipitation and one group prepared by impregnation of a certain carrier.
A temperature of 500°C is chosen for the reduction of catalysts prepared by coprecipitation in view of the basic nickel silicate formed.
The reduction of samples prepared by impregnation - except cog 1-4 in cl.
and MIG - is carried out at 300
o
e,
at which temperature the degree of re-duction is almost optimal. The sCJmp1es cog 1-4 incl. and MIG we re reducedat 500°C.
Analytical results of the catalysts in reduced state are given.
1.7 Literature
1. J. W. E. COENEN, Technische ikkelkatalysatoren op drager - thesis Delft 1958. 2. J.J. DE LANGE and G. H. VISSER, De Ingenieur, 58, 24 (1946).
3. J. J. B. VAN EIJK VAN VOORTHUYSEN and P. FRANZEN, Rec. Trav. Chim., 70,793 (1951).
4. Y. TRAMBouzE, Comt. Rend., 228, 1432 (1949). 5. S. TEICHNER,J. Chim. Phys., 47,244 (1950)
6. G. C. A. SCHUIT and L. L. VAN REYEN, Adv. in Cat. and Rel. Subjects, X, 242 (1958).
7. G. C. A. SCHUIT and N. H. DE BOER, J. Chim. Phys., 51, 482 (1954).
8. K. UMEMURA, Nippon Kagaku Zasshi, 81, 1793 (1960).
9. C. OKKERSE, Submicroporous and macroporous silica - thesis Delft 1961. 10. B. G. LINSEN,J. H. DE BOER and C. OKKERSE,J. Chim. Phys., 32, 439 (1960). 11. J. H. DE BOER, B. G. LINSEN and C. OKKERSE, Proc. Kon. Ned. Akad. Wet., B63, 360
(1960).
12. J. M. VLEESKENS, De rol van OH-groepen in silica - thesis Delft 1959. 13. E. WAGNER and H. BRÜNNER, Angew. Chem., 72, 744 (1960).
14. P. FRANZEN andJ.J. B. VAN EIJK VAN VOORTHUYSEN, Trans. 4th Intern. Congr. Soil Sci., Amsterdam, 3, 34 (1950).
15. J.J. B. VAN EIJK VAN VOORTI-lUYSEN and P. FRANZEN, Rec. Trav. Chim., 69, 666 (1950).
16. J. E. AI-lLBERG, Can. Pat. 514.229 (1945).
CHAPTER II
ADSORPTION OF HYDROGEN ON NICKEL AT ROOM TEMPERATURE*
n
.l
IntroductionIn practice, chemosorption may form an ideal basis for surface determinations, especially for metals. This chemosorption - realised by binding of valence electrons - can be considered as a chemical reaction between the gas to be adsorbed and the outer layer of atoms of the adsorbent. It is expected that this reaction stops as soon as a unimolecular adsorbed layer has been formed.
The high heat of adsorption characteristic of chemosorption may cause a high mobility in the crystallattice, owing to which the reaction is not confined to a unimolecular layer. A good example is the adsorption of oxygen on nickel [1, 2].
The continuation of the reaction beyond the unimolecular layer can be restricted by using a sufficiently low temperature. However, in this case another difficulty may present itself, namely the physical adsorption of the adsorbate on the unimolecular chemosorbed layer. If the chemosorption is sufficiently irreversible, the physically adsorbed gas can be pumped off. A classic al example of it is the chemosorption of CO on iron at -196
o
e
[3, 4].For the determination of the nickel surface area of the catalysts described in this thesis, the chemosorption of hydrogen has been applied.
Many research workers [5-14] have studied the chemosorption of hydrogen on nickel. From these investigations the conclusion can be drawn that - if the correct conditions are chosen - the chemosorption of hydrogen is excellently suitable for the surface determination of nick el and the difficulties mentioned above can be prevented.
If the correct conditions have been chosen, it will be quite simple to deter-mine the capacity of the unimolecular layer V m, for the isotherm will - as can
be expected for chemosorption - be of the so-called Langmuir type. The horizontal part of this isotherm will correspond with the saturation value of the unimolecular layer.
In spite of this, there will always be a certain uncertainty as regards the surface determination by means of chemosorption in connection with the
deter-* The experiments were carried out in co-operation with Mr. E. J. HOLSCHER and Mr. A. VAN DRIEL.
mination of the right value for the area occupied by an adsorbed molecule on
the surface, (J.
In contrast with the corresponding entities in physical adsorption phenomena,
this value will greatly dep end on the surface structure of the solid substance.
It is assumed that the surface atoms of the solid substance will retain their
original place in the crystal lattice during the chemical reaction with the
adsorbate, owing to which the chemosorption will be subject to the laws of stoichiometry. As aresuit, (J will vary for the va rio us crystal planes.
Metals on a carrier must be regarded as being built up of many metal
crystallites on the surface of the carrier. The metal surface is therefore the
total free metal surface area of these crystallites.
As a result of X-ray investigations, COENEN [15] suggested a model for such
crystallites having 333 Ni-atoms on the surface accessible to hydrogen. Of this
number 83 belong to cubical, 176 to octahedral and 74 to rhombic
dodecahe-dral crystal faces. By means of this model he calculated an average value of
6.33 Á2 for the place usually occupied by a Ni-atom. BEECK [16] calculated a value of 6.13 Á2 by comparing BET-surface are as determined by
krypton-adsorption and by hydrogen adsorption measurements to layers of nickel metal
deposited by vaporization. These layers have a completely different structure.
From investigations carried out by HORIUTI [17], BEECK and RITCHIE [18]
and BAKER and RIDEAL [19] the conclusion can be drawn that on a clean
nickel surfacc, hydrogen will be chemosorbed in the form of hydrogen atoms,
namely one hydrogen atom per nickel atom. Therefore, one hydrogen atom
will occupy one nickel atom on the surface, so that a value of 6.33 Á2 can be
taken for (JH, being the surface area of such an adsorbed hydrogen atom.
11.2 Apparatus and experirnental procedure
COENEN [15] has given a detailed description of the hydrogen adsorption on
nickel. According to the standard method described by this author, the
hydrogen adsorption measurement is carried out at room tcmperature and
at a pressure of 1 atm.
The apparatus required is shown in Fig. lI.l.
The gas adsorption apparatus mainly consists of a burette (B), a manometer
(M) and an adsorption vessel (C).
The burette (B) consists of a series of bulbs calibrated by weighing with
mercury before being connected to the apparatus.
In the manometer (M) is an indicator needie (i). Wh en reading or
ad-justing the pressure, the mercury level is always set on this point, so th at the
dead volume of the measuring space has a1ways a constant and well-known
high vacuum
t.
ct
p
M
" compressed oir
Fig. 11.1 Schematic representation of the apparatus for the hydrogen adsorption measure-ments.
For the calculation of the adsorbed volume, the volume Vk of the catalyst vessel (C) up to cock e and the volume
VI:
of the rest of the measuring space(B and the tubes up to cocks c and e and the mercury meniscus) must be known. These volumes can be easily determined by introducing helium into the
measuring space af ter evacuation. By a subsequent variation of pressure and volume (~V), Va; and Va;+ Vk can be calculated by means of the law of
Boyle-Gay Lussac.
The measuring procedure can be described as follows: The coprecipitates,
the impregnates cog 1-4 inclusive and MIG - as described in Chapter I - were
pre-reduced in a tube furnace at 500°C in a hydrogen current (60IJh) for 1 h. Subsequently, a passivation was carried out. A certain amount ofthe passivated
catalyst was reduced for 4 h in the gas adsorption apparatus at 500°C by passing over hydrogen via cocks a, c, e andj. Af ter closing cocks a andj, a two hours' evacuation was carried out at the same temperature via cock d.
With the impregnates Deoi-Deo6, Den-De16 and De22-De26, the reduction
was completely carried out in the gas adsorption apparatus. According to this procedure, the decomposition is carried out at 300°C for 1 h while passing
nitrogen over the samples via cocks b, c, e and
f
Coek b is then closed and coek a is opened, whereupon a reduction of 1 h is earried out at the sametemperature in a hydrogen stream (60 l/h). Af ter dosing coeks a and
J,
adegassing is subsequently earried out at 300°C via eoek d for 2 h.
Af ter degassing of the sample, cocks e, c and d are closed. Af ter this, a certain
amount of pure hydrogen is introdueed into the measuring spaee. The pressure PI in mm Hg, the volume of mereury in the burette Vbl in mI and the room temperature Tkl in oK are measured. From these data, the amount of hydrogen present in the measuring system can be calculated. Af ter opening cock e, the pressure deereases as a result of expansion in Vk and of adsorption
to the sample.
The mercury level in the burette is now increased in such a way that the
manometer indicates a pressure of 760 mmo Af ter 16 h, a possible deviation
of the pressure from 760 mm is corrected by means of the mercury level in the burette. Af ter this correetion, cock e is closed and the pressure and temperature
are precisely read. Subsequently, the mercury level in the burette is decreased
to the nearest calibration mark. The mercury volume in the burette is then V b2 , the new pressure P2 and the temperature Tk2 •
The amount of adsorbed hydrogen V m per g nickel in the catalyst - expressed in mI STP - can now be calculated according to
V m = 273XlOO[(Ve-V bl),P I _( V X - V b2),P2 760·G·a Tkl Tk2 Vk.760J T k2 • • (Il.l) l i l whieh:
G = the weight of the eatalyst sample in the state in which it was introduced into the adsorption vessel;
a = the percentage of niekel in the catalyst in the same state.
In the introduction of this chapter it has been shown that the value of the area on the surface occupied by one adsorbed hydrogen atom may be assumed to be aH
=
6.33 Á2. One molecule of H2 which is adsorbed, represents therefore 12.7 Á2. By means of this value and Vm , the nickel surface area ean now be calculated according toV
SN!
=
6.022 X 1023 X 12.7 X 10-20 X 224~4 m2/g Ni=
3.41· Vm m2/g Ni (Il.2)n.3
COInparison of X-ray crystallite size with that calculated frOin the hydrogen adsorptionX-ray investigations carried out by COENEN [15] showed that the technical nickel catalyst investigated by him consisted of fiat silica plates onto which
epitaxially grown - normally cubically crystallized - nickel crystallites are at-tached showing cubical, octahedral and rhombic dodecahedral faces. It
appeared there was a very good agreement between the crystallite size cal-culated from the line broadening and that which can be calculated from the hydrogen adsorption assuming that the crystallites can be described by hemi-spheres, attached with their equatorial plane on the silica. Generally, the deviation was not > 10%. This showed that the data obtained from the ad-sorption measurements support the picture of the structure pattern of the cata1yst obtained by X-ray analysis.
Inspired by these results, the crystallite sizes of some of the catalysts de-scribed in this thesis have also been determined by means of X-ray and com-pared with those calculated from the hydrogen adsorption, assuming again hemispheres attached with the equatorial plane on the carrier as a substitute for the crystallites. For the latter calculation, the following relations can be derived.
The volume V of a hemisphere with a radius ris equal to 2/3nr3. The weight of a nickel crystallite of this shape is therefore 2fanr3 X 8.91 g in which r is
expressed in cm. The surface area of this hemisphere is 2nr2. The total nick el surface area SNi per g Ni in m2 is, therefore, equal to
2nr2x 10-4 3x lO-4
SN' = = m2
/
g
(113)1 2fanr3x 8.91 8.9lr . . . . • . . . . . . .
The crystallite size determined by X-ray is defined as follows:
D=~V
,
in which D has the dimension of a length.
The radius, r, of a hemispherical crystallite is then:
13/
-r
=
DV;;
. . . .
. .
. .
.
. .
.
. .
.
. .
.
. . .
(II.4)If D is resolved from equations II.3 and II.4,
~18n 0.431 D
=
X 10-4= ---
X 10-4 cm 8.91SNi SNi or D = 4310A
. . .
(II.5) SNi11.4 Results and discussion
The results of the hydrogen adsorption measurements and the nickel surface areas calculated from them are given in Table 11.1.
Table Il.l Nickel surface area of the catalysts
Preparatian Reductian Hydrogen adsarptian
temperature Vm SNi NiJSiO,
(0C) (mi STPJgNi) (m'JgNi) (gJg) WANiG 500 12.9 44 2.00 ALNiG 500 15.7 54 1.76 ENiGl 500 15.1 51 1.82 ENiG, 500 19.8 67 1.79 NiWAG 500 12.3 42 1.98 NiALG 500 14.1 48 1.77 Pr 21 500 26.4 90 2.70 cag 1 500 6.9 24 0.103 cag 2 500 5.8 20 0.205 cag 3 500 4.8 16 0.307 cag 4 500 3.7 13 0.395 MIG 500 4.3 15 1.55 Deol 300 17.5 60 0.053 Deo. 300 16.8 57 0.108 Deo3 300 13.7 47 0.203 Deo. 300 11.3 39 0.294 Deo5 300 11.2 38 0.388 Deo6 300 11.2 38 0.492 Del l 300 35.9 122 0.060 Del. 300 26.3 90 0.113 Del3 300 20.8 71 0.212 De14 300 20.6 70 0.304 De l5 300 19.3 66 0.399 De l6 300 20.3 69 0.517 De., 300 20.4 70 0.105 De •• 300 17.7 60 0.321 De'5 300 15.3 52 0.415 De'6 300 16.3 56 0.515
The results from Table Il.l show that with the impregnation catalysts, the specific nickel surface area per g i decreases at an increasing NijSi02-ratio; in other words, the chance of sintering increases if less carrier surface is avail-able per volume unit of nickel.
In general, a larger carrier surface will therefore mean a larger nickel sur-face. For this reason it may be expected that the total surface area and the total nickel surface area will run parallel to a certain extent. Fig. Il.2 gives an illustration.
In this figure the codes of various catalysts have been indicated at equal
distances on the horizontal axis. For each catalyst, the total surface area SBET
total nickel surface area (dotted line) have been plotted vertically, correspond-ing with scales on the left and right hand axis respectively.
In this figure, the technical catalyst Pr 21 is an exception in the group of coprecipitates. Considering the way of preparation, this technical catalyst will contain a large amount of nickel silicate wh en compared with the other
co-2000 100 z ~ E z ~ (\IE ~ w... I {f)a:J 1000 "''-
I
Z sa (J) " I "'--.--.4t
'
Fig. II.2 Relatianship between nickel surf ace area and ta tal surf ace area.
precipitates. This, according to COENEN [15], leads to a nickel surface area which is relatively large with respect to the total surface area. For the same reason, the nickel surface areas of the other coprecipitates will greatly differ
from those of the impregnates which are also reduced at 500°C (cog-series).
The formation of nickel silicate mayalso be the reason why the preparations prepared by impregnation of Wipo 3 NRT and aerosil (Del and De2 prepara-tions) do not quite follow the generalline. As will be shown in Chapter lIl, the carrier material is chemically attacked during impregnation. This attack will presumably be accompanied by formation of nickel silicate which may
again cause a larger Ni-surface area than normally.
A comparison of the specific nickel surface areas of the cog-preparations
with those of the preparations Deo clearly shows th at larger nickel surface
areas are obtained by choosing moderate reduction conditions such as a low temperature and small amounts of catalyst.
In illustration of the influence of the reduction temperature, the specific nickel surface area has been determined for sample Deo6 as a function of the reduction temperature. The results are given in Table lI.2. The total surface
area of this preparation hardly changes as a function of the reduction tem-perature.
Table n.2 Influence of reduction temperature on nickel surface area
Preparation Reduction Hydrogen adsorption
temperature (OC) Vm (m1STP/gNi) SNi (m2/g Ni)
300 11.2 38
350 10.2 35
400 8.3 28
450 7.2 25
500 6.9 24
Fromthis table it appears that at a reduction temperature of 300°C, the nickel surface area will be larger than that at 500°C.
From the values found for the degree of reduction as a function of the degassing time at a temperature of300 °C (Chapter I), the amount ofhydrogen adsorbed by nickel at room temperature and 1 atm. can be calculated for De06 and compared with the value determined, using the method described in this chapter.
The degree of reduction is only an apparent one if no degassing is applied, for the gas formed during dissolution originates from the metallic nickel as well as from the adsorbed hydrogen. The degree of reduction af ter a 2 hours' degassing is the real one since it may be assumed that hydrogen is no longer adsorbed on the nickel surface. The amount of hydrogen adsorbed at room temperature and 1 atm. can be calculated from the difference between these two "degrees of reduction". For De06, this amount is 15.3 mI STPjg Ni. This value is distinctly higher than that stated in Table n.l. However, it should be realised th at during degassing of the pyrophoric catalyst, oxidation of the metallic nickel surface can be caused by the water liberated from the silica carrier [20, 21]. As aresult, the actuaUy measured degree of reduction wiU be too low and the amount of adsorbed hydrogen calculated from it, too high.
This oxidation will also occur by the method described in this chapter. According to SCHUIT and DE BOER [20, 21], the adsorption process wiU be very slow if the nickel surface area iSI somewhat oxidised but very rapid if it is clean. Although the amount of hydr~gen adsorbed is determined af ter 16 h according to the method described in this chapter, it may still be influenced by this low rate of adsorption. Therefore, the value found may be too low. From the literature [22], it is also known th at the formation of the unimolecular
layer of hydrogen will proceed very slowly when the degree of occupation of 1 is approached. The value found mayalso be too low for this reason.
In Table n.3 the crystallite size - determined by X-ray - of some of the catalysts is given and compared with the crystallite size calculated according to equation n.5.
Table II.3 Relationship between nickel surface area and crystallite size Preparation Deol Deo. Deo3 Deo. Deo5 Deo. DelG Dez• cog 1 cog 2 cog 3 NiWAG NiALG
*
a/b average 1.09 Crystallite size in Aa) from SNi b) from X-ray
72 54 75 59 92 84 112 110 113 88 113 95 62 86 77 71 183 175 219 206 267 209 79 102 86 90 a/b
*
1.33 1.27 1.10 1.02 1.28 1.19 0.72 1.08 1.05 1.06 1.28 0.77 0.96The results show that the average standard deviation is not larger than 20%. The agreement can be said to be good considering that:
- the X-ray crystallite size is measured to passivated catalysts, which may result in a decrease of the radius of the hemisphere of ca. 4
A;
the catalysts are heterodisperse as regards nickel crystallites;
the place occupied on an average by a nickel atom on the surface has been calculated on the basis of a model of the crystallite, owing to which devia-tions in the order of 10% from the re al value are not impossible;
for the shape of the crystallites a smooth hemisphere was taken with the equatorial plane as only plane of attachment to the carrier.
On the basis of the results described it can be established th at - in spite of all the uncertainties - the above figures for the nickel surface area available in the catalyst, obtained by means of the hydrogen adsorption, do probably not represent absolute values but they certainly have a relative value.
n.5
SUDunaryIn this chapter, the determination of the amount of hydrogen chemosorbed on nickel at room temperature and 1 atm. and the calculation of the total nickel surface area from it, are described in detail.
From these measurements it appears that the largest nickel surface are as are obtained for impregnation catalysts, if more carrier surface is available per unit of volume. A low reduction temperature and the use of small amounts of catalyst during the reduction have also a nickel surface increasing effect.
The conclusion of COENEN [15] that at a high nickel silicate content the nickel is more resistent to sin tering, has been confirmed.
For most of the catalysts described in this thesis, the crystallite size deter-mined by X-ray does on an average not deviate more than 20% from the crystallite size calculated from the hydrogen adsorption, assuming that the shape of the crystallites approach a smooth hemisphere with its equatorial plane attached to the carrier. In connection with this finding, it can be estab-lished that the nickel surface area calculated from the hydrogen adsorption is certainly a relative measure for the nickel surface area available in the catalyst.
II.6 Literature
1. O. D. GONZALEZ and G. PARRAVANO,]. Am. Chem. Soc., 78, 4533 (1956). 2. R. M. DELL, D. F. KLEMPERER and F. S. STONE,]. Phys. Chem., 60, 1586 (1956). 3. P. H. EMMETT and S. BRuNAuER,]. Am. Chem. Soc., 59, 310 (1937).
4. P. H. EMMETT and S. BRuNAuER, Trans. Electrochem. Soc., 71 (1937).
5. M. McD. BAKER, G. 1.]ENKINS and E. K. RIDEAL, Trans. Far. Soc., 51, 1592 (1955). 6. ].]. BROEDER, L. L. VAN REYEN, W. H. M. SACHTLER and G. C. A. SCHUIT, Z.
Electro-chem., 60, 838 (1956).
7. ].]. BROEDER, L. L. VAN REYEN and A. R. KORSWAGEN,]. Chim. Phys., 54,37 (1957).
8. P. M. GUNDRY and F. C. TOMPKINS, Trans. Far. Soc., 52, 1609 (1956). 9. P. M. GUNDRY and F. C. TOMPKINS, Trans. Far. Soc., 53, 218 (1957).
10. M. Prettre and O. GOEPFERT, Compt. Rend., 225, 681 (1947).
11. W. H. M. SACHTLER,]. Chem. Phys., 25,751 (1956).
12. G. C. A. SCHUIT, Proc. Int. Symp. Reac. Solids, Göthenburg 1952,571 (1954). 13. P. W. SELWOOD,]. Am. Chem. Soc., 78, 3893 (1956).
14. A. VAN ITTERBEEK, P. MARLENS and 1. VERPOORTEN, Mededel. Kon. Vlam. Acad. Weten
-schappen., Belg, 8,8 (1946).
15. ]. W. E. COENEN, Technische Nikkelkatalysatoren op drager - thesis Delft 1958.
16. O. BEEcK, Advances in Catalysis, 2, 151 (1950).
17. ]. HORlUTl, Proc. 2nd Intern. Congr. Surf. Act., 3, 71 (1957).
18. O. BEEcK and A. W. RITCHIE, Discussions Far. Soc., 8, 159 (1950). 19. M.McD. BAKER and E. K. RIDEAL, Trans. Far. Soc., 51, 1597 (1955). 20. G. C. A. Schuit and N. H. DE BOER, Rec. Trav. Chim., 72, 909 (1953). 21. G. C. A. SCHUIT and N. H. DE BOER, Nature, 168, 1040 (1951). 22. B. M. W. TRAPNELL, Chemisorption - Butterworths - London, 1955.
CHAPTER III
TEXTURE OF THE CATALYSTS INVESTIGATED
111.1 Introduction
As has been stated in the general introduction, the action in heterogeneous
catalysis occurs at the interface of 2 phases and the catalysts owe their activity
to the presence of a "catalytically active" surface. In order to obtain the
op-timum effect, a largest possible surface per unit of volume or of weight is
aimed at. Large specific surface are as can for instance be obtained by
applica-tion of aporous system, which consists of asolid substance having a network of
pores. Sometimes this network has been described, in an idealized way, as
consisting of uniform cylindrical small tubes, which have all the same diameter
and which are randomly distributed in the sub stance [1]. Other possible
arrangements are a network built up of globules with open spaces in between
being in contact with each other, or a network consisting of plane-parallel
plates with spaces in between running throughout the crystallite. In all such
cases, the catalyst may be characterized by its texture, a property describing
the assembly of the elcmentary solid partic1es and their sizes and shapes
together with the sizes and shapes of the open spaces between them.
The practical usefulness of a catalyst will mostly be determined by this
texture. For this reason, special attention will be paid to this aspect in this
chapter.
The literature describes various methods for the determination of the specific
surface area of a catalyst. Most of them are based on the adsorption of
sub-stances on the su·rface. One of the methods most frequently used at the
mo-ment is based on the adsorption of inert gases, such as nitrogen, argon and
krypton, at low temperature, namely that ofliquid nitrogen or oxygen.
The specific surface area alone is not sufficient to characterize a catalyst.
For a study of the accessibility to the surface, the shape and dimensions of the
pores present in the matrix should also be known. Again, the adsorption of
inert gases at low temperature may give important information on this question.
m.2
Apparatus and experimental procedureThe amount of nitrogen adsorbed by asolid substance at -19.6 °C at an
in-creasing or dein-creasing relative pressure can be determined volumetrically as
Espeeially the volumetrie teehnique is frequently used. A detailed deseription of the eonventional apparatus used for it has for instanee been given by
COENEN [3] and by LIPPENS [4]. This eonventional BET-apparatus has various
disadvantages:
a. the amount of adsorption heat to be removed is rather large as a result of the rather large amount of substanee to be used;
b. the point at whieh equilibrium is obtained is difficult to determine in eon-neetion with the large dead volume present;
c. the volumes adsorbed are ealculated eumulatively; d. leaking during the measurement is diffieult to deteet; e. the measurement is diffieult to interrupt.
All these disadvantages have led to the development of a miero BET-appara-tus [4, 5, 6]. A sehematie representation is given in Fig. IIl.1.
_high vocuum line
Fig. lIL1 Schematic representation of the apparatus for measuring nitrogen sorption isotherms
The prineipal parts of this gas adsorption apparatus are the following: 1. gasburette (A) filled with mereury (capaeity 100 mI subdivided in
1
/
10
mI); 2. adsorption vessel (C) dosed offfrom the measuring system by means ofsmall filter plate (b) in order to prevent eontamination of the apparatus by solid material ;
3. mercury pump (B) for transport of gas from burette (A) to adsorption vessel (C);
4. a capillary differential manometer (E);
5. vacuum and pressure chamber (H) to equal the pressure in the left part of the differential manometer (E) and that in the adsorption vessel (C); 6. manometer (G) indicating pressure in this left part;
7. manometer (F) connected to vessel (D) containing some pure condensed nitrogen, by which changes in temperature of the liquid nitrogen can be determined in the Dewar vessel placed around the adsorption vessel (C).
High vacuum is obtained by means of an oil pump and a mercury diffusion pump. The pressure is measured with the aid of a PhilipsfPenning manometer type Pw-7902.
The nitrogen used is purified by passing it over a reduced BTS-catalyst ex B.A.S.F. Uncondensed gases - such as hydrogen - are removed by repeated condensation and evaporation of the nitrogen.
A detailed description of the calibration of the micro BET-apparatus has been given by LIPPENS [5, 6]. For the calibration of the dead volume (C),
cocks c, d, e andj are opened, so that high vacuum can be obtained.
If the pressure has decreased below 10-5 mm Hg and af ter the apparatus
has been checked for possible leakages, cocks d, e andj are closed. Subsequent-ly, a Dewar vessel is filled with liquid nitrogen and placed around (C). By means of mercury pump (B), a certain amount of nitrogen is transferred from the gasburette (A) to the measuring system (C).
If the pressure in (C) increases, the mercury meniscus in the right hand part of (E) wiU decrease. By increasing the pressure in the left part of (E) by opening cocks g and h, the mercury meniscuses in both legs of (E) are bal-anced. If the pressure indicated by (G) is equal to
pG
mm Hg and the dif-ference between the mercury levels of (E) equal to PE mm Hg, the pressure in (C) is given byP =PG-PE . . . • • . • • • • . . • . . . (111.1)
The dead volume Va is now defined as the volume ofnitrogen (mI STP) giving a pressure in (C) of 1000 mm Hg under measuring conditions (room temper-ature 20°C; temperature of liquid nitrogen -196°C; level of liquid nitrogen in Dewar vessel adjusted at a fixed point of (C); mercury levels in both legs of (E) balanced). If the volume of gas (mI STP) introduced from gasburette (A) into (C) is equal to V,
V
=
O.OOlP( Va-K 'PE) . • • • • • . • • • • . . . (111.2) in which K is a constant given by the diameter of the capillary of (E)., - - --- - - - -- - - -- - - ,
When measuring the adsorption isotherm, Va must be decreased by an amount of gas of 1000 mm Hg under measuring conditions, namely the amount of gas which has been replaced by the volume of the sample to be measured.
This amount Vs is given by
1000 273
Vs = 760 X 78 X Vsp X G X 1.04 = 4.72 X Vsp X G. • • • • (lIL3) in which Vsp is the specific volume of the substance, G the weight in g and
1.04 a correction for the ideal gas law.
The adsorption isotherm is measured now in the same way as the dead volume. Ifthe total volume of gas in (C) is equal to V, the volume Va adsorbed by 1 g of the substance to be investigated is given by
1 [ Po- PE ]
Va = G V - 1000 X (Va- Vs-K·PE) . • . • • • • • (lIlA) The moment at which the pressure in (C) does no longer change more than 0.1 mm/min., is taken as equilibrium. This change will more or less correspond with an adsorbed volume of ab out 0.001 mI. The maximum equilibrium pressure measured in (C) is taken as saturation pressure. The variations in the saturation pressure during measuring are observed with the aid of the mano-meter (F).
111.3 DeterDlination of the specific surface area by Dleans of the Dlethod according Brunauer, EDlDlett and Teller
The method usually applied for the determination of the specific surface area of asolid sub stance is that according to BRUNAUER, EMMETT and TELLER [7].
Using this method, the surface area is calculated from the reversible part of the isotherm.
The equation derived by Brunauer, Emmett and Teller is based on the theory of the multimolecular adsorption of gases on the surface. This theory assumes that in the case of a normal adsorption by Van der Waals bonds, the bi, tri, etc. molecular layer can be formed already before the surface is com-pletely covered with the first layer. The conclusions arrived at by Brunauer, Emmett and Teller on the basis of the equation derived by them, have been formulated by DE BOER [8] as follows:
- the average time of adsorption of a molecule on the surface of the solid substance is independent of the degree of occupation;
- the average time of adsorption of a molecule on the layers already adsorbed is independent of the thickness of the adsorbed layer and of the degree of occupation.
The equation obtained by application of these assumptions is then:
p
/
po
1C
-
l
Va(l
-
P
/
po
)
=V
mC+
V
mC·P
/
po.
. .
.
.
. .
. (HL5)in which
Va
= the volume of gas adsorbed (mI STP/g substance) ;V m = the volume of gas required to cover the whole surface of the solid
sub-stance unimolecularly (mI STPfg substance) ;
C = a constant, indicating the ratio between the adsorption-time of the
molecules in the first layer and the adsorption-time of the molecules in
the second and following layers.
Although the assumptions used for the derivation of this equation are very
doubtful, practice has shown that the equation derived by Brunauer, Emmett
and Teller is no doubt useful when low relative pressures are concern ed.
The values obtained in this way for the specific surface area are in good
agreement with those obtained by other methods [9, 10, 16].
The surf ace area is obtained by plotting
y =
p
/
po
of equation (HL5) againstx
=p
f
po.
Va(l
-
P
/
po
)
In most cases, a straight line will be obtained with a sufficient number ofmeas-uring points between a relative pressure of 0.05 and 0.25. V m can be calculated from the slope of this straight line and the intercept from the y-axis.
For the calculation of the specific surf ace area from V m, the area occupied
by a molecule in the unimolecular layer should be known. EMMETT and
BRU-NAUER [11] assumed that the density of a physically adsorbed multilayer is equal to the density of the liquid at the same temperature.
If we assume the liquid to be a closest packing of spheres with a diameter of d, the specific surface area of a molecule is given by
s = id2y3
and the volume by
MV
v
=
J.d3Y2=
sp X 1024 A32 6.022 X 1023
From this volume and the surface of a molecule it follows that
s
=
i
y3 X iY2v2=
l.530iY
M2 Vsp 2 A2 . . . . . . (HL6) in which M is the molecular weight of the adsorbate and Vsp the specific volume of the liquid adsorbate in ml/g.
In this way a value of 16.27 Á2 is found for a nitrogen molecule. Using this value, the specific surface area of asolid substance is given by
V
mSBET
=
6.022 X 1023 X 16.27 X 10-20 X 22414 m2jg=
4.371 V m m2jg. . (lIl. 7)DE BOER and KRUYER [12] showed th at the picture given is not always correct,
for especially in the case of a very strong adsorption, the ni trog en molecule will 10se one of its rotation degrees of freedom and consequently occupy a different surface.
Experiments carried out by HARKINS and JURA [13] and LIVINGSTON [14]
showed indeed that the surface (s) of a nitrogen molecule can vary from 14
to 17 Á2.
The comparative measurements carried out by LIP PENS [SJ, MEIJS [15J and
others [16J with lauric acid adsorption make it acceptable, however, that a
value of 16.27 Á2 can be used very well for the surface determinations carried
out with the catalysts described in this thesis.
111.4 Calculation of the pore volume and the average po re radius
Technologically speaking, the total volume of the pores in a catalyst is a very
important datum.
This pore volume Vp may be calculated from the amount of nitrogen
ad-sorbed at a relative pressure of 0.999 by means of the formula
in which
M· Vsp
Vp = Vst x Mv
Vp
=
pore volume in cm3jg; .. . . (IIl.8a)
Vst = adsorbed volume ofnitrogen in mI STPjg at a relative pressure of 0.999 ;
M = molecular weight of nitrogen ;
Mv = molecular volume of nitrogen;
Vsp = specific volume of nitrogen.
For M = 28.016 and Vsp = 1.238 mljg
Vp = 0.1547x 10-2x Vst . . . • . . . (IlI.8b)
To characterize a catalyst, not only the specific surface area and pore volume
are important, but it is also usefu1 to know something about the size of the
pores.
In practice, the average pore radius i'p is frequently used. If it is assumed
that the pores are cylindrical or slit-shaped, have all the same dimension and
do not interseet, i'p for cylindrica1 pores or
d
p for slit-shaped pores is given by- 2Vp Á
i'p(dp) = - -X 104 • • • • • • • • • • • • • • • • (IlI.9) SBET
When deriving formula (IIl.9), the ~hape ofthe pores and their state has been
idealized. This ideal state will hardly ever occur in practice, so that lp(dp) should not be given a real value. For this reason, it will be necessary to apply
other methods of calculation for a deeper study of the pore distribution and
pore shape. For a comparison of related catalysts c.q. for the determination of
changes in catalysts at a certain treatment, the average lp(dp) is, however,
very useful.
m.5
Shape and width of the pores calculated frolD nitrogen sorption isotherlDsJIJ.5.l Shape of the pores and their volume
As discussed under III.4, the average pore radius lp has hardly any value for
the calculation of the texture of the catalyst. The introduction of a shape
factor c.q. an intersection factor according to WHEELER [17] and STEGGERDA
[18] did not give a re al improvement.
A better insight into the texture of a catalyst is obtained by means of the
theory of capillary condensation c.q. evaporation in or out of the pores at
pressures lower than the saturation pressure.
The saturation pressure above a liquid with curved surface is given by the
Kelvin equation.
2crV cos y
In plpo
=
RT . . . (lIl. 10) ·rkin which
plpo = relative vapour pressure above liquid;
cr = surface tension of the liquid;
y = contact angle between liquid surface and solid substance ;
V = molecular volume ofliquid;
rk = Kelvin radius of meniscus.
The Kelvin radius of the liquid surface is given by the two main radii or
2 1 1
- = -
+
-
. . . .
.
. . .
.
.
.
.
. .
. .
. .
. .
(111.11) rk r1 r2If the pressure is decreased, the completely filled pores will be emptied when
the relative pressure decreases below that given by equation (lIl. 1 0).
The volume of the emptied pores can be calculated from the amount of gas
formed by evaporation of the capillary condensed 1iquid af ter correction for
the remaining amount adsorbed.
Frorn eq uations (lIl. 1 0) and (lIl. 11 ) it follows that in some empty pores
the gas - depending on the shape of the pores - will condense at higher relative
phenomenon, it is not necessary that the adsorption branch of an isotherm
coincides with the desorption branch, in other words hysteresis is observed.
BARRER, McKENZIE and REAY [19] as weU as DE BOER [20] studied the
infiuence of the shape of pores on the shape of the hysteresis.
111.5.2 Calculation of pore distribution
In order to gain a better insight into the texture of a cata1yst, the pore
distribu-tion should be calcu1ated - as said already under III.4. This calculation is
based on the Kelvin equation.
The literature gives various methods. BARRETT, ]OYNER and HALENDA [21]
and CRANSTON and INKLEY [22] derived the method for cylindricaUy shaped
pores. The method for slit-shaped pores has been described by INNEs [23] and
STEGGERDA [18].
For the desorption branch of the isotherm, the Kelvin radius can be
cal-culated as a function of the relative pressure. The dimensions of the pores are now given by .
in which
r = rk+t
d = rk+ 2t
for cylindrical pores for slit-shaped pores
r = radius of cylindrical pores;
d
=
slit-width of slit-shaped pores;t
=
thickness of adsorbed layer.For the calculation ofboth types ofpores, the desorption branch ofthe isotherm
is divided into very small and equa1 steps in relative pressure.
A. Cylindrical pores
Coming from high values of the relative pressure and lowering this relative
pressure step by step, pores with a radius rn are stiU fiUed with liquid nitrogen
when the nth step is reached. If the relative pressure Pn/po is decreased to
Pn+l/PO, pores with a radius varying between rn and rn+l will be emptied (Pn+l
< pn, rn+l
<
rn). If the steps in relative pressure are very smaU it can besaid th at the pores mentioned have an average radius of 'in. At the same time,
the thickness of the adsorbed layer in pores, which are already empty, will
decrease from tn to tn+l.
The total volume Ll V~+l given oif in this step, is therefore given by
the volume of liquid evaporated from the pores with a radius offn ;
the volume of liquid evaporated by the decrease of the adsorbed layer in
pores emptied in previous steps at higher relative pressures;
hence by
