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CH4, C2H6 and C3H8 Formation in CO + H2 Reaction Over Supported on Silica Nickel and Nickel-Copper Catalysts
402
CH4 , C2 H 6 AND C3 H g FORMATION IN CO + H 2 REACTION OVER SUPPORTED ON SILICA NICKEL AND NICKEL-COPPER CATALYSTS J.A. DALMON and G.A. MARTIN Institut de Recherches sur la Catalyse - CNRS - 2 Avenue Einstein, 69626 Villeurbanne Cedex, France.
ABSTRACT:
CO hydrogenation into C 1, C and C 3 hydrocarbons 2 was investigated on well-defined Ni-Cu/Si0 2 and Ni/Si0 2 catalysts. On Ni-Cu catalysts the rates were shown to decrease when copper content x increases as (l_x)N
with Nl~12
for methane
and N2 ~ N ~ 20 for ethane and propane formation. On pure 3 Ni/Si0 catalysts the selectivity towards C C hydrocarbon 2 2(or 3)
formation was shown to increase as particle size increases, first
drastically up to ca. 6 nm then slightly above. Most of these results are discussed in term of ensembles of adjacent Ni atoms and suggest that hydrogenolysis can be considered as a reverse reaction of F.T. synthesis. 1.
I N'l'RODUCTION Owing to "geometric effects of dilution", alloying group VIII
metals with IB metals is expected to yield informations on the number of metallic atoms which are involved in the active site of catalytic reactions. In this respect, the CO hydrogenation into methane and heavier hydrocarbons on Ni-Cu catalysts has been re1,2). cently examined The results,. however, are not clear since the former work report an increase of the selectivity toward heavier hydrocarbons and the latter a decrease when the copper concentration increases. Moreover, little is known about the structure-sensitivity of CO hydrogenation over Ni. Another problem is the specificity of Ni catalysts towards CH4 formation in CO+H2 reaction by comparison with Fe or Co. This situation has prompted us to examine the problem of the selectivity of CO hydrogenation on well-defined Ni/Si0 2 and Ni-Cu/Si0 2 catalysts. 2. EXPERIMENTAL The morphological characteristics of the studied samples have been described in previous papers 3,4). Precursors were obtained 2/g) by reacting silica (Aerosil Degussa, 200 m with solutions
co + of nickel
H, Reaction over NijSiO, and Ni-CujSiO,
403
(or nickel and copper) nitrate hexamine. Reduction tem-
peratures (800-1300 K) and metallic loadings (4-23 wt % of Ni) of nickel precursors were varied to cover a large range of particle sizes (2.5-25 nm,
as deduced from hydrogen chemisorption and ma-
gnetic methods). The Ni-CujSi0 2 catalysts were prepared by reduction at 920 K. Adsorption and magnetic studies have shown that a homogeneous alloy was formed and that the surface composition of metallic particles (diameter, 6 nm) was very similar to the bulk one 4), in contrast with the surface enrichment in copper generally observed on Ni-Cu films and unsupported powders. Kinetic experiments were carried out in a flow system (35 mljmin) with a fixed-bed reactor at atmospheric pressure. High purity gases were used and gas analyses were performed by gas chromatography with catharometric and flame-ionization detectors. Conversion were smaller than a few percent. 3. RESULTS Three reactions were followed : CO
co
---'>
Reaction 1, intrinsic activity A
~
CO _
2,
A 2
3,
A 3
and the selectivity S was defined as S = A 2 j
(A 1 + A 2)
Kinetic parameters (apparent energy of activation E
orders with a, respect to reactants n H and nCO) were measured and seem to be, within experimental errors, identical for all the catalysts studied (table). TABLE Kinetic parameters (reactions were studied over the temperature range of 170 to 370°C, partial pressures varying from 80 to 300 Torr for CO and 150 to 500 Torr for H2) Reactions
Ea(kcal.mole
1
27 :!: 3 19 + - 3 - 4 19 +
2
3
-1
)
n
H
1 :!: 0.2 0.8 :!: 0.2 -
nCO -0.3 :!: 0.2 +0.3 :!: 0.2
-
These constant values suggest the same mechanism for each reaction on the different samples. Then, the observed changes of activity are probably due to changes in the number of active sites.
404
J.A. Dalman, G.A. Martin
3.1. Ni-CU/SiO Z Activities of Ni-Cu catalysts for Reactions 1, Z and 3 against copper content x of the alloy
are represented on figure
(o"x~0.4)
1.
0
(,)
Ql til
N~1011
0
0
0
(,)
-. til
A
~
:I
(,)
Ql
"0
E
-
[J
>-
~
(:i10
[]
9
<
0.1
0.3
x
Fig. 1. Activit¥ of Ni-Cu/Si0 catalysts against copper contentx. 2 T = Z50 C; HZ/CO = 4; 0 = CH4, 0 = CZH6, A = C 3Ha Activities vary with x as -x ) N (I ) A (x) = A (x=O) ( 1 where A(x) is the activity of the alloy of copper content x. Expe-
rimentals points fit this equation with the values of N : N = 13 ~ Z for Reaction 1 1 N = N = Z2 ~ 4 for Reactions 2 and 3 Z 3 The important difference in N between Reactions 1 and Z leads to a rapid decrease of selectivity S when adding copper. If it is assumed that the distribution of Ni and Cu atoms at the surface is random relation (I) shows that the activity of Reaction I decreases as the probability of finding ensembles of about 13 adjacent atoms of Ni, ensembles which would constitute according to this hypothesis, the "active site" associated with Reaction 1. Reactions Z and 3 would require ensembles of about ZO adjacent atoms. Such high values
co
+ H, Reaction over Ni/SiO, and Ni-Cu/SiO,
405
were already proposed (and supported by magnetic chemisorption measurements) in hydrocarbons reactions on nickelS). 3.2 Ni/Si0
with variable metallic particle size 2 Fig. 2 shows the activity of supported nickel for Reactions 1
and 2 as a function of particle size. In contrast with Reaction 1 which seems nearly "structure insensitive", Reaction 2 is strongly inhibited when the particle size decreases below ca. 6 nm, resulting in an important change of selectivity S (fig. 3).
100
200
D(A)
Fig. 2. Activity of Ni/Si0 catalysts for Reactions 1 (curve 1) 2 and 2 (curve 2) as a functlon of particle size. T = 21SoC; H = 4 2/CO 0.2
III
>-
::
.2:
tiQl
0.1
'i
III
200 Fig. 3. Selectivity S as a function of particle size. T H = 4 2/CO
406
J .A. Dalmon, G.A. Martin
Fig. 4 shows, in classical Arrhenius plots, the variations of activity of Reactions 1, 2 and 3 over the catalyst whose average diameter is 6 nm. Changes of selectivity S against temperature of reaction are represented on fig. 5 for different samples.
-1 ~-----------------t
01
o
...J
- 4 -'---_ _- - - ' - - - - - ' - - - - ' - - - - - ' - - - - - 1 2.1 2.2 1/T.103 2.0 Fig. 4. Log conversion as a function of the reciprocal temperature for sample D = 6 nm; H = 4 2/CO
0.2
en
>.
-
~0.1 o
Ql
Ql
en
o
25A 200
250
Fig. 5. Selectivity S against temperature for different samples 1I = 4 2/CO
co
+ Hz Reaction over Ni/SiO z and Ni-Cu/SiO z
407
4. DISCUSSION 4.1. Alloying effects Our results confirm that synthesis of hydrocarbons heavier than CH is, on silica-supported Ni catalysts, more sensitive to 4 Z). copper alloying than the methanation reaction It should be no1} ted that the behavior of Ni-Cu films is at variance with that of silica-supported alloys (this apparent discrepancy is perhaps due to the equilibrated copper-rich surface of films which can produce changes in mechanisms of CO + HZ reactions). The high N values observed in this work deserve some comments.
In previous works 6,7), it has been assumed that the intermediate of the methanation reaction is a superficial carbon bonded to 3 nickel atoms, Ni
3Ca d s'
NiCO a d s' NiZCO a d s
~
which is formed according to Ni 4COa d s
~
Ni
3Ca d s
+ NiO a d s
(the subscript ads means adsorbed) If only diluting effects of nickel atoms by inactive copper atoms are considered, we are expecting that N = 4 (3 Ni atoms for Ni 3C 1 plus one for NiO). As the observed value is larger, additional electronic effects are probably to be invoked : i/lt can be assumed that Ni 4COa d s' or Ni + NiO does require 4 nickel a d s, 3Ca d s atoms electronically imperturbed by the presence of neighboring copper atoms. Accordingly, the reactional site would be formed by 4 unperturbed nickel atoms surrounded with a certain number of other nickel atoms more or less perturbed by copper. ii/Another possibility is that the adsorption of CO in the molecular or cracked form inhibits the adsorption on surrounding atoms as does oxygen adsorption on Rhenium8), thus leading to N values larger than 4. Both hypotheses are in agreement with the following observations : i/lt is not possible to saturate the nickel surface with CO adsorption in moderate conditions 4)
iii. The relative abondance of the bridged
species NiZCO
decreases when the copper content x increases as ads (1 - x)N, with N~6, indicating that the number of adjacent nickel atoms of the site can be larger than the bond number 9). The comparizon of N N and N values is of special interest. 1, Z 3 NZ~ Z N1 suggests that ethane formation results
The fact that
from dimerization of monocarbon fragments on two neighboring ensembles of N1 nickel atoms. Let us recall that such polymerizations of unoxygenated monocarbon fragments were shown to be possible on
N3~ NZ' the mechanism of propane formation is probably different of the previous one (if not, we would expect
nickel 7,10). Since N3
~
3 N1). It suggests that the C intermediates are held on the 3
408
J.A. Dalman, G.A. Martin
surface by only two carbon atoms. Some hypotheses on this C for3 mation can be advanced : i) CO is directly involved in propane formation (CO insertion in an adsorbed dicarbon fragment could be a possible mechanism) ii) a monocarbon fragment, to which corresponds an ensemble N reacts with a dicarbon one bonded to the surface by l, only one carbon, this dicarbon species being associated with an N l ensemble adjacent to the previous one (a same kind of C interme3 diate, bonded to the surface by two carbons, was proposed in
~ C 2 + C hydrogenolysis 11)). Other mechanisms, however can be 3 l imagined. This change of mechanism for C hydrocarbon formation 3 (and probably heavier hydrocarbons) could be connected with the
C
fact that C l and C 2 hydrocarbons formations do not obey the Flory distribution law in Fischer-Tropsch synthesis.
Schult~
4.2. Structure effects The activity of catalysts with nickel particles sizes smaller than 6 nm, decreases when diameters decrease. The larger the N value, the more pronounced the particle size effect. This leads to a drastic decrease of the selectivity S when diameter decreases under 6 nm (fig. 3). A very similar behavior has been already reported l l) in the case of propane hydrogenolysis : reaction C ~ 3 C which 3 l requires higher N ensembles than C 3 ~ C + C is more affected by 2 l decreasing the particle size under 6 nm than C ~ C 2 + C l" This 3 phenomenon is probably due to the rapid decrease with size of the total number of available metallic surface atoms per particle. Reaction 2 which requires larger ensembles than Reaction 1 is more inhibited. This explanation however , is not fully satisfying since, according to this hypothesis, the critical diameter at which the activity should decrease is ca. 1 nm.
Hence, other factors, such as
the presence of inactive adspecies on the surface or the metalsupport interface, play probably an important role, and combine their effects with the previous one. For particle sizes larger than 6 nm,
it can be seen that the ra-
tes of Reactions 1 and 2 are little influenced by particle size. However they do not vary exactly in
aparal~l
way: the selectivity
S increases when diameters increase above 6 nm (fig. 3). It can be recalled that this same sequence of Ni/Si0 2 catalysts shows above 6 nm a sharp decrease of the rate of hydrogenolysis 12) which was attributed to the formation of high density surface planes thermodynamically more stable due to the high temperature treatments used in this work to obtain large diameters. The observed increase of S above 6 nm shows that this selectivity for hydrocarbons heavier than CH4 (C - C bond formation)
increases when the activity towards hy-
co +
H 2 Reaction over Ni/Si02 and Ni-Cu/Si02
409
drogenolysis, AH(C - C bond rupture) decreases. This seems to be a very general trend which is supported by two other observations i/When the temperature increases, A increases and S decreases H (Fig. 5). ii/ The activity of Fe/MgO catalysts towards hydrogenolysis is smaller than that of Ni/Si0 2 which are less selective for heavier hydrocarbons in CO + H2 reaction (unpublished results). Fischer-Tropsch synthesis and hydrogenolysis have probably C 1 and
C surface intermediates in common, these species C 1 and Cn being n more or less in equilibrium. Let us recall that the same C 1 surface intermediate, the so-called surface carbon Ni was already propo3C sed in CO + H reaction 7) and in C hydrogenolysis 5). 2 2H6 In conclusion, two kinds of effects are proposed to account for
variations of C 2(3)/C 1 formation in CO + H2 reaction over Ni/Si0 2 catalysts : i) "ensemble" effects : high dispersion of Ni and dilution of Ni by Cu leads preferentially to methane,
owing to the high number of
adjacent nickel atoms needed to form the active site in C - C formation. ii) effects due to the opposite characters of CO + H2 reaction (C - C formation) and hydrogenolysis (C - C rupture) a decrease in the
hydrogenoly~ing
character of the catalyst seems to corres-
pond to an increase in C - C
bond formation.
REFERENCES 1. W.A.A. Barneveld and V. Ponec, J. Catalysis, 51, 426 (1978). 2. L.J.M. Luyten, M.v. Eck, J.v. Grondelle and J-:H.C.v. Hoof, J. Phys. Chern., 82, 2000, (1978). 3. M. Primet, J.A. Dalmon and G.A. Martin, J. Catalysis, ~, 25 (1977) . 4. J.A. Dalmon, J. Catalysis, 60, 325 (1979). 5. J.A. Dalmon, J.P. Candy and~.A. Martin, Proc. Sixth Congress on Catalysis, Londres, Vol. 2, 903 (1976). G.A. Martin, J. Catal., 60,_345 (1979). 6. G.A. Martin, M. Primet and J.A. Dalmon, J. Catalysis, ~l, 321 (1978). 7. J.A. Dalmon and G.A. Martin, J. Chern. Soc. Farad. Trans. I, 75, 1011, (1979). 8. R: Ducros, J.J. Ehrhardt, M. Alnot and A. Cassuto, Surf. Sci., 55, 509 (1976). 9. ~A. Dalmon, M. Primet, G.A. Martin and B. Imelik, Surf. Sci., 50, 95 (1975). 10. ~A. Rabo, A.P. Risch and M.L. Poutsma, 9 th Central Regional Meeting of the American Chern. Soc., paper 54 (1977). 11. M.F. Guilleux, J.A. Dalmon and G.A. Martin, J. Catalysis, 62, 235 (1980). 12. G:A. Martin and J.A. Dalmon, C.R. acado Sci., 286 serie C, 127 (1978) .
410
J.A. Dalman, G.A. Martin
DISCUSSIDN K. Klier (Lehigh Univ., Bethlehem, U.S.A.) Data in Fig. 4 show that the apparent activation energy for the production of C ZH6 and C is not constant over the whole 3HS temperature range. Does this possibly indicate that the size of the ensembles, NZ' is temperature dependency? Do the values of E a for reactions Z and 3 and the values of NZ hold for a limited temperature range, such as T
~
SOOK?
J.A. Dalman For the three mentioned reactions, apparent activation energies and also orders indeed vary with the experimental conditions (temperature, partial pressures of reactants) and the values reported in the table are for given conditions.
However these
kinetic parameters do vary in the same manner when alloying copper with nickel, and consequently values for N hold for the range studied.
H. Arai (Kyushu Univ., Fukuaka) I agree with your discussion on C-C bond formation of adsorbed species including carbon. hydrogen activation.
C-H bond formation is dependent on
But the rupture of the C-C bond is strongly
inhibited by carbon monoxide, because carbon monoxide adsorption is very strong on the nickel surface and hydrocarbons adsorb with difficulty in the presence of carbon monoxide.
Have you
tried hydrogenolysis of propane or propylene in the presence of carbon monoxide?
Actually, I think you need not consider the
rupture mechanism in the presence of carbon monoxide.
J.A. Dalman We have not performed studies of hydrogenolysis in the presence of carbon monoxide, this kind of study would be probably made difficult because both reactions, hydrogenolysis and CO + HZ' run in the same temperature range on Ni and a labelled reactant will be probably necessary.
On the other hand we don't know if
such study will be interesting for our work because we think that the postulated equilibrium between C - C bond formation and rupture occurs only in an adsorbed state, that is there is no (proceedings from a C - C formation in CO + HZ ZH6 reaction) and subsequent read sorption followed by the cracking
desorption of C
of this molecule on Ni.
In our hypothesis, this surface equili-
brium would be influenced by different parameters such as tempe-
co
+ H, Reaction over NijSiO, and Ni-CujSiO,
411
rature, partial pressures and also by the hydrogenolyzing character of the catalyst.
M.A. Vannice (Pennsylvania State Univ.) 1) Are the specific activities in Fig. 1 based on total metal surface or only on nickel surface area?
If activities are based
on total metal area, how do activities compare based only on nickel surface area? 2) Are the selectivities in Figs. 3 & 5 compared at constant conversion?
If not, what is the effect of conversion on selecti-
vity?
J.A. Dalmon 1)
Specific activities on Fig. 1 are based on total metal
surface area.
As the same surface and bulk composition is postu-
lated, activities based on nickel surface area are obtained, assuming that copper is inactive, by dividing activities of Fig. 1 by (I-x).
As values of N are important, no drastic change
in activities versus x is observed. 2)
Figs. 2 and 4 show that the main product is always dis-
tinctly CH4, consequently the necessary corrections to give Figs. 3 and 5 at constant conversion are rather limited and the general trend given on these figures is maintained.
J.W.E. Coenen (Unilever Res., Vlaardingen) 1) The relation between formula 1 and Fig. 1 of the paper is not obvious.
Presumably In(l-X) is approximated by -x for small
values of X? 2)
In Fig. 2 the logarithmic activity scale is somewhat mis-
leading.
Below about 60
A,
activity seems to be proportional
to crystallite size, which agrees with what we found.
The
ensemble size available on a crystallite may will be smaller than the size of the crystallite, so that the critical crystal diameter below which there is a fall in activity may be larger than the ensemble size.
Also the crystallite size distribution
should be taken into account.
J.A. Dalman 1) This relation would be perhaps more clear if on Fig. 1 a law (l_x)N was drawn.
These laws in the range of N and x used
in this paper can be roughly represented by a series of straight lines very close to the experimental ones. 2) We agree with your result and statement.
As reported in
the discussion in the paper the critical particle diameter is
412
J.A. Dalman, G.A. Martin
probably higher than the size of the ensemble corresponding to the active site.
As deduced from magnetic granulometry (low
and high-field part of the magnetization curve) the size distribution seems rather narrow.
J.H.C. van Hooff (Eindhoven Univ. Tech.) I want to make a comment on the high N values as calculated by the authors.
The main reason for this result is the assump-
tion that the surface composition of the supported NijCu particles is equal to the bulk composition.
However in my opinion this
complete neglect of surface enrichment in copper is questionable; it is true that surface enrichment is much smaller in supported NijCu alloy particles than in films and unsupported powders but this does not mean the complete absence of this phenomenon, and because small deviations in surface Cu-concentration already drastically influence the N value it makes a big difference if the surface composition is taken equal to or very similar to the bulk composition.
My objections are supported by
our experiments with supported NijCu and RujCu catalysts (A part of the results have been reported in the paper mentioned in ref. 2) •
In the case of RU/CU no alloy formation occurs and consequently the more volatile Cu will be completely deposited at the surface of the Ru particles (Sinfelt).
Accordingly, we observe a
big influence of the addition of Cu to Ru on the activity for CO hydrogenation, and application of your equation I results in a value of about 5 for Nl'
Because there is no reason why the
ensemble for CH4 formation should be larger for Ni than for Ru we expect that an ensemble of about 5 adjacent Ni-atoms is also needed for this reaction.
This result can be obtained by appli-
cation of equation I if the surface Cu concentration is not taken equal to but about twice the bulk Cu concentration, as can be illustrated by the following example for x = 0.3: (1-0,3)13
=
0.0097 ~
(1-2xO.3)5
=
0.0102
J.A. Dalmon As indicated in the paper and in ref. 4 of the paper we have checked alloy formation (x Ray and magnetic methods) and studied the surface composition by chemisorption (volumetric and magnetic study of H2 and CO chemisorption), The results suggest that the surface composition of these supported NiCu alloys is very near the bulk one.
Accordingly, the use in equation I for x
Cu content) of the bulk value seems justified.
(surface
However the values
co
413
+ Hz Reaction over NijSiO z and Ni-CujSiOz
of N as obtained from I suppose that the distribution of the surface Ni and eu atoms is random; if some short-range ordering occurs N values could be over-estimated.
Nevertheless, we think
that whatever the surface composition and even if short-range ordering our conclusions in connection with the relation N 3 ~ 2 N hold. I
~
N2
R.B. Anderson (McMaster Univ., Ontario) To produce higher hydrocarbons, 2 necessary but not sufficient conditions are: selective poisoning by CO and a reaction pathway. Please comment on the effect of Cu on Ni in these terms.
J.A. Dalmon I think you want to speak of the effect of CO pressure and coverage on the products distribution.
We have at present time
a sutdy on this field in progress on nickel catalysts.
But the
main effect of copper seems to dilute Ni atoms and decrease the rate of the reactions requiring the largest ensembles (as C - C bond formation).
M. Baerns (Ruhr Univ., Bochum) In some recent work (U. Gantz, Ph. D. thesis, Ruhr University Bochum) we have observed a similar decrease in CO-conversion to C
and C at higher temperatures as is shown in Fig. 4 of 2H6 3HS your paper. Do you have any explanation for these findings?
J.A. Dalmon We have no definite explanation for these observations.
The
above-mentioned hydrogenolyzing character which probably increases with temperature is perhaps responsible for this decrease. Another explanation could be found in a model with a competition between adsorbed H and CO species. 2
H. Schulz (Univ. Karlsruhe) My first comment concerns your proposal to consider chain growth in Fischer-Tropsch synthesis as a reverse reaction of hydrogenolysis.
I want to point out that at temperatures and
partial pressures of the Fischer-Tropsch synthesis in a thermodynamic sense the hydrogenolysis of hydrocarbons in strongly favoured.
A statement like yours should then be enriched with
some limiting kinetic assumptions and specified further in order to avoid thermodynamic contradiction. your reaction conditions (2IS0C,
My further point concerns
H2/CO = 4, Ni catalyst). These are very much on the side of the methane synthesis and not of
414
J.A. Dalman, G.A. Martin
the Fischer-Tropsch synthesis.
Under your conditions hydro-
genolysis of hydrocarbons C yield methane is a fast reaction 2+to and no substantial insight for the Fischer-Tropsch reaction can be drawn from these experiments.
J.A. Dalman As mentioned in the reply to the question of Professor Arai
co + H2 reaction and chain-rupture in hydrogenolysis is postulated to occur only in the reverse character of chain-growth in
the adsorbed state during the C - C bond formation and not at all via desorption of C 2+ products and further cracking of these species. We agree with your statement nickel catalysts are not good F.T. catalysts.
However beside methane, C2+ hydrocarbons can
represent up to 30 per cent of the products and the conclusion on the opposite characters of C-C bond formation and rupture reactions is supported by the fact that Fe catalysts which are good F.T. catalysts are much less active in hydrogenolysis than Ni catalysts.
A. Sarkany (Inst. of Isotopes, Budapest) You have shown in your paper that there is some correlation between hydrogenolysis activity and the probability of C-C bond formation in the Fischer-Tropsch reaction on Ni catalysts. have some results that may support your suggestion.
We
Ni catalysts,
which are active in C-C bond rupture, are able to homologate saturated hydrocarbons.
On decreasing the hydrogenolysis
activity of Ni sites by poisoning with carbonaceous residues, formation of hydrocarbons with larger carbon number than the parent molecule could be observed (A. Sarkany, P. Tetenyi in JCS Chern. Comm., 1980).
The reaction seems to proceed by in-
corporation of C fragments into terminal olefins via a nickel l cyclobutane intermediate.
J.A. Dalman We thank Dr. Sarkany for this interesting comment. May we add that the mechanism you have proposed for the chain-growth could be applied to our results.
V. Panee (Univ. Leiden) Your results confirm (see Araki et al., V. Barneveld et al. on NijCu, Luyten on RujCU, NijCu) that CHy and higher hydrocarbon synthesis require a big ensemble. However, how reliable are the numerical values characterizing the size of ensembles?
co
415
+ H, Reaction over Ni/SiO, and Ni-Cu/SiO,
Before one accepts that the required (N up to 20) ensemble is of the size of whole planes of microcrystals, the following has to be carefully examined. 1) Is the magnetic method for determining the total alloy surface area that reliable (assumptions on the magnetic parameters as well as on particle size distribution must be made!) that we can conclude on basis of it that there is no Cu surface enrichment (the particles are big enough for that) although the thermodynamics requires such enrichment? 2)
Is the part inactivated by an inactive carbon layer the
same on Ni and on alloys? 3)
Is it not possible that alloys on carriers contain a small
amount of unalloyed Ni in the form of ultrasmall particles which are virtually inactive in the F.T. synthesis (F.T. runs worse on small particles) but active in hydrogenolysis? seems to run better on small particles.
(This reaction
Such effect would lead
to N (ethane) > N (methane». J.A. Dalmon
1) It has been shown that magnetic granulometry methods give results which accord well with other techniques (see e.g. W.E. Khun, J. Ehretsman, "Fine Particles" 1974 p.43).
Magnetic
granulometry needs the saturation magnetization of the sample, this parameter was obtained by using high fields
(up to 70 kOe)
and low temperature (4 K) in a superconductive coil; these conditions allOW us also to rule out the existence of a double size distribution with very small particles.
Moreover in this case
the total alloy surface area as deduced from this magnetic method is in good agreement with the particle size obtained from X-Ray line broadening. 2) Carbon deposition can indeed differ between pure nickel and its alloys.
In this study we observe no loss of activity against
time or when describing cycles of temperature or cycles of CO partial pressure: this reversibility suggests that if there is deposition its effects on the activities of nickel or alloys are probably limited. 3) A sutdy of C hydrogenolysis on the same range of Ni 2H6 catalysts show that the hydrogenolysis activity decreases when particle size becomes small (D < 50
A).
See ref. 12 of the
paper. As indicated in the reply to Prof. van Hooff values of N reported in the paper suppose there is no short-range ordering and even if this occurs conclusions in connection with N3
~
N2
~
2Nl
416
J.A. Dalman, G.A. Martin
always hold. ~
(Univ. Hong Kong)
If each carbon were associated with 13 surface Ni atoms (presumably these atoms could not be used by another carbon atom), then in the formation of C 2H6, for instance any pair of neighbouring Cads entities intending to form the C-C bond would be at least several interatomic distances apart.
Are you con-
sidering the two Cads entities may migrate to each other during the bond formation process?
If so, would you then entertain the
possibility that the Cu atoms may actually prevent effective migration of Cads species (in addition to deactivating some Ni atoms), thereby accounting for at least partially the high values of N,
(13 and 22) in your kinetic expression (I)?
J.A. Dalmon During C-C bond formation, we agree that two monocarbon species adsorbed on two neighboring Nl sites probably have to migrate to each other likely after partial hydrogenation of the surface carbon.
As proposed in the paper Cu could inhibit CO dissocia-
tion and can prevent this necessary C migration if two Nl sites are separated by Cu.
W. Palczewska (Inst. Phys. Chern., Warszawa) How do you explain the lack of copper segregation from the bulk to the surface of your alloy catalyst?
Is it to be attri-
buted perhaps to the presence of impurities in the surface layer (S.P.C. etc.)?
Such atoms could inhibit the achievement of
thermodynamic equilibrium between bulk and surface by closing the necessary diffusion paths (see e.g. Burton J.J. et aI, J. Chern. Phys.,
~'
1089 (1974».
J.A. Dalmon This problem has been discussed in the paper corresponding to the ref. 4 of the communication, but no definite explanation has been proposed.
Some AES experiments performed on these samples
by B.J. Wood and H. Wise have shown that they do not contain impurities of S or C (no lines were observed), and we think that the problem of these non-segregated surfaces still remains open.
I. Mochida (Kyushu Univ., Fukuoka) Questions to Figs. 2 and 3.
How does the number of coupled
sites for ethylene formation in comparison with the isolated sites for methane change with the particle size of the metal catalyst?
co +
H 2 Reaction over NijSi0 2 and Ni-CujSi02
417
J.A. Dalmon When the particle size decreases the ethane formation which requires ensembles larger than methane formation is more inhibited.
A quantitative correlation between the size of the metallic
particle and the available number of ensembles NZ and Nl at the surface would probably be made difficult by factors such as
metal-support interface, inactive species adsorbed on the surface, inactive crystallographic planes or atoms etc.
CH4 , C2 H 6 AND C3 H g FORMATION IN CO + H 2 REACTION OVER SUPPORTED ON SILICA NICKEL AND NICKEL-COPPER CATALYSTS J.A. DALMON and G.A. MARTIN Institut de Recherches sur la Catalyse - CNRS - 2 Avenue Einstein, 69626 Villeurbanne Cedex, France.
ABSTRACT:
CO hydrogenation into C 1, C and C 3 hydrocarbons 2 was investigated on well-defined Ni-Cu/Si0 2 and Ni/Si0 2 catalysts. On Ni-Cu catalysts the rates were shown to decrease when copper content x increases as (l_x)N
with Nl~12
for methane
and N2 ~ N ~ 20 for ethane and propane formation. On pure 3 Ni/Si0 catalysts the selectivity towards C C hydrocarbon 2 2(or 3)
formation was shown to increase as particle size increases, first
drastically up to ca. 6 nm then slightly above. Most of these results are discussed in term of ensembles of adjacent Ni atoms and suggest that hydrogenolysis can be considered as a reverse reaction of F.T. synthesis. 1.
I N'l'RODUCTION Owing to "geometric effects of dilution", alloying group VIII
metals with IB metals is expected to yield informations on the number of metallic atoms which are involved in the active site of catalytic reactions. In this respect, the CO hydrogenation into methane and heavier hydrocarbons on Ni-Cu catalysts has been re1,2). cently examined The results,. however, are not clear since the former work report an increase of the selectivity toward heavier hydrocarbons and the latter a decrease when the copper concentration increases. Moreover, little is known about the structure-sensitivity of CO hydrogenation over Ni. Another problem is the specificity of Ni catalysts towards CH4 formation in CO+H2 reaction by comparison with Fe or Co. This situation has prompted us to examine the problem of the selectivity of CO hydrogenation on well-defined Ni/Si0 2 and Ni-Cu/Si0 2 catalysts. 2. EXPERIMENTAL The morphological characteristics of the studied samples have been described in previous papers 3,4). Precursors were obtained 2/g) by reacting silica (Aerosil Degussa, 200 m with solutions
co + of nickel
H, Reaction over NijSiO, and Ni-CujSiO,
403
(or nickel and copper) nitrate hexamine. Reduction tem-
peratures (800-1300 K) and metallic loadings (4-23 wt % of Ni) of nickel precursors were varied to cover a large range of particle sizes (2.5-25 nm,
as deduced from hydrogen chemisorption and ma-
gnetic methods). The Ni-CujSi0 2 catalysts were prepared by reduction at 920 K. Adsorption and magnetic studies have shown that a homogeneous alloy was formed and that the surface composition of metallic particles (diameter, 6 nm) was very similar to the bulk one 4), in contrast with the surface enrichment in copper generally observed on Ni-Cu films and unsupported powders. Kinetic experiments were carried out in a flow system (35 mljmin) with a fixed-bed reactor at atmospheric pressure. High purity gases were used and gas analyses were performed by gas chromatography with catharometric and flame-ionization detectors. Conversion were smaller than a few percent. 3. RESULTS Three reactions were followed : CO
co
---'>
Reaction 1, intrinsic activity A
~
CO _
2,
A 2
3,
A 3
and the selectivity S was defined as S = A 2 j
(A 1 + A 2)
Kinetic parameters (apparent energy of activation E
orders with a, respect to reactants n H and nCO) were measured and seem to be, within experimental errors, identical for all the catalysts studied (table). TABLE Kinetic parameters (reactions were studied over the temperature range of 170 to 370°C, partial pressures varying from 80 to 300 Torr for CO and 150 to 500 Torr for H2) Reactions
Ea(kcal.mole
1
27 :!: 3 19 + - 3 - 4 19 +
2
3
-1
)
n
H
1 :!: 0.2 0.8 :!: 0.2 -
nCO -0.3 :!: 0.2 +0.3 :!: 0.2
-
These constant values suggest the same mechanism for each reaction on the different samples. Then, the observed changes of activity are probably due to changes in the number of active sites.
404
J.A. Dalman, G.A. Martin
3.1. Ni-CU/SiO Z Activities of Ni-Cu catalysts for Reactions 1, Z and 3 against copper content x of the alloy
are represented on figure
(o"x~0.4)
1.
0
(,)
Ql til
N~1011
0
0
0
(,)
-. til
A
~
:I
(,)
Ql
"0
E
-
[J
>-
~
(:i10
[]
9
<
0.1
0.3
x
Fig. 1. Activit¥ of Ni-Cu/Si0 catalysts against copper contentx. 2 T = Z50 C; HZ/CO = 4; 0 = CH4, 0 = CZH6, A = C 3Ha Activities vary with x as -x ) N (I ) A (x) = A (x=O) ( 1 where A(x) is the activity of the alloy of copper content x. Expe-
rimentals points fit this equation with the values of N : N = 13 ~ Z for Reaction 1 1 N = N = Z2 ~ 4 for Reactions 2 and 3 Z 3 The important difference in N between Reactions 1 and Z leads to a rapid decrease of selectivity S when adding copper. If it is assumed that the distribution of Ni and Cu atoms at the surface is random relation (I) shows that the activity of Reaction I decreases as the probability of finding ensembles of about 13 adjacent atoms of Ni, ensembles which would constitute according to this hypothesis, the "active site" associated with Reaction 1. Reactions Z and 3 would require ensembles of about ZO adjacent atoms. Such high values
co
+ H, Reaction over Ni/SiO, and Ni-Cu/SiO,
405
were already proposed (and supported by magnetic chemisorption measurements) in hydrocarbons reactions on nickelS). 3.2 Ni/Si0
with variable metallic particle size 2 Fig. 2 shows the activity of supported nickel for Reactions 1
and 2 as a function of particle size. In contrast with Reaction 1 which seems nearly "structure insensitive", Reaction 2 is strongly inhibited when the particle size decreases below ca. 6 nm, resulting in an important change of selectivity S (fig. 3).
100
200
D(A)
Fig. 2. Activity of Ni/Si0 catalysts for Reactions 1 (curve 1) 2 and 2 (curve 2) as a functlon of particle size. T = 21SoC; H = 4 2/CO 0.2
III
>-
::
.2:
tiQl
0.1
'i
III
200 Fig. 3. Selectivity S as a function of particle size. T H = 4 2/CO
406
J .A. Dalmon, G.A. Martin
Fig. 4 shows, in classical Arrhenius plots, the variations of activity of Reactions 1, 2 and 3 over the catalyst whose average diameter is 6 nm. Changes of selectivity S against temperature of reaction are represented on fig. 5 for different samples.
-1 ~-----------------t
01
o
...J
- 4 -'---_ _- - - ' - - - - - ' - - - - ' - - - - - ' - - - - - 1 2.1 2.2 1/T.103 2.0 Fig. 4. Log conversion as a function of the reciprocal temperature for sample D = 6 nm; H = 4 2/CO
0.2
en
>.
-
~0.1 o
Ql
Ql
en
o
25A 200
250
Fig. 5. Selectivity S against temperature for different samples 1I = 4 2/CO
co
+ Hz Reaction over Ni/SiO z and Ni-Cu/SiO z
407
4. DISCUSSION 4.1. Alloying effects Our results confirm that synthesis of hydrocarbons heavier than CH is, on silica-supported Ni catalysts, more sensitive to 4 Z). copper alloying than the methanation reaction It should be no1} ted that the behavior of Ni-Cu films is at variance with that of silica-supported alloys (this apparent discrepancy is perhaps due to the equilibrated copper-rich surface of films which can produce changes in mechanisms of CO + HZ reactions). The high N values observed in this work deserve some comments.
In previous works 6,7), it has been assumed that the intermediate of the methanation reaction is a superficial carbon bonded to 3 nickel atoms, Ni
3Ca d s'
NiCO a d s' NiZCO a d s
~
which is formed according to Ni 4COa d s
~
Ni
3Ca d s
+ NiO a d s
(the subscript ads means adsorbed) If only diluting effects of nickel atoms by inactive copper atoms are considered, we are expecting that N = 4 (3 Ni atoms for Ni 3C 1 plus one for NiO). As the observed value is larger, additional electronic effects are probably to be invoked : i/lt can be assumed that Ni 4COa d s' or Ni + NiO does require 4 nickel a d s, 3Ca d s atoms electronically imperturbed by the presence of neighboring copper atoms. Accordingly, the reactional site would be formed by 4 unperturbed nickel atoms surrounded with a certain number of other nickel atoms more or less perturbed by copper. ii/Another possibility is that the adsorption of CO in the molecular or cracked form inhibits the adsorption on surrounding atoms as does oxygen adsorption on Rhenium8), thus leading to N values larger than 4. Both hypotheses are in agreement with the following observations : i/lt is not possible to saturate the nickel surface with CO adsorption in moderate conditions 4)
iii. The relative abondance of the bridged
species NiZCO
decreases when the copper content x increases as ads (1 - x)N, with N~6, indicating that the number of adjacent nickel atoms of the site can be larger than the bond number 9). The comparizon of N N and N values is of special interest. 1, Z 3 NZ~ Z N1 suggests that ethane formation results
The fact that
from dimerization of monocarbon fragments on two neighboring ensembles of N1 nickel atoms. Let us recall that such polymerizations of unoxygenated monocarbon fragments were shown to be possible on
N3~ NZ' the mechanism of propane formation is probably different of the previous one (if not, we would expect
nickel 7,10). Since N3
~
3 N1). It suggests that the C intermediates are held on the 3
408
J.A. Dalman, G.A. Martin
surface by only two carbon atoms. Some hypotheses on this C for3 mation can be advanced : i) CO is directly involved in propane formation (CO insertion in an adsorbed dicarbon fragment could be a possible mechanism) ii) a monocarbon fragment, to which corresponds an ensemble N reacts with a dicarbon one bonded to the surface by l, only one carbon, this dicarbon species being associated with an N l ensemble adjacent to the previous one (a same kind of C interme3 diate, bonded to the surface by two carbons, was proposed in
~ C 2 + C hydrogenolysis 11)). Other mechanisms, however can be 3 l imagined. This change of mechanism for C hydrocarbon formation 3 (and probably heavier hydrocarbons) could be connected with the
C
fact that C l and C 2 hydrocarbons formations do not obey the Flory distribution law in Fischer-Tropsch synthesis.
Schult~
4.2. Structure effects The activity of catalysts with nickel particles sizes smaller than 6 nm, decreases when diameters decrease. The larger the N value, the more pronounced the particle size effect. This leads to a drastic decrease of the selectivity S when diameter decreases under 6 nm (fig. 3). A very similar behavior has been already reported l l) in the case of propane hydrogenolysis : reaction C ~ 3 C which 3 l requires higher N ensembles than C 3 ~ C + C is more affected by 2 l decreasing the particle size under 6 nm than C ~ C 2 + C l" This 3 phenomenon is probably due to the rapid decrease with size of the total number of available metallic surface atoms per particle. Reaction 2 which requires larger ensembles than Reaction 1 is more inhibited. This explanation however , is not fully satisfying since, according to this hypothesis, the critical diameter at which the activity should decrease is ca. 1 nm.
Hence, other factors, such as
the presence of inactive adspecies on the surface or the metalsupport interface, play probably an important role, and combine their effects with the previous one. For particle sizes larger than 6 nm,
it can be seen that the ra-
tes of Reactions 1 and 2 are little influenced by particle size. However they do not vary exactly in
aparal~l
way: the selectivity
S increases when diameters increase above 6 nm (fig. 3). It can be recalled that this same sequence of Ni/Si0 2 catalysts shows above 6 nm a sharp decrease of the rate of hydrogenolysis 12) which was attributed to the formation of high density surface planes thermodynamically more stable due to the high temperature treatments used in this work to obtain large diameters. The observed increase of S above 6 nm shows that this selectivity for hydrocarbons heavier than CH4 (C - C bond formation)
increases when the activity towards hy-
co +
H 2 Reaction over Ni/Si02 and Ni-Cu/Si02
409
drogenolysis, AH(C - C bond rupture) decreases. This seems to be a very general trend which is supported by two other observations i/When the temperature increases, A increases and S decreases H (Fig. 5). ii/ The activity of Fe/MgO catalysts towards hydrogenolysis is smaller than that of Ni/Si0 2 which are less selective for heavier hydrocarbons in CO + H2 reaction (unpublished results). Fischer-Tropsch synthesis and hydrogenolysis have probably C 1 and
C surface intermediates in common, these species C 1 and Cn being n more or less in equilibrium. Let us recall that the same C 1 surface intermediate, the so-called surface carbon Ni was already propo3C sed in CO + H reaction 7) and in C hydrogenolysis 5). 2 2H6 In conclusion, two kinds of effects are proposed to account for
variations of C 2(3)/C 1 formation in CO + H2 reaction over Ni/Si0 2 catalysts : i) "ensemble" effects : high dispersion of Ni and dilution of Ni by Cu leads preferentially to methane,
owing to the high number of
adjacent nickel atoms needed to form the active site in C - C formation. ii) effects due to the opposite characters of CO + H2 reaction (C - C formation) and hydrogenolysis (C - C rupture) a decrease in the
hydrogenoly~ing
character of the catalyst seems to corres-
pond to an increase in C - C
bond formation.
REFERENCES 1. W.A.A. Barneveld and V. Ponec, J. Catalysis, 51, 426 (1978). 2. L.J.M. Luyten, M.v. Eck, J.v. Grondelle and J-:H.C.v. Hoof, J. Phys. Chern., 82, 2000, (1978). 3. M. Primet, J.A. Dalmon and G.A. Martin, J. Catalysis, ~, 25 (1977) . 4. J.A. Dalmon, J. Catalysis, 60, 325 (1979). 5. J.A. Dalmon, J.P. Candy and~.A. Martin, Proc. Sixth Congress on Catalysis, Londres, Vol. 2, 903 (1976). G.A. Martin, J. Catal., 60,_345 (1979). 6. G.A. Martin, M. Primet and J.A. Dalmon, J. Catalysis, ~l, 321 (1978). 7. J.A. Dalmon and G.A. Martin, J. Chern. Soc. Farad. Trans. I, 75, 1011, (1979). 8. R: Ducros, J.J. Ehrhardt, M. Alnot and A. Cassuto, Surf. Sci., 55, 509 (1976). 9. ~A. Dalmon, M. Primet, G.A. Martin and B. Imelik, Surf. Sci., 50, 95 (1975). 10. ~A. Rabo, A.P. Risch and M.L. Poutsma, 9 th Central Regional Meeting of the American Chern. Soc., paper 54 (1977). 11. M.F. Guilleux, J.A. Dalmon and G.A. Martin, J. Catalysis, 62, 235 (1980). 12. G:A. Martin and J.A. Dalmon, C.R. acado Sci., 286 serie C, 127 (1978) .
410
J.A. Dalman, G.A. Martin
DISCUSSIDN K. Klier (Lehigh Univ., Bethlehem, U.S.A.) Data in Fig. 4 show that the apparent activation energy for the production of C ZH6 and C is not constant over the whole 3HS temperature range. Does this possibly indicate that the size of the ensembles, NZ' is temperature dependency? Do the values of E a for reactions Z and 3 and the values of NZ hold for a limited temperature range, such as T
~
SOOK?
J.A. Dalman For the three mentioned reactions, apparent activation energies and also orders indeed vary with the experimental conditions (temperature, partial pressures of reactants) and the values reported in the table are for given conditions.
However these
kinetic parameters do vary in the same manner when alloying copper with nickel, and consequently values for N hold for the range studied.
H. Arai (Kyushu Univ., Fukuaka) I agree with your discussion on C-C bond formation of adsorbed species including carbon. hydrogen activation.
C-H bond formation is dependent on
But the rupture of the C-C bond is strongly
inhibited by carbon monoxide, because carbon monoxide adsorption is very strong on the nickel surface and hydrocarbons adsorb with difficulty in the presence of carbon monoxide.
Have you
tried hydrogenolysis of propane or propylene in the presence of carbon monoxide?
Actually, I think you need not consider the
rupture mechanism in the presence of carbon monoxide.
J.A. Dalman We have not performed studies of hydrogenolysis in the presence of carbon monoxide, this kind of study would be probably made difficult because both reactions, hydrogenolysis and CO + HZ' run in the same temperature range on Ni and a labelled reactant will be probably necessary.
On the other hand we don't know if
such study will be interesting for our work because we think that the postulated equilibrium between C - C bond formation and rupture occurs only in an adsorbed state, that is there is no (proceedings from a C - C formation in CO + HZ ZH6 reaction) and subsequent read sorption followed by the cracking
desorption of C
of this molecule on Ni.
In our hypothesis, this surface equili-
brium would be influenced by different parameters such as tempe-
co
+ H, Reaction over NijSiO, and Ni-CujSiO,
411
rature, partial pressures and also by the hydrogenolyzing character of the catalyst.
M.A. Vannice (Pennsylvania State Univ.) 1) Are the specific activities in Fig. 1 based on total metal surface or only on nickel surface area?
If activities are based
on total metal area, how do activities compare based only on nickel surface area? 2) Are the selectivities in Figs. 3 & 5 compared at constant conversion?
If not, what is the effect of conversion on selecti-
vity?
J.A. Dalmon 1)
Specific activities on Fig. 1 are based on total metal
surface area.
As the same surface and bulk composition is postu-
lated, activities based on nickel surface area are obtained, assuming that copper is inactive, by dividing activities of Fig. 1 by (I-x).
As values of N are important, no drastic change
in activities versus x is observed. 2)
Figs. 2 and 4 show that the main product is always dis-
tinctly CH4, consequently the necessary corrections to give Figs. 3 and 5 at constant conversion are rather limited and the general trend given on these figures is maintained.
J.W.E. Coenen (Unilever Res., Vlaardingen) 1) The relation between formula 1 and Fig. 1 of the paper is not obvious.
Presumably In(l-X) is approximated by -x for small
values of X? 2)
In Fig. 2 the logarithmic activity scale is somewhat mis-
leading.
Below about 60
A,
activity seems to be proportional
to crystallite size, which agrees with what we found.
The
ensemble size available on a crystallite may will be smaller than the size of the crystallite, so that the critical crystal diameter below which there is a fall in activity may be larger than the ensemble size.
Also the crystallite size distribution
should be taken into account.
J.A. Dalman 1) This relation would be perhaps more clear if on Fig. 1 a law (l_x)N was drawn.
These laws in the range of N and x used
in this paper can be roughly represented by a series of straight lines very close to the experimental ones. 2) We agree with your result and statement.
As reported in
the discussion in the paper the critical particle diameter is
412
J.A. Dalman, G.A. Martin
probably higher than the size of the ensemble corresponding to the active site.
As deduced from magnetic granulometry (low
and high-field part of the magnetization curve) the size distribution seems rather narrow.
J.H.C. van Hooff (Eindhoven Univ. Tech.) I want to make a comment on the high N values as calculated by the authors.
The main reason for this result is the assump-
tion that the surface composition of the supported NijCu particles is equal to the bulk composition.
However in my opinion this
complete neglect of surface enrichment in copper is questionable; it is true that surface enrichment is much smaller in supported NijCu alloy particles than in films and unsupported powders but this does not mean the complete absence of this phenomenon, and because small deviations in surface Cu-concentration already drastically influence the N value it makes a big difference if the surface composition is taken equal to or very similar to the bulk composition.
My objections are supported by
our experiments with supported NijCu and RujCu catalysts (A part of the results have been reported in the paper mentioned in ref. 2) •
In the case of RU/CU no alloy formation occurs and consequently the more volatile Cu will be completely deposited at the surface of the Ru particles (Sinfelt).
Accordingly, we observe a
big influence of the addition of Cu to Ru on the activity for CO hydrogenation, and application of your equation I results in a value of about 5 for Nl'
Because there is no reason why the
ensemble for CH4 formation should be larger for Ni than for Ru we expect that an ensemble of about 5 adjacent Ni-atoms is also needed for this reaction.
This result can be obtained by appli-
cation of equation I if the surface Cu concentration is not taken equal to but about twice the bulk Cu concentration, as can be illustrated by the following example for x = 0.3: (1-0,3)13
=
0.0097 ~
(1-2xO.3)5
=
0.0102
J.A. Dalmon As indicated in the paper and in ref. 4 of the paper we have checked alloy formation (x Ray and magnetic methods) and studied the surface composition by chemisorption (volumetric and magnetic study of H2 and CO chemisorption), The results suggest that the surface composition of these supported NiCu alloys is very near the bulk one.
Accordingly, the use in equation I for x
Cu content) of the bulk value seems justified.
(surface
However the values
co
413
+ Hz Reaction over NijSiO z and Ni-CujSiOz
of N as obtained from I suppose that the distribution of the surface Ni and eu atoms is random; if some short-range ordering occurs N values could be over-estimated.
Nevertheless, we think
that whatever the surface composition and even if short-range ordering our conclusions in connection with the relation N 3 ~ 2 N hold. I
~
N2
R.B. Anderson (McMaster Univ., Ontario) To produce higher hydrocarbons, 2 necessary but not sufficient conditions are: selective poisoning by CO and a reaction pathway. Please comment on the effect of Cu on Ni in these terms.
J.A. Dalmon I think you want to speak of the effect of CO pressure and coverage on the products distribution.
We have at present time
a sutdy on this field in progress on nickel catalysts.
But the
main effect of copper seems to dilute Ni atoms and decrease the rate of the reactions requiring the largest ensembles (as C - C bond formation).
M. Baerns (Ruhr Univ., Bochum) In some recent work (U. Gantz, Ph. D. thesis, Ruhr University Bochum) we have observed a similar decrease in CO-conversion to C
and C at higher temperatures as is shown in Fig. 4 of 2H6 3HS your paper. Do you have any explanation for these findings?
J.A. Dalmon We have no definite explanation for these observations.
The
above-mentioned hydrogenolyzing character which probably increases with temperature is perhaps responsible for this decrease. Another explanation could be found in a model with a competition between adsorbed H and CO species. 2
H. Schulz (Univ. Karlsruhe) My first comment concerns your proposal to consider chain growth in Fischer-Tropsch synthesis as a reverse reaction of hydrogenolysis.
I want to point out that at temperatures and
partial pressures of the Fischer-Tropsch synthesis in a thermodynamic sense the hydrogenolysis of hydrocarbons in strongly favoured.
A statement like yours should then be enriched with
some limiting kinetic assumptions and specified further in order to avoid thermodynamic contradiction. your reaction conditions (2IS0C,
My further point concerns
H2/CO = 4, Ni catalyst). These are very much on the side of the methane synthesis and not of
414
J.A. Dalman, G.A. Martin
the Fischer-Tropsch synthesis.
Under your conditions hydro-
genolysis of hydrocarbons C yield methane is a fast reaction 2+to and no substantial insight for the Fischer-Tropsch reaction can be drawn from these experiments.
J.A. Dalman As mentioned in the reply to the question of Professor Arai
co + H2 reaction and chain-rupture in hydrogenolysis is postulated to occur only in the reverse character of chain-growth in
the adsorbed state during the C - C bond formation and not at all via desorption of C 2+ products and further cracking of these species. We agree with your statement nickel catalysts are not good F.T. catalysts.
However beside methane, C2+ hydrocarbons can
represent up to 30 per cent of the products and the conclusion on the opposite characters of C-C bond formation and rupture reactions is supported by the fact that Fe catalysts which are good F.T. catalysts are much less active in hydrogenolysis than Ni catalysts.
A. Sarkany (Inst. of Isotopes, Budapest) You have shown in your paper that there is some correlation between hydrogenolysis activity and the probability of C-C bond formation in the Fischer-Tropsch reaction on Ni catalysts. have some results that may support your suggestion.
We
Ni catalysts,
which are active in C-C bond rupture, are able to homologate saturated hydrocarbons.
On decreasing the hydrogenolysis
activity of Ni sites by poisoning with carbonaceous residues, formation of hydrocarbons with larger carbon number than the parent molecule could be observed (A. Sarkany, P. Tetenyi in JCS Chern. Comm., 1980).
The reaction seems to proceed by in-
corporation of C fragments into terminal olefins via a nickel l cyclobutane intermediate.
J.A. Dalman We thank Dr. Sarkany for this interesting comment. May we add that the mechanism you have proposed for the chain-growth could be applied to our results.
V. Panee (Univ. Leiden) Your results confirm (see Araki et al., V. Barneveld et al. on NijCu, Luyten on RujCU, NijCu) that CHy and higher hydrocarbon synthesis require a big ensemble. However, how reliable are the numerical values characterizing the size of ensembles?
co
415
+ H, Reaction over Ni/SiO, and Ni-Cu/SiO,
Before one accepts that the required (N up to 20) ensemble is of the size of whole planes of microcrystals, the following has to be carefully examined. 1) Is the magnetic method for determining the total alloy surface area that reliable (assumptions on the magnetic parameters as well as on particle size distribution must be made!) that we can conclude on basis of it that there is no Cu surface enrichment (the particles are big enough for that) although the thermodynamics requires such enrichment? 2)
Is the part inactivated by an inactive carbon layer the
same on Ni and on alloys? 3)
Is it not possible that alloys on carriers contain a small
amount of unalloyed Ni in the form of ultrasmall particles which are virtually inactive in the F.T. synthesis (F.T. runs worse on small particles) but active in hydrogenolysis? seems to run better on small particles.
(This reaction
Such effect would lead
to N (ethane) > N (methane». J.A. Dalmon
1) It has been shown that magnetic granulometry methods give results which accord well with other techniques (see e.g. W.E. Khun, J. Ehretsman, "Fine Particles" 1974 p.43).
Magnetic
granulometry needs the saturation magnetization of the sample, this parameter was obtained by using high fields
(up to 70 kOe)
and low temperature (4 K) in a superconductive coil; these conditions allOW us also to rule out the existence of a double size distribution with very small particles.
Moreover in this case
the total alloy surface area as deduced from this magnetic method is in good agreement with the particle size obtained from X-Ray line broadening. 2) Carbon deposition can indeed differ between pure nickel and its alloys.
In this study we observe no loss of activity against
time or when describing cycles of temperature or cycles of CO partial pressure: this reversibility suggests that if there is deposition its effects on the activities of nickel or alloys are probably limited. 3) A sutdy of C hydrogenolysis on the same range of Ni 2H6 catalysts show that the hydrogenolysis activity decreases when particle size becomes small (D < 50
A).
See ref. 12 of the
paper. As indicated in the reply to Prof. van Hooff values of N reported in the paper suppose there is no short-range ordering and even if this occurs conclusions in connection with N3
~
N2
~
2Nl
416
J.A. Dalman, G.A. Martin
always hold. ~
(Univ. Hong Kong)
If each carbon were associated with 13 surface Ni atoms (presumably these atoms could not be used by another carbon atom), then in the formation of C 2H6, for instance any pair of neighbouring Cads entities intending to form the C-C bond would be at least several interatomic distances apart.
Are you con-
sidering the two Cads entities may migrate to each other during the bond formation process?
If so, would you then entertain the
possibility that the Cu atoms may actually prevent effective migration of Cads species (in addition to deactivating some Ni atoms), thereby accounting for at least partially the high values of N,
(13 and 22) in your kinetic expression (I)?
J.A. Dalmon During C-C bond formation, we agree that two monocarbon species adsorbed on two neighboring Nl sites probably have to migrate to each other likely after partial hydrogenation of the surface carbon.
As proposed in the paper Cu could inhibit CO dissocia-
tion and can prevent this necessary C migration if two Nl sites are separated by Cu.
W. Palczewska (Inst. Phys. Chern., Warszawa) How do you explain the lack of copper segregation from the bulk to the surface of your alloy catalyst?
Is it to be attri-
buted perhaps to the presence of impurities in the surface layer (S.P.C. etc.)?
Such atoms could inhibit the achievement of
thermodynamic equilibrium between bulk and surface by closing the necessary diffusion paths (see e.g. Burton J.J. et aI, J. Chern. Phys.,
~'
1089 (1974».
J.A. Dalmon This problem has been discussed in the paper corresponding to the ref. 4 of the communication, but no definite explanation has been proposed.
Some AES experiments performed on these samples
by B.J. Wood and H. Wise have shown that they do not contain impurities of S or C (no lines were observed), and we think that the problem of these non-segregated surfaces still remains open.
I. Mochida (Kyushu Univ., Fukuoka) Questions to Figs. 2 and 3.
How does the number of coupled
sites for ethylene formation in comparison with the isolated sites for methane change with the particle size of the metal catalyst?
co +
H 2 Reaction over NijSi0 2 and Ni-CujSi02
417
J.A. Dalmon When the particle size decreases the ethane formation which requires ensembles larger than methane formation is more inhibited.
A quantitative correlation between the size of the metallic
particle and the available number of ensembles NZ and Nl at the surface would probably be made difficult by factors such as
metal-support interface, inactive species adsorbed on the surface, inactive crystallographic planes or atoms etc.