A. CIMINOAND G. PARRAVANO
706
\'VI.
66
PRODUCTION OF NICKEL CARBIDE BY DECOR.IYOSITION OF 12-HEXANE ON METALLIC NICKEL BY A. CIMINO~ A N D G. PARRAVANO' Deparlttient of Chernistiy, Universily of Rome, Rome, Italy Received J u l y Mt 1961
In receut years the formation and properties of metal carbides have been extensively studied.2 We wish to report on some data on the production of nickel carbide (Ni3C) by decomposition of n-hexane a t nickel surfaces. Experimental ~ O \ Y temperature
Essentially the method of Bahr and Bahra and Hofer and Peebles4 was used to follow the progress of the carburization process, the subsequent hydrogenation of the solid product, and for X-ray analysis of the solid phase formed. A stream of pure nitrogen was passed through n-hexane, kept at. constant temperature ( 5 0 . 5 ' ) and then through a powdered active nickel (ex-nitrate) which was loaded in a Bahr-type reaction vessel. In a run three or four vessels were fitted in an heated aluminum block (It 2'). One vessel was used to follow the reaction gravimetrically, a second in order to obtain data on t,he hydrogenation of the product, and a t8hird for X-ray analysis. This was performed with CuK, radiation (Ni filter) and a camera of 57 mm. in diameter, calibrated with pure sodium chloride (Rlerclr and Co., Inc.).
Results Some typical results are preselltetl ill Fig. 1 alld all the data are summarized in Tables I alld 11, TABLE I CARBURIZATION A N D HYDROGENATION OF NICKEL Reaction with n-hexane Hydrogenation Temp., Ctot., Temp., Ccnrb., Sample " C . Hours % "C. Hours % A1 250 109 1.43 275 220 1.12 A2 250 109 1.37 275 20 1.12 A3 250 109 1.37 . B1 270 77 3.55a 270 45 1.74 B2 270 77 3.09' .. .. .. C1 293 90 7.19 .. .. . C2 293 90 5.99 293 52 3.50 6.38 293 56 3.50 C3 293 90 D1 327 92 21.04 299 88 3.16 D2 327 92 22.54 299 130 3.16 D3 327 92 17.55
..
.
.
..
..
,.
After treatment a t 361' for 27 hours total carbon: B1 17.18%; B2 11.98%. a
6L t
TABLE I1 X-RAYANALYSISFOR SAMPLE C1
5
Ni dln obsd., A.
NilC d/n obFd., A.
NiaC d/n oaltd.,a A.
VS
2.030
2.291 2.165 2.030
2.295 2.165 2.027
S
1.7GO 1.572 1.325
1.575 1.325
1.221 1.132 1.109
1.221 1.130 1.100
0.984 .go1
0.979
.870 :850 .840
.8G7 .852 ,838
Intensity W W
in ni S
1.247
m 20
60 80 100 120 140 l 6 U Hours. Fig. la.-Carburi~atioii-hydl.ogenatioii of nickel: 0,t = 250"; Q Q, t = 293'.
40
111
w S
w w W
\V
in w
16
& a 12
111
6
a
8
4 80 100 120 140 160 Hours. Fig. 1b.-Carburization-hydrogenation of nickel: 1 = 32i '. 40
60
(1) Departinent of Cheiuistry, Princeton University, Princeton, N. J. (2) Hofer, Cohn and Peebles, THIS JOURNAL,54, I l G l (1050). (3) Balir and Bahr, Ber.. 61, 8177 (lYP8). (4) Hofer and Peebles, J . Am. Chem. Soo., 69, 893 (1U47).
,898
0.885
.812 .790 For (c = 255, b = 4.33.
111
(kkl)
(100) (002) (111)Ni(lol)Ni8C (200) . (102) (110) (220)
1.OM 1.021
111
24
20
..
X-Ray analysis Ni Ni Ni, NisC Ni Ni, NiaC Ni. NisC Ni Ni Ni Ni Ni, NiaC
(103) (112) (201) (311) ( 222 ) (104) (203) (400) (120) (121) (114) (331) (420)
Sample D2 was hydrogenated after treatment at 299" for 113 hours in an atmosphere of nitrogen. In one run a carburized sample was kept at room temperature for three months before hydrogenation. The content of carbide carbon of the sample remained unchanged. The following conclusions can be drawn from these results: (1) in the range of temperature investigated n-hexane is decomposed a t nickel surfaces; (2) the resulting solid phase is identified as nickel carbide, NiaC, and elemental carbon. As Balir a.nd Hvfer have already s1iow.n the differ-
June, 1952
KINETICS OF
C H A I N DEPOLYMMRIZATION
entiation of these two types of carbon products can be obtained upon treatment of the carburized phase with hydrogen; (3) under the present experimental conditions the decomposition of nickel carbide is not readily observed up to 300". This is in order with previous findings of Bahr3 and more recently of Hofer.2 It can be noticed that the amount of carbidic carbon, which is removed upon hydrogenation, is almost equal to the amount deposited during the first rapid stage of the carburization process. Further proof of this observation was obtained by treatment with hydrogen of a sample carburized for 23 hours at 291" (these conditions correspond to the first rapid stage of the process, see Fig. 1). It was found that the total amount of carbon deposited was 3.3% and carbidic carbon 3.1%, which is 95% of the total carbon deposited. The data of the first rapid stage of the carburization process can be fairly well fitted to a parabolic type 6f plot (Fig. 2), indicating that a diffusion process might be determining the rate of the overall process. From this plot an activation energy of 23.0 kcal./mole was derived. We conclude that,
X
10 15 20 Hours. Fig. 2.-Weight increase per unit weight of nickel ( A m / g ) during carburizatitn with n-hesane; parabolicjdot: 0, 1 = 250"; g,i! = 270 ; 0 , t = 293"; 0, t = 327
5
.
in the present case, carbide formatlion occurs through the following steps: (a) dissociative adsorption of n-hexane at nickel surfaces, and production of atomic carbon, (b) reaction to nickel carbide and material transport by diffusion through it. Acknowledgment.-We thank Dr. L. J. E. Hofer for reading the manuscript before publication.
KINETICS OF CHAIN DEPOLYMERIZATION1 BY ROBERT SIMHA AND L. A. WALL Ueparlnient of Chemical Engineering, New York University and National Bureau o j Standards, Washington, U . C. Received Julu 2 4 , I951
,
In the general theory of chain depolymerization reactions two parameters are important: the kinetic chain length 1/e uiid the transfer constant u. In terms of these the moleculsr weight distribution, average molecular weights and rates OF decomposition can be discussed. These rates decrease monotonously with conversion when the kinetic chain length is large in comparison with the D.P., regardless of the initiation mechanism. When it is small, there is a maximum rate, if the initiation is random or transfer is pronounced. If initiation is occurring only a t the chain ends, the reaction becomes of zero order. Although the general theory for any value of the kinetic chain length has been developed, numerical eolutions arc YO far available only for extreme cases. However, the initial rate has been given for all conditions. The theory is compared with experimental results on molecular weight change and rate of degradation for polystyrene and polymethylmethacrylate, respectively. In the former instance, an estimate of the kinetic chain length is made from the observed monomer yields. In the latter, e and the rate constant for initiation are obtained from molecular weight and rate curves. The observed molecular weight decrease in methyl methacrylate is more pronounced for the larger molecular weights and higher degrees of conversion, than the theory neglecting chain transfer would predict. This has been tentatively ascribed to chain transfer becoming more important a t later stages of the reaction. Thus, it would appear that the general state of affairs in thermal depolymerization of addition polymers is accounted for.2 However, discrepancies remain which can a t least qualitatively be explained in the framework of the general theory. Their quantitative interpretation requires a refinement of the reaction mechanism considered, in particular the initiation step, and further numerical evaluation of the rate equations. The theory was developed in order to interpret the thermal and photodecomposition of chain polymers. There are degradative processes which exhibit a striking similarity to the phenomena discussed here, a1though the detailed chemistry is, of course, entirely different. In the deterioration of textiles, some agents attack the fabric in such a way, as to leave the tensile strength of the residue unimpaired, while others produce a gradual decay in the tensile properties. Also, the breakdown of proteins by proteolytic enzymes seems to occur principally in two ways, one leaving intact material and small species, the other producing niolecules of intermediate sizes.s A kinetic description of these reactions could be similar to ours, which involves, in onc cxtreme a slow process, followed by more rapid ones, and in the other a single, moderately rapid step.
In recent years interest in the quantitative aspects of thermal depolymerization processes has I)een catalyzed by experimental investigations in this country and abroad.' The results obtained on a variety of polymers indicate .wide differences in respect to monomer yields, changes in molecular (1) Paper presented in part a t the Symposiu~non Degradation of R.Iacronioleeules. 119th Meeting of the American Chemical Society, Boston, Mass., Aprll 1-5, 1951. (2) For a more detailed analysis of experimental data see R. Simha, T m n s . iV. Y . Aead. Sei., 14, 151 (1952). (3) See the lecture imsented by IC. U. Linderstr@m-Langat tho 12th Iiitcrnational Congrcss of 1'1iru and .41)111icdClleillistry, SelJtcIllber Y-13, 1951, New York, N. T.
weight and rates of decomposition. Sometime ago a general theory was developed by the authors in collaboration with BlatzJ4a~b which permitted one to reduce these divergent results to a common basis. It is based on the concept of the degradation process as an inverse chain polymerization, characterized by initiation, propagation, chain transfer and termination acts. Previous theories6 had been built on the premise of a step reaction, special cases of which are the analog of a poly( 4 ) (a)
R. Sinilia, L. A . Wall a n d P. J. Blatz, J . Polurner Sei., 5 ,
G15 (1080); (b) R. Siitilia and L. A . Wall, ibid., 6 , 30 (1951). ( 5 ) H. Siiiilia, J . Appl. P h g s . , 12, 5CiU (IY41).
