3001
NOTES
Nitrogen Adsorption on Iridium and Rhodium’ by V. J. Mimeault and Robert S. Hansen Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa (Received February $4, 1966)
According to Bond,2 nitrogen is not chemisorbed on group VIII-B metals. We have found this to be the case for iridium and rhodium by the absence of a pressure increase on heating a sample filament (Ir or Rh) dosed with nitrogen a t 300°K. However, we have found some evidence that nitrogen adsorption does occur on iridium and rhodium if the nitrogen is thermally activated. The source of this thermal energy is the hot tungsten filaments present in conventional ionization gauges. This note is a summary of our findings. The vacuum system and experimental procedure are described elsewhere. Nitrogen gas was leaked into the vacuum system until the steady-state pressure was 4 x lo-’ torr, On flashing the filament, either iridium or rhodium, the total pressure increase was measured with a Bayard-Alpert ionization gauge containing a thoriated tungsten filament operating a t 0.4-ma emission, and the partial pressure increase was measured with a small bakeable mass spectrometer. The residual gases present in the background were less than 0.5% of the total nitrogen pressure. It was observed that under these conditions, no adsorption of nitrogen occurred as indicated by the absence of a pressure increase on flashing after dosing the filament for 15 min a t 300°K. At this pressure (4 X 10-7 torr), enough molecules strike the surface to form a monolayer in -6 sec (if the sticking probability is unity). Since the flash filament technique is capable of detecting 0.02 monolayer, the absence of a pressure increase on flashing implies thrtt the amount of adsorption was negligible. Nitrogen adsorption can be made to occur on iridium or rhodium if the tungsten filaments in the Nottingham ion gauge4 are operated at a temperature Ti greater than 2000°K. Since no direct line of sight exists between the tungsten filaments and the iridium or rhodium filament, a particle must make many collisions with the glass walls of the vacuum system in traveling from the tungsten filaments to the sample filament. The amount of nitrogen desorbed is proportional to the dosing interval for fixed Ti, and the proportionality constant increases strongly with T f (Figure 1). The adsorbing iridium or rhodium filament was held a t
300°K during dosing. A plot of the log of the amount of nitrogen desorbed for fixed dosing period (this amount is proportional to the adsorption rate constant) against 1/Tt is linear as shown in Figure 2. The slope of this plot corresponds to an activation energy of 58 kcal/mole, which is approximately one-fourth the dissociation energy of molecular nitrogen (226 kcal/mole) . 1
I
1
1
NITROGEN DESOWTION FROM IRIDIUM
1.5
-
Bp x 107mm
TIME (mid
Figure 1. Desorption of nitrogen from iridium as a function of dosing time and temperature of the ion gauge filament.
During the long adsorption time, there was appreciable adsorption of CO from the ambient background impurities. The low-temperature (short time) mass 28 peak in Figure 3 is due to carbon monoxide, as is shown by the coincident peak in the mass 12 fragment. Figure 3 also shows that the high-temperature peak consists of 14N2, 14N15N, and I5X2 in substantially statistical ratio although the dosing mixture contained only I4N2 and 1W2. Hence, substantially complete scrambling of isotopes has occurred in the adsorption-desorption process. This strongly indicates that the nitrogen has been adsorbed as independent atoms mobile on the surface a t a temperature below the desorption temperature. The high desorption temperature (-1000°K) indicates strong binding of the nitrogen to the surface which is also consistent with atomic adsorption. The adsorption of nitrogen on iridium and rhodium can be explained by a model based on the dissociation (1) Work was performed a t the Ames Laboratory of the U. 5. Atomic Energy Commission. Contribution No. 1862. (2) G. C. Bond, “Catalysis by Metals,” Academic Press, New York, N. Y., 1962. (3) V. J. Mimeault and R. S. Hansen, J . Chem. Phys., in press. (4) W. B. Nottingham, Natl. Symp. Vacuum Technol. Trans., 1 , 76 (1964).
Volume 70, Number 9 September 1966
NOTES
3002
of molecular nitrogen on the hot tungsten filament, with the atomic nitrogen produced largely recombined catalytically on the glass walls so that a steady-state atomic nitrogen concentration is maintained by the balance of thermal dissociation and catalytic recombination. The rate of adsorption on the rhodium or iridium filament is then proportional to the steady-state concentration of atomic nitrogen. Let N,, N,, and N G be the nitrogen atom concentrations (volume or surface as appropriate) in the gas phase and on the tungsten and glass surfaces, respectively, and let Nz(g) be the gas phase concentration of molecular nitrogen. The model is summarized in eq 1-3 K
'k
N , +N ,
'/2N2(g)
(thermal dissociation)
(1)
K'
Np
NG (adsorption)
k"
2Nc + N2(g) (catalytic recombination)
1 4.5
(2) (3)
plus the steady-state assumption that N g is produced by reaction 1 and consumed by reactions 2 and 3 at the same rate. The rate of production of atomic nitrogen due to the hot filament is
5.0
LCL~(.~,-I T Figure 2. Dependence of the rate of adsorption of activated nitrogen on iridium on the temperature of the activating source.
*
where AH is the effective activation energy and A is the preexponential term for the over-all reaction 1; presuming the transition state for the second reaction is very nearly desorbed atomic nitrogen, as seems likely, A H I * = 113 kcal, ie., half the dissociation energy of nitrogen. From reactions 2 and 3 we obtain for the rate of recombination of atomic nitrogen on the glass
-(2)>, =
k"No2
=
k"(K'Ng)2
(5)
For steady state, the production of atomic nitrogen according to eq 4 must be balanced by its consumption according to eq 5, Le.
ICPN,'" = kf'(K'Ng)2
(6)
or
10
20
30
40
TIME (SECI
Figure 3. Desorption of nitrogen isotopes from rhodium dosed a t 300°K with a mixture of 14Npand I6N2. The curves are translated vertically for clarity. The arrows in each case relate the curve to the appropriate scale.
The Journal of Physical Chemistry
The rate of adsorption is assumed proportional to N,. Of the rate constants on the right side of eq 7, only k changes as T fis varied. The effective activation energy for atomic nitrogen adsorption should hence be half
NOTES
3003
of these results, it is important to consider factors such as differences in the crystallite size and reducibility of the nickel in the catalysts. Useful information of this type can be obtained from magnetic Therefore, it was decided to obtain magnetic data on these particular catalysts, which include nickel supported on alumina, silica, and silica-alumina at concentrations of 1 and 10% nickel. I n this report, the catalytic properties of these samples are considered in the light of the information obtained from the magnetic studies.
1’
Properties of Nickel
Experimental Section Apparatus and Procedure. The Faraday method was used to determine the magnetic properties of the supported nickel catalysts. The apparatus is similar to others described in the l i t e r a t ~ r e . ~ The ~ ’ ~ sample, in a quartz bucket, was suspended from a Cahn electrobalance which was used to measure the force on the sample. A Varian 4-in. magnet with “constant gradient” pole pieces and a 2-in. gap provided magnetic fields up to 6500 gauss. The vacuum system was arranged so that samples could be reduced in situ in flowing hydrogen. Pressures of low6 torr were readily attainable in the apparatus. The standard procedure included reduction of the catalyst overnight at 370” in a hydrogen flow of 500 cc/min. The sample was then outgassed at the same temperature for 30 min. The magnetic measurements were made either in vacuo or in helium. If the measurements were made in vacuo, the approach to liquid nitrogen temperature was very slow, with a temperature of 85 to 90°K being the practical lower limit. In most cases a small amount of helium was added to facilitate the cooling to 80°K. The catalysts used in these studies have been described previously.’!
by J. L. Carter and J. H. Sinfelt
Results Typical data on t h e effect of field strength on specific
I 19
I 20
I 22
I
21 X
I
I
2400
23
lo-’ ( *K)
Figure 4. Equilibrium distribution of the nitrogen isotopes in the ambient of a flow system as a function of the temperature of the ion gauge filament.
that of IC or 56.5 kcal, which compares well with the 58 kcal observed. The ambient distribution of nitrogen isotopes is shown as a function of ion gauge filament temperature in Figure 4. The increase in 14N15Nand decrease in 14N2 and lSN2(the input gases) above 2100°K is apparent; part of the decrease is attributed to pumping by the glass walls.
Catalysis over Supported Metals. VI.
The
Application of Magnetic Studies in the Interpretation of the Catalytic
Esao Research and Enuineering Company, Linden, New Jersey (Received March 8, 1966)
I n previously reported studies from this laboratory, it was shown that the catalytic activity of nickel for ethane hydrogenolysis varied markedly when the nickel was supported on different oxide carriers.’I2 From hydrogen chemisorption measurements it was concluded that the variations in nickel surface areas on the different supports were small compared to the variations in catalytic activity. In other words, the specific catalytic activity of the nickel varied with the support. It was also shown that the specific catalytic On a given support varied activity Of with the nickel concentration. In the interpretation
(1) W. F. Taylor, D. J. C. Yates, and J. H. Sinfelt, J . Phys. Chem., 68, 2962 (1964). (2) W. F. Taylor, J. H. Sinfelt, and D. J. C. Yates, ibid.,69, 3857 (1965). (3) P. W. Selwood in “Catalysis,” Vol. I, Reinhold Publishing Corp., New York, N. Y., 1954, p 353. (4) P. W. Selwood, “Adsorption and Collective Paramagnetism,” Academic Press Inc., New York, N. Y., 1962. ( 5 ) P. W. Selwood, T. R. Phillips, and S. Adler, J . Am. Chem. SOC., 76, 2281 (1954). (6) P. W. Selwood, S. Adler, and T. R. Phillips, ibid., 77, 1462 (1955). (7) P. W. Selwood, ibid., 78, 3893 (1956). (8) D. Reinen and P. W. Selwood, J . Catdyaia, 2 , 109 (1963). (9) P. W. Selwood, “Magnetochemistry,” 2nd ed, Interscience Publishers, Inc., New York, N. Y., 1956, pp 11-13. (10) P. E. Jacobson and P. W. Selwood, J . Am. Chem. Soc., 7 6 , 2641 (1954).
Volume 70, Number 9 September 1968
