COMMUNICATIONS TO THE EDITOR
1179
Langmuir-Hinshelwood Mechanism as Revealed by Thermal Desorption Study. The Catalytic Decomposition of Methyl Formate on Copper Surface
Xir: Recently the thermal desorption method (tdm) has been applied to the study of adsorbed species and it has been found that various adsorbed states show different reactivities. However, these species observed by tdm are not necessarily identical with those of real intermediates during reactions catalyzed by solid surfaces. I n the present work we have applied this method to the study of the decomposition of methyl formate over copper surface.2 We have succeeded in revealing the details of this reaction which obeys the LangmuirHinshelwood mechanism by comparing the kinetic parameters determined by tdm with those obtained from conventional kinetics. The study of decomposition kinetics was carried out in a pressure range 5-25 mm a t temperatures between 100 and 170". The copper catalyst was prepared from cupric hydroxide by reducing with hydrogen a t 180" and was evacuated at 300". The products were found to be CO, H,, and traces of COz. The initial rate TO is expressed as ro
=
The study on thermal desorption was carried out on the same catalyst as was used for the kinetic study. After a known amount of methyl formate was adsorbed on the catalyst, the temperature of the catalyst was raised a t various rates (p) under evacuation of a constant speed (S). The pressure change due to desorbed gas was followed by means of a Pirani gauge. The composition of the gas phase was continuously analyzed by a mass spectrometer which was directly connected to the system. Large values of the S / p ratio were chosen to hold the condition under which the partial pressures of desorbed species are proportional to their respective rates of desorption.3 The desorption spectra shown in Figure 2 exhibit that there are several states in adsorbed methyl formate; since CO and Hz were generated simultaneously in peak B, adsorbed methyl formate corresponding to this peak is likely responsible to the decomposition which gives CO and H,.
+ KP)
kKP/(l
where P is the initial pressure of methyl formate and k and K are constants. From the temperature dependences shown in Figure 1, k and K are found to be 5.4 X loz7exp(-27,200/RT) molecules ern+ sec-l and 5.4 X 10-lO exp(14,300/RT) mm-l, respectively. When adsorption equilibrium is established for the reactant, it follows that K corresponds to the equilibrium constant, and the heat of "reactive" adsorption, &, is 14.3 kcal/mol. Accordingly, the constant k has the physical meaning of the rate constant of surface reaction with the activation energy Zr of 27.2 kcal/mol.
I
1
50
150 *C
100 Temperature
Figure 2. The desorption spectra of methyl formate (6 = 0.13 deg sec-1).
Both results of kinetics and of separated studies on thermal desorption of CO and Hz adsorbed on Cu show that the desorption rates of CO and Hz are much larger than that, of decomposition of adsorbed methyl formate. Therefore on raising the temperature, methyl formate adsorbed at room temperature is supposed to change in the following way HCOOCH,(g)
+HCOOCH3(a) --t kd
kr
CO(g)
+ Hz(g)
where k d and k, are the rate constants of the desorption and of the surface reaction, respectively. They are expressed in the forms k d = Vd exp(-Ed/RT) and k, = v, exp(-&/RT), where v's and E's are frequency factors and activation energies, respectively. When the adsorption of methyl formate is in a partial equilibrium ( k d >> kr), the following equations hold a t temperatures 'e2-
i3
I
I
2.4 I/T
x
I
2.5
103
Figure 1 . Temperature dependence of k and K .
(1) Y. Amenomiya and R. J. Cvetanovic, J. Catal., 9, 28 (1967). (2) E. Miyazaki and I. Yasumori, Bull. Chem. Soc. Jap., 40, 2012 (1967). U ~ 57, 641 (1961). (3) P. A. Redhead, Trans. F U T U ~Soc., Volume 73,Number 4 April 1969
COMMUNICATIONS TO THE EDITOR
1180
Table I
6.0 -
- 6.0
9
T
cn
0)
0
0
s
1
0
N
E,, kcal/mol Q or Ed, kcal/mol
Kinetics
Thermal desorption
2 7 . 2 =t 0 . 6 14.3 f 1.0
27.8 =t 1 . 3 14.8 f 0 . 7 (eq 1) 14.6 =k 1 . 0 (eq 2) 1 . 6 X lo6
L
I
- 5.8 f
5.8-
8
Ud,
...
sec-l
ur, molecules cmm2sec-1
5 . 4 x 1027
...
d
N
5.6
- 5.6
-
2.4
2.6
2.5
i/-rRor
(
~ 1 0 ~ 1
Figure 3. Plots of eq 1 and 2 for peak B.
TRand Tp, which correspond to peak maxima of methyl formate and carbon monoxide, respectively. * 2 log T R - log p
=
Ed/RTR -k log (Ed/VdR) ( 1 )
2 log TP - log fi
=
Ed/RTp f log (E,/VdR) ( 2 )
By measuring TR and Tp for various p(0.13 - 0.31°/ sec), the left-hand side of eq 1 (or 2) is plotted against ~/TR (or l / T p ) , as shown in Figure 3. From the slopes and the intercepts of these plots Ed, E,, and V d were estimated, and these values are summarized in Table I. There is good agreement with corresponding values in
The Journal of Physical Chemistry
the reaction scheme assumed above. The coincidence of a Q value with those of Ed suggests that the activation energy of adsorption for methyl formate is negligible. It is therefore concluded that the adsorbed state corresponding to peak B is the true intermediate in the catalytic decomposition of methyl formate to CO and Hz and that the surface reaction is the rate-determining step. Details and more general discussions for the case where the adsorption equilibrium is not established will be published later. (4) Reference 3, p 655.
DEPARTMENT O F CHEXISTRY TOKYO INSTITUTE OF TECHNOLOGY OOKAYAMA, MEGURO, TOKYO, JAPAN RECEIVED JUNE7, 1968
NAOHIRO MOMMA IWAOYASUMORI
