J . Phys. Chem. 1985, 89, 2493-2497
2493
Mechanism of Partial Oxidation of Methanol over MOO, As Studied by Temperature-Programmed Desorption William E. Farneth,* Fumio Ohuchi, Ralph H. Staley, Uma Chowdhry, and Arthur W. Sleight Central Research and Development Department, Experimental Station E356/231, E. I . du Pont de Nemours and Co.. Inc., Wilmington, Delaware 19898 (Received: November 19, 1984)
In this paper we demonstrate the utility of temperature-programmed desorption studies under high vacuum with simultaneous microbalance and mass spectrometry detection for obtaining detailed information about heterogeneous redox reactions. This method allows the chemisorption, reaction, and reoxidation stages of the mechanism to be separated in time and thereby examined independently. Changes in catalyst mass as a function of exposure, temperature, and sample history are followed along with mass spectral intensity profiles of desorbed gases. In the particular case of methanol over Moo3, it is shown that values for the number of active sites and the Arrhenius parameters for the rate-limiting C-H bond cleavage, both obtained by this method, can be combined to predict heterogeneous reaction rates in excelIent agreement with reactor data. A mechanism is proposed that is compatible with a wide range of experimental investigations of this reaction.
Introduction Bridging the gap between ultrahigh vacuum studies on model surfaces and chemical reaction studies on process catalysts is a challenge for surface science.' In order to make progress in this area, reactions must be examined over a range of conditions and by a combination of diagnostic methods. We have examined methanol oxidation over molybdate catalysts using a spectrum of techniques spanning pressures from UHV to several atmospheres and from single crystals to polycrystalline multicomponent catalysts.24 In this paper we report TPD (temperature-programmed desorption) and microbalance experiments' on this system. The complementarity of these results to previously described UPS, XPS,2 and reactor s t ~ d i e s ,will ~ , ~be demonstrated. A self-consistent mechanism incorporating information from all these sources is proposed. The balanced chemical reaction for methanol partial oxidation to formaldehyde can be written:
CH30H
+ '/202
+
CH2O
+ H20
The reaction is -36 kcal/mol exothermic at STP but requires a catalyst to proceed at a reasonable rate. One commercial catalyst is a mixture of MOO, and Fe2(Mo0,J3. Reactor studies have shown that the selectivities and rate laws for pure MOO, and the commercial catalyst mixture are ~ i m i l a r .These ~ kinetic studies have demonstrated that the catalytic process conforms to the general picture of partial oxidation first proposed by Mars and van Krevelen.5 This is a redox mechanism involving oxidation of the organic and simultaneous reduction of the catalyst. The reaction can be broken down into three elementary steps: (1) binding of alcohol at some surface site, A, (2) oxidation and desorption of the bound adsorbate with concurrent loss of oxygen from the catalyst, and (3) uptake of oxygen into the catalyst at site B with oxygen atom transport along the surface or through the lattice to replenish the vacancy caused by reaction at site A. Furthermore, the rate-limiting step for the reaction under process conditions has been shown to be cleavage of the CY C-H bond of methan01.~ The work reported here demonstrates that all three (1) Canning, N. D. S.; Madix, R. J. J . Phys. Chem. 1984,88, 2437 and reference therein. (2) Ohuchi, F.; Firment, L. E.; Chowdhry, U.; Ferretti, A. J. VUC.Sci. Technol. 1984, A2, 1022. Chowdhry, U.; Ferrehi, A,; Firment, L. E.; Machiels, c. J.; Ohvchi, F.; Sleight, A. w.; Staley, R. H. Appl. Surf. Sci. 1984, 19, 360. (3) Machiels, C. J.; Sleight, A. W. J . Card 1982, 76, 238. (4) Machiels, C. J.; Sleight, A. W. In "Proceedings of the 4th International Conference on the Chemistry and Uses pf Molybdenum, Golden, Co., 1982"; Barry, H.F.,Mitchell, P. C. H., Eds.; Climax Molybdenum Co.: Ann Arbor, MI, 1982, p 411. ( 5 ) Mars, R.; van Krevelen, D. W. Chem. Eng. Sci. 1954, 3, 41 (special supplement).
0022-3654/85/2089-2493$01.50/0
elementary steps in this reaction can be studied by a combination of microgravimetry and TPD. Experimental Section Adsorption data were obtained gravimetrically on a Cahn RG microbalance enclosed in a high-vacuum chamber. The system is pumped with a CTI-Cryotorr 7. Typical background pressures are 1 X lo-* torr. Gases can be admitted through an attached manifold and pressures during exposure cycles monitored with a Baratron 220 capacitance manometer. The system is capable of 1-rg resolution and typically 50-300-mg catalyst samples, in pressed pellets, were employed. The catalyst samples are placed on an aluminum weighing pan attached to the balance assembly via a hang-down wire. The catalyst chamber can be enclosed in a Kanthal furnace and heated at moderate rates with a VFI temperature programmer. Temperature-programmed desorption mass spectra were obtained in two ways. (1) The effluent gas stream from the microbalance chamber was monitored with a UTI 100 quadrupole residual gas analyzer. Heating rate was 5 OC/min, background pressure -1 X torr, and catalyst mass 50-300 mg. (2) A separate TPD apparatus employing a direct line-of-sight mass spectrometer and an external sample preparation chamber was also used. The mass spectrometer was differentially pumped with a CTI-Cryotorr 7. The mass spectrometer was a Leybold Heraeus Inficon IQ-200. Samples were translated from the preparation chamber into the TPD chamber with a linear sample manipulator through a gate valve. Within the TPD chamber, samples were raised vertically into a small oven, whose temperature could be ramped with a Data-trak 73-21 1 programmer. Heating rate was -0.5 "C/s. Background pressure was typically 1 X torr. The usual catalyst mass was -5 mg. In spite of the significant differences in the geometry and other conditions of the experiment, TPD data obtained by threse two methods were identical within the accuracy of the experiment. The latter method was employed for heating rate studies leading to activation parameters and the coverage dependence studies leading to the conclusion of first-order desorption. The former method was used for all adsorption data and for correlating product yields the catalyst mass changes. The MOO, powder used in this study was synthesized by decomposing molybdenum oxalate in air at 400 O C . The N 2 BET surface area of the powder was 10.2 m2/g and found to remain constant through the course of the experiment.
-
Results The general procedure in the experiments described herein is (1) mounting of the catalyst in the high-vacuum chamber of a modified Cahn RG microbalance and appropriate pretreatment and evacuation; (2) exposure of the catalyst to methanol for @ 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 12, 1985
2494
1
ao,
i
Farneth et al. TABLE I: Comparative Uptake of CH30H at Various Temperatures'
chemisorption, T, OC 25
B3
rg 124 4 72 f 4 -170 f 4
*
100 200
s 3
chemisorption,* no. of CH30H molecules/cm2 (9.6 f 0.3).X l O I 3 (5.6 f 0.3) X 10"
net wt. loss
(Ads/TPD), fig -30 f 2 -43 f 3 -280 f 5
'Catalyst mass 237.5 mg, 10.24 m2/g exposure 310 torr min. bAssume CH,OH adsorbs intact.
sf
,
I
._.
u
o
M
w
w
eo
no
(W
ZD
uo
zm
I
a00
Figure 1. Mass changes in Moo3 pellet during adsorption, TPD,and
reoxidation cycle. 133 0 1197 -1014
-
e11
-7 e k ? - 665 - 532F n - sse- 266
0
30
60
90
120
I50
180
210
240
270
500
TEMPERATURE (C)
Y
I 113
I5 00
Id
Id
w'
Figure 3. Mass spectral intensity profiles for CH30H ( m / e 32), CH20 ( m / e 30), and H20 ( m / e 18) during TPD (exposure = 2 X lo3langmuir at 25 "C). Intensities not corrected for mass spectral response factors.
Microbalancedata imply roughly equivalent weight losses in the a and
0peaks. Heating rate 0.37 K/s. Electron multiplier voltage = 2200 V .
chemisorption is 1.1 X 1014molecules/cm2. For reference, if the ~ o : u d R , ~ ~ L : ' ~ , M'~ - ~ ~ o ~ ~ ~ ~ ~ surface were entirely (010) face of MOO,, there would be 7.0 X
Figure 2. Methanol chemisorption vs. exposure at 25 OC on MOO,.
varying times and/or pressures and/or temperatures and gravimetric measurement of the uptake; (3) temperature-programmed desorption at a linear programming rate (generally 5 "C/min to 400 "C) and continuous monitoring of mass changes in the catalyst and mass spectral intensities of the effluent gases; (4) reoxidation in O2 at 400 "C while following changes in catalyst mass. This procedure has been applied many times to MOO, pellets of varying masses. Most work has been carried out on a single catalyst pellet of -200 mg with excellent reproducibility. For each cycle there is a mass gain during adsorption, a net mass loss during TPD, and a return to the original mass following reoxidation. This sequence of mass changes is shown schematically in Figure 1. In the following paragraphs, each leg of the cycle will be considered independently and its variation with experimental parameters described. Figure 2 (right-hand ordinate) shows mass changes observed for 166 mg of Moo3 as a function of exposure to CH30H at 25 OC. Exposure was varied both by changing pressure at constant time and vice versa. For example, points at approximately 1O'O langmuir on Figure 2 were generated at both 10 torr/20 min and 1 torr/200 min exposures. Following exposure, the system pressure is reduced to
