Langmuir 1991, 7, 13-15
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Partial Electrochemical Oxidation of Methane under Mild Conditions Karl W. Frese, Jr. Interfacial Sciences, Inc., 2362 Walsh Avenue, Santa Clara, California 95051 Received June 7, 1990. I n Final Form: October 24, 1990 We have made a preliminary study of the partial oxidation of methane at 25 "C in aqueous media. The reactions were performed by using gold, glassy carbon, Cu and Hg cathodes in an alkaline electrolyte containing dissolved 0 2 . The main oxidationproducts under these new reaction conditionsare formaldehyde and methanol. The rates of formaldehyde formation increased with cathodic current density. The CH20 formation rate at a Cu electrode at 50 mA cm-2 was 1.5 X 10-3 mol cm-2 h-* during the first 18 min of the reaction. The CH2O concentration oscillated with time, and CO and COZ were usually minor reaction products. The oxygen species, long-lived 02-, is suggested to play a role in CHI activation. Introduction Partial oxidation of methane is a topic of great interest to the gas and petroleum industries. Transformation of' abundant supplies of methane to useful chemicals, for example, CHzO, CH30H, and CzH4, is desired. We have made a preliminary study' of the partial oxidation of methane a t 25 "C in basic aqueous media. A somewhat related and independent approach has been recently published2 in which H2Oz is generated in the presence of Fe2+/Fe3+in acid. Well-known Fenton chemistry would lead to the formation of a significant quantity of OH radicals that could easily oxidize methane. Since OH is the most reactive oxidant that one could envision in this system, we believe its purposeful generation should be avoided to minimize destruction of products. The reactions were performed by using gold, glassy carbon, Cu, and Hg cathodes in an electrolyte containing dissolved 0 2 . Carbon, Hg, and Au were chosen as electrode materials because oxygen reduction to peroxide3 is known to occur with high efficiency on these surfaces under appropriate (low) overpotentials. The pathway to p e r ~ x i d eor, ~more accurately to OPH-,includes the species 0 2 - , in basic electrolyte. The stability of this intermediate is discussed below. Cu was chosen because it is an effective catalyst4 for partial oxidation of methane by 0 2 at the gas/solid interface. In the electrochemical systems we have heretofore studied, partial oxidation is not well-known. We have shown5 that some CO (5-10% yield) is formed along with CO2 (the other major product) when methane oxidation is carried out at low anodic overpotentials on Rh electrodes in 140 "C, 85% H3P04. CO2 is the exclusive product at Pt electrodes under similar fuel cell conditions. In basic solutions, reduced oxygen can exist in the forms 0 2 - and HOz-. These species may also be adsorbed on the electrode surface. We were prompted to test the validity of methane oxidation with electrochemically generated reduced oxygen by analogy to the know activity of reduced oxygen sites (1) (a) Frese, K. W., Jr. Electrochemistry of Small Carbon-Based Molecules. Quarterly Report No. l., Sept 1989 for The Gas Research Institute; CRI Contract No. 5083-260-0922. (b) Frese, K. W., Jr. These results were presented at the GRI Contractors' Inorganic Chemistry Review Meeting October 4-5, 1989, Denver, CO. (2) (a) Cooke, R. L.; Sammells, A. F. J. Electrochem. SOC.1990,137, 2007. (b) Cooke, R. L.; MacDuff, R. C.;Sammells,A. F. GRI Contractors' Inorganic Chemistry Review Meeting October 4-5, 1989, Denver, CO. (3) Yeager, E. J . Mol. Catal. 1986,5, 38. (4) Cesser, H. D.; Hunter, N. R.; Prakash, C. R. Chem. Reus. 1985,85, 235. (5) Summers, D. P.;Pound,B. G.;Frese,K. W., Jr. The Electrochemical Oxidation of Methane at Metal and Oxide Electrodes; Annual Report, Jan 1989; The Gas Research Institute: Chicago, IL, 1989.
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such as 0-, as discussed by Lundsford and co-workersqe We also found a positive correlation' between the rate of methane activation and the Sanderson partial negative charge on oxygen in oxides. Adding the electron to 0 2 by electrochemical means serves to both activate the 0 2 by creating a radical anion and, at the same time, partially satisfy the desire of 0 atoms to behave as two equivalent oxidizing agents. We might expect that partially reduced 0 2 may be a better choice for partial oxidation of CH4 than 0 , d or perhaps OH,,. The main oxidation product under these new reaction conditions is formaldehyde, although methanol could be obtained from formaldehyde by two-electron reduction. Indeed methanol has been observed in some experiments. It is important to be aware that, although the current is cathodic, a t the electrode potentials required to initiate 0 2 reduction, 11.0Vvs HESS, formaldehyde and methanol could be oxidized to CO or COz. Homogeneous pathways involving oxygen species can also contribute to formation and destruction of the desired products. The latter has been a major obstacle in high temperature schemes (200600 "C) of methane partial oxidation at the gas solid interface. A major advantage of our approach is that a low temperature (25 "C) is used, which greatly helps to overcome the problem of complete oxidation of CH4. Our observations of CHzO and CH30H formation (see below) suggest that deep electrochemical oxidation may be controlled even in batch processing of CH4. An unusual behavior that signals some reaction of CH2O in our system is the observation of oscillations in the concentration with time. Experimental Section The electrodes were a Cu foil (1 cm2),Au electroplate (3 cm2 geometrical area), reticulated glassy carbon (3 cm3 geometrical area), and Hg plated on a Cu screen (3 cm2 geometrical area). The Cu foil was cleaned by dipping in 10%HCl for 30 s to remove the oxide layer. The Au and Hg surfaces were fresh electroplates formed from acidified (HNOs) chlorides and were used without further pretreatment. The glassy carbon was rinsed with highpurity water before use. Methane, carbon monoxide, and carbon dioxide were determined by flame ionization detection gas chromatography using Porapak R and N. Formaldehyde and methanol were determined by thermal conductivity gas chromatography using APS 201 on T-port F column packing at 95-110 "C. The injector was operated
at 150 "C. The Pyrex electrolysiscell was two compartments with a fine glass frit separator. The total volume of the cell including head (6) Driscoll, D. J.; Campbell, K. D.; Lundsford, J. H. In Adu. Catal. 1987, 35, 139.
0 1991 American Chemical Society
Letters
14 Langmuir, Vol. 7,No. 1, 1991
Table I. Rates of CHI Partial Oxidation Products' in mol cm" h-* electrode
Hg/Cu
Au plate carbon Au plate Cu foil
electrolyte 2 M KOH 0.1 M NaClOlb 2 M NaOH 2 M KOH 0.01 M KOH
CHzO 3.4X 10" (0.36) 1.1 x 10-5 (1.2) 2 x 104 (0.2) 9.6 X 10" (1.0) 1.5 X (160)
CH30H 2.3 x 10-7 (0.012) 1.7 x 10-5 (0.91) 1.4X (0.75)
co
coz
12, where the lifetime of 0 2 - is longest.
Acknowledgment. The support of the Gas Research Institute is gratefully acknowledged.
