To recycle ethylene glycol (HOCH2CH2OH) fuel in alkaline fuel cells, active and selective catalysts for partially oxidizing HOCH2CH2OH to glycolic acid (HOCH2COOH) and oxalic acid ((COOH)2) are required at the anode; in other words, complete oxidation of HOCH2CH2OH to CO2 prevents ethylene glycol recycling. We investigate catalyst activity and selectivity for oxidizing HOCH2CH2OH to HOCH2COOH on Fe(0 0 1), Co(0 001), and Ni(1 1 1) via first-principles calculations. We calculate the oxidation reaction path from HOCH2CH2OH to HOCH2COOH without C–C bond dissociation to avoid CO2 generation. Partial oxidation of HOCH2CH2OH to HOCH2COOH without C–C bond dissociation proceeds as follows: O–H bond dissociation of HOCH2CH2OH to generate HOCH2CH2O; C–H bond dissociation of HOCH2CH2O to generate HOCH2CHO; C–H bond dissociation of HOCH2CHO to generate HOCH2CO; and OH addition to HOCH2CO to generate HOCH2COOH. The activation energies for O–H bond dissociation of HOCH2CH2OH and C–H bond dissociation of HOCH2CH2O and HOCH2CHO on Fe(0 0 1) are 20.2, 22.8, and 35.2 kcal/mol, respectively, which are the lowest of the three surfaces. Thus, Fe(0 0 1) is most active. To determine the selectivity, we compare the bond dissociation activation energies. The activation energies for C–C bond dissociation of HOCH2CH2OH and HOCH2CH2O on Fe(0 0 1) (66.7 and 39.5 kcal/mol, respectively) are higher than those for O–H bond dissociation of HOCH2CH2OH (20.2 kcal/mol) and C–H bond dissociation of HOCH2CH2O (22.8 kcal/mol), implying that the O–H bond of HOCH2CH2OH and C–H bond of HOCH2CH2O dissociate before the C–C bond dissociation during oxidation on Fe(0 0 1). In contrast, the activation energies for C–H and C–C bond dissociation of HOCH2CHO (35.2 and 32.8 kcal/mol, respectively) are similar. The C–H and C–C bonds therefore dissociate during HOCH2CHO oxidation. On Co(0 0 0 1) and Ni(1 1 1), the activation energies for C–C bond dissociation of HOCH2CH2O and HOCH2CHO are lower than those for their C–H bond dissociation. Therefore, Fe(0 0 1) is more active and selective than Co(0 0 0 1) and Ni(1 1 1) for the partial oxidation of HOCH2CH2OH to HOCH2COOH.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry