A First-Principles Comparison of Heterogeneous and Electrocatalytic Alcohol Oxidation Over Au and Pt Catalysts

A First-Principles Comparison of Heterogeneous and Electrocatalytic Alcohol Oxidation over Au and Pt Catalysts

David D. Hibbitts, Bhushan Zope, Robert J. Davis and Matthew Neurock

Abstract

In the emerging area of biorenewable-based chemicals, most processes are low-temperature, aqueous phase reactions, in stark contrast to the high-temperature, gas phase processes that dominate the current petroleum-based chemicals industry. The shift of environments allows for much-needed synergy in the areas of heterogeneous and electro-catalysis, a field which has already demonstrated the importance of the metal-solvent interface. One area in particular which has demonstrated similarities between electro- catalysis and aqueous-phase heterogeneous catalysis is the oxidation of CO and alcohols.

Recent work¹ on the alcohol oxidation over Au and Pt has demonstrated that the oxidation occurs through direct reaction with hydroxyl groups from the alkaline medium, summarized in Figure 1. These hydroxyl groups act as both nucleophiles and Brønsted bases in the selective oxidation of ethanol to acetic acid and isotopic labeling studies have confirmed the source of oxygen in the acid products as the solvent, rather than the gaseous O₂. The O₂ is still necessary for the reaction to proceed, as it must be reduced to reform the consumed hydroxyl groups and water. This reduction also removes excess electrons from the metal and closes the catalytic cycle. Writing the oxidation and reduction separately gives the following half-reactions:

RCH₂O₂ + 4¹⁸OH^- --> RCO¹⁸OH + 3H₂¹⁸O + 4e^-                                                                                  (Eq 1)

(1+x/2)O₂ + (2+x)H₂O + 4e^- --> xHOOH + 4OH^-                                                                                  (Eq 2)

As shown in Equation 2, a byproduct of this oxidation is the production of hydrogen peroxide, which can catalyze C-C cleavage through a variation of the Cannizzaro reaction.

            This mechanism shows that the mechanism during heterogeneous catalysis is no different from a short-circuited electrochemical cell, in which the oxidation and reduction are occurring on the same metal particles. In fact, electro-oxidation of alcohols and CO in alkaline media has been studied previously over Au and Pt electrodes.

            Of course, during electro-oxidation, there is an applied potential upon the metal catalyst, which causes significant changes in the reactivity of the catalyst. In order to further explore the similarities and differences between these two systems, the calculations performed upon the heterogeneous oxidation were redone using the double-reference method developed by Filhol, Taylor and Neurock². This allows for independent analysis of the effects of the metal, potential and mechanism.

Figure 1. Reaction scheme for the oxidation of alcohols to acids over a gold surface in water at high pH.

References

1.        B.N. Zope, D.D. Hibbitts, M. Neurock, and R.J. Davis. Science. 330. 74-77 (2010)

2.        C.D. Taylor, S.A. Wasileski, J-S. Filhol, and M. Neurock. Physical Review B. 73. 165402 (2006)