Platinum Monolayer Electrocatalysts for Anodic Oxidation of Alcohols

Many problems encountered in hydrogen-fuel-cell technology, particularly those of hydrogen storage and distribution, can be circumvented by replacing hydrogen with liquid fuels, such as methanol and ethanol. The slow and incomplete electrocatalytic oxidation reaction of these alcohol molecules occurring at the fuel cell anode is the main impediment to practical application of the direct alcohol fuel cell (DAFC). Andrzej Wieckowski made a lasting contribution to the studies of the mechanism of the adsorption and oxidation of formic acid and methanol on platinum electrodes using a radiochemical method from the start of his scientific work (1,2). Platinum and Pt-based materials are currently the best electrocatalysts for these reactions. The price and the limited reserves of Pt are the prime obstacles to adequately developing this major field.

 In this work we describe the electrocatalysts consisting of one platinum monolayer (Pt_ML) placed on extended or nanoparticle surfaces having the activity and selectivity for the oxidation of alcohol molecules that can be controlled with Pt-support interaction. Our study reveals the correlation between the substrate-induced lateral strain in Pt_MLand its reactivity (3).

 We synthesized these electrocatalysts by depositing a Pt_ML on five different single crystal substrates via the galvanic displacement of an underpotentially deposited (UPD) Cu monolayer. We demonstrated a significant enhancement in the catalytic activity associated with the tensile strain and decreased activity associated with the compressive strain. During methanol oxidation (Fig. 1a), the suitably expanded Pt_ML (i.e., Pt_ML/Au(111)) exhibited a negatively shifted potential at the onset of the reaction and over seven-fold enhancement in peak current density with respect to Pt(111) (the most active low-index plane of Pt). During ethanol oxidation (not shown), sizable enhancement was also observed from Pt_ML/Au(111).

 We carried out density functional theory (DFT) calculations (Fig. 1b) to gain better understanding of methanol oxidation on the surfaces of Pt_ML. The DFT-predicted trend in reactivity agreed well with the experimental observations, showing in decreasing sequence, Pt_ML/Au(111) > Pt(111) > Pt_ML/Pd(111) > Pt_ML/Ir(111) > Pt_ML/Rh(111) > Pt_ML/Ru(0001).

 In situ infrared reflection absorption spectroscopy (IRRAS) studies of methanol oxidation on Pt_ML/Au(111) showed that the enhanced activity was due to the formation of COH_ads instead of poisoning CO_ads and the promoted oxidation of COH_ads directly to CO₂. The high ethanol oxidation activity on Pt_ML/Au(111) was attributed to the fast kinetics of partial oxidation pathway generating acetic acid and acetaldehyde.

 Our recent efforts have been focused on the Pt_ML/Au system. An improved synthetic method was developed to ensure the growth of a smooth Pt_ML. The effect from crystallographic orientations of the underlysing Au substrates was studied with both Au single crystals and shape-controlled Au nanocrystals.

 Further analysis of above results and more details will be presented in the meeting. On the basis of the unique properties of Pt_MLelectrocatalysts, namely, an ultralow Pt content, tunable activity and selectivity, close to complete Pt utilization, it is likely that this approach will profoundly affect both future research and technologies in electrocatalysis of organic compounds.

Acknowledgement

 This work is supported by U.S. Department of Energy (DOE), Divisions of Chemical and Material Sciences, under the Contract No. DE-AC02-98CH10886. The DFT calculations were carried out using computational resources at the Center for Functional Nanomaterials at Brookhaven National Laboratory.

References

[1] A. Wieckowski, J. Sobkowski, A. J. Onska, J. Electroanal. Chem., 55 (1974) 383-389.

[2] A. Wieckowski, J. Sobkowski, J. Electroanal. Chem, 3 (1975) 365-377.

[3] M. Li, P. Liu, R. R. Adzic, J. Phys. Chem. Lett.3 (2012) 3480−3485.

Figure Caption: Fig. 1. Electrochemical (a) and DFT (b) investigations of methanol oxidation on Pt_ML supported on different substrates.