Application of In-situ Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy for the Understanding of Complex Reaction Mechanism and Kinetics: Formic Acid Oxidation on a Pt Film Electrode at Elevated Temperatures

Abstract

The potential of in-situ Fourier transform infrared (FTIR) spectroscopy measurements in an attenuated total reflection configuration (ATR-FTIRS) for the evaluation of reaction pathways, elementary reaction steps, and their kinetics is demonstrated for formic acid electrooxidation on a Pt film electrode. Quantitative kinetic information on two elementary steps, formic acid dehydration and CO(ad) oxidation, and on the contributions of the related pathways in the dual path reaction mechanism are derived from IR spectroscopic signals in simultaneous electrochemical and ATR-FTIRS measurements over a wide temperature range (25-80 degrees C). Linearly and multiply bonded CO(ad) and bridge-bonded formate are the only formic acid related stable reaction intermediates detected. With increasing temperature, the steady-state IR signal of CO(ad) increases, while that of formate decreases. Reaction rates for CO(ad) formation via formic acid dehydration and for CO(ad) oxidation as well as the activation energies of these processes were determined at different temperatures, potentials, and surface conditions (with and without preadsorbed CO from formic acid dehydration) from the temporal evolution of the IR intensities of CO(ad) during adsorption/reaction transients, using an IR intensity-CO(ad) coverage calibration. At potentials up to 0.75 V and temperatures from 25 to 80 degrees C, the "indirect" CO pathway contributes less than 5% (at potentials < or =0.6 V significantly below 1%) to the total Faradaic reaction current, making the "direct" pathway by far the dominant one under the present reaction conditions. Much higher activation energies for CO(ad) formation and CO(ad) oxidation compared with the effective activation energy of the total reaction, derived from the Faradaic currents, support this conclusion.

Oxidation Mechanism of Formic Acid on the Bismuth Adatom-Modified Pt(111) Surface

“…[8][9][10][11][12][13] Electrocatalytic reactions rates depend on the surface structure of the electrodes. So, Pt(111) electrodes exhibit low activity through the active intermediate and negligible CO formation, whereas Pt(100) surfaces show the highest activity for both routes.…”

“…20 It has been pointed out that, in the oxidation process of formic acid on pure platinum electrodes, adsorbed formate plays an important role. This specie has been detected by FTIR [9][10][11][12][13] and voltammetry, 21 coinciding the onset for the oxidation process with that for the adsorption of formate. 21 Also, DFT calculations indicate that adsorbed formate is a key element in the considered oxidation process.…”

Fundamental aspects of HCOOH oxidation at platinum single crystal surfaces with basal orientations and modified by irreversibly adsorbed adatoms

“…The scheme of this mechanism is the following: Regarding the nature of the active intermediate, the question is not yet fully resolved [3][4][5]. It has been proposed that the active intermediate is adsorbed formate, but other experiments seem to contradict that observation [6][7][8][9][10].…”

Bismuth‐Coated Mesoporous Platinum Microelectrodes as Sensors for Formic Acid Detection

“…However, platinum is susceptible to poisons such as CO, which strongly adsorbs onto the electrode surface active sites resulting in a dramatic decrease in catalytic efficiency [3,4,13]. Under given conditions, the activity of platinum towards the electrooxidation of HCOOH can be promoted by either using nanostructured platinum electrodes [13][14][15] or platinum electrodes modified by foreign metal ad-atoms [16][17][18][19][20][21][22][23][24].…”

“…Mesoporous platinum films have been employed for the electrooxidation of methanol and formic acid, and because of their high surface areas, they have displayed poisons-tolerance behaviour [13,25,26]. Moreover, modification of the Pt surface with ruthenium has provided further improvements of the electrode activity towards formic acid electrooxidation [21].…”