Enhanced Low-Temperature CO Oxidation on a Stepped Platinum Surface for Oxygen Pressures above 10^-5 Torr

Abstract

The rate of CO oxidation has been characterized on the stepped Pt(411) surface for oxygen pressures up to 0.002 Torr, over the 100-1000 K temperature range. CO oxidation was characterized using both temperature-programmed reaction spectroscopy (TPRS) and in situ soft X-ray fluorescence yield near-edge spectroscopy (FYNES). New understanding of the important role surface defects play in accelerating CO oxidation for oxygen pressure above 10(-5) Torr is presented in this paper for the first time. For saturated monolayers of CO, the oxidation rate increases and the activation energy decreases significantly for oxygen pressures above 10(-5) Torr. This enhanced CO oxidation rate is caused by a change in the rate-limiting step to a surface reaction limited process above 10(-5) Torr oxygen from a CO desorption limited process at lower oxygen pressure. For example, in oxygen pressures above 0.002 Torr, CO(2) formation begins at 275 K even for the CO saturated monolayer, which is well below the 350 K onset temperature for CO desorption. Isothermal kinetic measurements in flowing oxygen for this stepped surface indicate that activation energies and preexponential factors depend strongly on oxygen pressure, a factor that has not previously been considered critical for CO oxidation on platinum. As oxygen pressure is increased from 10(-6) to 0.002 Torr, the oxidation activation energies for the saturated CO monolayer decrease from 24.1 to 13.5 kcal/mol for reaction over the 0.95-0.90 ML CO coverage range. This dramatic decrease in activation energy is associated with a simple increase in oxygen pressure from 10(-5) to 10(-3) Torr. Activation energies as low as 7.8 kcal/mol were observed for oxidation of an initially saturated CO layer reacting over the 0.4-0.25 ML coverage range in oxygen pressure of 0.002 Torr. These dramatic changes in reaction mechanism with oxygen pressure for stepped surfaces are consistent with mechanistic models involving transient low activation energy dissociation sites for oxygen associated with step sites. Taken together these experimental results clearly indicate that surface defects play a key role in increasing the sensitivity of CO oxidation to oxygen pressure.

The new ambient-pressure X-ray photoelectron spectroscopy instrument at MAX-lab

“…This low-temperature activity is in line with the previously reported onset temperature for CO oxidation over Pt(111) of 305 K (Burnett et al, 2004). Moreover, low-temperature reactivity over surface steps is further enhanced at the oxygen pressures used here (Lewis et al, 2005), which might further contribute to the measured catalytic activity below 515 K.…”

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“…These findings demonstrate that defects on Ru catalysts allow the achievement of a high local oxygen coverage, thus producing O species active for CO oxidation; the same moieties may also be produced on non-defective Ru surfaces at high pressure and high temperature [348]. Similarly, evidence for a striking enhancement of low-temperature CO oxidation was reported on stepped Pt(411) for P(O 2 ) > 10 −5 Torr [349]. It was explained by a change in the rate limiting step of the reaction, passing from a CO desorption limited process below the critical oxygen pressure to a surface reaction limited process above it.…”

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“…The estimated apparent activation energies were 17.7 and 56.1 kJ mol −1 for NO reduction and CO oxidation, respectively. Among CO oxidation experiments, the activation energy can vary with sample preparation, substrate, and precious metal . However, the obtained activation energy of 56.1 kJ mol −1 is fairly consistent with previously reported values; thus, it is possible to consider that this reaction occurs according to the conventionally reported mechanism .…”