The decomposition of methanol on Ru(001) studied using laser induced thermal desorption

Abstract

The decomposition reaction of methanol on Ru(001) was studied using laser induced thermal desorption (LITD). The LITD studies, combined with temperature programmed desorption and Auger electron spectroscopy measurements, allowed absolute product yields for the three competing surface pathways to be determined over the entire range of chemisorbed methanol coverages at a heating rate of β=2.6 K/s. At the lowest methanol coverages of θ≤0.07θs, where θs is the surface coverage of a saturated chemisorbed layer, all the methanol reacted between 220–280 K. This methanol decomposition reaction yielded desorption-limited H2 and CO as reaction products. At higher coverages, molecular desorption and the second methanol decomposition reaction involving C–O bond breakage became increasingly important. At θ=θs, 50% of the initial methanol coverage desorbed, 24% produced H2 and CO and 26% left C on the surface. Isothermal LITD kinetic measurements were carried out at low methanol coverages of θ≤0.07θs at various temperatures from 180 to 220 K. The initial decomposition rates obtained from the isothermal LITD studies displayed first order kinetics. The decomposition kinetics at later times could not be fit by first order kinetics and suggested a self-poisoned reaction. Subsequent LITD studies revealed that CO inhibited the decomposition reaction. The product CO inhibition was modeled by first order kinetics with a CO-coverage dependent activation barrier. The observed first order reaction kinetics at low methanol coverage could be expressed by the pre-exponential ν=106 s−1 and the coverage-dependent activation barrier E=7 kcal/mol+αθCO/θCO,s, where α=20 kcal/mol and θCO/θCO,s is the dimensionless CO coverage normalized to the CO saturation coverage θCO,s. Isotopic LITD studies revealed that the decomposition kinetics of CH3OH, CD3OH, and CH3OD were identical. This equivalence suggested that the hindered rotation of the surface methoxy species is the reaction coordinate for the rate-limiting step in the decomposition reaction.

Ruthenium–Platinum Catalysts and Direct Methanol Fuel Cells (DMFC): A Review of Theoretical and Experimental Breakthroughs

“…Comparing Ru and Pt, solo ruthenium is more active in water dehydrogenation while methanol dehydrogenation is where solo platinum excels, supporting the above-mentioned hypothesis regarding their role in Ru-Pt catalysts [59,62]. Nevertheless, both present computational results indicating good performance in methanol dissociation, in agreement with previous experimental data [62,75,76].…”

“…For a platinum catalyst, the methanol oxidation presents a thermodynamic sink with adsorption of CO, though formation of surface carbon monoxide from methanol dissociation is easy and agrees with experimental data [59,62,75]. Further, the calculated thermodynamics of adsorbed carbon monoxide is in agreement with experimental evidence of carbon monoxide poisoning of Pt during methanol catalysis [59].…”

Photodissociation of adsorbed Mo(CO)6 induced by direct photoexcitation and hot electron attachment. II. Physical mechanisms

“…The normal component of the p wave on the vacuum side is different from that on the substrate side by a factor of Es' The s wave and the parallel component of the p wave are continuous across the vacuum-substrate interface. The amplitude square of the total electric field, normalized to the incident wave, is given by lel 2 = _1_ le s 12 + _r_ (le pl l 1 2 + lep1/E eff 1 2 ), (22) I+r I+r where r is the ratio of the number of incident photons between the p and s waves. The wavelength dependence of r was experimentally determined to be r = Fp/F, = -1.48 + 1.053 X 10-2 ;1, _ 7.16X 10-6 ;1, 2, where the wavelength ;1, is in nm.…”

Surface analysis by laser-induced desorption time-of-flight mass spectrometry

“…As a result, LID allows the surface to be probed at specific positions without modifying other areas of the sample, which is very useful in mapping the composition distribution on the surface 2,8 or monitoring adsorbate concentration at different times. 9,10 Other pulsed probe beams, such as ions, electrons, or fast atoms can also be employed for similar surface desorption analysis. 11 In order to obtain a representative mass spectrum for a multi-component sample less selective ionization methods are required.…”