Catalytic oxidation of carbon monoxide on palladium.  1.  Effect of pressure

Landry, S. M.; Betta, Ralph A. Dalla; Lu, Jia; Boudart, M.

doi:10.1021/j100366a037

Cited by 18 publications

(7 citation statements)

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“…10-6 to 102 mbar.21 No explanation for this increase in rate was given. 21 The data in Figure 6 also show that the oxidation rate at 445 K increases with an increase in the total pressure from 2.2 X 10"®to 1.6 X 1 (h6 Torr. This is simply because at low pressures, the CO desorption rate is competitive with the CO adsorption rate which, in turn, lowers the oxidation rate according to (7).…”

Section: Discussionmentioning

confidence: 83%

An infrared and kinetic study of carbon monoxide oxidation on model silica-supported palladium catalysts from 10-9 to 15 Torr

Xu¹,

Goodman²

1993

J. Phys. Chem.

133

100

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Section: Discussionmentioning

confidence: 83%

An infrared and kinetic study of carbon monoxide oxidation on model silica-supported palladium catalysts from 10-9 to 15 Torr

Xu¹,

Goodman²

1993

J. Phys. Chem.

133

100

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“…Significant changes in activation energy and reaction mechanism with varying reactant pressures in a flow environment have been reported previously for other transition metal surfaces. The activation energy for reaction decreased from 110 (26.3 kcal/mol) to 75 kJ/mol (17.9 kcal/mol) when the reaction pressure for CO oxidation was increased by 8 orders of magnitude on a Pd/Al 2 O 3 catalyst …”

Section: Discussionmentioning

confidence: 99%

“…The activation energy for reaction decreased from 110 (26.3 kcal/mol) to 75 kJ/mol (17.9 kcal/mol) when the reaction pressure for CO oxidation was increased by 8 orders of magnitude on a Pd/Al 2 O 3 catalyst. 54 However, in these experiments the active catalytic site could not be characterized in detail under reaction conditions. On the Pt(111) surface CO oxidation goes from a CO desorption limited regime at low temperatures to a reaction limited regime with large excesses of oxygen at elevated temperatures.…”

Section: Reaction Kineticsmentioning

confidence: 99%

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

et al. 2005

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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.

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“…This CO lattice model does, however, capture through-space co-adsorbate interactions and qualitatively describes the decrease in activation free energies (ΔG ⧧ ) with increasing coverage and the increase in measured enhancement factors (η, eq 16) with increasing CO pressure. Lattice models, such as this one, can therefore be used to determine qualitative effects of co-adsorbate coverage (whether barriers increase or decrease) for other reactions, such as CO oxidation on Pd, where inconsistent kinetic data 133,134 have complicated mechanistic interpretations, leading to unsupported proposals that "reverse spillover" of CO* suppresses oxidation rates. a Changes in ΔG ⧧ or ΔE lg⧧ as spectating CO* coverage (or the equivalent CO−CO distance in the CO lattice) changes from 0.00 to 1.00 ML and from 1.00 to 1.04 ML.…”

Section: Journal Of the American Chemical Societymentioning

confidence: 99%

Dense CO Adlayers as Enablers of CO Hydrogenation Turnovers on Ru Surfaces

2017

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High CO* coverages lead to rates much higher than Langmuirian treatments predict because co-adsorbate interactions destabilize relevant transition states less than their bound precursors. This is shown here by kinetic and spectroscopic data-interpreted by rate equations modified for thermodynamically nonideal surfaces-and by DFT treatments of CO-covered Ru clusters and lattice models that mimic adlayer densification. At conditions (0.01-1 kPa CO; 500-600 K) which create low CO* coverages (0.3-0.8 ML from in situ infrared spectra), turnover rates are accurately described by Langmuirian models. Infrared bands indicate that adlayers nearly saturate and then gradually densify as pressure increases above 1 kPa CO, and rates become increasingly larger than those predicted from Langmuir treatments (15-fold at 25 kPa and 70-fold at 1 MPa CO). These strong rate enhancements are described here by adapting formalisms for reactions in nonideal and nearly incompressible media (liquids, ultrahigh-pressure gases) to handle the strong co-adsorbate interactions within the nearly incompressible CO* adlayer. These approaches show that rates are enhanced by densifying CO* adlayers because CO hydrogenation has a negative activation area (calculated by DFT), analogous to how increasing pressure enhances rates for liquid-phase reactions with negative activation volumes. Without these co-adsorbate effects and the negative activation area of CO activation, Fischer-Tropsch synthesis would not occur at practical rates. These findings and conceptual frameworks accurately treat dense surface adlayers and are relevant in the general treatment of surface catalysis as it is typically practiced at conditions leading to saturation coverages of reactants or products.

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Catalytic oxidation of carbon monoxide on palladium. 1. Effect of pressure

Cited by 18 publications

References 0 publications

An infrared and kinetic study of carbon monoxide oxidation on model silica-supported palladium catalysts from 10-9 to 15 Torr

An infrared and kinetic study of carbon monoxide oxidation on model silica-supported palladium catalysts from 10-9 to 15 Torr

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

Dense CO Adlayers as Enablers of CO Hydrogenation Turnovers on Ru Surfaces

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Catalytic oxidation of carbon monoxide on palladium. 1. Effect of pressure

Cited by 18 publications

References 0 publications

An infrared and kinetic study of carbon monoxide oxidation on model silica-supported palladium catalysts from 10-9 to 15 Torr

An infrared and kinetic study of carbon monoxide oxidation on model silica-supported palladium catalysts from 10-9 to 15 Torr

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

Dense CO Adlayers as Enablers of CO Hydrogenation Turnovers on Ru Surfaces

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