Isothermal Kinetic Study of Nitric Oxide Adsorption and Decomposition on Pd(111) Surfaces:  Molecular Beam Experiments

Abstract

The kinetics of NO adsorption and dissociation on Pd(111) surfaces and the NO sticking coefficient (s(NO)) were probed by isothermal kinetic measurements between 300 and 525 K using a molecular beam instrument. NO dissociation and N2 productions were observed in the transient state from 425 K and above on Pd(111) surfaces with selective nitrogen production. Maximum nitrogen production was observed between 475 and 500 K. It was found that, at low temperatures, between 300 and 350 K, molecular adsorption occurs with a constant initial s(NO) of 0.5 until the Pd(111) surface is covered to about 70-80% by NO. Then s(NO) rapidly decreases with further increasing NO coverage, indicating typical precursor kinetics. The dynamic adsorption - desorption equilibrium on Pd(111) was probed in modulated beam experiments below 500 K. CO titration experiments after NO dosing indicate the diffusion of oxygen into the subsurface regions and beginning surface oxidation at > or = 475 K. Finally, we discuss the results with respect to the rate-limiting character of the different elementary steps of the reaction system.

Porous thin films toward bridging the material gap in heterogeneous catalysis

“…Changes observed in the partial pressures of O 2 was then converted into parameters, such as sticking coefficient (s), rate of adsorption, OSC by following a calibration procedure described in our earlier publications. 51,52 Results reported in the present manuscript is reproducible within an error margin of 10%. Results obtained in MBI on thin films are correlated to that of the results obtained on powder CZ materials at ambient pressure, and probably this is the first such report.…”

Kinetics of Nitric Oxide Adsorption on Pd(111) Surfaces through Molecular Beam Experiments: A Quantitative Study

“…All the relevant gas-phase species formed during the reaction are detected by a quadrupole mass spectrometer as a function of time. Typical experimental details are available in our earlier publications. ,,, In a typical molecular beam adsorption experiment, the NO molecular beam was first allowed to enter the UHV chamber with the shutter in a closed position to block the surface, so as to avoid the direct interaction of NO molecules and the Pd(111) surface (Figure a). Still, a small amount of NO adsorption from the background cannot be prevented.…”

“…At T g 400 K, the PSM is no longer adequate and the basin-like feature disappears. 12 A few words are in order regarding the estimation of the pumping constant, λ e , which is of great interest in the quantitative analysis. At t = 75 s, the doser is switched off, and hence the molecular beam flux is stopped.…”

“…It has been claimed that defect sites are particularly active for NO decomposition but nevertheless has been shown to occur even on atomically smooth Pd(111) planes. 9,11,12 There has been a paucity of studies in the fundamental aspects of NO adsorption, especially with techniques involving molecular beam instrument (MBI). 12,16,17 MBI can measure absolute sticking rates and coefficients with high accuracy, under ultrahigh vacuum (UHV) conditions needed for maintaining surface cleanliness.…”

“…9,11,12 There has been a paucity of studies in the fundamental aspects of NO adsorption, especially with techniques involving molecular beam instrument (MBI). 12,16,17 MBI can measure absolute sticking rates and coefficients with high accuracy, under ultrahigh vacuum (UHV) conditions needed for maintaining surface cleanliness.…”