Pulsed molecular-beam experiments oxidation

Heiz further examined the oxidation of CO in a recent paper using dual pulsed molecular beam experiments over Pd (n=8, 13, 30) [59]. The mole fractions of CO and O could be varied across the entire range a maximum in CO production occurring at a mole fraction x =0.6 at 478 K. As the temperature is... 

Catalytic Reactivity. The size-dependent cluster catalysis was first studied by pulsed molecular beams. In these experiments, a nitric oxide molecular pulse is injected onto the cluster catalysts and the product molecules CO2 and N2 are quantitatively detected by a mass spectrometer as a function of cluster size, temperature, and CO background pressure [468]. The catal3dic formation of CO2 on Pdso and Pd is shown in Fig. 1.95a and b for selected temperatures and for a constant CO partial pressure of 5 x 10 mbar and an NO effective pressure of 1 x 10 mbar. The width of the NO pulse was 100 ms. Pdg and Pdgo show almost no catalytic reactivity up to about 390 K. For Pdso, maximal reactivity is observed at 420 K whereas Pdg is most reactive at 450 K. At higher temperatures, the formation of CO2 decreases. The CO2 formation on both cluster sizes at temperature of maximal reactivity is stable even after hundreds of NO pulses. 

The breakthrough experiment was carried out by Whitham et al. [39,40] in France. They used a Smalley-type laser vaporization source (Fig. 4) to provide a molecular beam of Ca atoms entrained in He or Ar gas. The second harmonic (532 nm) from a pulsed Nd YAG laser was focused (Fig. 4) on a rotating calcium rod. About 500 jus prior to this, a pulsed valve (left side of Fig. 4) is opened and the plume of vaporized metal is entrained in Ar or He gas. The carrier gas is seeded with a few percent of the oxidant such as H20. The plume of excited- and ground-state metal atoms are carried down a short channel and react with the oxidant. At the end of the channel, the product molecules such as CaOH expand into the vacuum chamber and cool. After a short expansion, the pressure has dropped so low that the molecules are effectively in a collisionless, ultracold (<10K) environment. 

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