Removal Pathways of Surface Nitrogen in a Steady-State NO + CO Reaction on Pd(110) and Rh(110):  Angular and Velocity Distribution Studies

Abstract

The angular and velocity distributions of desorbing products N2 and CO2 were studied in a steady-state NO + CO reaction on Pd(110) and Rh(110) by cross-correlation time-of-flight techniques. The CO2 desorption sharply collimated along the surface normal on both surfaces. On the other hand, N2 desorption on Pd(110) sharply collimated along about 40° off the surface normal in the plane along the [001] direction below around 650 K, yielding a translational temperature of about 3600 K. At higher temperatures, the normally directed desorption was relatively enhanced. On Rh(110), desorbing N2 sharply collimated along the surface normal, yielding a translational temperature of about 2500 K. The inclined desorption was assigned to the decomposition of the intermediate, N2O(a) → N2(g) + O(a), and the normally directed component was proposed to be due to the associative desorption of adsorbed nitrogen atoms, 2N(a) → N2(g). The branching of these pathways was analyzed on Pd(110).

Spatial distributions of desorbing products in steady-state NO and N2O reductions on Pd(110)

“…The resultant mean kinetic energy is shown in the temperature units as T ͗E͘ = ͗E͘ /2k, where ͗E͘ is the mean kinetic energy and k is the Boltzmann constant. In the previous paper reporting the desorption angle dependence in the plane along the ͓001͔ direction, 21 this value peaked at around = 0°and = 40°, reaching about 3400 K. This time, the crystal-azimuth was varied at a fixed desorption angle of 40°. The translational temperature decreases quickly with increasing azimuth, showing that the desorption is fairly concentrated in the plane along the ͓001͔ direction ͓Fig.…”

“…21 The reaction chamber is equipped with x-ray photoelectron spectroscopy ͑XPS͒ optics, reverse-view lowenergy electron diffraction ͑LEED͒, an ion gun, and a quadrupole mass spectrometer ͑QMS͒ for angle-integrated ͑Al͒ measurements. The chopper house has a large pumping rate of about 7 m 3 s −1 .…”

“…Nevertheless, the reaction stoichiometry was already confirmed as xNO+CO→ yN 2 + zN 2 O+CO 2 where 2x = y + z. 21 At the pressure ratio of P NO / P CO = 1, the N 2 signal at = 0°was much less than that at 41°below 550 K, i.e., the product N 2 mostly comes from the intermediate N 2 O dissociation. Above 650 K, the normally directed N 2 signal overcomes the other.…”

“…14 Later, we showed that this inclined N 2 desorption is well reproduced in N 2 O decomposition on Pd͑110͒, Rh͑110͒, and Ir͑110͒ as well as in steady-state NO + CO ͑or H 2 ͒ and N 2 O+CO ͑or H 2 ͒ reactions on Pd͑110͒. 13,[15][16][17][18][19][20][21][22] Thus, the intermediate N 2 O͑a͒ formed from the NO͑a͒ +N͑a͒ fast reaction [23][24][25] has been proposed to be oriented along the ͓001͔ direction before dissociation in the course of the catalyzed NO reduction. In the present work, AR-product desorption measurements have been successfully performed at different crystal azimuths for the steady-state NO ͑or N 2 O͒ + CO reaction and the resultant spatial distributions of all products have been constructed in a three-dimensional way.…”

“…This decrement was evaluated and the signal intensity was corrected. 21 For the steady-state NO + CO reaction at different P NO / P CO ratios, Figs. 2͑a͒ and 2͑b͒ shows the AR signals at around the collimation angles ͑the maximum flux position͒ against T S , at = 0°for desorbing N 2 , CO 2 , and N 2 O, and at = 41°for the N 2 in the plane along the ͓001͔ direction.…”