ADSORPTION OF NITROGEN-DIOXIDE AND NITRIC-OXIDE ON PD(111)

The adsorption of NO2 on Pd(111) was studied by using HREELS and TPD. NO2 adsorbs molecularly at 110 K in a-mu-N,O nitrito bonding geometry. The assignment of this bonding geometry is made by comparison of the vibrational data obtained in this study with IR measurements of the NO2 symmetric and asymmetric stretching frequencies of a bridge-bonded nitro palladium complex. Chemisorbed NO2 dissociates upon heating to 180 K (at low coverages) into NO(a) and O(a). Below theta-NO2 = 0.25 ML, NO2 desorption is not detected; NO2 adsorption is irreversible. Above theta-NO2 = 0.25 ML, reversible NO2 adsorption occurs with adsorption energies of 14 and 16 kcal mol-1 as determined by desorption activation energies. NO adsorption on Pd(111) was also studied by HREELS and TPD, in order to further understand NO2 dissociation. The NO TPD data agree well with other studies of NO on Pd(111) with desorption activation energies of 38, 21, and 20 kcal mol-1. For saturation coverages of NO at 100 K, two NO species are observed with N-O stretching modes at 1575 and 1735 cm-1. These species are assigned as NO bonded at bridge and atop sites, respectively. We also note the lack of measureable adsorption of N2O on Pd(111) at 100 K.