Hysteresis phenomena in the NO-H-2 reaction over Ru(0001)

The NO-H-2 reaction was studied at NO pressures between 7.7 x 10(-8) and 7. 7 x 10(-7) mbar and for H-2/NO ratios varying from 6 to 127. Hysteresis phe nomena were observed during a heat and cool cycle dependent on the H-2/NO r atio, with the heating branch exhibiting less activity than the cooling bra nch. Apart from the H-2/NO ratio, the shape of the hysteresis also depends on the heating rate and the NO pressure. An almost 100% selectivity toward Nz is observed with only small amounts of NH3 formation at higher H-2/NO ra tios. From the AES and TDS experiments it was concluded that an O layer tha t is inactive to reaction with hydrogen is responsible for the low reaction rate on the heating branch. Once the O layer is removed, the total reactio n rate is determined by the removal of N-ads. The lateral interaction betwe en O-ads and N-ads turns out to be repulsive.