They suggested a thermal annealing mechanism that removed reaction sites from the surface at high temperatures. According to the hypothesis of Olander et al., as the temperature is reduced from a high value to a low value, the reaction sites are not regenerated fast enough, resulting in hysteresis. The scattering results with Ar suggest that thermal roughening had no impact on the scattering or reactive behavior , and it is not clear how the thermal annealing mechanism would be activated as the temperature is increased. Signatures of hysteresis were also seen in certain TPD results , although the authors did not use this terminology.