The isosteric phase transformation of the Ni(100)-c(5 2 × 2)R45°CO surface

A thermally activated isosteric phase transformation of CO on Ni(100) was identified and followed dynamically with temperature programmed reflection absorption infrared spectroscopy (TPRAIS). Carbon monoxide adsorbed to a coverage of 0.6 monolayers on Ni(100) at 190 K gives rise to a c(5 2 × 2)R45° LEED pattern and an infrared spectrum showing 2 3 of the molecules adsorbed in bridge sites and 1 3 of the molecules adsorbed in on-top sites. Heating the Ni crystal to 280 K results in little CO desorption and the same LEED pattern is observed. However, the infrared spectrrum shows a reversal of the site occupancy of CO after heating to 280 K so that 2 3 of the molecules are in on-top sites and 1 3 are in bridge sites. This phase transformation was followed dynamically with TPRAIS during temperature programming. The activation energy for the phase transformation is ≈ 64 kJ/mol. In addition the adsorbed CO trannsfers from on-top sites to bridge sites at 400 K before desorbing at 415 K.