Endothermic surface reconstruction

A few examples of adsorption processes accompanied by an endothermic step due to the deformation/reconstruction of the surface in interaction with molecules were illustrated. In the reported cases, the heat measured within the calorimetric cell was the combination of an exothermic (adsorption) and an endothermic (surface reconstruction) effect, which caused the calorimetrically measured heat to be lower than what expected on the basis of a plain adsorption. An extra-care in interpreting (at molecular level) the experimental calorimetric results should be addressed in several cases, and in this respect it is quite fhiitful to complement the molar volumetric-calorimetric data with results from other approaches, typically the various spectroscopic methods and/or the ab initio molecular modeling. 

Bolis et al. [46] recendy summarized their data concerning calorimetry of CO adsorption on oxides. They concluded that, CO adsorption calorimetry is not a good technique for the characterization of the surface Lewis acidity at least for aluminas because the adsorption exothermic signal is in part combined by an endothermic signal due to surface reconstruction phenomena. Due to the nonspecific interaction of CO2 with basic sites, Rossi et al. [47,48] proposed the use of hexa-fluoro isopropanol as an alternative for the characterization of basic sites. The use of sulfur dioxide as a probe is also successful [49]. 

On the GaAs(lll), a(2 x 2) reconstruction also exists but the formation of As vacancies would have an endothermic formation energy and the structure is energetically not stable [91]. Therefore, other atomic structure models were suggested to explain this surface and the model that fits best to the experimental results is the As-rich As trimer structure a structure characterized by three As... 

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