Energetics of CO2 and H2O adsorption on zinc oxide

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33 Scopus citations

Abstract

Adsorption of H2O and CO2 on zinc oxide surfaces was studied by gas adsorption calorimetry on nanocrystalline samples prepared by laser evaporation in oxygen to minimize surface impurities and degassed at 450 °C. Differential enthalpies of H2O and CO2 chemisorption are in the range150 ±10 kJ/mol and 110 ±10 kJ/mol up to a coverage of 2 molecules per nm2. Integral enthalpy of chemisorption for H2O is96.8 ±2.5 kJ/mol at 5.6 H 2O/nm2 when enthalpy of water condensation is reached, and for CO2 is96.6 ±2.5 kJ/mol at 2.6 CO2/nm 2 when adsorption ceases. These values are consistent with those reported for ZnO prepared by other methods after similar degas conditions. The similar energetics suggests possible competition of CO2 and H 2O for binding to ZnO surfaces. Exposure of bulk and nanocrystalline ZnO with preadsorbed CO2 to water vapor results in partial displacement of CO2 by H2O. In contrast, temperature-programmed desorption (TPD) indicates that a small fraction of CO2 is retained on ZnO surfaces up to 800 °C, under conditions where all H2O is desorbed, with adsorption energies near200 kJ/mol. Although molecular mechanisms of adsorption were not studied, the thermodynamic data are consistent with dissociative adsorption of H2O at low coverage and with several different modes of CO2 binding.

Original languageEnglish (US)
Pages (from-to)9091-9097
Number of pages7
JournalLangmuir
Volume30
Issue number30
DOIs
StatePublished - Aug 5 2014
Externally publishedYes

ASJC Scopus subject areas

  • Materials Science(all)
  • Condensed Matter Physics
  • Surfaces and Interfaces
  • Spectroscopy
  • Electrochemistry

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