Investigation of CO and CO₂ Adsorption on Tetragonal and Monoclinic Zirconia

Abstract

The adsorption of CO and CO2 on tetragonal and monoclinic ZrO2 was investigated using infrared spectroscopy and temperature-programmed desorption spectroscopy. For this study, samples were prepared of tetragonal ZrO2 (t-ZrO2) with surface areas of 20 and 187 m2/g and of monoclinic ZrO2 (m-ZrO2) with surface areas of 19 and 110 m2/g. The CO2 adsorption capacity of m-ZrO2 is more than an order of magnitude higher than that of t-ZrO2. The principal species observed on m-ZrO2 are HCO3- and m- and b-CO32-, whereas the principal species observed on t-ZrO2 are p- and b-CO3-. The higher CO2 adsorption capacity of m-ZrO2 is attributed to the higher concentration and basicity of the hydroxyl groups on this polymorph, as well as the stronger Lewis acidity of Zr4+ cations and the stronger Lewis basicity of O2- anions. Depending on the adsorption temperature, the CO adsorption capacity of m-ZrO2 is 5- to 10-fold higher than that of t-ZrO2. Below 450 K, CO is adsorbed as HCO3- and CO32- species on both polymorphs of ZrO2. At higher temperatures, only CO32- and HCOO species are observed. The CO adsorption capacity of m-ZrO2 is higher than that of t-ZrO2 because of the greater strength of the adsorption centers on this phase.