Investigation of CO and CO₂ Adsorption on Tetragonal and Monoclinic Zirconia

Abstract

The adsorption of CO and CO 2 on tetragonal and monoclinic ZrO 2 was investigated using infrared spectroscopy and temperature-programmed desorption spectroscopy. For this study, samples were prepared of tetragonal ZrO 2 (t-ZrO 2 ) with surface areas of 20 and 187 m 2 /g and of monoclinic ZrO 2 (m-ZrO 2 ) with surface areas of 19 and 110 m 2 /g. The CO 2 adsorption capacity of m-ZrO 2 is more than an order of magnitude higher than that of t-ZrO 2 . The principal species observed on m-ZrO 2 are HCO 3 - and m- and b-CO 3 2 -, whereas the principal species observed on t-ZrO 2 are p- and b-CO 3 - . The higher CO 2 adsorption capacity of m-ZrO 2 is attributed to the higher concentration and basicity of the hydroxyl groups on this polymorph, as well as the stronger Lewis acidity of Zr 4+ cations and the stronger Lewis basicity of O 2 - anions. Depending on the adsorption temperature, the CO adsorption capacity of m-ZrO 2 is 5- to 10-fold higher than that of t-ZrO 2 . Below 450 K, CO is adsorbed as HCO 3 - and CO 3 2 - species on both polymorphs of ZrO 2 . At higher temperatures, only CO 3 2 - and HCOO species are observed. The CO adsorption capacity of m-ZrO 2 is higher than that of t-ZrO 2 because of the greater strength of the adsorption centers on this phase.