Determining Heterogeneous Nucleation Rates of Mg(OH)₂ on MgO Using Multiharmonic QCM-D and X-ray Scattering methods

Mineral looping using MgO is a promising approach for direct air capture (DAC) of CO₂ from the atmosphere at the gigaton/yr scale. One initial step during the MgO carbonation process in the presence of humidity is brucite formation, Mg(OH)₂, as a possible passivating phase on MgO surfaces. The influence of temperature and relative humidity variation on heterogeneous nucleation and crystal growth kinetics of Mg(OH)₂ on MgO are not well understood under environmentally/DAC-relevant conditions. In this project, quartz crystal microbalance (QCM) commercial crystals were coated with a ~90nm-thick MgO film using pulsed laser deposition (PLD). Similar films were also deposited using RF magnetron sputtering. Atomic force microscopy characterization of PLD-deposited films showed a roughness of 0.21 nm and domain size of ~40-60 nm. Experiments to investigate brucite formation using multiharmonic QCM with Dissipation analysis (QCM-D) were conducted by flowing deionized water at 20 µl/min over the MgO film, as well as under water vapor exposure. This work allowed estimates of dissolution and growth rates by analyzing the time-dependence of mass loss under each experimental condition. Results of QCM analysis using first and second order kinetic models will be presented along with x-ray grazing incidence scattering and reflectivity results to relate molecular and nanoscale structural changes to the QCM studies.