Surface science of environmental interfaces (oxides and carbonates)

Surface science of environmental interfaces (oxides and carbonates)

(Left) Scanning-electron-microscopy image of a calcite (CaCO3) particle that was reacted with nitric acid at 40% RH. The arrows point to regions in which the Ca(NO3)2 deliquescent aqueous layer can be seen easily.

(Right) The proposed water-assisted surface segregation and island formation of micropuddles of calcium nitrate following water adsorption. FTIR spectroscopy has been used to probe these interactions. This kind of combined approach of spectroscopy and microscopy to study surface reactions under ambient conditions and pressure will provide a detail understanding of these reactions.



        The research described here falls into the broad area of environmental molecular surface science. The chemistry of single crystal surfaces of CaCO3, MgO and BaO with several pollutant molecules including NO2, HNO3 and SO2 at the gas-solid interface as a function of relative humidity is being investigated. Schematic cartoon (right) shows the complexities of an environmental interface. Oxide and carbonate surfaces under ambient conditions of temperature and relative humidity are usually terminated with hydroxyl groups and bicarbonate that can readily adsorb water.

Nitrogen and sulfur oxides represent major components of air pollution and there is a great deal of interest in these gases from several perspectives including the heterogeneous chemistry of these gases with aerosol particles in the atmosphere (e.g. mineral dust aerosol) and the environmental remediation of these gases from automotive exhaust and power plants. Our goal is to determine fundamental molecular-level aspects of the chemistry of nitrogen and sulfur oxides on the surface of oxides and carbonates under ambient conditions. It is well documented from a number of studies using a wide range of surface sensitive techniques that single crystal oxide and carbonate surfaces cleaved in air undergo facile reaction with atmospheric H2O to yield hydroxyl and bicarbonate, in the case of carbonates, groups on the surface. Water readily adsorb to these surfaces, most likely through hydrogen bonding to the surface OH and CO3H groups. It is our goal to understand the molecular level details of these reactions by using a combination of spectroscopy, Atomic Force Microscopy and Scanning-electron-microscopy to study these “wet” complex interfaces. This combined approach will give us important information about the molecular-level mechanistic aspects of these reactions including surface speciation, surface segregation and phase transitions.







The schematic diagram showing phase transitions of nitrate films on MgO(100) surfaces as a function of relative humidity at 296 K. The solid and dashed arrows are for increasing % RH and decreasing % RH, respectively.