Linking morphology and reactivity: growth and ligand-assisted dissolution of cobalt oxyhydroxide.

Abstract

Reactions at the interface of solid materials have a significant role in many fields of study, ranging from environmental science to industrial manufacturing. Identifying and quantifying the reactive surface area of these materials is vital to understanding the reactions in which they participate. The most basic effect of reactive surface area is governing the reaction rate at the surface but, in some cases, it is necessary to have a far more detailed understanding of the surface structure. Many reactions occur most efficiently, or even exclusively, at specific types of surface site. The ability to identify and measure these sites could dramatically improve the design of many applications, such as heterogeneous catalysts or waste remediation systems. One proposed method of measuring reactive surface area is the use of carefully selected probe molecules that are specifically reactive with the surface sites of interest. This work focuses on the development of a method for analyzing the surface characteristics of heterogenite (β-CoOOH) using the ligand iminodiacetic acid (IDA) as a probe. To investigate this system, first a range of model materials were necessary. The method of heterogenite synthesis was explored, revealing that a surprising amount of control can be exerted over the final particle morphology by altering simple factors such as reaction temperature or choice of oxidizing agent. The ligand-assisted dissolution of heterogenite by IDA produces a mixture of sfac and u-fac isomers of Co(IDA)2 –, and the relative amount of each isomer depends upon the surface characteristics of the heterogenite. When heterogenite particles were aged in suspension at room temperature, a rapid evolution of the number and type of surface site present was observed. This change was tracked by reacting the particles with IDA then separating and quantifying the resulting Co(IDA)2 – isomers. Through this method, it was found that the surface evolution occurs more slowly when aged in lower pH buffer. The connection between particle morphology and reactivity was strengthened when a link was found between the height of cylindrical heterogenite plates and the ratio of isomers formed during the dissolution reaction. From this, an empirical relationship between particle height and the relative amount of s-fac isomer was derived. This relationship allowed the tracking of particle growth via dissolution reactions rather than direct measurements. The final connection between morphology and reactivity was discovered when kinetic and thermodynamic studies were undertaken. The rate of reaction when the reactant concentrations are altered suggests that both surface diffusion and product desorption processes are involved in determining the overall reaction rate. Finally, it was hypothesized that the reactivity of some sites varies with temperature. Thus, the ratio of products produced depends not only on the number and type of surface site present, but also on the temperature of the reaction. Additional work is necessary to quantify this temperature dependence.