(761b) Impact of Surfactants On the Metal Distribution of Supported Catalysts

Supported catalysts are used in many industrial processes and applications, ranging from petrochemical and catalytic converters to fuel cells. These catalysts have many advantages, such as a high surface area, a low amount of the often expensive active component (Pd, Pt, etc.) and high mechanical and thermal stability. Clearly, the catalyst design has a pronounced effect on the performance of a catalytic process. With respect to the distribution of the active component in the support materials, four main categories of metal profiles can be distinguished, i.e., uniform, egg-yolk, egg-shell and egg-white profiles. The choice of the desired metal profile is determined by the required activity and selectivity, and tailored for specific reactions and/or processes. Although the development and preparation of supported catalysts have been investigated for many years, many aspects of the various catalyst manufacturing steps are still not fully understood, and in industry the design of catalysts is predominated by trial and error experiments,

Supported catalysts are usually prepared by impregnation, where a porous support is contacted with a liquid solution that contains the desired metal as a dissolved salt. This step is usually followed by the evaporation of the liquid solvent and that is drying. Previous work has shown that the metal distribution is controlled by the conditions of both the impregnation and drying steps.

In this work we have carried out experiments and simulations to investigate the effect of surfactants on the metal distribution of Ni/Alumina catalysts. We have tested two anionic surfactants, glycolic acid ethoxylate lauryl ether (GAE) and sodium dodecylbenzenesulfonate (SD), and one non-ionic surfactant, Brij 93 (BJ). GAE and BJ are liquid surfactants and SD is a powder surfactant. In the experiments, we found that SD cannot dissolve in water and the size of SD powders is much larger than the alumina support pore size. Thus SD cannot penetrate inside the support pores and its effect on the metal distribution is negligible. Without adding surfactants significant egg-shell distribution can be observed after drying at 80°C for low and moderate loading conditions. If 0.5% GAE or 2% BJ is added in the solutions and the impregnation time is sufficiently long, nearly uniform metal distribution can be obtained after drying for both low concentrations and relatively high concentrations. This is due to the two contributions of surfactants. The first contribution is that surfactants have a preference to accumulate on the interface between the bulk solution and the air to reduce the solution surface tension. The second contribution is that surfactants attach to the inside walls of the support to hold the metal ions at the walls if the surfactant molecules can interact with the metal ions. This will greatly reduce the movement of metal ions during drying. Since BJ is a non-ionic surfactant, the interaction between the BJ molecules and the nickel ions should be relatively weak. If we reduce the amount of BJ in the solution to 1%, egg-shell profiles have been observed after drying for low concentrations. Since GAE is negatively charged and nickel complex is positively charged in the solution, a strong interaction the between the surfactant molecules and the nickel ions is expected. The effect of impregnation time on the metal distribution is significant. By adjusting this parameter, we can get uniform, egg-shell and egg-white distributions when 1% GAE is added in the solutions. We also developed a computational model to simulate the drying process with and without surfactants. In the model, two parameters are important to capture the impact of surfactants:  the surfactant capability to capture the metal ions, and the surfactant capability to attach to the pore walls of the support.