Profiling Physicochemical Changes within Catalyst Bodies during Preparation: New Insights from Invasive and Noninvasive Microspectroscopic Studies

Cylindrical or spherical catalyst bodies with sizes ranging from tens of micrometers to a few millimeters have a wide variety of industrial applications. They are crucial in the oil refining industry and in the manufacture of bulk and fine chemicals. Their stability, activity, and selectivity are largely dependent on their preparation; thus, achieving the optimum catalyst requires a perfect understanding of the physicochemical processes occurring in a catalyst body during its synthesis. The ultimate goal of the catalyst researcher is to visualize these physicochemical processes as the catalyst is being prepared and without interfering with the system. In order to understand this chemistry and improve catalyst design, researchers need better, less invasive tools to observe this chemistry as it occurs, from the first stages in contact with a precursor all the way through its synthesis. In this Account, we provide an overview of the recent advances in the development of space- and time-resolved spectroscopic methods, from invasive techniques to noninvasive ones, to image the physicochemical processes taking place during the preparation of catalyst bodies. Although several preparation methods are available to produce catalyst bodies, the most common method used in industry is the incipient wetness impregnation. It is the most common method used in industry because it is simple and cost-effective. This method consists of three main steps each of which has an important role in the design of a catalytic material: pore volume impregnation, drying, and thermal treatment. During the impregnation step, the interface between the support surface and the precursor of the active phase at the solid liquid interface is where the critical synthetic chemistry occurs. Gas solid and solid solid interfaces are critical during the drying and thermal treatment steps. Because of the length scale of these catalyst bodies, the interfacial chemistry that occurs during preparation is space-dependent. Different processes occurring in the core or in the outer rim of the catalytic solid are enhanced by several factors, such as the impregnation solution pH, the metal ion concentration, the presence of organic additives, and the temperature gradients inside the body. Invasive methods for studying the molecular nature of the metal-ion species during the preparation of catalyst bodies include Raman, UV-vis-NIR, and IR microspectroscopies. Noninvasive techniques include magnetic resonance imaging (MRI). Synchrotron-based techniques such as tomographic energy dispersive diffraction imaging (TEDDI) and X-ray microtomography for noninvasive characterization are also evaluated.