Cooperative Catalysis for Selective Alcohol Oxidation with Molecular Oxygen

The activation of dioxygen for selective oxidation of organic molecules is a major catalytic challenge. Inspired by the activity of nitrogen-doped carbons in electrocatalytic oxygen reduction, we combined such a carbon with metal-oxide catalysts to yield cooperative catalysts. These simple materials boost the catalytic oxidation of several alcohols, using molecular oxygen at atmospheric pressure and low temperature (80 degrees C). Cobalt and copper oxide demonstrate the highest activities. The high activity and selectivity of these catalysts arises from the cooperative action of their components, as proven by various control experiments and spectroscopic techniques. We propose that the reaction should not be viewed as occurring at an active site', but rather at an active doughnut'-the volume surrounding the base of a carbon-supported metal-oxide particle

Understanding confinement effects and nano structuring in heterogeneous catalysis

During the COVID-19 lockdown we were all confined to our houses. While this puts a considerable strain on our lives, the chores around the house do tend to get done more efficiently. Catalytic reactions can benefit from confinement in a similar way: forcing the reactants closer to the catalyst, thereby controlling reactivity and selectivity. This dissertation investigates the nature of such confinement effects in heterogeneous catalysis, and provides a framework for analysing these effects. To establish confinement within our catalysts, we took two experimental routes: one where we actively created confinement by adding a barrier at different distances from the active site, and one where we used surface modification of novel two-dimensional materials (MXenes and MAX phases) to create confined spaces between the layers or on the surface. To identify the subtle changes in reactivity between catalysts, we have developed a novel device that precisely measures the kinetics of our gas-producing model reactions. We gained important insights about the role of the Arrhenius pre-exponential factor in surface catalysis and the range over which confinement effects are important. Finally, this dissertation establishes MXenes and MAX phases as a new types of catalytic materials and supports in heterogeneous catalysis. Using oxidative treatment, we could influence the type and number of acid sites on MXene, thereby controlling its reactivity and selectivity. The oxidation sensitivity of MXene also provides a new tool for tuning the electronic structure at metal particles, resulting in better catalysts for energy applications and on-demand hydrogen generation

Cooperative Surface‐Particle Catalysis: The Role of the “Active Doughnut” in Catalytic Oxidation

We consider the factors that govern the activity of bifunctional catalysts comprised of active particles supported on active surfaces. Such catalysts are interesting because the adsorption and diffusion steps, which are often discounted in “conventional” catalytic scenarios, play a key role here. We present an intuitive model, the so‐called “active doughnut” concept, defining an active catalytic region around the supported particles. This simple model explains the role of adsorption and diffusion steps in cascade catalytic cycles for active particles supported on active surfaces. The concept has two important practical implications. First, the reaction rate is no longer proportional to the number of active sites, but rather to the number of “communicative” active sites—those available to the reaction intermediates during their respective lifetimes. Second, it generates an important testable prediction concerning the dependence of the total reaction rate on the particle size. With these tools at hand, we examine six experimental examples of catalytic oxidation from the literature, and show that the active doughnut concept gives valuable insight even when detailed mechanistic information is hard to come by

Understanding Oxygen Activation on Metal- and Nitrogen-Codoped Carbon Catalysts

Multidoped carbons are often used for oxygen activation catalysis, both in heterogeneous catalysis and electrocatalysis. Identifying their catalytic sites is crucial for developing better catalysts. We now report an in-depth study into O2 activation on an important class of materials: carbons codoped by nitrogen and 10 different metals (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, and Pb). To identify catalytic sites, we studied their composition and structure (both bulk and surface) by a wide range of techniques, including temperature-programmed reduction, X-ray diffraction, electron microscopy, X-ray photoelectron spectroscopy, and N2 sorption porosimetry. The O2 activation step was studied by electrochemical oxygen reduction. To assign active sites, the electrocatalytic activity, selectivity, and stability were correlated to material composition and to known reactivity pathways. Two types of sites for O2 activation were identified and assigned for each multidoped material: (1) particles of partially reduced metal oxides and (2) metal–nitrogen clusters embedded into the carbon matrix. The detailed assignment correlates to activity in alcohol oxidation through similar volcano plots and leads to practical suggestions for catalyst development