On the role of different adsorption and reaction sites on supported nanoparticles during a catalytic reaction: NO decomposition on a Pd/alumina model catalyst

We have studied the catalytic decomposition of NO on a structurally well-defined Pd model catalyst, employing a combination of molecular beam techniques and time-resolved IR reflection absorption spectroscopy (TR-IRAS). In a first step, different active sites such as facets and edges/defects on the catalyst nanoparticles are identified spectroscopically. Subsequently, these spectroscopic signatures are utilized to monitor the site occupation by reactant species (NO) and products (atomic oxygen and nitrogen) under reaction conditions. Simultaneously, the kinetics of NO dissociation is investigated. It is found that atomic nitrogen and oxygen species are initially formed in the vicinity of edge or defect sites. At temperatures up to 300 K, the mobility of these atomic species is suppressed, whereas at higher temperature, diffusion onto the (111) facets of the particles can occur. It is shown both by IR spectroscopy of adsorbed NO under reaction conditions and by control experiments using CO as a probe molecule that nitrogen and oxygen species preferentially occupy particle edge and defect sites. By means of molecular beam CO titration experiments and TR-IRAS, it is demonstrated that the presence of these atomic species critically controls the NO dissociation activity. Specifically, the presence of strongly bound nitrogen in the vicinity of edge and defect sites gives rise to an enhanced dissociation probability under conditions of high adsorbate coverage