The selective catalytic reduction (SCR) of NOx with NH3 has been proven an efficient process for the removal of NOx from automotive exhaust emissions. Among the different catalytic systems that have been studied for this application, vanadates have shown promising properties with regard to their activity, durability and stability for application to the heavy duty mobile systems. In particular, iron vanadate FeVO4 has been proven a highly stable and selective catalyst that finds also further different applications including photocatalytic degradation of organic pollutants, gas sensor materials in detecting H2S traces in air, electrochromic electrodes, high energy density lithium batteries, and catalytic hydrogen combustion. Typically, the catalytic performance of catalysts strongly depends on their crystal structures and surface active sites of their active components, owing to the direct relationship with the exposed crystal planes, redox performances of the as-prepared catalysts. Therefore, control on the final structure and morphology already in the synthesis step is highly desired.
Nevertheless, thermal stability of these materials under operative conditions still remains an issue and further improvements are necessary for employment of these catalysts to mobile light-duty applications. The drawbacks that must be overcome are the necessary broadening of the operational window, the improvement of the low-temperature activity and high temperature stability. In this regard, doping is a viable route and, among the different catalyst formulations investigated, supported erbium vanadates were found to display higher activity. In addition, the connection between the choice of dopant and its loading in the vanadate and the catalytic performances are also relevant information to be pursued in order to optimize the catalyst performance.
In this context, we followed ex situ and time-resolved in situ XAS studies on hydrothermally prepared Fe-doped ErVO4, focusing on coordination geometry of the dopant in the vanadates, on the local environment of the VOx -species, as well as on the bond lengths between metal species. In particular, the influences of time and temperature of hydrothermal treatment, and of Er/Fe atomic ratio was assessed. Furthermore, in-situ experiments were performed within a heated closed capillary to mimic the hydrothermal conditions, to follow the structural evolution of the vanadates from the feedstock suspension up to the final crystalline compound.