RESUMO
This work describes a new operando surface enhanced Raman spectroscopy (SERS) platform that we developed for use with polymeric membranes that includes (1) a method for preparing SERS-active polymer membranes and (2) a permeation cell with optical access for SERS characterization of membranes under realistic operating conditions. This technique enables the direct correlation of membrane structure to its performance under realistic operating conditions by combining in situ SERS characterization of the molecular structure of polymer membranes and simultaneous measurement of solute permeation rates on the same sample. Using the new operando SERS technique, this work aims to clarify the unknown mechanisms by which reactive amines facilitate CO2 transport across polyvinylamine (PVAm), a prototypical facilitated transport membrane for CO2 separations. We show that a small amount of plasmonic silver particles added to the PVAm solution prior to knife-casting selectively enhances the sensitivity to detection of chemical intermediates (e.g., carbamate) formed in the PVAm film due to the surface-enhanced Raman scattering effect with only minimal effect on the CO2 permeance and selectivity of the membrane. Operando SERS characterization of PVAm during exposure to humidified CO2/CH4 biogas mixtures at room temperature shows that CO2 permeates across PVAm primarily as carbamate species. This work clarifies the previously unknown mechanism of CO2 facilitated transport across PVAm and establishes a new operando SERS platform that can be used with a wide range of polymer membrane systems. This technique can be used to elucidate fundamental transport mechanisms in polymer membranes, to establish reliable structure-performance relationships, and for real-time diagnostics of membrane fouling, among other applications.
RESUMO
Cu-based catalysts are highly active and selective for several CO2 conversion reactions; however, traditional monometallic Cu-based catalysts suffer poor thermal stability due to the aggregation of copper particles at high temperatures. In this work, we demonstrate a crystal engineering strategy to controllably prepare copper/silica (CuOx/SiO2) catalysts for the reverse water gas shift reaction (RWGS) at high temperatures. We show that CuOx/SiO2 catalysts derived from the in situ reduction of pure copper silicate nanotubes in a CO2 and H2 atmosphere exhibit superior catalytic activity with enhanced stability compared to traditional monometallic Cu-based catalysts for the RWGS at high temperatures. Detailed structural characterization reveals that there is a strong interaction between Cu and SiO2 in CuOx/SiO2 catalysts, which produces more Cu+ sites and smaller CuOx nanoparticles. Moreover, CuOx/SiO2 catalysts possess a unique dot core/rod shell structure, which could prevent the aggregation of Cu particles. This structural confinement effect, enhanced CO2 adsorption by Cu+, and small CuOx nanoparticles presumably caused the catalyst's extraordinary activity with enhanced stability at high temperatures.
RESUMO
Highly active and stable copper catalysts were successfully achieved by in situ self-reduction treatment of hierarchical double-shell copper silicate hollow nanofibers. The coexistence of Cu0 and Cu+ species in the as-prepared catalysts demonstrated the strong metal-support interactions and endowed them with outstanding catalytic performance for the RWGS reaction.
