Hydrothermal and Ambient Temperature Anchoring of Co (II) Oxygen Evolution Catalyst on Zeolitic Surfaces

Nature provides a way to transform the most abundant energetic source, sunlight, to chemical fuel. Artificial photosynthetic systems aim to convert solar energy into chemical fuels such as hydrogen through water splitting. Our group has proposed a zeolite-based system capable of mimicking the natural photosynthetic process such as light harvesting and charge separation. However, water splitting requires a catalyst that promotes the reactions. Catalyst that consist of earth abundant metals, such as cobalt, have been shown to catalyze water oxidation efficiently. We have developed two synthetic methods for the anchoring of cobalt catalyst onto zeolitic surfaces. The methods exploit the zeolite characteristic features such as pore size, high surface area and electronegative framework. The methods are optimized utilizing micron and nano-size zeolite crystals and later applied to zeolite membranes supported on polymeric supports. The first method consists of a three step hydrothermal synthetic route. The successful anchoring of 200 nm plate-like clusters onto the surface of micron-size zeolite is reported. Anchored beta-Co(OH) 2 clusters undergo a topotactic transformation to Co3O 4 at 125 °C. As a consequence, the catalytic activity of the material decreases due to the thermal oxidation. The hydrothermal method was performed using 40 nm zeolite Y particles. Characterization by Raman and XPS spectroscopy showed that the catalyst is an amorphous Co (II) hydroxide that "coats" the zeolite nanocrystals rather than separate clusters. A 5-fold increase in oxygen evolution yield over the micron-size catalyst was observed. The nano-supported clusters were found to be stable for more than one catalytic cycle. The second method exploits pore diameter as a way to control cobalt cluster size. The ambient temperature synthesis uses three bases which vary in cation size: NaOH, TMAOH and TBAOH. Successful size control was achieved as evidenced by "net-like" clusters in NaOH treated samples and 3 nm particles in TMAOH. No cluster formation was observed for TBAOH. The catalytic performance was found to be TMAOH > NaOH > TBAOH. TMAOH samples are effective due to their higher surface area. The oxygen evolution observed for TBAOH is the result of Co 2+ ions that remained in the zeolite cages. TMAOH was found to be unstable due to formation of Co(III)OOH during photo catalysis. The hydrothermal and ambient temperature methods were used to anchor Co(II) catalyst onto zeolite membranes surfaces. Upon hydrothermal anchoring of "plate-like" clusters, zeolite membrane layer is removed from the support resulting in removal of the catalyst. Formation of Co3O4 clusters using this method was observed due to poor anchoring of the clusters. We identified the ambient temperature method as the most effective for Co(II) catalyst anchoring. Catalytic performance for the membranes treated with the different bases was TMAOH > TBAOH > NaOH. The performance is dependent on the surface area of the cluster. TMAOH and TBAOH had spherical particles ranging from 30-60 nm on the surface whereas NaOH membranes showed a "film". Stability of the catalyst may be improved by transformation to Co3O4 via thermal annealing.