Development of novel nano-hydroxyapatite doped with silver as effective catalysts for carbon monoxide oxidation

A series of novel silver-containing catalysts were synthesized, characterized by X-ray diffraction, infrared spectroscopy, scanning and transmission electron microscopy, N-2 adsorption-desorption isotherms, thermogravimetric analysis and then tested for the oxidation of carbon monoxide (CO). The microwave-hydrothermal method was a useful synthesis pathway to structurally incorporate low amounts of silver (2.5-5.0 atomic percentage) to hydroxyapatite (HA) lattice, preserving the crystalline structure of the HA. Specific surface area and particle size (diameter) of the obtained materials were about 52-55 m(2)/g and 19-47 nm, respectively. Silver-catalysts showed higher catalytic activities than the unmodified HA sample at temperatures between 600 and 850 degrees C. The best catalytic results were achieved at around 700-800 degrees C with silver-containing HA samples. However, due to the significant decrease in the crystallinity of the materials exposed to 800 degrees C; then, 700 degrees C was established as the best thermal condition for producing CO2 and preserving the HA crystalline structure. Furthermore, cyclic tests demonstrated that silver-containing HA catalysts perform CO oxidation for 3 h through several consecutive catalytic tests at 700 degrees C without losing neither their activity nor their structural properties, evidencing their high thermal stability under the CO-O-2 atmosphere. Thus, the highest reaction rate values (r(CO2)) were obtained with HA containing 2.5% of silver at 700 degrees C during the five cycles performed, positioning it as a promissory catalyst with high activity during the CO oxidation at high temperatures. The proposed reaction mechanism was established using these silver-doped materials. This work constitutes the first assessment to add low amounts of silver to increase the activity of this kind of catalytic materials for the CO oxidation reaction.