Nanoparticles of CoAPO-5: synthesis and comparison with microcrystalline samples

The hydrothermal synthesis of a nanosized cobalt doped aluminum phosphate CoAPO-5 (CoAPO-5-N) in a water-surfactant-organic solvent mixture (emulsion method) is reported, along with its physico-chemical characterization and comparison with a sample obtained by conventional synthesis (CoAPO-5-C). Both XRD (X-ray Diffraction) peak widths and FESEM (Field Emission Scanning Electron Microscopy) pictures of CoAPO-5-N are in agreement with a nanoscale structure, although the aggregation of nanoparticles occurred. EDX analysis shows a more homogeneous distribution of cobalt in CoAPO-5-N, not attainable by conventional synthesis. The specific surface area, as measured by nitrogen adsorption at 77 K, shows a limited increase in CoAPO-5-N (242 m(2) g(-1)) with respect to CoAPO-5-C (216 m(2) g(-1)), whereas the external surface area is almost tripled. Such a definite increase in the outer surface of CoAPO-5-N is also evidenced by the fourfold increase in the rate of a reaction only involving the exterior surface of particles, the light-driven oxidation of water by persulfate anions, as activated by the bulky Ru(bipy)3(2+) complex, unable to enter CoAPO-5 micropores. Two new features were also noted, adding to the knowledge of CoAPO-5 systems: (i) tetrahedral Co(3+) species may coordinate ammonia molecules, assuming an octahedral configuration, as determined by UV-vis spectroscopy; (ii) Co(2+) species in trigonal coordination occur, able to coordinate either CO molecules at a low temperature or ammonia (or water) at room temperature, as evidenced by IR and UV-vis spectroscopy, respectively.

Surface interrogation of CoP(i) water oxidation catalyst by scanning electrochemical microscopy

Despite exhaustive spectroscopic investigations on the CoPi oxygen-evolving catalyst over the past several years, little is known about the surface cobalt sites and intermediates in direct contact with water that are responsible for the actual catalysis. Many studies thus far have been limited to ex situ characterizations or bulk film measurements, often in the absence of solvent. Here we describe an investigation of the CoPi catalyst by surface interrogation scanning electrochemical microscopy (SI-SECM). This method should allow us to selectively study surface atoms separately from the bulk in a solvent-filled environment. By SI-SECM, independent titrations of surface Co(III) and Co(IV) were performed, yielding a direct measurement of the surface active-site density of a CoPi electrode (11 Co/nm(2)). The pseudo-first-order reaction rate constants of Co(III) and Co(IV) with water were determined to be 0.19 and >2 s(-1), respectively, through time-dependent titration measurements.