The electrophotocatalytic synthesis of Glycine and Alanine from a simple gas mixture containing N-2, CO2, CH4 and H2O under mild reaction conditions (95 degrees C and 6 bar) was recently developed using a catalyst formed by permanently polarized hydroxyapatite, which is achieved using a thermally stimulated polarization process, coated with two layers of aminotris(methylenephosphonic acid) (ATMP) separated by an intermediate layer of zirconyl chloride (ZC). This work reports the optimization of the ATMPand ZCcoating content by examining the influence of their concentration of each component in each layer on the structural and electrochemical properties of the catalyst. After exhaustive analyses, such properties have been related with the efficiency of the catalysts prepared using different ATMPand ZCconcentrations to yield Gly and Ala amino acids by fixing nitrogen from N-2 and carbon from CO2 and CH4. Results show that, although the concentrations of ATMP and ZC in the first and the intermediate layers are important, the third layer plays a predominant role as is responsible of the apparition of supramolecular structures on the surface and the capacitive behavior of the coating. (C) 2020 Elsevier Inc. All rights reserved.