Publications

Polarized hydroxyapatite (HAp) scaffolds with customized architecture at the nanoscale have been presented as a green alternative to conventional catalysts used for carbon and dinitrogen fixation. HAp printable inks with controlled nanoporosity and rheological properties have been successfully achieved by incorporating Pluronic hydrogel. Nanoporous scaffolds with good mechanical properties, as demonstrated by means of the nanoindentation technique, have been obtained by a sintering treatment and the posterior thermally induced polarization process. Their catalytic activity has been evaluated by considering three different key reactions (all in the presence of liquid water): (1) the synthesis of amino acids from gas mixtures of N-2, CO2, and CH4; (2) the production of ethanol from gas mixtures of CO2 and CH4; and (3) the synthesis of ammonia from N-2 gas. Comparison of the yields obtained by using nanoporous and nonporous (conventional) polarized HAp catalysts shows that both the nanoporosity and water absorption capacity of the former represent a drawback when the catalytic reaction requires auxiliary coating layers, as for example for the production of amino acids. This is because the surface nanopores achieved by incorporating Pluronic hydrogel are completely hindered by such auxiliary coating layers. On the contrary, the catalytic activity improves drastically for reactions in which the HAp-based scaffolds with enhanced nanoporosity are used as catalysts. More specifically, the carbon fixation from CO2 and CH4 to yield ethanol improves by more than 3000% when compared with nonporous HAp catalyst. Similarly, the synthesis of ammonia by dinitrogen fixation increases by more than 2000%. Therefore, HAp catalysts based on nanoporous scaffolds exhibit an extraordinary potential for scalability and industrial utilization for many chemical reactions, enabling a feasible green chemistry alternative to catalysts based on heavy metals.

JTD Keywords: Amino acids, Amino-acids, Ammonium production, Bone, Carbon fixation, Composites, Constitutive phases, Decarbonization, Dinitrogen, Ditrogen fixation, Elastic-modulus, Electrophotosynthesis, Ethanol production, Hardness, Indentation, Nanoindentation, Pluronic hydrogel, Polarized hydroxyapatite

The processes related to the fixation of nitrogen in a catalyst able to produce glycine and alanine from a N2, CO2 and CH4 gas mixture at mild reaction conditions have been studied by combining experimental and theoretical investigations. Results have allowed to understand the role of different elements of the catalyst, which is constituted by permanently polarized hydroxyapatite (p-HAp), zirconia, and aminotris(methylenephosphonic acid) (ATMP). ATMP attracts N2 molecules towards the surface, maintaining them close to the zirconia and p-HAp components that are the most active from a catalytic point of view. On the other hand, the associative mechanism is thermodynamically favoured under mild reaction conditions with respect to the dissociative one, which is limited by the barrier associated to the Nsingle bondN bond cleavage. Because this reaction mechanism is similar to that employed in the nitrogen fixation by nitrogenase enzymes, these findings provide an opportunity to design new bioinspired catalysts.

JTD Keywords: Artificial photosynthesis, Carbon fixation, Hydroxyapatite, N[sbnd]N bond cleavage