Behavior of nickel supported on calcium-enriched hydroxyapatite samples for CCU-methanation and ICCU-methanation processes

Abstract

The viability of Ni supported on Ca-enriched hydroxyapatite (HAP) samples has been investigated for the intensification of CCU-methanation and ICCU-methanation decarbonization processes. Even working with a surface density of added Ca three times larger than that required for a deposition of a theoretical monolayer, there is no apparent segregation of its resulting phases. Moreover, the progressive addition of Ca dramatically increases both the number and the strength of basic sites. Likewise, this markedly increases the Ni dispersion and the density of Ni lattice defects. The addition of 1–3 wt% Ca induces a dramatic improvement of the CCU-methanation activity and selectivity, when compared with the Ni/HAP sample. The optimal catalyst shows a good resistance under a long-term stability test at 375 °C for 90 h TOS. A compromise between the surface basicity and the distribution of Ni active sites proves to be the key factor influencing the efficiency of the investigated materials. In the ICCU-methanation process the Ni/Ca(3)/HAP DFM with added Ca loading close to the theoretical monolayer exhibits the best performance. For instance, it proves to be the most efficient to convert the adsorbed CO2 to CH4, during the hydrogenation period, reaching a CH4 density close to 155 μmolCH4 g−1 at 400 °C. It is supposed that the methanation activity involves CO2 stored only on surface CaO species and it is most likely improved over DFMs exhibiting short Ni-CaO distances. Interestingly, the performance of our Ni/Ca(3)/HAP DFM clearly outperforms that reported over 15%Ni/15%CaO/Al2O3 and 9.3%Ni/CaO reference DFMs. We conclude that a dispersion on HAP support of Ca loadings close to the theoretical monolayer becomes a promising alternative to basic DFMs.