Artículos SCI

Nickel phosphide catalysts show a high level of selectivity for the reverse water-gas shift (RWGS) reaction, inhibiting the competing methanation reaction. This work investigates the extent to which suppression of methanation can be controlled by phosphidation and tests the stability of phosphide phases over 24-hour time on stream. Herein the synthesis of different phosphide crystal structures by varying Ni/P atomic ratios (from 0.5 to 2.4) is shown to affect the selectivity to CO over CH₄ in a significant way. We also show that the activity of these catalysts can be fine-tuned by the synthesis Ni/P ratio and identify suitable catalysts for low temperature RWGS process. Ni₁₂P₅-SiO₂ showed 80–100% selectivity over the full temperature range (i.e., 300–800 °C) tested, reaching 73% CO₂ conversion at 800 °C. Ni₂P-SiO₂ exhibited CO selectivity of 93–100% over a full temperature range, and 70% CO₂ conversion at 800 °C. The highest CO₂ conversions for Ni₁₂P₅-SiO₂ at all temperatures among all catalysts showed its promising nature for CO₂ capture and utilisation. The methanation reaction was suppressed in addition to RWGS activity improvement through the formation of nickel phosphide phases, and the crystal structure was found to determine CO selectivity, with the following order Ni₁₂P₅ >Ni₂P > Ni₃P. Based on the activity of the studied catalysts, the catalysts were ranked in order of suitability for the RWGS reaction as follows: Ni₁₂P₅-SiO₂ (Ni/P = 2.4) > Ni₂P-SiO₂ (Ni/P = 2) > NiP-SiO₂ (Ni/P = 1) > NiP₂-SiO₂ (Ni/P = 0.5). Two catalysts with Ni/P atomic ratios; 2.4 and 2, were selected for stability testing. The catalyst with Ni/P ratio = 2.4 (i.e., Ni₁₂P₅-SiO₂) was found to be more stable in terms of CO₂ conversion and CO yield over the 24-hour duration at 550 °C. Using the phosphidation strategy to tune both selectivity and activity of Ni catalysts for RWGS, methanation as a competing reaction is shown to be no longer a critical issue in the RWGS process for catalysts with high Ni/P atomic ratios (2.4 and 2) even at lower temperatures (300–500 °C). This opens up potential low temperature RWGS opportunities, especially coupled to downstream or tandem lower temperature processes to produce liquid fuels.

The SrCO3/SrO system has recently attracted interest for thermochemical energy storage due to the high energy densities potentially attainable. However, the high temperatures needed to promote calcination involve a sintering-induced deactivation of SrO to carbonation. In this work, SrO-based samples have been tested using a closed-loop pressure swing approach involving calcinations and carbonations at absolute pressures of 0.01 bar and 1 bar CO2, respectively. Using low CO2 absolute pressure for calcination decreases the reaction temperature to 900 degrees C, thus reducing the deactivation of SrO. Moreover, the use of additives further improves the reactivity of the samples. The addition of ZrO2 and MgO by mechanical mixing and acetic acid treatment, respectively, results in samples with very high multicycle performance, yielding material energy storage densities after twenty cycles above 5.0 GJ/m3. These results significantly improve those obtained for similar samples in which calcinations and carbonations were carried out at an absolute pressure of 1 bar CO2. Regarding the integration of the thermochemical energy storage into concentrating solar power plants, calcining SrO-based materials at low pressure increases the net thermal-to-electric efficiencies by up to 6 % points compared to CaO-based materials calcined at the same conditions. The importance of experimental conditions and precursors in the multicycle behaviour of SrO-based materials for thermochemical energy storage is emphasized.