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We present a comprehensive thermogravimetric analysis (TGA) of polyethylenimine (PEI)-impregnated resorcinol-formaldehyde (RF) aerogels. While numerous studies focus on PEI-impregnated SBA, RF materials have been less examined, despite their interest and specificities. As most articles on PEI-impregnated porous materials follow typical experimental methods defined for SBA, particularities of RF-PEI materials could remain unheeded. The design of nonisothermal TGA protocols, completed with nitrogen isotherms, based on the systematic filling of the matrix delivers a fundamental understanding of the relationship between the structure and function. This study demonstrates (i) the competition between the matrix and PEI for CO 2 -physisorption (φ) and CO 2 -chemisorption (χ), (ii) the hysteresis ([Formula: see text]) of CO 2 capture at low temperature attributed to the kinetic ( K ) hindrance of CO 2 diffusion ( D ) through PEI film/plugs limiting the chemisorption, and (iii) the thermodynamic (θ) equilibrium limiting the capture at high temperature. At variance with SBA-PEI materials, the first layers of PEI in RF are readily available for CO 2 capture given that this matrix does not covalently bind PEI as SBA. A facile method allows the discrimination between physi- and chemisorption, exhibiting how the former decreases with PEI coverage. The CO 2 capture hysteresis, while seldom introduced or discussed, underlines that the commonly accepted operating temperature of the "maximum capture" is based on an incomplete experiment. Through isotherm adsorption analysis, we correlate the evolution of this maximum to the morphological distribution of PEI. This contribution highlights the specificities of RF-PEI and the advantages of our TGA protocol in understanding the structure/function relationship of this kind of material by avoiding the typical direct applications of SBA-specific protocols. The method is straightforward, does not need large-scale facilities, and is applicable to other materials. Its easiness and rapidness are suited to high-volume studies, befitting for the comprehensive evaluation of interacting factors such as the matrix's nature, pore size, and PEI weight.

Lithium silicates have attracted great attention in recent years due to their potential use as high-temperature (450-700 °C) sorbents for CO 2 capture. Lithium orthosilicate (Li 4 SiO 4 ) can theoretically adsorb CO 2 in amounts up to 0.36 g CO 2 per g Li 4 SiO 4 . The development of new Li 4 SiO 4 -based sorbents is hindered by a lack of knowledge of the mechanisms ruling CO 2 adsorption on Li 4 SiO 4 , especially for eutectic mixtures. In this work, the structural, electronic, thermodynamic and CO 2 capture properties of monoclinic phases of Li 4 SiO 4 and a binary (Li 3 NaSiO 4 ) eutectic mixture are investigated using density functional theory. The properties of the bulk crystal phases as well as of the relevant surfaces are analysed. Likewise, the results for CO 2 -lithium silicates indicate that CO 2 is strongly adsorbed on the oxygen sites of both sorbents through chemisorption, causing an alteration not only in the chemical structure and atomic charges of the gas, as reflected by both the angles and bond distances as well as atomic charges, but also in the cell parameters of the Li 4 SiO 4 and Li 3 NaSiO 4 systems, especially in Li 4 SiO 4 (001) and Li 3 NaSiO 4 (010) surfaces. The results confirm strong adsorption of CO 2 molecules on all the considered surfaces and materials followed by CO 2 activation as inferred from CO 2 bending, bond elongation and surface to CO 2 charge transfer, indicating CO 2 chemisorption for all cases. The Li 4 SiO 4 and Li 3 NaSiO 4 surfaces may be proposed as suitable sorbents for CO 2 capture in wide temperature ranges.

Incipient wetness impregnation was employed to decorate two N-doped graphene-rich matrixes with iron, nickel, cobalt, and copper nanoparticles. The N-doped matrix was wetted with methanol solutions of the corresponding nitrates. After agitation and solvent evaporation, reduction at 800 °C over the carbon matrix promoted the formation of nanoparticles. The mass of the metal fraction was limited to 5 wt. % to determine if limited quantities of metallic nanoparticles catalyze the hydrogen capture/release of nanoconfined LiBH 4 . Isotherms of nitrogen adsorption afforded the textural characterization of the matrixes. Electronic microscopy displayed particles of definite size, evenly distributed on the matrixes, as confirmed by X-ray diffraction. The same techniques assessed the impact of LiBH 4 50 vol. % impregnation on nanoparticle distribution and size. The hydrogen storage properties of these materials were evaluated by differential scanning calorimetry and two cycles of volumetric studies. X-ray diffraction allowed us to follow the evolution of the material after two cycles of hydrogen capture-release. We discuss if limited quantities of coordination metals can improve the hydrogen storage properties of nanoconfined LiBH 4 , and which critical parameters might restrain the synergies between nanoconfinement and the presence of metal catalysts.

The feasibility of the reduction of CO2 to CH4 employing MgH2 in the presence and absence of cobalt as a catalyst was investigated for the first time, exploring different non-independent reaction conditions such as the grade of microstructural refinement, the molar ratio MgH2 : CO2, reaction time and temperature. For the un-catalyzed process a methane yield of 44.6% was obtained after 24 h of thermal treatment at 400 °C employing a molar ratio of 2 : 1, through a methanation mechanism that involves the direct reduction of CO2 and the generation of CH4via C as an intermediary. For the MgH2 catalyzed process a methane yield of 78% was achieved by heating at 350 °C for 48 h, 4 : 1 being the optimal molar ratio. The global mechanism corresponds to a Sabatier process favored by Co as an active catalyst, together with the reverse water gas shift reaction followed by methanation of CO in the presence of steam. On account of the fact that it was proved that the use of the catalyst allows lowering the operational temperature without collapsing the methane yield, this research provides interesting insight into a thermochemical method for CO2 reduction to CH4 employing a solid hydrogen storage medium as an H2 source.

CO 2 capture and conversion are a key research field for the transition towards an economy only based on renewable energy sources. In this regard, hydride materials are a potential option for CO 2 methanation since they can provide hydrogen and act as a catalytic species. In this work, Mg 2 NiH 4 complex hydride is synthesized by in situ monitoring of mechanical milling under a hydrogen atmosphere from a 2MgH 2 :Ni stoichiometric mixture. Temperature and pressure evolution is monitored, and the material is characterized, during milling in situ, thus providing a good insight into the synthesis process. The cubic polymorph of Mg 2 NiH 4 (S.G. Fm3[combining macron]m) starts to be formed in the early beginning of the mechanical treatment due to the mechanical stress induced by the milling process. Then, after 25 hours of milling, Mg 2 NiH 4 with a monoclinic (S.G. C12/c1) structure appears. The formation of the monoclinic polymorph is most likely related to the stress release that follows the continuous refinement of the material's microstructure. At the end of the milling process, after 60 hours, the as-milled material is composed of 90.8 wt% cubic Mg 2 NiH 4 , 5.7 wt% monoclinic Mg 2 NiH 4 , and 3.5 wt% remnant Ni. The as-milled Mg 2 NiH 4 shows high reactivity for CO 2 conversion into CH 4 . Under static conditions at 400 °C for 5 hours, the interactions between as-milled Mg 2 NiH 4 and CO 2 result in total CO 2 consumption and in the formation of the catalytic system Ni-MgNi 2 -Mg 2 Ni/MgO. Experimental evidence and thermodynamic equilibrium calculations suggest that the global methanation mechanism takes place through the adsorption of C and the direct solid gasification towards CH 4 formation.

CO2 emissions have been continuously increasing during the last half of the century with a relevant impact on the planet and are the main contributor to the greenhouse effect and global warming. The development of new technologies to mitigate these emissions poses a challenge. Herein, the recycling of CO2 to produce CH4 selectively by using Mg2FeH6 and Mg2NiH4 complex hydrides as dual conversion promoters and hydrogen sources has been demonstrated. Magnesium-based metal hydrides containing Fe and Ni catalyzed the hydrogenation of CO2 and their total conversion was obtained at 400 °C after 5 h and 10 h, respectively. The complete hydrogenation of CO2 depended on the complex hydride, H2 : CO2 mol ratio, and experimental conditions: temperature and time. For both hydrides, the activation of CO2 on the metal surface and its subsequent capture resulted in the formation of MgO. Investigations on the Mg2FeH6-CO2 system indicated that the main process occurs via the reversed water-gas shift reaction (WGSR), followed by the methanation of CO in the presence of steam. In contrast, the reduction of CO2 by the Mg-based hydride in the Mg2NiH4-CO2 system has a strong contribution to the global process. Complex metal hydrides are promising dual promoter-hydrogen sources for CO2 recycling and conversion into valuable fuels such as CH4.

Carbon capture and storage using regenerable sorbents are an effective approach to reduce CO 2 emissions from stationary sources. In this work, lithium orthosilicate (Li 4 SiO 4 ) was studied as a carbon dioxide sorbent. For a deeper understanding of the synthesis and carbonation mechanism of Li 4 SiO 4 , an in situ synchrotron radiation powder X-ray diffraction technique was used. The Li 4 SiO 4 powders were synthesized by a combination of ball milling of a Li 2 CO 3 and SiO 2 mixture followed by a thermal treatment process at low temperature. In situ studies showed that formation of Li 4 SiO 4 from the as-milled 2Li 2 CO 3 -SiO 2 mixture involves decomposition of Li 2 CO 3 by reaction with SiO 2 via Li 2 SiO 3 as an intermediate compound. No evidence of Li 2 Si 2 O 5 formation was obtained, in spite of thermodynamic predictions. The CO 2 capture by Li 4 SiO 4 was evaluated dynamically over a wide temperature range, reaching a maximum weight increase of 34 wt% and good cyclability after about 10 cycles. By thermogravimetric and microstructural analyses in combination with ex situ and in situ measurements, a two step carbonation mechanism and its influence on the final CO 2 capture was clearly elucidated. Under dynamical conditions up to 700 °C, the lower number of Li 2 CO 3 nuclei initially formed retards the double shell formation and the nucleation and growth of the Li 2 CO 3 particles remains the controlling step up to higher CO 2 capture capacity. Isothermal carbonation at 700 °C favours the formation of a higher number of Li 2 CO 3 nuclei that creates a thin carbonate shell. The CO 2 diffusion through this shell is the limiting step from the beginning and further carbonation is hindered as the reaction progresses.

Potassium-containing compounds, such as KH, KOH, KNH2 and different potassium halides, have shown positive effects on the dehydrogenation properties of the Li-Mg-N-H system. However, it is still discussed whether the K-compounds modify the thermodynamics of the system or if they have only a catalytic effect. In this work the impact of the addition of two K-containing compounds (0.08 mol% of KCl and KOH) on the hydrogen storage performance of the Mg(NH2)2-LiH composite was studied. The KOH incorporation reduced the dehydrogenation temperature from 197 °C to 154 °C, beginning the process at low temperature (∼70 °C). The doped sample was able to reversibly absorb and desorb 4.6 wt% of hydrogen with improved kinetics; dehydrogenation rates were increased four times, whereas absorptions required 20% less time to be completed in comparison to the pristine material. The thermodynamic destabilization of the Mg(NH2)2-2LiH composite by the addition of a small amount of KOH was demonstrated by an increment of 30% in the dehydrogenation equilibrium pressure. According to detailed structural investigations, the KH formed by the KOH decomposition through milling and thermal treatment, can replace LiH and react with Mg(NH2)2 to produce a mixed potassium-lithium amide (Li3K(NH2)4). The KH role is not limited to catalysis, but rather it is responsible for the thermodynamic destabilization of the Mg(NH2)2-LiH composite and it is actively involved in the dehydrogenation process.

The effect of different metal oxides (Co 3 O 4 and NiO) on the dehydrogenation reaction pathways of the Li 4 (NH 2 ) 3 BH 4 -LiH composite was investigated. The additives were reduced to metallic species i.e. Co and Ni which act as catalysts by breaking the B-H bonds in the Li-B-N-H compounds. The onset decomposition temperature was lowered by 32 °C for the Ni-catalysed sample, which released 8.8 wt% hydrogen below 275 °C. It was demonstrated that the decomposition of the doped composite followed a mechanism via LiNH 2 and Li 3 BN 2 formation as the end product with a strong reduction of NH 3 emission. The sample could be partially re-hydrogenated (∼1.5 wt%) due to lithium imide/amide transformation. To understand the role of LiH, Li 4 (NH 2 ) 3 BH 4 -LiH-NiO and Li 4 (NH 2 ) 3 BH 4 -NiO composites were compared. The absence of LiH as a reactant forced the system to follow another path, which involved the formation of an intermediate phase of composition Li 3 BN 2 H 2 at the early stages of dehydrogenation and the end products LiNH 2 and monoclinic Li 3 BN 2 . We provided evidence for the interaction between NiO and LiNH 2 during heating and proposed that the presence of Li facilitates a NH x -rich environment and the Ni catalyst mediates the electron transfer to promote NH x coupling.

Lithium fast-ion conductors have shown positive effects on the hydrogen storage properties of the Li-Mg-N-H system. In the present work, Li4(NH2)3BH4 doped Mg(NH2)2-2LiH was formed by milling the 2LiNH2-MgH2-0.2LiBH4 composite and posterior annealing under hydrogen pressure to reduce the kinetic barrier of the Li-Mg-N-H system. The effect of repetitive dehydrogenation/rehydrogenation cycles on the kinetic and thermodynamic performance was evaluated. The dehydrogenation rate in the doped composite was twice that in the un-doped sample at 200 °C, while hydrogenation was 20 times faster. The activation energy decreases by 9% due to the presence of Li4(NH2)3BH4 compared to the un-doped composite, evidencing its catalytic role. The presence of Li4(NH2)3BH4 in the composite stabilized the hydrogen storage capacity after successive sorption cycles. Thermodynamic studies revealed a variation in the pressure composition isotherm curves between the first dehydrogenation cycle and the subsequent. The Li4(NH2)3BH4 doped composite showed a sloped plateau region at higher equilibrium pressure in regard to the flat plateau of the un-doped composite. Detailed structural investigations revealed the effective influence of Li4(NH2)3BH4 in different reactions: the irreversible dehydrogenation in the presence of MgH2 and the reversible hydrogen release when it reacts with Li2Mg2(NH)3. The role of Li4(NH2)3BH4 in improving the dehydrogenation kinetics is associated with the weakening of the N-H bond and the mobile small ion mass transfer enhancement.

New amide-chloride phases were successfully synthesized by mechanical milling of the LiNH2-AlCl3 mixture at a molar ratio of 1 : 0.11 and further heating at 150 °C under argon (0.1 MPa) or under hydrogen pressure (0.7 MPa). Powder X-ray diffraction measurements as a function of milling time increase revealed that the milling of the LiNH2-0.11AlCl3 mixture results in the formation of a FCC solid solution with an excess of LiNH2. Subsequent heating of the LiNH2-0.11AlCl3 sample ball milled for 5 hours at 150 °C under argon or under hydrogen induces the appearance of an amide-chloride phase isostructural with cubic Li4(NH2)3Cl. This Li-Al-N-H-Cl phase transforms progressively into the trigonal phase after prolonged heating at 300 °C under hydrogen pressure. The thermal behaviour of the amide-chloride without and with LiH addition displays dissimilar decomposition pathways. The decomposition of amide-chloride alone involves the formation of ammonia and hydrogen from 120 to 300 °C. Conversely, the amide-chloride material in the presence of LiH only releases hydrogen avoiding the emission of ammonia. The resultant material is able to be rehydrogenated under moderate conditions (300 °C, 0.7 MPa H2), providing a new reversible hydrogen storage system.

Hydrogen adsorption on Pd/Ce(0.8)Zr(0.2)O(2) was studied by temperature-programmed reduction, volumetric measurements and IR spectroscopy. Hydrogen uptake and reduction rate at 353 K are strongly dependent on the hydrogen pressure. At relatively high hydrogen partial pressure, reduction involves PdO, the surface and a significant fraction of the bulk of the ceria based oxide. Formation of oxygen vacancies even at low temperature (<373 K) is observed. The hydrogen adsorption process is mainly irreversible, as is shown by an increase in the (2)F(5/2)-->(2)F(7/2) electronic transition of Ce(3+) with hydrogen pressure and surface dehydroxylation. This "severe" reduction has a negative effect on the subsequent hydrogen adsorption capability. The decrease of hydrogen uptake capacity and rate during adsorption can be associated with the partial loss of superficial OH and the presence of Ce(3+), which deactivates Pd electronically.