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Hydrogen is essential in a variety of large-scale chemical processes. As a carbon-free energy carrier, hydrogen has a potential for wide use within power production and transportation. However, most of the recent production methods involve the release of CO2 as a by-product. Hence, decarbonization of hydrogen production is one step to reduce CO2 emission into the Earth's atmosphere. Several process schemes have been suggested for low-carbon emission production of hydrogen. In this work, we show how to improve solid sorbents for the sorption-enhanced water-gas shift (SEWGS) process, which is a process that exploits a solid sorbent in the water-gas shift reactor to capture CO2 in situ and drive the process toward an improved hydrogen yield. We report herein a series of Co(x)Mg(3-)xAl materials based on hydrotalcites, promoted with various loadings of K. The materials have been characterized by BET, XRD, and NMR and tested for their CO2 adsorption performance in three adsorption-desorption cycles in a lab-scale fixed-bed reactor (20-22 bar, CO2 + steam as reactant gas, and isothermal conditions at 375 and 400 degrees C). The most promising material was subjected to a long-term test (120 adsorption-desorption cycles at similar conditions). This test indicates that a K-promoted Co1.5Mg1.5Al (22 wt % of added K2CO2 to the oxide) material has a higher cyclic capacity for CO2 than standard reference cases. We have estimated that the volumetric capacity (in mol/L unit) of this sorbent will be 23-26% higher than a standard reference material at 400 degrees C and 30-39% higher at 375 degrees C. This would, in fixed-bed columns, lead to significant reduction in the needed column volumes in the final process and reduce costs.

In order to improve the suitability of NaBH4 as a clean fuel, its decomposition temperature needs to be decreased to below 535 &deg, C, while its hydrogen release must be as high as possible. In this work, the influence of a collection of first and second period transition metal fluorides on the destabilization of NaBH4 is studied on samples produced by ball milling NaBH4 with 2 mol% of a metal fluoride additive. The effects obtained by increasing additive amount and changing oxidation state are also evaluated for NbF5, CeF3, and CeF4. The as-milled products are characterized by in-house power X-ray diffraction, while the hydrogen release and decomposition are monitored by temperature programmed desorption with residual gas analysis, differential scanning calorimetry, and thermogravimetry. The screening of samples containing 2 mol% of additive shows that distinctive groups of transition metal fluorides affect the ball milling process differently depending on their enthalpy of formation, melting point, or their ability to react at the temperatures achieved during ball milling. This leads to the formation of NaBF4 in the case of TiF4, MnF3, VF4, CdF2, NbF5, AgF, and CeF3 and the presence of the metal in CrF3, CuF2, and AgF. There is no linear correlation between the position of the transition metal in the periodic table and the observed behavior. The thermal behavior of the products after milling is given by the remaining NaBH4, fluoride, and the formation of intermediate metastable compounds. A noticeable decrease of the decomposition temperature is seen for the majority of the products, with the exceptions of the samples containing YF3, AgF, and CeF3. The largest decrease of the decomposition temperature is observed for NbF5. When comparing increasing amounts of the same additive, the largest decrease of the decomposition temperature is observed for 10 mol% of NbF5. Higher amounts of additive result in the loss of the NaBH4 thermal signal and ultimately the loss of the crystalline borohydride. When comparing additives with the same transition metal and different oxidation states, the most efficient additive is found to be the one with a higher oxidation state. Furthermore, among all the samples studied, higher oxidation state metal fluorides are found to be the most destabilizing agents for NaBH4. Overall, the present study shows that there is no single parameter affecting the destabilization of NaBH4 by transition metal fluorides. Instead, parameters such as the transition metal electronegativity and oxidation state or the enthalpy of formation of the fluoride and its melting point are competing to influence the destabilization. In particular, it is found that the combination of a high metal oxidation state and a low fluoride melting point will enhance destabilization. This is observed for MnF3, NbF5, NiF2, and CuF2, which lead to high gas releases from the decomposition of NaBH4 at the lowest decomposition temperatures.

Coke formation is a major reason in deactivation of acidic zeolite catalysts in industrial processes such as methanol to hydrocarbons conversion. Protecting the surface of acidic zeolite with an inert porous shell can greatly hinder the coke formation on the surface, and hence boost the lifetime of the catalyst. In this work, a solid-state steam-assisted method for synthesis of such optimized protective shell (silicate-1, ~15 nm thickness) is designed. This general and simple protocol can be applied to acidic zeolite catalysts to improve their catalytic lifetime. The silicalite-1 shell is synthesized on mesoporous ZSM-5 zeolite to explore its catalytic activity in methanol to hydrocarbons conversion. XPS and TEM analysis confirm the coverage of mesoporous zeolite crystals by non-acidic shell. In addition, nitrogen physisorption shows the accessibility of mesoporous ZSM-5 via microporous silicalite-1 network. Applying this protective shell increases the lifetime of the catalyst by 100% and its conversion capacity by 130%, in comparison to mesoporous ZSM-5 without the shell. The controlled formation of thin layer of microporous silicalite-1 around mesoporous ZSM-5 crystals (without growth of individual silicalite-1) accounts for enhanced catalytic improvement.

The sorbent enhanced water-gas shift (SEWGS) process has great potential for CO2 capture and is thereby a realistic alternative to conventional post-combustion capture by gas emission scrubbing. Nevertheless, better performing materials are required to make SEWGS competitive against state-of-the-art scrubbing technologies. Dawsonite, NaAl(OH)2CO3, has a high carbonate content and forms in situ during CO2 capture on alkali metal promoted alumina, thereby showing potential in variable temperature capture processes. For the isothermal SEWGS process the performance in pressure swing absorption is essential. We evaluate here synthetic Dawsonites [MAl(OH)2CO3, M = K, Na] as CO2 sorbents for the SEWGS process based on data from in situ X-ray diffraction, combined thermogravimetric analysis and mass spectrometry, magic-angle spinning nuclear magnetic resonance spectroscopy, as well as high pressure fixed bed reactor tests at isothermal conditions. The results show that synthetic Dawsonites are promising sorbents for the SEWGS process, however, with moderate cyclic CO2 capacities (∼0.3–0.7 mmol CO2/g sorbent) in comparison to alkali-metal promoted hydrotalcites. On the other hand, the materials exhibit relatively fast sorption rates at low temperatures (280–310 °C), favoring the water-gas shift reaction in the SEWGS process.

The deactivation of zeolite catalyst H-ZSM-5 by coking during the conversion of methanol to hydrocarbons was monitored by high-energy space- and time-resolved operando X-ray diffraction (XRD) . Space resolution was achieved by continuous scanning along the axial length of a capillary fixed bed reactor with a time resolution of 10 s per scan. Using real structural parameters obtained from XRD, we can track the development of coke at different points in the reactor and link this to a kinetic model to correlate catalyst deactivation with structural changes occurring in the material. The “burning cigar” model of catalyst bed deactivation is directly observed in real time.

In order to mitigate climate change driven by the observed high levels of carbon dioxide (CO2) in the atmosphere, many micro and nano-porous materials are being investigated for CO2 selectivity, capture and storage (CCS) purposes, including zeolites, metal organic frameworks (MOFs), functionalized polymers, activated carbons and nano-silicate clay minerals. Key properties include availability, non-toxicity, low cost, stability, energy of adsorption/desorption, sorbent regeneration, sorption kinetics and CO2 storage capacity. Here, we address the crucial point of the volumetric capture and storage capacity for CO2 in a low cost material which is natural, non-toxic, and stable. We show that the nano-silicate Nickel Fluorohectorite is able to capture 0.79 metric tons of CO2 per m3 of host material - one of the highest capacities ever achieved - and we compare volumetric and gravimetric capacity of the best CO2 sorbent materials reported to date. Our results suggest that the high capture capacity of this fluorohectorite clay is strongly coupled to the type and valence of the interlayer cation (here Ni2+) and the high charge density, which is almost twice that of montmorillonite, resulting in the highest reported CO2 uptake among clay minerals.

Pre-combustion CO 2 capture technologies are becoming viable alternatives to more conventional post-combustion capture by gas emissions scrubbing. The sorbent enhanced water-gas shift (SEWGS) process is a promising future technology for CO 2 capture. However, the process needs better performing materials than those available today to be competitive against state-of-the-art scrubbing technologies. Layered double hydroxides (LDH) are a promising class of materials to improve the performance of the SEWGS process. These materials have a general formula of M 2+ 1−x M 3+ x (OH) 2 (A n− ) x/n ·mH 2 O, and can be tuned by substituting the metal species, changing the M 2+ /M 3+ ratio, adjusting the synthesis parameters to influence morphology or by adding so-called promotors to improve performance. To aid an ongoing systematic study looking at several of these parameters we have developed a simple yet efficient way of screening materials for further in-depth studies. The method is highly suitable for a typical laboratory setting, and is based on thermogravimetric analysis combined with cyclic exposure to selected gases. In this article we present the results of applying the method to a selection of benchmark materials.

We have studied the structural behavior of SAPO-37 during calcination using simultaneous in situ powder X-ray diffraction (PXRD) and mass spectroscopy (MS) in addition to ex situ thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). A spike in the unit cell volume corresponding to template removal (tracked using the occupancy of the crystallographic sites in the SAPO-37 cages) is revealed from the XRD data and is strongly correlated with the DSC curve. The occupancy of the different template molecules in the faujasite (FAU) and sodalite (SOD) cages is strongly related to the two mass loss steps observed in the TGA data. The templates act as a physical stabilizing agent, not allowing any substantial unit cell response to temperature changes until they are removed. The FAU cages and SOD cages have different thermal response to the combustion of each template. The FAU cages are mainly responsible for the unit cell volume expansion observed after the template combustion. This expansion seems to be related with residual coke from template combustion. We could differentiate between the thermal response of oxygen and T-atoms. The T–O–T angle between two double 6-rings and a neighboring T–O–T linkage shared by SOD and FAU had different response to the thermal events. We were able to monitor the changes in the positions of oxygen and T-atoms during the removal of TPA+ and TMA+. Large changes to the framework structure at the point of template removal may have a significant effect on the long-term stability of the material in its activated form.

A good material for CO2 capture should possess some specific properties: (i) a large effective surface area with good adsorption capacity, (ii) selectivity for CO2, (iii) regeneration capacity with minimum energy input, allowing reutilization of the material for CO2 adsorption and (iv) low cost and high environmental friendliness. Smectite clays are layered nanoporous materials that may be good candidates in this context. Here we report experiments which show that gaseous CO2 intercalates into the interlayer nano-space of smectite clay (synthetic fluorohectorite) at conditions close to ambient. The rate of intercalation, as well as the retention ability of CO2 was found to be strongly dependent on the type of the interlayer cation, which in the present case is Li+, Na+ or Ni2+. Interestingly, we observe that the smectite Li-fluorohectorite is able to retain CO2 up to a temperature of 35°C at ambient pressure and that the captured CO2 can be released by heating above this temperature. Our estimates indicate that smectite clays, even with the standard cations analyzed here, can capture an amount of CO2 comparable to other materials studied in this context.

The development of a volumetric apparatus (also known as a Sieverts’ apparatus) for accurate and reliable hydrogen adsorption measurement is shown. The instrument minimizes the sources of systematic errors which are mainly due to inner volume calibration, stability and uniformity of the temperatures, precise evaluation of the skeletal volume of the measured samples, and thermodynamical properties of the gas species. A series of hardware and software solutions were designed and introduced in the apparatus, which we will indicate as f-PcT, in order to deal with these aspects. The results are represented in terms of an accurate evaluation of the equilibrium and dynamical characteristics of the molecular hydrogen adsorption on two well-known porous media. The contribution of each experimental solution to the error propagation of the adsorbed moles is assessed. The developed volumetric apparatus for gas storage capacity measurements allows an accurate evaluation over a 4 order-of-magnitude pressure range (from 1 kPa to 8 MPa) and in temperatures ranging between 77 K and 470 K. The acquired results are in good agreement with the values reported in the literature.

NaBH4 has long been identified as a viable hydrogen-storage material due to a theoretical gravimetric H2 capacity of 10.6 wt %. Because of the high enthalpy of decomposition of 108 ± 3 kJ mol–1, thermal decomposition of the pristine material does not occur until at least 500 °C, and thus NaBH4 has yet to be utilized in hydrogen-storage processes. In this study, NaBH4 has been milled with a variety of Ni-containing additives to investigate the effects on the temperatures required for thermal desorption of H2 by temperature-programmed desorption (TPD) measurements and the products characterized by powder X-ray diffraction (PXD). Ni-containing additives have been determined to significantly enhance the thermal desorption of H2 by at least 60 °C (Ni (65 wt %) on Si/Al2O3). PCT cycling experiments have been conducted to ascertain their effects on the reversible hydrogenation of the milled NaBH4. PXD analysis indicates that Ni reacts with B evolved during thermal decomposition to form NixBy species including Ni...

The identification of features in the Local Density of States (LDOS) of carbon nanotubes (CNTs) obtained by Scanning Tunneling Spectroscopy (STS) is of great importance in order to understand their properties. In this work, Single- and Multi-Wall Carbon Nanotubes are compared with Multi-Wall CNTs filled with tin nanowires (Sn@CNTs) in order to investigate the effect on morphological and electronic properties of the CNTs metallic filling. The LDOS of CNTs, together with topology changes, is investigated by using spatially resolved STM/STS at room temperature and in air and compared to the LDOS of highly oriented pyrolitic graphite (HOPG). The LDOS of CNTs is dominated from different electronic states filling the C 2p sigma-2p sigma* band gap. The appearance of those states is linked to the diameter and the defects of the CNTs. In fact, Sn-nanowires encapsulation induces changes in the structure of the CNTs and the appearance of electronic states in the LDOS inside the band gap. A more extensive description of the samples is obtained depicting the morphological features and the vibrational structure on wider areas using Scanning Electron Microscopy (SEM) and Raman spectroscopy, respectively. Journal of Nano Research