Christoph R. Müller

CaO‐Based CO2 Sorbents: From Fundamentals to the Development of New, Highly Effective Materials

The enormous anthropogenic emission of the greenhouse gas CO2 is most likely the main reason for climate change. Considering the continuing and indeed growing utilisation of fossil fuels for electricity generation and transportation purposes, development and implementation of processes that avoid the associated emissions of CO2 are urgently needed. CO2 capture and storage, commonly termed CCS, would be a possible mid-term solution to reduce the emissions of CO2 into the atmosphere. However, the costs associated with the currently available CO2 capture technology, that is, amine scrubbing, are prohibitively high, thus making the development of new CO2 sorbents a highly important research challenge. Indeed, CaO, readily obtained through the calcination of naturally occurring limestone, has been proposed as an alternative CO2 sorbent that could substantially reduce the costs of CO2 capture. However, one of the major drawbacks of using CaO derived from natural sources is its rapidly decreasing CO2 uptake capacity with repeated carbonation-calcination reactions. Here, we review the current understanding of fundamental aspects of the cyclic carbonation-calcination reactions of CaO such as its reversibility and kinetics. Subsequently, recent attempts to develop synthetic, CaO-based sorbents that possess high and cyclically stable CO2 uptakes are presented.

Synthesis of Highly Efficient, Ca‐Based, Al2O3‐Stabilized, Carbon Gel‐Templated CO2 Sorbents

www.MaterialsViews.com C O M M U Synthesis of Highly Effi cient, Ca-Based, Al 2 O 3 -Stabilized, Carbon Gel-Templated CO 2 Sorbents N IC A Marcin Broda and Christoph R. Müller * IO N The increasing concentration of greenhouse gases in the atmosphere is most likely linked to climate change. [ 1 ] Considering that the use of fossil energy carriers will continue and may even increase over the next decades, it is imperative to fi nd ways of using fossil fuels that limit the release of CO 2 into the atmosphere. One of the most promising short to mid-term strategies for mitigating climate change is CO 2 capture and storage (CCS). [ 2 ] However, the currently available CO 2 capture technology, i.e. scrubbing with amines, comes with a large penalty on plant effi ciency. Thus, there is a pressing need to develop more effi cient, less energy intensive CO 2 capture technologies. A promising, 2 nd generation CO 2 capture technology is the use of alkaline earth metal based CO 2 sorbents. [ 3 ] The selective capture and release of CO 2 is achieved by the following, theoretically reversible, reaction:

Highly efficient CO2 sorbents: development of synthetic, calcium-rich dolomites.

The reaction of CaO with CO(2) is a promising approach for separating CO(2) from hot flue gases. The main issue associated with the use of naturally occurring CaCO(3), that is, limestone, is the rapid decay of its CO(2) capture capacity over repeated cycles of carbonation and calcination. Interestingly, dolomite, a naturally occurring equimolar mixture of CaCO(3) and MgCO(3), possesses a CO(2) uptake that remains almost constant with cycle number. However, owing to the large quantity of MgCO(3) in dolomite, the total CO(2) uptake is comparatively small. Here, we report the development of a synthetic Ca-rich dolomite using a coprecipitation technique, which shows both a very high and a stable CO(2) uptake over repeated cycles of calcination and carbonation. To obtain such an excellent CO(2) uptake characteristic it was found to be crucial to mix the Ca(2+) and Mg(2+) on a molecular level, that is, within the crystalline lattice. For sorbents which were composed of mixtures of microscopic crystals of CaCO(3) and MgCO(3), a decay behavior similar to natural limestone was observed. After 15 cycles, the CO(2) uptake of the best sorbent was 0.51 g CO(2)/g sorbent exceeding the CO(2) uptake of limestone by almost 100%.

High-Purity Hydrogen via the Sorption-Enhanced Steam Methane Reforming Reaction over a Synthetic CaO-Based Sorbent and a Ni Catalyst

Sorbent-enhanced steam methane reforming (SE-SMR) is an emerging technology for the production of high-purity hydrogen from hydrocarbons with in situ CO2 capture. Here, SE-SMR was studied using a mixture containing a Ni-hydrotalcite-derived catalyst and a synthetic, Ca-based, calcium aluminate supported CO2 sorbent. The fresh and cycled materials were characterized using N2 physisorption, X-ray diffraction, and scanning and transmission electron microscopy. The combination of a Ni-hydrotalcite catalyst and the synthetic CO2 sorbent produced a stream of high-purity hydrogen, that is, 99 vol % (H2O- and N2-free basis). The CaO conversion of the synthetic CO2 sorbent was 0.58 mol CO2/mol CaO after 10 cycles, which was more than double the value achieved by limestone. The favorable CO2 capture characteristics of the synthetic CO2 sorbent were attributed to the uniform dispersion of CaO on a stable nanosized mayenite framework, thus retarding thermal sintering of the material. On the other hand, the cycled limestone lost its nanostructured morphology completely over 10 SE-SMR cycles due to its intrinsic lack of a support component.

Influence of the Calcination and Carbonation Conditions on the CO2 Uptake of Synthetic Ca-Based CO2 Sorbents

In this work we report the development of a Ca-based, Al(2)O(3)-stabilized sorbent using a sol-gel technique. The CO(2) uptake of the synthetic materials as a function of carbonation and calcination temperature and CO(2) partial pressure was critically assessed. In addition, performing the carbonation and calcination reactions in a gas-fluidized bed allowed the attrition characteristics of the new material to be investigated. After 30 cycles of calcination and carbonation conducted in a fluidized bed, the CO(2) uptake of the best sorbent was 0.31 g CO(2)/g sorbent, which is 60% higher than that measured for Rheinkalk limestone. A detailed characterization of the morphology of the sol-gel derived material confirmed that the nanostructure of the synthetic material is responsible for its high, cyclic CO(2) uptake. The sol-gel method ensured that Ca(2+) and Al(3+) were homogenously mixed (mostly in the form of the mixed oxide mayenite). The formation of a finely and homogeneously dispersed, high Tammann temperature support stabilized the nanostructured morphology over multiple reaction cycles, whereas limestone lost its initial nanostructured morphology rapidly due to its intrinsic lack of a support component.

Development of calcium-based, copper-functionalised CO2 sorbents to integrate chemical looping combustion into calcium looping

We experimentally demonstrate the feasibility of a novel process that integrates chemical looping combustion into the calcium looping scheme. Using a co-precipitation technique, calcium-based, copper-functionalised CO2 sorbents were developed. The material synthesized possessed stable CO2 uptake and oxygen carrying capacities, making it an attractive candidate for the modified calcium looping process.

Cooperativity and Dynamics Increase the Performance of NiFe Dry Reforming Catalysts

The dry reforming of methane (DRM), i.e., the reaction of methane and CO2 to form a synthesis gas, converts two major greenhouse gases into a useful chemical feedstock. In this work, we probe the effect and role of Fe in bimetallic NiFe dry reforming catalysts. To this end, monometallic Ni, Fe, and bimetallic Ni-Fe catalysts supported on a MgxAlyOz matrix derived via a hydrotalcite-like precursor were synthesized. Importantly, the textural features of the catalysts, i.e., the specific surface area (172-178 m2/gcat), pore volume (0.51-0.66 cm3/gcat), and particle size (5.4-5.8 nm) were kept constant. Bimetallic, Ni4Fe1 with Ni/(Ni + Fe) = 0.8, showed the highest activity and stability, whereas rapid deactivation and a low catalytic activity were observed for monometallic Ni and Fe catalysts, respectively. XRD, Raman, TPO, and TEM analysis confirmed that the deactivation of monometallic Ni catalysts was in large due to the formation of graphitic carbon. The promoting effect of Fe in bimetallic Ni-Fe was elucidated by combining operando XRD and XAS analyses and energy-dispersive X-ray spectroscopy complemented with density functional theory calculations. Under dry reforming conditions, Fe is oxidized partially to FeO leading to a partial dealloying and formation of a Ni-richer NiFe alloy. Fe migrates leading to the formation of FeO preferentially at the surface. Experiments in an inert helium atmosphere confirm that FeO reacts via a redox mechanism with carbon deposits forming CO, whereby the reduced Fe restores the original Ni-Fe alloy. Owing to the high activity of the material and the absence of any XRD signature of FeO, it is very likely that FeO is formed as small domains of a few atom layer thickness covering a fraction of the surface of the Ni-rich particles, ensuring a close proximity of the carbon removal (FeO) and methane activation (Ni) sites.

Effect of Pelletization and Addition of Steam on the Cyclic Performance of Carbon-Templated, CaO-Based CO2 Sorbents

In this work, we report the development of a synthetic CO2 sorbent that possesses a high cyclic CO2 uptake capacity and, in addition, sufficient mechanical strength to allow it to be used in fluidized-bed reactors. To overcome the problem of elutriation of the original powdered material, the synthetic CO2 sorbent was pelletized. An important aspect of this work was to assess the effect of steam on the cyclic CO2 capture capacity of the original, powdered CO2 sorbent and the pelletized material. After 30 cycles of repeated calcination and carbonation reactions conducted in a fluidized bed, the CO2 uptake of the pellets was 0.29 g of CO2/g of sorbent, a value that is 45% higher than that measured for the reference limestone. For the case that carbonation/calcination cycles were conducted in a thermogravimetric analyzer under steam-free carbonation conditions, the CO2 uptake of the best sorbent was 0.33 g of CO2/g of sorbent (after 10 cycles). Importantly, it should be noted that, after 10 cycles using wet carbonation conditions, the CO2 uptake of this material increased by 55% when compared to dry conditions. This observation was attributed to enhanced solid-state diffusion in the CaCO3 product layer under wet conditions. However, independent of the reaction conditions, the pelletized material showed a lower cyclic CO2 uptake when compared to the original powder. A detailed morphological characterization of the pellets indicated that the destruction of the primary, hollow micrometer-sized spheres during pelletization was responsible for the lower cyclic CO2 uptake of the pellets.

Synthesis of Cu-Rich, Al2O3-Stabilized Oxygen Carriers Using a Coprecipitation Technique: Redox and Carbon Formation Characteristics

Chemical looping combustion (CLC) is an emerging, new technology for carbon capture and storage (CCS). Copper-based oxygen carriers are of particular interest due to their high oxygen carrying capacity and reactivity, low tendency for carbon deposition, and exothermic reduction reactions. In this work, CuO-based and Al(2)O(3)-stabilized oxygen carriers with high CuO loadings were developed using a coprecipitation technique. The cyclic redox performance of the synthesized oxygen carriers was evaluated at 800 °C in a laboratory-scale fluidized bed reactor using a reducing atmosphere comprising 10 vol. % CH(4) and 90 vol. % N(2). The CuO content in the oxygen carrier was found to increase with the pH value at which the coprecipitation was performed. The oxygen carrying capacity of the oxygen carrier containing 87.8 wt % CuO was found to be high (5.5 mmol O(2)/g oxygen carrier) and stable over 25 redox cycles. Increasing the CuO content further, i.e. > 90 wt %, resulted in materials which showed a decreasing oxygen carrying capacity with cycle number. It was also shown that the incorporation of K(+) ions in the oxygen carrier can avoid the formation of the spinel CuAl(2)O(4) and significantly reduce carbon deposition.

Coprecipitated, Copper‐Based, Alumina‐Stabilized Materials for Carbon Dioxide Capture by Chemical Looping Combustion

Chemical looping combustion (CLC) has emerged as a carbon dioxide capture and storage (CCS) process to produce a pure stream of CO(2) at very low costs when compared with alternative CCS technologies, such as scrubbing with amines. From a thermodynamic point of view, copper oxide is arguably the most promising candidate for the oxygen carrier owing to its exothermic reduction and oxidation reactions and high oxygen-carrying capacity. However, the low melting point of pure copper of only 1085 °C has so far prohibited the synthesis of copper-rich oxygen carriers. This paper is concerned with the development of copper-based and Al(2)O(3)-stabilized oxygen carriers that contain a high mass fraction of CuO, namely, 82.4 wt %. The oxygen carriers were synthesized by using a coprecipitation technique. The synthesized oxygen carriers were characterized in detail with regards to their morphological properties, chemical composition, and surface topography. It was found that both the precipitating agent and the pH at which the precipitation was performed strongly influenced the structure and chemical composition of the oxygen carriers. In addition, XRD analysis confirmed that, for the majority of the precipitation conditions investigated, CuO reacted with Al(2)O(3) to form fully reducible CuAl(2)O(4). The redox characteristics of the synthesized materials were evaluated at 800 °C by using methane as the fuel and air for reoxidation. It was found that the oxygen-carrying capacity of the synthesized oxygen carriers was strongly influenced by both the precipitating agent and the pH at which the precipitation was performed; however, all oxygen carriers tested showed a stable cyclic oxygen-carrying capacity. The oxygen carriers synthesized at pH 5.5 using NaOH or Na(2)CO(3) as the precipitating agents were the best oxygen carriers synthesized owing to their high and stable oxygen transfer and uncoupling capacities. The excellent redox characteristics of the best oxygen carrier were interpreted in light of the detailed morphological characterization of the synthesized material and a synthesis-structure-performance relationship was developed.