Probing Surface Transformations of Lanthanum Nickelate Electrocatalysts during Oxygen Evolution Reaction.

Monitoring the spontaneous reconstruction of the surface of metal oxides under electrocatalytic reaction conditions is critical to identifying the active sites and establishing structure-activity relationships. Here, we report on a self-terminated surface reconstruction of Ruddlesden-Popper lanthanum nickel oxide (La2NiO4+δ) that occurs spontaneously during reaction with alkaline electrolyte species. Using a combination of high-resolution scanning transmission electron microscopy (HR-STEM), surface-sensitive X-ray photoelectron spectroscopy (XPS), and soft X-ray absorption spectroscopy (sXAS), as well as electrochemical techniques, we identify the structure of the reconstructed surface layer as an amorphous (oxy)hydroxide phase that features abundant under-coordinated nickel sites. No further amorphization of the crystalline oxide lattice (beyond the ∼2 nm thick layer formed) was observed during oxygen evolution reaction (OER) cycling experiments. Notably, the formation of the reconstructed surface layer increases the material's oxygen evolution reaction (OER) activity by a factor of 45 when compared to that of the pristine crystalline surface. In contrast, a related perovskite phase, i.e., LaNiO3, did not show noticeable surface reconstruction, and also no increase in its OER activity was observed. This work provides detailed insight into a surface reconstruction behavior dictated by the crystal structure of the parent oxide and highlights the importance of surface dynamics under reaction conditions.

Implications for the Hydrogenation of Propyne and Propene with Parahydrogen due to the in†situ Transformation of Rh2C to Rh0/C.

NMR spectroscopy studies using parahydrogen-induced polarization have previously established the existence of the pairwise hydrogen addition route in the hydrogenation of unsaturated hydrocarbons over heterogeneous catalysts, including those based on rhodium (Rh0). This pathway requires the incorporation of both hydrogen atoms from one hydrogen molecule to the same product molecule. However, the underlying mechanism for such pairwise hydrogen addition must be better understood. The involvement of carbon, either in the form of carbonaceous deposits on the surface of a catalyst or as a metal carbide phase, is known to modify catalytic properties significantly and thus could also affect the pairwise hydrogen addition route. Here, we explored carbon's role by studying the hydrogenation of propene and propyne with parahydrogen on a Rh2C catalyst and comparing the results with those for a Rh0/C catalyst obtained from Rh2C via H2 pretreatment. While the catalysts Rh2C and Rh0/C differ notably in the rate of conversion of parahydrogen to normal hydrogen as well as in terms of hydrogenation activity, our findings suggest that the carbide phase does not play a significant role in the pairwise H2 addition route on rhodium catalysts.

An investigation of the structural and electronic origins of enhanced chemical looping air separation performance of B-site substituted SrFe1-xCoxO3-δ perovskites.

Chemical looping air separation (CLAS) is a promising process intensification technology for extracting oxygen from air for oxygen enrichment in process streams. Co-doped strontium ferrites (SrFe1-xCoxO3-δ) have been found to have outstanding activities for CLAS processes. In this study, we explore the underlying factors driving the enhancement in oxygen uptake and release performance of perovskite structured SrFe1-xCoxO3-δ oxygen carriers for CLAS. Phase-pure perovskites, with B site substituted by up to 75 mol% Co, were prepared by a sol-gel method and systematically investigated through a wide range of well controlled experimental and computational approaches. While all SrFe1-xCoxO3-δ oxygen carriers showed excellent cyclic stability and structural reversibility over CLAS cycles, increased B site occupancy by Co resulted in monotonic decrease in onset temperature for oxygen release and increase in oxygen carrying capacity. These experimental trends can be fundamentally explained by an increase in the structural tolerance factor, an elevation in transition metal d-band, as well as an increased degree of hybridization between the metal d-band and the O p band. Therefore, these ab initio structural and electronic descriptors are useful design rationales for the hypothesis-driven synthesis of high-performing oxygen carriers for CLAS.

Peering into buried interfaces with X-rays and electrons to unveil MgCO3 formation during CO2 capture in molten salt-promoted MgO.

The addition of molten alkali metal salts drastically accelerates the kinetics of CO2 capture by MgO through the formation of MgCO3 However, the growth mechanism, the nature of MgCO3 formation, and the exact role of the molten alkali metal salts on the CO2 capture process remain elusive, holding back the development of more-effective MgO-based CO2 sorbents. Here, we unveil the growth mechanism of MgCO3 under practically relevant conditions using a well-defined, yet representative, model system that is a MgO(100) single crystal coated with NaNO3 The model system is interrogated by in situ X-ray reflectometry coupled with grazing incidence X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. When bare MgO(100) is exposed to a flow of CO2, a noncrystalline surface carbonate layer of ca. 7-Å thickness forms. In contrast, when MgO(100) is coated with NaNO3, MgCO3 crystals nucleate and grow. These crystals have a preferential orientation with respect to the MgO(100) substrate, and form at the interface between MgO(100) and the molten NaNO3 MgCO3 grows epitaxially with respect to MgO(100), and the lattice mismatch between MgCO3 and MgO is relaxed through lattice misfit dislocations. Pyramid-shaped pits on the surface of MgO, in proximity to and below the MgCO3 crystals, point to the etching of surface MgO, providing dissolved [Mg2+…O2-] ionic pairs for MgCO3 growth. Our studies highlight the importance of combining X-rays and electron microscopy techniques to provide atomic to micrometer scale insight into the changes occurring at complex interfaces under reactive conditions.

CO2 Capture at Medium to High Temperature Using Solid Oxide-Based Sorbents: Fundamental Aspects, Mechanistic Insights, and Recent Advances.

Carbon dioxide capture and mitigation form a key part of the technological response to combat climate change and reduce CO2 emissions. Solid materials capable of reversibly absorbing CO2 have been the focus of intense research for the past two decades, with promising stability and low energy costs to implement and operate compared to the more widely used liquid amines. In this review, we explore the fundamental aspects underpinning solid CO2 sorbents based on alkali and alkaline earth metal oxides operating at medium to high temperature: how their structure, chemical composition, and morphology impact their performance and long-term use. Various optimization strategies are outlined to improve upon the most promising materials, and we combine recent advances across disparate scientific disciplines, including materials discovery, synthesis, and in situ characterization, to present a coherent understanding of the mechanisms of CO2 absorption both at surfaces and within solid materials.

Advancing MXene Electrocatalysts for Energy Conversion Reactions: Surface, Stoichiometry, and Stability.

MXenes, due to their tailorable chemistry and favourable physical properties, have great promise in electrocatalytic energy conversion reactions. To exploit fully their enormous potential, further advances specific to electrocatalysis revolving around their performance, stability, compositional discovery and synthesis are required. The most recent advances in these aspects are discussed in detail: surface functional and stoichiometric modifications which can improve performance, Pourbaix stability related to their electrocatalytic operating conditions, density functional theory and advances in machine learning for their discovery, and prospects in large scale synthesis and solution processing techniques to produce membrane electrode assemblies and integrated electrodes. This Review provides a perspective that is complemented by new density functional theory calculations which show how these recent advances in MXene material design are paving the way for effective electrocatalysts required for the transition to integrated renewable energy systems.

Understanding the Synergy between Fe and Mo Sites in the Nitrate Reduction Reaction on a Bio-Inspired Bimetallic MXene Electrocatalyst.

Mo- and Fe-containing enzymes catalyze the reduction of nitrate and nitrite ions in nature. Inspired by this activity, we study here the nitrate reduction reaction (NO3 RR) catalyzed by an Fe-substituted two-dimensional molybdenum carbide of the MXene family, viz., Mo2 CTx : Fe (Tx are oxo, hydroxy and fluoro surface termination groups). Mo2 CTx : Fe contains isolated Fe sites in Mo positions of the host MXene (Mo2 CTx ) and features a Faradaic efficiency (FE) and an NH3 yield rate of 41 % and 3.2 µmol h-1 mg-1 , respectively, for the reduction of NO3 - to NH4 + in acidic media and 70 % and 12.9 µmol h-1 mg-1 in neutral media. Regardless of the media, Mo2 CTx : Fe outperforms monometallic Mo2 CTx owing to a more facile reductive defunctionalization of Tx groups, as evidenced by in situ X-ray absorption spectroscopy (Mo K-edge). After surface reduction, a Tx vacancy site binds a nitrate ion that subsequently fills the vacancy site with O* via oxygen transfer. Density function theory calculations provide further evidence that Fe sites promote the formation of surface O vacancies, which are identified as active sites and that function in NO3 RR in close analogy to the prevailing mechanism of the natural Mo-based nitrate reductase enzymes.

Altering Oxygen Binding by Redox-Inactive Metal Substitution to Control Catalytic Activity: Oxygen Reduction on Manganese Oxide Nanoparticles as a Model System.

Establishing generic catalyst design principles by identifying structural features of materials that influence their performance will advance the rational engineering of new catalytic materials. In this study, by investigating metal-substituted manganese oxide (spinel) nanoparticles, Mn3 O4 :M (M=Sr, Ca, Mg, Zn, Cu), we rationalize the dependence of the activity of Mn3 O4 :M for the electrocatalytic oxygen reduction reaction (ORR) on the enthalpy of formation of the binary MO oxide, Δf H°(MO), and the Lewis acidity of the M2+ substituent. Incorporation of elements M with low Δf H°(MO) enhances the oxygen binding strength in Mn3 O4 :M, which affects its activity in ORR due to the established correlation between ORR activity and the binding energy of *O/*OH/*OOH species. Our work provides a perspective on the design of new compositions for oxygen electrocatalysis relying on the rational substitution/doping by redox-inactive elements.

Elucidation of Metal Local Environments in Single-Atom Catalysts Based on Carbon Nitrides.

The ability to tailor the properties of metal centers in single-atom heterogeneous catalysts depends on the availability of advanced approaches for characterization of their structure. Except for specific host materials with well-defined metal adsorption sites, determining the local atomic environment remains a crucial challenge, often relying heavily on simulations. This article reports an advanced analysis of platinum atoms stabilized on poly(triazine imide), a nanocrystalline form of carbon nitride. The approach discriminates the distribution of surface coordination sites in the host, the evolution of metal coordination at different stages during the synthesis of the material, and the potential locations of metal atoms within the lattice. Consistent with density functional theory predictions, simultaneous high-resolution imaging in high-angle annular dark field and bright field modes experimentally confirms the preferred localization of platinum in-plane in the corners of the triangular cavities. X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and dynamic nuclear polarization enhanced 15 N nuclear magnetic resonance (DNP-NMR) spectroscopies coupled with density functional theory (DFT) simulations reveal that the predominant metal species comprise Pt(II) bound to three nitrogen atoms and one chlorine atom inside the coordination sites. The findings, which narrow the gap between experimental and theoretical elucidation, contribute to the improved structural understanding and provide a benchmark for exploring the speciation of single-atom catalysts based on carbon nitrides.

Gravitational instabilities in binary granular materials.

The motion and mixing of granular media are observed in several contexts in nature, often displaying striking similarities to liquids. Granular dynamics occur in geological phenomena and also enable technologies ranging from pharmaceuticals production to carbon capture. Here, we report the discovery of a family of gravitational instabilities in granular particle mixtures subject to vertical vibration and upward gas flow, including a Rayleigh-Taylor (RT)-like instability in which lighter grains rise through heavier grains in the form of "fingers" and "granular bubbles." We demonstrate that this RT-like instability arises due to a competition between upward drag force increased locally by gas channeling and downward contact forces, and thus the physical mechanism is entirely different from that found in liquids. This gas channeling mechanism also generates other gravitational instabilities: the rise of a granular bubble which leaves a trail of particles behind it and the cascading branching of a descending granular droplet. These instabilities suggest opportunities for patterning within granular mixtures.

Single-Atom-Substituted Mo2CTx:Fe-Layered Carbide for Selective Oxygen Reduction to Hydrogen Peroxide: Tracking the Evolution of the MXene Phase.

This work critically assesses the electrocatalytic activity, stability, and nature of the active phase of a two-dimensional molybdenum carbide (MXene) with single-atomic iron sites, Mo2CTx:Fe (Tx are surface terminating groups O, OH, and F), in the catalysis of the oxygen reduction reaction (ORR). X-ray absorption spectroscopy unequivocally confirmed that the iron single sites were incorporated into the Mo2CTx structure by substituting Mo atoms in the molybdenum carbide lattice with no other detectable Fe-containing phases. Mo2CTx:Fe, the first two-dimensional carbide with isolated iron sites, demonstrates a high catalytic activity and selectivity in the oxygen reduction to hydrogen peroxide. However, an analysis of the electrode material after the catalytic tests revealed that Mo2CTx:Fe transformed in situ into a graphitic carbon framework with dispersed iron oxyhydroxide (ferrihydrite, Fh) species (Fh/C), which are the actual active species. This experimental observation and the results obtained for the titanium and vanadium 2D carbides challenge previous studies that discuss the activity of the native MXene phases in oxygen electrocatalysis. Our work showcases the role of 2D metal carbides as precursors for active carbon-based (electro)catalysts and, more fundamentally, highlights the intrinsic evolution pathways of MXenes in electrocatalysis.

Tailoring Lattice Oxygen Binding in Ruthenium Pyrochlores to Enhance Oxygen Evolution Activity.

Ruthenium pyrochlores, that is, oxides of composition A2Ru2O7-δ, have emerged recently as state-of-the-art catalysts for the oxygen evolution reaction (OER) in acidic conditions. Here, we demonstrate that the A-site substituent in yttrium ruthenium pyrochlores Y1.8M0.2Ru2O7-δ (M = Cu, Co, Ni, Fe, Y) controls the concentration of surface oxygen vacancies (VO) in these materials whereby an increased concentration of VO sites correlates with a superior OER activity. DFT calculations rationalize these experimental trends demonstrating that the higher OER activity and VO surface density originate from a weakened strength of the M-O bond, scaling with the formation enthalpy of the respective MOx phases and the coupling between the M d states and O 2p states. Our work introduces a novel catalyst with improved OER performance, Y1.8Cu0.2Ru2O7-δ, and provides general guidelines for the design of active electrocatalysts.

Structural and thermodynamic study of Ca A- or Co B-site substituted SrFeO3-δ perovskites for low temperature chemical looping applications.

Perovskite-structured materials, owing to their chemical-physical properties and tuneable composition, have extended their range of applications to chemical looping processes, in which lattice oxygen provides the oxygen needed for chemical reactions omitting the use of co-fed gaseous oxidants. To optimise their oxygen donating behaviour to the specific application a fundamental understanding of the reduction/oxidation characteristics of perovskite structured oxides and their manipulation through the introduction of dopants is key. In this study, we investigate the structural and oxygen desorption/sorption properties of Sr1-xCaxFeO3-δ and SrFe1-xCoxO3-δ (0 ≤ x ≤ 1) to guide the design of more effective oxygen carriers for chemical looping applications at low temperatures (i.e. 400-600 °C). Ca A- or Co B-site substituted SrFeO3-δ show an increased reducibility, resulting in a higher oxygen capacity at T ≤ 600 °C when compared to the unsubstituted sample. The quantitative assessment of the thermodynamic properties (partial molar enthalpy and entropy of vacancy formation) confirms a reduced enthalpy of vacancy formation upon substitution in this temperature range (i.e. 400-600 °C). Among the examined samples, Sr0.8Ca0.2FeO3-δ exhibited the highest oxygen storage capacity (2.15 wt%) at 500 °C, complemented by excellent redox and structural stability over 100 cycles. The thermodynamic assessment, supported by in situ XRD measurements, revealed that the oxygen release occurs with a phase transition perovskite-brownmillerite below 770 °C, while the perovskite structure remains stable above 770 °C.

Na2CO3-modified CaO-based CO2 sorbents: the effects of structure and morphology on CO2 uptake.

Calcium looping (CaL) is a CO2 capture technique based on the reversible carbonation/calcination of CaO that is considered promising to reduce anthropogenic CO2 emissions. However, the rapid decay of the CO2 uptake of CaO over repeated cycles of carbonation and calcination due to sintering limits its implementation at the industrial scale. Thus, the development of material design strategies to stabilize the CO2 uptake capacity of CaO is paramount. The addition of alkali metal salts to CaO has been proposed as a strategy to mitigate the rapid loss of its cyclic CO2 uptake capacity. However, there are conflicting results concerning the effect of the addition of alkali metal carbonates on the structure and CO2 capacity of CaO. In this work, we aim at understanding the effect of the addition of Na2CO3 to CaO on the sorbent's structure and its CO2 uptake capacity. We demonstrate that under industrially-relevant conditions the addition of as little as 1 wt% of Na2CO3 reduces severely the CO2 uptake of CaO. Combining TGA, XAS and FIB-SEM analysis allowed us to attribute the performance degradation to the formation of the double salt Na2Ca(CO3)2 that induces strong sintering leading to a significant loss in the sorbent's pore volume. In addition, during the carbonation step the formation of a dense layer of Na2Ca(CO3)2 that covers unreacted CaO prevents its full carbonation to CaCO3.

Structural Evolution and Dynamics of an In2O3 Catalyst for CO2 Hydrogenation to Methanol: An Operando XAS-XRD and In Situ TEM Study.

We report an operando examination of a model nanocrystalline In2O3 catalyst for methanol synthesis via CO2 hydrogenation (300 °C, 20 bar) by combining X-ray absorption spectroscopy (XAS), X-ray powder diffraction (XRD), and in situ transmission electron microscopy (TEM). Three distinct catalytic regimes are identified during CO2 hydrogenation: activation, stable performance, and deactivation. The structural evolution of In2O3 nanoparticles (NPs) with time on stream (TOS) followed by XANES-EXAFS-XRD associates the activation stage with a minor decrease of the In-O coordination number and a partial reduction of In2O3 due to the formation of oxygen vacancy sites (i.e., In2O3-x). As the reaction proceeds, a reductive amorphization of In2O3 NPs takes place, characterized by decreasing In-O and In-In coordination numbers and intensities of the In2O3 Bragg peaks. A multivariate analysis of the XANES data confirms the formation of In2O3-x and, with TOS, metallic In. Notably, the appearance of molten In0 coincides with the onset of catalyst deactivation. This phase transition is also visualized by in situ TEM, acquired under reactive conditions at 800 mbar pressure. In situ TEM revealed an electron beam assisted transformation of In2O3 nanoparticles into a dynamic structure in which crystalline and amorphous phases coexist and continuously interconvert. The regeneration of the deactivated In0/In2O3-x catalyst by reoxidation was critically assessed revealing that the spent catalyst can be reoxidized only partially in a CO2 atmosphere or air yielding an average crystallite size of the resultant In2O3 that is approximately an order of magnitude larger than the initial one.

Single Site Cobalt Substitution in 2D Molybdenum Carbide (MXene) Enhances Catalytic Activity in the Hydrogen Evolution Reaction.

Two-dimensional (2D) carbides, nitrides, and carbonitrides known as MXenes are emerging materials with a wealth of useful applications. However, the range of metals capable of forming stable MXenes is limited mostly to early transition metals of groups 3-6, making the exploration of properties inherent to mid or late transition metal MXenes very challenging. To circumvent the inaccessibility of MXene phases derived from mid-to-late transition metals, we have developed a synthetic strategy that allows the incorporation of such transition metal sites into a host MXene matrix. Here, we report the structural characterization of a Mo2CTx:Co phase (where Tx are O, OH, and F surface terminations) that is obtained from a cobalt-substituted bulk molybdenum carbide (ß-Mo2C:Co)  through a two-step synthesis: first an intercalation of gallium yielding Mo2Ga2C:Co followed by removal of Ga via HF treatment. Extended X-ray absorption fine structure (EXAFS) analysis confirms that Co atoms occupy Mo positions in the Mo2CTx lattice, providing isolated Co centers without any detectable formation of other cobalt-containing phases. The beneficial effect of cobalt substitution on the redox properties of Mo2CTx:Co is manifested in a substantially improved hydrogen evolution reaction (HER) activity, as compared to the unsubstituted Mo2CTx catalyst. Density functional theory (DFT) calculations attribute the enhanced HER kinetics of Mo2CTx:Co to the favorable binding of hydrogen on the oxygen terminated MXene surface that is strongly influenced by the substitution of Mo by Co in the Mo2CTx lattice. In addition to the remarkable HER activity, Mo2CTx:Co features excellent operational and structural stability, on par with the best performing non-noble metal-based HER catalysts. Overall, our work expands the compositional space of the MXene family by introducing a material with site-isolated cobalt centers embedded in the stable matrix of Mo2CTx. The synthetic approach presented here illustrates that tailoring the properties of MXenes for a specific application can be achieved via substitution of the host metal sites by mid or late transition metals.

The effect of copper on the redox behaviour of iron oxide for chemical-looping hydrogen production probed by in situ X-ray absorption spectroscopy.

The production of high purity hydrogen with the simultaneous capture of CO2, can be achieved through a chemical looping (CL) cycle relying on an iron oxide-based oxygen carrier. Indeed, the availability of active and cyclically stable oxygen carriers is a key criterion for the practical implementation of this technology. In this regard, improving our understanding of the reduction pathway(s) of iron-based oxygen carriers and the development of concepts to increase the reduction kinetics are important aspects. The aim of this work is to evaluate the effect of the addition of copper on the redox behaviour of iron oxide based oxygen carriers stabilized on ZrO2. In situ pulsed-H2 XANES (Fe K-edge) experiments allowed for the determination of the reduction pathways in these materials, viz. the reduction of both Fe2O3 and CuFe2O4 proceeded via a Fe2+ intermediate: Fe2O3 (CuFe2O4) → Fe3O4 (Cu0) → FeO (Cu0) → Fe0 (Cu0). In the first step CuFe2O4 is reduced to Cu0 and Fe3O4, whereby Cu0 promotes the further reduction of iron oxide, increasing their rate of formation. In particular, the rate of reduction of FeO → Fe0 is accelerated most dramatically by Cu0. This is an encouraging result as the FeO → Fe0 transition is the slowest reduction reaction.

Molecularly Tailored Nickel Precursor and Support Yield a Stable Methane Dry Reforming Catalyst with Superior Metal Utilization.

Syngas production via the dry reforming of methane (DRM) is a highly endothermic process conducted under harsh conditions; hence, the main difficulty resides in generating stable catalysts. This can, in principle, be achieved by reducing coke formation, sintering, and loss of metal through diffusion in the support. [{Ni(µ2-OCHO)(OCHO)(tmeda)}2(µ2-OH2)] (tmeda = tetramethylethylenediamine), readily synthesized and soluble in a broad range of solvents, was developed as a molecular precursor to form 2 nm Ni(0) nanoparticles on alumina, the commonly used support in DRM. While such small nanoparticles prevent coke deposition and increase the initial activity, operando X-ray Absorption Near-Edge Structure (XANES) spectroscopy confirms that deactivation largely occurs through the migration of Ni into the support. However, we show that Ni loss into the support can be mitigated through the Mg-doping of alumina, thereby increasing significantly the stability for DRM. The superior performance of our catalytic system is a direct consequence of the molecular design of the metal precursor and the support, resulting in a maximization of the amount of accessible metallic nickel in the form of small nanoparticles while preventing coke deposition.

CaO-Based CO2 Sorbents Effectively Stabilized by Metal Oxides.

Calcium looping (i.e., CO2 capture by CaO) is a promising second-generation CO2 capture technology. CaO, derived from naturally occurring limestone, offers an inexpensive solution, but due to the harsh operating conditions of the process, limestone-derived sorbents undergo a rapid capacity decay induced by the sintering of CaCO3 . Here, we report a Pechini method to synthesize cyclically stable, CaO-based CO2 sorbents with a high CO2 uptake capacity. The sorbents synthesized feature compositional homogeneity in combination with a nanostructured and highly porous morphology. The presence of a single (Al2 O3 or Y2 O3 ) or bimetal oxide (Al2 O3 -Y2 O3 ) provides cyclic stability, except for MgO which undergoes a significant increase in its particle size with the cycle number. We also demonstrate a direct relationship between the CO2 uptake and the morphology of the synthesized sorbents. After 30 cycles of calcination and carbonation, the best performing sorbent, containing an equimolar mixture of Al2 O3 and Y2 O3 , exhibits a CO2 uptake capacity of 8.7 mmol CO2 g-1 sorbent, which is approximately 360 % higher than that of the reference limestone.

The development of effective CaO-based CO2 sorbents via a sacrificial templating technique.

A carbon-based sacrificial templating approach was employed to realize single-pot synthesis of cyclically stable CaO-based CO2 sorbents. The sacrificial carbonaceous template was formed through resorcinol-formaldehyde polymerization reaction. The resultant sorbents following the thermal decomposition of the carbonaceous template featured an inverse opal-like macrostructure composed of a highly porous nanostructured backbone. In addition to pure CaO, sorbents supported with Al2O3, MgO, Y2O3, and ZrO2 were synthesized. SEM and XRD were utilized to characterize the morphology and the chemical composition of the synthetic CO2 sorbents, respectively. The cyclic CO2 uptake performance of the synthetic sorbents was assessed by TGA and compared to limestone. All of the synthetic sorbents exhibited an improved CO2 uptake performance when compared to limestone. The performance enhancement became more pronounced in the case of supported sorbents. The sorbent with the best CO2 uptake performance was supported by a mixture of Al2O3 and Y2O3, and exhibited a CO2 uptake of 0.61 g CO2/g CaO after 10 cycles of calcination and carbonation under practically relevant operating temperatures, which exceeded the CO2 uptake of limestone by more than 350%.