The dry reforming of methane (DRM), i.e., the reaction of methane and CO 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 MgAlO matrix derived via a hydrotalcite-like precursor were synthesized. Importantly, the textural features of the catalysts, i.e., the specific surface area (172-178 m/g), pore volume (0.51-0.66 cm/g), and particle size (5.4-5.8 nm) were kept constant. Bimetallic, NiFe 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.

Transition metal nanoparticles (NPs) are typically supported on oxides to ensure their stability, which may result in modification of the original NP catalyst reactivity. In a number of cases, this is related to the formation of NP/support interface sites that play a role in catalysis. The metal/support interface effect verified experimentally is commonly ascribed to stronger reactants adsorption or their facile activation on such sites compared to bare NPs, as indicated by DFT-derived potential energy surfaces (PESs). However, the relevance of specific reaction elementary steps to the overall reaction rate depends on the preferred reaction pathways at reaction conditions, which usually cannot be inferred based solely on PES. Hereby, we use a multiscale (DFT/microkinetic) modeling approach and experiments to investigate the reactivity of the Ni/AlO interface toward water-gas shift (WGS) and dry reforming of methane (DRM), two key industrial reactions with common elementary steps and intermediates, but held at significantly different temperatures: 300 vs 650 °C, respectively. Our model shows that despite the more energetically favorable reaction pathways provided by the Ni/AlO interface, such sites may or may not impact the overall reaction rate depending on reaction conditions: the metal/support interface provides the active site for WGS reaction, acting as a reservoir for oxygenated species, while all Ni surface atoms are active for DRM. This is in contrast to what PESs alone indicate. The different active site requirement for WGS and DRM is confirmed by the experimental evaluation of the activity of a series of AlO-supported Ni NP catalysts with different NP sizes (2-16 nm) toward both reactions.

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(μ-OCHO)(OCHO)(tmeda)}(μ-OH)] (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.

The conversion of methane and carbon dioxide into a synthesis gas, the so-called dry reforming of methane (DRM), suffers from a stability issue caused by coke formation at the surface of the Ni-based catalysts. Using a colloidal approach, we demonstrate that supported 3−4 nm bimetallic NiFe nanoparticles with a Ni/Fe ratio of 3 have an enhanced stability compared to the corresponding pure Ni-based catalyst and a higher activity compared to conventional NiFe catalysts. The active sites for DRM are associated with Ni 0 , while FeO, observed by operando XAS under DRM conditions, allows for an effective decoking of the metal centers.

CaO is an effective high temperature CO 2 sorbent that, however, suffers from a loss of its CO 2 absorption capacity upon cycling due to sintering. The cyclic CO 2 uptake of CaO-based sorbents is improved by Ca 3 Al 2 O 6 as a structural stabilizer. Nonetheless, the initially rather stable CO 2 uptake of Ca 3 Al 2 O 6 -stabilized CaO yet starts to decay after around 10 cycles of CO 2 capture and sorbent regeneration, albeit at a significantly reduced rate compared to the unmodified reference material. Here, we show by a combined use of in situ XRD together with textural and morphological characterization techniques (SEM, STEM, and N 2 physisorption) and solid-state 27 Al NMR (in particular dynamic nuclear polarization surface enhanced NMR spectroscopy, DNP SENS) how microscopic changes trigger the sudden onset of deactivation of Ca 3 Al 2 O 6 -stabilized CaO. After a certain number of CO 2 capture and regeneration cycles (approximately 10), Ca 3 Al 2 O 6 transformed into Ca 12 Al 14 O 33 , followed by Al 2 O 3 segregation and enrichment at the surface in the form of small nanoparticles. Al 2 O 3 in such a form is not able to stabilize effectively the initially highly porous structure against thermal sintering, leading in turn to a reduced CO 2 uptake.