Contrasting the Role of Ni/Al₂O₃ Interfaces in Water–Gas Shift and Dry Reforming of Methane

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/Al₂O₃ 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/Al₂O₃ 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 Al₂O₃-supported Ni NP catalysts with different NP sizes (2–16 nm) toward both reactions

<i>In Situ</i> XRD and Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy Unravel the Deactivation Mechanism of CaO-Based, Ca₃Al₂O₆‑Stabilized CO₂ Sorbents

CaO is an effective high temperature CO₂ sorbent that, however, suffers from a loss of its CO₂ absorption capacity upon cycling due to sintering. The cyclic CO₂ uptake of CaO-based sorbents is improved by Ca₃Al₂O₆ as a structural stabilizer. Nonetheless, the initially rather stable CO₂ uptake of Ca₃Al₂O₆-stabilized CaO yet starts to decay after around 10 cycles of CO₂ capture and sorbent regeneration, albeit at a significantly reduced rate compared to the unmodified reference material. Here, we show by a combined use of <i>in situ</i> XRD together with textural and morphological characterization techniques (SEM, STEM, and N₂ physisorption) and solid-state ²⁷Al NMR (in particular dynamic nuclear polarization surface enhanced NMR spectroscopy, DNP SENS) how microscopic changes trigger the sudden onset of deactivation of Ca₃Al₂O₆-stabilized CaO. After a certain number of CO₂ capture and regeneration cycles (approximately 10), Ca₃Al₂O₆ transformed into Ca₁₂Al₁₄O₃₃, followed by Al₂O₃ segregation and enrichment at the surface in the form of small nanoparticles. Al₂O₃ in such a form is not able to stabilize effectively the initially highly porous structure against thermal sintering, leading in turn to a reduced CO₂ uptake

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 Mg_<i>x</i>Al_<i>y</i>O_<i>z</i> 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/g_cat), pore volume (0.51–0.66 cm³/g_cat), 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

Conformal Deposition of Conductive Single-Crystalline Cobalt Silicide Layer on Si Wafer via a Molecular Approach

The realization of metal–semiconductor contacts plays a significant role in ultrascaled integrated circuits. Here, we establish a low-temperature molecular approach for the conformal deposition of a 20 nm Co-rich layer on Si (100) wafers by reaction in solution of Co₂(CO)₈ with SiH₄. Postannealing at 850 °C under vacuum (∼10^–5 mbar) yields a crystalline CoSi₂ film with a lower surface roughness (<i>R</i>_rms = 5.3 nm) by comparison with the conventional physical method; this layer exhibiting a metallic conductive behavior (ohmic behavior) with a low resistivity (ρ = 11.6 μΩ cm) according to four-point probe measurement. This approach is applicable to trench-structured wafers, showing the conformal layer deposition on 3D structures and showcasing the potential of this approach in modern transistor technology