The nanoscale description of the reaction pathways and of the role of the intermediate species involved in a chemical process is a crucial milestone for tailoring more active, stable, and cheaper catalysts, thus providing “reaction engineering” capabilities. This level of insight has not been achieved yet for the catalytic hydrogenation of CO2 on Ni catalysts, a reaction of enormous environmental relevance. We present a thorough atomic-scale description of the mechanisms of this reaction, studied under controlled conditions on a model Ni catalyst, thus clarifying the long-standing debate on the actual reaction path followed by the reactants. Remarkably, formate, which is always observed under standard conditions, is found to be just a “dead-end” spectator molecule, formed via a Langmuir−Hinshelwood process, whereas the reaction proceeds through parallel Eley−Rideal channels, where hydrogen-assisted C−O bond cleavage in CO2 yields CO already at liquid nitrogen temperature.

We demonstrate that the key step for the reaction of CO 2 with hydrogen on Ni(110) is a change of the activated molecule coordination to the metal surface. At 90 K, CO 2 is negatively charged and chemically bonded via the carbon atom. When the temperature is increased and H approaches, the H-CO 2 complex flips and binds to the surface through the two oxygen atoms, while H binds to the carbon atom, thus yielding formate. We provide the atomic-level description of this process by means of conventional ultrahigh vacuum surface science techniques combined with density functional theory calculations and corroborated by high pressure reactivity tests. Knowledge about the details of the mechanisms involved in this reaction can yield a deeper comprehension of heterogeneous catalytic organic synthesis processes involving carbon dioxide as a reactant. We show why on Ni the CO 2 hydrogenation barrier is remarkably smaller than that on the common Cu metal-based catalyst. Our results provide a possible interpretation of the observed high catalytic activity of NiCu alloys.

Industrial catalysts are typically made of nanosized metal particles, carried by a solid support. The extremely small size of the particles maximizes the surface area exposed to the reactant, leading to higher reactivity. Moreover, the higher the number of metal atoms in contact with the support, the better the catalyst performance. In addition, peculiar properties have been observed for some metal/metal oxide particles of critical sizes. However, thermal stability of these nanostructures is limited by their size; smaller the particle size, the lower the thermal stability. The ability to fabricate and control the structure of nanoparticles allows to influence the resulting properties and, ultimately, to design stable catalysts with the desired characteristics. Tuning particle sizes provides the possibility to modulate the catalytic activity. Unique and unexpected properties have been observed by confining/embedding metal nanoparticles in inorganic channels or cavities, which indeed offers new opportunities for the design of advanced catalytic sytems. Innovation in catalyst design is a powerful tool in realizing the goals of more green, efficient and sustainable industrial processes. The present Review focuses on the catalytic performance of noble metal‐ and non precious metal‐based embedded catalysts with respect to traditional impregnated systems. Emphasis is dedicated to the improved thermal stability of these nanostructures compared to conventional systems.

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We present a combined experimental and theoretical study of the CO(2) interaction with the Ni(110) surface. Photoelectron spectroscopy, temperature-programmed desorption, and high-resolution electron energy loss spectroscopy measurements are performed at different coverages and for increasing surface temperature after adsorption at 90 K with the aim to study the competing processes of CO(2) dissociation and desorption. Simulations are performed within the framework of density functional theory using ab initio pseudopotentials, focusing on selected chemisorption geometries, determining the energetics and the structural and vibrational properties. Both experimental and theoretical vibrational frequencies yield consistent indications about two inequivalent adsorption sites that can be simultaneously populated at low temperature: short-bridge site with the molecular plane perpendicular to the surface and hollow site with the molecular plane inclined with respect to the surface. In both sites, the molecule has pure carbon or mixed oxygen-carbon coordination with the metal and is negatively charged and bent. Predicted energy barriers for adsorption and diffusion on the surface suggest a preferential adsorption path through the short-bridge site to the hollow site, which is compatible with the experimental findings. Theoretical results qualitatively support literature data concerning the increase of the work function upon chemisorption

La0.6Sr0.4Co1−y FeyO3−δ (y = 0.2, 0.5, 0.8) perovskite-type oxide powders were prepared by the citrate gel method with the aim of investigating the influence of cobalt/iron atomic ratio on the chemical and structural properties as well as on the catalytic activity. The samples were characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermal analysis, Kr physisorption surface area, scanning electron microscopy (SEM), and temperature programmed reduction (TPR). XRD outcomes reveal the formation of perovskite phase after calcination at 873 K. Depending on the Co/Fe atomic ratio, the perovskite exhibits two different structure: rhombohedral for y < 0.5, orthorhombic for y ≥ 0.5. However, other phases are also observed. The crystallite size increases with the calcination temperature, while decreases with the iron content. The specific surface area of the La0.6Sr0.4Co1−y FeyO3-δ perovskites (calcined at 1073 K) is low, but increases with the iron amount: it varies between 4.1 and 6.6 m2 g−1 moving from y = 0.2 to y = 0.8. XPS results reveal the presence of traces of Co(II) in the sample with lower cobalt amount. The scanning electron micrographs reveal nanoscaled near spherical particles which are clustered together, forming a highly porous microstructure. However, the samples with higher iron content show a more compact morphology due to the enhanced agglomeration. The perovskite phase is rather stable even after aggressive treatment as high temperature reduction. The influence of cobalt/iron atomic ratio on the catalytic activity toward alcohol steam reforming was investigated. All the samples present good alcohol dehydrogenation activity, as resulting from the methanol steam reforming experiments. In the steam reforming of ethanol, the La0.6Sr0.4Co0.5Fe0.5O3-δ presents the better performance (especially after reduction at 873 K) with the complete conversion of ethanol into syngas above 850 K and the almost complete suppression of the ethylene formation at lower temperature. The differences of the activity toward ethanol steam reforming are attributable to a different stabilization of Co(0) nanoparticles produced during the reduction at 873 K.

Despite the wide application of ceria-zirconia based materials in Three Way Catalysts (TWCs), Solid Oxides Fuel Cells (SOFCs), and H(2) production and purification reactions, an active debate is still open on the correlation between their structure and redox/catalytic performances. Existing reports support the need of either (i) a homogeneous solid solution or (ii) materials with nanoscale heterogeneity to obtain high activity and stability. Here we report on a simple and inexpensive approach to solve this problem taking advantage of the luminescence properties of Eu(III), used as a structural probe introduced either in the bulk or on the surface of the samples. In this way, the real structure of ceria-zirconia materials can be revealed even for amorphous high surface area samples. Formation of small domains is observed in catalytically important metastable samples which appear homogeneous by conventional XRD.

Ni(x wt.%) Cu(y wt.%)/Al2O3 samples were investigated as active and thermally stable catalysts for methanol and ethanol steam reforming. XRD data clearly evidenced the formation of a NiCu alloy under the adopted preparation procedure. The bimetallic systems exhibited improved activity in the methanol steam reforming with respect to the monometallic ones. The introduction of copper in the catalyst formulation showed a positive effect inhibiting the formation of methane, an undesirable by-product. On the other hand, in the ethanol steam reforming, the catalytic performance was less promising. Furthermore, the Ni : Cu ratio did not seem to affect the product distribution. However, enhanced stability was observed in the two subsequent run-up experiments, indicating the positive role played by the bimetallic systems