A theoretical overview on the prevention of coking in dry reforming of methane using non-precious transition metal catalysts

Ni/MSS@CeO2 sandwich catalysts for methane dry reforming: the role of reduction on oxygen vacancies

In-depth exploration of the key role of reduction temperature in the Ni-SiO2-CeO2 system DRM catalysts remains a huge challenge. This work deeply analyzes the role of reduction temperature on the oxygen vacancies and structure of the Ni/MSS@CeO2 (MSS refers to mesoporous SiO2 spheres with inverted cone pores) sandwich structure catalyst. Studies have shown that appropriate reduction temperature can significantly facilitate the Ni distribution, boost the Ni-CeO2 interaction, and thus decrease the CO2 and CH4 activation energy. As the reduction temperature rises, the crystallinity of Ce9.33(SiO4)6O2 increases, and catalyst's oxygen vacancy concentration increases. However, the Ce9.33(SiO4)6O2 weakens the interaction between Ni and Ce. Its excellent thermal stability and weak redox ability result in its oxygen vacancies being unable to effectively activate CO2. Moreover, the Ce9.33(SiO4)6O2 with good crystallinity will cover some Ni active sites. When the reduction temperature is 600 °C, its strong Ni-CeO2 interface promoted the cracking of CH4, and the active oxygen species provided by the adsorbed CO2 significantly inhibited carbon deposition. The carbon deposits after 30 h of high space velocity and low temperature reaction are minimal. In situ DRIFT experiments found that the active oxygen species may promptly react with methane cracking products in a timely manner, preventing CH4 from deeply breaking and reducing carbon deposits and graphitization. This work offers some guidelines for designing appropriate DRM catalysts for the Ni-SiO2-CeO2 system.

Synthesis of the NiO-Faujasite Nanocatalyst for Dry Reforming of Methane: The Effect of the Aniline Additive

Dry reforming of methane (DRM) using a heterogeneous catalyst presents one of the CO2 mitigation pathways to address global warming and climate change challenges. Such a suitable DRM catalyst with optimum activity and stability is still under intense research. We herein present a facile, slightly modified version of the conventional wet impregnation method to synthesize a NiO-faujasite nanocatalyst for DRM with the help of aniline, judiciously chosen based on the hard-soft acid-base (HSAB) principle. The resulting catalyst was characterized by the N2 adsorption isotherm, PXRD, SEM/TEM, XPS, 29Si solid-state NMR, H2-TPR, NH3-TPD, and DRM reaction, and its results were compared with those without aniline assistance. A smaller NiO nanoparticle with better dispersion was observed for our aniline-assisted sample resulting in a significant increase in activity (peaking at 86% CH4 conversion with a H2/CO ratio of 0.93) and stability for a 12 h time on stream. We hope that this work would pave the way to utilize the HSAB principle to synthesize more nanocatalysts with optimum overall performance.

Nanoparticle Exsolution from Nanoporous Perovskites for Highly Active and Stable Catalysts

Nanoporosity is clearly beneficial for the performance of heterogeneous catalysts. Although exsolution is a modern method to design innovative catalysts, thus far it is predominantly studied for sintered matrices. A quantitative description of the exsolution of Ni nanoparticles from nanoporous perovskite oxides and their effective application in the biogas dry reforming is here presented. The exsolution process is studied between 500 and 900 °C in nanoporous and sintered La0.52 Sr0.28 Ti0.94 Ni0.06 O3±δ . Using temperature-programmed reduction (TPR) and X-ray absorption spectroscopy (XAS), it is shown that the faster and larger oxygen release in the nanoporous material is responsible for twice as high Ni reduction than in the sintered system. For the nanoporous material, the nanoparticle formation mechanism, studied by in situ TEM and small-angle X-ray scattering (SAXS), follows the classical nucleation theory, while on sintered systems also small endogenous nanoparticles form despite the low Ni concentration. Biogas dry reforming tests demonstrate that nanoporous exsolved catalysts are up to 18 times more active than sintered ones with 90% of CO2 conversion at 800 °C. Time-on-stream tests exhibit superior long-term stability (only 3% activity loss in 8 h) and full regenerability (over three cycles) of the nanoporous exsolved materials in comparison to a commercial Ni/Al2 O3 catalyst.

Electronic structure and catalytic activity of exsolved Ni on Pd core-shell nanoparticles

This study reports first principles calculations performed to study the electronic structure and catalytic activity of exsolved Ni on Pd core-shell catalysts reported in recent experimental literature. The modification in the electronic and geometric properties of the Ni/Pd bimetallic system as successive layers of Ni are added on top of Pd is systematically investigated using the d-band model as well as the adsorption of O and CO on the surface of these core-shell structures. The results show that the adsorption of O and CO is more favourable on Ni/Pd core-shell catalysts compared to the pure Ni surface. As the dissociation of the O₂ molecule into atomic oxygen and CO oxidation are key steps in metal-catalysed oxidation reactions, we have examined the energetics of O₂ dissociation and CO oxidation reaction over the (111) faces of Ni as well as Ni/Pd structures. Our results suggest that both adsorption and dissociation are easier on Ni/Pd surfaces compared to a simple Ni surface. Unlike O₂ dissociation, we find that CO oxidation is unfavourable on Ni/Pd in comparison to Ni. The energetics of both reactions follow Brønsted-Evans-Polanyi relationships where the activation energy is linearly related to the reaction energy for all surfaces studied here. We found that a single monolayer of Ni on Pd, due to the synergistic effect of geometric and electronic factors, is the most active among the surfaces studied here towards the adsorption and dissociation of O₂. Both adsorption and dissociation become less favourable with an increase in the thickness of the Ni shell in these core-shell catalysts. A close analysis of the results indicates that both strain and ligand effects are active in the improved catalytic activity seen in Ni on Pd catalysts. Quite understandably, the ligand effect is only seen for the single monolayer of Ni on Pd and fades off as we go to two monolayers of Ni. The results reported here help us understand the connections between the electronic structure and catalytic activity of Ni/Pd core-shell nanoparticles, and these insights are expected to be useful in the development of core-shell catalysts.

Metal (Mo, W, Ti) Carbide Catalysts: Synthesis and Application as Alternative Catalysts for Dry Reforming of Hydrocarbons-A Review

Dry reforming of hydrocarbons (DRH) is a pro-environmental method for syngas production. It owes its pro-environmental character to the use of carbon dioxide, which is one of the main greenhouse gases. Currently used nickel catalysts on oxide supports suffer from rapid deactivation due to sintering of active metal particles or the deposition of carbon deposits blocking the flow of gases through the reaction tube. In this view, new alternative catalysts are highly sought after. Transition metal carbides (TMCs) can potentially replace traditional nickel catalysts due to their stability and activity in DR processes. The catalytic activity of carbides results from the synthesis-dependent structural properties of carbides. In this respect, this review presents the most important methods of titanium, molybdenum, and tungsten carbide synthesis and the influence of their properties on activity in catalyzing the reaction of methane with carbon dioxide.