Promotional effects in alumina and silica supported bimetallic Ni–Fe catalysts during CO2 hydrogenation

Engineered Catalyst Based on MIL-68(Al) with High Stability for Hydrogenation of Carbon Dioxide and Carbon Monoxide at Low Temperature

Today, the importance of decreasing and converting COx gases from the atmosphere into value-added chemicals by catalytic hydrogenation reactions has become one crucial challenge. In the current work, to facilitate the hydrogenation of COx, several mesoporous alumina catalysts with high efficiency and stability were synthesized using the MIL-68(Al) platform, a nanoporous MOF with a high surface area as a precatalyst, encapsulating nickel or nickel-iron nanoparticles (NPs). After removing the organic linker of MIL-68(Al) by calcination in air, two types of catalysts, promoted and unpromoted, were obtained with various loads of nickel and iron. A set of analyses (PXRD, BET-N2, TEM, FE-SEM, ICP-OES, EDX-map, CO2-TPD, H2-TPR, and H2-TPD) were performed to evaluate the physicochemical properties of catalysts. Based on the analysis results, the promoted catalyst had smaller particles and pores due to the effective and uniform distribution of nickel NPs. Also, H2-TPR and CO2-TPD results in samples containing Fe promoter demonstrated the facilitation of the reduction process and the adsorption and activation of CO2, respectively. The results of CO2 methanation indicated an improved catalytic performance for promoted samples, especially at low temperatures (200-300 °C), compared to unpromoted catalysts. 5Fe·15Ni@Al2O3 MIL-68(Al) catalyst displayed the best performance compared to other catalysts, with a conversion of 92.4% and selectivity of 99.6% at 350 °C and GHSV = 2500 h-1. Moreover, the 5Fe·15Ni@Al2O3 MIL-68(Al) catalyst facilitated the CO2 methanation reaction by reducing the activation energy to 42.5 kJ mol-1 compared with other reported catalysts. Both types of catalysts performed 100% hydrogenation of CO to CH4 with full selectivity at 250 °C and exhibited high stability for at least 100 h at 300 °C. Notably, such high significant catalytic performance is only achieved by the usage of the "MOFs templating strategy" due to the high surface area for the effective distribution of NPs, the strong metal-support interaction, and the formation of nickel aluminate species, preventing the sintering of NPs.

The role of surface oxidation and Fe-Ni synergy in Fe-Ni-S catalysts for CO2 hydrogenation

Increasing carbon dioxide (CO₂) emissions, resulting in climate change, have driven the motivation to achieve the effective and sustainable conversion of CO₂ into useful chemicals and fuels. Taking inspiration from biological processes, synthetic iron-nickel-sulfides have been proposed as suitable catalysts for the hydrogenation of CO₂. In order to experimentally validate this hypothesis, here we report violarite (Fe,Ni)₃S₄ as a cheap and economically viable catalyst for the hydrogenation of CO₂ into formate under mild, alkaline conditions at 125 °C and 20 bar (CO₂ : H₂ = 1 : 1). Calcination of violarite at 200 °C resulted in excellent catalytic activity, far superior to that of Fe-only and Ni-only sulfides. We further report first principles simulations of the CO₂ conversion on the partially oxidised (001) and (111) surfaces of stoichiometric violarite (FeNi₂S₄) and polydymite (Ni₃S₄) to rationalise the experimentally observed trends. We have obtained the thermodynamic and kinetic profiles for the reaction of carbon dioxide (CO₂) and water (H₂O) on the catalyst surfaces via substitution and dissociation mechanisms. We report that the partially oxidised (111) surface of FeNi₂S₄ is the best catalyst in the series and that the dissociation mechanism is the most favourable. Our study reveals that the partial oxidation of the FeNi₂S₄ surface, as well as the synergy of the Fe and Ni ions, are important in the catalytic activity of the material for the effective hydrogenation of CO₂ to formate.

Bimetallic Ni-Based Catalysts for CO2 Methanation: A Review

CO₂ methanation has recently emerged as a process that targets the reduction in anthropogenic CO₂ emissions, via the conversion of CO₂ captured from point and mobile sources, as well as H₂ produced from renewables into CH₄. Ni, among the early transition metals, as well as Ru and Rh, among the noble metals, have been known to be among the most active methanation catalysts, with Ni being favoured due to its low cost and high natural abundance. However, insufficient low-temperature activity, low dispersion and reducibility, as well as nanoparticle sintering are some of the main drawbacks when using Ni-based catalysts. Such problems can be partly overcome via the introduction of a second transition metal (e.g., Fe, Co) or a noble metal (e.g., Ru, Rh, Pt, Pd and Re) in Ni-based catalysts. Through Ni-M alloy formation, or the intricate synergy between two adjacent metallic phases, new high-performing and low-cost methanation catalysts can be obtained. This review summarizes and critically discusses recent progress made in the field of bimetallic Ni-M (M = Fe, Co, Cu, Ru, Rh, Pt, Pd, Re)-based catalyst development for the CO₂ methanation reaction.

Structural dynamics in Ni–Fe catalysts during CO2 methanation – role of iron oxide clusters

Operando XAS coupled with MES supported by DFT unravel the highly dynamic nature of Ni–Fe catalysts during CO₂ methanation and beneficial formation of iron oxide clusters.

The role of surface properties in CO2 methanation over carbon-supported Ni catalysts and their promotion by Fe

CO₂ methanation over activated carbon-supported Ni catalysts with enhanced surface chemistry properties and their improved performance by Fe promotion.

Ni Promotion by Fe: What Benefits for Catalytic Hydrogenation?

Metallic nickel is known to efficiently catalyze hydrogenation reactions, but one of its major drawbacks lies in its lack of selectivity, linked to side-reactions of hydrogenolysis and over-hydrogenation. More selective hydrogenations can be obtained upon the introduction of a second metal in combination with Ni. Fe is an interesting choice, as it is a cheap and abundant metal. This review aims at discussing the advantages and constraints brought by the preparation procedures of bimetallic supported Ni–Fe nanoparticles, and at analyzing the benefits one can draw by substituting Ni–Fe supported catalysts for Ni monometallic systems for the catalytic hydrogenation of organic molecules. Specific formulations, such as Ni75Fe25, will be singled out for their high activity or selectivity, and the various hypotheses behind the roles played by Fe will be summarized.

Deactivation mechanism of hydrotalcite-derived Ni–AlOx catalysts during low-temperature CO2 methanation via Ni-hydroxide formation and the role of Fe in limiting this effect

Ni–Fe/AlO_x with nanosheet structure, enhance the reducibility and stability of the Ni-hydroxide during the catalytic reaction due to the formation of spinel phase which stabilize smaller Ni nanoparticle with a weaker interaction with the support.

Hydrotalcite based Ni–Fe/(Mg, Al)Ox catalysts for CO2 methanation – tailoring Fe content for improved CO dissociation, basicity, and particle size

Advantages of hydrotalcite-like precursors and the synergistic effect of bimetallic Ni–Fe alloys are combined and the most appropriate amount of Fe identified with respect to activity, selectivity and stability.

Developing descriptors for CO2 methanation and CO2 reforming of CH4 over Al2O3 supported Ni and low-cost Ni based alloy catalysts

The catalytic performance of Ni can be modified by alloying with a suitable amount (25% of total metal loading) of another low-cost metal such as Fe, Co or Cu.

Direct versus hydrogen-assisted CO dissociation over stepped Ni and Ni3Fe surfaces: a computational investigation

The adsorption and dissociation of CO over stepped Ni and Ni3Fe surfaces were systematically studied using density functional theory slab calculations. Both (211)-like surface structure terminations (NiNi step and NiFe step, denoted as Ni3Fe(211)-AA and Ni3Fe(211)-AB) are considered for Ni3Fe. Direct scission of the C-O bond in CO is identified as the least likely one among the three proposed dissociation pathways and CO dissociation via a CHO intermediate appears to be most feasible at low CO coverage on pure and alloyed Ni(211) surfaces. The priority of H-assisted CO dissociation might originate from the more activated C-O bond in COH and CHO. Compared to Ni(211), the Ni3Fe(211)-AB surface could facilitate CO activation especially for the most possible CHO intermediate mechanism, whose rate-limiting step is found to be altered. The d-band center theory and Mulliken charge analysis are also employed to explain the activity difference between Ni3Fe(211)-AB and Ni3Fe(211)-AA. The significant structural sensitivity of CO dissociation highlights the importance of Fe locating in the step edge and the high reactivity of Ni3Fe(211)-AB is largely ascribed to the synergistic effect between Ni and Fe at the step edge.