Difference in reaction mechanism between ZnZrOx and InZrOx for CO2 hydrogenation

Oxide solid-solution catalysts, such as Zn-doped ZrO2 (ZnZrOx) and In-doped ZrO2 (InZrOx), exhibit distinctive catalytic capabilities for CH3OH synthesis via CO2 hydrogenation. We investigated the active site structures of these catalysts and their associated reaction mechanisms using both experimental and computational approaches. Electron microscopy and X-ray absorption spectroscopy reveal that the primary active sites are isolated cations, such as Zn2+ and In3+, dissolved in tetragonal ZrO2. Notably, for Zn2+, decomposition of the methoxy group, which is an essential intermediate in CH4 synthesis, is partially suppressed because of the relatively high stability of the methoxy group. Conversely, the methyl group strongly adsorbs on In3+, facilitating the conversion of the methoxy species into methyl groups. The decomposition of CH3OH is also suggested to contribute to CH4 synthesis. These results highlight the generation of CH4 as a byproduct of the InZrOx catalyst. Understanding the active site structure and elucidating the reaction mechanism at the atomic level are anticipated to contribute significantly to the future development of oxide solid-solution catalysts.

Engineering the Quaternary Hydrotalcite-Derived Ce-Promoted Ni-Based Catalysts for Enhanced Low-Temperature CO2 Hydrogenation into Methane

Ce-promoted NiMgAl mixed-oxide (NiCex-C, x = 0, 1, 5, 10) catalysts were prepared from the quaternary hydrotalcite precursors for CO₂ hydrogenation to methane. By engineering the Ce contents, NiCe5-C showed its prior catalytic performance in low-temperature CO₂ hydrogenation, being about three times higher than that of the Ce-free NiCe0-C catalyst (turnover frequency of NiCe5-C and NiCe0-C: 11.9 h^-1 vs. 3.9 h^-1 @ 225 °C). With extensive characterization, it was found that Ce dopants promoted the reduction of NiO by adjusting the interaction between Ni and Mg(Ce)AlOx support. The highest ratio of surface Ni⁰/(Ni²⁺ + Ni⁰) was obtained over NiCe5-C. Meanwhile, the surface basicity was tailored with Ce dopants. The strongest medium-strength basicity and highest capacity of CO₂ adsorption was achieved on NiCe5-C with 5 wt.% Ce content. The TOF tests indicated a good correlation with medium-strength basicity over the NiCex-C samples. The results showed that the high medium-strength and Ce-promoted surface Ni⁰ species endows the enhanced low-temperature catalytic performance in CO₂ hydrogenation to methane.

Advanced zeolite and ordered mesoporous silica-based catalysts for the conversion of CO2 to chemicals and fuels

For many years, capturing, storing or sequestering CO₂ from concentrated emission sources or from air has been a powerful technique for reducing atmospheric CO₂. Moreover, the use of CO₂ as a C1 building block to mitigate CO₂ emissions and, at the same time, produce sustainable chemicals or fuels is a challenging and promising alternative to meet global demand for chemicals and energy. Hence, the chemical incorporation and conversion of CO₂ into valuable chemicals has received much attention in the last decade, since CO₂ is an abundant, inexpensive, nontoxic, nonflammable, and renewable one-carbon building block. Nevertheless, CO₂ is the most oxidized form of carbon, thermodynamically the most stable form and kinetically inert. Consequently, the chemical conversion of CO₂ requires highly reactive, rich-energy substrates, highly stable products to be formed or harder reaction conditions. The use of catalysts constitutes an important tool in the development of sustainable chemistry, since catalysts increase the rate of the reaction without modifying the overall standard Gibbs energy in the reaction. Therefore, special attention has been paid to catalysis, and in particular to heterogeneous catalysis because of its environmentally friendly and recyclable nature attributed to simple separation and recovery, as well as its applicability to continuous reactor operations. Focusing on heterogeneous catalysts, we decided to center on zeolite and ordered mesoporous materials due to their high thermal and chemical stability and versatility, which make them good candidates for the design and development of catalysts for CO₂ conversion. In the present review, we analyze the state of the art in the last 25 years and the potential opportunities for using zeolite and OMS (ordered mesoporous silica) based materials to convert CO₂ into valuable chemicals essential for our daily lives and fuels, and to pave the way towards reducing carbon footprint. In this review, we have compiled, to the best of our knowledge, the different reactions involving catalysts based on zeolites and OMS to convert CO₂ into cyclic and dialkyl carbonates, acyclic carbamates, 2-oxazolidones, carboxylic acids, methanol, dimethylether, methane, higher alcohols (C₂₊OH), C₂₊ (gasoline, olefins and aromatics), syngas (RWGS, dry reforming of methane and alcohols), olefins (oxidative dehydrogenation of alkanes) and simple fuels by photoreduction. The use of advanced zeolite and OMS-based materials, and the development of new processes and technologies should provide a new impulse to boost the conversion of CO₂ into chemicals and fuels.

A Review on the Different Aspects and Challenges of the Dry Reforming of Methane (DRM) Reaction

The dry reforming of methane (DRM) reaction is among the most popular catalytic reactions for the production of syngas (H₂/CO) with a H₂:CO ratio favorable for the Fischer-Tropsch reaction; this makes the DRM reaction important from an industrial perspective, as unlimited possibilities for production of valuable products are presented by the FT process. At the same time, simultaneously tackling two major contributors to the greenhouse effect (CH₄ and CO₂) is an additional contribution of the DRM reaction. The main players in the DRM arena-Ni-supported catalysts-suffer from both coking and sintering, while the activation of the two reactants (CO₂ and CH₄) through different approaches merits further exploration, opening new pathways for innovation. In this review, different families of materials are explored and discussed, ranging from metal-supported catalysts, to layered materials, to organic frameworks. DRM catalyst design criteria-such as support basicity and surface area, bimetallic active sites and promoters, and metal-support interaction-are all discussed. To evaluate the reactivity of the surface and understand the energetics of the process, density-functional theory calculations are used as a unique tool.

Kinetic Study on CO-Selective Methanation over Nickel-Based Catalysts for Deep Removal of CO from Hydrogen-Rich Reformate

The CO-selective methanation process is considered as a promising CO removal process for compact fuel processors producing hydrogen, since the process selectively converts the trace of CO in the hydrogen-rich gas into methane without additional reactants. Two different types of efficient nickel-based catalysts, showing high activity and selectivity to the CO methanation reaction, were developed in our previous works; therefore, the kinetic models of the reactions over these nickel-based catalysts have been investigated adopting the mechanistic kinetic models based on the Langmuir chemisorption theory. In the methanation process, the product species can react with the reactant and also affect the adsorption/desorption of the molecules at the active sites. Thus, the kinetic parameter study should be carried out by global optimization handling all the rate equations for the plausible reactions at once. To estimate the kinetic parameters, an effective optimization algorithm combining both heuristic and deterministic methods is used due to the large solution space and the nonlinearity of the objective function. As a result, 14 kinetic parameters for each catalyst have been determined and the parameter sets for the catalysts have been compared to understand the catalytic characteristics.

Chemical route to prepare nickel supported on intermetallic Ti6Si7Ni16 nanoparticles catalyzing CO methanation

In this study, ternary intermetallic nickel silicide, Ti₆Si₇Ni₁₆, nanoparticles with a high surface area of 37.5 m² g^-1 were chemically prepared from SiO₂-impregnated oxide precursors, which were reduced at as low as 600 °C by a CaH₂ reducing agent in molten LiCl, resulting in the formation of single-phase Ti₆Si₇Ni₁₆ with a nanosized morphology. The intermetallic Ti₆Si₇Ni₁₆ phase in the nanoparticles was stabilized in air by surface passive oxide layers of TiO_x-SiO_y, which facilitated the handling of the nanoparticles. Considering our previous successful work of preparing single-phase LaNi₂Si₂ (39.3 m² g^-1) and YNi₂Si₂ (27.0 m² g^-1) nanoparticles in a similar manner, the proposed chemical method showed to be a versatile approach in preparing ternary silicide nanoparticles. In this study, we applied the obtained Ti₆Si₇Ni₁₆ nanoparticles as catalyst supports in CO methanation. The supported nickel catalyst showed an activation energy of 56 kJ mol^-1, which is half as low as that of common TiO₂-supported nickel catalysts. Also, Ni/Ti₆Si₇Ni₁₆ provided the lower activation energy more than any previous Ni-based catalyst. Since the measured work function of Ti₆Si₇Ni₁₆ (4.5 eV) was lower than that of nickel (5.15 eV), it was suggested that the Ti₆Si₇Ni₁₆ support can accelerate the rate-determining step of C-O bond dissociation in CO methanation due to its good electron donation capacity.

On the support dependency of the CO2 methanation – decoupling size and support effects

The influence of the support basicity, according to the Lewis and Brønsted definition, was investigated for the CO₂ methanation over isostructural Ru catalysts.

Tailoring the synergistic dual-decoration of (Cu–Co) transition metal auxiliaries in Fe-oxide/zeolite composite catalyst for the direct conversion of syngas to aromatics

Tailoring the crystal lattice and multiple phase interfaces via the feasible accommodation of Cu–Co into the host (Fe) structure, expedited the surface oxygen vacancies that modulated the reduction/chemisorption behavior of active Fe species.

Thermochemical and electrochemical aspects of carbon dioxide methanation: A sustainable approach to generate fuel via waste to energy theme

Sustainable generation of green energy and fine chemicals from carbon dioxide (CO₂) is the most desirable and promising route for justifying the atmospheric CO₂ build-up and carbon sequestration. This additionally serves to mitigate or defer global warming and avoid serious climate change. Renewable carbon is a possible source to reduce CO₂ emission and avoid the combustion of coal and petroleum products. In this context, there is a dire need to introduce modern industrial procedures to develop new carbon recycling strategies for CO_2, like spent carbon from CO₂. The role of diverse industrial processes for the proper utilization of renewable carbon would fruitfully simulate the natural procedure. For the past few decades, both heterogeneous and homogeneous catalysis approaches have been useful for the conversion of CO₂. Still, unfortunately, none of them addressed the current safety needs, cost-effectiveness, efficiency, reaction conditions, and selectivity. This review signifies the thermochemical and electrochemical approaches for the useful conversion of CO₂, in the presence of catalyst material, to some high-value products of industrial interests, such as fuels (methane). Furthermore, several suitable examples are discussed to represent the potential and perspective of these technologies. In summary, a highly efficient conversion of CO₂ to fuels and related high-value chemicals would fulfill the rising demands of diverse sectors of the modern world.

Ru nanoparticles supported on amorphous ZrO2 for CO2 methanation

The influence of support materials and preparation methods on CO₂ methanation activity was investigated using Ru nanoparticles supported on amorphous ZrO₂ (am-ZrO₂), crystalline ZrO₂ (cr-ZrO₂), and SiO₂.

Low Temperature Methanation of CO2 on High Ni Content Ni-Ce-ZrOδ Catalysts Prepared via One-Pot Hydrothermal Synthesis

Ni-Ce-Zr-Oδ catalysts were prepared via one-pot hydrothermal synthesis. It was found that Ni can be partially incorporated into the Ce-Zr lattice, increasing surface oxygen species. The catalysts possess high surface areas even at high Ni loadings. The catalyst with Ni content of 71.5 wt.% is able to activate CO2 methanation even at a low temperature (200 °C). Its CO2 conversion and methane selectivity were reported at 80% and 100%, respectively. The catalyst was stable for 48 h during the course of CO2 methanation at 300 °C. Catalysts with the addition of medium basic sites were found to have better catalytic activity for CO2 methanation.

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.

Methanol synthesis via CO2 hydrogenation over CuO–ZrO2 prepared by two-nozzle flame spray pyrolysis

Controlled CuO–ZrO₂ particle synthesis by tuning the flame spray pyrolysis conditions allow generating highly active and methanol selective CO₂ hydrogenation catalysts.

Sustainable Conversion of Carbon Dioxide: An Integrated Review of Catalysis and Life Cycle Assessment

CO₂ conversion covers a wide range of possible application areas from fuels to bulk and commodity chemicals and even to specialty products with biological activity such as pharmaceuticals. In the present review, we discuss selected examples in these areas in a combined analysis of the state-of-the-art of synthetic methodologies and processes with their life cycle assessment. Thereby, we attempted to assess the potential to reduce the environmental footprint in these application fields relative to the current petrochemical value chain. This analysis and discussion differs significantly from a viewpoint on CO₂ utilization as a measure for global CO₂ mitigation. Whereas the latter focuses on reducing the end-of-pipe problem "CO₂ emissions" from todays' industries, the approach taken here tries to identify opportunities by exploiting a novel feedstock that avoids the utilization of fossil resource in transition toward more sustainable future production. Thus, the motivation to develop CO₂-based chemistry does not depend primarily on the absolute amount of CO₂ emissions that can be remediated by a single technology. Rather, CO₂-based chemistry is stimulated by the significance of the relative improvement in carbon balance and other critical factors defining the environmental impact of chemical production in all relevant sectors in accord with the principles of green chemistry.

Catalytic behavior of metal catalysts in high-temperature RWGS reaction: In-situ FT-IR experiments and first-principles calculations

High-temperature chemical reactions are ubiquitous in (electro) chemical applications designed to meet the growing demands of environmental and energy protection. However, the fundamental understanding and optimization of such reactions are great challenges because they are hampered by the spontaneous, dynamic, and high-temperature conditions. Here, we investigated the roles of metal catalysts (Pd, Ni, Cu, and Ag) in the high-temperature reverse water-gas shift (RWGS) reaction using in-situ surface analyses and density functional theory (DFT) calculations. Catalysts were prepared by the deposition-precipitation method with urea hydrolysis and freeze-drying. Most metals show a maximum catalytic activity during the RWGS reaction (reaching the thermodynamic conversion limit) with formate groups as an intermediate adsorbed species, while Ag metal has limited activity with the carbonate species on its surface. According to DFT calculations, such carbonate groups result from the suppressed dissociation and adsorption of hydrogen on the Ag surface, which is in good agreement with the experimental RWGS results.

Physical mixing of TiO2 with sponge nickel creates new active sites for selective CO methanation

Ni–α-Al₂O₃, Ni–SiO₂, Ni–γ-Al₂O₃, Ni–TiO₂, and Ni–ZrO₂ were prepared by physical mixing of metal oxides with sponge Ni, and the effect of physical contact of the metal oxides with sponge Ni on selective CO methanation was examined.

Methanation of residual syngas after LPG synthesis: identifying the main effects on catalytic performance with Plackett–Burman screening design

Seven factors in catalyst development were selected and rated towards their impact on methanation as a downstream process.

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.

Improved Performance of Ru/γ-Al2O3 Catalysts in the Selective Methanation of CO in CO2-Rich Reformate Gases upon Transient Exposure to Water-Containing Reaction Gas

To better understand the role of water in the selective methanation of CO in CO2-rich reformate gases on Ru/Al2O3 catalysts, the influence of exposing these catalysts to H2O-rich reformate gases on their reaction characteristics in transient experiments was investigated by employing kinetic and in situ spectroscopic measurements as well as ex situ catalyst characterization. Transient exposure of the ruthenium catalyst to wet reaction gas (5 or 15% H2O) results in significantly enhanced activity and selectivity for CO methanation in subsequent reactions in dry reformate compared with activation and reaction in dry reformate directly. Operando X-ray absorption spectroscopy results reveal that this is in accordance with a significant decrease in ruthenium particle size, which is stable during subsequent reaction in dry reformate. The implications of these data and additional results from in situ IR spectroscopy on the role and influence of H2O on the reaction, also in technical applications, are discussed.