Effect of In2O3 particle size on CO2 hydrogenation to lower olefins over bifunctional catalysts

Recent advances in bifunctional synthesis gas conversion to chemicals and fuels with a comparison to monofunctional processes

In order to meet the climate goals of the Paris Agreement and limit the potentially catastrophic consequences of climate change, we must move away from the use of fossil feedstocks for the production of chemicals and fuels. The conversion of synthesis gas (a mixture of hydrogen, carbon monoxide and/or carbon dioxide) can contribute to this. Several reactions allow to convert synthesis gas to oxygenates (such as methanol), olefins or waxes. In a consecutive step, these products can be further converted into chemicals, such as dimethyl ether, short olefins, or aromatics. Alternatively, fuels like gasoline, diesel, or kerosene can be produced. These two different steps can be combined using bifunctional catalysis for direct conversion of synthesis gas to chemicals and fuels. The synergistic effects of combining two different catalysts are discussed in terms of activity and selectivity and compared to processes based on consecutive reaction with single conversion steps. We found that bifunctional catalysis can be a strong tool for the highly selective production of dimethyl ether and gasoline with high octane numbers. In terms of selectivity bifunctional catalysis for short olefins or aromatics struggles to compete with processes consisting of single catalytic conversion steps.

Zn promoted GaZrOx Ternary Solid Solution Oxide Combined with SAPO-34 Effectively Converts CO2 to Light Olefins with Low CO Selectivity

We proposed a new strategy for CO2 hydrogenation to prepare light olefins by introducing Zn into GaZrOx to construct ZnGaZrOx ternary oxides, which was combined with SAPO-34 to prepare a high-performance ZnGaZrOx/SAPO-34 tandem catalyst for CO2 hydrogenation to light olefins. By optimizing the Zn doping content, the ratio and mode of the two-phase composite, and the process conditions, the 3.5 %ZnGaZrOx/SAPO-34 tandem catalyst showed excellent catalytic performance and good high-temperature inhibition of the reverse water-gas shift (RWGS) reaction. The catalyst achieved 26.6 % CO2 conversion, 82.1 % C2 =-C4 = selectivity and 11.8 % light olefins yield. The ZnGaZrOx formed by introducing an appropriate amount of Zn into GaZrOx significantly enhanced the spillover H2 effect and also induced the generation of abundant oxygen vacancies to effectively promote the activation of CO2. Importantly, the RWGS reaction was also significantly suppressed at high temperatures, with the CO selectivity being only 46.1 % at 390 °C.

Structural Heredity in Catalysis: CO2 Self-Selective CeO2 Nanocrystals for Efficient Photothermal CO2 Hydrogenation to Methane

The chemical inertness of CO2 molecules makes their adsorption and activation on a catalyst surface one of the key challenges in recycling CO2 into chemical fuels. However, the traditional template synthesis and chemical modification strategies used to tackle this problem face severe structural collapse and modifier deactivation issues during the often-needed post-processing procedure. Herein, a CO2 self-selective hydrothermal growth strategy is proposed for the synthesis of CeO2 octahedral nanocrystals that participate in strong physicochemical interactions with CO2 molecules. The intense affinity for CO2 molecules persists during successive high-temperature treatments required for Ni deposition. This demonstrates the excellent structural heredity of the CO2 self-selective CeO2 nanocrystals, which leads to an outstanding photothermal CH4 productivity exceeding 9 mmol h-1 mcat -2 and an impressive selectivity of >99%. The excellent performance is correlated with the abundant oxygen vacancies and hydroxyl species on the CeO2 surface, which create many frustrated Lewis-pair active sites, and the strong interaction between Ni and CeO2 that promotes the dissociation of H2 molecules and the spillover of H atoms, thereby greatly benefitting the photothermal CO2 methanation reaction. This self-selective hydrothermal growth strategy represents a new pathway for the development of effective catalysts for targeted chemical reactions.

Intermediate Transfer Rates and Solid-State Ion Exchange are Key Factors Determining the Bifunctionality of In2O3/HZSM-5 Tandem CO2 Hydrogenation Catalyst

Identifying the descriptors for the synergistic catalytic activity of bifunctional oxide-zeolite catalysts constitutes a formidable challenge in realizing the potential of tandem hydrogenation of CO2 to hydrocarbons (HC) for sustainable fuel production. Herein, we combined CH3OH synthesis from CO2 and H2 on In2O3 and methanol-to-hydrocarbons (MTH) conversion on HZSM-5 and discerned the descriptors by leveraging the distance-dependent reactivity of bifunctional In2O3 and HZSM-5 admixtures. We modulated the distance between redox sites of In2O3 and acid sites of HZSM-5 from milliscale (∼10 mm) to microscale (∼300 μm) and observed a 3-fold increase in space-time yield of HC and CH3OH (7.5 × 10-5 molC gcat-1 min-1 and 2.5 × 10-5 molC gcat-1 min-1, respectively), due to a 10-fold increased rate of CH3OH advection (1.43 and 0.143 s-1 at microscale and milliscale, respectively) from redox to acid sites. Intriguingly, despite the potential of a three-order-of-magnitude enhanced CH3OH transfer at a nanoscale distance (∼300 nm), the sole product formed was CH4. Our reactivity data combined with Raman, Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) revealed the occurrence of solid-state-ion-exchange (SSIE) between acid sites and Inδ+ ions, likely forming In2O moieties, inhibiting C-C coupling and promoting CH4 formation through CH3OH hydrodeoxygenation (HDO). Density functional theory (DFT) calculations further revealed that CH3OH adsorption on the In2O moiety with preadsorbed and dissociated H2 forming an H-In-OH-In moiety is the likely reaction mechanism, with the kinetically relevant step appearing to be the hydrogenation of the methyl species. Overall, our study revealed that efficient CH3OH transfer and prevention of ion exchange are the key descriptors in achieving catalytic synergy in bifunctional In2O3/HZSM-5 systems.

Hydrogenation of CO2 to Light Olefins over ZnZrOx /SSZ-13

Converting CO2 to olefins is an ideal route to achieve carbon neutrality. However, selective hydrogenation to light olefins, especially single-component olefin, while reducing CH4 formation remains a great challenge. Herein, we developed ZnZrOx /SSZ-13 tandem catalyst for the highly selective hydrogenation of CO2 to light olefins. This catalyst shows C2 = -C4 = and propylene selectivity up to 89.4 % and 52 %, respectively, while CH4 is suppressed down to 2 %, and there is no obvious deactivation. It is demonstrated that the isolated moderate Brønsted acid sites (BAS) of SSZ-13 promotes the rapid conversion of intermediate species derived from ZnZrOx , thereby enhancing the kinetic coupling of the reactions and inhibit the formation of alkanes and improve the light olefins selectivity. Besides, the weaker BAS of SSZ-13 promote the conversion of intermediates into aromatics with 4-6 methyl groups, which is conducive to the aromatics cycle. Accordingly, more propene can be obtained by elevating the Si/Al ratio of SSZ-13. This provides an efficient strategy for CO2 hydrogenation to light olefins with high selectivity.

Mechanistic Understanding of the Use of Single-Atom and Nanocluster Catalysts for Syngas Production via Partial Oxidation of Methane

Single-atom and nanocluster catalysts presenting potent catalytic activity and excellent stability are used in high-temperature applications such as in structural composites, electrical devices, and catalytic chemical reactions. Recently, more attention has been drawn to application of these materials in clean fuel processing based on oxidation in terms of recovery and purification. The most popular media for catalytic oxidation reactions include gas phases, pure organic liquid phases, and aqueous solutions. It has been proven from the literature that catalysts are frequently selected as the finest in regulating organic wastewater, solar energy utilization, and environmental treatment applications in most catalytic oxidation of methane vis-à-vis photons and in environmental treatment applications. Single-atom and nanocluster catalysts have been engineered and applied in catalytic oxidations considering metal-support interactions and mechanisms facilitating catalytic deactivation. In this review, the present improvements on engineering single-atom and nano-catalysts are discussed. In detail, we summarize structure modification strategies, catalytic mechanisms, methods of synthesis, and application of single-atom and nano-catalysts for partial oxidation of methane (POM). We also present the catalytic performance of various atoms in the POM reaction. Full knowledge of the use of remarkable POM vis-à-vis the excellent structure is revealed. Based on the review conducted on single-atom and nanoclustered catalysts, we conclude their viability for POM reactions; however, the catalyst design must be carefully considered not only for isolating the individual influences from the active metal and support but also for incorporating the interactions of these components.

Effective synergies in indium oxide loaded with zirconia mixed with silicoaluminophosphate molecular sieve number 34 catalysts for carbon dioxide hydrogenation to lower olefins

Tandem catalysts consisting of metal oxides and zeolites have been widely studied for catalytic carbon dioxide (CO₂) hydrogenation to lower olefins, while the synergies of two components and their influence on the catalytic performance are still unclear. In this study, the composite catalysts composed of indium oxide loaded with zirconia (In₂O₃/ZrO₂) and silicoaluminophosphate molecular sieve number 34 (SAPO-34) are developed. Performance results indicate that the synergies between these two components can promote CO₂ hydrogenation. Further characterizations reveal that the chabazite (CHA) structure and acid sites in the SAPO-34 are destroyed when preparing In-Zr/SAPO by powder milling (In-Zr/SAPO-M) because of the excessive proximity of two components, which inhibits the activation of CO₂ and hydrogen (H₂), thus resulting in much higher methane selectivity than the catalysts prepared by granule stacking (In-Zr/SAPO-G). Proper granule integration manner promotes tandem reaction, thus enhancing CO₂ hydrogenation to lower olefins, which can provide a practicable strategy to improve catalytic performance and the selectivity of the target products.

Experience of Using DLS to Study the Particle Sizes of Active Component in the Catalysts Based on the Oxide and Non-Oxide Supports

The present study reports the use of the dynamic light scattering (DLS) method to analyze metal nanoparticle sizes in supported catalysts (as a model system for different metal-oxide nanocomposites, ceramics, etc.). The selective dissolution of matrices has been used to transform solids to sols for DLS analysis. DLS/STS (from solid to sol) technique was tested on a wide number of different sets of supported metal catalysts (Pt, Pd, Ru metals and Al2O3, SiO2, TiO2, C3N4, carbon and polymers as supports). The transmission electron microscopy and X-ray diffraction (TEM/XRD) results for the initial supported catalysts and the DLS results for the sols prepared from them showed good agreement with each other. Moreover, it has been shown that this approach can identify the minor contamination of catalysts by large particles or aggregates which are difficult to detect by TEM/XRD.

Switchable Tuning CO2 Hydrogenation Selectivity by Encapsulation of the Rh Nanoparticles While Exposing Single Atoms

The switch of CO₂ hydrogenation selectivity from CH₄ to CO over TiO₂ supported Rh catalysts is accomplished via selective encapsulation of Rh nanoparticles while exposing Rh single atoms by high-temperature reduction (HTR) according to their different strong metal-support interaction (SMSI) occurrence conditions, which can be reversed by subsequent oxidation treatment.

Light olefin synthesis from a diversity of renewable and fossil feedstocks: state-of the-art and outlook

Light olefins are important feedstocks and platform molecules for the chemical industry. Their synthesis has been a research priority in both academia and industry. There are many different approaches to the synthesis of these compounds, which differ by the choice of raw materials, catalysts and reaction conditions. The goals of this review are to highlight the most recent trends in light olefin synthesis and to perform a comparative analysis of different synthetic routes using several quantitative characteristics: selectivity, productivity, severity of operating conditions, stability, technological maturity and sustainability. Traditionally, on an industrial scale, the cracking of oil fractions has been used to produce light olefins. Methanol-to-olefins, alkane direct or oxidative dehydrogenation technologies have great potential in the short term and have already reached scientific and technological maturities. Major progress should be made in the field of methanol-mediated CO and CO₂ direct hydrogenation to light olefins. The electrocatalytic reduction of CO₂ to light olefins is a very attractive process in the long run due to the low reaction temperature and possible use of sustainable electricity. The application of modern concepts such as electricity-driven process intensification, looping, CO₂ management and nanoscale catalyst design should lead in the near future to more environmentally friendly, energy efficient and selective large-scale technologies for light olefin synthesis.

A Dual-Bed Strategy for Direct Conversion of Syngas to Light Paraffins

The authors studied the direct conversion of syngas to light paraffins in a dual-bed fixed-bed reactor. A dual-bed catalyst composed of three catalysts, a physically mixed methanol synthesis catalyst (CZA), and a methanol dehydration to dimethyl ether (DME) catalyst (Al2O3(C)) were put in the upper bed for direct conversion of syngas to DME, while the SAPO-34 (SP34-C) zeolite was put in the lower bed for methanol and DME conversion. The effects of the mass ratio of CZA to Al2O3(C), the H2/CO molar ratio, and the space velocity on catalytic performance of syngas to DME were studied in the upper bed. Moreover, a feed gas with a CO/CO2/DME/N2/H2 molar ratio of 9/6/4/5 balanced with H2 was simulated and studied in the lower bed over SP34-C; after optimizing the reaction conditions, the selectivity of light paraffins reached 90.8%, and the selectivity of propane was as high as 76.7%. For the direct conversion of syngas to light paraffins in a dual bed, 88.9% light paraffins selectivity in hydrocarbons was obtained at a CO conversion of 33.4%. This dual-bed strategy offers a potential route for the direct conversion of syngas to valuable chemicals.

Modulating p-Orbital of Bismuth Nanosheet by Nickel Doping for Electrocatalytic Carbon Dioxide Reduction Reaction

Electrochemical reduction of CO₂ (CO₂ RR) to value-added chemicals is an effective way to harvest renewable energy and utilize carbon dioxide. However, the electrocatalysts for CO₂ RR suffer from insufficient activity and selectivity due to the limitation of CO₂ activation. In this work, a Ni-doped Bi nanosheet (Ni@Bi-NS) electrocatalyst is synthesized for the electrochemical reduction of CO₂ to HCOOH. Physicochemical characterization methods are extensively used to investigate the composition and structure of the materials. Electrochemical results reveal that for the production of HCOOH, the obtained Ni@Bi-NS exhibits an equivalent current density of 51.12 mA cm^-2 at -1.10 V, which is much higher than the pure Bi-NS (18.00 mA cm^-2 at -1.10 V). A high Faradaic efficiency over 92.0 % for HCOOH is achieved in a wide potential range from -0.80 to -1.10 V, and particularly, the highest efficiency of 98.4 % is achieved at -0.90 V. Both experimental and theoretical results reveal that the superior activity and selectivity are attributed to the doping effect of Ni on the Bi nanosheet. The density functional theory calculation reveals that upon doping, the charge is transferred from Ni to the adjacent Bi atoms, which shifts the p-orbital electronic density states towards the Fermi level. The resultant strong orbital hybridization between Bi and the π* orbitals of CO₂ facilitates the formation of *OCHO intermediates and favors its activation. This work provides an effective strategy to develop active and selective electrocatalysts for CO₂ RR by modulating the electronic density state.

Dynamic transformation between bilayer islands and dinuclear clusters of Cr oxide on Au(111) through environment and interface effects

SignificanceFor oxide catalysts, it is important to elucidate and further control their atomic structures. In this work, well-defined CrO2 bilayer islands and Cr2O7 dinuclear clusters have been grown on Au(111) and unambiguously identified by scanning tunneling microscopy and theoretical calculations. Upon cycled redox treatments, the two kinds of oxide nanostructures can be reversibly transformed. It is interesting to note that both Cr oxides do not exist in bulk but need to be stabilized by the metal surface and the specific environment. Our results suggest that both redox atmosphere and interface confinement effects can be used to construct an oxide nanostructure with the specific chemical state and structure.

Techniques for the characterization of single atom catalysts

Single atom catalysts (SACs) are a hot research area recently. Over most of the SACs, the singly dispersed atoms are the active sites, which contribute to the catalytic activities significantly compared with a catalyst with continuously packed active sites. It is essential to determine whether SACs have been successfully synthesized. Several techniques have been applied for the characterization of the dispersion states of the active sites over SACs, such as Energy Dispersive X-ray spectroscopy (EDX), Electron Energy Loss Spectroscopy (EELS), etc. In this review, the techniques for the identification of the singly dispersed sites over SACs are introduced, the advantages and limitations of each technique are pointed out, and the future research directions have been discussed. It is hoped that this review will be helpful for a more comprehensive understanding of the characterization and detection methods involved in SACs, and stimulate and promote the further development of this emerging research field.