CeO2 Frustrated Lewis Pairs Improving CO2 and CH3OH Conversion to Monomethylcarbonate

B, N Codoped Defective Reduced Graphene Oxide as a Highly Efficient Frustrated Lewis Pairs Catalyst for the Selective Hydrogenation of α,β-Unsaturated Aldehydes to Unsaturated Alcohols

The development of all-solid-state frustrated Lewis pairs (FLPs) metal-free hydrogenation catalysts with excellent activity and stability remains a significant challenge. In this work, B, N codoped FLPs catalysts (De-rGO-NxBy) were prepared by the strategy of fabricating carbon defects and heteroatom doping on the surface of reduced graphene oxide and applied in the selective hydrogenation of α,β-unsaturated aldehydes to unsaturated alcohols. It was found that electron-rich pyridine-N (Lewis base) and adjacent electron-deficient B-N (Lewis acid) sites could be constructed on the surface of reduced graphene oxide using dicyandiamide and metaboric acid as N and B sources, thus forming FLPs sites. More importantly, the constructed carbon defects could facilitate the formation of pyridinic-N/B-N FLPs sites, thereby improving the catalytic hydrogenation activity and the selectivity to unsaturated alcohols. Furthermore, in situ DRIFTS and DFT calculations show that the pyridinic-N/B-N FLPs sites can efficiently activate the C═O of aldehydes and H2 molecules with only 0.53 eV dissociation energy of the H-H bond. Also, the catalyst presents excellent catalytic performance in the transfer hydrogenation reactions with cyclohexanol and its derivatives as hydrogen sources. This study provides new ideas for the design and preparation of all-solid-state FLPs metal-free catalysts and promotes the green synthesis of unsaturated alcohols.

Surface Structure and Chemistry of CeO2 Powder Catalysts Determined by Surface-Ligand Infrared Spectroscopy (SLIR)

ConspectusCerium is the most abundant rare earth element in the Earth's crust. Its most stable oxide, cerium dioxide (CeO2, ceria), is increasingly utilized in the field of catalysis. It can catalyze redox and acid-base reactions, and serve as a component of electrocatalysts and even photocatalysts. As one of the most commonly used in situ/operando characterization methods in catalysis, infrared (IR) spectroscopy is routinely employed to monitor reaction intermediates on the surface of solid catalysts, offering profound insight into reaction mechanisms. Additionally, IR vibrational frequencies of probe molecules adsorbed on solid catalysts provide detailed information about their structure and chemical states. Numerous studies in the literature have utilized carbon monoxide and methanol as IR probe molecules on ceria particles. However, assigning their vibrational frequencies is often highly controversial due to the great complexity of the actual surface of ceria particles, which include differently oriented crystal facets, reconstructions, defects, and other structural variations. In our laboratory, taking bulk ceria single crystals with distinct orientations as model systems, we employed a highly sensitive ultrahigh vacuum (UHV) infrared spectroscopy system (THEO) to study the adsorption of CO and methanol. It turns out that the theoretical calculations adopting hybrid functionals (HSE06) can bring the theoretical predictions into agreement with the experimental results for the CO frequencies on ceria single crystal surfaces. The obtained frequencies serve as reliable references to resolve the long-standing controversial assignments for the IR bands of CO and methanol adsorbed on ceria particles. Furthermore, these characteristic frequencies allow for the determination of orientations, oxidation states and restructuring of exposed crystal facets of ceria nanoparticles, which is applicable from UHV conditions to industrially relevant pressures of up to 1 bar, and from low temperatures (∼65 K) to high temperatures (∼1000 K). We also used molecular oxygen as a probe molecule to investigate its interaction with the ceria surface, crucial for understanding ceria's redox properties. Our findings reveal that the localization of oxygen vacancies and the mechanism of dioxygen activation are highly sensitive to surface orientations. We provided the first spectroscopic evidence showing that the oxygen vacancies on ceria (111) surfaces tend to localize in deep layers. In addition, we employed N2O as a probe molecule to elucidate the origin of the photocatalytic activity of ceria and showed that the photocatalytic activity is highly sensitive to the surface orientation (i.e., surface coordination structure). This Account shows that probe-molecule infrared spectroscopy serves as a powerful in situ/operando tool for studying the surface structure and chemistry of solid catalysts, and the knowledge gained through the "Surface Science" approach is essential as a crucial benchmark.

Green Synthesis of Urea from Carbon Dioxide and Ammonia Catalyzed by Ultraporous Permanently Polarized Hydroxyapatite

The sustainable synthesis of urea from ammonia (NH3) and carbon dioxide (CO2) using ultraporous permanently polarized hydroxyapatite (upp-HAp) as catalyst has been explored as an advantageous CO2-revalorization strategy. As the simultaneous activation of N2 and CO2 (single-step) demands an increase of the reaction conditions, we have re-visited the industrial two-step Bazarov reaction. upp-HAp has been designed as a stable multifunctional catalyst capable of promoting both CO2 and NH3 adsorption for their subsequent C-N bond formation. Herein we report the synthesis of 1 mmol/gcat of urea with a selectivity of 97 % under strictly mild conditions (95-120 °C and 1 bar of CO2; without applying any electrical currents or UV irradiation) which represents an efficiency of ~2 % and ~30 % with respect to the NH3 and CO2 content, respectively. The study of the NH3 content, products adsorbed in the catalyst, presence of intermediates and temperature of the reaction allows unveiling the great potential of upp-HAp as a green catalyst for sustainable Bazarov reactions. Results suggest that the double-step approach could be more advantageous for both synthesizing urea and as a CO2-revalorization strategy, which in turn promotes the development of specific technologies for the independent synthesis of green NH3.

Novel Lewis Acid-Base Interactions in Polymer-Derived Sodium-Doped Amorphous Si-B-N Ceramic: Towards Main-Group-Mediated Hydrogen Activation

Interest is growing in transition metal-free compounds for small molecule activation and catalysis. We discuss the opportunities arising from synthesizing sodium-doped amorphous silicon-boron-nitride (Na-doped a-SiBN). Na+ cations and 3-fold coordinated BIII moieties were incorporated into an amorphous silicon nitride network via chemical modification of a polysilazane followed by pyrolysis in ammonia (NH3) at 1000 °C. Emphasis is placed on the mechanisms of hydrogen (H2) activation within Na-doped a-SiBN structure. This material design approach allows the homogeneous distribution of Na+ and BIII moieties surrounded by SiN4 units contributing to the transformation of the BIII moieties into 4-fold coordinated geometry upon encountering H2, potentially serving as frustrated Lewis acid (FLA) sites. Exposure to H2 induced formation of frustrated Lewis base (FLB) N-= sites with Na+ as a charge-compensating cation, resulting in the in situ formation of a frustrated Lewis pair (FLP) motif (≡BFLA⋅⋅⋅Hδ-⋅⋅⋅Hδ+⋅⋅⋅:N-(Na+)=). Reversible H2 adsorption-desorption behavior with high activation energy for H2 desorption (124 kJ mol-1) suggested the H2 chemisorption on Na-doped a-SiBN. These findings highlight a future landscape full of possibilities within our reach, where we anticipate main-group-mediated small molecule activation will have an important impact on the design of more efficient catalytic processes and the discovery of new catalytic transformations.

Adducts of Lewis acidic stibanes with phosphane chalcogenides

Starting from ClSbRF2 (RF = C2F5, 3,5-(CF3)2C6H3) and H(E)P(tBu)2 (E = O, S), we prepared the oxy- and sulphanediyl-bridged adducts RF2Sb(Cl)-E-(H)P(tBu)2, which are stable against the elimination of HCl. The different electron-withdrawing substituents and chalcogen bridging units influence the size of the Sb-E-P angle. ClSb(C2F5)2 and nBu3SnH react to give HSb(C2F5)2, which seems to interact weakly with H(O)P(tBu)2 in solution as observed via NMR. All products were characterised by NMR spectroscopy and all the stable ones were additionally characterised by X-ray diffraction and elemental analyses.

CeO2-Based Frustrated Lewis Pairs via Defective Engineering: Formation Theory, Site Characterization, and Small Molecule Activation

Activation of small molecules is considered to be a central concern in the theoretical investigation of environment- and energy-related catalytic conversions. Sub-nanostructured frustrated Lewis pairs (FLPs) have been an emerging research hotspot in recent years due to their advantages in small molecule activation. Although the progress of catalytic applications of FLPs is increasingly reported, the fundamental theories related to the structural formation, site regulation, and catalytic mechanism of FLPs have not yet been fully developed. Given this, it is attempted to demonstrate the underlying theory of FLPs formation, corresponding regulation methods, and its activation mechanism on small molecules using CeO2 as the representative metal oxide. Specifically, this paper presents three fundamental principles for constructing FLPs on CeO2 surfaces, and feasible engineering methods for the regulation of FLPs sites are presented. Furthermore, cases where typical small molecules (e.g., hydrogen, carbon dioxide, methane oxygen, etc.) are activated over FLPs are analyzed. Meanwhile, corresponding future challenges for the development of FLPs-centered theory are presented. The insights presented in this paper may contribute to the theories of FLPs, which can potentially provide inspiration for the development of broader environment- and energy-related catalysis involving small molecule activation.

Dimethyl Carbonate Synthesis from CO2 over CeO2 with Electron-Enriched Lattice Oxygen Species

Direct synthesis of dimethyl carbonate (DMC) from CO2 plays an important role in carbon neutrality, but its efficiency is still far from the practical application, due to the limited understanding of the reaction mechanism and rational design of efficient catalyst. Herein, abundant electron-enriched lattice oxygen species were introduced into CeO2 catalyst by constructing the point defects and crystal-terminated phases in the crystal reconstruction process. Benefitting from the acid-base properties modulated by the electron-enriched lattice oxygen, the optimized CeO2 catalyst exhibited a much higher DMC yield of 22.2 mmol g-1 than the reported metal-oxide-based catalysts at the similar conditions. Mechanistic investigations illustrated that the electron-enriched lattice oxygen can provide abundant sites for CO2 adsorption and activation, and was advantageous of the formation of the weakly adsorbed active methoxy species. These were facilitating to the coupling of methoxy and CO2 for the key *CH3OCOO intermediate formation. More importantly, the weakened adsorption of *CH3OCOO on the electron-enriched lattice oxygen can switch the rate-determining-step (RDS) of DMC synthesis from *CH3OCOO formation to *CH3OCOO dissociation, and lower the corresponding activation barriers, thus giving rise to a high performance. This work provides insights into the underlying reaction mechanism for DMC synthesis from CO2 and methanol and the design of highly efficient catalysts.

Deciphering Local Structural Complexity in Zn/Ga-ZrO2 CO2 Hydrogenation Catalysts

In the last few decades, massive effort has been expended in heterogeneous catalysis to develop new materials presenting high conversion, selectivity, and stability even under high-temperature and high-pressure conditions. In this context, CO2 hydrogenation is an interesting reaction where the catalyst local structure is strongly related to the development of an active and stable material under hydrothermal conditions at T/P > 300 °C/30 bar. In order to clarify the relationship between catalyst local ordering and its activity/stability, we herein report a combined laboratory and synchrotron investigation of aliovalent element (Ce/Zn/Ga)-containing ZrO2 matrixes. The results reveal the influence of similar average structures with different short-range orderings on the catalyst properties. Moreover, a further step toward the comprehension of the oxygen vacancy formation mechanism in Ce- and Ga-ZrO2 catalysts is reported. Finally, the reported results illustrate a robust method to guide local structure determination and ultimately help to avoid overuse of the "solid solution" definition.

Advances in CO2 activation by frustrated Lewis pairs: from stoichiometric to catalytic reactions

The rise of CO2 concentrations in the environment due to anthropogenic activities results in global warming and threatens the future of humanity and biodiversity. To address excessive CO2 emissions and its effects on climate change, efforts towards CO2 capture and conversion into value adduct products such as methane, methanol, acetic acid, and carbonates have grown. Frustrated Lewis pairs (FLPs) can activate small molecules, including CO2 and convert it into value added products. This review covers recent progress and mechanistic insights into intra- and inter-molecular FLPs comprised of varying Lewis acids and bases (from groups 13, 14, 15 of the periodic table as well as transition metals) that activate CO2 in stoichiometric and catalytic fashion towards reduced products.

Multicomponent X-ray Shielding Using Sulfated Cerium Oxide and Bismuth Halide Composites

Lead is the most widely used X-ray-shielding material, but it is heavy (density ≈ 11.34 g/cm3) and toxic. Therefore, the replacement of Pb with lightweight, ecofriendly materials would be beneficial, and such materials would have applications in medicine, electronics, and aerospace engineering. However, the shielding ability of Pb-free materials is significantly lower than that of Pb itself. To maximize the radiation attenuation of non-Pb-based shielding materials, a high-attenuation cross-section, normal to the incoming X-ray direction, must be achieved. In this study, we developed efficient X-ray-shielding materials composed of sulfated cerium oxide (S-CeO2) and bismuth halides. Crucially, the materials are lightweight and mechanically flexible because of the absence of heavy metals (for example, Pb and W). Further, by pre-forming the doped metal oxide as a porous sponge matrix, and then incorporating the bismuth halides into the porous matrix, uniform, compact, and intimate composites with a high-attenuation cross-section were achieved. Owing to the synergetic effect of the doped metal oxide and bismuth halides, the resultant thin (approximately 3 mm) and lightweight (0.85 g·cm-3) composite achieved an excellent X-ray-shielding rate of approximately 92% at 60 kV, one of the highest values reported for non-heavy-metal shielding materials.