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.

Cyclohexane oxidation using advanced oxidation processes with metals and metal oxides as catalysts: a review

Selective oxidation of cyclohexane has gained substantial interest in the field of research due to the prominence of its products in industrial processes. Particularly, advanced oxidation processes (AOPs) constitute a positive technology for the oxidation of cyclohexane owing to their high oxidation potentials and environmental benign properties. This review entails to address the progress made in advanced oxidation of cyclohexane over nanostructured metals and metal oxides catalysts. The main focus is directed toward the photocatalysis, Fenton oxidation and ozonation as advanced oxidation processes. Mainly, the fundamental principles, prime factors of the AOPs in conjunction with metal and metal oxide catalysts and the mechanistic insight toward the oxidation of cyclohexane are highlighted. The affirmative effects of the metals and metal oxide catalysts mainly focusing on particle size, structure and elemental composition is stressed. Lastly, the advantages and disadvantages of the AOPs and the strategic approaches to counter the disadvantages are also clearly elucidated.

Reverse construction of dominant/secondary facets in Bi24O31Br10 photocatalysts for boosting electronic transfer

In this paper, it is found that the preferential growth of secondary {117} facets of Bi₂₄O₃₁Br₁₀ into dominant facets would lead to higher photocatalytic activity, although the original main {213} facet has a stronger molecular oxygen adsorption ability, which illustrates that the charge separation efficiency induced by dominant/secondary facet control plays a more important role than that of O₂ adsorptive performance in improving activity.

Electronic-State Manipulation of Surface Titanium Activates Dephosphorylation Over TiO2 Near Room Temperature

Dephosphorylation that removes a phosphate group from substrates is an important reaction for living organisms and environmental protection. Although CeO₂ has been shown to catalyze this reaction, cerium is low in natural abundance and has a narrow global distribution (>90 % of these reserves are located within six countries). It is thus imperative to find another element/material with high worldwide abundance that can also efficiently extract the phosphate out of agricultural waste for phosphorus recycle. Using para-nitrophenyl phosphate (p-NPP) as a model compound, we demonstrate that TiO₂ with a F-modified (001) surface can activate p-NPP dephosphorylation at temperatures as low as 40 °C. By probe-assisted nuclear magnetic resonance (NMR), it was revealed that the strong electron-withdrawing effect of fluorine makes Ti atoms (the active sites) on the (001) surface very acidic. The bidentate adsorption of p-NPP on this surface further promotes its subsequent activation with a barrier ≈20 kJ mol^-1 lower than that of the pristine (001) and (101) surfaces, allowing the activation of this reaction near room temperature (from >80 °C).

Surface Fingerprinting of Faceted Metal Oxides and Porous Zeolite Catalysts by Probe-Assisted Solid-State NMR Approaches

Acid catalysis in heterogeneous systems such as metal oxides and porous zeolites has been widely involved in various catalytic processes for chemical and petrochemical industries. In acid-catalyzed reactions, the performance (e.g., activity and selectivity) is closely associated with the acidic features of the catalysts, viz., type (Lewis vs Brønsted acidity), distribution (external vs internal surface), strength (strong vs weak), concentration (amount), and spatial interactions of acidic sites. The characterization of local structure and acidic properties of these active sites has important implications for understanding the reaction mechanism and the practical catalytic applications of acidic catalysts. Among diverse acidity characterization approaches, the solid-state nuclear magnetic resonance (SSNMR) technique with suitable probe molecules has been recognized as a reliable and versatile tool. Such a probe-assisted SSNMR approach could provide qualitative (type, distribution, and spatial interactions) and quantitative (strength and concentration) information on each acidic site. This Account aims to integrate our recent important findings in determining the structures and acidic characteristics of some typical metal oxide and zeolite catalysts by using the probe-assisted SSNMR technique, as well as clarifying the continuously evolving process of each discrete acidic site under hydrothermal or chemical treatments even at the molecular level with multiscale theoretical simulations.More specifically, we will describe herein the development and applications of the probe-assisted SSNMR methods, such as trimethylphosphine (TMP) and acetonitrile-d₃ (CD₃CN) in conjunction with advanced two-dimensional (2D) homo- and heteronuclear correlation spectroscopy, for characterizing the structures and properties of acidic sites in varied solid catalysts. Moreover, relevant information regarding the surface fingerprinting of various facets on crystalline metal oxide nanoparticles and active centers inside porous zeolites, the mapping of relevant spatial interactions, and the verification of structure-activity correlation were investigated as well. Relevant discussions are mainly based on the recent NMR experiments of our collaborating research groups, including (i) determining the acidic characterization with probe-assisted SSNMR approaches, (ii) mapping various active centers (or crystalline facets), and (iii) revealing their influence on catalytic performance of solid acid catalyst systems. It is anticipated that this information may provide more in-depth insights toward our fundamental understanding of solid acid catalysis.

Unusual Catalytic Properties of High-Energetic-Facet Polar Metal Oxides

ConspectusHeterogeneous catalysis is an area of great importance not only in chemical industries but also in energy conversion and environmental technologies. It is well-established that the specific surface morphology and structure of solid catalysts exert remarkable effects on catalytic performances, since most physical and chemical processes take place on the surface during catalytic reactions. Different from the widely studied faceted metallic nanoparticles, metal oxides give more complicated structures and surface features. Great progress has been achieved in controlling the shape and exposed facets of transition metal oxides during nanocrystal growth, usually by using surface-directing agents (SDAs). However, the effects of exposed facets remain controversial among researchers. It should be noted that high-energetic facets, especially polar facets, tend to lower their surface energy via different relaxation processes, such as surface reconstruction, redox change, adsorption of countercharged species, etc. These processes can subsequently lead to surface defect formation and break the surface stoichiometry, and the resulting changes in electronic configurations and charge migration properties all play important roles in heterogeneous catalysis. Because different materials prefer different relaxation methods, various surface features are created, and different techniques are required to investigate the different features from facet to facet. Conventional characterization techniques such as X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, etc. appear to be insufficient to elucidate the underlying principles of the facet effects. Consequently, an increasing number of novel techniques have been developed to differentiate the surface features, enabling greater understanding of the effects of facets on heterogeneous catalysis.In this Account, on the basis of previous studies by our own group, we will focus on the effects of tailored facets on heterogeneous catalysis introduced by engineered simple binary metal oxide nanomaterials primarily with exposed polar facets, in combination with detailed surface studies using a range of new characterization techniques. As a result, fundamental principles of the effects of facets are elucidated, and the structure-activity correlations are demonstrated. The surface features introduced by different relaxation processes are also investigated using a range of characterization techniques. For example, electron paramagnetic resonance spectroscopy is used to detect the oxygen vacancies, while probe-assisted solid-state NMR spectroscopy is shown to be facet-sensitive and able to evaluate the surface acidity. It is also shown that such different features influence the heterogeneous catalytic performances in different ways. With the help of first-principles density functional theory calculations, unique properties of the faceted metal oxides are discussed and unraveled. Besides, other materials such as transition metal chalcogenides and layered double hydroxides are also briefly discussed with regard to their application in facet-dependent catalysis studies.

Hydroxyl-enriched highly crystalline TiO2 suspensible photocatalyst: facile synthesis and superior H2-generation activity

A facile ethanol-controlled strategy was reported to simultaneously realize the excellent suspendability and high-crystallinity of a hydroxyl-enriched TiO₂ nanocrystal for efficient photocatalytic H₂ generation.

Trace-Level Fluorination of Mesoporous TiO2 Improves Photocatalytic and Pb(II) Adsorbent Performances

Fluorination is an effective way of tuning the physicochemical property and activity of TiO₂ nanocrystallites, which usually requires a considerable amount of hydrofluoric acid (or NH₄F) for a typical F/Ti molar ratio, R_F, of 0.5-69.0 during synthesis. This has consequential environmental issues due to the high toxicity and hazard of the reactants. In the present work, an environmentally benign fluorination approach is demonstrated that uses only a trace amount of sodium fluoride with an R_F of 10^-6 during synthesis. While it maintained the desirable high surface area (102.4 m²/g), the trace-level fluorination enabled significant enhancements on photocatalytic activities (e.g., a 56% increase on hydrogen evolution rate) and heavy metal Pb(II) removal (31%) of the mesoporous TiO₂. This can be attributed to enriched Ti³⁺ and localized spatial charge separation due to fluorination as proved by X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance spectroscopy (EPR), and density functional theory (DFT) analyses.

Nanoisozymes: The Origin behind Pristine CeO2 as Enzyme Mimetics

It is known that the interplay between molecules and active sites on the topmost surface of a solid catalyst determines its activity in heterogeneous catalysis. The electron density of the active site is believed to affect both adsorption and activation of reactant molecules at the surface. Unfortunately, commercial X-ray photoelectron spectroscopy, which is often adopted for such characterization, is not sensitive enough to analyze the topmost surface of a catalyst. Most researchers fail to acknowledge this point during their catalytic correlation, leading to different interpretations in the literature in recent decades. Recent studies on pristine Cu₂ O [Nat. Catal. 2019, 2, 889; Nat. Energy 2019, 4, 957] have clearly suggested that the electron density of surface Cu is facet dependent and plays a key role in CO₂ reduction. Herein, it is shown that pristine CeO₂ can reach 2506/1133 % increase in phosphatase-/peroxidase-like activity if the exposed surface is wisely selected. By using NMR spectroscopy with a surface probe, the electron density of the surface Ce (i.e., the active site) is found to be facet dependent and the key factor dictating their enzyme-mimicking activities. Most importantly, the surface area of the CeO₂ morphologies is demonstrated to become a factor only if surface Ce can activate the adsorbed reactant molecules.

Unravelling the Role of Structural Geometry and Chemical State of Well-Defined Oxygen Vacancies on Pristine CeO2 for H2O2 Activation

Although H₂O₂ has been often employed as a green oxidant for many CeO₂-catalyzed reactions, the underlying principle of its activation by surface oxygen vacancy (V_o) is still elusive due to the irreversible removal of postgenerated V_o by water (or H₂O₂). The metastable V_o (ms-V_o) naturally preserved on pristine CeO₂ surfaces was adopted herein for an in-depth study of their interplay with H₂O₂. Their well-defined local structures and chemical states were found facet-dependent affecting both the adsorption and subsequent activation of H₂O₂. It is concluded that a strong adsorption of H₂O₂ on ms-V_o may not guarantee its subsequent activation. The ms-V_o can be only free for the next catalytic cycle when the electron density of surface Ce is high enough to reduce/break the O-O bond of adsorbed H₂O₂. This explains the 211.8 and 35.8 times enhancement in H₂O₂ reactivity when the CeO₂ surface is changed from (111) and (110) to (100).

One-Step Hydrothermal Synthesis of P25 @ Few Layered MoS2 Nanosheets toward Enhanced Bi-catalytic Activities: Photocatalysis and Electrocatalysis

P25 loaded few layered molybdenum disulfide (MoS₂) nanosheets (P25@MoS₂) are successfully synthesized through a facile one-step hydrothermal process. The bi-catalytic activities, i.e., photocatalytic and electrocatalytic activities, of the as-prepared nanomaterials have been investigated. For the as-prepared products, the photocatalytic performances were investigated by degrading simulated pollutant under sunlight irradiation, and the hydrogen evolution reaction evaluated the electrocatalytic performances. The results indicate that P25@MoS₂ possesses excellent activities in both photocatalysis and electrocatalysis. The presence of MoS₂ broadens the light absorption range of P25 and improves the separation and transformation efficiency of photogenerated carriers, thus improving its photocatalytic performance. The existence of P25 inhibits the aggregation of MoS₂ to form more dispersed MoS₂ nanosheets with only few layers increasing its active sites. Thereby, the electrocatalytic performance is heightened. The excellent multifunction makes the as-prepared P25@MoS₂ a promising material in the fields of environment and energy.