Single-Facet Dominant Anatase TiO2 (101) and (001) Model Catalysts to Elucidate the Active Sites for Alkanol Dehydration

Effect of Tourmaline Addition on the Anti-Poisoning Performance of MnCeOx@TiO2 Catalyst for Low-Temperature Selective Catalytic Reduction of NOx

In view of the flue gas characteristics of cement kilns in China, the development of low-temperature denitrification catalysts with excellent anti-poisoning performance has important theoretical and practical significance. In this work, a series of MnCeOx@TiO2 and tourmaline-containing MnCeOx@TiO2-T catalysts was prepared using a chemical pre-deposition method. It was found that the MnCeOx@TiO2-T2 catalyst (containing 2% tourmaline) exhibited the best low-temperature NH3-selective catalytic reduction (NH3-SCR) performance, yielding 100% NOx conversion at 110 °C and above. When 100-300 ppm SO2 and 10 vol.% H2O were introduced to the reaction, the NOx conversion of the MnCeOx@TiO2-T2 catalyst was still higher than 90% at 170 °C, indicating good anti-poisoning performance. The addition of appropriate amounts of tourmaline can not only preferably expose the active {001} facets of TiO2 but also introduce the acidic SiO2 and Al2O3 components and increase the content of Mn4+ and Oα on the surface of the catalyst, all of which contribute to the enhancement of reaction activity of NH3-SCR and anti-poisoning performance. However, excess amounts of tourmaline led to the formation of dense surface of catalysts that suppressed the exposure of catalytic active sites, giving rise to the decrease in catalytic activity and anti-poisoning capability. Through an in situ DRIFTS study, it was found that the addition of appropriate amounts of tourmaline increased the number of Brønsted acid sites on the catalyst surface, which suppressed the adsorption of SO2 and thus inhibited the deposition of NH4HSO4 and (NH4)2HSO4 on the surface of the catalyst, thereby improving the NH3-SCR performance and anti-poisoning ability of the catalyst.

Ball Milling of La2O3 Tailors the Crystal Structure, Reactive Oxygen Species, and Free Radical and Non-Free Radical Photocatalytic Pathways

Non-free radical photocatalysis with metal oxide catalysts is an important advanced oxidation process that enables the removal of various emerging environmental pollutants, such as tetracycline. Here, four hexagonal La2O3 photocatalysts with different densities of oxygen vacancy and crystalline features are synthesized and then further treated by ball milling. Ball milling of these La2O3 photocatalysts is found to increase the amount of oxygen vacancies on their surfaces and thereby the amount of 1O2 species produced by them. The photocatalytic degradation of TC by these La2O3 photocatalysts depends on the oxygen vacancies present on them. Furthermore, the ones with a strong (101) diffraction peak remove tetracycline from water systems largely with 1O2 and •OH species, whereas those with a weak (101) diffraction peak do so mainly via 1O2 and direct electron transfer (DET) process. Their overall catalytic properties are also studied by density functional theory calculations. Moreover, the organic products produced from tetracycline by La2O3 photocatalysts containing a strong (101) diffraction peak are found to be less toxic than those produced by La2O3 photocatalysts containing a weak (101) diffraction peak. This study also provides convincing evidence that the structures of La2O3 determine the species that is produced by it and that end up mediating photocatalytic reaction pathways (i.e., free radical versus non-free radical) to degrade an emerging environment pollutant.

Full-spectrum nonmetallic plasmonic carriers for efficient isopropanol dehydration

Plasmonic hot carriers have the advantage of focusing, amplifying, and manipulating optical signals via electron oscillations which offers a feasible pathway to influence catalytic reactions. However, the contribution of nonmetallic hot carriers and thermal effects on the overall reactions are still unclear, and developing methods to enhance the efficiency of the catalysis is critical. Herein, we proposed a new strategy for flexibly modulating the hot electrons using a nonmetallic plasmonic heterostructure (named W₁₈O₄₉-nanowires/reduced-graphene-oxides) for isopropanol dehydration where the reaction rate was 180-fold greater than the corresponding thermocatalytic pathway. The key detail to this strategy lies in the synergetic utilization of ultraviolet light and visible-near-infrared light to enhance the hot electron generation and promote electron transfer for C-O bond cleavage during isopropanol dehydration reaction. This, in turn, results in a reduced reaction activation barrier down to 0.37 eV (compared to 1.0 eV of thermocatalysis) and a significantly improved conversion efficiency of 100% propylene from isopropanol. This work provides an additional strategy to modulate hot carrier of plasmonic semiconductors and helps guide the design of better catalytic materials and chemistries.

Electronic modulation of metal-support interactions improves polypropylene hydrogenolysis over ruthenium catalysts

Ruthenium (Ru) is the one of the most promising catalysts for polyolefin hydrogenolysis. Its performance varies widely with the support, but the reasons remain unknown. Here, we introduce a simple synthetic strategy (using ammonia as a modulator) to tune metal-support interactions and apply it to Ru deposited on titania (TiO₂). We demonstrate that combining deuterium nuclear magnetic resonance spectroscopy with temperature variation and density functional theory can reveal the complex nature, binding strength, and H amount. H₂ activation occurs heterolytically, leading to a hydride on Ru, an H⁺ on the nearest oxygen, and a partially positively charged Ru. This leads to partial reduction of TiO₂ and high coverages of H for spillover, showcasing a threefold increase in hydrogenolysis rates. This result points to the key role of the surface hydrogen coverage in improving hydrogenolysis catalyst performance.

Catalytic pathways of plastic waste valorization to lubricants are attractive avenues to foster circular economy. Tuning of catalyst electronic properties allows to significantly improve its activity due to boosted hydrogen storage on the surface.

Highly efficient synthesis of novel bio-based pentamethylene dicarbamate via carbonylation of pentanediamine with ethyl carbamate over well-defined titanium oxide catalysts

Highly efficient synthesis of bio-based pentamethylene dicarbamate from pentanediamine and ethyl carbamate was successfully achieved over the well-defined TiO2 catalysts, which provides a green and sustainable way for the production of bio-based isocyanates or polyurethane.

Construction and photocatalytic performance of fluorinated ZnO-TiO2 heterostructure composites

Titanium dioxide, as a promising photocatalytic material, has attracted extensive attention in the field of photocatalytic degradation of organic pollutants in sewage. However, the photocatalytic performance needs to be further improved. In this work, fluorinated ZnO–TiO₂ composites (F-ZTO) were prepared by a simple coprecipitation method. The photocatalytic performance of the samples was studied in detail with methyl orange as the target degradation product. The results indicated that under the same conditions, the degradation rates of 6% F-ZTO, F-TiO₂ and TiO₂ for methyl orange reached 93.75%, 76.56% and 62.89% respectively. This showed that the method used in this work could effectively improve the photocatalytic degradation performance of titanium dioxide. 6% F-ZTO showed an excellent photocatalytic activity, which was attributed to the small grain size, the large specific surface area and the effective inhibition of photoelectron–hole recombination due to fluorination and zinc oxide coupling. In three consecutive cycles, the photocatalytic activity was almost maintained, indicating that 6% F-ZTO had a good recycling performance.

Fluorinated ZnO-TiO₂ composites (F-ZTO) were prepared by a simple coprecipitation method, the method used could effectively improve the photocatalytic property of titanium dioxide, and 6% F-ZTO showed an excellent activity and recycling performance.

Selective suppression of {112} anatase facets by fluorination for enhanced TiO2 particle size and phase stability at elevated temperatures

Generally, anatase is the most desirable TiO₂ polymorphic phase for photovoltaic and photocatalytic applications due to its higher photoconductivity and lower recombination rates compared to the rutile phase. However, in applications where temperatures above 500 °C are required, growing pure anatase phase nanoparticles is still a challenge, as above this temperature TiO₂ crystallite sizes are larger than 35 nm which thermodynamically favors the growth of rutile crystallites. In this work, we show strong evidence, for the first time, that achieving a specific fraction (50%) of the {112} facets on the TiO₂ surface is the key limiting step for anatase-to-rutile phase transition, rather than the crystallite size. By using a fluorinated ionic liquid (IL) we have obtained pure anatase phase crystallites at temperatures up to 800 °C, even after the crystallites have grown beyond their thermodynamic size limit of ca. 35 nm. While fluorination by the IL did not affect {001} growth, it stabilized the pure anatase TiO₂ by suppressing the formation of {112} facets on anatase particles. By suppressing the {112} facets, using specific concentrations of fluorinated ionic liquid in the TiO₂ synthesis, we controlled the anatase-to-rutile phase transition over a wide range of temperatures. This information shall help synthetic researchers to determine the appropriate material conditions for specific applications.

Tuning the shape and crystal phase of TiO2 nanoparticles for catalysis

Synthesis of TiO₂ nanoparticles with tunable shape and crystal phase has attracted considerable attention for the design of highly efficient heterogeneous catalysts. Tailoring the shape of TiO₂, in the crystal phases of anatase, rutile, brookite and TiO₂(B), allows tuning of the atomic configurations on the dominantly exposed facets for maximizing the active sites and regulating the reaction route towards a specific channel for achieving high selectivity. Moreover, the shape and crystal phase of TiO₂ nanoparticles alter their interactions with metal species, which are commonly termed as strong metal-support interactions involving interfacial strain and charge transfer. On the other hand, metal particles, clusters and single atoms interact differently with TiO₂, because of the variation of the electronic structure, while the surface of TiO₂ determines the interfacial bonding via a geometric effect. The dynamic behavior of the metal-titania interfaces, driven by the chemisorption of the reactive molecules at elevated temperatures, also plays a decisive role in elaborating the structure-reactivity relationship.

Au/TiO2-Catalyzed Benzyl Alcohol Oxidation on Morphologically Precise Anatase Nanoparticles

Au nanoparticles (NP) on TiO₂ have been shown to be effective catalysts for selective oxidation reactions by using molecular oxygen. In this work, we have studied the influence of support morphology on the catalytic activity of Au/TiO₂ catalysts. Two TiO₂ anatase supports, a nanoplatelet-shaped material with predominantly the {001} facet exposed and a truncated bipyramidal-shaped nanoparticle with predominantly the {101} facet exposed, were prepared by using a nonaqueous solvothermal method and characterized by using DRIFTS, XPS, and TEM. Au nanoparticles were deposited on the supports by using the deposition-precipitation method, and particle sizes were determined by using STEM. Au nanoparticles were smaller on the support with the majority of the {101} facet exposed. The resulting materials were used to catalyze the aerobic oxidation of benzyl alcohol and trifluoromethylbenzyl alcohol. Support morphology impacts the catalytic activity of Au/TiO₂; reaction rates for reactions catalyzed by the predominantly {101} material were higher. Much of the increased reactivity can be explained by the presence of smaller Au particles on the predominantly {101} material, providing more Au/TiO₂ interface area, which is where catalysis occurs. The remaining modest differences between the two catalysts are likely due to geometric effects as Hammett slopes show no evidence for electronic differences between the Au particles on the different materials.

Elucidation of Active Sites in Aldol Condensation of Acetone over Single-Facet Dominant Anatase TiO2 (101) and (001) Catalysts

Aldol condensations of carbonyl compounds for C-C bond formation are a very important class of reactions in organic synthesis and upgrading of biomass-derived feedstocks. However, the atomic level understanding of reaction mechanisms and structure-activity correlation on widely used transition metal oxide catalysts are limited due to the high degree of structural heterogeneity of catalysts such as commercial TiO₂ powders. Here, we provide a deep understanding of the reaction mechanisms, kinetics, and structure-function relationships for vapor phase acetone aldol condensation through the controlled synthesis of two catalysts with high surface areas and clean, dominant facets, coupled with detailed characterization and kinetic studies that are further assisted by density functional theory (DFT) calculations. Temperature-dependent diffuse reflectance infrared Fourier transform spectroscopy showed the existence of abundant acetone bonded to surface hydroxyl groups (acetone-O_sH) and acetone bonded to Lewis acid sites (acetone-Ti_5c) on the surface of both {101} and {001} facet dominant TiO₂. Intermolecular C-C coupling of theenolate intermediate from acetone-Ti_5c and a vicinal acetone-O_sH is a kinetically relevant step, which is consistent with kinetic and isotopic studies as well as DFT calculations. The {001} facet showed a lower apparent activation energy (or higher activity) than the {101} facet. This is likely caused by the weaker Lewis acid and Brønsted base strengths of the {001} facet which favors the reprotonation-desorption of the coupled intermediate, making the C-C coupling step more exothermic on the {001} facet and resulting in an earlier transition state with a lower activation barrier. It is also possible that the {001} facet has a smoother surface configuration and less steric hindrance during intermolecular C-C bond formation than the {101} facet.