Catalytic oxidation of chlorinated organics over lanthanide perovskites: effects of phosphoric acid etching and water vapor on chlorine desorption behavior

In this article, the underlying effect of phosphoric acid etching and additional water vapor on chlorine desorption behavior over a model catalyst La3Mn2O7 was explored. Acid treatment led to the formation of LaPO4 and enhanced the mobility of lattice oxygen of La3Mn2O7 evidenced by a range of characterization (i.e., X-ray diffraction, temperature-programmed analyses, NH3–IR, etc.). The former introduced thermally stable Brönsted acidic sites that enhanced dichloromethane (DCM) hydrolysis while the latter facilitated desorption of accumulated chlorine at elevated temperatures. The acid-modified catalyst displayed a superior catalytic activity in DCM oxidation compared to the untreated sample, which was ascribed to the abundance of proton donors and Mn­(IV) species. The addition of water vapor to the reaction favored the formation and desorption of HCl and avoided surface chlorination at low temperatures. This resulted in a further reduction in reaction temperature under humid conditions (T90 of 380 °C for the modified catalyst). These results provide an in-depth interpretation of chlorine desorption behavior for DCM oxidation, which should aid the future design of industrial catalysts for the durable catalytic combustion of chlorinated organics

Efficient elimination of chlorinated organics on a phosphoric acid modified CeO2 catalyst: a hydrolytic destruction route

The development of efficient technologies to prevent the emission of hazardous chlorinated organics from industrial sources without forming harmful by-products, such as dioxins, is a major challenge in environmental chemistry. Herein, we developed a new hydrolytic destruction route for efficient chlorinated organics elimination and demonstrated that phosphoric acid modified CeO2 (HP-CeO2) can hydrolytically destruct chlorobenzene (CB) without forming polychlorinated congeners under the industry-relevant reaction conditions. The active site and origin of hydrolysis reactivity of HP-CeO2 were probed, which showed the surface phosphate groups can hydrolytically react with CB and water to form phenol and HCl, thus facilitating the chlorine desorption and ensuring a continual O2 activation. Subsequent density functional theory (DFT) calculations revealed a distinctly decreased formation energy of oxygen vacancy nearest (VO-1) and next-nearest (VO-2) to the bonded phosphate groups, which led to a significantly improved oxidizing ability of the catalyst. Significantly, no toxic dioxins were detected from the hydrolysis destruction of CB, which has been frequently cited as a significant challenge to avoid in the conventional oxidation route. This work not only reports an efficient route and corresponding phosphate active site for chlorinated organics elimination, but also illustrates that rational design of reaction route can solve some of the most important challenges in environmental catalysis

Catalytic Oxidation of Chlorobenzene over Mn_<i>x</i>Ce_1–<i>x</i>O₂/HZSM‑5 Catalysts: A Study with Practical Implications

Industrial-use catalysts usually encounter severe deactivation after long-term operation for catalytic oxidation of chlorinate volatile organic compounds (CVOCs), which becomes a “bottleneck” for large-scale application of catalytic combustion technology. In this work, typical acidic solid-supported catalysts of Mn_<i>x</i>Ce_1–<i>x</i>O₂/HZSM-5 were investigated for the catalytic oxidation of chlorobenzene (CB). The activation energy (<i>E</i>_a), Brønsted and Lewis acidities, CB adsorption and activation behaviors, long-term stabilities, and surficial accumulation compounds (after aging) were studied using a range of analytical techniques, including XPS, H₂-TPR, pyridine-IR, DRIFT, and O₂-TP-Ms. Experimental results revealed that the Brønsted/Lewis (B/L) ratio of Mn_<i>x</i>Ce_1–<i>x</i>O₂/HZSM-5 catalysts could be adjusted by ion exchange of H• (in HZSM-5) with Mnⁿ⁺ (where the exchange with Ce⁴⁺ did not distinctly affect the acidity); the long-term aged catalysts could accumulate ca. 14 organic compounds at surface, including highly toxic tetrachloromethane, trichloroethylene, tetrachloroethylene, <i>o</i>-dichlorobenzene, etc.; high humid operational environment could ensure a stable performance for Mn_<i>x</i>Ce_1–<i>x</i>O₂/HZSM-5 catalysts; this was due to the effective removal of Cl• and coke accumulations by H₂O washing, and the distinct increase of Lewis acidity by the interaction of H₂O with HZSM-5. This work gives an in-depth view into the CB oxidation over acidic solid-supported catalysts and could provide practical guidelines for the rational design of reliable catalysts for industrial applications

Palladium Encapsulated by an Oxygen-Saturated TiO2 Overlayer for Low-Temperature SO2-Tolerant Catalysis during CO Oxidation

The development of oxidation catalysts that are resistant to sulfur poisoning is crucial for extending the lifespan of catalysts in real-working conditions. Herein, we describe the design and synthesis of oxide-metal interaction (OMI) catalyst under oxidative atmospheres. By using organic coated TiO2, an oxide/metal inverse catalyst with non-classical oxygen-saturated TiO2 overlayers were obtained at relatively low temperature. These catalysts were found to incorporate ultra-small Pd metal and support particles with exceptional reactivity and stability for CO oxidation (under 21 vol% O2 and 10 vol% H2O). In particular, the core (Pd)–shell (TiO2) structured OMI catalyst exhibited excellent resistance to SO2 poisoning, yielding robust CO oxidation performance at 120 °C for 240 h (at 100 ppm SO2 and 10 vol% H2O). The stability of this new OMI catalyst was explained through density functional theory (DFT) calculations that interfacial oxygen atoms at Pd–O–Ti sites (of oxygen-saturated overlayers) serve as non-metal active sites for low-temperature CO oxidation, and change the SO2 adsorption from metal(d)-to-SO2(π*) back-bonding to much weaker σ(Ti–S) bonding

SO₂ Poisoning Structures and the Effects on Pure and Mn Doped CeO₂: A First Principles Investigation

SO_<i>x</i> poisoning effects in environmental catalysis have long been recognized as a challenge in development of efficient catalysts for industrial applications. In this paper, a theoretic method combining density functional theory and standard thermodynamic data (enthalpy and entropy) was applied to investigate the SO₂ poisoning to pure and Mn doped CeO₂ as model catalysts in realistic temperature and pressure. Surface Ce­(SO₄)₂ rather than Ce₂(SO₄)₃ was identified to be the most stable poisoning structure on pure CeO₂. The SO_<i>x</i> poisoning to the catalysts could not be surmounted simply by heteroatom doping, since the introduction of Mn will enhance the thermal stability of the surface sulfate. The results also indicated that the Lewis acidity of the catalysts could be enhanced by slightly sulfating, which might make some positive effect on catalytic performances for the abatement of environmentally sensitive species including NH₃, NO, CO, and hydrocarbons

Facile Approach for the Syntheses of Ultrafine TiO₂ Nanocrystallites with Defects and C Heterojunction for Photocatalytic Water Splitting

In this paper, a supercritical water (sc-H₂O) reaction medium was employed for the syntheses of ultrafine TiO₂ nanocrystallites (at ca. 5 nm) that were linked with lactate species at surface. The resulting hybrid material was then subjected to an aging at ca. 300 °C for 2 h under N₂ atmosphere. After subjected to spherical aberration corrected STEM and EPR analyses, it was noted that the aged sample was shown with highly distorted crystal lattice with oxygen vacancies at surface and Ti³⁺ in the bulk. The anoxic aging also caused incomplete combustion for lactate species, leading to the formation of C heterojunction with TiO₂. UV–vis, PL and transient photocurrent (TP) measurements revealed that the resulting surface oxygen vacancies and C heterojunction had conferred a combination of advantages in enhancing visible light absorption and promoting electron–hole pair separation for aged sample, which led to significantly promoted hydrogen production efficiency in photocatalytic water splitting under a full-spectrum irradiation (where the aged TiO₂ had yielded ca. 4-fold higher hydrogen production rate than the nonaged one and ca. 40–50-fold higher than commercial Degussa P25). We expected that the work conducted herein could provide a facile and controllable approach to produce simultaneously defects and C heterojunction for ultrafine TiO₂ nanocrystallites, which might lead to scale-up production of them for industry