Oscillatory Behavior of Pd-Au Catalysts in Toluene Total Oxidation

Current progress on catalytic oxidation of toluene: a review

Toluene is one of the pollutants that are dangerous to the environment and human health and has been sorted into priority pollutants; hence, the control of its emission is necessary. Due to severe problems caused by toluene, different techniques for the abatement of toluene have been developed. Catalytic oxidation is one of the promising methods and effective technologies for toluene degradation as it oxidizes it to CO₂ and does not deliver other pollutants to the environment. This paper highlights the recent progressive advancement of the catalysts for toluene oxidation. Five categories of catalysts, including noble metal catalysts, transition metal catalysts, perovskite catalysts, metal-organic frameworks (MOFs)-based catalysts, and spinel catalysts reported in the past half a decade (2015-2020), are reviewed. Various factors that influence their catalytic activities, such as morphology and structure, preparation methods, specific surface area, relative humidity, and coke formation, are discussed. Furthermore, the reaction mechanisms and kinetics for catalytic oxidation of toluene are also discussed.

Granular-carbon supported nano noble-metal (Au, Pd, Au-Pd): new dual-functional adsorbent/catalysts for effective removal of toluene at low-temperature and humid condition

Treatment of the volatile organic compounds (VOCs) especially aromatic compounds such as toluene at low temperature and in the highly humid condition is currently a great challenge. New dual functional adsorbent/catalysts have been developed in this study to minimize the mass-transfer limitation at low temperature. The ready-to-practically-use materials, which consisted nano-sized noble metal (Au, Pd, Au-Pd) supported on granular carbon (GC) have been prepared using a metal-sol method. The surface morphology, and structure of these granular materials were characterized to confirm the presence of nano-sized noble metal on the GC as well as the properties of the dual functional adsorbent/catalyst. The results of catalytic performance revealed that the presence of Pd played an important role in the formation of nano Au particles, which were the catalytic active-site for toluene oxidation. At 60% relative humidity and 150°C the dual functional adsorbent/catalyst, Au-Pd/GC, exhibited 97.2% toluene removal. Importantly, the kinetic analysis for the catalytic oxidation of nano-sized 0.5%Au-0.27%Pd/GC catalyst showed that the Langmuir-Hinshelwood mechanism provided a good fit towards the experimental data and allowed to determine the kinetic parameters of the reaction-rate law -rA=k⋅KA⋅CA1+KA⋅CA⋅KO2⋅CO21+KO2.CO2. The activation energy, adsorption enthalpy of toluene, and oxygen on the catalyst were reported.

Effect of Pd/Ce loading on the performance of Pd-Ce/γ-Al2O3 catalysts for toluene abatement

A single metal Pd/γ-Al₂O₃ catalyst and a bimetallic Pd-Ce/γ-Al₂O₃ catalyst were prepared by the equal-volume impregnation method to investigate the effect of CeO₂ loading on the catalytic oxidation of toluene. The specific surface area, surface morphology, and redox performance of the catalyst were characterized by N₂ desorption, scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), H₂-TPR, O₂-TPD, and electron paramagnetic resonance (EPR). The results showed that bimetal catalysts loaded CeO₂ had smaller nano-PdO particles than those of the Pd/γ-Al₂O₃ catalyst. Compared with the catalyst of 0.2Pd/γ-Al₂O₃ (percentage of mass, the same as below), the catalyst doped with 0.3CeO₂ had a stronger reduction peak, which was shifted to the low-temperature zone by more than 80 °C. The results of XPS and O₂-TPD showed that the introduction of CeO₂ provided more surface oxygen vacancy for the catalyst and enhanced its catalytic oxidation ability, and the amount of desorbed O₂ increased from 3.55 μmol/g to 8.54 μmol/g. The results of EPR were that the addition of CeO₂ increased the content of active oxygen species and oxygen vacancies on the surface of the catalysts, which might be due to the supply of electrons to the O₂ and PdO during the Ce³⁺toCe⁴⁺ conversion process. That could have accelerated the catalytic reaction process. Compared with the single precious metal catalyst, the T₁₀ and T₉₀ of the Pd-Ce/γ-Al₂O₃ catalyst were decreased by 22 °C and 40 °C, respectively.

Bimetallic Catalysts for Volatile Organic Compound Oxidation

In recent years, the impending necessity to improve the quality of outdoor and indoor air has produced a constant increase of investigations in the methodologies to remove and/or to decrease the emission of volatile organic compounds (VOCs). Among the various strategies for VOC elimination, catalytic oxidation and recently photocatalytic oxidation are regarded as some of the most promising technologies for VOC total oxidation from urban and industrial waste streams. This work is focused on bimetallic supported catalysts, investigating systematically the progress and developments in the design of these materials. In particular, we highlight their advantages compared to those of their monometallic counterparts in terms of catalytic performance and physicochemical properties (catalytic stability and reusability). The formation of a synergistic effect between the two metals is the key feature of these particular catalysts. This review examines the state-of-the-art of a peculiar sector (the bimetallic systems) belonging to a wide area (i.e., the several catalysts used for VOC removal) with the aim to contribute to further increase the knowledge of the catalytic materials for VOC removal, stressing the promising potential applications of the bimetallic catalysts in the air purification.

Catalysts for Oxidative Destruction of Volatile Organic Compounds

On a worldwide scale, concern for our environment has understandably gained high priority in many current political and social agendas [...]