Supported palladium catalysts in environmental catalytic technologies for gaseous emissions

Influence of the drying mode of support on the properties of Pd/Al2O3-ZrO2 materials used for methane combustion

This work constitutes a new trial to enhance the properties of palladium supported on alumina modified with zirconium used as catalysts for methane combustion. The effect of the support drying mode is studied. For this aim, Al2O3-ZrO2 binary oxides with zirconium loading of 2 and 5% in weight were prepared using sol-gel process then dried under ordinary or supercritical conditions. Palladium with a loading of 0.5% was deposited on the support by wet impregnation. Several techniques have been used to investigate differences between the two types of the derived catalysts.

Effects of thermal aging on the electronic and structural properties of Pt-Pd and toluene oxidation activity

Catalytic oxidation is a feasible method for remediating volatile organic compounds (VOCs), due to its lower energy consumption and mineralization of VOCs into H₂O and CO₂. Noble metal-based catalysts are preferred for the catalytic oxidation of VOCs because of their superior activity, but they are usually deactivated by thermal aging which sinters the metal particles. Here, we report that Pt-Pd/Al₂O₃ thermally aged at 700-900 °C in air showed enhanced catalytic activity for toluene oxidation in humid conditions. There were electronic and structural changes in the thermally aged Pt-Pd/Al₂O₃, as confirmed by numerous analyses. Both Pt and Pd existed in a metallic rather than oxidized state without additional reduction steps. The noble metal particles were assembled to form Pt-Pd alloy, in the form of isolated Pd atoms surrounded by Pt atoms. This specific alloy structure was found to be crucial to the observed enhancement in catalytic toluene oxidation at low temperature.

Mesoporous catalysts for catalytic oxidation of volatile organic compounds: preparations, mechanisms and applications

Volatile organic compounds (VOCs) are mainly derived from human activities, but they are harmful to the environment and our health. Catalytic oxidation is the most economical and efficient method to convert VOCs into harmless substances of water and carbon dioxide at relatively low temperatures among the existing techniques. Supporting noble metal and/or transition metal oxide catalysts on the porous materials and direct preparation of mesoporous catalysts are two efficient ways to obtain effective catalysts for the catalytic oxidation of VOCs. This review focuses on the preparation methods for noble-metal-based and transition-metal-oxide-based mesoporous catalysts, the reaction mechanisms of the catalytic oxidations of VOCs over them, the catalyst deactivation/regeneration, and the applications of such catalysts for VOCs removal. It is expected to provide guidance for the design, preparation and application of effective mesoporous catalysts with superior activity, high stability and low cost for the VOCs removal at lower temperatures.

Pd-based catalysts promoted by hierarchical porous Al2O3 and ZnO microsphere supports/coatings for ethyl acetate highly active and stable destruction

Developing economical and active materials is of great significance for VOC purification. Here, hierarchical porous Al₂O₃ and ZnO microspheres (Al₂O₃-pm and ZnO-pm) were synthesized by a facile hydrothermal strategy. The urchin-like Al₂O₃-pm and flower-like ZnO-pm possess high specific surface area (especially; external surface area) obviously boost the dispersion of Pd with 29.3 % and 30.1 % over Pd/Al₂O₃-pm and Pd/ZnO-pm, respectively, over 3.4 times higher than those of commercial Al₂O₃- and ZnO-supported counterparts. Pd/Al₂O₃-pm possesses excellent activity and CO₂ yield in ethyl acetate (EA) degradation, with TOF reaches 7.76 × 10^-3 s^-1 at 160 °C under GHSV of 50,000 h^-1. Moreover, Pd/Al₂O₃-pm exhibits satisfied performance in EA-contained binary VOCs oxidation and has high long-term stability under both dry and humid conditions. Both Pd sites and Brønsted acid sites participated in reaction process and initially react with EA to form ethylene and ethanol, respectively. Larger amount Brønsted acid sites over Pd/Al₂O₃-pm promote ethanol formation and C-C cleavage, resulting in different CO₂ yields and EA activation mechanisms. The coating greatly enhances Pd dispersion over Pd supported monolithic catalyst, endowing its desired activity and stability even with a much lower Pd loading. This work promotes the potential application of noble-metal-based monolithic materials in VOC degradation.

Recent advances in three-way catalysts of natural gas vehicles

This review presents recent advances in TWCs for NGVs, particularly for Pd-based catalysts and potential alternatives.

Hydrogen in Palladium and Storage Properties of Related Nanomaterials: Size, Shape, Alloying, and Metal-Organic Framework Coating Effects

One of the key issues for an upcoming hydrogen energy-based society is to develop highly efficient hydrogen-storage materials. Among the many hydrogen-storage materials reported, transition-metal hydrides can reversibly absorb and desorb hydrogen, and have thus attracted much interest from fundamental science to applications. In particular, the Pd-H system is a simple and classical metal-hydrogen system, providing a platform suitable for a thorough understanding of ways of controlling the hydrogen-storage properties of materials. By contrast, metal nanoparticles have been recently studied for hydrogen storage because of their unique properties and the degrees of freedom which cannot be observed in bulk, i. e., the size, shape, alloying, and surface coating. In this review, we overview the effects of such degrees of freedom on the hydrogen-storage properties of Pd-related nanomaterials, based on the fundamental science of bulk Pd-H. We shall show that sufficiently understanding the nature of the interaction between hydrogen and host materials enables us to control the hydrogen-storage properties though the electronic-structure control of materials.

Technologies for deodorization of malodorous gases

There is an increasing number of citizens' complaints about odor nuisance due to production or service activity. High social awareness imposes pressure on entrepreneurs and service providers forcing them to undertake effective steps aimed at minimization of the effects of their activity, also with respect to emission of malodorous substances. The article presents information about various technologies used for gas deodorization. Known solutions can be included into two groups: technologies offering prevention of emissions, and methodological solutions that enable removal of malodorous substances from the stream of emitted gases. It is obvious that the selection of deodorization technologies is conditioned by many factors, and it should be preceded by an in-depth analysis of possibilities and limitations offered by various solutions. The aim of the article is presentation of the available gas deodorization technologies as to facilitate the potential investors with selection of the method of malodorous gases emission limitation, suitable for particular conditions.

Effect of Y Modified Ceria Support in Mono and Bimetallic Pd–Au Catalysts for Complete Benzene Oxidation

Mono metallic and bimetallic Pd (1 wt. %)–Au (3 wt. %) catalysts were prepared using two ceria supports doped with 1 wt. % Y2O3. Yttrium was added by impregnation or co-precipitation. The catalyst synthesis was carried out by deposition–precipitation method, with sequential deposition–precipitation of palladium over previously loaded gold in the case of the bimetallic samples. The obtained materials, characterized by X-ray powder diffraction (XRD), High resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and temperature programmed reduction (TPR) techniques, were tested in the complete benzene oxidation (CBO). The results of the characterization analyses and the catalytic performance pointed to a close relationship between structural, redox, and catalytic properties of mono and bimetallic catalysts. Among the monometallic systems, Pd catalysts were more active as compared to the corresponding Au catalysts. The bimetallic systems exhibited the best combustion activity. In particular, over Pd–Au supported on Y-impregnated ceria, 100% of benzene conversion towards total oxidation at the temperature of 150 °C was obtained. Comparison of surface sensitive XPS results of fresh and spent catalysts ascertained the redox character of the reaction.

Promoter effect of Pd species on Mn oxide catalysts supported on rare-earth-iron mixed oxide

The loading of a small amount of Pd species dramatically promotes a catalytic performance of MnO_x species located on the hexagonal structure of YbFeO₃.

Support Morphology-Dependent Catalytic Activity of Pd/CeO₂ for Formaldehyde Oxidation

To eliminate indoor formaldehyde (HCHO) pollution, Pd/CeO2 catalysts with different morphologies of ceria support were employed. The palladium nanoparticles loaded on {100}-faceted CeO2 nanocubes exhibited much higher activity than those loaded on {111}-faceted ceria nanooctahedrons and nanorods (enclosed by {100} and {111} facets). The HCHO could be fully converted into CO2 over the Pd/CeO2 nanocubes at a GHSV of 10,000 h(-1) and a HCHO inlet concentration of 600 ppm at ambient temperature. The prepared catalysts were characterized by a series of techniques. The HRTEM, ICP-MS and XRD results confirmed the exposed facets of the ceria and the sizes (1-2 nm) of the palladium nanoparticles with loading amounts close to 1%. According to the Pd 3d XPS and H2-TPR results, the status of the Pd-species was dependent on the morphologies of the supports. The {100} facets of ceria could maintain the metallic Pd species rather than the {111} facets, which promoted HCHO catalytic combustion. The Raman and O 1s XPS results revealed that the nanorods with more defect sites and oxygen vacancies were responsible for the easy oxidation of the Pd-species and low catalytic activity.

Creation of Novel Solid-Solution Alloy Nanoparticles on the Basis of Density-of-States Engineering by Interelement Fusion

Currently 118 known elements are represented in the periodic table. Of these 118 elements, only about 80 elements are stable, nonradioactive, and widely available for our society. From the viewpoint of the "elements strategy", we need to make full use of the 80 elements to bring out their latent ability and create innovative materials. Furthermore, there is a strong demand that the use of rare or toxic elements be reduced or replaced while their important properties are retained. Advanced science and technology could create higher-performance materials even while replacing or reducing minor or harmful elements through the combination of more abundant elements. The properties of elements are correlated directly with their electronic states. In a solid, the magnitude of the density of states (DOS) at the Fermi level affects the physical and chemical properties. In the present age, more attention has been paid to improving the properties of materials by means of alloying elements. In particular, the solid-solution-type alloy is advantageous because the properties can be continuously controlled by tuning the compositions and/or combinations of the constituent elements. However, the majority of bulk alloys are of the phase-separated type under ambient conditions, where constituent elements are immiscible with each other. To overcome the challenge of the bulk-phase metallurgical aspects, we have focused on the nanosize effect and developed methods involving "nonequilibrium synthesis" or "a process of hydrogen absorption/desorption". We propose a new concept of "density-of-states engineering" for the design of materials having the most desirable and suitable properties by means of "interelement fusion". In this Account, we describe novel solid-solution alloys of Pd-Pt, Ag-Rh, and Pd-Ru systems in which the constituent elements are immiscible in the bulk state. The homogeneous solid-solution alloys of Pd and Pt were created from Pd core/Pt shell nanoparticles using a hydrogen absorption/desorption process as a trigger. Several atom percent replacements of Pd with Pt atoms resulted in a significantly enhanced hydrogen absorption capacity compared with Pd nanoparticles. AgxRh1-x and PdxRu1-x solid-solution alloy nanoparticles were also developed by nonequilibrium synthesis based on a polyol method. The AgxRh1-x nanoparticles demonstrated hydrogen storage properties, although pure metal nanoparticles of each constituent element do not adsorb hydrogen. AgxRh1-x is therefore considered to possess a similar electronic structure to Pd as a synthetic pseudo-palladium. The PdxRu1-x nanoparticles showed enhanced catalytic activity for CO oxidation, with the highest catalytic activity found using the equimolar Pd0.5Ru0.5 nanoparticles. The catalytic activity of the Pd0.5Ru0.5 nanoparticles exceeds that of the widely used and best-performing Ru catalysts for CO oxidation and is also higher than that of neighboring Rh on the periodic table. Our present work provides a guiding principle for the design of a suitable DOS shape according to the intended physical and/or chemical properties and a method for the development of novel solid-solution alloys.

Low temperature catalytic oxidation of volatile organic compounds: a review

Volatile organic compounds (VOCs) are toxic and recognized as one of the major contributors to air pollution.

Effect of pretreatment on pd/Al2O3 catalyst for catalytic oxidation of o-xylene at low temperature

The effect of pretreatment on Pd/Al2O3 catalysts for the catalytic oxidation of o-xylene at low temperature was studied by changing the pretreatment and testing conditions. The fresh and pretreated Pd/Al2O3 catalysts were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The results showed that the pretreatment dramatically changed the Pd/PdO ratio and then significantly affected the Pd/Al2O3 activity; while the pretreatment had not much influence on Pd particle size. The Pd/Al2O3 pre-reduced at 300 degrees C/400 degrees C, which has fully reduced Pd species, showed the highest activity; while the fresh Pd/Al2O3, which has fully oxidized Pd species, presented the worst performance, indicating the Pd chemical state plays an important role in the catalytic activity for the o-xylene oxidation. It is concluded that metallic Pd is the active species on the Pd/Al2O3 catalyst for the catalytic oxidation of o-xylene at low temperature.

The abatement of major pollutants in air and water by environmental catalysis

This review reports the research progress in the abatement of major pollutants in air and water by environmental catalysis. For air pollution control, the selective catalytic reduction of NO x (SCR) by ammonia and hydrocarbons on metal oxide and zeolite catalysts are reviewed and discussed, as is the removal of Hg from flue gas by catalysis. The oxidation of Volatile organic compounds (VOCs) by photo- and thermal-catalysis for indoor air quality improvement is reviewed. For wastewater treatment, the catalytic elimination of inorganic and organic pollutants in wastewater is presented. In addition, the mechanism for the procedure of abatement of air and water pollutants by catalysis is discussed in this review. Finally, a research orientation on environment catalysis for the treatment of air pollutants and wastewater is proposed.