Dispersed PdO Research Articles

Characterization of Pd-based FCC CO/NO x control additives by in situ FTIR and extended X-ray absorption fine structure spectroscopies

Pd n+ /Ce n+ /Na +/γ-Al 2O 3-type materials used as FCC additives for CO/NO x control were characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy and in situ FTIR. The EXAFS data indicate that in freshly prepared samples palladium is present in the form of highly dispersed PdO species. Reduction with H 2 at 500 °C leads to the formation of small Pd clusters incorporating on average approximately six to eight metal atoms at a Pd−Pd bond distance of 2.76 Å. All components of these materials can interact with NO and promote the formation of nitrate/nitrite species, essentially “trapping” NO x species on the catalyst surface. However, the Na + species dominate the surface chemistry and readily form sodium nitrates with a characteristic IR band at 1370–1385 cm −1. Finally, hydroxyls from the support are also actively participating in the formation of HNO x type compounds with characteristic stretching vibrations in the 3500–3572 cm −1 region.

Influence of solvents on the formation of Pd and PdO nanoparticles in SBA-15

SBA-15 supported Pd and PdO nanoparticles have been synthesized and compared in three different solvents, ethanol (EtOH), tetrahydrofuran (THF), and supercritical carbon dioxide (scCO 2). The particle size distribution and dispersion are strongly influenced by the nature of solvents. When the impregnation of Pd(hfac) 2 on SBA-15 without surface functionalization is carried out in ethanol, strong aggregation of Pd nanoparticles is induced on the external surface during evaporation and the size distribution is extremely poor. Loading of Pd precursors in THF induces PdO and Pd nanoparticles both in the mesoporous channels and on the external surface of the silica support. However, the particles even on the external surface are relatively well dispersed and the growth of particle is not significant. In contrast, the impregnation of Pd(hfac) 2 precursor in scCO 2 solvent results in well dispersed Pd and PdO nanoparticles in the mesoporous channels of SBA-15, where the particle size (∼6 nm) is limited by the mesopore diameter.

Characterization of PdO/Ce 0.8Y 0.2O 1.9 catalysts for carbon monoxide and methane oxidation

The Ce 0.8Y 0.2O 1.9 solid solution was prepared by nitrate sol–gel method, and a series of catalysts with different PdO loading were prepared using impregnation method. These catalysts were characterized by XRD, Raman, CO-TPR, CO 2- and O 2-TPD techniques. The PdO is highly dispersed on the surface of the solid solution when the loading is lower than 0.5 wt.%. As PdO loading increases to 2 wt.%, it begins to form the crystalline structure. CO 2-TPD profiles show that the CO adsorbed on highly dispersed PdO is more easily oxidized to CO 2 than that adsorbed on crystalline structure and O 2-TPD results indicate that it is more difficult to decompose for highly dispersed PdO than that for crystalline structure. CO-TPR profiles show that the highly dispersed PdO is easily reduced. Catalytic activities of these catalysts for CO and CH 4 oxidation indicate that both the highly dispersed and crystalline PdO are the active site for CO oxidation, while the crystalline structure is the active site for CH 4 oxidation.

PdO/Al2O3–(Ce1-X Zr X )O2 catalysts: effect of the sol-gel support composition

The contribution of (Ce1-X Zr X )O2 additives to alumina supports prepared by sol-gel and the catalytic properties of PdO/Al2O3–(Ce1-X Zr X )O2 catalysts (~0.3 wt% Pd, ~5 wt% (Ce1-X Zr X )O2) in CO oxidation was herein investigated. The addition of (Ce1-X Zr X )O2 to the support enhanced the surface area and decreased the size of Al2O3 particles. The UV–Vis bands of PdO particles and Pd2+ ions indicate that zirconia in (Ce1-X Zr X )O2 promotes palladium-support interactions by forming highly dispersed PdO particles. Temperature-programmed reduction (TPR) in hydrogen revealed that ceria enhanced the redox capacity of the supports while zirconia lowered the reduction temperature of palladium oxide species. The comprehensive study revealed that the Ce/Zr ratio was a key factor influencing the catalytic activity of samples in CO oxidation, because palladium oxide-support interactions had a significant effect in changing of the reducibility of samples. So, the PdO/Al2O3–(Ce0.5Zr0.5)O2 exhibited the highest catalytic activity.

Facile Synthesis of Monodispersed PdO Nanoparticles within Mesoporous Silica with Sonication

The synthesis and characterization of nanosized materials is an important issue in basic science research as well as in technological applications, as they have their own characteristic optical, electronic, magnetic, and catalytic properties greatly different from the bulk materials. Therefore, considerable effort has focused on the size and shape selective nanocrystal growth using a variety of synthetic methods including templating, electrochemistry, photochemistry, and seeding. Since powders and films of mesoporous silica have uniform pore diameters in the range of 4-30 nm and large surface area, these materials have been a promising template for the growth of various materials including polymers, semiconductors, metals, and metal oxides. Extensive efforts have been directed to the formation of metal nanoparticles inside the mesoporous structures via a solution-based infiltration process. Stucky and coworkers reported the preparation of isolated freestanding metal (Au, Ag, Pt) nanowires using SBA-15, followed by removal of the silica frame-work. Chemical vapor infiltration of volatile metal precursors and their subsequent decomposition in the channels of mesoporous silica have been also reported. Recently, a sonochemical method has been utilized for the preparation of SBA-15supported Ru nanoparticles by irradiating the Ru(III) mixture with intense ultrasound. Generally, the confined nanoparticles within the pores of mesoporous silica have shown an improved stability. However, the processes using mesoporous silica have sometimes resulted in poor morphology control and undesired particle formation outside silica surface. In this note, we describe a simple sonication-assisted synthetic route for the production of highly dispersed and uniform PdO nanoparticles within the pore channels of SBA-15 without any surface modification outside SBA-15. Most of PdO nanoparticles were imbedded inside the pores of SBA-15, which was confirmed by the analysis of a series of through-focused transmission electron microscope (TEM) images. In this study, PdO nanoparticles were prepared within the pore channels of mesoporous silica since it has been shown that these have catalytic applications in many reactions as well as sensor applications. It has been also reported that PdO nanostructures are easily converted to Pd, which has extensive catalytic applications, by H2 reduction process at an elevated temperature.

Spontaneous dispersion of PdO and generation of metal Pd cluster in zeolites studied by means of in situ DXAFS

Spontaneous dispersion and clustering processes of Pd were measured by means of energy-dispersive XAFS. The spontaneous dispersion of bulky metal Pd into highly dispersed PdO was directly observed on the H-type zeolite in the atmosphere of O2. Various clustering processes of metal Pd was observed over different kind of zeolite supports. The selective formation of Pd6 and Pd13 clusters were found over H-ZSM-5, H-Mordenite and H-Y supports, respectively. It was concluded that the crystal structure and acid sites of zeolites had significant influences on the dynamic behavior and the genesis of Pd clusters.

Energy-Dispersive XAFS Studies on the Spontaneous Dispersion of PdO and the Formation of Stable Pd Clusters in Zeolites

Spontaneous dispersion and clustering processes of Pd were measured by means of the energy-dispersive EXAFS method. The spontaneous dispersion of bulky metal Pd into highly dispersed PdO was directly observed on the H-type zeolite in the atmosphere of O2. In contrast to H-type zeolites, simple oxidation of the agglomerated Pd was observed on Na-ZSM-5. The structural change of Pd was followed in the atmosphere of hydrogen. The clustering processes of metal Pd depended on the kind of zeolite, and these were categorized into three groups. The first group, i.e., Na-ZSM-5 and H-beta, showed monotonic agglomeration of metal Pd by increasing the reduction temperature. The second group consisted of H-ZSM-5 and H-mordenite where the formation of Pd6 clusters was found. On these zeolites, the generation of Pd6 clusters was reversibly observed upon the repetition of reduction and oxidation treatments. The third group consisted of H-Y and USY zeolites where the formation of Pd13 clusters was observed. From these findings, it was concluded that the crystal structure and acid sites of zeolites had profound influences on the dynamic behavior and the genesis of Pd clusters with various structures.

Spontaneous Dispersion of PdO onto Acid Sites of Zeolites Studied by in situ DXAFS

Abstract The generation of highly dispersed PdO over zeolite supports was studied using in situ energy-dispersive XAFS (DXAFS) technique. From the comparison with the Na-ZSM-5, it was found that the oxidation as well as the spontaneous dispersion of Pd was promoted through the interaction between PdO and acid sites of H-form zeolites.

Remarkable improvement in the activity and stability of Pd/HZSM-5 catalyst for methane combustion

A novel Pd/HZSM-5 catalyst was prepared first by glow discharge plasma treatment followed by calcination thermally. Such prepared catalyst shows a higher activity and an enhanced stability for methane combustion. The XRD characterization and XPS analysis confirm that the plasma preparation leads to a better preparation of PdO active species over the HZSM-5 support. Especially, a plasma-enhanced acidity has been achieved upon the FT-IR analysis. The enhanced acidity plays an important role in stabilizing the dispersed PdO active species on the zeolite support.

Control of the Dispersion of Pd Through the Interaction with Acid Sites of Zeolite Studied by EXAFS

The structural change of Pd induced by acid sites of zeolite was studied by using EXAFS. The anchoring of dispersed PdO on the acid sites of ZSM-5 was evidenced from the reversible structural change of Pd upon the reduction and oxidation treatments. The novel method to regulate the dispersion of metal Pd and PdO through changing the acid concentration of zeolite was proposed.

Metal-Support Interaction Which Controls the Oxidation State, Structure and Catalysis of Pd

A metal-support interaction between Pd and various kinds of zeolite and metal oxide supports was studied, which were active in the catalytic combustion and selective reduction of NO. The acid-base properties of supports affected the oxidation state, structure and catalytic activity of Pd. The acid sites of zeolite played the role to anchor the dispersed PdO as evidenced by the dynamic structural change of Pd. It was found that the metal-support interaction is an important factor, affecting the oxidation state, structure and catalytic performance of Pd.

Infrared studies on SnO 2 and Pd/SnO 2

The peculiarities of the IR studies of adsorption on semiconducting oxides are reviewed with special attention to SnO 2. Absorption by free electrons is very strong in reducing atmospheres and well-defined conditions are required to get vibrational information on the nature of surface species. CO adsorption on SnO 2 at room temperature (r.t.) gives rise to CO–Sn 2+ and CO–Sn 4+ end-on species and to various carbonate entities. NO adsorption leads to the formation of mononitrosyls, nitrite and nitrate species. Pd/SnO 2 catalysts are prepared according to original procedures: photodeposition and grafting of a molecular complex; the grafting method leads to highly dispersed PdO which is reduced to well-dispersed Pd upon CO adsorption at r.t., whereas NO gives rise to nitrosyl species mainly adsorbed on ionic Pd. The specificity of SnO 2 is evidenced upon interaction of a stoichiometric (CO+NO) mixture at r.t.: formation of N 2O on SnO 2 alone, generation of N 2O and isocyanate on PdO/SnO 2. These results are explained by the presence of oxygen vacancies and may be related to the low-temperature catalytic activity of SnO 2-supported palladium in deNO x reactions reported elsewhere.

Regulation of the Dispersion of PdO through the Interaction with Acid Sites of Zeolite Studied by Extended X-ray Absorption Fine Structure

The structure of Pd interacted with acid sites of ZSM-5 was studied by using extended X-ray absorption fine structure (EXAFS). Dispersed PdO was generated on ZSM-5 upon the oxidation treatment, where the degree of dispersion was dependent on the acid amount of ZSM-5 support. The reduction of the dispersed PdO with H2 led to the formation of the agglomerated metal Pd, whose size was larger than ZSM-5 pore. The reoxidation of this metal Pd induced the disruption and the formation of the highly dispersed PdO in ZSM-5 (Si/Al2 = 24) again, which meant the formation of agglomerated metal Pd and dispersed PdO was reversible upon reduction and oxidation treatments. The reversible behavior of Pd was also found on Pd loaded on ZSM-5 with Si/Al2 ratio of 38 and 55, where the slightly aggregated PdO cluster was observed. The fact evidences the possibility to control the size of PdO simply through changing the acid amount of ZSM-5. The dispersion process of Pd from the agglomerated metal form to the dispersed PdO was followed over Pd/ZSM-5 (Si/Al2 = 24). The first step was the simple oxidation of the metal Pd to form agglomerated PdO, which occurred between 573 and 673 K. This was followed by the disruption of the agglomerated PdO into the dispersed PdO in the temperature range between 673 and 773 K. A novel method to regulate the degree of dispersion through the acid−base interaction between acid sites of zeolite and basic metal oxide was proposed.

X-ray Absorption Fine Structure Study of the Formation of the Highly Dispersed PdO over ZSM-5 and the Structural Change of Pd Induced by Adsorption of NO

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) were used to investigate the local structure of Pd supported on ZSM-5, which was affected by the acid sites of support and the adsorption of nitrogen oxide. After the thermal treatment of the initially ion-exchanged Pd amine complex, the formation of metal Pd particles was found, whose size was larger than zeolite pore. The subsequent oxidation led to the disruption of Pd particles and the formation of the dispersed PdO, where the degree of dispersion was dependent on the acid amount of ZSM-5. It was also found on Pd/HZSM-5 (Si/Al2 = 24) that the formation of metal Pd and dispersed PdO was reversible upon reduction and oxidation treatments. These facts prove the presence of strong interaction between acid sites and PdO. The role of acid sites of zeolite was considered to keep the dispersed state of PdO. The adsorption of NO on highly dispersed PdO induced a significant change in the local structure of Pd at r...

Studies on the Formation and Structure of Highly Dispersed PdO Interacted with Brønsted Acid Sites on Zeolite by EXAFS

Abstract The degree of the dispersion of PdO was highly dependent on the Al content of HZSM-5. This is due to the strong interaction between Brønsted acid sites and PdO as demonstrated by the reversible transformation from aggregated metal Pd to highly dispersed PdO upon the successive reduction and oxidation treatments. The role of Brønsted acid sites was considered to keep the dispersed state of PdO.

Methane combustion over the α-alumina supported Pd catalyst: Activity of the mixed Pd/PdO state

The effect of variations in temperature and oxygen partial pressure on the methane oxidation activity of α-alumina supported Pd catalysts was studied experimentally. It was found that after pretreatment at temperatures above 800°C, conditions where the metallic state of the catalyst is stable, “negative activation” occurred on cooling cycle leading to increase in conversion with decreasing temperature. The pretreatment at temperatures exceeding 800°C was found to be a necessary condition in order for this effect to take place. This phenomenon is attributed to appearance of highly dispersed PdO clusters in thermodynamic equilibrium with the previously formed metallic Pd surface. The contradictory results on the activity of Pd/PdO phase of the catalyst towards methane oxidation reported in the literature are explained by the proposed hypothesis.

Characterization study of CeO2 supported Pd catalyst for low-temperature carbon monoxide oxidation

Catalysts consisting of palladium supported on cerium dioxide (Pd/CeO2) were prepared and used for carbon monoxide oxidation in a stoichiometric mixture of carbon monoxide and oxygen. Pd/CeO2 exhibits high catalytic activity for the oxidation of CO, showing markedly enhanced catalytic activities due to the combined effect of palladium and cerium dioxide. The Pd/CeO2 catalyst is superior to Pd/ZrO2, Pd/Al2O3, Pd/TiO2, Pd/ZSM-5 and Pd/SiO2 catalysts with regard to the activity under the conditions examined. The catalysts were characterized by means of XRD and TPR. The position of the H2-TPR peak shifts to lower temperature with increasing Pd loading from 0.25 to 2.0%. CeO2 inhibits the hydrogen reduction of PdO. CO-TPR measurements have shown the existence of three peaks. The low-temperature peak (α) is due to the Pd hydroxide species. The β peak has been attributed to finely dispersed PdO. The high-temperature peak (γ) has been attributed to crystal phase PdO. Crystal phase PdO is more difficult to reduce by CO than finely dispersed PdO. On the basis of the catalytic activity and CO-TPR results, we conclude α species (Pd hydroxide) mainly contribute to the catalytic activity for low-temperature CO oxidation.

Evolution of Pd/SiO2 catalysts prepared from chlorine-free precursors

D. Łomot, W. Juszczyk, Z. Karpiński and F. Bozon-Verduraz, J. Chem. Soc., Faraday Trans., 1997, 93, 2015 DOI: 10.1039/A608616K