Over the past several years, cerium oxide and CeO2-containing materials have come under intense scrutiny as catalysts and as structural and electronic promoters of heterogeneous catalytic reactions. Recent developments regarding the characterization of ceria and CeO2-containing catalysts are critically reviewed with a special focus towards catalyst interaction with small molecules such as hydrogen, carbon monoxide, oxygen, and nitric oxide. Relevant catalytic and technological applications such as the use of ceria in automotive exhaust emission control and in the formulation of SO x reduction catalysts is described. A survey of the use of CeO2-containing materials as oxidation and reduction catalysts is also presented.

Catalytic Properties of Ceria and CeO2-Containing Materials

Interest in catalysis by metal nanoparticles (NPs) is increasing dramatically, as reflected by the large number of publications in the last five years. This field, "semi-heterogeneous catalysis", is at the frontier between homogeneous and heterogeneous catalysis, and progress has been made in the efficiency and selectivity of reactions and recovery and recyclability of the catalytic materials. Usually NP catalysts are prepared from a metal salt, a reducing agent, and a stabilizer and are supported on an oxide, charcoal, or a zeolite. Besides the polymers and oxides that used to be employed as standard, innovative stabilizers, media, and supports have appeared, such as dendrimers, specific ligands, ionic liquids, surfactants, membranes, carbon nanotubes, and a variety of oxides. Ligand-free procedures have provided remarkable results with extremely low metal loading. The Review presents the recent developments and the use of NP catalysis in organic synthesis, for example, in hydrogenation and C--C coupling reactions, and the heterogeneous oxidation of CO on gold NPs.

Nanoparticles as Recyclable Catalysts: The Frontier between Homogeneous and Heterogeneous Catalysis

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Advances in the development of novel cobalt Fischer-Tropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels.

Cerium dioxide (CeO2, ceria) is becoming an ubiquitous constituent in catalytic systems for a variety of applications. 2016 sees the 40th anniversary since ceria was first employed by Ford Motor Company as an oxygen storage component in car converters, to become in the years since its inception an irreplaceable component in three-way catalysts (TWCs). Apart from this well-established use, ceria is looming as a catalyst component for a wide range of catalytic applications. For some of these, such as fuel cells, CeO2-based materials have almost reached the market stage, while for some other catalytic reactions, such as reforming processes, photocatalysis, water-gas shift reaction, thermochemical water splitting, and organic reactions, ceria is emerging as a unique material, holding great promise for future market breakthroughs. While much knowledge about the fundamental characteristics of CeO2-based materials has already been acquired, new characterization techniques and powerful theoretical methods are dee...

Fundamentals and Catalytic Applications of CeO2-Based Materials

The effect of cerium introduction on vanadium-USY catalysts

The influence of the precursors on the promoting effect of ceria on Pd/Al2O3 catalyst, when ceria is coated over alumina was studied. The reaction of propane oxidation proceeded under different feed conditions and the surface active sites were characterized by X-ray photoelectron spectroscopy (XPS) and in situ diffuse reflectance spectroscopy (DRS). XPS and DRS results show that PdO/Pd 0 interface are the active sites independent of the precursor, while the catalysts containing CeO2 showed formation of palladium species in the highest oxidation state, probably PdO2 (338 eV) after the oxidation of propane. Besides, the O/Al and O/Ce ratios evidenced the increase of oxygen storage in the presence of CeO2. In addition, the precursor acetylacetonate favors the oxygen storage in the lattice.

https://www.scielo.br/pdf/aabc/v76n4/a14v76n4.pdf

The effect of precursors salts on surface state of Pd/Al2O3 and Pd/CeO2/Al2O3 catalysts

Co/Nb2O2, Co-Ru/Nb2O5 and Co-Re/Nb2O5 catalysts were prepared and characterized by TPR. The catalysts were investigated on CO hydrogenation under 0.4, 2 and 4Mpa. The total pressure has an important effect on the product distribution. The selectivity toward diesel and C5+ hydrocarbons achieves a maximum at reaction pressure of 2.0Mpa. The catalysts had good stability over 50h on stream at reaction conditions.

The effect of pressure on promoted RU and Re-Co/Niobia catalysts in the Fischer-Tropsch synthesis

Renewable ethylene can be obtained by dehydration of bio‐ethanol and used for production of vinyl acetate (VAM) through reaction with acetic acid (AcOH), using Pd−Cu catalysts. In the present manuscript, structural characterizations of Pd−Cu/ZrO2 catalysts show that these systems present cubic structure with different spatial distributions. Particularly, it is shown that combustion of ethylene and acetic acid can be inhibited below 180 °C, maximizing the rates of VAM formation, when the catalysts are modified with Ti+4. The effects of AcOH concentration on rates of VAM formation show that higher AcOH concentrations favor the formation of undesired byproducts, while lower AcOH concentrations favor effects related to O2 mobility, which can lead to surface decomposition. VAM formation is favored, with selectivities ranging from 0.8 to 1.0. XPS results indicate the existence of metallic Pd, CuO species and Zr species, in agreement with IR results. DRIFTS results also show that different Pd‐acetate intermediates can be present, depending on the electronic effects associated to Pd−Cu and Zr species.

Nanostructured Pd−Cu Catalysts Supported on Zr−Al and Zr−Ti for Synthesis of Vinyl Acetate

Pd/Nb 2 O 5 /Al 2 O 3 catalysts were prepared with several niobia loadings. XPS was used for determination of the monolayer coverage formation on Pd/Nb 2 O 5 /Al 2 O 3 samples. The spectra suggest that monolayer coverage occurred between 10 and 20 wt% of Nb 2 O 5 . Temperature-programmed sur face reaction (TPSR) measurements showed three distinct reactions for the oxidation of propane on palladium surface: complete oxidation, steam reforming and propane hydrogenolysis. Niobia addition inhibited the last two reactions and this was ascribed to the ensemble effect produced by NbO x reduced species on palladium. Even the propane oxidation has begun later on Pd/Nb 2 O 5 / Al 2 O 3 catalysts, but this was related to the Pd 0 surface instead of Pd 2+ observed on Pd/Al 2 O 3 . Infrared spectroscopy of adsorbed carbon monoxide was used to investigate the palladium surfaces before and after propane reaction. Immediately after reduction by H 2 , the spectra showed the linear and bridge adsorbed CO species on the Pd 0 surface. After reaction, one could observe a new band at 2130 cm -1 related to CO adsorption on Pd 2+ species. This oxidized site was more intense on Pd/Al 2 O 3 . It occurred with lower intensity on Pd/5%Nb 2 O 5 /Al 2 O 3 and could not be observed on Pd/20%Nb 2 O 5 /Al 2 O 3 catalyst. Carbon monoxide species adsorbed on Pd 0 were also observed in all samples after reaction. Our results suggest that niobia addition promoted different surface ratios of Pd 0 /PdO during the propane oxidation. Thus, this result could explain the deerease in the combustion reaction rate as opposed to morphological, poisoning or support effects.

Martin Schmal

Papers

The effect of cerium introduction on vanadium-USY catalysts

The effect of precursors salts on surface state of Pd/Al2O3 and Pd/CeO2/Al2O3 catalysts

The effect of pressure on promoted RU and Re-Co/Niobia catalysts in the Fischer-Tropsch synthesis

Nanostructured Pd−Cu Catalysts Supported on Zr−Al and Zr−Ti for Synthesis of Vinyl Acetate

Redox Properties of Pd/NbOx/Al2O3 Catalysts