Controlling Metal-Oxide Reducibility for Efficient C-H Bond Activation in Hydrocarbons

Knowing the structure of catalytically active species/phases and providing methods for their purposeful generation are two prerequisites for the design of catalysts with desired performance. Herein, we introduce a simple method for precise preparation of supported/bulk catalysts. It utilizes the ability of metal oxides to dissolve and to simultaneously precipitate during their treatment in an aqueous ammonia solution. Applying this method for a conventional VOx -Al2 O3 catalyst, the concentration of coordinatively unsaturated Al sites was tuned simply by changing the pH value of the solution. These sites affect the strength of V-O-Al bonds of isolated VOx species and thus the reducibility of the latter. This method is also applicable for controlling the reducibility of bulk catalysts as demonstrated for a CeO2 -ZrO2 -Al2 O3 system. The application potential of the developed catalysts was confirmed in the oxidative dehydrogenation of ethylbenzene to styrene with CO2 and in the non-oxidative propane dehydrogenation to propene. Our approach is extendable to the preparation of any metal oxide catalysts dissolvable in an ammonia solution.

Decoupling the Chemical and Mechanical Strain Effect on Steering the CO2 Activation over CeO2-Based Oxides: An Experimental and DFT Approach

Doped ceria-based metal oxides are widely used as supports and stand-alone catalysts in reactions where CO2 is involved. Thus, it is important to understand how to tailor their CO2 adsorption behavior. In this work, steering the CO2 activation behavior of Ce-La-Cu-O ternary oxide surfaces through the combined effect of chemical and mechanical strain was thoroughly examined using both experimental and ab initio modeling approaches. Doping with aliovalent metal cations (La3+ or La3+/Cu2+) and post-synthetic ball milling were considered as the origin of the chemical and mechanical strain of CeO2, respectively. Experimentally, microwave-assisted reflux-prepared Ce-La-Cu-O ternary oxides were imposed into mechanical forces to tune the structure, redox ability, defects, and CO2 surface adsorption properties; the latter were used as key descriptors. The purpose was to decouple the combined effect of the chemical strain (εC) and mechanical strain (εM) on the modification of the Ce-La-Cu-O surface reactivity toward CO2 activation. During the ab initio calculations, the stability (energy of formation, EOvf) of different configurations of oxygen vacant sites (Ov) was assessed under biaxial tensile strain (ε > 0) and compressive strain (ε < 0), whereas the CO2-philicity of the surface was assessed at different levels of the imposed mechanical strain. The EOvf values were found to decrease with increasing tensile strain. The Ce-La-Cu-O(111) surface exhibited the lowest EOvf values for the single subsurface sites, implying that Ov may occur spontaneously upon Cu addition. The mobility of the surface and bulk oxygen anions in the lattice contributing to the Ov population was measured using 16O/18O transient isothermal isotopic exchange experiments; the maximum in the dynamic rate of 16O18O formation, Rmax(16O18O), was 13.1 and 8.5 μmol g-1 s-1 for pristine (chemically strained) and dry ball-milled (chemically and mechanically strained) oxides, respectively. The CO2 activation pathway (redox vs associative) was experimentally probed using in situ diffuse reflectance infrared Fourier transform spectroscopy. It was demonstrated that the mechanical strain increased up to 6 times the CO2 adsorption sites, though reducing their thermal stability. This result supports the mechanical actuation of the "carbonate"-bound species; the latter was in agreement with the density functional theory (DFT)-calculated C-O bond lengths and O-C-O angles. Ab initio studies shed light on the CO2 adsorption energy (Eads), suggesting a covalent bonding which is enhanced in the presence of doping and under tensile strain. Bader charge analysis probed the adsorbate/surface charge distribution and illustrated that CO2 interacts with the dual sites (acidic and basic ones) on the surface, leading to the formation of bidentate carbonate species. Density of states (DOS) studies revealed a significant Eg drop in the presence of double Ov and compressive strain, a finding with design implications in covalent type of interactions. To bridge this study with industrially important catalytic applications, Ni-supported catalysts were prepared using pristine and ball-milled oxides and evaluated for the dry reforming of methane reaction. Ball milling was found to induce modification of the metal-support interface and Ni catalyst reducibility, thus leading to an increase in the CH4 and CO2 conversions. This study opens new possibilities to manipulate the CO2 activation for a portfolio of heterogeneous reactions.

Solvothermal synthesis of CeO2-ZrO2-M2O3 (M = La, Y, Bi) mixed oxide and their soot oxidation activity

CeO₂–ZrO₂-M₂O₃ (M = La, Y, Bi) mixed oxide has been prepared by a solvothermal synthesis method. The physico–chemical properties of the mixed oxide have been studied by X-ray powder diffraction (XRD), Raman spectroscopy, BET, X-ray photoelectron spectroscopy (XPS), TEM and temperature-programmed reduction (TPR), and the catalytic activity for soot oxidation has been studied by thermogravimetry (TG). La³⁺, Y³⁺ and Bi³⁺ exhibit positive effects on lowering the oxidation temperature of the soot. The XRD and Raman results showed formation of mixed oxides and TEM images suggested the nanosized nature of the particles. The benefit of yttrium or lanthana doping on the catalytic activity of ceria can be related to active oxygen formation provoked by the defective structure of ceria due to the presence of La³⁺ and Y³⁺. The benefit of Bi³⁺ doping on catalytic activity can be related to the reduction at low temperature both with Bi₂O₃ and ceria.

Nano CeO₂-based oxides with meso-pores have been synthesized by a solvothermal method.

Efficient photodegradation of azucryl red by copper-doped TiO2 nanoparticles-experimental and modeling studies

This research aims to investigate the effect of copper doping on the photocatalysis performance of TiO₂ nanoparticles for disposal wastewater from organic pollutants. X-ray diffraction analysis manifests the crystallization of a rutile phase for pure and copper-doped TiO₂ except for 2% resulting in a rutile-to-anatase phase transformation. The crystallite size is found less affected by Cu doping, i.e., ~30 nm. BET analysis indicates a decrease in the specific surface area as the doping loading increases. Scanning electron microscopy observations reveal spherical particles at the nanometer range for pure TiO₂ and then larger agglomerates of ultrafine particles with Cu doping. FTIR analysis notifies the existence of hydroxyl groups, which will promote the photocatalysis process. The photodegradation of azucryl red (AR) has been investigated under different conditions; i.e., Cu-loading, initial concentration of AR, and pH. The kinetics of the photodegradation process is further found to comply with the Lagergren kinetic law, regardless the experimental conditions. Nevertheless, the photodegradation process is not only controlled by the intra-particle diffusion mechanism, but also by mass transfer through a liquid film boundary. The maximum degradation of AR, i.e., 86%, has been achieved at pH = 5.0 during 60 min of contact time for the 2% Cu doping, with effective regeneration. The Freundlich model exhibits a better fitting for AR dye photodegradation equilibrium data, compared to Langmuir, Temkin, and Dubinin-Radushkevich.

Design Aspects of Doped CeO2 for Low-Temperature Catalytic CO Oxidation: Transient Kinetics and DFT Approach

CO elimination through oxidation over highly active and cost-effective catalysts is a way forward for many processes of industrial and environmental importance. In this study, doped CeO₂ with transition metals (TM = Cu, Co, Mn, Fe, Ni, Zr, and Zn) at a level of 20 at. % was tested for CO oxidation. The oxides were prepared using microwave-assisted sol-gel synthesis to improve catalyst's performance for the reaction of interest. The effect of heteroatoms on the physicochemical properties (structure, morphology, porosity, and reducibility) of the binary oxides M-Ce-O was meticulously investigated and correlated to their CO oxidation activity. It was found that the catalytic activity (per gram basis or TOF, s^-1) follows the order Cu-Ce-O > Ce-Co-O > Ni-Ce-O > Mn-Ce-O > Fe-Ce-O > Ce-Zn-O > CeO₂. Participation of mobile lattice oxygen species in the CO/O₂ reaction does occur, the extent of which is heteroatom-dependent. For that, state-of-the-art transient isotopic ¹⁸O-labeled experiments involving ¹⁶O/¹⁸O exchange followed by step-gas CO/Ar or CO/O₂/Ar switches were used to quantify the contribution of lattice oxygen to the reaction. SSITKA-DRIFTS studies probed the formation of carbonates while validating the Mars-van Krevelen (MvK) mechanism. Scanning transmission electron microscopy-high-angle annular dark field imaging coupled with energy-dispersive spectroscopy proved that the elemental composition of dopants in the individual nanoparticle of ceria is less than their composition at a larger scale, allowing the assessment of the doping efficacy. Despite the similar structural features of the catalysts, a clear difference in the O_lattice mobility was also found as well as its participation (as expressed with the α descriptor) in the reaction, following the order α_Cu > α_Co> α_Mn > α_Zn. Kinetic studies showed that it is rather the pre-exponential (entropic) factor and not the lowering of activation energy that justifies the order of activity of the solids. DFT calculations showed that the adsorption of CO on the Cu-doped CeO₂ surface is more favorable (-16.63 eV), followed by Co, Mn, Zn (-14.46, -4.90, and -4.24 eV, respectively), and pure CeO₂ (-0.63 eV). Also, copper compensates almost three times more charge (0.37e^-) compared to Co and Mn, ca. 0.13e^- and 0.10e^-, respectively, corroborating for its tendency to be reduced. Surface analysis (X-ray photoelectron spectroscopy), apart from the oxidation state of the elements, revealed a heteroatom-ceria surface interaction (O_a species) of different extents and of different populations of O_a species.

The role of precursor decomposition in the formation of samarium doped ceria nanoparticles via solid-state microwave synthesis

The impact on the final morphology of ceria (CeO2) nanoparticles made from different precursors (commercial: cerium acetate/nitrate) and in house: cerium tri(methylsilyl)amide (Ce-TMSA)) via a microwave solid state reaction has been determined. In all instances, powder X-ray diffraction indicated that the cubic fluorite CeO2 phase (PDF# 04–004-9150, with the space group Fm-3 m) had formed. Scanning electron microscopy (SEM) images revealed spherical nanoparticles were produced from the Ce-TMSA precursor. The commercial acetate and nitrate precursors produced particles with irregular morphology. The roles of the precursor decomposition and binding energy in the synthesis of the nanocrystals with various morphologies, as well as a possible growth mechanism, were evaluated based on experimental and computational data. The formation of spherical shaped nanoparticles was determined to be due to the preferential single-step decomposition of the Ce-TMSA as well as the low activation energy to overcome decomposition. Due to the complicated decomposition of the commercial precursors and high activation energy the resulting particles adopted an irregular morphology. Highly uniform samarium doped ceria (SmxCe1-xO2-δ) nanospheres were also synthesized from Ce-TMSA and samarium tri(methylsilyl)amide (Sm-TMSA). The effects of reaction time and temperature, on the final morphology were observed through SEM. The rapid single-step decomposition of TMSA-based precursors as observed through thermogravimetric analysis (TGA) and confirmed through the calculation of potential energy surfaces and binding energies from density functional theory (DFT) calculations, indicated that nanoparticle formation follows LaMer’s classical nucleation theory.

Enhancement of CO2 Reforming of CH4 Reaction Using Ni,Pd,Pt/Mg1−xCex4+O and Ni/Mg1−xCex4+O Catalysts

Catalysts Ni/Mg1−xCex4+O and Ni,Pd,Pt/Mg1−xCex4+O were developed using the co-precipitation–impregnation methods. Catalyst characterization took place using XRD, H2-TPR, XRF, XPS, Brunauer–Emmett–Teller (BET), TGA TEM, and FE-SEM. Testing the catalysts for the dry reforming of CH4 took place at temperatures of 700–900 °C. Findings from this study revealed a higher CH4 and CO2 conversion using the tri-metallic Ni,Pd,Pt/Mg1−xCex4+O catalyst in comparison with Ni monometallic systems in the whole temperature ranges. The catalyst Ni,Pd,Pt/Mg0.85Ce4+0.15O also reported an elevated activity level (CH4; 78%, and CO2; 90%) and an outstanding stability. Carbon deposition on spent catalysts was analyzed using TEM and Temperature programmed oxidation-mass spectroscopy (TPO-MS) following 200 h under an oxygen stream. The TEM and TPO-MS analysis results indicated a better anti-coking activity of the reduced catalyst along with a minimal concentration of platinum and palladium metals.

Temperature reversible synergistic formation of cerium oxyhydride and Au hydride: a combined XAS and XPDF study

In situ studies on the physical and chemical properties of Au in inverse ceria alumina supported catalysts have been conducted between 295 and 623 K using high energy resolved fluorescence detection X-ray absorption near edge spectroscopy and X-ray total scattering. Precise structural information is extracted on the metallic Au phase present in a 0.85 wt% Au containing inverse ceria alumina catalyst (ceria/Au/alumina). Herein evidence for the formation of an Au hydride species at elevated temperature is presented. Through modelling of total scattering data to extract the thermal properties of Au using Grüneisen theory of volumetric thermal expansion it proposed that the Au Hydride formation occurs synergistally with the formation of a cerium oxyhydride. The temperature reversible nature, whilst remaining in a reducing atmosphere, demonstrates the activation of hydrogen without consumption of oxygen from the supporting ceria lattice.

Mechanism of Synergistic Effect on Electron Transfer over Co-Ce/MCM-48 during Ozonation of Pharmaceuticals in Water

The same amount of metal was deposited on the surface of three-dimensional mesoporous MCM-48 by a facile impregnation-calcination method for catalytic ozonation of pharmaceutical and personal-care products in the liquid phase. At 120 min reaction time, Co/MCM-48 and Ce/MCM-48 showed 46.6 and 63.8% mineralization for clofibric acid (CA) degradation, respectively. Less than 33% mineralization was achieved with Co/MCM-48 and Ce/MCM-48 during sulfamethazine (SMZ) ozonation. In the presence of monometallic oxides modified MCM-48 catalysts, total organic carbon (TOC) removal of diclofenac sodium (DCF) was around 80%. The composite Co-Ce/MCM-48 catalyst exhibited significantly higher activity in terms of TOC removal of CA (83.6%), SMZ (51.7%) and DCF (86.8%). Co-Ce/MCM-48 inhibited efficiently the accumulation of small molecular carboxyl acids during ozonation. A detailed research was conducted to detect the nature of material structure and mechanism of catalytic ozonation by using a series of characterizations. The main reaction pathway of CA was determined by the analysis of liquid chromatography-mass spectrometry, in line with the results of frontier electron density calculations that reactive oxygen species (ROSs) were easy to attack negative regions of pharmaceuticals. The Si-O-Si, Co···HO-Si-O-Si-OH···Ce, and O₃···Co-HO-Si-O-Si-OH···Ce-OH···O₃ basic units in catalysts were constructed to detect the orbit-energy-level difference. The results revealed that a synergistic effect existed at the interface between cobalt and cerium oxides over MCM-48, which facilitated the ROSs sequence in solution with ozone. Therefore, the multivalence redox coupling of Ce⁴⁺/Ce³⁺ and Co³⁺/Co²⁺ along with electron transfer played an important role in catalytic ozonation process.

Enhanced plasma-catalytic decomposition of toluene over Co–Ce binary metal oxide catalysts with high energy efficiency

In-plasma catalysis has been considered as a promising technology to degrade volatile organic compounds. Heterogeneous catalysts, especially binary metal oxide catalysts, play an important role in further advancing the catalytic performance of in-plasma catalysis. This work investigates the toluene decomposition performance over Co–Ce binary metal oxide catalysts within the in-plasma catalysis. Co–Ce catalysts with different Co/Ce molar ratios are synthesized by a citric acid method. Results show that the catalytic activity of Co–Ce catalysts is obviously superior to those of monometallic counterparts. Especially, Co_0.75Ce_0.25O_x catalyst simultaneously realizes highly efficient toluene conversion (with a decomposition efficiency of 98.5% and a carbon balance of 97.8%) and a large energy efficiency of 7.12 g kW h⁻¹, among the best performance in the state-of-art literature (0.42 to 6.11 g kW h⁻¹). The superior catalytic performance is further interpreted by the synergistic effect between Co and Ce species and the significant plasma–catalyst interaction. Specifically, the synergistic effect can decrease the catalyst crystallite size, enlarge the specific surface area and improve the amount of oxygen vacancies/mobility, providing more active sites for the adsorption of surface active oxygen species. Meanwhile, the plasma–catalyst interaction is able to generate the surface discharge and reinforce the electric field strength, thereby accelerating the plasma-catalytic reactions. In the end, the plasma-catalytic reaction mechanism and pathways of toluene conversion are demonstrated.

This work demonstrates highly efficient plasma-catalytic decomposition of toluene over Co–Ce binary metal oxide catalysts with superior energy efficiency.

1,2-Dichloroethane Deep Oxidation over Bifunctional Ru/Ce x Al y Catalysts

Ru/Ce _x Al _y catalysts were synthesized with impregnation of RuCl₃ aqueous solution on Ce _x Al _y (Al₂O₃-CeO₂) and used in 1,2-dichloroethane (1,2-DCE) oxidation. Characterization by X-ray diffraction, Raman, NH₃-temperature-programmed desorption (TPD), CO₂-TPD, X-ray photoelectron spectroscopy, and H₂-temperature-programmed reduction indicates that CeO₂ exists as a form of face-centered cubic fluorite structure, whereas the chemical states and the structure of Ru species are dependent on the Ce content. The reducibility and acidity of catalysts increase with Ce/Ce + Al ratio. However, the latter is promoted only in a Ce/Ce + Al range of 0-0.25 and then decreases quickly. Ru/Ce _x Al _y catalysts have considerable activity for 1,2-DCE combustion. TOF_Ru of 1,2-DCE oxidation increases with strong acid, which is ascribed to a synergy of reducibility and acidity. Ru greatly inhibits the chlorination through the decreases in both Cl deposition and CH₂=CHCl formation. High stability of Ru/Ce₁₀Al₉₀ maintains at 280 °C for at least 25 h with CO₂ selectivity of 99% or higher. In situ Fourier transform infrared spectroscopy indicates that 1,2-DCE dissociates to form ClCH₂-CH₂O- species, which is an intermediate species for the production of CH₃CHO and CH₂=CHCl, the former responsible for deep oxidation.

Construction of Cu–Ce/graphene catalysts via a one-step hydrothermal method and their excellent CO catalytic oxidation performance

Cu–Ce/graphene catalysts show high dispersion of metal particles and excellent activity and stability for catalytic oxidation. In this study, a hydrothermal method was used to synthesize a series of bimetallic Cu–Ce/graphene catalysts, and the effects of the proportions of Cu and Ce on CO oxidation were investigated in detail. Indispensable characterizations such as XPS, XRD, TEM, BET, and H₂-TPR were conducted to explore the effect of the Cu/Ce molar ratio and the metal valence on the activity and determine the structure–performance relationship. The results showed that bimetallic supported catalysts, such as 3Cu5Ce/graphene, 1Cu1Ce/graphene, and 5Cu3Ce/graphene, possessed significant catalytic activity. Especially, the 5Cu3Ce/graphene catalyst showed highest catalytic activity for CO oxidation, the T₁₀₀ value was 132 °C, and the apparent activation energy was 68.03 kJ mol⁻¹. Furthermore, the stability of the 5Cu3Ce/graphene catalyst was outstanding, which could be maintained for at least 12 h. Moreover, the CeO₂ particles were well crystalline with the size 5–9 nm in these catalysts, and the CuO nanoparticles were well dispersed on CeO₂ and graphene. Notably, the ratio of Cu/Ce in the catalyst was higher, the interaction between the Ce species and the graphene was stronger, and the Cu species were more easily reduced; this was beneficial for the oxidation of CO.

Cu–Ce/graphene catalysts show high dispersion of metal particles and excellent activity and stability for catalytic oxidation.

Preparation and characterization of mesostructured cellular foam silica supported Cu–Ce mixed oxide catalysts for CO oxidation

A series of mesostructured cellular foam (MCF) silica supported CuO–CeO₂ catalysts with various total metal loadings (10–40 wt%) and various Cu/Ce ratios (Cu/Ce = 1/9, 2/8, and 3/7 wt/wt) were prepared and tested for CO oxidation.

Investigation on the structure–activity relationship of BaO promoting Pd/CeO2–ZrO2 catalysts for CO, HC and NOx conversions

The reasons for the variation in catalytic activity of BaO promoting Pd/CeO₂–ZrO₂ catalysts prepared via different methods are revealed.

Supported Pd nanoclusters with enhanced hydrogen spillover for NOx removal via H2-SCR: the elimination of “volcano-type” behaviour

The synthesis of a highly dispersed Pd catalyst that efficiently eliminates the “volcano-type” behavior of NO_x conversion in H₂-SCR with excess O₂.

Ceria/POLYMER hybrid nanoparticles as efficient catalysts for the hydration of nitriles to amides

We report the synthesis of ceria/polymer hybrid nanoparticles and their use as effective supported catalysts for the hydration of nitriles to amide, exemplified with the conversion of 2-cyanopiridine to 2-picolinamide. The polymeric cores, made of either polystyrene (PS) or poly(methyl methacrylate) (PMMA), are prepared by miniemulsion copolymerization in the presence of different functional comonomers that provide carboxylic or phosphate groups: acrylic acid, maleic acid, and ethylene glycol methacrylate phosphate. The functional groups of the comonomers generate a corona around the main polymer particle and serve as nucleating agents for the in situ crystallization of cerium(IV) oxide. The obtained hybrid nanoparticles can be easily redispersed in water or ethanol. The conversion of amides to nitriles was quantitative for most of the catalytic samples, with yields close to 100%. According to our experimental observations by high-performance liquid chromatography (HPLC), no work up is needed to separate the product from unreacted substrate. The substrate remains absorbed on the catalyst surface, whereas the product can be easily separated. The catalysts are shown to be recyclable and can be reused for a large number of cycles without loss in efficiency.

New insights into the structure of a CeO2–ZrO2–Al2O3 composite and its influence on the performance of the supported Pd-only three-way catalyst

CZA900 displays the most homogeneous structure, and consequently superior redox and catalytic performances are obtained for Pd/CZA900.

Low-temperature CO oxidation on CuO/CeO2catalysts: the significant effect of copper precursor and calcination temperature

The performance of CuO/CeO₂catalysts for CO oxidation strongly depends on the type of copper precursor and the calcination temperature.

Influence of Ce2O3 and CeO2 promoters on Pd/MgO catalysts in the dry-reforming of methane

In this study, the conversion of methane and CO₂ to synthesis gas using dry reforming over Pd/MgO catalysts using different concentrations of Ce³⁺ and Ce⁴⁺ was investigated.

Thermal and sonochemical synthesis of porous (Ce,Zr)O2 mixed oxides from metal β-diketonate precursors and their catalytic activity in wet air oxidation process of formic acid

Porous (Ce0.5Zr0.5)O2 solid solutions were prepared by thermolysis (T=285 °C) or sonolysis (20 kHz, I=32 W cm(-2), Pac=0.46 W mL(-1), T=200 °C) of Ce(III) and Zr(IV) acetylacetonates in oleylamine or hexadecylamine under argon followed by heat treatment of the precipitates obtained in air at 450 °C. Transmission Electron Microscopy images of the samples show nanoparticles of ca. 4-6 nm for the two synthetic approaches. The powder X-ray diffraction, scanning electron microscopy, energy dispersive X-ray and μ-Raman spectroscopy of solids obtained after heat treatment indicate the formation of (Ce0.5Zr0.5)O2 solid solutions with a metastable tetragonal crystal structure for the two synthetic routes. The specific surface area of the samples varies between 78 and 149 m(2) g(-1) depending on synthesis conditions. The use of Barrett-Joyner-Halenda and t-plot methods reveal the formation of mixed oxides with a hybrid morphology that combines mesoporosity and microporosity regardless of the method of preparation. Platinum nanoparticles were deposited on the surface of the mixed oxides by sonochemical reduction of Pt(IV). It was found that the materials prepared by sonochemistry exhibit better resistance to dissolution during the deposition process of platinum. X-ray photoelectron spectroscopy analysis shows the presence of Pt(0) and Pt(II) on the surface of mixed oxides. Porous (Ce0.5Zr0.5)O2 mixed oxides loaded with 1.5%wt. platinum exhibit high activity in catalytic wet air oxidation of formic acid at 40 °C.

Structural characterization of alumina-supported Rh catalysts: effects of ceriation and zirconiation by using metal-organic precursors

The effects of the addition of ceria and zirconia on the structural properties of supported rhodium catalysts (1.6 and 4 wt % Rh/γ-Al₂O₃) are studied. Ceria and zirconia are deposited by using two preparation methods. Method I involves the deposition of ceria on γ-Al₂O₃ from Ce(acac)₃, and the rhodium metal is subsequently added, whereas method II is based on a controlled surface reaction technique, that is, the decomposition of metal–organic M(acac)_x (in which M=Ce, x=3 and M=Zr, x=4) on Rh/γ-Al₂O₃. The structures of the prepared catalyst materials are characterized ex situ by using N₂ physisorption, transmission electron microscopy, high-angle annular dark-field scanning transmission election microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure spectroscopy (XAFS). All supported rhodium systems readily oxidize in air at room temperature. By using ceriated and zirconiated precursors, a larger rhodium-based metallic core fraction is obtained in comparison to the undoped rhodium catalysts, suggesting that ceria and zirconia protect the rhodium particles against extensive oxidation. XPS results indicate that after the calcination and reduction treatments, a small amount of chlorine is retained on the support of all rhodium catalysts. EXAFS analysis shows significant Rh—Cl interactions for Rh/Al₂O₃ and Rh/CeO_x/Al₂O₃ (method I) catalysts. After reaction with H₂/He in situ, for series of samples with 1.6 wt % Rh, the EXAFS first shell analysis affords a mean size of approximately 30 atoms. A broader spread is evident with a 4 wt % rhodium loading (ca. 30–110 atoms), with the incorporation of zirconium providing the largest particle sizes.