Rational Design of Transition Metal Co‐Doped Ceria Catalysts for Low‐Temperature CO Oxidation

Hydrothermal synthesis and characterization of transition metal (Mn/Fe/Cu) co-doped cerium oxide-based nano-additives for potential use in the reduction of exhaust emission from spark ignition engines

The goal of this work was to synthesize new cerium oxide-based nano-additives to minimise emissions from spark ignition (SI) engines fueled with gasoline blends, such as carbon monoxide (CO), unburned hydrocarbons (HC) and oxides of nitrogen (NO_x). To investigate the effect of transition metal dopants on their respective catalytic oxidation activity, nano-sized CeO₂ catalysts co-doped with Mn, Fe, Cu and Ag ions were successfully produced by a simple hydrothermal technique. The synthesis of nano-catalysts with cubic fluorite geometry was confirmed by XRD data. The addition of transition metal ions to the CeO₂ lattice increased the concentration of structural defects like oxygen vacancies and Ce³⁺ ions, which are advantageous for the catalytic oxidation reaction, as also supported by XAFS and RAMAN analysis. Further, nano-gasoline fuel emission parameters are measured and compared to straight gasoline fuel. The results demonstrated that harmful exhaust pollutants such as CO, HC and NO_x were significantly reduced. The high surface area, better redox characteristics and presence of additional oxygen vacancy sites or Ce³⁺ ions have been linked to the improved catalytic performance of the synthesized catalyst.

Illustrating the synthesis of doped and undoped CeO₂ nanomaterial and its potential application as a promising catalyst for additives to minimize emissions from spark ignition (SI) engines fueled with gasoline blends.

Effect of Ag doping on Pd/Ag-CeO2 catalysts for CO and C3H6 oxidation

To achieve high fuel efficiency and low emission in automobiles, it is necessary to develop highly active diesel oxidation catalysts (DOCs). Pd/CeO₂ catalysts have been widely used as active catalysts for CO and C₃H₆ oxidation reactions. Additionally, Ag has been reported to enhance the oxygen storage capacity (OSC) of CeO₂, which contributes to the oxidation ability of Pd/CeO₂. In this study, Pd/Ag-CeO₂ catalysts were used for CO and C₃H₆ oxidation reactions. When CeO₂ was doped with appropriate amounts of Ag, reducibility and CO desorption rate were increased, which confirmed the high OSCs of Ag-doped catalysts. However, Ag particles were formed and the Ce³⁺/Ce⁴⁺ ratio decreased when CeO₂ was doped with excess amounts of Ag. In addition, reduced Pd (Pd⁰), which is an active species for C₃H₆ oxidation, was formed and maintained even under oxidative reaction conditions. Since the removal of C₃H₆ is important for the oxidation of CO and C₃H₆, the catalyst with the highest Pd⁰ fraction (Pd/0.1Ag-CeO₂ and Pd/0.3Ag-CeO₂) presented improved catalytic activity. Consequently, the optimal amount of Ag enhanced the OSC of Pd/Ag-CeO₂ catalysts and formed active Pd⁰ species under oxidative conditions, which resulted in the excellent catalytic activity of Pd/Ag-CeO₂ for the CO and C₃H₆ oxidation reaction.

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.

Low-Temperature Selective Catalytic Reduction of NO x with NH3 over Mn-Ce Composites Synthesized by Polymer-Assisted Deposition

The Mn _x Ce _y binary catalysts with a three-dimensional network structure were successfully prepared via a polymer-assisted deposition method using ethylenediaminetetraacetic acid and polyethyleneimine as complexing agents. The developed pore structure could facilitate the gas diffusion and accelerate the catalytic reaction for NH₃ selective catalytic reduction (SCR). Moreover, the addition of Ce is beneficial for the exposure of active sites on the catalyst surface and increases the adsorption of the NH₃ and NO species. Therefore, the Mn₁Ce₁ catalyst exhibits the best catalytic activity for NO _x removal with a conversion rate of 97% at 180 °C, superior water resistance, and favorable stability. The SCR reaction over the Mn₁Ce₁ catalyst takes place through the E-R pathway, which is confirmed by the in situ diffuse reflectance Fourier transform analysis. This work explores a new strategy to fabricate multimetal catalysts and optimize the structure of catalysts.

Effects of Zr substitution on soot combustion over cubic fluorite-structured nanoceria: Soot-ceria contact and interfacial oxygen evolution

Ceria is widely used as a catalyst for soot combustion, but effects of Zr substitution on the reaction mechanism is ambiguous. The present work elucidates effects of Zr substitution on soot combustion over cubic fluorite-structured nanoceria. The nanostructured CeO₂, Ce_0.92Zr_0.08O₂, and Ce_0.84Zr_0.16O₂ composed of 5-6 nm crystallites display T_m-CO2 (the temperature at maximum CO₂ yield) at 383, 355, and 375°C under 10 vol.% O₂/N₂, respectively. The size of agglomerate decreases from 165.5 to 51.9-57.3 nm, which is beneficial for the soot-ceria contact. Moreover, Zr increases the amount of surface oxygen vacancies, generating more active oxygen (O₂^- and O^-) for soot oxidation. Thus, the activities of Ce_0.92Zr_0.08O₂ and Ce_0.84Zr_0.16O₂ in soot combustion are better than that of CeO₂. Although oxygen vacancies promote the migration of lattice O^2-, the enriched surface Zr also inhibits the mobility of lattice O^2-. Therefore, the T_m-CO2 of Ce_0.84Zr_0.16O₂ is higher than that of Ce_0.92Zr_0.08O₂. Based on reaction kinetic study, soot in direct contact with ceria preferentially decomposes with low activation energy, while the oxidation of isolated soot occurs through diffusion with high activation energy. The obtained findings provide new understanding on the soot combustion over nanoceria.

Synergistic Effect of Simultaneous Doping of Ceria Nanorods with Cu and Cr on CO Oxidation and NO Reduction

Ceria particles play a key role in catalytic applications such as automotive three-way catalytic systems in which toxic CO and NO are oxidized and reduced to safe CO2 and N2 , respectively. In this work, we explore the incorporation of Cu and Cr metals as dopants in the crystal structure of ceria nanorods prepared by a single-step hydrothermal synthesis. XRD, Raman and XPS confirm the incorporation of Cu and Cr in the ceria crystal lattices, offering ceria nanorods with a higher concentration of oxygen vacancies. XPS also confirms the presence of Cr and Cu surface species. H2 -TPR and XPS analysis show that the simultaneous Cu and Cr co-doping results in a catalyst with a higher surface Cu concentration and a much-enhanced surface reducibility, in comparison with either undoped or singly doped (Cu or Cr) ceria nanorods. While single Cu doping enhances catalytic CO oxidation and Cr doping improves catalytic NO reduction, co-doping with both Cu and Cr enhances the benefits of both dopants in a synergistic manner employing roughly a quarter of dopant weight.

Low temperature CO oxidation by doped cerium oxide electrospun fibers

We investigated CO oxidation behavior of doped cerium oxide fibers. Electrospinning technique was used to fabricate the inorganic fibers after burning off polymer component at 600 °C in air. Cu, Ni, Co, Mn, Fe, and La were doped at 10 and 30 mol% by dissolving metal salts into the polymeric electrospinning solution. 10 mol% Cu-doped ceria fiber showed excellent catalytic activity for low temperature CO oxidation with 50% CO conversion at just 52 °C. This 10 mol% Cu-doped sample showed unexpected regeneration behavior under simple ambient air annealing at 400 °C. From the CO oxidation behavior of the 12 samples, we conclude that absolute oxygen vacancy concentration estimated by Raman spectroscopy is not a good indicator for low temperature CO oxidation catalysts unless extra care is taken such that the Raman signal reflects oxide surface status. The experimental trend over the six dopants showed limited agreement with theoretically calculated oxygen vacancy formation energy in the literature.

Highly Active Transition Metal-Promoted CuCeMgAlO Mixed Oxide Catalysts Obtained from Multicationic LDH Precursors for the Total Oxidation of Methane

To improve the catalytic performance of an active layered double hydroxide (LDH)-derived CuCeMgAlO mixed oxide catalyst in the total oxidation of methane, it was promoted with different transition-metal cations. Thus, two series of multicationic mixed oxides were prepared by the thermal decomposition at 750 °C of their corresponding LDH precursors synthesized by coprecipitation at constant pH of 10 under ambient atmosphere. The first series of catalysts consisted of four M(3)CuCeMgAlO mixed oxides containing 3 at.% M (M = Mn, Fe, Co, Ni), 15 at.% Cu, 10 at.% Ce (at.% with respect to cations), and with Mg/Al atomic ratio fixed to 3. The second series consisted of four Co(x)CuCeMgAlO mixed oxides with x = 1, 3, 6, and 9 at.% Co, while keeping constant the Cu and Ce contents and the Mg/Al atomic ratio. All the mixed oxides were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) coupled with X-ray energy dispersion analysis (EDX), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption-desorption at −196 °C, temperature-programmed reduction under hydrogen (H2-TPR), and diffuse reflectance UV-VIS spectroscopy (DR UV-VIS), while thermogravimetric and differential thermal analyses (TG-DTG-DTA) together with XRD were used for the LDH precursors. The catalysts were evaluated in the total oxidation of methane, a test reaction for volatile organic compounds (VOC) abatement. Their catalytic performance was explained in correlation with their physicochemical properties and was compared with that of a reference Pd/Al2O3 catalyst. Among the mixed oxides studied, Co(3)CuCeMgAlO was found to be the most active catalyst, with a temperature corresponding to 50% methane conversion (T50) of 438 °C, which was only 19 °C higher than that of a reference Pd/Al2O3 catalyst. On the other hand, this T50 value was ca. 25 °C lower than that observed for the unpromoted CuCeMgAlO system, accounting for the improved performance of the Co-promoted catalyst, which also showed a good stability on stream.