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

Enhanced Replenishment of Active Lattice Oxygen Using Chiral Copper Oxide

Effective catalytic performance of the transition metal oxide is attributed to high specific surface areas, abundant surface oxygen atoms, and balanced valence ratios. Although the chirality of the transition metal has attracted attention, most studies have focused on optical application. A few chiral transition metal oxides were used as electrocatalysts and photocatalysts. The influence of the chiral catalysts on the thermal catalysis process has been less explored. In this study, Mn-loaded chiral (M/l-CuO and M/d-CuO) and achiral CuO (M/a-CuO) were synthesized and compared in the catalytic oxidization of toluene. Spectrally analyzed Mn was well-dispersed on both chiral and achiral CuO. l-CuO and d-CuO showed nanoflower-like chirality. The angles between each (001) plane of CuO were the source of chirality. The toluene turnover frequency (TOF) of the samples was in the order of Mn/d-CuO (5.6 × 10-5 s-1) > Mn/l-CuO (4.4 × 10-5 s-1) > Mn/a-CuO (3.2 × 10-5 s-1) at 240 °C, consistent with the order of the oxygen replenishment rate. The as-prepared catalysts had similar ratios of lattice oxygen/surface adsorbed oxygen, Mn3+/Mn4+, and Cu+/Cu2+. A higher TOF was attributed to chirality, which increased the lattice oxygen replenishment speed from the gaseous phase to the solid surface. Our study indicates gas-solid catalysis from a structure-activity viewpoint.

Different metal (Mn, Fe, Co, Ni, and Zr) decorated Cu/CeO2 catalysts for efficient CO oxidation in a rich CO2/H2 atmosphere

In this work, CuM/CeO2 (M = Mn, Fe, Co, Ni, and Zr) catalysts with a low Cu content of 1 wt% were purposely designed and prepared using the co-impregnation method. The samples were characterized using various techniques (TG-DTA, XRD, N2-adsorption/desorption measurements, H2-TPR, XPS and Raman spectroscopy) and CO preferential oxidation (CO-Prox) under H2/CO2-rich conditions was performed. The results have shown that enhanced catalytic performance was achieved upon the introduction of Mn, Co and Ni, and little impact was observed with Zr doping, but Fe showed a negative effect, as compared with the Cu/CeO2 catalyst. Characterization data revealed that the M doping strongly changed the surface composition, revealing the decreased Cu/Ce ratios on the surface, which could be accounted for by the formation of more M/Cu-O-Ce solid solution, or strong Cu-M interactions. When Mn was used, the obtained CuMn/CeO2 catalyst revealed the highest concentration of the oxygen vacancies and Ce3+ ions, which could be correlated well with its superior catalytic performance. Compared with the Cu/CeO2 catalyst, the CO conversion rate increased by 24.7% at a low temperature of 90 °C over the CuMn/CeO2 catalyst. At 130 °C, the maximum CO conversion was 94.7% and the CO2 selectivity was 78.9%. Conversely, the Fe doped Cu/CeO2 catalyst demonstrated the poorest catalytic activity, which was due to the blockage effect of Fe species on Cu showing a high Fe/Cu ratio of 1.9 on the surface.

Formation of superoxide and ozone-like species on Cu doped CeO2(111) and their CO oxidation reactivity: a DFT study

The adsorption of O2 on Cu/CeO2(111) and the CO oxidation reactivity of the formed oxygen species were studied using the DFT method. The results showed that superoxide species (O2δ-), which directly interacted with Cu, formed when O2 adsorbed on the surface oxygen vacancies, while O2 adsorbed on the subsurface oxygen vacancies gave rise to ozone-like O3δ- species by combining with the nearest surface lattice oxygen (O1). PDOS showed that hybridization of the 2p orbitals between O2 and O1 formed a delocalized π bond, confirming the formation of O3δ-. For O2δ-, electrons on Cu and O1 transferred to O2 while the charge of Ce remained unchanged. However, for O3δ-, the transferred electrons were mainly from O1, and partially from O2, Ce1 and Ce2. It was very interesting that Cu also received a few electrons in the latter case. Compared with CO directly adsorbed on lattice oxygen, the two oxygen species were active for CO oxidation, forming CO2 or carbonates, and higher absolute adsorption energy was obtained with the interaction between CO and O3δ-. The findings of this study provide new insight on the CO oxidation reaction mechanism, facilitating an in-depth understanding of Cu-doped CeO2 catalysts.

Catalysts Prepared from Atomically Dispersed Ce(III) on MgO Rival Bulk Ceria for CO Oxidation

Atomically dispersed cerium catalysts on an inert, crystalline MgO powder support were prepared by using both Ce(III) and Ce(IV) precursors. The materials were used as catalysts for CO oxidation in a once-through flow reactor and characterized by atomic-resolution scanning transmission electron microscopy, X-ray absorption near-edge structure spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction, among other techniques, before and after catalysis. The most active catalysts, formed from the precursor incorporating Ce(III), displayed performance similar to that reported for bulk ceria under comparable conditions. The catalyst provided stable time-on-stream performance for as long as it was kept on-stream, 2 days, increasing slightly in activity as the atomically dispersed cerium ions were transformed into ceria nanodomains represented as CeOx and having increased reducibility on the MgO support. The results suggest how highly dispersed supported ceria catalysts with low cerium loadings can be prepared and may pave the way for improved efficiencies of cerium utilization in oxidation catalysis.

Revealing the intrinsic nature of Ni-, Mn-, and Y-doped CeO2 catalysts with positive, additive, and negative effects on CO oxidation using operando DRIFTS-MS

The catalytic activity of a transition metal (host) oxide can be influenced by doping with a second cation (dopant), but the key factors dominating the activity of the doped catalyst are still controversial. Herein, CeO2 doped with Ni, Mn, and Y catalysts prepared using aerosol pyrolysis were used to demonstrate the positive, negative, and additive effects on CO oxidation as a model reaction. Various characterization results indicated that Ni, Mn, and Y had been successfully doped into the CeO2 lattice. The catalytic activities of each catalyst for CO conversion were in the order of Ni-CeO2 > Mn-CeO2 > CeO2 > Y-CeO2. Operando DRIFTS-MS and various characterization methods were applied to reveal the intrinsic nature of the doping effects. The accumulation rate of the surface bidentate carbonates determined the CO oxidation. A definition to evaluate the doping effect was proposed, which is anticipated to be useful for developing a rational catalyst with a high CO oxidation activity. The CO oxidation reactivities displayed strong correlations with the surface factors obtained from operando DRIFTS-MS analysis and the structure factors from XPS and Raman analyses.

The research on CO oxidation over Ce-Mn oxides: The preparation method effects and oxidation mechanism

A series of CeO₂-MnO_x for highly efficient catalytical oxidation of carbon monoxide were prepared by citrate sol-gel (C), hydrothermal (H) and hydrothermal-citrate complexation (CH) methods. The outcome indicates that the catalyst generated using the CH technique (CH-1:8) demonstrated the greatest catalytic performance for CO oxidation with a T₅₀ of 98 °C, and also good stability in 1400 min. Compared to the catalysts prepared by C and H method, CH-1:8 has the highest specific surface of 156.1 m² g^-1, and the better reducibility of CH-1:8 was also observed in CO-TPR. It is also observed the high ratio of adsorbed oxygen/lattice oxygen (1.5) in the XPS result. Moreover, characterizations by the TOF-SIMS method indicated that obtained catalyst CH-Ce/Mn = 1:8 had stronger interactions between Ce and Mn oxides, and the redox cycle of Mn³⁺+Ce⁴⁺ ↔ Mn⁴⁺+Ce³⁺ was a key process for CO adsorption and oxidation process. According to in-situ FTIR, the possible reaction pathway for CO was deduced in three ways. CO directly oxidize with O₂ to CO₂, CO adsorbed on Mn⁴⁺ and Ce³⁺ reacts with O to form intermediates (COO^-) (T > 50 °C) and carbonates (T > 90 °C), which are further oxidized into CO₂.

Fabrication of supported Pt/CeO2 nanocatalysts doped with different elements for CO oxidation: theoretical and experimental studies

Supported Pt/CeO₂ catalysts have been widely used in carbon monoxide (CO) oxidation; however, the high oxygen vacancy formation energy (E_vac) in the process leads to the poor performance of these catalysts. Herein, we explored different element (Pr, Cu, or N) doped CeO₂ supports using Ce-based metal-organic frameworks (MOFs) as precursors via calcination treatment. The obtained CeO₂ supports were used to load Pt nanoparticles. These catalysts were systematically characterized by various techniques, and they showed superior catalytic activity for CO oxidation compared to undoped catalysts which could be attributed to the formation of Ce³⁺, and high amounts of O_ads/(O_ads + O_lat) and Pt^δ+/Pt_total. Moreover, density functional theory calculations with on-site Coulomb interaction correction (DFT+U) were performed to provide atomic-scale insights into the reaction process by the Mars-van Krevelen (M-vK) mechanism, which revealed that the element-doped catalysts could simultaneously reduce the adsorption energies of CO and lower reaction energy barriers in the *OOCO associative pathway.

Fabrication and catalytic properties of nanorod-shaped (Pt-Pd)/CeO2 composites

Nanorod-supported (Pt-Pd)/CeO₂ catalysts were synthesized by a simple method of dealloying Al_91.7Ce₈ Pt _X Pd_0.3-X (X = 0, 0.075, 0.1, 0.15, 0.2, 0.3) alloy ribbons. SEM and TEM characterization implied that after calcination treatment, the achieved resultants exhibited interspersed nanorod structures with a rich distribution of nanopores. Catalytic tests showed that the (Pt_0.1-Pd_0.2)/CeO₂ catalyst calcined at 300 °C exhibited the highest catalyst activity for CO oxidation when compared with other catalysts prepared at different noble metal ratios or calcined at other temperatures, whose complete reaction temperature was as low as 100 °C. The outstanding catalytic performance is ascribed to the stable framework structure, rich gas pathways and collaborative effect between the noble Pt and Pd bimetals.

Atomically Incorporating Ni into Mesoporous CeO2 Matrix via Synchronous Spray-Pyrolysis as Efficient Noble-Metal-Free Catalyst for Low-Temperature CO Oxidation

Low-temperature catalytic CO oxidation is an important chemical process in versatile applications, such as the H₂ utilization for low-temperature H₂ air fuel cells. Pt-group metal catalysts are efficient but highly cost-consuming. This work demonstrates an excellent and sixpenny catalyst with earth-abundant Ni and Ce, in which Ni ions are atomically incorporated into the CeO₂ matrix (Ni-Ce-O_x) by synchronous spray-pyrolysis (SSP) of mixture nitrates of Ni and Ce. The Ni-Ce-O_x catalyst presents a mesoporous structure. Revealed by a model reaction of 1% CO, 1% O₂, and 98% balance He at a space velocity of 13,200 mL/g_cat/h, Ni-Ce-O_x catalysts display a typical volcano-shaped relationship between reactivity and Ni incorporation amount. The optimized Ni incorporation appears with a high Ni/Ce atomic ratio of 0.25, endowing the T₅₀ (temperature corresponding to a CO conversion of 50%), which is lower-shifted by 165 °C than that of pristine CeO₂ (266 °C). The density functional theory (DFT) calculations further indicate that the much-reduced oxygen vacancy formation energy at Ni-Ce single-atom sites boosted the adsorption activation of the CO molecule and therefore promoted the CO oxidation process. Besides, the₂ Ni-Ce-O_x from the SSP method presents better performance than the counterparts from immersion and hydrothermal methods. This work paves a way to access efficient noble-metal-free catalysts for low-temperature CO oxidation.

Uncovering the CO2 Capture Mechanism of NaNO3-Promoted MgO by 18O Isotope Labeling

MgO-based CO₂ sorbents promoted with molten alkali metal nitrates (e.g., NaNO₃) have emerged as promising materials for CO₂ capture and storage technologies due to their low cost and high theoretical CO₂ uptake capacities. Yet, the mechanism by which molten alkali metal nitrates promote the carbonation of MgO (CO₂ capture reaction) remains debated and poorly understood. Here, we utilize ¹⁸O isotope labeling experiments to provide new insights into the carbonation mechanism of NaNO₃-promoted MgO sorbents, a system in which the promoter is molten under operation conditions and hence inherently challenging to characterize. To conduct the ¹⁸O isotope labeling experiments, we report a facile and large-scale synthesis procedure to obtain labeled MgO with a high ¹⁸O isotope content. We use Raman spectroscopy and in situ thermogravimetric analysis in combination with mass spectrometry to track the ¹⁸O label in the solid (MgCO₃), molten (NaNO₃), and gas (CO₂) phases during the CO₂ capture (carbonation) and regeneration (decarbonation) reactions. We discovered a rapid oxygen exchange between CO₂ and MgO through the reversible formation of surface carbonates, independent of the presence of the promoter NaNO₃. On the other hand, no oxygen exchange was observed between NaNO₃ and CO₂ or NaNO₃ and MgO. Combining the results of the ¹⁸O labeling experiments, with insights gained from atomistic calculations, we propose a carbonation mechanism that, in the first stage, proceeds through a fast, surface-limited carbonation of MgO. These surface carbonates are subsequently dissolved as [Mg²⁺···CO₃ ^2-] ionic pairs in the molten NaNO₃ promoter. Upon reaching the solubility limit, MgCO₃ crystallizes at the MgO/NaNO₃ interface.

Synthesis and Oxygen Storage Capacities of Yttrium-Doped CeO2 with a Cubic Fluorite Structure

Doping CeO2 with Y cations was achieved in this study using three strategies: doping only during the hydrothermal process (H-Y-doped CeO2), doping only during the impregnation process (I-Y-doped CeO2), and doping during both the hydrothermal and impregnation processes (H/I-Y-doped CeO2). During the three synthesis strategies of Y-doped CeO2, these Y ions could be incorporated into the CeO2 lattice in the +3 state while holding the cubic fluorite structure, and no impurity phases were detected. Pure CeO2 crystal itself contained a certain number of intrinsic VO defects, and Y-doping was beneficial for the creation of extrinsic VO defects. The relative concentrations of VO defects were quantified by the values of A592/A464 obtained from Raman spectra, which were 1.47, 0.93, and 1.16 for the H-Y-, I-Y-, and H/I-Y-doped CeO2, respectively, and were higher than that of the undoped one (0.67). Moreover, the OSCs of the three Y-doped CeO2 were enhanced, and the sequence of OSCs was: H-Y-doped CeO2 (0.372 mmol/g) > H/I-Y-doped CeO2 (0.353 mmol/g) > I-Y-doped CeO2 (0.248 mmol/g) > Undoped CeO2 (0.153 mmol/g); this result was in good agreement with the Raman spectroscopy results.

Facile Microwave Synthesis of Hierarchical Porous Copper Oxide and Its Catalytic Activity and Kinetics for Carbon Monoxide Oxidation

The synthesis of copper oxide (CuO)-based nanomaterials has received a tremendous deal of interest in recent years. Particularly, the design and development of novel CuO structures with improved physical and chemical properties have attracted immense attention, especially for catalysis applications. We report on a rational, rapid, and surfactant-free microwave synthesis (MWS) of hierarchical porous copper oxide (HP-CuO) with a three-dimensional (3D) sponge-like topology using an MWS reactor. The activity of the microwave (MW)-synthesized HP-CuO catalysts for carbon monoxide (CO) oxidation was studied and compared to CuO prepared by the conventional heating method (CHM). Results showed that HP-CuO catalysts prepared by MWS for 10 and 30 min surpassed the CuO catalyst prepared by CHM, exhibiting T ₈₀ of 98 and 115 °C, respectively, as compared to 185 °C of CuO prepared by CHM (T₈₀ is the temperature corresponding to 80% CO conversion). In addition, the MW-synthesized HP-CuO catalysts outperformed the CHM-synthesized CuO, achieving a 100% CO conversion at 150 °C compared to 240 °C in the case of CuO prepared by CHM. Interestingly, the HP-CuO catalyst expressed workable CO conversion kinetics with a reaction rate of c.a.35 μmol s^-1 g^-1 at 150 °C and apparent activation energy (E _a) of 82 kJ mol^-1. The HP-CuO catalyst showed excellent cycling and long-term stabilities for CO oxidation up to 4 cycles and 72 h on the stream, respectively. The enhanced catalytic activity and stability of the HP-CuO catalyst appear to result from the unique topological and structural features of HP-CuO, which were revealed by SEM, XRD, Raman, BET, TGA, XPS, and TPR techniques.

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.

Promotional Effect of Pt-Doping on the Catalytic Performance of Pt−CeO2 Catalyst for CO Oxidation

Growing interest in the development of a hydrogen economy means that CO oxidation is increasingly important for upgrading H2-rich fuel gas streams for fuel cells. CeO2-supported catalysts are the most promising candidates for the catalytic oxidation of CO because of their high activity. In the present work, DFT+U calculations were performed to investigate the stability and CO oxidation reactivity of Ptn (n = 1−4) clusters supported on CeO2(111) (Pt/CeO2) and Pt-doped CeO2(111) (Pt/(Pt−Ce)O2) surfaces. The Pt clusters showed similar nucleation behavior on both CeO2 and (Pt−Ce)O2 surfaces. Further, the formation of oxygen vacancies (Ov) was facilitated because of surface charge depletion caused by the dopant Pt. Our DFT results suggest that the interfacial OV plays an important role in the CO oxidation reaction cycle, and the calculated energy barrier for the CO oxidation reaction on the Pt/(Pt−Ce)O2 surface is approximately 0.43 eV lower than that on the surface of the undoped catalyst, suggesting enhanced CO oxidation reactivity. Therefore, the chemical modification of the CeO2 support via doping is an effective strategy for improving the catalytic performance of Pt/CeO2.

Activating Surface Lattice Oxygen of a Cu/Zn1-xCuxO Catalyst through Interface Interactions for CO Oxidation

Surface lattice oxygen in metal oxides is a common participant in many chemical reactions. Given this, the structural design of catalysts to activate lattice oxygen and moreover investigations into the effect of lattice oxygen on reaction pathways are hot topics. With this in mind, herein we prepare CuO-Zn_1-xCu_xO (ZCO) nanofibers akin to the Trojan horse legend and via an in situ reduction obtain activated Cu/Zn_1-xCu_xO (Cu/ZCO) nanofibers. X-ray absorption spectroscopy and X-ray photoelectron spectroscopy reveal that surface lattice oxygen of Cu/ZCO is effectively activated from inert O^2- to reactive O^2-x. This activation stems from the enhanced covalence of metal-oxygen bonds and the electron transfer between Cu and the support. Online mass spectrometry reveals that Cu/ZCO with activated lattice oxygen exhibits a higher Mars-van Krevelen reaction efficiency during the CO oxidation process. This study offers a new avenue to engineer interface interactions, given, as highlighted here, the importance of surface lattice oxygen in oxide supports during the catalytic process.

Confinement of nano-gold in 3D hierarchically structured gadolinium-doped ceria mesocrystal: synergistic effect of chemical composition and structural hierarchy in CO and propane oxidation

Gadolinium-doped ceria hierarchical gold catalyst shows four-fold TOF increase compared to undoped non-hierarchical system, proving the synergistic effect of doping and structural hierarchy in propane oxidation.

Investigation of chlorine-poisoning mechanism of MnOx/TiO2 and MnOx-CeO2/TiO2 catalysts during o-DCBz catalytic decomposition: Experiment and first-principles calculation

The catalytic activity and stability of MnO_x/TiO₂ and MnO_x-CeO₂/TiO₂ catalysts for the oxidative degradation of 1,2-dichorobenzene (o-DCBz) at low temperatures (≤275 °C) were experimentally examined. The chlorine (Cl) poisoning mechanism of the catalysts was also clarified based on the catalyst characterization combined with theoretical calculations. Experimental results show that the MnO_x/TiO₂ catalyst is considerably deactivated during o-DCBz catalytic decomposition, mainly due to the chlorination of the catalytic active component. Ce addition and high temperature can effectively promote the resistance of MnO_x/TiO₂ catalyst to Cl poisoning. Density functional theory (DFT) calculations in the framework of first-principles reveal that Cl atom prefers to anchor on surface oxygen vacancy (OV) rather than on top site of Mn atom. The adsorption of Cl atom on surface OV hinders the dissociated adsorption of O₂ on surface OV and interrupts the regeneration of the surface reactive oxygen species. The adsorption of Cl atom on top site of Mn atom increases the formation energy of surface OV and damages the surface Lewis acid sites which act as the important adsorption sites for o-DCBz molecules. Ce addition causes Cl atom to adsorb preferentially onto the OV around Ce atom, which weakens the interaction between Cl atom and Mn atom. Consequently, the chlorination of the MnO_x species is prevented and the oxygen mobility of the catalyst is guaranteed to some extent.

An Insight into Geometries and Catalytic Applications of CeO2 from a DFT Outlook

Rare earth metal oxides (REMOs) have gained considerable attention in recent years owing to their distinctive properties and potential applications in electronic devices and catalysts. Particularly, cerium dioxide (CeO2), also known as ceria, has emerged as an interesting material in a wide variety of industrial, technological, and medical applications. Ceria can be synthesized with various morphologies, including rods, cubes, wires, tubes, and spheres. This comprehensive review offers valuable perceptions into the crystal structure, fundamental properties, and reaction mechanisms that govern the well-established surface-assisted reactions over ceria. The activity, selectivity, and stability of ceria, either as a stand-alone catalyst or as supports for other metals, are frequently ascribed to its strong interactions with the adsorbates and its facile redox cycle. Doping of ceria with transition metals is a common strategy to modify the characteristics and to fine-tune its reactive properties. DFT-derived chemical mechanisms are surveyed and presented in light of pertinent experimental findings. Finally, the effect of surface termination on catalysis by ceria is also highlighted.

Active Sites and Interfacial Reducibility of CuxO/CeO2 Catalysts Induced by Reducing Media and O2/H2 Activation

The development of a highly efficient and stable catalyst for preferential oxidation of CO for the commercialization of proton-exchange membrane fuel cells has been a result of continuous effort. The main challenge is the simultaneous control of abundant active sites and interfacial reducibility over the catalyst Cu_xO/CeO₂. Here, we report a strategy to modulate porosity, active sites, and O-vacancy sites (O_V) by reducing media and O₂/H₂ activation. O₂-pretreated CeO₂-supported Cu catalysts unequivocally demonstrate the low-temperature activity owing to the excess concentrations of Cu⁺ and Cu²⁺ as well as the relative population of Ce³⁺ and O-vacancy sites at the surface. O₂ activation improves the Cu²⁺ diffusion into the CeO₂ lattice to generate the synergistic effect and induces the formation of electron-enriched Cu²⁺-O_V-Ce³⁺ sites, which accelerate the activation and dissociation of CO/O₂ and the formation of reactive oxygen species during catalysis. Density function theory (DFT) calculations reveal that CO adsorbs on Cu₂O {110} and CuO {111} with relatively optimal adsorption energy and longer C-Cu lengths in contrast to that on Cu {111}, favoring the adsorption and desorption of CO. These are crucial for ongoing CO oxidation, producing CO₂ by the Mars-van Krevelen mechanism.

CO Oxidation at Near-Ambient Temperatures over TiO2-Supported Pd-Cu Catalysts: Promoting Effect of Pd-Cu Nanointerface and TiO2 Morphology

Significant improvement of the catalytic activity of palladium-based catalysts toward carbon monoxide (CO) oxidation reaction has been achieved through alloying and using different support materials. This work demonstrates the promoting effects of the nanointerface and the morphological features of the support on the CO oxidation reaction using a Pd-Cu/TiO2 catalyst. Pd-Cu catalysts supported on TiO2 were synthesized with wet chemical approaches and their catalytic activities for CO oxidation reaction were evaluated. The physicochemical properties of the prepared catalysts were studied using standard characterization tools including SEM, EDX, XRD, XPS, and Raman. The effects of the nanointerface between Pd and Cu and the morphology of the TiO2 support were investigated using three different-shaped TiO2 nanoparticles, namely spheres, nanotubes, and nanowires. The Pd catalysts that are modified through nanointerfacing with Cu and supported on TiO2 nanowires demonstrated the highest CO oxidation rates, reaching 100% CO conversion at temperature regime down to near-ambient temperatures of ~45 °C, compared to 70 °C and 150 °C in the case of pure Pd and pure Cu counterpart catalysts on the same support, respectively. The optimized Pd-Cu/TiO2 nanowires nanostructured system could serve as efficient and durable catalyst for CO oxidation at near-ambient temperature.

Barium promoted Ni/Sm2O3 catalysts for enhanced CO2 methanation

Low temperature CO₂ methanation is a favorable pathway to achieve high selectivity to methane while increasing the stability of the catalysts. A Ba promoted Ni/Sm₂O₃ catalyst was investigated for CO₂ methanation at atmospheric pressure with the temperature ranging from 200–450 °C. 5Ni–5Ba/Sm₂O₃ showed significant enhancement of CO₂ conversion particularly at temperatures ≤ 300 °C compared to Ni/Sm₂O₃. Incorporation of Ba into 5Ni/Sm₂O₃ improved the basicity of the catalysts and transformed the morphology of Sm₂O₃ from random structure into uniform groundnut shape nanoparticles. The uniformity of Sm₂O₃ created interparticle porosity that may be responsible for efficient heat transfer during a long catalytic reaction. Ba is also postulated to catalyze oxygen vacancy formation on Sm₂O₃ under a reducing environment presumably via isomorphic substitution. The disappearance of a high temperature (∼600 °C) reduction peak in H₂-TPR analysis revealed the reducibility of NiO following impregnation with Ba. However, further increasing the Ba loading to 15% formed BaNiO₃–BaNiO_2.36 phases which consequently reduced the activity of the Ni–Ba/Sm₂O₃ catalyst at low temperature. Ni was suggested to segregate from BaNiO₃–BaNiO_2.36 at high temperature thus exhibiting comparable activity with Ni/Sm₂O₃ at 450 °C.

Low temperature CO₂ methanation on 5Ni–5Ba/Sm₂O₃ is a favorable pathway to achieve high selectivity to methane while increasing the stability of the catalysts.