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.

Kinetic and Computational Studies of CO Oxidation and PROX on Cu/CeO2 Nanospheres

As supported CuO is well-known for low temperature activity, CuO/CeO2 nanosphere catalysts were synthesized and tested for CO oxidation and preferential oxidation of CO (PROX) in excess H2. For the first reaction, ignition was observed at 95 °C, whereas selective PROX occurred in a temperature window from 50 to 100 °C. The catalytic performance was independent of the initial oxidation state of the catalyst (CuO vs. Cu0), suggesting that the same active phase is formed under reaction conditions. Density functional modeling was applied to elucidate the intermediate steps of CO oxidation, as well as those of the comparably less feasible H2 transformation. In the simulations, various Cu and vacancy sites were probed as reactive centers enabling specific pathways.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11244-023-01848-x.

Study on the mechanism of NOx reduction by NH3-SCR over a ZnXCu1-XFe2O4 catalyst

Experimental evidence shows that CuFe₂O₄ exhibits excellent catalytic performance in the SCR reaction. However, there is a lack of in-depth research on its specific reaction mechanism. Our study begins by computing the adsorption model of molecules like NH₃ and then goes on to examine the SCR reaction mechanism of NH₃ on CuFe₂O₄ before and after Zn doping. The results indicate that NH₃ is chemically adsorbed (-1.26 eV) on the surface and has a strong interaction with the substrate. Importantly, Zn doping provides more favorable reactive sites for NH₃ molecules. Subsequent investigation into the NH₃ dehydrogenation and SCR reaction processes showed that incorporating Zn can greatly decrease the energy barrier of the most critical step in the reaction (0.58 eV). Additionally, the study also assesses the feasibility of the reaction of adsorbed NO with surface active O atoms to form NO₂ (barrier 0.86 eV). Lastly, the sulfur resistance of the catalyst before and after doping is calculated and analyzed, and it is found that Zn doping effectively improves the sulfur resistance. Our study provides valuable theoretical guidance for the development of ferrite spinel and doping modification.

In-situ Surface-Enhanced Raman Spectroscopy Reveals a Mars-van Krevelen-Type Gas Sensing Mechanism in Au@SnO2 Nanoparticle-Based Chemiresistors

Molecular-level understandings of gas sensing mechanisms of oxide-based chemiresistors are significant for designing high-performance gas sensors; however, the mechanisms are still controversial due to the lack of direct experimental evidence. This work demonstrates efficient in situ surface-enhanced Raman spectroscopy (SERS) tracing of the highly representative SnO₂-ethanol gas sensing using Au@SnO₂ nanoparticles (NPs), where the Au core and SnO₂ shell provide SERS activity and a gas sensing response, respectively. The in situ SERS evidence suggests that the sensing follows a Mars-van Krevelen mechanism rather than the prevailing adsorbed oxygen (AO) model. This mechanism is also observed in sensing other gases based on the Au@SnO₂ NPs, showing its universality. This work offers efficient in situ tracing for gas sensing and experimental elucidation of the specific gas sensing mechanism, potentially ending the long-term controversy over the gas sensing mechanisms. Therefore, it is highly significant to this field.

Relationship between hydrothermal temperatures and structural properties of CeO2 and enhanced catalytic activity of propene/toluene/CO oxidation by Au/CeO2 catalysts

A simple hydrothermal synthesis of CeO₂ was implemented to obtain a series of CeO₂-supported gold (Au) catalysts, used for the total oxidation of propene/toluene/CO gas mixtures and the oxidation of CO. CeO₂ preparation started from a cerium hydrogen carbonate precursor using a range of different hydrothermal temperatures (HT) from 120 to 180°C. High-resolution transmission electron microscopy, X-ray diffraction, and H₂-temperature-programmed reduction data indicated that CeO₂ morphology varied with the HT, and was composed of the more active (200) surface. Following Au deposition onto the CeO₂ support, this active crystal plane resulted in the most widely dispersed Au nanoparticles on the CeO₂ support. The catalytic performance of the CeO₂-supported Au catalysts for both oxidation reactions improved as the reducibility increased to generate lattice oxygen vacancies and the number of adsorbed peroxide species on the CeO₂ support increased due to addition of Au. The Au catalyst on the CeO₂ support prepared at 120°C was the most active in both propene/toluene/CO oxidation and independent CO oxidation.

Coadsorption Interfered CO Oxidation over Atomically Dispersed Au on h-BN

Similar to the metal centers in biocatalysis and homogeneous catalysis, the metal species in single atom catalysts (SACs) are charged, atomically dispersed and stabilized by support and substrate. The reaction condition dependent catalytic performance of SACs has long been realized, but seldom investigated before. We investigated CO oxidation pathways over SACs in reaction conditions using atomically dispersed Au on h-BN (AuBN) as a model with extensive first-principles-based calculations. We demonstrated that the adsorption of reactants, namely CO, O2 and CO2, and their coadsorption with reaction species on AuBN would be condition dependent, leading to various reaction species with different reactivity and impact the CO conversion. Specifically, the revised Langmuir-Hinshelwood pathway with the CO-mediated activation of O2 and dissociation of cyclic peroxide intermediate followed by the Eley-Rideal type reduction is dominant at high temperatures, while the coadsorbed CO-mediated dissociation of peroxide intermediate becomes plausible at low temperatures and high CO partial pressures. Carbonate species would also form in existence of CO2, react with coadsorbed CO and benefit the conversion. The findings highlight the origin of the condition-dependent CO oxidation performance of SACs in detailed conditions and may help to rationalize the current understanding of the superior catalytic performance of SACs.

Mechanism insights into CO oxidation over transition metal modified V2O5/TiO2 catalysts: A theoretical study

The V₂O₅/TiO₂ based selective catalytic reduction (SCR) catalysts possess not only promising capability on the denitrification of nitrogen oxides (NO_x), but also certain effects on the oxidation of carbon monoxide (CO) in the flue gas. Modification of traditional SCR catalysts with certain transition metals can further improve their catalytic oxidation ability of CO. Therefore, it is of great significance to reveal the catalytic oxidation mechanism of CO for developing modified SCR catalysts to achieve the co-removal of CO and NO_x. Theoretical calculations based on density functional theory (DFT) were performed to probe the comprehensive reaction mechanism of CO oxidation on M doped V₂O₅/TiO₂ catalysts (M = Mo, Fe, and Co). The whole CO oxidation cycles include three stages, i.e., the first CO oxidation, the re-oxidation of the surface, and the second CO oxidation. The terminal oxygen and the surface oxygen formed by the adsorbed O₂ all play vital roles in the whole CO oxidation cycles. The activation barriers of the rate-determining steps for CO oxidation on Fe-V₂O₅/TiO₂ and Co-V₂O₅/TiO₂ are much lower than that of Mo-V₂O₅/TiO₂, which indicates Fe and Co dopants can apparently promote the CO oxidation activities of the modified SCR catalysts. Meanwhile, the electronic structure analysis confirms that Fe and Co dopants can cause electron distribution change with strong oxidation ability at the active oxygen sites.

3D Tungsten Disulfide/Carbon Nanotube Networks as Separator Coatings and Cathode Additives for Stable and Fast Lithium-Sulfur Batteries

Commercial application of Li-S batteries is greatly restricted by their unsatisfactory cycle retention and poor cycling life originating from the lithium polysulfide (LiPS) shuttling effect and sluggish sulfur redox kinetics. Various strategies have been proposed to boost the performances of Li-S batteries, including nanostructured sulfur composites, functional separators/interlayers, electrode/electrolyte additives, and so on. However, how to combine two or more strategies to efficiently settle these challenging issues confronted by Li-S batteries is in desperate need. Here, we demonstrate a powerful combined strategy of introducing novel 3D WS₂/carbon nanotube (CNT) networks built by hybridization of 1D CNTs with 2D WS₂ into Li-S batteries, simultaneously serving as a functional cathode additive and separator coating. Such 3D WS2/CNTs networks with abundant edge sites, a large active surface, and a fast electron pathway twice perform functions from the cathode side and separator surface: (1) to suppress polysulfide diffusion through a physical barrier and chemical interactions; (2) to accelerate LiPS conversion reactions; and (3) to enhance conductivity for better sulfur reactivation and high utilization. As a result, the as-built WS2/CNTs-incorporated battery configuration achieves a commendable combination of capacity, rate, and cycle stability (1491 mA h g^-1 at 0.2 C, 754 mA h g^-1 at 5 C, and initial capacity of 1069 mA h g^-1 with an ultralow decay rate of 0.040% per cycle over 1000 cycles at 1 C) along with remarkably mitigated anode corrosion and low self-discharge.

Tunable Electronic Metal-Support Interactions on Ceria-Supported Noble-Metal Nanocatalysts in Controlling the Low-Temperature CO Oxidation Activity

A fundamental study on the metal-support interactions of supported metal catalysts is of great importance for developing heterogeneous catalysts with high performance, is still attracting and challenging in many heterogeneous catalytic reactions. In this work, we report the catalytic performances of CeO₂-supported noble-metal catalysts among single atoms, subnanoclusters (∼1 nm), and nanoparticles (2.2-2.7 nm) upon low-temperature CO oxidation reaction between 50 and 250 °C. The subnanoclusters and nanoparticles of Ru, Rh, and Ir showed much higher activities than those of the single atoms, while a Pd single-atom catalyst was more active than Pd subnanoclusters and nanoparticles. According to the results of multiple ex situ and in situ characterizations, the much different activities of Ru, Rh, Ir, and Pd were derived from the alterable electronic metal-support interactions (EMSI), which determine the concurrent reaction pathway including the famous Mars van Krevelen mechanism and carbonate-intermediate route on the most active metal sites of M^δ+ (0 < δ < 1) for Ru, Rh, and Ir and Pd²⁺ for Pd. Also, the moderate EMSI of CeO₂-supported Rh subnanoclusters furthest benefited activation of the adsorbed CO molecule and ensured it the highest activity among CeO₂-supported Ru, Rh, and Ir catalysts with similar metal deposit sizes.

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.

Potassium-incorporated manganese oxide enhances the activity and durability of platinum catalysts for low-temperature CO oxidation

Potassium-incorporated manganese oxide is demonstrated as an efficient support for fabricating highly active and stable Pt catalysts for low-temperature CO oxidation.

Carbon Monoxide Oxidation Promoted by Surface Polarization Charges in a CuO/Ag Hybrid Catalyst

Composite structures have been widely utilized to improve material performance. Here we report a semiconductor-metal hybrid structure (CuO/Ag) for CO oxidation that possesses very promising activity. Our first-principles calculations demonstrate that the significant improvement in this system's catalytic performance mainly comes from the polarized charge injection that results from the Schottky barrier formed at the CuO/Ag interface due to the work function differential there. Moreover, we propose a synergistic mechanism underlying the recovery process of this catalyst, which could significantly promote the recovery of oxygen vacancy created via the M-vK mechanism. These findings provide a new strategy for designing high performance heterogeneous catalysts.

Insight into the active site and reaction mechanism for selective oxidation of methane to methanol using H2O2 on a Rh1/ZrO2 catalyst

Five-coordinated Rh leads to the over-oxidation of CH₄, while four-coordinated Rh stabilizes CH₃ and facilitates methanol formation via the CH₃OOH intermediate.

Influence of gold on the reactivity behaviour of ceria nanorods in CO oxidation: combining operando spectroscopies and DFT calculations

In this combined Raman/UV-Vis and DFT study, structure-activity relations for CO oxidation over ceria nanorods (with/without gold) with CeO₂(110) and CeO₂(100) termination are elucidated using ceria nanocubes with CeO₂(100) termination as reference.

Divergent influence of {1 1 1} vs. {1 0 0} crystal planes and Yb3+ dopant on CO oxidation paths in mixed nano-sized oxide Au/Ce1−xYbxO2−x/2 (x = 0 or 0.1) systems

In this paper, the fundamental information on interactions in systems concerning nanocrystalline gold disperses on the shaped (octahedron-like or cube-like) Ce_1−xYb_xO_2−x/2 (x = 0 or 0.1) support has been discussed.

Well-defined palladium–ceria interfacial electronic effects trigger CO oxidation

The electron transfer from Pd cubes to CeO₂ rods via the interfaces triggered low-temperature CO oxidation.