Efficient detection of 45 ppb ammonia at room temperature using Ni-doped CeO2 octahedral nanostructures

To meet the requirements in air quality monitors for the public and industrial safety, sensors are required that can selectively detect the concentration of gaseous pollutants down to the parts per million (ppm) and ppb (parts per billion) levels. Herein, we report a remarkable NH3 sensor using Ni-doped CeO2 octahedral nanostructure which efficiently detects NH3 as low as 45 ppb at room temperature. The Ni-doped CeO2 sensor exhibits the maximum response of 42 towards 225 ppm NH3, which is ten-fold higher than pure CeO2. The improved sensing performance is caused by the enhancement of oxygen vacancy, bandgap narrowing, and redox property of CeO2 caused by Ni doping. Density functional theory confirms that O vacancy with Ni at Ce site (VONiCe) augments the sensing capabilities. The Bader charge analysis predicts the amount of charge transfer (0.04 e) between the Ni-CeO2 surface and the NH3 molecule. As well, the high negative adsorption energy (≈750 meV) and lowest distance (1.40 Å) of the NH3 molecule from the sensor surface lowers the detection limit. The present work enlightens the fabrication of sensing elements through defect engineering for ultra-trace detection of NH3 to be useful further in the field of sensor applications.

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.

A pH-responsive polymer-coated CaO2as oxygen-generating nanoparticlein situfor enhanced chemo-photodynamic synergistic therapy against tumors

Photodynamic therapy (PDT) has emerged as an efficient strategy for tumor treatment. However, Insufficient amounts of inherent hypoxia and intrinsic hydrogen peroxide (H2O2) in the tumor microenvironment severely constrained PDT, as oxygen is the critical substrate for photosensitivity reaction. Here, a pH-responsive H2O2and O2self-supplying hybrid nanoparticle was designed. Through, the calcium peroxide (CaO2) as carriers loading a chemotherapeutic drug a photosensitizer 5,10,15,20-tetrakis(4-aminophenyl) porphyrin (TAPP) and doxorubicin (DOX), was covered with polyacrylic acid (PAA) to build up a feature material DOX-TAPP-CaO2@OA@PAA (denoted as DTCOP) through the reverse microemulsion method. In the acidic tumor microenvironment conditions exposing the water-sensitive CaO2nanocore to generate hydrogen peroxide (H2O2) and O2, the self-supplied O2alleviates hypoxia to enhance the PDT, and releasing DOX and TAPP. Synthetic characterization shows that the succeeded synthesized Nanocarriers could effectively carry DOX and TAPP to the tumor site and release O2at the low pH of TME. And the experimental results demonstrated that this interpose exogenous oxygen strategy is efficient at inhibition of tumor growth bothin vitroandin vivo. The nanocomposite exhibits excellent biocompatibility and the ability to inhibit tumor growth and has significant potential for the treatment of hypoxic tumors.

Manganese Defective Clustering: Influence on the Spectroscopic Features of Ceria-Based Nanomaterials

The influence of manganese modification on the spectroscopic features of manganese-doped CeO₂ systems synthesized by the microwave-assisted hydrothermal route and their correlation with the presence of O defective structures were verified, focusing on their interaction with poisonous atmospheres. Raman and electron paramagnetic resonance studies confirmed the presence of defective clusters formed by dipoles and/or quadrupoles. The number of paramagnetic species was found to be inversely proportional to the doping concentration, resulting in an increase in the Mn²⁺ signal, likely due to the reduction of Mn³⁺ species after the interaction with CO. X-ray photoelectron spectroscopy data showed the pure system with 33% of its cerium species in the Ce³⁺ configuration, with an abrupt decrease to 19%, after the first modification with Mn, suggesting that 14% of the Ce³⁺ species are donating one electron to the Mn²⁺ ions, thus becoming nonparamagnetic Ce⁴⁺ species. On the contrary, 58% of the manganese species remain in the Mn²⁺ configuration with five unpaired electrons, corroborating the paramagnetic feature of the samples seen in the electron paramagnetic resonance study.

Exploration of the reaction mechanism of the LaFeO3 oxygen carrier for chemical-looping steam methane reforming: a DFT study

The CO conversion is expected to be controllable for chemical-looping steam methane reforming. Herein, density functional theory (DFT) calculations were employed to systematically explore the detailed reaction mechanism of CO conversion over the LaFeO₃ oxygen carrier. It is found that the FeO₂-terminated surface could exhibit better activity for CO adsorption than the LaO-terminated surface. In addition, the FeO₂-terminated surface is much more favorable for CO oxidation than the LaO-terminated surface and the Fe-O site is the main active site. The oxygen diffusion process is easier to proceed on the LaO-terminated surface compared with the FeO₂-terminated surface. Four pathways for the reaction process between the FeO₂-terminated surface and CO were proposed and oxygen diffusion was determined as the rate-limiting step. For the reaction of CO with the LaO-terminated surface, one pathway was considered and CO₂ desorption is the rate-limiting step. Comprehensively, the reactivity of CO conversion over the FeO₂-terminated surface is superior to that over the LaO-terminated surface. We could control the CO conversion by regulating the oxygen activity of LaFeO₃. This work provides guidance for the rational design of LaFeO₃ oxygen carriers in the CL-SRM process.

Boosting CO Catalytic Oxidation Performance via Highly Dispersed Copper Atomic Clusters: Regulated Electron Interaction and Reaction Pathways

Copper-loaded ceria (Cu/CeO₂) catalysts have become promising for the catalytic oxidation of industrial CO emissions. Since their superior redox property mainly arises from the synergistic effect between Cu and the CeO₂ support, the dispersion state of Cu species may dominate the catalytic performance of Cu/CeO₂ catalysts: the extremely high or low dispersity is disadvantageous for the catalytic performance. The nanoparticle catalysts usually present few contact sites, while the single-atom catalysts tend to be passivated due to their relatively single valence state. To achieve a suitable dispersion state, we synthesized a superior Cu/CeO₂ catalyst with Cu atomic clusters, realizing high atomic exposure and unit atomic activity simultaneously via favorable electron interaction and an anchoring effect. The catalyst reaches a 90% CO conversion at 130 °C, comparable to noble-metal catalysts. According to combined in situ spectroscopy and density functional theory calculations, the superior CO oxidation performance of the Cu atomic cluster catalyst results from the joint efforts of effective adsorption of CO at the electrophilic sites, the CO spillover phenomenon, and the efficient bicarbonate pathway triggered by hydroxyl. By providing a superior atomic cluster catalyst and uncovering the catalytic oxidation mechanism of Cu-Ce dual-active sites, our work may enlighten future research on industrial gaseous pollutant removal.

Partial Oxidation of Methane to Methanol on the M-O-Ag/Graphene (M = Ag, Cu) Composite Catalyst: A DFT Study

Partial oxidation of methane (CH₄) to methanol (CH₃OH) remains a great challenge in the field of catalysis due to its low selectivity and productivity. Herein, Ag-O-Ag/graphene and Cu-O-Ag/graphene composite catalysts are proposed to oxidize methane (CH₄) to methanol (CH₃OH) by using the first-principles calculations. It is shown that reactive oxygen species (μ-O) on both catalysts can activate the C-H bond of CH₄, and in addition to CH₄ activation, the catalytic activity follows the order of Ag-O-Ag/graphene (singlet) > Ag-O-Ag/graphene (triplet) ≈ Cu-O-Ag/graphene (triplet) > Cu-O-Ag/graphene (singlet). For CH₃OH* formation, the catalytic activity follows the order of Cu-O-Ag/graphene (triplet) > Ag-O-Ag/graphene (triplet) > Ag-O-Ag/graphene (singlet) > Cu-O-Ag/graphene (singlet). It can be inferred that the introduction of Cu not only reduces the use of noble metal Ag but also exhibits a catalytic effect comparable to that of the Ag-O-Ag/graphene catalyst. Our findings will provide a new avenue for understanding and designing highly effective catalysts for the direct conversion of CH₄ to CH₃OH.

Promoting the mechanism of OMS-2 for gas adsorption in different K+ concentrations

Adsorption performances between the OMS-2 catalyst and CH4 and O2 molecules are enhanced with increasing tunnel K+ concentration.

Density functional theory studies of transition metal carbides and nitrides as electrocatalysts

Transition metal carbides and nitrides are interesting non-precious materials that have been shown to replace or reduce the loading of precious metals for catalyzing several important electrochemical reactions. The purpose of this review is to summarize density functional theory (DFT) studies, describe reaction pathways, identify activity and selectivity descriptors, and present a future outlook in designing carbide and nitride catalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), nitrogen reduction reaction (N₂RR), CO₂ reduction reaction (CO₂RR) and alcohol oxidation reactions. This topic is of high interest to scientific communities working in the field of electrocatalysis and this review should provide theoretical guidance for the rational design of improved carbide and nitride electrocatalysts.

Study on the adsorption selection of CH$$_4$$ on CuO (110) versus (111) surfaces: a density functional theory study

Finding the active sites of suitable metal oxides is a key prerequisite for detecting CH$$_4$$ 4 . The purpose of the paper is to investigate the adsorption of CH$$_4$$ 4 on intrinsic and oxygen-vacancies CuO (111) and (110) surfaces using density functional theory calculations. The results show that CH$$_4$$ 4 has a strong adsorption energy of −0.370 to 0.391 eV at all site on the CuO (110) surface. The adsorption capacity of CH$$_4$$ 4 on CuO (111) surface is weak, ranging from −0.156 to −0.325 eV. In the surface containing oxygen vacancies, the adsorption capacity of CuO surface to CH$$_4$$ 4 is significantly stronger than that of intrinsic CuO surface. The results indicate that CuO (110) has strong adsorption and charge transfer capacity for CH$$_4$$ 4 , which may provide experimental guidance.

Oxygen vacancy engineering of cerium oxide for the selective photocatalytic oxidation of aromatic pollutants

The engineering of oxygen vacancies in CeO₂ nanoparticles (NPs) allows the specific fine-tuning of their oxidation power, and this can be used to rationally control their activity and selectivity in the photocatalytic oxidation (PCO) of aromatic pollutants. In the current study, a facile strategy for generating exceptionally stable oxygen vacancies in CeO₂ NPs through simple acid (CeO₂-A) or base (CeO₂-B) treatment was developed. The selective (or mild) PCO activities of CeO₂-A and CeO₂-B in the degradation of a variety of aromatic substrates in water were successfully demonstrated. CeO₂-B has more oxygen vacancies and exhibits superior photocatalytic performance compared to CeO₂-A. Control of oxygen vacancies in CeO₂ facilitates the adsorption and reduction of dissolved O₂ due to their high oxygen-storage ability. The oxygen vacancies in CeO₂-B as active sites for oxygen-mediated reactions act as (i) adsorption and reduction reaction sites for dissolved O₂, and (ii) photogenerated electron scavenging sites that promote the formation of H₂O₂ by multi-electron transfer. The oxygen vacancies in CeO₂-B are particularly stable and can be used repeatedly over 30 h without losing activity. The selective PCOs of organic substrates were studied systematically, revealing that the operating mechanisms for UV-illuminated CeO₂-B are very different from those for conventional TiO₂ photocatalysts. Thus, the present study provides new insights into the design of defect-engineered metal oxides for the development of novel photocatalysts.

Growth, sintering, and chemical states of Co supported on reducible CeO2(111) thin films: The effects of the metal coverage and the nature of the support

The growth, sintering, and interaction of cobalt with ceria were studied under ultrahigh vacuum conditions by vapor-deposition of Co onto well-defined CeO_x(111) (1.5 < x < 2) thin films grown on Ru(0001). Charge transfer from Co to ceria occurs upon deposition of Co on CeO_1.96 and partially reduced CeO_1.83 at 300 K. X-ray photoelectron spectroscopy studies show that Co is oxidized to Co²⁺ species at the cost of the reduction of Ce⁴⁺ to Ce³⁺, at a lesser extent on reduced ceria. Co²⁺ is the predominant species on CeO_1.96 at low Co coverages (e.g., ≤0.20 ML). The ratio of metallic Co/Co²⁺ increases with the increase in the Co coverage. However, both metallic Co and Co²⁺ species are present on CeO_1.83 even at low Co coverages with metallic Co as the major species. Scanning tunneling microscopy results demonstrate that Co tends to wet the CeO_1.96 surface at very low Co coverages at room temperature forming one-atomic layer high structures of Co-O-Ce. The increase in the Co coverage can cause the particle growth into three-dimensional structures. The formation of slightly flatter Co particles was observed on reduced CeO_1.83. In comparison with other transition metals including Ni, Rh, Pt, and Au, our studies demonstrate that Co on ceria exhibits a smaller particle size and higher thermal stability, likely arising from strong metal-support interactions. The formed particles upon Co deposition at 300 K are present on the ceria surface after heating to 1000 K. The Co-ceria interface can be tuned by varying the Co metal coverage, the annealing temperature, and the nature of the ceria surface.

Assessment of PBE+U and HSE06 methods and determination of optimal parameter U for the structural and energetic properties of rare earth oxides

Rare earth oxides are attracting increasing interest as a relatively unexplored group of materials with potential applications in heterogeneous catalysis and electrocatalysis; therefore, a credible and universal computational approach is needed for modeling their reactivity. In this work, we systematically assessed the performance of the PBE+U method against the results of the hybrid HSE06 method with respect to the description of structural parameters and energetic properties of the selected hexagonal lanthanide sesquioxides and the cubic fluorite-type cerium dioxide. In addition, we evaluated the performance of PBE+U in describing the electronic structure and adsorption properties of the CeO₂(111) and Nd₂O₃(0001) surfaces. The HSE06 method reproduces rather well the lattice parameters and selected energetic properties with respect to the experimental values. The PBE+U method is able to reproduce the results of HSE06 or the experimental values only if the U parameter is selected from an appropriate range of values. The U value around 3 eV gives the best description of the lattice parameters of most bulk oxides. 2 eV-3 eV is also found to be the optimal range of U for the reaction energies of bulk La₂O₃, Ce₂O₃, Nd₂O₃, Er₂O₃, and Ho₂O₃. U = 1 eV gives the best results for Pr₂O₃, Pm₂O₃, Eu₂O₃, Tm₂O₃, and Lu₂O₃, whereas Gd₂O₃ could not be accurately described by the PBE+U method. The U values (∼3 eV) found optimal for most bulk oxides also work well in the calculations of adsorption of small molecules on Nd₂O₃(0001) and CeO₂(111), although larger U values are required to obtain sufficient localization of 4f electrons.

Achieving Efficient Alkaline Hydrogen Evolution Reaction over a Ni5 P4 Catalyst Incorporating Single-Atomic Ru Sites

Developing efficient electrocatalysts for alkaline water electrolysis is central to substantial progress of alkaline hydrogen production. Herein, a Ni₅ P₄ electrocatalyst incorporating single-atom Ru (Ni₅ P₄ -Ru) is synthesized through the filling of Ru³⁺ species into the metal vacancies of nickel hydroxides and subsequent phosphorization treatment. Electron paramagnetic resonance spectroscopy, X-ray-based measurements, and electron microscopy observations confirm the strong interaction between the nickel-vacancy defect and Ru cation, resulting in more than 3.83 wt% single-atom Ru incorporation in the obtained Ni₅ P₄ -Ru. The Ni₅ P₄ -Ru as an alkaline hydrogen evolution reaction catalyst achieves low onset potential of 17 mV and an overpotential of 54 mV at a current density of 10 mA cm^-2 together with a small Tafel slope of 52.0 mV decade^-1 and long-term stability. Further spectroscopy analyses combined with density functional theory calculations reveal that the doped Ru sites can cause localized structure polarization, which brings the low energy barrier for water dissociation on Ru site and the optimized hydrogen adsorption free energy on the interstitial site, well rationalizing the experimental reactivity.

Partial oxidation of methane to methanol by isolated Pt catalyst supported on a CeO2 nanoparticle

Catalytic transformation of methane (CH₄) into methanol in a single step is a challenging issue for the utilization of CH₄. We present a direct method for converting CH₄ into methanol with high selectivity over a Pt/CeO₂ catalyst which contains ionic Pt²⁺ species supported on a CeO₂ nanoparticle. The Pt/CeO₂ catalyst reproducibly yielded 6.27 mmol/g of Pt with a selectivity of over 95% at 300 °C when CH₄ and CO are used as reactants, while the catalyst had a lower activity when using only CH₄ without CO. Active lattice oxygen created on the Pt and CeO₂ interface provides selective reaction pathways for the conversion of CH₄ to methanol. Based on high-angle annular dark-field scanning transmission electron microscopy, x-ray photoelectron spectroscopy, x-ray absorption near-edge structure, extended x-ray absorption fine structure, catalytic studies, and density functional theory calculations, we propose a mechanistic pathway involving CH₄ activation at the active site in the vicinity of Pt²⁺ species.

Approaches for Selective Oxidation of Methane to Methanol

Methane activation chemistry, despite being widely reported in literature, remains to date a subject of debate. The challenges in this reaction are not limited to methane activation but extend to stabilization of the intermediate species. The low C-H dissociation energy of intermediates vs. reactants leads to CO2 formation. For selective oxidation, nature presents methane monooxygenase as a benchmark. This enzyme selectively consumes methane by breaking it down into methanol. To assemble an active site similar to monooxygenase, the literature reports Cu-ZSM-5, Fe-ZSM-5, and Cu-MOR, using zeolites and systems like CeO2/Cu2O/Cu. However, the trade-off between methane activation and methanol selectivity remains a challenge. Density functional theory (DFT) calculations and spectroscopic studies indicate catalyst reducibility, oxygen mobility, and water as co-feed as primary factors that can assist in enabling higher selectivity. The use of chemical looping can further improve selectivity. However, in all systems, improvements in productivity per cycle are required in order to meet the economical/industrial standards.

Studies of CO2 hydrogenation over cobalt/ceria catalysts with in situ characterization: the effect of cobalt loading and metal–support interactions on the catalytic activity

Metal–oxide interactions affect the catalytic properties of Co/CeO₂ and can be used to control activity and selectivity.

Effects of alumina morphology on dry reforming of methane over Ni/Al2O3 catalysts

Ni-based catalysts supported on alumina with different morphologies exhibited different properties for the dry reforming of methane due to their specific lattice planes, 3D structures, and surface functional properties.