The impact of synthesis method of CNT supported CeZrO 2 and Ni-CeZrO 2 on catalytic activity in WGS reaction

Functionalization of Graphite with Oxidative Plasma

Surface-modified graphite is studied as an electrode material, an adsorbent, and a membrane component, among other applications. Modifying the graphite with plasma can be used to create relevant surface functionalities, in particular, various oxygen groups. The application of surface-oxidized graphite often requires its use in an aqueous environment. The application in an aqueous environment is not an issue for acid-oxidized carbons, but a discrepancy in the structure–activity relationship may arise because plasma-oxidized carbons show a time-dependent decrease in the degree of functionalization and related properties. Moreover, plasma-oxidized materials are often characterized in terms of their chemical and physical properties, most notably their degree of functionalization after plasma treatment, without contact with water. In this study, we used low-temperature plasma oxidation with pure oxygen and carbon dioxide and sample-washing with concentrated nitric and sulfuric acids. To evaluate the electronic properties of modified graphite, the work function changes and surface oxygen content were measured just after plasma modification and after water immersion. We show that water immersion drastically decreases the work function of plasma-treated samples, which is accompanied by a decrease in the number of radicals introduced by plasma. Our results demonstrate that the increase in stable work function as a result of plasma treatment, brought about by an increase in the surface oxygen species concentration, can be realized most effectively for the acid-washed graphite.

Dry Reforming of Methane over CNT-Supported CeZrO2, Ni and Ni-CeZrO2 Catalysts

In this work, the carbon nanotubes (CNT)-supported nanosized, well-dispersed, CeZrO2 and Ni-CeZrO2 catalysts were obtained and tested for the first time in the reaction of methane dry reforming (DRM). The performance of the hybrid materials was compared with the performance of Ni/CNT catalyst. The mechanism of the DRM reaction and the occurrence of reverse water gas shift reaction (RWGS) and CO2 deoxidation were discussed in terms of catalysts composition. The contribution of RWGS and CO2 deoxidation in the DRM process, demonstrating an increased CO2 consumption when compared to CH4, and H2/CO < 1, varied depending on the catalyst composition, was also studied.

Hybrid electrocatalytic ozonation treatment of high-salinity organic wastewater using Ni-Ce/OMC particle electrodes

Hybrid electrocatalytic ozonation is an efficient method for degrading high-salinity organic wastewater that has excellent oxidation ability and is environmentally friendly. Furthermore, the high salt content of the wastewater provides electrolyte to support the process, which avoids secondary pollution caused by the addition of electrolyte. In this work, Ni_0.2-Ce_0.2/ordered mesoporous carbon (OMC)/granular active carbon (GAC) particle electrodes with a Ni:Ce weight ratio of 1:1 were synthesized using a simple method. The electrodes were characterized by transmission electron microscopy and electron paramagnetic resonance spectroscopy, as well as other techniques. The catalytic performance was investigated using cyclic voltammetry and AC impedance. Higher reduction and oxidation peak currents were obtained with the Ni_0.2-Ce_0.2/OMC catalyst than with Ni_0.2/OMC or Ce_0.2/OMC, indicating that the bimetallic catalyst has higher activity for the reduction of O₂ to H₂O₂ and the oxidation of O₃ to ·OH. The order of the k values-which represent the mass-transfer rate-was Ni_0.2-Ce_0.2/OMC (0.157) > Ni_0.2/OMC (0.017) > Ce_0.2/OMC (0.014). The results show that cooperation between Ni, Ce, and OMC promoted the dispersion of Ni and Ce and improved the catalytic performance. Ni_0.2-Ce_0.2/OMC enhances the catalytic reduction of O₂ to H₂O₂, and, in addition, Ce is able to rapidly store and release oxygen through Ce³⁺/Ce⁴⁺ conversions and reacting with O₃ to generate ·OH, which increases the oxidation capacity of the material. Under the optimal conditions the chemical oxygen demand removal for high-salinity organic wastewater using Ni_0.2-Ce_0.2/OMC/GAC particle electrodes reached 93.7%.

Surface reduction properties of ceria-zirconia solid solutions: a first-principles study

Based on the density functional theory (DFT), the reduction properties of Ce_1-x Zr _x O₂ (110) surfaces were systematically calculated using CO as a probe for thermodynamic study, and a large supercell was applied to build the whole composition range (x = 0.125, 0.250, 0.375, 0.500, 0.625, 0.750, 0.875). From the calculated energy barriers of CO oxidation by lattice oxygen, we found that composition Ce_0.875Zr_0.125O₂ exhibited the most promising catalytic effectiveness with the lowest activation energy of 0.899 eV. Moreover, the active surface O_3c ions coordinated by two Zr ions and one Ce ion were facilely released from their bulk positions than the O_3c ions surrounded by two Ce ions and one Zr ion on Ce_0.625Zr_0.375O₂, Ce_0.500Zr_0.500O₂, and Ce_0.375Zr_0.625O₂ (110) surfaces. This difference could be explained by the binding strength of O_3c with different neighboring cations.

Enhanced catalytic ozonation by highly dispersed CeO2 on carbon nanotubes for mineralization of organic pollutants

Heterogeneous catalytic ozonation, in which ozone is activated by various catalysts to produce reactive oxygen species (ROS), is an effective approach to degrade persistent organic pollutants in water. However, catalyst with high activity and good stability for catalytic ozonation still remains rare. In this work, a highly dispersed cerium oxide on oxidized carbon nanotubes (CeO₂-OCNT) were prepared and characterized by X-ray diffractometer (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction using hydrogen (TPR-H₂) experiments. The as-synthesized CeO₂-OCNT showed significantly enhanced catalytic performance for degrading organic pollutants during catalytic ozonation. The removal efficiency of optimized CeO₂-OCNT for phenol mineralization was 2-3 times of pure CeO₂ and OCNT, and was also better than that of a composite with the same composition, which demonstrated a synergic effect between OCNT and CeO₂ on CeO₂-OCNT for catalytic ozonation. The TOC removal efficiency exhibited no obvious reduction after five cycling experiments, indicating the synthesized CeO₂-OCNT possessed good reusability. Moreover, electron paramagnetic resonance (EPR) and radicals quenching experiments revealed that hydroxyl radicals (OH) were the dominant ROS for organic pollutants degradation. The superior activity of CeO₂-OCNT for catalytic ozonation could be attributed to the well-dispersed CeO₂, the improved mass transfer, and the facilitated redox Ce³⁺/Ce⁴⁺ cycling.

CNT and H2 Production During CH4 Decomposition over Ni/CeZrO2. I. A Mechanistic Study

This work presents a new insight into the potential of a Ni/CeZrO2 catalyst in two separate processes: (i) Chemical Vapor Deposition (CVD) using methane as a feedstock to obtain carbon nanotubes (CNTs) and H2, and (ii) catalyst regeneration with H2O that yields H2. The direct reaction of methane with H2O (steam methane reforming (SMR)) leads to H2 and CO (and CO2), whereas carbon deposition—regardless of its type—is an unwanted reaction. The concept presented in this work assumes dividing that process into two reactors, which allows one to obtain two valuable products, i.e., CNTs and H2. The literature data on CNT production via CVD ignores the issue of H2 formation. Moreover, there is no data concerning CNT production in fluidized bed reactors over ceria-zirconia supported metal catalysts. The results presented in this work show that CNTs can be formed on Ni/CeZrO2 during CH4 decomposition, and that the catalyst can be easily regenerated with H2O, which is accompanied by a high production of H2. The ability of Ni/CeZrO2 to be regenerated is its main advantage over the Ni-MgO catalyst that is popular for CNT production. This paper also shows that the Ni/CeZrO2 catalyst has the potential to be used for CNT and H2 production in a larger scale process, e.g., in a fluidized bed reactor.

CNT and H2 Production during CH4 Decomposition over Ni/CeZrO2. II. Catalyst Performance and Its Regeneration in a Fluidized Bed

In this work, a ceria-zirconia supported nickel catalyst (Ni/CeZrO2) was for the first time used in a fluidized bed reactor in order to obtain carbon nanotubes (CNTs) and H2 in the reaction of the decomposition of CH4. The same catalyst was afterward regenerated with H2O, which was accompanied with the production of H2. The impact of catalyst granulation, temperature, and gas hourly space velocity (GHSV) on the amount and type of carbon deposits was determined using thermogravimetric analysis (TGA) and scanning and transmission electron microscopy (SEM and TEM). The presence of randomly oriented and curved CNTs with an outer diameter of up to 64 nm was proved. The Ni/CeZrO2 particles were loosely covered with CNTs, freely dispersed over CNTs, and strongly attached to the external CNT walls. TEM proved the presence of a Ni/CeZrO2@CNT hybrid material that can be further used as catalyst, e.g., in WGS or DRM reactions. The impact of GHSV on hydrogen production during catalyst regeneration was determined. The catalyst was subjected to cyclic tests of CH4 decomposition and regeneration. According to the obtained results, Ni/CeZrO2 can be used in CH4 conversion to CNTs and H2 (instead of CH4 combustion), e.g., in the vicinity of installations that require methane utilization.