Catalytic Behaviour of Flame-Made CuO-CeO2 Nanocatalysts in Efficient CO Oxidation

Effects of Electric Field on Chemical Looping Combustion: A DFT Study of CO Oxidation on CuO (111) Surface

Chemical looping combustion (CLC) is a promising and novel technology for carbon dioxide (CO2) capture with a relatively low energy consumption and cost. CuO, one of the most attractive oxygen carriers (OCs) for carbon dioxide (CO) oxidation, suffers from sintering and agglomeration during the reduction process. Applying an electric field (EF) may promote the CO oxidation process on the CuO surface, which could mitigate sintering and agglomeration by decreasing operating temperatures with negligible combustion efficiency loss. This study performs density functional theory (DFT) simulations to investigate the effects of EF on the oxidation of CO on the CuO (111) surface. The results indicate that both the orientation and strength of the EF can significantly affect the oxidation characteristics of CO on the CuO (111) surface such as total reaction energy, energy barriers of reactions, CO adsorption, and CO2 desorption. For the first time, this study reveals the role of EF in enhancing CO oxidation through CLC processes via first-principle calculations. Such findings could provide new strategies to improve the performance of CLC processes.

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.

Photothermal CO-PROX reaction over ternary CuCoMnOx spinel oxide catalysts: the effect of the copper dopant and thermal treatment

The purification of carbon monoxide in H₂-rich streams is an urgent problem for the practical application of fuel cells, and requires the development of efficient and economical catalysts for the preferential oxidation of CO (CO-PROX). In the present work, a facile solid phase synthesis method followed by an impregnation method were adopted to prepare a ternary CuCoMnO_x spinel oxide, which shows superior catalytic performance with CO conversion of 90% for photothermal CO-PROX at 250 mW cm^-2. The dopant of copper species leads to the incorporation of Cu ions into the CoMnO_x spinel lattice forming a ternary CuCoMnO_x spinel oxide. The appropriate calcination temperature (300 °C) contributes to the generation of abundant oxygen vacancies and strong synergetic Cu-Co-Mn interactions, which are conducive to the mobility of oxygen species to participate in CO oxidation reactions. On the other hand, the highest photocurrent response of CuCoMnO_x-300 also promotes the photo-oxidation activity of CO due to the high carrier concentration and efficient carrier separation. In addition, the in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) confirmed that doping copper species could enhance the CO adsorption capacity of the catalyst due to the generation of Cu⁺ species, which significantly increased the CO oxidation activity of the CuCoMnO_x spinel oxide. The present work provides a promising and eco-friendly solution to remove the trace CO in H₂-rich gas over CuCoMnO_x ternary spinel oxide with solar light as the only energy source.

The noble metals M (M = Pd, Ag, Au) decorated CeO2catalysts derived from solution combustion method for efficient low-temperature CO catalytic oxidation: effects of different M loading on catalytic performances

The noble metal nanoparticles have attracted attention due to their excellent catalytic performance for CO oxidation at low temperatures. M-CeO₂(M = Pd, Ag, Au) catalysts with different atomic ratios of M/Ce were deposited via solution combustion method. Among them, 3 at% Pd-CeO₂, 5 at% Ag-CeO₂and 1 at% Au-CeO₂catalysts have better catalytic performances. Especially, 5 at% Ag-CeO₂catalyst shows better low-temperature CO oxidation performance. The catalytic activity for CO oxidation follows the follows the following sequence: 5 at% Ag-CeO₂(T₅₀ = 69 °C) > 3 at% Pd-CeO₂(T₅₀ = 99 °C) >1 at% Au-CeO₂(T₅₀ = 115 °C). Meanwhile, the catalysts are characterized by means of powder x-ray diffraction, scanning electron microscope, transmission electron microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, Brunauer-Emmett-Teller and H₂-TPR. The characterization results show that the 5 at% Ag-CeO₂catalyst has excellent catalytic activity due to the good dispersion of Ag nanoparticles, the specific surface area of the material, and the reduction catalyst between different valence ions. Moreover, the surface of the catalyst enhances the mutual synergy, effectively promotes the generation of oxygen vacancies, and increases the active oxygen content of the catalyst surface. Finally, the catalytic mechanism of M-CeO₂catalysts is summarized.

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.

HKUST-1-Supported Cerium Catalysts for CO Oxidation

The synthesis method of metal–organic frameworks (MOFs) has an important impact on their properties, including their performance in catalytic reactions. In this work we report on how the performance of [Cu3(TMA)2(H2O)3]n (HKUST-1) and Ce@HKUST-1 in the reaction of CO oxidation depends on the synthesis method of HKUST-1 and the way the cerium active phase is introduced to it. The HKUST-1 is synthesised in two ways: via the conventional solvothermal method and in the presence of a cationic surfactant (hexadecyltrimethylammonium bromide (CTAB)). Obtained MOFs are used as supports for cerium oxide, which is deposited on their surfaces by applying wet and incipient wetness impregnation methods. To determine textural properties, structure, morphology, and thermal stability, the HKUST-1 supports and Ce@HKUST-1 catalysts are characterised using X-ray diffraction (XRD), N2 sorption, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), and thermogravimetric analysis (TGA). It is proven that the synthesis method of HKUST-1 has a significant impact on its morphology, surface area, and thermal stability. The synthesis method also influences the dispersion and the morphology of the deposited cerium oxide. Last but not least, the synthesis method affects the catalytic activity of the obtained material.

Facile preparation of highly active and stable CuO–CeO2 catalysts for low-temperature CO oxidation via a direct solvothermal method

CuO–CeO₂ catalysts prepared by a direct solvothermal method exhibit high activity and stability for low-temperature CO oxidation.