Inverse CeO2/CuO Catalyst As an Alternative to Classical Direct Configurations for Preferential Oxidation of CO in Hydrogen-Rich Stream

Single-and double transition metal atoms anchored C2N as a high-activity catalyst for CO oxidation: A first-principles study

The oxidation of CO has attracted great interest in recent years due to its important role in enhancing the catalyst durability in fuel cells and solving the growing environmental problems caused by CO emissions. Consequently, the catalytic oxidation of CO at double non-noble metal atoms anchored C2N is investigated using density functional theory (DFT) computations. All the screened Ti@C2N and Ti2@C2N are thermodynamically stable based on their binding energy calculations. The electronic characteristics, the natural bond orbital analyses (NBO), Frontier orbital, statistical thermodynamics, projected densities of states (PDOS) characteristics, non-covalent interactions (NCI), and quantum theory of atoms in molecules (QTAIM) descriptors of these systems have been examined to analyze the interaction process. Our comparative study suggested that the newly predicted double-atom catalyst (Ti2@C2N) is highly active for CO oxidation, which is a useful guideline for further development. The calculated static first-order hyperpolarizability (βo) illustrated that the double-atom catalyst under investigation can be considered a potential candidate for non-linear optical behavior and could be used for NLO applications. CO oxidation on Ti2@C2N along the Eley-Rideal (ER) mechanism with a low energy barrier of 0.16 eV, which is smaller than the maximum energy barrier (0.73 eV) of CO oxidation along the Langmuir-Hinshelwood (LH) mechanism. Consequently, the ER mechanism is more favorable both thermodynamically and dynamically. This work can provide useful insights and guidelines for future theoretical and experimental investigations to promote the design and development of highly effective and low-cost non-precious-metal Ti2@C2N nanocatalysts towards CO oxidation at ambient temperature.

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.

Modification of Electrospun CeO2 Nanofibers with CuCrO2 Particles Applied to Hydrogen Harvest from Steam Reforming of Methanol

Hydrogen is the alternative renewable energy source for addressing the energy crisis, global warming, and climate change. Hydrogen is mostly obtained in the industrial process by steam reforming of natural gas. In the present work, CuCrO₂ particles were attached to the surfaces of electrospun CeO₂ nanofibers to form CeO₂-CuCrO₂ nanofibers. However, the CuCrO₂ particles did not readily adhere to the surfaces of the CeO₂ nanofibers, so a trace amount of SiO₂ was added to the surfaces to make them hydrophilic. After the SiO₂ modification, the CeO₂ nanofibers were immersed in Cu-Cr-O precursor and annealed in a vacuum atmosphere to form CeO₂-CuCrO₂ nanofibers. The CuCrO₂, CeO₂, and CeO₂-CuCrO₂ nanofibers were examined by X-ray diffraction analysis, transmission electron microscopy, field emission scanning electron microscopy, scanning transmission electron microscope, thermogravimetric analysis, and Brunauer-Emmett-Teller studies (BET). The BET surface area of the CeO₂-CuCrO₂ nanofibers was 15.06 m²/g. The CeO₂-CuCrO₂ nanofibers exhibited hydrogen generation rates of up to 1335.16 mL min^-1 g-cat^-1 at 773 K. Furthermore, the CeO₂-CuCrO₂ nanofibers produced more hydrogen at lower temperatures. The hydrogen generation performance of these CeO₂-CuCrO₂ nanofibers could be of great importance in industry and have an economic impact.

Fabrication of Stable Cu-Ce Catalyst with Active Interfacial Sites for NOx Elimination by Flame Spray Pyrolysis

The complete conversion of NOx to harmless N2 without N2O formation is crucial for the control of air pollution, especially at low temperatures. Cu-based catalysts are promising materials due to their low cost and high activity in NO dissociation, even comparable to noble metals; however, they suffer from low stability. Here, we established a Cu-Ce catalyst in one step with strong metal–support interaction by the flame spray pyrolysis (FSP) method. Almost 100% NO conversion was achieved at 100 °C, and they completely transferred into N2 at a low temperature (200 °C) for the FSP-CuCe catalyst, exhibiting excellent performance in NO reduction by CO reaction. Moreover, the catalytic performance can stay stable, while 23% NO conversion was lost in the same condition for the one made by the co-precipitation (CP) method. This can be attributed to the synergistic effect of abundant active interfacial sites and more flexible surface oxygen created during the FSP process. The flame technology developed here provides an efficient way to fabricate strong metal–support interactions, exhibiting notable potential in the design of stable Cu-based catalysts.

Porous stainless-steel fibers supported CuCeFeOx/Zeolite catalysts for the enhanced CO oxidation: Experimental and kinetic studies

To develop novel catalysts of high-performance and cost-effectiveness, and to investigate the reaction kinetics of CO oxidation, ternary CuCeFeO_x catalysts supported on zeolite/PSF (porous stainless-steel fibers) were synthesized for the first time. Effects of different Ce/Fe ratios, loading amounts, calcination temperatures, and reaction kinetics were investigated. Remarkably improved catalytic performance was achieved in the PSF-supported catalysts compared to the granular ones, owing to the increased mass/heat transfer efficiency and the high dispersion of active metal oxide species anchored on the zeolite layer. The Cu₃Ce₁₂Fe₄-400 sample exhibited the best catalytic activity with a temperature difference in T₉₀ of almost 40 °C lower than the worst one. Characterization results from XRD, FTIR, TEM, XPS, H₂-TPR, etc. revealed that the promoted reducibility of metal oxides and formation of more oxygen vacancies significantly contributed to the enhanced catalytic activity. Furthermore, a generalized rate expression was derived from intrinsic and macro kinetic studies by assuming the conversion of CO to CO₂ as the rate-determining step, in which CO oxidation over the PSF-supported catalysts followed the pseudo-first-order kinetic established by the Langmuir-Hinshelwood type mechanism.

Trace CO elimination in H2-rich streams with a wide operation temperature window: Co deposited CuO-CeO2 catalysts

This work provides a new strategy to eliminate trace CO in H₂-rich gas in a wide operation temperature window for the application of hydrogen fuel cells. We engineered Co deposited CuO-CeO₂ catalysts with a Co/(Cu + Ce) molar ratio of 1/1 that manages to maintain the CO level at below 100 ppm from 85 to 240 °C in the H₂-rich reformate stream. CO-PROX and CO methanation reaction respectively occurred in the low and high temperature ranges. Multiple characterization techniques demonstrate that the molar ratio of Co/(Cu + Ce) significantly affects the synergistic interactions between the Cu, Co and Ce species, and ultimately the CO oxidation and CO methanation reactions. At low reaction temperatures, the Cu-Ce interaction mainly dominates the CO-PROX process, while at high reaction temperatures, CO methanation reaction takes place due to the reduction of Co₃O₄ to Co⁰ and the Co-Ce interaction takes charge of the CO methanation. Moreover, the increment of Co/(Cu + Ce) from 1/2 to 1 gives rise to the reprecipitation of the partially dissolved Cu species on Co₃O₄, which strengthens the Cu-Co interaction and stabilizes surface Cu⁺ and Co³⁺, thus promoting the low temperature CO-PROX catalytic performance.

Molecular immuno-imaging improves tumor detection in head and neck cancer

Detection and accurate delineation of tumor is important for the management of head and neck squamous cell carcinoma (HNSCC) but is challenging with current imaging techniques. In this study, we evaluated whether molecular immuno-imaging targeting myeloperoxidase (MPO) activity, an oxidative enzyme secreted by many myeloid innate immune cells, would be superior in detecting tumor extent compared to conventional contrast agent (DTPA-Gd) in a carcinogen-induced immunocompetent HNSCC murine model and corroborated in human surgical specimens. In C57BL/6 mice given 4-nitroquinoline-N-oxide (4-NQO), there was increased MPO activity in the head and neck region as detected by luminol bioluminescence compared to that of the control group. On magnetic resonance imaging, the mean enhancing volume detected by the MPO-targeting agent (MPO-Gd) was higher than that by the conventional agent DTPA-Gd. The tumor volume detected by MPO-Gd strongly correlated with tumor size on histology, and higher MPO-Gd signal corresponded to larger tumor size found by imaging and histology. On the contrary, the tumor volume detected by DTPA-Gd did not correlate as well with tumor size on histology. Importantly, MPO-Gd imaging detected areas not visualized with DTPA-Gd imaging that were confirmed histopathologically to represent early tumor. In human specimens, MPO was similarly associated with tumors, especially at the tumor margins. Thus, molecular immuno-imaging targeting MPO not only detects oxidative immune response in HNSCC, but can better detect and delineate tumor extent than nonselective imaging agents. Thus, our findings revealed that MPO imaging could improve tumor resection as well as be a useful imaging biomarker for tumor progression, and potentially improve clinical management of HNSCC once translated.

Bioinspired Synthesis of Ce1-x O2:x%Cu2+ Nanobelts for CO Oxidation and Organic Dye Degradation

Ce_1-x O₂:x%Cu²⁺ nanobelts were bioinspired, designed, and fabricated using commercial filter papers as scaffolds by adding Cu(NO₃)₂ in the original sol solution of CeO₂ nanobelts, which display excellent catalyst properties for CO oxidation and photocatalytic activity for organic dyes. Compared with pure CeO₂, CuO belts were synthesized using the same method and the corresponding Ce_0.5O₂:50%Cu²⁺ bulk materials were synthesized without filter paper as scaffolds; the synthesized Ce_1-x O₂:x%Cu²⁺ nanobelts, especially Ce_0.5O₂:50%Cu²⁺ nanobelts, can decrease the reaction temperature of CO to CO₂ at 100 °C with the conversion rate of 100%, much lower than the formerly reported kinds of Ce_1-x O₂:x%Cu²⁺ catalysts. Meanwhile, the synthesized Ce_1-x O₂:x%Cu²⁺ nanobelts also display better photocatalytic activity for organic dyes. All of these results provide useful information for the potential applications of the synthesized Ce_1-x O₂:x%Cu²⁺ nanobelts in catalyst fields.

Unraveling Catalytic Mechanisms for CO Oxidation on Boron-Doped Fullerene: A Computational Study

By means of spin-polarized density functional theory (DFT) computations, we unravel the reaction mechanisms of catalytic CO oxidation on B-doped fullerene. It is shown that O2 species favors to be chemically adsorbed via side-on configuration at the hex-C-B site with an adsorption energy of -1.07 eV. Two traditional pathways, Eley-Rideal (ER) and Langmuir-Hinshelwood (LH) mechanisms, are considered for the CO oxidation starting from O2 adsorption. CO species is able to bind at the B-top site of the B-doped fullerene with an adsorption energy of -0.78 eV. Therefore, CO oxidation that occurs starting from CO adsorption is also taken into account. Second reaction of CO oxidation occurs by the reaction of CO + O → CO2 with a very high energy barrier of 1.56 eV. A trimolecular Eley-Rideal (TER) pathway is proposed to avoid leaving the O atom on the B-doped fullerene after the first CO oxidation. These predictions manifest that boron-doped fullerene is a potential metal-free catalyst for CO oxidation.

A Photoactivated Cu-CeO2 Catalyst with Cu-[O]-Ce Active Species Designed through MOF Crystal Engineering

Fully utilizing solar energy for catalysis requires the integration of conversion mechanisms and therefore delicate design of catalyst structures and active species. Herein, a MOF crystal engineering method was developed to controllably synthesize a copper-ceria catalyst with well-dispersed photoactive Cu-[O]-Ce species. Using the preferential oxidation of CO as a model reaction, the catalyst showed remarkably efficient and stable photoactivated catalysis, which found practical application in feed gas treatment for fuel cell gas supply. The coexistence of photochemistry and thermochemistry effects contributes to the high efficiency. Our results demonstrate a catalyst design approach with atomic or molecular precision and a combinatorial photoactivation strategy for solar energy conversion.

Graphenylene-supported single-atom (Ru and Mo) catalysts for CO and NO oxidations

Based on density functional theory (DFT) calculations, the adsorption geometries, stability and catalytic properties of single-atom Ru and Mo anchored on graphenylene sheets (gra-Ru and gra-Mo) are comparatively investigated.

Recent advances in synergistic effect promoted catalysts for preferential oxidation of carbon monoxide

We reviewed recent advances in catalysts for PROX with emphasis on synergistic effects that contribute to enhanced catalytic performance.

Investigation of the re-dispersion of matrix Cu species in CuxCe1−xO2 nanorod catalysts and its effect on the catalytic performance in CO-PROX

The CO-PROX catalytic performance is sensitive to the re-dispersion state of matrix copper species, which separate out as highly dispersed CuO_x under thermal treatment.

Co3O4-CuCoO2 Nanomesh: An Interface-Enhanced Substrate that Simultaneously Promotes CO Adsorption and O2 Activation in H2 Purification

Nanomaterials are widely used as redox-type reaction catalysts, while reactant adsorption and O₂ activation are hardly to be promoted simultaneously, restricting their applications in many important catalytic fields such as preferential CO oxidation (CO-PROX) in H₂-rich stream. In this work, an interface-enhanced Co₃O₄-CuCoO₂ nanomesh was initially synthesized by a hydrothermal process using aluminum powder as a sacrificial agent. This nanomesh is systematically characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, N₂ adsorption, X-ray photoelectron spectroscopy, UV-vis absorption spectroscopy, Raman spectroscopy, X-ray absorption near-edge spectroscopy, hydrogen temperature-programmed reduction, and oxygen temperature-programmed desorption. It is demonstrated that the nanomesh possesses high-density nanopores, enabling a large number of CO adsorption sites exposed to the surface. Meanwhile, electron transfer from O^2- to Co³⁺/Co²⁺ and the weakened bonding strength of Co-O bond at surfaces promoted the oxygen activation and redox ability of Co₃O₄. When tested as a catalyst for CO-PROX, this nanomesh with an optimized pore structure and a surface electronic structure, exhibits a strikingly high catalytic oxidation activity at low temperatures as well as a broader operation temperature window (i.e., CO conversion >99.0%, 100-200 °C) in the CO selective oxidation reaction. The present finding should be highly useful in promoting the quest for better CO-PROX catalysts, a hot topic for proton exchange membrane fuel cells and automotive vehicles.

Comparison of Au–Ce and Au–Cu interaction over Au/CeO2–CuO catalysts for preferential CO oxidation

Au decelerates reduction of copper species, while it improves ceria reduction.

A Linear Scaling Relation for CO Oxidation on CeO2-Supported Pd

Resolving the structure and composition of supported nanoparticles under reaction conditions remains a challenge in heterogeneous catalysis. Advanced configurational sampling methods at the density functional theory level are used to identify stable structures of a Pd8 cluster on ceria (CeO2) in the absence and presence of O2. A Monte Carlo method in the Gibbs ensemble predicts Pd-oxide particles to be stable on CeO2 during CO oxidation. Computed potential energy diagrams for CO oxidation reaction cycles are used as input for microkinetics simulations. Pd-oxide exhibits a much higher CO oxidation activity than metallic Pd on CeO2. This work presents for the first time a scaling relation for a CeO2-supported metal nanoparticle catalyst in CO oxidation: a higher oxidation degree of the Pd cluster weakens CO binding and facilitates the rate-determining CO oxidation step with a ceria O atom. Our approach provides a new strategy to model supported nanoparticle catalysts.

Optimum Preferential Oxidation Performance of CeO2-CuO x-RGO Composites through Interfacial Regulation

Interfacial regulation offers a promising route to rationally and effectively design advanced materials for CO preferential oxidation. Herein, we initiated an interfacial regulation of CeO₂-CuO _x-RGO composites by adjusting the addition sequence of the components during the support formation. The presence of RGO along with the sequence tuning of the components is confirmed to survey the changes of the oxidation state of copper species, the content and distribution of the Cu⁺ site, and the synergistic interactions between Cu-Ce mixed oxides and reduced graphene oxide (RGO) over the catalysts. These catalysts were systematically characterized by inductively coupled plasma, X-ray diffraction, transmission electron microscopy/high-resolution transmission electron microscopy, hydrogen temperature-programmed reduction, X-ray photoelectron spectra, thermal gravimetric analysis, Raman spectra, and in situ diffuse reflectance infrared Fourier transform spectroscopy measurements. The results show that RGO is favorable for the generation of Cu⁺ and the dispersion of copper-cerium species in the as-prepared catalysts. Furthermore, by multi-interfacial regulation of the CeO₂-CuO _x-RGO composites, the catalyst CeO₂/CuO _x-RGO exhibits a strikingly high catalytic oxidation activity at a low temperature coupled with a broader operation temperature window (i.e., CO conversion >99.0%, 140-220 °C) in the CO-selective oxidation reaction, which has been attributed to the high content of the active species Cu⁺ enriched on the surface, the highly dispersed copper oxide clusters subjected to a strong interaction with ceria, and the synergistic interactions between Cu-Ce mixed oxides and RGO.

Dye-Assisted Transformation of Cu2O Nanocrystals to Amorphous Cu x O Nanoflakes for Enhanced Photocatalytic Performance

Amorphous Cu _x O nanoflakes with a thickness of 10-50 nm were synthesized through dye-assisted transformation of rhombic dodecahedral Cu₂O nanocrystals using a facile solution process. The morphology evolution observed by electron microscopy is highly dependent on the reaction between the surface and the dye. The crystal grain shrinks during the process until the formation of a purely amorphous nanoflake. The amorphous Cu _x O nanoflake consists of a combination of Cu(I) and Cu(II) with a ratio close to 1:1. It shows enhanced photocatalytic reactivity toward the degradation of methyl orange compared to that of rhombic dodecahedral Cu₂O nanocrystals with all active (110):Cu facets. The chemical composition and architecture remain the same after repeating degradation tests. The high surface-to-volume ratio contributes to its superior photocatalytic performance, whereas its low surface energy, confirmed by density functional theory simulations, explains its improved stability. The nanoflakes also show the ability of degrading nitrobenzene effectively, thus demonstrating great promise as a highly stable and active photocatalyst for environmental applications.