A First-Principle Study of Synergized O2 Activation and CO Oxidation by Ag Nanoparticles on TiO2(101) Support

Highly efficient and continuous activation of O2 by a novel FexP-FeCu composite for water purification and insights into the activation mechanisms through DFT calculation

Degradation of organic pollutants through O2 activation catalyzed by transitional metals is challenging without addition of external chemicals and input of energy. We prepare a novel Fe based catalyst by compositing carbon, iron phosphide (FexP), iron carbide (FexC), Fe0 and Cu NPs, which can continuously activate O2 to produce high amount of 1O2,·O2- and·OH radicals in a wide pH range. DFT calculation discloses that O2 molecules are dissociated into *O or exist as O-O in various configurations. The Fe-O2, Cu-O2 and FeP-O2 surfaces can react with H2O molecules to generate *OOH, *OH and/or OH-. The sorbed-O2 intermediates on FexC surface might be released as 1O2 or·O2-. The oxidative O2-sorbed surfaces and in-situ produced oxygen reactive species contribute to the efficient and pH-indenpendent degradation of organic pollutants. Cu NPs accelerate Fe2+/Fe3+ cycles and offer impetus to initiate O2 activation due to the potential difference between Fe and Cu. The recycling test and XPS results confirm that the mutual electron transferring among carbon, FexC, FexP, Fe and Cu maintains reactivity and stability of the catalysts.

Oxidation of methane and ethylene over Al incorporated N-doped graphene: A comparative mechanistic DFT study

It is generally recognized that developing effective methods for selective oxidation of hydrocarbons to generate more useful chemicals is a major challenge for the chemical industry. In the present study, density functional theory calculations are conducted to examine the catalytic partial oxidation of methane (CH₄) and ethylene (C₂H₄) by nitrous oxide (N₂O) over Al-incorporated porphyrin-like N-doped graphene (AlN₄-Gr). Adsorption energies for the most stable configurations of CH₄, C₂H₄, and N₂O molecules over the AlN₄-Gr catalyst are determined to be -0.25, -0.64, and -0.40 eV, respectively. According to our findings, N₂O can be efficiently split into N₂ and O_ads species with a negligible activation energy on the AlN₄-Gr surface. Meanwhile, CH₄ and C₂H₄ molecules compete for reaction with the activated oxygen atom (O_ads) that stays on the surface. The energy barriers for partial methane oxidation through the CH₄ + O_ads → CH₃° + HO_ads and CH₃° + HO_ads → CH₃OH reaction steps are 0.16 eV and 0.27 eV, respectively. Furthermore, the produced CH₃OH may be overoxidized by O_ads to give formaldehyde and water molecules by overcoming a relatively low activation barrier. The activation barriers for C₂H₄ epoxidation are small and comparable to those for CH₄ oxidation, implying that AlN₄-Gr is highly active for both reactions. The high energy barrier for the 1,2-hydrogen shift in the OCH₂CH₂ intermediate, on the other hand, makes the production of acetaldehyde impossible under normal conditions. According to the population analysis, the AlN₄-Gr serves as a strong electron donor to aid in the charge transfer between the Al atom and the O_ads moiety, which is necessary for the activation of CH₄ and C₂H₄. The findings of the present study may pave the way for a better understanding of the catalytic oxidation the CH₄ and C₂H₄, as well as for the development of highly efficient noble-metal free catalysts for these reactions.

Cobalt Phthalocyanine Supported on Mesoporous CeO2 as an Active Molecular Catalyst for CO Oxidation

Heterogenization of biomolecules by immobilizing on a metal oxide support could greatly enhance their catalytic activity and stability, but their interactions are generally weak. Herein, cobalt phthalocyanine (CoPc) molecules were firmly anchored on a Ce-based metal-organic framework (Ce-BTC) due to π-π stacking interaction between CoPc and aromatic frameworks of the BTC linker, which was followed by a calcination treatment to convert Ce-BTC to mesoporous CeO₂ and realize a molecular-level dispersion of CoPc on the surface of CeO₂. Various characterization results confirm the successful fabrication of molecular-based CoPc/CeO₂ catalysts which exhibited good CO oxidation performance. Importantly, we found that the mixing manner of Ce-BTC and CoPc remarkably affects the physicochemical properties which then determined the catalytic performance of the resultant CoPc/CeO₂ catalysts. In contrast, the direct physical mixing of CoPc and CeO₂ led to poor performance toward CO oxidation, manifesting that the Ce-BTC-mediated CoPc loading strategy is promising for the heterogenization of catalytic biomolecules.

Reduction of N2 to NH3 by TiO2-supported Ni cluster catalysts: a DFT study

Electrochemical techniques for ammonia synthesis are considered as an encouraging energy conversion technology to efficiently meet the challenge of nitrogen cycle balance. Herein, we find that TiO2(101)-supported Ni4 and Ni13 clusters can serve as efficient catalysts for electrocatalytic N2 reduction based on theoretical calculations. Electronic property calculations reveal the formation of electron-deficient Ni clusters on the TiO2 surface, which provides multiple active sites for N2 adsorption and activation. Theoretical calculation identifies the strongest activated configuration of N2* on the catalysts and confirms the potential-limiting step in the nitrogen reduction reaction (NRR). On Ni4-TiO2(101), N2* → NNH* is the potential-limiting step with a very small free energy increase (ΔG) of 0.24 eV (the corresponding overpotential is 0.33 V), while on Ni13-TiO2(101) the potential-limiting step occurs at NH* → NH2* with ΔG of 0.49 eV (the corresponding overpotential is 0.58 V). Moreover, the Nin-TiO2(101) catalyst, especially Ni13-TiO2(101), involves in a highly selective NRR even at the corresponding NRR overpotential. This work will enlighten material design to construct metal oxide supported transition metal clusters for the highly efficient NRR and NH3 synthesis.

Understanding the effect of interfacial interaction on metal/metal oxide electrocatalysts for hydrogen evolution and hydrogen oxidation reactions on the basis of first-principles calculations

Interfacial M–O and M–TM interactions of M/TMO contribute differently to the surface properties and HER/HOR pathway.

Low-temperature preferential oxidation of CO over Ag monolayer decorated Mo2C (MXene) for purifying H2

Selective removal of CO from reformate streams at low temperature is an important issue to be solved for anode catalysts of proton exchange membrane fuel cells (PEMFCs). Using density functional theory, we demonstrate that the Ag monolayer decorated Mo₂C can be used as an effective low cost catalyst with high activity, selectivity and stability for preferential oxidation of CO. Ag monolayer decorated Mo₂C can also be used as a filter membrane for separating H₂ and CO. Meanwhile, the high selectivity is achieved because H₂ hardly takes part in the reaction, and the good activity is obtained since the CO with moderate binding strength could be smoothly removed at various CO concentration. We propose a facile route for removing CO by injecting low O₂ supply because it can resist CO poisoning with promoted activity for CO oxidation at higher CO concentration. We anticipate that Ag monolayer decorated Mo₂C can be used as a promising H₂ purifying pretreater by preferential oxidation of CO connecting to the anode of PEMFCs. We hope the present results could advance the development of catalysts for purifying H₂ and inspire more researches on the application of MXene catalysts.

A Au monolayer on WC(0001) with unexpected high activity towards CO oxidation

Catalysts with weak adsorption yet high reactivity towards CO are urgently required to solve the serious problem of CO poisoning that occurs in many important reactions, e.g., in fuel cells. Using the combination of density functional calculations and ab initio molecular dynamic simulations, we found a promising electrocatalyst for this purpose: a Au monolayer on WC(0001) (Au_ML/WC), which has both high oxygen reduction activity and high tolerance to CO poisoning. The advantages of using Au_ML/WC as an electrocatalyst in fuel cells are demonstrated through analyses of energetics of different reaction steps as well as interaction properties of reactants and products. We anticipate that the present results are useful to advance the development of efficient catalysts with high tolerance to CO poisoning.

Defect Effects on TiO2 Nanosheets: Stabilizing Single Atomic Site Au and Promoting Catalytic Properties

Isolated single atomic site catalysts have attracted great interest due to their remarkable catalytic properties. Because of their high surface energy, single atoms are highly mobile and tend to form aggregate during synthetic and catalytic processes. Therefore, it is a significant challenge to fabricate isolated single atomic site catalysts with good stability. Herein, a gentle method to stabilize single atomic site metal by constructing defects on the surface of supports is presented. As a proof of concept, single atomic site Au supported on defective TiO₂ nanosheets is prepared and it is discovered that (1) the surface defects on TiO₂ nanosheets can effectively stabilize Au single atomic sites through forming the Ti-Au-Ti structure; and (2) the Ti-Au-Ti structure can also promote the catalytic properties through reducing the energy barrier and relieving the competitive adsorption on isolated Au atomic sites. It is believed that this work paves a way to design stable and active single atomic site catalysts on oxide supports.

Phosphomolybdic acid supported single-metal-atom catalysis in CO oxidation: first-principles calculations

Highly active phosphomolybdic acid supported single-metal-atom catalysts for CO oxidation.

Ag/AgxH3-xPMo12O40 Nanowires with Enhanced Visible-Light-Driven Photocatalytic Performance

Photocatalysis, a promising technology platform to address the environmental problems, has been attracting considerable attention. In this paper, Ag/Ag_xH_3-xPMo₁₂O₄₀ (simplified as Ag/AgHPMo₁₂) nanowires have been synthesized by a facile solid reaction route and in situ photodeposited method. The results of SEM and TEM indicate that the diameters of AgHPMo₁₂ nanowires are about 45 ± 10 nm, and Ag nanoparticles with diameters in the range of 5-15 nm are uniformly anchored on the surface of AgHPMo₁₂ nanowires. The Ag content in the Ag/AgHPMo₁₂ composite was manipulated by the light irradiation time (Ag/AgHPMo₁₂-x; x stands for the irradiation time; x = 2, 4, 6, 8 h, respectively). With increasing irradiation time, the light absorption of as-synthesized samples in the visible region was gradually enhanced. The Ag/AgHPMo₁₂-4 exhibits the best photocatalytic performance for the degradation of methyl orange and reduction of Cr₂O₇^2- under visible-light (λ > 420 nm) irradiation. The study of the photocatalytic mechanism reveals that both Ag and AgHPMo₁₂ can be excited by visible light. The photoinduced electrons were transferred from AgHPMo₁₂ to metallic Ag, and combined with the Ag plasmonic holes. The Ag plasmonic electrons were trapped by O₂ to form ·O₂^-, or directly reduced Cr₂O₇^2- to Cr³⁺. Meanwhile, the ·O₂^- species and the photogenerated holes of AgHPMo₁₂ were used to oxidize MO or i-PrOH; thus, they showed highly efficient and recyclable photocatalytic performance for removing the organic and inorganic pollutants.