CeO2 nanorods supported CuOx-RuOx bimetallic catalysts for low temperature CO oxidation

Bimetallic catalysts often outperform monometallic catalysts due to changeable structural orientation, synergistic effects, and integration of two different metal or metal oxide properties. Here, a series of CeO2 nanorods (NR) supported bimetallic CuOx and RuOx catalysts (Cu: Ru ratios of 9:1, 7:3, and 5:5) were prepared using a wet impregnation method. In situ DRIFTS, H2 temperature programmed reduction (H2-TPR), CO temperature programmed desorption (CO-TPD), and other characterization techniques were used to investigate the effect of the Cu:Ru ratio on the activity of low-temperature CO oxidation. Among three catalysts, CeO2 NR supported 7 wt% Cu-3 wt% Ru catalyst after a reduction activation treatment showed the best performance with 100 % CO conversion at 166 °C and the lowest activation energy of 18.37 kJ mol-1. Raman and XPS profiles revealed that the origin of the superior performance is at least partially related to the high surface oxygen vacancy concentration and other distinct oxygen species (physi-/chemi-sorbed oxygen and bulk lattice oxygen), leading to outstanding adsorption and oxidation property of CO.

Evolution hydrothermal aging mechanism for Ag/CeO2 catalysts in regeneration of catalytic diesel particulate filter with DFT calculation

In order to avoid the high cost of existing precious metal catalyst like Pt, Ag/CeO₂ was the most promising catalysts for mobile source soot emission control technologies, but there was a clear trade-off between hydrothermal aging resistance and catalytic oxidation performance hindered the application of this catalyst. In order to reveal the hydrothermal aging mechanism of Ag/CeO₂ catalysts, the TGA (thermogravimetric analysis) experiments were investigated to reveal the mechanism of Ag modification on catalytic activity of CeO₂ catalyst between fresh and hydrothermal aging and were also characterized with the related characterization experiments to in-depth research the lattice morphology and valence changes. The degradation mechanism of Ag/CeO₂ catalysts in vapor with high-temperature was also explained and demonstrated based on density functional and molecular thermodynamics theories. The experimental and simulation data showed that the catalytic activity of soot combustion within Ag/CeO₂ decreased more significantly after hydrothermal aging than CeO₂ due to the less agglomerated, which caused by the decreased in O_II/O_I and Ce³⁺/Ce⁴⁺ compared with CeO₂. As shown in density function theory (DFT) calculation, the decreased surface energy and the increased oxygen vacancy formation energy of the low Mille index surface after Ag modification led to the instability structure and the high catalytic activity. Ag modification also increased the adsorption energy and Gibbs free energy of H₂O on the low Miller index surface compared to CeO₂, indicating that the desorption temperature of H₂O molecules in (1 1 0) and (1 0 0) was higher than (1 1 1) in CeO₂ and Ag/CeO₂, which led to the migration of (1 1 1) crystal surfaces to (1 1 0) and (1 0 0) in the vapor environment. These conclusions can provide a valuable addition to the regenerative application of Ce-based catalysts in diesel exhaust aftertreatment system the aerial pollution.

A computational study on the adsorption of arsenic pollutants on graphene-based single-atom iron adsorbents

Integrated gasification combined cycle (IGCC) is a promising clean technology for coal power generation; however, the high volatility and toxicity of arsenic pollutants (As₂, As₄, AsO and AsH₃) released from an IGCC coal plant cause serious damage to human health and the ecological environment. Therefore, highly efficient adsorbents for simultaneous treatment of multiple arsenic pollutants are urgently needed. In this work, the adsorption characteristics and competitive adsorption behaviors of As₂, As₄, AsO, and AsH₃ on four kinds of graphene-based single-atom iron adsorbents (Fe/GA) were systematically investigated through density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. The results suggest that single-vacancy Fe/GA doped with three nitrogen atoms has the largest adsorption ability for As₂, As₄, AsO and AsH₃. The adsorption energies of As₂, AsO and As₄ on Fe/GA depend on both charge transfer and orbital hybridization, while the adsorption energy of AsH₃ is mainly decided by electronic transfer. The adsorption differences of As₂, As₄, AsO and AsH₃ on four Fe/GA adsorbents can be explained through the obvious linear relationship between the adsorption energy and Fermi softness. As₂, As₄, AsO and AsH₃ will compete for adsorption sites when they exist on the same adsorbent surface simultaneously, and the adsorption capacities of AsO and As₂ are relatively stronger. After the competitive adsorption between AsO and As₂, AsO occupies the adsorption site at 300-900 K. This theoretical work suggests that Fe/GA is a promising adsorbent for the simultaneous removal of multiple arsenic pollutants with high adsorption capacity and low cost.

Mechanism analysis of Au, Ru noble metal clusters modified on TiO2 (101) to intensify overall photocatalytic water splitting

Accelerating the separation and migration of photo-carriers (electron-hole pairs) to improve the photo-quantum utilization efficiency in photocatalytic overall water splitting is highly desirable. Herein, the photo-deposition of Ru or Au noble metal clusters with superior electronic properties as a co-catalyst on the (101) facet of anatase TiO2 and the mechanism of intensifying the photocatalysis have been investigated by calculation based density functional theory (DFT). As a result, the as-synthesized Ru/TiO2 and Au/TiO2 exhibit high hydrogen evolution reaction (HER) activity. Such a greatly enhanced HER is attributed to the interfacial interactivity of the catalysts due to the existence of robust chemical bonds (Ru-O-Ti, Au-O-Ti) as electron-traps that provide the photogenerated electrons. In addition, the formation of new degenerate energy levels due to the existence of Ru-4d and Au-5d electronic impurity states leads to the narrowing of the band gap of the catalysts. In addition, the as-synthesized Au/TiO2 exhibits more faster HER rate than Ru/TiO2, which is attributed to the effects of surface plasmon resonance (SPR) as a synergistic effect of plasmon-induced 'hot' electrons that enhance the harvesting of the final built-in electric field and promote the migration and separation of the photo-carriers, which efficiently facilitates hydrogen evolution from the photocatalytic overall water splitting reaction.

CeO2(111) electronic reducibility tuned by ultra-small supported bimetallic Pt-Cu clusters

Controlling Ce4+ to Ce3+ electronic reducibility in a rare-earth binary oxide such as CeO2 has enormous applications in heterogeneous catalysis, where a profound understanding of reactivity and selectivity at the atomic level is yet to be reached. Thus, in this work we report an extensive DFT-based Basin Hopping global optimization study to find the most stable bimetallic Pt-Cu clusters supported on the CeO2(111) oxide surface, involving up to 5 atoms in size for all compositions. Our PBE+U global optimization calculations indicate a preference for Pt-Cu clusters to adopt 2D planar geometries parallel to the oxide surface, due to the formation of strong metal bonds to oxygen surface sites and charge transfer effects. The calculated adsorption energy values (Eads) for both mono- and bimetallic systems are of the order of 1.79 up to 4.07 eV, implying a strong metal cluster interaction with the oxide surface. Our calculations indicate that at such sub-nanometer sizes, the number of Ce4+ surface atoms reduced to Ce3+ cations is mediated by the amount of Cu atoms within the cluster, reaching a maximum of three Ce3+ for a supported Cu5 cluster. Our computational results have critical implications on the continuous understanding of the strong metal-support interactions over reducible oxides such as CeO2, as well as the advancement of frontier research areas such as heterogeneous single-atom catalysts (SAC) and single-cluster catalysts (SCC).

Effects of ensembles, ligand, and strain on adsorbate binding to alloy surfaces

Alloying elements with strong and weak adsorption properties can produce a catalyst with optimally tuned adsorbate binding. A full understanding of this alloying effect, however, is not well-established. Here, we use density functional theory to study the ensemble, ligand, and strain effects of close-packed surfaces alloyed by transition metals with a combination of strong and weak adsorption of H and O. Specifically, we consider PdAu, RhAu, and PtAu bimetallics as ordered and randomly alloyed (111) surfaces, as well as randomly alloyed 140-atom clusters. In these alloys, Au is the weak-binding component and Pd, Rh, and Pt are characteristic strong-binding metals. In order to separate the different effects of alloying on binding, we calculate the tunability of H- and O-binding energies as a function of lattice constant (strain effect), number of alloy-substituted sublayers (ligand effect), and randomly alloyed geometries (ensemble effect). We find that on these alloyed surfaces, the ensemble effect more significantly tunes the adsorbate binding as compared to the ligand and strain effects, with the binding energies predominantly determined by the local adsorption environment provided by the specific triatomic ensemble on the (111) surface. However, we also find that tuning of adsorbate binding from the ligand and strain effects cannot be neglected in a quantitative description. Extending our studies to other bimetallics (PdAg, RhAg, PtAg, PdCu, RhCu, and PtCu), we find similar conclusions that the tunability of adsorbate binding on random alloys is predominately described by the ensemble effect.