Selective catalytic oxidation of DMF over Cu-Ce/H-MOR by modulating the surface active sites.

Selective catalytic oxidation of the hazardous DMF exhaust gas presents a significant challenge in balancing oxidation activity and products selectivity (CO, NOx, N2, etc.). It is found that Cu/H-MOR demonstrates superior performance for DMF oxidation compared to CuO on other supports (γ-Al2O3, HY, ZSM-5) in terms of product selectivity and stability. The geometric and electronic structures of CuO active sites in Cu/H-MOR have been regulated by CeO2 promoter, leading to an increase in the ratio of active CuO (highly dispersed CuO and Cu+ specie). As a result, the oxidation activity and stability of the Cu/H-MOR catalyst were enhanced for DMF selective catalytic oxidation. However, excessive CuO or CeO2 content led to decreased N2 selectivity due to over-high oxidation activity. It is also revealed that Ce3+ species, active CuO species, and surface acid sites play a critical role in internal selective catalytic reduction reaction during DMF oxidation. The 10Cu-Ce/H-MOR (1/4) catalyst exhibited both high oxidation activity and internal selective catalytic reduction activity due to its abundance of active CuO specie as well as Ce3+ species and surface acid sites. Consequently, the 10Cu-Ce/H-MOR (1/4) catalyst demonstrated the widest temperature window for DMF oxidation with high N2 selectivity. These findings emphasize the importance of surface active sites modification for DMF selective catalytic oxidation.

Hydrogen fluoride adsorption and reaction on the α-Al2O3(0001) surface: a density functional theory study.

The adsorption and reaction behaviors of HF on the α-Al(2)O(3)(0001) surface are systematically investigated using density functional theory method. By increasing the number of HF molecules in a p(2 × 1) α-Al(2)O(3)(0001) slab, we find that HF is chemically dissociated at low coverage; while both physical and dissociative adsorption occurs at a 3/2 monolayer (ML) coverage. At the same coverage (1.0 ML), diverse configurations of the dissociated HF are obtained in the p(2 × 1) model; while only one is observed in the p(1 × 1) slab due to its smaller surface area compared with the former one. Preliminary fluorination reaction study suggests that the total energy of two dissociated HF in the p(2 × 1) slab increases by 1.00 and 0.72 eV for the formation and desorption of water intermediate, respectively. The coadsorption behaviors of HF and H(2)O indicate that the pre-adsorbed water is unfavorable for the fluorination of Al(2)O(3), which is well consistent with the experimental results. The calculated density of states show that the peak of σ(H-F) disappears, while the peaks of σ(H-O) and σ(Al-F) are observed at -8.4 and -5 to -3 eV for the dissociated HF. Charge density difference analysis indicates that the dissociated F atom attracts electrons, while no obvious changes on electrons are observed for the surface Al atoms.

A DFT study of the structures of Au(x) clusters on a CeO2(111) surface.

Studying the structures of metal clusters on oxide supports is challenging due to their various structural possibilities. In the present work, a simple rule in which the number of Au atoms in different layers of Au(x) clusters is changed successively is used to systematically investigate the structures of Au(x) (x=1-10) clusters on stoichiometric and partially reduced CeO(2)(111) surface by DFT calculations. The calculations indicate that the adsorption energy of a single Au atom on the surface, the surface structure, as well as the Au-Au bond strength and arrangement play the key roles in determining Au(x) structures on CeO(2)(111). The most stable Au(2) and Au(3) clusters on CeO(2)(111) are 2D vertical structures, while the most stable structures of Au(x) clusters (x>3) are generally 3D structures, except for Au(7). The 3D structures of large Au(x) clusters in which the Au number in the bottom layer does not exceed that in the top layer are not stable. The differences between Au(x) on CeO(2)(111) and Mg(100) were also studied. The stabilizing effect of surface oxygen vacancies on Au(x) cluster structures depends on the size of Au(x) cluster and the relative positions of Au(x) cluster and oxygen vacancy. The present work will be helpful in improving the understanding of metal cluster structures on oxide supports.

Study of defect sites in Ce1-xMxO2-δ (x = 0.2) solid solutions using Raman spectroscopy.

A series of Ce(1-x)M(x)O(2-δ) (M = Gd, Zr, La, Sm, Y, Lu, and Pr) samples were characterized by Raman spectroscopy to investigate the evolution of defect sites (oxygen vacancies and MO(8)-type complex) and their distributions in the samples. It was found that the evolution of oxygen vacancies was due to the different ionic valence state of dopant from that of Ce(4+), while the evolution of the MO(8)-type complex was due to the different ionic radius of dopant from that of Ce(4+). The distributions of defect sites were investigated using 325 and 514 nm excitation laser lines, indicating that the defect sites were surface enriched. Moreover, the increasing ordering level of the sample led to a decline in the concentration of the MO(8)-type complex in the sample but the constant concentration of oxygen vacancies, implying that the metastable MO(8)-type complex species were more disordered compared to the oxygen vacancies.

Raman spectroscopic study on the structure in the surface and the bulk shell of Ce(x)Pr(1-x)O(2-delta) mixed oxides.

The difference between the surface and the bulk shell of Ce(x)Pr(1-x)O(2-delta) mixed oxides was studied by Raman spectroscopy with four different excitation lasers. Two Raman peaks appear at 465 and 570 cm(-1) under all of the four lasers. The former is attributed to the Raman active F(2g) mode of CeO2, while the latter is attributed to oxygen vacancy. On the basis of the fact that the laser with shorter wavelength is closer to the electronic adsorption of samples, it is found that the Raman information detected by excitation laser with shorter wavelength is more sensitive to the surface region of samples. An inflection is observed in the relationship of the value I570/I465 to the Ce content in Ce(x)Pr(1-x)O(2-delta). With the increase in the wavelength of excitation laser, the Ce content corresponding to the inflection decreases. Combined with the surface concentration obtained by XPS, it can be deduced that the composition of Ce(x)Pr(1-x)O(2-delta) mixed oxide particles in the surface region and the bulk shell are different, the former is enrichment of Pr component and the latter is enrichment of Ce component. The thickness of the surface layer with rich Pr component decreases with the increase in the Ce content.

[XRD and Raman spectroscopic comparative study on phase transformation of gamma-Al2O3 at high temperature].

The phase transformation of gamma-Al2O3 at high temperature was characterized by XRD and Raman techniques. Raman results show that under the excitation at 632.8 nm, the Raman shift bands at 1175 and 1241 cm(-1) and the other two bands at 1370 and 1 400 cm(-1) are attributed to the Cr3+ and (or) Fe3+ fluorescence bands in theta-Al2 03 and alpha-Al2O3 environments respectively. Compared with XRD measurements, it is more sensitive to use these two groups of Raman bands to estimate the phase formation and transformation of theta-Al2O3 and alpha-Al2O3. Therefore, Raman spectroscopy can deduce the high temperature phase transformation effectively. Experiments results show that gamma-Al2O3 starts to transform to theta-Al2O3 and alpha-Al2O3 synchronously at 800 degrees C, but the nature of the sample determines the final temperature of transforming to alpha-Al2O3.