Search

Ir nanoparticles (NPs) were synthesized viaa colloidal method using tetradecyltrimethyl ammonium bromide (TTAB) as the ligand, and the supported Ir/BN catalysts were employed for selective hydrogenation of crotonaldehyde (CRAL). It was found that by proper thermal treatments, the TTAB-capped Ir NPs were very active and selective for the reaction. The Ir/BN catalyst calcined at 300 °C with surface TTAB residue gave a quasi-steady state turnover frequency (TOF) for crotyl alcohol (CROL) formation of 0.02 s−1and a CROL selectivity of 94.4%, while that calcined at 500 °C with clean surface gave a TOF of 0.004 s−1and a CROL selectivity of 56.2%. The Ir-TTAB interface was essential for the enhanced performance. In-situIR spectra along with kinetic investigation revealed that TTAB improved CRAL adsorption and strengthened C=O adsorption, which accounted for high activity and selectivity. However, high coverage of TTAB on the Ir surface suppressed the H2adsorption and consequently lowered the activity. Thus the findings provided useful information on the design of efficient catalysts for selective hydrogenation.

Various Cu2O nanocrystals (NCs), including cubes exposing {100} crystal planes (c-Cu2O), octahedra exposing {111} crystal planes (o-Cu2O), and rhombic dodecahedra exposing {110} crystal planes (d-Cu2O), were employed to study the morphology-dependent reduction kinetics and surface copper species evolution of Cu2O catalyst under a reductive CO atmosphere. Remarkable morphology dependence of Cu2O NCs reduced by CO was observed that varies with CO concentration. When CO concentrations are 0.1 and 1%, the reduction of Cu2O is dominated by the density of CO reduction sites and thus o-Cu2O NCs exhibit the strongest reducibility due to their better CO adsorption capacity, while the strongest reducibility was observed on c-Cu2O NCs at CO concentrations of 5 and 10% due to the reduction dominantly switching to the diffusivity of subsurface/bulk lattice O into the surface. X-ray photoelectron spectroscopy (XPS) results without exposure to air display that metallic Cu is easily enriched on the o-Cu2O surface but the surface copper species is exclusively composed of Cu(I) on c-Cu2O NCs during the CO reduction process. Moreover, the reducibility of o-Cu2O NCs was significantly influenced by H2O while that of c-Cu2O NCs was hardly affected, which was mainly ascribed to their distinctly different surface compositions. These results not only clearly reveal the morphology-dependent reducibility of Cu2O NCs in different CO concentrations but also provide direct evidence in efficiently guiding the in situ adsorbate-induced restructuring process of Cu2O in CO-involved reduction reactions.

A series of Co–Cr–O mixed oxides with different Co/Cr molar ratios are synthesized and tested for the total oxidation of propane. The reaction behaviors are closely related to the structural features of the mixed oxides. The catalyst with a Co/Cr molar ratio of 1:2 (1Co2Cr) and a spinel structure has the best activity (with a reaction rate of 1.38 μmol g−1s−1at 250 °C), which is attributed to the synergistic roles of its high surface acidity and good low-temperature reducibility, as evidenced by the temperature-programmed desorption of ammonia, reduction of hydrogen, and surface reaction of propane. Kinetic study shows that the reaction orders of propane and oxygen on the 1Co2Cr catalyst (0.58 ± 0.03 and 0.34 ± 0.05, respectively) are lower than those on the 2Co1Cr catalyst (0.77 ± 0.02 and 0.98 ± 0.16, respectively) and 1Co5Cr (0.66 ± 0.05 and 1.30 ± 0.11, respectively), indicating that the coverages of propane and oxygen on 1Co2Cr are higher than those on the other catalysts due to its higher surface acidity and higher reducibility. In addition, in-situ diffuse reflectance infrared spectroscopic investigation reveals that the main surface species on 1Co2Cr during the reaction are polydentate carbonate species, which accumulate on the surface at low temperatures (< 250 °C) but decompose at relatively high temperatures.

A series of FeOx-promoted Ir/SiO2catalysts were prepared and tested for gas phase selective hydrogenation of crotonaldehyde. It was found that a catalyst containing Ir nanoparticles contacting with highly dispersed FeOxclusters (3Ir/0.1Fe/SiO2) showed excellent activity and stability, with a 5-fold enhanced steady state crotyl alcohol yield (59.6%) compared to that of the bare Ir/SiO2(12.4%), while a catalyst promoted with high content of Fe (3Ir/3.5Fe/SiO2) had high initial activity but deactivated rapidly. A catalyst with similar highly dispersed FeOxclusters but prepared by an inverse impregnation sequence (0.1Fe/3Ir/SiO2) also suffered severe deactivation. Various characterizations revealed that the observed behaviors were closely related to the Ir–FeOxinteractions induced by different morphologies of the catalysts. The active sites generated at Ir–FeOxinterface were responsible for the better performance. The catalyst deactivation was attributed to the deposit of heavy compound and strong adsorption of CO on the surface, which was induced by the strong Ir–FeOxinteraction due to heavy decoration of FeOxon the Ir surface.

Three CuO–MnOx–CeO2catalysts with different impregnation sequences (i.e. MnOx/CuO/CeO2, CuO/MnOx/CeO2and CuO–MnOx/CeO2) were prepared and the effects of impregnation sequences on the structures and catalytic behaviors of these catalysts were investigated. It was found that the MnOx/CuO/CeO2possessed the largest amount of oxygen vacancies but the lowest reducibility; the CuO/MnOx/CeO2had the largest Cu+contents but the lowest amount of oxygen vacancies; the CuO–MnOx/CeO2catalyst had the highest CuO dispersion and the best reducibility, along with moderate amount of oxygen vacancies and Cu+contents on the surface. The kinetic studies revealed that the apparent activation energies of CO oxidation over the CuO–MnOx/CeO2, MnOx/CuO/CeO2and CuO/MnOx/CeO2were 49.5, 51.8 and 73.8 kJ mol−1, in order, and the activities followed an order of CuO–MnOx/CeO2> MnOx/CuO/CeO2> CuO/MnOx/CeO2. The highest performance of the CuO–MnOx/CeO2was ascribed to the highly dispersed CuO species and the mobility of lattice oxygen.

Catalytic selective hydrogenation of α,β-unsaturated aldehydes to α,β-unsaturated alcohols is very important in the synthesis of various fine chemicals. However, the development of highly efficient catalyst systems is challenging because of the low selectivity and severe deactivation of the currently employed catalysts such as Pt and Au. In this work, a series of CrOx- and FeOx-promoted Ir/SiO2catalysts were prepared by a sequential impregnation method and tested for gas phase selective hydrogenation of crotonaldehyde. It was found that the addition of promoters could greatly enhance the catalytic performance. The Ir/SiO2catalyst promoted with combined CrOx–FeOxwith a (Cr + Fe)/Ir ratio of 0.05 showed the highest steady state crotyl alcohol yield of 64.5% at a selectivity of 86%, which is 4-fold higher than that of the unpromoted Ir/SiO2(15.5%) catalyst. Such an enhancement was due to the formation of new active sites generated at the Ir–CrOxand/or Ir–FeOxinterfaces. Also, catalysts with low loadings of promoters showed excellent stability due to their appropriate electronic properties, while the catalyst with a high loading of promoters deactivated quickly due to the strong adsorption of products on the surface.

Abstract: Ru catalysts supported on ZnO with different Ru contents were prepared by an impregnation method and were applied to the vapor-phase selective hydrogenation of crotonaldehyde. The catalysts were characterized by X-ray powder diffraction (XRD), NH3 temperature-programmed desorption (NH3-TPD), transmission electron microscopy (TEM) and temperature-programmed oxidation (TPO). It was found that with increasing Ru contents in the Ru/ZnO catalysts, the activity (TOF), surface acidity amount and deactivation rate increased and the selectivity to crotyl alcohol increased first and then decreased. The 3Ru/ZnO catalyst showed the highest selectivity to crotyl alcohol (up to 88.0%) for the hydrogenation of crotonaldehyde. The initial TOF values of the catalysts depended on the strength of surface acidity and the Ru particle sizes. The more Lewis acid sites made catalysts deactivate more easily. It was assumed that the deactivation was due to the formation of organic compounds deposition and poison effect of CO strongly adsorbed on the Ru atoms. [Copyright &y& Elsevier]

The PdO/Ce1–xPdxO2−δcatalyst prepared by a solution-combustion method contained free surface PdO species and PdO species in Ce1–xPdxO2−δsolid solution, whereas the PdO/CeO2catalyst prepared by an impregnation method contained only free surface PdO species. The free surface PdO species could be removed by nitric acid. Contributions of the PdO species to catalytic CO oxidation were quantitatively evaluated. The free surface PdO species in the PdO/Ce1–xPdxO2−δcatalyst had the highest activity (969.3 μmolCOgPd–1s–1), those in the PdO/CeO2catalyst had medium activity (109.0 μmolCOgPd–1s–1), and the PdO species in the Ce1–xPdxO2−δsolid solution had the lowest activity (13.2 μmolCOgPd–1s–1). Synergetic effects of PdO species were responsible for the enhanced reactivity of the PdO/Ce1–xPdxO2−δcatalyst, as the free surface PdO species provided CO chemisorption sites and the Ce1–xPdxO2−δsolid solution generated more oxygen vacancies for oxygen activation.

A series of CuO/CeO2and inverse CeO2/CuO catalysts were prepared by an incipient wetness impregnation method and tested for CO oxidation. Crystallite sizes of CeO2and CuO were evaluated by X-ray diffraction and N2O chemisorption, as well as transmission electron microscopy. It was found that a CuO(5)/CeO2-500 catalyst with a CuO crystallite size of 4.1 nm and a CeO2(5)/CuO-500 catalyst with a CeO2crystallite size of 4.0 nm had identical activities, indicating that the reaction may occur at the interface of CuO−CeO2. According to the turnover frequency based on CuO sites located on the CuO−CeO2interface, the activity on the larger CuO crystallite was much higher than that on the smaller one, indicating that CuO−CeO2catalyst for CO oxidation is structure-sensitive. The enhanced activity was ascribed to a higher density of chemisorbed CO on the active sites for the larger CuO crystallite.

Abstract: A dual-functional Pd/CuO-CeO2-Y2O3 (Pd/CCY) using CuO-CeO2 as precursor was prepared and tested for catalytic combustion of toluene and ethyl acetate. It was found that the catalyst is much higher active and thermally stable for combustion of both toluene and ethyl acetate, compared with the catalyst prepared with the conventional method. Therefore, the formation of the CuO-CeO2 solid solution before coating is the key to high activity and thermal stability. Moreover, Pd/PdO species are the active phase for oxidation of toluene, while CuO or CuO-CeO2 is responsible for oxidation of ethyl acetate, and the formation of the Pd probably impels the enhancement of activity for the catalyst calcined at 1000°C. [Copyright &y& Elsevier]

Ce 0.9Pr 0.1O 2-δ, Ce 0.95Cu 0.05O 2-δ, and Ce 0.9Pr 0.05Cu 0.05O 2-δmixed oxides and pure CeO 2were prepared with a sol−gel method and were characterized by XRD, in situ Raman, and in situ DRIFTS techniques. The XRD results confirmed the formation of Ce−Pr−O solid solution. The Raman results indicated that a higher concentration of oxygen vacancies was obtained on the Pr-doped samples compared to the Ce 0.95Cu 0.05O 2-δand pure CeO 2samples. Surface chemical states of the Ce 0.9Pr 0.1O 2-δand Ce 0.9Pr 0.05Cu 0.05O 2-δmixed oxides were determined by in situ Raman spectroscopy, which indicated that the surfaces of the two mixed oxides were both close to oxidation state during the reaction, despite of the presence of reducing reactant CO in the gas mixture. The in situ DRIFTS results evidenced the chemisorption of CO in the Cu-containing samples. The catalysts were tested for CO oxidation, and it was found that the enhanced reactivity was closely related to the higher concentrations of the oxygen vacancies and the chemisorbed CO in the catalysts, due to the fact that the oxygen vacancies provide activation centers for O 2and the Cu +ions provide chemisorption sites for CO.

A Ce0.9Pr0.1O2-solid solution was prepared by a sol−gel method. Changes in microstructure of the solid solution under different atmospheres (O2, He, and H2) and temperatures were characterized by an in situ X-ray diffraction (XRD) technique. Raman peaks at 460 cm-1ascribed to the F2gvibration mode of CeO2in the fluorite structure and at 570 cm-1ascribed to oxygen vacancies in the solid solution were studied by in situ Raman spectroscopy using 785- and 514-nm excitation laser lines, providing bulk and surface information, respectively. With a 785-nm laser line, the A570/A460ratio reflecting the oxygen vacancies concentration increased under O2and He while it first increased and then decreased under H2with increasing temperature. With a 514-nm excitation laser line, the A570/A460ratio decreased with increasing temperature under all atmospheres. The growth of the A570/A460ratio under the 785-nm laser line was due to the positive effects of high temperature and high concentration of oxygen vacancies and the negative effect of reduction of the sample under reducing atmospheres (He and H2), while the decline in the A570/A460ratio under 514 nm was due to the dominant negative effect of the migration of surface Pr from surface to bulk during the heating process.

Nanosized CuO−CeO2catalysts with high surface area (>90 m2g-1) were prepared by a modified citrate sol−gel method with incorporation of N2thermal treatment. CO temperature-programmed reduction results indicated that there are three CuO species in the catalyst, namely, the finely dispersed CuO, the bulk CuO, and the Cu2in the CeO2lattice. Using CO oxidation as a model reaction, catalytic activity of each species was evaluated. It was found that the finely dispersed CuO species had the highest activity (183.3 mmolCOgCu-1h-1) and the bulk CuO had medium activity (100.4 mmolCOgCu-1h-1), while the Cu2in the CeO2lattice had the lowest activity (21.3 mmolCOgCu-1h-1). Furthermore, the Cu2species in the CeO2lattice could migrate to the catalyst surface to form finely dispersed CuO under high-temperature calcination or under reaction conditions, which could consequently enhance the catalytic activity.