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/SiO2 catalysts 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/SiO2 catalyst promoted with combined CrOx–FeOx with 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–CrOx and/or Ir–FeOx interfaces. 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.

A series of FeOx-promoted Ir/SiO2 catalysts were prepared and tested for gas phase selective hydrogenation of crotonaldehyde. It was found that a catalyst containing Ir nanoparticles contacting with highly dispersed FeOx clusters (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 FeOx clusters 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–FeOx interactions induced by different morphologies of the catalysts. The active sites generated at Ir–FeOx interface 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–FeOx interaction due to heavy decoration of FeOx on the Ir surface.

Highly ordered 2D and 3D-Co3O4 catalysts were prepared using SBA-15 and KIT-6 as templates. Nano-Co3O4 catalyst was obtained by calcination of cobalt nitrate as a comparison. The BET surface area of nano- Co3O4, 2D-Co3O4 and 3D-Co3O4 catalysts was 16.2, 63.9 and 75.1 m2/g, respectively. All the catalysts were tested for the total combustion of methane and their catalytic performance was in order of 3D-Co3O4(T90=355 °C)>2D-Co3O4 (T90=383 °C)>nano-Co3O4(T90=455 °C). It was also found that the order of the areal specific reaction rates for the combustion of methane follow the same order of total activity. The characterization result demonstrated that enhanced catalytic performance of methane of the 2D-Co3O4 and 3D-Co3O4 catalysts was due to their pronounced reducibility and abundant active Co3+ species which was caused by the preferential exposure of {220} crystal planes in 3D-Co3O4 and 2D-Co3O4 catalysts compared to the nano-Co3O4.

The manufacture of 1,1-dichloroethylene(1,1-DCE) usually employs liquid phase method to perform the dehydrochlorination of 1,1,2-trichloroethane(TCE), where large amounts of high-concentration salty wastewater is produced inevitably. It has been a long-term goal to achieve the gas phase synthesis of 1,1-DCE via supported catalysts. In this work, the gas-phase synthesis of 1,1-DCE from TCE was studied in the presence of pentaethylenehexamine( PEHA) supported on silica. High and stable selectivity to 1,1-DCE(up to 98%) was obtained, which could be ascribed to the relatively strong basicity of PEHA according to a proposed E2 mechanism. The formation of PEHA chloride from the HCl generated in situ was detected and was considered to be the main reason for the deactivation of PEHA catalyst.

A series of Pd/AlF3 catalysts was prepared by an impregnation method and tested for vapor-phase dehydrofluorination of 1,1,1,3,3-pentafluoropropane(HFC-245fa) to synthesize 1,3,3,3-tetrafluoropropene(HFO-1234ze). The highest activity was obtained over Pd/AlF3 catalyst containing 1.0%(mass fraction) of Pd, with an HFC-245fa conversion of 79.5% and an HFO-1234ze selectivity of 99.4% after the reaction at 300 °C for 100 h. The reactivity was related to the surface acidity, as AlF3 provided active sites for the reaction. With the addition of Pd, the catalyst stability could be significantly improved. Raman spectroscopic and thermal-gravimetric analysis results reveal that there was less carbon deposit on spent Pd/AlF3 catalyst surface because Pd could effectively pyrolyse it. Thus, Pd/AlF3 catalysts were bi-functional for dehydrofluorination of HFC-245fa.

A series of Pd/La-Al2O3 (PLA) catalysts with La-Al2O3 (LA) support calcined at different temperatures (500, 700, 900 and 1050 °C) were prepared using an incipient wetness impregnation method. The activity of the fresh and hydrothermally aged PLA catalysts were tested for total oxidation of CO and C3H8. The activity of the fresh PLA catalysts for CO and C3H8 oxidation increased with increasing calcination temperature of the support, while the activities of the aged catalysts declined and became essentially the same. CO chemisorption results revealed that the suppressed activities of the aged catalysts were mainly due to the decline of palladium dispersion. The turnover frequency (TOF) of CO oxidation increased with increasing reduction ability of the catalysts, with a fresh catalyst calcined at 1050 °C having the highest value (0.048 s−1). However, the TOF of C3H8 total oxidation was affected by not only the redox properties of catalysts but also the size of Pd particle, and large Pd particles possessed higher TOF value of C3H8 oxidation, with the highest value (0.125 s−1) being obtained on an aged catalyst calcined at 500 °C.CO and C3H8 total oxidation over fresh and aged PLA catalysts

•Reduction temperature has crucial impact on hydrogenation of crotonaldehyde.•The Ru–Ir/ZnO-200 catalyst shows the highest activity and stability.•Proper surface acidity and metal charge density is helpful to catalytic performance.•Catalysts deactivation is due to deposition of carbon and strong adsorption of CO.A Ru–Ir/ZnO catalyst with metal loadings of 3% Ru and 3% Ir was reduced at different temperatures (150 to 400 °C) and tested for vapor-phase selective hydrogenation of crotonaldehyde at 80 °C. It was found that with increasing reduction temperature, the crotonaldehyde conversion over the catalysts first increased and then decreased. A conversion of 93.5% and the selectivity to crotyl alcohol of 86.6% was observed after 10 h reaction on a Ru–Ir/ZnO catalyst reduced at 200 °C. Various characterizations such as X-ray photon spectroscopy (XPS) and ammonia temperature-programmed desorption (NH3-TPD) results demonstrated that moderate interaction between the CO bond and the M0 (M = Ru, Ir or Ru–Ir alloy) due to the proper charge density of M0, and surface acidity of the catalysts played decisive roles in the enhanced activity and selectivity obtained on the catalyst. In addition, the deactivation of the catalyst was due to the carbon deposit (including organic compounds) on the catalyst surface, as evidenced by Raman spectroscopy and temperature-programmed oxidation over the spent catalyst. Also, the strong adsorption of CO on the catalyst surface generated by a decarbonylation reaction could be another reason for catalyst deactivation, as evidenced by a CO poisoning experiment.

Cr2O3 catalysts were prepared by a precipitation method and tested for vapor phase fluorination of HCFC-1233xf (2-chloro-3,3,3-trifluoropropene) to HFC-1234yf (2,3,3,3-tetrafluoropropene) to investigate the effect of calcination temperature on the catalytic performance. The catalysts were characterized by XRD, Raman, NH3-TPD, and BET techniques. The results show, with increasing calcination temperature, the crystallite size of the catalyst increased while the surface acid sites decreased. It was found that the catalyst calcined at 500 °C exhibited the highest catalytic activity, with a HCFC-1233xf conversion of 63.3% and selectivies to HFC-1234yf and HFC-245eb (1,1,1,2,3-pentafluoropropane) of 59 and 38%, respectively, at a reaction temperature of 320 °C. Moreover, it was found that the carbon deposit on the surface was responsible for the deactivation of the catalyst during the reaction.

Mesocellular silica foam (MCF) materials with different pore volumes were prepared and modified with tetraethylenepentamine (TEPA) as sorbents for CO2 capture. The as-prepared sorbents were characterized by XRD, TEM, SEM, nitrogen adsorption/desorption, and FTIR. CO2 capture performances of the adsorbents were tested in a fixed-bed reactor equipped with an online MS. The results indicated that the pore volume of supports has great effect on the CO2 capture performance. With the increasing of pore volume as well as the window and cell sizes of the MCF, more TEPA can be loaded into the pores of MCF. For MCF with larger pore volume, more unoccupied space is left after the same amount of TEPA was loaded into the pores. The unoccupied space is beneficial for higher CO2 uptake because the mass transfer limitation can be reduced to some extent and the interaction between CO2 and TEPA may be easier. MCF material with largest pore volume exhibited the largest CO2 uptake of 4.34 mmol/g of adsorbent with a 70 wt % TEPA loading tested by the fixed-bed reactor and at least 4.57 mmol/g tested by thermogravimetric analysis (TGA) under the conditions of 10.0% (v/v) CO2 in N2 at 75 °C. Repeated adsorption/desorption cycles revealed that its high CO2 capacity can be regenerated via temperature swing adsorption and so it may be useful for CO2 capture via TEPA functionalized MCFs.

Ru/C catalysts were prepared by an impregnation method and their catalytic properties were tested for hydrogenolysis methyl difluoroacetate to difluoroethanol. The catalysts were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), CO chemisorption and H2 temperature-programmed reduction (H2-TPR). The effects of reaction temperature, Ru content and reduction temperature of the Ru/C catalysts on the reaction were investigated. It was found that with increasing Ru contents in the Ru/C catalysts, the methyl difluoroacetate conversion, the selectivity to difluoroethanol and the TOF value first increased and then decreased. A 3Ru/C catalyst reduced at 400 °C exhibited the highest selectivity to difluoroethanol (93.5%) and the highest activity (39.5%). It was also found that the Ru/C catalyst showed a good stability of catalytic hydrogenolysis of methyl difluoroacetate within 100 h.Graphical abstractA process and catalyst for synthesis of difluoroethanol are shown in this study.Highlights► 240 °C was the optimum reaction temperature for hydrogenolysis methyl difluoroacetate to difluoroethanol. ► 3Ru/C catalyst reduced at 400 °C exhibited the highest activity and selectivity to difluoroethanol. ► Ru/C catalyst showed a good reaction stability within 100 h.

•Promoter M (M = Y, Co, La, Zn) can change the catalytic performance of Cr2O3 catalyst.•La-Cr2O3(F) catalyst exhibited the highest activity and selectivity and it showed a good reaction stability.•Surface acid sites density of M-Cr2O3 catalysts have a larger impact on the activity of the catalyst.The vapor phase fluorination of perchloroethylene (PCE) to synthesize 2,2-dichloro-1,1,1-trifluoroethane (HCFC-123), 1-chloro-1,2,2,2-tetrafluoroethane (HCFC-124) and pentafluoroethane (HFC-125) was carried out on M-Cr2O3 catalysts with different promoters (M = Y, Co, La, Zn). The catalysts were characterized by X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H2-TPR), Raman spectrum, Ammonia temperature-programmed desorption (NH3-TPD) and X-ray photoelectron spectroscopy (XPS) techniques. It was found that in the pre-fluorination process CrOx (x ≥ 1.5) in M-Cr2O3 catalysts could be transformed into CrOxFy species. The highest activity was obtained on La-Cr2O3(F) catalyst with 90.6% of PCE conversion and 93.7% to total selectivity (HCFC-123 + HCFC-124 + HFC-125) at 300 °C. The decline in surface acid sites density of the catalyst could improve the specific reaction rate, and the formation of surface CrOxFy species could enhance the selectivities to HCFC-123, HCFC-124 and HFC-125 for gas phase fluorination of PCE.The conversion of PCE is 90.6% and selectivity to effective product is 93.7% at 300 °C over La-Cr2O3(F) catalyst.

•Free PdO and Pd2+ ions were segregated efficiently by nitric acid treatment.•Turnover frequency of PdO and Pd2+ ions was quantitatively evaluated.•CO oxidation activity of free PdO was higher than that of Pd2+ ions.•Synergetic effect could improve CO catalytic activity of PdO-CeO2/SiO2 catalyst.Aiming at comparing the CO oxidation activity of free PdO and Pd2+ ions, a series of PdO-CeO2/SiO2 catalysts were prepared via an impregnation method and the turnover frequencies of them were quantitatively evaluated. The catalysts were further characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), temperature-programmed reduction (TPR), Raman technique and CO chemisorption. It was found that free PdO species and Pd2+ ions could be segregated efficiently by nitric acid treatment. The PdO-CeO2/SiO2 catalyst calcined at 600 °C showed the highest activity. The results of CO chemisorption suggested that free PdO species had a higher activity than Pd2+ ions, and the turnover frequencies of them were 3.27 × 10−2 s−1 and 0.49 × 10−2 s−1, respectively. The activity of PdO-CeO2/SiO2 catalyst was higher than that of PdO/SiO2 catalyst. Moreover, the CO catalytic activity of PdO-CeO2/SiO2 catalyst could be assigned to the synergetic effect of free PdO and Pd2+ ions, and this effect could improve the CO catalytic activity of catalysts.

Vapor-phase hydrogenation of crotonaldehyde was carried out over Pt/CeO2 and Pt/ZrO2 catalysts. It was found that both catalysts suffered deactivation, with the conversion of crotonaldehyde decreasing from 31 to 6% over the Pt/CeO2 catalyst and 20 to 7% over the Pt/ZrO2 catalyst, which was due to the formation of organic compounds on the catalyst surface as revealed by temperature programmed oxidation technique on the spent catalysts, and the poisoning effect of CO chemisorptions on Pt atoms via decarbonylation reaction. For the Pt/ZrO2 catalyst, selectivity to crotyl alcohol reached 48% and kept stable during the reaction, while for the Pt/CeO2 catalyst, the selectivity to crotyl alcohol decreased dramatically from 51 to 33%. The decrease of selectivity for the Pt/CeO2 catalyst was attributed to carbon deposit formed on the catalyst surface by the reaction between CO and reduced Ce3+ ion in CeO2. However, for the Pt/ZrO2, no carbon deposit was formed on the catalyst surface, which could account for the stable selectivity during the reaction process.Graphical abstractHighlight► Organic compounds covered active sites and led to catalyst deactivation. ► CO formed via decarbonylation chemisorption on Pt atoms also suppressed activity. ► PtC interfaces on Pt/CeO2 inhibited selectivity to crotyl alcohol.

Vapor-phase selective hydrogenation of crotonaldehyde was carried out over Ir/TiO2 catalysts with different Ir contents prepared by an impregnation method. The catalysts were characterized by X-ray powder diffraction (XRD), temperature-programmed reduction (TPR), diffuse reflectance infrared Fourier transform spectra of CO adsorption (CO-DRIFTS), NH3 temperature-programmed desorption (NH3-TPD), Raman spectroscopy and temperature-programmed oxidation (TPO). It was found that with increasing Ir content in Ir/TiO2, both the activity (TOF) and selectivity to crotyl alcohol first increased and then slightly decreased. The 3 % Ir/TiO2 catalyst showed the highest activity, (with a TOF of 9.3 × 10−3 s−1) and the highest selectivity to crotyl alcohol (74.6 %) in the hydrogenation of crotonaldehyde. The results of CO-DRIFTS indicated that the reduced catalyst contains a mixture of Ir0 and Irδ+. It was concluded that the catalytic performance of the catalysts depended on the strength of surface acidity and the Ir particles size for the selective hydrogenation of crotonaldehyde to crotyl alcohol.

Well-crystallized β-AlF3 with high surface area was synthesized by a carbon template method. Porous γ-Al2O3 was filled with carbon and transformed into AlF3 by fluorination. After removing the carbon template by thermal combustion, the resulting β-AlF3 had a surface area of 114 m2 g−1. Temperatures for fluorination and thermal combustion were crucial for the phase composition of the resulting sample. The high surface area β-AlF3 was very active for dismutation of CF2Cl2 due to its large amount of surface acid sites.

This study focuses on developing a catalyst for the removal of volatile organic compounds (VOCs) accompanied chlorinated volatile organic compounds (CVOCs). By a combination of deposition–precipitation and impregnation methods, a Pd/Cr2O3–ZrO2 catalyst was prepared and tested for catalytic oxidation of dichloromethane, ethyl acetate, and toluene. It was found that the Pd/Cr2O3–ZrO2 catalyst are very active for the catalytic oxidation of all these three different organics, due to the bifunctional catalysis of Pd and Cr. By comparing the catalytic performance of Cr2O3–ZrO2, Pd/ZrO2, and Pd/Cr2O3–ZrO2 catalysts, it suggested that the Cr species are more active for dichloromethane and ethyl acetate oxidation, while the Pd species play a very important role in toluene oxidation.

The effects of MoO3 content and calcination temperature on the solid-state reaction of MoO3–CeO2 complex oxide were studied by Raman spectroscopy and XRD. Depending on the MoO3 content, molybdenum oxide species were found to exist as the highly dispersed molybdate, crystalline Ce8Mo12O49 and Ce2Mo4O15 for the MoO3–CeO2 complex oxide calcined at 500 °C resulting from the solid-state reaction between MoO3 and CeO2. Such a solid–state reaction is promoted at higher temperature. However, Ce2Mo3O13 is observed for 20% MoO3–CeO2 sample under O2 atmosphere, which indicates Ce2Mo3O13 is formed firstly and then it is transformed into Ce8Mo12O49 at higher temperature during the solid-state reaction.Highlights► The solid-state reaction of MoO3–CeO2 was studied by Raman and XRD. ► Molybdenum oxide species exist as molybdate, Ce8Mo12O49 and Ce2Mo4O15. ► The solid-state reaction is promoted at higher temperature and under O2 atmosphere.

The PdO/Ce1–xPdxO2−δ catalyst prepared by a solution-combustion method contained free surface PdO species and PdO species in Ce1–xPdxO2−δ solid solution, whereas the PdO/CeO2 catalyst 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−δ catalyst had the highest activity (969.3 μmolCO gPd–1 s–1), those in the PdO/CeO2 catalyst had medium activity (109.0 μmolCO gPd–1 s–1), and the PdO species in the Ce1–xPdxO2−δ solid solution had the lowest activity (13.2 μmolCO gPd–1 s–1). Synergetic effects of PdO species were responsible for the enhanced reactivity of the PdO/Ce1–xPdxO2−δ catalyst, as the free surface PdO species provided CO chemisorption sites and the Ce1–xPdxO2−δ solid solution generated more oxygen vacancies for oxygen activation.

A series of Ce1–xMxO2−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 MO8-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 Ce4+, while the evolution of the MO8-type complex was due to the different ionic radius of dopant from that of Ce4+. 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 MO8-type complex in the sample but the constant concentration of oxygen vacancies, implying that the metastable MO8-type complex species were more disordered compared to the oxygen vacancies.

Surface properties of rare-earth (RE) doped ceria (RE = Sm, Gd, Pr, and Tb) were investigated by UV (325 nm) and visible (514, 633, and 785 nm) Raman spectroscopy, combined with UV−vis diffuse reflectance spectroscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectra techniques. It was found that the optical absorption property of samples, the wavelength of detecting laser line, and the inhomogeneous distribution of the dopants significantly affected the obtained surface information, namely, the peak intensity and shape at ca. 460 and 570 cm−1, as well as the observed oxygen vacancy concentration (A570/A460). The UV laser line detected the surface information of RE-doped ceria and disclosed the presence of many oxygen vacancies in the samples. The visible laser lines penetrated into the inner layer of the Sm- or Gd-doped CeO2 and reflected the whole information of samples because of their weak absorptions of the visible laser. However, the Pr- or Tb-doped CeO2 absorbed visible light strongly; thus, the laser can only determine the outer surface information of the sample.

Using Cerium chloride octahydrate as a precursor and dodecyl sodium sulfate as a template, nano-sized CeO2 with extra-high surface area were synthesized by a surfactant-templated method. The highest surface area of the synthesized CeO2 was 457 m2 g−1. Combined with XRD, TEM and BET results, it was found that when the calcination temperature was lower than 600 °C, the particle size of CeO2 hardly changed while the surface area declined due to the co-action of the sintering and agglomeration of the particles. However, with a high calcination temperature (above 600 °C), the sintering was the mainly reason to the decline of the BET surface area and the rapid growth of the particles size. Moreover, by the test for CO oxidation, it was found that the reactivity of the CeO2 sample remarkably increased with decreasing particle size, which was due to the fact that smaller particles provided more structural defects.

A series of CuO/CeO2 and inverse CeO2/CuO catalysts were prepared by an incipient wetness impregnation method and tested for CO oxidation. Crystallite sizes of CeO2 and 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 CeO2 crystallite 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−CeO2 interface, the activity on the larger CuO crystallite was much higher than that on the smaller one, indicating that CuO−CeO2 catalyst 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.

A series of La-doped Al2O3 catalysts were prepared and tested for the vapor phase hydrofluorination of C2H2 to vinyl fluoride (CH2CHF, VF). It was found that the La-doped catalyst gave a stable catalytic performance and a higher selectivity to the desired VF and a lower selectivity to coke deposition compared with the pure Al2O3 catalyst. The enhancement in VF selectivity on the La-doped catalyst was due to the elimination of acidic sites on the Al2O3 surface by the addition of La2O3, evidenced by NH3–TPD results, which could also explain the declined selectivity to coke deposition on the catalyst. Raman result indicated there were two different vibration forms of CH distortion and CC expansion for the coke deposition.It is found that the La-doped catalyst gives a stable catalytic performance and a higher selectivity to the desired vinyl fluoride (VF) and a lower selectivity to coke deposition compared with the pure Al2O3 catalyst. The enhancement in VF selectivity on the La-doped catalysts is due to the elimination of acidic sites on the Al2O3 surface by the addition of La2O3, evidenced by NH3–TPD results.

Ce0.8Sm0.2O2 − δ electrolytes were prepared by sol–gel methods with different thermal treatment conditions. It was found that the particle size of the powders thermally treated in N2 could be controlled less than 10 nm, due to the fact that carbons that enwrapped the precursors could inhibit their crystallization. At the same time, the nano-sized precursors could form larger nuclei at the initial stage of sintering, resulting in the formation of electrolyte with higher density and larger grain size. At the same consent of oxygen vacancies, the enhancement of electrical properties for the electrolyte was correlated to its microstructure obtained by the N2 thermal treatment.

A series of CexGd1−xO2−δ samples were prepared by a citrate method. Phase structure and AC impedance spectroscopy of these electrolytes were studied. It was found that doping of Gd3+ into CeO2 could help to form solid solutions, which is beneficial to the formation of more oxygen vacancies and consequently improves electrical conductivity of the electrolytes. The highest conductivity was obtained on a Ce0.6Gd0.4O2−δ electrolyte. Arrhenius plot of the Ce0.6Gd0.4O2−δ electrolyte shows an inflexion at 600 °C, implying that the electrolyte has dual mechanisms of electrical conductivity. It was also found that the phase transition has influence on the electrical conductivity, as the activation energy increases when the electrolytes transit from a mixed CeO2-based and Gd2O3-based solid solution phases to only CeO2-based solid solution phase.

The catalytic behavior of CeO2-based solid solution catalysts (Ce0.7Zr0.3O2, Ce0.7Pr0.3O2−δ, and Ce0.7Gd0.3O1.85) had been studied for soot combustion. These catalysts were characterized using X-ray diffraction (XRD), UV-visible, Raman, and H2 temperature-programmed reduction (H2-TPR) measurements. The Zr, Pr, and Gd cations replaced Ce cations in the CeO2 lattice to form nanocrystalline CeO2-based solid solutions. The crystallite size of Ce0.7Zr0.3O2, Ce0.7Pr0.3O2−δ, and Ce0.7Gd0.3O1.85 is 7.3, 7.8, and 11.1 nm, respectively, which are much smaller than pure CeO2 (36.2 nm). The substituting process can promote the formation of oxygen vacancies, which can hasten the diffusion rate of oxygen, then improve the combustion activity. According to the catalytic soot combustion results, the ranking in activity of these catalysts is Ce0.7Zr0.3O2 > Ce0.7Pr0.3O2−δ > Ce0.7Gd0.3O1.85 ∼ CeO2, which is well consistent with the reducibility sequence of the samples. The mass of catalyst plus soot mixture have effect on soot combustion: the Tig and Tmax decrease with the increasing of sample weight. Combustion activity of catalysts decreases slowly with cycled runs, which can be attributed to the activity lost of catalyst.

Ce0.9Pr0.1O2-δ, Ce0.95Cu0.05O2-δ, and Ce0.9Pr0.05Cu0.05O2-δ mixed oxides and pure CeO2 were 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 Ce0.95Cu0.05O2-δ and pure CeO2 samples. Surface chemical states of the Ce0.9Pr0.1O2-δ and Ce0.9Pr0.05Cu0.05O2-δ mixed 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 O2 and the Cu+ ions provide chemisorption sites for CO.

Mesoporous PrOy–ZrO2 materials were obtained by coprecipitation process. XRD patterns and TEM images reveal pore size growth with increase in the calcining temperature. N2 adsorption–desorption isotherms of the PrOy–ZrO2 samples are indicative of type IV behavior characteristic of mesoporous materials (as evidenced by low-angle XRD patterns). PrOy–ZrO2 calcined at 500 and 650 °C show well-developed H2 type hysteresis loops, indicating the existence of ink-bottle-like pores with a narrow entrance and a large cavity. PrOy–ZrO2 calcined at 800 and 950 °C show H1 type hysteresis loops, which is often associated with porous materials known to consist of agglomerates of approximately uniform spheres, and hence to have narrow distributions of pore size. The phase analysis by Rietveld refinement method clearly shows the formation of cubic PrOy–ZrO2 solid solution from 500 to 950 °C.

Ce0.8Pr0.2OY solid solutions with ultrafine crystalline sizes and high specific surface area were prepared by an improved citrate precursor method, where a nitrogen treatment was added prior to calcinations in air. The samples were characterized by TG-DSC, Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM) and BET nitrogen adsorption. XRD and Raman results show that the formation of Ce0.8Pr0.2OY solid solutions typical of the fluorite-like cubic structure with oxygen vacancies occurs when the Ce–Pr citrate precursors are heated at high temperature in the nitrogen atmosphere. Subsequent calcinations at a low temperature in air to remove carbon species have no apparent effects on the formed solid solutions. Ce0.8Pr0.2OY solid solution prepared by the improved citrate precursor method at 800°C has ultrafine nanoparticles of less than 10 nm and high specific surface area of 92.1 m2/g, while the sample prepared by the conventional citrate precursor method has mean particle size of 62.1 nm and specific surface area of 18.1 m2/g. Furthermore, Ce–Pr solid solution by the improved method have the mesoporous structure, more lattice defects and oxygen vacancies, which will have a promising application in the catalyst region as well as SOFC field.

A series of CuO/Ce1−xCuxO2−δ catalysts were prepared, and corresponding Ce1−xCuxO2−δ catalysts were obtained with nitric acid treatment. X-ray diffraction and Raman spectroscopic results revealed the presence of surface CuO species and CuxCe1−xO2−δ solid solution in the catalysts. CO oxidation testing found that the CO conversion was proportional to the concentrations of chemisorbed CO and oxygen vacancies in the CuO/Ce1−xCuxO2−δ catalysts, suggesting synergetic effects of the surface CuO species and CuxCe1−xO2−δ solid solution on the reactivity, as the former provided sites for CO chemisorption and the latter promoted reducibility of the catalyst for oxygen activation. Kinetic studies showed that the apparent activation energy was 42 kJ mol−1 for CuO/Ce1−xCuxO2−δ and 95 kJ mol−1 for Ce1−xCuxO2−δ. The power-law rate expression was rCO=k1PCO0.74PO20 for CuO/Ce1−xCuxO2−δ and rCO=k2PCO1.04PO20 for the Ce1−xCuxO2−δ catalyst, indicating that the reaction pathway followed a Mars–van Krevelen type mechanism.Synergetic effects of surface CuO species and CuxCe1−xO2−δ solid solution on CO oxidation over CuO/Ce1−xCuxO2−δ catalysts were found, as the former provided sites for CO chemisorption and the latter promoted reducibility of the catalyst for oxygen activation.Download high-res image (67KB)Download full-size image

Chlorinated volatile organic compounds (CVOCs) are hazardous and potent environmental pollutants. Catalytic combustion has been regarded as one of the most promising technologies to eliminate CVOCs emissions. CH2Cl2 is difficult to be oxidized among CVOCs. A catalyst with high activity for CVOCs oxidation is desirable. In this paper, CrOx/Al2O3 catalysts with different Cr contents were prepared using a deposition–precipitation method and tested for CH2Cl2 oxidation. The highest activity was obtained over the catalyst with 18% Cr content, with a complete oxidation of CH2Cl2 at 350 °C. Characterization results indicated that both high oxidation state Cr species (Cr(VI) species) and crystalline Cr2O3 existed in the CrOx/Al2O3 catalysts, and the average valence of Cr species in the catalysts decreased with Cr content. It was found that the reaction rate for CH2Cl2 oxidation increased with increasing average valence of Cr, which indicated that high oxidation state Cr species were probably the active phase for the reaction.Graphical abstractDownload high-res image (123KB)Download full-size imageHighlights► The average valence of Cr species in the catalysts decreased with Cr content. ► The reaction rate for CH2Cl2 oxidation increased with increasing average valence of Cr species. ► The enhanced reactivity of the catalysts was due to high oxidation state Cr species.

High-surface area nanosized CuO–CeO2 catalysts were prepared by a surfactant-templated method and tested for CO oxidation. The catalysts were characterized by XRD, TEM, N2 sorption, H2-TPR, and CO-TPR. The surfactant method can be used for preparing CuO–CeO2 mixed oxides with a crystallite size of about 5 nm. The highest BET surface area of the catalysts was 215 m2 g−1, achieved over a 3.3% CuO content catalyst. XRD results indicated that the absence of a CuO phase with <12% CuO content may partially incorporate in the CeO2 lattice to form CuxCe1−xO2−δ solid solution, whereas a higher CuO content causes the formation of bulk CuO. These high-surface area nanosized catalysts were found to be very active for CO oxidation reaction; the lowest T90 was 80 °C, achieved over a 12.0% CuO content catalyst. In addition, the CuO–CeO2 catalysts also show high catalytic activity for selective oxidation of CO in excess H2 at relatively low temperature. H2-TPR results reveal three reduction peaks for these catalysts, which could be attributed to reduction of the highly dispersed CuO, the Cu2+ in the CeO2 lattice, and the bulk CuO. Removal of the finely dispersed CuO in the catalyst by acid treatment resulted in a decline in catalytic activity for CO oxidation, indicating that the finely dispersed CuO species are the active sites for the reaction.

A series of CrOx–Y2O3 catalysts were prepared by a deposition–precipitation method and tested for the fluorination of 2-chloro-1,1,1-trifluoroethane (CF3CH2Cl) to synthesize 1,1,1,2-tetrafluoroethane (CF3CH2F). The highest activity was obtained on a pre-fluorinated catalyst calcined at 400 °C, with 19% of CF3CH2Cl conversion at 320 °C. The effect of the calcination temperature on the CrOx species was investigated. X-ray diffraction and Raman results indicated that the CrOx species (Cr(VI)) were well dispersed on the catalyst surface when the catalyst was calcined at 400 °C. With increasing calcination temperature, most of the CrOx species changed from high oxidation state Cr(VI) to low oxidation state Cr(V) or Cr(III) species, which resulted in difficulty in pre-fluorination of the catalyst. It was also found that the CrFx, CrOxFy or Cr(OH)xFy phases originated from high oxidation state Cr(VI) species were the active sites for the fluorination reaction.

Vapor-phase selective hydrogenation of crotonaldehyde was conducted over Ir/SiO2 catalysts to investigate the effects of Ir loading on the catalytic behaviors. The catalysts were characterized by X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), CO chemisorption, temperature-programmed reduction (TPR), diffuse reflectance infrared Fourier transform spectra of CO adsorption (CO-DRIFTS), NH3 temperature-programmed desorption (NH3-TPD), and temperature-programmed oxidation (TPO). It was found that the particle size of Ir in the catalyst increased with Ir loading, being 2.1, 3.9 and 6.7 nm for the 1Ir/SiO2, 3Ir/SiO2 and 5Ir/SiO2 catalyst, respectively. The Ir species on the Ir/SiO2 catalysts consisted of Ir0 and Irδ+, but the average valence of Ir species decreased with increasing Ir loading. Also, catalytic testing results revealed that the reactivity of the catalyst increased with Ir loading. Interestingly, it was found that the reaction underwent an induction period, with the conversion of crotonaldehyede and the selectivity to crotyl alcohol gradually increasing during the reaction, and eventually reaching a steady state. The highest selectivity (77.6%) to crotyl alcohol was obtained over the 3Ir/SiO2 catalyst, and the conversion increased gradually to 15.6%. The catalytic behavior of these stable catalysts could be attributed to the proper Ir particle size, the existence of Ir0 and Irδ+ species on the surface, and high amount of surface acid sites in these catalysts.Graphical abstractHighlights► Selective hydrogenation of crotonaldehyde was conducted over Ir/SiO2 catalysts. ► The catalysts did not deactivate during the reaction and reached steady state in 9 h. ► The Ir/SiO2 catalysts contained Ir0 and Irδ+ species. ► Chemical nature of Ir species and surface acidity were important for the reaction.

Well-crystallized β-AlF3 with high surface area was synthesized by a carbon template method. Porous γ-Al2O3 was filled with carbon and transformed into AlF3 by fluorination. After removing the carbon template by thermal combustion, the resulting β-AlF3 had a surface area of 114 m2 g−1. Temperatures for fluorination and thermal combustion were crucial for the phase composition of the resulting sample. The high surface area β-AlF3 was very active for dismutation of CF2Cl2 due to its large amount of surface acid sites.

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/SiO2 catalysts 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/SiO2 catalyst promoted with combined CrOx–FeOx with 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–CrOx and/or Ir–FeOx interfaces. 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.