Potassium-doped Co3O4, prepared via the sol–gel method, was pretreated by water washing and subsequently the fresh, washed catalysts were compared with pure Co3O4 in order to investigate the existing states of potassium species and their influence on the activities for NO and soot oxidation. The samples were characterized by N2 adsorption–desorption, XRD, FT-IR, NOx-TPD, CO2-TPD, O2-TPD and XPS. It was found that two types of K species existed in K-Co3O4, i.e., the “free” K and the “stable” K. The “free” K species, which were present as carbonates and nitrates with high mobility, could improve the contact state of catalyst-soot and thus significantly accelerate soot oxidation, but they also greatly covered the active surface of Co3O4 and were adverse for NO oxidation. The “stable” K species, which interacted strongly with Co3O4 and acted as electronic and structural modifiers, facilitated the formation of O−/O2− species and oxygen vacancies, exposed more surface Co3+ sites and increased the amount of the medium/strong basic sites, and as a result, they enhanced the intrinsic activities of Co3O4 for soot and NO oxidation. Both K species improved the NOx storage capacities of Co3O4, and the NOx species adsorbed on medium/strong basic sites with appropriate stability, caused by the effect of the “stable” K, were more beneficial for soot oxidation than those adsorbed on “free” K with high stability.

•Doping of Bi2O3 enhances the activity and stability of Co3O4 for soot oxidation.•Bi2O3 doping improves the oxygen vacancy formation and lattice oxygen reactivity.•The channels of oxygen activation are extended over Bi2O3-Co3O4.•Bi2O3-Co3O4 can work efficiently under condition containing H2O and SO2.Bi2O3-Co3O4 catalysts were prepared by sol-gel method and tested for soot oxidation by O2. The composite oxides showed excellent activity under both tight and loose contact when compared with individual Co3O4 or Bi2O3, and the maximum activity was obtained over catalyst with Bi/Co molar ratio of 0.2. The samples were characterized by means of XRD, N2 adsorption, FE-SEM, XPS, FT-IR, C-TPR and O2-TPD. It was found that Bi2O3 with low melting point deposited on Co3O4 surface could not only promote the contact state between soot and catalyst, but also produce more oxygen species with high mobility and reactivity at Bi-Co interface layer. Oxygen activation channel and reaction pathway were discussed based on the results of isothermal anaerobic titrations and 18O-isotopic tests, which confirmed that soot was more likely to react with lattice oxygen species rather than O2, especially at low temperatures. The high mobility of lattice oxygen species was attributed to a combination of the O2− conductivity of Bi2O3 and the accelerative formation of oxygen vacancies at Bi-Co interface. A feasible reaction mechanism over the binary catalysts for soot oxidation was proposed. The stability tests were also studied and the results indicated that Bi-modified Co3O4 showed prominent tolerance against thermal shock, H2O and SO2, thus being a promising active component for practical application.Download high-res image (119KB)Download full-size image

CO adsorption and O2 activation played important roles in CO oxidation on a supported Pd catalyst, which were dependent on the chemical state of Pd. A series of Pd catalysts supported on Al2O3 with different Pd states were prepared: metal Pd (NCR), PdO (NC), Pd2+ coordinated with Cl− (Pd2+–Cl−, CF), and a mixture of PdO and Pd2+–Cl− (CC) and the activity of CO oxidation was in the order NCR ~ CF > CC ≫ NC. The catalysts were characterized by Brunauer, Emmett and Teller (BET) surface area analysis, X-ray diffraction (XRD), temperature programmed reduction by hydrogen (H2-TPR), X-ray photoelectron spectroscopy (XPS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The results showed that metal Pd (Pd0) could be partially oxidized to Pd+ in the presence of O2, which produced new CO adsorption sites and decreased the CO adsorption strength simultaneously. The cooperation between the enhanced CO adsorption and the decrease in the CO adsorption strength led to high activity for CO oxidation on NCR. For high chemical valence Pd in the species (Pd2+), the chemical environment or coordinated ligand of the Pd species showed large effects on the CO oxidation. The lowest CO oxidation activity on NC occurred when PdO hardly adsorbed CO, meanwhile, PdO was not easily reduced by CO. However, the presence of Cl− significantly promoted the reduction of Pd2+ to Pd+, which increased the amount of CO adsorption and resulted in the higher activity for CO oxidation on CF and CC than PdO. Tuning the CO adsorption by adjusting the chemical state of Pd may be a useful approach to prepare a highly efficient supported Pd catalyst.

A mathematic model of carbonyl sulfide (COS) removal at low temperature with fouling of catalyst has been developed based on experimental results. Kinetic studies were conducted in a fixed bed reactor under atmospheric pressure and at low temperature (40-70 °C). Experimental results of breakthrough curves were used to obtain kinetic parameters accounting for axial dispersion, external and internal mass-transfer resistances as well as the sulphur deposition on inner-face of catalyst. Initial bulk porosity of particle (ɛP0), deactivation coefficient (α), sulfide deposition coefficient (β) were used to quantify the behavior of COS removal at different operating conditions. Adsorption heat of H2O and activation energy of COS removal was 21.5 and 62.4 kJ/mol respectively. The effects of flow rate, COS inlet concentration, temperature and relative humidity(RH) were analyzed, and it was found that relative humidity carried a heavier weight than temperature on εP0, α, β within our experimental conditions. The model agreed well with the experimental breakthrough curves and satisfactorily predicted the fixed-bed reactor performance, and this model can be used as a reliable tool for process design and scaling-up of similar system.

A mathematical model describing the rate of carbon disulfide (CS2) removal has been developed. Kinetic studies were carried out in a fixed-bed reactor under atmospheric pressure and a range of temperatures (30–70 °C). The effects of flow rate, CS2 inlet concentration, temperature and relative humidity were analyzed. A kinetic model based on axial dispersion, external and internal mass-transfer resistances, as well as effects of S deposition on the inner-face of the catalyst was in agreement with the CS2 experimental breakthrough curves. The mathematical model can be used for process design and scale-up of similar systems.

The Co3O4 and Bi2O3–Co3O4 were prepared by precipitation and co-precipitation method. The samples were characterized by XRD, BET, H2-TPR, Raman, XPS and EPR. The low-temperature CO oxidation on the catalysts was also investigated. The results showed the deposition of Bi2O3 enhanced the activity and stability of Co3O4 for CO oxidation. 20 wt.% Bi2O3–Co3O4 could completely convert CO as low as −89 °C, and maintain the complete oxidation of CO at −75 °C for 10 h. XRD and Raman results showed Bi2O3 could enter the lattice of Co3O4, and promote the formation of the lattice distortion and structural defect. H2-TPR results showed that reduction of Co3O4 was promoted and the diffusion of oxygen was accelerated. XPS and EPR results showed the surface richness of Co3+ and the increase of Co2+ in 20 wt.% Bi2O3–Co3O4. The formation of more Co2+ in 20 wt.% Bi2O3–Co3O4 could produce structure defects and lead to the formation of more oxygen vacancy, which was suggested to play the critical role in promoting the catalytic activity and stability of 20 wt.% Bi2O3–Co3O4.Graphical abstractDownload high-res image (152KB)Download full-size imageHighlights► The deposition of Bi2O3 enhances the activity and stability of Co3O4 for CO oxidation. ► 20 wt.% Bi2O3–Co3O4 can completely convert CO as low as −89 °C. ► Bi2O3 promotes the formation of the lattice distortion and structural defect. ► Oxygen vacancy plays the critical role in CO oxidation on 20 wt.% Bi2O3–Co3O4.

This work investigated the effect of potassium (K) on the catalytic activity of Co3O4 prepared by sol–gel method for soot combustion under loose contact. The catalysts were characterized by XRD, TPR, Raman, FT-IR, and XPS. The results showed the deposition of 4.4 wt.% K accelerated the activity of Co3O4 for soot combustion under loose contact, the temperature of maximum soot oxidation rate (Tm) decreased from 490 °C over Co3O4 to 417 °C over K(0.1)/Co3O4(600). It was discovered that there is an exact linear relationship between Tm and the value of active Co3+/Co for the first time, where Co represented the total amount Co3+ and Co2+. Our findings strongly suggested that potassium, as a promoter, not only favors CB–catalyst contact, but also produces more active Co3+ with the increase of K content. Doped K entered the crystal lattice and aroused the lattice distortion. The Tm in TPO decreased linearly with the increase of microstrain. The lattice distortion and the interaction of K and Co3O4 promoted the formation of the surface active Co3+ and active O−, and improved the activity of lattice oxygen.Graphical abstractDownload high-res image (140KB)Download full-size imageHighlights► The deposition of K accelerated the activity of Co3O4 for soot combustion under loose contact. ► An exact linear relationship between Tm and the value of active Co3+/Co. ► An exact linear relationship between Tm and crystal microstrain of Co3O4 ► K aroused lattice expansion and weakened the Co–O bonds. ► K promoted the formation of the surface active Co3+ sites.

CO adsorption and O2 activation played important roles in CO oxidation on a supported Pd catalyst, which were dependent on the chemical state of Pd. A series of Pd catalysts supported on Al2O3 with different Pd states were prepared: metal Pd (NCR), PdO (NC), Pd2+ coordinated with Cl− (Pd2+–Cl−, CF), and a mixture of PdO and Pd2+–Cl− (CC) and the activity of CO oxidation was in the order NCR ~ CF > CC ≫ NC. The catalysts were characterized by Brunauer, Emmett and Teller (BET) surface area analysis, X-ray diffraction (XRD), temperature programmed reduction by hydrogen (H2-TPR), X-ray photoelectron spectroscopy (XPS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The results showed that metal Pd (Pd0) could be partially oxidized to Pd+ in the presence of O2, which produced new CO adsorption sites and decreased the CO adsorption strength simultaneously. The cooperation between the enhanced CO adsorption and the decrease in the CO adsorption strength led to high activity for CO oxidation on NCR. For high chemical valence Pd in the species (Pd2+), the chemical environment or coordinated ligand of the Pd species showed large effects on the CO oxidation. The lowest CO oxidation activity on NC occurred when PdO hardly adsorbed CO, meanwhile, PdO was not easily reduced by CO. However, the presence of Cl− significantly promoted the reduction of Pd2+ to Pd+, which increased the amount of CO adsorption and resulted in the higher activity for CO oxidation on CF and CC than PdO. Tuning the CO adsorption by adjusting the chemical state of Pd may be a useful approach to prepare a highly efficient supported Pd catalyst.