A comparative study of differently prepared rare earths-modified ceria-supported gold catalysts for preferential oxidation of CO

The preferential oxidation of CO in H₂-rich gas was studied over gold catalysts supported on ceria modified by rare earths (RE = La, Sm, Gd and Y). The ceria supports were prepared by mechanochemical activation or co-precipitation. The amount of RE₂O₃ was 10 wt%. Gold (2 wt%) was added by the deposition-precipitation method. The samples were characterized using XRD, HRTEM, HAADF, TPR, and Raman spectroscopy. It was established that catalysts prepared by co-precipitation were more active than samples made by mechanochemical activation. A gold catalyst on yttrium-modified ceria, prepared by co-precipitation, exhibited the highest catalytic activity and selectivity, and high stability. No substantial differences in the size distribution and average size of the nanogold particles in the studied catalysts were observed. The main reason for the differences in PROX activity of these gold catalysts was searched into the role of the ceria supports, depending on the preparation method, and the nature of the modifier.     

Electrical properties of ceria Co-doped with Sm and Nd

Ceria co-doped with Sm³⁺ and Nd³⁺ powders are successfully synthesized by citric acid–nitrate low-temperature combustion process. In order to optimize the electrical properties of the series of ceria co-doped with Sm³⁺ and Nd³⁺, the effects of co-doping, doping content and sintering conditions on grain and grain boundary conductivity are investigated in detail. For the series of Ce_0.9(Sm_xNd_1−x)_0.1O_1.95 (x = 0, 0.5, 1) and Ce_1−x(Sm_0.5Nd_0.5)_xO_δ (x = 0.05, 0.10, 0.15, 0.20) sintered under the same condition, Ce_0.9(Sm_0.5Nd_0.5)_0.1O_1.95 exhibits both higher grain and grain boundary conductivity. Compared with Ce_0.9Gd_0.1O_1.95 and Ce_0.8Sm_0.2O_1.9, Ce_0.9(Sm_0.5Nd_0.5)_0.1O_1.95 sintered at 1350–1400 °C shows higher total conductivity with the value of 1.0 × 10⁻² S cm⁻¹ at 550 °C. In addition, it can be found the trends of grain and grain boundary activation energies of Ce_1−x(Sm_0.5Nd_0.5)_xO_δ are both consistent with those of Ce_1−xNd_xO_δ, but different from those of Ce_1−xSm_xO_δ, which can be explained as: the local ordering of oxygen vacancies maybe occurs more easily in Nd-doped ceria than in Sm-doped ceria; the segregation amount of Sm³⁺ is more than that of Nd³⁺ to the grain boundaries in ceria co-doped with Sm³⁺ and Nd³⁺, which is confirmed by X-ray photoelectron spectroscopy (XPS).     

Dual-templating fabrication of three-dimensionally ordered macroporous ceria with hierarchical pores and its use as a support for enhanced catalytic performance of preferential CO oxidation

In this study, several three-dimensionally ordered macroporous (3DOM) CeO₂ having hierarchical pore structure were successfully prepared via a dual ‘hard-soft’ templating strategy using Ce(NO₃)₃·6H₂O containing phenol-formaldehyde (PF) resol or/and sucrose as the ceria precursor. The resulting CeO₂ samples were characterized by N₂ adsorption-desorption analysis, scanning electron microscopy, transmission electron microscopy and X-ray diffraction, which showed that the hierarchical 3DOM CeO₂ possessed interconnected networks of the ordered macropore structures with large mesopores, and both the BET surface area and pore volume increased significantly compared with the conventional 3DOM CeO₂. The improved textural parameters should be attributed to the emergence of mesopores in the interconnected three-dimensional skeleton, which were formed by oxidative removal of carbon produced from carbonization of PF resol or sucrose. The hierarchical 3DOM CeO₂ exhibited a superior performance to the conventional 3DOM or bulk CeO₂ when used as supports for Ir catalysts in preferential CO oxidation.     

Compositional dependence of electrical conductivity of Ce1−xTbxO2−δ (0x1)

Well dispersed Ce_1−xTb_xO_2−δ nano-powders were synthesized by using the carbonate coprecipitation method in an entire compositional range of 0x1, which allowed the preparation of highly dense pellets by sintering at 1673 K and the systematical study of the electrical conductivity in such a wide compositional range. It was found that the conductivity increased with increasing Tb concentration except that of x=0.80. Secondary phase were observed by using X-ray diffraction in the samples with x0.80, which might have negative impact on the conductivity of the samples.     

Effect of tungsten oxide on ceria nanorods to support copper species as CO oxidation catalysts

In this work,tungsten oxide with different concentrations(0,0.4 at%,2.0 at% and 3.2 at%) was introduced to the ceria nanorods via a deposition-precipitation(DP) approach,and copper species of ca.10 at% were sequentially anchored onto the modified ceria support by a similar DP route.The aim of the study was to investigate the effect of the amount of tungsten oxide(0,0.4 at%,2.0 at%,and 3.2 at%) modifier on the copper-ceria catalysts for CO oxidation reaction and shed light on the structure-activity relationship.By the aids of multiple characterization techniques including N_2 adsorption,high-resolution transmission electron microscopy(HRTEM),powder X-ray diffraction(XRD),X-ray absorption fine structure(XAFS),and temperature-programmed reduction by hydrogen(H_2-TPR) in combination with the catalytic performance for CO oxidation reaction,it is found that the copper-ceria samples maintain the crystal structure of the fluorite fcc CeO_2 phase with the same nanorod-like morphology with the introduction of tungsten oxide,while the textural properties(the surface area,pore volume and pore size) of ceria support and copper-ceria catalysts are changed,and the oxidation states of copper and tungsten are kept the same as Cu~(2+)and W~(6+)before and after the reaction,but the introduction of tungsten oxide(WO_3)significantly changes the metal-support interaction(transfer the CuO_x clusters to Cu-[O_x]-Ce species),which delivers to impair the catalytic activity of copper-ceria catalysts for CO oxidation reaction.     

Ceria supported platinum catalyst for CO oxidation reaction:Importance of metallic active species——Commemorating the 100th anniversary of the birth of Academician Guangxian Xu

Ceria supported platinum catalyst has now been widely studied due to its excellent activity for CO oxidatio n.However,the electron state of active metal center is still an open question.In this work,a ce ria nanorod support was prepared and platinum(Pt) with 0.9 at% was deposited using an impregnation method to obtain Pt/CeO_2 catalyst.With the help of "light-off" experiment and temperatureprogrammed reduction under CO(CO-TPR) test,the conclusion is proposed that the process of hydrogen reduction can enhance the activity of CO oxidation reaction for the generation of optimal active Pt site.An innovative near-situ X-ray absorption fine structure(XAFS) technique was used to investigate the chemical state of central Pt atom during the reaction process,clearly demonstrating that the high oxidized state of Pt does harm to the activity for CO oxidation while the relatively reductive Pt exhibits high activity,and the different oxidized state and chemical environment of Pt during every process has been identified.Furthermore,the activity of our Pt/CeO_2 catalyst is superior to that of most of the previous reports about CO catalytic oxidation by Pt based catalyst.Moreover,the optimal active species(Pt-O_4) have been identified after hydrogen reduction,which could be a possible key strategy to control the oxidation of Pt.     

Shape-selective alkylation of biphenyl catalyzed by H-Mordenites

Liquid phase alkylation of biphenyl was studied over large pore zeolites. Selective formation of the narrowest products, 4,4′’-diisopropylbiphenyl (4,4′’-DIPB), occurred only over HM among the zeolites with twelvemembered pore openings. These shape-selective catalyses are ascribed to steric restriction of transition state and to entrance of bulky substrates into the pores. The dealumination of HM enhanced catalytic activity and the selectivity of 4,4′’-DIPB because of the decrease of coke-deposition, while the activity and the selectivity were low over HM with the low SiO₂/Al₂O₃ ratio. Non-regioselective catalysis occurs on external acid sites because severe cokedeposition deactivates the acid sites inside the pores by blocking pore openings. The selectivity of DIPB isomers was changed with propylene pressure and/or with reaction temperature. 4,4′’-DIPB yielded selectively under high propylene pressure (<0.3 MPa) at 250 °C, while the selectivity of 4,4′’-DIPB decreased under such low propylene pressure as 0.2MPa. Selective formation of 4,4′’-DIPB was observed at moderate temperature such as 250 °C, whereas the decrease of the selectivity of 4,4′’-DIPB occurred at higher temperature as 300 °C. However, 4,4′’-DIPB was almost exclusively isomer in the encapsulated DIPB isomers inside the pores under every pressure and temperature. These decreases of the selectivity of 4,4′’-DIPB are due to the isomerization of 4,4′’-DIPB on the external acid sites. The deactivation of external acid sites of HM was examined to reduce non-regioselective alkylation and isomerization. External acid sites were deactivated by calcination after impregnation of cerium on HM without the decrease in pore radii. Selectivities of 4,4′’-DIPB were improved even at high temperatures in the isopropylation of biphenyl because of the suppression of non-regioselective alkylation and isomerization at the external acid sites. The selectivity of 4,4′’-diethylbiphenyl (4,4′’-DEBP) in the ethylation of biphenyl was much lower than that in the isopropylation. Among the DEBP isomers, 4,4′’-DEBP has the highest reactivity for the ethylation to polyethylbiphenyls inside the pores, whereas the isopropylation of 4,4′’-DIPB was negligibly low inside the pores. These differences are ascribed to the difference in steric restriction at the transition state composed of substrate, alkylating agent, and acid sites inside the pores.     

Development of a transient kinetic model for the CO oxidation by O2 over a Pt/Rh/CeO2/γ-Al2O3 three-way catalyst

A transient kinetic model was developed for the CO oxidation by O₂ over a Pt/Rh/CeO₂/γ-Al₂O₃ three-way catalyst. The experiments which were modelled consisted of periodically switching between a feed stream containing 0.5 mol% CO in helium and a feed stream containing 0.5 mol% O₂ in helium, with a frequency from 0.1 to 0.25 Hz, in the temperature range 393–433 K. These temperatures are representative for cold start conditions. The transient experiments yield information about the reaction mechanism. A transient kinetic model based on elementary reaction steps was developed which describes the experimental data in the above mentioned range of experimental conditions adequately. The kinetic model consists of two monofunctional and one bifunctional contribution. The first monofunctional reaction path comprises competitive adsorption of CO and O₂ on the noble metal surface followed by a surface reaction. The second monofunctional reaction path consists of CO adsorption on an oxygen atom adsorbed on the noble metal surface, followed by a reaction to CO₂. The bifunctional reaction path involves a reaction between CO adsorbed on the noble metal surface and oxygen from ceria at the noble metal/ceria interface. Also, reversible adsorption of carbon dioxide on the support is taken into account. The kinetic parameters, i.e. preexponential factors and activation energies for the different elementary reaction steps, and the oxygen storage capacity were estimated using multi-response non-linear regression analysis of the oxygen, carbon monoxide and carbon dioxide outlet concentrations.     

Comments on “Ionic transport in (nano)composites for fuel cells,Int. J. Hydrogen Energy 41 (2016) 7666–7675”

An article recently published in this journal on the phenomenon of ionic conductivity enhancement proves to be another example of misinterpretation due to intolerable handling of literature data. As a consequence, ionic conductivity enhancement remains a speculative assertion without evidence.