Four different synthetic routes (co-precipitation, oxidation–precipitation, citric acid sol–gel and reversed microemulsion) are adopted to prepare barium modified Pd/CeO2–ZrO2 catalysts and their catalytic activity towards CO, HC and NOx conversions is studied. The surface and bulk properties of these catalysts are characterized via XRD, N2 adsorption, XPS, UV-Raman, H2-TPR, and in situ DRIFTS. The catalyst prepared via the co-precipitation method exhibits the optimum three-way catalytic behavior, which is mainly due to its superior redox ability, whereas the oxidation–precipitation synthesis renders the catalyst with the best homogeneity and thermal resistance. However, for the catalyst prepared via the sol–gel route, its worst NOx reduction capacity is verified by the scarce appearance of negatively charged Pd0–NOδ− species, which is related to the faster dissociation of NO based on in situ DRIFTS, and the abundance of surface CO–Pd+ species reveals its unsatisfactory deep oxidizability of the HC reactant.

•Bifunctional y(Ce,Cr)xO2/HZSM-5 catalysts with oxidizing and acid sites are prepared.•Synergy between (Ce,Cr)xO2 and HZSM-5 strongly promotes the elimination of Cl-VOCs.•Catalyst with equivalent mass of (Ce,Cr)xO2 and HZSM-5 shows the best activity.•Partial deactivation of (Ce,Cr)xO2/HZSM-5 is due to coke, H2O and Cl adsorption.•Addition of (Ce,Cr)xO2 significantly reduces the coke formation on HZSM-5.A series of y(Ce,Cr)xO2/HZSM-5 composite materials (with different mass ratios of (Ce,Cr)xO2 to the HZSM-5 zeolite) are firstly constructed via traditional deposition–precipitation method, and then evaluated for eliminating low concentration chlorinated organic pollutants with different molecule structures. Compared to single (Ce,Cr)xO2 or HZSM-5, the synergistic catalytic effect between (Ce,Cr)xO2 and HZSM-5 improves the destructive efficiency of y(Ce,Cr)xO2/HZSM-5. This is due to that the abundant strong/weak acid centers on HZSM-5 firstly favor the adsorption and dechlorination of the chlorinated organic molecules, while the superior oxidation property of (Ce,Cr)xO2 promotes the oxidative destruction of these Cl-VOCs (chlorinated volatile organic compounds) and the by-products, and also suppresses the surface deposition of chlorine and carbon species. Particularly, the (Ce,Cr)xO2/HZSM-5 catalyst with the same weight of (Ce,Cr)xO2 and HZSM-5 (50%:50% for (Ce,Cr)xO2:HZSM-5) exhibits the highest oxidative activity, since its appropriate concentration of acid and oxidative sites can play to the largest degree. Moreover, adding slight benzene or water into the reaction system slightly decreases the catalytic activity owing to the competitive adsorption effect on the active centers, whereas water reduces the formation of by-products, since it can accelerate the eliminating of the surface chlorine species. The preferable catalytic activity and durability in the continuous long-time reaction reveals that the (Ce,Cr)xO2/HZSM-5 catalyst deserves more attention and is potential for industrial application to eliminate the chlorinated organic pollutants.Download high-res image (128KB)Download full-size image

•CuCe(rod) with varying copper loadings were prepared by ethanol thermal method.•Phase quantification reveals the gradually increased copper coverage on CeO2(rod).•Moderate increase of Cu coverage promotes the interaction between CuO and CeO2(rod).•Over high Cu coverage impairs the dispersion of CuO with larger size on CeO2(rod).•Formation of bulk phase CuO would not influence the catalytic activity of CuCe(rod).A series of CuO/CeO2(rod) catalysts with varying copper loadings (5, 7, 9, 11 and 15 wt %) are prepared by an ethanol thermal method and applied to the preferential oxidation of CO in excess hydrogen gas. The phase quantification obtained by XRD Rietveld refinement reveals the gradually increased copper coverage on the surface of CeO2(rod) with the increase of copper content from 5% to 15%. Moderate increase of surface copper coverage results in the increased amount of highly dispersed CuO strongly interacting with CeO2(rod) and greatly promotes the catalytic performance of CuCe(rod) for CO-PROX. However, H2-TPR profiles before/after N2O isothermal oxidation of the catalysts reveals that the over high surface coverage of Cu atoms on CeO2(rod) leads to the reduced dispersion of CuO particles with larger size, which weakens the interfacial Cu-Ce interaction and leads to the decline of their catalytic performance for CO-PROX. Moreover, the formation of bulk phase CuO (copper loading ≥ 9%) has no influence on the catalytic performance of CuCe(rod) catalyst, supported by the similar catalytic performance of CuCe(rod)-11% and CuCe(rod)-15% with significantly different amounts of bulk phase CuO.Download high-res image (211KB)Download full-size image

•CeO2(rod) behaves as a hexagonal prism exposed four {111} and two {100} planes.•CeO2(rod) is formed by the first oriented attachment and subsequent Ostwald ripening.•Copper oxides are highly dispersed on CeO2(rod) with different hydrothermal time.•The same architecture of Cu-Ce interaction with different strength has been formed.•Strongly bound Cu-[Ox]-Ce promotes the catalytic activity of CuCe(rod) for CO-PROX.CeO2(rod) has been found to be exposed four {111} planes and two {100} planes with a hexangular cross section, and the growth mechanism follows to the oriented attachment of the cube-like basic grains with a [110] growth direction and the subsequent Ostwald ripening, corresponding to the increasing lateral size and longitudinal size with prolonging the hydrothermal time. Equal amount of copper oxide totally highly distributed on the surface of CeO2(rod) supports with different abundance of oxygen vacancies facilitate to produce the strongly bound Cu-[Ox]-Ce species to different degrees (supported by XPS, H2-TPR and in situ DRIFTs results), which is encouraged by prolongation of hydrothermal time of CeO2(rod) supports. The essential role of interfacial Cu-Ce interaction in CuCe(rod) catalysts for CO-PROX was identified by the enhanced catalytic performance of CuCe(rod)-48h, on account of much stronger Cu-Ce interaction generated in it. Moreover, we proposed a potential structural model of strongly bound Cu-[Ox]-Ce to interpret the synergetic effect of copper and ceria species in CuO/CeO2 catalysts and generalized the possible reaction mechanism for CO-PROX over the CuCe(rod) catalyst.Download high-res image (245KB)Download full-size image

We prepared four ceria supports with different morphologies (rod, cube, plate and polyhedra) by using hydrothermal methods and supported copper oxide catalysts, which are used for selective oxidation of CO in hydrogen-rich gas and characterized by various techniques. The results show that CuO/CeO2-rod and CuO/CeO2-polyhedra exhibit a higher low-temperature catalytic oxidation activity coupled with a broader operating temperature “window” (CO conversion > 99.0%, 95–125 °C and 90–125 °C, respectively) in CO selective oxidation reaction, due to the higher content of the active species Cu+, the smaller copper oxide clusters subjected to a strong interaction with ceria and more oxygen vacancies on the surface of the catalysts. In addition, higher specific surface area, smaller average pore size and concentrated pore-size distribution of mesopores are also beneficial to improve the catalytic performance. However, CuO/CeO2-cube shows the worst performance for CO-PROX, mainly due to the weak interaction between copper oxide particles and the exposed [100] facet of nanocube-ceria.

Pd/CeO2–ZrO2–Nd2O3 (CZN) catalysts with different CeO2/ZrO2 molar ratios were synthesized and have been characterized by multiple techniques, e.g. XRD in combination with Rietveld refinement, UV-Raman, XPS and in situ DRIFTS. The XRD pattern of CZN with CeO2/ZrO2 molar ratios ≥1/2 can be indexed satisfactorily to the fluorite structure with a space group Fmm, while the XRD patterns of CZ12 only display diffraction peaks of the tetragonal phase (S.G. P42/nmc). Nd addition can effectively stabilize the cubic structure of the CZN support and increase the enrichment of defect sites on the surface, which may be related to the better catalytic activity of Pd/CZN12 catalysts compared with Pd/CZ12. The presence of moderate ZrO2 can increase the concentration of O* active species, leading to accelerate the formation of nitrate species and thus enhance the catalytic activity of NOx and HC elimination. The Pd-dispersion decreases with the increasing Zr content, leading to the decreased CO catalytic activity, especially for the aged catalysts. The change regularity of the OSC value is almost the same with the in situ dynamic operational window, demonstrating that the in situ dynamic operational window is basically affected by the OSC value.

HZSM-5, Al2O3, TiO2 and SiO2 supported CeO2-ZrO2-CrOx catalysts were prepared by deposition-precipitation method and tested for deep catalytic oxidation of 1,2-dichloroethane (DCE), as one of the common chlorinated organic pollutants. All the catalysts were characterized by means of N2 adsorption-desorption, X-ray photoelectron spectroscopy (XPS), ammonia-temperature-programmed desorption (NH3-TPD) and hydrogen temperature-programmed reduction (H2-TPR). The characterization results revealed that there was strongly synergistic effect between the oxidizability of CZCr species and the acidity of supports, which obviously promoted the catalytic activity for DCE degradation. 20%CZCr/HZSM-5 showed the highest activity and good durability during the long-term continuous test. The catalytic activity decreased in the order: 20%CZCr/HZSM-5>CZCr>20%CZCr/TiO2>20%CZCr/Al2O3>20%CZCr/SiO2.Catalytic performance for DCE degradation over fresh catalysts (a) Conversion curve of DCE; (b) Concentration of byproduct C2H3Cl; (c) Conversion curves of DCE over the various supports

Pd/CeO2-ZrO2-BaO (Pd/CZB) catalysts with 5.0 wt % BaO and different CeO2/ZrO2 molar ratio were synthesized. The modification with BaO significantly promotes the catalytic activity of HC and NOx conversions when the molar ratio of CeO2 to ZrO2 is 2/1, because the substitution of Cex+/Zr4+ ions by Ba2+ promotes the formation of oxygen vacancy as revealed by XRD refinement analysis results and improves the mobility of active oxygen as a result of electronic and structural modifications. Among the Ba-doped catalysts, the crystalline form of the CZB supports gradually transform from cubic and pseudocubic to stable tetragonal phase as the increase of ZrO2 content. Meanwhile, the Raman results show that addition of moderate ZrO2 promotes the formation of structural defects, resulting in higher OSC and NOx storage efficiency. On the other hand, the introduction of moderate ZrO2 facilitates the dissociative adsorption of NO and promotes the deep oxidation of HC. Therefore, Pd/CZB catalysts with CeO2/ZrO2 molar ratio of 3/1–1/1, especially Pd/CZB21, show a much better catalytic activity of HC and NOx eliminations and a wider dynamic operation window. Zr-rich catalysts present worse catalytic activity of CO oxidation compared with Ce-rich catalysts, which is mainly arising from the decreased amount of active sites. After thermal aging treatment, Zr-rich catalysts (CeO2/ZrO2 < 1) undergo less severe deterioration of the catalytic activity compared with Ce-rich catalysts (CeO2/ZrO2 ≥ 1) as a result of better thermostability, and Pd/CZB11-a presents the best catalytic performance and widest dynamic operation window.

•We study the effect of calcination temperature on CuO–MnOx–CeO2 catalyst for CO-PROX.•CuMnCe mixed oxide calcined at 500 °C shows the highest catalytic activity (T50% = 74 °C).•CuCMn(500) exhibits the broadest temperature window (110–140 °C).•A suitable calcination temperature leads to forming a stable Cu–Mn–Ce–O solid solution.•More active compounds (Cu+/Mn4+ species and oxygen vacancies) generate in CuCMn(500).Manganese doped CuO–CeO2 catalysts (CuO–MnOx–CeO2) with Mn/Cu molar ratio of 1:5 and variable calcination temperatures were prepared by a hydrothermal method and used for selective oxidation of CO in hydrogen-rich gas, characterized by various techniques. An appropriate calcination temperature shows an essential stimulative effect on CuO–MnOx–CeO2 catalysts for the CO-PROX. The catalyst calcined at 500 °C displays the highest low-temperature catalytic activity (T50% = 74 °C) and the broadest operating temperature “window” (CO conversion >99.0%, 110–140 °C). XRD and UV-Raman results reveal that an appropriate calcination temperature can promote the formation of Cu–Mn–Ce–O ternary oxide solid solution, adjust the degree of crystallinity of CeO2 and enhance the formation of oxygen vacancies. H2-TPR and XPS demonstrate that calcining at 500 °C improves the active reducing species in both bulk and surface of the catalyst. In situ DRIFTS suggests that Cu+ species which possess strong interaction with ceria also can be facilitated with a suitable calcination temperature.

A series of Pd/CeO2–ZrO2 (Pd/CZ) catalysts modified with barium oxide with different contents were prepared and characterized by XRD, BET, XPS, H2-TPR and NOx adsorption/desorption-MS. In particular, the effect of a structural promoter (Ba) on the catalytic and surface behaviour was investigated by means of in situ DRIFTS. Doped Ba enters into the CeO2 lattice, and thus results in the formation of more homogenous Ce–Zr–Ba ternary solid solution, which enhances the textural/structural thermostability and NOx storage efficiency of the catalysts. DRIFTS results indicate that the introduction of Ba weakens the strong adsorption of HC reactants on the surface of the catalysts. Furthermore, the addition of Ba accelerates the dissociation of NO and facilitates the formation of intermediates (NCO and CN) due to the outstanding electron-donating ability of Ba. Therefore, the Ba modified catalysts present much higher catalytic activity for HC and NOx conversions compared with Pd/CZ. However, the addition of Ba inhibits the oxidation of CO at low temperatures resulting from the decreased amount of active sites. In addition, the NOx operation window is widened with increasing Ba content due to the enhanced NOx storage capacity.

To reveal the role of alkaline earths modification in promoting the catalytic activity of HC and NOx conversion, catalytic activity tests and DRIFT experiments under different reaction conditions were performed. The introduction of alkaline earths promotes the catalytic activities of both HC and NOx elimination under stoichiometric NOx–CO–HC–O2 conditions, especially for Pd/CZBa, whereas it inhibits CO oxidation. In addition, the operation window of NOx conversion is also obviously widened as a result of enhanced NOx storage ability. DRIFTS results demonstrate that modification with alkaline earths improves the formation rate of nitrites and nitrates due to enhanced electron transfer and oxygen mobility, as well as the dissociation of NO species on the active metal, which increases the storage ability and reduction performance of NOx. On the other hand, the doping of alkaline earths could weaken the inhibiting effect arising from strongly bonded HC species on NO adsorption and dissociation due to the promoted catalytic activity of HC deep oxidation, which promotes the reaction of CO and NOx at low temperature.

The influence of oxidizing, reducing and reacting pretreatment gases on an automotive Pd/Ce0.67Zr0.33O2–Al2O3 catalyst is examined and characterized using catalytic performance tests, in situ DRIFTS, CO chemisorption, HRTEM, XPS, H2-TPR and OSC. The pretreatment gas impacts both the Pd particle size and chemical state, giving oxidized state and less growth of Pd in oxidizing gas. In contrast, Pd is mainly in the metallic state and exhibits more growth in reducing gas. In addition, reducing gas pretreatment also increases the concentration of oxygen vacancies. As a result, the catalyst pretreated with oxidizing gas shows good catalytic performance for HC oxidation. Pretreatment with reducing gas not only promotes the conversion of CO, NO and NO2, but also broadens the operation window. However, for the Pd/CZA-RG catalyst pretreated with reacting gas, based on the analysis of in situ DRIFTS, some active sites in the catalyst surface are blocked by strongly adsorbed HC species, hindering NO conversion.

Pd/CeO2–ZrO2–Pr2O3 (CZP) catalysts were synthesized with different Ce/Zr molar ratios and 8.0 wt % Pr2O3 doping. Their structures were characterized by various techniques, especially using X-ray diffraction (XRD) in combination with Rietveld refinement and Raman analysis. The XRD pattern of CZP with Ce/Zr molar ratios >1 can be indexed satisfactorily to the fluorite structure with a space group Fm-3m. When the Ce/Zr molar ratio reaches 1/2, the XRD patterns only display diffraction peaks of the tetragonal phase (S.G. P42/nmc). The Pr additive in the crystal lattice mainly replaces the position of Ce, which improves the redox reaction activity of catalysts when compared with Pr-free catalysts. Raman results reveal the enrichment of defect sites on the surface. The presence of moderate ZrO2 can increase both oxygen vacancies and the ZrO8-type complex in supports, which is in accord with the change regularity of their oxygen storage capacity (OSC) values. Moreover, the addition of moderate Zr and Pr promotes the catalytic activity of NOx, HC, and CO conversions and thermal stability and obviously widens the operational window due to their larger number of oxygen vacancies and higher OSC value, especially for Pd/CZP catalysts with Ce/Zr molar ratios from 2/1 to 1/2.

•Ce–Cr mixed oxides prepared by different methods show special texture/structure.•CeO2–CrOx–C exhibits high catalytic activity and stability for DCE decomposition.•Only trace of byproduct (C2H3Cl) is detected over CeO2–CrOx catalysts.•Strong interaction between CeO2 and CrOx improves the catalytic performance.Four CeO2–CrOx mixed oxide catalysts were synthesized by different preparation methods and investigated for deep oxidation of 1,2-dichloroethane (DCE), as a typical representative of the chlorinated volatile organic compounds (CVOCs). The results show that the CeO2–CrOx–C and CeO2–CrOx–M catalysts prepared by coprecipitation and microemulsion methods exhibit higher catalytic activity for DCE decomposition coupled with higher selectivity to CO2 and HCl formation and only trace of chlorinated byproduct is detected, probably due to their relatively more total acidity, higher ratio of strong/weak acidity, more Cr6+ species with strong oxidizing ability and more accessible oxygen species. Moreover, higher specific surface area and wider pore-size distribution of mesopores are also favorable for the improvement of catalytic performances.

A series of CrOx–CeO2/Ti-PILC (PILC is pillared interlayered clay) catalysts for n-butylamine oxidation were prepared using an impregnation method, and the structures, surface acidity distributions, and redox properties of the catalysts were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, H2 temperature-programmed reduction, and NH3 temperature-programmed desorption. The results show that addition of an appropriate amount of CeO2 enhances the interactions between Cr and Ce, and this increases the acid strength and mobility of active oxygen species on the catalyst. 8CrCe(6:1)/Ti-PILC(12,20) exhibits the best catalytic performance and control of NOx in n-butylamine oxidation.

CeO2–ZrO2 (CZ) doped with different transition metals was prepared through a co-precipitation and supercritical drying method and the corresponding Pd-only TWCs were characterized. Pd/CZFe and Pd/CZCo exhibit the best catalytic activity and the widest operation window, in accordance with the decrease of the light-off and full-conversion temperature. However, Pd/CZCr restrains the catalytic property. It may be due to the different effect of transition metals on the property of CZ. The introduction of Fe and Co promotes the surface atom ratios of Ce/Zr, makes part of Ce4+ transferred into Ce3+ in order to maintain the electrical neutrality and seems to facilitate the reduction of Ce4+ → Ce3+ or the formation of oxygen vacancies of CZ. The increasing concentration of oxygen vacancies for CZFe leads to the enhancement of DOSC. Moreover, the introduction of Fe favors the transfer of oxygen in different atmospheres.Graphical abstractHighlights► The introduction of Fe or Co promotes the surface atom ratios of Ce/Zr. ► The introduction of Fe or Co makes the part of Ce4+ transferred into Ce3+. ► The increasing of oxygen vacancies for CZFe leads to the enhancement of DOSC. ► Pd/CZFe and Pd/CZCo are active in promoting the three-way catalytic performance.

Selective oxidation of CO in H2-rich streams is performed over a series of CuO–MnOx–CeO2 catalysts prepared by hydrothermal (CuMC-HY), co-precipitation (CuMC-CP), impregnation (CuMC-IM) and citrate sol–gel (CuMC-SG) methods. The catalysts are characterized by N2 adsorption/desorption, XRD, SEM, HR-TEM, TPR and XPS techniques. The results show that the catalyst prepared by a hydrothermal method exhibits the best catalytic activity, especially at low temperatures. The temperature of 50% CO conversion (T50) is only 74 °C and the temperature window of CO conversions up to 99.0% is about 40 °C wide, from 110 to 140 °C. Moreover, the temperature window is still maintained 20 °C wide even at lower temperatures when there are 15% CO2 and 7.5% H2O in the reaction gas. The superior catalytic performance of CuMC-HY is attributed to the formation of Mn–Cu–Ce–O solid solution, the unique pore structure and the existence of more Cu+ and Mn4+ species as well as oxygen vacancies. The sequence of catalytic activity is as follows: CuMC-HY > CuMC-SG > CuMC-IM > CuMC-CP. The worst catalytic activity, obtained from the catalyst prepared by the co-precipitation method, is possibly related to the existence of independent CuOx and MnOx oxides, which weakly interact with ceria in the catalyst.Graphical abstractHighlights► CuMC-HY exhibits the best catalytic activity for CO PROX in H2-rich streams. ► The temperature range of CuMC-HY for CO complete conversion is from 110 to 140 °C. ► CuMC-HY maintains superior catalytic performance facing 15% CO2 and 7.5% H2O. ► The solid solution of Mn–Cu–Ce–O is formed over the CuMC-HY catalyst. ► A large number of Cu+ and Mn4+ species exist in CuMC-HY.

Different contents of praseodymia were introduced to modify Ce0.2Zr0.8O2 (CZ) and the supported Pd-only three-way catalysts before and after aging were also prepared. The influence of praseodymia doping on the structural/textural properties of CZ and the effect on the three-way catalytic activity were investigated. Structural and textural characterizations reveal that the addition of praseodymia results in the formation of Ce–Zr–Pr ternary solid solution (CZP) with higher specific surface area, better thermal stability and larger oxygen storage capacity (OSC) than that of CZ. It is more credible that Zr is replaced by Pr during the formation of CZP. The modified Pd-only three-way catalysts present relatively higher catalytic activity to the main target pollutants in gasoline engine exhaust and exhibit wider air/fuel operation window due to the improved properties of CZP.Graphical abstractThis figure displays the CO2 response peaks over Pd/CZ, Pd/CZP5, Pd/CZa and Pd/CZP5a derived from the dynamic oxygen storage capacity (DOSC) testing. The results show that the addition of Pr into Ce0.2Zr0.8O2 (CZ) leads to enhanced DOSC of its supported Pd-only three-way catalyst no matter before and after aging.Highlights► The effect of Pr doping on Ce0.2Zr0.8O2 solid solution is investigated. ► The doping effect on the supported Pd-only three-way catalyst is also studied. ► The Pr doping results in enhanced oxygen storage capacity and thermal stability. ► The Pr doping results in improved three-way catalytic activity. ► The Pr doping leads to relatively wide air/fuel operation window.

In this paper, five types of ceria–zirconia modified alumina (CZA) were prepared by co-precipitation with supercritical drying (CPS), co-precipitation with common drying (CPC), sol–gel (SG), micro-emulsion (ME) and impregnation (IM) methods, respectively. The corresponding supported Pd-only three-way catalysts (TWC) were also prepared and evaluated in the simulative gasoline engine exhaust. The influence of different preparation methods on the physicochemical properties of CZA mixed oxide and its supported TWC was characterized by X-ray diffraction (XRD), N2 adsorption/desorption and transmission electron microscopy (TEM) techniques. The redox behavior was investigated with H2 temperature-programmed reduction (H2-TPR) experiments. The results reveal that the CZA mixed oxides derived from CPS and ME methods exhibit the relatively higher textural and structural properties as well as better redox behavior, which lead to the better catalytic activity and wider air-to-fuel operation window of the corresponding Pd-only three-way catalysts.Graphical abstractHighlights► The ceria–zirconia modified alumina (CZA) was prepared by five different methods. ► The effect of preparation methods on the structural properties of CZA was studied. ► Coprecipitation with supercritical drying leads to good thermal stability of CZA. ► The corresponding Pd-only three-way catalyst exhibits higher catalytic performance.

The influence of rare earth elements (La, Nd, Pr, Sm and Y) addition to Ce0.2Zr0.8O2 and its supported Pd-only three-way catalysts has been investigated by X-ray diffraction (XRD), N2 adsorption/desorption, X-ray photoelectron spectroscopy (XPS) and H2 temperature programmed reduction (H2-TPR) techniques, and that the dynamic oxygen storage capacity (DOSC) has also been evaluated under transient conditions. Special attention was given to the information of structural modification and the effect of doping on the three-way catalytic performance. The phase for all the ceria–zirconia-rare earth ternary solid solution is single tetragonal, irrespective of the treatment temperature applied. The presence of La, Nd and Pr results in enhanced thermal stability, improved reducibility and increased strong metal–support interaction, leading to the relatively higher three-way catalytic activity for all the target pollutants over the corresponding catalysts. The air/fuel operation window is also enlarged due to the increased dynamic oxygen storage capacity.Graphical abstractHighlights► The effect of rare earth doping on Ce0.2Zr0.8O2 solid solution is investigated. ► The doping effect on the supported Pd-only three-way catalyst is also studied. ► The addition of La, Nd and Pr results in improved textural/structural properties. ► The addition of La, Nd and Pr leads to improved three-way catalytic performance.

The CeO2 or/and CuO modified USY zeolite catalysts were prepared and investigated in the catalytic behavior for chlorinated volatile organic compounds (CVOCs) decomposition as well as the durability during 100 h exposure to 1,2-chloroethane (DCE). The results reveal that modified USY catalysts show good catalytic activity for CVOCs decomposition, and high selectivity to the formation of CO2 and HCl. The better catalytic activity of the CeO2 or/and CuO modified USY catalysts can be ascribed to the high dispersion of active phases (CeO2 or CuO), the improved mobility of active oxygen species and the increment of Lewis acidity. The addition of CeO2 or/and CuO improves the durability of the catalysts during the long term exposure to DCE due to the slight coke deposition and preserved high density of acid sites.Graphical abstractHighlights► The catalytic activity for CVOCs destruction is evidently enhanced over modified USY catalysts. ► The high activity is due to high dispersion of CeO2 or CuO, good oxygen mobility and Lewis acidity. ► Modified USY catalysts present high selectivity to HCl and CO2 formation. ► Interaction between CuO and CeO2 improves the durability of the catalyst in long term reaction.

Automotive exhaust emission is a major cause of air pollution. Three-way catalyst (TWC) which can eliminate CO, HC (hydrocarbons), and NOx simultaneously has been used to control exhaust emissions. Ceria-zirconia is a key component in TWC and most researchers pay attention to Ceria-Zirconia (Ce-rich) solid solution. The research presented in this paper is focused on the intrinsic structure of Ceria-Zirconia (Zr-based) solid solution and its application in TWC. A series of Ce0.2Zr0.8O2 modified with rare earths (La, Nd, Pr, Sm, and Y) have been prepared by coprecipitation method combined with supercritical drying technique. All samples showed single tetragonal solid solution, indicating that the rare earth ion inserted into the lattice structure completely, and an approximately linearly relationship between lattice parameter a and the ionic radius of doped rare earth was observed. The catalytic performances of corresponding Pd-only catalysts were investigated in simulated exhaust gas. The presence of La, Nd, and Pr was favorable to the catalytic activity and wide air/fuel operation window. The relationship between the intrinsic structure of the Zr-based ceria-zirconia solid solution and catalytic activity was discussed in detail, which has some reference value for catalyst design and application.

Ce0.67Zr0.33O2 (CZ) mixed oxide was prepared by coprecipitation and effect of the synthesis conditions on CZ properties were investigated in this work, as well as the catalytic performance of Pd-only three-way catalyst (TWC) supported on the prepared CZ. With the increase of aging temperature during precipitation, the crystallite size of CZ increases significantly, while the oxygen storage capacity complete (OSCC) drops and no notable effect on porosity of CZ is observed. The porosity is mainly determined by drying temperature and pH value during precipitation. Prepared under optimized conditions (precipitation at the pH of 9.5, aging at 25 °C and then drying under supercritical condition in alcohol), the CZ samples exhibit good properties. This preparation procedure leads to CZ with proper pore size distribution, larger average pore size (20.42 nm), higher pore volume (0.69 cm3/g) and enhanced OSCC value (383.0 μmol/g). Furthermore, the Pd-only TWC supported on the CZ synthesized under the optimized conditions exhibits satisfactory catalytic activity and very low reduction temperature (53 °C).Research highlights▶ In this paper, influence of synthesis condition on properties of Ce0.67Zr0.33O2 (CZ) mixed oxides was firstly studied in detail and coprecipitation was taken as the standard method. ▶ With the increase of aging temperature during precipitation, the crystallite size of CZ increases significantly, while the oxygen storage capacity complete (OSCC) drops and no notable effect on porosity of CZ is observed. Drying temperature has a significant influence on the porosity rather than on the structural properties of CZ. Moreover, the textural and structural properties of CZ are affected by the pH value during precipitation. ▶ The catalytic activity of Pd-only three-way catalyst (TWC) is related to the properties of CZ mixed oxides and the reducibility of PdO species. The best synthesis condition of CZ was obtained and the Pd-only TWC supported on this support exhibits the best catalytic activity.

CeO2–ZrO2–Al2O3 (CZA) mixed oxides were synthesized using sol–gel and supercritical drying methods, and their physicochemical properties were characterized by BET surface area (BET), X-ray diffraction (XRD), H2-temperature-programmed reduction (H2-TPR) and O2-temperature-programmed desorption (O2-TPD) techniques. The catalytic performances of Pd/CZA catalysts with different thermal treatment were also investigated. The results indicate that catalytic activities are affected by thermal treatment of support or catalyst and Pd/CZA catalysts exhibit good thermal stability. H2-TPR results show that the effect of thermal treatment on the dispersion state of PdO is different between catalyst and support after calcination. However, O2-TPD results show that the influence of thermal treatment on the desorption performance of oxygen is similar in both cases. Desorption of surface oxygen is closely related to surface area of the support. During the calcination of the supports or the catalysts at high temperature, the surface oxygen desorption capacity and the mobility of lattice oxygen decrease due to the sintering of supports, which is harmful to the dispersion of active species, corresponding to the decrease of three-way catalytic activities.

Three supported CeO2–USY catalysts were prepared by different CeO2 loading methods and evaluated for the deep oxidation of 1,2-dichloroethane (DCE). All the catalysts were characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), ammonia temperature-programmed desorption (NH3-TPD), diffuse reflectance infrared spectra of pyridine adsorption (DRIFT), hydrogen temperature-programmed reduction (H2-TPR), oxygen storage capacity complete (OSCC) and X-ray photoelectron spectroscopy (XPS) techniques. The results show that a strong synergy effect occurs in the CeO2–USY catalysts prepared by impregnation method and mechanical grinding of USY and Ce(NO3)3·6H2O. Moreover, higher dispersion of CeO2 species, better mobility of oxygen species of the catalysts as well as better catalytic activity for DCE decomposition are obtained over these two catalysts. All the CeO2–USY catalysts show a high selectivity towards the formation of HCl. In addition, the CeO2–USY catalyst prepared by impregnation method maintains a high conversion in the 100 h test of DCE decomposition.The figure presents the H2-TPR profiles of samples. It is noting that the strong interaction between CeO2 and USY in CeO2–USY-IM and CeO2–USY-M1 evidently improves the mobility of oxygen species.

Three Y zeolites supported CeO2 catalysts (CeO2/USY, CeO2/HY, CeO2/SSY) were prepared and used for deep oxidation of 1,2-dichloroethane (DCE) in low concentration (about 1,000 ppm). The catalysts were characterized by XRD, N2 adsorption/desorption and H2-TPR. The results showed that the catalytic activity of the supported CeO2 catalysts was much higher than that of Y zeolites, in particular, CeO2/USY exhibited the highest activity, T98% values of DCE was about 270 °C. And the catalytic activity was strongly related to the interaction between CeO2 and Y zeolites.

Influence of residual K+K+ on the preferential oxidation of CO in excess hydrogen (PROX) over CuO–CeO2CuO–CeO2 catalysts was investigated. CuO–CeO2CuO–CeO2 catalysts were characterized by BET, ICP, XRD, UV-Raman and TPR techniques. The results showed that the existence of residual K+K+ made αα peak in TPR of CuO–CeO2CuO–CeO2 catalysts shift to higher temperatures and depressed the PROX in the absence of CO2CO2 and H2OH2O in the feed over CuO–CeO2CuO–CeO2 catalysts. However, small amount of residual K+K+ was beneficial to the catalytic performance of CuO–CeO2CuO–CeO2 catalysts in the PROX in the presence of CO2CO2 and H2OH2O in the feed. Consequently, residual K+K+ with an appropriate content was beneficial to improve the catalytic performance of CuO–CeO2CuO–CeO2 catalysts in the presence of CO2CO2 and H2OH2O.

Alumina pillared clays (Al-PILC) were impregnated with Ce and Pd, and then studied as catalysts in the deep oxidation of benzene of low concentration (130–160 ppm). The influence of synthesis conditions on pore structure of Al-PILC was investigated, and also the relationship between structure of the supports and catalytic properties in benzene oxidation of the catalysts was studied. The supports and catalysts were characterized by X-ray powder diffraction, N2 adsorption, atomic force microscopy, transmission electron microscopy, and temperature programmed reduction techniques. The results showed that pillaring caused a strong increase in basal spacing (d001), surface area, micropore volume (Vmic) and mesopore surface area (Ames). Doping with Ce on Al-PILC yielded a super structure and a reduction in Vmic and Ames. Activity tests showed that optimized synthesis conditions of pillaring solution and doping with Ce can obviously improve the activity of Pd/Al-PILC catalysts. The optimized structure of the supports can strengthen the interaction between CeO2 and Al-PILC, improve the dispersion of Pd particles and increase the active sites, which enhances the catalytic activity of catalysts for the deep oxidation of low concentration of benzene.

Pt–Ni/CNTs catalysts are prepared by different impregnation techniques and different reduction methods (H2, HCHO, and KBH4) for the selective hydrogenation of cinnamaldehyde (CMA) to hydrocinnamaldehyde (HCMA) and investigated by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and temperature programmed reduction (H2-TPR) techniques. The results show that the catalytic selectivity and activity of the Pt–Ni/CNTs catalysts would significantly be improved by using KBH4 as a reducing agent, due to the electronic synergetic effect of Pt–Ni–B, and 96% for conversion of CMA and 88% for selectivity of HCMA are obtained over Pt–Ni/CNTs catalyst reduced by KBH4. Furthermore, the hydrogenation rate of CMA and selectivity of CMA to HCMA over Pt–Ni/CNTs catalyst are significantly improved in the presence of trace base or acid promoters again. The best result (92% for conversion of CMA and 96% for selectivity of HCMA) is obtained when NaOAc is used as base promoter.

The methane combustion behavior is investigated over Pd catalysts supported on Ce–Zr/Al2O3 and Ce–Zr–M/Al2O3 (M = Mg, Ca, Sr, Ba). Characterization of the catalysts is performed by BET, XRD, TEM, H2-TPR, O2-TPO and CH4/O2-TPSR techniques. Activity tests in methane combustion show that the addition of trace alkaline earths to Pd/Ce–Zr/Al2O3 catalyst obviously increases the catalytic activity of the catalysts under lower reaction temperature conditions, and Pd/Ce–Zr–Ca/Al2O3 exhibits the highest catalytic activity and thermal stability among all catalysts. The addition of Ca to Pd/Ce–Zr/Al2O3 inhibits the site growth and decomposition of PdO particles and improves the reduction–reoxidation properties of the active PdO species, which increases the catalytic activity and thermal stability of the Pd/Ce–Zr/Al2O3 catalyst.The effects of alkaline earths (Mg, Ca, Sr and Ba) addition to Pd/Ce–Zr/Al2O3 catalyst on methane combustion behavior have been investigated. The addition of Ca to Pd/Ce–Zr/Al2O3 inhibits the site growth and decomposition of PdO particles and improves the reduction–reoxidation properties of the active PdO species, which increases the catalytic activity and thermal stability of the Pd/Ce–Zr/Al2O3 catalyst.

Ce–Zr–M (M = La, Pr, Nd, Sm and Y) ternary mixed oxides were first prepared by coprecipitation method combined with supercritical drying technique. The Pd-only three-way catalysts (TWC) were obtained by the incipient impregnation method, in which Ce–Zr–M ternary mixed oxides were used as supports. The samples were characterized by nitrogen adsorption–desorption, X-ray powder diffraction, X-ray photoelectron spectroscopy, oxygen storage capacity complete, H2-temperature programmed reduction techniques and transmission electron microscopy. The effects of rare earth addition on Pd-only three-way catalytic activities have also been investigated. The results show that rare earth doping improves remarkably structural and textural stability of mixed oxide. After aging treatment, the crystallite size of CeO2–ZrO2 is 21.4 nm, but it decreases to 9.6, 9.3, 9.6, 10.4 and 9.4 nm with doping of La, Pr, Nd, Sm and Y into CeO2–ZrO2, respectively. The incorporation of La, Nd, Pr, Sm and Y into ceria–zirconia lattice enhances the oxygen storage capacity and markedly improves the interaction between PdO species and support. After aging treatment, Pd-only three-way catalyst supported on Ce–Zr–M ternary mixed oxide exhibits superior activity for HC (C3H6 + C3H8) and NOx conversion. Furthermore, compared to Y, Sm or La, more homogeneous ternary solid solution of Ce–Zr–M (M = Nd, Pr) is formed and the aged catalyst supported on CZ doped by Nd or Pr exhibits better influence on the thermal stability. Therefore, Ce–Zr–M (M = Nd, Pr) should be more suitable as support for TWC especially after aging treatment.

High performance CuO-CeO2 catalysts for selective oxidation of CO in excess hydrogen were prepared by a hydrothermal method under different preparation conditions and evaluated for catalytic activities and selectivities. By changing the nCTAB/nCe ratio and hydrothermal aging time, the catalytic activity of the CuO-CeO2 catalysts increased and the operating temperature window, in which the CO conversion was higher than 99%, was widened. XRD results showed no peaks of CuOx species and Cu-Ce-O solid solution were observed. On the other hand, Cu+ species in the CuO-CeO2 catalysts, which was associated with a strong interaction between copper oxide clusters and cerium oxide and could be favorable for improving the selective oxidation performance of CO in excess H2, were detected by H2-TPR and XPS techniques.

Doping of different rare-earth metals (Pr, Nd, Y and La) had an evident influence on the catalytic performance of CuO-CeO2 for the preferential oxidation (PROX) of CO in excess hydrogen. As for Pr, the doping enhanced the catalytic activity of CuO-CeO2 for PROX. For example, the CO conversion over the above catalyst for PROX was higher than 99% at 120 °C. Especially, the doping of Pr widened the temperature window by 20 °C over CuO-CeO2 with 99% CO conversion. For Nd, Y, and La, the doping depressed the catalytic activity of CuO-CeO2 for PROX. However, the doping of transition metals markedly improved the selectivity of CuO-CeO2 for PROX.

CuO-CeO2 catalyst prepared with co-precipitation showed high catalytic performance for the preferential oxidation of CO in excess hydrogen (PROX). Influence of pH values in the preparation of CuO-CeO2 on its catalytic performance was investigated in this work. The CuO-CeO2 catalyst prepared at pH = 13.03 had the smallest particle size (5.4 nm), the largest surface areas (138 m2/g) and the highest activity with CO conversion of 99.6% at 130 °C. The CuO-CeO2 catalyst was characterized using BET, XRD and TPR techniques. The results showed that when the pH value of the mixed solution containing Cu and Ce species was properly adjusted, both the adsorption layers and diffusion layers of the formed colloidal particles in hydroxide precursor of CuO-CeO2 were modified, resulting in the better catalytic performance for PROX on the final CuO-CeO2 catalyst

AbstractIn the present study, we have investigated the reducibility of CuO species on CuO-CeO2 catalysts and the influence of CuO species on the catalytic performance for CO preferential oxidation (CO PROX) in excess hydrogen. It is revealed that the smaller the difference of reduction temperature (denoted as T) for two adjacent CuO species is, the higher the catalytic activity of CuO-CeO2 for the PROX in excess hydrogen may be obtained. It means that if the reduction energy of Cu0-Cu2+ pairs matched better, the reduction-oxidation recycle of Cu0-Cu2+ pairs would go on more easily, then the transferring energy of Cu0-Cu2+ pairs would be lesser. Therefore, the CuO-CeO2 catalysts will be largely improved in their catalytic performance if the different CuO species on the catalysts have matched the reduction energy, which would allows them to cooperate effectively.

Influence of three different preparation methods, i.e. impregnation, coprecipitation, and inverse coprecipitation, on the preferential oxidation of CO in excess hydrogen (PROX) over CuO-CeO2 catalysts has been investigated and CuO-CeO2 catalysts are characterized using BET, XPS, XRD, UV Raman, and TPR techniques. The results show that the catalysts prepared by coprecipitation have smaller particle sizes, well-dispersed CuOx species, more oxygen vacancies, and are more active in the PROX than those prepared by the other methods. However, the inverse coprecipitation depresses the catalytic performance of CuO-CeO2 catalysts and causes the growth of CuO-CeO2 because of different pH value in the precipitation process.

AbstractThe CuO-CeO2 catalyst prepared by chelating method has a superior catalytic performance for the preferential oxidation of CO in rich hydrogen, compared with the CuO-CeO2 catalyst prepared by coprecipitation method. The CO conversions over these catalysts, at 120 °C and 120000 ml/(g·h) in the absence of CO2 and H2O, are 99.6% and 88.6%, respectively, and the selectivity of O2 over these catalysts is very close (i.e. 51.3% and 55.8%, respectively). The influence of certain factors such as hydrogen concentration, carbon monoxide concentration, H2O, O2/CO ratios, and space velocity on the catalytic performance of CuO-CeO2 catalyst prepared by chelating method is also studied. The results show that the addition of hydrogen and H2O has a negative effect on the catalytic performance of CuO-CeO2 catalyst, however, the variation of space velocity and the O2/CO ratio causes a comparatively slight influence.

Ce0.67Zr0.33O2 (CZ) doped by nickel oxide with different content and the corresponding Pd-only three-way catalysts before and after aging has been prepared and characterized. The investigations show that CZ doped with nickel oxide obviously results in more active fresh catalysts with enhanced textural properties, but suffers from a net loss of activity after aging. The introduction of Ni promotes the reducibility of samples, causing the enhancement of oxygen storage capacity (OSC) of fresh samples. CZNi(3%) exhibits better textural and structural properties because of the formation of more homogeneous Ce–Zr–Ni–O ternary solid solution, which promotes the interaction between Ce–Zr and Ni. However, the thermal aging leads to a loss of surface area and a significant decrease of the reducibility and OSC, except for CZNi(3%). Only Pd/CZNi(3%) represents better catalytic activity after aging. It reveals that the catalytic behavior of these bimetallic systems is strongly affected by the nature of support.

Four types of Ce0.67Zr0.33O2 (CZCP, CZH, CZHP and CZM) mixed oxides as supports were prepared by coprecipitation, hydrothermal, homogeneous precipitation and microemulsion methods, respectively. The results show that both the preparation methods and the aging procedures have significant influences on the physicochemical properties of Ce0.67Zr0.33O2 mixed oxides. All these fresh Ce0.67Zr0.33O2 samples develop mesopore structure, especially that, CZCP obtains the largest diameter (20.87 nm) and volume (0.455 cm3/g). The XRD results indicate that fresh CZHP and CZCP samples contain cubic and tetragonal phases, while only cubic solid solution is found in fresh CZH and CZM samples, which show better low-temperature reducibility. Moreover, higher oxygen storage capacity complete (OSCC) values and faster O2 uptake are observed in CZH and CZM samples compared with those of fresh CZCP and CZHP. After calcination at 1100 °C, it is found that the OSCC values and the kinetics of the O2 uptake for CZM and CZHP samples are deteriorated due to the coexistence of cubic and tetragonal phases as well as strong sintering of samples. On the contrary, the aged CZCP and CZH samples keep satisfactory low-temperature reducibility and oxygen storage capacity. For fresh catalysts, the pore-size distribution of mixed oxides seems to be important for catalytic activities. In contrast, for aged catalysts, their activities are related to both the redox properties and the textural properties of aged mixed oxides. Therefore, the Pd-only three-way catalyst supported on the CZCP exhibits the most promising amplified amplitude of stoichiometric window.

CZ doped by nickel oxide with different loadings was prepared through a co-precipitation and supercritical dried method and the corresponding Pd-only TWCs were prepared and characterized. The results demonstrate that Pd/CZNi(3%) catalyst exhibits the best catalytic activity and the widest operation window before and after aging, compared with that of other catalysts. Moreover, the introduction of nickel oxide with 3% loading increases the average pore diameter and broadens the range of pore size distribution of sample, which is propitious to the formation of accumulative pore favoring the adsorption/desorption of reaction species. The introduction of nickel oxide with suitable loading (3%) promotes the thermal stability of CZ corresponding to the better surface composition and more concentration of Ce3+ or oxygen vacancy. Furthermore, the introduction of nickel oxide with 3% loading exhibits the best DOSC compared with other content doping, possibly due to the better structure and better surface composition and better thermal stability of CZNi(3%).Graphical abstractDownload high-res image (114KB)Download full-size imageHighlights► Pd/CZNi(3%) catalyst exhibits the best catalytic activity before and after aging. ► The introduction of Ni with 3% loading promotes the thermal stability of CZ. ► CZNi(3%) exhibits better surface composition and more concentration of Ce3+. ► The increasing of oxygen vacancies for CZNi leads to the enhancement of DOSC.

•The increase of oxygen vacancies quantity enhances the metal-support interaction.•Zr accelerates the formation rate of nitrate/carboxylate/carbonate species.•Higher Zr amount favors catalytic activity of HC and NO eliminations.•Pd/CZP catalysts with Ce/Zr molar ratio of 2:1 ∼ 1:2 exhibit good thermal stability.•Ce-rich catalysts display better CO and NO2 elimination performance.Pd/CeO2–ZrO2–Pr2O3 (CZP) catalysts with different Ce/Zr molar ratios were synthesized and systematically investigated by XRD, N2 adsorption–desorption, XPS, H2-TPR, OSC and in situ DRIFTS techniques. The results of XPS, in situ DRIFTS, etc., show that the number of oxygen vacancies increases with the increasing Zr content and thus leads to the enhanced metal-support interaction and the accelerative formation rate of nitrate, formate, acetate and carbonate species, resulting in improving catalytic performance for HC and NO elimination, especially for Pd/CZP catalysts with Ce/Zr from 1/2 to 1/3. While Pd/CZP catalysts with higher OSC value (Ce/Zr = 4/1–1/2) exhibit better catalytic activity of CO and NO2 elimination. An appropriate concentration of Zr facilitates the diffusion of Pr from the surface to the bulk of the CZP supports, thus forming more homogeneous CZP solid solution and improving the structure/textual stability, which promotes the thermal stability of catalysts. Pd/CZP catalysts with Ce/Zr from 2/1 to 1/2 exhibit good thermal stability.

In this work, supercritical drying technology is investigated as a new method contrast with the conventional drying techniques to obtain La modified ceria–zirconia solid solution (CZL) with advanced textural/structural properties, and its supported Pd-only three-way catalysts (TWCs) were also prepared and studied. The results demonstrate that the CZL sample prepared by supercritical drying method shows relatively larger specific surface area, better thermal stability and higher redox properties, as well as the prominent oxygen storage capacity compared with samples prepared by conventional drying method. Moreover, it also exhibits remarkable porosity and wide pore size distribution due to the elimination of vapor–liquid interface in the process of supercritical drying, which is beneficial to the adsorption/desorption of pollutant in TWCs. The excellent structural/textural properties of the forenamed CZL support lead to the outstanding catalytic activity, wide air-to-fuel operation window of the corresponding three-way catalyst, indicating its tremendous potential possibilities.Graphical abstractDownload full-size imageResearch highlights▶ CeO2–ZrO2–La2O3 (CZL) was prepared by advanced supercritical drying technique. ▶ Supercritical drying results in special structural/textural property of CZL. ▶ The supported three-way catalyst exhibits superior catalytic activity. ▶ The supported three-way catalyst shows wide air/fuel operation window.

Large pore Al/Ce pillared clays (AlCe-PILC) were synthesized and used as supports for Pd catalysts in deep oxidation of low concentration of benzene (130–160 ppm). The supports and catalysts are characterized by X-ray powder diffraction (XRD), N2 adsorption/desorption, high resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy (EDX) and temperature-programmed reduction techniques (TPR). The results showed that AlCe-PILC are characterized by basal spacings of 1.79–2.83 nm and have high surface areas of 343.6–377.4 m2/g. Catalytic activity tests show that the catalytic activity of Pd catalysts in benzene deep oxidation is greatly dependent on the type of supports. Temperature for the complete benzene conversion using the following catalysts decreases in the order: Pd/Na-mmt (>400 °C) > Pd/Al-PILC (340 °C) > Pd/AlCe-PILC (≤300 °C). And the effects of hydrothermally treated time and different loading of Ce of Al/Ce pillaring solution on the activity of Pd/AlCe-PILC catalysts were also investigated, results of which show that high mesopore surface area and large pore structures of the supports are the key factors in improving the activity of the Pd/AlCe-PILC catalysts. The temperature for complete oxidation of benzene with Pd/AlCe-PILC (5;30) catalyst was 250 °C, exhibiting the highest catalytic activity.

We prepared four ceria supports with different morphologies (rod, cube, plate and polyhedra) by using hydrothermal methods and supported copper oxide catalysts, which are used for selective oxidation of CO in hydrogen-rich gas and characterized by various techniques. The results show that CuO/CeO2-rod and CuO/CeO2-polyhedra exhibit a higher low-temperature catalytic oxidation activity coupled with a broader operating temperature “window” (CO conversion > 99.0%, 95–125 °C and 90–125 °C, respectively) in CO selective oxidation reaction, due to the higher content of the active species Cu+, the smaller copper oxide clusters subjected to a strong interaction with ceria and more oxygen vacancies on the surface of the catalysts. In addition, higher specific surface area, smaller average pore size and concentrated pore-size distribution of mesopores are also beneficial to improve the catalytic performance. However, CuO/CeO2-cube shows the worst performance for CO-PROX, mainly due to the weak interaction between copper oxide particles and the exposed [100] facet of nanocube-ceria.

A series of Pd/CeO2–ZrO2 (Pd/CZ) catalysts modified with barium oxide with different contents were prepared and characterized by XRD, BET, XPS, H2-TPR and NOx adsorption/desorption-MS. In particular, the effect of a structural promoter (Ba) on the catalytic and surface behaviour was investigated by means of in situ DRIFTS. Doped Ba enters into the CeO2 lattice, and thus results in the formation of more homogenous Ce–Zr–Ba ternary solid solution, which enhances the textural/structural thermostability and NOx storage efficiency of the catalysts. DRIFTS results indicate that the introduction of Ba weakens the strong adsorption of HC reactants on the surface of the catalysts. Furthermore, the addition of Ba accelerates the dissociation of NO and facilitates the formation of intermediates (NCO and CN) due to the outstanding electron-donating ability of Ba. Therefore, the Ba modified catalysts present much higher catalytic activity for HC and NOx conversions compared with Pd/CZ. However, the addition of Ba inhibits the oxidation of CO at low temperatures resulting from the decreased amount of active sites. In addition, the NOx operation window is widened with increasing Ba content due to the enhanced NOx storage capacity.

Pd/CeO2–ZrO2–Nd2O3 (CZN) catalysts with different CeO2/ZrO2 molar ratios were synthesized and have been characterized by multiple techniques, e.g. XRD in combination with Rietveld refinement, UV-Raman, XPS and in situ DRIFTS. The XRD pattern of CZN with CeO2/ZrO2 molar ratios ≥1/2 can be indexed satisfactorily to the fluorite structure with a space group Fmm, while the XRD patterns of CZ12 only display diffraction peaks of the tetragonal phase (S.G. P42/nmc). Nd addition can effectively stabilize the cubic structure of the CZN support and increase the enrichment of defect sites on the surface, which may be related to the better catalytic activity of Pd/CZN12 catalysts compared with Pd/CZ12. The presence of moderate ZrO2 can increase the concentration of O* active species, leading to accelerate the formation of nitrate species and thus enhance the catalytic activity of NOx and HC elimination. The Pd-dispersion decreases with the increasing Zr content, leading to the decreased CO catalytic activity, especially for the aged catalysts. The change regularity of the OSC value is almost the same with the in situ dynamic operational window, demonstrating that the in situ dynamic operational window is basically affected by the OSC value.

Four different synthetic routes (co-precipitation, oxidation–precipitation, citric acid sol–gel and reversed microemulsion) are adopted to prepare barium modified Pd/CeO2–ZrO2 catalysts and their catalytic activity towards CO, HC and NOx conversions is studied. The surface and bulk properties of these catalysts are characterized via XRD, N2 adsorption, XPS, UV-Raman, H2-TPR, and in situ DRIFTS. The catalyst prepared via the co-precipitation method exhibits the optimum three-way catalytic behavior, which is mainly due to its superior redox ability, whereas the oxidation–precipitation synthesis renders the catalyst with the best homogeneity and thermal resistance. However, for the catalyst prepared via the sol–gel route, its worst NOx reduction capacity is verified by the scarce appearance of negatively charged Pd0–NOδ− species, which is related to the faster dissociation of NO based on in situ DRIFTS, and the abundance of surface CO–Pd+ species reveals its unsatisfactory deep oxidizability of the HC reactant.

To reveal the role of alkaline earths modification in promoting the catalytic activity of HC and NOx conversion, catalytic activity tests and DRIFT experiments under different reaction conditions were performed. The introduction of alkaline earths promotes the catalytic activities of both HC and NOx elimination under stoichiometric NOx–CO–HC–O2 conditions, especially for Pd/CZBa, whereas it inhibits CO oxidation. In addition, the operation window of NOx conversion is also obviously widened as a result of enhanced NOx storage ability. DRIFTS results demonstrate that modification with alkaline earths improves the formation rate of nitrites and nitrates due to enhanced electron transfer and oxygen mobility, as well as the dissociation of NO species on the active metal, which increases the storage ability and reduction performance of NOx. On the other hand, the doping of alkaline earths could weaken the inhibiting effect arising from strongly bonded HC species on NO adsorption and dissociation due to the promoted catalytic activity of HC deep oxidation, which promotes the reaction of CO and NOx at low temperature.