CeO2 ZrO2 Catalysts Research Articles

Soot oxidation over Pt-loaded CeO2-ZrO2 catalysts under gasoline exhaust conditions: Soot-catalyst contact efficiency and Pt chemical state

Decreasing the ignition temperature of soot combustion is a key of the improved catalyst performances, but a much lower concentration of oxygen from gasoline vehicle exhaust makes the research of the catalyst extremely challenging. Improving the adsorption and activation of oxygen molecules on the catalyst surface and the effective migration rate of active oxygen species are the key factors to enhance soot oxidation. In this work, Pt/CeO2-ZrO2-re catalyst was synthesized by synergistically adjusting CeO2-ZrO2 pore structure and Pt chemical state, as a result, the conversion temperature (T50, 50% soot was oxidized) was reduced by 113 ℃ in 1% O2/N2. The pore structure of CeO2-ZrO2 was optimized by copolymerization microsphere template assisted method, and better soot-contact efficiency and intrinsic properties were obtained, resulting in a higher availability of reactive oxygen species. In addition, adjusting Pt chemical state by using the liquid phase reduction method is significant for the adsorption and activation of oxygen molecules and catalytic cycle of soot oxidation. Structure-activity relationship studies showed that soot oxidation performance is highly related to the initial oxidation state of Pt and pore structure in 1% O2/N2. Better soot-contact efficiency, improved intrinsic properties and the existence of metallic Pt are responsible for the higher soot oxidation activity of Pt/CeO2-ZrO2-re catalyst.

Catalytic oxidation of soot by CeO2–ZrO2 catalysts: Role of Zr

Human health has been adversely affected by the soot particles in the exhaust of diesel engines. In this study, ceria-zirconia supported 1, 3, and 5 wt% zirconium (Zr) catalysts were prepared using the co-precipitation method and tested for soot combustion. The chemical and physical properties of catalysts were evaluated through NH3-temperature-programmed desorption (NH3-TPD), scanning electron microscopy (SEM), physisorption analysis, Raman spectroscopy, H2-temperature programmed reduction (H2-TPR), and X-ray diffraction (XRD). The catalytic activity of the catalysts for soot combustion was tested in loose contact through thermo-gravimetric analysis (TGA). Nitrogen and air atmosphere were used for the combustion of model soot. 3 and 5 wt% Zr/CeO2–ZrO2 showed higher soot oxidation than 1 wt% Zr/CeO2–ZrO2 in the nitrogen atmosphere, whereas percentage weight loss of model soot was high with 1 wt% Zr/CeO2–ZrO2 than 3 and 5 wt% Zr/CeO2–ZrO2.

Dimethyl carbonate synthesis from CO2 and methanol over CeO2-ZrO2 catalyst

CexZr1-xO2 catalysts with different Ce/Zr mole ratios were prepared by the sol-gel method and used for the direct synthesis of dimethyl carbonate (DMC) from CO2 and methanol. Among a series of CexZr1-xO2 catalysts, Ce0.8Zr02O2 catalyst exhibited the highest catalytic activity. Characterization results demonstrated that doping Zr into CeO2 can lead to the formation of the Ce-O-Zr solid solution, which would strengthen the interaction between ZrO2 and CeO2. Ce0.8Zr02O2 catalyst possessed the highest concentration of oxygen vacancy sites, thus exhibiting the highest activity for the direct synthesis of DMC.

Boosting diesel soot catalytic combustion via enhancement of solid(catalyst)-solid(soot) contact by tailoring micrometer scaled sheet-type agglomerations of CeO2-ZrO2 catalyst

Solid(catalyst)-solid(soot) contact is a crucial factor in diesel soot catalytic combustion, and rationally optimizing solid(catalyst)-solid(soot) contact is a promising yet challenging pathway for improving catalytic diesel soot combustion performance. Here, we report a micrometer scaled sheet-type CeO2-ZrO2 soot oxidation catalyst. The nitrogen adsorption-desorption result indicates that soot particulates are barely able to enter into the inner pore of the catalyst (average pore size < 20 nm) due to the large size of filamentous soot agglomeration (reaching up to 1 μm). Thus the catalyst-soot contact mainly depends on the catalyst external surface and morphology. Electron microscope images confirm the presence of microsheet agglomerations of the prepared CeO2-ZrO2 catalyst, which is an advantage for enhancing the adherence of soot filaments on catalyst. Compared to common CeO2-ZrO2 particle catalyst, the CeO2-ZrO2 microsheet shows a remarkably higher macrolevel soot combustion performance. Interestingly, the single catalytic site activity (intrinsic activity) of CeO2-ZrO2 microsheet is not enhanced by the micro-structure. Thus the enhanced soot combustion efficiency could be ascribed to the improving of catalyst-soot contact. And therefore, this work suggests that constructing structured catalyst at micrometer level is an efficient way to improve the solid(catalyst)-solid(soot) contact efficiency and thereby boost the diesel soot combustion performance.

Supercritical Water Gasification of Lignin and Cellulose Catalyzed with Co-precipitated CeO2–ZrO2

Lignin and cellulose were gasified at 500 and 600 °C in supercritical water over co-precipitated CeO₂–ZrO₂ catalyst. The addition of CeO₂–ZrO₂ improved the gasification efficiency and hydrogen production, and the catalytic effect was more significant at lower temperatures. The H₂ yield from cellulose gasification at 500 °C increased by over 2.5 times to 8.50 mol/kg with the presence of CeO₂–ZrO₂. The gas chromatography-mass spectrometry (GC-MS) analysis showed that the catalyst reduced the content of cyclopentenone and furan derivatives in the aqueous product but increased the content of refractory phenols. We characterized the fresh and the used catalysts with H₂-temperature-programmed reduction (H₂-TPR), X-ray diffraction (XRD), and scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS) analysis, showing that CeO₂ was the main active component, which catalyzed gasification through redox reactions. ZrO₂ enhanced the catalytic activity of CeO₂ by lowering the reduction temperature in hydrogen, increasing the dispersion of CeO₂, and facilitating H₂ adsorption on the catalyst surface.

Acetalization of glycerol and benzaldehyde to synthesize biofuel additives using SO42-/CeO2–ZrO2 catalyst

Synthesis of 1,3- dioxane and 1,3-dioxolane, using sulfated CeO2–ZrO2 catalyst for acetalization of glycerol with benzaldehyde, is the focus of present work. SO42-/CeO2–ZrO2 catalyst was synthesized using combustion method. Experiments were carried out to analyze the effect of various solvents (n-hexane, toluene, tert-butyl alcohol, pentanol), molar ratios (1:3, 1:5, 1:7), catalyst loadings (3 wt%, 5 wt%, 9 wt %) and temperatures (80 °C, 90 °C, 100 °C) on glycerol conversion and selectivity of the products. Selectivity of 87.20% dioxolane and 12.80% dioxane was obtained at molar ratio of 1:3, 9 wt% catalyst loading and temperature of 100 °C.Strong NH3 desorption peak from NH3-TPD study indicated the high acidic strength of sulphated catalyst. Strong surface acidity and surface porosity (observed from TEM and SEM analysis) contributed to an enhanced activity of the catalyst for glycerol acetalization reaction. The kinetics of the reaction was studied using an elementary kinetic law. A correlation coefficient of 0.98 from the selected kinetic model proved that the rate of acetalization reaction was dependent on glycerol concentration and acetal formation was instantaneous. The study demonstrated the application of an environmentally benign, inexpensive, thermally stable, active SO42-/CeO2–ZrO2 catalyst for glycerol acetalization reaction to synthesize 1,3-dioxolane as the desired product.

An eco-friendly synthesis of polyhydroquinoline derivatives using MoO3 promoted CeO2-ZrO2 solid heterogeneous catalyst

The molybdenum (8–20 wt%) (MoO3) promoted CeO2-ZrO2 (ceria-zirconia) solid solutions were prepared using precipitation and impregnation method, calcined at 500 °C. The catalytic potency of synthesized materials were tested for the synthesis of polyhydroquinoline derivatives using a mixture of various aromatic aldehydes, dimedone, and ethyl acetoacetate and ammonium acetate in ethanol under reflux condition. Amazingly, it was observed that with increasing Mo wt% loading increases the excellent yield of product this may be due to acidic nature of MoO3. Particularly, the 20 wt% MoO3 promoted CeO2-ZrO2 catalyst exhibited promising catalytic activity in terms of excellent yield of the products with short reaction time. The present methods significantly contributed as efficient and eco-friendly method for the synthesis of polyhydroquinoline derivatives.

Tailored Alkali Resistance of DeNOx Catalysts by Improving Redox Properties and Activating Adsorbed Reactive Species

SummaryIt is still challenging to develop strongly alkali-resistant catalysts for selective catalytic reduction of NOx with NH3. It is generally believed that the maintenance of acidity is the most important factor because of neutral effects of alkali. This work discovers that the redox properties rather than acidity play decisive roles in improving alkali resistance of some specific catalyst systems. K-poisoned Fe-decorated SO42−-modified CeZr oxide (Fe/SO42−/CeZr) catalysts show decreased acidity but reserve the high redox properties. The higher reactivity of NHx species induced by K poisoning compensates for the decreased amount of adsorbed NHx, leading to a desired reaction efficiency between adsorbed NHx and nitrate species. This study provides a unique perspective in designing an alkali-resistant deNOx catalyst via improving redox properties and activating the reactivities of NHx species rather than routinely increasing acidic sites for NHx adsorption, which is of significance for academic interests and practical applications.

Hydrogen Production via the Oxy-Steam Reforming of LNG or Methane on Ni Catalysts

Ni catalysts supported on ZrO2, 5%CeO2-ZrO2, and 5%La2O3-ZrO2 were prepared via the impregnation method and tested in the oxy-steam reforming of methane and liquified natural gas (LNG). All tested catalysts exhibited high catalytic activity in the studied process at 700 and 900 °C. The improvement of the stability of Ni catalysts after the addition of CeO2 oxide in the studied oxy-steam reforming of LNG process was confirmed. In addition, high activity and selectivity towards hydrogen was proven in the oxy-steam reforming process at 900 °C over a 20%Ni/5%CeO2-ZrO2 catalyst. It was also proved that the addition of CeO2 onto a ZrO2 carrier leads to a decrease in the NiO and metallic Ni crystallite sizes that were detected by the X-Ray diffraction (XRD) technique. The solid solution formation between NiO and ZrO2 and/or NiO and CeO2 was proved. Superior reactivity in the oxy-steam reforming of CH4 and the LNG process exhibited a 20%Ni/ZrO2 catalyst, which showed the highest methane conversions at 500 and 600 °C, equal to 63% and 89%, respectively. In addition, also in the case of the LNG reforming reaction, the most active catalyst was the 20%Ni/ZrO2 system, which demonstrated 46.3% and 76.9% of the methane conversion value at 500 and 600 °C and the total conversion of others hydrocarbons (ethane, propane and butane). In addition, this catalytic system exhibited the highest selectivity towards hydrogen formation in the oxy-steam reforming of the LNG reaction equal to 71.2% and 71.3% at 500 and 600 °C, respectively. The highest activity of this system can be explained by the uniform distributions of Ni species and their highest concentration compared to the rest of the monometallic Ni catalysts. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) results also confirmed a strong interaction of NiO with ZrO2 in the case of the 20%Ni/ZrO2 catalysts. The presence of selected NiZrO+ ions emitted from the investigated surface of the 20%Ni/ZrO2 system was detected.

Catalytic depolymerization of alkaline lignin to value-added phenolic-based compounds over Ni/CeO2-ZrO2 catalyst synthesized with a one-step chemical reduction of Ni species using NaBH4 as the reducing agent

Alkaline lignin obtained from paper manufacturing is a potential feedstock for upgrading to produce value-added chemicals. This study demonstrates the efficiencies of synthesized Ni supported CeO2-ZrO2 catalysts to convert this low-cost alkaline lignin into value-added phenolic-based chemicals for further commercial applications. Lignin depolymerization was performed under mild reaction conditions, namely, reaction temperature of 200–300 °C, and reaction time of 1–5 h. Three different catalyst preparation methods (simple wet-impregnation, wet-impregnation assisted reduction in H2 atmosphere and one-step chemical reduction) were investigated and used to evaluate possible correlations between the active Ni phase and catalytic performance in lignin depolymerization. The active Ni phase was successfully formed by one-step chemical reduction method which showed a significant improvement on product yield, especially lignin oil. The best result obtained in lignin depolymerization was by using 10%Ni/CeO2-ZrO2-red catalyst which resulted in producing a high yield of lignin oil (46.8%) from lignin feedstock under mild condition of 250 °C. The major phenolic compounds in lignin oil were: guaiacol, methyl-guaiacol, ethyl-guaiacol, vanillin, acetovanillone, and homovanillic acid. Furthermore, the reaction temperature affected the product yield and product distribution resulting in higher diversity of phenolic compounds in lignin oil.

Preparation of nanostructured CeO2-ZrO2-based materials with stabilized surface area and their catalysis in soot oxidation

By using different precipitants to control the crystallization rate, a set of nanostructured CeO2-ZrO2-based materials with varying crystallite size, textural properties and thermal stability were synthesized, and were employed in soot catalytic oxidation. We investigated the physic-chemical properties of the materials using complementary techniques, including DSC-TGA, TEM, N2 adsorption-desorption, XRD, XPS and Hydrogen-Temperature Programmed Reduction (H2-TPR). The results indicated that the decompositions of hydroxides (M(OH)x) and carbonates (M(CO3)y) generated small and large oxide grains, respectively. Thus, the thermal stability was enhanced with increasing (NH4)2CO3 concentration, considering that the specific surface area and pore volume increased from 33 (CZ-Na) to 52 m2·g−1 (CZ-C5a) and from 0.15 to 0.34 ml·g−1, respectively, after aging at 1000 °C for 4 h. Although CZ-Na exhibited the preferable lattice oxygen mobility, it showed the worst soot oxidation activity, resulting from its sharp declined surface area and a lack of surface available oxygen. In contrast, CZ-5a with the largest amount of total surface available oxygen showed the optimal catalytic performance. Moreover, nearly no deterioration of activity was detected in CZ-5a after a long-term stability test. Therefore, soot oxidation is a surface-dependent reaction, and CeO2-ZrO2 catalyst design for this purpose should allow for stabilized high surface area.

A facilitated synthesis of hierarchically porous Cu–Ce–Zr catalyst using bacterial cellulose for VOCs oxidation

Highly active CuO–CeO2–ZrO2 catalysts were prepared by sol-gel method, using environmentally friendly bacterial cellulose (BC) as structure directing regent.The catalyst designed with commercial BC (Com-BC) exhibited catalytic performances in toluene (T100 = 220 °C) and ethyl acetate oxidation (T100 = 170 °C) superior to the catalysts prepared by traditional methods. Furthermore, excellent stability was obtained and no deactivation was observed during the 100 h on stream in toluene and ethyl acetate oxidation at T100. The excellent activity and stability of Com-BC can be explained by the hierarchically porous structure, abundant oxygen vacancies, and good reducibility.

Production of H2 and CO from Refuse Derived Fuels Over Ni-Doped CeO2–ZrO2 Catalyst

Production of H₂ and CO from Refuse Derived Fuels Over Ni-Doped CeO₂–ZrO₂ Catalyst

A new understanding of CeO2-ZrO2 catalysts calcinated at different temperatures: Reduction property and soot-O2 reaction

CexZr(1-x)O2 (x = 0.27, 0.50 and 0.73) mixed oxides were prepared by co-precipitation method and then aged at 700–1000 °C. The reducibility of the aged catalysts was investigated by H2-TPR. It was found that the changes in reduction properties are related to the Ce/Zr ratio and Ce0.27Zr0.73O2 shows an increased H2 consumption with aging temperature increasing, even 1000 °C-aged samples show an increased reduction ability. Raman, XPS and HRTEM reveal that the promotion on reduction property of the aged samples should be ascribed to the increase of surface oxygen species, surface Ce/Zr ratio, O-vacancies and (110) and (100) planes. However, this promotion can not obviously improve soot oxidation activity. XRD, BET and O2-TPD results reveal that textural, structural and oxygen desorption properties are crucial to soot oxidation activity. The poor contact between soot and active sites would be an important factor that leads to a contradiction between H2-TPR and soot-O2 reaction activity.

The oxidative dehydrogenation of ethane: Convectional vs microwave heating of Ba - containing catalysts

The influence of heating method on oxidative dehydrogenation of ethane (ODE) over BaO–CeO2–ZrO2 and BaCl2–TiO2–SnO2 catalysts was studied. Noticeable differences were observed at the ODE under conventional heating and microwave (MW) heating when only the granules of catalyst but not the surrounding gas mixture are heated. The MW heating of the BaO–CeO2–ZrO2 catalyst resulted in a decrease in selectivity to carbon dioxide and in an increase in selectivity to carbon monoxide. Supposedly, the conventional heating led to gas-phase complete oxidation of carbon monoxide to carbon dioxide. The MW heating of the BaCl2–TiO2–SnO2 catalyst suppressed almost completely parallel and consecutive deep oxidation of ethylene but selectivity to ethylene increased remarkably. The study demonstrates potentialities of the MW heating in enhancing the efficiency of ODE. The observed phenomena can be accounted for the effective “quenching” of the target ODE product via inhibiting undesirable homogeneous reactions in the gas phase.

Achievement of hydrogen production from autothermal steam reforming of methanol over Cu-loaded mesoporous CeO2 and Cu-loaded mesoporous CeO2–ZrO2 catalysts

Hydrogen production by oxidative steam reforming of methanol (OSRM) or autothermal steam reforming of methanol (ASRM) was investigated over Cu-loaded mesoporous CeO2 and Cu-loaded mesoporous CeO2–ZrO2 catalysts, synthesized via a nanocasting process using MCM-48 as a hard template, followed by a deposition–precipitation technique. Various Cu contents were loaded on the mesoporous CeO2 and CeO2–ZrO2 supports. The fresh and spent catalysts were characterized by N2 adsorption–desorption, X-ray diffraction, temperature-programmed oxidation, and X-ray photoelectron spectroscopy. The ASRM results showed that 9 wt% Cu loading onto mesoporous CeO2 and CeO2–ZrO2 provided the best catalytic performance with 100% methanol conversion and 60% H2 yield at 350° and 300 °C, respectively. Furthermore, the time-on-stream stability testing of the 9 wt% Cu loading catalyst was at 168 h, and the CO selectivity of these two catalysts indicated that the addition of ZrO2 into the catalyst reduced the CO selectivity during the ASRM process.

Influence of sulfation on CeO2-ZrO2 catalysts for NO reduction with NH3

CeO2-ZrO2 (CeZr) and sulfated CeO2-ZrO2 (S-CeZr) catalysts were prepared for the selective catalytic reduction of NO with NH3. The CeZr catalysts exhibited higher activity at low temperatures (< 200 °C) and lower activity at high temperatures (> 200 °C) than the S-CeZr catalysts. The sulfation of CeZr was studied in terms of surface acidity, redox properties and NO adsorption-desorption by temperature-dependent experiments and in situ infrared spectroscopy. S-CeZr displayed high concentrations of acidic sites and increased surface acidities, but poor reducibility compared with CeZr. The high acidity of S-CeZr was attributed to the presence of Brønsted acid sites, arising mainly from the surface sulfates. Because the surface was covered with sulfate species, S-CeZr showed lower NO adsorption and weaker oxidation ability than CeZr. The adsorption of NH3 on the Brønsted acid sites restricted the reaction with NO at low temperatures, but the selective catalytic reduction cycle occurred easily at relatively low temperatures (150 °C), and the weakly bound nitrite was partially activated on the S-CeZr catalyst at relatively high temperatures (300 °C). The catalytic mechanisms for the CeZr and S-CeZr catalysts at 150 and 300 °C were also studied.

Nanoflake-assembled Al2O3-supported CeO2-ZrO2 as an efficient catalyst for oxidative dehydrogenation of ethylbenzene with CO2

An Al2O3 material assembled by nanoflakes was used to prepare supported CeO2-ZrO2 catalyst via a deposition-precipitation method for oxidative dehydrogenation of ethylbenzene with CO2. Both unsupported and commercial Al2O3-supported CeO2-ZrO2 were prepared for comparison. It was found that the CeO2-ZrO2/nanoflake-assembled Al2O3 catalyst exhibited the best catalytic activity. The characterization results revealed that the slit-shape pores existing in nanoflake-assembled Al2O3 were responsible for the small particle size and high Ce/Zr surface ratio of supported CeO2-ZrO2 species. The dispersion of Ce1-xZrx(OH)4 precursors onto Al2O3 support surface during the deposition-precipitation process was proposed. The high dispersion and large numbers of surface oxygen vacancies of the CeO2-ZrO2 species on nanoflake-assembled Al2O3 contributed to the excellent catalytic performance in oxidative dehydrogenation of ethylbenzene with CO2. This kind of special Al2O3 is expected to be a promising support for preparing highly dispersed metal/metal oxide catalysts.

Structure dependence and reaction mechanism of CO oxidation: A model study on macroporous CeO2 and CeO2-ZrO2 catalysts

Three-dimensionally ordered macroporous (3DOM) CeO2 and CeO2-ZrO2 oxides with different particle sizes, oxygen vacancy concentrations, oxygen mobility and preferentially exposed surface planes were synthesized, which, as model catalysts, give a new approach for understanding the structural dependence and reaction mechanism of CO oxidation over CeO2-based catalysts. The prepared 3DOM CeO2 and CeO2-ZrO2 catalysts exhibited much higher catalytic activity for CO oxidation than the nonporous samples. Although the textural and reducible features of catalysts could affect the catalytic performance, the preferentially exposed surface plane is crucial for determining the catalytic activity. The {110} plane of CeO2 could create more active sites for CO adsorption, resulting in relatively high activity for CO oxidation. Higher concentration of oxygen vacancy would also enhance the reactivity for CO oxidation. Langmuir-Hinshelwood mechanism should be the crucial reaction pathway for CO oxidation over the 3DOM CeO2-based catalysts, although the Mars-van Krevelen mechanism cannot be ignored.

Effect of calcination/reduction temperature of Ni impregnated CeO2–ZrO2 catalysts on hydrogen yield and coke minimization in low temperature reforming of ethanol

CeO2–ZrO2 was proven to be a quite effective catalyst support for steam reforming of ethanol (SRE). In this study, properties of Ni impregnated mesoporous CeO2–ZrO2 catalysts of different compositions were optimized by changing the calcination/reduction temperature. Increase of calcination/reduction temperature from 400 °C to 650 °C caused significant decrease in surface area and redox ability of the catalysts, yielding higher coke formation and lower hydrogen yields. Results proved that, very high hydrogen selectivity values with negligible coke formation can be achieved at a SRE reaction temperature of 450 °C, over the Ni impregnated ceria zirconia mixed oxide catalyst, which was also calcined/reduced at 450 °C. Considering its high hydrogen selectivity values with negligible coke formation, the catalyst containing a CeO2/ZrO2 ratio of 6:1 was found as the best catalyst for low temperature SRE. Achievement of high hydrogen yields with negligible coke formation at SRE temperatures as low as 400 °C was quite promising from the point of view of hydrogen economy.