Ceria Catalysts Research Articles

Py-GC/MS study of prot lignin with cobalt impregnated titania, ceria and zirconia catalysts

In the present study, pyrolysis–gas chromatography and mass spectrometry (Py–GC/MS) analysis were employed to characterize the structure and composition of evolving gas while using prot lignin as feedstock. Non-catalytic and catalytic (Co/TiO2, Co/CeO2 and Co/ZrO2) pyrolysis were performed at 300, 400, 500 and 600 °C. The pyrolysis products were grouped as phenolics (S, G and H-type), heterocyclic, other type, aromatic hydrocarbon and aliphatic hydrocarbon. Among all the catalysts, Co/CeO2 effectively promoted the formation of S-type phenolics (30.8%) at 300 °C from the depolymerization of lignin, and showed maximum selectivity towards ethanone, 1-(2-hydroxy-5-methylphenyl) (26.3%) and acetosyringone (13.5%) at 300 °C. Maximum amount of H-type phenolics (49.7%) was observed in the presence of Co/TiO2 catalyst at 600 °C. In another set of experiments, non-catalytic and catalytic (Co/CeO2) hydropyrolysis of lignin were also investigated using 1, 5, 10 and 15 bar pressure of hydrogen at 300 °C. At 1 bar H2 pressure, Co/CeO2 catalyst effectively led to the production of maximum amount of aromatic hydrocarbon (46.8%) from the hydropyrolysis of lignin, while, 23.9% of aromatic hydrocarbon was obtained from the non-catalytic hydropyrolysis of lignin.

Robust Multi-Objective Optimization for Response Surface Models Applied to Direct Low-Value Natural Gas Conversion Processes

The high proportion of CO2/CH4 in low aggregated value natural gas compositions can be used strategically and intelligently to produce more hydrocarbons through oxidative methane coupling (OCM). The main goal of this study was to optimize direct low-value natural gas conversion via CO2-OCM on metal oxide catalysts using robust multi-objective optimization based on an entropic measure to choose the most preferred Pareto optimal point as the problem’s final solution. The responses of CH4 conversion, C2 selectivity, and C2 yield are modeled using the response surface methodology. In this methodology, decision variables, e.g., the CO2/CH4 ratio, reactor temperature, wt.% CaO and wt.% MnO in ceria catalyst, are all employed. The Pareto optimal solution was obtained via the following combination of process parameters: CO2/CH4 ratio = 2.50, reactor temperature = 1179.5 K, wt.% CaO in ceria catalyst = 17.2%, wt.% MnO in ceria catalyst = 6.0%. By using the optimal weighting strategy w1 = 0.2602, w2 = 0.3203, w3 = 0.4295, the simultaneous optimal values for the objective functions were: CH4 conversion = 8.806%, C2 selectivity = 51.468%, C2 yield = 3.275%. Finally, an entropic measure used as a decision-making criterion was found to be useful in mapping the regions of minimal variation among the Pareto optimal responses and the results obtained, and this demonstrates that the optimization weights exert influence on the forecast variation of the obtained response.

A Highly Selective and Stable Ruthenium‐Nickel Supported on Ceria Catalyst for Carbon Dioxide Methanation

AbstractThe performance of nickel, ruthenium, and nickel‐ruthenium impregnated on cerium oxide catalysts was tested in the methanation of carbon dioxide reaction. The nickel and ruthenium contents were 4 and 0.4 wt%, respectively. The properties of the catalysts were studied using elementary analysis, Brunauer‐Emmet‐Teller specific surface area measurements, X‐ray diffraction, temperature programmed reduction, temperature programmed desorption, H2 chemisorption and transmission electron microscopy. The results showed that the addition of ruthenium improves the catalytic performance by promoting the dispersion of nickel species over the surface of the cerium oxide support. The ruthenium‐nickel combination produced a stable catalyst that did not show any deactivation even after 75 hours on‐stream. At high feed gas total pressures (5 and 10 bar), the catalytic conversions of CO2 were close to the thermodynamic equilibrium values with a 100 % selectivity towards CH4 formation.

Cobalt doped ceria catalysts for the oxidative abatement of gaseous pollutants and colorimetric detection of H2O2

The development of cost-effective catalysts prepared via simple methods is an urge for their use in pollutant abatement. Here, cobalt doping was used to improve the oxidizing nature of nanoceria, and the catalysts prepared via precipitation followed by wet impregnation were effectively used in the simultaneous oxidation of gaseous pollutants carbon monoxide and hydrocarbon as well as in the colorimetric sensing of H2O2. Material characterization studies revealed the effective incorporation of Co to the nano CeO2 lattice at lower concentrations. The improved oxygen storage capacity and reducibility of CeO2 upon Co doping were evident from the Raman spectroscopic analysis and temperature-programmed reduction of hydrogen. 100% activity is observed in the simultaneous CO and propane oxidation at 450 °C and the Co doped system additionally displayed a wide detection range and low detection limit for H2O2, which is relevant in biochemical sensing.

Impact of HAN Ternary Propellant System Decomposition on Catalytic Sustainability

AbstractAqueous solution of Hydroxylammonium nitrate (HAN) in combination with a suitable fuel has been explored as a promising high performing green monopropellant in recent times. The current study primarily explores the effect of the addition of a fuel component into HAN solution on the sustainability of a recently developed catalyst which was examined for the only binary system consisting of HAN and water. The HAN ternary system examined was HAN: methanol: water in 70 : 15 : 15 (w/w) composition. The endurance of an ‘iridium over alumina’ catalyst and an indigenously developed cobalt doped ceria‐based catalyst was analysed using a batch reactor wherein a pressure rise during decomposition of propellant whilst a pulse mode operation was monitored. Catalysts were also characterized via SEM/EDS, XRD, and XPS to identify changes over the period of sequential injection in the batch reactor. Doped ceria catalyst was found to be far robust and resistant to any poisoning when compared to iridium catalyst at run time of the operation involving nearly 45 sequential injections of propellant over the catalyst.

Catalytic role of assembled Ce Lewis acid sites over ceria for electrocatalytic conversion of dinitrogen to ammonia

CeO2-based catalysts are emerging as novel candidates for catalyzing nitrogen reduction reaction (NRR). However, despite the increasing amount of experimental and theoretical research, the design of more efficient ceria catalysts for NRR remains a challenge due to the poor knowledge of the catalytic mechanism, particularly the nature of the active sites and how they catalyze NRR. Here, using first-principle calculations, we investigated the NRR catalysis process involving adjacent Ce Lewis acid clusters formed on (111), (110), and (100) facets of CeO2 as active sites. Our results revealed that the assembled structures of the Ce Lewis acid as active centers after the oxygen vacancies (Ovs) were opened. The exposed Ce sites on CeO2(111), CeO2(110), and CeO2(100) can cause N2 to be adsorbed in a “lying-down” manner, which facilitates the N2 activation and thus leads to much higher NRR activity. Furthermore, from the perspective of electronic structure, we establish two useful descriptors for assessing the NRR activity on ceria with Ovs: The N–N bond strength of the adsorbed N2 and the adsorption energy of the *N2H intermediate. This work thus provides direct guidance for the design of more-effective oxide catalysts without the use of scarce metals.

Methanol reforming by nanostructured Pd/Sm-doped ceria catalysts

Catalysts of 2 wt% Pd/Sm-doped Ceria (SDC) were prepared and evaluated for hydrogen production by methanol reforming. Three preparation methods were used: a one-step method in which formation of the SDC phase and Pd incorporation took place in a single citrate-complexation process (Method A); impregnation of pre-prepared SDC nanopowder supports using Pd(NO3)2 solution (Method B); and impregnation of pre-prepared SDC from H2PdCl4 solution (Method C). Both methanol conversion and hydrogen yield increased as: bare supports<<Method A < Method C < Method B, the latter achieving values of up to 97.4 % and 236.2 %, respectively. Pd was well dispersed as fine nanoparticles by Methods B and C, but larger particles were resolved for Method A. Method B resulted in a high surface concentration of Pd, further improving catalytic activity. Undesirable Cl was retained in samples of Method C. Method B resulted in catalysts with superior nanostructure, resulting in the best activity and CO2 selectivity.

Kinetics and Mechanism of Thermal and Catalytic Decomposition of Hydroxylammonium Nitrate (HAN) Monopropellant

AbstractHydroxylammonium nitrate (HAN) monopropellant with its low sensitivity, volatility and toxicity, not only promises a safer substitute to hydrazine but also offers a higher specific impulse and a positive oxygen balance. This study primarily explores the decomposition mechanism and underlying kinetics for a newly developed cerium oxide‐based catalyst. The chemical kinetics involved in the thermal and catalytic decomposition was examined through an isoconversional method using thermogravimetric analysis (TG) data. The activation energy (Ea) was evaluated using differential isoconversional method (Friedman's method) and advanced integral isoconversional method (Popescu‐Ortega's method). To find frequency factor, compensation effect method (ACE) and intercept method (A0) were used. Advanced integral method could not provide realistic kinetic parameters while differential method predicted values similar to reported values for thermal decomposition. The values obtained for thermal decomposition were found matching with reported values obtained by other techniques which validated the methodology adopted. The reaction mechanism for both thermal and catalytic decomposition were proposed based on evolved gas analysis. The mechanism proposed was able to describe the variable kinetic parameters extracted from isoconversional method. Overall Ea of thermal decomposition of HAN was ∼95 kJ/mol. The decomposition over ceria‐based catalyst was distributed over three stages namely inception, peak and decay with Ea of ∼33 kJ/mol, ∼170 kJ/mol and ∼130 kJ/mol, respectively. Under identical conditions, iridium‐based catalyst show two decomposition stages with Ea of ∼65 kJ/mol and ∼120 kJ/mol while final stage being evaporation of HNO3. The ceria catalyst favoured N2 formation, increased decomposition of HNO3 and enhanced decomposition enthalpy.

Insights into the electronic and redox behavior of surface-phosphated ceria catalysts in correlation with their propane oxydehydrogenation performance.

In situ electrical conductivity measurements (ECMs) have been employed to gain insights into the redox and electronic behavior of ceria and surface-phosphated ceria catalysts with phosphorus contents lower than 2.2 at%. Temperature-programmed reduction under hydrogen (H2-TPR) was used to analyze the reducibility of the catalysts. Their propane oxydehydrogenation performance both in terms of activity and selectivity has been explained. It has been unambiguously shown that all the catalysts function via a heterogeneous redox mechanism involving only surface and subsurface lattice oxygen species whose availability and reactivity decrease with increasing phosphorus content with consequences on the catalytic performance.

The facet effect of ceria nanoparticles on platinum dispersion and catalytic activity of methanol partial oxidation.

The effect of platinum-supported nano-shaped ceria catalysts on methanol partial oxidation and methyl formate product selectivity has been investigated. A Pt-supported CeO2 nanocube catalyst had a higher turnover frequency than nanosphere catalysts; however, nanosphere catalysts showed higher selectivity towards methyl formate. The observed ceria shape effect in catalysis was associated with the shape-dependent Pt dispersion and its oxidation states. Furthermore, in situ studies revealed that the reduced platinum and mono-dentate methoxy group were responsible for the higher turnover frequency.

Microemulsion vs. Precipitation: Which Is the Best Synthesis of Nickel–Ceria Catalysts for Ethanol Steam Reforming?

Ethanol steam reforming is one of the most promising ways to produce hydrogen from biomass, and the goal of this research is to investigate robust, selective and active catalysts for this reaction. In particular, this work is focused on the effect of the different ceria support preparation methods on the Ni active phase stabilization. Two synthetic approaches were evaluated: precipitation (with urea) and microemulsion. The effects of lanthanum doping were investigated too. All catalysts were characterized using N2-physisorption, temperature programmed reduction (TPR), XRD and SEM, to understand the influence of the synthetic approach on the morphological and structural features and their relationship with catalytic properties. Two synthesis methods gave strongly different features. Catalysts prepared by precipitation showed higher reducibility (which involves higher oxygen mobility) and a more homogeneous Ni particle size distribution. Catalytic tests (at 500 °C for 5 h using severe Gas Hourly Space Velocity conditions) revealed also different behaviors. Though the initial conversion (near complete) and H2 yield (60%, i.e., 3.6 mol H2/mol ethanol) were the same, the catalyst prepared by microemulsion was deactivated much faster. Similar trends were found for La-promoted supports. Catalyst deactivation was mainly related to coke deposition as was shown by SEM of the used samples. Higher reducibility of the catalysts prepared by the precipitation method led to a decrease in coke deposition rate by facilitating the removal of coke precursors, which made them the more stable catalysts of the reaction.

Efficient Imine Formation by Oxidative Coupling at Low Temperature Catalyzed by High-Surface-Area Mesoporous CeO2 with Exceptional Redox Property.

High-surface-area mesoporous CeO2 (hsmCeO2 ) was prepared by a facile organic-template-induced homogeneous precipitation process and showed excellent catalytic activity in imine synthesis in the absence of base from primary alcohols and amines in air atmosphere at low temperature. For comparison, ordinary CeO2 and hsmCeO2 after different thermal treatments were also investigated. XRD, N2 physisorption, UV-Raman, H2 temperature-programmed reduction, O2 temperature-programmed desorption, EPR spectroscopy, and X-ray photoelectron spectroscopy were used to unravel the structural and redox properties. The hsmCeO2 calcined at 400 °C shows the highest specific surface area (158 m2 g-1 ), the highest fraction of surface coordinatively unsaturated Ce3+ ions (18.2 %), and the highest concentration of reactive oxygen vacancies (2.4×1015 spins g-1 ). In the model reaction of oxidative coupling of benzyl alcohol and aniline, such an exceptional redox property of the hsmCeO2 catalyst can boost benzylideneaniline formation (2.75 and 5.55 mmol h-1 based on >99 % yield at 60 and 80 °C, respectively) in air with no base additives. It can also work effectively at a temperature of 30 °C and in gram-scale synthesis. These are among the best results for all benchmark ceria catalysts in the literature. Moreover, the hsmCeO2 catalyst shows a wide scope towards primary alcohols and amines with good to excellent yield of imines. The influence of reaction parameters, the reusability of the catalyst, and the reaction mechanism were investigated.

Design of an Ultrastable and Highly Active Ceria Catalyst for CO Oxidation by Rare-Earth- and Transition-Metal Co-Doping

Catalyst design with good stability beyond simply having high activity is crucial for a variety of reactions. Here, we evaluate the ceria catalyst for CO oxidation as a model reaction to rationally...

Enhanced Performance of Zirconium-Doped Ceria Catalysts for the Methoxycarbonylation of Anilines.

The methoxycarbonylation of anilines stands as an attractive method for the phosgene-free production of carbamates. Despite the high yields obtained for ceria catalysts, the reduction of the amount of side products and the prevention of catalyst deactivation still represent major hurdles in this chemistry. One advantage of ceria is the possibility of tuning its reactivity by doping its lattice with other metals. In the present work, a series of doped ceria-based materials, prepared by substitution with metals, are evaluated in the methoxycarbonylation of 2,4-diaminotoluene with dimethyl carbonate. Among all catalysts, containing Eu, Hf, La, Pr, Sm, Tb, Y or Zr, ceria promoted with 2 mol % Zr exhibited 96 % selectivity towards the desired carbamates, improving the pure CeO2 catalyst. Density functional theory demonstrates that two descriptors are needed: 1) a geometric factor that governs the reduction of energy barriers for carbamate formation through ureas; 2) catalyst basicity as N-H bonds need to be activated. Assessment in subsequent reaction cycles revealed that the CeO2 -ZrO2 catalyst is more stable than bulk CeO2 , along with the reduction of fouling processes.

CO Catalytic Oxidation Performances of Pd-Doped BaCeO3

A series of palladium doped barium-based cerium (BaCe1-xPdxO3-δ) catalysts were prepared by the sol–gel method. The catalytic properties of BaCe1-xPdxO3-δ for the oxidation of carbon monoxide were studied. The results show that Pd doping can effectively inhibit the crystal size of catalyst powder, and the specific surface area of the catalyst increases with the increase of doping ratio. Palladium doping significantly improved the catalytic activity of barium ceric acid-based catalyst for CO oxidation. BaCe0.96Pd0.04O3-δ had the best catalytic activity for CO oxidation. After calcination at high temperature, the BaCeO3 perovskite is stable.

Mechanochemical Synthesis as an Alternative Effective Technique for the Preparation of the Composite Catalysts

—Mechanochemical synthesis in a ball mill was applied for the nanocomposite Cu(CuO)–CeO2 catalyst preparation from CeO2 and following dopants: Cu metal and copper oxides of different morphology and composition (CuO pure and CuO containing 4 or 16.5 wt % of Cu2O). The materials obtained were examined with the use of X-ray phase analysis, scanning electron microscopy, temperature-programmed reduction in CO, H2, C2H6 (TPR-СО, TPR-Н2, and TPR-С2Н6), and tested as catalysts in reactions of selective CO oxidation in H2 excess (CO-PROX) and total C2H6 oxidation. New forms of oxygen with high low-temperature reactivity towards CO, H2, and C2H6 were found by TPR in the samples synthesized. It was shown that CO conversion was slightly affected by the dopant nature in the dopant-CeO2 mixture. Contrary, C2H6 conversion at low temperatures depends on dopant composition. The highest C2H6 conversion at 400°С (91.4%) was observed on Cu–CeO2. The lowest one (54.2%) was observed on CuO–CeO2. As was demonstrated, mechanochemical synthesis is a universal technique to produce copper oxide–ceria catalysts.

NiMo-calcium-doped ceria catalysts for inert-substrate-supported tubular solid oxide fuel cells running on isooctane

A calcium-doped ceria (Ce1-xCaxO2−δ, 0 ≤ x ≤ 0.3) has been applied as a ceramic support in NiMo-based catalysts for an internal reforming tubular solid oxide fuel cell running on isooctane. Introducing calcium into the CeO2-based ceramic was found to improve conductivity of Ce1-xCaxO2−δ. The Ce0.9Ca0.1O2−δ (x = 0.10) sample exhibited an optimum conductivity of 0.045 S cm−1 at 750 °C. The transport of oxygen ions in Ce1-xCaxO2−δ promoted the catalytic partial oxidation of isooctane in the NiMo–Ce1-xCaxO2−δ catalyst, which increased the fuel conversion as well as H2 and CO yields. As a result, the NiMo–Ce0.9Ca0.1O2−δ (x = 0.10) catalyst exhibited a high isooctane conversion of 98%, and the H2 and CO yields achieved 74% and 83%, respectively, for reforming of isooctane and air at the O/C ratio of 1.0 at 750 °C. Furthermore, the NiMo–Ce0.9Ca0.1O2−δ catalyst has been applied as an internal reforming layer for an inert-substrate-supported tubular solid oxide fuel cell running on isooctane/air. Due to its high reforming activity, the single cell presented an initial maximum power density of 355 mW cm−2 in isooctane/air at 750 °C and displayed stable electrochemical performance during ~30 h operation. These results demonstrated the application feasibility of the NiMo–Ce0.9Ca0.1O2−δ catalyst for direct internal reforming solid oxide fuel cells running on isooctane/air.

Morphology-Sensitive Sulfation Effect on Ceria Catalysts for NH3-SCR

In this work, sulfation effect on NH3-SCR performance over CeO2 with different morphology (nanocubes and nanorods) was carefully studied. A morphology-sensitive sulfation effect on NH3-SCR activity was observed on CeO2. Interestingly, the NO removal efficiencies of CeO2 nanocubes and CeO2 nanorods got greatly enhanced after the sulfation at low-temperature range ( 350 °C). A series of characterizations such as N2 adsorption–desorption, XRD, HR-TEM, TG–DTA, Raman and in situ DRIFTS of SO2 + O2 adsorption were conducted to investigate the interactions between SO2 and nano-shaped CeO2. It was revealed that the morphology of CeO2 nanorods was damaged severely when sulfated at high temperature (> 350 °C), while the structure of CeO2 nanocubes was relatively stable under the same condition. Furthermore, more sulfate species were found on CeO2 nanorods than on CeO2 nanocubes, in particular of bulk like sulfate species. As a result, the NO removal efficiency of CeO2 nanorods declined greatly when sulfated at high temperature.

Transformations of Biomass, Its Derivatives, and Downstream Chemicals over Ceria Catalysts

In the past two decades, on account of the energy and environmental crisis brought by the decline in fossil resources, price volatility, and climate change, the high-value utilization of biomass fe...

Effect of oxygen vacancies on ceria catalyst for selective catalytic reduction of NO with NH3

The effect of oxygen vacancies on ceria catalyst for selective catalytic reduction with NH3 (NH3-SCR) of NO was investigated. The characterizations indicated that CeO2-Ar catalyst calcined in Ar atmosphere possessed more oxygen vacancies than CeO2 catalyst calcined in ambient air. XPS results showed that the surface oxygen contents of CeO2 and CeO2-Ar catalysts were 70.2 and 67.9% respectively, indicative of the loss of surface oxygen and existence of oxygen vacancies on CeO2-Ar catalyst. The H2-TPR and O2-TPO experiments suggested that the presence of abundant oxygen vacancies improved the redox behavior, providing CeO2-Ar catalyst a superior ability to oxygen uptake and release. Furthermore, the adsorption capacity for NH3 and NO of CeO2-Ar catalyst was enhanced by oxygen vacancies. As a result, CeO2-Ar catalyst showed a quite higher NO conversion (>90% above 260 °C) than CeO2 catalyst at 120 – 400 °C. The proposed mechanism suggested that the oxygen vacancies accelerated the acidity cycle and redox cycle during NH3-SCR reaction. Under the action of surface adsorbed oxygen on oxygen vacancies, the adsorption and activation of NH3 and NO were promoted and the restore of Ce4+ from Ce3+ was facilitated, which were responsible for the enhancement of catalytic performance of CeO2-Ar catalyst.