Sn-induced CuO–CeO2 catalysts with improved performance for CO preferential oxidation in H2-rich streams

Sn modified CuO–CeO₂ catalysts with different Sn loadings were prepared by a facile, green and solvent-free method. The effect of Sn/Ce ratio over Sn–Cu–Ce-x (x = 0, 1, 2.5, 5, 7.5) samples on CO activity and O₂ selectivity was investigated. The samples were characterized by various techniques using N₂-adsorption/desorption, XRD, H₂-TPR, XPS, Raman and in-situ DRIFTS. It was revealed that stronger interaction between acitve sites and support, higher amounts of Sn²⁺ and Ce³⁺, associated with increased amount of oxygen vacancies, were observed on the catalyst of Sn–Cu–Ce-5. As a result, the optimized catalyst displayed an excellent catalytic performance even in the presence of CO₂ and H₂O. In this sense, probing the Sn modified CuO–CeO₂ catalyst can elucidate some useful keys for the development of high CO₂ and H₂O-resistance catalyst during CO-preferential oxidation in H₂-rich streams.     

Pressure loss and compensation in the combustion process of Al–CuO nanoenergetics on a microheater chip

This study investigates pressure loss and compensation in the combustion process of Al–CuO metastable intermolecular composite (MIC) on a microheater chip. A ball cell model of pressure change in the combustion process is proposed to show the effects of pressure loss on the reaction rate and efficiency of energy output at microscale. An effective compensation method for pressure loss is then developed by integrating Al–CuO MIC with CL-20 (2,4,6,8,10,12-hexanitrohexaazaisowurtzitane) onto a SiO₂/Cr/Pt/Au microheater chip. The combustion processes of Al–CuO MIC with different weight percentages of fine CL-20 particles on the microheater chips are observed by high-speed video recording. Results indicate that the reaction of Al–CuO MIC is a slow combustion process that turns into intense deflagration after adding fine CL-20 particles to Al–CuO MIC. The pressure–time characteristics indicate higher maximum pressure and pressurization rate for Al–CuO/CL-20 because the pressure loss at microscale is well compensated by the addition of fine CL-20. This study proves the importance of pressure loss in the combustion process of MIC at microscale and provides an efficient compensation strategy for pressure loss to improve the reaction rate and efficiency of energy output at microscale environment.     

Investigation of effects of sulfur on dry reforming of biogas over nickel–iron based catalysts

The aim of this study is to maintain and increase the activity of the catalyst in the presence of H₂S with the addition of iron to the Ni catalyst. Alumina-supported monometallic iron and bimetallic nickel-iron catalysts with different weight percentages (8% Fe, 3% Ni – 3% Fe and 8% Ni – 8% Fe) were synthesized using the wet impregnation method in this study. Alumina was prepared by the sol-gel method. The activities of these synthesized catalysts in the methane dry reforming reaction were investigated at different H₂S concentrations (0 ppm, 2 ppm, and 50 ppm) with a total flow rate of 60 mL/min containing an equimolar ratio of CH₄, CO₂, and Ar at 750 °C and atmospheric pressure. To investigate the effect of sulfur compounds on the catalytic activity, the catalysts were also exposed to different gas compositions such as the mixture of H₂S + He, H₂S + CO₂ + He, and H₂S + CO₂ + CH₄ + He. In this case, FT-IR with a gas cell was used to determine the components in the gas stream at the reactor outlet. To explain catalytic performance, characterization studies were carried out using XRD, N₂ adsorption/desorption, DRIFT, SEM, TGA, and XPS analysis. All-synthesized materials showed Type-IV isotherm with a hysteresis loop corresponding to an ordered mesoporous structure. The DRIFT analysis showed a decrease in the Lewis acid sites after the addition of iron into the Ni-catalysts. In the activity test carried out in the presence of 50 ppm H₂S, it was observed that the iron-containing 8Ni–8Fe@SGA catalyst increased the sulfur resistance slightly, compared to the monometallic 8Ni@SGA catalyst. TGA analysis showed that Fe addition reduced coke deposition, as the Ni–Fe catalyst had a lower nickel crystal size than the Ni-based catalyst. FTIR analysis with a gas cell showed that sulfur in H₂S transformed to other sulfur compounds such as COS and/or SO₂ during dry reforming of biogas over alumina-supported Ni–Fe catalysts.     

The effect of copper oxide on the CuO–NiO/CeO2 structure and its influence on the CO-PROX reaction

The effect of copper oxide species on the CuO–NiO/CeO₂ structure and the influence on the preferential CO oxidation in H₂ excess (CO-PROX) reaction at low and high temperatures were investigated. Temperature-programmed surface reaction (TPSR), In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and X-ray photoelectron spectroscopy (XPS) results allowed determining the surface species. The maximum temperature of CO₂ formation or selectivity decreased about 40 °C for the CuO–NiO/CeO₂ catalyst compared to the NiO/CeO₂, which suggests that the addition of Cu⁺ species increases the active sites due to interaction with the Ni–Ce structure. Therefore, the activity of the catalyst was closely related to the oxygen in vacancies and the formation of Cu⁺-carbonyl species of the redox mechanism. Besides, the superior selectivity towards CO₂ below 150 °C depends on the carbonyl stabilization at the surface, inhibiting the adsorption and subsequent oxidation of H₂. Using TPSR and spectroscopic analyzes by DRIFTS and XPS allowed us to propose the reaction mechanisms for low and high temperatures.     

Effects of nano-Ag on the combustion process of Al–CuO metastable intermolecular composite

Effects of nano-Ag with high thermal conductivity on the combustion wave behavior of Al–CuO MIC (metastable intermolecular composite) are studied in this paper by incorporating Al–CuO MIC with nano-Ag particles in different weight proportions. The physical and chemical characteristics of Al–CuO MIC are determined using scanning electron microscope (SEM), X-ray diffraction (XRD) and differential scanning calorimeter (DSC). The combustion wave behavior is identified by high-speed video recording (HSVR). The experimental observations confirm that the presence of nano-Ag particles improves the heat transfer efficiency. With nano-Ag increasing from 1 wt% to 10 wt%, the first exothermic peak temperature decreases from 607.8 °C to 567.6 °C, and average combustion speed (ACS) increases at first and then reduces. The most suitable amount of nano-Ag is 2 wt% with the ACS and instantaneous combustion velocity on the order of 954.0 m/s, 1562.5 m/s. Moreover, heat transfer mechanisms in the combustion process of Al–CuO MIC are better understood, especially by distinguishing conduction from convection during the combustion propagation. Furthermore, three stages (ignition, acceleration and steady combustion) of reactive propagation are observed in the combustion process. And the corresponding dominative heat transfer mechanisms in the three stages are conduction, conduction to convection transition, and convection, respectively.     

Investigation of combustion and emission characteristics in a TBC diesel engine fuelled with CH4–CO2–H2 mixtures

In this study, an experimental investigation was performed to reveal combustion and emission characteristics of common-rail four-cylinder diesel engine run with CH₄, CO₂ and H₂ mixtures. The engine pistons were thermally coated with zirconia and Ni–Al bond coat by plasma spray method. With a small amount of the pilot diesel, port fuelled methane (100% CH₄), synthetic biogas (80% CH₄ + 20% CO₂), and hydrogen presented (80% CH₄+10% CO₂+10% H₂) mixtures were used as main fuel at different loads (50 Nm, 75 Nm, and 100 Nm) at a constant speed of 1750 min^?1. Comparative analysis of the combustion (cylinder pressure, PRR, HRR, CHR, ringing intensity, CA10, CA50, and CA90), BSFC, and emissions (CO₂, HC, NO_x, smoke, and oxygen) at the various engine loads with and without piston coating was made for all fuel combinations. It was found that coating the engine pistons enhanced the examining combustion characteristics, whereas it slightly changed BSFC and most of the emissions. As compared to the sole diesel fuel, the gaseous fuel operations showed higher in-cylinder pressure, PRR, and ringing intensity values, earlier combustion starting and CAs, and lower diesel injection pressure at the same engine operating conditions. Dramatic increase in the ringing intensity was particularly found by the hydrogen introduced mixture under the tests with coated piston. HC and CO₂ emissions increased in operation with the synthetic biogas; however, hydrogen introduction reduced HC emissions by 4.97–30.92%, and CO₂ emissions by 5.16–10%.     

Demonstrating the dual functionalities of CeO2–CuO composites in solid oxide fuel cells

Nowadays, lowering the operating temperature of solid oxide fuel cells (SOFCs) is a major challenge towards their widespread application. This has triggered extensive material studies involving the research for new electrolytes and electrodes. Among these works, it has been shown that CeO₂ is not only a promising basis of solid oxide electrolytes, but also capable of serving as a catalytic assistant in anode. In the present work, to develop new electrolytes and electrodes for SOFCs based on these features of CeO₂, a new type of functional composite is developed by introducing semiconductor CuO into CeO₂. The prepared composites with mole ratios of 7:3 (7CeO₂–3CuO) and 3:7 (3CeO₂–7CuO) are assessed as electrolyte and anode in fuel cells, respectively. The cell based on 7CeO₂–3CuO electrolyte reaches a power outputs of 845 mW cm^?2 at 550 °C, superior to that of pure CeO₂ electrolyte fuel cell, while an Ce_0.8Sm_0.2O_2-δ electrolyte SOFC with 3CeO₂–7CuO anode achieves high power density along with open circuit voltage of 1.05 V at 550 °C. In terms of polarization curve and AC impedance analysis, our investigation manifests the developed 7CeO₂–3CuO composite has good electrolyte capability with a hybrid H⁺/O^2? conductivity of 0.1–0.137 S cm^?1 at 500–550 °C, while the 3CeO₂–7CuO composite plays a competent anode role with considerable catalytic activity, indicative of the dual-functionalities of CeO₂–CuO in fuel cell. Furthermore, a bulk heterojunction effect based on CeO₂/CuO pn junction is proposed to interpret the suppressed electrons in 7CeO₂–3CuO electrolyte. Our study thus reveals the great potential of CeO₂–CuO to develop functional materials for SOFCs to enable low-temperature operation.     

Investigation of oxidation and electrical behavior of AISI 430 steel coated with Mn–Co–CeO2 composite

Ferritic stainless steels, under the working conditions of solid oxide fuel cells, form a chromium oxide layer. This layer has a low electrical conductivity and consequently reduces the efficiency of these energy converters. An action to improve the properties of the connecting plates is to use a conductive and protective layer of coating. In this study, AISI 430 stainless steel was coated with Mn–Co–CeO₂ through electroplating technique. To evaluate the oxidation behavior, isothermal and cyclic oxidation tests were used at 800 °C. Area specific resistance (ASR) of uncoated and coated specimens was also compared as a function of time during oxidation at 800 °C. Coating microstructure and oxidized samples were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD) device. In isothermal oxidation, uncoated samples had more weight gain than the Mn–Co–CeO₂ coated samples. The coating layer improved oxidation resistance by limiting the diffusion of chromium cation and oxygen anion. The cyclic oxidation results showed that the Mn–Co–CeO₂ coated samples had a very good resistance to cracking and spallation. Also, the results of ASR showed that formation of MnCo₂O₄ and MnFe₂O₄ spinels and also the presence of CeO₂ resulted in reduction of area specific resistance. ASR for samples coated with Mn–Co–CeO₂ and uncoated samples was 12.4 mΩ.cm² and 38.7 mΩ.cm², respectively after 200 h of oxidation at 800 °C.     

Study of Ni/CeO2–ZnO catalysts in the production of H2 from acetone steam reforming

This paper reports the study of new Ni/ZnO-based catalysts for hydrogen production from substoichiometric acetone steam reforming (ASR). The effect of CeO₂ introduction is analyzed regarding the catalytic behavior and carbon deposits formation. ASR was studied at 600 °C using a steam/carbon ratio S/C = 1. Ni/xCeZnO (x = 10, 20, 30 CeO₂ wt %) catalysts showed a better performance than the bare Ni/ZnO. Ni/xCeZnO generated a lower amount and less ordered carbon deposits than Ni/ZnO. The higher the CeO₂ content in Ni/xCeZnO, the lower the amount of carbon deposits in the post-reaction catalyst. The highest H₂ production under ASR at the experimental conditions used was achieved for the Ni/xCeZnO catalysts. In-situ DRIFTS-MS experiments under ESR conditions showed different reaction pathways over Ni/20CeZnO and Ni/ZnO catalysts.     

The deactivation effect of Na2O and NaCl on CeO2–TiO2 catalysts for selective catalytic reduction of NO with NH3

The effect of Na₂O and NaCl on CeO₂–TiO₂ catalyst for the selective catalytic reduction of NO with NH₃ was investigated with BET, XRD, XPS, NH₃-TPD, H₂-TPR, in-situ DRIFT and catalytic activity measurements. The results showed that both Na species could deactivate the CeO₂–TiO₂ catalyst and Na₂O had a stronger effect than NaCl. The more serious deactivation by Na₂O could be ascribed to smaller surface area, fewer surface Ce³⁺ and chemical adsorbed oxygen, lower surface acidity, and worse reducibility. The introduction of NaCl and Na₂O facilitated the formation of new surface NO_x adspecies, but were inactive in NH₃-SCR reaction. The adsorption of NH₃ were inhibited. The NH₃-SCR reaction over the CeO₂–TiO₂ catalyst was governed by both E-R and L-H mechanisms. The introduction of NaCl and Na₂O didn't change the NH₃-SCR reaction mechanisms.     

Ce promoting effect on the activity and coke formation of Ni catalysts supported on mesoporous nanocrystalline γ-Al2O3 in autothermal reforming of methane

In this study methane autothermal reforming (ATR) was investigated over Ni/Al₂O₃ and Ni/Al₂O₃–CeO₂ catalysts. The catalyst carriers were prepared through a facile one-step method, which produced mesoporous nanocrystalline carriers for Ni catalysts. The samples were characterized by XRD, TPR, BET, TPO and SEM characterization techniques and the catalytic activity and stability were also studied at different conditions (GHSV and feed ratio) in methane ATR. It was found that the nickel catalyst supported on 3 wt.% Ce–Al₂O₃ exhibited higher activity compared to the catalysts supported on the Al₂O₃ and promoted Al₂O₃ with 1 and 6 wt.% Ce. The results also showed that the nickel catalyst supported on 3 wt.% Ce–Al₂O₃ possessed the highest resistance against carbon deposition in ATR reaction.     

CuO–CeO2/SiO2 coating on ceramic monolith: Effect of the nature of the catalyst support on CO preferential oxidation in a H2-rich stream

Powder and structured catalysts based on CuO–CeO₂ nanoparticles dispersed on different silica are studied in CO preferential oxidation. Silica of natural origin (Celite) and fumed silica (aerosil), both commercial materials, and synthesized mesoporous SBA-15 with 20, 200 and 650 m²g^-1 respectively, are selected as supports. CuCe/Celite coated on cordierite monolith displays the highest activity, reaching CO conversion above 90% between 140 and 210 °C and more than 99% around 160 °C. The addition of 10% CO₂ and 10% H₂O partially deactivates the monolithic catalyst.The lower surface area of CuCe/Celite favors the contact between CuO and CeO₂ nanoparticles promoting a better interaction of Cu⁺²/Cu⁺ and Ce⁺³/Ce⁺⁴ redox couples. Raman spectroscopy reveals oxygen vacancies and XPS results show high metal lattice surface oxygen concentration and surface enrichment of Cu and Ce which promote the catalytic activity.     

Monoliths washcoated with AuCu catalysts for CO removal in an ethanol fuel processor: Effect of CeO2–SiO2 dual support on the catalytic performance and reactor cost

CO removal from a syngas was evaluated in a single catalytic unit using structured AuCu/CeO₂–SiO₂ catalysts. Initially, AuCu/CeO₂–SiO₂ powder catalysts with several SiO₂ loadings (0–100 mol%) were assessed. SiO₂ addition into AuCu/CeO₂ increased the surface area by up to 3.4 times and reduced its cost by up to 60%, but reduced CO conversion. On the other hand, the AuCu/CeO₂–SiO₂ catalyst washcoated on monolith outperformed the powder samples at temperatures above 260 °C. In particular, outlet CO concentrations adequate to feed high temperature fuel cells were obtained with monoliths washcoated with AuCu/CeO₂ (0.2% CO) and AuCu/CeO₂–SiO₂ with 75 mol% of SiO₂ (3.6% CO). Furthermore, the Au/CeO₂–SiO₂ monolith (14 $/g_cat) is 36% cheaper than the AuCu/CeO₂ monolith, paving the road to the development of compact and economic H₂ cleanup processes.     

The effect of La2O3 and CeO2 modifiers on properties of Ni–Al catalysts for LNG prereforming

Ni/La–Al₂O₃ and Ni/Ce–Al₂O₃ catalysts with a small amount of promoters intended for prereforming of LNG were characterized by XRF, N₂ adsorption-desorption, XRD, H₂ chemisorption, HRTEM and XPS. The catalytic activity was evaluated in methane steam reforming both in the kinetic and diffusion regime, at temperatures characteristic of pre-reforming. Carbonaceous deposit was analysed by TPO-MS method. The nature and location of the coke were studied by HRTEM.La or Ce addition into Ni–Al system causes the increase of the active surface area of Ni by enhancing its dispersion. Studies at kinetic regime have shown that the promoted catalysts have almost twice the activity than reference Ni–Al catalyst. This effect was not confirmed by measurements in the diffusion regime on whole catalyst tablets. Almost identical textural properties of catalysts and diffusive limitations related to them but not the catalytic properties of the material itself appeared to be crucial factors. The presence of La (but not Ce) causes a significant increase in resistance to coking.     

Low-temperature partial oxidation of methane over Pd–Ni bimetallic catalysts supported on CeO2

Monometallic Pd and Ni and bimetallic Pd–Ni catalysts supported on CeO₂ are prepared via mechanochemical and conventional incipient wetness impregnation methods and tested for the production of syngas by the partial oxidation of methane. Compared with monometallic Ni/CeO₂ and Pd/CeO₂, bimetallic Pd–Ni/CeO₂ catalysts show considerable higher methane conversion and syngas yield. Additionally, the bimetallic catalysts prepared by ball milling produce syngas at lower temperature. Different preparation parameters, such as metal loading, Pd/Ni ratio, milling energy, milling time and order of incorporation of the metals are examined. The best performance is obtained with a bimetallic catalyst prepared at 50 Hz for 20 min with only 0.12 wt% Pd and 1.38 wt% Ni. Stability tests demonstrate superior stability for bimetallic Pd–Ni/CeO₂ catalysts prepared by a mechanochemical approach. From the characterization results, this is explained in terms of an impressive dispersion of metal species with a strong interaction with the surface of CeO₂.     

Effects of metal loading and support modification on the low-temperature steam reforming of ethanol (LTSRE) over the Ni–Sn/CeO2 catalysts

This article presents the effect of metal loading and support modification with MgO on low-temperature steam reforming of ethanol (LTSRE) over Ni–Sn/CeO₂ catalysts prepare by a single-pot solution combustion synthesis (SCS) method. Atmospheric pressure activity study of these catalysts (0.5 g) is performed at different temperatures (200–400 °C), H₂O:EtOH = 12: 1 mol ratio, and feed flow rate 0.1 ml/min. After 10 h TOS at 400 °C, NiSn(5)/CM12 catalyst with 5 wt.% total metal loading, optimal Sn (Ni:Sn = 14:1), and Ce:Mg = 1:2 mol ratio shows EtOH conversion 100% and H₂ selectivity 70% with low coke deposition. Physicochemical characterizations (XRD, Raman, FESEM, TEM, and N₂ adsorption-desorption) reveal that addition of MgO in CeO₂ and an optimal amount of Sn decrease both Ni and support particle sizes while oxygen storage capacity (OSC) of the support increases (by XPS). Alkaline characteristics of MgO reduces support's acidity and improves active metal-support interaction, as evaluated by NH₃-TPD and H₂-TPR.     

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.     

Effect of zirconium addition on the structure and properties of CuO/CeO2 catalysts for high-temperature water–gas shift in an IGCC system

A series of Ce_xZr_1−xO₂-based Cu catalysts was synthesized by the co-precipitation method. The influences of copper content, zirconium addition, and ratio of ceria to zirconia on the catalytic activity were investigated. BET, N₂O decomposition, XRD, TEM, SEM, EDS, Raman spectroscopy, H₂-TPR, TG/DTA, and XPS were used to characterize the catalysts. The catalytic activity was tested in terms of CO conversion and H₂ selectivity in H₂-rich coal-derived synthesis gas, simulating the actual gas composition of an integrated gasification combined cycle (IGCC) system. The long-term catalyst stability was also examined at 450 °C for 196 h. The addition of zirconium was found to be very important in enhancing catalytic performance. The surface area, copper dispersion, oxygen storage and mobility capacity, reducibility, as well as resistance to sintering all improved after zirconium addition.     

Partial oxidation of methane on co-precipitated Ni–Mg/Al catalysts modified with copper or iron

Methane transformation to hydrogen and synthesis gas (CO + H₂) by heterogenous catalysts can play an important role to secure the supply of energy, chemicals and fuels in the future. Methane is the main constituent of natural gas and biogas and it is also found in crystalline hydrates at the continental slopes of many oceans. In view of this vast reserves and resources, the use of methane as chemical feedstock has to be intensified. In this present work, (NiMg)Al catalysts doped with Fe or Cu, prepared by co-precipitation method and characterized by different techniques, were studied in the partial oxidation of methane (T_reaction = 750 °C, CH₄/O₂ ratio = 2). The effect of catalyst composition and pre-treatment conditions of these catalysts were investigated. Also, these catalysts show a very high activity and selectivity in the partial oxidation reaction, which depends on the conditions of catalysts preparation. The obtained results indicated increasing of activity and selectivity with decreasing calcination temperature and increasing nickel and aluminium contents in the catalysts composition. The solid doped with iron constituted the best catalyst for the total oxidation of methane and for the water–gas shift reaction. On the other hand, the addition of copper was remarkably improved the catalytic performances of the (NiMg)Al solid. So, the presence of this element supported the partial oxidation of methane with production of syngas (CO + H₂). With the addition of iron or copper for the catalyst composition, we were observed (in our previous work) the possibility of formation of NiM (M = Fe or Cu) alloy which increased nickel particles dispersion. In the case of copper, the reducibility of NiO was also assisted (TPR results) which increased catalytic activity in partial oxidation of methane.