Temperature-programmed Reduction Research Articles

Investigation of microstructure evolution and redox behavior of Ni-GDC cermet by cyclic re-oxidation in CO2

The microstructure and reactivity of Ni-GDC cermet under cyclic oxidation in CO2 conditions were investigated using temperature-programmed reduction (TPR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The XRD results revealed that NiO was formed during the re-oxidation process in CO2. SEM and TPR results demonstrated that nanostructured NiO aggregates were formed on the surface of the sample after being re-oxidized at 400 °C for 1200 min, resulting in an α peak with high reduction activity and a conversion of only 16% from Ni to NiO. In contrast, the samples re-oxidized at 600 °C for 600 min and 800 °C for 240 min were completely oxidized, resulting in reconstructed and sintered NiO with larger particle size, and a β peak with lower reactivity and a conversion of 100%. Redox cycling was further conducted on completely oxidized samples, which showed a decrease in β peak temperature from the first to seventh cycle. The migration and redistribution of sintered NiO towards the exterior were confirmed by comparing SEM images after the first and fourth cycles, which favored an increase in the reactive area. Therefore, this study reveals that lower re-oxidation temperatures (≤400 °C) and an appropriate increase in redox cycles are beneficial for forming surficial nanostructures while hindering the formation of large sintered NiO particles.

Hydrogen Production by Steam Reforming of Ethanol and Dry Reforming of Methane with CO2 on Ni/Vermiculite: Stability Improvement via Acid or Base Treatment of the Support.

In this study, vermiculite was explored as a support material for nickel catalysts in two key processes in syngas production: dry reforming of methane with CO2 and steam reforming of ethanol. The vermiculite underwent acid or base treatment, followed by the preparation of Ni catalysts through incipient wetness impregnation. Characterization was conducted using various techniques, including X-ray diffraction (XRD), SEM-EDS, FTIR, and temperature-programmed reduction (H2-TPR). TG-TD analyses were performed to assess the formation of carbon deposits on spent catalysts. The Ni-based catalysts were used in reaction tests without a reduction pre-treatment. Initially, raw vermiculite-supported nickel showed limited catalytic activity in the dry reforming of methane. After acid (Ni/VTA) or base (Ni/VTB) treatment, vermiculite proved to be an effective support for nickel catalysts that displayed outstanding performance, achieving high methane conversion and hydrogen yield. The acidic treatment improved the reduction of nickel species and reduced carbon deposition, outperforming the Ni over alkali treated support. The prepared catalysts were also evaluated in ethanol steam reforming under various conditions including temperature, water/ethanol ratio, and space velocity, with acid-treated catalysts confirming the best performance.

Enhanced CO2 methanation over LaNiO3/CeO2 derivative catalyst with high activity and stability

The conversion of CO2 into synthetic natural gas offers a promising approach for the long-term storage of renewable energy, addressing the challenge of its intermittent supply. This study successfully developed a catalyst comprised of LaNiO3 supported by CeO2, synthesized via a citric acid-aided impregnation method. This catalyst demonstrated efficiency and stability as a precursor for the CO2 methanation reaction. Upon reduction, it transforms into a Ni–La2O3/CeO2 configuration, featuring dispersed Ni nanoparticles. Characterization through hydrogen temperature-programmed reduction (H2-TPR) revealed that this catalyst exhibits enhanced reducibility and smaller particle sizes, which promote hydrogen activation at lower temperatures. X-ray photoelectron spectroscopy (XPS) analysis further clarified that the increased catalytic activity results from electron transfer interactions between Ni, CeO2, and La2O3. Moreover, the catalyst displayed an augmented count of weak to medium basic sites, which, along with improved hydrogen activation, elevated the CO2 conversion rate to 83.8% within the kinetic control region. The collaborative effect of Ni, La2O3, and CeO2 substantially outperformed the conventional Ni/CeO2 catalyst in CO2 methanation efficiency. Additionally, in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) analysis revealed that the methanation process predominantly follows the HCOO* pathway. Notably, the LaNiO3/CeO2 catalyst exceeded the performance of Ni–La/CeO2 in CO2 adsorption, HCOO* intermediate decomposition, and CHO* conversion, significantly enhancing low-temperature methanation activity.

Biogas dry reforming over Li–Ni–Al LDH-derived catalysts

Biogas dry reforming or dry reforming of methane (DRM) is an alternative to produce hydrogen, in the context of development of sustainable routes for energy production, since this route uses CH4 and CO2, two greenhouse gases as reactants. However, the deactivation of catalysts due to carbon deposition and sintering is the main drawback of this reaction. In this work, with the aim of minimizing carbon formation and sintering during DRM, the effect of lithium as a promoter in catalysts Ni–Al derived from layered double hydroxides was investigated. The catalysts were prepared by co-precipitation and characterized by different techniques: BET surface area (SBET), X-ray diffraction (XRD), temperature-programmed reduction (TPR), CO2 and H2 temperature-programmed desorption (CO2-TPD and H2-TPD), Scanning Electron Microscopy (SEM-EDS), temperature-programmed oxidation (TPO). The catalytic tests were carried out in a fixed bed reactor using a synthetic biogas (60% CH4 and 40% CO2) in the temperature range of 500–750 °C. The incorporation of Li into the Ni–Al-LDH changed the reducibility and basicity of the catalysts. The Li2 sample containing an intermediary amount of Li (5 wt%) showed the highest reduction temperature and highest density of basic sites, which resulted in the smallest Ni crystallite size before and after the reactions. As a result, this catalyst presented the greater resistance to sintering and a lower rate of carbon produced. For the reaction carried out at 700 °C, only the Li2 catalyst remaining active during the test and reaching a maximum of 80% and a final CH4 conversion of 54% after 480 min of reaction, whereas CO2 conversion was higher than 90%. All other samples deactivated at different rates due to carbon deposition and sintering.

Enhancing Interaction between Lanthanum Manganese Cobalt Oxide and Carbon Black through Different Approaches for Primary Zn-Air Batteries.

Due to the need for decarbonization in energy generation, it is necessary to develop electrocatalysts for the oxygen reduction reaction (ORR), a key process in energy generation systems such as fuel cells and metal-air batteries. Perovskite-carbon material composites have emerged as active and stable electrocatalysts for the ORR, and the interaction between both components is a crucial aspect for electrocatalytic activity. This work explores different mixing methods for composite preparation, including mortar mixing, ball milling, and hydrothermal and thermal treatments. Hydrothermal treatment combined with ball milling resulted in the most favorable electrocatalytic performance, promoting intimate and extensive contact between the perovskite and carbon material and improving electrocatalytic activity. Employing X-ray photoelectron spectroscopy (XPS), an increase in the number of M-O-C species was observed, indicating enhanced interaction between the perovskite and the carbon material due to the adopted mixing methods. This finding was further corroborated by temperature-programmed reduction (TPR) and temperature-programmed desorption (TPD) techniques. Interestingly, the ball milling method results in similar performance to the hydrothermal method in the zinc-air battery and, thus, is preferable because of the ease and straightforward scalability of the preparation process.

Remarkable enhancement of CO oxidation over Pt1/CeO2 catalysts by atomically dispersed tungsten

Ceria-supported atomically dispersed Pt is of great interest in CO oxidation due to their unique catalytic performance and high metal utilization. However, the intrinsic activity of the Pt1/CeO2 single atom catalysts may not be desirable due to the overstabilization of Pt2+ species in the CeO2 matrix. Herein, the local environment of Pt2+ species was tailored by introducing a tungsten oxide additive into the Pt1/CeO2 catalyst, achieving a dramatic improvement in catalytic performance for CO oxidation at low temperatures. The optimized catalyst with 2.5 wt% W loading achieved 100 % CO conversion at 150 °C, which was 120 °C lower than that of the unmodified Pt/CeO2 catalyst. Through in situ infrared spectroscopy and temperature-programmed reduction characterizations, as well as theoretical calculations, we concluded that the tungsten additive modulated the interactions between Pt2+ species and CeO2. Thus, the reducibility of Pt was enhanced, which facilitated the formation of catalytically active metallic Pt0 species during the reaction.

Highly Efficient PtSn/Al2O3 and PtSnZnCa/Al2O3 Catalysts for Ethane Dehydrogenation: Influence of Catalyst Pretreatment Atmosphere

Increased demand for ethylene has motivated direct ethane dehydrogenation over Pt-based catalysts. PtSn/γ-Al2O3 and PtSnZnCa/γ-Al2O3 catalysts were investigated with the aim of understanding the effect of the pretreatment environment on the state of dispersed Pt for ethane dehydrogenation. The catalysts were prepared by the impregnation method and pretreated in different environments like static air (SA), flowing air (FA), and nitrogen (N2) atmospheres. A comprehensive characterization of the catalysts was performed using Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), Temperature-Programmed Reduction (TPR), NH3 Temperature-Programmed Desorption (NH3-TPD), X-ray photoelectron spectroscopy (XPS), and Transmission Electron Microscopy (TEM) techniques. The results reveal that the PtSn on Al2O3 catalyst pretreated in the static air environment (PtSn-SA) exhibits 21% ethylene yield with 95% selectivity at 625 °C. XPS analysis found more platinum and tin on the catalyst surface after static air treatment. The overall acidity of the catalysts decreased after thermal treatment in static air. Elemental mapping demonstrated that Pt agglomeration was pronounced in catalysts calcined under flowing air and nitrogen. These factors are responsible for the enhanced activity of the PtSn-SA catalyst compared to the other catalysts. The addition of Zn and Ca to the PtSn catalysts increases the yield of the catalyst calcined in static air (PtSnZnCa-SA). The PtSnZnCa-SA catalyst showed the highest ethylene yield of 27% with 99% selectivity and highly stable activity at 625 °C for 10 h.

Tri-reforming of methane over Ni/ZrO2 catalyst derived from Zr-MOF for the production of synthesis gas.

The increasing concentration of CO2 and CH4 in the environment is a global concern. Tri-reforming of methane (TRM) is a promising route for the conversion of these two greenhouse gases to more valuable synthesis gas with an H2/CO ratio of 1.5-2. In this study, a series of Zr-MOF synthesized via the solvothermal method and impregnation technique was used to synthesize the nickel impregnated on MOF-derived ZrO2 catalyst. The catalyst was characterized by various methods, including N2-porosimetry, X-ray diffraction (XRD), temperature programmed reduction (TPR), CO2-temperature programmed desorption (CO2-TPD), thermo-gravimetric analysis (TGA), chemisorption, field-emission scanning electron microscopy (FE-SEM), and high-resolution transmission electron microscopy (HR-TEM). Characterization results confirmed the formation of the Zr-MOF and nickel metal dispersed on MOF-derived ZrO2. Further, the tri-reforming activity of the catalyst developed was evaluated in a downflow-packed bed reactor. The various catalysts were screened for TRM activity at different temperatures (600-850°C). Results demonstrated that TRM was highly favorable over the NZ-1000 catalyst due to its desirable physicochemical properties, including nickel metal surface area (2.3 m2/gcat-1), metal dispersion (7.1%), and nickel metal reducibility (45%), respectively. Over the NZ-1000 catalyst, an optimum H2/CO ratio of ~ 1.6-2 was achieved at 750°C, and it was stable for a longer period of time.

Ammonia and toluene oxidation: Mutual activating effect of copper and cerium on catalytic efficiency

Copper and cerium containing hydrotalcite-like precursors Cux-Cey-Mg-Al were synthesized by mutual co-precipitation followed by calcination at 800 °C. The obtained mixed metal oxides were studied as catalysts for selective ammonia oxidation (NH3-SCO) and toluene combustion. Various techniques, such as temperature-programmed reduction with hydrogen, temperature-programmed desorption of toluene, scanning electron microscopy and reactive frontal chromatography were used to characterize the catalysts. Series 800-Cux-Ce2.5 showed the highest catalytic efficiency in both reactions, allowing complete NH3 and C7H8 conversion below 350 °C and 500 °C, respectively. CeO2 crystallites (12–18 nm), as well as dispersed CuO oxide forms were found in the most active samples. Both phases affect pollutants conversion, however, for ammonia oxidation the occurrence of Cu2+ is essential, while for toluene oxidation, the formation of Ce4+ is sufficient. Over the most active sample (800-Cu5-Ce2.5) the complete conversion in mutual oxidation of NH3 and toluene was achieved below 450 °C.

Magnesium borate whiskers/La2-XCeXZr2O7 catalysts preparation on cordierite honeycomb ceramic surfaces for soot catalytic oxidation͏

Magnesium borate whiskers were used to prepare a hierarchical microstructure on cordierite ceramic substrate through the precipitation-molten salt method. The La2-XCeXZr2O7 catalyst was then loaded onto the magnesium borate whiskers to form the hierarchical microstructure. The Ce-doped catalysts exhibited a significant improvement in the catalytic activity of soot combustion compared to the La2Zr2O7 catalyst. The study employed various techniques to characterize the structural morphology and phase composition, including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Fourier transform-infrared spectroscopy (FT-IR), and X-ray diffraction (XRD). The growth of Magnesium borate whiskers (Mg2B2O5) with diameters of 100–200 nm and lengths of 2–5 μm were grown and tightly bonded to the cordierite ceramic substrate to mimic the structure of human respiratory system cilia. This could improve the filtration efficiency of soot in car exhaust. BET analysis showed that the specific surface area of cordierite increased from 0.684 m2/g to 1.363 m2/g after whisker growth. The study confirmed that the whiskers were firmly attached to the cordierite ceramic substrate, as demonstrated by the binding strength experiments. The addition of Ce metal has been found to significantly enhance soot combustion due to the presence of more active oxygen species and superior mobility of lattice oxygen species with more oxygen vacancies. The catalyst's intrinsic redox property was characterized using X-ray photoelectron spectroscopy (XPS) and H2 temperature-programmed reduction (H2-TPR). The semi-quantitative thermogravimetric analysis (TG-DTG) and the temperature-programmed oxidation (TPO) of catalysts showed that Z-La1.9Ce0·1Zr2O7 exhibited the highest catalytic activity for soot combustion, with an ignition temperature of 268 °C, a peak temperature of 418 °C, and a temperature of 90 % soot conversion of 464 °C. Furthermore, cyclic stability experiments demonstrate that the Z-La1.9Ce0·1Zr2O7 catalyst exhibits excellent structural stability and consistent catalytic performance. This distinctive microstructure holds great potential for applications in the field of DPF.

Optimization of syngas production via methane bi-reforming using CeO2 promoted Cu/MnO2 catalyst

Currently, syngas plays an important role in renewable and sustainable energy production. The idea of manufacturing syngas via bi-reforming methane, which involves the combination of methane (CH4), carbon dioxide (CO2), and steam, appears very promising. As a result, the goal of this research is to improve syngas output by improving process parameters in methane bi-reforming using a 3%Ce-15%Cu/MnO2 catalyst. Optimization analysis was performed using response surface methodology (RSM). The ultrasonic impregnation (UI) method was employed to synthesize the catalysts used in this study. Following that, the catalyst was characterized using several techniques such as Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption (TPD), and temperature programmed oxidation (TPO). The findings of the characterization show that the presence of CeO2 promoters has a dual effect on the size of CuO crystallites. Firstly, it reduces the size from 19.07 nm to 13.66 nm because to the dilutive effect generated by the inclusion of CeO2. Second, the presence of CeO2 promoter accelerates the transition from CuO to Cu0 metallic phase. Furthermore, the addition of CeO2 boosts the CH4 and CO2 conversion rates by 23.65% and 24.93%, respectively. As a result, the H2 yield increases significantly when compared to the unpromoted catalyst. The study investigates the influence of process parameters, specifically the reaction temperature (700–900℃), CO2 ratio (0.2–1), and gas hourly space velocity (GHSV) (16–36 L g cat−1 hr−1), on the conversion of CH4 and CO2, as well as the H2/CO ratio. The optimization study finds that the highest conversion rates for CH4 and CO2 are 78.32% and 72.45%, respectively, when the reaction temperature is 800 °C, the CO2 ratio is 0.6, and the gas hourly space velocity (GHSV) is 26 L g cat−1 hr−1. The optimum conditions result in the highest syngas ratio of 1.77. The results of the optimization are then assessed using the mean errors. The H2/CO ratio, as well as the average errors for CH4 and CO2 conversions, are discovered to be 0.15%, 0.95%, and 0%, respectively.

Performance of NiO doped on alkaline sludge from waste photovoltaic industries for catalytic dry reforming of methane.

Alkali sludge (AS) is waste abundantly generated from solar photovoltaic (PV) solar cell industries. Since this potential basic material is still underutilized, a combination with NiO catalyst might greatly influence coke resentence, especially in high-temperature thermochemical reactions (Arora and Prasad, RSC Adv. 6:108,668-108688, 2016). This paper investigated alkaline sludge containing 3CaO-2SiO2 doped with well-known NiO to enhance the dry reforming of methane (DRM) reaction. The wet-impregnation method was used to prepare the xNiO/AS (x = 5-15%) catalysts. Subsequently, all catalysts were tested by using X-ray diffraction (XRD), nitrogen adsorption/desorption (BET), temperature-programmed reduction of hydrogen (H2-TPR), temperature-programmed desorption of carbon dioxide (TPD-CO2), field emission scanning electron microscopy (FESEM-EDX), and X-ray photoelectron spectroscopy (XPS). The spent catalysts were analyzed by thermogravimetric analysis (TGA/DTG), transmission electron microscopy (TEM), and temperature-programmed oxidation (TPO). The catalytic performance of xNiO/AS catalysts was investigated in a fixed bed reactor connected with gas chromatography thermal conductivity detector (GC-TCD) at a CH4:CO2 flow rate of 30 mL-1 during a 10-h reaction by following (Shamsuddin et al., Int. J. Energy Res. 45:15,463-15,480, 2021d). For optimization parameters, the effects of NiO concentration (5, 10, and 15%), reaction temperature (700, 750, 800, 850, and 900 °C), catalyst loading (0.1, 0.2, 0.3, 0.4, and 0.5 g), and gas hourly space velocity (GHSV) range from 3000, 6000, 9000, 12,000, and 15,000 h-1 were evaluated. The results showed that physical characteristics such as BET surface area and porosity do not significantly impact NiO percentages of dispersion, whereas chemical characteristics like reducibility are crucial for the catalysts' efficient catalytic activity. Due to the active sites on the catalyst surface being more accessible, increased NiO dispersion resulted in higher reactant conversion. The catalytic performance on various parameters that showed 15%NiO/AS exhibited high reactant conversion up to 98% and 40-60% product selectivity in 700 °C, 0.2 g catalyst loading, and 12,000 h-1 GHSV. According to spent catalyst analyses, the catalyst was stable even after the DRM reaction. Meanwhile, increased reducibility resulted in more and better active site formation on the catalyst. Synergetic effect of efficient NiO as active metal and medium basic sites from AS enhanced DRM catalytic activity and stability with low coke formation.

Revealing the effect of synthetic method on integrated CO2 capture and conversion performance of Ni-Na2ZrO3 bifunctional materials

Integrated CO2 capture and utilization by reverse water gas shift (ICCU-RWGS) reaction for syngas production for C1 chemical industry has gained increasing attention. Designing highly active bifunctional materials that enables CO2 adsorption and in-situ conversion is essential. In this work, Ni-Na2ZrO3 bifunctional materials are prepared by different synthetic methods for ICCU-RWGS. N2 physisorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), CO2 temperature-programmed desorption (CO2-TPD) and H2 temperature-programmed reduction (H2-TPR) analysis reveals that Ni-Na2ZrO3-CP prepared by the co-precipitation (CP) method shows small Ni crystalline size, moderate surface basicity, and excellent reducibility. ICCU-RWGS performance is investigated under isothermal conditions of 650 °C, 10 %CO2 and 35 %H2. The results indicate that Ni-Na2ZrO3-CP demonstrates excellent performance in ICCU-RWGS, with a notable CO2 capture capacity of 3.76 mmol CO2/g and a CO production rate of 3.10 mmol CO/g. Additionally, it exhibits relatively good operational stability. Detailed reaction paths of the ICCU-RWGS procedure for the Ni-Na2ZrO3 bifunctional materials are proposed. The findings will shed new perspectives on the design of synthetic bifunctional catalysts for ICCU-RWGS.

Investigating the Catalytic Influence of Boron on Ni-Co/Ca Catalysts for Improved Syngas Generation from Rice Straw Pyrolysis.

A series of boron-promoted Ni-Co/Ca catalysts were synthesized by the sol-gel method to enhance syngas generation from biomass pyrolysis. The efficiency of these catalysts was evaluated during the pyrolysis of rice straw in a fixed-bed reactor, varying the Ni/Co ratio, boron addition, calcination temperature, and residence time. The catalysts underwent comprehensive characterization using X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) analysis, scanning electron microscopy (SEM), and hydrogen temperature-programmed reduction (H2-TPR). The results indicated that the Ni-Co/Ca catalysts yielded superior syngas compared to singular Ni or Co catalysts, suggesting a synergistic interplay between nickel and cobalt. The incorporation of 4% boron significantly decreased the particle size of the active metals, enhancing both the catalytic activity and stability. Optimal syngas production was achieved under the following conditions: a biomass-to-catalyst mass ratio of 2:1, a Ni-Co ratio of 1:1, a calcination temperature of 400 °C, a pyrolysis temperature of 800 °C, and a 20 min residence time. These conditions led to a syngas yield of 431.8 mL/g, a 131.28% increase over the non-catalytic pyrolysis yield of 188.6 mL/g. This study not only demonstrates the potential of Ni-Co/Ca catalysts in biomass pyrolysis for syngas production but also provides a foundation for future catalyst performance optimization.

Enhancing CO and H2 Production in Propane Dry Reforming in Excess of CO2.

This study focuses on addressing the challenges in the dry reforming of propane, a process historically marked by low syngas yields and only moderate conversions of CO2 and propane. The primary objective was to enhance CO2 utilization and boost the selectivity of syngas (CO and H2) production using titania-based catalysts. For synthesizing these catalysts, an impregnation method was employed with subsequent characterization through X-ray diffraction (XRD), N2 adsorption-desorption, ammonia temperature-programmed desorption (TPD), and hydrogen temperature-programmed reduction (TPR). The titania-based catalysts generally possess weak acidic strength, with each catalyst displaying a unique reduction profile. The dry reforming process using these catalysts resulted in varying levels of propane conversion, with V/Ti, Ir/Ti, Al/Ti, and Zr/Ti catalysts showing distinct efficiencies. Notably, the Ir/Ti and V/Ti oxide catalysts achieved the lowest selectivity for generating intermediate byproducts such as methane, ethane, ethylene, and propylene while successfully promoting higher syngas CO and H2 production alongside stable propane conversion. When exposed to excess CO2, each catalyst consumed differing amounts of CO2 molecules. Particularly, the Ir/Ti and V/Ti oxide catalysts demonstrated enhanced activity in promoting CO2 reactions with intermediate radical species, facilitating carbon-carbon (C-C) bond dissociation and leading to increased syngas production. This study offers valuable insights into the potential of titania-based catalysts in improving the efficiency and selectivity of propane dry reforming processes for blue hydrogen.

Effect of Copper Particle Size on the Surface Structure and Catalytic Activity of Cu–CeO2 Nanocomposites Prepared by Mechanochemical Synthesis in the Preferential CO Oxidation in a H2-Rich Stream (CO-PROX)

An effect of Cu powder dispersion and morphology on the surface structure and the physical–chemical and catalytic properties of Cu–CeO2 catalysts prepared by mechanochemical synthesis was studied in the preferential CO oxidation in a H2-rich stream (CO-PROX). Two catalysts, produced by 30 min ball-milling from CeO2 and 8 mass% of copper powders and with particle sizes of several tens (dendrite-like Cu) and 50–200 nm (spherical Cu obtained with levitation-jet method), respectively, were characterized by X-ray diffraction and electron microscopy methods, a temperature-programmed reduction with CO and H2, and with Fourier-transform infrared spectroscopy. The catalyst synthesized from the “large-scale” dendrite-like Cu powder, whose surface consisted of CuxO (Cu+) agglomerates located directly on the surface of facetted CeO2 crystals with a CeO2(111) and CeO2(100) crystal planes exposition, was approximately two times less active at 120–160 °C than the catalyst synthesized from the fine Cu powder, whose surface consisted of CuxO (Cu2+) clusters of 4–6 nm in size located on the steps of facetted CeO2 nanocrystals. Although a large part of CO2 reacted with a ceria surface to give carbonate-like species, no blockage of CO-activating centers was observed due to the surface architecture. The surface structure formed by the use of highly dispersed Cu powder is found to be a key factor responsible for the catalytic activity.

Modeling the temperature-programmed reduction of metal oxide catalysts by considering the particle-size distribution effect

Hydrogen temperature-programmed reduction (H2-TPR) has become a very useful and common technique for the chemical characterization of solids as it is sensitive to the study of reducible species in catalysis and is considered to be a fingerprint for the reducibility of metal oxide catalysts. However, although modeling of H2-TPR patterns has been extensively studied, little attention has been paid to the effect of particle-size distribution (PSD). The complexity of modeling H2-TPR patterns arises from the fact that the chemistry of metal oxide reduction depends on several factors, including particle size, nature of the support material and confinement within the porous structure, amongst others. In order to identify the kinetic reaction model governing the reduction of certain metal oxides and to explore the effect of PSD, pure metal oxides that only exhibited the particle size difference effect were used to model the H2-TPR patterns. Kinetic and thermodynamic data, which are very useful for characterizing heterogeneous catalysts, were obtained from this study. This work presents a simple procedure for modeling H2-TPR patterns of various metal oxides (i.e., CuO, Ag2O, and NiO) used as active phases in several reactions of environmental and energetic interest using several solid-state reaction kinetic models and considering their PSDs. The results obtained show that modeling the H2-TPR profiles provides information regarding the PSD of metal oxide catalysts that undergo a single-step reduction and only present the particle size difference effect.

Industrial waste-based Ni-catalysts for ammonia decomposition to produce clean hydrogen

The high mineral containing solid waste materials generated during aluminum production currently lacks any practical application. In this work, the potential of solid waste with/without additional nickel loading (10 to 20 wt%) was investigated as an active catalyst for low temperature ammonia decomposition to produce high purity hydrogen. The X-ray diffraction (XRD) analysis of the samples reveals that iron oxide was the primary component of solid waste with the ability to catalyze the ammonia decomposition reaction. The reducibility, metal support interaction and catalytic activity of the solid waste supported Ni-based catalysts were correlated by the reduction kinetics of the catalysts using temperature-programmed reduction (H2-TPR) data and nucleation/nuclei growth models. Based on the statistical indicators, it was concluded that the random nucleation model describes catalyst reduction adequately. The estimated activation energy for the reduction of the solid waste itself was significantly higher (108.5 kJ/mol) than that of solid waste supported catalyst containing 10 wt% Ni (86.6 kJ/mol). The increase in nickel loading also slightly decreased the activation energy of catalyst reduction, indicating an improvement in the reducibility of the nickel-containing catalysts. These reduction behaviors were also reflected in the ammonia decomposition reaction in a flow reactor. The addition of nickel onto the solid waste resulted in the creation of additional active nickel sites. Consequently, the nickel loaded solid waste supported catalysts exhibited significant improvements in ammonia decomposition activity. Among the three solid waste supported Ni-based catalysts, the catalyst containing 15 wt% Ni displayed the highest activity, that also remained unchanged over extended period, indicating the catalyst stability.

Effect of Nb as dopant of hydrotalcite catalysts during the ethanol condensation to optimize n-Butanol production

The present study optimizes Cu-supported catalysts on hydrotalcite for the synthesis of n-butanol whose interest is in its use as biofuel, due to its superior performance in comparison with other short-chain alcohols. The catalytic performance was improved by adding Nb to the catalytic formulation. In this way, several Cu-Nb catalysts supported on hydrotalcites (Mg/Al) with different Nb loadings were synthesized by incipient wetness impregnation and characterized by H2−temperature-programmed reduction, X-ray diffraction, N2 physisorption, CO2− and NH3−thermo-programmed desorption and X-ray photoelectron spectroscopy. It was shown that the incorporation of Nb promoted a suitable balance between the acid-basic and redox sites, modulating the Cu0/Cu+ dehydrogenation/hydrogenation properties for the n-BuOH production. The synthesis of multiple value-added products (n-BuOH, acetaldehyde, ethyl acetate, and 1-hexanol) was achieved in both the liquid− (at 180 ºC) and gas−phase (150−350 ºC), where it was demonstrated that both the n-BuOH selectivity and ethanol conversion improved with the Nb addition. The performance of the catalysts under time on stream and their recycling is also described.

Ex situ and in situ studies on structural features of Pt/Ce0.75Zr0.25O2 catalyst for water gas shift reaction

Pt/Ce1-xZrxO2 catalysts have shown high activity in water gas shift (WGS) reaction, but their structural organization has been studied insufficiently. This work represents a detailed structural study on the supported Pt species and the metal/support interface in a 5 wt% Pt/Ce0.75Zr0.25O2 catalyst in the initial state, after reductive activation, and after WGS reaction in H2-enriched reformate-simulating mixture. A wide range of methods was used: ex situ and in situ X-ray diffraction (XRD) studies, high resolution transmission electron microscopy (HRTEM), X-ray atomic pair distribution function (PDF) method, pseudo in situ X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2-TPR). The Ce0.75Zr0.25O2 mixed oxide was shown to provide a highly dispersed state of Pt species. XPS revealed that the initial catalyst contains Pt2+species. PDF analysis allowed us to propose the structure of ultrafine PtO particles and to elucidate the metal-support interaction with fixation of Pt ions on the support surface. In situ XRD, pseudo in situ XPS, and H2-TPR studies revealed the reduction of ultrafine PtO particles in H2 atmosphere at low temperatures (20–90°С) with the formation of metallic Pt0 particles and simultaneous partial reduction of the support surface due to the hydrogen spillover. Catalytic tests of the as-prepared and poisoned with chlorine 5wt% Pt/Ce0.75Zr0.25O2 catalysts in the WGS reaction showed an important role of the oxygen vacancies in the oxide support as active sites. These findings contribute to the understanding catalytic performance of Pt/Ce1-xZrxO2 systems.