Temperature-programmed Reduction Experiments Research Articles

Towards detailed combustor wall kinetics: An experimental and kinetic modeling study of hydrogen oxidation on Inconel

Gas-phase hydrogen combustion is ubiquitous in industrial processes, and the associated surface kinetics on heat-resistant alloys plays a crucial role in designing efficient low-carbon technologies. We conducted new temperature programmed reduction experiments to determine the reducibility of materials following an oxidation cycle. These experiments were modeled using a thermoconsistent multi-site microkinetic model for H2 heterogeneous oxidation on Inconels, which was validated against literature experiments. This competitive adsorption model considers iron bulk content, hydrogen spillover and subsurface oxygen migration on hydrogen surface oxidation kinetics. New X-ray diffraction experiments confirmed the postulated crystallographic structures in the Inconel samples, suggesting their presence on the surface scale. The phenomenological model was coupled with several state-of-the-art gas-phase oxidation mechanisms to assess gas/surface reactions interaction as a function of material and temperature. The results reveal a complex interaction in which surface removes H radicals from the gas-phase, while the Bradford (H2+OH) reaction converts OH into H2O, promoting water adsorption on Inconel. This interaction was found to give rise to gas-phase thermokinetic oscillations. The model predicts a non-monotonous effect of reactor area-to-volume ratio on reactivity and emphasizes the impact of Inconel composition on product selectivity. Overall, the multi-site model provides new insight into the contrasting reactivities among active sites, bridging the gap between material science and heterogeneous combustion modeling.

Investigation of the synergistic effect of O2/H2O pretreatment and Na addition on the NO-semichar reaction

The surface physicochemical features of semichar play a crucial role in determining its reactivity, dominating the subsequent reaction between semichar and NO. Therefore, the synergistic effect of O2/H2O pretreatment and Na addition on the microstructural characteristics of semichar and NO reduction characteristics is revealed through experimental and density functional theory (DFT) methods. Experimental results indicate that O2/H2O pretreatments with Na addition promote the development of pores. Micropores are considered the major pore structures in semichar. Based on Raman and laser-induced breakdown spectroscopy (LIBS) results, the combined effect of O2/H2O pretreatments and Na promotes the formation of small aromatic rings, exhibiting a positive correlation with molecular emissions. Temperature-programmed reduction experiments (TPR) demonstrate that the inclusion of Na in O2/H2O pretreatments enhances the reduction ability of NO. The reduction mechanism of NO-semichar is determined using the DFT method. The simulation results indicate that the adsorption of H2O during O2/H2O pretreatment is chemisorption and independent of Na atom. The additional Na reduces the energy gap of the NO-semichar, facilitating the decomposition of N2 by weakening the conjugated components of the aromatic structures.

Understanding the Reduction Behavior of VOx/CeO2 on a Molecular Level: Combining Temperature-Programmed Reduction with Multiple In-Situ Spectroscopies and X-ray Diffraction

As catalytic processes become more important in academic and industrial applications, an intimate understanding is highly desirable to improve their efficiency on a rational basis. Because thorough mechanistic investigations require an elaborate and expensive spectroscopic and theoretical analysis, it is a major goal to link mechanistic insights to simple descriptors, such as the reducibility, that are accessible by temperature-programmed reduction (TPR) experiments, to bridge the gap between fundamental understanding and application of catalysts. In this work, we present a detailed in-situ spectroscopic analysis of TPR results from loading-dependent VOx/CeO2 catalysts, using in-situ multiwavelength Raman, IR, UV–vis, and quasi-in-situ X-ray photoelectron spectroscopy as well as in-situ X-ray diffraction. The catalyst reduction shows a complex network of different processes, contributing to the overall reducibility, which are controlled by the unique interaction at the vanadia–ceria interface. The temperatures at which they occur depend significantly on the nuclearity of the surface vanadia species. By elucidating the temperature- and vanadia loading-dependent behavior, we provide a fundamental understanding of the underlying molecular processes, thus developing an important basis for interpretation of the reduction behavior of other oxide catalysts.

Influence of Zn and Fe promoters on Ni-Bi/γ-Al2O3 catalyst for oxidative dehydrogenation of n-butane to butadiene

This study investigated the effects of Zn and Fe metal oxide species as promoters on NiO-Bi2O3/γ-Al2O3 in the oxidative dehydrogenation (ODH) of n-butane to produce butadiene. Ni-Bi-O, Ni-Zn-Bi-O, Ni-Fe-Bi-O, and Ni-Zn-Fe-Bi-O catalysts supported on γ-Al2O3 were synthesized, characterized, and evaluated in a packed bed flow reactor at different temperatures and O2/n-butane molar ratios. Substituting 50 wt.% of Ni with one or both promoters were found to enhance both the butadiene selectivity. Among the synthesized catalysts, Ni-Zn-Fe-Bi-O/γ-Al2O3 (10 wt.% Ni-5 wt.% Zn-5 wt.% Fe-30 wt.% Bi) showed the highest butadiene selectivity of 49.0% with 18.6% n-butane conversion at 450 °C and a molar feed ratio of O2/n-butane = 2.0. This enhanced performance was attributed to the synergetic effects of Ni, which simultaneously improved the first-step dehydrogenation selectivity via a promotion with Zn and the second-step dehydrogenation selectivity by the incorporation of Fe. Moreover, Zn allowed the surface reduction to occur at lower temperatures, while Fe balanced the densities of strong basic sites with weak and moderate acid sites (0.183 mmol CO2/g-cat and 0.188 mmol NH3/g-cat, respectively). The aforementioned results were confirmed with CO2/NH3 temperature-programmed desorption and H2 temperature-programmed reduction experiments. Under this reaction condition, the catalyst maintains good stability over 15 h of time-on-stream.

Reactivity of Lattice Oxygen in Ti-Site-Substituted SrTiO3 Perovskite Catalysts

An environmental catalyst in which a transition metal (Mn, Fe, or Co) was substituted into the Ti site of the host material, SrTiO3, was synthesized, and the reactivity of lattice oxygen was evaluated. For CO oxidation, Mn- and Co-doped SrTiO3 catalysts, which provided high thermal stabilities, exhibited higher activities than Pt/Al2O3 catalysts despite their low surface areas. Temperature-programmed reduction experiments using X-ray absorption fine structure (XAFS) measurements showed that the lattice oxygen of Co-doped catalyst was released at the lowest temperature. Isotopic experiments with CO and 18O2 revealed that the lattice oxygen was involved in CO oxidation on Fe- and Co-doped catalysts; that is, CO oxidation on these catalysts proceeded via the Mars-van Krevelen mechanism. On the other hand, for Mn-doped catalyst, the contribution of lattice oxygen to CO oxidation was relatively negligible, indicating that the reaction proceeded according to the Langmuir-Hinshelwood mechanism. This paper clearly demonstrates that the catalytic mechanism can be adjusted by substituting transition metals into SrTiO3.

Understanding C-H activation in light alkanes over Cu-MOR zeolites by coupling advanced spectroscopy and temperature-programmed reduction experiments.

The direct activation of methane to methanol (MTM) proceeds through a chemical-looping process over Cu-oxo sites in zeolites. Herein, we extend the overall understanding of oxidation reactions over metal-oxo sites and C-H activation reactions by pinpointing the evolution of Cu species during reduction. To do so, a set of temperature-programmed reduction experiments were performed with CH4, C2H6, and CO. With a temperature ramp, the Cu reduction could be accelerated to detect changes in Cu speciation that are normally not detected due to the slow CH4 adsorption/interaction during MTM (∼200 °C). To follow the Cu-speciation with the three reductants, X-ray absorption spectroscopy (XAS), UV-vis and FT-IR spectroscopy were applied. Multivariate curve resolution alternating least-square (MCR-ALS) analysis was used to resolve the time-dependent concentration profiles of pure Cu components in the X-ray absorption near edge structure (XANES) spectra. Within the large datasets, as many as six different CuII and CuI components were found. Close correlations were found between the XANES-derived CuII to CuI reduction, CH4 consumption, and CO2 production. A reducibility-activity relationship was also observed for the Cu-MOR zeolites. Extended X-ray absorption fine structure (EXAFS) spectra for the pure Cu components were furthermore obtained with MCR-ALS analysis. With wavelet transform (WT) analysis of the EXAFS spectra, we were able to resolve the atomic speciation at different radial distances from Cu (up to about 4 Å). These results indicate that all the CuII components consist of multimeric CuII-oxo sites, albeit with different Cu-Cu distances.

Pelletized SiO2-supported La0.5Ba0.5FeO3 for conversion of CO2 to CO by a reverse water-gas shift chemical looping process

Perovskite oxides, such as La0.5Ba0.5FeO3 (LBF), are promising materials for converting CO2 to CO through low-temperature reverse water–gas shift chemical looping (RWGS-CL). To transition toward industrialization, this study focused on forming perovskite-oxide/silica composite pellets (25 wt% LBF) with sufficiently high crushing strength, with materials prepared by extrusion and tableting methods. X-ray fluorescence (XRF) and diffraction (XRD) confirmed that the resulting material is 26.6 wt% LBF on SiO2 with a combined structure of LBF and SiO2 components. Temperature-programmed reduction (TPR) and oxidation (TPO) experiments revealed that CO2 is converted to CO at 550 °C by LBF/SiO2 pellets, higher than LBF by 50 °C. CO2 chemisorption was determined to be 62.2 μmol/gLBF. DRIFTS-MS experiments confirmed that adsorbed CO2 is dissociated to CO. High material stability is proven by long-term RWGL-CL experiments combined with XRD and XPS. The CO2 to CO yields were 2.21, 2.41, and 2.35 mmol/gLBF of 10 mm tableting pellets, 6 mm tableting pellets, and extrusion pellets, respectively. Deactivated pellets are able to be regenerated by air thermal treatment following intentional reduction. All pellets showed stable redox properties in 50 cycles of semi-batch reactor experiments, indicating that LBF/SiO2 pellets are candidate materials for further scale-up evaluation.

Exploration of Optical, Redox, and Catalytic Properties of Vanadia-Mayenite Nanocomposites

The present paper continues the exploration of the physicochemical and catalytic properties of vanadia-mayenite composites. The samples were prepared by an impregnation of calcium aluminate Ca12Al14O33 (mayenite, C12A7) with a solution of vanadium precursor. Pure mayenite and V/C12A7 nanocomposites were characterized by Raman and diffuse reflectance UV–Vis spectroscopies. The reducibility of the samples was examined in temperature-programmed reduction experiments performed in a hydrogen atmosphere. The catalytic performance of vanadium-containing systems was studied in the non-oxidative dehydrogenation of ethane. As found, the low-loaded sample (5%V/C12A7 sample) contains vanadium predominantly in the form of Ca3(VO4)2, while for the 10%V/C12A7 sample, two types of calcium vanadates (Ca2V2O7 and Ca3(VO4)2) are registered. The presence of these phases defines the spectroscopic characteristics and the redox properties of nanocomposites. Both the samples, 5%V/C12A7 and 10%V/C12A7, exhibit comparable catalytic activity, which is mainly connected with the amount of the Ca3(VO4)2 phase. The uniqueness of the studied catalysts is their excellent tolerance toward coke formation under the reaction conditions.

CO Oxidation over Alumina-Supported Copper Catalysts

CO oxidation, one of the most important chemical reactions, has been commonly studied in both academia and the industry. It is one good probe reaction in the fields of surface science and heterogeneous catalysis, by which we can gain a better understanding and knowledge of the reaction mechanism. Herein, we studied the oxidation state of the Cu species to seek insight into the role of the copper species in the reaction activity. The catalysts were characterized by XRD, N2 adsorption-desorption, X-ray absorption spectroscopy, and temperature-programmed reduction. The obtained results suggested that adding of Fe into the Cu/Al2O3 catalyst can greatly shift the light-off curve of the CO conversion to a much lower temperature, which means the activity was significantly improved by the Fe promoter. From the transient and temperature-programmed reduction experiments, we conclude that oxygen vacancy plays an important role in influencing CO oxidation activity. Adding Fe into the Cu/Al2O3 catalyst can remove part of the oxygen from the Cu species and form more oxygen vacancy. These oxygen vacancy sites are the main active sites for CO oxidation reaction and follow a Mars-van Krevelen-type reaction mechanism.

Experimental and density functional theory investigation of the NO reduction mechanism by semichars preheated in Ar and CO2/Ar atmospheres

The extensive utilization of coal-based solid fuels in thermal power plants has led to massive emissions of greenhouse gases (GHG) and gaseous pollutants, resulting in serious environmental issues. In this study, the NO reduction reactivity of semichars preheated in Ar and Ar/CO2 atmospheres was investigated via temperature-programmed reduction (TPR) experiments. Based on the TPR-DTPR method, the NO initial temperature, the maximum NO reduction rate, and the NO reduction indices of different semichars were determined. Compared with CO2/Ar pretreatment, the inert pretreatment enhanced the NO reduction index of the semichar. The CO2/Ar pretreatment was beneficial for decreasing the total Raman area and the ratio of I(Gr+VL+Vr)/ID due to the preferential consumption of high-reactivity structures and the condensation of aromatic ring structures. Furthermore, the detrimental effect of CO2/Ar treatment on semichar reactivity was relevant to the volumetric fraction, exerting an extraordinary correspondence with Rr. Density functional theory (DFT) was employed further to analyse the NO reduction mechanisms by semichars derived from Ar and CO2/Ar atmospheres. For the inert pretreatment, the NO reduction pathways consisted of two pathways (path 1 and path 2). Path 1 was supported by thermodynamics and dynamics. The energy barrier of the rate-determining step was 280.53 kJ/mol. Owing to the difference in the evolutionary pathways of C(O) on the semichar surface, the reaction between NO and the semichars derived from the CO2/Ar atmospheres could be summarized as two pathways (path 3 and path 4). Path 3 was identified as the better method, and the energy barrier of the rate-determining step was 285.39 kJ/mol. The slightly adverse effects of CO2/Ar pretreatment on NO heterogeneous reduction were determined by coupling the experimental and theoretical methods, providing new insights into nitrogen migration and transformation mechanisms.

The effect of the incorporation of catalase mimetic activity cations on the structural, thermal and chemical durability properties of the 45S5 Bioglass®

Understanding the effect of the incorporation of doping ions into the structure, thermal properties and chemical durability of bioactive glasses is fundamental for the design of new compositions with tailored biological functions and applications. In this work, we have applied a combined experimental and computational approach to unravel the effect of adding metal oxides of Ce, Ti, V, Mn, Fe, Co, Cu, and Zr that impart catalase mimetic activity to the 45S5 Bioglass® on its density, thermal properties and chemical durability. UV-Vis-NIR spectroscopy and temperature-programmed reduction (TPR) experiments allowed us to determine the oxidation states of the doping cations in the bulk of the glasses, Differential Thermal Analysis has been used to determine the glass transition and crystallization temperature, whereas the chemical durability in water was determined by following the hydrolytic resistance of glass grains at 98 °C standard method. The experimental results have been interpreted at the atomic level by exploiting reliable bulk and surface structural models of the investigated glasses generated by using Molecular Dynamics Simulations. Structure-property relationships helpful for the rational design of new glass compositions have also been inferred.

Plasma‐Modified Coal Fly Ash Zeolites with Enhanced Catalytic Efficiency toward the Total Oxidation of Volatile Organic Compounds as Low‐Cost Substitutes for Platinum Group Metals Catalysts

The initial coal fly ash zeolites (CFAZ) are obtained from raw coal fly ash with two SiO2/Al2O3 ratios of 2.0 and 2.2 by hydrothermal activation or by two‐stage process. Microwave plasma surface treatment of CFAZ is performed to modify their surface and therefore their catalytic activity in total VOC oxidation. CHF3 and SF6 are used as plasma agents at two different treatments times—30 s and 60 s. The obtained hierarchal CFAZ are characterized by X‐ray diffraction, N2‐physisorption, temperature programmed reduction experiments, UV–vis spectroscopy and X‐ray photoelectron spectroscopy to clarify the effect of plasma treatment on the surface and reduction properties, morphology and structure of the modified samples. The catalytic performance of the modified CFAZ is investigated in a flow system toward toluene oxidation in the temperature range of 250–520 °C. It has been observed that CHF3 favors the catalytic activity of CFAZ at shorter treatment duration, while SF6 inhibits their catalytic properties. The development of efficient low‐cost catalyst will contribute to the saving of critical raw materials such as platinum group metals, and to the utilization of the enormous resource of coal ash instead of its landfilling in accordance to the development of circular economy and smart waste management.

Synthesis and characterization of the flower-like LaxCe1−xO1.5+δ catalyst for low-temperature oxidative coupling of methane

Lanthanum-based oxides are promising candidates for low-temperature oxidative coupling of methane (OCM). To further lower the OCM reaction temperature, the Ce doped flower-like La2O2CO3 microsphere catalysts were synthesized, achieving a significantly low reaction temperature (375 °C) while maintaining high C2+ hydrocarbon selectivity (43.0%). Doping Ce into the lattice of La2O2CO3 created more surface oxygen vacancies and bulk lattice defects, which was in favor of the transformation and migration of oxygen species at 350–400 °C. The designed H2 temperature-programmed reduction (H2-TPR) experiments provided strong evidence that the low reaction temperature of LaxCe1−xO1.5+δ can be attributed to the transformation and migration of oxygen species, which dynamically generated surface oxygen vacancies for continuous oxygen activation to selectively convert methane. Moreover, designed temperature-programmed surface reaction (TPSR) clarified that two kinds of surface oxygen species in LaxCe1−xO1.5+δ catalysts were concerned with catalytic performance, that is, the surface chemisorbed oxygen species for the activation of CH4 and the formation of CH3• intermediates, surface La-Ce-O lattice oxygen species that caused the excessive oxidation of CH3• intermediates. Finally, the factors affecting the transformation and migration of oxygen species were explored.

Effects of Treatment Conditions on Pd Speciation in CHA and Beta Zeolites for Passive NO x Adsorption.

The structure and evolution of Pd species in Pd-exchanged zeolite materials intended for use as passive NOx adsorbers were examined under various pretreatment conditions. Using in situ CO-diffuse reflectance infrared spectroscopy, Pd structures were characterized after 500 °C pretreatments in inert (Ar), water (1–2% H2O in Ar), oxidizing (air), and reducing (H2, CO) atmospheres. Two zeolites of similar Si/Al ratios but different framework topologies (Beta, CHA) were found to show different distributions of Pd species, depending on the reducing agent used. Reduction in H2 (500 °C; 10% H2 in Ar) followed by re-oxidation (500 °C; air) led to higher amounts of single-site Pd ions on Pd-CHA than Pd-Beta, whereas high-temperature reduction in CO (500 °C; 1000 ppm CO in Ar) followed by re-oxidation (500 °C; air) led to significant loss of ionic Pd on both Pd-CHA and Pd-Beta, albeit H2 temperature-programmed reduction and XPS experiments suggest that this phenomena may be limited to surface Pd. High-temperature treatments with water (500 °C; 1–2% H2O in Ar) are shown to form either Pd metal or PdO particles, with Pd-Beta being more susceptible to these effects than Pd-CHA. This work suggests that the effects of CO are especially problematic with respect to the durability of these materials in passive NOx adsorption applications, especially in the case of Beta zeolite.

Effect of Ni/Fe ratio in Ni–Fe catalysts prepared under external magnetic field on CO2 methanation

BackgroundThe power-to-gas (PtG) method has significant potential as a strategy for storing excess renewable energy in the form of synthetic natural gas. PtG involves the hydrogenation of CO2 to CH4 from H2 obtained from renewable energy. MethodsIn this study, Ni–Fe catalysts with different Ni/Fe ratios were prepared for high-efficiency CO2 methanation using chemical reduction with an external magnetic field. The Ni–Fe catalysts were characterized by XRD, inductively coupled plasma, and SEM-EDS measurements, and their performance and chemical properties were measured by H2 temperature programmed reduction experiments. Significant findingsThe catalyst with the best performance for CO2 methanation had a ratio of Ni to Fe of 8/2 (Ni8Fe2). The Ni8Fe2 catalyst could convert over 80% of CO2 to methane at 150 °C. In addition, the CH4 selectivity of CO2 methanation was 95% when the reaction conditions were controlled at H2/CO2 = 4 and GHSV ∼1000 h−1. The low cost and simple preparation of these Ni–Fe catalysts have high potential for future commercialization.

Role of Ba in low temperature thermochemical conversion of carbon dioxide with LaFeO3 perovskite oxides

Reverse water-gas shift chemical looping (RWGS-CL) is a low temperature thermochemical process to convert CO2 to CO by perovskite oxides. In this research, perovskite oxides of the form of La1-xBaxFeO3 (x = 0.25−0.75) are explored for enhancing CO yield while maintaining stability over multiple cycles. DFT was utilized to calculate the oxygen vacancy formation energies (Evac) identifying Evac of La0.75Ba0.25FeO3 (3.3 eV) and La0.5Ba0.5FeO3 (3.0 eV) are in the ideal range of 3.4 ± 0.5 eV. Additional Ba resulted in instability. XRD confirmed cubic perovskite structure and XRF analysis established the precise composition. XPS revealed that the materials had Ba enrichment at the surface, compared to the overall bulk. Temperature-programmed reduction (TPR) and oxidation (TPO) experiments determined that CO2 is converted to CO in the low temperature range of 400–600 °C for all compositions, which is lower than temperatures of many thermochemical CO2 processes. Isothermal cycling between hydrogen and CO2 at 600 °C indicated La0.5Ba0.5FeO3 and La0.4Ba0.6FeO3 were stable (verified by XRD and XPS) and produced CO at high yields of 200 and 500 μmol/gLBF, respectively. The CO2 conversion step fits to Langmuir–Hinshelwood type kinetics based on its partial pressure dependency and is consistent with DRIFTS studies. Therefore, multiple materials in this study enhance performance, indicating their suitability for industrial scale adaptation upon energy integration as a possible strategy for renewable energy storage in chemical form.

The role of ruthenium on the acidity of mixed alumina and silica phases and its impact on activity for ethanol dehydration

AbstractRuthenium catalysts anchored on three distinct supports (γ‐Al2O3, γ‐χ‐Al2O3, and silica [SiO2]) are tested for ethanol dehydration at 250°C. The catalytic materials were characterized using X‐ray diffraction (XRD), N2 physisorption, ammonia temperature programmed desorption (NH3‐TPD), FTIR, UV‐Vis spectroscopy, in situ UV‐Vis spectroscopy, H2 temperature programmed reduction experiments (H2‐TPR), and thermogravimetric analysis (TGA) after reaction. The activity was correlated to the number of acid sites present in the support. However, the addition of Ru increased the activity for alumina supported catalysts. Product selectivity depends on the nature of the acid sites present on the catalytic material, with ruthenium playing a critical role on the acid properties of the catalysts.

Surface reactions during temperature-programmed desorption and reduction experiments with oxygen-functionalized carbon blacks

Carbon black was functionalized by gas-phase oxidation using nitric acid vapor at 150 °C, and temperature-programmed desorption (TPD) and temperature-programmed reduction (TPR) experiments were performed in a plug-flow reactor to analyze the decomposition mechanisms of oxygen-containing surface groups by monitoring evolved H2O, CO2, and CO quantitatively. Subsequent TPD measurements detected an enrichment of acidic surface groups with increasing duration of the HNO3 functionalization from 2 h to 24 h. A significant amount of H2O was released during the TPD experiments, yielding H2O evolution profiles which were deconvoluted into two Gaussian peaks at 162 °C and 228 °C. The combined analysis of the CO2 and H2O profiles indicates that desorbed H2O originates from chemisorbed water bound to carboxylic acid groups and from condensation reactions of carboxylic acids and phenols. Phenols and carbonyls were found to be reduced selectively by H2 during TPR, generating a pronounced H2O peak at 650 °C. A new peak in the CO2 evolution profile appeared at 575 °C in reducing atmosphere, which is assigned to the hydrolysis of anhydrides and lactones with subsequent decomposition. Thus, taking H2O into account is mandatory for a complete quantitative analysis of the decomposition mechanisms occurring during TPD and TPR experiments.

Optimizing Temperature Treatment of Copper Hollow Fibers for the Electrochemical Reduction of CO2 to CO

Copper hollow fibers were prepared via dry-wet spinning of a polymer solution of N-methylpyrrolidone, Polyetherimide, Polyvinyl Pyrolidone, and copper particles of sizes in the range of 1–2 µm. To remove template molecules and to sinter the copper particles, the time of calcination was varied in a range of 1–4 h at 600 °C. This calcination temperature was determined based on Thermal Gravimetric Analysis (TGA), showing completion of hydrocarbon removal at this temperature. Furthermore, the temperature of the subsequent treatment of the fibers in a flow of 4% H2 (in Ar) was varied in the range of 200 °C to 400 °C, at a fixed time of 1 h. Temperature programmed reduction experiments (TPR) were used to analyze the hydrogen treatment. The Faradaic Efficiency (FE) towards CO in electrochemical reduction of CO2 was determined at −0.45 V vs. RHE (Reversible Hydrogen Electrode), using a 0.3 M KHCO3 electrolyte. A calcination time of 3 h at 600 °C and a hydrogen treatment temperature of 280 °C were found to induce the highest FE to CO of 73% at these constant electrochemical conditions. Optimizing oxidation properties is discussed to likely affect porosity, favoring the CO2 gas distribution over the length of the fiber, and hence the CO2 reduction efficiency. Treatment in H2 in the range of 250 to 300 °C is proposed to affect the content of residual (subsurface) oxygen in Cu, which leads to favorable properties on the nanoscale.

In situ X-ray absorption study of Cu species in Cu-CHA catalysts for NH3-SCR during temperature-programmed reduction in NO/NH3

Ammonia-mediated selective catalytic reduction (NH3-SCR) using Cu-exchanged chabazite zeolites as catalysts is one of the leading technologies for NOx removal from exhaust gases, with CuII/CuI redox cycles being the basis of the catalytic reaction. The amount of CuII ions reduced by NO/NH3 can be quantified by the consumption of NO during temperature-programmed reduction experiments (NO-TPR). In this article, we show the capabilities of in situ X-ray absorption near-edge spectroscopy (XANES), coupled with multivariate curve resolution (MCR) and principal component analysis (PCA) methods, in following CuII/CuI speciation during reduction in NO/NH3 after oxidation in NO/O2 at 50 °C on samples with different copper loading and pretreatment conditions. Our XANES results show that during the NO/NH3 ramp CuII ions are fully reduced to CuI in the 50–290 °C range. The number of species involved in the process, their XANES spectra and their concentration profiles as a function of the temperature were obtained by MCR and PCA. Mixed ligand ammonia solvated complexes [CuII(NH3)3(X)]+ (X = OH−/O− or NO3−) are present at the beginning of the experiment, and are transformed into mobile [CuI(NH3)2]+ complexes: these complexes lose an NH3 ligand and become framework-coordinated above 200 °C. In the process, multiple CuII/CuI reduction events are observed: the first one around 130 °C is identified with the reduction of [CuII(NH3)3(OH/O)]+ moieties, while the second one occurs around 220–240 °C and is associated with the reduction of the ammonia-solvated Cu-NO3− species. The nitrate concentration in the catalysts is found to be dependent on the zeolite Cu loading and on the applied pretreatment conditions. Ammonia solvation increases the number of CuII sites available for the formation of nitrates, as confirmed by infrared spectroscopy.