Temperature-programmed Reduction Data Research Articles

The chemical state and Cu+ stability for three-way catalytic performance of Cu-added Al2O3 catalysts

Cu-added γ-Al2O3 (CuAl) catalysts having varying Cu loadings (3, 5, 8, and 10 mol%) were prepared at 900 °C. The effects of the copper oxidation states and the stability of these states under oxidizing and reducing atmospheres were examined. The use of these materials as three-way catalysts (TWCs) was also assessed. Analyses by X-ray diffraction and transmission electron microscopy indicated that copper oxide species were well dispersed and formed a solid solution in a surface layer of the γ-Al2O3. The TWC activity was found to increase with the addition of copper, and the 8 mol% CuAl catalyst showed very high performance despite being heat-treated at high temperatures. In the case of the 10 mol% CuAl catalyst, the excess Cu loading resulted in the precipitations of copper species on the γ-Al2O3 surface, which decreased catalytic activity. The oxidation states also differed between these catalysts. X-ray absorption fine structure and temperature-programmed reduction data demonstrated variations in copper oxidation states among the catalysts before and after oxidation and reduction treatments. The 8 mol% CuAl catalyst had a high Cu+/Cu2+ ratio and contained stable Cu+ species during both oxidation and reduction trials. The stability of the copper oxidation states is evidently a factor affecting catalytic activity.

Rate Constant Approximation with Cubic Splines for Kinetic Analysis of Temperature-Programmed Reduction Data

Temperature-programmed reduction (TPR) is a widely used method for characterization of oxide-based catalysts, sorbents, and functional materials, but its results lack quantitative assessment. Here, we present a novel approach to kinetic analysis of the TPR that can be applied to a large variety of systems involving multiple limiting stages. Implementation of cubic splines to approximate rate constant vs. conversion dependencies obtained from several TPR curves recorded at different heating rates yields in a set of kinetic parameters (activation energy and preexponential factors) for all reduction stages. Relationship between preexponential factor of the first reduction stage and the specific surface area of the sample has been shown. Reduction of hematite has been studied to prove the performance of the developed kinetic analysis technique.

The Influence of Cu and Al Additives on Reduction of Iron(III) Oxide: In Situ XRD and XANES Study.

The reduction of Fe-based nanocomposite catalysts doped with Al and Cu has been studied using in situ X-ray diffraction (XRD), in situ X-ray absorption near-edge structure (XANES), and temperature-programmed reduction (TPR) techniques. The catalysts have been synthesized by melting of iron, aluminum, and copper salts. According to XRD, the catalysts consist mainly of Fe2O3 and Al2O3 phases. Alumina is in an amorphous state, whereas iron oxide forms nanoparticles with the protohematite structure. The Al3+ cations are partially dissolved in the Fe2O3 lattice. Due to strong alumina-iron oxide interaction, the specific surface area of the catalysts increases significantly. TPR and XANES data indicate that copper forms highly dispersed surface CuO nanoparticles and partially dissolves in iron oxide. It has been shown that the reduction of iron(III) oxide by CO proceeds via two routes: a direct two-stage reduction of iron(III) oxide to metal (Fe2O3 → Fe3O4 → Fe) or an indirect three-stage reduction with the formation of FeO intermediate phases (Fe2O3 → Fe3O4 → FeO → Fe). The introduction of Al into Fe2O3 leads to a decrease in the rate for all reduction steps. In addition, the introduction of Al stabilizes small Fe3O4 particles and prevents further sintering of the iron oxide. The mechanism of stabilization is associated with the formation of Fe3- xAl xO4 solid solution. The addition of copper to the Fe-Al catalyst leads to the formation of highly dispersed CuO particles on the catalyst surface and a mixed oxide with a spinel-type crystalline structure similar to that of CuFe2O4. The low-temperature reduction of Cu2+ to Cu0 accelerates the Fe2O3 → Fe3O4 and FeO → Fe transformations but does not affect the Fe3O4 → FeO/Fe stages. These changes in the reduction properties significantly affect the catalytic performance of the Fe-based nanocomposite catalysts in the low-temperature oxidation of CO.

Iron precipitated onto ceria-zirconia nanoparticle mixtures for the production of hydrogen via two-step thermochemical water splitting

Several novel materials were synthesized by precipitating iron oxide (using the previously optimized 10% Fe loading by weight) onto mixtures of nanoparticle zirconia and ceria to investigate the effects of adding CeO2 to FeOx/ZrO2 materials in the thermochemical water splitting reaction. At water splitting temperatures of 1000 °C (after thermal reduction at 1450 °C), the stability of the CeO2-containing materials was lower than for the FeOx/ZrO2 material, and there was no advantage to adding CeO2 to the FeOx/ZrO2 material. However, when operating at a water splitting (WS) temperature of 1200 °C, the stability increased and the hydrogen production was significantly higher over most materials compared with a water splitting temperature of 1000 °C. At a WS temperature of 1200 °C the FeOx/Zr75Ce25O2 (75% Zr75O2 and 25% CeO2 by weight) and FeOx/Zr50Ce50O2 materials performed slightly better than the FeOx/ZrO2 material, and X-ray photoelectron spectroscopy data revealed that the surface concertation of iron is less important compared with water splitting at 1000 °C. The temperature programmed reduction data indicated that the FeOx-CeO2 interactions were weaker compared with FeOx-ZrO2 interactions, since the FeOx reduction occurred at lower temperatures for the CeO2-containing materials. The weaker interactions can explain why the stability was lower for the materials containing CeO2 (sintering of FeOx was likely more pronounced) The X-ray diffraction data revealed that ZrO2-CeO2 solid solutions formed after activation at 1450 °C and lattice volume calculations indicated that iron did incorporate into the ZrO2-CeO2 matrices. More incorporation was observed after water-splitting at 1200 °C compared with a lower temperature (1000 °C), and likely explains why the materials were more stable during water-splitting at 1200 °C.

Calculation of MoS2 slabs morphology descriptors from transmission electron microscopy data revisited. Case study of the influence of citric acid and treatment conditions on the properties of MoS2/Al2O3

Molybdenum sulfide MoS2 slabs length L and stacking S obtained from the transmission microscopy (TEM) images are the major characteristics of hydrotreating sulfide catalysts, widely used as morphology descriptors. In this work we demonstrate that the formulae routinely used to estimate mean L and S should be revisited. To obtain physically meaningful values L and S, both should be calculated as surface weighted averages. Moreover, correlation between L and S must be taken into account. Using surface weighted values of mean L and S, MoS2 phase morphology was studied for a series of MoS2/Al2O3 catalysts. The effects of Mo loading, addition of citric acid and sulfidation by H2S/N2 or H2S/H2 mixtures at different temperatures from 350 to 550°C were studied. The catalysts were evaluated in hydrodesulfurization of thiophene (HDS) and studied by temperature programmed reduction (TPR). Thorough statistical treatment of TEM, HDS and TPR data suggests that the intrinsic properties of MoS2 active phase depend primarily on the treatment atmosphere and temperature rather than on the addition of citric acid. If the impregnated catalyst is dried at temperature above 120°C, citric acid allows moderate improvement of MoS2 morphology. However the edge-specific activity of supported sulfide remains unaffected.

Reduction specifics of rare-earth orthovanadates (REE = La, Nd, Sm, Dy, Ho, Er, Tm, Yb, and Lu)

The reduction specifics of REE orthovanadates LnVO4 (Ln = La, Nd, Sm, Dy, Ho, Er, Tm, Yb, and Lu) have been studied using the temperature-programmed reduction (TPR) method. Hydrogen and carbon monoxide were chosen as reducing agents. The reduction temperature is found to depend both on the REE and the reducing agent. REE orthovanadates are reduced in the range 1033–1153 K not forming phases that contain vanadium in intermediate oxidation states. In CO, the reduction temperature is found to be higher than in H2 for all orthovanadates. TPR data have been used to calculate the activation energies of reduction of REE orthovanadates using the Kissinger equation. The effective activation energies of reduction depend on the REE and the reducing agent and are in the range 41–147 kJ/mol.

Methanol steam reforming on LaCo1−x−yPdxZnyO3±δ

Perovskite-type oxides LaCo1−x−yPdxZnyO3±δ are studied for the methanol steam reforming (MSR) reaction. LaCo1−x−yPdxZnyO3±δ with and without Co substitution by Pd and/or Zn are synthesized by the amorphous citrate method. The effect of substitution on the reducibility of the materials is investigated by in situ XRD and differential thermal/thermogravimetric analysis (DTA/TG) during temperature programmed reduction (TPR) with hydrogen. From this, the phase composition evolving as a function of temperature is determined. The TPR data indicate that the reducibility of the materials strongly depends on the substituent, i.e., Pd or Zn. The former increases the reducibility, while substitution by Zn decreases the reducibility. Materials containing both Pd and Zn exhibit intermediate reducibility as compared to the un-substituted LaCoO3±δ (LCO) and mono-metal substituted LaCo0.873Pd0.127O3±δ (LCPO) and LaCo0.89Zn0.11O3±δ (LCZO). In general, the metal ions in the materials are completely reduced at 590°C, except La2O3. After reductive pre-treatment at 590°C for 2h, the MSR performance of the materials is evaluated between 150 and 400°C while keeping comparable CH3OH conversion to assess the product-concentration-dependent CO2 selectivity. These results confirm that the CO2 selectivity is strongly dependent on the material composition. For example, the un-substituted LCO, LCPO and LCZO show a CO2 selectivity of around 13% at 300°C, while it is improved to 80% on both Pd and Zn containing LaCo0.85Pd0.075Zn0.075O3±δ (LCPZO-15).

Low-Temperature Selective Catalytic Reduction of NOx with NH3 over Novel Mn–Zr Mixed Oxide Catalysts

Novel Mn–Zr mixed oxide catalysts have been prepared by the citric acid method for the low-temperature selective catalytic reduction (SCR) of NOx with ammonia in the presence of excess oxygen. They have been characterized by a series of techniques, specifically N2 adsorption–desorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). It was found that an Mn(0.5)–ZrOx-450 (Mn/(Mn + Zr) mole ratio of 0.5) catalyst showed the highest activity, giving 100% NOx conversion at 100 °C with a space velocity of 30 000 h–1. XRD results suggested that an Mn–Zr solid solution was formed in the Mn(0.5)–ZrOx-450 catalyst, with highly dispersed MnOx. TPR data indicated a strong interaction between the zirconium oxide and manganese oxide, which improved the reduction ability of the MnOx. The TPD results indicated that an appropriate NH3 adsorption ability was beneficial for the low-temperature SCR. The catalyst showed a c...

Nanosized Ce1−xLaxO2−δ/Al2O3 solid solutions for CO oxidation: Combined study of structural characteristics and catalytic evaluation

Nanosized lanthanum-doped ceria (Ce1−xLaxO2−δ) and the same dispersed on alumina support (Ce1−xLaxO2−δ/Al2O3) were synthesized following a simple and high yielding procedure. Formation of solid solutions and interaction with the support were monitored by X-ray diffraction (XRD), Raman spectroscopy (RS; UV-RS and Vis-RS), high-resolution electron microscopy (HREM), Brunauer–Emmett–Teller (BET) surface area, X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and diffuse reflectance spectroscopy (UV-DRS) techniques. XRD results confirmed the formation of ceria–lanthana solid solutions. HREM studies revealed well distributed and small sized Ce–La–oxide particles of ∼7nm size over the surface of alumina support. Formation of defect sites or oxygen vacancies was investigated by RS using 325 and 632.81nm excitation laser lines which confirmed their presence in the investigated samples. RS studies further deduced that evolution of oxygen vacancies (∼570cm−1) was due to different ionic valence state of dopant (La3+) from that of Ce4+, while the development of MO8-type complex (∼600cm−1) was due to different ionic radius of dopant (La3+, 0.110nm) from that of Ce4+ (0.097nm). XPS results suggested that there is no significant change in the Ce3+ amount during thermal treatments due to the absence of undesirable cerium aluminates formation. TPR data revealed that surface modification with support enhances the reducibility of the doped ceria. Catalytic evaluation showed a 50% CO conversion temperature of 634K per 66mg of CL for the CL/A catalyst, which is lower than the CL sample (636K per 100mg of CL), possibly on account of an increased dispersion, more oxygen vacancies and easy reducibility.

Selective oxidation of alcohols over Si3N4-supported silver catalysts

Silver-containing catalysts supported on ceramic silicon nitrides and modified with Zr or Al are considered. The catalytic activities of the catalysts in the dehydrogenation and oxidative dehydrogenation of ethanol into acetaldehyde are compared. The introduction of oxygen into the reaction mixture decreases the temperature of 100% ethanol conversion to 270°C and increases the acetaldehyde selectivity to 95% for all of the catalysts. According to temperature-programmed reduction data, the simultaneous presence of Fe and Ag on the catalyst surface enhances the activity of the supported catalysts. It is hypothesized that the Ag/Fe interface plays the key role in the formation of active sites on the catalyst surface.

The influence of the amount and impregnation sequence of potassium on the catalytic performance of VOx/Al2O3 for the non-oxidative dehydrogenation of n-butane

The non-oxidative dehydrogenation of n-butane was studied over potassium-promoted supported vanadium alumina catalysts (at 823 K under atmosphere pressure) to explore the influence of the amount and the order of potassium aggregation on the catalytic activity and selectivity towards olefins. The non-oxidative dehydrogenation activity and selectivity decreased with increasing K/V molar ratio. The best catalytic performance was obtained with 1 wt.% of potassium loading and when the promoter was first aggregated in the supported vanadium alumina catalysts. The order of promoter aggregation has effects in the catalytic activity. TPR (temperature programmed reduction) data indicate that less vanadium species are accessible to the reduction when low amount (1 wt.%) of alkali metal promoter was first introduced in these samples. After regeneration treatment, these promoted catalysts did not recover the full activity. Taking in account TPR results, this catalyst deactivation may be related to the irreversible structural transformation of surface vanadium oxide species by potassium.

Ni-based xero- and aerogels as catalysts for nitroxidation processes

Porous nanocomposites made out of nickel dispersed on silica or alumina matrices were prepared as prospective catalysts for the nitroxidation of hydrocarbons in the form of aerogel or xerogel by adopting either a supercritical or a conventional gel drying procedure. The structural and textural features of the materials were investigated by X-ray diffraction, transmission electron microscopy and N2 physisorption and combined to the acid/base and reducibility data as deduced by adsorption microcalorimetry and temperature programmed reduction (TPR) profiles. The alumina-based samples are made out of nanocrystalline nickel aluminate and are mesoporous, although the aerogel has larger pore volumes and surface area than the xerogel. On the other hand, in the silica-based samples nickel oxide nanocrystals are dispersed on amorphous silica, the size of the nanocrystals being around 5 nm in the microporous xerogel and 14 nm in the mainly mesoporous aerogel. TPR data point out that the alumina-based samples have similar reducibility, whereas significant differences were observed in the silica-supported composites, the NiO–SiO2 aerogel exhibiting improved reducibility at low temperature. The NO-catalyst interaction was monitored by temperature programmed NO reaction coupled to mass spectrometry and preliminary tests on the use of the NiO–SiO2 xerogel and aerogel nanocomposites for the catalytic nitroxidation of 1-methyl-naphthalene to 1-naphthonitrile were obtained in a fixed-bed continuous-flow reactor. The data indicate that the aerogel exhibits larger selectivity than the corresponding xerogel, pointing out the importance of tuning the sol–gel parameters in the design of porous composite materials for catalytic applications.

The effect of copper catalyst reducibility on low temperature carbon fiber synthesis

A temperature programmed reduction (TPR) investigation of copper supported catalysts was undertaken to establish the role of reduction temperature on copper and its effect on the synthesis of shaped carbonaceous material (spiral, straight or formless carbon nanofibers) produced by the CCVD method. Catalyst reducibility was found to be strongly related to the type of support used (TiO 2 < MgO < CaO), the copper precursor (Cu(NO 3) 2 < CuCl 2 < Cu(acac) 2) and to a lesser extent solvent utilized. The most favorable yields of carbon fibers were obtained by reducing catalysts at temperatures inferred from TPR data. The reduction temperatures used impacted upon the morphology of the carbon fibers produced (straight, spiral or curled). We have found that carbon fibers synthesized from copper catalysts at low temperature (250 °C) utilizing Cu(NO 3) 2/TiO 2/water gave the best mass yield (340%) and selectivity towards carbon material with helical morphology (70%).

Production of granulated-copper oxide nanoparticles for catalytic application

Ultra fine CuO nanoparticles in the range of 2 ± 0.2 nm were synthesized by the supercritical hydrothermal method in a batch reactor. It was demonstrated that elevating the pH of the Cu2+ precursor solution to around 6 (neutral condition) not only does not lead to excessive agglomeration of the particles, but also reduces particle size and in general promotes their nanoscale characteristics. Prepared nanoparticles were immobilized in the biopolymeric matrix of barium alginate and calcined at different temperatures resulting in microspherical granules of high porosity and elevated mechanical strength. The fabricated samples were characterized using x-ray diffractometry (XRD), transmission and scanning electron microscopy (TEM and SEM), nitrogen adsorption analysis (BET), mechanical testing, and temperature programmed reduction (TPR). It was found that topochemical models based on a nucleation growth mechanism fail in proper fitting of the TPR data. Instead, a generalized Sestak model in which different physicochemical mechanisms such as the mass action law are taken into account gives a satisfactory regression of the kinetics behavior.

Dehydrogenation of propane over In 2O 3–Al 2O 3 mixed oxide in the presence of carbon dioxide

Dehydrogenation of propane (PDH) to propylene in the presence of CO 2 was performed over binary In 2O 3–Al 2O 3 mixed oxides prepared by alcoholic coprecipitation. Synergistic effect of different composition on the catalytic performance has been observed. Characterization by X-ray diffraction and X-ray photoelectron spectroscopy revealed that the combination of the two components can result in significant modification in the surface properties and bulk dispersion of the In 2O 3 phase, which in turn leads to a higher reducibility of the In–Al–O composite. A correlation between the temperature-programmed reduction data and the PDH activity for In 2O 3–Al 2O 3 revealed that the creation of surface metallic indium species during the reaction is crucial for the dehydrogenation performance. The promoting effect of CO 2 on the yield of propylene has been attributed to a facilitation of simple dehydrogenation by its coupling with the reverse water gas shift reaction based on the results from temperature-programmed reaction of CO 2 with H 2.

Desempenho de catalisadores com Nb 2 O 5 para produção de hidrogênio a partir da reforma de etanol com vapor d’água

Cu/Nb 2 O 5 and Cu/Nb 2 O 5 /Al 2 O 3 were studied as catalysts in ethanol steam-reforming for hydrogen production. They were prepared through impregnation, with different copper and Nb 2 O 5 concentrations. Cu/ZnO/Al 2 O 3 (G66), a commercial catalyst for methanol reforming was used for comparison. Temperature-programmed reduction data showed incomplete reduction of the copper, resulting in two reduction peaks, in opposition to CuO bulk, whose reduction occurred in one step (only one peak), it evidences a copper modification through Nb 2 O 5 . Results of diffuse reflectance spectra also evidenced the interaction. The catalytic tests showed that the niobium oxide dispersion on alumina improved the catalyst action; the ternary catalysts are the ones that show higher hydrogen production. They also produce ethyl ether as side product. The binary catalysts don't present parallel reaction, probably because when Cu/Nb 2 O 5 is dispersed on alumina there is another type of catalytic site production, which produces ethyl ether.

Catalytic combustion of benzene on Co/CeO2/SBA-15 and Co/SBA-15 catalysts

Catalytic oxidation of benzene was investigated on Co/CeO2/SBA-15 and Co/SBA-15 catalysts. These catalysts were characterized by X-ray diffraction, N2 sorption and temperature programmed reduction (TPR) methods. The results show that Co3O4 are not well dispersed on CeO2/SBA-15. The specific surface area and pore volume of CeO2/SBA-15 are significantly decreased upon impregnation, which is may be ascribed to the serious blockage of channel. Meanwhile, the TPR data show that the reduction of Co3O4 to Co0 is more difficult on CeO2/SBA-15 than on SBA-15. Consequently, catalyst Co/CeO2/SBA-15 shows lower catalytic activity than Co/SBA-15 in the oxidation of benzene.

Forms of oxygen in La1 − x Ca x MnO3 + δ (x = 0–1) perovskites and their reactivities in oxidation reactions

The effects of substitution in the cationic sublattice and of the synthesis procedure on the reactivity of different forms of oxygen in La1 − xCaxMnO3 + δ perovskites synthesized by mechanochemical and ceramic processing was studied by temperature-programmed reduction (TPR) with hydrogen. As the calcium content of the perovskite is raised, the maxima of the TPR peaks shift to lower temperatures and the extent of reduction of the perovskite increase, implying an increase in the reactivity of the system. Conversely, raising the calcination temperature or extending the calcination time shifts the maxima of the peaks to higher temperatures and diminishes the extent of reduction of the sample. TPR data for the intermediate-composition samples can be explained in terms of the dependence of microstructure on the synthesis procedure (near-surface calcium segregation in the mechanochemically synthesized samples and the microheterogeneous structure of the ceramic samples). The reduction process Mn4+ → Mn2+ takes place in the low- and medium-temperature regions. According to the literature, the bulk reduction process Mn3+ → Mn2+ occurs at high temperatures. The activity of the system in CO oxidation is correlated with the amount of the most reactive surface oxygen, which is eliminated in hydrogen TPR runs below 250–300°C.

An alternative method for parameter identification from temperature programmed reduction (TPR) data

The experimental method of temperature programmed reduction (TPR) for the investigation of gas–solid reactions is well established and widely used since the 1970s. With regard to the temporal and financial effort for TPR measurements, the quantitative model-based analysis of the data is not adequately developed. The TPR data analysis is comprised of two aspects: a discrete model identification and a continuous parameter optimisation. In this contribution, a general model for TPR experiments is introduced and a strategy for the model identification is proposed. This results in a large number of continuous optimisation problems which can be solved very efficiently by a proposed optimisation algorithm that is especially tailored for the problem at hand. The applicability of the method is demonstrated using TPR measurements of iron oxide in hydrogen gas.

The kinetic study of temperature-programmed reduction of nickel oxide in hydrogen atmosphere

The powder samples of nickel oxide were synthesized by sol–gel procedure. The non-isothermal reduction of nickel oxide using hydrogen was investigated by temperature-programmed reduction (TPR) at four different heating rates (2.5, 5, 10 and 20 ∘ C min - 1 ). Reaction kinetics parameters ( E a , ln A ) were determined using a few methods: Stationary point (SP), Kissinger, Friedman, Flynn–Wall–Ozawa and Kissinger–Akahira–Sunose methods. The Málek's kinetic procedure was used for the determination of the most suitable kinetic model and calculation of a complete set of kinetic parameters. The kinetic model of the investigated reduction process was determined, and this model corresponds to the empirical two-parameter Šesták–Berggren equation which gives a more quantitative description. The kinetic triplet obtained for the investigated process is E a = 90.8 kJ mol - 1 ; ln A = 19.50 ( A expressed in min - 1 ) and f ( α ) = α 0.63 ( 1 - α ) 1.39 . The reliability of kinetics parameters calculated from TPR data is tested by comparing calculated and experimental non-isothermal data. Comparing both experimental and calculated rate curves at constant heating rate assessed the adequate consistency of the kinetic triplet. In agreement with the obtained results, the mechanism of nickel oxide reduction by hydrogen was considered.