Temperature-programmed Reduction Studies Research Articles

A Highly Efficient and Selective Nickel/Clay Catalyst for Liquid Phase Hydrogenation of Maleic Anhydride to Succinic Anhydride

Three Ni-based catalysts with different clay as support were prepared and tested in the hydrogenation of maleic anhydride, among which Ni/clay1 showed best activity and selectivity. Over Ni/clay1 catalyst prepared by impregnation method, 97.14% conversion of maleic anhydride and 99.55% selectivity to succinic anhydride were obtained at 180 .deg. C under a pressure of 1 MPa. Catalytic activity was greatly influenced by the temperature and weighted hourly space velocity. Catalyst deactivation studies showed that this catalyst have a long life time, the yield of MA still higher than 90% even after a reaction time of 60 h. X-ray diffraction (XRD) and H{sub 2} temperature programmed reduction (TPR) were use to investigate the properties of the catalyst. XRD and TPR studies showed that Ni was present as Ni{sup 2+} on the support, which indicated that there was no elemental nickel (Ni{sup 0}) and Ni{sub 2}O{sub 3} in the unreduced samples. The formation of Ni was strong impact on catalytic activity.

Hydrogen production through the aqueous phase reforming of ethylene glycol over supported Pt-based bimetallic catalysts

The catalytic activities of supported Pt-based bimetallic catalysts (Pt-M) were studied for hydrogen production via aqueous phase reforming (APR) using a 10 wt% ethylene glycol solution. The catalysts and supports used were characterized via X-ray powder diffraction (XRD), transmission electron microscopy (TEM), nitrogen adsorption-desorption, CO chemisorption, and temperature programmed reduction (TPR) techniques. It was found that the Pt–Mn (Pt:Mn = 1:1, molar ratio) bimetallic catalyst significantly enhanced the catalytic performances such as the hydrogen yield and hydrogen production when compared with monometallic catalysts and other bimetallic catalysts that were examined. The XRD and TPR studies confirmed the interaction between the Pt and Mn species, leading to the Pt–Mn alloys supported on CMK-3. Related to the effect of the type of support, the CMK-3 support demonstrated better performance than the commercial activated carbon and alumina. Accordingly, it can be understood that the better catalytic performance of the APR reaction over Pt–Mn/CMK-3 catalyst is dependent on the alloy effect as well as the structural properties and nature of support given by the addition of the second metal.

One-step synthesis of a Pt–Co–SWCNT hybrid material from a Pt–Co–MCM-41 catalyst

In this paper, we present a direct one-step synthesis of Pt–Co–SWCNT from the catalytic CO disproportionation over Pt–Co–MCM-41 catalysts with different Pt loadings. The SWCNT produced by Pt–Co–MCM-41 shows higher yield than the SWCNT produced by Co–MCM-41 under the same reaction conditions, and also SWCNTs with diameter larger than 2 nm have been identified in the products. X-ray absorption and temperature programmed reduction studies indicate that Pt exists as metallic nanoparticles in the Pt–Co–MCM-41 catalysts, while Co is atomically dispersed and tetrahedrally coordinated in the MCM-41 framework as a Co2+ cation. Detailed structural study also suggested that in the Pt–Co–SWCNT hybrids produced by this method, the Pt–Co bimetallic nanoparticles consist of a Pt core and Co shell with no detectable alloy phase. This study provides a method for the direct synthesis of a Pt–M–CNT catalyst, where M is a non-precious metal.

A new view on the relations between tungsten and vanadium in V2O5[sbnd]WO3/TiO2 catalysts for the selective reduction of NO with NH3

The relation between tungsten and vanadium oxide species in V2O5WO3/TiO2 catalysts for the selective catalytic reduction of nitrogen oxides by ammonia was studied with two series of catalysts containing 0.5–5wt.% V2O5 and 0 or 10wt.% WO3 impregnated onto a high surface-area titania hydrate of anatase structure to keep theoretical surface coverages of the transition metal oxides ⩽1. The catalysts, which were investigated by XRD, nitrogen physisorption, Raman and EPR spectroscopy, temperature-programmed reduction (TPR), and catalytic reduction studies, all exhibited the well-known acceleration of the SCR reaction by the tungsten promoter. From the EPR spectra and the TPR profiles, it was concluded that the presence of tungsten induces the formation of a surface oxide phase with intimate mixing between vanadium and tungsten oxide species instead of pronounced surface vanadium oxide island formation, which was found in the absence of tungsten. This suggests that the promotional effect of tungsten may originate from a direct influence on the neighboring vanadium oxide species or from the disruption of too large surface vanadium oxide ensembles.

Surface Optimization and Redox Behavior of Vanadium Oxides Supported on γ-Al2O3

This report describes optimization of the synthesis of γ-Al2O3 supported vanadium oxides using three different vanadium precursors: OV(OiPr)3, OV(OEt)3 and OV(OPr)3. It was observed that the actual loading of the metal oxide is controlled by a number of factors, including the kinetics and thermodynamics of the grafting process, the time allowed for the grafting reaction to proceed, the amount of available precursor, the concentration of the grafting solution, and temperature at which grafting is pursued. From TPR (temperature programmed reduction) and XPS (X-ray photoelectron spectroscopy) studies it was observed that V4+ and V5+ were the predominant oxidation states present under both normal and rigorous reducing conditions simulating those prevalent in the steam reforming process.

Temperature-programmed and X-ray diffractometry studies of hydrogen-reduction course and products of WO 3 powder: Influence of reduction parameters

The hydrogen-reduction course and products of synthetic tungsten(VI) oxide (WO 3) were examined by means of temperature-programmed reduction (TPR) and X-ray powder diffractometry (XRD) studies. A set of model tungsten compounds was procured and examined similarly for reference purposes. Results obtained could help resolving two subsequent reduction stages: (i) a low-temperature stage (<1050 K) through which WO 3 is reduced to the tetravalent state (WO 2) via formation and subsequent reduction of intermediate WO 2.96, WO 2.9, WO 2.72 oxides; and (ii) a high-temperature stage (>1050 K) through which WO 2 thus produced is reduced to the metallic state (W o) via two intermediate oxide species (tentatively, WO and W 2O–W 3O). Reduction events involved in the high-temperature stage were found to be relatively more sensitive to the reduction parameters; namely, the starting oxide mass, heating temperature and rate, and gas flow rate and composition. They were also found to require lower activation energies than those required by events occurring throughout the low-temperature stage, a fact that may suspect compliance of the high-temperature reduction events to autocatalytic effects.

Catalytic activities of cobalt, nickel and copper ferrospinels for sulfuric acid decomposition: The high temperature step in the sulfur based thermochemical water splitting cycles

The catalytic decomposition of sulfuric acid is the most endothermic step of the sulfur based water splitting thermochemical cycles, which are promising technologies for large scale hydrogen production in future. In the present study the catalytic activities of three ferrospinels AFe 2O 4 (A = Co, Ni, Cu) were evaluated for high temperature sulfuric acid decomposition reaction. Catalyst characterization by Mössbauer spectroscopy confirmed the occupancy of octahedral and tetrahedral sites by Fe 3+ ions in all three inverse spinels. The temperature programmed reduction studies revealed that the reducibility of Fe 3+ was greatly enhanced in CuFe 2O 4 as compared to other ferrites. Copper ferrite was found to be the most active catalyst for the reaction with ∼78% conversion at 800 °C. Presence of sulfate species on the spent catalysts was revealed by an ex situ analysis of the spent catalyst samples by FTIR, SEM and evolved gas analysis (EGA). FTIR spectra of all the three spent catalyst samples exhibit four prominent peaks in the region 950–1200 cm −1, which is an indicative of C 2v symmetry and bidentate sulfate coordination. A plausible mechanism for the sulfuric acid decomposition over spinel ferrites was proposed via the metal sulfate formation and then decomposition followed with an oxygen evolution step with the sulfate decomposition being the rate determining step at higher temperatures. EGA showed evolution of SO 2 as a decomposition product from existed sulfate of spent catalysts at high temperatures, with the rate of SO 2 evolution following the order: CuFe 2O 4 > NiFe 2O 4 > CoFe 2O 4. The enhanced rate of decomposition of the sulfates of copper ferrite can be attributed to the higher electronegativity of Cu 2+ as compared to Ni 2+ and Co 2+, which renders the S–O bond in the mixed metal sulfate weaker than others and thus more susceptible to dissociation. The lower thermal stability of sulfate and better reducibility are responsible for the improved catalytic properties of copper ferrite among the three ferrospinels investigated for sulfuric acid decomposition.

Synthesis and physicochemical characterization of nanocrystalline cobalt doped lanthanum strontium ferrite

Nanosized crystallites of La 0.6Sr 0.4Fe 0.8Co 0.2O 3− δ (LSCF), a promising cathode material for Intermediate Temperature-Solid Oxide Fuel Cells (IT-SOFCs) has been synthesized by alternative methods like ceramic route, polymerisable complex process and gel-combustion method and calcined at different temperatures. X-ray diffraction studies were used for the determination of phase purity, crystal structure and average crystallite size of the samples. Microstructure of LSCF samples was studied by SEM. Temperature-programmed reduction studies were done for evaluating the redox behavior of the samples prepared by alternative methods. The electrical conductivity measurements of sintered samples were carried out at elevated temperatures using four-probe method. The electrical conductivity of sample synthesized by gel-combustion method was more as compared to sample prepared by polymerisable complex process and ceramic route. The electrical conductivity decreased at high temperature due to loss of oxygen and the formation of oxygen vacancies. Thermo-dilatometry was used to study the linear thermal expansion behavior of the samples which shows that there is an observable increase in thermal expansion at high temperatures.

Influence of Ba precursor on structural and catalytic properties of Pt–Ba/alumina NOx storage-reduction catalyst

The effect of the choice of barium precursor (nitrate or acetate), in the preparation of Pt–Ba/γ-Al2O3 NSR catalysts, on their structure and catalytic behavior has been investigated by means of X-ray diffraction (XRD), temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and catalytic tests. XRD patterns showed that the type of Ba precursor and repeated NSR cycles influence the crystallinity of Ba species. Depending on the Ba precursor, remarkable differences in Ba(NO3)2 crystallite sizes as well as availability of the Pt for hydrogen chemisorption were observed. TPD and TPR studies showed that the catalyst prepared using the Ba acetate-precursor route is more efficient in the decomposition and reduction of Ba nitrates at lower temperatures. The catalyst prepared by the Ba acetate-precursor route showed both enhanced NOx storage ability and selectivity toward nitrogen in the reduction process, which is attributed to the intimate contact of the small crystallites of the Ba-component and the Pt particles. Environmental transmission electron microscopy (ETEM) measurements revealed that introduction of H2O vapor induces morphological changes of the Ba phase leading to partial liquefaction as a result of the formation of hygroscopic barium hydroxides. The interplay of this liquid-like behavior of the Ba phase together with the chemical transformations occurring during NOx storage and reduction can lead to remarkable structural changes affecting the NSR performance.

Study on Redox, Structural and Electrical Properties of CexZr1 − xO2 for Applications in SOFC Anodes

The effect of redox cycles on the reducibility, structural, and electrical properties of dense pellets of ceria-zirconia solid solutions has been investigated by temperature programmed reduction studies, X-ray diffraction technique, Raman spectroscopy, and electrochemical impedance spectroscopy. Similarities and differences among the compositions with high , intermediate , and low amounts of ceria were evaluated in view of their application as active components for solid oxide fuel cell (SOFC) anodes. Cycles of reduction and reoxidation, respectively, at 1273 and , promote the reduction of all the compositions investigated, shifting it at a lower temperature. Moreover, redox treatments cause a significant change in the crystal structure of , ultimately leading to the formation of cubic pyrochlore phases as the reduction temperature increases. The correlations among structural changes, redox, and electrical properties of these materials are discussed.

Hydrogenation of Maleic Anhydride to Succinic Anhydride over Ni/HY-Al2O3

The hydrogenation of maleic anhydride to succinic anhydride was carried out over Ni/Al2O3-HY catalysts. The catalysts, which contained 3−30 wt % Ni, were prepared via a kneading method. The catalysts were characterized by an array of techniques, including X-ray diffraction (XRD) and H2 temperature-programmed reduction (TPR). The nature of the Ni species on the support, and their interaction with the support, were studied. XRD and TPR studies showed that nickel was present as Ni2+ and Ni3+ species on the support, and that there was no elemental nickel (Ni0) in the unreduced samples. Pretreatment using pure H2 at different temperatures affected the reactivity. Pretreatment at 350 °C for 3 h was the best among the conditions tested.

Nanocrystalline Cerium−Bismuth Oxides: Synthesis, Structural Characterization, and Redox Properties

The hydrothermal oxidation of Ce3+ in basic aqueous solution by NaBiVO3 at 240 °C produces fine powders of cerium−bismuth oxides with crystallite sizes of less than 10 nm, as observed by transmission electron microscopy (TEM). Analysis of X-ray absorption near-edge structure at the Bi LIII and Ce LIII edges confirms the reduction of Bi5+ to Bi3+ and the oxidation of Ce3+ to Ce4+, consistent with a general chemical composition CeIV1-xBiIIIxO2-(x/2) (x ≤ 0.6) for the mixed oxides. The refined cubic lattice parameters from powder neutron diffraction indicate a fluorite-like solid solution with volume that increases with Bi content. Pair distribution functions derived from total neutron scattering yield estimated values for refined particle diameters that agree with TEM, showing a decreasing crystallite diameter with increasing bismuth content, and a full analysis reveals that despite average long-range structure being described well as a fluorite solid-solution, the local structure is distorted. Thus, the short-range structure can be simulated as a mixture of symmetric eight-coordinate Ce sites, as seen in CeO2, and distorted Bi sites, as seen in β-Bi2O3. Temperature programmed reduction studies reveal a large hydrogen uptake in the mixed oxides, which is reproducible on a second subsequent cycle. In situ powder XRD under hydrogen flow, however, reveals that this is due to partial phase separation to give bismuth metal. After six cycles, a material of initial composition Ce0.5Bi0.5O1.75 is completely phase-separated and its hydrogen uptake is reduced with high-resolution TEM analysis confirming the permanent reduction of the sample.

Hydrogen production by water–gas shift reaction over bimetallic Cu–Ni catalysts supported on La-doped mesoporous ceria

Water–gas shift reaction was investigated over a series of Ni, Cu and Cu–Ni bimetallic catalysts supported on La-doped mesoporous ceria. The catalysts were characterized by powder X-ray diffraction (XRD), high-temperature XRD (HTXRD), transmission electron microscopy (TEM), N 2 adsorption analysis, and H 2 temperature-programmed reduction (TPR). Combined results of powder XRD, HTXRD and TPR studies provided evidence of the formation of CuNi alloys in the CuNi/CeLaO x bimetallic catalysts. The catalytic activity of supported Cu–Ni catalysts was evaluated in the water–gas shift (WGS) reaction at 150–400 °C. The effects of the Cu/Ni ratio on the physicochemical properties and the catalytic performance were examined. The presence of copper in bimetallic catalysts resulted in suppression of undesirable methanation side-reaction, while the Ni component was important for high WGS activity. These findings strongly suggested that the bimetallic Cu–Ni compositions are highly promising as WGS catalysts.

Complete catalytic oxidation of o-xylene over Mn–Ce oxides prepared using a redox-precipitation method

A series of Mn–Ce oxides were prepared using a redox-precipitation method and the complete catalytic oxidation of o-xylene was examined. Catalytic activity was evaluated in terms of both o-xylene conversion and CO 2 yield. The effects of the Mn at/Ce at atomic ratio and calcination temperature on the features of catalyst structure and catalytic behavior were examined. When the Mn at/Ce at ratio was 1.5 and the catalyst was calcined at 400 °C, the conversion of o-xylene was 100% and the CO 2 yield was 100% at 240 °C. X-ray diffraction (XRD), X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), and hydrogen temperature-programmed reduction (H 2-TPR) studies revealed that the catalysts prepared via the redox-precipitation method possessed a homogenous dispersion of amorphous MnO 2 and CeO 2 nanoparticles; in the main active phase, MnO 2 provided available oxygen species and CeO 2 enhanced oxygen mobility. The synergistic effects between MnO 2 and CeO 2 potentiated the catalytic activity necessary for the complete catalytic oxidation of o-xylene.

Indium as a chemical promoter in Fe-based Fischer-Tropsch synthesis

The ability of indium to act as a chemical promoter in Fe-based FTS reaction was evaluated and compared to that of copper. Indium was chosen for study due its relationship to Cu through the ‘ Knight' s Move’ relationship. N 2 physisorption, temperature programmed reduction (TPR), X-ray diffraction (XRD) and diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) were employed to characterise the catalysts. “ In situ” Fischer-Tropsch synthesis (FTS) reactions were also performed in a DRIFTS reactor. It was found via TPR studies that indium exhibited a similar ability to reduce Fe as found for copper. Results obtained from XRD and N 2 physisorption experiments showed indium promoted catalysts give comparable results to those of copper promoted catalysts. However, indium decreased the % reducibility and CO adsorption ability of the Fe catalyst. Indium also lowered the FTS activity of the Fe-based catalyst, an effect ascribed to be associated with the low melting point of In. The effect of adding indium to a multi-promoted Fe-based FTS catalyst was also investigated. The addition of indium suppressed the reduction properties of the catalyst when H 2 was employed as a reductant. When CO was employed as a reductant, the reduction/carburization properties were improved and resulted in a selectivity shift to heavy weight hydrocarbons during the FTS reaction. Adding indium to the multi-promoted catalyst also lowered the catalyst surface area which resulted in a decrease to the FTS activity. It appears that while the ‘ Knight' s Move’ relationship is consistent with Cu and In both acting to enhance Fe reduction, the low In melting point determines the poor activity of the FT catalysts.

Microwave-assisted polyol synthesis of Pt/H-ZSM5 catalysts

Three H-ZSM5 solids with Si/Al atomic ratios of 22, 71 and 96 were synthesized and characterized by X-ray diffraction (XRD), ICP-AES chemical analysis and nitrogen physisorption at −196 °C. These were employed as supports in the synthesis of Pt/H-ZSM5 catalysts with platinum contents around 1 wt.%, by means of the microwave-assisted polyol method: platinum salt solutions in ethylene glycol were irradiated at powers of 184 and 461 W for time periods of 30 and 60 s in order to obtain Pt nanoparticles suspensions which were mixed with the supports and ultrasonically irradiated for 30 min to obtain the catalysts. This methodology allowed the synthesis of the catalysts efficiently and in relatively short times. Characterization of the catalysts by XRD showed that the ultrasonic irradiation did not affect the support structure. X-ray photoelectron spectroscopy revealed the presence of Pt(0) species on the solids surfaces. Temperature-programmed reduction studies suggested an effect of the time and power of microwave irradiation on the reducibility of the catalysts, i.e., at the shorter times and lower irradiation powers, the reducibility of the platinum species decreases. Catalytic testing through the hydrogenation of toluene indicated that there is an effect of time and power of microwave irradiation on the initial hydrogenation activity of the Pt/H-ZSM5 catalysts. The catalyst prepared from suspensions irradiated at 461 W for 60 s showed the highest activity, which seems to be related to the lower particle size and better dispersion of Pt.

Improvement of physico-chemical properties by addition of H2O2: An extensive case study on the RE-doped ceria system (RE = Gd, Sm)

Effect of H2O2 on synthesis and powder properties such as surface area and agglomerate size of nanocrystalline Ce0.8M0.2O1.90 (M: Sm, Gd) was explored by treating cerium nitrate and rare-earth nitrate with NaOH in the presence/absence of H2O2. The resultant products were characterized by x-ray diffraction, Raman spectroscopy, thermo-gravimetry–differential thermal analysis, dynamic light scattering, surface area analysis, high-resolution transmission electron microscopy, and x-ray photoelectron spectroscopy. The presence of H2O2 was found to have a profound effect on powder properties such as surface area and particle size of these doped ceria samples and results in smaller crystallite size, softer agglomerates, and larger surface area. A mechanism is proposed to explain the observed better powder properties of the samples. It was also shown that the samples prepared in the presence of H2O2 can lower the conversion temperature of CO to CO2, proving these to be better catalysts. Interestingly, temperature-programmed reduction studies on Sm3+-doped samples showed that the doping in conjunction with the use of H2O2 leads to enhanced reduction properties of the samples over multiple cycles.

Determination of Fe(IV) species in partially substituted perovskite La0.6Ca0.4FeO3

The partial substitution for lanthanum in LaFeO3 perovskite with calcium, a cation with lower oxidation state, causes an electronic unbalance. The X-ray diffraction (XRD) and temperature-programmed reduction (TPR) results suggest that, consequently, the elevation of the iron oxidation state to Fe(IV) occurs. The Mossbauer spectroscopy studies confirm the onset of Fe(IV) to maintain the structure electroneutrality. The quantification of Fe(IV) species is in good agreement with the estimated amount from the hydrogen consumption in the TPR studies. This shows that TPR—a very simple technique widely used to characterize heterogeneous catalysts—is an appropriate technique to quantify Fe(IV) species in LaFeO3 substituted perovskites. To verify the correct assignment of step corresponding to the Fe(IV) reduction, the different iron species after each reduction step, during the TPR experiment, were confirmed by Mossbauer spectroscopy in controlled atmosphere.

Fabrication of CeZrO 2 on Ni/SiO 2 and promoted catalysis for methane autothermal reforming in a fluidized bed reactor

The methane autothermal reforming in the present of oxygen was studied over cerium- and zirconium-promoted Ni/SiO 2 catalysts in a fluidized bed reactor. The addition of CeZrO 2 resulted in a significant improvement in the initial activity of the catalysis as well as an increase in the stability. The long-term activity of the promoted catalyst was dependent upon the rapid redox properties between the oxidative zone and the reductive zone in a fluidized bed reactor. H 2 temperature-programmed reduction (H 2-TPR) and temperature-programmed surface reaction (TPSR) studies demonstrated that addition of the CeZrO 2 resulted in an increase in the reducibility and oxygen transfer ability of the support, Ni/Ce 0.5ZrO 0.5O 2-SiO 2 showed improved redox properties compared with Ni/SiO 2 due to a low-temperature reduction. X-ray diffraction (XRD) of the fresh and spent catalysts showed that the promoter enhanced the nickel dispersion and retarded metal particle growth during reaction at high temperature, and surface Ni was gradually oxidized by remaining O 2, leading to Ni deactivation.

(Z)-cyclooctene epoxidation and cyclohexane oxidation on Ni/alumina-pillared clay catalysts

This work reports the synthesis of Ni-clay catalysts by impregnation of Al-pillared saponite with various Ni salts, using several Ni loadings. The catalysts were characterized by X-ray diffraction, infrared spectroscopy, thermogravimetric and differential thermal analysis, nitrogen adsorption, temperature-programmed reduction studies, and ultraviolet–visible spectroscopy. All the solids displayed good structural properties. FTIR spectroscopy confirmed the total removal of the anionic precursors after calcination of the solids at 500 °C. The presence of NiO and NiAl 2O 4 in the nickel particles formed on the surface of the solids was demonstrated. Deconvolution of TPR curves allowed to identify three Ni-reduction effects, their temperature and relative intensity depended on the preparation of each sample. The catalytic behaviour of the solids for (Z)-cyclooctene epoxidation and cyclohexane oxidation by either iodosylbenzene (a model oxidant) or hydrogen peroxide (a clean oxidant) was studied. In the case of the epoxidation reactions, there was 100% selectivity for the epoxide. As for cyclohexane oxidation, only cyclohexanone was formed, whereas no cyclohexanol was detected. The use of radical scavengers indicated that the high-valent oxo-nickel species is responsible for the oxygen transfer between the oxidant and the substrates.