Temperature-programmed Techniques Research Articles

Synthesis Ce-doped biomass carbon-based g-C3N4 via plant growing guide and temperature-programmed technique for degrading 2-Mercaptobenzothiazole

In the present study, we constructed the Ce-doped biomass carbon-based mesoporous g-C3N4 photocatalyst via alternanthera philoxeroides griseb growing guide using calcination method. The Ce-C@g-C3N4 exhibited more superior photocatalytic performance for 2-mercaptobenzothiazole (96 %) degrading than that of pure g-C3N4 (39 %) and C@g-C3N4 (60 %). As a consequence, the improved degradation efficiency of Ce-C@g-C3N4 attributed to the biomass carbon can significantly enhance the mass transport channels of electron-hole pairs. Meanwhile, the doped Ce4+ ions acting as the redox center can sufficiently utilize the photogenerated carriers for generating more active radicals. Also, the possible photocatalytic reaction mechanisms including transfer behaviors of charge carriers, generation of reactive species, intermediate degradation products are also discussed in detail. This work may provide a promising approach for synthesizing g-C3N4-based photocatalyst by the plant growing guide and application in wastewater treatment.

Enhanced catalytic performance of cobalt and iron co-doped ceria catalysts for soot combustion

A series of promising Ce–Co–Fe catalysts were successfully synthesized using a cetyl-trimethylammonium-bromide-assisted co-precipitation method and investigated for diesel soot combustion. The surface morphological and structural properties were systematically examined using various techniques: X-ray diffraction, scanning electron microscope, N2 adsorption–desorption, Raman spectroscopy, temperature-programmed reduction and in situ diffuse reflection infrared Fourier transform spectroscopy. The catalyst–soot combustion activities were tested in O2 and NO + O2 using a temperature-programmed technique. Nanometer crystalline solid solutions were formed with high surface areas when the Fe and Co cations were co-doped in the ceria lattice. Transition metals doping played a key role in increasing oxygen vacancies and promoting the redox performance of Ce–Co–Fe catalysts. Co–Fe co-doping accelerated the oxidation of soot under both “tight” and “loose” contact conditions. Among all the ceria-based catalysts, Ce80Co15Fe5 showed superior activity with T10 = 256 °C and high selectivity with $$ S_{{{\text{CO}}_{ 2} }} \, = \,100\% $$ under tight a contact mode. The observed high catalytic activity following co-doping was proved to have occurred because of various reasons such as improved redox properties, increased oxygen vacancies and high surface area. The presence of NO in O2 also promoted soot oxidation, which follows the NO2-assisted mechanism. Moreover, the in situ DRIFTS performed under an isothermal condition in NO + O2 confirmed the strong adsorption capacity for NOx species on the doped ceria catalyst.

Surface enrichment phenomenon in the Ba-doped cobalt catalyst for ammonia synthesis

Barium promoted cobalt catalysts (Co/Ba) for NH3 synthesis were synthesized by co-precipitation of a mixture of cobalt and barium carbonates. Various pretreatment method of the obtained precursors were applied: subsequent calcination and in situ reduction or in situ reduction only (without calcination). The catalytic materials were subjected to detailed characterization studies by nitrogen physisorption, X-ray powder diffraction (XRPD), temperature-programmed techniques (TPR-MS, TPD-H2, TPD-N2, TPSR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM-EDX). The Co/Ba catalysts were tested in catalytic ammonia synthesis reaction under conditions close to the industrial ones (400 °C, 6.3 MPa, H2/N2 = 3). Studies revealed that the method of catalysts precursors pretreatment has a significant influence on the properties of the final Co/Ba systems. The more effective method is the one without subsequent calcination – this material exhibited the most favorable catalytic properties. Detailed surface studies showed that calcination process of the co-precipitated carbonates mixture adversely affects the final properties of the catalyst, because it favors a migration of the barium promoter to the catalyst's surface during its reduction (surface enrichment phenomenon) limiting the amount of active phase (metallic cobalt) available to reactants during the catalytic reaction and thus substantially decreasing the catalyst's activity.

Using the 33S Nuclide for Determining the Particle Size of the Molybdenum Disulfide Phase Supported on Mesoporous Silica

A new approach has been proposed for determining the particle size of the sulfide phase of hydrotreating catalysts. The method includes sulfiding the catalyst with hydrogen sulfide containing the 33S nuclide; carrying out the thiophene hydrodesulfurization reaction; and, at the final stage, determining the isotopic composition of sulfur on the catalyst using a combination of temperature-programmed oxidation and mass spectrometry techniques (TPO–MS). It has been shown that during hydrodesulfurization, part of the most labile sulfur 33S on the faces of hexagonal prisms of MoS2 is replaced by 32S from thiophene, and the degree of this substitution can be determined by the TPO–MS method. It has been found that with a decrease in the particle size of the sulfide phase, which is confirmed by transmission electron microscopy (TEM), the proportion of labile sulfur capable of substitution during hydrodesulfurization increases.

Simple strategy synthesizing stable CuZnO/SiO2 methanol synthesis catalyst

The metal particle size distribution of a catalyst can affect the rate of deactivation in many chemical reaction. This work developed a facile method to prepare a well dispersed CuZnO/SiO2 catalyst via rotary evaporation assisted deposition-precipitation method (RE-CZS). By comparison, conventional deposition-precipitation method and ultrasound assisted deposition-precipitation method were applied to synthesize other catalysts (Im-CZS and Ul-CZS), in order to clarify the influence of Cu particle size distribution of the catalyst on catalytic stability of methanol synthesis via CO2 hydrogenation. RE-CZS catalyst possessed the narrowest Cu particle distribution (ca. 4.9–9.1 nm) and thus exhibited the best catalytic stability because of the least Cu particle growth. Im-CZS catalyst showed the broadest Cu particle distribution (ca. 5.3–28.7 nm) and suffered from the most serious particle growth resulting in the least catalytic stability. The Cu particle distribution and the related catalytic stability of the Ul-CZS catalyst lied between RE-CZS catalyst and Im-CZS catalyst. The origin of the tremendous effect of the Cu particle size distribution on the catalytic stability was studied via XRD, N2 adsorption/desorption, N2O chemisorption, ICP-AES, XPS, TEM and temperature-programmed techniques, which suggested that catalyst deactivation was mainly attributed to the growth of Cu particles originating from Ostwald ripening effect.

Temperature-programmed studies of isobutene oxidation over α-Bi2Mo3O12: Active oxygen species and reaction mechanism

In this work, the redox and reaction properties of α-Bi2Mo3O12 during isobutene (i-C4=) oxidation are probed by a series of temperature-programmed techniques, including temperature-programmed reduction (i-C4=-TPR), oxidation (TPO), and oxidation reaction (TPOR). The results of i-C4=-TPR suggest the initial reduction of α-Bi2Mo3O12 by isobutene with the phase transformation to γ-Bi2MoO6 and formation of reduced MoO2, and the further deep reduction to form metallic bismuth. On the fully oxidized α-Bi2Mo3O12 sample, isobutene oxidation is only able to proceed at high temperature with lattice oxygen as the active oxygen species via Mars-van Krevelen mechanism. On the fresh and pre-reduced catalyst samples, however, the oxygen species formed by the chemisorption of gas-phase oxygen on surface defects and vacancies are also active for isobutene oxidation at low temperature via Langmuir-Hinshelwood mechanism. The combined Langmuir-Hinshelwood and Mars-van-Krevelen mechanism is thus proposed for isobutene oxidation on α-Bi2Mo3O12.

Experimental Evidence of the Mechanism of Selective Catalytic Reduction of NO with NH3 over Fe-Containing BEA Zeolites.

Various temperature-programmed techniques were used as tools in mechanistic studies of selective catalytic reduction (SCR) of NO with ammonia in the presence of Fe-containing BEA zeolites. Moreover, FTIR studies of adsorbed NH3 and NO were conducted to determine the interactions of reactants with the catalyst surface. Iron was introduced into BEA zeolite by three different methods: i) two-step post-synthesis; ii) conventional wet impregnation; iii) ion exchange. The catalytic activity was dependent on the method used for iron introduction. The reactivities of NH3 and NO adsorbed on iron-modified zeolites obtained by impregnation and ion-exchange methods were higher than those measured for the catalyst obtained by a two-step post-synthesis method. The activity of Fe-containing zeolites in SCR was related to the form of deposited iron species, as well as to the nature, strength, and concentration of acid sites. Possible reaction pathways of NO reduction over the FeBEA zeolite catalysts were presented and discussed.

Sensitivity of nanocrystalline tungsten oxide to CO and ammonia gas determined by surface catalysts

Nanocrystalline tungsten oxide with variable particle size and surface area was synthesized by aqueous deposition and heat treatment for use in resistive gas sensors. Surface modification with 1 wt.% Pd and Ru was performed by impregnation to improve the sensitivity to CO and ammonia. Acid and oxidation surface sites were evaluated by temperature-programmed techniques using probe molecules. The surface acidity dropped with increasing particle size, and was weakly affected by additives. Lower crystallinity of WO3 and the presence of Ru species favoured temperature-programmed reduction of the materials. Modifying WO3 increased its sensitivity, to CO at ambient condition for modification by Pd and to NH3 at elevated temperature for Ru modification. An in situ infrared study of the gas – solid interaction showed that the catalytic additives change the interaction route of tungsten oxide with the target gases and make the reception of detected molecules independent of the semiconductor oxide matrix.

The Use of Temperature-Programmed Desorption to Explain the Electrochemical Behaviour of PGM-Free PEFC Cathode Catalysts

Oxygen reduction catalysts in PEFCs have traditionally contained relatively high amounts of platinum or other platinum group metals. There has been a drive towards reducing the loading, but a promising alternative has been discovered in the form of iron and nitrogen-doped carbon. However, their activity is still not acceptable, and their improvement is hampered by the lack of knowledge about the chemical structure of the materials and in particular the active site(s). There is still disagreement over whether the active site is organic or metallic, or even both. Temperature-programmed techniques such as temperature-programmed desorption (TPD) and temperature-programmed reaction spectroscopy (TPRS) are powerful techniques used in conventional catalysis for probing active sites. Molecules which interact with the active sites are adsorbed on the catalyst, then the system is heated and the desorption of the initial molecule or products can be measured by mass spectrometry. These techniques allow quantification of number of active sites, heats of desorption and reaction, assessment of reaction mechanisms and studies of poisoning behaviour. This study reports extensive TPD and TPRS work done on an iron and nitrogen doped carbon-based oxygen reduction catalyst. Interactions between various molecules and the active site(s) on the catalyst have been studied and quantified. Correlations between TPRS and TPD studies and electrochemical studies have been made from fresh, aged and poisoned materials, and these have been compared to studies on similar materials containing either no iron, or no iron and nitrogen. In addition, ex situ measurements including EXAFS and Mössbauer on the same materials will be reported on. Together these experiments are used to draw conclusions about the nature of the active site(s) in these materials. Figure 1

Simultaneous oxidation of Hg0 and NH3-SCR of NO by nanophase Ce x Zr y Mn z O2 at low temperature: the interaction and mechanism.

Simultaneous oxidation of Hg0 and NH3-SCR of NO by catalyst is one of the key methods for co-purification of coal-fired flue gas. Till now, the interaction between the oxidation of Hg0 and NH3-SCR of NO and its mechanism have not clarified. In this study, a series of nanophase Ce x Zr y Mn z O2 was prepared for the simultaneous oxidation of Hg0 and NH3-SCR of NO at low temperature. The catalysts were characterized using surface area analysis, X-ray diffraction, temperature-programmed techniques, and several types of microscopy and spectroscopy. The experimental results indicated that the Ce0.47Zr0.22Mn0.31O2 exhibited superior Hg0 removal efficiency (> 99%) and NO conversion efficiency (> 90%) even at 150°C, and it also exhibited a good durability in the presence of SO2 and H2O. The excellent performance of Ce0.47Zr0.22Mn0.31O2 on co-purifying Hg0 and NO was due to the stronger synergistic effects of Ce-Zr-Mn in Ce0.47Zr0.22Mn0.31O2 than that of the others, which was illustrated by the characterization results of XPS, XRD, and FT-IR. Moreover, it was found that the NO conversion of Ce0.47Zr0.22Mn0.31O2 could be slightly influenced by Hg0 and was decreased about 4% to the max, while that of Hg0 could rarely be affected by the selected catalytic reduction process of NO. It might be due to the co-purification mechanism of NO and Hg0. The mechanism of the simultaneous oxidation of Hg0 and NH3-SCR of NO was mainly due to the synergetic effect on the mobility of surface oxygen and the activation of lattice oxygen of Ce0.47Zr0.22Mn0.31O2. The effect of the oxidation of Hg0 on the NH3-SCR of NO was mainly due to the absorbed Hg0/Hg2+ on the surface of Ce0.47Zr0.22Mn0.31O2, which attenuated the formation of NH3(ad), -NH2(ad), and NH4+ on its acid sites. Similarly, the NH3-SCR of NO process could hardly influence the oxidation of Hg0 when NO and Hg0 were co-purified.

Ru/FeOx catalyst performance design: Highly dispersed Ru species for selective carbon dioxide hydrogenation

A series of Ru/FeOx catalysts were synthesized for the selective hydrogenation of CO2 to CO. Detailed characterizations of the catalysts through X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and temperature-programmed techniques were performed to directly monitor the surface chemical properties and the catalytic performance to elucidate the reaction mechanism. Highly dispersed Ru species were observed on the surface of FeOx regardless of the initial Ru loading. Varying the Ru loading resulted in changes to the Ru coverage over the FeOx surface, which had a significant impact on the interaction between Ru and adsorbed H, and concomitantly, the H2 activation capacity via the ability for H2 dissociation. FeOx having 0.01% of Ru loading exhibited 100% selectivity toward CO resulting from the very strong interaction between Ru and adsorbed H, which limits the desorption of the activated H species and hinders over-reduction of CO to CH4. Further increasing the Ru loading of the catalysts to above 0.01% resulted in the adsorbed H to be easily dissociated, as a result of a weaker interaction with Ru, which allowed excessive CO reduction to produce CH4. Understanding how to selectively design the catalyst by tuning the initial loading of the active phase has broader implications on the design of supported metal catalysts toward preparing liquid fuels from CO2.

The Effect of Using H4P2O7 as Phosphorus Source for Synthesizing Vanadyl Pyrophosphate Catalysts

<p>Vanadyl pyrophosphate (VO)<sub>2</sub>P<sub>2</sub>O<sub>7</sub> catalysts synthesized via VOPO<sub>4</sub>·2H<sub>2</sub>O were investigated by using BET surface area measurement, X-ray Diffraction (XRD), Scanning Electron Microscope (SEM), and Temperature-Programmed Techniques (TPD and TPRS). H<sub>3</sub>PO<sub>4</sub> and H<sub>4</sub>P<sub>2</sub>O<sub>7</sub> were used as the phosphorus source. Only pyrophosphate phase was observed for both final catalysts after 75 hours of calcination in a reaction flow of <em>n</em>-butane/air mixture (0.75% <em>n</em>-butane/air). However, catalyst derived from H<sub>4</sub>P<sub>2</sub>O<sub>7</sub> based preparation (denoted VPD<sub>pyro</sub>) exhibit better crystallinity and slightly higher BET surface area compared to the H<sub>3</sub>PO<sub>4</sub> based preparation (denoted VPD<sub>ortho</sub>). The nature of the oxidants for both catalysts was investigated by O<sub>2</sub>-TPD. For VPD<sub>pyro</sub>, TPD showed an oxygen peak maximum at 986 K and a shoulder at 1003 K, whereas for VPD<sub>ortho</sub>, the oxygen was desorbed as two peaks maxima at 966 and 994 K. The total amount of oxygen desorbed thermally from VPD<sub>pyro</sub> (3.60×10<sup>20</sup> atom×g<sup>-1</sup>) is higher than that obtained for VPD<sub>ortho</sub> (3.07×10<sup>20</sup> atom×g<sup>-1</sup>). VPD<sub>pyro</sub> displayed a slightly improved activity and selectivity for <em>n</em>-butane oxidation. A proper amount of V<sup>5+</sup> species may have an effect on the enhancement of the catalytic activity.</p>

Synthesis of a Ti-SBA-15-NiMo Hydrodesulfurization Catalyst: The Effect of the Hydrothermal Synthesis Temperature of NiMo and Molybdenum Loading on the Catalytic Activity

We report herein the investigation of the catalytic activity on the hydrothermal synthesis temperature (HTsynT; 60–120 °C) and molybdenum loading (5–13 wt %) of NiMo catalyst supported over titanium-modified mesoporous silica (Ti-SBA-15) for the simultaneous hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4-methyldibenzothiophene (4-MDBT) in dodecane. It was found that the dispersion of an NiMo active phase in a one-pot synthesis is size-dependent and mostly controlled by the HTsynT for an efficient HDS catalytic performance. The as-developed catalysts were characterized by N2 physiosorption, X-ray diffraction (XRD), temperature-programmed techniques, and scanning electron microscopy. XRD analysis revealed that MoO3 particles agglomerate to form a high crystalline phase, as a function of the hydrothermal temperature and molybdenum loading, which, in turn, affects the textural properties, number of active sites, and reduction temperature. The catalytic performance for the simultaneous HDS of DBT a...

On the use of infrared spectrometer as detector for Temperature Programmed (TP) techniques in catalysts characterization

IR spectrometer has been successfully applied as detector for temperature-programmed techniques, i.e. NH3-TPD and H2-TPR, demonstrating an excellent flexibility, robustness and effectiveness in the proposed study. IR allowed also to detect unexpected or coproduced compounds and to avoid in many cases the expensive coupling of different techniques. For NH3-TPD, a successful qualitative and quantitative determination has been achieved with an excellent signal/noise ratio and on materials characterized by different: Brønsted/Lewis acidity, thermal pretreatment, doping elements and oxidation activity. For H2-TPR, the reducibility of different supported Ni-based catalytic systems has been successfully investigated following vapor formation and identifying different Ni species.

Single-pot synthesis of Ti-SBA-15-NiMo hydrodesulfurization catalysts: Role of calcination temperature on dispersion and activity

A novel approach was systematically investigated for developing single-pot (SP) NiMo-supported Ti-SBA-15 catalysts for hydrodesulfurization (HDS). The technical challenge associated with the SP approach is the formation of crystalline NiMoO4, and this was avoided by reducing the calcination temperature of the oxide catalyst from 550°C to 300°C to form a highly dispersed and efficient HDS catalyst. The dispersion, textural properties, surface acidity, oxide reducibility, surface chemical and elemental compositions of developed catalysts were measured by XRD, Raman spectroscopy, N2-physisorption, and temperature-programmed techniques (TPR and TPD), X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma (ICP). Scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HRTEM) were employed as additional characterization methods to investigate the catalysts’ morphology and microstructure. Catalyst performance was evaluated in a batch autoclave reactor using a simulated feed containing 2500 parts per million sulfur (ppm-S) (dibenzothiophene, DBT) in dodecane. The catalyst prepared by the SP approach and calcined at a lower temperature (300°C) showed significantly improved HDS activity and direct desulfurization selectivity compared to the catalyst prepared by the conventional impregnation approach. An increase in the calcination temperature to 550°C, however, resulted in a decrease in the HDS activity of the catalysts prepared by both methods. The dispersion, surface acidity, and textural properties of the catalysts prepared using the SP method combine to improve the catalytic efficiency. The use of a complexing agent to aid the dispersion of the active phase was unnecessary when the SP method was used for catalyst preparation.

Synthesis, Characterization, and Activity Pattern of Ni–Al Hydrotalcite Catalysts in CO2 Methanation

Two nickel–aluminum hydrotalcite samples (HTLCs) were prepared by a coprecipitation method at different pH values and investigated as catalysts for the hydrogenation of carbon dioxide. The newly synthesized samples have been compared with a reference alumina supported nickel-based commercial catalyst, with equal nickel content. The as-prepared and commercial samples were characterized by BET analysis, atomic adsorption spectroscopy (AAS), X-ray diffraction (XRD), and temperature-programmed techniques (H2-TPR and CO-TPD). Catalytic activity of the analyzed samples was investigated toward hydrogenation of CO2 at atmospheric pressure by varying reaction temperature between 250 and 400 °C. The maximum CO2-to-CH4 conversion value achieved by hydrotalcyte was ≈86% at 300 °C. The superior performance of HTLCs has been put in relationship with the major catalysts reducibility nature and with the higher metal surface area and metal dispersion. The stability of the HTLCs was investigated through long-term tests, re...

Selective ketonization of acetic acid over HZSM-5: The importance of acyl species and the influence of water

The ketonization of acetic acid over HZSM-5 is investigated via a combination of temperature-programmed techniques (TP), IR, and reaction kinetics. TP experiments demonstrate the generation of stable acyl intermediates that require higher temperatures to facilitate C–C coupling with a second acetic acid molecule. Near-exclusive selectivities for the ketonization reaction devoid of significant sequential aldol condensation are observed at temperatures ranging from 270 to 330°C. The rate-determining step is proposed to involve the interaction of an acyl group with a second acid, with an apparent reaction order of 1.6. This order can be explained by a second-order rate-determining step that is reduced as a result of acid-derived surface species. Isotope labeling experiments reveal that the rate-determining step is the formation of the C–C bond rather than the activation of the second acid. The ketonization reaction exhibits a −0.46 order with respect to water, but this loss in activity is accompanied by a significant increase in catalyst stability.

Selective hydrogenation of 1,3-butadiene in presence of 1-butene under liquid phase conditions with NiPd/Al2O3 catalysts

The catalytic performance of Al2O3-supported monometallic and bimetallic catalysts in selective hydrogenation of 1,3-butadiene in the presence of 1-butene under liquid phase conditions was studied. Bimetallic catalysts were prepared by the coimpregnation method with the required amounts of the precursors salts [Ni(NO3)2·6H2O and Pd(NH3)4Cl2·H2O] over pellet-form γ-Al2O3 with a constant content of Pd (0.5 wt%) and varying Ni/Pd atomic ratio (0.25, 0.5, 0.75, and 1) obtaining egg-shell profiles of the active components. The catalysts were characterized by X-ray diffraction, temperature-programmed techniques, such as reduction in hydrogen and desorption of ammonia, N2 physisorption, and transmission electron microscopy. The catalytic test showed that the 1,3-butadiene was selectively hydrogenated when bimetallic catalysts were used. The addition of Ni to the Pd-based catalysts suppressed n-butane formation and increased recovery of 1-butene at medium conversion. Therefore, it was observed an improved catalytic performance of the bimetallic catalysts being highest in the case of the 1NiPd/Al2O3.

Исследование термического разложения природных карбонатов кальция методом температурно-программированной масс-спектрометрии

Исследование термического разложения природных карбонатов кальция методом температурно-программированной масс-спектрометрии

Investigation of Coke Formation in Dry Methane Reforming over Nickel-based Monolithic Catalysts

Coking accumulations via dry methane reforming (DMR) over 10NAM monolithic catalyst and pelletized catalyst was investigated. 10NAM catalyst was synthesized and coated on a wall of monolithic reactor. Pelletized catalyst of 10NAM was also prepared for the comparison. Consequently, catalyst was characterized by BET, <TEX>$H_2-TPR$</TEX> and <TEX>$H_2-TPD$</TEX>. The catalytic reaction was undergone at <TEX>$600^{\circ}C$</TEX> under atmospheric pressure and <TEX>$CH_4$</TEX> to <TEX>$CO_2$</TEX> reactant ratio of 1:2. The coking formation over spent catalyst was then carried out in the hydrogen flow using temperature programmed technique (TPH). According to the results, DMR over 10NAM monolithic catalyst exhibits a minimized coking formation comparing to the use of pelletized catalyst. This could be attributed to a prominent heat transfer efficiency of the monolithic catalyst.