Showing papers on "Temperature-programmed reduction published in 2018"

ZrO2 support imparts superior activity and stability of Co catalysts for CO2 methanation

Abstract: Screening of various supports reveals that Co catalysts supported on ZrO2 and Al2O3 show good initial activity for CO2 methanation. Co/ZrO2 and Co/Al2O3 catalysts prepared by impregnation with different metal loadings were further examined comparatively. The 10Co/ZrO2 catalyst showed high activity with CO2 conversion of 92.5% and CH4 selectivity of 99.9% without deactivation after 300 h time on stream (TOS). However, the 10Co/Al2O3 catalyst gave a lower CO2 conversion of 77.8% which decreased to 38.6% after 300 h TOS. The catalysts were characterized by STEM/EDS (scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy), in situ XRD(X-ray diffractometer), H2-TPR(temperature programmed reduction), XPS (X-ray photoelectron spectroscopy), chemisorption of H2, CO, CH4, CO2 and NH3-TPD (temperature programmed desorption). Re-dispersion of Co species on the ZrO2 support during reduction by H2 was observed by STEM/EDS. New Co-Zr phase formed on the Co-ZrO2 interface was directly observed by TEM for the first time; the Co/ZrO2 catalyst exhibited high stability with high activity for CO2 conversion. In situ XRD, H2-TPR and XPS results indicate the promoting effect of ZrO2 on the reduction of Co3O4 to Co metal along with the negative effect of Al2O3. The oxygen vacancies on the ZrO2 detected by XPS may help to activate CO2 and H2O and resist deactivation. Co/Al2O3 catalyst deactivates rapidly due to coke deposition and spinel formation.

Tuning Ni-catalyzed CO2 hydrogenation selectivity via Ni-ceria support interactions and Ni-Fe bimetallic formation

Abstract: CO2 hydrogenation over Fe-modified Ni/CeO2 catalysts was investigated in a batch reactor using time-resolved in situ FTIR spectroscopy. Low loading of Ni/CeO2 was associated with high selectivity to CO over CH4, while higher Ni loading improved CO2 hydrogenation activity with a reduced CO selectivity. X-ray absorption near-edge structure (XANES) analysis revealed Ni to be metallic for all catalysts including the CO-selective low loading 0.5% Ni catalyst, suggesting that the selectivity trend is due to structural rather than oxidation state effects. The loading amount of 1.5% Ni was selected for co-impregnation with Fe, based on the significant shift in product selectivity towards CH4 for that loading amount, in order to shift the selectivity towards CO while maintaining high activity. Temperature programmed reduction (TPR) results indicated bimetallic interactions between Ni and Fe, and XANES analysis showed that about 70% of Fe in the bimetallic catalysts was oxidized. The Ni-Fe catalysts demonstrated improved selectivity towards CO without significantly compromising activity, coupling the high activity of Ni catalysts and the high CO selectivity of Fe. The general trends in Ni loading and bimetallic modification should guide efforts to develop non-precious metal catalysts for the selective production of CO by CO2 hydrogenation.

Activity and stability of powder and monolith-coated Ni/GDC catalysts for CO2 methanation

Abstract: The methanation of CO2 via the Sabatier process is gaining interest for power-to-gas (P2G) application. In this work, CO2 methanation activity and stability were investigated over Ni/GDC (gadolinium-doped-ceria) catalysts at atmospheric pressure varying reaction temperature (TSET = 300–600 °C) and space velocity (GHSV = 10,000–50,000 h−1). Powder catalysts with different Ni content (15–50 wt.%) were synthesized by the solution combustion synthesis (SCS). The same method was adopted to in situ deposit the Ni/GDC (50 wt.%Ni) coating layer on the cordierite monolith (500 cpsi). The catalysts were characterized by N2 adsorption-desorption, X-ray diffraction (XRD), H2 temperature programmed reduction (H2-TPR), CO2 temperature programmed desorption (CO2-TPD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Temperature profiles along the structured catalytic bed were discussed to interpret the experimental results. Catalytic performance increased by increasing the Ni content due to enhanced metal-to-support interaction, basicity and oxygen vacancies. Uniform, thin and high-resistance catalytic layers were in situ deposited on the cordierite monoliths by the fully reproducible SCS method. Structured catalysts showed high methane productivity per unit weight of catalyst due to simultaneous low catalytic loading and high flow rate. Excellent stability was observed over 200 h of time-on-stream. The results reported in this manuscript pinpointed on the important aspects of realizing CO2 methanation on structured catalysts, providing a platform for further optimization studies.

Complete oxidation of formaldehyde over TiO2 supported subnanometer Rh catalyst at ambient temperature

Abstract: Catalytic oxidation of formaldehyde (HCHO) to CO2 and H2O under ambient condition is highly desired for purifying indoor air quality. In this work, it was found for the first time that TiO2 supported subnanometer Rh catalyst exhibited a remarkable activity with complete removal of HCHO at room temperature. The humidity under ambient condition can significantly improve the activity, stability and specific rates but result in lower activation energy for the oxidation of HCHO on this catalyst. A combination of characterizations, such as H2 temperature programmed reduction, adsorption microcalorimetry and in situ diffuse reflectance infrared Fourier transform spectroscopy, demonstrated the importance of Rh species for the dissociative adsorption of O2 and the key role of water on the oxidation of HCHO. The O atoms facilitated the transformation of HCHO adsorbed on TiO2 into intermediate of formates, while the presence of water led to the production of hydroxyl probably from the reaction with the adsorbed O species, which promoted the decomposition of formates to CO2. This study can provide an important implication into the design and fabrication of more economic Rh-based catalysts for ambient HCHO oxidation.

Influence of the support on Ni catalysts performance in the in-line steam reforming of biomass fast pyrolysis derived volatiles

Abstract: The influence the support has on the performance of Ni catalysts used in the reforming of biomass fast pyrolysis volatiles has been assessed. Accordingly, five catalysts have been prepared by wet impregnation method, namely Ni/Al2O3, Ni/SiO2, Ni/MgO, Ni/TiO2 and Ni/ZrO2. These catalysts have been characterized by nitrogen adsorption/desorption, X-ray fluorescence spectroscopy, temperature programmed reduction and X-ray diffraction techniques. The pyrolysis-reforming runs have been performed in a bench scale unit operating in continuous regime. The biomass (pine wood sawdust) pyrolysis step has been carried out in a conical spouted bed reactor at 500 °C, with the volatiles produced (a mixture of gases and bio-oil) being reformed in-line on the prepared catalysts in a fluidized bed reactor at 600 °C. Remarkable differences have been observed amongst the catalyst prepared, with Ni/Al2O3, Ni/MgO and Ni/ZrO2 being those leading to the most encouraging results, whereas Ni/TiO2 and, especially Ni/SiO2, having a limited reforming activity. The performance of each catalyst has been related to its properties determined in the characterization.

High stability and activity of solution combustion synthesized Pd-based catalysts for methane combustion in presence of water

Abstract: Pd-ceria and Pd-ceria-zirconia catalysts were prepared by solution combustion synthesis (SCS) and tested for methane oxidation in presence of 10 vol% H2O. Their behavior was compared with that of traditional incipient wetness impregnated catalysts (IW). Both SCS samples showed better stability and complete recovery of catalytic activity after prolonged time-on-stream at 723 K in cycling wet/dry atmosphere compared to IW ones. During transient light-off experiments, the increase in the temperature to achieve 30% methane conversion was of only 34 K for Pd/Ce0.75Zr0.25O2 SCS. The better performance of SCS samples has been attributed to their strong Pd-ceria interaction and to their higher oxygen exchange capability, as measured by Temperature Programmed Reduction experiments and oxygen storage capacity measurements. For Pd/Ce0.75Zr0.25O2 SCS, CO chemisorption, DRIFT and TEM characterization revealed the presence of very small Pd nanoparticles also after the aging cycle, indicating that on this sample palladium is present in a finely dispersed form that remains stable during cycling wet/dry atmosphere.

Highly porous monolith/TiO2 supported Cu, Cu-Ni, Ru, and Pt catalysts in methanol steam reforming process for H2 generation

Abstract: A comprehensive investigation on the steam reforming of methanol (SRM) process using a microstructure monolith in conjunction with a synthesized nanostructure of TiO2 is presented. The surface of the designed monolith/TiO2 structure was coated with copper, copper/nickel, ruthenium, and platinum as the catalyst. The prepared catalysts were then characterized with FE-SEM, AFM, BET surface area measurement, FT-IR, far-infrared, temperature programmed reduction (TPR), and X-ray diffraction (XRD). The obtained results show that in this process, the designed Monolith/TiO2 structure has a remarkable impact on methanol conversion (99%) and carbon monoxide selectivity (5%). These results are interpreted by the high surface area and superb mass transfer in this micro/nano engineered structure. These results also indicated that the catalyst activity and dispersion are influenced by the metal-support interaction, which is more pronounced in the TiO2 supports. The ruthenium catalyst presented the highest conversion and selectivity for this process. However, the economically viable property of the Cu-Ni catalyst supposes it as an alternative for the noble ruthenium catalyst.

Hydrotreating of Guaiacol: A Comparative Study of Red Mud-Supported Nickel and Commercial Ni/SiO2-Al2O3 Catalysts

Abstract: Upgrading of bio-oil through catalytic hydrotreating was investigated with guaiacol as a model compound. A nickel supported on red mud (Ni/RM) hydrotreating catalyst was developed and compared to the standard Ni/SiO2-Al2O3 catalysts under similar experimental conditions. The Ni/RM catalyst was characterized by inductively coupled plasma atomic emission spectrometry (ICP-AES), X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), BET specific surface area, and temperature programmed reduction (TPR). The effects of reaction temperature (300, 350, 400 °C) and initial hydrogen pressure (4.83 MPa (700 psi), 5.52 MPa (800 psi), and 6.21 MPa (900 psi)) on products distribution were investigated. The major products of hydrotreating process were catechol, anisole, phenol, cyclohexane, hexane, benzene, toluene, and xylene. Increasing the reaction temperature and hydrogen pressure improved HDO reactions. Complete HDO was achieved at reaction temperature of 400 °C and initial hydrogen pressure of 6.21 MPa (900 psi). Under these conditions, the selectivity to cyclohexane, benzene, toluene, and xylene over Ni/RM catalyst were 38.8, 24.5, 18.1, and 7.9% respectively, whereas these values were 62.2, 15.9, 8.4, and 4.5% respectively over Ni/SiO2-Al2O3. Reaction network and the kinetics of guaiacol HDO were proposed according to analysis of the products. The Ni/RM catalyst was more effective for deoxygenation reactions than hydrogenation while commercial Ni/SiO2-Al2O3 was more effective for hydrogenation than deoxygenation. Thus, hydrogen consumption per gram of bio-oil was lower for the Ni/RM catalyst compared to the Ni/SiO2-Al2O3. There was less hydrocracking and coke formation for the Ni/RM compared to the commercial catalyst and this resulted in higher liquid yield for the new catalyst.

Regeneration of NiAl2O4 spinel type catalysts used in the reforming of raw bio-oil

Abstract: The regenerability of Ni catalysts in reforming reactions is a key factor for process viability. Accordingly, this study addresses the regeneration of two spinel NiAl2O4 type catalysts by reaction-regeneration cycles in the oxidative steam reforming (OSR) of raw bio-oil. The spinel type catalysts were prepared by different methods including a supported Ni/La2O3-αAl2O3 catalyst and a bulk NiAl2O4 catalyst. The experimental set-up consists of two units connected in series for i) the thermal treatment of bio-oil at 500 °C, in order to control the deposition of pyrolytic lignin, followed by; ii) the oxidative steam reforming (OSR) of the remaining oxygenates in a fluidized bed catalytic reactor. The conditions in the OSR reaction step were: 700 °C; oxygen/steam/carbon ratio (O/S/C), 0.34/6/1; space time, 0.75 gcatalysth/gbio-oil (for supported catalyst) and 0.15 gcatalysth/gbio-oil (for bulk catalyst). Three different strategies have been studied in the regeneration step by coke combustion, including the in situ regeneration inside the reactor at 650 °C and 850 °C, and the ex situ regeneration in an external oven at 850 °C, for 4 h in all the cases. The behavior of the fresh and regenerated catalysts has been explained according to their metallic properties, determined by different characterization techniques (temperature programmed reduction (TPR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electronic microscopy (TEM)). According to these results, the combustion ex situ of the catalyst at 850 °C is able to completely regenerate the bulk catalyst, since these regeneration conditions lead to the total recovery of the NiAl2O4 spinel phase together with negligible loss of Ni on the surface in the catalyst. These novel results are crucial for future industrial implementation of the process.

Complete methane oxidation over Ba modified Pd/Al2O3: The effect of water vapor

Abstract: The effect on complete methane (CH4) oxidation activity by an addition of up to 2 wt.% barium (Ba) promoter to alumina supported palladium (Pd/Al2O3, 2 wt.% Pd) was investigated. The catalyst samples were characterized with various techniques; temperature programmed oxidation (TPO), temperature programmed reduction with CH4 (CH4-TPR), X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM) and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). Flow reactor was used to investigate the CH4 oxidation activity in the presence and in the absence of water vapor and also to evaluate the possibility to regenerate the catalytic activity after water vapor exposure. The results from the TPO and the CH4-TPR experiments together with the XPS analysis gave no evidences for electronic promotion of the catalytic activity by addition of 0.5–2 wt.% Ba. This goes in line with the CH4 conversion in dry gas condition, which was not affected by the Ba addition. However, we observed that an addition of Ba to Pd/Al2O3 enhances the CH4 oxidation activity in the presence of water vapor, hence mitigates the effect of water deactivation. Interestingly, it was also seen that after water vapor exposure, the CH4 oxidation activity could be regenerated to greater extent for the Ba promoted samples, particularly for the regeneration temperatures of 500–600 °C. Our results clearly show that the support influences the water deactivation of the active palladium sites and that the addition of barium is beneficial for the catalyst regeneration.

Formation of NH3 and N2O in a modern natural gas three-way catalyst designed for heavy-duty vehicles: the effects of simulated exhaust gas composition and ageing

Abstract: An increasing number of heavy-duty vehicles are using liquefied natural gas (LNG) as a fuel due to the expanding refuelling station network for LNG and lower overall emissions compared to diesel vehicles. The latest EURO VI regulation or natural gas fuelled vehicles set a limit for NH3 of 10 ppm, and N2O exhaust is expected to be restricted in Europe in the near future. Poisonous and corrosive NH3 and the greenhouse gas N2O are formed as by-products in a three-way catalyst used to minimize the emissions of stoichiometric heavy-duty engines. In this work, we studied how high temperature NH3 and N2O formed in modern, fresh and aged bimetallic Pd/Rh three-way catalysts in simulated exhaust gas. More precisely, the exhaust gas composition and temperature were examined. Decreases in NO concentration and increases in temperature lowered the formation of NH3 and N2O, whereas a decrease in CH4 concentration reduced only NH3 formation. According to Raman and powder X-ray diffraction experiments, the structure of the catalyst changed during the ageing, and this reputedly affected the function of cerium-zirconium mixed oxides and thus the formation of NH3 and N2O. Temperature programmed reduction (H2-TPR) measurements showed changes in cerium-zirconium mixed oxide performance after ageing supporting Raman spectroscopy findings. Catalyst ageing in oxidizing conditions increased the formation of N2O. This study showed that exhaust gas composition plays an important role in the formation of undesired NH3 and N2O emissions.

Tuning Y-zeolite Based Catalyst with Copper for Enhanced Activity and Selectivity in Vapor Phase Hydrogenolysis of Glycerol to 1,2-Propanediol

Abstract: Highly dispersed copper oxide species supported on a Y-zeolite with different Si/Al ratios (5.1, 12, 30, and 60) were synthesized by an incipient wet impregnation method. The catalysts were characterized by X-ray diffraction (XRD), N2 adsorption (BET), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV–vis diffused reflectance spectroscopy (UV-DRS), N2O chemisorption, Pyridine-adsorption Fourier transform Infrared Spectroscopy (Py-FTIR), NH3 temperature programmed desorption (NH3-TPD) and temperature programmed reduction (TPR). Copper dispersion and metallic area were measured by the N2O decomposition method. The amount of Bronsted and Lewis acidic sites was analyzed by Pyridine adsorption followed by FT-IR. Performance of the prepared catalysts in vapor phase hydrogenolysis of glycerol to 1,2-propanediol was evaluated by using a fixed-bed stainless-steel reactor at 0.2 MPa and 210 °C. The studies revealed that the 3 wt.% CuO/Y-zeolite (Si/Al = 5.1) catalyst was highly active in glycerol hydrogenolysis showing 92% conversion of glycerol and 83% selectivity to 1,2-propanediol. This seems to originate from high dispersion of Cu, smaller Cu crystallite size, and the presence of a high amount of surface beneficial acido-basic properties.

Effect of CO2 in the oxidative dehydrogenation reaction of propane over Cr/ZrO2 catalysts

Abstract: Investigation was made on the effect of chromium content, and method of hydrothermal preparation of Cr/ZrO2 catalysts on their catalytic properties for CO2 oxidative dehydrogenation of propane (ODP). The catalysts were characterized by nitrogen physisorption, X-ray diffraction, Raman spectroscopy, and temperature programmed reduction and desorption of CO2 (TPD-CO2). The Cr/ZrO2 catalysts containing various chromium contents between 2.5 and 15 wt.% of Cr were prepared by conventional and microwave-assisted hydrothermal methods. Tetragonal ZrO2 was formed, with higher Cr contents, and smaller crystallite sizes obtained using the microwave-assisted method. The relationship between selectivity to propene and propane conversion suggested that independent of the preparation method, the catalytic properties could be classified in two groups, with low and high chromium contents. The TPD-CO2 results showed that a fraction of the CO2 was desorbed at temperatures higher than 500 °C, with simultaneous reduction of Cr(VI) species. The presence of CO2 in the reactants caused strong decreases of activity, selectivity, and yield towards propene. These results suggested that CO2 adsorbed strongly on the chromium oxide active sites for dehydrogenation of propane. The Cr/ZrO2 catalysts were active and selective for dehydrogenation of propane in the absence of CO2 (CDP), and became deactivated with time on stream. The activity was reestablished by thermal treatment with CO2 or O2 after deactivation in CDP catalytic cycles, with the activity always being reestablished by treatment in O2. The deactivation occurred by reduction of Cr(VI) species and by deposition of carbonaceous species produced in oligomerization reactions.

Citric Acid-Assisted Synthesis of Nanoparticle Copper Catalyst Supported on an Oxide System for the Reduction of Furfural to Furfuryl Alcohol in the Vapor Phase

Abstract: The simplest method of synthesizing metallic Cu (111) nanoparticles on the zinc–aluminum mixed oxides was found to be easily obtained by solid state grinding of all the three metal nitrates and organic compounds in suitable proportions. Organic compounds such as citric acid, formic acid, and hydrazine were used, which act as reducing agents for Cu. The catalysts were characterized by various physicochemical methods such as X-ray diffraction, X-ray fluorescence, temperature programmed reduction, high-resolution transmission electron spectroscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy and N2-physisorption. The use of citric acid resulted in Cu(111) nanoparticles with higher surface area and better dispersion on the surface of mixed oxides. The catalysts were used for the selective hydrogenation of furfural to furfuryl alcohol under vapor phase conditions. Among the various catalysts studied, citric acid-assisted catalysis showed better conversion with higher selectivi...