Effect Of Catalyst Pretreatment Research Articles

NOx storage and reduction by H2 over highly dispersed Pt on Co1Mg2Al1Ox-LDO for stationary applications: A transient kinetic study

The performance of a highly dispersed Pt/Co1Mg2Al1Ox-LDO catalyst towards NOx-storage and reduction by H2 for NOx control in stationary applications was investigated under forced feed composition cycling (“swing-reactor” system). The influence of 7 vol% CO2 and 5 vol% H2O in the NO-containing and H2-containing feeds on the dynamic evolution of adsorbed NOx-s and their reduction performance (conversion and N2-selectvity) were determined in the 200–400 oC range. The effects of catalyst pretreatment with 20 or 50 ppm SO2/He gas mixture on its NOx-s storage and reduction by hydrogen dynamics were also investigated. Reduction of NOx-s formed on the support involves a hydrogen spillover effect from Pt to the metal-support interface and support NOx-s sites. A very stable NSR performance was obtained at 350 oC for 2 h of successive NOx-adsorption (3 min)/reduction by H2 (1 min) steps. The nature of active and inactive NOx-s in H2 was probed by in-situ DRIFTS.

H2 Production by Methane Oxy-Reforming: Effect of Catalyst Pretreatment on the Properties and Activity of Rh-Ce0.5Zr0.5O2 Synthetized by Microemulsion

Green hydrogen introduction in hard-to-abate processes is held back by the cost of substituting steam reforming plants with electrolyzers. However, green hydrogen can be integrated in properly modified reforming processes. The process proposed here involves the substitution of steam reforming with oxy-reforming, which is the coupling of the former with catalytic partial oxidation (CPO), exploiting the pure oxygen coproduced during electrolysis to feed CPO, which allows for better heat exchange thanks to its exothermic nature. With the aim of developing tailored catalysts for the oxy-reforming process, Ce0.5Zr0.5O2 was synthetized by microemulsion and impregnated with Rh. The Ce-based supports were calcined at different temperatures (750 and 900 °C) and the catalysts were reduced at 750 °C or 500 °C. Tuning the calcination temperature allowed for an increase in the support surface area, resulting in well-dispersed Rh species that provided a high reducibility for both the metal active phase and the Ce-based support. This allowed for an increase in methane conversion under different conditions of contact time and pressure and the outperformance of the other catalysts. The higher activity was related to well-dispersed Rh species interacting with the support that provided a high concentration of surface OH* on the Ce-based support and increased methane dissociation. This anticipated the occurrence and the extent of steam reforming over the catalytic bed, producing a smoother thermal profile.

Effects of Catalyst Pretreatment on Carbon Nanotube Synthesis from Methane Using Thin Stainless-Steel Foil as Catalyst by Chemical Vapor Deposition Method.

Synthesis of carbon nanotubes (CNTs) was carried out using methane as a carbon source via the chemical vapor deposition (CVD) method. A thin stainless-steel foil was used as catalyst for CNT growth. Our results revealed that pretreatment step of the stainless-steel foil as a catalyst plays an important role in CNT formation. In our experiments, a catalyst pretreatment temperature of 850 °C or 950 °C was found to facilitate the creation of Fe- and Cr-rich particles are active sites on the foil surface, leading to CNT formation. It is noted that the size of metallic particles after pretreatment is closely related to the diameter of the synthesized CNTs. It is interesting that a shorter catalyst pretreatment brings the growth of semiconducting typed CNTs while a longer pretreatment creates metallic CNTs. This finding might lead to a process for improving the quality of CNTs grown on steel foil as catalyst.

CO2-oxidative ethane dehydrogenation over highly efficient carbon-resistant Fe-catalysts

Ethylene is a very important raw material for chemical industry, which can be produced from the underutilized ethane, originating from shale gas, through the oxidative ethane dehydrogenation (EDH) using CO2 as a mild oxidant. Two 5 wt%Fe-based catalysts, supported on mixed MgO-ZrO2 and NiO-MgO-ZrO2 oxides, prepared by auto-combustion sol-gel method, were investigated for CO2-EDH. The crystallographic structure of the catalysts was examined by X-ray diffraction (XRD), while the surface composition and oxidation state were identified using X-ray photoelectron spectroscopy (XPS). The effect of catalyst pre-treatment, reduction by H2, on catalytic activity, selectivity and carbon-resistance was evaluated for the studied samples.Both catalysts provided high ethylene selectivity and activity during CO2-oxidative dehydrogenation of C2H6 at 873 K, atmospheric pressure and equimolar CO2/C2H6 ratio. The addition of NiO in the support material had a promoting effect, since it resulted in a catalyst with a twofold higher activity. Similarly, the selectivity towards C2H4 remained at high levels, above 70%, slightly increased, as it was exemplified during an iso-conversion experiment. The best candidate lost 25% of its initial activity during the first 3 h of a stability test, while it remained unchanged from 3 to 17 h. Deposited carbon was negligible, whereas sintering was identified as the main deactivation reason. Systematic XPS analysis revealed that deposition of Fe on NiO-MgO-ZrO2 support significantly affected the local environment of Ni, contributing to the enhanced catalytic performance. Increased Fe2+ concentration was found for the “used” Fe-catalyst supported on NiO-MgO-ZrO2, compared to the “as-prepared”, implying partial reduction of Fe during CO2-EDH reaction, as it was also observed by HRTEM.

Hydrogen Photo-Production from Glycerol Using Nickel-Doped TiO2 Catalysts: Effect of Catalyst Pre-Treatment

In the present piece of research, hydrogen production via the photo-reforming of glycerol (a byproduct from biodiesel generation) is studied. Catalysts consisted of titania modified by Ni (0.5% by weight) obtained through deposition–precipitation or impregnation synthetic methods (labelled as Ni-0.5-DP and Ni-0.5-IMP, respectively). Reactions were performed both under UV and solar irradiation. Activity significantly improved in the presence of Ni, especially under solar irradiation. Moreover, pre-reduced solids exhibited higher catalytic activities than untreated solids, despite the “in-situ” reduction of nickel species and the elimination of surface chlorides under reaction conditions (as evidenced by XPS). It is possible that the catalyst pretreatment at 400 °C under hydrogen resulted in some strong metal–support interactions. In summary, the highest hydrogen production value (ca. 2600 micromole H2·g−1) was achieved with pre-reduced Ni-0.5-DP solid using UV light for an irradiation time of 6 h. This value represents a 15.7-fold increase as compared to Evonik P25.

Use of Plasma-Synthesized Nano-Catalysts for CO Hydrogenation in Low-Temperature Fischer⁻Tropsch Synthesis: Effect of Catalyst Pre-Treatment.

A study was done on the effect of temperature and catalyst pre-treatment on CO hydrogenation over plasma-synthesized catalysts during the Fischer–Tropsch synthesis (FTS). Nanometric Co/C, Fe/C, and 50%Co-50%Fe/C catalysts with BET specific surface area of ~80 m2 g–1 were tested at a 2 MPa pressure and a gas hourly space velocity (GHSV) of 2000 cm3 h−1 g−1 of a catalyst (at STP) in hydrogen-rich FTS feed gas (H2:CO = 2.2). After pre-treatment in both H2 and CO, transmission electron microscopy (TEM) showed that the used catalysts shifted from a mono-modal particle-size distribution (mean ~11 nm) to a multi-modal distribution with a substantial increase in the smaller nanoparticles (~5 nm), which was statistically significant. Further characterization was conducted by scanning electron microscopy (SEM with EDX elemental mapping), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The average CO conversion at 500 K was 18% (Co/C), 17% (Fe/C), and 16% (Co-Fe/C); 46%, 37%, and 57% at 520 K; and 85%, 86% and 71% at 540 K respectively. The selectivity of Co/C for C5+ was ~98% with 8% gasoline, 61%, diesel and 28% wax (fractions) at 500 K; 22% gasoline, 50% diesel, and 19% wax at 520 K; and 24% gasoline, 34% diesel, and 11% wax at 540 K, besides CO2 and CH4 as by-products. Fe-containing catalysts manifested similar trends, with a poor conformity to the Anderson–Schulz–Flory (ASF) product distribution.

HDO activity of carbon-supported Rh, Ni and Mo-Ni catalysts

This work reports the performance of low metal loading (about 3wt.%) Rh, Ni and Mo-Ni catalysts supported on carbon in the hydrodeoxygenation (HDO) of phenol. The catalytic tests were carried out in a flow microreactor at 310°C and at a total hydrogen pressure of 3MPa. A commercial NiMoP/γ-Al2O3 sulfided catalyst was used as reference. The effect of catalyst pre-treatment (reduction against sulfidation) was investigated in the case of the noble metal catalyst. The thermally treated in inert atmosphere, fresh reduced and spent catalysts have been characterized by several physicochemical techniques: chemical analysis, N2 adsorption-desorption isotherms, Raman spectroscopy, Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and High Resolution Transmission Electron Microscopy (HRTEM).The HDO of phenol was found to be favored on the bifunctional Rh/C and Ni/AC catalysts which exhibit a large amount of well dispersed metallic species (from HRTEM). Contrary to a commercial sulfided NiMo/Al2O3 catalyst, the hydrogenation of aromatic ring of phenol was favored being the CO bond hydrogenolysis a minor reaction route. Catalytic HDO of phenol over Rh/C reveal that the catalyst activation by reduction is advantageous compared to that by sulfidation. The partial substitution of Ni by Mo in the Mo-Ni/AC catalyst led to enhance of the catalyst stability but decrease of overall activity occurs due to the decrease of amount of metallic nickel species. The role of the MoO3 species in the inhibition of deactivation by coke is discussed.

Biomass Catalytic Pyrolysis on Ni/ZSM-5: Effects of Nickel Pretreatment and Loading

In this work, Ni/ZSM-5 catalysts with varied nickel loadings were evaluated for their ability to produce aromatic hydrocarbons by upgrading of pine pyrolysis vapors. The effect of catalyst pretreatment by hydrogen reduction was also investigated. Results indicate that the addition of nickel increases the yield of aromatic hydrocarbons while simultaneously increasing the conversion of oxygenates, relative to ZSM-5, and these effects are more pronounced with increasing nickel loading. Additionally, while initial activity differences were observed between the oxidized and reduced forms of nickel on ZSM-5 (i.e., NiO/ZSM-5 versus Ni/ZSM-5), the activity of both catalysts converges with increasing time on stream. These reaction results coupled with characterization of pristine and spent catalysts suggest that the catalysts reach similar active states during catalytic pyrolysis, regardless of pretreatment, as NiO undergoes in situ reduction to Ni by biomass pyrolysis vapors. This reduction of NiO to Ni was confi...

The effects of catalyst pretreatment, growth atmosphere and temperature on carbon nanotube synthesis using Co–Mo/MgO catalyst

The growth of high quality and high yield carbon nanotubes (CNTs) by catalytic chemical vapor deposition (CVD) of CH4 over Co–Mo/MgO catalyst was investigated for different growth temperatures and H2 flow rates. It was observed that CNT yield decreased with the H2 flow rate, however, quality increased with increasing H2 flow rate. CNT yield increased for the temperatures 850–950°C but dropped significantly above 950°C. In this study, the highest yield of 1526% was obtained at the growth temperature of 950°C. The optimum H2 flow rate was 200sccm; this rate gave both high graphitization and high yield of product. Various CNT growth atmospheres including Ar, H2 and the mixture of both gases were also analyzed and it was observed that the highest quality CNTs were obtained for both pretreatment and growth carried out with H2. This gave a high yield of 292%. On the other hand, CNT growth carried out under Ar atmosphere gave higher CNT yield of 368%, however, the CNTs grown with Ar were more defective and had larger diameters. Prime novelty statementWe demonstrate a sorbitol added catalysis synthesis method and importance of the ideal growth conditions to improve high quality single walled carbon nanotube yield up to 1500%.

Ru/TiO2 for the preferential oxidation of CO in H2-rich stream: Effects of catalyst pre-treatments and reconstruction of Ru sites

The preferential oxidation (PROX) of CO is a promising strategy for trace CO clean up in H2-rich stream to fuel cells. In the present study, a series of TiO2 supported clusters were prepared and studied for the PROX of CO. Amongst, Ru/TiO2 catalyst exhibited remarkably high PROX activity in the operation temperature range of fuel cells. The effects of catalyst preparation and pre-treatment on the catalytic performance of Ru/TiO2 were investigated in detail. Ru/TiO2 catalyst prepared by photo-deposition and pre-treated under H2–CO atmosphere was found to be the most promising one and complete CO oxidation could be achieved at >373K. Ru/TiO2 pre-treated under different reducing atmospheres were characterized by high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) of CO adsorption. The surface reconstruction of Ru sites during catalyst pre-treatment was observed and isolated metallic Ru species was identified as preferred active sites for PROX reaction. Based on the catalytic and characterization results, the possible mechanism for PROX of CO over Ru/TiO2 was proposed.

Effect of Catalyst Pretreatment on Chirality-Selective Growth of Single-Walled Carbon Nanotubes

We show that catalyst pretreatment conditions can have a profound effect on the chiral distribution in single-walled carbon nanotube chemical vapor deposition. Using a SiO2-supported cobalt model catalyst and pretreatment in NH3, we obtain a comparably narrowed chiral distribution with a downshifted tube diameter range, independent of the hydrocarbon source. Our findings demonstrate that the state of the catalyst at the point of carbon nanotube nucleation is of fundamental importance for chiral control, thus identifying the pretreatment atmosphere as a key parameter for control of diameter and chirality distributions.

Fischer–Tropsch synthesis on Co/ZnO catalyst—Effect of pretreatment procedure

The effect of catalyst pretreatment, using hydrogen or carbon monoxide, on the activity and selectivity of cobalt on ZnO catalyst (10 wt% Co/ZnO) during Fischer–Tropsch synthesis was studied in a fixed bed reactor. Catalyst reduced with hydrogen had higher activity and higher olefin content, and produced less methane than the CO activated catalyst. Catalysts were characterized by means of Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), temperature programmed reduction (TPR), and hydrogen chemisorption (H 2-TPD) with pulse re-oxidation. Reduction of Co 3O 4 occurs in two stages during H 2-TPR and CO-TPR. In the latter case CoO, metallic cobalt, cobalt carbides (Co x C; x = 2 or 3) and carbon (from CO disproportionation) are formed during the reduction process. The existence of Co 0 along with CoO and Co x C, indicates a possibility of two reaction paths for formation of Co x C from CoO: the direct path (CoO → Co x C) and a series reaction via Co 0 (i.e. CoO → Co 0 → Co x C).

Hydrogenation of α-Methylstyrene in a Piston-Oscillating Monolith Reactor

The hydrogenation of α-methylstyrene (AMS) to cumene was investigated in a novel piston-oscillating monolith reactor (POMR). Low-frequency (0−17.5 Hz) and -amplitude (2.5 mm) mechanical oscillations were applied to the three-phase system at 46 °C and 0.44 MPa of H2. For comparison purposes, the reaction was also carried out in a stirred tank at identical temperatures and pressures and at similar power input per volume. Results show activity improvements of up to 84% for 17.5 Hz, 2.5 mm piston oscillations over low-frequency gas flow pulsing conditions, and significantly greater improvements when compared to trickle beds or conventional monolith reactors. The POMR also gives as good or better selectivity toward cumene than a stirred tank at identical conditions. While the effects of catalyst pretreatment and its impact on the Pd crystallite size also have an important role in determining the catalyst activity and long-term stability in AMS hydrogenation, these results suggest that the impact of low-frequency and -amplitude oscillations, applied to structured reactors, is considerable.

Nano-Structured Gold Catalysts Supported on CeO2 and CeO2-Al2O3 for NOx Reduction by CO: Effect of Catalyst Pretreatment and Feed Composition

Gold catalysts supported on ceria and ceria-alumina were studied in NOx reduction by CO. Gold was loaded using deposition-precipitation method. The ceria-alumina (20 wt% alumina) support was synthesized by co-precipitation. The average size of gold and ceria nano-particles was bellow 10 nm. It was established that the type of pretreatment do not have a substantial effect on the catalytic activity. The presence of O2 in the feed leads to a high conversion of CO to CO2 but no NO conversion was registered. Both NO and CO conversion was increased adding H2 to the feed. The catalytic activity became higher upon adding higher amount of H2. Supplementary to the main reaction parallel reactions took place. Bellow 200 degrees C N2O formation and at 250 degrees C and above the NH3 formation was detected. At around 200 degrees C it was established 100% selectivity to N2. The addition of water to the feed influenced positively the CO conversion and did not influence negatively the conversion of NO. The selectivity to N2 at around 200 degrees C remained 100% independent of the presence of moisture. Alumina in the mixed support prevents the sintering of both gold and ceria nano-particles. The results obtained make the catalysts containing gold supported on ceria-alumina promising for practical application.

Effects of catalyst pre-treatment on the growth of single-walled carbon nanotubes by microwave CVD

Layers of carbon nanotubes were deposited by microwave CVD on oxidized silicon substrates coated with Al–Fe–Mo catalyst films. To achieve a tube growth at about 973K, the ion bombardment of the catalyst surface has to be avoided. The appropriate pre-treatment of the substrates is essential for the deposition of single-walled carbon nanotubes. Annealing in air is preferable to the frequently used reducing pre-treatment prior to the deposition as a higher area density of the tubes and a better reproducibility of deposition can be obtained. To figure out this finding, selected samples were investigated by analytical transmission electron microscopy and Raman spectroscopy. It is shown that the pre-treatment has a strong effect on the size and distribution of the catalyst particles.

Characterization and catalytic activity of differently pretreated Pd/Al 2O 3 catalysts: the role of acid sites and of palladium–alumina interactions

The effect of catalyst pretreatment on the performance of chlorine-free γ-alumina-supported palladium catalysts with metal loadings in the range of 0.3 to 2.77 wt% has been investigated in the hydroconversion of n-hexane (nH) and 2,2-dimethylbutane (22DMB) at 290 °C and atmospheric pressure. The catalyst properties were modified by varying the catalyst pretreatment: low-temperature reduction (LTR) at 300 °C; high-temperature reduction (HTR) at 600 °C; and oxidation of highly reduced catalysts followed by LTR (regeneration). The isomerization activity for both nH and 22DMB conversion is enhanced by HTR of the Pd/Al 2O 3 as compared to LTR and regeneration. It is concluded that HTR increases the Lewis acidity of the alumina and the participation of Lewis acid sites improves catalytic performance. The influence of Lewis acid catalyst sites on the macroscopic kinetics overshadows the modest catalytic contribution from palladium. Ammonia effectively blocks the acid centers and limits the overall catalytic performance to metal-only catalysis. The role of support acid sites in the conversion of C 6-hydrocarbons was further demonstrated by isomerization activity associated with 22DMB hydroconversion catalyzed by an alumina-supported Ni catalyst. Based on the results of FTIR studies of adsorbed CO and results of temperature-programmed palladium hydride phase decomposition (TPHD), it was concluded that a part of the alumina-supported Pd transforms into a Pd–Al alloy during the HTR pretreatment. The catalytic performance of the regenerated catalysts in the C 6-alkane conversion is consistent with the suggestion that the Pd–Al alloy formed during HTR decomposes during the regeneration treatment to form Al 2O 3 moieties on the Pd surface. These moieties, most likely, interact strongly with nearby palladium species, creating sites which are active for alkane isomerization and, simultaneously, resistant to coking.

Effect of catalyst pretreatment on the growth of carbon nanotubes

Carbon nanotubes is synthesized by thermal chemical vapor deposition using acetylene as hydrocarbon gas on Ni-coated silicon substrate. The ‘worm-like’ carbon fibers and ‘spaghetti-like’ carbon nanotubes is grown at 700 °C with the Ni catalyst treated by thermal annealing and H 2 plasma, respectively. It is found that the H 2 plasma treatment is a crucial step to maintain the activity of catalyst prior to the reaction of acetylene gas for the nanotube growth. After clarifying this phenomenon, the carbon nanofibers can successfully be grown at low temperature (500 °C). The field emission test shows both the carbon nanotubes and carbon nanofibers have low turn-on voltage and high emission current, which is sufficient for practical application in the carbon nanotube-/nanofiber-based field emission displays.

Hydrodechlorination of 1,2-dichloroethane catalyzed by Pt–Cu/C: effect of catalyst pretreatment

The catalytic performance of Pt–Cu/C with Pt/Cu atomic ratios of 1:1 and 1:3 in the reaction of 1,2-dichloroethane dechlorination at 200°C and atmospheric pressure was investigated to understand the molecular phenomena governing the change in ethylene selectivity with time on stream (TOS). When the Pt1–Cu3/C catalyst is prereduced in H2 at 220°C, ethane is the major product at early TOS, while ethylene is the major product at longer TOS. Pretreatment in flowing He at 220°C eliminates the transient behavior and significantly increases steady state selectivity toward ethylene for both Pt–Cu catalysts. The transient behavior is also eliminated after reduction at 220°C followed by treatment with HCl at the same temperature. Quite different behavior is observed with a Pt/C catalyst in that ethane is the major product regardless of the type of pretreatment. The results are interpreted via a molecular scheme including the formation of mixed PtCuClx species during the non-reductive pretreatment. These PtCuClx particles are converted into the corresponding alloy during subsequent reduction by H2 or reaction mixture. The enrichment of the catalysts’ surface with Cu results from the pretreatment with HCl.

Selective hydrogenation of 2-butyne-1,4-diol to 2-butene-1,4-diol: roles of ammonia, catalyst pretreatment and kinetic studies

The selective hydrogenation of 2-butyne-1,4 diol to cis-2-butene-1,4 diol has been studied using different palladium-supported catalysts. With 1% Pd/C catalyst, the major product formed was 1,4-butanediol, along with other side products such as butenediol, γ-hydroxy butyraldehyde, n-butyraldehyde and n-butanol. Role of supports and ammonia was very significant in achieving a high selectivity to the intermediate, 2-butene-1,4-diol. A selective hydrogenation of 2-butyne-1,4-diol to give cis-2-butene-1,4-diol was achieved using 1% Pd/CaCO 3–NH 3 catalyst system. Effects of catalyst pretreatment with both 2-butyne-1,4-diol and H 2 on the catalyst activity and selectivity were investigated. It was found that the catalyst activity increased substantially, while the selectivity to 2-butene-1,4-diol decreased slightly with hydrogen-pretreated catalyst. The effects of hydrogen pressure, catalyst loading, initial concentrations of 2-butyne-1,4-diol and ammonia on the initial rate of hydrogenation were also studied in a batch reactor in a temperature range of 323–353 K. All the data obtained were in the kinetic regime under the conditions of the present work, based on which a Langmuir–Hinshelwood (L–H) type rate model was proposed. To verify the applicability of the kinetic model over a wide range of conditions, a batch reactor model was also developed. The agreement between the predicted concentration vs. time profiles with the experimental results was excellent.

Aromatization ofn-butane over Ni-ZSM-5 and Cu-ZSM-5 zeolite catalysts prepared by using Ni and Cu impregnated silica, fiber

The reaction ofn-butane aromatization was carried out over Ni-ZSM-5 and Cu-ZSM-5 zeolite catalysts prepared by using Ni and Cu impregnated silica fiber during the process of ZSM-5 zeolite synthesis. The catalysts prepared were characterized by X-ray powder diffraction, nitrogen adsorption and X-ray fluorescency. The acidic properties of the catalysts were investigated by temperature-programmed desorption of ammonia using a mass spectrometer equipped with a QTMD detector. The effect of catalyst pretreatment, reaction temperature, and time on stream on the reaction ofn-butane to aromatic hydrocarbons were investigated. The modification of ZSM-5 by Ni and Cu increased the selectivity to aromatic hydrocarbons. The state of Ni and Cu and their stabilization in the ZSM-5 structure was highly influenced by the mode of catalyst pretreatments.