Deposition Of Carbonaceous Species Research Articles

Unpredicted porosity improvement upon thermal reactivation of a fouled multi-walled carbon nanotube

Deposition of carbonaceous species on a solid catalyst’s surface during a catalysed processes is quite usual. This causes fouling, or depletion of the textural features, with a performance decay. In this work, preliminary results for an ex situ thermal oxidative reactivation of a fouled MWCNT are presented. The coke is nearly completely removed and, as a side effect, some of the MWCNT is combusted. Remarkably, the textural features are improved; 23 % higher BET area and 29 % higher total pore volume, while maintaining the isotherm shape. These improvements are attributed to two factors. Firstly, the removal during reactivation of non-porous (or less-porous) carbon domains present in the starting MWCNT, that positively influences the porosity in various ways; a control experiment employing the fresh MWCNT strongly suggests this hypothesis. Secondly, the new microporosity also contributes to the better BET.

On the applicability of PdAg membranes in propane dehydrogenation processes

The applicability of PdAg membranes in the propane dehydrogenation process has been evaluated in this work. The integration of membranes into the propane dehydrogenation process has the potential of making a significant step forward in energy and resource efficiency of the selective production of propylene. To assess the compatibility of the PdAg membrane with the process conditions of propane dehydrogenation, two double-skin PdAg membranes were exposed to the emulated process conditions of two cases: 1. A fully integrated membrane-reactor system (higher temperature) and 2. A cascade of catalyst and membrane separator units (lower temperature). The results of this study showed that at high temperature (475–500 °C) strong reduction of the hydrogen flux is observed during exposure to reactant and product containing mixtures. The reason for this is the deposition of carbonaceous species on the active sites of the membrane. Additionally, also the lifetime of the membrane (in terms of selectivity) was significantly lower due to the regeneration performed at high temperature in diluted air. The lower temperature experiments (300–350 °C) showed a much lower degree of deactivation and at the same time the membrane could be regenerated in air repeatedly without visible reduction of its selectivity. These findings show that the DS-PdAg membrane can be repeatedly exposed to reaction-regeneration cycles at acceptable activity and without significant loss of selectivity when operated in a cascade configuration at a temperature at or below 350 °C.

Elucidation of radical- and oxygenate-driven paths in zeolite-catalysed conversion of methanol and methyl chloride to hydrocarbons

Understanding hydrocarbon generation in the zeolite-catalysed conversions of methanol and methyl chloride requires advanced spectroscopic approaches to distinguish the complex mechanisms governing C–C bond formation, chain growth and the deposition of carbonaceous species. Here operando photoelectron photoion coincidence (PEPICO) spectroscopy enables the isomer-selective identification of pathways to hydrocarbons of up to C14 in size, providing direct experimental evidence of methyl radicals in both reactions and ketene in the methanol-to-hydrocarbons reaction. Both routes converge to C5 molecules that transform into aromatics. Operando PEPICO highlights distinctions in the prevalence of coke precursors, which is supported by electron paramagnetic resonance measurements, providing evidence of differences in the representative molecular structure, density and distribution of accumulated carbonaceous species. Radical-driven pathways in the methyl chloride-to-hydrocarbons reaction(s) accelerate the formation of extended aromatic systems, leading to fast deactivation. By contrast, the generation of alkylated species through oxygenate-driven pathways in the methanol-to-hydrocarbons reaction extends the catalyst lifetime. The findings demonstrate the potential of the presented methods to provide valuable mechanistic insights into complex reaction networks.

The nature of the deactivation of hydrothermally stable Ni/SiO2–Al2O3 catalyst in long-time aqueous phase hydrogenation of crude 1,4-butanediol

Abstract The deactivation of Ni/SiO2–Al2O3 catalyst in hydrogenation of crude 1,4-butanediol was investigated. During the operation time of 2140 h, the catalyst showed slow activity decay. Characterization results, for four spent catalysts used at different time, indicated that the main reason of the catalyst deactivation was the deposition of carbonaceous species that covered the active Ni and blocked mesopores of the catalyst. The TPO and SEM measurements revealed that the carbonaceous species included both oligomeric and polymeric species with high C/H ratio and showed sheet. Such carbonaceous species might be eliminated through either direct H2 reduction or the combined oxidation–reduction methodologies.

Catalyst deactivation in the direct synthesis of dimethyl ether from syngas over CuO/ZnO/Al2O3 and γ-Al2O3 mechanical mixtures

Direct synthesis of dimethyl ether from syngas over mixed catalysts constitutes a novel route aimed to replace the traditional two-step process. Many previous studies about this one-step process showed that catalyst deactivation is unavoidable. The present study wants to characterize the deactivation of CuO/ZnO/Al2O3 and γ-Al2O3 mechanical mixtures, and develop a deactivation model for predicting catalyst performance in presence of deactivation.It was demonstrated that water adsorbs over the γ-Al2O3 surface, blocking its active sites and causing a sharp conversion drop (mainly observed during the first hours on stream). This effect was reversible and could be avoided by increasing temperature (270 °C or above). The other deactivation mechanism was the deposition of carbonaceous species over the catalyst surface. A deactivation model was proposed and fitted to the experimental data.

Air Regeneration of Ethanol-Laden Pellet NaY-SiO2 and Pt/NaY-SiO2: Effects of Air Flow Rate on Pt Morphology and Regeneration Efficiency

Regeneration process and adsorbent performance were investigated by a fixed-bed adsorber at 300 °C. Surface species, zeolite structure, and Pt morphology were characterized by FT-IR, XRPD and EXAFS, respectively. Performance test results indicated that ethanol adsorption capacity of Pt/NaY-SiO2 is about 2.5 times that of NaY-SiO2. After regeneration, adsorption-capacity loss is 2.5 and 43%, respectively, for Pt/NaY-SiO2 regenerated at superficial velocity of 13.2 (PtR(HF)) and 5.3 cm/min (PtR(LF)); in contrast, it is 8 and 21%, respectively, for NaYR(HF) and NaYR(LF). The appearance of absorption bands in the CH stretching region (υCH) of the IR spectra characterizing the regenerated NaY-SiO2 suggested that the adsorption-capacity loss for NaY-SiO2 was mainly caused by the deposition of carbonaceous species formed in regeneration, which cannot be burned off readily at 300 °C. In contrast, no υCH bands have been observed for the IR spectra of PtR(HF) and PtR(LF), indicating that Pt helps to burn off carbonaceous species. However, Pt agglomeration was observed in TEM and EXAFS for Pt/NaY-SiO2(LF). The appearance of a υCO band at about 2085 cm−1 of the IR spectra characterizing PtR(LF) suggested that Pt agglomeration was induced by CO adsorption. The growth of Pt particles decreases the ethanol adsorbed on Pt together with the conversion of ethanol to ethoxides and aldehyde, leading to a decrease of adsorption capacity.

Methane bi-reforming over boron-doped Ni/SBA-15 catalyst: Longevity evaluation

A highly active and stable boron-promoted catalyst was successfully prepared by using the sequential incipient wetness impregnation technique and examined for methane bi-reforming reaction. The initial investigation found that the NiO and B2O3 particles were dispersed on the outer surface of the high surface area SBA-15 support. In addition, the catalytic activity was increased linearly with the tested reaction temperature due to the endothermic nature of the reaction. In fact, the catalyst achieved the CH4 conversion and H2/CO molar ratio of approximately 67.3% and 2.7, respectively at 1073 K. The resulting product ratio is highly suitable for downstream Fischer-Tropsch (FT) synthesis. The B-promoted catalyst showed the lowest degree of catalyst deactivation (4%) at 1023 K. Additionally, the XPS measurements unveiled that the boron facilitates the adsorption of CO2 by donating electrons to the neighbouring Ni cluster and thus improved its catalytic performance. Furthermore, Raman and XRD analysis revealed that the boron promotion on 10%Ni/SBA-15 could prevent the reoxidation and deposition of carbonaceous species.

Development of Cell-Protecting Methods for Carbon-Removal from Porous Ni-YSZ Anodes and Regeneration of the Cell Performance

Solid oxide fuel cells are highly efficient units, which directly convert the chemical energy of gaseous fuels into electrical energy without additional conversion steps. The overall efficiency can be increased by using heat losses occurring during the operation at high temperatures between 600–1000°C. SOFCs further offer a great fuel flexibility and compared to other fuel cell types they can internally reform hydrocarbons. Internal conversion of carbon-containing fuels over nickel as the most widely used catalyst tends to promote carbonaceous deposits on the anode surface as well as inside the anode, causing deactivation of the fuel cells. Nevertheless, undisturbed operation with conventional fuels and early detection of the cell degradation are highly important for the commercial usage of solid oxide fuel cells in industry, such as in auxiliary power units or especially for mobile application. Formation and deposition of carbonaceous species on the anode surface of solid oxide fuel cells may be prevented under specific operating conditions such as high steam/carbon ratio or high current density. The increasing amount of steam in fuel can chemically remove carbon adsorbed on the anode surface thus reducing the total amount of carbon deposits. However, this would significantly dilute the fuel thus reducing the overall power and the cell efficiency. Further possibility to avoid carbon formation is to operate the cells under load higher than the critical current density. However, increase of the steam amount or variation of the cell load is neither realistic nor advantageous for the commercial purpose. Without the need for the variation of the operating conditions, such as higher amount of steam or higher load, early detection of degradation phenomenon and applying of appropriate cell-protecting regeneration strategies can improve the operating reliability of the SOFC systems. In order to prolong the cell’s lifetime and to accelerate the market access of SOFCs, diverse approaches for carbon removal in a cell-protecting manner and regeneration of the cell performance were developed and detailed examined. They are outlined within this study. The practicability of the developed methods for industrial application was also considered. The regeneration strategies are based on application of: (1) gaseous components, for the gasification of carbon deposits, and (2) electrochemical methods, which remove carbon during the cell loading. Operation under gaseous components as hydrogen, water vapour or oxygen-enriched gas mixtures allowed complete carbon removal. Nevertheless, under operation in a fuel cell mode, only the approach that combined hydrogen and water vapour removed carbon from the anode while also protecting it from further undesired degradation. It is important to note that very high amount of water can deteriorate the cell performance and even provoke unwanted nickel oxidation, which thus must be considered. However, the allowed water vapor amount must thus me determined. Furthermore, feeding the anode with a very low amount of oxygen in a wide temperature range showed high carbon removal rates but resulted in further deterioration of the cell performance, and eventually led to mechanical degradation. Both methods for carbon removal in a cell-protecting manner and approaches that induced further microstructure degradation are presented within this study.

Forced deactivation and postmortem characterization of a metallic microchannel reactor employed for the preferential oxidation of CO (PROX)

This manuscript is one of the few works presenting evidences of the effect of prolonged use of a microreactor. Our reactor has been designed for the PROX reaction. Near to 550h of operation under different feed-streams, including CO2 and H2O, in the 100–300°C temperature range, and several regeneration cycles, and a final forced deactivation during ∼360h resulted in the permanent loss of activity of the microreactor. This could be attributed to some phenomena whose have compromised the chemical nature of the catalyst and that of the reactor including: displacement of the coating to the mouth of the channels, detachments and cracks of the catalytic layer, migration of some elements of the metallic substrate to the surface (Fe, Cr, Y), and deposition of carbonaceous species from the reaction over the catalytic layer and/or the metallic substrate. Furthermore, sulfur compounds were detected in both inlet and outlet zones of the microreactor, coming probably from a lubricant applied over the screws that sealed the assembling of the microreactor.This is a first approach for understanding possible effects of deactivation during long-term applications of a microreactor in the PROX reaction that could be considered as a case study useful for future designs of this kind of devices. The presented information could be extrapolated to similar reactions where thermal treatments along with highly corrosive atmospheres would be applied, in order to carry out a more appropriate design of future generations of microreactors, with a longer useful life. For that purpose not only the adequate selection of the catalysts must be done, but also the adequate choice of the fabrication material of the reactors is needed.

Characterization of Bulk and Surface Chemical States on Electrochemically Cycled LiFePO4: A Solid State NMR Study

Bulk and surface chemical states were both investigated for electrochemically delithiated LixFePO4 using 7Li and 31P MAS NMR spectroscopy. The quantitative lithium extraction/insertion from LiFePO4 and the reversible two-phase reaction behavior between LiFePO4 and FePO4 were confirmed on electrochemical operation. The 7Li and 31P NMR spectra of the fully charged LixFePO4 evidenced a single Li-poor phase Li0.05FePO4 instead of a biphasic mixture of 0.05LiFePO4 and 0.95FePO4. Simultaneous growth of LixPOyFz was also shown as a surface film component on the charged electrodes, which dynamically increased and decreased in intensity during charging and discharging reactions, respectively. The electrode surface was further characterized with XPS to discuss the surface film formation during electrochemical cycles. Combined chemical state analyzes by NMR and XPS spectroscopies suggested that the degradation of LiPF6 salt occurred from the initial redox cycles, but that of solvent occurred after multiple cycling. The deposition of carbonaceous species would be related to a small capacity fading observed after the multiple charge-discharge cycles.

Deposition of carbonaceous species over Ag/alumina catalysts for the HC-SCR of NOx under lean conditions: a qualitative and quantitative study

The deposition of carbonaceous species on fresh and aged Ag/Al2O3 catalysts used for the selective catalytic reduction of NOx (HC-SCR) was investigated by a combination of analytical techniques including SEM-EDXA, nitrogen physisorption, GC-MS and TGA. Time-on-stream (TOS) experiments were carried out to determine the level of deactivation over the catalysts. Short-term deactivation was monitored with a MicroGC with a short retention time. In addition, long-term deactivation was also investigated. Hexadecane, a paraffinic component that can be produced by decarboxylation and/or decarbonylation of natural oils and fats, and a standard Finnish commercial diesel fuel were used as the reducing agents. Finally, the influence of hydrogen on the carbonaceous deposition over the catalyst was also investigated. The NO-to-N2 activity improved in the temperature range 200–500 °C as hexadecane and H2 were used as reducing agents. In the case of commercial diesel fuel, a similar positive effect of H2 on the activity was also observed, however, at somewhat higher temperature, e.g., 350–500 °C. The results from the qualitative and quantitative analyses of the carbon deposits on the catalysts are presented and discussed. Indications of decreased activity over the catalyst were found, especially at low temperatures (<350 °C). Nevertheless, only small amounts of carbon were found on the catalytic surface. It is suggested that the deactivation phenomenon could not only be attributed to the deposition of carbonaceous species on the catalysts surface but the dependence of the adsorption kinetics of the reacting species as a function of temperature and partial pressure should also be carefully taken into account.

Wet Air Oxidation of Phenol over Ru Based Catalysts – Effects of Support

AbstractCatalytic wet air oxidation (CWAO) of aqueous solutions of phenol was performed on ruthenium catalysts supported on different oxides: TiO2, ZrO2 and their doped ceria mixtures. Phenol was chosen as a model pollutant molecule because of its wide use in industrial processes. All the samples were found to be highly active for phenol oxidation and the various titania-ceria mixtures were the most efficient for total organic carbon (TOC) removal. ICP analysis of the remaining solution after reaction revealed that ruthenium has not leached. Moreover, elementary analysis of the used catalysts showed that the deposition of carbonaceous species on the surface of the catalysts was rather low and was dependent on the nature of the support.

Raman Spectroscopy for Carbon Based Amorphous Thin Films

ABSTRACTVibrational spectroscopies and in particular Raman spectroscopy are known to be among the most useful tools to characterize and control the properties of carbon materials. This is because of the possibility to detect and study the carbon bonding state and because of the strong correlation between shape and width of the vibrational signals and the structure of the amorphous network. The aim of this work is to present some experiments in which the formation and evolution of carbon films obtained by energetic particle irradiation of solid targets or by deposition of carbonaceous species onto suitable surfaces, are controlled and studied by these methods. Particular attention will be given to pure amorphous carbon films and carbon based binary alloys.

CO2 reforming of propane over supported Rh

The adsorption, decomposition, and reaction of propane with CO2 have been investigated on Rh catalysts, deposited on various supports. The strong interaction of propane with Rh was noticed above 273 K. By means of Fourier transform infrared spectroscopy, π-bonded propylene, di-σ-bonded propylene, and propylidyne have been identified. Propane underwent dehydrogenation and cracking on supported Rh at 824–923 K. Propylene formed with a selectivity of 50–60%. The other major products were ethylene and methane. The deposition of carbonaceous species was also observed, the hydrogenation of which occurred only above 700–750 K, with a peak temperature of 900–950 K. The amount of carbon was more than one order of magnitude higher than that of surface Rh atoms, suggesting its diffusion from the Rh onto the support. The presence of CO2 basically altered the reaction pathway of propane, and the formation of H2 and CO with a ratio of 0.42–0.59 came into prominence. Propylene was detected only in traces. This led to the assumption that propylene reacted quickly with CO2 over Rh after its formation. This idea was confirmed by separate study of the reaction of propylene with CO2. Taking into account the rates of decomposition of propane and CO2 on Rh catalysts, as well as the reaction orders, we came to the conclusion that the CO2 is involved in the rate-determining step of the dry reforming of propane. The highest specific rates for the production of H2 and CO were measured for Rh/TiO2; this was explained by the extended dissociation of CO2 due to the electronic interaction between Rh and TiO2.

Decomposition of Propane and Its Reactions with CO2 Over Alumina-Supported Pt Metals

Propane underwent dehydrogenation and cracking on alumina-supported Pt metals at 823-923 K. Propylene formed with highest selectivity (50-80%) on Pt/Al2O3. The deposition of carbonaceous species was observed on every sample, the hydrogenation of which occurred only above 700-750 K with a peak temperature of 900-950 K. The presence of CO2 basically altered the reaction pathway of propane, and the formation of CO and H2 came into prominence. The highest specific rates for the production of H2 and CO were measured for the Ru and Rh catalysts.

Activity and Stability of a &lt;&lt;Pt/AIGaPON&gt;&gt; Oxynitride in the Dehydrogenation of Isobutane

Isobutane dehydrogenation was studied on platinum impregnated mixed aluminium gallium phosphorus oxide and oxynitride, in a continuous, flow micro-reactor at 500-550 degrees C. Comparison of the > and > shows the importance of nitridation on the acido-basic properties of the catalyst. A deactivation of the catalyst, due to the deposition of carbonaceous species on the surface,was observed. As the properties of the oxynitride would be altered by a regeneration treatment at high temperature with flowing oxygen, the possibility of decreasing the deactivation rate by decreasing the reaction temperature and by adding hydrogen to the reactant mixture was explored. Catalytic tests, carried out at different hydrogen partial pressures, showed that the hydrogen inhibits the carbon deposition on the surface of the catalyst and thus increases the catalytic stability.

Changes in porosity of graphite caused by radiolytic gasification by carbon dioxide

Methods have been developed to study porosity in nuclear grade graphite. The changes induced during the radiolytic gasification of graphite in carbon dioxide have been investigated. Porosity in radiolytically gasified graphite (0–22.8% wt. loss) was examined by optical microscopy and scanning electron microscopy (SEM). Each sample was vacuum impregnated with a slow-setting resin containing a fluorescent dye. Optical microscopy was used to study pores > 2 μm 2 c.s.a. A semi-automatic image analysis system linked to the optical microscope enabled pore parameter data including cross-sectional areas, perimeters, Feret's diameters and shape factors, to be collected. The results showed that radiolytic gasification produced a large increase in the number of pores <100 μ 2 c.s.a. New open pores <2 μm 2 c.s.a. were developed by gasification of existing open porosity into the closed porosity (<2 μm 2 c.s.a.) within the binder-coke. Open pores, 2–100 μm 2 c.s.a., which were gasified within the coarse-grained mosaics of the binder-coke. In the gasification process to 22.8% wt. loss, the apparent open pore volume increased from 6.6 to 33.8% and the apparent closed pore volumes decreased from ~3% to 0.1%. The increase in apparent open porosity from 6.6% (virgin) to 33.8% resulted from gasification within original open porosity and by the opening and development of closed porosity. There was no evidence for creation of porosity from within the “bulk” graphite, it being developed from existing fine porosity. The structure of pores > 100 μm 2 c.s.a. showed no change because of the inhibition of oxidation by deposition of carbonaceous species from the CH 4 inhibitor. Such species diffuse to the pore wall and are sacrificially oxidised.