Formation Of Carbonaceous Species Research Articles

Synthesis and Structure of Carbon‐doped TiO2 by Carbothermal Treatment

AbstractCarbon modified titanium dioxide (TiO2) is a promising candidate for catalytic applications or fuel cells, where the modified oxide could replace currently used catalyst support materials. Carbothermally treated TiO2 was successfully prepared by annealing under acetylene/nitrogen gas flow in a rotary tube furnace. The carbon content in the TiO2 samples ranged from 5 to 14.5 wt.‐% as determined by thermogravimetric measurements. The powders showed suppression of the phase transition from anatase to rutile up to a treatment temperature of 825°C. Above 600°C rutile is the thermodynamically stable phase, therefore the suppression must be attributed to either carbon in the lattice or the reducing atmosphere in the furnace. Raman spectra revealed the characteristic G and D bands, indicating the formation of carbonaceous species in the samples. In addition, a shift of the anatase Eg(1) band was observed indicating a lattice disorder pointing toward carbon incorporation into the lattice. Diffuse reflectance spectra show sub band gap absorption together with a shift of the absorption edge. Depending on the extraction method of band gaps from spectra, the band gap values show a decrease or increase with increasing carbon content. Details of the evaluation and interpretation of the spectra are discussed.

Operando DRIFT and In situ Raman Spectroscopic Studies on Aspects of CO2 Fischer‐Tropsch Synthesis Catalyzed by Bulk Iron Oxide‐Based Catalysts

AbstractHematite is a suitable precursor to obtain catalytically active iron (oxide) phases for CO2 Fischer‐Tropsch synthesis after a reductive pretreatment. As concluded from in situ Raman spectroscopy, in hydrogen atmosphere the transformation from α‐Fe2O3 to Fe3O4 is faster than the further reduction from Fe3O4 to Fe (metallic iron). The rate of these steps highly depends on the temperature. Starting from pure hematite, surface formate species are formed at a CO2/H2 gas mixture at elevated temperatures, observable by DRIFT‐spectroscopy. The exposure to CO2 and CO led to surface carbonate‐carboxylate surface species. Reduced samples with varying contents of Fe3O4 and Fe did not show any observable adsorbates at reaction conditions. The same behavior was found during the dosage of the single gases CO2 and CO to these reduced catalysts. The formation of carbonaceous species, detected by Raman spectroscopy, could indirectly hint to the occurrence of carbidation and was especially observed for a good performing catalyst with a medium reduction degree.

Theoretical insights into effect of surface composition of Pt-Ru bimetallic catalysts on CH3OH oxidation: mechanistic considerations

A deeper mechanistic understanding on CH3OH oxidation on Pt-Ru alloys with different Ru surface compositions is provided by DFT-based theoretical studies in this paper. The present results show that alloying and surface compositions of Ru can change CH3OH oxidation pathway and activity. The optimal surface composition of Ru is speculated to be ca. 50% since the higher Ru surface composition can lead to formation of carbonaceous species that can poison surface. Our present calculated Ru surface composition of ca. 50% exhibits excellent consistency with experimental studies. The origin of enhanced catalytic activity of Pt-Ru alloys is determined. The significantly decreased surface work functions after alloying suggest more electrons are transferred into adsorbates. The calculated lower electrode potentials after alloying imply that lower overpotentials are required for CH3OH oxidation. The excellent consistency with experimental study on decreased onset potentials after alloying further confirms accuracy of our present calculated results. It is hoped that a systematic understanding of the atomic- and molecular-level processes on CH3OH oxidation mechanisms on Pt-Ru alloys will result in the ultimate goal of the explanation of origin of enhanced electrocatalytic activity and design of improved Pt-based alloy electrocatalysts for DMFCs.

Emissions of Carbonaceous Particulate Matter and Ultrafine Particles from Vehicles-A Scientific Review in a Cross-Cutting Context of Air Pollution and Climate Change.

Airborne particulate matter (PM) is a pollutant of concern not only because of its adverse effects on human health but also on visibility and the radiative budget of the atmosphere. PM can be considered as a sum of solid/liquid species covering a wide range of particle sizes with diverse chemical composition. Organic aerosols may be emitted (primary organic aerosols, POA), or formed in the atmosphere following reaction of volatile organic compounds (secondary organic aerosols, SOA), but some of these compounds may partition between the gas and aerosol phases depending upon ambient conditions. This review focuses on carbonaceous PM and gaseous precursors emitted by road traffic, including ultrafine particles (UFP) and polycyclic aromatic hydrocarbons (PAHs) that are clearly linked to the evolution and formation of carbonaceous species. Clearly, the solid fraction of PM has been reduced during the last two decades, with the implementation of after-treatment systems abating approximately 99% of primary solid particle mass concentrations. However, the role of brown carbon and its radiative effect on climate and the generation of ultrafine particles by nucleation of organic vapour during the dilution of the exhaust remain unclear phenomena and will need further investigation. The increasing role of gasoline vehicles on carbonaceous particle emissions and formation is also highlighted, particularly through the chemical and thermodynamic evolution of organic gases and their propensity to produce particles. The remaining carbon-containing particles from brakes, tyres and road wear will still be a problem even in a future of full electrification of the vehicle fleet. Some key conclusions and recommendations are also proposed to support the decision makers in view of the next regulations on vehicle emissions worldwide.

Experimental and modeling-based analysis of reaction pathways on catalysts for natural gas engines under periodic lean/rich oscillations

Methane (CH4) has the prospective of becoming one of the major energy carriers for heavy-duty mobility, but its catalytic abatement remains a challenge. CH4 abatement reaction pathways were analyzed using targeted periodic lean/rich oscillations, which resulted in higher CH4 conversion than under static conditions. At both the lean-to-rich and rich-to-lean transitions of such oscillations, local maxima of CH4 concentration were followed by local minima during each lean and rich phase. The CH4 concentration profile during one oscillation displayed a 'W' shape. Four reaction regimes were identified corresponding to the four sections of the characteristic concentration shape. Entering the rich phase, Steam reforming (SR) started, which was then hampered by the formation of carbonaceous species and CO interaction with the catalytic surface. By entering the lean phase, SR is again enhanced as more active sites became available due to oxygen related reactions. However, increasing chemisorbed oxygen slowed down the oxidation of CH4. The transition between the consecutive regimes was linked to changes of the active Ce3+ coverage ratio, which plays an important role in SR reaction. Similar 'W' shapes were observed under various oscillation conditions and at different measurement points along the catalyst axis. The CH4 concentration profile was found to shift downwards with decreasing oscillation frequencies until a frequency threshold, which is marked by the equivalence of oscillation rich/lean phase length and the time needed to completely deplete/replenish the Oxygen storage capacity (OSC). The conversion characteristics were reproduced by numerical simulations, which revealed the spatial distribution of reactions in the catalyst under these dynamic conditions. Optimal overall CH4 conversion was reached for an even distribution of SR along the catalyst.

Synergies of Mn oxidative ability and ZSM-5 acidity for 1, 2-dichloroethane catalytic elimination

Four kinds of ZSM-5, γ-Al2O3, and commercial SiO2 loaded with 1 wt% Mn were prepared for selective catalytic oxidation of 1,2-dichloroethane. The 1 Mn/ZSM-5-27 (SiO2/Al2O3 = 27) catalyst showed the lowest T90 of 248 °C and the highest selectivities to HCl (67 %) and CO2 (31 %). By varying the Mn loading, it was found that 10 Mn/ZSM-5-27 exhibits the best performance owing to its suitable redox property associated with its high acidity. The formation of carbonaceous species, organic chloride, and metallic chlorine oxide (MnOxCly) on the surface is responsible for deactivation. The mechanism of catalytic oxidation of 1,2-dichloroethane was studied by in situ DRIFTS and GC–MS. Chloroethanol and vinyl alcohol were found to be the vital intermediates. Additionally, vinyl chloride can easily be attacked by chlorine radical to produce several kinds of undesirable polychlorinated ethylenes (C2HxCl4-x).

Bi-reforming of methane on Ni/SBA-15 catalyst for syngas production: Influence of feed composition

Bi-reforming of methane (BRM) was evaluated for Ni catalyst dispersed on SBA-15 support prepared by hydrothermal technique. BRM reactions were conducted under atmospheric condition with varying reactant partial pressure in the range of 10–45 kPa and 1073 K in fixed-bed reactor. The ordered hexagonal mesoporous SBA-15 support possessing large specific surface area of 669.5 m2 g−1 was well preserved with NiO addition during incipient wetness impregnation. Additionally, NiO species with mean crystallite dimension of 14.5 nm were randomly distributed over SBA-15 support surface and inside its mesoporous channels. Thus, these particles were reduced at various temperatures depending on different degrees of metal-support interaction. At stoichiometric condition and 1073 K, CH4 and CO2 conversions were about 61.6% and 58.9%, respectively whilst H2/CO ratio of 2.14 slightly superior to theoretical value for BRM would suggest the predominance of methane steam reforming. H2 and CO yields were significantly enhanced with increasing CO2/(CH4 + H2O) ratio due to growing CO2 gasification rate of partially dehydrogenated species from CH4 decomposition. Additionally, a considerable decline of H2 to CO ratio from 2.14 to 1.83 was detected with reducing H2O/(CH4 + CO2) ratio due to dominant reverse water-gas shift side reaction at H2O-deficient feedstock. Interestingly, 10%Ni/SBA-15 catalyst was resistant to graphitic carbon formation in the co-occurrence of H2O and CO2 oxidizing agents and the mesoporous catalyst structure was still maintained after BRM. A strong correlation between formation of carbonaceous species and catalytic activity was observed.

Theoretical investigation of conversion between different C2Hxspecies over Pd–Ag/Pd(100) surface alloys: influence on the selectivity and transformation of carbonaceous species

Full reaction pathways for acetylene hydrogenation on model catalysts are important for understanding the influence of ethylene selectivity and the formation of carbonaceous species.

Promotional mechanism of propane on selective catalytic reduction of NOx by methane over In/H-BEA at low temperature

Effects of propane/methane ratios on NOx reduction by mixtures of methane and propane over In/H-BEA catalyst were investigated at temperatures ranging from 250 to 550°C. The higher catalytic activity of In/H-BEA was exhibited for CH4-SCR at high temperatures above 450°C, while the higher NOx conversion was achieved in C3H8-SCR at below 425°C. A broadened temperature window and enhanced CO2 selectivity were achieved by combining of methane and propane as the co-reductant. The mixtures with propane/methane of 1/2 showed the most superior T50 (325°C) and T90 (500°C) temperatures for NOx reduction over In/H-BEA catalyst. For the promotion mechanism of propane on NO reduction by methane at low temperature, the formation of carbonaceous species (e.g. R-COOH) were enhanced by the activation of C3H8 on Brønsted acid sites at low temperature, and further promoted the generation of NCO species, which was a crucial determining step for NO reduction.

In‐situ Raman spectroscopy study of Ru/TiO2 catalyst in the selective methanation of CO

Raman spectroscopic technique has been used to characterize a Ru/TiO2 catalyst and to follow in situ their structural changes during the CO selective methanation reaction (S‐MET). For a better comprehension of the catalytic mechanism, the in‐situ Raman study of the catalysts activation (reduction) process, the isolated CO and CO2 methanation reactions and the effect of the composition of the reactive stream (H2O and CO2 presence) have been carried out. Raman spectroscopy evidences that the catalyst is composed by islands of TiO2–RuO2 solid solutions, constituting Ru–TiO2 interphases in the form of RuxTi1 − xO2 rutile type solid solutions. The activation procedure with H2 at 300 °C promotes the reduction of the RuO2–TiO2 islands generating Ruo–Ti3+ centers. The spectroscopic changes are in agreement with the strong increase in chemical reactivity as increasing the carbonaceous intermediates observed. The selective methanation of CO proceeds after their adsorption on these Ruo–Ti3+ active centers and subsequent C―O dissociation throughout the formation of CHx/CnHx/CnHxO/CHx―CO species. These intermediates are transformed into CH4 by a combination of hydrogenation reactions. The formation of carbonaceous species during the methanation of CO and CO2 suggests that the CO presence is required to promote the CO2 methanation. Similar carbonaceous species are detected when the selective CO methanation is carried out with water in the stream. However, the activation of the catalysts occurs at much lower temperatures, and the carbon oxidation is favored by the oxidative effect of water. Copyright © 2015 John Wiley & Sons, Ltd.

Catalytic conversion of trichloroethylene on nickel containing beta zeolites into value added products

Nickel containing BEA zeolites, NiSiBEA and NiHAlBEA, with 2wt% of Ni were prepared by a two-step postsynthesis method and conventional wet impregnation. The calcination of NiSiBEA and NiHAlBEA at 773K for 3h in air led to the formation of C-NiSiBEA and C-NiHAlBEA. After reduction at 873K for 3h in flowing 10% H2/Ar, red-C-NiSiBEA and red-C-NiHAlBEA were obtained and investigated as the catalysts in hydrodechlorination of trichloroethylene at 503K. Comparative study performed on red-C-NiSiBEA and red-C-NiHAlBEA showed very high stability of the catalyst prepared by a two-step postsynthesis method and strong deactivation of the catalyst prepared by conventional wet impregnation during ~17h of reaction. Application of both catalysts resulted in high selectivity to unsaturated hydrocarbons that are the most desired products in HDC processes. TPH of the catalysts after kinetic run have shown that the main cause of deactivation of NiHAlBEA is the formation of carbonaceous species during hydrodechlorination of trichloroethylene.

Regeneration of Silica‐Supported Silicotungstic Acid as a Catalyst for the Dehydration of Glycerol

The dehydration reaction of glycerol to acrolein is catalyzed by acid catalysts. These catalysts tend to suffer from the formation of carbonaceous species on their surface (coking), which leads to substantial degradation of their performances (deactivation). To regenerate the as-deactivated catalysts, various techniques have been proposed so far, such as the co-feeding of oxygen, continuous regeneration by using a moving catalytic bed, or alternating between reaction and regeneration. Herein, we study the regeneration of supported heteropolyacid catalysts. We show that the support has a strong impact on the thermal stability of the active phase. In particular, zirconia has been found to stabilize silicotungstic acid, thus enabling the nondestructive regeneration of the catalyst. Furthermore, the addition of steam to the regeneration feed has a positive impact by hindering the degradation reaction by equilibrium displacement. The catalysts are further used in a periodic reaction/regeneration process, whereby the possibility of maintaining long-term catalytic performances is evidenced.

Preferential oxidation of CO (CO-PROX) over CuO x/CeO 2 coated microchannel reactor

The general aspects of the synthesis and characterization results of a CuO x /CeO 2 catalyst were presented. In addition the principal steps for manufacturing a microchannel reactor and for the coating of the CuO x /CeO 2 catalyst onto the microchannels walls, were also summarized. The catalytic activity of this microchannel reactor during the preferential oxidation of CO (CO-PROX) was evaluated employing a feed-stream that simulates a reformate off-gas after the WGS unit. Two activation atmospheres were studied (H 2/N 2 and O 2/N 2). The reducing pretreatment improved the resistance to deactivation by formation of carbonaceous species over the catalyst surface at high temperatures. The presence of H 2O and CO 2 in the feed-stream was also analyzed indicating that the adsorption of CO 2 inhibited the conversion of CO at lower temperatures because these compounds modified the active sites through the formation of carbonaceous species on the catalyst surface. Finally, the experimental results of the microreactor performance were compared with CFD simulations that were carried out using a kinetic for the CuO x /CeO 2 powder catalyst. The experimental results were reasonably well described by the model, thus confirming its validity.

In situ characterization of carbonaceous deposits formed on chromia/zirconia during isobutane dehydrogenation

The formation of carbonaceous species during isobutane dehydrogenation on chromia/zirconia was studied by in situ diffuse reflectance Fourier transform infrared spectroscopy combined with mass spectrometry for on-line product gas analysis. The reactions of the hydroxyl groups of zirconia support with the hydrocarbons and the reduction of the chromates resulted in the formation of adsorbed acetone, formate and acetate. However, they were suspected not be involved in the dehydrogenation. The reduced chromia/zirconia catalysts were active in the dehydrogenation of isobutane but deactivated due to formation of coke. These unsaturated/aromatic hydrocarbon species were suggested to form through the adsorbed dehydrogenation intermediates or products.

Dichlorodifluoromethane Decomposition to CO2 with Simultaneous Halogen Fixation by Calcium Oxide Based Materials

The decomposition of CCl2F2 to CO2 and accompanying halogen fixation by a CaO based material was studied. To improve the low reactivity of CaO, a consequence of its low surface acidity, transition metal oxides were added. Impregnation of metal acetylacetonate followed by removal of the ligand under vacuum was found to be an effective method. This method resulted in the formation of carbonaceous species and the reduction of metal oxide to metal, both of which were thought to initiate the decomposition reaction. The reactivity of these materials (MOx(a)/CaO-vac) was found to be in the following order: M = Ni > Cu > V = Fe > Mn > Co > Ca. In particular, nickel supported on CaO was most effective for the decomposition of CCl2F2. During the preparation, nickel oxide was reduced to the metal phase. CCl2F2 was decomposed to CO2 with a small amount of CO, and halogens were fixed as CaFCl, without significant deactivation at 723 K.

Benzene Selective Oxidation with N 2 O on Fe/MFI Catalysts: Role of Zeolite and Iron Sites on the Deactivation Mechanism

The catalytic performances of Fe-zeolites having MFI structures and in which the Fe introduced either by ion exchange or during the hydrothermal synthesis has undergone partial framework to extra-framework migration induced by controlled heat treatment are reported. In particular, the catalytic behavior as function of time-on-stream and the formation of carbonaceous species were studied. The results suggest that only a small fraction of the iron is active in the selective oxidation of benzene to phenol in the presence of N2O. It is suggested that the active fraction is formed by isolated iron ions in a pseudo-octahedral configuration with the sites positioned in hydroxyl nests (defects) of the zeolite and is selective in phenol formation as a result of in situ reduction during the catalytic tests. Two possible pathways of carbonaceous species were identified, the first through the intermediate further hydroxylation of phenol and the second through the coupling of phenol with benzene or another phenol molecule. This second pathway is the dominant mechanism of formation of carbonaceous species, although the relative rate of the two pathways depends on the zeolite characteristics and iron loading. It is also suggested that the second pathway depends on the strong chemisorption of phenol, probably on Lewis acid sites, which hinders the fast back-desorption of phenol out from the zeolite channels and thus favors the formation of carbonaceous species. Catalysts prepared by hydrothermal treatment show a lower rate of deactivation than those prepared by ion exchange, although the latter show a comparable productivity to phenol for amounts of iron in extra-framework positions around 20 to 30 times lower. The results also indicate that the presence of Al in the zeolite framework is beneficial for reducing the rate of deactivation as compared to that of Fe-silicalite samples.

Marked Role of Carbon Monoxide in the Formation of Benzene During CH4-CO Reaction Over Rh/SiO2 Catalysts

Temperature-programmed desorption (He-TPD) and temperature-programmed reaction with hydrogen (H2-TPR), carbon monoxide (CO-TPR) or methane (CH4-TPR) were carried out to elucidate the benzene formation mechanism as well as the role of CO during CH4-CO reaction over SiO2-supported Rh catalysts. The steady-state surface for the CH4-CO reaction was different from that of the CH4 decomposition reaction. The existence of benzene-like adsorbed species as “building blocks” was demonstrated on the CH4-CO reaction surface, while no such higher hydrocarbon adsorbed species was detected in the case of the CH4 decomposition surface. On the contrary, in CO-TPR experiments various unsaturated hydrocarbons were released from the steady-state CH4 decomposition surface, which was not the case from the CH4-CO reaction surface. It is concluded that adsorbed CO may play an important role to enhance the C-C bond formation of carbonaceous species, which correlates deeply with the novel phenomenon of selective benzene formation in the CH4-CO reaction.

Direct Oxidation of Propylene with Molecular Oxygen Over Ti-Modified Sulfated Zirconia Catalyst, 2. In Situ Diffuse Reflectance FT-IR Study

During the reaction of propylene with O2, in situ Diffuse Reflectance FT-IR measurements were performed over Ti-modified SZ and SZ catalysts. Without O2, the main bands characteristic of (branched) hydrocarbons, formed by oligomerization leading to, finally, carbonaceous residue, appeared within the range of 3900-2750 cm-1, which was affected by the bands of surface OH groups. Investigation of these IR bands showed the role of molecular oxygen not only to limit the formation of carbonaceous species on the catalyst surface, but also to form oxygenates and these findings were in good agreement with the results of catalytic reaction.

Influence of the Hydrogenation Step on Selectivity during the Nonoxidative Oligomerization of Methane to Alkanes on Pt/SiO2Catalysts (EUROPt-1)

Methane oligomerization to alkanes can be accomplished on supported platinum via a two-step procedure: formation of carbonaceous species on the metallic surface by methane adsorption, followed by hydrogenation of these species. Temperature-programmed oxidation (TPO) experiments performed after hydrogenation steps of various durations show that the hydrogenation of a carbonaceous deposit obtained at 300°C on the reference Pt/SiO2catalyst EUROPt-1 is not a fast process. Two groups of surface carbonaceous species have been characterized through their different reactivities toward oxygen, but at 300°C their reactivities toward hydrogen are similar. Among alkanes up to C5, methane is the main product of hydrogenation, corresponding to one-half of the surface carbon reactive toward hydrogen; linear and branched alkanes are produced from the other half of the reactive carbonaceous species. On EUROPt-1, mainly ethane andn-pentane are produced during the first minutes of reaction, while on a sintered catalyst the initial production inn-pentane is negligible. The release ofn-pentane during an intermediate purge with inert gas on EUROPt-1 shows that C–C bonds can form already during methane adsorption, leading to C5precursors on specific active sites of this catalyst maybe coordinately unsaturated platinum atoms. A model of formation of C5precursors is proposed by analogy with the organometallic chemistry of molecular hydrocarbon platinacycles. The subsequent production of alkanes (C2>C3>C4>C5) could be described through a statistical model of dynamic coupling between carbonaceous species involving hydrogen, rather than by hydrogenolysis of heavier carbonaceous species. However, this latter mechanism is likely to predominate for the production of C6–C8compounds.

The Formation of Carbonaceous Species from Carbonmonoxide and from Carbonmonoxide/Hydrogen on Cobalt Catalysts. Indications of Metal‐Support Interaction

AbstractThe reaction of carbon monoxide and carbon monoxide/hydrogen mixtures with two catalysts, a pure cobalt catalyst and a cobalt on alumina catalyst has been studied, mainly in the thermobalance. It appears that the support prevents carbon diffusion into the cobalt crystallites, whereas on the pure cobalt catalysts the carbide Co3C is formed as the first product. Hydrogen efficiently prevents the formation of carbides in the pure cobalt catalysts; on the supported catalysts relatively large quantities of intermediates are deposited from carbon monoxide/hydrogen mixtures. The observed effects are explained by an interaction between alumina and cobalt that reduces the cobalt‐carbon bond strength and the hydrogen activity on the supported catalyst.