Effect of co-feeding carbon dioxide on Fischer–Tropsch synthesis over an iron–manganese catalyst in a spinning basket reactor

The effect of co-feeding CO₂ on the catalytic properties of an Fe–Mn catalyst during Fischer–Tropsch synthesis (FTS) was investigated in a spinning basket reactor by varying added CO₂ partial pressure in the feed gas. It was found that co-feeding CO₂ to syngas did not decrease the activity of the catalyst, on the contrary, a dramatic increase of the activity and an increase of methane selectivity were observed over the catalyst after removal of CO₂ from the feed gas. The addition of CO₂ led to an increase in olefin/paraffin ratios of low carbon hydrocarbons and a slight decrease in C₁₉⁺ selectivity. It also slightly decreased CO₂ formation rate on the catalyst by increasing the rate of reverse step of the water–gas shift (WGS) reaction and pushing the reaction towards equilibrium, and did not remarkably influence the hydrocarbon formation rate. However, the co-feeding CO₂ can significantly increase the water formation rate and the overall oxygenate formation rate under these reaction conditions.     

CO2 absorption rate in an algal culture: Effect of pH

In a CO₂-limited algal culture, grown in a tubular loop photobioreactor, the maximum rate of CO₂ absorption increased about 1.5-fold when the culture pH value was increased from 6.5 to 7.5 with a fixed initial P_CO2. The mean volumetric CO₂ transfer coefficient (K_La) increased about 1.8-fold. The bicarbonate ion concentration would be increased 10-fold by the pH increase. The effect of pH on the absorption rate is attributed to changes in either the CO₂ diffusivity, the gas bubble size, or the CO₂ reaction kinetics at the gas/liquid boundary.     

Reduction of CO2 on bismuth modified Pt(1 1 0) single-crystal surfaces. Effect of bismuth and poisoning intermediates on the rate of hydrogen evolution

The reduction of CO₂ on bismuth modified Pt(1 1 0) single crystal surfaces has been studied voltammetrically. The effect of bismuth and the amount of formed CO species on the rate of hydrogen evolution has also been investigated. A decrease in the rate of CO₂ reduction is observed due to the modification of the surface with bismuth adsorption. This decrease goes beyond the simple third body effect expected from the blockage of active sites on the platinum surface after bismuth adsorption. However, the hydrogen evolution reaction is relatively insensitive to the presence of adsorbed species, in contrast with previous result reported for Pt(1 1 1) and Pt(1 0 0) surfaces.     

CH4/CD4 isotope effect on the reaction of adsorbed hydrocarbon species in CO2-reforming over Ni/Al2O3 catalyst

By replacing CH₄ with CD₄, the isotope effect on the reaction of adsorbed hydrocarbon species with CO₂ over Ni/Al₂O₃ catalyst was studied using pulse surface reaction rate analysis (PSRA). The first-order rate constant for this step was 1.45 times larger for CH₄ than for CD₄. The observed isotope effect suggests that the reaction of adsorbed hydrocarbon species with CO₂ (or adsorbed oxygen) is rate-controlling for the reforming of CH₄.     

The phenol steam reforming reaction towards H2 production on natural calcite

The steam reforming of phenol towards H₂ production was studied in the 650–800 °C range over a natural pre-calcined (air, 850 °C) calcite material. The effects of reaction temperature, water, hydrogen, and carbon dioxide feed concentrations, and gas hourly space velocity (GHSV, h⁻¹) were investigated. The increase of reaction temperature in the 650–800 °C range and water feed concentration in the 40–50 vol% range were found to be beneficial for catalyst activity and H₂-yield. A similar result was also obtained in the case of decreasing the GHSV from 85,000 to 30,000 h⁻¹. The effect of concentration of carbon dioxide and hydrogen in the phenol/water feed stream was found to significantly decrease the rate of phenol steam reforming reaction. The latter was probed to be related to the reduction in the rate of water dissociation as evidenced by the significant decrease in the concentration of adsorbed bicarbonate and OH species on the surface of CaO according to in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)-CO₂ adsorption experiments in the presence of water and hydrogen in the feed stream. Details of the CO₂ adsorption on the CaO surface at different reaction temperatures and gas atmospheres using in situ DRIFTS and transient isothermal adsorption experiments with mass spectrometry were obtained. Bridged, bicarbonate and unidentate carbonate species were formed under CO₂/H₂O/He gas mixtures at 600 °C with the latter being the most populated. A substantial decrease in the surface concentration of bicarbonate and OH species was observed when the CaO surface was exposed to CO₂/H₂O/H₂/He gas mixtures at 600 °C, result that probes for the inhibiting effect of H₂ on the phenol steam reforming activity. Phenol steam reforming reaction followed by isothermal oxygen titration allowed the measurement of accumulated “carbonaceous” species formed during phenol steam reforming as a function of reaction temperature and short time on stream. An increase in the amount of “carbonaceous” species with reaction time (650–800 °C range) was evidenced, in particular at 800 °C (4.7 vs. 6.7 mg C/g solid after 5 and 20 min on stream, respectively).     

On-line extraction-reaction of canola oil using immobilized lipase in supercritical CO2

Lipase-catalyzed hydrolysis of canola oil in supercritical CO₂ (SCCO₂) was studied as a model reaction to develop an on-line extraction–reaction process to extract oil from oilseeds and convert the oil to other valuable products using SCCO₂. Immobilized lipase from Mucor miehei was used as the catalyst and the process was carried out at 24 MPa and 35°C. Product composition was analyzed using supercritical fluid chromatography. The effect of enzyme load, CO₂ flow rate and canola flake load on the amount of product and its composition was investigated. Hydrolysis occurred to a larger extent to free fatty acids and glycerol with an increase in enzyme load, a decrease in CO₂ flow rate or a decrease in canola load. On-line extraction-reaction process using SCCO₂ shows great potential for new process design to obtain products from agricultural commodities for use as ingredients in food and other industries.     

High-pressure phase equilibrium for the hydroformylation of 1-octene to nonanal in compressed CO2

The hydroformylation reaction in supercritical carbon dioxide or CO₂-expanded liquids (CXLs) has many advantageous properties. However, accurate phase behavior and equilibrium must be known to properly understand and engineer these systems. In this investigation, the vapor-liquid equilibrium and mixture critical points of CO₂ systems with 1-octene, nonanal, 1-octene and nonanal mixtures, and mixtures of 1-octene, nonanal and syngas (CO/H₂) were measured at 60 °C up to 120 bar of pressure. The Peng-Robinson equation of state with van der Waals two-parameter mixing rule was employed successfully to correlate the binary mixture data and predict the ternary mixture data. The presence of CO/H₂ pressure increased the mixture critical points and decreased the volume expansion at any given pressure. In an actual reaction, the mixture critical point would increase throughout the reaction, while the volume of the liquid phase would decrease. These data will aid the understanding and reaction engineering for the hydroformylation reaction in CO₂-expanded liquids and supercritical fluids.     

Effects of zirconia promotion on the activity of Cu/SiO2 for methanol synthesis from CO/H2 and CO2/H2

The effect of zirconia promotion on Cu/SiO₂ for the hydrogenation of CO and CO₂ at 0.65 MPa has been investigated at temperatures between 473 and 573 K. With increasing zirconia loading, the rate of methanol synthesis is greatly enhanced for both CO and CO₂ hydrogenation, but more significantly for CO hydrogenation. For example, at 533 K the methanol synthesis activity of 30.5 wt% zirconia-promoted Cu/SiO₂ is 84 and 25 times that of unpromoted Cu/SiO₂ for CO and CO₂ hydrogenation, respectively. For all catalysts, the rate of methanol synthesis from CO₂/H₂ is higher than that from CO/H₂. The apparent activation energy for methanol synthesis from CO decreases from 22.5 to 17.5 kcal/mol with zirconia addition, suggesting that zirconia alters the reaction pathway. For CO₂ hydrogenation, the apparent activation energies (~12 kcal/mol) for methanol synthesis and the reverse water-gas shift (RWGS) reaction are not significantly affected by zirconia addition. While zirconia addition greatly increases the methanol synthesis rate for CO₂ hydrogenation, the effect on the RWGS reaction activity is comparatively small. The observed effects of zirconia are interpreted in terms of a mechanism which zirconia serves to adsorb either CO or CO₂, whereas Cu serves to adsorb H₂. It is proposed that methanol is formed by the hydrogenation of the species adsorbed on zirconia.     

Analysis of Plasma Reaction for Decomposition of CHF3 in a Dielectric Barrier Discharge

The decomposition of CHF₃ in a mixture with O₂ and Ar was investigated in a coaxial dielectric barrier discharge at atmospheric pressure. CHF₃ decomposition increased linearly in regard to specific energy input (SEI), whereas energy efficiency decreased. The main product was CO_2, and its selectivity increased with high SEI and the presence of O₂ in the feed, but an increase of O₂ in the feed led to a decrease in decomposition rate. An increase in total flow rate led to an increase of the absolute amount of CHF₃ decomposition and energy efficiency; however, the decomposition of CHF₃ decreased. A complete CHF₃ decomposition occurred under an SEI of 1.54 kJ/L with the selectivity of CO₂ and CO as 89.87% and 7.00%, respectively. Optical emission spectroscopic analysis could explain the available reaction pathways for CHF₃ decomposition in the CHF₃/O₂/Ar atmospheric plasma and show the possibility of F₂ and HF formation.     

CO2 corrosion of carbon steel in the presence of acetic acid at higher temperatures

The corrosion behaviour of X 65 carbon steel in the presence of acetic acid in N₂- and CO₂-saturated systems has been investigated using electrochemical techniques. The presence of acetic acid does not influence the anodic reaction but strongly accelerates the cathodic reaction. The cathodic reaction and consequently the corrosion rate of mild steel in the CO₂-saturated system increase with increase in acetic acid concentration and temperature. From the values of the apparent activation energies, the corrosion reaction in the absence of acetic acid was found to be under mixed interfacial reaction/diffusion control while interfacial reaction control dominates in the presence of acetic acid. The reduction of adsorbed undissociated acetic acid on the metal surface is proposed as the key species primarily responsible for accelerated corrosion rate at all temperatures.     

Isotope Effects in Methanol Synthesis and the Reactivity of Copper Formates on a Cu/SiO2 Catalyst

Here we investigate isotope effects on the catalytic methanol synthesis reaction and the reactivity of copper-bound formate species in CO₂–H₂ atmospheres on Cu/SiO₂ catalysts by simultaneous IR and MS measurements, both steady-state and transient. Studies of isotopic variants (H/D, ¹²C/¹³C) reveal that bidentate formate dominates the copper surface at steady state. The steady-state formate coverages of HCOO (in 6 bar 3:1 H₂:CO₂) and DCOO (in D₂:CO₂) are similar and the steady-state formate coverages in both systems decrease by ~80% from 350 K to 550 K. Over the temperature range 413 K–553 K, the steady-state methanol synthesis rate shows a weak H/D isotope effect (1.05 ± 0.05) with somewhat higher activation energies in H₂:CO₂ (79 kJ/mole) than D₂:CO₂ (71 kJ/mole) over the range 473 K–553 K. The reverse water gas shift (RWGS) rates are higher than methanol synthesis and also shows a weak positive H/D isotope effect with higher activation energy for H₂/CO₂ than D₂/CO₂ (108 vs. and 102 kJ/mole) The reactivity of the resulting formate species in 6 bar H₂, 6 bar D₂ and 6 bar Ar is strongly dominated by decomposition back to CO₂ and H₂. H₂ and D₂ exposure compared to Ar do not enhance the formate decomposition rate. The decomposition profiles on the supported catalyst deviate from first order decay, indicating distributed surface reactivity. The average decomposition rates are similar to values previously reported on single crystals. The average activation energies for formate decomposition are 90 ± 17 kJ/mole for HCOO and 119 ± 11 kJ/mole for DCOO. By contrast to the catalytic reaction rates, the formate decomposition rate shows a strong H/D kinetic isotope effect (H/D ~8 at 413 K), similar to previously observed values on Cu(110).     

Carbon dioxide pressure induced heterogeneous and homogeneous Heck and Sonogashira coupling reactions using fluorinated palladium complex catalysts

The Heck reaction of iodobenzene and methyl acrylate was investigated with CO₂-philic Pd complex catalysts having fluorous ponytails and the organic base triethylamine (Et₃N) in the presence of CO₂ under solventless conditions at 80 °C. The catalysts are not soluble in the organic phase in the absence of CO₂ and the reaction occurs in a solid-liquid biphasic system. When the organic liquid mixture is pressurized by CO₂, CO₂ is dissolved into the organic phase and this promotes the dissolution of the Pd complex catalysts. As a result, the Heck reaction occurs homogeneously in the organic phase, which enhances the rate of reaction. This positive effect of CO₂ pressurization competes with the negative effect that the reacting species are diluted by an increasing amount of CO₂ molecules dissolved. Thus, the maximum conversion appears at a CO₂ pressure of around 4 MPa under the present reaction conditions. The catalysts are separated in the solid granules by depressurization and are recyclable without loss of activity after washing with n-hexane and/or water. When the washing is made with hexane alone, the catalytic activity tends to increase on the repeated Heck reactions, probably due to the accumulation of such a base adduct as Et₃NHI on the catalysts. When the washing is further made with water, however, the base adduct is taken off from the catalysts and they show similar activity levels in the repeated runs. The potential of CO₂ pressure tunable heterogeneous/homogeneous reaction system has also been investigated for Sonogashira reactions of iodobenzene and phenylacetylene under similar conditions.     

Effect of process parameters on circulating fluidized bed coal gasification using 3D full-loop CFD simulation

This article presents a 3D full-loop computational fluid dynamics (CFD) simulation of a circulating fluidized bed gasifier (CFBG). The simulation results are validated against the experimental data and found to be in good agreement. Thereupon, the effect of the process parameters, ie, temperature, pressure, air/coal (A/C) ratio, and steam/coal (S/C) ratio, on the performance of the gasifier is analyzed. The effect of temperature on the hydrodynamics was found to be small. The CO and H₂ increase, whereas the CO₂ and H₂O decrease with an increase in temperature. While the effect of pressure on the outlet species mole fraction is negligible, the gas and solid axial velocity decrease with an increase in pressure. With an increasing A/C ratio or decreasing S/C ratio, the combustion products (CO₂ and H₂O) increase, whereas the gasification products (CO and H₂) decrease due to the increase in the O₂ concentration. In addition, temperature increases with an increase in the A/C ratio or a decrease in the S/C ratio. The feed velocity increases with an increasing A/C or S/C ratio, and, accordingly, the pressure increases and bed height decreases. The CH₄ decreases in all of the cases as it is being consumed in gasification as well as combustion reactions.     

Molecular beam mass spectrometry investigations of low temperature diamond growth using CO2/CH4 plasmas

Diamond films have been successfully deposited at substrate temperatures as low as 435°C using CO₂/CH₄ gas mixtures in a microwave plasma chemical vapour deposition (CVD) reactor. In order to understand why it is possible to grow diamond at these low temperatures using these gases, we have performed the first in situ molecular beam mass spectrometry studies to measure, simultaneously, the concentrations of the dominant gas phase species present during growth over a wide range of plasma gas mixtures (0–80% CH₄, balance CO₂). Optical emission spectroscopy has also been used to investigate gas phase species present in the microwave plasma. These experimental measurements give further evidence that CH₃ radicals may be the key growth species and suggest that CO may be of greater importance to the plasma chemistry of CO₂/CH₄ gas mixtures than previously thought.     

Selective Removal of Carbon Dioxide from Wet CO2/H2 Mixtures via Facilitated Transport Membranes containing Amine Blends as Carriers

The selective separation of carbon dioxide (CO₂) from a wet gaseous mixture of CO₂/H₂ through facilitated transport membranes containing immobilized aqueous solutions of monoethanolamine (MEA), diethanolamine (DEA), ethylenediamine (EDA) and monoprotonated ethylenediamine (EDAH⁺) and their blends was experimentally investigated. The effect of CO₂ partial pressure, amine concentration, feed side pressure and amine species on the CO₂ and H₂ permeances were studied. The CO₂ permeability through amine solution membranes decreased with increasing CO₂ feed partial pressure but the H₂ permeance was almost independent of the H₂ partial pressure. A comparison of experimental results showed that single or blended amines with low viscosity and a moderate equilibrium constant, i.e., large forward and reverse reaction rate of CO₂‐amine, are suitable for effective separation of CO₂. The permeability of CO₂ generally increased with an increase in amine concentration, although this increase may be compromised by the salting out effect and decrease in diffusivities of species. The results obtained indicated that CO₂ permeance across a variety of amines are in the order of DEA (2 M) > MD (2 M) > MD (1 M) > MEA (2 M) > MEA (4 M) > MD (4 M) > DEA (1 M) > DEA (4 M) > MEA (1 M) for various concentrations of MEA + DEA blend and are in the order of EDAH⁺ (2 M) > DEA (2 M) > MH (2 M) > DH (2 M) > ED (2 M) > EDA (2 M) > MEA (2 M) for various blends of amine.     

Decomposition of phenols in aqueous solution by a UV/O3 process

The decomposition of several non‐biodegradable phenols by the UV/O₃ and ozonation processes was studied and compared under various solution pH values, O₃ input mass flow rates and UV intensities to investigate the removal efficiencies of reactants and organic intermediates. The decomposition rate of phenols by the UV/O₃ process was found to increase with increasing O₃ input dosage, light intensity and solution pH value. The mineralization efficiencies of phenols in aqueous solution would be above 98% under adequate reaction conditions within three hours, but would be retarded for alkaline solutions because of the dissolution of CO₂ formed by mineralization of phenols. The increment of ozone input dosage had little effect on the mineralization of organic intermediates at the latter course of the reaction. The order of the decomposition rate of the phenols used in this research was 2,4‐dichlorophenol > 2‐chlorophenol > 2‐nitrophenol for low and neutral pH solutions, whereas they were nearly alike for alkaline solutions. The two‐step consecutive kinetic model was found to fit well in modeling the behavior of species during the decomposition of phenols in aqueous solutions by the UV/O₃ process.     

Spatiotemporal patterns in a porous catalyst during light-off and quenching of carbon monoxide oxidation

A detailed numerical model was used to simulate the behavior of carbon monoxide oxidation within a porous platinum/alumina catalyst during temperature ramps. The model was validated in previous work by fitting step-response experiments which were performed over a range of temperatures and in which concentration gradients over the catalyst layer were directly measured. As a result of the low CO and O₂ concentrations used, the catalyst layer could be considered isothermal. The numerical experiments performed with the model in this work reveal complex spatial patterns of species and local reaction rate which change with time and temperature.As temperature is increased, CO desorbs and reaction rapidly increases, reacting adsorbed CO off the Pt surface and producing a peak in CO₂ production during catalyst light-off. Over a nonporous surface of the same material, the reaction rate would be an order-of-magnitude lower and no CO₂ peak would be produced. At steady state after reaction light-off has been obtained, reaction occurs in a narrow zone below the external face of the layer which is exposed to the constant feed gas composition. As temperature is then decreased, the CO₂ production rate decreases gradually as the front of the region covered with adsorbed CO penetrates further and pushes the reaction zone deeper into the catalyst layer. When the adsorbed CO front reaches the internal face, the CO₂ production rate drops abruptly as the reaction “quenches”.Catalyst layer thickness was changed over the range 0.06-1.0 mm at constant total Pt content. As the layer thickness was decreased, the steady-state CO₂ production rate after light-off increased, however the range of temperatures in which the catalyst was active decreased. Three qualitatively different sets of spatiotemporal patterns were obtained as the layer thickness was changed from relatively thin, to medium, to thick. Analysis of the patterns provides understanding of the temperature-dependent behavior of the catalyst and how this behavior varies with catalyst layer thickness.     

The absorption of CO2 by amine-potash solutions

CO₂ has been absorbed in a stirred vessel into aqueous solutions of MEA and DEA containing K₂CO₃, K₂SO₄ and Na₂SO₄ at 25° and 11°C. While electrolytes in general appear to increase the rate of reaction between CO₂ and both MEA and DEA, K₂CO₃ increases the rate of reaction of DEA much more than that of MEA. The addition of K₂CO₃ to solutions of DEA increases the absorption rate, in spite of the decrease in the solubility and diffusivity of CO₂. The observations help to explain why DEA is an effective promoter for the absorption and desorption of CO₂ by hot, concentrated potash solutions.     

Kinetic study of the catalytic reforming of methane with carbon dioxide to synthesis gas over Ni/La2O3 catalyst

The kinetic behavior of the Ni/La₂O₃ catalyst in the reforming reaction of methane with carbon dioxide was investigated as a function of temperature and partial pressures of CH₄ and CO₂. The apparent activation energy of the reforming reaction was estimated to be 13.2 kcal/mol. It was also found that increase of the H₂ partial pressure leads to a continuous enhancement of the rate of CO formation, due to the simultaneous occurrence of the water-gas shift reaction. The mechanism of the CH₄/CO₂ reaction has been investigated using steady-state isotopic tracing and transient experiments, as well as FTIR, XRD, XPS and HR-TEM techniques. Based on the mechanistic results, a kinetic model was developed, which was found to predict satisfactorily the kinetic measurements. Methane cracking and the surface reaction between C and oxycarbonate species, are suggested to be the rate determining steps of the CH₄/CO₂ reaction over the Ni/La₂O₃ catalyst.     

Cobalt Particle Size Effects in the Fischer–Tropsch Synthesis and in the Hydrogenation of CO2 Studied with Nanoparticle Model Catalysts on Silica

We have investigated the effect of cobalt nanoparticle size in Fischer–Tropsch synthesis (CO/H₂) and have compared it to data obtained for carbon dioxide hydrogenation (CO₂/H₂) using model catalysts produced by colloidal methods. Both reactions demonstrated size dependence, in which we observed an increase of the turnover frequency with increasing average particle size. In both case, a maximum activity was found for cobalt particles around 10–11 nm in size. Regarding the selectivity, no size-dependent effect has been observed for the CO₂ hydrogenation, whereas CO hydrogenation selectivity depends both on the temperature and on the size of the particles. The hydrogenation of CO₂ produces mainly methane and carbon monoxide for all sizes and temperatures. The Fischer–Tropsch reaction exhibited small changes in the selectivity at low temperature (below 250 °C) while at high temperatures we observed an increase in chain growth with the increase of the size of cobalt particles. At 250 °C, large crystallites exhibit a higher selectivity to olefin than to the paraffin equivalents, indicating a decrease in the hydrogenation activity.