Various Partial Pressures Of CO Research Articles

Kinetics of CO Oxidation over Unloaded and Pd-Loaded α-Fe2O3 Spherical Submicron Powder Catalysts: Photoacoustic Investigations at Low Pressure

In this study, α-Fe2O3 spherical particles with an average diameter of approximately 200 nm were synthesized by a solvothermal method for use as both a catalyst and medium for a Pd catalyst. The kinetics of CO oxidation over powders of α-Fe2O3 spherical particles and 14 wt % Pd/α-Fe2O3 spherical particles were measured in a static reactor by using a CO2 laser-based photoacoustic technique. The total pressure was fixed at 40 Torr for the CO/O2/N2 mixture for temperatures in the range of 225–350 °C. The variation in the CO2 photoacoustic signal with the CO2 concentration during CO oxidation was recorded as a function of time, and the CO2 photoacoustic data at the early reaction stage was used to estimate the rates of CO2 formation. Based on plots of ln(rate) vs. 1/T, apparent activation energies were calculated as 13.4 kcal/mol for the α-Fe2O3 submicron powder and 13.2 kcal/mol for the 14 wt % Pd/α-Fe2O3 submicron powder. Reaction orders with respect to CO and O2 were determined from the rates measured at various partial pressures of CO and O2 at 350 °C. The zero-order of the reaction with respect to Po2 was observed for CO oxidation over α-Fe2O3 submicron powder, while 0.48 order to Po2 was observed for CO oxidation over Pd/α-Fe2O3 submicron powder. The partial orders with respect to PCO were determined as 0.58 and 0.54 for the α-Fe2O3, and the Pd/α-Fe2O3 submicron powders, respectively. The kinetic results obtained from both catalysts were compared with those for the α-Fe2O3 fine powder catalysts and were used to understand the reaction mechanism.

Kinetics of Fischer-Tropsch synthesis on supported cobalt: Effect of temperature on CO and H2 partial pressure dependencies

Kinetic data were measured for Fischer-Tropsch Synthesis (FTS) on a cobalt catalyst supported on silica-modified alumina at various partial pressures of CO and H2 and at four different temperatures (210, 220, 230, and 240°C). The data were sufficient to determine power law rate expressions at each of the four temperatures which indicate that the dependence of rate on PH2 increases with increasing temperature while the rate coefficient for PCO decreases with temperature going from positive order (+0.3) at 210°C to negative order (−0.6) at 240°C. Several mechanisms were explored and Langmuir–Hinshelwood (LH) rate models derived in an attempt to explain the data trends. Traditional FT rate expressions have a denominator term, KCO*PCO which, since the denominator is squared in LH expressions, can allow for an overall negative order PCO dependence. However, since KCO is the equilibrium constant for the adsorption of CO, its value decreases with increasing temperature which causes the overall PCO dependence to become more positive with increasing temperature instead of more negative. In this work we propose a mechanism based on parallel hydrogen-assisted mechanistic pathways that leads to a LH model with the denominator term, k’CO*PCO where k’CO is effectively an activated rate constant instead of an equilibrium constant and therefore increases with increasing temperature instead of decreasing. This model, which fits the data extremely well, also explains the presence of atomic carbon on the surface of the catalyst.

Influence of CO 2 on the oxygen permeation performance of perovskite-type BaCo xFe yZr zO 3− δ hollow fiber membranes

The influence of CO 2 on the oxygen permeation performance of perovskite-type BaCo x Fe y Zr z O 3−δ (BCFZ, x + y + z = 1) hollow fiber membrane under different experimental conditions is presented. Helium with various partial pressures of CO 2 was applied as sweep gas at different temperatures on the shell side of the hollow fiber leading to a decrease and finally drop to zero of the oxygen permeation while feeding air on the core side due to carbonate formation. Repeated cycles of changing the sweep gas between the CO 2/He mixture and pure He were conducted in order to check the reversibility of the carbonate formation. The analysis of the microstructure after permeation experiments was carried out by X-ray diffraction (XRD) as well as by scanning electron microscope (SEM). It was found that both microstructure as well as oxygen permeation are recovered in a CO 2-free atmosphere like pure He on the shell side while feeding air on the core side. Under exposure of 50% CO 2 in He for 5 h, the perovskite structure is impaired up to a depth of ca. 15 μm and approximately 30 μm after 10 h, respectively.

Photoacoustic investigations of CO 2–CH 4 reaction catalyzed by nickel particles embedded into SBA15 mesopores

A photoacoustic spectroscopy technique was employed to the kinetic study of the CO 2/CH 4 reaction catalyzed by Ni particles embedded into the mesochannels of SBA15. The catalytic CO 2/CH 4 reaction was performed in a closed-circulating reactor system at various partial pressures of CO 2 and CH 4 (40 Torr total pressure) in the temperature range of 500–700 °C. The CO 2 photoacoustic signal that varied with the concentration of CO 2 during the catalytic reaction was recorded as a function of time by using a differential photoacoustic cell. Under the reaction conditions, the CO 2 photoacoustic measurements showed the SBA15 compound used as support to be inactive for the reaction. The CO 2/CH 4 reaction carried out in the presence of the H 2-reduced Ni/SBA15 catalyst showed significant time-dependent changes in the CO 2 photoacoustic signal, while the reaction performed in the presence of the as-prepared Ni/SBA15 catalyst did not. The CO 2 photoacoustic signal obtained at early reaction times provided precise data of the CO 2 disappearance rate. The rate of CO 2 disappearance was observed to increase with increasing temperature in the range of 500–700 °C. The apparent activation energy for the CO 2 consumption in the Ni/SBA15 catalyzed reaction was calculated to be 6.2 kcal/mol. Reaction orders, determined from initial rates of CO 2 disappearance at various P C O 2 ’s and P C H 4 ’s at 700 °C, were found to be 0.28 for CH 4 and 0.39 for CO 2, respectively. The kinetic results were compared with those previously reported and were used to infer a catalytic reaction mechanism for the CO 2/CH 4 reaction at low pressures.

Determination of open-circuit potentials at gas/electrode/YSZ boundary versus molten carbonate reference electrode at medium temperatures: I. Potentials of Au and Pt in O 2 and H 2 + H 2O atmospheres

A double gas concentration cell as combination of the cell with the yttria stabilized zirconia (YSZ) electrolyte and the cell with molten Li 2CO 3 + Na 2CO 3 eutectics is proposed as an alternative cell system with a standard reference electrode for measurements of the open-circuit potential (OCP) values of electrodes in oxygen concentration cell with the yttria stabilized zirconia (YSZ) electrolyte. In this double-cell one electrode is common for the two cells and the reference electrode is the standard molten carbonate half-cell with 0.33O 2 + 0.67CO 2 atmosphere. This reference electrode should enable the monitoring of OCP and overpotential values in polarization studies in the three-electrodes configuration. If the possible reaction between the solid YSZ and liquid molten carbonates electrolyte is very slow, the measured values of the open-circuit-voltage (OCV) of this cell may be considered equal to the respective reversible electromotive forces (EMF). Very good resistance of the smooth YSZ products to the corrosion in highly dehydrated Li/Na molten carbonates has been shown in experiments lasting few 1000 h. Hence, the consistency of OCV values with the respective EMF values have been tested at various partial pressures of CO 2 and O 2 in the gas mixtures above the molten carbonate electrolyte and at various partial pressures of O 2 + Ar or H 2 + H 2O gas mixtures at the Au or Pt electrodes/YSZ interface. The results have shown the reliability of the double-cell in determination of the open-circuit potentials (OCP) of gas electrodes at the YSZ surface as measured versus the reference electrode with molten carbonate electrolyte. The consistency of OCP and EMF values has been shown satisfying and enhances to use the proposed double-cell in further investigations of OCP and overpotential values at TPB of electrode/YSZ/mixture of reacting gases. At high differences of O 2 partial pressures on both sides of the YSZ membrane some permeation of this gas through the YSZ membrane has been observed. Probably, this effect has an electrochemical character.

Kinetic mechanism of acetyl-CoA synthase: steady-state synthesis at variable Co/Co2 pressures.

Steady-state initial rates of acetyl-CoA synthesis (upsilon/[E(tot)]) catalyzed by acetyl-CoA synthase from Clostridium thermoaceticum (ACS) were determined at various partial pressures of CO and CO2. When [CO] was varied from 0 to 100 microM in a balance of Ar, rates increased sharply from 0.3 to 100 min(-1). At [CO] > 100 microM, rates declined sharply and eventually stabilized at 10 min(-1) at 980 microM CO. Equivalent experiments carried out in CO2 revealed similar inhibitory behavior and residual activity under saturating [CO]. Plots of upsilon/[E(tot)] vs [CO2] at different fixed inhibitory [CO] revealed that Vmax/[E(tot)] (kcat) decreased with increasing [CO]. Plots of upsilon/[E(tot)] vs [CO2] at different fixed noninhibitory [CO] showed that Vmax/[E(tot)] was insensitive to changes in [CO]. Of eleven candidate mechanisms, the simplest one that fit the data best had the following key features: (a) either CO or CO2 (at a designated reductant level and pH) activate the enzyme (E' + CO right arrow over left arrow E, E' + CO2/2e-/2H+ right arrow over left arrow E); (b) CO and CO2 are both substrates that compete for the same enzyme form (E + CO right arrow over left arrow ECO, E + CO2/2e-/2H+ right arrow over left arrow ECO, and ECO --> E + P); (c) between 3 and 5 molecules of CO bind cooperatively to an enzyme form different from that to which CO2 and substrate CO bind (nCO + ECO right arrow over left arrow (CO)nECO), and this inhibits catalysis; and (d) the residual activity arises from either the (CO)nECO state or a heterogeneous form of the enzyme. Implications of these results, focusing on the roles of CO and CO2 in catalysis, are discussed.

CO/CO2 potentiometric titrations of carbon monoxide dehydrogenase from Clostridium thermoaceticum and the effect of CO2.

Acetogenic carbon monoxide dehydrogenases catalyze the reversible oxidation of CO to CO2 and the synthesis of acetyl-coenzyme A, utilizing two novel Ni-Fe-S active sites (the C- and A-clusters, respectively) and an [Fe4S4]2+/1+ cluster (the B-cluster) that serves to transfer electrons. Enzyme samples were titrated under equilibrium conditions using various partial pressures of CO in Ar and CO2 atmospheres. EPR signal intensities from each cluster were analyzed as a function of potential using the Nernst equation. The presence of CO2 raised the reduction potentials of the A-, B-, and C-clusters, and it appeared to increase the strength of CO (substrate for acetyl-CoA synthesis) binding to the reduced A-cluster. Carbon dioxide also appeared to stabilize an intermediate EPR-silent state of the C-cluster and alter the saturation/relaxation properties of the reduced B-cluster. Simulations assuming n values (number of e- involved in reduction) larger than appropriate for the individual reactions generally fit better to the titration data than those which assumed the appropriate n, indicating positive redox cooperativity. Carbon dioxide did not inhibit 1,10-phenanthroline from removing the labile Ni from the A-cluster, but it did inhibit the CO/acetyl-coenzyme A exchange activity, probably by causing CO to bind more tightly to the A-cluster. Taken together, these results indicate a significant CO2-dependent conformational change affecting the properties of all three clusters and both subunits. Since the enzyme operates in vivo in a CO2 environment, the CO2-induced conformation may be mechanistically important.

Dissolution kinetics of calcium carbonate minerals in H 2OCO 2 solutions in turbulent flow: The role of the diffusion boundary layer and the slow reaction H 2O + CO 2 → H + + HCO 3−

Dissolution and precipitation of calcium carbonate minerals in aqueous solutions with turbulent flow are controlled by a diffusion boundary layer (DBL) adjacent to the surface of the mineral, across which mass transfer is effected by molecular diffusion. A rotating disk technique was used to investigate the effect of the DBL on the dissolution rates of CaCO 3. This technique allows an exact adjustment of the thickness of the DBL by controlling the rotation speed of a circular sample of CaCO 3. Measurements of the dissolution rates in H 2OCO 2Ca 2+-solutions in equilibrium with various partial pressures of CO 2 from 1·10 −3 up to 1 atm showed a dependence of the rates R on the rotation frequency ω, given by R ∝ ω n . The exponent n varies from 0.25 at low P co 2 to about 0.01 at a P co 2 of 1 atm. This reveals that the rates are not controlled by mass transport only, which would require n = 0.5. The experimental data can be explained employing a theoretical model, which also takes into account the slow reaction CO 2 + H 2O → H + + HCO 3 − and the chemical reactions at the surface (Dreybrodt and Buhmann, 1991). Interpretation of the experimental data in view of this model reveals that conversion of CO 2 plays an important role in the control of the rates. At high P CO 2 and large DBL thickness (ϵ > 0.001 cm), conversion of CO 2 occurs mainly in the DBL and, therefore, becomes rate limiting. This is corroborated by the observation that upon addition of the enzyme carbonic anhydrase, which catalyzes CO 2-conversion, the dissolution rates are enhanced by 1 order of magnitude. From our experimental observations we conclude that the theoretical model above enables one to predict dissolution rates with satisfactory precision. Since the precipitation rates from supersaturated solutions are determined by the same mechanisms as dissolution, we infer that this model is also valid to predict precipitation rates. The predicted rates for both dissolution and precipitation can be approximated by a linear rate law R = α · ( c eq − c), where c eq is the equilibrium concentration with respect to calcite and a a rate constant, dependent on temperature, P co 2 , DBL thickness (ϵ), and the thickness of the water sheet flowing on the mineral. Values of α are listed that can be used for a variety of geologically relevant conditions.

Thermodynamic Considerations for Eliminating P and Si from Molten Copper by Na<SUB>2</SUB>CO<SUB>3</SUB>–Based Slag

Thermodynamic properties of NaO 0.5 -CO 2 -XO υ/2 slag (X: P or Si, υ: valence of X) have been investigated by conducting the following measurements: (1) Solubility measurements of Na 3 PO 4 (s) into sodium carbonate melts at 1423 to 1523 K under various partial pressures of CO 2 and oxygen. (2) Activity measurements of NaO 0.5 under various partial pressures of CO 2 . (3) Solubility measurements of CO 2 in the slags. Based on the results, the equilibrium distribution ratios of P and Si between the slags and the molten copper have been estimated as a function of partial pressures of CO 2 and oxygen and the slag composition at 1523 K

Thermodynamic Considerations for Eliminating Ni and Zn from Molten Copper by Na<SUB>2</SUB>CO<SUB>3</SUB>-Base Slag

Considering the application of sodium carbonate slag to the elimination of Ni and Zn from molten copper, thermodynamic properties of NaO 0.5 -CO 2 -XO slag (X: Ni or Zn) and Cu-Zn-O melt have been investigated by conducting the following measurements. (1) Solubilities of NiO(s) and ZnO(s) into sodium carbonate melts at 1323 to 1523 K under various partial pressures of CO 2 and oxygen. From the results obtained, the equilibrium phase relationship of the slags were determined. (2) Electrochemical measurements of activities of NaO 0.5 and Na(CO 3 ) 0.5 in the (solid+liquid) coexistence region to confirm the dissolution mechanism of NiO and ZnO into Na 2 CO 3 melt. (3) Equilibrium distribution experiments of Ni and Zn between the homogeneous liquid slags and copper to determine the activities of NiO and ZnO. (4) Equilibrium experiments between Cu-Zn-O melt and ZnO(s) were also conducted at 1523 K and the first order interaction parameter of Zn with respect to O in molten copper was determined as: e ZN O =-106(1523 K). Finally based on the results, the equilibrium distribution ratios of these elements between the slags and the molten copper have been estimated as a function of partial pressures of CO 2 and oxygen at 1523 K.

Thermodynamic Properties of NaO<SUB>0.5</SUB>–CO<SUB>2</SUB>–FeO<SUB>1.5</SUB> Slag

Thermodynamic properties of NaO 0.5 -CO 2 -FeO 1.5 slag have been investigated for the application of sodium carbonate slag to the elimination of Fe from molten copper. The activities of NaO 0.5 and Na(CO 3 ) 0.5 were measured by the EMF method, using β″-alumina as a solid electrolyte for various partial pressures of CO 2 at 1423 to 1523 K. There is a stable solid compound NaFeO 2 in this system, and the solubility measurement of this compound into sodium carbonate melt at 1523 K revealed that the solubility is highly limited and controlled by the partial pressure of CO 2 . The dissolution reaction seems to proceed according to the following reaction and Fe exists as FeO 3 3− anion in the sodium carbonate-based melt

Mass transfer of CO 2 across membranes: Facilitation in the presence of bicarbonate ion and the enzyme carbonic anhydrase

A theoretical and experimental analysis of facilitated transport of CO 2 across membranes containing NaHCO 3 and the enzyme carbonic anhydrase (carbonate hydro-lyase EC 4.2.1.1) is presented. The necessary diffusion reaction equations are derived and the system constraints defined. For the CO 2HCO 3 − system, mathematical simplifications based on the magnitude of various reaction and concentration terms are made to make the equations tractable to solution. The resultant equations are solved by a number of analytical and numerical techniques, each having a limited, though useful, range of validity. The experimental arrangement consists of a liquid membrane (created by soaking a porous filter paper in the test solution), a diffusion chamber, and gas metering and analysis equipment. Conditions were selected to cover the range from diffusion- to reaction-dominated behavior. The flux of CO 2 across a membrane containing 1 M NaHCO 3 was measured at various partial pressures of CO 2 (2–28 in Hg) and with membrane thicknesses of 0.02, 0.06 and 0.10 cm. The extent of facilitation (defined as the ratio of reaction-related flux to the expected Fick's Law flux in the absence of reaction) ranged from near zero to nearly 5 in these experiments. The agreement between model calculations and experimental observation was found to be excellent over the entire range of near-diffusion to near-equilibrium behavior. In the presence of enzyme carbonic anhydrase (0.10 mg/ml, activity approx. 80%) and 1 M NaHCO 3, the CO 2 flux across a 0.02 cm membrane was 3–10-fold higher than the corresponding flux in the absence of enzyme. From experiments at various enzyme concentrations and membrane thicknesses, it appeared that the apparent CO 2 reaction rate was directly proportional to the enzyme concentration. The model calculations for the enzyme-catalyzed reactions agreed with the experimental observations to within ±10%.