Reaction Rate Of CO Research Articles

HO + CO Reaction Rates and H/D Kinetic Isotope Effects: Master Equation Models with ab Initio SCTST Rate Constants

Ab initio microcanonical rate constants were computed using Semi-Classical Transition State Theory (SCTST) and used in two master equation formulations (1D, depending on active energy with centrifugal corrections, and 2D, depending on total energy and angular momentum) to compute temperature-dependent rate constants for the title reactions using a potential energy surface obtained by sophisticated ab initio calculations. The 2D master equation was used at the P = 0 and P = ∞ limits, while the 1D master equation with centrifugal corrections and an empirical energy transfer parameter could be used over the entire pressure range. Rate constants were computed for 75 K ≤ T ≤ 2500 K and 0 ≤ [He] ≤ 10(23) cm(-3). For all temperatures and pressures important for combustion and for the terrestrial atmosphere, the agreement with the experimental rate constants is very good, but at very high pressures and T ≤ 200 K, the theoretical rate constants are significantly smaller than the experimental values. This effect is possibly due to the presence in the experiments of dimers and prereactive complexes, which were not included in the model calculations. The computed H/D kinetic isotope effects are in acceptable agreement with experimental data, which show considerable scatter. Overall, the agreement between experimental and theoretical H/D kinetic isotope effects is much better than in previous work, and an assumption of non-RRKM behavior does not appear to be needed to reproduce experimental observations.

Study on potential biphasic solvents: Absorption capacity, CO2 loading and reaction rate

For its high potential in reducing energy penalty of power plant, the biphasic absorption system for CO2 capture has attracted more and more attentions. Only few solvents, however, are reported to be suitable for this system. Meanwhile, as the previous researchers have pointed out, besides CO2 loading and reaction rate, absorption capacity should also be considered when we evaluate new solvents. In this paper, therefore, 12 potential single or mixed amine solutions for biphasic system are studied by comparing their maximum CO2 loading, absorption capacity, and the reaction rate. The results could be useful in the biphasic CO2 capture system in the future.

Effect of CO2 and steam gasification reactions on the oxy-combustion of pulverized coal char

For oxy-combustion with flue gas recirculation, elevated levels of CO2 and steam affect the heat capacity of the gas, radiant transport, and other gas transport properties. A topic of widespread speculation has concerned the effect of gasification reactions of coal char on the char burning rate. To asses the impact of these reactions on the oxy-fuel combustion of pulverized coal char, we computed the char consumption characteristics for a range of CO2 and H2O reaction rate coefficients for a 100μm coal char particle reacting in environments of varying O2, H2O, and CO2 concentrations using the kinetics code SKIPPY (Surface Kinetics in Porous Particles). Results indicate that gasification reactions reduce the char particle temperature significantly (because of the reaction endothermicity) and thereby reduce the rate of char oxidation and the radiant emission from burning char particles. However, the overall effect of the combined steam and CO2 gasification reactions is to increase the carbon consumption rate by approximately 10% in typical oxy-fuel combustion environments. The gasification reactions have a greater influence on char combustion in oxygen-enriched environments, due to the higher char combustion temperature under these conditions. In addition, the gasification reactions have increasing influence as the gas temperature increases (for a given O2 concentration) and as the particle size increases. Gasification reactions account for roughly 20% of the carbon consumption in low oxygen conditions, and for about 30% under oxygen-enriched conditions. An increase in the carbon consumption rate and a decrease in particle temperature are also evident under conventional air-blown combustion conditions when the gasification reactions are included in the model.

High-Pressure Char Gasification Kinetics: CO Inhibition of the C–CO2 Reaction

Results of high-pressure (up to 3.0 MPa) CO-inhibited char–CO2 reaction rate measurements are presented and analyzed using a Langmuir–Hinshelwood (LH) kinetic model commonly used to describe gasification kinetics at atmospheric pressure. Despite previous success by the authors and others in using this reaction scheme to describe high-pressure kinetics of uninhibited gasification reactions, this work shows that it is unable to describe well CO-inhibited reaction kinetics at (relatively) high partial pressures of CO and CO2. Measurements of the char surface area suggest significant impact of CO and CO2 partial pressures on the development of the char surface area during the reaction (and, therefore, most likely on the term describing the number of active sites on the char surface in the LH rate equation). This term is considered to be a constant in the standard LH formulation; therefore, an alternative LH approach to describe the kinetics is presented, whereby rates are described using the LH equation on a ...

Kinetic study of the catalytic dry reforming of CH 4 with CO 2 over La 2− xSr xNiO 4 perovskite-type oxides

Dry reforming of methane by carbon dioxide was investigated over La 2− x Sr x NiO 4 perovskite-type oxides. The catalysts exhibited promising activity and very good stability at equimolar methane and carbon dioxide feed. Small amounts of Sr increase carbon deposition on the catalyst surface but addition of higher amounts of Sr suppresses coking and increases the CO 2 conversion and the CO production. The kinetics of the reaction over LaSrNiO 4 was also investigated by variation of the partial pressures of the reactants. The experimental data were fitted using the semi-empirical power rate law equation as well as using a Langmuir–Hinshelwood kinetic model. The activation energy for methane consumption estimated from both interpretations was ∼41 kJ/mol. Lower values of activation energy estimated for carbon dioxide elimination, when compared to values obtained for methane consumption, correspond to higher reaction rate of CO 2 in comparison to CH 4 reaction rate.

The Significance of Char Morphology to the Analysis of High-Temperature Char−CO2Reaction Rates†

The reaction rates of coal chars with CO2 is an important aspect of coal performance under gasification conditions. Most studies of the char−CO2 reaction are undertaken at temperatures (typically ∼1200 K or below) significantly lower than those found in entrained flow applications, to allow detailed investigations into the surface reaction processes. Application of these data to high temperatures therefore requires consideration of how these reaction rates are affected by gas diffusion through the pore structure of reacting particles, yet there are very few char−CO2 rate data at high temperatures and pressures against which such applications can be tested or verified. This paper presents results of measurements of the char−CO2 reaction rate at high pressures (2.0 MPa) and high temperatures (up to 1673 K) using an entrained-flow reactor and also presents analyses of the morphology of the chars sampled during reaction. Using an effectiveness-factor-based approach, the effects of pore-diffusion and surface r...

Optimizing the calculation of alveolar capillary conductance and pulmonary capillary blood volume with carbon monoxide and nitric oxide in humans

The classic method for determining alveolar‐capillary membrane conductance (DM) and pulmonary capillary blood volume (VC) involves measurement of lung diffusing capacity for carbon monoxide (DLCO) at multiple inspired O2 tensions (FIO2). Recently, the technique has been simplified by simultaneously measuring DLNO (nitric oxide) to determine values at a single FIO2. Controversy exists over equations for calculation of θCO (reaction rate of CO with Hgb) and the correct diffusion ratio for NO and CO (α) in the calculation of DMCO. The purpose of this study was to investigate how various published equations and ratios influence DM & VC using both the multiple and single FIO2 methods. Eight healthy males were studied. DLCO & DLNO were measured using the rebreathe method (inspired vol. ~75% of inspiratory capacity, FIO2 18, 35, & 55%). DM & VC were first calculated using five reported equations for θCO. Once DM & VC were optimized based on reported predicted values, the DLNO method was compared using a range of α values. Using the multiple FIO2 method, depending on the equation for θCO, DM ranged from 47 to 103 ml/min/mmHg, and VC from 74 to 102 ml. For the calculation of DM & VC using the DLNO method, α ranged from 1.5 to 4.0 in order to optimize DM & VC values to the classic method. The determination of DM & VC is critically dependent on the equations used for θCO which in turn affects the optimal ratio used to correct the DLNO method. NIH HL71478

Methanol partial oxidation on Cu–Zn thin films grown on Ni(1 0 0) surface

Methanol partial oxidation on Cu–Zn thin films epitaxially grown on Ni(1 0 0) surface has been investigated in an ultra high vacuum (UHV) system combined with a high-pressure reaction cell. According to temperature programmed reaction (TPR) and temperature programmed desorption (TPD) in UHV, surface reaction between methanol and pre-adsorbed oxygen on epitaxially grown Cu thin films mainly produced H 2 and CO together with HCHO, although addition of Zn greatly suppressed the formation of HCHO. Annealing of the thin films at 420 K prior to admission of methanol and oxygen reactants dramatically accelerated reaction rate of CO 2 product and the maximum formation of H 2 and CO 2 was observed on the surface of 1 ML Cu with 0.25 ML Zn. On the contrary, the rate of methanol partial oxidation in the high-pressure reaction cell monotonously increased with the thickness of Cu films up to 3 ML. The addition of 0.25 ML Zn increased the reaction rate independent of Cu film thicknesses of 1–6 ML. Post-reaction surface analysis showed Cu agglomeration on the Ni substrate. Cu 2p X-ray photoelectron spectroscopy (XPS) and LMM Auger data reveal the presence of Cu + species in addition to metallic Cu atoms abundant in the films corresponding to the higher reactivity. It is proposed that the oxidation number of Cu is controlled by oxided Zn in Cu–Zn alloy.

Catalytic Carbon Dioxide Reforming of Methane to Synthesis Gas over Activated Carbon Catalyst

The catalytic activity and kinetic behavior of catalytic reforming of methane with carbon dioxide over activated carbon were investigated as a function of reaction temperature, gas hourly space velocity (GHSV), and partial pressures of CH4 and CO2. The CH4 and CO2 conversions were greatly influenced by the reaction temperature in the range of 850−1050 °C. The apparent activation energies for CH4 and CO2 consumption and CO and H2 production were 32.63 ± 1.06, 25.54 ± 1.79, 24.81 ± 3.06, and 32.99 ± 2.58 kcal/mol, respectively. The curves of reaction rates versus GHSV showed various trends at different temperatures and indicated 7500 mL/h·g-cat was sufficient for operation in the kinetic regime. The reaction rate of methane and carbon dioxide over activated carbon was affected significantly by the partial pressures. Under a higher CO2 pressure, the excess CO2 reacted with H2 through the reverse water−gas shift (RWGS) reaction. The predictions of the CH4 and CO2 reaction rates based on a semiexperimental for...

Chemical reactivity analysis of the CO + OH and CO + HO 2 reactions

The kinetics and reactivity associated with the CO + OH and CO + HO 2 reactions were analyzed based on ab initio molecular orbital theory, with the interest of identifying the difference in their reaction dynamics that leads to the slower rate of the latter. Two hydrogen-bonded pre-reactive complexes, OOH···OC and OOH···CO, were found on the potential energy surface of CO + HO 2, having energies that are higher than those of the entrance reactants by 1.2 and 2.9 kcal mol −1, respectively. Through intrinsic reaction coordinate analysis, it was further found that these two complexes hinder the forward reaction, HO 2 + CO → products, by forcing a reorientation of the reactants. The configurations of the transition state structures in forming the trans/ cis intermediates in the title reactions were rationalized by virtual representation of the electrostatic potentials of the reactants from self-consistent field calculations at the CCSD(T)/AUG-cc-pVTZ level, with the different reactivities of the two reactions ascribed to spatial interactions between the local electrostatic potentials of the entrance reactants. The energy gap of the frontier molecular orbitals for the CO + OH entrance channel was found to be ca. 29 kcal mol −1 lower than that of the CO + HO 2 entrance channel by time-dependent coupled cluster theories, which quantitatively explains the difference in their reactivities. Furthermore, the condensed Fukui functions were calculated using Mulliken atomic charges through natural population analysis to identify the most reactive sites of the entrance reactants, with the results again supporting the slower reaction rate of CO + HO 2.

용융 아연과 WC-Co 코팅층 내 코발트의 반응거동에 미치는 아연욕 중의 Al 첨가 영향

The objective of this study is to investigate the effect of Al addition on the reaction behavior of cobalt with molten zinc. Pure cobalt specimen was immersion tested in the three kinds of molten zinc (pure, 0.12%Al added and 0.24%Al added) baths at <TEX>$460^{\circ}C,\;490^{\circ}C\;and\;520^{\circ}C$</TEX>. For the understanding of degradation processes, specimens were analyzed with scanning electron microscope (SEM) and energy dispersive spectrum (EDS), and electrochemical stripping method. When 0.12% and 0.24% Al was added in molten zinc baths, three intermetallic compounds layers of <TEX>${\gamma},\;{\gamma}_1,\;and\;{\gamma}_2$</TEX> were formed on the Co matrix and <TEX>${\beta}_1$</TEX> layer was not formed between the Co matrix and the <TEX>${\gamma}$</TEX> layer. Particles of CoAl intermetallic compound were formed at the interface between the <TEX>${\gamma}_2$</TEX> layer and zinc melt and they did not adhere to the Co-Zn intermetallic layer. Weight loss of the Co specimen increased as Al content in the molten zinc increased and the relationship of weight loss vs. immersion time followed parabolic rate law. Rate controlling process for the reaction rate of Co with Al added molten zinc was analyzed as the diffusion process of Al atom through a boundary layer between the <TEX>${\gamma}_2$</TEX> layer and the Al added zinc melt.

Comparison of the Binding and Reactivity of Plant and Mammalian Peroxidases to Indole Derivatives by Computational Docking

The oxidation of melatonin by the mammalian myeloperoxidase (MPO) provides protection against the damaging effects of reactive oxygen species. Indole derivatives, such as melatonin and serotonin, are also substrates of the plant horseradish peroxidase (HRP), but this enzyme exhibits remarkable differences from MPO in the specificity and reaction rates for these compounds. A structural understanding of the determinants of the reactivity of these enzymes to indole derivatives would greatly aid their exploitation for biosynthetic and drug design applications. Consequently, after validation of the docking procedure, we performed computational docking of melatonin and serotonin to structural models of the ferric and compound I and II (co I and co II, respectively) states of HRP and MPO. The substrates dock at the heme edge on the distal side, but with different orientations in the two proteins. The distal cavity is larger in MPO than in HRP; however, in MPO, the substrates make closer contacts with the heme involving ring stacking, whereas in HRP, no ring stacking is observed. The observed differences in substrate binding may contribute to the higher reaction rates and lower substrate specificity of MPO relative to those of HRP. The docking results, along with the previously measured heme-protein reduction potentials, suggest that the differentially lowered reaction rates of co II of HRP and MPO with respect to those of co I could stem from as yet undetermined conformational or electrostatic differences between the co I and co II states of MPO, which are absent in HRP.

Meso-scale modelling of CO oxidation in digitally reconstructed porous [formula omitted] catalyst

In this paper we present a methodology for modelling of local reaction-transport processes in a digitally reconstructed, porous heterogeneous catalyst. Microkinetics of the CO oxidation on Pt / γ - Al 2 O 3 with an explicit consideration of the surface-deposited species has been employed in the model. The reaction takes place on the Pt sites located on the γ - Al 2 O 3 surface and in the meso-pores, simultaneously with the transport of gaseous reaction components. Several 3D porous structures have been digitally reconstructed from the typical Scanning electron microscope images of Pt / γ - Al 2 O 3 catalytic washcoats of monoliths. Typical dependences of overall CO reaction rate and the effectiveness factor on the temperature and properties of the porous Pt / γ - Al 2 O 3 structure (washcoat macro-porosity, characteristic size of the macro-pores, the size of γ - Al 2 O 3 particles, noble metal loading and distribution) are evaluated. Non-monotonous dependence of the averaged reaction rate on the macro-porosity of the washcoat is observed. It is shown that the dependence of the average reaction rate on the amount of Pt in the porous layer is strongly non-linear under certain conditions and the spatial distribution of the catalytically active sites within the porous structure of the support controls the effectiveness factor to a significant extent. In the combination with semi-deterministic methods of the reconstruction (simulation of the catalyst preparation process) the results can be used for the optimisation of the washcoat structure. The methodology presented in this work is also a contribution to the development of multi-scale simulation techniques whereby volume-averaged parameters calculated at one spatial scale are used as input values for simulations at a larger scale.

被覆造粒炭添加による新コークス製造法の開発

New cokemaking by blending of agglomerated coal particles was developed for making high strength and high reactivity coke. Agglomerated coal particles were produced by coating the slightly caking coal particles with caking coal particles. In the case of conventional cokemaking process, at the high blending ratio of the slightly caking coal, coke reactivity is raised, however, coke strength reduces. Cracks can be easily generated at the interface between coke textures, because the slightly caking coal particles have low fluidity at the carbonization and the semi-coke has high contraction.The slightly caking coal particles (1-5 mm) coated with the caking coal were carbonized in coal blend. From microfocus X ray CT images, the generation of the cracks at the interface between coke textures was not observed. When the -0.5 mm of Ro =1.1% caking coal was used as a coating coal and the slightly caking coal (MF value; 2 ddpm) was used as a central coal, coke strength and coke size were enhanced. At 1373-1773K, the CO2 reaction rate of the coke blended with 20% agglomerated particles was higher than that of coke without adding agglomerated particles.

Natural Geochemical Analogues for Carbon Dioxide Storage in Deep Geological Porous Reservoirs, a United Kingdom Perspective

Elevated concentrations of atmospheric CO2 are implicated in global warming. Mitigation of this requires capture of CO2 from fossil fuel power sources and storage in subsurface aquifers or depleted hydrocarbon fields. Demonstration projects and financial analysis suggest that this is technologically feasible. CO2 must retained below ground for 104 y into the future to enable the surface carbon cycle to reduce atmospheric CO2 levels. To provide robust predictions of the performance of disposal sites at the required timescale, one approach is to study natural CO2 accumulations, which give insight into rock-CO2 -brine interactions over timescales of 103 - 5.106 y. In contrast to geochemical modelling predictions, natural CO2 fields in the North Sea (Brae, Miller, Magnus, Sleipner), at 4.0 km and deeper, do not show the mineral products which are predicted to form. Calcite and feldspar still comprise 5-20% of the rock, and dawsonite is absent. SE Australian and Arizona reservoir sandstones also do not fit to geochemical predictions. A state of disequilibrium possibly exists, so that existing geochemical modelling is not capable of accurately predicting kinetic-controlled and surface-chemistry controlled mineral dissolution or precipitation in natural subsurface sandstones on the required timescales. Improved calibration of models is required. Geochemical evidence from laboratory experiments (months to years duration), or from enhanced oil recovery (30 y duration) are again too short in timescale. To help to bridge the 104 y gap, it may be useful to examine natural analogues (103-106 y), which span the timescale required for durable disposal. The Colorado Plateau is a natural CO2 system, analogous to an hydrocarbon system, where 100 Gm3 CO2 fields occur, sourced from 0-5 Ma volcanics. Deep erosion has exposed the sediments which formed CO2 source, CO2 carrier, CO2 reservoir, CO2 trap, CO2 seal. Some very large CO2 traps are now exhumed, and some are currently leaking to form cool travertine springs at the surface. Natural examples at Salt Wash Green River, and at Moab Fault are briefly described. These show extensive bleaching of haematite which may be locally redeposited, carbonate cementation ?13C -70 around point sources, and silica precipitation, which may seal leak-off on buried anticline crests. Accurate geochemical modelling of the long-term performance of CO2 storage sites, requires improved understanding of CO2 reaction paths and reaction rates with aquifer reservoirs and with overlying seals. Robust prediction of disposal site performance is not possible without this.

Electrocatalytic reforming of carbon dioxide by methane in SOFC system

The reaction of carbon dioxide catalytic reforming with methane is an attractive route because these greenhouse gases can be converted into variable feedstocks. However this reaction is a highly energy consuming and coke forming process. These problems were improved by the electrocatalytic reforming of CO 2 with CH 4 in a solid oxide fuel cell (SOFC) membrane reactor system, which generates high electrical power and synthesis gases. The single cell consists of catalyst electrode (NiO–MgO), counter electrode ((La,Sr)MnO 3) and Y 2O 3 stabilized ZrO 2 (YSZ) electrolyte. The reaction rates of CO 2 and CH 4, and the electrochemical properties were investigated by an on-line GC and impedance-analyzer under open- and closed-circuit conditions, respectively. It was found that reaction rates of CO 2 and CH 4 under the closed-circuit condition were more stable than those of the open-circuit. The results were interpreted that the stability of catalyst anode was maintained by the reaction of oxygen ion transferred from the cathode with the surface carbon formed in the internal CO 2 reforming by CH 4 in SOFC system.

Photoacoustic measurements of water–gas shift reaction on ferric oxide catalyst

A photoacoustic spectroscopic (PAS) technique was applied to the kinetic study of the water–gas shift reaction in the presence of α-Fe 2O 3 catalyst. The catalytic reaction was performed in the temperature range from 523 to 673 K at various partial pressures of carbon monoxide and water–vapor in a closed-circulation reactor system using a differential photoacoustic cell. The variation of CO 2 photoacoustic signal with the concentration of CO 2 produced during the reaction was recorded as a function of time. The time-resolved photoacoustic spectra obtained for the early reaction stage provided precise data of CO 2 formation rate. The results of CO 2 photoacoustic spectra indicated that the catalytic water–gas shift reaction over α-Fe 2O 3 occurred when α-Fe 2O 3 was partially reduced to Fe 3O 4 by heating it in vacuum. The vacuum-pretreated α-Fe 2O 3 catalyst showed catalytic activity for the water–gas shift reaction and its rate of CO 2 formation increased with increasing temperature in this temperature range. The rate of CO 2 formation on the vacuum-activated α-Fe 2O 3 catalyst was highly enhanced when the catalyst was treated in situ with H 2O(g) prior to the admittance of CO(g)/H 2O(g) reaction mixture. The reaction orders determined from the initial reaction rates of CO 2 formation were found to be 0.80 and 0.08 with respect to CO and H 2O, respectively. From the experimental results, the active sites and the reaction mechanism for the water–gas shift reaction on the vacuum-activated α-Fe 2O 3 catalyst could also be discussed.

Modelling Analysis for the Carbonylation Stage of Two-Step Methanol Synthesis

A comprehensive modelling analysis for the carbonylation stage of low temperature two-step methanol synthesis in an integrated two-stage reactor was conducted based on chemical reaction equilibrium and phase equilibrium. Dynamic behavior of the reaction process, the composition of the liquid and the gas phases, the effects of total pressure, space velocity, temperature on CO conversion, and CO reaction rate were simulated. The main features of the reactor behavior can be correctly predicted. In general, the composition of every component predicted corresponds well with the experimental data. The result shows that a high space velocity favors CO reaction rates at the steady state.

Stability and reactivity of electrogenerated cobalt(I) towards aryl halides in the presence of additives such as vinyl acetate or methyl vinyl ketone: Application to the electrosynthesis of arylzinc compounds

The electrochemical reduction of CoBr 2 in acetonitrile+pyridine (v/v=9:1) has been investigated. In this solvent mixture, the disproportionation reaction rate of the electrogenerated Co(I) is about twice as slow as in dimethylformamide+pyridine (9:1) and the reaction between Co(I) and arylhalides, ArX, also is slowed down by the same factor. We show that the use of vinyl acetate (vinyl-OAc) or methyl vinyl ketone (MVK) as additives in acetonitrile+pyridine increases the stability of Co(I). Thus, in the presence of vinyl-OAc the disproportionation reaction rate of Co(I) is divided by seven whereas the rate of the oxidative addition of Co(I) with aryl halides is divided only by two. Preparative-scale electrolyses of arylzinc compounds confirm this result since the quantity of catalyst, CoBr 2, can be divided by four as compared to the amount used in the absence of additives. In the two cases (presence of vinyl-OAc or MVK) the presence of pyridine is required to stabilize Co(I). In these reactions, the rate constants for both chemical reactions (disproportionation reaction and oxidative addition) have been determined and the stabilization of Co(I) is discussed in terms of formation of π-complexes with the additives.

Estimation of hydrocarbon biodegradation rates in gasoline-contaminated sediment from measured respiration rates

An open microcosm method for quantifying microbial respiration and estimating biodegradation rates of hydrocarbons in gasoline-contaminated sediment samples has been developed and validated. Stainless-steel bioreactors are filled with soil or sediment samples, and the vapor-phase composition (concentrations of oxygen (O 2), nitrogen (N 2), carbon dioxide (CO 2), and selected hydrocarbons) is monitored over time. Replacement gas is added as the vapor sample is taken, and selection of the replacement gas composition facilitates real-time decision-making regarding environmental conditions within the bioreactor. This capability allows for maintenance of field conditions over time, which is not possible in closed microcosms. Reaction rates of CO 2 and O 2 are calculated from the vapor-phase composition time series. Rates of hydrocarbon biodegradation are either measured directly from the hydrocarbon mass balance, or estimated from CO 2 and O 2 reaction rates and assumed reaction stoichiometries. Open microcosm experiments using sediments spiked with toluene and p-xylene were conducted to validate the stoichiometric assumptions. Respiration rates calculated from O 2 consumption and from CO 2 production provide estimates of toluene and p-xylene degradation rates within about ±50% of measured values when complete mineralization stoichiometry is assumed. Measured values ranged from 851.1 to 965.1 g m −3 year −1 for toluene, and 407.2–942.3 g m −3 year −1 for p-xylene. Contaminated sediment samples from a gasoline-spill site were used in a second set of microcosm experiments. Here, reaction rates of O 2 and CO 2 were measured and used to estimate hydrocarbon respiration rates. Total hydrocarbon reaction rates ranged from 49.0 g m −3 year −1 in uncontaminated (background) to 1040.4 g m −3 year −1 for highly contaminated sediment, based on CO 2 production data. These rate estimates were similar to those obtained independently from in situ CO 2 vertical gradient and flux determinations at the field site. In these experiments, aerobic conditions were maintained in the microcosms by using air as the replacement gas, thus preserving the ambient aerobic environment of the subsurface near the capillary zone. This would not be possible with closed microcosms.