Mechanism Of Carbon Monoxide Oxidation Research Articles

The Surface/Interface Modulation of Platinum Group Metal (PGM)-Free Catalysts for VOCs and CO Catalytic Oxidation.

Effective management of volatile organic compounds (VOCs) and carbon monoxide (CO) is critical to human health and the ecological environment. Catalytic oxidation is one of the most promising technologies for achieving efficient VOCs and CO emission control. Platinum group metal (PGM)-free catalysts are recently receiving sustainable attention in catalyzing VOCs and CO removal due to their low cost, superior catalytic activity, and excellent stability, but PGM-free catalysts face challenges in low-temperature catalytic efficiency. In this mini-review, starting with discussing the catalytic mechanism of VOCs and CO oxidation, we summarize the surface/interface modulation strategies of PGM-free catalysts to promote oxygen and VOCs/CO molecule activation for enhanced low-temperature oxidation activity, including oxygen vacancy engineering, heteroatom doping, surface acidity modification, and active interface construction. We highlight the currently remaining challenges and prospects of advanced PGM-free catalyst development for highly efficient VOCs and CO emission control in practical applications.

Gold Decorated Hydroxyapatite–CeO2 Enabled Surface Frustrated Lewis Pairs for CO Oxidation

The activation of molecular oxygen (O2) holds the fundamental step for reaction mechanism of carbon monoxide oxidation. Surface frustrated Lewis pairs (SFLPs) with sterically hindered Lewis acid–base pairs are proven to activate small molecules, such as hydrogen. However, the activation of molecular O2 on SFLPs remains unexplored. Herein, the construction of SFLPs in Au decorated hydroxyapatite (HAP)–CeO2 heterojunction is reported. The coordinately unsaturated Lewis acidic Ce3+ and the Lewis basic OH− group in HAP enable the activation of O2 molecules to –Ce3+ species for carbon monoxide (CO) oxidation. The active site is identified based on the comprehensive in situ electron paramagnetic resonance, in situ diffuse reflectance infrared Fourier transform spectroscopy, X‐Ray absorption spectroscopy, and theoretical simulation. The results provide a new strategy to construct SFLPs sites for O2 molecule activation and the subsequent CO oxidation.

Unraveling the Reaction Mechanism of Mo/Cu CO Dehydrogenase Using QM/MM Calculations

Some microorganisms, like the aerobic soil bacteria, Oligotropha carboxidovorans, have the capability to oxidize the highly toxic atmospheric gas carbon monoxide (CO) into CO2 through CO dehydrogenase enzymes, whose active site contains a bimetallic MoCu center. Over the last decades, a number of experimental and theoretical investigations were devoted to understanding the mechanism of CO oxidation and, in particular, the role of a very stable thiocarbonate intermediate that may be formed during the catalytic cycle. The occurrence of such an intermediate was reported to make the CO2 release step kinetically difficult. In this work, by using an accurate QM/MM approach and energy refinement by means of the BigQM method, we were able to determine the role of such an intermediate and propose a novel mechanism for the oxidation of CO into CO2 by Mo/Cu CO dehydrogenase. Surprisingly, we found that the detachment of CO2 occurs directly from the product of the Mo═O nucleophilic attack reaction on the carbon of CO aided by the transient coordination of the active site glutamate to the Mo ion. The estimated activation barrier is in good agreement with the experimental one, while the thiocarbonate turned out to not interfere with the CO-oxidation catalytic cycle. The results highlight the importance of the environmental effects in the assembly of the molecular model and in the choice of the computational protocol. Our accurate modeling of the enzyme also allowed us to exclude the involvement of a frustrated Lewis pair in the CO-oxidation mechanism, which has recently been suggested based on an analysis of structural and electronic features of synthetic mimics of the Mo/Cu CO dehydrogenase active site.

Mechanism insights into CO oxidation over transition metal modified V2O5/TiO2 catalysts: A theoretical study

The V2O5/TiO2 based selective catalytic reduction (SCR) catalysts possess not only promising capability on the denitrification of nitrogen oxides (NOx), but also certain effects on the oxidation of carbon monoxide (CO) in the flue gas. Modification of traditional SCR catalysts with certain transition metals can further improve their catalytic oxidation ability of CO. Therefore, it is of great significance to reveal the catalytic oxidation mechanism of CO for developing modified SCR catalysts to achieve the co-removal of CO and NOx. Theoretical calculations based on density functional theory (DFT) were performed to probe the comprehensive reaction mechanism of CO oxidation on M doped V2O5/TiO2 catalysts (M = Mo, Fe, and Co). The whole CO oxidation cycles include three stages, i.e., the first CO oxidation, the re-oxidation of the surface, and the second CO oxidation. The terminal oxygen and the surface oxygen formed by the adsorbed O2 all play vital roles in the whole CO oxidation cycles. The activation barriers of the rate-determining steps for CO oxidation on Fe–V2O5/TiO2 and Co–V2O5/TiO2 are much lower than that of Mo–V2O5/TiO2, which indicates Fe and Co dopants can apparently promote the CO oxidation activities of the modified SCR catalysts. Meanwhile, the electronic structure analysis confirms that Fe and Co dopants can cause electron distribution change with strong oxidation ability at the active oxygen sites.

Selective Oxidation of H2 and CO by NiIr Catalyst in Aqueous Solution: A DFT Mechanistic Study.

One of the challenges in utilizing hydrogen gas (H2) as a sustainable fossil fuel alternative is the inhibition of H2 oxidation by carbon monoxide (CO), which is involved in the industrial production of H2 sources. To solve this problem, a catalyst that selectively oxidizes either CO or H2 or one that co-oxidizes H2 and CO is needed. Recently, a NiIr catalyst [NiIICl(X)IrIIICl(η5-C5Me5)], (X = N,N'-dimethyl-3,7-diazanonane-1,9-dithiolate), which efficiently and selectively oxidizes either H2 or CO depending on the pH, has been developed (Angew. Chem. Int. Ed. 2017, 56, 9723-9726). In the present work, density functional theory (DFT) calculations are employed to elucidate the pH-dependent reaction mechanisms of H2 and CO oxidation catalyzed by this NiIr catalyst. During H2 oxidation, our calculations suggest that dihydrogen binds to the Ir center and generates an Ir(III)-dihydrogen complex, followed by subsequent isomerization to an Ir(V)-dihydride species. Then, a proton is abstracted by a buffer base, CH3COO-, resulting in the formation of a hydride complex. The catalytic cycle completes with electron transfer from the hydride complex to a protonated 2,6-dichlorobenzeneindophenol (DCIP) and a proton transfer from the oxidized hydride complex to a buffer base. The CO oxidation mechanism involves three distinct steps, i.e., (1) formation of a metal carbonyl complex, (2) formation of a metallocarboxylic acid, and (3) conversion of the metallocarboxylic acid to a hydride complex. The formation of the metallocarboxylic acid involves nucleophilic attack of OH- to the carbonyl-C followed by a large structural change with concomitant cleavage of the Ir-S bond and rotation of the COOH group along the NiIr axis. During the conversion of the metallocarboxylic acid to the hydride complex, intramolecular proton transfer followed by removal of CO2 leads to the formation of the hydride complexes. In addition, the barrier heights for the binding of small molecules (H2, OH-, H2O, and CO) to Ir were calculated, and the results indicated that dissociation from Ir is a faster process than the binding of H2O and H2. These calculations indicate that H2 oxidation is inhibited by CO and OH- and thus prefers acidic conditions. In contrast, the CO oxidation reactions occur more favorably under basic conditions, as the formation of the metallocarboxylic acid involves OH- attack to a carbonyl-C and the binding of OH- to Ni largely stabilizes the triplet spin state of the complex. Taken together, these calculations provide a rationale for the experimentally observed pH-dependent, selective oxidations of H2 and CO.

CO oxidation over supported gold nanoparticles as revealed by operando grazing incidence X-ray scattering analysis.

The mechanism of carbon monoxide oxidation over gold was explored using a model planar catalyst consisting of monodisperse gold nanoparticles periodically arranged on single crystal SiO2/Si(111) substrates using a combination of Grazing Incidence Small Angle X-ray Scattering and Grazing Incidence X-ray Diffraction (GISAXS/GIXD) under reaction conditions. It is shown that nanoparticle composition, size and shape change when the catalyst is exposed to reactive gases. During CO oxidation, the particle's submergence depth with respect to the surface decreases due to the removal of gold oxide at the metal-support edge, meanwhile the particle 'flattens' to maximise the number of the reaction sites along its perimeter. The effect of the CO concentration on the catalyst structure is also discussed. Our results support the dual catalytic sites mechanism whereby CO is activated on the gold surface whereas molecular oxygen is dissociating at the gold-support interface.

Mechanism of carbon monoxide oxidation on supported copper catalysts modified with cerium and platinum

The mechanism of CO oxidation on copper oxide catalysts without additives and with cerium and platinum additives supported on γ-Al2O3 was studied. It was found that the presence of Ce and Pt facilitated the reduction of copper and increased the activity of the catalysts. It was established that the parent substances participated in the reaction from an adsorbed state; in this case, the surface coverage of all of the catalysts with oxygen was greater than the coverage with CO, and the reaction system components exhibited different adsorption capacity. The introduction of Ce increases the bond strength of CO with the surface and weakens the bond of oxygen with the surface. The presence of Pt increases the bond strengths of CO, O2, and CO2 with the surface.

A temperature-programmed desorption and IR spectroscopic study of the mechanism of carbon monoxide oxidation on copper-containing catalysts

With the use of the temperature-programmed desorption of CO and IR spectroscopy, it was found that, after the adsorption of carbon monoxide on the oxidized 5% CuO/CeO2, 5% CuO/ZrO2, 5% CuO/Al2O3, and 5% CuO/SiO2 samples at 20°C, the greatest amount of adsorbed molecules (2.5 × 1020 g−1) was present on 5% CuO/ZrO2. Of these molecules, 1.0 × 1020 and 1.4 × 1020 g−1 formed carbonyl and carbonate adsorption complexes with the participation of copper-containing sites, respectively. The oxidized sample of 5% CuO/ZrO2 contained only the oxidation sites Cu2+O2−, which participated in the formation of carbonates upon the adsorption of CO. A portion of Cu2+ cations was reduced to Cu+ in the course of reaction, and two types of the carbonyl complexes Cu+CO were formed on them. They were characterized by the presence of absorption bands at 2110 and 2107 cm−1 in the IR spectrum and decomposed with the desorption of CO at 100 and 170°C. Carbonyls were oxidized by adsorbed oxygen at 20°C to carbonates. The temperature of their decomposition accompanied by the desorption of CO2 (Tmax = 170°C) was lower than that of carbonates (240 and 350°C) formed upon the adsorption of CO on the oxidized surface in the absence of oxygen from a gas phase. The properties of adsorption complexes and their participation in the reaction CO + O2 → CO2 at low temperatures, in particular, with the use of a hydrogen-containing mixture, were considered. The oxidation of CO on CuO clusters in 5% CuO/CeO2 and 5% CuO/ZrO2 were discussed.

First-principles study of CO oxidation on bismuth-promoted Pt(111) surfaces

Vehicle emission control regulations necessitate the removal of carbon monoxide (CO) from engine exhausts via CO oxidation. Although bismuth (Bi)-promoted platinum (Pt) catalysts show improvement in CO oxidation performance over pure Pt, it is still not known whether Bi acts simply as a site blocker to reduce CO poisoning or whether it is an active participant in the catalytic reactions. In this study, we report density functional theory-based CO oxidation energetics and kinetics in the presence of different Bi dopant configurations. Adsorption energies for both CO and oxygen atoms are found to be reduced by Bi doping. Bi dopant also creates surface areas in its vicinity where CO adsorption is prohibited, whereas molecular oxygen dissociation is promoted. Significant reduction is shown for CO oxidation barriers on Bi-modified Pt(111) surfaces leading to exothermic CO2 formation. Our results elucidate that promoting Pt with Bi affects both the electronic properties of the catalyst and alters the Pt ensemble size available for elementary reactions within CO oxidation mechanism.

On the mechanism of carbon monoxide oxidation on the surface of gold nanoclusters supported on titanium oxide

The process of carbon monoxide (CO) oxidation on the surface of a system comprising nanodimensional gold clusters deposited onto thin films of titanium oxide of variable stoichiometry formed on a Re(1000) single crystal surface has been studied by methods of thermodesorption, IR, and X-ray photoelectron spectroscopy. It is established that oxygen contained in titanium oxide plays an important role in the conversion of CO into CO2. The efficiency of this process on the Au/TiOx (x < 2) system surface is significantly higher that that on the Au/TiO2 system.

The CO Oxidation Mechanism Over Ag-Co And Co-Ce Mixed Oxide Catalysts

Carbonmonoxide oxidation mechanism was studied over the 50/50 Ag-Co and 50/50 Co-Ce mixed oxide catalysts. The temperature programmed methods (TPR-H2 and TPD-CO) were used to characterize the catalysts. FTIR spectrums were collected under different feed conditions in order to determine the reaction mechanism. The carbonates adsorption bands and gas phase CO2 bands were obtained for both of the catalysts. CO linearly bounded to Ag+ sites at 2164 cm-1 and to Co3+ sites at 2105 cm-1 for 50/50 Ag-Co mixed oxide catalyst. CO linearly bounded to Co3+ sites at 2173 cm-1 and 2108 cm-1 for 50/50 Co-Ce mixed oxide catalyst. The CO adsorption studies showed that the CO reacted with surface oxygen species. The CO2 adsorbed on the catalyst surface as surface carbonate species. The oxidation of CO with surface oxygen species is probably via a non-competitive Langmuir-Hinshelwood mechanism on the catalysts.

Near-ambient X-ray photoemission spectroscopy and kinetic approach to the mechanism of carbon monoxide oxidation over lanthanum substituted cobaltites

Abstract We have studied the oxidation of carbon monoxide over a lanthanum substituted perovskite (La 0.5 Sr 0.5 CoO 3− d ) catalyst prepared by spray pyrolysis. Under the assumption of a first-order kinetics mechanism for CO, it has been found that the activation energy barrier of the reaction changes from ∼80 to ∼40 kJ mol −1 at a threshold temperature of ca. 320 °C. In situ XPS near-ambient pressure (∼0.2 torr) shows that the gas phase oxygen concentration over the sample decreases sharply at ca. 300 °C. These two observations suggest that the oxidation of CO undergoes a change of mechanism at temperatures higher than 300 °C.

Kinetics and mechanism of carbon monoxide oxidation on the supported metal complex catalyst PdCl2-CuCl2/Al2O3

The kinetics of carbon monoxide oxidation with atmospheric oxygen on a PdCl2-CuCl2/γ-Al2O3 catalyst was studied at T = 27°C and an N2-O2-CO mixture pressure of 1 atm. The catalyst was prepared by cold impregnation. Three groups of mechanistic hypotheses are considered, and two of them are demonstrated to be consistent with kinetic data, although they differ in the roles of water and oxygen in carbon monoxide oxidation.

A Study of NO+CO Reaction over Various Supported Catalysts in the Presence of O2and H2O

NO and CO are the main gases emitted into the atmosphere from both stationary and mobile combustion systems. These toxic gases represent major form of pollutants and contribute mainly to the formation of smog, acid rain and ozone depletion in the stratosphere. The simultaneous elimination of these two toxic gases leading to non-toxic compounds such as N2 and CO2 is considered an efficient process. Although the studied reaction is spontaneous, it is well known that the presence of a catalyst is necessary. For the catalytic reduction of NOx, reducing agents such as carbon monoxide (CO), hydrogen (H2), ammonia (NH3), urea ((NH2)2CO) and different hydrocarbons are usually being employed. Many supports such as alumina, titania, zirconia, ceria were usually used for dispersing the active phase for the elimination of nitrogen oxide. It is well known that supports play an important role in determining the nature and number of active sites, and consequently in the activity of the catalysts. The metal oxides are advantageous for the stability of the catalyst support and/or for enhancement of the catalyst activity under dynamic operations. Among the transition metals studied, copper show a potential activity in the NO+CO reaction and have been extensively studied. Sakurai et al. tested catalytic activities of cupric oxide and alumina-supported copper oxides in NO reduction by CO. Metal oxides such as Ce and Ni are commonly employed O2 storage component, which are frequently added to the catalyst to enhance the catalytic active sites. These oxides improve the performance of the catalyst, further making it less sensitive to temporary variations in the air-fuel ratio by getting reversibly oxidized or reduced at typical operating temperatures. Loof et al. stated that at higher temperatures, the O2 storage capacity in catalysts containing Pt, Rh, Ni and Ce supported on Al2O3, Ni is found to be the dominant storage metal and Ce predominates over Pt and Rh. The reduced surface of Ce in contact with reduced noble metals is the most effective state of the catalyst and greater activity is detected with decreasing CeO2 crystallite size. Addition of cerium oxide play an important role in providing oxygen storage by transforming between Ce2O3 under reducing conditions and CeO2 under oxidizing conditions. It is further believed that doped ceria inhibits the deactivation of active components in the reaction due to the sintering of the metals and surface deterioration of the support. Garcia et al. pointed out that the Cu-Ce interaction affected the reactivity of the catalysts towards NO thereby increasing the amount of adsorbed species which in turn lead to high NO adsorption at low temperature. The NO dissociation peaks were shifted towards low temperature upon addition of ZrO2 onto CuO/TiO2, indicating NO decomposition activity was higher in CuO-ZrO2/ TiO2 than by CuO/TiO2. Hernandez et al. suggested that the presence of acid-base sites on Pt/ZrO2-CeO2 seemed to be important for the NO+CO reaction. The Cu-Ce synergism for NO reduction with CO in the presence of O2, H2O and SO2 has been investigated by Wen and He. The results suggested that the synergistic effect between Cu and Ce ions could enhance the performance of catalyst both in activity and resistance to H2O and SO2. Redox Mechanism involving lattice oxygen/oxygen vacancy participation was proposed for CO oxidation on CeO2. Sayle et al. conducted atomic simulation of a CO oxidation mechanism on CeO2 and suggested that the surface oxygen of CeO2 is more active than the bulk oxygen. In the present study the importance of (i) method involved in preparation of catalyst, (ii) metal support interaction of catalyst and (iii) Cu-Ce synergistic effect in the NO reduction by CO are discussed.

Interaction of Potassium Cyanide with the [Ni-4Fe-5S] Active Site Cluster of CO Dehydrogenase from Carboxydothermus hydrogenoformans

The Ni-Fe carbon monoxide (CO) dehydrogenase II (CODHII(Ch)) from the anaerobic CO-utilizing hydrogenogenic bacterium Carboxydothermus hydrogenoformans catalyzes the oxidation of CO, presumably at the Ni-(micro(2)S)-Fe1 subsite of the [Ni-4S-5S] cluster in the active site. The CO oxidation mechanism proposed on the basis of several CODHII(Ch) crystal structures involved the apical binding of CO at the nickel ion and the activation of water at the Fe1 ion of the cluster. To understand how CO interacts with the active site, we have studied the reactivity of the cluster with potassium cyanide and analyzed the resulting type of nickel coordination by x-ray absorption spectroscopy. Cyanide acts as a competitive inhibitor of reduced CODHII(Ch) with respect to the substrate CO and is therefore expected to mimic the substrate. It inhibits the enzyme reversibly, forming a nickel cyanide. In this reaction, one of the four square-planar sulfur ligands of nickel is replaced by the carbon atom of cyanide, suggesting removal of the micro(2)S from the Ni-(micro(2)S)-Fe1 subsite. Upon reactivation of the inhibited enzyme, cyanide is released, and the square-planar coordination of nickel by 4S ligands is recovered, which includes the reformation of the Ni-(micro(2)S)-Fe1 bridge. The results are summarized in a model of the CO oxidation mechanism at the [Ni-4Fe-5S] active site cluster of CODHII(Ch) from C. hydrogenoformans.

Interaction of Carbon Monoxide with Anatase Surfaces at High Temperatures: Optimization of a Carbon Monoxide Sensor

Sensing the presence of particular gases in harsh environments, such as at high temperatures, poses challenges in the choice of materials as well as in measurements of the appropriate sensing-related property of the material. In this study, we examine the sensing of carbon monoxide (CO) in a nitrogen background at temperatures up to 600 °C using the anatase phase of TiO2 as the sensing material. In particular, the change in resistance of anatase is used to detect the presence of CO. Copper oxide (CuO) is added to anatase to increase the sensitivity toward CO detection. However, the presence of CuO led to partial transformation of anatase to rutile at temperatures of 800 °C used for bonding the sensor material to the sensing platform. By adding La2O3 to the CuO/anatase, the anatase phase is maintained under all thermal treatments. Diffuse reflectance infrared spectroscopy is used to examine the mechanism of CO oxidation. Interaction of lanthanum with the anatase increased the reactivity of the anatase surface toward CO. In addition, the presence of CuO led to increased adsorption of CO as well as enhanced desorption of CO2, explaining the enhancement of the sensitivity of the CuO-containing anatase toward sensing of CO. Electron microscopy has provided information on the microstructure of the sensor material. An effective medium approximation theory is used to model the observed resistivity data over the temperature range 400−600 °C. The energies of adsorption of CO and the reaction of CO with adsorbed oxygen to form CO2 are extracted. These values are consistent with the role of CuO acting as a catalyst. This study demonstrates that anatase doped with lanthanum along with the presence of surface/CuO is an effective sensor for CO at temperatures as high as 600 °C.

Mechanism of carbon monoxide oxidation by the carbon monoxide dehydrogenase/acetyl-CoA synthase from Clostridium thermoaceticum: kinetic characterization of the intermediates.

Carbon monoxide dehydrogenase/acetyl-CoA synthase (CODH/ACS) from Clostridium thermoaceticum catalyzes (i) the synthesis of acetyl-CoA from a methylated corrinoid protein, CO, and coenzyme A and (ii) the oxidation of CO to CO2. CO oxidation occurs at a Ni- and FeS-containing center known as cluster C. Electrons are transferred from cluster C to a separate metal center, cluster B, to external acceptors like ferredoxin. In the work described here, we performed reductive titrations of CODH/ACS with CO and sodium dithionite and monitored the reaction by electron paramagnetic resonance (EPR) spectroscopy. We also performed pre-steady-state kinetic studies by rapid freeze-quench EPR spectroscopy (FQ-EPR) and stopped-flow kinetics. Redox titrations of CODH/ACS revealed the existence of a UV-visible and EPR-silent electron acceptor denoted center S that does not appear to be associated with any of the other metal centers in the protein. Our results support the previous proposals [Anderson, M. E., & Lindahl, P. A. (1994) Biochemistry 33, 8702-8711; Anderson, M. E., & Lindahl, P. A. (1996) Biochemistry 35, 8371-8380] that the Cred2 form of cluster C is two electrons more reduced than the Cred1 form. The combined results from titrations and pre-steady-state studies were used to formulate a mechanism for CO oxidation, composed of the following steps: (i) CO binding to the [Cred1,Box, Xox] state to yield a Cred1-CO complex; (ii) two-electron reduction of Cred1 to Cred2 concerted with CO2 release; (iii) binding of a second CO molecule to the [Cred2,Box,Xox] state to form a Cred2-CO complex; (iv) electron transfer from Cred2-CO to cluster B to form [Cred2,Bred,Xred] with concerted release of the second CO2. Step iii competes with internal electron transfer from Cred2 to Box and Xox. At high CO concentrations, step iii is favored, whereas at low concentrations, only one CO molecule per turnover binds and undergoes oxidation. Closure of the catalytic cycle involves electron transfer from reduced enzyme to an electron acceptor protein, like ferredoxin. Xox is a yet-uncharacterized electron acceptor that may be an intermediate in the reduction of center S. The Cred2 state appears to be the predominant state of cluster C during steady-state turnover. The rate-determining step for the first half-reaction is step iv, while during steady-state turnover, it appears to be electron transfer to external electron acceptors.

A Laboratory Study of Heterogeneous Catalysis in Ultrahigh Vacuum

A surface science experiment has been developed that can be used in both the undergraduate and graduate curriculum. Students study the mechanism of carbon monoxide oxidation using a platinum foil catalyst, and they also get hands-on experience with ultrahigh vacuum equipment. Using temperature programmed desorption, students study the desorption of carbon monoxide and oxygen from platinum foil in vacuum. The study allows students to determine that the Langmuir-Hinshelwood reaction mechanism operates in this catalytic system.

Manganese nodules as catalyst and adsorbent. 4. Catalytic activity of manganese nodules for oxidation of carbon monoxide.

To investigate the possible use as catalyst of the manganese nodule, which can be a new source of metals, the oxidation of carbon monoxide with the nodule was carried out and its catalytic properties were compared with those of a commercial Pt-Al2O3 catalyst.The manganese nodule showed some catalytic activity at temperatures below 210°C and it gradually increased with increasing temperature, while the Pt-Al2O3 catalyst showed a lower activity than that of the nodule catalyst below 210°C but it suddenly increased at above this temperature. The nodule had a greater resistance to poisoning by H2S than Pt-Al2O3. It was also found that the excess oxygen on the nodule surface was closely related with the reaction of carbon monoxide oxidation. From these results the reaction mechanism of carbon monoxide oxidation was proposed.

Carbon monoxide oxidation by Clostridium thermoaceticum and Clostridium formicoaceticum.

Cultures of Clostridium formicoaceticum and C. thermoaceticum growing on fructose and glucose, respectively, were shown to rapidly oxidize CO to CO(2). Rates up to 0.4 mumol min(-1) mg of wet cells(-1) were observed. Carbon monoxide oxidation by cell suspensions was found (i) to be dependent on pyruvate, (ii) to be inhibited by alkyl halides and arsenate, and (iii) to stimulate CO(2) reduction to acetate. Cell extracts catalyzed the oxidation of carbon monoxide with methyl viologen at specific rates up to 10 mumol min(-1) mg of protein(-1) (35 degrees C, pH 7.2). Nicotinamide adenine dinucleotide, nicotinamide adenine dinucleotide phosphate and ferredoxin from C. pasteurianum were ineffective as electron acceptors. The catalytic mechanism of carbon monoxide oxidation was "ping-pong," indicating that the enzyme catalyzing carbon monoxide oxidation can be present in an oxidized and a reduced form. The oxidized form was shown to react reversibly with cyanide, and the reduced form was shown to react reversibly with alkyl halides: cyanide inactivated the enzyme only in the absence of carbon monoxide, and alkyl halides inactivated it only in the presence of carbon monoxide. Extracts inactivated by alkyl halides were reactivated by photolysis. The findings are interpreted to indicate that carbon monoxide oxidation in the two bacteria is catalyzed by a corrinoid enzyme and that in vivo the reaction is coupled with the reduction of CO(2) to acetate. Cultures of C. acidi-urici and C. cylindrosporum growing on hypoxanthine were found not to oxidize CO, indicating that clostridia mediating a corrinoid-independent total synthesis of acetate from CO(2) do not possess a CO-oxidizing system.