Kinetics Of CO Dissociation Research Articles

Constrained Chemical Dynamics of CO Dissociation/Hydrogenation on Rh Surfaces.

Among noble metal catalysts, rhodium (Rh) is unique in its ability to perform a one-step synthesis of ethanol from syngas. The first steps following the adsorption of syngas on Rh surfaces are assumed to be responsible for the conversion of CO and the selectivity effects between C1 , C2 , and oxygenated species. In the current work, constrained ab initio molecular dynamics are applied to investigate the kinetics of CO dissociation and hydrogenation over flat and stepped Rh surfaces. The obtained barriers for the Rh(111) surface are in good agreement with the literature data. On the stepped Rh(211) surface, a large site-dependent variation in barrier height is shown, with the upper terrace exhibiting behavior comparable to the Rh(111) surface, whereas the barriers over the lower terrace site are generally significantly lower. The rate constants are calculated using transition state theory for both surfaces, and are applied successfully in a microkinetic model, confirming the predicted impact on CO conversion and CH4 /C1 -oxygenate/C2 Hn selectivity. In addition to the high-accuracy energetics and rate constants reported for CO dissociation/hydrogenation and the presentation of an updated microkinetic mechanism for Rh catalysts, the applicability of constrained molecular dynamics for reaction barrier calculation is confirmed, and sensitive pathways affecting the selectivity between formaldehyde/methanol over Rh catalysts are highlighted.

Mechanistic Insights into the Activation of Soluble Guanylate Cyclase by Carbon Monoxide: A Multistep Mechanism Proposed for the BAY 41-2272 Induced Formation of 5-Coordinate CO-Heme.

Soluble guanylate cyclase (sGC) is a heme-containing enzyme that catalyzes cGMP production upon sensing NO. While the CO adduct, sGC-CO, is much less active, the allosteric regulator BAY 41-2272 stimulates the cGMP productivity to the same extent as that of sGC-NO. The stimulatory effect has been thought to be likely associated with Fe-His bond cleavage leading to 5-coordinate CO-heme, but the detailed mechanism remains unresolved. In this study, we examined the mechanism under the condition including BAY 41-2272, 2'-deoxy-3'-GMP and foscarnet. The addition of these effectors caused the original 6-coordinate CO-heme to convert to an end product that was an equimolar mixture of a 5- and a new 6-coordinate CO-heme, as assessed by IR spectral measurements. The two types of CO-hemes in the end product were further confirmed by CO dissociation kinetics. Stopped-flow measurements under the condition indicated that the ferrous sGC bound CO as two reversible steps, where the primary step was assigned to the full conversion of the ferrous enzyme to the 6-coordinate CO-heme, and subsequently followed by the slower second step leading a partial conversion of the 6-coordinate CO-heme to the 5-coordinate CO-heme. The observed rates for both steps linearly depended on CO concentrations. The unexpected CO dependence of the rates in the second step supports a multistep mechanism, in which the 5-coordinate CO-heme is led by CO release from a putative bis-carbonyl intermediate that is likely provided by the binding of a second CO to the 6-coordinate CO-heme. This mechanism provides a new aspect on the activation of sGC by CO.

Catalytic CO Hydrogenation: Mechanism and Kinetics from Chemical Transients at Low and Atmospheric Pressures

The hydrogenation of CO over Co model catalysts was studied using relaxation-type methods operating in situ either at atmospheric pressures or under surface science conditions. Emphasis was laid on providing information on the surface composition and on how it changes with time under catalytic reaction conditions. Using pressure forcing in chemical transient kinetics (CTK), the build-up of the steady-state was studied at 503 K and atmospheric pressure to demonstrate that the active catalyst surface is not metallic but covered with carbon, oxygen and hydrogen in excess of a monolayer equivalent. Both build-up and backward transients suggest CO to act as the “monomer” which probably inserts into an O–H bond to form the primary surface complex necessary for hydrocarbon and oxygenate formation. Repetitive electric field pulses (pulsed field desorption mass spectrometry, PFDMS) at low pressures have allowed the CO dissociation kinetics on a nano-sized Co 3D crystal (“tip”) to be monitored in the millisecond time range. No evidence for the occurrence of the Boudouard reaction was obtained in either PFDMS or CTK. Adsorbed CHx (x = 1–3) species were detected in small amounts demonstrating that CO dissociation is fast compared to carbon hydrogenation. Adsorbed Co-subcarbonyl species, Co(CO)x were also detected by PFDMS and possibly mediate the necessary surface mobility during the initial restructuring of the catalyst. Surface carbon seems to inhibit Co-subcarbonyl formation.

Novel Anionic Heterocyclic Thiolate Complexes of Group VIB Metal Carbonyls: Synthesis and Reactivity

The syntheses and characterization of novel Group VIB metal carbonyl complexes containing heterocyclic thiolate ligands is described. The reactions of the photogenerated intermediate, M(CO)5THF (M = W, Mo, Cr), with the thiolate anions RS− in [(n‐Bu)4N][SR] [SR− = 2‐mercaptobenzimidazolyl (a), 2‐mercaptobenzothiazolyl (b), 2‐mercaptobenzoxazolyl (c)], afforded exclusively [(n‐Bu)4N][M(CO)5SR] in fairly good yields. Homo‐ and heterobimetallic complexes of the type [(n‐Bu)4N][M(CO)5(μ‐SR)M′(CO)5] (M, M′ = W, Mo, Cr), in which both the exocyclic S‐atom and the endocyclic N‐atom are involved in coordination to two metal centers, have been prepared by the reaction of [(n‐Bu)4N][M(CO)5SR] with M′(CO)5·THF at ambient temperatures. The methods of preparation and characterization of these complexes are described. CO‐substitution reactions in [(n‐Bu)4N][W(CO)5SR] [SR− = (b) and (c)] with tri(iso‐propyl)phosphite (L) afforded the complexes [(n‐Bu)4N][cis‐W(CO)4(SR){P(OPr‐i)3}] as the sole reaction product. The kinetics of CO dissociation from the pentacarbonyl anions under pseudo‐first‐order conditions confirm that these reactions are first‐order in substrate and independent of the entering group (L) concentration. The facile CO dissociation in these complexes is another example of neighboring‐group participation in assisting cis‐CO loss in octahedral metal carbonyls. From the kinetics data, the anionic ligands (b) and (c) are classified as cis‐CO labilizing groups. Rate constants and activation parameters for the CO‐substitution reactions in these complexes are reported.

Atomic resonance absorption spectroscopy measurements on high-temperature CO dissociation kinetics

The dissociation rate coefficient of CO was measured behind reflected shock waves in the temperature range 5500 < J<9000 K. By the use of the atomic resonance absorption spectroscopy (ARAS), the time-dependent formation of C and O atoms could be directly observed in the postshock reaction zone. The experiments were performed in mixtures of CO highly diluted in argon at pressures between 0.5 and 1.3 bar. The initial CO concentrations varied from 100 to 10,000 ppm. Assuming vibrational equilibrium, the data for the thermal decomposition of CO, (CO + Ar ^ C + O + Ar), can be represented by the following modified Arrhenius expression: k = 4.3 X1027 J-3-1 exp(-129,000 K/J) cn^mol-^-1 ±50%. Computer simulations based on a two-temperature model were not able to explain observed reaction induction times. Vibrational relaxation alone seems to have a negligible influence under the present conditions. The effect of impurities was minimized by keeping background pressure in the shock tube below 5xlO~ 7 mbar.

Dissociation and oxidation of carbon monoxide over Rh/Al 2O 3 catalysts

The activity of Rh/Al 2O 3 catalysts for CO oxidation was investigated by transient isotopic pulse experiments using a packed-bed reactor. This transient experimental scheme revealed significant CO dissociation activity during CO oxidation over Rh/Al 2O 3 catalysts. Results indicate that the oxidation of CO proceeds via dissociative oxidation by its own oxygen as well as via direct oxidation by gas-phase oxygen on well-dispersed Rh/Al 2O 3 catalysts. The rate of CO dissociation is on the same order of magnitude as the rate of CO oxidation; under steady-state conditions at 300 °C, the rate of CO dissociation is approximately half that of direct oxidation. Differences in CO dissociation activity between single-crystal Rh surfaces and well-dispersed supported Rh particles are explained in terms of the molecular bonding and adsorption characteristics on these two different surfaces. The importance of CO dissociation kinetics in the overall CO oxidation activity of Rh/Al 2O 3 catalysts is further discussed in view of the reaction lightoff behavior.

The kinetics of CO dissociation on Ru(001): Time-resolved vibrational spectroscopy at elevated pressures

Time-resolved Fourier transform-infrared reflection absorption spectroscopy (FT-IRAS) has been utilized to measure the kinetics of CO dissociation on a Ru(001) surface at elevated pressures (10−3 to 10 Torr) and temperatures (500–700 K). The reaction of CO with Ru(001) is found to be a nonsteady state and results in CO disproportionation, i.e., 2CO→C+CO2. The decrease in total CO coverage follows first order kinetics and exhibits Arrhenius behavior with an activation energy of 20.6 kcal and a preexponential factor of 102 s−1. Comparison of the overall reaction rate with that of CO2 formation (O+CO→CO2) confirms that CO dissociation is the rate-limiting step in the disproportionation reaction. The in situ reaction rate constant exhibits a weak dependence on CO pressure (&amp;lt;first order). However, the determination of local CO coverages during reaction reveals a linear dependence of the dissociation rate with CO coverage. This confirms that the chemisorbed state of the molecule is a precursor to dissociation and that a high pressure is required to maintain a steady state surface coverage of CO at reaction temperature. In situ vibrational spectra demonstrate the formation of carbon islands under reaction conditions which prevent further CO adsorption and result in a decrease in total CO coverage at constant local CO coverage. Post-reaction spectroscopy confirms the formation of two-dimensional islands of carbon whose reactivity toward oxidation is found to be between that of amorphous carbon and three-dimensional graphite.

Summary Abstract: The kinetics of CO dissociation on Fe(111)

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Summary Abstract: The kinetics of CO dissociation on Mo(110) and effects of coadsorbates

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The kinetics of CO dissociation on Mo(001)

The kinetics of CO dissociation on Mo(001)

The kinetics of CO dissociation on Mo(001)

The thermal dissociation of molecular CO adsorbed on the Mo(001) surface has been studied using ultraviolet photoemission methods. Results derived from isothermal measurements indicate that the conversion to atomic β-CO does not obey simple first order kinetics. The observed rate behavior can be explained, however, by an activation energy which increases with adsorbate coverage from a value of 0.60 eV near half coverage. Adsorbate-induced rearrangement of Mo atoms may be responsible for the coverage-dependent changes in the reaction rate.