Steady-state decomposition

Kieken LD, Neurock M, Mei DH. 2005. Screening by kinetic Monte Carlo simulation of Pt-Au(lOO) surfaces for the steady-state decomposition of nitric oxide in excess dioxygen. J Phys Chem B 109 2234-2244. 

A simple test rig was set up consisting of pressurization system, monolith catalyst, and control and diagnostic system [6]. The purpose of the tests is to focus on four principal characteristics of H2O2 decomposition (i) transient time to reach steady-state decomposition, (ii) initial temperature of the decomposition chamber, (iii) steady-state decomposition temperature, and (iv) H2O2 mass flow rate. The efficiency of the decomposition was assessed from the final temperature. The concentration of hydrogen peroxide was 70 wt.-%. Figure 2 is an example of test output for sample F. The calculated adiabatic decomposition temperature for 70 wt.-% hydrogen peroxide is 220°C. 

The transient interaction of NO with Cu-exchanged ZSM5 at 80°C has been studied to investigate the redox chemistry of Cu in these catalysts. The unsteady-state formation of gaseous N2O upon NO adsorption on He pretreated zeolite (550°C) has been observed and associated to the reoxidation from Cu to Cu" of a fraction of Cu sites. A similar dependence on the Cu content for NO steady-state decomposition runs at 500°C and N2O transient production at 80°C has been obtained for Cu-ZSM5 catalysts at Si/Al = 25 and 80, showing the ability of this transient analysis in titrating Cu sites active in NO decomposition. 

The steady-state decomposition of hydrogen peroxide in bromine -f- bromide solutions (Lmngston, J. Amer. Chem. Soc. 1926, 48, 54)... 

There are no fundamental reasons that could prevent the application of these vaporization equations to cases of thermal decomposition (dissociative vaporization) of a substance, except for the requirement that the rate J must be measured under a steady-state decomposition mode, corresponding to the deceleratory period in the a—t kinetic curve. 

Here, as in Eq. 3.14, the quantity Pb is expressed in bar. Under steady-state decomposition conditions, the low-volatility product condenses totally in the reactant/product reaction zone. 

L. D. Kieken, M. Neurock, and D. H. Mei,/. Phys. Chem. B, 109, 2234 (2005). Screening by Kinetic Monte Carlo Simulation of Pt-Au(lOO) Surfaces for the Steady-State Decomposition of Nitric Oxide in Excess Dioxygen. 

New radicals are introduced by thermolysis of the hydroperoxide by chain-branching decomposition (eq. 4). Radicals are removed from the system by chain-termination reaction(s) (eq. 5). Under steady-state conditions, the production of new radicals is in balance with the rate of radical removal by termination reactions and equation 8 appHes for the scheme of equations 1—5 where r. = rate of new radical introduction (eq. 4). 

Although the decomposition rate of peroxide is thus increased, the consequent lowering of steady-state peroxide concentration leaves the effective rate unchanged in the simple peroxide cycle kinetic scheme (25). In real systems, at certain critical levels, a catalyst can become an inhibitor (2,180). 

The above mechanism, together with the assumptions that initiator decomposition is rate controlling and that a steady state in chain radicals exists, results in the classical expressions (eqs. 8 and 9) for polymerization rate, and number-average degree of polymerization, in a homogeneous,... 

STRATEGY Construct the rate laws for the elementary reactions and combine them into the overall rate law for the decomposition of the reactant. If necessary, use the steady-state approximation for any intermediates and simplify it by using arguments based on rapid pre-equilibria and the existence of a rate-determining step. 

In the above reactions, I signifies an initiator molecule, Rq the chain-initiating species, M a monomer molecule, R, a radical of chain length n, Pn a polymer molecule of chain length n, and f the initiator efficiency. The usual approximations for long chains and radical quasi-steady state (rate of initiation equals rate of termination) (2-6) are applied. Also applied is the assumption that the initiation step is much faster than initiator decomposition. ,1) With these assumptions, the monomer mass balance for a batch reactor is given by the following differential equation. 

A full development of the rate law for the bimolecular reaction of MDI to yield carbodiimide and CO indicates that the reaction should truly be 2nd-order in MDI. This would be observed experimentally under conditions in which MDI is at limiting concentrations. This is not the case for these experimements MDI is present in considerable excess (usually 5.5-6 g of MDI (4.7-5.1 ml) are used in an 8.8 ml vessel). So at least at the early stages of reaction, the carbon dioxide evolution would be expected to display pseudo-zero order kinetics. As the amount of MDI is depleted, then 2nd-order kinetics should be observed. In fact, the asymptotic portion of the 225 C Isotherm can be fitted to a 2nd-order rate law. This kinetic analysis is consistent with a more detailed mechanism for the decomposition, in which 2 molecules of MDI form a cyclic intermediate through a thermally allowed [2+2] cycloaddition, which is formed at steady state concentrations and may then decompose to carbodiimide and carbon dioxide. Isocyanates and other related compounds have been reported to participate in [2 + 2] and [4 + 2] cycloaddition reactions (8.91. 

Steady-state solutions are found by iterative solution of the nonlinear residual equations R(a,P) = 0 using Newton s methods, as described elsewhere (28). Contributions to the Jacobian matrix are formed explicitly in terms of the finite element coefficients for the interface shape and the field variables. Special matrix software (31) is used for Gaussian elimination of the linear equation sets which result at each Newton iteration. This software accounts for the special "arrow structure of the Jacobian matrix and computes an LU-decomposition of the matrix so that qu2usi-Newton iteration schemes can be used for additional savings. 

The effect of catalyst supports on methane conversions and hydrogen yield in the methane decomposition at 998 K and GHSV of2700 h at steady state. 

Steady state measurements of NO decomposition in the absence of CO under potentiostatic conditions gave the expected result, namely rapid self-poisoning of the system by chemisorbed oxygen addition of CO resulted immediately in a finite reaction rate which varied reversibly and reproducibly with changes in catalyst potential (Vwr) and reactant partial pressures. Figure 1 shows steady state (potentiostatic) rate data for CO2, N2 and N2O production as a function of Vwr at 621 K for a constant inlet pressures (P no, P co) of NO and CO of 0.75 k Pa. Also shown is the Vwr dependence of N2 selectivity where the latter quantity is defined as... 

The SiC diluent did not contribute to the N2O decomposition at the reaction temperatures studied. Prior to each run, the catalyst was subjected to calcination by heating the catalyst in He at 30 K/min to 923 K and maintaining this temperature for one hour. Subsequently, the temperature was decreased to the desired value and the feed mixture was passed over the bed. Temperature and feed composition were varied in a random order in the experiments. Generally, 40 to 50 min after a change of conditions the conversion levels were constant and considered as the steady-state. At least five analyses were averaged for a data point. 

In this reaction scheme, the steady-state concentration of peroxyl radicals will be a direa function of the concentration of the transition metal and lipid peroxide content of the LDL particle, and will increase as the reaction proceeds. Scheme 2.2 is a diagrammatic representation of the redox interactions between copper, lipid hydroperoxides and lipid in the presence of a chain-breaking antioxidant. For the sake of clarity, the reaction involving the regeneration of the oxidized form of copper (Reaction 2.9) has been omitted. The first step is the independent decomposition of the Upid hydroperoxide to form the peroxyl radical. This may be terminated by reaction with an antioxidant, AH, but the lipid peroxide formed will contribute to the peroxide pool. It is evident from this scheme that the efficacy of a chain-breaking antioxidant in this scheme will be highly dependent on the initial size of the peroxide pool. In the section describing the copper-dependent oxidation of LDL (Section 2.6.1), the implications of this idea will be pursued further. 

Moden, B., da Costa, P., Fonte, B. et al. (2002) Kinetics and mechanism of steady-state catalytic NO decomposition reactions on Cu-ZSM5,./. Catal., 209, 75. 

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