The dehydrogenation of isobutane over supported chromium oxide catalysts was studied by a combination of in situ UV-Vis diffuse reflectance spectroscopy and on line GC analysis. A well-defined set of experiments, based on an experimental design, was carried out to develop mathematical models, which quantitatively relate Cr-speciation and dehydrogenation activity with reaction temperature and time, support composition, gas composition and Cr loading. It will Ž .2qr3q be shown that: 1 the dehydrogenation activity is proportional with the amount of in situ measured surface Cr , and Ž . 3q maximum for a 7.5 wt.% CrrAl O catalyst operating at 5008C in 2% isobutane in N ; and 2 Cr -sites are more active in 2 3 2 alkane dehydrogenation than Cr 2q -sites. This paper is the first example of the use of experimental design for deriving structure-activity relationships in the field of heterogeneous catalysis. q

In earlier work on the room temperature oxidation of CzHz by 0 atoms, two distinct sources of methylene radicals have been identified: (i) direct, primary production via channel l b of the CzHz + 0 reaction, and (ii) delayed formation via the secondary reaction 3 involving the products HCCO and H of the other primary channel la:Presently, it was confirmed by a detailed sensitivity analysis that the precise shapes of the resulting total methylene concentration-versus-time profiles in C2H2/0 systems depend strongly on the kla/klb branching ratio. Along that line, the important parameter kla/klb was determined from relative CH2 concentration-versus-time profiles measured in a variety of CzH2/0/H systems using Discharge FlowMolecular Beam sampling Mass Spectrometry techniques (DF-MBMS). The data analysis was carried out by deductive kinetic modelling; the method, as applied to profile shapes, is discussed at length. Via this novel, independent approach, the C H Z (~B~) yield of the two-channel C2Hz + 0 reaction was determined to be k l b / k 1 = 0.17 5 0.08. The indicated 2v error includes possible systematic errors due to uncertainties in the rate constants of other reactions that influence the shapes of the CH2 profiles. The present result, which translates to an HCCO yield kl,/kl = 0.83 i-0.08, is in excellent agreement with other recent determinations. The above mechanism, with the subsequent reactions that it initiates, also reproduces the measured absolute [CzHz], [O], and [HI profiles with an average accuracy of 5%, thus validating the consistency of the CzHz/O/H reaction model put forward here.

Many industrial response surface experiments are deliberately not conducted in a completely randomized fashion. This is because some of the factors investigated in the experiment are hard to change. The resulting experimental design then is of the split-plot type and the observations in the experiment are in many cases correlated. A proper analysis of the experimental data therefore is a mixed model analysis involving generalized least squares estimation.Many people, however, analyze the data as if the experiment was completely randomized, and estimate the model using ordinary least squares. The purpose of the present paper is to quantify the differences in conclusions reached from the two methods of analysis and to provide the reader with guidance for analyzing split-plot experiments in practice. The problem of choosing the number of degrees of freedom for significance tests in the mixed model analysis is discussed as well.

A minor CH(a4Z-and/or X2n) plus COZ producing channel of the reaction between ketenyl radicals and oxygen atoms was identified as the probable source of CH(a4Z-) radicals in room temperature CzHd0 systems: HCCO + 0 -CH(a4Z-and/or XZn) + COz (r2b). Using the discharge-flow/molecular beam sampling mass spectrometry technique, reaction r2b was found to be the dominant COZ source in such systems. Hence, the observed COZ production could be used as a tracer to quantify CH formation by the reaction between HCCO and 0. The only other critical parameter, the rate constant kl, of the primary reaction channel CZHZ + 0 -HCCO + H, is well established. Contributions from other, minor COZ sources were accounted for by kinetic modeling. In this way, the COZ plus CH yield of reaction r2 at T = 290 K was found to be kzdkz = 0.062 f 0.024 (95% overall confidence region); using the known kz = 1.3 x 10-lo cm3 molecule-' s-l, a value of k2b = (8.0 f 3.1) x cm3 molecule-' s-l is derived. Although an experimental proof for CH(a4Z-) formation in the newly identified reaction r2b was not obtained, an examination of the different reaction pathways on the HCCO + 0 potential energy surfaces, calculated at the QCISD(T)/6-3ll++G-(d,p)//UMP2/6-3 lG(d,p)+ZPE level of molecular orbital theory, leads one to expect that a sizeable fraction of the methylidyne radicals arises in the quartet a4Z-state. Each of the characterized CH-producing pathways of HCCO + 0 should become faster at elevated temperatures; therefore, it is suggested that C H ( a 4 Z and/or X2n) + C02 production by HCCO + 0 can be an important process in hydrocarbon flames.

The kinetics of the title reactions have been studied at T = 290 K and p = 2 Torr He using the fast‐flow technique combined with molecular‐beam sampling mass spectrometry. – In our novel approach, the total rate constant k1 of RO2 + NO → RO + NO2/RONO2 (1) is determined from the shape of the NO2‐growth profile. The validity of the method is demonstrated by the nearly identical results for k1, (CF3O2 + NO) obtained from CF3O2‐decays: k1 = (1.54 ± 0.35) · 10−11 and from NO2‐growths: k1 = (1.51 ± 0.4) · 10−11 cm3 s−1, in excellent agreement also with previous determinations. – Rate coefficient data of alkylperoxy + NO reactions, derived from NO2‐profiles, are reported for i‐C3H7O2 + NO: k1 = (5.0 ± 1.2) · 10−12 and for t‐C4H9O2 + NO: k1 = (4.0 ± 1.1) · 10−12 cm3 s−1. Our results, in combination with literature values for smaller peroxy radicals, indicate a marked decrease of the rate coefficient with increasing CH3‐substitution. Including data on haloalkyl‐ and acetylperoxy reactions, the reactivity of RO2 towards NO is shown to correlate with the electron acceptor/donor properties of substituents on the α‐carbon.