J . Phys. Chem. 1987, 91, 692-694
692
Kinetics of the Reaction between Formyl Radicals and Atomic Hydrogen Raimo S. Timonen,+ Emil Ratajczak,t and David Gutman* Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois 60616 (Received: May 19, 1986; I n Final Form: August 25, 1986)
The kinetics of the reaction of formyl radicals with atomic hydrogen was studied as a function of temperature. The reaction was isolated for direct investigation in a tubular reactor coupled to a photoionization mass spectrometer. Rate constants (296 K), 1.3 X (350 K), and 0.96 were obtained for the overall reaction H + HCO products. They are 1.4 X X (418 K) cm3 molecule-' s-I. Estimated error limits are f30%. These measured rate constants are in close ag:eement with recently calculated values obtained by Harding and Wagner using an ab initio potential energy surface for HzCO characterized along the reactive pathways of this reaction.
-
A three-dimensional ab initio potential energy surface for HzCO has recently been calculated by Harding and Wagner with special emphasis placed on characterizing the regions of the surface which correspond to the addition and abstraction pathways of the H H C O reaction:I-*
+
H
+ HCO
e
H,CO*
4
H2
+ CO
(la)
b+ H2C0 H
+ HCO
---*
H,
+ CO
(2)
Rate constants for both routes were calculated by using variational transition-state theory with vibrational frequencies, structures, and energetics along the reaction coordinate obtained by using this potential energy surface. The calculations predicted, among other things, an overall H H C O rate constant of 0.96 X for the process
+
H
+ HCO
-
products
(3)
which is independent of temperature from 300 to 1000 K and which has essentially no pressure dependence below 1 atm (98% of the total flow). Gas was sampled through a 0.08-cm-diameter tapered hole in the wall of the reactor and formed into a beam by a conical skimmer before it entered the vacuum chamber containing the photoionization mass spectrometer. (This reactor has a larger sampling orifice than those which have been used in prior investigations in order to increase detection sensitivity). As the gas beam traversed the ion source, a portion was photoionized and then mass selected. Temporal ion signal profiles were recorded from a short time before each laser pulse to 13-28 ms following the pulse by using a multichannel scalar. Typically data from 5000 to 30 000 repetitions of the experiment were accumulated before the data were analyzed. The only major modification of the experimental facility which was required for these experiments was the installation of an H-atom source attached to the tubular reactor. Hydrogen atoms were generated by a microwave discharge through pure hydrogen gas which flowed slowly through a secondary I-cm4.d. Pyrex tube which was attached to the tubular reactor a short distance before it enters the photoionization mass spectrometer vacuum chamber (38 cm from the gas sampling point). The H-Hz gas mixture flowed rapidly through a constricted section (1-mm i.d., 10-cm long) of the secondary tube located just before the point where it joins the tubular reactor. This section isolated the H-atom source from the gases in the reactor. H-atom concentrations were varied by placing the microwave discharge at different distances along the secondary tube or by changing the H2 flow rate in the secondary tube. Most of the hydrogen atoms produced in the microwave discharge recombined before the flow from the H-atom source reached the tubular reactor. The [H]/[Hz] ratio was typically 0.1. Hydrogen-atom concentrations were measured by using the NOz titration method.8 At the beginning and end of each experiment conducted to measure k3,NO2was added to the tubular reactor through a permanent gas inlet located just downstream from the point where flow from the H-atom source joins the main flow of gas in the tubular reactor. H-atom concentrations were determined repeatedly from the measured NOz depletion and the NO formation when the discharge was turned on and off. Measured H-atom concentrations before and after each experiment generally agreed to better than 15%. The H-atom concentration used to determine k3 was the average of the two measurements. During the study of reaction 3, H-atom concentrations were measured by titrations conducted near the point where the H-atom source joined the tubular reactor. These measured values had to be corrected for H-atom loss along the tubular reactor to the center of the 15-cm-long uniformly heated section just before the point from which gas is sampled during an experiment. For each temperature and each flow condition (flow velocity and gas density) used to measure k3,ancillary experiments were conducted to determine the degree of H-atom loss along the reactor. For these experiments, a thin (0.35-cm-diameter) movable gas inlet tube was inserted along the axis of the tubular reactor in order to introduce NOz into the main flow near the sampling orifice. Then H-atom concentrations were determined at the two titration positions by introducing NOz alternately through the end of the movable gas inlet tube and through the permanent gas inlet located near the H-atom source. The direct comparisons of the H-atom concentrations at these two positions under each set of experimental conditions provided the knowledge needed to determine H-atom concentrations in the reaction zone from the titrations farther upstream. The extent of H-atom attenuation along the flow tube between the two positions was typically 30% and never greater than 50%. The H-atom loss across the reaction zone was below 10% (determined by titrations using the movable inlet tube at different positions in this region of the tubular reactor). A significant fraction of the H-atom loss occurred in a stainless steel (8) Clyne, M. A. A,; Nip, W. S. Reactive Intermediates in the Gas Phase; Setser, D. W., Ed.; Academic: New York, 1979; Chapter 1.
The Journal of Physical Chemistry, Vol. 91, No. 3, 1987 693
I
I I
I
I
I
I
11
Symbol [MI [CH$HO]
0
1.0
[HI x
2.0
(molec cm-3)
Figure 1. Plot of first-order decay constant (k') for HCO vs. [HI for experiments conducted at 418 K. Superimposed on the same plot are the results of the principal experiments (open circles) and those of the ancillary experiments (other symbols) conducted at different densities and different [CH,CHO] to demonstrate the independence of k'on these two variables. Results of other experiments conducted at different laser intensities are superimposable on the open circles. Units of concentration in the table are 10-I6[M]and 10-'3[CH3CHO]molecule ~ m - ~The . decay constant for the experiment shown in the insert is 120 s-'.
coupler which joined two sections of the flow tube together near the titration point. The factor which restricted this study to a rather narrow temperature range was the increasing importance with rising temperature of a reaction which occurs along the length of the tubular reactor between the hydrogen atoms and the free-radical precursor, CH3CHO: H CH3CHO Hz + CH3CO (5)
+
-
Up to =400 K, this reaction consumes less than 5% of the H atoms during the transit time along the reactor to the sampling point (based on calculations using measured values of k59). H-atom concentrations could not be measured with C H 3 C H 0 present to verify the likely negligible additional loss of H atoms caused by reaction 5 because the C H 3 C 0 radicals produced by reaction 5 also react rapidly with NOz during the H-atom titration and also produce NO. This interference during a titration obscures any indication of H-atom loss due to reaction 5. We assume that H-atom attenuation along the tubular reactor is unaffected by reaction 5. At each temperature and gas density used in this study, the H C O decay constant was measured with hydrogen atoms present using different C H 3 C H 0 concentrations which were at least a factor of 2 apart. At the temperatures of the reported experiments, the H C O decay constant was unaffected by [CH3CHO] (see Figure 1). These observations support the validity of the assumption. Formyl radical decay profiles were measured as a function of H-atom concentration. H atoms were always in large excess over [HCOIo. The actual HCO concentrations could not be measured because the extent of C H 3 C H 0 decomposition was too small to observe (
