Infrared Chemiluminescence Technique Research Articles

Recombination versus Disproportionation Reactions of Hydrogen Atoms with ClCF2CHF, ClC2F4, BrC2H4, BrC2F4, and BrCF2CFBr Radicals and Unimolecular Reactions of the Haloethane Molecules from Recombination

The reactions of hydrogen atoms with the ClCF2CHF, ClC2F4, BrC2H4, BrC2F4, and BrCF2CFBr radicals have been studied at room temperature and 1 Torr pressure of Ar by an infrared chemiluminescence technique in a flow reactor. The H + CF3CH2 recombination reaction was also examined to provide a reference point to earlier experiments from this laboratory. The recombination step generates vibrationally excited molecules that undergo HX(X = Br, Cl, F) elimination at 1 Torr of pressure. The characteristic low vibrational excitation, 〈fV(HX)〉 ≈ 0.15, with a monotonically declining distribution from unimolecular 1,2-HX elimination reactions versus 〈fV(HX)〉 ≈ 0.35 with an inverted distribution from disproportionation, or direct halogen atom abstraction, reactions is used as a diagnostic test for recombination versus disproportionation mechanisms. Upon the basis of the observed HBr vibrational distributions, the H + BrC2F4 reaction has a small Br atom abstraction component that is superimposed upon the HBr vibration...

分子線赤外発光法によるPd単結晶表面上における定常的CO＋NO反応

The infrared chemiluminescence technique was applied to the steady-state CO oxidation by NO on Pd(111) and Pd(110) to measure the internal energy of CO2 desorbed from surfaces. In this paper, on the basis of the results of CO oxidation by O2, we elucidate the CO2 formation mechanism of CO + NO reaction. From the comparison of IR emission spectra of CO2 between CO + NO and CO + O2 reactions, it is found that the vibrational energy states of CO2 in CO + NO reaction are similar to those in CO + O2 reaction. This indicates that the reaction path of CO2 formation in CO + NO is the same as that in CO + O2. The vibrational states are much dependent on the surface structure: the CO2 molecules produced on flat Pd(111) were highly excited (especially bending mode) than those on Pd(110) which has atomically rough surface.

Structure Sensitivity in the Kinetics and the Dynamics of CO Oxidation over Stepped Pd(335) Studied by the Molecular Beam Infrared Chemiluminescence Technique: Determination of Working Sites during the Steady-State Reaction

Kinetics and dynamics of CO oxidation have been studied on a stepped Pd(335) surface at a steady-state condition and compared with those on flat Pd(111). The infrared (IR) chemiluminescence technique was applied to determine where the active catalytic sites are on the Pd(335) surface. Since the vibrational energy state of the product CO2 is sensitive to the structures of the reaction sites on Pd surfaces, information about the working reaction sites during the steady-state CO oxidation can be obtained from the IR emission spectra of the product CO2. The production rate of CO2 was higher on Pd(335) than on Pd(111), indicating that the steps on the surface enhance the catalytic activity for CO oxidation under the steady-state condition. However, the rate data do not necessarily show the real active sites for the CO + O recombination reaction. At a surface temperature of 850 K, the vibrational Boltzmann temperature (TV) of the product CO2 on Pd(335) was quite different from (much lower than) that on Pd(111), although the Pd(335) surface has four-atom wide (111) terraces. The lower TV value on Pd(335) was similar to that on Pd(110)(1 × 1), indicating that a relatively linear activated CO2 complex was formed. Therefore, during the steady-state CO oxidation on Pd(335), the reaction does not take place on the (111) terrace sites, but mostly on the step sites at 850 K. On the contrary, as the CO coverage increased at a lower surface temperature and at a high CO/O2 ratio, the TV values on Pd(335) tend to approach those on Pd(111), indicating that the contribution of the active sites on the steps is decreased and the working reaction sites shift to the (111) terrace sites.

Dynamics and kinetics of CO oxidation on Pd(335): infrared chemiluminescence of CO 2

Infrared chemiluminescence technique has been applied to steady-state carbon monoxide oxidation on a stepped Pd(335) surface, which consists of four-atom wide terraces of (111) orientation and monatomic-height steps, and the results are compared with those of a flat Pd(111) surface. The activity for CO oxidation was higher on Pd(335) than on Pd(111). The vibrational temperatures ( T v) of the product CO 2 molecules were substantially different on the stepped Pd(335) surface from on the Pd(111) surface, and the difference in T v between on Pd(335) and on Pd(111) is much larger at the surface temperature ( T s) of 850 K than at T s=650 K. These results suggest that, for the steady-state CO oxidation on the Pd(335) surface, the reaction takes place mostly on the step sites at T s=850 K, but the contribution from the terrace sites is increased significantly at T s=650 K.

The Dynamics of the Steady-state CO Oxidation on Single Crystal Pd Surfaces. Identification of Reaction Site Using Infrared Chemiluminescence Technique.

The internal energy distribution of C02 produced and desorbed in the steady-state CO oxidation (∼10-2 Torr) on well-defined single crystal Pd surfaces were successfully measured by using infrared (IR) chemiluminescence technique. The CO2 molecules produced on flat surface such as Pd(111) were highly excited (especially in terms of bending mode) than those on Pd(110) (1×1) which has atomically rough surface. These results show that the dynamics of CO oxidation on Pd surfaces is structure-sensitive. The difference in the vibrational distribution of the desorbing CO2 indicates that a structure of activated complex of CO2 is more bent on Pd(111) and relatively linear on Pd(110). Also, the kinetics of the steady-state CO oxidation is structure-sensitive. Furthermore, the IR chemiluminescence technique was applied to the study of CO2 on Pd(335) having both (111) terrace and step properties. The IR study on Pd(335) revealed that the steady-state CO oxidation is proceeding at step sites under higher surface temperature condition (850 K), and the active site of CO+O recombination reaction shifts to the terrace site with increasing CO coverage.

The dynamics of CO oxidation on Pt(110) studied by infrared chemiluminescence of the product CO 2 : effect of CO coverage

The infrared chemiluminescence technique has been applied to the catalytic oxidation of CO on a Pt(110)(1×2) surface. The vibrational and the rotational states of CO2 formed on the reconstructed Pt(110)(1×2) surface are more excited than those on the terrace Pt(111) surface. The vibrational state of the product CO2 strongly depends on the CO coverage: the vibrational temperature (TV) of the product CO2 becomes higher, as the coverage of CO increases.

Infrared chemiluminescence study of vibrationally excited CO 2 formed by catalytic oxidation of CO over single crystal and polycrystalline Pt surfaces

The infrared chemiluminescence technique has been applied to the catalytic oxidation of CO on Pt(110), Pt(111) and polycrystalline Pt surfaces. The CO 2 molecules produced by the oxidation of CO on Pt(110)-(1 × 2) and Pt(111) were vibrationally excited but rotationally rather cool. However, the vibrational temperature ( T v) is higher on Pt(110)-(1 × 2) than on Pt(111), while T v is much lower on Pd(110) than on Pd(111). On polycrystalline Pt surfaces, the vibrational and rotational energy states of product CO 2 were altered by the pretreatment (annealing in vacuo or in air), which strongly affects the surface structure. These results suggest that the vibrational and rotational states of the CO 2 product are sensitive to the surface structure, i.e. the transition state of CO oxidation is strongly affected by the structure of the reaction sites.

Infrared chemiluminescence study of the dynamics of catalytic oxidation of CO and HCOOH on Pd(111) and polycrystalline Pd surfaces

The infrared chemiluminescence technique was applied to CO oxidation, HCOOH decomposition and HCOOH + O 2 reaction on Pd(111) single crystal and polycrystalline Pd surfaces for the study of the dynamics of the catalytic reactions. The vibrational temperature ( T V) of CO 2 formed by CO oxidation on Pd(111) was much higher than that on polycrystalline Pd foil, indicating that the dynamics of CO oxidation are structure-sensitive. The vibrational temperature ( T V) and the rotational temperature ( T R) of CO 2 formed by CO oxidation on Pd(111) increased with increasing the surface temperature, but this may not be due to the oxygen coverage effect because little change in T V was observed by changing the O 2 CO ratio. In the case of HCOOH oxidation, the rate of CO 2 formation was much more enhanced in the presence of O 2 than decomposition of HCOOH without O 2. However, the intensities of IR emission of CO 2 produced by the HCOOH decomposition and oxidation on both Pd surfaces were very weak, suggesting that the product CO 2 was not so excited vibrationally and that the internal energy distribution of the product CO 2 was quite different from that of CO 2 formed by CO oxidation (bimolecular reaction between CO(ad) and O(ad)).

Structure-sensitivity of the Dynamics of CO Oxidation on Pd(111), Pd(110) and Polycrystalline Pd Surfaces: Infrared Chemiluminescence Study of the Product CO2

Abstract The infrared chemiluminescence technique has been applied to the catalytic oxidation of CO on Pd(111), Pd(110) and polycrystalline Pd surfaces. The product CO2 molecules were vibrationally and rotationally excited in all cases. However, the vibrational energy states of CO2 were different among these three surfaces. This result means that the structure of the activated CO2 complex (i.e., the dynamics of CO oxidation) depends on the surface structures. In addition, the steady-state reaction rates of CO oxidation as a function of surface temperature were also different among the Pd surfaces. These results show that the CO oxidation on Pd is structure-sensitive not only in the kinetics but also in the dynamics.

Molecular-Beam Infrared Chemiluminescence Study of the Dynamics of Catalytic Reactions: Decomposition and Oxidation of Methanol and Formic Acid and Partial Oxidation of Butane on Pd Surface.

The infrared chemiluminescence technique was applied to catalytic oxidation of CH3OH, HCOOH and i-C4H10 on a Pd surface, and the reaction dynamics and mechanism were discussed in terms of internal energy states of nascent product CO2 and CO molecules. The vibrational and rotational states of CO2 produced by CH3OH oxidation were quite similar to those of CO2 produced by CO oxidation, indicating that the final step of the CO2 formation in the CH3OH oxidation involved a bimolecular process (i.e., COad+Oad). On the contrary, the intensity of IR emission of CO2 produced by HCOOH oxidation was very weak, suggesting that most of CO2 was not so vibrationally excited and that CO2 was formed via decomposition of HCOOad (unimolecular decomposition of formate). The selective production of CO and H2O was observed by the partial oxidation of i-C4H10 on Pd, and the vibrational state of product CO was very different between the Pd and Pt surfaces. These differences in internal energy states of the CO and CO2 products can provide us with new information on the dynamics and mechanism of catalytic reactions.

Singlet molecular oxygen generation from the decomposition of sodium peroxotungstate and sodium peroxomolybdate

Singlet oxygen is formed by thermal decomposition of peroxomolybdates and peroxotungstates in basic aqueous solution. As shown by both infrared chemiluminescence and chemical trapping, the yield of 1 O 2 released by this system is a total of about 160% (80% based on O 2 released). The infrared chemiluminescence technique is particularly useful for studying reaction kinetics in this system

Flowing afterglow infrared chemiluminescence studies of vibrational energy disposal in the ion–molecule reactions F−+HBr,DBr→HF,DF+Br−

Product vibrational state distributions for the ion–molecule reactions F−+HBr,DBr→HF(v≤4), DF (v≤6)+Br− are determined using the flowing afterglow infrared chemiluminescence technique. The nascent distributions are (0.09±0.04)v=1: (0.29±0.04)v=2: (0.34±0.04)v=3: (0.28±0.04)v=4 for the HF product, and (0.05±0.04)v=1: (0.12±0.04)v=2: (0.16±0.04)v=3: (0.25±0.04)v=4: (0.22±0.04)v=5: (0.20±0.04)v=6 for the DF product. The fractions of the available energy deposited in product vibration are 0.60±0.04 and 0.63±0.05 for the proton transfer and deuteron transfer reactions, respectively. A surprisal analysis suggests that less than 5% of the product molecules are formed in v=0. The HF distribution is somewhat hotter than that reported previously, while the DF distribution is measured for the first time. Both distributions are remarkably similar to those reported for the analogous neutral processes, which suggests that direct collisions dominate the reactive encounters despite the presence of a deep attractive well in the potential surface for the ion–molecule reactions.

Energy partitioning in atom–radical reactions: The reaction of F atoms with NH2

An extension of the low-pressure infrared chemiluminescence technique has allowed the measurement of energy partitioning in the atom/radical reactions: F+NH2→HF+NH, F+ND2→DF+ND. A complete numerical model of the experiment is described in detail including its parametrization. This model allows the unambiguous determination of the primary energy distribution of the above reactions. These reactions give inverted product energy distributions, in contrast to the isoelectronic F+OH→HF+O reaction. The inverted primary energy distribution for F+NH2/ND2 indicates a direct abstraction mechanism. Ab initio quantum chemical computations on some features of the relevant potential energy surfaces support this direct abstraction route. An energetically accessible transition state, having approximately zero barrier, is found on the triplet surface which directly correlates reagents and products. The geometry of this triplet transition state is also suggestive of strong HF vibrational excitation. Abstraction on the triplet surface provides an alternative pathway to reaction on the lowest singlet surface, which contains a deep potential energy well corresponding to NH2F.

Energy distributions in the HF and CO products of the reaction of F atoms with HCO

The initial energy distributions in both products of the title reaction have been measured using a variant of the low-pressure infrared chemiluminescence technique. The vibrational populations of both products decrease with increasing vibrational level, indicating of the formation of a long-lived HFCO intermediate. The HF vibrational population is well represented by a (restricted phase space) statistical calculation. The CO vibrational excitation is considerably colder than this calculation predicts. These results, combined with ab initio reaction coordinate calculations suggest that free exchange of energy among the modes of the HFCO intermediate ceases early in the exit channel. Thereafter, an excess of energy is partitioned into relative translation of the products.

Vibrational relaxation of highly excited diatomics. VII. DF(v=9–12) and HF(v=5–7)+HF(v=0), DF(v=0) in all combinations

Vibrational relaxation rate constants are measured for DF(v=9–12) and HF(v=5–7) by ground state HF and DF using the fast-flow infrared chemiluminescence technique. The 14 rate constants range from (1.4 to 5.5)×10−10 cm3 s−1, i.e., probabilities per gas-kinetic (Lennard–Jones) collision from 0.47 to 1.72. The rate constants are independent of the magnitude or sign of the vibrational (V→V) energy defect which indicates that these are V→R,T processes. They are slightly faster for HF than for DF quencher. These results are discussed in the context of theoretical trajectory calculations and comparisons with other highly excited diatomics as well as other quenchers.

Vibrational relaxation of highly excited diatomics. VI. DF(9≤v≤12)+N2, CO, CO2, and N2O and HF(v=5–7)+CO

Vibrational relaxation rate constants are determined for DF(v=9–12) by Q=N2, CO, CO2, and N2O and for HF(v=5–7) by CO using the fast flow infrared chemiluminescence technique. The rate constants range from 0.6 to 35×10−11 cm3 s−1, and with the exception of Q=N2, the energy transfer probabilities P per gas-kinetic (Lennard-Jones) collision are large 0.4<P<1.0. The vibrational energy gap ΔE is small for the DF(v) relaxation processes and the v dependence of kQv, v−1 is much weaker than that for HF(v). The data suggest independent effects based on ΔE and on v which partially cancel each other for DF, but not for HF. The initial, unrelaxed v distribution for the D+F2→DF(v≤13)+F reaction is found to peak at v=9, in disagreement with earlier experimental studies but in agreement with recent theoretical work.

Evaluation of HX (X = Cl, Br, O) product distributions from chemiluminescence. I. H atom reactions

The HX product state distributions from the H+Cl2, Br2, NO2Cl, PCl3, and NO2 reactions have been studied by the infrared chemiluminescence technique in two different laboratories with two types of reactors; a fast-flow system with = 1 Torr of Ar buffer gas and a low-pressure, cold-wall system (usually called the cold-wall arrested-relaxation method). The same Einstein coefficients were used in both laboratories to convert intensities to populations and emphasis is placed upon evaluation of the reliability of the resulting vibrational-rotational HX distributions. Good agreement was found between the HX distributions from the cold-wall reactors from the two laboratories and for both types of reactors for all of the reactions, except PCl3. For the H+Cl2, Br2 and NO2 reactions, our general results are in good accord with presently accepted data; but, our experiments provide somewhat more detail than in the literature. The NO2Cl results are new and = 0.40 and = 0.01. The H+PCl3 reaction appears to proceed by two channels and the HCl chemiluminescence cannot be assigned only to HCl formation via direct Cl atom abstraction.

Vibrational relaxation of highly excited diatomics. IV. HF(v=1–7) + CO2, N2O, and HF

Vibrational relaxation rate constants are measured for HF(v=1–4) with Q=CO2, N2O, and HF by the fast flow infrared chemiluminescence technique using four HF(v) generating reactions whose initial vibrational distributions are found be be unrelaxed. The data are combined with earlier results for v=5, 6, and 7 to provide information on v dependence and quenching mechanism. The rate constants, kQv,v−1 range from 1.2×10−12 to 4.5×10−10 cm3 s−1 and increase with power law exponents n of 2.7 to 3.0 in k∝vn for all three quenchers. The relaxation is principally V–V for CO2 and N2O, but mainly V-R,T for HF, at least for the higher v levels. The relaxation rate constants are compared with theoretical estimates and form a valuable data base for future theoretical work.

Energy partitioning in some atom/radical reactions found in atmospheric pollution systems

A modification of the arrested relaxation infrared chemiluminescence technique has been used to study the energy disposal in two atom–radical reactions. Vinyl and formyl radicals were generated chemically and their subsequent reactions with O atoms (in the case of vinyl) and F atoms (in the case of formyl) were followed. Product CO emission was observed in the system O/C2H3. Emission from both CO and HF was seen in the F/HCO system. The O/C2H3 reaction gives rise to substantially higher excitation in the CO product than does the F/HCO case. In each case, bound intermediates may be formed; the differences in energy diposal are discussed in terms of the possible influence of these intermediates.

Vibrational relaxation of highly excited diatomics. III. HF(v = 5, 6, 7)+H2, D2, N2, HF, CO2, N2O, CH4, and C2H6

Vibrational relaxation rate constants kQv are reported for HF (v = 5, 6, 7) collisions with eight quencher molecules Q = H2, D2, N2, HF, CO2, N2O, CH4, and C2H6, at 298 K under conditions of rotational equilibrium, using the fast flow infrared chemiluminescence technique. The rates are faster than had been reported by some investigators, the energy transfer probability PQv ranging from PH25 = 3.2×10−3 to PHF7 = 1.45. The rate constants rise with increasing v, i.e., n = 2.0 to 8.4 for different Q in the vn correlation. For all Q except H2 and HF, V-V transfer is suggested, but the Lambert–Salter plots have different slopes for different Q. The data are compared with published measurements and with theory, the latter especially for Q = HF.