H2CO Dissociation Research Articles

Structure Sensitivity of CO2 Conversion over Nickel Metal Nanoparticles Explained by Micro-Kinetics Simulations.

Nickel metal nanoparticles are intensively researched for the catalytic conversion of carbon dioxide. They are commercially explored in the so-called power-to-methane application in which renewably resourced H2 reacts with CO2 to produce CH4, which is better known as the Sabatier reaction. Previous work has shown that this reaction is structure-sensitive. For instance, Ni/SiO2 catalysts reveal a maximum performance when nickel metal nanoparticles of ∼2-3 nm are used. Particularly important to a better understanding of the structure sensitivity of the Sabatier reaction over nickel-based catalysts is to understand all relevant elementary reaction steps over various nickel metal facets because this will tell as to which type of nickel facets and which elementary reaction steps are crucial for designing an efficient nickel-based methanation catalyst. In this work, we have determined by density functional theory (DFT) calculations and micro-kinetics modeling (MKM) simulations that the two terrace facets Ni(111) and Ni(100) and the stepped facet Ni(211) barely show any activity in CO2 methanation. The stepped facet Ni(110) turned out to be the most effective in CO2 methanation. Herein, it was found that the dominant kinetic route corresponds to a combination of the carbide and formate reaction pathways. It was found that the dissociation of H2CO* toward CH2* and O* is the most critical elementary reaction step on this Ni(110) facet. The calculated activity of a range of Wulff-constructed nickel metal nanoparticles, accounting for varying ratios of the different facets and undercoordinated atoms exposed, reveals the same trend of activity-versus-nanoparticle size, as was observed in previous experimental work from our research group, thereby providing an explanation for the structure-sensitive nature of the Sabatier reaction.

Tunneling Rate Constants for H2CO+H on Amorphous Solid Water Surfaces

Abstract Formaldehyde (H2CO) is one of the most abundant molecules observed in the icy mantle covering interstellar grains. Studying its evolution can contribute to our understanding of the formation of complex organic molecules in various interstellar environments. In this work, we investigated the hydrogenation reactions of H2CO yielding CH3O, CH2OH, and the hydrogen abstraction resulting in H2+HCO on an amorphous solid water (ASW) surface using a quantum mechanics/molecular mechanics (QM/MM) model. The binding energies of H2CO on the ASW surface vary broadly, from 1000 to 9370 K. No correlation was found between binding energies and activation energies of hydrogenation reactions. Combining instanton theory with QM/MM modeling, we calculated rate constants for the Langmuir–Hinshelwood and the Eley–Rideal mechanisms for the three product channels of H+H2CO surface reactions down to 59 K. We found that the channel producing CH2OH can be ignored, owing to its high activation barrier leading to significantly lower rates than the other two channels. The ASW surface influences the reactivity in favor of formation of CH3O (branching ratio ∼80%) and hinders the H2CO dissociation into H2+HCO. In addition, kinetic isotope effects are strong in all reaction channels and vary strongly between the channels. Finally, we provide fits of the rate constants to be used in astrochemical models.

Adsorption and decomposition of formaldehyde on the B12N12 nanostructure: a density functional theory study

Minnesota functionals computations DFT/M06-2X is carried out to study the adsorption and breakdown of formaldehyde (H2CO) on the top of a boron nitride nanostructure, B12N12, BNn. Two chemical and physical adsorption types and two reaction routes are recognized. The electrical properties of the nanostructure can be changed when the adsorption of H2CO occurs on the top of BNn. The routes of H2CO dissociation via breaking of the C–O and C–H bonds are investigated. For the H2CO dissociation on the top of B12N12, the rate determining step follows the reaction CH2O → CHO + H. The computations suggest that the C–H bond cleavage is the most desirable route on the top of BNn.

Improved virtual orbitals in state specific multireference perturbation theory for prototypes of quasidegenerate electronic structure.

The state-specific multireference perturbation theory (SSMRPT) with an improved virtual orbital complete active space configuration interaction (IVO-CASCI) reference function [called as IVO-SSMRPT] is used to investigate the energy surface, geometrical parameters, molecular properties of spectroscopic interest for the systems/situations [such as BeH2, BeCH2, MgCH2, Si2H4, unimolecular dissociation of H2CO, and intramolecular reaction pathways of 1,3-butadiene] where the effect of quasidegeneracy cannot be neglected. The merit of using the IVO-CASCI rather than complete active space self-consistent field (CASSCF) is that it is free from iterations beyond those in the initial SCF calculation and the convergence difficulties that plague CASSCF calculations with increasing size of the CAS. While IVO-CASCI describes the non-dynamical correlation, the SSMRPT scheme is a good second-order perturbative approximation to account for the rest of the correlation energy. Our IVO-SSMRPT method is instrumental in avoiding intruder states in an size-extensive manner and allows the revision of the content of wave function in the model space. It can treat model as well as real systems with predictive accuracy, as is evident from the fairly nice accordance between our estimates, and high-level theoretical results. Our estimates also corroborate well with some experimental findings.

Experimental and Theoretical Investigation of Triple Fragmentation in the Photodissociation Dynamics of H2CO

The photodissociation dynamics of H2CO molecules at energies bracketing the triple fragmentation threshold were investigated using velocity map ion imaging of the H-atom fragments. An algorithm was developed to model the experimental results as a two-step process: initially barrierless C-H bond fission on the S0 potential energy surface to form H + HCO, followed by secondary fragmentation of those HCO radicals with sufficient internal energy to overcome the small exit channel barrier on the HCO surface to form H + CO. Our model treats the first step using phase space theory (PST) and the second using a combined PST-impulsive model, with a tunneling correction. Experimentally, triple fragmentation reaches 25% of the radical (H + HCO) channel photochemical yield at energies about 1500 cm(-1) above the barrier for breaking the second bond. In addition, the triplet (T1) channel appears to reduce in importance after the barrier on the T1 surface is exceeded, slowly decreasing to <10% of the total radical yield at higher energy. The double PST-impulsive model provides a good fit to the experimental H-atom speed and energy distributions for H2CO dissociation on S0 spanning >7000 cm(-1) of available energy.

THE FAR-ULTRAVIOLET SPECTRAL SIGNATURES OF FORMALDEHYDE AND CARBON DIOXIDE IN COMETS

Observations of four comets made with the Far Ultraviolet Spectroscopic Explorer show the rotational envelope of the (0,0) band of the CO Hopfield-Birge system (C - X) at 1088 A to consist of both "cold" and "hot" components, the "cold" component accounting for ~75% of the flux and with a rotational temperature in the range 55-75 K. We identify the "hot" component as coming from the dissociation of CO2 into rotationally "hot" CO, with electron impact dissociation probably dominant over photodissociation near the nucleus. An additional weak, broad satellite band is seen centered near the position of the P(40) line that we attribute to CO fluorescence from a non-thermal high J rotational population produced by photodissociation of formaldehyde into CO and H2. This process also leaves the H2 preferentially populated in excited vibrational levels which are identified by fluorescent H2 lines in the spectrum excited by solar OVI 1031.9 and solar Lyman-alpha. The amount of H2 produced by H2CO dissociation is comparable to the amount produced by photodissociation of H2O. Electron impact excitation of CO, rather than resonance fluorescence, appears to be the primary source of the observed (B - X) (0,0) band at 1151 A.

The unimolecular dissociation of H2CO on the lowest triplet potential-energy surface

The unimolecular dissociation reaction of H2CO on the triplet potential-energy surface has been studied via ab initio electronic structure theory. The stationary point geometries for the equilibrium and transition state are determined employing the configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory with large basis sets up to the correlation consistent (cc)-pVQZ basis. With the best method, cc-pVQZ CCSD(T), the first excited triplet (ã 3A″) state lies 72.2 kcal/mol (25 260 cm−1) above the ground (X̃ 1A1) state of H2CO, which is in excellent agreement with the experimental observation of 72.03 kcal/mol (25 194 cm−1). The dissociation limit (H⋅+HCO⋅) is located at 86.3 kcal/mol (30 170 cm−1) above the ground state of H2CO, which is again in good agreement with the two experimentally determined values of 86.57 kcal/mol (30 280 cm−1) and 86.71 kcal/mol (30 328.5 cm−1). With the same method the triplet dissociation transition state lies 92.4 kcal/mol (32 300 cm−1) above the ground state. Consequently, the activation energy for the dissociation reaction of H2CO on the triplet surface is determined ab initio to be 18.9–20.1 kcal/mol (6620–7040 cm−1) (including an estimated error bar of 1.2 kcal/mol or 420 cm−1). The zero-point vibrationally corrected exit barrier height is predicted to be 4.9–6.1 kcal/mol (1710–2130 cm−1). These newly predicted energies are consistent with the recent experimental observations by the Moore group at University of California-Berkeley (1987) and by the Wittig group at University of Southern California (1997).

A Comparative Study of CO and CO2Hydrogenation over Rh/SiO2

The hydrogenation of CO and CO2over Rh/SiO2have been investigated for the purpose of identifying the similarities and differences between these two reaction systems.In situinfrared spectroscopy was used to characterize the surface of the catalyst. Exposure of the catalyst to CO or CO2produced very similar infrared spectra in which the principal features are those for linearly and bridge-bonded CO. In the case of CO2adsorption, a band for weakly adsorbed CO2could also be observed. For identical reaction conditions the rate of CO2hydrogenation to methane is higher than that for CO hydrogenation. The activation energy for CO hydrogenation is 23.2 kcal/mol and that for CO2hydrogenation is 16.6 kcal/mol. The partial pressure dependances on H2and COz(z=1, 2) are 0.67 and −0.80, respectively, for CO hydrogenation, and 0.53 and −0.46, respectively, for CO2hydrogenation. Infrared spectroscopy reveals that under reaction conditions the catalyst surface is nearly saturated by adsorbed CO. The spectra observed during CO and CO2hydrogenation are similar, the principal difference being that the CO coverage during CO hydrogenation is somewhat higher than that during CO2hydrogenation. The CO coverage is insensitive to H2partial pressure, but increases slightly with increasing COzpartial pressure. Transient-response experiments demonstrate that the adsorbed CO is a critical intermediate in both reaction systems. It is proposed that the rate determining step in the formation of methane is the dissociation of H2CO, produced by the stepwise hydrogenation of adsorbed CO. A rate expression derived from the proposed mechanism properly describes the experimentally observed reaction kinetics both under steady-state and transient-response conditions.

Rotation–vibration state resolved unimolecular dynamics of highly excited CH3O (X̃ 2E). II. Intramolecular vibrational dynamics of excited ‘‘C–O stretch’’ states

The stimulated emission pumping (SEP) spectra of highly excited CH3O (X̃ 2E) reported in the preceding paper [A. Geers, J. Kappert, F. Temps, and J. Wiebrecht, J. Chem. Phys. 101, 3618 (1994); paper I] are analyzed to extract quantitative data on the collision-free intramolecular vibrational dynamics of the molecules. Attention is focused on the spectra and dynamics of the excited C–O stretch vibration (ν3) states at energies between 3 000 cm−1≤E≤10 000 cm−1. The spectra are found to exhibit a rather sudden transition at E≊5000 cm−1 from the regular, assignable properties at low energies to the ‘‘statistical’’ region at high energies. This IVR ‘‘threshold’’ is approximately 2000 cm−1 below the asymptotic H–H2CO dissociation limit of the molecule. The observed densities of states at these energies approach the full J- and symmetry-sorted rovibronic densities estimated from harmonic state counts. The nearest-neighbor level spacing distribution in the spectra and the magnitude of the inferred root mean square level coupling matrix elements support the assumption of nearly ‘‘ergodic’’ molecular properties in this region. From computed time autocorrelation functions, the IVR lifetimes of the highly excited C–O stretch states are found to be of the order of τ≊0.2–0.3 ps (v3=5 to 6). Evidence is obtained from some spectra for two-tier IVR processes with characteristic time scales of 0.2 and 2 ps, respectively. The highest excited C–O stretch states at energies above ≊8000 cm−1 (v3≥8) appear to be coupled only weakly to neighboring background states. This last observation hints at the existence of quasistable periodic orbits, which are embedded in the quasicontinuum of dissociative states at these energies.

Rotation–vibration state resolved unimolecular dynamics of highly vibrationally excited CH3O (X̃ 2E). I. Observed stimulated emission pumping spectra

Using the technique of stimulated emission pumping (SEP) spectroscopy, highly excited vibration–rotation states of the CH3O (X̃ 2E) molecule were probed up to energies of E≤10 000 cm−1. The highest excitation energies exceed the asymptotic H–H2CO dissociation limit of the molecule [ΔrH00(H–H2CO)≊6900 cm−1]. Work was carried out at different experimental resolutions. First, low resolution survey SEP spectra were found to exhibit persistent vibrational structure up to energies far above the dissociation limit. The observed main features were found to be assignable, in a zero-order picture that leaves aside possible mode-to-mode couplings, to the progression of the excited C–O stretch vibration states (ν3). The widths of the respective features correspond to localized short-time vibrational motion for times of ≥0.3 ps (≥10 C–O vibrational periods). Second, in high resolution scans over the coarse vibrational features, characteristic clumps of individual vibration–rotation eigenstates were revealed. These clumps are ascribed to distinctive Franck–Condon active bright zero-order levels which are mixed with the large number of Franck–Condon inactive dark bath states. Under carefully selected conditions, the clumps could be attributed to states with defined and well known values of the total angular momentum quantum number J, which remains as a good quantum number in different coupling cases. These clump spectra will be analyzed quantitatively in the following paper with respect to their bearing for the intramolecular vibrational dynamics of highly excited CH3O (X̃) as a function of vibrational and rotational excitation. From the observed spectra, quantitative data can be obtained on the rate and extent of collision-free intramolecular vibrational and rovibrational energy redistribution (IVR and IRVR) processes, which would result after coherent ultrashort pulse excitation of the molecules.

ChemInform Abstract: The Impact of Vibrationally Excited Levels of the Transition State on CO (υ = 0,J) Distributions Resulting from Dissociation of H2CO.

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The impact of vibrationally excited levels of the transition state on CO (v=0,J ) distributions resulting from dissociation of H2CO

The dissociation of formaldehyde into molecular products is known to proceed through vibrationally excited, quantum ergodic levels of the ground electronic state (H2CO*) and then a transition state (H2CO‡) according to the following mechanism: H2CO(S0)+hν→H2CO(S1)→H2CO*(S0) →H2CO‡→H2+CO. This paper reports the distribution of rotational energy in the CO (v=0) fragment following excitation to single rotational levels of the S1 2141 band of formaldehyde. The maxima of the rotational distributions resulting from photolysis on different rotational lines are observed to vary between J=38 and 49. Variations in the widths of the distributions are also observed. These variations show no systematic trends in the quantum indices or radiative lifetime of the parent molecule. Distributions calculated using an infinite-order-sudden approximation-based algorithm show fluctuations similar to those observed experimentally when each of the vibrational levels of the transition state through which the molecule can decay is randomly weighted. Thus it is understood that the ergodic character of each H2CO* S0 eigenstate uniquely projects onto the vibrational levels of the transition state, and since different levels of the transition state give rise to differing product distributions, decay through different S0 levels produces differing rotational distributions.

Stimulated-emission pumping for state-selective initiation of unimolecular reactions

Stimulated-emission pumping is successfully applied to the study of unimolecular dissociation dynamics of HFCO → HF + CO in a pulsed jet. Dissociative levels of the ground electronic state are prepared by stimulated-emission pumping, and the CO fragments formed from these levels are probed by vacuum-ultraviolet laser-induced fluorescence spectroscopy. The rotational energy distribution of the CO fragments indicates modest differences between the dissociation dynamics of HFCO and those of H2CO. However, the Doppler width of CO fragments from HFCO indicates that the total kinetic energy of the fragments from HFCO is ~65% of total available energy, the same fraction as in the dissociation of H2CO. The linewidths of the dissociative levels are also measured with high-resolution fluorescence-dip-detected stimulated-emission pumping spectroscopy.

The photodissociation of formaldehyde: A coupled cluster study including connected triple excitations of the transition state barrier height for H2CO→H2+CO

A b initio molecular quantum mechanics has been applied to the unimolecular dissociation of H2CO. Basis sets as large as triple zeta plus double polarization (TZ+2P) were used in conjunction with complete optimization of all stationary point geometries. The classical barrier height is predicted with the TZ+2P basis set to be 101.9 (SCF), 95.0 (CISD), 90.4 (CCSD), and 86.8 kcal/mol (CCSDT-1). With correction for zero-point vibrational energies, the activation energy is predicted to be 81.4 kcal/mol, in good agreement with experimental estimates.

Coherent anti-Stokes Raman scattering diagnostics of combustion and other reactive media at ONERA

Coherent anti-Stokes Raman scattering (CARS) is well adapted to the measurement of temperatures and concentrations of molecules in reactive gaseous media. The general principles are given along with the major technical advantages and requirements. Characteristic applications are described in order to present state-of-the-art performance levels: measurement time, spatial resolution, detection sensitivity. Results are presented of instantaneous temperature measurements in combustion and in piston engines, measurements of rotational–vibrational populations of H2 in a magnetic multicusp discharge, and measurements of the vibrational excitation of the prompt H2 formed in H2CO dissociation.