Multiphoton breakdown of acetylene; formation of organic building block fragments.

Mass resolved REMPI spectra and electron and ion velocity map images were recorded for REMPI of acetylene in the case of two-photon resonant excitations to low lying 3p and 4p Rydberg states. Combined data analysis of ion signal intensities and electron and ion kinetic energy release distribution revealed multiphoton-fragmentation processes in terms of photodissociation and photoionization channels to form the molecular ion, C2H2+ and the fragment ions H+, C+, CH+, CH2+, C2+ and C2H+. The ratio of fragment ion formation over the parent ion formation increases with excitation energy. To a large extent, multiphoton-fragmentation involves the initial breakdown of the molecule into ground and excited state neutral fragments by two-, three- and four-photon dissociation processes prior to multiphoton ionization. The three-photon dissociation processes via superexcited molecular state(s) are found to be the most important and electronically excited fragment species playing a significant role in the overall multiphoton-fragmentation. Furthermore, the data are indicative of the involvement of secondary photodissociation processes and provide information on fragment energetics as well as state interactions. The question, whether acetylene could be an important source of building block fragments for the formation of organic molecules in interstellar space, is addressed.

High energy state interactions, energetics and multiphoto-fragmentation processes of HI.

Mass resolved multiphoton ionization data for two-photon resonant excitations (REMPI) in the 69 000-79 000 cm-1 region were recorded for HI. REMPI spectra of fragment and molecular ions were derived from the data and analysed to obtain information relevant to the state interactions, energetics and photofragmentation processes of intermediate Rydberg and ion-pair states (HI**). Spectral perturbations observed as line shifts and intensity anomalies acted complementarily to demonstrate the effects of the state interactions. The interaction strength and character mixing of Rydberg states and Rydberg and ion-pair states of different interaction types and the states energetics were quantified by deperturbation analysis for the high energy region of 75 000-79 000 cm-1, which is dense in states. Energetics of new, not previously observed, Rydberg states, detected in the lower energy excitation region of 69 000-75 000 cm-1 was characterized by simulation calculations. Ion intensity borrowing effects, found in the spectra of interacting states, are evidence of alterations in two-photon transition probabilities due to state mixing. Based on variations in relative spectral line intensities the major photofragmentation processes involved are proposed. These involve one-photon excitation of the intermediate states (HI**) to form repulsive superexcited states (HI#) followed by autoionization, dissociation, photodissociation and photoionization processes to form ions. The importance of state interactions in multiphoton-fragmentation processes is evident from the work.

Formation of highly excited iodine atoms from multiphoton excitation of CH3I.

Mass resolved REMPI spectra, as well as CH3+and I+ ion and photoelectron images, were recorded for two-photon resonant excitations of CH3I via s, p and d Rydberg states (CH3I**) in the excitation region of 55 700 to 70 000 cm-1. Photoelectron (PE) and ion kinetic energy release spectra (KERs) were derived from the images. The data revealed that after the two-photon resonant excitation, an additional photon is absorbed to form one or more superexcited state(s) (CH3I#), followed by branching into three pathways. The major one is the dissociation of CH3I# to form excited Rydberg states of iodine atoms (I**) along with CH3(X), a phenomenon not commonly observed in methyl halides. The second (minor) pathway involves autoionization of CH3I# towards CH3I+(X), which absorbs another photon to form CH3+ along with I/I* and the third one (minor) is CH3I# dissociation towards the ion pair, CH3+ + I-, prior to I- electron ejection. Furthermore, one-photon non-resonant dissociation of CH3I to form CH3(X) and I/I* prior to three-photon ionization of the fragments is also detected.

High energy Rydberg and ion-pair states, state mixing and excitation dynamics of HI.

Mass resolved multiphoton ionization data for two-photon resonant excitations (REMPI) in the region of 74 000-75 000 cm-1 were recorded for HI. Spectra structures of fragment and molecular ions derived from the data were analyzed to derive information relevant to the energetics and state mixing of ion-pair and Rydberg resonance states as well as for the excitation dynamics. Four new ion-pair vibrational states (V1Σ+(v' = m + i); i = 16-19) and two Rydberg states (j3Σ-(0+; v' = 1) and N1Π1(v' = 2)) were identified and characterized. Spectral perturbations allowed characterization and quantization of both homogeneous and heterogeneous state interactions and mixing of the Rydberg states and ion-pair states. Intensity alterations, with respect to energy level excitations and ion masses, are found to be clear indications of state mixing as well as branching into different fragmentation (both photodissociation and photoionization) channels.

Calculations of NO reduction with CO over a Cu1/PMA single-atom catalyst: a study of surface oxygen species, active sites, and the reaction mechanism.

Density functional theory (DFT) calculations have been employed to probe the reaction mechanism of NO reduction with CO over a Cu1/PMA (PMA is the phosphomolybdate, Cs3PMo12O40) single-atom catalyst (SAC). Several important aspects of the catalytic system were addressed, including the generation of oxygen vacancies (Ov), formation of N2O2 intermediates, scission of the N-O bond of N2O2 intermediates to form N2O or N2, and decomposition of N2O to form N2. Unlike most previous theoretical studies, which tend to explore the reaction mechanism of polyoxometalate (POM) systems based on the isolated anionic unit, here, we build a model of the catalytic system with neutral species by introduction of counter cations to model the solid structure of the Cu1/PMA SAC. The major findings of our present study are: (1) CO adsorption on Cu sites leads to the formation of cationic Cu carbonyl species; (2) the Oc atom at the surface of the PMA support can easily react with the adsorbed CO to generate a Cu-Ov pair; (3) the Cu-Ov pair embedded on PMA is found to be the active site, not only for the formation of N2O2* by the reaction of two NO molecules via an Eley-Rideal pathway but also for the decomposition of N2O to form N2; (4) the adsorption of a NO molecule on the Cu-Ov pair with a bridging model results in charge transfer from the Cu atom to the π* antibonding orbital of the NO molecule; (5) IR spectroscopy of the key intermediates has been identified based on our DFT calculations; and (6) the Cu atom serves as an electron acceptor in Ov formation steps and an electron donor in N2O2 decomposition steps, and thus represents an electron reservoir. These results suggest that the POM-supported SAC with the cheaper Cu element is an efficient catalyst for the reaction between CO and NO.

Computational Study on M1/POM Single-Atom Catalysts (M = Cu, Zn, Ag, and Au; POM = [PW12O40]3-): Metal-Support Interactions and Catalytic Cycle for Alkene Epoxidation.

Geometrical structures, metal-support interactions, and infrared (IR) spectroscopy of a series of M1/POM (M = Cu, Zn, Ag, and Au; POM = [PW12O40]3-) single-atom catalysts (SACs), and catalytic cycle for alkene epoxidation catalyzed by M1/POM SACs were studied using density functional theory (DFT) calculations. The calculations demonstrate that the most probable anchoring sties for the isolated single atoms studied here in the M1/POM SACs are the fourfold hollow sites on the surface of POM support. The bonding interaction between single metal atom and surface of POM support comes from the molecular orbitals with a mixture of d atomic orbital of metal and 2p group orbital of surface oxygen atoms of POM cage. The calculated adsorption energy of isolated metal atoms in these M1/POM SACs indicates that the early transition metals (Cu and Zn) have high thermal stability. The DFT-derived IR spectra show that the four characteristic peaks of free Keggin-type POM structure split into six because of introduction of isolated metal atom. Compared with other metal atoms, the Zn1/POM SAC has the high reactivity for activity of dioxygen molecule, because the dioxygen moiety in Zn1/POM SAC displays O2-· radical feature with [POM4-·Zn2+O2-·]3- configuration. Finally, a catalytic cycle for ethylene epoxidation by O2 catalyzed by Zn1/POM SAC was proposed based on our DFT calculations. Supported noble-metal SACs are among the most important catalysts currently. However, noble metals are expensive and of limited supply. Development of non-noble-metal SACs is of essential importance. Therefore, the reported Zn1/POM SAC would be very useful to guide the search for SACs into non-noble metals.

New insight into the catalytic cycle about epoxidation of alkenes by N2O over a Mn-substituted Keggin-type polyoxometalate.

Although epoxidation of alkenes by N2O catalyzed by Mn-substituted polyoxometalates (POMs) has been studied both experimental and theoretical methods, a complete catalytic cycle has not been established currently. In the present paper, density functional theory (DFT) calculations were employed to explore possible reaction mechanism about this catalytic cycle. Our DFT studies reveal that the reaction pathway starts from a low-valent Keggin-type POM aquametal derivative [PW11O39MnIIIH2O]4-. In the presence of N2O pressure, the formation of the active catalytic species [PW11O39MnVO]4- involves a ligand-substituted reaction about replacement of the aqua ligand with N2O to generation of POM/N2O adduct [PW11O39MnIIION2]4- and dissociation of N2 from this adduct. The calculated free energy indicates that the ligand-substituted reaction is endergonic both in gas phase or various solvents. The partial optimization method reveals that the dissociation of N2 from [PW11O39MnIIION2]4- involves crossing of the quintet state with a low-lying triplet state. Due to the high reactivity, the high-valent MnV-oxo species, [PW11O39MnVO]4-, may react with the excess N2O and alkenes. Thus, two alternative reaction pathways corresponding to activation of N2O and epoxidation of alkenes have been considered in this work. The calculated free energy profile indicates that epoxidation of alkenes pathway is the favorable routes. Finally, a complete catalytic cycle for this reaction has been proposed. The rate-determining step in this catalytic cycle is the dissociation of N2 from the low-valent POM/N2O adduct according to our DFT-M06L calculations.