Efficiency Of H2 Formation Research Articles

Strong Enhancement in Cobalt(II)‐TPMA Aqueous Hydrogen Photosynthesis through Intramolecular Proton Relay

AbstractPhotosynthetic hydrogen generation by cobalt(II) tris(2‐pyridylmethyl)amine (TPMA) complexes is mainly limited by protonation kinetics and decomposition routes involving demetallation. In the present work we have explored the effects of both proton shuttles and improved rigidity on the catalytic ability of cobalt(II) TPMA complexes. Remarkably, we demonstrate that, while a small enhancement in the catalytic performance is attained in a rigid cage structure, the introduction of ammonium groups as proton transfer relays in close proximity to the cobalt center allows to reach a 4‐fold increase in the quantum efficiency of H2 formation, and a surprising 22‐fold gain in the maximum turnover number, at low catalyst concentration. The beneficial role of the ammonium relays in promoting faster intramolecular proton transfer to the reduced cobalt center is documented by transient absorption spectroscopy, showcasing the great relevance of tuning the catalyst periphery to achieve efficient catalysis of solar fuel formation.

Strong Enhancement in Cobalt(II)-TPMA Aqueous Hydrogen Photosynthesis through Intramolecular Proton Relay.

Photosynthetic hydrogen generation by cobalt(II) tris(2-pyridylmethyl)amine (TPMA) complexes is mainly limited by protonation kinetics and decomposition routes involving demetallation. In the present work we have explored the effects of both proton shuttles and improved rigidity on the catalytic ability of cobalt(II) TPMA complexes. Remarkably, we demonstrate that, while a small enhancement in the catalytic performance is attained in a rigid cage structure, the introduction of ammonium groups as proton transfer relays in close proximity to the cobalt center allows to reach a 4-fold increase in the quantum efficiency of H2 formation, and a surprising 22-fold gain in the maximum turnover number, at low catalyst concentration. The beneficial role of the ammonium relays in promoting faster intramolecular proton transfer to the reduced cobalt center is documented by transient absorption spectroscopy, showcasing the great relevance of tuning the catalyst periphery to achieve efficient catalysis of solar fuel formation.

Effect of surface H2 on molecular hydrogen formation on interstellar grains

ABSTRACT We investigate how the existence of hydrogen molecules on grain surfaces may affect H2 formation efficiency in diffuse and translucent clouds. Hydrogen molecules are able to reduce the desorption energy of H atoms on grain surfaces in models. The detailed microscopic Monte Carlo method is used to perform model simulations. We found that the impact of the existence of H2 on H2 formation efficiency strongly depends on the diffusion barriers of H2 on grain surfaces. Diffuse cloud models that do not consider surface H2 predict that H atom recombination efficiency is above 0.5 over a grain temperature (T) range 10 and 14 K. The adopted H2 diffusion barriers in diffuse cloud models that consider surface H2 are 80${{\ \rm per\ cent}}$ H2 desorption energies so that H2 can be trapped in stronger binding sites. Depending on model parameters, these diffuse cloud models predict that the recombination efficiency is between nearly 0 and 0.5 at 10 ≤T≤ 14 K. Translucent cloud model results show that H2 formation efficiency is not affected by the existence of surface H2 if the adopted average H2 diffusion barrier on grain surfaces is low (194 K) so that H2 can diffuse rapidly on grain surfaces. However, the recombination efficiency can drop to below 0.002 atT≥ 10 K if higher average H2 diffusion barrier is used (255 K) in translucent cloud models.

Highly Dispersed Co Nanoparticles Prepared by an Improved Method for Plasma-Driven NH3 Decomposition to Produce H2

Previous studies reveal that combining non-thermal plasma with cheap metal catalysts achieved a significant synergy of enhancing performance of NH3 decomposition, and this synergy strongly depended on the properties of the catalyst used. In this study, techniques of vacuum-freeze drying and plasma calcination were employed to improve the conventional preparation method of catalyst, aiming to enhance the activity of plasma-catalytic NH3 decomposition. Compared with the activity of the catalyst prepared by a conventional method, the conversion of NH3 significantly increased by 47% when Co/fumed SiO2 was prepared by the improved method, and the energy efficiency of H2 production increased from 2.3 to 5.7 mol(kW·h)−1 as well. So far, the highest energy efficiency of H2 formation of 15.9 mol(kW·h)−1 was achieved on improved prepared Co/fumed SiO2 with 98.0% ammonia conversion at the optimal conditions. The improved preparation method enables cobalt species to be highly dispersed on fumed SiO2 support, which creates more active sites. Besides, interaction of Co with fumed SiO2 and acidity of the catalyst were strengthened according to results of H2-TPR and NH3-probe experiments, respectively. These results demonstrate that employing vacuum-freeze drying and plasma calcination during catalyst preparation is an effective approach to manipulate the properties of catalyst, and enables the catalyst to display high activity towards plasma-catalytic NH3 decomposition to produce H2.

Adsorption energies of H and H2: a quantum-chemical study

The chemical composition of interstellar grain mantle is mostly dependent on adsorption energies of the surface species. Since hydrogen is widespread either in atomic or in molecular form, our aim in this work is to review (by quantum chemical calculations) the variation of the adsorption energies of H and H2 depending on the nature of the adsorbents. Choice of absorbents was based on relative abundances of interstellar materials. Since carbonaceous and silicate grains are very abundant, we used them as our absorbents. To save computational time, benzene (smallest structure sample of PAHs) is employed as carbonaceous material and for silicate grain, simple cluster of silicon dioxide (silica) (SiO2)3 is used. Around dense cloud regions, water is the major constituent of a grain mantle, therefore, usage of binding energies with bare grains is immaterial. To mimic the water as the adsorbents, we use a water-cluster ((H2O)6). We found that, for all types of adsorbents considered here, binding energies of H are always lower than those of H2, whereas, some of the experimental values are just the other way around. Assuming a steady state solution to the rate equation method, we also present the H2 formation efficiency window in various cases.

H2FORMATION IN DIFFUSE CLOUDS: A NEW KINETIC MONTE CARLO STUDY

We used the continuous-time random-walk Monte Carlo technique to study anew the formation of H2 on the surfaces of interstellar dust grains in diffuse interstellar clouds. For our study, we considered three different grain materials, olivine (a polycrystalline silicate), amorphous silicate, and amorphous carbon, as well as a grain temperature that depends on granular size. For some runs, we included temperature fluctuations. Four different granular surfaces were used, one with one type of binding site due to physisorption, one with five different types of physisorption binding sites due to lateral forces, and two with sites for chemisorption, one in which chemisorption sites are entered through precursor physisorption sites, and one in which chemisorption is direct but occurs with a barrier for the adsorption of the first hydrogen atom. We found that on flat and rough olivine surfaces, molecular hydrogen is formed at low efficiencies, with smaller grains contributing very little despite their large numbers due to high temperatures. For flat amorphous carbon and amorphous silicate surfaces, the efficiency increases, reaching unity for the largest grains. For models with barrierless chemisorption, the efficiency of formation of H2 is near unity at all grain sizes considered, while for direct chemisorption via a barrier, we found efficiencies of 0.13-0.6 depending upon the barrier, but independent of grain size. Treating the flat olivine and amorphous silicate surfaces with temperature fluctuations increases the efficiency of H2 formation.

When sticking influences H2formation

Aims. Interstellar dust grains, because of their catalytic properties, are crucial to the formation of H2, the most abundant molecule in the Universe. The formation of molecular hydrogen strongly depends on the ability of H atoms to stick on dust grains. In this study we determine the sticking coefficient of H atoms chemisorbed on graphitic surfaces, and estimate its impact on the forma- tion of H2. Methods. The sticking probability of H atoms chemisorbed onto graphitic surfaces is obtained using a mixed classical-quantum dynamics method. In this, the H atom is treated quantum- mechanically and the vibrational modes of the surface are treated classically. The implications of sticking for the formation of H2 are addressed by using Kinetic Monte Carlo simulations that follow how atoms stick, move and associate with each other on dust surfaces of different temper- ature. Results. In our model, molecular hydrogen forms very efficiently for dust temperatures lower than 15 K through the involvement of physisorbed H atoms. At dust temperatures higher than 15 K and gas temperatures lower than 2000 K, H2 formation differs strongly if the H atoms com- ing from the gas phase have to cross a square barrier (usually considered in previous studies) or a barrier obtained by DFT calculations to become chemisorbed. The product of sticking times efficiency can be increased by many orders of magnitude when realistic barriers are considered. If graphite phonons are taken into account in the dynamics calculations, then H atoms stick better on the surface at high energies, but the overall H2 formation efficiency is only slightly affected. Our results suggest that H2 formation can proceed efficiently in photon dominated regions, X-ray dominated regions, hot cores and in the early Universe when the first dust is available.

Modelling of surface chemistry on an inhomogeneous interstellar grain

Context. Many interstellar molecules are formed through grain surface reactions. These reactions are usually modelled using rate equations, while considering a homogeneous grain with only one type of binding site for each species. However, amorphicity and the irregular character of interstellar dust grains make inhomogeneous grain surfaces much more likely.Aims. The aim of this study is to investigate the effect of surface inhomogeneity on surface reaction rates. The formation of molecular hydrogen is here taken as an example. Methods. The continuous-time random-walk Monte Carlo method is used to study the dependence of the H2 formation rate on surface size, fraction of strong binding sites and the distribution of these sites for different temperatures and bond strengths. Classical rate equations and modified rate equations are applied to try to reproduce these results.Results. The H2 formation efficiency is strongly affected by the introduction of a second type of binding site. This effect depends on the strength of the binding energy and the fraction of strong binding sites. The way in which the sites are distributed can change the formation rate by as much as four orders of magnitude. Classical rate equations fail to reproduce the formation rate for all tested situations. Modified rate equations are able to obtain a reasonable agreement with the Monte Carlo results for inhomogeneous surfaces with randomly distributed sites consisting of less than 30% strong sites.Conclusions. Since the different types of binding sites on interstellar grains are probably randomly distributed, we recommend the use of modified rate equations in gas grain models to include the effect of surface inhomogeneity. This method reproduces the most important features while being computationally inexpensive.

ERRATUM: “H2FORMATION ON GRAIN SURFACES” (2004, ApJ, 604, 222)

As noted by Bai & Goodman (2009), the equations for the transmission coefficients to go from a site i to a site j in Cazaux & Tielens (2004) erroneously omitted a key factor and also contained a typo. The correct equations are These equations differ from Equations (1) and (2) of the original paper in the additional factor s{{E-B_{ij}}/{E}}, which is the ratio of the transmitted over the incident wave numbers, with a change of variable in order to define the energy E as being zero at the bottom of the potential well i (e.g., for T pc, the energy of the atom is defined from the physisorbed site). The relevant energies Bi , Bj , and Bij are summarized in Table 1 where—compared to the original paper—a small typo in the expression for the Bij has been corrected as well. Note that Bij is negative with these definitions and the omitted factor can greatly increase the values of the transmission coefficient for an H atom to go from a physisorbed site to a chemisorbed site. As a result, the probability for a physisorbed H atom to become chemisorbed increases. By the same token, the H2 formation efficiency will also increase. Using the parameters for the potential energy curves as described in Figure 15 of Cazaux & Tielens (2004), Figure 1 shows the corrected efficiencies for H2 formation on olivine and carbonaceous surfaces. For grain temperatures higher than 25 K, these efficiencies are ~3.5 times higher than without the correction. For lower temperatures, where H2 formation from physisorbed H atoms dominates, the efficiencies remain unchanged. The general expressions for the H2 formation efficiency obtained in Cazaux & Tielens (2002, Equation (9)) and Cazaux & Tielens (2004, Equation (19)) remain valid. However, the expression for αpc is calculated from the transmission coefficients that were previously underestimated. Therefore, the corrected expression for αpc is where the first term is the mobility through tunneling and the second term through thermal hopping. In the original paper, the tunneling term was not taken into account because tunneling was only important at low dust temperatures and, for those conditions, direct recombination of physisorbed atoms dominates H2 formation. With the corrected expression, this term has to be taken into account.

H 2 Formation on Grain Surfaces

Molecular hydrogen is the most abundant molecule in the Universe and dominates the mass budget of the gas, particularly in regions of star formation. H2 is also an important chemical intermediate in the formation of larger species and can be an important gas coolant when the medium lacks metals. Because of the inefficiency of gas-phase reactions to form H2, this molecule is generally thought to form on grain surfaces. Observations of H2 in a wide variety of objects showed that this molecule could form efficiently over a wide range of physical conditions. To understand the mechanism responsible for such an efficient formation, we developed a model for molecular hydrogen formation on grain surfaces. This model considers the interaction between atom and surface as beeing either weak (Van der Waals interaction—physisorption) or strong (covalent bound—chemisorption), as well as the mobility of the atom on a surface due toquantum mechanical diffusion and thermal hopping. This model solves the time-dependent kinetic rate equation for the formation of molecular hydrogen and its deuterated forms. Our results have been benchmarked with laboratory experiments on silicates, carbonaceous and graphitic surfaces. This comparison allowed us to derive some characteristics of the considered surfaces. An extension of our model to astrophysical conditions gives an estimate of H2 formation efficiency for a wide range of physical conditions. One of our main results is the efficient formation of molecular hydrogen for gas and grain temperatures up to several hundreds of kelvins. We also compared our predictions to observations in astrophysical objects such as photodissociation regions (PDRs). The addition of deuterium in our model for the formation of HD and D2 molecules is also discussed.

Molecular Hydrogen formation on grain surfaces

We reconsider H2 formation on grain surfaces. We develop a rate equation model which takes into account the presence of both physisorbed and chemisorbed sites on the surface, including quantum mechanical tunnelling and thermal diffusion. In this study, we take into consideration the uncertainties on the characteristics of graphitic surfaces. We calculate the H2 formation efficiency with the Langmuir Hinshelwood and Eley Rideal mechanisms, and discuss the importance of these mechanisms for a wide range of grain and gas temperatures. We also develop a Monte Carlo simulation to calculate the H2 formation efficiency and compare the results to our rate equation models. Our results are the following: (1) Depending on the barrier againt chemisorption, we predict the efficiency of H2 formation for a wide range of grain and gas temperatures. (2) The Eley-Rideal mechanism has an impact on the H2 formation efficiency at high grain and gas temperatures. (3) The fact that we consider chemisorption in our model makes the rate equation and Monte Carlo approaches equivalent.

Exact results for hydrogen recombination on dust grain surfaces.

The recombination of hydrogen in the interstellar medium, taking place on surfaces of microscopic dust grains, is an essential process in the evolution of chemical complexity in interstellar clouds. Molecular hydrogen plays an important role in absorbing the heat that emerges during gravitational collapse, thus enabling the formation of structure in the universe. The H2 formation process has been studied theoretically, and in recent years also by laboratory experiments. The experimental results were analyzed using a rate equation model. The parameters of the surface that are relevant to H2 formation were obtained and used in order to calculate the recombination rate under interstellar conditions. However, it turned out that, due to the microscopic size of the dust grains and the low density of H atoms, the rate equations may not always apply. A master equation approach that provides a good description of the H2 formation process was proposed. It takes into account both the discrete nature of the H atoms and the fluctuations in the number of atoms on a grain. In this paper we present a comprehensive analysis of the H2 formation process, under steady state conditions, using an exact solution of the master equation. This solution provides an exact result for the hydrogen recombination rate and its dependence on the flux, the surface temperature, and the grain size. The results are compared with those obtained from the rate equations. The relevant length scales in the problem are identified and the parameter space is divided into two domains. One domain, characterized by first order kinetics, exhibits high efficiency of H2 formation. In the other domain, characterized by second order kinetics, the efficiency of H2 formation is low. In each of these domains we identify the range of parameters in which, due to the small size of the grains, the rate equations do not account correctly for the recombination rate and the master equation is needed.

Molecular Hydrogen along Two Lines of Sight throughthe Large Magellanic Cloud

ABSTRACTWe have remeasured the H2 column densities along lines of sight to the Large Magellanic Cloud (LMC) using spectra taken by the Hopkins Ultraviolet Telescope (HUT). Three lines of sight were analyzed using the pair method and previously determined Fitzpatrick & Massa extinction parameters to allow H2 absorption models to be fitted. Substantial improvements over the values first reported by Clayton et al. were made for two of the three lines of sight. Comparisons between LMC and Galactic environments suggest that the factors which lead to extinction curve variations (e.g., metallicity or shocks) do not affect the efficiency of H2 formation on the surfaces of the dust grains: NH2/E(B - V) values are approximately the same in both galaxies despite large differences in NHI/E(B - V).