The transformation of cuboctahedral to icosahedral nanoparticles: atomic structure and dynamics

The rearrangement of transition metal nanoparticles from cuboctahedral to icosahedral structures is studied for up to 923 atoms. The atomic structure and temperature dependence of the transition are investigated with a well-defined collective variable. This collective variable describes the folding of the square fcc(100) facets into two triangular facets through a linear combination of the diagonals of all fcc(100) facets of all shells of the particle. Activation barriers are determined through harmonic transition state theory and constrained molecular dynamics simulations based on force field potentials. These calculations predict an activation entropy larger than 1 meV K$^{-1}$, leading to strongly temperature dependent activation barriers. Density functional theory calculations were additionally performed both as single point calculations and as full optimizations. Cu, Ag, Au and Ni clusters show low barriers for concerted, symmetric transition up to the 309-atomic clusters. In contrast, for Pd, Pt, Rh and Ir higher barriers are required, already for the 147-atomic clusters. With increasing barriers, an asymmetric but still concerted rearrangement becomes energetically more favorable than the fully symmetric transformation. The material-dependence of the transition can be correlated with the melting point of the bulk metals

Theoretical investigation of the side-chain mechanism of the MTO process over H-SSZ-13 using DFT and calculations

The side-chain mechanism of the methanol-to-olefins process over the H-SSZ-13 acidic zeolite was investigated using periodic density functional theory with corrections from highly accurate ab intio calculations on large cluster models. Hexa-, penta- and tetramethylbenzene are studied as co-catalysts for the production of ethene and propene. The highest barrier, both of ethene and propene formation, is found for the methylation of the side-chain towards the formation of an ethyl or isopropyl group. All other barriers are found to be substantially lower. This leads to a clear selectivity for ethene since the elimination of ethene with a rather low barrier competes with methylation towards propene which requires a barrier that is more than 100 kJ mol$^{-1}$ higher

Effect of Impurities on the Initiation of the Methanol-to-Olefins Process: Kinetic Modeling Based on Ab Initio Rate Constants

The relevance of a selection of organic impurities for the initiation of the MTO process was quantified in a kinetic model comprising 107 elementary steps with ab initio computed reaction barriers (MP2:DFT). This model includes a representative part of the autocatalytic olefin cycle as well as a direct initiation mechanism starting from methanol through CO-mediated direct C–C bond formation. We find that the effect of different impurities on the olefin evolution varies with the type of impurity and their partial pressures. The reactivity of the considered impurities for initiating the olefin cycle increases in the order formaldehyde < di-methoxy methane < CO < methyl acetate < ethanol < ethene < propene. In our kinetic model, already extremely low quantities of impurities such as ethanol lead to faster initiation than through direct C–C bond formation which only matters in complete absence of impurities. Graphic Abstract: [Figure not available: see fulltext.

Influence of Confinement on Barriers for Alkoxide Formation in Acidic Zeolites

The influence of the confinement imposed by eight different zeotypes on the formation of the alkoxides of 13 primary alcohols is studied using dispersion corrected density functional theory calculations with the PBE‐D3 functional. Adsorption energies of the alcohols are computed along with barriers for formation of the alkoxides, which is the first step of the stepwise dehydration mechanism. We find that variations in the adsorption and transition state energies are largely governed by van der Waals interactions between substrates and the zeolite framework. Trends between different reactants, on the other hand, are largely due to the size of the molecules, which can be described quantitatively by the number of atoms constituting them. We find that the stabilization of adsorbates is largest for frameworks that are neither too small, leading to repulsive interaction, nor too spacious leading only to weak interaction

Toward Computing Accurate Free Energies in Heterogeneous Catalysis: a Case Study for Adsorbed Isobutene in H-ZSM-5

Herein, we propose a novel computational protocol that enables calculating free energies with improved accuracy by combining the best available techniques for enthalpy and entropy calculation. While the entropy is described by enhanced sampling molecular dynamics techniques, the energy is calculated using ab initio methods. We apply the method to assess the stability of isobutene adsorption intermediates in the zeolite H-SSZ-13, a prototypical problem that is computationally extremely challenging in terms of calculating enthalpy and entropy. We find that at typical operating conditions for zeolite catalysis (400 °C), the physisorbed π-complex, and not the tertiary carbenium ion as often reported, is the most stable intermediate. This method paves the way for sampling-based techniques to calculate the accurate free energies in a broad range of chemistry-related disciplines, thus presenting a big step forward toward predictive modeling

Cost-effectiveness of price subsidies on fortified packaged infant cereals in reducing iron deficiency anemia in 6-23-month-old children in urban India

Introduction: Iron deficiency anaemia (IDA) is a major public health problem in India and especially harmful in early childhood due to its impact on cognitive development and increased all-cause mortality. We estimate the cost-effectiveness of price subsidies on fortified packaged infant cereals (F-PICs) in reducing IDA in 6-23-monthold children in urban India. Materials and Methods: Cost-effectiveness is estimated by comparing the net social cost of price subsidies with the disability-adjusted life-years (DALYs) averted with price subsidies. The net social costs correspond to the cost of the subsidy minus the monetary costs saved by reducing IDA. The estimation proceeds in three steps: 1) the current lifetime costs of IDA are assessed with a health economic model combining the prevalence of anemia, derived from a large population survey, with information on the health consequences of IDA and their costs in terms of mortality, morbidity, and DALYs. 2) The effects of price subsidies on the demand for F-PICs are assessed with a market survey among 4801 households in 12 large Indian cities. 3) The cost-effectiveness is calculated by combining the findings of the first two steps with the results of a systematic review on the effectiveness of F-PICs in reducing IDA. We compare the cost-effectiveness of interventions that differ in the level of the subsidy and in the socio-economic strata (SES) eligible for the subsidy. Results: The lifetime social costs of IDA in 6-23-month-old children in large Indian cities amount to production losses of 3222 USD and to 726,000 DALYs. Poor households incur the highest costs, yet even wealthier households suffer substantial losses. The market survey reveals that few households currently buy F-PICs, with the share ranging from 14% to 36%. Wealthier households are generally more likely to buy FPICs. The costs of the subsidies per DALY averted range from 909 to 3649 USD. Interventions targeted at poorer households are most effective. Almost all interventions are cost saving from a societal perspective when taking into account the reduction of future production losses. Return per DALY averted ranges between gains of 1655 USD to a cost of 411 USD. Conclusion: Price subsidies on F-PICs are a cost-effective way to reduce the social costs of IDA in 6-23-month-old children in large Indian cities. Interventions targeting poorer households are especially cost-effective