Palladium-based catalysts are known to promote the selective hydrogenation of acetylene to ethylene. Unfortunately, coupling reactions between the numerous surface intermediates generated in this process occur alongside. These side reactions are undesired, generating the so-called "green oil", i.e., C 4 + hydrocarbons that poison the active sites of the catalyst. The current work assesses the energetic and kinetic aspects of C 4 side products formation from the standpoint of computational chemistry. Our results demonstrate that the CC coupling of common surface species, in particular acetylene, vinylidene and vinyl, are competitive with selective hydrogenation. These CC couplings are particularly easy for intermediates where the C-Pd bond can largely remain intact during the coupling. Furthermore, the thus formed oligomers tend to be hydrogenated more easily, consuming hydrogen normally spent on acetylene hydrogenation. The analysis of site requirement suggests that isolated Pd 2 ensembles are sufficient for selective hydrogenation and would suppress oligomerization. However, upon aging, the PdAg alloy is likely to undergo reverse segregation and in this case, our computations suggest that the selectivity of the catalyst is lost, with enhanced CC couplings interfering even more strongly. Hence, small Pd ensembles are crucial to avoid oligomerisation side reactions of acetylene.
ABSTRACT:Restructuring of alloy surfaces induced by strongly bound adsorbates is a well-establish phenomenon occurring in catalysis and membrane science. In catalytic processes this restructuring can have profound effects since it alters the ensemble distribution between the as-prepared state of the catalyst and the catalytic surface under operando conditions. This work assesses the restructuring of Pd-Ag alloys induced by adsorption of acetylene in the framework of the ensemble formalism. A detailed Ising-type model Hamiltonian of the (111) surface plane is fitted to extensive Density Functional Theory computations. The equilibrium distributions under a realistic environment are then evaluated by a Monte Carlo approach as a function of temperature and alloy composition. Acetylene induces a strong reverse segregation within the relevant range of temperature. Therefore, the surface of Pd-Ag catalysts is almost entirely covered by Pd for bulk ratios < 0.8 Ag/Pd, which is, in general, detrimental to the selectivity of Pd-Ag catalysts. Despite the very strong vertical segregation, acetylene only induces marginal in-plane ordering, i.e., the surface triangular ensembles follow random distributions as a function of the surface layer Ag-fraction quite closely. This can be explained by two factors: first, triangular sites are not sufficient to fully capture the diversity of acetylene binding energies on Pd-Ag alloy surfaces. Rather, an extended environment including the first coordination sphere is necessary, and leads to an overlap in terms of binding energy between weakling binding Pd 3 ensembles and strongly binding Pd 2 Ag ensembles. The second critical aspect is related to lateral interactions, which preclude adsorption of acetylene molecules on nearest neighbor triangular sites. Therefore, in a Pd 3 island, roughly two thirds of Pd 3 sites would be lost. Our study suggests that the equilibrium structure of these alloy catalysts under operando conditions are far from the state targeted by catalyst design, revealing a nearly unavoidable reason for loss of selectivity of the catalyst with time of operation.
_________________________________________The accurate description of the energy of adsorbate layers is crucial for the understanding of chemistry at interfaces. For heterogeneous catalysis not only the interaction of the adsorbate with the surface, but also adsorbate-adsorbate lateral interactions significantly affect activation energies of reactions. Modeling the interactions of adsorbates with the catalyst surface and with each other can be efficiently achieved in the cluster expansion Hamiltonian formalism, which has recently been implemented in a graph-theoretical kinetic Monte Carlo (kMC) scheme to describe multi-dentate species. Automating the development of the cluster expansion Hamiltonians for catalytic systems is challenging and requires the mapping of adsorbate configurations for extended adsorbates onto a graphical lattice. The current work adopts machine learning methods to reach this goal. Clusters are automatically detected based on formalized, but intuitive chemical concepts. The corresponding energy coefficients for the cluster expansion are calculated by an inversion scheme. The potential of this method is demonstrated for the example of ethylene adsorption on Pd (111), for which we propose several expansions, depending on the graphical lattice. It turns out that for this system the best description is obtained as a combination of single molecule patterns and a few coupling terms accounting for lateral interactions. _______________________________________
S2As a separate file, the supporting information contain our Fortran code, called SurfaceEquilibrium.f90. It can be compiled by running:gfortran -ffree-line-length-none -o SurfaceEquilibrium.x SurfaceEquilibrium.f90The default version does not account for lateral interactions, but simple instructions in the code are given to turn lateral interactions on. The input file (called fort.10), needs to contain the following three lines:Pressure in Pa Temperature in K The silver molar fraction Then, you can run ./SurfaceEquilibrium.x and obtain the output file (fort.11). As mentioned in the "Theoretical Background" section of the manuscript, the system of equation requires us to specify the "randomness ansatz" in order to close the system of equations: Mass balance and ensemble equilibrium equations are solved for the molar fractions of three ensembles as a function of the molar fraction of the fourth one. The randomness ansatz suggests that the ensemble distribution of the uncovered part follows Martins' distribution for the silver molar fraction of this uncovered fraction of the surface, which in general is not the same as for the total surface. Whenever the randomness ansatz is violated (due to stoichiometric constraints and the adsorption equilibrium with acetylene), the algorithm minimizes the deviation. Embedding Energies and
For poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) films exhibiting nanoporous-crystalline (NC) phases, c^ orientation (i.e., crystalline polymer chain axes being preferentially perpendicular to the film plane) is obtained by crystallization of amorphous films, as induced by sorption of suitable low-molecular-mass guest molecules. The occurrence of c^ orientation is relevant for applications of NC PPO films because it markedly increases film transparency as well as guest diffusivity. Surprisingly, we show that the known crystallization procedures lead to c^ oriented thick (50–300 μm) films and to unoriented thin (£20 μm) films. This absence of crystalline phase orientation for thin films is rationalized by fast guest sorption kinetics, which avoid co-crystallization in confined spaces and hence inhibit formation of flat-on lamellae. For thick films exhibiting c^ orientation, sigmoid kinetics of guest sorption and of thickening of PPO films are observed, with inflection points associated with guest-induced film plasticization. Corresponding crystallization kinetics are linear with time and show that co-crystal growth is poorly affected by film plasticization. An additional relevant result of this study is the linear relationship between WAXD crystallinity index and DSC melting enthalpy, which allows evaluation of melting enthalpy of the NC α form of PPO (DHmo = 42 ± 2 J/g).
Using DFT methods we describe the fluxional behavior of mono-, bi-, and tripodal W(CH3)(6) species grafted on partially dehydroxylated silica. Our results show that the umbrella inversion mechanism described to explain the fluxionality of the parent W(CH3)(6) molecule is not operative after grafting on silica. Low-energy turnstile rotations are instead operative for all considered species, correlating the experimental values found in solid-state NMR
Guest molecular features determining the formation of α and β phases of poly(2-6-dimethyl-1,4-phenylene) oxide (PPO) are explored by collecting literature data and adding many new film preparations, both by solution casting and by guest sorption in amorphous films. Independently of the considered preparation method, the α-form is favored by the hydrophobic and bulky guest molecules, while the hydrophilic and small guest molecules favor the β-form. Furthermore, molecular modeling studies indicate that the β-form inducer guests establish stronger dispersive interactions with the PPO units than the α-form inducer guests. Thus, the achievement of co-crystalline (and derived nanoporous crystalline) α- and β-forms would result from differences in energy gain due to the host–guest interactions established at the local scale.
The synthesis of the triple-calix[6]arene derivative 6 in which three calix[6]arene macrocycles are linked to a central 1,3,5-trimethylbenzene moiety is reported. Derivative 6 is able to give multiple-threading processes in the presence of dialkylammonium axles. The formation of pseudo[2]rotaxane, pseudo[3]rotaxane, and pseudo[4]rotaxane by threading one, two, and three, respectively, calix-wheels of 6 has been studied by 1D and 2D NMR, DOSY, and ESI-FT-ICR MS/MS experiments. The use of a directional alkylbenzylammonium axle led to the stereoselective formation of endo-alkyl pseudo[n]rotaxane stereoisomers.
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