Showing papers by "Vladimir Galvita published in 2017"

The Positive Role of Hydrogen on the Dehydrogenation of Propane on Pt(111)

Abstract: Propane dehydrogenation on a Pt-based catalyst can be accelerated by cofeeding hydrogen. An extensive reaction network for propane dehydrogenation on Pt(111), including side reactions and deep dehydrogenation reactions, is proposed to explain the effect of cofeeding hydrogen. Simulations at 873 K and 1 bar total pressure reproduce the experimental trends at increasing H2/C3H8 inlet ratios and allow exploration of the origin of the positive effect of cofeeding hydrogen. Increasing hydrogen pressure leads to a lower coverage of deeply dehydrogenated coke precursors on the surface: in particular, CCH3 (ethylidyne) and CH (methylidyne). In addition, it increases hydrogen coverage, which decreases the propylene adsorption strength while the energy barriers for the further dehydrogenation of propylene increase. The combined effect of a decreasing coke precursor coverage, facilitated propylene desorption, and increasing deep dehydrogenation barriers explains the higher catalytic activity when hydrogen is cofed.

Controlling the stability of a Fe-Ni reforming catalyst : structural organization of the active components

Abstract: Fe–Ni catalysts present high activity in dry reforming of methane, with high carbon resistance, but suffer from deactivation via sintering and Fe segregation. Enhanced control of the stability and activity of Fe–Ni/MgAl 2 O 4 was achieved by means of Pd addition. The evolution of the catalyst structure during H 2 Temperature Programmed Reduction (TPR) and CO 2 Temperature Programmed Oxidation (TPO) was investigated using time-resolved in situ X-ray diffraction (XRD). During reduction of Fe–Ni–Pd supported on MgAl 2 O 4 , a core shell alloy forms at the surface, where Fe–Ni is in the core and Fe–Ni–Pd in the shell. A 0.2 wt% Pd loading or Ni:Pd molar ratio as high as 75:1 showed the best performance in terms of both activity and stability of the catalyst at 1023 K and total pressure of 101.3 kPa. Experimental results and DFT calculations showed that Pd addition to bimetallic Fe–Ni reduces the tendency of Fe to segregate to the surface of the alloy particles under methane dry reforming (DRM) conditions, due to the formation of a thin Fe–Ni–Pd surface layer. The latter acts as a barrier for Fe segregation from the core. Segregation of Fe from the trimetallic shell still occurs, but to a lesser extent as the Fe concentration is lower. This Ni:Pd molar ratio is capable of controlling the carbon formation and hence ensure high catalyst activity of 24.8 mmol s −1 g metals −1 after 21 h time-on-stream.

CO production from CO2 via reverse water–gas shift reaction performed in a chemical looping mode: Kinetics on modified iron oxide

Abstract: Carbon monoxide production from carbon dioxide via isothermal reverse water–gas shift chemical looping (RWGS-CL) is studied with a modified iron oxide oxygen carrier material (80 wt% Fe2O3–Ce0.5Zr0.5O2). The material is characterized by TEM, XRD and thermogravimetry at temperatures from 750 °C to 850 °C and gas mole fractions of H2 and CO2 from 0.05 to 0.75, respectively. High temperature and high reactant concentrations favor the oxidation and reduction of the material during repeated redox cycles. The reaction rate of reduction is always faster than that of oxidation applying the same gas concentration of H2 and CO2, respectively. The long term stability of the material is investigated with 500 redox cycles in a plug flow reactor. The material shows gradual deactivation lowering the CO yield during the first 100 redox cycles. After that, a steady state CO yield is reached for the next 400 redox cycles. Deactivation is attributed to surface sintering which results in slower reaction kinetics. TG data was used for a kinetic analysis applying the master plot method. The experimental data for oxidation and reduction indicated reaction mechanisms, which are well described by a reaction order and a geometrical contraction model. After parameter estimation, a good agreement between the model and the TG data was achieved with the reaction order and geometrical contraction model for oxidation and reduction, respectively. The RWGS-CL process can be used for sustainable CO production from CO2 if the energy for the process and for H2 production is supplied by renewable sources.

A core-shell structured Fe2O3/ZrO2@ZrO2 nanomaterial with enhanced redox activity and stability for CO2 conversion

Abstract: A novel Fe2O3/ZrO2@ZrO2 nanomaterial with core-shell structure is proposed, where first Fe2O3 nanoparticles are loaded onto a ZrO2 support as a core and afterwards the core is coated with a thin and porous layer of ZrO2. Such combination of nanocoating and impregnation methods has been applied to synthesize core-shell oxygen storage nanomaterials with different iron oxide loading. 2D in-situ XRD patterns recorded during isothermal redox cycles at different temperatures (550–650 °C) show the evolution of Fe3O4 to metallic iron in Fe2O3/ZrO2@ZrO2 as a function of temperature. A detailed characterization of fresh and spent samples demonstrates that the Fe2O3/ZrO2@ZrO2 materials exhibit excellent structural stability (stable pore structure, specific surface area and core-shell morphology) and strong capability to resist sintering after 100 redox cycles at 650 °C for CO2 conversion to CO compared to the samples prepared by impregnation only. The strong thermal stability of ZrO2 coating materials contributes to keep up the activity of active phase during high-temperature environments.

Study of butanol conversion to butenes over H-ZSM-5: Effect of chemical structure on activity, selectivity and reaction pathways

Abstract: To evaluate the viability of the use of butanol as a green chemical key molecule, the effects of temperature and site time on the transformation of the three butanol isomers, 1-butanol, 2-butanol and iso -butanol, towards butenes over H-ZSM-5 have been studied in search of the most promising isomer. Under dehydration conditions, 2-butanol is by far the most active and intermediate ether formation is only observed for 1-butanol. On the other hand, only isobutanol allows the direct formation of isobutene, the most valuable of the butene isomers. Hence, it is especially interesting to further stimulate the biomass derived isobutanol production, e.g. via genetic modification of appropriate microorganisms, in order to allow thereafter the formation of green drop-in isobutene upon dehydration with H-ZSM-5 in existing refineries. Due to the large potential of isobutanol, a reaction path analysis via ab-initio based microkinetic simulations is conducted for this molecule. Comparing these results with simulations on 1-butanol indicates that the shift is occurring due to a shift of the dominant reaction pathway towards the direct dehydration via anti elimination. A Gibbs free energy analysis shows that the large distortion of the transition state for isobutanol etherification renders this path far less favorable, resulting in the lack of formation of the di-alkyl ether, whilst the increased degree of substitution of the alkyl chain in isobutanol is found to promote the direct dehydration path, leading to an increase of the overall activity of isobutanol as compared to 1-butanol.

Role of intermediates in reaction pathways from ethene to hydrocarbons over H-ZSM-5

Abstract: Insight in ethene to hydrocarbon transformation over a H-ZSM-5 catalyst was obtained by means of temporal analysis of products (TAP) in the temperature range 598–698 K with pulses of higher olefins, dienes, cyclodienes and aromatics. Pulses of propene, 1-butene and 1-hexene allowed to identify the cracking routes from ethene oligomerization products. When pulsing benzene or ethylbenzene, only accumulation of aromatics occurred. In-situ temperature programmed desorption (TPD) experiments after pulsing identified aromatics as long-lived surface species. The role of intermediates was assessed by means of pre-adsorption of the different feeds before pulsing ethene, in so-called pump-probe experiments. Butene enhanced propene formation, while all other olefins favored butene production via aliphatic surface intermediates. The latter were also intermediates in the conversion of hexadiene to butene and aromatics, while cyclohexadiene was converted to propene and aromatics via aromatic surface intermediates. In contrast to ethylbenzene pulses alone, aromatics alkylation participated towards light olefin production via sidechain/paring mechanisms. Isotope experiments of 13C2H4 over a catalyst coked during continuous flow experiments with 12C only showed scrambling in both propene and butene products, stressing the role of long-lived aromatic surface intermediates.

Size- and composition-controlled Pt─Sn bimetallic nanoparticles prepared by atomic layer deposition

Abstract: Pt–Sn bimetallic nanoparticles (BMNPs) are used in a variety of catalytic reactions and are widely accepted as a model system for Pt-based bimetallics in fundamental catalysis research. Here, Pt–Sn BMNPs were prepared via a two-step synthesis procedure combining atomic layer deposition (ALD) and temperature programmed reduction (TPR). In situ X-ray diffraction measurements during TPR and ex situ X-ray absorption spectroscopy at the Pt LIII-edge revealed the formation of Pt–Sn bimetallic alloys with a phase determined by the Pt/(Pt + Sn) atomic ratio of the as-deposited bilayer. The size of the BMNPs could be tuned by changing the total thickness of the bilayers, while keeping the Pt/(Pt + Sn) atomic ratio constant. Due to the exceptional control over BMNP size and crystalline phase, the proposed method will enable highly systematic studies of the relation between the structure and the performance of Pt–Sn bimetallic catalysts.

Combined Chemical Looping: New Possibilities for Energy Storage and Conversion

Abstract: A novel concept of energy storage and conversion was demonstrated in a laboratory-scale test. The proposed combined chemical looping is able to store and release energy from chemical looping combustion for heat generation and hydrogen production by steam–iron processes integrated in one reactor. The reactor contains two concentric chambers, which are both filled with iron-based material. In a first step, all material is reduced to the metallic form, thus “charging” the reactor. For the second step or “discharging”, steam is fed to the inner chamber and air is fed to the outer chamber. The inner chamber is used for hydrogen production, and the external chamber is used for heat generation. In addition to iron, the external chamber contains a highly pyrophoric Ni-based layer at the air entry point to enable the startup of heat generation at room temperature. This concept of combined chemical looping was successfully tested: heat was generated by metal oxidation in air, and H2 was produced following contact o...

(Invited) Atomic Layer Deposition of Nanoalloys of Noble and Non-Noble Metals

Abstract: We present a novel ALD-based approach for the synthesis of bimetallic materials consisting of a noble metal along with a nonnoble metal such as Pt-M (M = In, Ga, Sn, etc.), with a precise control on the composition and size. First, a bilayer consisting of a metal oxide and a Pt film of the desired thickness is deposited on to the substrate. The film is then subjected to a temperature programmed reduction (TPR) under H2 atmosphere. In situ X-ray diffraction (XRD) measurements during TPR revealed the formation of Pt±M bimetallic alloys with a phase determined by the Pt/(Pt + M) atomic ratio of the as-deposited bilayer. Scanning electron microscopic (SEM) analysis revealed the formation nanoparticles after annealing, with the particle size controlled by the initial total thickness of the bilayer.