Effect of Hydrogen Addition on Coke Formation and Product Distribution in Catalytic Coupling of Methane.

The effect of hydrogen addition on catalytic nonoxidative coupling of methane at 1000 °C was investigated. Experiments were performed at varying ratios between the catalyst and the postcatalytic volume to discern the effect of hydrogen on the catalytic reaction as well as on the gas-phase reaction. Adding 10% H2 decreases the methane conversion by a factor of 2, almost independent of the ratio between the catalyst and the postcatalytic residence time. The effect on the conversion is mostly determined by gas-phase chemistry. Hydrogen addition has no influence on the C2 hydrocarbon yield, whereas aromatic selectivity is significantly reduced. Changes in selectivity are attributed to changes in methane conversion. Quantitative determination of the amount of coke deposited on the catalyst reveals a decrease by 1 order of magnitude when dosing up to 10% H2, while carbon deposits-downstream of the catalyst bed are suppressed to a much lower extent. These results suggest a process in which the produced hydrogen is partly recycled, maximizing the carbon selectivity to C2 hydrocarbons while minimizing both aromatics and, most crucially, formation of coke on the catalyst as well as further deposits-downstream.

Leveraging Expertise in Thermal Catalysis to Understand Plasma Catalysis.

Best practices in testing heterogeneous catalysts are translated to plasma-catalytic experiments. Independent determination of plasma-catalytic and plasma-chemical contributions is essential. Non-porous catalyst particles are preferred because active sites inside sub-micron pores cannot contribute. Temperature variation is needed to determine kinetics, despite the complexity of thermal effects in plasma. Rigorous checks on catalyst deactivation and mass balance are needed. Plasma enhanced reversed reactions should be minimized by keeping conversion low and far from thermodynamic equilibrium, preventing underestimation of the rate of forward reaction. In contrast, plasma-catalytic studies often aim at conversions surpassing thermodynamic equilibrium, not obtaining any information on kinetics. Calculation of catalyst activity per active sites (turn-over-frequency) requires also appropriate characterization to determine the number of active sites. The relationship between kinetics and thermodynamics for plasma-catalysis is discussed using endothermic decomposition of CO2 and exothermic synthesis of ammonia from N2 and H2 as examples. Assuming Langmuir-Hinshelwood and Eley-Rideal mechanisms, the effect of excitation of reactant molecules on activation barriers and surface coverages are discussed, influencing reaction rates. The consequences of reversed reactions are considered. Plasma-catalysis with catalysts applied for thermal catalysis at much higher temperature should be avoided, as adsorbed species are bonded too strongly resulting in low rates.

Correction: From the Birkeland-Eyde process towards energy-efficient plasma-based NOX synthesis: a techno-economic analysis.

[This corrects the article DOI: 10.1039/D0EE03763J.].

Technoeconomic Evaluation of the Industrial Implementation of Catalytic Direct Nonoxidative Methane Coupling.

This paper presents a process design for catalytic nonoxidative natural gas conversion to olefins and aromatics, highlighting the opportunities and challenges concerning industrial implementation. The optimal reactor conditions are 5 bar and 1000 °C. Heat exchange over the reactor is challenging due to the high temperature and low gas pressure. Recovery of ethylene is economically unattractive due to the low ethylene concentration in the product stream, leading to a methane-to-aromatics process, recycling ethylene. Benzene is the most valuable product at an efficiency of 0.31 kgbenzene/kgmethane with hydrogen as a major valuable byproduct. Naphthalene, with a low value, is unfortunately the dominant product, at 0.52 kgnaphthalene/kgmethane. It is suggested to hydrocrack the naphthalene to more valuable BTX products in an additional downstream process. The process is calculated to result in a 107 $ profit per ton CH4.

Effect of ethane and ethylene on catalytic non oxidative coupling of methane.

The effect of addition of ethane and ethylene (C2) on methane coupling at 1000 °C was investigated. A Fe/SiO2 catalyst was used to determine the contributions of catalytic as well as C2 initiated methane activation. The catalyst load as well as the residence times at 1000 °C downstream of the catalyst bed were varied. C2 addition significantly increases methane conversion rates, similarly for both ethane and ethylene, although ethylene is more effective when operating with long residence times in the post-catalytic volume. Methane activation via C2 addition proceeds dominantly in the gas-phase whereas catalytic C2 activation is negligible. The catalyst has no effect on methane conversion when the feed contains more than 2 vol% C2. Product selectivity distribution as well as total hydrocarbon yield at 10% conversion is not influenced by C2 addition, but is influenced by the amount of catalyst as well as residence time in the post-catalytic volume at high temperature. It is proposed that C2 impurities in natural gas change from a nuisance to an advantage by enhancing methane conversion and simplifying purification of the natural gas feed. A process is proposed in which ethylene is recycled back into the reactor to initiate methane coupling, leading to a process converting methane to aromatics.

From the Birkeland-Eyde process towards energy-efficient plasma-based NO X synthesis: a techno-economic analysis.

Plasma-based NO X synthesis via the Birkeland-Eyde process was one of the first industrial nitrogen fixation methods. However, this technology never played a dominant role for nitrogen fixation, due to the invention of the Haber-Bosch process. Recently, nitrogen fixation by plasma technology has gained significant interest again, due to the emergence of low cost, renewable electricity. We first present a short historical background of plasma-based NO X synthesis. Thereafter, we discuss the reported performance for plasma-based NO X synthesis in various types of plasma reactors, along with the current understanding regarding the reaction mechanisms in the plasma phase, as well as on a catalytic surface. Finally, we benchmark the plasma-based NO X synthesis process with the electrolysis-based Haber-Bosch process combined with the Ostwald process, in terms of the investment cost and energy consumption. This analysis shows that the energy consumption for NO X synthesis with plasma technology is almost competitive with the commercial process with its current best value of 2.4 MJ mol N-1, which is required to decrease further to about 0.7 MJ mol N-1 in order to become fully competitive. This may be accomplished through further plasma reactor optimization and effective plasma-catalyst coupling.

Influence of the Catalyst Particle Size on the Aqueous Phase Reforming of n-Butanol Over Rh/ZrO2.

Butanol is a by-product obtained from biomass that can be valorized through aqueous phase reforming. Rh/ZrO2 catalysts were prepared and characterized, varying the size of the support particles. The results showed a relatively mild effect of internal mass transport on butanol conversion. However, the influence of internal transport limitations on the product distribution was much stronger, promoting consecutive reactions, i.e., dehydrogenation, hydrogenolysis, and reforming of propane and ethane. Hydrogen consuming reactions, i.e., hydrogenolysis, were more strongly enhanced than hydrogen producing reactions due to internal concentration gradients. Large support particles deactivated faster, attributed to high concentrations of butyraldehyde inside the catalyst particles, enhancing deposit formation via aldol condensation reactions. Consequently, also the local butyric acid concentration was high, decreasing the local pH, enhancing Rh leaching. The influence of internal transfer limitation on product distribution and stability is discussed based on a reaction scheme with three main stages, i.e., (1) formation of liquid intermediates via dehydrogenation, (2) formation of gas via decarbonylation/decarboxylation reactions, and (3) hydrocarbon hydrogenolysis/reforming/dehydrogenation.

Catalytic Performance of Ni/CeO2/X-ZrO2 (X = Ca, Y) Catalysts in the Aqueous-Phase Reforming of Methanol.

In this study, we reported on the effect of promoting Ni/ZrO2 catalysts with Ce, Ca (two different loadings), and Y for the aqueous-phase reforming (APR) of methanol. We mainly focused on the effect of the redox properties of ceria and the basicity provided by calcium or yttrium on the activity and selectivity of Ni in this reaction. A systematic characterization of the catalysts was performed using complementary methods such as XRD, XPS, TPR, CO2-TPD, H2 chemisorption, HAADF-STEM, and EDS-STEM. Our results reveal that the improvement in reducibility derived from the incorporation of Ce did not have a positive impact on catalytic behaviour thus contrasting with the results reported in the literature for other Ce-based catalytic compositions. On the contrary, the available Ni-metallic surface and the presence of weak basic sites derived from Ca incorporation seem to play a major role on the catalytic performance for APR of methanol. The best performance was found for a Ce-free catalyst with a molar Ca content of 4%.

Competitive Adsorption of Nitrite and Hydrogen on Palladium during Nitrite Hydrogenation.

Nitrite hydrogenation is studied in steady-state as well as transient operation using a Pd catalyst in a tubular membrane contactor reactor. A negative reaction order in hydrogen in steady state operation proofs that hydrogen and nitrite adsorb competitively. In transient operation, feeding nitrite to the Pd surface fully covered with hydrogen results initially in very low conversion of nitrite, speeding up once hydrogen is removed from part of the Pd surface. Additional proof for competitive adsorption between hydrogen and nitrite is provided by the observation that exposure of a nitrite-covered catalyst to hydrogen induces desorption of nitrite. Formation of ammonia in these experiments proceeds via two pathways, first via a fast reaction followed by extremely slow hydrogenation of adsorbed N atoms, which is kinetically not relevant. This information is relevant for designing effective and selective catalysts when operating at very low nitrite concentration.

Water and carbon oxides on monoclinic zirconia: experimental and computational insights.

Zirconium oxide (ZrO2, zirconia) is an interesting catalytic material to be used in biomass conversion, e.g., gasification and reforming. In this work, we show that reducing and hydrating pretreatments affect the surface sites on monoclinic zirconia. The multitechnique approach comprises temperature-programmed surface reactions (TPSR) under CO and CO2 at 100-550 °C, in situ DRIFTS investigations of the surface species and density functional theory (DFT) calculations. The key findings of the work are: (1) formates are formed either directly from gas-phase CO on terminal surface hydroxyls or via the linear CO surface species that are found exclusively on the reduced zirconia without water treatment; (2) formates are able to decompose at high temperature either reversibly to CO or reductively to CO2 and H2via surface reaction between formates and multicoordinated hydroxyls; and (3) a new weak reversible binding state of CO is found exclusively on ZrO2 that is first reduced and subsequently hydrated.

Exposed surfaces on shape-controlled ceria nanoparticles revealed through AC-TEM and water-gas shift reactivity.

Aberration-corrected transmission electron microscopy and high-angle annular dark field imaging was used to investigate the surface structures and internal defects of CeO2 nanoparticles (octahedra, rods, and cubes). Further, their catalytic reactivity in the water-gas shift (WGS) reaction and the exposed surface sites by using FTIR spectroscopy were tested. Rods and octahedra expose stable (111) surfaces whereas cubes have primarily (100) facets. Rods also had internal voids and surface steps. The exposed planes are consistent with observed reactivity patterns, and the normalized WGS reactivity of octahedra and rods were similar, but the cubes were more reactive. In situ FTIR spectroscopy showed that rods and octahedra exhibit similar spectra for -OH groups and that carbonates and formates formed upon exposure to CO whereas for cubes clear differences were observed. These results provide definitive information on the nature of the exposed surfaces in these CeO2 nanostructures and their influence on the WGS reactivity.

Towards stable catalysts for aqueous phase conversion of ethylene glycol for renewable hydrogen.

Aqueous-phase reforming of ethylene glycol over alumina-supported Pt-based catalysts is reported. Performance of the catalysts is investigated by conducting kinetics and in situ attenuated total reflectance (ATR)-IR spectroscopic analysis. Pt/γ-Al2 O3 is unstable under APR conditions (270 °C, 90 bar) and undergoes phase transformation to boehmite [AlO(OH)]. This conversion of alumina is studied in situ by using ATR-IR spectroscopy; transition into boehmite proceeds even at milder conditions (210 °C, 40 bar). Pt/γ-Al2 O3 deactivates irreversibly because the Pt surface area decreases owing to an increasing metal particle size and coverage with boehmite. However, Pt supported on boehmite itself shows stable activity. Surprisingly, the rate of formation of hydrogen per Pt surface atom is significantly higher on boehmite compared to an alumina-supported catalyst. This observation seems correlated to both increased concentration of surface OH groups as well as to enhanced oxidation of Pt when comparing Pt/γ-Al2 O3 with Pt/AlO(OH).

Valorization of humin-based byproducts from biomass processing-a route to sustainable hydrogen.

The synthesis of biomass-based top value-added chemical platforms, for example, 5-hydroxymethyl furfural, furfural, or levulinic acid from the acid-catalyzed dehydration of sugars results in high yields of insoluble by-products, referred to as humin. Valorization of humin by steam reforming for H2 is discussed. Both thermal and catalytic steam gasification were investigated systematically. Humin undergoes drastic changes under thermal pre-treatment to the gasification temperature. Alkali-metal-based catalysts were screened for the reactions. Na2 CO3 showed the highest activity and was selected for further study. The presence of Na2 CO3 enhances the gasification rate drastically, and gas-product analysis shows that the selectivity to CO and CO2 is 75% and 25%, respectively, which is a H2 /CO ratio of 2 (corresponding to 81.3% H2 as compared to the thermodynamic equilibrium). A possible process for the complete, efficient conversion of humin is outlined.

Building microscopic soccer balls with evaporating colloidal fakir drops.

Evaporation-driven particle self-assembly can be used to generate three-dimensional microstructures. We present a unique method to create colloidal microstructures in which we can control the amount of particles and their packing fraction. To this end, we evaporate colloidal dispersion droplets on a special type of superhydrophobic microstructured surface, on which the droplet remains in Cassie-Baxter state during the entire evaporative process. The remainders of the droplet consist of a massive spherical cluster of the microspheres, with diameters ranging from a few tens up to several hundreds of microns. We present scaling arguments to show how the final particle packing fraction of these balls depends on the dynamics of the droplet evaporation, particle size, and number of particles in the system.

How water droplets evaporate on a superhydrophobic substrate.

Evaporation of water droplets on a superhydrophobic substrate, on which the contact line is pinned, is investigated. While previous studies focused mainly on droplets with contact angles smaller than 90°, here we analyze almost the full range of possible contact angles (10°-150°). The greater contact angles and pinned contact lines can be achieved by use of superhydrophobic carbon nanofiber substrates. The time evolutions of the contact angle and the droplet mass are examined. The experimental data are in good quantitative agreement with the model presented by Popov [Phys. Rev. E 71, 036313 (2005)], demonstrating that the evaporation process is quasistatic, diffusion-driven, and that thermal effects play no role. Furthermore, we show that the experimental data for the evolution of both the contact angle and the droplet mass can be collapsed onto one respective universal curve for all droplet sizes and initial contact angles.

Light at the interface: the potential of attenuated total reflection infrared spectroscopy for understanding heterogeneous catalysis in water.

IR spectroscopy has been an important tool for studying detailed interactions of reactants and reaction-intermediates with catalyst surfaces. Studying reactions in water is, however, far from trivial, due to the excessive absorption of infrared light by water. One way to deal with this is the use of Attenuated Total Reflection spectroscopy (ATR-IR) minimizing the path length of infrared light through the water. Moreover, ATR-IR allows for a direct comparison of reactions in gas and water on the same sample, which bridges the gap between separate catalyst investigations in gas and liquid phase. This tutorial review describes recent progress in using ATR-IR for studying heterogeneous catalysts in water. An overview is given of the important aspects to be taken into account when using ATR-IR to study heterogeneous catalysts in liquid phase, like the procedure to prepare stable catalyst layers on the internal reflection element. As a case study, CO adsorption and oxidation on noble metal catalysts is investigated with ATR-IR in gas and water. The results show a large effect of water and pH on the adsorption and oxidation of CO on Pt/Al(2)O(3) and Pd/Al(2)O(3). From the results it is concluded that water affects the metal particle potential as well as the adsorbed CO molecule directly, resulting in higher oxidation rates in water compared to gas phase. Moreover, also pH influences the metal particle potential with a clear effect on the observed oxidation rates. Finally, the future outlook illustrates that ATR-IR spectroscopy holds great promise in the field of liquid phase heterogeneous catalysis.

Drop impact upon micro- and nanostructured superhydrophobic surfaces.

We experimentally investigate drop impact dynamics onto different superhydrophobic surfaces, consisting of regular polymeric micropatterns and rough carbon nanofibers, with similar static contact angles. The main control parameters are the Weber number We and the roughness of the surface. At small We, i.e., small impact velocity, the impact evolutions are similar for both types of substrates, exhibiting Fakir state, complete bouncing, partial rebouncing, trapping of an air bubble, jetting, and sticky vibrating water balls. At large We, splashing impacts emerge forming several satellite droplets, which are more pronounced for the multiscale rough carbon nanofiber jungles. The results imply that the multiscale surface roughness at nanoscale plays a minor role in the impact events for small We less than or approximately equal 120 but an important one for large We greater than or approximately equal 120. Finally, we find the effect of ambient air pressure to be negligible in the explored parameter regime We less than or approximately equal 150.

The influence of water and pH on adsorption and oxidation of CO on Pd/Al2O3--an investigation by attenuated total reflection infrared spectroscopy.

Adsorption and oxidation of carbon monoxide over a Pd/Al2O3 catalyst layer was investigated both in gas phase and water. Both adsorption and oxidation of CO are significantly affected by the presence of liquid water. Water influences the potential of the metal particles as well as the dipole moment of the adsorbed CO molecule directly, which is reflected both in large red shifts and a higher infrared intensity when experiments are carried out in water. Furthermore, the rate of CO oxidation increases significantly by both the presence of water and by increasing the pH. Enhancement of the oxidation rate is attributed to a weakening of the CO bond by increasing potential of the metal particle, similar to CO oxidation over Pt/Al2O3 as recently published [S. D. Ebbesen et al., J. Catal., 2007, 246, 66]. However, on Pd/Al2O3 the oxidation of palladium is clearly promoted at increasing pH, further enhancing the oxidation of CO over Pd/Al2O3.

Presence of lithium ions in MgO lattice: surface characterization by infrared spectroscopy and reactivity towards oxidative conversion of propane.

The surface morphology of Li-promoted MgO catalysts prepared using the sol-gel method (sg) and wet impregnation procedure (imp), respectively, has been studied by low-temperature infrared spectroscopy of adsorbed CO molecules. The results show that step sites, as unselective catalytic centers, are the major features existing on the surface of pure MgO, and those are active toward the oxidative conversion of propane. However, the concentration of these sites is drastically reduced by the incorporation of lithium ions in the MgO lattice. In fact, the incorporated Li (+) ions tend to move into the surface region and occupy sites associated with lower coordination number (e.g., step sites). Li/MgO-sg catalysts are characterized by a higher concentration of incorporation of lithium compared to Li/MgO-imp. In the case of oxidative dehydrogenation/cracking of propane, Li/MgO-sg catalysts show higher activity and selectivity to olefins compared to materials prepared using wet impregnation. Catalytic performance differs strongly regarding (i) the amount of olefins formed, and (ii) the ratio of C(3)H(6)/C(2)H(4). It is shown that high density of active sites is essential for further oxidative dehydrogenation of propyl radicals to propylene and suppression of cracking reactions pathway.