Display omitted
•Ru promotes reducibility and oxygen vacancy concentration of CeO2.•Oxygen vacancy concentration order: Ru/CeO2/r > Ru/CeO2/o > Ru/CeO2/c.•Ru/CeO2/r, with highest reducibility and ...defect concentration, is most active.
In this study, CeO2 rods (CeO2/r), cubes (CeO2/c) and octahedra (CeO2/o) supported catalysts with identical Ru particle size were prepared. Trend in the activity of these catalysts for CO2 methanation was compared with the trend in their oxygen vacancy concentration measured after calcination. Ru/CeO2/r outperforms the other two catalysts with a reaction rate of 11.0 × 10−8 mols-1mRu-2 and selectivity to methane of 99% at 250 °C. Temperature-programmed reduction (TPR), Raman and X-ray photoemission spectroscopy (XPS) results confirms that Ru addition enhances reduction of CeO2. Also, Ru/CeO2/r is more reducible and contains more oxygen vacancies as compared to Ru/CeO2/o and Ru/CeO2/c, both after calcination as well as under reducing conditions. H2 consumption during TPR shows removal of oxygen equivalent to about 3 monolayers, implying diffusion of vacancies into the subsurface or bulk of CeO2. The catalyst with the highest concentration of oxygen vacancies is also the most active catalyst, suggesting that reactive adsorption CO2 at an oxygen vacancy is the rate determining step.
Plasma-based NOX 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 NOX synthesis. Thereafter, we discuss the reported performance for plasma-based NOX 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 NOX 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 NOX 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.
Display omitted
•The rate of calcium carbonate decomposition increases when steam is applied.•A catalytic effect occurs since the activation energy decreases without by-products formation.•Steam ...opens a new pathway via formation of surface bicarbonate.•Bicarbonate decomposes in two steps, via surface calcium hydroxide.
The search for cheap solutions for carbon dioxide capture in order to prevent global warming is still challenging. Calcium oxide may be a suitable sorbent, but the regeneration process from calcium carbonate requires too high temperatures, causing sintering and decreasing sorption capacity. In this study the effect of steam on the decomposition of the carbonate is investigated. A clear rate-enhancing effect up to a factor of 4 is observed when steam concentrations up to 1.25% are applied during isothermal reactions at temperatures between 590 and 650 °C. This results in a decrease of the apparent activation barrier from 201 to 140 kJ mol−1, caused by the opening of a new reaction pathway. The kinetics of steam catalyzed decomposition of CaCO3 is discussed and a simple reaction scheme is proposed, including estimation of kinetic constants. The new pathway proceeds via formation of a stable surface bicarbonate followed by decomposition to surface OH groups, which then decompose by desorbing H2O.
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.
The 2020 plasma catalysis roadmap Bogaerts, Annemie; Tu, Xin; Whitehead, J Christopher ...
Journal of physics. D, Applied physics,
10/2020, Letnik:
53, Številka:
44
Journal Article
Recenzirano
Odprti dostop
Plasma catalysis is gaining increasing interest for various gas conversion applications, such as CO2 conversion into value-added chemicals and fuels, CH4 activation into hydrogen, higher hydrocarbons ...or oxygenates, and NH3 synthesis. Other applications are already more established, such as for air pollution control, e.g. volatile organic compound remediation, particulate matter and NOx removal. In addition, plasma is also very promising for catalyst synthesis and treatment. Plasma catalysis clearly has benefits over 'conventional' catalysis, as outlined in the Introduction. However, a better insight into the underlying physical and chemical processes is crucial. This can be obtained by experiments applying diagnostics, studying both the chemical processes at the catalyst surface and the physicochemical mechanisms of plasma-catalyst interactions, as well as by computer modeling. The key challenge is to design cost-effective, highly active and stable catalysts tailored to the plasma environment. Therefore, insight from thermal catalysis as well as electro- and photocatalysis is crucial. All these aspects are covered in this Roadmap paper, written by specialists in their field, presenting the state-of-the-art, the current and future challenges, as well as the advances in science and technology needed to meet these challenges.
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 ≲ 120 but an important one for large We ≳ 120. Finally, we find the effect of ambient air pressure to be negligible in the explored parameter regime We ≲ 150.
Plasma-enhanced catalytic ammonia synthesis has been proposed as an alternative pathway for green nitrogen fixation in the case of medium- and small-scale operation. Recently, Mehta et al. Nat. ...Catal. 2018, 1 (4), 269−275 postulated that plasma-induced vibrational excitations of N2 decrease the dissociation barrier, without influencing the subsequent hydrogenation reactions and ammonia desorption at atmospheric conditions. In this paper, this postulation is substantiated with experimental data of unpromoted and promoted, alumina-supported ruthenium ammonia synthesis catalysts. Within the temperature regime for plasma-enhanced catalytic ammonia synthesis over ruthenium-based catalysts (>200 °C), synergy is experimentally observed between the catalyst and the plasma by a lowered apparent activation energy. While the apparent activation energy for thermal-catalytic ammonia synthesis typically ranges from ∼60 to ∼115 kJ mol–1 depending on the promoters, the apparent activation energy for plasma-enhanced catalytic ammonia synthesis ranges from ∼20 to ∼40 kJ mol–1, consistent with the hypothesis that ammonia synthesis is enhanced via plasma-induced vibrational excitations of N2. Further support follows from the observation that the effects of promoters and supports on activity are similar for thermal catalysis and plasma-enhanced catalysis. As promoter and support influence activity via enhancing dissociation of N2, it follows that breaking the N–N bond is still relevant in plasma-enhanced catalytic ammonia synthesis.
Display omitted
•Preparation of structured DBD plasma reactors by loading μm-thin catalytic layers.•Catalytic wall reactors keep plasma conditions and conversion constant.•Methane is activated by DBD ...plasma while Pd catalyst modifies product distribution.•Formation of undesired carbon deposits reduced by factor of 2.•Structured reactors outperform packed bed reactors.
Non-oxidative coupling of methane has been performed in DBD plasma reactors with a catalytic layer with varying thickness loaded on the reactor wall. These structured reactors allow to study the effect of the thickness of the catalyst layer, including the blank plasma reactor, without significant modification of plasma properties, SEI and residence time. Moreover, it allows analysis of the catalytic effect of Pd/Al2O3. The catalyst layer decreases the methane conversion only mildly, which is attributed to hydrogenation of CHx radicals at the outer surface of the catalyst layer. This results in typically 34 % methane conversion at 2.8 W at room temperature with 6% CH4 in Ar, independently of the layer thickness. In contrast, the thickness of the catalyst layer strongly influences the product distribution, assigned to hydrogenation of acetylenes at external and internal surfaces in the catalyst layer. The formation of undesired deposits is suppressed by a factor of 2 with value-added hydrocarbons selectivity of 70 % and a carbon balance of 93 %. In addition, catalytic-wall reactors was compared with packed bed reactors. The synergistic effect is much more evident in the structured reactor than in the packed bed reactor, independently of the position of the catalytic bed.
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.
This Scientific Perspective outlines best practice in the design of plasma catalysis experiments. A water pumping model depicts the limitations of an exergonic reaction that surpasses thermodynamic equilibrium, where the pump represents the plasma. The leak back causes energy dissipation, depending on the water level in the upper reservoir (representing conversion) and the size of the leak (representing plasma activation of the product).
Carbonaceous materials are abundant, affordable and simple to implement for a wide range of applications. Its utilization as catalyst support for oxidation reactions seems counterintuitive due to the ...instability of carbonaceous materials under depollution conditions. The current research work demonstrates that the properties of carbon as support can be fine-tuned via the introduction of heteroatoms. The effect of N-doping together with the anchoring of platinum nanoparticles on the catalytic performance is systematically studied. Factors influencing the performance for CO oxidation are elucidated using X-ray scattering, N2 physisorption, transmission electron microscopy, elemental analysis, Raman spectroscopy, X-ray photoelectron spectroscopy, temperature programmed reduction and thermogravimetric analysis. The doping of nitrogen in the carbon framework improves the stability of the carbon support, while adding nitrogen and oxygen improves the stability and performance. Doped carbonaceous materials can be a promising support for low and medium temperature range applications (100–250 °C).
Display omitted
PDF
