Ni Catalyst Surface Research Articles

Effect of Electrochemical Surface Oxidation on Intrinsic Activity Towards the Hydrogen Evolution Reaction of Ultra-Smooth Nanocrystalline Ni Thin Films

Green hydrogen has emerged as a leading contender in the pursuit of a sustainable energy transition, driving the need for optimized catalysts in hydrogen production. Therefore, considerable effort is taken to replace the highly active, yet expensive Pt-group metals with non-noble alternatives such as Ni. Understanding the hydrogen evolution reaction (HER) mechanism on well-performing Ni surfaces is essential for gaining fundamental knowledge and implementing this in practical applications. While high surface area structures are typically designed for enhanced efficiency, there is limited research available on the fundamental aspects of thin films and smooth surfaces, which are crucial for benchmarking intrinsic catalytic activity and elucidating catalytic mechanisms. To address this gap, we perform a combined microstructural and electrochemical evaluation of smooth Ni thin film electrodes to analyze HER in alkaline media.This study aimed to evaluate the intrinsic catalytic properties of nanocrystalline Ni thin films and the effect of electrochemical oxidation on the catalytic behavior, while precisely monitoring the evolution of the film surface state and roughness. In addition to experimental work, Density Functional Theory (DFT) calculations and Kinetic Monte Carlo (KMC) simulations are used to understand HER on oxide-modified Ni surfaces and the role of different elementary processes involved in the HER mechanism.Nanocrystalline low-roughness Ni electrodes for HER were produced via magnetron sputter deposition to minimize the difference between geometrical and electrochemical surface area. This means we find intrinsic material properties directly, bypassing the need for additional measurements for real surface area determination. The analysis of catalyst activities towards HER as well as the oxidation process [1] was performed via cyclic voltammetry, using a three-electrode setup. Within the conducted studies, we can report a significant lowering of the overpotential for HER upon in-situ oxidation of the electrodes due to the existence of NiO, and Ni(OH)2 on the Ni catalyst surface [2], leading also to a limitation of the deactivation processes. Further analysis including topography evaluation (AFM), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) in combination with selected area diffraction (SAED) of the as-deposited and oxidized electrodes confirmed that the observed effect can be assigned to the availability of oxides on the surface assuredly (Figure 1). Additionally, theoretical calculations and simulations (DFT, KMC) support the experimental data and derived theories.In conclusion, the beneficial effect of NiO and Ni(OH)2 presence on Ni surfaces was examined and successfully proven by a combination of electrochemical measurements, material characterization, theoretical calculations, and a careful experimental design that minimizes the influence of interfering factors, such as surface roughness.

Polarization Resistance of Ceria-Containing Fuel Electrodes in Solid Oxide Cells Studied By Impedance and DRT Analysis

Introduction Solid oxide electrolysis cell (SOEC), which enables steam electrolysis by reversely operating solid oxide fuel cell (SOFC), is capable of highly efficient hydrogen production. Whilst SOFC and SOEC are using similar materials and cell structures, electrode reactions in SOEC operation have to be studied in more details. In our previous study [1], Ni-ScSZ cermet, widely used as SOFC fuel electrodes, exhibited a tendency where electrode resistance increases at a low absolute current density in SOEC operation, while Ni-GDC co-impregnated cells fabricated by the impregnation method exhibited sufficient performance in a wide current density range. Here in this study, various model fuel electrode materials were evaluated and compared in a wide current density range in the SOFC and SOEC modes under the same measurement conditions. Electrode reactions are evaluated through measurements of electrochemical impedance spectra (EIS) and subsequent analysis of distribution of relaxation times (DRT). Experimental Three types of cells with different fuel electrodes were prepared: Ni-ScSZ cermet fuel electrode; Ni-GDC cermet fuel electrode; and Ni-GDC co-impregnated fuel electrode. The Ni-GDC cermet fuel electrodes were fabricated using gadolinia-doped ceria (GDC), widely known as a mixed ionic electronic conductor, besides a pure ionic conductor ScSZ. The Ni-GDC co-impregnated cell was fabricated by preparing a composite electrode backbone of GDC and La-doped SrTiO3 (LST), followed by impregnation of the catalyst particles, Ni and GDC. La-Sr-Ti oxides are known to exhibit sufficient stability in both oxidizing and reducing atmospheres [2, 3]. Moreover, co-impregnated fuel electrodes have shown high tolerance against highly-humidified hydrogen fuel and redox durability [4].Electrode resistances were separated into the ohmic resistance and the nonohmic polarization resistance through electrochemical impedance spectroscopy. Impedance measurements were made by using an impedance analyzer (1255WB, Solartron, UK) in a frequency range between 0.1 Hz and 1 MHz. The fuel, 50%-humidified H2, was supplied to the fuel electrode. The operating temperature was 800 °C. To characterize the electrode processes in both SOFC and SOEC modes, impedance measurements were performed by applying stepwise changes in current density up to ±1.2 A cm-2, for every 0.2 A cm-2. The DRT analysis of the EIS data was performed using a numerical calculation program based on Tikhonov regularization (Z-Assist, Toyo Corporation, Japan) to separate electrochemical processes involved in the electrode reactions. Results and discussion The polarization resistances obtained by the EIS measurements are shown in Fig. 1. The Ni-ScSZ cermet fuel electrode (Fig. 1 (a)) exhibited unique polarization resistance in the SOEC mode. With increasing the absolute value of current density in the SOEC mode, polarization resistance increased significantly at first, reaching a maximum value at -0.4 A cm-2, then decreased. In contrast, for the Ni-GDC cermet fuel electrode (Fig. 1 (b)), polarization resistance increased only slightly with increasing current density in the SOEC mode, and overall polarization resistance remained low. This tendency was also seen for the Ni-GDC co-impregnated fuel electrode (Fig. 1 (c)), where such increase in polarization resistance was further suppressed.Such increase and decrease in polarization resistance observed for the Ni-ScSZ cermet have to be explained by different mechanisms (at least two opposite factors). One possible explanation could be that the reduced and recovered catalytic activity due to e.g. oxidation and reduction of the Ni catalyst surface. The presence of GDC in the fuel electrode has a positive effect reducing the polarization resistance in the SOEC mode. The electrode reaction area may be extended beyond the Ni surface and/or the triple phase boundaries. Furthermore, the Ce ion in ceria undergoes a valence change, which may promote flexible surface exchange reactions, and surface reactions on the Ni catalysts. For the SOEC operation, it will be desirable to clarify the effects of GDC in the r-SOC electrode reactions. Acknowledgments A part of this study was supported by “Research and Development Program for Promoting Innovative Clean Energy Technologies Through International Collaboration” of the New Energy and Industrial Technology Development Organization (NEDO) (Contract No.20001460-0). Collaborative support by Prof. H. L. Tuller, Prof. B. Yildiz, and Prof. J. L. M. Rupp at Massachusetts Institute of Technology (MIT) is gratefully acknowledged. References N. Endo, T. Fukumoto, R. Ushijima, K. Natsukoshi, Y. Tachikawa, J. Matsuda, S. Taniguchi, and K. Sasaki, ECS Trans., 103(1), 1981 (2021).Q. Ma and F. Tietz, Solid State Ionics, 225, 108 (2012).G. Chen, H. Kishimoto, K. Yamaji, K. Kuramoto, and T. Horita, J. Electrochem. Soc., 162(3), F223 (2014).S. Futamura, A. Muramoto, Y. Tachikawa, J. Matsuda, S. M. Lyth, Y, Shiratori, S. Taniguchi, and K. Sasaki, Int. J. Hydrogen Energy, 44(16), 8502 (2019). Figure 1

Investigation of Ni catalyst activation during plasma-assisted methane oxidation

Atmospheric pressure plasma has shown promise in improving thermally activated catalytic reactions through a process termed plasma-catalysis synergy. In this work, we investigated atmospheric pressure plasma jet (APPJ)-assisted CH4 oxidation over a Ni/SiO2 .Al2O3 catalyst. Downstream gas-phase products from CH4 conversion were quantified by Fourier transform infrared spectroscopy. The catalyst near-surface region was characterized by in-situ diffuse reflectance infrared Fourier transform spectroscopy. The catalyst was observed to be activated at elevated temperature (500 °C) if it was exposed to the APPJ operated at large plasma power. ‘Catalyst activation’ signifies that the purely thermal conversion of CH4 using catalysts which had been pre-exposed to plasma became more intense and produced consistently CO product, even if the plasma was extinguished. Without the application of the APPJ to the Ni catalyst surface this was not observed at 500 °C. The study of different exposure conditions of the activated catalyst indicates that the reduction of the catalyst by the APPJ is likely the cause of the catalyst activation. We also observed a systematic shift of the vibrational frequency of adsorbed CO on Ni catalyst when plasma operating conditions and catalyst temperatures were varied and discussed possible explanations for the observed changes. This work provides insights into the plasma-catalyst interaction, especially catalyst modification in the plasma catalysis process, and potentially demonstrates the possibility of utilizing the surface CO as a local probe to understand the plasma-catalyst interaction and shed light on the complexity of plasma catalysis.

Enhance coking tolerance of high-performance direct carbon dioxide-methane solid oxide fuel cells with an additional internal reforming catalyst

Carbon deposition can be observed on the surface of Ni catalyst surface because of its high catalytic activity of hydrocarbon cracking, and thus cause the degradation of cell performance. In this work, anode supported single cells without and with the additional internal reforming catalyst NiFe 2 O 4 -GDC are successful assembled, and then used to characterize electrochemical performance and carbon tolerance in H 2 and 50%CO 2 –50%CH 4 fuels. Both single cells without and with NiFe 2 O 4 -GDC demonstrate similarly high electrochemical performance and low electrode polarization resistance fueled in H 2 , and the peak power densities are 1037.15 and 1163.36 mW/cm 2 at 800 °C, respectively, the difference of which derived from the ohmic impedance and the electrode polarization resistance. Interestingly, to better understand the contribution of different electrode polarization impedances, the distribution of relaxation time (DRT) analysis is used to distinguish different rate-limiting steps with high resolution. While 50%CO 2 –50%CH 4 fuel is applied, single cells with internal reforming catalyst NiFe 2 O 4 -GDC demonstrates high electrochemical performance (1046.72 mW/cm 2 at 800 °C), long-term stability and excellent carbon tolerance from the Raman scanning analysis. All preliminary experimental results suggest that high performance and coking tolerance can be obtained for direct carbon dioxide-methane solid oxide fuel cells with an additional internal reforming catalyst. • The fuel pre-reforming by CO 2 is an effective method for carbon removal. • High-performance direct carbon dioxide-methane solid oxide fuel cells can be realized. • Additional internal reforming catalyst is used to enhance coking tolerance. • DRT analysis is used to distinguish different rate-limiting steps of electrode process. • Raman spectra analysis is performed to evaluate the carbon deposits of anode catalyst.

Partial oxidation of phenolic wastewater using NaOH and Ni addition for hydrogen production and phenolics degradation in supercritical water

Phenolic wastewater is a challenge for traditional microbial treatment because of complex pollutant constituents and high concentration of aromatic hydrocarbons. Using H2O2 as oxidant, partial oxidation of phenolic wastewater using NaOH and Ni addition for hydrogen production and phenolics degradation in supercritical water was explored. It was found that higher oxygen ratio was advantageous for phenolics and other organic pollutants converting to gaseous products from supercritical water partial oxidation without any catalyst addition, but hydrogen production peaked at 0.5 oxygen ratio. Hydrogen gasification efficiency were raised to 75.68% within 0.5 oxygen ratio and 3.0% NaOH addition. Although separate Ni catalyst effectively enhanced the production of the gasifiable intermediate, hydrogen gasification efficiency appeared to a balance due to the enhancement of hydrogenation and methanation reactions. Both organic pollutants removal and hydrogen production enhancement were significantly improved under the combined NaOH-Ni catalyst, 85.24% TOC removal and 86.39% hydrogen gasification efficiency achieved within 1.0 NaOH + 2.0 Ni catalyst. There was very little cyclic compounds detected in liquid phase from combined NaOH-Ni catalytic partial oxidation, compared with individual addition. S, O and Si elements deposited in the surface of Ni catalyst, which have some negative effect on reuse. The positive effect of combined NaOH-Ni catalyst on organic pollutants removal and hydrogen production enhancement was demonstrated, which provided novel results and information that help to solve the problems in the process of toxic and high concentration organic wastewater treatment and facilitate the engineering application of supercritical water partial oxidation technology in the future.

A two-dimensional modeling of solid oxide fuel cell button cells with detailed electrochemistry mechanism

Abstract A two-dimensional model of nickel/yttria-stabilized zirconia (Ni/YSZ) solid oxide fuel cell (SOFC) was developed for a button cell system. The model integrates the detailed catalytic, electrochemical elementary reactions with ionic/electronic conduction and multiple gas transport processes in SOFC. The model is validated using published experimental data for H 2 –H 2 O fuel gas under different cell sizes and operating conditions. The distributions of gas/surface phase species concentration and current density were predicted and the effects of operating temperature, fuel gas composition and fuel channel tube design on the cell performance were studied. The results show that the electrochemical reaction processes occurs mainly within a 20 μm distance from the anode/electrolyte interface and that the Ni catalyst surface is covered mainly by H(s). For the chamber channel design, the calculations show that the tube chamber should have a diameter no smaller than the cathode electrode to obtain the best SOFC performance.

Effect of pollutants on biogas steam reforming

This study reports the influence of biogas poisoning on a Ni based catalyst working under steam reforming conditions (atmospheric pressure, T = 1073 K and H2O/CH4 = 2 mol/mol). A biogas stream composed by CH4 and CO2 with a ratio 55/45 vol.%, added with different chemical species (H2S, hydrocarbons mixture and D5) as contaminants, was used as inlet gas stream.First, effect of poisoning on Ni catalyst were separately evaluated and the boundary concentrations for each contaminants were revealed (0.4 ppm, 200 ppm and 0.5 ppm for H2S, hydrocarbons and D5 respectively) to assure Ni stable performances on time on stream (100 h at 50,000 h−1 of GHSV). Successively, a comparison between Ni catalytic behaviors in presence of two combined poisoning in the biogas (H2S + Hydrocarbons and Hydrocarbons + D5) was carried out.It was found that the effect of combined poisoning, even though it considered in moderate concentration, is harmful for Ni catalyst activity. Methane conversion on time on stream was reduced from 86% to 40% after 50 h, when the couple of poisoning Hydrocarbons + D5 was added to the inlet gas stream, while a lower deactivation pattern (about 73%) was leaded by couple H2S + Hydrocarbons. Both poisoning mixtures promoted coke deposition on Ni catalyst surface (about ≥0.5 mgC/gcat·h) independently by poisoning chemical characteristics probably due to adsorption/deposition of contaminants on catalytic sites.

The electronic structure of the α–Ni(OH)2 films: Influence on the production of the high–performance Ni–catalyst surface

In a view of the importance of the solid-state properties of the oxide/hydroxide films on the efficiency of Ni–catalysts, this work is focused on the electronic structure of the passive α–Ni(OH)2 film that behaves as an n-type semiconductor (with anion vacancies as dominant point defects).The solid-state transformation of the n-type α–Ni(OH)2 film to Ni was discussed in the terms of Gerischer's approach for a reductive decomposition of a classical semiconductor. The kinetics of the α–Ni(OH)2 decomposition to Ni was studied in situ and the decomposition mechanism was proposed. The nickel metal nucleation begins at the anion vacancies on the film|solution interface and finally the whole oxide film matrix transforms to the new metallic phase. The hydrogen evolution reaction (h.e.r.) takes place at a potential more positive than Er(H2/H+) at a freshly, in situ extracted Ni surface.The semiconducting properties of the passive film were compared to those of an oxide film, also n-type semiconductor that is formed on Ni by ion-implantation method.The latter film mentioned shows compact, more ordered and less amorphous structure than the first one what is in accordance to the X-ray photoelectron spectroscopy (XPS) results.

Direct vs. indirect pathway for nitrobenzene reduction reaction on a Ni catalyst surface: a density functional study.

Density functional theory (DFT) calculations are performed to understand and address the previous experimental results that showed the reduction of nitrobenzene to aniline prefers direct over indirect reaction pathways irrespective of the catalyst surface. Nitrobenzene to aniline conversion occurs via the hydroxyl amine intermediate (direct pathway) or via the azoxybenzene intermediate (indirect pathway). Through our computational study we calculated the spin polarized and dispersion corrected reaction energies and activation barriers corresponding to various reaction pathways for the reduction of nitrobenzene to aniline over a Ni catalyst surface. The adsorption behaviour of the substrate, nitrobenzene, on the catalyst surface was also considered and the energetically most preferable structural orientation was elucidated. Our study indicates that the parallel adsorption behaviour of the molecules over a catalyst surface is preferable over vertical adsorption behaviour. Based on the reaction energies and activation barrier of the various elementary steps involved in direct or indirect reaction pathways, we find that the direct reduction pathway of nitrobenzene over the Ni(111) catalyst surface is more favourable than the indirect reaction pathway.

TG/DTA-MS evaluation of methane cracking and coking on doped nickel–zirconia based cermets

Cermet materials based on metallic nickel and cubic zirconia are the key material for applications on solid oxide fuel cells and high temperature water electrolysis. The main advantage is the possibility of direct feeding a hydrocarbon fuel, like methane, or even an alcohol as a source of hydrogen. The reforming reaction on the Ni catalyst surface can produce hydrogen continuously. However, the resulting catalyst poisoning by carbon deposition (coking) imparts their broad application. The work shows the evaluation of coking tolerance of some cermets prepared by mechanical alloying techniques and compares new additives specially chosen in order to avoid coking and increase the catalytic activity. Refractory metal additives besides copper were added to the basic cermet. While copper is a known doping agent that avoids coking, the refractory metals (Mo and W) have a twofold effect: promote sintering at lower temperatures and increase Cu activity due to their mutual immiscibility. Results of TG/DTA-MS analysis demonstrate both refractory metals have increased the coking tolerance as well as the catalytic activity during diluted methane cracking. Molybdenum and tungsten additives are promised regarding the improvement of these cermet materials for high temperature electrochemical devices.

Preliminary Studies about Carbon Deposition Behavior of Ni-Metal/YSZ Bimetallic Cermet Anode of SOFC

Nickel/Yttrium-stabilized zirconia (Ni/YSZ) composite is the standard anode material with high stability and catalytic activity in both oxidation of H2 and reforming hydrocarbons. However, carbon is deposited on the surface of Ni catalyst in hydrocarbon atmosphere which is called coking. It leads to decrease in percolation, power density, and eventually crack. In this paper, 5 mol% of titanium Oxide (TiO2) and manganese oxide (MnO) are added to NiO/YSZ powder. Prepared mixed anode powder is applied on Ni/YSZ support button cell as functional anode. Hydrogen oxidation catalytic activities of sample anodes were measured by IV curve, and polarization resistances were measured by impedance spectroscopy analysis at 800 ˚C in dry H2 atmosphere. Ni/YSZ, 5 mol% MnO Ni/YSZ and 5mol% TiO2 Ni/YSZ cells showed power density of 628mW/cm2, 458mW/cm2 and 382mW/cm2 at 0.6V respectively. Cells survived in dry CH4 atmosphere for 16, 27 and 32 minutes without performance drop.

Modeling of solid oxide electrolysis cell for syngas generation with detailed surface chemistry

A solid oxide electrolysis cell model for syngas generation is developed using a button cell test system as a physical base. The model coherently bridges the multi transport processes of charge, mass, momentum, and energy with detailed surface chemistry. The model is validated using experimental polarization curves. Upon the validation, extensive simulations are performed to elucidate surface electrolysis processes. Results indicate that the electrolysis processes of CO2 and H2O are pretty much independent with each other in the H2 electrode (electrolysis cathode). The carbon coking effect is mainly determined by the content of CO2 in the H2 electrode. Increasing the applied cell voltage may improve syngas production, it also causes the enhancement of the surface coverage of C(s) and the deposition of carbon on the surface of Ni catalyst. High operating temperature may effectively improve adsorption/desorption rate and enhance surface electrolysis process as well as potentially mitigate carbon deposition on Ni surface.

Top-Down Heating for Low Substrate Temperature Synthesis of Carbon Nanotubes

A top-down heating method to allow for low-temperature large area synthesis of carbon nanotubes using plasma-enhanced chemical vapour deposition is introduced in this paper. The approach utilizes top-down electromagnetic heating rather than conventional heating from a substrate heater under the electrode. A temperature gradient is created between the Ni catalyst surface and the substrate using a metal thermal control barrier layer, on which carbon nanotubes are grown as a function of the bias voltage, hydrocarbon concentrations and growth conditions. The heat during growth is provided by the plasma or energy coupling to the catalyst via top-down heating, which based on the coupled power density and the cooling of the substrate, in addition to the thermal 'barrier layer' properties will dictate the temperature of the growth surface. This unique approach of top-down heating with suitable cooling schemes, coupled with thermal barriers allows for the low substrate temperature synthesis of carbon nanotubes, scalable to large areas.

Dynamical Observation of Bamboo-like Carbon Nanotube Growth

The growth dynamics of bamboo-like multiwalled carbon nanotubes (BCNTs) via catalytic decomposition of C2H2 on Ni catalyst at 650 degrees C was observed in real time using an in situ ultrahigh vacuum transmission electron microscope. During BCNT growth, the shape of the catalyst particle changes constantly but remains metallic and crystalline. Graphene sheets (bamboo knots) within the nanotube preferentially nucleate on the multistep Ni-graphite edges at the point where the graphene joins the catalyst particle, where it is stabilized by both the graphene walls and the Ni catalyst surface. The growth of a complete inner graphene layer growth prior to contraction of the Ni catalyst particle due to restoring cohesive forces will result in a complete BCNT knot whereas partial growth of the inner wall will lead to an incomplete BCNT knot.

Deposition of carbon nanotubes by capillary-type atmospheric pressure PECVD

In this study, the growth behavior of carbon nanotubes(CNT) deposited from C 2H 2 by an atmospheric pressure plasma enhanced chemical vapor deposition (AP-PECVD) method was investigated. The deposition was performed at 400 °C using a modified dielectric barrier discharge using He/C 2H 2 with additive gases such as N 2 and NH 3. The effect of the pretreatment on the surface morphology of the Ni catalyst was also investigated. Treatment of the Ni catalyst surface with a He (6 slm)/NH 3 (90 sccm) atmospheric pressure plasma for 5 min at 400 °C transformed the Ni surface into more uniformly agglomerated small Ni particles required for the enhanced carbon diffusion during the CNT formation. The CNTs grown by using He (6 slm)/C 2H 2 (90 sccm) AP-PECVD with additive gases (NH 3, N 2) for 5 min at 400 °C after the pretreatment showed uniform 1.5–2 μm long and 20–50 nm diameter multi-layer CNTs. By the application of − 1.2 kV of dc bias to the substrate during the growth CNTs by the AP-PECVD, more vertical CNTs could be obtained. FT-Raman data showed the grown CNTs are muti-wall CNTs and have the intensity ratio of D-band/G-band was 0.8–0.9, so defects were involved in the grown CNTs. This study provides a new method growing CNTs at atmospheric pressure and at low temperature, which has special advantages for large scale applications using conventional glass substrates.

An investigation of plasma chemistry for dc plasma enhanced chemical vapour deposition ofcarbon nanotubes and nanofibres

The role of plasma in plasma enhanced chemical vapour deposition of carbonnanotubes and nanofibres is investigated with both experimental and computationaldiagnostic techniques. A residual gas analysis (RGA) of a 12 mbar dc discharge with aC2H2/NH3 gas mixture is conducted near the Ni catalyst surface employed for carbon nanofibre growth.The results are corroborated with a 1D dc discharge model that solves for species densities,ion momentum, and ion, electron and neutral gas thermal energies. The effect ofvarying the plasma power from 0 to 200 W on the gas composition is studied. Thedissociation efficiency of the plasma is demonstrated where over 50% of the feedstock isconverted to a mixture of hydrogen, nitrogen and hydrogen cyanide at 200 W.Finally, the important role that endothermic ion–molecule reactions play in thisconversion is, for the first time, established. Of these reactions, dissociative protonabstraction and collision-induced dissociation are of the greatest significance.

Carbon nanotubes and nanostructures grown at below 400°C

AbstractIn this paper, we report clear evidence for the growth of carbon nanotubes and nanostructures at low substrate temperatures, using direct-current plasma-enhanced chemical vapour deposition. The catalyst particles are mounted on a titanium layer which acts as a thermal barrier, and allows for a larger temperature gradient between the Ni catalyst surface and the substrate. A simple thermodynamic simulation shows that the temperature differential between the substrate growth surface and the growth electrode is determined by the thickness of the titanium layer. This facilitates the growth of nanotubes, as opposed to nanofibres with herring-bone or amorphous structures. The growth properties are discussed as a function of the bias voltage and hydrocarbon concentration. The heating during growth provided solely by the plasma is below 400°C and is dependent on the process conditions and the electrode configuration in the growth chamber. These conditions need to be taken into account when comparing processes across different growth methods and instruments. The novel approach based on the use of a thermal barrier ensures the synthesis of carbon nanotubes at room temperature substrate conditions, which can be attained with a suitable cooling scheme.

Catalyst performance of MoS 2 and WS 2 for the CO 2-reforming of CH 4 Suppression of carbon deposition

The catalyst performance of MoS 2 and WS 2 was verified for the CO 2-reforming of CH 4 through the comparison with that of Ni SiO 2 . No catalyst deactivation was observed on the disulfides, although the rate was much smaller than that on Ni. It was found that the disulfides exhibited a poor ability for the direct decomposition of CH 4. The poor ability did not lead to carbon deposition during reforming reaction, which assured a stable activity. It was also observed that the rate equation was completely different between disulfide catalysts and Ni catalyst. The reaction order was positive with respect to the CH 4 partial pressure and negative with respect to the CO 2 partial pressure on the former catalysts, while negative with respect to CH 4 partial pressure and positive with respect to the CO 2 partial pressure on the latter. The observed reaction kinetics suggested that the surface of the disulfide catalyst was abundant in adsorbed CO 2, while the surface of Ni catalyst was abundant in adsorbed CH 4. The abundance in adsorbed CO 2 was considered an unfavorable condition for the direct decomposition of CH 4 and as a result, carbon deposition was suppressed during CO 2-reforming on disulfide catalyst. Sulfur tolerance was found to be an additional performance of the disulfide catalyst.

Estimating the density of carbon atoms on a Ni catalyst surface in equilibrium with a carbonaceous gas

The amount of carbon adsorbed on the surface of Ni in contact with carbonaceous gas mixtures such as CH 4/H 2 and CO/CO 2, is estimated from equilibrium segregation data. The results are displayed on “gas composition versus temperature” plots for the above two gas mixtures. These plots provide basic thermodynamic information relevant to reactions such as steam reforming of hydrocarbons on supported Ni catalysts. For example, the plot for CO/CO 2 gas mixtures represents the Boudouard equilibrium on a single crystal Ni catalyst, whilst the plot for CH 4/H 2 gas mixtures provides information relevant to the equilibrium hydrogenation of adsorbed C to CH 4.