Unsupported Nickel Catalysts Research Articles

Selective Hydrogenation of Aromatic Nitro Compounds Using Unsupported Nickel Catalysts

AbstractHydrogenation of nitroarenes has attracted renewed attention since it was discovered that non‐noble metal catalysts could selectively reduce nitroarenes with sensitive reducible groups. Here, we report the results of the chemoselective catalytic hydrogenation of nitroarenes with unsupported Ni NPs. Initial catalytic assessments showed high activity and selectivity, reaching conversions up to 99 % and 100 % selectivity towards the substituted anilines containing reducible groups such as halogens and nitrile. The recyclability test showed impressive stability of the catalyst up to the fifth run for the target product. Preliminary studies on the catalytic mechanism of the Ni NPs indicate that the nitrosobenzene might not participate as an intermediate in this reaction. Instead, a hydroxylamino adsorbed on the Ni surface is proposed. This work displays an accessible approach for synthesizing an active and selective catalyst for reducing nitroarenes in the presence of facile reducible substituents.

The Characterisation of Hydrogen on Nickel and Cobalt Catalysts

We have investigated a series of supported and unsupported nickel and cobalt catalysts, principally using neutron vibrational spectroscopy (inelastic neutron scattering, INS). For an alumina supported Ni catalyst we are able to detect hydrogen on the metal for the first time, all previous work has used Raney Ni. For an unsupported Ni foam catalyst, which has similar behaviour to Raney Ni but with a much lower density, the spectra show that there are approximately equal numbers of (100) and (111) sites, in contrast to Raney Ni that shows largely (111) sites. The observation of hydrogen on cobalt catalysts proved to be extremely challenging. In order to generate a cobalt metal surface, reduction in hydrogen at 250–300 °C is required. Lower temperatures result in a largely hydroxylated surface. The spectra show that on Raney Co (and probably also on a Co foam catalyst), hydrogen occupies a threefold hollow site, similar to that found on Co(10bar{1}0). The reduced surface is highly reactive: transfers between cells in a high quality glovebox were sufficient to re-hydroxylate the surface.

Unsupported nickel catalyst prepared from nickel foam for methane decomposition and recycling the carbon deposited spent catalyst

Catalytic methane decomposition (CMD) receives increasing attention for co-production of COx-free hydrogen and valuable carbon by-product, and the catalyst plays a crucial role on methane conversion and the product features. Unsupported nickel catalysts derived from commercial nickel foam (NF) were prepared for CMD by mild pre-treatment. Effects of the pre-treatment method (acid treatment, thermal treatment, acid-thermal treatment and hydrogen reduction) and reaction temperature were explored on the NF morphology and CMD reactivity in a fixed-bed reactor. It is found that catalytic performance of the NF-based catalyst is highly dependent on the pre-treatment and reaction temperature. The thermal and acid-thermal treatments could greatly promote the catalytic activity (with methane conversion up to 74.6% and 91.8%, respectively) at 850 °C. To fully release potential abilities of the catalyst, the carbon deposited spent catalyst was recycled as a fresh catalyst in the CMD test by several strategies. High and stable methane conversion (up to around 90%–93%) can be achieved by simulating the operation model in a fluidized-bed reactor for a continuous CMD process. Besides, the carbon deposited spent catalyst could serve as a promising candidate of supercapacitor electrode material.

Catalytic Ethanolysis of Enzymatic Hydrolysis Lignin over an Unsupported Nickel Catalyst: The Effect of Reaction Conditions

The effect of reaction conditions on ethanolysis of enzymatic hydrolysis lignin (EHL) with an unsupported nickel catalyst, that is, Ni(220H), was investigated. The two-dimensional heteronuclear sin...

Catalytic Depolymerization of Enzymatic Hydrolysis Lignin into Monomers over an Unsupported Nickel Catalyst in Supercritical Ethanol

Lignin is the largest renewable source of aromatic compounds in nature, and the production of value-added aromatic compounds from lignin can reduce the dependence on fossil fuel feedstocks. Here, a...

The reactivity and initial carbonization of an Unsupported Nickel catalyst in dry reforming of methane

High speed product gas analysis during the dry reforming of CH4 allows the observation of transient elevated H2 and CO production rates, combined with high initial conversions of CH4 and CO2. The transients are observed in the first 120 s of the reaction, followed by a lower steady-state production rate after 3–5 min. The catalytic reactivity in the initial 120 s is important in understanding the catalyst performance and determining the reactions responsible for carbon deposition. Monitoring the changes in the H2 and CH4 partial pressures from the start, through the transient production regime, and on through the steady-state regime, shows that the transient reactivity is directly related to the decomposition of CH4. This finding is further supported by the linear trend found between the H2/CO ratio and the carbon deposition in the peak region of the product gas profile. Although significant amounts of carbon are deposited on the Ni surface, it is not completely deactivated at any of the reaction temperatures in the 500–800 °C range explored herein. Characterizing the initial reactivity in terms of carbon deposition and catalyst performance in dry reforming of CH4 may lead to changes in the sampling technique for researchers working with reforming reactions, and optimized catalysts that exhibit reduced initial carbon deposition.

Field-assisted suppression of coke in the methane steam reforming reaction

Electric fields were applied to an unsupported nickel catalyst during methane steam reforming with a low steam-to-methane ratio of 2:1. As expected, catalysts operated in standard mode (without an applied field) experienced high levels of coking. Coke formation was almost completely suppressed by the application of a positive electric field. This is explained using density functional theory computations which indicate that positive electric fields (1) increase the affinity for water adsorption on the surface, thereby increasing the amount of oxygen available to remove carbon, (2) stabilize individual carbon fragments, thus reducing their tendency to polymerize, and (3) increase the oxygen storage capabilities of the catalyst by inducing sub-surface oxygen formation and bulk oxidation.

Promotion of unsupported nickel catalyst using iron for CO2 methanation

To facilitate the understanding of the Ni-Fe bimetallic system, unsupported nickel, iron and a series of Ni-Fe catalysts were synthesized, characterized and tested for the CO2 methanation relevant for power to gas technology. Characterizations revealed the formation of various types of Ni-Fe alloys in the reduced catalysts. The activity results revealed that Ni-Fe catalysts with weight ratio of Ni:Fe > 1 showed a significantly improved rate (∼2–5 times) of CO2 conversion and CH4 formation compared to Ni and Fe due to the formation of suitable nickel-rich Ni-Fe alloy. A change in surface morphology of these alloy catalysts was also detected. The Ni-rich Ni-Fe alloy formed in 90Ni10Fe and 75Ni25Fe differed from each other. The former was the most active due to the absence of unalloyed Fe, in contrast to the presence of unalloyed Fe in the later. Such observations would be helpful to design better performing Ni-Fe catalysts.

Production of COx Free Hydrogen and Nanocarbon via Methane Decomposition Over Unsupported Porous Nickel and Iron Catalysts

Methane decomposition is the most effective method to produce greenhouse gas free hydrogen and nanocarbon. In this work, unsupported nickel and iron catalysts were successfully synthesized via a facile solid state citrate fusion method and used for the non-oxidative thermocatalytic decomposition of methane. The formation of phase pure crystalline structure of NiO and Fe2O3 with high surface porosity and improved textural properties was observed by various characterization techniques. The release of huge amount of gases from the bulk of the fused sample during the annealing process could result in the porosity. The catalysts exhibited high catalytic activity and stability for methane decomposition at three different reaction temperatures of 700, 800 and 900 °C without any catalyst deactivation. The yield of hydrogen and nanocarbon increased significantly with increasing the reaction temperature. A maximum hydrogen yield of 67% was obtained for the NiO at 900 °C, with a slight deactivation with time on stream. After 180 min of time on stream, the hydrogen yield was observed to be more or less same (49 ± 2%), at all of the temperatures. The iron catalyst exhibited quite high catalytic stability, where the hydrogen yield remained more or less same. The structural characterization of spent catalysts indicated the formation of metal encapsulated carbon nanoonions and few layered graphene sheets over Ni and Fe catalysts respectively. The crystallinity of the nanocarbon was found to be increasing with increasing the reaction temperature and a high graphitization degree was observed for the nanocarbon deposited at 900 °C, over both of the unsupported catalysts.

Low-temperature Selective CO Methanation over Unsupported Nickel Catalyst Covered by Silica Thin Layer

The catalytic performance of unsupported Ni catalyst covered by a silica thin layer with core-shell structure for selective CO methanation was investigated. The thin silica layer prepared with a silane coupling agent as the silica source effectively suppressed Ni-sintering even at quite low Si/Ni ratio. Due to the low Si/Ni ratio, the Ni surface area of the catalyst was increased by 20 % compared to the conventional unsupported Ni catalyst and outlet CO concentration reached less than 10 ppm even at low temperature (155 °C). The durability test revealed that outlet CO concentration was maintained at less than 10 ppm during operation for 3000 h over the developed catalyst even under severe conditions (SV 4800 h–1, inlet CO concentration 0.5 %). Further improvement of the catalyst selectivity is necessary because the operating window for the developed catalyst, in which outlet CO concentration is less than 10 ppm and CH4 formation is less than 1 %, was almost the same as that for the conventional catalyst.

Chemical catalytic vapor deposition (CCVD) synthesis of carbon nanotubes by decomposition of ethylene on metal (Ni, Co, Fe) nanoparticles

A simple method for producing unsupported nickel catalyst that can be used to synthesize multi-wall carbon nanotubes (MWNT) has been developed. The yield of purified MWNTs is about 1.8 gmwnt/(gcat×h).

Steam reforming of methane over unsupported nickel catalysts

This paper describes a study of steam reforming of methane using unsupported nickel powder catalysts. The reaction yields were measured and the unsupported nickel powder surface was studied to explore its potential as a catalyst in internal or external reforming solid oxide fuel cells. The unsupported nickel catalyst used and presented in this paper is a pure micrometric nickel powder with an open filamentary structure, irregular ‘fractal-like’ surface and high external/internal surface ratio. CH 4 conversion increases and coke deposition decreases significantly with the decrease of CH 4:H 2O ratio. At a CH 4:H 2O ratio of 1:2 thermodynamic equilibrium is achieved, even with methane residence times of only ∼0.5 s. The CH 4 conversion is 98 ± 2% at 700 °C and no coke is generated during steam reforming which compares favorably with supported Ni catalyst systems. This ratio was used in further investigations to measure the hydrogen production, the CH 4 conversion, the H 2 yield and the selectivity of the CO, and CO 2 formation. Methane-rich fuel ratios cause significant deviations of the experimental results from the theoretical model, which has been partially correlated to the adsorption of carbon on the surface according to TEM, XPS and elemental analysis. At the fuel: water ratio of 1:2, the unsupported Ni catalyst exhibited high catalytic activity and stability during the steam reforming of methane at low-medium temperature range.

Energetic Mapping of Ni Catalysts by Detailed Kinetic Modeling

Temperature-programmed desorption (TPD) of CO has been performed on supported and unsupported nickel catalysts. The unsupported Ni catalyst consists of a Ni(14 13 13) single crystal which has been studied under ultrahigh vacuum conditions. The desorption energy for CO at low CO surface coverage was found to be 119 kJ/mol, and the binding energy of C to the Ni(111) surface of the crystal was 703 kJ/mol. The supported catalysts consist of nickel supported on hydrotalcite-like compounds with three different Mg2+/Al3+ ratios. The experimental results show that for the supported Ni catalysts TPD of CO leads to desorption of both CO and CO2, with the latter being dominant. Dissociation of CO takes place, and considerable amounts of residue C are left on the surface. The residue C is removed by temperature-programmed oxidation (TPO). The results show that a low Mg2+/Al3+ ratio in the hydrotalcite precursor seems to result in more steplike sites, kinks, and defects for carbon monoxide dissociation. A detailed kinetic modeling of the TPO results based on elementary reaction steps has been conducted to give an energetic map of supported Ni catalysts. Experimental results from the ideal Ni surface fit nicely with literature values, providing useful information for identifying active sites on supported Ni catalysts.

Impact of the structure and reactivity of nickel particles on the catalytic growth of carbon nanofibers

Catalytically grown fishbone carbon nanofibers (CNF), are prepared by the decomposition of carbon-containing gases (CH 4, CO/H 2 or C 2H 4/H 2) over a silica-supported nickel catalyst and an unsupported nickel catalyst at 550 °C. It turns out that both the nickel particle size and the nature of the carbon-containing gas significantly affects the CNF growth process. We demonstrate that at the chosen temperature small supported nickel particles need a carbon-containing gas with a relatively low reactivity, like CH 4 or CO/H 2, to produce CNF. The resulting fishbone CNF have a uniform and small diameter (25 nm). The CNF thus synthesized hold great potential, e.g. as catalyst support material. However, the large unsupported nickel particles only produce CNF using a reactive carbon-containing gas, like C 2H 4/H 2. The CNF thus obtained show a variety of morphologies with a large range of diameters (50–500 nm). The CNF yield is a subtle interplay between the nickel particle size and consequently the exposed crystal planes on the one hand and the reactivity of the carbon-containing gas on the other.

ChemInform Abstract: Modification of Supported and Unsupported Nickel Catalysts by α‐ Amino and α‐Hydroxy Acids for Chiral Reactions

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Low temperature oxidative dehydrogenation of ethane over Ni-based catalysts

In contrast to supported palladium, nickel-based catalysts give interesting results in the oxidative dehydrogenation of ethane. Unsupported nickel catalysts are active at temperatures as low as 490 K. Reduced nickel particles are present during the reaction.

Chemisorption of Oxygen on Supported Nickel Catalysts

One unsupported nickel catalysts (about 55 wt.% Ni) supported on SiO2, Al2O3, TiO2, Nb2O5, ZrO2, V2O5 and ZnO were characterized by pulse dynamic oxygen chemisorption measured at -78 °C and evaluated in the reaction of reductive amination of diethylene glycol. Correlation was obtained between their oxygen uptake and catalytic activity.

Comparative deactivation study of nickel and palladium catalysts in liquid phase hydrogenation

The step by step deactivation by mercuric iodide and thiophenol of unsupported nickel and palladium catalysts during liquid phase 1-hexene and styrene hydrogenation was studied. Both poisons are strongly adsorbed on active sites and, although their toxicity changed with catalyst and reactant, it was always found to be very high. As we already showed it, the poison molecules number required for total deactivation is close to the kink atoms number of metallic cristallites.

Metal boride catalysts in methanation of carbon monoxide: III. Sulfur resistance of nickel boride catalysts compared to nickel and raney nickel

The methanation activity of unsupported and alumina supported nickel and nickel borides, prepared by sodium borohydride reduction, was determined during in situ exposure to 10 ppm H 2S at 525 K and 1.3 atm as a function of time. Using a recently developed deactivation model, deactivation rate constants and estimates of catalyst life were determined. The results indicate that nickel boride and Raney nickel catalysts are significantly more sulfur resistant, i.e. deactivate much more slowly in the presence of H 2S, compared to unsupported nickel and nickel alumina catalysts. The superior sulfur resistance of the boron and aluminium promoted catalysts is explained by (i) their intrinsically low deactivation rate constants and (ii) their large capacity for sulfur adsorption.

The Effect of Crystallite Size of Nickel on the Enantioselectivity of Modified Nickel Catalysts

Abstract Enantioselective hydrogenation of methyl acetoacetate to methyl 3-hydroxybutyrate with various unsupported nickel catalysts modified by (2R,3R)-tartaric acid was studied under mild conditions in order to elucidate the dependence of the selectivity upon the surface states of the catalysts. The catalyst with the larger crystallite size has the higher enantioselectivity, presumably because of the higher probability to obtain large ensembles of regularlyarranged nickel atoms in the catalyst surface for the modifier to adsorb strongly and regularly. The nickel boride catalyst has very low enantioselectivity, mainly due to its amorphous structure. The detrimental effect of additives (B, P, and Al in Ni–B, Ni–P, and R–Ni catalysts) was explained as the effect of their lowering the crystallinity of the catalysts.