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The composition of the reducing gas in the activation of Co Fischer-Tropsch synthesis catalysts determines the nature of the catalytically active Co species. This study reports on the effect of H2 versus syngas (H2/CO = 2) on the reducibility of Co3O4 nanoparticles supported on hollow carbon spheres, using ex situ and in situ high-resolution aberration-corrected analytical electron microscopy. High-resolution images revealed twinned fcc Co particles encapsulated in carbon from syngas treatment while H2-treated particles were mostly CoO. Moreover, the electron energy loss of the Co-L3,2 and O-K edge fine structures show improved reducibility in syngas than in H2 at 350 °C. The effect of high temperature on the reducibility of the Co3O4 nanoparticles is also explored. Carbon fiber encapsulation of twinned fcc Co particles observed during the syngas treatment provides sinter resistance at high temperatures. Both ex situ and in situ results indicate that syngas activation is efficient for obtaining highly reduced Co nanoparticles at lower temperatures.

Carbon materials have been widely used as microwave susceptors in many chemical processes because they are highly effective at transforming incoming electromagnetic energy for local (hot spot) heating. This property raises the intriguing possibility of using the all-pervasive carbonaceous deposits in operating heterogeneous catalytic processes to augment the catalytic performance of microwave-initiated reactions. Here, the catalytic activities of a range of carbon materials, together with carbon residues produced from a "test" reaction-the dehydrogenation of hexadecane under microwave-initiated heterogeneous catalytic processes, have been investigated. Despite the excellent microwave absorption properties observed among these various carbons, only activated carbons and graphene nanoplatelets were found to be highly effective for the microwave-initiated dehydrogenation of hexadecane. During the dehydrogenation of hexadecane on a Fe/SiC catalyst, active carbon species were formed at the early stage of the reactions but were subsequently transformed into filamentous but catalytically inert carbons that ultimately deactivated the operating catalyst.

Well-geometric-confined yolk-shell catalysts can act as nanoreactors that are of benefit for the antisintering of metals and resistance to coke formation in high-temperature reactions such as the CO2 reforming of methane. Notwithstanding the credible advances of core/yolk-shell catalysts, the enlarged shell diffusion effects that occur under high space velocity can deactivate the catalysts and hence pose a hurdle for the potential application of these types of catalysts. Here, we demonstrated the importance of the shell thickness and porosity of small-sized Ni@SiO2 nanoreactor catalysts, which can vary the diffusional paths/rates of the diffusants that directly affect the catalytic activity. The nanoreactor with an ∼4.5 nm shell thickness and rich pores performed the best in tolerating the shell diffusion effects, and importantly, no catalytic deactivation was observed. We further proposed a shell diffusion effect scheme by modifying the Weisz-Prater and blocker model and found that the "gas wall/hard blocker" formed on the openings of the shell pores can cause reversible/irreversible interruption of the shell mass transfer and thus temporarily/permanently deactivate the nanoreactor catalysts. This work highlights the shell diffusion effects, apart from the metal sintering and coke formation, as an important factor that are ascribed to the deactivation of a nanoreactor catalyst.

The importance of extracted and refined fossil carbonaceous fuels (petroleum, diesel etc.) to the development of human society cannot be overestimated. These natural resources have improved billions of lives, worldwide, in providing accessible, relatively inexpensive energy at nearly every scale. Notwithstanding the credible advances in renewable energy production over the past decade or so, the aerial combustion of coal, natural gas and liquid fossil fuels, given humankinds insatiable demand for power, will continue to be the ready source of more than 85% of the world's energy in the foreseeable and possibly the distant future. Human activities based on the combustion of fossil fuels, however, has led to significant anthropogenic emissions of carbon dioxide (CO2) to the atmosphere – and that fact is now seen as the major contributor to global warming and climate change. To stabilise global mean temperatures will depend on the ultimate transformation of humankind's energy system to one that does not introduce CO2 into the atmosphere. The hydrogen economy has long been mooted as a route to achieving the required net-zero emissions energy future. Paradoxically, fossil fuel sources such as petroleum, crude and extra-heavy crude oil, petrol, diesel and methane are reported here to produce high volumes of high-purity hydrogen through their microwave-initiated catalytic dehydrogenation using fine iron particles. The co-product of this dehydrogenation process, solid carbon, can be safely stored underground in perpetuity or converted in future to valuable hydrocarbons and other materials. Through their catalytic dehydrogenation to yield carbon-free hydrogen – rather than through their aerial combustion to produce carbon dioxide – petroleum and other fossil fuels can now serve as an energy pathway to stabilising global mean temperatures.

The ubiquitous challenge of plastic waste has led to the modern descriptor plastisphere to represent the human-made plastic environment and ecosystem. Here we report a straightforward rapid method for the catalytic deconstruction of various plastic feedstocks into hydrogen and high-value carbons. We use microwaves together with abundant and inexpensive iron-based catalysts as microwave susceptors to initiate the catalytic deconstruction process. The one-step process typically takes 30–90 s to transform a sample of mechanically pulverized commercial plastic into hydrogen and (predominantly) multiwalled carbon nanotubes. A high hydrogen yield of 55.6 mmol g−1plastic is achieved, with over 97% of the theoretical mass of hydrogen being extracted from the deconstructed plastic. The approach is demonstrated on widely used, real-world plastic waste. This proof-of-concept advance highlights the potential of plastic waste itself as a valuable energy feedstock for the production of hydrogen and high-value carbon materials.

Funder: King Abdulaziz City for Science and Technology (KACST); doi: https://doi.org/10.13039/501100004919, With mounting concerns over climate change, the utilisation or conversion of carbon dioxide into sustainable, synthetic hydrocarbons fuels, most notably for transportation purposes, continues to attract worldwide interest. This is particularly true in the search for sustainable or renewable aviation fuels. These offer considerable potential since, instead of consuming fossil crude oil, the fuels are produced from carbon dioxide using sustainable renewable hydrogen and energy. We report here a synthetic protocol to the fixation of carbon dioxide by converting it directly into aviation jet fuel using novel, inexpensive iron-based catalysts. We prepare the Fe-Mn-K catalyst by the so-called Organic Combustion Method, and the catalyst shows a carbon dioxide conversion through hydrogenation to hydrocarbons in the aviation jet fuel range of 38.2%, with a yield of 17.2%, and a selectivity of 47.8%, and with an attendant low carbon monoxide (5.6%) and methane selectivity (10.4%). The conversion reaction also produces light olefins ethylene, propylene, and butenes, totalling a yield of 8.7%, which are important raw materials for the petrochemical industry and are presently also only obtained from fossil crude oil. As this carbon dioxide is extracted from air, and re-emitted from jet fuels when combusted in flight, the overall effect is a carbon-neutral fuel. This contrasts with jet fuels produced from hydrocarbon fossil sources where the combustion process unlocks the fossil carbon and places it into the atmosphere, in longevity, as aerial carbon - carbon dioxide.

The insatiable—and ever-growing—demand of both the developed and the developing countries for power continues to be met largely by the carbonaceous fuels comprising coal, and the hydrocarbons natural gas and liquid petroleum. We review the properties of the chemical elements, overlaid with trends in the periodic table, which can help explain the historical—and present—dominance of hydrocarbons as fuels for power generation. However, the continued use of hydrocarbons as fuel/power sources to meet our economic and social needs is now recognized as a major driver of dangerous global environmental changes, including climate change, acid deposition, urban smog and the release of many toxic materials. This has resulted in an unprecedented interest in and focus on alternative, renewable or sustainable energy sources. A major area of interest to emerge is in hydrogen energy as a sustainable vector for our future energy needs. In that vision, the issue of hydrogen storage is now a key challenge in support of hydrogen-fuelled transportation using fuel cells. The chemistry of hydrogen is itself beautifully diverse through a variety of different types of chemical interactions and bonds forming compounds with most other elements in the periodic table. In terms of their hydrogen storage and production properties, we outline various relationships among hydride compounds and materials of the chemical elements to provide some qualitative and quantitative insights. These encompass thermodynamic and polarizing strength properties to provide such background information. We provide an overview of the fundamental nature of hydrides particularly in relation to the key operating parameters of hydrogen gravimetric storage density and the desorption/operating temperature at which the requisite amount of hydrogen is released for use in the fuel cell. While we await the global transition to a completely renewable and sustainable future, it is also necessary to seek CO 2 mitigation technologies applied to the use of fossil fuels. We review recent advances in the strategy of using hydrocarbon fossil fuels themselves as compounds for the high capacity storage and production of hydrogen without any CO 2 emissions. Based on these advances, the world may end up with a hydrogen economy completely different from the one it had expected to develop; remarkably, with ‘Green hydrogen' being derived directly from the hydrogen-stripping of fossil fuels. This article is part of the theme issue ‘Mendeleev and the periodic table'.

The historical roots, the discovery and the modern relevance of Dmitri Mendeleev's remarkable advance have been the subject of numerous scholarly works. Here, with a brief overview, we hope to provide a link into the contents of this special issue honouring the great scientist. Mendeleev's advance, announced in March 1869, as he put it in 1889, to the ‘…then youthful Russian Chemical Society…’, first set out the very basis of the periodic law of the chemical elements, the natural relation between the properties of the elements and their atomic weights. This was, and still is, the centrepiece of a historical journey for chemistry to today's position as a pre-eminent science. This article is part of the theme issue ‘Mendeleev and the periodic table'.

The demarcation of the chemical elements into metals and non-metals dates back to the dawn of Dmitri Mendeleev's construction of the periodic table; it still represents the cornerstone of our view of modern chemistry. In this contribution, a particular emphasis will be attached to the question ‘Why do the chemical elements of the periodic table exist either as metals or non-metals under ambient conditions?’ This is perhaps most apparent in the p-block of the periodic table where one sees an almost-diagonal line separating metals and non-metals. The first searching, quantum-mechanical considerations of this question were put forward by Hund in 1934. Interestingly, the very first discussion of the problem—in fact, a pre-quantum-mechanical approach—was made earlier, by Goldhammer in 1913 and Herzfeld in 1927. Their simple rationalization, in terms of atomic properties which confer metallic or non-metallic status to elements across the periodic table, leads to what is commonly called the Goldhammer–Herzfeld criterion for metallization. For a variety of undoubtedly complex reasons, the Goldhammer–Herzfeld theory lay dormant for close to half a century. However, since that time the criterion has been repeatedly applied, with great success, to many systems and materials exhibiting non-metal to metal transitions in order to predict, and understand, the precise conditions for metallization. Here, we review the application of Goldhammer–Herzfeld theory to the question of the metallic versus non-metallic status of chemical elements within the periodic system. A link between that theory and the work of Sir Nevill Mott on the metal-non-metal transition is also highlighted. The application of the ‘simple’, but highly effective Goldhammer–Herzfeld and Mott criteria, reveal when a chemical element of the periodic table will behave as a metal, and when it will behave as a non-metal. The success of these different, but converging approaches, lends weight to the idea of a simple, universal criterion for rationalizing the instantly-recognizable structure of the periodic table where … the metals are here, the non-metals are there … The challenge of the metallic and non-metallic states of oxides is also briefly introduced. This article is part of the theme issue ‘Mendeleev and the periodic table’.

Coal tar, a major by-product of the coal industry, presents considerable difficulties in its refining and conversion into fuels due to its complex chemical composition and physical properties, such as high viscosity, corrosiveness, thermal instability, etc. Here we report a new route for producing hydrogen-rich gases together with carbonaceous materials, including carbon nanotubes, through the microwave-initiated catalytic deep dehydrogenation of coal tar using inexpensive iron catalysts. The resulting carbonaceous materials generated over the catalyst were investigated using a variety of techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature programmed oxidation (TPO) and Raman spectroscopy. Importantly, we have found that an aqueous emulsion feed of the coal tar enables considerably easier handling and an enhanced hydrogen production whilst also significantly reducing the extent of catalyst deactivation. This behaviour is shown to be assisted by the phenomenon of micro-explosion that enhances mass and heat transfer during the catalytic reactions.

Microwave - assisted catalysis has previously been shown to offer unique advantages in the rapid production of hydrogen from hydrocarbons fuels. In this work, we have prepared a series of supported catalysts with varying iron content and support materials, and evaluated the resulting H2 production from hexadecane, a key representative component of diesel fuel. These catalysts, under microwave activation, exhibit high selectivity for H2 over hydrocarbons in the effluent gas. Microwave-assisted Fe catalysts supported on SiC at low Fe loading levels (

The thermodynamics of jet fuel range alkane synthesis from carbon dioxide and hydrogen has been investigated. The hydrocarbon synthesis process is divided into three elementary steps, including light olefins formation (C2-C6) from hydrogenation of CO2, α-olefins (C7-C16) synthesis from oligomerization of light olefins, and hydrogenation of α-olefins (C8-C16) to straight-chain paraffin (C8-C16). The enthalpy changes and Gibbs free energy changes of the relevant reactions were calculated, and the equilibrium products distribution was computed based on the Gibbs free energy minimization method. The calculation results show that lower temperature (below 673.15 K), higher pressure (3 MPa), higher molar ratio of H2 to CO2 (3:1-4:1), and introducing a small amount of N2 in the reactants are favorable for the hydrogenation of CO2; lower temperature (below 500 K), higher pressure (2-3 MPa), addition of N2 is favorable for the oligomerization of light olefins; and lower temperature (below 700 K), higher pressure (2-3 MPa), and addition of N2 is favorable for the hydrogenation of α-olefins. The overall positive effect of introducing N2 results from its heat dilution of the process. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.

Hydrogen as an energy carrier promises a sustainable energy revolution. However, one of the greatest challenges for any future hydrogen economy is the necessity for large scale hydrogen production not involving concurrent CO2 production. The high intrinsic hydrogen content of liquid-range alkane hydrocarbons (including diesel) offers a potential route to CO2 -free hydrogen production through their catalytic deep dehydrogenation. We report here a means of rapidly liberating high-purity hydrogen by microwave-promoted catalytic dehydrogenation of liquid alkanes using Fe and Ni particles supported on silicon carbide. A H2 production selectivity from all evolved gases of some 98 %, is achieved with less than a fraction of a percent of adventitious CO and CO2 . The major co-product is solid, elemental carbon.

Thermodynamic calculations and analysis were carried out for a rational understanding of the results from selected laboratory MTH reactions. Simulations without solid carbons (coke), CO, CO2 and light alkanes target on the yield of olefin and aromatic products, which has been found better referenced to the real experimental observations that occur in time-on-stream (TOS). The confliction between simulated data and real experimental results is presumably ascribed to the limited dwelling time of products in the reaction system. Hydrocarbon pool based reactions donate olefins and methyl-benzenes as primary products in a continuous-flow MTH reaction; when the dwelling time of product extends intra-conversions (H2 transfers) between products would further adjust the composition of MTH yield, in which case alkanes and aromatic products (cokes precursors) increase. In the case of intra-conversions are ignored due to limited product dwelling time, thermodynamic calculation on Gibbs free energy change of selected sub reactions shows fairly close results to the real experimental data, which well supports the above explanations. This work highlights the importance of proper choosing target products and/or sub reactions for a rational thermodynamic prediction of MTH product distribution obtained in time-on-stream.

A rapid and non-invasive method to determine the dispersity of emulsions is developed based on the interrelationship between the droplet size distribution and the dielectric properties of emulsions. A range of water-in-oil emulsions with different water contents and droplet size distributions were analysed using a microwave cavity perturbation technique together with dynamic light scattering. The results demonstrate that the dielectric properties, as measured by non-invasive microwave cavity analysis, can be used to characterise the dispersity of emulsions, and is also capable of characterizing heavy oil emulsions. This technique has great potential for industrial applications to examine the sedimentation, creaming and hence the stability of emulsions.

In this work, a series of SiO2 supported Co catalysts have been prepared by adding urea or ruthenium to the cobalt precursor solution compared with the catalysts prepared without using urea. The characterization results show that cobalt is present as Co3O4 over the silica supports. The addition of urea to the catalyst precursor helps to generate smaller crystallite even at very high cobalt loading, while impregnation without using urea results in larger crystallite size as Co loading increases. Likewise, the addition of Ru to the catalyst precursor leads to a more uniform crystallite of Co3O4, and can help to lower the Co3O4 crystallite size to some extent, which has the similar effect on the catalyst prepared with urea. We have also shown that the support pore size and volume do have significant effect on the crystallite size of the cobalt oxide. The catalytic performances of the Co/SiO2 catalysts prepared with and without adding urea have been evaluated for Fischer Tropsch synthesis and the catalyst prepared using urea demonstrates significantly improved catalyst activity as well as selectivity to C5+.

In order to obtain the most favorable pyrolysis process for producing the highest yield of syngas from a range of leaf plants, the techniques of slow pyrolysis, fast pyrolysis and microwave pyrolysis of apple tree leaves, bamboo leaves, cypress leaves, sycamore tree leaves, and winter green leaves were investigated. The results showed that microwave pyrolysis produced the highest gaseous product yields (hydrogen rich gas) on all the 5 different kinds of leaves The apple tree leaves, sycamore leaves and wintergreen leaves showed higher yields of gas products than bamboo leaves and cypress leaves across all forms of pyrolysis. The experimentally determined higher heating values (HHV) of bio-chars from different leaves ranged from 20 to 28 MJ/Kg, interestingly, which are comparable to some coals. For the same variety of leaves, the HHV of bio-chars via different pyrolysis processes did not change significantly. However, the HHV of bio-chars from different leaves behaved differently, with the bio-char from wintergreen leaves showing the highest HHV, which reached around 27 MJ/Kg; on the contrary, the HHV of bio-chars from bamboo leaves gave the lowest values, some 20–22 MJ/Kg. For the gaseous products, the total amount of H2, CO, CO2 and CH4 reached 95% from all 3 pyrolysis treatments. Importantly, compared with slow and fast pyrolysis, the microwave pyrolysis of different leaves shows the highest content of H2 and CO and the lowest contents of CO2. The HHV of gaseous products of different leaves, using the same pyrolysis treatment, showed similar values but noticeably different when the pyrolysis process is for the same leaves is changed. The HHV of gaseous products via microwave pyrolysis are the highest amongst the different pyrolysis processes with the gaseous products HHV via fast pyrolysis being the lowest. We therefore conclude that microwave pyrolysis is a highly effective and also a highly selective process for H2 rich gas production from natural leaves. Furthermore, the addition of acid, bases and salts as catalysts for the pyrolysis did not noticeably result in any significant activity and products changes during the microwave pyrolysis process.

Hydrogen is often described as the fuel of the future, especially for application in hydrogen powered fuel-cell vehicles (HFCV’s). However, its widespread implementation in this role has been thwarted by the lack of a lightweight, safe, on-board hydrogen storage material. Here we show that benign, readily-available hydrocarbon wax is capable of rapidly releasing large amounts of hydrogen through microwave-assisted catalytic decomposition. This discovery offers a new material and system for safe and efficient hydrogen storage and could facilitate its application in a HFCV. Importantly, hydrogen storage materials made of wax can be manufactured through completely sustainable processes utilizing biomass or other renewable feedstocks.

The dehydrogenation of C3H8 in the presence of CO2 is an attractive catalytic route for C3H6 production. In studying the various possibilities to utilise CO2 to convert hydrocarbons using the sustainable energy source of solar thermal energy, thermodynamic calculations were carried out for the dehydrogenation of C3H8 using CO2for the process operating in the temperature range of 300–500 °C. Importantly, the results highlight the enhanced potential of C3H8 as compared to its lighter and heavier homologues (C2H6 and C4H10, respectively). To be utilised in this CO2 utilisation reaction the Gibbs free energy (ΔrGθm) of each reaction in the modelled, complete reacting system of the dehydrogenation of C3H8 in the presence of CO2 also indicate that further cracking of C3H6 will affect the ultimate yield and selectivity of the final products. In a parallel experimental study, catalytic tests of the dehydrogenation of C3H8 in the presence of CO2 over 5 wt%-Cr2O3/ZrO2 catalysts operating at 500 °C, atmospheric pressure, and for various C3H8 partial pressures and various overall GHSV (Gas Hourly Space Velocity) values. The results showed that an increase in the C3H8 partial pressure produced an inhibition of C3H8 conversion but, importantly, a promising enhancement of C3H6 selectivity. This phenomenon can be attributed to competitive adsorption on the catalyst between the generated C3H6 and inactivated C3H8, which inhibits any further cracking effect on C3H6 to produce by-products. As a comparison, the increase of the overall GHSV can also decrease the C3H8 conversion to a similar extent, but the further cracking of C3H6 cannot be limited.

The intrinsic reaction kinetics of methane aromatization under non-oxidative conditions over modified Mo/HZSM-5 catalysts was studied in the quartz pipe-reactor under ordinary pressure with the temperature ranging from 913.15 to 973.15 K and the space velocity from 700 to 2100 ml/(g·h). The Langmuir-Hinshelwood model was chosen to describe the intrinsic kinetics while Levenberg-Marquardt method was selected to determine the parameters in the kinetic model. Statistical test and residual error distribution diagrams showed that experimental data were in good agreement with calculated data, and Langmuir-Hinshelwood model was suitable for the description of the intrinsic kinetics of methane aromatization under the reaction conditions discussed in this article.