Experimental studies and molecular modelling of catalytic steam gasification of brown coal containing iron species

Fuel 93 (2012) 404–414

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Experimental studies and molecular modelling of catalytic steam gasification of brown coal containing iron species G. Domazetis a,b,⇑, B.D. James b, , J. Liesegang c, M. Raoarun b, M. Kuiper d, I.D. Potter b, D. Oehme b a

Clean Coal Technology Pty. Ltd., Victoria 3111, Australia Chemistry Department, La Trobe University, Victoria 3086, Australia c Physics Department, La Trobe University, Victoria 3086, Australia d Victorian Partnership for Advanced Computing, Victoria 3053, Australia b

a r t i c l e

i n f o

Article history: Received 15 October 2010 Received in revised form 1 March 2011 Accepted 1 September 2011 Available online 17 September 2011 Keywords: Catalytic brown coal steam gasification Reaction mechanisms Molecular modelling

a b s t r a c t The paper presents experimental data of catalytic steam gasification of brown coal containing aqua-iron species, and the chemical mechanism(s) at a molecular level. Experimental techniques provided weight loss from catalysed reaction of char with steam of 17 wt% at 800 °C and 40 wt% at 900 °C, over 15 min, on a dry ash free basis (daf). Inorganic and organic oxygen, identified using XPS in the char samples, was derived from reactions with steam. High yields of H2 resulted from catalysed reactions between char and steam. Semi-empirical (SE) quantum molecular modelling using MOPAC, of reaction routes for high temperature pyrolysis and steam gasification, provided results consistent with experimental data for weight loss, iron species, and the distribution of inorganic and organic oxygen in char samples after reaction with steam. The catalysis mechanism(s) that have been examined are considered to be a hybrid of organometallic and heterogeneous chemistry, involving iron hydride species that precede H2 formation; oxygen insertion into [Fe–C–] to form [Fe–O–C–] followed by elimination of CO, creating another [Fe–C] site to continue the catalytic cycle. SE modelling indicates concerted reactions were more energetically favoured. Initial results from molecular dynamics (MD) show a higher concentration of H2O molecules about the active site [Fe–C]. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Catalytic steam gasification of coal is a technology that offers increased H2 in synthesis gas with lower CO2 emissions and considerable potential for an improved environmental footprint [1–10]. The prospect of an affordable source of hydrogen has motivated research into catalytic steam gasification of low-rank coal. Development of this technology requires an understanding at the molecular level of the catalytic reaction mechanisms that lead to enhanced hydrogen production, which are also related to the characteristics of the particular coal. Water soluble inorganic species added to brown coal can enhance coal reactivity and act as catalysts for steam gasification. Iron compounds are a relatively cheap source of catalyst and may be added to hydrophilic low-rank coals as aqua-iron species [4,6]. Catalytic coal gasification chemistry is a relatively novel example of heterogeneous catalysis; the inorganic species are added to the coal molecular matrix, which forms the solid phase, and these ⇑ Corresponding author at: Chemistry Department, La Trobe University, Victoria 3086, Australia. Tel.: +61 3 9841 9142; fax: +61 3 9479 1399. E-mail address: [email protected] (G. Domazetis).   Deceased. 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.09.001

eventually form the catalytic species as part of the thermolytic chemistry of the coal macromolecular matrix. The molecular transformations of the added inorganic species are part of the molecular changes of the coal, and the resulting char is also the reactive substrate. A large number of reaction routes occur during mediated low-rank coal pyrolysis chemistry that can ultimately form catalytically active sites within char [11]. It is well understood that the formation of active sites is a major feature of heterogeneous catalysis theory and these are often formed in situ for a particular catalytic system. Thus it is necessary to both identify the species that act as active sites and how they are formed during the thermal treatment of the catalytic system. There are many complicated factors related to the nature of intermediate chemical interactions of reactive molecules with the surface of solid catalysts, and identifying the active sites is especially difficult, made more so by the paucity of sensitive techniques to investigate in situ transformation under real catalytic conditions. Additional challenges on the theoretical aspects of heterogeneous catalysis are posed by the variety of mechanisms, which may be either step (discrete) reactions or concerted mechanism(s); the latter are characterised by higher rates of chemical transformations and lower activation energies [12–14]. In the particular case of steam gasification of coal containing metal species, in addition to the complicated reaction routes

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involving inorganics leading to the formation of actives sites, the inorganic species also impact on the char morphology formed during coal pyrolysis [15]. The chemistry involving H2O with the char/catalyst is a particular case of heterogeneous catalysis in which H2O is adsorbed onto an active site, followed by reaction with the char substrate, to yield mainly H2 and CO. The overall chemistry includes the formation of active iron sites as the coal is heated to a relatively high temperature, albeit these are part of the char substrate which is consumed by reaction with steam. Studies of catalytic steam gasification have been undertaken by various workers over a number of years and various mechanisms have been suggested, including oxygen transfer from H2O, the transport of C via metal-carbide type species, and of carbon reacting directly with metallic species [7–10]; a large number of inorganic salts have been examined for various coals, including salts of potassium, nickel, iron, and eutectic mixtures [16–19]. Global chemical kinetics modelling has often been used to describe coal gasification under conditions found in industrial coal gasifiers. Such treatments consider the many chemical and physical processes that combine to influence the conversion rate of coal char, including gas diffusion through the pores of the char, reaction at the surface, and diffusion of the products away from the reaction site. These global treatments need to include changes in pore structure and the chemical composition of the char during heating and gasification; a combination of process conditions will determine the overall rate of char conversion, such as temperature, reactant and product gas composition, particle size and char morphology. Such treatments are phenomenological, dealing mainly with coal gasification carried out using mixtures of oxygen and steam under conditions found in industrial coal gasifiers; e.g. entrained flow and fluid-bed gasification technology [20,21]. We have focussed on molecular reaction mechanisms that deal specifically with the formation of catalytically active sites within char molecules, to distinguish these from the numerous events that occur during coal gasification. This is done because overall phenomenological treatments cannot deal with the molecular reaction mechanisms nor provide specific insights into the nature of the catalytically active species. We have previously discussed metal mediated pyrolysis at lowtemperatures, and also high-temperature pyrolysis that leads to the formation of active sites. The present paper deals with molecular aspects of catalytic steam gasification of brown coal, under laboratory conditions, in an atmosphere containing steam. Brown coal samples were treated to contain increasing amounts of multi-nuclear iron hydroxyl species, and our previous studies under nitrogen or helium have examined low-temperature pyrolysis chemistry that transformed the char/iron species, and also hightemperature pyrolysis preceding the formation of active sites. In this paper, we examine the reactions under steam at increasing temperatures to ascertain catalytic reactions. We also discuss high-level SE molecular modelling, to elucidate the major reaction routes and to identify the active sites for iron catalysed steam gasification to yield H2 and CO. We discuss initial results of the first reported molecular dynamics modelling of steam molecules reacting with a molecule of char containing the iron catalytic site.

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though a mesh 10 sieve; the iron content has been shown to vary depending on the size and nature of coal particles, and analysis of iron content was also obtained for <30 microns and >30 micron coal particles; full details of sample preparation and the nature of polynuclear iron hydroxyl species are given in [24]. Pyrolysis and steam gasification experiments were carried out using a quartz tube reactor placed inside a Lindberg furnace in an atmosphere of nitrogen (pyrolysis) or helium with steam (gasification). A known mass of sample was placed into the heated furnace and measurements taken for prescribed periods of time. Pyrolysis data was obtained at low temperatures (200–600 °C) and high temperature (600–900 °C); details are given in [25]. The concentration of CO2 and CO gases from low-temperature pyrolysis was monitored periodically over a total period of 300–400 min using a gas cell and a Perkin Elmer 1720-X FTIR instrument, calibrated using standard mixtures of CO2 and CO [25]. The gases H2, CH4, CO and CO2 during steam gasification were monitored by gas chromatography (GC), using a Shimadzu GC – 4B PTF Gas Chromatograph or a Shimadzu GC 2010 Gas Chromatograph; both employed a thermal conductivity detector. The GC was calibrated using standard gas mixtures of H2, CO, CO2, and CH4. Data for weight loss and gases composition from steam gasification were mainly obtained after a period of 15 min; all weight loss has been reported on a dry ash free basis (daf). In addition, for some samples, data was also collected until complete weight loss was achieved, taking periods of between 15 and 120 min. Elemental composition of coal and char samples were obtained using microanalysis (elemental composition); X-ray Photoelectron Spectroscopy (XPS) analysis (the distribution from XPS are reported as at% values for C, O and Fe for coal and char samples), and X-ray Diffraction (XRD) analysis of iron phases in char. The total weight loss and concentrations of gases were measured at given temperatures over the range of 200–900 °C; this was done by adding the sample to the heated furnace and after 15 min, the quartz tube was removed and cooled rapidly under helium. The experimental data was reproducible to 10%; data for both catalytic and non-catalytic steam gasification was obtained; the average of three data sets was used to plot Fig. 2. TG and DTG data were also obtained using Perkin Elmer, TGA7 Thermogravimetric Analyser, under nitrogen. The experimental results for pyrolysis may be summarised as follows: at low temperatures, acid washed brown coal yields mainly CO2, CO and H2O, and at higher temperature, yields H2,

2. Experimental Experimental methodology has been previously discussed [6,22–27] and is only mentioned briefly here. The coal sample was a well mixed acid washed (aw) German brown coal and the same batch of aw coal treated to contain iron hydroxyl complexes at 1.7 wt% Fe, 4.4 wt% Fe, 7.8 wt% Fe, 12.4 wt% Fe. The elemental analysis of the acid washed coal, on a dry basis (db) is: C = 64.1%; H = 4.3%; O = 31.1%; ash = 0.4%. The coal has been crushed to pass

Fig. 1. Char molecular model with the [Fe3] cluster (purple = Fe, red = O, blue = N, black = C, white = H). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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3. Computations 3.1. Molecular modelling

Fig. 2. Weight loss (daf) from coal samples heated in steam for 15 min (weight loss for aw coal under He also shown for comparison; R2 values for graphs 0.98–0.99; catalytic 7.8 wt% Fe, 12.4 wt% Fe; non-catalytic aw, 1.7 wt% Fe, d 4.4 wt% Fe; . . . pyrolysis data aw coal ).

CO, and small amounts of CO2, light hydrocarbons and tar. The same coal sample containing significant amounts of iron species at low-temperature pyrolysis underwent a slightly higher weight loss compared to aw coal, but produced a relatively larger amount of CO2, without significant yields of tar. With increase in temperature more CO is observed relative to CO2 for all coal samples, and ultimately the products were mainly CO and H2. Our results, and those from numerous other publications by various groups, have been reviewed in [11]. XRD data from char samples prepared in an inert atmosphere over the temperature range 200–900 °C identified various iron species, ultimately leading to mainly reduced iron species at higher temperatures. The XRD data are considered to be from extraneous crystalline particles mixed with the char. The XRD identified iron oxide species when these samples were heated in Helium/steam. XPS data of the char sample containing iron species, obtained at low-temperature under Helium, identified inorganic oxygen and carbon consistent with an iron/carbonate intermediate species in char; at high temperatures the XPS shows less oxygen and mainly C and H. The XPS of samples heated under an atmosphere containing steam provided inorganic oxygen from organic oxygen and provides evidence of steam reacting with char. TG and DTG data for the coal samples containing iron hydroxyl species (7.8 wt% Fe) when heated under He, underwent weight losses over regions centred at 223 °C, 322 °C and 402 °C, and a sharp, distinct weight loss at 696 °C. The observed molar weight loss at 696 °C was equivalent to the amount of CO2 lost from our proposed iron/carbonate intermediate [25]. The acid washed coal sample underwent weight losses centred at 320 °C and 410 °C, and no distinct weight loss similar to that observed for the sample containing iron, was observed for acid washed coal at 700 °C. The distribution of aqua-polynuclear hydroxyl iron species chemically bound to brown coal, and their transformations into the iron clusters, has been discussed previously [24]. The distinction between small iron clusters bound to the coal/char molecular matrix, and the behaviour of larger iron compounds present as non-bonded species has also been discussed and will not be repeated here [27]. In this paper, we show catalytic activity of brown coal containing larger amounts of iron, and model this chemistry at a molecular level to show that active iron sites are formed in the char molecular matrix containing iron clusters.

SE and density functional theory (DFT) quantum mechanics (QM) molecular computations face major challenges stemming from the large molecular model. We have previously discussed the development of a suitable molecular model of low-rank coal, and have examined reaction routes for metal mediated pyrolysis chemistry leading to active sites [11,22–27]. This molecular model of brown coal was developed using experimentally measured values of brown coal, and the molecular model of char formed by performing changes to the coal molecular model that were consistent with experimental data obtained from pyrolysis of brown coal, and the same coal containing iron species. The modelling results discussed here are a continuation of our previous modelling; the present results are confined to high temperature pyrolysis and catalytic steam gasification that is observed after the formation of catalytically active sites. Molecular modelling studies commenced with SE computations of a singe coal molecule (500–600 atoms) containing various aqua iron species. The transformations were performed manually, to follow specific reaction routes for low-temperature pyrolysis and also the formation of iron/carbonate intermediates, into char molecules containing various iron oxides, and ultimately into a char model containing iron clusters [11,25]. Detailed discussion of the distribution of the aqua polynuclear iron hydroxyl species in brown coal, the formation of iron oxide clusters, and the various mechanisms for low-temperature pyrolysis modelling, have been provided previously [24,25,27]. High temperature pyrolysis has been modelled using char models containing various iron clusters (size of 260–300 atoms, iron clusters FemOn, m = 3–5, O = 0–5) were used to perform ab initio DFT computations. We performed molecular dynamics computations using a similar but smaller char molecule (175 atoms) containing one [Fe3] cluster. SE computations consisted of single point self consistent field (1scf-SE) and SE-QM computations performed using CAChe [28] and MOPAC undertaken on the NCI National Facility in Canberra, Australia, which is supported by the Australian Commonwealth Government [29]. Density functional theory (DFT) computations and 1scf-DFT were performed using Jaguar in the Schrödinger package at the Victorian Partnership for Advanced Computing facility [30]. SE computations with the PM5 Hamiltonian were used to obtain the ground state structures of all coal and char molecular models, and ab initio DFT geometry computations were used to obtain the energy profile for various char molecular models. 1scfPM5 and 1scf-DFT calculations were used to obtain relative changes in the calculated energy for the same molecule that had undergone an internal transformation. The data obtained from SE calculations include total energy, the heat of formation (DHf), bond lengths, bond angles, bond orders, partial charges, and contributions of r and p components to bonding with iron clusters. DHf, is defined in MOPAC as: DHf = Eelect + Enuc  Eisol + Eatom, Eelect is the electronic energy, Enuc is the nuclear–nuclear repulsion energy, Eisol is the energy required to strip all the valence electrons off all the atoms in the system, and Eatom is the total heat of atomization of all the atoms in the system; calculated in eV and converted into kcal/mol in MOPAC. Numerous reaction routes and molecular structures were examined for hydrogen and carbon monoxide formation during metal mediated high-temperature pyrolysis and also for catalytic steam gasification. These calculations obtained the difference in DHf (1scf-SE) and difference in total energy (1scf-DFT) for each single change for the particular reaction sequence relative to the starting molecule. The changes in DHf values for each change in the

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structure for a particular mechanism were used to indicate if each step was energetically favourable or un-favourable by comparing it with the DHf value of the molecular system used to initiate the reaction route. The overall difference in DHf of the reaction sequence was an indication of the exothermic or endothermic nature of the particular reaction sequence. Two transition states were examined for the reactions yielding H2 using iron clusters and H2O: (1) a linear [Fem  O–H–H] structure, or (2) an iron hydride [Fem   H–H(O)] structure (m = 3 and 5). Calculations were also performed to obtain the relative energies of intermediate structures reflecting (1) and (2) using the molecular model [Char[Fe3]  (H2O)]. The results from these computations indicated the iron hydride structure (2) was energetically favoured, and subsequently all reaction routes examined followed the formation of iron hydride species. The catalytic steam coal gasification reactions were examined for char containing iron complexes yielded H2 and CO, the major products from catalytic steam gasification. 3.2. Molecular dynamics (MD) The molecular model of char containing a Fe3 cluster used for MD simulations is shown in Fig. 1; this is similar but smaller than the molecular model used in the SE and DFT molecular computations (MF C95H71NO8Fe3, MW 1522.15, Fe = 11.0 wt%). The char molecule consisted of two fragments, each with chemical bonds to the Fe3 cluster. The first char fragment was bonded to two Fe centres by char oxygen, and the second had formed one [Fe OH] coordination bond with a char [OH] group. A full set of force-field parameters, required to perform MD simulation, were not available for this char model. For atoms that were a part of the Fe3 cluster, geometric parameters (bonds, bond angles, dihedrals) and atomic charges needed to be defined, while atomic charges also had to be defined for all atoms of the two char fragments. Parameter and atomic charge development was performed by splitting the char model into three sub-models: the Fe3 cluster with three oxygen atoms bound and capped by methyl groups, and the two char fragments mentioned above. The oxygen atoms which link the char fragments to the Fe3 cluster were included in the char fragment sub-models and were capped with N-methyl groups. Atomic charges for the char fragments were calculated using the RESP methodology [33] with ESP generated by Gaussian [32] QM calculations at the HF/6-31G⁄ level of theory. The approach used for calculating the charges of the Fe3 cluster consisted of first performing QM calculations at the HF/STO-3G level of theory. Atomic charges were then defined by comparing charges generated by both the Mulliken and RESP methodologies. Atomic charges for the Fe atoms in the Fe3 cluster were assigned the same charge of +0.452892. The oxygen atoms directly bonded to the Fe atoms were assigned a charge of 0.700000, while the hydroxyl oxygen that was coordinated to the Fe3 cluster was assigned a charge of 0.750000. Van der Waals (vdW) parameters for Fe were extracted from AMBER with the vdW radius set to 0.12 nm and the 6–12 potential well depth set to 0.05 kcal/mol. Geometric parameters for the Fe3 cluster were generated by analysing the SE and DFT molecular models, and by comparison with other force-field data. Energy minimisations and molecular dynamics (MD) simulations were carried out using the AMBER 10 suite of programs [31]. The Cartesian coordinates for the char model were obtained from SE and DFT molecular models, and then immersed in a box of SPC/E [35] water molecules, which extended at least 1.2 nm from the molecule. These H2O molecules act as a solvent surrounding the char molecule, but also move to simulate the dynamics of steam at high temperatures. Explicit solvent simulations were performed at 300 K (27 °C) and 843 K (570 °C). For the 300 K simulation, a three phase

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protocol was followed whereby the first phase was a 2000 step minimisation (500 steps of steepest descent followed by 1500 step of conjugate gradient). A heating phase (10 ps) followed where the system was heated to its target temperature using a Langevin dynamics heating scheme [34] with a collision frequency of 1 ps1. A 2.0 kcal/mol restraint was placed on all non-hydrogen atoms to restrict movement while heating, and a standard cut-off of 1.2 nm was used to limit vdW and electrostatic calculations. The final phase was a 1 ns production phase, which was performed to sample structures of the conformational space of the system. Coordinates were output to a trajectory file every 1 ps. SHAKE [36] was used to constrain all bonds involving hydrogen, while the PME method [37] with a cut-off of 1.2 nm was used to calculate electrostatic interactions. Heating was performed in the NVT ensemble, while the production was performed in the NTP ensemble [31,33–35]. A slightly different protocol had to be followed to perform the MD simulations at 843 K. The char model was solvated with fewer water molecules (655 instead of 2277) and a thorough six step equilibration had to be performed after heating so that the production phase could be performed at an appropriate density. Due to the increased temperature, the MD time step was reduced from 1 fs to 0.5 fs, while a restraint was also placed on the molecule to prevent it from ‘‘falling apart’’. Two different production phases were produced; one with a restraint of 1.0 kcal/mol, and the other 0.5 kcal/mol. The trajectory files from AMBER were used to visualise the dynamics of steam molecules about the char molecule containing the iron cluster. This was achieved with the molecular visualisation and analysis program VMD [38].

4. Results and discussion Catalytic steam coal gasification discussed in this paper differs from the usual heterogeneous catalysts because the solid phase, char, is also the reacting substrate. This chemistry, at a molecular level, may be considered as either heterogeneous catalysis, organometallic reactions, or a hybrid of these, because the substrate and the catalytic species form one solid phase with metal–oxygen and metal–carbon bonds to the char matrix. Other chemistry may be attributed to the coal/iron system if any iron hydroxide had precipitated in the coal pores; such chemistry is not considered to participate in char/catalysis reactions because such species are unlikely to form molecular bonds with the coal/char macromolecule (such iron species, however, would exist as extraneous particles within the coal/char matrix and may be involved in other events during gasification). In brown coal, the catalytic precursors are metal–ligand complexes which are part of the coal molecular substrate, and some are transformed into active sites on heating, during the formation of the char substrate. These precursors are transformed into catalytic sites containing metal–oxygen and metal–carbon bonds within the char/inorganic molecular matrix. Thus the chemistry may be considered similar to hybrid systems discussed in [39,40] which consider the catalytic fragment on a surface as an organometallic species bound to a ligand which is from the solid phase. The experimentally measured weight loss for the samples in this study is due to pyrolysis, and also due to reaction of char with steam; the latter is significant for those samples undergoing catalytic reactions with steam. Weight loss due to pyrolysis has been fully analysed previously [25]. Pyrolysis chemistry has been classified as low-temperature (200–600 °C) and high temperature (600– 900 °C); low temperatures yield mainly CO2 and CO, and at high temperatures mainly CO and H2. For samples containing iron species, the chemistry also leads to the formation of catalytic sites that preceded catalytic steam gasification [11,27]. We have shown a

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correlation between experimental and modelling results of lowtemperature pyrolysis for weight loss and the yield of CO2 and CO, and we will not repeat this here. The present modelling includes high temperature pyrolysis, because this chemistry leads to the formation of catalytic sites that take part in catalytic steam gasification to yield the main products H2 and CO [11,27]. The weight loss for all samples studied, over the temperature range 200–600 °C, is very similar under He and under He/steam, showing weight loss in this region would be attributed mainly to pyrolysis/thermolytic chemistry. Metal mediated pyrolysis yields a higher CO2:CO ration when compared to that for aw coal. The weight loss above 700 °C (daf) under identical conditions, for samples containing 8.8% Fe and 12.4% Fe, increased under an atmosphere of steam compared to that for all other samples, due to catalytic reaction between the coal substrate and steam. The weight loss for samples not displaying significant catalytic activity under the same conditions was similar to the weight loss observed in an inert atmosphere and is attributed to pyrolysis. The experimental data has been obtained under conditions that enable a direct comparison of weight loss for non-catalytic with catalytic systems. The data has been obtained using the same well mixed sample of German brown coal, to which increasing amounts of aqua-iron species were added. All experimental data enabled a direct comparing between acid washed coal and the same coal containing the iron species. Data of the weight loss of each sample was obtained from 200 °C to 900 °C, for 100° increase in temperature, under an inert atmosphere (N2 or He, for pyrolysis) and also under an atmosphere of the same inert gas saturated with steam. The conditions used were the same in terms of initial weight sample, flow rate, and the time (15 min) each sample was heated. As a result, if a greater increase in weight loss was observed for any sample when heated under He/steam, this would be directly attributed to reactions with steam. The catalytic activity was ascertained from the various weight losses under the same conditions, the total yield of gases exiting the reactor, and the analysis of the gases for H2, CO, CH4 and CO2. The molecular model of brown coal was also based on experimental data for brown coal, and containing the aqua-iron species [Fe(H2O)6]3+, [Fe2(OH)2 (H2O)4]4+ and [Fe3(OH)6 (H2O)10]3+. Molecular modelling of the reaction routes for pyrolysis of these samples of brown was consistent with experimental data for weight loss and yields of gaseous products. The mechanism for H2 and CO formation included hydrogen abstraction and elimination of CO from [Fe–O–C–(C)] groups to form the [Fe–C] active site for reactions with H2O. The importance of a suitable molecular model, and reaction routes of pyrolysis, have been discussed previously [6,11,22–27].

Table 1 The mole% of gases measured at 700 °C for coal samples containing iron. Coal sample

H2

CO2

CO

CH4

Total

1.7 wt% Fe 4.4 wt% Fe 7.8 wt% Fe 12.4 wt% Fe

ND ND 14.7 14.6

2.4 3.9 18.9 21.5

1.2 1.6 3.7 2.7

0.6 0.7 1.4 1.1

4.2 6.2 38.7 39.9

all similar; the weight loss for aw coal for pyrolysis is also given in the Figure, clearly showing that the weight losses for these samples were similar to those observed for pyrolysis, as discussed in [25]. Catalytic gasification is indicated above 700 °C for coal samples containing 7.8 wt% and 12.4 wt% Fe, with the weight loss increasing considerably with increasing temperature (17 wt% additional loss at 800 °C, and P32 wt% additional loss at 900 °C). The reactions between the char and steam for the samples that contained 7.8 wt% and 12.4 wt% Fe lead to high yields of the gases H2, CO, CO2 and CH4. The %mole of each gas measured at 700 °C is shown in Table 1 (ND = not detected; the remaining proportion is carrier gas; error at 10% of measured value). The gas samples from the coals with 12.4 wt% Fe contained between 52 and 55 mol% (average 54.5 mol%) of H2 at 900 °C, consistent with the greater weight loss observed. The yield of gaseous products and their composition, and the weight loss, for the coal samples containing 1.7% and 4.4% Fe, were consistent with non-catalytic activity. The XPS results for the coal sample containing 7.8 wt% Fe heated in steam at 900 °C are shown in Figs. 3a and 3b; the data is presented as the percentage of atoms (at%) assigned to the stated groups. The Figures show the samples heated under steam contain a greater proportion of inorganic and organic oxygen compared to the same sample heated under Helium. The increase in organic oxygen groups in the char samples heated in steam was accompanied by a decrease in the proportion of [C(C–H)] groups. The observed increases in oxygen were due to extraction of oxygen to both the iron species and the char molecule from reaction with H2O. The product gases from the coal samples containing 7.8 wt% and 12.4 wt% Fe, heated under steam at 900 °C over a 15 min period, provided the following molar percentages (the average of two measurements for each coal sample): H2 = 54.5 mol%, CO = 18.6 mol%; CO2 = 30.7 mol%, CH4 = 0.8 mol%. These yields of H2 were greater than the theoretical yield calculated for the mass of char consumed by steam. A yield similar to the experimental data can be explained by additional chemistry between CO and H2O; this is done by: (1) calculating the moles of H2, CO, CO2 attributed to the weight loss from the coal pyrolysis, and the moles of H2 and CO from reaction

4.1. Experimental results for catalytic steam gasification The weight loss for brown coal samples resulting from reactions with steam over a 15 min period is shown in Fig. 2. The data show that under an atmosphere of He/H2O, over the same period of 15 min, the coal samples containing 7.8 wt% and 12.4 wt% iron hydroxyl species underwent a considerably greater weight loss over the temperature 700–900 °C, and this is due to catalysed reaction between steam and char. The yields and composition of the gases from these samples were consequently the result from reaction between steam and char; catalysed reaction was observed for the samples containing the larger amounts of iron. Under the same conditions, the weight losses, and gaseous composition for the other three samples (Table 1 below), indicates zero, or negligible reaction with steam, and thus are classified as non-catalytic. The data points in Fig. 2 are averages from three measurements, with the deviation for shown by the error bars. The weight losses for aw coal, and also containing 1.7 wt% Fe and 4.4 wt% Fe, were

Fig. 3a. XPS data of char samples with Fe that displayed catalytic activity heated at 900 °C, showing the increase in inorganic oxygen bound to iron resulting from reaction with steam.

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Fig. 3b. XPS data of char samples that displayed catalytic activity to steam heated at 900 °C, showing organic groups in char containing organic oxygen from reaction with steam.

between char with steam at 900 °C; i.e. C + H2O ? H2 + CO, and (2) assuming between 50 and 60 mol% of the CO had undergone postgasification reaction with steam, (CO + H2O ? H2 + CO2). The ratio of H2 and CO2 to CO for the measured gases were: 3.6 (H2:CO); 2.1 (CO2:CO), and <0.1 (CH4:CO), which compares to the calculated rations (excluding any CH4 formation) for 50–60 mol% of CO reacting with steam, of: 3.1–4.2 (H2:CO), and 1.5–2.1 (CO2:CO). 4.2. High-level molecular modelling high temperature pyrolysis The major features of low-temperature pyrolysis reactions for brown coal containing chemically bound iron hydroxyl species may be summarised as loss of carboxyl groups that are chemically associated with iron species, resulting in CO2 (and some CO); the loss of CO2 has been modelled via iron/carbonate species that ultimately form iron oxide clusters. As the temperature increases, further weight loss occurs to yield CO, H2O and H2; H2 yield is modelled by hydrogen abstraction from adjacent char-hydroxyl groups. Hydrogen abstraction from char hydroxyl groups coordinated to iron centres [Fe–(OH)–C] would form [Fe–O–C–C] groups. Loss of CO from these [Fe–O–C–C] groups forms [Fe–C] bonds. The overall results of SE and DFT molecular modelling for ironmediated pyrolysis chemistry have been consistent with the experimental results, in terms of the yields of products, the weight loss of the coal sample, and on the observed nature of the metal species; these have been thoroughly reviewed in [11]. High temperature pyrolysis chemistry will be considered here as this leads to the formation of the [Fe–C] catalytic sites while predominately yielding H2 and CO. The various mechanisms that may be examined for H2 and CO formation from high temperature pyrolysis are discrete, parallel, or concerted reactions. Discrete chemistry consists of a reaction sequence of a given number of single steps yielding one product; e.g. H2 formation could be a distinct reaction sequence and this would be followed by another distinct reaction sequence to yield CO. Parallel reactions signify different active sites that would lead to different reactions taking place in parallel, and the final products would nonetheless consist of a mixture of H2 and CO; we postulate the [Fe–C] site for catalysis and this would negate parallel reaction. Modelling results were obtained to compare discrete reaction sequences with reaction routes that mimic concerted mechanisms. The oxidative addition of an [Fe] centre with [C–H] to form [Fe–C] and [Fe–H] in a transition state, is characteristic of the concerted mechanisms (e.g. the breaking of [C–H] and the formation of [Metal-C] and [Metal-H] in the transition state [41]). The output from molecular modelling included values of the relative changes in DHf for: (i) discrete reaction sequences

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that yielded exclusively either H2, or CO, and (ii) reactions that mimicked concerted chemistry, in which both H2 and CO were formed during the reaction sequence. SE and DFT for various reactions routes mimicking concerted chemistry, included the formation of H2 and CO, were modelled by forming an intermediate that yields both H2 and CO; i.e. an iron (hydride) intermediate that yields H2 and also acts as an iron (carbonyl) intermediate to yield CO. The reactions that mimic concerted routes gave lower energy changes than those for discrete reaction schemes. A rigorous treatment of concerted reaction mechanisms however, including mapping of the reaction trajectory, and identification of a specific intermediate for the various char molecular models, is beyond our present computational capabilities. SE results were obtained for the char model containing the iron as either [FemO] or [Fem] clusters (m = 3 or 5); these computations used the {char[Fe3O]} model. For the discrete formation and loss of H2, these computations provided a near thermo-neutral result, with the largest energy barrier of +20 kcal/mol for the reaction sequence; the discrete reaction sequence that yielded CO gave an energy change of +66 kcal/mol. The results for the same reaction sequence modelled as concerted chemistry yielding CO gave the largest energy change of +32 kcal/mol, and a near thermo-neutral overall result, indicating this as the likely chemistry. These pyrolysis reaction routes are illustrated by the 1scf-SE results shown in Fig. 4, which shows the results of a pyrolysis reaction sequence mimicking concerted chemistry yielding H2 and CO. The various species considered are labelled with numbers in Fig. 4 to designate steps in the reaction sequence; hydride structures are labelled (  H2) while chemisorbed and molecular hydrogen is labelled (H2). This reaction commences with abstraction of H from the char [OH] groups coordinated to the iron cluster [Fe3O]: i.e. {char[(HO)2 ? Fe3O]}. The reaction sequence leads to the iron (hydride) and (carbonyl) intermediate labelled {charFe3O(H2)(CO)3}. This sequence contains iron (hydride) and (carbonyl) intermediates; the iron hydride intermediate labelled [charFe3O(  H2)(CO)3] in Fig. 4, is shown in Fig. 5a while b shows the molecule of H2 adsorbed to the iron centre and the iron (carbonyl) intermediate. In this modelling sequence relatively small energy changes were observed for the formation of the iron hydride. The H2 molecule remained chemisorbed onto the char model until the complex labelled {charFe3O (H2)(CO)6}. Two iron carbonyl species were examined prior to loss of CO, which are labelled with the numeral (11) and (11a) in Fig. 4; species (11) contains CO bonded in parallel to the Fe, and species (11a) contains the CO bonded ‘end-on’ to the iron-CO. The results indicate that at high temperatures, CO would be formed by loss of CO from species (11). The results also indicate that formation of iron-carbonyl compounds is energetically favoured from species (11); an overall endothermic result of about +50 kcal/mol was observed for the concerted formation of H2 and CO. DFT calculations for H2 and CO formation during coal pyrolysis were consistent with these reaction routes; energy changes were obtained for: (i) the formation of H2 by H abstraction from [O–H] and [C–H] groups, to form char[(C–O)–Fe3(–C)] species, (ii) cleavage of two [OH] groups, to form char[–O–C)2(Fe3)], and (iii) elimination of CO from an [Fe–O–C–C] group to form the [Fe–C] reaction centre. DFT geometry optimisation of the various structures {Char(Fe3O)} with H abstraction and loss of H2 and CO were also similar to the SE modelling. 4.3. High-level molecular modelling – catalytic steam gasification SE and DFT molecular modelling of catalytic steam gasification for the model {char[Fe3]} may commence with chemisorption of

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Fig. 4. Pyrolysis reaction route to model concerted chemistry for the yield of H2 and CO.

Fig. 5. (a) Iron (hydride) intermediate, and (b) iron (carbonyl) intermediate prior to loss of H2 (white – hydrogen; red – oxygen; grey – carbon; purple – iron). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

H2O on either the [Fe–O] or the [Fe–C] site by forming a coordination bond between oxygen and the Fe centre [H2O ? Fe]. The reaction routes included H abstraction from H2O to form the iron hydride, and ultimately yielding H2. The reaction routes leading to CO formation included insertion of oxygen from H2O into the [Fe–C] group to form [Fe–O–C–C]; CO is eliminated and another [Fe–C] is formed. Additional reaction routes may be contemplated in char containing various iron oxide clusters, such as [Fe3O], [Fe3O2], [Fe5O2] and also iron clusters, such as [Fe3], [Fe4] and [Fe5]. The final steps in these reaction routes are the elimination of H2 and CO as gases; these may be modelled to occur separately, or concurrently. It is impractical to examine all possible reactions routes, and we have focussed mostly on reactions that yield the major products H2 and CO, using the char model containing the iron cluster [Fe3]. The experimental data show catalytic activity is observed when brown coal contains a substantial amount of iron species and negligible (or no) catalytic activity for coal containing small amounts of iron species; this indicates catalytic activity is related to the number of active sites that may form within the char. An estimate of the number of [Fe–C] active sites would depend on the type of iron cluster, and also on the number of these cluster present in the char. If all of the iron were assumed to consist of only [Fe3] clusters, the char molecular model would contain an estimated two clusters per char molecule, with a total of 2–4 active iron sites. If the iron were present as larger clusters (e.g. [Fe6]) than fewer active sites would be available for H2O chemisorption. We have pre-

viously shown that iron species would be distributed as a number of poly-nuclear iron complexes, and less than 60% of the iron would be present as small iron clusters; this indicates that a substantial amount of iron species would need to be added to the coal molecule to provide a significant number of active sites for catalytic steam gasification. SE and DFT results show chemisorption of H2O onto the reactive site of the iron centre in {char[Fe3]} was energetically favoured by between 12 kcal/mol and 30 kcal/mol per H2O molecule. Molecular modelling of chemisorption on sterically hindered iron sites however, shows these sites are not energetically favoured. The chemisorbed water molecule would form a coordination bond to a Fe centre from Fe3 clusters containing either [Fe–O] or [Fe–C], as shown in Fig. 6a and b. The ionic attraction between the [Fed +(–C)] group and [Od(–H2)] would lead to attraction of the H2O molecule while QM modelling shows the formation of the [(Fe2)(C–Fe OH2)] coordination bond is energetically favoured. A SE optimised {char[Fe2(Fe OH2)]} molecular structure, contained a distorted octahedral Fe centre (with a coordinated H2O ligand), with one Fe  Fe distance in the cluster of 0.31 nm, and the other two Fe centres forming distorted tetrahedral structures. From this, reaction routes that would yield H2 include H abstraction from the coordinated H2O, leading to the intermediate structure containing an H atom situated between the two Fe centres; the Fe–H bond lengths in this structure were 0.18 nm and 0.16 nm. The reaction sequence would be insertion of O (from H2O) into the [Fe–C]

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Fig. 6. (a) Coordination of H2O to [Fe–O], (b) coordination of H2O to [Fe–C] (white – hydrogen; red – oxygen; grey – carbon; purple – iron). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

group to form [Fe–O–C–], followed by elimination of CO and formation of another [Fe–C] group, establishing the catalytic cycle. 1scf-DFT calculated energy changes for a discrete reaction scheme, with H2O forming a coordination bond to the Fe centre of char[Fe3], provided a maximum energy change of +92 kcal/mol (cf. 1scf-SE result of +84 kcal/mol) to yield H2, indicating a relatively large energy barrier. The subsequent discrete reaction sequence yielding CO from this molecule, gave a further overall energy change of about +130 kcal/mol. These energy changes were greater than those obtained for concerted reaction routes. It must be emphasised that rigorous molecular modelling of reaction schemes of concerted chemistry for catalytic steam gasification is extremely difficult, because the size of the char molecule makes it impossible to carry out the highest level ab initio computations. Concerted mechanisms have been discussed for numerous reactions; for example a concerted metallation–deprotonation mechanism is discussed for palladium-catalysed direct arylation for a range of aromatic substrates [42]. Various molecular-level aspects of classical supported metal catalysts preparation and molecularlevel characterisation have been reviewed [12], and often several possible active sites may compete on the surface of the particular solid. The present studies have examined reaction routes that include the major features of concerted chemistry. Fig. 7 provides the SE data obtained for a scheme that mimics concerted chemistry with simultaneous loss of H2 and CO (each number in the data labels in the Figure designates a step in the reaction sequence).

The largest energy change of +110 kcal/mol was for the simultaneous loss of both CO and H2 from the iron cluster. The formation of the hydride [Fe  H] intermediate leading to H2 was energetically favoured, as was the insertion of O into [Fe–C] to form [Fe–O–C]; the energy change of +78 kcal/mol was due to elimination of CO from [Fe–O–C] to yield CO and a new [Fe–C] group. The simultaneous loss of both H2 and CO from an iron centre has been included in this reaction route to illustrate the impact this would have on the overall energy change. A scheme that mimics concerted chemistry in which the loss of CO occurs from an iron hydride intermediate, instead of simultaneous loss of both CO and H2, gave a more favourable energetic change than that shown in Fig. 7. The results for this scheme are shown in Fig. 8; the reaction sequence includes insertion of [O] from H2O, into an [Fe–C], to give the [Fe–O–C] group, followed by formation of the {char[Fe3]   (CO)} structure in which CO is chemisorbed onto Fe. Ultimately the loss of CO is accompanied with a new [Fe–C] group to continue the catalytic cycle. The largest energy change for this sequence is +55 kcal/mol, and the overall reaction is almost thermo-neutral, due to the favourable energy change accompanying the formation of the iron hydride intermediate {char[Fe3  H]} during the loss of CO. A number of additional reaction schemes were examined that mimicked various concerted reaction routes, including the formation of an iron-hydride by abstraction of [H] from a –CH3 group in char during steam gasification. This scheme formed H2 by abstracting [H] from char and [H] from the H2O molecule, with

Fig. 7. Reaction route to mimic concerted chemistry for simultaneous formation of CO and H2 from reaction of (char[Fe3]) and H2O.

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Fig. 8. Reaction of steam involving iron hydride in CO formation and loss (after H2 yield).

[O] insertion into the resulting [Fe–C] group to yield CO. This reaction sequence yielded H2 and CO with an overall energy change of +89 kcal/mol. We carried out additional molecular modelling to examine reaction routes for yields of CO and H2 using char molecules that contained iron clusters at different positions (and thus different configurations); while the specific data from this modelling differs for different configurations, overall the results all indicate that reactions routes that mimic concerted chemistry, with the formation of iron hydride and carbonyl intermediates, provide pathways with lower energy changes. Generally the molecular modelling to yield H2 and CO provided overall energy changes of between +40 and +90 kcal/ mol. Chemical mechanisms have been postulated [9,10] for reactions between steam and petroleum coke (and PVC derived char) and the experimentally determined activation energy for CO formation from char containing iron has been reported at +93 kcal/ mol, which compares favourably with the results that mimicked concerted chemistry yielding CO. A simplified modelling scheme is outlined below for H2 and CO formation and the cyclic nature of the catalysis chemistry:

intermediates formed at the second [Fe3] cluster undergoing pyrolysis reactions. The largest energy change for H2 formation in the sequence was +13 kcal/mol and the largest energy change for CO formation was +46 kcal/mol; these results indicate relatively lower energy barriers for these complicated reaction schemes. A large number of inorganic compounds have been tested as catalysts for coal gasification, including, Fe, Ni, K, Ca, and mixtures of main group and transition metal species, such as Fe/Ca and Fe/K. Inorganics and minerals in lignite appear to increase the reactivity to steam, but at elevated temperatures, reactions involving silicates with alkalis, such as Ca and K, and reactions involving sulphur and inorganics, would increase the complexity of metal mediated pyrolysis, and often may inhibit catalytic activity [4,43–45,17,46,47]. The greater yield of H2 observed experimentally, with the lower proportion of CO, and higher amount of CO2, indicates additional reactions have occurred. In practical systems, all events that occur in a gasifier are generally considered coal gasification. We have focussed on the molecular chemistry that deals with catalytic reactions involving char and steam, and to distinguish this molecular chemistry from the numerous events in a gasifier, we have used the term post-gasification chemistry when discussing the higher yield of H2 and CO2. As char is consumed by reacting with steam, the relative proportion of iron oxide in the remaining char would increase. Those particles containing larger amounts of iron oxide species, and larger iron oxide particles that from any precipitated iron hydroxide, may be involved in this post-gasification chemistry with the synthesis gas. The post-gasification chemistry may be similar to the water–gas-shift reaction, or may be due to additional reactions involving iron oxides e.g.: 2FeO + H2 + CO ? 2Fe + CO2 + H2O ? Fe2O + H2 + CO2. In practice, all of these events would constitute the overall gasification of the coal to determine the composition of the synthesis gas. 4.4. Major aspects of the mechanism of catalytic steam gasification of brown coal We have identified the major aspect at the molecular level for iron catalysed steam gasification of brown coal, namely the formation of active iron sites [Fe–C] and the reactions routes with H2O. The number of active sites would be related to the number of

Char[Fe3( OH)2] ? Char[Fe(Fe–O)2] + H2 Char[Fe(Fe–O–C)2] ? Char[Fe2(Fe–O–C)] + CO Char[Fe2(Fe–O–C)] ? Char[(Fe–C)Fe2] + CO Char[Fe3] + H2O ? Char[Fe2(Fe OH2)] Char[Fe3( OH2)] ? Char[Fe2(Fe–O–C)] + H2 Char[Fe2(Fe–O–C)] ? Char[Fe3] + CO Char[Fe3O] + H2O ? Char[Fe3O( H2O)] Char[Fe3O( H2O)] ? Char[Fe3O(–O–C)] + H2 Char[Fe3O(–O–C)] ? Char[Fe3O] + CO Char[Fe3O] ? Char[Fe2(Fe–O–C)] Char[Fe2(Fe–O–C)] ? Char[Fe3] + CO Char[Fe3] + H2O ? Char[Fe2 (Fe OH2)] . . . chemisorption of H2O to continue the cycle

Molecular modelling was also carried out for char molecular models containing a larger amount of iron; for these the char molecular model contained two [Fe3] clusters at different positions in the molecule. This reaction route also yielded H2 and CO from the reaction of one molecule of H2O with char. Reaction schemes were also examined in which the yield of H2 was from pyrolysis chemistry, to illustrate the possible overlap between pyrolysis and steam gasification. The entire reaction sequence consisted of 23 steps; CO formation was also accompanied with iron hydride

(pyrolysis chemistry) (pyrolysis chemistry) (active site) (chemisorption of H2O) (steam gasification, O insertion) (steam gasification and new active site) (chemisorption of H2O) (steam gasification) (steam gasification and new active site) (O insertion) (CO, steam gasification, and new Fe site)

poly-nuclear iron hydroxyl species added to the brown coal molecular matrix. Overall the catalytic reaction mechanism may be viewed as hybrid organometallic chemistry and heterogeneous catalysis; the organometallic aspect stems from the active site consisting of an iron-carbon bond formed in situ within the reacting substrate, while the heterogeneous catalysis is indicated by the dynamics of steam molecules chemisorbed on the active site of the solid phase yielding the gaseous products H2 and CO. The aqua-polynuclear iron hydroxyl species added to the brown coal

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intermediates are energetically favoured over discrete reaction routes. The major routes have been examined using molecular models of char containing the [Fe3] and the [Fe3O] clusters; these commenced with chemisorption of H2O onto the iron centres of [Fe–O] and [Fe–C] sites to form the [Fe OH2] coordination bond, followed by H abstraction from H2O via iron hydride intermediates, and O insertion into the [Fe–C] bond to form [Fe–O–C–], with CO elimination from the [Fe–O–C–] group to form another [Fe–C] sites, and continue the catalytic cycle. MD modelling shows H2O molecules interact with the [Fe–C] site; further modelling is required to provide additional insights into this novel chemistry. Acknowledgements

Fig. 9. Snapshot showing steam molecules about the active site (green = Fe, red = oxygen, white = hydrogen, light blue = carbon). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

molecular matrix have been transformed to a large extent into a range of various sizes of iron clusters. The smaller iron clusters are present within the char molecular matrix where they form active sites for catalysis. The very large iron clusters, and any iron hydroxide that may precipitate within the coal pores, would not be expected to form a significant number of [Fe–C] active sites. Modelling of relatively large iron species shows that these would cause larger pores to form within the char molecular model. These larger iron species are more likely to participate in gas phase reactions discussed as post-gasification chemistry above. The increase in char porosity resulting from these larger iron species would improve the accessibility of steam molecules to active sites [11,26,27]. While SE molecular modelling of these larger molecules at a high level requires an excessive amount of computer time, a small number of computations were carried out at a lower accuracy to get an indication on the configurations when two and three active sites were present in the char molecule. The results indicate that the accessibility of some active sites to H2O molecules may vary throughout the char substrate. Further insights into this complicated chemistry are obtained from MD modelling; the present results show that a greater number of steam molecules impact at the catalytic active site, as illustrated by a snapshot of the system in Fig. 9 (steam molecules not interacting with the iron centres surround the char but are at larger distances from the char in this snapshot). The present results also show that the [Fe–C] site would incur a larger number of impacts. The involvement of higher energy states for the substrate will also need to be examined because these reactions occur at elevated temperatures. These relatively novel mechanistic aspects for this system are due mainly due to the active sites created in situ and part of the solid char substrate. This aspect provides a considerable challenge to MD modelling. 5. Conclusions The pyrolysis of brown coal containing polynuclear iron hydroxyl species at high temperatures has been shown to form the active [Fe–C–] sites for reactions with steam molecules. Experimental data has shown that a relatively large amount of iron must be added to the brown coal to achieve significant reaction with steam, indicating a greater number of active sites would form in coal samples containing larger amounts of iron species. Computer molecular modelling has shown that reaction routes that mimic concerted chemistry involving iron hydride/carbonyl

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