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Emily C. Hayward, Glen J. Smales, Brian R. Pauw, [Masaki Takeguchi](https://orcid.org/0000-0002-0282-6020), Alexander Kulak, Robert D. Hunter, Zoe Schnepp

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[The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of cellulose](https://mdr.nims.go.jp/datasets/935b2cf9-788e-4f64-b056-a8117fc866f2)

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The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of celluloseRSCSustainabilityPAPEROpen Access Article. Published on 14 October 2024. Downloaded on 12/9/2024 12:12:41 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineView Journal  | View IssueThe effect of cataaSchool of Chemistry, University of Birminghac.ukbBundesanstalt für Materialforschung und -p12205, GermanycCenter for Basic Research on Materials, NatSengen, Tsukuba, Ibaraki, 305-0047, JapandSchool of Chemistry, University of Leeds, LeDepartment of Chemical Engineering, Imper† Electronic supplementary informahttps://doi.org/10.1039/d4su00365aCite this: RSC Sustainability, 2024, 2,3490Received 8th July 2024Accepted 30th September 2024DOI: 10.1039/d4su00365arsc.li/rscsus3490 | RSC Sustainability, 2024, 2, 3lyst precursors on the mechanismof iron-catalysed graphitization of cellulose†Emily C. Hayward, *a Glen J. Smales,b Brian R. Pauw,b Masaki Takeguchi, cAlexander Kulak,d Robert D. Huntere and Zoe Schnepp aIron-catalysed graphitization of biomass is a simple and sustainable route to carbons with high graphiticcontent. It uses abundant precursors and moderate processing temperatures and generates carbonswith high porosity. Recently, it has been demonstrated that the choice of biomass precursor can havea significant impact on the textural and compositional properties of the resulting carbon. In this paper,we demonstrate that the choice of catalyst is also critical to the carbon structure. Aqueous iron(III) nitrateand iron(III) chloride convert cellulose to carbons with very different textural properties. This is due to thechoice of iron catalyst changing the mechanism of cellulose decomposition and also the nature of theactive graphitization catalyst.Sustainability spotlightClean energy depends on many technologies that use carbons. This includes electrode materials for lithium and sodium batteries and electrocatalyst supports.An exciting route to carbons with high graphitic content is iron-catalysed graphitization, which uses simple iron salts to convert biomass to carbon at moderatetemperatures. Numerous authors have employed iron and biomass to generate carbons, but the wide range of precursors and conditions hinders understandingof how to tune the properties of the carbons. This paper presents a comparative study of how different iron salts can have a dramatic effect on the texturalproperties of carbonized cellulose. Such understanding is essential for investigation and scale-up of iron-catalysed graphitization as a technology.IntroductionCarbon materials have a broad range of applications includinglithium and sodium ion batteries,1,2 adsorbents for waterremediation3 and supercapacitors.4 Many of these applicationsrequire a balance of properties including porosity, surface area,conductivity and the presence of graphitic or graphene-likefeatures. Biomass is a particularly attractive precursor tocarbon materials, due to the diversity of sources and potentialvalorisation of agricultural and forestry waste streams.5 Manydifferent types of biomass have been investigated for carbonproduction, including raw lignocellulosic biomass6,7 andbiomass extracts such as cellulose8 or glucose.9 These can beconverted to carbons via pyrolysis,10 catalytic graphitization11 orhydrothermal carbonization.12 One of the most importantam, B15 2TT, UK. E-mail: ech620@bham.rüfung (BAM), Unter den Eichen 87, Berlinional Institute for Materials Science, 1-2-1eeds, LS2 9JT, UKial College London, London SW7 2AZ, UKtion (ESI) available. See DOI:490–3499factors for widespread application of carbons from biomass isthe ability to reliably tune the structure and properties. Whilethe effects of processing parameters on pyrolysis13,14 andhydrothermal carbonization15 have been studied in detail,catalytic graphitization has received a lot less attention.Catalytic graphitization is the use of metal catalysts toproduce graphitic carbons at moderate temperatures. Biomassis combined with a metal-containing compound and heated inan inert atmosphere to temperatures above 700 °C. Metalnanoparticles are generated in situ and catalyse the conversionof biomass-derived amorphous carbon to graphitic nano-structures such as nanotubes.7 The impact of biomass type onthe structure and properties of carbons produced by catalyticgraphitization is now quite well understood.3,16,17 However, theinuence of different graphitization catalysts has received a lotless attention. Across the literature, the numerous examples ofcatalytic graphitization use a wide range of catalyst precursors,including chloride, nitrate, citrate and acetate salts of iron,cobalt, and nickel, or metal/metal oxide nanoparticles. Thismakes it very difficult to make general conclusions about howdifferent precursors impact the properties of the resultingcarbon.In this paper, we investigate of the effect of different iron saltcatalysts on the mechanism of graphitization of cellulose.Cellulose is one of the most promising biomass precursors forgraphitization, as it is the most abundant biopolymer on the© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://crossmark.crossref.org/dialog/?doi=10.1039/d4su00365a&domain=pdf&date_stamp=2024-10-30http://orcid.org/0009-0004-9211-954Xhttp://orcid.org/0000-0002-0282-6020http://orcid.org/0000-0003-2171-067Xhttps://doi.org/10.1039/d4su00365ahttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4su00365ahttps://pubs.rsc.org/en/journals/journal/SUhttps://pubs.rsc.org/en/journals/journal/SU?issueid=SU002011Paper RSC SustainabilityOpen Access Article. Published on 14 October 2024. Downloaded on 12/9/2024 12:12:41 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlineplanet, with 1.5 × 1012 tons produced in nature each year.18 Itcan be extracted readily and on a very large scale from wood andagricultural waste, making it ideal as a sustainable precursor formaterials and as a subject for this study. Previous reports ongraphitization of cellulose have used nickel, cobalt and ironsalts and shown that it is possible to generate carbons withgraphitic nanostructures such as hollow graphitic shells,19ribbon-like morphologies20,21 or multi-walled graphitic carbonnanotubes.19 It is also apparent that different iron salts canimpact carbon structure. For example, cellulose spheres treatedwith iron nitrate produced carbons with amixture of micro- andmesoporosity, whereas iron chloride produced highly micro-porous carbons.20 The authors suggested that the higher Lewisacidity of iron chloride was responsible for the different struc-tures but the mechanism of cellulose decomposition withdifferent iron salts was not studied. In this paper, we demon-strate that different iron salts change the structure and prop-erties of cellulose-derived carbons by changing the cellulosedecomposition mechanism and the size distribution of thegraphitization catalyst. This detailed mechanistic and process-ing insight is critical if cellulose graphitization is to providea real option for sustainable carbons.Results and discussionCharacteristics of graphitic carbonCarbons were produced by combining microcrystalline cellu-lose (MCC) with aqueous Fe(NO3)3 or FeCl3 and heating to 800 °C under nitrogen for 1 h. Powder X-ray diffraction (p-XRD)patterns show a sharp peak at 26.1° for both samples, charac-teristic of the interplanar spacing of graphitic carbon (Fig. 1a).This synthesis is highly reproducible, with separate samplesproducing very similar p-XRD patterns when heated under thesame conditions (Fig. S1†).16,17 In contrast, a control sample ofcellulose with no catalyst shows only two very broad peaks at23.5° and 43.6°, indicative of amorphous carbon with somelocal stacking but no long-range order. Nitrogen porosimetrydata of the two iron-catalysed cellulose samples show type IVisotherms indicating a porous structure consisting of bothmesopores and micropores (Fig. 1b). In contrast, the controlFig. 1 (a) p-XRD pattern (CuKa source) and (b) vertically offset N2 adsorppre-treatment, or treatment with aqueous Fe(NO3)3 or FeCl3.© 2024 The Author(s). Published by the Royal Society of Chemistrysample shows a type I isotherm, indicating a microporousstructure. More detailed information on the surface area andpore volume is provided in Table 1.22 The presence of meso-pores is consistent with the formation of graphitic structuressuch as shells23 and nanotubes,7 which have been observed inprevious reports of iron-catalysed graphitization. These shellsand nanotubes are formed by catalyst nanoparticles dissolvingamorphous carbon and reprecipitating hollow graphitic nano-structures. The nanotube diameter is dependent on the size ofthe nanoparticle catalyst and nanoparticles within the 10–50 nm size range thus produce mesoporous carbons. Interest-ingly, the carbon produced from cellulose and FeCl3 containsmacropores, as indicated by the continuing adsorption at p/p0=1 in the isotherm. This suggests that the catalyst particles arelarger in the sample synthesized with FeCl3 than the onesynthesized with Fe(NO3)3. The BET surface area is lower forboth graphitized samples, presumably as the microporousamorphous carbon is converted to mesoporous graphiticnanostructures.Raman microscopy was used to further investigate thenature of the graphitic carbon in each sample. Fig. 2 shows twoprominent peaks at 1325 cm−1 and 1600 cm−1, correspondingto the D and G peaks respectively. In perfect graphite, only the Gpeak is allowed (at ∼1581 cm−1) and the D peak is forbidden.Therefore, the presence of the D peak and the deviation of the Gpeak from the position of perfect graphite indicates that allthree cellulose-derived carbons exhibit disorder. To extract peakposition and full width half maximum (FWHM) values, a 4 peakVoigt function was used, where peaks are attributed to the G,D1, D3 and D4 bands. The information acquired from thismethod is detailed in Table 2. The position of the G peak isconrmed to be shied to higher wavenumber for all threesamples, which is consistent with nanocrystalline domains. Theratio of the intensity of the D peak to the G peak (ID/IG) iscommonly used to provide information on the level of graphi-tization. However, in this case, there is little difference betweenvalues of any of the carbon structures, likely owing to itssensitivity to other factors, such as surface defects. However, theratio of intensity for the D3 peak compared to that of the G peak(ID3/IG) has been shown as a good indicator for the ratio oftion isotherms of microcrystalline cellulose heated to 800 °C withoutRSC Sustainability, 2024, 2, 3490–3499 | 3491http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4su00365aTable 1 Adsorptive properties of carbons produced frommicrocrystalline cellulose using no additive, Fe(NO3)3 or FeCl3 and pyrolyzed at 800 °CIron salt Specic surface area (m2 g−1) Total pore volume (cm3 g−1) Micropore volume (cm2 g−1) % MicroporesNone 470 0.19 0.16 86Fe(NO3)3 360 0.24 0.077 32FeCl3 370 0.30 0.086 34Fig. 2 Raman spectra for microcrystalline cellulose treated with (a) no iron salt, (b) Fe(NO3)3 and (c) FeCl3 and held at 800 °C for 1 h – includingthe deconvoluted Raman spectra, fitted using a 4-point Voigt function.Table 2 Information derived from Raman microscopy after fitting,including G peak position, full width half maximum (FWHM) of G peak,and the intensity ratios of the D and G (ID/IG) peaks and the D3 and Gpeaks (ID3/IG)Iron saltG peak position(cm−1)G peak FWHM(cm−1) ID/IG ID3/IGNone 1599 � 1 66.6 � 3 2.55 1.03Fe(NO3)3 1595 � 1 72.2 � 2 2.50 0.43FeCl3 1595 � 1 72.6 � 1 2.60 0.40RSC Sustainability PaperOpen Access Article. Published on 14 October 2024. Downloaded on 12/9/2024 12:12:41 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlineamorphous to graphitic carbon.24 Considering these ratios, it isclear that iron catalysts signicantly increase graphitic content,but there is little difference between the two iron salts. Giventhe small differences between the Raman data for the two iron-containing samples, we elected not to analyse the different peakratios any further.To investigate the reason for the different textural propertiesof carbons produced from different iron salts, we used ther-mogravimetric analysis coupled with mass spectrometry (TGA-MS) to probe the reaction mechanism. For all samples, theTGA data (Fig. 3a) show a single large mass loss between 300 °Cand 400 °C, corresponding to thermal decomposition of thecellulose polymer. The onset of mass loss and peak mass loss isat a lower temperature for both the iron-containing samples.The differential thermogravimetric (DTG) data for the FeCl3-containing sample (Fig. 3b) also shows a shoulder at 230–280 °C. This correlates to a peak form/z = 18 in the MS data (Fig. 3c),3492 | RSC Sustainability, 2024, 2, 3490–3499indicating loss of water. Cellulose pyrolysis involves multiplechemical reactions, including dehydration, depolymerizationand ring opening.25 Char formation can occur via directcarbonization of the cellulose or by secondary polymerization ofvolatile decomposition products.26 Catalysts such as metal saltsare known to inuence cellulose decomposition and the DTGdata suggest that FeCl3 promotes dehydration reactions ata lower temperature than Fe(NO3)3.27 This is consistent with theability of FeCl3 to act as a Lewis acid and suppress the formationof levoglucosan (Fig. 3f) by promoting dehydration in cellulose,in addition to depolymerisation of the biopolymer.28–31 Furtherevidence for this comes from MS data for m/z = 60, the mostprominent fragment for levoglucosan (Fig. 3d). Both iron saltsdrive formation of levoglucosan at lower temperatures thanpure cellulose and in smaller amounts, but the peak for thecellulose–FeCl3 system is particularly small. The release of COand CO2 are more challenging to track as CO is masked by theN2 atmosphere of the experiment (m/z = 28) and CO2 is overlaidby N2O (m/z= 44). The sharp peak form/z= 44 for the cellulose–Fe(NO3)3 sample (Fig. 3e) is likely to be a mixture of N2O andCO2 as the highly oxidizing nitrate reacts with the cellulose. CO2release from the FeCl3–cellulose system starts earlier than forthe cellulose control and the peak is smaller. This providesfurther evidence that the FeCl3 is changing the decompositionreactions signicantly. Regardless of the gases released, theyield of the carbon produced using FeCl3 (16%) is higher thanthat produced using Fe(NO3)3 (11%). Identical Fe:cellulosemolar ratios were used so this indicates that the FeCl3 ispromoting cellulose decomposition pathways that lower the© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4su00365aFig. 3 (a) TGA and (b) DTG data for cellulose, Fe(NO3)3–cellulose and FeCl3–cellulose pyrolysis in N2, mass spectrometry data showingtemperature-dependent release of ion fragments of m/z = (c) 18, (d) 60 and (e) 44 and (f) structure of levoglucosan.Paper RSC SustainabilityOpen Access Article. Published on 14 October 2024. Downloaded on 12/9/2024 12:12:41 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlineamount of carbon lost as CO/CO2 and other volatiles like levo-glucosan. This in turn will affect the evolution of pores in thecarbon.Further evidence for the early dehydration of cellulose in thepresence of FeCl3 comes from p-XRD data. For raw cellulose andcellulose with Fe(NO3)3, the characteristic peaks for the cellu-lose structure (Fig. S2a†) can be seen clearly even aer heatingto 300 °C (Fig. 4a and b), whereas the peaks disappear between250 °C and 300 °C for cellulose with FeCl3 (Fig. 4c). The crystalstructure of cellulose involves a lot of intra and intermolecularhydrogen bonds, which are broken during dehydration.32Therefore the loss of peaks below 300 °C for the cellulose–FeCl3system is consistent with early onset of dehydration catalysed bythe FeCl3. It should be noted that no crystalline iron phases areobservable in the p-XRD data up to 400 °C. Investigations ofrelated systems have shown that very small amorphous iron–oxygen clusters or iron oxide nanoparticles evolve during heat-ing of biomass with iron salts so it is possible that similarstructures exist alongside the decomposing cellulose but can'tbe detected by p-XRD.33Fourier transform infrared spectroscopy (FTIR) offers moreinsight into the molecular transformations involved in thedecomposition of cellulose in the three systems. The FTIRspectrum for the control (no iron) sample up to 300 °C (Fig. 4d)shows peaks corresponding to –OH, C–O (ether) and C–O(pyranose), which are all present in the cellulose structure(Fig. S2b†). Most of the peaks disappear by 400 °C, in line withthe major mass loss observed at 300–400 °C in the TGA and theloss of peaks in the p-XRD patterns. In their place, small peaks© 2024 The Author(s). Published by the Royal Society of Chemistryfor C]C and C]O emerge, consistent with the formation ofdecomposition products like levoglucosenone. For both theiron-containing samples (Fig. 4e and f), there is a signicantloss of functionality below 300 °C, as indicated by a decrease inFTIR peak intensity between 250 °C and 300 °C. The OH peak isnot present at 300 °C, consistent with dehydration processes.The C–O ether peak also disappears by 300 °C, which suggestsa signicant amount of depolymerization in both iron-containing systems. Interestingly, for the cellulose–Fe(NO3)3system, the C–O pyranose peak disappears completely by 300 °C, suggesting ring opening is one of the decomposition path-ways. Iron oxides have been shown to catalyse ring-openingduring cellulose decomposition as well as participate in depo-lymerization and dehydration.34,35 This could indicate thatsmall iron oxide clusters are formed during pyrolysis of cellu-lose–Fe(NO3)3, as has been observed in other biomass–Fe(NO3)3systems.33 For both iron-containing systems, C]C and C]Opeaks are observed from 300 °C, again showing how the ironpromotes carbonization in these systems.The later stages of cellulose carbonization were investigatedusing ex situ p-XRD. Fig. 5a shows that carbonization of purecellulose does not produce any crystalline graphitic carbon, asexpected. In contrast, the carbonization of cellulose–Fe(NO3)3produces a characteristic peak for graphitic carbon at 26.1°(Fig. 5b). This occurs alongside peaks for Fe3C and a-Fe, whichis consistent with the formation of Fe/Fe3C catalyst nano-particles. At 600 °C and 500 °C, there are no peaks in the p-XRDpatterns for the cellulose–Fe(NO3)3 system, suggesting that ironis present as very small nanoparticles or only as amorphousRSC Sustainability, 2024, 2, 3490–3499 | 3493http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4su00365aFig. 4 p-XRD patterns for microcrystalline cellulose treated with (a) no iron salt, (b) Fe(NO3)3 and (c) FeCl3 and heated to various temperatures.FTIR spectra for microcrystalline cellulose treated with (d) no iron salt, (e) Fe(NO3)3 and (f) FeCl3, and heated to various temperatures.RSC Sustainability PaperOpen Access Article. Published on 14 October 2024. Downloaded on 12/9/2024 12:12:41 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlineclusters. In contrast, the pyrolysis of cellulose with FeCl3produces sharp peaks for iron oxide (primarily Fe3O4) from500 °C (Fig. 5c). The lack of signicant peak broadening indi-cates the iron oxide particles are relatively large. From 700 °C,the iron oxide peaks have been replaced by sharp peaks for a-Fewith a very small presence of graphitic carbon and by 800 °C thegraphitic carbon peak is fully developed with some a-FeFig. 5 Ex situ p-XRD patterns of microcrystalline cellulose treated with (a700 °C and 800 °C for 1 h. Samples are cooled prior to analysis, thereforetemperature are not seen in the ex situ p-XRD patterns. Peaks marked *correspond to Fe0.911O.3494 | RSC Sustainability, 2024, 2, 3490–3499replaced by Fe3C. The ex situ nature of the experiments meansthat we cannot make conclusions as to the identity of thecatalyst in the two systems. However, the data strongly suggeststwo different routes to reaching the active graphitization cata-lysts with much larger crystalline species formed in the FeCl3system. This would be consistent with the observation of mac-ropores in the cellulose–FeCl3-derived carbon.) no catalyst, (b) Fe(NO3)3 and (c) FeCl3 and heated to 500 °C, 600 °C,, any high temperature phases (e.g. g-Fe) which may be present at highcorrespond to a reference pattern for Fe (ferrite) and peaks marked ^© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4su00365aFig. 6 TEM images of cellulose treated with Fe(NO3)3 and heated to (a) 600 °C and (b) 700 °C and (c) SEM image of cellulose–Fe(NO3)3 afterheating at 800 °C. TEM images of cellulose treated with FeCl3 and heated to (d) 600 °C and (e) 700 °C and (f) SEM image of cellulose–FeCl3 afterheating at 800 °C.Fig. 7 (a) Fitted SAXS data for microcrystalline cellulose treated with Fe(NO3)3 and held at 600 °C, 700 °C and 800 °C for 1 h. Particle sizehistograms coupled with visibility limits (black dots, left y-axis) and cumulative distribution functions (right y-axis) for microcrystalline cellulosetreated with Fe(NO3)3 and held at (b) 600 °C, (c) 700 °C and (d) 800 °C for 1 h.© 2024 The Author(s). Published by the Royal Society of Chemistry RSC Sustainability, 2024, 2, 3490–3499 | 3495Paper RSC SustainabilityOpen Access Article. Published on 14 October 2024. Downloaded on 12/9/2024 12:12:41 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4su00365aRSC Sustainability PaperOpen Access Article. Published on 14 October 2024. Downloaded on 12/9/2024 12:12:41 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineTransmission electron microscopy (TEM) was used to furtherprobe the evolution of catalyst particles in the two cellulose–Fesystems. Aer heating to 600 °C (Fig. 6a), the cellulose–Fe(NO3)3sample shows very small (<5 nm in diameter) dark spots of moreelectron-dense material, indicating iron-rich nanoparticles. Noevidence of lattice fringes could be seen, which could indicate thatthe nanoparticles are amorphous. This is consistent with the lackof peaks in the p-XRD data. The nanoparticles are larger andmoreclearly dened with increasing temperature (Fig. 6b), consistentwith the appearance of Fe3C/Fe peaks in the p-XRD data. From700 °C, stacked layers of graphitic carbon can be seen, charac-teristic of hollow shells and nanotubes formed by catalyticgraphitization. Scanning electron microscopy (SEM) with back-scattered electron detector shows the Fe3C/Fe nanoparticles aresmall and evenly distributed across the carbon (Fig. 6c, full imageincluded in Fig. S3†). In contrast, the cellulose–FeCl3 sampleshowed large (>100 nm in diameter), faceted particles aerheating to 600 °C, many of which had become detached from theunderlying carbon when dispersed on the TEM grid (Fig. 6d). Thisis consistent with the sharp peaks for Fe3O4 in the p-XRD. Wepropose that the large particles are a result of halide vapourhydrolysis, where volatile FeCl3$6H2O vaporises and reacts withwater evolved during heating. The result is the deposition of largemagnetite (Fe3O4) particles.36 In contrast, Fe(NO3)3 decomposesduring heating and so remains spread homogeneouslythroughout the sample.37 Aer heating to 700 °C (Fig. 6e), theFig. 8 (a) Fitted SAXS data for microcrystalline cellulose treated with FeClcoupled with visibility limits (black dots, left y-axis) and cumulative distribFeCl3 and held at (b) 600 °C, (c) 700 °C and (d) 800 °C for 1 h.3496 | RSC Sustainability, 2024, 2, 3490–3499FeCl3–cellulose sample appears to be similar to the Fe(NO3)3–cellulose system, with dark iron-rich particles within layers ofgraphitic carbon nanostructures. However, the SEM shows thatthe particles are much more variable in size, with some particleshundreds of nm in diameter (Fig. 6f, full image included inFig. S4†). This is consistent with porosimetry data, which indi-cated a mixture of meso- and macroporosity in the carbonproduced using FeCl3.Given that SEM and TEM are microscopic techniques, smallangle X-ray scattering (SAXS) was performed on bulk amounts ofsample to provide averaged structural information. For carbonsprepared from cellulose and Fe(NO3)3 there is a clear increase inscattering between 600 °C and 700 °C (Fig. 7a). At 600 °C, thebroad peak in the scattering data around q = 1 nm−1 corre-sponds to scattering features around 5 nm in diameter,consistent with the small particles/clusters observed in TEMimages. The data in q range 0.027# q (nm−1)$ 9.87 were ttedand analysed using McSAS, a Monte Carlo method to extractform-free size distributions (full details in ESI,† including fullrange of collected in Fig. S5† and data with t lines inFig. S6†).38 The size histogram for the scattering structurespresent in the Fe(NO3)3–cellulose system at 600 °C (Fig. 7b)shows three main features. The peak at very low radius (<1 nm)can be assigned to micropores and surface roughness (causedby scattering from the carbon–air interface). The central peak(between 2–3 nm) is ascribed to the developing iron particles3 and held at 600 °C, 700 °C and 800 °C for 1 h. Particle size histogramsution functions (right y-axis) for microcrystalline cellulose treated with© 2024 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4su00365aPaper RSC SustainabilityOpen Access Article. Published on 14 October 2024. Downloaded on 12/9/2024 12:12:41 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlineand the features at large radius (>20 nm) are likely caused bylarger pores from the original cellulose structure. At 700 °C and800 °C (Fig. 7c and d), the main contribution to the scatteringcomes from features in the 3–40 nm size range, consistent withboth the catalyst nanoparticles and the mesopores in thegraphitic nanostructures. The FeCl3–cellulose system showsa similar increase in scattering from 600 °C to 800 °C (Fig. 8a).However, the broad peak centred around q = 1 nm−1 is notpresent at 600 °C and there is a corresponding lack of featuresin the 3–40 nm size range in the histogram (Fig. 8b). This isconsistent with TEM, which only showed large, faceted crystals,unlike the small nanoparticles/clusters seen in the Fe(NO3)3–cellulose system. The large crystals would be well outside of thesize range probed by SAXS. At 700 °C, the histogram (Fig. 8c)indicates the emergence of some scattering features around10 nm in radius, consistent with the observation of nano-particles in the TEM. At 800 °C, the scattering prole andhistogram (Fig. 8d) are both similar to those of the Fe(NO3)3–cellulose system, again consistent with the observation ofnanoparticle catalysts and graphitic nanostructures withmesopores.ConclusionsThis study has comprehensively examined the impact of usingFe(NO3)3 and FeCl3 as catalysts for the iron-catalysed graphiti-zation of cellulose. The inuence of the counterion (NO3− orCl−) on yield and textural properties is considerable, with FeCl3promoting a higher yield of carbon with nitrogen porosimetrydata providing evidence that porosity extends from micro tomacropores. This is in comparison to a primarily mesoporouscarbon produced from cellulose with Fe(NO3)3. The reason forthe difference in material properties is that the iron saltspromote different decomposition pathways, presumably due tothe different Lewis acidity of the salts. Iron chloride promotesdehydration of cellulose and strongly suppresses the formationof levoglucosan, a volatile decomposition product. This in turnmeans that the iron chloride generates a higher carbon yieldthan iron nitrate. Another major difference between the twosystems is the volatility of FeCl3. Vaporization of the chlorideand subsequent hydrolysis results in the deposition of largeiron oxide crystallites. These in turn produce large Fe/Fe3Ccatalyst particles which generate graphitic macropores. This isin contrast to the Fe(NO3)3–cellulose system, which remainsamorphous until >600 °C, with the iron highly dispersed. Theresulting catalyst particles are much smaller, resulting ina mesoporous graphitic structure. Thus, a simple change inmetal salt can have a signicant impact on material propertiesin pyrolysis. This is a powerful tool for tuning carbon porosityand structure.ExperimentalMaterialsMicrocrystalline cellulose (<20 mm), iron nitrate nonahydrateand iron(III) chloride hexahydrate were purchased from Sigma-Aldrich and used without any further modication.© 2024 The Author(s). Published by the Royal Society of ChemistryPreparation of cellulose samples with iron saltsIn all cases, 0.68 mmol iron salt was dissolved in 40 mL of waterat room temperature and added to 20 g of microcrystallinecellulose (MCC). The mixture was stirred manually until theliquid was absorbed and dried in a 70 °C oven overnight. Forpyrolysis, 2 g of sample was placed in an alumina crucible andheated to desired temperature (250, 300, 400, 500, 600, 700 and800 °C) at a rate of 5 °C min−1 under N2 atmosphere with owrate 1 L min−1. The samples were held at this temperature for1 h before cooling to room temperature.Fourier transformed infrared spectroscopyInfrared spectroscopy was collected using Bruker Alpha FT-IRspectrometer with an ATR attachment.Thermogravimetric analysisThermogravimetric analysis was carried out using Netzsch STA449 F3 Jupiter and mass spectrometry data collected by QMS403 Aeolos Quadro. Thermograms were collected with averagesample mass between 5–10 mg and heated at 10 °C min−1between 40–800 °C under N2 atmosphere.Powder X-ray diffractionSamples were ground into a ne powder and placed on low-background silicon wafer sample holders. p-XRD experimentswere performed using a PANalytical Empyrean diffractometerwith a copper anode (wavelengths: Ka1 = 1.5406 Å, Ka2 =1.5443 Å) and a Pixel 2D detector. The diffractometer did nothave a monochromator but the Kb radiation was removed witha nickel lter.Raman spectroscopySamples were ground to a ne powder and placed on a glassslide. Raman spectroscopy was taken using Renishaw inViaRaman microscope using a red laser at 10% power with wave-length of 633 nm. Peak tting was completed using assuminga 4-peak Voigt function. The 5-peak t was not used owing tosignicant overlap between the G and D2 peaks in the spectra.Scanning electron microscopySamples were mounted on an SEM stub using an adhesivecopper tape. Samples were viewed with a FEM-SEM FEI Nova450 using a CBS detector, operating at 5 kV with decelerationmode.Small angle X-ray scatteringSamples were ground into a ne powder and distributed acrossa hole in a paper sample holder between two pieces of ScotchMagic tape. The wide-range SAXS experiments were performedusing the Multi-scale Analyser for Ultrane Structures (MAUS)at the Federal Institute for Materials Research and Testing(BAM), Berlin. Copper and molybdenum anodes (8 eV and 17 eVphotons, respectively) were used tomeasure over a wide q range.RSC Sustainability, 2024, 2, 3490–3499 | 3497http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4su00365aRSC Sustainability PaperOpen Access Article. Published on 14 October 2024. Downloaded on 12/9/2024 12:12:41 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineTransmission electron microscopyApprox. 50 mg of sample was dispersed in approx. 1 mL ethanoland sonicated for around 10 minutes. Subsequently sampleswere drop cast onto an E-chip for the Protochips FUSIONheating holder. Samples were observed using a JEOL JEM-2100Fin TEM mode at 200 kV acceleration voltage.Nitrogen sorptionNitrogen sorption measurements were carried out usinga Quantachrome Nova 1000 series volumetric gas sorptionanalyser at 77 K. 50–200 mg sample was ground to ne powderand degassed at 100 °C for 24 h under vacuum. Isotherms werecarried out with sample tubes calibrated for ller rods over thepressure range p/p0 0.015–0.095. BET surface areas were calcu-lated using the Rouquerol correction to select range p/p0 0.015–0.04 using the method recommended by the InternationalOrganization for Standardization (ISO) 9277.39 The total porevolume was obtained from the isotherm plateau and themicropore volume was obtained using the t-plot method,according to ISO 15901–2.40Data availabilityAll data associated with this paper are openly available fromhttps://doi.org/10.25500/edata.bham.00001143.Conflicts of interestThere are no conicts to declare.AcknowledgementsThe authors acknowledge the Leverhulme Trust (Grant RPG-2020-076) and the University of Birmingham (EH PhDstudentship) for funding. This work made use of Aalto Univer-sity Bioeconomy Facilities.References1 H. Zhu, F. Shen, W. Luo, S. Zhu, M. Zhao and B. Natarajan,Nano Energy, 2017, 33, 37–44.2 M. Drews, J. Büttner, M. Bauer, J. Ahmed, R. Sahu, C. Scheu,S. Vierrath, A. Fischer and D. Biro, Chemelectrochem, 2021, 8,4750–4761.3 R. D. Hunter, J. Davies, S. J. A. 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See DOI: https://doi.org/10.1039/d4su00365a The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4su00365a The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4su00365a The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4su00365a The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4su00365a The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4su00365a The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4su00365a The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4su00365a The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4su00365a The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4su00365a The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4su00365a The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4su00365a The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4su00365a The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4su00365a The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4su00365a The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4su00365a The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4su00365a The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4su00365a The effect of catalyst precursors on the mechanism of iron-catalysed graphitization of celluloseElectronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4su00365a