# Catalytic conversion of spent coffee grounds into graphitic and nanotube biocarbon

Gloria Ngene, Ange Nzihou

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Gloria Ngene, Ange Nzihou. Catalytic conversion of spent coffee grounds into graphitic and nanotube biocarbon. Materials Today Communications, 2025, 47, pp.113240. 10.1016/j.mtcomm.2025.113240. hal-05143855

HAL Id: hal-05143855

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Submitted on 4 Jul 2025

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# Catalytic conversion of spent coffee grounds into graphitic and nanotube biocarbon

Gloria I. Ngeneª, Ange Nzihouª,ᵇ,ᶜ

## ARTICLE INFO

**Keywords:**

Biomass waste

Graphitic-biocarbon

Metal catalyst

Catalytic graphitization

Biocarbon nanostructure

## ABSTRACT

Graphitic biocarbon comprising of graphene-like shells, nanotube-like structures, spherical clusters of graphitic biocarbon encapsulated Fe, and amorphous biocarbon, were simultaneously synthesized from spent coffee grounds. The study investigated the influence of two distinct types of non-toxic, easily accessible metal catalysts on graphitization, and the evolution of the catalyst-impregnated biocarbon nanostructures with temperature. The findings show that using Ca as a catalyst produces a highly porous graphitic structure with a larger crystallite size ($L_c$ = 14.38 nm) and nanotube-like structures that become denser with temperature, increasing in diameter from 58.48 nm at 1200 °C to 93.64 nm at 1800 °C. Conversely, Fe as catalyst produces non-porous graphitic structures with a smaller crystallite size ($L_c$ = 4.06 nm). Notably, unlike previous literature reports, nanotube-like structures were observed only in Ca-catalyzed biocarbon, not in the Fe-catalyzed ones.

## 1. Introduction

Following the successful extraction of planer single layer graphene from graphite by mechanical exfoliation two decades ago [35], research has intensified into the nanostructure modification, and functionalization of this remarkable carbon nanomaterial [1,26,40,7,59].

Studies and applications that were initially challenging to carry out with graphite, due to the restrictions linked to its condensed structure are now realized with graphene and graphene derivatives [36]. In 2006, barely two years after graphene synthesis via exfoliation technique, 2-dimentional multilayer graphene was synthesized by chemical vapor deposition (CVD) [48]. This opened exciting new route in graphene synthesis research, focused on producing high-quality graphene with fewer defects and improved control over the number of layers. Although the CVD has grown into a scalable production method, the graphene obtained by the exfoliation technique have been reported to possess better quality [32]. This can be explained by the complexities associated with the nucleation and crystal growth mechanisms which are likely to induce defects; missing atoms in the carbon lattice (vacancies), folds or dislocations in the structure, and misaligned regions (grain boundaries) [32,41]. So far, precursors for CVD include gases (methane, ethylene, acetylene); liquids (toluene); and solids (polystyrene, poly (methyl methacrylate), sucrose [32,48,6]. The diversity of these precursors and

catalysts available for this process contributes to its success. Another successful approach to synthesizing graphene-like carbon is by applying high temperature treatment (2000°C - 3000°C) to carbon rich fossil substrates [15,49,53,55]. Examples of this highly graphitizable carbon substrates include, coke, pitch, coal, fossil-derived plastics and carbon fibers [15,49,53]. However, in-line with the growing emphasis on eco-friendly and sustainable technologies and processes, coupled with the challenges posed by the increasing population, diminishing resources and the environmental impact of fossil fuel dependence, the synthesis of graphene-like carbon from biomass (a sustainable alternative) is gaining momentum [4,9,28,38,39,47,51,60]). The biomass route to advanced carbon synthesis especially graphitic structures offers both environmental and economic benefits. It offers a more practical and sustainable approach to the realization of catalytic graphitization for commercial production or scale-up. It is both low-cost and readily available, making it a feasible biocarbon source. In addition, converting biomass waste into graphitic biocarbon provides a sustainable approach to solid waste management, while promoting a circular economy [44].

Graphitic biocarbon is synthesized from biomass via metal catalyst assisted high-temperature treatment (pyrolysis), carried out at temperatures between 1000°C to 3000°C [12,5,9]. Similar to CVD, the biomass graphitization process is dependent on temperature and on the presence of metal catalyst particularly transition metals. These metals play a

<footnote>ª Université de Toulouse, IMT Mines Albi, RAPSODEE CNRS UMR 5302, Campus Jarlard, Albi Cedex 09 F.81013, France</footnote>

<footnote>ᵇ Andlinger Center for Energy and the Environment, Princeton University, Princeton, NJ 08544, USA</footnote>

<footnote>* Corresponding author at: Université de Toulouse, IMT Mines Albi, RAPSODEE CNRS UMR 5302, Campus Jarlard, Albi Cedex 09 F.81013, France.
E-mail addresses: ange.nzihou@mines-albi.fr, ange.nzihou@princeton.edu (A. Nzihou).</footnote>
crucial role in facilitating the graphitization process. Commonly used
metals include cobalt (Co), copper (Cu), iron (Fe), manganese (Mn),
molybdenum (Mo), and nickel (Ni) [25,38,47,45,60,62]. Currently,
graphene-like materials synthesized from biomass are of lower quality
compared to those synthesized by CVD or exfoliation techniques, and
this could be directly linked to the higher degree of defects associated
with the process [15,22]. This is because the synthesis of graphitic
biocarbon from biomass presents greater complexity and challenges in
controlling atomic and molecular reactions within the bulk biocarbon
matrix. As a result of the poor precision and control of the reactions
occurring at the molecular level, the process is characterized by uneven
nucleation and growth of the carbon crystals which give rise to defects
[14,22,5]. Furthermore, the highly heterogeneous nature of biomass
waste, especially lignocellulosic materials, contributes to this challenge.
Thus, depending on the type of biomass used, choice of metal catalyst,
and graphitization conditions (temperature, duration), the produced
graphitic material may exhibit a mixture of carbon structures like tur-
bostratic graphite, graphene nanosheets, graphene fiber, graphene
nanospheres, carbon nanotubes and amorphous carbon, resulting in a
material that can be best described as a heterogenous biocarbon mate-
rial with graphene-like properties [24,25,31,38]. Although this is often
viewed as a limitation, it is not necessarily a drawback as the peculiar
properties of these materials could be exploited in a wider range of
applications [25].

A recent report highlighted that defects within carbon materials
contribute to enhanced performance in lithium-ion batteries by
increasing lithium-ion diffusion sites within anode materials [61].
Similarly, the synthesis of nitrogen-doped carbon/ carbon nanotubes
(CNT) composite material obtained by coating (CNTs) with N-doped
carbon has been shown to improve the capacitance of CNTs by intro-
ducing active sites available for improved electron transport [42]. The
deliberate creation of defects in crystalline carbon materials like gra-
phene, graphite and carbon nanotubes to enhance reactivity and per-
formance particularly in electrocatalysis has been reported [59,62].
These defects such as vacancy or edges can be introduced through
methods like electron irradiation, plasma or chemical etching. Among
these methods, plasma and oxidative etching are preferred because they
offer better control over the defect formation process [59]. Attempts to
use single-layer amorphous carbon as a substitute for defective graphene
in electrocatalysis have not been successful due to its high defect density
and lack of long-range periodic structure [50,59]). In contrast,
graphene-like carbon nanostructures synthesized via catalytic graphiti-
zation contain fewer defects and retain areas of long-range crystalline
order [18,33], which makes it a promising candidate for applications in
catalysis especially electrocatalysis [47,58,63]). Although achieving
precise control over nanostructure development can be challenging, as
the process is relatively bulk-oriented compared to chemical vapor
deposition (CVD) [18,32]. However, studies indicate that adjusting the
temperature, catalyst type and concentration, and curing time of the
graphitization process can provide some degree of control over the de-
gree of graphitization achieved [12,13,28,46]). Furthermore, catalytic
graphitization offers a more facile and eco-friendly approach, making
scaling up for industrial applications more feasible.

Several studies have explored the synthesis of graphitic biocarbon from biomass using single and bimetallic catalysts [16,38,46,60]. Most of these studies utilized transition metals such as nickel (Ni), cobalt (Co), molybdenum (Mo), iron (Fe), Manganese (Mn), as catalyst for the process. Usually, the emphasis is on obtaining quality crystalline structures and not on the cost or toxicity of the catalyst [20,46,60]. Compared to other transition metals, iron is less toxic and low-cost, making it a more sustainable choice for biomass graphitization [12,18]. Furthermore, recent studies have shown promising results with iron (Fe) as a catalyst, achieving substantial levels of graphitization [28,33,38,46]. In the Fe-catalyzed graphitization of biomass, both the dissolution-precipitation mechanism and the carbide formation-decomposition pathway are reported to play a role in driving

the process [14,16,19].

Recently, the use of calcium as catalyst for the heterogenous
graphitization of cellulose was reported, showing notable levels of
graphitization between 1400°C and 1800°C[4]. The biocarbon nano-
structure obtained was described as “crumbled-paper-like” [5,6], which
was attributed to the vaporization of the catalyst (calcium) supporting
the graphene-like shells at 1800°C. The proposed mechanism for the
graphitization process involved the formation of the metastable CaC2
intermediate which decomposes with increasing temperature, releasing
calcium (vapor), while leaving behind dissolved carbon that precipitates
into structured biocarbon [5]. This finding was very interesting for two
main reasons, first, calcium is not a typical catalyst used for graphiti-
zation but showed promising potential. Secondly, the metal catalyst
vaporizes during the graphitization process which is of benefit in certain
applications where pristine structured biocarbon is required without the
inconvenience of the acid washing step.

Unlike previous studies carried out by our research group that uti-
lized microcrystalline cellulose as the carbon source with calcium (Ca)
as catalyst [5] or iron (Fe) as catalyst [14], the present study focuses on
raw biomass waste, specifically coffee grounds. It evaluates the catalytic
effects of calcium and iron separately on a more complex and heterog-
enous carbon source. The motivation for this study comes from the
abundance of coffee grounds, a widely produced food waste across
Europe. Compared to microcrystalline cellulose, coffee grounds is a
more practical and sustainable precursor for the catalytic graphitization
of biomass. Furthermore, to the best of our knowledge, there has been no
prior investigation in the literature exploring the performance of Ca and
Fe as catalysts in the graphitization of raw biomass, particularly coffee
grounds.

Spent coffee grounds consist of approximately 39 % hemicellulose,
12 % cellulose, 24 % lignin and about 17 % proteins [2]. Each of these
components degrades at different rates and produces distinct
by-products [10,43,65]. Consequently, the composition of the resulting
char differs significantly from that derived from a single-component
biopolymer, such as cellulose. The variations in the char composition
could have a direct impact on the efficiency and characteristics of the
catalytic graphitization process [10,34]. Additionally, the substantial
and uneven distribution of inorganics (mineral content) in biomass may
impact the graphitization process. Ultimately, the complex mix of
biopolymer, and the relatively high mineral content in raw biomass may
introduce changes in the process that may not occur during the catalytic
graphitization of a single-component biopolymer. Thus, studying cata-
lytic graphitization using raw biomass is crucial for gaining a more
comprehensive understanding of the process.

Finally, the synthesis of Ca and Fe catalyzed graphene-like biocarbon
allotropes/hybrid multifunctional materials highlighted in this study,
holds potential for applications where the complementary properties of
diverse carbon nanostructures enhance performance, as demonstrated in
batteries [42,46,61], and catalysis [40,47,59,62].

## 2. Results and discussion

### 2.1. *Thermal behavior of precursor material*

The thermal behavior of the non-impregnated and metal-
impregnated precursors were studied in the temperature range of 30°C
to 900°C (Fig. 1). This analysis is crucial as it provides valuable insights
into the thermal behavior of the precursors. Even at the early stages of
pyrolysis, it highlights the possible impact of the two distinct classes of
metal catalysts calcium, an alkaline earth metal, and iron, a transition
metal used in the graphitization process. Metal catalysts have been re-
ported to promote the depolymerization reactions of the amorphous
components, and the fragmentation and carbonization of the crystalline
portion of lignocellulosic biomass [56,66]. These catalysts ultimately
influence the overall product distribution and the rearrangement of the
resulting aromatic hydrocarbon produced during the pyrolysis process.
<figure><img src="image_2.png" /><figcaption>Fig. 1. Thermogravimetric analysis (TGA) of non-impregnated coffee grounds (CG), Ca-impregnated (CG Ca) and Fe-impregnated (CG Fe) under N₂ atmosphere, (a) TGA and (b) dTG curve.</figcaption></figure>

A higher yield of polycyclic aromatic hydrocarbon (naphthalene, fluorene, and phenanthrene) was recorded for iron-impregnated corn straw pyrolyzed at 800°C and calcium-impregnated pine wood at 900°C compared to the non-impregnated sample [27,66].

As illustrated in Fig. (1a), the residual masses obtained at the end of the pyrolysis at 900°C were 25.36 %, 21.78 % and 17.99 % for the non-impregnated biomass, Ca-impregnated and Fe-impregnated biomass respectively. This observation might be attributed to the distinct catalytic effects of Ca and Fe on biomass decomposition during pyrolysis. Ca-impregnated biomass tends to promote the formation of non-condensable gases and char over tar (the liquid fraction), whereas Fe-impregnated biomass facilitates the production of greater amounts of tar and gases relative to char [56,66].

The first derivative of the TGA (dTG) curve (Fig. 1b) shows three distinct peaks corresponding to the decomposition of hemicellulose, cellulose and lignin components of the biomass. A noticeable shift to lower degradation temperatures is observed with hemicellulose decomposing at 262°C (from 291°C) and cellulose 307°C (from 334°C) for the Ca-impregnated samples. In contrast, Fe-impregnated samples exhibit an earlier onset of the decomposition, with the amorphous component of the biomass beginning to degrade around 150°C compared to 200°C for Ca-impregnated and the reference sample. This can be explained by the decomposition of iron (III) nitrate ($Fe(NO_3)_3$ to iron (III) oxide ($Fe_2O_3$) which occurs between 157 and 269 °C [11,30]. Since $Fe_2O_3$ is more stable and catalytically active than $Fe(NO_3)_3$ [12], its presence below 200°C in biomass during pyrolysis promotes the dehydration and cleavage of weak bonds in the hemicellulose structure as observed in the Fe-impregnated samples. In contrast, this effect was absent in the Ca-impregnated samples, as the decomposition of the calcium nitrate $Ca(NO_3)_2$ to the more active calcium oxide (CaO) species occurs at a much higher temperature (> 400 °C) [67]. Hence the onset activation was only observed in the Fe-impregnated samples. Overall, the result highlights the catalytic effect of calcium and iron which reduces the activation energy of the decomposition reaction [54,57]. Having shown the variations in the thermal degradation profiles of Ca- and Fe-impregnated biomass, attributed to the difference in their catalytic activity, which impacts the degradation process, char composition and yield, the next section will cover the evolution of the metal catalyst and the carbon matrix at elevated temperature (1200°C-1800°C).

## 2.2. Structural evolution of the metal catalyzed graphitic biocarbon at elevated temperatures

### 2.2.1. Effect of temperature treatment on micro and nanostructure analysis demonstrated by SEM and TEM images

Graphene-like carbon synthesis from unconventional carbon precursors such as carbonaceous biomass materials are usually performed

at elevated temperatures around 2800°C [53,9]. However, in this study, a maximum temperature of 1800°C was considered sufficient to achieve the desired outcome. The SEM images depicting the microstructure of the graphitic biocarbon obtained at 1800°C is shown in Fig. 2. Focusing on images (a), (b) and (c), a noticeable distinction was observed in the microstructures that can be directly linked to the type of catalyst employed in the process. Firstly, the microstructure of the coffee grounds biomass, characterized by its large shallow pores is retained even after exposure to high-temperature treatments of up to 1800°C (Fig. 2a).

The microstructure of the Ca-impregnated graphitic biocarbon (Fig. 2b) appears spongey and porous unlike images of the non-impregnated (Fig. 2a) and Fe-impregnated samples (Fig. 2c). On closer examination, the Ca-impregnated graphitic biocarbon reveals a distinct honeycomb-like structure [64], characterized by a mix of open and closed pores with varying sizes (Fig. 2e). Conversely, the Fe-impregnated graphitic biocarbon depicted in (Fig. 2c) resembles the reference sample (without impregnation). However, a closer inspection shows that its microstructure is enveloped by clusters of spherical formation that can be identified as Fe particles encased within the graphitic biocarbon [37].

Moreover, the SEM images provide valuable insights into the behavior of the metal catalyst during the graphitization process as well as structural transformation occurring in the sample. Fig. 3 shows the effect of graphitization temperature on the metal catalysts. For the Ca-impregnated samples, as the temperature rises from 1200°C to 1800°C, the CaO nanoparticles, identifiable as bright spots in Fig. 3a, almost entirely disappear by 1800°C (Fig. 3b). This suggests a series of reactions involving CaO and amorphous carbon that leads to the formation of the unstable $CaC_2$ or $CaCN_2$ which decomposes releasing Ca vapor while precipitating graphene-like carbon with increasing temperature [5,23,64]. In contrast, the Fe-impregnated samples reveal scattered bright spots throughout the carbon matrix corresponding to Fe particles even at 1800°C (Fig. 3c), indicating that the Fe particles were not decomposed even at elevated temperatures. In addition, Fig. 3f depicts clusters of spherical graphitic biocarbon encapsulated Fe particles of varying sizes along with raptured encased Fe particles which offers insight into the graphitization process. The thermal behavior of these catalyst highlighted by the SEM images offers some degree of understanding into their catalytic roles during the graphitization process.

Notably, nanotube-like structures were observed for Ca-catalyzed graphitic biocarbon samples at both temperatures 1200°C and 1800°C (Fig. 3d and e), that was not the case for Fe-catalyzed samples. These nanotube-like structures appear to be sparsely dispersed within the carbon matrix (Figure S1). At 1200°C, the structures exhibit longer length, (over 15 µm) with narrower diameter (58.48 nm) while at 1800°C, they appear denser with an increased diameter of around
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** SEM images depicting the difference in structural morphology of the reference sample (non-impregnated biocarbon) (a), Ca impregnated biocarbon (b), Fe impregnated biocarbon (c), graphitized at 1800°C; the red dotted rectangles on the SEM images represent magnified area; non-impregnated biocarbon (d), Ca impregnated biocarbon (e), Fe impregnated biocarbon (f).</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** SEM images showing the distribution of the metal catalyst within the biocarbon microstructure of (a) Ca-impregnated biocarbon graphitized at 1200°C, (b) Ca-impregnated biocarbon graphitized at 1800°C, (c) Fe impregnated biocarbon graphitized at 1800°C; SEM images showing the occurrence of nanotube-like structures in (d) Ca-impregnated biocarbon graphitized at 1200°C, (e) Ca-impregnated biocarbon graphitized at 1800°C (the red dotted rectangles on the SEM image highlights the formation of the nanotube-like structures); SEM image illustrating the rupture of the graphitic biocarbon capsule enclosing the Fe particles (f).</figcaption></figure>

93.64 nm and a reduced length of approximately 5 µm.

Once again, the impact of temperature on the evolution of the morphology of the graphitic biocarbon is evident but the mechanism underlying the formation of these nanotube-like structures in the Ca-catalyzed samples, which based on the diameter (58.48 nm – 93.64 nm) are likely carbon nanofibers (CNF), remains unclear [17]. On the other hand, the synthesis of CNTs from Fe catalyzed organic substrates is known [24,25,38,47,68]. This mechanism has also been described as the tunnelling of the molten iron droplets through the carbon matrix [21] or more recently, as the burrowing of solid Fe crystals through the amorphous carbon matrix [19].

To further evaluate the structural changes occurring during the graphitization process, HRTEM, TEM images and EDS mapping of the metal catalyzed graphitic biocarbon are illustrated in Fig. 4. The influence of temperature on the metal catalyst is evident. Sintering of the spherical shaped Fe nanoparticles is observed, resulting in an increase in their average particles size as temperatures rises from 1200°C to 1800°C (Fig. 4a - d) [14,37]. In contrast to the Fe catalyst, the CaO nanoparticles undergoes a series of reactions that results in the vaporization of Ca with increase in temperature (Fig. 4e – h). Moreover, the EDS mapping depicted in Fig. (4i – l) corroborates the behavior of the Ca catalysts observed in the TEM images as temperature rises from 1200°C to
<figure><img src="image_5.png" /><figcaption>Fig. 4. TEM images illustrating the evolution of the metal catalyst (Fe) in the iron impregnated biocarbon with increasing temperature of graphitization from 1200°C to 1800°C (a-d); Dark field images of calcium impregnated biocarbon showing the distribution of the metal catalyst (Ca) with increasing temperature from 1200°C – 1800°C (e-h); and EDS mapping analysis highlighting the distribution of (Ca) with increasing temperature from 1200°C – 1800°C (i-l).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 5. Lattice fringe images of Fe- and Ca- impregnated biocarbon obtained via HR-TEM showing the vertical stacking height (yellow line) of the graphitic biocarbon generated as temperature increases from 1200°C to 1800°C.</figcaption></figure>
1800°C. Based on the evolution of the metal catalyst with temperature, valuable insights into the mechanism of graphitization can be gained. As the disappearance of the Ca with increasing temperature suggests a carbide formation-decomposition pathway that results in the vaporization of calcium and the precipitation of graphene-like carbon [5]. On the other hand, the agglomeration of the Fe catalyst with increasing temperature aligns with the proposed dissolution-precipitation mechanism, which is known to be facilitated by the growth of Fe particle size [12,28]. Finally, the HRTEM/TEM images complement the previously seen SEM images, collectively illustrating the evolution of the metal catalyst with increasing temperature.

Fig. 5 shows a closer examination of the changes in the lattice fringes

of both Fe- and Ca-catalyzed graphitic biocarbon as the temperature increases. These changes in the lattice fringes indicate a transition in the crystal structure as temperature rises from 1200°C to 1800°C. It is evident that they become clearer and more well-defined as the severity of the heat treatment increases. The interlayer spacing derived from the TEM analysis on Gatan of the Fe-catalyzed graphitic biocarbon decreased from 0.378 nm at 1200°C to 0.343 nm at 1800°C (Fig. 5a–d). Concurrently, the stacking order (Lc) increased from 5.25 nm at 1200°C to 7.53 nm at 1800°C. Additionally, the estimated number of graphene-like sheets increased with values of 13.8, 19.12, 20.14 and 21.95 at 1200°C, 1400°C, 1600°C and 1800°C respectively. These findings suggest that a substantial transition from amorphous to crystalline

<figure><img src="image_7.png" /><figcaption>Fig. 6. Raman spectra of the non-impregnated biocarbon and metal impregnated biocarbon at different temperatures. (a) non-impregnated biocarbon, (b) Calcium impregnated biocarbon, and (c) Iron impregnated biocarbon.</figcaption></figure>
biocarbon occurs even at 1200°C. This transformation appears to stabilize by 1400°C, as evidenced by only a minimal increase in crystallite size and the number of graphene-like sheets at higher temperatures.

On the other hand, a detailed examination of the lattice fringes of the Ca-catalyzed graphitic biocarbon reveals a higher interlayer spacing compared to the Fe-catalyzed samples, decreasing from 0.397 nm at 1200°C to 0.358 nm at 1800°C. The stacking order ($L_c$) exhibited a steady increase from 1.96 nm at 1200°C to 2.88 nm at 1400°C, followed by a sharp rise to 10.98 nm at 1600°C, and then dropping to 3.07 nm. Similarly, the estimated number of graphene-like sheets increased from 4.93 at 1200°C to 7.61 at 1400°C, peaked at 1600°C with a value of 20.05, and then decreased to 8.57 at 1800°C. Overall, analysis of HR-TEM images suggests that significant transformation of the carbon lattice begins around 1400°C and peaks at 1600°C. However, at higher temperatures (1800°C), distortions and defects emerge, impacting the lattice fringes. The emergence of these defects at 1800°C for Ca-catalyzed sample is likely due to the disappearance of the metal catalyst core, that serves as an anchor or support around which the graphene-like shells forms [5]. Conversely, the Fe-catalyzed samples exhibits better stability at temperatures above 1600°C, as the metal catalyst undergoes sintering, reenforcing the core structure around which the graphitic biocarbon forms [14].

Furthermore, the evolution of the carbon lattice, particularly the stacking order $L_c$ with temperature further supports the proposed graphitization mechanism inferred from the behavior of the metal catalyst (Fig. 4). For Fe-catalyzed biocarbon, $L_c$ shows a steady rise, consistent with the growth of Fe particle size, supporting the dissolution-precipitation pathway. Conversely, for Ca-catalyzed biocarbon, $L_c$ increases until 1600°C and then exhibits a decline, aligning with the vaporization of calcium and thus backing the carbide formation-decomposition mechanism.

### 2.2.2. Effect of metal catalyst and treatment temperature on the graphitic nanostructure defects

Raman spectral analysis was conducted to gain further insights into the nanostructure of the synthesized graphene-like material, particularly its degree of structural defects. As shown in Fig. 6, three prominent peaks corresponding to the D, and G bands (first-order region), and 2D bands (second-order region) [53] were observed in the spectra of all three samples: (a) non-impregnated, (b) Ca-impregnated, and (c) Fe-impregnated graphitic biocarbon. The D band appearing around 1340 – 1345 cm⁻¹, is regarded as the defect band and the intensity decreases with increase in the structural organization. The G band on the other hand is attributed to vibrational stretching in the aromatic layers of graphite. It occurs around 1565 – 1580 cm⁻¹ and appears as the only peak in the first order region in well-ordered graphite. The appearance of the 2D peak around 2653 – 2680 cm⁻¹ is indicative of a higher degree of order in the sp² hybridized carbon.

The Raman analysis results reveal that at 1200°C, the 2D band is present as a broad, low-intensity peak exclusively in the Fe-catalyzed samples, indicating a higher degree of structural organization compared to the non-impregnated and Ca-catalyzed samples at the same temperature. Additionally, the $I_D/I_G$ ratio, which provides an indication of the level of defects in the nanostructure, shows that the Fe-catalyzed sample exhibits the lowest level of defects at 1200°C compared to the other samples.

Furthermore, as the graphitization temperature increased to 1800°C, the influence of the catalyst becomes more pronounced, with the level of defects following the order: Fe-impregnated < Ca-impregnated < non-impregnated. Although the 2D band is present at 1200°C, 1400°C, 1600°C, and 1800°C in the Fe-catalyzed samples, it becomes prominent only at 1800°C. In contrast, the Ca-catalyzed samples exhibit prominent 2D peaks across a wider temperature range, from 1400°C to 1800°C. This indicates that the Ca catalyst enhances the structural organization of carbon over a broader temperature range compared to the Fe catalyst.

As the temperature increased from 1200°C to 1400°C, the $I_D/I_G$ ratio

of the Ca-catalyzed sample decreased significantly from 1.54 to 0.2. However, a further increase in temperature to 1600°C resulted to a slight increase in the $I_D/I_G$ ratio to 0.25. This trend persisted at 1800°C, with the $I_D/I_G$ ratio increasing further to 0.43. The increase in the defects levels with rising temperature is likely due to the vaporization of the catalyst, which diminishes its stabilizing effect on the graphene-like shells. For the Fe-catalyzed samples, the $I_D/I_G$ ratio appears to increase with temperature between 1200°C and 1600°C. This behavior might be attributed to interference caused by Fe induced fluorescence during the data acquisition. The degree of graphitization (DOG) calculated from the Raman results follow a similar trend as the degree of defects ($I_D/I_G$) (Table S1).

### 2.2.3. Impact of metal catalyst and temperature treatment on crystalline structure analysis

Fig. 7 shows the evolution of the XRD profiles for the reference sample (non-impregnated), Ca-catalyzed and Fe-catalyzed biocarbon subjected to increasing heat treatment temperatures from 1200°C to 1800°C. The XRD pattern for the reference sample (non-impregnated) comprise three broad prominent carbon peaks that narrow with increasing temperature. Compared to samples treated at lower temperatures, carbon treated at 1800°C exhibits slightly narrower peaks at 25.88°, 43.58° and 77.82°, corresponding to the (002), (100), and (110) graphite peaks, respectively (Fig. 7a). The (002) peak is associated with the stacking of graphene layers. In addition, peaks corresponding to CaO and $CaC_2$ were also observed at this temperature.

For the Ca-catalyzed biocarbon, the prominent peaks at 1200°C were of CaO and $Ca(OH)_2$. However, as the treatment temperature increases to 1400°C, the carbon peaks at 26.15°, 43.38°, and 77.75° corresponding to the (002), (100), and (110) peaks of graphite respectively, were observed along with some calcium species (Fig. 7b). The intensity of the carbon peaks increases as the treatment temperature reaches 1600°C with the appearance of the 004 peak at 55° suggesting an improvement in the quality of the stacking order [53]. As the treatment temperature approaches 1800°C, the intensity of the (002), (100), (110), and (004) peaks increases, indicating the influence of temperature on the graphitization process [14,5,53]. In addition, similar to the (002) peak observed in the non-impregnated sample at 1800°C, the (002) peaks of the Ca-catalyzed samples are asymmetric. This asymmetry points to the coexistence of both crystalline and turbostratic form of carbon [5].

As indicated by previously seen SEM and TEM/EDS analyses, the presence of calcium species decreases as the temperature increases. While traces of calcium are detected at 1600°C, no evidence of calcium species is found at 1800°C in the Ca-catalyzed graphitic biocarbon. This is likely due to a series of chemical reactions occurring at elevated temperatures, including the reaction of CaO with amorphous carbon to form $CaC_2$ [5,23], the reaction of $CaC_2$ with CaO leading to the release of calcium [23], or the decomposition of $CaC_2$ [5,23], which also releases calcium and precipitates graphene-like carbon. A summary of the possible reactions and products involved in the release (vaporization) of the calcium catalysts are as follows;

$$3Ca_{amorphous} + CaO \rightarrow CaC_2 + CO \quad (1)$$

$$CaC_2 + 2CaO \rightarrow 3Ca + 2CO \quad (2)$$

$$CaC_2 \rightarrow Ca + 2C_{structured} \quad (3)$$

These equations offer insight into the mechanism of the formation of graphene-like carbon when calcium is employed as catalyst. The reactions suggest that graphitic biocarbon formation is most likely favored after CaO is consumed in the process that generates $CaC_2$. Since the reaction between amorphous carbon and CaO to form $CaC_2$ occurs around 1400°C [23], this could explain why the (002) peak corresponding to the stacking of the graphene layers is observed at 1400°C and not at 1200°C. The significant production of CO at elevated temperatures during this process likely contributes to the highly porous
<figure><img src="image_8.png" /><figcaption>**Fig. 7.** X-ray diffraction patterns of non-impregnated and metal impregnated biocarbon at different temperatures. (a) reference (non-impregnated biocarbon), (b) Calcium impregnated biocarbon, and (c) Iron impregnated biocarbon.</figcaption></figure>

structure of the Ca-catalyzed graphitic biocarbon, as evidenced by SEM images. Additionally, this CO generation accounts for the notable reduction in product yield observed with this catalyst, particularly at 1600°C and 1800°C (Fig. 9e).

Analysis of the XRD profiles of the Fe-catalyzed samples reveal four prominent peaks corresponding to the graphite peaks: (002), (100), (110), and (004) (Fig. 7c). Unlike the Ca-catalyzed samples, the Fe species comprising Fe and Fe₃C were observed in the samples at all treatment temperatures. This indicates that the Fe catalyst exhibits higher thermal stability than the Ca catalyst within the temperature range (1200°C – 1800°C) investigated in this study.

Interestingly, Fe₃C an intermediate product formed during the graphitization process, reported to be unstable and only present at low temperature < 800°C [16,18,19,21], was detected at all temperatures. The presence of both Fe and Fe₃C in the graphitized samples suggests

that both Fe species play a role in the graphitization process. This indicates that the mechanism of graphitization likely involves both Fe-carbide decomposition and the Fe/C dissolution/precipitation pathways. The presence of Fe₃C in graphene-like carbon synthesized at elevated temperatures has been reported at 1200°C during the graphitization of saccharose-based char [8] and at 1600°C during the graphitization of ferrocene [3]. These findings demonstrate that Fe₃C can persist and remain stable even at elevated temperatures.

The slightly broad (002) peak observed in the graphitic biocarbon across all temperatures (1200°C–1800°C) suggests some level of disorder in the carbon structure, probably consisting primarily of turbostratic carbon (stacked sheets of sp²-hybridized carbon with rotational disorder) rather than well-ordered graphitic biocarbon. This structural disorder likely accounts for the low crystallite size (Lc) observed in the Fe-catalyzed samples, which range from 2.99 at 1200°C to 4.06 nm at
<figure><img src="image_9.png" /><figcaption>**Fig. 8.** Temperature dependence of in-plane ($L_a$) and out-of-plane ($L_c$) crystallite sizes determined from XRD for (a) Ca-catalyzed samples, (b) Fe-catalyzed samples; Comparison of the temperature dependence of in-plane ($L_a$) crystallite sizes calculated from XRD and Raman analysis for (c) Ca-catalyzed samples, (d) Fe-catalyzed samples, (where CG represents coffee grounds).</figcaption></figure>

1800°C, compared to the Ca-catalyzed samples, which exhibit significantly larger $L_c$ values ranging from 6.73 nm at 1400°C to 14.38 nm at 1800°C (Fig. 8a and b). In contrast, the $L_a$ values for the Fe-catalyzed samples, which range from 24.27 nm at 1200°C to 13.56 nm at 1800°C samples were notably higher than those of the Ca-catalyzed samples ranging from 4.17 nm at 1400°C to 5.71 nm at 1800°C. These finding reveal that the Ca-catalyst favors the vertical growth/stacking degree of the biocarbon crystals while the Fe-catalyst promotes the lateral growth of the crystals in the horizontal direction. Overall, the substantial disparity between the $L_c$ and $L_a$ obtained for the Fe-catalyzed samples indicates that the graphene-like material formed was poorly ordered (Table S2). Furthermore, as the temperature increases, the values of $L_c$ and $L_a$ for both Ca- and Fe-catalyzed samples converge especially in the Fe-catalyzed samples. This suggests that higher temperature promotes more homogenous growth [14].

Fig. 8(c) and (d), compares the $L_a$ values determined from XRD and Raman analysis. For the Ca-catalyzed samples, both techniques show a similar temperature-dependent trend for $L_a$, indicating coherence between the methods (Table S3). However, the $L_a$ values estimated by Raman were higher than those for XRD, which is a phenomenon attributed to differences in the measurement principles of the two techniques. In contrast, for the Fe-catalyzed samples synthesized at 1200°C to 1600°C, the $L_a$ values obtained from XRD were unusually higher than those determined by Raman. This anomaly is likely due to Fe-induced fluorescence interfering with Raman measurements. At 1800°C, however, the fluorescence effect diminishes, and the $L_a$ values determined by Raman become higher than those estimated by XRD, aligning with literature reports [53].

### 2.2.4. Insight into catalyst behavior during graphitization of raw biomass (coffee grounds)

In order to have a clearer understanding of the catalyst behavior or mechanism involved in both Ca- and Fe-catalyzed graphitization of this raw biomass, it is important to consider other characteristics of the synthesized graphene-like material, such as the specific surface area (nitrogen adsorption), and the surface chemical composition determined by X-ray photoelectron spectroscopy (XPS). By analyzing how the two catalyst influence specific properties of the synthesized graphitic biocarbon, as well as the nuances in the correlations between these properties, valuable insights into the underlying mechanism involved in the process can be gained.

Fig. 9a and b depict the relationship between BET specific surface area (m²/g) and % sp² bonded carbon (estimated from XPS) of the Ca-catalyzed and Fe-catalyzed graphitic biocarbon respectively. It can be seen that the Ca-catalyzed biocarbon exhibit lower sp² hybridized carbon content compared to the Fe-catalyzed biocarbon (Figure S2 and S3). The enrichment of the sp²-bonded carbon content in the biocarbon is one of the key indicators of the efficiency of the graphitization process. Thus, an increase in the sp²-bonded carbon content with rising temperature is usually anticipated. Furthermore, the sp²-bonded carbon is responsible for the unique properties of graphitic materials especially their exceptional electrical and thermal conductivity [29]. The samples shown in Fig. 9(a, b) were produced at 1200°C, 1400°C and 1800°C. The Ca-catalyzed samples show a positive correlation with a moderately strong linear relationship between specific surface area and sp² hybridized carbon content. In contrast, the Fe-catalyzed samples exhibit a negative correlation with a very strong linear relationship between these two parameters. This result demonstrates that the graphitic biocarbon
synthesized can be easily tailored to exhibit a highly porous micro-
structure or non-porous structure, simply by the choice metal catalyst
used.

Fig. 9c and d show the correlation between the degree of graphitization determined from Raman analysis and the sp² bonded carbon content estimated from XPS analysis. It is worth noting that a high degree of graphitization was achieved with both catalysts. Both Ca and Fe-catalyzed samples exhibit a positive correlation, with the Ca-catalyzed samples showing a weak linear relationship and the Fe-catalyzed samples displaying a strong linear relationship. The positive correlation observed for both catalysts suggest that the sp² bonded carbon content influences the degree of graphitization. However, it is important to note that a high sp² hybridized carbon content does not necessary correspond to a highly ordered carbon lattice. Furthermore, the weak linear correlation obtained between the two parameters for the Ca-catalyzed

biocarbon may be linked to the vaporization of the Ca-catalyst with
increasing graphitization temperature.

In Fig. 9e the influence of temperature on the yield of graphitization for both Ca and Fe-catalyzed samples is illustrated. In agreement with SEM images, and other previously discussed results, the impact of temperature on the yield loss was more pronounced in the Ca-catalyzed samples. As mentioned earlier this is plausibly linked to the series of reactions leading up to the vaporization of the metal catalyst and the subsequent precipitation of the crystalline carbon. Comparing the Metal/Carbon (M/C) ratio for the two catalysts (Fig. 9f), the Fe/C ratio remains relatively stable across the investigated graphitization temperature range (1200°C to 1800°C). In contrast, the Ca/C ratio evolves with temperature, disappearing rapidly at 1800°C. These findings are critical for understanding the mechanisms involved in the graphitization process for Ca and Fe-catalyzed biocarbon (Figure S4). As the evolution

<figure><img src="image_10.png" /><figcaption>**Fig. 9.** Correlation between key parameters of the graphitic biocarbon determined by diverse characterization techniques: Relationship between BET specific surface area and sp2 bonded carbon (determined from XPS data) (a) Ca-catalyzed graphitic biocarbon, (b) Fe-catalyzed graphitic biocarbon; Relationship between the degree of graphitization (determined from Raman analysis) and the sp2 bonded carbon (determined from XPS data) (c) Ca-catalyzed graphitic biocarbon, (d) Fe-catalyzed graphitic biocarbon; Temperature dependence of (e) product yield, (f) metal catalyst to carbon ratio; (CG stands for coffee grounds).</figcaption></figure>
of the metal catalyst during the process, whether through agglomeration or decomposition at elevated temperature, impact the progression of the graphitization process. Moreover, the degree of graphitization obtained in this study with Fe/C below 0.1 is considerably higher than that of a similar study conducted with Fe/C of 1 [12].

### 3. Conclusions

The study investigated the roles of Ca, an unconventional catalyst, and Fe in the graphitization process. The findings indicate that using Ca as catalyst produces a highly porous graphitic structure, whereas Fe as a catalyst result in non-porous structures. The catalysts significantly influenced both the in-plane crystallite size and graphene stacking, with Ca-catalyzed samples producing larger crystals compared to Fe-catalyzed samples. Interestingly, the formation of nanotube-like structure typically associated with Fe-catalyzed organic substrates, were not observed for this biomass (spent coffee grounds). Rather, they were observed only when Ca was utilized as catalyst. Furthermore, the difference in the resulting nanostructures obtained were attributed to the carbon crystallization mechanisms associated with each catalyst (Ca verse Fe). For Ca the carbide formation-decomposition mechanism was proposed, while for Fe, both carbide formation-decomposition and the dissolution-precipitation mechanisms were suggested. Overall, high degree of graphitization was achieved with both catalysts. The exceptional properties of these materials combined with the approach developed, opens exciting opportunities for the valorization of biomass waste streams into advanced carbon materials for applications in energy storage, catalysis and environmental remediation.

### 4. Materials and methods

#### 4.1. Graphitic biocarbon synthesis

##### 4.1.1. Sample preparation

Dried spent coffee grounds (CG) powder collected in the UK, was used as the carbon precursor in this study. The ash content of the biomass was around 2 % mainly composed of potassium, calcium, magnesium, phosphorus and silicon. The impregnation process involved dispersing the biomass in 0.423 M/L solution of Ca(NO₃)₂·4H₂O (referred to as CG-Ca) and 0.049 M/L solution of Fe(NO₃)₃·9H₂O (referred to as CG-Fe). These mixtures were continuously stirred using a magnetic stirrer for 6 h and then filtered and oven dried at 105°C overnight. The choice of metal catalyst concentration used in this study were based on previous investigation conducted [14,4]. The Ca and Fe mineral content were determined by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) while the elemental content was determined by the CHNS-O analyzer. Table 1 shows the chemical composition of the carbon precursor (coffee grounds) along with the Ca and Fe contents.

##### 4.1.2. Pyrolysis and graphitization process

Samples were pyrolyzed in a vertical tubular reactor (Carbolite) at 800°C under a nitrogen atmosphere, with a heating rate of 5 °C/min, and a holding time of 1 h at the target temperature. To convert the

<table><caption>**Table 1**

Chemical composition of the coffee grounds before and after impregnation.</caption><thead><tr><th>Samples</th><th>Coffee grounds (CG)</th><th>Ca impregnated Coffee grounds (Ca-CG)</th><th>Fe impregnated Coffee grounds (Fe-CG)</th></tr></thead><tbody><tr><td>Fe (%)</td><td>0.01</td><td>0.01</td><td>1.10</td></tr><tr><td>Ca (%)</td><td>0.14</td><td>2.74</td><td>0.14</td></tr><tr><td>C (%)</td><td>51.3</td><td>45.7</td><td>50.0</td></tr><tr><td>H (%)</td><td>7.2</td><td>6.6</td><td>6.8</td></tr><tr><td>N (%)</td><td>2.4</td><td>4.0</td><td>2.6</td></tr><tr><td>S (%)</td><td>0</td><td>0</td><td>0</td></tr></tbody></table>

resulting amorphous carbon into graphitic biocarbon, the biochar produced was subjected to further pyrolysis under identical conditions but at higher temperatures, ranging from 1200°C to 1800°C, using a horizontal tubular furnace (Nabertherm).

#### 4.2. Characterization of the graphitic biocarbon

##### 4.2.1. Characterization of the crystalline properties by X-ray diffraction (XRD)

To study the crystalline properties of the powdered samples, XRD was utilized. It was carried out on a Phillips PANalytical X'pert Pro MPD, equipped with Cu-Kα radiation source (λ = 1.543 Å) operating at 45 kV and 40 mA. Diffraction peaks along 2θ were obtained between 10° and 90°, at a scan speed of 0.033°/s. The Carbon peaks were fitted with the Origin Lab software, using the pseudo-Voigt function. Bragg's equation was used in the calculation of the interlayer spacing (Eq. 1), while Scherrer's equation (Eq. 2) was applied for the calculation of the crystallite size (in-plane crystallite size/lateral size/width, L<sub>a</sub>) and stacking height/thickness, L<sub>c</sub>).

$$d_{002} \ (nm) = \frac{K}{2\sin\theta} \qquad (4)$$

$$L(nm) = \frac{K\lambda}{(\beta - s)\cos\theta} \qquad (5)$$

Where d<sub>002</sub> represents the interlayer spacing, θ the peak position in radians, and β and s are the Full Width at Half Maximum (FWHM) in radians of the studied peaks and silica, respectively. Silica was used as a reference to correct for broadening. K is a constant related to crystallite shape taken as 0.89 for L<sub>c</sub> and 1.84 for L<sub>a</sub>.

##### 4.2.2. Characterization of graphitic structure by Raman spectroscopy

Raman spectroscopy was used to investigate the graphitic nature, and defect density of the produced biocarbon. The analysis was carried out using the Raman-AFM, Alpha 300 R, WITec microscope. The samples were thoroughly mixed with KBr in a weight ratio of 2 mg of biocarbon to 200 mg of KBr and then compressed into a flat disk of about 12 mm prior to the analysis. Spectra were recorded using the 532 nm wavelength, covering a spectral range of 0–4000 cm<sup>-1</sup>. Two spectra were acquired for each sample and curve fitting was performed using the Lorentzian profile on MagicPlot software. The in-plane crystallite size (La) was calculated using the formula in Eq. 3 proposed by Tuinstra and Koenig [52]. Where I<sub>D</sub> and I<sub>G</sub> represent the intensity of the D and G bands, E<sub>L</sub> represents the laser energy (2.3308 ev). The degree of graphitization (DOG) was estimated using Eq. 4 [12].

$$L_a \ (nm) = \frac{4.4}{\frac{I_D}{I_G}} * \left(\frac{2.41}{E_L}\right)^4 \qquad (6)$$

$$\text{DOG}(\%) = \frac{I_G}{(I_D + I_G)} * 100 \qquad (7)$$

#### 4.2.3. Characterization of surface chemistry by X-ray photoelectron spectroscopy (XPS)

The surface chemistry information of the graphitic biocarbon was acquired by the K-ALPHA ThermoFisher Scientific apparatus equipped with a monochromatized Al source (1486.7 eV). The X-ray spot size was 400 µm, and the pass energy was fixed at 40 eV. The obtained spectra were treated/deconvoluted using MagicPlot software.

#### 4.2.4. Characterization of nanotexture and microstructure

TEM and HRTEM images were acquired using the JEOL JEM ARM 200 F equipment operated at 200 kV. The samples were first homogenized by grinding and then dispersed in ethanol before being placed on a copper grid for analysis. As for the EDS spectra, they were recorded on a JEOL CENTURIO detector. The surface morphology and microstructure
were investigated using a ThermoFisher Scientific Quattro S-SEM instrument. The samples were analyzed in low vacuum mode, between 0.5 and 0.6 mbar (depending on the samples and the magnification); with a voltage of 6 kV. The images were treated using the Gatan digital micrograph software to obtain the Lc, and d002. The number of sheets (N) was determined by dividing the crystallite size (Lc) by the interlayer space (d002).

### 4.2.5. Other characterizations

The specific surface area (BET method) of samples was determined by N₂ gas adsorption at 77 K using the TriStar II 3020 (Micrometrics) analyzer, while the sample's thermal degradation profile was studied by the LABSYS evo DTA-DSC-Setaram instrument. The samples were analyzed under a nitrogen atmosphere, with a heating rate of 5 °C/min, and a holding time of 15 min.

The yield of graphitization was determined using the formula

$$yield of graphitization = \frac{M_{final}}{M_{initial}} * 100 \qquad (8)$$

Where Mfinal is the mass of graphitic biocarbon after heat treatment, and Minitial is the mass of carbon source (biochar) before heat treatment.

### CRediT authorship contribution statement

**Ngene Gloria Ifunanya:** Writing – review & editing, Writing – original draft, Validation, Methodology, Investigation, Formal analysis, Conceptualization.
**Ange Nzihou:** Writing – review & editing, Validation, Supervision, Resources, Conceptualization.

### Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

The authors acknowledge colleagues at RAPSOdee Center- IMT Mines Albi (Nathalie LYCZKO, Christine ROLLAND, Sylvie DEL CONFETTO, Lauréne HAURIE), Centre de MicroCaractérisation Raimond Castaing (Teresa HUNGRIA), and PLACAMAT (Christine LABRUGERE SARROSTE).

## Appendix A. Supporting information

Additional data supporting this study has been provided. It includes tables presenting Raman and XRD parameters, figures showing XPS data and SEM image of Ca-catalyzed biocarbon (word document). Supplementary data associated with this article can be found in the online version at doi:10.1016/j.mtcomm.2025.113240.

### Data availability

Data will be made available on request.

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