Introduction

Over the last few decades, climate change has emerged as one of the most pressing global challenges due to its devastation in every corner of the world. Among the contributing factors, carbon dioxide has played a significant role as a most influenced factor to global warming. The atmospheric carbon dioxide content has continuously rising from 280 ppm before the industrial revolution to the current level of 420 ppm (Cianconi et al. 2020; Kemp et al. 2022; Clarke et al. 2022; Dziejarski et al. 2023). The surge was primarily due to human activities like industrial processes, vehicle emissions, and agricultural practices (Yoro and Daramola 2020; Lynch et al. 2021). While certain activities are essential for global consumption, efforts to mitigate excessive carbon dioxide emissions have led to the development of various innovative technologies, including direct capture methods and catalytic reactions (Burns and Nicholson 2017; Donnison et al. 2020; Gadikota 2021; Loomis et al. 2022).

Emphasizing the global requirement for more efficient carbon dioxide capture, this study explored the application of plasma technology, specifically metal arc for carbon dioxide reduction. Unlike traditional methods, metal arc not only offers an effective means of capturing carbon dioxide but also concurrently produces valuable carbon-based materials, such as titanium carbide. The use of microwave plasma technology, known for its speed, efficiency, and versatility in various chemical syntheses, plays a pivotal role in this approach. Microwave plasma technology has been widely adopted for synthesizing titanium-based materials, demonstrating its efficacy in producing nanoparticles and functional materials (Komarneni et al. 1995, 1999; Jokisaari et al. 2005; Garino et al. 2019). In this work, the metal arc for carbon dioxide reduction system employed carbon dioxide as the sole carbon source for producing titanium carbide (TiC), holding the potential to significantly reduce carbon dioxide levels in various circumstances. The microwave plasma mechanism utilized in this study is further discussed in supplementary material which provided more detail including physical and chemical approach to deal with titanium carbide.

Experimental

$${{\text{Ti}}+{{\text{CO}}}_{2}\to {\text{TiC}}+{\text{O}}}_{2}$$
(1)

For the reaction of titanium and carbon dioxide to obtain titanium carbide, the reaction is shown in Eq. 1. In this study, the enthalpy and Gibbs energy of titanium were assumed to be 0 kJ/mol, while for carbon dioxide, values were 393.5 kJ/mol and 394.4 kJ/mol, respectively. Titanium carbide exhibited an enthalpy of − 184.1 kJ/mol and a Gibbs energy of − 180.3 kJ/mol. Given oxygen’s elemental form, both its enthalpy and Gibbs energy were considered as zero. The reaction’s enthalpy and Gibbs energy were determined to be 209.4 kJ/mol and 214 kJ/mol, respectively, indicating an endothermic and thermodynamically unfavorable process that required an additional mechanism to drive it forward. To overcome this challenge, microwave plasma was employed, facilitating the reaction in the form of ionized atoms.

Figure 1a shows a 99.5% high-purity titanium rod drilled into sharp-edged scrap, suitable for plasma generation with microwave radiation. (Subannajui 2016; Khotmungkhun et al. 2022). Figure 1b illustrates the metal arc for rapid carbon dioxide reduction system with an alumina container connected to gas tubes, powered by four magnetrons at 750 watts each. Sequential magnetron activation prevented overheating. Titanium shaving scrap was loaded into the container. Without microwave radiation, there was no reaction. Upon exposure as shown in Fig. 1c, sharp edges triggered intense plasma, ionizing titanium atoms, carbon dioxide, nitrogen, and oxygen ions. Under a circumstance that various gases became ions and ready to react, the reaction with lowest activation energy took place, and titanium carbide were formed facilitating the formation of titanium carbide with the lowest activation energy (Ertl et al. 1997; Du et al. 2000). Regarding the conversion to form titanium carbide, starting the reaction from titanium was preferred due to its lower activation energy compared to titanium dioxide (Swift and Koc 1999). The robust titanium dioxide passivation layer impedes continuous oxidization of titanium shaving scrap; however, the interaction with carbon dioxide enables carbothermal reduction, eliminating the oxide film and promoting an uninterrupted carbide formation reaction (Woo et al. 2007). In Figure S1b, metallic titanium scrap transforms into black titanium carbide within 30 s under carbon dioxide flooding, demonstrating process efficiency. Microwave plasma ionization rapidly lowers carbon dioxide concentration to 385 ppm in 30 s, which was even lower than atmospheric levels. Comprehensive characterization of the titanium carbide is detailed in the supplementary material.

Fig. 1
figure 1

a Metallic titanium (Ti) was drilled, and the titanium shaved lathe was collected, b titanium shaved lathe placed in the microwave system with carbon dioxide (CO2), c magnetron radiated microwave to arc on titanium shaved lathe and form titanium carbide (TiC) after ignition, d carbon dioxide concentration during the spark

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Results and discussion

Carbon dioxide conversion

Since the carbon dioxide was eliminated within a short time, this could be an extremely fast method to eliminate carbon dioxide deserving of further development. The reduction following exponential regression is shown in Eq. 2, where carbon dioxide was in ppm and t was time in seconds. The conversion efficiency of carbon dioxide elimination is calculated from Eq. 3.

$${{\text{CO}}}_{2}=300.58+1699.42{{\text{e}}}^{-0.1{\text{t}}}$$
(2)
$${\text{Efficiency}} = \frac{{{\text{CO}}_{{2}} \, {\text{that}}\,{\text{was}}\, {\text{eliminated}} }}{{{\text{CO}}_{{2}} \,{\text{ that}}\, {\text{was}}\,{\text{ added}}}} \times 100$$
(3)

With an initial atmospheric carbon dioxide level of 420 ppm, the introduced amount in the system was (2000-420) ppm. After 30 s of plasma arc, the residual carbon dioxide was 385 ppm, resulting in the removal of (2000-385) ppm in Fig. 1d. This system could eliminate all input carbon dioxide already. Hence the conversion efficiency reached 100%. The energy efficiency of the process, calculated by comparing theoretical energy requirements to actual energy consumption, reached an impressive 72% (Yin et al. 2021). This surpassed other plasma technologies with similar energy efficiency, as many of them exhibited lower carbon dioxide conversion rates and slower processing speeds, often incompatible with normal atmospheric pressure conditions (Bogaerts and Centi 2020).

Plasma spectrum

To study the ionization of titanium with gases, even though the elimination of carbon dioxide was the main study in this work, the arc plasma spectrum from other gas conditions was also observed for mechanism comparison. Plasma spectra of different gases and electrodes are observed in Fig. 2a–d, with the chamber containing 90% by volume of the respective feed gas. As a result, oxygen spectrum vanished in the carbon dioxide atmosphere, and carbon dioxide spectrum disappeared in the oxygen atmosphere. All spectra displayed an identical carbon peak, indicating graphite ionization. Each peak in Fig. 2a–c was the reference for peaks of ionized atoms in Fig. 2d. The absence of titanium spectrum peaks in Fig. 2c and Figure S2b suggested that titanium ionization might not occur without carbon dioxide. In contrast, Fig. 2d shows the presence of various spectrum peaks for carbon dioxide, carbon monoxide, carbon, oxygen, nitrogen, and titanium in arc plasma (Kuroda et al. 2005; Rezaei et al. 2014; Hrycak et al. 2014; Kostrin et al. 2017; Ray et al. 2017). Ionization of titanium and carbon spectrum peaks in arc plasma was crucial for titanium carbide formation. The spectrum suggested that microwaves could sufficiently provide sufficient energy to the plasma to break carbon dioxide and titanium bonds. Figure 2d displays multiple ionized titanium and carbon peaks, facilitating the completion of the titanium carbide formation reaction (Eq. 1). The reaction product also included oxygen as carbon was extracted from carbon dioxide.

Fig. 2
figure 2

Arc plasma spectrum: a arc plasma of graphite carbon (C) in oxygen (O2), b arc plasma of graphite in carbon dioxide (CO2), c arc plasma of titanium in oxygen, d arc plasma of titanium in carbon dioxide. Nitrogen (N2) also exists in the plasma

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Titanium carbide for gas sensing application

Titanium carbide, a by-product in the carbon dioxide elimination process, was investigated for its potential as a gas sensor. In Fig. 3a, a rectangular titanium carbide piece which obtained from microwave plasma formation was assembled with a three-dimensional printed carbon interdigitated sample, forming a gas sensor chip. The chip, composed of a polylactic acid filament-printed substrate and interdigitated leads, underwent experimental testing with various gases shown in Figure S13 and Fig. 3b. The three-dimensional printed sensor demonstrated exceptional selectivity for liquefied petroleum gas, with the highest signal response. This unique liquefied petroleum gas sensor, utilizing waste carbon dioxide and carbon nanotube three-dimensional printing, showed promising results in terms of selectivity. Figure 3c illustrates varying resistance increasing rates over time at different liquefied petroleum gas concentrations, while Fig. 3d shows higher sensitivity with increased liquefied petroleum gas concentration. The scalability of the sensor is feasible with mass production of the carbon nanotube three-dimensional printed array, considering the potential mass production of titanium carbide with a cost under 10 USD. Further fabrication details are available in the supplementary material.

Fig. 3
figure 3

a Three-dimensional printed interdigitate and titanium carbide integration for a sensor chip, b resistant response of sensor chip with liquefied petroleum gas, c resistance of liquefied petroleum gas with different concentrations; % represented percent by volume, d sensitivity of liquefied petroleum gas with different concentrations. (% volume) represented percent by volume. Rg/Ra stands for the sensor resistance ratio of liquefied petroleum gas and air, respectively

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Characterization of titanium carbide

The proves of titanium carbide formation and extensive investigation of titanium carbide properties were implemented using various techniques, including scanning electron microscopy, transmission electron microscopy, X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), UV–Vis absorption, photoluminescence (PL), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), microhardness, and scratch tests. The rough surface of titanium carbide resulted from carburization, introducing carbon into the structure. XRD indicated characteristic crystallography of titanium carbide, while EDS and Raman spectroscopy explained carbon incorporation. XPS provided information on carbon characteristics, confirming carbide formation. UV–Vis absorption and photoluminescence revealed optical properties. DSC and TGA demonstrated titanium carbide oxidization at high temperatures, transforming titanium carbide into oxide. Microhardness and scratch tests illustrated its mechanical properties as an exceptionally hard material as detailed in the supplementary material (Figure S3–S7).

The study further explored titanium carbide applications, including the possibility of reimbursing carbon dioxide and pure metallic titanium, and electro-fuel possibilities, as detailed in the supplementary material (Figure S8–S12). The findings highlighted the versatility of titanium carbide and its potential beyond carbon dioxide elimination.

Conclusion

While compressing carbon dioxide into underground rock layers to conclude the carbon cycle is another challenging idea, since carbon dioxide could be possibly leaving the rock layer one day, our study introduces an alternative method. We suggest ending the carbon cycle by converting carbon into metal carbide, specifically titanium carbide in this work. The demonstrated technique utilizes atmospheric microwave plasma with a metallic arc, operating without the need for a vacuum system. The carbon dioxide reduction rate is rapid, as indicated by exponential regression, implying an extremely fast elimination of carbon dioxide in which carbon atoms are trapped in the form of carbide. If the input is carbon dioxide, the by-products of this reaction under microwave plasma include titanium carbide, carbon monoxide, and oxygen, as carbon is captured by titanium. Although microwave plasma can be generated by various metallic materials, such as iron (Fe), gold (Au) and platinum (Pt), titanium proves stable and selectively effective for carbide formation. Titanium sacrificially forms titanium carbide, a highly conductive refractorily ceramic material.