Non-thermal plasma-assisted catalytic CO₂ conversion over Zn-TCPP 2D catalyst

The synthesis of ZnTCPP was performed using similar procedure to described in (Wiśniewski et al. 2019). Briefly, Zn(NO₃)₂ 6H₂O (POCh, 0.250 g) and TCPP (tetrakis(4-carboxyphenyl)porphyrin), TCI chemicals, 0.079 g) in a solvent mixture of DMF (N,N-Dimethylformamide, ACROS Organics™)/EtOH (POCh) 99.8% (15 ml/7.5 ml) were placed in a 30 ml screw-capped glass vial and mixed by about 15 min, until total dissolution of both reagents. Then the vial was placed into oven at 80 °C for 24 h. Purple with glitter precipitate was then obtained by filtration on nylon membrane filters (pore size 0.45 μm). The product was washed with 5 ml mixture of DEF (N N-Diethylformamide) and EtOH (2:1), and 3 times with 10 ml EtOH 96%. The solid was then evacuated at 50 °C and in vacuum for 24 h.

The full characteristic of ZnTCPP including: elemental and thermogravimetric analysis, SEM, PXRD was presented recently (Wiśniewski et al. 2019). The results are consistent with the results presented in the literature (Zhao et al. 2015) and correspond to a 2D chemical structure.

Additionally, material was characterized with scanning electron microscopy (SEM), using a Quanta 3D FEG (EHT = 30 kV) instrument and HRTEM to show its 2D structure.

Carbon dioxide adsorption–desorption measurements were performed at the temperature of 25 °C using a typical gravimetric adsorption apparatus equipped with the Baratron pressure transducers (MKS Instruments, Germany). Before the measurements sample was thermally desorbed (at 200 °C) under high vacuum (10^–3 Torr) until a constant mass was obtained (usually after 3 days).

CO₂ adsorption was performed under isobaric conditions (p(CO₂) = 25 hPa by flowing CO₂/He 2.5% gas mixture) changing the temperature of the process up and down from 25 to 250 °C, with a period of 1 h at each temperature (period of 25 °C) in order to assure that the equilibrium was achieved. A Praying Mantis in situ cell from Harrick Scientific Corporation was used as reactor for the infrared (IR) spectroscopic DRIFT studies (Fig. 1). The construction of this cell enables the thermal treatment of the powdered sample up to 600 °C in any controlled atmosphere or in a vacuum. The IR spectra for the samples were recorded (using Nicollet S10) at adsorption temperature, i.e. without cooling down to the room temperature. Spectral changes accompanying CO₂ adsorption were established by comparing IR spectra of the same sample recorded in a vacuum and those recorded under a defined adsorption temperature. The respective gas phase was the background for each spectrum. This simple operation enables observation of spectral changes of surface, without perturbation from the gas phase. Once the equilibrium was reached, the band at ca. 2340 cm⁻¹ was integrated (I). Experimental results were presented as I/I_max, where I_max is the band of maximum intensity taken at the lowest temperature. Thus the I/I_max value represents the experimental value of relative adsorption.

The in-situ FTIR studies were carried out in a vacuum cell, described previously (Zawadzki and Wiśniewski 2007, 2002; Wiśniewski 2014), plugged into the vacuum line (Fig. 1). The pressure of CO₂ was constant, and equal 0.38 Torr. The application of this technique makes it possible to determine in what extent physical or chemical adsorption occurs. Moreover, we can easily determine the absence or presence of any surface oxygen compounds formed by chemisorption. We can also, based on the spectra of tested material, check in what extent a material changes its structure during the process and in what extent a sample is affected by possible impurities. The construction of the in-situ FTIR cell enables the thermal treatment of the film sample up to 1200 K in any controlled atmosphere or in a vacuum. The spectra were recorded using the Mattson Genesis II FTIR spectrophotometer. Spectral changes accompanying the tested process were investigated. The respective gas phase was a background for each sample spectrum, enabling the observation of spectral changes of a sample surface only, i.e. without perturbation from the gas phase. A period of at least 0.5 h (monitoring continuously) at each power was held in order to be sure that the equilibrium was achieved.

The gases (CO₂ and CO) were analyzed quantitatively with using a multiple-pass gas cell (with the pathlength of 2 m) attached on-line to the flow system (Fig. 1).

The results from the SEM and HRTEM structure determination studies are presented in Fig. 2. Based on these results it can be concluded that ZnTCPP are built mainly of wafers/layers. Their square-shaped sheets visible in both SEM as well as HRTEM pictures are the reflection of organic part of the MOF. Interestingly HRTEM results reveal the square units with distance of 1.2 nm which is in perfect agreement with theoretical predictions. Moreover, porphyrin angles are extended to the micro-scale and observed as a flat sheets with right angles.

These results confirm our previous observations (Wiśniewski et al. 2019) and mean that Zn-TCPP forms 2D material with the distance between particular sheets ca. 0.5 nm based on XRD measurements.

Four nitrogen atoms as well as aromatic units in each porphyrin structure cause that the material is highly thermally stable, even in oxidative atmosphere. Thermogravimetric measurements show its stability up to ca. 400 °C in air (Wiśniewski et al. 2019). Additionally, these 4 N-atoms are the main reason of its Lewis-base properties, causing that CO₂ molecules should adsorb strongly.

To examine this idea adsorption isotherms were measured at temperature range of 25–100 °C. In parallel, DRIFT investigations were performed to discover the adsorption mechanism. The experimental adsorption data for pure CO₂ are shown in Fig. 3. As in the case of “ordinary” microporous materials, the adsorption of CO₂ is mainly caused by interactions in very narrow micropores.

The rise in adsorption temperature causes diminish in the adsorbed CO₂ amount, meaning that CO₂ does not chemisorb on the Zn-TCPP surface. The in-situ IR adsorption studies confirm this statement. The observed spectral changes are caused only by physically adsorbed CO₂.

The presence of adsorbed CO₂ in micropores of Zn-TCPP at slightly higher temperatures, its thermal stability and basic nature make the tested material perfect for CO₂-plasma treatment. It is well known from the literature (Snoeckx and Bogaerts 2017) that, in non-thermal plasmas the temperature of the electrons is the highest, followed by the temperature of vibrationally excited molecules, while the lowest temperature is shared by the ions and neutral species. The temperature of electrons is in the order of 1 eV (ca. 10 000 K), while the neutral species temperature being initially close to the room temperature can rise slightly due to the collision energy. Moreover, the phenomena generates radicals, ions and vibrationally and electronically excited species not normally present in at room temperature (Aerts et al. 2012; Christensen et al. 2018; Whitehead 2016).

Constructed analytical system (Fig. 1) makes it possible to observe substrates, intermediates as well as products of the plasma-assisted CO₂ conversion due to parallel, in-situ and ex-situ investigations. As the former reveal the presence of the unstable intermediates the latter show stable products enabling quantitative analysis. The results are collected in Fig. 4.

According to the literature (Fridman and Rusanov 1994), almost 97% of the CO₂, exposed to low temperature plasmas, can be converted into vibrational excitation of the gas feed molecules (CO₂*). It is confirmed clearly in Fig. 4, observed as a 63 cm^–1 red-shifting of CO₂ asymmetric stretching vibrations. The CO₂* molecules relax quite easily, being observed outside the plasma zone as typical CO₂ molecules (2349 cm^–1).

As it was expected, adding a catalyst speed up the chemical processes in a plasma and/or offer alternative mechanistic pathways with lower activation energies. Reactive species are produced and interact with catalyst at temperatures at which most thermally-driven process would be slow and impossible to be observed. Moreover, vibrationally excited molecules cannot have markedly different adsorptive properties. Their acid–base properties should remain constant contrary to the reactivity on the surface of catalysts also being in contact with plasma (Chen et al. 2008) thus it should be possible to enhance targeted reactions and suppress others by controlling the number density of electrons and electron and gas temperatures. That’s way, both catalyst addition as well as power increase (Fig. 4) shift the reaction forward, i.e. to CO formation.

The vibrationally excited form of the CO₂ ground electronic state, i.e. CO₂*, is generally accepted as being the intermediate in the plasma-induced reduction of CO₂ to CO and O₂ (Alliati et al. 2018). However, the analysis of Fig. 4 lead to the conclusion that without catalyst the process is highy inefficient. Thus the in-situ analysis of catalyst surface seem to be reasonable.

As it is shown in Fig. 5, CO₂ adsorption (without plasma) causes formation only physically adsorbed CO₂ species (2336 and 660 cm^–1). Contrary, on the Zn-TCPP in CO₂ plasma atmosphere new, interesting surface species appear. Their exact assigned needs further studies, nevertheless the presence of –C=O, –C–O, as well as their intermediates cannot be excluded due to the appearance of the IR bands in the region of 1650–800 cm^–1.

The wide band at ca. 2264 cm^–1 could be attributed to physically adsorbed CO₂*. The signals at 1605 and 1411 cm^–1 most probably are response for the presence of carbonate species in the pores. IR peaks observed at 1107 and 999 cm^–1 confirm the existence of –C–O.

Taking into accordance the above, the overall process could be summarized as follows (s denotes the surface active center):

Vibrationally excited CO₂ formed in Eq. 1 is observed directly in plasma zone (Fig. 4). Its adsorption on Zn-TCPP surface (Eq. 2) causes the appearance of broad bands below 2300 cm^–1 and the decomposition of this surface complex leads to CO liberation (Eq. 3). Formed thus surface oxides s–O can recombine, producing O₂ or adsorb CO₂ creating surface carbonates (Eqs. 4 and 5). Due to the fact that no CO physically adsorbed on the material surface was observed the last process (Eq. 7) should be much faster than the others. The simplified mechanism proposed above, includes that at temperatures as low as 100 °C under high vacuum all formed surface species should be desorbed.