Abstract
A novel approach of titanium nitride (TiN) incorporated into SBA-15 framework was developed using one-step hydrothermal synthesis method. TiN contents up to ~18 wt% were directly dispersed in a synthetic gel under a typical strong acidic condition. The physico-chemical characteristics and the surface properties were investigated by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), N2 adsorption-desorption, field emission scanning electron microscope (FESEM) equipped with energy dispersive X-ray spectroscopy (EDS), wavelength dispersive X-ray fluorescence (WDXRF) and CO2-temperature programmed desorption (CO2-TPD). The results indicated that the highly ordered mesostructured was effectively maintained with high specific surface area of 532–685 m2g−1. The basicity of the modified SBA-15 increased with rising TiN loading. These modified materials were applied as a support of Ni catalyst in dry reforming of methane (DRM). Their catalytic behavior possessed superior conversions for both CO2 and CH4 with the highest H2/CO ratio (0.83) as well as 50 % lower carbon formation, compared to bare SBA-15 support.
Introduction
Mesoporous silica materials such as SBA-15 are those with a uniform, well-ordered structure with a pore diameter in the range of 2–50 nm (2–15 nm for SBA-15). The most prominent characteristics, namely, high surface area, high thermal stability, and thick pore walls, make them utilized efficiently in many applications i.e. adsorption, sensing technology, catalysis, and drug delivery systems [1], [2], [3], [4], [5], [6]. Many research attempts have focused on the development of mesoporous silica by designing the preferred properties and using in a wide range of applications. Thus, the ability to attach a variety of functional groups into mesoporous silica materials is interesting and challenging. The highlight is not only to maintain the primary structure but also present new characteristics to the fabricated material.
In the recent years, remarkable progress has been achieved on the studies of transition metal nitrides properties which can behave like platinum metals or noble metals because the similarities in the electronic structure for most of the reactions [7], [8], [9]. Titanium nitride (TiN) is one of the important technological materials that possesses many advantages such as high melting point, excellent thermal and chemical stability, high oxidation resistance, low costs and high availability [10], [11], [12]. TiN is not only widespread on protective coverings, it has been used in the catalytic applications, such as oxidation, alkylation and hydrogenolysis [13], [14], [15], [16]. Interest in TiN and its applicability has increased with a highly desirable to modify and improve its performance for various usages.
Owing to these unique physicochemical properties of mesoporous silica and TiN, a combination of both materials has attracted immense attention. The benefits of complement their extensive traditional utilization can provide alternative noble metal free materials. To our knowledge, just a few works have reported a several-step procedure for the synthesis of TiN incorporated in SBA-15 mesoporous silica [17]. In this work, TiN-inserted ordered mesoporous silica (TiN–SBA-15) is prepared by a facile one-pot synthesis. The initial substance is non-hazardous, no additional treatment is required, that can reduce auxiliary substances, and the process is safe and inexpensive which is efficient in the economy and energy conservation viewpoints.
It is quite well-known that the supported noble metal catalysts contributed excellent catalytic activity and less coking for syngas production in dry reforming of methane reaction (DRM) (CH4+CO2→→2CO+2H2). However, they cannot be taken into consideration for industrial-scale production because of their relatively high cost and limited availability. Many efforts have been made to achieve high-performance Ni-based catalysts with durability but lower cost comparable to those of noble metals [18]. Therefore, the modification of TiN–SBA-15 as non-noble metal composites could apply to support Ni catalyst for DRM reaction. However, there are few studies about using noble metal-free to increase the performance and endurance of Ni-based catalysts in DRM such as NiMo2C/Al2O3, Co3Mo3N, Ni/Si3N4 and Ni/BN@mSiO2 [19], [20], [21], [22].
In this work, we report a convenient and green route for the synthesis of TiN–SBA-15 with various TiN concentrations (5–18 wt%). The synthesized materials successfully retained their mesostructure along with a well co-condensation of TiN and silica species. To our knowledge, this work is among the pioneer reports about one-step synthesis with catalytic performance test on DRM of these materials. In DRM testing, TiN–SBA-15 supported Ni catalyst, exhibited high catalytic activity and superior material stability over pure SBA-15 support. This reveals the feasibility to enhance the performance of Ni-based catalysts in DRM which may apply in other related catalytic reactions. It is a promising prospect for the sustainable development of noble-metal-free catalysts in practical purposes.
Experimental
Catalyst preparation
A mesoporous SBA-15 was prepared according to the method described by Zhao et al. [23]. The TiN–SBA-15 was synthesized using a modified hydrothermal method of the typical synthesis of SBA-15. Different amounts of the TiN powder (particles size <3 μm, Sigma-Aldrich) were dispersed in acidic micellar solution of P123 (EO20PO70EO20, Mn~5800) after the addition of tetraethyl orthosilicate (TEOS). Next, the mixture was vigorously stirred at 35°C for 20 h and aged at 90°C for 24 h; then, a similar procedure of the typical synthesis of SBA-15 was used. The organic template of as-synthesized TiN–SBA-15 was successfully removed by calcination at 550°C under N2 atmosphere for 4 h. The obtained TiN–SBA-15 with TiN loading of 5, 10 and 18 wt%, were denoted as S1, S2 and S3, respectively.
Material characterizations
For structural characterization, X-ray diffractograms were recorded on a Rigaku DMAX-2200 Ultima+ Diffractometer. The crystal structure was collected on a Panalytical X’Pert Pro diffractometer, and then, a Rietveld-based quantitative analysis was done, using the Topas3 program to calculate the amount of TiN/TiO2. The X-ray photoelectron spectroscopy (XPS) was performed on a JEOL JPS-9010MC with an Al Kα (1487 eV) X-ray source to identify the incorporation of TiN on the silica surface. The FTIR KBr-pellet technique was measured on Nicolet 6700 for study of the surface functional groups. Nitrogen adsorption-desorption was done by using a BELSORP-mini instrument at 77 K to obtain the surface area values. The morphology of the material was observed using JEOL JSM-7610F FESEM equipped with an EDS. The elemental analysis was performed using Bruker S8 Tiger WDXRF. The microstructure of the specimens was investigated using JEOL JEM-2010 TEM at an accelerated voltage of 200 kV. For the study of CO2 adsorption capacity, the temperature-programmed desorption (TPD) of CO2 was carried out by using Thermo Finnigan, TPDRO 1100. The catalyst was treated at 400°C in N2 for 1 h before raised up to 950°C (10°C/min) and annealing in CO2 to saturate their surface for 0.5 h. Then the samples were flushed with helium at a flow rate 30 mL/min. The amount of CO2 desorption was measured and recorded as a function of the temperature.
Catalytic performance test
Toward practical application in DRM, SBA-15 and TiN–SBA-15 supported 7.5 wt% of Ni-based catalysts were prepared by wet impregnation method using an aqueous solution of Ni(NO3)2·6H2O. The catalysts without and with TiN loading were nominated as Ni/SBA-15, Ni/S1, Ni/S2 and Ni/S3. The prepared catalysts were dried in air at 80°C before calcined in N2 atmosphere at 500°C for 3 h. The measurement of catalytic performance was carried out in a fixed bed 316L stainless steel reactor (1/4″ ID and 50 cm in length) with the molar ratio of CH4/CO2=1, at atmospheric pressure and 700°C. Prior test, the catalyst was reduced in 10% H2 at 700°C for 1 h. The gaseous reactants and products were investigated by an online gas chromatograph (PR2100) with thermal conductivity detector (10″×1/8″ SS Hayesep Q 80/100). After reaction, the amount of coke deposition on the catalysts was determined by Thermogravimetric Analyzer (209 F3 Tarsus). However, detailed of catalyst structure and catalytic behavior were described in our previous publication [24].
DRM==CO2+CH4→→2H2+2CO
%CH4 conversion=100×([CH4]in–[CH4]out)/[CH4]in
% CO2 conversion=100×([CO2]in–[CO2]out)/[CO2]in
% H2 yield=100×1/2[H2]out/[CH4]in
% CO yield=100×[CO]out/([CH4]in+[CO2]in)
H2/CO==H2 yield/CO yield
Results and discussion
Structural characterization
X-ray diffractograms at low 2θ region of SBA-15 and TiN–SBA-15 with various TiN concentrations (sample S1–S3) were shown in Fig. 1a. All materials exhibited three well-resolved diffraction peaks, which can be indexed to (100), (110), and (200) of the mesoporous material structure. These peaks corresponded to the well-ordered structure of two-dimensional hexagonal space group (P6mm) of the SBA-15 characteristics [23]. However, the diffraction peaks of TiN–SBA-15 showed a significant reduction in the intensity and shifted toward a higher angle with increasing TiN loading. This indicates a slightly decreasing of lattice parameters of mesoporous TiN‒SBA-15 samples. It is caused by the dilution of the TiN content with the silica oxide framework on the order of the mesoporous structure [25]. However, those TiN‒SBA-15 can still maintain mesoscopic pore structure of SBA-15.
X-ray diffractograms of pure SBA-15 and TiN–SBA-15 at (a) low 2θ region and (b) 2θ between 10° and 80°.
Figure 1b showed the X-ray diffractograms at 2θ between 10° and 80° of TiN–SBA-15 with various TiN concentrations. The broad peak around 22° was the typical signature peak of amorphous silica for all the samples [26]. The diffraction peaks can be assigned to (111), (200), (220), (311), and (222) planes of TiN (PDF-01-071-0299). High intensity and sharp peaks are related to high TiN loading, confirming the residence of TiN on the surface of SBA-15. Moreover, the apparent of small amount of rutile TiO2 (PDF-01-072-1148) was detected. This is due to the oxidation of TiN during calcinations process at high temperatures [16].
The EDS elemental mapping illustrated in Fig. 2a–c indicated the presence of Si, O, and Ti on the prepared TiN‒SBA-15 samples. The amount of TiN loading was calculated from the wt% of Ti, which determined by WDXRF analysis as shown in Table 1. The aggregation of TiN increased with increasing TiN amount, as seen in Fig. 2c. The weight fraction TiN/TiO2 of the prepared TiN–SBA-15 samples were calculated using the Topas3 software, also shown in Table 1. It was found that highly dispersed TiN particles could be easily oxidized to form TiO2. This was confirmed by the TiN/TiO2 weight ratio, which was almost equal to 1/1 of sample S1 and S2 but nearly 3/1 for sample S3. Therefore, the increase of TiN amount has a tendency to aggregate on the surface of the prepared TiN–SBA-15 samples.
SEM-EDS elemental maps of TiN modified SBA-15 (a) sample S1 (b) sample S2 (c) sample S3.
Textural parameters of SBA-15 and TiN modified SBA-15.
| Sample | Surface area SBET (m2g−1) | Pore volume Vt(BJH) (cm3g−1) | Pore diameter D(BJH) (nm) | Theoretical loading of TiN (wt%) | Composition By WDXRF (wt% of Ti) | TiN/TiO2 wt% (Topas3) |
|---|---|---|---|---|---|---|
| SBA-15 | 703 | 1.1 | 8.2 | – | – | – |
| S1 | 685 | 0.8 | 7.2 | 4.9 | 3.8 | 44:56 |
| S2 | 592 | 0.6 | 7.1 | 9.3 | 7.2 | 43:57 |
| S3 | 532 | 0.5 | 7.1 | 17.9 | 13.9 | 74:26 |
The XPS was used to investigate Ti and O of TiN–SBA-15 samples and referenced TiN particles to confirm the incorporation of TiN on the silica surface. The XPS were calibrated based on the C1s peak at 284.8 eV. Figure 3a showed the Ti 2p XPS spectra by comparison with the previous study. For pure TiN, the peaks can be defined to both TiN and TiO2 phases; TiN (Ti 2p3/2: 455.7 eV, Ti 2p1/2: 460.0 eV) and TiO2 (Ti 2p3/2: 458.0 eV, Ti 2p1/2: 464.0 eV) [27], [28], [29]. In case of TiN–SBA-15 samples (S1–S3), only the TiO2 phase was identified. The shift toward lower binding energy of TiN–SBA-15 sample at Ti 2p3/2 (from 458.4 eV to 458.2 eV) and Ti 2p1/2 (from 464.2 eV to 463.9 eV) was observed. This can be due to the gathering of TiN particles that against the oxidation. High loading of TiN was aggregated whereas the small amount was distributed and easily oxidized to form TiO2 as revealed by the EDS results. From Fig. 3b, the native oxide O1s peaks of TiN at 531.0 eV and 529.0 eV, can also be noticed. This suggests the general formation of oxide species on the surface of TiN particles [30]. A peak at about 533 eV can be assigned to Si–O–Si whereas a peak at around 529 eV is belonging to Si–O–Ti, indicating titanium was incorporated into SBA-15 framework [30], [31], [32]. It should be noted that the latter peak intensity was increased with an increasing of the titanium concentration [33].
XPS spectra of (a) Ti 2p and (b) O 1s of pure TiN and TiN–SBA-15.
The FTIR spectra of both SBA-15 and TiN–SBA-15 were shown in Fig. 4. All samples demonstrated three characteristics peaks of SBA-15 at 1089, 806, and 470 cm−1, which can be attributed to the condensed silica (Si–O–Si and O–Si–O bending vibration). The peak shoulder around 970 cm−1 was indicated as the stretching vibration of the silanol group (Si–OH) [34], [35]. The O–H bending and stretching vibrations of the adsorbed water were indexed at 1640 cm−1 and 3460 cm−1, respectively [35]. The characteristic peak at 1380 cm−1 of pure TiN was used as a fingerprint to prove the existing incorporated TiN in the SBA-15 matrix, which could not be found in the normal SBA-15 spectrum [11]. This is also another proved evidence of TiN containing in SBA-15 structure.
FTIR spectra of TiN–SBA-15 with various TiN concentrations (S1–S3), in comparison with pure TiN and pure SBA-15.
The nitrogen adsorption–desorption isotherms of the synthesized samples, shown in Fig. 5a. The graph represented a type IV isotherm with an H1-hysteresis loop, indicating that the incorporation of TiN preserved the mesoporous structure of SBA-15. However, loading TiN into siliceous SBA-15 resulted in a reduction of surface area, pore diameter and pore volume of pure SBA-15 as indicated in Fig. 5b and Table 1. The adsorption step of TiN modified SBA-15 (sample S1–S3) also occurred at lower P/P0 than that of pure SBA-15. This showed the increasing amount of TiN can influence the SBA-15 framework by partial pore blocking and develop more disordered structure which was consistent with the result from XRD.
(a) N2 adsorption-desorption isotherms and (b) pore size distributions of SBA-15 and TiN–SBA-15.
About the microstructure of TiN–SBA-15, the scanning and transmission electron micrographs of sample S3 were shown in Fig. 6a and b, respectively. In Fig. 6a, the scanning electron micrograph revealed a rod-liked structure, with a short particle size of ca. 0.6 μm, which was the typical shape of SBA-15. Transmission electron micrograph in Fig. 6b showed TiN clusters condensed on the external surface of SBA-15, which can be identified by EDS. Thus, these small clusters may cause the partial pore blockage of the mesoporous which was consistent with the N2 adsorption–desorption results. This micrograph also confirmed that the appropriate amount of incorporated TiN did not disrupt the microstructure of primary mesoporous structure of SBA-15.
(a) Scanning electron and (b) transmission electron micrographs of TiN–SBA-15 (Sample S3).
Moreover, TiN‒SBA-15 exhibited total quantity of CO2 adsorption higher than pure TiN and SBA-15 as indicated in Table 2. The CO2 adsorption capacity increased with increasing TiN loading (sample S3>S2>S1) as shown in Fig. 7. This is essential information to prove that the basicity of the prepared materials was increased by the combination of both TiN and SBA-15.
CO2 adsorption capacity of TiN, SBA-15 and TiN-SBA-15.
| Sample | CO2 adsorbed on different basic sites (μmol/g) |
||
|---|---|---|---|
| Weak (∼120°C) | Strong (500–950°C) | Total | |
| TiN (pure) | 2.6 | – | 2.6 |
| SBA-15 | 62.4 | 50.6 | 113.0 |
| S1 | 27.8 | 133.4 | 161.2 |
| S2 | 47.6 | 191.8 | 239.4 |
| S3 | 64.0 | 298.8 | 362.8 |
CO2-TPD profile of TiN (pure), SBA-15, S1, S2 and S3.
Catalytic performance
The feasibility of using TiN–SBA-15 supported Ni catalyst in DRM reaction was investigated, in comparison with Ni catalyst on SBA-15 (Ni/SBA-15). The catalytic performance result of all the prepared catalysts was exhibited in Fig. 8a–c. The measurement results showed that all TiN‒SBA-15 supported Ni catalysts (Ni/S1, Ni/S2 and Ni/S3) had higher conversions of CO2 and CH4 and higher H2/CO than that without TiN (Ni/SBA-15). The catalysts with TiN–SBA-15 support also possessed high stability during the reaction period of 4 h, CH4 conversion decreased by 4.3%, 1.7% and 2.0% for Ni/S1, Ni/S2 and Ni/S3, respectively, while CO2 conversions decreased 1.5% for Ni/S1 but increased 0.8% and 2.7% over Ni/S2 and Ni/S3, respectively. A gradually decrease in both CO2 and CH4 conversions was obviously noticed on unmodified SBA-15 support, being by 44.3% and 49.7%, respectively.
Catalytic performance (a) CO2 conversion (b) CH4 conversion and (c) H2/CO ratio.
The H2/CO ratio from all catalysts was also found less than 1. These were due to the occurrence of reverse water gas shift (RWGS) reaction [36]. However, the modified supports showed H2/CO ratio higher than unmodified support, especially the highest H2/CO ratio (0.83) close to 1 which obtained from sample Ni/S3. Interestingly, the incorporation of TiN showed the positive effect to improve the catalytic performance of Ni-based catalysts similar to the results that using Ru, a noble metal as a promoter which reported by Yasyerli et al. [37].
It can be concluded that TiN can enhance and prolong the catalytic performance also the lifetime of Ni catalyst. This ascribed to the presence of the basicity by TiN incorporation in SBA-15 framework which can adsorb CO2 and promote the Boudourd reaction leading to low carbon deposition on the catalyst [36], [38]. In addition, the effects of geometric and electronic structures of two active metals are highly associated with good catalytic performance [39], [40]. Suggesting by a different in catalytic performance of TiN−Ni over TiO2−Ni in hydrogenolysis reaction which contributes towards metal-support electronic interactions [41], [42]. The strong metal-support interaction of TiN−Ni was induced by TiN which significantly alter the property of material. The electron transfer between Ni and TiN suggests by the partial oxidation of TiN as well as the oxidative stability of Ni [16]. Thus, the improvement of Ni catalyst in this study could similarly occur by the incorporation of TiN.
After the reaction, TGA analysis was further performed to investigate the amount of coke formation during the reaction (see Fig. 9). The highest weight loss (72%) was obtained from Ni/SBA-15, indicating the severe deactivation of the catalyst which was related to low catalytic activity after the first hour of the reaction. In case of Ni/TiN-SBA-15 catalysts, % weight loss was lower than Ni/SBA-15, in the order; Ni/S1 (40%)>Ni/S3 (35%)>Ni/S2 (21%). These catalysts did not present any evidences of deactivation, they were notably active towards the reaction time of 4 h. For this reason, the main point of Ni/SBA-15 catalyst deactivation was caused by the deposition of coke which was the results of methane decomposition and CO bifurcation reaction [37], [38].
TGA profile of the catalysts after 4 h reaction.
Compared to Ni/SBA-15 catalyst, Ni/TiN–SBA-15 catalyst exhibited better catalytic performance, stable activity and lower carbon formation. These results verified the positive effect of TiN containing SBA-15 structure. The addition of TiN not only increase the basicity of SBA-15 but also enhance the performance of Ni catalyst in DRM reaction. Therefore, TiN‒SBA-15 has a potential to apply in other related catalytic reactions as noble metal free catalyst.
Conclusions
TiN-modified SBA-15 mesoporous materials with various TiN contents were successfully confined in the SBA-15 framework by a convenient direct synthesis. The co-condensation of the TiN and the silanol groups during the gel formation could sustain the mesoporous structure and the morphology of SBA-15. The enhanced basicity was obtained from loading TiN into SBA-15 structure. The increase in TiN loading resulted in particle agglomeration, which consequently led to a decrease in the surface area and pore volume. The new material showed higher CO2 adsorption than pure TiN or bare SBA-15. This can provide the adsorption capacity of CO2 on Ni/TiN-SBA-15 catalyst. This one-step hydrothermal synthesis is a cost-effective and environmental-friendly method to fabricate a composite material of metal nitride-modified mesoporous silica. Moreover, the modified mesoporous TiN-SBA-15 supported Ni catalyst showed the improvement in the catalytic activity for DRM reaction. Thus, the basicity of catalyst is also increased which urged to convert Boudouard reaction and reduced carbon formation on Ni catalysts. The coke formation was diminished due to an increase in support basicity. High conversions of both CH4 and CO2, as well as maintaining H2/CO ratio were achieved due to higher catalytic stability. Hence, the presence of TiN offered a promising catalytic mesoporous support for Ni-based catalysts in DRM reaction and possibly for other reactions.
Article note
A collection of invited papers based on presentations at the 8th IUPAC International Conference on Green Chemistry (ICGC-8), Bangkok, Thailand, 9–14 September 2018.
Acknowledgment
This research has been supported by grants from Science Achievement Scholarship of Thailand (SAST), CU-56-912-AM, the International Research Integration: Chula Research Scholar, Ratchadaphiseksomphot Endowment Fund and the 90th Anniversary of Chulalongkorn University Fund.
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