Currently, in situ transmission IR spectroscopy (TIRS), diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, and Fourier transform infrared attenuated total reflection (FTIR/ATR) spectroscopy are very popular tools in the study of heterogeneous catalysis. These techniques provide molecular- level information related to the surface active sites and the species adsorbed on heterogeneous catalysts ^[1-15]. However, all of these commercial FTIR spectrometers operate in the single- beam (SB-FTIR) mode, and require the measurement of reference (background) spectra to remove variations caused by changes in background absorption and instrument conditions ^[16]. They are not suitable for real-time study of the dynamic processes involved in gas/solid heterogeneous catalysis because they cannot collect the reference and sample spectra simultaneously. Another major problem with SB-FTIR spectrometers is that they are strongly influenced by gas-phase molecular vibration and heat irradiation at high temperatures. However, there is an urgent need for real-time surface characterization methods in the field of heterogeneous catalysis ^[17]. This is because the fundamental understanding of heterogene-ous catalytic processes, and thus the rational design of advanced catalysts, requires information related to the transient species involved in the reaction. These transient species can be captured only under real reaction conditions. In principle, a dual-beam Fourier transform infrared spectrometer (DB-FTIR) could simultaneously eliminate the influence of both gas-phase molecular vibration and heat irradiation at real reaction temperatures. This technique is therefore expected to be a useful operando characterization method for heterogeneous catalysis.

Kuehl, Dignam and Debreczeny et al ^{[16, 18, 19]} have tried to develop a DB-FTIR spectrometer by redesigning the optical layout of a commercial spectrometer to simulate dual-beam conditions. However, this is too difficult to be of practical use. To the best of our knowledge, the application of DB-FTIR in heterogeneous catalysis has not been reported until now.

Here, we report for the first time a facile method for the development of an operando DB-FTIR spectrometer. First, a special high-temperature IR reactor cell with two identical cells (for the reference and sample) was designed. This IR reactor cell enables the simultaneous collection of reference spectra, which result from gas-phase molecular vibration and heat irradiation, in the reference cell (without catalyst), and of sample spectra, which result from not only gas-phase molecular vibration and heat irradiation but also the catalyst surface species, in the sample cell (with catalyst). Then, two identical commercial infrared spectrometers were coupled together to construct a DB-FTIR spectrometer. The DB-FTIR spectrometer can perform time-resolved collection of both reference and sample spectra, and thus produce real-time spectra of the catalyst surface species under real heterogeneous reaction conditions (continuous gas flow and temperatures of up to 550 ℃) by simultaneous subtraction of the reference spectra from the sample spectra.

In this study, the IR reactor cell was developed to have identical CaF₂ windows to ensure total elimination of the background interference (Fig. 1). Two Nicolet 10 s infrared spectrometers equipped with mercury cadmium telluride (MCT) detectors were used to construct the DB-FTIR spectrometer (Fig. 2). Special software was designed to ensure that the two spectrometers collected the spectra simultaneously. The catalysts were pressed into self-supporting thin wafers (1 cm²) that were placed in the sample beam, and the reference beam was vacant. The sample was activated in the IR reactor cell at 400 ℃ for 4 h under vacuum (10⁻³ Pa), and the spectra were recorded at a resolution of 4 cm⁻¹ with 64 scans in the region of ṽ = 4000-1000 cm⁻¹. The intensities of the reference and sample beams were adjusted to the same level.

First, the equivalence of the reference and sample cells of the in-situ reactor, and the synchronism of the two spectrometers making up the DB-FTIR instrument were confirmed under working conditions by a blank test, as shown in Fig. 3. The spectra in Fig. 3 indicate that, in the absence of the catalyst, the differential spectra of the sample and reference cells were essentially straight lines, independent of the temperature of the cells (r.t. or 400 ℃) and the atmosphere (still air or a flowing mixture of isobutene and nitrogen gas) used.

To demonstrate the advantages of the DB-FTIR instrument as an operando spectrometer, an ordinary SB-FTIR spectrometer was compared with the DB-FTIR spectrometer by studying isobutene and isobutane adsorptions on nano-sized HZSM-5 zeolite (SiO₂/Al₂O₃ molar ratio = 26, crystal size = 20-50 nm), as shown in Figs. 4 and 5. Fig. 4 shows that during the adsorption of isobutene, the DB-FTIR spectrum contained only four obvious absorption bands, which could be attributed to the species adsorbed on the surface of the zeolite. According to literature ^[13], the bands at 2959 and 2873 cm⁻¹ are attributed to the asymmetric and symmetric C-H stretching vibrations of the -CH₃ group, respectively. The band at 2934 cm⁻¹ is associated with the asymmetric C-H stretching vibration of the -(CH₂)- group, and the band at 2860 cm⁻¹ is attributed to the symmetric C-H stretching vibration of the -(CH₂)- group. In contrast, when the ordinary SB-FTIR spectrometer was used, the obtained spectrum exhibited interference from gas-phase molecular vibration absorptions. This interference included not only weak absorptions in the region of 3072-3099 cm⁻¹, but also very strong absorptions in the region of 2800-3000 cm⁻¹; the latter strong absorptions were directly overlaid on the absorption bands of the surface species. Similar results were observed with the DB-FTIR and SB-FTIR spectrometers when isobutane was used as the adsorbate. However, adsorption of the less basic isobutane is more difficult than adsorption of isobutene on the protonic zeolite, so the surface-adsorbed isobutane afforded considerably weaker IR absorption bands at 2960, 2933, 2872, and 2860 cm⁻¹, as shown in the DB-FTIR spectrum in Fig. 5. Consequently, when the ordinary SB-FTIR spectrometer was used, the weak absorptions of the surface species were completely overwhelmed by the absorptions from gas-phase molecular vibrations. These two examples thus indicate that, owing mainly to the influence of the gas-phase molecular spectrum, it is difficult to obtain the correct spectrum of the catalyst surface species under real reaction conditions using an ordinary SB-FTIR spectrometer. In contrast, the spectrum is easily obtained from the DB-FTIR spectrometer because the influence of the gas-phase molecular vibrations is completely eliminated by simultaneous subtraction of the reference spectrum.

To examine the possibility of using the DB-FTIR spectrometer in a real-time operando study, it was used to follow the dynamic process of isobutene adsorption on nano-sized HZSM-5 at 150 ℃. Time-resolved spectra were collected at intervals of 1.27 min. Fig. 6 shows that at the beginning of the adsorption (first 4 min), three absorption bands were observed on the catalyst surface. The band at 3610 cm⁻¹ is attributed to the bridging hydroxyl group (Si(OH)Al), that at 3726 cm⁻¹ to free internal silanol (SiOH), and that at 3740 cm⁻¹ to isolated external SiOH ^[20]. After 6.35 min, the band corresponding to the bridging hydroxyl group disappeared, and five new bands appeared. The new bands at 2956, 2933, 2872, and 2860 cm⁻¹ are attributed to C-H stretching vibrations. As mentioned above, the bands at 2956 and 2872 cm⁻¹ are respectively attributed to asymmetric and symmetric C-H stretching of the -CH₃ group, whereas those at 2933 and 2860 cm⁻¹ are assigned to the asymmetric and symmetric C-H stretching vibrations of the -(CH₂)- group, respectively. The band at 1467 cm⁻¹ is attributed to the C-H bending vibration of the -(CH₂)- group, and that at 1458 cm⁻¹ to the asymmetric C-H bending vibration of the -CH₃ group. With an increase in the adsorption time from 6.35 to 8.89 min, the intensities of these six absorption bands increased slightly. However, increasing the adsorption time beyond 8.89 min did not lead to any further changes in the intensities of the bands, and the isobutene adsorption can be considered to have reached equilibrium. There is thus no doubt that the operando DB-FTIR spectrometer can be used to study dynamic surface processes in gas/solid heterogeneous catalysis under real conditions.

Furthermore, the operando DB-FTIR spectrometer was used to study isobutene transformation over the nano-sized HZSM-5 zeolite at 300 ℃. Fig. 7 shows the spectra of the surface species on the catalyst in the first 15.24 min of the reaction (spectra collected at 1.27-min intervals). The bridging hydroxyl groups can be observed in the first 3.81 min, but their intensities clearly decrease with time. In the 3000-3100 cm⁻¹ region, three bands were observed at 3080, 3085, and 3092 cm⁻¹. These bands are ascribed to the =C-H stretching vibration of an aromatic ring ^[13]. In addition, an absorption band consisting of many overlapping signals, and thus caused by many different adsorbates, appears in the C-H stretching region (2800-3000 cm⁻¹). This band is much stronger than the bands observed between 3000 and 3100 cm⁻¹. There are three major absorption bands in the C-H bending region (1400-1600 cm⁻¹). Among these, the band at 1505 cm⁻¹ is related to the aromatic ring ^[13]. These spectra suggest that isobutene aromatization occurred very easily over the nano-sized HZSM-5 zeolite, even though the reaction temperature was as low as 300 ℃.

Fig. 8 shows the spectra of the surface species on the catalyst over a wide time span of 0-50 min. As the reaction time increases, the intensities of the absorption bands related to the aromatic ring, namely the C-H stretching vibrations at 3070, 3085, and 3091 cm⁻¹, and the C-H bending vibration at 1505 cm⁻¹, gradually increase. This means that isobutene aromatization over nano-sized HZSM-5 zeolite was enhanced at prolonged reaction time. Furthermore, the highly overlapped absorption band in the C-H stretching region at 2800-3000 cm⁻¹ becomes stronger and wider as the reaction time increases. This band is most likely ascribed to various aromatic intermediates, which implies that multiple intermediates in the aromatization of isobutene were observed at the same time by the DB-FTIR spectrometer. These intermediates might help to establish a pool of precursors for the formation of aromatics. The observation of these intermediates agrees with the accepted viewpoint that the aromatization of olefins proceeds through a number of intermediates and carbenium ion transition states. This process involves the oligomerization-cracking of small olefins to give C₄-C₁₀ olefins, hydrogen transfer from the bigger olefins to the smaller olefins (mainly C₂-C₃) to give dienes, cyclization of the dienes to give cycloalkenes, and finally hydrogen transfer from the cycloalkenes to the small olefins to form the aromatic products ^[21-25]. Development of a precursor pool is likely to accelerate the formation of aromatics.

As expected, when the DB-FTIR spectrometer was replaced with an ordinary SB-FTIR spectrometer to study the same isobutene transformation process, no valuable information was obtained concerning the surface changes (Fig. 9). In addition to the interference from the gas-phase molecular vibration absorptions, a strong effect of the heat irradiation on the bending vibrations of the C-H bonds (1400-1600 cm⁻¹) was also observed at the high reaction temperature (300 ℃).

In summary, an operando DB-FTIR spectrometer has been developed for the first time using a facile method. This spectrometer is expected to be a powerful tool for the in-situ study of heterogeneous catalysis. It is very convenient for eliminating the effects of gas-phase molecular vibration and heat irradiation, and can provide important information on surface active sites, intermediate or spectator species and the reaction mechanism under real reaction conditions. Further work on the dynamic processes of several important reactions involving heterogeneous catalysis, including the aromatization of short-chain alkanes over HZSM-5 and Zn/HZSM-5 zeolite, is currently underway in our group.