Evaluation of CO2 and H2O Adsorption on a Porous Polymer Using DFT and In Situ DRIFT Spectroscopy

Numerous hyper-cross-linked polymers (HCPs) have been developed as CO2 adsorbents and photocatalysts. Yet, little is known of the CO2 and H2O adsorption mechanisms on amorphous porous polymers. Gaining a better understanding of these mechanisms and determining the adsorption sites are key to the rational design of improved adsorbents and photocatalysts. Herein, we present a unique approach that combines density functional theory (DFT), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and multivariate spectral analysis to investigate CO2 and H2O adsorption sites on a triazine–biphenyl HCP. We found that CO2 and H2O adsorb on the same HCP sites albeit with different adsorption strengths. The primary amines of the triazines were identified as favoring strong CO2 binding interactions. Given the potential use of HCPs for CO2 photoreduction, we also investigated CO2 and H2O adsorption under transient light irradiation. Under irradiation, we observed partial CO2 and H2O desorption and a redistribution of interactions between the H2O and CO2 molecules that remain adsorbed at HCP adsorption sites.

where m a represent the atomic mass of an element. S3

Section S2 -Vibrational analysis and band assignment
To assign the triazine-biphenyl HCP vibrational bands one first needs to understand how intermolecular interactions impact the infrared spectrum of a triazine molecule and that of a biphenyl molecule. To do this, we evaluated the effect of triazine intermolecular interactions on its infrared spectrum by gradually increasing the number of triazine interactions in the computer simulations. The latter was carried out by computing the infrared spectra of one and six triazine molecules. Given the increasing computing cost, we studied the effect of more intermolecular interactions experimentally by collecting the ATR-IR spectrum of an 80 nM triazine solution and that of crystallized triazine powder. We focused the initial vibrational analysis on the spectral region ranging from 1800 to 1000 cm -1 where interactions with the substrate dominate. We discuss the vibrational analysis of other spectral regions below.
As Figure S6a and Table S4 show, triazine intermolecular interactions generate prominent spectral changes. An increase in intermolecular interactions, specifically in the number of their configurations, gives rise to band broadening. Among the measured spectra, the triazine powder infrared spectrum displaying the broadest band shapes resulting from an increased number of dipole-dipole interactions between the amino groups present in the triazine monomer. When increasing the number of intermolecular interactions, we also observe various band shifts. Bands 1, 2, and 5 of Figure S6a, which correspond to NH 2 stretching vibrations coupled with C-NH 2 , C-C and C-N vibrations, according to the normal mode analysis (Table   S4), respectively, gradually shifted towards higher wavenumbers. Additionally, we note the appearance of new vibrational bands (bands 3 and 4) that correspond to N-H vibrations coupled with N-C=N vibrations. Overall, as Figures S6a shows, by gradually capturing the spectral implications of triazine intermolecular interactions, we can achieve a precise assignment of the triazine powder vibrational bands. As triazine powder is used as precursor for the HCP synthesis, the spectral features generated by strong intermolecular interaction will likely be present in the infrared spectrum of the HCP. A detailed assignment of the triazine powder vibrational bands can be found in Table S4.
To investigate the effect of the biphenyl intermolecular interactions on the IR spectral features, we followed a similar approach. The absence of strong polar groups in the biphenyl molecule and therefore, the lack of strong dipole-dipole intermolecular interactions, facilitated the assignment process. As Figure S6b shows, the simulated infrared spectrum of a single biphenyl molecule agrees well with the measured infrared spectrum of the biphenyl powder.
The vibrational band at 1088 cm -1 (band 3, Figure S6b and Table S5) corresponds to the stretching vibration of the methoxy group. We observe two new vibrational bands at 1718 and 1697 cm -1 (bands 1 and 2, Figure S6b). As both vibrational bands do not correspond to any of the ones present in the computed infrared spectra of a single biphenyl molecules, they likely originate from biphenyl intermolecular interactions. The broad band tailing and their location at around 1700 cm -1 point to C=O bond vibrations. We assign these two vibrational bands to the interaction of an aromatic carbon with an oxygen atom of a methoxy group present in another biphenyl molecule. A detailed assignment of the biphenyl powder vibrational bands can be found in Table S5.
The band assignment of the triazine and biphenyl powder, and the spectral implications of intramolecular interaction being confirmed, a precise analysis of the triazine-biphenyl HCP vibrational bands is now possible. As shown by Figure S6c, HCP exhibits characteristic vibrational bands of both triazine and biphenyl powder. Bands 1 and 2 of Figure S6c correspond to the vibrations of the intermolecular biphenyl C=O bond, while bands 3 to 9 correspond to the triazine component. Bands 3 and 4 correspond to NH 2 stretching vibrations coupled with C-N, and C-C ring vibrations. Lastly, band 10 corresponds to the vibration of unreacted methoxy groups arising from partially crosslinked biphenyl molecules. A more detailed assignment of the other HCP vibrational bands can be found in Table S6.
We note that all the aforementioned experimental infrared spectra were acquired using ATR-IR spectroscopy. As each infrared technique uses different sampling configurations, they will display different surface and bulk sensitivities and certain spectral features might differ. 7 To achieve a precise band assignment, one must compare spectra that are measured using identical IR techniques and necessary precautions needs to be taken when comparing spectra acquired using different sampling configurations, e.g., ATR-IR and DRIFTS. As Figure S7 shows, in the 1800 to 1000 cm -1 spectral region, the vibrational bands observed by ATR-IR and DRIFT spectra of the HCP are similar. We can therefore unequivocally transpose the obtained HCP band assignment based on the ATR-IR studies to that of the DRIFT spectra.
So far, we focused the vibrational analysis on the 1800 to 1000 cm -1 region. Yet, the 3650 to 3275 cm -1 region is also of interest as it corresponds to the 'pure' amino region (i.e. not coupled with aromatic vibrations). Interestingly, as Figure S6d shows, the measured ATR-IR spectrum S13 of the triazine-biphenyl HCP displays little amine vibrational bands. However, when performing DRIFTS analysis, reported below, we observe vibrational bands at 3530 and 3415 cm -1 , corresponding to the primary NH 2 asymmetric and symmetric stretching vibrations, respectively. 8 As mentioned above, this variance likely arises from the difference in measurement technique used. In ATR-IRS, the penetration depth of the infrared light depends on the incoming wavelength. At smaller wavelength, the penetration depth decreases, resulting in a loss of sensitivity at higher wavenumbers. Such phenomenon could explain the apparent absence of the primary amine bands at 3530 and 3415 cm -1 .
Though tangentially relevant to the objective of the study, we note that DRIFTS provided insights into the reaction mechanism linked to HCP formation. In brief, it supports the S N 1 route speculated in earlier studies, as shown in Figure S8. 2,9 Upon protonation by the acid catalyst, a methoxy group of the biphenyl monomer leaves as methanol, forming a carbocation.
The carbocation then undergoes nucleophilic attack from the triazine monomer and finally regeneration of the acid catalyst occurs by proton transfer. As we detected primary amine groups and no secondary amines, we conject that the phenyl ring of the triazine monomer, and not the primary amines, act as nucleophile. Overall, the stretching vibration of the primary amines observed using DRIFTS unveiled the synthesis mechanism of the triazine-biphenyl HCP. Table S4. Comparison between the experimental ATR infrared spectrum of triazine powder and the simulated infrared spectrum of one triazine molecule, enabling assignment of the triazine monomer vibrational bands. To allow comparison between the simulated and the measured infrared spectra, a scaling factor of 0.973 was applied to the simulated one.
Band number as labeled in Figure S6a Experimental vibrational frequency of triazine powder  Table S5. Comparison between the experimental ATR infrared spectra of biphenyl powder and the simulated infrared spectra of one biphenyl molecule, enabling assignment of the biphenyl monomer vibrational bands. To allow comparison between the simulated and the measured infrared spectra a scaling factor of 0.975 was applied to the simulated one.
Band number as labeled in Figure        S26 Figure S12. Comparison between the measured difference spectrum under a CO 2 atmosphere at 50 °C and the calculated one. The calculated difference spectrum is obtained by mathematically shifting the background spectrum by -8 cm -1 , and the latter is subtracted to the original background spectra.

Section S4 -Vibrational band analysis during H 2 O adsorption
As Figure S13a shows, in the presence of H 2 O at 50 °C, the difference spectrum displays negative and positive bands, suggesting that some of the HCP vibrational bands are shifted due to interactions between HCP and H 2 O. Following the same reasoning as for CO 2 adsorption, as negative bands precede positive ones, we infer that some of the HCP vibrational bands are red shifted. We then processed the time-resolved spectra using MCR to determine which HCP vibrational bands shifted upon interactions with H 2 O. As water vapor absorbs infrared light, it creates an undesirable background noise in the 2000-1300 cm -1 region and one should separate this effect from that of water adsorption in HCP. Thanks to the different kinetic behavior of water vapor and that of adsorbed water, MCR can resolve the water vapor contribution and separate it from the other spectral components ( Figure S13a).
As Figure S13a shows, when processing the difference spectrum obtained under a H 2 O atmosphere using MCR, we extract three spectrally significant and kinetically pure components. The first component (purple, Figure S13b) corresponds to the water vapor present in the DRIFTS cell, the second component (red, Figure S13c) to the positive spectral features of the difference spectrum, and the third component (blue, Figure S13d) to the negative spectral features of the difference spectrum. As shown in Figure S13c Figure S13d) Figure S13c and d show, at 100 °C, the positive vibrational bands linked to the triazine monomer (bands 3 and 7 and of Figure S13c and d), decrease faster than those of the intermolecular biphenyl C=O bond (bands 1 and 2). Hence,

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unlike CO 2 , H 2 O adsorbs more strongly to the biphenyl site than to the triazine one ( Figure   S13e). This phenomenon likely results from strong hydrogen bonds between the water molecules and the intermolecular biphenyl C=O bonds. At 150 °C, due to very low H 2 O adsorption, we did not extract meaningful data.

Section S5 -Vibrational band analysis in the presence of CO 2 and H 2 O under irradiation
Isotopic study to study intermediates: Compared to 12 CO 2 , the vibrational bands of 13 CO 2 should be red-shifted. As Figure S14a shows, under irradiation, when switching from a 12 CO 2 /H 2 O to a 13 CO 2 /H 2 O atmosphere, the spectroscopic signature of CO 2 shifted. Such shift is clearly visible for the combination bands located at 3725, 3700, 3627, and 3600 cm -1 , and the symmetric and asymmetric CO 2 stretching bands located at 2350 and 2330 cm -1 , respectively