Visible‐Light‐Driven CO2 Reduction by Mesoporous Carbon Nitride Modified with Polymeric Cobalt Phthalocyanine

Abstract The integration of molecular catalysts with low‐cost, solid light absorbers presents a promising strategy to construct catalysts for the generation of solar fuels. Here, we report a photocatalyst for CO2 reduction that consists of a polymeric cobalt phthalocyanine catalyst (CoPPc) coupled with mesoporous carbon nitride (mpg‐CNx) as the photosensitizer. This precious‐metal‐free hybrid catalyst selectively converts CO2 to CO in organic solvents under UV/Vis light (AM 1.5G, 100 mW cm−2, λ>300 nm) with a cobalt‐based turnover number of 90 for CO after 60 h. Notably, the photocatalyst retains 60 % CO evolution activity under visible light irradiation (λ>400 nm) and displays moderate water tolerance. The in situ polymerization of the phthalocyanine allows control of catalyst loading and is key for achieving photocatalytic CO2 conversion.


Souvik Roya nd Erwin Reisner*
Abstract: The integration of molecular catalysts with low-cost, solid light absorbers presents apromising strategy to construct catalysts for the generation of solar fuels.H ere,w er eport aphotocatalyst for CO 2 reduction that consists of apolymeric cobalt phthalocyanine catalyst (CoPPc) coupled with mesoporous carbon nitride (mpg-CN x )a st he photosensitizer.T his precious-metal-free hybrid catalyst selectively converts CO 2 to CO in organic solvents under UV/Vis light (AM 1.5G, 100 mW cm À2 , l > 300 nm) with ac obalt-based turnover number of 90 for CO after 60 h. Notably,t he photocatalyst retains 60 %C Oe volution activity under visible light irradiation (l > 400 nm) and displays moderate water tolerance.The in situ polymerization of the phthalocyanine allows control of catalyst loading and is key for achieving photocatalytic CO 2 conversion.
Photocatalytic reduction of CO 2 to produce storable fuels offers an attractive path to capture and utilize the greenhouse gas CO 2 and ultimately implement ac arbon-neutral energy cycle.T he development of efficient, sustainable,a nd economically viable catalysts and light-absorbers lies at the nexus of solar-fuel research on CO 2 utilization. Hybrid photosynthetic systems with molecular catalysts immobilized on solid supports (light-absorbing semiconductors or dye-sensitized semiconductors) have recently emerged as ap romising approach for suspension-based photoreactor applications, because they combine the selectivity of molecules with the durability of heterogeneous materials. [1] While many earth abundant metal based molecular complexes have been reported for CO 2 reduction in homogeneous solution, there are relatively few examples of heterogenization of these catalysts on solid light-absorbers. [2] Thed evelopment of new robust catalyst-photosensitizer interfaces remains achallenge that offers the key for improved photocatalytic activity of colloidal material-molecule hybrid systems.
Graphitic carbon nitride (g-CN x )has recently emerged as ap romising semiconductor for photocatalytic applications, [3] including water splitting [4] and CO 2 reduction, [5] because of its nontoxicity,facile synthesis,capability to absorb UV as well as visible light, and durability under photochemical conditions. Ar elatively narrow band gap and sufficiently negative conduction band energy minimum (À1.10 Vv s. NHE at pH 6.6) [4b, 6] allow g-CN x to harvest UV/Vis light and subsequently reduce as urface-bound molecular catalyst via photoinduced electron transfer. In CN x -based photocatalytic systems for CO 2 reduction, different types of co-catalysts have been used, including weakly anchoring phosphonic acid functionalized Ru complexes or Ru-Re dyads, [6,7] molecular cobalt and iron complexes in solution, [8] metalloporphyrins covalently grafted on CN x , [9] single-atom cobalt sites incorporated in the material, [10] and sodium niobite nanowires. [11] Despite encouraging reports with CN x -porphyrin hybrid catalysts, [9,12] aC N x /molecular catalyst system that consists of only earth-abundant elements and is entirely heterogeneous,d urable,e fficient, and selective for CO production remains an elusive target. Cobalt phthalocyanine is ak nown electrocatalyst for CO 2 reduction [13] but has rarely been explored in photocatalysis. [14] Herein, we report photocatalytic reduction of CO 2 to CO by ar obust organic-inorganic hybrid material in which mesoporous carbon nitride (mpg-CN x )h arvests solar energy and activates as urface-deposited polymeric cobalt phthalocyanine (CoPPc;P Pc denotes polymeric phthalocyanine) catalyst toward CO 2 reduction ( Figure 1). CoPPc is deposited on mpg-CN x via an in situ polymerization method, which represents an effective strategy for catalyst immobilization. This study demonstrates that the polymer-CN x interface plays akey role in catalysis by enabling the transfer of photoexcited electrons from CN x to the attached CoPPc catalyst.
Mpg-CN x was prepared by heating cyanamide in air using colloidal silica as ah ard template,w hich was subsequently etched with aqueous ammonium bifluoride. [15] Thempg-CN x j CoPPc hybrid was synthesized by microwave-assisted polymerization of 1,2,4,5-tetracyanobenzene (TCNB) with Co 2+ ions in the presence of mpg-CN x dispersed in 1-pentanol. [16] Formation of the polymeric catalyst was observed by ac olor change from pale orange to green. Aw eak p-p stacking interaction between the polymeric phthalocyanine sheet and tri-s-triazine units of mpg-CN x may contribute toward the facile charge transport through the interface. [17] Thec obalt content in the hybrids (mpg-CN x j CoPPc a ; a = mmol Co g À1 ) was modulated by controlling the amount of TCNB during the synthesis.C atalyst loadings,d etermined by inductively coupled plasma optical emission spectrometry (ICP-OES), were in the range of 4.1-107 mmol Co g À1 with higher Co content causing av isible intensification of green color of the solid ( Figure S1).
Successful formation of CoPPc on mpg-CN x was confirmed by diffuse reflectance UV/Vis spectroscopy (DRS), attenuated total reflectance infrared (ATR-IR), Raman, and X-ray photoelectron spectroscopy (XPS). DRS of mpg-CN x j CoPPc shows the characteristic (S 0 !S 1 )Qband of CoPPc at 700 nm, which is red-shifted compared to the monomeric cobalt phthalocyanine (CoPc) (670 nm in DMF), consistent with its polymeric structure ( Figure 2A). [16a, 18] Figure 2B shows that the Raman spectrum of mpg-CN x is featureless, whereas that of the hybrid materials displays bands originating from CoPPc. [19] TheA TR-IR spectrum of mpg-CN x j CoPPc is dominated by mpg-CN x peaks,w hich mask the weaker CoPPc stretches ( Figure S2).
Thei ntroduction of CoPPc leads to al ower Brunauer-Emmett-Teller (BET) surface area of the hybrid material (104 m 2 g À1 for mpg-CN x j CoPPc 21.7 vs.1 34 m 2 g À1 for bare mpg-CN x ,F igure S3), suggesting that the catalyst deposition occurs not only on the surface,but also inside the mesoporous structure.However,the integration of CoPPc does not affect the periodic stacking of the lamellar structure of CN x ,a s demonstrated by powder X-ray diffraction ( Figure S4). XPS spectra of pure CoPPc polymer and mpg-CN x j CoPPc at three different catalyst loadings are shown in Figure 2C and Figure S5. TheC o2pr egion of all samples consists of peaks at 796.2 and 780.8 eV associated with Co 2p 1/2 and Co 2p 3/2 transitions,r espectively.T he satellite features at % 802 and % 786 eV,w hich are characteristic of Co II paramagnetic species,a re clearly discernible for CoPPc and mpg-CN x j CoPPc 107 ,b ut are less pronounced for lower cobalt contents. TheC 1s XPS spectra of mpg-CN x j CoPPc feature am ore intense peak at 284.8 eV,w hich can be attributed to the C(sp 2 )-C(sp 2 )b onds of CoPPc ( Figure S6). Thei ntensity of this peak increases with higher catalyst loading.
Theo verpotential required for CO 2 reduction by CoPPc was estimated by cyclic voltammetric analysis of CoPPc/ carbon nanotube (CNT) composite electrodes. [13c] Thec yclic voltammogram of CoPPc j CNT in am ixture of acetonitrile (MeCN) and triethanolamine (TEOA) (4:1 v/v) displays alarge catalytic wave under CO 2 with an onset at À0.91 Vvs. NHE ( Figure S11), which suggests that CO 2 reduction by CoPPc occurs at am ore positive potential compared to the conduction band of CN x .
Thep hotocatalytic activity of the mpg-CN x j CoPPc hybrid was studied in CO 2 -saturated MeCN under UVfiltered simulated solar light irradiation (100 mW cm À2 ,A M 1.5G, l > 400 nm) with TEOAasasacrificial electron donor. While bare mpg-CN x generates only trace amounts of H 2 and CO (headspace analysis by gas chromatography), the CoPPcmodified material (mpg-CN x j CoPPc) exhibits considerably higher activity toward CO 2 reduction to CO (red trace, Figure 3A). Mechanically mixed pure CoPPc and mpg-CN x is inactive toward CO 2 reduction (blue trace,F igure 3A), highlighting the importance of the in situ polymerization. Only trace amounts of formate (< 1 mmol g À1 )w ere detected in all systems by ion chromatography.
This hypothesis is further supported by the low CO evolution activity of as uspension of mpg-CN x with monomeric CoPc in solution (green trace). CO was not observed in control experiments without mpg-CN x ,TEOA, CO 2 ,orlight.
Isotope labeling studies with 13 CO 2 using mass spectrometry and infrared spectroscopy confirmed that CO was produced from CO 2 (Figures S12 and S13). Irradiation of mpg-CN x j CoPPc under visible light equipped with al ong-pass filter (l > 455 nm) produced very little CO,which is consistent with mpg-CN x acting as the light absorber.T oc onfirm that the cobalt phthalocyanine units in the polymer are the active catalyst and not single-site cobalt ions coordinated to the tri-striazine moieties of CN x , [10] mpg-CN x j CoCl 2 was synthesized by heating CoCl 2 and mpg-CN x in the absence of TCNB under identical reaction conditions (weight ratio 1:120;e quivalent to CoCl 2 used for mpg-CN x j CoPPc 26.2 synthesis). This material displays minimal activity toward CO 2 reduction (purple trace). An on-mesoporous carbon-nitride-based hybrid (g-CN x j CoPPc) is also inactive under identical condition (orange trace).
Theh ybrid catalysts change color from pale green to purple during photocatalysis,indicating photoreduction of the Co centers of CoPPc ( Figure 3B,i nset). UV/Vis absorption spectra of an acetonitrile suspension (20 %T EOA, v/v) of mpg-CN x j CoPPc 107 display ar ed-shift of the Q-band of CoPPc from 712 to 750 nm and appearance of an ew charge transfer band at 528 nm, upon visible light irradiation under CO 2 ( Figure 3B,r ed trace). Thes pectral change suggests formation of ar educed CoPPc species,w hich was corroborated by spectroelectrochemical analysis of aCoPPc thin film deposited on conductive FTO( fluorine-doped tin oxide)coated glass electrode (CoPPc j FTO, Figure S14). An electrochemically reduced CoPPc film (À1.0 Vv s. NHE) exhibits two absorption bands at 529 and 735 nm, which is consistent with the spectrum of Co I PPc. [20] This indicates transfer of the photoexcited electron from the conduction band of CN x to CoPPc to yield Co I centers that subsequently bind CO 2 and convert it to CO through as econd electron transfer from (mpg-CN x )* or (mpg-CN x ) À ( Figure S15). [13c, 21] Photocatalytic activity of mpg-CN x j CoPPc is largely dependent on the catalyst loading as illustrated in Figure 4A. Thea mount of CO generated increased linearly with cobalt loading until % 12 mmol Co g À1 .F urther increase in cobalt content (> 20 mmol g À1 )r esulted in adecrease in activity and only at race amount of CO was detected for the highest loading sample (107 mmol Co g À1 ). At high cobalt concentrations,the carbon nitride surface is completely sheathed by the CoPPc layer, which blocks the incoming light and reduces the accessibility of the mpg-CN x surface to TEOA, thereby hindering photocatalysis.T he amount of CO evolved vs.C o loading profile fits well with the selectivity toward CO exhibited by mpg-CN x j CoPPc ( Figure 4A,T able S2). Longterm photocatalysis experiments show that the catalyst remains active for 4days and only displays am arginal decrease of CO selectivity,h ighlighting the stability and robustness of the photocatalyst assembly (Table S2 and Figure S16).
Under full solar spectrum irradiation (l > 300 nm), mpg-CN x j CoPPc 11.9 generated 1000 mmol CO g À1 after 48 hw ith 85 %s electivity (TON Co = 84), which corresponds to a6 5% increase in activity compared to that under visible light alone (607 mmol CO g À1 after 48 h, TON Co = 51) ( Figure 4B,F igure S17, and Table S3). Similarly,m pg-CN x j CoPPc 17.4 exhibited a45% increase in activity under UV/Vis irradiation and asmall improvement in selectivity toward CO.T his observa-  tion is consistent with the high UV absorbance (< 400 nm) of mpg-CN x .H owever,t his enhancement becomes less pronounced at higher Co loading as demonstrated by the similar TON Co values observed for mpg-CN x j CoPPc 21.7 under visible and UV/Vis irradiation ( Figure 4B,r ed traces). Thee xternal quantum efficiency( EQE) for CO formation by mpg-CN x j CoPPc 11.9 was calculated to be 0.11 %and 0.03 %atl ex = 360 and 400 nm, respectively (see Table S4 and Figure S18 for more information).
Thea mount of CO evolved in three 12 hr ecycling tests displays excellent agreement with that produced during continuous irradiation under visible light, consistent with heterogenous catalysis ( Figure 4C). When the runtime for each cycle was shortened to 4h,t he catalyst displayed as ignificant induction period and the CO evolution peaked during the 5 th cycle with subsequent gradual loss of activity. Theuse of alarger amount of catalyst is likely responsible for the delay as light scattering in concentrated suspension becomes al imiting factor. However,t he catalyst retains its excellent selectivity up to the 11 th cycle (44 h). In along-term experiment, the catalyst was recycled after 26 hv isible light photocatalysis and it retained 84 %activity in the second run for 38 h( Figure S19).
Monitoring the cobalt content of the mpg-CN x j CoPPc 17.4 under visible light photocatalysis reveals % 20 %loss of cobalt over the initial 4h ( Figure S20). However,v ery little Co subsequently leached from the photocatalyst between 4a nd 48 h. XPS analysis of the catalyst after 24 hp hotocatalysis confirms that Co remains on the mpg-CN x surface (Figure S21).
Addition of water to the reaction medium affects the performance of the photocatalyst. In comparison to the photocatalytic experiments in which CO is produced by mpg-CN x j CoPPc 21.7 under visible light in MeCN,the activity drops to 41 %a nd 26 %i nt he presence of 10 %a nd 20 %w ater, respectively ( Figures S22 and 23). Ther educed activity in water is likely caused by the phase separation of MeCN/H 2 O/ TEOAmixture during photocatalysis,with mpg-CN x j CoPPc being partitioned into the bottom aqueous layer (Figure S22 C). [22] When dimethylacetamide (DMA) was used as the solvent, [23] the reaction mixture remained monophasic and the photocatalyst exhibited markedly improved water tolerance ( Figures S24 and 25). Compared to the experiments in DMA, mpg-CN x j CoPPc 11.9 retains 90 %a nd 78 %C Oe volution activity in the presence of 10 %and 20 %water, respectively. Under fully aqueous conditions,t he photocatalyst produced 62 mmol CO g À1 ,c orresponding to % 5.1 turnovers per Co. Water-tolerance is an important feature as it may enable the photoreduction of CO 2 using water as an electron donor in the future. [24] Previously reported CN x -based photocatalysts for CO 2 reduction commonly employed either molecular co-catalysts that remained in the solution phase or phosphonic acid functionalized Ru complexes and Ru/Re dyads that interact weakly with the CN x via its surface -NH 2 groups. [7b, 8a, 23] Only af ew examples of hybrid materials with immobilized molecular catalysts have been reported, including aC oporphyrin covalently attached to CN x , [9] and am echanically mixed Fe-porphyrin/CN x hybrid. [12] In the former case,t he TON Co and selectivity were reported to be < 1a nd % 80 % under 80 kPa CO 2 ,r espectively,w hereas in the latter case, high CO selectivity (98 %) was observed with aTON of 5.7. In comparison, the mpg-CN x j CoPPc system described here performs well over longer time periods with 90 turnovers after 60 ha nd displays moderate tolerance toward water. However,t his rate is still significantly slower than electrocatalytic CO evolution rates displayed by CoPPc j CNT composite electrodes (TON 11 240, 24 he lectrolysis), [13c] suggesting that photocatalysis is likely limited by the rate of transfer of electron from mpg-CN x to the catalyst and not the inherent CO 2 reduction capability of CoPPc. This is supported by the linear decrease of photocatalytic activity of mpg-CN x j CoPPc with light intensity,w hile the selectivity remained unaffected ( Figure S26). Notably,t he mesoporosity of CN x plays ak ey role in catalysis by facilitating electron-hole separation through shortening of the migration distance. [25] In summary,wehave interfaced 2D cobalt phthalocyanine sheets with mesoporous carbon nitride via an in situ polymerization technique to fabricate ah ybrid catalyst for use in selective CO 2 reduction under visible light irradiation. Photocatalysis and spectroscopic studies demonstrate that molecular cobalt phthalocyanine units act as the catalytic centers and that the catalysis is enabled by the immobilization of the polymer in the porous carbon nitride.T his work provides ar are example of an effective and robust heterogenous CO 2 reduction photocatalyst featuring inexpensive, earth-abundant components,a nd provides av ersatile platform for catalyst immobilization on heterogeneous light absorbers.