TiO2@PEI-Grafted-MWCNTs Hybrids Nanocomposites Catalysts for CO2 Photoreduction

Anatase (TiO2) and multiwalled carbon nanotubes bearing polyethylenimine (PEI) anchored on their surface were hybridized in different proportions according to a sol-gel method. The resulting nanocomposites (TiO2@PEI-MWCNTs), characterized by BET, XRD, XPS, SEM, and UV techniques, were found efficient catalysts for CO2 photoreduction into formic and acetic acids in water suspension and under visible light irradiation. PEI-grafted nanotubes co-catalysts are believed to act as CO2 activators by forming a carbamate intermediate allowing to accomplish the first example in the literature of polyamines/nanotubes/TiO2 mediated CO2 photoreduction to carboxylic acids.

A gas burette was used to evaluate the adsorption/desorption ability of amine-grafted MWCNTs. The gas volume of CO2 was measured in a thermostated schlenk tube connected with the burette (Fig. S1). The sample was degassed under vacuum for 2 hours at room temperature (25°C). When adsorption step began, a CO2 flow was maintained for 6 hours at 25°C. When adsorption step was complete, a flow of inert N2 was introduced into the flask to remove residual carbon dioxide. Then, the temperature was increased to 80 °C for 3 hours to accomplish the desorption step. Figure S1. Schematic representation of apparatus for measuring CO2 adsorption/desorption ability

Detailed XRD analyses of hybrids nanocomposites
The diffraction pattern of PEI-grafted MWCNTs 4 displayed two probable low intensive and widened peaks ( fig. S4a), indicating a very low degree of crystallinity of the sample. Due to its similarity with the diffraction pattern of purified MWCNTs described in the paper by Xia et al. [14], the two broad peaks can be associated to the (0 0 2) and (1 0 0) reflections of the MWCNTs. The diffraction patterns of the two samples 1A and 4A, consisting of TiO2 with minimal amounts (2%) of carbon nanotubes, showed similar features and were processed by the qualitative analysis software QUALX2.0 that identified the anatase TiO2 phase.
In fig. 4b the Miller indices of anatase TiO2 reflections associated to the major diffraction peaks are shown.
As expected, on increasing the PEI-MWCNTs content up to 10% (sample 4B), the anatase polymorphic form did not persist anymore ( fig. 4c), most likely because the increase of the almost amorphous nanotubes hinders the agglomeration of TiO2 particles.

Detailed XPS analyses with High-resolution N1s signal
The N1s signal could be curve fitted with two components a dominant component at 400.5 ± 0.2 eV ascribed to amino groups and a minor component centred at 402.2 ± 0.2 eV that could be due to protonated amino groups or to amino groups involved in hydrogen bonding (Fig. S2).

PEI-MWCNTs-TiO2
PEI-MWCNTs-TiO2/after reaction Figure S3. High-resolution N1s signal  (Fig. S4) shows heterogeneous samples consisting of carbon nanotubes randomly dispersed into TiO2 agglomerates. In particular, image of 4C at the highest level of magnification ( Fig. S4D) better highlights the close interfacial contact between CNTs and TiO2.

Schematic of CO2 photoreduction apparatus
Apparatus for photocatalytic experiments consisted of a three necked Pyrex batch photoreactor of cylindrical shape equipped with a jacket for cooling water circulation and ports in its upper section useful for gas inlet (and outlet), sampling and pH and temperature measurements. Two lamps were placed in proximity of the reactor (5-6 cm): HRC UV-VIS lamp 300W (Sanolux) and Xe-Halogen lamp 400W (Radium) In a typical procedure, in 20 ml of an aqueous suspension of photocatalyst, argon was bubbled for 30 minute, to avoid the presence of air, and then CO2 for approximately 60 minutes before switching on the lamps. Two catalyst amounts chosen for optimization experiments were 150 and 500 mg corresponding to loadings of 7.5 g/ml and 25 g/ml, respectively. The initial measured pH value was 4.58 and temperature inside the reactor was kept at approximately 25 °C by a continuous water circulation through the photoreactor jacket. The photoreactivity runs lasted 5.0 h and products were detected by Ion Cromatography.