Gas phase photocatalytic reduction of CO2 was carried out on the synthesized GO materials under ambient conditions using a halogen lamp as the light source. The commercial TiO2 was separately tested for gas phase photocatalytic CO2 reduction under the similar conditions. According to the procedure described in the Experimental section, the methanol (MeOH) formation rate, RMeOH (mmol g cat1 h1), was calculated and plotted in Fig. 6 as a function of reaction time. Activity of the methanol formation was found to follow the order: GO-3 > GO-1 > GO-2 > TiO2 (P-25). GO-3 displays a moderate methanol conversion rate of 0.172 mmol g cat1 h1 at a four-hour reaction time with a better stability. In GO-1 and GO-2 samples, the RMeOH initially increased very fast until it reached the maximum of 0.110 and 0.089 mmol g cat1 h1, respectively, and then it
leveled off after 2 hours of testing, whereas a methanol conversion rate of only 0.03 mmol g cat1 h1 was observed with pure TiO2 (P-25). Additionally, to eliminate the surface area effect on the methanol formation rate, we calculated the methanol conversation rate (mmol per surface cat1 h1) based on the active sites of GO-3 (53.7 m2 g1) and TiO2 (55 m2 g1).
The calculated methanol formation rates were 3.2 103 and 0.54 103 mmol per surface cat1 h1 for GO-3 and TiO2, respectively. This analysis supports the previous observation that the photocatalytic CO2 to methanol conversion rate of the modied graphene oxide (GO-3) is six-fold higher than that of the pure TiO2. Using RMeOH as a measure of performance, assynthesized GO-3 has denitely outplayed the TiO2, as observed in Fig. 6.