Boosting the H2 Production Efficiency via Photocatalytic Organic Reforming: The Role of Additional Hole Scavenging System

Abstract

The simultaneous photocatalytic H2 evolution with environmental remediation over semiconducting metal oxides is a fascinating process for sustainable fuel production. However, most of the previously reported photocatalytic reforming showed nonstoichiometric amounts of the evolved H2 when organic substrates were used. To explain the reasons for this phenomenon, a careful analysis of the products and intermediates in gas and aqueous phases upon the photocatalytic hydrogen evolution from oxalic acid using Pt/TiO2 was performed. A quadrupole mass spectrometer (QMS) was used for the continuous flow monitoring of the evolved gases, while high performance ion chromatography (HPIC), isotopic labeling, and electron paramagnetic resonance (EPR) were employed to understand the reactions in the solution. The entire consumption of oxalic acid led to a ~30% lower H2 amount than theoretically expected. Due to the contribution of the photo-Kolbe reaction mechanism, a tiny amount of formic acid was produced then disappeared shortly after the complete consumption of oxalic acid. Nevertheless, a much lower concentration of formic acid was generated compared to the nonstoichiometric difference between the formed H2 and the consumed oxalic acid. Isotopic labeling measurements showed that the evolved H2, HD, and/or D2 matched those of the solvent; however, using D2O decreased the reaction rate. Interestingly, the presence of KI as an additional hole scavenger with oxalic acid had a considerable impact on the reaction mechanism, and thus the hydrogen yield, as indicated by the QMS and the EPR measurements. The added KI promoted H2 evolution to reach the theoretically predictable amount and inhibited the formation of intermediates without affecting the oxalic acid degradation rate. The proposed mechanism, by which KI boosts the photocatalytic performance, is of great importance in enhancing the overall energy efficiency for hydrogen production via photocatalytic organic reforming.

Application of EPR Spectroscopy in TiO2 and Nb2O5 Photocatalysis

“…Although the authors did not observe the formation of • OH, however, they could not exclude their contribution in the photocatalytic process. However, the authors in another publication [97] performed EPR spin-trap experiments as described before but with the presence of KI as a second hole scavenger in the addition to oxalic acid. They noticed that DMPO • -CO 2 -spin adduct is the lonely signal detected in the presence of KI with much lower EPR intensities as presented in Figure 22.…”

“…EPR spin trap spectrum in N 2 atmosphere for a UV-irradiated suspension contained oxalic acid (5.5 mmol L −1 ), 20 mmol L −1 KI, DMPO (0.8 mmol L −1 ), and Pt/TiO 2 (1 g L −1). Adapted with permission from[97].…”

The Role of Metal Nanoparticles in Promoting Photocatalysis by TiO2

“…Most of the work in this area uses methanol as the hole scavenger, combined with a photoactive support, such as titania, which then limits the useable wavelength of solar radiation, and a so-called co-catalyst, usually metal NPs [21][22][23][24][25][26][27]. An example of results from such a system is given below, in this case with Pd as the metal (Fig.…”

How Can Hybrid Materials Enable a Circular Economy?

“…Hybrid materials can be successfully used for water and wastewater treatment as filter components [39], catalysts [40] or photocatalysts [41] (which could even be coupled with hydrogen production [42,43] discussed in the following section).…”