Reports: DNI1050801-DNI10: Novel CO2 Reduction Catalysts Driven by Visible Light

Feng Jiao, University of Delaware

In the first year of study, we investigated copper oxide based catalysts as good candidates for photocatalytic CO2 reduction, because copper is cheap, abundant, and its oxides are active under visible light. In this report, we will present our initial results on fabrication and structural characterization of copper oxide nanoclusters. Some preliminary results for CO2photoreduction will also be discussed.

As an initial task, we synthesized mesoporous silica supported CuxOy catalyst. Cu was loaded into the pores of SBA-15 through incipient wetness impregnation. Copper chloride (CuCl2) was mixed with SBA-15 in ethanol (EtOH) in order to ensure that the CuCl2 was well-mixed throughout the SBA-15. The amount of CuCl2 was calculated based on the desired ratio of Cu to SBA-15. So far, 1, 4.8, and 10 percent weight-Cu/weight-SBA-15 were synthesized. Each of these weight ratios will be tested in order to determine which has the highest activity. N-hexane (C6H14) was added in order to force EtOH into the pores of SBA-15. EtOH moves preferentially into the hydrophilic pores of SBA-15 because it is more hydrophilic than n-hexane.

After incipient wetness impregnation, CuCl2/SBA-15 was exposed to ammonium hydroxide (NH4OH) vapor in order to form a copper ammonia complex. The complex was calcinated at 400°C in air for three hours to form CuO. Then, the catalyst was reduced  under the flow of H2 in order to form CuxOy particles. This reduction process converted the CuO, an inactive catalyst for CO2 reduction, into Cu2O, which was reported to be active for CO2 reduction. The temperatures chosen for the reduction process ranged from 125°C to 250°C.

We first confirmed the morphology of as-prepared copper oxide nanoclusters. TEM results show the ordered structure of SBA-15 and the CuxOy particles. The darker particles on the TEM images are the CuxOy particles. The ordered structure of SBA-15 is visible in all samples. The average pore size of SBA-15 is about 10 nm. As the weight ratio of Cu/SBA-15 increases, the number of CuxOy particles increases, but the shape of the CuxOy particles does not change. It can be seen that there are CuxOyparticles formed within the mesoporous structure of SBA-15 for all weight ratios.

Powder XRD were performed. The low angle powder XRD patterns show the (100), (110), and (200) peaks of the p6mm hexagonal structure for samples before and after the impregnation of copper and after the reduction under the flow of H2. This demonstrates that the mesoporous structure of SBA-15 was maintained throughout the catalyst preparation procedure. Turning to the wide angle PXRD data, CuO and CuCl2 phases are present in the CuxOy/SBA-15 catalyst before reduction under the flow of H2. The broad nature of the peaks for these phases indicates small particle size. At the reduction temperatures of 250°C and 150°C, only Cu and Cu2O phases can be seen. The peaks at these reduction temperatures were also broad, indicating that small particle size was maintained. At the reduction temperature of 125°C, the CuCl2 phase, is still seen which means that not all of the Cu was reduced while under the flow of H2 The PXRD patterns show that, at reduction temperatures of 150°C and higher, the desired catalyst is formed. At a reduction temperature of 125°C, the Cu precursor CuCl2is still present. 

In preliminary CO2 photoreduction experiments, we investigated the CO2 reduction activity of copper oxide nanoclusters in mesoporous silica by using Ru(bpy)32+ as sensitizer and TEA as sacrificial electron donor under visible light. The CO2 photoreduction experiments were carried out as follows: triethylamine (TEA) is dissolved in anhydrous CH3CN, followed by bobbling with pure nitrogen and CO2. Then, the solution is mixed with Ru(bpy)32+ and catalyst in a nitrogen filled glovebox, which is filled into an FTIR liquid cell with CaF2 windows. FTIR spectra are recorded before and after irradiation at 476 nm (Ar ion laser, 400 mW). In the preliminary experiments, we used CO2 and its isotopes, such as 12C16O2, 13C16O2, and 12C18O2, to identify the products. The FTIR spectra shown in Figure 3 are the results collected in the 12C18O2 experiments. After one-hour photolysis, two peaks appeared at 1725 and 1693 cm-1. The peak at 1725 cm-1, which was also observed in the 12C16O2 experiment, probably correspond to C=16O double bond stretching in the TEA oxidation or Ru(bpy)32+ decomposition products. The peak at 1693 cm-1, associated with the C=18O double bond stretching, was observed in the 12C18O2 experiment, while it didn’t appeared in other parallel experiments under identical conditions with different CO2 isotopes. This observation clearly confirms the dissociation of CO2 in the presence of copper oxide nanoclusters catalyst under visible light. In this following year, we will try to identify and measure the CO2 reduction products in a quantitative way.