57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

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 Cu_xO_y 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 Cu_xO_y 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 Cu_xO_y particles. The darker particles on the TEM images are the Cu_xO_y 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 Cu_xO_y particles increases, but the shape of the Cu_xO_y particles does not change. It can be seen that there are Cu_xO_yparticles 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 Cu_xO_y/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.