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

Objective

Photocatalytic reduction of CO₂ with water by sunlight provides a new opportunity for simultaneous mitigation of greenhouse gases and production of energy-bearing compounds that can be further converted to substitutes for petroleum products. However, a low CO₂ conversion rate and the lack of efficient and inexpensive catalysts have impeded the development of this technology. The objective of this research was and to design novel nanostructured composite materials to achieve high CO₂-to-fuels conversion efficiencies and to understand the mechanism of CO₂ photoreduction with water.

Results

Over the past one and a half years, we have achieved significant progress towards realization of the project goal. Specifically, we have developed three novel materials for CO₂ photoreduction: (1) Cu-modified TiO₂ nanoparticles with engineered surface defect sites; (2) Ag-deposited TiO₂ nanoparticles assembled to form porous microspheres; (3) Ce-doped TiO₂ dispersed on ordered mesoporous SBA-15 support. All three novel catalysts have demonstrated much more effective activities in CO₂ photoreduction to fuels compared with bare TiO₂ nanoparticles.

(1) Surface defective Cu/TiO₂ nanoparticles

Cu/TiO₂ with surface defect sites, Cu(I)/TiO_2-x, was prepared by a simple precipitation method followed by heat treatment in an inert gas environment (e.g., helium) at moderate temperature (e.g. 300 °C). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) identified the surface defect sites to be Ti³⁺ and oxygen vacancies, and Cu(I) dominated the Cu species on TiO₂. The in situ DRIFTS analyses also demonstrated that CO₂^- species, generated upon an electron attachment to CO₂, are spontaneously dissociated into CO even in the dark over the Cu(I)/TiO_2-x catalyst. This dissociation process is associated with the oxygen vacancies that not only provide the electron centers but also the sites for the adsorption of oxygen atom from CO₂. Isotopic carbon-labeling experiments confirmed that the produced CO is indeed derived from CO₂ (see Figure 1). Compared with in the dark, CO₂ activation and dissociation under photo-illumination is remarkably improved, possibly due to sustained electron supply and partial regeneration of surface oxygen vacancy induced by irradiation. The results from DRIFTS analyses were verified by the measurement of catalytic activity using gas chromatography (GC). These findings are important to advancing the understanding in the chemistry of CO₂ adsorption and photocatalytic reduction on the surface of metal oxide catalysts.

(2) Porous microspheres assembled by Ag/TiO₂ nanoparticles

A simple ultrasonic spray pyrolysis method was used to prepare mesoporous Ag/TiO₂ composite in the morphology of microspheres using TiO₂ (P25) and AgNO₃ as the precursors. Ag nanoparticles in the size of 1-2 nm were deposited on TiO₂ (see Figure 2). The activity of the Ag/TiO₂ catalyst was measured for solar fuel production from CO₂ photoreduction with H₂O in the presence of methanol as a hole scavenger. CO and H₂ were the major products from the photocatalytic reactions. The optimal Ag concentration on TiO₂ was 2 wt%. The H₂ production rate of the 2% Ag/TiO₂ prepared by the spray pyrolysis method exhibited a six-fold enhancement compared with that prepared by a conventional wet-impregnation method. The ratio of H₂/CO (in the range from 2 to 10) can be controlled by adjusting the ratio of reactants (see Table 1). This controllable H₂/CO ratio in the reaction products and negligible CH₄ production indicates that it is a promising technology to produce syngas (the mixture of H₂ and CO) through photocatalytic CO₂ reduction on the engineered Ag/TiO₂ samples.

(3) Ce-doped TiO₂ dispersed on SBA-15

Ce-TiO₂ and Ce-TiO₂/SBA-15 composites were prepared by a sol-gel method and tested for CO₂ photoreduction with water. Compared with pristine TiO₂, TiO₂ doped by 1 or 3% Ce improved the production of CO by four times. The reason may be due to the facilitated charge transfer induced by the doped Ce ions, the higher surface area of the catalyst, as well as the stabilization of anatase phase. Dispersing Ce-TiO₂ nanoparticles on the mesoporous SBA-15 support further enhanced both CO and CH₄ production (see Figure 3). Particularly the CH₄ production was enhanced by up to 115 times compared with unsupported Ce-TiO₂. The superior catalytic activity may be related to the partially embedded Ce-TiO₂ in the ordered 1-D pores in SBA-15 that form synergies between the different components and enhance the diffusion and adsorption of CO₂. This mechanism also correlates well the results that using SBA-15 as the support led to more than 10 times higher activity in CO₂ photoreduction than using amorphous silica as the support.

Future Plan

In the second year of the project, research efforts will be dedicated to further improving the overall CO₂ conversion rate and control of product selectivity. Cu/TiO₂ catalysts with controllable Cu oxidation state (Cu⁰, Cu⁺, or Cu²⁺) will be synthesized and the effect of the different Cu species will be examined by combined GC/MS measurement of catalytic activity and in situ DRIFTS study. Nonmetal doping in the TiO₂ will be engineered to enhance visible light activity.

Impact

So far, the research supported by ACS-PRF has resulted in three journal articles published in Journal of Physical Chemistry C, International Journal of Hydrogen Energy, and Catalysis Science & Technology, respectively. The results were also disseminated in several national conferences. The ACS-PRF project has trained one postdoc and one graduate student. It also provided research experience for one undergraduate student who was co-author of one of the journal publications. The ACS-PRF grant has served as important seed funding for the PI to secure other funding to conduct research in the field of renewable energy.

Figure 1. In situ DRIFT spectra for ¹³CO₂ interaction with defective Cu(I)/TiO_2-x in the dark

Figure 2. SEM and HRTEM images of the Ag/TiO₂ catalyst

Figure 3. Production of CO and CH₄ over Ce-TiO₂/SBA-15 catalysts under UV-vis illumination for 4 h

Table 1. Production rate and product selectivity of the 2%Ag/TiO₂ sample under simulated solar illumination