58th Annual Report on Research 2013 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

For the previous reporting period (1/1/2011 ¨C 8/31/2012), 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.

For the current reporting period (9/1/2012 ¨C 8/31/2013), we have fabricated new nanocomposites and further advanced the study in CO₂ photoreduction with water from the following two aspects: (1) Tailoring Cu valence and oxygen vacancy in Cu/TiO₂ catalysts and understanding their effects in CO₂ photoreduction, (2) Synthesizing porous microspheres of MgO patched TiO₂ and studying the temperature dependent activity and stability for MgO/TiO₂ catalysts

(1) Tailoring Cu valence and oxygen vacancy in Cu/TiO₂ catalysts

The incorporation of Cu species in TiO₂ photocatalysts is beneficial to photocatalytic CO₂ reduction to fuels, but the effect of Cu valence is poorly understood. In this work, Cu/TiO₂ (P25) nanoparticle catalysts were prepared by a simple precipitation and calcination method. The as-prepared Cu/TiO₂ sample was dominated by Cu²⁺ species. Thermal pretreatment of the as-prepared samples in He and H₂ atmosphere resulted in the transition to a surface dominated by Cu⁺ and mixed Cu⁺/Cu⁰, respectively. The Cu valences were confirmed by in situ X-ray photoelectron spectroscopy (XPS) (Figure 1) and diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses (Figure 2). These thermal pretreatments in reducing atmospheres also induced the formation of defect sites such as oxygen vacancies and Ti³⁺, confirmed by UV-vis spectra and DRIFTS analyses. The various  Cu/TiO₂ catalysts were tested in CO₂ photoreduction with water vapor under simulated solar irradiation, and their activities were in the order of as-prepared (unpretreated) < He-pretreated < H₂-pretreated. Compared with unpretreated TiO₂ (P25), the H₂-pretreated Cu/TiO₂ demonstrated a 10-fold and 189-fold enhancement in the production of CO and CH₄, respectively (Figure 3). This significant enhancement was mainly attributed to the synergy of the following two factors: (1) the formation of surface defect sites promoting CO₂ adsorption and subsequent charge transfer to the adsorbed CO₂; (2) the existence of Cu⁺/Cu⁰ couples that facilitate electron and hole trapping at different sites.

(2) Porous microspheres MgO patched TiO₂ for enhanced activity and stability

A novel MgO-patched TiO₂ microsphere photocatalyst (Figure 4) was fabricated through a simple ultrasonic spray pyrolysis method using TiO₂ (P25) nanoparticles and Mg(NO₃)₂ as the precursors. Conventional photocatalysis experiments are conducted at room temperature; in this work, we have investigated the effect of reaction temperature in the range of 50 to 170 ^oC. As shown in Figure 5, key results from this work are: (1) the activity of CO₂ photoreduction to CO increased as the temperature increased for both TiO₂ and MgO/TiO₂, and reached optimum at 150 ^oC; (2) the incorporation of MgO to TiO₂ enhanced both production yield and the stability of the catalyst. The possible reason for the superb activity at 150 ^oC for MgO/TiO₂ is easier desorption of reaction intermediates and the enhanced CO₂ adsorption by MgO. A higher temperature favors intermediates/products desorption but limits CO₂ adsorption, as confirmed by in situ DRIFTS analysis data. The weakened CO₂ adsorption ability, however, is compensated by adding MgO to the TiO₂ surface. The finding in this work is of great importance as it provides a new reaction scheme that may be suitable for power plant flue gas CO₂ capture and conversion where the temperature is in the range of 130 to 150 ^oC.

Impact

This two-year research supported by ACS-PRF has resulted in a total of five journal articles published in Journal of Physical Chemistry C, International Journal of Hydrogen Energy, and Catalysis Science & Technology, Applied Catalysis B: Environment, and Chemical Communications, respectively. The results were also disseminated in several national conferences. The findings of this project have contributed to the field of solar fuel production by providing significant insights in materials design and reaction mechanism. The ACS-PRF project has trained one postdoc and two graduate students. It also provided research experience for one undergraduate student who also co-authored some 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, including a recent NSF CAREER Award in 2013.

Figure 1. In situ XPS spectra for Cu 2p of 1%Cu/TiO₂ samples

Figure 2. In situ DRIFTS spectra of CO adsorption on unpretreated and pretreated 1%Cu/TiO₂ sample;

Figure 3. The production of (a) CO and (b) CH₄ on the unpretreated and pretreated TiO₂ and Cu/TiO₂ catalysts under photoirradiation for 6.5 hr.

Figure 4. SEM images (a, b), X-ray elemental mapping image (c), and HRTEM image (d) for MgO/TiO₂ catalysts.

Figure 5. Rate of CO production from CO₂ photoreduction with H₂O vapor on (a) TiO₂ and (b) MgO/TiO₂ at different reaction temperatures