A thermodynamic analysis of continuous fuel production by redox cycling of ceria in a single solar reactor under isothermal conditions is presented. Ceria is partially reduced in a sweep gas flow of purified nitrogen and reoxidized with either steam or carbon dioxide to produce hydrogen or carbon monoxide, respectively. The sweep gas and oxidizer flows are preheated by the product gases. The influence of selected process parameters, including operating temperature, pressure, and the effectiveness of heat recovery, on the solar-to-fuel conversion efficiency is determined. For a solar concentration ratio of 3000, typical of state-of-the-art solar dish concentrators, and operating temperature of 1773 K, 95.5% of the available gas-phase heat must be recovered to reach conversion efficiencies of 10% and 18% for hydrogen and carbon monoxide production, respectively, assuming the flow rate of inert sweep gas is equivalent to that in a counter-current flow arrangement of gas and ceria. The efficiency depends strongly on the gas-phase heat recovery effectiveness and the sweep gas flow rate. Introducing a temperature swing of 150 K between reduction and oxidation steps strongly reduces the sweep gas flow rate and increases the efficiency from 10% to 31.6% for hydrogen production.

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An isothermal thermochemical cycle to split CO 2 based on nonstoichiometric reduction and oxidation of ceria is demonstrated. Carbon monoxide is produced via an oxygen partial pressure swing by alternating inert sweep gas and CO 2 flows over the ceria. The rates of reduction and oxidation at 1500 °C in a porous ceria particle bed are measured for sweep gas and CO 2 flow rates from 50 to 600 mL min −1 g −1 and analyzed to identify cycle operating conditions (gas flow rates and reduction and oxidation durations) that maximize process efficiency. For a solar reactor assumed to operate at 3000 suns concentration and with 90% of the sensible heat of the gases recovered, the optimal cycle uses 150 mL min −1 g −1 sweep gas and 50 mL min −1 g −1 CO 2 at reduction and oxidation periods of 100 and 155 s, respectively. This cycle is demonstrated in an IR imaging furnace over 102 cycles, yielding a stable average rate of CO production of 0.079 μmol s −1 g −1 and a projected reactor efficiency of 4%. The optimal conditions apply at large scale if the flow rates are scaled in proportion to the ceria mass.

Finding pathways to renewable generation of fuels is a crucial step toward mitigating the ecological impacts of fossil fuel combustion. A renewable fuel requires a sustainable energy input and abundant feedstocks. One promising route is through the use of concentrated solar energy to drive the thermochemical splitting of H 2 O and CO 2 . The splitting of H 2 O generates hydrogen fuel and oxygen. Additionally, splitting both H 2 O and CO 2 generates syngas (CO and H 2 ) that can be converted into hydrocarbon fuels through the FischerÀTropsch process. Direct splitting of H 2 O and CO 2 in a single step is extremely endergonic (ΔG H 2 > 0 at T < 4700 K; ΔG CO > 0 at T < 3200 K), and it is difficult to separate the product gases at the required temperature. 1,2 Therefore, direct "one-step" splitting remains impractical.Metal oxides can be used to circumvent the challenges of one step thermochemical fuel production by breaking the process into two steps. 3 First, the solid metal oxide is thermally reduced at high temperatures (>1200 °C), releasing O 2 . The reactive material is then reoxidized by H 2 O or CO 2 at lower temperatures, producing H 2 or CO. This cyclic process allows for lower requisite temperatures for splitting, permits recycling of the metal oxide, and provides intrinsic separation of product gases (O 2 in one step and H 2 or CO in the other). 4,5 The most commonly investigated two-step metal oxide cycles are the zinc (ZnOÀZn) and ferrite (FeOÀFe 3 O 4 ) cycles. However, problems are encountered in both systems. Notably, facile recombination of zinc vapor with O 2 during thermal reduction occurs in the zinc cycle, 6,7 and an inert zirconium oxide phase is needed to stabilize the active materials in the ferrite cycle. 8,9 Cerium oxide is an attractive alternative for solar thermochemical fuel production (eqs 1À3) from metal oxides. Both water splitting and CO 2 splitting with CeO 2 have been investigated, first in catalytic systems 10,11 and then as a solar thermochemical process. 5,12À17 CeO 2 has found use in automotive three-way catalysis and other catalytic systems, due to its ability to reversibly store and release lattice oxygen. 18À20 This mechanism occurs due to the partial reduction of the Ce 4+ cations in CeO 2 to Ce 3+ , and it results in the formation of nonstoichiometric, cubic phases via the formation of oxygen vacancies without significant reorganization of the lattice. 20,21 When compared to other metal oxide materials for fuel production, CeO 2 has a higher melting point (2400 °C), improved thermal stability, and lack of crystal reordering phase transitions in the operating temperature range. 16 CeO 2 h CeO 2Àδ þ 0:5δO 2

ABSTRACT. This experimental study extends prior studies to consider the influences of discharge polarity, current, relative humidity, air temperature, and wire diameter and material on ozone generation rate in two-stage, wire-plate indoor air cleaners. Promising methods of decreasing the quantity of ozone released into living and work spaces are identified. Use of positive corona discharge is imperative since ozone generation rates are nearly an order of magnitude higher with negative discharge. For a specific precipitator design, the most important parameter in predicting ozone generation rate is current level. Changes in temperature and relative humidity of the inlet air stream over the range of ambient conditions expected in typical homes have less impact. In the commercial air cleaner studied, a 40% reduction in current from 1.08 to 0.60 mA, reduces ozone generation rate by nearly 50% from 0.005 to 0.0025 mg s-'. This reduction in current reduces particle collection efficiency by 20%. An increase in relative humidity from 17 to 55% decreases ozone generation rate 17%. An increase in air temperature from 293 to 301K decreases ozone generation rate by 6%. Ozone production can be controlled by the selection of wire diameter and material. At a fixed voltage, use of 0.10 mm rather than 0.20 mm tungsten discharge wires reduces ozone generation rate by 40%. The accompanying reduction in current does not cause a reduction in collection efficiency as long as the voltage in the collection stage is held constant. The benefit of controlling ozone generation rate by selection of wire material is that the electrical characteristics of the air cleaner are not affected. With a positive corona discharge, ozone generation rate is decreased by 30% with copper wires and by 50% with silver wires as compared to the rate with standard tungsten wires.