New catalysts enable photocatalytic version of water gas shift reaction for H2 production
18 June 2013
Researchers at the Univ. Politécnica de Valencia (Spain) have found that noble metal nanoparticles supported on titanium dioxide or cerium dioxide can catalyze the industrially important water gas shift (WGS) reaction for hydrogen production at ambient temperatures using visible light irradiation. An open access paper on their discovery is published in the RSC journal Energy and Environmental Science.
Currently, most hydrogen is produced via the steam reforming of natural gas, hydrocarbons and coal. Additional amounts of hydrogen are generated by the reaction of CO with water (the water gas shift reaction)—which also leads to the formation of CO2. WGS is an endothermic process typically carried out in industry at high temperatures (about 350 °C) with either an iron oxide- or copper-based catalyst to achieve almost complete CO conversion.
A conventional two-stage industrial gas shift is capable of converting approximately 96% of CO initially in the syngas, according to the US National Energy Technology Laboratory. An Argonne National Laboratory life cycle assessment of a Shell gasification-based multi-product stream in 2001 found that a dual-bed approach yielded a 76% CO conversion in the first bed, with 98% conversion in the second.
In this context, in the present manuscript we report the photocatalytic version of the WGS performed at ambient temperature with sunlight and visible light. When this process is carried out with sunlight, no additional energy consumption is required, and hydrogen is obtained from CO using the sun as the sustainable energy resource. As far as we know there are no precedents on the photocatalytic WGS.
—Sastre et al.
The team investigated a number of photoactive catalysts including six TiO2 containing metal nanoparticles (NPs) and three CeO2 having different loadings of Au NPs.
Results showed that although TiO2 and CeO2 show a low activity for promoting the photocatalytic version of WGS, the presence of noble metals considerably increases their photoactivity. The most active photocatalyst tested was Au/TiO2 which achieves CO conversion of about 40% in 4 hours under the tested reaction conditions.
Longer irradiation times lead to higher conversions of up to 71% in 22 hours and lower light fluencies lead to lower conversions.
The team also carried out a series of irradiation experiments under analogous conditions to those using sunlight, but employing the quasi monochromatic light from an LED lamp emitting at 450 nm as the excitation source. They found that conversion with LED are lower than those obtained with the solar simulator.
In the present article we have reported our finding on a novel photocatalytic hydrogen generation from water using CO as a reducing agent in the presence of TiO2 or CeO2 as photocatalysts containing noble metal NPs. The process, which takes place at ambient temperature, can be promoted by solar light and the most efficient Au/TiO2 photocatalyst shows a significant photoactivity with visible light. In the context of hydrogen technology and considering the current importance of WGS, our results open up the way to perform a sunlight-driven, near ambient temperature WGS process.
—Sastre et al.
Resources
Francesc Sastre, Marica Oteri, Avelino Corma and Hermenegildo García (2013) Photocatalytic water gas shift using visible or simulated solar light for the efficient, room-temperature hydrogen generation. Energy Environ. Sci. doi: 10.1039/C3EE40656C
"..can catalyze the industrially important water gas shift (WGS) reaction for hydrogen production at ambient temperatures using visible light irradiation." sounds important.
Can a chemist comment?
Posted by: kelly | 18 June 2013 at 12:25 PM
It sounds like this is similar to photo-dissociation of water, only using light to provide the much smaller amount of energy needed to extract hydrogen using carbon monoxide as the oxygen sink.
Posted by: Engineer-Poet | 18 June 2013 at 09:40 PM
probably interesting chemistry, but I wonder if it's worth the effort.
By far most of the energy still comes from oxidizing carbon to CO2, and the infrastructure to collect enough solar light will probably be much to expensive and unreliable for the small gain in efficiency.
If one would invest the same effort and money in simply installing solar photovoltaic cells, the produced completely carbon-free electricity can be used to produced H2 from water.
I am quite confident the amount of H2 produced this way per invested euro will be much higher than by installing these photocatalists with combined CO-infrastructure.
Still, interesting chemistry, but H2 should be produced without carbon at all.
Posted by: Alain | 21 June 2013 at 04:23 AM
It seems at first rather like a waste of carbon monoxide, which can otherwise be hydrogenated to form hydrocarbon fuels using the Fischer-Tropsch reaction. The water gas shift consumes CO:
H2O + CO -> H2 + CO2
However, to create hydrocarbon fuels, the basic desired reaction is:
3 H2 + CO -> CH4 + H2O
Further iterations lead to longer-chain hydrocarbons (gasoline, etc.). So it seems the ideal setup would have two feed-ins: H2 from water and CO from atmospheric (or aqueous) carbon dioxide, with renewable energy being used at all stages to drive the reaction forward. Aka 'artificial photosynthesis.'
(H2 is much harder to store and transport than hydrocarbon fuels like natural gas, gasoline and diesel.)
Posted by: photosymbiont | 24 June 2013 at 07:40 PM