Hydrogen has been attracting increasing attention as an energy source, able to be produced from water by way of artificial synthesis that uses a photocatalyst together with water electrolysis. And while its production does involve a myriad of technical issues, hopes are there that the costs involved can be significantly reduced through the construction of a large-scale system.
One such system in play is that from the New Energy and Industrial Technology Development Organization (NEDO), which is currently running a project aimed at synthesizing chemicals using hydrogen and CO2. R&D themes here include (1) a photocatalyst for decomposing water into oxygen and hydrogen through sunlight; (2) a separation membrane to safely separate the generated hydrogen and oxygen; and (3) a catalyst for highly selective synthesis of olefins from the hydrogen and CO2.
To look into photocatalyst development, NEDO has been working with the likes of the University of Tokyo and Shinshu University – bringing in also the Japan Technological Research Association of Artificial Photosynthetic Chemical Process (ARPChem) – to develop by this year a photocatalyst that achieves an energy conversion efficiency of 10 percent. Such efficiency would be equivalent to around 50 times that of plant photosynthesis. To put this into perspective, if an artificial hydrogen gas field were to be installed across 3 percent of the Sahara Desert, it would be capable of supplying enough energy to support the entire globe.
One idea that some have looked at to achieve this is that of two-stage catalysts combining an oxygen production catalyst and a hydrogen production catalyst. And among these, particular attention has gone to tandem catalysts, which have already been used to achieve a conversion efficiency of 7 percent – showing that 10 percent is well within reach. Still, with the tandem type having a high level of activity, it is not well suited to area increases and cost reduction. Given this, promise is being identified in the adoption of parallel-type catalysts or, alternatively, a single-stage catalyst using powder semiconductor photocatalysis.
The latter of these, an acid sulfide photocatalyst (Y2Ti2O5S2), sees water decomposed in visible light to make for efficient use of solar energy. Given the fine particles involved here, a photocatalyst sheet can be created by way of coating, thereby allowing for an increase in area while also reducing cost. Development efforts are currently under way to increase conversion efficiency from 1.3 percent up to 2–3 percent by the end of this year. If the 3 percent mark can be achieved with a powder catalyst, it would be possible to start looking at industrial-scale use.
Still, given that hydrogen and oxygen are generated as a mixed gas in each of the single-, dual-, and parallel-stage types, the need is there to ensure safe separation of the hydrogen without explosion. Having looked at a separation process that utilizes a zeolite membrane from Mitsubishi Chemical Corp., NEDO is currently pursuing reduced costs with this in view of real-world application.
Then for developing a process to selectively produce olefins through solar hydrogen reaction with recovered CO2, efforts are centering on research efforts out of the Tokyo Institute of Technology. The process here involves methanol being produced from hydrogen and CO2, with olefin production then coming from said methanol – the goal being to selectively produce ethylene, propylene and butene with a targeted olefin yield of 70 percent.
The aim here is to make this compatible with innovative C1 processes for producing methanol at low temperature, low pressure and high yield. It would then be possible to achieve stable plant operations that allow for a gradual shift in the raw materials used to make hydrogen, with the point being to transition from methane to water.
Following the end of the NEDO project in fiscal 2021, there will be a need for further efforts that allow for CO2 emissions to be gradually reduced. First here is a follow-up project that is slated to slide in from fiscal 2021 with the aim of commercialization. This is projected to see the practical application of an innovative C1 process for utilizing fossil resources by the mid-2020s.
For the backend of the 2020s, the aim is to shift some petrochemical processes over to using biomass raw materials. Then from the 2030s, aims are to get started on the practical application of a solar hydrogen-utilizing chemical process; followed by the 2040s and onward with full-scale carbon capture and utilization (CCU), with the intent of using the CO2 recovered here as a chemical raw material.
Expectations are that this multi-stage plan will allow for chemical complexes to achieve virtually zero emissions by way of transitioning current naphtha cracker practices over to a biomass raw material process that could be expanded at large, as well as by also adopting a methanol-to-olefin (MTO) process coming via artificial photosynthesis.