In their paper, they suggested that this is a first step towards the development of small-scale low-pressure devices for CO₂ reduction to methanol.

Today, methanol is produced in large facilities from CO, CO₂ and H₂ (derived from fossil resources) in a high-pressure (50–100 bar) process using a Cu/ZnO/Al2O₃ catalyst. If hydrogen production is to be distributed and produced in small-scale devices, it would be attractive if the subsequent conversion of H₂ into a liquid fuel could also be performed in simpler, low-pressure decentralized units. This is not, however, simply a case of reengineering the technology currently optimized for high-pressure conversion of syngas into methanol, because a low-pressure CO₂ reduction process may require a different catalyst.

Another challenge arises with the use of CO-free CO₂, which will lead to CO as a by-product of methanol via the reverse water–gas shift (rWGS) reaction. The production of CO not only reduces the yield of methanol—it also has a negative effect when methanol is used in fuel cells because CO poisons the Pt catalyst used. Using the industrial Cu/ZnO/Al2O₃ catalyst (which is optimized for different reaction conditions including a CO-rich feed) in low-pressure methanol synthesis leads to significant CO production, so new catalysts are needed to advance this field.

… we report the discovery of a new, non-precious metal catalyst working at low pressure with similar or higher methanol yield than the current Cu/ZnO/Al2O₃ methanol synthesis catalyst. We use a computational descriptor-based approach to guide us towards a new class of Ni-Ga catalysts and show experimentally that it has the unique property that it reduces CO₂ to methanol without producing large amounts of CO via the rWGS reaction.

—Studt et al.

Worldwide, about 65 million metric tons of methanol are produced each year for use in the manufacture of paints, polymers, glues and biofuels. In a typical methanol plant, natural gas and water are converted to synthesis gas (syngas), which consists of carbon monoxide, carbon dioxide and hydrogen. The syngas is then converted into methanol in a high-pressure process using a catalyst made of copper, zinc and aluminum.

Methanol is processed in huge factories at very high pressures using hydrogen, carbon dioxide and carbon monoxide from natural gas. We are looking for materials than can make methanol from clean sources, such as sunshine, under low-pressure conditions, while generating low amounts of carbon monoxide. We spent a lot of time studying methanol synthesis and the industrial process. It took us about three years to figure out how the process works and to identify the active sites on the copper-zinc-aluminum catalyst that synthesize methanol.

—lead author Felix Studt, SLAC

The ultimate goal is to develop a large-scale manufacturing process that is nonpolluting and carbon neutral using clean hydrogen, the authors said.

Imagine if you could synthesize methanol using hydrogen from renewable sources, such as water split by sunlight, and carbon dioxide captured from power plants and other industrial smokestacks. Eventually we would also like to make higher alcohols, such as ethanol and propanol, which, unlike methanol, can be directly added to gasoline today.

—co-author Jens Nørskov, a professor of chemical engineering at Stanford.

Instead of testing a variety of compounds in the lab, Studt and his colleagues searched for promising catalysts in a massive computerized database that he and co-author Frank Abild-Pedersen developed at SLAC—i.e., computational materials design.

Studt compared the copper-zinc-aluminum catalyst with thousands of other materials in the database. The most promising candidate turned out to be a little-known compound called nickel-gallium.

The researchers turned to a research group at the Technical University of Denmark led by Ib Chorkendorff, a co-author of the research paper, for testing of the compound. First, the Danish team carried out the task of synthesizing nickel and gallium into a solid catalyst. Then the scientists conducted a series of experiments to see if the new catalyst could actually produce methanol at ordinary room pressure.

The lab tests confirmed that at high temperatures, nickel-gallium produced more methanol than the conventional copper-zinc-aluminum catalyst, and considerably less of the carbon monoxide byproduct.

You want to make methanol, not carbon monoxide. You also want a catalyst that’s stable and doesn’t decompose. The lab tests showed that nickel-gallium is, in fact, a very stable solid. We’d like to make the catalyst a little more clean. If it contains just a few nanoparticles of pure nickel, the output drops quite a bit, because pure nickel is lousy at synthesizing methanol. In fact, it makes all sorts of chemical byproducts that you don’t want.

—Ib Chorkendorff

Nickel is relatively abundant, and gallium, although more expensive, is widely used in the electronics industry. This suggests that the new catalyst could eventually be scaled up for industrial use, according to the authors. But to make methanol synthesis a truly carbon-neutral process will require overcoming many additional hurdles, they noted.

Other co-authors of the study are Jens Hummelshøj of SLAC; and Irek Sharafutdinov, Christian Elkjaer and Søren Dahl of the Technical University of Denmark.

The research was supported by the USDepartment of Energy, the Danish National Research Foundation and the Danish Ministry of Science, Technology and Innovation.

Resources

• Felix Studt, Irek Sharafutdinov, Frank Abild-Pedersen, Christian F. Elkjær, Jens S. Hummelshøj, Søren Dahl, Ib Chorkendorff & Jens K. Nørskov (2014) “Discovery of a Ni-Ga catalyst for carbon dioxide reduction to methanol” Nature Chemistry doi: 10.1038/nchem.1873