Figure 1. The nickel-gallium active site synthesizes hydrogen and carbon dioxide into methanol. In this artist's rendering, the nickel atoms are light grey, gallium atoms are dark grey, and oxygen atoms are red. Source: Jens Hummelshoj/SLAC.
The scientists, hailing from Stanford University,
Stanford, Calif.; SLAC National Accelerator Laboratory
, Menlo Park, Calif.; and the Technical University of Denmark
, Kongens Lyngby, Denmark, note their ultimate goal is to develop a large-scale, low-cost, non-polluting, carbon-neutral manufacturing process using clean hydrogen to produce methanol for use as a potential fuel and chemical feedstock."Eventually we would also like to make higher alcohols, such as ethanol and propanol, which, unlike methanol, can be directly added to gasoline today," says Jens Nørskov, a professor of chemical engineering at Stanford who worked on the project."Methanol is processed in huge factories at very high pressures using hydrogen, CO2
and CO from natural gas," adds Felix Studt, a staff scientist at SLAC. "We are looking for materials than can make methanol from clean sources, such as sunshine, under low-pressure conditions, while generating low amounts of CO."The group spent three years studying methanol synthesis at the molecular level to identify the active sites on the copper-zinc-aluminum catalyst that synthesize methanol. Next, the team searched for a new catalyst that would react at low pressures using only hydrogen and CO2
. A massive computerized database at SLAC compared the copper-zinc-aluminum catalyst with thousands of other materials and found nickel-gallium to be a potential candidate.The group then turned to the Technical University of Denmark led by Ib Chorkendorff, to test the compound. Nickel and gallium were synthesized into a solid catalyst, which then was tested at ambient pressure. It produced more methanol at high temperatures than the conventional catalyst and significantly less CO byproduct. "You want to make methanol, not carbon monoxide," Chorkendorff notes. "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." More details appear in a recent article in Nature Chemistry
.While these results show promise, a great deal of work lies ahead, the researchers say. Currently, the group is focused on optimizing both the reaction conditions as well as the catalyst. "There hasn't really been a lot of catalyst optimization done so far and we'd like to make the catalyst more phase pure (and with less pure nickel). We are also looking at promoters as we try to find ways of suppressing the formation of CO further. We think that this could be achieved by selectively poisoning the sites that are active for CO formation while we keep the sites that make methanol," notes Studt.In addition, because the reaction mechanisms for CO2
hydrogenation to methanol and the reverse water gas shift to make CO are quite different, Studt believes optimizing the catalyst in a way that is only active for methanol formation could result in high selectivity, but adds it's very hard to make any predictions in numbers. Further challenges include taking CO2
from air or power plant operations and hydrogenating that using H2
made from renewable resources. "I think there are quite some challenges ahead for all of the different processes. It has to be shown that CO2
can be captured efficiently (that might be the biggest challenge here). But we would also need efficient production of H2
, and there is actually a lot of research being done right now on the topic of both electrochemical and photo-electrochemical H2
production (this is actually done both by our group at Stanford and by Ib Chorkendorffs group at the Technical University in Denmark)," says Studt.Once the catalyst is optimized, the group plans to find an external partner to run tests on a pilot-plant scale.