Nanoparticle catalysts, drivers of most of the world’s giant chemical processes, often lose their catalytic oomph because their active sites become clogged from a buildup of carbon gunk known as coke and because the tiny active particles fuse into larger less active ones. By nanoengineering catalyst particles and the materials on which they sit, researchers have come up with a way to shut down both deleterious processes, and used the method to make robust catalysts that convert carbon dioxide to valuable products (Science 2020, DOI: 10.1126/science.aav2412A).
One strategy for reducing atmospheric levels of CO2 involves using the ubiquitous compound as a feedstock in major industrial chemical processes. Dry reforming of methane is one promising candidate. That process reacts CO2 with methane and generates a mixture of carbon monoxide and hydrogen. Known as synthesis gas, or syngas, the CO-H2 mixture is used commercially to produce transportation fuels and chemicals. Several catalysts, including ones based on low-cost nickel, can do the reforming job. But they aren’t durable due to coking and particle fusing, also known as sintering.
Youngdong Song and Cafer T. Yavuz of the Korea Advanced Institute of Science and Technology and coworkers suspected that certain types of highly active catalyst sites might be causing the trouble. So they set out to prepare uniform catalysts largely free of crystal defects, the typical culprits for that hard-to-control reactivity.
Through experimentation, the team found that molybdenum-doped nickel nanoparticle catalysts—anchored to a solid such as magnesium oxide—actively drove the methane reforming reaction. Some common methods for making such catalysts left them riddled with defects. So the team turned to less common ones.
The researchers began by reacting magnesium chips with CO2, producing nearly perfect MgO crystals. Then they reacted nickel and molybdenum salts in a polyol-mediated process using hydrazine as a reducing agent and a surfactant to control the size of the Ni-Mo particles. Lab tests showed that the new catalysts were more active than many industrial and research catalysts. And unlike catalysts the team made via common preparation methods, the new catalysts showed no signs of coke buildup and performance loss even after 850 hours of reaction.
Synchrotron studies showed why. Synthesis conditions led to formation of roughly 3-nm-diameter Ni-Mo crystals that latched onto the relatively few highly reactive defect-like sites on MgO. Under reaction conditions, additional particles migrated to these high-energy step-edge sites, controllably fused into somewhat larger particles, stopped growing, and stayed in place, blocking unwanted reactions at these potential trouble spots.
Qiang Xu, a catalysis specialist at the National Institute of Advanced Industrial Science and Technology in Osaka, Japan, notes that “this discovery will likely drive rapid development of active and stable nanocatalysts for many reactions and may pave the way for industrial application.”.