Self-Renewing Catalyst Turns Greenhouse Gases Into Valuable Syngas

Dry reforming of methane is a chemical process that converts two abundant greenhouse gases, methane and carbon dioxide, into syngas, a mixture of hydrogen and carbon monoxide that can be a feedstock for fuels and a wide range of industrial chemicals. Despite its promise, however, dry reforming of methane has not been industrialized or commercialised. This is because the process requires high temperatures of up to 800° Celsius, which cause catalysts to quickly fail.

Now a team of researchers from three U.S. Department of Energy (DOE) national laboratories - Oak Ridge, Brookhaven and Argonne National Laboratories – with the University of Tennessee Knoxville and Yale University have demonstrated a catalyst that can function at these high temperatures long-term. In fact, the catalyst’s active sites were generated during the high-temperature process.

At high temperatures metal nanoparticles in catalysts tend to cluster together forming less-active clumps and carbon deposits, known as coke, that cover the active surface, hindering the reaction. These problems deactivate the catalyst.

To address this problem, the scientists developed a metal oxide solid solution catalyst from which active nickel–copper alloy nanoparticles continuously emerge during the dry reforming reaction; a process known as exsolution. Their work was published in ACS Catalysis.

To create the catalyst, nickel, magnesium, copper and zinc chloride salts were ball-milled together and then heated to 900°C for four hours. This created a homogeneous solid oxide solution, with the nickel and copper tightly bound within its crystal structure. The nanoparticles could then be generated via a controlled reduction.

To explore how the solid solution transformed under reaction conditions, the research team conducted synchrotron X-ray pair distribution function (PDF) scattering measurements at beamline 1-ID-C of the Advanced Photon Source (APS) at Argonne and X-ray absorption spectroscopy (XAS) measurements at beamline 6-BM of the National Synchrotron Light Source II (NSLS-II) at Brookhaven. The APS and NSLS-II are DOE Office of Science user facilities.

These experiments revealed structural changes in the catalyst samples that suggested the formation of a new alloy phase. They showed that that copper ions reduce relatively quickly to their metallic state during the reaction, while the nickel ions reduced more slowly. Ultimately the two metals formed nickel-copper bimetallic nanoparticles on the surface of the bulky solid oxide, with a composition of roughly 80 per cent nickel and 20 per cent copper.

Further tests on spent catalysts showed that there was little coke accumulation on the active catalyst particles. After 140 hours of operation, analysis showed that the nickel-copper bimetallic nanoparticles carried only 0.17 per cent weight of coke. The researchers concluded that coke is expelled from the catalyst surface as the reaction proceeds rather than building up and blocking active sites. Additional studies showed that magnesium becomes enriched at the surface of the catalyst during reactions, with the study authors suggesting that this reordering of the topmost layer of the oxide solid solution as the nickel–copper nanoparticles exsolve weakens carbon adhesion.

These attributes produced a catalyst that showed little deactivation over long periods of use. Under a concentrated 50/50 reactant feed of methane and carbon dioxide at 800°C, the metal oxide solid solution catalyst lost just 8 per cent of its peak activity after 87 hours.

The researchers say that their approach tackles the high-temperature instability of catalysts that has so far limited dry reforming of methane. By embedding active metals in a stable oxide that enables controlled nanoparticle exsolution and clearing of coke from active sites, they directly address the dual deactivation problem. The study authors add that their insights provide guidance on generating resistant and flexible catalysts via in situ active site formation from easily produced metal oxide solid solutions. They also provide a blueprint for producing durable, scalable catalysts for industrial syngas production from methane and carbon dioxide. – Michael Allen