Hydrochloric acid boosts catalyst activity

X-ray investigation reveals altered structure of enhanced catalyst material

Nickel molybdenum sulfide catalyst

Nickel molybdenum sulfide catalyst after acid treatment. (Credit: TUM, Manuel Wagenhofer) [Source]

A research team from the Technical University of Munich (TUM) has developed a synthesis process which drastically increases the activity of catalysts for the desulfurisation of crude oil. PETRA III measurements show the structure of the enhanced material, revealing the reason for the increased efficiency after treatment with hydrochloric acid. The new process could perhaps also be used for catalysts in fuel cells, as the team led by TUM scientists Johannes Lercher and Hui Shi reports in the journal Science Advances.

Crude oil contains a great deal of sulfur. To turn the crude oil into fuel, the sulfur compounds must be removed using hydrogen. Experts call this process hydrotreating. The process is carried out using catalysts. Hydrotreating is one of the most important catalytic processes – both with regard to the quantity of catalyst used and the quantity of processed raw material. With highly pressurized hydrogen, impurities such as sulfur or nitrogen compounds are removed from the crude oil as completely as possible.

“These kinds of impurities would later combust to form sulfur dioxide and nitrogen oxides, which would result in negative effects on the environment especially the air quality,” says Manuel Wagenhofer, first author of the study. In addition, sulfur and nitrogen compounds would also damage precious metals in catalytic converters in modern vehicles, and drastically reduce their effectiveness.

The TUM chemists examined such mixed metal sulfide catalysts for their effectiveness in hydrotreating by first synthesizing nickel molybdenum sulfides over several process stages, and then treating them with acid. “It was amazing how much adding concentrated hydrochloric acid increased the catalytic performance,” says Wagenhofer. “Hydrochloric acid improves the accessibility of active centers in the catalysts by removing less active components, mainly nickel sulfides. Purer, and therefore more active, mixed metal sulfides are formed.”

The scientists examined the improved catalyst material at beamline P65 of DESY's X-ray light source PETRA III: “Thanks to the X-ray absorption spectroscopy fine-structure XAFS, a structural model for the active phases of the catalyst can be developed,” says DESY researcher Edmund Welter, head of the beamline. “This shows that the active phase is nickel incorporated into the molybdenum sulfide structure, and not finely distributed pure nickel sulfide phases.”

The results are also very important for fundamental research. The purified mixed metal sulfides are also easier to examine, scientifically. “For example, we were able to identify and quantify active centers on the catalysts that were treated in this way,” explains Lercher. “This was only possible because the surface was no longer covered in nickel sulfide.”

In principle, the acid treatment could apparently be used as an investigation instrument for a series of similar catalysts, to optimize these, for example, for use with oils from renewable raw materials which are to be transformed into climate-friendly fuels in the future via a refining process. “If we understand mixed metal sulfide catalysts better, we can perhaps improve them considerably for use in other important fields of the future, such as water electrolysis or hydrogen fuel cells,” says Johannes Lercher.

Parts of this work were funded by Chevron Energy Technology Company and the Federal Ministry of Education and Research (BMBF) in the framework of the MatDynamics joint project.

(from DESY news)


Enhancing hydrogenation activity of Ni-Mo sulfide hydrodesulfurization catalysts; Manuel F. Wagenhofer, Hui Shi, Oliver Y. Gutierrez, Andreas Jentys, Johannes A. Lercher; Science Advances, 2020; DOI: 10.1126/sciadv.aax5331