Using XRD to Determine Crystallite Sizes and Phase Composition of Molybdate-Type Catalysts

Molybdenum catalysts

Similar to molybdenum-bearing enzymes which are common bacterial catalysts that break down the chemical bond of molecular nitrogen in the atmosphere in the process of biological nitrogen fixation, which is critical to plant life and agriculture, molybdates are molybdenum compounds in high oxidation states. Used as catalysts in selective oxidation of hydrocarbons, molybdates are precursor compounds for catalysts used in the hydrodesulfurization of refined petroleum products and natural gas. Hydrodesulfurization is a critical step in refining fossil and biofuels to remove sulfur, a known pollutant responsible for smog. The process is also used to create ultra-low sulfur diesel fuel and to upgrade the octane rating of intermediate hydrocarbons in the production of petroleum.

Types of molybdate catalysts

Nickel molybdate (NiMoO₄) has attractive structures, higher specific capacitance, higher electrochemical activity and magnetic properties compared to other molybdates (Zn, Co, Mg, etc.) because it displays high density of states near the top of the valence band. It is used in the oxidative dehydrogenation of light alkanes such as propane and has been studied for use as a high-performance NiMoO₄xH₂O nanorod electrodes for supercapacitors. It can be found in three crystalline forms under atmospheric pressure, α-NiMoO₄ at low temperature, β-NiMoO₄ at high temperature, NiMoO₄nH2O hydrate, and another allotrope (NiMoO₄-II) at high pressure. (ref. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6017352/) X-ray diffraction (XRD) is a method that is very sensitive to changes in the crystallographic structure of solid materials and commonly used to distinguish different crystalline forms.

Cobalt molybdate heterostructures (CoMoO₄ and CoO₂MoO₃O₈) create a high-performance hydrogen evolution reaction, which has been suggested as a promising clean and renewable energy source. These molybdate heterostructures were found to create efficient and inexpensive electrocatalysts for catalytic reactions that produced energy with high voltage efficiency, excellent long-term stability, and superior conductivity. The materials were shown to reduce hydrogen adsorption energy and significantly enhance electrocatalytic activity (ref. https://pubag.nal.usda.gov/catalog/6539770).

XRD analysis of catalyst particles

The catalytic activity of such molybdenum materials strongly depends on the particle size and the ratio to the support material. For these types of materials, Al₂O₃ is often used as a support material and appears as a mixture of amorphous and crystalline fractions. The most commonly used method to determine both the crystallite size of nanoparticles and the quantitative composition of solid mixtures is XRD.

One of our XRD application specialists, Dr. Simon Welzmiller, conducted XRD measurements of NiMoO₄ and CoMoO₄ powders to determine quantitative phase analysis and crystallite size of these candidate molybdate catalysts. Using our ARL EQUINOX 100 benchtop X-ray diffractor, the qualitative phase analysis revealed that both samples contain a significant amount of Al₂O₃, which acts as a support for the actual β-CoMoO₄ and β-NiMoO₄ phases. The analysis was performed in 45 minutes. This analysis revealed significant amounts of active molybdate catalysts with crystallite size of 10 nm for both molybdates.

For more information on this, you can download our whitepaper. Download “Characterization of molybdate type catalysts using benchtop ARL EQUINOX 100 XRD”.

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