Bulk Composition of Catalysts

Bulk composition of catalysts can be analysed in terms of the phases and / or the elements present. A combination of both approaches is advisable for a full description of bulk composition.

Phase analysis by X-ray diffraction (XRD) goes beyond elemental composition to characterise the crystalline forms present, e.g. an active metallic element might be present as pure metal, oxide, mixed oxide, carbonate, carbide, etc. Even support materials can undergo changes under reaction conditions, e.g. transition aluminas transforming to corundum. Phase identification is based on the the International Centre for Diffraction Data PDF database. Quantification of phases can be based on traditional methods with calibration mixtures but our preference is to use standardless methods where possible. If crystal structures are available, as they are for most inorganic phases, the Rietveld method can be used based on these structures.

X-ray fluorescence is a particularly useful technique for examining the bulk composition of catalyst samples, often with minimal sample preparation. It can be used to provide an overall assessment of the elements present on a new or used catalyst, and can estimate the concentrations present using a re-iterative semi-quantitative calculation. This is useful to identify poisoning by elements such as sulphur, chlorine or unwanted heavy metals which may have caused catalyst failure in service. Support systems may be screened for impurities in a similar way. If there is a requirement  to check that actual concentrations present match those in a specification, then quantitative analysis by XRF is appropriate for the accurate determination of major and minor concentrations. XRF is an effective tool for examining catalysts for the elements Na to U in the approximate ranges of 0.01 to 100% with results expressed as elements or oxides (calculated).

For the determination of trace levels of most elements it may be more appropriate to use ICP techniques (OES or MS), for which it is usually necessary to bring the sample into aqueous solution using microwave digestion with acids, or alkaline fusions. The extra sensitivity offered by ICP-OES and particularly ICP-MS more than compensates for the dilution effect of bringing the sample into solution. With these techniques very low concentrations may be observed in one catalyst compared to another e.g. for comparing competitor’s formulations. Similarly it is possible to examine the distribution of heavy metal poisoning throughout a catalyst bed.

The determination of some non-metals is poor using XRF or ICP techniques, though some laboratories may estimate their concentrations using these techniques. Combustion analysers are a much better solution for the determination of carbon, hydrogen, nitrogen, sulphur, halogens and oxygen in suitable matrices. We have several instruments with different detection methods depending upon the levels of analytes expected.