Characterizing Catalyst Failure Modes with Analytical XAFS

The eventual failure of catalysts is an unavoidable fact, whether in research and development, scale-up, or operations. Understanding the fundamental causes for catalyst failure is the critical first step toward finding improved catalyst formulations and processes that extend lifetimes, as well as improve the commercial potential and actual operations budgets for industrial-scale reactors.

Until now, many of the underlying mechanisms that lead to deactivation have been analytically difficult to characterize, but with laboratory-based X-ray Absorption Fine Structure (XAFS), easyXAFS is able to provide key insights into catalyst degradation pathways.

The 6 Mechanisms for Catalyst Failure Modes

There are six mechanisms for catalyst failure that focus on supported heterogenous catalysts, as shown in the figure below.  

Two failure mechanisms, coking and substrate sintering, introduce diffusion barriers that prevent reactants from reaching otherwise functioning metal centers.  

One mechanism is specific to industrial-scale process management, i.e., the problem of mechanical failure of catalyst pellets.

The other three failure mechanisms share a changing local environment at the active metal species, which poses long-standing, largely unmet analytical challenges in industry. 

Industrial catalysts and real-world catalysts seeking to use inexpensive feedstocks often fail because of poisoning where some species in the input reactants chemisorbs or reacts at the active metal sites, resulting in catalyst poisoning.  

Metal sintering occurs when the single-atom or larger metal atom clusters are mobile under processing conditions and decrease their surface energy by forming larger clusters, thus decreasing catalytic activity.  

Finally, at long operating times some catalysts degrade via formation of a catalytically inactive phase that nucleates and rimes to consume the previously catalytically active metal species on the support surface.

While coking, support sintering, and mechanical breakdown often present as diffusional limitations or structural failures, poisoning, metal sintering, and inactive phase formation involve changes in the local chemical environment of the catalytically active metal species — typically before long-range structural changes become detectable.

The Limits of Conventional X-Ray Techniques

X-ray diffraction is blind to the onset of these short-range structural or isolated site changes. 

X-ray photoelectron spectroscopy can detect oxidation state changes that accompany poisoning and the formation of inactive phases, but requires ultra-high vacuum (UHV)  conditions and is notoriously sensitive to both sample preparation and data analysis assumptions.

Transmission electron microscopy (TEM) again requires UHV conditions and can only image small sample, often non-representative portions after tedious sample preparation.

The Solution: Laboratory-Based Analytical XAFS

The development of laboratory-based X-ray Absorption Fine Structure (XAFS) systems has significantly increased accessibility to a technique that is well-established for quantifying changes in oxidation state and local atomic structure of a specific chemical element.

While XAFS has traditionally been seen as an expert-method with highly restricted access at synchrotron light sources, easyXAFS is the leader in the ongoing rebirth of laboratory-based XAFS using conventional x-ray sources. Our spectrometers and mail-in measurement service give rapid characterization of changing oxidation state and local environment at metal sites in supported heterogeneous catalysts. This can be accompanied by customized analysis software for repetitive assays, providing turn-key solutions for industry.

This new, analytical XAFS access for the catalyst R&D and industrial communities is accelerating materials discovery cycles, strengthening intellectual property filings, solving process issues in catalyst scale-up, and offering a new path for improving catalyst lifetime from the benchtop to the industrial scale.

To learn more about how easyXAFS can support your catalyst development and failure analysis efforts, contact us to discuss your specific challenges and how XAFS can deliver actionable insights.