How the in situ monitoring of bulk crystalline phases during catalyst activation results in a better understanding of heterogeneous catalysis.

Simon Penner

doi:10.1039/d1ce00817j

Abstract

The present Highlight article shows the importance of the in situ monitoring of bulk crystalline compounds for a more thorough understanding of heterogeneous catalysts at the intersection of catalysis, materials science, crystallography and inorganic chemistry. Although catalytic action is widely regarded as a purely surface-bound phenomenon, there is increasing evidence that bulk processes can detrimentally or beneficially influence the catalytic properties of various material classes. Such bulk processes include polymorphic transformations, formation of oxygen-deficient structures, transient phases and the formation of a metal–oxide composite. The monitoring of these processes and the subsequent establishment of structure–property relationships are most effective if carried out in situ under real operation conditions. By focusing on synchrotron-based in situ X-ray diffraction as the perfect tool to follow the evolution of crystalline species, we exemplify the strength of the concept with five examples from various areas of catalytic research. As catalyst activation studies are increasingly becoming a hot topic in heterogeneous catalysis, the (self-)activation of oxide- and intermetallic compound-based materials during methanol steam and methane dry reforming is highlighted. The perovskite LaNiO3 is selected as an example to show the complex structural dynamics before and during methane dry reforming, which is only revealed upon monitoring all intermediate crystalline species in the transformation from LaNiO3 into Ni/La2O3/La2O2CO3. ZrO2-based materials form the second group, indicating the in situ decomposition of the intermetallic compound Cu51Zr14 into an epitaxially stabilized Cu/tetragonal ZrO2 composite during methanol steam reforming, the stability of a ZrO0.31C0.69 oxycarbide and the gas-phase dependence of the tetragonal-to-monoclinic ZrO2 polymorphic transformation. The latter is the key parameter to the catalytic understanding of ZrO2 and is only appreciated in full detail once it is possible to follow the individual steps of the transformation between the crystalline polymorphic structures. A selected example is devoted to how the monitoring of crystalline reactive carbon during methane dry reforming operation aids in the mechanistic understanding of a Ni/MnO catalyst. The most important aspect is the strict use of in situ monitoring of the structural changes occurring during (self-)activation to establish meaningful structure–property relationships allowing conclusions beyond isolated surface chemical aspects.

Highlights

Introduction into the conceptThe central goal in heterogeneous catalytic research is the determination of and knowledge about catalytically active sites.[1]
We have shown the capabilities of this approach for a number of intermetallic compounds in a variety of reactions,[22] but exemplify it for Cu51Zr14 in the methanol steam reforming reaction.[8]
CrystEngComm decomposition of such intermetallic compounds especially during methanol steam reforming has been known for some time,[8,22] it is exactly the crystalline nature of the Cu51Zr14 compound that allows the transition of the precursor structure into the active phase by, e.g., in situ X-ray diffraction correlated to catalytic selectivity and activity to be monitored

Summary

Introduction into the concept

The central goal in heterogeneous catalytic research is the determination of and knowledge about catalytically active sites.[1]. If it is carried out under in situ reaction conditions, it allows the monitoring of the development of crystalline species as a function of temperature and reaction gas environment and, by correlation with catalytic profiles, the establishment of structure– activity correlations.[7] It is worth noting that the in situ monitoring allows the potential influence of transient crystalline species on the catalytic properties to be unraveled, which cannot inherently be studied if ex situ characterization of the spent catalyst state is carried out This phenomenon is pronounced if the active phase is formed via a pathway of self-activation, e.g., as observed in the decomposition of intermetallic compounds[8] or the exsolution of metallic particles from perovskite materials.[9,10].

In situ activation of DRM catalysts

In situ activation of methanol steam reforming catalysts

Stability of oxycarbides

Gas phase dependency of the polymorphic ZrO2 structure transformation

Carbon reactivity

Summary and outlook