Trends and Advances in Operando Methodology
Atsushi Urakawa*
Abstract

After the introduction of the term, operando, the catalysis community has taken
significant steps forward to understand chemistry and physics taking place within catalyst
body and catalytic reactor on different length and time scales with great motivation to firmly
establish catalyst structure vs. catalytic performance relationships. Herein recent advances,
current trends and possible future directions in operando methodology are briefly
summarized.

Introduction
Nearly 15 years has passed since the Latin word, operando, was coined by Miguel A.
Bañares, Eric M. Gaigneaux, Gerhard Mestl, and Bert M. Weckhuysen to distinguish the
experimental conditions from in situ and the first papers mentioning operando appeared in
2002 [1-4]. As described in detail by Bañares [5] and Weckhuysen [6], this historical action
was motivated by the necessity to highlight the importance of studying catalyst materials
under working conditions (i.e. showing practically relevant catalytic activity) by analytical
and visualization techniques. The definition of catalysts’ working condition naturally requires
verification of catalytic activity and selectivity. Hence, the condition of in situ study of
catalyst materials and/or catalytically active species together with simultaneous evaluation of
catalytic performance is generally defined as operando methodology. The term has now
widespread to various research fields such as gas sensors [7,8] and battery [9-11] when

1

spectroscopic measurements and spectro-imaging are performed under technologically
relevant conditions to elucidate structure-performance relationships.
The birth of operando methodology in catalysis was urged by the increasing number
of examples showing large temperature, pressure and materials gaps where catalytically
active species and sites could be uniquely formed under specific reaction conditions, typically
a combination of high-pressure, high-temperature, and specific nano-structured catalyst
materials deposited over inorganic materials which are shaped in a pellet form (Figure 1).
Nowadays, it is widely recognized that the structure of catalyst surface is dynamic and often
undergoes reconstruction when reactants interact with the outer surface. A notable example is
the atmospheric-pressure STM study of CO oxidation by Hendriksen and Frenken [12].
When the partial pressure of O2 to CO was sufficiently high, the Pt surface was reconstructed,
forming PtO2-like islands which were remarkably more active in CO oxidation than the clean
Pt surface [13,14]. When O2 concentration became low, the Pt surface reconstructed back to
flat Pt surface, showing the dynamic nature of the catalytic surface.[12,15] Similar insights
were also gained for Pd surface and the widely observed oscillatory behavior of CO oxidation
activity could be elegantly explained by formation and disappearance of the highly active
nano-oxide phase of Pd.[16] These studies show that well-defined materials under operando
conditions can yield precise and important physical and chemical insights on the atomic
scale.
Obtaining the key information on catalyst materials and reaction conditions directly
linked to the reactivity of catalysts on the laboratory or industrial scale requires, however,
further efforts in simulating and understanding the complexity of the catalyst material,
diffusional characteristics, and hydrodynamics on different length scales, i.e. those of atom,
catalyst particle, pellet and reactor [17,18]. When catalyst pellets are packed in a reactor and
a fluid of reactant(s) passes through it, there are always concentration gradients of reactants,
2

products and possible intermediates in the fluid and over catalyst surface along the axial and
possibly radial direction of the reactor. On a smaller scale, within the catalyst pellet, reactions
take place and concentration gradients of reactants, intermediates and products arise. On the
same scale, elemental and concentrational heterogeneities of catalytically active materials
within the pellet may be present deliberately or accidentally. On the smallest scale, catalytic
activity is induced and influenced by various physical and chemical phenomena such as
atomic/molecular adsorption, surface reaction, desorption, surface reconstruction, active site
formation, and/or surface poisoning, which also create concentrational heterogeneities
because actual catalyst surfaces are never flat like perfect single crystals but complexly
shaped mountains and valleys composed of various chemical elements. It is important to
realize that the above gradients on the scales of reactor and catalyst pellet can directly
influence the atomic scale surface chemistry because they impact on the concentrations of
chemical species in the fluid phase, which can be even time-dependent [19]. Moreover,
temperature may vary on each length scales and understanding these gradients can be of
direct relevance for full comprehension of heterogeneous catalytic processes [20].
Furthermore, even when technology allows studying physicochemical processes on
the different length and desired time scales, there is another issue of studying catalytically
active sites and species selectively under operando conditions without the information buried
in the overwhelming signals of spectator chemical species, solvent and catalyst support
materials. This is inherent to any measurement techniques and becomes serious under
operando conditions, where surface chemical species are present at high concentrations and
selection of detection methods is limited by the typical operational conditions of high
temperature and pressure.
Over the past years, significant progresses were made to tackle the above challenging
aspects of operando methodology and catalysis research in general. Recent trends and
3

advances made in the field from my personal view are briefly highlighted in Figure 2 and
summarized below.
Multi-probe approach
Using a single characterization technique often fails to give a sufficiently precise
picture of catalytic processes and materials because it is limited in probing area (spaceresolution), speed (time-resolution), transition (type of excitation/relaxation), element,
electronic structure and/or geometrical structure (long/short-range ordering). Therefore,
combined use of complementary techniques is desired to gain holistic views on catalytic
processes (Figure 2a) [21]. Powerful combination to gain molecular, electronic and structural
information

is

vibrational

(IR/Raman)

spectroscopy

combined

with

X-ray

spectroscopy/diffraction [22]. Newton has pioneered the combination of diffuse reflectance
Fourier transform infrared spectroscopy (DRIFTS) and X-ray absorption spectroscopy
(XAS), the former providing information about chemical bonds of surface species and the
latter providing geometrical and electronic structures of active metals [23]. The advent of
low-noise, high-resolution 2D detectors has enabled X-ray diffraction (XRD) to be combined
with other techniques like XAS using the same capillary reactor cell [21,22,24,25]. Another
combination gaining popularity recently using X-ray is XAS/X-ray emission spectroscopy
(XES), which can yield a detailed picture of electronic states (unoccupied and occupied
orbitals). Also, local coordination (e.g. adsorbate or ligand) environment can be studied by
the latter technique more precisely. An elegant use of the combination was recently
demonstrated by Jacob and Grunwaldt to elucidate the chemistry around active Fe center
located in a zeolite structure for selective catalytic reduction (SCR) of NO by NH3 [26].
Possibilities for the combination of operando probes are infinite and it is important to choose
the most appropriate techniques to investigate the key information under question.

4

Space-resolved spectroscopy/diffraction, imaging and tomography
Seeing the reality by eyes (with the help of light) is often worth more than a number
of catalytic experiments. This approach is very rapidly evolving and has been recently
demonstrated by several research groups under operando conditions. Generally, three
different modes are used to add space-resolution to a technique on reactor, pellet and atomic
scale with/without the help of post-mathematical reconstruction of images.
The first one (Figure 2b) makes use of a focused light and one gains a spaceresolution by moving a pellet/reactor. Confocal Raman spectroscopy operates in this mode
and has been applied to study water transport within proton-conducting polymer electrolyte
under operando condition [27]. Along entire catalyst beds, operando space- and timeresolved DRIFTS-Raman [19], XAFS [28] and XRD [29] were used to study catalyst
materials in deNOx chemistry and methanol-to-olefin (MTO) process. The second mode
(Figure 2c) is to obtain the full picture in one shot using a 2D detector with a great advantage
in the time-resolution [30]. Such a fast 2D detector was used also for single-molecule
counting based on total internal reflection fluorescence microscopy (TIRFM) to prove that a
cycloaddition reaction took place over the surface of Cu nanoparticles and not in the solution
by possible leached Cu species [31].
The third mode (Figure 2d) for chemical/structural imaging or tomography, slice by
slice, is widely used in magnetic resonance imaging (MRI) well known for medical
applications. MRI is also actively employed in heterogeneous catalysis research by Koptyug
to gain unique insights into mass transport and heat transfer on the pellet and reactor scales
[32]. Similar slicing approach using computed tomography (CT) of XRD , XAS, and X-ray
fluorescence (XRF) have been pioneered by Beale, Jacques, Di Michiel, and Weckhuysen
among others using mainly synchrotron facilities [33,34] and even in the lab [35]. Inside the
complex membrane reactors for oxidative coupling of methane (OCM) reaction could be seen
5

through [36] and in another study even a catalyst particle in liquid phase chemistry could be
studied [37]. Impressive time-resolution (>1 Hz) was demonstrated in battery research under
operando thermal runaway using CT-tomography and radiography combined with thermal
imaging [38].

Multivariate analysis
Analysis of operando spectroscopic data often suffers from three issues. One is
related to the detection selectivity towards what we would like to hunt as described above.
Another challenge is the interpretation of spectra, which is often hindered by overlapping
signals of coexisting multiple components in the bulk and/or on the surface of catalyst solids.
Under operando conditions, the situation is significantly more challenging compared to that
under well-defined UHV conditions. Even worse, appropriate reference spectra are often
unavailable because catalytically active species and phases may be only present stably or
transiently under operando conditions.
The problem of detection selectivity may be elegantly solved by exerting a
perturbation of an external parameter (temperature, concentration, light flux, etc.) to a
catalytic system such that the reaction rates and concentrations of species/phase of interest
vary. Such step/pulse-response experiments are widely used. Among them, steady-state
isotopic transient kinetic analysis (SSITKA) is an experimental technique to follow the
kinetics of catalytic reactions by varying reactant concentration in the form of isotopomers
without disturbing the chemistry [40]. Another approach to add selectivity with drastic
improvement in detection sensitivity is modulation excitation spectroscopy (MES) [41]. MES
uses periodic change of an external parameter (e.g. varying reactant concentration, ideally
from that of steady-state one for operando studies). Spectra are continuously measured over a
number of stimulus-relaxation cycles and averaged into one cycle (first S/N improvement)

6

and then the one-cycle data are further processed by phase-sensitive detection (PSD) to filter
out static signal (e.g. due to solvent and catalyst support) and noise (second S/N
improvement) and to kinetically differentiate evolving signals in the so-called phase domain
[41]. MES can be combined with any techniques such as IR [42] and XAFS [43]. Recently,
the theory of MES for diffraction has been developed and the technique is called modulation
enhanced diffraction (MED) [44]. By MED one can study active sublattices [45] and
element-specificity can be achieved by applying X-ray energy MED [46].
SSITKA and MES can extract valuable information, but both often suffer from signal
overlaps and occasional unavailability of spectral references for interpretation. Recently, an
increasing number of applications of multivariate spectral analysis suited for blind source
separation has been witnessed. In such analysis, one measures spectra over a period of time
with/without external perturbation and directly post-process the spectroscopic data by the
mathematical engine to separate them into kinetically pure components and respective
concentration profiles (Figure 2e). Blind means there is no reference required for the spectral
separation process. Band-target entropy minimization (BTEM) actively applied in
homogeneous catalysis [47] was also employed to identify surface species measured by
DRIFTS during the reaction of CO and NO over Pd [48]. Non-negative matrix factorization
(NNMF) was used to separate overlapping UV-Vis spectra to identify deactivating species
during MTO process over H-SSZ-13 [49]. Multivariate curve resolution (MCR) became very
popular across different disciplines for blind source separation and it has been successfully
used for XAS [50,51] and IR spectroscopy [52] with kinetic analysis of heterogeneous
catalytic reactions with a strong emphasis to simplify the spectral analysis by the conversion
of numerous spectra to a few to some component spectra and respective concentration
profiles.

Outlook
7

Categorizing specific areas of in situ into operando has promoted the awareness of
researchers about the importance of operando methodology in catalysis research to study
catalysts under technologically relevant conditions and to look at important spots within
catalytic reactors and catalyst materials. The intentional use of the term, operando, urged by
the awareness of the “gaps” has obviously driven the great technological advances in
demanding and sophisticated space- and time-resolved measurements on different length
scales. There are clear next steps in the three main streams of operando methodology
described here by employing more complementary probes measured simultaneously,
enhancing further the space/time resolution, and improving the blind source spectral
separation methods. Yet, the challenges of operando community still remain as indicated by
the generally poor penetration of operando methodology in industry or by scarce
collaboration between academic and industry researchers using operando methodology from
the author’s view. The point is less scientific but more opportunities for dialogs between
researchers in academia and industry are crucial to enhance the relevance of research work to
know exactly what kind of aspects are more relevant in industrial processes and to know
conditional constraints in commercial processes. Along the same line, the relevance is best
ascertained when operando studies are performed in a reactor where catalytic performance
makes practical sense. For this, chemical engineering aspects like hydrodynamics, mass
transport, and reaction kinetics are ideally taken into account in operando spectroscopic
reactor design. These aspects are actually best clarified by also operando methodologies
using spectroscopic and visual inspection to see through operando reactor mimicking
industrial processes or envisioned future catalytic processes.
Acknowledgements
MINECO is acknowledged for support through Severo Ochoa Excellence Accreditation
2014−2018 (SEV-2013-0319) and CTQ2012-34153.
8

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13

Figure 1. Three major gaps (material gap, temperature gap and pressure gap) often causing
problems in practical relevance of spectroscopic and diffraction studies of catalyst materials.
These gaps have to be filled to address real situation of catalyst material at work under
technologically relevant conditions. UHV stands for ultra-high vacuum.

material
gap

P gap

T

Industrial
reaction

UHV

T gap
in situ

P

14

Figure 2. Trends in operando methodology: (a) multi-probe approach, (b-d) space- and
possibly time-resolved approach, where spatial resolution is achieved by (b) moving a sample
(reactor/catalyst particle), (c) using 2D detector, (d) first sliced 2D then 3D spectroimaging
by sample translation, and (e) multivariate analysis to identify spectroscopically “pure”
components and their concentration profiles often without the need of reference sample
measurements. For (a-d) the approach can be used for studying a single catalyst particle or
pellet.

(a)

(b)

(c)

(d)

Time

Concentration

Multivariate
analysis

Intensity

Intensity

(e)

Energy

Energy

15

Time