Heterogeneous catalysis is gaining importance in everyday life: it is estimated that more than 80% of the goods or objects surrounding us have taken advantage, at least once, of a heterogeneous catalytic process during their production. Moreover, with the increasing concern over environmental protection and the demand forever-increasing life quality, politicians and public institutions consistently push towards more stringent regulations on gas emissions, forcing industrial and private subjects to limit waste and to treat exhaust.
These regulations are often complied with catalytic processes designed for a sustainable energy economy and/or for emission abatement. The time of catalysts formulation by empirical ‘‘trial and error’’ methods is already well behind us, and even the implementation of technological demanding and expensive high-throughput screening methods has not been able to produce significant advances in catalyst discovery, though it has provided advances in material enhancement. Both academia and industry are convinced that in order to have an efficient catalyst design, a rational approach is necessary. Obviously, this rational design of a material needs a thorough knowledge of the ideal working way of the catalysts itself, in terms of active centers and local molecular interactions.
This is why a marked increase in the number of papers dealing with spectroscopic investigations of catalytic reactions has been observed in recent years. It reflects the desire of scientists to monitor at the molecular level the details of chemical reactions. In fact, traditional microkinetic studies can offer reasonable models of the processes taking place, using calculations partly based on thermodynamics and other theoretical considerations.
However, such analyses are in some instances merely a curve-fitting exercise, based on the measure of the concentrations of reactants and products under varying experimental conditions, without any actual direct observation inside the reactor. The use of spectroscopy can be very beneficial in supporting microkinetic analysis by literally turning on the light inside the reactor, which is no longer a ‘‘black box’’ to the scientist. Spectroscopic techniques had been mostly used in the past to characterize fresh or used catalysts, in order to obtain structural information relating to the bulk and surface of the solids. In the second half of the last century, and in particular at the end of the 1970’s, a few scientists realized that such characterization would be so much more useful when used in the conditions of temperature and pressure relevant to the catalytic tests.
This was the dawn of in situ analyses, which went on spreading to most fields of the chemical sciences, supported by the increased availability of commercial in situ cells. One of the great achievements of in situ techniques was the realization that the behavior of a catalytic surface in the presence of reagents under relevant pressures and temperatures was often significantly different from that observed ex situ, for example, under vacuum or room temperature conditions. A molecular approach to the study of chemical reactions requires the determination of active sites, reaction elementary steps and, when possible, intermediate species. Such investigations are only relevant when the catalyst is studied in situ and heterogeneous catalytic systems are now routinely investigated under continuous flow conditions at concentrations, pressures, temperatures and contact times typical of use, i.e. modus operandi.
Therefore, in order to identify these more focused analytical techniques a new expression, i.e. operando, was pushed forward. We should recall that the term operando spectroscopy refers to spectroscopic measurements of catalysts under (or as close as possible to) working conditions (appropriate temperature, pressure and reactant composition) with simultaneous on-line product analysis. This term was introduced in the literature in 2002 [see, for example: B.M. Weckhuysen, Chem. Commun. (2002) 97; M.A. Bañares, et al., Chem. Commun. (2002) 1292] with the aim to distinguish work in which on-line activity measurement was performed alongside spectroscopic measurements (i.e. operando) from those in which only spectroscopic data were recorded (i.e. in situ). The on-line analysis of the reaction effluents is needed to ensure that the activity data obtained in the operando reactor are consistent with those observed in a conventional reactor. Therefore, real-time spectroscopy during reaction coupled to simultaneous activity measurements in a kinetically relevant cell (operando methodology) provides the insight for a knowledge-based catalyst design. Spectroscopic techniques during reaction uncover how a catalytic surface behaves and which are the relevant catalytic steps, active sites and reaction intermediates during a given process. The determination of kinetically relevant data requires that the chemical engineering aspects of operando reactors are fully considered. Once the mechanism and actual working phases are established, the pathway to a successful formulation and improvement of catalytic systems is paved.