In situ and operando spectroscopic techniques are crucial for our understanding of complex heterogeneously catalyzed reactions. Under actual reaction conditions, however, many phenomena such as adsorption, (by)-product formation, and desorption of various species in different phases occur simultaneously, leading to crowded spectra that are difficult to interpret. About 15 years ago, modulation excitation spectroscopy (MES) was introduced to the heterogeneous catalysis community and has been increasingly applied since then. The periodic perturbation of a given system, in combination with phase-sensitive detection (PSD) analysis, significantly reduces noise, distinguishes between active and spectator species, and enables extraction of kinetic information. In this review article, we discuss the origin and theory of MES, summarize different application examples (with an emphasis on heterogeneous catalysis), and suggest future developments of the technique.

The origin recognition complex (ORC) binds to the specific positions on chromosomes that serve as DNA replication origins. Although ORC is conserved from yeast to humans, the DNA sequence elements that specify ORC binding are not. In particular, metazoan ORC shows no obvious DNA sequence specificity, whereas yeast ORC binds to a specific DNA sequence within all yeast origins. Thus, whereas chromatin must play an important role in metazoan ORC's ability to recognize origins, it is unclear whether chromatin plays a role in yeast ORC's recognition of origins. This study focused on the role of the conserved N-terminal bromo-adjacent homology domain of yeast Orc1 (Orc1BAH). Recent studies indicate that BAH domains are chromatin-binding modules. We show that the Orc1BAH domain was necessary for ORC's stable association with yeast chromosomes, and was physiologically relevant to DNA replication in vivo. This replication role was separable from the Orc1BAH domain's previously defined role in transcriptional silencing. Genome-wide analyses of ORC binding in ORC1 and orc1bahD cells revealed that the Orc1BAH domain contributed to ORC's association with most yeast origins, including a class of origins highly dependent on the Orc1BAH domain for ORC association (orc1bahD-sensitive origins). Orc1bahD-sensitive origins required the Orc1BAH domain for normal activity on chromosomes and plasmids, and were associated with a distinct local nucleosome structure. These data provide molecular insights into how the Orc1BAH domain contributes to ORC's selection of replication origins, as well as new tools for examining conserved mechanisms governing ORC's selection of origins within eukaryotic chromosomes. The first step in genome duplication for any organism is the appropriate selection of origins, the chromosomal sites where DNA replication begins. In eukaryotic cells, the origin recognition complex (ORC), an evolutionarily conserved heterohexamer, selects DNA replication origins by binding to specific positions on chromosomes (Bell 2002). During G1 phase of the cell cycle, ORC recruits additional factors to assemble the prereplication complex (pre-RC), culminating in the loading of the MCM replicative helicase (Diffley 2004;Stillman 2005). When cells enter S phase, phosphorylation of pre-RC components by S-phase kinases triggers the recruitment of additional proteins and the unwinding of DNA replication origins (Stillman 2005;Sclafani and Holzen 2007). The proteins, including ORC, and the steps in assembly and activation of origin complexes are fairly well conserved from budding yeast to humans (Bell and Dutta 2002). However, it is unclear whether mechanisms involved in ORC's selection of 8 These authors contributed equally to this work.

In recent years, the on-purpose production of 1,3-butadiene (BD) from renewable resources such as ethanol has received increased attention. In that context, Lewis acid catalysts play an important role, especially in the two-step process, i.e. when a mixture of acetaldehyde and ethanol is used as substrate. As the reaction mechanism consists of many intermediates and occurs over different catalytic functionalities, it is notoriously difficult to gain molecular-level insights into the mechanism. Here, we present a study on Lewis acidic Ta-BEA and propose a plausible reaction mechanism. We developed an operando DRIFTS-MS setup that allows for precise control and analysis of changes in the gas-phase composition as well as surface species. Using this tool, we found a surface intermediate with a vibrational frequency at 1690 cm–1 that is only formed in the presence of both ethanol and crotonaldehyde and that is likely involved in the production of BD. Our data further suggests that a subtle control of the ratio of ethanol to acetaldehyde is crucial to keep a high ethoxy coverage and to desorb the intermediate crotyl alcohol in order to achieve high BD productivity. To the best of our knowledge, this is the first dynamic operando spectroscopic study on this re-emerging reaction.

Snβ zeolites are amongst the most effective heterogeneous catalysts for the Baeyer–Villiger (BV) oxidation of ketones with aqueous hydrogen peroxide (H2O2). The high selectivity is rooted in the activation of the carbonyl substrate through interaction with the isolated SnIV sites. However, these sites are also accessible to other molecules in the reaction mixture (in particular the co‐solvent and product). In this contribution, we report the impact of competitive adsorption on the Snβ‐catalyzed BV oxidation of cyclohexanone with aqueous H2O2. We furthermore prepared a series of Snβ zeolites with varying amounts of framework silanols and quantified their hydrophilicities with water adsorption and IR experiments. By correlating the results with catalytic data, we show that Snβ zeolites with an intermediate hydrophilicity achieve the highest activity. Our adaptable post‐synthetic synthesis allows us to tune the material synthesis, resulting in enhanced activity compared with conventional hydrothermal Snβ.

The discovery of new materials tailored for a given application typically requires the screening of a large number of compounds and this process can be significantly accelerated by computational analysis. In such an approach the performance of a compound is correlated to a materials property, a so called descriptor. Here we develop a descriptor-based approach for the adsorption of CO and NO to Cu, Ni, Co and Fe sites in zeolites. We start out by discussing a possible design strategy for zeolite catalysts, define the studied test set of sites in the zeolites SSZ-13 and Mordenite, and define a 1 set of appropriate descriptors. In a subsequent step we use these descriptors in single-, two-and multi-parameter regression analysis and finally use a machine-learning genetic algorithm to reduce the number of variables. We find that one or two descriptors are not sufficient to accurately capture the interactions between molecules and metal centers in zeolites and indeed a multi-parameter approach is necessary. Even though many of the descriptors are directly correlated, we identify the position of the s-orbital and the number of valence electrons of the active site as well as the HOMO-LUMO gap of the adsorbate as most important descriptors. Furthermore the reconstruction of the active sites upon adsorption plays a crucial role and when it is explicitly included in the analysis, correlations improve significantly. In the future we expect that the fundamental methodology developed here will be adapted and transferred to selected problems in adsorption and catalysis and will assist the rational design of materials for the given application.