Quantification Of Active Sites Research Articles

Kinetics of chemical processes: From molecular to industrial scale

Combining spectroscopic and transient kinetic techniques provides access to the identification and quantification of active sites and corresponding turnover frequencies of catalyzed reactions [1]. Ab initio calculated rate coefficients combined with network generation codes allow so-called microkinetic models accounting for all relevant elementary steps of complex reaction networks. Computational Fluid Dynamics (CFD) account for the scale-dependent transport of mass, energy and momentum and, hence, renders the design of an industrial process based on the intrinsic chemical kinetics possible. These theoretical and experimental achievements cannot be a substitute for the insights provided by the analysis of a catalytic cycle in terms of a limited number of kinetically significant steps [2]. We analyse the kinetics of a single-path reaction represented as a closed sequence of two steps [3]. We discuss some examples of chemical looping processes providing, in contrast to catalytic technology, the possibility to perform the steps of a closed sequence at different conditions.

Toward quantification of active sites and site-specific activity for polyaromatics hydrogenation on transition metal sulfides

Quantification of active sites is indispensable for understanding a catalytic material, ultimately enabling meaningful comparisons across different catalysts and discerning the underlying intrinsic kinetics. At present, however, few methods are available for quantitatively probing the catalytic surface of transition metal sulfides (TMS) during catalysis. TMS catalysts expose Brønsted acidic (sulfhydryls, SH) and Lewis acidic sites (coordinatively unsaturated sites, CUS) at their surfaces and, thus, are conceptually amenable to in situ acid-base titration with judicious choices of bases that strongly bind to the acidic entities. In this proof-of-concept study, in situ titration using several N-containing organic bases allowed us to determine the surface concentration of active sites and their specific activities for polyaromatics hydrogenation on unsupported TMS catalysts. Ni-promoted sulfide catalysts were more active than the unpromoted ones, because of a modest increase in the site density and a 2- to 3-fold enhancement in the TOF averaged over all titrated sites. By further discriminating between the titrated CUS and SH with site-selective titrants, a substantial difference in the TOF was found between CUS and SH among the titrated pool of sites. In addition to the titrated sites that contributed to the majority (80–95%) of the hydrogenation activity, some active sites remained unoccupied by the studied base molecules. The contribution of these residual sites, likely inaccessible to large-sized and less mobile titrants, depended on the temperature, H2 pressure and the catalyst, but was unaffected by the space time, titrant concentration, and the type or concentration of the polyaromatic reactant, for a given catalyst. This work leads to several emerging opportunities to better understand complex TMS catalysts.

Methanol conversion on borocarbonitride catalysts: Identification and quantification of active sites.

Borocarbonitrides (BCNs) have emerged as highly selective catalysts for the oxidative dehydrogenation (ODH) reaction. However, there is a lack of in-depth understanding of the catalytic mechanism over BCN catalysts due to the complexity of the surface oxygen functional groups. Here, BCN nanotubes with multiple active sites are synthesized for oxygen-assisted methanol conversion reaction. The catalyst shows a notable activity improvement for methanol conversion (29%) with excellent selectivity to formaldehyde (54%). Kinetic measurements indicate that carboxylic acid groups on BCN are responsible for the formation of dimethyl ether, while the redox catalysis to formaldehyde occurs on both ketonic carbonyl and boron hydroxyl (B─OH) sites. The ODH reaction pathway on the B─OH site is further revealed by in situ infrared, x-ray absorption spectra, and density functional theory. The present work provides physical-chemical insights into the functional mechanism of BCN catalysts, paving the way for further development of the underexplored nonmetallic catalytic systems.

Cover Picture: Quantification of Active Sites and Elucidation of the Reaction Mechanism of the Electrochemical Nitrogen Reduction Reaction on Vanadium Nitride (Angew. Chem. Int. Ed. 39/2019)

Cover Picture: Quantification of Active Sites and Elucidation of the Reaction Mechanism of the Electrochemical Nitrogen Reduction Reaction on Vanadium Nitride (Angew. Chem. Int. Ed. 39/2019)

Quantification of Active Sites and Elucidation of the Reaction Mechanism of the Electrochemical Nitrogen Reduction Reaction on Vanadium Nitride

AbstractDespite recent intense interest in the development of catalysts for the electrochemical nitrogen reduction reaction (ENRR), mechanistic understanding and catalyst design principles remain lacking. In this work, we develop a strategy to determine the density of initial and steady‐state active sites on ENRR catalysts that follow the Mars–van Krevelen mechanism via quantitative isotope‐exchange experiments. This method allows the comparison of intrinsic activities of active sites and facilitates the identification and improvement of active‐site structures for ENRR. Combined with detailed density functional theory calculations, we show that the rate‐limiting step in the ENRR is likely the initial N≡N bond activation via the addition of a proton and an electron to the adsorbed N2 on the N vacancies to form N2H. The methodology developed and mechanistic insights gained in this work could guide the rational catalyst design in the ENRR.

Quantification of Active Sites and Elucidation of the Reaction Mechanism of the Electrochemical Nitrogen Reduction Reaction on Vanadium Nitride.

Despite recent intense interest in the development of catalysts for the electrochemical nitrogen reduction reaction (ENRR), mechanistic understanding and catalyst design principles remain lacking. In this work, we develop a strategy to determine the density of initial and steady-state active sites on ENRR catalysts that follow the Mars-van Krevelen mechanism via quantitative isotope-exchange experiments. This method allows the comparison of intrinsic activities of active sites and facilitates the identification and improvement of active-site structures for ENRR. Combined with detailed density functional theory calculations, we show that the rate-limiting step in the ENRR is likely the initial N≡N bond activation via the addition of a proton and an electron to the adsorbed N2 on the N vacancies to form N2 H. The methodology developed and mechanistic insights gained in this work could guide the rational catalyst design in the ENRR.

Methods for determining the intrinsic kinetic characteristics of irreversible adsorption processes

To benchmark the performance of a different of catalytic materials it is essential to know the adsorption characteristics such as the kinetic order of adsorption, the apparent rate constant and the total number of active sites. It is advantageous to normalize the reaction rate to the number of active sites; a quantity commonly referred to as the turnover frequency (TOF). Methods such as chemisorption and SSITKA (steady state isotopic transient kinetic analysis) are conventionally used to quantify the number of active sites, both have advantages and drawbacks. The TAP (Temporal Analysis of Products) pulse response technique provides a distinct method for precise (10 nmol) quantification of active sites at elevated temperatures. Using irreversible reaction processes, mathematical techniques for analysis of pulsed active site titration are described herein. Whereas previous methods simply consider the total capacity for gas adsorption, these new methods take into account the dynamics and interdependence of the conversion, the number of available sites and the adsorption rate as they change over the course of the titration experiment. Moreover, these methods enable an independent fitting of the kinetic order and stoichiometry of the adsorption process directly from experimental data. In comparison with experimental data for the incremental oxidation of a reduced platinum, we found the analytical method of linear rectification was most robust and efficient. In fitting the kinetic order to experimental data collected between 125 and 175 °C we found a clear indication over the range of 45–95% conversion of a second order rate dependence and 2:1 balance between active sites and O2.

Molecular Probes for the Identification and Quantification of Active Sites in PGM-Free ORR Catalysts

The nature of the active site(s) of platinum group metal-free (PGM-free) electrocatalysts for the oxygen reduction reaction (ORR) remains a topic of intense debate. This is mainly due to the complex heterogeneous character produced during the high-temperature synthesis process required for the generation of electrocatalytically active materials. Quantum chemical modeling of possible active site structures has proposed heme-like moieties in which pyridinic nitrogen groups are coordinated to transition-metal center forming a Fe-Nx-type of structure (1-3). Nevertheless, unlike heme structures that easily poison with strongly bound molecules such as CO, PGM-free electrocatalysts have shown extreme resilience to a number of commonly known poisoning molecules, e.g., CO, H2S, SO2 (4, 5). Thus, the difficulty of developing a characterization technique based on molecular probes will not only rely on the identification of non-traditional probes but also on the successful recovery of electrocatalytic activity required to estimate active site density based on a proposed electrochemical reaction mechanism of poisoning and stripping. A molecular probe approach in which a particular molecule interacts specifically with the active site by forming a stable chemical adduct causing a decrease in electrocatalytic activity, and when removed fully recovers its initial activity, can be a powerful tool for the characterization and quantification of the active site(s). Development of such technique will not only provide experimental evidence about the nature of the active site(s), vital for the synthesis of next-generation PGM-free catalyst with more durable and increased concentration of active sites, but also will become a valuable tool for the further understanding of more complex processes such as reaction pathways and degradation mechanisms. Due to the extraordinary tolerance of PGM-free electrocatalysts to a number of commonly used poisons, nitrogen-containing molecules: nitrite (NO2 -) and nitric oxide (NO), are proposed as molecular probes as they have been reported to interact with Fe-heme structures in electrochemical environments (6). Furthermore, recent ex-situ studies reported by our group and collaborators showed that the NO probe molecule directly interacts with FexNy structures (7, 8). In this work, the poisoning extent of ORR activity in acidic electrolytes caused by the formed chemical adducts is investigated using electrochemical half-cell experiments. The specificity of the probe molecules was studied via X-ray spectroscopic techniques: XPS, EXAFS, and XANES, to better understand their interaction with Fe-centered FexNy moieties. Quantum chemical models of proposed active site interactions with probe molecules are used to determine possible binding motifs and relative binding energies of different sites/molecules. The combination of experimental and computational approaches in this work will yield a powerful suite of tools for interpreting the nature and density of active sites in PGM-free electrocatalysts. Acknowledgments This research is supported by DOE Fuel Cell Technologies Office, through the Electrocatalysis Consortium (ElectroCat). References E. F. Holby, G. Wu, P. Zelenay and C. D. Taylor, The Journal of Physical Chemistry C, 118, 14388 (2014).Q. Y. Jia, N. Ramaswamy, U. Tylus, K. Strickland, J. K. Li, A. Serov, K. Artyushkova, P. Atanassov, J. Anibal, C. Gumeci, S. C. Barton, M. T. Sougrati, F. Jaouen, B. Halevi and S. Mukerjee, Nano Energy, 29, 65 (2016).E. F. Holby and C. D. Taylor, Sci Rep, 5, 9286 (2015).D. Malko, T. Lopes, E. Symianakis and A. R. Kucernak, J Mater Chem A, 4, 142 (2016).T. Reshetenko, A. Serov, K. Artyushkova, I. Matanovic, S. Stariha and P. Atanassov, Journal of Power Sources, 324, 556 (2016).V. Rosca, M. Duca, M. T. de Groot and M. T. M. Koper, Chemical Reviews, 109, 2209 (2009).P. Zelenay and D. Myers, 2017 Annual Merit Review and Peer Evaluation Meeting (2017).J. L. Kneebone, S. L. Daifuku, J. A. Kehl, G. Wu, H. T. Chung, M. Hu, E. E. Alp, K. L. More, P. Zelenay, E. F. Holby and M. L. Neidig, The Journal of Physical Chemistry C, acs.jpcc.7b03779 (2017).

Quantification of Active Sites at Spinel Type Metal Oxide Catalysts by Surface Interrogation Scanning Electrochemical Microscopy

Nanostructured spinel type metal oxides are promising catalysts for the oxygen reduction reaction (ORR) in fuel cells as well as for the oxygen evolution reaction (OER) in electrolysers under alkaline conditions [1]. For a more comprehensive understanding of the oxygen electrochemistry the enlightenment of the interaction between catalyst structure and occurring surface reaction will be a major step [2]. Therefore, new experimental approaches are needed to identify material patterns that determine the electrocatalytic activity. Especially, following the oxidation states of metal atoms as function of potential and time will provide new insights into the ORR and OER mechanisms. A versatile and powerful method in the assessment of electrocatalysts is scanning electrochemical microscopy (SECM). The spatial activity and selectivity of a catalytic material is immediately visualized by analyzing the current response of a movable probe electrode [3]. Recently, Rodríguez-López et al. [4] described a surface interrogation mode of SECM that allows the detection of adsorbates depending on time and substrate potential. This method is based on a transient feedback of a redox mediator with which a coulometric titration is performed. By integrating the current response at the tip electrode the quantification of active catalytic sites is accessible. In this study, we apply the surface interrogation mode to detect the amount of active sites at spinel type metal oxide catalyst under alkaline fuel cell conditions. Therefore, the catalyst powder is supplied in cavity-microelectrodes which were prepared by electrochemical etching of a metal wire [5]. Through the quantification of active sites and the examination of their reactivity new insights to the ORR of alkaline fuel cells are achieved. The results will be compared to a commercial carbon-supported platinum fuel cell catalyst. 1. X. Deng and H. Tüysüz, ACS Catal., 4, 3701 (2014). 2. I. Katsounaros, S. Cherevko, A. R. Zeradjanin and K. J. J. Mayrhofer, Angew. Chem. Int. Ed., 53, 102 (2014). 3. G. Wittstock, M. Burchardt, S. E. Pust, Y. Shen and C. Zhao, Angew. Chem. Int. Ed., 46, 1584 (2007). 4. J. Rodríguez-López, M. A. Alpuche-Avilés and A. J. Bard, J. Am. Chem. Soc., 130, 16985 (2008). 5. A. Minguzzi, C. Locatelli, O. Lugaresi, A. Vertova and S. Rondinini, Electrochim. Acta, 114, 637 (2013).

Oxidative Dehydrogenation on Nanocarbon: Identification and Quantification of Active Sites by Chemical Titration

Nanostructured carbon-based materials have shown high catalytic activity in several important reactions and related chemical industrial processes, such as direct or oxidative dehydrogenation of hydrocarbons and Friedel–Crafts reactions. Nanocarbon materials exhibit significant advantages over traditional metal or metal oxide based catalysts because of their tunable acidity/basicity, electron density, and convenient recycling and reusability, and they have been shown to be potential alternatives to conventional catalysts to meet the requirements of sustainable chemistry. As a result, the field of nanocarbon catalysis has been experiencing an unparalleled development of new catalyst synthesis or their applications in new reaction systems. However, there is only slow growth of mechanistic interpretation of carbon-catalyzed reactions, which is even more urgent to advance our knowledge in related fields. Present research on the mechanism of carbon catalysis suggests that oxygen containing functional groups, especially ketonic carbonyl groups on nanocarbon, which are rich in electrons, may act as the catalytic active sites for oxidative dehydrogenation (ODH) of alkanes to corresponding alkenes. The reaction process is assumed to be similar to that for transition-metal oxide catalysts. The C H bonds of alkanes dissociate at active oxygen functional groups, and the hydrogen atoms are abstracted by Lewis base sites. After the desorption of alkene products, gas-phase O2 reacts with the abstracted hydrogen to form H2O, then the active catalytic sites are regenerated to finish one catalytic cycle. The above unspecific catalytic mechanism is only based on the qualitative characterization of carbon catalysts, while the identity of the active sites or a detailed kinetic study has never been executed with direct and convincing chemical evidence. One of the most critical problems that limits the quantitative description of the catalytic mechanism is the uncertainty of the chemical structure of nanocarbon materials. The coexistence of several kinds of surface functional groups (such as hydroxyl, carbonyl, and carboxylic acid groups) is unavoidable, as the synthesis or the following surface modification procedures of nanocarbon catalysts are normally realized by a severe physical or chemical process, such as laser irradiation and oxidation by HNO3, O2, and O3. [8] There are still lack of reliable quantification methods for the surface functional groups on nanostructured carbon materials because of their complexity in type and quantity. As a result, turnover frequency (TOF), the ultimate parameter to evaluate the intrinsic activity of heterogeneous catalysts, is also rarely reported in the case of nanocarbon catalysts, making it impossible to study the detailed reaction kinetics or compare the activity of carbon catalysts bearing different structures fairly and objectively. The quantitative surface composition analysis is also desirable for the application of nanostructured carbon as a catalyst support or electrochemical devices, which takes an even larger proportion in the field of carbon materials, as the surface structure of nanocarbon materials is essential for their physical or chemical properties (for example, affinity for a certain metal or metal ion). In view of the quantification methods of oxygen functional groups, herein we propose a chemical titration method to determine the surface concentration of three kinds of typical oxygen functional groups ( C=O, C OH, and COOH) on the surface of carbon nanotubes (CNTs) (Scheme 1). Through selective deactivation of these specific oxygen functional groups and the assessment of the catalytic activity of different CNT derivatives for ethylbenzene (EB) ODH reactions, we provided chemical evidence to show that

Accessing Quantitative Degrees of Functionalization on Solid Substrates via Solid-State NMR Spectroscopy

The development of a solid-state nuclear magnetic resonance (NMR) method allowing the quantification of active sites (i.e., residual vinyl groups) accessible for chemical functionalization on the surface of poly(divinyl benzene) microspheres is presented. Residual vinyl groups of poly(divinyl benzene) microspheres (PDVB55 and PDVB80) were quantified via solid-state 13C cross-polarization magic-angle spinning (CP-MAS) NMR spectroscopy. In addition, 13C CP-MAS NMR spectroscopy allows the comparison of core and grafted microspheres functionalization on the same (arbitrary) scale in a short measuring time. This scale was calibrated by an extended absolute quantification of the vinyl groups using 13C single pulse excitation (SPE) MAS NMR spectroscopy. The degree of cross-linking of the microspheres was calculated to be 30 and 50% for PDVB55 and PDVB80 microspheres, respectively. The number of active groups per nominal surface area is 110 and 179 groups per nm2 for PDVB55 and PDVB80 microspheres, respectively. The loading capacities of the microspheres (e.g., 0.61 and 0.65 mmol·g−1) are not too far removed from those found in Merrifield resins of comparable sizes.