CO-PROX Reaction Research Articles

Ceria-supported Pd catalysts with different size regimes ranging from single atoms to nanoparticles for the oxidation of CO

Supported metal catalysts are the most widely used in industrial processes and the metal particle size plays a crucial factor in determining the catalytic performance. Herein, CeO2-supported Pd catalysts with different Pd size regimes ranging from single atoms, to nanoclusters (1–2 nm), and to nanoparticles (>2 nm) were used for both CO oxidation and preferential oxidation of CO in H2 (CO-PROX). Compared to Pd nanoclusters and nanoparticles, CeO2-supported single Pd atoms (PdSA/CeO2) are the most intrinsically active in CO oxidation, with an apparent activation energy of ca. 40 kJ mol−1. Results of kinetic investigations and in situ diffuse reflectance infrared Fourier transformed spectroscopy demonstrate the CO oxidation proceeding through a Langmuir-Hinshelwood mechanism with the decomposition of formate species acting dominantly as the rate-determining step. The CO reaction rate is exclusively promoted on PdSA/CeO2 catalysts for the CO-PROX reaction, which could be ascribed to a stronger H-spillover effect on isolated Pd sites to produce bridged-OH on CeO2 surface, simultaneously facilitating the consumptions of bicarbonate and formate species. There results greatly deepen the fundamental understanding of the Pd size regimes over Pd/CeO2 catalysts for the oxidation of CO.

The effect of copper oxide on the CuO–NiO/CeO2 structure and its influence on the CO-PROX reaction

The effect of copper oxide species on the CuO–NiO/CeO 2 structure and the influence on the preferential CO oxidation in H 2 excess (CO-PROX) reaction at low and high temperatures were investigated. Temperature-programmed surface reaction (TPSR), In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and X-ray photoelectron spectroscopy (XPS) results allowed determining the surface species. The maximum temperature of CO 2 formation or selectivity decreased about 40 °C for the CuO–NiO/CeO 2 catalyst compared to the NiO/CeO 2 , which suggests that the addition of Cu + species increases the active sites due to interaction with the Ni–Ce structure. Therefore, the activity of the catalyst was closely related to the oxygen in vacancies and the formation of Cu + -carbonyl species of the redox mechanism. Besides, the superior selectivity towards CO 2 below 150 °C depends on the carbonyl stabilization at the surface, inhibiting the adsorption and subsequent oxidation of H 2 . Using TPSR and spectroscopic analyzes by DRIFTS and XPS allowed us to propose the reaction mechanisms for low and high temperatures. • Synergistic interaction between dispersed CuO species and Ni–Ce–O solid solution. • The presence of Ce 3+ in CeO 2 indicate formation of oxygen vacancies on the surface. • The activity was closely related to oxygen vacancies and Cu + -carbonyl species. • The temperature of CuNiCeO catalyst decreased about 40 °C compared to the NiCeO. • The oxygen vacancies and Cu + -carbonyl species suggest a multiple-step process.

Trace CO elimination in H2-rich streams with a wide operation temperature window: Co deposited CuO-CeO2 catalysts.

This work provides a new strategy to eliminate trace CO in H2-rich gas in a wide operation temperature window for the application of hydrogen fuel cells. We engineered Co deposited CuO-CeO2 catalysts with a Co/(Cu + Ce) molar ratio of 1/1 that manages to maintain the CO level at below 100 ppm from 85 to 240 °C in the H2-rich reformate stream. CO-PROX and CO methanation reaction respectively occurred in the low and high temperature ranges. Multiple characterization techniques demonstrate that the molar ratio of Co/(Cu + Ce) significantly affects the synergistic interactions between the Cu, Co and Ce species, and ultimately the CO oxidation and CO methanation reactions. At low reaction temperatures, the Cu-Ce interaction mainly dominates the CO-PROX process, while at high reaction temperatures, CO methanation reaction takes place due to the reduction of Co3O4 to Co0 and the Co-Ce interaction takes charge of the CO methanation. Moreover, the increment of Co/(Cu + Ce) from 1/2 to 1 gives rise to the reprecipitation of the partially dissolved Cu species on Co3O4, which strengthens the Cu-Co interaction and stabilizes surface Cu+ and Co3+, thus promoting the low temperature CO-PROX catalytic performance.

Isotopic evidence for the tangled mechanism of the CO-PROX reaction over mixed and bare cobalt spinel catalysts

The catalytic performance of the bare Co3O4 and mixed cobalt-spinel catalysts (MxCo3−xO4; M = Cr, Mn, Fe, Ni, Cu, Zn) in the CO-PROX process was investigated in the temperature-programmed surface reaction (TPSR) mode using 18O2 as an oxidant.

Investigations of the Effect of H2 in CO Oxidation over Ceria Catalysts

The preferential CO oxidation (so-called CO-PROX) is the selective CO oxidation amid H2-rich atmospheres, a process where ceria-based materials are consolidated catalysts. This article aims to disentangle the potential CO–H2 synergism under CO-PROX conditions on the low-index ceria surfaces (111), (110) and (100). Polycrystalline ceria, nanorods and ceria nanocubes were prepared to assess the physicochemical features of the targeted surfaces. Diffuse reflectance infrared Fourier-transformed spectroscopy (DRIFTS) shows that ceria surfaces are strongly carbonated even at room temperature by the effect of CO, with their depletion related to the CO oxidation onset. Conversely, formate species formed upon OH + CO interaction appear at temperatures around 60 °C and remain adsorbed regardless the reaction degree, indicating that these species do not take part in the CO oxidation. Density functional theory calculations (DFT) reveal that ceria facets exhibit high OH coverages all along the CO-PROX reaction, whilst CO is only chemisorbed on the (110) termination. A CO oxidation mechanism that explains the early formation of carbonates on ceria and the effect of the OH coverage in the overall catalytic cycle is proposed. In short, hydroxyl groups induce surface defects on ceria that increase the COx–catalyst interaction, revealed by the CO adsorption energies and the stabilization of intermediates and readsorbed products. In addition, high OH coverages are shown to facilitate the hydrogen transfer to form less stable HCOx products, which, in the case of the (110) and (100), is key to prevent surface poisoning. Altogether, this work sheds light on the yet unclear CO–H2 interactions on ceria surfaces during CO-PROX reaction, providing valuable insights to guide the design of more efficient reactors and catalysts for this process.

Cobalt oxide decorated porous silica particles: Structure and activity relationship in the catalytic oxidation of carbon monoxide

Porous (0.6–30 nm) and non-porous spherical silica nanoparticles are synthesized. These silicas with different morphologies are decorated with cobalt (2.9 wt%) using Co(NO3)2·6H2O as precursor. Co-SiO2 particles are studied by N2 sorption, TEM, XRD, XPS, FTIR and TPR-H2 methods. Specific surface area (SSA) of the prepared materials is 10–1200 m2/g. The main crystalline phase is identified as Co3O4. The activity of Co-SiO2 composites in the CO oxidation and preferential oxidation of CO (PROX) in a H2-rich gas mixture improves with increasing atomic ratios of Co/Si and Co3+/Co2+ on the silica surface. Catalysts with the large mesopores containing mainly Co2+ are the least active. Co-SiO2 composites based on synthetic opal grown by sedimentation of non-porous silica particles with the lowest SSA demonstrate the highest catalytic activity. The CO conversion reaches 100% at 150 °C in the absence of hydrogen, while in PROX the 90% conversion of CO is achieved at 170 °C. The large Co3O4 aggregates between non-porous spherical silica particles of opal are the most active and stable during CO-PROX reaction. The lower interaction of Co3O4 nanoparticles with this support promotes the stabilization of the oxidized state Co3+ and provides a higher yield of CO2 in CO-PROX reaction.

Unravelling the role of Fe in trimetallic Fe-Cu-Pt/Al2O3 catalysts for CO-PROX reaction

This work proposes a trimetallic Fe-Cu/Pt/Al2O3 catalyst as an appealing system for preferential oxidation of CO (CO-PROX) reaction. The excellent conversion rates achieved by the Fe-Cu/Pt/Al2O3 catalysts under realistic reforming-surrogated feed streams along with the catalyst stability, reproducibility, and scalability showcase a very competitive system for CO-PROX reaction units. Furthermore, the systematic analysis conducted for Pt/Al2O3, Cu/Pt Al2O3, and Fe-Cu/Pt/Al2O3 catalysts enabled establishing meaningful relationships between catalytic behaviour and the catalyst surface to reactants interactions. Thus, the enhanced CO oxidation performances attained by the incorporation of Fe species into bimetallic Cu/Pt/Al2O3 catalysts were associated to superior surface electron densities and inhibited CO adsorption process over Pt surfaces. Remarkably, operando-DRIFTS spectroscopy evidenced significantly larger H-containing surface species developed over the trimetallic system. The enhanced abilities for developing thermally instable intermediates favoured by small amounts of Fe should indeed determine the enhanced catalysts behaviours displayed by the trimetallic Fe-Cu/Pt/Al2O3 catalyst.

In Situ Raman Spectroscopic Study of Species in Co-MnCeOx Catalysts under COPrOx Reaction Conditions

This work is focused on the study of Co-MnCeOx catalysts by in situ Raman spectroscopy, under COPrOx conditions, and aims to establish a correlation with the catalytic results of different catalytic formulations. Through XRD, the catalysts structures were explored, and the oxidation states and relative concentrations of surface species were analyzed by XPS. Catalysts were prepared with Co (10 wt %) and different Mn concentrations. The more active catalyst contains 2.5 wt % of Mn and Co/Ce ≈ 1 (CO conversion 99.5%, 150–180 °C). The Co-Mn2.5Ce catalyst is composed of Co3O4 spinel, Mn inserted in the CeO2 lattice, and probably MnOx dispersed, which remains stable up to 250 °C under reaction conditions. XRD results confirm the crystalline structure of Co3O4 spinel; however, there was no evidence of crystalline phases of MnOx oxides. A lower lattice parameter suggests the insertion of Mnn+ ions in the ceria framework. The catalytic surface is composed of Co3+-Co2+ belonging to Co3O4 and Ce4+ of ceria and Mn4+-Mn3+ species.

New findings regarding the role of copper entity particle size on the performance of Cu/ceria-based catalysts in the CO-PROX reaction

The preferential oxidation of CO in H2-rich mixtures has been studied using Ce0.8Zr0.2O2-supported Cu catalysts containing increasing amounts of copper [1 wt% (Cu1/CZ), 2 wt% (Cu2/CZ) and 4 wt% (Cu4/CZ)]. The oxide support was prepared by co-precipitation and the copper was incorporated using the wetness impregnation method. The catalysts were characterized using N2 adsorption, XRD, STEM-XEDS, H2-TPR, H2 chemisorption and XPS techniques. In particular, the complementarity of electron microscopy and hydrogen chemisorption techniques was found to be very useful for elucidating one of the most challenging properties of copper catalysts, namely their dispersion. The results show that the Cu1/CZ catalyst is the most active in the CO-PROX reaction at low temperatures (<125 °C), despite having a lower copper content. The results are discussed and correlated with the textural properties of the catalysts and with parameters such as metal dispersion and the binary interfacial active sites.

The Impact of Ceria Loading on the CuOx−CeO2 Interaction and Performance of AuCu/CeO2−SiO2 Catalysts in CO‐PROX Reaction

AbstractThe use of CeO2 as a promoter for non‐reducible supports, such as SiO2, enhances the catalytic activity by facilitating the activation of species such as O2 and H2O. However, in the case of the bimetallic nanoparticles (NPs), the presence of promoters may also influence the migration of the metallic species, dramatically impacting the stabilization of catalytic sites and as consequence, the overall catalytic performance. In the case of AuCu alloy supported catalysts, the reduced form, i. e., AuCu alloy, or the oxidized form, i. e., Au−CuOx species, present different activities/selectivities. Moreover, several parameters, such as reactional conditions, AuCu particle size, support nature, and pretreatments dictate which species are stabilized on the catalyst surface. The preferential oxidation of CO (CO‐PROX) reaction, that takes place under the H2‐rich stream, favors the stabilization of the reduced form of the AuCu catalysts. However, the Cu distribution in the crystal lattice and the final composition of the AuCu NPs are dependent on the mobility of the Cu species on the support. In this work, we show that the loading of the CeO2 as a promotor in AuCu/SiO2 catalysts impacts the CeOx species and the Cu reincorporation into the Au lattice and consequently, the activity and stability of the catalysts under CO‐PROX reaction.

Effect of manganese promotion on the activity and selectivity of cobalt catalysts for CO preferential oxidation

The preferential oxidation of CO in H2-rich mixtures (COPrOx) is a major catalytic reaction utilized for hydrogen purification. In the exploration of alternatives to noble metals, cobalt-based catalysts appear to be a very promising choice. The activity and stability of cobalt in the COPrOx reaction can be improved by the addition of transition metals and manganese is maybe the most prominent among them. Yet, the arrangement of the two components in the catalytically active state is largely unknown, which hinders in-depth understanding of the manganese promotion effect. Here, we compare pure and Mn-modified cobalt catalysts and correlate their structural and chemical characteristics with their COPrOx performance. The Mn-promoted cobalt catalyst is significantly more active than pure cobalt especially at intermediate reaction temperatures (around 200 °C). The addition of Mn improves the structural stability of the catalyst and helps to maintain higher specific surface areas. Chemical and microstructural analysis using various operando and in situ techniques revealed that Mn promotes CO conversion by partially stabilizing CoO phase during reaction conditions. It is also suggested that at high temperature, Mn suppress CO methanation reaction but promotes H2 oxidation. Apart of the particular interest in COPrOx reaction, in a general context, this work shows how the spatial distribution of the different catalyst components at nanoscopic level, may affect the surface chemistry and consequently control the reactivity.

Blocking the non-selective sites through surface plasmon-induced deposition of metal oxide on Au/TiO2 for CO-PROX reaction

CO preferential oxidation (PROX) in excess hydrogen is a promising strategy for CO removal in hydrogen-rich reformate. However, the CO conversion and CO 2 selectivity in the PROX reaction dramatically decrease with rising temperature, resulting in a major challenge to complete CO removal across a wide temperature window. Here, we developed a photoexcited deposition of metal oxide (PDMO) method based on surface plasmonic resonance of Au nanoparticles, which can tune the interfacial properties by depositing lead oxide at the Au-TiO 2 interface. The interface modified Pb 3 O 4 /Au/TiO 2 catalyst can successfully eliminate CO in a wide temperature range (70°C–140°C) and exhibit superior stability in the PROX reaction. Further experiments demonstrated that the improved PROX performance is due to the severely depressed hydrogen oxidation through simultaneously inhibiting the activation of hydrogen and oxygen. The results further reveal that the interface of Au/TiO 2 is the superactive but non-selective site for H 2 and CO oxidation. • Photoexcited deposition of metal oxide method is employed for interface engineering • Interface-engineered Au-TiO 2 helps to identify the active site in CO and H 2 oxidation • Blocking the non-selective sites greatly improves the CO-PROX performance • Interface anchoring with inert lead oxide enhances long-term thermal stability Against the background of the carbon-neutral era, the liquid sunshine industry is booming due to the consumption of greenhouse gas (CO 2 ) and the solution of storage and transportation of H 2 energy. However, before the terminal application of H 2 energy, CO must be removed in the H 2 -rich reformate to avoid poisoning the noble metal catalysts. Gold-catalyzed CO preferential oxidation (PROX) has great potential in the removal of CO from H 2 -rich reformate. The competition oxidation of CO and H 2 at the interface of gold support always results in a decrease in CO oxidation conversion and selectivity, especially at high temperature. However, it is a major challenge to distinguish the interfacial sites from the other parts of nanoparticle surface. We show here that the interface of Au-TiO 2 is modified by our photoexcited deposition of metal oxide (PDMO) method, which greatly affects the performance of the CO-PROX reaction. The interfacial area of metal support is the ultrahigh active site in catalysis. However, the ultrahigh active sites are normally adverse for chemoselective oxidation or hydrogenation reaction. Interface-engineered catalyst is prepared based on a selective photoexcited deposition of metal oxide method and shows greatly improved performance in CO preferential oxidation in H 2 -rich stream (PROX reaction).

CO Preferential Oxidation in a Microchannel Reactor Using a Ru-Cs/Al2O3 Catalyst: Experimentation and CFD Modelling

This work presents an experimental and modelling evaluation of the preferential oxidation of CO (CO PROX) from a H2-rich gas stream typically produced from fossil fuels and ultimately intended for hydrogen fuel cell applications. A microchannel reactor containing a washcoated 8.5 wt.% Ru/Al2O3 catalyst was used to preferentially oxidise CO to form CO2 in a gas stream containing (by vol.%): 1.4% CO, 10% CO2, 18% N2, 68.6% H2, and 2% added O2. CO concentrations in the product gas were as low as 42 ppm (99.7% CO conversion) at reaction temperatures in the range 120–140 °C and space velocities in the range 65.2–97.8 NL gcat−1 h−1. For these conditions, less than 4% of the H2 feed was consumed via its oxidation and reverse water-gas shift. Furthermore, a computational fluid dynamic (CFD) model describing the microchannel reactor for CO PROX was developed. With kinetic parameter estimation and goodness of fit calculations, it was determined that the model described the reactor with a confidence interval far greater than 95%. In the temperature range 100–200 °C, the model yielded CO PROX reaction rate profiles, with associated mass transport properties, within the axial dimension of the microchannels––not quantifiable during the experimental investigation. This work demonstrates that microchannel reactor technology, supporting an active catalyst for CO PROX, is well suited for CO abatement in a H2-rich gas stream at moderate reaction temperatures and high space velocities.

AuCu/TiO2 Catalysts prepared using electron beam irradiation for the preferential oxidation of carbon monoxide in hydrogen-rich mixtures

The major part of the world production of hydrogen (H2) is originated from a combination of methane steam reforming and water-gas shift reaction resulting in an H2-rich mixture known as reformate gas, which contains about 1% vol (10,000 ppm) of carbon monoxide (CO). The preferential oxidation reaction of CO in H2-rich mixtures (CO-PROX) has been considered a very promising process for H2 purification, reducing CO for values below 50 ppm allowing its use in PEMFC Fuel Cells. Au nanoparticles supported on TiO2 (Au/TiO2) catalysts have been shown good activity and selectivity for CO-PROX reaction in the temperature range between 20-80 ºC; however, the catalytic activity strongly depends on the preparation method. Also, the addition of Cu to the Au/TiO2 catalyst could increase the activity and selectivity for CO-PROX reaction. In this work, AuCu/TiO2 catalysts with composition 0.5%Au0.5%Cu/TiO2 were prepared in a single step using electron beam irradiation, where the Au3+ and Cu2+ ions were dissolved in water/2-propanol solution, the TiO2 support was dispersed and the obtained mixture was irradiated under stirring at room temperature using different dose rates (8 – 64 kGy s-1) and total doses (144 – 576 kGy). The catalysts were characterized by energy dispersive X-ray analysis, X-ray diffraction transmission electron microscopy, temperature-programmed reduction and tested for CO-PROX reaction. The best result was obtained with a catalyst prepared with a dose rate of 64 kGy s-1 and a total dose of 576 kGy showed a CO conversion of 45% and a CO2 selectivity of 30% at 150 oC.

Mineral Manganese Oxides as Oxidation Catalysts: Capabilities in the CO-PROX Reaction.

Cryptomelane is an abundant mineral manganese oxide with unique physicochemical features. This work investigates the real capabilities of cryptomelane as an oxidation catalyst. In particular, the preferential CO oxidation (CO-PROX), has been studied as a simple reaction model. When doped with copper, the cryptomelane-based material has revealed a great potential, displaying a comparable activity to the high-performance CuO/CeO2. Despite stability concerns that compromise the primary catalyst reusability, CuO/cryptomelane is particularly robust in the presence of CO2 and H2O, typical components of realistic CO-PROX streams. The CO-PROX reaction mechanism has been assessed by means of isotopic oxygen pulse experiments. Altogether, CuO/CeO2 shows a greater oxygen lability, which facilitates lattice oxygen enrolment in the CO-PROX mechanism. In the case of CuO/cryptomelane, in spite of its lower oxygen mobility, the intrinsic structural water co-assists as active oxygen species involved in CO-PROX. Thus, the presence of moisture in the reaction stream turns out to be beneficial for the stability of the cryptomelane structure, besides aiding into the active oxygen restitution in the catalyst. Overall, this study proves that CuO/cryptomelane is a promising competitor to CuO/CeO2 in CO-PROX technology, whose implementation can bring the CO-PROX technology and H2 purification processes a more sustainable nature.

Design and fabrication of integral carbon monoliths combining 3D printing and sol–gel polymerization: effects of the channel morphology on the CO-PROX reaction

A new method to synthesize integral carbon monoliths with a controlled channel morphology has been developed in this work by combining 3D-printing technology and sol–gel polymerization.

Customizable Heterogeneous Catalysts: Nonchanneled Advanced Monolithic Supports Manufactured by 3D-Printing for Improved Active Phase Coating Performance.

Three-dimensional (3D)-printed catalysts are being increasingly studied; however, most of these studies focus on the obtention of catalytically active monoliths, and thus traditional channeled monolithic catalysts are usually obtained and tested, losing sight of the advantages that 3D-printing could entail. This work goes one step further, and an advanced monolith with specifically designed geometry has been obtained, taking advantage of the versatility provided by 3D-printing. As a proof of concept, nonchanneled advanced monolithic (NCM) support, composed of several transversal discs containing deposits for active phase deposition and slits through which the gas circulates, was obtained and tested in the CO-PrOx reaction. The results evidenced that the NCM support showed superior catalytic performance compared to conventional channeled monoliths (CMs). The region of temperature in which the active phase can work under chemical control, and thus in a more efficient way, is increased by 31% in NCM compared to the powdered or the CM sample. Turbulence occurs inside the fluid path through the NCM, which enhances the mass transfer of reagents and products toward and from the active sites to the fluid bulk favoring the chemical reaction rate. The nonchanneled monolith also improved heat dispersion by the tortuous paths, reducing the local temperature at the active site. Thus, the way in which reactants and products are transported inside the monoliths plays a crucial role, and this is affected by the inner geometry of the monoliths.

Active copper species of co-precipitated copper-ceria catalysts in the CO-PROX reaction: An in situ XANES and DRIFTS study

Properties of co-precipitated CuO/CeO2 catalysts with nominal content of 1–3 wt.% copper were investigated by EDS, XRD, BET, STEM, EDS map, H2-TPR, in situ DRIFTS and in situ XANES. By comparing results of DRIFTS, XANES and catalytic performance in the CO-PROX reaction, it was evidenced a relationship between the catalytic behavior and the copper oxidation state of the materials. In situ DRIFTS and XANES analyses shed light on the oxidation state of active copper species involved in the CO-PROX reaction between 120 °C and 200 °C. The investigated catalysts present a balance between Cu° and Cu+ species under PROX conditions and the Cu+ is the main species liable to adsorb CO. Furthermore, our studies indicated that the PROX reaction on CuO/CeO2 is impacted by the nature of Cu species, which in turn may vary with reaction temperature, depending on the Cu content. From these results, we propose a new mechanism of PROX reaction on CuCe catalysts, including Cu°/Cu+ and Ce3+/Ce4+ redox pairs and direct CO oxidation on Cu°.

CO oxidation and CO-PROX reactions over Au catalysts supported on different metal oxides: a comparative study

Comparative research about the total CO oxidation and CO-PROX reaction over Au-based materials supported on different oxides was carried out. CeO2, MnO2, TiO2 and SiO2 with nominal content of 1 wt.% of Au were prepared by a modified deposition–precipitation method. The samples were characterized by XRF, XRD, BET, H2-TPR and HR-TEM techniques. Catalytic results of the CO oxidation in the absence and presence of hydrogen showed that the supports and Au/SiO2 were inactive. For the Au/MnO2 and Au/TiO2 samples, CO conversion was lower in the presence of H2, while for Au/CeO2 the conversion of CO in the CO-PROX was higher compared to that in the total CO oxidation. These results show that the impact of H2 on CO oxidation over Au catalysts is dependent on the charactersitics of the materials that, in turn, are influenced by the nature of the support.

CO–PROX reactions on copper Y2O3-ZrO2 catalysts prepared by a single step co-precipitation technique

Nano-sized CuYZr ternary oxides obtained through an original single step co-precipitation synthesis were submitted to an in depth characterization and further tested in the catalytic CO PROX reaction. The tetragonal phase was identified as the main one whatever the calcination temperature. The study of the copper distribution within the crystals through complementary STEM EDX chemical mapping, UV–vis and H2 Thermo Programmed Reduction (TPR) analysis allowed the identification of at least three distinct sites. The PROX tests revealed interesting properties with CO conversion as high as 95 % and relatively high O2 selectivity towards CO2 in the high temperature range. Finally, interesting relationships between relative abundance of copper sites and catalytic activity/selectivity were obtained allowing suggesting relevant active sites.