CrOx Species Research Articles

Atomically Dispersed CrOX on Pd Metallene for CO-Resistant Methanol Oxidation.

The effective design and construction of high-performance methanol oxidation reaction (MOR) electrocatalysts are significant for the development of direct methanol fuel cells. But the active sites of the MOR electrocatalysts are susceptible to being poisoned by CO, resulting in poor durability. Herein, we report an atomically dispersed CrOX species anchored on Pd metallene through bridging O atoms. This catalyst shows an outstanding MOR performance with 7 times higher mass activity and 100 mV lower CO electrooxidation potential than commercial Pd/C. The results of operando electrochemical Fourier transform infrared spectroscopy demonstrate the rapid removal of CO* on CrOX-Pd metallene. Theoretical calculations reveal that atomically dispersed CrOX can lower the adsorption energy of CO* on Pd sites and enhance that of OH* through the formation of a hydrogen bond, decreasing the formation energy of COOH*. This work provides a new strategy for improving MOR performance via atomically engineering oxide/metal interfaces.

CrOx/Ce1−xZrxO2 for chemical looping propane oxidative dehydrogenation: The redox interaction between CrOx and the support

Propane dehydrogenation is an attractive alternative for propylene production to petroleum-based routes. However, overcoming deactivation and toxicity of catalysts in propane direct dehydrogenation (PDH), and over oxidation in oxidative dehydrogenation (ODHP) remain challenging. Here, a chromium-based catalyst supported on a Ce-Zr solid solution is constructed for chemical looping oxidative propane dehydrogenation (CL-ODHP). A dual modulation of oxygen species by Zr doping was unraveled by characterizations and DFT calculation: the oxygen of surface CrOx was constrained while the oxygen mobility of the Ce-Zr support was promoted. Based on these properties, the interaction between surface CrOx and the support further enables an oxygen “donor–acceptor”, which potentially functions as in situ activation and regeneration of redox CrOx species as well as blockage of non-selective oxygen released from the Ce-Zr support. The optimized material (7.5Cr/Ce0.8Zr0.2O2) achieves propane conversion up to 50% and propylene yield of 31% at 600 °C, and has the lowest Cr content among previously reported Cr-based catalysts that have comparable performance. These results provide insights for the design of highly active and selective redox catalysts for chemical looping as a reaction engineering strategy for the upgrading of propane dehydrogenation processes.

NiO and CrOx interaction promoting in situ generation of the coordinatively unsaturated Cr–O acid–base for oxygen-lean propane dehydrogenation

The interaction between NiO and CrOx species increases the reducibility of Cr–O–Ni lattice oxygen, which is consumed to form an oxygen vacancy and a nearby unsaturated Cr–O acid–base pair as the possible active sites for PDH reaction.

Effects of CrOx species doping on V2O5-WO3/TiO2 catalysts on selective catalytic reduction of NOx by NH3 at low temperature

Effects of CrOx species doping on V2O5-WO3/TiO2 catalysts on selective catalytic reduction of NOx by NH3 at low temperature

Effect of alumina surface chemistry on chromia dispersion and dehydrogenation activity of CrOx/η-Al2O3 catalysts with high Cr content

In this work, effect of alumina surface chemistry on physicochemical properties and isobutane dehydrogenation activity of CrOx/η-Al2O3 catalysts with ca. 13 wt. % Cr was investigated. For this purpose, three catalysts were synthesized from η-Al2O3 samples which have identical textural properties but different density of Lewis acid sites (LAS) and OH groups as revealed by IR spectroscopy of adsorbed CO and pyridine. Systematic investigation of CrOx/η-Al2O3 catalysts with a number of techniques (chemical analysis, N2 adsorption, X-ray diffraction, Raman spectroscopy and transmission electron microscopy) showed that (a) Cr6+ and Cr3+ quantities as well as surface areas are the same; (b) quantity of low-active large α-Cr2O3 (> ca. 5 nm) and very small CrOx species (< ca. 1 nm) diminishes while (c) content of partially crystallized Cr2O3 particles from ca. 1 to about 5 nm size increases with the LAS density on parent η-Al2O3 and its dehydroxylation. Correspondingly, isobutane dehydrogenation activity at low times on stream (up to 40 min at 570 °C) increases. This activity difference was observed in the course of 50 dehydrogenation-regeneration cycles. Therefore, alumina surface chemistry strongly affects the Cr2O3 dispersion and dehydrogenation performance of high-loaded chromia/alumina catalysts. On the basis of the results, a model of surface structure for CrOx/Al2O3 catalysts was proposed including surface migration of chromium being dependent upon LAS and OH groups of Al2O3.

Facile Redox Synthesis of Novel Bimetallic Crn+/Pd0 Nanoparticles Supported on SiO2 and TiO2 for Catalytic Selective Hydrogenation with Molecular Hydrogen

The bimetallic Crn+/Pd0 nanoparticles have been synthesized for the first time by a two-step redox method. The method includes the deposition of Pd0 nanoparticles on the surface of SiO2 and TiO2 carriers followed by the deposition of Crn+ on the surface of Pd0 nanoparticles using the redox procedures, which are based on the catalytic reduction of Crn+ with H2 in aqueous suspensions at ambient conditions. Transmission (TEM) and scanning (SEM) electron microscopy, X-ray photoelectron spectroscopy (XPS), Fourie-transformed infrared spectroscopy of adsorbed CO (FTIR-CO), and CO chemisorption studies were performed to characterize the morphology, nanoparticle size, element, and particle distribution, as well as the electronic state of deposited metals in the obtained catalysts. A decrease in nanoparticle size from 22 nm (Pd/SiO2) to 2–6 nm (Pd/TiO2) makes possible deposition of up to 1.1 wt.% Cr most likely as Cr3+. The deposition of CrOx species on the surface of Pd nanoparticles was confirmed using FTIR of adsorbed CO and the method of temperature-programmed reduction with hydrogen (TPR-H2). The intensive hydrogen consumption in the temperature ranges from −50 °C to 40 °C (Cr/Pd/SiO2) and from −90 °C to −40 °C (Cr/Pd/TiO2) was first observed for the supported Pd catalysts. The decrease in the temperature of β-PdHx decomposition indicates the strong interaction between the deposited Crn+ species and Pd0 nanoparticle after reduction with H2 at 500 °C. The novel Crn+/Pd/TiO2 catalysts demonstrated a considerably higher activity in selective hydrogenation of phenylacetylene than the Pd/TiO2 catalyst at ambient conditions.

Enhanced Catalytic Performance of Cr/MOR for Ethane Dehydrogenation Through Dealumination

Dehydrogenation of ethane to ethylene assisted by CO2 was investigated over CrOx catalysts supported on MOR zeolites. The catalysts were characterized by XRD, N2 adsorption, SEM, 27Al and 29Si MAS NMR, XPS, DRIFTS, H2-TPR and laser Raman spectroscopy. The catalysts exhibit enhanced activity and stability after MOR dealumination, with an optimal ethane conversion of ~ 38% and ethylene yield of ~ 33%. Characterization results indicate that the improved catalytic performance derives from the increased silanol nests of MOR support created by acidic dealumination, which are in favor of dispersion and stabilization of CrOx species, resulting in more reducible Cr(VI) considered as the active site for dehydrogenation.

Impact of precursor sequence of addition for one-pot synthesis of Cr-MCM-41 catalyst nanoparticles to enhance ethane oxidative dehydrogenation with carbon dioxide

In this work, the effect of sequence addition of silica and chromium precursors of one-pot synthesis of Cr/MCM-41catalysts on their structural properties and catalytic performance for oxidative dehydrogenation of ethane (ODH) to ethylene using CO2 was investigated. The developed catalysts have been characterized by several techniques such as N2 sorption analyzer, XRD, TEM, SEM, UV–Vis, FTIR, TPD, and XPS. Results showed that the pore and surface characteristics of the developed catalysts strongly depend on the employed sequence. Catalyst prepared by sequence of addition of tetraethyl orthosilicate (TEOS) then Cr, exhibited similar catalytic property to the one prepared using conventional impregnation method. On the other hand, prepared catalyst with sequence of Cr then TEOS, exhibited the highest catalytic ethylene conversion and yield ~58% and 53%, respectively at 700 °C. The enhanced performance arose from the small sizes of mesoporous silica nanoparticles that promoted the high degree of dispersion for CrOx species into meso-channels and surface of porous silica support. Moreover, those small sizes of mesoporous silica nanoparticles have enhanced the interaction of reactants with CrOx species, thus facilitating a faster reduction-oxidation cycle. Various chromium species including Cr6+, Cr3+ and crystalline Cr2O3 were found in all prepared samples; however the highest Cr6+/Cr3+ ratio, that reportedly to be highly active one, was recorded for 8 wt%Cr/MCM-41-Cr-TEOS catalyst. Furthermore, the impact of different Cr loading contents based on Cr-TEOS sequence of addition on the conversion of ethane, yield and selectivity to ethylene has been investigated as well.

Oxidative dehydrogenation of ethane with carbon dioxide over Cr2O3/SBA-15 catalysts: the influence of sulfate modification of the support

Unmodified and sulfate-modified SBA-15-supported Cr2O3 catalysts were prepared by impregnation method. The physico-chemical properties of the supports and catalysts were determined by nitrogen adsorption/desorption, powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), laser-Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), UV–Vis diffuse reflectance spectroscopy (UV-DRS), inductively coupled plasma optical emission spectroscopy (ICP-OES), transmission electron microscopy (TEM) and temperature-programmed reduction (TPR) techniques. Oxidative dehydrogenation of ethane to ethylene (ODE) with CO2 as oxidant was carried out on these catalysts in a fixed-bed reactor at temperatures in the range of 600–700 °C and at atmospheric pressure. The changes in structural and textural properties because of sulfate modification were identified. Sulfate modification affected the nature of interaction of CrOx species with the SBA-15 support. During the evaluation, it was observed that sulfate modification enhances ethane conversion and ethylene selectivity of the catalyst. Better dispersion of CrOx and the increase in Cr6+/Cr3+ ratio seem to be the reasons for the higher performance of the sulfate-modified catalysts compared to that of the unmodified catalyst.

Characterization and catalytic property of nano Cr-based catalysts for ethane dehydrogenation in CO2

ABSTRACTA series of CeO2 promoted nano Na-ZSM-5 supported chromium oxide catalysts were prepared using impregnation method and characterized by XRD, FE-SEM, DTA-TG, and CO2-TPD techniques. Their catalytic performance of ethane dehydrogenation to ethylene in the presence of CO2 was investigated. The addition of Ce species obviously improve the dispersion of active component Cr species on Na-ZSM-5 catalysts, leading to increase of the activity and stability of the catalysts, and reduction of the amount of carbon deposition on the catalyst surface at the same time. The nano 7.5Ce-7.5Cr/Na-ZSM-5 catalyst exhibits both high activity and stability, with an ethane conversion of ∼20% and ethylene yield of ∼19% without any obvious trend of deactivation in 360 min. Characterization results show that the excellent catalytic performance comes from well dispersion of CrOx species on nano ZSM-5 surface, proper acidic-basic property, and less carbon deposition.

Chromium-based catalysts for ethane dehydrogenation: Effect of SBA-15 support

A series of SBA-15 with varying textural and surface properties were systematically prepared by post-synthesis and followed by modification with chromium oxide using incipient wetness impregnation method. They were characterized by SAXS, XRD, N2 adsorption, XPS, 29Si MAS NMR, TG, UV-vis and Raman. The catalytic performances of these CrOx supported catalysts in ethane dehydrogenation in the presence of CO2 were investigated. The amount of silanols on SBA-15 surface was found to be crucial for high activity in ethane dehydrogenation. The surface silanols can be eliminated by calcination at high temperature, and recovered by rehydroxylation with diluted H2O2. High catalytic activity comes from good dispersion of CrOx species, which can only be obtained on the silanol-abundant surface.

Oxidative dehydrogenation of ethane with CO2 over Cr supported on submicron ZSM-5 zeolite

A series of submicron ZSM-5-supported chromium oxide catalysts were prepared and characterized by XRD, N2 adsorption, 27Al MAS NMR, SEM, XPS, laser Raman spectroscopy and diffuse reflectance UV-Vis spectroscopy. The catalytic performance of these materials during ethane dehydrogenation in the presence of CO2 was investigated. The catalysts exhibited both high activity and stability, with an ethane conversion of ∼65% and ethylene yield of ∼49% without any obvious deactivation following 50 h. Characterization results show that the excellent catalytic performance results from the high degree of dispersion of CrOx species on the submicron ZSM-5 surface. Both a high Si/Al ratio and the use of the Na-form of the ZSM-5 support were found to favor CrOx dispersion. The promotional effect of CO2 on the dehydrogenation reaction was quite evident and can be attributed to the reverse water-gas shift reaction.

Active, selective and robust Pd and/or Cr catalysts supported on Ti-, Zr- or [Ti,Zr]-pillared montmorillonites for destruction of chlorinated volatile organic compounds

Pd and/or Cr catalysts supported on Ti-, Zr- or mixed [Ti,Zr]-pillared montomorillonite clays (PILC), for use in combustion of dichloromethane (DCM) and trichloroethylene (TCE), were synthesized, and characterized with XRD, SEM, TEM/HRTEM, XPS, ESR and Raman spectroscopies, N2 adsorption/desorption at −196°C, NH3 TPD–MS, and FTIR of pyridine adsorption. The catalysts are porous solids with high surface area (240–390m2/g). Pd exists as fine PdOx nanoparticles, predominantly of ≤10nm size, while Cr is present as monomeric CrOx species. The highest (Pd4+ and Cr6+), or next to highest (Cr5+), oxidation states dominate at the catalysts surface. The active phases are most reducible when supported on Ti-PILC and most difficult to reduce on [Ti,Zr]-PILC. Catalysts supported on [Ti,Zr]-PILC show highest total acidity, of predominantly Lewis character. The excellent performance of the catalysts in combustion of DCM and TCE is attributed to the combination of highly porous structure of PILC supports, good dispersion of the active phases, and appropriate blend of redox and acid–base functions. In both reactions, for a given type of support, the catalysts with Cr-only active phase show best performance in terms of activity, which is assigned to the presence of well dispersed, highly oxidized Cr6+/Cr5+ species. In DCM combustion, catalysts supported on Ti-PILC show superior activity, attributed to much better reducibility of active phases deposited on this carrier. In TCE oxidation, best activity is obtained for [Ti,Zr]-PILC supported catalysts, which is correlated with the highest Lewis acidity of these samples. Pd-only catalysts are most selective, showing 100% selectivity to HCl and CO2 at T90 for both studied reactions. Cr-only catalysts are least selective, and emit substantial amounts of Cl2 and CO. Mixed Pd,Cr-catalysts represent an attractive, cheaper alternative to purely Pd systems. At T90 their selectivity to CO2 is 100% in both reactions, with the selectivity to HCl ranging from 87 to 100%. PILC-supported catalyst proved very stable in the reaction environment, both in terms of the catalytic performance and the structural identity.

Investigation of chromium oxide clusters grafted on SBA-15 using Cr-polycation sol

A comparative study of chromium oxide clusters grafted on mesoporous silica SBA-15 was carried out using samples synthesized by one-pot, impregnation and Cr-polycation sol grafting methods. The nature of CrOx species incorporated into SBA-15 by direct hydrothermal one-pot method as well as impregnation of Cr(NO3)3·9H2O and Cr-polycation precursors was characterized by XRD, BET isotherms, UV–Vis DRS, FTIR, TGA, O1s XPS, 29Si-MAS NMR, H2-TPR, NH3-TPD, SEM and TEM. Powder XRD did not show the presence of Cr2O3 in the calcined samples obtained by one-pot method. It, however, shows the rhombohedral clusters of α-Cr2O3 dispersed over SBA-15 for CrOx/SBA-15 samples prepared by impregnation and polycation sol grafting methods. The absorption band at 296 nm, observed for Cr3+ in solution, is absent for the aqueous Cr-polycation sol. There is evidence that the presence of chromium precursor in the reaction medium can influence the morphology of SBA-15 as seen in the SEM micrographs. Charge transfer transitions demonstrate the insertion of CrOx species on SBA-15 matrix synthesized by one-pot method. Cr-polycation grafted SBA-15 sample shows unique vibrational features at 573 and 624 cm−1 attributed to extra-framework CrOx species. The ratio of Cr6+/Cr3+ species present in CrOx/SBA-15 samples depends on the Cr-precursor employed for grafting on SBA-15. One-pot synthesized samples predominantly contain coordinated water (δH–O–H at 1,635 cm−1) on SBA-15 while impregnated samples show water molecules associated with CrOx species (δH–O–H at 1,594 cm−1).

Effect of Cr Species in CrO&lt;sub&gt;&lt;em&gt;x&lt;/em&gt;&lt;/sub&gt;-Y&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Catalyst on Chlorine/Fluorine Exchange Reactions

CrOx-Y2O3catalysts were prepared by a deposition-precipitation method and tested by fluorinating 2-chloro-1,1,1-trifleuoroethane (HCFC-133a) to synthesize 1,1,1,2-tetrafluoroethane (HFC-134a). Using Raman spectrum and X-ray powder diffraction (XRD) technique, we found that the atmosphere and the temperature during the calcination process greatly influenced the CrOx species in the catalysts. When the catalyst was calcined in nitrogen and then in air at different temperatures (T) (NAT), the Cr species changed from CrO3 to YCrO4 and YCrO3 with increasing calcination temperature. The NA500 catalyst showed better stability than the catalyst calcined in air at 350 ℃ (the A350 catalyst), because the YCrO4 species formed during calcination could transform into an active species during the pre-fluorination process and the coke deposition was inhibited on the catalyst surface during the reaction.

Aberration-Corrected STEM for Understanding of the Catalytic Mechanisms and Development of New Catalysts

Aberration-corrected Z-contrast (HAADF) STEM has proven an invaluable tool for catalysis research due to its superior resolution and high sensitivity. ORNL’s 300 kV VG Microscopes’ HB603U STEM with Nion aberration corrector was shown to resolve lattice spacings as small as 0.78 A [1] and routinely allows us to image single atoms inside the materials and on their surfaces as well . The ability to simultaneously collect EELS spectra provides additional possibilities for characterization. All these aspects have proven instrumental for the studies of catalytic activity and degradation in Cr-doped transition aluminas, which are widely used for dehydrogenation of alkanes [2]. Even though structural difference between γ-Al2O3 and η-Al2O3 is minimal and only amounts to different distribution of cation vacancies within spinel framework, Cr-doped η-Al2O3 catalysts can last several years, while Cr/γ-Al2O3 catalysts degrade in weeks [3]. Our STEM studies revealed that Cr dopant is distributed differently in these two systems. On γ-Al2O3, Cr is segregated into extended “patches” of Cr2O3 (Fig. 1). On η-Al2O3, on the other hand, no segregation was observed. Contrast in the STEM images was mostly uniform, with the thinnest areas of the alumina flakes sometimes displaying faint spots (Fig.2a)). The spots have similar intensity (Figs. 2b)-d)) of about the right level for Cr. EELS studies (Fig. 2e)) also indicate uniform distribution of dopant on η-Al2O3 surface. Our first-principles calculations suggest that catalytic processes primarily happen on isolated CrOx species, and preserving a high density of these active sites is necessary to retain activity. Due to a unique surface reconstruction resulting from its specific distribution of cation vacancies, γ-Al2O3 (110C) surface can accommodate growth of extended Cr2O3 “patches’, while on η-Al2O3 Cr remains isolated, resulting in much slower degradation.

97/01802 Optimization of deliveries of raw-material components and process conditions of coke manufacture

Al2O3 and SiO2 were used to load chromium oxides with incipient-wetness impregnation method. These supported chromia catalysts were evaluated for the dealkylation of C10+ heavy aromatics in hydrogen flow and were characterized with BET, XRD, UV–vis DRS and H2-TPR techniques. The results indicate that the catalytic activity of supported chromia catalysts on SiO2 is higher than that of on Al2O3. Structural characterization results show that surface reduced CrOx species are the active sites for hydrodealkylation of C10+ heavy aromatics. (C10+ heavy aromatics denote the aromatics whose carbon numbers are equal to or greater than 10.) The structural models of CrOx species on the Al2O3 or SiO2 support surface were also proposed.