Surface Chromium Species Research Articles

Effect of alumina polymorph on the dehydrogenation activity of supported chromia/alumina catalysts

In this work, the effect of surface properties of different alumina supports for CrOx/Al2O3-catalysts on Cr-speciation and dehydrogenation activity was studied. For this purpose, a series of CrOx/Al2O3-catalysts with a monolayer coverage of CrOx (ca. 4.4 at. Cr/nm2) was prepared with γ-, δ-, (δ + θ)-, η-, θ-Al2O3 as supports. Systematic investigation of fresh catalysts with a set of techniques (chemical analysis, N2 adsorption, XRD, Raman and ESR spectroscopy) showed that (i) chemical state distribution of Cr (Cr6+, Cr5+, Cr3+) was the same for all of the catalysts; (ii) CrOx-species were in X-ray amorphous state; (iii) surface chromate species were virtually of the same oligomerization degree. However, aggregation of Cr3+ ions both in calcined and reduced catalysts is support-dependent. ESR data and infrared spectroscopic results of adsorbed CO showed that increase of Lewis acid sites (LAS) surface density on bare aluminas promotes the growth of the relative amount of Cr3+ ions in a relatively large Cr2O3-like clusters in the fresh catalysts. Catalytic testing in a cycling dehydrogenation-regeneration mode leads to catalyst deactivation. Investigation of spent catalysts revealed that deactivation is accompanied with sintering of Cr3+Ox-species with partial migration of Cr3+ ions inside alumina. It was observed that increase of LAS surface density on aluminas promotes the growth of both the initial dehydrogenation activity and stability upon cycling. On the basis of experimental results, it was concluded that (i) Cr3+ ions on the surface of relatively large Cr2O3-like clusters are more active than isolated and poorly agglomerated Cr3+ ions; (ii) increase in LAS concentration on alumina induces growth of surface coverage by Cr2O3-like clusters with concomitant modification of intrinsic activity of active sites through their interaction with support.

Deactivation Studies of the CrOx/Al2O3 Dehydrogenation Catalysts under Cyclic Redox Conditions

In this work, the deactivation of a CrOx/Al2O3 dehydrogenation catalyst in an industrial reactor has been systematically studied under cyclic dehydrogenation–regeneration conditions from start up to end of run. The results showed that the CrOx/Al2O3 catalyst deactivation occurred through three major processes. The first process is related to the complex transformation of surface chromium species, the second process refers to the phase transformation and/or sintering of support alumina, and the third process is the interfacial CrOx/Al2O3 transformation into the solid solution of α-(Al, Cr)2O3. In addition, the reduction of the chromium oxide surface area and the migration of Cr3+ into the alumina support resulted in the increase of the surface Cr6+/Cr3+ ratio at the end of the run which increased the specific catalyst acidity and reduced the catalyst selectivity. The main causes of irreversible deactivation of the CrOx/Al2O3 catalyst are the decrease of chromium oxide surface area, the formation of α-(Al, ...

High-resolution 29Si CP/MAS solid state NMR spectroscopy and DFT investigation on the role of geminal and single silanols in grafting chromium species over Phillips Cr/silica catalyst

Phillips Cr/silica catalyst is industrially important in ethylene polymerization. The high-resolution solid state 1H MAS NMR and 29Si CP/MAS NMR allowed the identification of the type and amount of silanols: geminal vs. single (isolated and vicinal) silanols on Phillips catalysts calcined at different temperatures, which were compared with those of the bare silica gel counterparts. The residual silanols on the catalyst and silica gel samples were all decreased with increasing calcination temperatures from 120 to 800°C. For the catalysts treated at temperatures lower than 300°C, the amount of residual silanol groups were much lower than those of the silica gel counterparts. It suggested that the chromium species were mainly grafted on the silica gel through esterification reaction with surface silanols below 300°C. The geminal silanols almost disappeared on the catalysts at 120°C, while that for the silica gel occurred at 300°C. Further increasing the calcination temperatures from 300 to 800°C, the amount of single silanols were slower decreased for the catalysts than that for the silica gel samples. It indicated that the presence of the grafted chromate species obstructed further removal of the residual single silanols. The role of silanols on the formation of surface chromate species on the well-defined polyoligomericsilsesquioxane (POSS) models containing various types of silanols was theoretically studied by density functional theory (DFT) method. It was shown that one silanol of the geminal pair and another adjacent single silanol was the most thermodynamically favored for grafting chromium species. The priority of the reaction between chromium species and different types of surface silanol groups during calcination for Phillips catalysts were experimentally and theoretically elucidated for the first time.

Investigating the CrOx/Al2O3 dehydrogenation catalyst model: I. identification and stability evaluation of the Cr species on the fresh and equilibrated catalysts

In this study a series of the fresh and equilibrated CrOx/Al2O3 catalysts with the Cr loading from 0.58wt% to 13.2wt% were in detail studied using combined techniques, including Raman spectroscopy, TEM, XPS, O2 chemisorption, TPR, XRD and chemical analysis, to identify surface chromium species and to evaluate their stability under the cyclic dehydrogenation-regeneration (redox) conditions. The study showed that the stable chromium species present in the equilibrated CrOx/Al2O3 catalyst can be achieved through complete surface restructuring of initial chromium species in the fresh one under the redox conditions. This study also showed that the fresh CrOx/Al2O3 catalyst contained not only previously known chromium species of ‘isolated’ mono/poly aluminum chromate(s), amorphous and crystalline Cr2O3 clusters with different sizes, but also chromium-chromate (Cr6+) species on the surface of Cr2O3 clusters and aluminum-chromium-chromate (Cr6+) species coupled with Cr2O3 clusters. The ‘isolated’ aluminum chromate(s) and small Cr2O3 clusters on the catalyst surface were not stable under the cyclic redox conditions and disappeared after the break-in-period of the catalyst operation. The paper presents the mechanism of the surface restructuring of chromium species under the redox conditions and the revised model for the CrOx/Al2O3 catalyst with the consideration of the change of surface chromium species caused by the cyclic redox aging. The model can be applied to the commercial CrOx/Al2O3 catalysts.

High-resolution spectroscopy (XPS, 1H MAS solid-state NMR) and DFT investigations into Ti-modified Phillips CrO x/SiO 2 catalysts

Ti-modified Phillips catalyst is a most important industrial catalyst widely used in ethylene polymerization, but the mechanism still remains mysterious. In this work, Ti-modified Phillips catalysts were characterized by high-resolution X-ray photoelectron spectroscopy (XPS) and 1H magic-angle-spin solid-state nuclear magnetic resonance ( 1H MAS solid-state NMR) combined with density functional investigations into the effects of Ti-modification on promotion of polymerization activity and regulation of microstructures of the polymer chains. XPS data revealed Ti-modification caused increase in electron-deficiency and photo-stability of the surface chromate species. 1H NMR provided the first direct evidence of surface residual Ti–OH groups. Modeling results rationalized well the effects of Ti-modification on promotion of polymerization activity, extension of molecular weight distribution (MWD) to lower MW region, improvement of the distribution of inserted co-monomer and enhancement of 2,1-insertion in regioselectivity of Phillips catalysts. It was the first time the Ti-modification on Phillips catalysts were theoretically elucidated.

Effect of Phosphorous Additive on the Surface Chromium Oxide Species on Alumina for Propane Oxidation to Propylene

Chromium oxide deposited on phosphorus-loaded alumina is used in the oxidative dehydrogenation (ODH) of propane to propylene. Operando Raman-GC studies allowed evaluating the molecular structure of surface Cr oxide species, their reactivity and their changes undergone during operation. The addition of phosphorous to the alumina substrate decreases propane conversion, mainly due to the decrease of CO x , there is a concomitant increase in the selectivity to propylene formation. The addition of phosphorous affects the nature of the Cr oxide species, which do not form of a new phase. The surface interaction between dispersed chromate species and phosphorous weakens the Cr=O bond and retards the aggregation of surface chromate species as Cr2O3 during reaction in ca. 170 °C.

Ethane dehydrogenation over pore-expanded mesoporous silica-supported chromium oxide: 2. Catalytic properties and nature of active sites

Pore-expanded mesoporous MCM-41 silica-supported chromium oxide catalysts with different chromia loadings were synthesized using three different methods. The catalysts were thoroughly characterized and used for ethane dehydrogenation (DH). Reaction studies revealed that the ethane conversion and coke formation on the catalyst depend on the chromia loading and the catalyst preparation method. All catalysts were highly selective toward ethylene. However, among the three catalysts containing 5 wt% chromia, the 5 wt% Cr/PE-MCM-41 catalyst exhibited the best performance with an ethane conversion of 23% and ∼99% ethylene selectivity at 650 °C (C 2H 6/N 2 = 0.176, total contact time = 0.0032 g min/mL). The chromia catalysts prepared using as-synthesized pore-expanded mesoporous silica (Cr/PE-MCM-41) also exhibited the highest stability. XPS analysis revealed the presence of three different chromium, namely Cr(II), Cr(III) and Cr(VI), whose overall surface concentration as well as relative content varied with the reaction time-on-stream (TOS). Furthermore, the nature and relative amounts of surface chromium species were monitored by in situ XPS measurements as a function of TOS. Correlation of the XPS findings with ethane DH data showed clearly that the Cr(III) species are the main active sites.

Concentration and purification of chromate from electroplating wastewater by two-stage electrodialysis processes

A designed two-stage electrodialysis system is proposed to concentrate and purify chromate from a low pH electroplating wastewater using monovalent selective electrodialysis membranes. With low pH of the raw water (pH 2.2) in the first stage, chromate was presented as HCrO 4 − and monovalent ions (HCrO 4 −, NH 2SO 3 −, Na + and Cl −) were able to pass through the membrane thus chromate was concentrated up to 191%. Higher current density, flowrate and more membrane area all increased the chromium recovery. When pH was adjusted to 8.5 before entering the second stage, the chromate species was presented as divalent CrO 4 2− and retained in the concentrated stream, and the rest monovalent ions (NH 2SO 3 −, Na + and Cl −) were separated by passing through the membrane. For example, 45% of the chlorides were separated in this study. The separation efficiencies in the second stage were also increased when the current density, flowrate and membrane area were increased. Electron Spectroscopy for Chemical Analysis was used to examine the surface chromate species for stage 1, and anion exchange membrane showed more chromate fouling comparing to cation exchange membrane due to more adsorption and concentration polarization effects for the anion exchange membrane.

Vibrational Properties of CrII Centers on Reduced Phillips Catalysts Highlighted by Resonant Raman Spectroscopy

Surface chromate species on Phillips catalysts are probed directly using resonant Raman spectroscopy (see figure). Under pre-resonance conditions, for the first time, the vibrational features of the CrII-active centers and their evolution upon interaction with N2 and CO are observed.

Correlation of oxidation states of surface chromium species with ethylene polymerization activity for Phillips CrO x/SiO 2 catalysts modified by Al-alkyl cocatalyst

The real oxidation states of active sites on the important industrial Phillips type catalysts are still unclear. In this study, Phillips catalysts calcined at different temperatures (400, 600 and 800 °C) were modified by triethylaluminum(TEA) at different Al/Cr molar ratio and subsequently characterized by X-ray photoelectron spectroscopy (XPS) method and ethylene polymerization tests in order to elucidate the real oxidation states of active sites. Activity of each catalyst increases up to a maximum value then drops down with the increasing of Al/Cr molar ratio. The TEA-modified catalyst calcined at higher temperature got a larger maximum value of polymerization activity at a lower Al/Cr molar ratio. XPS method quantified the mixed Cr oxidation states including +2, +3, +5 and +6 of surface chromium species, in which only the +2 and +6 Cr species were found to be relating to the polymerization activity. The real active chromium precursor might be a chromium cluster ( Scheme 1) named as Cr 2+·2Cr 6+ composed of one Cr(II)O x,surf species and two Cr(VI)O x,surf species, in which the former acts as the real center of active Cr cluster and the latter acts as the necessary neighboring ligand environment. The relative concentration of active sites is estimated to be around 14.4–24.9 (mol % Cr) for the TEA-modified Phillips catalysts depending on the calcination temperature and Al/Cr molar ratio.

Characterization of Cr/SiO 2 catalysts and ethylene polymerization by XPS

Cr/SiO 2 catalysts with 1 or 3 wt.% Cr loadings and different chromium precursors were characterized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). A method to determine chromium species in the sample was developed through the decomposition of the Cr 2p XPS spectrum in Cr 6+ and Cr 3+ standard spectra. The results of the binding energy from the Cr 2p region and of the distribution of chromium species allowed to evaluate the dynamic photo-reduction of the surface chromium species during XPS analysis. Photo-reduction of surface Cr 6+ to Cr 3+ species was verified for all samples supported in silica, depending on the precursor and chromium content. Bulk CrO 3 and Cr 2O 3 standards did not reveal variation in the binding energy of Cr 2p 3/2, but a physical mixture of CrO 3 with SiO 2 presented photo-reduction. The behavior of this mixture resembled to the catalysts and suggests the participation of the surface hydroxyls of silica in the photo-reduction process. XPS intensity measurements for assessing dispersion of chromium oxide were used to compare the calcined and reduced catalysts to different chromium precursors. Polyethylene chains were detected by in situ XPS, while oligomerization products were not observed.

Photo-stability of surface chromate species on Phillips CrO x/SiO 2 catalysts isothermally calcined at various temperatures, probed by high resolution X-ray photoelectron spectroscopy

Phillips CrO x /SiO 2 catalyst is still an important industrial catalyst for ethylene polymerization. Poor understanding of the electronic states of surface chromate species has hindered further mechanistic exploration and improvement of this catalyst system. The objective of this work is to systematically study the photo-stability of Phillips catalyst by X-ray photoelectron spectroscopy (XPS) method, since such photo-stability has not been well documented yet in the literature. Five catalysts: namely, PC200, PC300, PC400, PC600 and PC800 with 1.0 wt.% of Cr loading were prepared from the industrial Phillips catalyst precursor PCP120 at 200, 300, 400, 600 and 800 °C, respectively. The photo-stability in terms of electronic stability for the surface chromate species (Cr(VI)O x,surf ) on these catalysts was investigated by a high resolution XPS method. The resistance to photo-reduction into supported Cr(III)O x , surf species induced by X-ray irradiation during XPS measurement was studied. The photo-stability of surface chromate species on Phillips catalyst was found to be significantly dependent on the calcination temperature for preparation of the catalysts. A correlation between the photo-stability of surface chromate species and the calcination temperatures for catalyst preparation has been established. Deeper insight on the electronic states of surface chromate species on Phillips type catalysts has been achieved by high resolution XPS methods.

High resolution X-ray photoelectron spectroscopic analysis of transformation of surface chromium species on Phillips CrOx/SiO2 catalysts isothermally calcined at various temperatures

Abstract Phillips CrO x /SiO 2 catalyst is still producing several million tons of polyethylene each year. However, insufficient understanding of the surface chemistry of this important industrial catalyst hindered its further development. In this work, surface analytical approach on transformation of chromium(III) acetate into supported chromate species (Cr(VI)O x ,surf ) of Phillips CrO x /SiO 2 catalysts isothermally calcined at various temperatures was carried out by high resolution XPS method. High resolution XPS characterizations have elucidated the specific transformation process of chromium(III) acetate into bulky CrO 3 and subsequently into Cr(VI)O x ,surf as a function of calcination temperature between 120 and 800 °C. The specific surface components in various catalyst samples versus calcination temperature were clarified. It was evident that chromium(III) acetate could be very easily oxidized and decomposed into bulky CrO 3 during 120 °C isothermal drying process. The bulky CrO 3 started to be transformed into supported chromate species at 200 °C and could be completely stabilized on silica gel surface as chromate species at 400 °C. Significant amount of bulky pentavalent Cr oxide (Cr 2 O 5 or (Cr 2 O 7 ) 4− ) due to partial thermal decomposition of bulky CrO 3 was only observed on 200 °C-calcined sample due to the incomplete stabilization of bulky CrO 3 into chromate species at 200 °C. As the precursor of active site on calcined Phillips catalyst, the chromate species (Cr(VI)O x ,surf ) was found to be gradually becoming more and more electron-deficient with increasing calcination temperature of the catalyst from 200 to 800 °C. Only a slight thermal-induced partial reduction of chromate species into Cr(III)O x ,surf was observed at high temperature (600–800 °C).

Low-temperature adsorption of oxygen on calcined chromia: IR spectroscopic and sorptiometric evidence for oxygen-assisted topochemical reduction of surface chromate species

The evolution with calcination of chromia samples of catalytic interest, prepared from gels with or without added ammonium nitrate, has been studied by thermogravimetry (TG) and differential thermal analysis (DTA). The low-temperature (77 K) adsorption of oxygen on alumina- and silica-supported samples of the former type has revealed the presence of monolayer and multilayer types of irreversibly adsorbed species. Low-temperature in situ infrared (IR) spectra show that the adsorption of oxygen is associated with the removal of a multiplicity of absorptions between 1100 and 730 cm −1. Probable types of surface species removed are CrO groups of Cr(VI)–O surface species, dichromates and chromates together with possibly peroxide; (O 2) 2−, the latter being associated with prominent bands near 800 cm −1. Room-temperature ex situ infrared spectra on an alumina-supported sample shows at lower frequencies the presence of the growth of α-Cr 2O 3 surface species after oxygen adsorption.

Raman spectroscopic study of supported chromium(vi) oxide catalysts

Raman spectroscopy has been used to characterise Cr(VI) oxide species supported on the surfaces of SiO2, Al2O3, SiO2–Al2O3 and TiO2, as a function of Cr loading and calcination temperature. The interpretation of the spectra has been facilitated by Lorentzian band deconvolution and ab initio calculations employing the hybrid SCF-DFT method B3-LYP, in conjunction with the effective core potential LanL2DZ basis set. These calculations have enabled the elucidation of the geometries of surface chromate species by comparison with reference Cr(VI) compounds, and also detailed force constant calculations and vibrational assignments in terms of potential energy distributions. For all of the surface chromate(VI) species only Raman bands due to stretching vibrations involving CrO bonds are observed. Other chromate vibrations are predicted to be strongly mixed with vibrational coordinates of the support, giving rise to composite modes which have much lower Raman intensity.

Effect of modifiers on the reactivity of Cr2O3/Al2O3 and Cr2O3/TiO2 catalysts for the oxidative dehydrogenation of propane

Chromium oxide supported on alumina and titania supports was modified with oxides of sodium, vanadium and molybdenum. The modified and unmodified chromium oxide catalysts were characterized by several techniques. The presence of surface chromium oxide and surface molybdenum and vanadium oxide species was detected in the unmodified and molybdenum and vanadium oxide modified supported chromium oxide catalysts. The reducibility (Tmax and H/Cr ratio) of the surface chromium species was not affected for the vanadium and molybdenum oxide modified catalysts; however, the reducibility changed noticeably for sodium modified supported chromium oxide catalysts. Studies of the reactivity of the ODH of propane revealed the effect of modifiers on the reactivity properties of the surface chromium oxide species. The activity and propene selectivity decreased for sodium modified supported chromium oxide catalysts. However, the activity increased for vanadium oxide modified catalysts and was similar for molybdenum oxide modified catalysts irrespective of the support. The propene selectivity was higher for molybdenum oxide modified chromium oxide catalysts. However, the propene selectivity for vanadium oxide modified catalysts depends on the support since it appears that the inherent selectivity of the surface vanadium oxide species is reflected.

New aspects of the induction period of ethene polymerization using Phillips CrO x/SiO 2 catalyst probed by XPS, TPD and EPMA

In this work, an industrial Phillips CrO x /SiO 2 catalyst engaged in the induction period through interaction with ethene at room temperature (RT) was studied by the combination of surface analytical methods: X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and electron probe microanalysis (EPMA). XPS method disclosed the reduction of surface chromate species (expressed as Cr(VI)O x,surf ) into surface-stabilized Cr 3+ and Cr 2+ species (expressed as Cr(III)O x,surf and Cr(II)O x,surf , respectively) by monomer during the induction period and the simultaneous formation of two kinds of surface carbon species, namely, formaldehyde and unsaturated hydrocarbons. The hydrocarbon species were supposed to be composed of not only coordinated ethene, but also adsorbed longer olefin chains. TPD measurement confirmed the formation of formaldehyde. EPMA characterization revealed that the surface Cr aggregation occurred in a low extent during the induction period resulting in a few Cr islands in sizes of several microns on catalyst surface. It was demonstrated that reduction and alkylation of Cr species as well as initiation of ethene insertion had already occurred even in the induction period. A mechanism concerning the reactions occurred during the induction period on the Phillips catalyst was proposed. The Cr(II)O x,surf species coordinated with one formaldehyde and one ethene is postulated to be the active sites precursor for the alkylation as well as initiation of the ethene insertion. The coordinatively-adsorbed formaldehyde molecules on the catalyst are supposed to be the very reason for the hindered ethene oligomerization rather than normal polymerization, as well as for the induction of the surface Cr aggregation.

Characterisation of ceria supported chromia catalysts

A series of ceria supported chromia catalysts of different loading were prepared and the effect of support in stabilising various chemical and molecular states of chromium with the calcination temperature were studied. XRD results showed that the chromia phase is randomly dispersed, disordered, and the crystallite size may be smaller than the detection limit. BET surface area and pore volume results suggested some plugging of pores on the support by chromia phase. EGA analysis suggested the existence of the surface heterogenity involving weakly and strongly chemisorbed CO 2. EGA and NIR DRS studies revealed the bulk-like behaviour in the catalysts having chromia loading of more than 5 wt.%. EGA, FTIR and UV–VIS–DRS studies supported the formation of anchored surface chromium species. Moreover, the nature of molecular states of chromium and the coordinative environment under various thermal conditions were investigated using FTIR and UV–VIS–DRS techniques.

Influence of Lanthana on the Nature of Surface Chromium Species in La2O3-Modified CrOx/ZrO2 Catalysts

Infrared spectroscopy of the adsorbed probe molecules CO, NO, and NH3, photoelectron spectroscopy, temperature-programmed desorption of NH3, X-ray diffraction, and BET surface analysis have been used to study surface and bulk characteristics of lanthana-modified CrOx/ZrO2 catalysts. La2O3 addition to CrOx/ZrO2 strongly modifies the structure of supported chromia species and stabilizes coexisting isolated Cr3+ and highly dispersed Cr2O3 particles. Unlike that, chromia in La2O3-free CrOx/ZrO2 undergoes considerable sintering at 823 K, which is the necessary reduction and reaction temperature for n-octane aromatization, applied as a test reaction. The strong interaction between chromia and La2O3-ZrO2 could be due to an increased number of nonacid hydroxyl groups, which are possibly responsible for a more efficient anchoring of chromia species on the La2O3-containing support rather than on pure zirconia. The performances of La2O3-promoted and La2O3-free CrOx/ZrO2 catalysts in n-octane aromatization do not differ so dramatically as would be expected from the large differences in the structure of surface chromia species.

Dehydrogenation of ethane with carbon dioxide over supported chromium oxide catalysts

The oxidative dehydrogenation of ethane into ethylene by carbon dioxide over an unsupported Cr 2O 3 and several supported Cr 2O 3 catalysts on metal oxides such as Al 2O 3, SiO 2, TiO 2, and ZrO 2 was investigated and the effect of support on the catalytic activity was studied. The unsupported Cr 2O 3 shows medium catalytic activity in this reaction; the support will exert a quite different effect on catalytic behavior. The catalytic activity varies with the nature of supports. Cr 2O 3/SiO 2 catalysts exhibit an excellent performance in this reaction. Cr 2O 3 loading also affects the catalytic activity; 8 wt.% Cr 2O 3/SiO 2 catalysts can produce 55.5% ethylene yield at 61% ethane conversion at 650°C. Characterization indicates that the distribution of chromium oxide on supports and surface chromium species structure are influenced by the nature of supports. The acidity/basicity and redox property of catalysts determines the catalytic activity in the dehydrogenation of ethane by carbon dioxide.