Oxygen Chemisorption Capacity Research Articles

The evolution of catalytically active calcium catalyst during steam gasification of lignite char

Coal or coal char/pyrocarbon gasification is a viable way to produce cheaper hydrogen. The gasification of three different lignites and their pretreated samples showed that either the intrinsic calcium in lignite or the extrinsic calcium added by ion exchange had high catalytic activity. It was proven that most of the calcium in the lignite was organically associated, which was mainly bound to carboxyl groups and highly dispersed at an atomic level. Furthermore, Yuxi lignite was demineralized to investigate the catalytic characteristics of calcium and potassium catalysts on steam gasification. The changes of gasification rate, gas compositions and oxygen chemisorption capacity with carbon conversion hinted that the catalytic mechanism of calcium was different from that of potassium. X-ray absorption near edge structure (XANES) analysis revealed that carboxyl-bound calcium was the active form of the catalyst, and the deactivation of the calcium catalyst in the gasification process was due to the transformation of carboxyl-bound calcium to other chemical forms. The organically bound calcium also led to the porous char structures at the early gasification stage, which improved the catalytic activity.

Kinetic study on in-situ and cooling char combustion in a two-step reaction analyzer

Cooling char, which is prepared in an inert atmosphere and then reacted with oxygen after cooling, has been widely used in the kinetic study of char combustion. However, there is no cooling process in commercial combustion systems and thus the impacts of the cooling process on char reactivity remain unclear. To illustrate this question, a two-step reaction analyzer was developed to study the combustion kinetics of in-situ char, which was produced at a constant high temperature in an inert atmosphere and then directly reacted with oxygen at different temperatures. Two types of Chinese coal, Shenhua sub-bituminous coal and Yangquan anthracite, were selected as samples. The results showed that the reaction rate of in-situ char was about 1.1–1.25 times of that of rapid cooling char, suggesting that the cooling process has a significant effect on char reactivity. Then the combustion rate and structure of chars cooled at different cooling rates were investigated to determine the reason for char deactivation during the cooling process. The cooling char reactivity decreased with the decreasing cooling rate, but the specific surface area did not decreased considerably. This result revealed that char deactivation during the cooling process was independent of specific surface area. As the cooling rate decreased, the oxygen chemisorption capacity (active surface area) of cooling char decreased for Shenhua coal, indicating that the observed char deactivation during the cooling process might be caused by the decrease of active surface area. In addition, Yangquan anthracite, as a high rank coal, was less sensitive to the cooling process due to its higher carbon crystallites and provision of fewer active sites for oxidation.

Activation of n-Heptane: A Study with VMgO Catalysts

Vanadium-magnesium oxide (VMgO) catalysts with vanadium loadings of 15 and 20 wt% were prepared by the method of wet impregnation. The prepared catalyst were characterized by IR and Raman spectroscopy, Powder XRD, ICP-OES, N2 adsorption desorption isotherms, oxygen chemisorption, TPR, TPD, SEM, SEM-EDX and TEM. From XRD and IR analyses, vanadium was found to be present in two phases i.e. orthovanadate and pyrovanadate. Dispersion of vanadium on the MgO was in the range of 30–40 % and oxygen chemisorption capacity increased with the vanadium loading. The catalysts were tested in a continuous flow fixed bed reactor at a GHSV of 4,000 h−1 over a temperature range of 350–550 °C at an n-heptane to oxygen molar ratio of 1:1. Oxidative dehydrogenation of n-heptane showed that product selectivity depends on the phase of vanadium present on the MgO catalysts and conversion is proportional to the amount of chemisorbed oxygen on the surface of the catalyst. Carbon oxides are the major products over the MgO support, whereas heptene isomers are the major products over the VMgO catalysts. 2-Heptene, toluene and 2-heptanol are the dominant products in the heptene isomers, aromatics and oxygenate products respectively.

Hydrocarbon reactions on MoS 2 revisited, II: Catalytic properties in alkene hydrogenation, cis–trans isomerization, and H 2/D 2 exchange

MoS 2 prepared by thermal decomposition of ammonium tetrathiomolybdate in inert gas at temperatures up to 773 K was activated by treatments involving thermoevacuation at 723 K and/or reduction at 573 K. The effect of different activations on the activity in ethylene hydrogenation, cis–trans isomerization of 2-butene, and H 2/D 2 isotope exchange (all measured at temperatures around 473 K) was compared, taking into account the extent of Mo exposition as measured by oxygen chemisorption. It was found that activation had a widely varying impact on the test reactions, indicating that these proceed on sites with different numbers of vacancies. Hydrogenation activity was boosted by two orders of magnitude when reduction of the catalyst at 573 K was followed by thermoevacuation at 723 K, whereas this change was moderate with H 2/D 2 exchange and negative with cis–trans isomerization. For the latter reaction, mere thermoevacuation was a suitable activation, and the impact of subsequent catalyst reduction was negative, whereas appreciable activity in the former reactions was obtained only when thermoevacuation was combined with a subsequent reduction. The data suggest that all three reactions proceeded on different sites, probably 3 vacancies per Mo for olefin hydrogenation, 2 vacancies per Mo for H 2/D 2 exchange, and 1 vacancy per Mo for cis–trans isomerization, with the latter two reactions involving adjacent SH groups. Sites with greater Mo exposure than stated may be suitable for H 2/D 2 exchange but not for cis–trans isomerization. With increasing severity of the activation treatments, sites with a small number of vacancies appeared to combine and migrate toward the rims or to escape into the bulk. Therefore, very high hydrogenation activity may be seen on surfaces with an oxygen chemisorption capacity far from the maximum value occurring after less drastic treatments. Our findings imply that a combination of chemisorption studies with test reactions may be a valuable tool for surface characterization of sulfide catalysts.

Hydrocarbon reactions on MoS 2 revisited, I: Activation of MoS 2 and interaction with hydrogen studied by transient kinetic experiments

MoS 2 was prepared by thermal decomposition of ammonium tetrathiomolybdate in inert gas at 723–773 K and activated by procedures involving evacuation at 723 K and/or reduction at different temperatures, predominately 573 K. Bulk and surface properties of these samples were studied by X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), nitrogen physisorption, oxygen chemisorption (273 K, pulse mode), and isotope exchange with D 2 to determine the quantity of exchangeable hydrogen. In addition, the quantity of hydrogenation-active surface hydrogen was determined by hydrogenation of ethylene in absence of gas-phase hydrogen. No obvious relationship was found between the S/Mo stoichiometry and the degree of Mo exposure; depending on the pretreatment, exposed Mo ions can be detected by oxygen chemisorption at widely varying quantities while the S/Mo ratio remains ⩾2. At more severe reduction treatments to remove additional sulfur, the oxygen chemisorption capacity decreases, likely due to migration of defects into the bulk. The reactivity of MoS 2, as expressed by the temperatures required to expose Mo ions and the quantity of exchangeable hydrogen, varies significantly between batches prepared by similar routes. Therefore, thorough surface characterization by a set of techniques including reactivity studies is an indispensable prerequisite for any meaningful study investigating the relationship between catalytic properties with preparation procedures. After activation, MoS 2 holds hydrogen, which is able to hydrogenate ethylene in absence of gas-phase H 2; the quantity of this hydrogenation-active surface hydrogen is not correlated with the hydrogenation activity, however.

Influence of CuO x additives on CO oxidation activity and related surface and bulk behaviours of Mn 2O 3, Cr 2O 3 and WO 3 catalysts

CuO x -modified and unmodified Mn 2O 3, Cr 2O 3 and WO 3 catalysts were prepared by impregnation with Cu(NO 3) 2 solution and/or calcination at 700°C of appropriate precursor compounds. Bulk phase composition and thermochemical stability were characterized by X-ray diffractometry, infrared spectroscopy and thermogravimetry. The surface area, chemical composition, oxidation state and chemisorption capacity were determined by X-ray photoelectron spectroscopy and sorptometry of N 2, O 2 and CO gas molecules. The catalytic CO oxidation activity was tested, using a gas circulating system equipped with a fixed-bed microreactor and a gas chromatograph. The CuO x additives were found to promote markedly the otherwise insignificant CO oxidation activity of WO 3 at 250–400°C. This was related to concomitant improvement primarily in the lattice oxygen reactivity, and in the O 2 chemisorption capacity. The already high CO oxidation activities of Mn 2O 3 and Cr 2O 3 catalysts at 150–250°C were found to be, respectively, slightly and hardly improved by CuO x additives, despite a marked improvement in oxygen chemisorption capacity. This was attributed to establishment on the unmodified catalysts of a favourable electron-mobile environment. In oxygen-rich reaction atmosphere, both the modified and unmodified catalysts were thermochemically stable. However, in lean oxygen atmosphere, Cr 2O 3 and CuO x -modified WO 3 were relatively the most stable test catalysts.

Interaction of oxygen with lead-precovered nickel(100)

The interaction of oxygen with lead-precovered Ni(100) has been investigated by Auger electron spectroscopy (AES). The oxygen chemisorption capacity decays linearly with lead coverage and is completely suppressed above 0.5 lead atom per nickel atom; lead acts as an inert site blocker for the chemisorption stage. Preadsorbed lead slows down the oxidation kinetics, but does not prevent substrate oxidation; nickel oxidation occurs in the bare areas as well as under the lead deposit. For lead, the oxidation mechanism is different according to whether the atoms are in a two-dimensional configuration (submonolayer range) or in a three-dimensional one (crystallites); for crystallites, oxidation occurs from direct impingement of oxygen molecules; for a submonolayer deposit, the oxidation results from a charge transfer between the lead atoms and the underlying nickel oxide.

Correlation between the Stretching Frequency of Carbon Monoxide Adsorbed and the Fermi Level Local Density of States at Surfaces of Platinum Catalysts

We have studied the stretching frequency (extrapolated to zero coverage) of adsorbed CO and the 195Pt NMR (before adsorption of CO) of Pt catalysts on zeolite-NaY (three dispersions), on zeolite-NaHY (two dispersions), and on zeolite-NaX (one dispersion); we have also measured the hydrogen and oxygen chemisorption capacity of these samples. For comparison, we studied three samples on an oxidic carrier (unpromoted and promoted by Li or K salts, all from the same initial batch) by infrared and by NMR. In the zeolite case, we find a direct correlation between the CO stretching frequency and the density of states at the Fermi energy on surface sites (surface Ef-LDOS) derived from the clean-surface NMR data, but the chemisorption results seem to be determined by steric as well as electronic effects, and show no simple relation to the two other quantities. The values for the oxidic samples appear to form a distinct set, with probably some promotor-specific effects. The zeolite data support an interpretation of ...

Characterization of supported vanadium oxide catalysts by a low-temperature oxygen chemisorption technique: III. TheV 2/O 5ZrO 2 system

Low-temperature oxygen chemisorption (LTOC) has been applied to characterize a series ofV 2/0 5ZrO 2 catalysts containing up to 10.38% of V 20 5. The catalysts were also characterized by X-ray diffraction (XRD), and electron spin resonance (ESR). Oxygen chemisorption results suggest the formation of a monolayer of vanadium oxide at a concentration of about 6.53 wt.-% with a high degree of dispersion of vanadia on ZrO 2. X-ray diffraction studies revealed that the phase transition of ZrO 2 (monoclinic to tetragonal) is catalyzed by the presence of vanadia on the support. The ESR results have also shown that the chemical species on the surface and their structure are more affected by the vanadia loading. Oxygen chemisorption capacities of the catalysts were found to correlate with the activities of the catalysts for the partial oxidation of methanol to formaldehyde. A comparison of LTOC results of vanadia supported on zirconia with those of vanadia supported on alumina, silica and titania suggests that the vanadium oxide disperses better on zirconia.

An Evaluation of the Oxygen Chemisorption Capacity of Mild Gasification Char at Various Burnoff Levels

Abstract Our previous results addressed the reactivity of coal chars prepared at mild and severe conditions. The active site concentration of coal chars as determined by oxygen chemisorption capacity is known to serve as a predictive parameter to reflect the reactivity of these chars. However, it is not certain whether the unoccupied active surface area at a given conversion can effectively describe the corresponding burnoff rate. In this study, changes in the oxygen chemisorption capacity of char as a function of burnolT are determined in a thermogravimetric analysis system for low-lemperaturc (500°C) chars. The oxygen chemisorption capacity undergoes changes as a function of char burnoff. The results demonstrate that the changes in oxygen chemisorption capacity does not follow the same trend as the reactivity profile. suggesting that the stable carbon-oxygen complex (present on the surface but not measured during chemisorption) may play a key role in determining the overall gasification reactivity. The changes in the oxygen chemisorption capacity as a function of burnoff can be attributed to three competing mechanisms: (a) formation of stable C-O complexes, (b) opening of previously closed micropores, and finally, (c) loss of micropores because of pore coalescence and burnoff at a high level of oxidation. This study also demonstrates that the reactivity data can be normalized by any reference time (between 10- to 70-percent burnoff). Therefore, arbitrarily defined reference times used in the literature do not necessarily signify parameters of fundamental importance.

Low temperature oxygen chemisorption: a facile technique for characterization of vanadium oxide catalysts supported on anatase and rutile polymorphs of TiO2

Oxygen chemisorption at –78 °C has been determined on the reduced surface of vanadium oxide catalysts supported on anatase and rutile modifications of TiO2; a correlation is found between oxygen chemisorption capacities with the activity of the catalysts for vapour-phase oxidation of methanol to formaldehyde.

Characterization of γ-alumina-supported tungsten sulfide hydroprocessing catalysts: I. Low-temperature oxygen chemisorption

A series of γ-alumina-supported tungsten sulfide catalysts containing up to ca. 10 μmol m −2 tungsten has been characterized by oxygen chemisorption (static) at −78°C, X-ray diffraction and surface area measurements. They have also been evaluated by two probe reactions, namely hydrodesulfurization (HDS) of thiophene and hydrogenation (HYD) of cyclohexene. HDS and HYD activities of these catalysts are found to correlate with their oxygen chemisorption capacities. Nickel is found to have considerable promotional effect on these reactions; however, cobalt shows no such effect on the HDS activity, and only marginal promotional effect on HYD activity of the W/Al 2O 3 catalysts. Based on the results, a surface structural model, showing a highly dispersed patchy monolayer of amorphous tungsten sulfide phase at lower tungsten loadings and a bulk WS 2 phase, in addition to the monolayer phase at higher tungsten loadings, is proposed.

Significance of char active surface area for appraising the reactivity of low- and high-temperature chars

Contemporary char reactivity studies have focussed primarily on coal chars prepared under severe (high-temperature) conditions. In this study, the reactivity of chars prepared under mild (low-temperature) conditions has been addressed. A thermogravimetric analysis system (TGA) was used to determine the reactivity of chars in oxidizing atmosphere using isothermal or non-isothermal techniques. Coal chars were prepared in a TGA or in a slow heating rate organic devolatilization reactor (SHRODR) at a temperature range between 500 ° and 950 °C. The chars prepared by mild pyrolysis of coal at 500 °C are shown to be highly reactive. Comparison of reactivities of low- and high-temperature chars shows that the low-temperature chars exhibit higher reactivity than either the parent coals or the high-temperature chars. Correlation between isothermal reactivity results (e.g. time) and non-isothermal reactivity data (e.g. temperature) has been obtained. Hydrogen contents of chars correlate well with the reactivity of the chars. The study confirms the importance of oxygen chemisorption capacity as a significant reactivity parameter for both low- and high-temperature chars. A new approach has been used for calculating the oxygen chemisorption capacity of chars by accounting for the carbon surface sites occupied by hydrogen (and, therefore, these sites were unavailable for oxygen chemisorption). The occupied sites are readily freed during reactivity measurements and thus were available for participation in carbon-oxygen reactions.

Gasification reactivity of Chilean coals

Gasification reactivities of raw and acid-washed coal chars obtained from the three most important coal-bearing regions in Chile have been determined in 0.1 MPa of oxygen using a thermobalance. Oxygen chemisorption capacities of the demineralized chars were also measured gravimetrically at 373 K in 0.1 MPa of oxygen. The subbituminous coals of Catamutun and Peckett are more reactive than the bituminous coals of Lota and Trongol due to the catalytic effect of their inorganic constituents. However, in the absence of catalytically significant mineral matter, coal rank is not an important parameter of char reactivity. The reactivity of chars based on carbon active surface areas, estimated from gravimetric chemisorption measurements, agreed very well with the previously reported value based on active surface areas obtained in a volumetric system. These results supply additional evidence that active surface area is the fundamental parameter that can explain most of the observed differences in the kinetic behaviour of coal chars and carbons in general.

Evolution of pore structure and active surface areas of coal and char during hydrogenation

A Belgian coal (Beringen) has been used in a fundamental study of the changes in pore structure and active surface area during hydrogenation. The coal was devolatilized under N 2 and then hydrogenated at constant temperature up to various conversion yields. Reactivities of the chars were determined by isothermal thermogravimetric analysis. Changes in total surface area, calculated from CO 2 adsorption isotherms at 298 K, and in macropore distribution, obtained by mercury penetration, were found to lead to the same conclusion. In the first reaction stage, the preponderant phenomenon was the opening of previously closed micropores, induced by the material consumed by the reaction. This stopped completely when conversion yield exceeded 55 wt% when the maximum surface area was observed. Then removal of material from pore walls became predominant, pores were enlarged and pore walls disappeared gradually with increasing hydrogenation yields. Oxygen chemisorption capacities were measured at 383 K and 0.1 MPa air to give a relative indication of the concentration of carbon active sites. Active surface area results were compared to total surface area values and correlated with reaction rates.

Relation between reduction and sulphidation process during the activation of a Co-Mo/Al2O3 hydrodesulphurization catalyst

The course of activation at 673 and 773 K of an industrial hydrodesulphurization catalyst CHEROX 36-01 was followed gravimetrically. Reduction with hydrogen, reductive sulphidation with a H2S/H2 mixture, non-reductive sulphidation with a H2S/He mixture and sulphidation of prereduced catalysts were compared. All sulphidation procedures are much faster than the reduction alone, the latter has, however, a higher apparent activation energy. In the reductive sulphidation with H2S/H2, reduction and sulphidation take place simultaneously, sulphidation being a predominant process at the beginning of the activation, especially at 673 K. It has been shown how the two processes can be decoupled or superposed. The catalytic activity in ethylene hydrogenation develops much more slowly when the catalyst is activated only by reduction than when it is activated by reductive sulphidation, in accordance with the relative velocities of these two activation procedures. The hydrogenation activity of catalysts reduced to a different extent correlates with their oxygen chemisorption capacity at 195 K.

Oxygen chemisorption: Its relationship to hydrotreating activity of alumina-supported nickel-molybdenum catalysts

Oxygen chemisorption capacities of sulfided alumina-supported NiMo catalysts were determined by pulse and static adsorption methods at ambient temperatures. The pulse method resulted in lower chemisorption uptakes compared to the static method. Static to pulse uptake ratios of 2.12 to 3.03 were observed for freshly sulflded catalysts while values of 1.70 to 1.92 were obtained for catalysts which had been tested for hydrodenitrogenation (HDN) activity. A general trend of increasing O 2 chemisorption uptake with increasing activity for removal of basic nitrogen, total nitrogen, and sulfur was observed for NiMo catalysts. However, because variations in the specific O 2 chemisorption capacity per activity unit among these three reactions were small (±10%), it does not appear that O 2 adsorption is selective for the active sites of any one reaction, rather, it is probably proportional to the sum of these different sites. Comparison of the O 2 adsorption capacity with the activity data (reproducibility ±3%) indicates the use of O 2 adsorption can distinguish large differences in catalyst activity (>25%) but not smaller differences.

Importance of carbon active sites in the gasification of coal chars

A demineralized lignite has been used in a fundamental study of the role of carbon active sites in coal char gasification. The chars were prepared in N 2 under a wide variety of conditions of heating rate (10 K min −1 to 10 4 K s −1), temperature (975–1475 K) and residence time (0.3 s–1 h). Both pyrolysis residence time and temperature have a significant effect on the reactivity of chars in 0.1 MPa air, determined by isothermal thermogravimetric analysis. The chars were characterized in terms of their elemental composition, micropore volume, total and active surface area, and carbon crystallite size. Total surface area, calculated from C0 2 adsorption isotherms at 298 K, was found not to be a relevant reactivity normalization parameter. Oxygen chemisorption capacity at 375 K and 0.1 MPa air was found to be a valid index of char reactivity and, therefore, gives an indication, at least from a relative standpoint, of the concentration of carbon active sites in a char. The commonly observed deactivation of coal chars with increasing severity of pyrolysis conditions was correlated with their active surface areas. The importance of the concept of active sites in gasification reactions is illustrated for carbons of increasing purity and crystallinity including a Saran char, a graphitized carbon black and a spectroscopically pure natural graphite.

Comparative characterization of monodisperse platinum, palladium and nickel in a faujasite X matrix by dynamic oxygen chemisorption and electron microscopy

A comparison of particle size distributions from electron micrographs and oxygen chemisorption capacities on reduced platinum and palladium faujasite X confirms the existence of monodisperse metal phases of narrow aggregate size distribution and excludes a grape structure for the occluded metal particles. For reduced, monodisperse nickel faujasites a degree of reduction can be obtained by this comparison.