Temperature-programmed Oxidation Studies Research Articles

Ozone catalytic oxidation of toluene over 13X zeolite supported metal oxides and the effect of moisture on the catalytic process

This paper reports the behavior of 13X zeolite supported Ce, Cu, Co, Ag and Mn metal oxides toward ozone catalytic oxidation (OZCO) of toluene and the influence of moisture on the decomposition process. The simple impregnation method was adapted to disperse the metal oxides and found highly active for toluene oxidation in the presence of ozone. The steady-state activities and ozone decomposition data reveal that the activity is in the order of Mn/13X>Ce/13X>Cu/13X>Ag/13X>Co/13X and Mn/13X>Cu/13X>Ce/13X>Ag/13X>Co/13X, respectively. The extent of ozone decomposition is responsible for the degree of oxidative decomposition of toluene over the Mn/13X catalyst. The addition of moisture (Relative Humidity of 25%) to the reaction mixture considerably enhanced the conversion of toluene and selectivity to carbon oxides from 49% to 61% and 38% to 53% respectively, on the Mn/13X catalyst. The two sets of experiment results reveal that the surface adsorbed by-products such as benzene, benzaldehyde, p-methyl phenol and oxalic acid are considerably oxidized to CO2 in the presence of moisture whereas in the absence of moisture these by-products are slowly oxidized. The activity data in the presence of ozone and moisture also reveal that the moisture has considerably enhanced the activation of surface adsorbed by-products than that of initial toluene oxidation. Based on the temperature programmed desorption and temperature programmed oxidation studies, the addition of moisture decreased the by-products accumulation thereby, reduced the catalyst deactivation and enhanced the extended oxidation of toluene on the Mn/13X zeolite.

Oxidation Reactivity and Structure of LDPE-Derived Solid Carbons: A Temperature-Programmed Oxidation Study

A wide set of solid carbons obtained by low-density polyethylene (LDPE) pyrolysis has been extensively analyzed by temperature programmed oxidation (TPO) technique. These samples showed different TPO behavior depending on pyrolysis conditions employed, having a wide range of TPO profiles, from one predominant peak to several overlapping curves. The evolution of the oxygen reactivity of LDPE-derived solid carbons heat treated to 1300, 2000, 2300, and 2600 °C was also examined by TPO. The deconvolution of TPO curves of starting and heat-treated carbons allowed the identification of six different peaks consistently occurring in different samples, which was related to groups of carbon domains with equivalent reactivity. Four of them mainly appeared in the starting carbons and were related with more reactive and less ordered carbon domains. The heat-treatment of the carbons allowed the study of the evolution of carbon domain groups from higher to lower oxygen reactivity. A TPO parameter, Tmax has been defined as a weight peak temperature oxidation and compared to other characterization techniques (X-ray diffraction (XRD), Raman spectroscopy, elemental analysis) in an attempt to correlate TPO reactivity and structural parameters. The TPO technique showed a high sensibility to analyze carbon structure.

Pyrophoricity and stability of copper and platinum based water-gas shift catalysts during oxidative shut-down/start-up operation

The pyrophoricity of Cu/ZnO-based and Pt-based catalysts was studied during oxidative shut-down/start-up of the low-temperature water-gas shift (LT-WGS) reaction to assess whether these catalysts are suitable for fuel cell application. The Cu/ZnO-based catalysts were observed to display high levels of pyrophoricity manifested as a sharp temperature rise of the catalyst bed upon air introduction. This promoted severe sintering of the bulk and metallic phases of the catalyst facilitating catalyst deactivation. No pyrophoricity was observed for any of the Pt-based catalysts; however, sintering of the metallic phase in Pt/TiO 2 and Pt/ZrO 2 persisted, leading to a decrease in activity. It was likely that the sintering of Pt occurred during LT-WGS operation itself. In contrast, Pt/CeO 2 was the only catalyst which retained its activity, displaying no loss in specific surface area or metal dispersion throughout the entire process making it the most suitable candidate of the materials investigated for fuel cell systems. Temperature-programmed oxidation studies indicated deactivation by the oxidative shut-down/start-up operation did not result from the build-up of carbonaceous species.

Temperature-programmed oxidation studies of carbon materials prepared from anthracites by high temperature treatment

Temperature-programmed oxidation (TPO) was used to analyse carbon materials prepared from two different anthracites by high temperature treatment. TPO data indicated the presence of two carbon species with different oxidation reactivity in all the materials prepared from both anthracites. The influence of the heat treatment temperature on the evolution and structure of different carbon species was evaluated by using both the TPO data and the XRD parameters. The less ordered (more reactive) carbon species were gradually converted into the more ordered (less reactive) species by increasing the heat treatment temperature, thus, giving rise to the formation of materials with a higher overall degree of graphitic order as determined by XRD. Within the set of materials prepared from one of the anthracites (ATO), a consistent relationship was found between the oxygen reactivity of the carbons species and the treatment temperature in the range of 2300–2600 °C. In addition, there was also a good correlation between the proportion of the more reactive carbon component in the materials and their XRD parameters (particularly L a).

Catalyst support effects in the growth of structured carbon from the decomposition of ethylene over nickel

The characteristics of carbon nanofibers generated during ethylene decomposition over supported nickel can be readily manipulated by a judicious choice of the support material. The occurrence and ramifications of Ni/support interaction(s), in terms of Ni particle size/morphology/orientation, are considered and related to the carbon structure/dimensions and yield. A 7±1% w/w Ni loading was achieved by standard impregnation of SiO 2, Al 2O 3, MgO, Ta 2O 5, activated carbon (AC), and graphite: the reduced catalysts have been characterized by H 2 chemisorption, CO chemisorption/temperature-programmed desorption (TPD), and high-resolution transmission electron microscopy (HRTEM). The reaction of ethylene with hydrogen over these catalysts also generated ethane via hydrogenation, a step that was favored over Ni/Al 2O 3 and Ni/MgO. Carbon yield (where T<800 K) increased in the sequence Ni/Al 2O 3 ∼ Ni/MgO < Ni/AC < Ni/graphite< Ni/Ta 2O 5 < Ni/SiO 2; at higher temperatures (>850 K), Ni/AC and Ni/graphite delivered the highest yields. With the exception of Ni/graphite and Ni/Ta 2O 5, which produced helical and highly curved fibers, the other supported Ni catalysts generated a relatively straight (limited curvature) fibrous growth. The occurrence of Ni fragmentation and secondary fiber growth from such fragments is illustrated and discussed. The influence of H 2 content in the feed was investigated with respect to both carbon yield and structure; an increased H 2 content served to enhance fiber structural order. Temperature-programmed oxidation studies have been used to probe the graphitic nature of the carbon product; the results are consistent with HRTEM analysis.

Structural characterization of coke deposits on industrial spent paraffin dehydrogenation catalysts

Coke deposition on spent noble metal catalysts used in petroleum/petrochemical industries is of serious concern on account of its impact on the catalyst deactivation and consequent loss in the production yield. In order to counteract the effects of coke deposition, it is vital to know the location as well as the nature and composition coke deposited on the spent catalysts. In the present study, spent Pt-Sn/Al 2O 3 catalysts used in the industrial reactors for selective dehydrogenation of C 10–C 13 n-paraffins to mono-olefins at different coke loading (approximately 7–9%, w/w) were characterized. The characterization of coke deposits were analyzed by the combination of analytical techniques including HPLC, solid-state 13C CP/MAS NMR, and TGA. Average structural information has been obtained from the quantitative analysis of NMR data. The results indicate that the nature of coke present in the soluble coke extracts of spent catalysts is rich in alkylated mono- and diaromatics with low percentage of polyaromatics whereas the nature of insoluble coke is highly polyaromatic (aromaticity, f a>0.95). In addition, temperature programmed oxidation studies by TGA reveals that the coke is deposited on the dispersed metal as well as on the support.

Coke Formation on Pt/ZrO2/Al2O3 Catalysts during CH4 Reforming with CO2

The CO2 reforming of methane was studied over Pt supported on Al2O3, ZrO2, and x % ZrO2/Al2O3 (1 ≤ x ≤ 20 wt %). The Pt/Al2O3 deactivated very quickly during 20 h onstream at 1073 K and a CH4/CO2 ratio of 1:1, while the catalysts with a ZrO2 content above 5 wt % presented improved stability during 60 h. Temperature-programmed oxidation studies showed that the amount of carbon on Pt/Al2O3 is much larger than that on zirconia-containing catalysts. Deactivation is attributed to carbon formation surrounding the metal−support perimeter. The high stability of zirconia-based catalysts is probably due to strong Pt−Zrn+ interactions, which reduce carbon formation during the reaction by promoting CO2 dissociation. High ratios of CH4/CO2 were also used in an attempt to accelerate deactivation, but even under these severe deactivation conditions, the catalyst with 10 wt % ZrO2 exhibited excellent stability.

Growth of Filamentous Carbon from the Surface of Ni/SiO2 Doped with Alkali Metal Bromides

The growth of ordered filamentous carbon, catalytically generated from the decomposition of ethylene, has been studied over the temperature range 673–898 K using an 11% w/w Ni/SiO2 catalyst doped to varying degrees (0.1–9.3% w/w) with a range of alkali metal bromides. The effect of these alkali metal/halogen adatoms in promoting/inhibiting carbon growth has been assessed and variations in the associated carbon structural characteristics have been examined. The introduction of Li consistently promoted filamentous carbon growth (where 723 K<T<823 K) while the presence of Na, K, Rb, or Cs resulted in an equivalent or lower carbon yield. The degree of carbon deposition was strongly dependent on the nature and loading of the alkali metal, the Ni/Br ratio in the activated catalyst, and reaction temperature; conditions for optimum carbon growth are identified. The response of carbon yield and structural order to alkali bromide doping is discussed in terms of Ni particle electronic structure and metal/support interaction(s). High-resolution transmission electron microscopy (HRTEM) has been used to probe the filamentous carbon structure and the dispersion/morphology/size of the supported Ni crystallites. Highly curved and helical filaments predominated over the doped (particularly CsBr) samples and this is attributed to a disruption in carbon diffusion through the Ni particle caused by a spreading/coating of the particle by the alkali adatom. Temperature-programmed oxidation studies have highlighted the changes in the graphitic nature of the carbon due to catalyst doping; the results are consistent with the TEM analysis.

Pressurized Dehydrocondensation of Methane toward Benzene and Naphthalene on Mo/HZSM-5 Catalyst: Optimization of Reaction Parameters and Promotion by CO2 Addition

The rate of benzene formation with time on stream is remarkably stabilized under pressurized condition in the dehydrocondensation reaction of methane. Under a methane pressure of more than 0.2 MPa and a suitable CO2 concentration in methane feed, the high activity and stability were achieved at elevated reaction temperatures of more than 1023 K and methane space velocities of more than 5400 ml/(g·h) due to minimized coke formation on the Mo/HZSM-5 catalyst. A high and stable benzene formation rate of 4000 nmol/ (g-cat·s) was obtained on the 6% Mo/HZSM-5 catalyst under the optimized reaction conditions, e.g., 1073 K, 0.3 MPa, and 7200 ml/(g·h), and with 5% CO2 addition. A temperature-programmed oxidation study demonstrated that both addition of CO2 to the methane feed and increasing of the methane pressure greatly suppressed the deposition of coke on the Mo/HZSM-5 catalyst. Moreover, the H/C ratio of the coke on the catalyst increased due to CO2 addition and methane pressure increment, implying that the surface carbon had relatively high hydrogen content at the prevailing reaction conditions. It suggests that the added CO2 reacted with coke before its transformation to inactive graphitic carbon blocking the microporous channels of HZSM-5 zeolite, which is more detrimental for methane dehydrocondensation reaction.

CO2 Reforming of CH4 over Pt/ZrO2 Catalysts Promoted with La and Ce Oxides

The CO2 reforming of CH4 was studied over cerium- and lanthanum-promoted Pt/ZrO2 catalysts at 1073 K and a CH4 : CO2 ratio of 2 : 1. The high ratios of CH4 : CO2 were used in an attempt to accelerate deactivation of the catalyst and make a faster comparison of the different samples. The addition of Ce resulted in a significant improvement in the stability, with no decrease in either CH4 or CO2 conversion observed. The promotion with La also resulted in an increase in the stability of the catalyst as well as an increase in the initial activity. The mechanism of reaction has been studied on the promoted catalysts, and it has been shown that it is not altered by the presence of the promoters. The long-term activity of the catalyst is dependent upon the balance between the rate of CH4 decomposition and the rate of dissociation of CO2. The results of pulse studies demonstrate that addition of the cerium results in an increase in the reducibility and oxygen transfer ability of the support. Temperature-programmed oxidation studies demonstrated that although the total amount of carbon deposits on the Ce-promoted catalyst is not lower than that on the unpromoted one, these deposits are eliminated at much lower temperature, indicating the ability of the catalyst to clean the active sites. On the other hand, the TPO studies on the La-doped catalyst showed a much lower extent of carbon deposition. Interestingly, the ability of La to inhibit carbon formation is only observed in the presence of CO2. Under pulses of pure CH4, the La-promoted catalyst was the one which showed the highest tendency for carbon deposition. X-ray diffraction and BET area characterization of this catalyst showed that the promoter increases the thermal stability of the support. Finally, it was shown that the promoters also retard metal particle growth during reaction at high temperature. Smaller Pt particles result in an increase in the interfacial area and subsequent increase in the removal of the carbon from the metal. The increase in the efficiency of the cleaning mechanism resulted in catalysts that were stable for the dry reforming reaction under severely deactivating conditions (i.e., 1073 K, CH4 : CO2 ratio=2 : 1) for over 20 h.

Effects of the addition of lanthana on the thermal stability of alumina-supported palladium

Promotional effects of lanthana on the catalytic property of Pd/A1 2O 3 toward methane combustion were examined. The results revealed that Pd/A1 2O 3 became active on raising the temperature above 767 K but its activity decayed considerably at high temperatures. Addition of La 2O 3 to the Pd/A1 2O 3 catalyst inhibited this activity decay. Hydrogen chemisorption and BET measurements revealed that the addition of La 2O 3 to Pd/A1 2O 3 not only enhanced the thermal stability of γ-A1 2O 3, but also retarded the supported Pd from sintering upon calcinations at T> 923 K. Temperature-programmed oxidation studies further demonstrated that the retardation in Pd sintering resulted from an increase in the bond strength of Pd-O. Therefore, the increase in the bond strength of Pd-O may account for the promotional effect of lanthana and the long lifetime of La-added Pd/A1 2O 3 catalyst in methane combustion.

Temperature-programmed oxidation studies of aged V-containing hydroprocessing catalysts

Temperature-programmed oxidation (TPO) of a NiMo/alumina hydrotreating catalyst aged with Kuwait resid showed the presence of two distinct types of coke. A low-temperature coke (type I), showing a CO 2 peak around 460°C, is attributed to a ‘normal’ coke, which is found on catalysts aged with lighter feeds. A high-temperature coke (type II), showing a CO 2 peak around 620°C, is associated with vanadium deposits and appears to be consistent with the presence of asphaltenes. The presence of COS in the TPO profiles is taken as evidence of sulfur in the coke. In addition, a low-temperature COS peak is observed, which is ascribed to adsorbed CH x species in the inorganic sulfide phase. The coke effectively covers the inorganic sulfide phase. Hence, appreciable loss of catalytic activity could be due to coverage of active sites by coke, aside from any specific poisoning effects of metal deposits.

Temperature-programmed oxidation studies of aged hydroprocessing catalysts

Temperature-programmed oxidation tests were carried out on catalysts which were aged in a bench-scale unit, using either a vacuum gas oil (VGO) feed or a Kuwait atmospheric feed. The VGO-aged catalysts displayed one CO 2 peak from oxidation of coke and two SO 2 peaks from oxidation of inorganic sulfur on the catalyst. The temperature at which the CO 2 peak appeared increased with the coke content. For the Kuwait-aged catalysts, a second, high-temperature carbon oxidation peak was observed when moderate amounts of vanadium were present. The carbon oxidation peaks were closely accompanied by SO 2 peaks. The coke in the latter catalysts essentially covered the inorganic sulfur phase as well as adsorbed nitrogen compounds. It was found that the aged catalysts could lose a considerable amount of sulfur during the extraction procedure if too high a temperature was used during post-drying.

Characterization of nitrogen on aged hydroprocessing catalysts by temperature-programmed oxidation

Temperature-programmed oxidation studies were carried out on: (1) sulfided hydroprocessing catalysts treated with-ammonia or aniline; (2) catalysts coked with ethylcarbazole or pyrene; and (3 ) aged catalysts derived from a vacuum gas oil feed. Oxidation of nitrogen on the catalysts gave nitric oxide, while oxidation of carbon gave carbon dioxide and carbon monoxide. Two types of nitrogen on the catalysts were detected, a low-temperature form, ascribed to adsorbed NH x species, and a high-temperature form, ascribed to strongly adsorbed organic-N species. It is proposed that the high-temperature form of nitrogen consists of either strongly adsorbed organic-N species with a separate coke phase, or coke-N species, in which the coke covers the N-species. In either case, the nitrogen atom appears to be attached to the catalyst surface and is not in the bulk of the coke itself.