Sulphide-induced deactivation of Pd/Al2O3 as hydrodechlorination catalyst and its oxidative regeneration with permanganate

Pd-based catalysts have become important in environmental catalysis for their ability to hydrodechlorinate a wide range of chlorinated organic contaminants in water under ambient conditions. The success of their application in the remediation practice, e.g. for groundwater treatment, is often hindered by the sensitivity of Pd to poisoning by sulphur compounds. In this study, the stability and sulphide-induced deactivation behaviour of a highly active Pd/Al₂O₃ catalyst was investigated. The specific activity of Pd for the hydrodechlorination of chlorobenzene corresponds to rate coefficients up to k_Pd = 350 L g⁻¹ min⁻¹. The totally deactivated catalyst, resultant of sulphide poisoning, was regenerated with potassium permanganate. The pH value, as a key parameter which may influence the degree of deactivation as well as the efficiency of catalyst regeneration, was evaluated. Results show that in clean water the Pd/Al₂O₃ catalyst showed no inherent deactivation regardless of the ageing time and the pH value of the catalyst suspension. The degree of catalyst poisoning effected by 1.8–5.4 μM sulphide, corresponding to molar ratios of S:Pd_surface = 1.5–8.5, was observed to be higher under neutral and alkaline than under acidic conditions. The exposure of the catalyst to higher sulphide concentration of 14.2 μM resulted in complete catalyst deactivation regardless of the pH conditions. However, the efficacy of permanganate as oxidative regenerant for the fouled catalyst showed strong pH-dependence. A regeneration time of 10–30 min at low pH was sufficient to recover completely the high catalytic activity of Pd/Al₂O₃ for the hydrodechlorination reaction.     

Deactivation of V2O5-WO3-TiO2 SCR catalyst at biomass fired power plants: Elucidation of mechanisms by lab- and pilot-scale experiments

In this work, deactivation of a commercial type V₂O₅-WO₃-TiO₂ catalyst by aerosols of potassium compounds was investigated in two ways: (1) by exposing the catalyst in a lab-scale reactor to a layer of KCl particles or fly ash from biomass combustion; (2) by exposing full-length monolith catalysts to pure KCl or K₂SO₄ aerosols in a bench-scale reactor. Exposed samples were characterized by activity measurements, SEM-EDX, BET/Hg-porosimetry, and NH₃ chemisorption. The work was carried out to support the interpretation of observations of a previous study in which catalysts were exposed on a full-scale biomass fired power plant and to reveal the mechanisms of catalyst deactivation.Slight deactivation (about 10%) was observed for catalyst plates exposed to a layer of KCl particles at 350 °C for 2397 h. No deactivation was found for catalyst plates exposed for 2970 h to fly ash (consisting mainly of KCl and K₂SO₄) collected from an SCR pilot plant installed on a straw-fired power plant. A fast deactivation was observed for catalysts exposed to pure KCl or K₂SO₄ aerosols at 350 °C in the bench-scale reactor. The deactivation rates for KCl aerosol and K₂SO₄ aerosol exposed catalysts were about 1% per day and 0.4% per day, respectively.SEM analysis of potassium-containing aerosol exposed catalysts revealed that the potassium salt partly deposited on the catalyst outer wall which may decrease the diffusion rate of NO and NH₃ into the catalyst. However, potassium also penetrated into the catalyst wall and the average K/V ratios (0.5–0.75) in the catalyst structure are high enough to explain the level of deactivation observed. The catalyst capacity for NH₃ chemisorption decreased as a function of exposure time, which reveals that Brønsted acid sites had reacted with potassium compounds and thereby rendered inactive in the catalytic cycle. The conclusion is that chemical poisoning of active sites is the dominating deactivation mechanism, but physical blocking of the surface area may also contribute to the loss of activity in a practical application. The results support the observation and mechanisms of deactivation of SCR catalysts in biomass fired systems proposed in a previous study [Y. Zheng, A.D. Jensen, J.E. Johnsson, Appl. Catal. B 60 (2005) 253].     

Roles of interaction between components in CZZA/HZSM-5 catalyst for dimethyl ether synthesis via CO2 hydrogenation

The roles of interaction between two catalyst components in CuO–ZnO–ZrO₂–Al₂O₃ (CZZA)/HZSM-5 bifunctional catalyst for dimethyl ether (DME) synthesis via carbon dioxide hydrogenation were investigated. It was found that CZZA catalyst showed excellent stability during methanol (MeOH) synthesis for 100 h, while there was a severe loss of catalytic activity in the bifunctional catalyst for DME synthesis. Hence, the effects of different degrees of intimacy of two catalyst components were studied for DME synthesis, including mixed and separated modes. For the mixed mode, the particle size of catalysts and the amount of reaction intermediates were proven to influence the catalyst deactivation. For the separated mode, the catalysts showed rapid deactivation within a short time. Various characterizations indicated that the remarkable deactivation of separated mode was mainly caused by the decrease of copper active centers (e.g., sintering and oxidation) and blockage of acid sites via increased coke deposition on HZSM-5.     

Dry Reforming of Methane with a Ni/Al2O3‐YSZ Catalyst: The Role of the Catalyst Preparation Protocol

Many studies of Ni based ceramic supporting reforming catalysts are found in the literature. A synthesis of the reported results shows that their efficiency and durability are significantly affected by their fabrication protocol. This research has been aimed at evaluating how the conditions of 1) the ceramic support preparation and 2) the Ni deposition, through an impregnation‐drying‐calcination‐reduction protocol, affect the catalytic activity and the catalyst deactivation over time during methane dry reforming. The catalyst support used in this study was obtained by the mixing and pressing of alumina and YSZ (Yttria Stabilized Zirconia) powders, then calcining the mixtures at high temperature to form pellets of limited porosity (specific surface of 1.5‐10 m²/g), without inducing change to the crystalline phases. The results show that the surface density of the nickel particles, the catalyst activity, and its life span are highly dependent upon the catalyst preparation protocol. The initial nitrate solution concentration, the duration of the impregnation and the specific surface of the ceramic support have, all of them, a considerable influence on the size range of the deposited nickel particles. The surface density, the amount and the size of the latter highly affect the catalytic activity. It has been also shown that an increase in the ratio CH₄/CO₂ is detrimental to the catalytic activity of the tested formulations; a small excess of methane is enough to initiate the deactivation process of the catalyst very quickly for all of the composition tested in this study. A phenomenological deactivation kinetics model has been built and optimized. Although there are differences in deactivation rates among the different formulations tested, the model shows that the deactivation rate is highly dependent upon the reaction rate constant and that zero‐ and first‐order kinetics give statistically the same prediction error; the latter is always lower or equal to the experimental error.     

EFFECTS OF CARBON DEPOSITS ON THE SPECIFIC ACTIVITY OF NICKEL AND NICKEL BIMETALLIC CATALYSTS

The effects of carbon formation on methanation activity of nickel and nickel bimetallic catalysts were investigated. Carbon was deposited on these catalysts at 675-700 K, 1 atm, H₂/CO = 2 and space velocities of 80,000 to 200,000 h^?1 over a period of 6-24 hours. Specific methanation activities were measured before and after carbon depositing treatments at 500-575 K, 1 atm H₂/CO = 4 and space velocities of 100,000 h^?1. The results show that Ni/Al₂O₃ loses 20-60% of its initial activity within 10-15 hours of treatment. Platinum and cobalt promoted nickel are significantly more resistant to deactivation by carbon. However, Ni-MoO₂ is highly susceptible to deactivation, losing essentially all of its activity within a few hours. Data showing the effects of reaction conditions, metal concentration and catalyst composition on the extent of deactivation and the effects of deactivation on catalyst strength are presented and discussed     

The effect of toluene-insoluble fraction of coal on catalytic activities of a Ni-Mo-γ-Al2O3 catalyst in the hydrotreating of coal liquids

Feedstocks prepared with 723 K⁺ distillation residues obtained from the Australian brown coalderived oil and hydrogenated creosote oil were hydrotreated to elucidate the effects of toluene-insoluble fractions of coal on the catalytic activities of a Ni-Mo-γ-Al₂O₃ catalyst. The toluene-insoluble fractions exerted harmful effects on the catalyst by deactivation due to carbonaceous deposits formed on the catalyst surfaces. The deactivation of the catalyst was more significant in hydroconverting of 623 K⁺ residues to 623 K⁻ oil fractions and hydrodenitrogenation reactions than in hydrodesulfurization reaction.The analyses of carbonaceous deposits on the spent catalyst surfaces by using an EPMA and a CP/MAS ¹³C-NMR indicated that the toluene-insoluble fractions were too refractory to be hydrogenated on the catalyst surfaces and hindered the reactant molecules from accessing to the active sites compared to asphaltenes.     

Correlation of the deactivation of CoMo/Al2O3 in hydrodesulfurization with surface carbon species

The deactivation of CoMo/Al₂O₃ in the hydrodesulfurization (HDS) of dibenzothiophene (DBT) was investigated under laboratory conditions that allowed the accelerated deposition of coke on the catalyst. The coke deposition was enhanced at low H₂ pressures and when naphthalene was added to the reaction solution. Characterization of deactivated catalysts by elemental analysis (EA) and temperature-programmed oxidation (TPO) identified two types of carbonaceous species deposited on the catalysts, the reactive and the refractory species. The refractory deposit, or hard coke, was a major contributor to the deactivation and, therefore, the amounts of hard coke present on the catalyst determined the overall activity. A correlation was established in this study between the activity and the amounts of deposited hard coke based on the results of accelerated deactivation treatment. A similar relation was also observed between the two parameters when the catalyst was used in an industrial process for long periods. The above findings suggest that the reaction periods of two different scales, i.e., in laboratory and industrial processes, can be correlated with each other based on the amounts of hard coke when coking is the major mechanism of catalyst deactivation.     

Deactivation of H-mordenite and ZrO2/SO2−4 during n-butane isomerization

The catalyst deactivation in the isomerization of n-butane was studied on H-mordenite and ZrO₂ promoted by SO²⁻₄ ion. The deactivation of H-mordenite was the result of coke deposition on the catalyst pores and the deactivation of ZrO₂/SO²⁻₄ resulted from the decrease in the oxidation state of sulphur in the surface complex. The reoxidation of sulphur, producing again the SO²⁻₄ ion, led to the recovery of the catalytic activity of ZrO₂/SO²⁻₄ and coke burning resulted in the recovery of the H-mordenite activity.     

Prevention of catalyst deactivation caused by coke formation in the methanation of carbon oxides

Prevention of catalyst deactivation in carbon monoxide methanation on a highly active Ni-based composite catalyst has been investigated. The composite catalyst, Ni-La₂O₃-Ru supported on silica, has greater activity than that of a Ni catalyst, but the decrease of the catalyst activity with reaction time is greater than that of the Ni catalyst, especially when the CO conversion is low. The reason for this behaviour is found in the relation between the amount of surface-carbon species and the degree of deactivation. When the CO methanation reaction is operated at above the temperature of complete CO conversion, catalyst deactivation is avoided. At such high temperatures the amount of surface-carbon species is small. The catalyst deactivation is considerably suppressed with a low concentration, e.g. 1–3 kPa, of additional CO₂ or CO₂ + H₂O. The cause of this suppression is considered to be the renewal of the covered surface with the carbon-species by the competitive adsorption of these additives.     

Low methane selectivity using Co/MnO catalysts for the Fischer-Tropsch reaction: Effect of increasing pressure and Co-feeding ethene

CO hydrogenation using cobalt/ manganese oxide catalysts is described and discussed. These catalysts are known to give low methane selectivity with high selectivity to C₃ hydrocarbons at moderate reaction conditions (GHSV < 500 h^–1, < 600 kPa). In this study the effect of reaction conditions more appropriate to industrial operation are investigated. CO hydrogenation at 1–2 MPa using catalyst formulations with Co/Mn = 0.5 and 1.0 gives selectivities to methane that are comparable to those observed at lower pressures. At the higher pressure the catalyst rapidly deactivates, a feature that is not observed at lower pressures. However, prior to deactivation rates of CO + CO₂ conversion > 8 mol/1-catalyst h can be observed. Co-feeding ethene during CO hydrogenation is investigated by the reaction of¹³C0-¹²C₂H₄-H₂ mixtures and a significant decrease in methane selectivity is observed but the hydrogenation of ethene is also a dominant reaction. The results show that the co-fed ethene can be molecularly incorporated but in addition it can generate a C, species that can react further to form methane and higher hydrocarbons.     

Improvement of coke resistance of Ni/Al2O3 catalyst in CH4/CO2 reforming by ZrO2 addition

This work investigates the improvement of Ni/Al₂O₃ catalyst stability by ZrO₂ addition for H₂ gas production from CH₄/CO₂ reforming reactions. The initial effect of Ni addition was followed by the effect of increasing operating temperature to 500–700 °C as well as the effect of ZrO₂ loading and the promoted catalyst preparation methods by using a feed gas mixture at a CH₄:CO₂ ratio of 1:1.25. The experimental results showed that a high reaction temperature of 700 °C was favored by an endothermic dry reforming reaction. In this reaction the deactivation of Ni/Al₂O₃ was mainly due to coke deposition. This deactivation was evidently inhibited by ZrO₂, as it enhances dissociation of CO₂ forming oxygen intermediates near the contact between ZrO₂ and nickel where the deposited coke is gasified afterwards. The texture of the catalyst or BET surface area was affected by the catalyst preparation method. The change of the catalyst texture resulted from the formation of ZrO₂–Al₂O₃ composite and the plugging of Al₂O₃ pore by ZrO₂. The 15% Ni/10% ZrO₂/Al₂O₃ co-impregnated catalyst showed a higher BET surface area and catalytic activity than the sequentially impregnated catalyst whereas coke inhibition capability of the promoted catalysts prepared by either method was comparable. Further study on long-term catalyst stability should be made.     

Partial Oxidation of H2S to Sulfur Over Mo and/or W-Containing Bronzes with a TTB-Structure

Mo- and W-based oxidic bronzes, with and without Te-atoms in framework positions are active and selective, for the partial oxidation of H₂S to sulfur in the 160?C220 °C temperature range. The catalysts have been prepared by a hydrothermal synthesis and heat-treated in N₂ at 600 °C, and characterized by several physico-chemical techniques, i.e. AAS, S_BET, XRD, SEM?CEDX, DR UV?Cvis and XPS. Te-free catalysts are active, selective and stable for the partial oxidation of H₂S to sulfur. V-atoms in Mo?CO?CV pairs can be proposed as the active sites. Moreover, Te-containing materials show fast catalyst decay. This catalyst deactivation can be related to the presence of Te⁰ and MoS₂, which are observed in used catalyst.     

Effect of Ni loading and calcination temperature on catalyst performance and catalyst deactivation of Ni/SiO2 in the hydrodechlorination of 1,2‐dichloropropane into propylene

The hydrodechlorination of 1,2‐dichloropropane (DCPA), a chlorinated organic waste which is produced in the epichlorohydrin process, to propylene was carried out over Ni/SiO₂ catalysts. The effects of Ni loading and calcination temperature on catalyst performance and catalyst deactivation of Ni/SiO₂ were systematically investigated. The Ni/SiO₂ catalysts efficiently converted DCPA into propylene in 95% selectivity or higher. The particle size of Ni on SiO₂ was strongly related to the catalyst stability. In terms of the effect of Ni loading, the largest Ni particles on SiO₂ showed the best durability against deactivation. A series of TPR and UV‐DRS measurements revealed that nickel hydrosilicate was formed as the result of the interaction between Ni and SiO₂. Nickel hydrosilicate was found to be responsible for the catalyst stability leading to low catalyst deactivation. HCl adsorption on Ni/SiO₂ was the main reason for catalyst deactivation. HCl modified the crystal structure of metallic Ni to NiCl₂ and led to irreversible deactivation and metal sintering. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cumene Hydroperoxide Hydrogenation on a Pd/Al2O3 Catalyst in a Trickle Bed Reactor – Kinetics of Hydrogenation and Deactivation

Liquid‐phase hydrogenation using a Pd/Al₂O₃ catalyst provides a potential technique for the reduction of cumene hydroperoxide (CHP) to α‐cumyl alcohol (CA). In this paper, CHP hydrogenation was carried out in a cocurrent downflow trickle‐bed reactor over a wide range of reaction conditions to study the reaction and deactivation kinetics. The proposed intrinsic rate expression for CHP hydrogenation is based on an Eley‐Rideal mechanism that accounts for an irreversible surface reaction between the absorbed CHP with nonabsorbed hydrogen molecules. During CHP hydrogenation, an exponential decay in activity of the Pd/Al₂O₃ catalyst and the presence of residual activity were observed. A kinetic deactivation model with residual activity was developed. Based on reaction and deactivation kinetics, catalyst deactivation was attributed to oxidation of the catalyst surface by CHP. The presence of residual activity was due to the partial reduction of oxidized catalyst surface by hydrogen.     

Esterification over rare earth oxide and alumina promoted SO4/ZrO2

Solid superacid catalysts including SO₄²⁻/ZrO₂ (SZ), rare earth (RE) oxide-promoted SZ and RE oxides together with alumina-promoted SZ were prepared. Their catalytic performances in the esterification reaction of ethanol and acetic acid were investigated. The textural property, crystalline phase and surface acidity of the prepared catalysts were characterized by using nitrogen adsorption–desorption isotherms, X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy of pyridine adsorption techniques, respectively. Effects of the reaction time and catalyst reuse cycle as well as catalyst regeneration on the catalytic behaviors were studied. Experimental results showed that Yb₂O₃–Al₂O₃ promoted SZ (designated as SZAY) catalyst exhibited an optimal esterification performance; the Lewis acid sites with moderate and super strong strength could mainly be responsible for the esterification reaction; and doping both Yb₂O₃ and Al₂O₃ on SZ not only boosted the esterification activity but also alleviated catalyst deactivation resulted from the surface sulfur loss by solvation.     

Raising Co/Al2O3 catalyst lifetime in Fischer–Tropsch synthesis by using a novel dual-bed reactor

Accelerated deactivation of 15 wt.% Co/Al₂O₃ catalyst in Fischer–Tropsch synthesis (FTS) in a single-bed and a dual-bed reactor is reported. Water was found to have a remarkable effect on the deactivation of Co/Al₂O₃ catalyst during FTS. Synthesis at higher temperatures and lower space velocities resulted in higher values of P_H2O/(P_CO + P_H2) and P_H2O/P_CO and higher catalyst deactivation rates. Water-induced back-oxidation of cobalt, cobalt–alumina interactions, irreducible cobalt aluminates formation and refractory coke formation are the main sources of deactivation. When the water to carbon monoxide plus hydrogen ratio P_H2O/(P_CO + P_H2) is greater than about 0.55 or water to carbon monoxide ratio P_H2O/P_CO is greater than about 1.5, it is not uncommon to find rapid catalyst deactivation. Separation of water and heavy hydrocarbons between the two catalytic beds of the dual-bed reactor, resulted in 62% lower catalyst deactivation rate than that of the single-bed reactor. The amount of refractory coke formation on the catalysts of the dual-bed reactor is 34% lower than that of the single-bed reactor. It was revealed that activity recovery of the used catalysts of the dual-bed is higher than that of the single-bed reactor.     

Pd‐Ag/α‐Al2O3 Catalyst Deactivation in Acetylene Selective Hydrogenation Process

The selective hydrogenation of acetylene to ethylene over Pd‐Ag/α‐Al₂O₃ catalysts prepared by different impregnation/reduction methods was studied. The best catalytic performance was achieved with the sample prepared by sequential impregnation. A kinetic model based on first order in acetylene and 0.5th order in hydrogen for the main reaction and second‐order independent decay law for catalyst deactivation was used to fit the conversion time data and to obtain quantitative assessment of catalyst performances. Fair fits were observed from which the reaction and deactivation rate constants were evaluated. Coke deposition amounts showed a good correlation with catalyst deactivation rate constants, indicating that coke formation should be the main cause of catalyst deactivation.     

Coking and regeneration of palladium-doped H3PW12O40/SiO2 catalysts

The coking during propene oligomerisation and subsequent regeneration of both silica-supported heteropoly acid H₃PW₁₂O₄₀ (PW) and its palladium-modified form (1.6–2.5 wt% Pd) have been studied. ³¹P MAS NMR studies have revealed that the Keggin structure of the catalyst was unaffected by coke deposition in both unmodified PW/SiO₂ and Pd-modified form. As shown by ¹³C MAS NMR and TGA/TPO, the Pd modification affects the nature of the coke formed: for the standard catalyst (PW/SiO₂) both soft coke, comprising mainly high molecular weight aliphatic oligomers, and hard coke, comprising polynuclear aromatics, are formed whilst on the Pd-modified catalyst only the soft coke is observed. Coke formation causes strong deactivation of the catalyst in the oligomerisation of propene. The aerobic burning of coke on the unmodified PW/SiO₂ occurs in the temperature range of 470–520°C. Doping the catalyst with Pd significantly decreases this temperature to allow catalyst regeneration at temperatures as low as 350°C without loss of catalytic activity. This revised version was published online in July 2006 with corrections to the Cover Date.     

The deactivation of Co/SiO2 catalyst for Fischer–Tropsch synthesis at different ratios of H2 to CO

The deactivation of Co/SiO₂ catalyst for Fischer–Tropsch synthesis (FTS) at different H₂ / CO ratios was investigated by XRD, FTIR, BET, XPS, TPR and H₂ chemisorption. It was found that the deactivation rate of the catalyst increased with the rise of the H₂ / CO ratio. The generation of silicates and/or hydrosilicates species was evidenced by TPR and XPS, and their amounts were monotonously enhanced with increasing H₂ / CO ratio, which suggested that the deactivation was caused by the transformation of metallic cobalt into inactive silicates and the high partial pressure of H₂ facilitated the formation of the silicates. Moreover, the percentage loss of the surface cobalt was larger than that of bulk cobalt, suggesting that the cobalt silicates and/or hydrosilicates species were formed mainly on the surface of the catalyst or in the small crystallites. For the catalyst run at H₂ / CO ratio of 1, it was observed that the sintering also contributed to the catalyst deactivation, but it was a less important factor for the deactivation.     

Continuous acylation of anisole by acetic anhydride in mesoporous solid acid catalysts: Reaction media effects on catalyst deactivation

The acylation of anisole with acetic anhydride was carried out in a continuous slurry reactor over mesoporous supported Nafion^® (SAC-13) and heteropolyacid (HPA) catalysts. At 70 °C, using an anisole-rich feed molar ratio of 5:1 and a space velocity of 1.6 g_{acetic anhydride} g⁻¹_cat. h⁻¹, acetic anhydride conversions of 40–50% with excellent selectivity (>95%) toward the primary product, p-methoxyacetophenone (p-MOAP), were observed at time on stream (TOS) of a few hours. However, all the catalysts deactivated completely during liquid-phase operation in less than 24 h. It was observed that the Keggin ions from the supported HPA-based catalyst (70% HPA/SiO₂) leached out into the solution, as confirmed by elemental analysis. The 50% Cs_2.5-HPA/SiO₂ catalyst, on the other hand, was more leach-resistant, yet deactivated rapidly during liquid-phase operation. SAC-13-type catalysts, which displayed the best combination of stability and leach resistance during liquid-phase operation, were evaluated in CO₂-expanded liquids (CXLs) to better enhance the transport properties and potentially mitigate deactivation. It is observed that the CXL media gave lower conversion and surprisingly, faster deactivation compared with liquid-phase operation, indicating that CO₂ had a detrimental effect despite the use of polar cosolvents like nitromethane. The spent catalysts were subjected to Soxhlet extraction with polar solvents like nitromethane. Such treatment did not restore catalyst activity. BET surface area, pore volume of the fresh and spent catalysts, GC/MS analysis of the Soxhlet extract, and IR analysis of the spent catalyst (before and after Soxhlet extraction) indicate that the deactivation could be caused by the primary product, p-MOAP and/or multiply acetylated products in the micropores of Nafion^® catalyst aggregates. Treating the spent catalyst with boiling HNO₃ solution restored complete activity of the SAC-13-type catalysts. The high TON (∼400) achieved with these catalysts before deactivation and their ability to regain complete activity for acylation reactions indicate that Nafion^® catalysts are promising alternatives to the conventional homogeneous Lewis acids like AlCl₃.