Sintering Of Active Phase Research Articles

Low-temperature reverse water gas-shift reaction over highly efficient Cu-hydrotalcites: Mechanistic insights on the role of malachite phase

Carbon dioxide (CO2) transformation into valuable fuels and chemicals is in most cases a challenge far from readiness nowadays. One possible route for its conversion is the reverse water gas shift reaction (rWGS), crucial for syngas generation and required for the chemical conversion of CO2 to fuels and platform chemicals. In this paper, well organized Cu/Zn/Al structures were proposed as efficient catalysts for rWGS reaction at low temperatures. The results of in situ XRD revealed the formation of layered structures such malachite and hydrotalcite. The operando DRIFTS-MS studies of those structure suggests a participation of Cu2+/Cu+ pair in the reaction, promoting the redox mechanism and enhancing the activity at lower temperature. This work also provides a new strategy to design Cu-based rWGS catalysts able to prevent the sintering of active phase.

H2-rich syngas production from biogas reforming: Overcoming coking and sintering using bimetallic Ni-based catalysts

Dry reforming of methane is a very appealing catalytic route biogas (mainly composed by greenhouse gases: carbon dioxide and methane) conversion into added value syngas, which could be further upgraded to produce liquid fuels and added value chemicals. However, the major culprits of this reaction are coking and active phase sintering that result in catalysts deactivation. Herein we have developed a highly stable bimetallic Ni–Rh catalyst supported on mixed CeO2–Al2O3 oxide using low-noble metal loadings. The addition of small amounts of rhodium to nickel catalysts prevents coke formation and improves sintering resistance, achieving high conversions over extended reaction times hence resulting in promising catalysts for biogas upgrading.

Surface Dopants‐Induced Interfacial Bonding Greatly Enhances Active Phase‐Support Interaction of Sintering‐Resistant Catalyst for Automotive CO Oxidation

AbstractSupported nanoparticle catalysts are widely used to remove CO pollution in automobile emission control. During vehicle use, catalysts need to experience high‐temperature environments, leading to sintering of active phase and loss of activity. Synthesizing cost‐effective catalysts with simultaneously high catalytic activity and sintering resistance is of great importance but remains challenging. In this study, we show that Cu phase supported on commercial antimony‐doped tin oxide (ATO) is relatively resistant to sintering and can operate at temperatures below 200 °C without any performance degradation after high temperature aging. Specifically, T50 (the temperature of 50 % conversion) of CO conversion over the Cu/ATO sample is maintained at 148 °C before and after high‐temperature aging treatment at 800 °C. The strong active phase‐support interactions could be used to stabilize Cu active phase by taking advantage of extra interfacial bonding stability between Cu islands and surface antimony dopants in ATO, and finally tune activity and stability of the catalysts. The Cu‐on‐ATO ensemble shows excellent resistance against sintering, with conversion efficiency remaining nearly unchanged upon high‐temperature aging at 800 °C for 2 to 10 h. In contrast, strong sintering resistance characteristic of catalysts cannot be attained by supporting Cu on undoped tin oxide (SnO2) or conventional Al2O3 due to absence of specific anchoring sites on the support oxide surfaces. Our findings open up the path in heterogeneous catalysis for the design of active and sintering‐resistant catalysts utilizing surface engineering strategies.

Size effect in propane dehydrogenation on PtIn/Sn-SBA-15

The mesoporous Sn-SBA-15 (SSF) with varied pore size from was synthesized by changing the hydrothermal temperature, and used to disperse the PtSn oxides (PtSn/SSF) through a typical incipient wetness impregnation method. The pore size had the profound effects on porosity, particle size, the redox property, dispersion and surface chemical state etc. by applying N2 adsorption/desorption isotherms, XRD, HRTEM, TPR, H2-chemisorption, XPS and in situ CO-DRIFT etc. techniques. The catalytic deactivation was mainly caused by sintering of active phase and coke deposition which was identified and quantified by combining Raman and TG analysis. The optimal pore size in this study was 8 nm, corresponding to the sample PtSn/SSF#8 over which the highest propane conversion of 46.9% and the least coke 16.8 mgC/gcat. were achieved.

Remarkably stable and efficient Ni and Ni-Co catalysts for CO2 methanation

• Efficient Ni and Ni-Co on alumina catalysts were prepared and characterized. • CO 2 methanation process is greener when using our Ni and Ni-Co catalysts. • The addition of cobalt improved the reducibility of nickel species. • The addition of cobalt improved activity and selectivity towards CH 4 . • Ni and Ni-Co catalysts showed high stability over 200 h of continuous reaction. CO 2 methanation is one of the most promising ways to store energy based on the power-to-gas concept. In this study, efficient nickel (Ni) and nickel-cobalt (Ni-Co), supported on alumina catalysts with different amounts of Ni and Co were prepared, characterized and used for CO 2 methanation under the atmospheric pressure. The catalysts were prepared in the form of extrudates for catalytic tests. The effect of Ni content and the influence of Co on Ni catalysts were studied in a packed bed methanation reactor at laboratory scale. An optimal Ni-Co content was identified based on CO 2 and H 2 conversion and CH 4 selectivity and yield. The addition of Co improved the reducibility of Ni species and Ni particles’ dispersion over the support. Therefore, the presence of Co enhanced the catalyst activity and selectivity towards CH 4 . Moreover, it was observed that low reaction temperatures (lower than 350 °C) can be used when lowering the Ni content (i.e. 10% wt.). This major fact is favourable to the reduction of the overall process energy consumption and to the decrease of the catalysts’ deactivation at high temperature (>400 °C), such as the active phase sintering. Finally, the Ni and Ni-Co catalysts prepared in this work for CO 2 methanation presented a remarkably high stability over 200 h of continuous reaction.

Effect of support on hydrogen production from chemical looping steam reforming of ethanol over Ni-based oxygen carriers

The Ni-based oxygen carriers (OCs) with different supports including alloy Ni/Al2O3, lamellar Ni/MMT, mesoporous Ni/Al-MCM-41 and mesoporous Ni/SBA-15 were synthesized. The effect of support on hydrogen production from chemical looping steam reforming (CLSR) of ethanol was investigated in a fixed-bed reactor. The oxygen carriers were characterized by some techniques, including N2 adsorption–desorption, XRD, TEM, ICP-OES, H2 pulse chemisorption, H2-TPR, and TG-DTG. It was observed that Ni/SBA-15 exhibited most efficient confinement effect followed by Ni/Al-MCM-41, Ni/MMT and Ni/Al2O3 via: 1) small nickel particle size and high dispersion as well as strengthened metal-support interaction; 2) sintering resistance due to spatial restriction of support; 3) anti-coke capability derived from small nickel particles and ordered diffusion routes for reactants and products. In addition, the silica supported OCs were conductive to promote water gas shift (WGS) reaction but the supports containing Al atoms were prone to coke deposition due to the formation of acid sites. The ‘dead time’ and oxygen transfer capacity reflected that the redox performance of oxygen carriers was listed in the following order: Ni/SBA-15, Ni/Al-MCM-41, Ni/MMT and Ni/Al2O3. Ni/Al-MCM-41 exhibited excellent activity in initial cycle of CLSR but the collapse of the Ni/Al-MCM-41 structure for its weak thermo stability leaded to the deactivation and sintering of active phase. The Ni/SBA-15 and lamellar Ni/MMT OCs exhibited superior activity and stability but the performance of Ni/Al2O3 OCs was mediocre.

Steam reforming of iso-octane toward hydrogen production over mono- and bi-metallic Cu–Co/CeO2 catalysts: Structure-activity correlations

Τhe feasibility of tailoring the iso-octane steam reforming activity of Cu/CeO2 catalysts through the use of Co as a second active metal (Cu20−xCox, where x = 0, 5, 10, 15, 20 wt%), is investigated. Characterization studies, involving N2 adsorption–desorption at −196 °C (BET), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) and Temperature Programmed Reduction (H2-TPR), were carried out to reveal the impact of the morphological, structural and surface properties of the catalysts on the reforming performance. The results showed that reforming activity was monotonically increased upon increasing cobalt loading. The Co/CeO2 catalyst demonstrated the optimum performance with a H2 yield of 70–80% in the 600–800 °C temperature interval. The Co/CeO2 catalyst exhibited also excellent stability at temperatures above 700 °C, while Cu-based catalysts rapidly deactivated in long term stability tests. A close correlation between surface/redox properties and steam reforming efficiency was established. The lower reducibility of Co/CeO2 catalysts, associated with the formation of Co3+ species, in Co3O4-like phase, can be accounted for the enhanced carbon tolerance of Co-based catalysts. Furthermore, the high concentration of surface oxygen species on Co/CeO2 catalysts can be considered for their enhanced performance. On the other hand, the Cu-induced easier reducibility of bimetallic catalysts, in conjunction with carbon deposition and active phase sintering can be accounted for their inferior steam reforming performance. Irreversible changes in the redox properties of Cu-based catalysts, taking place under reaction conditions, could be resulted to ceria deactivation thus hindering the redox process to keep on.

Bucky paper with improved mechanical stability made from vertically aligned carbon nanotubes for desulfurization process

Bucky paper (BP) made from vertically aligned carbon nanotubes exhibits a high mechanical strength and flexibility owing to the high aspect ratio of the carbon nanotubes. Such macroscopic material with nanoscopic properties was efficiently used as a catalyst support in the gas-phase desulfurization reaction. The Fe 2O 3 (3 wt.%) supported on BP exhibits superior stability and activity in H 2S oxidation to sulphur compared with other catalysts supported on activated charcoal and carbon felt. The high stability of the Fe 2O 3/BP catalyst was attributed to the presence of a strong active phase-support interaction which prevents the active phase sintering during operation.

Hydroprocessing catalyst deactivation in commercial practice

Hydroprocessing catalysts are susceptible to deactivation by several mechanisms, e.g. coke formation, metals deposition and active phase sintering, in their life cycle. Typical examples of catalyst deactivation are presented, showing the phenomena that may be encountered in commercial hydroprocessing units. Spent samples of hydroprocessing catalysts were obtained from various commercial units, as well as pilot plants. The cause(s) of deactivation were identified by analyzing spent catalyst samples for coke and metal content, pore size distribution and the distribution of metals (by SEM-EDX and STEM-EDX). Catalysts applied in the hydroprocessing of light feeds (middle distillates) mainly deactivate due to coke deposition and possibly sintering. Poisoning by metal deposition becomes significant during hydroprocessing of heavy feeds (like VGO and residue) as well as upgraded streams (e.g. coker products). Once the possible causes are known, deactivation mitigation strategies can be defined, which comprise optimizing the catalyst properties, catalyst loading and reactor operation.

Performance of carbon nanofibres, high surface area graphites, and activated carbons as supports of Pd-based hydrodechlorination catalysts

Palladium catalysts (1 wt%) supported on three different carbonaceous supports (activated carbon, AC; carbon nanofibre, CNF; and high surface area graphite, HSAG) were prepared and tested for tetrachloroethylene (TTCE) catalytic hydrodechlorination at 523 K and 195 h −1 space velocity (WHSV). Organic solutions (0.9 mol/L of TTCE in toluene) were used as feed, simulating real chlorinated wastes. Pd/activated carbon and Pd/CNF have shown a fast deactivation on stream, whereas the HSAG-supported catalyst showed the best performance. Characterization of fresh and used catalysts by BET, TEM, XRD and XPS revealed that the deactivation of Pd/AC catalyst is caused by the micropore blockage because of coke formation, whereas in the case of Pd/CNF, deactivation is caused by a combination of coke deposition (in lower extent than in the case of activated carbon), active phase sintering and chlorine poisoning. The best behaviour of the HSAG-based catalysts is caused by the absence of sintering and coke formation, although chlorine poisoning is still present.

Life cycle of hydroprocessing catalysts and total catalyst management

Hydroprocessing catalysts based on Ni, Co, Mo and W are used in various refinery processing applications where several deactivation mechanisms become of importance (coke formation, active phase sintering, metals deposition, poisoning) in the catalyst's life cycle. The life cycle of commercial hydroprocessing catalysts is very complex and includes the catalyst production, sulfidation, use, oxidative regeneration followed by re-sulfidation and reuse or, if reuse is not possible, recycling or disposal. To understand the changes in catalyst properties taking place during a life cycle, the catalyst quality in the different stages can be best monitored by using advanced analytical techniques. The catalyst's life cycle is further complicated by numerous technical, environmental and organizational issues involved. In principle, different companies can be involved in each of the life cycle steps. Leading catalyst manufacturers, together with specialized firms, offer refineries a total catalyst management concept, starting with the purchase of the fresh catalyst and ending with its final recycling or disposal. Total catalyst management includes a broad range of services, ensuring optimal timing during the change-out process, reliable, smooth and safe operations, minimal downtime and maximum catalyst and unit performance.

Influence of reduction temperature and metal loading on the performance of molybdenum phosphide catalysts for dibenzothiophene hydrodesulfurization

Two series of alumina-supported molybdenum phosphide (MoP) catalysts with low and high metal loadings were prepared by temperature-programmed reduction of the oxidic catalyst precursors in hydrogen to different temperatures (823, 923, 1023 and 1123 K, respectively). Effects of reduction temperature and metal loading on the surface distribution and the type of species formed were studied by TPR, SBET, XRD, HRTEM, 31P NMR, 27Al NMR and in the reaction of dibenzothiophene (DBT) hydrodesulfurization (HDS) performed in a flow reactor at 553 K and total hydrogen pressure of 3.4 MPa. HRTEM and 31P NMR confirmed formation of MoP phase on all catalysts. The 9.9 wt% Mo catalyst activated at lowest reduction temperature (823 K) was found to be most active among the catalysts studied. The presence of a low amount of Mo0 species on the surface of this catalyst does not appear to be a drawback for the catalytic activity. The increase in both metal loading (from 9.9 to 15 wt% Mo and from 3.2 to 4.8 wt% P) and reduction temperature (from 823 to 1123 K) was found to be detrimental for HDS activity due to sintering of active phase, and also to decrease in specific area and formation of phosphate species.

Synergistic effect of Pd in methane combustion PdMnO /Al2O3 catalysts

Abstract The catalytic methane combustion was investigated over alumina-supported monometallic and bimetallic palladium and manganese oxide catalysts. The catalytic activity of these systems showed that palladium incorporation on MnOx/Al2O3 catalyst leads to an enhancement in methane combustion. The higher catalytic activity of the PdMn/Al2O3 catalysts is related to a greater mobility of lattice oxygen in manganese oxide in the presence of palladium. These bimetallic catalysts also showed a significant improvement in catalysts stability with respect the monometallic ones. Surface analysis of the used catalysts revealed less amount of coke and Mn/Al and Pd/Al atomic ratios almost unchanged, which is indication of absence of active phase sintering.

Part I. n-Butane dehydrogenation on unsupported carbon modified MoO 3 (MoO xC y): effect of steam on the catalyst stability

MoO 3 activated under butane and hydrogen mixture at 623 K was tested for the reactions of simultaneous isomerization and dehydrogenation of n-butane at 823 K. The active phase, a mixture of oxycarbide and oxide of molybdenum, was active but unstable under the reaction conditions, i.e. high temperatures and a reducing environment, leading to the formation of a superficial carbide layer and carbon-rich oxycarbide which were active but completely unselective, i.e. 100% selectivity towards methane. The introduction of steam into the feed largely stabilized the catalyst selectivity at around 80% by avoiding the extensive carburization. However, the introduction of steam or oxygen led to a lower dehydrogenation activity by favoring the active phase sintering. Along with sintering an additional deactivation due to coke formation was also observed. This latter deactivation process becomes more pronounced as the hydrogen to hydrocarbon molar ratio decreased. However, the deactivated catalyst can be totally recovered by applying a short oxidative regeneration process at the same reaction temperature.

Regeneration of a nickel/silica catalyst poisoned by thiophene

The regeneration of a Ni/SiO 2 catalyst that was completely poisoned by thiophene during the benzene hydrogenation was studied. The regeneration treatments consisted of a sequence of oxydation-reduction treatments using low oxygen partial pressure mixtures and pure hydrogen, with intermediate argon stream purging at the same treatment temperature. The influence of treatment time, oxygen partial pressure, number of cycles and treatment temperature on the sulphur removal and recovering of both hydrogenation activity and metallic surface area was studied. This new method is easy to carry out and quick, allowing to remove up to 90% of the sulphur of the completely deactivated catalyst. Non-equilib- rium states between oxydation and reduction steps seem to be responsible for sulphur removal. The regeneration efficiency reached in a particular treatment depended on the balance between two opposite effects: the sulphur and the active phase sintering.

Regeneration of a nickel/silica catalyst poisoned by thiophene

The regeneration of a Ni/SiO 2 catalyst that was completely poisoned by thiophene during the benzene hydrogenation was studied. The regeneration treatments consisted of a sequence of oxydation-reduction treatments using low oxygen partial pressure mixtures and pure hydrogen, with intermediate argon stream purging at the same treatment temperature. The influence of treatment time, oxygen partial pressure, number of cycles and treatment temperature on the sulphur removal and recovering of both hydrogenation activity and metallic surface area was studied. This new method is easy to carry out and quick, allowing to remove up to 90% of the sulphur of the completely deactivated catalyst. Non-equilib- rium states between oxydation and reduction steps seem to be responsible for sulphur removal. The regeneration efficiency reached in a particular treatment depended on the balance between two opposite effects: the sulphur and the active phase sintering.