Inhibit Carbon Deposition Research Articles

Novel nonmetal catalyst of supported tetraphenylphosphonium bromide for acetylene hydrochlorination

HCl is preferentially activated by accepting electrons from TPPB, which can promote catalytic performance and inhibit carbon deposition.

Insight into CaO addition on coking resistance of Ni surface for sorption enhanced methane steam reforming: A density functional study

As a promising technology for hydrogen production, the sorption enhanced methane steam reforming progress (SESMR) has received much attention. Ni/CaO based bi-functional catalysts were widely used for SESMR due to their high CO2 adsorption capacity, high activity and low material cost. Currently, most of investigations were focused on improving the stability of carbonation reaction at absorptive sites during cyclic utilization, while many other aspects such as attrition resistance, sulfur tolerance, and coking resistance of the material were poorly explored. This study focused on investigating the potential change induced by Ca addition in terms of coking resistance of Ni using density functional theory. The key steps determining coke accumulation over the Ni surface were studied systematically, mainly including the generation of monatomic C by methane cracking, elimination of C atoms by oxidation, supplying of oxygen intermediates by H2O dissociation, C2 formation by surface migration and so on. The results showed that Ca presence could enhance the mobility of O atoms over surface, thus accelerating the elimination of C atoms through the oxidation to from CO. Moreover, due to Ca presence, the H2O dissociation became more easily to produce sufficient O intermediates for C oxidation. Additionally, introducing Ca has dramatically slowed down the C diffusion and CC formation rate, thus inhibiting carbon deposition. Therefore, Ca incorporation could boost the coking resistance of Ni surface effectively. These results are helpful in understanding the mechanisms of promoters for increasing coking resistance, which can provide valuable information for the rational design of the bi-functional catalysts.

Improvement of low temperature activity and stability of Ni catalysts with addition of Pt for hydrogen production via steam reforming of ethylene glycol

Hydrogen production by steam reforming of ethylene glycol (EG) at 300 °C was investigated over SiO2 and CeO2 supported Pt–Ni bimetallic catalysts prepared by incipient wetness impregnation methods. It was observed that impregnation sequence of Pt and Ni can affect the performance of catalysts apparently. Catalyst with Pt first and then Ni addition showed higher EG conversion and H2 yield owing to the Ni enrichment on the surface and the proper interaction between Pt and Ni. It was observed that although SiO2 supported catalysts exhibited better activity and H2 selectivity, CeO2 supported ones had better stability. This is attributed to the less coke formation on CeO2. Increasing Pt/Ni ratio enhanced the reaction activity, and Pt3–Ni7 catalysts with 3 wt% Pt and 7 wt% Ni showed the highest activity and stability. Ni surficial enrichment facilitated the C—C bond rupture and water gas shift reactions; and Pt addition inhibited methanation reaction. Electron transfer and hydrogen spillover from Pt to Ni suppressed carbon deposition. These combined effects lead to the excellent performance of Pt3–Ni7 supported catalysts.

Inhibition of carbon deposition using iron ore modified by K and Cu in chemical looping hydrogen generation

Inhibition of carbon deposition using iron ore modified by K and Cu in chemical looping hydrogen generation

NiMo-ceria-zirconia-based anode for solid oxide fuel cells operating on gasoline surrogate

A multi-functional NiMo-ceria-zirconia composite was developed as an anode for a solid oxide fuel cell (SOFC) running on isooctane, a commonly used gasoline surrogate. The anode layer was fabricated on an electrolyte-supported single cell using yttria-stabilized zirconia (YSZ) as the electrolyte and La0.6Sr0.4Co0.2Fe0.8O3-δ as the cathode. Our results indicate that the single NiMo-ceria-zirconia layer possesses a dual-functionality: firstly to internally convert complex hydrocarbons into synthesis gas with a high resistance to coking and secondly to electrochemically oxidize the synthesis gas mixture for electric power generation. Compared with the conventional Ni-YSZ anode single cell, the application of the ceria-zirconia in the anode significantly suppresses carbon deposition and improves the performance stability of the single cell. Furthermore, the addition of Mo in the Ni-ceria-zirconia appears to facilitate a higher degree of sintering for the anode, which increases the electronic conductivity and maximum power density of the single cell. Consequently, at 800 °C the single cell with 5 wt.% Mo in the Ni-ceria-zirconia-based anode displayed a higher maximum power density of 212 mW cm−2 at 0.48 V and significantly enhanced performance stability than the conventional Ni-YSZ anode single cell in the isooctane/air operating mode.

Electron microscopy study of the formation mechanism of catalytic nickel-rich particles and the role of carbonyl sulphide in the suppression of carbon deposition on 20Cr-25Ni steel

Austenitic stainless steel is used as fuel cladding in advanced gas-cooled nuclear reactors (AGR). At elevated temperatures, when the steel is exposed to CO2 based environments filamentary carbon deposits form on the surface of the steel. This filamentary carbon deposition is known to be catalysed by metallic nickel-rich particles. Adding trace amount of carbonyl sulphide (COS) into the gas mixtures suppresses the carbon deposition. In this current work, it has been shown that at 600 °C, the formation of filamentary carbon was suppressed by the addition of 215 ppb COS to a depositing gas mixture (containing approximately 1000 vppm C2H4/1% CO/bal. CO2) which was known to provide the environment suitable for carbon deposition. Samples exposed to the gas mixtures with and without 215 ppb COS were characterised using electron microscopy techniques to understand the formation mechanism of the nickel-rich particles and the inhibition mechanism due to the addition of COS. Electron diffraction study shows that the nickel-rich particles in the oxide layers assume the same crystallography as that of the austenitic metal underneath, regardless of the COS addition. The current observations also show that the metal-oxide interfaces was nickel-rich and a simple model has been proposed to explain the formation of nickel-rich particles within the subsurface oxide. Furthermore, it was found that when COS was added the surface of the nickel-rich particles in the oxide layer was found to be sulphur-rich by energy dispersive spectroscopy (EDS) on a scanning transmission electron microscope (STEM). It is believed that the surface sulphur adsorption onto the nickel-rich particles, rather than bulk sulphide formation, resulted in the inhibition of carbon deformation on the steel.

Effects of cerium and lanthanum on Ni-based catalysts for CO2 reforming of toluene

The dry reforming of toluene (DRT) has been studied on Cerium and Lanthanum-promoted Ni–Al catalysts derived from hydrotalcites precursors in the temperature range 300–800 °C. X-ray powder diffraction (XRD) and thermogravimetric and differential thermal analyses (TGA-DSC) of as-prepared samples confirmed the formation of the hydrotalcite structure. Temperature programmed reduction (TPR) and desorption (CO2-TPD) indicated that Ni phase reducibility and basicity of the catalysts were influenced by the presence of the promoters. Ce-promoted catalyst was reducible at lower temperatures and had higher total basicity than the La-promoted and non-promoted catalysts. Furthermore textural property of solid was modified after cerium incorporation. These factors played a key role in the enhancement of the catalytic activity and suppression of carbon deposition. Post-reaction characterization of the catalysts by Temperature Programmed Oxidation (TPO) and TGA-DSC evidenced a better resistance towards carbon deposition for the Ce and La promoted catalysts. The promoted and non-promoted catalysts showed similar toluene conversion but promoted catalysts presented lower carbon deposition after test.Benefit effect of Cerium promotion catalyst was related by its characteristic which greatly influenced the activity. Lanthanum promoted exhibited a similar textural and physico-chemical characteristic than non-promoted catalyst but showed a higher performance than this latter. Promotion-activity relationship of the La containing catalyst was related by the presence of La2O3 on Ni based catalyst which could influence the transport and the adsorption of the CO2 onto the active sites surface.

Characteristics of tar abatement by thermal cracking and char catalytic reforming in a fluidized bed two-stage reactor

With respect to the newly proposed fluidized bed two-stage (FBTS) gasification process, the researches on tar thermal cracking and catalytic removal by char bed layer were conducted on a FB two-stage reaction apparatus. It shows that the effect of char on tar removal was much related to the reaction temperature and residence time of tar-containing fuel gas in the tar reformer. Under the tested experimental conditions, increasing reaction temperature and residence time can effectively promote tar abatement and generate more H2, CO and CH4. For catalytic reforming by char, the higher reaction temperature and the longer residence time, the lower tar content in fuel gas. Compared to thermal cracking, catalytic reforming of tar by char was more effective in tar removal efficiency and inhibition of carbon deposition, upgrading the quality of fuel gas. The researches indicated that the suitable operational conditions for tar removal by char in the FB two-stage gasification process were at reforming temperature of 1000 °C, and residence time above 0.9 s.

Understanding the performance and mechanism of Mg-containing oxides as support catalysts in the thermal dry reforming of methane.

Dry reforming of methane (DRM) is one of the more promising methods for syngas (synthetic gas) production and co-utilization of methane and carbon dioxide, which are the main greenhouse gases. Magnesium is commonly applied in a Ni-based catalyst in DRM to improve catalyst performance and inhibit carbon deposition. The aim of this review is to gain better insight into recent developments on the use of Mg as a support or promoter for DRM catalysts. Its high basicity and high thermal stability make Mg suitable for introduction into the highly endothermic reaction of DRM. The introduction of Mg as a support or promoter for Ni-based catalysts allows for good metal dispersion on the catalyst surface, which consequently facilitates high catalytic activity and low catalyst deactivation. The mechanism of DRM and carbon formation and reduction are reviewed. This work further explores how different constraints, such as the synthesis method, metal loading, pretreatment, and operating conditions, influence the dry reforming reactions and product yields. In this review, different strategies for enhancing catalytic activity and the effect of metal dispersion on Mg-containing oxide catalysts are highlighted.

The Effect of N‐Doping in Activated Carbon‐Supported Au‐Sr Catalysts for Acetylene Hydrochlorination to Vinyl Chloride

AbstractWe used N‐doped activated carbon (NAC) as support to prepare a novel Au−Sr catalyst and evaluated the effect of N‐dopants on the performance of Au−Sr catalyst in acetylene hydrochlorination. A promoting effect was observed when N atoms were incorporated into AC framework, and the prepared SrAu/NAC catalyst demonstrated optimal activity compared with the undoped Au/AC and SrAu/AC catalysts. The SrAu/NAC also showed an excellent stability during the 500 h test, with acetylene conversion and vinyl chloride selectivity both exceeding 99.9%. A detailed characterization was investigated through XPS, TPD, TPR, TGA, PXRD, TEM, AAS, and BET to reveal the physicochemical properties of the related catalysts. It is demonstrated that the NAC can function as an effective support to enhance activity and life of Au‐based catalysts with unique electronic property, and the presence of doped nitrogen species as anchor sites is beneficial to increasing the electron density of active Au(III) center through electron transfer from doped N atoms to the active Au(III) species, and stabilizing active Au(III) ions and retarding its reduction within catalyst preparation and use process. Moreover, the combination of N‐dopants and strontium additive may promote a synergistic effect to make active Au species disperse better and significantly enhance the adsorption of HCl, remarkably suppress the reduction of active cationic Au(III), and effectively inhibit carbon deposition in the catalytic reaction.

Acetoxylation of acetylene to vinyl acetate monomer over bimetallic Zn-Ni/AC catalysts

Zinc and bimetallic zinc-nickel catalysts for acetylene acetoxylation were prepared with Zn(OAc)2·2H2O and Ni(OAc)2·4H2O as precursors, and characterized by ICP-AES, BET, XPS, TEM, TPD-MS and TG. The catalytic performances of the obtained Zn-Ni/AC catalysts were tested in a fixed-bed reactor. The results indicated that the addition of nickel decreased the size of Zn(OAc)2 particles and inhibited carbon deposition on the catalyst surface; there was a synergistic effect between the two metal components, which modified the electronic properties of Zn(OAc)2 particles and modulated the catalytic adsorption of acetylene and acetic acid, thereby enhancing the activity and stability of the zinc catalysts.

Stability of Ni-YSZ Anode for SOFCs in Methane Fuel: The Effects of Infiltrating La0.8Sr0.2FeO3-δ and Gd-Doped CeO2 Materials

Improving carbon deposition resistance of Ni/YSZ anode in hydrocarbon fuels is crucial for the stability operation of solid oxide fuel calls (SOFCs). In this work, La0.8Sr0.2FeO3-δ (LSF) and Gd-doped CeO2 (GDC) are infiltrated into Ni-YSZ anodes for carbon deposition resistance, while LSF is infiltrated into YSZ cathode backbone for oxygen reduction reaction. The structure design allowed simultaneously fabrication of cathode and anode catalyst, greatly simplifying the preparation process. The distribution of relaxation times (DRT) method is applied to isolate the contributions of electrode polarization processes and illuminate the role of infiltrated LSF and GDC. It is found that LSF nano particles layer reduces the electrocatalysis of Ni, which increases the resistance associating to mass transfer process in H2 or CH4 conversion. As compared with LSF, GDC can improves the electrochemical performance of cell, due to its high ionic conductivity, extending the three phase boundary and facilitating the charge transfer process. For the stability test in wet CH4, cells with LSF and GDC modified anode show excellent carbon deposition resistance, where no significant performance degradation is occurred in more than 100 h operation at 0.25 A·cm−2 and 750°C. This is ascribed to the LSF and GDC inhibiting carbon deposition process.

NiCo@SiO2 core-shell catalyst with high activity and long lifetime for CO2 conversion through DRM reaction

A novel NiCo@SiO2 core-shell catalyst with single NiCo alloy nanoparticle encapsulated by SiO2 shell was synthesized by microemulsion method. During dry reforming of CH4 with CO2 (DRM) reaction, this catalyst exhibited high activity and selectivity, superior over either the Ni@SiO2 or the Co@SiO2 core-shell catalyst or the NiCo/SiO2 supported catalyst. The CO2 and CH4 (1:1) could be absolutely converted into CO and H2 with molar ratio around 1:1. More importantly, this catalyst displayed very long lifetime (> 1000h) in the DRM reaction at 800°C, much better than either the Ni@SiO2 or the Co@SiO2 core-shell catalyst or the NiCo/SiO2 supported catalyst. Kinetic studies revealed that the catalyst with small metal nanoparticle size exhibited higher activity, which could also inhibit carbon deposition. The encapsulation of metal nanoparticles by SiO2 shell could effectively inhibit the agglomeration of active sites. Meanwhile, the enhanced activity of NiCo alloy catalyst could also diminish the surface carbon deposition. Therefore, the NiCo@SiO2 core-shell catalyst displayed excellent stability in DRM reaction, showing good potential in industrial application.

Highly Selective Production of p-Xylene from 2,5-Dimethylfuran over Hierarchical NbOx-Based Catalyst

A hierarchical NbOx-based catalyst with both Bronsted acid and Lewis acid sites was synthesized in the absence of corrosive hydrofluoric acid, exhibiting high catalytic activity for biobased p-xylene (PX) production from 2,5-dimethylfuran (DMF). The as-prepared composite was composed of Nb2O5 and NbOPO4 crystals, and the densities of Bronsted acid and Lewis acid were determined to be 232.9 and 80.4 μmol/g, respectively. The well-balanced Bronsted/Lewis acidity and the hierarchical structure with small mesopores (3 nm) and large mesopores (48 nm) contributed to the high activity and stability: a conversion of 87.2% with the PX selectivity of 92.7%, and a carbon balance of 94.6% was achieved after 6 h of reaction at 523 K. In comparison with Sn-Beta, NbOx-based catalyst prepared in this work showed obvious advantages in suppressing carbon deposition: 90.9 and 54.7 mmol of PX were obtained over the NbOx-based catalyst and the Sn-Beta, respectively, after 24 h. Spent catalysts were regenerated through calcina...

Bi-functional composite oxides M(Na, K)-Ni/La-Al2O3 catalysts for steam reforming of n-decane

n-Decane steam reforming towards hydrogen has been researched in a fixed-bed reactor containing spherical particle of Ni supported La-Al2O3 catalyst. The as-prepared Ni/La-Al2O3 catalyst was modified by the alkaline promoters M(Na, K) in order to inhibit carbon deposition. The physicochemical properties of these catalysts were characterized by Nitrogen physisorption, X-ray diffraction, X-ray photoelectron spectroscopy, Infrared spectroscopy, Temperature programmed reduction/desorption, Scanning electron microscopy-energy dispersive X-ray spectroscopy. The results showed that the addition of alkaline modifiers had a slightly impact on the surface area, crystallinity and morphology of Ni/La-Al2O3 catalyst, but improved the activity and stability, as well as reduced the rate of carbon deposition formation and thereby increased the working life of the catalysts. Meaningfully, K was shown an excellent catalytic performance in inhibiting carbon deposition, and the presence of K-modifier prevented metal sintering and controlled Ni particle size and dispersion, strengthened the metal-support interaction, and decreased the acidity of catalysts.

Durability studies of limonite ore for catalytic decomposition of phenol as a model biomass tar in a fluidized bed

The development of highly active, durable, tar decomposition catalysts is desirable for practical application of biomass gasification at low temperature. In this study, limonite ore calcinated at 900 °C was used as bed material in a fluidized bed for catalytic decomposition of phenol (biomass tar model) at 650 °C. The limonite showed catalytic activity for the decomposition of phenol to produce combustible gases (H2 and CO). A long-term durability test of phenol decomposition with O2 regeneration treatment was conducted for 25 h using the limonite catalyst. The O2 treatment repeatedly removed the deposited carbon, allowing H2 and CO to be generated constantly for 25 h. Steam reforming of phenol using the limonite catalyst was also conducted with various steam/carbon ratios (S/C = 0, 0.4, and 0.8), of which S/C = 0.4 was the best for suppressing carbon deposition and producing higher volumes of H2 and CO gases. Continuous steam reforming for 24 h could be achieved using a limonite catalyst with S/C = 0.4, while maintaining the production of H2 and CO gases, even though carbon deposition occurred steadily. These results indicated that limonite ore is a promising bed material for use in internally circulating fluidized-bed systems for long-term catalytic decomposition of biomass tar.

CO2 reforming of methane over supported LaNiO3 perovskite-type oxides

This work investigated the performance of supported LaNiO3 perovskite-type oxides for the CO2 reforming of methane. The methane and CO2 conversion increased at the beginning of reaction for LaNiO3 and LaNiO3/Al2O3 catalysts. On the other hand, conversion remained quite constant for LaNiO3/CeSiO2. in situ XPS experiments under reaction conditions revealed that the metallic Ni particles were oxidized by CO2 from the feed for LaNiO3 and LaNiO3/Al2O3 catalysts. Ceria support was preferentially oxidized, limiting the oxidation of the metallic phase. Raman spectroscopy and thermogravimetric analysis showed that carbon was formed mainly over LaNiO3 and LaNiO3/Al2O3 catalysts. Supporting LaNiO3 over CeSiO2 almost completely suppressed carbon deposition. in situ XPS experiments showed a continuous change of ceria oxidation states between Ce4+ and Ce3+ under reaction conditions. This result in a high oxygen mobility of ceria support that reacts with carbon, inhibiting the formation of nickel carbide and consequently the nucleation and growth of carbon filaments.

Effective hydrogen production from propane steam reforming using M/NiO/YSZ catalysts (M = Ru, Rh, Pd, and Ag)

This study has investigated the propane steam reforming (PSR) performance of M/Ni/YSZ catalysts (M=Ru, Rh, Pd, and Ag) for effective hydrogen production. To improve the catalytic performance, particularly with respect to the duration of the catalyst, YSZ (yttria-stabilized zirconia) is used as a support and noble metals such as Ru, Rh, Pd, and Ag are introduced as promoters. YSZ is used to suppress carbon deposition as it provides lattice oxygen to carbon coke on the catalyst. Noble metals provide synergy, assisting Ni metal in improving dehydrogenation. Preferentially, the PSR performance of 1.0wt.%M/Ni/YSZ catalysts are measured to find which noble metal is suitable as a promoter. The result confirms that the Rh component is the best promoter for M/Ni/YSZ. This study also demonstrates the appropriate amount of Rh to use, based on the PSR performance over Ni/YSZ and 0.1, 0.5, and 1.0wt.% Rh/Ni/YSZ for 100h. The results confirm that Ni/YSZ becomes deactivated at 76h, but the catalysts with the Rh promoter have much higher durability. Thus, this study concludes that 0.5wt.% Rh/Ni/YSZ is the best catalyst because it does not have excessive Rh content and also does not show any discernable decrease in the PSR performance. Moreover, the Rh/Ni/YSZ catalysts had a higher resistance to carbon deposition because of the presence of the Rh promoter, which could improve lattice oxygen mobility of the Rh/Ni/YSZ catalyst.

Insight into carbon deposition associated with NiCo/MgO catalyzed CH4/CO2 reforming by using density functional theory

A density-functional theory method has been conducted to investigate carbon deposition associated with NiCo/MgO catalyzed CH4/CO2 reforming. In the process of carbon deposition formation, CH4 dissociation to produce C, further C accumulates to form carbon deposition. In addition, C and its precursor can be eliminated by oxygen species from CO2 dissociation. Based on above analysis about carbon deposition, three possible ways are considered to contribute to resist carbon deposition (1) reducing CH4 dissociation to decrease C produced; if not, (2) accelerating CHx(x=0∼3) oxidation to eliminate C, (3) blocking C accumulation, further suppressing carbon deposition, that is, the rate of CHx oxidation is more than that of C accumulation. The calculation results show that the dissociation of CH4 has almost equal activation energy on perfect and defective NiCo/MgO, and they are lower than those on Ni/MgO and NiCo(111), indicating that it is impossible to reduce C produce through reducing CH4 dissociation when addition a second metal Co to Ni/MgO or using different support MgO; CH oxidation is favorable to produce CHO compared to CH dissociation into C and H, further it is favorable for CHO dissociation into CO and H on perfect and defective NiCo/MgO catalyst. In addition, C oxidation is easier on defective NiCo/MgO than that on perfect NiCo/MgO, meanwhile it is easier on NiCo/MgO than that on NiCo(111), indicating that using defective MgO support accelerates C oxidation, further elimination carbon deposition. Carbon accumulation is easier on perfect NiCo/MgO than that on defective NiCo/MgO. The results indicate that the defective MgO surface support NiCo bimetal catalyst is more resistant to carbon deposition than perfect one. One can modify the Ni-based catalyst behavior via addition a second metal Co or using MgO support with oxygen-vacancy. This might be given a clue for experimental catalyst to develop Ni-based catalyst with the property of resisting carbon deposition.

Coke Resistant and Sulfur Tolerant Ni-based Cermet Anodes for Solid Oxide Fuel Cells

Conventional Ni-based cermet anodes of solid oxide fuel cells (SOFCs) are highly susceptible to deactivation from carbon deposition and sulfur poisoning in sulfur-containing hydrocarbon fuels. Here we propose a viable approach to substantially enhance the carbon deposition resistance and sulfur tolerance of the Ni-gadolinia doped ceria (Ni-GDC) anode. Perovskite BaCe0.9Yb0.1O3- δ (BCYb) is adopted to modify Ni-GDC anode via wet impregnation. Since BCYb nanoparticles could selectively deposit on Ni surfaces in a Ni-GDC anode backbone, a slight amount of infiltrates could lead to the excellent modification of the Ni grains without blocking the gas diffusion channels. The deposited BCYb nanoparticles decrease the exposed surfaces of Ni grains, showing promise to inhibit carbon deposition and sulfur species adsorption. BCYb has high oxygen ion conductivity, beneficial to the transportation of oxygen ions to remove the adsorbed carbon and sulfur (Fig. 1). Therefore, the stability of conventional Ni-GDC anode in wet methane fuel (3% H2O in CH4) is remarkably improved with modification of BCYb nanoparticles. The voltage of a full cell with conventional Ni-GDC anode decreases rapidly from 0.58 to 0.15 V within 6 h at 200 mA cm-2 and 750 °C. While in the case of a cell with BCYb modified Ni-GDC anode, designated as B+Ni-GDC, the cell voltage is essentially constant at around 0.65 V over a period of 48 h tested under identical conditions. Furthermore, the impregnation of BCYb nanoparticles can also improve sulfur poisoning resistance of Ni-GDC anode. The enhanced stability is observed in 500 ppm of H2S/H2 at 650 °C. The B+Ni-GDC anode manages to retain its microstructure in CH4 and H2S-containing H2 fuels, resulting in stable performance output. In addition, the electrochemical performance of Ni-GDC anode is also improved with BCYb modification in both wet H2 and CH4 fuels. The BCYb impregnation approach shows promising potential in the development of highly active and stable Ni-GDC based anodes for direct hydrocarbon SOFCs. Figure 1. Schematic diagram showing carbon and sulfur removal processes on BCYb modified Ni-GDC anode. Figure 1