Pore diameters of Ni/ZrO2 catalysts affect properties of the coke in steam reforming of acetic acid

Pore structures of support affect not only the dispersion of metals species, but also the catalytic stabilities and the properties of the generated coke in steam reforming reaction of acetic acid. This has been demonstrated in this study with Ni/ZrO₂ catalysts of varied pore diameters. The results showed that the Ni/ZrO₂ catalyst with the support calcined at 700 °C had the largest diameter, which facilitated the entering of nickel species into the pores, suppressing the migration and agglomerate in reduction of the catalysts. Although the catalysts with the varied pore diameters showed little impacts on the catalytic performances, the properties of the coke varied substantially. The catalyst with the large pore diameter favored the growth of the crystal carbon in the coke to a bigger size and was more tolerant to coke accumulation while maintained the catalytic stability. In addition, thermal stability, resistivity to oxidation, aromaticity, functionalities and the morphologies of the coke (i.e. carbon nanofibers, carbon nanotubes and bamboo-like structure) were all connected with the porous structures of the Ni/ZrO₂ catalysts.     

Sequence of Ni/SiO2 and Cu/SiO2 in dual catalyst bed significantly impacts coke properties in glycerol steam reforming

Coke formation is a major challenge in steam reforming reactions. In addition to development of robust catalyst for tackling coking, in this study we explored the approach of using dual catalyst bed with the catalyst on top as the guard or sacrifice catalyst while with the bottom catalyst to catalyze the steam reforming. The rationale is that some oxygen-containing reactants are prone to polymerize on heating, and the polymeric coke could directly fall on surface of catalyst and leads to the rapid deactivation. Hence, glycerol was selected as the reactant for steam reforming in the catalyst bed with the Cu/SiO₂ placed on the top of Ni/SiO₂ catalyst. Our results demonstrate that first contact of glycerol to Cu/SiO₂ on top changed abundance/type of small intermediates and the π-conjugated oligomers reached the Ni/SiO₂ catalyst, rendering the Ni catalyst with a higher resistivity towards coking and deactivation. In addition, the carbon nanotube form of coke over Ni/SiO₂ was thinner in wall thickness and larger in inner diameter of the cavity due to the impact of D-Cu/SiO₂. Substantial polymeric coke with amorphous structure and low thermal stability formed over Cu/SiO₂ via polymerisation of reaction intermediates. The characterization (in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)) for the glycerol steam reforming indicated that the Cu/SiO₂ and Ni/SiO₂ catalyst induced the formation of the very different functionalities of the reaction intermediates.     

Influence of La2O3 addition on activity and coke formation over Ni/SiO2 for acetic acid steam reforming

The influence of the addition amount of lanthanum oxide on the activity and coke formation over Ni/SiO₂ catalyst for hydrogen production from AcOH steam reforming was studied in the paper. Through the catalyst characterization and experimental research, it was found that with the addition of La₂O₃ the particle size of the active metal Ni decreased, and the active Ni-support interaction was enhanced. With the optimal addition amount of La₂O₃ (30 wt %), Ni/30LaSi catalyst displayed the highest activity with the high carbon conversion rate of 98.8%. The surface morphology and thermogravimetric analysis of the spent catalyst revealed that the carbon deposition was mainly in the form of filamentous coke and the diameter of the carbon fiber was related with the Ni particle size. The addition of suitable La₂O₃ can inhibit the formation of carbon deposition, decrease the content of encapsulating coke, and improve the catalyst stability.     

Hydrogen production from acetic acid steam reforming over nickel-based catalyst synthesized via MOF process

In our earlier work, we have reported that Ni supported on γ-Al₂O₃–La₂O₃–CeO₂ (ALC) catalyst prepared via metal organic framework (MOF) was more active for acetic acid steam reforming (AASR) [1]. Here we report detailed study on the performance of this catalyst for AASR. Effects of operating conditions such as temperatures (400–650 °C), steam to carbon molar ratio (S/C) and feed flow rate (1.5–5.5 mL/h) were evaluated and optimized. Results showed an excellent activity for AASR at the molar ratio S/C = 6.5, feed flow rate = 2.5 mL/h and, at 600 °C with almost total conversion and more than 90% of H₂ yield. The ordered porous structure of embedded nickel supported catalyst promotes excellent steam reforming activity and water gas shift reaction even at low temperatures, which leads to the good stable behaviour up to 36 h of TOS. The coke formation was also significantly suppressed by ALC support. Catalyst regenerated by passing oxygen at 500 °C and followed by reduction in hydrogen also show a good activity. Catalysts were characterized by DT-TGA, XRD, TEM, H₂-TPR and N₂-adsorption-desorption to understand the micro structure and coke deposition behaviour.     

Effect of ceria addition on NiRu/CeO2Al2O3 catalysts in steam reforming of acetic acid

NiRu bimetallic catalysts with different amount of CeO₂ loaded on the γ-Al₂O₃ support were prepared. The properties of catalysts were characterized by means of N₂ adsorption-desorption, XRD, H₂-TPR and XPS techniques. Catalytic activities for the steam reforming of acetic acid over these catalysts were investigated at the temperature range from 650 °C to 750 °C. The addition of CeO₂ dramatically improved the activity and stability of the catalyst. Among these catalysts, the NiRu/10CeAl catalyst showed the highest catalytic activity as well as a good stability owing to the abundant Ce³⁺ on the surface of catalyst. The existence of Ce³⁺ promoted the formation of CO₂ from CO because of the mobilizable oxygen, which was favorable for the formation of hydrogen. The coke amount and species deposited on the catalysts after the activity tests were analyzed by DTG. As expected, the NiRu/10CeAl catalyst showed the best resistance to carbon formation. The temperature stepwise steam decoking experiment of the spent catalysts was conducted to elucidate the relationship between the existence of Ce³⁺ and the decoking abilities of various catalysts. It was verified that the existence of Ce³⁺ significantly promoted the decoking abilities of the catalysts.     

Catalytic steam reforming of acetic acid for hydrogen production

A variety of supported metal catalysts were tested under conditions of steam reforming of acetic acid (HAc), which was selected as a model compound for pyrolysis oil. The influence of several parameters on catalytic activity and selectivity were examined, including catalyst composition, i.e. nature of the metal and the carrier, reaction temperature and time on stream. The metallic phase of such catalysts was comprised of various metals, such as Pt, Pd, Rh, Ru and Ni, which were supported on metal oxides carriers, such as _Al2O3${\mathrm{Al}}_2{\mathrm{O}}_3$, _La2O3/_Al2O3${\mathrm{La}}_2{\mathrm{O}}_3/{\mathrm{Al}}_2{\mathrm{O}}_3$, MgO/_Al2O3$\mathrm{MgO}/{\mathrm{Al}}_2{\mathrm{O}}_3$ and _CeO2/_Al2O3${\mathrm{CeO}}_2/{\mathrm{Al}}_2{\mathrm{O}}_3$. It was found that Ni-based and Ru-based catalysts present high activity and selectivity toward hydrogen production. Ru catalysts supported on _La2O3/_Al2O3${\mathrm{La}}_2{\mathrm{O}}_3/{\mathrm{Al}}_2{\mathrm{O}}_3$ and MgO/_Al2O3$\mathrm{MgO}/{\mathrm{Al}}_2{\mathrm{O}}_3$ carriers, showed good long-term stability as a function of time on stream. However, Ni catalysts were not as stable as Ru catalysts. The amount of carbon deposited on each catalyst was estimated, and it was found that it depends strongly on the nature of the catalyst.     

Highly active Ni/CeO2 for the steam reforming of acetic acid using CTAB as surfactant template

A series of CTAB-templated Ni/Ce catalysts were synthesized by adding CTAB during the hydrothermal synthesis of ceria to improve the pore structure of catalysts. Their catalytic performance was evaluated in the steam reforming of acetic acid and the effects of CTAB concentration on the porous structures, reducibilities, morphology, and activity of catalysts were studied. The catalysts were characterized by BET, XRD, H₂-TPR, XPS, HRTEM, in-situ DRIFTS, DTG, FTIR, and temperature-programmed reaction to elucidate the structure-activity relationship of the catalyst. The results showed that owing to the CTAB assistance, a high surface area of ceria could be achieved, which induced a better Ni dispersion with a smaller Ni size, strengthened the interaction between Ni and CeO₂, and promoted the reducibility of Ni, obtaining higher activity and lower methane yield than Ni/Ce. Among these prepared samples, Ni/Ce–C6 showed the highest surface area and the best catalytic performance with a hydrogen yield of up to 82.5% even at a low temperature (550 °C). Owing to the stronger Ni-ceria interaction of Ni/Ce–C6, the lattice oxygen in ceria migrates easily to the Ni surface, interacts with the reaction intermediates, and thus improves the CO₂/CO ratio in the products. Much more CO and CO₂ and less CH₄ were observed over Ni/Ce–C6 during the temperature-programmed reaction, indicating its high activity. In-situ DRFITS characterization demonstrated that the two types of catalysts had similar reaction intermediates but various adsorption and conversion abilities toward the acetic acid. More reaction intermediates were adsorbed at low temperatures and a higher conversion was obtained over Ni/Ce–C6 owing to its better Ni dispersion. The CTAB assistance inhibited the formation of amorphous carbon but facilitated the formation of graphitic carbon at ～637 °C which did not induce catalyst deactivation.     

Steam reforming of acetic acid for hydrogen production over attapulgite and alumina supported Ni catalysts: Impacts of properties of supports on catalytic behaviors

In this study, the potential of attapulgite (ATTP) as the support of nickel catalysts for steam reforming of acetic acid to produce hydrogen were evaluated. Ni/Al₂O₃ was prepared and evaluated for comparison. The results showed that ATTP had a much lower specific surface area and a lower thermal stability than alumina. Nevertheless, the interaction between nickel and ATTP was much weaker than that of nickel with alumina. As a result, the Ni/ATTP catalyst had superior activity than the Ni/Al₂O₃ catalyst, especially at low nickel loading. Ni/Al₂O₃ was more stable than Ni/ATTP. The fibrous coke, which was probably catalytic coke, formed over Ni/Al₂O₃ did not cause the rapid deactivation of the catalyst, while the amorphous coke formed over Ni/ATTP catalyst, which was probably the polymeric coke, rapidly deactivated the catalyst. The coke species contained COOH, CO, aliphatic structure and aromatic ring structures. In addition, the effects of these two carriers on the steam reforming mechanism were investigated by the in-situ DRIFTS.     

Effect of Co,Fe and Rh addition on coke deposition over Ni/Ce0.5Zr0.5O2 catalysts for steam reforming of ethanol

Ni-based monometallic and bimetallic catalysts (Ni, NiRh, NiCo and NiFe) supported on Ce_0.5Zr_0.5O₂ support were evaluated on the steam reforming of ethanol (SRE) performance. The supports of Ce_0.5Zr_0.5O₂ composite oxide was prepared by co-precipitation method with Na₂CO₃ precipitant and assigned as CeZr(N). The monometallic catalyst was prepared by incipient wetness impregnation method and assigned as Ni/CeZr(N). The bimetallic catalysts were prepared by co-impregnation method to disperse the metals on the CeZr(N) support and assigned as NiM/CeZr(N). All samples were characterized by using XRD, TPR, BET, EA and TEM techniques at various stages of the catalyst. The results indicated that the facile reduction and smaller particle size of Ni/CeZr(N) (T₉₉ = 300 °C) and NiRh/CeZr(N) (T₉₉ = 250 °C) catalysts were preferential than the NiFe/CeZr(N) (T₉₉ = 325 °C) and NiCo/CeZr(N) (T₉₉ = 375 °C) catalysts. Also, both the Ni/CeZr(N) and NiRh/CeZr(N) catalysts displayed better durability among these catalysts over 100 h and 400 h, respectively. Since the serious coke formation for the NiCo/CeZr(N) catalyst, the activity only maintained around 6 h, the durability on the NiFe/CeZr(N) catalyst approached 50 h.     

Nickel catalyst auto-reduction during steam reforming of bio-oil model compound acetic acid

Transition metal catalysts widely used in refineries are provided as oxides and require pre-reduction to become activated. The auto-reduction of a NiO/Al₂O₃ catalyst with acetic acid (HAc) followed by HAc steam reforming was investigated in a packed bed reactor. Effects of temperature and molar steam to carbon ratio (S/C) on reduction kinetics and catalyst performance were analysed. Results showed that a steady steam reforming regime along with complete NiO reduction could be obtained after a coexistence stage of reduction and reforming. A 2D nucleation and nuclei growth model fitted the NiO auto-reduction. The maximum reduction rate constant was attained at S/C = 2. Steam reforming activity of the auto-reduced catalyst was just below that of the H₂-reduced catalyst, probably attributed to denser carbon filament formation and larger loss of active Ni. Despite this, a H₂ yield of 76.4% of the equilibrium value and HAc conversion of 88.97% were achieved at 750 °C and S/C = 3.     

Steam reforming of acetic acid over nickel catalysts: Impacts of fourteen additives on the catalytic behaviors

This study investigated the effects of a series of additives ranging from alkali, alkali earth, transition and rare earth metals (Na, Mg, K, Ca, Sc, Cr, Mn, Fe, Co, Cu, Zn, Zr, La, Ce) on catalytic behaviors of Ni/Al₂O₃ in steam reforming of acetic acid. The addition of alkali and alkaline metals led to the sintering of alumina and nickel species. Cr, Ce or Zr addition filled pores, while Mn addition enhanced the specific area by creating more pores. Cu and Zn addition probably led to merging of small pores. In addition, K or Na promoted reduction of nickel oxides, promoting the catalytic activity. La, Ce or Co also enhanced activity of the catalysts, while Mg, Ca, Sc, Cu, Zn or Zr decreased the activities. The K or Na modified catalysts was much more stable than that of the La or Ce modified catalysts, although the coking was more serious. The coke formed in K or Na modified catalysts was mainly in fibrous form (probably the catalytic coke), while that in La or Ce modified catalysts was mainly amorphous form (probably the polymeric coke). The fibrous coke did not lead to the fast deactivation of the nickel catalysts as the amorphous coke did. The TG-MS and DRIFTS study showed that the coke contained the aliphatic structures with the functionalities such as methyl group, carbonyl groups, carboxylic structures as well as the structures of aromatic rings. The amorphous coke formed in La or Ce modified catalysts contained more aliphatic structures. The in situ DRIFTS study of steam reforming of acetic acid showed that a number of reaction intermediates including the –OH, CO, aliphatic CC, aromatic CC, COO, C–H, –CH3, aliphatic C–O–C structure, and absorbed CO₂ involved in the reforming reaction.     

Ceria-promoted Ni@Al2O3 core-shell catalyst for steam reforming of acetic acid with enhanced activity and coke resistance

A series of Ni@Al₂O₃ core-shell catalysts with ceria added to the surface of Ni nanoparticles or inside the alumina shell were prepared, and the effect of ceria addition on the performance of the catalyst in the steam reforming of acetic acid was investigated. The prepared catalysts were characterized by BET, XRD, HRTEM, H₂-TPR and DTG. The addition of ceria to the surface of nickel nanoparticles greatly enhanced the activity of catalyst owing to the presence of the mobile oxygen, which migrated from the ceria lattice. Among the prepared catalysts, the Ni@Al10Ce catalyst showed the highest activity with a conversion of acetic acid up to 97.0% even at a low temperature (650 °C). The molar ratio of CO₂/CO was also improved due to the oxidation of CO by the mobile oxygen into CO₂. The coke formation on the core-shell catalysts was significantly inhibited by the addition of ceria to the surface of nickel nanoparticles due to the oxidation of carbon species by the mobile oxygen in the ceria lattice. However, the Ni@Al10Ce-a catalyst with ceria added to the alumina shell showed a low activity and the formation of a large amount of coke. It is suggested that only the ceria in close to the Ni surface has the promoting effect on the catalytic performance of the Ni@Al₂O₃ catalyst in the steam reforming of acetic acid.     

Catalytic steam reforming of acetic acid in a fluidized bed reactor with oxygen addition

Catalytic steam reforming of bio-oil is a promising process for producing hydrogen in a sustainable environmentally friendly way that can improve the utilization of local resources (natural sources or wastes). However, there remain drawbacks such as coke formation that produce operational problems and deactivation of the catalysts. Coprecipitated Ni/Al catalysts are here used in a fluidized bed for reforming at 650 °C of acetic acid as a model compound of bio-oil–aqueous fraction. Different strategies are applied in order to study their effects on the catalytic steam reforming process: modification of the catalyst by increasing the calcination temperature or adding promoters such as calcium. The addition of small quantities of oxygen is also tested resulting in an optimum percentage to achieve a high carbon conversion process with less coke and without a hydrogen yield penalty production. The results for catalytic steam reforming are compared with other ones from literature.     

The kinetics of acetic acid steam reforming on Ni/Ca-Al2O3 catalyst

As a significant by-product of many thermochemical and biological waste conversion processes, acetic acid (AcOH) is often investigated as model feedstock in the production of sustainable hydrogen from non-fossil sources. The kinetics of its steam reforming were extracted from packed bed reactor experiments over an industrially produced 14 wt% Ni/Ca-Al₂O₃ catalyst at atmospheric pressure. The model consisting of AcOH steam reforming producing CO₂ and H₂, AcOH decomposition to CO and H₂, and water gas shift, achieved the best fit, reflected in the lowest average relative errors (ARE) with experimental results, with ARE values below 5.4% and 6.4% on AcOH and water conversions respectively, and below 4% on H₂ mol fraction. This model was validated away from equilibrium using additional experimental points, as well as for a wide range of equilibrium conditions with varying temperature (600–700 °C) and feed molar steam to carbon ratios (3–8) at atmospheric pressure using an independent method.     

Highly active Nd-promoted Ni@Al2O3 core-shell catalyst for steam reforming of acetic acid: Insights of the remarkable coke resistance

Aiming at enhancing the coke resistance of Ni-based catalysts, the Nd-doped Ni@A₂O₃ core-shell catalysts were prepared and their catalytic performance was evaluated in the steam reforming of acetic acid (SRAA). The catalysts were characterized by BET, XRD, XRF, HRTEM, H₂-TPR, NH₃-TPD, and DTG. The catalytic performance was greatly enhanced by the incorporation of Nd, with decreased yields of CO, CH₄, and acetone, increased yield of CO₂, and remarkable carbon resistance. The decoking behavior of the spent catalyst was elucidated by H₂O¹⁸-TSD. A low reaction temperature facilitates the formation of amorphous carbon, leading to catalyst deactivation. The decoking ability of the catalyst is greatly improved by the Nd incorporation but is also catalyzed by the exposed Ni surface. The Ni0.01Nd@Al catalyst greatly balanced the exposed Ni surface and the mobile lattice oxygen, showing the highest catalytic activity, lowest coke deposition, and superb decoking ability.     

Promoted activity of porous silica coated Ni/CeO2ZrO2 catalyst for steam reforming of acetic acid

Porous silica coated Ni/CeO₂ZrO₂ catalysts were synthesized for steam reforming of acetic acid. The silica coated Ni/CeO₂ZrO₂ catalyst showed a significantly enhanced activity (95% acetic acid conversion) than the Ni/CeO₂ZrO₂ catalysts (62% acetic acid conversion) at a low temperature (550 °C). Interaction between Ni/CeO₂ZrO₂ and silica layer was proved to be a crucial role on enhancing of catalytic activities. Further characterization (XPS, H₂-TPR) indicates this interaction facilitates the steam reforming reaction and raises the selectivity of CO by modifying the surface Ni electronic structure. In addition, the coated catalyst also exhibited a good stability and no obvious deactivation was detected at 550 °C and 600 °C within 30 h.     

Steam reforming of toluene: Impacts of externally added oxygen-containing intermediates on property of coke

Toluene is a typical tar compound resulting from biomass gasification. Toluene reforming could generate abundant carbonaceous intermediates that easily aggregate to form coke. Herein, the co-reforming of toluene with methanol or formic acid is conducted, aiming to understand impacts of oxygen-containing intermediates from dissociation of methanol or formic acid on coking tendency and properties of coke formed. The results indicated that the coke percentage in the used catalyst from 34.7% in toluene reforming to 14.7% with co-addition of formic acid while 22.3% with addition of methanol. This results from presence of abundant oxygen-containing intermediates (CO_x and ¹OH) generated from the dissociation of methanol or formic acid, which effectively gasify the C=C species from toluene. Additionally, dissociation of formic acid generates intermediates bearing the C=O functionality that involve in polymerization, forming amorphous coke of aliphatic nature and high tendency to oxidation, which is opposite to that of co-feeding methanol.     

Hydrogen production by steam reforming of acetic acid: Comparison of conventional supported metal catalysts and metal-incorporated mesoporous smectite-like catalysts

The activities of various metal catalysts were tested in steam reforming of acetic acid for the production of H₂, using conventional metal oxides and transition metal-incorporated mesoporous smectite-like materials as supports. It has been found that Pt is superior to Ni, Co, and Fe among Al₂O₃ supported catalysts, Al₂O₃ is more effective than ZrO₂ and SiO₂ as support for Pt, Ni incorporated smectite (SM(Ni)) support is more effective than Fe and Co incorporated ones for Pt, and SM(Ni) is also active in the absence of Pt. The total activity for the conversion of acetic acid is in the order of Pt/Al₂O₃ > Pt/SM(Ni) > SM(Ni) but the ability of H₂ production is comparable among these catalysts. These catalysts (and the other ones) were observed to lose their activities during the reforming reactions. The activity of Pt/Al₂O₃ decreased during the whole course of reaction up to 10 h. In contrast, the activity of SM(Ni) also decreased within 2 h but it showed a stable activity in the following stage of reaction. The initial activity of the used Pt/SM(Ni) and SM(Ni) was able to be almost completely restored by thermal treatment with H₂ but less effectively for the used Pt/Al₂O₃. The catalyst deactivation was shown to occur by the formation and deposition of carbon materials on the catalysts (XRD, TEM, thermal analysis). The properties of carbon deposits formed on Pt/Al₂O₃ and SM(Ni) catalysts should be different and this may be responsible for the differences in the extent of deactivation and in the regeneration behavior between the two catalysts.     

Production of hydrogen via steam reforming of acetic acid over Ni and Co supported on La2O3 catalyst

Nickel (Ni)-cobalt (Co) supported on lanthanum (III) oxide (La₂O₃) catalyst was prepared via impregnation technique to study the steam reformation of acetic acid for hydrogen generation by using one-step fixed bed reactor. Moreover, in order to specify the physical and the chemical attributes of the catalyst, X-ray diffraction (XRD), nitrogen physisorption, temperature-programmed reduction (TPR), temperature-programmed desorption of ammonia and carbon dioxide (TPD-NH₃ and CO₂), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) methods were employed. The nitrogen physisorption analysis showed that the presence of Co on Ni/La₂O₃ improved the textural properties of the catalyst by increasing the surface area, the pore diameter and the pore volume of the catalyst. This improved the dispersion of metal particle and caused a reduction in the size of metal particle, and consequently, increased the catalytic activity, as well as the resistance to coke formation. On top of that, the condensation and the dehydration reactions during acetic acid steam reforming created carbon deposition on acidic site of the catalyst, which resulted in the deactivation of catalyst and the formation of coke. Besides, in this study, Ni/La₂O₃ contributed to a high acetic acid conversion (100%) at 700 °C, but it produced more coking compared to Ni–Co/La₂O₃ and Co/La₂O₃ catalysts.