Hydrogen production from steam reforming of acetic acid over Ni-Fe/Palygorskite modified with cerium

Abstract

The steam reforming of acetic acid (SRA) was carried out in a fixed-bed tubular reactor with Ni-Fe/ceria-palygorskite (CPG) catalysts. The as-prepared catalysts were analyzed by N2 adsorption-desorption, scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS), H2 temperature programmed reduction (H2-TPR), and X-ray diffraction (XRD). The results of H2-TPR and XRD showed that the addition of CeO2 increased the hydrogen consumption of catalysts and the interaction force between active component (Ni-Fe alloy) and carrier. Moreover, the Ni-Fe alloys were successfully synthesized in the Ni-Fe/CPG catalysts and their crystallite sizes were decreased by adding CeO2. In addition, these catalysts were employed to SRA at 600 °C, GHSV = 14427 h-1 and different molar ratio of S/C. The experimental results revealed that the Ni-Fe/C0.4PG0.6 catalyst can achieve the highest yield of H2 (87%) and HOAc conversion (95%), as well as the highest stability during the process of steam SRA. Additionally, the spent catalysts were characterized by XRD, SEM, and thermogravimetric analysis (TGA). The results showed that the addition of CeO2 enhanced the stability and activity of Ni-Fe/palygorskite catalyst and reduced the coke deposition rate on the catalyst surface.

Full Article

Hydrogen Production from Steam Reforming of Acetic Acid over Ni-Fe/Palygorskite Modified with Cerium

Yishuang Wang,^a,b Mingqiang Chen,^a,b,* Jie Yang,^b Shaomin Liu,^a Zhonglian Yang,^b Jun Wang,^b and Tian Liang ^b

The steam reforming of acetic acid (SRA) was carried out in a fixed-bed tubular reactor with Ni-Fe/ceria-palygorskite (CPG) catalysts. The as-prepared catalysts were analyzed by N₂ adsorption-desorption, scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS), H₂ temperature programmed reduction (H₂-TPR), and X-ray diffraction (XRD). The results of H₂-TPR and XRD showed that the addition of CeO₂ increased the hydrogen consumption of catalysts and the interaction force between active component (Ni-Fe alloy) and carrier. Moreover, the Ni-Fe alloys were successfully synthesized in the Ni-Fe/CPG catalysts and their crystallite sizes were decreased by adding CeO₂. In addition, these catalysts were employed to SRA at 600 °C, GHSV = 14427 h^-1 and different molar ratio of S/C. The experimental results revealed that the Ni-Fe/C_0.4PG_0.6 catalyst can achieve the highest yield of H₂ (87%) and HOAc conversion (95%), as well as the highest stability during the process of steam SRA. Additionally, the spent catalysts were characterized by XRD, SEM, and thermogravimetric analysis (TGA). The results showed that the addition of CeO₂ enhanced the stability and activity of Ni-Fe/palygorskite catalyst and reduced the coke deposition rate on the catalyst surface.

Keywords: Steam reforming; Acetic acid; Hydrogen; Ni-Fe/CPG; Palygorskite; Carbon deposition

Contact information: a: School of Earth Science and Environmental Engineering and b: School of Chemical Engineering, Anhui University of Science and Technology, 232001, Huainan, PR China;

* Corresponding author: mqchen@aust.edu.cn

INTRODUCTION

With declining fossil fuel reserves and growing ecological issues at present, clean and renewable energy sources are very favorably regarded. The utilization of hydrogen energy, which has the advantages of high calorific value and environmental friendliness, already meets the needs of clean energy. Hydrogen is also an important raw material used in ammonia synthesis and petroleum refineries (Kathe et al. 2016). However, most hydrogen is produced from fossil fuels (e.g., coal, naphtha, and natural gas) or light hydrocarbons by reformer technology. This process always releases carbon dioxide into the atmosphere, causing the global warming problems and depleting fossil fuel reserves (Esteban-Díez et al. 2016). To reduce the consumption of these non-renewable resources, hydrogen production via pyrolysis, gasification, and steam reforming of biomass feedstock, which is carbon neutral and renewable, has been extensively studied (Rapagná et al. 2002; Bulushev and Ross 2011; Ma et al. 2014). Wu and co-workers investigated the effects of Ni content in catalysts and biomass components (cellulose, xylan, and lignin) on the hydrogen and syngas production via the steam pyrolysis-gasification technology in the two-stage fixed-bed. They found that hydrogen content was increased from 30.1 to 50.6 vol.% ( the volume fraction in total gas composition) with the increase of Ni loading from 5 to 40 wt.% during the wood sawdust pyrolysis-gasification (Wu et al. 2011). They also revealed the compositions of biomass, gasification temperature, and the addition of steam had significant influence on hydrogen production (Wu et al. 2013). Additionally, the 2D dynamic model of biomass pyrolysis or steam reforming in two-stage fixed-bed reactors had been posed by Olaleye et al. (2014), and it agreed well with the experimental data in predicting the product and hydrogen yields from pyrolysis/steam reforming of different biomass feedstock at different reaction conditions. Although hydrogen production from these technologies is feasible and potential, the tar and particulates accompanying the product gas are always leading to blocking and fouling of process equipment, such as pipelines and turbines (Zhang et al. 2010). Furthermore, producing hydrogen from the above mentioned processes is of low economical value and relatively ineffective due to the high expenditure of collecting and delivering raw biomass feedstock and the low energy density of them (Chattanathan et al. 2012). Therefore, a promising scheme to generate hydrogen is to first carry out pyrolysis/gasification technology that converts biomass to bio-oil (Shen et al. 2015; Kan et al. 2016), which is subsequently reformed with steam for producing hydrogen. The viability of such a process is attributed to the fact that bio-oil is easier to transport and handle and has a higher energy density than biomass feedstock. It is well known that bio-oil is a complex mixture of alcohols, acids, ketones, esters, aldehydes, phenols, sugars, syringols, furans, guaiacols, and multifunction compounds (Trane et al. 2012). These oxygenated organic compounds endow bio-oil with many disadvantages, such as high viscosity, low hydrothermal stability, high corrosivity, and high oxygen content, all of which are important for large-scale utilization (Wang et al. 2013). Steam reforming (SR) can be applied to convert the real bio-oil or each of the bio-oil components to H₂-rich gas. Thermodynamic calculations and experimental results of reforming of model compounds of bio-oil, ethanol (Iulianelli et al. 2016), acetic acid (HOAc), acetone (Li et al. 2012), ethylene glycol (Mei et al. 2016), and phenol (Koike et al. 2015) show that the components in the presence of steam can be easily converted to H₂-rich gas, reaching the maximum H₂ yield (65 to 90%) at different temperatures under ambient pressure and a steam/carbon (S/C) ratio greater than 1.

Hydrogen production has been examined during the steam reforming of acetic acid (SRA), which is a major component in bio-derived compounds and constitutes 12 to 30% of bio-oil (Basagiannis and Verykios 2007; Vagia and Lemonidou 2007; Basagiannis and Verykios 2008; Hu and Lu 2010; Li et al. 2012; Mohanty et al. 2012; Pant et al. 2013; Wang et al. 2015; Esteban-Díez et al. 2016; Wang et al. 2016). The effect of transition metals (e.g., Ni, Co, Fe, Cu, and Zn) and noble metals (e.g., Pd, Pt, Rh, and Ru) and the individual/mixture of the support (e.g., Al₂O₃,CeO₂, La₂O₃, and MgO) on SRA has been abundantly studied. Noble metals, especially Ru, loaded on different carriers have more significant activity than transition metals for SRA. Basagiannis and Verykios (2007) demonstrated that Ru-based and Ni-based catalysts loaded on different metal oxides carriers, such as Al₂O₃, MgO/Al₂O₃, CeO₂/Al₂O₃, and La₂O₃/Al₂O₃, showed high activity and selectivity to H₂. They also revealed that the stability of Ru-based catalysts was higher than that of Ni-based catalysts in long-term SR. Bossola et al. (2016) conducted the experiment of SRA over Mg(Al)O supported Ru-based and Rh-based catalysts, which had very low loading of active metal (0.5 wt.%), at 700 °C and S/C = 3. The Ru catalysts reached 100% conversion of HOAc and hydrogen yield higher than 70%. Although noble metal catalysts are highly efficient for SRA, their expensive price makes them not suitable for industrial manufacture. Therefore, lower-cost transition metal catalysts have been investigated (Basagiannis and Verykios 2006; Hu and Lu 2010; Wang et al. 2012; Zhang et al. 2014; Wang et al 2015, 2016; Pandey and Deo 2016). Among the transition metals, Ni-based catalysts have high-activity in SRA for H₂ production. Hu and Lu (2010) investigated the reaction of SRA for hydrogen production over transition metals, such as Ni, Co, Fe, and Cu supported on Al₂O₃. The Ni/Al₂O₃ and Co/Al₂O₃ catalysts had a significant activity among these catalysts, which was attributed to their high activity for cracking C-C and C-H bonds, but the performance of Ni/Al₂O₃ catalyst was much more stable than that of Co/Al₂O₃ in a long-term reaction. However, monometallic nickel can be sintered easily at high temperature and suffers low anti-carbon deposition performance, resulting in Ni-based catalysts easily becoming deactivated (An et al. 2011). To solve these problems, bimetallic catalysts, such as Ni-Fe (Koike et al. 2015; Wang et al. 2015), Ni-Co (Pant et al. 2013; Yue et al. 2015), and Ni-Cu (Lytkina et al. 2015), have been applied in catalytic reforming for H₂ production and many catalyst carriers with high oxygen storage capacity (e.g., CeO₂ and La₂O₃) (Remiro et al. 2013; Zhang et al. 2014; Osorio-Vargas et al. 2016) have been introduced into Ni-based catalysts.

For the sake of economic efficiency, there are also many low-cost mineral material carriers; perovskite (Ramesh et al. 2015), olivine (Świerczyński et al. 2007), and palygorskite (attapulgite) (Wang et al. 2015, 2016) that have been used in nickel-based catalysts for steam-reforming oxygenated hydrocarbons. Among them, Palygorskite (PG), also called attapulgite, has obtained more and more attention in recent studies, which is mainly because it is a natural nonmetallic mineral with a 1-D fibrous morphology and has a unique porous crystalline structure containing tetrahedral and octahedral layers. In addition, due to its special structure, low cost, and easy availability, PG clay is widely applied in chemicals, surface coating, and catalysis (Li et al. 2013; Laosiripojana et al. 2014; Guo et al. 2015). Cao et al. (2008) studied low-temperature CO oxidation over a series of CuO supported on PG catalysts, which showed interesting catalytic activity compared with the reported TiO₂, Fe₂O₃, and Ce_0.8Zr_0.2O₂ supported CuO catalysts. Notwithstanding PG (as a catalytic carrier) has these unique structure and physicochemical properties, it is rarely applied to SRA. In order to extend the application of PG and optimize the product distribution of SRA, in our recent research (Wang et al. 2016), PG was selected as a catalyst support to prepare different Ni/ATP (attapulgite) catalysts for SRA, which exhibited an outstanding HOAc conversion efficiency (95%) and H₂ yield (82%). Unfortunately, there was some amount of coke deposited on the catalyst surface, and their stabilities were decreased after 4 h of reaction time. This can be attributed to two major drawbacks of metallic nickel supported on attapulgite; such combinations have a high tendency to sinter and to form carbonaceous species, leading to severe deactivation. These undesirable attributes can be minimized by adding other metals/ metallic oxide as promoters to the catalytic system. Among these promoters, as has been reported by Laosiripojana et al. (2014), metallic iron and CeO₂ are available and efficient for Ni-based catalysts. Iron species, as one of the ingredient of active phase, not only can change the redox properties of Ni-based catalysts by forming Fe₂O₃ or Fe₃O₄, but also produce the Ni-Fe alloy upon the interaction between Ni and Fe (Abelló et al. 2013; Bolshak et al. 2013). Additionally, CeO₂ also has promising redox properties and high oxygen vacancy and mobility, which can promote the gasification of carbonaceous species deposited on the catalyst (Perrichon et al. 1994; Zhang et al. 2014; Li et al. 2015). Therefore, the specialties of iron and ceria could restrain the sintering of active metallic nickel and the forming of coke precursor, and then enhance the activity and stability of Ni-based catalyst.

However, these effective and promising facilitators have not been studied in the Ni/PG system. According to our previous study (Wang et al. 2015), it was shown that the Ni-Fe/PG catalyst with the Ni:Fe molar ratio of 1:1 exhibited outstanding activity and H₂ selectivity. In this paper, we focus on the addition of CeO₂ and the effect of Ni-Fe/PG catalyst on the catalytic performance and gaseous product distribution. Firstly, we modified the raw PG with CeO₂ by using mechanical blending method to produce carriers, and then they were recorded as C_xPG_1-x. Subsequently the Ni-Fe bimetallic catalysts loaded on C_xPG_1-x were prepared using a co-precipitation method. Furthermore, the prepared catalysts were applied into SRA at 600 °C, GHSV = 14427 h^-1, and different S/C (1~9). The yield of H₂, the conversion of HOAc and the distribution of gaseous products in the process of SRA over these catalysts were detected. In addition, the effect of CeO₂ on the reactive activity, the stability, and the anti-carbon deposition ability were also investigated.

EXPERIMENTAL

Raw Materials

All precursors and chemicals used for catalyst preparation were of high purity (99.9%) and purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Ni(NO₃)₂·6H₂O and Fe(NO₃)₃·xH₂O were used for preparing the catalysts precursors of nickel and iron. CeO₂ was commercial and Palygorskite was collected from SuZhou of Anhui Province.

Catalysts Preparation

Mechanical blending method for CeO₂/ Palygorskite (CPG) supports

First, 10 g of PG and n g CeO₂ (n = 1.11, 2.22, 4.29, and 6.67, respectively) were dispersed into absolute ethyl alcohol to form turbid liquid. The mixture was placed in an agate jar assembled in an ND7-1L ball grinder and ground for 4 h at a revolving speed of 200 r/min. Filter cakes were collected from the suspension by suction filtration, oven-dried at 105 °C for 12 h, ground into powders, and finally calcined at 550 °C with the heating rate of 2 °C/min for 2 h. A series of CPG carriers were prepared though these procedures, which were represented as C_xPG_1-x (x = 0.1, 0.2, 0.3, 0.4; representing weight percent).

Co-precipitation method for Ni-Fe/ CeO₂/ Palygorskite (NF/ CPG) catalysts

In previous work (Wang et al. 2015), a Ni-Fe bimetallic catalyst, with a molar ratio of Ni:Fe = 1:1, showed outstanding activity for steam reforming of bio-oil and achieved superior yield of H₂. Therefore, the nickel and iron species (1:1 of Ni:Fe molar ratio) were loaded on the series of CPG carriers via co-precipitation. Ni(NO₃)₂·6H₂O and Fe(NO₃)₃·xH₂O as nickel and iron precursors were thoroughly mixed 100 mL of deionized water in a round-bottom flask and then placed in thermostat water bath with magnetic stirring at 80 °C for 2 h. A certain amount (10 g) of CPG was added into the system before being fully stirred at 80 °C for 2 h. Next, 2.5 M ammonia hydroxide was added dropwise into this solution until the pH reached 7.5 ± 0.5 and the precipitate was obtained. The precipitate was filtered, washed with deionized water, dried at 105 °C for 12 h, calcined at 550 °C with the heating rate of 2 °C/min for 2 h, pelletized, and crushed into 80 mesh with a sifter. The prepared fresh catalysts were denoted NF/C_xPG_1-x (x = 0.1, 0.2, 0.3, 0.4).

Catalyst Characterization

The crystal phases of catalysts were identified with a DanDong HaoYuan DX-2800 X-ray Diffractometer (Dandong, China), using Cu-Kα radiation (λ = 1.5406 Å, 40 kV, 30 mA) in the 2θ range of 5 to 70° at a scanning step of 0.03°. The morphologies were observed by Hitachi S-4800 Scanning Electron Microscope (SEM; Hitachi, Japan) coupled with Energy Dispersive Spectroscopy (EDS). The element compositions of all as-prepared catalysts were detected with Inductively Coupled Plasma Optical Emission Spectrum (ICP-OES, Varian, USA). The textual properties of all fresh catalysts were measured by the N₂ adsorption-desorption isotherms at liquid nitrogen temperature (77 K), using an ASAP2020 surface area and porosity analyzer (Norcross, USA). The surface area of catalyst was calculated by the Brunauer-Emmett-Teller (BET) method, the cumulative volume of catalyst pores was calculated from BJH desorption isotherm, and the average pore diameter was calculated from BJH desorption isotherm. Temperature Programmed Reduction (H₂-TPR) was performed using a TianJin PengXaing PX200 chemical adsorption instrument (Tianjin, China), in which the H₂ consumption was determined with a Thermal Conductivity Detector (TCD). A total of 100 mg of catalyst was added into the bed-reactor and reduced with a 5% H₂/Ar (v/v) mixture flowing at 40 mL·min^-1 for each measurement. In this process, the reaction temperature was raised from room temperature to 900 °C at a heating rate of 10 °C·min^-1. Thermogravimetric analysis (TGA) of spent catalysts was performed on the TGA/DSC1 STAR^e System instrument (Mettler-Toledo, Switzerland) under air (50 mL/min) heating from room temperature to 900 °C at a heating rate of 10 °C/min to detect the amount and types of coke deposit.

Experimental Setup and Methods

The SRA was carried out in a fixed-bed tubular reactor under atmospheric pressure at 600 °C. This reactor was made of high-temperature resistance stainless steel, with an inner diameter of 30 mm and an overall length of 300 mm. A porous distributor was placed 200 mm away from the top of the reactor to support the catalytic material. The reaction temperature was monitored and controlled by two thermocouples: one was inserted into the center of the reactor while the other was placed between the furnace and the tube. The flow rates of carrier gas (99.99% N₂) and reducing gas (10% H₂/N₂, v/v) were measured by two glass rotameters. The raw material was introduced into the system by a constant flow pump. The schematic layout of the experimental device is shown in Fig. 1.

Fig. 1. Schematic diagram of experimental device

Prior to every experiment, some amount of catalyst was placed on the porous distributor and activated under the H₂/N₂ (v/v = 10%, flow velocity, 0.32 L·min^-1) atmosphere at 600 °C for 2 h. The gaseous products were collected by airbags and analyzed with gas chromatography (Shanghai Huaai, GC9160) equipped with 5 Å molecular sieve (measures H₂, CH₄, and CO) and Porapak Q column (measures CO₂) by using Thermal Conductivity Detector (TCD) The condensed liquid phases were analyzed by Agilent 6820 60m DB-Wax capillary column. The liquid products were mainly acetone and un-reacted HOAc.

The overall process of SRA is complex, which includes a set of elementary steps involving several organic intermediates and various secondary reactions, according to different catalysts and operating conditions. The thermal decomposition mechanisms of HOAc can be dated back to the year of 1969 and were reported by Blake and Jackson (1969). After that Wang et al. (1996) and Marquevich et al. (1999) studied the involved reactions during the process of SRA. Meanwhile, according to the resent studies (Mohanty et al. 2012; Nogueira et al. 2014; Wang et al. 2015), the favorable reactions can be summarized by R1-R7.

In the process of SRA for hydrogen generation, the theoretical maximum yield of H₂ is 4 mole per mole of acetic acid. On the basis of elemental balance, the conversion of HOAc and the yield of H₂ could be calculated with equations, and the related assumptions on fuel have been reported by Pimenidou et al. (2010).

The molar flow rate of total dry outlet gas (n_out,dry) can be calculated based on nitrogen balance (Eq. 1). In which y is represent the molar flow of i species (such as H₂, CH₄, CO, CO₂, C₂, and C₃) in outlet gas.

(1)

The conversion of HOAc (X_HOAc) can be estimated based on carbon balance, with the total molar flow of carbon in the gaseous products being divided by the molar flow of carbon in feed, as described in Eq. 2.

(2)

The yield of H₂ (Y_H2) is defined as the percentage of experimental H₂ yield in theoretical maximum H₂ yield according to (R3) (Eq. 3).

(3)

In order to remove the effect of N₂ and compare the gas concentration of component, the distribution (f_i, %) of gas product can be obtained by Eq. 4.

(4)

RESULTS AND DISCUSSION

Catalyst Characterizations

The textural properties and chemical compositions of all samples

The textural properties (e.g. the surface area, cumulative volume of pores, and the average pore diameter) of fresh catalysts were measured by N₂ adsorption-desorption, and the calculated values are presented in Table 1. Additionally, the N₂ adsorption-desorption isotherms and pore size distributions of PG, NF/PG and NF/C_0.4PG_0.6 were displayed in Fig. 2. From these information, it can be seen that the series of NF/C_xPG_1-x showed advantageous surface area (125 to 175 m²·g^-1) and regular pore diameter (7 to 10 nm).

Table 1. The Textural Properties of All Fresh Samples

^a Surface area (S_BET) calculated by Brunauer-Emmett-Teller (BET) method; ^b V is cumulative volume of pores calculated from BJH desorption isotherm. ^c d_pore is average pore diameter calculated from BJH desorption isotherm.

Fig. 2. (a) N₂ adsorption-desorption isotherms of PG, NF/PG and NF/C_0.4PG_0.6 samples; (b) pore size distribution of NF/PG and NF/C_0.4PG_0.6 catalysts

The chemical content of catalyst carriers were shown in Table 2, from which it could be seen that PG was be made up of SiO₂, Al₂O₃, MgO, and Fe₂O₃. Notably, the CeO₂ content in C_0.1PG_0.9, C_0.2PG_0.8, C_0.3PG_0.7, and C_0.4PG_0.6 samples was 11.3%, 18.8%, 27.1%, and 38.8%, respectively, they were almost equal to the nominal content of CeO₂.

Table 2. Chemical Composition of Catalyst Supports (wt.%)

In addition, the chemical compositions of all catalysts are presented in Table 3. The contents of NiO and Fe₂O₃ in Ni-Fe bimetallic catalysts were roughly equal to each other. It is worth noting that the content of Fe₂O₃ was higher than that of NiO in one catalyst. In other words, the practical content of Ni:Fe molar ratio was lower than that of nominal mole ratio (Ni:Fe = 1:1). This was caused by the small amount of additional Fe₂O₃ content in PG (Table 1), although the mole proportion of Ni to Fe added into catalyst was 1 during the process of catalyst preparation. In order to further investigate the actual content of Ni and Fe in every catalyst, the catalysts were analyzed by ICP-OES and the result was shown in Table 4. It also testified the actual content of Ni:Fe was little lower than that of nominal mole ratio 1.

Table 3. Chemical Composition of All Fresh Catalysts (wt.%)

Table 4. Element Composition of Calcined Catalysts Obtained by ICP-OES

Scanning electron microscopy (SEM)

The SEM micrographs of raw PG and fresh prepared NF/C_0.4PG_0.6 are shown in Figs. 3a and 3b. Raw PG was fibrous or clubbed structure in its surface, every fiber or rod was smooth, and there were some small ravines present among different ribbons and interlaminations. Thus, the unique structure of PG could supply a high specific surface area (145 m²·g^-1, as shown in Table 1) (Guo et al. 2015). Figure 3b shows that the surface of NF/C_0.4PG_0.6 sample was relatively rougher than the raw PG. There were many spherical rough particles adhered on the surface of PG, and they were attributed to nickel-iron and CeO₂ particles were located among different ribbons and interlaminations. In addition, the form of spherical rough particles was ascribed to the synthetic interaction among nickel-iron oxide, cerium dioxide, and PG particles. This situation could cause the catalyst specific area to decrease from 145 m²·g^-1 to 125 m²·g^-1 and pore volume to reduce from 0.444 cm³·g^-1 to 0.228 cm³·g^-1, as shown in Table 1.

Fig. 3. SEM images of catalysts: (a) raw PG; (b) NF/C_0.4PG_0.6

Temperature programmed reduction (H₂-TPR)

The reducibility of supports and catalysts was analyzed with H₂-TPR, and the results were presented in Fig. 4. There are two reduction peaks related to CeO₂, which have been described in the literature (Yao et al. 1984; Perrichon et al. 1994). The one located at 570 °C (peak ) was attributed to the reduction of the CeO₂ surface oxygen; the other peak at 890 °C (peak ) was associated with the reduction of bulk part of CeO₂, which emerged through removing the reactive oxygen species (O^2-) from the reticulum and forming the Ce₂O₃ (2CeO₂ + H₂ → Ce₂O₃ + H₂O). Obviously, there was no significant peak of PG support, as shown in Fig. 4a, which demonstrated that PG could not be easily reduced. It also can be seen from Fig. 4a, the center temperature of the two peaks and , which belong to the reduction peaks of CeO₂, was removed to a lower temperature than those described in the literature. Furthermore, the intensities of peaks and of C_xPG_1-x supports (Fig. 4a) tended to increase with increasing CeO₂, which indicated that the addition of CeO₂ increased the hydrogen consumption and surface oxygen of catalyst carriers. This point will be further certified by TG-DSC analysis for spent catalyst in a later section.

Additionally, there were two apparent peaks (α and γ) that emerged in calcined catalysts (Fig. 4b). In the sample of NF/PG, the peak α was a broad overlapping peak within the temperature range of 350 °C to 700 °C, which represented the reduction of Ni-Fe oxides that had weak interaction with carriers to form Ni-Fe alloy. This result was also demonstrated by XRD of reduced catalysts (Fig. 5b). After the insertion of CeO₂, the center temperature of reduction peak α was slightly increased, and the temperature range was slightly constricted with the increased CeO₂. This phenomenon was ascribed to addition of CeO₂ into NF/PG catalyst enhanced the interaction of nickel-iron alloys and improved the coactions among different catalyst components of the series of NF/CPG catalysts. The other peak γ located at 650 °C was attributed to the reduction of Ni-Fe oxides that had strong interaction with carriers to produce Ni-Fe alloy. In addition, the center temperature of peak γ was increased with the addition of CeO₂. This finding further demonstrated that CeO₂ could promote the interaction between active metal and carrier and increase the catalyst stability, which had been demonstrated in the stability test section. Obviously, the characteristic peaks of PG at 2 = 50.06° and 59.83° gradually disappeared with the increase of CeO₂, and the intensities of characteristic peaks at 2 = 20.33° and 26.66° were weakened. This result revealed that the crystallization of PG changed slightly after inserting CeO₂ into its structural framework.

Fig. 4. H₂-TPR profiles: (a) the catalyst carriers and (b) the calcined catalysts

X-ray diffraction analysis

Figure 5 shows the XRD patterns of all fresh catalysts and reduced catalysts after the H₂ reduction at 600 °C for 2 h. As shown in Fig. 5a, the peaks at 2 = 20.33° and 26.66° were the typical characteristic peaks of PG (JCPDS 31-0783), and the peaks 50.06° and 59.83° were attributed to dolomite (JCPDS 99-0046) (Wang et al. 2016). Their intensities were decreased and even disappeared with the addition of CeO₂. This may have been due to the interaction between PG and CeO₂ (You et al. 2010). The diffraction peaks at 2 = 28.55°, 33.08°, 47.51°, 56.36°, 59.01°, and 69.44° were well defined and belonged to CeO₂ (JCPDS 81-0792) with a fluorite structure presented in the catalysts.

In NF/PG, the characteristic diffraction peaks of PG were not changed, and the characteristic diffraction peaks of nickel oxide and iron oxide or their synthesis could not be detected, as shown in Fig. 4a. When the catalysts were reduced with H₂ at 600 °C for 2 h, the characteristic diffraction peaks of PG and CeO₂ were not significantly changed (Fig. 4b), which was mainly because CPG supports were hard to reduce at 600 °C. This result was confirmed by H₂-TPR (Fig. 3). This further demonstrated the interaction between CeO₂ and PG. However, when Ni and Fe species were introduced, a broad and low-intensity diffraction peak at 2 = 44.01° was observed, as shown in Fig. 4b. In previous studies, Laosiripojana et al. (2014) and Pandey and Deo (2016) developed the Ni-Fe bimetallic based catalysts supported by different carriers for reforming biomass tar and catalyzing CO₂ methanation reaction. During analyzing the as-prepared catalysts by XRD, they also found the diffraction peaks at ~44° in reduced catalysts and established that they belong to the characteristic peak of Ni-Fe (111) alloy. Therefore, the peaks at 44.01° in reduced NF/PG and NF/C_xPG_1-x were attributed to the Ni-Fe alloys with crystal face (111). These results indicated that the absence of nickel and iron diffraction peaks before reduction (Fig. 5a) could be due to the high dispersion of nickel oxide and iron oxide particles on the surface of CPG supports or to the formation of nickel and iron amorphous crystallization, which could not be detected with XRD. In addition, as shown in Table 5, the Ni-Fe alloy particle size, which was calculated by the Scherrer equation based on the (111) lattice plane parameters in XRD analysis, decreased from 15.5 nm (NF/PG,) to 8.9 nm (NF/C_0.4PG_0.6,) with the addition of CeO₂. The variation further revealed that CeO₂ addition enhanced the dispersion of catalyst active phases (Ni-Fe alloy) on the carrier. Previous studies uncovered that Ni-Fe alloys are successfully synthesized on the surface of catalyst support using nickel and iron as raw materials because nickel and iron atoms have a similar electronic orbit structure and metallic magnetism (Świerczyński et al. 2007; Wang et al. 2011; Pandey and Deo 2016). Therefore, the emerged broad diffraction peaks at 2 = 44.01° in these synthetic catalysts were due to the formation of Ni-Fe alloy phases.

Fig. 5. XRD patterns of catalysts: (a) calcined catalysts; (b) reduced catalysts

Table 5. The Calculated Values of Ni-Fe Alloy Size at 2θ = 44.01° in Reduced and Spent Catalysts by Scherrer Equation from XRD Data

Catalytic Activity Tests

Activities of all catalysts

The operation of SRA was performed under the following conditions: temperature at 600 °C, S/C = 3, GHSV = 14427 h^-1 , and time on steam for 4 h. Figures 6a and 6b show the time-dependent variations of X_HOAc and Y_H2 for all catalysts during SRA, and Fig. 6c shows the distribution of gaseous products (H₂, CO, CO₂, and CH₄). The X_HOAc and Y_H2 resulting from the reaction in the presence of NF/PG catalyst were approximately 80% and 52%, respectively, which were remarkably higher than those in the blank test using PG as a catalyst (approximately 50% and 5%, respectively). The concentrations of CH₄ and CO₂ (Fig. 6c) reached the highest levels, while the concentration of H₂ was at its lowest value in the PG sample. Figure 6c also shows that the concentrations of H₂ and CO increased to 45% and 32% over the NF/PG catalyst. This was because PG had no obvious activity for SRA, and the temperature was the only major active factor in the situation, in which the thermal decomposition of HOAc (R4) and ketonization (R5) became predominant under this temperature, as these reactions were endothermic.

Fig. 6. Effect of different catalysts on (a) the conversion of HOAc and (b) the H₂ yield with time on steam; (c) the production distribution over all catalysts during the process of SRA

In addition, the X_HOAc and Y_H2 increased markedly as nickel-iron species were introduced into PG accompanied with the increases of H₂ and CO concentrations. This result reflected that nickel and iron species accelerated the decomposition of HOAc and facilitated the steam-reforming reaction of HOAc and intermediate species, such as acetone and CH₄ (R3, R8-R10) (Laosiripojana et al. 2014); Ni has a strong ability for breaking C-C bonds, and Fe could promote a steam-reforming reaction of intermediates. When different amounts of CeO₂ were added into the NF/PG catalyst, the X_HOAc increased from 80% to 95%, accompanied by an increase in H₂ yield from 52% to 87% (Figs. 6a, b). The concentrations of CH₄ and CO noticeably decreased (Fig. 6c). This phenomenon was attributed to the superior properties of CeO₂ in terms of its high oxygen storage capacity and the high oxygen mobility (as proven by H₂-TPR), which improved carbon gasification (R12) on the catalyst surface (Zhang et al. 2012), and, thus, improved the methane decomposition (R11) and the gasification of carbon deposition.

The effects of S/C on HOAc conversion and gaseous product distributions

The influence of the S/C molar ratios ranging from 1 to 9 on the SRA was investigated at atmospheric and at 600 °C. Table 6 shows the X_HOAc, Y_H2, as well as the gaseous products concentrations as a function of S/C. At S/C = 1, the X_HOAc and Y_H2 were rather low, and furthermore, there were significant amounts of CO and CH₄. Apparently, higher S/C molar ratios increased the X_HOAc, Y_H2, as well as the distribution of H₂, while they decreased the CO and CH₄ concentrations. The concentration of CO₂ increased slightly with the S/C. Esteban-Díez et al. (2016) and Noor et al. (2014) had demonstrated that low S/C molar ratios reduced the methane SR (R9, R10) and WGSR (R2), while they enhanced methanation (R6, R7) simultaneously. Consequently the CH₄ concentration increased as the S/C molar ratio decreased. Therefore, the CH₄ content was a key factor for determining the H₂ selectivity during the SRA process. Hu and Lu (2009) and co-workers pointed out that the high partial pressure of steam at high S/C could promote the steam adsorption on the active sites of catalysts, and consequently inhibited the feedstock degradation and prevented caking. In addition, the high steam pressure was also beneficial for WGSR to reduce the CO content, leading to higher H₂ concentration. Kechagiopoulos et al. (2006) had indicated that an excess steam (beyond the stoichiometric limit) was favorable for the suppression of carbon deposition during SR of bio-oil because it promoted the partial gasification of carbon formed, consequently enhancing the conversion of HOAc and intermediates. The results of the present work showed that the catalyst activity in SRA was high with the S/C molar ratio changing from 6 to 9, as in these conditions the Y_H2 and H₂ content showed high values together with low CH₄ and CO contents.

Table 6. Influence of S/C on HOAc Conversion and Gaseous Products Distributions *

* –T= 600 °C; GHSV = 14427 h^-1; atmospheric pressure; time on steam for 4 h

Characterizations of Spent Catalysts

In order to investigate the carbon deposition resistance and sintering extent of these catalysts under harsh operating conditions, the catalysts were tested at low S/C molar ratio (S/C = 1) and 600 °C, GHSV = 14427 h^-1 for 4 h steam reforming. After reaction, these spent catalysts were analyzed by XRD, SEM, as well as TG-DSC.

Figure 7 shows the XRD diffraction patterns of all spent catalysts. The diffraction peaks at approximately 2θ = 27° in samples PG and NF/PG belong to the carbon with graphitic structure on these catalysts (Koo et al. 2008; Therdthianwong et al. 2008). The peaks disappeared after the addition of CeO₂, which was ascribed to the interaction between CeO₂ and PG, promoting the migration of oxygen ions from the interior of CeO₂ to its surface, leading to the gasification of carbon deposition. In addition, the peaks related to the PG and CeO₂ in the XRD patterns were consistent with those found in the XRD patterns before reaction (Fig. 5b). This was explained by the CeO₂ restraining the sintering of active metals during SRA. Furthermore, the diffraction peak located at 2θ = 44.01° was attributed to the Ni-Fe alloy (111) plane and was consistent with the XRD patterns before the reaction (Fig. 5b). Based on the (111) plane of the formed Ni-Fe alloy, the estimated crystallite sizes are shown in Table 5. It was observed that crystallite sizes of formed Ni-Fe alloy were 15.4 nm and 26.3 nm in reduced and spent NF/PG samples, respectively, while they were 8.9 nm and 10.0 nm in reduced and spent NF/C_0.4PG_0.6 samples, respectively. The Ni-Fe alloy in NF/PG underwent significant sintering during SRA, while the Ni-Fe alloy sintering was suppressed by CeO₂ addition NF/C_0.4PG_0.6. It was worth noting that the peaks at 2θ = 51.4° (Fig. 7) presented in all spent catalysts and they were attributed to the characteristic peak of Ni-Fe alloy at (200) lattice plane (Xie et al. 2017), while they were unobserved in the after reduced catalysts in Fig. 5b.

Fig. 7. XRD diffraction patterns of all catalysts after reaction for 4 h at 600 ℃ with S/C = 3 and GHSV = 14427 h^-1

Theofanidis and colleagues (2015) designed the carbon-resistant dry reforming Fe-Ni catalyst. After the in-situ XRD analysis for reduced sample, they found the Ni-Fe alloy at 2θ = 44° and 51.4°, which was attributed to the (111) and (200) lattice planes, respectively. Kang et al. (2016) had also demonstrated the (111) and (200) lattice planes of Ni-Fe alloy emerged in bimetallic Ni-Fe/Al₂O₃ catalysts after the H₂ pre-treatment, which were applied into propane steam reforming for hydrogen production. Therefore, there was no apparent Ni-Fe alloy peak at (200) lattice plane in the present work (Fig. 5b), because Ni-Fe species were highly dispersed on the PG surface and there was strong interaction with the PG. This was mainly due to the high surface area of PG which enhanced the dispersion of active metal, and then, resulted in the Ni-Fe oxides which were difficult to completely reduce. Additionally, the CeO₂ addition also enhanced the interaction between nickel-iron species and carrier, as demonstrated by H₂-TPR (Fig. 4b), leading to no obvious diffraction peak of Ni-Fe alloy of (200) plane in reduced catalysts.

For the purpose of investigating and comparing the morphology of spent catalysts, the SEM images of fresh and spent catalysts are presented in Fig. 8. Comparison of Figs. 8a and 8c shows that there was a large area of caking and metallic sintering on the surface of the NF/PG sample. The metallic sintering had also been indicated by XRD analysis, and the results are presented in Fig. 5, Fig. 7, as well as Table 5. Consequently the active sites and the filamentous basic structure of PG were packaged by some amount of amorphous carbon, but the filamentous fundamental structure of NF/C_0.4PG_0.6 was clearly shown in Fig. 8d. Thus, the formation of carbon deposition and the caking of catalysts were inhibited by adding CeO₂ (Chiou et al. 2014; Vicente et al. 2014) because it was well known that CeO₂ has unique redox properties and high oxygen vacancy and mobility, which can promote the gasification of carbonaceous species deposited on a catalyst (Perrichon et al. 1994; Zhang et al. 2014; Li et al. 2015). Figures 8c and 8d also showed that some spherical particles were attached on the surface of catalysts after reaction, wherein the particles attached on the surface of NF/C_0.4PG_0.6 catalyst were in a more uniform distribution than those attached on NF/PG. These particles were attributed to the aggregation of Ni-Fe alloy particles. This result was consistent with previous studies (Tian et al. 2013) and was confirmed by the XRD diffraction patterns in Fig. 7.

Fig. 8. SEM images of catalysts: (a) fresh NF/PG; (b) fresh NF/C_0.4PG_0.6; (c) spent NF/PG; and (d) spent NF/C_0.4PG_0.6

Metallic nickel is known to be an active catalyst component for SR of hydrocarbons and oxygenates to hydrogen or synthesis gas. Moreover, alloying of Ni with other metals can enhance the catalytic activity and stability, and the iron is a suitable modifier. In addition, the smaller and more uniform Ni-Fe alloy nanoparticles are needed at even high temperature during SR process (Koike et al. 2015). Consequently, the catalytic activity sequence: NF/C_0.4PG_0.6 > NF/C_0.3PG_0.7 > NF/C_0.2PG_0.8> NF/C_0.1PG_0.9 > NF/PG (Fig. 6a) during 4 h on steam was attributed to the Ni-Fe alloy crystallite size sequence: NF/C_0.4PG_0.6 < NF/C_0.3PG_0.7 < NF/C_0.2PG_0.8 < NF/C_0.1PG_0.9 < NF/PG (Table 5). Moreover, the activity of every catalyst after 60 min on steam was higher than that in 0 ~ 60 min, as shown in Fig. 6a. This was due to the formation of Ni-Fe alloy with (200) plane (Fig. 7), that increased the active sites of the catalyst. The Ni-Fe alloy (200) came from those unreduced nickel-iron oxides during the active process by reducing gas (as mentioned above) and were further reduced by H₂ in gaseous product.

From the results of XRD and SEM, there was some amount of coke deposited on the surface of spent catalysts. According to Perez-Lopez et al. (2006) and Luo et al. (2016), the combustion of carbon deposition started at a temperature above 400 °C. So the spent catalysts were analyzed by TGA analysis, in which they were heated in air from room temperature to 900 °C with a ramp of 10 °C/min, to determine the quantity and structure of coke deposited on the catalysts (Nogueira et al. 2014). The TG-DSC profiles and the carbon deposition rate are presented in Fig. 9. From the inset map in Fig. 9, the blank test over PG experienced a severe carbon deposited rate of 90.5 mg/(g_cat.·h), while the carbon deposition rate on NF/PG and NF/_0.4PG_0.6 were 12.5 mg/(g_cat.·h) and 9.6 mg/(g_cat.·h) respectively. The lower carbon deposition rate for used NF/_0.4PG_0.6 was mainly because the coke deposited on the catalyst surface was gasified by the surface oxygen of catalyst, which came from the interior of CeO₂. Since CeO₂ was a superior oxygen carrier and increased the catalyst surface oxygen (as proven by H₂-TPR in Fig. 4). The found carbon deposition rates of NF/PG and NF/_0.4PG_0.6 were better than that reported by our previous research (Wang et al. 2016), in which 25.9 mg/(g_cat.·h) coke rate was formed on IM-Ni/ATC at 650 °C. Moreover, the carbon deposition amount (3.8%) of NF/_0.4PG_0.6 catalyst was lower than the coke amount (9.6%) of spent Ni-Co/La₂O₃ catalyst was researched by Nabgan et al. (2016) during the process of SRA. As far as anyone knows, TG-DSC analysis is a complementary technique for estimating the structural order of carbon deposits, because the more ordered carbon structure needs the higher temperature for gasification (Chen et al. 2010; Nogueria et al. 2014). Fig. 9 shows the DSC profiles of spent catalysts, where it can be seen that carbon deposition oxidation for PG started from 100 °C to 750 °C and the peaks appeared at about 598 °C and 707 °C. According to Nogueria et al. (2014), the peak at 220 to 350 °C was assigned to the thermal desorption of absorbed species (H₂O and CO₂) on spent sample and the oxidation of carbonaceous species with low ordered structure like an amorphous type, and the peak at above 500 °C was attributed to the removal of the graphitic or filamentous carbon. Therefore, the exothermic peaks of spent catalysts in this study (Fig. 9) belong to the graphitic or filamentous carbon. However, there were no peaks assigned to the oxidation of amorphous at low temperature. This is because the spent samples had been pre-treated in air by heating to 300 °C to exclude the absorbed species (H₂O and CO₂) affecting the carbon deposition rate. Ultimately, the CeO₂ added into NF/PG not only decreased the carbon deposition rate from 12.5 mg/(g_cat.·h) for NF/PG to 9.6 mg/(g_cat.·h) for NF/_0.4PG_0.6, but also partially removed the the graphitic or filamentous structural carbon (also be demonstrated by XRD). Curiously, the exothermic peak of NF/_0.4PG_0.6 at 488 °C, which was higher than the oxidation temperature of amorphous type carbon (300 °C) and lower than that of graphitic or filamentous carbon (500 °C). It can be attributed to the carbide with the moderate ordered structure. In other word, this structural carbon was higher ordered carbon compared with amorphous type carbon and a little lower than that of graphitic or filamentous carbon (An et al. 2011). Additionally, it had no influence on the activity and stability of catalyst.

Fig. 9. TGA and DSC profiles of the spent PG, NF/PG, as well as NF/C_0.4PG_0.6.

Stability Tests of NF/PG and NF/C_0.4PG_0.6

Developing highly stable catalysts is one of the major factors in SR reactions. It was found that NF/C_0.4PG_0.6 showed the best performance at 600 °C, S/C of 3, and GHSV = 14427 h^-1 for 20 h. Moreover, the NF/PG was selected as a counterpart to provide better insight into the catalytic behaviors of NF/C_0.4PG_0.6. Results are shown in Fig. 10. The NF/C_0.4PG_0.6 presented superior stability during the entire time on steam. The H₂ yield and conversion was maintained at about 85% and 95% for 20 h on steam, as shown in Fig. 10b. Conversely, the NF/PG catalyst deactivated significantly after 10 h on steam. The X_HOAc and Y_H2 decreased from about 80% and 55% at the initial test to about 55% and 18 % at the end of the test, respectively (Fig. 10a).

Fig. 10. Stability tests of NF/PG & NF/C_0.4PG_0.6 catalysts: T = 600 °C; S/C = 3; GHSV = 14427 h^-1.

DISCUSSION

The catalysts employed by SR technology are invariably subject to serious deactivation mechanisms including coking, sintering of the active phase, and oxidation of the metallic phase, resulting in the decrease of the active sites on the surface of catalyst and lowering of the catalytic performance to convert carbonaceous compound to C₁ product and hydrogen (Hu and Lu 2010). From the results of XRD and TG-DSC characterizations for reduced and spent catalysts, the CeO₂ not only decreased the crystallite size of Ni-Fe alloy, but also suppressed the carbon deposition rate of catalyst. In addition, the H₂-TPR results further showed the interaction between active phase (Ni-Fe alloy) and CPG support increased by the addition of CeO₂. Consequently, NF/C_0.4PG_0.6 catalyst showed outstanding catalytic activity and stability compared with NF/PG catalyst. In this study, the highest values of X_HOAc (95%) and Y_H2 (85%) were obtained over NF/C_0.4PG_0.6 at 600 °C, S/C of 3, and GHSV = 14427 h^-1. Mohanty et al. (2012) designed the Cu-Zn/Ca-Al catalyst for SRA and achieved 81% of X_HOAc and 80% of Y_H2 at 800 °C, whereas the results were lower than those found in this study. In addition, the 100% of X_HOAc and 73% of Y_H2 were realized by Bossola and co-workers (2016) on Ru supported on Mg-Al oxides at 700 °C, while the H₂ yield was a little lower than that in this paper. However, the carbon deposition rate in their report (1.9 to 4.9 mg/(g_cat.·h)) was smaller compared with the corresponding result on NF/C_0.4PG_0.6 catalyst (9.6 mg/(g_cat.·h)), because noble metal was always higher than transition metal in terms of anti-carbon deposition. Among the series of Ni/γ-Al₂O₃ with different Ni loading prepared by An et al. (2011), 12 wt% Ni/γ-Al₂O₃ showed the best catalytic performance and obtained the highest X_HOAc (49%) and Y_H2 (29%) at 600 °C. In the Ni-Fe bimetallic catalysts, the 10Ni-Fe/ZDA catalyst achieved 100% of X_HOAc and 89.6% of Y_H2 at 700 °C respectively, which was reported by Wang et al. (2015) in SRA, whereas they were all slightly decreased after 11 h on steam. Obviously, NF/C_0.4PG_0.6 catalyst had superior catalytic performance among the previous designed steam reforming catalysts and potential value for the industrialization of HOAc steam reforming for hydrogen production.

CONCLUSIONS

1. The results from H₂ temperature programmed reduction (H₂-TPR) and X-ray diffraction (XRD) analyses showed that the addition of CeO₂ increased the hydrogen consumption of catalysts and the interaction force between active component (Ni-Fe alloy) and carrier. Moreover, the Ni-Fe alloys were successfully synthesized in the Ni-Fe/CPG catalysts and their crystallite sizes were decreased by adding CeO₂.
2. The catalyst Ni-Fe/C_0.4PG_0.6 achieved the highest HOAc conversion (95%), highest H₂ yield (87%), and highest H₂ concentration (63%) in gaseous products and enjoyed the most excellent catalytic stability compared with other catalysts in the SRA reaction.
3. The addition of CeO₂ enhanced the stability and activity of Ni-Fe/PG catalysts and decreased the coke deposition rate on the catalyst surface.

ACKNOWLEDGEMENTS

The authors thank the National Natural Science Foundation of China (21376007) and National Science and Technology Support Project of China (2014BAD02B03) for the financial support.

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Article submitted: November 23, 2016; Peer review completed: January 28, 2017; Revised version received and accepted: March 29, 2017; Published: May 16, 2017.

DOI: 10.15376/biores.12.3.4830-4853