CuZnAl-oxides nanocatalysts

Applied Energy 254 (2019) 113022

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Experimental investigation of steam reforming of methanol over La2CuO4/ CuZnAl-oxides nanocatalysts ⁎

Yidian Zhanga,b, Shaopeng Guoa,c, Zhenyu Tiana,b, Yawen Zhaod, , Yong Haoa,b,

T

⁎⁎

a

Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China c School of Energy and Environment, Inner Mongolia University of Science and Technology, Baotou 014010, PR China d China Resources Power Holdings Co., Ltd. (CR Power), Shenzhen 518000, PR China b

H I GH L IG H T S

design method using synergistic effects among components was developed. • AA catalyst of favorable catalysts were designed using new method. • Thegroup catalytic performance was analyzed and improvement was disclosed. • A PVTC system was developed to validate the benefits of the catalysts in application. •

A R T I C LE I N FO

A B S T R A C T

Keywords: Steam reforming of methanol Nanocatalyst Perovskite-like Compound metal oxides Hybrid power generation system

Nanocatalysts of compound metal oxides (La2CuO4)x(CNZ-1)1−x (x = 0.3, 0.5, 0.7) were prepared. Steam reforming of methanol (SRM) over these nanocatalysts was experimentally studied at a H2O/methanol molar ratio of 1.6. The results showed that the methanol solution catalyzed by all catalysts synthesized in this work could be completely converted into H2, CO2 and a small amount of CO below a reaction temperature of 270 °C with a liquid hourly space velocity (LHSV) of 1.2 ml/(g·h). The catalysts of La2CuO4 and CuO/ZnO/Al2O3 were tested under the same operating conditions. Compared with La2CuO4, LCOx-CNZ showed better performance with a higher methanol conversion rate and H2 yield. Conversely LCO5-CNZ had better CO and H2 selectivity compared with CuO/ZnO/Al2O3. LCO3-CNZ showed good competitiveness in all four above aspects when operated at 150–270 °C. It could be concluded that LCOx-CNZ with special structures provided a significant improvement in catalytic performance of SRM benefiting from the synergistic effect among La2CuO4 and CuZnAl oxides. Thermodynamics analysis and experiments using a hybrid power generation system were applied with the above catalysts. Under direct normal irradiation at 915 W/m2 and a reaction temperature of 230 °C, LCO3-CNZ showed 9.7% higher H2 yield and 3.9% higher net solar power generation efficiency than did Cu/Zn/Al oxides.

1. Introduction In recent years, hydrogen has been considered one of the most ideal substitutions for traditional fossil fuels in future sustainable energy system because of its high energy density and lack of pollution. However, the major impediments in using hydrogen as an energy source are the relatively high energy consumption in production and the difficulty and high cost in its storage and transport [1]. As a liquid fuel that is easy to store and that has low vaporization and reforming temperatures, methanol offers a reliable solution for hydrogen

⁎

generation. Additionally, it can conveniently utilize alternative energy (such as solar energy) to provide the heat needed in the process of producing hydrogen, for example, by using thermochemical methods. There are three main ways to convert methanol into hydrogen-rich gas, which are the decomposition of methanol (DM), the steam reforming of methanol (SRM) and the partial oxidative methanol (POM) reforming reaction, which are shown as follows (Eqs. (1)–(3)) [2–5]:

CH3 OH → 2H2 + CO

θ ΔH298 = 22.4 kJ mol−1

CH3 OH + H2 O→ 3H2 + CO2

θ ΔH298

= 49.4 kJ mol−1

Corresponding author. Corresponding author at: Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, PR China. E-mail addresses: [email protected] (Y. Zhao), [email protected] (Y. Hao).

⁎⁎

https://doi.org/10.1016/j.apenergy.2019.04.018 Received 23 November 2018; Received in revised form 27 February 2019; Accepted 7 April 2019 Available online 21 August 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.

(1) (2)

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ηnet net solar power generation efficiency LHVi low calorific value of component i, kJ/mol XMeOH conversion rate of methanol, % Fi flow rate of component i in the gaseous effluents LCOx-CNZ La2CuO4/CuO/ZnO/Al2O3 samples ̇ absorbed solar heat rate of the heater Qabs Q̇ loss heat loss rate of the upper surface of the heater Slen area of Fresnel lens, m2 K incident angle modifier τ transmissivity of the lens T′ Temperature of the heater-receiving surface, K T0 Temperature of the surroundings, K hrea molar reaction enthalpy, kJ/mol molar enthalpy of the outlet flow of the heater hout Tin/Tout temperature of inlet/outlet flow of the heater, K Δhvap latent heat of vaporization, kJ/mol PPV power generation of PV cells ηfc power generation efficiency of fuel cell, Sm selectivity of product m in reaction

Nomenclature LHSV PEMFC ̇ Qgain I ηopt γ α A T h ηconv ṅin,tot h in cp,l/cp,v Tb

liquid hourly space velocity, ml/(g·h) proton-exchange membrane fuel cells enthalpy change rate of the heater direct normal irradiation, W/m2 optical efficiency of the lens and the heater intercept factor absorptivity of the selective absorption coating of the heater-receiving surface area of the heater-receiving surface, m2 temperature of the reaction, K convective heat transfer coefficient between the air and the receiving-surface of the heater, W/(m2·K) conversion rate of methanol total molar inlet flow rate, mol/min molar enthalpy of the inlet flow of the heater molar heat capacity at constant pressure of liquid/gaseous methanol boiling point of the methanol solution, K

CH3 OH + 0.5O2 → 2H2 + CO2

θ ΔH298 = −192.2 kJ mol−1

further increase the catalytic activity [21]. For example, Jiang et al. [22] found that the partial replacement of Ni3+ by Cu2+ improved the reactivity of LaNiO3. Oxygen vacancies were generated as a result of the reduction of positive charge and, therefore, promoted the performance of the catalyst. Additionally, the electronic imbalance generated by the structural disorder and oxygen vacancies may result in the valence variation of a part of metal ions, indicating an effect on the performance of the perovskite. For example, Tejuca et al. [23,24] indicated that the perovskite compound had great catalytic potential due to its variety and good stability with mixed oxidation states. In the same period of perovskite explorations, the perovskite-like compound was also studied. Longo et al. [25] found that the alternate layering of perovskite (ABX3) and rock-salt (AX) made it possible for perovskite-like compounds to process a higher tolerance for vacancies, disorder and A-site or B-site cation replacement by other metal ions. Furthermore, it has been reported that the high dispersion of active metal sites reduced from perovskite [15,26–30] and perovskite-like compounds [10,31] may benefit both methanol synthesis and SRM, showing great potential for catalysis. Therefore, perovskite and perovskite-like oxides are worth considering for catalytic reactions like SRM. However, the high pre-treatment temperature of perovskite over 873 K usually leads to sintering of the reduced metal elements, which may cause deactivation of the catalysts [32–34]. Additionally, the perovskite and perovskite-like compounds also have disadvantages in their thermal and chemical stability [10]. One way to promote the performance of material is utilizing the synergistic effects among different components. Zhang et al. [35] claimed that the synergistic reaction between Al2O3 and HZSM-5 may cause an improvement of bifunctional Al2O3/HZSM-5 catalysts in catalytic performance for the CO2 desorption. The specific ionic groups of AlO2− and MEACOO− were supposed to account for this phenomenon, which might participate in different steps of the reaction. Efforts were also made in terms of perovskite. For example, Lee et al. [36] promoted the CO2 reduction and CH4 selectivity when converting CO2 into CH4 by loading Ni and Pt on a perovskite CaTiO3 support; Li et al. [37] elaborated the synergy between carbon and the perovskite oxide LaMnO3, which was responsible for the remarkable improvement of oxygen reduction reaction (ORR) activity. In this paper, a group of synergistic SRM nanocatalysts consisting of perovskite-like La2CuO4 and CuZnAl oxides were originally proposed and synthesized, which integrated the advantages of CuZnAl oxides and perovskite-like catalysts. Catalyst characterization methods such as Xray diffraction (XRD), field emission scanning electron microscope

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Among these reactions, SRM is the most suitable for supplying fuel for highly efficient proton-exchange membrane fuel cells (PEMFC) because it has a higher H2/CO2 ratio and produces less CO which will deactivate the anodic catalyst of fuel cells. The relative activity of the anodic platinum catalyst for100 ppm CO is approximately 35% higher than that for 20 ppm CO at 80 °C [6]. Therefore, SRM has received wide attention, since the production of electricity by fuel cells is seen as a promising environmentally friendly technology and PEMFCs are suitable for small-scale generation and transport applications [7,8]. As tangential reactions occur along with SRM, other by-products (methane, formaldehyde, methyl formate and dimethyl ether) also present [9]. Therefore, improved performance of the catalysts is urgently needed to improve the H2 yields and reduce CO selectivity. Generally, copper-containing catalysts are most commonly used for the SRM reaction. These catalysts, especially CuO/ZnO or CuO/ZnO/ Al2O3, are typically used because of their relatively favorable catalytic activity and selectivity [3,10]. However, several problems persist, such as the high equilibrium temperature required to convert methanol completely into hydrogen-rich gas, thermal sintering and increases in the CO yield along with the temperature [11]. Additionally, the pyrophoric issue of copper-containing catalysts remains to be resolved [12–14]. Other commonly used catalysts, group 8–10 catalysts, have also been widely studied in the literature because of their highly stable selectivity. However, they introduce the disadvantage of low catalytic activity. It is now critical to develop a catalyst with good performance for SRM to satisfy the need to produce H2 with low CO ratio. To circumvent the above issues, perovskite (ABX3) and perovskitelike (AB2X4) compounds have been widely investigated by researchers regarding their special catalytic characteristics. Since the most commonly studied perovskite and perovskite-like compounds are oxides, they are also usually referred to as ABO3 and AB2O4 [15,16]. Parravano et al. [17,18] first used perovskite compounds in chemical reactions as catalysts in 1952. Subsequently, the compound began to draw increasing attention for its high catalytic performance, sulfur tolerance and coking resistance [19]. It is proved that 12 large coordinated rare earth metal cations at A-site and 6 small coordinated transition metal cations at B-site are consisted in the structure of perovskite. Metal ions or oxygen vacancies and disorder, which may serve as reactive centers for reactions involve CO2 or H2O [20], are ensured by outstanding structural stability of the compound and, therefore, benefit the catalysis. Besides, it is easy to control the property by doping at A or B-site in perovskite oxides [19]. Vacancies created after the substitution can 2

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2.2. Characterization of catalyst

(FESEM), transmission electron microscopy (TEM) and the BET specific surface area were used to analyze the prepared catalysts. The experimental results of the catalytic activity of the synergistic novel catalysts were compared with both conventional CuO/ZnO/Al2O3 and pure La2CuO4 catalysts, showing promoted performance. The aim of this paper is to seek a new path for catalyst design to obtain outstanding catalysts for SRM. The ultimate goal of generating power using SRM with renewable energy sources based on these high-performing catalysts can then be discussed. For example, catalysts with outstanding CO selectivity can produce hydrogen for PEMFC and the ones with superior conversion rate or H2 yield can be suitable for other power-generation equipment. Therefore, this study should be considered the first step of a series of work. Furthermore, to validate the advantages of the novel catalysts in the power generation application, a hybrid system combining photovoltaics and mid-/low-temperature methanol thermochemistry was developed. Thermodynamic analysis and modeling of the system were conducted, and experiments were performed.

Textural and structural properties of the prepared catalysts were studied by XRD, FESEM, TEM and BET. X-ray diffraction (XRD) patterns were performed on a SmartLab X-ray diffractometer utilizing Cu Ka radiation (40 kV, 40 mA) in the range from 10° to 90°, with a scanning step speed of 20°. The morphology of the samples was investigated using a Hitachi·S-4800 field emission scanning electron microscope (FESEM) with an accelerating voltage of 5.0 kV. The resolutions of its secondary electron probe were 1.0 nm with 15 kV and 2.0 nm with 1 kV. To address the morphology of the samples, transmission electron microscopy (TEM) analysis was also used with a JEOL-2100F microscope. The BET specific surface area of the catalysts was obtained using N2 adsorption at −196 °C on a Quantachrome, AutoSorb-iQ3 system. 2.3. Evaluation of catalytic performance The methanol steam reforming reaction was carried out in a fixed bed microreactor with a cylindrical glass tube with an inner diameter of 8 mm (Fig. 3(a)). The catalysts (1000 mg) in the form of particles of 30–60 mesh were loaded into the reactor in each test. A thermocouple was inserted into the reactor to detect the reaction temperature, which was supposed to vary between 150 and 320 °C throughout the test. The molar ratio of H2O and methanol (AR) was set at 1.6 to ensure a relatively high conversion rate, and the LHSV (liquid hourly space velocity) of the reaction was set within a range from 0.6 ml/(g·h) to 2.1 ml/(g·h). The gaseous products were analyzed by an on-line gas chromatograph (Nanjing Hope Analytical Equipment Co., Ltd, GC-9860–5 V), abbreviated as GC (Fig. 3 (b)). All the lines and valves through the methanol/ water feed were heated up to above 110 °C to avoid the condensation of methanol and water before the condenser. During the test, the catalysts were first reduced under a mixed gas flow of H2/Ar (15/75 ml/min) during pre-treatment. Then, the reactor was purged by Ar, and steam reforming was started by introducing a gaseous mixture of CH3OH/ H2O/Ar/N2, in which a constant flow of N2 was used as an internal standard to calculate the conversions of methanol and yields of products. The N2 flow was detected and shown on GC as its concentration in the mixed gas without reaction, which could be conveniently used to calculate the total flow rate of the mixed gas and then of other components. Based on these flow rates and the amount of methanol used, the conversion rate and other performance such as CO selectivity can be obtained.

2. Experiments 2.1. Catalyst preparation The La2CuO4 perovskite-like oxide, denoted as LCO in this paper, was used as a reference for the test. It was prepared using a co-precipitation method. Instead of using Na2CO3 solution as the titrating solution, the aqueous solution of metal nitrates (AR, 99%) was dripped into the aqueous solution of Na2CO3 (AR, 99%) with vigorous stirring and the pH was adjusted to and maintained at 9 in this work. This process ensure that La3+ ions and Cu2+ ions were precipitated simultaneously, because La(OH)3 and Cu(OH)2 are formed at a pH of 8.4 and 6, respectively. Therefore, a more uniform precipitation could be obtained. The solution was then stirred for another 1.5 h to maintain the pH at a constant level to ensure the stable existence of co-precipitate. The obtained precipitate was then filtered, washed successively with distilled water and ethanol, and dried completely in air at 333 K for 12 h, followed by calcination at 1123 K for 5 h and grinding to obtain La2CuO4 powders. A commercial CuO/ZnO/Al2O3 catalyst, CNZ1 (Southwest Research & Design Institute of Chemical Industry, Chengdu), was also used for comparison after being crushed to fine particles and ground. CNZ-1 and La2CuO4 powders were observed to have sizes less than 100 nm at least in one dimension (see FESEM and TEM images in Fig. 6(a) and (b)), thus proving to be nanopowders. The original properties of the CNZ-1 catalyst were tested and are shown in Table 1. It should be noted that the CNZ-1 catalyst, similar to all other catalysts, was pressed, crushed and sieved into particles using a 30–60 mesh, and tested under identical conditions. A high-energy ball mill (Shanghai Fritch Facilities and Instruments Co., Ltd., PULUERISETTE 6) was used to mix the above nanopowders. The nanopowders of different composition were mixed with ethanol as an abradant in a steel container in air, opened after a period of ball milling for 40 min at a rotate speed of 400 r/min, and then dried under a sodium lamp or in a centrifuge. The obtained flakelets or powders were then well ground, subsequently pressed to a disc with a tablet machine (Tianjin Botian Shengda Technology Development Co., Ltd., FW-5A), crushed roughly and sieved to particles using a 30–60 mesh. Through the above processes, the ball-milled samples were prepared. According to the different component mass ratios, three compound catalysts, (La2CuO4)0.3(CNZ-1)0.7, (La2CuO4)0.5(CNZ-1)0.5 and (La2CuO4)0.7(CNZ-1)0.3, were prepared in this work. The preparation processes are shown in Fig. 1. The subscripts represent their nominal mass compositions. The catalysts were then denoted as LCO3-CNZ, LCO5-CNZ and LCO7-CNZ respectively. LCO and CNZ-1 were pressed, crushed, sieved and tested under the same conditions with LCOx-CNZ (x = 3, 5, 7).

3. Thermodynamic analysis and experimental test of the solar fuel production system To evaluate the actual application of the above novel catalysts, a solar power generation system combining photovoltaics and mid-/lowtemperature methanol thermochemistry was conducted. 3.1. Description of the system A schematic diagram of the hybrid photovoltaics and thermochemistry (PVTC) system is shown in Fig. 2. Detailed descriptions of the hybrid system have been first provided by Li in our previous publication [38]. Briefly, the hybrid system consisted of two subsystems: the PV Table 1 Properties of the CNZ-1 catalyst.

3

Parameters

Value

Mass fraction of CuO (%) Mass fraction of ZnO (%) Mass fraction of Al2O3 (%) Specific surface area (m2/g) Original equivalent diameter of the catalyst (mm)

70.42 10.97 18.61 90.25 5.00

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Fig. 1. The processes of preparation of the catalysts: (a) weighing, (b) dissolving, (c) titration and stirring, (d) aging, (e) filtration, (f) drying, (g) calcination, (h) mixing in high-energy ball mill, (i) centrifuging and drying and (j) obtaining the catalysts.

After SRM in the reactor, a mixed gas (steam 6, consisting of H2, CO2, CO, H2O, and unreacted methanol vapor) is sent to condenser A. The flow of H2O and methanol is condensed and removed, while the rest of the mixed gas (steam 8–1) can be sent to GC for testing with a constant flow of N2 (steam 9).

subsystem, including PV cells fastened on the pre-reactor and reactor and necessary circuits, which provided power directly; the other subsystem, the thermochemical subsystem, is mainly composed of pre-reactor, reactor, fuel cell and condensers. Methanol solution is sequentially pumped into the pre-reactor (after passing through the heat exchanger). It is heated and vaporized by the pre-reactor, and then it reacts with the catalyst present in the reactor to produce H2. After condensation, the flow of pure H2 is sent to fuel cell to produce power. Both subsystems use solar irradiation as an energy source. A coupling relationship exists between the two subsystems. When concentrated solar light reaches the pre-reactor and reactor, it is preferentially utilized by PV cells on their upper surfaces. PV cells produce heat while absorbing light to produce power, and therefore, they can be used as the heat source for vaporizing the methanol solution in the pre-reactor and SRM in the reactor. A hybrid system is also shown in the schematic diagram in Fig. 2.

3.2. Analytical methodology Thermodynamic modeling and simulation were carried out based on the hybrid system. More details about the modeling can be found in our previous work [39]. For simplicity, consider the pre-reactor and reactor as a whole, denoted as “heater” in the following statement. Suppose heat flow is uniform and the other boundaries are adiabatic except the upper surface of the heater. Concisely, the thermal equilibrium is given as follows:

Fig. 2. Schematic diagram of the hybrid system. 4

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(

)

̇ (I ) = Qgain ̇ Qabs T , ṅin,CH3OH + Q̇ loss (T )

(4)

̇ (I ) is the absorbed solar heat rate of the heater, where Qabs ̇ (T , ṅin,CH3OH) is the enthalpy change rate considering the reaction Qgain and preheating enthalpy change rate of the methanol solution, and Q̇ loss (T ) is the heat loss rate of the upper surface of the heater. They are expressed respectively as follows: ̇ (I ) = I ·Slen·ηopt = I ·Slen·K ·γ ·τ ·α Qabs

(

)

(5)

̇ Qgain T , ṅin,CH3OH = Q̇pre (ṅin,tot , Tin, Tout ) + ṅin,CH3OH ·ηconv ·hrea

(6)

Q̇ loss (T ) = A·h·(T ′ − T0 ) + A·ε ·σ·(T ′ 4 − T04 )

(7)

where Q̇pre (ṅin,tot , Tin, Tout ) in Eq. (6) can be further expressed as follows:

Q̇pre (ṅin,tot , Tin, Tout ) = ṅin,tot ·(hout − h in )

(8)

where the difference between h out and h in can be measured by the following polynomial [40]:

hout − h in = c p,l (Tb − Tin ) + c p,v (Tout − Tb ) + Δh vap

Fig. 4. Maximum power generation of the three-junction gallium arsenide PV cell with different temperatures (when the direct normal irradiance is 915 W/ m2 and the concentrated area is 0.1 m2).

(9)

The net solar power generation efficiency is an important indicator in evaluating the performance of the hybrid PVTC system, which is defined as follows:

ηnet =

power generation efficiency of fuel cell. A novel catalyst of LCO3-CNZ was applied to the hybrid PVTC system as a representation, and CNZ-1 was used as a reference catalyst.

(LHVH2 − LHVCH3OH)·ηfc + PPV Slen·I

(10)

4. Results and discussions

The physical meanings of all the items mentioned above are explained in the nomenclature at the end of this paper.

4.1. Catalytic characterization The XRD patterns of CNZ-1, La2CuO4, LCO3-CNZ, LCO5-CNZ and LCO7-CNZ are shown in Fig. 5(a)–(e) respectively. The patterns of La2CuO4 (Fig. 5(b)) and LCOx-CNZ (Fig. 5(c)–(e)) without reduction exhibited narrow characteristic peaks of La2CuO4, confirming the presence of the desired perovskite-like compound. After pre-treatment, strong peaks related to lanthanum oxide La2O3 and Cu metal appeared while traces of CuO vanished, indicating that La2CuO4 and CuO had been mostly or even completely reduced. It is also shown in Fig. 5(c)–(e) that the structures of the LCOx-CNZ catalysts remained stable after 50 h of reaction, showing a good thermal and chemical stability. It should be noted that in all the XRD patterns shown in Fig. 5, the peaks of Al2O3 were rather weak due to possible reasons that it may have been covered by other oxides, it has been well dispersed or some of it may has formed in amorphous phase [41]. FESEM (Fig. 6) and TEM (Fig. 7) images of catalysts revealed the structural properties of the five catalysts. The components were distinguished by their unique lattice spacing in Fig. 7. Each catalyst clearly had a size less than 100 nm at least in one dimension; therefore, they

3.3. Experimental verification An experimental testbed of the thermochemical subsystem of the hybrid PVTC system was conducted. The reactor is shown in Fig. 3(c). The heat provided by the PV cells was conveniently replaced by heater bands. To ensure the feasibility of this measure, the power of the heater bands was set according to the test data for a three-junction gallium arsenide PV cell (Tianjin Blue Sky Sun Technology Co., Ltd.). The maximum efficiency of the PV cell with the temperature was also tested, and therefore, the power generation as a dependent variable of efficiency and direct normal irradiation could be measured. Fig. 4 shows the maximum power generation of a 1 cm2 PV cell with the varying of temperature under a directive normal irradiation of 915 W/ m2 concentrated by a 0.1 m2 Fresnel lens. Additionally, the gaseous exhaust of condenser A (Fig. 2) was sent to GC through line 10 to measure the concentration ratio of each component. The solar power generated by the thermochemical subsystem was calculated by the product of the increment in calorific value from methanol to H2 and the

Fig. 3. Experimental equipment: (a) evaluation system of catalytic performance, (b) GC and (c) reactor of PVTC system with heater bands on the upper surface. 5

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Fig. 5. XRD patterns of (a) CNZ-1, (b) LCO, (c) LCO3-CNZ, (d) LCO5-CNZ and (e) LCO7-CNZ.

remained uncovered. The structural properties of these catalysts may have an influence on their catalytic performance. Table 2 shows the BET specific surface area of the five catalysts. The BET specific surface area of La2CuO4 was 2.896 m2/g, which is close to the value of 2.7 m2/g reported in other research [10]. This relatively low surface area may be due to the sintering of La2CuO4 at high temperature (750 °C) required for the formation of its single-phase crystal structure [42]. The specific surface area of CNZ-1 was the largest among the catalysts evaluated in this work. The BET surface area of these catalysts was observed to decrease nonlinearly with the increase in the molar ratio of La2CuO4 (Fig. 8).

were all nanocatalysts by definition. CNZ-1 had the smallest particles with a diameter less than 20 nm (Fig. 7 (a)); the sizes of the La2CuO4 particles were uneven, but after ball-milling the catalysts of LCOx-CNZ (x = 3, 5, 7) demonstrated more uniform diameters with small metal oxides particles covering the surface. When the molar ratio of La2CuO4 was low, for example, when x = 3 (Figs. 6(c) and 7(c)), the CuO, ZnO and Al2O3 particles exceed the amount needed to completely cover the surface of La2CuO4 and, thus, overlapped and even conglomerated with each other. When x = 5 (Figs. 6(d) and 7(d)), the oxides particles more evenly covered the surface of the perovskite-like compound. When x = 7 (Figs. 6(e) and 7(e)), the ratio of La2CuO4 was higher than that of CuZnAl oxides. The CuZnAl oxides particles were clearly less than in LCO3-CNZ and LCO5-CNZ, and some of the La2CuO4 particles surfaces

Fig. 6. FESEM images of (a) CNZ-1, (b) LCO, (c) LCO3-CNZ, (d) LCO5-CNZ and (e) LCO7-CNZ. 6

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Fig. 7. TEM images of (a) CNZ-1, (b) LCO, (c) LCO3-CNZ, (d) LCO5-CNZ and (e) LCO7-CNZ. Table 2 BET specific surface area of catalysts.

2

BET surface area (m /g)

Table 3 Pre-treatment temperature of catalysts.

CNZ-1

LCO

LCO3-CNZ

LCO5-CNZ

LCO7-CNZ

90.250

2.896

51.328

19.264

13.892

pre-treatment temperature (°C)

4.2. Catalytic activity and selectivity The catalytic performance including activity and selectivity were tested in the evaluation system in Fig. 3(a). The conversion rate of CH3OH and the selectivity of CO were used to evaluate the catalytic activities, as expressed in the following Eqs. (11) and (12), respectively. The H2 selectivity (Eq. (13)) and H2 yield of the catalysts are also given in this work. The latter is defined as the amount of H2 produced by 1000 mg catalyst per second.

FCO + FCO2 × 100% FMeOH

(11)

SCO =

FCO × 100% FCO + FCO2

(12)

SH2 =

FH2 × 100% FH2 + FCO + FCO2

(13)

LCO

LCO3-CNZ

LCO5-CNZ

LCO7-CNZ

300

500

400

450

470

could be found based on the XRD patterns of the reduced catalysts, as shown in Fig. 5. The pre-treatment temperatures of prepared catalysts are listed in Table 3. The pre-treatment temperatures were set to reduce the catalysts to a uniform state (that the characteristic peaks in XRD patterns disappeared) and, therefore, deeply depended on the specific mass ratio of the components. As a result, they were not the same for different catalysts. It should be noted that normally the percentage of a composition less than 5 wt% cannot be detected in XRD. Therefore, the disappearance of characteristic peaks cannot ensure the complete reduction of catalyst. The pre-treatment temperatures of LCOx-CNZ catalysts were lower than that of La2CuO4, supporting the presence of synergistic effects among the components and, thus, facilitating the pretreatment of the La2CuO4 component of these catalysts. In the absence of a synergistic effect, the pre-treatment temperature would be 500 °C for every catalyst provided the improvement of La2CuO4. Another conclusion could be drawn by comparing Tables 2 and 3. As shown in Fig. 8, the BET surface area and pre-treatment temperatures demonstrated a nominally opposite tendency with an increase in x. Therefore, the former property may have an influence on the latter one. The results with a LHSV of 1.2 ml/(g·h) were taken as a typical example to study the performance of the catalysts (Fig. 9). As seen in Fig. 9(a), LCO3-CNZ showed a higher methanol conversion rate than any other catalysts assessed in this study, especially when the reaction temperature was below 230 °C. It also showed advantages in selectivity and yield of H2 at lower temperatures of approximately 220 °C and below, as shown in Fig. 9(c) and (d). However, the advantages disappeared and became almost the same as the other four catalysts as the temperature was increased. Additionally, Fig. 9(b) shows that the LCO3-CNZ catalyst had a nearly identical selectivity as CO for CNZ-1 throughout the temperature range from 150–270 °C. Therefore, to obtain a good conversion rate, H2 selectivity, H2 yield and a normal CO selectivity, the LCO3-CNZ catalyst is better for use in situations where the system operates at or below 270 °C. Another arresting catalyst was LCO5-CNZ, which demonstrated a noticeable advantage in selectivity against CO to benefit the utilization of PEMFC. As the temperature increased, the LCO5-CNZ catalyst gradually showed a better H2 selectivity and H2 yield than the other four catalysts. Specifically, the CO selectivity of LCO5-CNZ was 0.8–3.3 percentage points lower than that of CNZ-1 within the tested temperature range from 200 °C to 320 °C; the H2 selectivity of LCO5-CNZ was 5.7 percentage points higher than CNZ-

Fig. 8. BET surface area and pre-treatment temperatures of catalysts.

XMeOH =

CNZ-1

The SRM experiments were performed at reaction temperatures ranging from 150–320 °C. The active sites had to be reduced from the compound metal oxides during pre-treatment, which was mostly influenced by the reduction temperature and reaction time. Here, the pretreatment time was set to 3 h and the appropriate reaction temperature 7

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Fig. 9. The conversion rate (a), selectivity of CO (b) and H2 (c), and H2 yield (d), LHSV = 1.2 ml/(g·h).

increase, indicating a decline in the catalytic performance. This phenomenon may be due to the quick sintering of reduced La2CuO4 particles because of their low surface area, as shown in Table 2. In contrast to La2CuO4, LCOx-CNZ catalysts could function at temperatures of 300 °C and above with good performance, suggesting that the presence of ZnO and Al2O3 in LCOx-CNZ could stabilize La2CuO4 and prevent it from sintering. Additionally, the synergistic effects of the perovskitelike La2CuO4 and CuZnAl oxides could be confirmed to improve the performance of LCOx-CNZ with a much lower BET surface area than CNZ-1, as shown in Table 1.

1 at a lower temperature of 200 °C and close to it at a temperature above 250 °C. When the temperatures exceeded 270 °C, a favorable H2 yield could be obtained with the catalyst of LCO5-CNZ, providing 3.4 percentage points more product than that of CNZ-1 at temperatures above 285 °C. In contrast, although the conversion rate of methanol catalyzed by LCO5-CNZ was lower than that catalyzed by LCO3-CNZ, LCO7-CNZ and CNZ-1 at temperatures below 250 °C, it showed competitive values at relatively higher temperatures and was closed to 100% at approximately 270 °C. Based on the above phenomena, the LCO5-CNZ catalyst demonstrated good performance between 250 and 300 °C, with favorable selectivity and yield and acceptable activity. As analyzed above, the superiority of LCO5-CNZ was mainly observed for CO selectivity, becoming increasingly obvious as the temperature rose. Additionally, though the other three characteristics were not as competitive as the other catalysts (except LCO), they were equal to and even exceeded the counterparts of other catalysts at higher temperatures. In contract, LCO3-CNZ was optimal low temperatures where its CO selectivity was almost the same as CNZ-1 and LCO5-CNZ and showed superiority in terms of other characteristics. Therefore, it can be concluded that the two catalysts, LCO3-CNZ and LCO5-CNZ, are optimal for different situations. When CO selectivity is the first priority or the operating temperature is greater than 270 °C, LCO5-CNZ should be chosen. Otherwise, LCO3-CNZ is more suitable for a wide temperature range. In comparison, as shown in Fig. 9(a), (c) and (d), LCO7-CNZ was the second-best among the five catalysts, demonstrating a competitive conversion rate, H2 selectivity and H2 yield throughout the temperature range from 150 °C to 320 °C. However, it was not suitable for PEMFC because it had the worst CO selectivity among the catalysts examined in this study. Therefore, the LCO7-CNZ catalyst is better used in systems in which the CO selectivity is not a primary concerns. The small surface area may be responsible for the inferior performance of pure LCO, as shown in Fig. 9(a)–(d), The presence of inflection points on the profiles of La2CuO4 at 300 °C should also be noted. When the reaction temperature was higher than 300 °C, the conservation rate, H2 selectivity and H2 yield dropped in contrary to their previous trends. Simultaneously, the CO selectivity demonstrated a faster

4.3. Calculated and experimental results of hybrid PVTC system As analyzed above, LCO5-CNZ has advantage in CO selectivity. A hydrogen production system with SRM and PEMFC over LCO5-CNZ has been studied in our previous work [43]. It is disclosed that with a direct normal irradiation of 1000 W/m2 and a LHSV of 2.1 ml/(g·h), the net solar power generation efficiency of LCO5-CNZ is 10.4 percentage points higher than that of CNZ-1. The reason of this result is that the much lower CO ratio of LCO5-CNZ products led to a higher efficiency of PEMFC and then a higher net solar power generation efficiency. It should be noted that the temperature was set at 300 °C to ensure a uniform methanol conversion rate for both catalysts. Therefore, it was concluded that LCO5-CNZ as SRM catalyst is suitable for PEMFC. LCOx-CNZ catalysts also show benefits in terms of conversion rate. As proposed above, LCO3-CNZ has the best conversion rate among all five catalysts while the CO selectivity is similar to commercial CNZ-1. A solid oxide fuel cell (SOFC) was applied in the hybrid PVTC system to measure this advantage. The efficiency of SOFC was set at 50% [44]. Numerical calculation and test verification of the catalysts in the hybrid PVTC system were conducted based on the above catalytic performance data. Table 4 shows the calculated and experimental results for LCO3CNZ and CNZ-1 catalysts under specific conditions. The results with a direct normal irradiation of 915 W/m2 and a reaction temperature of 230 °C were considered as a typical example. It could be initially concluded that the equilibrium LHSVs of the methanol solution of LCO3CNZ were lower than CNZ-1 in both the calculated and experimental 8

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use in actual power generation systems.

Table 4 Calculated and experimental results for LCO3-CNZ and CNZ-1 catalysts. CNZ-1 Direct normal irradiation (W/m2) Reaction temperature (°C)

Acknowledgements

LCO3-CNZ

915 230

LHSV equilibrium of methanol solution (ml/(g·h))

Calculated Experimental

2.08 2.00

2.02 1.93

H2 yield (ml/min)

Calculated Experimental

1263.31 1143.95

1379.58 1254.95

Net solar power generation efficiency (%)

Calculated Experimental

34.9 31.1

36.2 32.3

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results. However, because of the higher conversion rate resulting from both the better performance and lower LHSV, the hybrid system using LCO3-CNZ provided a better H2 yield and net solar power generation efficiency than that using the CNZ-1 catalyst. Specifically, the experimental results for LCO3-CNZ were relatively 9.7% higher in terms of H2 yield and 3.9% in terms of net solar power generation efficiency compared with CNZ-1. It should also be noted that the experimental LHSV equilibrium of the methanol solution was lower than that based on the calculated results. The phenomenon may be a consequence of the much higher heat loss in the experimental test than in the numerical calculation, resulting in less available heat for the reaction. Since the reaction temperature remained the same, the decrease in LHSV would clearly lead to a higher conversion rate, which might benefit the H2 yield and net solar power generation efficiency. However, the tendency towards a decline caused by the higher heat loss and lower LHSV together exceeded the effect of LHSV. As a result, the experimental H2 yield and net solar power generation efficiency were lower than the calculated ones. 5. Conclusion In this work, a group of novel nanocatalysts of compound metal oxides prepared by co-precipitation and high-energy ball milling methods were developed and tested for hydrogen production by steam reforming of methanol. Catalyst characterizations performed by XRD, FESEM and TEM were used to analyze the catalysts. Characterization properties and comparisons among the test data for LCOx-CNZ (x = 3, 5, 7), La2CuO4 and CNZ-1 confirmed the presence of synergistic effects of the perovskite-like compound La2CuO4 and CuZnAl oxides and showed that the effects might promote the performance of the compound metal oxides. These results support the proposal of a new way to design high-performance catalysts through the use of synergistic effects among the components. The present results indicated that the LCO5-CNZ catalyst has better performances between 250 °C and 300 °C with a LHSV of 1.2 ml/(g·h). In this case, the CO selectivity is 0.2–0.5 percentage points lower and H2 selectivity 1–2 percentage points higher than that of CuZnAl oxides catalyst; the conversion rate is greater than 85%. Another promising catalyst, LCO3-CNZ, was confirmed to provide better results in situations where the system operates at 150–270 °C. In conclusion, LCO5CNZ is suitable for reactions with high requirements for CO selectivity or that are performed above 270 °C. Alternately, LCO3-CNZ is more favorable due to its wide temperature range. With superior catalytic performance, these novel catalysts can provide more H2 with higher purity using the same amount of methanol solution when used in an energy supply system. A hybrid PVTC system was also used in this study. Under a direct normal irradiation of 915 W/m2 and a reaction temperature of 230 °C, the novel catalyst of LCO3-CNZ showed a 9.7% and 3.9% relatively increase in H2 yield and net solar power generation efficiency, respectively, compared with commercial CNZ-1. This experimental result demonstrates the advantages of better performing novel catalysts for 9

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