Synthesis of CuO/ZnO/Al₂O₃/ZrO₂/CeO₂ nanocatalysts via homogeneous precipitation and combustion methods used in methanol steam reforming for fuel cell grade hydrogen production

Abstract

Homogeneous precipitation and urea-nitrate combustion methods have been comparatively investigated for the synthesis of CeO₂ promoted CuO/ZnO/Al₂O₃/ZrO₂ nanocatalysts. The catalytic performance of the samples was studied in the methanol steam reforming process for fuel cell grade hydrogen production. The physicochemical properties of the prepared nanocatalysts were characterized by XRD, FESEM, PSD, EDX, BET and FTIR analysis. XRD analysis shows that the homogeneous precipitation method and CeO₂ addition improves the dispersion and decreases the relative crystallinity of CuO and ZnO species. FESEM images illustrate that the homogeneous precipitation method is more capable of the synthesis of smaller particles than the urea-nitrate combustion method. Addition of ceria to the fabricated catalysts decreases the particle size and enhances surface homogeneity. EDX analysis demonstrates that Cu and Zn elements are homogeneously dispersed on the catalyst surface, which is synthesized by a homogeneous precipitation method. The catalytic performance demonstrates that applying the homogeneous precipitation method improves activity while the ceria addition leads to lower CO selectivity.

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1 Introduction

In recent years, fuel cells have been nominated as a promising alternative to the internal combustion engine, though hydrogen supply as their feed is not solved yet.^1,2 Hydrogen can be produced from a variety of feedstocks such as methane, methanol, ethanol, etc.^3–5 However, it should be noted that the application of fuel cells in vehicles forces us to produce on board hydrogen. Among various alternatives, methanol due to its advantages such as high H/C ratio, no C–C bonds and low operation temperature is most attractive.^6–10 Hydrogen can be produced via a methanol feed through four catalytic pathways:^11,12

• Methanol decomposition.

• Steam reforming of methanol.

• Partial oxidation of methanol.

• Oxidative steam reforming of methanol.

Among these methods, steam reforming of methanol (SRM) with high hydrogen yield and low CO production is more desirable.^13,14 Catalysts for this process are divided into two groups; copper based catalysts and group VIII metals.^15–17 Copper based catalysts are the most appropriate catalysts for the steam reforming of methanol reaction as they are more active and selective for hydrogen.^18,19

In order to improve the activity and reducibility of copper based catalysts, most investigations have focused on employing an appropriate promoter and synthesis method.^20–22 ZnO is known as the most desirable promoter for enhancing the reducibility and dispersion of Cu species.^23,24 Al₂O₃ as a structural promoter has a positive effect on the reactivity of the CuO/ZnO catalysts.^14,25 It was added to the catalysts to increase the surface area, mechanical resistance and thermal stability.^8,26 However, alumina at high loading has a negative effect on the activity and hinders the reforming reaction.^8,27 In addition, other promoters such as CeO₂ and ZrO₂ have been interestingly studied due to their positive influence on the methanol steam reforming process.²² The improvement in activity, stability and dispersion of CuO and ZnO, as well as reduction in CO selectivity, are some of the significant promoting effects of CeO₂.^8,22 Furthermore, CeO₂ prevents reducibility of the SRM catalyst.⁸ ZrO₂ is introduced as a reducing agent in the active phase and improves the dispersion of active sites. It can also diminish CO yield and delay catalyst deactivation.^8,26–29

In addition to promoter effects, the physicochemical properties of the catalyst employed in each process can be affected by the synthesis method.^30–32 Conventionally, precipitation and impregnation are known as common methods for the synthesis of copper based catalysts. Much research has been performed to compare these methods with other novel fabrication pathways.²¹ One of the new techniques that has been recently applied in catalyst synthesis is the urea-nitrate combustion (UNC) method. Unlike conventional precipitation and impregnation methodologies, the UNC method is fast and simple due to the elimination of some time-consuming steps such as washing, filtering and drying.³³ This method has two agents; an oxidizing agent such as nitrate and sulfate salts and an organic fuel such as glycine, citric acid or urea. Rapid and facile synthesis as well as producing a homogeneous powder from cheap precursors are the advantages of the UNC method.^34–36 On the other hand, some work has focused on improving the conventional precipitation method by altering the original pathway. The homogeneous precipitation (HP) method is known to be an effective synthesis method for the production of highly dispersed active sites. Also, previous results have shown that a high degree of crystallinity can be achieved using this method.^37,38 Comparing the homogeneous precipitation method with the co-precipitation method shows that nanocatalysts synthesized using the homogeneous precipitation method are more active because of its highly dispersion and smaller particles of active phases.³⁷

Although so much work has been performed to characterize catalysts fabricated by the homogeneous precipitation and urea-nitrate combustion methods individually, there is little research that compares these methods. Therefore, the aim of this paper is to compare the characteristic and catalytic properties of CuO/ZnO/Al₂O₃/ZrO₂ nanocatalysts synthesized by urea-nitrate combustion and homogeneous precipitation methods. Moreover, the fabricated catalysts were promoted by ceria (10% wt) to investigate its influence on the characterization of the prepared nanocatalysts. The characteristics of the fabricated catalysts were evaluated by XRD, FESEM, PSD, EDX, BET and FTIR analysis. The catalytic performance of CuO/ZnO/Al₂O₃/ZrO₂ (CZAZ) and CuO/ZnO/Al₂O₃/ZrO₂/CeO₂ (CZAZC) nanocatalysts were studied in a fixed bed reactor through the methanol steam reforming reaction.

2 Materials and methods

2.1 Materials

Nitrate solutions used as nanocatalyst precursors were obtained by mixing Cu(NO₃)₂·3H₂O, Zn(NO₃)₂·3H₂O, Zr(NO₃)₄·5H₂O and Ce(NO₃)₃·6H₂O which were supplied by Merck Company, Germany. Aluminium hydroxide and Al(NO₃)₃·9H₂O were used as alumina precursors in the UNC and HP method, respectively. These precursors, as well as urea as a precipitant in the HP method and fuel in the UNC method were supplied by Merck Company, Germany. Distilled water was used for nanocatalyst preparation stages. All of the reagents were used as received without further purification.

2.2 Nanocatalyst preparation and procedures

To investigate the effect of preparation method on the synthesis of CeO₂ promoted CuO/ZnO/Al₂O₃/ZrO₂ nanocatalysts, two samples with different compositions were synthesized via urea-nitrate combustion (UNC) and homogeneous precipitation (HP) methods. In this paper, CuO/ZnO/Al₂O₃/ZrO₂ and CuO/ZnO/Al₂O₃/ZrO₂/CeO₂ nanocatalysts are named CZAZ and CZAZC, respectively. UNC and HP after the name of the catalysts refers to urea-nitrate combustion and homogeneous precipitation, respectively.

In the UNC method (Fig. 1s in the ESI†), an aqueous solution of Cu, Zn, Zr and Ce nitrate was made to the desired composition. Aluminium hydroxide (Al(OH)₃) was heated under an air flow at 400 °C for 4 h to produce boehmite (AlOOH) as an Al precursor. In the next step, the nitrate solution of precursors, boehmite and urea as the combustion fuel were mixed for 45 min. The urea/nitrate ratio was determined as 3. The resulting solution was heated at 80 °C to evaporate excess water and form a viscous gel. The gel was put in a muffle furnace where it started to boil and froth at 400 °C and form a black powder. Finally, the resulting powder was calcined at 350 °C for 4 h under an air flow.

In the HP method (Fig. 2s in the ESI†), an aqueous solution of Cu, Zn, Zr, Ce and Al nitrate precursors was mixed with urea (urea/nitrate = 20:1 molar ratio). The urea was hydrolysed by heating the mixture to 90 °C for 24 h to release the precipitant agent (hydroxide ions). The precipitate was washed with deionized water and dried under an air flow at 80 °C for 24 h. The samples were calcined at 350 °C for 5 h. Finally, the prepared nanocatalysts from both methods were shaped for catalytic performance studies toward methanol steam reforming.

2.3 Nanocatalysts characterization techniques

X-ray diffraction (XRD) measurements were used to study the crystal structure of the synthesised nanocatalysts. They was carried out using a D5000 Siemens X-ray diffractometer with a Cu Kα radiation source (0.154056 nm) which was operated at 30 kV and 40 mA. The resulting patterns were compared with the Joint Committee on Powder Diffraction Standards (JCPDS) to identify the crystallite phases of metal oxides. Moreover, the crystallite size was calculated using the Scherrer equation as follows:
where τ is the mean size of the ordered crystallite. K, λ and θ are the shape factor, X-ray wavelength and Bragg angle, respectively. β is known as the line broadening at half the maximum intensity (FWHM), after subtracting the instrumental line broadening. Furthermore, for more statistical analysis of XRD patterns the relative crystallinity was calculated by the selection of a single peak of the specific material. The highest intensity of the selected peak was assumed to be 100% of relative crystallinity and the others which surely had lower intensity were calculated compared to the basis value.

The surface morphology of CZAZ and CZAZC nanocatalysts were studied using a field emission scanning electron microscope (FESEM, HITACHI S-4160). Before each test for more accurate images, the samples were coated with an ultrathin layer of gold to increase the conductivity of the surface. The particle size distribution histograms were depicted by ImageJ software by analysing the FESEM images. EDX and dot mapping were conducted for surface elemental analysis (VEGA II, TESCAN). Specific surface area measurements were investigated by nitrogen adsorption at 77 K and desorption at room temperature using a Quantachrome ChemBET 3000 analyser. Before each measurement, the samples were degassed at 200 °C for 30 min. The FTIR spectra were acquired for surface functional groups by means of a Unicam 4000 FTIR spectrometer, using a KBr plate in the range of 400–4000 cm⁻¹.

2.4 Experimental setup for catalytic performance test

A schematic flowchart of the experimental setup for catalytic performance test toward the methanol steam reforming reaction is illustrated in Fig. 1. The setup consists of a gas feeding section, a fixed bed reactor and an analytical section. A methanol and water mixture was fed into the reactor by argon stream through a saturator. The flow rate of this stream was 70 mL min⁻¹ which was controlled by a mass flow controller. The H₂O/MeOH ratio was considered as 1.5 in the feed stream. The steam reforming of methanol reaction was carried out in a U-shape Pyrex tube packed bed reactor (ID = 8 mm and length = 32 cm) with 0.4 g of nanocatalyst loading. The heat of the reaction was provided by placing the reactor inside an electrical furnace equipped with an electronic temperature controller. Prior to the catalytic and stability tests, the nanocatalyst was reduced under a stream of hydrogen with a flow rate of 100 mL min⁻¹ at 300 °C under atmospheric pressure for 3 h. Experiments were carried out over a temperature range of 200–300 °C to evaluate catalytic performance at atmospheric pressure. The gas hourly space velocity (GHSV) was set at 10 000 cm³ g_cat⁻¹ h⁻¹ as the literature reports that this is the most efficient.^39–41 An on-line gas chromatograph (GC Chrom, Teif Gostar Faraz, Iran) equipped with a Plot-U Column (Agilent Technologies) as well as TCD and FID detectors was used to analyse reactor products. Argon was used as the carrier gas in the GC column.

Fig. 1 Experimental setup for activity test of CuO/ZnO/Al₂O₃/ZrO₂ and CuO/ZnO/Al₂O₃/ZrO₂/CeO₂ nanocatalysts used in hydrogen production via methanol steam reforming.

The conversion of methanol and selectivity of the products are defined as follows:

where F_i is the flow rate of component i in the gaseous effluent.

3 Results and discussion

3.1 Nanocatalyst characterization

3.1.1 XRD analysis. The XRD patterns of the CZAZ–UNC, CZAZ–HP, CZAZC–UNC and CZAZ–HP nanocatalysts are shown in Fig. 2. As is shown, most of the known peaks are at 2θ = 30–40°. The indexed peaks of monoclinic CuO (JCPDS 01-080-1268) at 2θ = 35.5°, 38.8° and 48.8°, hexagonal ZnO (JCPDS 01-076-0704) at 2θ = 31.7°, 34.4°, 36.2° and 56.5° and tetragonal ZrO₂ (JCPDS 01-080-0784) at 2θ = 30.2° and 35.3° are clearly seen in the spectra of CZAZ–UNC and CZAZC–UNC nanocatalysts. However, for the CZAZ–HP and CZAZC–HP nanocatalysts, applying the homogeneous precipitation method led to a decrease of peak intensity and relative crystallinity of all phases, but improved their dispersion. The indexed peaks of CeO₂ (JCPDS 01-075-0076) are present at 2θ = 28.8°, 33.2°, 47.7° and 56.6°. The presence of ceria in CZAZ samples led to a reduction of nanocatalyst crystallinity. This decrease in crystal size is more obvious for the CZAZC–HP nanocatalyst. This is related to the nature of the homogeneous precipitation method to decrease the crystal size and instead enhance dispersion, which is improved by the addition of ceria. Glancing at the diffraction patterns of all the samples, no Al₂O₃ (JCPDS 00-004-0880) phase can be seen. This can be interpreted to be due to its special crystallite state, low loading and high dispersion.¹⁴ Studies have confirmed that spinel compounds are formed at high calcination temperatures (>600 °C), so no form of copper or zinc aluminate was detected.⁴² One of the advantages of the homogeneous precipitation method, as seen in the XRD results, is improving the dispersion of elements which happened for CuO, ZnO and ZrO₂ crystals compared to the UNC method. Table 1 demonstrates the relative crystallinity (RC) and the crystallite size of the observed phases. According to the results, the highest relative crystallinity of all species can be detected in samples which were fabricated by the urea-nitrate combustion method. However, this was predictable according to the qualitative interpretation of the XRD patterns which show that crystals with low intensity were formed by homogeneous precipitation. Also, addition of ceria to CZAZ samples in both synthesis methods led to lower relative crystallinity of other species due to overlapping of ceria crystals with other ones. This result was proven by crystallite size calculations via Scherrer’s equation. According to calculated crystallite size of CuO species, application of the HP method resulted in smaller crystallites than the UNC method. Furthermore, Scherrer calculations prove the nanocrystallite properties of the synthesized catalysts. Small crystallite size in the homogeneous precipitation method can effect the production of small particles and improve accessibility for methanol and water molecules.

Fig. 2 XRD patterns of the synthesised nanocatalysts: (a) CZAZ–UNC, (b) CZAZ–HP, (c) CZAZC–UNC and (d) CZAZC–HP.

Table 1 Chemical composition, specific surface area and structural properties of synthesized nanocatalysts

Nanocatalyst	Synthesis method	Cu (%)	Zn (%)	Al (%)	Zr (%)	Ce (%)	BET (m^2 g^−1)	Relative crystallinity^a					Crystallite size^b (nm)
								CuO	ZnO	Al2O3	ZrO2	CeO2	CuO^c	ZnO^d	Al2O3^e	ZrO2^f	CeO2^g
a Relative crystallinity: XRD relative peak intensity.b Crystallite size estimated by Scherrer’s equation.c Crystallite phase: monoclinic (JCPDS: 01-080-1268, 2θ = 35.5, 35.6, 38.8, 38.9, 48.8, 61.6, 68.2).d Crystallite phase: hexagonal (JCPDS: 01-076-0704, 2θ = 31.7, 34.4, 36.2, 47.5, 56.5, 62.8, 67.9, 69.0).e Crystallite phase: cubic (JCPDS: 00-004-0880, 2θ = 37.4, 39.7, 42.8, 45.8, 67.3).f Crystallite phase: cubic (JCPDS: 01-080-0784, 2θ = 30.2, 34.6, 35.3, 43.0, 50.3, 50.7, 53.9, 59.4, 60.2).g Crystallite phase: cubic (JCPDS: 01-075-0076, 2θ = 28.8, 33.2, 47.7, 56.6, 59.4, 69.8, 77.1, 79.5, 88.9).
CZAZ–UNC	Combustion	40	30	10	20	0	26.0	100	100	0	98.9	—	15.6	15.9	—	9.8	—
CZAZ–HP	Precipitation	40	30	10	20	0	132.9	34.2	0.0	0	11.2	—	8.4	—	—	—	—
CZAZC–UNC	Combustion	30	30	10	20	10	26.7	55.4	88.2	0	100	100	16.5	17.2	—	8.7	30.7
CZAZC–HP	Precipitation	30	30	10	20	10	149.9	10.9	11.8	0	0.0	50.0	—	—	—	—	—

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3.1.2 FESEM analysis. FESEM images of the synthesized nanocatalysts are shown in Fig. 3. Combustion gases produced through the UNC method create micropores, which is one of the main advantages of the UNC method over other synthesis methods and facilitate the accessibility of feed molecules to catalytic sites. But on the other hand, it can be seen that the catalytic particles produced by the HP method are smaller and it seems that this kind of nanocatalyst has a higher specific surface area compared to those made using the UNC method. Addition of ceria has a positive effect on improving the homogeneity and reducing particle size in both HP and UNC catalysts. However, their effect on HP nanocatalysts was more significant. All FESEM images prove that the particles of all synthesized catalysts are nanometers in size. But the particles of the CZAZC–HP sample are smaller than those of the others. Therefore, it seems that the CZAZC–HP sample may have a higher specific surface area and as a result achieves a higher activity in the catalytic experiments.

Fig. 3 FESEM images of synthesised nanocatalysts: (a) CZAZ–UNC, (b) CZAZ–HP, (c) CZAZC–UNC and (d) CZAZC–HP.

3.1.3 PSD analysis. Size distribution histograms of CZAZC–UNC and CZAZC–HP nanocatalysts, which were calculated using ImageJ software, are illustrated in Fig. 4. To compare the effect of the synthesis method on particle size, two ceria promoted CZAZ samples which were fabricated by the HP and UNC methods were studied using ImageJ software. The particle sizes of the CZAZC–UNC and CZAZC–HP nanocatalysts are distributed between 16.9–66.5 nm and 9.8–54.1 nm, respectively. In the CZAZC–UNC nanocatalysts, most of the particles are in the range from 30–40 nm and for CZAZC–HP nanocatalysts, they are in the range from 20–30 nm. This means that the HP method is capable of synthesising smaller particles than the UNC method.

Fig. 4 Surface particle size distribution histogram of synthesised nanocatalysts: (a) CZAZC–UNC and (b) CZAZC–HP.

3.1.4 EDX analysis. EDX spectra of the CZAZ and CZAZC nanocatalysts are presented in Fig. 5. All of the elements used in the fabrication method can be observed clearly in the EDX spectra. As is seen, the main Cu active site for the steam methanol reforming process in both CZAZ–HP and CZAZC–HP nanocatalysts is well dispersed. Also, Zn species have a very low aggregation and are well dispersed on the surface of this sample. From the EDX results, a comparison between parent solutions and surface elements was carried out and is shown in Fig. 6. Producing homogeneous powders is a feature of the UNC method; the surface analysis of CZAZ–UNC and CZAZC–UNC nanocatalysts does not fit well with its composition, but the results of EDX dot mapping and parent solution are in good accordance for the CZAZ–HP and CZAZC–HP nanocatalysts.

Fig. 5 EDX analysis of synthesised nanocatalysts: (a) CZAZ–UNC, (b) CZAZ–HP, (c) CZAZC–UNC and (d) CZAZC–HP.

Fig. 6 Parent solution vs. surface chemical analysis of synthesised nanocatalysts: CZAZ–UNC, CZAZ–HP, CZAZC–UNC and CZAZC–HP.

3.1.5 BET analysis. The specific surface area of the synthesised nanocatalysts is illustrated in Table 1. As is shown by XRD, FESEM and PSD results, the crystallite size and particle size of HP nanocatalysts compared to UNC nanocatalysts are smaller and narrower. This result as an influential parameter is in accordance with BET analysis in that the specific surface area of nanocatalysts synthesised by HP have the highest specific area. Besides, due to the negative effect of the UNC method in reducing specific surface area, alumina’s effect on the enhancement of surface area is neutralized. This is because of the nature of the UNC method to provide a foam with more micropores than nanopores.

3.1.6 FTIR analysis. Fig. 7 illustrates the FTIR spectra of the fabricated nanocatalysts showing their functional groups. Generally, it can be understood that the spectra reveals significant peaks for all the samples, especially the CZAZC ones. This is due to same functional groups being present in the synthesized nanocatalysts. As has been considered in the literature, metal oxides lead to stretching frequencies in the range of 480–800 cm⁻¹.³³ More precisely, the absorption bands at around 480 cm⁻¹ and 590 cm⁻¹ can be assigned to Cu–O and Zn–O species, respectively.^43,44 The band at 1640 cm⁻¹ can be assigned to interlayer water molecules.^5,45,46 Hydroxide groups are also detected by the broad stretching band around 3450 cm⁻¹; these may be loosely adsorbed by the nanocatalysts.^47–49 The presence of surface water in the nanostructured catalysts is due to the adsorption of water molecules during different synthesis steps.^50–52 Urea that was used in both synthesis methods consists of hydrogen and carbon atoms. So, it can produce carbon dioxide during its reaction with nitrates or thermal decomposition. Therefore, it was expected to find carbonated species formed by adsorbed CO₂ on the nanocatalyst surface which appear at 1405 cm⁻¹.^53–55

Fig. 7 FTIR spectra of synthesised nanocatalysts: (a) CZAZ–UNC, (b) CZAZ–HP, (c) CZAZC–UNC and (d) CZAZC–HP.

3.2 Catalytic performance study toward methanol steam reforming

3.2.1 Methanol conversion. The catalytic performance of the nanocatalysts fabricated by homogeneous precipitation and urea-nitrate combustion methods were investigated in a fixed bed reactor through the steam reforming of methanol reaction. The methanol conversion of synthesized nanocatalysts is shown in Fig. 8. Fig. 8a illustrates the effect of synthesis method on the methanol conversion of CZAZ nanocatalysts; it was also investigated for the ceria promoted CZAZ nanocatalysts as shown in Fig. 8b. It can be seen that application of the homogeneous precipitation method for the synthesis of CZAZ or CZAZC nanocatalysts leads to higher conversion. However, this effect is more significant for the CZAZ samples which reached complete conversion at about 220 °C. So, it can be concluded that the synthesis method has a more important role on the methanol conversion of the fabricated nanocatalysts. However, this was predictable according to the prominent characteristic properties of the nanocatalysts synthesized by homogeneous precipitation. The differences between these two synthesis methods are more apparent at low temperatures where the role of nanocatalyst is more significant. These results are much better than in previous work in which this kind of nanocatalyst was fabricated using different synthesis methods, especially co-precipitation ones. Some examples are listed in Table 2. It can be seen that all previous works which fabricated CZAZ or CZAZC catalysts by the conventional co-precipitation method had a lower methanol conversion than the present work. This difference exists even between the samples prepared by the impregnation method, a popular and simple synthesis method. So, comparison between these works and the CZAZ–HP sample shows the higher efficiency of this fabricated nanocatalyst for converting methanol to products, especially hydrogen, at low temperatures.

Fig. 8 Influence of synthesis method and CeO₂ addition on methanol conversion over synthesised nanocatalysts: (a) CZAZ–UNC vs. CZAZ–HP and (b) CZAZC–UNC vs. CZAZC–HP.

Table 2 Catalytic performance comparison of promoted CuO/ZnO/Al₂O₃ catalysts synthesized by different methods

Sample	Synthesis method	Composition (wt%)	T (°C)	Methanol conversion	Reference
CuO/ZnO/ZrO2/Al2O3	Co-precipitation	30/30/30/10	270	92.4	8
CuO/ZnO/CeO2/ZrO2/Al2O3	Co-precipitation	30/30/9/27/10	270	89.4	8
CuO/ZnO/ZrO2/Al2O3	Co-precipitation	30/40/20/10	270	80	57
CuO/ZnO/CeO2/ZrO2	Co-precipitation	45/20/17.5/17.5	260	100	56
CuO/ZnO/ZrO2/Al2O3	Impregnation	15/15/10/60	270	70	27
CuO/ZnO/CeO2/Al2O3	Impregnation	10/10/40/40	350	96	58
CuO/ZnO/CeO2/ZrO2/Al2O3	Homogeneous precipitation	30/30/10/20/10	240	100	Present study
CuO/ZnO/CeO2/ZrO2/Al2O3	Urea-nitrates combustion	30/30/10/20/10	280	100	Present study
CuO/ZnO/ZrO2/Al2O3	Homogeneous precipitation	40/30/20/10	220	100	Present study
CuO/ZnO/ZrO2/Al2O3	Urea-nitrates combustion	40/30/20/10	260	100	Present study

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On the other hand, it can be understood that addition of ceria had no significant influence on enhancing methanol conversion while in some cases it partially decreased the conversion values. Some reasons can account for this behaviour of ceria in CZAZ nanocatalysts. As mentioned in previous work, CeO₂ improves methanol conversion⁸ by ameliorating the dispersions of CuO and ZnO in the nanocatalysts, but the steam methanol reforming reaction was clearly weakened as CeO₂ was doped. The interactions between CeO₂ and Al₂O₃ have a negative effect on CeO₂ and ZnO interaction as an effective parameter in the steam methanol reforming reaction.⁸ Therefore, the methanol conversion values of the CZAZC nanocatalysts are lower than the CZAZ samples due to the negative effect of ceria on ZnO as an effective parameter on methanol conversion. On the other hand, the interaction between ceria and zirconia can have an influence on the good interaction between CuO and ZnO as the main active phases of the SRM reaction to decrease methanol conversion.⁵⁶

3.2.2 Product selectivity. Fig. 9 depicts the effect of synthesis method on product selectivity. High H₂ selectivity and low CO selectivity are two important parameters for selecting suitable steam methanol reforming catalysts. An at-a-glance look demonstrates that all the samples have suitable performance for the production of hydrogen (around stoichiometric value) as the main desired product. At higher temperatures the amount of CO increases because the reverse water–gas shift reaction proceeds. In this research, it is observed that the addition of CeO₂ reduces CO selectivity in accordance with previous work.⁸ It is worth noting that selection of a good catalyst for the steam reforming of methanol reaction should be done with regard to methanol conversion and product selectivity. It means that a catalyst must be chosen which reaches complete conversion at a lower temperature and has an appropriate selectivity for products. So, according to methanol conversion results it can be concluded that the CZAZC–HP nanocatalyst which reached complete conversion at 240 °C is the best sample in this work. This choice is made in terms of hydrogen and carbon monoxide selectivity. Therefore, it can be concluded that the samples prepared by the homogeneous precipitation and promoted by ceria have better performance for the production of fuel cell grade hydrogen with a low amount of CO from the steam methanol reforming process.

Fig. 9 Influence of synthesis method and CeO₂ addition on product selectivity over the synthesised nanocatalysts: (a) CZAZ–UNC, (b) CZAZ–HP, (c) CZAZC–UNC and (d) CZAZC–HP.

3.2.3 Time on stream performance. The results of stability tests for the CZAZC–HP nanocatalyst for 1200 min are shown in Fig. 10. As seen, the methanol conversion remained fixed for 1200 min. Also, the CO and H₂ selectivity stayed approximately constant. The fine particle size distribution, good dispersion and high surface area of the CZAZC–HP nanocatalyst improve its stability. Also, CeO₂ is able to oxidize the carbon deposited on the nanocatalyst surface. The selectivity of the products stays nearly constant.

Fig. 10 Time on stream performance of the CZAZC–HP nanocatalyst.

4 Conclusions

The effects of preparation method and promoter addition on the physicochemical and catalytic properties of CuO–ZnO–Al₂O₃–ZrO₂ (CZAZ) nanocatalysts were studied. XRD patterns show that the homogeneous precipitation method improves Cu dispersion compared to the urea-nitrate combustion method. FESEM analysis indicates that HP-nanocatalysts have more homogeneous and smaller particles than UNC-nanocatalysts. EDX dot mapping analysis shows a good dispersion of Cu and Zn elements on the HP-nanocatalysts. FTIR analysis depicts that metal oxides were formed during both the preparation methods and no unwanted functional groups were found on the synthesized nanocatalysts. Catalytic performance tests demonstrate that the HP-nanocatalysts, due to their characteristic advantages, gave low CO selectivity and high methanol conversion. Moreover, CeO₂ addition decreases CO selectivity and modifies Cu dispersion, however, CeO₂ affects the interaction between ZnO and CuO, and reduces methanol conversion.

Acknowledgements

The authors gratefully acknowledge Sahand University of Technology for financial support of the research as well as the Iran Nanotechnology Initiative Council for complementary financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03858a

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