multi-composition cu-zn-al catalyst supported on...

European Journal of Scientific Research ISSN 1450-216X Vol.28 No.1 (2009), pp.141-154 © EuroJournals Publishing, Inc. 2009 http://www.eurojournals.com/ejsr.htm

Multi-Composition Cu-Zn-Al Catalyst Supported on ZSM-5 for

Hydrogen Production

Zahira Yaakob Faculty of Chemical and Process Engineering, Faculty of Engineering

National University of Malaysia (UKM), Bangi, Selangor, 43600 E-mail: [email protected]

Tel: (603) 8921-6025

M.N. Satheesh Kumar Faculty of Chemical and Process Engineering, Faculty of Engineering

National University of Malaysia (UKM), Bangi, Selangor, 43600

M.A. Ibrahim Faculty of Chemical and Process Engineering, Faculty of Engineering


W.R.W. Daud Faculty of Chemical and Process Engineering, Faculty of Engineering


A.A.H. Kadhum Faculty of Chemical and Process Engineering, Faculty of Engineering


Abstract

Multi-composition Cu-Zn-Al (CZA) catalyst system supported on aluminosilicate zeolite (ZSM-5) was prepared by the impregnation method. The experiments were performed by varying the metal composition using statistical experimental design method namely Simplex Centroid Design. A series of catalysts were prepared by varying the different metal content in the composition up to 15 % supported on 85% ZSM-5. Brunauer-Emmett-Teller method (BET) was adopted to characterise the surface area of the prepared catalyst. The reduction properties of the catalyst were ascertained using temperature programmed reduction (TPR) technique. The CO adsorption capabilities of the catalysts at 60, 200, 225, 250, 275 and 300 °C have been investigated using chemisorption method. X-ray diffraction (XRD) technique was adopted to evaluate the structure of the catalysts. The shape and morphology of the catalysts were analysed using scanning electron microscopy–energy dispersive x-ray (SEM-EDX). The multifunctional Cu, Zn and Al catalyst supported on ZSM-5 showed a high BET surface area of about 132 ~ 196 m2/g. The TPR profiles showed the occurrence of reduction in a temperature range of 230 – 260ºC. The performance of the catalyst in stream reforming methanol (SRM) and auto thermal steam reforming methanol (ATRM) processes suggested that, a maximum hydrogen yield of 73.09 and 76.88 mol% could be obtained with Cu6Zn7Al2/ZSM-5 catalyst in SRM and

Multi-Composition Cu-Zn-Al Catalyst Supported on ZSM-5 for Hydrogen Production 142

ATRM process respectively. The observed carbon monoxide (CO) formation in SRM and ATRM process was 0.12 and 0.77 mol% respectively. Keywords: Cu-Zn-Al/ZSM-5 catalyst, hydrogen, PEMFC, SEM-EDX, SRM, ATRM,

XRD, Fuel Cell Introduction The diminution of fossil fuel reserves and increased green house gas emission has directed the global research to capture the potential benefits of hydrogen economy from renewable resources [1]. The increased emission from the internal combustion engines suggests for alternative power sources [2]. Hydrogen fuel cells are an alternative to internal combustion engines which run on petroleum based hydrocarbon fuels. As the hydrogen fuel cells are energy efficient and produce only water and heat, it can minimize the global energy and environmental problems [3]. The proton exchange (or polymer electrolyte) membrane fuel cells are becoming popular due to its low operating temperatures for on board power generation [4]. It is undesirable to store the hydrogen as liquid or high pressure gas on board. It is convenient and preferable to have a hydrogen source that can supply hydrogen whenever is needed. Though the hydrogen fuel cells are well established, the infrastructure for the onboard catalytic generation and supply of hydrogen from liquid or liquefied fuels needs to be addressed [5].

The methanol appears to be the best source for hydrogen fuel among other high density liquid fuels due to (i) renewable resource (ii) low boiling point (iii) high H/C ratio and (iv) easy storage [6-8]. The fuel processing technologies such as steam reforming (SRM) [9, 10], partial oxidation of methanol (POM) [11, 12] and combination of SRM and POM that is auto thermal steam reforming methanol (ATRM) [13, 14] have been employed for the hydrogen production from methanol. The ATRM process is known to produce relatively high hydrogen concentration. But the ATRM also produces a CO as a by-product which has negative impact when used in polymer electrolyte or membrane modified fuel cells (PEMFC). The formed CO is expected to poison the platinum electrode and weakening the anode catalyst of PEMFC. It has been reported in the literature that the level of CO produced in SRM process over copper based catalysts can be influenced by factors such as reaction temperature [15], molar ratio of methanol and water [16], contact time, conversion of methanol, oxygen feed to methanol steam mixture [17], heterogeneity of the copper surface, particle size and mechanical treatment of the catalyst [18]. The catalysts employed in fuel processing technology plays a major role and it should have the requirements such as low cost, high activity, good durability, and electrical conductivity.

It is very essential to prevent the entry of CO into fuel cell in order to protect the electrodes. The possibilities of prevention of CO entry into fuel cell are (i) attaching an extra unit between the steam reforming reactor and the fuel cell (ii) integration of purification system in the reformer reactor and (iii) development of new catalyst which has the capability to produce little or no CO [18]. The selection of copper based catalyst was reported as the more effective and suitable in terms of activity and hydrogen-selectivity for SRM [19]. The utilisation of various copper mixed oxide catalysts such as Cu-Zn-Al [20] Cu-Zn-Al-Zr [21] Cu-ZnO-ZrO2 [22] Cu/CeO2/Al2O3 [23], Cu-ZnO-ZrO2-Al2O3 [24], Cu-Zn-V-Al [25] and Cu-Zn-Pd-Al [26] have been reported in the literature.

The systematic investigation reported in the literature revealed that the appropriate composition of Cu-Zn-Al catalyst has high selectivity for hydrogen production through SRM [15, 19, 20 & 27] and ATRM [27]. It is also important to select the proper support system for the catalyst which has vital role of maintaining the high surface area. The present research study utilised the ZSM-5 as a support material for Cu-Zn-Al catalyst system. ZSM-5 is a zeolite with a high silica and low aluminum content. Its structure is based on channels with insecting tunnels [28]. The ZSM-5 zeolite catalyst is used in the petroleum industry for hydrocarbon interconversion. Although there are many reports available in the literature on different support system, the literature lacks the information on Cu-Zn-Al system supported on ZSM-5. Hence the present research investigation is concentrated to prepare a series of

143 Zahira Yaakob, M.N. Satheesh Kumar, M.A. Ibrahim, W.R.W. Daud and A.A.H. Kadhum

Cu-Zn-Al catalyst supported on ZSM-5. The prepared catalysts were characterised using different techniques and evaluated the performance of catalysts in both SRM and ATRM processes. Experimental Materials

The support material ZSM-5 having Si/Al ratio of 23 and surface area of 425 m2/ was procured from Zeolyst International Inc. USA. The source of copper, zinc and aluminium are Cu(NO3)2.3H2O, Zn(NO3)2.6H2O and Al(NO3)3.9H2O respectively. Catalyst Preparation

Catalysts based on Cu-Zn-Al supported on ZSM-5 were prepared by impregnation method. The weight percentage of Cu-Zn-Al and ZSM-5 maintained in the catalyst composition was 15 and 85%. A series of catalysts having different weight percentage of Cu, Zn and Al were prepared. The experiments were designed to have the catalyst composition with different metal combination using Design Expert Software (Table 1). The catalysts were prepared slowly by adding ZSM-5 powder into a solution containing copper nitrate, zinc nitrate and aluminium nitrate. The resultant mixture was heated at 50-70°C until it dry followed by overnight drying at 110ºC and calcination at 550ºC for 4 hours. Table 1: Catalyst compositions of Cu-Zn-Al/ZSM-5 with 15 wt. % metal loading

Compositions (wt. %) No. Sample Cu Zn Al ZSM-5 1. Cu8Zn5Al2/ZMS-5* 8.0 5.0 2.0 85.0 2. Cu67Zn57Al27/ZMS-5 6.7 5.7 2.7 85.0 3. Cu6Zn7Al2/ZMS-5 6.0 7.0 2.0 85.0 4. Cu6Zn5Al4/ZMS-5 6.0 5.0 4.0 85.0 5. Cu53Zn63Al33/ZMS-5 5.3 6.3 3.3 85.0 6. Cu47Zn77Al27/ZMS-5 4.7 7.7 2.7 85.0 7. Cu47Zn57Al47/ZMS-5 4.7 5.7 4.7 85.0 8. Cu4Zn9Al2/ZMS-5* 4.0 9.0 2.0 85.0 9. Cu4Zn7Al4/ZMS-5* 4.0 7.0 4.0 85.0

10. Cu4Zn5Al6/ZMS-5* 4.0 5.0 6.0 85.0 Catalyst Characterisation

The surface area of the prepared catalysts were characterised using BET measurements. The samples were treated at 120 ºC for 30 min to ensure a clean surface prior to the adsorption isotherm test. The specific surface area of the calcined catalyst samples were calculated from the acquired nitrogen (N2) adsorption-desorption data. A cross-sectional area of 16.2Ǻ of the N2 molecule was assumed in the calculations of the specific surface area using the BET method.

The reduction properties of the prepared calcinied catalyst samples were measured using temperature programmed reduction (TPR) method. About 100 to 150 mg of the calcined samples were placed in U-quartz tube and Argon was used to sweep through the sample treated at 120 ºC for 1 hour. After this, the temperature was reduced to 50 ºC before the TPR evaluation. The samples were reduced up to 600 ºC for 1 hour in a gas mixture consisting of 5 % H2/Ar at heating rate of 10oC/min. The reduction signal was detected using thermal conductivity detector (TCD).

The chemisorptions method was adopted to evaluate the CO adsorption capability of the catalysts at 60, 200, 225, 250, 275 and 300 °C. The shape and distribution of metal in the catalysts were determined using Scanning Electron Microscopy (SEM); LEO 1450VP Variable Pressure Scanning Electron Microscope (VPSEM) with Energy Dispersive X-ray Microanalysis System (EDS).


The powder X-ray Diffraction (XRD) patterns were recorded with SIEMENS diffractometer (Model D5000). The radiation source was CuKα (nickel filtered). The selected scan angle (2θ) speed was 2 – 90° and 2.4°/ min respectively. Catalytic Test

Catalytic activity measurements were carried out in a in a stainless steel tubular packed-bed micro reactor (6.35mm i.d.) at atmospheric pressure in the temperature range of 150 to 300°C (with 25ºC intervals). The catalyst samples were reduced at 400°C for 2 hours in 10 % H2–N2 stream prior to the activity evaluation. 500 mg catalyst was held between the quartz wool inside the micro reactor. A premix of water and methonal was fed into the custom-built vaporiszer using a syringe pump. The purified air and nitrogen (carrier) flow was adjusted using mass flow controllers and mixed with the water-methanol vapor in a static mixer. The adjusted total flow was 100 ml / min with H2O/CH3OH and O2/CH3OH ratio of 1.3 and 0.1 respectively. All pipe lines were heated to avoid liquid condensation in the system. Product Analysis

The gas composition from the outlet of the reactor after the reaction were analyzed using gas chromatograph (Perkin Elmer, Autosystem XL) with a thermal conductivity detector (TCD), flame ionization detector (FID) and methanizer. A Hayesep Q column was used for separation of CO2, H2O and CH3OH. A 13× column molecular sieve was used for the separation of H2, O2, N2, CH4 and CO. The FID and methanizer were used for the detection of CO concentration less than10 ppm. Results and Discussion BET Surface Area

The BET surface area, bulk compsoition and atomic ratio of the prepared catalysts having different composition are given in Table 2. The BET surface area of all the prepared catalyst found to be lower than that of ZSM-5 support. This can be attributed to the fact that, the strong acid character of metal nitrates present in the catalyst composition may destroy the ZSM-5 structure which is less resistant to acid [29]. In addition to this the thermal and chemical treatment may also impart some degree of defects in ZSM-5 during catalyst preparation. The observed highest and lowest BET surface area is 196 m2/g and 132 m2/g for the catalysts Cu4Zn5Al6/ZSM-5 and Cu4Zn7Al4/ZSM-5 respectively. The high surface area in case of Cu4Zn5Al6/ZSM-5 compared to other catalyst composition could be due to the presence of high amount of Al content (6 wt%) which can minimize the reduction of BET surface area [30]. It can be noticed from the Table 1 that, the surface area of the catalysts have decreased with increasing the copper or zinc content in the formulation. A similar behaviour was noticed by Gervasini and co-workers [31] in copper-exchanged ZSM-5 and ETS-10 catalyst system.


Table 2: BET surface area, Bulk Compositions and Calculation of Atomic Ratio of catalysts

Bulk Compositions* (wt. %) Atomic Ratio** No Sample SBET (m2/g) Cu Zn Al (Cu+Zn) (Cu+Zn) / Al 1. Cu4Zn5Al6/ZSM-5 196 4.80 4.79 5.22 9.59 1.84 2. Cu8Zn5Al2/ZSM-5 185 10.14 5.83 2.22 15.97 7.19 3. Cu53Zn63Al33/ZSM-5 183 7.44 6.17 3.45 13.61 3.94 4. Cu47Zn77Al27/ZSM-5 173 5.05 7.59 2.88 12.64 4.39 5. Cu47Zn57Al47/ZSM-5 170 5.10 5.57 4.50 10.67 2.37 6. Cu4Zn9Al2/ZSM-5 167 3.90 9.38 4.37 13.28 3.04 7. Cu6Zn7Al2/ZSM-5 164 5.33 4.23 3.50 9.56 2.73 8. Cu67Zn57Al27/ZSM-5 155 10.85 5.70 3.06 16.55 5.41 9. Cu6Zn5Al4/ZSM-5 154 6.88 5.72 4.49 12.60 2.81 10. Cu4Zn7Al4/ZSM-5 132 4.31 6.80 4.16 11.11 2.67

Note (*) – Bulk compositions determined by SEM-EDX. (**) – Atomic ratio derived by SEM-EDX.

Temperature Programmed Reduction (TPR)

The TPR is a useful technique to ascertain the state of the metallic components and reduction properties of the metal oxide precursor [32].The low reduction temperature is an indication for the presence of more active CuO and its involvement in the reaction is expected to enhance the methanol conversion for hydrogen production [24]. The TPR profiles of all the prepared Cu-Zn-Al catalyst having different metal composition supported on ZSM-5 is shown in Figure 1. All catalyst samples found to reduce at a temperature less than 260 °C indicated that CuO species dispersed on Cu-Zn-Al/ZSM-5 were easily reduced. These observations are in agreement with the results reported by Takeguschi and co-workers [33]. From Figure 1, it can be noticed that, catalysts have a clear peak temperature of reduction from 230 to 260 °C when the Cu loading was decreased from 8 to 4 wt. %. In case of Cu8Zn5Al2/ZSM-5 catalyst, a starting reduction at 210 °C followed by a shoulder peak at 230 °C and reduction completion at 300 °C was noticed. This result explains that the (i) Cu species were essentially identical irrespective of Cu loading with the catalysts containing 8 wt. % Cu and (ii) dispersed Cu species may require longer time to reduce completely. The clear single peak in case of all catalyst samples revealed one step reduction of Cu2+ directly to Cu0. Bulanek et al [34] noticed a one step reduction with two different peak temperature in case of Cu supported on zeolites prepared via ion-exchange method. The observed TPR trend is almost similar irrespective of different preparation methods [34] and multi-component metal composition.

Figure 1: TPR Profile of Cu-Zn-Al/ZSM-5 Catalysts with Different Composition Content


CO Adsorption (Chemisorption)

CO adsorption is one of the useful technique to ascertain the nature of dispersed transition metal species over catalytic surfaces [35]. The CO adsorption capabilities of the catalysts were investigated at both low and high temperature. The adsorption of CO was measured at a temperature of 60, 200, 225, 250, 275 and 300 ºC. The CO adsorption results for Cu-Zn-Al/ZSM-5 catalysts having different metal composition are shown in Table 3. All the prepared catalyst has shown the CO adsorption capability at both low and high temperature. This may be due to the fact that, the CO is a weak σ-donor and π-acceptor may interact specifically with cationic Lewis acid sites in ZSM-5. Also, CO bonds to the surface of the cation may depend on the charge and coordination symmetry of the metal cation [36]. The CO adsorption of the catalysts at 60 ºC followed a sequence: Cu4Zn7Al4/ZSM-5 (122.9µmol/g) > Cu6Zn7Al2/ZSM-5 (105.5µmol/g) > Cu67Zn57Al27/ZSM-5 (102.8 µmol/g). The CO adsorption capabilities of the prepared catalyst at higher temperature above 60 ºC are relatively low. Generally CO is expected to adsorb at lower temperature than at higher temperature. This phenomenon of reduced CO adsorption with increase in temperature is inline with the results revealed elsewhere [37, 38]. Table 3: CO Chemisorptions of Catalysts at Different Temperatures.

CO Chemisorptions (µmol/g) No. Sample 60 ºC 200 ºC 225 ºC 250 ºC 275 ºC 300 ºC 1. Cu8Zn5Al2/ZSM-5 54.7 20.1 28.6 84.1 46.5 38.5 2. Cu67Zn57Al27/ZSM-5 102.8 23.1 33.8 22.8 20.8 57.5 3. Cu6Zn7Al2/ZSM-5 105.5 10.5 31.4 21.8 86.6 60.9 4. Cu6Zn5Al4/ZSM-5 65.1 41.4 42.3 15.7 36.0 7.9 5. Cu53Zn63Al33/ZSM-5 53.6 44.7 77.1 18.2 52.7 69.3 6. Cu47Zn77Al27/ZSM-5 34.1 37.1 26.1 25.8 13.4 25.9 7. Cu47Zn57Al47/ZSM-5 21.3 45.3 44.9 28.5 67.3 23.3 8. Cu4Zn9Al2/ZSM-5 72.7 24.0 8.0 54.8 89.4 18.9 9. Cu4Zn7Al4/ZSM-5 122.9 43.4 32.5 45.9 8.7 44.1 10. Cu4Zn5Al6/ZSM-5 43.5 42.2 37.8 27.6 15.4 60.0

Table 4: Catalyst performance in steam methanol reforming (SRM) experiments at 300 ºC.

No. Catalyst H2 Yield (mol %) CO2 (mol %) CO (mol %) Methanol Conversion (%)

1. Cu6Zn7Al2/ZSM-5 73.09 14.62 0.12 82.80 2. Cu6Zn5Al4/ZSM-5 70.91 14.18 0.23 85.49 3. Cu67Zn57Al27/ZSM-5 68.68 13.74 0.21 82.83 4. Cu4Zn5Al6/ZSM-5 68.20 13.64 2.11 84.12 5. Cu4Zn9Al2/ZSM-5 58.61 11.7 0.29 71.14 6. Cu53Zn63Al33/ZSM-5 49.92 9.98 0.19 60.48 7. Cu47Zn77Al27/ZSM-5 33.41 6.68 0.16 40.38 8. Cu47Zn57Al47/ZSM-5 28.78 5.76 0.45 35.05 9. Cu4Zn7Al4/ZSM-5 21.12 4.22 0.22 25.55

10. Cu8Zn5Al2/ZSM-5 18.67 3.73 0.20 22.77 SEM-EDX Study

SEM-EDX was employed to investigate how the active material was dispersed on the ZSM-5 supports. The SEM microphotgraphs of Cu8Zn5Al2/ZSM-5, Cu4Zn9Al2/ZSM-5, Cu4Zn7Al4/ZSM-5 and Cu4Zn5Al6/ZSM-5 catalysts are shown in Figure 1. The catalysts have showed an unclear morphology and aggregates with dimensions of about 1-2 µm. The co-existence of copper and zinc as mixed oxides on the Cu-Zn-Al/ZSM-5 catalyst surface was noticed. This observation was similar to the observation made by Lindstrom et al [15]. The EDX analysis (Table 2) has shown an even distribution of copper on the surface of the catalyst. The size of the copper particles found to have a direct link to the catalytic


composition. High amount of copper in the composition may lead to have larger particles more in the system. The presence of copper content in a range 6-8% in the catalyst may form large crystal lattice.

Figure 2: SEM Micrographs of Cu-Zn-Al/ZSM-5

Cu8Zn5Al2/ZSM-5 Cu4Zn9Al2/ZSM-5

Cu4Zn7Al4/ZSM-5 Cu4Zn5Al6/ZSM-5

X-ray Diffraction (XRD)

The XRD patterns for the Cu-Zn-Al/ZSM-5 are shown in Figure 3. The introduction of metal content into ZSM-5 found to reduce the peak intensity. The 2θ values for calcinied ZSM-5 was reported in the literature as 7.94°, 8.01° and 8.90° [39]. The well-defined peaks of shifted ZSM-5 was recorded at 7.80°, 7.82°, 7.84°, 7.90°, 7.93°, 7.94°, 8.74°, 8.75°, 8.80°, 8.86°and 8.99° compared to original peak of calcined ZSM-5 at 7.94°, 8.01° and 8.90° [40]. This X-ray diffraction with a low angle of incidence may be due to the acid character of metal solution which presents within the ZSM-5 structure. A similar trend was observed by Hagey & Lasa [40] with respect to ZSM-5. In general, Cu and Zn existed as separate oxides (CuO and ZnO) in Cu-Zn-Al/ZSM-5 catalysts.. The diffraction line for Cu53Zn63Al33/ZSM-5 catalyst at 2θ of 35.48°, 38.7°, 51.4° and 61.5° could be attribute to the presence of CuO [41]. CuO species were also detected in Cu67Zn57Al27/ZSM-5, Cu6Zn5Al4/ZSM-5, Cu4Zn9Al2/ZSM-5 and Cu4Zn7Al4/ZSM-5 catalysts at 2θ of 35.48° and 38.8°. The detection of CuO in the catalysts suggested that CuO is in amorphous state on the support [42]. The Nanba and co-workers [43] have noticed a peak for CuO with the Cu content around 6.4 wt% in the Cu-ZSM-5 catalyst prepared via ion-exchange method. The value of 2θ at 36.1° for Cu67Zn57Al27/ZSM-5, Cu6Zn5Al4/ZSM-5, Cu4Zn9Al2/ZSM-5 and Cu4Zn7Al4/ZSM-5 corresponds to ZnO.


Figure 3: XRD patterns of Cu-Zn-Al/ZSM-5 catalysts where Δ represent shifted ZSM-5; � represent CuAl2Si10O29.51.2H2O; represent CuO; ⌂ represent ZnO; represent Al 4.2Cu3.2Zn0.7 and represent CuSiO3.H2O.

Catalyst Performance

In Steam Methanol Reforming (SRM) Process The performance of the catalysts in SRM process is given in Table 4. It is evident from the results that Cu6Zn7Al2/ZSM-5 catalyst has superior performance compared to other catalyst formulations. The observed maximum H2 yield and methanol conversion at 300 °C for Cu6Zn7Al2/ZSM-5 catalyst was 73.09 mol % and 82.80 % respectively. Also it can be observed that, Cu6Zn7Al2/ZSM-5 catalyst has generated highest CO2 with lowest CO concentration (0.12 mol %) compared to other catalyst formulations. The H2 yield, CO2 yield and methanol conversion for Cu6Zn7Al2/ZSM-5 catalyst is shown in Figure 4. The CO concentration found to increase with increase in methanol conversion and temperature. The observed methanol conversion for Cu6Zn7Al2/ZSM-5 catalyst was 82.80 % at 300°C. The H2 yield, CO2 yield and methanol conversion was low and less than 10.0 mol % at a temperature of 175 - 225 °C. From these results, it can be inferred that, the catalyst activity increases when the reaction temperature rises to 250 °C and above. Due to the low reduction temperature of CuO (230 - 260 °C) as observed in TPR profile (Figure 1), the actual activity of reaction becomes active at 275 - 300 °C. The reduction of catalyst at less than 260 °C is an indication for the proper dispersion of CuO in Cu6Zn7Al2/ZSM-5 [34].


Figure 4: Activity of Cu6Zn7Al2/ZSM-5 Catalyst for SRM.

In Auto Thermal Reforming Methanol (ATRM) Process The performance of the catalysts in ATRM process is given in Table 5. The Cu4Zn7Al4/ZSM-5 catalyst has showed the better performance compared to other catalyst system. The complete methanol conversion was noticed at a temperature of 300°C for Cu4Zn7Al4/ZSM-5 catalyst. The maximum H2 and CO2 yield noticed in this process was 76.88 and 21.97 mol % at 300 °C. Like SRM, ATRM has also shown an increased CO concentration with increase in the reaction temperature and methanol conversion. The H2 yield, CO2 yield and methanol conversion for Cu4Zn7Al4/ZSM-5 catalyst is shown in Figure 6. The activity of the catalyst found to be slow initially with only 15 mol % conversion at 150-200°C. The activity found to increase with the reaction temperature above 225 °C. This may be due to the reduction of the catalyst at around 200 °C itself (Figure 1) before reaching the maximum reduction peaks at around 225 to 260 °C. At 225 °C, the activity H2 yield and methanol conversion of Cu4Zn7Al4/ZSM-5 catalyst was almost 73 and 94 mol % respectively. Then, observed H2 yield was moderate with increasing the temperature from 225 to 300 °C. Table 5: Catalyst Performance in Auto Thermal Methanol Reforming (ATRM) Experiments at 300 ºC.

No. Catalyst H2 Yield (mol %) CO2 (mol %) CO (mol %) Methanol Conversion (%)

1. Cu4Zn7Al4/ZSM-5 76.88 21.97 0.77 100.00 2. Cu67Zn57Al27/ZSM-5 75.29 21.51 1.52 98.91 3. Cu53Zn63Al33/ZSM-5 75.16 21.47 1.53 98.56 4. Cu4Zn5Al6/ZSM-5 73.96 21.13 3.33 98.60 5. Cu4Zn9Al2/ZSM-5 70.99 20.28 1.24 96.44 6. Cu6Zn5Al4/ZSM-5 66.31 18.94 0.98 85.99 7. Cu47Zn77Al27/ZSM-5 62.51 17.86 0.63 60.88 8. Cu6Zn7Al2/ZSM-5 54.00 15.43 0.43 70.31 9. Cu47Zn57Al47/ZSM-5 46.90 13.40 0.28 60.88 10. Cu8Zn5Al2/ZSM-5 29.49 8.43 0.61 39.62

Performance comparison of the catalyst in SRM and ATRM process The observed methanol conversion in ATRM is relatively better compared to SRM process. Among the series of catalysts studied, the catalyst Cu6Zn7Al2/ZSM-5 and Cu4Zn7Al4/ZSM-5 have showed better methanol conversion (Figure 5a) in SRM and ATRM process respectively. The observed methanol conversion in ATRM process (close to 100%) is superior compared to that of SRM. The


observed CO concentration is high in ATRM process at all temperatures compared to SRM as shown in Figure 5b. The obtained methanol yield in ATRM process with Cu4Zn7Al4/ZSM-5 catalyst is higher compared to SRM with Cu6Zn7Al2/ZSM-5. The observed CO formation was more in ATRM compared to SRM process.

Figure 5: Performance of reforming process on (a) methanol conversion (b) CO concentration.

(a) (b)

The effect of atomic ratio [(Cu + Zn)/Al] of the catalysts (Table 2) on the methanol conversion, H2 yield, CO2 yield and CO selectivity in both SRM and ATRM is shown in Figure 6a and 6b. CO2 selectivity is defined as the ratio between the concentration of CO2 and the sum of the concentrations of CO and CO2. Generally, it can be observed from the Figure 6a and 6b that, the catalytic performance in terms of methanol conversion, H2 yield, CO2 yield and CO selectivity improved with increasing the atomic ratio (that is by decreasing the Al content) in the sample. The three major observations can be inferred though this evaluation. Firstly, catalytic performance in terms of H2 yield and methanol conversion was improved with increasing the atomic ratio from 2.73 to 5.41 in case of SRM and 2.37 to 5.41 in case of ATRM process. This observation clearly explains that the performance of the catalyst becomes better when the Al content in the composition decreases. The catalyst Cu6Zn7Al2/ZSM-5 having 6 wt % copper and an atomic ratio of 2.73 showed a better performance in SRM process. The catalyst Cu4Zn7Al4/ZSM-5 having 4 wt % copper and 2.67 atomic ratio was found to be better performed in ATRM process. The high performance of these catalysts can be attributed to the high copper surface area and copper dispersion [44]. The CO2 selectivity remained above 95.0% and the recorded CO selectivity was less than 5.0% in both SRM and ATRM process.


Figure 6: Evaluation of the effect of atomic ratio on the catalytic performance 300 °C in the (a)SRM and (b)ATRM process.

The Cu8Zn5Al2/ZSM-5 catalyst having the highest atomic ratio of 7.19 (having 8wt% copper) has shown the poor performance in both SRM and ATRM process. The catalyst having atomic ratio of 7.10 exhibited a lowest H2 yield of about 18.67 mol % and a methanol conversion of less than 23 % in SRM process. A similar trend was noticed for ATRM process with an H2 yield of about 29.49 mol % and a methanol conversion less than 40%. The high loading of copper in the catalyst may be unsuitable for the impregnation method which has negative impact on the catalyst performance. The performance of the catalyst found to reduce with increase in the atomic ratio could be due to the reduction in Cu metal surface area, Cu metal dispersion and increased Cu particle size [44]. Lindstrom et al. [45] made a similar observation and reported that the lower catalytic activity arises due to the presence of high copper content which causes formation of agglomerates and reduce the metal surface area.

The observed catalyst performance in terms of CO2 and CO selectivity found to be low with decreasing the atomic ratio from 7.19 to 1.84 in both SRM and ATRM process. In SRM process, CO2 selectivity has declined from 99.19 to 86.60 % and CO selectivity has increased from 0.81 to 13.40 %.


In case of ATRM process, CO2 selectivity was reduced from 97.95 to 86.39 % and CO selectivity increased from 2.05 to 13.61 %. These results suggested that aluminum content in the Cu-Zn-Al/ZSM-5 catalysts has a strong influence on the CO selectivity. The principal role of aluminum oxide is expected to enhance the dispersion of copper oxide species on its surface [46]. High loading of Al content is favorable for the stabilization of copper structure on the catalyst surfaces [47]. Hence the existence of Al content in a limited quantity in catalyst composition may be useful. The improved catalytic activity with decreasing Al content may be due to the enhanced copper reducibility, surface area and dispersion [48]. Conclusions The Cu-Zn-Al/ZSM-5 catalysts having varied metal content in the composition up to 15 wt. % supported on 85% ZSM-5 have been prepared through impregnation method and characterized with different techniques. The observed BET surface area of the catalyst lie in the range 132 - 196 m2/g. The TPR experiments showed a low reduction temperature in the range 230 - 260 °C. A much lower reduction temperature can be observed with increasing the Cu content from 4 to 8 wt% in the catalyst. The observed CO chemisorption of the catalyst was in the range of 7.9 - 122.9 μmol/g at all test temperatures. The SEM-EDX investigation revealed the shape, morphology of active metals, metal distribution and also composition on the supports. The crystalline structure of CuO and ZnO species in different catalysts system was identified through XRD analysis.

The multifunctional catalyst prepared in the present study based on Cu-Zn-Al/ ZSM-5 have been characterised and analysed to ascertain the hydrogen yield and CO formation in SRM and ATRM process. The catalytic performance test identified the two different best catalyst composition for SRM and ATRM process. The Cu6Zn7Al2/ZSM-5 and Cu4Zn7Al4/ZSM-5 catalyst composition are selected as the best catalyst for SRM and ATRM processes respectively. The Cu4Zn7Al4/ZSM-5 catalyst having 4 wt. % copper in ATRM process showed the highest activity in terms of H2 yield (76.88 mol %) and CO concentration (0.77 mol %). In case of SRM process the catalyst Cu6Zn7Al2/ZSM-5 having 6 wt% copper exhibited an H2 yield of 73.09 mol % and CO concentration of 0.12 mol %. The CO concentration was found to be lower in all catalyst system with SRM process.


References [1] P. Kruger: Int. Hydrogen Energy, 26, 1137 (2001). [2] B.D. McNicol, D.A.J. Rand, K.R. Williams: J. Power Sources 100, 47 (2001). [3] M. Conte, A. Iacobazzi, M. Ronchetti, R. Vellone:, J. Power Sources, 100, 171 (2001). [4] R.L. Borup, M.A. Inbody, T.A. Semelsberger, J.I. Tafoya, D.R. Guidry: Catal. Today, 99, 263

(2005) [5] N. Edwards, S.R. Ellis, J.C. Frost, S.E. Golunski, A.N.J. Keulen, N.G. Lindewald, J.G.

Reinkingh: J. Power Sources, 71, 123 (1998). [6] S. Velu, K. Suzuki, T. Osaki: Catal. Lett. 69, 43 (2000). [7] J.O.M. Bockris: Int. J. Hydrogen Energy, 24, 1 (1999). [8] S.T. Yong, K. Hidajat, S. Kawi: Journal of Power Sources 131, 91 (2004) [9] B. Lindstrom, L.J. Pettersson: J. Power Sources 106, 264 (2002). [10] J.Y. Xi, Z.F. Wang, G.X. Lu: Appl. Catal. A 225, 77 (2002) [11] J.P. Breen, J.R.H. Ross: Catal. Today, 51, 521 (1999) [12] S.P. Asprey, B.W. Wojciechowski, B.A. Peppley: Appl. Catal.: Gen., 179, 70 (1999). [13] K. Geissler, E. Newson, F. Vogel, T.B. Truong, P. Hottinger, A. Wokaun,: Phys. Chem. Chem.

Phys., 2001,º3, 289 (2001). [14] S. H. Chan, H. M. Wang: Journal of Power Sources, 126, 8 (2004) [15] B. Lindstrom, L.J. Petterson, P.G. Menon: Appl. Catal. A: General, 234, 111 (2002). [16] Y. Choi, H.G. Stenger, Catal. Today. 124, 432 (2003). [17] J. Agrell, M. Boutonnet, I. Melian-Cabrera, J.L.G. Fierro: Appl. Catal. A: General, 253, 201

(2003) [18] H. Purnama, T. Ressler, R.E. Jentoft, H. Soerijanto, R. Schlogl, R. Schomacker: Appl. Catal. A:

General, 259, 83 (2004). [19] M. Matsukata, S.Uemiya, E. Kikuchi: Chem. Lett., 761, (1988). [20] J.C. Amphlett, R.F. Mann, R.D. Weir: Canad. J. Chem. Eng., 66, 950 (1988). [21] S. Velu, K. Suzuki, M. Okazaki, M.P. Kapoor, T.Osaki, F. Ohashi: J. Catal., 194, 373 (2000). [22] J. Sloczynski, R. Grabowski, A. Kozlowska, P. Olszewski, M. Lachowska, J.Skrzypek, J.

Stoch: Appl. Catal. A: General, 249, 129 (2003). [23] X. Zhang, P. Shi: J. Mol. Catal. A. Chemical, 194, 99 (2003). [24] J. Agrell, H. Birgersson, M. Boutonnet, I. Melian-Cabrera, R.M. Navarro, J.L.G. Fierro: J.

Catal., 219, 389 (2003). [25] M.S. Mahmud, Z. Yaakob, W.R.W. Daud, A.B. Mohamad: Proceedings of the 18th

Symposium of Malaysian Chemical Engineers, Vol. I: 222-227 (2004). [26] J.M. Lorna, Z. Yaakob, W.D. Wan Ramli, M. Abu Bakar: Prosiding Seminar Pelajar Siswazah

FKEJ UKM, p173-178 (2004). [27] M. Turco, G. Bagnasco, U. Costantino, F. Marmottini, T. Montanari, G. Ramis, G. Busca: J.

Catal., 228, 43 (2004). [28] H.E.C. Heng, O.B. Yang: Chinese Chemical Letters, 15, 368 (2004). [29] J.P. Shen, C. Song: Catal. Today. 77, 89 (2004). [30] Z.Yaakob, M.A. Ibrahim, W.R.W. Daud & A.A.H. Kadhum: Proceeding of 2nd International

Conference on Chemical and Bioprocess Engineering in conjunction with 19th Symposium of Malaysian Chemical Engineers (SOMChe 2005), Kota Kinabalu, Sabah, Malaysia, p852-857 (2005).

[31] A. Gervasini, C. Picciau, A. Auroux: Micro porous and Mesoporous Mat. 35, 457 (2000). [32] F. Pinna: Catal. Today, 41, 129 (1998). [33] T. Takeguchi, Y. Kani, M. Inoue, K.Eguchi: Catal. Lett., 83, 49-53 (2002). [34] R. Bulanek, B. Wichterlova, Z. Sobalik: J. Tichy, Appl. Catal. B: Env., 31, 13 (2001). [35] S. Velu, K. Suzuki, M. Okazaki, Catalysis Letters 62, 159 (1999). [36] K. Hadjiivanov, G. Vayssilov: Advances in Catalysis, 47, 307 (2002). [37] K. Hadjiivanov, H. Knozinger; J. Catal., 191, 480 (2000).


[38] O. Bludsky, P. Nachtigall, P. Cicmanec, P. Knotek, R. Bulanek: Catalysis Today, 100, 385 (2005).

[39] M.M.J. Treacy, J.B. Higgins, J.B. Collection of Simulated XRD powder patterns for zeolites. 4th Rev. Ed. Amsterdam: Elsevier (2001).

[40] L. Hagey, H. Lasa: Chemical Engineering Science. 54, 3391 (1999). [41] Y.J. Huang, H.P. Wong, J.F.Lee: Chemsphere 39, 1347 (1999). [42] C.H. Wang, H.S. Weng: Applied Catalysis A: General, 170, 73 (1988). [43] T. Nanba, S. Masukawa, A. Ogata, J. Uchisawa, A. Obuchi, Applied Catalysis B:

Environmental, 61, 288 (2005). [44] S. Velu, K. Suzuki, M. Okazaki: Catalysis Letters, 62, 159 (1999). [45] B. Lindstrom, L.J.Pettersson: International Journal of Hydrogen Energy, 26, 923 (2001). [46] S. Murcia-Mascaros, R.M. Navarro, L. Gomez-Sainero, U. Costantino, M. Nocchetti,

J.L.G.Fierro: Journal of Molecular Catalysis A: Chemical, 194: 99 (2001). [47] X. Zhang, P. Shi: Journal of Molecular Catalysis A: Chemical, 194, 99 (2003). [48] S. Velu, K. Suzuki, M. Okazaki, M.P. Kapoor, T.Osaki, F. Ohashi: Applied Catalysis A:

General, 213, 47 (2001).

multi-composition cu-zn-al catalyst supported on...

Documents