Synthesis and Activity Test of Cu/ZnO/Al2O3 for the Methanol Steam Rforming as a Fuel Cell’s Hydrogen Supplier

IGBN Makertihartha, Subagji & Melia Laniwati Gunawan

Abstract：The synthesis of hydrogen from hydrocarbons through the steam reforming of methanol on Cu/ZnO/Al₂O₃ catalyst has been investigated. This process is assigned to be one of the promising alternatives for fuel cell hydrogen process source, Hydrogen synthesis from methanol can be carried out by means of methanol steam reforming which is a gas phase catalytic reaction between methanol and water, in this research, the Cu/ZnO/Al₂O₃ catalyst prpated by the fry impregnation was used. The specific surface area of catalyst was 194.69 m²/ gram, the methanol steam reforming (SRM) reaction was carried out by means of the injection of gas mixture containing g methanol and water with 1:1.2 mol ratio an 20-90 ml/minute feed flow rate to a faxed bed reactor loaded by 1g of catalyst, the reaction temperature was 200-300 ℃, and the reactor pressure was 1 atm. Preceding the reaction , catalyst was reduced in the H₂/N₂ mixture at 160℃. This study shows that at 300℃ reaction temperature, methanol conversion reached 100% at 28 ml/minute gas flow rate. This conversion decreased significantly with the increase of gas flow rate. Meanwhile, the catalyst prepared for STM was stable in 36 hours of operation at 260℃. The catalyst exhibited a good stability although the reason condition was shifted to a higher gas flow rate.

Keywords: Cu/ZnO/Al₂O_3; fuel cell; hydrogen production; methanol steam reforming.

1 Introduction

Fuel cell powered electric vehicles and power plants using hydrogen as fuel are currently being developed in an effort to protect the environment and sustainable development. Hydrogen produced by steam reforming of methanol(SRM) is an increasing worldwide interest, the equilibrium conversion of SRM reaction reaches around 100% at 150 ℃ at atmospheric pressure. Unfortunately, a considerable amount of CO (>100 ppm) as a by-product is produced during the reaction at temperature above 300°C. As for the application of Polymer Electrolyte Fuel Cell (PEFC), even traces of CO (>20 ppm) in the reformed gases deteriorate the Pt electrode and the cell performance is worsened. An ideal method to produce hydrogen with lower amount of CO from SRM greatly requires a high performance catalyst, which must be highly active and selective for hydrogen production and also stable for a long period in a continuous operation. Now the most widely used catalysts for this reaction are copper containing catalysts since copper has been found to be high activity and selectivity for hydrogen production ^{[1, 2]}.

For fuel cell applications there are three processes available for extracting H₂ from methanol, namely: (i) steam reforming of methanol (SRM), (ii) partial oxidation of methanol (POM) and (iii) combined reforming of methanol (CRM). The reactions of these processes are given as follows:

SRM: CH₃OH + H₂O → 3H₂ + CO₂

POM: CH₃OH + 1/2O₂ → 2H₂ + CO₂

CRM: CH₃OH + (1 – X) H₂O + 1/2x O₂→  (3-x)H₂ + CO₂  

The SRM reaction is a highly developed and thoroughly studied process it is possible to produce a product gas containing up to 75% hydrogen while containing a high selectivity towards carbon dioxide. However, it is a slow and endothermic process. Therefore a significant amount of heat has to be provided to maintain the reforming reaction.

The SRM reaction is the reverse reaction of methanol synthesis from the mixture of hydrogen and CO₂ ^{[1, 3, 4]}. Hence, it is assumed that the catalyst for the methanol synthesis is also active for its reverse reaction. Traditionally, Cu/ZnO/Al₂O₃ is used to catalyze a low temperature gas shift reaction. Although the thermal stability of Cu/ZnO/Al₂O₃ catalyst is relatively low, its activity and selectivity are considerably high. Moreover, the raw materials to produce catalyst Cu/ZnO/Al₂O₃ could be found easily in Indonesia.

2 Experiment

2.1 Catalyst Preparation

A copper-based catalyst was derived from hydroxynitrate precursors prepared by impregantion of metal nitrates solutions inγ-Al₂O₃ . In short, to prepare 10 gram of catalyst, 4.155 gram of Cu(NO₃)₂ .3H₂O and 1.755 gram of Zn(NO₃)₂ .4H₂O were needed. All these salts were diluted in 5.6 mL of aquadest. This volume of solution was determined from the pore volume of 8.361 gram of γ-Al₂O₃ catalyst support. The salts solution was impregnated intoγ-Al₂O₃ and then was dried at 120℃ for 12 hours. Afterward, the catalyst was calcined at 360℃ for 1 hour^[5,6].

The characterization of Cu/ZnO/Al₂O₃ catalyst was carried out by means of XRD analysis in order to analyze the catalyst crystal structure, and BET isotherm adsorption to measure the catalyst surface area.

2.2  Activity Test

Hydrogen synthesis from methanol on Cu/ZnO/Al₂O₃ catalyst was carried out in the laboratory fixed bed reactor made of stainless steel with 8 mm internal diameter. For this experiment, 1 gram catalyst was used. Preceding of the reaction, the catalyst was reduced at temperature of 300°C in the H₂ /N₂ gas mixture for about 2 hours converted the CuO to Cu. The SRM reaction was carried out by means of introducing the gas mixture methanol-water with water to methanol ratio of 1.2. The total flow rate of gas was 20-90 mL/minute. In all experimental runs, the methanol concentration was kept at 17.8%. The reaction temperature was controlled at 200-300°C. The experimental set up is shown in Figure 1. The gas products were analyzed by a Shimadzu gas chromatography. The GC was equipped with a thermal conductivity detector and two packed columns in parallel (one for separation of polar components i.e. CO₂, water, and methanol, and a molecular sieve for separation of H₂, O₂, N₂, and CO). The temperature of column was controlled at 50°C. Argon was used as the carrier gas.

Figure 1 Process flow diagram of experimental set-up for SRM reaction[6].

3 Result and Discussion

3.1 Catalyst Characterization

The X-ray diffractogram of catalyst aster calcinations is shown in Figure 2. The characteristic peaks of CuO and Al₂O₃ have been indicated in the diffractogram of  catalyst after calcinations. No other features ate observed except the characteristic peaks of CuO appears at 2θ≈36-38° indicating relatively large crystallites. This indicates that CuO is highly dispersed on the catalyst surface. Signals from  crystalline ZnO in the catalyst matrices, or the fact that ZnO is present in highly disorder or amorphous states because of the relatively low calcinations temperature(360℃). However, features associated with Al₂O₃ were detected.

2θ

Figure 2 XRD difratogram of Cu/ZnO/Al₂O₃ catalyst

 The catalyst surface area was determined by BET(Brunauer-Emmet-Teller) method by means of using Gas Surption Analyzer NOVA 100 Quantachrome. The catalyst surface area compared with the γ-Al₂O₃ are shown in Table 1.

Table 1 Catalyst specific surface area

The Al₂O₃ loosed 15% of its surface area due to the impregnation process with u and Zn species and heat treatment. However, this surface area was much greater than that of catalyst prepared by the co-precipitation method. A copper based catalyst prepared by Agrell et al (2003) by means of co-precipitation method has 92 m²/g specific surface area. Most of copper based catalysts prepared by the co-precipitation method have specific surface area less than 100 m²/g ^{[7, 8, 9]}.

3.2 Catalyst Activity Test

Catalytic experiments were performed at atmospheric pressure in a laboratory packed-bed tubular reactor. The catalyst was diluted in glass wool to avoid adverse thermal effects. Both catalyst and diluents were sieved in order to minimize the pressure drop over the catalyst bed. The methanol and water mixture was fed to the evaporator by means of a liquid syringe pump. In this experiment, nitrogen was added as an inert. The reaction conditions are described in Table 2. One set of experiments was carried out under different weight hourly speed velocity (WHSV) and temperature but on a similar reactor setup.

Table 2 Reaction Conditions for Steam Reforming of Methanol

Prior to each experiment, the catalyst was reduced in situ at 166-300℃ in a stream of 50% H₂ in N₂. The dwell time was at least 5 h all cases. The temperature scheme of catalyst reduction is shown in Figure 3. The catalyst reduction aims to convert CuO to active metal Cu which is used for the reformation reaction of methanol.

Figure 3 Temperature scheme for catalyst reduction.

The SRM experiments were conducted with steam in excess of stoichiometry (H₂O/CH₃OH = 1.2), ensuring the complete methanol conversion and suppressing the CO formation by the reverse water–gas shift (RWGS) reaction. The product gas composition was analyzed by a GC-8A Shimadzu gas chromatograph. In the following text, the product gas composition refers to the composition of the gas stream leaving the reactor, including unconverted methanol and O₂, but excluding N₂ used for dilution.

The catalytic activity was evaluated from the data collected between 5 h and 6 h of on-stream operation for methanol conversion (X(MeOH)), H₂ selectivity (S(H₂ )), and CO₂ selectivity (S(CO₂)), and they are given as follows:

3.2.1 Methanol Conversion

The catalyst activity was measured as a conversion of methanol in steam reformation reactor obtained from the concentrations of methanol in feed and product streams. Figure 3 shows the catalyst activity as a function of gas flow rate and reaction temperature. Analysis of the effluent gas indicated that H₂ and CO₂ were major components with a minor amount of CO. Other products such as formaldehyde, formic acid, methyl formate and dimethyl ether formed during reactions of methanol on Cu-based catalysts could not be detected under the reaction conditions.

Figure 4 The catalyst activity in different WHSV and temperatures

Catalyst shows similar trend of activity in various reactor temperatures. The greater the WHSV, the smaller the methanol conversion obtained from the catalyst activity test. It is clear that the catalyst activity was a function of reaction temperature. Kinetically, the reaction rate is significantly influenced by the heat supplied by the reactor heater. Hence, at high temperature, the methanol conversion could be similar with the thermodynamic conversion. Above 275℃, the SRM thermodynamic conversion is 100%. At 300℃ and low WHSV, the methanol conversion reached 99.8%.

Nevertheless, some studies reported that at temperature above 300℃, the hydrogen production rate of SRM catalyzed by Cu/ZnO/Al₂O₃ decreased ^[1,4,5] due to the carbon monoxide production at high temperature. In this condition, some side reactions producing carbon monoxide were favored. Furthermore, the profile of yield of hydrogen in SRM resulted from this experiment is similar with the profile observed by Turco, et al. ^[10].

CO is produced mainly from the reversed water gas shift reaction. With the presence of oxygen in the feed, CO is also produced from methanol decomposition, probably through the formation of intermediate oxygenated species, such as adsorbed formaldehyde or dioxymethylene that are bonded to metallic Cu or to cationic sites of the oxide matrix. These intermediate species are probably common also to the mechanism of SRM, and can be desorbed as gaseous formaldehyde or, if retained on the catalytic surface, further dehydrogenated giving rise to CO or CO₂. Thus it is expected that if the catalyst retains these intermediates strongly, it will favour the formation of CO. Turco et al observed that the production of CO increases with increasing aluminum content suggesting a correlation of CO with the acidity of the catalysts, in particular with the concentration of the strongest acid sites. The formation of CO was suppressed by means of using excessive water in the feed to avoid the reversed water gas shift reaction. At this reaction condition, the yield of hydrogen could also be decreased because of the reaction of methanol condensation producing formaldehyde, due to the acid sites of Al₂O₃ and/or aluminates. However, the condensation product was not detected. Hence, the yield of hydrogen was significantly decreased at higher WHSV mainly due to the lower methanol conversion at this reaction condition.