Gold Based Catalyst Preparation and Reaction Pathway on Partial Oxidation of Methanol to Produce Hydrogen Reaction

Gold Based Catalyst Preparation and Reaction Pathway on Partial Oxidation of Methanol to Produce Hydrogen Reaction

Bake Aibibula1, Sharif F. Zaman1

1. Chemical and Materials Engineering Department,

2. Faculty of Engineering, King Abdulaziz University

Jeddah, Saudi Arabia

Abstract Hydrogen production from partial oxidation of methanol (POM) is one of the most promising process for the on-board fuel cell automobile application. Gold supported catalyst will significantly increase H2 selectivity and decrease the CO formation in POM reaction. In this mini review paper briefly summarizes the catalyst preparation process and possible reaction mechanism which is take place in the POM reaction process.

Keywords Gold catalysts, Reaction mechanism, partial oxidation

1. INTRODUCTION

   Hydrogen is regarded as pure energy source, due to it has high energy efficiency and zero environment pollution, combustion of hydrogen is produce water. Hydrogen can be used directly fuel for the automobile, but challenge is when using hydrogen as a direct fuel for car, people will face safety, storage, refilling and transportation problem. Best solution for that using methanol to produce hydrogen for on-board vehicles, because of methanol has relatively high energy density and high hydrogen to carbon ratio. Hydrogen can be produced from methanol by methanol decomposition[14], methanol steam reforming[57], methanol partial oxidation[813] and oxidative steam reforming of methanol[1416]. Following are chemical reaction equation for these four reaction:

   CH3OH CO + 2H2(1) CH3OH + H2O CO2 + 3H2(2)

   CH3OH + 1/2O2 CO2 +2H2(3)

   2CH3OH + 1/2O2 + H2O 2CO2 + 5H2(4)

   Partial oxidation of methanol (Eq.3) has several advantages compared to other three reactions. Partial oxidation of methanol is exothermic reaction, which is doesnt require extra energy for the reaction[17], more excitingly air or oxygen can be used as oxidant instead of steam, and also it is main reaction products are hydrogen and carbon dioxide. Methanol decomposition (Eq.1) are not

   suggested for fuel cell application, because it is main reaction products include CO, and it is harmful for the expensive platinum anode electrodes in proton exchange membrane fuel cell (PEMFC). CO concentration must be lower than 20 ppm level, otherwise it will rapidly decrease fuel cell efficiency[18-19]. Methanol steam reforming (Eq.

   2) and oxidative steam reforming (Eq. 4) are require extra steam generation unit, it needs extra energy to produce steam, and also it will make system complicate and costly.

   People have been studied mainly copper[2022] and palladium[23-24] based catalysts for POM reaction to produce hydrogen, but main problem in this two type of catalysts are produce considerable amount of CO and low stability. Since M. Haruta and his co-workers [25] found that gold supported on TiO2,-Fe2O3 and Co3O4 catalysts has superior catalytic activity for the CO oxidation at low temperature, gold catalysts attracted more attention in different reaction system. Feg-Wen Chang and his co- workers [26]have studied Au/TiO2 catalyst for partial oxidation of methanol to produce hydrogen at lower temperature, it shows negligible amount of CO. Also his group further studied different types of supported gold catalysts for selectively produce hydrogen with a target of low emission of CO[10, 11, 27], and they successfully decrease the CO formation in POM reaction. From previously studied relevant literature can be seen, hydrogen production via partial oxidation of methanol over gold based catalyst are effective way to CO free hydrogen generation for fuel cell application in the future.

   This review briefly organized gold catalyst preparation for POM and possible reaction mechanism involved in POM are will be discussed. This will helps reader to easy understand of POM reaction to produce hydrogen.

2. CATALYST PREPARATION

   In the beginning gold catalysts assumed as poorly active, because of larger particle size[28]. Since G. C.Bond and P. A. Sermon found that Au/SiO2 catalyst is active for olefin hydrogenation, because ofsmall gold nanoparticles, peoples mind start changing for gold catalysts. In 1989, M. Haruta and his co-workers discovered gold supported

   catalyst shows very good catalytic activity towards to CO complete oxidation at low temperature[29]. More and more research focus on preparing highly dispersed gold nanoparticles for different preparation method and optimize the preparation parameter. In the following will give some detailed information for the active gold catalyst preparation.

   1. Selecting Suitable Catalyst support

      Selecting suitable active site for gold catalysts are most important. According to the literature, nature of the support strongly affected on gold dispersion on the surface[30], and state of gold[31]. Also using reducible support rather than the non-reducible support for the oxidation reaction are suggested[25].

   2. Catalyst preparation method

      Recent year most of the gold catalyst have been prepared their catalyst deposition-precipitation[27, 32], co- precipitation[31], impregnation[8] and PVP stabilized reduction method[33-34]. Among these different preparation method, deposition-precipitation method are favorable for gold catalyst preparation. Because, the catalysts which are prepared by deposition precipitation method, gives small gold particle size that is main effecting factor for the gold catalyst activity. M. Haruta[28]found that deposition-precipitation method are most effective and easy way to prepare active catalyst for CO oxidation, and this method extensively used in POM reaction gold catalyst preparation as well[27, 35].

   3. Catalyst preparation parameters

   Proper catalyst preparation parameters also main influencing factor for gold catalyst performance. Such as, best control of pH in catalyst preparation, using of suitable precipitation reagent, amount of gold loading to the surface and catalyst calcination temperature are the most effecting

   occurs highly exothermic reaction. In Au/TiO2[26] and Au- Pd/SBA-16-CeO2[37] catalysts are shows similar trends.

   CH3OH + 1.5O2 CO2 + 2H2O (5)

   When increase temperature, the formed water will consumed by steam reforming reaction (Eq. 2), that will increase H2 selectivity and decrease water selectivity[21, 26].

   There can also be methanol partial oxidation (Eq. 3) instead of methanol combustion in case of no excess oxygen, which is good for hydrogen generation.

   Further increasing temperature, no more O2 was left and methanol decomposition (Eq. 1) reaction will occur, and it will increase H2 and CO formation. CO formation will be harmful for platinum anode electrode in fuel cell.

   At higher reaction temperature both methanol and oxygen are diminished from the system, now reverse water gas shift reaction (Eq. 6) startsto take place over the surface, that will consume H2 and increase CO formation. This is why most of studied catalysts shows, after optimum reaction temperature, desired reaction products are shows decreasing trend.

   CO2+H2 CO + H2O (6)

   This explains the decrease in hydrogen selectivity at higher temperature. Moreover at low temperature low selectivity of hydrogen suggests that surface oxidation of hydrogen happened more selectively over the gold catalysts.When oxygen is excess in the system, CO oxidation (Eq. 7) and H2complete combustion reaction (Eq.

   8) will occur. Because gold supported catalyst is very active for CO oxidation when presence of oygen[34] and hydrogen combustion.

   CO + 0.5O CO (7)

   parameters. According to the literature, best pH for the gold 2 2

   catalyst preparation is about 8[28, 32], and calcination at lower temperature are suggested[36]. Au/Fe2O3[31] catalyst preparation process, higher catalytic activity of catalyst using Na2CO3 as reagent rather than the K2CO3, because catalyst used Na2CO3 precipitant has lower gold particle size (2.3 nm) than the K2CO3 as precipitant (4.5 nm). Most of studied result shows that, gold loading between 1-3 wt.% is effective for gold catalyst, this also matches with gold catalyst for partial oxidation of methanol.

3. REACTION MECHANISM

   In Partial oxidation of methanol reaction, several reaction mechanism are involved, which are complete combustion of methanol, methanol steam reforming, methanol decomposition, water gas shift reaction, reverse water gas shift reaction, CO oxidation and hydrogen combustion reaction. These reactions mainly depend on reaction operation temperature, catalyst and reactant & reaction compositions in the system.

   Partial oxidation of methanol reaction strongly depend on temperature. At lower temperature, methanol combustion reaction (Eq. 5) will take place, methanol consume oxygen

   H2 + 0.5O2 H2O(8)

   Operating POM reaction at mild reaction condition not only have advantages in cost effective manner but also it will helps to eliminate some by products and avoid undesired reaction. CO will form and hydrogen will consume via reverse water gas shift reaction at high temperature. Operating at lower temperature, also increase catalyst stability. Another reason is that high temperature is not favorable for gold catalyst, gold catalyst start sintering at relatively low temperature compared to other catalysts.

4. SUMMARY AND OUTLOOK

Selecting suitable support for the catalyst preparation and improving preparation procedure and parameters, and also optimizing reaction condition are the main effecting parameters in this reaction. The studies on gold catalysts are shows better catalytic performance regarding with the CO free rich hydrogen gas compared to the commercial copper zinc based catalyst. However, these catalysts are still need to be improve, specially, catalysts stability of gold based catalyst. It is the main drawbacks of gold based catalyst[12].

Methanol is ideal hydrogen carrier, which can be catalytically convertible to hydrogen. Partial oxidation of methanol over gold based catalyst is promising way to produce hydrogen for on-board vehicles. Based on developed catalysts and studied reaction pathway, it needs to continuous effort on this study, and develop suitable and practical solution for PEMFC engine.

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