Catalytic study of copper based catalysts for steam reforming of methanol

Abstract: The aim of this work is to study the catalytic properties of copper based catalysts used in the steam reforming of methanol. This method is known as one of the most favourable catalytic processes for producing hydrogen on-based. The catalysts investigated in this work are CuO/ZrO₂ catalyst, which were prepared using different kind of preparation methods and a commercial CuO/ZnO/Al₂O₃ catalyst which was used as a reference. The results of the studies can be divided into three sections:

1 The catalytic study reported in chapter 4 is focused on the investigation of the CO formation during steam reforming of methanol on a commercial CuO/ZnO/Al₂O₃ catalyst. The reaction schemes considered in this work are the methanol stream reforming reaction and the reverse water gas shift reaction. The experimental results of CO partial pressure as a function of contract time at different reaction temperatures show very clearly that CO was formed as a consecutive product. The implications of the the reaction scheme, in particular with respect to the production of CO as a secondary product, are discussed in the framework of onboard production of H₂for fuel cell applications in automobiles. Potential chemical engineering solutions for minimizing CO production are outlined.

2  In chapter 5, the catalytic properties of a CuO/ZrO₂ catalyst synthesized by a templating technique were investigated with respect to activity, long term stability, CO formation, and response to oxygen addition to the feed. It is shown that, depending on the time on stream, the

temporary addition of oxygen to the feed has a beneficial effect on the activity of the CuO/ZrO₂ catalyst. After activation, the CuO/ZrO₂ catalyst is found to be more active (per copper mass) than the CuO/ZnO/A1₂O₃ catalyst, more stable during time on stream, and to produce less CO.

3  In chapter 6, the study of the catalytic behaviours has been carried out on the six

CuO/ZrO₂ catalysts. The catalysts were synthesized with different preparation methods, i.e.incorporation of CuO in ZrO₂-nanopowder, in mesoporous ZrO₂ and in macroporous ZrO₂.

The activity of CuO/ZrO₂ catalysts can be improved by oxygen treatment. The catalystswhich have been used in the reaction provide a much larger value of the Sa than the freshcatalysts. This indicates that the new CuO/ZrO₂ catalysts provide much higher stability withrespect to the sintering of metal particles in comparison to the commercial CuO/ZnO/A1₂O₃catalyst. The result concerning the increase of Sa correlates well with the increase of theactivity of the used catalysts compared to the fresh catalysts. No linear correlation was foundbetween the activity and copper surface area. However, the activity of the catalysts can becorrelated with thepreparation methods. In comparison to the commercial CuO/ZnO/A1₂O₃,the CuO/ZrO₂ catalysts are more active. The CO concentration determined as a function ofmethanol conversion shows very clearly that less amount of CO was formed overCuO/ZrO₂catalysts than the commercial CuO/ZnO/A1₂O₃ catalyst.

1 .Introduction

1 .1 Motivation and Strategy

Air pollution and continuous global warming are serious environmental problems, which cancause the change of climate and the damage to environment. Pollutants such as carbonmonoxide, hydrocarbons, sulphur dioxide and nitrogen oxides are ofimportance because theyinfluence the formation of smog. Carbon dioxide, methane and certain nitrogen oxides are ofglobal significance. In the urban areas, the transport sector is one of the main contributors tothe air pollution. For examples, in Athens, Los Angeles, and Mexico City almost 100% ofcarbon monoxide emissions come from road vehicles, whereas NO_X-emissions are caused byroad transport at between 75% and 85%. The suffering of worldwide some 1.1 billionurban citizens from severe air pollution is related to about 700.000 death cases,reported fromthe World Bank. The other problem caused by the emission of the pollutants is the increase of

the global temperature. It is reported that each of the first eight months of 1998 new recordhighs for global temperatures is recorded. Carbon dioxide is thought to be the maincontributor for the greenhouse effect. Every gallon of gasoline burned in an automobileproduces 20 pounds of carbon dioxide. Transportation sector isresponsible for one-third of allcarbon dioxide emissions. Efforts to minimise the environmental damage of rapidly growingautomobiles use have focused on end-of-pipe technologies such as catalytic converters andparticle traps and recently  on producing  cleaner gasoline.  This  strategy has  shown asignificant decrease of the emissions from the newest cars being put on the road, butthestrategy has its limitation. In order to provide ultra low emission vehicles or zero emissionvehicles use of fuel cell technology is one of the most prominent solutions. Hydrogen is used

as fuel to power the fuel cell. The generating of the electricity by the chemical process,combining hydrogen and oxygen to form water, produces  no emissions  at all.  Otheradvantages of using hydrogen in the fuel cell in comparison to the conventional internalcombustion engine are higher energy efficiency, low noise, no formation of soot particle,which can impact the human health. The most promising type of fuel cell for application inthe automobile is the low temperature proton exchange membrane (PEFC) fuel cell. Theprototype of such passenger cars have been successfully demonstrated by many automobileindustries. The on-board supply ofhydrogen for the vehicles can generally be divided intothree groups:

1.Storage of high pressure hydrogen and liquid hydrogen.

2. Using of metal-hydride as hydrogen storage.

3. Reforming of hydrocarbon, such as methanol, ethanol, dimethylether, gasoline, diesel,etc.

The lack of a hydrogen refuelling infrastructure, combined with the complexity of on-board storage and handling of the hydrogen, are the drawbacks of applying pure hydrogen on-board. Furthermore, the weight and the volume of the hydrogen tank on board are much greater than of gasoline of diesel. This is a problem of the space limitation in the automobile and the increase of weight causes the increase of fuel consumption. The comparison of the weight and the volume of different fuels based on same energy equivalent of 50 litre gasoline are depicted in Figure 1 .1.

The alternative to the use of either liquid hydrogen or high pressure hydrogen on board is to carry liquidfuels that have high energydensities and covert them to a hydrogen-rich gas( reformate) via an on-board fuel cell processor. Oneto produce hydrogen on board is methanol. This dueto the following superior advantages of using methanol in comparison to other liquid fuels in particular with respect to the on boardreforming process:

low reaction temperature and atmospheric pressure

simple molecule with high molar ratio of hydrogen to carbon

low CO concentration (CO is poison to the fuel cell performance)

no emission of pollutants, such as NO_X, SO_X

no formation of soot particles

minor effort of changing the fuelling station (from gasoline or diesel)

Another advantage of using methanol as fuel that should also be taken into account is thatthere are many sources to produce methanol such as natural gas, oil, coal, biomass. Inaddition, methanol is the third commodity chemical after ammonia and ethylene, with aproduction excess of 25 million tons. The production of hydrogen from methanol isperformed in a reformer reactor. The catalyst used for this reaction is a copper based catalyst.

Two main problems using commercial CuO/ZnO/A1₂O₃ catalyst for this process are high COformation and poor long term stability. Using the Polymer Electrolyte Membrane Fuel Cell(PEMFC) as one of the favourable kinds of fuel cells in the passenger car, CO is found to bethe poison to the fuel cell which occupies the active surface of Pt electrode. Dams et al. performed series concentrations of CO from 30 ppm to 1000 ppm introduced into the gasmixture of hydrogen and carbon dioxide. They found that only CO with 30 ppm showed asatisfactory result of the decrease of the performance over a limited period. The influence ofthe CO concentrations to the decrease of the voltage has been carried out by Lemons. Itrevealed that the increase of the CO concentration resulted in the monotonically decrease ofthe voltagedetermined over a wide range of current density which related directly to thedecay of the fuel cell performance, Figure 1.2.

Another drawback of using the commercial CuO/ZnO/A1₂O₃ catalyst is the poor long termstability. One of the main factors which cause the decay of the catalyst activity with time onstream is the sintering of the metal particles that result in a decrease of the surface area of theactive site. In order to solve these problems, high CO formation and poor long-term stability,many strategies concerning the improvement of the catalyst properties have been followed i.e.synthesis of other metal based catalysts (Pd), synthesis of copper based catalysts promotedwith different metal oxides and synthesis of copper based catalysts with various kinds ofpreparation methods.

The objective of this work is to study the catalytic behaviours of novel Cu/ZrO₂ catalystswhich were prepared with different preparation methods. The catalytic properties of thesecatalysts, such as activity, long term stability, CO selectivity, were studied by means of afixed bed reactor. In order to evaluate the catalytic properties of the Cu/ZrO₂ catalysts, acommercial CuO/ZnO/A1₂O₃ catalyst was used as areference. A kinetic study of thecommercial CuO/ZnO/A1₂O₃ catalyst was also performed in this work.

1.2 Hydrogen production from methanol

There are three process alternatives to produce hydrogen through the conversion of methanol:

decomposition

partial oxidation

steam reforming

The decomposition reaction is the most simple process from a chemical point of view assolely methanol is used as feedstock.

CH₃OH→2H₂+CO   ΔHr=128 kJ/mol

However, the reaction is strongly endothermic which means that it requires a lot of energy foroperating. Furthermore, the decomposition yields product gas containing up to 67% hydrogenand 33% carbon monoxide. The high content of CO requires a CO clean-up system if thisreaction would be used in the fuel cell system. The CO clean-up system is regarded to be themost complicated part in the fuel cell system. Because of these drawbacks, the decompositionof methanol is found to be unsuitable for fuel cell applications.

In contrast to the decomposition reaction, partial oxidation is a fast and exothermic reaction.

CH₃OH+0.5O₂→2H₂+CO₂     ΔHr=-192 kJ/mol

Several studies on this reaction have been published in the last few years. Theadvantage of this process with respect to the exothermic nature is that an additional energysupply for the reaction is not necessary. However, the exothermic behaviour should be takeninto account when designing the reactor. The fast increase of temperature in the reactor canform hot spots, which can cause the deactivation of the oxidation catalyst through sintering ofthe metal particles. The hydrogen concentration up to 67% in a product stream can beachieved when methanol is partially oxidised with pure oxygen in the feed. The oxygenrequired for the automobile application would most likely be supplied from air.  Due to thehigh content of nitrogen in the air, this causes dilution of the product gas with nitrogen. As aresult, the maximum theoretical hydrogen content in such a system is lowered to 41%. Thedecrease of the hydrogen content in the product stream influences strongly the performance ofthe electricity production in fuel cell.

The steam reforming of methanol (SRM) is known as a reverse reaction of methanol

synthesis.

CH₃OH+H₂O→3H₂+CO₂ ΔHr=50 kJ/mol

SRM is considered to be the most favourable process of hydrogen production in comparisonto the decomposition and partial oxidation of methanol. This is because of the ability toproduce gas with high hydrogen concentration (75%) and highselectivity for carbon dioxide.

SRM is an endothermic reaction. The energy needed for the reaction can be supplied from acatalytic burner device, Figure 1.3. Because of the superiority of this process with respect tohigh methanol conversion, high hydrogen concentration and mild reaction conditions, studiesof this reaction have been carried out intensively by many research groups.

Another additional alternative to produce hydrogen from methanol is to combine the partialoxidation with the steam reforming. The advantage of this process is that heat requirement forthe reaction can be supplied by the reaction itself (autothermal reaction). However, theconcentration of hydrogen in gas product and methanol conversion is lower than that in theSRM.

1.3 Methanol steam reforming

The general reaction conditions of SRM are as follows:

      reaction temperature: 250-300℃

      pressure: 1 bar

      1:1 to 1:1.3 molar ratio of methanol to water

The main products of SRM are hydrogen, carbon dioxide and a low content of carbon monoxide is produced in this process (up to 2 volume% in dry product stream when using acopper based catalyst). The reaction schemes for the formation of carbon monoxide in SRMwill be discussed later. Hydrogen production based on SRM for fuel cell drive system consistsof the following main devices: a methanol steam reformer, a catalytic burner which providesheat for the reformer and converts all burnable gases in the flue gas into water and carbondioxide, a gas cleaning unit which reduces CO concentration of the hydrogen-rich product andfeeds to the Proton Exchange Fuel Cell (PEFC). A gas storage system is also integrated in thefuel cell system in order to feed the fuel cell during the start-up and speed-up phases. Ascheme of the fuel cell drive system based on SRM is shown in Figure 1.3.