Catalytic Combustion Of Methanol

Published Date: 02 Nov 2017

Proceedings of the ASME 2013 International Mechanical Engineering Congress & Exposition

IMECE2013

November 13-21, 2013, San Diego, California, USA

IMECE2013-62776

DRAFT: ON-CHIP POWER GENERATION: MICROFLUIDIC-BASED REACTOR FOR

CATALYTIC COMBUSTION OF METHANOL

Sheng Tian Thomas Thundat Subir Bhattacharjee

Department of Chemical and Materials Department of Chemical and Materials Department of Mechanical

Engineering, University of Alberta, Engineering, University of Alberta Engineering, University of Alberta,

Edmonton, Alberta, Canada Edmonton, Alberta, Canada Edmonton, Alberta, Canada

Email: [email protected]

Kenneth Cadien Sushanta Mitra

Department of Chemical and Materials Department of Mechanical

Engineering,University of Alberta, Engineering, University of Alberta

Edmonton, Alberta, Canada Edmonton, Alberta,Canada

Email: [email protected]

ABSTRACT PPR positive photo resist

In this paper, a batch of microfluidic-based reactors with in- HDMS hexamethyldisilazane

tegrated micropillars was fabricated on silicon wafer with stan- RPM revolutions per minute

dard optical lithography and deep reactive-ion etching (DRIE) UV Ultraviolet

technique. A Bosch process of DIRE was used to obtain 85 m D diameter of the pillar

etching depth and undulating sidewall profiles on the surface of H height of the pillar

those micropillars. Such porous structures boost surface-area- S internal suface area

to-volume-ratio as well as enhance heat and mass transfer co- V internal volume

efficients. Platinum (Pt) nanoparticles (NPs) stabilized by poly- S/V surface-area-to-volume ration

acrylate sodium in a water-ethonal-based suspension were de- SQ square arrangement

posited on the reactor surface using a surface-selective infiltra- ST staggered arrangement

tion method. By introducing methanol vapor/air gas mixture into

AX axially flow direction

the reactors, stable catalytic combustion of methanol over Pt NPs

DI diagonally flow direction

starting from room temperature can be achieved.

RIE reactive-ion etching

SEM scanning electron microscope

NOMENCLATURE TEM transmission electron microscopy

DRIE deep reactive-ion etching

Pt Platinum

NPs nanoparticles INTRODUCTION

GC gas chromatographic In the past decade, along with an increasing number of

power-hungry hand-held electronic devices entering people’s

daily life, such as communicating, computing and entertain-

Address all correspondence to this author.

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ment equipment, there is a huge demand of portable power over Pt nanoparticles dispersed on an anodized alumina wafer in

generating and supplying systems with higher energy density a microburner. [7] By adjusting the air flow rate and methanol

and longer-operational life. However, the research development mole fraction, the highest temperature of the burner wall could

of rechargeable batteries, which are the main choice of power be increased to more than 600 C. They also proposed that high

source nowadays, is quite slow compared with the rapid evo- flow rates and relatively lower fuel content would significantly

lution of electronic devices, making the "portable power gap" reduce the start-up time for reaching the steady state of the

even larger. Furthermore, environmental issues caused by heavy system.

metals and toxic chemicals contained in discarded batteries

restrict its future progress. Therefore, the researchers had to turn In this paper, a design of entire operating system for

their attention to other energy conversion methods. microfluidic-based reactors is presented in details. We fabricated

a batch of microfluidic-based reactors with integrated micropil-

Hydrocarbon fuels, the energy density of which is several lars with standard optical lithography and deep reactive-ion etch-

orders of magnitude higher than conventional batteries. For ing (DRIE) technique. By using such reactors as the heat source

example, the energy density of methanol (19.7MJ/kg) is nearly for commercial thermoelectric modules, on-chip power genera-

27 times higher than Li-ion batteries (0.72MJ/kg). Also, tion is expected to be achieved. Also a chip-holder and the entire

hydrocarbon fuels possess the advantages of availability and experimental set-up are shown here. This system was designed

portability. Massive research efforts have been made to design for a goal of generating a 0.1W output power with very low con-

and create portable power generators based on hydrocarbon sumption rate of methanol.

fuels, which are considered as appropriate alternatives for

conventional batteries. Taking methanol for instance, there are

already several existing types of fuel cell systems that make REACTOR FABRICATION

use of the high energy density of methanol, such as reformed Microfluidic-based Reactors Fabricated on Silicon

hydrogen fuel cells [1, 2] and direct fuel cell [3, 4]. However, Based on a fabrication procedure devepoled by our group

they still suffer from poor performance of separation membrane, [8], 16 microfluidic-based reactors with integrated micropillars

methanol cross-over and catalyst poisoning, especially CO. were fabricated on one silicon wafer with standard optical lithog-

raphy and deep reactive-ion etching (DRIE) technique, which

Direct thermal-to-electric energy conversion using a is briefly described in Fig. 1. Different models with integrated

thermoelectric platform is an promising method for power gen- micropillars were designed in L-Edit (Tanner EDA Inc., USA).

eration due to its higher energy density, containing no moving Each model consists of a region of 8.4mm 8.4mm with an

parts, quite and environmentally friendly features compared array of 82 82 micropillars arranged in either square or stag-

with other conventional energy conversion methods such as gered arrangement, inlet and exit regions, and microfluidic ports.

the internal combustion engine or a chemical battery. Because These microfluidic-based reactors were designed for different

of Seebeck effect, if a combustor as the heat source is placed pillar diameters (90 m, 70 m, 50 m, 30 m), different pillar ar-

on the hot side of a thermoelectric device, electricity can be rangement (square, sttaggered) and different flow directions (axi-

produced from the temperature difference between the hot and ally, diagonally). The parameters of the designed reactors on one

cool regions conjuncted by thermoelectrical material. A detailed silicon wafer is shown in Table 1. D is the diameter of the pillar,

review about thermoelectric device and its power generation H is the height of the pillar or the etching depth, S is the internal

principle can be found in [5]. surface area, V is the internal volume, S/V is the surface-area-

to-volume-ratio. SQ and ST are for square and staggered pillar

It is long known that Platinum (Pt) particles can facili- arrangement respectively. AX is the notation for axially flow di-

tate combustion of methanol. Hu Z. et al. achieved stable rection while DI is for diagnoally flow direction.

and reproducible spontaneous self-ignition and self-supporting

combustion of methanol at room temperature by introducing A 4-inch-diameter Si substrate with 500nm silicon

methanol/air vapor into a glass tube reactor filled with Pt dioxide layer on top was first cleaned in a standard Piranha

nanoparticles (NPs) on quartz glass wool. [6] Gas chromato- solution (H SO : H O 3 : 1) and then dried with Nitrogen

2 4 2 2

graphic (GC) analysis of the combustion gas products confirms gas. Before spin-coating of the positive photo resist (PPR)

the complete reaction of methanol, which is shown in Eqn. (1): HPR 506 (Fuji-film Electronic Materials Inc., USA), the sicon

wafer was placed in YES HMDS Oven for one cycle to be

3 coated with a layer of HDMS (hexamethyldisilazane), which

CH OH l O CO 2H O H 639kJ mol (1)

3 2 2 2 2 promotes adhesion of photo resist to silicon dioxide layer. PPR

is spin-coated on the substrate using the following parameters:

Karim A.M. et al. studied the catalytic self-ignition of methanol (1)Spread: 500RPM for 10s; and (2) Spin: 4000RPM for 40s.

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TABLE 1. GEOMETRICAL PARAMETERS OF DIFFERENT RE-

ACTORS DESIGNED

2 3

Notations D ( m) H ( m) S (mm ) V (mm ) S/V

90 SQ AX 90 85 190.81 2.36 80.80

90 ST AX 90 85 190.81 2.36 80.80

90 SQ DI 90 85 190.81 2.36 80.80

90 ST DI 90 85 190.81 2.36 80.80

70 SQ AX 70 85 171.80 3.80 45.23

70 ST DI 70 85 171.80 3.80 45.23

70 SQ DI 70 85 171.80 3.80 45.23

70 ST DI 70 85 171.80 3.80 45.23

50 SQ DI 50 85 148.56 4.88 30.47

50 ST DI 50 85 148.56 4.88 30.47

50 SQ DI 50 85 148.56 4.88 30.47

50 ST DI 50 85 148.56 4.88 30.47

30 SQ DI 30 85 121.10 5.59 21.65

30 ST DI 30 85 121.10 5.59 21.65

30 SQ DI 30 85 121.10 5.59 21.65

30 ST DI 30 85 121.10 5.59 21.65

FIGURE 1. PROCESS FLOW DIAGRAM OF THE MICROFABRI-

CATION TECHNIQUE USED FOR GENERATING MICROPILLARS of those micropillars fabricated on the silicon wafer in Fig. 2,

ON SILICON SUBSTRATE undulating sidewall profiles on the surface of those micropillars

resulting from Bosch DRIE process were obtained. As 70 cycles

of DRIE process was used with STS ICP-RIE system to get

After post bake was done using Solitec vaccum hotplate at 115 85 m etching depth, one single "ripple" width of the undulating

C for 90s, the integrated micropillars patterns were transferred feature is about 1.21 m.

onto PPR with standard optical photolithography by exposing

Si substrate to Ultraviolet (UV) light on ABM Mask Aligner.

Development was done to remove the UV-exposed photo resist Catalyst Deposition

before further etching process. SiO2 layer was etched using STS In order to keep the reactor top surface clean for subsequent

RIE system, which was followed by anisotropic etching of Si for sealing via anodic bonding, a surface-selective infiltration

85 m using Bosch DRIE method with STS ICP-RIE system. method similar to [8] was adopted. Compared to other con-

As we observed, PPR was also etched during this process due ventional coating procedures [10], such approach can precisely

to the selectivity of the etching systems. The residual PPR was control the amount of catalyst loaded into the reactors.

cleaned by oxygen plasma process using Branson Barrel etcher

before removing the SiO2 layer underneath using STS RIE An hydrophobic Parafilm sealing film (American National

system again. Can Co., USA) was pressed onto the top surface of the silicon

wafer. By heating the wafer on a hot plate at 80 C for 10 min,

As shown in scanning electron microscope (SEM) pictures the hydrophobic film became soft gradually. After peeling off,

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FIGURE 2. SEM PICTURES OF MICROPILLARS FABRICATED FIGURE 3. SEM MORPHOLOGY CHANGE OF CATALYST BE-

ON SILICON WAFER (a) 50 SQ DI (b) MAGNIFIED IMAGE OF FORE AND AFTER CALCINATION (a) 30 SQ AX AFTER CALCI-

50 SQ DI (c) 70 ST DI (d) MAGNIFIED IMAGE OF 70 ST AX NATION (b) MAGNIFIED IMAGE OF 30 SQ AX (c) 30 ST AX AF-

TER DRYING (d) MAGNIFIED IMAGE OF 30 ST AX

a residual layer from the Parafilm was still remained on the

surface of the wafer. As the Platinum (Pt) nanoparticles (NPs)

are stabilized in a water-based suspension, they will therefore

ogy of Pt NPs deposited on the reactors. After 16 l of water-

selectively coat only the inner surface of the silicon wafer but

ethonal-based Pt NPs suspension was dropped onto the top sur-

not the top surface. The Parafilm sealing film can be easily

face, 30 SQ AX and 30 ST AX were dried in the air overnight.

removed during the calcination later.

30 SQ AX was then put in a furnace at 500 C for 15 min. The

resulting samples were characterized by SEM. From Fig. 3 (c)

Pt NPs stabilized by polyacrylate sodium in a water-based

(d), we observed that after drying in the air, the Pt NPs started

suspension were purchased from Sciventions Inc., Canada, the

to grow and aggregate, forming micro-scale particles, especially

concentration of which is 1.5mg/ml. Less than 10nm particle

those on the bottom surface of the reactor around the micropillars

size is observed from transmission electron microscopy (TEM).

like ring-shape. Such morphology is not good for catalytic reac-

The surface of the silicon wafer with integrated micropillars is

tion, since the surface area of Pt NPs is reduced tremendously.

superhydrophobic, as the contact angle of Pt suspension droplet

¨ Aggregation phenomenon became more severe after 15 min cal-

on it meassured by Kruss Drop Shape Analysis System DSA100

¨ cination as shown in Fig. 3 (a)(b), the distribution of Pt catalyst

(KRUSS GmbH., Germany) is more than 145 . In order to

turned nonuniform as they aggregate to form a thin film structure

increase the wettability of Pt suspension on the surface of Si

in some area. However, the hydrophobic Parafilm sealing film

substrate, same volume of absolute ethyl alcohol (SIGMA-

was gone during calcination, as top surface of Si substrate of the

ALDRICH Inc., USA) was added into the original suspendsion,

30 SQ AX sample became clean again afterward.

as the surface tension of ethanol is much smaller. After mixing

by a Vortex Mixer (Fisher Scientific, Canada) for 15 min, a

certain volume of the water-ethonal-based Pt NPs suspension

was measured by a micropipette. After dropping of the Pt NPs Reactor Sealing

suspension onto the Si substrate, the liquid would fill into the

For experimental investigation purpose, a borofloat glass

space between those micropillars very soon. The contact angle of

layer which consists of inlet and outlet ports drilled using

water-ethonal-based Pt suspension droplet was reduced to 40 .

abrasive water-jet cutter (2652 Jet Machining Center, USA) was

bonded to the top surface of the silicon wafer in order to form a

As the top surface of the Si substrate remained hydropho-

closed microfluidic device.

bic during the drying of the catalyst, more cycles of Pt NPs de-

position can be done, so that more amount of catalyst will be

loaded into the reactor. Two samples, 30 SQ AX and 30 ST AX The schematic of fabricated microfluidic-based reactors

coated with Pt NPs were chosen for calcination experiments in with integrated micropillars based on the procedure described

order to determine how calcination would change the morphol- above is shown in Fig. 4.

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module tightly in the meanwhile providing heat if the methanol

vapor/air mixture is introduced, so that on-chip power genera-

tion is expected to be achieved. Thermocouple wires can placed

on the hot or cool side of the thermoelectric module through the

holes cutted to measure the temperature difference. Such an ex-

perimental set-up can not only serve as a performance test sys-

tem for the microfluidic-based reactors, but also pressure drop

meassurement appartus. All the data of temperature, power and

pressure are recorded by the data acquisition and visualization

system simultaneously.

FIGURE 4. SCEMATIC OF FABRICATED MICROFLUIDIC-

CONCLUSIONS AND FUTURE WORK

BASED REACTORS WITH INTERATED MICROPILLARS

A batch of microfluidic-based reactors with integrated

micropillars was fabricated on silicon wafer with standard

optical lithography and DRIE technique. Undulating sidewall

profiles on the surface of those micropillars were obtained

intentionally by using a Bosch process. Such roughness feature

served as support for catalyst. Pt NPs stabilized by polyacrylate

sodium in a water-ethonal-based suspension were deposited on

the reactor surface using a surface-selective infiltration method.

The addition of ethonal can increase the wettability of the Pt

suspendsion on superhydrophobic micropillar-patterned silicon

substrate significantly. The liquid droplet filled into the volume

between micropillar within a few minutes. Calcination of Pt

NPs at 500 C for 15 min removed all the hydrophobic Parafilm

sealing film coatted on the top surface of the Si substrate,

making the surface clean and ready for further anodic bonding.

However, the aggregation of Pt NPs was severe, even forming

a thin film structure on the bottom surface of the reactor after

calcination. Such morphology is bad for catalytic reaction, since

the surface area of Pt NPs is reduced tremendously.

Optimized catalyst deposition should be explored. The

performance as well as the pressure drop of the fabricated

microfluidic-based reactors will be studied using the chip-holder

and the experimental set-up designed very soon. By introducing

methanol vapor/air gas mixture into the reactors, stable catalytic

combustion of methanol over Pt NPs can be achieved. Such re-

actors are promising heat source for thermoelectric module so as

to achieve on-chip power generation.

FIGURE 5. CHIP-HOLDER AND EXPERIMENTAL SET-UP DE-

SIGHED

ACKNOWLEDGMENT

CHIP-HOLDER AND EXPERIMENTAL SET-UP Financial support of Natural Sciences and Engineering Re-

The chip-holder and the entire experimental set-up are search Council (NSERC) of Canada as Strategic Project Grants

shown in Fig. 5. The chip-holder possesses two slots, one for funding for this work is gratefully acknowledged. Sheng Tian

microfluidic-based reactor, the other for commercial thermoelec- thanks Naga Siva Kumar Gunda, Dr. Prashant R. Waghmare and

tric module. The reactor will get contact with the thermoelectric Dr. Natalia Semagina for helpful discussions.

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