A Comparison of Hydrogen Gas Production of

Copper and Gold Catalysts for the Water-Gas

Shift Reaction

By: Roman Hodson (rh28397)

Mullins Research Group

Abstract

This experiment used gas chromatography (GC) to determine the H2 production rate for

the Water-Gas Shift Reaction (WGSR) using HiFUEL W220 commercial copper, gold supported

on vanadium oxide (Au/V2O5), and gold supported on cerium oxide (Au/CeO2) catalysts. The

results show that the gold catalysts on a per gram metal basis produced more H2 than the

commercial copper catalyst, which gives credence to the notion that nanoparticle sized gold

displays good catalytic activity for the WGSR. Both the Au/V2O5 and Au/CeO2 display similar

WGSR catalytic behavior.

Introduction

Catalysts are an important aspect of chemical reactions as they increase the rate of

reaction and lower the amount of free energy needed to reach the transition state for a reaction.1

A catalyst provides an alternate pathway for a reaction with a lowered activation energy, and

therefore increases the rate of reaction with respect to the uncatalyzed process. Figure 1 provides

a hypothetical energy diagram for non-catalyzed and catalyzed pathways.

Figure 1. Activation Energy Barrier1

Catalysts have made their way into industrial processes, as they allow for the creation of

products with the addition of less energy to overcome the activation energy barrier. For example,

industry implements the Haber-Bosch process which uses an iron catalyst treated with potassium

hydroxide as a promoting agent to produce ammonia, an important compound in fertilizer. 2 The

overall reaction for ammonia production is shown in Scheme I, and the flow scheme for the

Haber-Bosch process is outlined in Figure 2.2

N2(g) + 3H2 ↔ 2NH3 (g) ΔH = -92 kJ mol-1 (I)

Figure 2. Outline of Haber-Bosch process3

The Haber-Bosch process combines nitrogen from the air and hydrogen gas, derived mainly from

methane, in a reversible exothermic reaction.3 In addition, the creation of ammonia is performed

at high temperatures even though the reaction is exothermic, in order to overcome kinetic

limitations, as at lower temperatures the reaction proceeds at a slower rate. 3 Also, the reaction is

held at high pressures, around 200 atm, so that the nitrogen and hydrogen molecules have a

greater chance of interacting, which ultimately increases the production rate of ammonia.3

In 2007, scientist Gerhard Ertl elucidated the mechanism and reaction energy diagram for

the synthesis of ammonia.2 The reaction energy diagram for the synthesis of ammonia is

provided in Figure 3.

Figure 3. Reaction energy diagram for synthesis of ammonia2

Scientific findings like Ertl’s provide better insight into reaction kinetics, which help to provide

information as to how to maximize products for reactions by using the least amount of energy

needed.

An increased rate of reaction can imply a lowered activation energy. However, when

comparing two catalysts, an increase in the observed rate of reaction may also indicate a greater

number of active sites. An active site is a location on a catalyst where the reactant or reactant

binds, which facilitates the reaction by providing a more suitable chemical environment for the

reaction to occur.4 For example, in biological processes, a substrate binds to an enzyme (the

catalyst), which facilitates a reaction.4 This induced fit model is shown in Figure 4.

Figure 4. Induced fit model4

While the number of active sites and lowered activation energy cannot be totally distinguished

from one another, Equations 1 and 2 relate the rate of production to the activation energy

rate=k [ A ] [ B ] (1)

k=A e−E a

RT (2)

where k is the rate constant (M-1s-1), [A] and [B] are the concentration of reactants (M), Ea is the

activation energy (kJ mol-1), R is a constant (0.0083145 kJ K-1 mol-1), T is the temperature (K),

and the rate is given in M s-1. By using an Arrhenius plot, the ln(k) versus 1/T is plotted, which

provides a linear fit with a slope of –Ea/R, from which the activation energy can be calculated. A

generalized Arrhenius plot is provided in Figure 5.

Figure 5. Ideal Arrhenius Plot5

For many years, gold was believed to not have any catalytic properties, as it is normally

an inert material.5 However, in recent years, gold has shown catalytic properties when the

particle sizes are on the nano-scale.6 Gold catalysis is normally performed using gold

nanoparticles deposited on a support material.6 Normally, the support material is a metal oxide

which can have catalytic properties of its own. A study by Wu et. al states that a metal oxide

support with a metal deposited on it creates new catalytic activity, which can be engineered

geometrically to facilitate oxidation, hydrogenation, and coupling reactions.7

One of the techniques used to deposit the gold nanoparticles on the support material is

known as strong electrostatic adsorption, which is based on attraction due to differences in

electrostatic charge.8 Strong electrostatic adsorption is a wet laboratory technique performed at a

pH where the electrostatic interaction between the catalyst and support material is strongest.8

Figure 6 shows the adsorption of metals based on the pH of solution, as indicated by the Jiao et.

al study.

Figure 6. Strong electrostatic adsorption8

Strong electrostatic adsorption consists of suspending a support material, such as V2O5

(vanadia) on which the gold can deposit, in solution of dissolved gold precursor. Then, by

adjusting the pH, the metal precursor can adsorb to the support material due to electrostatic

interactions.8 Vanadia in solution forms hydroxyls on its surface when dissolved in water.

Therefore, at pH values above the isoelectric point, these hydroxyls become deprotonated, and

the vanadia surface becomes negatively charged. However, vanadia has a low isoelectric point,

and its hydroxyls become deprotonated at pH values near 3. By using a positively charged gold

precursor, the gold can deposit successfully onto the support material. In the case of the strong

electrostatic adsorption performed in this report, the gold precursor employed was Au(en)2Cl3.

These findings have led to the testing of nanoparticle sized gold as a catalyst with

processes such as the Water-Gas Shift Reaction (WGSR). The WGSR is an industrial process

which converts carbon monoxide and water into carbon dioxide and hydrogen gas, as shown in

Scheme II.9

CO + H2O → H2 + CO2 (II)

The WGSR is an important industrial process, as it is a component of fuel processing for fuel cell

applications.10 Industrially, this process is carried out in two steps, each using a different metal

catalyst. The first stage of the process is carried out at high temperatures, ranging from 573-673

K, using an iron catalyst.9 However, this stage is thermodynamically hindered as the WGSR is an

exothermic process (∆H = -41.4 kJ mol-1).9 To increase the production, a second lower

temperature step, ranging from 473-523 K, is run using a copper catalyst.9 This second step is

kinetically limited. Because of these thermodynamic and kinetic obstacles, gold has been tested

as a catalyst for the WGSR at lower temperatures in this particular experiment, and has shown

good catalytic activity.

The goal of this particular experiment is to compare the rate of H2 production for the

WGSR for Au/V2O5, Au/CeO2, and copper catalysts, as well as to determine the WGSR

activation energy for each catalyst. By calculating H2 production on a per gram basis for each

catalyst, the rate of production can be determined. This rate of production can then be used to

determine the WGSR activation energy for each catalyst.

Experimental

The first part of this experimental procedure consisted of depositing gold on to the

support materials through strong electrostatic adsorption (SEA). For the Au/V2O5 catalyst, the

first step in the SEA procedure was dissolving 43 mg of a Au(en)2Cl3 precursor in 150 mL H2O,

and placing the resulting solution in a roundbottom flask. Then, the pH was adjusted to a pH of 6

by the addition of 1 M Na2CO3. After adjusting the pH, 2 g of vanadia was added to the solution,

and was stirred for 2 hours while keeping the pH near a value of 6. After the two hours passed,

the solution was then centrifuged, and its supernatant was discarded. The resulting solid was then

washed and centrifuged with deionized water three times, and was then placed in a vacuum oven

at room temperature overnight. The SEA procedure for the Au/CeO2 catalyst was the same for

the Au/V2O5, except the gold precursor was 1.66 mg of HAuCl4 in 35 mL H2O, with 1 g of the

CeO2 support material.

After the synthesis of the catalysts, the WGSR was analyzed using the gas chromatogram

(GC). For the Au/V2O5 catalyst WGSR, 100 mg of the catalyst was loaded into a quartz tube and

placed in the reactor system, with a water trap placed at the base to collect water vapor. After

loading the catalyst, it underwent a pretreatment step where the temperature of the system was

raised to 300 oC at a ramp rate of 5 oC/min in a flow rate of 60 standard cubic centimeters per

minute (sccm) of H2 and 16 sccm of H2O. The temperature was held at 300 oC for a total of 2

hours. After the two hours passed, the system was cooled to 100 oC in an Ar flow of 131 sccm.

After the pretreatment steps, the WGSR was run. First, the flow rates of the gases were changed

to 3 sccm CO, 16 sccm H2O, and 131 sccm Ar, with 11.8 μL/min H2O. The effluent was then

sampled every 10 minutes at temperatures of 100 oC, 200 oC, and 300 oC, with its H2 production

observed on the chromatogram. This process was repeated for both the Au/CeO2 and copper

catalysts. After the chromatograms for each catalyst were collected, they were analyzed using

Origin software. By integrating the chromatogram peaks corresponding to the different products,

the values were converted into H2 production in units of cc/(g-Au-hr) using a calibration curve.

Results and Discussion

In the WGSR run in this report, an indicator of a good catalytic activity is determined by

the rate of production of H2. The H2 rate of production of the catalysts on a per gram basis is

shown in Figures 1-3.

300.2 300.4 300.6 300.8 301 301.2 301.4 301.60

2

4

6

8

10

12

100 C

200 C

300 C

Temperature (°C)

H2 R

ate

of P

rodu

ction

(cc/

(g-C

u-hr

))

Figure 1. H2 rate of production of Cu commercial catalyst

250 300 350 400 450 500 550 600 6500

2

4

6

8

10

12

100 C

200 C

300 C

Temperature (°C)

H2 R

ate

of P

rodu

ction

(cc/

(g-A

u-hr

))

Figure 2. H2 rate of production of Au/CeO2 catalyst

50 100 150 200 250 300 3500

2000

4000

6000

8000

10000

12000

14000

100 C

200 C

300 C

Temperature (°C)

H2 R

ate

of P

rodu

ction

(cc/

(g-A

u-hr

))

Figure 3. H2 rate of production of Au/V2O5 catalyst

Figures 1-3 show that the Au/V2O5 catalyst produces the most H2 on a per gram basis, and that

the commercial copper catalyst produces the least H2 on a per gram basis. Figures 4-6 provide

the Arrhenius plots used to find the activation energies of the reactions, shown in Table 1.

0.0015 0.0017 0.0019 0.0021 0.0023 0.0025 0.0027 0.00292

3

4

5

6

7

8

9

10

f(x) = − 4683.79357435562 x + 16.1027630844204R² = 0.902402831873495

1/T (Kelvin)

ln(k

)

Figure 4. Copper catalyst Arrhenius plot

0.0015 0.0017 0.0019 0.0021 0.0023 0.0025 0.0027 0.00292

3

4

5

6

7

8

9

10

f(x) = − 3722.79267198897 x + 15.3052033659255R² = 0.993779446516828

1/T (Kelvin)

ln(k

)

Figure 5. Au/CeO2 catalyst Arrhenius plot

0.0015 0.0017 0.0019 0.0021 0.0023 0.0025 0.0027 0.00292

3

4

5

6

7

8

9

10f(x) = − 4751.71555659119 x + 17.8073948629763R² = 0.992184994015426

1/T (Kelvin)

ln(k

)

Figure 6. Au/V2O5 catalyst Arrhenius plot

Table 1. Activation energies of catalysts

Catalyst Activation Energy (kJ mol-1)

Copper 38.94346

Au/CeO2 30.95322

Au/V2O5 39.51039

Table 1 shows that the activation energy of the Au/CeO2 catalyst is the lowest, and the activation

energy of Au/V2O5 is the highest. However, the R2 value for the copper catalyst is not close to 1,

which means that the activation energy calculated from the Arrhenius plot in Figure 4 is not

reliable. While the activation energy for Au/V2O5 is greater than the activation energy for

Au/CeO2, the H2 rate of production is greater for Au/V2O5 than that of Au/CeO2. A greater H2

rate of production indicates a greater number of active sites. On another note, the activation

energies are similar to the activation energy found in literature, of 40 kJ mol -1.11 In essence, these

results are promising as they show that the gold catalysts are more productive than the

commercial copper catalyst on a per gram basis.

Conclusion

The data from this experiment provides good information concerning the WGSR. The

results show that on a per gram basis, the gold catalysts have a greater H2 production rate than

the commercial copper catalyst, which strengthens the argument that gold nanoparticles are a

viable option as catalysts. In addition, the greatest activation energy was found to be for the

Au/V2O5, and the smallest activation energy was found to be for the Au/CeO2 catalyst. However,

the Au/V2O5 catalyst may have more active sites, as the H2 production rate observed over

Au/V2O5 is higher than Au/CeO2.

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minority role of formates as reaction intermediates. Journal of Catalysis, 247(2), 277-287.