FRI Nano Stream
Summer 2013

CATALYTIC EVALUATION OF DENs
OBJECTIVE
For the remaining weeks of the semester, you will perform experiments aimed at (I)
assessing the capacity of dendrimers to bind metal ions using UV-Vis spectroscopy, (II)
using dendrimers as a template to synthesize metal nanoparticles, and (III) measuring
nanoparticle catalytic activity using a model reaction monitored by UV-Vis spectroscopy.
INTRODUCTION
Re-cap: Catalysis

A catalyst is any substance that speeds up
a chemical reaction but is neither produced nor consumed during the reaction. Many
nanomaterials have enhanced catalytic properties compared to corresponding bulk
materials because of their increased surface-area-to-volume ratio and altered geometric
and electronic structures.1
The reduction of 4-nitrophenol (4-NP) serves as a good model reaction for the evaluation
of catalysts because the concentration of 4-NP can be easily monitored quantitatively by
UV-Vis spectroscopy. When 4-NP is reduced to form 4-aminophenol (4-AP), the reaction
can be observed as a change in the solution color from yellow to colorless. Being able to
quantify the reaction rate makes it possible to compare catalysts quantitatively. This
reaction must be carried out at basic pH, so that the 4-NP is in its deprotonated form,
which has the strongest color.

Re-cap: Kinetics
Nanoparticle III: Kinetics

FRI Nano Stream
Summer 2013

For a reaction in which
aA + bB ? cC

(1)

the rate law expression has the general formula
rate =k [A]x[B]y

(2)

where [A] and [B] are the concentrations of reactants, x and y are the order of reaction with
respect to each reactant, and k is the rate constant. Note that the rate is dependent on the
concentrations of reactants while k is not. The rate constant k is only affected by
temperature, catalyst, and catalyst concentration. The values of the exponents (x and y)
and the rate constant k bear no necessary relationship to the coefficients (a and b) of the
balanced chemical equation. For the overall reaction: k, x, and y must be determined
experimentally.2
The reduction of 4-NP can be expressed by the following:
3H2 + 4-NP ? 4-AP + H2O (3)
The source of H2 gas in this reaction is NaBH4, and it is present at 1000x the concentration
of 4-NP, in our experimental set-up. The large excess of NaBH 4 means that effectively the
concentration of NaBH4 does not change during the course of the reaction and therefore
does not effect the rate of the reaction. In other words, the reaction appears to be first
order with respect to [4-NP], as has been shown by Endo et al. 3 and Feng et al.4
Because the reaction has been manipulated in order to make it appear first order, it is
considered “pseudo first-order” with respect to 4-NP and can be represented by the first
order rate law:
rate = k [4-NP]

(4)

Integrating this equation with respect to time (t) yields the following integrated rate law
equation:
ln[4-NP] = - k t + ln[4-NP]0

(5)

where [4-NP]0 is the initial concentration of 4-NP and [4-NP] is the concentration of 4-NP at
time t.
According to the Beer-Lambert Law, the concentration of a substance in solution is
proportional to absorbance of the solution. Hence, the Beer-Lambert law for 4-NP is:
A400 = ?l[4-NP]

(5)

Where A400 is the absorbance at 400 nm, ? is the molar absorptivity in L mol-1 cm-1, l is the
pathlength in cm, and [4-NP] is the concentration of 4-NP. This can be re-arranged to yield
[4-NP] = A400/(?l)
Nanoparticle III: Kinetics

(6)

FRI Nano Stream
Summer 2013

Combining the Beer-Lambert Law (6) with the first order integrated rate law (5) yields the
following equation:
ln(A400) = - kt

+ ln(A400,t=0)

(7)

Note that the molar absorptivity and pathlength factors cancel out. This equation (7) is in
the form y = mx + b, where x is time (t), y is the ln(A400), and the absolute value of the
slope is the rate constant (k).
CHEMICALS AND SAFETY ISSUES
Chemical

Hazards

e.g.: NaOH

Highly corrosive, hygroscopic, toxic

Sodium
Borohydride
(NaBH4)

corrosive, EVOLVING H2 GAS!!!!!

Precautions
Wear gloves and
goggles;
clean up spills right
away
DO NOT CAP NaBH4
SOLUTIONS AFTER
ADDING WATER,
THEY WILL
EXPLODE!!

PROCEDURE
You will write your own procedure for this experiment. Complete the pre-lab questions
and then write your procedure. Be sure to include the following sections:
Before you start
? Read the reference paper by Feng et al.4
Review this statistics tutorial: https://sites.google.com/site/frinanostream/tutorials/statistics
? Answer the following pre-lab questions and then write a step-wise procedure
1. Why is it important that your nitrophenol solutions in pH 12 water?
2. What is the concentration of OH- in a solution with a pH of 12? (Hints: pH +pOH
= 14 and pH=-log[H+] and C1V1=C2V2)
3. How much 0.1 M NaOH is needed to prepare 50 ml of water at pH 12?
4. What mass of 4-NP do you need in order to make 10ml of 6mM 4-NP solution?
5. What was the concentration of the nitrophenol added to the cuvette for the
reduction reaction done by Feng et al. (hint: look at Figure 3)? 4
a. How much 6mM nitrophenol do you need in order to prepare 20 ml of the
concentration of nitrophenol used in Feng et al.?
b. How much additional water do you need to add? (Note: what pH should
the water be?)
6. What was the concentration of the NaBH4 added to the cuvette in Feng et al.?4
a. What mass of NaBH4 do you need in order to prepare 10 ml of the
concentration of NaBH4 used?
Nanoparticle III: Kinetics

FRI Nano Stream
Summer 2013

b. Why is it important never to cap a solution of NaBH 4?
7. After the water, DENs, nitrophenol, and NaBH 4 have been added to the cuvette,
a. What will the final, total volume in the cuvette be?
b. What will the nitrophenol concentration in the cuvette be?
c. What will the NaBH4 concentration in the cuvette be?
d. What will the concentration of DENs in the cuvette be? (assume an initial
concentration of 10 ?M DENs, and the same volume as Feng et al.)
8. What wavelength will you monitor over time in order to follow the course of the
reduction of 4-NP?
9. Why does the reaction appear first order with respect to 4-nitrophenol?
10. How will you determine the rate constant (k) for each trial? (Hint: see eq. 7)
11. The mass of the piece of bulk copper you will be given to test as a catalyst is ~510 mg. What is the total mass of copper that you will be adding to each of your
kinetic trials? Which do you hypothesize will be a better catalyst? Why?
Make sure your procedure includes the following:
? Prepare pH 12 water
? Prepare 4-NP solutions
? Prepare NaBH4 solution (Note: Use this solution from 5 - 30 minutes after adding
water.)
? Perform at least 5 kinetic trials with your DENs
? Control: perform a kinetic trial substituting water for DENs and adding a piece of
copper wire
RESULTS – must be completed in your lab notebook before you may start your proposal
project
1. Analyze the data from your kinetic trials.
a. For each of your trials, correct for background absorbance by subtracting the
minimum absorbance: A400-min = A400 – min(A400).
b. Graph 1: Prepare a graph for each trial showing A 400-min vs. time (s). Put all
graphs in your lab notebook, and include one graph in your report as an
example.
c. Graph 2: Prepare a graph for each trial according to equation (7). Put all
graphs in your lab notebook, and include one graph in your report as an
example.
d. From the second set of graphs, determine the rate constant for each trial.
e. Find the average and standard deviation of your trials for each type of DEN,
and record them in your notebook. Use a Q-test to reject possible outliers.
2. Do you think that your copper DEN was an active catalyst for the reduction of 4-NP?
Why? Support your answer with data.
REFERENCES
Nanoparticle III: Kinetics

FRI Nano Stream
Summer 2013

(1)

Roduner, E. Chemical Society Reviews. 2006, 35, 583–92.

(2)

Whitten, K. W.; Davis, R. E.; Peck, L. M. General Chemistry, 5th Editio.; Saunders
College Publishing; 1996.

(3)

Endo, T.; Yoshimura, T.; Esumi, K. Journal of colloid and interface science. 2005,
286, 602–9.

(4)

Feng, Z. V.; Lyon, J. L.; Croley, J. S.; Crooks, R. M.; Bout, D. A. Vanden; Stevenson,
K. J. Journal of Chemical Education. 2009, 86, 368–372.

Nanoparticle III: Kinetics