Literature review done on applications of metal nanoparticles in photocatalysis

1. `
CH 2150 PARTICLE TECHNOLOGY
Assignment
Applications of Metal Nanoparticles in Photocatalysis
NAME : BENARAGAMA B.V.C.M.
INDEX NO : 170070D
DATE OF SUB : 26/12/2019
Department of Chemical and Process Engineering
University of Moratuwa

2. • Introduction
For the past few decades, Nanotechnology has been a frequently published term in most
of the scientific literature. In fact, Nanotechnology is considered as the next era of
technology, which has reached its exponential rate of growth at present. According to
Drexler and K. Eric (Drexler, n.d.), Nanotechnology can be defined as the manipulation of
material in the scale of nanometres for different applications. Since the material are
manipulated at an atomic scale, chemical engineers have to play a very important role in
the development of applications related to nanotechnology.
The particles under control in nanotechnology are known as nanoparticles, which are in
the size range of 1-100 nanometres. The objective of the following review written on the
applications of nanoparticles in the area of photocatalysis is to provide a general
understanding of common applications of nanoparticles as photocatalyst in the field of
chemical engineering.
(Kuznetsov, 2017)

3. • Photocatalysis
The purpose of a catalyst is to increase the rate of a reaction to provide a higher yield in
a given time. Photocatalysis is a phenomenon where light or radiation energy is used for
catalysis of a reaction. The phenomenon may or may not need a chemical catalysis
material. Originally, metal oxide nanoparticles were used as catalysts for most of the
reactions in the past. But with the discoveries involving the enhancements that can be
done to reaction rates and conditions with the use of metal nanoparticles (MNP) the
attention has been mainly paid to use of nanoparticles as catalysts and cocatalysts.
Similarly, some instances related to the use of MNPs directly for catalysis of certain
reactions could also be found.
According to Sarina, Waclawik, & Zhu, (2013), MNPs can act in two ways to enhance the
photocatalytic reaction rates. Firstly, the MNPs act as light absorbers to provide the
energy needed for the chemical transformation of the reactant molecules. Secondly, they
also provide sites for products to be formed, especially in gaseous products. In order to
understand the mechanism of photocatalysis, a thorough knowledge of electromagnetic
radiations are needed. In brief, there are two modes of absorption of light in MNPs
namely, the light absorption of electrons in metallic lattice without ionization at short
wavelength radiations, (generally UV, near X-ray) and light absorption due to Localized
Surface Plasmon Resonance (LSPR) in longer wavelengths (UV-Visible) which can only be
seen in few MNP types (e.g: Au, Ag, Cu). The second mode is being experimented to bring
into practice, in order to use renewable solar energy for chemical transformations.
(Peiris, McMurtrie, & Zhu, 2016) (Peiris et al., 2016)

4. • Common Instances for Applications of MNPs as Catalysts
.1. Selective Oxidation Reactions
.1.1. Oxidation of Alcohols
Even though there are numerous techniques to oxidize alcohols, the normal
catalytic reactions being not selective enough to get a product with admirable
purity has always been a problem. The usual catalysts used need high temperature
and pressure conditions to achieve sufficient production rates, which negatively
affects the selectivity and over-oxidation could be a result. (Sun, Blatter, & Frei,
1994). On the other hand, there are sophisticated reaction methods for the
purpose, but they could be expensive or toxic (Lee & Spitzer, 1970). However,
methods have been found to use Au MNPs on zeolite to achieve a selectivity of
99% at temperature 40oC for alcohol oxidation reactions to carbonyl compounds
(Zhang, Ke, & Zhu, 2012). The importance is zeolite alone could not show any
photocatalytic properties. Use of Pt with ZrO2, showing very low selectivity for the
same reaction proved that the plasmonic MNPs light absorption in Au, can show
higher efficiency in photocatalysis than in normal electron light absorption
phenomenon (Han, Martens, Waclawik, Sarina, & Zhu, 2018).
(Peiris et al., 2016)

5. .1.2. Oxidation of Aliphatic Alcohols to Esters
The conventional conversion of acid and alcohols into esters has very low
productivity and gives a considerable amount of undesired products (conversion:
62%). Methods have been developed to produce esters in largescale using readily
available non-toxic alcohols through eco-friendly methods involving
photocatalytic reactions with Au/Pd MNP alloy (conversion: 94%, selectivity: 76%)
under visible light and ambient temperature (Xiao et al., 2015).
.2. Selective Reduction Reactions
.2.1. Reduction and reductive coupling of nitroarenes
Aromatic azo compounds are commonly used as dyes in most industries, such as
in textiles, food, polymer, and, paper. Because these compounds cannot be
processed under high pressure and temperature conditions, in order to reduce
them into an intermediate state, they are reduced to a lower level and are
oxidized to the needed oxidation state (Grirrane, Corma, & García, 2008).
However, this method produced a large number of waste products which are
harmful to the atmosphere. New methods have been developed using Au MNPs
on ZrO2 to partially reduce nitroarenes to azo compounds with a 100% yield, at
400C temperature under visible light.
(Peiris et al., 2016)

6. .3. Cross-coupling Reactions
.3.1. Suzuki-Miyaura C-C coupling
Palladium phosphine complexes, used as catalysts for Suzuki reaction caused
numerous problems in recovery of the catalysts (since the catalyst is
homogeneous), and releases several types of pollutants as bi-products. The
reaction could be carried out with heterogeneous catalysts, but the process is not
cost-effective due to the extreme conditions needed. With the recently developed
Au−Pd alloy MNPs on ZrO2, the needed temperature could be reduced to 400C
from 1000C, under visible light with a 96% conversion.
(Peiris et al., 2016)

7. • Conclusion
The discovery of MNPs has been a huge turning point in chemical engineering. The MNPs
as catalysts and co-catalysts has transformed the field of chemical synthesis into a new
path involving green synthesis, where the reactant wastage has been minimized, reaction
conditions simplified, and environmental problems reduced. The feasibility of using solar
energy for reactions has led to a revolution in solar panel technologies to create durable,
cheaper equipment to harness solar energy. Also, the use of MNPs to decompose
wastewater organic substances effectively has directed Green engineering fields to a new
extreme. Due to the word limit, the review has to be concluded only with a brief summary
of the information found. Further research of the topic can be performed by following
the references.

8. • References
Drexler, K. E. (n.d.). Engines of Creation : The Coming Era of Nanotechnology Chapter 1 :
ENGINES OF CONSTRUCTION. Retrieved from
http://www.foresight.org/EOC/EOC_Chapter_1.html
Grirrane, A., Corma, A., & García, H. (2008). Gold-catalyzed synthesis of aromatic azo
compounds from anilines and nitroaromatics. Science, 322(5908), 1661–1664.
https://doi.org/10.1126/science.1166401
Han, P., Martens, W., Waclawik, E. R., Sarina, S., & Zhu, H. (2018, June 1). Metal
Nanoparticle Photocatalysts: Synthesis, Characterization, and Application. Particle and
Particle Systems Characterization, Vol. 35. https://doi.org/10.1002/ppsc.201700489
Kuznetsov, A. P. (2017). Evolution of methods of assessing the accuracy of metal-cutting
machines. Russian Engineering Research, 37(3), 171–179.
https://doi.org/10.3103/S1068798X17030157
Lee, D. G., & Spitzer, U. A. (1970). Aqueous dichromate oxidation of primary alcohols. The
Journal of Organic Chemistry, 35(10), 3589–3590. https://doi.org/10.1021/jo00835a101
Peiris, S., McMurtrie, J., & Zhu, H. Y. (2016). Metal nanoparticle photocatalysts: Emerging
processes for green organic synthesis. Catalysis Science and Technology, 6(2), 320–338.
https://doi.org/10.1039/c5cy02048d
Sarina, S., Waclawik, E. R., & Zhu, H. (2013, July). Photocatalysis on supported gold and
silver nanoparticles under ultraviolet and visible light irradiation. Green Chemistry, Vol.
15, pp. 1814–1833. https://doi.org/10.1039/c3gc40450a
Sun, H., Blatter, F., & Frei, H. (1994). Selective Oxidation of Toluene to Benzaldehyde by
O2 with Visible Light in Barium(2+)- and Calcium(2+)-Exchanged Zeolite Y. Journal of the
American Chemical Society, 116(17), 7951–7952. https://doi.org/10.1021/ja00096a084
Xiao, Q., Liu, Z., Bo, A., Zavahir, S., Sarina, S., Bottle, S., … Zhu, H. (2015). Catalytic
Transformation of Aliphatic Alcohols to Corresponding Esters in O 2 under Neutral
Conditions Using Visible-Light Irradiation. Journal of the American Chemical Society,

9. 137(5), 1956–1966. https://doi.org/10.1021/ja511619c
Zhang, X., Ke, X., & Zhu, H. (2012). Zeolite-Supported Gold Nanoparticles for Selective
Photooxidation of Aromatic Alcohols under Visible-Light Irradiation. Chemistry - A
European Journal, 18(26), 8048–8056. https://doi.org/10.1002/chem.201200368