Journal of Photochemistry & Photobiology A: Chemistry
Short note
1H NMR as a quick screen for photocatalytic reaction efficiency
Mark P. Croxalla,1, Reece T. Lawrenceb,1, M. Cynthia Goha,b,
a Department of Chemistry, University of Toronto, 80 St. George St., Toronto, Ontario, M5S 3H6, Canada
b Department of Material Science and Engineering, University of Toronto, 184 College St., Toronto, Ontario, M5S 3E4, Canada
A B S T R A C T
1H NMR is a common technique for tracking chemical reactions; here it is demonstrated that it can be used effectively to evaluate efficiency of photocatalysts. The
photocatalytic degradation of acetylsalicylic acid (Aspirin) by P25 TiO2 and its photolytic degradation by UV light were examined; and they were found to be
comparable, suggesting that this breakdown reaction does not benefit greatly from the catalyst’s presence. Due to its chemical specificity, 1H NMR enables iden-
tification of breakdown products, which is a useful consideration in wastewater treatment and mechanistic studies.
1. Introduction
organic degradation and properly monitor reaction intermediates [5].
However, the high operating cost and relatively difficulty of LC–MS
makes it undesirable for screening large numbers of new photocatalysts.
Adding to the difficulty is the possibility that nanoparticle catalysts can
damage LC columns and MS instrumentation.
Pharmaceuticals and personal care products (PPCPs) represent a
class of compounds introduced to wastewater through anthropogenic
means. Advanced oxidative processes, such as UV photolysis and pho-
tocatalysis, have been identified as powerful techniques for treating
such contaminants [1]. A large number of pharmaceuticals are sus-
ceptible to direct UV photolysis, but the efficiency of this process is
heavily dependent on each compound’s molar extinction coefficient
[2]. Photocatalytic materials, such as nanosized TiO2 and other metal
oxides, absorb UV light and use photogenerated electrons and holes to
catalyze the degradation of organic species to benign CO2 and H2O [3].
The most common method of testing photocatalytic efficiency is the
degradation of dyes monitored by UV–vis absorbance spectroscopy.
However, if the UV–vis spectrum of the analyte and intermediates in the
photocatalytic breakdown are similar, it can be difficult to properly
calculate degradation efficiency. While it is simple to monitor the de-
coloration of a dye using UV–vis spectroscopy, this approach does not
give much information beyond the first bond breaking step. In addition,
most photocatalytic materials strongly absorb UV light which makes
analysis of organic analytes impossible if even small concentrations of
the catalyst remain present in the solution.
Nuclear Magnetic Resonance Spectroscopy (NMR) is commonly
used in organic and inorganic chemistry to monitor reaction progress
and can provide powerful chemical information using common in-
strumentation; it is also resilient to the presence of nanocatalysts. We
have empirically noticed that a small amount of P25 TiO2 in the NMR
tube does not affect the observed peaks. NMR has not been utilized
much in the context of photocatalysis; it has been used to show the
removal of a dye [6], to quantify adsorption onto a solid surface [7],
and to supplement LC–MS results in mechanistic studies [8].
In this paper, we show how NMR can be utilized in the evaluation of
photocatalysts for breakdown of organic matter. We illustrate this using
aspirin as a model PPCP analyte: it is present in wastewater across the
globe and its breakdown has been characterized by LC–MS [9]. The first
step in the photocatalytic breakdown of aspirin is the deacetylation to
salicylic acid and acetic acid (Fig. 1]) [9].
We monitor the conversion of aspirin to products using 1H NMR to
demonstrates its potential for quick screening of the photocatalytic
process.
Total Organic Carbon (TOC) and Chemical Oxygen Demand (COD)
measurements are also used to monitor concentrations of organic
compounds in solution but they lack chemical sensitivity [4] which is
important when studying complex systems like wastewater. The pre-
sence of catalyst can give false results if it interacts with either of the
methodologies.
2. Methods and materials
25 mg of Aeroxide P25 TiO2 (Evonik) was added to a 6.7 mM so-
lution of aspirin in D2O in glass vials with UV cutoff of 315 nm. A si-
milar sample was prepared without TiO2 as a photolysis comparison.
The reaction vials were vigorously stirred 20 cm from an unfiltered
Liquid chromatography mass spectrometry (LC–MS) provides pow-
erful chemical information and can reliably measure efficiency of
⁎ Corresponding author at: Department of Chemistry, University of Toronto, 80 St. George St., Toronto, Ontario, M5S 3H6, Canada.
1 These authors contributed equally.
Received 15 February 2019; Received in revised form 20 June 2019; Accepted 7 July 2019
1010-6030/©2019TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense
(http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Croxall, Mark P.
