Heterogeneous
catalysis holds great promise for oxidizing or reducing
a range of pollutants in water. A well-recognized, but understudied,
barrier to implement catalytic treatment centers around fouling or
aging over time of the catalyst surfaces. To better understand how
to study catalyst fouling or aging, we selected a representative bimetallic
catalyst (Pd–In supported on Al2O3),
which holds promise to reduce nitrate to innocuous nitrogen gas byproducts
upon hydrogen addition, and six model solutions (deionized water,
sodium hypochlorite, sodium borohydride, acetic acid, sodium sulfide,
and tap water). Our novel aging experimental apparatus permitted single
passage of each model solution, separately, through a small packed-bed
reactor containing replicate bimetallic catalyst “beds”
that could be sacrificed weekly for off-line characterization to quantify
impacts of fouling or aging. The composition of the model solutions
led to the following gradual changes in surface composition, morphology,
or catalytic reactivity: (i) formation of passivating species, (ii)
decreased catalytic sites due to metal leaching under acid conditions
or sulfide poisoning, (iii) dissolution and/or transformation of indium,
(iv) formation of new catalytic sites by the introduction of an additional
metallic element, and (v) oxidative etching. The model solution water
chemistry captured a wide range of conditions likely to be encountered
in potable or industrial water treatment. Aging-induced changes altered
catalytic activity and provided insights into potential strategies
to improve long-term catalyst operations for water treatment.
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