Amino-alkylphosphonic acid-grafted TiO2 materials
are
of increasing interest in a variety of applications such as metal
sorption, heterogeneous catalysis, CO2 capture, and enzyme
immobilization. To date, systematic insights into the synthesis–properties–performance
correlation are missing for such materials, albeit giving important
know-how towards their applicability and limitations. In this work,
the impact of the chain length and modification conditions (concentration
and temperature) of amino-alkylphosphonic acid-grafted TiO2 on the surface properties and adsorption performance of palladium
is studied. Via grafting with aminomethyl-, 3-aminopropyl-, and 6-aminohexylphosphonic
acid, combined with the spectroscopic techniques (DRIFT, 31P NMR, XPS) and zeta potential measurements, differences in surface
properties between the C1, C3, and C6 chains are revealed. The modification
degree decreases with increasing chain length under the same synthesis
conditions, indicative of folded grafted groups that sterically shield
an increasing area of binding sites with increasing chain length.
Next, all techniques confirm the different surface interactions of
a C1 chain compared to a C3 or C6 chain. This is in line with palladium
adsorption experiments, where only for a C1 chain, the adsorption
efficiency is affected by the precursor concentration used for modification.
The absence of a straightforward correlation between the number of
free NH2 groups and the adsorption capacity for the different
chain lengths indicates that other chain-length-specific surface interactions
are controlling the adsorption performance. The increasing pH stability
in the order of C1 < C3 < C6 can possibly be associated to a
higher fraction of inaccessible hydrophilic sites due to the presence
of folded structures. Lastly, the comparison of adsorption performance
and pH stability with 3-aminopropyl(triethoxysilane)-grafted TiO2 reveals the applicability of both grafting methods depending
on the envisaged pH during sorption.
While synthesis‐properties‐performance correlations are being studied for organophosphonic acid grafted TiO2, their stability and the impact of the exposure conditions on possible changes in the interfacial surface chemistry remain unexplored. Here, the impact of different ageing conditions on the evolution of the surface properties of propyl‐ and 3‐aminopropylphosphonic acid grafted mesoporous TiO2 over a period of 2 years is reported, using solid‐state 31P and 13C NMR, ToF‐SIMS and EPR as main techniques. In humid conditions under ambient light exposure, PA grafted TiO2 surfaces initiate and facilitate photo‐induced oxidative reactions, resulting in the formation of phosphate species and degradation of the grafted organic group with a loss of carbon content ranging from 40 to 60 wt %. By revealing its mechanism, solutions were provided to prevent degradation. This work provides valuable insights for the broad community in choosing optimal exposure/storage conditions that extend the lifetime and improve the materials′ performance, positively impacting sustainability.
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