Influence of UV and water treatment on the photocatalytic activity of cement-based samples

The effect of medium term exposure to UV radiation and water washing was studied using two photocatalytic cement-based materials. The samples were exposed at 740±70 ?W cm-2 UV radiation for ~3000 h into an irradiation chamber supplied with untreated ambient air. The exposure period was interrupted periodically in order to measure the photocatalytic activity as NO degradation rate. During the exposure experiment two washing cycles (by immersion in pure water for 2 and 5 minutes respectively) were also performed. After each washing cycle the samples were allowed to equilibrate their humidity content by exposing them to untreated ambient air in the dark. The photocatalytic activity evolution during the experiment was studied measuring the degradation of nitric oxide (NO) in air at typical ambient concentration (75 ppb) using a special constant- concentration analytical method [1] that allows to reach the desired reactor internal NO concentration (under UV irradiation) by modulating the inlet pollutant flow in a successive approximation trial. The photocatalytic NO degradation rates measured for the two cement-based samples demonstrate relevant variations after the various UV exposure periods and the two water treatment cycles. The two samples also show a different variation pattern, possibly indicating the presence of several mechanisms operating concurrently. These early results suggest that the previous sample story can have a deep effect on the photocatalytic reaction rate and that the actual activity of a catalytic surface operating outdoor can be very different from the value measured in the laboratory with a fresh specimen.