ATR-FTIR Flow Cell Reactor


ATR-FTIR Flow Cell Reactor

          A combined approach of QCM measurements and ATR-FTIR spectroscopy, in a custom-built flow system, allows for both quantification of the amount of uptake and spectroscopic data that provides information on speciation of adsorbed products. A Thermo Nicolette 6700 FTIR spectrophotometer and a QCM200, 5 MHz quartz crystal microbalance are integrated into the flow system for the analysis of water uptake and surface adsorption of  reactive gases on metal oxides and carbonates. A commercial attenuated total reflection (ATR) horizontal liquid cell apparatus (Pike  Technology) is placed in the internal compartment of the spectrometer. The bottom portion of the liquid cell, supplied by Pike Technology, is used as is. The top portion of the liquid cell is redesigned to contain inlet and outlet ports for humidified air, reactive gases, and a Honeywell HIH-3602-L relative humidity sensor for in situ measurements of relative humidity. For ATR-FTIR experiments, a sample of known mass (15 mg) is deposited evenly onto the AMTIR (or ZnSe or Ge) element either through a suspension or dry powder.

Schematic of the flow system for surface adsorption of H2O and HNO3 using ATR-FTIR spectroscopy and QCM measurements.

This QCM purchased from SRS has a gold/Cr polished quartz crystals in the center of the flow cell. Initially, before applying a thin coating of a sample, a baseline frequency is established that is ±0.1 Hz from the initial fundamental frequency. The top of the flow cell is then removed in order to coat the QCM with the sample of interest. The powder samples are first suspended in water and then this is sprayed onto the QCM crystal using a glass atomizer. A dry air flow is passed through the colloidal suspension and pushed through a small opening where a mist containing particles of interest is delivered to the QCM. This method created a uniform thin film of particles covering the entire active area of the balance. Typical sample weights on the order of 10 to 18 mg over the 0.4 cm2 active area of the crystal isare determined exactly by measuring changes in frequency. The frequency change of the resonator can be related to the increase in mass on the crystal as given by the Sauerbrey equation, Δf = – Cf Δm

Here, Δf is the change in frequency of the resonator, Δis the change in mass related to the change in frequency (μg/cm2), and Cf is the sensitivity factor which is a constant of 56.6 Hz cm/μg for a 5 MHz AT-cut quartz crystal. This can then be used to determine quantitative adsorption measurements. For example, the uptake of water at each humidity level was observed using FTIR and QCM measurements while the relative humidity was recorded using the humidity sensor and a custom-built digital readout.

Other reactive acids (e.g. HNO3, HCOOH) - Uptake is conducted in a similar way to the water uptake measurements but at a single steady flow of dry air over the top of the acid solution. A calibration is done to convert the units of flow into units of pressure. While monitoring the uptake using FTIR and QCM measurements, the reversible and irreversible uptake components are analyzed using the QCM and the corresponding ATR-FTIR spectrum to quantify and characterize irreversibly and reversibly bound species.