TPD experiments

2.4.1. Setup

TPD experiments were carried out using a tubular quartz reactor with an inner diameter of 28 mm (Fig. 2-18). Below, this setup is called “TPD reactor” in short. The gas-mixing unit was quite similar to the one described in chapter 2.1.2: Electronic mass-flow controllers (Brooks 5850S) provided a model gas stream that typically consisted of 5% or 0% H₂O,

A This chapter is partly based on the publication:

A. M. Bernhard, I. Czekaj, M. Elsener, A. Wokaun, O. Kröcher „Evaporation of Urea at

2.5 10.0 15.0 22.5

10% O2 in N2. H2O was added by a water saturator.^A The total gas flow was 431 L/h at STP in the TPD experiments and 215 L/h at STP in the isothermal experiments except when otherwise stated. The heating rate was always 10 K/min.

The heating of the reactor was divided into three sections (entrance, center and exit), which could be heated individually. The entrance and center temperatures were measured by thermocouples placed in the gas stream;

the center-thermocouple was placed downstream of the sample. The mantle temperature in the exit section was measured below the heating wire. Fig. 2-18 shows the placement of the thermocouples in the reactor.

Fig. 2-18. Scheme of the quartz reactor for TPD experiments with two monoliths (M1 and M2) in the metal adaptor.

The temperature of the reactor entrance was always set to the same value as the center, whereas the reactor exit was usually heated 50 K higher to avoid condensation of product gases. Lower exit temperatures resulted in a tailing of the gas-phase components due to re-adsorption. On the other hand, higher exit temperatures did not influence the measured urea nor the HNCO concentrations (Fig. 2-19), indicating that neither urea re-adsorption nor decomposition in the gas phase occurred due to heat transfer from the reactor wall.

A The two last biuret decomposition experiments and the preparation of DRIFT samples were performed with catalytic H2 oxidation for water generation.

Fig. 2-19. Urea evaporation and decomposition at 140°C with different reactor exit temperatures. Model gas: 10% O2, 5% H2O in N2, gas flow = 215 L/h at STP, GHSV = 9400 h^-1.

The gaseous low-molecular-weight compounds NH₃, CO₂, H₂O, NO, NO₂, N₂O, CO, HNCO, formic acid, HCN, formaldehyde, methanamide and HNO3 were measured with a multi-component analysis method developed in-house [25]. The quantification method was quite similar to the method used at the spray reactor (see chapter 2.1.7), but it was developed on a Nexus FTIR spectrometer from ThermoNicolet. A similar liquid-quench method as for the spray rector (chapter 2.2) was also applied for TPD experiments.

2.4.2. Monoliths

For most TPD experiments, small cuboid monoliths with 20.5 mm length, 17.5 mm width, 12.4 mm height, 400 cpsi and 9 · 13 = 117 channels were used. Besides, cuboid monoliths with doubled length (40 mm) were used as well. The monoliths were inserted into a metal adaptor to fit the round reactor, as shown in Fig. 2-18. In some experiments, a large inert cylindrical monolith with dimensions 27 mm diameter, 42 mm length, 400 cpsi and

0 100 200 300 400

190

190 190

=>240

190=>240 240 240 T reactor exit, °C

ppm Urea

HNCO

293 channels was used. The resulting space velocities were 97’000 h^-1 for the small monoliths at 431 L/h (at STP) gas flow and 9400 h^-1 at 215 L/h (at STP) for the large monolith. The catalyst preparation and characterization will be described in chapter 2.6 on page 69.

Before starting an experiment, a monolith was impregnated with the reactant by dipping it into a solution of urea, biuret or CYA. For biuret and CYA, the solutions had to be heated to solubilize these reactants.

Melamine, which is poorly soluble in water, was suspended.^A Next to dipping, the excess solution was blown out of the monolith channels, the outer monolith surface was cleaned with a tissue and the monolith was weighted in the wet state. Finally, the monolith was gently dried using a blow dryer.

2.4.3. Experiment types

In Fig. 2-18, two small cuboid monoliths are shown in the metal adaptor.

In fact, different arrangements were made for different types of experiments:

 Evaporation and non-catalytic decomposition with one inert cordierite monolith only, impregnated with the reactant. All the three monolith sizes mentioned above were used, namely: small cuboid, long cuboid and large cylindrical. These experiments were either performed at a temperature ramp of 10 K/min or isothermally [43].

 Catalytic decomposition with two catalyst-coated monoliths, the first one impregnated with the reactant and the second one clean to complete the

A For comparison, CYA was suspended as well in selected experiments.

reaction. Theses experiments were performed at a temperature ramp of 10 K/min.

 Catalytic decomposition with one inert monolith, impregnated with the reactant (M1 in Fig. 2-18), followed by a clean, catalyst-coated monolith (M2 in Fig. 2-18) to perform the catalytic reaction. Theses experiments were performed at a temperature ramp of 10 K/min or isothermally. For the isothermal experiments, a long cuboid monolith was preferably used as the first monolith to slow down the depletion of the reactant. In the context of the DRIFT experiments reported in chapter 5, the clean second monolith was loaded with gaseous urea and then washed to check the surface species by HPLC analysis.

 Isothermal catalytic decomposition on one impregnated and catalyst-coated monolith. The reaction was quenched after a certain time by pulling the monolith out of the hot reactor and letting it cool down at ambient conditions. Remained reactant, reaction intermediates and non-volatile reaction products were then washed off the monolith by immersing it into the HPLC eluent and leaving it overnight at room temperature. Finally, the washing solution was analyzed by HPLC (chapter 2.3).

To check the washing efficiency, some catalyst-coated monoliths were washed a second time by boiling in de-ionized water. Additionally, one monolith was washed once and then used for a TPD experiment. In conclusion, the washing efficiency (first washing) was very high if urea or biuret was the reactant, or about 90% (based on the N-balance) if melamine was the reactant. The stability of the byproducts in the eluent was checked by boiling an HPLC standard solution containing 10 ppm of biuret, triuret,

melamine, ammeline, ammelide and 100 ppm of CYA for 15 min. The boiling decreased the triuret concentration significantly, but the other compounds appeared to be stable. Since the predominant share of the reactants and products were washed off from the monolith with eluent already at RT, further decomposition could be neglected.^A