Temperature Programmed Studies

Temperature programmed reduction (TPR), temperature programmed oxidation (TPO), temperature programmed desorption (TPD) and pulse chemisorption experiments were conducted using a Micromeritics Autochem II 2920 Analyzer. In each experiment, 100 to

200 mg of catalyst sample was placed in a U-shape quartz tube and the tube was installed inside the heating chamber of the analyzer.

3.3.3.1 Temperature Programmed Reduction (TPR)

The TPR test provides valuable information about the reduction characteristics of a catalyst. Catalysts are subjected to repeated oxidation and reduction cycles in a gasifier. Regenerability of a catalyst depends on its metal reducibility and reduction temperature. TPR was performed to determine the amount of reducible species and the temperature range at which reduction occurs.

Before the hydrogen TPR experiments, the sample was pre-oxidized using a gas containing 5% oxygen in helium at 750 °C. The oxidized sample was, then, cooled down under argon flow to remove any gas phase oxygen trapped in the catalyst particles. Following this step, the sample reduction was performed using a gas containing 10% hydrogen in argon. This gas was circulated throughout the catalyst bed at a rate of 50 ml/min. While the gas was flown through the particle bed, the bed temperature was raised progressively from ambient to 950 °C at a rate of 10 °C/min. The gas leaving the quartz reactor was circulated through a cooling loop in order to remove the water (produced during the reduction reaction) before it reached a thermal conductivity detector (TCD). The cooling loop was refrigerated using a mixture of liquid nitrogen and iso- propanol. Once the bed temperature reached the reduction temperature, hydrogen reacted with the oxide(s) present in the sample. A thermal conductivity detector (TCD) was used to analyze the water-free exit gas concentration. The amount of hydrogen consumed (

2 H V

) in the reduction of the catalyst sample was determined from the TCD signal. The reacted hydrogen was further related to the number of reducible species (Ni) in the catalyst sample as follows:

g H Ni Ni v V MW W  2  (3.1)

where, represents the molecular weight of the reducible species (g), stands for the volume of H2 consumed at STP (cm3), denotes the gas molar volume at STP (cm3/mol) and represents the actual metal amount on the oxygen carrier (g). ν stands for the stoichiometric number based on the following reaction stoichiometry:

.

The percentage reduction (R) was then calculated as follows:

% 100   O Ni W W (%) R (3.2)

where, Wo represents the actual metal amount in the catalyst sample.

3.3.3.2 Temperature Programmed Oxidation (TPO)

The TPO examines the extent to which a catalyst can be oxidized or was previously reduced. During the present research, TPO experiments were developed following a TPR experiment. This was done, in order to re-oxidize the sample previously reduced in the TPR cycle. Before starting a TPO run, the system was cooled down to room temperature. During the cooling period, an inert gas (helium) flow was maintained to flush out any unreacted hydrogen from the system.

The steps of TPO were exactly the same as the ones of a TPR with the exceptions that in this case, the flowing gas stream had a composition of 5% O2 and 95% He and the bed temperature was increased up to 700 °C. As with the TPR, the total amount of consumed O2 calculated from processed TCD data, was used to measure the percentage of metal oxidation.

3.3.3.3 H2 Pulse Chemisorption

H2 pulse chemisorption was conducted to determine the active metal surface, the percent metal dispersion and the average active metal crystal size based on the monolayer of gas adsorbed on the catalyst. Regarding metal dispersion, it is important to mention that it can vary depending on several factors, such as: a) the type of metal/support selected, b) the

specific surface area of the support chosen, c) the sample preparation methods and d) the effects of the promoter employed.

Figure 3.2: Typical TCD profile for H2 pulse chemisorption experiments, where each peak pepresents the eluted hydrogen after each injection

H2 pulse chemisorption was performed at ambient temperature following the TPR experiments. After reduction, a stream of argon gas was flown through the sample bed at a rate of 50 ml/min. Hydrogen gas was then injected as a series of consecutive pulses containing 1.0 ml each using a calibrated loop with a 1.5 min delay between each gas sequential injection. Each pulse generated a TCD peak which was recorded at the exit of the gas stream as shown in Figure 3.2. Peak areas changed for each injection as a result of the H2 chemisorbed amount. When two consecutive peaks yielded essentially the same area (less than 1% difference), the sample was considered saturated with hydrogen. As a result, the total amount of hydrogen required for saturation was calculated as

. This X value describes the total hydrogen amount chemically

adsorbed on the active sites of the catalyst. X can be used to calculate the percent metal dispersion as follows: R W X D  117 % (3.3) Time (min) 0 5 10 15 20 25 TC D Signal (a. u. ) 0.00 0.01 0.02 0.03 0.04 0.05 0.06

where D is the metal dispersion, X stands for the total hydrogen chemisorbed (mol of H2 / g of catalyst), W denotes the metal wt% in the sample and R constitutes the percentage of metal reducibility.

Furthermore, the average crystal size (dv) of the metal on the support was calculated from the percent metal dispersion using the following equation:

D S V d m m v % 1   (3.4)

where, represents the particle shape constant, stands for the volume of metal atoms (nm3) and denotes the average surface area (nm2) of metal particles exposed per surface metal atom.

3.3.3.4 NH3/CO2 Temperature Programmed Desorption (TPD)

Determining the quantity and strength of the acid sites on the support phase (γ-Al2O3) is of great importance for understanding and predicting the performance of the supported Ni catalyst. NH3-TPD is one of the most widely used techniques to characterize the acid sites on oxide surfaces. Before the TPD experiment, the catalyst sample was pre-treated by flowing He or H2 (in case of Ni loaded samples) through the bed at 700 oC. The catalyst sample was then brought to saturation by flowing a stream of gas containing 5% NH3 in Helium at 50 oC for 1 hr. After NH3 adsorption, the sample was purged by He again for 1 hr at the adsorption temperature. During the desorption, the temperature in the bed was raised at a linear rate (15 °C/min) from ambient to 950 °C while a stream of inert He gas was flown through the bed. Once the temperature in the bed overcame the energy of desorption, NH3 was desorbed from the sample surface. A TCD detector was used to analyze the gas leaving the catalyst sample. The amount of desorbed NH3 was calculated from the calibrated TCD signal. The total acidity of the catalyst sample is related directly to the amount of desorbed NH3.

In order to establish the basicity and CO2 adsorption capacity of the catalyst samples, CO2-TPD was performed using a similar procedure as used for NH3-TPD. After the pre-

treatment, CO2 was chemisorbed onto the samples by flowing a stream of gas containing 10% CO2 in Helium at 45 oC for 1 hr. Then, the samples were purged under He flow for another 30 min. Following this, CO2-TPD profiles were recorded using a TCD detector up to 950 °C. In this case, a heating rate of 20 °C/min was used.