Toluene Decomposition in Plasma–Catalytic Systems with Nickel Catalysts on CaO-Al2O3 Carrier
Round 1
Reviewer 1 Report
(1) What is the experimental uncertainty in toluene decomposition, shown in Figure 1 and Figure 2?
(2) Would it be possible that NiO was reduced in situ to Ni? If so, what would happen to the decomposition of toluene in presence of Ni/(CaO-Al2O3) and NiO/(CaO-Al2O3), if the residence time is long enough or the reaction time is sufficiently long?
(3) As indicated in Line 187, tar formation was observed with the use of metallic nickel catalyst. If the discharge power is increased or the hydrogen composition in the inlet is increased, can the deactivation of nickel catalyst be relieved?
Author Response
Dear Reviewer,
Thank you for reviewing our paper, below the answers to your comments and questions:
(1) What is the experimental uncertainty in toluene decomposition, shown in Figure 1 and Figure 2?
Measurement error related to gas chromatography uncertainty was 5%. Error bars were added to fig 1 and 2
(2) Would it be possible that NiO was reduced in situ to Ni? If so, what would happen to the decomposition of toluene in presence of Ni/(CaO-Al2O3) and NiO/(CaO-Al2O3), if the residence time is long enough or the reaction time is sufficiently long?
It could not be excluded that NiO was reduced to Ni during the process. Below text was added in lines 104-115
In previous studies, a reduction of NiO to Ni was observed when G-0117 [23] catalyst was used. This had a positive effect since Ni was more active than NiO in the decomposition of tar. However, in the case of this study, this might have had a negative influence. The reason was the different Ni/Ca ratio in the catalyst bed. A very low addition of calcium (Ca/Ni below 0.2) increased the resistance of the Ni/CaO-Al2O3 catalyst to sintering and the formation of carbon deposits, while a higher ratio resulted in greater soot formation. This was due to the growth of Ni crystallites and the coverage of the catalyst surface with Ca, which hindered the interaction of Ni with the catalyst bed [26, 30]. The optimum Ca/Ni ratio for NiO/CaO-Al2O3 catalyst might be different from that of Ni/CaO-Al2O3 catalyst, which would explain why the addition of calcium did not decrease toluene conversion rate in the plasma-catalytic system with NiO/CaO-Al2O3 [29].
(3) As indicated in Line 187, tar formation was observed with the use of metallic nickel catalyst. If the discharge power is increased or the hydrogen composition in the inlet is increased, can the deactivation of nickel catalyst be relieved?
Explanation added to the text (lines 224-232)
A higher hydrogen concentration in the initial gas and an increase in discharge power resulted in a higher amount of hydrogen radicals which could then react with other particles during the process. In the current study, the deactivation of Ni/CaO-Al2O3 could be caused by a too high Ni/Ca ratio on the catalyst bed, which led to a decrease in the number of active sites on the catalyst. A higher amount of hydrogen radicals did not led to a higher conversion rate or resistance to deactivation in previous studies [24]. Instead, the radicals were used in the hydrogenation of toluene decomposition intermediates due to lack of the active sites on the catalyst surface, which lowered C7H8 conversion rate.
Reviewer 2 Report
The authors investigated Toluene Decomposition in Plasma-Catalytic Systems with 2 Nickel Catalysts on CaO-Al2O3 Carrier.
1. On line page 33-34 "The main disadvantage of using catalyst was quick 33 deactivation of the catalysts, mostly as a result of carbon deposits on its surface" which catalyst is the author referring to?
2. Please, check line 38 "However, the most of these studies were.." for language error.
3. The authors need to compare the performance of the catalysts with those in the literature.
4. What evidence can be provided to support the proposed mechanism.
Author Response
Dear Reviewer,
Thank you for reviewing our paper, below the answers to your quenstions and comments:
- On line page 33-34 "The main disadvantage of using catalyst was quick 33 deactivation of the catalysts, mostly as a result of carbon deposits on its surface" which catalyst is the author referring to?
We are referring to nickel catalysts in general, carbon deposits formation is known problem present on those catalysts in tar decomposition process. The examples are in related papers [13-16].
Comment was adjusted in lines (34-36)
The main disadvantage of using nickel catalysts has been deactivation of the catalyst, mostly due to the formation of carbon deposits on its surface [13-16].
- Please, check line 38 "However, the most of these studies were.." for language error.
Line 40 was adjusted and the whole paper received a language check.
However, most studies have been conducted in small-scale reactors…
- The authors need to compare the performance of the catalysts with those in the literature.
A comparison of toluene conversion rate was added to the energy efficiency paragraph (lines 130-140)
Tar imitator conversion rates obtained in this study were lower than those re-ported in other groups in which as much as even 95.7% of toluene and 83.4% of naphthalene were decomposed when Ni-Co/γ-Al2O3 catalyst was used in plasma-catalytic with gliding arc plasma system [31]. Other studies also reported a very high conversion of toluene up to 95.2% when Ni/γ-Al2O3 catalyst with rotating gliding arc discharge was used [32]. The energy efficiency values obtained in the studies were in a similar range to those reported in the literature i.e. 6.7 g/kWh [20], 3.6 g/kWh [33]. However, when GA discharge plasma was used, much higher EE values were obtained, up to 46.3 g/kWh [34]. The difference between EE and toluene conversion rates reported in the literature and this study could be due to the lower gas flow rate (up to 0.54 Nm3/h) [32], which allowed longer residence time and contact of excited radicals with toluene particles.
- What evidence can be provided to support the proposed mechanism.
The mechanism was proposed based on toluene decomposition gaseous and solid products adsorbed on catalyst, that were analyzed by gas chromatography and mass spectroscopy. Other groups studies were also used to propose the mechanism [27, 31-36].
Explanation added to text (lines 199-205)
The catalysts were rinsed with acetone after the process then the obtained solution was analyzed with MS to identify toluene decomposition products (Figure 5). At retention time (RT) between 0,48 and 0,54 min were methanol, acetone, water and C2-C4 hydrocarbons. 2-Pentanone was identified (RT = 2,75 min) as a product of acetone condensation reaction. The proposed reaction mechanism is based on previous [20-21, 24] and current studies toluene decomposition intermediate products as well as decomposition mechanisms proposed in the literature [27, 31-36].
