Study on Ni/ZrO 2 catalyst preparation

. In this work, the influence of catalyst preparation temperature on its structure was investigated. We have synthesized 12 different Ni/ZrO 2 catalysts by varying the calcination temperature, time, and active metal content, and these catalysts will be further used in the carbon dioxide methanation reaction. Structure and properties of the catalysts were determined using XRD and SEM analysis. Therefore, Ni content of the catalysts were measured by ICP-OES. Regarding to the crystal size calculation using XRD data by Scherer equation, when calcination time was increased the average crystal size of nickel oxide was decreased from 42.38 nm to 38.93 nm whereas it decreased to 39.23 nm when the calcination temperature was increased. This shows that the distribution of active metals in the catalyst increases when the heat treatment parameters are increased. In addition, it can be assumed that the activity of the catalyst can be enhanced when the calcination temperature and time were increases.


Introduction
Coal makes 24 percent of resources consumed in world energy production and almost 40 percent of total electricity production is solely based on coal [1]. Even though world economy strongly depends on coal, its utilization is considered to be one of the main source of environmental pollution Due to human activities such as industrialization and urbanization, the concentration of CO 2 released into the atmosphere has exceeded the limit that could cause the global warming, i.e. it has reached more than 440 ppm [2], [3]. Therefore, the carbon capture and storage (CCS) process has been developing rapidly in recent years to mitigate carbon dioxide emissions [3]- [7].
The main reaction used in the process is the CO 2 methanation reaction discovered by Sabatier, which is also being developed for use in combination with renewable energy production [2]- [6], [8]- [10]. CO 2 methanation is a catalytic reaction. Noble metals such as Ru, Rh, Pd and Ni are the most suitable and widely studied for this reaction, and among them, Ni is drawing much attention due to its high activity, low cost and high abundance [4], [5], [7]- [13]. The only problem with Ni-based catalysts is deactivation due to carbon deposition. Therefore, in order to solve this problem and improve the activity and stability of Ni catalysts, researchers have studied various support materials such as Al 2 O 3 , SiO 2 , CeO 2 , ZrO 2 , TiO 2 and hydrotalcite. Among these supports, ZrO 2 reduced the carbon deposition by hydrogen spillover phenomenon, and thus increased the activity and stability of the catalyst, so ZrO 2 support was widely studied [2], [4], [5], [9]- [11], [14]. Ni/ZrO 2 catalyst has a high degree of CO 2 conversion and nearly 100% selectivity to methane, thus a number of work has been done on this catalyst. For instance, adding promoters, varying precursor salts, comparing preparation methods and varying reaction parameters, etc. [2], [5], [6], [8], [11], [15]- [17]. The activity, structure, and properties of a catalyst depend greatly on the conditions under which it was synthesized, especially the calcination process. In order to synthesize a catalyst suitable for implementation to industrial scale, it is necessary to establish suitable parameters for its preparation. The calcination temperature of a catalyst is one of the main factors determining its activity, and the activity of supported metal catalysts strongly depends on the temperature and time of heat treatment [12], [18]. Therefore, in this work, we aimed to determine the effect of calcination temperature and time of Ni/ZrO 2 catalyst to its structure and properties.

Experimental
Following preparation parameters were varied in catalyst synthesis. Herein: 1. Nickel content: 5%, 20% and 50% 2. Calcination temperature: 400 о С and 600 о С 3. Calcination time: 4 and 6 hours Metal concentration of the catalyst were analyzed by inductively coupled with plasma-optical emission spectrometer (ICP-OES, iCAP6500, Thermo Scientific). Xray diffraction of the samples were performed by X-ray diffractometer (MAXima_X XRD-7000, Shimadzu Scientific Instruments) with Cu-Kα radiation (λ=1.541874 Å). The operating voltage and current were 40kV and 30mA, respectively. SEM images were taken by Scanning electron microscopy with energy dispersive spectroscopy TM-1000 at Field Research laboratory of Mongolian University of Science and Technology associated with Nagoya University of Japan.

Ni/ZrO2 catalyst preparation by impregnation method
Catalysts were developed by the well-known impregnation method [5], [6], [13], [19], [20]. Nickel nitrate (Ni(NO 3 ) 2 ⸱6H 2 O, 99.9%, Tianjin Factory No.3 of Chemicals and reagents, China) and zirconium dioxide (ZrO 2 , 99.9%, China) were used as received. Calculated amount of Ni(NO 3 ) 2 ⸱6H 2 O was dissolved in a small amount of Study on Ni/ZrO2 catalyst preparation 3 deionized water. After 30 minutes, measured amount of zirconium dioxide was added to the solution gradually with continuous stirring. The mixture were stirred in mortar and pestle until the solution containing active metal has completely deposited in the pores of support. Formerly the mixture was dried at 110 о С for 12 hours and calcined at different temperatures and times. The lower and upper limits of calcination temperature and duration were chosen based on previous literature [2], [5], [7]- [9], [13], [14], [16] and heat treatment conditions were illustrated in Table 1.
Totally 12 different catalysts were prepared with nominal nickel contents of 5%, 20% and 50%, and at calcination temperatures of 400 о С and 600 о С for 4 and 6 hours. Name of the catalysts corresponding to the preparation conditions are shown below.

Results and discussion
Ni/ZrO 2 catalysts were synthesized by impregnation method at different preparation conditions mentioned above. Synthesized catalysts were in a form of grey powder and the color was changed from light to dark grey as the nickel content increases.
Study on Ni/ZrO2 catalyst preparation 5

X-ray diffractogram of Ni/ZrO2 catalysts processed at different calcination temperatures
. XRD patterns for 5%Ni/ZrO 2 catalysts calcined at 400 о С and 600 о С temperatures for 6 hours were shown in (а) and (b), for 20%Ni/ZrO 2 catalysts calcined at 400 о С and 600 о С temperatures for 6 hours were shown in (c) and (d), and for 50%Ni/ZrO 2 catalysts calcined at 400 о С and 600 о С temperatures for 6 hours were shown in (e) and (f) of Fig.3, respectively. From the diffractogram, it can be seen that the diffractogram peaks became sharper when the calcination temperature increases. It might indicate that the crystallinity of the phases were enhanced when increasing the parameter.

X-ray diffractogram of Ni/ZrO2 catalysts processed for different calcination time
XRD patterns of 5%Ni/ZrO 2 catalysts calcined at 600 о С for 4 and 6 hours were shown in (а) and (b), for 20%Ni/ZrO 2 catalysts calcined at 600 о С for 4 and 6 hours were shown in (c) and (d), and for 50%Ni/ZrO 2 catalysts calcined at 600 о С for 4 and 6 hours were shown in (e) and (f) of Fig.4, respectively. It can be seen that, the diffractogram peaks were also sharpened when increasing the calcination time which also may indicate that the phase crystallinity was increased when increasing the duration of heat treatment. Crystal sizes of NiO and ZrO 2 phases were calculated using FWHM values from XRD result by Scherer equation and it's shown in Table 2. According to the table, the crystal size of NiO and ZrO 2 phases were decreased when calcination temperature and time are increased. For instance, the crystal size of NiO of Cat-1 (5% Ni, calcined at 400 o C for 4 hours) was 42.38 nm whereas the crystal size of Cat-3 (5% Ni, calcined at 400 o C for 6 hours) were 38.93 nm. Similarly, the crystal size was decreased when increasing the calcination temperature to 600 о С (Cat-5) to 39.23 nm. Same trend was observed for catalysts with 20% of nickel content and interestingly Study on Ni/ZrO2 catalyst preparation 7 the opposite trend was observed for catalysts with 50% nickel content. This corresponds to the fact that, as mentioned in the literature [4], high metal loading reduces its dispersion when preparing catalysts by impregnation method.
It was reported that in few studies, the calcination temperature had shown significant impact on the crystallite sizes of the active metal as well as its dispersion and therefore it has positively affected on the catalyst activity and stability [12], [18]. Moreover, it has been noted that the activity of the catalyst depends on particle size of the nickel oxide [15]. Summing up from this, it can be considered that the distribution of metals in our synthesized catalyst were increased when the calcination temperature and time were increased. In addition, further research will be held to determine the effect of the change in crystal size to the catalyst activity. Regarding to the figure, no other elements were observed other than nickel and zirconium, which means that the final catalysts doesn't have any impurities. Therefore, as the nickel content increases the corresponding peak intensity were shown to be increased.
Elemental composition of catalysts determined by SEM-EDS was shown in the following table. In the figure below, SEM images and SEM-EDS spectra of catalysts with same nickel content and prepared at 400 o C for 4 hours and 600 o C for 6 hours, were compared. According to the image, the catalyst prepared under high temperature for long period of time has more homogenous structure and smaller particles than the catalyst prepared at lower temperature for a short period of time. Therefore, it's in agreement with the XRD result and crystal size calculation.

ICP-OES analysis result of Ni/ZrO2 catalyst
The nominal nickel contents of catalysts were estimated to be 5%, 20% and 50%, and its content were determined using ICAP-6500 ICP-OES at Central Geological Laboratory. The experimental value of nickel content usually tends to be less than the nominal value [6]. According to the result of ICP-OES measurement in Table 4, the experimental value of nickel content was less than the calculated value for this work. However, the difference between experimental and nominal value was not that big meaning that the active metal has deposited on the support successfully.

Conclusion
According to the results of XRD, SEM and ICP-OES analysis, the catalysts we intend to obtain were formed free of any impurities and have metal content close to the nominal value.
From the X-ray diffractogram and crystal size calculation results of the catalysts, increasing the calcination temperature and time have led to a rise in crystallinity of the phases and a decrease in the crystal size. This indicates an increase in active metal dispersion, and it has been mentioned in previous works that increased dispersion has a positive effect on catalyst activity. Therefore, in our next research work, we will continue this work and investigate the effect of calcination temperature, time, and further crystallite size on the catalytic activity.