EXPLOITATION PROPERTIES OF Ni-Pd / Al 2 O 3 CATALYST SUPPORTED ON CERAMIC FOAM

The goal of this research was to examine exploitation properties of Ni-Pd/Al2O3 catalyst supported on α-Al2O3 based foam in the dry methane reforming. The catalyst was prepared by using aerosol method. Chloride precursors for Ni and Pd were reduced by hydrogen at low temperature of 533 K, without previous calcination. The reforming experiment was performed for 3 h, with standing time of 1 h for each of the following temperatures: 873, 973 and 1023 K. Conclusions on selectivity, catalytic activity and stability were made on the basis of CO and H2 yields.


Introduction
Dry reforming of methane (DRM) is a significant technology in the field of sustainable development due to simultaneous processing of CO 2 and CH 4 , as the main greenhouse gases.In the DRM process, noble or non-noble metal based catalysts and temperatures from 973 to 1173 K are required in order to produce synthesis gas.Synthesis gas (CO + H 2 ) is an important product which is used for liquid fuels production [1,2].The DRM process occurs according to the following chemical reaction [3]:  Corresponding author: Vesna Nikolić, vnikolic@tmf.bg.ac.rsNi-based catalysts, mainly Ni/Al 2 O 3 , are suitable for utilization in industry because of high reactivity and favorable price of Ni [1,2].Ni-based catalysts are prone to rapid deactivation, caused by deposition of by-products on active surfaces and sintering of active metallic particles at high temperatures.In order to avoid deactivation, these catalysts can be modified by only 0.4 to 0.5 wt.% of Pd [4,5].
Catalyst supports such as reticulated ceramic foams exhibit low pressure drops during the fluid flow, good mass transfer and good heat conductivity [6].In the earlier work, the authors prepared Ni-Pd/Al 2 O 3 catalyst supported on α-Al 2 O 3 based foam by using aerosol method [7].The foam was obtained as previously described [8].Using of chloride precursors for Ni and Pd resulted in elimination of calcination step and very low reduction temperature -533 K [7].Furthermore, oxides formed by calcination have lower reducibility than chlorides [9].
The aim of this research is to examine exploitation properties of Ni-Pd/Al 2 O 3 catalyst in the DRM process.Yields of H 2 and CO enabled to reach conclusions on selectivity, activity and stability of the catalyst.

Experimental
To prepare the catalyst, NiCl 2 ×6H 2 O, PdCl 2 (MERCK, pro analysis) and α-Al 2 O 3 based foam were used.The foam was synthesized earlier by polymer replication [8].According to previously presented method [7], chloride solution was ultrasonically nebulized and deposited to the foam at 473 K.After drying at 473 K for 1 h, the sample was reduced with H 2 (flow rate: 20 L h -1 ) at 533 K for 1.5 h.Metal content was 20 wt.% of Ni modified with 0.1 wt. of % Pd.After the reduction, microstructure of the catalyst was investigated by using a JEOL JSM-5800LV scanning electron microscope (SEM).
Apparatus and technological scheme for the DRM experiment are presented in Fig. 1 (a) and (b).
The DRM experiment was performed in a quartz reactor at temperatures of 873, 973 and 1023 K, without changing the catalyst.After reaching 873 K, inlet stream of CH 4 and CO 2 was introduced into the reactor.Process temperatures were changed during the experiment.CO and H 2 concentrations were measured with Testo 340 Flue Gas Analyzer while flowing N 2 was used to dilute outlet stream.In order to investigate stability of the catalyst, CO and H 2 concentrations were measured at 1023 K at the end of the experiment.Yields of CO and H 2 were determined by using equations ( 2) and (3), considering the dilution with N 2 .Calculation was carried out by taking into account theoretical values of the yields for complete inlet gas reforming, equation (1).Based on CO and H 2 yields, the authors reached conclusions on activity, selectivity and stability of the catalyst.

Results and discussion
Surface morphology of the reduced Ni-Pd/Al 2 O 3 catalyst is presented in Fig. 2.

Fig. 2. Micrograph of Ni-Pd/Al 2 O 3 reduced at 533 K for 1.5 h, metal content (wt. %): 20 % Ni and 0.1 % Pd
After the reduction at 533 K, relatively smooth metallic film was formed over the whole foam surface (Fig. 2).No undesirable agglomerates were detected and obtained surface morphology could be suitable for catalytic processes.Reduction of chloride precursors for Ni and Pd resulted in almost complete transfer to metallic phases.The chlorides reached a reduction degree of 98.2 wt.% [7].Our previous study showed that only 20.0 wt.% of oxide precursors for Ni and Pd, obtained by calcination, was reduced at the same reduction conditions.Therefore, the synthesis of catalytic components from chloride instead of oxide precursors can provide energy savings due to elimination of the calcination step [7].In addition, after kinetic analysis of NiO and NiCl 2 reduction by H 2 , the authors concluded that the chloride phase has higher reducibility [9].
Results of the DRM process are presented in Fig. 3.

Fig. 3. CO and H 2 yields at: 873 K, 973 K and 1023 K
The results obtained at 873 K varied in a great extent.CO yield ranged from 18.8 and 67.1 %, with mean value of 41.5 %.Values of H 2 yield ranged from 16.6 and 54.8 %, ant the mean value was 32.2 %.Nevertheless, nearly constant H 2 /CO molar ratios were achieved during the period of 20 minutes, with the mean value of 0.8.Yields of CO and H 2 during the reforming at 873 K indicated the requirement for higher process temperature.The reforming results significantly improved at the process temperature of 973 K. Obtained values varied in the following ranges: CO yield -from 57.7 to 68.0 % (mean value: 62.8 %) and H 2 yield -from 50.6 to 58.8 % (mean value: 54.8 %).Those yields were relatively constant.After 1.5 minutes, concentrations of CO and H 2 increased and then slightly declined after 3 minutes.Nearly constant and more favorable molar ratio of H 2 and CO was achieved than at 873 K, with the mean value of 0.9.Performance of the tested catalyst was the most pronounced at 1023 K, where yields of the main products reached nearly maximal values.CO yielded from 85.0 to 98.1 %, with the mean value of 91.0 %.H 2 yielded between 84.3 and 93.2 %, and the mean value was 88.5 %.A slight decrease of CO and H 2 yields was noted after 2 minutes.After 2.5 minutes, the yields reached close values and remained relatively constant until the end of the investigation.H 2 /CO molar ratio had the most favorable mean value of 1.0 and was almost constant during the measuring.It was concluded that the catalyst had good selectivity for CO and H 2 .High yields of those gases after 3 h of testing indicate that the catalyst possesses high activity and stability.

Conclusions
Selectivity, activity and stability of Ni-Pd/Al 2 O 3 catalyst supported on α-Al 2 O 3 based foam were examined in the dry methane reforming experiment.The catalyst was previously prepared by using aerosol method.Ni and Pd were synthesized from noncalcined chloride precursors by direct hydrogen reduction at 533 K. Metallic coating, obtained over the foam surface, was relatively smooth.The reforming process lasted for 3 h, without changing the catalyst.Mean results at process temperature of 873 K were the following: CO yield -41.5 %, H 2 yield -32.2 % and H 2 /CO molar ratio -0.8.Significant improvement of the process efficiency was achieved at 973 K. Mean values of CO and H 2 yields reached 62.8 and 54.8 %, respectively, and mean H 2 /CO molar ratio was 0.9.The highest catalytic performance was achieved at 1023 K -mean yields of CO and H 2 reached values of 91.0 % and 88.5 %, respectively.Mean H 2 /CO molar ratio was near 1.0.On the basis of obtain results, it was concluded that the catalyst had high activity, stability and selectivity for CO and H 2 .