TiO2 Nanorods

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Kinetics, Catalysis, and Reaction Engineering

Enhanced Metal-support Interaction in Ni/Co3O4 /TiO2 Nanorods Toward Stable and Dynamic Hydrogen Production from Phenol Steam Reforming Tariq Abbas, Muhammad Tahir, and Nor Aishah Saidina Amin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03542 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on November 1, 2018

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Industrial & Engineering Chemistry Research

Enhanced Metal-support Interaction in Ni/Co3O4/TiO2 Nanorods Toward Stable and Dynamic Hydrogen Production from Phenol Steam Reforming Tariq Abbas, Muhammad Tahir*, Nor Aishah Saidina Amin Chemical Reaction Engineering Group (CREG), Faculty of Chemical & Energy Engineering, Universiti Teknologi Malaysia (UTM), 81310 UTM Johor Bahru, Johor, Malaysia. *Corresponding author: [email protected]

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

Fabrication of highly stable Ni and Co3O4 nanocubes supported TiO2 nanorods (NRs) catalyst with improved metal support interaction for steam reforming of phenol (SRP) has been investigated. Enhanced catalytic performance of Ni-Co3O4/TiO2 NRs with H2-rich gas and hardly formed coke was achieved and was rationalized with spectroscopic results and structural assessments. Over 10 % Ni-5% Co3O4/TiO2 NRs, SRP and WGS reactions dominate, producing mostly H2 and CO2, yet Ni/TiO2 MPs promoted CO. With catalyst loading, S/C and GHSV, H2 yield was improved with higher CO2/CO ratio and low CO yield. Ni-C3O4/TiO2 NRs catalyst possesses excellent stability, which prevailed for more than 100 h without obvious deactivation. The high performance was due to 1D TiO2 NRs/CO3O4 nanocubes heterojunction, good metal dispersion and higher reducibility, thereby provides strong interaction of bimetallic active sites. This study reveals that Co3O4 promotes Ni/TiO2 NRs activity and stability towards H2 production and is promising for commercial applications. Keywords: TiO2 nanorods; Ni/Co3O4, Hydrogen production; Phenol steam reforming; High stability 1. INTRODUCTION The necessity of cleaner and sustainable production of fuels and chemicals for the growing world population is increasing. Mostly conventional fossil fuels have been used as a main global energy consumption source in the past years, yet they are nonrenewable source of energy 1. Consequently, it is important to develop an economical process for the production of renewable fuels using waste sources. Petrochemical industries largely produced phenolic compounds as by-product and these compounds are also considered as waste of bio-mass gasification, reforming, and tar process, yet main quantity is obtained in the process of bio-oil pyrolysis 2. Phenolic compounds are produced in the form of mixture in water, which is present in excess amounts, thus uneconomical to purify by physical or chemical separation processes. Besides, phenol is considered as a serious environmental pollutant and harmful for both the aquatic and human life. In the recent years, a lot of research has been conducted to transfer these compounds into clean and sustainable source of energy and industrial chemicals via economical and 2 ACS Paragon Plus Environment

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ecofriendly processes. Many techniques are being used to convert a phenol mixture into renewable fuel and chemicals such as aqueous phase reforming (APR) and steam reforming (SR) 3. Using SR technique, large quantity of water has been required, thus researchers have extensively investigated SR of model compounds produced from biomass and bio-oil like methanol 4, ethanol 5-6, acetic acid 7, glycerol 8, acetone 9, and phenol 10. Since, phenol has attracted much interest owing to its high H2 content, it has much potential to be converted to renewable fuels through steam reforming of phenol (SRP). However, this process requires efficient and stable catalyst for production of hydrogen. In catalytic SRP, aluminum oxide (Al2O3) is one of the most commonly used support because of its high chemical and mechanical stability, cost-effective and high metal dispersion because of its high surface area

11.

Furthermore, Ni supported on MgO and

Al2O3 has shown better performance in terms of conversion with prevailed H2 yield

12.

Among the metals, Ni is mostly used as it has a relatively low cost compared to rare earth and noble metals 13. Noble metals like Rhodium (Rh), Ruthenium (Ru) and Palladium (Pd) can resist carbon formation with increased catalyst stability

14-16.

However, these metals

are rare in nature and have high cost which makes them less feasible for commercial use. Although, Ni has a good activity towards C–C bond cleavage, but it also has high rates of methane (CH4) formation as well as sintering of catalyst along with coke formation 17-19. Carbon formation can be minimized using bimetallic catalysts like nickel (Ni) and cobalt (Co) on different supports such as Al2O3, La2O3 and ZrO2 20. With the addition of Co into Ni containing catalyst, carbon formation and carbon growth can be minimized. Moreover, Ni-Co catalyst can enhance the stability of the catalyst and have better resistance towards the oxidation of active metals

21-23.

Zhao and co-workers reported 68.7 %

conversion of ethanol at 350 oC using Ni and Co as catalyst supported on Al2O3 for SR of ethanol. The group further stated that higher dispersion of catalyst shows higher stability and lower carbon deposition

24.

The interaction between support and metal is vital to

maximize process efficiency with improved yield and stability. Therefore, it is imperative to develop structured nanocatalyst towards achieving high catalyst stability. Recently, TiO2 as a catalyst support exhibit exceptional electronic interfaces between the active metal and support due to its property of being reducible oxide and presents in various structures 25-27. The activity and stability of TiO2 nanoparticles can 3 ACS Paragon Plus Environment


be further improved using structured TiO2 nanorods (NRs) with tunable size and shape. Therefore, using modified one dimensional (1D) TiO2 NRs will provide higher surface area compared to conventional TiO2 in addition of minimum mass transfer limitations. The structured catalysts can further improve the activity due to high dispersion of active metals. Kho and co-workers used Ni/TiO2 for steam reforming of methane and achieved 45 % methane conversion at 500 oC. The catalyst was deactivated after 54 h of reaction due to carbon deposition

28.

Moreover, addition of Co into Ni containing catalyst,

provides more stability to support because Co can break the actual surface on which Ni ensembles, which apparently reduces the particle size of Ni. According to Wang and co-workers 29, the Co3O4 displays unique magnetic features related to surface electrons spin with a larger super capacitance that is useful to improve the interaction with the catalyst TiO2 support whereas, the large specific surface area of nanoporous structure of Co3O4 also promotes the adsorption of carbon monoxide (CO) onto its surface and thus enhancing the water gas shift (WGS) reaction with minimized carbon formation. Ni-Co catalyst can enhance stability of catalyst and have better resistance towards oxidation of active metal

22.

According to current literature, the use of 1D TiO2 based

structured catalysts for steam reforming of phenol has never been reported. The synergistic effects of bimetal Ni/Co supported on TiO2 nanorods (NRs) would be effective to give prolong stability while promoting hydrogen due to its electronic properties with the use of nanoporous structure of Co3O4. In this study, facile fabrication of Ni/Co3O4 modified TiO2 NRs nanostructures for enhanced hydrogen production via SRP has been investigated. The interaction of different support structures of TiO2 such as MPs and NRs with different active metals particle sizes have been critically investigated for enhanced hydrogen production with controlled growth of carbon nanotubes and graphene sheets. The catalysts were prepared using hydrothermal and wet impregnation methods. Initially, the catalytic activity of different support structures of TiO2 (MPs and NRs) with various Ni loadings under the same operating conditions towards hydrogen generation has been discussed. The effect of parameters such as temperature (600-900 oC), phenol concentration (05-10 wt. %) and the feed flow rate (5-15 ml/h) were also elucidated. The interaction of Ni/Co or Co3O4 amount onto TiO2 and carbon formation and surface texture of resultant catalysts 4 ACS Paragon Plus Environment

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was examined. The results showed that Ni/Co3O4 loaded TiO2 NRs found to be very efficient in SRP for H2-rich gas production with high activity, selectivity and stability. 2.

EXPERIMENTAL

2.1.

Materials

Cobalt (II) chloride (CoCl2), Nickel(II) nitrate hexahydrate (99.999% Ni(NO3)2.6H2O), Titanium (IV) oxide, anatase ≥ 99% (TiO2) and urea (CO(NH2)2) were obtained from Sigma-Aldrich. Phenol (80% w/w in Water) was purchased from Qrec (Asia). Gas cylinders of Air, Nitrogen and Argon (99.995 % purity) were purchased from Mega Mount Industrial Gases Sdn. Bhd. Johor, Malaysia. All chemicals were of analytical grade and were used without any supplementary refinement. 2.2.

Preparation of TiO2 nanorods To prepare TiO2 nanorods (NRs), 1 g of commercial anatase TiO2 microparticles

(MPs) (Sigma-Aldrich) were added into 70 mL of NaOH aqueous solution (10 M) and stirred for 1 h until a homogeneous suspension was achieved. The suspension was transferred to 100 mL Teflon-lined stainless steel autoclave. The autoclave was placed in a muffle furnace and temperature was maintained at 200 oC for 24 h and cooled to room temperature. The obtained product was washed several times with a 0.1 M aqueous solution of HCl, distilled water, and ethanol, respectively until the pH of the suspension becomes ~7. The suspension was recovered by vacuum filtration and the filtrate was oven dried at 110 oC for 12 h. Subsequently, the dried product was crushed and calcined in a muffle furnace at 550 oC for 5 h at a heating rate of 5 oC/min. 2.3

Preparation of Co3O4 nanocubes In a typical synthesis procedure of Co3O4 nanocubes, 1.2 g of CoCl2 were dissolved

in 20 mL of distilled water to form homogeneous solution. Then, 0.06 g of urea dissolved in 20 mL distilled water was added dropwise into the CoCl2 solution under stirring. The reaction mixture was transferred into a 75 mL Teflon-lined autoclave and placed in a furnace and the temperature was set to 110 °C for 6 h. The resulting pink precipitates were separated via centrifugation, followed by washing with water and ethanol, respectively. Finally, the black colored powder was obtained by oven drying for 12 h at 100 oC and calcination at 300 °C for 3 h. 2.4

Preparation of Ni/ Co3O4-TiO2 NRs 5 ACS Paragon Plus Environment


The preparation of Ni-Co3O4/TiO2 NRs catalysts was conducted by the incipient wetness impregnation of Ni(NO3)2·6H2O (Sigma-Aldrich) and Co3O4 onto a TiO2 NRs support. In a typical catalyst preparation process, 1 g of TiO2 NRs and 0.05 g of Co3O4 were dispersed in 70 ml of distilled water under vigorous stirring for 1 h. Afterwards, 0.5 g of Ni(NO3)2·6H2O was added to the stirring mixture and continuously stirred at 100 oC for 6 h. The slurry was dried in an oven overnight at 100 oC. Finally, the sample was calcined at 550 oC with heating rate of 5 oC/min for 5 h. The resulting powder was named as 10% Ni5% Co3O4/TiO2 NRs. A similar procedure was used to prepare 5 % and 15 % Ni loaded Co3O4/TiO2 NRs. Besides, 1 and 7 % Co3O4 loaded samples were prepared by loading 0.01 and 0.07 g of Co3O4, respectively. Detailed schematic representation of catalyst preparation is shown in Figure 1. 2.5

Catalyst characterization

All the catalyst samples were characterized using various physico-chemical methods. The powder X-ray diffraction (XRD) of samples was used to analyze the structure crystallinity and phase transition by using Bruker D8 advance diffractometer equipped with a Cu kα radiation source, where λ =1.5 Å, at 40 kV and 40 mA. XRD patterns were measured in steps of 1° min-1 in the range of 20–60° (2θ). Brunauer-Emmett-Teller (BET) method was used to measure the surface properties of catalyst that include BET and BJH surface area, BJH pore volume and pore size. The BET experiments were carried out by means of N2 (99.995%) adsorption and desorption in a Micrometrics ASAP 2020. The operating conditions consist of N2 adsorption at -196 oC and degasification at 250 oC held under vacuum for four hours. Transformation of various species on the surface of fresh and spent catalysts before and after SRP were analyzed by Fourier transformed infrared (FTIR) spectroscopy. In this technique, solid catalyst samples were subjected to a beam of infrared spectrum. Perkin Elmer with vacuum and flow module was used to carry out FTIR spectroscopy. The structural morphology of specimens of fresh and spent catalyst was examined by a high resolution – transmission electron microscope (HR-TEM 120 KV) by Hitachi HT7700 coupled with selected area electron diffraction (SAED) and an energy dispersive X-ray (EDX) detector. To check thermal stability, weight loss and amount of carbon deposition on fresh and spent samples was done by TGA on a PerkinElmer TGA/DTG up to 1000 oC under operating conditions of N2 flow (50 mL min−1) and heating 6 ACS Paragon Plus Environment

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rate of 10 oC min−1. The H2-TPR experiments were performed to analyze reducibility of fresh catalyst using Micrometrics AutoChem 2910 instrument. The operating conditions were 50 to 800 oC temperature range, 10 oC min-1 heating rate and 10 vol. % H2 in Ar gas as reducing feed stream composition with 20 ml min-1 as the total flow rate. 2.6

Steam Reforming of Phenol The schematic of experimental setup for steam reforming of phenol is presented in

Figure 2. The phenol feed (0.1667 mL min-1 flow rate) was fed into the reactor using the liquid syringe pump (model: KDS-100, KD Scientific USA). Then, the nitrogen was controlled by digital mass flow controller (Alicat Scientific Mc-1Slpm-D (N2)) at a flow rate of 20 mL min-1, mixed with phenol solution, and pre-heated at 200 oC. The mixture was then vaporized at atmospheric pressure and fed into the stainless steel fixed bed reactor. The temperature of reactor was set in the range of 600 to 900 oC, whereas the 0.3 g of catalyst was placed in a stainless steel reactor (12.9 mm O.D and 9 mm I.D) with a Ktype thermocouple placed inside a fixed bed reactor to control the temperature of the catalyst bed. The catalyst and reactor was purged before the experiment with nitrogen at 20 mL min-1 flow rate and 700 oC for 60 min. Then, the reaction products obtained from the reactor were flowed through a condenser to condense the unreacted liquid. The gas products were analyzed using an online gas chromatograph (GC) system (model No: Agilent 6890N) connected with a thermal conductivity detector (TCD) and CARBOXEN 1010 PLOT capillary column (0.53 mm x 30 m). The temperature of the GC system column was set at 275 oC. The liquid products were analyzed using an offline gas chromatograph (GC) system (model No: Agilent 7820N) connected with a Flame-ionization detection (FID) and a DB–WAX capillary column (0.25 mm x 30 m). 2.7

Calculation of catalytic performance in SRP The performance of the SRP in a fixed bed reactor using different catalysts were

analyzed by equations (1) to (5) to calculate phenol conversion, selectivity and yield of products, respectively.

Phenol Conversion (%) 

 Phenolin    Phenolout  100  Phenolin 


(1)


# moles of gas product 100  moles of all gas products

(2)

moles of H 2 produced 100 moles of H 2 stoichiometric potential

(3)

Product Selectivity (%)  H 2 Yield (%) 

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Yield of CO (%) 

moles of CO produced 100 6  moles of phenolin

Yield of CO 2 (%) 

(4)

moles of CO 2 produced 100 6  moles of phenolin

(5)

The flow rate of gaseous products (ml min-1) is converted into (mmol min-1). At 25 °C and atmospheric pressure, the volume of 1 mmol is equal to 24.04 ml 30. 3.

RESULTS AND DISCUSSION

3.1

Characterization of catalysts and support The XRD patterns of TiO2-NRs, 10% Ni/TiO2-NRs, 5 % Co3O4/TiO2-NRs and 10

% Ni-5 % Co3O4/TiO2-NRs catalyst samples, calcined at 550 oC up to 5 h are shown in Figure 3. The peaks for TiO2 particles were indicated at 2-theta (degree) 25.4o, 38.0o, 48.1o, 53.0o, 55.2o and 62.9o, which corresponds to (101), (004), (200), (105), (211) and (204) planes associated with tetragonal anatase, all in pure anatase phase

27.

There were also

diffraction peaks at 2-theta of 30.0o, 33.1o, 36.9o, 44.4o, 39.4o and 65.4o, corresponding to Co3O4 nanoparticles. Similarly, XRD diffraction peaks at 37.2o and 43.3o reflection of Ni as NiO in the composite samples 31. Figure 4 (a) shows H2-TPR profile of the Ni-Co3O4 supported on TiO2 NRs. This shows the differences of Ni-Co3O4 species comparable properties to disperse the Ni-Co structures. The pre-calcined catalyst of NiO/TiO2 displays two broad reduction peaks within the temperature spectrum of 512 oC and 596 oC. The first peak at about 512 oC could be recognized due to the reduction of Ni, while the other peaks detected over 600 oC could be attributed to the partial reduction of TiO2 support due to SMSI (strong metal surface interaction) effect, where x < 2

32-33.

Similarly, three peaks were obtained for Ni-Co3O4

loading onto TiO2 at 402 oC, 524 oC and above 600 oC which could be induced by the reduction of NiO, Co3O4 and TiO2, respectively24, 34. The observed temperature of the NiCo3O4/TiO2 NRs catalyst was higher which indicates a reduction process is taking place at 8 ACS Paragon Plus Environment

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a lower rate. This observation together with the simultaneous decreased of NiO reducibility were resulted by the partial reduction of bulk NiO-Co3O4/TiO2 NRs composite catalyst 3536.

Based on the TPR results, it is evidence that the different degree of interaction between

the support species and metals was obtained which ultimately alter the characterization of the reduction profile. Furthermore, shifting of the reduction profile for Ni-Co3O4 supported on TiO2 NRs to a higher temperature was due to the strong interaction between active metals and support. Whereas, low temperature (< 600 oC) with lower intensity peaks for Ni/TiO2 NRs depicts a lower interaction between active metal and support 37. The CO2-TPD peaks of 10 % NiO/TiO2 NRs and 10 % NiO-5 % Co3O4/TiO2 NRs catalysts are shown in Figure 4 (b) which was performed up to 800 oC after the process of reduction with flow of pure hydrogen for an hour. In catalytic phenol steam reforming, the nature of active metal sites plays a key role in the activity of catalyst where as a higher basic sites of active metals on catalyst support favors the CO2 adsorption (acidic characteristic of CO2 molecule). Thus, the gasification of the carbon containing deposits produced when reaction takes place, as compared to other catalyst with strong acidity will promotes the growth of carbonaceous deposits onto the surface of catalyst 7. As per literature, the basic nature of active sites for CO2 desorption peaks was categorized into three temperature ranges i.e., 50 to 150 oC, 200 to 400 oC and higher than 400 oC corresponds to weak, moderate and strong basic sites, respectively37-39. Therefore, the CO2-TPD peaks at 200 oC exhibits moderate basic sites, whereas at 530 oC and 773 oC are the strong basic sites for Ni-Co3O4/TiO2 – NRs, which has strong basicity than the 10 wt % of Ni/TiO2 – NRs. Hence, the active metal acts as a source of active oxygen, which has greater reactivity that prevented the coke deposition to occur on the catalyst surface. The CO2-TPD peak profiles of the Ni-Co3O4/TiO2 NRs catalysts were obtained after 10 min of CO2 adsorption conducted at 25 oC. The desorption of CO2 was highly active most probably because the quantity of CO2 desorbed was found to be lower in quantity and CO2 formed during the reaction was strongly adsorbed onto the surface of the TiO2. This was contributed by the TiO2 structure because it was not decomposed within the range of temperature under consideration. On the top of that, a group of strong peaks of CO2 was identified which were desorbed from the Ni-Co3O4/TiO2 NRs. Hence, it could be


Industrial & Engineering Chemistry Research 1 2 3 concluded that the basicity of catalyst has been remarkably increased by addition of Co3O4 4 5 along with Ni. 6 HR-TEM micrographs of Ni/Co3O4-TiO2 NRs catalysts are depicted in Figure 5. 7 8 Evidently, a homogenous composite catalyst could be seen in Figure 5 (a). Figure 5 (b-c) 9 10 depicts a good interaction of CO3O4 nanocubes with TiO2 NRs with uniform dispersion of 11 NiO nanoparticles. Figure 5 (d-e) reveals successful development of Co3O4/TiO2 NRs 12 13 heterojunction interaction between metal and support to get higher activity and stability of 14 15 support. Selected area (electron) diffraction (SAED) shown in Figure 5(f) confirms the 16 17 production of cobalt particles having facets near to spherical shape with a cubic crystal 18 structure in addition of the reflection of anatase phase of TiO2–NRs. The same crystal 19 20 structure was also reported for production of cobalt nanoparticles in previous studies 40. 21 22 The N2 adsorption and desorption isotherms of the calcined 10 % NiO/TiO2 - NRs 23 24 and 10 % NiO - 5 % Co3O4/TiO2 –NRs are shown in Figure 6. Both samples show increase 25 in absorption above 0.9 partial pressure (P/P0). The analysis summary of BET and BJH 26 27 surface area, t-plot pore volume and average pore size of Ni/TiO2 micro particle (TMP), 28 29 Ni/TiO2 NRs, and Ni-Co3O4/TiO2 NRs are presented in Table 1. The BET surface area of 30 Ni/TiO2 MPs was 8.34 m2/g, while for Ni/TiO2 NRs, a surface area of 25.43 m2/g was 31 32 recorded. Addition of Co3O4 on Ni/TiO2 NRs have no significant effect on the surface area. 33 34 Similar trends were observed in the BJH surface of Ni/TiO2 NRs (21.06 m2/g), which was 35 36 recorded higher as compared to Ni/TiO2 MPs (8.53 m2/g). However, a significant 37 increment in pore volume was obtained in Ni and Co loading TiO2 NRs samples. 38 39 40 41 Table 1: Summary of surface properties of catalyst samples. 42 43 BET surface BJH surface t-plot micro BJH pore t-plot micro2 44 area (m /g) area (m2/g) pore area volume pore volume Samples 45 (m2/g) (cm3/g) (cm3/g) 46 10% Ni-TiO2 MPs 8.34 8.53 9.442 0.0738 0.0036 47 48 10% Ni-TiO2 NRs 25.43 21.06 27.602 0.1447 0.0118 49 50 10% Ni-5%Co3O4- TiO2 NRs 25.44 21.05 27.602 0.1446 0.0119 51 52 53 3.2 Phenol steam reforming for hydrogen production 54 55 3.2.1 Effects of Ni and Co3O4 loading onto TiO2 NRs for phenol steam reforming 56 57 58 10 59 ACS Paragon Plus Environment 60

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The steady state activity testing over the Ni/TiO2 MPs, Ni/TiO2 NRs and Ni-Co3O4/TiO2 NRs was performed to investigate the role of nickel and cobalt on the effectiveness of different structures of TiO2 support for SRP at 700 oC, steam to carbon ratio (S/C=15/1) and atmospheric pressure. Table 2 compares the performance of different catalysts in terms of phenol conversion, products yield and selectivity. Using blank reactor experiments (homogenous system) with same feed conditions, no appreciable phenol conversion and products yield were obtained. Therefore, gas phase SRP for the production of H2 in homogenous system was negligible under the experimental operating conditions. Using SRP process in the presence of catalysts, phenol conversion and hydrogen yield were obviously increased. Using all types of catalysts, the products formed include H2, CO and CO2, yet CH4 and hydrocarbon production were not detectable. Support structure and Ni/Co3O4 metals was seen to significantly affect the product distributions and phenol conversion. Among the supports, TiO2 NRs were more efficient than the commercial TiO2 MPs, indicated by the higher phenol conversion and H2 yield. Using TiO2 NRs, phenol conversion was increased from 34.07 to 42.12 %, while the H2 yield of 12.4 % was achieved compared to 5.6 % in the presence of TiO2 MPs, respectively. Similarly, the lower yield of CO was produced over the TiO2 NRs than using its micro particles. This increased in the catalytic performance of TiO2 NRs could be attributed to one dimensional structure (1D) with high surface area and pore volume (Table 1), thus provides higher residence time for the catalytic reaction as compared to TiO2 MPs. Modifying TiO2 NRs with Ni as active metal, both phenol conversion and H2 yield were significantly improved as shown in Table 2. It has been observed that the catalytic performance of the TiO2 NRs catalyst without active metal loading was much lower than Ni/TiO2 NRs. The yield and selectivity of H2, CO and CO2 in product stream was initially increased and then gradually decreased with Ni loading from 5 to 15%. The maximum phenol conversion of 69.32 % and H2 yield of 76.6 % were obtained with 10 % Ni/TiO2 NRs, whereas at 15% Ni loading catalytic performance starts declining. Although, for a Ni loaded catalyst, the amount of Ni active sites and metal dispersion are the significant factors for the enhanced catalytic activity. With the increase of Ni-loading, there would be more active metal sites, while a proper dispersion of Ni would be decreased due to the agglomeration of active sites41. With the 11 ACS Paragon Plus Environment


increase in Ni contents from 5 to 10%, the yield and selectivity of H2 and CO2 were increased, yet CO yield was decreased. Water gas shift (WGS, CO + H2O⟶CO2 + H2) could be one of the reasons to increase H2 and CO2 yield and selectivity, because nickel is one of the promising active metals which can promote WGS reaction 42. With the lower amount of Ni loading, active sites for SR reaction could be insufficient whereas higher Ni loading might form agglomerates crystals. Mostly, amount of Ni loading on to conventional catalysts support surfaces used in steam reforming of phenol does not exceed 10 wt. % to avoid accumulation and to reduce sintering effect on Ni particles. Both Nabgan et al.

43

and Wang et al.

44

applied a 10 wt. % Ni loading on Al2O3–La2O3 and γ-Al2O3

support in SRP and acetic acid, respectively for H2 production. More interestingly, performance of 10% Ni/TiO2 MPs was lower when compared with 10% Ni/TiO2 NRs. This was obviously due to higher dispersion of Ni over 1D TiO2 NRs to develop more active sites over the support surface with a trivial agglomeration of active sites to reduce sintering effect of Ni particles 41. Furthermore, after establishing 10 wt. % Ni as an optimal loading on the catalyst support of TiO2 NRs, the effects of Co3O4 nanocubes on the catalytic performance was analyzed. The amounts of Co3O4 nanocubes loading were tested in the range of 1 to 7 wt. % over the 10 % Ni/TiO2 NRs catalyst under the same experimental conditions. Modification in structural and chemical properties of Co3O4 nanocubes onto TiO2 NRs greatly influenced the catalyst activity. The catalytic performance of 5 wt. % Co3O4 nanocubes as an active metal promoter on the 10 wt. % Ni/TiO2 NRs showed higher H2 and lower CO yield compared to 1 and 7 wt. % Co3O4 nanocubes. Thus, 5 wt. % Co3O4 nanocubes is the optimal loading to get higher activity contributed to the uniform dispersion of Ni over TiO2 NRs and suppressed CO formation by converting it into CO2 and H2 through WGS reaction. By comparing performance with 10 % Ni/TiO2 NRs, it was evidence that phenol conversion was increased from 69.32 to 92 %, while H2 yield was improved from 76.6 to 83.5 % with Co3O4 loading into Ni/TiO2 NRs. Furthermore, CO yield was reduced from 6.9 to 3.1 % over 10% Ni-5% Co3O4/TiO2 NRs catalyst. Thus, higher CO yield over the Ni/TiO2 NRs was reordered, however, with Co3O4 nanocubes it was greatly reduced. This was obvious, due to higher metal dispersion, improved reducibility and formation of Co3O4/TiO2 NRs heterojunction which provides more active sites with minimum carbon 12 ACS Paragon Plus Environment

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formation. Therefore, Co with Ni as bimetallic catalysts could provide resistance to carbon formation on the catalyst surface

45-47.

Davidson and co-workers described that Co

efficiently improved the catalytic activity in a bimetallic Ni-Co catalyst due to the better nickel dispersion and resistance towards carbon deposition

31.

Therefore, further

experiments were conducted in the presence of 10 % Ni-5% Co3O4/TiO2 NRs samples to investigate the effects of temperature, catalyst loading, steam to carbon ratio (S/C), phenol concentration and stability analysis. Table 2: Screening of TiO2 structure and Ni loading with phenol conversion, yield and selectivity of products Yield (%)*

Catalyst

Selectivity (%)

H2

CO

CO2

Homogenous

3.1

1.4

TiO2 MPs

5.6

CO

CO2

1.0

64.8 20.0

14.2

6.55

3.9

1.7

55.4 25.6

10.7

34.07

TiO2 NRs

12.4 2.5

8.9

63.7

8.3

27.6

42.12

5% Ni/TiO2 NRs

72.1 8.1

7.0

66.9 16.7

14.6

62.60

10% Ni/TiO2 NRs

76.6 6.9

24.1

72.5

3.6

20.3

69.32

15% Ni/TiO2 NRs

69.2 8.4

25.6

72.3

3.7

25.9

60.01

10% Ni/TiO2 MPs

69.9 6.5

18.3

69.8

8.3

24.7

65.4

10%Ni-1%Co3O4/TiO2 NRs

81.3 4.6

19.2

71.3

6.5

22.2

88.6


83.5 3.1

24.5

72.8

2.4

24.8

92.0


81.1 3.4

25.3

72.2

3.0

24.0

90.5

*Reaction

H2

Phenol conversion (%)

conditions: 1 atm., 700 oC, 5 wt. % phenol, S/C = 15 (mol/mol), Feed flow = 10

ml/h, 8 h reaction time; GHSV=1525 h-1. 3.2.2 Effect of reaction temperature The comparison of phenol conversion, products yields and selectivity under different temperatures in the range of 600 to 900 oC using 10 % Ni-5% Co3O4/TiO2 NRs at steam to carbon ratio 15 (S/C=15/1) and atmospheric pressure is presented in Table 3. At 600 oC, phenol conversion and H2 was lower, but was gradually increased when the temperature was increased. Highest reforming activity was obtained at 800 oC, with H2 yield of 87.65 13 ACS Paragon Plus Environment


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%. More interestingly, yields of CO and CO2 while phenol conversion were increased using temperature 600 to 900 oC as similarly reported previously in the literature 48. These results could be explained on the SRP and WSG reactions. The steam reforming of phenol can be simply presented using stoichiometric reaction as explained in Equation 6 and WGS reaction as presented in Equation 7 49. C6 H 5OH+5H 2 O   6CO  8H 2

H  710.91 kJ / mol

(6)

  CO 2  H 2 CO+H 2 O  

H  41.15 kJ / mol

(7)

As SRP in Equation 6 is an endothermic reaction, thus elevated temperature is encouraging for SRP reactions to generate higher H2 yield

50.

Water gas shift reaction (Equation 7)

consists of exothermic reaction and thus, high temperature supports the reverse WGS reaction, leading to increasing CO yield. Furthermore, with the increase in temperature, phenol conversion was improved, while H2 yield was dropped. This was obvious, many other side reactions can play a vital role to vary the outlet gas products composition. CO2 methanation (Equation 8) and methane cracking (Equation 9) reactions are some important examples of these side reactions.

CO 2 +4H 2  CH 4 +2H 2 O

(8)

CH 4  H 2 O  CO+3H 2

(9)

Although, CO2 methanation is one of the favorable reaction due to its production at lower temperature, yet CH4 was not identified in all the experiments. This was evidently, highly activity of catalysts promoted steam reforming of methane with the production of CO and H2 at low temperature

51.

However, at very high temperature, RWSG reaction was

favorable thus the production of H2 was decreased. Hence, to promote higher phenol conversion, elevated temperature is required, and relatively lower temperature is favorable to get higher H2 yield. Although, Co3O4 loaded into 1D TiO2 NRs provides higher Ni dispersion, yet at higher temperature, decomposition and C-containing products reaction would not be avoided, resulting in higher coking on the catalyst surface. This phenomenon is much similar to the one reported in the literature, in which cobalt based catalysts exhibits higher activity despite the formation of carbon. This was also because of higher bimetallic interaction, good metal-supports interaction and conductive features of the structured TiO2 support. 14 ACS Paragon Plus Environment

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Table 3: Effect of temperature on catalytic performance of 10%Ni/5%Co3O4/TiO2 NRs for phenol conversion and production of H2, CO and CO2. Yield (%)*

Temperature

Selectivity (%)

Phenol conversion

(oC)

H2

CO

CO2

CO

CO2

(%)

600

63.39

2.18

24.44 69.44 2.78

28.78

76

700

83.51

3.10

24.59 69.31 2.43

27.83

92

800

87.65

5.57

29.55 68.84 3.40

28.52

96

900

86.56

9.03

30.98 68.88 5.47

25.86

98

*Reaction

H2

conditions: 1 atm., 5 wt. % phenol, S/C = 15 (mol/mol), Feed Flow = 10 ml/h, 8

h reaction time; GHSV=1525 h-1. 3.2.3 Effect of catalyst loading In addition to reaction temperature, catalytic activity was further explored by investigating the effect of different catalyst loading of 0.1, 0.15 and 0.3 g. Loading of the catalyst in the reactor directly affects the concentration of the products mixture during SRP. As we already performed steam reforming of phenol without catalyst (homogenous) in the reactor bed, which showed very low phenol conversion and H2 yield (Table 2). However, when catalyst was introduced into reactor, as a fixed bed, a significant enhancement in H2 yield and selectivity was achieved. As anticipated, for all three loading amounts of catalyst, the yield and selectivity of H2 increased as loading of catalyst was increased. With the increase in catalyst loading from 0.1 to 0.3 g, selectivity was improved from 65.62 to 71.87 % (Figure 7a), while H2 yield was increased from 63.92 to 81.45 % (Figure 7b), respectively, an evident of enhanced SRP reaction with higher catalyst loading. Thus, increased in activity was obviously due to more active sites available with longer residence time for SRP conversion towards selective H2 production. However, using the lower catalyst loading (0.10 g), more CO was produced, while its yield was decreased and CO2 yield was enhanced with more catalyst loading. These results reveal that during SRP reactions, a significant amount of CO was produced, however, its yield was controlled by 15 ACS Paragon Plus Environment


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the available catalyst active sites. When the catalyst loading was increased, more active sites would be available for CO conversion with H2O to CO2 and H2 through WGS reaction (Equation 7). When catalyst loading was increased to 0.30 g, the amount of CO was also increased while CO2 yield was decreased, evidently, with excessive catalyst loading, reversed water gas shift (RWGS) reaction would be activated. Therefore, higher loading of Ni-Co3O4/TiO2 NRs is favorable to promote hydrogen production with higher catalyst loading due to enhanced metal dispersion over the 1D TiO2 NRs structure and adsorption limitations inside the catalyst surface. 3.2.4 Effect of steam to carbon ratio and gas hourly space velocity The effect of steam to carbon ratio was investigated using S/C ratios of 7, 11 and 15 at reaction temperature 700 oC and atmospheric pressure and composition of results after the reactions are presented in Figure 8. The products yield and phenol conversion at different S/C ratios has been illustrated in Figure 8 (a). The main products detected were H2, CO and CO2 in all types of the reaction systems. The yield of H2 and phenol conversion was somewhat increased with an increase of S/C ratio 7 to 15. With the increase of carbon in the feed mixture, for example, using lower H2O/C ratio, more production of CO and CO2 were obtained comparative to H2 yield. Figure 8 (b) reveals products selectivity at different initial S/C ratios during SRP reactions. Therefore, H2 selectivity was reduced with higher carbon contents in the feed mixture using lower H2O/C ratios in the feed mixture. In addition, CO2/CO ratio was increased with the increase of S/C ratio, indicating the importance of water gas shift reaction. With the increase in partial pressure of water (higher H2O/C), more steam will react with CO for the production of H2 and CO2, thus more steam in the feed mixture promotes WGS reaction (Equation 7). When steam used was higher, more H2O molecules could be absorbed to the active sites of the catalyst, and therefore the absorption of the phenol molecules could be prevented. As a result, higher phenol conversion and H2 yield will take place over the highly efficient catalyst

52.

This also

reveals that Ni-Co3O4/TiO2 NRs promotes enhanced phenol conversion and H2 yield using higher S/C ratio, which enables less coke formation and provides more stability to catalyst. The effect of catalyst structure and morphology was further investigated by varying the feed flow rates, which were measured as a gas hourly space velocity (GHSV). The different GHSV were obtained by varying phenol feed flow rate from 5 to 15 mL h-1, while 16 ACS Paragon Plus Environment

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keeping the flow rate of N2 of 20 mL min-1. The experimental results were obtained with catalyst loading 0.30 g, S/C ratio 15, temperature 700 oC and atmospheric pressure. Figure 9 presents the effect of GHSV on the yield and selectivity of the products. Figure 9 (a) demonstrates that H2 selectivity was increased with the increase of GHSV and highest H2 selectivity can be obtained with GHSV of 1532 h-1, however, H2 yield was increased with the increase of GSHV as presented in Figure 9 (b). When lower GHSV, higher yield of CO was produced, while CO2/CO was enhanced with higher GHSV as depicted in Figure 9 (c). Besides, with the increase of GHSV, yield and selectivity of CO2 were increased, while it has adverse effects on CO production as shown in Figure 9 (d). The increase in H2 yield with GHSV reveals that SRP reaction over structured Ni-CO3O3/TiO2 NRs is dependent on mass transfer towards the catalyst surface instead of adsorption-desorption process inside the catalyst channels and over the active sites. When GHSV was increased, mass transfer of phenol towards the catalysts surface would be higher to promote SRP reaction, resulting in higher catalyst activity for H2 production. In addition, at higher GSHV, due to efficient mass transfer, less possibilities of RWGS reactions over the catalysts surface. This also confirms that good metal support interactions with high dispersion of active metal over the structured Co3O4 nanocubes and TiO2 NRs, surface reactions were very fast and the rate of reaction would be dependent only on the mass transfer from the bulk flow towards the catalyst surface. However, more coke formation would be expected due to cracking reactions with more carbon available at the catalyst surface, possibly due to formation of other sides reactions, for example, methane which would be converted to coke during the efficient SRP reaction 53. 3.2.5 Stability analysis of catalyst The stability analysis for 10% Ni/TiO2 NRs and 5 % Co3O4–10 % Ni/TiO2 NRs catalyst was performed at 700 oC with S/C =15, in the feed stream, 0.30 g catalyst loading, 700 oC and atmospheric pressure and the results are presented in Figure 10. When 10 % Ni-5 % Co3O4/TiO2 NRs catalyst was kept on stream during 100 h of time, continuous and selective production of H2 was recorded as shown in Figure 10 (a). Similarly, phenol conversion was consistent throughout the reaction time, which suggest catalyst have good activity for both phenol conversion and H2 production. This enhanced stability for continuous production of H2 throughout the reaction time was evidently due to less coke 17 ACS Paragon Plus Environment


formation, thus revealed high stability in metal-support interaction. This also discloses that catalyst is exceptionally worthy in thermal stability and has high potential for commercial applications. After 70 h of time on stream, there was a slight increase in CO formation, obviously due to the reversed WGS reaction of CO2 and H2 into CO due to the absorption of CO2 and H2 on the catalyst surface. A comparative study was conducted over 10 % Ni/TiO2 NRs to prove the synergistic effect of Co3O4 in catalytic activity. As shown in Figure 10 (b), the catalytic activity in terms of H2 yield was highest initially, then gradually lost activity over time on stream. More importantly, gas flow blockage (choking) and pressure buildup in the reactor was also observed during the long run experiments over a Ni/TiO2 NRs catalyst. This was obviously due to significant carbon deposition over the catalyst surface, resulting in hindrance of gas flow and sintering of Ni active sites would be occurred over the catalyst surface. Therefore, TiO2 NRs accompanied by Co3O4 nanocubes could be a good candidate to be used as an active metal support because it has high mechanical and chemical stability, economical and large surface area for active metal dispersion 27. Thus, enhanced activity and stability of the composite catalyst is due to promotional effects of Co3O4 in Ni/TiO2–NRs due to controlled morphology, strong interaction of bimetallic active sites with the catalyst support and exceptional electronic interfaces between the active metal and support due to its property of being reducible oxide. 3.3

Characterization of spent catalysts TGA curves of fresh 10 % Ni-5 % Co3O4/TiO2NRs catalyst sample and its

corresponding spent has been presented in Figure 11. It is evident from Figure 11 (a) that fresh catalyst exhibited catalytic degradation in three steps. The initial degradation was occurred from 50 to 450 oC referred as removal of moisture from the catalyst surface with weight loss of ~2%. A weight loss of ~ 0.75 % was recorded in the second phase of degradation in the temperature range 450 to 700 oC, this might be due to eradication of organic residues and dehydration. The third phase of degradation occurs after 800 °C with 3 % of weight loss was noted, which shows the thermal stability of the catalyst. Likewise, for the spent catalyst sample as depicted in Figure 11 (b), 4 % of total weight loss was detected in spent catalyst within the temperature range of 50 to 800 oC. The lower weight 18 ACS Paragon Plus Environment

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loss of the catalyst is an indication toward high thermal stability due to good metal-support interactions, cubic structure of Co3N4 and 1D structure of TiO2 NRs with reducible properties. Furthermore, both spent and fresh samples showed a single reverse peak for the carbon curve in respective DTG profiles. The only peak in both profiles is an indicator that only a single form of carbon exists on the catalyst surface. Moreover, the carbon peak of the spent catalyst sample was recorded more sharp and deep downwards, arising at temperatures near to 700 °C, clarifying formation of a single and most stable form of carbon species on the surface of spent catalyst after 100 h of time on stream. To examine possible carbon deposition and catalyst particles structural changes during phenol steam reforming, post-reaction characterization of 10% Ni/TiO2 NRs and 10% Ni-5% Co3O4/TiO2 NRs catalyst sample was executed. Figure 12 illustrates the IR spectra achieved for different fresh and spent catalyst samples. Evidently, the samples profile show similarity of chemical composition on the catalyst surfaces of both fresh and spent samples, exhibiting almost parallel features of IR spectra. This confirms that the spent sample of both Ni/TiO2 NRs and Ni-Co3O4/TiO2 NRs has same type of carbon residue. IR spectra corresponds to the stretching vibrational peaks at 3250 cm-1 of the hydroxyl -OH group and some of the ambient water from surrounding atmosphere might be re-adsorbed. IR profiles peaks recorded at 1500 cm-1 relates to the C=O group vibrations. The sharp vibration corresponds to hydroxyl bonds of molecular water were recorded at 1250 and 900 cm-1 of IR spectra. Minor peaks of Ti−O vibrations were observed near 550 cm-1. HR-TEM analysis of spent catalyst shown in Figure 13 supports the finding from HR-TEM analysis of fresh catalyst as discussed previously. It can be seen in the micrographs that the formation of carbon was not occurring on the Co3O4 surface (Figure 13 a-b). This could be concluded Co3O4 resist carbon deposition, which leads to prolonged stability of the catalyst during SRP reaction. Furthermore, prepared Co3O4 doped Ni/TiO2 NRs catalyst has the ability to produce commercially valuable carbon products like multiwall carbon nanotubes (MWCNTs), carbon nanofibers and graphene sheets during the SRM process (Figure 13 c). As observed in the EDX elemental analysis, Ti, Co, Ni and C elements have been detected in the spent catalyst (Figure 13 d). Therefore, Co3O4 loaded Ni/TiO2 NRs



provides good stability and high activity for selective hydrogen production in SRP and can be further used for commercial production of hydrogen and other energy applications. 4.

CONCLUSION Ni/CO3O4 supported TiO2 NRs were successfully developed and investigated for

SRP towards enhanced and continuous H2 production. The variable gas concentration (CO, CO2 and H2) during phenol reforming was obtained using different S/C ratios, catalyst loading and GSHV. Using 5 wt. % phenol with a feed rate of 10 ml h−1, reforming temperature of 700°C and 0.3g of catalyst loading, H2 yield of 83.5 %, selectivity of 72.8 % and phenol conversion of 92 % was achieved. Higher temperature (> 700°C) would be used for greater yields of H2, while CO2/CO ratio was decreased at temperature above 800 oC

due to reversed WGS reaction. The good stability in terms of H2 production after 100

hours of SRP over 10% Ni-5% Co3O4/TiO2 NRs was achieved compared to 10% Ni/TiO2 NRs having no stability over the time on stream. The strong interaction of metal-support effect and efficient WGS reaction resulting in prolonged stability with minimum coke formation. Therefore, 1D nanorods TiO2 interaction with Co3O4 nanocubes, higher bimetallic interaction, improved metal dispersion and enhanced reducibility are the critical factors to contribute enhanced activity and stability. Steam reforming of phenol offers a clean and renewable production of hydrogen, while the use of newly developed catalyst would be promising in other energy applications. ACKNOWLEDGEMENTS The authors would like to extend their deepest appreciation to Ministry of Education (MOHE) Malaysia for financial support of this research under FRGS (Fundamental Research Grant Scheme, Vot 4F876) and University Technology Malaysia (UTM), Malaysia for financial support of this research under (Research University Grant, Vot 17H06). REFERENCES (1) BP BP Statistical Review of World Energy 2017; London, UK, 2017. (2) Zheng, C. W.; Wang, Q.; Li, C. Y., An overview of medium- to long-term predictions of global wave energy resources. Renew. Sust Energ. Rev. 2017, 79, 1492-1502. (3) Peng, Q. Q.; Tao, Y. W.; Ling, H. J.; Wu, Z. Y.; Zhu, Z. L.; Jiang, R. L.; Zhao, Y. M.; 20 ACS Paragon Plus Environment

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(47) Mizuno, S. C. M.; Braga, A. H.; Hori, C. E.; Santos, J. B. O.; Bueno, J. M. C., Steam reforming of acetic acid over MgAl2O4-supported Co and Ni catalysts: Effect of the composition of Ni/Co and reactants on reaction pathways. Catal. Today 2017, 296, 144153. (48) Meng, J.; Zhao, Z.; Wang, X.; Zheng, A.; Zhang, D.; Huang, Z.; Zhao, K.; Wei, G.; Li, H., Comparative study on phenol and naphthalene steam reforming over Ni-Fe alloy catalysts supported on olivine synthesized by different methods. Energ. Convers. Manage. 2018, 168, 60-73. (49) Wang, S. R.; Cai, Q. J.; Zhang, F.; Li, X. B.; Zhang, L.; Luo, Z. Y., Hydrogen production via catalytic reforming of the bio-oil model compounds: Acetic acid, phenol and hydroxyacetone. Int. J. Hydrogen Energy 2014, 39 (32), 18675-18687. (50) Karnjanakom, S.; Guan, G. Q.; Asep, B.; Du, X.; Hao, X. G.; Samart, C.; Abudula, A., Catalytic steam reforming of tar derived from steam gasification of sunflower stalk over ethylene glycol assisting prepared Ni/MCM-41. Energ. Convers. Manage. 2015, 98, 359-368. (51) Itkulova, S. S.; Nurmakanov, Y. Y.; Kussanova, S. K.; Boleubayev, Y. A., Production of a hydrogen-enriched syngas by combined CO2-steam reforming of methane over Cobased catalysts supported on alumina modified with zirconia. Catal. Today 2018, 299, 272279. (52) Tao, J.; Lu, Q.; Dong, C. Q.; Du, X. Z.; Dahlquist, E., Effects of electric current upon catalytic steam reforming of biomass gasification tar model compounds to syngas. Energ. Convers. Manage. 2015, 100, 56-63. (53) Fu, P.; Yi, W. M.; Li, Z. H.; Bai, X. Y.; Zhang, A. D.; Li, Y. M.; Li, Z., Investigation on hydrogen production by catalytic steam reforming of maize stalk fast pyrolysis bio-oil. Int. J. Hydrogen Energy 2014, 39 (26), 13962-13971.


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Page 25 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figures Captions Figure 1

Detailed schematic representation of 10 % Ni – 5 % Co3O4/TiO2 NRs catalyst preparation.

Figure 2

Schematic representation of Experimental Setup for SRP.

Figure 3

XRD patterns of TiO2, 10% Ni/TiO2 NRs, 5 % Co3O4/TiO2 and 10 % Ni 5 % Co3O4/TiO2 NRs catalyst samples.

Figure 4

10 % NiO / TiO2 NRs and 10 % NiO - 5 % Co3O4 / TiO2 NRs catalysts (a) temperature-programmed reduction (TPR) profiles of 10% vol H2/Ar, and a heating rate at 10 oC/min and; (b) temperature-programmed desorption (TPD) profiles of 10% vol CO2/Ar, and a heating rate at 10 oC/min, respectively.

Figure 5

HR-TEM micrographs: (a-e) fresh 10 % Ni - 5% Co3O4/TiO2 NRs and; (f) Selected area (electron) diffraction (SAED).

Figure 6

N2 adsorption-desorption isotherms of 1o % Ni/TiO2 MPs, 10% Ni/TiO2 NRs and 10% Ni-5% Co3O4/TiO2 NRs samples.

Figure 7

Effect of different weights of 10 % Ni-5% Co3O4/TiO2 NRs catalyst loading on steam reforming of phenol; (a) H2 selectivity (%), (b) H2 yield (%), (c) CO and CO2 yield (%) and (d) CO and CO2 selectivity (%). Reaction conditions: 1 atm., 700 oC, 5 wt.% phenol, S/C=15 (mol/mol), 8 h reaction time.

Figure 8

: Effect of steam to carbon ratio on the performance of 10% Ni-5% Co3O4/TiO2 NRs composite catalyst; (a) product yield and phenol conversion (%); (b) products selectivity (%). Reaction conditions: 1 atm., 700 oC, Feed Flow = 10 ml/ h, S/C for 5, 7 and 10 wt. % phenol = 15, 11 and 7 (mol/mol), respectively, 8 h reaction time.

Figure 9

Effect of gas hourly space velocity (GHSV) on the performance of 10% Ni5% Co3O4/TiO2 NRs composite catalyst; (a) H2 selectivity (%); (b) H2 yield (%); (c-d) Selectivity (%) and Yield (%) of CO and CO2, respectively. Reaction conditions: 1 atm., 700 oC, 5 wt.% phenol, S/C = 15 (mol/mol), 8 h reaction time. 25 ACS Paragon Plus Environment


Figure 10

(a) Stability test of 10% Ni-5% Co3O4/TiO2 NRs composite catalyst; (b) comparison of H2 yield between 5% Co3O4 -10% Ni/TiO2 NRs and 10% Ni/TiO2 NRs; Reaction conditions: 1 atm., 700 °C, Phenol = 5 wt.%, Feed Flow= 10 ml/h, S/C = 15/1.

Figure 11

TGA plots of 10 % Ni - 5 % Co3O4 / TiO2 NRs catalyst; (a) fresh catalyst, (b) spent catalyst.

Figure 12

FTIR spectra of fresh and spent 10% Ni/TiO2 NRs and 10% Ni-5% Co3O4/TiO2 NRs catalyst samples.

Figure 13

HR-TEM micrographs: (a-c) spent 10% Ni/5% Co3O4-TiO2 NRs; (d) EDX elemental analysis of Ni/Co3O4-TiO2 NRs.


Page 26 of 41

Page 27 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Figure 1: Detailed schematic representation of 10 % Ni – 5 % Co3O4/TiO2 NRs catalyst preparation.


Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Figure 2: Schematic representation of Experimental Setup for SRP


Page 28 of 41

Page 29 of 41

O

O ---- TiO2

TiO2

 ---- Co3O4

10% NiO/TiO2

 ---- NiO

5% Co3O4/TiO2 10% NiO-5% Co3O4/TiO2



Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


  O

10

20

30



40

O

O

50

 O

60

O

70

80

2-theta (degree) Figure 3: XRD patterns of TiO2, 10% Ni/TiO2 NRs, 5 % Co3O4/TiO2 NRs and 10 % Ni 5 % Co3O4/TiO2 NRs catalyst samples.



o

Intensity (mW)

(a)

o

(524 C)

(603 C)

o

402 C

10 % Ni- 5% Co3O4/TiO2 NRs

o

(596 C)

o

(512 C)

10 % NiO-TiO2 NRs 100

200

300

400

500

600

700

800

o

Temperature ( C)

(b)

o

(773 C)

10% NiO-TiO2 NRs 10% NiO-5 % Co3O4/TiO2 NRs o

(200 C)

o

(529 C)

Intensity (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 41

o

(639 C)

o

(189 C)

100

200

300

400

500

o

600

700

800

Temperature ( C) Figure 4: 10 % NiO/TiO2 NRs and 10 % NiO - 5 % Co3O4 / TiO2 NRs catalysts (a) temperature-programmed reduction (TPR) profiles of 10% vol H2/Ar, and a heating rate at 10 oC/min and; (b) temperature-programmed desorption (TPD) profiles of 10% vol CO2/Ar, and a heating rate at 10 oC/min, respectively.


Page 31 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Figure 5: HR-TEM micrographs: (a-e) fresh 10 % Ni - 5% Co3O4/TiO2 NRs and; (f) Selected area (electron) diffraction (SAED).



0.005

10% Ni/TiO2 MPs 10% Ni/TiO2 NRs 10% Ni-5% Co3O4/TiO2 NRs

-1

0.004

na/mol g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 41

0.003

0.002

0.001

0.000 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Relative pressure (P/Po)

0.8

0.9

1.0

Figure 6: N2 adsorption-desorption isotherms of 1o % Ni/TiO2 MPs, 10% Ni/TiO2 NRs and 10% Ni-5% Co3O4/TiO2 NRs samples.


Page 33 of 41

100

90

(a)

0.30 g 0.15 g 0.10 g

70

60

50

(b)

80

H2 Yield (%)

H2 Selectivity (%)

80

40

60

0.30 g 0.15 g 0.10 g

40

20

30 1

2

3

4

5

6

7

0

8

1

T.O.S (h) 35

35

(c)

CO,

30

CO2

2

3

(d)

4

5

T.O.S (h)

CO,

30

6

7

8

CO2

25 Selectivity (%)

25 Product yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 15 10

20 15 10 5

5

0

0 0.10 g

0.15 g

0.10 g

0.3 g

0.15 g

0.3 g

Catalyst loading (g)

Catalyst loading (g)

Figure 7: Effect of different weights of 10 % Ni-5% Co3O4/TiO2 NRs catalyst loading on steam reforming of phenol; (a) H2 selectivity (%), (b) H2 yield (%), (c) CO and CO2 yield (%) and (d) CO and CO2 selectivity (%). Reaction conditions: 1 atm., 700 oC, 5 wt.% phenol, S/C=15 (mol/mol), 8 h reaction time.



X,

(a)

H2,

CO,

100

CO2

90

90

80

80

70

70

60

60

50

50

40

40

30

30

20

20

10

10

Phenol conversion, X (%)

Product Yield (%)

100

0

0 7:1

11:1

15:1

S/C Ratio

90

(b)

H2,

CO,

CO2

80

Product Selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 41

70 60 50 40 30 20 10 0

7:1

11:1

S/C Ratio

15:1

Figure 8: Effect of steam to carbon ratio on the performance of 10% Ni-5% Co3O4/TiO2 NRs composite catalyst; (a) product yield and phenol conversion (%); (b) products selectivity (%). Reaction conditions: 1 atm., 700 oC, Feed Flow = 10 ml/ h, S/C for 5, 7 and 10 wt. % phenol = 15, 11 and 7 (mol/mol), respectively, 8 h reaction time.


Page 35 of 41

80 75

100

(a)

90

65

-1

-1

1525 h ,

1532 h ,

1538 h

H2 Yield (%)

H2 Selectivity (%)

(b)

80

70 -1

60 55

70 60 50 40 30

-1

-1

1525 h ,

50

1532 h ,

1538 h

-1

20

45

10

40

0

1 30

2

3

(c)

4

5

6

T.O.S (h)

CO,

7

8

1

CO2

35

25

20

15

10

5

0

2

3

(d)

4

5

6

T.O.S (h)

CO,

7

8

CO2

30 Product yield (%)

Selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25 20 15 10 5

-1

1525 h

-1

1532 h

0

-1

1538 h

-1

1525 h

-1

-1

1532 h

-1

GHSV (h )

-1

1538 h

GHSV (h )

Figure 9: Effect of gas hourly space velocity (GHSV) on the performance of 10% Ni-5% Co3O4/TiO2 NRs composite catalyst; (a) H2 selectivity (%); (b) H2 yield (%); (c-d) Selectivity (%) and Yield (%) of CO and CO2, respectively. Reaction conditions: 1 atm., 700 oC, 5 wt.% phenol, S/C = 15 (mol/mol), 8 h reaction time.



Ni-Co 3O4/TiO2 NRs 100

Stable

90

90

80

80

70

70

SH ,

60

SCO

2

50

SCH ,

SCO

50

YH ,

XPhenol

40

4

40

60

2

2

30

30

20

20

10

10

0

0 0

10

20

30

40

50

60

70

80

90

Phenol Conversion (%)

(a)

H2 Yield (%)

Selectivity (%)

100

100

TOS (h) 100

Stable

(b) 80

H2 Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 41

60

Unstable

40

10% Ni - 5% Co 3O4/ TiO2 NRs 10% Ni / TiO2 NRs

20

0 0

10

20

30

40

50

60

70

80

90

100

TOS (h)

Figure 10: (a) Stability test of 10% Ni-5% Co3O4/TiO2 NRs composite catalyst; (b) comparison of H2 yield between 5%Co3O4 -10% Ni/TiO2 NRs and 10% Ni/TiO2 NRs; Reaction conditions: 1 atm., 700 °C, Phenol = 5 wt.%, Feed Flow= 10 ml/h, S/C = 15/1.


Page 37 of 41

0.4

99.5

Weight %

(a)

Weight % Derivative Weight %

99.0

0.2

98.5

0.0

98.0

-0.2

97.5

-0.4 o

450 C

Deriv. Weight (%/min)

-0.6

97.0

-0.8

96.5 100

200

300

400

500

600

700

800

Temperature ( °C) 0.2

100

(b)

0.1

99

Weight %

0.0 98 -0.1

Weight % (%) Derivative Weight % (%/min)

97

-0.2 o

725 C

Deriv. Weight (%/min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.3

96 -0.4 100

200

300

400

500

600

700

800

Temperature (°C) Figure 11: TGA plots of 10 % Ni - 5 % Co3O4 / TiO2 NRs catalyst; (a) fresh catalyst, (b) spent catalyst.



100

80

Trasmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 41

1500

1250

60

3250

40

2250 10 % Ni/TiO2 NRs-Fresh

20

10 % Ni/TiO2 NRs-Spent

900

10% Ni-5 % Co3O4/TiO2 NRs-Fresh

0

10% Ni-5 % Co3O4/TiO2 NRs-Spent 500

1000

1500

2000

2500

-1

3000

3500

4000

Wavenumber (cm ) Figure 12: FTIR spectra of fresh and spent 10% Ni/TiO2 NRs and 10% Ni-5% Co3O4/TiO2 NRs catalyst samples.


Page 39 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


(b)

(a)

t

t

Multiwall CNT

e

(c)

t

Carbon nanofibers

Co3O4

Co3O4

Graphene nanosheets

Graphene nanosheets

e Co3O4

Co3O4

x e

MCNT

Carbon nanofibers

MCNT

(d)

t x e t

Figure 13: HR-TEM micrographs: (a-c) spent 10%Ni/5%Co3O4-TiO2 NRs; (d) EDX elemental analysis of Ni/Co3O4-TiO2 NRs.

39

ACS Paragon Plus Environment


Highlights 

Ni-doped Co2O3/TiO2 Nano-rods tested for steam reforming of phenol to produce selective hydrogen.



Co2O3 increased catalytic activity of Ni/TiO2 nano-rods with improved H2 yield, selectivity and phenol conversion.



Catalyst structure, composition and reaction conditions showed significant effect on H2 production.



Ni-doped Co2O3/TiO2 Nano-rods exhibit long run stability as compared Ni-doped TiO2 Nano-rods in SRP.


Page 40 of 41

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Graphical Abstract


TiO2 Nanorods

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