Î±-Al2O3 Catalysts: A New Technique

Subscriber access provided by University of South Dakota

Article 2

2

3

NO Pulse Titration of Ni/#-AlO Catalysts: A New Technique Applicable to Nickel Surface Area Determination of Nickel-Based Catalysts Shohei Tada, Misato Yokoyama, Ryuji Kikuchi, Takahide Haneda, and Hiromichi Kameyama J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp404291k • Publication Date (Web): 21 Jun 2013 Downloaded from http://pubs.acs.org on June 23, 2013

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

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

The Journal of Physical Chemistry

N2O Pulse Titration of Ni/α-Al2O3 Catalysts: A New Technique Applicable to Nickel Surface Area Determination of Nickel-based Catalysts Shohei Tada,a,b Misato Yokoyama,a Ryuji Kikuchi,a,* Takahide Haneda,c Hiromichi Kameyamac

a

Department of Chemical System Engineering, School of Engineering, The University of Tokyo,7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan b

c

Research Fellow of Japan Society for the Promotion of Science

Tokyo Gas Co., Ltd., 1-7-7, Suehiro-cho, Tsurumi-ku, Yokohama-shi, Kanagawa 230-0045, Japan

*Corresponding author: Tel. & Fax: +81-3-5841-1167 E-mail address: [email protected]

ACS Paragon Plus Environment

1


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 2 of 30

ABSTRACT Transmission electron microscopy (TEM), CO pulse, and N2O pulse titration were applied to measuring Ni particle sizes on Ni/α-Al2O3 catalysts. It was clarified for the first time that the N2O pulse titration could estimate the size at titration temperature between 50 ºC and 100 ºC, which was confirmed by TEM observation. This agreement in the Ni particle sizes by N2O titration and TEM observation means that the Ni surface was fully covered with the monolayer of oxygen formed by N2O decomposition and the adsorption of N2 and N2O on the catalysts was negligible. On the other hand, the Ni particle sizes estimated from the CO pulse technique was larger than those measured by the TEM observations. This is because the stoichiometry of CO to Ni was assumed to be one despite the complicated adsorption states of CO on Ni surface. On the contrary, the stoichiometry of oxygen to Ni using N2O pulse titration was one, which indicates that the stoichiometry measurements were not required for this method. KEYWORDS Chemisorption, nickel, CO pulse, transmission electron spectroscopy, turnover frequency, FTIR


2

Page 3 of 30

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


INTRODUCTION Supported nickel and nickel-based catalysts are effective for various reactions, such as reforming of hydrocarbons, 1,2,3,4 methanation of carbon oxides,5,6 hydrogenation of aldehydes,7 primary amine synthesis,8 and anodic reaction in a solid oxide fuel cell.9,10 It is well accepted that the nickel dispersion and nickel surface area of the catalysts mostly determines catalytic activity and selectivity for the structure sensitive reactions. One of the conventional methods for nickel dispersion determination is a statistical approach using particle size distribution of nickel on catalysts, which is obtained from direct observations by transmission electron microscopy (TEM). However, the distinction of metal particles on supported catalysts is still problematic in the case of insufficient contrast difference between the metal particles and support materials. Many researchers have also investigated the static-volumetric and dynamic chemisorption methods to measure the surface area of nickel on nickel-based catalysts in the following manner: i) probe gas molecules, such as H2 11,12,13,14,15 and CO,16,17 are adsorbed on nickel-based catalysts, ii) the number of surface nickel atoms are calculated from the adsorbed amount of the molecules and the stoichiometry between nickel and chemisorbed gas molecule, and then iii) the number of surface nickel atoms thus calculated is converted to the nickel surface area using the area occupied by one surface nickel atom.18,19 With a static-volumetric chemisorption method the amount of the gas adsorbed at constant temperature is given as a function of the equilibrium pressure (adsorption isotherm). The apparatus for this method are equipped with an appropriative vacuum device. The pulse technique is also carried out as one of the dynamic chemisorption methods. In this method, a pulse of adsorbate gas is injected into the flow of inert gas over the well reduced catalyst under ambient pressure. The quantity of the gas consumed after pulse


3


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 4 of 30

injection is measured easily using thermal conductivity detector in the pulse experimental apparatus which responds in proportion to the amount of the adsorbate gas. Each probe molecule has its own disadvantage for the chemisorption method, which means that suitable probe molecules should be chosen for nickel surface area determination. In H2 chemisorption, a part of H2 molecules dissociated on the nickel surface may be migrated to subsurface layers or to the support. This phenomenon is known as hydrogen spillover effect.18,19,20 While CO is strongly and stably chemisorbed on nickel surface compared to H2, there are many complex adsorbed states of CO on nickel surface, such as terminal and bridging CO, poly-nuclear Nix(CO)y with x = 2 or 3 and low y values.21,22 The reactive chemisorption of N2O, as a probe molecule, on metal surface of supported metal catalysts is an adequate method for measuring specific metal surface area. This technique is frequently applied to the measurement of copper,23,24,25,26,27 silver,28 and platinum 29 surface area. Each N2O molecule adsorbed on metal surface was decomposed to gaseous N2 molecule and chemisorbed oxygen atom, and then the metal surface is covered with the monolayer of oxygen. Thereby the metal surface area can be estimated from the amount of consumed N2O and produced N2. Evans et al. investigated the relationship between adsorption temperature and N2O adsorption over copper catalysts.24 At temperatures up to 65 ºC N2O was not decomposed. On the contrary, at 160 ºC and higher bulk oxidation of metallic copper probably occurred. These results indicate that the appropriate temperature of N2O titration for the copper catalysts was between 65 ºC and 160 ºC

(actually the authors adopted 90 ºC as the titration temperature), and

there is a suitable temperature range for N2O titration depending on chemical property of active metals. In this study, Ni particle size on Ni/α-Al2O3 was estimated by TEM observation, CO pulse chemisorptions, and N2O titration, and Ni crystallite size by XRD. The temperature


4

Page 5 of 30

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


dependence of N2O pulse titration for Ni surface on Ni/α-Al2O3 was also studied, which determined a suitable temperature for this technique. In addition, the Ni surface area estimation by the N2O titration was validated by activity tests of CO methanation.

EXPERIMENTAL METHODS

Catalyst Preparation Ni/α-Al2O3 was prepared by the impregnation method. As support materials, α-Al2O3, prepared by calcining γ-Al2O3 (JRC-ALO6) at 1200 ºC for 6 h, provided by Catalysis Society of Japan was used. The specific surface areas of Al2O3 were evaluated by the BET method using nitrogen adsorption (Micromeritics, ASAP2000). High temperature calcination process decreased the surface area from 165 to 4.5 m2 g-1. It was impregnated with an aqueous solution of Ni(NO3)2·6H2O (Wako, 99.9%), dried at 100 ºC and calcined at 700 ºC for 2 h. The Ni loadings of Ni/α-Al2O3 were 5, 10, 15, and 20 wt%. Characterization N2O titration, developed by Evans et al,24 was carried out in the flow system to measure Ni particle size on well-reduced Ni/α-Al2O3 (Quantachrome, CHEMBET-3000). The catalysts of 50-200 mg were placed in the quartz tube and reduced at 650 ºC for 1 h in 5% H2/Ar prior to the test. He gas was used as carried gas at 15 ml min-1, and the successive doses of 10% N2O/He gas were subsequently introduced into He stream by means of a calibrated injection valve (25 µlN2O pulse-1) at desired temperature. The product gases (N2 and N2O) were separated by Porapak T column (GL science) and analyzed by a thermal conductivity detector (TCD). The number of Ni surface atom was estimated according to eq. (1).


5


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

Ni + N2O → NiO + N2

Page 6 of 30

(1)

Surface area of Ni SNi (m2 gNi-1) and Ni particle size dNi (nm) were calculated by eq. (2) and (3), respectively, assuming that Ni particles on the supports are spherical in uniform volume and density.

S Ni =

Y × Nav WNi × A

d Ni =

6 S Ni × ρNi

(2) (3)

, where Y = the amount of N2O consumption or N2 production (mol gcat-1), Nav = Avogadro’s constant (6.02 × 1023 mol-1), WNi = Ni loading (gNi gcat-1), A = the number of Ni surface atoms per unit area (1.54 × 1019 m-2),30 and ρNi = Ni density (8.91 g cm-3). The Ni particle size on Ni/α-Al2O3 was also estimated by CO pulse method (Quantachrome, CHEMBET-3000). After the reduction of the sample by 5% H2/Ar at 650 ºC for 1 h, the samples were cooled to 50 ºC under He flow, and then several CO pulses (25 µlCO pulse-1) were passed over the samples. Particle size of Ni was calculated under the assumption that the stoichiometry of CO/Ni was equal to 1. Surface area of Ni SNi of each catalyst was derived from substitution of CO adsorption amount to Y in eq. (2), and then dNi was calculated from eq. (3) and obtained SNi. The crystalline phase of catalysts was determined by X-ray diffraction (XRD, Rigaku, RINT2400 instrument) at voltage of 40 kV and current of 100 mA. The morphology of the Ni particles of the Ni/α-Al2O3 catalysts was monitored using transmission electron microscopy


6

Page 7 of 30

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


(TEM, JEOL, JEOL-2010F). Additionally, X-ray energy dispersive spectrometry (EDS) was used to determine the elemental composition. Fourier transform infrared spectroscopy (FTIR) measurements were performed to identify the CO adsorption state on Ni. The FTIR cell equipped with high purity CaF2 windows and capability for heating and cooling was placed in a JASCO FTIR 6100 instrument with a mercury cadmium telluride detector (JASCO, MCT6000M). The powdered samples, reduced at 650 ºC for 1 h under 5% H2/Ar flow, were pressed into a thin self-supporting disk of about 30 mg cm-2 and set in the cell. The samples were oxidized during the disk preparation, but it was confirmed by temperature programmed reduction by H2 that the oxide layers could be reduced below 300 ºC.31 All samples were pretreated in the cell by reduction at 300 ºC in 5% H2/Ar for 30 min, and cooling to room temperature under N2 atmosphere. Then background spectra were collected at room temperature, followed by exposure to 10% CO/He for 5 min. Finally, their spectra were recorded after the mixed gas had been removed by flushing 1, 3, and 5 min with N2. Typically, 100 scans were collected for one spectrum. Activity Test The catalytic performance for CO methanation was evaluated in a 4-mm I.D. fixed-bed quartz tubular reactor at atmospheric pressure. A quantity of 50 mg of catalyst powder was placed in the reactor, and then reduced at 650 ºC for 1 h in 5% H2/Ar flow prior to each run. The feed gas consists of 5% CO/H2. A W/F value (expressed in terms of the ratio of catalyst weight to the total feed rate) was 0.37 gcat h mol-1. The gas composition at the reactor outlet was analyzed with a micro gas chromatograph (Varian, CP-4900) equipped with MS-5A column and a TCD. The conversion and conversion rate of CO and turnover frequency (TOF) of CO methanation are defined as follows:


7


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

CO conversion = 1 −

FCO,out

CO conversion rate = TOF =

where

FCO,in

and

(4)

FCO,in

FCO,in × COconversion W

CO conversion rate S Ni

FCO,out

Page 8 of 30

(5)

(6)

are the inlet and outlet molar flow rates of CO (mol s-1), respectively, W

is the catalyst weight (g), and SNi is Surface area of Ni (m2 gNi-1).

RESULTS AND DISCUSSION Characterizations by TEM, XRD, and CO Chemisorption TEM As illustrated in Figure 1, the transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) were used to examine reduced Ni/α-Al2O3. In the TEM images, a lot of darker spots can be observed, and the darker spots are identified as Ni by EDS analysis. Mean average diameter of Ni particles on 5, 10, 15, and 20wt%Ni/α-Al2O3 are shown to be 35, 37, 47, and 58 nm, respectively, as summarized Table 1. The Ni particles grew as the Ni loading increased.


8

Page 9 of 30

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


Figure 1. TEM images of Ni/α-Al2O3. Ni loading: (a) 5, (b) 10, (c) 15, and (d) 20 wt%. Bar scale in (a-d) is 50 nm. Particle distribution of Ni on (e) 5, (f) 10, (g) 15, and (h) 20 wt% Ni/α-Al2O3. More than 100 Ni particles were counted.


9


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 10 of 30

XRD The crystalline phases present in as-prepared and well-reduced Ni/α-Al2O3 catalysts were analyzed using XRD, and all peaks were identified with ICDD files, as shown in Figure 2. All catalysts had a lot of peaks assignable to corundum α-Al2O3. The as-prepared Ni/α-Al2O3 exhibited similar diffraction patterns with a peak at 37º, which correspond to (101) plane of NiO. The peak at 43º is attributed to (111) plane of NiO, and overlapped one of α-Al2O3. As for reduced Ni/α-Al2O3 peaks at 44 and 52º were assignable to (111) and (200) plane of metallic Ni. The mean crystallite size of Ni was estimated by the diffraction peak of Ni (111) plane and Scherrer equation, D=

Kλ β cos θ

(7)

where K is the shape factor (0.89), λ is X-ray wavelength (0.154 nm), β is the line broadening at half the maximum intensity in radians, and θ is Bragg angle.32 The sizes of Ni metal on 5, 10, 15, and 20 wt% Ni/α-Al2O3, as summarized in Table 1, were 42, 45, 49, and 55 nm, respectively, as shown in Table 1. It is concluded from these results that the crystallite size was similar to the Ni particle size measured from TEM observations, and also increased with increase in Ni loading.


10

Page 11 of 30

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


Table 1. Ni Particle Size of 5, 10, 15, and 20w% Ni/α-Al2O3 Estimated by TEM Observation, N2O Pulse Titration, CO Chemisorption, XRD Measurement Loading weight Ni particle size / nm Ni (111) / wt% Crystallite size*** / nm N2O titration* TEM CO pulse ** 5 31 35 93 42 10 37 37 99 45 15 53 47 128 49 20 61 58 139 55 * At 75 ºC. ** At 50 ºC. *** Estimated by XRD patterns.

Figure 2. XRD patterns of 5, 10, 15, and 20 wt% Ni/α-Al2O3 (a) before and (b) after reduction at 650 ºC. ICDD-card number: α-Al2O3 (45-0946), NiO (44-1159), and Ni (04-0850).

CO Chemisorption To obtain the CO adsorption state on Ni/α-Al2O3, the surface species after dosing 10% CO/He onto 10wt% Ni/α-Al2O3 catalyst surface were examined by Fourier transform infrared spectroscopy (FTIR) measurements, as shown in Figure 3. In all spectra adsorption peaks at 1650, 1430, and 1230 cm-1 were observed, which are attributed to the bicarbonate groups on Al2O3 surface. 33,34 Also in all spectra two bands between 2200 and 2100 cm-1 can be seen,


11


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 12 of 30

ascribed to CO adsorbed on Al2O3 35 or the CO adsorption over Ni2+.36 The intensity of them decreased with flushing time, indicating that the CO weakly adsorbed on Al2O3 or Ni2+. In addition, the bands of linear CO adspecies at 2050 cm-1 and bridge CO adspecies at 1950 and 1905 cm-1 were observed.37,38,39 As for the well-reduced Ni/α-Al2O3 samples, Ni particle sizes were estimated by CO chemisorptions under assumption that CO was linearly adsorbed on Ni surface, and the estimated sizes were actually larger than the Ni particle size monitored using TEM observations due to surface carbonyl formation with more than one CO molecule being adsorbed per surface Ni atom.22 If Ni surface area of nickel-based catalysts is assessed using CO chemisorptions, it is necessary to know the CO surface coverage and the CO adsorption state on nickel. For instance, Fisher et al. reported how to determine the CO surface coverage and adsorption state on Rh/SiO2.40 Initially they measured infrared spectra during the exposure of the catalyst to 250 Torr of CO and 510 Torr of He, and attained the following 2 peaks: linearly bonded CO on Rh (Rh-CO, 2067-2039 cm-1) and bridge-bonded CO on Rh (Rh2-CO, 1895-1856 cm-1). Then the CO surface coverage of Rh-CO and Rh2-CO was calculated by the integrated Beer-Lambert relation. In addition, Ni(CO)4 can be formed in CO chemisorptions on Ni species,41,42 which makes it difficult to determine the CO surface coverage.


12

Page 13 of 30

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


Figure 3. FTIR spectra of CO adsorbed at room temperature (ca. 25 ºC) on 10wt% Ni/α-Al2O3 recorded during N2 flushing for (a) 1, (b) 3, and (c) 5 min.

Investigation of N2O Titration Condition for Ni Surface Area Estimation Oxygen adsorption via N2O decomposition to N2 on well-reduced Ni/α-Al2O3 catalysts

was

considered to determine the Ni surface area of the catalysts. Firstly, N2O decomposition on Ni/αAl2O3 was confirmed. Figure 4 and Table 2 show the results of N2O titration measurement for 10wt% Ni/α-Al2O3 at 50 ºC. As for the first 3 pulses, a strong peak appeared at ca. 1350 sec after dosing N2O/He mixed gas. Concerning the next 2 pulses, two peaks were seen at ca. 1350 and 2120 sec after pulse injection. Furthermore, the intensity of the former peak decreased and that


13


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 14 of 30

of the latter peak increased as the pulse number increased from 4 to 5. About 6th and 7th pulses, a sharp peak was measured longer than 2100 sec after pulse injection. These results indicate that Porapak T column can separate N2 (ca. 1350 sec) from N2O (> 2100 sec), and that N2O was decomposed to N2 on 10wt%Ni/α-Al2O3 at 50 ºC.

Table 2 Injection Time and Peak Position of N2O Titration Method for 10wt% Ni/α-Al2O3 at 50 ºC Pulse No. Injection time t1/ sec Peak position t2/ sec t2-t1 /sec 1 0 1370 1370 2 1320 2700 1380 3 2580 3920 1340 4 3930 5260 1330 6060 2130 5 6430 7790 1360 8550 2120 6 8740 11160 2420 7 10920 13250 2330

Figure 4. N2O titration sequence for 10wt% Ni/α-Al2O3 at 50 ºC.


14

Page 15 of 30

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


Next, temperature dependence of N2O titration for 10wt% Ni/α-Al2O3 was studied, as summarized in Table 3. As for all 5 measurements, the amount of produced N2 (VN2) mostly accorded with that of consumed N2O (VN2O), which means that N2O adsorbed on α-Al2O3 was negligible and decomposed to N2 on the metallic Ni surface. There was a slight increase in VN2 and VN2O from 28ºC to 50 ºC. The VN2O remained unchanged up to 100ºC, and then started to increase. Suitable titration temperatures are between 50 ºC and 100 ºC and the Ni surface is expected to be covered with the monolayer of oxygen. This is supported by the fact that the Ni particle size estimated from VN2O in this temperature range agreed well with that measured from TEM image. It is considered that N2O decomposition rate was not high enough to cover all Ni surface at low titration temperature such as 28 ºC, leading to the higher estimate of Ni particle size than the actual size. On the other hand, N2O titration at 150 ºC oxidized not only metallic Ni surface but also metallic Ni bulk, resulting in the lower estimate of the size. Particle sizes of Ni on Ni/α-Al2O3 with different Ni loadings were estimated by N2O pulse titration at 75 ºC and summarized in Table 1. The sizes on 5, 10, 15, and 20wt% Ni/α-Al2O3 were 31, 37, 53, and 61 nm, respectively. This estimation was in good agreement with the Ni particle size measured from TEM observations. In addition, the stoichiometry of Ni/N2O was equal to 1 as shown in eq. (1). Thus there is no need to measure the stoichiometry of Ni and a probe molecule before using N2O pulse titration for Ni catalysts, which is differently from the CO pulse chemisorption technique. In this study, N2O pulse titration technique was applied to the surface area determination of 2050 nm Ni particles. The applicability of this technique to Ni particles less than 10 nm will be further examined in the next stage, as the Ni particles less than 10 nm are frequently utilized in catalytic reaction.


15


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 16 of 30

Table 3. Temperature Dependence of N2O Titration Method for 10wt% Ni/α-Al2O3 Titration temperature VN2 * VN2O ** Ni particle size*** / ºC / µmol gcat / µmol gcat / nm 28 38 37 47 50 48 47 37 75 47 47 37 100 44 47 38 150 84 87 20 * Amount of produced N2. ** Amount of consumed N2O. *** Calculated by N2O titration method.

Activity Test of CO Methanation on Ni/α-Al2O3 Catalysts In this section, the Ni surface area estimation by the N2O titration was validated by activity test of CO methanation. Table 4 lists the turnover frequency (TOF) of CO methanation on Ni catalysts at 275 ºC. As for all our catalysts the conversion of CO at 275 ºC was less than 10%. The TOF of our Ni/α-Al2O3 catalysts was ca. 1×105 s-1, and was almost equal to the reported values. 22, 45 The TOF was slightly increased with Ni loadings, which means that CO methanation tends to prefer Ni/α-Al2O3 catalysts with large Ni particles. As for catalytic hydrogenation reactions, this tendency was reported by several researchers.43,44 While small particles possess the surface atoms of active metal with low coordination numbers, such as edge and corner, large particles was occupied with the atoms having high coordination numbers, such as terrace. Thus these results support the validity of the Ni surface area estimation by N2O titration.


16

Page 17 of 30

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


Table 4 Turnover Frequency of CO Methanation at 275 ºC* Catalysts Gas composition of TOF / 103 s-1 CO/H2 5wt%Ni/α-Al2O3 5/95 87 10wt%Ni/α-Al2O3 5/95 94 15wt%Ni/α-Al2O3 5/95 126 20wt%Ni/α-Al2O3 5/95 131 8.8wt%Ni/η-Al2O3 25/75 85 5wt%Ni/ZrO2

25/75

90-100

Method** N2O titration N2O titration N2O titration N2O titration H2 chemisorption H2 chemisorption

Ref. This study This study This study This study [22] [45]

* H2/CO = 3. ** Method for Ni surface area estimation.

CONCLUSION In summary, as for Ni/α-Al2O3 Ni particle size estimation by TEM, CO pulse, and N2O pulse titration was investigated, and using N2O pulse titration at the temperature between 50 and 100 ºC Ni particle size could be measured for the first time. Metallic Ni on Ni/α-Al2O3 was expected to be covered with the monolayer of oxygen using the method in this temperature range. Furthermore, the amount of N2 production on Ni/α-Al2O3 was almost equal to that of N2O consumption, leading to N2O decomposition on only metallic Ni surface and negligible N2O adsorption on the α-Al2O3 support. This estimation was in good agreement with the Ni particle size measured from TEM observations. While Ni surface was not completely covered by oxygen from N2O at the temperature below 50 ºC, N2O oxidized not only Ni surface but also bulk Ni at the temperature above 100 ºC. Titration technique of N2O for Ni characterization has the following merits: 1) easy-to-use approach compared to TEM observation, and 2) no need to measure the stoichiometry of Ni and a probe molecule differently from the CO pulse chemisorption technique. The TOFs of Ni/α-Al2O3 for CO methanation were evaluated by


17


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 18 of 30

activity tests and N2O pulse titration, and were ca. 1×105 s-1. The values were close to the reported TOFs.

ACKNOWLEDGMENTS TEM observation was conducted in Center for Nano Lithography & Analysis, The University of Tokyo, supported by the Ministry of Education, Culture, Science, and Technology, Japan.

REFERENCES

(1) Urasaki, K.; Sekine, Y.; Kawabe, S.; Kikuchi, E.; Matsuoka, M. Catalytic Activities and Coking Resistance of Ni/perovskites in Steam Reforming of Methane. Appl. Catal. A: Gen. 2005, 286, 23-29. (2) Takehira, K.; Shishido, T.; Wang, P.; Kosaka, T.; Takaki, K. Autothermal reforming of CH4 over supported Ni catalysts prepared from Mg–Al hydrotalcite-like anionic clay. J. Catal. 2004, 221, 43-54. (3) Soliman, M. A.; Adris, A. M.; Al-Ubaid, A. S.; El-Nashaie, S. S. E. H. Intrinsic kinetics of nickel/calcium aluminate catalyst for methane steam reforming. J. Chem. Tech. Biotechnol. 1992, 55, 131-138.


18

Page 19 of 30

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


(4) Xu, S.; Zhao, R.; Wang, X. Highly coking resistance and stable Ni/Al2O3 catalysts prepared by W/O microemulsion for partial oxidation of methane. Fuel Process. Technol. 2004, 86, 123133. (5) Urasaki, K.; Tanpo, Y.; Nagashima, Y.; Kikuchi, R.; Satokawa, S. Effects of preparation conditions of Ni/TiO2 catalysts for selective CO methanation in the reformate gas. Appl. Catal. A: Gen. 2013, 452, 174-178. (6) Tada, S.; Shimizu, T.; Kameyama, H.; Haneda, T.; Kikuchi, R. Ni/CeO2 catalysts with high CO2 methanation activity and high CH4 selectivity at low temperatures. Int. J. Hydrogen Energy 2012, 37, 5527-5531. (7) Schimpf, S.; Louis, C.; Claus, P. Ni/SiO2 catalysts prepared with ethylenediamine nickel precursors: influence of the pretreatment on the catalytic properties in glucose hydrogenation. Appl. Catal. A: Gen. 2007, 318, 45-53. (8) Shimizu, K.; Kon, K.; Onodera, W.; Yamasaki, H.; Kondo, J. N. Heterogeneous Ni Catalyst for Direct Synthesis of Primary Amines from Alcohols and Ammonia. ACS Catal. 2013, 3, 112117. (9) Sunagawa, Y.; Yamamoto, K.; Muramatsu, A. Improvement in SOFC Anode Performance by Finely-Structured Ni/YSZ Cermet Prepared via Heterocoagulation. J. Phys. Chem. B 2006, 110, 6224-6228.


19


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 20 of 30

(10) Xu, J.; Sakanoi, R.; Higuchi, Y.; Ozawa, N.; Sato, K.; Hashida, T.; Kubo, M. Molecular Dynamics Simulation of Ni Nanoparticles Sintering Process in Ni/YSZ Multi-Nanoparticle System. J. Phys. Chem. C 2013, 117, 9663-9672. ( 11 ) Geyer, R.; Hunold, J.; Keck, M.; Kraak, P.; Pachulski, A.; Schödel, R. Methods for Determination the Metal Crystallite size of Ni Supported Catalysts. Chem. Ing. Tech. 2012, 84, 160-164. (12) Rouabah, D.; Fraissard, J. 1H and

129

Xe nuclear magnetic resonance studies of hydrogen

chemisorption on supported platinum Application to the metal dispersion and spillover. Solid State Nucl. Magn. Reson. 1994, 3, 153-162 (13) Velu, S.; Gangwal, S. K. Synthesis of alumina supported nickel nanoparticle catalysts and evaluation of nickel metal dispersions by temperature programmed desorption. Solid State Ionics 2006, 177, 803-811. (14) Zhang, Z.L.; Verykios, X.E. Carbon dioxide reforming of methane to synthesis gas over supported Ni catalysts. Catal. Today 1994, 21, 589-595. ( 15 ) Mihaylov, M.; Tsoncheva, T.; Hadjivanov, K. Structure sensitive of methanol decomposition on Ni/SiO2 catalysts. J. Mater. Sci. 2011, 46, 7144-7151. (16) Verma, A.; Ruthven, D. M. A pulse chromatographic method of determing the specific activity of supported metal catalysts. J. Catal. 1970, 19, 401-405. (17) Brooks, C. S.; Kehrer, V. J. Chemisorption of carbon monoxide on metal surfaces by pulse chromatography. Anal. Chem. 1969, 41, 103-106.


20

Page 21 of 30

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


( 18 ) Fierro, J. L. G. Spectroscopic Characterization of Heterogeneous Catalysts Part B: Chemisorption of Probe Molecules. Stud. Surf. Sci. Catal. 1990, 57, B1-B66. (19) Lemaitre, J. L.; Menon, P. G.; Delannay, F. The Measurement of Catalyst Dispersion in Characterization of Heterogeneous Catalysts; Delannay. F., Eds.; Marcel Dekker, Inc.: New York, 1984; pp 299-365. (20) Techner, S. J.; Mazabrard, A. R.; Pajonk, G.; Gardes, G. E. E.; Hoang-Van, C. Hydrogen spillover in catalytic reactions: I. Activation of alumina. J. Colloid Interface Sci. 1977, 55, 8899. (21) Jensen, M. B.; Morandi, S.; Prinetto, F.; Sjåstad, A. O.; Olsbye, U.; Ghiotti, G. FT-IR characterization of supported Ni-catalysts: Influence of different supports on the metal phase properties. Catal. Today 2012, 197, 38-49. (22) Vannice, M. A. The catalytic synthesis of hydrocarbons from H2/CO mixtures over the group VIII metals: IV. The kinetic behavior of CO hydrogenation over Ni catalysts. J. Catal. 1976, 44, 152-162. (23) Osinga, Th. J.; Linsen, B. G.; van Beek, W. P. The determination of the specific copper surface area in catalysts. J. Catal. 1967, 7, 277-303. (24) Evans, J. W.; Wainwright, M. S.; Bridgewater, A. J.; Young, D. J. On the determination of copper surface area by reaction with nitrous oxide. Appl. Catal. 1983, 7, 75-83. (25) Nishimura, S; Shishido, T.; Ohyama, T.; Teramura, K.; Takagaki, A.; Tanaka, T.; Ebitani, K. In situ observation of the dynamic behavior of Cu-Al-Ox catalysts for water gas shift reaction


21


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 22 of 30

during daily start-up and shut down (DSS)-like operation. Catal. Sci. Technol. 2012, 2, 16851693. (26) Wang, L.- C.; Liu, Q.; Chen, M.; Liu, Y.- M.; Cao, Y.; He, H.- Y. Fan, K.- N. Structure Evolution and Catalytic Properties of Nanostructured Cu/ZrO2 Catalysts Prepared by Oxalate Gel-Coprecipitation Technique. J. Phys. Chem. C 2007, 111, 16549-16557. (27) Bravo-Suárez, J. J.; Subramaniam, B.; Chaudhari, R. V. Ultraviolet-Visible Spectroscopy and Temperature-Programmed Techniques as Tools for Structural Characterization of Cu in CuMgAlOx Mixed Metal Oxides. J. Phys. Chem. C 2012, 116, 18207-18221. (28) Seyedmonir, S. R.; Strohmayer, D. E.; Geoffroy, G. L.; Vannice, M. A. Characterization of supported silver catalysts: I. Adsorption of O2, H2, N2O, and the H2-titration of adsorbed oxygen on well-dispersed Ag on TiO2. J. Catal. 1984, 87, 424-436. (29) Dawody, J.; Eurenius, L.; Abdulhamid, H.; Skoglundh, M.; Olsson, E.; Fridell, E. Platinum dispersion measurements for Pt/BaO/Al2O3, NOx storage catalysts. Appl. Catal. A: Gen. 2005, 296, 157-168. (30) Kington, G. L.; Holmes, J. M. Adsorption by evaporated copper films at 78 ºK. Part 1.Kyrpton and hydrogen. Trans. Faraday Soc. 1953, 49, 417-425. (31) Tada, S.; Kikuchi, R.; Takagaki, A.; Sugawara, T.; Oyama, S. T.; Urasaki, K.; Satokawa, S. Study of Ru-Ni/TiO2 catalysts for selective CO methanation. Appl. Catal. B: Environ. 2013, 140141, 258-264.


22

Page 23 of 30

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


(32) Bragg, W. H.; Bragg, W. L. Crystal Texture in The Crystalline state; Bragg, W.L., Eds.; G. Bell and Sons, Ltd.: London, 1962; pp 188-207. (33) Parkyns, N. D. Influence of thermal pretreatment on the infrared spectrum of carbon dioxide adsorbed on alumina. J. Phys. Chem. 1971, 75, 526-531. ( 34 ) Rege, S. U.; Yang, R. T. A novel FTIR method for studying gas adsorption at low concentrations: H2O and CO2 on NaX zeolite and γ-alumina. Chem. Eng. Sci. 2001, 56, 37813796. (35) Gribov, E. N.; Zavorotynska, O.; Agostini, G.; Vitillo, J. G.; Ricchiardi, G.; Spoto, G.; Zechina, A. FTIR spectroscopy and thermodynamics of CO and H2 adsorbed on γ-, δ-, and αAl2O3. Phys. Chem. Chem. Phys. 2010, 12, 6474-6482. (36) Gonzalez-Delacruz, V. M.; Pereñiguez, R.; Ternero, F.; Holgado, J. P.; Caballero, A. Modifying the Size of Nickel Metallic Particles by H2/CO Treatment in Ni/ZrO2 Methane Dry Reforming Catalysts. ACS Catal. 2011, 1, 82-88. (37) Primet, M.; Dalmon, J. A.; Martin, G. A. Adsorption of CO on well-defined Ni/SiO2 catalysts in the 195-373 K range studied by infrared spectroscopy and magnetic methods. J. Catal. 1977, 46, 25-36. (38) Hu, C.; Chen, Y.; Li, P.; Min, H.; Chen, Y.; Tian, A. Temperature-programmed FT-IR study of the adsorption of CO and co-adsorption of CO and H2 on Ni/Al2O3. J. Mol. Catal. A: Chem. 1996, 110, 163-169.


23


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 24 of 30

(39) Morlanés, N. Reaction mechanism of naphta steam reforming on nickel-based catalysts, and FTIR spectroscopy with CO adsorption to elucidate real active sites. Int. J. Hydrogen Energy 2013, 38, 3588-3596. (40) Fisher, I. A.; Bell, A. T. A Comparative Study of CO and CO2 Hydrogenation over Rh/SiO2. J. Catal. 1996, 162, 54-65. ( 41 ) García － Diéguez, M.; Finocchio, E.; Larrubia, M. A.; Alemany, L. J.; Busca, G. Characterization of alumina-supported Pt, Ni and PtNi alloy catalysts for the dry reforming of methane. J. Catal. 2010, 274, 11-20. (42) Rossetti, I.; Biffi, C.; Bianchi, C. L.; Nichele, V.; Signoretto, M.; Menegazzo, F.; Finocchio, E.; Ramis, G.; Michele, A. D. Ni/SiO2 and Ni/ZrO2 catalysts for the steam reforming of ethanol. Appl. Catal. B: Environ. 2012, 117-118, 384-396. (43) Ruta, M.; Semagina, N.; Kiwi-Minsker, L. Monodispersed Pd Nanoparticles for Acetylene Selective Hydrogenation: Particle Size and Support Effects. J. Phys. Chem. C 2008, 112, 1363513641. ( 44 ) Silvestre-Albero, J.; Rupprechter, G.; Freund, H.-J. Atmospheric pressure studies of selective 1,3-butadiene hydrogenation on well-defined Pd/Al2O3/NiAl(110) model catalysts: Effect of Pd particle size. J. Catal. 2006, 240, 58-65. (45) Della Betta, R. A.; Piken, A. G.; Shelef, M. Heterogeneous methanation: Initial rate of CO hydrogenation on supported ruthenium and nickel. J. Catal. 1975, 35, 54-60.


24

Page 25 of 30

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


TOC IMAGE


25


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

246x422mm (72 x 72 DPI)


Page 26 of 30

Page 27 of 30

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


299x305mm (72 x 72 DPI)



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

290x288mm (72 x 72 DPI)


Page 28 of 30

Page 29 of 30

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


469x208mm (72 x 72 DPI)



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

270x105mm (72 x 72 DPI)


Page 30 of 30

Î±-Al2O3 Catalysts: A New Technique

Recommend Documents