Ind. Eng. Chem. Res. 2009, 48, 5239–5249
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Ultradeep Hydrodesulfurization and Adsorptive Desulfurization of Diesel Fuel on Metal-Rich Nickel Phosphides Miron V. Landau,*,† Moti Herskowitz,† Tali Hoffman,† David Fuks,‡ Edward Liverts,§ Dima Vingurt,‡ and Natali Froumin‡ Department of Chemical Engineering, Blechner Center for Industrial Catalysis and Process DeVelopment, Department of Materials Engineering, and Department of Mechanical Engineering, Ben-Gurion UniVersity of the NegeV, Beer-SheVa, 84105, Israel
Three materials containing Ni2P, Ni12P5, and Ni3P phases on silica gel with surface area 320 m2/g at loadings of 32-37 wt % and the crystal size of NixP phases 30, 9, and 13 nm, respectively, were prepared by a combination of impregnation and TPR methods and tested in hydrodesulfurization (HDS) and adsorptive desulfurization (ADS) of diesel fuel. There were established opposite trends in changing the DS efficiency in two processes: The HDS rate constant decreased while the ADS sulfur capacity (breakthrough at 1 ppmw) increased with increasing the Ni to P ratio in NixP from 2 to 3. The observed behavior was attributed to the specific features of the densities of states (DOS) obtained from the density functional theory calculations of total and partial DOS for Ni and P in NixP phases and revealed in XPS measurements of binding energy of Ni 2p3/2- and P 2p-electrons. This attribution was consistent with the analysis of the relative part of d-electrons of Ni participating in bonding with p-electrons of phosphorus in these phases. 1. Introduction The stringent requirements for sulfur removal from hydrocarbon feedstocks reflect the strict environmental laws for transportation fuels and the demand of on-board and on-site portable and mobile fuel-cell applications.1 The maximum allowable sulfur content in highway diesel fuel will be reduced in the U.S., Europe, Japan, and Korea to less than 10 ppmw by 2010.2-4 The fuel processing systems for hydrogen production with its subsequent application in fuel cells requires reduction of sulfur content to 1 means that nanocrystals of guest nickel phosphide phases in prepared reduced samples did not block the supports mesopores and even had a significant contribution to the total surface area of composite materials.42 Therefore, comparison of the intrinsic performance of NixP phases by normalizing the measured performance characteristics to the specific surface area or concentration of surface nickel atoms calculated from the crystal size of corresponding phosphides is valid. 3.2. HDS/ADS Performances of Supported NixP Phases. The results of kinetic measurements in HDS of dibenzothiophene are presented in Figure 3. The first-order rate constants [kW(HDS)] calculated from these data are listed in Table 4. Their values decreased with increasing of Ni/P ratio. The results of ADS tests performed with low-sulfur diesel fuel are shown in Figure 4. The amount of adsorbed sulfur increased linearly with the time of stream up to the breakthrough where the outlet sulfur content in diesel fuel increased sharply. The S-adsorption rate was high, reducing the sulfur content from 12 to Ni12P5/SiO2 > Ni3P/SiO2, reflecting higher S-adsorption rates with materials having higher Ni/P ratio in phosphide phases. The sulfur capacity listed in Table 4 increased from Ni2P to Ni3P. Comparison of intrinsic catalytic activities and sulfur adsorption capacities of different NixP phases requires information about surface areas of active phosphide phases in NixP/SiO2 composites. There is no quantitative method for estimation of specific surface areas of metal phosphides phases based on the specific adsorption of probe molecules. Chemisorption of CO
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Table 4. Dispersion and HDS-ADS Performance of NixP Phases Supported on Silica-Gel NixP crystal size
estimated specific surface area m2/g solid
TEM NixP phase in Ni-P/SiO2
theoretical density, g/cm3
XRD average, nm
size range, nm
% nanocrystals
XRD
Ni2P
7.35
30
7.53
9
Ni3P
7.82
13
29 39 19 11 9 75 16 26 29 40 5
9
Ni12P5
40-50 30-40 20-30 10-20 15-20 10-15 5-10 20-25 15-20 10-15 5-10
at room temperature widely used for determination of the surface concentration of active sites in metal phosphides correlates with the total surface area of bulk nickel phosphides.43-47 But its amount measured for Ni2P and WP were an order of magnitude lower compared with the number of metal atoms exposed on the phosphides surface.45,46 The detailed spectroscopic investigation detected several forms of CO adsorbed on nickel phosphide surface: CO terminally bonded to coordinatively unsaturated Ni and P atoms, CO in the form of bridged carbonyl, Ni(CO)4, and even as surface carbonate.27,48 CO chemisorption stoichiometry that takes in account all the types of surface atoms and their coordination, for different NixP compositions at monolayer coverage, still needs to be determined. Therefore, intrinsic HDS and ADS performances of NixP/SiO2 materials were determined using the specific surface area of nickel phosphide phases. That was estimated based on their average crystal sizes (D, nm), crystal size distributions, and theoretical densities (F, g/cm3) derived from XRD and TEM data (Table 4). The average crystal sizes of NixP phases calculated from the XRD peaks broadening using the Sherrer algorithm are shown in Table 4. The surface area of Ni-phosphide phases was calculated according to eq 3 assuming globular shape of their nanocrystals proven by HRTEM (Figure 5 and reference 49): SANixP[m2/g NixP ] ) 6000 /FD
(3)
The specific surface areas of NixP in NixP/SiO2 materials [m2/gNixP/SiO2] were obtained by multiplying the calculated SANixP values by weight fractions of corresponding phases shown in Table 2. Furthermore, the specific surface area of NixP in NixP/SiO2 solids was estimated on the basis of the TEM data. There were 10 different micrographs that were obtained from 85 × 85 µm2 areas
Figure 4. ADS performance (effluent sulfur concentration and sulfur capacity) measured with Ni-P/SiO2 materials.
TEM
m2/g NixP XRD
TEM
KW(HDS) cm3 g-1 h-1
CW(ADS) mmol S · g-1 × 102
27
25
44.0
6.6
32
26
88
72
35.7
21.5
19
16
59
50
14.6
29.7
of three NixP/SiO2 materials containing Ni2P, Ni12P5, and Ni3P phosphides. The nanocrystals of corresponding nickel phosphide phases were clearly observed as dark spots displayed in typical TEM images shown in Figure 5. This is a result of incoherent elastic scattering of electrons and diffraction contrast created at NixP crystal areas of ordered atomic layers. Only black spots that contained Ni and P elements according to the local EDAX analysis and displayed parallel fringes at HRTEM images after proper titling of the sample reflecting the atomic layers in NixP phases were included (insets at Figure 5a-c). NixP crystal size was calculated with accuracy of (1 nm. The analysis detected relatively narrow size distributions in the range 5-25 nm with sharp maximum at 10-15 nm for Ni12P5 and Ni3P and a wider distribution in range of 10-50 nm for Ni2P. The nanocrystals of all three NixP phases had a globular shape even at sizes close to 50 nm as observed for the Ni2P phase (Figure 5). It means that the specific surface area of corresponding NixP phases based on size distributions of their nanocrystals shown in Table 4 was calculated according to the following equation: N
SANixP[m2/g NixP] ) [6000/F(
∑ 1/D )]/N i
(4)
i)1
where Di is the diameter of nanocrystal i, and N is the number of nanocrystals that was used in the calculation. The specific surface area estimated for a specific phase according to XRD and TEM data and presented in Table 4 are rather similar considering the errors involved in each method. The surface area of NixP phases, determined from TEM micrographs was used for calculations of specific catalytic activities and sulfur adsorption capacities. The specific HDS activities calculated as reaction rate constants normalized per m2 of NixP active phase [KS(HDS)] and specific ADS capacities as mmols of sulfur adsorbed per gram of NixP/SiO2 materials corresponded to their sulfur capacities (Scap, eq 1) and normalized per m2 of NixP active phase [CS(ADS)] are compared in Figure 6. The intrinsic ADS capacity gradually increased with increasing the Ni/P ratio in phosphide phase. The value for Ni3P exceeded that of N2P by a factor of 2.4. The opposite trend was obtained for intrinsic HDS activity with the maximum for Ni2P phase. Recording the XRD patterns of spent samples after ADS and HDS tests showed that all three metal-rich phases were stable during ADS runs. Only in spent Ni3P/SiO2 was a small (
