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SiO2 Doped with Alkali Metal Bromides
Journal of Colloid and Interface Science 250, 37–48 (2002) doi:10.1006/jcis.2002.8298, available online at http://www.idealibrary.com on
Growth of Filamentous Carbon from the Surface of Ni/SiO2 Doped with Alkali Metal Bromides Colin Park∗ and Mark A. Keane†,1 ∗ Department of Chemical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom; and †Department of Chemical and Materials Engineering, University of Kentucky, Lexington, Kentucky 40506-0046 Received October 22, 2001; accepted February 12, 2002; published online April 23, 2002
exploited in adsorption (13), catalysis (14–16), hydrogen storage (17–19), and electronics applications (20, 21). The work reported herein focuses on the application of alkali bromide doped Ni-supported catalysts to generate structured carbon filaments from a model C2 H4 /H2 feedstock. The use of both electropositive and electronegative promoters is widespread in catalysis. In catalytic reforming, for instance, the consensus of opinion is that the level of (unwanted) carbon deposition is directly related to the Cl content of the catalyst (22–24). Indeed, careful doping of metallic catalysts has been routinely used to alter both activity and selectivity, a result attributed in part to induced electronic perturbations in the metal. The influence of electronegative adatoms, S in particular, on the catalytic growth of carbonaceous materials has been studied with a view to altering the nature and characteristics of the deposited carbon (25–27). Chambers and Baker (28) deliberately introduced ppm levels of Cl2 to a C2 H4 /H2 reactant feed, over Fe and Co catalysts, in an attempt to gauge the influence of an electronegative additive on carbon production. The authors proposed that the adsorption of Cl2 leads to a reconstruction of the metal surface to one that enhanced hydrocarbon decomposition. Tibbetts and co-workers (29) surmised that low levels of S were sufficient to liquefy catalytic Fe particles, thereby enhancing carbon filament nucleation while high S concentrations inhibited carbon formation. Fan and co-workers (25), using thiophene as the S source, observed a critical dependence of carbon yield and nanofiber width on the S/C ratio. It is established (30–33) that alkali doping of supported metal catalysts results in an electron enrichment of the metal particles, which in turn modifies adsorption and reactivity. The direct synthesis of a carbonaceous material containing a known amount of alkali metal and/or halogen is highly desirable for many applications, notably in the development of effective nanoscale electronic devices and batteries (34, 35). Such a direct route would circumvent the problematic intercalation step that is generally used to introduce dopants into carbon materials but which suffers the decided drawback of a rapid exfoliation (36). An added advantage to such a direct synthesis is that the dopant is more likely to be in a stable form, built into and dispersed throughout the carbon structure, a feature that is critical in enhancing conductivity properties (37, 38). The use of an electropositive
The growth of ordered filamentous carbon, catalytically generated from the decomposition of ethylene, has been studied over the temperature range 673–898 K using an 11% w/w Ni/SiO2 catalyst doped to varying degrees (0.1–9.3% w/w) with a range of alkali metal bromides. The effect of these alkali metal/halogen adatoms in promoting/inhibiting carbon growth has been assessed and variations in the associated carbon structural characteristics have been examined. The introduction of Li consistently promoted filamentous carbon growth (where 723 K < T < 823 K) while the presence of Na, K, Rb, or Cs resulted in an equivalent or lower carbon yield. The degree of carbon deposition was strongly dependent on the nature and loading of the alkali metal, the Ni/Br ratio in the activated catalyst, and reaction temperature; conditions for optimum carbon growth are identified. The response of carbon yield and structural order to alkali bromide doping is discussed in terms of Ni particle electronic structure and metal/support interaction(s). High-resolution transmission electron microscopy (HRTEM) has been used to probe the filamentous carbon structure and the dispersion/morphology/size of the supported Ni crystallites. Highly curved and helical filaments predominated over the doped (particularly CsBr) samples and this is attributed to a disruption in carbon diffusion through the Ni particle caused by a spreading/coating of the particle by the alkali adatom. Temperature-programmed oxidation studies have highlighted the changes in the graphitic nature of the carbon due to catalyst doping; the results are consistent with the TEM analysis. C 2002 Elsevier Science (USA) Key Words: filamentous carbon; Ni/silica; alkali metal; bromine; ethylene decomposition.
INTRODUCTION
The controlled synthesis of carbonaceous materials that possess a well-defined and -ordered structure has gained considerable momentum in the last 2 decades with the discovery and characterization of buckminsterfullerene (1), multi-/singlewalled carbon nanotubes (2–9), graphitic nanofibers (10, 11), and related materials (3, 12). Such ordered carbon structures possess unique chemical and physical properties that can be
1 To whom correspondence should be addressed. Fax: (859) 323 1929. E-mail: [email protected].
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adatom (Cs) to induce ordering and reconstruction of amorphous carbon at room temperature has been reported by Stevens and co-workers (39, 40). We have reported (41) the growth of ordered filamentous carbon at 553 K from a modified Ni/SiO2 catalyst during the gas-phase hydrodechlorination of chlorobenzene. The “modification” resulted in the incorporation of an appreciable K and Br component onto the catalyst surface. Filamentous carbon growth was achieved at a temperature at least 150 K lower than that normally employed in conventional catalytic routes. We attributed this low-temperature carbon growth to a combination of an electron withdrawal/donation by the halogen/alkali metal on the catalyst surface that serves to favor the destructive chemisorption of chlorobenzene and subsequent precipitation of ordered carbon. We have extended this work to consider the growth of filamentous carbon from an ethylene feedstock and recently communicated (42) an appreciable enhancement in carbon growth from Ni/SiO2 doped with LiBr. In this paper, we consider the effects of varying the nature and loading of the alkali metal on the carbon yield and structural characteristics. EXPERIMENTAL
Catalyst Preparation, Activation, and Characterization The “parent” Ni/SiO2 catalyst was prepared by a standard incipient wetness technique where the silica support (Aldrich fumed silica, surface area >200 m2 g−1 ) was impregnated with a 2-butanolic solution of Ni(NO3 )2 to yield an 11% w/w Ni loading. The promoted catalysts were prepared by subsequently impregnating this parent Ni/SiO2 with methanolic solutions of the respective alkali metal bromides to realize the desired loading; each catalyst precursor was dried overnight at 383 K. The Ni and alkali metal contents were determined (to within ±2%) by atomic absorption spectrophotometry (VarianSpectra AA-10). The catalyst precursors were activated by heating at 10 K min−1 in 100 cm3 min−1 dry 20% v/v H2 /He to a final temperature of 873 K that was maintained for 2 h. The Br content of the activated doped catalysts was measured by heating a known weight of the sample under reflux for 16 h and analyzing the aqueous supernatant potentiometrically using a Metrohm 716 automatic titration unit (AgNO3 titrant, combined Ag electrode). Subsequent EDX elemental mapping of the refluxed solid did not reveal even trace quantities of Br on the surface. Carbon monoxide chemisorption was employed to characterize the supported Ni sites where the catalyst was cooled, following the reduction step, to 298 K in dry He and a fixed volume (10 µ1) of CO was pulsed into the He carrier gas stream; the concentration of CO exiting the reactor was measured using an on-line thermal conductivity detector (TCD). The injection of CO was repeated until the downstream peak area was constant, indicating surface saturation with CO. The catalyst was thoroughly flushed with dry He for 1 h to remove any physisorbed CO and then ramped (25 K min−1 ) to 1073 K with a continual monitoring of the CO content in the exiting gas. Data acquisition and analysis were
performed using the JCL 6000 data collection/manipulation package while the catalyst temperature was continuously monitored by means of a data logging system (Pico Technology, Model TC-08); the latter ensured an accurate measure of the desorption temperature. Upon completion of the CO TPD sequence, a series of calibration peaks were taken at ambient temperature to quantify CO uptake; reproducibility was better than ±3%. Catalytic Reactor System and Carbon Characterization All catalytic reactions were carried out (in situ directly after catalyst activation) under atmospheric pressure in a fixedbed continuous flow silica reactor over the temperature range 673 K ≤ T ≤ 873 K using a 4/1 v/v C2 H4 /H2 reactant mixture. The reaction temperature (±1 K) was monitored continuously by means of a thermocouple inserted in the catalyst bed. The catalytic measurements were made at a W/Q ratio = 2 × 10−5 g h cm−3 , where W represents the weight of activated catalyst and Q is the inlet volumetric feed rate of ethylene. The product stream was sampled at regular intervals by means of a heated gas-sampling valve and analyzed by on-line gas chromatography (Cambridge GC94 chromatograph) employing a DB-1 50 m × 0.20 mm i.d., 0.33-µm capillary column (J&W Scientific) in conjunction with the JCL6000 software. The reactor was cooled to ambient temperature and the catalyst/carbon was passivated in a 2% v/v O2 /He mixture before any weight changes to the catalyst were determined. Temperature-programmed oxidation (TPO) profiles of the carbon generated from the parent and doped Ni/SiO2 catalysts were obtained from throughly washed, demineralized samples to avoid any confusion arising from a catalyzed gasification of carbon by any residual Ni, Br, or alkali metal (43, 44). A known weight (ca. 100 mg) of the demineralized sample was heated from room temperature to 1203 K (at 25 K min−1 ) in a 5% v/v O2 /He mixture with on-line TCD analysis of the exhaust gas. The catalyst bed temperature was again independently monitored using the TC-08 data logger. These profiles were assessed against those generated for model carbon samples: graphite (Sigma-Aldrich, synthetic powder); amorphous carbon (Darco G-60, 100 mesh). The model carbons underwent the same demineralization process prior to oxidation to facilitate a direct comparison with the catalytically generated carbon. The structural characteristics of each catalyst and the carbon growth were probed by high-resolution transmission electron microscopy (HRTEM) using a Phillips CM200 FEGTEM microscope equipped with a UTW energy-dispersive x-ray (EDX) detector (Oxford Instruments) operated at an accelerating voltage of 200 kV. Suitable specimens for TEM were prepared by ultrasonic dispersion in 2-butanol where a drop of the resultant suspension was evaporated on a holey carbon support grid. The ultrasonic dispersion was limited to few seconds to avoid sample damage and ensure that the filaments were not detached from the substrate, a feature of extended ultrasonic treatment.
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GROWTH OF FILAMENTOUS CARBON FROM Ni/SiO2
TABLE 1 Chemical Composition, CO Uptake/Desorption Temperatures (Tmax ), and the Surface Weighted Average Ni Particle Size Associated with the Activated Parent and Alkali Bromide Doped Ni/SiO2
Ni/SiO2 Ni/SiO2 –Li-I Ni/SiO2 –Li-II Ni/SiO2 –Li-III Ni/SiO2 –Li-IV Ni/SiO2 –Na Ni/SiO2 –K Ni/SiO2 –Rb Ni/SiO2 –Cs-I Ni/SiO2 –Cs-II a b
Ni w/w (%)
Alkali Metal w/w (%)
Ni/Br mole ratio
CO uptake (µmol g−1 Ni )
Tmax (K)
Average Ni particle sizea (nm)
11 9 9 8 8 9 10 10 11 8
— 0.1 0.4 1.0 6.0 0.5 0.5 0.4 0.5 9.3
— 25.3 6.8 2.4 7.0 16.1 15.9 53.7 35.2 46.2
105 87 74 56 11 79 66 62 59 74
663 383, 513, 1073 378, 508, 1073 518, 1073 525, 883, 1073 1073 348, 1073 371, 758, 1073 453, 791, 1073 553, 888, 1043
12.9 7.1 13.0 10.5 20.3 9.3 10.5 8.7 7.4 —b
Based on TEM analysis. Alkali metal layer obstructed analysis by TEM.
All gasses [He (99.99%), C2 H4 (99.95%), H2 (99.99%), and O2 /He (99.99%)] were supplied by BOC and dried before use by passage through activated molecular sieves. The alkali bromide dopants (>98%) were supplied by Sigma-Aldrich and used without any further purification. RESULTS AND DISCUSSION
The chemical composition of the activated parent and alkali bromide doped Ni/SiO2 catalysts and the pertinent CO chemisorption results are given in Table 1. The surface weighted average Ni particle sizes, based on TEM analysis of over 500 individual particles, are also included in Table 1. The CO uptake on the doped samples was significantly lower and, taking the Ni/SiO2 –Li system, declined further with increasing Li loading. This effect could be ascribed to an increase in the average Ni particle size (lower Ni dispersion) due to the formation of surface metal halide that is known to be particularly mobile and prone to sintering (45, 46). However, TEM analysis only revealed significant (average) Ni particle growth at high doping levels, i.e., Ni/SiO2 –Li-IV. Inconsistencies in the dimensions of supported metal particles derived from different analytical techniques continue to bedevil heterogeneous catalysis research (47). The observed decrease in CO uptake may be due to a spreading of the alkali metal over the Ni surface that serves to occlude the supported Ni crystallites from incoming CO sorbent. Such a masking by an alkali adlayer when introduced by impregnation has been noted elsewhere (48). Representative lowmagnification TEM images of selected freshly activated catalyst samples, illustrating the surface morphology/dispersion of the metal phase, are shown in Fig. 1. The TEM/EDX analyses of both freshly activated Rb and the Cs-doped catalysts revealed a thin and highly mobile layer of alkali metal covering sections of the surface, a feature particularly pronounced at higher alkali metal loadings (≥5% w/w). The presence of needle-like structures, with large lattice spacings (ca. 4 nm), was observed in the
case of the Cs-rich Ni/SiO2 –Cs-II sample and is shown in the TEM image presented in Fig. 2; we have identified the latter by EDX elemental mapping as a surface crystalline Cs phase. The Ni particle size histograms for the family of Li–Ni/SiO2 samples, shown in Fig. 3, are more informative and reveal a clear shift in size distribution to higher values for the Li-concentrated sample. Where the Li loading did not exceed 1% w/w, there was little variation in the average Ni particle diameters. Indeed, Arena and co-workers (33) only observed a measurable increase in the size of (19% w/w) Ni supported on MgO where the Li doping level exceeded 0.5% w/w and this was accompanied by a decrease in CO chemisorption capacity. Moreover, thermal desorption spectroscopy (TDS) has shown that the mode of CO adsorption and adsorption energetics are strongly influenced by the incorporation of alkali metals (49–51). Carbon monoxide desorption from the parent Ni/SiO2 catalyst occurred in a single step with a distinct temperature-related maximum (Tmax ) at 663 K. The occurrence of desorption peaks for the doped samples at a lower temperature than that associated with the parent Ni/SiO2 is diagnostic of weaker CO/Ni interaction(s) due to electron donation from the alkali metal, in keeping with conclusions drawn by Arena and co-workers (33). Desorption of CO at higher temperatures can then be ascribed to the presence of residual electronegative Br that serves to strengthen the Ni/CO bond. It is evident that the chemisorption/desorption behavior of Ni/SiO2 has been disrupted due to the incorporation of alkali bromide and this can be linked to changes in the electronic structure of the surface Ni sites. The effects of these modifications to the supported metal phase in terms of ethylene decomposition to solid carbon can be assessed from the data presented in Fig. 4: carbon yield is plotted as a function of temperature for the parent Ni/SiO2 and the doped samples that contained a roughly equivalent alkali metal content. In addition, the carbon yields from each doped catalyst (Ydoped ) relative to that generated from the parent Ni/SiO2 (Yparent ) at three selected temperatures are recorded in Table 2. It can be
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FIG. 1.
Representative low-magnification TEM images of freshly activated (a) Ni/SiO2 and (b) Ni/SiO2 –Li-II.
readily seen that, of all the alkali metal dopants, only the introduction of Li resulted in any significant increase in carbon production. The carbon yields that have been reported in the literature are highly dependent upon reaction conditions and the
FIG. 2. Low-magnification TEM image showing the crystalline needle-like structures associated with activated Ni/SiO2 –Cs-II.
FIG. 3. Nickel particle size distribution in the freshly activated Ni/SiO2 catalyst (downward hatched bars), Ni/SiO2 –Li-I (open bars), Ni/SiO2 –Li-II (crosshatched bars), Ni/SiO2 –Li-III (solid bars), and Ni/SiO2 –Li-IV (upward hatched bars).
GROWTH OF FILAMENTOUS CARBON FROM Ni/SiO2
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FIG. 4. Carbon yield (expressed per gram of Ni) as a function of reaction temperature for the decomposition of ethylene over Ni/SiO2 (), Ni/SiO2 –Li–II (), Ni/SiO2 –Na (), Ni/SiO2 –K (), Ni/SiO2 –Rb (), and Ni/SiO2 –Cs-I ().
type of catalyst used. The use of unsupported mono- or bimetallic transition metal catalysts has been reported to lead to phenomenal yields, i.e., in excess of 200 gc g−1 metal (52). Avdeeva et al. (53) and Ermakova and co-workers (54) obtained carbon yields of up to 384 and 161 gc g−1 Ni from co-precipitated silica and alumina, respectively, over the temperature range 573–973 K. Nagy and co-workers (9, 55), in examining the growth of filamentous carbon from zeolite and silica substrates, recorded much lower yields (up to 65 gc g−1 metal ) from 2.5% w/w Co and Fe loaded samples. Similarly, Anderson and Rodriguez (56), using a series of silica-supported bimetallic Fe : Ni catalysts, reported a maximum yield of 21 gc g−1 metal 873 K from a CO/H2 reactant mixture while Ermakova et al. (57) recently reported a maxiTABLE 2 Ratio of Carbon Yield from the Doped Samples (Ydoped ) to That Generated from the Parent Ni/SiO2 (Yparent ) at Three Representative Temperatures Ydoped /Y parent
Ni/SiO2 Li–Ni/SiO2 -I Li–Ni/SiO2 -II Li–Ni/SiO2 -III Li–Ni/SiO2 -IV Na–Ni/SiO2 K–Ni/SiO2 Rb–Ni/SiO2 Cs–Ni/SiO2 -I Cs–Ni/SiO2 -II
723 K
773 K
873 K
1 1.2 2.0 1.5 1.0 1.2 0.6 0.9 0.9 0.1
1 1.1 1.7 1.3 1.7 0.8 0.6 0.6 0.7 0.1
1 0.6 0.8 1.1 0.9 0.5 0.8 0.4 0.3 0.1
mum value of 45 gc g−1 Fe for the decomposition of methane over Fe/SiO2 . The carbon yields generated in this study fall within the range of values cited above. The maximum yield (roughly twice that obtained from Ni/SiO2 ) was achieved from Ni/SiO2 – Li-II operating at 723 K. In the case of the other alkali bromide treated samples, with the exception of Ni/SiO2 –Na at 723 K, doping served to inhibit carbon formation to varying degrees. In the decomposition of ethylene to carbon, the principal competing reactions are hydrogenation to ethane and hydrogenolysis to methane. The presence of alkali metal dopants has been found elsewhere (30, 31) to suppress catalytic hydrogenation reactions. In this study, although the hydrogenation step was a secondary reaction over the parent Ni/SiO2 (selectivity <8%), there was still a discernible decrease in the conversion to ethane (selectivity <3%), over all the doped samples. The latter observation, allied to the substantial promotion of ethylene decomposition over the Li-based catalysts, supports the contention that the Ni crystallites underwent some reconstruction to an orientation that favors carbon production at the expense of hydrogenation. In the case of the other doped catalysts, a concomitant decrease was observed in carbon deposition and ethane hydrogenation activities. Hydrogenolysis to methane was only observed in the first 10 min of the reaction over the parent Ni/SiO2 catalyst (selectivity <3%) while no hydrogenolysis activity was noted for any of the doped catalysts. We have shown elsewhere (58) that doping Ni/SiO2 with alkali hydroxide had no effect on the overall carbon yield from a C2 H4 /H2 feed. Moreover, the hydroxide-doped samples were characterized by a similar average Ni particle size/size distribution to that associated with the parent catalyst. The appreciable promotion of carbon yield recorded here for Ni/SiO2 –Li-II
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suggests a critical involvement of both electropositive (Li) and electronegative (Br) adatoms. A reducion of the Ni d-electron density, induced by residual surface Br, should result in a stronger interaction with the olefin (28), weakening the C–C bond in the adsorbed ethylene and favoring decomposition. An enhanced decomposition of the hydrocarbon on the free metal surface generates carbon atoms with a concomitant desorption of molecular hydrogen. It must, however, be stressed that the level of carbon deposition was strongly dependent not only on the nature of the dopant(s) but also on the dopant loading and reaction temperature. Indeed, reaction temperature has been shown elsewhere to have a considerable impact on the growth of ordered filamentous carbon (57, 59–62). As LiBr doping produced the only appreciable promotion of carbon growth, the effect of Li loading (from 0.1 to 6% w/w) was considered in some depth and the results in terms of the temperature dependence of yield are presented in Fig. 5. An increase in Li content appears to have shifted the temperature for maximum carbon yield to higher values. Such a temperature-related maximum has been noted elsewhere (53, 60, 63) and the optimum temperature appears to be strongly dependent on the nature of the catalyst and the feedstock. The Li-dilute Ni/SiO2 –Li-I sample generated a roughly equivalent temperature profile to the parent Ni/SiO2 . In this case, the level of LiBr doping can be regarded as insufficient to induce any significant promotional effect. As a general observation, in those instances where the alkali bromide treatment did not result in any significant enhancement of carbon yield (see Table 2), the activated catalyst bore a higher Ni/Br mole ratio (see Table 1), i.e., lower relative residual Br content. The latter is a strong indicator of the crucial role played by the electronegative Br in promoting strong ethylene/surface interactions that must precede
dissociation/carbon deposition. The surface Ni/Br ratio accordingly emerges as a critical variable in determining the extent of carbon growth. While the mechanism for filamentous carbon growth is still open to question, the consensus of opinion is that the dissolution and diffusion of carbon through the metal particle is driven forward by a concentration gradient (64–66). The formation of NiC3 has been proposed as the nucleation species (67, 68) but the involvement of (metastable) metal carbide remains a supposition without any direct evidence for its formation. The movement of carbon atoms through the metal lattice necessitates a displacement of Ni atoms as the C atom diameter (0.142 nm) exceeds the available spacing (0.101 nm) (68). The growth of carbon filaments can be considered to result from a stable but fluid supersaturated solution of carbon in the metal. Doping with alkali bromide must induce a restructuring of the active centers to enhance the destructive chemisorption of ethylene and/or facilitate carbon diffusion to the metal/graphite interface. Any promotional effect is strongly dependent on dopant concentration, and at higher doping levels, the masking of surface Ni by the mobile alkali metal phase (particularly evident with Ni/SiO2 –Cs-II) must serve to inhibit filament production. The carbon generated in these studies is filamentous in nature, grown in a whisker-like mode where the active metal particle could be found at the growing tip/center of the filament; see Figs. 6 and 7. Zhang and Amiridis (69) likewise reported Ni particles on the tip of carbon filaments grown during methane decomposition over an impregnated Ni/SiO2 while Boellaard and co-workers also observed a detachment of metal particles from the support in their study of carbon monoxide decomposition in the presence of hydrogen at elevated temperatures (70). As the Ni interaction(s) with the silica support is(are)
FIG. 5. Carbon yield (expressed per gram of Ni) as a function of reaction temperature for the decomposition of ethylene over Ni/SiO2 (), Ni/SiO2 –Li-I (), Ni/SiO2 –Li-II (), Ni/SiO2 –Li-III (), and Ni/SiO2 –Li-IV ().
GROWTH OF FILAMENTOUS CARBON FROM Ni/SiO2
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FIG. 6. Representative TEM images illustrating the nature of the filamentous carbon growth from the parent Ni/SiO2 catalyst: (a) low-magnification micrograph and (b) higher magnification image of a single (large diameter) nanofilament grown at 773 K. Inset: HRTEM image showing the lattice fringes of a section of an individual carbon nanofilament.
relatively weak in these impregnated samples (71), the pressure exerted on the metal/support interface due to graphite formation is of sufficient magnitude to extract Ni particles from the support. Once the metal particle is detached from the silica, a fresh surface is exposed to ethylene and growth continues with the Ni particle located on the filament tip. The dimensions of the seed Ni particle should determine the width of the catalytically generated carbon filaments. The distribution of filament diameters grown from the parent and representative Li- and Cs-doped samples is compared in Fig. 8 with the Ni particle size distribution in the freshly activated samples. In every instance the average carbon filament diameter exceeded that of the initial Ni particle diameter, e.g., Ni particle diameter in the freshly activated Ni/SiO2 –Cs-I sample (=7.4 nm) < the associated average carbon filament diameter (=19.5 nm). These results are indicative of a Ni particle growth/sintering during ethylene decomposition. Anderson and Rodriguez, in a recent study (72), have likewise observed a wider range of filament diameters relative to the Ni particle size distribution in their activated Ni/SiO2 . While there is not a direct match between initial metal diameter and final filament width in our study, metal dispersion does
influence the final filament dimensions in that smaller initial metal particles ultimately yield a narrower overall filamentous product. Low-magnification TEM analysis (Figs. 6a and 7a) revealed that when compared with the carbon growth from Ni/SiO2 , a degree of filament curvature was introduced with the incorporation of alkali metal dopants. The nanofilaments generated from Ni/SiO2 (Fig. 6a) possess a hollow core that runs parallel to the filament axis. The occurrence of a central hollow core in the growing filament has been attributed to a deformation or faceting of the supported metal particle that alters the relative rate of carbon diffusion and filament nucleation (73, 74). Metal particle restructuring can lead to differences in the carbon diffusional path lengths which impact growth characteristics. A change to the Ni particle morphology, from rounded edges and an indistinct shape to well-defined geometrical entities, was also apparent after doping, a change intimately associated with stronger metal– support interaction (75, 76). Upon closer examination at high resolution, the graphite platelets associated with the parent Ni/SiO2 are on the whole aligned parallel to the filament axis; this feature is well-illustrated in the inset to Fig. 6b. In addition, a
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FIG. 7. Representative TEM images of carbon filaments grown at 773 K from (a) Ni/SiO2 –Li-II (inset: TEM detail showing Ni particle trapped in the center of a filament) and (b) Ni/SiO2 –Cs-I (inset: TEM detail showing multidirectional filament growth from one Ni particle).
lesser carbon component showed a lattice alignment at an angle to the filament axis, in a “herringbone” configuration. The carbon grown from Ni/SiO2 –Li-II (Fig. 7) showed an appreciable structural diversity where “straight” filaments similar to those grown from Ni/SiO2 are in evidence in addition to carbon that possesses an open helical structure. The growth of helical carbon structures has been attributed to modifications in the diffusion characteristics of the metal particle (77), usually achieved by the introduction of additives that cause a diffusional imbalance, resulting in a faster transport through one side of the particle. The presence of S and P has been shown to induce filamentous carbon structures possessing a coiled/helical structure (26, 27, 78). The latter was attributed to the adsorption of the adatoms at specific crystallographic faces at the exposed rear of the metal particle that altered the precipitation step in such a manner to create anisotropic growth. The incorporation of bulky volatile alkali dopants can be expected to have a significant impact on the diffusion characteristics of the supported Ni crystallites where CsBr doping (in particular), on the basis of TEM analysis, generated a predominantly curved/helical product. Park and Baker have reported a similar helical growth upon the addition of CO to
a C2 H4 /H2 feed over iron-rich Fe : Ni bimetallic catalysts (52). One interesting feature observed in this study was multidirectional growth of filamentous carbon from highly geometric particles in the CsBr-doped samples, as shown in the inset to Fig. 7b. Metal particles are known to adopt well-defined faceted entities during the catalytic step (66), features that are characteristic of the supporting substrate, reaction conditions, and particle composition (79). In general, these features are often not observed since the particle relaxes to adopt the more energetically favorable spherical form when the reactant gas is removed. However, when the particle is locked within a carbon filament, it can retain its geometrical morphology upon cooling and passivation. The growth of filamentous carbon from the parent Ni/SiO2 catalyst was both mono- and bi-directional where the particle was generally either rhombahedral or cubic in nature. Temperature-programmed oxidation (TPO) profiles generated for carbon deposited on selected catalysts are provided in Fig. 9A and can be assessed against the TPO profiles obtained from model amorphous and graphitic carbon, shown in Fig. 9B. The most noticeable feature is that the oxidation profiles associated with carbon grown from the doped samples are broader
GROWTH OF FILAMENTOUS CARBON FROM Ni/SiO2
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FIG. 8. Nickel particle size distribution (open bars) in the freshly activated catalysts and the filament diameter distributions (solid bars) associated with the carbon grown from the same catalysts at 773 K: (a) Ni/SiO2 ; (b) Ni/SiO2 –Li-II; (c) Ni/SiO2 –Cs-I.
than that recorded for the parent Ni/SiO2 growth, a feature that is diagnostic of a wider range of carbon structures, as borne out by the TEM results. All the TPO profiles for the alkali-doped catalysts possess two (albeit ill-defined) peaks, suggesting two stages of oxidation, one which coincides (in terms of the Tmax range) with the single TPO maximum that characterizes the carbon deposited on the parent catalyst. The second oxidation peak arises at a higher temperature, indicative of a more ordered, i.e., more graphitic, carbon. The relative intensity of this hightemperature peak (T > 1150 K) decreased in traversing down the
alkali group from Li to Cs. The high-temperature peak predominated in the Li-doped sample while the majority of the carbon growth from Ni/SiO2 –Cs-I was clearly of a less ordered nature. From repeated TEM analyses, it is clear that the occurrence of coiled/helical nanofilaments was more prevalent from Ni/SiO2 – Cs when compared with that from Ni/SiO2 –Li. The latter can be attributed to the greater volatility and size of the Cs adatom (80) that translates into a more substantial modification of the metal centers in terms of carbon precipitation/growth. Linking this effect with the TPO results suggests that more curved
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FIG. 9. (A) Temperature-programmed oxidation (TPO) profiles for the filamentous carbon grown from (a) Ni/SiO2 –Li-II, (b) Ni/SiO2 –K, (c) Ni/SiO2 –Cs-I, and (d) Ni/SiO2 at 773 K. (B) TPO profiles for model (e) graphite and (f ) amorphous carbon.
filamentous structures are less resistant to oxidation. The influence of the alkali metal loading on the nature of the deposited filamentous carbon is highlighted in Fig. 10 for the series of LiBr-doped samples. The presence of 0.1% w/w Li had no real effect on the carbon oxidation characteristics in terms of the overall temperature range wherein oxidation occurred. Increasing the Li loading to 0.4 and 1% w/w resulted in a significant elevation in the onset temperature of carbon gasification that can be related to a substantial increase in the order or graphitic nature. The TPO profiles for both these samples were noticeably broadened, suggesting a range of carbon filaments with different gasification properties that again ties in with the TEM results. A further increase in the Li loading to 6% w/w lead to a sharpening of the gasification peak with an associated Tmax that is significantly higher than that recorded for the parent Ni/SiO2 . Overall, doping with Li ≥ 0.4% w/w increased the graphitic nature of the deposited carbon at all the ethylene decomposition temperatures studied; where the reaction temperature exceeded 873 K, these characteristic features were still prominent but less pronounced.
A change in the graphitic nature of this filamentous carbon due to alkali bromide doping necessitates a concomitant change at the metal/deposited carbon interface. On the basis of the TPO studies, the presence of each of these dopants introduced some degree of order to the deposited carbon. The arrangement of Ni atoms at the face where the carbon is deposited ultimately regulates the nature of the precipitated carbon. If the atoms are arranged in such a manner that they are consistent with those of the basal plane structure of graphite, then the carbon that dissolves in and diffuses through the particle will be precipitated as an ordered structure. Conversely, if there is little or no match between the atomic arrangements of the depositing face and graphite, a more disordered carbon will be generated. The nature of the Ni/support interaction as it impacts surface Ni electronic structure is critical in determining carbon growth characteristics. Angermann and Horz (81) demonstrated that strong metal–support interaction favors the formation of highly ordered carbonaceous structures in that the wetting and spreading of the metal particle cause it to adopt a form compatible with the generation of ordered carbon, i.e., a complimentary
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GROWTH OF FILAMENTOUS CARBON FROM Ni/SiO2
halide are dispersed across the carbon filaments but we are unable at this time to confidently state whether these adatoms are on the surface or built into the carbon. CONCLUSIONS
FIG. 10. Temperature-programmed oxidation (TPO) profiles for the filamentous carbon grown from (a) Ni/SiO2 –Li-I, (b) Ni/SiO2 –Li-II, (c) Ni/SiO2 – Li-III, (d) Ni/SiO2 –Li-IV, and (e) Ni/SiO2 at 773 K.
atomic arrangement at the metal/carbon interface. Electron donation from the alkali metal can serve to enhance Ni/support interaction and so contribute to a structured carbon deposition. Moreover, Stevens et al. (40) have shown that the presence of Cs can promote the rearrangement of amorphous carbon into a graphitic product through electron donation, leading to carboanionic formation that facilitates polymerization. In a related study, Albers and co-workers have reported (12) an unexpected growth of carbon filaments on Pt/Al2 O3 and Pd/SiO2 catalysts used for prolonged periods to promote the synthesis of hydrogen cyanide and the vinyl chloride process, respectively. The generation of this ordered carbon was attributed to the presence of Fe and Cl impurities while the involvement of Na and K was also invoked. High levels of alkali metals and halides were found located inside the coke deposits. The authors concluded that these species were incorporated into the carbon structures during the deposition process and not at a later time as confirmed by XPS and oxidation studies. We have shown in a previous communication (41) that any alkali metal or halogen on the catalyst surface is intimately associated with the filamentous carbon grown from that surface. A preliminary elemental mapping of the carbon generated in this study has shown that the alkali metal and the
The introduction of alkali bromides to a standard impregnated Ni/SiO2 catalyst brought about significant changes in Ni particle size/dispersion and electronic structure, a feature highlighted by suppressed CO uptakes and additional low- and high-temperature CO desorption peaks. Passage of a C2 H4 /H2 reaction mixture over Ni/SiO2 in the temperature range 673– 873 K generated filamentous carbon with secondary hydrogenation/hydrogenolysis reactions. Doping the catalyst with alkali bromide inhibited the secondary reactions in every instance and a 0.4–0.5% w/w loading was either beneficial (Li) or detrimental (Na/K/Rb/Cs) to the degree of carbon deposition. At higher dopant concentrations, the Ni crystallites were masked by the mobile surface alkali, leading to an overall suppression of ethylene conversion/carbon deposition, an effect that was particularly prevalent in the case of CsBr. The optimum temperature in terms of carbon yield was dependent on the nature and loading of the dopant; a maximum carbon yield (125 gc g−1 Ni ) was recorded for a 0.4% w/w Li-doped sample where the Ni/Br mol ratio = 6.8. The residual Br content (or Ni/Br ratio) on the activated catalyst surface appears to be crucial in determining carbon yield where charge transfer to the electronegative Br must serve to render the adsorbed ethylene more susceptible to decomposition/carbon precipitation. Significant changes in the nature of the filamentous carbon were imposed by the introduction of the alkali bromide, i.e., the occurrence of helical structures in addition to an increase in the overall degree of nanofiber curvature. The latter was most pronounced with the addition of CsBr and is attributed to a coating/blocking of the supported Ni that disrupts the process of carbon diffusion through the metal particle. Doping with alkali metal (>0.1% w/w) resulted in an increase (to varying degrees) in the overall order of the solid carbon product. ACKNOWLEDGMENTS This work was supported by a grant (RSRG 20833) from the Royal Society. The authors are grateful to Dr. R. Brydson for his assistance with the TEM analysis.
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Growth of Filamentous Carbon from the Surface of Ni/SiO2 Doped with Alkali Metal Bromides Colin Park∗ and Mark A. Keane†,1 ∗ Department of Chemical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom; and †Department of Chemical and Materials Engineering, University of Kentucky, Lexington, Kentucky 40506-0046 Received October 22, 2001; accepted February 12, 2002; published online April 23, 2002
exploited in adsorption (13), catalysis (14–16), hydrogen storage (17–19), and electronics applications (20, 21). The work reported herein focuses on the application of alkali bromide doped Ni-supported catalysts to generate structured carbon filaments from a model C2 H4 /H2 feedstock. The use of both electropositive and electronegative promoters is widespread in catalysis. In catalytic reforming, for instance, the consensus of opinion is that the level of (unwanted) carbon deposition is directly related to the Cl content of the catalyst (22–24). Indeed, careful doping of metallic catalysts has been routinely used to alter both activity and selectivity, a result attributed in part to induced electronic perturbations in the metal. The influence of electronegative adatoms, S in particular, on the catalytic growth of carbonaceous materials has been studied with a view to altering the nature and characteristics of the deposited carbon (25–27). Chambers and Baker (28) deliberately introduced ppm levels of Cl2 to a C2 H4 /H2 reactant feed, over Fe and Co catalysts, in an attempt to gauge the influence of an electronegative additive on carbon production. The authors proposed that the adsorption of Cl2 leads to a reconstruction of the metal surface to one that enhanced hydrocarbon decomposition. Tibbetts and co-workers (29) surmised that low levels of S were sufficient to liquefy catalytic Fe particles, thereby enhancing carbon filament nucleation while high S concentrations inhibited carbon formation. Fan and co-workers (25), using thiophene as the S source, observed a critical dependence of carbon yield and nanofiber width on the S/C ratio. It is established (30–33) that alkali doping of supported metal catalysts results in an electron enrichment of the metal particles, which in turn modifies adsorption and reactivity. The direct synthesis of a carbonaceous material containing a known amount of alkali metal and/or halogen is highly desirable for many applications, notably in the development of effective nanoscale electronic devices and batteries (34, 35). Such a direct route would circumvent the problematic intercalation step that is generally used to introduce dopants into carbon materials but which suffers the decided drawback of a rapid exfoliation (36). An added advantage to such a direct synthesis is that the dopant is more likely to be in a stable form, built into and dispersed throughout the carbon structure, a feature that is critical in enhancing conductivity properties (37, 38). The use of an electropositive
The growth of ordered filamentous carbon, catalytically generated from the decomposition of ethylene, has been studied over the temperature range 673–898 K using an 11% w/w Ni/SiO2 catalyst doped to varying degrees (0.1–9.3% w/w) with a range of alkali metal bromides. The effect of these alkali metal/halogen adatoms in promoting/inhibiting carbon growth has been assessed and variations in the associated carbon structural characteristics have been examined. The introduction of Li consistently promoted filamentous carbon growth (where 723 K < T < 823 K) while the presence of Na, K, Rb, or Cs resulted in an equivalent or lower carbon yield. The degree of carbon deposition was strongly dependent on the nature and loading of the alkali metal, the Ni/Br ratio in the activated catalyst, and reaction temperature; conditions for optimum carbon growth are identified. The response of carbon yield and structural order to alkali bromide doping is discussed in terms of Ni particle electronic structure and metal/support interaction(s). High-resolution transmission electron microscopy (HRTEM) has been used to probe the filamentous carbon structure and the dispersion/morphology/size of the supported Ni crystallites. Highly curved and helical filaments predominated over the doped (particularly CsBr) samples and this is attributed to a disruption in carbon diffusion through the Ni particle caused by a spreading/coating of the particle by the alkali adatom. Temperature-programmed oxidation studies have highlighted the changes in the graphitic nature of the carbon due to catalyst doping; the results are consistent with the TEM analysis. C 2002 Elsevier Science (USA) Key Words: filamentous carbon; Ni/silica; alkali metal; bromine; ethylene decomposition.
INTRODUCTION
The controlled synthesis of carbonaceous materials that possess a well-defined and -ordered structure has gained considerable momentum in the last 2 decades with the discovery and characterization of buckminsterfullerene (1), multi-/singlewalled carbon nanotubes (2–9), graphitic nanofibers (10, 11), and related materials (3, 12). Such ordered carbon structures possess unique chemical and physical properties that can be
1 To whom correspondence should be addressed. Fax: (859) 323 1929. E-mail: [email protected].
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adatom (Cs) to induce ordering and reconstruction of amorphous carbon at room temperature has been reported by Stevens and co-workers (39, 40). We have reported (41) the growth of ordered filamentous carbon at 553 K from a modified Ni/SiO2 catalyst during the gas-phase hydrodechlorination of chlorobenzene. The “modification” resulted in the incorporation of an appreciable K and Br component onto the catalyst surface. Filamentous carbon growth was achieved at a temperature at least 150 K lower than that normally employed in conventional catalytic routes. We attributed this low-temperature carbon growth to a combination of an electron withdrawal/donation by the halogen/alkali metal on the catalyst surface that serves to favor the destructive chemisorption of chlorobenzene and subsequent precipitation of ordered carbon. We have extended this work to consider the growth of filamentous carbon from an ethylene feedstock and recently communicated (42) an appreciable enhancement in carbon growth from Ni/SiO2 doped with LiBr. In this paper, we consider the effects of varying the nature and loading of the alkali metal on the carbon yield and structural characteristics. EXPERIMENTAL
Catalyst Preparation, Activation, and Characterization The “parent” Ni/SiO2 catalyst was prepared by a standard incipient wetness technique where the silica support (Aldrich fumed silica, surface area >200 m2 g−1 ) was impregnated with a 2-butanolic solution of Ni(NO3 )2 to yield an 11% w/w Ni loading. The promoted catalysts were prepared by subsequently impregnating this parent Ni/SiO2 with methanolic solutions of the respective alkali metal bromides to realize the desired loading; each catalyst precursor was dried overnight at 383 K. The Ni and alkali metal contents were determined (to within ±2%) by atomic absorption spectrophotometry (VarianSpectra AA-10). The catalyst precursors were activated by heating at 10 K min−1 in 100 cm3 min−1 dry 20% v/v H2 /He to a final temperature of 873 K that was maintained for 2 h. The Br content of the activated doped catalysts was measured by heating a known weight of the sample under reflux for 16 h and analyzing the aqueous supernatant potentiometrically using a Metrohm 716 automatic titration unit (AgNO3 titrant, combined Ag electrode). Subsequent EDX elemental mapping of the refluxed solid did not reveal even trace quantities of Br on the surface. Carbon monoxide chemisorption was employed to characterize the supported Ni sites where the catalyst was cooled, following the reduction step, to 298 K in dry He and a fixed volume (10 µ1) of CO was pulsed into the He carrier gas stream; the concentration of CO exiting the reactor was measured using an on-line thermal conductivity detector (TCD). The injection of CO was repeated until the downstream peak area was constant, indicating surface saturation with CO. The catalyst was thoroughly flushed with dry He for 1 h to remove any physisorbed CO and then ramped (25 K min−1 ) to 1073 K with a continual monitoring of the CO content in the exiting gas. Data acquisition and analysis were
performed using the JCL 6000 data collection/manipulation package while the catalyst temperature was continuously monitored by means of a data logging system (Pico Technology, Model TC-08); the latter ensured an accurate measure of the desorption temperature. Upon completion of the CO TPD sequence, a series of calibration peaks were taken at ambient temperature to quantify CO uptake; reproducibility was better than ±3%. Catalytic Reactor System and Carbon Characterization All catalytic reactions were carried out (in situ directly after catalyst activation) under atmospheric pressure in a fixedbed continuous flow silica reactor over the temperature range 673 K ≤ T ≤ 873 K using a 4/1 v/v C2 H4 /H2 reactant mixture. The reaction temperature (±1 K) was monitored continuously by means of a thermocouple inserted in the catalyst bed. The catalytic measurements were made at a W/Q ratio = 2 × 10−5 g h cm−3 , where W represents the weight of activated catalyst and Q is the inlet volumetric feed rate of ethylene. The product stream was sampled at regular intervals by means of a heated gas-sampling valve and analyzed by on-line gas chromatography (Cambridge GC94 chromatograph) employing a DB-1 50 m × 0.20 mm i.d., 0.33-µm capillary column (J&W Scientific) in conjunction with the JCL6000 software. The reactor was cooled to ambient temperature and the catalyst/carbon was passivated in a 2% v/v O2 /He mixture before any weight changes to the catalyst were determined. Temperature-programmed oxidation (TPO) profiles of the carbon generated from the parent and doped Ni/SiO2 catalysts were obtained from throughly washed, demineralized samples to avoid any confusion arising from a catalyzed gasification of carbon by any residual Ni, Br, or alkali metal (43, 44). A known weight (ca. 100 mg) of the demineralized sample was heated from room temperature to 1203 K (at 25 K min−1 ) in a 5% v/v O2 /He mixture with on-line TCD analysis of the exhaust gas. The catalyst bed temperature was again independently monitored using the TC-08 data logger. These profiles were assessed against those generated for model carbon samples: graphite (Sigma-Aldrich, synthetic powder); amorphous carbon (Darco G-60, 100 mesh). The model carbons underwent the same demineralization process prior to oxidation to facilitate a direct comparison with the catalytically generated carbon. The structural characteristics of each catalyst and the carbon growth were probed by high-resolution transmission electron microscopy (HRTEM) using a Phillips CM200 FEGTEM microscope equipped with a UTW energy-dispersive x-ray (EDX) detector (Oxford Instruments) operated at an accelerating voltage of 200 kV. Suitable specimens for TEM were prepared by ultrasonic dispersion in 2-butanol where a drop of the resultant suspension was evaporated on a holey carbon support grid. The ultrasonic dispersion was limited to few seconds to avoid sample damage and ensure that the filaments were not detached from the substrate, a feature of extended ultrasonic treatment.
39
GROWTH OF FILAMENTOUS CARBON FROM Ni/SiO2
TABLE 1 Chemical Composition, CO Uptake/Desorption Temperatures (Tmax ), and the Surface Weighted Average Ni Particle Size Associated with the Activated Parent and Alkali Bromide Doped Ni/SiO2
Ni/SiO2 Ni/SiO2 –Li-I Ni/SiO2 –Li-II Ni/SiO2 –Li-III Ni/SiO2 –Li-IV Ni/SiO2 –Na Ni/SiO2 –K Ni/SiO2 –Rb Ni/SiO2 –Cs-I Ni/SiO2 –Cs-II a b
Ni w/w (%)
Alkali Metal w/w (%)
Ni/Br mole ratio
CO uptake (µmol g−1 Ni )
Tmax (K)
Average Ni particle sizea (nm)
11 9 9 8 8 9 10 10 11 8
— 0.1 0.4 1.0 6.0 0.5 0.5 0.4 0.5 9.3
— 25.3 6.8 2.4 7.0 16.1 15.9 53.7 35.2 46.2
105 87 74 56 11 79 66 62 59 74
663 383, 513, 1073 378, 508, 1073 518, 1073 525, 883, 1073 1073 348, 1073 371, 758, 1073 453, 791, 1073 553, 888, 1043
12.9 7.1 13.0 10.5 20.3 9.3 10.5 8.7 7.4 —b
Based on TEM analysis. Alkali metal layer obstructed analysis by TEM.
All gasses [He (99.99%), C2 H4 (99.95%), H2 (99.99%), and O2 /He (99.99%)] were supplied by BOC and dried before use by passage through activated molecular sieves. The alkali bromide dopants (>98%) were supplied by Sigma-Aldrich and used without any further purification. RESULTS AND DISCUSSION
The chemical composition of the activated parent and alkali bromide doped Ni/SiO2 catalysts and the pertinent CO chemisorption results are given in Table 1. The surface weighted average Ni particle sizes, based on TEM analysis of over 500 individual particles, are also included in Table 1. The CO uptake on the doped samples was significantly lower and, taking the Ni/SiO2 –Li system, declined further with increasing Li loading. This effect could be ascribed to an increase in the average Ni particle size (lower Ni dispersion) due to the formation of surface metal halide that is known to be particularly mobile and prone to sintering (45, 46). However, TEM analysis only revealed significant (average) Ni particle growth at high doping levels, i.e., Ni/SiO2 –Li-IV. Inconsistencies in the dimensions of supported metal particles derived from different analytical techniques continue to bedevil heterogeneous catalysis research (47). The observed decrease in CO uptake may be due to a spreading of the alkali metal over the Ni surface that serves to occlude the supported Ni crystallites from incoming CO sorbent. Such a masking by an alkali adlayer when introduced by impregnation has been noted elsewhere (48). Representative lowmagnification TEM images of selected freshly activated catalyst samples, illustrating the surface morphology/dispersion of the metal phase, are shown in Fig. 1. The TEM/EDX analyses of both freshly activated Rb and the Cs-doped catalysts revealed a thin and highly mobile layer of alkali metal covering sections of the surface, a feature particularly pronounced at higher alkali metal loadings (≥5% w/w). The presence of needle-like structures, with large lattice spacings (ca. 4 nm), was observed in the
case of the Cs-rich Ni/SiO2 –Cs-II sample and is shown in the TEM image presented in Fig. 2; we have identified the latter by EDX elemental mapping as a surface crystalline Cs phase. The Ni particle size histograms for the family of Li–Ni/SiO2 samples, shown in Fig. 3, are more informative and reveal a clear shift in size distribution to higher values for the Li-concentrated sample. Where the Li loading did not exceed 1% w/w, there was little variation in the average Ni particle diameters. Indeed, Arena and co-workers (33) only observed a measurable increase in the size of (19% w/w) Ni supported on MgO where the Li doping level exceeded 0.5% w/w and this was accompanied by a decrease in CO chemisorption capacity. Moreover, thermal desorption spectroscopy (TDS) has shown that the mode of CO adsorption and adsorption energetics are strongly influenced by the incorporation of alkali metals (49–51). Carbon monoxide desorption from the parent Ni/SiO2 catalyst occurred in a single step with a distinct temperature-related maximum (Tmax ) at 663 K. The occurrence of desorption peaks for the doped samples at a lower temperature than that associated with the parent Ni/SiO2 is diagnostic of weaker CO/Ni interaction(s) due to electron donation from the alkali metal, in keeping with conclusions drawn by Arena and co-workers (33). Desorption of CO at higher temperatures can then be ascribed to the presence of residual electronegative Br that serves to strengthen the Ni/CO bond. It is evident that the chemisorption/desorption behavior of Ni/SiO2 has been disrupted due to the incorporation of alkali bromide and this can be linked to changes in the electronic structure of the surface Ni sites. The effects of these modifications to the supported metal phase in terms of ethylene decomposition to solid carbon can be assessed from the data presented in Fig. 4: carbon yield is plotted as a function of temperature for the parent Ni/SiO2 and the doped samples that contained a roughly equivalent alkali metal content. In addition, the carbon yields from each doped catalyst (Ydoped ) relative to that generated from the parent Ni/SiO2 (Yparent ) at three selected temperatures are recorded in Table 2. It can be
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FIG. 1.
Representative low-magnification TEM images of freshly activated (a) Ni/SiO2 and (b) Ni/SiO2 –Li-II.
readily seen that, of all the alkali metal dopants, only the introduction of Li resulted in any significant increase in carbon production. The carbon yields that have been reported in the literature are highly dependent upon reaction conditions and the
FIG. 2. Low-magnification TEM image showing the crystalline needle-like structures associated with activated Ni/SiO2 –Cs-II.
FIG. 3. Nickel particle size distribution in the freshly activated Ni/SiO2 catalyst (downward hatched bars), Ni/SiO2 –Li-I (open bars), Ni/SiO2 –Li-II (crosshatched bars), Ni/SiO2 –Li-III (solid bars), and Ni/SiO2 –Li-IV (upward hatched bars).
GROWTH OF FILAMENTOUS CARBON FROM Ni/SiO2
41
FIG. 4. Carbon yield (expressed per gram of Ni) as a function of reaction temperature for the decomposition of ethylene over Ni/SiO2 (), Ni/SiO2 –Li–II (), Ni/SiO2 –Na (), Ni/SiO2 –K (), Ni/SiO2 –Rb (), and Ni/SiO2 –Cs-I ().
type of catalyst used. The use of unsupported mono- or bimetallic transition metal catalysts has been reported to lead to phenomenal yields, i.e., in excess of 200 gc g−1 metal (52). Avdeeva et al. (53) and Ermakova and co-workers (54) obtained carbon yields of up to 384 and 161 gc g−1 Ni from co-precipitated silica and alumina, respectively, over the temperature range 573–973 K. Nagy and co-workers (9, 55), in examining the growth of filamentous carbon from zeolite and silica substrates, recorded much lower yields (up to 65 gc g−1 metal ) from 2.5% w/w Co and Fe loaded samples. Similarly, Anderson and Rodriguez (56), using a series of silica-supported bimetallic Fe : Ni catalysts, reported a maximum yield of 21 gc g−1 metal 873 K from a CO/H2 reactant mixture while Ermakova et al. (57) recently reported a maxiTABLE 2 Ratio of Carbon Yield from the Doped Samples (Ydoped ) to That Generated from the Parent Ni/SiO2 (Yparent ) at Three Representative Temperatures Ydoped /Y parent
Ni/SiO2 Li–Ni/SiO2 -I Li–Ni/SiO2 -II Li–Ni/SiO2 -III Li–Ni/SiO2 -IV Na–Ni/SiO2 K–Ni/SiO2 Rb–Ni/SiO2 Cs–Ni/SiO2 -I Cs–Ni/SiO2 -II
723 K
773 K
873 K
1 1.2 2.0 1.5 1.0 1.2 0.6 0.9 0.9 0.1
1 1.1 1.7 1.3 1.7 0.8 0.6 0.6 0.7 0.1
1 0.6 0.8 1.1 0.9 0.5 0.8 0.4 0.3 0.1
mum value of 45 gc g−1 Fe for the decomposition of methane over Fe/SiO2 . The carbon yields generated in this study fall within the range of values cited above. The maximum yield (roughly twice that obtained from Ni/SiO2 ) was achieved from Ni/SiO2 – Li-II operating at 723 K. In the case of the other alkali bromide treated samples, with the exception of Ni/SiO2 –Na at 723 K, doping served to inhibit carbon formation to varying degrees. In the decomposition of ethylene to carbon, the principal competing reactions are hydrogenation to ethane and hydrogenolysis to methane. The presence of alkali metal dopants has been found elsewhere (30, 31) to suppress catalytic hydrogenation reactions. In this study, although the hydrogenation step was a secondary reaction over the parent Ni/SiO2 (selectivity <8%), there was still a discernible decrease in the conversion to ethane (selectivity <3%), over all the doped samples. The latter observation, allied to the substantial promotion of ethylene decomposition over the Li-based catalysts, supports the contention that the Ni crystallites underwent some reconstruction to an orientation that favors carbon production at the expense of hydrogenation. In the case of the other doped catalysts, a concomitant decrease was observed in carbon deposition and ethane hydrogenation activities. Hydrogenolysis to methane was only observed in the first 10 min of the reaction over the parent Ni/SiO2 catalyst (selectivity <3%) while no hydrogenolysis activity was noted for any of the doped catalysts. We have shown elsewhere (58) that doping Ni/SiO2 with alkali hydroxide had no effect on the overall carbon yield from a C2 H4 /H2 feed. Moreover, the hydroxide-doped samples were characterized by a similar average Ni particle size/size distribution to that associated with the parent catalyst. The appreciable promotion of carbon yield recorded here for Ni/SiO2 –Li-II
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suggests a critical involvement of both electropositive (Li) and electronegative (Br) adatoms. A reducion of the Ni d-electron density, induced by residual surface Br, should result in a stronger interaction with the olefin (28), weakening the C–C bond in the adsorbed ethylene and favoring decomposition. An enhanced decomposition of the hydrocarbon on the free metal surface generates carbon atoms with a concomitant desorption of molecular hydrogen. It must, however, be stressed that the level of carbon deposition was strongly dependent not only on the nature of the dopant(s) but also on the dopant loading and reaction temperature. Indeed, reaction temperature has been shown elsewhere to have a considerable impact on the growth of ordered filamentous carbon (57, 59–62). As LiBr doping produced the only appreciable promotion of carbon growth, the effect of Li loading (from 0.1 to 6% w/w) was considered in some depth and the results in terms of the temperature dependence of yield are presented in Fig. 5. An increase in Li content appears to have shifted the temperature for maximum carbon yield to higher values. Such a temperature-related maximum has been noted elsewhere (53, 60, 63) and the optimum temperature appears to be strongly dependent on the nature of the catalyst and the feedstock. The Li-dilute Ni/SiO2 –Li-I sample generated a roughly equivalent temperature profile to the parent Ni/SiO2 . In this case, the level of LiBr doping can be regarded as insufficient to induce any significant promotional effect. As a general observation, in those instances where the alkali bromide treatment did not result in any significant enhancement of carbon yield (see Table 2), the activated catalyst bore a higher Ni/Br mole ratio (see Table 1), i.e., lower relative residual Br content. The latter is a strong indicator of the crucial role played by the electronegative Br in promoting strong ethylene/surface interactions that must precede
dissociation/carbon deposition. The surface Ni/Br ratio accordingly emerges as a critical variable in determining the extent of carbon growth. While the mechanism for filamentous carbon growth is still open to question, the consensus of opinion is that the dissolution and diffusion of carbon through the metal particle is driven forward by a concentration gradient (64–66). The formation of NiC3 has been proposed as the nucleation species (67, 68) but the involvement of (metastable) metal carbide remains a supposition without any direct evidence for its formation. The movement of carbon atoms through the metal lattice necessitates a displacement of Ni atoms as the C atom diameter (0.142 nm) exceeds the available spacing (0.101 nm) (68). The growth of carbon filaments can be considered to result from a stable but fluid supersaturated solution of carbon in the metal. Doping with alkali bromide must induce a restructuring of the active centers to enhance the destructive chemisorption of ethylene and/or facilitate carbon diffusion to the metal/graphite interface. Any promotional effect is strongly dependent on dopant concentration, and at higher doping levels, the masking of surface Ni by the mobile alkali metal phase (particularly evident with Ni/SiO2 –Cs-II) must serve to inhibit filament production. The carbon generated in these studies is filamentous in nature, grown in a whisker-like mode where the active metal particle could be found at the growing tip/center of the filament; see Figs. 6 and 7. Zhang and Amiridis (69) likewise reported Ni particles on the tip of carbon filaments grown during methane decomposition over an impregnated Ni/SiO2 while Boellaard and co-workers also observed a detachment of metal particles from the support in their study of carbon monoxide decomposition in the presence of hydrogen at elevated temperatures (70). As the Ni interaction(s) with the silica support is(are)
FIG. 5. Carbon yield (expressed per gram of Ni) as a function of reaction temperature for the decomposition of ethylene over Ni/SiO2 (), Ni/SiO2 –Li-I (), Ni/SiO2 –Li-II (), Ni/SiO2 –Li-III (), and Ni/SiO2 –Li-IV ().
GROWTH OF FILAMENTOUS CARBON FROM Ni/SiO2
43
FIG. 6. Representative TEM images illustrating the nature of the filamentous carbon growth from the parent Ni/SiO2 catalyst: (a) low-magnification micrograph and (b) higher magnification image of a single (large diameter) nanofilament grown at 773 K. Inset: HRTEM image showing the lattice fringes of a section of an individual carbon nanofilament.
relatively weak in these impregnated samples (71), the pressure exerted on the metal/support interface due to graphite formation is of sufficient magnitude to extract Ni particles from the support. Once the metal particle is detached from the silica, a fresh surface is exposed to ethylene and growth continues with the Ni particle located on the filament tip. The dimensions of the seed Ni particle should determine the width of the catalytically generated carbon filaments. The distribution of filament diameters grown from the parent and representative Li- and Cs-doped samples is compared in Fig. 8 with the Ni particle size distribution in the freshly activated samples. In every instance the average carbon filament diameter exceeded that of the initial Ni particle diameter, e.g., Ni particle diameter in the freshly activated Ni/SiO2 –Cs-I sample (=7.4 nm) < the associated average carbon filament diameter (=19.5 nm). These results are indicative of a Ni particle growth/sintering during ethylene decomposition. Anderson and Rodriguez, in a recent study (72), have likewise observed a wider range of filament diameters relative to the Ni particle size distribution in their activated Ni/SiO2 . While there is not a direct match between initial metal diameter and final filament width in our study, metal dispersion does
influence the final filament dimensions in that smaller initial metal particles ultimately yield a narrower overall filamentous product. Low-magnification TEM analysis (Figs. 6a and 7a) revealed that when compared with the carbon growth from Ni/SiO2 , a degree of filament curvature was introduced with the incorporation of alkali metal dopants. The nanofilaments generated from Ni/SiO2 (Fig. 6a) possess a hollow core that runs parallel to the filament axis. The occurrence of a central hollow core in the growing filament has been attributed to a deformation or faceting of the supported metal particle that alters the relative rate of carbon diffusion and filament nucleation (73, 74). Metal particle restructuring can lead to differences in the carbon diffusional path lengths which impact growth characteristics. A change to the Ni particle morphology, from rounded edges and an indistinct shape to well-defined geometrical entities, was also apparent after doping, a change intimately associated with stronger metal– support interaction (75, 76). Upon closer examination at high resolution, the graphite platelets associated with the parent Ni/SiO2 are on the whole aligned parallel to the filament axis; this feature is well-illustrated in the inset to Fig. 6b. In addition, a
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FIG. 7. Representative TEM images of carbon filaments grown at 773 K from (a) Ni/SiO2 –Li-II (inset: TEM detail showing Ni particle trapped in the center of a filament) and (b) Ni/SiO2 –Cs-I (inset: TEM detail showing multidirectional filament growth from one Ni particle).
lesser carbon component showed a lattice alignment at an angle to the filament axis, in a “herringbone” configuration. The carbon grown from Ni/SiO2 –Li-II (Fig. 7) showed an appreciable structural diversity where “straight” filaments similar to those grown from Ni/SiO2 are in evidence in addition to carbon that possesses an open helical structure. The growth of helical carbon structures has been attributed to modifications in the diffusion characteristics of the metal particle (77), usually achieved by the introduction of additives that cause a diffusional imbalance, resulting in a faster transport through one side of the particle. The presence of S and P has been shown to induce filamentous carbon structures possessing a coiled/helical structure (26, 27, 78). The latter was attributed to the adsorption of the adatoms at specific crystallographic faces at the exposed rear of the metal particle that altered the precipitation step in such a manner to create anisotropic growth. The incorporation of bulky volatile alkali dopants can be expected to have a significant impact on the diffusion characteristics of the supported Ni crystallites where CsBr doping (in particular), on the basis of TEM analysis, generated a predominantly curved/helical product. Park and Baker have reported a similar helical growth upon the addition of CO to
a C2 H4 /H2 feed over iron-rich Fe : Ni bimetallic catalysts (52). One interesting feature observed in this study was multidirectional growth of filamentous carbon from highly geometric particles in the CsBr-doped samples, as shown in the inset to Fig. 7b. Metal particles are known to adopt well-defined faceted entities during the catalytic step (66), features that are characteristic of the supporting substrate, reaction conditions, and particle composition (79). In general, these features are often not observed since the particle relaxes to adopt the more energetically favorable spherical form when the reactant gas is removed. However, when the particle is locked within a carbon filament, it can retain its geometrical morphology upon cooling and passivation. The growth of filamentous carbon from the parent Ni/SiO2 catalyst was both mono- and bi-directional where the particle was generally either rhombahedral or cubic in nature. Temperature-programmed oxidation (TPO) profiles generated for carbon deposited on selected catalysts are provided in Fig. 9A and can be assessed against the TPO profiles obtained from model amorphous and graphitic carbon, shown in Fig. 9B. The most noticeable feature is that the oxidation profiles associated with carbon grown from the doped samples are broader
GROWTH OF FILAMENTOUS CARBON FROM Ni/SiO2
45
FIG. 8. Nickel particle size distribution (open bars) in the freshly activated catalysts and the filament diameter distributions (solid bars) associated with the carbon grown from the same catalysts at 773 K: (a) Ni/SiO2 ; (b) Ni/SiO2 –Li-II; (c) Ni/SiO2 –Cs-I.
than that recorded for the parent Ni/SiO2 growth, a feature that is diagnostic of a wider range of carbon structures, as borne out by the TEM results. All the TPO profiles for the alkali-doped catalysts possess two (albeit ill-defined) peaks, suggesting two stages of oxidation, one which coincides (in terms of the Tmax range) with the single TPO maximum that characterizes the carbon deposited on the parent catalyst. The second oxidation peak arises at a higher temperature, indicative of a more ordered, i.e., more graphitic, carbon. The relative intensity of this hightemperature peak (T > 1150 K) decreased in traversing down the
alkali group from Li to Cs. The high-temperature peak predominated in the Li-doped sample while the majority of the carbon growth from Ni/SiO2 –Cs-I was clearly of a less ordered nature. From repeated TEM analyses, it is clear that the occurrence of coiled/helical nanofilaments was more prevalent from Ni/SiO2 – Cs when compared with that from Ni/SiO2 –Li. The latter can be attributed to the greater volatility and size of the Cs adatom (80) that translates into a more substantial modification of the metal centers in terms of carbon precipitation/growth. Linking this effect with the TPO results suggests that more curved
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FIG. 9. (A) Temperature-programmed oxidation (TPO) profiles for the filamentous carbon grown from (a) Ni/SiO2 –Li-II, (b) Ni/SiO2 –K, (c) Ni/SiO2 –Cs-I, and (d) Ni/SiO2 at 773 K. (B) TPO profiles for model (e) graphite and (f ) amorphous carbon.
filamentous structures are less resistant to oxidation. The influence of the alkali metal loading on the nature of the deposited filamentous carbon is highlighted in Fig. 10 for the series of LiBr-doped samples. The presence of 0.1% w/w Li had no real effect on the carbon oxidation characteristics in terms of the overall temperature range wherein oxidation occurred. Increasing the Li loading to 0.4 and 1% w/w resulted in a significant elevation in the onset temperature of carbon gasification that can be related to a substantial increase in the order or graphitic nature. The TPO profiles for both these samples were noticeably broadened, suggesting a range of carbon filaments with different gasification properties that again ties in with the TEM results. A further increase in the Li loading to 6% w/w lead to a sharpening of the gasification peak with an associated Tmax that is significantly higher than that recorded for the parent Ni/SiO2 . Overall, doping with Li ≥ 0.4% w/w increased the graphitic nature of the deposited carbon at all the ethylene decomposition temperatures studied; where the reaction temperature exceeded 873 K, these characteristic features were still prominent but less pronounced.
A change in the graphitic nature of this filamentous carbon due to alkali bromide doping necessitates a concomitant change at the metal/deposited carbon interface. On the basis of the TPO studies, the presence of each of these dopants introduced some degree of order to the deposited carbon. The arrangement of Ni atoms at the face where the carbon is deposited ultimately regulates the nature of the precipitated carbon. If the atoms are arranged in such a manner that they are consistent with those of the basal plane structure of graphite, then the carbon that dissolves in and diffuses through the particle will be precipitated as an ordered structure. Conversely, if there is little or no match between the atomic arrangements of the depositing face and graphite, a more disordered carbon will be generated. The nature of the Ni/support interaction as it impacts surface Ni electronic structure is critical in determining carbon growth characteristics. Angermann and Horz (81) demonstrated that strong metal–support interaction favors the formation of highly ordered carbonaceous structures in that the wetting and spreading of the metal particle cause it to adopt a form compatible with the generation of ordered carbon, i.e., a complimentary
47
GROWTH OF FILAMENTOUS CARBON FROM Ni/SiO2
halide are dispersed across the carbon filaments but we are unable at this time to confidently state whether these adatoms are on the surface or built into the carbon. CONCLUSIONS
FIG. 10. Temperature-programmed oxidation (TPO) profiles for the filamentous carbon grown from (a) Ni/SiO2 –Li-I, (b) Ni/SiO2 –Li-II, (c) Ni/SiO2 – Li-III, (d) Ni/SiO2 –Li-IV, and (e) Ni/SiO2 at 773 K.
atomic arrangement at the metal/carbon interface. Electron donation from the alkali metal can serve to enhance Ni/support interaction and so contribute to a structured carbon deposition. Moreover, Stevens et al. (40) have shown that the presence of Cs can promote the rearrangement of amorphous carbon into a graphitic product through electron donation, leading to carboanionic formation that facilitates polymerization. In a related study, Albers and co-workers have reported (12) an unexpected growth of carbon filaments on Pt/Al2 O3 and Pd/SiO2 catalysts used for prolonged periods to promote the synthesis of hydrogen cyanide and the vinyl chloride process, respectively. The generation of this ordered carbon was attributed to the presence of Fe and Cl impurities while the involvement of Na and K was also invoked. High levels of alkali metals and halides were found located inside the coke deposits. The authors concluded that these species were incorporated into the carbon structures during the deposition process and not at a later time as confirmed by XPS and oxidation studies. We have shown in a previous communication (41) that any alkali metal or halogen on the catalyst surface is intimately associated with the filamentous carbon grown from that surface. A preliminary elemental mapping of the carbon generated in this study has shown that the alkali metal and the
The introduction of alkali bromides to a standard impregnated Ni/SiO2 catalyst brought about significant changes in Ni particle size/dispersion and electronic structure, a feature highlighted by suppressed CO uptakes and additional low- and high-temperature CO desorption peaks. Passage of a C2 H4 /H2 reaction mixture over Ni/SiO2 in the temperature range 673– 873 K generated filamentous carbon with secondary hydrogenation/hydrogenolysis reactions. Doping the catalyst with alkali bromide inhibited the secondary reactions in every instance and a 0.4–0.5% w/w loading was either beneficial (Li) or detrimental (Na/K/Rb/Cs) to the degree of carbon deposition. At higher dopant concentrations, the Ni crystallites were masked by the mobile surface alkali, leading to an overall suppression of ethylene conversion/carbon deposition, an effect that was particularly prevalent in the case of CsBr. The optimum temperature in terms of carbon yield was dependent on the nature and loading of the dopant; a maximum carbon yield (125 gc g−1 Ni ) was recorded for a 0.4% w/w Li-doped sample where the Ni/Br mol ratio = 6.8. The residual Br content (or Ni/Br ratio) on the activated catalyst surface appears to be crucial in determining carbon yield where charge transfer to the electronegative Br must serve to render the adsorbed ethylene more susceptible to decomposition/carbon precipitation. Significant changes in the nature of the filamentous carbon were imposed by the introduction of the alkali bromide, i.e., the occurrence of helical structures in addition to an increase in the overall degree of nanofiber curvature. The latter was most pronounced with the addition of CsBr and is attributed to a coating/blocking of the supported Ni that disrupts the process of carbon diffusion through the metal particle. Doping with alkali metal (>0.1% w/w) resulted in an increase (to varying degrees) in the overall order of the solid carbon product. ACKNOWLEDGMENTS This work was supported by a grant (RSRG 20833) from the Royal Society. The authors are grateful to Dr. R. Brydson for his assistance with the TEM analysis.
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