Understanding the “Activation” of the Nanocluster [HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)98-y(EtOH)y] for Low Temperature Growth of Carbon Nanotubes

Abstract

(FeMoC), was the first molecular catalyst precursor (pro-catalyst) that promised controlled growth of carbon nanotubes (CNTs); however, temperatures in excess of * 900°C or the addition of excess iron were required as catalyst promoters for CNT growth. To understand these disappointing results the ''activation'' of FeMoC for CNT growth was studied by systematic investigation of H 2 gas concentration and growth temperature. The pathway for ''activation'' of FeMoC occurs through the sufficient reduction of both metal oxide components in the pro-catalyst. By ensuring pro-catalyst reduction prior to introduction of growth gases, we demonstrate for the first time, growth of CNTs at temperatures as low as 600°C without the use of catalyst promoters using the single molecular precursor, FeMoC. To understand the role of catalyst promoters used in prior work, thermogravimetric analysis experiments were performed. The addition of an iron catalyst promoter is observed to play two key roles in the ''activation'' of FeMoC: (1) to replenish sublimated metal atoms, and (2) to reduce the reduction temperature required for reduction of FeMoC into an ''active'' catalyst. These results caution the conditions employed in many earlier studies for CNT growth, and create new possibilities for molecular pro-catalysts.

Catalytic Growth of Carbon Nanotubes by Direct Liquid Injection CVD Using the Nanocluster [HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)98-y(EtOH)y]

“…The increase in growth temperature appears to provide a more favorable growth environment for carbon nanotubes. This transition in CNT growth at temperatures >900 • C agrees with prior work using FeMoC [16,17,23,24,44], suggesting that FeMoC has a temperature dependent activation mechanism for CNT growth [26]. Product yield for the growth temperature experiments is shown to increase when the growth temperature is increased from 700 • C to 900 • C. This is followed by a decrease in product yield when increasing the growth temperature to 1000 • C. This growth temperature relationship with product yield likely arises from the temperature dependent behavior of the kinetics of hydrocarbon decomposition.…”

“…This transition in CNT growth at temperatures >900 °C agrees with prior work using FeMoC [16,17,23,24,44], suggesting that FeMoC has a temperature dependent activation mechanism for CNT growth [26]. Product yield for the growth temperature experiments is shown to increase when the growth temperature is increased from 700 °C to 900 °C.…”

“…When considering that both Mo and Fe have demonstrated effective catalytic performance both independently and as mixed heterogenous nanoparticles [19][20][21][22], FeMoC should be catalytically active, but under the growth conditions reported in prior work it is not [16,17,23,24]. We have recently shown that this is in part due to both incomplete catalyst activation and the sublimation of a portion of the Mo component during the slow catalyst activation process [25,26]. However, the growth conditions (temperature, gas composition, carbon source, and catalyst concentration) also play role in the catalytic activity [27][28][29][30].…”

Investigation of the Reduction of a Molybdenum/Iron Molecular Nanocluster Single Source Precursor

“…The product was refluxed in EtOH for 12 h and a dark green solution was collected. [20,24,25]. In brief, 1.00 g (5.03 mmol) of iron (II) chloride tetrahydrate was dissolved in 75 mL of Millipore water in a 500 mL round-bottom flask containing a stir rod.…”

“…We have previously employed the large molecular cluster, [HxPMo12O40H4Mo72Fe30(O2CMe)15O254(H2O)98−y(EtOH)y] (1) [20] (Figure 1), as a catalyst precursor for the growth of carbon nanotubes (CNTs) [21][22][23][24][25]. During our study, we demonstrated that the composition of the catalyst, and the subsequent ability to grow CNTs, was highly dependent on the activation step, which involved reduction at elevated temperatures [24]. This, in turn, led to a detailed study of the kinetics of the reduction from 1 to the active catalyst species [26].…”