JP5187844B2 - CO oxidation catalyst - Google Patents

Description

The present invention relates to a catalyst that oxidizes CO to CO ₂ , and more particularly relates to a catalyst that exhibits catalytic activity at a temperature of 250 ° C. or higher.

As shown in the following patent documents, the conventional CO oxidation catalyst mainly uses expensive noble metals such as Pt and Au, and rare elements such as rare earths.
The use of these precious metals has been one of the factors that make it difficult to recover CO ₂ by CO oxidation and hinders activities to prevent global warming.
JP-A-2005-230616 JP 2001-220103 A JP 2002-058924 A

  In view of such circumstances, the present invention is characterized by providing a catalyst that expresses a CO oxidation function using Ni and Al that are generally easily obtained without using noble metals or rare metals.

  The catalyst of the invention 1 is characterized by comprising Ni-Al intermetallic compound nanoparticles.

Invention 2 is characterized in that in the catalyst of Invention 1, the intermetallic compound has at least one of Ni ₃ Al, NiAl, and Ni phases.

Invention 3 is characterized in that, in the catalyst of Invention 1 or 2, the nanoparticle has a BET specific surface area of 50 m ² / g or more.

Invention 4 is characterized in that, in the catalyst according to any one of Inventions 1 to 3, a NiAl alloy ingot is nanoparticulated by a vacuum arc plasma deposition method.

The catalyst of the present invention showed high catalytic activity for the oxidation reaction of carbon monoxide (CO), although it contained no precious metal or rare element.
Furthermore, since it is stable in the air, it is easier to handle than conventional Raney Ni catalysts.

A preferable range of the particle size of the intermetallic compound nanoparticles is 1 nm to 100 nm. This is because particles having a size within that range can exist stably and have a high specific surface area.
The preferred range of the Ni-Al weight ratio is 76-95 wt% for Ni and 5-24 wt% for Al. In this range, there is an intermetallic compound phase of Ni ₃ Al and NiAl according to the Ni—Al binary phase diagram.
The phase structure of the intermetallic compound nanoparticles need not have all of the Ni ₃ Al, NiAl, Ni, NiO, and Al ₂ O ₃ phases, and is one of Ni ₃ Al, NiAl, Ni having catalytic activity. There may be more than one phase.

Production of Ni-Al Intermetallic Compound Nanoparticles An alloy ingot having the following composition was produced using Ni (nickel) and Al (aluminum) in an arc melting furnace.


Using the above-described NiAl alloy ingot, nanoparticle samples of each composition were prepared as shown in Table 2 by vacuum arc plasma deposition. When the composition of the phases of these nanoparticle samples was confirmed by powder X-ray diffraction measurement, as shown in FIG. 1, these nanoparticle samples contained Ni ₃ Al, NiAl, Ni, NiO and Al ₂ O ₃ phases. It was the main phase.
In addition, the abbreviation in a figure shows the abbreviation of the alloy used as a raw material.

The overall particle size distribution was measured by laser diffraction / scattering method using a microtrack particle size distribution analyzer. No. in Table 2 No. 2 nanoparticles range from 1 nm to 600 nm. 5 nanoparticles were found to range from 1 nm to 800 nm. No. 1 in Table 2 prepared by a transmission electron microscope (TEM) and a scanning transmission electron microscope (STEM). The size, shape and composition of the two nanoparticles were analyzed. It was found that the particle size was distributed mainly in the range from 1 nm to 100 nm (FIG. 2).
The specific surface area was measured by nitrogen gas adsorption. The specific surface area (BET method) of these nanoparticle samples was found to be 50 to 112 m ² / g (Table 2). This is a large specific surface area (50-100 m ² / g) comparable to conventional Raney Ni catalysts.
Furthermore, this nanoparticle catalyst has a great merit that it is more stable than Raney Ni. Raney Ni catalyst oxidizes and burns strongly in the air, so it must be stored in a liquid such as water. Therefore, it can be mainly applied only to liquid reactions. On the other hand, the present nanoparticle catalyst is stable even in air and does not burn, so it is easy to handle and can be applied to high temperature gas reactions.

The catalytic properties of the nanoparticles were measured for the following CO oxidation reaction.
2CO + O ₂ → 2CO ₂
Reactor system The catalytic reaction was carried out by a fixed bed flow type catalytic reactor. A nanoparticle sample of about 5 to 20 mg is introduced into a quartz reaction tube having an inner diameter of 8 mm, and a sample layer is fixed by filling quartz wool with a thickness of about 10 mm above and below the sample layer. The reaction tube was heated by an electric furnace to carry out a catalytic reaction at a predetermined temperature. Temperature control was performed by a thermocouple in contact with the sample layer. The upper part of the reaction tube was connected to a mixed gas line of CO + O ₂ + He. A gas chromatograph and a gas flow meter were connected to the lower part of the reaction tube, and the composition and amount of the reaction product were measured. FIG. 3 shows the reactor system.

Catalytic activity in the oxidation reaction of CO The oxidation reaction of CO was performed using nanoparticle samples of compositions Ni22.4Al, Ni25Al and Ni27Al, respectively. A mixed gas of CO, O ₂ and He (volume ratio CO: O ₂ : He = 1: 2: 97) was introduced into the reaction tube at a flow rate of 100 ml / min, and gas chromatography was performed in a temperature range from room temperature to 575 ° C. Was used to measure the concentrations of CO, O ₂ , CO ₂ , and He gas components at the outlet of the reaction tube. For comparison, the same catalytic reaction was carried out using a commercially available pure Pt powder sample (Furuya Metal Co., Ltd.) and a Ni-25 atomic% Al powder sample (manufactured by High Purity Chemical Co., Ltd.) prepared by the rotary atomization method. It was. In the case of Pt powder, 0.15 g of powder having a particle size of 150 μm or less collected by a sieve was introduced into the reaction tube. In the case of atomized Ni ₃ Al powder, 0.40 g of powder having a particle size of 32 to 75 μm obtained by sieving was introduced into the reaction tube. From the measured concentrations of CO and CO ₂ at each temperature, the CO conversion rate was determined by the following equation.
CO conversion (%) = CO ₂ concentration / (CO concentration + CO ₂ concentration) × 10 ²

Table 3 shows the calculated CO conversion at each temperature.
FIG. 4 graphically illustrates these CO conversions as a function of reaction temperature. Ni22.4Al, Ni25Al, and Ni27Al started to show activity at 250 ° C., and the conversion increased with increasing temperature, and a CO conversion of about 100% was obtained at 400 ° C.
The catalytic activity of these Ni—Al nanoparticles is almost the same as that of pure Pt powder (shown in FIG. 4).
On the other hand, Ni25Al produced by atomization started to show activity at 375 ° C., and the CO conversion increased with increasing temperature, but only 86% was obtained even at a high temperature of 575 ° C. Therefore, it shows that the nano NiAl particle | grains of this technique have higher activity than the NiAl powder produced by the atomization method. Furthermore, from this result, it was found that a nanoparticle catalyst composed of Ni and Al not containing a noble metal element exhibits an activity similar to that of pure Pt powder in the CO oxidation reaction.

The graph which shows the powder X-ray-diffraction measurement result of a NiAl nanoparticle sample. The photograph which shows the TEM observation result of a NiAl nanoparticle. Schematic which shows a catalyst reaction apparatus system. The graph which showed the conversion of CO of each sample of Table 3 as a function of reaction temperature.

Claims (4)

A catalyst that oxidizes CO to CO ₂ and comprises nanoparticles of a Ni-Al intermetallic compound. The catalyst according to claim 1, wherein the intermetallic compound has at least one phase of Ni ₃ Al, NiAl, and Ni phase. The catalyst according to claim 1 or 2, wherein the BET specific surface area of the nanoparticles is 50 m ² / g or more.   The catalyst according to any one of claims 1 to 3, wherein the NiAl alloy ingot is nanoparticulated by a vacuum arc plasma deposition method.