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 仪器的设计特征对应用的影响

摘要

化学吸附是一种高度专业的分析方法，因此，需要高度专业的设备，与表面积和孔隙率析仪不同，化学吸附器涉及到化学反应，由于这些反应可能使用具有腐蚀性的气体，因此对仪器本身就存在着严格的要求。本文针对一些可能影响化学吸附仪使用的仪器的设计特征进行了一系列的讨论。便于您更能了解你的设备，能够更大程度的利用仪器性能。

Application  Note—58

 

Design  Characteristics  Impact  A Applications

Instrument Type: √     2900,2000,2010

Technique: √   Chemisorption

Chemisorption is a highly specialized branch of materials analysis,and thus requires highly specialized equipment.

Unlike surface area analyzers and porosimeters,chemisorptions

analyzers must examine chemical reactions.Since these

reactions may involve corrosive gases,harsh demands are

placed on the analyzers itself.Perhaps the following

discussion will help clarify these and other considerations

that impact the utility of chemisorptions analyzers.

Stainless Steel Tubing

The type of tubing used in chemisorptions analyzers has a direct

impact on the type of analysis that can be performed.

Stainless steel tubing,for example,is impervious to the corrosive gases,(namely H2S,NH3,SO2,and NO).This allows the

Scientists to carry out many types of analyses including

Determination of active site density,acid sites by ammonia

titration,catalyst regeneration.

Copper tubing has a different impact on the scientists work.

Copper will corrode when using the corrosive gases.Ultimately,

the corrosion process interferes with the analysis in that

the data will incorporate some unspecified amout of chemisored

species from the copper tubing.This shows up in the graphs

as delays and distorted peaks.Under these circumstances,the

types of analysis that can be performed are severely limited.

Degassing In Situ

Most chemisorptions studies are especially sensitive to air

contamination .Even a small amount  of oxygen,like a few 

parts per million,will reoxidize a reduced (degassed) sample

and cause false data.This occurs if the preparation and

analysis ports are separated.Simply moving the sample tube

from the analysis port introduces some amout of air and reoxidizes

the sample just prior to analysis.

Some analyzers use a one-port design allowing sample

preparation to occur in situ.By turning a valve,the

preparation port is converted into an analysis port,

thus preventing air contamination.This meanscritical chemisorption

studies requiring thoroughly degassed samples can be accurately

performed.

Calibration

Chemsorption on analyzers can be calibrated by either a manual syringe

Or by a built-in calibration loop.The significance of these methods

Lies in the degree to which they contribute convenience and accuracy.

For instance,calibrating by syringe injection is simple enough to

perform,but hazardous  when using noxious gases like CO and NO .The

operator either siphons the gas from an air-borne stream or uses a

septum<197>both methods are potentially dangerous.Taking precautions

against exposure to the gas add inconvenience to the hand-to-eye

coordination of the operator.

The built-in calibration loop solves the problems associated with

Syringe calibration.Use of noxious gases is not restrictive since

the calibration involves a closed loop.Gas cannot escape into

the air.Furthermore,this method supports chemisorptions studies

that require exact calibrations each and every time as is necessary

for most R&D applications.

Thermostatically  Controlled

Thermal Conductivity Detectors(TCDs) are designed to respond to slight

changes in the concentration of a given gas mixture.The degree of change

will reflect the amount of gas adsorbed by the sample.But,the TCDs are

so sensitive to temperature changes that they will also respond to

changes in ambient temperature fluctuations can interfere with the

analysis.

A change in the room temperature during a working day results in

a temperature-induced drift;that is,the baseline shifts downward

or upward.Data generated from an offset baseline is not useful.

-chemisorption analyzers designed with thermostatically controlled

TCDs maintain a scientist can focus on the analysis instead of the

Baseline.