Elsevier

Applied Catalysis A: General

Volumes 439–440, 10 October 2012, Pages 80-87
Applied Catalysis A: General

Optimization of bimetallic dry reforming catalysts by temperature programmed reaction

https://doi.org/10.1016/j.apcata.2012.06.041Get rights and content

Abstract

Catalytic reforming of CH4 with CO2 was investigated on mono and bimetallic Pd(or Pt)-Ni based samples supported on Al2O3 and ZrO2. A simple temperature-programmed procedure was used to study both reforming and coke formation via CH4 or CO decomposition. Catalysts supported on zirconia are more active than alumina-supported samples. The addition of Pt or Pd to Ni/ZrO2, preferably by co-impregnation, prevents coke formation. The co-impregnated Ni-Pd/ZrO2 catalyst looks the most promising for a possible industrial application.

Highlights

► A simple temperature-programmed procedure was used. ► Both CH4/CO2 reforming and coke formation via CH4 or CO decomposition were studied. ► Bimetallic Pd(or Pt)-Ni catalysts on zirconia and alumina were investigated. ► The co-impregnated Ni-Pd/ZrO2 catalyst looks the most promising for an industrial application.

Introduction

The catalytic reforming of CH4 with CO2, rather than steam, for the production of synthesis gas (CO + H2) has attracted a considerable interest in the past 20 years for both environmental and commercial reasons. In fact it reduces CO2 and CH4 emissions, which are greenhouse gases, and it yields, in the product gas, a lower H2/CO ratio (H2:CO:1:1 or less) compared to steam reforming (H2:CO:3:1). This synthesis gas with a high CO concentration is a suitable feed for Fischer–Tropsch plants and for the synthesis of acetic acid, dimethyl ether and oxoalcohols [1]. An additional advantage of dry reforming is in those cases in which the reactants are simultaneously available at low cost, or even at negative prices [2]. The major problem preventing commercialization of the CO2 reforming is finding a suitable catalyst that will not deactivate under the conditions needed for this reaction. In fact, due to the endothermic nature of the process, high temperatures (∼1100 K) are required to reach high conversions. Besides this process works in presence of high concentrations of carbon-containing compounds. These conditions lead to quick deactivation of the catalyst due to carbon deposition, which originates mainly from two reactions, i.e., methane decomposition (CH4  C + 2H2) and carbon monoxide disproportionation (2CO  C + CO2). Although in literature the authors agree that carbon formation is the primary reason for the catalyst deactivation, disagreement exists on the source of the carbon. In fact, in principle, all three carbon-containing species present in the reforming reaction (CH4, CO and CO2) can contribute to carbon deposition. Some authors [3], [4] reported that CH4 decomposition is the main source of carbon, others showed that carbon originates mainly from CO [5], [6].

To solve the problem of coking the following points were investigated: (i) the increase of the H/C content of the feed through the addition of water (coupling with steam reforming); (ii) the increase of the O/C content of the feed through the addition of oxygen (coupling with partial oxidation); (iii) the use of a catalyst which minimize the rate of coking.

Typically, supported Ni or noble metals are reported as potential catalysts for the CH4/CO2 reforming. It has been shown [7] that Pt-based catalysts have high activity in the dry reforming reaction and are less sensitive to carbon deposition compared to Ni-based catalysts, but the latter is the metal most widely studied because of its high activity and low price, pointing to possible industrial application. The addition of alkali or alkali earth dopants to nickel [8], [9], or the use of supports with basic characteristics [10], [11] were reported as methods to minimize carbon deposition. Some authors [12], [13], [14], [15], [16], [17], [18], [19], [20] have investigated the effects of bimetallic samples in the dry reforming of methane. A very recent review has summarized some examples of the catalysts for methane steam reforming and the additive effect of noble metals [21]. With regard to dry reforming, Ni-Ru and Ni-Pd on different supports [12], [17], [19], [20], Ni-Cu/Al2O3 [15], Ni-Rh on alumina [13], [16], on silica [14], on zirconia [18], have shown to enhance the catalytic performances of the monometallic Ni sample. Bimetallic Ni-Pt samples on nanofibrous alumina [22], [23] and ZSM5 [24] have been lately investigated.

In the present work we have studied the promotion of Ni by a small amount of platinum or palladium in order to improve the stability of Ni-based samples. Ni-Pt or Ni-Pd bimetallic catalysts were supported on both alumina and zirconia, which are the most used and suitable supports for these catalysts. We have investigated the complex system of reactions by temperature programmed (TP) tests in order to shorten the experimental work, as required by the industry.

So, a simple TP procedure was developed for both reforming and coking reactions after temperature programmed reduction (TPR) of the catalysts. In particular, with the aim of studying carbon deposition, all catalysts were tested in coking reaction either via CH4 or CO temperature programmed decomposition (TPD). Then temperature programmed oxidations (TPO) were performed in order to characterize the reactivity of the coke. The set of TPR–TP–TPD–TPO provides an easy and fast way to obtain helpful information on both catalytic activity and reaction mechanism.

Section snippets

Catalyst preparation

Zirconia support (Z) was prepared by precipitation from ZrOCl2·8H2O (Fluka) at constant pH (pH 10), aged under reflux conditions [25], [26], washed free from chloride (AgNO3 test), dried at 383 K overnight and finally calcined at 773 K in flowing air (30 mL/min STP). Al2O3 (Alu D-Akzo) was used as received (A). Supports were impregnated by incipient wetness with Ni(NO3)26H2O or H2PtCl6 or H2PdCl4 aqueous solutions to give a nominal 5 wt% Ni or 0.5 wt% Pt (or Pd) loaded catalyst. Bimetallic 5 wt%

Characterization of the catalysts

Table 1 reports the metal loadings of the catalysts investigated, together with the mean metal particle sizes calculated by chemisorptions analyses. After metal deposition and calcination, a TPR analysis was performed to investigate the metal oxidation state and in the case of bimetallic samples to identify possible interactions between platinum (palladium) and nickel.

Fig. 1(a) shows the TPR analyses of alumina supported samples. The TPR profile (not shown) of alumina support (A) is completely

Conclusions

The advantages of using a temperature programmed set of reactions, i.e. reduction, CH4/CO2 reforming, CH4 and CO decomposition, oxidation of the formed coke, were successfully proved for fast development of dry reforming catalysts.

The results presented in this work point out that the support plays a decisive role on the activity of the catalyst. Zirconia supported catalysts are more active than the alumina supported ones. Temperature-programmed decomposition tests show the formation of coke on

Acknowledgement

We thank Mrs. Tania Fantinel for technical assistance.

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